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Liquid phase epitaxy of gallium arsenide on zinc selenide
Journal of Crystal
Growth 15 (1972) 204-210
LIQUID
PHASE EPITAXY
J. W. BALCH Department Received
0 North Holland Publishing
Co.
OF GALLIUM
ARSENIDE
ON ZINC SELENIDE
and W. W. ANDERSON
of Electrical 15 October
Engineering,
1971; revised
The Ohio State
manuscript
University,
received
Columbus,
28 February
Ohio 43210,
U.S.A.
1972
Gallium arsenide has been grown upon (11 I), (IiT) and (110) surfaces of ZnSe by liquid phase epitaxy from Ga and Sn solutions. GaAs films grown from Ga solutions were usually characterized by rough junctions or solvent inclusions. These films were always degenerate n-type, with electron concentrations of about 10t9/cm3. GaAs films grown from Sn solutions had smooth junctions free of gross defects, and uniformly thin films of GaAs up to 20 urn thick were obtained for (111) oriented substrates. Films grown from undoped Sn solutions were n-type with electron concentrations of about 10”/cm3. By adding Zn to the Sn solutions, p-type films with hole concentrations of 1019/cm3 were obtained.
Mach et a1.6) have recently reported some success in using such a technique. Consequently, a liquid phase epitaxy process has been developed for growing thin layers of GaAs on ZnSe.
1. Introduction The GaAs-ZnSe heterojunction is of interest for studies of several potential devices including heterojunction transistors’) and solar cells’) and as an optical waveguide for investigating nonlinear optical interactions in GaAs3). To investigate devices which require a thin layer of GaAs (l-10 urn - such as the GaAsZnSe optical waveguide) an epitaxial process of growing GaAs on ZnSe is desirable. Various HCl chemical vapor deposition processes have been used for growing ZnSe upon GaAs“,5), but experience here and in other laboratories has shown that similar processes cannot easily be employed to grow GaAs on ZnSe, although
2. Experimental
The modified system’) used to grow GaAs on ZnSe substrates is shown schematically in fig. 1. The high temperature parts of the system consisted of a synthetic quartz furnace tube, boat holder and slider, and a boat and substrate holder made of spectra-pure graphite (nominal purity 99.9995 %). A quartz window in the small tipping furnace allowed the saturation temperature to be determined by visual observation
View Quartz
Quartz
methods
Port F
Slider
Substrate
Holder
and Substrate
D-4
Chamber Graphite
Fig. 1.
Schematic
diagram
Boat
of liquid
204
Cold Trap 190’K
J
phase
epitaxy
apparatus.
Oil Bubbler
LIQUID
PHASE
EPITAXY
OF GALLIUM
and also established a slight vertical temperature gradient. At the front of the quartz tube, the boat holder was supported by a gas-tight stainless steel coupling with a Viton O-ring which allowed the boat to be inserted or removed from the furnace. A high purity hydrogen or argon atmosphere was supplied to the quartz chamber through stainless steel cold traps cooled respectively by liquid nitrogen or dry ice-acetone, Pyrex-teflon metering valves, Pyrex-sapphire flow meters and teflon tubing with walls 4 in. thick. The exhaust of the system was through a dry iceacetone cold trap and oil bubbler. Joints between the glass, metal, or teflon tubes were made with high density heat shrinkable polyethylene tubing. The growth solution was contained in a well in the graphite boat and remained stationary during the tipping of the furnace. When the furnace was tipped, the quartz slider moved the ZnSe substrate onto the surface of the saturated solution where growth occurred. Both GaAs and ZnSe are less dense than the Ga or Sn solutions so that the substrate floats on the solution. To insure that the quartz slider would remove the sample from the solution when the furnace was tipped back after the GaAs had been grown, the top surface of the solution had to be even with the graphite floor on which the quartz slider moved. A solution of slightly larger volume than the well was used so that when the furnace was initially tipped, the quartz slider and substrate holder removed a thin layer of the solution and deposited the substrate on a clean solution surface that was even with the graphite floor. The Ga or Sn solution was prepared by placing a mixture of metallic solvent and GaAs in the graphite boat. Gallium of 7N purity was used as supplied. Tin of 6N purity was cut from an ingot, etched in a HFHNO, solution and rinsed in distilled water. Two types of GaAs were used : (1) polycrystalline p-type (77.5 Q-cm) and (2) single crystalline Te-doped n-type with a 4.8 x 10’6/cm3 carrier concentration. Preparation of the GaAs involved a polishing solution of 3 H,SO,-1 H,O,-1 H,O followed by rinses in cold distilled water and hot methanol. When Zn was used to dope the Sn solutions, the Zn was etched in an HNO, solution. The amounts of metallic solvent and GaAs needed to make approximately a 1 g solution of a given liquidus temperature were determined from the publish-
ARSENIDE
ed solubility
ON
ZINC
curves
205
SELENIDE
of GaAs
in Ga8,9)
and
Sn9).
Liquidus temperatures used for Ga solutions were in the range 725-870 “C and for Sn solutions 460-640 “C. After being loaded, the solution was baked in a hydrogen atmosphere for 1 hr at 150 “C above the liquidus temperature before any epitaxial growth runs were attempted. GaAs was grown on the (1 IO), (11 I), and surfaces of ZnSe substrates which were either insulating or semiconducting. The substrates were prepared from insulating boules of single crystalline melt-grown ZnSe
(iii)
supplied by Dr. Y. S. Park of the Aero-Space Research Laboratories. Semiconducting substrates with a resistivity of about 1 Q-cm were obtained by firing large area wafers approximately 0.5 mm thick in molten Zn or Zn vapor at 900 “C for 3-5 hr. The (110) surface was obtained by cleavage. Wafers with approximately --(111) and (111) surfaces were cut from the boules. For final orientation these wafers were mouted on a precision polishing jig and optically aligned using light reflection patterns from etch pits created by ahot NaOH etch. The oriented wafers were lapped and mechanically polished to a thickness of 0.4-0.5 mm. Following mechanical polishing, the substrates were washed in several hot baths of acetone, methanol, and trichloroethylene, slightly etched in a K,Cr,O, + H,SO, polishing solution and rinsed in distilled water. Immediately before growing GaAs, a substrate was cleaved to proper size (3 x 5 mm) to fit the graphite sample holder, given a final chemical polish for l-2 min in 90 “C dichromate solution (1.25 g K,Cr,O,, 24 ml H,O) and thoroughly 16 ml cont. H,SO,, rinsed in deionized water. After final chemical polishing, the substrate was immediately placed in the quartz chamber, which usually was hot and filled with argon. Epitaxial growth occurred in a hydrogen atmosphere. Typical temperaturetime growth schedules for a Ga solvent and a Sn solvent are shown in fig. 2. Initial liquidus temperatures of about 775 “C for Ga solutions and 620 “C for Sn solutions were the most common. Cooling rates of 10 “C/min or 1 “C/min were used during growth. 3. Physical characteristics Over seventy-five films of GaAs have been grown on ZnSe, about fifty from Ga solutions and the remainder from Sn solutions. The Sn solution-grown
206
J.
W.
BALCH
AND
N
35OC/min
(minutes) (a)
Time
W.
ANDERSON
solutions
Tea Tb (776T),
Time
W.
( minutes 1
(b) Fig. 2.
Typical growth temperature schedule for liquid phase epitaxy from (a) Ga solution and (b) Sn solution. Solution composition (atom fraction) (a) 0.984 Ga, 1.6 x lo-’ As, (b) 0.953 Sn, 2.35 x lo-’ Ga, 2.35 x lo-* As. T, = time and temperature at which the sample first contacted the solution. T,, = time and temperature at which programmed cooling was begun. T, = time and temperature at which the sample was removed from the solution.
films are most recent. Many of the early films were grown while the epitaxial system and growth procedures were being developed, and consequently these films were inferior to those described below. The most convenient method found for assessing the structural quality of the GaAs epitaxial layers was microscopic investigation of a (110) plane cleaved at right angles to the GaAs-ZnSe junction plane. This plane is perpendicular to all three ZnSe surfaces used in these experiments. Study of the cleavage plane allowed the quality of the epitaxy at the junction to be examined, permitted the visible structural flaws to be observed and presented good profiles of the GaAs growth habits on the different ZnSe surfaces. The crosssectional view was used to measure the thickness and thereby determine the growth rates for various surfaces and growth conditions. Because the junction is between different materials, no etchant was needed to expose the junction. In fig. 3, photomicrographs are shown of the (110) cleavage planes of GaAs films grown from Ga and Sn
on the three ZnSe surfaces.
These films are
representative of those grown using the best growth conditions found. A cooling rate of 1 “C/min was used to grow all the films except LE-25, which was grown with a cooling rate of 10 “C/min. No difference in growth morphology was seen for films grown on insulating or semiconducting substrates. A comparison of the six photographs shows that the GaAs film grown from a Sn solution on (111) ZnSe (fig. 3a) is the most uniform in thickness and has the fewest visible structural flaws. The junction is seen to be smooth and straight, indicating that good epitaxy has been obtained. Films of comparable structure were obtained for growth temperatures between 640 and 580 “C. Layers as thick as 20 urn and as thin as 1 urn have been grown. As the starting growth temperature was lowered, the (111) films became less uniform in thickness and thus similar to those grown from Sn solutions on (iii) or (110) surfaces. Fig. 4 shows the (110) cleavage plane of a (111) film grown from a Sn solution between 509 and 484 “C. Although GaAs layers grown from Sn solutions on (iii) or (110) surfaces were characterized by nonuniformity of thickness (figs. 3b and 3c), the junctions were smooth and straight. Since such nonuniform layers were difficult to work with, little experimentation was done with these samples. For GaAs films grown from Ga melts, the (iii) ZnSe surface was found to yield the best epitaxy. Fig. 3e shows a lo-30 urn thick layer with a very straight and smooth junction. However, the films on this surface were not uniform in thickness. No improvement in uniformity could be obtained by varying the starting growth temperature from 749 to 852 “C. The minimum growth temperature was limited by the low solubility of GaAs in Ga. Growth from Ga solutions upon (111) and (110) surfaces yielded similar layers (figs. 3d and 3f). Although these layers were more uniform than those grown from Ga solutions on the (iii) surface, poor epitaxy occurred at the ZnSe surface, causing structural flaws to propagate from the interface plane into the layers. Another problem of these layers was solvent inclusions which were almost always present at the interface plane and sometimes in the layers themselves. The arrow in fig. 3f points to a large solvent inclusion. For these samples, the GaAs-ZnSe junction was very
LIQUID
PHASE
EPITAXY
OF
GALLIUM
ARSENIDE
ON
ZINC
207
SELENIDE
(c)
(4 Fig. 3.
Cleaved
cross sections
of GaAs films grown
from Sn and Ga solutions
on three surfaces
Sample
Solution
ZnSe Surface
Sample
Solution
(a) LE-68 (b) LE-55 (c) LE-63
Sn Sn Sn
(111) --(111) (110)
(d) LE-42 (e) LE-36 (f) LE-25
Ga Ga Ga
jagged. Variation of the starting growth temperature from 741 to 920 “C failed to yield layers free of solvent inclusions or with smoother junctions. To obtain a better picture of the GaAs-ZnSe interface, several samples were angle lapped from the GaAs side. Fig. 5 shows two photomicrographs illustrating the smoothness of the junctions obtainable for GaAs --grown from a Ga solution on (111) ZeSe (fig. 5a) and from a Sn solution on (111) ZnSe (fig. 5b). The junctions are seen to be smooth within 0.1 urn. (The coarseness of the surface in fig. 5a is due to scratches caused by the 3 urn Al,O, particles used for polishing. The sample of fig. 5b was polished with 0.3 urn particles.) The smoothness of junctions for GaAs grown from Sn solutions on (iii) and (110) ZnSe was comparable to that shown in fig. 5b for the (111) surface. GaAs grown from Ga solutions on (111) and (110) ZnSe always had an interface roughness of 3 urn or more.
of ZnSe. ZnSe
Surface (111) (Iii) (110)
Average growth rates of the epitaxial layers shown in figs. 3a-3f are given in table 1. These rates were
Fig. 4. Cross section ZnSe showing unstable temperatures of 509484
of Sn solution-grown GaAs on (111) GaAs growth morphology for growth “C.
J. W.
BALCH
AND
W.
W.
ANDERSON
TABLE
1
Approximate growth rates of GaAs solutions Sample
onto Orientation
ZnSe
grown from substrates
Growth temperature (“C)
Sn solutions
LE-68 LE-55 LE-63
Ga solutions
LE-42 LE-36 LE-25
Ga
and
Approximate growth rate* (umlmin)
(110)
621-611 61 l-593 640-632
0.5 0.7 1.0
(111) (TIT) (110)
764-757 776-766 782-775
3.0 4.5 16.0
(111)
(iii)
Sn
* A cooling rate of approximately 1 “C/min was used for all samples except LE-25, which was grown with a 10 “C/min cooling rate.
substrate was almost always much thicker than which grew on the oriented flat surface. To ascertain that true orientational dependent taxy of the GaAs on the ZnSe substrate was being tained, an X-ray back Laue diffraction pattern taken on a 40 urn thick layer grown on a (iii) strate. The distinct spots of the pattern confirmed single crystallinity of the layer.
that epiobwas subthe
4. Electrical characteristics
Angle-lapped (2”) cross sections of GaAs-ZnSe heteroFig. 5 Juncttons. (a) GaAs grown from a Ga solution on (iiT) ZnSe. (b) GaAs grown from a Sn solution on (111) ZnSe.
computed from the maximum film thicknesses and total cooling times. Thus, for nonuniform thicknesses maximum growth rates are listed. The Ga solutiongrown films had the highest growth rates and were the poorest structurally. The growth rate for a particular solution, substrate orientation, and cooling rate was found to be larger for a higher starting growth temperature, due to the apdependence of the soluproximate e -“IT temperature bility of GaAs in both Ga and Sn solutions. Other factors which affected growth rate were the size and geometry of the solution and the substrate orientation. The GaAs layer deposited on the edges and sides of a
The resistivity, p, and Hall coefficient, R,,, of GaAs films grown on insulating substrates were measured by the dc van der Pauw technique”). Measurements were made at room temperature in the dark with a magnetic field strength of one tesla. Electrical contacts were pure Sn spheres (0.25 mm dia.) to the n-type films, and Ino,9ssGaeoo,Zno,o, spheres (0.25 mm dia.) to the p-type films. The spheres were alloyed in a H,-HCl atmosphere at a maximum temperature of 350 “C. The contacts generally showed linear currentvoltage characteristics. Carrier type, concentration, and Hall mobility for several samples are listed in table 2. GaAs grown from Ga solutions was always degenerate n-type with carrier concentrations of about 1019/cn13. Attempts to grow p-type GaAs from Ga solutions doped with up to 1.2 at ‘A, Zn were unsuccessful. Unlike Ga solutions, Sn solutions yielded both n- and p-type GaAs films. Films grown from undoped Sn solutions were n-type with carrier concentrations of the order of 10t7/cm3 in agreement with Nelson’s original work’ ‘) on growth from Sn solutions on GaAs substrates. Also consistent
LlQUlD
PHASE
EPITAXY
OF GALLIUM
ARSENIDE
ON
ZINC
209
SELENIDE
TABLE 2
Carrier Sample
Substrate orientation
concentrations Starting melt composition (atom fraction)
and Hall mobilities Growth temperature range (“C)
LE-9
110
LE-42
111
LE-49
ITT
LE-54
ii1
LE-57
111
LE-64
111
LE-69
111
Xoa Xn. xo. X& xoa X&
= = = = = =
xo. ,yar xsn Xoa Xas Xsn *oa XaJ
= = = = = = = =
Xzn Xsn xGa xAY
= = = =
x$..=
Xzn =
0.966 3.4 x 10-z 0.986 1.4x10-2 0.966 3.4 x 10-z 0.95 2.5 x lO-2 2.5 x lo-’ 0.96 2.0 x 1o-2 2.0 x 10-Z 0.939 3.0x 10-Z 3.0x 1o-2 1.0x 10-a 0.953 2.3 x IO-’ 2.3 x lo-’ 1.0x 10-S
of GaAs
grown
Thickness epitaxial layer
-
(pm)
830-600
60
764-751
21
830-811
48-65
624-595
20-40
599-580
14.5
640-620
12
on ZnSe substrates of
Carrier concentration and type (cm- ‘) 1.1 x 10’9 n-type 1.4x 10’9 n-type 1.3x10’9 n-type 1.9x10” n-type 7.0 x 10’6 to 4.0 x 101’ n-type 1.6 x 1Or9
Hall mobility (cm*/V-set)
166 65 515 128
115
4.2
p-type
619-602
with Nelson’s results is the ability to grow p-type GaAs from Zn-doped Sn solutions. Sn solutions doped with 0.1 at % Zn, the only ones used, have consistently yielded hole concentrations on the order of 1019/cm3. The degenerate n-type carrier concentrations for the Ga solution-grown GaAs are presumably due to doping with Se, which together with Zn is introduced into the Ga solutions by partial dissolution of the ZnSe substrates. Since the solutions are about 97% Ga (see table 2), the amount of Zn and Se introduced can be estimated from the solubility of ZnSe in pure Ga which Wagner and Lorenzl’) give as about 0.5 at% at 800 “C. A comparison of the net carrier concentrations and mobilities (table 2) shows that GaAs grown from Sn solutions is more heavily compensated than GaAs grown from Ga solutions. This higher compensation occurs even though ZnSe has a lower solubility in Sn solutions than in Ga solutions at the respective growth temperatures [- 0.05 at “/, in Sn at 600 ‘C13) versus 0.5 at % in Ga at 800 C”)] and presumably could be due to a greater difference in Se and Zn distribution coefficients for Ga solutions than for Sn solutions and to the amphoteric behavior of Sn in GaAs.
11
3.0 x 10’9
3.7
p-type
5. Conclusions GaAs could easily be grown upon ZnSe by Ga or Sn solution liquid phase epitaxy. Sn solutions produced GaAs of better physical structure and carrier concentration control than Ga solutions. GaAs grown from Sn solutions always had a very smooth junction with the ZnSe substrate, and uniformly thin l-20 urn films were obtained by growth upon the (111) ZnSe surface. GaAs grown from Ga solutions was often characterized by a rough junction and Ga inclusions at the ZnSe substrate. For electrical or optical characterization and for device development, GaAs films of uniform thickness and good structural quality are desirable. Therefore, for the growth conditions used to date, only the films grown on (111) ZnSe from Sn solutions are useful. Both n- and p-type GaAs could be grown from Sn solutions. Electron concentrations as low as 10r7/cm3 and hole concentrations of about 10’ ‘/cm3 were obtained. From Ga solutions, only degenerate (> 1019/cm3) n-type GaAs could be grown. Solvent dissolution of the ZnSe substrate results in highly compensated GaAs with a much lower mobility than bulk GaAs of comparable carrier concentration.
210 While some control
J. W.
over the growth
BALCH
AND
habit of GaAs
onto ZnSe was achieved in this investigation, it is obvious that much more work needs to be done before any of the devices mentioned in the Introduction can be constructed with practically useful characteristics.
References 1) A. G. Mimes and D. L. Feucht, NASA Contract No. NGR 39-087-002 Quarterly Report, January 3 1, 1970, available from the National Technical Information Service, Springfield, Va., Accession No. N 70-21403.
W.
W.
ANDERSON
2) R. Sahai and A. G. Milnes, Solid State Electron. 13 (1970) 1289. 3) Y. Suematsu, Japan. J. Appl. Phys. 9 (1970) 798. 4) A. Baczewski, J. Electrochem. Sot. 112 (1965) 577. 5) H. J. Hovel and A. G. Milnes, J. Electrochem. Sot. 116 (1969) 843. 6) R. Mach, W. Ludwig, G. Eichhorn and H. Arnold, Phys. Status Solidi (a) 2 (1970) 701. 7) M. B. Panish, I. Hayashi and S. Sumski, IEEE J. Quantum Electron. QE-5 (1969) 210. 8) R. N. Hall, J. Electrochem. Sot. 110 (1963) 385. 9) M. Rubenstein, J. Electrochem. Sot. 113 (1966) 752. 10) L. J. van der Pauw, Philips Tech. Rev. 20 (1958) 220. 11) H. Nelson, RCA Rev. 24 (1963) 603. 12) P. Wagner and M. R. Lorenz, J. Phys. Chem. Solids 27 (1966) 1749. 13) S. Kimura, J. Chem. Thermodynamics 3 (1971) 7.
Growth 15 (1972) 204-210
LIQUID
PHASE EPITAXY
J. W. BALCH Department Received
0 North Holland Publishing
Co.
OF GALLIUM
ARSENIDE
ON ZINC SELENIDE
and W. W. ANDERSON
of Electrical 15 October
Engineering,
1971; revised
The Ohio State
manuscript
University,
received
Columbus,
28 February
Ohio 43210,
U.S.A.
1972
Gallium arsenide has been grown upon (11 I), (IiT) and (110) surfaces of ZnSe by liquid phase epitaxy from Ga and Sn solutions. GaAs films grown from Ga solutions were usually characterized by rough junctions or solvent inclusions. These films were always degenerate n-type, with electron concentrations of about 10t9/cm3. GaAs films grown from Sn solutions had smooth junctions free of gross defects, and uniformly thin films of GaAs up to 20 urn thick were obtained for (111) oriented substrates. Films grown from undoped Sn solutions were n-type with electron concentrations of about 10”/cm3. By adding Zn to the Sn solutions, p-type films with hole concentrations of 1019/cm3 were obtained.
Mach et a1.6) have recently reported some success in using such a technique. Consequently, a liquid phase epitaxy process has been developed for growing thin layers of GaAs on ZnSe.
1. Introduction The GaAs-ZnSe heterojunction is of interest for studies of several potential devices including heterojunction transistors’) and solar cells’) and as an optical waveguide for investigating nonlinear optical interactions in GaAs3). To investigate devices which require a thin layer of GaAs (l-10 urn - such as the GaAsZnSe optical waveguide) an epitaxial process of growing GaAs on ZnSe is desirable. Various HCl chemical vapor deposition processes have been used for growing ZnSe upon GaAs“,5), but experience here and in other laboratories has shown that similar processes cannot easily be employed to grow GaAs on ZnSe, although
2. Experimental
The modified system’) used to grow GaAs on ZnSe substrates is shown schematically in fig. 1. The high temperature parts of the system consisted of a synthetic quartz furnace tube, boat holder and slider, and a boat and substrate holder made of spectra-pure graphite (nominal purity 99.9995 %). A quartz window in the small tipping furnace allowed the saturation temperature to be determined by visual observation
View Quartz
Quartz
methods
Port F
Slider
Substrate
Holder
and Substrate
D-4
Chamber Graphite
Fig. 1.
Schematic
diagram
Boat
of liquid
204
Cold Trap 190’K
J
phase
epitaxy
apparatus.
Oil Bubbler
LIQUID
PHASE
EPITAXY
OF GALLIUM
and also established a slight vertical temperature gradient. At the front of the quartz tube, the boat holder was supported by a gas-tight stainless steel coupling with a Viton O-ring which allowed the boat to be inserted or removed from the furnace. A high purity hydrogen or argon atmosphere was supplied to the quartz chamber through stainless steel cold traps cooled respectively by liquid nitrogen or dry ice-acetone, Pyrex-teflon metering valves, Pyrex-sapphire flow meters and teflon tubing with walls 4 in. thick. The exhaust of the system was through a dry iceacetone cold trap and oil bubbler. Joints between the glass, metal, or teflon tubes were made with high density heat shrinkable polyethylene tubing. The growth solution was contained in a well in the graphite boat and remained stationary during the tipping of the furnace. When the furnace was tipped, the quartz slider moved the ZnSe substrate onto the surface of the saturated solution where growth occurred. Both GaAs and ZnSe are less dense than the Ga or Sn solutions so that the substrate floats on the solution. To insure that the quartz slider would remove the sample from the solution when the furnace was tipped back after the GaAs had been grown, the top surface of the solution had to be even with the graphite floor on which the quartz slider moved. A solution of slightly larger volume than the well was used so that when the furnace was initially tipped, the quartz slider and substrate holder removed a thin layer of the solution and deposited the substrate on a clean solution surface that was even with the graphite floor. The Ga or Sn solution was prepared by placing a mixture of metallic solvent and GaAs in the graphite boat. Gallium of 7N purity was used as supplied. Tin of 6N purity was cut from an ingot, etched in a HFHNO, solution and rinsed in distilled water. Two types of GaAs were used : (1) polycrystalline p-type (77.5 Q-cm) and (2) single crystalline Te-doped n-type with a 4.8 x 10’6/cm3 carrier concentration. Preparation of the GaAs involved a polishing solution of 3 H,SO,-1 H,O,-1 H,O followed by rinses in cold distilled water and hot methanol. When Zn was used to dope the Sn solutions, the Zn was etched in an HNO, solution. The amounts of metallic solvent and GaAs needed to make approximately a 1 g solution of a given liquidus temperature were determined from the publish-
ARSENIDE
ed solubility
ON
ZINC
curves
205
SELENIDE
of GaAs
in Ga8,9)
and
Sn9).
Liquidus temperatures used for Ga solutions were in the range 725-870 “C and for Sn solutions 460-640 “C. After being loaded, the solution was baked in a hydrogen atmosphere for 1 hr at 150 “C above the liquidus temperature before any epitaxial growth runs were attempted. GaAs was grown on the (1 IO), (11 I), and surfaces of ZnSe substrates which were either insulating or semiconducting. The substrates were prepared from insulating boules of single crystalline melt-grown ZnSe
(iii)
supplied by Dr. Y. S. Park of the Aero-Space Research Laboratories. Semiconducting substrates with a resistivity of about 1 Q-cm were obtained by firing large area wafers approximately 0.5 mm thick in molten Zn or Zn vapor at 900 “C for 3-5 hr. The (110) surface was obtained by cleavage. Wafers with approximately --(111) and (111) surfaces were cut from the boules. For final orientation these wafers were mouted on a precision polishing jig and optically aligned using light reflection patterns from etch pits created by ahot NaOH etch. The oriented wafers were lapped and mechanically polished to a thickness of 0.4-0.5 mm. Following mechanical polishing, the substrates were washed in several hot baths of acetone, methanol, and trichloroethylene, slightly etched in a K,Cr,O, + H,SO, polishing solution and rinsed in distilled water. Immediately before growing GaAs, a substrate was cleaved to proper size (3 x 5 mm) to fit the graphite sample holder, given a final chemical polish for l-2 min in 90 “C dichromate solution (1.25 g K,Cr,O,, 24 ml H,O) and thoroughly 16 ml cont. H,SO,, rinsed in deionized water. After final chemical polishing, the substrate was immediately placed in the quartz chamber, which usually was hot and filled with argon. Epitaxial growth occurred in a hydrogen atmosphere. Typical temperaturetime growth schedules for a Ga solvent and a Sn solvent are shown in fig. 2. Initial liquidus temperatures of about 775 “C for Ga solutions and 620 “C for Sn solutions were the most common. Cooling rates of 10 “C/min or 1 “C/min were used during growth. 3. Physical characteristics Over seventy-five films of GaAs have been grown on ZnSe, about fifty from Ga solutions and the remainder from Sn solutions. The Sn solution-grown
206
J.
W.
BALCH
AND
N
35OC/min
(minutes) (a)
Time
W.
ANDERSON
solutions
Tea Tb (776T),
Time
W.
( minutes 1
(b) Fig. 2.
Typical growth temperature schedule for liquid phase epitaxy from (a) Ga solution and (b) Sn solution. Solution composition (atom fraction) (a) 0.984 Ga, 1.6 x lo-’ As, (b) 0.953 Sn, 2.35 x lo-’ Ga, 2.35 x lo-* As. T, = time and temperature at which the sample first contacted the solution. T,, = time and temperature at which programmed cooling was begun. T, = time and temperature at which the sample was removed from the solution.
films are most recent. Many of the early films were grown while the epitaxial system and growth procedures were being developed, and consequently these films were inferior to those described below. The most convenient method found for assessing the structural quality of the GaAs epitaxial layers was microscopic investigation of a (110) plane cleaved at right angles to the GaAs-ZnSe junction plane. This plane is perpendicular to all three ZnSe surfaces used in these experiments. Study of the cleavage plane allowed the quality of the epitaxy at the junction to be examined, permitted the visible structural flaws to be observed and presented good profiles of the GaAs growth habits on the different ZnSe surfaces. The crosssectional view was used to measure the thickness and thereby determine the growth rates for various surfaces and growth conditions. Because the junction is between different materials, no etchant was needed to expose the junction. In fig. 3, photomicrographs are shown of the (110) cleavage planes of GaAs films grown from Ga and Sn
on the three ZnSe surfaces.
These films are
representative of those grown using the best growth conditions found. A cooling rate of 1 “C/min was used to grow all the films except LE-25, which was grown with a cooling rate of 10 “C/min. No difference in growth morphology was seen for films grown on insulating or semiconducting substrates. A comparison of the six photographs shows that the GaAs film grown from a Sn solution on (111) ZnSe (fig. 3a) is the most uniform in thickness and has the fewest visible structural flaws. The junction is seen to be smooth and straight, indicating that good epitaxy has been obtained. Films of comparable structure were obtained for growth temperatures between 640 and 580 “C. Layers as thick as 20 urn and as thin as 1 urn have been grown. As the starting growth temperature was lowered, the (111) films became less uniform in thickness and thus similar to those grown from Sn solutions on (iii) or (110) surfaces. Fig. 4 shows the (110) cleavage plane of a (111) film grown from a Sn solution between 509 and 484 “C. Although GaAs layers grown from Sn solutions on (iii) or (110) surfaces were characterized by nonuniformity of thickness (figs. 3b and 3c), the junctions were smooth and straight. Since such nonuniform layers were difficult to work with, little experimentation was done with these samples. For GaAs films grown from Ga melts, the (iii) ZnSe surface was found to yield the best epitaxy. Fig. 3e shows a lo-30 urn thick layer with a very straight and smooth junction. However, the films on this surface were not uniform in thickness. No improvement in uniformity could be obtained by varying the starting growth temperature from 749 to 852 “C. The minimum growth temperature was limited by the low solubility of GaAs in Ga. Growth from Ga solutions upon (111) and (110) surfaces yielded similar layers (figs. 3d and 3f). Although these layers were more uniform than those grown from Ga solutions on the (iii) surface, poor epitaxy occurred at the ZnSe surface, causing structural flaws to propagate from the interface plane into the layers. Another problem of these layers was solvent inclusions which were almost always present at the interface plane and sometimes in the layers themselves. The arrow in fig. 3f points to a large solvent inclusion. For these samples, the GaAs-ZnSe junction was very
LIQUID
PHASE
EPITAXY
OF
GALLIUM
ARSENIDE
ON
ZINC
207
SELENIDE
(c)
(4 Fig. 3.
Cleaved
cross sections
of GaAs films grown
from Sn and Ga solutions
on three surfaces
Sample
Solution
ZnSe Surface
Sample
Solution
(a) LE-68 (b) LE-55 (c) LE-63
Sn Sn Sn
(111) --(111) (110)
(d) LE-42 (e) LE-36 (f) LE-25
Ga Ga Ga
jagged. Variation of the starting growth temperature from 741 to 920 “C failed to yield layers free of solvent inclusions or with smoother junctions. To obtain a better picture of the GaAs-ZnSe interface, several samples were angle lapped from the GaAs side. Fig. 5 shows two photomicrographs illustrating the smoothness of the junctions obtainable for GaAs --grown from a Ga solution on (111) ZeSe (fig. 5a) and from a Sn solution on (111) ZnSe (fig. 5b). The junctions are seen to be smooth within 0.1 urn. (The coarseness of the surface in fig. 5a is due to scratches caused by the 3 urn Al,O, particles used for polishing. The sample of fig. 5b was polished with 0.3 urn particles.) The smoothness of junctions for GaAs grown from Sn solutions on (iii) and (110) ZnSe was comparable to that shown in fig. 5b for the (111) surface. GaAs grown from Ga solutions on (111) and (110) ZnSe always had an interface roughness of 3 urn or more.
of ZnSe. ZnSe
Surface (111) (Iii) (110)
Average growth rates of the epitaxial layers shown in figs. 3a-3f are given in table 1. These rates were
Fig. 4. Cross section ZnSe showing unstable temperatures of 509484
of Sn solution-grown GaAs on (111) GaAs growth morphology for growth “C.
J. W.
BALCH
AND
W.
W.
ANDERSON
TABLE
1
Approximate growth rates of GaAs solutions Sample
onto Orientation
ZnSe
grown from substrates
Growth temperature (“C)
Sn solutions
LE-68 LE-55 LE-63
Ga solutions
LE-42 LE-36 LE-25
Ga
and
Approximate growth rate* (umlmin)
(110)
621-611 61 l-593 640-632
0.5 0.7 1.0
(111) (TIT) (110)
764-757 776-766 782-775
3.0 4.5 16.0
(111)
(iii)
Sn
* A cooling rate of approximately 1 “C/min was used for all samples except LE-25, which was grown with a 10 “C/min cooling rate.
substrate was almost always much thicker than which grew on the oriented flat surface. To ascertain that true orientational dependent taxy of the GaAs on the ZnSe substrate was being tained, an X-ray back Laue diffraction pattern taken on a 40 urn thick layer grown on a (iii) strate. The distinct spots of the pattern confirmed single crystallinity of the layer.
that epiobwas subthe
4. Electrical characteristics
Angle-lapped (2”) cross sections of GaAs-ZnSe heteroFig. 5 Juncttons. (a) GaAs grown from a Ga solution on (iiT) ZnSe. (b) GaAs grown from a Sn solution on (111) ZnSe.
computed from the maximum film thicknesses and total cooling times. Thus, for nonuniform thicknesses maximum growth rates are listed. The Ga solutiongrown films had the highest growth rates and were the poorest structurally. The growth rate for a particular solution, substrate orientation, and cooling rate was found to be larger for a higher starting growth temperature, due to the apdependence of the soluproximate e -“IT temperature bility of GaAs in both Ga and Sn solutions. Other factors which affected growth rate were the size and geometry of the solution and the substrate orientation. The GaAs layer deposited on the edges and sides of a
The resistivity, p, and Hall coefficient, R,,, of GaAs films grown on insulating substrates were measured by the dc van der Pauw technique”). Measurements were made at room temperature in the dark with a magnetic field strength of one tesla. Electrical contacts were pure Sn spheres (0.25 mm dia.) to the n-type films, and Ino,9ssGaeoo,Zno,o, spheres (0.25 mm dia.) to the p-type films. The spheres were alloyed in a H,-HCl atmosphere at a maximum temperature of 350 “C. The contacts generally showed linear currentvoltage characteristics. Carrier type, concentration, and Hall mobility for several samples are listed in table 2. GaAs grown from Ga solutions was always degenerate n-type with carrier concentrations of about 1019/cn13. Attempts to grow p-type GaAs from Ga solutions doped with up to 1.2 at ‘A, Zn were unsuccessful. Unlike Ga solutions, Sn solutions yielded both n- and p-type GaAs films. Films grown from undoped Sn solutions were n-type with carrier concentrations of the order of 10t7/cm3 in agreement with Nelson’s original work’ ‘) on growth from Sn solutions on GaAs substrates. Also consistent
LlQUlD
PHASE
EPITAXY
OF GALLIUM
ARSENIDE
ON
ZINC
209
SELENIDE
TABLE 2
Carrier Sample
Substrate orientation
concentrations Starting melt composition (atom fraction)
and Hall mobilities Growth temperature range (“C)
LE-9
110
LE-42
111
LE-49
ITT
LE-54
ii1
LE-57
111
LE-64
111
LE-69
111
Xoa Xn. xo. X& xoa X&
= = = = = =
xo. ,yar xsn Xoa Xas Xsn *oa XaJ
= = = = = = = =
Xzn Xsn xGa xAY
= = = =
x$..=
Xzn =
0.966 3.4 x 10-z 0.986 1.4x10-2 0.966 3.4 x 10-z 0.95 2.5 x lO-2 2.5 x lo-’ 0.96 2.0 x 1o-2 2.0 x 10-Z 0.939 3.0x 10-Z 3.0x 1o-2 1.0x 10-a 0.953 2.3 x IO-’ 2.3 x lo-’ 1.0x 10-S
of GaAs
grown
Thickness epitaxial layer
-
(pm)
830-600
60
764-751
21
830-811
48-65
624-595
20-40
599-580
14.5
640-620
12
on ZnSe substrates of
Carrier concentration and type (cm- ‘) 1.1 x 10’9 n-type 1.4x 10’9 n-type 1.3x10’9 n-type 1.9x10” n-type 7.0 x 10’6 to 4.0 x 101’ n-type 1.6 x 1Or9
Hall mobility (cm*/V-set)
166 65 515 128
115
4.2
p-type
619-602
with Nelson’s results is the ability to grow p-type GaAs from Zn-doped Sn solutions. Sn solutions doped with 0.1 at % Zn, the only ones used, have consistently yielded hole concentrations on the order of 1019/cm3. The degenerate n-type carrier concentrations for the Ga solution-grown GaAs are presumably due to doping with Se, which together with Zn is introduced into the Ga solutions by partial dissolution of the ZnSe substrates. Since the solutions are about 97% Ga (see table 2), the amount of Zn and Se introduced can be estimated from the solubility of ZnSe in pure Ga which Wagner and Lorenzl’) give as about 0.5 at% at 800 “C. A comparison of the net carrier concentrations and mobilities (table 2) shows that GaAs grown from Sn solutions is more heavily compensated than GaAs grown from Ga solutions. This higher compensation occurs even though ZnSe has a lower solubility in Sn solutions than in Ga solutions at the respective growth temperatures [- 0.05 at “/, in Sn at 600 ‘C13) versus 0.5 at % in Ga at 800 C”)] and presumably could be due to a greater difference in Se and Zn distribution coefficients for Ga solutions than for Sn solutions and to the amphoteric behavior of Sn in GaAs.
11
3.0 x 10’9
3.7
p-type
5. Conclusions GaAs could easily be grown upon ZnSe by Ga or Sn solution liquid phase epitaxy. Sn solutions produced GaAs of better physical structure and carrier concentration control than Ga solutions. GaAs grown from Sn solutions always had a very smooth junction with the ZnSe substrate, and uniformly thin l-20 urn films were obtained by growth upon the (111) ZnSe surface. GaAs grown from Ga solutions was often characterized by a rough junction and Ga inclusions at the ZnSe substrate. For electrical or optical characterization and for device development, GaAs films of uniform thickness and good structural quality are desirable. Therefore, for the growth conditions used to date, only the films grown on (111) ZnSe from Sn solutions are useful. Both n- and p-type GaAs could be grown from Sn solutions. Electron concentrations as low as 10r7/cm3 and hole concentrations of about 10’ ‘/cm3 were obtained. From Ga solutions, only degenerate (> 1019/cm3) n-type GaAs could be grown. Solvent dissolution of the ZnSe substrate results in highly compensated GaAs with a much lower mobility than bulk GaAs of comparable carrier concentration.
210 While some control
J. W.
over the growth
BALCH
AND
habit of GaAs
onto ZnSe was achieved in this investigation, it is obvious that much more work needs to be done before any of the devices mentioned in the Introduction can be constructed with practically useful characteristics.
References 1) A. G. Mimes and D. L. Feucht, NASA Contract No. NGR 39-087-002 Quarterly Report, January 3 1, 1970, available from the National Technical Information Service, Springfield, Va., Accession No. N 70-21403.
W.
W.
ANDERSON
2) R. Sahai and A. G. Milnes, Solid State Electron. 13 (1970) 1289. 3) Y. Suematsu, Japan. J. Appl. Phys. 9 (1970) 798. 4) A. Baczewski, J. Electrochem. Sot. 112 (1965) 577. 5) H. J. Hovel and A. G. Milnes, J. Electrochem. Sot. 116 (1969) 843. 6) R. Mach, W. Ludwig, G. Eichhorn and H. Arnold, Phys. Status Solidi (a) 2 (1970) 701. 7) M. B. Panish, I. Hayashi and S. Sumski, IEEE J. Quantum Electron. QE-5 (1969) 210. 8) R. N. Hall, J. Electrochem. Sot. 110 (1963) 385. 9) M. Rubenstein, J. Electrochem. Sot. 113 (1966) 752. 10) L. J. van der Pauw, Philips Tech. Rev. 20 (1958) 220. 11) H. Nelson, RCA Rev. 24 (1963) 603. 12) P. Wagner and M. R. Lorenz, J. Phys. Chem. Solids 27 (1966) 1749. 13) S. Kimura, J. Chem. Thermodynamics 3 (1971) 7.