CA1200468A - Crystal of germanium and gallium arsenide - Google Patents

Abstract

CRYSTAL OF GERMANIUM AND GALLIUM ARSENIDE
Abstract A crystal of Ga(1-0.5x)Gex As(1-0.5x). The crystals are grown by epitaxial chemical vapor deposition with varying concentrations of the germanium which may be positioned on either or both the gallium and the arsenic sites in the respective sublattices.

Description

CRYSTAL OF GE~MANIUM AND GALLIUM ARSENIDÆ
DESCRIPTION
. .
In the field of devices useful in energy conversion applications, crystals are being souyht with different types of physical properties. Efforts are being directed toward the formation of crystals with different bandgaps which will have electrical and electro-optical properties not hexetofore available. As one example, in the field of light emitting devices there has been a subs~antial need in the art for devices that exhibit light emission ir. the 1 to 1.3 ~m wavelength range.
There has been effort in the art as set forth in Physical Review, Vol. 108, No. 4, p. 965, November 15, 1957 to identify many of the physical properties of intermetallic compounds made of elements from Group III and Group V o~ the Periodic Table of Elements.
Solid solutions of gallium arsenide with appreciable concentrations of germanium would exhibit light emission in the 1-1.3 ~m wavelenyth range. Further compounds of Gal xAs Gex in film thicknesses suitable for semi-conductor device purposes have been manufactured in accordance with the teachings of U.S. Patent No. 3,979,271 by the physical process of cosputtering the various ingrPdients. In this work, however, the relative composi-tions were not reported. At this state of the art thecrystals produced by the physical process of sputtering have imperfection problems that serve as a limit to device applications.

YOg-77-064XX ,~_S,~q~3,~

The elements germanium has been employed as a dopant in GaAs semiconductor devices as set forth in Journal of Applied Physics, Vol. 41, No. 1, Jan. 1970, p. 264.
However, the solid solubility limit in equilibrium processes is as set forth in the Physical Review, Vol. 108, No. 4, p. 965 is of the order of 0.5 atomic % or 0.005 atomic fraction so that as a dopant the germanium concentration is of that order. The germanium is known to be amphoteric as set forth in Journal of Electrochemical Society, Vol. 108, No. 8, p. 716, 1961.
. . . ~ .
In accordance with the invention there is produced a crystal of the Group III-V intermetallic compound gallium arsenide containing the Group IV element germanium selectively positioned on sites in either or both the gallium or the arsenic sublattice. The crystal may also be identified as a Group III-IV-V crystal. The invention involves a technique oE producing a semiconductor quality GaAs crystal with selectively positioned germanium which can be placed on either or both of the gallium or arsenide sublattice in selective concentrations by control of concentration of the Ge-Ga and the Ge-As ratios during the process of chemical vapor growth. The crystal of the invention is described by the designation Ga(l-0 5x)GexAs(l-O.5x) ;
In this designation x is the germanium atom fraction.

The atom fraction may be contrasted with atom percent in that the atom percent is 100 times the atom fraction.
As an illustration, assuming x, the germanium fraction in a particular crystal to be 0.3, the designation for such a crystal would then be Ga(0 85)Ge(0 3)As(0 85)~

Yo~-77-064~x ~ n~33~ ~ ~

which means that 15 gallium atoms are taken from every 100 gallium atoms and 15 arsenic atoms are taken from every 100 arsenic atoms to provide room for 30 germanium atoms.
Another aspect of this invention includes: In a crystal of gallium arsenide containing germanium the improvement comprising the positioning of germanium atoms on both the gallium and on the arsenic sites in the crystal in preselected relative concentrations, and including a quantity of conductivity type determining impurity in said crystal.

Brief Description of the Drawings FIG. 1 is an atomic lattice illustration of the Ga(1-0 5X~GeXAS(1 0 5x) Crystal of the inventiOn FIG. 2 is a graph of the var.iation in bandgap with germanium concentration for the crystal of the invention.
E'IG. 3 is an illustration of one apparatus for practicing the invention.
FIG. 4 is a temperature profile illustrating the temperature within the apparatus of FIG. 3.
E`IG. 5 is an illustration of another apparatus for practicing the invention.
FIG. 6 is a temperature profile illustrating the temperature within the apparatus of FIG. 5.
FIG. 7 is an illustration of still another apparatus for perfoxmin~ the invention r FIG. 8 is a temperature profile illustrating the temperature within the apparatus of FIG. 7.

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Y~9-77~064XX

Referring to FIG. 1 an illustration is provided of a section of the lattice structure of the crystal of the invention. The lattice structure is of the zinc-blende type having a first periodic sublattice of Ga atoms shown ~s ~ bonded to each other, a second sublattice of arsenic atoms shown as O bonded to each other, with both sublattices in t~.rn bonded into a single crystal. The germanium atoms introduced in accordance with t~.e invention are shown as ~
occupying selectively both in location and concentratiOn sites that are either on the gallium or on the arsenic sublattices, or both.
The crystal of the invention may further be doped with conductivity type determining impurities which may enter ; the crystal either substitutionally at sites ir, the lattice such as ar;y atom site in FIG. 1 or where the impurity atom is of the proper size smallex than the lattice spacing the conductivity type determining impurities may enter the la-ttice interstitially between lattice sites. The crystal may be doped with a Group II impurity such as Zn or Cd to impart p conductivity type semiconductor performance and it may be doped with a Group VI impurity such as Te or Se to produce n conductiv.ity type semiconductor performance.
The germanium atoms can be directed to the desired sublattice sites ~y control of the relative ratios of -the Ge-As and Ge-Ga vapors in a chemical vapor deposition : op,eration.

(l-0.5x)GexAs(l_o.sx) crystal exhibits the ; property of varying bandgap with concentration. ~he varia-tion in bandgap is illustrated in a graph in FIG. 2 wherein both the direct and the indirect bandgap variation with germanium fraction (x) at the temperature of 300Ko From the graph one skilled in the art will be able to select a crystal composition for light emission in a desired fre~uency.

The invention is a Ga(l_0~5x)Gex As(l 0.5x) Y
which has a number of properties not available hereto-fore in the axt. The preferred metnod of formation is that of chemical vapor growth prlncipally because the many parameters involved in growth may be precisely controlled, the resulting crystals are of high quality and are free of imperfections that could be semiconductor carrier traps. In the light of the principles set forth, however, it will be apparent to one skilled in the art that physical processes currently under development in the art such as molecular beam epitaxy may be adapted to produce the invention.

FIGS. 3, 5 and 7 illustrate three types of vapor growth apparatus in which the invention can be practiced. In each apparatus conditions are provided for the trans-port of each element entering the reaction in a separate vapor to the site of the growth substrate ; 20 which has the lattice periodicity established, where a condition is maintained that is a departure from equilibrium so that the gas decomposes and as the decomposition takes place, the reaction cannot reverse and the atoms of each ingredient element assume a lattice position and grow on the substrate.

The departure from equilibrium can be achieved with any parameter to which the decomposition reaction is sensitive and should be such that the reaction is essentially irreversible. The departure from equili-brium in each of FIGS. 4, 6 and 8 is provided as areduced temperature at the substrate. Where a differ ence in temperature is employed about 25% of the total temperature is satisfactory.

c~

Referring now to FIG. 3~ The apparatus is a furnace for chemical vapor growth. The furnace is equipped with the standard multizone external heat source shown as a jacket 1 and within the heat source 1 a reac~ion chamber 2, generally made uf quartz, is positioned. The chamber 2 has at least two introduc-tion ports, 3 and 4, and at least one exhaust port 5.
The element gallium is liquid at most working tempera-tures and provision is made for ingredien-t 6 which is the gallium in an open top container upstream o~ the substrate with the port 3 positioned to permit one gas to flow over the ingredient 6 and the port 4 being sufficiently long that vapors introduced therein are rnixed with vapors from ingredient 6 downstream or ingredient 6 in a deposition zone in the vicinity of the substrates 7. The substrates 7 are surrounded by a heat zone in jac~et 1 which permi~s the substrate temperature to be established at some level different from the temperature of the region surrounding the ingredients, thereby providing an equilibrium shift.

EXAMPLE I

In a particular example referring to FIGS. 3 and 4, the ingredient of the gallium arsenide crystal that is liquid, the gallium, will be positioned as ingredient 6. The gases introduced will be diluted with hydrogen to maintain a reducing atmosphere and for concentra-tion control. Hydrogen chloride, with a bypass so that it can be di.luted with hydrogen, is introduced at port 3 to form GaCl. At port 4 there will be three branches which are not shown. The first branch includes select-able sources of hydrogen and heliu~.. The second branch is a source of arsenic trichloride AsC13, usually maintained at room tempe~ature, with a controllable bypass of hydrogen. ~he third branch is a source of germanium tetrachloride GeC14, maintained generally at 0C, with a hydrcgen bypassO With this arransement JI~
each gas concentration and flow rate as well as the overall flow rate may be maintained.

Assuming the chamber 2 to have a 55 mm inside diameter and the ports 3 and 4 each ha~il.g a 4 mm inside diameter, the following specifications ln the process steps will apply.

The first step is to purge. This is done by bypassing, first with helium then with hydrogen, the source of gerrnanium, the source of arsenic and the source of hydrogen chloride.

In the second step, a small flow of hydrogen is kept going all the time during the following reaction steps.

Step 3. The flows through the hydrogen chloride lS source, the germanium source and the arsenic source are next established.

The flow through port 3 of hydrogen chloxide plus hydrogen will be 50 milliliters per minute, producing thereby concentration of hydrogen chloride of 7xlO 6 moles per/milliliter.

The flow through the germanium tetrachloride, mai~-tained at 0C, will be established at a concentration of lO 6 moles per milliliter.

The flow through the arsenic trichloride, maintained at room temperature, will be established at a con-centration of 6x10 7 moles per/milliliter.

With these concentrations the flow of germanium tetrachloride and arsenic trichloride will nominally be 25 milliliters per/minute each including the H2 carrier gas.

6~

Since, as may be seen from FIG. 3, the tube from port 4 passes through the high temperature zone, a total hydrogen flow of lO0 milliliters is provided to make sure that no decomposition of any of the GeC14 or As~13 with resultlng deposit occurs in the tu~e in the high temperature zone.

Referring to FIG. 4 the temperature in the high temperature region is maintained in the range of from 750 to 900C whereas in the lower temperature region the temperature is maintained in the range of 600 to 750C with a differential maintained at about 150~C.

Under these conditions qermanium will be incorporated in the gallium arsenide as the gallium arsenide crystal lattice grows epitaxially on the substrates 7.

The germanium prefers the gallium sites in the lattice, if the concentrations of the As and Ge gasseous species are appro~imately equal.

The concentration of germanium and the location of the germanium can be altered by forcing the germanium from the gallium site to the arsenic sites. This is done by decreasing the ratio of the gases of the individual elements in the reaction zone. For the Example l under discussion, the ratios would he altered in the following way.

The hydrogen flow through the arsenic trichloride is reduced to about 15 milliliters while the hydrogen 1OW through the germanium tetrachloride source is increased to about 35 milliliters. IJnder these con-ditions the germanium occupies the arsenic sites ar.dgal]ium sites appro~imately equallv.

~7~

The growth rate requires monitoring, since the germanium galli~ arsenide crystal is metastable, if an attempt is made to grow too fast, two phases will result in which there will be a separate gallium 5 arsenide and a germanium pnase.

Consistent with maintaining a single phase the lower the substrate temperature the larger quantity or (~) fraction of germanium will be incorporated into the crystal.

The crystal is inherently n conductlvity type. Con-ductivity type determining impurities may be intro-duced to direct conductivity type and to determine resistivity.

A conductivity type determining impurity from Group II
will impart p type conductivity or from Group VI will impart n type conductivity. The impurities can be introduced either as a separate branch going in through port 4 or in the alternative as a source positioned inside the chamber 2.

EXP~LE II

The con-trol of quantity and sublattlce location of the germanium may be exercised by providing a differ-ence in surface area o germanium and gallium arsenide solid sources in a chemical vapor growth operation.

Referring to ~IG. 5 an apparatus similar to E'IG. 3 and bearing the same reference numerals is provided.
The dlfference between the apparatus of FIGS. 3 and 5 is that in FIG. 5 the ingredient elements are provided in solid or liquid form so that different surface 30` areas are available. The elements Ga and As are provided as a solid Ga~s compound source 6A and the 06~X~

germanium is a solid labelled 6B. Port 3 is the only one used. As may be seen from FIG. 6 the departure from equilibrium is provided by maintaining the substrates at a lower temperature.

A diluted halogen carrier such as bromine plus hydrogen are introduced at port 3. The germanium quanti-ty 6B is physically smaller than the quantity 6A of gallium-arsenide and the halogen carrier flows freely over both.

In the process the first step would be to establish a flow rate of bromine and hydrogen through port 3 at about 50 milliliters per mlnute and with a tube inside diameter of 55 mm and a port 3 diameter of 4 mm, a concentration of bromine of 3xlO 6 moles will be rnain~ained.

The germanium sample 6B will be about 20cm2 and the gallium arsenide sample will have a surface area of about 80cm2.

Referring to FIG. 6 a high temperature is maintainecl in the region of the ingxedients 6A and 6B and a differential ~o a lower temperature is maintained in the region of the substrates 7. The ingredlents 6A
and 6B are maintained in the range of 750 to 900C
and the substrates are maintained in the range of 600C to 750C with a 150 differential.

At the high temperature region gallium bromide, arsenic bromide and germanium bromide each will be formed and each will decompose in the vicinity of , the substrate 7.

With this size ratio of elements 6A and 6B the ratio of germanium bromide to gallium bromide and arsenic bromide is such that a crystal of Ga(l o 5x)Ge~

YO9~77-064XX

As(l 0 5x) is grown. An adjustment ln the size of the germanium sample relative to the size of the GaAs sample will change the ratio of GeBr2, AsBr3 gases and can produce a crystal with a larger germanium ,ract~on occupying the As sites.

While elements 6A and 6B are shown positioned vertically, it will be clear to one skilled in the art that they may be positioned side by side.

EX~PLE III

The ingredient elements may all be delivered as gases so that the relative concentrations may be readily controlled over the substrates by varying the rates.

In FIG. 7 an apparatus similar to both FIGS. 3 and 5 is provided with modifications that port 4 only is used but it now has two counterparts. The ports are shown as 4A, 4B and 4C. The Ga ingredient 6 shown as a solid or liquid is positioned in port 4B.

The temperature in the chamber 2 in the vicinity of the ports 4A, 4B and 4C is maintained around 750 to 900C and the temperature in the vicinity of the sub-strates is maintained about 600~ to 750C with a 150C differential between regions. Through port 4B
HCl or Br2 diluted with H2 is passed, similar to the preceding e~amples.

Through port 4A, GeC14 diluted with H2 at a flow rate of 25 ml/min at a concentration of 10-6 moles per milliliter is introduced.

Through port 4C, AsC 3 diluted with H2 is passed over the Ga source 6 at a concentration of 6xlO moles/
milliliter.

YO~-77-064XX
,?,~046E3 Under these conditions a crystal of Ga(l 0 5x)Ge~
As(l o 5x) is formed wherein the majority of the Ge i5 incorporated on the Ga sublattice. The ratio o AsCl3 to GeCl4 may be changed by reducing the flow of AsCl3 ~o 15 ml/min and increasir.~ the GeC'4 f 1W to 25 ml/min resulting in a crystal with Ge dlstributed approximately equally on the Ga and As sublattices.

The above examples are not exhaustive of process approaches and in the light of the principles set forth, one skilled in the art can select the desired parameters. The process must provide a departure from equilibrium at deposition and the further from equlibrium the more Ge can be incorporated. Other types of suitable processes are the organometallic and the molecular beam epitaxy processes.

YO9-77~064XX ~,?,~

The following table of specific crystals and proper-ties is provided as an indication of the results of process parameter variations:

Ge CARRIER 2 FRACTION CONDUCTIVITY DENSITy cm 5S~lPLE (x) TYPE PER cmJ MOBILITY Vs 1 0.0027 n 6X1017 348

2 0.0020 n 6X1017 300

3 0.0038 n 8.4x1017 338

4 0.0023 n l.9x1018 388 10 5 0.025 n 6.3x1017 376 6 0.011 n 1x1018 450 7 0.011 n 5X1017 225 8 0.004 n 4.5x1017 348 9 0.004 n 7X1017 373 15 10 0.004 n 6X1016 338 11 0.004~ p 5.2x1017 282 12 0.0045 n 13 0.0047 n 14 0.003 n 9xlol7 353 20 15 0.061 n Ge substrate 16 0.003 n 8.6x1017 399 17 0.0079 n 18 0.0085 n 19 0.0080 n 25 20 0.0069 n 2.4x1017 306 21 0.035 n Ge substrate 22 0.0026 n 1.8x1018 408 23 0.023 n 2X1017 995 24 0.0454 n 30 25 0.036 n 26 0.025 n 8.2x1017 383 27 0.026 n 28 0.026 n 7.2x1017 727 29 0.035 n 0.109 n 31 0.0207 Zn p l.lx1019 19.4 doped YO9-77-064~X

Ge CARRIER 2 F~ACTION CONDUCTIVITY DE~SIT~ cm S~IPLE (~) TYPE PER cm MOBILITY vs __ 32 0.020~Zn p

5 33 0.043~doped p 34 0.ol0J p - 6.3x10l8 93.6 0.99 n Ge substrate 36 0.299 n What has been described is a new semiconductor crystal and techniques for vapor qrowing that crystal where-in the concentrations of arsenic and qallium with respect to germanium are adjusted such that the germanium is controllably positioned on the qallium or the arsenic sublattice with the contro] being both by location and by concentration~

Claims (14)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A semiconductor crystal of gallium arsenide having germanium atoms in an atom fraction greater than 0.005 essentially evenly occupying both gallium and arsenic sublattice sites.

2. A crystal Ga(1-0.5x)GexAs(1-0.5x) wherein the x quantity of Ge is positioned on each of the Ga and the As sublattices of said crystal.

3. The crystal of Claim 2 wherein said x fraction of Ge is evenly divided between said Ga and said As.

4. In a crystal of gallium arsenide containing germanium the improvement comprising the positioning of germanium atoms on both the gallium and on the arsenic sites in the crystal in preselected relative concentrations, and including a quantity of conductivity type determining impurity in said crystal.

5. The process of forming a crystal of gallium arsenide containing germanium comprising the steps of providing separate gaseous compounds of each of said gallium, said arsenic and said germanium;
providing separate control of the ratio of said germanium gaseous compound with respect to said gallium gaseous compound and with respect to said arsenic gaseous compound;
providing a deposition zone in which there is positioned at least one substrate compatible for epitaxial growth;
maintaining a deposition condition in said deposition zone such that each said gaseous compound of said gallium, said arsenic and said germanium irre-versibly decomposes.

6. The process of Claim 5 wherein each said gaseous compound is halogen compound.

7. The process of Claim 6 wherein said deposition condition is a lower temperature.

8. The process of Claim 5 wherein said separate control is achieved by providing different areas for at least one of the sources of said gaseous compounds.

9. The process of Claim 5 wherein said separate control is achieved by varying the individual gaseous compound rates.

10. The process of Claim 5 including the step of introducing into said deposition zone a conductivity deter-mining impurity.

11. In a process of forming a crystal of gallium arsenide containing germanium by decomposing over an epitaxial crystal growth substrate maintained at a lower temperature a plurality of gaseous compounds each containing at least one of germanium, gallium and arsenic ingredients, the improvement for maintaining a selected ratio of said ingredients in gaseous form during deposition comprising passing a gaseous carrier over separate quantities of said ingredients in non-gaseous form, each of said quantities having a surface area with respect to the others of said quantities in proportion to said selected ratio.

12. The process of claim 11 wherein one of said quantities is solid gallium arsenide and another of said quantities is solid germanium.

13. The process of claim 12 wherein said gaseous carrier is bromine plus hydrogen.

14. The process of claim 12 for producing a gallium arsenide crystal having germanium occupying essentially equal gallium and arsenic sublattice sites wherein said germanium quantity is 20 cm2, said gallium arsenide quantity is 80 cm2 and said substrate is maintained at essentially at a 150°C lower temperature than said quantities.