RHEED によるGaAs 表面構造の研究
9
解 説� 1．は じ め に 2．なぜ RHEED か？ 反射高速電子回折法（reflection high-energy electron dif- fraction: RHEED）を使った GaAs 表面の研究と聞くと， 分子線エピタキシー成長（molecular beam epitaxy: MBE） 中に起こる RHEED 強度振動を連想される読者が多いこ とと思う。実際，試料表面上に広い空間が確保できると いう特徴を持つ RHEED は，MBE 成長過程の評価に最 適な手法といえる。一方，表面構造解析手法としての RHEED（ロッキングカーブ解析）の認知度は決して高 くはなく，低速電子回折法（low-energy electron diffraction: LEED）や X 線回折法（x-ray diffraction: XRD）などの後 塵を拝しているとの印象は拭いきれない。しかしながら， GaAs などの化合物半導体の表面構造を研究対象とした 場合，MBE 成長室内で利用可能という RHEED の特徴 がもたらす利点は計り知れない。 本稿では，著者が最近行った GaAs 表面研究の一部を 紹介する。まず，GaAs 表面の研究に RHEED が適して いる理由（2 節）と実験装置（3 節）について簡単に述 べた後，具体例として GaAs （001） -c （4×4）と c （8×2） 表面構造および GaAs （001） - （2×4）表面上の Zn の吸着 構造を評価した結果について説明する（4 節）。 GaAs 表面上では表面組成に依存して様々な再配列構 造の現れることが知られている。例えば（001）表面の構 造は，表面の組成が As リッチから Ga リッチになるに つれて，c （4×4）→（2×4）→（2×6） ／ （3×6）→c （8×2）と 変化する。これらの表面構造を MBE を用いて作製する 場合，通常，As 分子線強度と基板温度によってその組 成を制御する。これは As の蒸気圧が Ga のそれよりも 桁違いに大きいことを利用したもので，基板温度（As 分子線強度）の増加とともに表面は Ga （As）リッチとな る。ところが，こうして準備した表面を分析しようとす ると，やや面倒な問題に直面する。試料を分析室へ移送 するために MBE 成長室から取り出す際に，基板温度が 十分に高い状態で As 分子線の供給を中止すると表面か ら As が脱離してしまう。逆に，As 雰囲気下で基板温度 表面科学 Vol. 24, No. 3, pp. 136―144, 2003 研究紹介 RHEED による GaAs 表面構造の研究 大 竹 晃 浩 独立行政法人 物質・材料研究機構ナノマテリアル研究所 ! 305―0047 茨城県つくば市千現 1―2―1 （2002 年 12 月 6 日受理） RHEED Studies of GaAs Surface Structure Akihiro OHTAKE National Institute for Materials Science, Nanomaterial Laboratory 1―2―1 Sengen, Tsukuba, Ibaraki 305―0047 (Received December 6, 2002) Rocking-curve analysis of reflection high-energy electron diffraction (RHEED) based on dynamical diffraction theory is a powerful method to study atomic structures of solid surfaces. Also, the grazing-angle geometry of the RHEED elec- tron gun makes itself suitable for in-situ monitoring of molecular-beam epitaxy (MBE). This is of great advantage in evaluating the real surface structure of GaAs using RHEED rocking-curve analysis, because the experimental data can be analyzed without considering the effect of the adsorption and／or desorption of As, which might occur while the sam- ple is transferred from an MBE chamber. This paper briefly reviews recent RHEED studies on the atomic structures of GaAs(001) surfaces. Results for the As-stabilized c(4×4) and Ga-stabilized c(8×2) surfaces, and the adsorption struc- ture of Zn on the (2×4) surface are presented. E-mail: [email protected] 10
-
Upload
yamigamikou2 -
Category
Documents
-
view
180 -
download
3
Transcript of RHEED によるGaAs 表面構造の研究
Vol. 24, No. 3, pp. 136144, 2003
RHEED GaAs
! 3050047 121
2002 12 6
RHEED Studies of GaAs Surface Structure Akihiro OHTAKENational Institute for Materials Science, Nanomaterial Laboratory 121 Sengen, Tsukuba, Ibaraki 3050047 (Received December 6, 2002)
Rocking-curve analysis of reection high-energy electron diffraction (RHEED) based on dynamical diffraction theory is a powerful method to study atomic structures of solid surfaces. Also, the grazing-angle geometry of the RHEED electron gun makes itself suitable for in-situ monitoring of molecular-beam epitaxy (MBE). This is of great advantage in evaluating the real surface structure of GaAs using RHEED rocking-curve analysis, because the experimental data can be analyzed without considering the effect of the adsorption andor desorption of As, which might occur while the sample is transferred from an MBE chamber. This paper briey reviews recent RHEED studies on the atomic structures of GaAs(001) surfaces. Results for the As-stabilized c(44) and Ga-stabilized c(82) surfaces, and the adsorption structure of Zn on the (24) surface are presented.
1
2 3 GaAs 001 44 82 -c c GaAs 001 24 Zn 4
reection high-energy electron diffraction: RHEED GaAs molecular beam epitaxy: MBE RHEED RHEED MBE RHEED low-energy electron diffraction: LEED X x-ray diffraction: XRD GaAs MBE RHEED GaAs GaAs RHEED E-mail: [email protected] 10
2 RHEED GaAs 001 As Ga c c 442426 36 82 MBE As As Ga As Ga As MBE As As As
137
As MBE RHEED MBE GaAs RHEED
Torr As4 580 550 0.05 MLs As4 GaAs 001 24 MBE II As4 RHEED Staib EK-35-R 07 10 RHEED multislice 1 RHEED 25
33. 1
Fig. 1 MBE scanning tunneling microscopy: STM X x-ray photoelectron spectroscopy: XPS MBE II RHEED MBE GaAs RHEED X total-reectionangle x-ray spectroscope; TRAXS reectance difference spectroscope; RDS 3. 2 epi-ready GaAs 001 MBE I 1106
4GaAs 4. 1 GaAs 001 44 -c 2
GaAs 001 24 2 246 1990 7 As 443 As-As c Fig. 2a 10
STM XPSLoad Lock
AFM
Pre-Heat
TRAXS
RHEED
beam-rocking RHEED1m
MBE (II)
RDS
MBE (I)
Fig. 1
Dual-chamber MBE system.11
138(a) three As-dimer model
24 3 2003(b) two As-dimer model
(c) Ga-As dimer model [110] : As [110] : Ga
Fig. 2
Surface reconstruction models for GaAs ( 001 ) c(44).
[110]
00-rod intensity (arb. units)
(a) 520C (with As)Fig. 4
[110]
(b) 430C (with As)
Filled state STM images obtained from the GaAs (001)-c(44) surface with a tunneling current of 0.3 nA and a sample voltage of 3 V. Image dimensions are 3615512 (a) and 80522 (b). The solid lines in (b) show the c(44) lattice mesh.
(c) 250C (with As) (d) 250C (without As)
11 c 44 As 913 c 44 44 c As
0Fig. 3
1 2 3 4 5 6 7 glancing angle (degree)
MBE RHEED c 44 GaAs 001 44 As4 -c 2.5107 Torr 24 RHEED Fig. 3 550 450 44 c 200 RHEED 200450 As4 250 51011 Torr 12
RHEED rocking curve measured from the GaAs(001) surfaces at [110] incidence.
813 GaAs 001 44 -c XRD 3 As-As 2 As-As Fig. 2b 10 STM 89
LEED
12
139
(a) 0 0
(f) 0 0
(b) RHEED intensity (arb. units)
(c) 1 0
RHEED intensity (arb. units)
1 0 2
(g) 0
1 2
(h) 0 1
(d)
3 0 2
(i) 0
3 2
(e) 2 0
(j) 0 2
0 1 2 3 4 5 6 7 glancing angle (degree)Fig. 5
0 1 2 3 4 5 6 7 glancing angle (degree)
RHEED rocking curves (solid curves) measured from the GaAs (001)-c(44) surface at 250 . The dashed and dotted curves are calculated from the Ga-As dimer and As-As dimer models, respectively.
RHEED As 44 c 44 STM Fig. 4a c b 44 c Fig. 4b 110 110Fig. 4 STM As Fig. 2a As-As Ga-As Fig. 2 c Fig. 4b STM RHEED Fig. 5 44 c Ga-As Fig. 6 As-As 13
[110] (y) 3 1 4 2 [110] (x) As 3 4
1 3 3 4 4
2
3 4
1
2 3 4
Fig. 6
Optimized structure model of the GaAs(001)-c(44) surface.
Ga-As 5 R AsAs R 30 Table 1 Fig. 6 Ga RHEED 2
140
24 3 2003
Table 1
Atomic displacements from bulk positions in the optimized structure model for the GaAs(001)c(44) surface (in ). x RHEED analysis y 0.550.15 1.090.14 0.480.28 1.210.27 0.290.14 0.190.15 0.140.14 0.140.09 z 0.150.07 0.850.05 0.220.13 0.810.11 0.250.10 0.210.09 0.010.07 0.200.05 First-principles calculations x y z 0.14 0.08 0.23 0.02 0.01 0.04 0.53 1.09 0.47 1.15 0.13 0.05 0.01 0.06 0.13 0.80 0.17 0.81 0.21 0.18 0.03 0.25
As1 Ga1 As2 Ga2 As3 As3 As4 As4
0.280.07 0.130.13 0.280.16 0.090.16 0.010.15 0.180.10
(a)
(b)
(c)
(d)
(e)
[110] : As [110] : Ga
Fig. 7
Surface reconstruction models for GaAs(001)-c(82). 26 36 82 Ga c As 44 c 24 24 82 c 24 450 26 36 600 82 c 82 600 c 26 36 STM XRD Fig. 8 610 Fig. 7d Fig. 8 R 14. 4 Kumpf Fig. 7e Ga Ga 2 Ga Ga 1 R 8.9 Fig. 8 Table 2 Fig. 9 14
Ga-As Table 1 As-As Ga-As 4. 2 GaAs 001 82 -c 3 Ga GaAs 001 82 -c 42 As 24 2 24 Ga As 2 42 Fig. 7a14 3 Ga 42Fig. 7b15 Ga Fig. 7d 16 LEEDSTMXRD 1617 XRD Fig. 7e 18 Ga 82 c
141
(a) 0 0
(g)
6 0 4
(b) RHEED intensity (arb. units)
1 0 4 RHEED intensity (arb. units)
(h)
7 0 4
(c)
2 0 4
(i) 2 0
(d)
3 0 4
(j) 0 0
(e) 1 0
(k) 0 1
(f)
5 0 4
(l) 0 2
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
glancing angle (degree)Fig. 8
glancing angle (degree)
RHEED rocking curves (solid curves) measured from the GaAs(001)c(82) surface at 610 . The dashed and dotted curves are calculated from the optimized structures based on the models proposed by Kumpf et al.18and Lee et al.16 respectively. ,
Table 2
Atomic coordinates of the optimized structure model for the GaAs(001)-c(82) surface. The atomic coordinates x and y are given as fraction of unit cells along the [110] and [110], directions in Fig. 9. The z coordinates refers to the [001] direction with magnitude equal to the bulk (001) spacing of 5.6538 , the origin of which is at the fth atomic layer. This study x y 0.5300.150 0.4570.030 0.2480.105 0.000 1.000 0.000 1.000 0.4630.032 0.6230.018 z 0.7550.026 0.6380.016 0.4560.032 0.5540.021 0.5770.030 0.5280.023 0.5270.021 0.2830.012 0.2180.017
0.270.09 0.470.13
x 0.000 1.460 2.000 1.117 1.130 0.526 0.539 0.516 1.474
Kumpf et al. (ref. 18) y z 0.500 0.489 0.293 0.000 1.000 0.000 1.000 0.470 0.670 0.827 0.639 0.475 0.571 0.579 0.515 0.509 0.256 0.212
0.19 0.63
Ga(1) As(1) Ga(2) Ga(3) Ga(4) As(2) As(3) Ga(5) Ga(6)
0.000 1.5010.020 2.000 1.1080.040 1.1250.040 0.5050.045 0.5930.033 0.4830.030 1.4630.040
Fig. 7d Fig. 7e 19 Ga 2 1 Ga 15
XRD RHEED Fig. 7d c 82 26 36 c 82
142
24 3 2003
Ga 82 c
4. 3
GaAs 001 Zn 4
II-VI ZnSe GaAs 001 24 Zn 20 24 Zn
3 1 5 4 6
[110] 2 [110] As
STM XPS 2123 Zn 24 XPS Zn 2122 STM 2223 24 Zn 3 5 2
1
1
4 3 5 6
2
4
200300 Zn Zn ZnSe Zn 24 Zn RHEED
6
Fig. 9
Optimized structure model of the GaAs(001)-c(82) surface.
(a) GaAs(001) (250C)
(b) Zn/GaAs(001) (250C)
(c) Zn/GaAs(001) (200C)
00 1 4
00 1 4
00
0
0
0
1 4 2 4 3 4
0
2 4
0
2 4 3 4
0
RHEED intensity (arb. units)
RHEED intensity (arb. units)
0
3 4
RHEED intensity (arb. units)
0
0
01
01
01
02
02
02
00
00
00
10
10
10
20
20
20
0 1 2 3 4 5 glancing angle (degree)
0 1 2 3 4 5 glancing angle (degree)
0 1 2 3 4 5 glancing angle (degree)
Fig. 10
(a) RHEED rocking curves (solid curves) measured from the GaAs(001)(24) surface. The dashed curves are calculated for the optimized structure based on the 2(24) model. (b) and (c) are same as in Fig. 10 (a), but for the surfaces exposed to the Zn beam at 250 and 200 , respectively.16
143
Fig. 10 Zn Zn RHEED Zn 24 2 24 45 Fig. 10a Zn
Zn TRAXS 4 STM Zn Zn Ga STM Zn Zn II STM Zn
Fig. 10bc Fig. 11 2 24 2 As As A D Zn Zn Table 3 250 24 Zn C D Ga A B 200 C D A B Zn As-As 4 Table 3 24 6
5
RHEED GaAs RHEED InSb InAs RHEED 2527 MBE RHEED MBE
A
C[110] (x)
[110] (y)
2830 ZnGaAs 001
D
As
B A,B A,B C,D[110] (y) [001] (z)
RHEED
Fig. 11
Surface reconstruction model for GaAs(001)-(24). Adsorption sites of Zn are indicated by the letters A, B, C, and D.
RHEED
Table 3
The atomic coordinates (x, y, and z) and the site occupancies ( ) of Zn atoms on the GaAs(001)-(24) surface. x 0.69 4.37 2.74 6.60 y 2.25 0.56 12.00 12.00 200 z 0.56 1.08 1.63 1.41
Notations in Fig. 8 A B C D
0.60 (0.410.77) 0.41 (0.350.46) 1.00 (0.721.00) 0.99 (0.611.00)17
x 1.07 4.45 2.94 6.22
y 2.41 0.48 12.00 12.00
250 z 0.88 0.55 1.71 1.38
0.14 (0.020.24) 0.03 (0.000.12) 0.59 (0.400.82) 0.49 (0.180.79)
144
24 3 2003
1) A. Ichimiya: Jpn. J. Appl. Phys. Part 1 22, 176 (1983); 24, 1365 (1985). 2) A. Ohtake, J. Nakamura, S. Tsukamoto, N. Koguchi and A. Natori: Phys. Rev. Lett. 89, 206102 (2002). 3) A. Ohtake, S. Tsukamoto, M. Pristovsek, N. Koguchi and M. Ozeki: Phys. Rev. B 65, 233311 (2002). 4) A. Ohtake, T. Komura, T. Hanada, S. Miwa, T. Yasuda, K. Arai and T. Yao: Phys. Rev. B 59, 8032 (1999); A. Ohtake, T. Hanada, T. Yasuda and T. Yao: Appl. Phys. Lett. 74, 2975 (1999). 5) A. Ohtake, M. Ozeki, T. Yasuda and T. Hanada: Phys. Rev. B 65, 165315 (2002). 6) D.J. Chadi: J. Vac. Sci. Technol. A 5, 834 (1987). 7) T. Hashizume, Q.K. Xue, A. Ichimiya and T. Sakurai: Phys. Rev. B 51, 4200 (1995). 8) D.K. Biegelsen, R.D. Bringans, J.E. Northrup and L.-E. Swarts: Phys. Rev. B 41, 5701 (1990). 9) A.R. Avery, D.M. Holmes, J. Sudijono, T.S. Jones and B. A. Joyce: Surf. Sci. 323, 91 (1995). 10) M. Sauvage-Simkin, R. Pinchaux, J. Massies, P. Calverie, N. Jedrecy, J. Bonnet and I.K. Robinson: Phys. Rev. Lett. 62, 563 (1989). 11) A. Nagashima, M. Tazima, A. Nishimura, Y. Takagi and J. Yoshino: Surf. Sci. 493, 227 (2001). 12) N. Moll, A. Kley, E. Pehlke and M. Schefer: Phys. Rev. B 54, 8844 (1996). 13) C. Sasaoka, Y. Kato and A. Usui: Surf. Sci. 265, L 239 (1992). 14) Q. Xue, T. Hashizume, J.M. Zhou, T. Sakata, T. Ohno and T. Sakurai: Phys. Rev. Lett. 74, 3177 (1995).
15) J. Cerd, F.J. Palomares and F. Soria: Phys. Rev. Lett. 75, 665 (1995). 16) S.H. Lee, W. Moritz and M. Shefer: Phys. Rev. Lett. 85, 3890 (2000). 17) D. Paget, Y. Garreau, M. Sauvage, P. Chiaradia, R. Pinchaux and W . G . Schmidt : Phys . Rev . B 64, 161305 (2001). 18) C. Kumpf, L.D. Marks, D. Ellis, D. Smilgies, E. Landemark, M. Nielsen, R. Feidenhansl, J. Zegenhagen, O. Bunk, J.H. Zeysing, Y. Su and R.L. Johnson: Phys. Rev. Lett. 86, 3586 (2001);C. Kumpf, D. Smilgies, E. Landemark , M . Nielsen , R . Feidenhansl , O . Bunk , J . H . Zeysing, Y. Su, R.L. Johnson, L. Cao, J. Zegenhagen, B. O. Fimland, L.D. Marks and D. Ellis: Phys. Rev. B 64, 075307 (2001). 19) M.D. Pashley: Phys. Rev. B 40, 10481 (1989). 20) L.H. Kuo, K. Kimura, T. Yasuda, S. Miwa, C.G. Jin, K. Tanaka and T. Yao: Appl. Phys. Lett. 68, 2413 (1996). 21) S. Heun, J.J. Paggel, S. Rubini and A. Franciosi: J. Vac. Sci. Technol. B 14, 2980 (1996). 22) S. Miwa, L.H. Kuo, K. Kimura, T. Yasuda, A. Ohtake, C. G. Jin and T. Yao: Appl. Phys. Lett. 73, 939 (1998). 23) S. Ahsan, A. Kahn and M.D. Pashley: Appl. Phys. Lett. 71, 2178 (1997). 24) R. Miotto, G.P. Srivastava and A.C. Ferraz: Appl. Phys. Lett. 76, 3735 (2000); Phys. Rev. B 62, 13623 (2000). 25) A. Ohtake, T. Komura, T. Hanada, S. Miwa, T. Yasuda, K. Arai and T. Yao: Phys. Rev. B 59, 8032 (1999). 26) A. Ohtake, T. Hanada, T. Yasuda, K. Arai and T. Yao: Phys. Rev. B 60, 8326 (1999). 27) A. Ohtake, J. Nakamura, T. Komura, T. Hanada, T. Yao, H. Kuramochi and M. Ozeki: Phys. Rev. B 64, 045138 (2001). 28) H. Nakahara and A. Ichimiya: Surf. Sci. 241, 124 (1991). 29) Y. Fukaya, Y. Shigeta and K. Maki: Phys. Rev. B 61, 13000 (2000). 30) A. Ohtake, M. Ozeki and J. Nakamura: Phys. Rev. Lett. 84, 4665 (2000).
18
