Growth, physical properties and fabrication of the opto-electronic device of II-VI compound semiconductor self-assembled quantum dots

• Main Authors: Chu-Shou Yang, 楊祝壽
• Other Authors: Wu-Ching Chou
• Format: Others
• Language: en_US
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/h84efe

博士 === 中原大學 === 應用物理研究所 === 91 === Epilayers, quantum dots, and light emitting diode structures, which were made of the binary and ternary II-VI compound semiconductors, were grown by the molecular beam epitaxy. The reflectivity, photoluminescence, high pressure Raman scattering, ambient and high-pressure x-ray diffraction measurement, and atomic force microscopy were used to investigate the interesting physical properties. Five topics were studied. Firstly, the lattice vibration of the ZnSe1-xTex epilayers was investigated. The dependence of the longitudinal optical (LO) and transverse optical (TO) phonon frequency on the Te concentration was found to follow previous theoretical predictions. However, an additional vibration mode was observed at the energy between that of the LO and TO phonons. The microscopic force constants FZnTe and FZnSe were evaluated to be 6.46×106 amu•(cm-2) and 2.91×106 amu•(cm-2), respectively. In addition, the Raman spectra were recorded at high pressure up to 20 GPa. The pressure at which the semiconductor to metal transition occurred was characterized by the disappearance of the LO phonon and was found to decrease with the Te concentration. Current results imply the decreasing crystal stability with the Te concentration. Secondly, from the high pressure x-ray measurement, it was found that the zinc blende (ZB) to rock salt (RS) phase transition pressures of the Zn0.93Mn0.07Se and Zn0.76Mn0.24Se bulk crystals are around 11.8±1.5 GPa and 9.9±0.5 GPa, respectively. Their respective bulk moduli are 61.8±0.8 GPa and 62.4±0.8 GPa. The pressure-induced ZB to RS structure phase transition is proposed as a signature of the semiconductor to metal transition for Zn1-xMnxSe. The above proposal is further corroborated by the observation of the disappearance of the longitudinal optical (LO) phonon at the pressure where the ZB to RS structure transition occurs. Thirdly, the effect of substrate misorientation angle (SMA) on the band gap energies of the II-VI compound semiconductor epilayers, Zn1-xMgxSe, Zn1-xMnxSe, ZnSe1-xTex, Zn1-xCdxSe and ZnSe1-xSx, was studied by the optical spectroscopy. The band gap energies were found to increase with the substrate misorientation (tilting) angle for the Zn1-xMgxSe, ZnSe1-xTex, Zn1-xCdxSe, and ZnSe1-xSx epilayers. For the Zn1-xMnxSe epilayers, however, the band gap energies decrease with SMA. Both the decrease (for the Zn1-xMnxSe epilayers) and increase (for the other epilayers) in band gap energy were attributed to the increasing incorporation of smaller ion as the SMA was increased. The dependence of energy gap on SMA is almost linear for the ZnSe1-xTex and Zn1-xMnxSe epilayers. While, for the Zn1-xMgxSe and Zn1-xCdxSe epilayers, the energy gap increases abruptly at small tilt angle, then becomes insensitive to the SMA at the tilting angle larger than 10 degrees. In the case of ZnSe1-xSx epilayers, the situation is opposite. At small angle, the band gap energy is almost not influenced by the SMA. At SMA larger than 10 degrees, the band gap energy increases suddenly. The sudden increase with the SMA at large tilting angle results from the abrupt increase in kink density which is a function of the square of tilt angle. The energy difference between the energy gap of the epilayer grown on (001) substrate and the energy gap of the epilayer grown on (001) substrate with 15 degrees tilt toward [010] is largest (18 meV) for the Zn1-xMgxSe epilayers. While, for the Zn1-xMnxSe, ZnSe1-xSx and ZnSe1-xTex epilayers, the energy differences are about 5 meV. Fourthly, the morphology and the size dependent PL spectra of the type II ZnTe QD grown in the ZnSe matrix were obtained. The coverage of ZnTe was varied from 2.5 to 4.0 mono-layers (MLs). The PL peak energy is found to decrease with the dot size. For the 2.5 and 3.0 MLs samples, the PL peak energy decreases monotonically with the increasing temperature. However, for the 3.5 MLs sample, as the temperature is increased, the PL peak energy exhibits blue shift first then red shift as the temperature is increased above 40K. Carrier thermalization and carrier transfer between QDs are used to explain the experimental data. Temperature dependent line width broadening model is employed to fit the high temperature data. The activation energy, which was found by the simple PL intensity quenching model, of the 2.5, 3.0, and 3.5 MLs are 6.35, 9.40, and 18.87 meV, respectively. Finally, the orange light emitting diodes (LEDs) have been successfully fabricated by using the type II ZnTe QDs. The LEDs structure, which coupled with the ZnCdSe quantum well in the active layer, has a higher luminescence efficiency than that one without ZnCdSe quantum well. The room temperature electroluminescence of the LEDs emits wavelengths at 582.1 and 629.3 nm. The turn on voltage decreases with the thickness of active layer.