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Oscillatory As4 surface reaction rates during molecular beam epitaxy of AlAs, GaAs and InAs
Journal of Crystal Growth 111 (1991) 125—130 North-Holland
125
Oscillatory As4 surface reaction rates during molecular beam epitaxy of AlAs, GaAs and InAs J.Y. Tsao, T.M. Brennan and B.E. Hammons Sandia National Laboratories, Albuquerque, New Mexico 87185-5800, USA
We report oscillations in the surface reaction rate of As4 during molecular beam epitaxy of AlAs, GaAs and InAs. The oscillations are periodic with the bilayer-by-bilayer growth cycle, and correspond to a slightly lower reaction rate approximately 1/4 of the way, and a slightly higher reaction rate approximately 3/4 of the way, through that cycle.
Crystal growth has long been known to be mediated by surface microstructural defects such as adatoms, steps and kinks [1]. Measurements sensitive to that defect microstructure have therefore been central to advances in our understanding of crystal growth. For molecular beam epitaxy (MBE) of 111/V compounds, however, surface defect microstructure is only half the picture. In these systems, the growth species typically include stable molecules, such as As4, which must chemically react with the surface before growth can occur. Surface chemistry is therefore a crucial second half of the picture. Indeed, we expect the overall dynamics of crystal growth to be determined by a complex interplay between surface chemical reactivity and surface defect microstructure [2]. Thus far, however, very little is known about the dependence of surface chemical reactivity on surface defect microstructure during Ill/V MBE. Nearly all previous direct studies of surface chemistry during Ill/V MBE have been performed on surfaces having an “average” microstructure characteristic of steady-state growth [3,4]. In this paper, we report reflection mass spectrometry (REMS) studies of reactive sticking of As4 and desorption of As2 during MBE of AlAs, GaAs and InAs on carefully prepared, smooth (001) surfaces. We find, interestingly, that in all these systems the reaction rate of As4 with the surface oscillates initially, before settling to an 0022-0248/91/$03.50 © 1991
—
average value characteristic of steady-state growth. These “REMS” oscillations are analogous to those seen in reflection-high-energy diffraction (RHEED) intensities [5,6], photoemission [7], secondary electron emission [81 and optical reflectance difference spectroscopy (RDS) [9], in that they are periodic with the bilayer-by-bilayer growth cycle. They are unique, though, in that they establish, for the first time, a periodic variation in the chemical reactivity of the surface during MBE. Our experimental geometry is nearly the same as that described previously [10]. An unapertured UTI-100C mass spectrometer, recessed in a liquidnitrogen-cooled housing, is attached directly to an effusion-cell port of a state-of-the-art (Riber 32}~~ Ill/V MBE chamber. Due to improved liquidnitrogen transfer to the cryoshrouds of this chamber, background signals from volatile As4 not originating from the wafer surface were somewhat lower (approximately one-sixth of the line-of-sight signals) than in our previous studies. In this growth chamber, we routinely observe REMS oscillations during MBE of AlAs, GaAs and InAs on carefully smoothened vicinal surfaces of the same material. The oscillations are most intense, however, during MBE on “nearly singular” substrates. Therefore, in this paper we restrict ourselves to presenting measurements during MBE on a GaAs wafer miscut by less jhan one-twentieth of a degree from (001). We also restrict ourselves
Elsevier Science Publishers B.V. (North-Holland)
126
J. Y. Tsao et aL
/
Oscillatory As
4 surface reaction rates during MBE of AlAs, GaAs and InAs
to measurements during AlAs MBE, which is the system we have studied most systematically thus far, and in which REMS oscillations are quite reproducible. Even under such favorable conditions, of course, careful surface preparation is important: prior to these measurements, a 1 ~sm thick GaAs buffer layer was grown at 580 °C, followed by a 1 ~tm thick buffer layer of AlAs at 620°C. During buffer layer growth, and during all of the measurements described here, the surface was bathed in a flux of pure molecular As4. From previous studies [11], the As fluxes leaving such a surface are thought to consist exclusively of As4 and As2. A superposition of the cracking patterns of these two parent molecules, weighted by their relative fluxes, determines the measured mass spectrum. Hence, to deduce the fluxes of these two
parent molecules, it is necessary to measure ion currents at two mass/charge (m/q) ratios. In this study, by multiplexing the mass spectrometer at 0.025 s intervals, we measured ion currents “simultaneously” at m/q = 150 amu/e (nominally Ask), and m/q 300 amu/e (nominally As~).These ion currents can be linearly related to the fluxes leaving the surface according to: •out t
•REMS
—
j
~15O
—
m22 ~iAs2)
•REMS 13Q~
—m~~JAs
-~
, A ~OUt
m24 ~‘JAs4
-r
•out
—
The three matrix elements can be determined from the cracking pattern of known outgoing As4 fluxes, and from the responses of outgoing As4 and As2 fluxes to variations in temperature and group III fluxes during MBE [14]. For the operating condi-
AIAs/AlAs(OO1): 0.32 BL/s (eg3015) 1.2—
-
I
A ~out ,,.out ~JAS4~~JAS5
-
‘1’ I
-,
-
0.32 ML/s 0.8
~0.6
0.4
-
_____
-
-
-
-
0.32
.~~out ‘~JAs2
s/BL
-
0.2
-
0.0
I
25
30
I
35 Time (s)
I
40
45
Fig. 1. Time-resolved As4 and As2 fluxes leaying the surface during AlAs MBE at 0.32 bilayers/s. Initially, the Al shutter is closed, so that all the As4 that strikes the surface leaves, either as “reflected” As4, or as “desorbing” As2. At t = 30 s, the Al shutter is opened, As must now incorporate into the growing AlAs crystal, and both As fluxes leaving the surface decrease. Finally, at t = 45 s, the Al shutter is closed, and the As fluxes leaving the surface return to their initial steady-state values. The vertical lines indicate the 0, 2~r,4sr and 6~rpoints in the bilayer growth cycle, relative to the initiation of growth. The solid line drawn through the 4J~ data is a nonlinear-least-squares fit to eq. (3).
J. 1’. Tsao et al.
/ Oscillatory As4
surface reaction rates during MBE of AlAs, GaAs and InAs
127
ALAs/A1As(001): 0.45 BL/s (eg3OlZ) I
1.2
-
1.0
-
I
I
I
-
t
,,.
0.4
-
0.45 ML/s
-
i —s/BL 0.45
-
2
00
-
~
•out 2JAs 0.2
-
4j~+2j~
-
~0.6
I
-
-
~
25I
30I
-
35I
40I
I 45
Time (s) Fig. 2. Same as fig. 1, except at a growth rate of 0.45 bilayers/s.
tions of this study, they were found to be m22 6.1 nA s/ML, m24 1.2 nA s/ML, and m~ 1.7 nA s/ML, where 1 ML represents the number density of As (or Al) atoms on a bulk-terminated As (or Al) surface of AlAs (001). Then, by inverting eqs. 1 and 2, outgoing fluxes can be deduced from measured ion currents. Note that the ion current at m/q = 150 amu/e is especially sensitive to the As2 flux leaving the surface, while the ion current at m/q = 300 amu/e is sensitive exclusively to the As4 flux leaving the surface [12]. In figs. 1 and 2, we show two examples of outgoing As fluxes during AlAs MBE on AlAs (001) at 620°C, deduced from REMS measurements and eqs. (1) and (2). For both figures the As4 flux incident on the surface in units of equivalent 1.1 flux ML/s (~6.9 on x 1014 2 As s ~atoms )~Forwas fig.4j~4 1 the Al incident the cm surface was 0.32 ML/s (~2.0 x 1014 cm2 s~)for fig. 2 it was jj 0.45 ML/s (~2.8 x 1014 cm2 s~). The sequence of events in both figures is as follows. Initially, for t ~ 30 s, the Al shutter is
closed. In the absence of growth, all the As4 that strikes the surface ultimately leaves. Not all the As leaving the surface, though, leaves as As4. At this relatively high temperature, approximately 36% of the incident As4 cracks and ultimately leaves the surface as As2. At t = 30 s, the Al shutter is opened, AlAs MBE is initiated, and both the reflecting As4 and desorbing As2 fluxes decrease. The total As flux leaving the surface decreases by 0.32 ML/s in fig. 1 and by 0.45 ML/s in fig. 2, so as to consume exactly the Al fluxes incident on the surface. Finally, at t =45 s, the Al shutter is closed, and all the outgoing As fluxes recover to their initial steady-state values. Note that in neither figure is a “tail” observed in those recoveries. The absence of such a been tail indicates that the by incident Allower had already entirely consumed As. For V/Ill flux ratios, not all the incident Al need be so consumed, and pronounced tails are observed in the recovery of the outgoing As fluxes [14]. In this study, we deliberately avoided V/Ill flux ratios (defined as f~4/J~)lower than 0.53, both
128
J. 1’. Tsao et aL
/
Oscillatory As
4 surface reaction rates during MBE of AlAs, GaAs and InAs
to avoid such tails, as well as to avoid deviations from the usual As-stabilized 2 x 4 surface reconstruction during growth. The overall features seen in figs. 1 and 2 can be qualitatively understood on the basis of a balance between (a) cracking of incident As4 followed by chemisorption of As, which increases the As coverage, and (b) desorption of As2 and chemisorption of Al, which decrease the As coverage [13]. Turning on the Al flux decreases the As coverage, and thereby increases the (presumably Al-coverage-dependent) cracking of As4 and decreases the (also presumably As-coverage-dependent) desorption of As2. These overall features are always present, even on vicinal surfaces, or on nearly-singular surfaces imperfectly prepared. They therefore probably represent some sort of average behavior of a quasi-steady-state distribution of surface defects. However, on well-prepared, nearly-singular surfaces, we see, superimposed on this average behavbr, oscillations in the As4 flux leaving the surface. Qualitatively, these “REMS oscillations” are most intense under approximately the same conditions that RHEED oscillations are most intense (for AlAs, in the temperature range 590—640°C).They are, however, somewhat more sensitive than are RHEED oscillations to the smoothness of the starting surface. We have found, e.g., that oscillations during AlAs MBE can only be reproduced consistently at 620°C if the surface is first smoothened by a growth interruption greater than 30 s. At lower growth temperatures, either longer interruptions or higher temperature annealing is required. To quantify the oscillations in figs. 1 and 2, we have performed nonlinear least-squares fits (shown as solid curves through the decaying portions of the outgoing As4 fluxes) to the empirical form, f(t)
=
A
+
B
e1(tt~
xcos[2~Tv(t
—
+ t0)
C —
e2(tto)
(3)
where t0 29.92 s is the time at which the Al shutter was opened. The first two terms correspond to the non-oscillatory part of the overall decay of the outgoing As4 flux. The third term corresponds to an exponentially damped sinusoi-
a I
I
I
—
-(5 2 —
0 I
b ~
300
I
-
-
~ ~ 200 ~ ioo
—
—
—
I
0
1
2
3
—
—
-
I
I
4
5
Bilayer Growth Time (s) Fig. 3. REMS oscillation periods 1/v (a) and phase offsets 4~ (b) deduced from fits to time-resolved reflected As4 fluxes such as those shown in figs. 1 and 2, as a function of bilayer growth periods known from thickness and RHEED oscillation calibrations. The dashed lines represent equivalence between the two periods (a) and 1130 (b).
dal oscillation wth frequency v and phase offset ~.
The best-fit oscillation periods, 1/i’, for measurements at various growth rates are shown in fig. 3a. As can be seen, they agree quite well with growth rates deduced from the periodicity of RHEED oscillations. They also agree, from inspection of figs. 1 and 2, with the decrease in the total As flux leaving the surface upon opening the Al shutter. Therefore, from a technological point of view, REMS oscillations and As REMS “deficits” are both potentially useful alternatives to RHEED oscillations for real-time measurement of growth rate. It is evident from figs. 1 and 2, however, that the REMS oscillations are relatively weak. They damp quickly (on a time scale comparable to the oscillation periods themselves), and their initial amplitudes are small (roughly 5% of the total As4 flux incident on the surface). It is therefore not implausible that they be caused directly by step density oscillations during growth, since the am-
J. Y. Tsao et aL
/ Oscillatory
As
4 surface reaction rates during MBE of AlAs, GaAs and InAs
plitude of such oscillations might be expected to be of that same order of magnitude [15]. Interestingly, over the range of growth conditions studied here, the phase offset of the oscillations relative to the initiation of growth is ~ 113°, as shown in fig. 3b. In other words, As4 reacts with the surface least rapidly approximately 1/4 of the way through the bilayer growth cycle, and most rapidly approximately 3/4 of the way through the bilayer cycle. Although there is currently some uncertainty as to the interpretation of phase offsets observed in RHEED oscillations, one anticipates that at the 1/4 bilayer point the creation rate of steps might be highest, and that at the 3/4 bilayer point the annihilation rate of steps might be highest [16]. If so, then the creation of new steps would seem, counterintuitively, to be associated with a decrease in surface As, while the annihilation of steps would seem to be associated with an increase in surface As. Finally, it is interesting to note that, although the reflected As4 flux oscillates measurably, the desorbing As2 flux does not. Partly, this may be because at these growth temperatures the As2 desorption fluxes are much lower than As4 reflected fluxes; superimposed oscillations might be too weak to observe with our present signal-to-noise ratio. However, it also may be that As2 desorbs only slightly differently from steps as from terraces In summary, we have observed oscillations in As4 incorporation rates during molecular beam epitaxy of AlAs, GaAs and InAs. The oscillations are periodic with the bilayer-by-bilayer growth cycle, and suggest that AlAs GaAs and InAs surface chemistry depend on surface microstructure. It will be exciting to explore and understand the origin of that dependence. In particular, it will be interesting to understand the atomic structure of steps on AlAs, GaAs and InAs (001) at a level of detail comparable to our current understanding of the terrace reconstructions [17,18]. It might then be possible to apply electron counting arguments [19} to deduce likely sequences of intermediate structures during incorporation at steps [20]. Such electron countmg arguments have already been used to deduce likely intermediate structures for “homogeneous” incor-
129
poration of Ga and As species on terraces [21]. Indeed, the measurements reported here appear to be consistent with such a mechanism. However, experimental evidence appears to favor the dominance, at least for some growth conditions, of mechanisms for “hetergeneous” incorporation at defects such as steps [22,23]. Perhaps both mechanisms are important, but under different growth conditions. Ultimately, it will of course be interesting to finally combine a microscopic understanding of microstructure-dependent elementary attachment mechanisms with rate-equation models [24] or Monte Carlo simulations [25] of the large-scale evolution of Ill/V surface microstructure, and to eventually predict macroscopic measurements such as those reported here. We would like to acknowledge helpful conversations with Jack Houston and Paul Peercy. This work, performed at Sandia National Laboratories, was supported by the United States Department of Energy under Contract No. DEACO4-76DP00789. References [1] W..K. Burton, N. Cabrera and F.C. Frank, Phil. Trans. Roy. Soc. London A243 (1951) 299. [2] A. Madhukar and S.V. Ghaisas, CRC Critical Rev. Solid State Mater. Sci. 14 (1988) 1. [3] See, e.g., C.T. Foxon and B.A. Joyce, in: Current Topics in Materials Science, Vol. 7, Ed. E. Kaldis (North-Holland, Amsterdam, 1981) ch. 1. [4] J.Y. Tsao, T.M. Brennan and B.E. Hammons, Appl. Phys. Letters 53 (1988) 288. [5] J.J. Harris, B.A. Joyce and P.J. Dobson, Surface Sci. 103 (1981) L90. [6] C.E.C. Wood, Surface Sci. 108 (1981) L441. [7] J.N. Eckstein, C. Webb, S.-L. Weng and K.A. Bertness, Appi. Phys. Letters 51 (1987) 1833. [8] L.P. Erickson, M.D. Longerbone, R.C. Youngman and BE. Dies, J. Crystal Growth 81 (1987) 55. [9] J.P. Harbison, D.E. Aspnes, A.A. Studna, L.T. Florez and M.K. Kelly, Appl. Phys. Letters 52 (1988) 2046. [10] J.Y. Tsao, T.M. Brennan, J.F. Kkm and B.E. Hammons, Appi. Phys. Letters 55 (1989) 777. [11] C.T. Foxon, J.A. Harvey and B.A. Joyce, J. Phys. Chem. Solids 34 (1973) 1693. [12] We neglect possible dimer recombination in the mass spectrometer itself.
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Oscillatory As
4 surface reaction rates during MBE of AlAs, GaAs and InAs
[13] J.Y. Tsao, T.M. Brennan, J.F. Kiem and B.E. Hammons, J. Vacuum Sci. Technol. A7 (1989) 2138. [14] T.M. Brennan, J.Y. Tsao and B.E. Hammons, in preparation. [15] C.S. Lent and P.1. Cohen, Phys. Rev. B33 (1986) 8329. [16] S. Clarke and D.D. Vvedensky, J. Appl. Phys. 63 (1988) 2272. [17] D.J. Chadi, J. Vacuum Sd. Technol. A5 (1987) 834. [18] D.K. Biegelsen, R.D. Bringans, J.E. Northrup and L.-E. Swartz, Phys. Rev. B41 (1990) 5701. [19] J.A. Appelbaum, G.A. Baraff and D.R. Hamann, Phys. Rev. 14 (1976) 1623.
[20] E. Kaxiras, O.L. Alerhand, J.D. Joannopoulos and G.W. Turner, Phys. Rev. Letters 62 (1989) 2484. (21] H.H. Farrell, J.P. Harbison and L.D. Peterson, J. Vacuum Sci. Technol. B5 (1987) 1482. [22] P.M. Petroff, A.C. Gossard and W. Wiegmann, Appl. Phys. Letters 45 (1984) 620. [23] J.H. Neave, PJ. Dobson, B.A. Joyce and J. Zhang, Appl. Phys. Letters 47 (1985) 100. [24] T. Shitara and T. Nishinaga, Japan. J. Appl. Phys. 28 (1989) 1212. [25] M. Thomsen and A. Madhukar, J. Crystal Growth 80 (1987) 275.
125
Oscillatory As4 surface reaction rates during molecular beam epitaxy of AlAs, GaAs and InAs J.Y. Tsao, T.M. Brennan and B.E. Hammons Sandia National Laboratories, Albuquerque, New Mexico 87185-5800, USA
We report oscillations in the surface reaction rate of As4 during molecular beam epitaxy of AlAs, GaAs and InAs. The oscillations are periodic with the bilayer-by-bilayer growth cycle, and correspond to a slightly lower reaction rate approximately 1/4 of the way, and a slightly higher reaction rate approximately 3/4 of the way, through that cycle.
Crystal growth has long been known to be mediated by surface microstructural defects such as adatoms, steps and kinks [1]. Measurements sensitive to that defect microstructure have therefore been central to advances in our understanding of crystal growth. For molecular beam epitaxy (MBE) of 111/V compounds, however, surface defect microstructure is only half the picture. In these systems, the growth species typically include stable molecules, such as As4, which must chemically react with the surface before growth can occur. Surface chemistry is therefore a crucial second half of the picture. Indeed, we expect the overall dynamics of crystal growth to be determined by a complex interplay between surface chemical reactivity and surface defect microstructure [2]. Thus far, however, very little is known about the dependence of surface chemical reactivity on surface defect microstructure during Ill/V MBE. Nearly all previous direct studies of surface chemistry during Ill/V MBE have been performed on surfaces having an “average” microstructure characteristic of steady-state growth [3,4]. In this paper, we report reflection mass spectrometry (REMS) studies of reactive sticking of As4 and desorption of As2 during MBE of AlAs, GaAs and InAs on carefully prepared, smooth (001) surfaces. We find, interestingly, that in all these systems the reaction rate of As4 with the surface oscillates initially, before settling to an 0022-0248/91/$03.50 © 1991
—
average value characteristic of steady-state growth. These “REMS” oscillations are analogous to those seen in reflection-high-energy diffraction (RHEED) intensities [5,6], photoemission [7], secondary electron emission [81 and optical reflectance difference spectroscopy (RDS) [9], in that they are periodic with the bilayer-by-bilayer growth cycle. They are unique, though, in that they establish, for the first time, a periodic variation in the chemical reactivity of the surface during MBE. Our experimental geometry is nearly the same as that described previously [10]. An unapertured UTI-100C mass spectrometer, recessed in a liquidnitrogen-cooled housing, is attached directly to an effusion-cell port of a state-of-the-art (Riber 32}~~ Ill/V MBE chamber. Due to improved liquidnitrogen transfer to the cryoshrouds of this chamber, background signals from volatile As4 not originating from the wafer surface were somewhat lower (approximately one-sixth of the line-of-sight signals) than in our previous studies. In this growth chamber, we routinely observe REMS oscillations during MBE of AlAs, GaAs and InAs on carefully smoothened vicinal surfaces of the same material. The oscillations are most intense, however, during MBE on “nearly singular” substrates. Therefore, in this paper we restrict ourselves to presenting measurements during MBE on a GaAs wafer miscut by less jhan one-twentieth of a degree from (001). We also restrict ourselves
Elsevier Science Publishers B.V. (North-Holland)
126
J. Y. Tsao et aL
/
Oscillatory As
4 surface reaction rates during MBE of AlAs, GaAs and InAs
to measurements during AlAs MBE, which is the system we have studied most systematically thus far, and in which REMS oscillations are quite reproducible. Even under such favorable conditions, of course, careful surface preparation is important: prior to these measurements, a 1 ~sm thick GaAs buffer layer was grown at 580 °C, followed by a 1 ~tm thick buffer layer of AlAs at 620°C. During buffer layer growth, and during all of the measurements described here, the surface was bathed in a flux of pure molecular As4. From previous studies [11], the As fluxes leaving such a surface are thought to consist exclusively of As4 and As2. A superposition of the cracking patterns of these two parent molecules, weighted by their relative fluxes, determines the measured mass spectrum. Hence, to deduce the fluxes of these two
parent molecules, it is necessary to measure ion currents at two mass/charge (m/q) ratios. In this study, by multiplexing the mass spectrometer at 0.025 s intervals, we measured ion currents “simultaneously” at m/q = 150 amu/e (nominally Ask), and m/q 300 amu/e (nominally As~).These ion currents can be linearly related to the fluxes leaving the surface according to: •out t
•REMS
—
j
~15O
—
m22 ~iAs2)
•REMS 13Q~
—m~~JAs
-~
, A ~OUt
m24 ~‘JAs4
-r
•out
—
The three matrix elements can be determined from the cracking pattern of known outgoing As4 fluxes, and from the responses of outgoing As4 and As2 fluxes to variations in temperature and group III fluxes during MBE [14]. For the operating condi-
AIAs/AlAs(OO1): 0.32 BL/s (eg3015) 1.2—
-
I
A ~out ,,.out ~JAS4~~JAS5
-
‘1’ I
-,
-
0.32 ML/s 0.8
~0.6
0.4
-
_____
-
-
-
-
0.32
.~~out ‘~JAs2
s/BL
-
0.2
-
0.0
I
25
30
I
35 Time (s)
I
40
45
Fig. 1. Time-resolved As4 and As2 fluxes leaying the surface during AlAs MBE at 0.32 bilayers/s. Initially, the Al shutter is closed, so that all the As4 that strikes the surface leaves, either as “reflected” As4, or as “desorbing” As2. At t = 30 s, the Al shutter is opened, As must now incorporate into the growing AlAs crystal, and both As fluxes leaving the surface decrease. Finally, at t = 45 s, the Al shutter is closed, and the As fluxes leaving the surface return to their initial steady-state values. The vertical lines indicate the 0, 2~r,4sr and 6~rpoints in the bilayer growth cycle, relative to the initiation of growth. The solid line drawn through the 4J~ data is a nonlinear-least-squares fit to eq. (3).
J. 1’. Tsao et al.
/ Oscillatory As4
surface reaction rates during MBE of AlAs, GaAs and InAs
127
ALAs/A1As(001): 0.45 BL/s (eg3OlZ) I
1.2
-
1.0
-
I
I
I
-
t
,,.
0.4
-
0.45 ML/s
-
i —s/BL 0.45
-
2
00
-
~
•out 2JAs 0.2
-
4j~+2j~
-
~0.6
I
-
-
~
25I
30I
-
35I
40I
I 45
Time (s) Fig. 2. Same as fig. 1, except at a growth rate of 0.45 bilayers/s.
tions of this study, they were found to be m22 6.1 nA s/ML, m24 1.2 nA s/ML, and m~ 1.7 nA s/ML, where 1 ML represents the number density of As (or Al) atoms on a bulk-terminated As (or Al) surface of AlAs (001). Then, by inverting eqs. 1 and 2, outgoing fluxes can be deduced from measured ion currents. Note that the ion current at m/q = 150 amu/e is especially sensitive to the As2 flux leaving the surface, while the ion current at m/q = 300 amu/e is sensitive exclusively to the As4 flux leaving the surface [12]. In figs. 1 and 2, we show two examples of outgoing As fluxes during AlAs MBE on AlAs (001) at 620°C, deduced from REMS measurements and eqs. (1) and (2). For both figures the As4 flux incident on the surface in units of equivalent 1.1 flux ML/s (~6.9 on x 1014 2 As s ~atoms )~Forwas fig.4j~4 1 the Al incident the cm surface was 0.32 ML/s (~2.0 x 1014 cm2 s~)for fig. 2 it was jj 0.45 ML/s (~2.8 x 1014 cm2 s~). The sequence of events in both figures is as follows. Initially, for t ~ 30 s, the Al shutter is
closed. In the absence of growth, all the As4 that strikes the surface ultimately leaves. Not all the As leaving the surface, though, leaves as As4. At this relatively high temperature, approximately 36% of the incident As4 cracks and ultimately leaves the surface as As2. At t = 30 s, the Al shutter is opened, AlAs MBE is initiated, and both the reflecting As4 and desorbing As2 fluxes decrease. The total As flux leaving the surface decreases by 0.32 ML/s in fig. 1 and by 0.45 ML/s in fig. 2, so as to consume exactly the Al fluxes incident on the surface. Finally, at t =45 s, the Al shutter is closed, and all the outgoing As fluxes recover to their initial steady-state values. Note that in neither figure is a “tail” observed in those recoveries. The absence of such a been tail indicates that the by incident Allower had already entirely consumed As. For V/Ill flux ratios, not all the incident Al need be so consumed, and pronounced tails are observed in the recovery of the outgoing As fluxes [14]. In this study, we deliberately avoided V/Ill flux ratios (defined as f~4/J~)lower than 0.53, both
128
J. 1’. Tsao et aL
/
Oscillatory As
4 surface reaction rates during MBE of AlAs, GaAs and InAs
to avoid such tails, as well as to avoid deviations from the usual As-stabilized 2 x 4 surface reconstruction during growth. The overall features seen in figs. 1 and 2 can be qualitatively understood on the basis of a balance between (a) cracking of incident As4 followed by chemisorption of As, which increases the As coverage, and (b) desorption of As2 and chemisorption of Al, which decrease the As coverage [13]. Turning on the Al flux decreases the As coverage, and thereby increases the (presumably Al-coverage-dependent) cracking of As4 and decreases the (also presumably As-coverage-dependent) desorption of As2. These overall features are always present, even on vicinal surfaces, or on nearly-singular surfaces imperfectly prepared. They therefore probably represent some sort of average behavior of a quasi-steady-state distribution of surface defects. However, on well-prepared, nearly-singular surfaces, we see, superimposed on this average behavbr, oscillations in the As4 flux leaving the surface. Qualitatively, these “REMS oscillations” are most intense under approximately the same conditions that RHEED oscillations are most intense (for AlAs, in the temperature range 590—640°C).They are, however, somewhat more sensitive than are RHEED oscillations to the smoothness of the starting surface. We have found, e.g., that oscillations during AlAs MBE can only be reproduced consistently at 620°C if the surface is first smoothened by a growth interruption greater than 30 s. At lower growth temperatures, either longer interruptions or higher temperature annealing is required. To quantify the oscillations in figs. 1 and 2, we have performed nonlinear least-squares fits (shown as solid curves through the decaying portions of the outgoing As4 fluxes) to the empirical form, f(t)
=
A
+
B
e1(tt~
xcos[2~Tv(t
—
+ t0)
C —
e2(tto)
(3)
where t0 29.92 s is the time at which the Al shutter was opened. The first two terms correspond to the non-oscillatory part of the overall decay of the outgoing As4 flux. The third term corresponds to an exponentially damped sinusoi-
a I
I
I
—
-(5 2 —
0 I
b ~
300
I
-
-
~ ~ 200 ~ ioo
—
—
—
I
0
1
2
3
—
—
-
I
I
4
5
Bilayer Growth Time (s) Fig. 3. REMS oscillation periods 1/v (a) and phase offsets 4~ (b) deduced from fits to time-resolved reflected As4 fluxes such as those shown in figs. 1 and 2, as a function of bilayer growth periods known from thickness and RHEED oscillation calibrations. The dashed lines represent equivalence between the two periods (a) and 1130 (b).
dal oscillation wth frequency v and phase offset ~.
The best-fit oscillation periods, 1/i’, for measurements at various growth rates are shown in fig. 3a. As can be seen, they agree quite well with growth rates deduced from the periodicity of RHEED oscillations. They also agree, from inspection of figs. 1 and 2, with the decrease in the total As flux leaving the surface upon opening the Al shutter. Therefore, from a technological point of view, REMS oscillations and As REMS “deficits” are both potentially useful alternatives to RHEED oscillations for real-time measurement of growth rate. It is evident from figs. 1 and 2, however, that the REMS oscillations are relatively weak. They damp quickly (on a time scale comparable to the oscillation periods themselves), and their initial amplitudes are small (roughly 5% of the total As4 flux incident on the surface). It is therefore not implausible that they be caused directly by step density oscillations during growth, since the am-
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plitude of such oscillations might be expected to be of that same order of magnitude [15]. Interestingly, over the range of growth conditions studied here, the phase offset of the oscillations relative to the initiation of growth is ~ 113°, as shown in fig. 3b. In other words, As4 reacts with the surface least rapidly approximately 1/4 of the way through the bilayer growth cycle, and most rapidly approximately 3/4 of the way through the bilayer cycle. Although there is currently some uncertainty as to the interpretation of phase offsets observed in RHEED oscillations, one anticipates that at the 1/4 bilayer point the creation rate of steps might be highest, and that at the 3/4 bilayer point the annihilation rate of steps might be highest [16]. If so, then the creation of new steps would seem, counterintuitively, to be associated with a decrease in surface As, while the annihilation of steps would seem to be associated with an increase in surface As. Finally, it is interesting to note that, although the reflected As4 flux oscillates measurably, the desorbing As2 flux does not. Partly, this may be because at these growth temperatures the As2 desorption fluxes are much lower than As4 reflected fluxes; superimposed oscillations might be too weak to observe with our present signal-to-noise ratio. However, it also may be that As2 desorbs only slightly differently from steps as from terraces In summary, we have observed oscillations in As4 incorporation rates during molecular beam epitaxy of AlAs, GaAs and InAs. The oscillations are periodic with the bilayer-by-bilayer growth cycle, and suggest that AlAs GaAs and InAs surface chemistry depend on surface microstructure. It will be exciting to explore and understand the origin of that dependence. In particular, it will be interesting to understand the atomic structure of steps on AlAs, GaAs and InAs (001) at a level of detail comparable to our current understanding of the terrace reconstructions [17,18]. It might then be possible to apply electron counting arguments [19} to deduce likely sequences of intermediate structures during incorporation at steps [20]. Such electron countmg arguments have already been used to deduce likely intermediate structures for “homogeneous” incor-
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poration of Ga and As species on terraces [21]. Indeed, the measurements reported here appear to be consistent with such a mechanism. However, experimental evidence appears to favor the dominance, at least for some growth conditions, of mechanisms for “hetergeneous” incorporation at defects such as steps [22,23]. Perhaps both mechanisms are important, but under different growth conditions. Ultimately, it will of course be interesting to finally combine a microscopic understanding of microstructure-dependent elementary attachment mechanisms with rate-equation models [24] or Monte Carlo simulations [25] of the large-scale evolution of Ill/V surface microstructure, and to eventually predict macroscopic measurements such as those reported here. We would like to acknowledge helpful conversations with Jack Houston and Paul Peercy. This work, performed at Sandia National Laboratories, was supported by the United States Department of Energy under Contract No. DEACO4-76DP00789. References [1] W..K. Burton, N. Cabrera and F.C. Frank, Phil. Trans. Roy. Soc. London A243 (1951) 299. [2] A. Madhukar and S.V. Ghaisas, CRC Critical Rev. Solid State Mater. Sci. 14 (1988) 1. [3] See, e.g., C.T. Foxon and B.A. Joyce, in: Current Topics in Materials Science, Vol. 7, Ed. E. Kaldis (North-Holland, Amsterdam, 1981) ch. 1. [4] J.Y. Tsao, T.M. Brennan and B.E. Hammons, Appl. Phys. Letters 53 (1988) 288. [5] J.J. Harris, B.A. Joyce and P.J. Dobson, Surface Sci. 103 (1981) L90. [6] C.E.C. Wood, Surface Sci. 108 (1981) L441. [7] J.N. Eckstein, C. Webb, S.-L. Weng and K.A. Bertness, Appi. Phys. Letters 51 (1987) 1833. [8] L.P. Erickson, M.D. Longerbone, R.C. Youngman and BE. Dies, J. Crystal Growth 81 (1987) 55. [9] J.P. Harbison, D.E. Aspnes, A.A. Studna, L.T. Florez and M.K. Kelly, Appl. Phys. Letters 52 (1988) 2046. [10] J.Y. Tsao, T.M. Brennan, J.F. Kkm and B.E. Hammons, Appi. Phys. Letters 55 (1989) 777. [11] C.T. Foxon, J.A. Harvey and B.A. Joyce, J. Phys. Chem. Solids 34 (1973) 1693. [12] We neglect possible dimer recombination in the mass spectrometer itself.
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[13] J.Y. Tsao, T.M. Brennan, J.F. Kiem and B.E. Hammons, J. Vacuum Sci. Technol. A7 (1989) 2138. [14] T.M. Brennan, J.Y. Tsao and B.E. Hammons, in preparation. [15] C.S. Lent and P.1. Cohen, Phys. Rev. B33 (1986) 8329. [16] S. Clarke and D.D. Vvedensky, J. Appl. Phys. 63 (1988) 2272. [17] D.J. Chadi, J. Vacuum Sd. Technol. A5 (1987) 834. [18] D.K. Biegelsen, R.D. Bringans, J.E. Northrup and L.-E. Swartz, Phys. Rev. B41 (1990) 5701. [19] J.A. Appelbaum, G.A. Baraff and D.R. Hamann, Phys. Rev. 14 (1976) 1623.
[20] E. Kaxiras, O.L. Alerhand, J.D. Joannopoulos and G.W. Turner, Phys. Rev. Letters 62 (1989) 2484. (21] H.H. Farrell, J.P. Harbison and L.D. Peterson, J. Vacuum Sci. Technol. B5 (1987) 1482. [22] P.M. Petroff, A.C. Gossard and W. Wiegmann, Appl. Phys. Letters 45 (1984) 620. [23] J.H. Neave, PJ. Dobson, B.A. Joyce and J. Zhang, Appl. Phys. Letters 47 (1985) 100. [24] T. Shitara and T. Nishinaga, Japan. J. Appl. Phys. 28 (1989) 1212. [25] M. Thomsen and A. Madhukar, J. Crystal Growth 80 (1987) 275.