WO2002071562A9 - Quantum dot vertical cavity surface emitting laser - Google Patents
Quantum dot vertical cavity surface emitting laserInfo
- Publication number
- WO2002071562A9 WO2002071562A9 PCT/US2002/006221 US0206221W WO02071562A9 WO 2002071562 A9 WO2002071562 A9 WO 2002071562A9 US 0206221 W US0206221 W US 0206221W WO 02071562 A9 WO02071562 A9 WO 02071562A9
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- mirror
- layers
- layer
- dbr
- vcsel
- Prior art date
Links
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B82—NANOTECHNOLOGY
- B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
- B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B82—NANOTECHNOLOGY
- B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
- B82Y10/00—Nanotechnology for information processing, storage or transmission, e.g. quantum computing or single electron logic
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B82—NANOTECHNOLOGY
- B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
- B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
- H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
- H01S5/18308—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL] having a special structure for lateral current or light confinement
- H01S5/18311—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL] having a special structure for lateral current or light confinement using selective oxidation
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
- H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
- H01S5/18341—Intra-cavity contacts
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/04—Processes or apparatus for excitation, e.g. pumping, e.g. by electron beams
- H01S5/042—Electrical excitation ; Circuits therefor
- H01S5/0421—Electrical excitation ; Circuits therefor characterised by the semiconducting contacting layers
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/04—Processes or apparatus for excitation, e.g. pumping, e.g. by electron beams
- H01S5/042—Electrical excitation ; Circuits therefor
- H01S5/0425—Electrodes, e.g. characterised by the structure
- H01S5/04256—Electrodes, e.g. characterised by the structure characterised by the configuration
- H01S5/04257—Electrodes, e.g. characterised by the structure characterised by the configuration having positive and negative electrodes on the same side of the substrate
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
- H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
- H01S5/18308—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL] having a special structure for lateral current or light confinement
- H01S5/18322—Position of the structure
- H01S5/1833—Position of the structure with more than one structure
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
- H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
- H01S5/18344—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL] characterized by the mesa, e.g. dimensions or shape of the mesa
- H01S5/1835—Non-circular mesa
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
- H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
- H01S5/18358—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL] containing spacer layers to adjust the phase of the light wave in the cavity
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
- H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
- H01S5/18361—Structure of the reflectors, e.g. hybrid mirrors
- H01S5/18369—Structure of the reflectors, e.g. hybrid mirrors based on dielectric materials
- H01S5/18372—Structure of the reflectors, e.g. hybrid mirrors based on dielectric materials by native oxidation
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
- H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
- H01S5/18383—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL] with periodic active regions at nodes or maxima of light intensity
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
- H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
- H01S5/18397—Plurality of active layers vertically stacked in a cavity for multi-wavelength emission
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/30—Structure or shape of the active region; Materials used for the active region
- H01S5/305—Structure or shape of the active region; Materials used for the active region characterised by the doping materials used in the laser structure
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/30—Structure or shape of the active region; Materials used for the active region
- H01S5/305—Structure or shape of the active region; Materials used for the active region characterised by the doping materials used in the laser structure
- H01S5/3054—Structure or shape of the active region; Materials used for the active region characterised by the doping materials used in the laser structure p-doping
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/30—Structure or shape of the active region; Materials used for the active region
- H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
- H01S5/341—Structures having reduced dimensionality, e.g. quantum wires
- H01S5/3412—Structures having reduced dimensionality, e.g. quantum wires quantum box or quantum dash
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10S977/00—Nanotechnology
- Y10S977/902—Specified use of nanostructure
- Y10S977/932—Specified use of nanostructure for electronic or optoelectronic application
- Y10S977/949—Radiation emitter using nanostructure
- Y10S977/95—Electromagnetic energy
- Y10S977/951—Laser
Definitions
- the present invention relates generally to self -assembled semiconductor quantum dot lasers. More particularly, the present invention is directed towards quantum dot vertical cavity surface emitting lasers (QD- VCSELs).
- QD- VCSELs quantum dot vertical cavity surface emitting lasers
- VSELs Vertical cavity surface emitting lasers
- Some of the advantages of a conventional VCSEL include surface emission, a nearly round emission pattern, a low threshold current, and the potential for high-yield, low cost manufacturing and packaging.
- FIG. 1 illustrates some of the features of a conventional VCSEL 100.
- a bottom mirror 105 is disposed on a substrate 102.
- An active region 110 is disposed between the bottom mirror 105 and a top mirror 120.
- a conventional VCSEL typically includes a quantum well active region for providing optical gain.
- a quantum well active region typically includes one or more quantum wells capable of providing a comparatively high optical gain.
- Optical feedback is typically provided by top and bottom distributed bragg reflector (DBR) mirror structures.
- DBR distributed bragg reflector
- the mirrors typically comprise pairs of alternating high index and low index semiconductor layers, with each layer typically being approximately a quarter wavelength in optical thickness.
- the active region is typically a high index region approximately an integer number of half wavelengths in thickness having a gain region disposed in its center.
- Quantum dot (QD) VCSELs are of potential interest for a variety of applications.
- Each quantum dot consists of an island of low bandgap material surrounded on all sides by a higher bandgap material.
- the low bandgap island of each quantum dot is sufficiently small that each dimension (length, width, and height) is smaller than the thermal deBroglie wavelength over operating temperatures of interest.
- the quantum dot has its energy states quantum confined in three dimensions, resulting in a delta-like density of states (e.g., a high density of states in a finite energy band around each permissible optical transition, analogous to a density of states for atoms).
- Quantum dot active regions have a variety of characteristics that make them of interest for VCSELs, such as potential advantages in regards to temperature sensitivity and high-speed modulation.
- QD-VCSELs there are several technical barriers that have hindered the commercial exploitation of QD-VCSELs.
- QD-VCSELs One barrier to the commercial exploitation of QD-VCSELs is that conventional quantum dot active regions typically have a peak optical gain that is low compared with quantum wells due to the small fill factor of quantum dots. Moreover, the optical gain at the ground state energy level saturates in quantum dots. The optical gain available from a layer of quantum dots is typically about an order of magnitude lower than that which can be achieved from a quantum well. For example, in edge-emitting lasers, the maximum ground state gain that can be achieved from a single layer of quantum dots is typically in the range of about 5 to 10 cm" 1 . [0010] Another barrier to the commercial use of QD-VCSELs is that many commercial applications have demanding operational requirements.
- ten-gigabit Ethernet 10-GigE
- an extended temperature range e.g., up to about 85 °C
- the maximum ground state optical gain decreases with increasing operating temperature this requirement further exacerbates the difficulty of designing a QD-VCSEL having sufficient optical gain to operate within ambient temperature ranges of commercial interest.
- a quantum dot vertical cavity surface emitting laser has a low cavity loss and a correspondingly low threshold gain.
- at least one of the mirrors of the laser cavity is an ultrahigh reflectivity distributed bragg reflector (DBR) mirror with mirror pairs comprised of alternating layers of high refractive index semiconductor and low refractive index oxide.
- DBR distributed bragg reflector
- Doped intracavity contact layers between the DBR mirrors provide current to a quantum dot active region.
- the contact layers have a thickness of about a half a wavelength or less to reduce free carrier loss. In one embodiment, about a quarter of a wavelength or less of each contact layer is heavily doped.
- the heavily doped portions of the contact layer may be positioned to have a low optical overlap with the longitudinal mode to reduce the free carrier loss.
- additional mode control layers are disposed between the DBR mirrors and the active region to reduce the optical overlap of the mode in doped regions and increase the optical confinement in the active region.
- the mode control layers are approximately quarter wavelength thick regions, have a refractive index different than adjacent layers, and are positioned to produce resonant reflections that beneficially increase the optical confinement of the longitudinal optical mode in the quantum dot active region and reduce optical confinement in heavily doped contact regions.
- each ultrahigh reflectivity DBR mirror is formed using a lateral oxidation process to convert oxidizable semiconductor layers into low refractive index oxides.
- delamination of laterally oxidized mirror layers is inhibited by including intermediate composition layers to reduce residual stress.
- one or more openings is arranged to permit lateral oxidation of bottom mirror regions while preserving lateral support regions to support the bottom mirror layers.
- FIG. 1 illustrates a prior art vertical cavity surface emitting laser design.
- FIGS. 2A, 2B, and 2b illustrate vertical cavity surface emitting lasers in accord with embodiments of the present invention.
- FIG. 3. illustrates one embodiment of a mirror pair for a DBR mirror having laterally oxidizable layers.
- FIG. 4A is a top view illustrating a VCSEL having openings formed through oxidizable bottom DBR mirror layers which have been used to laterally oxidize a bottom DBR mirror while retaining regions for laterally supporting the bottom DBR mirror of the VCSEL.
- FIG. 4B is a cross sectional view through line A-A of FIG. 4A.
- FIG. 4C is a cross sectional view through line B-B of FIG. 4A.
- FIGS. 5A and 5B show layer sequences of VCSELs having an active region including a mode control layer with different intracavity contact layer implementations.
- FIG. 6A shows a layer sequence of an embodiment having two mode control layers.
- FIG. 6B illustrates in more detail an embodiment of a layer sequence for an active region and mode control layers along with illustrative thicknesses in terms of the optical wavelength in the laser.
- FIG. 7 shows a sequence of epitaxially grown layers for one embodiment of a VCSEL for producing light with a wavelength around about
- FIG. 8 shows plots of the refractive index of key layers for the layer sequence VCSEL of FIG. 7 and the calculated intensity of the longitudinal mode.
- FIG. 9 is a plot of quantum dot density versus growth temperature for self-assembled InAs quantum dots grown by molecular beam epitaxy on
- FIG. 10 shows plots of modal gain versus current density for two different quantum dot densities.
- FIG. 11 is a plot illustrating a jump in gain associated with excited states of the quantum dots at high current densities.
- FIG. 12A illustrates preferred quantum dot growth parameters for InAs quantum dots and FIG. 12B illustrates a technique for embedding quantum dots in quantum wells.
- FIG. 13 illustrates an embodiment in which quantum dot layers are placed proximate a single antinode.
- FIG. 14 illustrates an embodiment in which quantum dot layers are placed proximate two antinodes.
- FIG. 15A is a perspective view illustrating a processed VCSEL.
- FIG. 15B illustrates a top mirror mesa etch mask step to etch to a first contact layer and holes for vertical isolation of the two contact layers using lateral oxidation.
- FIG. 15C illustrates a first metal deposition step.
- FIG. 15D illustrates an etch step to etch to a second contact layer.
- FIG. 15E illustrates a second metal deposition step.
- FIG. 15F illustrates a bottom mirror opening etch step.
- FIG. 15 G illustrates a top view of a fabricated VCSEL.
- FIG. 16A is an illustrative graph of longitudinal mode intensity in a
- FIG. 16B is an illustrative graph of longitudinal mode intensity in a
- the present invention is directed towards quantum dot vertical cavity surface emitting lasers (QD-VCSELs) having a low cavity loss and a correspondingly low threshold gain.
- QD-VCSELs quantum dot vertical cavity surface emitting lasers
- One application of the VCSELs of the present invention is for high data rate communication systems with an emission wavelength greater than about 1290 nanometers (nm) in which the VCSEL must lase over an extended range of ambient temperatures (e.g., 0 °C to 85 °C).
- ambient temperatures e.g., 0 °C to 85 °C.
- the VCSELs of the present invention may be utilized in a variety of applications.
- a QD-VCSEL 200 of the present invention has a quantum dot active region 210 disposed between a bottom surface 204 of a top mirror 220 and a top surface 208 of a bottom mirror 205.
- Each mirror 205 and 220 is a distributed bragg reflector (DBR) mirror with a ⁇ /2 refractive index variation associated with a sequence of mirror pairs, where ⁇ is the emission wavelength of laser light inside the laser cavity.
- DBR distributed bragg reflector
- layer thicknesses referred to in reference to " ⁇ " or “wavelength” refers to a desired optical thickness with respect to the wavelength of the laser light within the laser, with the wavelength in the laser being ⁇ - ⁇ o/n., where ⁇ ois the free space wavelength and n. is the effective refractive index in the laser. It will also be understood that thicknesses referred to in terms of fractions of wavelengths (e.g., ⁇ /4, ⁇ /2, ⁇ ) are desired nominal target thicknesses but that some variation in actual thicknesses about the target thicknesses is consistent with the optical physics of operation. [0044] Bottom mirror 205 is disposed on a substrate layer 202.
- each mirror has a corresponding top, bottom, and side with respect to a longitudinal optical mode reflected between the two mirrors.
- at least one of the mirrors is a high reflectivity oxide/semiconductor DBR mirror formed by a lateral oxidation process.
- Contact layers 240 and 230 have doped regions to permit electron- hole pairs to be injected to the active region 210 responsive to a current.
- Contact layers 230 and 240 are also known as "intracavity" contact layers because in the processed device they permit current to be provided from contact layers disposed within the optical cavity.
- a current aperture layer (e.g., a selectively oxidizable layer that may be oxidized outside of the VCSEL to reduce deleterious currents) is preferably included as part of at least one of the contact layers.
- mesa etching may be used to expose the contact layers and suitable metal contact layers deposited to form ohmic contacts to portions of the contact layers.
- VCSEL 200 is grown using a suitable epitaxial growth technique for growing self-assembled III-V semiconductor quantum dot active regions, such as molecular beam epitaxy (MBE) or metal-organic vapor phase epitaxy
- MBE molecular beam epitaxy
- the quantum dots may be selected to have a ground state transition energy corresponding to a wavelength in the range of about 1290 nm to 1330 nm or 1480 to 1620 nm.
- the VCSEL is grown on a GaAs substrate 202 using MBE
- the mirrors 205 and 220 are grown as AlGaAs layers having aluminum molar fractions selected to form mirror pairs with a ⁇ /2 variation in refractive index (e.g., two ⁇ /4 layers having different refractive indices)
- the quantum dot active region 210 comprises one or more quantum dot layers, with each layer of quantum dots being a layer of self-assembled InAs quantum dots embedded in an InGaAs quantum well having GaAs quantum well barriers.
- active regions utilizing InAs quantum dots may be grown to have ground state emission wavelengths over a range of wavelengths.
- each layer of quantum dots has only a limited maximum gain at the ground state transition energy due to the deltalike density of states function of quantum dots.
- For self-assembled quantum dots there is also typically a limit on the number of quantum dot layers that can be employed without generating deleterious strain.
- the optical gain decreases with increasing active region temperature.
- the saturated ground state gain depends upon several parameters. Studies by the inventors indicate that a saturated ground state gain of as high as 25 cm -1 may be achieved using a quantum dot active region having several InAs quantum dot layers. Thus, it is desirable to have a VCSEL with a threshold gain below about 25 cm -1 .
- the threshold lasing condition for a quantum dot VCSEL similar to that shown in FIG. 2 is given by:
- Tqd is the optical confinement of the quantum dot layers
- gqd is the gain of a quantum dot layer
- Tc is the optical confinement in the contact layers
- ⁇ - is the free carrier loss associated with doping the contact layers
- L eff is the effective cavity length of a longitudinal mode reflected between the two mirrors
- Reff is the effective mirror reflectivity associated with the top and bottom mirror layers and is conventionally the product of the top and bottom
- mirror loss The mirror reflectivity, active layer thickness, and contact layer thicknesses will also affect the optical confinement of the quantum dot layers and the optical confinement in contact layers.
- the ground state transition energy has a saturable gain that is temperature dependent.
- the saturated gain must be greater than the threshold gain for lasing to occur at the ground state energy level.
- the expression may be approximated as: [0052] Low Mirror Loss Design
- R kN ⁇ n/n
- N the number of mirror pairs
- n the average index of refraction of the two layers
- k is a constant (or a function of ⁇ n/n)
- ⁇ n is the difference in index of refraction for the two layers.
- the refractive index is typically small, such that a large number of mirror pairs are required to achieve a high DBR mirror reflectivity.
- a GaAs/AlAs mirror pair has a refractive index step of only about 0.6. Consequently, to form an ultra high reflectivity DBR mirror (e.g., a mirror reflectivity of greater than 99.99%) would require growing a large number of mirror pairs, which would result in extremely thick DBR mirrors that would be impractical to grow and process.
- the effective reflectivity of the mirrors 205 and 220 is increased if at least one of the mirrors in the processed VCSEL has oxide/semiconductor mirror pairs with a high index step between adjacent mirror layers of each mirror pairs.
- the epitaxially grown layers of at least one of the mirrors 205 or 220 is grown to have a sequence of mirrors pairs of oxidizable semiconductor layers and substantially nonoxidzable semiconductor layers.
- the oxidizable semiconductor layer is laterally oxidized in a post-growth process to convert it into a metal oxide having a substantially lower refractive index than the as- grown layer.
- the oxidation rate increases with increasing aluminum molar fraction.
- AlGaAs layer with an aluminum molar fraction greater than about 0.90 when exposed to steam and nitrogen at a temperature of about 450 °C, the arsenic is converted to arsine leaving behind an amorphous mixture of aluminum oxides, gallium oxides, and residual hydrogen.
- the rate of oxidation is highly dependent upon the aluminum molar fraction, with AlAs oxidizing extremely rapidly.
- Alo.9sGao.02As oxidizes about three times faster than Alo.96Gao.04As and ten times faster than Alo.92Gao.osAs.
- Metal oxides typically have a low refractive index compared with III-V semiconductors.
- the as- grown DBR mirror may comprise a sequence of pairs of high/low aluminum composition AlGaAs layers, such as AlAs/ AlGaAs or AlAs/GaAs layers.
- the refractive index step is increased to about 2.0 by selectively oxidizing the AlAs layer into AlOx (refractive index of about 1.6) in a post- growth oxidation process.
- the corresponding reflectivity for a 1.3 micron wavelength emission laser is calculated to be 99.9341% for five DBR mirror pairs and 99.99922% for eight mirror pairs.
- the top-mirror 220 may comprise an oxide/semiconductor DBR mirror whereas the bottom mirror 205 comprises a semiconductor DBR mirror.
- the bottom mirror 205 may comprise an oxide/semiconductor DBR mirror while the top mirror 220 comprises a semiconductor DBR mirror.
- both the top and bottom mirrors 205 and 220 may comprise oxide/semiconductor mirrors.
- Oxide/semiconductor DBR mirrors formed by laterally oxidizing high Al composition layers have a tendency to delaminate, particularly if large unsupported areas are completely oxidized.
- the semiconductor mirror structure and mirror oxidation process is selected to inhibit mirror delamination during processing and subsequent operation of the VCSEL.
- FIG. 3 is a diagram illustrating a sequence of grown DBR mirror pair layers (prior to lateral oxidation).
- an oxidizable semiconductor layer 305 is connected to a substantially non-oxidizable semiconductor layer 315 by an intermediate layer 310.
- the composition of the oxidizable semiconductor layer is preferably selected to have a controllable oxidation rate in a lateral oxidation process.
- Intermediate layer 310 preferably has a composition selected to inhibit delamination of layer 305 from layer 315.
- the relative thickness of layers 305, 310, and 315 are selected to form DBR mirror pairs with subsequent lateral oxidation of the oxidizable layers.
- the non-oxidizable semiconductor layer 315 comprises a layer of AlGaAs having a first molar fraction of aluminum while the oxidizable layer 305 comprises a layer of AlGaAs having a second, higher molar fraction of aluminum.
- AlAs oxidizes extremely rapidly.
- AlGaAs with an aluminum molar fraction below about 0.95 oxidizes comparatively slowly. Consequently, in one embodiment the oxidizable layer has an aluminum molar fraction of between about 0.97 to 0.99, with 0.98 being preferred.
- intermediate layer 310 may comprise a region in which the aluminum, molar fraction is graded between the aluminum composition of layers 305 and 315 (e.g., Alo.92Gao.08 As).
- the intermediate layer improves adhesion and is believed to reduce mechanical instabilities associated with residual strain at the interfaces.
- side portions of the mirror layers must be exposed for oxidation.
- the same mesa etch used to form the top mirror mesa is sufficient to expose side portions of mirror layers for oxidation.
- lateral oxidation of bottom mirror layers is more difficult.
- a laterally oxidized bottom DBR mirror tends to have significant residual strain energy due to the fact that it may be larger in area than the top mirror and because the bottom mirror, which supports other portions of the VCSEL, cannot relieve strain from exposed surfaces as readily as the top mirror. Thus, a laterally oxidized bottom DBR mirror is of particular concern in regards to delamination.
- the lateral oxidation process is performed through one or more openings (e.g., etched trenches) formed in the bottom mirror layers. As illustrated in the top view of FIG. 4A, the openings 410 are spaced far enough apart that the VCSEL's bottom DBR mirror 420 is laterally supported.
- the oxidation conditions are adjusted such that the lateral oxidation 440 spreads throughout the desired bottom DBR mirror region.
- the spacing between the openings 410 and their location relative to the intended lasing region of the bottom mirror may be selected to retain connection portions 430 of the DBR mirror layers that connect the oxidized DBR mirror to unoxidized mirror layers 450.
- a process for laterally oxidizing a bottom mirror includes opening oxidation windows 410 proximate a side portion of the VCSEL's bottom DBR mirror 420.
- one or more trench openings may be formed that expose a portion of at least one side of oxidizable DBR mirror layers for the bottom DBR mirror.
- two parallel trenches may be formed near the sides of the bottom VCSEL mirror 420.
- the oxidizable mirror layers are oxidized laterally about the openings 410.
- FIG. 4B is a cross sectional view through line A-A of FIG. 4A. Proximate trenches 410 the bottom mirror 205 is laterally oxidized.
- a top mirror 220 may be defined by a mesa etch and suitable p contact 490 and n-contact layers 480 deposited on the contact layers 230 and 240. In one embodiment, top mirror 220 is laterally oxidized in the same oxidation step.
- FIG. 4C is a cross-sectional view through line B-B of FIG. 4A. Note that lateral support to the bottom DBR mirror layer is provided in regions where the oxidized bottom mirror (outside of the VCSEL) is connected to unoxidized mirror material, thereby supporting the oxidized mirror and inhibiting delamination. Referring to FIG. 4A, it will be understood that it is desirable to select process conditions that oxidize the bottom DBR mirror in VSCEL areas that will emit light while also niinimizing the total oxidized area 440 consistent with oxidizing bottom VCSEL mirror 420.
- the longitudinal optical mode will be tightly confined between the DBR mirrors 205 and 220.
- the contact layers 230 and 240 require a sufficient doping-thickness product to achieve an acceptable oh ic resistance. However, if the contact layers 230 and 240 are heavily doped, this can result in substantial optical losses due to free-carrier losses in the contact layers unless the thickness and doping profile of the contact layers is appropriately selected. Consequently, in one embodiment of the present invention the contact layers have a thickness and doping profile selected to permit a reasonable ohmic resistance to be achieved with a comparatively low optical loss.
- the electrical contact layers 230 and 240 are designed to provide electron hole pairs into the quantum dot active region layers.
- the contact layers form a p-n diode junction for injecting electron hole pairs into quantum dot active region 210.
- contact layer 240 may include a heavily doped p-type layer whereas contact layer 230 may include a heavily doped n- type layer.
- Additional current aperture layers are preferably included to limit current injection to intended laser regions.
- the current aperture layers may comprise a layer of AlGaAs that is also oxidized in regions disposed away from the active region of the VCSEL, such as under contact pad metallizations.
- the contact layers and associated aperture layers have a thickness of about a half of a wavelength.
- the contact layers are not uniformly doped but instead are doped most heavily in regions where the optical field has the lowest intensity in the contact layers.
- Doped contact layers 230 and 240 have an optical loss associated with free carrier absorption.
- the free carrier absorption increases with dopant concentration and the magnitude of the electric field of the longitudinal optical mode.
- the optical mode intensity outside of the active region 210 between DBR mirrors approximates an envelope function within which the intensity varies with a periodicity determined by the wavelength.
- Selecting each contact layer to have an optical thickness of less than about ⁇ /2 facilitates placing the peak doping proximate an optical node of the longitudinal mode (e.g., a region having a low intensity).
- the precise free carrier loss may be minimized by using a computer analysis technique to integrate the loss through the contact layer based upon the dopant concentration and field strength at each point within the contact layer.
- each contact layer has a thickness of about ⁇ /2 or less and includes a heavily doped layer having a thickness of about ⁇ /4 or less. Selecting the heavily doped portion of the contact layers 230 and 240 to have a thickness of about ⁇ /4 facilitates reducing the optical losses because the most heavily doped portion may be placed proximate a node in the optical intensity, e.g., the overlap of the field intensity is reduced. Consequently, in a preferred embodiment of the present invention the thickness of the contact layers is selected to be about ⁇ /2 or less. In one embodiment, heavily doped contact layers have a thickness of about ⁇ /4 or less.
- the optical absorption of the contact layers may also be reduced by grading the doping concentration to have a higher doping concentration in regions where the longitudinal mode has a lower intensity.
- the p-contact layers 240 may comprise a highly doped p-type GaAs layer 505 proximate the bottom 204 of the top DBR mirror 220 and having a thickness of about ⁇ /4.
- the remaining ⁇ /4 thickness closest to the active region comprises a p-type GaAs layer 510 and AlGaAs current aperture layer 515 (e.g., a layer that can be selectively oxidized in contact pad regions to reduce parasitic conduction).
- a suitable n-type contact layer 230 may comprise a heavily n-doped GaAs layer 540 proximate the top surface of the bottom DBR mirror 205 and an AlGaAs current aperture layer 545.
- An additional benefit of reducing the thickness of the contact layer thickness to an optical thickness of about ⁇ /2 or less is that it increases the relative fraction of the mode confined to the active region.
- a reduction in the confinement factor of the mode in the contact layers results in a corresponding increase in the optical confinement factor within the active region.
- the active region has an at least one associated mode control layer to further reduce optical loss in the contact layers.
- the function of the mode control layer is to adjust the shape of the longitudinal mode to beneficially reduce r_ and achieve a high Tqd.
- a mode control layer is any layer having a refractive index profile that adjusts the optical mode to place a longitudinal node proximate heavily doped contact layers such as to reduce the free carrier loss associated with the contact layers.
- the mode control layers create reflections selected to produce a resonance effect that beneficially alters the longitudinal mode intensity distribution between the top and bottom mirrors 205 and 220.
- each mode control layer may comprise a single layer functioning as a mirror layer and permitting the passage of an electrical current, with the mirror layer reducing the optical intensity within the contact layers.
- the active region is an integral number of half -wavelengths in thickness and includes a plurality of quantum dot layers, with each quantum dot layer being a pluraHty of quantum dots embedded in a quantum well. It should be understood that other VCSEL designs could result in the presence of quarter-wave phase shift layer with concomitant change in the other layer thicknesses.
- the mode control layers comprise an approximately quarter wavelength thickness region of low index or high index layers.
- each of the mode control layers 610 are regions disposed between the first or second ends 602 or 604 of the active region 210 and a respective mirror.
- each mode control layer is a ⁇ /4 thick layer having a different refractive index than adjacent layers (e.g., functions a partial DBR mirror). The difference in refractive indices between the mode control layer 610 and adjacent layers creates optical reflections.
- a resonance condition is established with the additional reflections of the mode control layers 610 beneficially altering the longitudinal mode intensity profile between the mirrors.
- Active region 210 includes quantum dot layers 655 with InAs quantum dots 690.
- the mode control layers 610 may comprise layers having either a lower or higher refractive index than adjacent layers. If the refractive index is higher, the thickness of the active region is preferably an integer number of half wavelengths. If the refractive index is lower, than the active region is preferably an odd multiple of quarter wavelengths in thickness, and GaAs barrier layers 660.
- the resonant reflections can be used to create large changes in longitudinal mode intensity between top and bottom DBR mirrors. FIG.
- 16A illustrates optical field intensity 1600 versus distance across the active and contact layers of a VCSEL having high reflectivity top and bottom DBR mirrors in a structure that does not have mode control layers 1610.
- VCSEL cavity components are superimposed on the plot.
- the refractive indices of the layers is approximately uniform over optical-scale distances. Consequently, the longitudinal optical mode is essentially periodic between the two mirrors. This results in a high overlap of the field in contact layers, resulting in high cavity losses.
- FIG. 16B is a plot 1650 of longitudinal mode intensity versus distance in a VCSEL that illustrates the effect of mode control layers 610 having a refractive index profile and placement selected to produce a resonance condition.
- the mode control layers are quarter wavelength thick mode control layers comprised of a lower index material (e.g., AlGaAs layers).
- the resonant reflections from the mode control layers simultaneously increases the optical confinement factor of the active region while also reducing the confinement of the mode in the contact layers.
- FIG. 7 shows an exemplary sequence of grown layers for a quantum dot VCSEL 700 including DBR mode control layers. Exemplary thicknesses, dopings, and MBE growth temperatures are shown.
- FIG. 8 shows plots of mode intensityy 810 and refractive indices 820 through the VCSEL (with oxidized AlAs mirror layers).
- the active region includes three InGaAs quantum wells approximately 9 nm in thickness. Approximately 2.4 monolayers of indium are deposited under growth conditions selected to form quantum dots, e.g., 1 nm of the quantum well is grown, approximately 2.4 equivalent monolayers of Indium Arsenide are deposited to form InAs islands, and a top 8 nm InGaAs layer is deposited to embed the InAs islands.
- the mode control layers 610 comprise 107 nm thick regions of Alo.92Gao.08As.
- the top and bottom DBR mirrors 205 and 220 comprise alternating nominally AIAs/GaAs layers.
- the AlAs layer is preferably Alo.9sGao.02As, since this facilitates controlling the rate of lateral oxidation.
- an intermediate layer of Alo.92Gao.osAs to inhibit delamination.
- the contact layers are preferably doped to reduce ohmic contact resistance.
- Layers which are to be oxidized are preferably lightly doped or undoped. Electrical interfaces between regions having different AlGaAs compositions may be graded in composition, if desired, to reduce electrical resistance. Note that the number of mirror pairs of each mirror does not have to be identical, e.g., in this example the bottom mirror 205 has more mirror pairs than the top mirror 220.
- a mode control layer 610 may be placed between one or more of the contact layers.
- the mode control layer may be placed between the current aperture layer and highly doped contact layers.
- the longitudinal mode intensity 810 is tightly confined about the active region with an antinode centered in the quantum dot layers.
- the longitudinal mode has an antinode proximate each contact layer, which reduces optical absorption.
- the doping within each contact layer may also be graded to have a peak doping concentration in regions having a low optical intensity, thereby further reducing optical absorption.
- the modal quantum dot gain may be increased by selecting growth parameters that increase the quantum dot density, increasing the number of quantum dot layers consistent with strain limitations, and arranging the quantum dot layers to increase the optical confinement factor of the quantum dots.
- a preferred growth technique is molecular beam epitaxy (MBE) with the quantum dot layer grown at a temperature between about 450 °C to 540 °C.
- the other layers top mirror, bottom mirror, and contact layers
- a conventional optical pyrometer may be used to determine the temperature.
- the arsenic flux is preferably chosen to achieve an arsenic stabilized surface.
- the quantum dots form as self- assembled islands.
- a Stranski-Krastanow (S- K) growth mode occurs once a sufficient number of equivalent monolayers of InAs are deposited.
- S- K growth mode the driving force for the formation of islands is the reduction in strain energy afforded by elastic deformation, i.e., for S-K growth it is more energetically favorable to increase surface energy by islanding than by relaxing strain by dislocation generation.
- S-K growth mode the growth becomes three dimensional after a critical thickness of the larger lattice constant material is grown upon an initial wetting layer.
- FIG. 9 is a plot of quantum dot density versus MBE growth temperature for quantum dots grown on two different InGaAs well layer compositions. It can be seen that the dot density depends strongly upon temperature and also upon the composition of the bottom well layer. Dot densities of greater than 1 x 10 11 cm -2 may be achieved at a growth temperature of about 470 °C. The dot density can be adjusted by more than a factor of five by selecting a growth temperature between 470 °C to 540 °C. Experiments indicate that the dot density is at least a factor of two higher when the dots are grown on an InGaAs layer compared with a GaAs layer at a comparable temperature.
- the dot density also increases when the InGaAs alloy composition is increased from Ino.1Gao.9As to In0.2Ga0.sAs.
- the thickness of the bottom InGaAs well layer may be extremely thin and still have the same effect as a thick layer in regards to the nucleation of quantum dots on the bottom InGaAs layer.
- the bottom well layer need only have a thickness consistent with it having a reproducible thickness and alloy composition.
- the bottom well layer may have a thickness as low as 0.5 nm, although a thickness of about one nanometer may be easier to reproducibly grow.
- FIG. 10 is a plot of modal gain versus current density of edge emitting lasers for two different dot densities. It can be seen in FIG. 10 that the modal gain of each quantum dot layer increases with increasing dot density. Consequently, in one embodiment a quantum dot growth temperature is selected to increase the quantum dot density.
- the ground state gain of a quantum dot has a maximum (saturated) optical gain due to the delta-like density of states of quantum dots. However, at high pumping levels additional excited states may be accessible to provide additional gain at shorter wavelength, as indicated in the plot of FIG. 11.
- FIGS. 12A and 12B shows exemplary growth temperatures and thicknesses for a quantum dot layer.
- FIG. 12A shows embedded quantum dots (QDs) and FIG. 12B shows a corresponding growth layer sequence.
- the quantum dots 1210 are formed on a first well layer 1205 of InGaAs and embedded in a second well layer 1215 of InGaAs.
- a bottom InGaAs quantum well layer of between about 0.5 to 2 nm in thickness is grown on top of a GaAs barrier layer.
- An InAs floating layer is preferably initially deposited on the GaAs layer to raise the surface indium concentration close to its equilibrium concentration at the growth temperature, thereby improving the compositional uniformity of subsequent InGaAs layers.
- the equilibrium concentration of segregated (floating) indium depends on temperature and InGasAs composition but is typically about 0.5 to 1 monolayers over a range of common growth parameters.
- An InGaAs quantum well composition of between about Ino.1Gao.9As to about Ino.2Gao.8As is preferred. Higher indium molar fractions tend to increase the dot density and the depth of the energy barrier for confining electrons and holes in the quantum well. However, higher indium concentrations also increase the strain associated with each layer. Typically about 1 to 3 monolayers of InAs is deposited to form InAs islands. A top well layer of about 6 to 11 nanometers of InGaAs may be used to embed the InAs.
- a GaAs layer of about 10-40 nm is grown to form a second quantum well barrier.
- a desorption step is performed after growth of the top InGaAs layer to planarize any residual InAs islands that protrude above the top InGaAs layer.
- the time and temperature of the desorption step are preferably selected to rapidly planarize protruding InAs regions but to preserve InGaAs.
- several monolayers of GaAs are deposited before the desorption step to facilitate maintaining a stable top InGaAs well layer during the desorption step.
- the number of quantum dot layers and their spacing is limited, in part, by strain effects.
- the strain thickness product of an individual layer of quantum dots should be sufficiently low to prevent the formation of deleterious dislocation and defects. Additionally, the cumulative strain associated with all of the layers should be sufficiently low to prevent the formatting of deleterious defects.
- an average strain-thickness product should be below a threshold average strain (e.g., 0.5%).
- the strain thickness product of an individual quantum dot layer is EwTw, where Ew is the strain of a well layer and Tw is the thickness of the well.
- the strain thickness product of an individual barrier layer is EbTb, where Eb is the strain of the barrier layer and Tb is the thickness of the barrier layer.
- Eav is:
- Equation 2 can be re-expressed as a relationship between the barrier thickness, well thickness, modified average strain, strain in the barriers, and strain in the well:
- ⁇ b ⁇ (E ⁇ -E ) L J (n + ⁇ )(Eav -Eb)
- Equation 5 can be used to derive a relationship for a minimum barrier layer thickness. If the average strain is selected to be less than a maximum average strain (for example, and average strain less than about
- the rninimurn barrier thickness for a structure with 6 quantum dot layers is about 20 nanometers.
- the quantum dot layers within the active region are preferably placed proximate an anti-node (a region of peak optical intensity) of the longitudinal mode, since this beneficially increases the optical confinement in each quantum dot layer.
- approximately three-to-six ⁇ quantum dot layers are placed about each antinode. Using less than three quantum dot layers per antinode typically produces less gain than desired for many applications.
- a preferred number of quantum dot layers per antinode is three- to-four, since using more quantum dot layers tends to increase the threshold current.
- the active region has a thickness selected to generate a single antinode centered within the active region and mode control layers.
- the quantum dot layers 1310 are positioned proximate the single antinode 1310 of the longitudinal mode.
- the active region and mode control layers are centered on a node and has a thickness selected such that there are at least two antinodes within the active region.
- quantum dot layers are disposed proximate each antinode.
- FIG. 14 illustrates a VCSEL having two sets of quantum dot layers 1420 with each set centered about one of two antinodes 1410.
- each set of of quantum dot layers 1420 may comprise three or four quantum dot layers.
- FIG. 15A is a perspective view of an exemplary processed VCSEL 1500 fabricated in accord with one embodiment of the present invention.
- the processing includes steps for etching down to the p-type contact layers 240 in regions outside of the desired top mirror 220 of the VCSEL.
- Conventional photolithography processes are used.
- An exemplary top mirror area is about 14 to 30 microns square.
- a suitable mask layer e.g., a photoresist mask having a mask region 1550 for protecting the top mirror during the first etch process is illustrated in FIG. 15B.
- the etch process may use any suitable wet or dry etch process.
- ICP inductively coupled plasma
- FIG. 15 C shows an exemplary p-metallization.
- Examples of p-metal layers include Au/Zn/Au metallization.
- a ring 1552 of p-contact metal is formed on the p-contact layer around the top mirror mesa to provide a low electrical resistance.
- the ring 1552 is about ten microns wide and connected to a pad 1556 by a neck 1554 about ten microns wide and about forty microns long.
- An exemplary p-contact pad is about 100 microns by 100 microns in area.
- the p-contact layer includes holes 1558 on a pad region to permit via holes to be etched down to a current aperture layer, which is oxidized during the mirror oxidation process. As one example, each hole 1558 may be about ten microns square.
- a cavity mesa etch is used to etch down to the n-type contact layer 230.
- a suitable mask layer is shown in FIG. 15D to protect the top mirror mesa and p-contact metallization.
- a n-metal contact layer 1520 is deposited on the n-contact layer.
- the n-metal contact layer be a AuGe/Ni/Au contact.
- a suitable mask is shown in FIG. 15E.
- two trenches 1570 are included. In one embodiment, each trench may be about seven microns wide and about seventy microns long.
- An oxygen plasma or other cleaning step may be used to clean the sample prior to lateral oxidation in a water vapor oxidation process.
- an additional mask may be used to etch a trench down through the bottom mirror.
- a single lateral oxidation step may be used to simultaneously oxidize the top mirror, contact pad isolation, and the bottom mirror.
- FIG. 15 G shows a top view of a completed VCSEL.
- the threshold gain may be selected to be below the saturated ground state gain over an extended temperature range.
- the ground state transition energy of a layer of quantum dots has a maximum gain at which the gain saturates.
- the saturated ground state gain decreases with increasing temperature.
- reducing the threshold gain increases the ambient temperature operating range, e.g., a low threshold gain permits a VCSEL to be operated at higher temperatures.
- the combination of features of the present invention permits an approximately order of magnitude reduction in threshold gain compared with a conventional quantum well VCSEL.
- optical absorption in contact layers is reduced due to the comparatively thin doped contact layers (e.g., contact layers with heavily doped regions less than ⁇ /2 in thickness), the mode control layers which reduce the optical intensity in contact layers, and the grading of doping profiles in contact layers, which places the highest doping concentrations in regions with the lowest optical intensity. Consequently, the optical absorption in the contact layers by at least a factor of two compared with conventional VCSELs having ⁇ thick layers.
- the anti-delamination features of the present invention f acilitates the use of ultra-high reflectivity DBR mirrors, which also reduces the threshold gain.
- the anti-delamination features of the present invention permit oxide/semiconductor DBR mirrors to be manufactured that have about a factor of ten lower mirror loss than conventional semiconductor DBR mirrors.
- the arrangement of quantum dot layers within the active region facilitates achieving a high optical confinement factor of quantum dot layers, further reducing the threshold gain requirements.
- the high reflectivity mirrors in combination with the mode control layers results in an increase in the available optical gain by about a factor of 1.5 to 2 due to the increased optical confinement factor for the quantum dots. This, in combination with the approximately factor of two reduction in the optical loss in the contact layers results in the VCSEL having an approximately 3-to-4 fold improvement in gain versus loss.
- a typical ground-state saturated gain of a multiple layer quantum dot active layer may be 25 cm -1 or more at room temperature.
- the cavity loss is only about 10 cm 4 . This means that there is a 15 cm -1 margin. This permits, for example, the laser to be operated at an elevated temperature for which the saturated gain decreases by more than a factor of two (e.g., to 12.5 cm -1 ).
- the reflectivity of top and bottom oxidized mirrors exceeds 99.9%.
- an oxidized DBR mirror with eight AlO/GaAs DBR mirror pairs has a calculated longitudinal mode reflectivity of 99.999943% while an AlO/GaAs DBR mirror with five mirror pairs has a calculated longitudinal mode reflectivity of 99.97%.
- An optimized VCSEL structure similar to that shown in FIG. 8 has a quantum dot optical confinement factor of between about 1-2%, depending upon the number of quantum dot layers. The optical losses associated with contact layers has been reduced to a value about the same as the loss due to the mirror transmission.
- the calculated differential efficiency is about 50%.
- QD-VCSELS lasing in the ground state transition energy over an extended temperature range may be designed.
- a laser operating between a first temperature (e.g., 0°C) to a second temperature (e.g., 85 °C) the saturated ground state gain at each temperature may be calculated and the cavity loss is selected to permit lasing over the temperature range.
- An edge-emitting quantum dot laser may be used to empirically determine the range of QD gain between two temperatures.
- the effects of the DBR mirrors may be simulated by using an external cavity laser configuration in which a diffraction grating mirror provides wavelength selective feedback to a Fabry-Perot laser ("gain chip").
- the threshold gain/cavity losses for the external cavity may be determined by characterizing the external grating mirror reflectivity and coupling optics. This threshold gain is the value that the gain chip must satisfy. At the selected wavelength and its associated threshold gain, the maximum operating temperature of the laser chip can be assessed by varying its temperature alone. Thus, the relationship between gain and temperature can be incorporated into the design of the VCSEL cavity. [00109] The VCSEL design may then be adjusted to achieve a threshold gain less than the saturated quantum dot gain at the highest operating temperature. For example, if an extended operating temperature range is desired, the mirror loss may be reduced by increasing the number of DBR mirror layers and/or using oxidized mirrors in both the top and bottom DBR mirror. Alternatively, the number of quantum dot layers may be increased.
- the present invention is not limited to AlGaAs materials.
- III-V materials systems include AlGalnAs and related ternary alloys; AlInAs and GalhAs on InP substrates; and AlGaAsSb and AlAsSb, and GaAsSb and associated ternary alloys on InP.
- mode control layers generating resonant reflections may be incorporated in VCSELs fabricated from a variety of materials.
- lateral oxidation occurs in a variety of III-V materials, such as alloys formed from digital alloys having AlAs layers, the lateral oxidation of bottom DBR mirrors through trench openings may be applied to a variety of III-V materials.
Abstract
Description
Claims
Applications Claiming Priority (6)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US27230701P | 2001-03-02 | 2001-03-02 | |
US60/272,307 | 2001-03-02 | ||
US27618601P | 2001-03-16 | 2001-03-16 | |
US60/276,186 | 2001-03-16 | ||
US31630501P | 2001-08-31 | 2001-08-31 | |
US60/316,305 | 2001-08-31 |
Publications (3)
Publication Number | Publication Date |
---|---|
WO2002071562A2 WO2002071562A2 (en) | 2002-09-12 |
WO2002071562A3 WO2002071562A3 (en) | 2004-03-18 |
WO2002071562A9 true WO2002071562A9 (en) | 2004-04-22 |
Family
ID=27402465
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/US2002/006221 WO2002071562A2 (en) | 2001-03-02 | 2002-03-01 | Quantum dot vertical cavity surface emitting laser |
Country Status (2)
Country | Link |
---|---|
US (1) | US6782021B2 (en) |
WO (1) | WO2002071562A2 (en) |
Families Citing this family (48)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20060151428A1 (en) * | 2002-12-30 | 2006-07-13 | Reiner Windisch | Method for roughening a surface of a body, and optoelectronic component |
US7433381B2 (en) * | 2003-06-25 | 2008-10-07 | Finisar Corporation | InP based long wavelength VCSEL |
US7282732B2 (en) * | 2003-10-24 | 2007-10-16 | Stc. Unm | Quantum dot structures |
FI20031691A (en) * | 2003-11-20 | 2005-05-21 | Modulight Inc | semiconductor laser |
US20060043395A1 (en) * | 2004-08-26 | 2006-03-02 | National Inst Of Adv Industrial Science And Tech | Semiconductor light-emitting element and method of producing the same |
KR100673499B1 (en) * | 2005-01-13 | 2007-01-24 | 한국과학기술연구원 | Distributed Bragg's Reflector Made By Digital-Alloy Multinary Compound Semiconductor |
KR100668328B1 (en) * | 2005-02-15 | 2007-01-12 | 삼성전자주식회사 | Quantum dot vertical cavity surface emitting laser and fabrication method of the same |
US20060227825A1 (en) * | 2005-04-07 | 2006-10-12 | Nl-Nanosemiconductor Gmbh | Mode-locked quantum dot laser with controllable gain properties by multiple stacking |
US8008215B2 (en) * | 2005-05-12 | 2011-08-30 | Massachusetts Institute Of Technology | Integration of buried oxide layers with crystalline layers |
DE102005057800B4 (en) * | 2005-11-30 | 2009-02-26 | Technische Universität Berlin | Single photon source and method for its production and operation |
US7561607B2 (en) * | 2005-12-07 | 2009-07-14 | Innolume Gmbh | Laser source with broadband spectrum emission |
JP2009518833A (en) | 2005-12-07 | 2009-05-07 | インノルメ ゲゼルシャフト ミット ベシュレンクテル ハフツング | Laser light source with broadband spectral emission |
US7835408B2 (en) * | 2005-12-07 | 2010-11-16 | Innolume Gmbh | Optical transmission system |
US8003979B2 (en) * | 2006-02-10 | 2011-08-23 | The Research Foundation Of State University Of New York | High density coupling of quantum dots to carbon nanotube surface for efficient photodetection |
JP5095260B2 (en) * | 2006-05-15 | 2012-12-12 | 富士通株式会社 | Manufacturing method of semiconductor light emitting device |
JP4985954B2 (en) * | 2006-06-27 | 2012-07-25 | セイコーエプソン株式会社 | Surface emitting semiconductor laser |
JP5082344B2 (en) * | 2006-08-31 | 2012-11-28 | 富士ゼロックス株式会社 | Surface emitting semiconductor laser and manufacturing method thereof |
JP4110181B2 (en) * | 2006-09-01 | 2008-07-02 | キヤノン株式会社 | Semiconductor laser device |
US20100097691A1 (en) * | 2006-09-28 | 2010-04-22 | Research Foundation Of The City University Of New York | Spin-coated polymer microcavity for light emitters and lasers |
JP2008098299A (en) | 2006-10-10 | 2008-04-24 | Mitsubishi Electric Corp | Semiconductor optical device and manufacturing method thereof |
US7499481B2 (en) * | 2006-11-14 | 2009-03-03 | Canon Kabushiki Kaisha | Surface-emitting laser and method for producing the same |
US20100019618A1 (en) * | 2007-07-05 | 2010-01-28 | Eliade Stefanescu | Transversal quantum heat converter |
US20090007951A1 (en) * | 2007-07-05 | 2009-01-08 | Eliade Stefanescu | Quantum injection system |
US20090007950A1 (en) * | 2007-07-05 | 2009-01-08 | Eliade Stefanescu | Longitudinal quantum heat converter |
JP4709259B2 (en) * | 2007-10-12 | 2011-06-22 | キヤノン株式会社 | Surface emitting laser |
JP5279392B2 (en) * | 2008-07-31 | 2013-09-04 | キヤノン株式会社 | Surface emitting laser and method for manufacturing the same, method for manufacturing surface emitting laser array, and optical apparatus including surface emitting laser array |
EP2371044B1 (en) * | 2008-12-03 | 2019-08-28 | Innolume GmbH | Semiconductor laser with low relative intensity noise of individual longitudinal modes and optical transmission system incorporating the laser |
US8349712B2 (en) | 2011-03-30 | 2013-01-08 | Technische Universitat Berlin | Layer assembly |
KR101941170B1 (en) * | 2011-12-12 | 2019-01-23 | 삼성전자주식회사 | Transmissive image modulator using multi Fabry-Perot resonant modes and multi absorption modes |
JP6216785B2 (en) * | 2012-07-11 | 2017-10-18 | コーニンクレッカ フィリップス エヌ ヴェKoninklijke Philips N.V. | VCSEL with contact in cavity |
EP2878048B1 (en) * | 2012-07-27 | 2022-02-09 | Thorlabs, Inc. | Amplified widely tunable short cavity laser |
CN103107287A (en) * | 2013-02-19 | 2013-05-15 | 中国科学院理化技术研究所 | Application of heteroatom doped carbon quantum dot in solar cell |
US9082637B2 (en) | 2012-08-17 | 2015-07-14 | The University Of Connecticut | Optoelectronic integrated circuit |
US10256368B2 (en) * | 2012-12-18 | 2019-04-09 | Sk Siltron Co., Ltd. | Semiconductor substrate for controlling a strain |
US20140175287A1 (en) * | 2012-12-21 | 2014-06-26 | Jarrod Vaillancourt | Optical Antenna Enhanced Infrared Detector |
US20140175286A1 (en) * | 2012-12-21 | 2014-06-26 | Jarrod Vaillancourt | High Operating Temperature Quantum Dot Infrared Detector |
US9203215B2 (en) * | 2013-07-03 | 2015-12-01 | Inphenix, Inc. | Wavelength-tunable vertical cavity surface emitting laser for swept source optical coherence tomography system |
RU2554302C2 (en) * | 2013-11-06 | 2015-06-27 | Федеральное государственное бюджетное учреждение высшего профессионального образования и науки Санкт-Петербургский Академический университет - научно-образовательный центр нанотехнологий Российской академии наук | Vertically emitting laser with bragg mirrors and intracavity metal contacts |
JP6488299B2 (en) | 2013-11-26 | 2019-03-20 | インフェニックス インコーポレイテッドInphenix, Inc. | Tunable MEMS Fabry-Perot filter |
GB201503498D0 (en) * | 2015-03-02 | 2015-04-15 | Univ Lancaster | Vertical-cavity surface-emitting laser |
US10868407B2 (en) | 2015-06-04 | 2020-12-15 | Hewlett Packard Enterprise Development Lp | Monolithic WDM VCSELS with spatially varying gain peak and fabry perot wavelength |
WO2016198282A1 (en) * | 2015-06-09 | 2016-12-15 | Koninklijke Philips N.V. | Vertical cavity surface emitting laser |
GB2555100B (en) * | 2016-10-14 | 2020-07-08 | Toshiba Res Europe Limited | A photon source and a method of fabricating a photon source |
CN107221574B (en) * | 2017-07-19 | 2023-04-18 | 中山德华芯片技术有限公司 | Composite DBR structure applied to multi-junction solar cell and preparation method thereof |
EP3470912B1 (en) * | 2017-10-10 | 2022-02-02 | Samsung Electronics Co., Ltd. | Quantum dot light modulator and apparatus including the same |
TWI742714B (en) * | 2019-06-11 | 2021-10-11 | 全新光電科技股份有限公司 | Semiconductor laser diode |
KR20210035377A (en) * | 2019-09-23 | 2021-04-01 | 삼성전자주식회사 | light modulator, beam steering device including the same and electronic device including the beam steering device |
US11876348B2 (en) | 2020-09-25 | 2024-01-16 | Apple Inc. | Trench process for dense VCSEL design |
Family Cites Families (20)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0535293A1 (en) | 1991-01-29 | 1993-04-07 | Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. | A method of fabricating a compositional semiconductor device |
SE501722C2 (en) * | 1993-09-10 | 1995-05-02 | Ellemtel Utvecklings Ab | Surface emitting laser device with vertical cavity |
EP0665578B1 (en) | 1993-11-25 | 2002-02-20 | Nippon Telegraph And Telephone Corporation | Semiconductor structure and method of fabricating the same |
DE4431827A1 (en) * | 1994-09-07 | 1996-03-14 | Lautenschlaeger Mepla Werke | Furniture hinge tab in form of supporting arm |
JP3468866B2 (en) * | 1994-09-16 | 2003-11-17 | 富士通株式会社 | Semiconductor device using three-dimensional quantum confinement |
US5710436A (en) | 1994-09-27 | 1998-01-20 | Kabushiki Kaisha Toshiba | Quantum effect device |
US5614435A (en) | 1994-10-27 | 1997-03-25 | The Regents Of The University Of California | Quantum dot fabrication process using strained epitaxial growth |
US5541949A (en) | 1995-01-30 | 1996-07-30 | Bell Communications Research, Inc. | Strained algainas quantum-well diode lasers |
US5557627A (en) | 1995-05-19 | 1996-09-17 | Sandia Corporation | Visible-wavelength semiconductor lasers and arrays |
US5881086A (en) | 1995-10-19 | 1999-03-09 | Canon Kabushiki Kaisha | Optical semiconductor device with quantum wires, fabrication method thereof, and light source apparatus, and optical communication system using the same |
FR2744292B1 (en) | 1996-01-29 | 1998-04-30 | Menigaux Louis | MULTI-WAVELENGTH LASER EMISSION COMPONENT |
JP3033517B2 (en) | 1997-04-17 | 2000-04-17 | 日本電気株式会社 | Semiconductor tunable laser |
CA2242670A1 (en) | 1997-07-14 | 1999-01-14 | Mitel Semiconductor Ab | Field modulated vertical cavity surface-emitting laser with internal optical pumping |
US5953356A (en) | 1997-11-04 | 1999-09-14 | Wisconsin Alumni Research Foundation | Intersubband quantum box semiconductor laser |
JP4138930B2 (en) | 1998-03-17 | 2008-08-27 | 富士通株式会社 | Quantum semiconductor device and quantum semiconductor light emitting device |
US6117699A (en) | 1998-04-10 | 2000-09-12 | Hewlett-Packard Company | Monolithic multiple wavelength VCSEL array |
US5991326A (en) * | 1998-04-14 | 1999-11-23 | Bandwidth9, Inc. | Lattice-relaxed verticle optical cavities |
US6177884B1 (en) * | 1998-11-12 | 2001-01-23 | Hunt Technologies, Inc. | Integrated power line metering and communication method and apparatus |
US6931042B2 (en) | 2000-05-31 | 2005-08-16 | Sandia Corporation | Long wavelength vertical cavity surface emitting laser |
US6329668B1 (en) | 2000-07-27 | 2001-12-11 | Mp Technologies L.L.C. | Quantum dots for optoelecronic devices |
-
2002
- 2002-03-01 US US10/087,408 patent/US6782021B2/en not_active Expired - Lifetime
- 2002-03-01 WO PCT/US2002/006221 patent/WO2002071562A2/en not_active Application Discontinuation
Also Published As
Publication number | Publication date |
---|---|
US20020176474A1 (en) | 2002-11-28 |
WO2002071562A2 (en) | 2002-09-12 |
US6782021B2 (en) | 2004-08-24 |
WO2002071562A3 (en) | 2004-03-18 |
Similar Documents
Publication | Publication Date | Title |
---|---|---|
US6782021B2 (en) | Quantum dot vertical cavity surface emitting laser | |
US6931042B2 (en) | Long wavelength vertical cavity surface emitting laser | |
US5493577A (en) | Efficient semiconductor light-emitting device and method | |
EP1182756B1 (en) | Semiconductor laser device having lower threshold current | |
Choquette et al. | Fabrication and performance of selectively oxidized vertical-cavity lasers | |
JP4047718B2 (en) | Quantum dash device | |
EP1389813B1 (en) | Wavelength tunable VCSEL | |
US20020131462A1 (en) | Intracavity contacted long wavelength VCSELs with buried antimony layers | |
US20050249254A1 (en) | Current-confinement heterostructure for an epitaxial mode-confined vertical cavity surface emitting laser | |
WO2002017445A1 (en) | Heat spreading layers for vertical cavity surface emitting lasers | |
US20050063440A1 (en) | Epitaxial mode-confined vertical cavity surface emitting laser (VCSEL) and method of manufacturing same | |
US9917421B2 (en) | P-type isolation regions adjacent to semiconductor laser facets | |
EP1145395A2 (en) | Compound semiconductor structures for optoelectronic devices | |
EP2686922B1 (en) | Multi-section quantum cascade laser with p-type isolation regions | |
EP1228557A2 (en) | Long wavelength pseudomorphic inganpassb type-i and type-ii active layers for the gas material system | |
Illek et al. | Low threshold lasing operation of narrow stripe oxide-confined GaInNAs/GaAs multiquantum well lasers at 1.28 µm | |
US7816163B2 (en) | Radiation-emitting semiconductor body for a vertically emitting laser and method for producing same | |
WO2001093387A2 (en) | Long wavelength vertical cavity surface emitting laser | |
Ustinov et al. | Quantum dot VCSELs | |
US6931044B2 (en) | Method and apparatus for improving temperature performance for GaAsSb/GaAs devices | |
US6859474B1 (en) | Long wavelength pseudomorphic InGaNPAsSb type-I and type-II active layers for the gaas material system | |
WO2020231714A1 (en) | Vertical cavity surface emitting laser with buried tunnel junction as current confinement aperture | |
von Würtemberg et al. | Performance optimisation of epitaxially regrown 1.3-μm vertical-cavity surface-emitting lasers | |
Iwai et al. | High-performance 1.3-/spl mu/m InAsP strained-layer quantum-well ACIS (Al-oxide confined inner stripe) lasers | |
CN112189288A (en) | Vertical cavity surface emitting device with buried index guiding current confinement layer |
Legal Events
Date | Code | Title | Description |
---|---|---|---|
AK | Designated states |
Kind code of ref document: A2 Designated state(s): AE AG AL AM AT AU AZ BA BB BG BR BY BZ CA CH CN CO CR CU CZ DE DK DM DZ EC EE ES FI GB GD GE GH GM HR HU ID IL IN IS JP KE KG KP KR KZ LC LK LR LS LT LU LV MA MD MG MK MN MW MX MZ NO NZ PL PT RO RU SD SE SG SI SK SL TJ TM TR TT TZ UA UG UZ VN YU ZA ZW |
|
AL | Designated countries for regional patents |
Kind code of ref document: A2 Designated state(s): GH GM KE LS MW MZ SD SL SZ TZ UG ZM ZW AM AZ BY KG KZ MD RU TJ TM AT BE CH CY DE DK ES FI FR GB GR IE IT LU MC NL PT SE TR BF BJ CF CG CI CM GA GN GQ GW ML MR NE SN TD TG |
|
121 | Ep: the epo has been informed by wipo that ep was designated in this application | ||
REG | Reference to national code |
Ref country code: DE Ref legal event code: 8642 |
|
122 | Ep: pct application non-entry in european phase | ||
COP | Corrected version of pamphlet |
Free format text: PAGES 1/28-28/28, DRAWINGS, REPLACED BY NEW PAGES 1/13-13/13; DUE TO LATE TRANSMITTAL BY THE RECEIVING OFFICE |
|
NENP | Non-entry into the national phase |
Ref country code: JP |
|
WWW | Wipo information: withdrawn in national office |
Country of ref document: JP |