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

本计画的目标为（i）制作ZnSe基II-VI族及GaN基III-V族宽禁带半导体的低维结构，（ii）评估此类结构中的基本激子性质，以及（iii）研究实现高量子效率发光二极管（LED）的关键点，低阈值的激光二极管和利用激子系统的大振荡器强度的高开关速度的光子器件。1997至1998年期间取得的成果可概述如下。(1)采用分子束外延（MBE）技术，在超高真空下，在GaAs（110）表面制备了自组织的CdSe/ZnSe量子点。CdSe薄膜的生长模式为Stranski-Krastanow生长模式。通过显微荧光光谱分析，量子点的宽荧光峰被分解为许多尖锐的荧光峰，表明所制备的CdSe岛具有准零维δ函数的态密度。(2)采用双腔MBE技术在GaAs（100）衬底上相干生长了CdSe（1，2，3 ML）单量子威尔斯（SQW）/ZnSe波导（每个50 nm）/ZnSSe（每个0.85 μ m）/GaAs缓冲层。局域激子在这种结构中的动力学行为进行了评估，使用时间分辨光致发光光谱。结果表明，在低温下，局域双激子的辐射复合对1 ML SQW结构的光学增益有贡献。(3)研究了不同In组分的InGaN量子阱器件中激子的维数。结果表明，蓝光LED的辐射复合时间几乎与温度无关（4 - 5ns），这表明发射中心可以归因于零维性质。

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.