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InP grown by atmospheric pressure MOCVD
Applied Surface Science 44 (1990) 161-164 North-Holland
161
LETTER AES S P U T I ~ R D E P T H P R O F I L E S APPLIED T O INTERFACE ANALYSIS O F G a l n A s / I n P G R O W N BY A T M O S P H E R I C P R E S S U R E MOCVD Jonder MORAIS IFGI~, UNICAMP, Campinas, S.P., Brazil
Aldionso M. MACHADO, Marco A. SACILOTTI CPqD / TELEBRAS, S.P., Brazil
and Richard LANDERS 1FGW, UNICAMP, Campinas, S.P., Brazil Received 19 December 1989; accepted for publication 26 December 1989
Interfaces between InP and GalnAs layers, grown by atmospheric pressure MOCVD, have been studied using Auger electron spectroscopy and Ar + sputtering. The abruptness of the interfaces in this kind of epitaxial growth depends on the microscopic morphology of the growing surface (roughness) and on the way in which the active gases are substituted when changing the composition of the layers. We have investigated the influence of interrupting the growth at hetero-interfaces on the quality of the interface. It is shown that the abruptness of the interface improved with longer growth interruptions.
I. Introduction
Structures using Ga0.47In0.s3As have received increasing attention due to their properties such as: high free-carrier mobility at room temperature (# = 14000 cm2/V • s [1]) and small energy gap; these are very attractive for device applications, for example, field effect transistors (FET) and low noise photo-detectors in the range of 1.0 to 1.6 #m, used in fiber optics communications. MOCVD (metal organic chemical vapor deposition) and CBE (chemical beam epitaxy) have been used for production of high-quality G a l n A s / InP structures [2-5]. It must be pointed out that the growth of As-containing compounds on InP, presents special problems in relation to systems where only As is present in both layers. The 0169-4332/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
transition required between the Group V compounds must be done with great care, since to obtain high-quality devices, it is necessary to have abrupt interfaces between layers with different composition, in addition to materials with good optical and electrical quality. Both in MBE and MOCVD, a growth interruption between layers of different composition can improve the interfaces. The length of the interruptions can be very different depending on the growth method employed: from minutes in the case of MBE [6], to fractions of seconds for low pressure MOCVD [7]. To avoid strains at the interfaces, it is essential to grow the Gaxln~_xAs with x exactly equal to 0.47 so as to obtain lattice match to InP. Using Auger electron spectroscopy and low-en-
162
J. Morais et al. / Interface analysis of GalnAs / lnP grown by atmospheric pressure M O C VD
ergy Ar + sputtering, we have investigated the influence of growth interruptions on the interface quality of G a I n A s / I n P structures.
2. Experimental The samples were grown in an atmospheric pressure M O C V D horizontal reactor shown schematically in fig. 1. The reactor consists of an external cylindrical quartz tube (60 mm diameter) that surrounds an internal tube with square cross section (40 × 40 mm) which supports the graphite susceptor that is wedge shaped, with an angle of approximately 15 °. The susceptor is heated by radio frequency. Triethylgallium (TEG), trimethylindium (TMI), pure arsine (ASH3) and pure phosphine (PH3) were used as source materials. Hydrogen was used exclusively as the carrier gas and the total flow rate was 5 slm with an average gas velocity over the substrate of 5.2 c m / s . The growth parameters (gas flow, substrate temperature, metal organic bubbler temperature, e t c . . . ) were similar to those required for obtaining thick unstrained layers, that is, with Ga0.47 ln0.53As composition as verified by photoluminescence and X-ray diffraction. The T E G and TMI bubblers were kept at 0 and 18°C, respectively. The substrate was heated to the deposition temperature of 6 5 0 ° C in a PH 3 atmosphere. The growth rates were 4 / ~ m / h for InP and 2 . 5 / z m / h for GaInAs. In this MOCVD system we obtained InP with/~ = 95 000 cm2/V - s (77 K) and n = 10 a4 cm 3, and GaInAs lattice matched to InP with / ~ = 5 6 0 0 0 c m Z / V . s (77 K) and n = 2 × 1 0 1 5 cm
beam of 3000 eV and 1.5/xA, and argon ions with 500 eV energy and 2 . 5 / x A / c m 2. The base pressure prior to sputtering was 2 × 10-10 Torr. Depth profiling of different samples was used to discuss the transition from G a l n A s to InP at the G a l n A s / I n P interface.
3. Results The samples consisted of a thick lnP layer (1000 ,~) grown on an InP substrate; over the InP a 100 ,~ thick GalnAs layer was grown (fig. 2a); the inverse structure was also grown, that is a thick layer of GalnAs and a 100 A cap of InP (fig. 2b). To minimize the loss of depth resolution which is inherent to Ar ÷ sputtering, and caused by knock-on mixing and preferential sputtering, only the first interface was investigated. Knock-on mixing can induce a modified layer at the surface due to the large number of collision cascades that take place in the solid. The thickness of this modified layer is related to the energy of the colliding ions and the angle of incidence [8,9]. Preferential sputtering is related to differences in the energy of adhesion of the different atomic species on the surface [10]. The interface width obtained is largely determined by the way the gases are switched at the growing surface, and by the microscopic growth morphology (roughness). A growth interruption
a Galn
3
b
As IOO
InP
100 .~
The Auger analysis was performed in a commercial Varian system using a primary electron AsH~
MFC
InP
ooo<. o, PH~
MFC
IH 2
Fig. 1. Schematic representation of the AP-MOCVD reactor.
1000;
S.L ]riP SUBSTRATE
Gain
As 1000~ .53
S.1. InP SUBSTRATE
Fig. 2. Schematic diagram of the structures grown for Auger sputter profile experiments: (a) GalnAs/InP and (b) InP/ GalnAs.
J. Morais et aL / Interface analysis of GalnAs/ lnP grown by atmospheric pressure MOCVD
\'\
/
~
! /
~a
.,,~
Rf~---'----
As _
!! ~~
Z3
.... 0
n0
Osec of growth interruption
'
sec of growth interruption
~..,//," ~ ,".",..h-' .... 50
I O0 DEPTH (a.u.)
, .... 150
200
Fig. 3. Depth profiles of two OalnAs/lnP structures with 0 and 30 s growth interruptions at the interface. The enrichment of In at the surface is probably due to segregation [13,14].
can act on both these effects. First, an interruption, depending on its length, will diminish the concentration of the reacting gases at the interface before the next gas front arrives. The following factors must be considered: (i) Transit time of the reactive gas front from the manifold to the substrate. (ii) Residence time of molecules trapped in eddy and dead volumes. (iii) Time interval for a molecule to cross the stagnant layer. The greatest difficulty is in establishing the residence time which depends on reactor design, geometry, gas velocity, etc. Simultaneously, the roughness is diminished by an accommodation of the residual atoms left on the interface, driven by surface energy, reducing the number of incomplete layers present. In our case the growth interruption consisted of maintaining the H : flow while interrupting both the reactive gases (hydrides and metal organics). Fig. 3 shows composition profiles of two G a I n A s / I n P samples grown with interruptions of 0 and 30 s. Defining the interface width as the region where the concentration of a given element changes from 84% to 16% or vice-versa [11], the
163
longer interruption produced a slightly sharper interface. Fig. 4 shows the superposition of the In composition profile of three I n P / G a I n A s samples with different growth interruptions of 0, 15 and 30 s, respectively. As before, there is a reduction of the interface width with longer growth interruption, from 35 to 14 .A, which is close to the m a x i m u m expected resolution for the A u g e r / s p u t t e r i n g technique [12]. This resolution is limited by the roughness of the sputtered surface and the nonnegligible mean free path of Auger electrons. The relatively long growth interruptions necessary to obtain the best results tend to indicate the existence of secondary gas sources for: (i) The reordering of the interface due to roughness is probably very quick. In L P - M O C V D where the transient times are very short the best interfaces are obtained with growth interruption as short as 0.5 s [7] and as the growth mechanisms are similar in A P - M O C V D growth one would expect the reordering of the interface to take less than 0.5 s. It must be said that the growth mechanism in MBE is different, leading to rougher interfaces and thus the use of relatively long growth interruption [6]. (ii) The time to cross the stagnant layer is of the order of 1 s with our growth conditions. (iii) Lastly the mean transit time from manifold to sample is 8 s.
\
-..~
'~-- ~ . ~ . ~ .
\\'-.
30 seconds
~
\'~.
15seconds 0 seconds
~"-
,
0
,
,
,
I
50
,
,
,
,
I
,
,
,
,
I
,
,
,
,
I O0 I 50 200 DEPTH (a.u.) Fig. 4. Three In profiles of InP/GalnAs structures with 0, 15 and 30 s of growth interruptions at the interface.
164
J. Morais et al. / Interface analysis of GalnAs / lnP grown by atmospheric pressure MOCVD
As the growth i n t e r r u p t i o n s necessary were of 30 s o n e can c o n c l u d e that the d e a d v o l u m e s an d eddies act as s e c o n d a r y sources long after the m a i n gas flow has b e e n switched off.
4. Conclusion G a l n A s / I n P a n d I n P / G a l n A s structures were g r o w n by a t m o s p h e r i c pressure M O C V D with different g r o w t h i n t e r r u p t i o n s at the hetero-interfaces. A E S sputter d e p t h profiling showed a considerable i m p r o v e m e n t on the q u a li ty o f the interface wi d t h with longer g r o w t h interruptions.
Acknowledgements T h e authors w o u l d like to thank V.S. S u n d a r a n for useful discussions at the b e g i n n i n g of this work, and T.A. F a z a n a n d L.K. H i o r u s h i for technical assistance. W e gratefully a c k n o w l e d g e financial s u p p o r t f r o m C N P q , F A P E S P and T E L E B R A S (Brazil).
References [l] E.J. Thrush, C.G. Cureton and A.T.R. Briggs, J. Cryst. Growth 93 (1988) 870. [2] M. Razeghi, J.P. Hirtz, U.O. Ziemelis, C. Delalande, B. Etienne and M. Voos, Appl. Phys. Lett. 43 (1983) 585. [3] W.T. Tsang and E.F. Schubert, Appl. Phys. Lett. 49 (1986) 220. [4] T.Y. Wang, K.L. Fry, A. Persson, E.H. Reihlen and G.B. Stringfellow, J. Appl. Phys. 63 (1988) 2674. [5] B.I. Miller, E.F. Schubert, U. Koren, A. Ourmazd, A.H. Dayem and R.J. Capik, Appl. Phys. Lett. 49 (1986) 1384. [6] M. Tanaka and H. Sakaki, J. Cryst. Growth 81 (1987) 159. [7] H. Kamei, T. Katsuyama, K. Ono and K. Yoshida, Inst. Phys. Conf. Ser. No. 96 (1989) 125. [8] H.H. Andersen, Appl. Phys. 18 (1979) 131. [9] F. Matsunaga, H. Kakibayachi, T. Mishima and S. Kawase, Jpn. J. Appl. Phys. 27 (1988) 149. [10] M.P. Seah, Thin Solid Films 81 (1981) 279. [11] S. Hofmann, Surf. Interface Anal. 2 (1980) 148. [12] M.P. Seah and C.P. Hunt, Surf. Interface Anal. 5 (1983) 33. [13] J.T. Ebner and J.R. Arthur, J. Vac. Sci. Technol. A 5 (1987) 2007. [14] F. Houzay, J.M. Moison, C. Guille, F. Barthe and M. Van Rompay, J. Cryst. Growth 95 (1989) 35.
161
LETTER AES S P U T I ~ R D E P T H P R O F I L E S APPLIED T O INTERFACE ANALYSIS O F G a l n A s / I n P G R O W N BY A T M O S P H E R I C P R E S S U R E MOCVD Jonder MORAIS IFGI~, UNICAMP, Campinas, S.P., Brazil
Aldionso M. MACHADO, Marco A. SACILOTTI CPqD / TELEBRAS, S.P., Brazil
and Richard LANDERS 1FGW, UNICAMP, Campinas, S.P., Brazil Received 19 December 1989; accepted for publication 26 December 1989
Interfaces between InP and GalnAs layers, grown by atmospheric pressure MOCVD, have been studied using Auger electron spectroscopy and Ar + sputtering. The abruptness of the interfaces in this kind of epitaxial growth depends on the microscopic morphology of the growing surface (roughness) and on the way in which the active gases are substituted when changing the composition of the layers. We have investigated the influence of interrupting the growth at hetero-interfaces on the quality of the interface. It is shown that the abruptness of the interface improved with longer growth interruptions.
I. Introduction
Structures using Ga0.47In0.s3As have received increasing attention due to their properties such as: high free-carrier mobility at room temperature (# = 14000 cm2/V • s [1]) and small energy gap; these are very attractive for device applications, for example, field effect transistors (FET) and low noise photo-detectors in the range of 1.0 to 1.6 #m, used in fiber optics communications. MOCVD (metal organic chemical vapor deposition) and CBE (chemical beam epitaxy) have been used for production of high-quality G a l n A s / InP structures [2-5]. It must be pointed out that the growth of As-containing compounds on InP, presents special problems in relation to systems where only As is present in both layers. The 0169-4332/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
transition required between the Group V compounds must be done with great care, since to obtain high-quality devices, it is necessary to have abrupt interfaces between layers with different composition, in addition to materials with good optical and electrical quality. Both in MBE and MOCVD, a growth interruption between layers of different composition can improve the interfaces. The length of the interruptions can be very different depending on the growth method employed: from minutes in the case of MBE [6], to fractions of seconds for low pressure MOCVD [7]. To avoid strains at the interfaces, it is essential to grow the Gaxln~_xAs with x exactly equal to 0.47 so as to obtain lattice match to InP. Using Auger electron spectroscopy and low-en-
162
J. Morais et al. / Interface analysis of GalnAs / lnP grown by atmospheric pressure M O C VD
ergy Ar + sputtering, we have investigated the influence of growth interruptions on the interface quality of G a I n A s / I n P structures.
2. Experimental The samples were grown in an atmospheric pressure M O C V D horizontal reactor shown schematically in fig. 1. The reactor consists of an external cylindrical quartz tube (60 mm diameter) that surrounds an internal tube with square cross section (40 × 40 mm) which supports the graphite susceptor that is wedge shaped, with an angle of approximately 15 °. The susceptor is heated by radio frequency. Triethylgallium (TEG), trimethylindium (TMI), pure arsine (ASH3) and pure phosphine (PH3) were used as source materials. Hydrogen was used exclusively as the carrier gas and the total flow rate was 5 slm with an average gas velocity over the substrate of 5.2 c m / s . The growth parameters (gas flow, substrate temperature, metal organic bubbler temperature, e t c . . . ) were similar to those required for obtaining thick unstrained layers, that is, with Ga0.47 ln0.53As composition as verified by photoluminescence and X-ray diffraction. The T E G and TMI bubblers were kept at 0 and 18°C, respectively. The substrate was heated to the deposition temperature of 6 5 0 ° C in a PH 3 atmosphere. The growth rates were 4 / ~ m / h for InP and 2 . 5 / z m / h for GaInAs. In this MOCVD system we obtained InP with/~ = 95 000 cm2/V - s (77 K) and n = 10 a4 cm 3, and GaInAs lattice matched to InP with / ~ = 5 6 0 0 0 c m Z / V . s (77 K) and n = 2 × 1 0 1 5 cm
beam of 3000 eV and 1.5/xA, and argon ions with 500 eV energy and 2 . 5 / x A / c m 2. The base pressure prior to sputtering was 2 × 10-10 Torr. Depth profiling of different samples was used to discuss the transition from G a l n A s to InP at the G a l n A s / I n P interface.
3. Results The samples consisted of a thick lnP layer (1000 ,~) grown on an InP substrate; over the InP a 100 ,~ thick GalnAs layer was grown (fig. 2a); the inverse structure was also grown, that is a thick layer of GalnAs and a 100 A cap of InP (fig. 2b). To minimize the loss of depth resolution which is inherent to Ar ÷ sputtering, and caused by knock-on mixing and preferential sputtering, only the first interface was investigated. Knock-on mixing can induce a modified layer at the surface due to the large number of collision cascades that take place in the solid. The thickness of this modified layer is related to the energy of the colliding ions and the angle of incidence [8,9]. Preferential sputtering is related to differences in the energy of adhesion of the different atomic species on the surface [10]. The interface width obtained is largely determined by the way the gases are switched at the growing surface, and by the microscopic growth morphology (roughness). A growth interruption
a Galn
3
b
As IOO
InP
100 .~
The Auger analysis was performed in a commercial Varian system using a primary electron AsH~
MFC
InP
ooo<. o, PH~
MFC
IH 2
Fig. 1. Schematic representation of the AP-MOCVD reactor.
1000;
S.L ]riP SUBSTRATE
Gain
As 1000~ .53
S.1. InP SUBSTRATE
Fig. 2. Schematic diagram of the structures grown for Auger sputter profile experiments: (a) GalnAs/InP and (b) InP/ GalnAs.
J. Morais et aL / Interface analysis of GalnAs/ lnP grown by atmospheric pressure MOCVD
\'\
/
~
! /
~a
.,,~
Rf~---'----
As _
!! ~~
Z3
.... 0
n0
Osec of growth interruption
'
sec of growth interruption
~..,//," ~ ,".",..h-' .... 50
I O0 DEPTH (a.u.)
, .... 150
200
Fig. 3. Depth profiles of two OalnAs/lnP structures with 0 and 30 s growth interruptions at the interface. The enrichment of In at the surface is probably due to segregation [13,14].
can act on both these effects. First, an interruption, depending on its length, will diminish the concentration of the reacting gases at the interface before the next gas front arrives. The following factors must be considered: (i) Transit time of the reactive gas front from the manifold to the substrate. (ii) Residence time of molecules trapped in eddy and dead volumes. (iii) Time interval for a molecule to cross the stagnant layer. The greatest difficulty is in establishing the residence time which depends on reactor design, geometry, gas velocity, etc. Simultaneously, the roughness is diminished by an accommodation of the residual atoms left on the interface, driven by surface energy, reducing the number of incomplete layers present. In our case the growth interruption consisted of maintaining the H : flow while interrupting both the reactive gases (hydrides and metal organics). Fig. 3 shows composition profiles of two G a I n A s / I n P samples grown with interruptions of 0 and 30 s. Defining the interface width as the region where the concentration of a given element changes from 84% to 16% or vice-versa [11], the
163
longer interruption produced a slightly sharper interface. Fig. 4 shows the superposition of the In composition profile of three I n P / G a I n A s samples with different growth interruptions of 0, 15 and 30 s, respectively. As before, there is a reduction of the interface width with longer growth interruption, from 35 to 14 .A, which is close to the m a x i m u m expected resolution for the A u g e r / s p u t t e r i n g technique [12]. This resolution is limited by the roughness of the sputtered surface and the nonnegligible mean free path of Auger electrons. The relatively long growth interruptions necessary to obtain the best results tend to indicate the existence of secondary gas sources for: (i) The reordering of the interface due to roughness is probably very quick. In L P - M O C V D where the transient times are very short the best interfaces are obtained with growth interruption as short as 0.5 s [7] and as the growth mechanisms are similar in A P - M O C V D growth one would expect the reordering of the interface to take less than 0.5 s. It must be said that the growth mechanism in MBE is different, leading to rougher interfaces and thus the use of relatively long growth interruption [6]. (ii) The time to cross the stagnant layer is of the order of 1 s with our growth conditions. (iii) Lastly the mean transit time from manifold to sample is 8 s.
\
-..~
'~-- ~ . ~ . ~ .
\\'-.
30 seconds
~
\'~.
15seconds 0 seconds
~"-
,
0
,
,
,
I
50
,
,
,
,
I
,
,
,
,
I
,
,
,
,
I O0 I 50 200 DEPTH (a.u.) Fig. 4. Three In profiles of InP/GalnAs structures with 0, 15 and 30 s of growth interruptions at the interface.
164
J. Morais et al. / Interface analysis of GalnAs / lnP grown by atmospheric pressure MOCVD
As the growth i n t e r r u p t i o n s necessary were of 30 s o n e can c o n c l u d e that the d e a d v o l u m e s an d eddies act as s e c o n d a r y sources long after the m a i n gas flow has b e e n switched off.
4. Conclusion G a l n A s / I n P a n d I n P / G a l n A s structures were g r o w n by a t m o s p h e r i c pressure M O C V D with different g r o w t h i n t e r r u p t i o n s at the hetero-interfaces. A E S sputter d e p t h profiling showed a considerable i m p r o v e m e n t on the q u a li ty o f the interface wi d t h with longer g r o w t h interruptions.
Acknowledgements T h e authors w o u l d like to thank V.S. S u n d a r a n for useful discussions at the b e g i n n i n g of this work, and T.A. F a z a n a n d L.K. H i o r u s h i for technical assistance. W e gratefully a c k n o w l e d g e financial s u p p o r t f r o m C N P q , F A P E S P and T E L E B R A S (Brazil).
References [l] E.J. Thrush, C.G. Cureton and A.T.R. Briggs, J. Cryst. Growth 93 (1988) 870. [2] M. Razeghi, J.P. Hirtz, U.O. Ziemelis, C. Delalande, B. Etienne and M. Voos, Appl. Phys. Lett. 43 (1983) 585. [3] W.T. Tsang and E.F. Schubert, Appl. Phys. Lett. 49 (1986) 220. [4] T.Y. Wang, K.L. Fry, A. Persson, E.H. Reihlen and G.B. Stringfellow, J. Appl. Phys. 63 (1988) 2674. [5] B.I. Miller, E.F. Schubert, U. Koren, A. Ourmazd, A.H. Dayem and R.J. Capik, Appl. Phys. Lett. 49 (1986) 1384. [6] M. Tanaka and H. Sakaki, J. Cryst. Growth 81 (1987) 159. [7] H. Kamei, T. Katsuyama, K. Ono and K. Yoshida, Inst. Phys. Conf. Ser. No. 96 (1989) 125. [8] H.H. Andersen, Appl. Phys. 18 (1979) 131. [9] F. Matsunaga, H. Kakibayachi, T. Mishima and S. Kawase, Jpn. J. Appl. Phys. 27 (1988) 149. [10] M.P. Seah, Thin Solid Films 81 (1981) 279. [11] S. Hofmann, Surf. Interface Anal. 2 (1980) 148. [12] M.P. Seah and C.P. Hunt, Surf. Interface Anal. 5 (1983) 33. [13] J.T. Ebner and J.R. Arthur, J. Vac. Sci. Technol. A 5 (1987) 2007. [14] F. Houzay, J.M. Moison, C. Guille, F. Barthe and M. Van Rompay, J. Cryst. Growth 95 (1989) 35.