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: OHTAKE.Akihiro@nims.go.jp 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)

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