- Email: [email protected]
Epitaxial growth of thin GaAs layers by hot-wall epitaxy on transparent substrates
Journal of Crystal Growth 70 (1984) 103—107 North-Holland, Amsterdam
103
EPITAXIAL GROWTh OF ThIN GaAs LAYERS BY HOT-WALL EPITAXY ON TRANSPARENT SUBSTRATES M. SADEGHI, H. SITFER and H. GRUBER Institutfür Experimentaiphysik, Universität Linz, A-4040 Linz, Austria
Hot-Wall Epitaxy (HWE) is a vacuum-evaporation technique whose main characteristic is the possibility to grow epitaxial layers with a minimum loss ofmaterial and under conditions closer to thennodynamic equilibrium than most other evaporation methods. Until now the HWE technique was used to grow layers of IV—VI and Il—VI compounds. In this work we report for the first time on the capability of this technique of producing epitaxial GaAs films with controlled stoichiometry on SrF 2 substrates. The growth of GaAs layers on transparent substrates allowed optical investigations to characterize the layers. Finally we report our first results ofdoping experiments with Mn.
1. Introduction
2. Growth apparatus and growth parameters
The Hot-Wall Epitaxy (HWE) technique was previously used to prepare high quality layers of IV—VI and Il—VI compounds [1,2]. Two important growth conditions, high substrate temperature and low supersaturation can be obtained by this technique and consequently high crystalline quality of the epitaxial layers was obtained in the past. We slightly modified this simple technique to grow GaAs by HWE. The HWE also satisfies the requirements for the growth of GaAs films and represents the advantages of high flexibility and little loss of material. To our knowledge, GaAs has not been reported to be grown on monocrystalline SrF2 substrates previously. That provides the possibility to perform transmission experiments and therewith the use of the wide range of optical characterization of the grown layers. The main emphasis of this work was to obtain parameters for growing reproduceable GaAs layers. We worked also on the problem of doping the material with Mn. Until now we achieved semi-instiltating layers obviously by unintentional doping with unknown compensating impurities. However, the GaAs layers are of high crystalline quality and can be used as a thin substrate on a transparent support for the growth ofmore complicated layer structures.
In simple terms we can say that the HWE is a vacuum-film-deposition technique whose main characteristic is the growth of epitaxial films under conditions as near as possible to thermodynamic equilibrium [3]. This is achieved by the insertion of a heated wall between source and substrate, acting as a liner to direct the evaporating molecules. In this manner, (a) loss of evaporating materials is avoided, (b) a clean environment is kept within the growth tube, even when the system is operated in non-UHV conditions, (c) relative high pressure of the evaporating materials can be maintained inside the tube and, as a result, (d) the difference between the source and the substrate temperature can be significantly reduced. A schematic diagram of the apparatus is shown in fig. 1. It consists of two heated concentric graphite tubes sealed at one end containing different sources in different positions within the tubes. A source of polycrystalline GaAs is situated at the bottom of the wide tube. A source of pure Ga contained in a boat in the upper half of the wide tube is used to adjust the stoichiometry and the growth rate of the GaAs layers. The source for the doping material is situated in the thinner tube. The substrate rests on the upper
0022—0248/84/sO3 .00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
104
M. Sadeghi eta!.
iR JJ~I
9 9
:~ Il4iLSubsfrate
/ Expitaxial growth
of thin GaAs layers by HWE on transparent substrates
higher As2 partial pressure than Ga partial pressure. Since the sticking coefficient of As2 is very low the reevaporating As gave an additional cleaning effect for the substrate surface. By increasing the temperature of the Ga source the partial pressure of Ga was increased and as a consequence, nucleation followed by layer growth took place. The growth rate could be controlled to some extent by the temperature of the Ga source. The GaAs layers were grown on (111) oriented SrF2 crystals cleaved in air immediately before deposition. For comparison we also used chemically polished (100) oriented GaAs wafers as substrate material.
z .
—
~
• II! • j~
/
Ga source
: •
I
/ J
/
:
J
~{ :
GaAs Source Heaters
3. Results
Doping source
lI~~’:111 ~if
_________________
1(z)
500
(°C)
700
900
• Fig. I. Schematic setup of the HWE apparatus and corresponding temperature profile.
end of the tube, and acts as a lid to close the tube, so the system can be considered as semiclosed. Each of the three sources and the substrate are heated independently by means of resistance ovens. Several radiation shields are used in order to prevent overheating of the walls of the bell jar where the whole apparatus is contained, The main advantage of a HWE system such as that described here is that it combines the flexibility of a typical evaporation system with the closed nature of a typical vapor growth system. That is, one can grow a large number of films under different growth conditions by simply bringing new substrates to the reactor from a storage section within the bell jar without breaking the vacuum. A typical temperature profile is also shown in fig. 1. The range over which the temperature in the growth apparatus was varied are the following: 900—925 °Cfor the GaAs source; 850—950 °C for the Ga source and 560—6 10 °Cfor the substrate. A typical growth sequence for a GaAs layer consisted of the following steps. The substrate was inserted into the system under conditions with much
Our results indicate that it is possible to grow consistently reproduceable monocrystalline films by HWE using the same source for approximately 20 films with a thickness of 2 ~tm. The growth rate was in the range of 1—2 jim/h depending on the temperature of the Ga source (TGa). We increased TGa from 870 to 950 °Cresulting in an increase of the growth rate by a factor oftwo. For TGa> 950 C the usually mirrorlike and structureless surfaces became slightly blue. The investigation of the surface morphology in a scanning electron microscope showed little droplets on the surface, as shown in fig. 2. The droplets were analysed by a microprobe as pure Ga and are due to the high Ga partial pressure during the growth. The composition of the GaAs films was tested by a microprobe and compared with bulk material. Only the elements of the layer and of the substrate could be detected. We used the ratio of the intensity of the two isolated peaks of Ga-K~xand As-KfJ, to compare the composition in different spots on the film surface. We analysed several different points on a GaAs bulk sample and on our epitaxial layers. The deviation between film and bulk material was less than the statistical scattering ofthe results within one kind of material. As a test of the crystalline quality we performed X-ray-diffraction measurements. The results, depicted in fig. 3, showed sharp peaks for the SrF2 substrates as well as for the GaAs layers. Even the 0
M. Sadeghi eta!.
/ Expitaxial growth
of thin GaAs layers by HWE on transparent substrates
105
[333)-GaAs
~kSrFfl~
___
90.0
89,5
89,0
07,5
07,0
27,0
26,5
~ei] Fig. 3. X-ray diffraction spectrum for a GaAs film grown on monocrystalline SrF 2.
i
sphttmg of the hnes due to the Cu-Kx radiation used is clearly reproduced in the GaAs peaks. The transparent substrate material gave the possibility of per-
p in
forming optical transmission experiments. Fig. 4 shows the transmission curve for a typical GaAs layer at room temperature. The oscillations are
Fig. 2. Scanning electron micrograph of Ga droplets on the GaAs surface.
80
ii
I
/
/
II
~3aAs/SrF2 ~6O
drl,2j.sm — experiment - ——— calculated •
VI
I I I
g g
•
I I
I
I—
i
I S
I
/ /
S
20 /
0’
~
I
700
800 WAVELE~TH
900
InmI
Fig. 4. Transmission as a function of wavelength for a 1.2 ~tmthick GaAs film.
1000
M. Sadeghi eta!. / Expttaxial growth of thin GaAs layers by HWE on transparent substrates
106
caused by multiple interference in the GaAs layer. The dashed curve was calculated by the use of a model taking into account the energy gap and the thickness of the material as well as the dielectric constant and the oscillator strength. The best agreement between experiment and theory was achieved with the assumption of an energy gap of 1.450 eV which deviates by 27 rneV from the values known from the literature [3]. All GaAs layers grown on SrF2 substrates showed a larger energy gap than expected for bulk material. This effect could be due to the strain in the GaAs layers because of the lattice mismatch between the SrF2 substrates and the GaAs layers. The good agreement between theory and experiment indicates that the layers consist of pure GaAs free from metallic inclusions. After optimizing the parameters for the growth of stoichiometric monocrystalline layers we introduced the Mn source into the HWE system for doping experiments. Our first results showed a clear dependence of the carrier concentration on the ternperature of the Mn source, as depicted in fig. 5. The carrier concentration could be shifted over two orders ofmagnitude from 1017 up to 1019 cm Since the degree of compensation is unknown, the carrier concentration is not a direct measure of the Mn content in the layers. The carrier concentration and the mobility of the carriers were investigated as a
I
I
• 2
•
P
~ 8 ~
10
•
~
.
s
~
•
~ I
2 .
I
I
~ .
Fig. 6. Carrier concentration and Hall mobility as a function of inverse temperature for a Mn doped GaAs layer grown on SrF2.
~.
______________________________ I I I •
• •
~
~•
function oftemperature. The results are summarized in fig. 6. The activation energy calculated from the least square fit to the carrier concentration data is 139 meV, which is in the vicinity of a Mn level of 113 meV reported2/V’ by sIlegems et al. [4]. The low of the carriers is attributed mobility 10 cmof compensation. Structural imperto a high of degree fections are unlikely to be the reason for the low mobility. The results of the X-ray diffraction and optical transmission experiments described above show a high degree of crystalline perfection. Also inspection by electron microscopy does not reveal any indication of crystalline imperfections. In the absence of further information we can only attribute tentatively these low mobilities to compensation.
Ix Ix
4. Conclusion 600
650 700 TEMPERATURE OF Mn-SOURCE TMnE°C I
Fig. 5. Carrier concentration versus the temperature of the Mn doping source,
The usefuiness of HWE for the growth of monocrystalline GaAs layers on transparent SrF 2 substrates was clearly demonstrated. The results of the optical investigations confirm the stoichiornetry of
M. Sadeghi et al.
/ Expizaxial growth
of thin GaAs layers by HWE on transparent substrates
the samples and also that the films are free from metallic inclusions. The electrical characterization of the GaAs layers proved that it is possible to control the carrier concentration by the use of an extra doping source. The high resistivity ofthe layers make them applicable as substrate materials for further growth steps.
Acknowledgements
The authors would like to thank Professor H. Heinrich for valuable discussions and his critical reading of the manuscript. We gratefully acknow-
107
ledge the technical assistance of 0. Fuchs, C. Hrdlicka and C. Leitner. The workwas supported by the “Fonds zur Forderung der wissenschaftlichen Forschung in Osterreich”. References [1] A. Lopez-Otero, Thin Solid Films 49 (1978) 3. [2] A. Lopez-Otero and W. Huber, 3. Crystal Growth 45 (1978) 214. [3] J. S. Blakeinore, 1. App!. Phys. 53(1982) Rl23. [4] M. Ilegems, P.., Dingle and L. W. Rupp J. AppI. Phys. 46 (1975) 3059.
103
EPITAXIAL GROWTh OF ThIN GaAs LAYERS BY HOT-WALL EPITAXY ON TRANSPARENT SUBSTRATES M. SADEGHI, H. SITFER and H. GRUBER Institutfür Experimentaiphysik, Universität Linz, A-4040 Linz, Austria
Hot-Wall Epitaxy (HWE) is a vacuum-evaporation technique whose main characteristic is the possibility to grow epitaxial layers with a minimum loss ofmaterial and under conditions closer to thennodynamic equilibrium than most other evaporation methods. Until now the HWE technique was used to grow layers of IV—VI and Il—VI compounds. In this work we report for the first time on the capability of this technique of producing epitaxial GaAs films with controlled stoichiometry on SrF 2 substrates. The growth of GaAs layers on transparent substrates allowed optical investigations to characterize the layers. Finally we report our first results ofdoping experiments with Mn.
1. Introduction
2. Growth apparatus and growth parameters
The Hot-Wall Epitaxy (HWE) technique was previously used to prepare high quality layers of IV—VI and Il—VI compounds [1,2]. Two important growth conditions, high substrate temperature and low supersaturation can be obtained by this technique and consequently high crystalline quality of the epitaxial layers was obtained in the past. We slightly modified this simple technique to grow GaAs by HWE. The HWE also satisfies the requirements for the growth of GaAs films and represents the advantages of high flexibility and little loss of material. To our knowledge, GaAs has not been reported to be grown on monocrystalline SrF2 substrates previously. That provides the possibility to perform transmission experiments and therewith the use of the wide range of optical characterization of the grown layers. The main emphasis of this work was to obtain parameters for growing reproduceable GaAs layers. We worked also on the problem of doping the material with Mn. Until now we achieved semi-instiltating layers obviously by unintentional doping with unknown compensating impurities. However, the GaAs layers are of high crystalline quality and can be used as a thin substrate on a transparent support for the growth ofmore complicated layer structures.
In simple terms we can say that the HWE is a vacuum-film-deposition technique whose main characteristic is the growth of epitaxial films under conditions as near as possible to thermodynamic equilibrium [3]. This is achieved by the insertion of a heated wall between source and substrate, acting as a liner to direct the evaporating molecules. In this manner, (a) loss of evaporating materials is avoided, (b) a clean environment is kept within the growth tube, even when the system is operated in non-UHV conditions, (c) relative high pressure of the evaporating materials can be maintained inside the tube and, as a result, (d) the difference between the source and the substrate temperature can be significantly reduced. A schematic diagram of the apparatus is shown in fig. 1. It consists of two heated concentric graphite tubes sealed at one end containing different sources in different positions within the tubes. A source of polycrystalline GaAs is situated at the bottom of the wide tube. A source of pure Ga contained in a boat in the upper half of the wide tube is used to adjust the stoichiometry and the growth rate of the GaAs layers. The source for the doping material is situated in the thinner tube. The substrate rests on the upper
0022—0248/84/sO3 .00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
104
M. Sadeghi eta!.
iR JJ~I
9 9
:~ Il4iLSubsfrate
/ Expitaxial growth
of thin GaAs layers by HWE on transparent substrates
higher As2 partial pressure than Ga partial pressure. Since the sticking coefficient of As2 is very low the reevaporating As gave an additional cleaning effect for the substrate surface. By increasing the temperature of the Ga source the partial pressure of Ga was increased and as a consequence, nucleation followed by layer growth took place. The growth rate could be controlled to some extent by the temperature of the Ga source. The GaAs layers were grown on (111) oriented SrF2 crystals cleaved in air immediately before deposition. For comparison we also used chemically polished (100) oriented GaAs wafers as substrate material.
z .
—
~
• II! • j~
/
Ga source
: •
I
/ J
/
:
J
~{ :
GaAs Source Heaters
3. Results
Doping source
lI~~’:111 ~if
_________________
1(z)
500
(°C)
700
900
• Fig. I. Schematic setup of the HWE apparatus and corresponding temperature profile.
end of the tube, and acts as a lid to close the tube, so the system can be considered as semiclosed. Each of the three sources and the substrate are heated independently by means of resistance ovens. Several radiation shields are used in order to prevent overheating of the walls of the bell jar where the whole apparatus is contained, The main advantage of a HWE system such as that described here is that it combines the flexibility of a typical evaporation system with the closed nature of a typical vapor growth system. That is, one can grow a large number of films under different growth conditions by simply bringing new substrates to the reactor from a storage section within the bell jar without breaking the vacuum. A typical temperature profile is also shown in fig. 1. The range over which the temperature in the growth apparatus was varied are the following: 900—925 °Cfor the GaAs source; 850—950 °C for the Ga source and 560—6 10 °Cfor the substrate. A typical growth sequence for a GaAs layer consisted of the following steps. The substrate was inserted into the system under conditions with much
Our results indicate that it is possible to grow consistently reproduceable monocrystalline films by HWE using the same source for approximately 20 films with a thickness of 2 ~tm. The growth rate was in the range of 1—2 jim/h depending on the temperature of the Ga source (TGa). We increased TGa from 870 to 950 °Cresulting in an increase of the growth rate by a factor oftwo. For TGa> 950 C the usually mirrorlike and structureless surfaces became slightly blue. The investigation of the surface morphology in a scanning electron microscope showed little droplets on the surface, as shown in fig. 2. The droplets were analysed by a microprobe as pure Ga and are due to the high Ga partial pressure during the growth. The composition of the GaAs films was tested by a microprobe and compared with bulk material. Only the elements of the layer and of the substrate could be detected. We used the ratio of the intensity of the two isolated peaks of Ga-K~xand As-KfJ, to compare the composition in different spots on the film surface. We analysed several different points on a GaAs bulk sample and on our epitaxial layers. The deviation between film and bulk material was less than the statistical scattering ofthe results within one kind of material. As a test of the crystalline quality we performed X-ray-diffraction measurements. The results, depicted in fig. 3, showed sharp peaks for the SrF2 substrates as well as for the GaAs layers. Even the 0
M. Sadeghi eta!.
/ Expitaxial growth
of thin GaAs layers by HWE on transparent substrates
105
[333)-GaAs
~kSrFfl~
___
90.0
89,5
89,0
07,5
07,0
27,0
26,5
~ei] Fig. 3. X-ray diffraction spectrum for a GaAs film grown on monocrystalline SrF 2.
i
sphttmg of the hnes due to the Cu-Kx radiation used is clearly reproduced in the GaAs peaks. The transparent substrate material gave the possibility of per-
p in
forming optical transmission experiments. Fig. 4 shows the transmission curve for a typical GaAs layer at room temperature. The oscillations are
Fig. 2. Scanning electron micrograph of Ga droplets on the GaAs surface.
80
ii
I
/
/
II
~3aAs/SrF2 ~6O
drl,2j.sm — experiment - ——— calculated •
VI
I I I
g g
•
I I
I
I—
i
I S
I
/ /
S
20 /
0’
~
I
700
800 WAVELE~TH
900
InmI
Fig. 4. Transmission as a function of wavelength for a 1.2 ~tmthick GaAs film.
1000
M. Sadeghi eta!. / Expttaxial growth of thin GaAs layers by HWE on transparent substrates
106
caused by multiple interference in the GaAs layer. The dashed curve was calculated by the use of a model taking into account the energy gap and the thickness of the material as well as the dielectric constant and the oscillator strength. The best agreement between experiment and theory was achieved with the assumption of an energy gap of 1.450 eV which deviates by 27 rneV from the values known from the literature [3]. All GaAs layers grown on SrF2 substrates showed a larger energy gap than expected for bulk material. This effect could be due to the strain in the GaAs layers because of the lattice mismatch between the SrF2 substrates and the GaAs layers. The good agreement between theory and experiment indicates that the layers consist of pure GaAs free from metallic inclusions. After optimizing the parameters for the growth of stoichiometric monocrystalline layers we introduced the Mn source into the HWE system for doping experiments. Our first results showed a clear dependence of the carrier concentration on the ternperature of the Mn source, as depicted in fig. 5. The carrier concentration could be shifted over two orders ofmagnitude from 1017 up to 1019 cm Since the degree of compensation is unknown, the carrier concentration is not a direct measure of the Mn content in the layers. The carrier concentration and the mobility of the carriers were investigated as a
I
I
• 2
•
P
~ 8 ~
10
•
~
.
s
~
•
~ I
2 .
I
I
~ .
Fig. 6. Carrier concentration and Hall mobility as a function of inverse temperature for a Mn doped GaAs layer grown on SrF2.
~.
______________________________ I I I •
• •
~
~•
function oftemperature. The results are summarized in fig. 6. The activation energy calculated from the least square fit to the carrier concentration data is 139 meV, which is in the vicinity of a Mn level of 113 meV reported2/V’ by sIlegems et al. [4]. The low of the carriers is attributed mobility 10 cmof compensation. Structural imperto a high of degree fections are unlikely to be the reason for the low mobility. The results of the X-ray diffraction and optical transmission experiments described above show a high degree of crystalline perfection. Also inspection by electron microscopy does not reveal any indication of crystalline imperfections. In the absence of further information we can only attribute tentatively these low mobilities to compensation.
Ix Ix
4. Conclusion 600
650 700 TEMPERATURE OF Mn-SOURCE TMnE°C I
Fig. 5. Carrier concentration versus the temperature of the Mn doping source,
The usefuiness of HWE for the growth of monocrystalline GaAs layers on transparent SrF 2 substrates was clearly demonstrated. The results of the optical investigations confirm the stoichiornetry of
M. Sadeghi et al.
/ Expizaxial growth
of thin GaAs layers by HWE on transparent substrates
the samples and also that the films are free from metallic inclusions. The electrical characterization of the GaAs layers proved that it is possible to control the carrier concentration by the use of an extra doping source. The high resistivity ofthe layers make them applicable as substrate materials for further growth steps.
Acknowledgements
The authors would like to thank Professor H. Heinrich for valuable discussions and his critical reading of the manuscript. We gratefully acknow-
107
ledge the technical assistance of 0. Fuchs, C. Hrdlicka and C. Leitner. The workwas supported by the “Fonds zur Forderung der wissenschaftlichen Forschung in Osterreich”. References [1] A. Lopez-Otero, Thin Solid Films 49 (1978) 3. [2] A. Lopez-Otero and W. Huber, 3. Crystal Growth 45 (1978) 214. [3] J. S. Blakeinore, 1. App!. Phys. 53(1982) Rl23. [4] M. Ilegems, P.., Dingle and L. W. Rupp J. AppI. Phys. 46 (1975) 3059.