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Surface processes in metalorganic molecular beam epitaxial growth of GaAs
576
Journal of Crystal Growth 110 (1991) 576—586 North-Holland
Surface processes in metalorganic molecular beam epitaxial growth of GaAs M. Uneta
“,
Y. Watanabe and Y. Ohmachi
*
NTT Applied Electronics Laboratories, 3-9-11, Midori-cho, Mu,sashino-shi, Tokyo 180, Japan
Received 6 March 1990; manuscript received in final form 10 August 1990
Surface processes in metalorganic molecular beam epitaxial (MOMBE) growth of GaAs. which uses triethylgallium (TEGa) and arsenic (As 4), are investigated from the results of the substrate temperature, TEGa flow rate and As pressure dependences of the growth rate. Surface reactions on the Ga- and As-terminated surfaces are also independently studied by measuring the growth rate in the TEGa—As4 alternating supply mode. Results are qualitatively explained by considering the TEGa adsorption, TEGa decomposition and TEGa desorption processes.
1. Introduction There has been considerable interest in the growth of 111-V epitaxial layers by metalorganic molecular beam epitaxy (MOMBE). MOMBE uses metalorganic compounds as source materials in high-vacuum systems. Since little reaction in the gas phase is expected in a high vacuum, MOMBE growth is mainly determined by the chemical reactions of source materials on the heated substrate surface. However, MOMBE growth introduces much more complex surface processes during growth than molecular beam epitaxy (MBE), which uses atomic beams of group-Ill atoms. MBE growth rate is determined by the quantity of supplied group-Ill atoms, because almost all of them incorporate epitaxially into the lattice under typical conditions. Conversely. MOMBE growth rate depends on the relative ratio of the desorption and decomposition of the adsorbed alkyl-gallium. These reactions, namely the adsorption. decomposition and desorption of alkyl-gallium. are sensitive to the chemical nature of the surface. These *
Present address: NTT Opto-electronics Laboratories, 3-1. Morinosato Wakamiya. Atsugi-shi, Kanagawa Pref. 243-01. Japan.
0022-0248/91/$03.50 ~ 1991
—
issues make the MOMBE growth rate strongly dependent on the growth conditions. Recently, Robertson et al. have proposed a growth model of chemical beam epitaxy (CBE) [I.]. This model assumes sequential cleavage of alkyl groups in the decomposition reaction of TEGa molecules into atomic Ga. It well explains the substrate temperature dependence of the growth rate. However, the As-pressure dependence of the growth rate [2] and the different incorporation rate of TEGa on the Ga- and As-terminated surfaces [3] cannot be deduced from this model. Furthermore, deviations from the experimental resuits clearly exist at low growth temperatures in this model. This is because TEGa decomposition is independently considered, and effects of As atoms or the surface condition on the TEGa decomposition are neglected in this model. This paper systematically investigates surface processes in MOMBE GaAs growth which uses triethylgalhum (TEGa) and arsenic (As4). These investigations are based on the results of the substrate temperature. TEGa flow rate and As pressure dependences of the growth rate. It also investigates surface reactions of TEGa on the Ga- and As-terminated surfaces from the growth rates in the TEGa—As4 alternating supply mode.
Elsevier Science Publishers B.V. (North-Holland)
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2. Experimental The MOMBE system used here consisted of an exchange chamber, a preparation chamber and a growth chamber. The base pressures in the preparation and growth chambers were in the low-mid 10— 10 Torr range. TEGa and solid arsenic were used as sources of Ga and As, respectively. TEGa vapor was directly introduced into the growth chamber without a carrier gas. The flow rate of TEGa vapor was precisely controlled with a mass-flow controller. The arsenic vapor (As 4) was evaporated from a conventional effusion cell. The partial pressure was monitored by a nude ion gauge placed adjacent to the substrate holder. Semi-insulating GaAs substrates exactly oriented (001) were used. The substrate temperature was measured with an optical pyrometer. The thickness of the grown layer was measured with a stylus profiler. Reflection high energy electron diffraction (RHEED) observations were made using a 25 keV electron beam. The growth rate in the TEGa-As4 alternating supply mode was measured from the growth thickness, and that in the conventional growth mode was measured from the period of the RHEED intensity oscillation. It was confirmed that the growth rate was not enhanced by the irradiation of the electron beam by comparing the growth thicknesses in the irradiated and nonirradiated areas.
700
600 TEGo
~ -~
500 400 (C) 4 Torr As4 2.0x10
4 3
7~’ ‘~‘
/
2 1
1
5
~o.4 0.3 02
0.1
~
13
14
15
16
-t
10 /Tsub (K Fig. 1. MOMBE growth rates as a function of substrate ternperature [3].
in two ways. Between 560 and 680°C(region III), the growth rate gradually decreases, whereas above 680°C (region IV), it falls rapidly. Region III is characteristic of GaAs MOMBE, and this gradual decrease is caused by the desorption of adsorbed TEGa (adTEGa) [3,6]. The rapid fall-off above 680°C (region IV) is also observed in the solid source MBE growth of GaAs [7,8]. This fall-off is due to the desorption of Ga atoms.
3. Results
3.2. TEGa flow rate dependence
3.1. Substrate temperature dependence
Fig. 2 shows the TEGa flow rate dependence of the growth rate at different growth temperatures. At high temperatures above 466°C(regions II, III and IV), the growth rate increases linearly with the TEGa flow rate, indicating no saturation of. the surface in these regions. It also suggests that the limiting processes of the growth rate (the adTEGa desorption (region III) and the atomic Ga desorption (region IV)) are first-order reactions. Region II is the TEGa transport limiting region. On the other hand, the growth rate tends to saturate as the TEGa flow rate increases at low temperatures (region I). This tendency becomes
The MOMBE growth rate has a strong substrate temperature dependence, as reported previously [3—6],The relationship between the GaAs growth rate and the substrate temperature is shown in fig. 1. There are four regions of substrate ternperature dependence of growth rate. Below 450°C (region I in fig. 1), the growth rate decreases as the substrate temperature decreases. In this region, the growth rate is limited by the decomposition of TEGa molecules adsorbed on the surface. Between 450 and 560°C (region II), the growth rate is constant. Above 560°C,the growth rate decreases
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As~2.0x10’ Torr
2
—
calculated curve
E LU
2
--~ - --
_
1
—416C
,~~—4~o•c
1
___________
2
TEGa FLOW RATE (sccm) Fig. 2. MOMBE growth rates as a function of TEGa flow rate,
stronger at lower temperatures. This phenomenon is well explained by a model that considers a limit in the surface coverage of adTEGa on the Asterminated surface [9]. As the substrate temperature decreases, it is expected that the adTEGa coverage on the surface increases, because of the decreasing decomposition rate of adTEGa. As this coverage increases, the number of sites available for TEGa adsorption decreases due to the limited adTEGa coverage. This causes the saturation of the growth rate with the TEGa flow rate. This model well reproduces the TEGa flow rate dependence of the growth rate, as shown in fig. 2 (solid curve) (cf. appendix). In this model, the limited adTEGa surface coverage is assumed to be the same as the surface site number of Ga atom on the GaAs(100) surface. This means that the numher of TEGa adsorption sites equals that of Ga surface sites. Also, this limited coverage suggests that no TEGa adsorption occurs at the TEGa-adsorbed sites on the As-terminated surface. By fitting the experimental results to this model, the activation energy of the adTEGa decomposition is obtained as 51 kcal/rnol.
occur above 560°C as shown in fig. 1, little adTEGa desorption is expected at these temperatures. Thus, this phenomenon probably results from the suppression of the TEGa adsorption or the adTEGa decomposition by the excessive As. At 550°C,the adTEGa decomposition rate is high enough relative to the TEGa supply rate, thus the growth rate at this temperature is insensitive to a change ,in the adTEGa decomposition rate. Therefore, this phenomenon is considered to result from .
.
than suppression the from that of the the TEGa adTEGa adsorption decomposition. rather Furthermore, it has been reported that adTEGa decomposition is mainly a thermal process [4,10], suggesting that the adTEGa decomposition rate is not affected by the presence of excessive As. This also supports the above~consideration. Further investigation of the effect of excessive As is described in section 3.5. Unlike the result obtained in the low-temperature region, the growth rate at 650°C decreases with decreasing As pressure (region III). At this temperature, the growth rate is limited by the adTEGa desorption. Thus, this result shows that the adTEGa desorption is enhanced by decreasing As pressure. Since As atoms desorb rapidly from the surface at this temperature, the As surface coverage becomes dependent on the As pressure. Actually, the As-stabilized (2 x 4) RHEED reconstruction becomes unclear as the As pressure de-
TEGO 119
E
1.0
~
0.5
650.C
~37~C
3.3. As-pressure dependence
Fig. 3 shows the As pressure dependences of the growth rate at different substrate temperatures. At 379°C(region I) and 550°C(region II), the growth rate decreases as the As pressure increases. Since the adTEGa desorption begins to
_________________
0
5 10 15 A~ FLUX (x10°Torr) Fig. 3. MOMBE growth rates as a function of As 4 flux. TEGa flow rate is 0.9 SCCM.
M. Uneta et a!.
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[1 1 01
/ Surface
terns
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processes in MOMBE growth of GaAs
‘“3.i
__________ ‘a’ ~
p
‘ ~ Fig. 4. RHEED patterns growth at 630°Cwith 4 during Torr and (b) 3.5x105 Torr. As4 flux of (a) 2.0x10
creases, as shown in fig. 4, indicating a decrease in the As surface coverage. Therefore, this enhancement of the adTEGa desorption is considered to result from decreasing As surface coverage. This suggests that the adTEGa desorption is stronger on the Ga-terminated surface than on the Asterminated surface. The higher adTEGa desorption rate on the Ga-terminated surface can also explain the fact that the adTEGa desorption was enhanced or suppressed by the presence of In or Al atoms, respectively [6], because the As surface coverage is expected to be lower for InGaAs and higher for AlGaAs in accordance with 111—As binding energy [11].
to appear alternately. Fig. 5 showssupply RHEED obtained in the alternating modepatat 550°C. The RHEED pattern changed from (2 X 4) to (4 X 2) when TEGa molecules were supplied, and changed back to (2 x 4) when As4 molecules were resupplied. This verifies that the Ga- and As-terminated surfaces appear alternately. Furthermore, the sharp (4 x 2) reconstruction during TEGa supply indicates that the first Ga layer is formed two-dimensionally on the As-terminated surface. This means that TEGa molecules supplied before the first Ga layer formation adsorb and decompose on the As-terminated surface, and excess TEGa supplied after this adsorbs and decompose on the Ga-terminated surface. Therefore, we can independently investigate TEGa reactions on the As- and Ga-terminated surfaces from the respective growth rates be low and above 1 monolayer/cycle in the the growth alternating Here, by measuring ratessupply in themode. alternating supply mode at high temperatures, the adTEGa desorption on the As- and Ga-terminated surfaces was studied. Next, the adTEGa decomposition and TEGa adsorption on these surfaces were studied from the growth rates at low temperatures.
1 10
1
T
(a)
3.4. TEGa—As 4 alternating supply growth
Different surface reactions of TEGa molecules on the Ga- and As-terminated surfaces were suggested by the As pressure dependence of the growth rate. To investigate MOMBE processes on the Ga- and As-terminated surfaces independently we measured the growth rates in the TEGa-As4 alternating supply mode. In this mode, the Ga- and As-terminated surfaces are expected
(b) ..
~.
.
. l-io. RHFFD patterns in the TI-.Ga-As4 alternating supply mode at 550°C: (a) during As4 supply; (b) during TEGa supply.
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3.4.1. Growth rates at high temperatures
‘~3
Fig. 6 shows the growth rates above 500 C in the alternating supply mode as a function of the amount of TEGa supplied in one cycle. At these temperatures, the growth rate is limited by the adTEGa desorption. Thus, we can estimate the adTEGa desorption on the Ga- and As-terminated surface. The amount of TEGa in one cycle was varied by changing the TEGa flow rate in the fixed TEGa supply during 3 s. As shown in fig. 6, the growth rate above 1 monolayer/cycle is smaller than that below I monolayer/cycle. This difference becomes significant as the temperature increases. This result clearly demonstrates that the adTEGa desorption rate on the Ga-terminated surface is higher than that on the As-terminated surface, which is consistent with the result that the growth rate decreases with decreasing As coverage at a high temperature (fig. 3). The temperature dependence of the growth rate in excess of I monolayer/ cycle has the apparent activation energy of 17 kcal/mol. The difference in the growth rate appears before the growth rate reaches 1 monolayer/cycle in fig. 6. This is probably because of 75% As surface coverage on the (2 X 4) As-terminated surface [12] Also, it may results from the As desorption after
4
___________________________ alternating supply T~e 500:C
/ 2
/
~
/ / / ~/ //
/
/
/
/
/
/
,..•‘
//
/
~ ~
•..—
•~-~
/,:/_i—~~~~
/ 0 TEGa FEED
RATE
in MOMBE growth of GaAs
2 3 (pmol/CYCIe)
Fig. 6. MOMBE growth rates as a function of TEGa feed rate in the alternating supply mode above 500°C.
~‘
alternating Supply
/
400•C •
340 •C
§ w
2
/
.D
// 0
1 2T 3T TEGa SUPPLY TIME (s/cycle)
Fig. 7. MOMBE growth rates as a function of TEGa supply time in the alternating supply mode below 400°C.
the As cell shutter closed in the alternating supply mode. 3.4.2. Growth rate at low temperatures
Fig. 7 shows the growth rates in the alternating supply mode below 400°C. In this measurement, the amount of TEGa in one cycle was varied by changing TEGa supply time. The TEGa flow rate was fixed at 0.3 SCCM. In fig. 7, T represents the normalized time required for the first Ga layer formation. At 400°C, T is 5 s, and at 340°C, T 15 s. After reaching 1 monolayer/cycle at T, the growth rate continues to increase linearly with the TEGa supply time at 400°C.Since the growth rate is limited by the adTEGa decomposition at this temperatures, as shown in fig. 1, this linear relation indicates that the adTEGa decomposition rate is the same on the Ga- and As-terminated surfaces. At 340°C, however, the slope of the growth rate with TEGa supply time increases above I monolayer/cycle, indicating higher adTEGa decomposition rate on the Ga-terminated surface than on the As-terminated surface. Although this seems unlikely considering the chemical affinity of TEGa, a similar phenomenon has been observed in CBE [13] and in metalorganic chemical vapor deposition (MOCVD) [14].
is
g M. Uneta et aL
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g A
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/
581
Surface processes in MOMBE growth of GaAs
~
(a) I
I
(b)
then form small droplets on the Ga-terminated surface. On the other hand, ring patterns appeared with excess TEGa supply on the Ga-terminated surface at 340°C,as shown in fig. 8c. This indicates that multiple-layer adsorption of TEGa occurs on the Ga-terminated surface. This phenomenon is in contrast with the TEGa adsorption on the As-terminated surface, where no TEGa adsorption occurs on the TEGa-adsorbed As-terminated surface, as indicated in section 3.2. The enhancement of the growth rate in excess of 1 monolayer/cycle at 340°C,shown in fig. 6, can be understood if we consider the TEGa multiple-layer adsorption on the Ga-terminated surface, because effective surface coverage of adTEGa increases when the multiple-layer adsorption occurs. 3.5. Effect of excessive As
At low temperatures, excessive As adsorption occurs on the As-terminated surface, which results in deviation from stoichiometory at the surface [15]. Here, to directly investigate the effect of excessive As on the TEGa adsorption, which is suggested in section 3.3, we compare the RHEED intensity variations during TEGa supply on the
(c)
As-terminated surface which shows (2 X 4) recon-
Fig. 8. RI-WED patterns during iEGa-As
4 alternating suppl~ growth at 340°C:(a) during As4 supply; (b) at Ga-monolayer formation time; (c) when excess-TEGa is supplied after Gamonolayer formation.
To get further insight into this phenomenon, we observed RHEED patterns during the TEGa—As4 alternating supply growth at 340°C, and compared them with those at a higher temperature (550°C). At both temperatures, the As- and Gaterminated surfaces appear alternately corresponding to the As4 and TEGa supply, respectively, as shown in fig. S and figs. 8a and 8b. However, a different phenomenon is observed when excessive TEGa molecules are supplied on the Ga-terminated surface. At 550°C,the (4 X 2) reconstruction, which shows the Ga-terminated surface, did not change by supplying excessive TEGa. This is because excess TEGa molecules rapidly decompose into Ga atoms at high temperatures, which
—
[1 1 0 1
F
110
I.
a
lb Fig 9. RIIF.Fl) patterns at 340°C-(a) without As deposition during the cooling process; (b) after Ga and As deposition.
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struction, and on the excess-As-adsorbed surface at low temperatures. The (2 x 4) As-terminated, surface at low temperatures was prepared by cbsing the As-cell shutter in the cooling process after annealing the surface in As4 flux at 600°C. The RHEED pattern of this surface showed clear (2 x 4) reconstruction even at 340°C, as shown in fig. 9. The excess-As adsorbed surface was prepared by the deposition of GaAs at low substrate temperatures. This deposition caused the (2 x 4) reconstruction to become unclear, as shown in fig. 9b, indicating that excessive As adsorption occurred on the surface [15]. RHEED intensity vanations during TEGa supply on the (2 X 4) surface and on the excessAs-adsorbed surface at 340 C are shown in figs. lOa and lob, respectively. In fig. 10a, the intensity rises to a maximum after T0 seconds from the start of the TEGa supply. As reported previously, this peak corresponds to the Ga-monolayer formation time [3,9]. On the other hand, two peaks appear in the RHEED intensity variation when TEGa was supplied on the excess-As-adsorbed surface, as shown in fig. lob. The intensity increases at the start of TEGa supply instead of decreasing as shown in fig. lOa. After reaching a maximum at T~,it rises to another maximum at 1. The measurement of the growth rate in the TEGa—As4 alternating supply mode at 340°C(fig. 6) shows
a
TEGa ON
Z
TEGa OFF ~
o
~
ON
~.
TIME b
\
TEGa AS~OFF ON
LiJTEG~ON )~
________________________ iii
2Or~
AS4 4.0x10’ Torr o
\
To
10
\
~\ \ N
—
0
3~O
SUBSTRATE
400
450
TEMPERATURE (C)
Fig. 11. Substrate temperture dependence of T0 and 1~. Penods of the RHEED oscillation (P) are also shown.
that the RHEED intensity peak at 1 corresponds to the Ga-monolayer formation time. The other peak at 7~probably arose from the desorption of excessive As on the surface by TEGa adsorption. In both cases (figs. lOa and lOb), As4 supply on the Ga-deposited surface causes the intensity to rise to a peak and then to decrease instead of recovering to the level of the (2 x 4) As-terminated surface. This results from the deviation in the surface stoichiometory caused by the excess-As adsorption [15]. The Ga-monolayer formation times on the excess-As-adsorbed surface and on the conventional
as
(2 x 4) As-terminated surface were compared using T and T0 measures of Ga-monolayer formation time, respectively. Fig. 11 shows T1. and T0 as a function of substrate temperature. Both 1. and T0 increase with decreasing temperature, indicating that the adTEGa decomposition becomes energetically unfavorable at lower temperatures. It is evident that T~becomes longer than T0 below 450°C, and that the difference becomes larger as the temperature decreases. This result directly demonstrates the presence of excessive As on This the surface that suppresses the Ga-layer formation. verifies that the TEGa adsorption is suppressed by
~ o~ ~ TIME -
-
-
-
-
Fig. 10. RHEED intensity variation during TEGa supply at 340°C; (a) on the (2x 4) reconstructed surface (fig. 9a) and (b) on the excess-As-adsorbed surface (fig. 9b).
the excessive As on the surface, which was mdicated in section 3.3. Fig. 11 shows that the effect of excessive As adsorption appears below 450°C. This temperature is considered to depend on the As4-flux inten-
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/ Surface processes in MOMBE growth of GaAs
sity, because the excess-As coverage varies with the As4-flux intensity [15], In fig. 11, the periods of RHEED intensity oscillations (R), obtained in the TEGa—As4 simultaneous supply mode, are also indicated. These periods are exactly equal to the GaAs-monolayer formation times [16]. Since the MOMBE growth rate of GaAs is generally determined by the number of Ga atoms released from TEGa molecules, the GaAs-monolayer formation time (R) is expected to be equal to the Ga-monolayer formation time. As shown in fig. 11, R agrees well with rather thank with T0, especially below 400°C. This demonstrates that the growth rates in the simultaneous supply mode are also reduced by excessive As at low growth temperatures, consistent with the result of As pressure dependence of the growth rated at low growth temperatures (fig. 3).
4.
within the surface resident life-time
583 Tdsp.
Some
R5Ga decomposes into atomic Ga with the time constant Tdcp. The other R~Gadesorbs from the surface after Tdsp. As a starting point, we consider that all Ga atoms released by adTEGa molecules on the surface incorporate epitaxially into the lattice. This means that the substrate temperature region where the desorption of Ga atoms occurs is excluded. The growth rate in this region can be explained in the same manner as in MBE [7]. Thus, the MOMBE growth rate is determined by s and by the relationship between ‘Tdcp and Tdsp. In the following, each surface process (adsorption, desorption and decomposition) is independently discussed based on the above obtained results. Furthermore, from the following discussion, we derive the rate equation for the MOMBE growth rate of GaAs in the TEGa—As4 simultaneous supply mode in the appendix.
Discussion
4.1. Model of surface processes To explain the qualitative behavior of the above results, we consider three simple processes for supplied TEGa molecules to decompose into atomic Ga on the GaAs surface. They are: (1) Adsorption of TEGa on the surface, (2) Desorption of adTEGa. (3) Decomposition of adTEGa. These surface processes are schematically described in fig. 12. The supplied TEGa molecules (R3Ga) adsorb on the surface with an efficiency s. AdTEGa molecules (R~Ga)exist on the surface
~3 a R3
R3
OdSO~PtIOfl\
decomposition Rn
C) ~
Rn’
/esorPtlon F~n
~-
C)
III~à~II~/ Q Ga
Fig. 12. Schematic figure of MOMBE surface processes, where R C2H6 and R0 C2H5, (C2H5)2 or (C2H5)3. GaR0 comesponds to adTEGa.
4.1.1. TEGa adsorption Suppression of TEGa adsorption by the presence of excessive As on the surface is indicated by the results that the growth rate decreases with increasing As pressure at low growth temperatures (fig. 3), and that the Ga-monolayer formation time becomes long on the excess-As-adsorbed surface (fig. 9). This suggests that the TEGa-adsorption efficiency is small at excess-As-adsorbed sites. Also, the RHEED intensity behavior during TEGa supply on the excess-As-adsorbed surface (fig. 10(b)) showed that the TEGa adsorption occurs after excess-As desorbs from the surface when TEGa molecules are supplied on the excess-As-adsorbed surface. The saturation of the growth rate with TEGa flow rate at low growth temperatures (fig. 2) mdicated that no TEGa occurs surface. at the TEGa-adsorbed sites on adsorption the As-terminated Conversely, the growth rate measurements (fig. 7) and RHEED observation at 340°C(fig. 8) in the the multiple-layer adsorption fTEG the Ga-terminated surface at low temperatures. This can be explained as follows. Ga in TEGa has empty electron states that can accept electrons. Since the dangling bonds of the
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growth of GaAs
As-terminated surface have excess electrons to denote, the TEGa adsorption occurs easily. Also, it is expected that the adTEGa on the Asterminated surface is chemically stable. Thus, it seems likely that no TEGa adsorption occurs at the TEGa-adsorbed sites on the As-terminated surface. On the other hand, dangling bonds of
the model which considers this effect, the activation energy of the adTEGa decomposition was obtained as 51 kcal/mol (section 3.2). This value agrees well with the mean dissociation energy of ethyl—Ga bonds in TEGa (48 kcal/mol) [17]. The linear relationship between the growth rate and the TEGa feed rate in the TEGa—As4 alter-
Ga-terminated surface are electron deficient. Thus, it is expected that adTEGa on the Ga-terminated surface is chemically unstable. Thereby, it becomes energetically favorable that adTEGa cornbines with supplied TEGa, which then forms the multiple layer of TEGa.
nating supply mode at 400°C revealed that the adTEGa decomposition rate is similar on the Gaand As-terminated surfaces. The differences in the MOMBE surface processes on the Ga- and Asterminated surfaces were explained by the adsorption and desorption processes (sections 4.1.1 and 4.1.2).
4.1.2. adTEGa desorption
The decreasing growth rate in the alternating supply mode above 500°C(fig. 6) and the decreasing growth rate with decreasing As pressure at 650°C(fig. 3) demonstrate that Tdsp is smaller on the Ga-terminated surface than on the Asterminated surface. This probably results from the smaller stabilization energy of the TEGa adsorption on the Ga-terminated surface than on the As-terminated surface. This is consistent with the above discussion about the TEGa adsorption. The activation energy of 17 kcab/mol obtained from fig. 6 is considered to corresponds to the difference between the desorption and decomposition energy of adTEGa on the Ga-terminated surface. This is because the adTEGa decomposition rate, as well as the adTEGa desorption rate, is expected to be increase as the temperature increases. Also, the fact that the gradual decrease above 550°C (region III) is not observed in the MOMBE growth of AlGaAs [6], indicates that the adTEGa desorption is negligible on the Asterminated surface, because the As-terminated surface is maintained at much higher temperatures during the growth of Al-GaAs than GaAs. 4.1.3. adTEGa decomposition
The substrate temperature dependence of the growth rate (fig. 1) has an apparent activation energy of 38 kcal/mol in region I. However, the saturation of the surface with undecomposed TEGa occurs as the temperature decreases; thus this value itself does not correspond to the activation energy of the adTEGa decomposition. Using
4.2. Atomic layer epitaxy
Atomic layer epitaxy (ALE) of GaAs, in which self-limiting deposition of 1 monolayer of GaAs occurs in one cycle of TEGa and As source alternating supply, has been successfully achieved using Ar laser irradiation (laser ALE) [18]. Futhermore, GaAs ALE without using laser (thermal ALE) was also reported [19,20]. The self-limiting process of the laser ALE has been well explained by the site selective decomposition model, which is based on the different decomposition rates of TEGa on the Ga- and As-terminated surfaces [18]. However, the mechanism has not been entirely clarified for the thermal ALE. Ohno et al. have proposed the site selective decomposition model [19]. On the other hand, Nishizawa et al. have proposed a self-limiting adsorption model, which is based on the limiting adsorption of TEGa on the As-terminated surface [20]. According to the site selective decomposition model, it is required that the TEGa decomposition rate on the As-terminated surface is 25 times larger than on the Ga-terminated surface to achieve ALE [18]. However, the result showed that the adTEGa decomposition rate is similar on the Ga- and As-terminated surfaces. Furthermore, it was also shown that no TEGa adsorption occurs on the TEGa-adsorbed sites on the As-terminated surface. These results support the self-limiting adsorption model for the thermal ALE. When the growth temperature is low enough in the TEGaAs4 alternating supply mode, adTEGa on the
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/ Surface processes in MOMBE growth of GaAs
585
As-terminated surface stably exists, not decomposing into atomic Ga. This stable adsorbed layer, which forms a GaAs monolayer when the As source is supplied, prevents further adsorption of TEGa. ALE is probably caused by these processes.
where X, Y and Z are the surface coverages of adTEGa, As and excessively adsorbed As, respectively, F is the incident flux of TEGa and s is the TEGa-adsorption efficiency. In this equation, s is assumed to be zero at the excess-As-adsorbed sites. Also, the maximum s is assumed to be unity [21]. The calculated curves in fig. 2 are obtained
5. Conclusion
by assuming ‘r~ c~and Z ysis will be given elsewhere. —,
=
0. A detailed anal-
Surface processes in MOMBE growth of GaAs, which uses TEGa and As4, were investigated from the results of the substrate temperature, TEGa flow rate and As pressure dependences of the growth rate. Surface reactions on the Ga- and As-terminated surfaces were also independently studied by measuring the growth rate in the TEGa—As4 alternating supply mode. The results showed that the TEGa desorption rate is higher on the Ga-terminated surface than on the Asterminated surface, and that the TEGa adsorption is suppressed in the presence of excessive As and undecomposed TEGa on the As-terminated surface. Furthermore it was indicated that the multiple-layer adsorption of TEGa occurs on the Gaterminated surface at low growth temperatures. These were discussed based on the chemical nature of TEGa molecules and the growth surface.
Acknowledgments The authors would like to thank Dr. Yoshiji Horikoshi and Dr. Naoki Kobayashi for helpful discussions.
Appendix: Rate equation Considering the points disscussed in section 4.1, we derive a rate equation for the MOMBE GaAs growth rate (R): d X/dt
=
sF
—
X/Td
—
x(l
—
i’ )/Td S~t
with S
11 Z~ R
— —
—
‘1
—
X~’
~,
— .~
/
Tdcp,
References [1] A. Robertson, Jr., T.H. Chiu, W.T. Tsang and J.E. Cunningham, J. App!. Phys. 64 (1988) 877. [2] T.H. Chiu, J.E. Cunningham and A. Robertson, Jr., J. Growth 95 (1989) 136. [3] Caystal M. Uneta, Y. Watanabe and Y. Ohmachi, AppI. Phys. Letters 54 (1989) 2327. [4] T.H. Chiu, W.T. Tsang, J.E. Cunningham and A. Robertson, Jr., J. App!. Phys. 62 (1987) 2302. [5] W.T. Tsang, T.H. Chiu, J.E. Cunningham and A. Robertson, Jr., Appl. Phys. Letters 50 (1987) 1376. [6] N. Kobayashi, J.L. Benchimol, F. Alexandre and Y. Gao, Appi. Phys. Letters 51(1987) 1907. [7] R. Heckingbottorn, J. Vacuum Sci. Technol. B3 (1985) 572. [8] S. Chika, H. Kato, M. Nakayama and N. Sano, Japan. J. Appl. Phys. 25 (1986) 1’~1. [9] M. Uneta, Y. Watanabe and Y. Ohmachi, in: Extended Abstracts 20th Conf. on Solid State Devices and Materials, Tokyo, 1988 (Japan Society of Applied Physics, Tokyo, 1988) p. 371. [10] M. Uneta, Y. Watanabe and Y. Ohmachi, Japan. J. Appi. Phys. 27 (1988) L2411. [11] R.C. Weast, Ed., Handbook of Chemistry and Physics (CRC Press, Boca Rato, FL, 1984) p. E-89. [12] M.D. Pashley, K.W. Haberemn, W. Friday, J.M. Woodall and PD. Kirchner, Phys. Rev. Letters 60 (1988) 2176; M.D. Pashley, K.W. Habemern and J.M. Woodall, J. Vacuum Sci. Technol. B6 (1988) 1468. [13] K. Nagata, Y. limuma, Y. Aoyagi and S. Namba, J. Crystal Growth 95 (1989) 142. [14] H. Ohno, S. Ohtsuka, A. Ohuchi, T. Matsubara and H. Hasegawa, J. Crystal Growth 93 (1989) 342. [15] Y. Honikoshi and M. Kawashima, Japan. J. Appl. Phys. 28 (1989) 200. [16] For example, B.A. Joyce, P.J. Dobson, J.H. Neave, K. Woodbridge, J. Zhang, P.K. Larsen and B. Bolger, Surface Sci. 168 (1986) 423. [17] G.P. Smith and R. Patrick, Intern. Chem. Kinetics 15 [18] Phys. (1983) A. Doi,27(1988) 167. S. Iwai, 795. T. Meguro and S. Namba, Japan. J. App!.
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[19] H. Ohno, S. Ohtsuka, H. Ishii, Y. Matsubara and H. Hasegawa, App!. Phys. Letters 54 (1989) 200. [20] J. Nishizawa, H. Kurabayashi, H. Abe and N. Sakurai, J. Vacuum Sci. Technol. A5 (1987) 1572.
[21] V.M. Donnelly, CM. Tu, J.C. Beggy, YR. McCrary, MG. Lamont, T.D. Harris, F.A. Baiocch and R.C. Farrow, AppI. Phys. Letters 52 (1988) 1065.
Journal of Crystal Growth 110 (1991) 576—586 North-Holland
Surface processes in metalorganic molecular beam epitaxial growth of GaAs M. Uneta
“,
Y. Watanabe and Y. Ohmachi
*
NTT Applied Electronics Laboratories, 3-9-11, Midori-cho, Mu,sashino-shi, Tokyo 180, Japan
Received 6 March 1990; manuscript received in final form 10 August 1990
Surface processes in metalorganic molecular beam epitaxial (MOMBE) growth of GaAs. which uses triethylgallium (TEGa) and arsenic (As 4), are investigated from the results of the substrate temperature, TEGa flow rate and As pressure dependences of the growth rate. Surface reactions on the Ga- and As-terminated surfaces are also independently studied by measuring the growth rate in the TEGa—As4 alternating supply mode. Results are qualitatively explained by considering the TEGa adsorption, TEGa decomposition and TEGa desorption processes.
1. Introduction There has been considerable interest in the growth of 111-V epitaxial layers by metalorganic molecular beam epitaxy (MOMBE). MOMBE uses metalorganic compounds as source materials in high-vacuum systems. Since little reaction in the gas phase is expected in a high vacuum, MOMBE growth is mainly determined by the chemical reactions of source materials on the heated substrate surface. However, MOMBE growth introduces much more complex surface processes during growth than molecular beam epitaxy (MBE), which uses atomic beams of group-Ill atoms. MBE growth rate is determined by the quantity of supplied group-Ill atoms, because almost all of them incorporate epitaxially into the lattice under typical conditions. Conversely. MOMBE growth rate depends on the relative ratio of the desorption and decomposition of the adsorbed alkyl-gallium. These reactions, namely the adsorption. decomposition and desorption of alkyl-gallium. are sensitive to the chemical nature of the surface. These *
Present address: NTT Opto-electronics Laboratories, 3-1. Morinosato Wakamiya. Atsugi-shi, Kanagawa Pref. 243-01. Japan.
0022-0248/91/$03.50 ~ 1991
—
issues make the MOMBE growth rate strongly dependent on the growth conditions. Recently, Robertson et al. have proposed a growth model of chemical beam epitaxy (CBE) [I.]. This model assumes sequential cleavage of alkyl groups in the decomposition reaction of TEGa molecules into atomic Ga. It well explains the substrate temperature dependence of the growth rate. However, the As-pressure dependence of the growth rate [2] and the different incorporation rate of TEGa on the Ga- and As-terminated surfaces [3] cannot be deduced from this model. Furthermore, deviations from the experimental resuits clearly exist at low growth temperatures in this model. This is because TEGa decomposition is independently considered, and effects of As atoms or the surface condition on the TEGa decomposition are neglected in this model. This paper systematically investigates surface processes in MOMBE GaAs growth which uses triethylgalhum (TEGa) and arsenic (As4). These investigations are based on the results of the substrate temperature. TEGa flow rate and As pressure dependences of the growth rate. It also investigates surface reactions of TEGa on the Ga- and As-terminated surfaces from the growth rates in the TEGa—As4 alternating supply mode.
Elsevier Science Publishers B.V. (North-Holland)
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ic
2. Experimental The MOMBE system used here consisted of an exchange chamber, a preparation chamber and a growth chamber. The base pressures in the preparation and growth chambers were in the low-mid 10— 10 Torr range. TEGa and solid arsenic were used as sources of Ga and As, respectively. TEGa vapor was directly introduced into the growth chamber without a carrier gas. The flow rate of TEGa vapor was precisely controlled with a mass-flow controller. The arsenic vapor (As 4) was evaporated from a conventional effusion cell. The partial pressure was monitored by a nude ion gauge placed adjacent to the substrate holder. Semi-insulating GaAs substrates exactly oriented (001) were used. The substrate temperature was measured with an optical pyrometer. The thickness of the grown layer was measured with a stylus profiler. Reflection high energy electron diffraction (RHEED) observations were made using a 25 keV electron beam. The growth rate in the TEGa-As4 alternating supply mode was measured from the growth thickness, and that in the conventional growth mode was measured from the period of the RHEED intensity oscillation. It was confirmed that the growth rate was not enhanced by the irradiation of the electron beam by comparing the growth thicknesses in the irradiated and nonirradiated areas.
700
600 TEGo
~ -~
500 400 (C) 4 Torr As4 2.0x10
4 3
7~’ ‘~‘
/
2 1
1
5
~o.4 0.3 02
0.1
~
13
14
15
16
-t
10 /Tsub (K Fig. 1. MOMBE growth rates as a function of substrate ternperature [3].
in two ways. Between 560 and 680°C(region III), the growth rate gradually decreases, whereas above 680°C (region IV), it falls rapidly. Region III is characteristic of GaAs MOMBE, and this gradual decrease is caused by the desorption of adsorbed TEGa (adTEGa) [3,6]. The rapid fall-off above 680°C (region IV) is also observed in the solid source MBE growth of GaAs [7,8]. This fall-off is due to the desorption of Ga atoms.
3. Results
3.2. TEGa flow rate dependence
3.1. Substrate temperature dependence
Fig. 2 shows the TEGa flow rate dependence of the growth rate at different growth temperatures. At high temperatures above 466°C(regions II, III and IV), the growth rate increases linearly with the TEGa flow rate, indicating no saturation of. the surface in these regions. It also suggests that the limiting processes of the growth rate (the adTEGa desorption (region III) and the atomic Ga desorption (region IV)) are first-order reactions. Region II is the TEGa transport limiting region. On the other hand, the growth rate tends to saturate as the TEGa flow rate increases at low temperatures (region I). This tendency becomes
The MOMBE growth rate has a strong substrate temperature dependence, as reported previously [3—6],The relationship between the GaAs growth rate and the substrate temperature is shown in fig. 1. There are four regions of substrate ternperature dependence of growth rate. Below 450°C (region I in fig. 1), the growth rate decreases as the substrate temperature decreases. In this region, the growth rate is limited by the decomposition of TEGa molecules adsorbed on the surface. Between 450 and 560°C (region II), the growth rate is constant. Above 560°C,the growth rate decreases
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As~2.0x10’ Torr
2
—
calculated curve
E LU
2
--~ - --
_
1
—416C
,~~—4~o•c
1
___________
2
TEGa FLOW RATE (sccm) Fig. 2. MOMBE growth rates as a function of TEGa flow rate,
stronger at lower temperatures. This phenomenon is well explained by a model that considers a limit in the surface coverage of adTEGa on the Asterminated surface [9]. As the substrate temperature decreases, it is expected that the adTEGa coverage on the surface increases, because of the decreasing decomposition rate of adTEGa. As this coverage increases, the number of sites available for TEGa adsorption decreases due to the limited adTEGa coverage. This causes the saturation of the growth rate with the TEGa flow rate. This model well reproduces the TEGa flow rate dependence of the growth rate, as shown in fig. 2 (solid curve) (cf. appendix). In this model, the limited adTEGa surface coverage is assumed to be the same as the surface site number of Ga atom on the GaAs(100) surface. This means that the numher of TEGa adsorption sites equals that of Ga surface sites. Also, this limited coverage suggests that no TEGa adsorption occurs at the TEGa-adsorbed sites on the As-terminated surface. By fitting the experimental results to this model, the activation energy of the adTEGa decomposition is obtained as 51 kcal/rnol.
occur above 560°C as shown in fig. 1, little adTEGa desorption is expected at these temperatures. Thus, this phenomenon probably results from the suppression of the TEGa adsorption or the adTEGa decomposition by the excessive As. At 550°C,the adTEGa decomposition rate is high enough relative to the TEGa supply rate, thus the growth rate at this temperature is insensitive to a change ,in the adTEGa decomposition rate. Therefore, this phenomenon is considered to result from .
.
than suppression the from that of the the TEGa adTEGa adsorption decomposition. rather Furthermore, it has been reported that adTEGa decomposition is mainly a thermal process [4,10], suggesting that the adTEGa decomposition rate is not affected by the presence of excessive As. This also supports the above~consideration. Further investigation of the effect of excessive As is described in section 3.5. Unlike the result obtained in the low-temperature region, the growth rate at 650°C decreases with decreasing As pressure (region III). At this temperature, the growth rate is limited by the adTEGa desorption. Thus, this result shows that the adTEGa desorption is enhanced by decreasing As pressure. Since As atoms desorb rapidly from the surface at this temperature, the As surface coverage becomes dependent on the As pressure. Actually, the As-stabilized (2 x 4) RHEED reconstruction becomes unclear as the As pressure de-
TEGO 119
E
1.0
~
0.5
650.C
~37~C
3.3. As-pressure dependence
Fig. 3 shows the As pressure dependences of the growth rate at different substrate temperatures. At 379°C(region I) and 550°C(region II), the growth rate decreases as the As pressure increases. Since the adTEGa desorption begins to
_________________
0
5 10 15 A~ FLUX (x10°Torr) Fig. 3. MOMBE growth rates as a function of As 4 flux. TEGa flow rate is 0.9 SCCM.
M. Uneta et a!.
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[1 1 01
/ Surface
terns
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processes in MOMBE growth of GaAs
‘“3.i
__________ ‘a’ ~
p
‘ ~ Fig. 4. RHEED patterns growth at 630°Cwith 4 during Torr and (b) 3.5x105 Torr. As4 flux of (a) 2.0x10
creases, as shown in fig. 4, indicating a decrease in the As surface coverage. Therefore, this enhancement of the adTEGa desorption is considered to result from decreasing As surface coverage. This suggests that the adTEGa desorption is stronger on the Ga-terminated surface than on the Asterminated surface. The higher adTEGa desorption rate on the Ga-terminated surface can also explain the fact that the adTEGa desorption was enhanced or suppressed by the presence of In or Al atoms, respectively [6], because the As surface coverage is expected to be lower for InGaAs and higher for AlGaAs in accordance with 111—As binding energy [11].
to appear alternately. Fig. 5 showssupply RHEED obtained in the alternating modepatat 550°C. The RHEED pattern changed from (2 X 4) to (4 X 2) when TEGa molecules were supplied, and changed back to (2 x 4) when As4 molecules were resupplied. This verifies that the Ga- and As-terminated surfaces appear alternately. Furthermore, the sharp (4 x 2) reconstruction during TEGa supply indicates that the first Ga layer is formed two-dimensionally on the As-terminated surface. This means that TEGa molecules supplied before the first Ga layer formation adsorb and decompose on the As-terminated surface, and excess TEGa supplied after this adsorbs and decompose on the Ga-terminated surface. Therefore, we can independently investigate TEGa reactions on the As- and Ga-terminated surfaces from the respective growth rates be low and above 1 monolayer/cycle in the the growth alternating Here, by measuring ratessupply in themode. alternating supply mode at high temperatures, the adTEGa desorption on the As- and Ga-terminated surfaces was studied. Next, the adTEGa decomposition and TEGa adsorption on these surfaces were studied from the growth rates at low temperatures.
1 10
1
T
(a)
3.4. TEGa—As 4 alternating supply growth
Different surface reactions of TEGa molecules on the Ga- and As-terminated surfaces were suggested by the As pressure dependence of the growth rate. To investigate MOMBE processes on the Ga- and As-terminated surfaces independently we measured the growth rates in the TEGa-As4 alternating supply mode. In this mode, the Ga- and As-terminated surfaces are expected
(b) ..
~.
.
. l-io. RHFFD patterns in the TI-.Ga-As4 alternating supply mode at 550°C: (a) during As4 supply; (b) during TEGa supply.
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M. Uneta et a!.
/ Surface processes
3.4.1. Growth rates at high temperatures
‘~3
Fig. 6 shows the growth rates above 500 C in the alternating supply mode as a function of the amount of TEGa supplied in one cycle. At these temperatures, the growth rate is limited by the adTEGa desorption. Thus, we can estimate the adTEGa desorption on the Ga- and As-terminated surface. The amount of TEGa in one cycle was varied by changing the TEGa flow rate in the fixed TEGa supply during 3 s. As shown in fig. 6, the growth rate above 1 monolayer/cycle is smaller than that below I monolayer/cycle. This difference becomes significant as the temperature increases. This result clearly demonstrates that the adTEGa desorption rate on the Ga-terminated surface is higher than that on the As-terminated surface, which is consistent with the result that the growth rate decreases with decreasing As coverage at a high temperature (fig. 3). The temperature dependence of the growth rate in excess of I monolayer/ cycle has the apparent activation energy of 17 kcal/mol. The difference in the growth rate appears before the growth rate reaches 1 monolayer/cycle in fig. 6. This is probably because of 75% As surface coverage on the (2 X 4) As-terminated surface [12] Also, it may results from the As desorption after
4
___________________________ alternating supply T~e 500:C
/ 2
/
~
/ / / ~/ //
/
/
/
/
/
/
,..•‘
//
/
~ ~
•..—
•~-~
/,:/_i—~~~~
/ 0 TEGa FEED
RATE
in MOMBE growth of GaAs
2 3 (pmol/CYCIe)
Fig. 6. MOMBE growth rates as a function of TEGa feed rate in the alternating supply mode above 500°C.
~‘
alternating Supply
/
400•C •
340 •C
§ w
2
/
.D
// 0
1 2T 3T TEGa SUPPLY TIME (s/cycle)
Fig. 7. MOMBE growth rates as a function of TEGa supply time in the alternating supply mode below 400°C.
the As cell shutter closed in the alternating supply mode. 3.4.2. Growth rate at low temperatures
Fig. 7 shows the growth rates in the alternating supply mode below 400°C. In this measurement, the amount of TEGa in one cycle was varied by changing TEGa supply time. The TEGa flow rate was fixed at 0.3 SCCM. In fig. 7, T represents the normalized time required for the first Ga layer formation. At 400°C, T is 5 s, and at 340°C, T 15 s. After reaching 1 monolayer/cycle at T, the growth rate continues to increase linearly with the TEGa supply time at 400°C.Since the growth rate is limited by the adTEGa decomposition at this temperatures, as shown in fig. 1, this linear relation indicates that the adTEGa decomposition rate is the same on the Ga- and As-terminated surfaces. At 340°C, however, the slope of the growth rate with TEGa supply time increases above I monolayer/cycle, indicating higher adTEGa decomposition rate on the Ga-terminated surface than on the As-terminated surface. Although this seems unlikely considering the chemical affinity of TEGa, a similar phenomenon has been observed in CBE [13] and in metalorganic chemical vapor deposition (MOCVD) [14].
is
g M. Uneta et aL
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g A
[1
/
581
Surface processes in MOMBE growth of GaAs
~
(a) I
I
(b)
then form small droplets on the Ga-terminated surface. On the other hand, ring patterns appeared with excess TEGa supply on the Ga-terminated surface at 340°C,as shown in fig. 8c. This indicates that multiple-layer adsorption of TEGa occurs on the Ga-terminated surface. This phenomenon is in contrast with the TEGa adsorption on the As-terminated surface, where no TEGa adsorption occurs on the TEGa-adsorbed As-terminated surface, as indicated in section 3.2. The enhancement of the growth rate in excess of 1 monolayer/cycle at 340°C,shown in fig. 6, can be understood if we consider the TEGa multiple-layer adsorption on the Ga-terminated surface, because effective surface coverage of adTEGa increases when the multiple-layer adsorption occurs. 3.5. Effect of excessive As
At low temperatures, excessive As adsorption occurs on the As-terminated surface, which results in deviation from stoichiometory at the surface [15]. Here, to directly investigate the effect of excessive As on the TEGa adsorption, which is suggested in section 3.3, we compare the RHEED intensity variations during TEGa supply on the
(c)
As-terminated surface which shows (2 X 4) recon-
Fig. 8. RI-WED patterns during iEGa-As
4 alternating suppl~ growth at 340°C:(a) during As4 supply; (b) at Ga-monolayer formation time; (c) when excess-TEGa is supplied after Gamonolayer formation.
To get further insight into this phenomenon, we observed RHEED patterns during the TEGa—As4 alternating supply growth at 340°C, and compared them with those at a higher temperature (550°C). At both temperatures, the As- and Gaterminated surfaces appear alternately corresponding to the As4 and TEGa supply, respectively, as shown in fig. S and figs. 8a and 8b. However, a different phenomenon is observed when excessive TEGa molecules are supplied on the Ga-terminated surface. At 550°C,the (4 X 2) reconstruction, which shows the Ga-terminated surface, did not change by supplying excessive TEGa. This is because excess TEGa molecules rapidly decompose into Ga atoms at high temperatures, which
—
[1 1 0 1
F
110
I.
a
lb Fig 9. RIIF.Fl) patterns at 340°C-(a) without As deposition during the cooling process; (b) after Ga and As deposition.
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processes in MOMBE growth of GaAs
struction, and on the excess-As-adsorbed surface at low temperatures. The (2 x 4) As-terminated, surface at low temperatures was prepared by cbsing the As-cell shutter in the cooling process after annealing the surface in As4 flux at 600°C. The RHEED pattern of this surface showed clear (2 x 4) reconstruction even at 340°C, as shown in fig. 9. The excess-As adsorbed surface was prepared by the deposition of GaAs at low substrate temperatures. This deposition caused the (2 x 4) reconstruction to become unclear, as shown in fig. 9b, indicating that excessive As adsorption occurred on the surface [15]. RHEED intensity vanations during TEGa supply on the (2 X 4) surface and on the excessAs-adsorbed surface at 340 C are shown in figs. lOa and lob, respectively. In fig. 10a, the intensity rises to a maximum after T0 seconds from the start of the TEGa supply. As reported previously, this peak corresponds to the Ga-monolayer formation time [3,9]. On the other hand, two peaks appear in the RHEED intensity variation when TEGa was supplied on the excess-As-adsorbed surface, as shown in fig. lob. The intensity increases at the start of TEGa supply instead of decreasing as shown in fig. lOa. After reaching a maximum at T~,it rises to another maximum at 1. The measurement of the growth rate in the TEGa—As4 alternating supply mode at 340°C(fig. 6) shows
a
TEGa ON
Z
TEGa OFF ~
o
~
ON
~.
TIME b
\
TEGa AS~OFF ON
LiJTEG~ON )~
________________________ iii
2Or~
AS4 4.0x10’ Torr o
\
To
10
\
~\ \ N
—
0
3~O
SUBSTRATE
400
450
TEMPERATURE (C)
Fig. 11. Substrate temperture dependence of T0 and 1~. Penods of the RHEED oscillation (P) are also shown.
that the RHEED intensity peak at 1 corresponds to the Ga-monolayer formation time. The other peak at 7~probably arose from the desorption of excessive As on the surface by TEGa adsorption. In both cases (figs. lOa and lOb), As4 supply on the Ga-deposited surface causes the intensity to rise to a peak and then to decrease instead of recovering to the level of the (2 x 4) As-terminated surface. This results from the deviation in the surface stoichiometory caused by the excess-As adsorption [15]. The Ga-monolayer formation times on the excess-As-adsorbed surface and on the conventional
as
(2 x 4) As-terminated surface were compared using T and T0 measures of Ga-monolayer formation time, respectively. Fig. 11 shows T1. and T0 as a function of substrate temperature. Both 1. and T0 increase with decreasing temperature, indicating that the adTEGa decomposition becomes energetically unfavorable at lower temperatures. It is evident that T~becomes longer than T0 below 450°C, and that the difference becomes larger as the temperature decreases. This result directly demonstrates the presence of excessive As on This the surface that suppresses the Ga-layer formation. verifies that the TEGa adsorption is suppressed by
~ o~ ~ TIME -
-
-
-
-
Fig. 10. RHEED intensity variation during TEGa supply at 340°C; (a) on the (2x 4) reconstructed surface (fig. 9a) and (b) on the excess-As-adsorbed surface (fig. 9b).
the excessive As on the surface, which was mdicated in section 3.3. Fig. 11 shows that the effect of excessive As adsorption appears below 450°C. This temperature is considered to depend on the As4-flux inten-
M. Uneta et aL
/ Surface processes in MOMBE growth of GaAs
sity, because the excess-As coverage varies with the As4-flux intensity [15], In fig. 11, the periods of RHEED intensity oscillations (R), obtained in the TEGa—As4 simultaneous supply mode, are also indicated. These periods are exactly equal to the GaAs-monolayer formation times [16]. Since the MOMBE growth rate of GaAs is generally determined by the number of Ga atoms released from TEGa molecules, the GaAs-monolayer formation time (R) is expected to be equal to the Ga-monolayer formation time. As shown in fig. 11, R agrees well with rather thank with T0, especially below 400°C. This demonstrates that the growth rates in the simultaneous supply mode are also reduced by excessive As at low growth temperatures, consistent with the result of As pressure dependence of the growth rated at low growth temperatures (fig. 3).
4.
within the surface resident life-time
583 Tdsp.
Some
R5Ga decomposes into atomic Ga with the time constant Tdcp. The other R~Gadesorbs from the surface after Tdsp. As a starting point, we consider that all Ga atoms released by adTEGa molecules on the surface incorporate epitaxially into the lattice. This means that the substrate temperature region where the desorption of Ga atoms occurs is excluded. The growth rate in this region can be explained in the same manner as in MBE [7]. Thus, the MOMBE growth rate is determined by s and by the relationship between ‘Tdcp and Tdsp. In the following, each surface process (adsorption, desorption and decomposition) is independently discussed based on the above obtained results. Furthermore, from the following discussion, we derive the rate equation for the MOMBE growth rate of GaAs in the TEGa—As4 simultaneous supply mode in the appendix.
Discussion
4.1. Model of surface processes To explain the qualitative behavior of the above results, we consider three simple processes for supplied TEGa molecules to decompose into atomic Ga on the GaAs surface. They are: (1) Adsorption of TEGa on the surface, (2) Desorption of adTEGa. (3) Decomposition of adTEGa. These surface processes are schematically described in fig. 12. The supplied TEGa molecules (R3Ga) adsorb on the surface with an efficiency s. AdTEGa molecules (R~Ga)exist on the surface
~3 a R3
R3
OdSO~PtIOfl\
decomposition Rn
C) ~
Rn’
/esorPtlon F~n
~-
C)
III~à~II~/ Q Ga
Fig. 12. Schematic figure of MOMBE surface processes, where R C2H6 and R0 C2H5, (C2H5)2 or (C2H5)3. GaR0 comesponds to adTEGa.
4.1.1. TEGa adsorption Suppression of TEGa adsorption by the presence of excessive As on the surface is indicated by the results that the growth rate decreases with increasing As pressure at low growth temperatures (fig. 3), and that the Ga-monolayer formation time becomes long on the excess-As-adsorbed surface (fig. 9). This suggests that the TEGa-adsorption efficiency is small at excess-As-adsorbed sites. Also, the RHEED intensity behavior during TEGa supply on the excess-As-adsorbed surface (fig. 10(b)) showed that the TEGa adsorption occurs after excess-As desorbs from the surface when TEGa molecules are supplied on the excess-As-adsorbed surface. The saturation of the growth rate with TEGa flow rate at low growth temperatures (fig. 2) mdicated that no TEGa occurs surface. at the TEGa-adsorbed sites on adsorption the As-terminated Conversely, the growth rate measurements (fig. 7) and RHEED observation at 340°C(fig. 8) in the the multiple-layer adsorption fTEG the Ga-terminated surface at low temperatures. This can be explained as follows. Ga in TEGa has empty electron states that can accept electrons. Since the dangling bonds of the
584
M. Uneta et a!.
/ Surface processes in MOMBE
growth of GaAs
As-terminated surface have excess electrons to denote, the TEGa adsorption occurs easily. Also, it is expected that the adTEGa on the Asterminated surface is chemically stable. Thus, it seems likely that no TEGa adsorption occurs at the TEGa-adsorbed sites on the As-terminated surface. On the other hand, dangling bonds of
the model which considers this effect, the activation energy of the adTEGa decomposition was obtained as 51 kcal/mol (section 3.2). This value agrees well with the mean dissociation energy of ethyl—Ga bonds in TEGa (48 kcal/mol) [17]. The linear relationship between the growth rate and the TEGa feed rate in the TEGa—As4 alter-
Ga-terminated surface are electron deficient. Thus, it is expected that adTEGa on the Ga-terminated surface is chemically unstable. Thereby, it becomes energetically favorable that adTEGa cornbines with supplied TEGa, which then forms the multiple layer of TEGa.
nating supply mode at 400°C revealed that the adTEGa decomposition rate is similar on the Gaand As-terminated surfaces. The differences in the MOMBE surface processes on the Ga- and Asterminated surfaces were explained by the adsorption and desorption processes (sections 4.1.1 and 4.1.2).
4.1.2. adTEGa desorption
The decreasing growth rate in the alternating supply mode above 500°C(fig. 6) and the decreasing growth rate with decreasing As pressure at 650°C(fig. 3) demonstrate that Tdsp is smaller on the Ga-terminated surface than on the Asterminated surface. This probably results from the smaller stabilization energy of the TEGa adsorption on the Ga-terminated surface than on the As-terminated surface. This is consistent with the above discussion about the TEGa adsorption. The activation energy of 17 kcab/mol obtained from fig. 6 is considered to corresponds to the difference between the desorption and decomposition energy of adTEGa on the Ga-terminated surface. This is because the adTEGa decomposition rate, as well as the adTEGa desorption rate, is expected to be increase as the temperature increases. Also, the fact that the gradual decrease above 550°C (region III) is not observed in the MOMBE growth of AlGaAs [6], indicates that the adTEGa desorption is negligible on the Asterminated surface, because the As-terminated surface is maintained at much higher temperatures during the growth of Al-GaAs than GaAs. 4.1.3. adTEGa decomposition
The substrate temperature dependence of the growth rate (fig. 1) has an apparent activation energy of 38 kcal/mol in region I. However, the saturation of the surface with undecomposed TEGa occurs as the temperature decreases; thus this value itself does not correspond to the activation energy of the adTEGa decomposition. Using
4.2. Atomic layer epitaxy
Atomic layer epitaxy (ALE) of GaAs, in which self-limiting deposition of 1 monolayer of GaAs occurs in one cycle of TEGa and As source alternating supply, has been successfully achieved using Ar laser irradiation (laser ALE) [18]. Futhermore, GaAs ALE without using laser (thermal ALE) was also reported [19,20]. The self-limiting process of the laser ALE has been well explained by the site selective decomposition model, which is based on the different decomposition rates of TEGa on the Ga- and As-terminated surfaces [18]. However, the mechanism has not been entirely clarified for the thermal ALE. Ohno et al. have proposed the site selective decomposition model [19]. On the other hand, Nishizawa et al. have proposed a self-limiting adsorption model, which is based on the limiting adsorption of TEGa on the As-terminated surface [20]. According to the site selective decomposition model, it is required that the TEGa decomposition rate on the As-terminated surface is 25 times larger than on the Ga-terminated surface to achieve ALE [18]. However, the result showed that the adTEGa decomposition rate is similar on the Ga- and As-terminated surfaces. Furthermore, it was also shown that no TEGa adsorption occurs on the TEGa-adsorbed sites on the As-terminated surface. These results support the self-limiting adsorption model for the thermal ALE. When the growth temperature is low enough in the TEGaAs4 alternating supply mode, adTEGa on the
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/ Surface processes in MOMBE growth of GaAs
585
As-terminated surface stably exists, not decomposing into atomic Ga. This stable adsorbed layer, which forms a GaAs monolayer when the As source is supplied, prevents further adsorption of TEGa. ALE is probably caused by these processes.
where X, Y and Z are the surface coverages of adTEGa, As and excessively adsorbed As, respectively, F is the incident flux of TEGa and s is the TEGa-adsorption efficiency. In this equation, s is assumed to be zero at the excess-As-adsorbed sites. Also, the maximum s is assumed to be unity [21]. The calculated curves in fig. 2 are obtained
5. Conclusion
by assuming ‘r~ c~and Z ysis will be given elsewhere. —,
=
0. A detailed anal-
Surface processes in MOMBE growth of GaAs, which uses TEGa and As4, were investigated from the results of the substrate temperature, TEGa flow rate and As pressure dependences of the growth rate. Surface reactions on the Ga- and As-terminated surfaces were also independently studied by measuring the growth rate in the TEGa—As4 alternating supply mode. The results showed that the TEGa desorption rate is higher on the Ga-terminated surface than on the Asterminated surface, and that the TEGa adsorption is suppressed in the presence of excessive As and undecomposed TEGa on the As-terminated surface. Furthermore it was indicated that the multiple-layer adsorption of TEGa occurs on the Gaterminated surface at low growth temperatures. These were discussed based on the chemical nature of TEGa molecules and the growth surface.
Acknowledgments The authors would like to thank Dr. Yoshiji Horikoshi and Dr. Naoki Kobayashi for helpful discussions.
Appendix: Rate equation Considering the points disscussed in section 4.1, we derive a rate equation for the MOMBE GaAs growth rate (R): d X/dt
=
sF
—
X/Td
—
x(l
—
i’ )/Td S~t
with S
11 Z~ R
— —
—
‘1
—
X~’
~,
— .~
/
Tdcp,
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