- Email: [email protected]
Growth dynamics of InGaAsGaAs by MBE
Journal
ELSEVIER
of Crystal
Growth
175/176 (1997) 203-210
Growth dynamics of InGaAs/GaAs
by MBE
Fraqoise Fourniera,*, Robert A. Metzgera, Alan Doolittle”, April S. Brown”, Carrie Carter-Comana, Nan Marie Jokersta, Robert Bicknell-Tassiusb aSchool of Electrical and Computer Engineering, Microelectronics
Research Center, Georgia Institute qf Technology, Atlanta, Georgia 30332.0269, USA b Georgia Tech Research Institute, Georgia Institute qf Technology, Atlanta. Georgia 30332.0269, USA
Abstract The growth dynamics of the InGaAs/GaAs system have been investigated by desorption mass spectrometry (DMS). Indium desorption spectra indicate the presence of one or two desorption mechanisms depending on the V/III beam equivalent pressure ratio. The activation energy associated with one of the desorption processes is found to be 1.3 eV and independent of V/III ratio and arsenic species. Analysis of the decay curve allows the calculation of the indium surface population during growth. This population is compared for the different growth conditions investigated. Indium incorporation coefficient curves as a function of substrate temperature are presented. Indium incorporation is found to be enhanced using high V/III ratio and the arsenic dimer, Asz.
1. Introduction The
large
band
gap
and
lattice
constant
differ-
GaAs and InAs makes the InGaAs/GaAs system of technical interest for electronic and optical applications. In addition, growth kinetic processes which depend on strain enable the production of quantum-confined structures and can be exploited by varying the indium composition of the InGaAs alloy. Several studies have shown that the dynamics during the growth of InGaAs are complex [l], particularly at temperatures where indium desorption, segregation and ence
between
*Corresponding author. [email protected].
Fax:
0022-0248/97/$17.00 Copyright PII SOO22-0248(96)00888-3
+ 1 404 894 5028; e-mail: four-
0
1997 Published
incorporation are all important. A good understanding of the surface cation kinetics and the influence of growth parameters on indium incorporation are necessary in order to adequately control the growth of InGaAs. Another motivation for the study of InGaAs growth dynamics is to enable the full understanding and exploitation of the strain modulated epitaxy (SME) approach [2,3]. This new technology utilizes thin compliant substrates which are bottom-patterned. The pattern in the compliant bonded substrate modulates the growing overlayer to substrate thickness ratio. The strain at the surface of the growing film, which depends on this ratio, is thus laterally controlled. Thus, one aspect of this study was to measure the dependence of InGaAs growth dynamics on strain. Some previous
by Elsevier Science B.V. All rights reserved
204
F. Foumier
et al. i Journal ~f‘Ctysta1 Growth 175176
reports [4, 51 have shown that the activation energy for desorption has a significant strain dependence. In order to fully exploit indium kinetics in SME, we need to understand the relations existing between strain and surface mechanisms, such as segregation, desorption and migration for the InGaAs/GaAs system. By repositioning the mass spectrometer in a direct line of sight with the growing surface, several authors [S, 61 have shown that desorption mass spectrometry (DMS) is a useful in situ surface characterization tool. We report in this paper the result of an extensive DMS study during the growth of InGaAs on GaAs.
(1997) 203-210
not rotated, and the mass spectrometer desorption signal, which was assumed to be directly proportional to the indium desorbed flux, Fn(In), was recorded for further data analysis. We reference the mass spectrometer signal of 3.485 x lo-*’ A to an incident indium flux of Fi = 0.192 urn/h. After deposition of the InGaAs layer and a return to zero for the mass spectrometer desorption signal, the substrate temperature was raised to 580-C and a 600 A GaAs layer was grown, in order to smooth the surface and bury the preceding InGaAs layer. A new set of measurements was then taken at the next highest InGaAs growth temperature.
3. Results and discussions 2. Experimental
procedure
A UT1 mass spectrometer, tuned on the charge to mass peak ratio 115 of indium was positioned in line-of-sight to the substrate holder via an aperatured nipple. Substrate temperatures were measured by a thermocouple in direct contact to the back of the substrate holder. Oxide desorption was monitored by RHEED and the thermocouple reading was calibrated by assigment of the observed oxide desorption temperature to 580°C. (1 0 0)-oriented GaAs substrates were mounted on an indium-free holder. An EPI arsenic cracker was used to produce fluxes of both arsenic tetramers and dimers. For the desorption spectra recorded in presence of As,, the temperature of the arsenic cracker was increased to 900°C but the temperature of the evaporator was kept constant to 335°C. Thus, only the As2 and As4 molecular fluxes were modified but the arsenic flux was not changed. The growth experiment was repeated as a function of substrate temperature (510_63O”C), indium composition (5-21%), InGaAs growth rate (0.43-0.9 12 urn/h), V/III beam equivalent pressure ratio (17 : 1, 36 : l), measured with AL, and arsenic species. After substrate oxide removal, a 1200 A GaAs buffer layer was deposited at 580°C. The substrate temperature was then lowered to the first temperature of the investigated range, 510°C and In,Ga, _,As was grown for 25 s (approximately, 63A of In,.,,Ga 0,79A~ in the case of 21% In). During the InGaAs deposition, the substrate was
The desorption spectra (indium desorption as a function of time during InGaAs growth) were recorded for different growth temperatures and growth conditions (Fig. 1 shows desorption spectra for growth with AQ). Each desorption spectrum has the same profile as described by Evans [S]; an exponential rise at growth initiation (t = 0), followed by the realization of a steady-state signal. The amplitude of the steady-state signal increases with increasing substrate temperature. Upon termination of the InGaAs growth (r = 25 s), the desorption signal drops back to zero with a time constant which depends on the growth conditions. For low substrate temperatures (5 10~52O‘Q no detectable mass spectrometer signal was recorded. The indium desorption signal begins to appear around 53O’C. The data presented (see Fig. 1) were obtained with GaAs and InGaAs growth rates of 0.714 and 0.912 urn/h, respectively. The indium composition was approximately 21%. Fig. la shows data for growth under a low V/III (17 : 1) ratio, while Fig. lb shows data obtained for growth with a higher V/III ratio (36 : 1). The shape of these two sets of desorption spectra is quite different. The series of curves at low V/III ratio is square-shaped, and we can define two desorption mechanisms in the decay curve, with two different rate constants Kdl and Kd2. In the case of the higher V/III ratio, the shape is more rounded, and the decay curve can be modeled by a single desorption mechanism. These two sets of measurements were repeated using
F. Fournier et al. : Journal of Cnz~tul Growth 175:‘176 (I 997) 203-~210
-___,_______ --..--....-._.._. _-.;-_ ” _ .-.. - _____ _ ” I
205
_ -._ -.___
1
9$
2; i
Fig. 1. Series of desorption
spectra
presence of As4, for low V/III ratio shows one mechanism.
as a function
of substrate
temperature
during
growth
of In, zlGa,) irAs on GaAs MBE. in the mechanisms while(b) only
(- 17 : 1) (a) and high V!III ratio ( - 36 : 1) (b). (a) presents two desorption
on the dimer AsZ, with the same dependence Vi111 ratio observed. Other studies report data which may appear similar but arc interpreted differently from the data in this study. Evans et al. [7] observe two desorption phenomena for InAs deposited on GaAs. They attribute the lowest-temperature desorption mechanism (620 C) to indium desorption from a segregated surface desorption layer, and the higher-temperature (700°C) to indium evolving from the lattice. The two mechanisms observed in our data occur at temperatures less than 640°C. Kao et al. [8] in their DMS analysis of InGaAs growth, observed two separable signals in their decay curve. They associated one part of the signal to desorption. while the other part was interpreted as indium reflection due to the observed insensitivity of the signal to substrate temperature. In our case, below 530 C, no measurable desorbed signal is noted in the presence of an open or closed indium shutter. For temperatures above 610 C, we observed a saturation of the steady-state amplitude (see Fig. la). The saturation amplitude is dependent on the V/III ratio. In the presence of higher V,/III ratio, the saturation appears at a temperature greater than 63O’C. For high V/III ratio and high temperatures we note similar behaviour for the growth of InGaAs utilizing AsL. In order to extract more information from the mass desorption spectra, we utilize a general equation describing the indium surface population during growth. By analogy to the dopant incorporation model presented by Wood and Joyce [9]. we can write the temporal variation of the indium surface population, N,, as dN,(In)/dt
= Fi - K,(Ns(ln))P
- K,(Ns(ln)Y,
(I)
where Fi is the incident indium flux in atoms/cm2, the second term defines the indium desorbed flux, F,( In), and the final term represents the incorporation flux, F,(In). Kd and K, are. respectively, the desorption and incorporation rate contants. The units of K depend on the reaction order. Finally, p and (1 are, respectively, the desorption and incorporation order. In the case of several desorption mechanisms different Kd need to be defined. In the case of two mechanisms, the K expressions can be-
written as a function of activation strate temperature, as follows: &i
= &lo
&2 = &o
energy and sub-
exp ( - EAiIkT),
(2)
exp ( - E,,/kT).
(3
In order to take into account the two separate desorption mechanisms which clearly appear in our desorption spectra, we have introduced two terms in the desorption part of Eq. (1) each related to one desorption phenomenon and each with an associated activation energy, EAl and EA2. We asto the desorption sume that EA, corresponds activation energy of an indium segregated layer, which does not appear in the presence of a high V/III ratio. E,, represents the desorption activation energy for indium atoms from a more tightly bound state. The initial rise in signal apparent in the desorption spectra corresponds to the creation of an indium surface population. During the establishment of this population, indium incorporation, segregation and desorption are present. This situation can be described by Eq. (1). When the steady state is reached (dNs(In),/dt = 0), Eq. (1) gives: Fi = Kd(Ns (In)Y’ + Ki(Ns(In))q. The transient rise and saturation characteristics of the desorption spectra are difficult to analyse because the creation of an indium surface population, as well as incorporation and desorption, are all taking place simultaneously. Upon termination of growth, the analysis is made easier due to the presence of a single surface process: desorption. The fit of the decay curve allows us to determine the desorption rate, K,,, as a function of substrate temperature. Because of the sharp variation present in the first part of the decay curve, no data fit was obtained in this region; thus. we were unable to extract information about the desorption rate, Kdl associated with the first mechanism. An Arrhenius plot of the Kd2 gives the next activation energy associated with the second desorption process, Ek2. We found an average value of 1.3 eV for EA2, which agrees well with the previous values [10-l 21 reported for indium desorption. We find that the value of EAz seems to be relatively independent of the V/III ratio and arsenic species.
F. Fournier
et al. ,/ Journal
of Cqstal
Growth
1751’176 (1997) X)3-2/0
207
3
2.5
0.5
0
(a)
-0.5 3.5
3
2.5
2 T
0.5
(b)
-0.5
Fig. 2. Desorption spectra for T = 630-C, the indium percentage and InGaAs growth rate are. respectively, 21% and 0.912 pm/h. The arsenic tetramer, As4, is used. Area A (a) and B(b) are proportional to the indium surface population from the most tightly bound s#late. (a,b) correspond to a low V/III ratio (- 17 : 1) and higher V/III ratio (- 36 : I).
03
02
01
(a)
0 500
520
540
560
580
600
620
640
Tcmperature[C]
09
06
07
06
05
04
03
02
01
0
500
5.70
540
560
560
600
620
640
Temperature[C] (b)
Fig. 3. Indium incorporallon coefficient as a function of substrate presence of As, (a) and for V:III ratio (- 17 : 1) for both arsenic
In addition, we observe little dependence of the activation energy on the indium composition in the growing InGaAs film. None of the spectra recorded for different indium compositions and the same V/III ratio show a significant variation of Ed2, although the measurements were taken for different but very similar InGaAs growth rates of 1.11 urn/h and 0.912 urn&. respectively. for 5 and 21 %I in-
temperature species (b).
for low
(- 17 : I) and high ( - 36 : I) V.:III ratio. in the
dium. This result indicates that the link between strain and growth kinetics needed to exploit the SME approach will be found in surface mechanisms such as segregation and migration, rather than in indium desorption. The integration of the area defined by the decay curve gives us information about the indium surface population (see Fig. 2). The average
F. Fourniev et al. i Journal qf Crystal Growth 1751176 (1997) 203-210
population in the case of low V/III ratio (16 : 1, 20 : l), given by area A, is found to be around 0.3 ML [13] (Fig. 2a) in the case of 21% In, for both arsenic species, and for 14% In with As,. For the high V/III ratio case (36 : l), the average population is given by area B (Fig. 2b). For 21% In, we have calculated a surface population of around 1.O ML [ 131 and 0.5 ML, respectively, for Ash and As2. Evans et al., measure between 1.0 and 2.0 monolayers of segregated indium, independent of As2 overpressure [7]. Finally, the steady-state part of the desorption spectra (dN,/dt = 0) is used to calculate the indium incorporation coefficient rln, defined by: rln = 1 - FD(In)/Fi. We compare in Fig. 3a the indium incorporation coefficient as a function of substrate temperature for high and low V/III ratio. For T = 52O”C, Al,”is very near unity. For higher temperatures, the incorporation coefficient decreases steadily. For a higher V/III ratio of 36 : 1, 3,” drops from 0.85 to 0.4 between 55O’C and 600°C. The magnitude of the incorporation coefficient is greater for growth under a higher V/III ratio. However, in the presence of a low V,/III ratio, the decrease of qn with temperature is not as great as the decrease under a high ratio, which is in agreement with indium segregation studies [l, 141. This results appears to be true for both As, and AsZ. For high temperatures the situation seems to be reversed; for 0.3 because low V/III ratio, x,~ stays around of the saturation phenomenon mentioned previously, whereas c+” continues to drop for high V/III ratio. For a higher V/III ratio, the results show a higher indium incorporation coefficient and, concurrently, a higher indium surface population. This may appear contradictory but is not, since both the desorbed and incorporated fluxes, F,JIn) and F,(In), are proportional to the indium surface population, Ns. The presence of a higher Ns does not automatically imply poorer incorporation. When examining the interplay of the dynamics, the surface population, Ns, is apparently strongly dependent on the V/III ratio, while the activation energy for desorption is apparently independent of growth condition. Thus, it appears that the dominant mechanism controlling the observed
209
differences in growth result from the varying surface population. Finally, as shown in Fig. 3b, a comparison of the incorporation coefficient curves for both arsenic species shows significantly greater indium incorporation for growth under As, for both low and high V/III ratios.
4. Conclusions A DMS study of indium desorption during InGaAs growth by MBE has been presented for a wide range of growth conditions (substrate temperature,V/III ratio, arsenic species and In percentage). For low V/III ratio (- 17 : 1) two distinct desorption mechanisms were observed in the decay curve of the mass spectrometer data, whereas for higher ratio (- 36 : l), only one mechanism was present. The fit of the decay curves give us a average value of 1.3 eV for one of the desorption mechanisms. This activation energy is found to be independent of both V/III ratio, arsenic species and indium composition. This result gives us the new directions to choose in order to fully exploit kinetics in our SME approach. Additionally, we confirm that the indium incorporation is generally improved in the presence of high V/III ratio. The data also indicates that incorporation is probably controlled by the indium surface population, Ns, which depends significantly on the V/III ratio and As species. Indium incorporation is dramatically enhanced by the use of the arsenic dimer.
Acknowledgements This work is supported by ARO/ARPA No. DAAH 04-95-l -0367
contract
References Cl1 K. Woodbridge, Appl. Phys. Lett. 60 (1992). A.S. Brown, R. Bicknell-Tassius, I21 C. Carter-Coman,
N.M. Joker% F. Fournier and D.E. Dawson, J. Vat. Sci. Technol. B 14 (1996). AS. Brown, R. Bicknell-Tassius, N.M. [31 C. Carter-Coman, Jokerst and M. Allen, Appl. Phys. Lett. 69 (1996).
210
F. Fournier et al. 1 Journal of Cr)istal Growth I75 1176 (I 997) 203-210
[4] J.-P. Reithmaier, H. Riechert and H. Schlotterer. J. Crystal Growth 111 (1990) 407. [S] K.R. Evans, C.E. Stutz, E.N. Taylor and J.E. Ehret, J. Vat. Sci. Technol. B 9 (1991). [6] A.J. Spring Thorpe and P. Mandeville, J. Vat. Sci. Technol. B 6 (1988). [7] K.R. Evans, R. Kaspi, J.E. Ehret and M. Skowronski, J. Vat. Sci. Technol. B i3 (1995). [S] Y.C. Kao, F.G. Celii and H.Y. Liu, J. Vat. Sci. Technol. B 11 (1993). [9] C.E.C. Wood and B.A. Joyce, J. Appl. Phys. 49 (1978).
[lo] [ll]
CT. Foxon and B.A. Joyce, J. Crystal Growth 44(1978) 75. Yu.0. Kanter, A.K. Gutakovsky, A.A. Fedorov, M.A. Revenko, S.V. Rubanov and S.I. Stenin, Thin Solid Films 163 (1988) 497. [12] K. Radhakrishnan and S.F. Yoon, R. Gapalakrishnan and K.L. Tan, J. Vat. Sci. Technol A 12 (1994). [ 131 The definition of one monolayer (1 ML) used for the calculations is the following: 1 ML = a (In,Ga, _.As)/2, where a(In,Ga, _,As) is the lattice parameter in angstrom. 1141 J. Nagle, J.P. Landesman, M. Larive, C. Mottet and P. Bois, J. Crystal Growth 127 (1993) 550.
ELSEVIER
of Crystal
Growth
175/176 (1997) 203-210
Growth dynamics of InGaAs/GaAs
by MBE
Fraqoise Fourniera,*, Robert A. Metzgera, Alan Doolittle”, April S. Brown”, Carrie Carter-Comana, Nan Marie Jokersta, Robert Bicknell-Tassiusb aSchool of Electrical and Computer Engineering, Microelectronics
Research Center, Georgia Institute qf Technology, Atlanta, Georgia 30332.0269, USA b Georgia Tech Research Institute, Georgia Institute qf Technology, Atlanta. Georgia 30332.0269, USA
Abstract The growth dynamics of the InGaAs/GaAs system have been investigated by desorption mass spectrometry (DMS). Indium desorption spectra indicate the presence of one or two desorption mechanisms depending on the V/III beam equivalent pressure ratio. The activation energy associated with one of the desorption processes is found to be 1.3 eV and independent of V/III ratio and arsenic species. Analysis of the decay curve allows the calculation of the indium surface population during growth. This population is compared for the different growth conditions investigated. Indium incorporation coefficient curves as a function of substrate temperature are presented. Indium incorporation is found to be enhanced using high V/III ratio and the arsenic dimer, Asz.
1. Introduction The
large
band
gap
and
lattice
constant
differ-
GaAs and InAs makes the InGaAs/GaAs system of technical interest for electronic and optical applications. In addition, growth kinetic processes which depend on strain enable the production of quantum-confined structures and can be exploited by varying the indium composition of the InGaAs alloy. Several studies have shown that the dynamics during the growth of InGaAs are complex [l], particularly at temperatures where indium desorption, segregation and ence
between
*Corresponding author. [email protected].
Fax:
0022-0248/97/$17.00 Copyright PII SOO22-0248(96)00888-3
+ 1 404 894 5028; e-mail: four-
0
1997 Published
incorporation are all important. A good understanding of the surface cation kinetics and the influence of growth parameters on indium incorporation are necessary in order to adequately control the growth of InGaAs. Another motivation for the study of InGaAs growth dynamics is to enable the full understanding and exploitation of the strain modulated epitaxy (SME) approach [2,3]. This new technology utilizes thin compliant substrates which are bottom-patterned. The pattern in the compliant bonded substrate modulates the growing overlayer to substrate thickness ratio. The strain at the surface of the growing film, which depends on this ratio, is thus laterally controlled. Thus, one aspect of this study was to measure the dependence of InGaAs growth dynamics on strain. Some previous
by Elsevier Science B.V. All rights reserved
204
F. Foumier
et al. i Journal ~f‘Ctysta1 Growth 175176
reports [4, 51 have shown that the activation energy for desorption has a significant strain dependence. In order to fully exploit indium kinetics in SME, we need to understand the relations existing between strain and surface mechanisms, such as segregation, desorption and migration for the InGaAs/GaAs system. By repositioning the mass spectrometer in a direct line of sight with the growing surface, several authors [S, 61 have shown that desorption mass spectrometry (DMS) is a useful in situ surface characterization tool. We report in this paper the result of an extensive DMS study during the growth of InGaAs on GaAs.
(1997) 203-210
not rotated, and the mass spectrometer desorption signal, which was assumed to be directly proportional to the indium desorbed flux, Fn(In), was recorded for further data analysis. We reference the mass spectrometer signal of 3.485 x lo-*’ A to an incident indium flux of Fi = 0.192 urn/h. After deposition of the InGaAs layer and a return to zero for the mass spectrometer desorption signal, the substrate temperature was raised to 580-C and a 600 A GaAs layer was grown, in order to smooth the surface and bury the preceding InGaAs layer. A new set of measurements was then taken at the next highest InGaAs growth temperature.
3. Results and discussions 2. Experimental
procedure
A UT1 mass spectrometer, tuned on the charge to mass peak ratio 115 of indium was positioned in line-of-sight to the substrate holder via an aperatured nipple. Substrate temperatures were measured by a thermocouple in direct contact to the back of the substrate holder. Oxide desorption was monitored by RHEED and the thermocouple reading was calibrated by assigment of the observed oxide desorption temperature to 580°C. (1 0 0)-oriented GaAs substrates were mounted on an indium-free holder. An EPI arsenic cracker was used to produce fluxes of both arsenic tetramers and dimers. For the desorption spectra recorded in presence of As,, the temperature of the arsenic cracker was increased to 900°C but the temperature of the evaporator was kept constant to 335°C. Thus, only the As2 and As4 molecular fluxes were modified but the arsenic flux was not changed. The growth experiment was repeated as a function of substrate temperature (510_63O”C), indium composition (5-21%), InGaAs growth rate (0.43-0.9 12 urn/h), V/III beam equivalent pressure ratio (17 : 1, 36 : l), measured with AL, and arsenic species. After substrate oxide removal, a 1200 A GaAs buffer layer was deposited at 580°C. The substrate temperature was then lowered to the first temperature of the investigated range, 510°C and In,Ga, _,As was grown for 25 s (approximately, 63A of In,.,,Ga 0,79A~ in the case of 21% In). During the InGaAs deposition, the substrate was
The desorption spectra (indium desorption as a function of time during InGaAs growth) were recorded for different growth temperatures and growth conditions (Fig. 1 shows desorption spectra for growth with AQ). Each desorption spectrum has the same profile as described by Evans [S]; an exponential rise at growth initiation (t = 0), followed by the realization of a steady-state signal. The amplitude of the steady-state signal increases with increasing substrate temperature. Upon termination of the InGaAs growth (r = 25 s), the desorption signal drops back to zero with a time constant which depends on the growth conditions. For low substrate temperatures (5 10~52O‘Q no detectable mass spectrometer signal was recorded. The indium desorption signal begins to appear around 53O’C. The data presented (see Fig. 1) were obtained with GaAs and InGaAs growth rates of 0.714 and 0.912 urn/h, respectively. The indium composition was approximately 21%. Fig. la shows data for growth under a low V/III (17 : 1) ratio, while Fig. lb shows data obtained for growth with a higher V/III ratio (36 : 1). The shape of these two sets of desorption spectra is quite different. The series of curves at low V/III ratio is square-shaped, and we can define two desorption mechanisms in the decay curve, with two different rate constants Kdl and Kd2. In the case of the higher V/III ratio, the shape is more rounded, and the decay curve can be modeled by a single desorption mechanism. These two sets of measurements were repeated using
F. Fournier et al. : Journal of Cnz~tul Growth 175:‘176 (I 997) 203-~210
-___,_______ --..--....-._.._. _-.;-_ ” _ .-.. - _____ _ ” I
205
_ -._ -.___
1
9$
2; i
Fig. 1. Series of desorption
spectra
presence of As4, for low V/III ratio shows one mechanism.
as a function
of substrate
temperature
during
growth
of In, zlGa,) irAs on GaAs MBE. in the mechanisms while(b) only
(- 17 : 1) (a) and high V!III ratio ( - 36 : 1) (b). (a) presents two desorption
on the dimer AsZ, with the same dependence Vi111 ratio observed. Other studies report data which may appear similar but arc interpreted differently from the data in this study. Evans et al. [7] observe two desorption phenomena for InAs deposited on GaAs. They attribute the lowest-temperature desorption mechanism (620 C) to indium desorption from a segregated surface desorption layer, and the higher-temperature (700°C) to indium evolving from the lattice. The two mechanisms observed in our data occur at temperatures less than 640°C. Kao et al. [8] in their DMS analysis of InGaAs growth, observed two separable signals in their decay curve. They associated one part of the signal to desorption. while the other part was interpreted as indium reflection due to the observed insensitivity of the signal to substrate temperature. In our case, below 530 C, no measurable desorbed signal is noted in the presence of an open or closed indium shutter. For temperatures above 610 C, we observed a saturation of the steady-state amplitude (see Fig. la). The saturation amplitude is dependent on the V/III ratio. In the presence of higher V,/III ratio, the saturation appears at a temperature greater than 63O’C. For high V/III ratio and high temperatures we note similar behaviour for the growth of InGaAs utilizing AsL. In order to extract more information from the mass desorption spectra, we utilize a general equation describing the indium surface population during growth. By analogy to the dopant incorporation model presented by Wood and Joyce [9]. we can write the temporal variation of the indium surface population, N,, as dN,(In)/dt
= Fi - K,(Ns(ln))P
- K,(Ns(ln)Y,
(I)
where Fi is the incident indium flux in atoms/cm2, the second term defines the indium desorbed flux, F,( In), and the final term represents the incorporation flux, F,(In). Kd and K, are. respectively, the desorption and incorporation rate contants. The units of K depend on the reaction order. Finally, p and (1 are, respectively, the desorption and incorporation order. In the case of several desorption mechanisms different Kd need to be defined. In the case of two mechanisms, the K expressions can be-
written as a function of activation strate temperature, as follows: &i
= &lo
&2 = &o
energy and sub-
exp ( - EAiIkT),
(2)
exp ( - E,,/kT).
(3
In order to take into account the two separate desorption mechanisms which clearly appear in our desorption spectra, we have introduced two terms in the desorption part of Eq. (1) each related to one desorption phenomenon and each with an associated activation energy, EAl and EA2. We asto the desorption sume that EA, corresponds activation energy of an indium segregated layer, which does not appear in the presence of a high V/III ratio. E,, represents the desorption activation energy for indium atoms from a more tightly bound state. The initial rise in signal apparent in the desorption spectra corresponds to the creation of an indium surface population. During the establishment of this population, indium incorporation, segregation and desorption are present. This situation can be described by Eq. (1). When the steady state is reached (dNs(In),/dt = 0), Eq. (1) gives: Fi = Kd(Ns (In)Y’ + Ki(Ns(In))q. The transient rise and saturation characteristics of the desorption spectra are difficult to analyse because the creation of an indium surface population, as well as incorporation and desorption, are all taking place simultaneously. Upon termination of growth, the analysis is made easier due to the presence of a single surface process: desorption. The fit of the decay curve allows us to determine the desorption rate, K,,, as a function of substrate temperature. Because of the sharp variation present in the first part of the decay curve, no data fit was obtained in this region; thus. we were unable to extract information about the desorption rate, Kdl associated with the first mechanism. An Arrhenius plot of the Kd2 gives the next activation energy associated with the second desorption process, Ek2. We found an average value of 1.3 eV for EA2, which agrees well with the previous values [10-l 21 reported for indium desorption. We find that the value of EAz seems to be relatively independent of the V/III ratio and arsenic species.
F. Fournier
et al. ,/ Journal
of Cqstal
Growth
1751’176 (1997) X)3-2/0
207
3
2.5
0.5
0
(a)
-0.5 3.5
3
2.5
2 T
0.5
(b)
-0.5
Fig. 2. Desorption spectra for T = 630-C, the indium percentage and InGaAs growth rate are. respectively, 21% and 0.912 pm/h. The arsenic tetramer, As4, is used. Area A (a) and B(b) are proportional to the indium surface population from the most tightly bound s#late. (a,b) correspond to a low V/III ratio (- 17 : 1) and higher V/III ratio (- 36 : I).
03
02
01
(a)
0 500
520
540
560
580
600
620
640
Tcmperature[C]
09
06
07
06
05
04
03
02
01
0
500
5.70
540
560
560
600
620
640
Temperature[C] (b)
Fig. 3. Indium incorporallon coefficient as a function of substrate presence of As, (a) and for V:III ratio (- 17 : 1) for both arsenic
In addition, we observe little dependence of the activation energy on the indium composition in the growing InGaAs film. None of the spectra recorded for different indium compositions and the same V/III ratio show a significant variation of Ed2, although the measurements were taken for different but very similar InGaAs growth rates of 1.11 urn/h and 0.912 urn&. respectively. for 5 and 21 %I in-
temperature species (b).
for low
(- 17 : I) and high ( - 36 : I) V.:III ratio. in the
dium. This result indicates that the link between strain and growth kinetics needed to exploit the SME approach will be found in surface mechanisms such as segregation and migration, rather than in indium desorption. The integration of the area defined by the decay curve gives us information about the indium surface population (see Fig. 2). The average
F. Fourniev et al. i Journal qf Crystal Growth 1751176 (1997) 203-210
population in the case of low V/III ratio (16 : 1, 20 : l), given by area A, is found to be around 0.3 ML [13] (Fig. 2a) in the case of 21% In, for both arsenic species, and for 14% In with As,. For the high V/III ratio case (36 : l), the average population is given by area B (Fig. 2b). For 21% In, we have calculated a surface population of around 1.O ML [ 131 and 0.5 ML, respectively, for Ash and As2. Evans et al., measure between 1.0 and 2.0 monolayers of segregated indium, independent of As2 overpressure [7]. Finally, the steady-state part of the desorption spectra (dN,/dt = 0) is used to calculate the indium incorporation coefficient rln, defined by: rln = 1 - FD(In)/Fi. We compare in Fig. 3a the indium incorporation coefficient as a function of substrate temperature for high and low V/III ratio. For T = 52O”C, Al,”is very near unity. For higher temperatures, the incorporation coefficient decreases steadily. For a higher V/III ratio of 36 : 1, 3,” drops from 0.85 to 0.4 between 55O’C and 600°C. The magnitude of the incorporation coefficient is greater for growth under a higher V/III ratio. However, in the presence of a low V,/III ratio, the decrease of qn with temperature is not as great as the decrease under a high ratio, which is in agreement with indium segregation studies [l, 141. This results appears to be true for both As, and AsZ. For high temperatures the situation seems to be reversed; for 0.3 because low V/III ratio, x,~ stays around of the saturation phenomenon mentioned previously, whereas c+” continues to drop for high V/III ratio. For a higher V/III ratio, the results show a higher indium incorporation coefficient and, concurrently, a higher indium surface population. This may appear contradictory but is not, since both the desorbed and incorporated fluxes, F,JIn) and F,(In), are proportional to the indium surface population, Ns. The presence of a higher Ns does not automatically imply poorer incorporation. When examining the interplay of the dynamics, the surface population, Ns, is apparently strongly dependent on the V/III ratio, while the activation energy for desorption is apparently independent of growth condition. Thus, it appears that the dominant mechanism controlling the observed
209
differences in growth result from the varying surface population. Finally, as shown in Fig. 3b, a comparison of the incorporation coefficient curves for both arsenic species shows significantly greater indium incorporation for growth under As, for both low and high V/III ratios.
4. Conclusions A DMS study of indium desorption during InGaAs growth by MBE has been presented for a wide range of growth conditions (substrate temperature,V/III ratio, arsenic species and In percentage). For low V/III ratio (- 17 : 1) two distinct desorption mechanisms were observed in the decay curve of the mass spectrometer data, whereas for higher ratio (- 36 : l), only one mechanism was present. The fit of the decay curves give us a average value of 1.3 eV for one of the desorption mechanisms. This activation energy is found to be independent of both V/III ratio, arsenic species and indium composition. This result gives us the new directions to choose in order to fully exploit kinetics in our SME approach. Additionally, we confirm that the indium incorporation is generally improved in the presence of high V/III ratio. The data also indicates that incorporation is probably controlled by the indium surface population, Ns, which depends significantly on the V/III ratio and As species. Indium incorporation is dramatically enhanced by the use of the arsenic dimer.
Acknowledgements This work is supported by ARO/ARPA No. DAAH 04-95-l -0367
contract
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