以擴散法將矽摻雜入p型氮化鎵之中使其轉變為n型氮化鎵，此種n型氮化鎵的電子移動率為90-150 cm2V-1s-1，此時鎂在n型氮化鎵中將會成為一種缺陷，因此載子的傳輸以跳躍的傳遞方式為主，載子跳躍的方式可經由這些缺陷中心進行或經由電子波函數之間的重疊所產生。在此論文中用四種不同的溫度進行擴散，其溫度分別是800℃、900℃、1000℃以及1100℃，而得到的補償比例分別為0.3、0.45、0.6以及0.75。在此種n型氮化鎵之中，電子移動率便受到補償比例的限制，導致電子跳躍為主要的傳輸機制。
氧化鋅為另一個新興的寬能隙材料，在此論文中以分子束磊晶將氧化鋅磊晶層成長於氧化鋅基板或氮化鎵基板上，以氦鎘雷射為光源所量測的光激發光譜中，氧化鋅磊晶層只發出受束縛的激子所放出的光及其聲子複製，在氧化鋅基板上所觀察到的綠光在氧化鋅磊晶層並未觀察到。以高功率的氮氣雷射為光源所量測的光激發光譜中，在成長於氮化鎵基板上的氧化鋅磊晶層可觀察到激子非彈性碰撞，在氧化鋅基板及成長於氧化鋅基板上的氧化鋅磊晶層並未觀察到激子非彈性碰撞，此不同的現象可能是由於氧化鋅基板與氮化鎵基板的品質不同所造成。
在此論文中也用不同通量比的成長條件下在氮化鎵基板上成長出氧化鋅磊晶層，以氦鎘雷射為光源所量測的光激發光譜中，在室溫下可得到激子所放出其波長為376 nm的光，其半高寬為10 nm (90 meV)。由X光繞射的量測結果可得知這些氧化鋅磊晶層中在[0002]的方向上存在著0.2％的壓縮應力，其應力的來源可能是由晶粒的邊界所產生。

The characteristics of p-type Mg-doped GaN films diffused with Si are studied. N-type conductivity is achieved, and the carrier mobility of diffused GaN is 90-150 cm2V-1s-1, higher than of p-GaN but less than that of epitaxially grown n-GaN. The Mg acceptor states could become deep compensating defects, and the compensation ratio NA/ND is 0.3, 0.45, 0.6, and 0.75 for 800, 900, 1000, and 1100°C diffused GaN, respectively. The carrier transport may be dominated by electron hopping through these deep compensating centers or through diffusion. The results of temperature-dependent carrier concentration indicate that thermal annealing may induce defects at the surface, leading to an additional activation energy Ed ~ 10 meV in the 200-500 K region in diffused GaN.
Photoluminescence (PL) of homoepitaxial and heteroepitaxial ZnO films grown by plasma-assisted molecular beam epitaxy is studied. Homoepitaxial ZnO layers were grown on an O-face melt-grown ZnO (0001) substrate. Heteroepitaxial ZnO layers were grown on an epitaxial GaN template predeposited by metalorganic chemical vapor deposition on a c-plane sapphire substrate. The low-excitation PL spectra of ZnO epilayers excited by a He-Cd laser exhibit only bound-exciton emission with phonon replicas. There are green luminescence from the ZnO substrate but not from the ZnO epilayers. However, under high-excitation by a N2 pulse laser, the emission due to exciton-exciton scattering dominates the PL spectrum from the heteroepitaxial ZnO layer but is not observed from the homoepitaxial ZnO layer. The difference is probably due to the different quality of the ZnO substrate and GaN template.
We have also investigated heteroepitaxial ZnO films grown under various O/Zn flux ratios. PL spectra of ZnO epilayers excited by a He-Cd laser exhibit exciton emission at 376 nm with a full width at half maximum (FWHM) of 10 nm (90 meV) at room temperature. The exciton emission intensity of stoichiometric condition is 2 times greater than that of O-rich and Zn-rich conditions. Samples grown under stoichiometric and Zn-rich conditions do not exhibit defect-related green luminescence, but samples grown under O-rich condition do. In these heteroepitaxial ZnO layers there exists interstitial Zn and Zn vacancies. X-ray diffraction measurements revealed that there exists a residual compressive strain, ε ~ -0.2%, in the [0002] direction of the ZnO epilayer. The residual strain might be attributed to grain boundaries of ZnO.

Chapter 1 Introduction....................1
Chapter 2 Electron transport in Si-diffused p-GaN....................8
2-1 Motivation...............8
2-2 Diffusion process.............9
2-3 Results and discussions...............10
2-4 Summary...............14
Chapter 3 Plasma-assisted molecular beam epitaxy....................25
3-1 Effusion cell...............25
3-2 Plasma source...............26
3-3 Growth procedure...............27
3-4 Calibration of O/Zn flux ratios...............28
Chapter 4 Optical properties of homoepitaxial and heteroepitaxial ZnO..............36
4-1 Motivation...............................36
4-2 Experimental procedure...................37
4-3 Results and discussions..................38
4-4 Summary..................................41
Chapter 5 Heteroepitaxial ZnO grown on GaN under various O/Zn flux ratios................53
5-1 Motivation...............................53
5-2 Experiments..............................54
5-3 Results and discussions..................55
5-4 Summary..................................60
Chapter 6 Conclusions.........................69
Publication list...............................71
Awards.........................................73

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