Basic Research for the Development of Laser Diode with Low-threshold Current Density Utilizing Large Oscillator Strength of Excitonic System

本项目的目标是(i)制备基于znse的ii - vi和基于gan的iii - v宽禁带半导体的低维结构，（ii）评估这些结构的基本激子特性，（iii）研究利用激子系统的大振荡器强度实现高量子效率发光二极管（led），低阈值激光二极管（ld）和高开关速度光子器件的关键点。1997年至1998年期间取得的结果可总结如下。(1)在超高真空条件下，利用分子束外延（MBE）在解裂诱导的GaAs（110）表面制备了自组织CdSe/ZnSe量子点（QDs）。CdSe层表现为Stranski-Krastanow生长模式。通过显微PL光谱分析，将量子点的宽PL峰分解为许多尖峰，表明所制备的CdSe岛具有准零维δ函数态密度。(2)采用双室MBE技术在GaAs（100）衬底上相干生长CdSe （1,2,3 ml）单量子阱(SQWs) / ZnSe波导（每个50nm） / ZnSSe（每个0.85 nm） / GaAs缓冲液。用时间分辨光致发光光谱法评估了这种结构中局域激子的动力学行为。在低温条件下，局域双激子的辐射复合可以增加1 ML SQW结构的光学增益。(3)研究了不同铟成分的ingan量子阱器件中激子的维数。研究发现，蓝色led的辐射复合时间几乎与温度无关（4-5ns），表明其发射中心可归因于零维性质。

The objective of this project is (i) fabrication of low-dimensional structures of both ZnSe-based II-VI and GaN-based III-V wide-bandgap semiconductors, (ii) assessment of fundamental excitonic properties in such structures, and (iii) study of key points for the realization light emitting diodes (LEDs) with high quantum efficiency, laser diodes (LDs) with low threshold and photonic devices with high switching speed utilizing large oscillator strength of excitonic system. The results obtained in the period of 1997 to 1998 can be summarized as follows.(1) Self-organized CdSe/ZnSe quantum dots (QDs) were fabricated on the cleavage-induced GaAs (110) surface in ultra high vacuum by molecular beam epitaxy (MBE). CdSe layers showed the Stranski-Krastanow growth mode. By using the microscopic PL spectroscopy, the broad PL peak of QD's was resolved into a lot of sharp peaks, indicating that the fabricated CdSe islands have quasi-zero-dimensional delta-function like density of states.(2) CdSe (1, 2, 3ML) single quantum wells (SQWs) / ZnSe waveguides (50nm in each) / ZnSSe (0.85 mum in each) / GaAs buffers were coherently grown on GaAs (100) substrates by means of two-chamber MBE technique. Dynamical behavior of localized excitons in such structures was assessed using time-resolved photoluminescence spectroscopy. It was found that radiative recombination of localized bi-excitons contributes to the optical gain in the structure with 1 ML SQW at low-temperature.(3) Dimesionality of excitons was investigated in InGaN-quantum-well devices with various In compositions. It was found that the radiative recombination times of blue LEDs were almost independent on temperature (4-5ns) suggesting that the emission centers can be attributed to zero-dimensional nature.