交互式激發光調制反射光譜研究

Abstract

本論文研究旨在提出一種新的光調制反射光譜(photoreflectance，PR)技術。我們以兩束強度相當，明暗相位相反的雷射光照射在半導體樣品上，使冷激光(photoluminescence，PL)的交流強度下降了近一百倍，從而抑制了PR實驗在低溫下的PL雜訊。我們以此交互式激發光調制反射光譜(alternative PR，APR)系統測量分析Al0.3Ga0.7As/100A-GaAs/Al0.3Ga0.7As多重量子井的能階。我們證明在樣品溫度為100K時仍可測得高能階處之PR訊號，而傳統PR技術則否。我們比較了APR和PR的雜訊，亦證明了前者的雜訊在任何溫度下皆小於後者的，並從而探討雷射被光截波器(chopper)切割而造成之波形變化及對APR應用的限制。另外，我們證實PR振幅與調制強度I之關係為振幅正比於ln(γI+1)；在有限調制範圍內，振幅亦正比於I^(1/3+1)。

In this study, we pursued a new and novel method to overcome the intrinsic problem occurred when using the photoreflectance(PR) technique to measure high luminant samples, such as semiconductor quantum wells at low temperature. Many experiments showed that the intensive photoluminscence(PL) light may deteriorate the PR spectrum features. In principle, we use two laser pump beams alternatively to modulate the reflectance of probe beam incident to samples. By using this alternative PR (APR) mechanism, we demonstrated that the magnitude of the unwanted PL signal could be significantly reduced in two order compared with which is presented in a conventional PR technique. The noise levels associated with APR at various sample temperatures were measured and found to be less than those associated with conventional PR under the same conditions. We applied APR technique to measure the photoreflectance spectrum of Al0.3Ga0.7As/100A-GaAs/Al0.3Ga0.7As quantum well. The spectrum features match with the calculated quantum-well energy levels. Our measurement results reveal that the APR features are clearer than those of conventional PR spectra, especially for low-temperature cases.