|Publication number||US3718511 A|
|Publication date||Feb 27, 1973|
|Filing date||Dec 15, 1970|
|Priority date||Dec 17, 1969|
|Also published as||DE2062041A1, DE2062041B2, DE2062041C3|
|Publication number||US 3718511 A, US 3718511A, US-A-3718511, US3718511 A, US3718511A|
|Original Assignee||Thomson Csf|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (1), Non-Patent Citations (1), Referenced by (16), Classifications (24)|
|External Links: USPTO, USPTO Assignment, Espacenet|
United States Patent [191 Moulin Feb. 27, 1973 PROCESS FOR EPITAXIALLY GROWING SEMICONDUCTOR CRYSTALS  Inventor: Michel Moulin,
France  Assignee: Thomson-CSF  Filed: Dec. 15, 1970  Appl. No.: 98,262
 Field of Search..'.148/l7l, 17 2, 1.5; 252/623 T; 117/113, 160, 200, 227, 201
 References Cited UNITED STATES PATENTS 3,403,133 9/1968 Fredrick et al ..252/62.3 T
OTHER PUBLICATIONS Hiscoclgs et al., Crystal Pulling and ggn stitution in PbixSnxTeJournal of Materials Science, Vol. 3, 1968, pages 76-79.
Primary Examiner-Robert D. Edmonds Attorney-Karl F. Ross [5 7 ABSTRACT A semiconductor with a PIN junction, adapted to be used as a detector or emitter of luminous radiation, is grown from a bath consisting of a liquefied mixture of three elements which are the constituents of two alloys or solid solutions taken from Groups lI/VI and/or IV/Vl of the Periodic Table. The proportions of the three elements in the mixture are so chosen that the solidus curve of the temperature/composition diagram intersects the stoichiometric line at a point lying along the boundary between the solid and the solid/liquid phase on the side of the lower concentrations of the element common to the two alloys. In a state of thermodynamic equilibrium for the liquid/solid phase, the bath is slowly cooled in a temperature range above or below a critical temperature corresponding to the point of intersection with resulting growth of an N- type or P-type layer on a substrate immersed in the bath. By a change in the composition, with or without a shift in temperature, the conductivity type of the layer is altered with formation of a PIN junction.
9 Claims, 7 Drawing Figures PATENIEUFEBZYIW 3,718,511
SHEET 15F 2 FIG. I
Michel Moulin Attorney PROCESS FOR EPITAXIALLY GROWING SEMICONDUCTOR CRYSTALS My present invention relates to a process for epitaxially growing semiconductor crystals of predetermined conductivity type, more specifically a unitary crystal body with at least two zones of opposite conductivity types forming a P/N junction therebetween.
Conventional processes for producing such semiconductors, by introducing impurities or altering the lattice structure of the crystalline body through alloying or diffusion, do not yield crystals of exactly predictable and reproducible quality. This quality depends on the electronic properties and other parameters of the starting material, such as the width of the forbidden band, which cannot always be accurately predetermined and which require a corrective heat treatment; however, the subsequent handling of the material at elevated temperatures tends to alter the characteristics of the product in an unfavorable manner, as by objectionably increasing the number of charge carriers in the n-type of p-type zone.
The general object of my invention is to provide an improved process for making such semiconductors with avoidance of the aforestated drawbacks.
A more specific object is to provide a process for reproducibly manufacturing a crystal structure adapted to be used as a detector or emitter of luminous radiation.
In accordance with my present invention, I utilize the phenomenon of expitaxial crystal growth from a suitable solution on a compatible substrate immersed therein, with control of the bath composition and the operating temperatures, to obtain a crystalline layer of the desired conductivity type which may be the same as or different from that of the substrate and which may be followed by the formation of a second crystal layer, of opposite conductivity type, upon a modification of the bath composition and without removal of the treated substrate therefrom. Thus, the resulting crystal body may have a junction between the portion thereof constituted by the original substrate and a layer of opposite conductivity type grown thereon, and/or between two such layers grown successively in the same bath. It is also possible to replace or supplement such a PIN junction by a so-called homojunction formed between zones of like conductivity type but different concentration of charge carriers to produce a resistance differential, the transition from one zone to the next being again accomplished by a suitable modification of the bath composition.
More particularly, the process according to my invention starts with the selection of two semiconductive compositions of two constituents each, these constituents being normally solid elements taken from Groups Il/VI and/or IV/Vl of the Periodic Table and including one element common to both compositions. It is preferred to utilize either selenium or tellurium, both from Group VI, as one of the elements and to choose the other two elements of the composition from among such metals as lead and tin (Group IV) and/or cadmium, zinc or mercury (Group II).
An important consideration for the choice of these elements is the requirement that they form alloys or solid solutions which can be represented by a tempera ture/composition diagram whose stoichiometric line is intersected by the solidus curve in certain compositions described hereinafter, the point of intersection lies along the boundary between the solid and the solid/liquid phases on the side of the lower concentrations of the common element, the liquidus curve of the diagram diverging from that boundary in the direction of these lower concentrations so as to be substantially spaced therefrom at the temperature level of this point of intersection.
Next, a bath consisting of a liquefied mixture of these three constituents is prepared in proportions corresponding to a point on the liquidus curve which is spaced from a neutral point on that curve, i.e., the one lying on the temperature level of the aforementioned point of intersection, in a direction consistent with the desired conductivity type of the crystal layer to be grown; this is the point of saturation and incipient solidification occurring upon a suitable lowering of the bath temperature. Now, a substrate of compatible crystal structure (such as a conventionally produced semiconductor body of the same basic composition) is immersed in the bath which is thereupon cooled at a controlled rate resulting in the growing of the desired layer on the substrate. Upon the attainment of a final temperature, still remote from the level of the neutral point on the liquidus curve, the controlled cooling is terminated at least temporarily, with or without immediate removal of the coated substrate from the bath according to the number of layers to be formed.
If a second layer is required, the proportion of the bath constituents is modified before further cooling. Thus, if a PIN junction is to be formed between the two layers, the modification may be such as to shift the saturation point on the liquidus curve to a location on the opposite side of the neutral point, generally in the direction of decreasing temperatures with resulting reliquefaction of the bath mixture at the aforementioned final temperature; alternatively, the modification may vary the position of the solidus curve so as to displace its point of intersection with the stoichiometric line to a location a the opposite side of the final temperature level previously reached, thus again with a reversal of the relative positions of the neutral point and the saturation point on the liquidus curve. In the first instance, the proportion of the common element with reference to the combined proportion of the two other elements may be reduced, preferably by the introduction of added amounts of these latter two elements with substantially no change of their relative proportion in the bath; in the second instance, the relative proportion of the last-mentioned elements may be varied with substantially no change in the proportion of the common element with reference to these other two. Both measures could, however, also be used jointly.
The invention will be described in greater detail hereinafter with reference to the accompanying drawing in which:
FIGS. 1 and 2 are temperature/composition diagrams of different two-component mixtures to be used as starting materials for a process according to the invention;
FIGS. 3 and 4 are similar diagrams for a three-component composition consisting of the constituents of the mixtures of FIGS. 1 and 2; and
FIGS. 5 '7 are somewhat schematic cross-sectional views of semiconductor bodies obtained by the process according to my invention.
In FIG. 1 I have shown the phase diagram of a leadtellurium alloy, with the proportion of lead in terms of atomic concentration decreasing from 100 percent to percent from left to right and with the corresponding proportion of tellurium similarly decreasing from right to left. The diagram shows, at the 50 percent value, a stoichiometric line l (denoting the intrinsic semiconductor material corresponding to this composition) along with a solidus curve 11 and a liquidus curve Ill separating a liquid phase (liq), a liquid-solidphase (ligsol) and a solid phase (sol). To the left of line l there is shown a region N representing n-type conductivity for compositions with a relatively low tellurium content, the opposite region P on the right denoting p-type conductivity with a high proportion of tellurium. It will be noted that solidus curve II does not intersect the stoichiometric line I at any temperature plotted on the diagram, except at the common vertex of the two curves.
FIG. 2 shows a corresponding diagram for a mixture of tin and tellurium. Here the solidus curve 11 is offset to the right from stoichiometric line I, i.e., in the direction of increasing percentages oftellurium (region P), without ever"intersecting that line.
The diagram of FIG. 3 relates to a mixture of the two compositions represented in FIGS. 1 and 2, i.e., a three-component alloy consisting of lead, tin and tellurium. The combined proportion of lead and tin decreases from 100 to 0 percent from left to right, the proportion of tellurium (the common element of the two starting compositions) decreasing again from right to left in this diagram. We may consider the composition of the mixture as given by with the parenthetical terms expressing the respective atomic concentrations.
The solidus curve 11 of FIG. 3 intersects the stoichiometric line I at a point lying on atemperature level T, (about 470 C); this point of intersection 10 is located on the boundary between the liquid-solid and the solid phase at the left of the diagram, thus on the side of the lower percentages of the common component Te. On the same temperature level T, there exists on the liquidus curve 111 a neutral point 20 corresponding to a relatively low proportion u, of (Te) and (Pb Sn). The relative proportion x of lead and tin in that mixture does not affect the location of points 10 and 20 so long as the overall proportion u is maintained constant.
If the bath composition were such as to correspond to the ratio u,, a cooling of the liquid from a more elevated temperature to the level T, would lead to incipient solidification at the neutral point 20, with a growth ofa layer of intrinsic (high-resistance) material on an immersed substrate. In accordance with this invention, however, l choose an initial composition corresponding to either a ratio such as u,,, with a saturation point 21 on a temperature level T, (in a range of 500 to 550 C) corresponding to a point 11 on solidus curve 11 well above point 10 and within the n-type region N of the diagram, or a ratio such as u with a saturation point 22 on a temperature level T, (in a range of 400 to 450 C) corresponding to a point 12 on curve 11 well below point 10 and within the p-type region P.
As illustrated in FIG. 4, a change in the relative proportion of lead and tin with maintenance of the stoichiometrically effective overall ratio 11, results in a shifting of the solidus curve ll either toward the left, thus into a position II' closer to that of the Pb/Te diagram of FIG. I, or toward the right, i.e., into a position ll closer to that of the Sn/Te diagram of FIG. 2. Curve II intersects the line I at a point 10' and the temperature level T, at a point 11' within the n-type region N; curve Il" intersects the line I at a point 10" and the temperature level Te, at a point 12" within the p-type region P.
Thus, modifying x instead of u also shifts the relative position of the saturation point and the neutral point on the liquidus curve. With a displacement of the point of intersection 10 to either the position 10 or the position 10" in FIG. 4, the neutral point 20 is similarly displaced to a position 20' or 20" so that either n-type or p-type deposits can be obtained with an initial bath composition u, and with a temperature in the neighborhood of level T,.
The following Examples serve to illustrate the several aspects of my invention:
EXAMPLE I It is desired to form a junction between a p-type monocrystalline substrate of composition (Pb Sn, )1 and.
The substrate is a wafer out along a privileged crystal plane from a suitably doped p-type body of the composition stated, produced by a conventional crystaldrawing process.
The bath chosen or the formation of the junction has the composition (Pb Sn Se corresponding to x 0.1 land u 0.05. This mixture is heated in a protective atmosphere of argon to a temperature of 800 C, above the liquidus curve on the PbSn side of the associated phase diagram which is generally similar to that of FIGS. 3 and 4.
This bath is cooled to a level of about 700 C corresponding to the point 21 in the diagram of FIG. 3. At
this point, the substrate is immersed into the saturated solution from which a layer consisting predominantly of lead and tin begins to crystallize on the substrate. Next, the bath temperature is progressively lowered over a period of about 10 minutes to about 650 C, well above the level T,. The substrate is then removed and is found to have an epitaxial layer of the same basic composition (Pb,S, Se) as the substrate, with a lead/tin ratio corresponding to X=0.06, which is of n-type conductivity. The growth rate of this layer is on the order of 2 microns per minute.
EXAMPLE II It is desired to produce a semiconductor of the same general type as that obtained in Example I, with two epitaxial layers ofp and n type, respectively, separated by a P/N junction. The first, n-type layer is produced in the same manner as in the preceding Example. When the final bath temperature of 650 C is reached, however, the substrate is not removed but the composition of the bath is modified by the introduction of a sufficient quantity of a lead/tin alloy to change the propor tion of selenium from a value u 0.05 (corresponding to u,, in FIG. 3) to a value u =0.03 (corresponding to u, in FIG. 3), without change in the magnitude of x. With these altered proportions, the operating point is shifted to the left of the liquidus curve III so that the mixture is reliquefied, requiring further cooling to about 600 C restore saturation at a new point of incipient solidification corresponding to point 22 of FIG. 3.
Thereafter, controlled cooling is resumed for a period of, say, 20 minutes with formation of a second, p-type layer (with x 0.075) on the n-type layer already present on the substrate which is then removed from the bath.
EXAMPLE III The semiconductor body described in the preceding Example can be produced by modifying the bath concentration, after formation of the n-type first layer in the manner. described, by introducing a sufficient amount of selenium and tin to change the value of x from 0.11 to 0.15, with u remaining at its original value of 0.05. This establishes a new solidus curve, similar to curve II of FIG. 4, to the right of the original curve whereby the deposit obtained upon further controlled cooling is of p-type conductivity as explained above.
EXAMPLE IV To produce a semiconductor akin to that of Example I but with the selenium replaced by tellurium, a bath consisting of lead, tin and tellurium as discussed in conjunction with FIGS. 3 and 4 is used with a composition (Pb Sn Te i.e., with x 0.30 and u 0.05. The substrate, in this case, is a conventionally drawn monocrystal composed of lead, tin and tellerium. The n-type layer grown on that substrate is of substantially the same composition, with x 0.20 and Controlled cooling takes place from 550 to about 500 Cl Again, a second (p-type) layer may be deposited on the n-type first layer by the technique of Example [I or III.
In FIGS. 5 7 I have shown several types of semiconductor obtainable with the process described above. According to FIG. 5, a substrate 30 of p-type conductivity is covered by an epitaxially grown n-type layer 31 forming therewith a junction 32, the two major faces of being electrically-energized in the forward direction of one of their junctions. To obtain a laser-type stimulation of this emission, it is desirable to subdivide an exposed layer, as illustrated in FIG. 7 for the layer 31, in two dimensions into a multiplicity of small segments of approximately cubic shape measuring, for example, 0.5 mm on each side. These cubes, which may be produced by etching through a mask of silicon oxide, are provided on all lateral faces with semireflecting coatings 37 to intensify the emission of substantially monochromatic light from the plane of junction 32.
The concentration of charge carriers and the physical thickness of the various layers depends on the intended use of the semiconductor. For photovoltaic detection, for example, the light-receiving layer (31 in FIG. 5) should be only limitedly conductive, i.e., should deviate only slightly in its composition from the stoichiometric relationship, and should have a thickness depending on the absorptivity of the material for the wavelengths to be detected, the adjoining zone of opposite conductivity type being given a strong concentration of charge carriers to minimize current flow in the nonilluminated state. The use of homojunction, as described above, in the position of boundary 36 (thus with additional n-type material of different carrier concentration on the segments of FIG. 7) may help solve the problem of guiding the radiation in an emitter of light, particularly if this emitter has an active layer 31 insufficiently transparent to this radiation.
The process described hereinabove does not exclude, of course, the possibility of additionally doping one or more of the layers by conventional techniques to modify their carrier concentrations.
For the detection and emission of infrared radiation, particularly suitable compositions include (Cd, Hg)Te in addition to the lead/tin/tellurium and lead/tin/selenium alloys discussed above. For the visible spectrum, Sn(Se,Te) is preferred.
l. A process for epitaxially growing a semiconductor crystal of predetermined conductivity type, comprising the steps of:
selecting two semiconductive compositions of two normally solid elements each, including one element common to both compositions, with constituents from Groups II/VI or IV/VI of the Periodic Table, said constituents being cadmium or mercury in Group II, lead or tin in Group IV and selenium or tellurium in Group VI, said common element being selenium, tellurium or zinc, the other two elements being members of the same Group;
preparing a bath consisting of a liquefied mixture of the constituents of said compositions in proportions giving rise to a temperature/composition diagram with a stoichiometric line and with a solidus curve intersecting said line at a first point lying along the boundary between the solid and the solid/liquid phases on one side of the solidus curve, said diagram having a liquidus curve diverging from said boundary and defining a second point on the temperature level of said first point, said proportions being chosen to correspond to a third point on said liquidus curve spaced from said second point in a direction consistent with said predetermined conductivity type;
lowering the temperature of said bath from an elevated level to a level of incipient solidification corresponding to said third point;
immersing into said bath a substrate of a crystal structure compatible with that of a solid mixture of said constituents;
progressively cooling said bath at a controlled rate with growth of a layer of said predetermined conductivity type on said substrate;
and terminating the controlled cooling of said bath at a final temperature remote from the level of said first and second points with subsequent removal of said substrate therefrom.
2. A process as defined in claim 1 wherein, following termination of controlled cooling and prior to removal of said substrate, the proportion of said constituents is modified in said bath to reverse said conductivity type, with subsequent continuation of controlled cooling and formation of another layer of opposite conductivity type on said substrate.
3. A process as defined in claim 2 wherein the modification of the bath composition involves a diminution of the proportion of said common element with reference to the combined proportion of the other two elements, with substantially no change in the relative proportion of said other two elements and with resulting reliquefaction of the mixture at said final temperature above a fourth point on said liquidus curve which is spaced from said second point in a direction opposite said third point and consistent with said opposite conductivity type, the bath temperature being lowered to said fourth point prior to resumption of controlled cooling to form said other layer.
4. A process as defined in claim 2 wherein the modification of the bath composition involves the substantial maintenance of the original proportion of said common element with reference to the combined proportion of the other two elements, with a change in the relative proportion of said other two elements resulting in a shifting of said boundary and corresponding displacement of said first point sufficient to move said second point onto the opposite side of said third point, controlled cooling being resumed without reliquefaction of the mixture.
5. A process as defined in claim 1 wherein said common element is tellurium or selenium and the other two elements are lead and tin.
6. A process as defined in claim 5 wherein the initial bath composition is substantially (Pb Sn Te 7. A process as defined in claim 5 wherein the initial bath composition is substantially (Pb Sn Se 8. A process as defined in claim 1 wherein said common element is tellurium and the other two elements are cadmium and mercury.
9. A process as defined in claim 1 wherein said common element is zinc and the other two elements are selenium and tellurium.
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|U.S. Classification||117/56, 117/956, 117/67, 257/441, 438/95, 148/DIG.630, 257/461, 117/939|
|International Classification||H01L21/00, H01L21/208, H01L29/24, H01L31/00, H01L33/00|
|Cooperative Classification||H01L21/00, H01L33/00, Y10S148/063, H01L29/24, H01L31/00, H01L21/208|
|European Classification||H01L21/00, H01L33/00, H01L29/24, H01L21/208, H01L31/00|