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Carbon incorporation in GaAs layer grown by atomic layer epitaxy
Journal of Crystal Growth 93 (1988) 557.-561 North-Holland, Amsterdam
557
CARBON INCORPORATION IN GaAs LAYER GROWN BY ATOMIC LAYER EPITAXY K. M O C H I Z U K I , M. OZEKI: K. K O D A M A and N. O H T S U K A Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, A tsugi 243-01, Japan
The mechanism by which carbon is incorporated into GaAs layers by atomic layer epitaxy using trimethylgallium or triethylgaUium was investigated. The carbon density varied from 1 × 1013 to 8 × 10 Is cm -3 with the trimethylgallium (or triethylgallium) and arsine pulse durations and mole fractions. It was also observed that the carbon incorporation drastically changed at the pulse duration and mole fraction where the growth rate per gas cycle started to saturate to one monolayer (0.283 nm/cycle for a (It)0) substrate). The results were explained by the selective adsorption of carbon on surface gallium, the reaction of methylgallium with arsine, and the exchange interaction between arsenic and carbon atoms. Even when the trimethylgallium source was used, the epitaxiai layers grown under the optimized growth conditions exhibited an electron concentration of 1 × t0:4 cm- 3 and a mobility of 80000 cm2/V.s at 77 K, a photoluminescence spectrum with several sharp excitonic lines at the band gap energy, and an extremely low level of carbon related peaks.
1. Introduction The control of residual impurities is importr:r~t for the development of crystal growth technolog¢. Although atomic layer epitaxy [1-6] (ALE) is a promising technique for the growth of heterostructures where the alloy composition and dopant atoms can be changed at the atomic level, there are still some technological barriers to be overcome. In particular, carbon contamination is a serious problem for A L E where organometallic source compounds are used [7,8]. The purpose of this study was to obtain a better understanding of the carbon incorpora:ic~n mechanism in ALE using trimethyl3 ~llium (2 MG) or triethylgallium (TEG) and arsirc in order to reduce the contamina~io~ in GaAs c~itaxi:-d layers.
Fig. 1 shows a diagram of the growth system. For ALE, it is necessary to (~uickly change from one source gas to another to prevent mixing. Growth was performed in a low pressure chimney-type reactor [9], where the carrier gas was introduced from the bottom of the reactor and exhausted from the top. We used a gas manifold with a pressure-balanced vent-and-run system [10]. Due to these improvements° a gas exchange time of less than 0.1 s was achieved ;or the growth system. The substrate was located in a fast gas stream from a gas nozzle to suppress the decomposition of T M G in the boundary layer.
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2. Experimenta! The gas source for gallium atoms was either T M G or TEG, and arsine gas for trsenic atoms. Hydrogen was used as the cartier gas, and a vapor pressure of 20 "l'orr (2600 Pa) was maintained during the epitaxia! growth. The growth temperature was 500 o C, and the substratc was (I09) GaAs.
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0022-0248/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
55 8
K. Mochizuki et al. / Carbon incorporation in Ga.4s layer grown by ALE
~ TMGa + H=I (TEGa) I AsHa + H= H=
1 cycle
ALE GaAs
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Time Fig. 2. The gas flow sequence for GaAs ALE.
i
810
|
820
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830
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840
850
Wavelength (nm)
Fig. 2 shows the gas flow sequence used in our experiments. The alternating T M G or T E G and arsine gas pulses are separated by hydrogen gas purge pulses which prevent the mixing of reaction gases. Here, one gas cycle is defined as shown in fig. 2. The mole fraction for T M G was changed from 6.25 × 10 - 6 tO 1.75 × 10 -3 and that for arsine from 5 × 10 -4 to 2.4 x 10 -2. The growth thickness was obtained by measuring the step height between the epitaxial surface and substrate. The substrate surface was covered with a silicon-dioxide film which was removed prior to measurement. Low-temperature (4.2 K) photoluminescence measurements and SIMS were used to identify the residual impurities. The freecartier concentration was obtained from C - V and Hall effect measurements. The deep level was also measured, using DLTS.
3. Results and discussion Fig. 3 shows the photoluminescence spectrum obtained for an ALE-grown GaAs layer, where the mole fraction of T M G was 1 x 10 -3 and that for ursine was 2.4 × 10 - 2 . A pulse duration of 4 s was used for T M G and ursine. A ~uminescence peak with a relatively broad half-width is observed at a wavelength of 830 nm. The specLru:m is due to the donor-accept~;r pair emission associated with the carbon acceptor, although the peak is shifted to higher energy due to high acceptor concentration [11]. The temperature dependence of carrier (hole) concentration was found to be determined by a shallow acceptor level with an ionization en,ergy of about 20 me~,'. These results indicate
Fig. 3. Photoluminescence spectrum of ALE GaAs, at a TMG mole fraction of 1 × 10- 3 a TMG pulse duration of 4 s, a n arsine mole fraction of 2.4 × 10-2 a n d a n arsine pulse duration of 4 s.
that the d o m i n a n t impurity in the layer was carbon. Figs. 4 and 5 show the variations ir growth rate per gas cycle and in hole (carbon) concentration when the T M G mole fraction and the pulse duration were varied. The growth raze per cycle increased until the thickness per cycle reached .one monolayer and saturated. The hole concentration was observed to increase with the T M G mole fraction or pulse duration. The rate of change of carbon concentration appeared to change at the mole fraction or pulse duration where the growth
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TMG mole fraction {X 10 -4) Fig. 4. The variations in growth thickness per gas cycle and hole concentration when the TMG mole fraction was varied. The TMG pulse duration was 4 s. The arsine pulse duration was 4 s and the ursine mole fraction was 2.4 × l0 2.
K. Mochizuki et a L / Carbon incorporation in GaAs layer grown by A L E
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Fig. 5. The variations in growth thickness per gas cycle and hole concentration when the T M G pulse duration was varied. The T M G mole fraction was 1 x 10 -3. The arsine pulse duration was 4 s and the arsine mole fraction was 2.4 x 1 0 - 2
rate per cycle started to saturate at one monolayer. Although DLTS measurements were made for the sample, we could not observe any deep levels in the film. Thi~ suggests that the change in hole concentration was not caused by an increase in compensation. 'I'he variation in hole (carbon) concentration as a function of the arsine mole fraction or pulse duration was also investigated. The results are shown in figs. 6 and 7. The growth rate did not change with the arsine mole fraction or pulse
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AsH= pulse duration (s) Fig. 7. The variations of growth thickness per gas cycle and hole concentration when arsine pulse duration of was varied. The T M G pulse duration was ¢ s. The T M G mole fraction was 2 × 10 - 4 and the arsine mole fraction was 2.4× 10 -2.
duration after the thickness quickly rose to one monolayer. The hole concentration decreased to 1 × 1017cm-3 as the arsine mole fraction or pulse duration increased. The carbon concentration of the films was also directly measured by SIMS measurement. The carbon concentration increased with the T M G mole fraction and pulse duration. The increase was proportional to the increase of the hole concentration. These results provided further evidence that the main impurity was carbon. If the carbon atoms were incorporated from the background, the carbon concentration of the layer should increase with the purge time. However, the hole concentration was not observed to change (table 1) when the purge time after T M G or arsine pulse was increased. As shown in fig. 5, the hole
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Fig. 6. The ,.,,_.,5:ations of growth thickness p'~- +,as cycle aad hole concentration when tire arsine ~no!': f . . c a o n was varied. The T M G pulse duration was 4 s. The T M G mole fraction was 2 × 10 -a and the arsine pulse duration was 4 s.
Sample n u m b e r
A
B
C
Growth sequence T M G pulse duration (s) Purge time after "rMG pulse (s) Arsine pulse duration (s) Purge time after arsine pulse (s~
4 6 ~ 1
4 1 4 ,
4 1 .I ~,
Holeconcentrationat30OK(×10Scm
~) 1.08 1.41 2.01
560
K. Mochizuki et al. / Carbon incorporation in GaAs layer grown b.v ALE
concentration increased with the TMG pulse duration. In the case of the arsine pulse, the hole concentration decreased with the pulse duration (fig. 7). These results suggest that the carbon impurity is incorporated during the T M G pulse duration. In the saturation region of the growth rate, that is, when the mole fraction is above 2 x 10 -4 in fig. 4 or when the pulse duration is above 4 s in fig. 5, the observed T M G molecules on the surface were considered to be decomposed and the surface was covered with gallium atoms. However, the carbon concentration was found to increase in the saturation region, as shown in figs. 4 and 5. This indicates that the carbon contamination does not directly come from T M G adsorbed on arsenic site, but from the hydrocarbon or carbon in the vapor. The number of hydrocarbon molecules or carbon atoms in the vapor is considered to be proportional to the mole fraction or the pulse duration of TMG. To explain the variation in hole concentration, it was necessary to consider the effect of the selective adsorption of carbon or hydrocarbon onto surface gallium atoms. A carbon atom tends to occupy the arsenic lattice site and acts as an acceptor in GaAs, while it would be difficult to occupy the gallium lattice site according to ,he electroncgativity arg)~ments [12]. In fact, there has been no definite observation concerning ,:arbon atoms occupying gallium lattice sites in Ca.As and GaP. In our model, a carbon atom is assumed to adsorb on the surface gallium a t o m avd v~ot on the arsenic atom. This assumption is consistent with the previous report that the carbon incorporation is affected by the density of surface gallium atoms when G~As films are g r o v a by metalorgan!c chemical vapor deDosi)i~, !131. The carbon concentra".ion N c in a film can be expressed as +
1
re.A,
t s< ,),,, ]
fl) where N is the number of carbc,~a atoms adsorbed duriag the T M G pulse duration, R is the growth thi,.kness per cycie, tV~tG is :he pulse duration of
TMG, x ( t ) is the surface gallium coverage determined from the experimental results of the growth rate, and is varying from 0 (10f'¢o arsenic coverage) to 1 (100% gallium coverage), rc.oa is the adsorption time of hydrocarbon on the gallium surface, "re.As is the adsorption time of hydrocarbon on the arsenic surface, and Sc is the number of hydrocarbon molecules that reached the surface during the T M G pulse, and is proportional to the T M G mole fraction. The growth rate R is directly related to the gallium coverage and is expressed as T = 0.283x(txMG) [nml [14]. The upper curve in fig. 4 was calculated from eq. (1) using the assumption of selective adsorption, i.e., assummg that ~'C.A~ was infinitely large. For x(t), the measured data shown by the lower curve of fig. 4 were used, and Sc/~'c.oa [cm-2 s - l ] was 5.52 × 10 24 [cm -1 s -!] times the mole fraction of TMG. The curve obtained by optimum fitting seems to describe the experimental results very well. When the value of "re.As was finite and comparab e to ~C.G~, the ot~served sharp drop of hole concentration for small values of T M G mole fraction could not be reproduced by eq. (1) at all. This indicates that ads;)rption of hydrocarbon or carbon takes place selectively on surface gallium atoms. A,', e played an important role in the incorporation of carbon. The film had a b_igh impurity concentration, above 3 x 1018 c m - 3 when insufficient arsine was supplied 0)'. the surface (fig. 6). Observatiofi~ showed that as the arsine pulse duration incr.eased, the carbon concentration decreas~:d. It is t h u s concludec~ that the arsine removes the caff.on atoms from the growth surface. Tlae the llytllk)~r~i~ ~. . . . . . . . . . . . .l¢i~At~al ~.... decomposed t~om" the arsine p r o b a b l y plays an important role in this reaction. When arsenic was supplied beyond the mole fraction of ~ x 10 -z in fig. 6. the carbon concentration did not change and saturated to 1 .Y, 1017 c m -a. T~-lis m a y b e caused by the exchange interaction be,ween the carbon atoms on the surface and the a~senic atoms in the lattice. Gnce the carbon atoms occupy sites in the bulk lattice, they can not be easily removed. When T E G was used as the gallium source in the ALE, we had similar results as those obtained
K. Machizuki et al. / Carbon incorporation in GaA s laver grown by A L E
ALE GaAs
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/I/l
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'81o'
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' 950
Fig. 8, The photolum;,nescence spectrum of the film grown under the optimized growth condition.
for growth using TMG, except that the hole concentration of the films using TEG was less than that using TMG. The carbon incorporated was drastically decreased when the T M G mole fraction (pulse duration) was decreased and the arsine mole fraction (pulse duration) was increased. In order to get high purity layers, ALE growth should be carried o~ut under the condition of a shorter duration of T M G pulse with a higher mole fraction and a sufficient supply of arsine. The epitaxial layer grown under such a growth condition was found to be n-type. The layer grown under the condition of T M G mole fraction of 2 × 10-3, T M G pulse duration of 0.5 s, arsine mole fraction of 2.4 × 10 - ~and arsine pulse duration of 4 s had an electron concentration of 1 × 10 ~4 c m - 3 and a mobility of 80 000 cm2/V • s at 77 K. Fig. 8 shows the photoh:minescence spectrum of the film with several sharp excitot:ie lines at the band gap energy and an extremely low level of carbon related peaks refle,zt;r,g t.be high purity of the layer.
4. Summary The T M G source is the origin of carbon impurity. The carbon ;.n selectively adsorbed on the surface gallium atom. Arsine removes the carbon on the surface. The a m o u n t of carbon incorporated in the epitaxial layers was decreased when the T M G supply was decreased ar, d the arsine
561
supply was increased. When the growth was done under the condition that the T M G pulse duration was short with increased mole fraction and that sufficient arsine was supplied, the epitaxial layer was n-type. The layer exhibited and electron concentration of 1 × 10 t4 cm -3 and a mobility of 80000 cmZ/V • s at 77 K, and a photoluminesce:,.ce spectrum with several sharp excitonic lines at the band gap energy and an extremely low level of carbon related peaks.
Acknowledgernents The authors are grateful to Dr. O. Ohtsuki for his continuous encouragement and to E. Taguchi for valuable technical assistance.
References [1] T. Suntola and J. Antson, Finnish Palcnt No. 52359 (t974) and US Patent No. 4,058,431) (1977). [2] J. Nishizawa, H. Abe and T. Kurabayashi, J. Electrochem. Soc, 132 (1985) 1197. [3] A. Usui and H. Sunakawa, Japan. J. Appl. Phys. 25 (1086) 1,21'2 [4] S.M. Bedair, M.A. Tischler, T. Katsuyama and N.A. EIMasry, Appl. Phys. Letters 47 (1985) 51. [5] A. Dot, Y. Aoyagi and S, Namba, Appl. Phys. I.etters 48 (1986) 1787. [6] N. Kobayashi, T. Makimoto and Y. I-lorikoshi, Japan. J. Appl. Phys. 24 (1985) L962. [7] J. Nishizawa, H. Abe, T. Kurabayashi and N. Sakurai, J. Vacuum Sci. Techno,. A4 ~1986) 706. [8] M.A, Tischler and S.M. Bedair, J. Cr3stal Growth 77 (1986; 89. [9] M.R. Leys, C. van Opdrop, M.P.A. Viegers anc~ H.J. Talen-van der Mheen, J. Crystal Growth 68 (1984) 431. [10] J.S. Roberts, N.J. Mason and M. Robinson, J. Crystal Growth 68 (1984) 422. [11] M Ozeki~ N. Nakai~ K_ Dazai and O. Ryuzan. Japan. J. Appl. Phys. 12 (1974) 1121. [121 J.C. Phillips, Bands and Bonds in Semiconductors (Academic Press, New York, 1973). [13] K. Famamura, J. Ogawa, K. Akimoto, Y. Mort and ('. Kojima, Appl. Phys. Letters 50 (1987) 1149. [14] The growth rate is described by a somewhat complex expression using sorption and desorption time constants and is to be published elsewhere.
557
CARBON INCORPORATION IN GaAs LAYER GROWN BY ATOMIC LAYER EPITAXY K. M O C H I Z U K I , M. OZEKI: K. K O D A M A and N. O H T S U K A Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, A tsugi 243-01, Japan
The mechanism by which carbon is incorporated into GaAs layers by atomic layer epitaxy using trimethylgallium or triethylgaUium was investigated. The carbon density varied from 1 × 1013 to 8 × 10 Is cm -3 with the trimethylgallium (or triethylgallium) and arsine pulse durations and mole fractions. It was also observed that the carbon incorporation drastically changed at the pulse duration and mole fraction where the growth rate per gas cycle started to saturate to one monolayer (0.283 nm/cycle for a (It)0) substrate). The results were explained by the selective adsorption of carbon on surface gallium, the reaction of methylgallium with arsine, and the exchange interaction between arsenic and carbon atoms. Even when the trimethylgallium source was used, the epitaxiai layers grown under the optimized growth conditions exhibited an electron concentration of 1 × t0:4 cm- 3 and a mobility of 80000 cm2/V.s at 77 K, a photoluminescence spectrum with several sharp excitonic lines at the band gap energy, and an extremely low level of carbon related peaks.
1. Introduction The control of residual impurities is importr:r~t for the development of crystal growth technolog¢. Although atomic layer epitaxy [1-6] (ALE) is a promising technique for the growth of heterostructures where the alloy composition and dopant atoms can be changed at the atomic level, there are still some technological barriers to be overcome. In particular, carbon contamination is a serious problem for A L E where organometallic source compounds are used [7,8]. The purpose of this study was to obtain a better understanding of the carbon incorpora:ic~n mechanism in ALE using trimethyl3 ~llium (2 MG) or triethylgallium (TEG) and arsirc in order to reduce the contamina~io~ in GaAs c~itaxi:-d layers.
Fig. 1 shows a diagram of the growth system. For ALE, it is necessary to (~uickly change from one source gas to another to prevent mixing. Growth was performed in a low pressure chimney-type reactor [9], where the carrier gas was introduced from the bottom of the reactor and exhausted from the top. We used a gas manifold with a pressure-balanced vent-and-run system [10]. Due to these improvements° a gas exchange time of less than 0.1 s was achieved ;or the growth system. The substrate was located in a fast gas stream from a gas nozzle to suppress the decomposition of T M G in the boundary layer.
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2. Experimenta! The gas source for gallium atoms was either T M G or TEG, and arsine gas for trsenic atoms. Hydrogen was used as the cartier gas, and a vapor pressure of 20 "l'orr (2600 Pa) was maintained during the epitaxia! growth. The growth temperature was 500 o C, and the substratc was (I09) GaAs.
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0022-0248/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
55 8
K. Mochizuki et al. / Carbon incorporation in Ga.4s layer grown by ALE
~ TMGa + H=I (TEGa) I AsHa + H= H=
1 cycle
ALE GaAs
I I
I-1
I1 -1
r-l
~
4.2K Tg=5OO~
.q
?q .I-"l I--
/L
C ¢D
Time Fig. 2. The gas flow sequence for GaAs ALE.
i
810
|
820
|
830
i
840
850
Wavelength (nm)
Fig. 2 shows the gas flow sequence used in our experiments. The alternating T M G or T E G and arsine gas pulses are separated by hydrogen gas purge pulses which prevent the mixing of reaction gases. Here, one gas cycle is defined as shown in fig. 2. The mole fraction for T M G was changed from 6.25 × 10 - 6 tO 1.75 × 10 -3 and that for arsine from 5 × 10 -4 to 2.4 x 10 -2. The growth thickness was obtained by measuring the step height between the epitaxial surface and substrate. The substrate surface was covered with a silicon-dioxide film which was removed prior to measurement. Low-temperature (4.2 K) photoluminescence measurements and SIMS were used to identify the residual impurities. The freecartier concentration was obtained from C - V and Hall effect measurements. The deep level was also measured, using DLTS.
3. Results and discussion Fig. 3 shows the photoluminescence spectrum obtained for an ALE-grown GaAs layer, where the mole fraction of T M G was 1 x 10 -3 and that for ursine was 2.4 × 10 - 2 . A pulse duration of 4 s was used for T M G and ursine. A ~uminescence peak with a relatively broad half-width is observed at a wavelength of 830 nm. The specLru:m is due to the donor-accept~;r pair emission associated with the carbon acceptor, although the peak is shifted to higher energy due to high acceptor concentration [11]. The temperature dependence of carrier (hole) concentration was found to be determined by a shallow acceptor level with an ionization en,ergy of about 20 me~,'. These results indicate
Fig. 3. Photoluminescence spectrum of ALE GaAs, at a TMG mole fraction of 1 × 10- 3 a TMG pulse duration of 4 s, a n arsine mole fraction of 2.4 × 10-2 a n d a n arsine pulse duration of 4 s.
that the d o m i n a n t impurity in the layer was carbon. Figs. 4 and 5 show the variations ir growth rate per gas cycle and in hole (carbon) concentration when the T M G mole fraction and the pulse duration were varied. The growth raze per cycle increased until the thickness per cycle reached .one monolayer and saturated. The hole concentration was observed to increase with the T M G mole fraction or pulse duration. The rate of change of carbon concentration appeared to change at the mole fraction or pulse duration where the growth
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K. Mochizuki et a L / Carbon incorporation in GaAs layer grown by A L E
559
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Fig. 5. The variations in growth thickness per gas cycle and hole concentration when the T M G pulse duration was varied. The T M G mole fraction was 1 x 10 -3. The arsine pulse duration was 4 s and the arsine mole fraction was 2.4 x 1 0 - 2
rate per cycle started to saturate at one monolayer. Although DLTS measurements were made for the sample, we could not observe any deep levels in the film. Thi~ suggests that the change in hole concentration was not caused by an increase in compensation. 'I'he variation in hole (carbon) concentration as a function of the arsine mole fraction or pulse duration was also investigated. The results are shown in figs. 6 and 7. The growth rate did not change with the arsine mole fraction or pulse
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AsH= pulse duration (s) Fig. 7. The variations of growth thickness per gas cycle and hole concentration when arsine pulse duration of was varied. The T M G pulse duration was ¢ s. The T M G mole fraction was 2 × 10 - 4 and the arsine mole fraction was 2.4× 10 -2.
duration after the thickness quickly rose to one monolayer. The hole concentration decreased to 1 × 1017cm-3 as the arsine mole fraction or pulse duration increased. The carbon concentration of the films was also directly measured by SIMS measurement. The carbon concentration increased with the T M G mole fraction and pulse duration. The increase was proportional to the increase of the hole concentration. These results provided further evidence that the main impurity was carbon. If the carbon atoms were incorporated from the background, the carbon concentration of the layer should increase with the purge time. However, the hole concentration was not observed to change (table 1) when the purge time after T M G or arsine pulse was increased. As shown in fig. 5, the hole
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The vaiiaficm in hole cor~r,--'-ation when purge ~ime was varied
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Fig. 6. The ,.,,_.,5:ations of growth thickness p'~- +,as cycle aad hole concentration when tire arsine ~no!': f . . c a o n was varied. The T M G pulse duration was 4 s. The T M G mole fraction was 2 × 10 -a and the arsine pulse duration was 4 s.
Sample n u m b e r
A
B
C
Growth sequence T M G pulse duration (s) Purge time after "rMG pulse (s) Arsine pulse duration (s) Purge time after arsine pulse (s~
4 6 ~ 1
4 1 4 ,
4 1 .I ~,
Holeconcentrationat30OK(×10Scm
~) 1.08 1.41 2.01
560
K. Mochizuki et al. / Carbon incorporation in GaAs layer grown b.v ALE
concentration increased with the TMG pulse duration. In the case of the arsine pulse, the hole concentration decreased with the pulse duration (fig. 7). These results suggest that the carbon impurity is incorporated during the T M G pulse duration. In the saturation region of the growth rate, that is, when the mole fraction is above 2 x 10 -4 in fig. 4 or when the pulse duration is above 4 s in fig. 5, the observed T M G molecules on the surface were considered to be decomposed and the surface was covered with gallium atoms. However, the carbon concentration was found to increase in the saturation region, as shown in figs. 4 and 5. This indicates that the carbon contamination does not directly come from T M G adsorbed on arsenic site, but from the hydrocarbon or carbon in the vapor. The number of hydrocarbon molecules or carbon atoms in the vapor is considered to be proportional to the mole fraction or the pulse duration of TMG. To explain the variation in hole concentration, it was necessary to consider the effect of the selective adsorption of carbon or hydrocarbon onto surface gallium atoms. A carbon atom tends to occupy the arsenic lattice site and acts as an acceptor in GaAs, while it would be difficult to occupy the gallium lattice site according to ,he electroncgativity arg)~ments [12]. In fact, there has been no definite observation concerning ,:arbon atoms occupying gallium lattice sites in Ca.As and GaP. In our model, a carbon atom is assumed to adsorb on the surface gallium a t o m avd v~ot on the arsenic atom. This assumption is consistent with the previous report that the carbon incorporation is affected by the density of surface gallium atoms when G~As films are g r o v a by metalorgan!c chemical vapor deDosi)i~, !131. The carbon concentra".ion N c in a film can be expressed as +
1
re.A,
t s< ,),,, ]
fl) where N is the number of carbc,~a atoms adsorbed duriag the T M G pulse duration, R is the growth thi,.kness per cycie, tV~tG is :he pulse duration of
TMG, x ( t ) is the surface gallium coverage determined from the experimental results of the growth rate, and is varying from 0 (10f'¢o arsenic coverage) to 1 (100% gallium coverage), rc.oa is the adsorption time of hydrocarbon on the gallium surface, "re.As is the adsorption time of hydrocarbon on the arsenic surface, and Sc is the number of hydrocarbon molecules that reached the surface during the T M G pulse, and is proportional to the T M G mole fraction. The growth rate R is directly related to the gallium coverage and is expressed as T = 0.283x(txMG) [nml [14]. The upper curve in fig. 4 was calculated from eq. (1) using the assumption of selective adsorption, i.e., assummg that ~'C.A~ was infinitely large. For x(t), the measured data shown by the lower curve of fig. 4 were used, and Sc/~'c.oa [cm-2 s - l ] was 5.52 × 10 24 [cm -1 s -!] times the mole fraction of TMG. The curve obtained by optimum fitting seems to describe the experimental results very well. When the value of "re.As was finite and comparab e to ~C.G~, the ot~served sharp drop of hole concentration for small values of T M G mole fraction could not be reproduced by eq. (1) at all. This indicates that ads;)rption of hydrocarbon or carbon takes place selectively on surface gallium atoms. A,', e played an important role in the incorporation of carbon. The film had a b_igh impurity concentration, above 3 x 1018 c m - 3 when insufficient arsine was supplied 0)'. the surface (fig. 6). Observatiofi~ showed that as the arsine pulse duration incr.eased, the carbon concentration decreas~:d. It is t h u s concludec~ that the arsine removes the caff.on atoms from the growth surface. Tlae the llytllk)~r~i~ ~. . . . . . . . . . . . .l¢i~At~al ~.... decomposed t~om" the arsine p r o b a b l y plays an important role in this reaction. When arsenic was supplied beyond the mole fraction of ~ x 10 -z in fig. 6. the carbon concentration did not change and saturated to 1 .Y, 1017 c m -a. T~-lis m a y b e caused by the exchange interaction be,ween the carbon atoms on the surface and the a~senic atoms in the lattice. Gnce the carbon atoms occupy sites in the bulk lattice, they can not be easily removed. When T E G was used as the gallium source in the ALE, we had similar results as those obtained
K. Machizuki et al. / Carbon incorporation in GaA s laver grown by A L E
ALE GaAs
4.2K
(D°,X) Tg=500*C --4
(D',X)
/I/l
LP
u~ (A*,Xl
-_=
810
'81o'
830 ' '81o Wavalength (nm)
' 950
Fig. 8, The photolum;,nescence spectrum of the film grown under the optimized growth condition.
for growth using TMG, except that the hole concentration of the films using TEG was less than that using TMG. The carbon incorporated was drastically decreased when the T M G mole fraction (pulse duration) was decreased and the arsine mole fraction (pulse duration) was increased. In order to get high purity layers, ALE growth should be carried o~ut under the condition of a shorter duration of T M G pulse with a higher mole fraction and a sufficient supply of arsine. The epitaxial layer grown under such a growth condition was found to be n-type. The layer grown under the condition of T M G mole fraction of 2 × 10-3, T M G pulse duration of 0.5 s, arsine mole fraction of 2.4 × 10 - ~and arsine pulse duration of 4 s had an electron concentration of 1 × 10 ~4 c m - 3 and a mobility of 80 000 cm2/V • s at 77 K. Fig. 8 shows the photoh:minescence spectrum of the film with several sharp excitot:ie lines at the band gap energy and an extremely low level of carbon related peaks refle,zt;r,g t.be high purity of the layer.
4. Summary The T M G source is the origin of carbon impurity. The carbon ;.n selectively adsorbed on the surface gallium atom. Arsine removes the carbon on the surface. The a m o u n t of carbon incorporated in the epitaxial layers was decreased when the T M G supply was decreased ar, d the arsine
561
supply was increased. When the growth was done under the condition that the T M G pulse duration was short with increased mole fraction and that sufficient arsine was supplied, the epitaxial layer was n-type. The layer exhibited and electron concentration of 1 × 10 t4 cm -3 and a mobility of 80000 cmZ/V • s at 77 K, and a photoluminesce:,.ce spectrum with several sharp excitonic lines at the band gap energy and an extremely low level of carbon related peaks.
Acknowledgernents The authors are grateful to Dr. O. Ohtsuki for his continuous encouragement and to E. Taguchi for valuable technical assistance.
References [1] T. Suntola and J. Antson, Finnish Palcnt No. 52359 (t974) and US Patent No. 4,058,431) (1977). [2] J. Nishizawa, H. Abe and T. Kurabayashi, J. Electrochem. Soc, 132 (1985) 1197. [3] A. Usui and H. Sunakawa, Japan. J. Appl. Phys. 25 (1086) 1,21'2 [4] S.M. Bedair, M.A. Tischler, T. Katsuyama and N.A. EIMasry, Appl. Phys. Letters 47 (1985) 51. [5] A. Dot, Y. Aoyagi and S, Namba, Appl. Phys. I.etters 48 (1986) 1787. [6] N. Kobayashi, T. Makimoto and Y. I-lorikoshi, Japan. J. Appl. Phys. 24 (1985) L962. [7] J. Nishizawa, H. Abe, T. Kurabayashi and N. Sakurai, J. Vacuum Sci. Techno,. A4 ~1986) 706. [8] M.A, Tischler and S.M. Bedair, J. Cr3stal Growth 77 (1986; 89. [9] M.R. Leys, C. van Opdrop, M.P.A. Viegers anc~ H.J. Talen-van der Mheen, J. Crystal Growth 68 (1984) 431. [10] J.S. Roberts, N.J. Mason and M. Robinson, J. Crystal Growth 68 (1984) 422. [11] M Ozeki~ N. Nakai~ K_ Dazai and O. Ryuzan. Japan. J. Appl. Phys. 12 (1974) 1121. [121 J.C. Phillips, Bands and Bonds in Semiconductors (Academic Press, New York, 1973). [13] K. Famamura, J. Ogawa, K. Akimoto, Y. Mort and ('. Kojima, Appl. Phys. Letters 50 (1987) 1149. [14] The growth rate is described by a somewhat complex expression using sorption and desorption time constants and is to be published elsewhere.