- Email: [email protected]
Characterization of p-GaAs by low pressure MOCVD using DEZ as dopant
472
Journal of Crystal Growth 67 (1984) 472—476 North-Holland, Amsterdam
CHARACTERIZATION OF p-GaAs BY LOW PRESSURE MOCVD USING DEZ AS DOPANT Y.K.
SU, C.Y. CHANG, T.S. WU and Y.C. CHOU
Research Institute of Electronic and Electrical Engineering, National Cheng Kung Unicersity, Tainan, Taiwan, Rep. of China
and C.Y. NEE Material Research Laboratories. ITRI, Hsinchu, Taiwan, Rep. of China Received 20 February 1984; manuscript received in final form 20 April 1984
Zinc-doped GaAs epilayers grown by low pressure metalorganic chemical vapor deposition (LP-MOCVD) are studied. Triethylgalhum (TEG) and arsine (AsH
1) are used as Ga and As source, respectively. Diethylzinc (DEZ) is used as p-type dopant. Layers of high crystalline quality can be obtained. The influence of growth parameters such as DEZ mole fraction, growth temperatures and AsH1 mole fraction on hole concentration are measured and discussed. These results can be explained well by a simple qualitative model. The hole concentration is proportional to the concentration of gallium vacancies. The I—V characteristics of Schottky diodes and p—n junctions are discussed. The ideality factor is about 1.3.
1. Introduction
duced using trimethylgallium (TMG) as the Ga source [3—5]; few workers utilized triethylgallium
Since the first demonstration of the growth of GaAs by metalorganic chemical vapor deposition [1], a general interest in MOCVD technique has persisted. During the last decade, a great deal of effort has been made to study the growth of GaAs epilayers. Such efforts have been stimulated by the potential applications in microwave oscillators, integrated optics, field effect transistors, and laser diodes. Most of the early MOCVD growth was carried out at atmospheric pressure; however, Duchemin et al. [2] investigated the growth of InP at reduced pressure and found that the advantages of silicon growth at low pressure were also evident in the growth of 111—V compounds by low pressure metalorganic chemical vapor deposition (LPMOCVD). At low pressure, the concentration gradient across the stagnant layer is greater for species diffusing out from the substrate, thus less of the volatile dopant is contained in the stagnant layer and impurity grading due to autodoping is reduced. Almost all of the high purity GaAs was pro-
(TEG) as the Ga source [6—81. In an effort to obtain low carbon contamination, TEG is used as the Ga source in this report, instead of TMG. Because of the many advantages mentioned above in the low pressure process, reduced pressure is also utilized. Chang, Su and co-workers [9—111 have studied the characterization of undoped and Sn-doped GaAs epitaxial layers by low pressure MOCVD. In this paper, Zn-doped GaAs epilayers were grown on (100)-oriented GaAs: Si wafers. The metalorganic sources of Zn are dimethylzinc (DMZ) and diethyizinc (DEZ). Both ignite spontaneously or detonate upon exposure to air and decompose violently in water. Hence one must be very careful when these sources are detached. The vapor pressures for DMZ and DEG at 0°C are 125 and 4 Torr, respectively. Due to the high vapor pressure of DMZ, bubbler temperatures between —100 and —75°C were necessary [12], however, the trap temperature for DEZ is kept at 0°C. Hence DEZ is considered as a better zinc source for this experiment. The effects of growth
0022-0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Y.K. Suet al.
/
Characterization of p-GaAs by LP-MOCVD
parameters on the hole concentration and their explanations are investigated. The Schottky diode and p—n homojunction are made and discussed,
2. Experimental A vertical reactor system [9—11] is used. The apparatus consists of a single vertical quartz tube and a graphite susceptor which is heated inductively by a radio-frequency source, and can be easily detached from the quartz tube to facilitate substrate loading and unloading. Metalorganic sources of TEG and DEZ purchased from Furnace & Material Trading Ltd. with a stated purity of 99.999% are used as Ga source and p-type dopant, respectively. The TEG and DEZ are carried into the reactor by bubbling H2 through the liquid. The AsH3 gas used for the experiments is diluted down to 1% with high purity H2. Polished (100)-oriented GaAs: Si wafers with ~ 2/V. s are used as substrates. They are rinsed cm in hot trichloroethylene, etched 3—5 mm in hot
473
tion on mole fraction of DEZ. The carrier concentration increases with DEZ concentration. For a given mole fraction of DEZ, the doping level is observed to increase as the growth temperature decreases as shown in fig. 2. These phenomena can be interpreted as Zn evaporating from the surface of the epitaxial layer more at higher temperatures. For given TEG and DEZ mole fractions, the relationship of hole concentration versus AsH 3 mole fraction is illustrated in fig. 3. Now we consider an elementary model of the TEG: AsH3 system. The reactions leading to the formation of GaAs can be expressed as Ga(C2H5)3 AsH3
+ ~
+ VA.,
H2
+ VGA
GaGa +
3 C2H6, (1)
~ H2.
A5A,, +
(2)
where k1 and k2 are the equilibrium constants of the above reactions, then 1~A.,H~‘3C
[VGa] [VA.,]
~7
k2
2H6
~
(65°C) 3H 2S04: 1H202(30%): 1H20, rinsed in DI water and CH3OH, and blown dry in filtered high purity N2. The substrates are placed on the RF heated susceptor and the reactor is then evacuated, The reactor chamber is backfilled with very high purity H2 and a flow of 2 1/mm is established. AsH3 flow is started when the substrate temperature reaches about 500°C in order to prevent substrate decomposition. The deposition temperature varies from 600 to 700 °C. After deposition, the AsH3 flow is continued until the substrate temperature drops below 500°C. The deposition pressure is maintained at 20 Torr.
3. Results The Zn-doped epilayers grown on Si-doped fl type GaAs substrates appeared to be mirror smooth. The X-ray back reflection Laue method is used for confirmation of epitaxy. It indicates that good crystallinity can be obtained by low pressure MOCVD with DEZ as the Zn dopant. Fig. 1 shows the dependence of hole concentra-
An increase in the ratio ~A,,H. ~TEG would increase the gallium vacancy concentration, hence, the incorporation of zinc would be increased. The hole concentration is thus increased when the AsH3 mole fraction is increased. The relationship can be 20
10 ~SH3
MF
TEG
MF = 70 I0~
=
5a
big
~I8
~
Growth Temperature 550°C
•
Growth Temperature
=
700°C
A
Growth Temperature
=
750°C
I0~~ lOG
i0~ 057
i6°
l0~
Mole Fraction
Fig. 1. Hole concentration of zinc doped GaAs as a function of DEZ mole fraction in the vapor.
474
Y.K. Suet al.
/
Characterization of p-GaAs by LP-MOCVD
epitaxial film. The I—V characteristic of this Schottky shownepitaxial in fig. 4.layer The ishole centrationdiode of theisp-type 6 X conlO~~
T° (C
750
10’ g
700 I
6.50
600
560
3 and the diode area is 3.14 x iO~cm2. These cm curves show weak breakdown, which may be due .5
to heavy doping of the n~ substrate or the epic9
JQIB
-
~MF=5
taxial layer. After growing a Se-doped GaAs epilayer (dcctron concentration 5 X 1017 cm3) on a (100)oriented n-type GaAs substrate, a Zn-doped GaAs layer of about 2—3 ~.tm thick is grown on the
xlQ~3/min TEG
MF=7x104/m,n
067
MF2xlcj~’m,n
0
iO’~ 995
n-type layer. The hole concentration of this layer is 5 x lO~~cm3. The experimental procedures are similar to those described in the above section. hut the epitaxial layers are processed by the mesa-etch/00
(05 110 bO0o(’~) T
115
120
ing technique [14]. The I—V characteristic of this
Fig. 2. Effect of growth temperature on the hole concentration.
expressed as follows: 2
(4)
flh6CPA.,H.
The above result is different from that of selenium
6. o (N
0
doping [13]. After the Zn-doped epilayer is grown, a Au—Ge film is evaporated onto the back side of the substrate, and an aluminium film is evaporated on the
(0
20 .
1V/Div T= 700C TEG MF=7x1Q~4/m,n
.5 (2
OEZ
ME=2x106/mio
~ a 0 0
.~ I 1018
-
bO~
lo~
/0.~
I0~
,6sH 3 Mole Fraction
1V/D,v
Fig. 3. Hole concentration as a function of AsH3 mole fraction
Fig. 4. I—V characteristics of Al—GaAs Schottky diode: (a)
in the vapor,
as-deposited; (b) after sintering in N2 at 450 ° C.
YK. Su et al.
/
Characterization of p-GaAs br LP-MOCVD
475
4. Conclusion
__________________________________________ 0
1 V/Div 4’-GaAs
of Zn—GaAs/Se—GaAs/n’
Fig. 5. I—V characteristics homodiode.
homodiode is illustrated in fig. 5. The result shows very good rectification. The current—voltage characteristic exhibits an exp(qv/nkT) dependence with n = 1.3. Fig. 6 shows forward and reverse bias behavior for this GaAs p—n junction.
O~4 08
12
l~ 2~3~24
/
temperature, AsH3 flow rate and DEZ flow rate on the hole concentrations are studied and discussed. The results are qualitatively explained well by a simple model. The hole concentration is proportional to the concentration of gallium vacancies. . A Schottky diode is made and investigated. The . . I— V curves show weak breakdown, possibly due to either the heavily doped n + substrate or the epitaxial layer. The current—voltage characteristic for a p—n homojunction is also illustrated. It shows a very good rectification. The ideality factor is about 1.3.
Acknowledgements the authors would like to express their gratitude to Dr. M.K. Lee of the National Chun-Shan University for fruitful discussions. The authors would also like to thank Dr. J.Y. Lee, Chairman of the Department of EE, NCKU, for his stimulation and assistance during the work. The financial sup-
Voltages ~~jao
Diethylzinc can be used as a source of Zndopant in GaAs epitaxial growth by low pressure metalorganic CVD using TEG and AsH 3 as Ga and As sources, respectively. The Zn-doped epilayers appear to be mirror smooth. The effects of growth parameters such as growth
/ -
port of the National Science Council, ROC, is deeply appreciated.
~ bo~-
-
References
~ lO~ -
-
[I] H.M. Manasevit and WI. Simpson, J. Electrochem. Soc. 116 (1969) 1729.
-
[2] J.P. Duchemin, M. Bonnet, F. Koelsch and D. Huyghe. J. Crystal Growth 45 (1978) 181. [3] S. Ito, T. Shinohara and Y. Seki, J. Electrochem. Soc. 120
/
b0-~ -
/
I-’ 108
-
Io~-
-
F?everse s,as
IO’° 0
1
-2
3
~
‘~
(1973) 1419. [4] J.P. Duchemin, M. Bonnet, F. Koehsch and D. Huyghe. J. Ehectrochem. Soc. 126 (1979) 1134. [5] N.J. Nelson, K.K. Thomson, R.L. Moon, H.A. Vander and LW. Apph. Phys. Letters 33 (1978) 26.
~
Voltages
reverse p—n junction.
Fig. 6. Forward and MOCVD GaAs
bias I—V characteristics
for a
James,
[6] Y. Seki, K. Tanno, K. Lida and E. Ichiki, J. Electrochem. Soc. 122 (1975) 1108.
476
Y.K. Suet al. / Characterization of p-GaAs by LP-MOCVD
[7] GB. Stringfehhow and H.T. Hall, Jr., J. Crystal Growth 43 (1978) 47. [8] GB. Stringfellow and H.T. Hall, Jr., J. Electron. Mater. 8 (1979) 201. [9] C.Y. Chang, Y.K. Su, M.K. Lee, L.G. Chen and M.P. l-loung. J. Crystal Growth 55 (1981) 24. [10] M.K. Lee, C.Y. Chang and Y.K. Su, AppI. Phys. Letters 42 (1983) 88.
[11] C.Y. Chang, M.K. Lee, Y.K. Su and W.C. Hsu. J. AppI. Phys. (1983). [12] S.J. Bass, J. Crystal Growth 31 (1975) 172. [13] Y.K. Su, T.S. Wu, C.Y. Chang and Y.C. Chou, J. AppI. Phys., to be published. [14] R.L.V. Tuyh, CA. Liechti, RE. Lee and E. Growen, IEEE J. Solid State Circuit SC-12 (1977) 485.
Journal of Crystal Growth 67 (1984) 472—476 North-Holland, Amsterdam
CHARACTERIZATION OF p-GaAs BY LOW PRESSURE MOCVD USING DEZ AS DOPANT Y.K.
SU, C.Y. CHANG, T.S. WU and Y.C. CHOU
Research Institute of Electronic and Electrical Engineering, National Cheng Kung Unicersity, Tainan, Taiwan, Rep. of China
and C.Y. NEE Material Research Laboratories. ITRI, Hsinchu, Taiwan, Rep. of China Received 20 February 1984; manuscript received in final form 20 April 1984
Zinc-doped GaAs epilayers grown by low pressure metalorganic chemical vapor deposition (LP-MOCVD) are studied. Triethylgalhum (TEG) and arsine (AsH
1) are used as Ga and As source, respectively. Diethylzinc (DEZ) is used as p-type dopant. Layers of high crystalline quality can be obtained. The influence of growth parameters such as DEZ mole fraction, growth temperatures and AsH1 mole fraction on hole concentration are measured and discussed. These results can be explained well by a simple qualitative model. The hole concentration is proportional to the concentration of gallium vacancies. The I—V characteristics of Schottky diodes and p—n junctions are discussed. The ideality factor is about 1.3.
1. Introduction
duced using trimethylgallium (TMG) as the Ga source [3—5]; few workers utilized triethylgallium
Since the first demonstration of the growth of GaAs by metalorganic chemical vapor deposition [1], a general interest in MOCVD technique has persisted. During the last decade, a great deal of effort has been made to study the growth of GaAs epilayers. Such efforts have been stimulated by the potential applications in microwave oscillators, integrated optics, field effect transistors, and laser diodes. Most of the early MOCVD growth was carried out at atmospheric pressure; however, Duchemin et al. [2] investigated the growth of InP at reduced pressure and found that the advantages of silicon growth at low pressure were also evident in the growth of 111—V compounds by low pressure metalorganic chemical vapor deposition (LPMOCVD). At low pressure, the concentration gradient across the stagnant layer is greater for species diffusing out from the substrate, thus less of the volatile dopant is contained in the stagnant layer and impurity grading due to autodoping is reduced. Almost all of the high purity GaAs was pro-
(TEG) as the Ga source [6—81. In an effort to obtain low carbon contamination, TEG is used as the Ga source in this report, instead of TMG. Because of the many advantages mentioned above in the low pressure process, reduced pressure is also utilized. Chang, Su and co-workers [9—111 have studied the characterization of undoped and Sn-doped GaAs epitaxial layers by low pressure MOCVD. In this paper, Zn-doped GaAs epilayers were grown on (100)-oriented GaAs: Si wafers. The metalorganic sources of Zn are dimethylzinc (DMZ) and diethyizinc (DEZ). Both ignite spontaneously or detonate upon exposure to air and decompose violently in water. Hence one must be very careful when these sources are detached. The vapor pressures for DMZ and DEG at 0°C are 125 and 4 Torr, respectively. Due to the high vapor pressure of DMZ, bubbler temperatures between —100 and —75°C were necessary [12], however, the trap temperature for DEZ is kept at 0°C. Hence DEZ is considered as a better zinc source for this experiment. The effects of growth
0022-0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Y.K. Suet al.
/
Characterization of p-GaAs by LP-MOCVD
parameters on the hole concentration and their explanations are investigated. The Schottky diode and p—n homojunction are made and discussed,
2. Experimental A vertical reactor system [9—11] is used. The apparatus consists of a single vertical quartz tube and a graphite susceptor which is heated inductively by a radio-frequency source, and can be easily detached from the quartz tube to facilitate substrate loading and unloading. Metalorganic sources of TEG and DEZ purchased from Furnace & Material Trading Ltd. with a stated purity of 99.999% are used as Ga source and p-type dopant, respectively. The TEG and DEZ are carried into the reactor by bubbling H2 through the liquid. The AsH3 gas used for the experiments is diluted down to 1% with high purity H2. Polished (100)-oriented GaAs: Si wafers with ~ 2/V. s are used as substrates. They are rinsed cm in hot trichloroethylene, etched 3—5 mm in hot
473
tion on mole fraction of DEZ. The carrier concentration increases with DEZ concentration. For a given mole fraction of DEZ, the doping level is observed to increase as the growth temperature decreases as shown in fig. 2. These phenomena can be interpreted as Zn evaporating from the surface of the epitaxial layer more at higher temperatures. For given TEG and DEZ mole fractions, the relationship of hole concentration versus AsH 3 mole fraction is illustrated in fig. 3. Now we consider an elementary model of the TEG: AsH3 system. The reactions leading to the formation of GaAs can be expressed as Ga(C2H5)3 AsH3
+ ~
+ VA.,
H2
+ VGA
GaGa +
3 C2H6, (1)
~ H2.
A5A,, +
(2)
where k1 and k2 are the equilibrium constants of the above reactions, then 1~A.,H~‘3C
[VGa] [VA.,]
~7
k2
2H6
~
(65°C) 3H 2S04: 1H202(30%): 1H20, rinsed in DI water and CH3OH, and blown dry in filtered high purity N2. The substrates are placed on the RF heated susceptor and the reactor is then evacuated, The reactor chamber is backfilled with very high purity H2 and a flow of 2 1/mm is established. AsH3 flow is started when the substrate temperature reaches about 500°C in order to prevent substrate decomposition. The deposition temperature varies from 600 to 700 °C. After deposition, the AsH3 flow is continued until the substrate temperature drops below 500°C. The deposition pressure is maintained at 20 Torr.
3. Results The Zn-doped epilayers grown on Si-doped fl type GaAs substrates appeared to be mirror smooth. The X-ray back reflection Laue method is used for confirmation of epitaxy. It indicates that good crystallinity can be obtained by low pressure MOCVD with DEZ as the Zn dopant. Fig. 1 shows the dependence of hole concentra-
An increase in the ratio ~A,,H. ~TEG would increase the gallium vacancy concentration, hence, the incorporation of zinc would be increased. The hole concentration is thus increased when the AsH3 mole fraction is increased. The relationship can be 20
10 ~SH3
MF
TEG
MF = 70 I0~
=
5a
big
~I8
~
Growth Temperature 550°C
•
Growth Temperature
=
700°C
A
Growth Temperature
=
750°C
I0~~ lOG
i0~ 057
i6°
l0~
Mole Fraction
Fig. 1. Hole concentration of zinc doped GaAs as a function of DEZ mole fraction in the vapor.
474
Y.K. Suet al.
/
Characterization of p-GaAs by LP-MOCVD
epitaxial film. The I—V characteristic of this Schottky shownepitaxial in fig. 4.layer The ishole centrationdiode of theisp-type 6 X conlO~~
T° (C
750
10’ g
700 I
6.50
600
560
3 and the diode area is 3.14 x iO~cm2. These cm curves show weak breakdown, which may be due .5
to heavy doping of the n~ substrate or the epic9
JQIB
-
~MF=5
taxial layer. After growing a Se-doped GaAs epilayer (dcctron concentration 5 X 1017 cm3) on a (100)oriented n-type GaAs substrate, a Zn-doped GaAs layer of about 2—3 ~.tm thick is grown on the
xlQ~3/min TEG
MF=7x104/m,n
067
MF2xlcj~’m,n
0
iO’~ 995
n-type layer. The hole concentration of this layer is 5 x lO~~cm3. The experimental procedures are similar to those described in the above section. hut the epitaxial layers are processed by the mesa-etch/00
(05 110 bO0o(’~) T
115
120
ing technique [14]. The I—V characteristic of this
Fig. 2. Effect of growth temperature on the hole concentration.
expressed as follows: 2
(4)
flh6CPA.,H.
The above result is different from that of selenium
6. o (N
0
doping [13]. After the Zn-doped epilayer is grown, a Au—Ge film is evaporated onto the back side of the substrate, and an aluminium film is evaporated on the
(0
20 .
1V/Div T= 700C TEG MF=7x1Q~4/m,n
.5 (2
OEZ
ME=2x106/mio
~ a 0 0
.~ I 1018
-
bO~
lo~
/0.~
I0~
,6sH 3 Mole Fraction
1V/D,v
Fig. 3. Hole concentration as a function of AsH3 mole fraction
Fig. 4. I—V characteristics of Al—GaAs Schottky diode: (a)
in the vapor,
as-deposited; (b) after sintering in N2 at 450 ° C.
YK. Su et al.
/
Characterization of p-GaAs br LP-MOCVD
475
4. Conclusion
__________________________________________ 0
1 V/Div 4’-GaAs
of Zn—GaAs/Se—GaAs/n’
Fig. 5. I—V characteristics homodiode.
homodiode is illustrated in fig. 5. The result shows very good rectification. The current—voltage characteristic exhibits an exp(qv/nkT) dependence with n = 1.3. Fig. 6 shows forward and reverse bias behavior for this GaAs p—n junction.
O~4 08
12
l~ 2~3~24
/
temperature, AsH3 flow rate and DEZ flow rate on the hole concentrations are studied and discussed. The results are qualitatively explained well by a simple model. The hole concentration is proportional to the concentration of gallium vacancies. . A Schottky diode is made and investigated. The . . I— V curves show weak breakdown, possibly due to either the heavily doped n + substrate or the epitaxial layer. The current—voltage characteristic for a p—n homojunction is also illustrated. It shows a very good rectification. The ideality factor is about 1.3.
Acknowledgements the authors would like to express their gratitude to Dr. M.K. Lee of the National Chun-Shan University for fruitful discussions. The authors would also like to thank Dr. J.Y. Lee, Chairman of the Department of EE, NCKU, for his stimulation and assistance during the work. The financial sup-
Voltages ~~jao
Diethylzinc can be used as a source of Zndopant in GaAs epitaxial growth by low pressure metalorganic CVD using TEG and AsH 3 as Ga and As sources, respectively. The Zn-doped epilayers appear to be mirror smooth. The effects of growth parameters such as growth
/ -
port of the National Science Council, ROC, is deeply appreciated.
~ bo~-
-
References
~ lO~ -
-
[I] H.M. Manasevit and WI. Simpson, J. Electrochem. Soc. 116 (1969) 1729.
-
[2] J.P. Duchemin, M. Bonnet, F. Koelsch and D. Huyghe. J. Crystal Growth 45 (1978) 181. [3] S. Ito, T. Shinohara and Y. Seki, J. Electrochem. Soc. 120
/
b0-~ -
/
I-’ 108
-
Io~-
-
F?everse s,as
IO’° 0
1
-2
3
~
‘~
(1973) 1419. [4] J.P. Duchemin, M. Bonnet, F. Koehsch and D. Huyghe. J. Ehectrochem. Soc. 126 (1979) 1134. [5] N.J. Nelson, K.K. Thomson, R.L. Moon, H.A. Vander and LW. Apph. Phys. Letters 33 (1978) 26.
~
Voltages
reverse p—n junction.
Fig. 6. Forward and MOCVD GaAs
bias I—V characteristics
for a
James,
[6] Y. Seki, K. Tanno, K. Lida and E. Ichiki, J. Electrochem. Soc. 122 (1975) 1108.
476
Y.K. Suet al. / Characterization of p-GaAs by LP-MOCVD
[7] GB. Stringfehhow and H.T. Hall, Jr., J. Crystal Growth 43 (1978) 47. [8] GB. Stringfellow and H.T. Hall, Jr., J. Electron. Mater. 8 (1979) 201. [9] C.Y. Chang, Y.K. Su, M.K. Lee, L.G. Chen and M.P. l-loung. J. Crystal Growth 55 (1981) 24. [10] M.K. Lee, C.Y. Chang and Y.K. Su, AppI. Phys. Letters 42 (1983) 88.
[11] C.Y. Chang, M.K. Lee, Y.K. Su and W.C. Hsu. J. AppI. Phys. (1983). [12] S.J. Bass, J. Crystal Growth 31 (1975) 172. [13] Y.K. Su, T.S. Wu, C.Y. Chang and Y.C. Chou, J. AppI. Phys., to be published. [14] R.L.V. Tuyh, CA. Liechti, RE. Lee and E. Growen, IEEE J. Solid State Circuit SC-12 (1977) 485.