GaNAs材料磊晶成長與AlAs濕氧化膜之研究

Abstract

本論文探討GaNAs/GaAs的磊晶成長，以及AlAs濕氧化之研究。在GaNAs/GaAs的磊晶成長方面，主要使用有機金屬氣相磊晶法在GaAs基板上成長GaNAs，研究磊晶參數，包括成長溫度、[DMHy]/([DMHy]+[AsH3])比例、成長速率等對GaNAs薄膜中N含量的影響。GaNAs中N含量隨著[DMHy]/([DMHy]+[AsH3])比例變大而增加，隨著成長溫度Tg升高而減少，而成長速率也會影響N含量，但是趨勢比較複雜。由SIMS結果得到，在低溫、高DMHy流量的成長條件下，GaNAs薄膜內的碳、氫原子濃度非常高，而在高溫、高DMHy流量的條件下，所成長的GaNAs薄膜可能會產生相分離的現象，形成GaNAs與GaN。熱退火處理可以大幅度提昇GaNAs薄膜的PL發光強度，所需熱退火溫度只要比成長溫度高100℃左右即可得到良好的效果。但是要有效降低GaNAs薄膜中的H原子濃度，必須將熱退火溫度升高到800℃以上，才有明顯的效果。研究中也發現在成長GaNAs時通入NH3不會改變GaNAs中N含量，但是卻會提高PL發光強度，這可能是加入NH3可以抑制本質缺陷的產生。但是在熱退火處理後，成長時通入NH3的樣品其PL發光強度反而降低，推測可能是因NH3純度不足，反而將雜質帶入薄膜所造成。在AlAs濕氧化方面，研究中使用了Al0.4Ga0.6As與Ga0.51In0.49P作為氧化位障層，避免GaAs表面因濕氧化產生大量表面缺陷；同時也比較在不同濕氧化條件下，Al0.4Ga0.6As與Ga0.51In0.49P抗氧化的穩定性，發現在各種濕氧化條件下Ga0.51In0.49P非常穩定，而Al0.4Ga0.6As在高溫長時間的濕氧化條件下則會逐漸被氧化，失去保護GaAs的效果。實驗中也利用AlAs濕氧化的方式，在GaAs太陽電池表面形成Al-oxide抗反射膜，由於Al-oxide/GaAs界面缺陷濃度很高，因此GaAs太陽電池的能量轉換效率很差，但是利用Al0.4Ga0.6As或是Ga0.51In0.49P氧化位障層，可大幅提昇GaAs太陽電池的能量轉換效率。在AM1.5G、100 mW/cm2的照光條件下，GaAs太陽電池的能量轉換效率最高可達19%以上。

The OMVPE growth of GaNAs on GaAs and the wet oxidation of AlAs have been investigated in this dissertation. For GaNAs/GaAs epitaxial growth, we studied the influences of the major growth parameters, including growth temperature Tg, the ratio of [DMHy]/([DMHy]+[AsH3]) and growth rate, on N content of GaNAs films. The N content of GaNAs increases as the ratio of [DMHy]/([DMHy]+[AsH3]) increases, and decreases as the growth temperature rises. The growth rate also affects the N content of GaNAs, but the relation is more complex. According the results of SIMS measurements, the concentrations of carbon and hydrogen atoms in GaNAs films is very high under the growth condition of low growth temperature and high DMHy flow rate. The phase separation will occur under the growth condition of high growth temperature and high DMHy flow rate. The treatment of thermal annealing can increase the PL peak intensity greatly, and the temperature needed for thermal annealing is only 100℃ higher than the growth temperature. However, the temperature of thermal annealing must reach 800℃ to effetely reduce the hydrogen concentration in GaNAs. Adding NH3 during epitaxial growth of GaNAs will not change the composition of GaNAs but increase the PL peak intensity. We suggest that adding NH3 during epitaxial growth of GaNAs will prevent the formation of native defects in GaNAs. However, after thermal annealing treatment, the PL intensity of the sample grown with NH3 is lower than that without NH3. It might be that the quality of NH3 used in experiments is not good enough, and adding NH3 during epitaxial growth of GaNAs will introduce impurities into the GaNAs films. For studies of wet oxidation of AlAs, Ga0.51In0.49P and Al0.4Ga0.6As were used as anti-oxidation barrier layer materials to protect the GaAs surface during wet oxidation process. The effects and stability of Ga0.51In0.49P and Al0.4Ga0.6As barrier layers are compared under varied wet oxidation conditions. Experimental results indicate that the Ga0.51In0.49P barrier layer is more stable than the Al0.4Ga0.6As layer at higher oxidation temperature and longer period of oxidation time. The Al-oxide formed by wet oxidation of AlAs is also used as the anti-reflection (AR) coating of the GaAs solar cell. Results from spectral response measurement and illuminated I-V measurement suggest that the interfacial recombination velocity is extremely high at the Al-oxide/GaAs interface. Adding a 20-nm-thick Al0.4Ga0.6As or Ga0.51In0.49P barrier layer between the Al-oxide and the GaAs emitter can reduce the influence of interfacial recombination and enhance the performance of the GaAs solar cells. The energy conversion efficiency of the GaAs solar cell utilizing the wet oxidation of AlAs to form the AR coating can reach 19% under the luminescence condition of one-sun, AM 1.5G and 100 mW/cm2.