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Selective area etching of III–V semiconductors using TDMAAs and TDMASb in metalorganic molecular beam epitaxy chamber
J. . . . . . . . C R Y S T A L QIROWTH
Journal of Crystal Growth 175/176 (1997) 1236-1241
ELSEVIER
Selective area etching of III-V semiconductors using TDMAAs and TDMASb in metalorganic molecular beam epitaxy chamber K. Yamamoto, H. Asahi*, T. Hayashi, K. Hidaka, S. Gonda The Institute of Scientific and Industrial Research, Osaka University, 8-1, Mihogaoka, Ibaraki, Osaka 567, Japan
Abstract
Selective area etching of SiO2-masked GaAs using trisdimethylaminoarsenic (TDMAAs) is studied. GaAs substrates are partly masked with stripe shaped SiO2 films along the [0 1 1] and [0 1 1] directions. Cross-sectional scanning electron microscopy observation shows that the etched shape depends on both the direction of the mask and substrate temperature, and is almost independent of the TDMAAs flow rate. At low substrate temperatures, the (1 1 1)B surface is preferentially formed and a V-shaped groove is formed for the [0 1 1] mask direction. With increasing substrate temperature, the V-groove changes to the channel with vertical side walls due to the drastic increase of the etching rate of (1 1 1)B. For the [0 T 1] mask direction, the vertical (0 1 1) plane is formed at all temperatures. It is found that etched shapes are determined by the difference of etching rates and that etching rates are influenced by the crystal structure on the surface. On the other hand, the etching rate of GaSb by TDMASb is independent of substrate surface orientation. This is because the bond strength of GaSb is weak and the etching proceeds along the direction of molecular beams. Enhanced etching rate is observed for InP by TDMAAs, which is explained by the formation of thin InAs layer on InP due to the irradiation of As released from TDMAAs.
1. I n t r o d u c t i o n
Selective area etching and regrowth using patterned substrates are emerging as a very promising technique for the fabrication of nanoscale structures, such as quantum dots or wires [1]. Considering these multiple processes, the clean environment is required since the optical and electrical
* Corresponding author. E-mail:[email protected].
properties of devices will be affected by a contamination due to an air exposure or surface residues. Furthermore, the accurate control of etched profiles with a minimal damage is necessary. For this purpose, metalorganic molecular beam epitaxy (MOMBE) is a useful method because the etching and regrowth can be conducted in the same vacuum chamber by switching from etching to growth and vice versa as described below. An As precursor, trisdimethylaminoarsenic {AsI-N(CH3)2]3, TDMAAs}, has been proposed as an alternative candidate in the M O M B E growth of
0022-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PH S 0 0 2 2 - 0 2 4 8 ( 9 6 ) 0 0 8 6 3 - 9
1C Yamamoto et al. / Journal of Crystal Growth 175/176 (1997) 1236-1241
GaAs and AIGaAs [2-9] instead of the extremely toxic and high thermally stable hydride-like ASH3. Since TDMAAs releases As atoms even at low temperatures (300-450°C) compared with other precursors [5, 6], thermal precracking in a gas cell before the supply on the substrate surface is not needed. This leads to a low carbon incorporation by TDMAAs in the growth of GaAs with both triethylgallium (TEGa) and trimethylgallium (TMGa) [3, 7]. It was also reported that GaAs and GaSb substrates are etched by TDMAAs and by trisdimethylaminoantimony {Sb[N(CHa)2]a, TDMASb} [9-12]. The etching by these metalorganics was considered to be due to the reaction of surface Ga atoms with TDMAAs and TDMASb related species (probably, amine species) resulting in the formation of volatile Ga species although the exact mechanism has not been understood yet. The etching proceeds in the monolayer-by-monolayer mode, which was confirmed by the reflection highenergy electron diffraction (RHEED) intensity oscillations [12]. For the regrowth, TDMAAs is also a suitable As source because the selective area epitaxy of GaAs was easily achieved by the combination of TEGa and TDMAAs at lower temperatures than by the use of TEGa and solid As [8]. In this paper, we report on the metalorganic molecular beam etching of SiO2-masked GaAs substrates by TDMAAs and discuss the mechanism of the formation of etched shapes. In addition, the dependence of etching characteristics on etched substrate (GaSb, GaAs, GaP, InP) and metalorganic molecules (TDMAAs, TDMASb) is also described.
2. Experimental procedure The etching experiments were conducted using a modified ULVAC MBE 300 apparatus equipped with gas cells for TDMAAs and TDMASb. The growth chamber was evacuated by an oil diffusion pump with a liquid nitrogen trap, yielding a pressure of less than 5 × 10 -6 Torr during etching. Non-precracked TDMAAs and TDMASb were directly introduced into the growth chamber
1237
through mass flow controllers (MFCs) without any carrier gases. Source cylinder, gas line and gas cell were heated at proper temperatures in order to avoid their condensation and to get reasonable gas flow rates. The substrates used were (1 0 0)-GaAs partly masked with SiO2 films for selective area etching, and non-masked (1 0 0) and (n 1 1) {n = 5, 4, 3, 2, 7, 1}, GaAs and GaSb. Stripe mask patterns aligned along [0 1 1] and [0 i 1] directions were formed by a conventional photolithography and a wet chemical etching. The substrate temperature (T~ub) was monitored by an optical pyrometer calibrated by the melting point of InSb at 525°C. The substrate holder was rotated intermittently during etching. Cross-sectional etched profiles were studied by the scanning electron microscopy (SEM).
3. Results and discussion 3.1. Selective area etching on Si02-masked GaAs substrate
Selective area etching using TDMAAs was performed on SiO2-masked GaAs substrates. Fig. 1 shows typical cross-sectional SEM photographs of etched substrates with a stripe mask aligned along the [0 1 1] and [0 T 1] directions. It was found that etched shapes were influenced by the direction of a stripe mask and T~ub. At Tsub lower than 550°C, for the [0 1 1] mask direction, only(1 1 1)B surfaces were observed and the V-shaped groove was formed (Fig. la). With elevating T~ub up to 600°C, (1 1 1)B surfaces disappeared and only vertical (0 1 1) side walls emerged (Fig. lb). On the other hand, for the [0 T 1] mask direction, the etched side walls were a combination of (1 1 1)A and vertical (0 1 1) at low temperatures (Fig. lc), and in addition to these surfaces (4 1 1)A facets emerged at Tsu b higher than 600°C (Fig. ld). But etched shapes were almost independent of a TDMAAs flow rate. These results indicate the possibility of controlling the etched profiles by selecting the etching condition. In order to study the mechanism for the formation of etched side walls, the ER variations with Tsu b w e r e measured for (1 0 0), (4 1 1), and (1 1 1)
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K. Yamamoto et al. /Journal of Crystal Growth 175/176 (1997) 1236-1241
5 trn Fig. 1. Cross-sectional SEM photographs of GaAs with (a), (b) [0 1 1] and (c), (d) [0 ] 1] oriented SiO 2 stripe masks after etching by TDMAAs at (a), (c) low and (b), (d) high T~,b.
surfaces as shown in Fig. 2. These (n 1 1) substrates were mounted on a same Mo holder, and the etched thickness (or etching rate (ER)) was measured by the step height between the etched area and the masked one with a small Ta metal strip. As can be seen, ERs were increased with Ts,b for all surfaces. Our previous work [12] showed that the etching occurs when amino-species react with surface G a atoms, which are formed by the desorption of outermost As atoms from the surface. This suggests that the ER increases at higher Ts,b due to the the low surface coverage of group V atoms, where the desorption of As is enhanced. Now, we consider the dependence of the ER on the substrate orientation. At low T~ub, the ER of the (1 1 1)B was smallest, indicating that the (1 1 1)B is
2.~]
,
n .
i
,
i
.
i
,
(a) [011] direction
(b') i011] 'direction
1 5 O(lOO) "E
"
A(411)B
~/~k
~ 0.5 TDMAAs
"~;o -~oo 660 Tsub (°C)
= 0.2 SCCM
7 0 0 4 0 6 ' '5()0 ' '660' ' 700 Tsub (°C)
Fig. 2. Etching rate dependence of GaAs by TDMAAs on T~.b as a function of surface orientation.
K. Yamamoto et al. / Journal of Crystal Growth 175/176 (1997) 1236-1241
difficult to be etched. However, by increasing Tsub, it drastically increased and approached those for other surfaces. On the other hand, that of the (1 1 1)A was slightly smaller than that of the (1 0 0) at all Tsub, but did not increase above 600°C. These differences in ERs are explained by taking into account the crystal structure of three surfaces, i.e., (1 1 1)A, (1 0 0) and (1 1 1)B. Although the ERs for other (n 1 1) surfaces were not considered here, they are assumed to be similar to those for the (100) and (1 1 1) surfaces because the (n 1 1) surfaces are composed of (1 0 0) and (1 1 1) surfaces. The (1 1 1)A and (1 1 1)B are terminated by Ga and As atoms, respectively. That is, As atoms on the (1 1 1)B surface are strongly bound with three back-bonds to the underlying Ga atom, whereas those on the (1 0 0) and (1 1 1)A have two and only one backbonds, respectively. At low Tsub, the surface coverage of As on the (1 1 1)B surface is highest due to the strong strength of As-bonds and a little desorption of As, resulting in the slow ER. From the above discussion, the surface coverage of As on the (1 1 1)A is lowest and the ER is considered to be the highest. However, the ER of (1 1 1)A surface was lower than that of the (1 0 0) surface. This discrepancy might be due to the strong bonding of Ga atoms on the (1 1 1)A surface, where Ga atoms have three back-bonds to the underlying As atoms. At higher T~ub above 600°C, the R H E E D pattern indicated that the desorption of As molecules is enhanced and that the surface is terminated by the Ga surface or covered with Ga droplets. Therefore, ERs are independent of the surface coverage of As and depend on only the strength of Ga atoms. It is also understood that the ER of (1 1 1)B, where Ga atoms are bound to surface As atoms with only one bond, drastically increases with increasing Tsub and that the ER of (1 1 1)A, where Ga atoms with three back-bonds, is lower than those of the other surfaces. Now, we discuss the dependence of etched profiles on the etching condition (Fig. 1). For the [0 1 1] mask direction, the (1 1 1)B surface are selectively formed due to the low ER of the (111)B at low Tsu b. However, as the Ts,b increases, ER for the (1 1 1)B plane increases rapidly and approaches those for the other planes. Therefore, at higher Tsub, vertical (0 1 1) side walls are formed because the
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etching proceeds along the direction of molecular beams which is perpendicular to the surface. On the other hand, for the [-0 T 1] mask direction, the ER of the (1 1 1)A plane is slightly smaller than those for the other surfaces at all Tsu b. Therefore, vertical etching is mainly expected, but (1 1 1)A planes are also formed at the bottom region of the etched walls due to the low etching rate of (1 1 1)A. But (4 1 1)A planes also appeared at the base of the walls. This is not clear, but it might be related to the reflection of TDMAAs molecules from the (1 1 1)A planes inside the etched channel or to the higher stability of the (4 1 1)A surface [13]. 3.2. Comparison of etching rates between GaAs and GaSb
We investigated the etching process on GaAs and GaSb substrates. The ER of GaAs by TDMAAs increased with Tsub for all surface orientations. In addition, the (1 0 0) surface was etched with the highest ER and the ER was decreased in accordance with an increase of the tilt angle ((1 0 0) > (4 1 1)> (1 1 1)) as shown in Fig. 2. From the decomposition study of TDMAAs, it is completely pyrolized above 350°C [-5, 6]. Since etching experiments here were conducted at Tsub higher than 450°C, the decomposition efficiency is almost unity, producing no orientation dependence. Therefore, ERs on high index substrates are considered to be determined by the crystal structure of the surface. Fig. 3 shows the substrate orientation (tilt angle) dependence of the ER of GaSb by TDMASb as a function of Tsub. As can be seen in this figure, the ER is almost independent of the tilt angle. Solid circles in Fig. 4 show the ER of (1 00)-GaSb by TDMASb. The ER of GaSb saturated at Tsu b 550°C, whereas that of GaAs by TDMASb increased with Tsu b as shown in Fig. 4. This saturation of the ER is caused by the weak bond of GaSb compared with that of GaAs. In the case of GaSb, Sb atoms are easily desorbed from the top surface and Ga droplets exist on the Sb sublayer at T~ub higher than 500°C [14]. On the surface with Ga droplets, the ER depends neither on the surface coverage of Sb, nor on the tilt angle because T D M A S b easily reacts with Ga droplets on the surface under the Sb free condition. Furthermore,
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K. Yamamoto et al. / Journal of Crystal Growth 175/176 (1997) 1236-1241
0.3
1.0 GaSh substrate
•
TDMASb : 0.1 SCCM
•
'$ 0.2
'
Tsub = 450 °C
411A
2~B
i
I
,
i
'
t
,
TDMAAs -- 0.1SCCM
• +sub=sl0 °¢
I. 'ILIA
i
Tsub = 5 7 0 ° C
IIIB"
~e
0.8i (t07/~
0.6
.-~ o.1 l
~
l
~
511A
5 1 ~ l B
3lIB
.~ 0.4
aAs
311A
0"06~0 ' 40 ' 2'0 '
0
20
40
60
0.2
Tilt Angle (degree)
Fig. 3. Etching rate dependence of GaSh by TDMASb on surface orientation as a function of T~ub. .2 [
TDMASb = 0.1 SCCM
(100)" G a S b / ~ 4 ~ ' ~ . G a A s $
~ 0.1 ._= e-
(lO0)-lnP
0.0 450
500
fl
(IO0)-GaP
550 600 Ts,,. (°C)
650
Fig. 4. Etching rate dependence of (1 0 0)-GaSb, GaAs, GaP and InP on T~ubfor the etching by TDMASb.
the ER is limited by the supply of TDMASb, resulting in the saturation of the ER. 3.3. Etching o f different substrates by TDMAAs and TDMASb
We studied the influence of the ER on the etched material, (100)-GaSb, GaAs, G a P and InP substrates. Fig. 4 shows the ER dependence of various substrates etched by T D M A S b on the T~ub. All substrates were etched and the order of the ER is
,
I
J
I
i
I
i
I
,
L
,
440 460 480 500 520 540 Tsub (°C) Fig. 5. Etching rate dependence of(l 0 0)-GaSb,GaAs, and InP on T~.bfor the etching by TDMAAs.
G a S b > I n P > GaAs > GaP. In order to explain this result, we consider the difference of bond strength. The bond of G a S b (45.9 kcal/mol) is the weakest a m o n g them, resulting in the highest etching rate, while the etching rate of G a P is relatively low because of the strongest bond strength (54.9 kcal/mol) [15]. Therefore, the order of bond strength, which is G a - S b < In-P (47.3 kcal/mol) < G a - A s (50.1 kcal/mol) < G a - P , corresponds to the order of the ER. However, the ER of I n P is higher than that of G a S b when T D M A A s was used for etching instead of T D M A S b as shown in Fig. 5. The SEM observation indicated that surface morphologies of I n P were rough after etching. In solid source MBE, it has been known that the surface of I n P is replaced by thin InAs layers by the irradiation of As beam due to the high vapor pressure of P [16]. Hence, the surface of I n P is exchanged into InAs by As atoms released from TDMAAs, followed by etching by amino-groups. Therefore, the ER o f I n P substrates increases due to the etching of InAs instead of InP.
4. S u m m a r y
Selective area etching of SiO2-masked GaAs using T D M A A s was studied. Cross-sectional SEM
K. Yamamoto et al. / Journal of Crystal Growth 175/176 (1997) 1236-1241
observation showed that the etched shape depends on b o t h the direction of the mask and the substrate temperature, and is almost independent of the T D M A A s flow rate. It was found that these etched shapes are determined by the ER for each surfaces and that the E R is influenced by the crystal structure on the surface. Therefore, it is possible to control the etched profiles by choosing etching conditions. T D M A A s is not only a promising arsenic source for g r o w t h but also is a quite useful in situ etching gas source. In contrast, the E R of G a S b etched by T D M A S b was independent of the crystal structure and was similar for all high index substrates. This is due to the weak b o n d strength of GaSb. W h e n the I n P substrate was etched by T D M A A s , the surface of I n P was replaced by thin InAs layers due to the irradiation of As released from T D M A A s and the growth rate of I n P was enhanced.
Acknowledgements This w o r k was supported in part by the Scientific Research Grant-in-Aid # 07650014 from the Ministry of Education, Sport, Science and Culture of Japan, and the Research and Development Association for Future Electron Devices.
References [1"1 W.T. Tsang, R. Kapre and P.F. Sciortino, Jr., J. Crystal Growth 136 (1994) 42.
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[21 C.R. Abernathy, P.W. Wisk, S.J. Pearton, F. Ren, D.A. Bohling and G.T. Muhr, J. Crystal Growth 124 (1992) 64. [3] C.R. Abernathy, P.W. Wisk, D.A. Bohling and G.T. Muhr, Appl. Phys. Lett. 60 (1992) 2421. [4] G. Zimmermann, H. Protzmann, T. Marschner, O. Zseb~Sk, W. Stolz, E.O. G6bel, P. Gimmnich, J. Lorberth, T. Filz, P. Kurpas and W. Richter, J. Crystal Growth 129 (1993) 37. [5"1 K. Hamaoka, I. Suemune, K. Fujii, T. Koui, A. Kiahimoto and M. Yamanishi, Jpn. J. Appl. Phys. 30 (1991) L1579. [6"1 S. Salim, J.P. Lu, K.F. Jensen and D.A. Bohling, J. Crystal Growth 124 (1992) 16. [7"1 S. Goto, Y. Nomura, Y. Morishita, Y. Katayama and H. Ohno, J. Crystal Growth 149 (1995) 143. [8"1 X.F. Liu, H. Asahi, K. Inoue, D. Marx, K. Asami, K. Miki and S. Gonda, J. Appl. Phys. 77 (1995) 1952. [9] H. Asahi, X.F. Liu, K. Inoue, D. Marx, K. Asami, K. Miki and S. Gonda, J. Crystal Growth 145 (1994) 668. [10] X.F. Liu, H. Asahi, K. Inoue, D, Marx, K. Asami, K. Miki and S. Gonda, Appl. Phys. Lett. 65 (1994) 1027. [11] D. Marx, H. Asahi, X.F. Liu, M. Higashiwaki, A.B. Villaflor, K. Miki, K. Yamamoto, S. Gonda, S. Shimomura and S. Hiyamizu, J. Crystal Growth 150 (1995) 551. 1-12] A.B. Villaflor, H. Asahi, D. Marx, K. Miki, K. Yamamoto and S. Gonda, J. Crystal Growth 150 (1995) 638. [13] S. Shimomura, A. Wakejima, A. Adachi, Y. Okamoto, N. Sano, K. Murase and S. Hiyamizu, Jpn. J. Appl. Phys. 32 (1993) L1728. [14] M. Yano, K. Yamamoto, T. Utatsu and M. Inoue, J. Vac. Sci. Technol. B 12 (1994) 1133. [15] CRC Handbook of chemistry and Physics, 70th ed. (CRC Press, Boca Ration, 1990) p. 197. [16] M. Yano, H. Yokose, Y. Iwai and M. Inoue, J. Crystal Growth 111 (1991)609.
Journal of Crystal Growth 175/176 (1997) 1236-1241
ELSEVIER
Selective area etching of III-V semiconductors using TDMAAs and TDMASb in metalorganic molecular beam epitaxy chamber K. Yamamoto, H. Asahi*, T. Hayashi, K. Hidaka, S. Gonda The Institute of Scientific and Industrial Research, Osaka University, 8-1, Mihogaoka, Ibaraki, Osaka 567, Japan
Abstract
Selective area etching of SiO2-masked GaAs using trisdimethylaminoarsenic (TDMAAs) is studied. GaAs substrates are partly masked with stripe shaped SiO2 films along the [0 1 1] and [0 1 1] directions. Cross-sectional scanning electron microscopy observation shows that the etched shape depends on both the direction of the mask and substrate temperature, and is almost independent of the TDMAAs flow rate. At low substrate temperatures, the (1 1 1)B surface is preferentially formed and a V-shaped groove is formed for the [0 1 1] mask direction. With increasing substrate temperature, the V-groove changes to the channel with vertical side walls due to the drastic increase of the etching rate of (1 1 1)B. For the [0 T 1] mask direction, the vertical (0 1 1) plane is formed at all temperatures. It is found that etched shapes are determined by the difference of etching rates and that etching rates are influenced by the crystal structure on the surface. On the other hand, the etching rate of GaSb by TDMASb is independent of substrate surface orientation. This is because the bond strength of GaSb is weak and the etching proceeds along the direction of molecular beams. Enhanced etching rate is observed for InP by TDMAAs, which is explained by the formation of thin InAs layer on InP due to the irradiation of As released from TDMAAs.
1. I n t r o d u c t i o n
Selective area etching and regrowth using patterned substrates are emerging as a very promising technique for the fabrication of nanoscale structures, such as quantum dots or wires [1]. Considering these multiple processes, the clean environment is required since the optical and electrical
* Corresponding author. E-mail:[email protected].
properties of devices will be affected by a contamination due to an air exposure or surface residues. Furthermore, the accurate control of etched profiles with a minimal damage is necessary. For this purpose, metalorganic molecular beam epitaxy (MOMBE) is a useful method because the etching and regrowth can be conducted in the same vacuum chamber by switching from etching to growth and vice versa as described below. An As precursor, trisdimethylaminoarsenic {AsI-N(CH3)2]3, TDMAAs}, has been proposed as an alternative candidate in the M O M B E growth of
0022-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PH S 0 0 2 2 - 0 2 4 8 ( 9 6 ) 0 0 8 6 3 - 9
1C Yamamoto et al. / Journal of Crystal Growth 175/176 (1997) 1236-1241
GaAs and AIGaAs [2-9] instead of the extremely toxic and high thermally stable hydride-like ASH3. Since TDMAAs releases As atoms even at low temperatures (300-450°C) compared with other precursors [5, 6], thermal precracking in a gas cell before the supply on the substrate surface is not needed. This leads to a low carbon incorporation by TDMAAs in the growth of GaAs with both triethylgallium (TEGa) and trimethylgallium (TMGa) [3, 7]. It was also reported that GaAs and GaSb substrates are etched by TDMAAs and by trisdimethylaminoantimony {Sb[N(CHa)2]a, TDMASb} [9-12]. The etching by these metalorganics was considered to be due to the reaction of surface Ga atoms with TDMAAs and TDMASb related species (probably, amine species) resulting in the formation of volatile Ga species although the exact mechanism has not been understood yet. The etching proceeds in the monolayer-by-monolayer mode, which was confirmed by the reflection highenergy electron diffraction (RHEED) intensity oscillations [12]. For the regrowth, TDMAAs is also a suitable As source because the selective area epitaxy of GaAs was easily achieved by the combination of TEGa and TDMAAs at lower temperatures than by the use of TEGa and solid As [8]. In this paper, we report on the metalorganic molecular beam etching of SiO2-masked GaAs substrates by TDMAAs and discuss the mechanism of the formation of etched shapes. In addition, the dependence of etching characteristics on etched substrate (GaSb, GaAs, GaP, InP) and metalorganic molecules (TDMAAs, TDMASb) is also described.
2. Experimental procedure The etching experiments were conducted using a modified ULVAC MBE 300 apparatus equipped with gas cells for TDMAAs and TDMASb. The growth chamber was evacuated by an oil diffusion pump with a liquid nitrogen trap, yielding a pressure of less than 5 × 10 -6 Torr during etching. Non-precracked TDMAAs and TDMASb were directly introduced into the growth chamber
1237
through mass flow controllers (MFCs) without any carrier gases. Source cylinder, gas line and gas cell were heated at proper temperatures in order to avoid their condensation and to get reasonable gas flow rates. The substrates used were (1 0 0)-GaAs partly masked with SiO2 films for selective area etching, and non-masked (1 0 0) and (n 1 1) {n = 5, 4, 3, 2, 7, 1}, GaAs and GaSb. Stripe mask patterns aligned along [0 1 1] and [0 i 1] directions were formed by a conventional photolithography and a wet chemical etching. The substrate temperature (T~ub) was monitored by an optical pyrometer calibrated by the melting point of InSb at 525°C. The substrate holder was rotated intermittently during etching. Cross-sectional etched profiles were studied by the scanning electron microscopy (SEM).
3. Results and discussion 3.1. Selective area etching on Si02-masked GaAs substrate
Selective area etching using TDMAAs was performed on SiO2-masked GaAs substrates. Fig. 1 shows typical cross-sectional SEM photographs of etched substrates with a stripe mask aligned along the [0 1 1] and [0 T 1] directions. It was found that etched shapes were influenced by the direction of a stripe mask and T~ub. At Tsub lower than 550°C, for the [0 1 1] mask direction, only(1 1 1)B surfaces were observed and the V-shaped groove was formed (Fig. la). With elevating T~ub up to 600°C, (1 1 1)B surfaces disappeared and only vertical (0 1 1) side walls emerged (Fig. lb). On the other hand, for the [0 T 1] mask direction, the etched side walls were a combination of (1 1 1)A and vertical (0 1 1) at low temperatures (Fig. lc), and in addition to these surfaces (4 1 1)A facets emerged at Tsu b higher than 600°C (Fig. ld). But etched shapes were almost independent of a TDMAAs flow rate. These results indicate the possibility of controlling the etched profiles by selecting the etching condition. In order to study the mechanism for the formation of etched side walls, the ER variations with Tsu b w e r e measured for (1 0 0), (4 1 1), and (1 1 1)
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K. Yamamoto et al. /Journal of Crystal Growth 175/176 (1997) 1236-1241
5 trn Fig. 1. Cross-sectional SEM photographs of GaAs with (a), (b) [0 1 1] and (c), (d) [0 ] 1] oriented SiO 2 stripe masks after etching by TDMAAs at (a), (c) low and (b), (d) high T~,b.
surfaces as shown in Fig. 2. These (n 1 1) substrates were mounted on a same Mo holder, and the etched thickness (or etching rate (ER)) was measured by the step height between the etched area and the masked one with a small Ta metal strip. As can be seen, ERs were increased with Ts,b for all surfaces. Our previous work [12] showed that the etching occurs when amino-species react with surface G a atoms, which are formed by the desorption of outermost As atoms from the surface. This suggests that the ER increases at higher Ts,b due to the the low surface coverage of group V atoms, where the desorption of As is enhanced. Now, we consider the dependence of the ER on the substrate orientation. At low T~ub, the ER of the (1 1 1)B was smallest, indicating that the (1 1 1)B is
2.~]
,
n .
i
,
i
.
i
,
(a) [011] direction
(b') i011] 'direction
1 5 O(lOO) "E
"
A(411)B
~/~k
~ 0.5 TDMAAs
"~;o -~oo 660 Tsub (°C)
= 0.2 SCCM
7 0 0 4 0 6 ' '5()0 ' '660' ' 700 Tsub (°C)
Fig. 2. Etching rate dependence of GaAs by TDMAAs on T~.b as a function of surface orientation.
K. Yamamoto et al. / Journal of Crystal Growth 175/176 (1997) 1236-1241
difficult to be etched. However, by increasing Tsub, it drastically increased and approached those for other surfaces. On the other hand, that of the (1 1 1)A was slightly smaller than that of the (1 0 0) at all Tsub, but did not increase above 600°C. These differences in ERs are explained by taking into account the crystal structure of three surfaces, i.e., (1 1 1)A, (1 0 0) and (1 1 1)B. Although the ERs for other (n 1 1) surfaces were not considered here, they are assumed to be similar to those for the (100) and (1 1 1) surfaces because the (n 1 1) surfaces are composed of (1 0 0) and (1 1 1) surfaces. The (1 1 1)A and (1 1 1)B are terminated by Ga and As atoms, respectively. That is, As atoms on the (1 1 1)B surface are strongly bound with three back-bonds to the underlying Ga atom, whereas those on the (1 0 0) and (1 1 1)A have two and only one backbonds, respectively. At low Tsub, the surface coverage of As on the (1 1 1)B surface is highest due to the strong strength of As-bonds and a little desorption of As, resulting in the slow ER. From the above discussion, the surface coverage of As on the (1 1 1)A is lowest and the ER is considered to be the highest. However, the ER of (1 1 1)A surface was lower than that of the (1 0 0) surface. This discrepancy might be due to the strong bonding of Ga atoms on the (1 1 1)A surface, where Ga atoms have three back-bonds to the underlying As atoms. At higher T~ub above 600°C, the R H E E D pattern indicated that the desorption of As molecules is enhanced and that the surface is terminated by the Ga surface or covered with Ga droplets. Therefore, ERs are independent of the surface coverage of As and depend on only the strength of Ga atoms. It is also understood that the ER of (1 1 1)B, where Ga atoms are bound to surface As atoms with only one bond, drastically increases with increasing Tsub and that the ER of (1 1 1)A, where Ga atoms with three back-bonds, is lower than those of the other surfaces. Now, we discuss the dependence of etched profiles on the etching condition (Fig. 1). For the [0 1 1] mask direction, the (1 1 1)B surface are selectively formed due to the low ER of the (111)B at low Tsu b. However, as the Ts,b increases, ER for the (1 1 1)B plane increases rapidly and approaches those for the other planes. Therefore, at higher Tsub, vertical (0 1 1) side walls are formed because the
1239
etching proceeds along the direction of molecular beams which is perpendicular to the surface. On the other hand, for the [-0 T 1] mask direction, the ER of the (1 1 1)A plane is slightly smaller than those for the other surfaces at all Tsu b. Therefore, vertical etching is mainly expected, but (1 1 1)A planes are also formed at the bottom region of the etched walls due to the low etching rate of (1 1 1)A. But (4 1 1)A planes also appeared at the base of the walls. This is not clear, but it might be related to the reflection of TDMAAs molecules from the (1 1 1)A planes inside the etched channel or to the higher stability of the (4 1 1)A surface [13]. 3.2. Comparison of etching rates between GaAs and GaSb
We investigated the etching process on GaAs and GaSb substrates. The ER of GaAs by TDMAAs increased with Tsub for all surface orientations. In addition, the (1 0 0) surface was etched with the highest ER and the ER was decreased in accordance with an increase of the tilt angle ((1 0 0) > (4 1 1)> (1 1 1)) as shown in Fig. 2. From the decomposition study of TDMAAs, it is completely pyrolized above 350°C [-5, 6]. Since etching experiments here were conducted at Tsub higher than 450°C, the decomposition efficiency is almost unity, producing no orientation dependence. Therefore, ERs on high index substrates are considered to be determined by the crystal structure of the surface. Fig. 3 shows the substrate orientation (tilt angle) dependence of the ER of GaSb by TDMASb as a function of Tsub. As can be seen in this figure, the ER is almost independent of the tilt angle. Solid circles in Fig. 4 show the ER of (1 00)-GaSb by TDMASb. The ER of GaSb saturated at Tsu b 550°C, whereas that of GaAs by TDMASb increased with Tsu b as shown in Fig. 4. This saturation of the ER is caused by the weak bond of GaSb compared with that of GaAs. In the case of GaSb, Sb atoms are easily desorbed from the top surface and Ga droplets exist on the Sb sublayer at T~ub higher than 500°C [14]. On the surface with Ga droplets, the ER depends neither on the surface coverage of Sb, nor on the tilt angle because T D M A S b easily reacts with Ga droplets on the surface under the Sb free condition. Furthermore,
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K. Yamamoto et al. / Journal of Crystal Growth 175/176 (1997) 1236-1241
0.3
1.0 GaSh substrate
•
TDMASb : 0.1 SCCM
•
'$ 0.2
'
Tsub = 450 °C
411A
2~B
i
I
,
i
'
t
,
TDMAAs -- 0.1SCCM
• +sub=sl0 °¢
I. 'ILIA
i
Tsub = 5 7 0 ° C
IIIB"
~e
0.8i (t07/~
0.6
.-~ o.1 l
~
l
~
511A
5 1 ~ l B
3lIB
.~ 0.4
aAs
311A
0"06~0 ' 40 ' 2'0 '
0
20
40
60
0.2
Tilt Angle (degree)
Fig. 3. Etching rate dependence of GaSh by TDMASb on surface orientation as a function of T~ub. .2 [
TDMASb = 0.1 SCCM
(100)" G a S b / ~ 4 ~ ' ~ . G a A s $
~ 0.1 ._= e-
(lO0)-lnP
0.0 450
500
fl
(IO0)-GaP
550 600 Ts,,. (°C)
650
Fig. 4. Etching rate dependence of (1 0 0)-GaSb, GaAs, GaP and InP on T~ubfor the etching by TDMASb.
the ER is limited by the supply of TDMASb, resulting in the saturation of the ER. 3.3. Etching o f different substrates by TDMAAs and TDMASb
We studied the influence of the ER on the etched material, (100)-GaSb, GaAs, G a P and InP substrates. Fig. 4 shows the ER dependence of various substrates etched by T D M A S b on the T~ub. All substrates were etched and the order of the ER is
,
I
J
I
i
I
i
I
,
L
,
440 460 480 500 520 540 Tsub (°C) Fig. 5. Etching rate dependence of(l 0 0)-GaSb,GaAs, and InP on T~.bfor the etching by TDMAAs.
G a S b > I n P > GaAs > GaP. In order to explain this result, we consider the difference of bond strength. The bond of G a S b (45.9 kcal/mol) is the weakest a m o n g them, resulting in the highest etching rate, while the etching rate of G a P is relatively low because of the strongest bond strength (54.9 kcal/mol) [15]. Therefore, the order of bond strength, which is G a - S b < In-P (47.3 kcal/mol) < G a - A s (50.1 kcal/mol) < G a - P , corresponds to the order of the ER. However, the ER of I n P is higher than that of G a S b when T D M A A s was used for etching instead of T D M A S b as shown in Fig. 5. The SEM observation indicated that surface morphologies of I n P were rough after etching. In solid source MBE, it has been known that the surface of I n P is replaced by thin InAs layers by the irradiation of As beam due to the high vapor pressure of P [16]. Hence, the surface of I n P is exchanged into InAs by As atoms released from TDMAAs, followed by etching by amino-groups. Therefore, the ER o f I n P substrates increases due to the etching of InAs instead of InP.
4. S u m m a r y
Selective area etching of SiO2-masked GaAs using T D M A A s was studied. Cross-sectional SEM
K. Yamamoto et al. / Journal of Crystal Growth 175/176 (1997) 1236-1241
observation showed that the etched shape depends on b o t h the direction of the mask and the substrate temperature, and is almost independent of the T D M A A s flow rate. It was found that these etched shapes are determined by the ER for each surfaces and that the E R is influenced by the crystal structure on the surface. Therefore, it is possible to control the etched profiles by choosing etching conditions. T D M A A s is not only a promising arsenic source for g r o w t h but also is a quite useful in situ etching gas source. In contrast, the E R of G a S b etched by T D M A S b was independent of the crystal structure and was similar for all high index substrates. This is due to the weak b o n d strength of GaSb. W h e n the I n P substrate was etched by T D M A A s , the surface of I n P was replaced by thin InAs layers due to the irradiation of As released from T D M A A s and the growth rate of I n P was enhanced.
Acknowledgements This w o r k was supported in part by the Scientific Research Grant-in-Aid # 07650014 from the Ministry of Education, Sport, Science and Culture of Japan, and the Research and Development Association for Future Electron Devices.
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