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Reflectance-difference study of surface chemistry in MOVPE growth
68
Journal of Crystal Growth 107 (1991) 68—72 North-Holland
Reflectance-difference study of surface chemistry in MOVPE growth L. Samuelson, K. Deppert
~,
S. Jeppesen, J. Jönsson, G. Paulsson and P. Schmidt
*
Department of Solid State Physics, University of Lund, Box 118, S-221 00 Lund, Sweden
With the advent of the reflectance-difference (RD) technique the electronic configurations of Ill—V surfaces can be probed also under the non-ultra-high vacuum (UHV) conditions prevailing during metalorganic vapour phase epitaxy (MOVPE). In this paper the surface chemistry involved in the MOVPE growth of GaAs from triethylgallium and arsine is studied in a growth system where the pressure can be changed from high-vacuum (10—i Torr) to low-pressure (1 Torr) conditions. Studies of the GaAs surface during its exposure to triethylgallium and to arsine are presented. Growth oscillations detected in real time are used to characterize growth and to investigate three-dimensional island formation during Ga saturation. Finally, growth oscillations are used to study the kinetics of the regeneration of ideal, As-terminated, surface conditions.
1. Introduction At the present time, a strong trend can be observed towards the development of epitaxial techniques with in situ monitoring of the epitaxial process. Until recently, it was primarily by RHEED (reflectance high-energy electron diffraction) that stationary as well as periodic changes in the character of the growing surfaces during molecular beam epitaxy (MBE) could be followed in real time [1]. Unfortunately, the pressure conditions of metalorganic vapour phase epitaxy (MOVPE) growth do not permit the application of RHEED in studies or in the control of the growth. For a long time a need has been apparent for a method which gives detailed information on the surface chemistry during MOVPE growth. The introduction of the reflectance-difference (RD) technique [21 seems to be a major step forward with regard to in situ control during MOVPE growth. In this paper we demonstrate that RD measured at a fixed wavelength can be used to follow the formation of single monolayers of Ga and As in a pure TEG ((C2H5)3Ga) and arsine (AsH3) environment, respectively. The formation *
. . Permanent address: Central Institute of Optics and Spectroscopy, Academy of Sciences of the GDR, Rudower Chaussee 6, DDR-1199 Berlin, German Dem. Rep.
0022-0248/91/$03.50 © ~991
—
of monolayers can also be studied in real time by RD and be detected as growth oscillations during continuous growth, where the formation of one bilayer of GaAs corresponds to one period of the oscillating RD signal [3]. Results are also presented which show that growth oscillations detected by RD can be used to evaluate the status of the coherency of the surface and to find out conditions under which the surface looses and regains its coherency.
2. Experimental details 2.1. The reflectance-difference set-up and the highvacuum MO VPE system The idea behind RD is to measure the relative difference in reflectance for light polarized along the two principal axes, namely [110] and [110], of the reconstructed surface [2]. The change in the reflectance can be traced back to the absorption by the dimer bonds in the outermost atomic layer. Since the Ga—Ga and As—As dimer bonds are oriented parallel to the [110] and [110] axes, respectively, and have optical absorptit~nmaxima at different wavelengths the RD signal can be used to follow the relative coverage of Ga and As in the monolayer terminating the crystal.
Elsevier Science Publishers B.V. (North-Holland)
L Samuelson eta!.
Detector
RD study of surface chemistry in MOVPE growth
X-t recorder
Ratio
Analyzer ~5 ~ M
/
Lock-in amplifier
o~
Polarizer Quartz window
Growth
0
chamber
—
Ill 0 El IDE
but pressures up MOVPE-like to 1 Torr cangrowth be used,coni.e. withgrowth low-pressure (LP) ditions. 2.2. Monolayer saturation experiments
Mirror HeNe Laser
I
69
Substrate
Fig. 1. Schematic diagram of the RD set-up situated on top of the stainless steel vacuum chamber. The relative azimuths of the polarizer, the photoelastic modulator, the analyzer and the sample are indicated,
The experimental set-up [4] used in this study is shown schematically in fig. 1. The RD equipment is situated on top of the stainless steel vacuum chamber containing the growth cell. We use a 0.5 mW HeNe laser, lasing at A = 6328 A, as the light source and a Si diode as a detector. The relative azimuths of the optical components are also mdicated in the figure. The substrate is placed on a temperature-controlled susceptor positioned inside an inner cell in a high-vacuum MOVPE system, which has been described elsewhere [5]. Briefly, this growth chamber allows us to grow uniform smooth layers of GaAs from TEG and arsine with 3. Uniformity and doping levels aroundare1015 cm by the use of a cracking efficiency achieved temperature-controlled inner cell, which allows multiple impingement of the source molecules onto the surface. The flux of AsH3 used in the present study is, typically, 1—2 mi/mm. The pressure in the inner cell is around iO~ Torr during growth
Starting with an AsH3-stabilized surface, the AsH3 flux was stopped and an amount of TEG corresponding to one monolayer was injected. The RD signal, in fig. 2, first increases linearly but falls off from the linear behaviour just before the termination of the TEG flux. When the AsH3 flow is resumed, the RD signal exhibits an nearly exponential recovery towards the initial level. The lower part of fig. 2 shows the same valve switching sequence but in this case the TEG valve is kept open several times longer. After the expected mitial transient, resulting from the formation of a monolayer of Ga, partially decomposed TEG species clearly contribute to the RD signal. It should be noted that the physisorbed layer(s) must be at least partially oriented by the Ga-reconstructed surface since only preferentially oriented TEG species will give contributions. The times it would have taken to complete the first, second and third Ga monolayers by continuous growth are indicated by arrows. At the end of the TEG exposure, the RD signal shows satura_____ TEG Off~
T,55Or~C
TEG I I 2 00 300 40 0 Time (sec) Fig. 2. RD signal of a (100) GaAs surface, initially stabilized with AsH 3, when TEG is injected into the chamber. The amount of TEG corresponds to the growth of 1 ML (upper curve) and 5 ML (lower curve). Each of the traces is terminated by exposure of the surface to AsH3 by which As stabilization is regained.
0
.
I 1 00
70
L. Samue!son et a!.
/ RD study ofsurface chemistry in MOVPE growth
tion, which can either be due to some self-limiting step or might indicate that the additional deposition is, from an optical point of view, isotropic. The large feature observed when the flow of AsH3
bilities for growth control using non-ultra-high vacuum techniques. In the following sections we will demonstrate new examples of surface chem-
is resumed clearly corresponds to the excess amount of TEG products on the surface.
istry investigations for which detection of growth oscillations by RD can be used to probe surface conditions.
2.3. RD oscillations during continuous epitaxial growth
2.4. Loss of surface coherency during TEG saturation
The RD signal can also be used to follow the changes on the surface during continuous growth in a RHEED-like fashion [3]. Fig. 3, curve d, shows a damped oscillatory behaviour of the RD signal obtained by injecting TEG after the AsH3 surface stabilization, but without interrupting the AsH3 flow. Depending on growth conditions, up to 30 oscillations can be observed, after which the signal settles at a level characteristic of the V/Ill ratio used. By demonstrating this possibility to measure growth ocillations in real time by optical techniques, we have clearly opened up new possi-
From previous studies of atomic layer epitaxial (ALE) growth of GaAs from TEG and AsH3 it is known that on top of a Ga-stabilized GaAs surface, at least one physisorbed layer of TEG or derivatives thereof can be adsorbed [6]. This behaviour was also clearly demonstrated in the experiments described above for T = 550°Cin section 2.2 and the corresponding studies previously reported for T = 410°C [5]. From the point of view of using TEG as a Ga precursor to perform ALE growth, this is an unwanted behaviour. Apart from the tendency of TEG to form more than one
T=550°C AsH3 on~
AsH3 off A TEG Ofl T
4
AsH3 on
I
I
0
40
AsH3 off A TEG ~ T
AsH3 off TEG on AsI-1
TEG on
4 3 on
I
I
I
I
I
I
120
160
200
240
280
Time (sec) Fig. 3. RD detection of the coherence of the GaAs surface after AsH 3- stabilization and different degrees of coverage with TEG. The amount of TEG corresponds to the growth of (a) 5—6 ML, (b) 3 ML, (c) 1 ML and (d) only AsH3 stabilization. The presence of growth oscillations in curve (c) shows that the suface coherence remained during this TEG exposure of the surface.
L Samuelson et a!. / RD study of surface chemistry in MOVPE growth
monolayer on GaAs, it is also of great importance to understand the mechanisms by which excess Ga is adsorbed on the surface. The way in which the physisorbed Ga species are adsorbed on the surface is investigated in the experiment described in fig. 3, curves a—d. Curve 3a shows the RD response during the oversaturation of the surface to a dose of TEG corresponding to approximately 4 monolayers of Ga. Contrary to the experiments described in section 2.2, we now keep the flux of TEG on while AsH3 is added to the surface. The RD surface response observed in curve 3a is that, after an initial, rapid transient, the RD signal quickly approaches the level corresponding to epitaxial growth (compare curve 3d). However, in this case the growth conditions are reached without any traces of growth oscillations being observable. Clearly during the Ga saturation the surface has lost its coherency, which we propose to be related to the formation of three-dimensional islands of TEG derivates. In order to determine during which phase of Ga adsorption the coherency was lost, we have performed the same experiments using shorter exposure times during the “TEG-only” period. In the case illustrated by curve 3b, AsH3 was added during the TEG exposure exactly at the minimum of the RD response. With this way of returning to epitaxial growth, again a very strong positive feature is observed in the RD response, which is followed, also in this case, by a rapid and smooth approach to growth, but still without growth oscillations being observed. Clearly, already at the time when the RD response passes through its lowest part (between 2 and 4 monolayers of Ga exposure), the surface coherency has been lost. Curve 3c describes the experiment when the Ga exposure is only extended to the maximum point, i.e. to the time when one monolayer of Ga is believed to have been formed on the surface. When AsH3 is added (together with the TEG) to the surface at this moment, the behaviour shown in curve 3c is observed; directly from the positive peak the RD transient goes into continuous growth oscillations. This observation clearly shows that at the point we have attributed to the formation of one chemically bonded monolayer of Ga, the surface is in complete coherency and no three-di-
71
mensional Ga deposits are formed. The sequence described and measured in real time is thus an optically controlled experiment of surface manipulation which should be very useful for monitoring atomic layer epitaxial growth. 2.5. Surface regeneration of As-stabilized surface The growth oscillations as observed in fig. 3, curve d (or fig. 4, curve a) are most clearly observed after the growth of a buffer layer which seems to smoothen the surface, followed by an _________________________________
(d)
(C)
~
~
A
(b)
U
-~
(a)
51
__________________________________ ‘
50
‘
100
‘
150
‘
200
Time (sec) Fig. 4. RD detection of the regeneration of an As-terminated surface after growth at T = 550°C.The height of the oscillations when growth is resumed measures the amount of coherency regained. Arrows indicate the events when TEG was switched off (arrow downwards) and on (arrow upwards). The time allowed for regeneration is for (a) very long, (b) 60 s, (c) 25
and (d) 10 s.
72
L. Samuelson a a!.
/ RD study of surface chemistry in MOVPE growth
extended exposure to AsH3. The relative amplitudes of the oscillations are then, for growth conditions like those in the experiments described here, typically 20%—30% of the total RD transition at the initiation of growth. After the decay of the growth oscillation amplitude, which most often occurs after 20—30 oscillation periods, the (001) growing surface displays a certain average admixture of As and Ga dimers which does not vary with time. The coherent Asstabilized surface can, however, be regenerated by exposure to AsH3. Examples of this surface regeneration are shown in fig. 4, curves a—d, where curve a, with the very large growth oscillation amplitude, is for an extended exposure to AsH3. The left part of curve d in fig. 4 shows the ending of a growth period during which growth oscillations have decayed completely. This is followed by a 10 s period during which only the AsH3 flux is kept on. The later part of the sequence of events in curve d shows the re-initiation of growth during which a few oscillations can be observed, but with only approximately 10% of the oscillation amplitude of that in curve a. Similarly, curve c shows the further regeneration of growth oscillations following 25 s of exposure to AsH3. In this case about 25% of the oscillation amplitude is regained. In curve b, finally, the AsH3 treatment is performed for 60 ~ which is sufficient to regenerate more than 80% of the growth oscillation amplitude of curve a. The important quality of the GaAs surface for the detection of growth oscillations by RD is thus a perfect As-terminated and As-stabilized surface. The kinetics of this surface regeneration can directly be followed and measured by the RD growth oscillations. 2.6. Relevance of these results for conventional MO VPE The growth system used in these investigations allows the increase of the chamber pressure from the mTorr level used here up to the Torr range. In preliminary experiments performed in this LP MOVPE regime, we have applied RD detection
during the growth of smooth layers at the higher pressures but have, so far, not obtained growth oscillations under these growth conditions. Our high-vacuum MOVPE growth conditions, in terms of the V/Ill balance prevailing on the surface, are of course optimized for RD studies of surface chemistry in epitaxial growth. We are, however, convinced that the knowledge gained during the studies under these optimized conditions is directly transferable to LP-MOVPE growth, since sources and surface chemistries in the two cases have very strong similarities.
3. Conclusions We have shown that reflectance-difference is able to follow with high sensitivity the formation of overlayers of Ga and As during growth from triethylgallium and arsine. Detection of growth oscillations by RD during epitaxy have been used to study how surface coherency is lost during overexposure to Ga and to measure the kinetics of regeneration of As-stabilized surfaces.
Acknowledgements This work was supported by the Swedish National Board for Technical Development and the Swedish Natural Science Research Council. References [1] J.H. Neave, BA. Joyce, P.J. Dobson and N. Norton, Appi. Phys. A31 (1983) 1. [2] D.E. Aspnes, R. Bhat, E. Colas, E.T. Florez, J.P. Harbison. M.K. Kelly, V.G. Keramidas, MA. Koza and A.A. Studna, Proc. SPIE 1037 (1989) 2. [3] J. Jönsson, K. Deppert, S. Jeppesen, G. Paulsson, L. Samuelson and P. Schmidt, Appl. Phys. Letters 56 (1990) 2414. [4] G. Paulsson, K. Deppert, S. Jeppesen, J. Jonsson, L. Samuelson and P. Schmidt, J. Crystal Growth 105 (1990) 312. [5] P. Schmidt, K. Deppert, S. Jeppesen, J. Jdnsson, G. Paulsson and L. Samuelson, J. Crystal Growth 105 (1990) 306. [6] T.H. Chiu, AppI. Phys. Letters 55 (1989) 1244.
Journal of Crystal Growth 107 (1991) 68—72 North-Holland
Reflectance-difference study of surface chemistry in MOVPE growth L. Samuelson, K. Deppert
~,
S. Jeppesen, J. Jönsson, G. Paulsson and P. Schmidt
*
Department of Solid State Physics, University of Lund, Box 118, S-221 00 Lund, Sweden
With the advent of the reflectance-difference (RD) technique the electronic configurations of Ill—V surfaces can be probed also under the non-ultra-high vacuum (UHV) conditions prevailing during metalorganic vapour phase epitaxy (MOVPE). In this paper the surface chemistry involved in the MOVPE growth of GaAs from triethylgallium and arsine is studied in a growth system where the pressure can be changed from high-vacuum (10—i Torr) to low-pressure (1 Torr) conditions. Studies of the GaAs surface during its exposure to triethylgallium and to arsine are presented. Growth oscillations detected in real time are used to characterize growth and to investigate three-dimensional island formation during Ga saturation. Finally, growth oscillations are used to study the kinetics of the regeneration of ideal, As-terminated, surface conditions.
1. Introduction At the present time, a strong trend can be observed towards the development of epitaxial techniques with in situ monitoring of the epitaxial process. Until recently, it was primarily by RHEED (reflectance high-energy electron diffraction) that stationary as well as periodic changes in the character of the growing surfaces during molecular beam epitaxy (MBE) could be followed in real time [1]. Unfortunately, the pressure conditions of metalorganic vapour phase epitaxy (MOVPE) growth do not permit the application of RHEED in studies or in the control of the growth. For a long time a need has been apparent for a method which gives detailed information on the surface chemistry during MOVPE growth. The introduction of the reflectance-difference (RD) technique [21 seems to be a major step forward with regard to in situ control during MOVPE growth. In this paper we demonstrate that RD measured at a fixed wavelength can be used to follow the formation of single monolayers of Ga and As in a pure TEG ((C2H5)3Ga) and arsine (AsH3) environment, respectively. The formation *
. . Permanent address: Central Institute of Optics and Spectroscopy, Academy of Sciences of the GDR, Rudower Chaussee 6, DDR-1199 Berlin, German Dem. Rep.
0022-0248/91/$03.50 © ~991
—
of monolayers can also be studied in real time by RD and be detected as growth oscillations during continuous growth, where the formation of one bilayer of GaAs corresponds to one period of the oscillating RD signal [3]. Results are also presented which show that growth oscillations detected by RD can be used to evaluate the status of the coherency of the surface and to find out conditions under which the surface looses and regains its coherency.
2. Experimental details 2.1. The reflectance-difference set-up and the highvacuum MO VPE system The idea behind RD is to measure the relative difference in reflectance for light polarized along the two principal axes, namely [110] and [110], of the reconstructed surface [2]. The change in the reflectance can be traced back to the absorption by the dimer bonds in the outermost atomic layer. Since the Ga—Ga and As—As dimer bonds are oriented parallel to the [110] and [110] axes, respectively, and have optical absorptit~nmaxima at different wavelengths the RD signal can be used to follow the relative coverage of Ga and As in the monolayer terminating the crystal.
Elsevier Science Publishers B.V. (North-Holland)
L Samuelson eta!.
Detector
RD study of surface chemistry in MOVPE growth
X-t recorder
Ratio
Analyzer ~5 ~ M
/
Lock-in amplifier
o~
Polarizer Quartz window
Growth
0
chamber
—
Ill 0 El IDE
but pressures up MOVPE-like to 1 Torr cangrowth be used,coni.e. withgrowth low-pressure (LP) ditions. 2.2. Monolayer saturation experiments
Mirror HeNe Laser
I
69
Substrate
Fig. 1. Schematic diagram of the RD set-up situated on top of the stainless steel vacuum chamber. The relative azimuths of the polarizer, the photoelastic modulator, the analyzer and the sample are indicated,
The experimental set-up [4] used in this study is shown schematically in fig. 1. The RD equipment is situated on top of the stainless steel vacuum chamber containing the growth cell. We use a 0.5 mW HeNe laser, lasing at A = 6328 A, as the light source and a Si diode as a detector. The relative azimuths of the optical components are also mdicated in the figure. The substrate is placed on a temperature-controlled susceptor positioned inside an inner cell in a high-vacuum MOVPE system, which has been described elsewhere [5]. Briefly, this growth chamber allows us to grow uniform smooth layers of GaAs from TEG and arsine with 3. Uniformity and doping levels aroundare1015 cm by the use of a cracking efficiency achieved temperature-controlled inner cell, which allows multiple impingement of the source molecules onto the surface. The flux of AsH3 used in the present study is, typically, 1—2 mi/mm. The pressure in the inner cell is around iO~ Torr during growth
Starting with an AsH3-stabilized surface, the AsH3 flux was stopped and an amount of TEG corresponding to one monolayer was injected. The RD signal, in fig. 2, first increases linearly but falls off from the linear behaviour just before the termination of the TEG flux. When the AsH3 flow is resumed, the RD signal exhibits an nearly exponential recovery towards the initial level. The lower part of fig. 2 shows the same valve switching sequence but in this case the TEG valve is kept open several times longer. After the expected mitial transient, resulting from the formation of a monolayer of Ga, partially decomposed TEG species clearly contribute to the RD signal. It should be noted that the physisorbed layer(s) must be at least partially oriented by the Ga-reconstructed surface since only preferentially oriented TEG species will give contributions. The times it would have taken to complete the first, second and third Ga monolayers by continuous growth are indicated by arrows. At the end of the TEG exposure, the RD signal shows satura_____ TEG Off~
T,55Or~C
TEG I I 2 00 300 40 0 Time (sec) Fig. 2. RD signal of a (100) GaAs surface, initially stabilized with AsH 3, when TEG is injected into the chamber. The amount of TEG corresponds to the growth of 1 ML (upper curve) and 5 ML (lower curve). Each of the traces is terminated by exposure of the surface to AsH3 by which As stabilization is regained.
0
.
I 1 00
70
L. Samue!son et a!.
/ RD study ofsurface chemistry in MOVPE growth
tion, which can either be due to some self-limiting step or might indicate that the additional deposition is, from an optical point of view, isotropic. The large feature observed when the flow of AsH3
bilities for growth control using non-ultra-high vacuum techniques. In the following sections we will demonstrate new examples of surface chem-
is resumed clearly corresponds to the excess amount of TEG products on the surface.
istry investigations for which detection of growth oscillations by RD can be used to probe surface conditions.
2.3. RD oscillations during continuous epitaxial growth
2.4. Loss of surface coherency during TEG saturation
The RD signal can also be used to follow the changes on the surface during continuous growth in a RHEED-like fashion [3]. Fig. 3, curve d, shows a damped oscillatory behaviour of the RD signal obtained by injecting TEG after the AsH3 surface stabilization, but without interrupting the AsH3 flow. Depending on growth conditions, up to 30 oscillations can be observed, after which the signal settles at a level characteristic of the V/Ill ratio used. By demonstrating this possibility to measure growth ocillations in real time by optical techniques, we have clearly opened up new possi-
From previous studies of atomic layer epitaxial (ALE) growth of GaAs from TEG and AsH3 it is known that on top of a Ga-stabilized GaAs surface, at least one physisorbed layer of TEG or derivatives thereof can be adsorbed [6]. This behaviour was also clearly demonstrated in the experiments described above for T = 550°Cin section 2.2 and the corresponding studies previously reported for T = 410°C [5]. From the point of view of using TEG as a Ga precursor to perform ALE growth, this is an unwanted behaviour. Apart from the tendency of TEG to form more than one
T=550°C AsH3 on~
AsH3 off A TEG Ofl T
4
AsH3 on
I
I
0
40
AsH3 off A TEG ~ T
AsH3 off TEG on AsI-1
TEG on
4 3 on
I
I
I
I
I
I
120
160
200
240
280
Time (sec) Fig. 3. RD detection of the coherence of the GaAs surface after AsH 3- stabilization and different degrees of coverage with TEG. The amount of TEG corresponds to the growth of (a) 5—6 ML, (b) 3 ML, (c) 1 ML and (d) only AsH3 stabilization. The presence of growth oscillations in curve (c) shows that the suface coherence remained during this TEG exposure of the surface.
L Samuelson et a!. / RD study of surface chemistry in MOVPE growth
monolayer on GaAs, it is also of great importance to understand the mechanisms by which excess Ga is adsorbed on the surface. The way in which the physisorbed Ga species are adsorbed on the surface is investigated in the experiment described in fig. 3, curves a—d. Curve 3a shows the RD response during the oversaturation of the surface to a dose of TEG corresponding to approximately 4 monolayers of Ga. Contrary to the experiments described in section 2.2, we now keep the flux of TEG on while AsH3 is added to the surface. The RD surface response observed in curve 3a is that, after an initial, rapid transient, the RD signal quickly approaches the level corresponding to epitaxial growth (compare curve 3d). However, in this case the growth conditions are reached without any traces of growth oscillations being observable. Clearly during the Ga saturation the surface has lost its coherency, which we propose to be related to the formation of three-dimensional islands of TEG derivates. In order to determine during which phase of Ga adsorption the coherency was lost, we have performed the same experiments using shorter exposure times during the “TEG-only” period. In the case illustrated by curve 3b, AsH3 was added during the TEG exposure exactly at the minimum of the RD response. With this way of returning to epitaxial growth, again a very strong positive feature is observed in the RD response, which is followed, also in this case, by a rapid and smooth approach to growth, but still without growth oscillations being observed. Clearly, already at the time when the RD response passes through its lowest part (between 2 and 4 monolayers of Ga exposure), the surface coherency has been lost. Curve 3c describes the experiment when the Ga exposure is only extended to the maximum point, i.e. to the time when one monolayer of Ga is believed to have been formed on the surface. When AsH3 is added (together with the TEG) to the surface at this moment, the behaviour shown in curve 3c is observed; directly from the positive peak the RD transient goes into continuous growth oscillations. This observation clearly shows that at the point we have attributed to the formation of one chemically bonded monolayer of Ga, the surface is in complete coherency and no three-di-
71
mensional Ga deposits are formed. The sequence described and measured in real time is thus an optically controlled experiment of surface manipulation which should be very useful for monitoring atomic layer epitaxial growth. 2.5. Surface regeneration of As-stabilized surface The growth oscillations as observed in fig. 3, curve d (or fig. 4, curve a) are most clearly observed after the growth of a buffer layer which seems to smoothen the surface, followed by an _________________________________
(d)
(C)
~
~
A
(b)
U
-~
(a)
51
__________________________________ ‘
50
‘
100
‘
150
‘
200
Time (sec) Fig. 4. RD detection of the regeneration of an As-terminated surface after growth at T = 550°C.The height of the oscillations when growth is resumed measures the amount of coherency regained. Arrows indicate the events when TEG was switched off (arrow downwards) and on (arrow upwards). The time allowed for regeneration is for (a) very long, (b) 60 s, (c) 25
and (d) 10 s.
72
L. Samuelson a a!.
/ RD study of surface chemistry in MOVPE growth
extended exposure to AsH3. The relative amplitudes of the oscillations are then, for growth conditions like those in the experiments described here, typically 20%—30% of the total RD transition at the initiation of growth. After the decay of the growth oscillation amplitude, which most often occurs after 20—30 oscillation periods, the (001) growing surface displays a certain average admixture of As and Ga dimers which does not vary with time. The coherent Asstabilized surface can, however, be regenerated by exposure to AsH3. Examples of this surface regeneration are shown in fig. 4, curves a—d, where curve a, with the very large growth oscillation amplitude, is for an extended exposure to AsH3. The left part of curve d in fig. 4 shows the ending of a growth period during which growth oscillations have decayed completely. This is followed by a 10 s period during which only the AsH3 flux is kept on. The later part of the sequence of events in curve d shows the re-initiation of growth during which a few oscillations can be observed, but with only approximately 10% of the oscillation amplitude of that in curve a. Similarly, curve c shows the further regeneration of growth oscillations following 25 s of exposure to AsH3. In this case about 25% of the oscillation amplitude is regained. In curve b, finally, the AsH3 treatment is performed for 60 ~ which is sufficient to regenerate more than 80% of the growth oscillation amplitude of curve a. The important quality of the GaAs surface for the detection of growth oscillations by RD is thus a perfect As-terminated and As-stabilized surface. The kinetics of this surface regeneration can directly be followed and measured by the RD growth oscillations. 2.6. Relevance of these results for conventional MO VPE The growth system used in these investigations allows the increase of the chamber pressure from the mTorr level used here up to the Torr range. In preliminary experiments performed in this LP MOVPE regime, we have applied RD detection
during the growth of smooth layers at the higher pressures but have, so far, not obtained growth oscillations under these growth conditions. Our high-vacuum MOVPE growth conditions, in terms of the V/Ill balance prevailing on the surface, are of course optimized for RD studies of surface chemistry in epitaxial growth. We are, however, convinced that the knowledge gained during the studies under these optimized conditions is directly transferable to LP-MOVPE growth, since sources and surface chemistries in the two cases have very strong similarities.
3. Conclusions We have shown that reflectance-difference is able to follow with high sensitivity the formation of overlayers of Ga and As during growth from triethylgallium and arsine. Detection of growth oscillations by RD during epitaxy have been used to study how surface coherency is lost during overexposure to Ga and to measure the kinetics of regeneration of As-stabilized surfaces.
Acknowledgements This work was supported by the Swedish National Board for Technical Development and the Swedish Natural Science Research Council. References [1] J.H. Neave, BA. Joyce, P.J. Dobson and N. Norton, Appi. Phys. A31 (1983) 1. [2] D.E. Aspnes, R. Bhat, E. Colas, E.T. Florez, J.P. Harbison. M.K. Kelly, V.G. Keramidas, MA. Koza and A.A. Studna, Proc. SPIE 1037 (1989) 2. [3] J. Jönsson, K. Deppert, S. Jeppesen, G. Paulsson, L. Samuelson and P. Schmidt, Appl. Phys. Letters 56 (1990) 2414. [4] G. Paulsson, K. Deppert, S. Jeppesen, J. Jonsson, L. Samuelson and P. Schmidt, J. Crystal Growth 105 (1990) 312. [5] P. Schmidt, K. Deppert, S. Jeppesen, J. Jdnsson, G. Paulsson and L. Samuelson, J. Crystal Growth 105 (1990) 306. [6] T.H. Chiu, AppI. Phys. Letters 55 (1989) 1244.