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Heteroepitaxial growth of GaAs on sapphire substrates by a three-step method using low pressure MOCVD
Journal of Crystal Growth 77 (1986) 524—529 North-Holland, Amsterdam
524
HETEROEPITAXJAL GROWTh OF GaAs ON SAPPHIRE SUBSTRATES BY A THREE-STEP METhOD USING LOW PRESSURE MOCVD Akihiko SUGIMURA Department ofElectrical Engineering College of Engineering, University of Osaka Prefecture, Mozu, Sakai. Osaka 591, Japan
Takashi HOSOI and Kokichi ISHIBITSU Central Research Laboratories, Kyocera Corporation, Yamashina, Kyoto 607, Japan
and Takao KAWAMURA Department of Electrical Engineering, College ofEngineering, University of Osaka Prefecture, Mozu, Sakai, Osaka 591, Japan
Single crystal films of (11l)GaAs have been grown onto (0001)A1
203 substrates by a low pressure MOCVD method. Using a three-step growth procedure, high quality heteroepitaxial growth of GaAs has been achieved by using an annealed buffer layer of 40 nm in thickness deposited on the substrate. Gallium arsenide films grown at a temperature of 620°Chave a2/V mirror-like s and a surface carrier Concentration of 4x 1016 GaAs cm3 films at room temperature for a film thickness of 6 an ~m.electron mobility of 4000 cm morphology. Silicon-doped exhibit n-type conductivity and typically
1. Introduction High quality GaAs is one of the most attractive compounds for the semiconductor device applications. The epitaxial growth of GaAs by MOCVD has received more intensive study following the recent improvement of source gases. Heteroepitaxial GaAs films grown on various substrates such as a-Al203 [1—5], Si [6—8], Ge [9], and MgAl2O4 [1,2,10,11] have been reported. Heteroepitaxial growth will be useful for the preparation of large area GaAs films with the potential for low production costs. Growth of GaAs on ~-Al2O3 substrates is advantageous for the production of crack-free thick film growth because the linear coefficients of thermal expansion (about 5 x i0~ 1 at room temperature) of both materials are o c similar and because there is generally limited interdiffusion of impurities from the substrate [12]. An additional advantageous aspect of GaAs film growth on an insulating sapphire substrate is the improvement in crystallinity that can be achieved compared with growth on a Si or Ge substrate; a
consequence, it appears, of the considerably larger mismatch on GaAs compared with the other substrates. In this paper, heteroepitaxy of high quality (111)GaAs grown on a (0001)Al 203 substrate was achieved by the use of an intermediate annealed buffer layer in the interface between the final GaAs film and the substrate. First, an initial polycrystalline GaAs layer was formed on the substrate. Next, this initial layer was recrystallized by annealing to form a buffer layer. Finally, a single crystal film was grown on this recrystallized buffer layer. This growth procedure will be referred to as the three-step growth (TSG) method. It has been shown by optimizing the growth conditions in the TSG electrical procedureproperties that the and finalsurface epitaxial film had good morphology. 2. Experimental Heteroepitaxial growth using the TSG method in a low pressure MOCVD was carried out follow-
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
A. Sugimura et al.
2nd Step
P
~1
620
-
3rd Step ~taI Growth
1st Step
JecrystaL
420
/ Hereroepitaxial
\
lization
525
growth of GaAs on sapphire
Table 1 Growth conditions Pressure Temperature [ASH [Si2H6l/[TMG] 3]/[TMG]
100 Torr 610—660°C 5 80 (1.7—4.2)x10
Total gas flow
3500 SCCM
grown onto (0001)Al substrates. Homoepitaxial growth was also203 carried out on GaAs sub-
Nucleation
TIME Fig. 1. The three step growth (TSG) method for heteroepitaxy in a low pressure MOCVD apparatus. First step: deposition of the initial polycrystalline GaAs layer of ~- 40 nm in thickness on the substrate at 420°C.Second step: annealing the initial layer at 620°C for 20 mm to bring about recrystallization. Third step: growth of GaAs film,
ing the procedure shown in fig. 1. An initial polycrystalline GaAs layer, 40 nm in thickness, was nucleated on the substrate at 420°C. After heating the wafer up to the growth temperature (620°C)in flowing arsine gas, the GaAs layer was allowed to recrystallize by annealing at the same temperature. Crystal growth of the final film was then started. Vapor growth of GaAs was performed in a horizontal hot-wall quartz tube 450 mm in length and 74 mm in diameter. An RF-coupled SiC coated graphite susceptor was used to heat the substrate. The substrate was oriented horizontally, this being the optimum condition for homogeneous deposition in our system. A thermocouple was set inside —
strates oriented 2°off the (100) towards the (011) orientation in order to compare with heteroepitaxial layers. The thickness of the growth film was standardized at 6 J.Lm by controlling the growth time. The fixed epilayer thickness allowed cornparison of the electrical properties of growth films under the same conditions. Carrier concentration and mobilities were determined by Hall effect measurement at room temperature on samples using the Van der Pauw method. 3. Experimental results Growth conditions in the TSG method were optimized as follows. An initial GaAs layer was 5001.
~~
/
4000
~ 0
the susceptor in order to control the substrate temperature. The reactant gases used were tnmethylgallium (TMG) as the source for the gallium, together with 10% arsine in H2 as a source for the arsenic mixed in a carrier gas of Pd diffused hydrogen. All the flow gases were controlled by precision electric mass flow controllers. All gas lines were 316 stainless steel with VCR connections. Typical growth conditions are shown in table 1. The growth temperature was in the range of 610 to 660 °C.The mole fraction of the source gases and the growth pressure were fixed at [ASH3]/[TMG] = 80 to 100 Torr, respectively. Total gas flow rate was 3500 SCCM. Heteroepitaxial layers were
3000
I -~ °~
I
2000
0
20
40
60
THICKNESS
80
100
(nm)
Fig. 2. Electron mobility as a function of the thickness of the initial layer. The GaAs layer is deposited at a constant temperature of 420°C. The properties of the growth films are affected by the thickness of the initial layer. The last micrometer of the constant 6 ~m film is doped with Si.
526
A. Sugimura et aL
/ Heteroepitaxial growth of GaAs on sapphire
deposited on the substrate at a constant temperature of 420°C in order to study the effect of the recrystallization process. The properties of subsequent films as affected by the variation in the thickness of the initial layer were investigated. Fig. 2 shows the electron mobility as a function of the thickness of the initial layers. In order to estimate the electrical properties of heteroepitaxial films, the last 1 p~mof the film was doped with Si donors under the condition of 6 p.m constant film thickness. Disilane (Si2H6) gas (2 ppm in H2) was used as the dopant gas. The carrier concentration of the doped heteroepitaxial films was proportional to the mole fraction of the 2 H6 I to [TMG] ratio. This result was similar to that reported by Kuech et a!. [11]. However, Si2H6 gas showed a doping efficiency dependent on the growth temperature in the range of our experiments. Following the initial layer of 40 nrn in thickness, the 6 p.m film which was doped with Si in the last 1 p.m of growth indicated maximum 2/V. s ata room ternelectron mobility of 4000 cm perature. The thickness of the initially deposited layer evidently influenced the properties of growth
In the second step, the initial layer was recrystallized by annealing in an AsH3 flow at a temperature of 620°C. It was confirmed by X-ray diffraction measurements that all the initial layers recrystallized on annealing to produce a (111) orientation. Fig. 3 shows the electron mobility of the last 1 ,.Lm layer of growth, which was Si-doped, as a function of annealing time of the initial layer. The mobility increased proportionally with the logarithm of the annealing time for annealing times between 0 and 80 mm. It would be expected from the results as shown in fig. 3 that the quality of the final layer would depend strongly on the effectiveness of the reorientation achieved in the recrystallization of the initial layer. The annealing time of the initial layer in the TSG method was standardized at 20 mm based on the above mentioned results. In the third step, after the initial layer had been deposited on the sapphire substrate and recrystallized, constant 6 p.m of GaAs was grown. 4 shows athe relation between electron mobilityFig. and carrier concentration of GaAs films grown at 620°C. The closed circles indicate the properties
films as shown in this figure. 1000
5000 •~
o
(11 1)GaAsI/(0001)A1
•
203 (100)GaAs//(100)GaAs
8000
~
2000
0 •
[Si2H6J/[’rMG]:4.2x10-5 {Si2i-l~]i[TMG]. 1 .7x 10 i015
ANNEALING 0 5 TIME 20 (mm80 Fig. 3. Room temperature electron mobility of the last mlcrometer layer of growth which was Si-doped as a function of annealing time of the initial layer. All the initial polycrystalline layers recrystallized to give a (111) Orientation by annealing in AsH 3 flow at 620°C.
1016
i017
io~
1019
3) Fig. 4. The CARRIER relation CONCENTRATION between electron mobility (cth and carrier concentration of GaAs films grown at 620°C. The closed circles indicate the properties of heteroepitaxial films and the open circles those of homoepitaxial films. The solid line shows the analytical relation in each case for compensation ratios of I and 5 as found in ref. [13].
A. Sugimura et aL
/ Heteroepitaxial growth of GaAs on sapphire
of heteroepitaxial films and the open circles those of homoepitaxial films. The solid lines show mobility carrier concentration relationships for an analytical model [13] representing GaAs having compensation ratios of 1 and 5. The homoepitaxial films exhibit high quality and low cornpensation even at low carrier concentration. However, the mobility of the heteroepitaxial films decreased with decreasing flow of Si2 6 for carrier concentrations less than 4 x 1016 cm This undesirable tendency was evident for heteroepitaxial GaAs films grown by the TSG method and doped with Si for their entire thickness as well as by layers grown using the standard method. These electrical characteristics would be consistent with the presence of a high resistivity (>i0~ ~2 cm) p-layer. Fig. 5 shows the relation between mobility and film thickness. The circles indicate films grown by the TSG method and the squares indicate those grown by the standard method in which there was no annealing stage. To compare the properties of the GaAs films grown by each of these growth methods, the entire GaAs layer was Si doped. This
527 °
.
_____
-.
~.
6000
______________________________________
_____
__________________ _______
___________________
o Three Step Growth o Standard Growth
I
_______
4000
________
___
~: 2000
FILM THICKNESS (pm) Fig. 5. The relation between mobility and film thickness prepared by the TSG method and a non-TSG heteroepitaxial or standard (single deposition) method. The circles indicate the properties of the films grown by the TSG method and the squares those by the standard method.
______
_________________
_____
—
Fig. 6. Scanning electron micrographs of epitaxial (111)GaAs films grown on (0001)Al203 substrate at several growth ternperatures: (a) 660°C; (b) 630°C; (c) 620°C; (d) 610°C. Marker represents 10 tim.
528
A. Sugirnura et of
/ Heteroepitaxial growth of GaAs on sapphire
~
______________
__________________________________
~ ______________
______________
__________
__________ _____
figure shows that the annealing process which produces a reorientated layer in the TSG method is a useful way of producing films with improved electrical properties. The surface morphology of the GaAs films depends on the growth temperature. Fig. 6 shows SEM photographs of epitaxial (111)GaAs films grown on a (0001)A1203 substrate. Mirror like surface morphology was obtained at a growth temperature of 620°C. However, in the case of higher (630°C) and/or lower (610°C) growth temperatures, the films had a milky appearance and exhibited a triangular faceted morphology. For this range of the growth temperatures, the mobility was the same for layers having the same constant carrier concentration by control of Si2 H6 flow. The growth on Al 203 substrates having surfaces tilted from the (0001) orientation toward the (1102) orientation was also studied. The mobilities of films grown on these substrates showed almost constant values for tilt angles below 3°,but decreased for larger tilt angles of 5°and 10°.Fig. 7 shows the surface morphology of the films grown for various angles of surface tilt from the (0001). The best morphologies were obtained using substrates having surfaces tilted 2°off the (0001) as shown in fig. 7.
4. ___________
——
—,
Discussion
The growth conditions for heteroepitaxial growth of (111)GaAs on (0001)Al203 substrate were optimized so that the properties of the layers were comparable with homoepitaxial films grown onto GaAs substrates. Homoepitaxial undoped films grown under typical conditions exhibited good quality electrical properties electron 2/V s andwith carrier conmobilities of 8000 cm centrations of 1.1 x 1014 crn3 at room temperature. In our growth experiments, high quality epitaxial growth films were produced by restricting the introduction of impurities from the atmosphere of the growth system. In the case of the heteroepitaxial study, however, the growth layer -
— —
Fig. 7. Scanning electron micrographs of epitaxial GaAs films grown on vanous substrates oriented at vanous angles to the (0001) towards the (1102) orientation: (a) 0° (i.e., on the (0001)): (b) 2°: (Cl 3°: (d) 5°. Marker represents 10 Jim.
.
•
.
had a high density of defects which formed in the vicinity of the interface due to a large lattice
A. Sugimura et aL
/
Heteroepitaxial growth of GaAs on sapphire
mismatch of about 13.6% between the layer and the substrate. As reported also by Manasevit and Thorsen [3], the first few micrometers of growth exhibited p-type behavior, and the acceptor concentration decreased with an increase of the distance from the substrate. In our experiment, the pto n-type transition would be expected to occur at a distance greater than 6 p.m from the interface by analogy with the electrical properties of homoepitaxial films grown by the same system, since they had a low content of donor impurities. The analysis of the properties of films is consistent with the fact that undoped heteroepitaxial films of 6 p.m in thickness had a high resistivity, greater than i03 f~ cm.
5.
Conclusion
High quality (111)GaAs films have been grown on (0001)Al 203 substrate by a three step growth method. An initial deposit on annealing formed a buffer layer which confined the defects to the substrate interface and enabled the subsequent deposition of high quality layers. This final layer had a mirror like surface, good electrical properties, of which an electron mobility of 4000 cm2/V. s and a carrier concentration of 4X 1016 cm3. These improvements of the film properties enabled a comparison with the properties of the standard growth films to be made. The significant feature of the three-step heteroepitaxial —
—
529
growth method is the formation of a recrystallized buffer layer between the final growth film and the substrate.
Acknowledgement The authors wish to thank Dr. H. Takenoshita for many helpful discussions and encouragements through the course of this work.
References [1] H.M. Manasevit, Appl. Phys. Letters 12 (1968) 156. [2] H.M. Manasevit, J. Electrochem. Soc. 116 (1969) 1725. [3] H.M. Manasevit and A.C. Thorsen, Met. Trans. 1 (1970) 623. [4] AC. Thorsen and H.M. Manasevit, J. Appi. Phys. 42 (1971) 2519. [5] H.M. Manasevit, J. Crystal Growth 55 (1981) 1. [6] K. Okamoto and T. Imai, Appl. Phys. Letters 42 (1983) 972. [7] M. Akiyama, Y. Kawarada and K. Kaminishi, Japan. J. Appl. Phys. 23 (1984) L917. [8] M. Akiyama, Y. Kawarada and K. Kaminishi, J. Crystal Growth 68 (1984) 21. [9] K. Morizaki, J. Crystal Growth 38 (1977) 249. [10] H.M. Manasevit, J. Crystal Growth 13 (1972) 306. [11] T.F. Kuech, E. Veuhoff and B.S. Meyerson, Appl. Phys. 44 (1984)in:986. [12] Letters G.W. Cullen, Heteroepitaxial Semiconductors for Electronic Devices, Eds. G.W. Cullen and C.C. Wang (Springers, New York, 1978). [13] DL. Rode and S. Knight, Phys. Rev. B3 (1971) 2534.
524
HETEROEPITAXJAL GROWTh OF GaAs ON SAPPHIRE SUBSTRATES BY A THREE-STEP METhOD USING LOW PRESSURE MOCVD Akihiko SUGIMURA Department ofElectrical Engineering College of Engineering, University of Osaka Prefecture, Mozu, Sakai. Osaka 591, Japan
Takashi HOSOI and Kokichi ISHIBITSU Central Research Laboratories, Kyocera Corporation, Yamashina, Kyoto 607, Japan
and Takao KAWAMURA Department of Electrical Engineering, College ofEngineering, University of Osaka Prefecture, Mozu, Sakai, Osaka 591, Japan
Single crystal films of (11l)GaAs have been grown onto (0001)A1
203 substrates by a low pressure MOCVD method. Using a three-step growth procedure, high quality heteroepitaxial growth of GaAs has been achieved by using an annealed buffer layer of 40 nm in thickness deposited on the substrate. Gallium arsenide films grown at a temperature of 620°Chave a2/V mirror-like s and a surface carrier Concentration of 4x 1016 GaAs cm3 films at room temperature for a film thickness of 6 an ~m.electron mobility of 4000 cm morphology. Silicon-doped exhibit n-type conductivity and typically
1. Introduction High quality GaAs is one of the most attractive compounds for the semiconductor device applications. The epitaxial growth of GaAs by MOCVD has received more intensive study following the recent improvement of source gases. Heteroepitaxial GaAs films grown on various substrates such as a-Al203 [1—5], Si [6—8], Ge [9], and MgAl2O4 [1,2,10,11] have been reported. Heteroepitaxial growth will be useful for the preparation of large area GaAs films with the potential for low production costs. Growth of GaAs on ~-Al2O3 substrates is advantageous for the production of crack-free thick film growth because the linear coefficients of thermal expansion (about 5 x i0~ 1 at room temperature) of both materials are o c similar and because there is generally limited interdiffusion of impurities from the substrate [12]. An additional advantageous aspect of GaAs film growth on an insulating sapphire substrate is the improvement in crystallinity that can be achieved compared with growth on a Si or Ge substrate; a
consequence, it appears, of the considerably larger mismatch on GaAs compared with the other substrates. In this paper, heteroepitaxy of high quality (111)GaAs grown on a (0001)Al 203 substrate was achieved by the use of an intermediate annealed buffer layer in the interface between the final GaAs film and the substrate. First, an initial polycrystalline GaAs layer was formed on the substrate. Next, this initial layer was recrystallized by annealing to form a buffer layer. Finally, a single crystal film was grown on this recrystallized buffer layer. This growth procedure will be referred to as the three-step growth (TSG) method. It has been shown by optimizing the growth conditions in the TSG electrical procedureproperties that the and finalsurface epitaxial film had good morphology. 2. Experimental Heteroepitaxial growth using the TSG method in a low pressure MOCVD was carried out follow-
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
A. Sugimura et al.
2nd Step
P
~1
620
-
3rd Step ~taI Growth
1st Step
JecrystaL
420
/ Hereroepitaxial
\
lization
525
growth of GaAs on sapphire
Table 1 Growth conditions Pressure Temperature [ASH [Si2H6l/[TMG] 3]/[TMG]
100 Torr 610—660°C 5 80 (1.7—4.2)x10
Total gas flow
3500 SCCM
grown onto (0001)Al substrates. Homoepitaxial growth was also203 carried out on GaAs sub-
Nucleation
TIME Fig. 1. The three step growth (TSG) method for heteroepitaxy in a low pressure MOCVD apparatus. First step: deposition of the initial polycrystalline GaAs layer of ~- 40 nm in thickness on the substrate at 420°C.Second step: annealing the initial layer at 620°C for 20 mm to bring about recrystallization. Third step: growth of GaAs film,
ing the procedure shown in fig. 1. An initial polycrystalline GaAs layer, 40 nm in thickness, was nucleated on the substrate at 420°C. After heating the wafer up to the growth temperature (620°C)in flowing arsine gas, the GaAs layer was allowed to recrystallize by annealing at the same temperature. Crystal growth of the final film was then started. Vapor growth of GaAs was performed in a horizontal hot-wall quartz tube 450 mm in length and 74 mm in diameter. An RF-coupled SiC coated graphite susceptor was used to heat the substrate. The substrate was oriented horizontally, this being the optimum condition for homogeneous deposition in our system. A thermocouple was set inside —
strates oriented 2°off the (100) towards the (011) orientation in order to compare with heteroepitaxial layers. The thickness of the growth film was standardized at 6 J.Lm by controlling the growth time. The fixed epilayer thickness allowed cornparison of the electrical properties of growth films under the same conditions. Carrier concentration and mobilities were determined by Hall effect measurement at room temperature on samples using the Van der Pauw method. 3. Experimental results Growth conditions in the TSG method were optimized as follows. An initial GaAs layer was 5001.
~~
/
4000
~ 0
the susceptor in order to control the substrate temperature. The reactant gases used were tnmethylgallium (TMG) as the source for the gallium, together with 10% arsine in H2 as a source for the arsenic mixed in a carrier gas of Pd diffused hydrogen. All the flow gases were controlled by precision electric mass flow controllers. All gas lines were 316 stainless steel with VCR connections. Typical growth conditions are shown in table 1. The growth temperature was in the range of 610 to 660 °C.The mole fraction of the source gases and the growth pressure were fixed at [ASH3]/[TMG] = 80 to 100 Torr, respectively. Total gas flow rate was 3500 SCCM. Heteroepitaxial layers were
3000
I -~ °~
I
2000
0
20
40
60
THICKNESS
80
100
(nm)
Fig. 2. Electron mobility as a function of the thickness of the initial layer. The GaAs layer is deposited at a constant temperature of 420°C. The properties of the growth films are affected by the thickness of the initial layer. The last micrometer of the constant 6 ~m film is doped with Si.
526
A. Sugimura et aL
/ Heteroepitaxial growth of GaAs on sapphire
deposited on the substrate at a constant temperature of 420°C in order to study the effect of the recrystallization process. The properties of subsequent films as affected by the variation in the thickness of the initial layer were investigated. Fig. 2 shows the electron mobility as a function of the thickness of the initial layers. In order to estimate the electrical properties of heteroepitaxial films, the last 1 p~mof the film was doped with Si donors under the condition of 6 p.m constant film thickness. Disilane (Si2H6) gas (2 ppm in H2) was used as the dopant gas. The carrier concentration of the doped heteroepitaxial films was proportional to the mole fraction of the 2 H6 I to [TMG] ratio. This result was similar to that reported by Kuech et a!. [11]. However, Si2H6 gas showed a doping efficiency dependent on the growth temperature in the range of our experiments. Following the initial layer of 40 nrn in thickness, the 6 p.m film which was doped with Si in the last 1 p.m of growth indicated maximum 2/V. s ata room ternelectron mobility of 4000 cm perature. The thickness of the initially deposited layer evidently influenced the properties of growth
In the second step, the initial layer was recrystallized by annealing in an AsH3 flow at a temperature of 620°C. It was confirmed by X-ray diffraction measurements that all the initial layers recrystallized on annealing to produce a (111) orientation. Fig. 3 shows the electron mobility of the last 1 ,.Lm layer of growth, which was Si-doped, as a function of annealing time of the initial layer. The mobility increased proportionally with the logarithm of the annealing time for annealing times between 0 and 80 mm. It would be expected from the results as shown in fig. 3 that the quality of the final layer would depend strongly on the effectiveness of the reorientation achieved in the recrystallization of the initial layer. The annealing time of the initial layer in the TSG method was standardized at 20 mm based on the above mentioned results. In the third step, after the initial layer had been deposited on the sapphire substrate and recrystallized, constant 6 p.m of GaAs was grown. 4 shows athe relation between electron mobilityFig. and carrier concentration of GaAs films grown at 620°C. The closed circles indicate the properties
films as shown in this figure. 1000
5000 •~
o
(11 1)GaAsI/(0001)A1
•
203 (100)GaAs//(100)GaAs
8000
~
2000
0 •
[Si2H6J/[’rMG]:4.2x10-5 {Si2i-l~]i[TMG]. 1 .7x 10 i015
ANNEALING 0 5 TIME 20 (mm80 Fig. 3. Room temperature electron mobility of the last mlcrometer layer of growth which was Si-doped as a function of annealing time of the initial layer. All the initial polycrystalline layers recrystallized to give a (111) Orientation by annealing in AsH 3 flow at 620°C.
1016
i017
io~
1019
3) Fig. 4. The CARRIER relation CONCENTRATION between electron mobility (cth and carrier concentration of GaAs films grown at 620°C. The closed circles indicate the properties of heteroepitaxial films and the open circles those of homoepitaxial films. The solid line shows the analytical relation in each case for compensation ratios of I and 5 as found in ref. [13].
A. Sugimura et aL
/ Heteroepitaxial growth of GaAs on sapphire
of heteroepitaxial films and the open circles those of homoepitaxial films. The solid lines show mobility carrier concentration relationships for an analytical model [13] representing GaAs having compensation ratios of 1 and 5. The homoepitaxial films exhibit high quality and low cornpensation even at low carrier concentration. However, the mobility of the heteroepitaxial films decreased with decreasing flow of Si2 6 for carrier concentrations less than 4 x 1016 cm This undesirable tendency was evident for heteroepitaxial GaAs films grown by the TSG method and doped with Si for their entire thickness as well as by layers grown using the standard method. These electrical characteristics would be consistent with the presence of a high resistivity (>i0~ ~2 cm) p-layer. Fig. 5 shows the relation between mobility and film thickness. The circles indicate films grown by the TSG method and the squares indicate those grown by the standard method in which there was no annealing stage. To compare the properties of the GaAs films grown by each of these growth methods, the entire GaAs layer was Si doped. This
527 °
.
_____
-.
~.
6000
______________________________________
_____
__________________ _______
___________________
o Three Step Growth o Standard Growth
I
_______
4000
________
___
~: 2000
FILM THICKNESS (pm) Fig. 5. The relation between mobility and film thickness prepared by the TSG method and a non-TSG heteroepitaxial or standard (single deposition) method. The circles indicate the properties of the films grown by the TSG method and the squares those by the standard method.
______
_________________
_____
—
Fig. 6. Scanning electron micrographs of epitaxial (111)GaAs films grown on (0001)Al203 substrate at several growth ternperatures: (a) 660°C; (b) 630°C; (c) 620°C; (d) 610°C. Marker represents 10 tim.
528
A. Sugirnura et of
/ Heteroepitaxial growth of GaAs on sapphire
~
______________
__________________________________
~ ______________
______________
__________
__________ _____
figure shows that the annealing process which produces a reorientated layer in the TSG method is a useful way of producing films with improved electrical properties. The surface morphology of the GaAs films depends on the growth temperature. Fig. 6 shows SEM photographs of epitaxial (111)GaAs films grown on a (0001)A1203 substrate. Mirror like surface morphology was obtained at a growth temperature of 620°C. However, in the case of higher (630°C) and/or lower (610°C) growth temperatures, the films had a milky appearance and exhibited a triangular faceted morphology. For this range of the growth temperatures, the mobility was the same for layers having the same constant carrier concentration by control of Si2 H6 flow. The growth on Al 203 substrates having surfaces tilted from the (0001) orientation toward the (1102) orientation was also studied. The mobilities of films grown on these substrates showed almost constant values for tilt angles below 3°,but decreased for larger tilt angles of 5°and 10°.Fig. 7 shows the surface morphology of the films grown for various angles of surface tilt from the (0001). The best morphologies were obtained using substrates having surfaces tilted 2°off the (0001) as shown in fig. 7.
4. ___________
——
—,
Discussion
The growth conditions for heteroepitaxial growth of (111)GaAs on (0001)Al203 substrate were optimized so that the properties of the layers were comparable with homoepitaxial films grown onto GaAs substrates. Homoepitaxial undoped films grown under typical conditions exhibited good quality electrical properties electron 2/V s andwith carrier conmobilities of 8000 cm centrations of 1.1 x 1014 crn3 at room temperature. In our growth experiments, high quality epitaxial growth films were produced by restricting the introduction of impurities from the atmosphere of the growth system. In the case of the heteroepitaxial study, however, the growth layer -
— —
Fig. 7. Scanning electron micrographs of epitaxial GaAs films grown on vanous substrates oriented at vanous angles to the (0001) towards the (1102) orientation: (a) 0° (i.e., on the (0001)): (b) 2°: (Cl 3°: (d) 5°. Marker represents 10 Jim.
.
•
.
had a high density of defects which formed in the vicinity of the interface due to a large lattice
A. Sugimura et aL
/
Heteroepitaxial growth of GaAs on sapphire
mismatch of about 13.6% between the layer and the substrate. As reported also by Manasevit and Thorsen [3], the first few micrometers of growth exhibited p-type behavior, and the acceptor concentration decreased with an increase of the distance from the substrate. In our experiment, the pto n-type transition would be expected to occur at a distance greater than 6 p.m from the interface by analogy with the electrical properties of homoepitaxial films grown by the same system, since they had a low content of donor impurities. The analysis of the properties of films is consistent with the fact that undoped heteroepitaxial films of 6 p.m in thickness had a high resistivity, greater than i03 f~ cm.
5.
Conclusion
High quality (111)GaAs films have been grown on (0001)Al 203 substrate by a three step growth method. An initial deposit on annealing formed a buffer layer which confined the defects to the substrate interface and enabled the subsequent deposition of high quality layers. This final layer had a mirror like surface, good electrical properties, of which an electron mobility of 4000 cm2/V. s and a carrier concentration of 4X 1016 cm3. These improvements of the film properties enabled a comparison with the properties of the standard growth films to be made. The significant feature of the three-step heteroepitaxial —
—
529
growth method is the formation of a recrystallized buffer layer between the final growth film and the substrate.
Acknowledgement The authors wish to thank Dr. H. Takenoshita for many helpful discussions and encouragements through the course of this work.
References [1] H.M. Manasevit, Appl. Phys. Letters 12 (1968) 156. [2] H.M. Manasevit, J. Electrochem. Soc. 116 (1969) 1725. [3] H.M. Manasevit and A.C. Thorsen, Met. Trans. 1 (1970) 623. [4] AC. Thorsen and H.M. Manasevit, J. Appi. Phys. 42 (1971) 2519. [5] H.M. Manasevit, J. Crystal Growth 55 (1981) 1. [6] K. Okamoto and T. Imai, Appl. Phys. Letters 42 (1983) 972. [7] M. Akiyama, Y. Kawarada and K. Kaminishi, Japan. J. Appl. Phys. 23 (1984) L917. [8] M. Akiyama, Y. Kawarada and K. Kaminishi, J. Crystal Growth 68 (1984) 21. [9] K. Morizaki, J. Crystal Growth 38 (1977) 249. [10] H.M. Manasevit, J. Crystal Growth 13 (1972) 306. [11] T.F. Kuech, E. Veuhoff and B.S. Meyerson, Appl. Phys. 44 (1984)in:986. [12] Letters G.W. Cullen, Heteroepitaxial Semiconductors for Electronic Devices, Eds. G.W. Cullen and C.C. Wang (Springers, New York, 1978). [13] DL. Rode and S. Knight, Phys. Rev. B3 (1971) 2534.