將氧化鋅/氧化鎂鋅單量子井成長在c-面藍寶石基板之光學特性探討

Abstract

我們成功的利用PLD成長出三種不同量子井厚度的分別為 2.0nm、4.0nm、5.2nm，並探討不同量子井厚度的光電性質，如：(1) 聲子的交互作用， (2) 激子束縛能， 和(3) 內建電場。由樣品在低溫光譜與變溫光譜隨著厚度的變化，在2nm量子井樣品確實能看到有藍移的現象，並且激子束縛能大於60meV，而在4.0nm與5.2nm的激子束縛能小於60meV，而且隨著厚度的增加而減少，LO聲子與激子的耦合強度也隨著厚度的增加而下降。這些能證實具有量子侷限效應與量子侷限史塔克效應，符合有內建場的理論趨勢上。另外在厚度5.2nm的激子發光峰附近有一能量約為3.34eV譜峰，為了探討其為何，我們固定溫度在80K，做改變激發能量密度之ＰＬ光譜實驗，其結果顯示在低激發能量密度下，與激子發光峰相差約45meV，約等於厚度5.2nm量子井的激子束縛能；但隨著激發能量密度增大，這個能量差減少至約30meV，約為此量子井激子束縛能的四分之三。這個結果表示這個譜峰可能是來自激子-激子散射，即所謂的P-band發光，與激子譜峰差45meV之發光來自P(無窮大)躍遷，而與激子譜峰差30meV之發光來自P2躍遷。

We have successfully grown three thicknesses of single quantum wells (SQWs) by pulsed laser deposition (PLD), which are 2nm, 4nm, and 5.2nm,respectively. Using temperature and pumping power dependent photoluminescence (PL) spectroscopy, we analyze these SQW samples and discuss the properties of (1) phonon interaction, (2) exciton binding energy, and (3) built-in electric field on these samples. From the low-temperature PL spectra of the SQWs have blue shifted D0X emission for 2 nm with the exciton binding energy large than 60meV, whereas, the exciton binding energies for 4nm and 5.2nm SQWs are 54 and 48 meV, respectively, which are less than 60 meV of the bulk ZnO and the phonon interaction decreases with decreasing well width. These results reveal the SQWs possess not only quantum confinement effect but also quantum confined Stark effect with built-in electric field. Furthermore, there is an apparent extra spectral peak in the PL spectrum of the 5.2nm SQW at 3.34eV about 45 meV below the exciton emission. In order to explore the origin of this peak, we made excitation density dependent PL measured at 80K. The results reveal that this extra peak shifts from 45 meV below the exciton emission to about 30 meV at high excitation density. We therefore attribute this peak to the exciton-exciton scattering or so-called the P-band emission that shows changing from P(infinity) emission (45 meV below exciton emission) at low excitation to P2 emission about 30 meV below exciton emission at high excitation density.