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Coordinatively saturated Ga compounds — A new type of group III precursor for the MOCVD of GaAs
290
Journal of Crystal Growth 102 (1990) 290—292 North-Holland
COORDINATIVELY SATURATED Ga COMPOUNDS FOR THE MOCVD OF GaAs V. FRESE, G.K. REGEL, H. HARDTDEGEN
*
-
A NEW TYPE OF GROUP III PRECURSOR
A. BRAUERS and P. BALK
Institute of Semiconductor Electronics, Technical University Aachen, D-5]OO Aachen. Fed. Rep. of Germany
and M. HOSTALEK, M. LOKAI and L POHL F. Merck. D-6100 Darmstadt, Fed, Rep. of Germany
Received 21 June 1989~manuscript received in final form 28 September 1989
The Ga sources 1-3-dimethyl-aminopropyl-1 -galla-cyclohexane (C 5H10Ga-(CH2 )~)-N(CH~),) and trimethylgallium-diisopropylamine adduct (Ga(CH3)~.. .N(C~H7),H)were succesfully utilized for the epitaxial growth of GaAs in low pressure (1000—2000 Pa) MOCVD. AsH~was used as the group V source. GaAs layers obtained from these novel precursors at optimized conditions exhibit good morphological qualities. Electrical and photoluminescence data indicate that the intrinsic impurity (C,N) uptake from the source materials is low.
Conventional metalorganic group III sources, like TMG (trimethyl-Ga, Ga(CH3)3), TMA (tnmethyl-Al, Al(CH3)3) and the corresponding ethyl compounds, are quite reactive towards oxygen and even pyrophoric. With trace amounts of oxygen, alkoxy formation occurs during preparation and handling. Particularly in the deposition of GaAlAs this may lead to oxygen contamination of the films. For this reason it is worthwhile looking for precursors that are less sensitive in this respect. The reactive mentioned trialkyls is mainlynature causedofby the the above free orbital of the group III metal atom, which is easily occupied by electrons from the donor-like element oxygen. The reactivity towards oxygen can be eliminated by occupying the orbital with electrons from a different donor element, for example nitrogen. This can be achieved by using adducts, like Ga(CH 3)3 N (C3 72 H (trimethyl Ga-diisopropylamine) or intramolecular coordination compounds, like •
*
.
Permanent address: ISI Kernforschungsanlage Jülich, Postfach 1913, D-5170 JOlich. Fed. Rep. of Germany.
0022-0248/90/503.50
‘~.
1990
—
C5H10Ga-(CH2)3-N(CH3)2 (1-3-dimethyl-aminopropyl-1-galla-cyclohexane, see insert fig. 1). These compounds are indeed stable in air. The present communication reports on a study showing the basic feasibility of using the two compounds in the MOCVD of Ill—V semiconductor films. To this end these starting materials, which were synthesized at Merck research laboratories, were used in combination with AsH3 (Phoenix Plus) and a H2 carrier (Pd diffused) for the deposition 2) of GaAs in areactor horizontal, rectangular (3 x 6holder cm lamp heated with graphite substrate at low total pressure ((1—2) x i0~Pa). Because of the low vapor pressures of the group III precursors (approximately 1.7 Pa for the intramolecularly coordinated ring compound, 17 Pa at 293 K for the adduct), this low total pressure was necessary in order to obtain acceptable growth rates. Substrates were semi-insulating (100) GaAs wafers (20 off towards (110), Showa Denko). The growth rate was determined by weighing; the films were characterized by Hall measurements and photoluminescence (PL).
Elsevier Science Publishers BY. (North-Holland)
V. Frese et al.
1100
/ Coordinativelt’ saturated Ga compounds
1000
900
800
291
700
T(K)
3Pa
p-Go, ~,p5-H =017 Pa,
=
2xlO
p~238Pa, v =lm/s 5.0
op
01 Pa, p
=lO3Pa
tot RAsH
‘ ~=
97 Pa,
v
=
2 rn/s
3
.__.•____•_____•___.._S,.,,,
-±
2.0 U---
/
1.0 D_DDD~~D~A
~
a
~ Ci,
-~
CS,
0.5 I
09
I
I
I
1.2 1.3 1./. 1/I (103K1) Fig. 1. GaAs growth rates versus temperature; Ga source; (a) ring compound (see insert) (C Pa, Pu
=
0.1 Pa,
PAoI-I~97
Pa. v
1.0
=
11
200 cm/s); (b) adduct Pa.((CH3)3Ga...N(C3H7),H)(p11 v =100 cm/s.
Fig. 1 shows the growth rate as a function of temperature using the ring compound at 0.10 Pa (total gas pressure 1 X iO~Pa at a velocity of 2 m/s; AsH 3 pressure 97 Pa) and the adduct at 0.17 Pa (total gas pressure 2 x iO~Pa at a velocity of I m/s; pressure 238 slowly Pa). For the800ring pound AsH3 the rate increases from to corn1020 K. In the case of the adduct it remains basically constant from 870 to 1050 K. This implies that over the entire range of interest for the deposition of GaAs and GaA1As the rate is essentially diffusion-controlled and that prereactions upstream of the substrate holder and consequently depletion in the hot zone do not play a significant role. This behavior is desirable since it tends to produce uniform film properties over the entire substrate [1]. Films obtained from both Ga sources between 850 and 1050 K exhibited smooth surfaces with a low defect density. All layers obtained from the ring compound were n-type at growth temperatures above 840 K and exhibited high resistivity below this temperature. As an example, satisfactory electrical properties were obtained at 870 K; for a 2 ~tm thick film, 4 x 10~~ cm3, ~L 2/V s; ~ 7100 cm2/V~s. cm 8 x io’~cm3 p..~ 3o0 51 000 Although higher mobiltties have been reported in the literature, the electron mobility at room tempera•
• .
=
=
•
5H10-Ga(CH2)3-N-(CH3),) ~ 3 Pa, Pu = 0.17 Pa, PASH, = 2 IO
10~ 238
ture, where interface scattering in thin films is not a dominant process, is comparable to that obtamed on GaAs from conventional MOCVD sources. At higher growth temperatures, the background appears to be higher: 2 x 1015 3, ~doping 25,000 cm2/V s, againn77 pointing to a cm modest level of compensation. Using the adduct source generally high resistivity material was obtamed. In one case a layer grown at 870 K could be evaluated by Hall measurement. The values obtained (n 3, ~ 51,000 77 3 xan10~~ cm ionized impurity cm2/V. s) point to overall concentration around I x 10~cm3 [2]. =
=
=
=
3 2 K 2 -=
~ .2~1
7meV
~ —
I
1.48
=
=
1.5
.
I
149
Energy
(eV)
Fig. 2. 2 K PL spectrum of GaAs film grown from ring compound 1-3-dimethyl-aminopropyl-1-galla-cyclohexane as Ga source at 870 K, P 11 = i0~Pa.
292
V. Frese et al.
/
Coordinatively saturated Ga compound.s
spectra were recorded at different temperatures and excitation densities. Therefore they do not permit drawing quantitative conclusions, e.g. on T-4 >..
C SI
‘U
them
1460
1480 1.500 1.520 Energy (eV) Fig. 3. 4.2 K PL spectrum of GaAs deposited from adduct trimethyl gallium-disopropyl amine at 870 K and p 1~,1 =
2 1O~
Pa.
In fig. 2 the 2 K FL spectrum of the above mentioned film grown at 870 K from the ring compound is shown. The resolution of the cxcitonic transitions is somewhat inferior to that of the optically best layers grown from standard precursors. A full width at half maximum of 1.7 meV was measured. The energy position of the band edge transition shows that nitrogen from the Ga precursor is not incorporated within the limit of 3). A low intensity signal detection x 1019 impurities cm caused by (1 acceptor (probably (e, Zn) or (D, CA~)) is visible at 1.489 eV. In the films deposited with the adduct precursor the acceptor related emission dominates the FL spectrum (fig. 3), which was obtained on a high resistivity layer. However, as may be seen from the insert of the figure, the resolution of the excitonic transitions points to the high optical quality of the material. It should he noted that the two different FL
Ga sources. An important criterion for the usefulness of these novel content precursors is the feasibility using the carbon originating from the of different for high quality GaAIAs growth. Preliminary experiments indicate that indeed the tendency towards oxygen uptake in this ternary compound is distinctly reduced [3]. Only limited qualities of the precursors were available for this initial evaluation. Extensive efforts at purification were not made. Nonetheless, our first results demonstrate the correctness of our considerations and prove the basic suitability of coordinatively saturated group III precursors for MOCVD. A study of the structural modification of the precursors to increase their vapor pressure, which is necessary for growth at higher total pressures, is in progress. The authors would like to thank K. Werner (TU Delft, Netherlands) and F. Reinhardt for the low temperature PL measurements. Part of this work was supported by the Bundesministerium für Forschung und Technologie, tract No 415-7291-NT 2717 E3.FRG. Under con-
References [1] R.J.
Field and S.K. Ghandhi, J. Crystal Growth 69 (1984)
581. [21W Walukiewicz, J. Lagowski and H C. Gatos. J AppI.
131
Phys. 53 (1982) 769. V. Frese. G.K. Regel, H. Hardtdegen. A. Brauers, P. Balk, M. Hostalek, M. Lokai and L. PohI, to be published.
Journal of Crystal Growth 102 (1990) 290—292 North-Holland
COORDINATIVELY SATURATED Ga COMPOUNDS FOR THE MOCVD OF GaAs V. FRESE, G.K. REGEL, H. HARDTDEGEN
*
-
A NEW TYPE OF GROUP III PRECURSOR
A. BRAUERS and P. BALK
Institute of Semiconductor Electronics, Technical University Aachen, D-5]OO Aachen. Fed. Rep. of Germany
and M. HOSTALEK, M. LOKAI and L POHL F. Merck. D-6100 Darmstadt, Fed, Rep. of Germany
Received 21 June 1989~manuscript received in final form 28 September 1989
The Ga sources 1-3-dimethyl-aminopropyl-1 -galla-cyclohexane (C 5H10Ga-(CH2 )~)-N(CH~),) and trimethylgallium-diisopropylamine adduct (Ga(CH3)~.. .N(C~H7),H)were succesfully utilized for the epitaxial growth of GaAs in low pressure (1000—2000 Pa) MOCVD. AsH~was used as the group V source. GaAs layers obtained from these novel precursors at optimized conditions exhibit good morphological qualities. Electrical and photoluminescence data indicate that the intrinsic impurity (C,N) uptake from the source materials is low.
Conventional metalorganic group III sources, like TMG (trimethyl-Ga, Ga(CH3)3), TMA (tnmethyl-Al, Al(CH3)3) and the corresponding ethyl compounds, are quite reactive towards oxygen and even pyrophoric. With trace amounts of oxygen, alkoxy formation occurs during preparation and handling. Particularly in the deposition of GaAlAs this may lead to oxygen contamination of the films. For this reason it is worthwhile looking for precursors that are less sensitive in this respect. The reactive mentioned trialkyls is mainlynature causedofby the the above free orbital of the group III metal atom, which is easily occupied by electrons from the donor-like element oxygen. The reactivity towards oxygen can be eliminated by occupying the orbital with electrons from a different donor element, for example nitrogen. This can be achieved by using adducts, like Ga(CH 3)3 N (C3 72 H (trimethyl Ga-diisopropylamine) or intramolecular coordination compounds, like •
*
.
Permanent address: ISI Kernforschungsanlage Jülich, Postfach 1913, D-5170 JOlich. Fed. Rep. of Germany.
0022-0248/90/503.50
‘~.
1990
—
C5H10Ga-(CH2)3-N(CH3)2 (1-3-dimethyl-aminopropyl-1-galla-cyclohexane, see insert fig. 1). These compounds are indeed stable in air. The present communication reports on a study showing the basic feasibility of using the two compounds in the MOCVD of Ill—V semiconductor films. To this end these starting materials, which were synthesized at Merck research laboratories, were used in combination with AsH3 (Phoenix Plus) and a H2 carrier (Pd diffused) for the deposition 2) of GaAs in areactor horizontal, rectangular (3 x 6holder cm lamp heated with graphite substrate at low total pressure ((1—2) x i0~Pa). Because of the low vapor pressures of the group III precursors (approximately 1.7 Pa for the intramolecularly coordinated ring compound, 17 Pa at 293 K for the adduct), this low total pressure was necessary in order to obtain acceptable growth rates. Substrates were semi-insulating (100) GaAs wafers (20 off towards (110), Showa Denko). The growth rate was determined by weighing; the films were characterized by Hall measurements and photoluminescence (PL).
Elsevier Science Publishers BY. (North-Holland)
V. Frese et al.
1100
/ Coordinativelt’ saturated Ga compounds
1000
900
800
291
700
T(K)
3Pa
p-Go, ~,p5-H =017 Pa,
=
2xlO
p~238Pa, v =lm/s 5.0
op
01 Pa, p
=lO3Pa
tot RAsH
‘ ~=
97 Pa,
v
=
2 rn/s
3
.__.•____•_____•___.._S,.,,,
-±
2.0 U---
/
1.0 D_DDD~~D~A
~
a
~ Ci,
-~
CS,
0.5 I
09
I
I
I
1.2 1.3 1./. 1/I (103K1) Fig. 1. GaAs growth rates versus temperature; Ga source; (a) ring compound (see insert) (C Pa, Pu
=
0.1 Pa,
PAoI-I~97
Pa. v
1.0
=
11
200 cm/s); (b) adduct Pa.((CH3)3Ga...N(C3H7),H)(p11 v =100 cm/s.
Fig. 1 shows the growth rate as a function of temperature using the ring compound at 0.10 Pa (total gas pressure 1 X iO~Pa at a velocity of 2 m/s; AsH 3 pressure 97 Pa) and the adduct at 0.17 Pa (total gas pressure 2 x iO~Pa at a velocity of I m/s; pressure 238 slowly Pa). For the800ring pound AsH3 the rate increases from to corn1020 K. In the case of the adduct it remains basically constant from 870 to 1050 K. This implies that over the entire range of interest for the deposition of GaAs and GaA1As the rate is essentially diffusion-controlled and that prereactions upstream of the substrate holder and consequently depletion in the hot zone do not play a significant role. This behavior is desirable since it tends to produce uniform film properties over the entire substrate [1]. Films obtained from both Ga sources between 850 and 1050 K exhibited smooth surfaces with a low defect density. All layers obtained from the ring compound were n-type at growth temperatures above 840 K and exhibited high resistivity below this temperature. As an example, satisfactory electrical properties were obtained at 870 K; for a 2 ~tm thick film, 4 x 10~~ cm3, ~L 2/V s; ~ 7100 cm2/V~s. cm 8 x io’~cm3 p..~ 3o0 51 000 Although higher mobiltties have been reported in the literature, the electron mobility at room tempera•
• .
=
=
•
5H10-Ga(CH2)3-N-(CH3),) ~ 3 Pa, Pu = 0.17 Pa, PASH, = 2 IO
10~ 238
ture, where interface scattering in thin films is not a dominant process, is comparable to that obtamed on GaAs from conventional MOCVD sources. At higher growth temperatures, the background appears to be higher: 2 x 1015 3, ~doping 25,000 cm2/V s, againn77 pointing to a cm modest level of compensation. Using the adduct source generally high resistivity material was obtamed. In one case a layer grown at 870 K could be evaluated by Hall measurement. The values obtained (n 3, ~ 51,000 77 3 xan10~~ cm ionized impurity cm2/V. s) point to overall concentration around I x 10~cm3 [2]. =
=
=
=
3 2 K 2 -=
~ .2~1
7meV
~ —
I
1.48
=
=
1.5
.
I
149
Energy
(eV)
Fig. 2. 2 K PL spectrum of GaAs film grown from ring compound 1-3-dimethyl-aminopropyl-1-galla-cyclohexane as Ga source at 870 K, P 11 = i0~Pa.
292
V. Frese et al.
/
Coordinatively saturated Ga compound.s
spectra were recorded at different temperatures and excitation densities. Therefore they do not permit drawing quantitative conclusions, e.g. on T-4 >..
C SI
‘U
them
1460
1480 1.500 1.520 Energy (eV) Fig. 3. 4.2 K PL spectrum of GaAs deposited from adduct trimethyl gallium-disopropyl amine at 870 K and p 1~,1 =
2 1O~
Pa.
In fig. 2 the 2 K FL spectrum of the above mentioned film grown at 870 K from the ring compound is shown. The resolution of the cxcitonic transitions is somewhat inferior to that of the optically best layers grown from standard precursors. A full width at half maximum of 1.7 meV was measured. The energy position of the band edge transition shows that nitrogen from the Ga precursor is not incorporated within the limit of 3). A low intensity signal detection x 1019 impurities cm caused by (1 acceptor (probably (e, Zn) or (D, CA~)) is visible at 1.489 eV. In the films deposited with the adduct precursor the acceptor related emission dominates the FL spectrum (fig. 3), which was obtained on a high resistivity layer. However, as may be seen from the insert of the figure, the resolution of the excitonic transitions points to the high optical quality of the material. It should he noted that the two different FL
Ga sources. An important criterion for the usefulness of these novel content precursors is the feasibility using the carbon originating from the of different for high quality GaAIAs growth. Preliminary experiments indicate that indeed the tendency towards oxygen uptake in this ternary compound is distinctly reduced [3]. Only limited qualities of the precursors were available for this initial evaluation. Extensive efforts at purification were not made. Nonetheless, our first results demonstrate the correctness of our considerations and prove the basic suitability of coordinatively saturated group III precursors for MOCVD. A study of the structural modification of the precursors to increase their vapor pressure, which is necessary for growth at higher total pressures, is in progress. The authors would like to thank K. Werner (TU Delft, Netherlands) and F. Reinhardt for the low temperature PL measurements. Part of this work was supported by the Bundesministerium für Forschung und Technologie, tract No 415-7291-NT 2717 E3.FRG. Under con-
References [1] R.J.
Field and S.K. Ghandhi, J. Crystal Growth 69 (1984)
581. [21W Walukiewicz, J. Lagowski and H C. Gatos. J AppI.
131
Phys. 53 (1982) 769. V. Frese. G.K. Regel, H. Hardtdegen. A. Brauers, P. Balk, M. Hostalek, M. Lokai and L. PohI, to be published.