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GaN基III-V宽带隙半导体的低维结构，(Ii)评估这种结构中的基本激子性质，以及(Iii)研究利用激子系统的大振子强度实现高量子效率的发光二极管(LED)、低阈值的激光二极管(LD)和高开关速度的光子器件的关键技术。主要研究成果如下：(1)利用分子束外延技术，在超高真空下解理诱导的GaAs(110)表面制备了自组织的CdSe/ZnSe量子点。Cd Se层呈STranski-Krastanow生长模式。利用显微光致发光光谱，量子点的光致发光峰被分解成许多锐利的峰，表明所制备的CdSe岛具有准零维的类三角函数态密度。(2)利用两室分子束外延技术在GaAs100衬底上相干生长了单量子阱(1，2，3ml)单量子阱(SQW)/ZnSe(50 Nm)/ZnSSe(0.85微米)/GaAs缓冲层。利用时间分辨光致发光光谱研究了这种结构中局域激子的动力学行为。发现在低温下，局域双激子的辐射复合对1ML量子阱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度。研究发现，蓝光LED的辐射复合时间几乎与温度无关(4-5 ns)，这表明发光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.