Polarity control of InN thin film on Si(111) substrate
5.2 Results and Discussion
In this section, the effects of thickness of the initial Al-layer and the lattice
discussed. Recently, X. Y. Liu et al. [47] investigated the polarity of GaN layer grown on Si(111). It was found that the polarity depends on the thickness of initially deposited Al-layers before the growth of AlN buffer layer. Above 1.3 ML of Al-layer, the Ga-polar film is obtained. Otherwise, the film is N-polar.
According the previous study of polarity control of GaN [47], the InN polarity was also controlled by adjusting the thickness of Al initial deposition layer. Fig. 5-1 shows the real time RHEED patterns of Sample A surface under the Al beam exposure of 0, 10 and 20 sec at 850°C. Fig. 5-1 (a) is the clean Si (111) 7×7 pattern at the substrate temperature of 850°C. After Al exposure of 10 sec with the BEP of 3.49×10-8 torr, √3×√3 pattern appeared, as shown in Fig. 5-1 (b). In previous report,
√3×√3 phase of Al on Si (111) was observed with the Al coverage of 1/3 ML and each Al atom terminates three Si dangling bonds [48]. After Al exposure of 20 sec, sharp streaks accompanying sub-streaks were observed. The pattern remains the same after long exposure to Al beam [Fig. 5-1 (c)]. The pattern was attributed to the so-called Al/Si (111) γ-phase, which indicates the saturated phase with the coverage of 0.6 ML Al-layer [49]. Fig. 5-1 (d) shows the streaky RHEED pattern of AlN buffer layer grown on Si(111) at 850oC. The streaky RHEED pattern suggests a two-dimensional growth mode and a smooth surface. Subsequently, the substrate temperature was decreased to 560oC for the growth of InN film. Fig. 5-1 (e) shows the
streaky RHEED pattern of InN film.
According to the phase formation diagram of Al on Si (111), only 1 ML of Al-layer can be deposited on Si(111) surface for the temperature above 600oC [50]. In order to deposit more than 1 ML on Si (111), low-temperature-growth below 600 °C is required. Fig. 5-2 (a) and (b) show RHEED patterns of Sample B surface under the Al beam exposure of 0 and 60 sec at 600 °C, respectively. Compared to the clean surface of Fig. 5-2 (a), the 7×7 fractional diffraction streaks disappeared completely in Fig.5-2(b). It implies the formation of Al islands when the Al beam exposure exceeding 60 sec, thickness of Al-layer is more than 1 ML. After the growth of 60 sec Al-layer, the growth of AlN was started. RHEED pattern obtained after 200 sec of AlN deposition at 600 °C was shown in Fig. 5-2 (c). It is more diffusive as compared to the sharp streak patterns observed on the AlN buffer. After 200 sec of low temperature AlN deposition, 60 min of AlN buffer growth was started by increasing the temperature up to 850oC. Fig. 5-2 (d) shows the spotty RHEED pattern of AlN buffer layer grown at 850oC, which indicates the three-dimensional growth mode and rough surface. Subsequently, the growth temperature was decreased to 460C for the growth of InN layer. Fig. 5-2 (e) shows the RHEED pattern after the growth of InN layer at 460oC.
for 90 min at room temperature. After etching, polygonal structures were observed in sample A, as shown in Fig. 5-3(b). However, for sample B, the sample surface was more smooth after etching, as shown in Fig. 5-4(b). The appearance of polygonal structures was attributed to the N-polarity. On the other hand, smooth surface is the character of In-polarity InN. Therefore, we concluded that the polarity of InN layers can be changed from N-polarity to In-polarity by increasing the thickness of Al initial deposition layer prior to the growth of the AlN buffer layer.
In order to understand the change of lattice polarity on the initially deposited Al-layer, the following growth mechanism is proposed. The selection of AlN polarity is schematically shown in Fig. 5-5. The polarity of AlN grown on the Al covered surface depends on bonding configuration among N atoms and Al atoms. If the N atom takes position A, Al polarity occurs; if the N atoms take position B, N polarity occurs. When a N atom enters position B, it needs to cooperate three Al atoms.
Therefore, the formation of the N-polarity bonding configuration seems kinetically unfavorable process on the Al covered surface [51].
5.3 Conclusions
In summary by controlling the thickness of initially deposited Al-layer prior to AlN buffer layer, we demonstrated that the polarity of InN thin films on Si(111)
substrate by PA-MBE. The InN thin film changed from N-polar to In-polar when Al layer exceeding one ML. The lattice polarities of InN layers were determined by chemical wet etching. N-polar InN surface is rough and pyramid structure exist after etching. However, the surface of In-polar InN is smooth.
Fig. 5-1 RHEED patterns of Si (111) at 850 °C exposed to Al beam for (a) 0 sec, (b) 10 sec, (c) 20 sec, (d) after 60 min AlN growth and (e) after 2hr InN deposition at 560oC. The incident electron beam directions are along Si[11-2] (∥AlN [-1100]) and Si[-110] (∥AlN [11-20]) directions, respectively.
Fig. 5-2 RHEED patterns of Si (111) at 600 °C, exposed to Al beam for (a) 0 sec, (b) 60 sec, (c) after 200 sec AlN deposited, (d) after 60 min AlN growth and (e) after 2hr InN deposition at 460oC. The incident electron beam directions are along Si[11-2]
Fig. 5-3 The SEM images of sample A before and after etching in a 10 mol/l KOH solution at room temperature.
Fig. 5-4. The SEM images of sample B before and after etching in a 10 mol/l KOH solution at room temperature.
\
Fig. 5-5. Polarity selection process of AlN on the Al layer.
Chapter 6
Conclusion
In this dissertation, the spontaneous growth of GaN and InN nano-rods on Si(111) substrate by PA-MBE were studied. In the case of GaN nano-rods, the morphology of nano-rods strongly affected by the N/Ga ratio and the growth temperature. The density/diameter of GaN nano-rods increases/decreases with the growth temperature and the N/Ga ratio increase. From the results of XRD and Raman measurements, one can conclude that nano-rods are strain-free single crystals.
For the case of InN nano-rods, we demonstrate that vertically aligned InN nano-rods can be grown on Si(111) by PA-MBE. From the results of XRD one can conclude that nano-rods are strain-free single crystals.
In addition, strain-free GaN overgrown on GaN nano-rods on Si substrate is realized. The strain-free condition was identified by the strong free A exciton (FXA) photoluminescence (PL) peak at 3.478 eV and the E2 high phonon Raman shift of 567cm-1. It is clearly demonstrated that the critical diameter of GaN nano-rods is
and phonon Raman energy with decreasing the diameter of nano-rod result from the strain relaxation of overgrowth GaN.
On other hand, InN overgrowth on InN nano-rods on Si (111) substrate, exhibit film delimitation and cracking occurs during growth. Due to the poor adhesion between the rod and the Si substrate, However, from the result of PL measurement, we found that the InN film quality has been enhanced compare to the growth on AlN/Si(111) substrate.
Furthermore, In- and N-polar InN layers grown on Si(111) substrate by MBE were investigated. We found that the lattice polarity of InN can be controlled by the thickness of initial Al-layer that were deposited prior to AlN buffer layer. A change from N-polar to In-polar was observed when Al-layer exceeded one monolayer (ML).
In this study, the lattice polarity of InN was determined by chemical wet etching method. In the case of N-polar InN, the sample’s surface became rough and pyramids appeared after etching. On the other hand, In-polar InN remained smooth surface.
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Publications Journal Papers :
1. Y. C. Lin, H. L. Chung, J. T. Ku, C. Y. Chen, K. F. Chien, W. C. Fan, L. Lee, J. I.
Chyi, W. C. Chou, W. H. Chang, and W. K. Chen, “Optical characterization of isoeletronic ZnSe1-xOx semiconductors”. J. Cryst. Growth 323, 122 (2011).
2. Yuen-Yee Wong, Edward Yi Chang, Yue-Han Wu, Mantu K. Hudait, Tsung-Hsi Yang, Jet-Rung Chang, Jui Tai Ku, Wu-Ching Chou, Chiang-Yao Chen, Jer-Shen Maa, Yueh-Chin Lin, “Dislocation reduction in GaN film using Ga-lean GaN buffer layer and migration enhanced epitaxy”. Journal of Thin Solid Film, vol. 509, issue 19, pp 6208-6213, (2011).
3. W. C. Fan, J. T. Ku, W. C. Chou, W. K. Chen, W. H. Chang, C. S. Yang, C. H.
Chia, “Magneto-optical properties of ZnMnTe/ZnSe quantum dots”. Journal of Crystal Growth 323, 380 (2011).
4. Jui Tai Ku, Tsung-Hsi Yang, Jet-Rung Chang, Yuen-Yee Wong, Wu-Ching Chou, Chun-Yen Chang and Chiang-Yao Chen, “Epitaxial overgrowth of gallium nitride nano-rods on silicon(111) substrates by RF-plasma-assisted molecular beam epitaxy”.
Japanese Journal of Applied Physics 49, 04DH06, (2010).
5. L. Lee, W. C. Fan, J. T. Ku, W. H. Chang, W. K. Chen, W. C. Chou, C. H. Ko, C.
H. Wu, Y. R. Lin, C. H. Wann, C. W. Hsu, Y. F. Chen, and Y. K.
Su,“Cathodoluminescence studies of GaAs nano-wires grown on shallow trench patterned Si”. Nanotechnology 21, 465701 (2010).
6. Yuen-Yee Wong, Edward Yi Chang, Tsung-Hsi Yang, Jet-Rung Chang, Jui Tai
Dislocations on Electrical Properties of AlGaN/GaN Heterostructure Grown by MBE”. Journal of the Electrochemical Society, 157, pp H746-H749, (2010).
7. Yuen-Yee Wong, Edward Yi Chang, Tsung-Hsi Yang, Jet-Rung Chang, Yi-Cheng Chen and Jui Tai Ku, “The Effect of AlN Buffer Growth Parameters on the Defect Structure of GaN Grown on Sapphire by Plasma-assisted Molecular Beam Epitaxy”.
Journal of Crystal Growth, 311, pp 1487-1492, (2009).
8. Tsung Hsi Yang, Jui Tai Ku, Jet-Rung Chang, Shih-Guo Shen, Yi-Cheng Chen, Yuen Yee Wong, Wu Ching Chou, Chien-Ying Chen and Chun-Yen Chang,
“Growth of free-standing GaN layer on Si(111) substrate”. Journal of Crystal Growth, 311, pp 1997-2001, (2009).
9. C. H. Chia, C. T. Yuan, J. T. Ku, S. L. Yang, W. C. Chou, J. Y. Juang, S. Y. Hsieh, K. C. Chiu, J. S. Hsu, and S. Y. Jeng, “Temperature dependence of excitonic emission in cubic thin film”. Journal of Luminescence, 128, 123 (2008).
10. Y. C. Lin, W. C. Chou, W. C. Fan, J. T. Ku, F. K. Ke, W. J. Wang, S. L. Yang, W. K. Chen, W. H. Chang, and C. H. Chia, “Time-resolved photoluminescence of isoelectronic traps in ZnSe1-xTex semiconductor alloys”. Appl. Phys. Lett. 93, 241909 (2008).
11. Jet Rung Chang, Tsung Hsi Yang, Jui Tai Ku, Shih Guo Shen, Yi Cheng Chen, Yuen Yee Wong, and Chun Yen Chang, “GaN Growth on Si(111) Using Simultaneous AlN/α-Si3N4 Buffer Structure”. Japanese Journal of Applied Physics, vol. 47, no. 7, pp 5572–5575, (2008).