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Title Page

ABSTRACT

Contents

1. Introduction 11

2. Backgrounds 13

2.1. Gallium Arsenide (GaAs) for spintronics 13

2.2. Surface defects in GaAs growth 16

2.2.1. Mound shape defects 16

2.2.2. Triangle shape defects 19

2.3. How to suppress the surface defects? 21

2.3.1. Growth temperature and Ga/As flux ratio 21

2.3.2. Vicinal substrates 24

3. Experimental 26

3.1. Basic concept of molecular beam epitaxy 26

3.2. Epitaxial growth of GaAs(110) 30

3.3. Surface morphology analysis 32

3.4. Microstructure analysis 33

4. Results and Discussion 35

4.1. An effect of a layer thickness on the GaAs(110) layer 35

4.1.1. An observation of surface morphology evolution 35

4.1.2. An observation of the Ga or As agglomeration 42

4.1.3. An observation of underlying crystal defects 43

4.2. An effect of a growth temperature on the GaAs(110) layer 51

5. Conclusions 54

References 56

List of Figures

Fig. 1. Schematic of gallium arsenide structure. 13

Fig. 2. Atomic arrangement of the surfaces (a) GaAs(001) and (b) GaAs(110). 15

Fig. 3. Surface and underlying defects in GaAs(001) called "mound (a)" and "pyramid (b)" respectively. 17

Fig. 4. Schematic of Ehrlich-Schwoebel barrier energy to inhibit downward movement of adatoms. 18

Fig. 5. 5 Reported triangle shaped surface defects (a) and〈112〉directional step edge (b). 20

Fig. 6. AFM image of a 1,000 ㎚ thick GaAs(001) layer grown at a substrate temperature of (a) 280 ℃ and (b) 210 ℃. 22

Fig. 7. Normarski microscopic images (90 ㎛ × 90 ㎛) of the GaAs(110) surface after deposition 1000ML of GaAs at five different growth conditions using As₄.... 23

Fig. 8. Schematic of the islands formation mechanism on the relatively longer terrace of the GaAs surface. 24

Fig. 9. Cross-sectional electron microscopy bright field images of LT-GaAs layers on different substrate types (a) nominal substrate and (b) vicinal substrate. 25

Fig. 10. Schematic of a typical MBE system. 26

Fig. 11. Mechanisms of RHEED intensity oscillations during growth of a monolayer. 28

Fig. 12. 12 Different surface elemental processes occurring in MBE growths. 29

Fig. 13. Schematic of homoepitaxial GaAs(110) growth process and conditions. 31

Fig. 14. Schematic of AFM instrument showing "beam bounce" method of detection using a laser and position sensitive photodiode detector. 32

Fig. 15. Interactions between electrons and material. 33

Fig. 16. Schematic illustration of cross-sectional TEM sampling to be have [110] zone axis using a FIB to investigate the crystal defects in GaAs(110) layer.(이미지참조) 34

Fig. 17. 10 ㎛ × 10 ㎛ AFM images (a)-(e) of GaAs(110) surface depending on different layer thickness (a) 200 ㎚, (b) 300 ㎚, (c) 350 ㎚, (d) 500 ㎚, and (e)... 36

Fig. 18. AFM image of elongated line defects along with [110] on the surface.(이미지참조) 38

Fig. 19. (a) measured roughness data of surface defects by AFM analysis, (b) height, and (c) size (length × width). 40

Fig. 20. 20 Scanning electron microscopy images with different layer thickness (a) 200 ㎚, (b) 300 ㎚, (c) 500 ㎚, and (d) 1,000 ㎚ (Insets are AFM images observed). 41

Fig. 21. Electron probe micro-analyzer results (a) SEM image, (b) Ga distribution, and (c) As distribution. 42

Fig. 22. (a) SEM and (b) cross-sectional TEM images sampled by FIB 44

Fig. 23. Comparison of twin density between (a) out of pyramid region and (b) inner pyramid region. 45

Fig. 24. Cross-sectional TEM image and diffraction patterns of GaAs(110) substrate and homoepitaxial layer in [110] zone axis (a) TEM image, (b) substrate...(이미지참조) 46

Fig. 25. (a) cross-sectional TEM image and diffraction patterns for three different regions (b) region 1 (between long legs), (c) region 2 (out of pyramid), and (d)... 47

Fig. 26. Illustration of stacking fault formation in zinc-blende structure (a) zinc-blende (111) surface, (b) wurtzite(0001) surface, and (c) inserted wurtzite 1... 48

Fig. 27. (a) cross-sectional TEM image and (b) Balls and sticks model of the pyramid defect formation in GaAs(110). 49

Fig. 28. Cross-sectional TEM images and diffraction patterns of long leg (a) TEM image, (b) HRTEM image, (c) FFT image out of long leg, and (d) FFT image of... 50

Fig. 29. 10 ㎛ × 10 ㎛ AFM images (a)-(e) of GaAs(110) surface depending on different growth temperature (a) 360 ℃, (b) 400 ℃ , (c) 420 ℃, (d) 440 ℃, and... 53

초록보기

 The evolution of surface topology and underlying multiple twin defects in homoepitaxial GaAs(110) growths has been investigated with respect to the layer thickness and growth temperature by mainly using a transmission electron microscope and an atomic force microscope. Single triangular-shaped surface defects appear at the layer thickness of 300 nm which is considered as a critical thickness upon the advent of underlying pyramidal defects of stacking faults and twins. With the increase of the layer thickness, the triangular-shaped defects further evolve into paired triangular defects of which individual apexes contact pointing in the [00-1] crystallographic direction on the surface. Additional increase of the layer thickness above 500 nm makes the surface defect shape further change to a dagger with four side edges parallel to〈112〉and〈113〉directions and the more entangled underlying multiple twins. It is also found that the rather higher growth temperature of 420℃ retards the evolution of the defects and significantly reduces the surface defect density because of enhanced Ga migration. Higher temperatures above 420℃, however, lead to surface roughness due to the low As incorporation.

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