S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Materials Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
Numerical Studies on the GaN Thin Film Layer Grown on Sapphire Wafer by Multi-Wafer Hydride Vapor Phase Epitaxy Equipment
사파이어 웨이퍼에서 멀티 웨이퍼 수소화 기상증착 에피택시 장비로 성장된 질화갈륨 박막에 대한 수치 해석 연구
- 공과대학 재료공학부
- Issue Date
- 서울대학교 대학원
- 학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2016. 2. 이경우.
In this study, experimental analysis and numerical simulation analysis have been exploited to investigate the effects of operating parameters on the uniformity of deposition thickness in a new multi-wafer hydride vapor phase epitaxy (HVPE) device. The numerical calculation results have shown the same trend with the experimental results demonstrating that increasing the carrier gas flow rate could shift the maximum value position of the deposition rate to increase the uniformity of the deposition rate distribution within the wafer. Moreover, the temperature effect, pressure effect, susceptor rotation effect, wafer rotation effect and carrier gas composition effect on the uniformity of the deposition thickness are evaluated through the analysis of standard deviation. Among the operating parameters, wafer rotation could not be directly simulated due to its complexity. However, we employed a method to track the trajectory of the points at the wafers to obtain the deposition rate distribution, which has considered the coupled effects of the susceptor rotation and wafer rotation. Through the numerical analysis, it is concluded that with regard to the uniformity of deposition thickness along the radial direction, high carrier gas flow rate, high temperature, low pressure, low rotation speed and nitrogen as the carrier gas are the optimized conditions.
Moreover, in order to investigate the factors in affecting the distribution of the density of the hillocks along radial direction, we have conducted calculations to observe the fluid flow, mass transfer and deposition rate distribution. The numerical calculation results have shown that due to rotation of the susceptor, the wafers have experienced high and low growth rate alternately. So the growth rate fluctuations at different distances from the inlets are compared by standard deviation analysis. The calculation results show that the standard deviations of deposition rates along the azimuthal direction increase from the center to the periphery, which might explain why the density of the hillocks increases from the center to the periphery in the experiments. Moreover, it is found that the non-uniform deposition rates are the result of low speed rotation of the susceptor. Increasing rotation speed of the susceptor increases the uniformity of the gas flow pattern and deposition rate, which means that the high rotation speed can decrease the standard deviation of the deposition rate along azimuthal direction. Consequently, the density of the hillocks can be decreased. Thus, through the numerical analysis, we predict that the high speed rotation will prevent the hillock formation in the multi-wafer horizontal HVPE equipment.
We also proposed a 3D multi-susceptor model for analyzing the GaN deposition thickness distribution and V/III ratio distribution at the GaN deposition surface. The GaN thin film is grown in the multi-susceptor HVPE equipment at 1213 K and 1 Bar. The deposition thickness distributions from the calculations have been compared with the experimental results. Moreover, the standard deviations of deposition thickness of the films achieved from calculations and experiments have been compared. Besides, in the calculation results, we found that the V/III ratio at the GaN deposition surface increases from the center to the periphery and from low susceptor to high susceptor. Our calculation results have also been verified by LMM (Laser measuring microscope) observation of the surface morphology of the GaN thin film. In according with the calculation results, the density of the pits also decreases from the center to the periphery as well as from low susceptor to high susceptor, demonstrating that our calculation model has the capability to predict the distribution of the pits at the surface of the GaN thin films.