MOCVD成長GaAsN/GaAs量子井的深層缺陷能階與能帶研究

Abstract

本篇論文主要研究GaAs1-XNX量子井的物理特性，對半導體量子結構而言，能帶圖是其一個非常重要的特性，量子井的能帶即代表著侷限電子與電洞的位勢，所以能帶圖對理論計算上是非常有用的，在本研究中，利用各種不論是光性或是電性的量測技術，藉由比較不同的量子井厚度與濃度，以及低溫成長所產生的缺陷，可以讓我們了解到GaAs1-XNX的量子井能帶結構。

當摻雜N於GaAs中，在PL光譜中除了量子井的訊號之外，產生了額外的深層能階其發光能量大約在1.1 eV附近，這個深層能階產生了與量子井相同的紅移現象，而這個現象是因為量子井的尺寸效應所產生的，加入氮時主要是下降CB(conduction band)，所以這個深層能階的訊號主要是由量子井與在VB上0.2 eV的VGa缺陷能階之間的躍遷，利用實驗上所量測得到這個缺陷能階與GaAsN CB及GaAs CB間躍遷的能量值，我們可以發現所量測的樣品能帶結構為type-II的結構，量測不同的氮濃度可以計算得其VB offset分別為0.6%為0.022 eV而1.8%為0.003 eV，可以初步推測當氮濃度增加時，其能帶結構由type-II慢慢轉變到type-I。而樣品熱退火的結果也符合深層能階為我們所推論的VGa，但是在較低溫的熱退火溫度下會發現氮的成分波動效應將會加強，如果將熱退火溫度增加到800 ℃便可以降低成分波動效應的影響。如前面所說的摻雜一點點氮時主要會下降CB edge但對VB的影響很小，而這個情形下的能帶結構為雜質的型態，故可以利用微擾理論下的BAC模型去計算GaAs1-XNX的能隙，而這個計算結果也與我們實驗所量測的結果相當接近。

除了光學特性上量測的結果之外，我們也利用電性量測技術去量測出樣品的缺陷能階。首先在C-V量測上可以發現在接近樣品表面的區域存在EL2的缺陷對樣品產生空乏，而當量子井厚度增加的時候，量子態會降低使載子所面對到的能量位障較高，在較高的位障以及EL2缺陷的空乏影響下，量子態中的電子被空乏，當我們再較厚的量子井結構下，會有較大的EL2空乏，此時DLTS與電容導納頻譜中我們會量測到二到三個缺陷訊號，而其中之一便是由氮的成分波動效應所表現出來的訊號，藉由量測與分析各個不同厚度與氮濃度的量子井，我們可以分析出各個不同氮濃度所產生的成分波動效應能階，最後我們將電性量測與PL的數據相比，會發現到在電性以及光性我們分別量測到成分波動效應的訊號且相當接近。

在這個研究當中我們量測出GaAsN/GaAs量子井結構的能帶，且利用因為低溫成長所造成的深層能階量測證實了這是一個type-II的結構，藉由缺陷的出現，本研究得以分析量子井的能帶結構，且在高溫的熱退火作用下便可將其去除掉，而成份波動效應也出現在訊號當中，且低溫熱退火會增加成分波動效應的現象。在電性量測上除了量測到EL2缺陷的出現，而電性上與光性可以量測到同樣接近的氮的成分波動效應訊號，所以本研究是利用了缺陷量測去分析樣品的能帶結構，與探討氮的成分波動效應。

This thesis studies the physical properties of GaAs1-XNX /GaAs quantum wells with emphasis on the experimental determination of the electronic band structure. When N is incorporated into the GaAs, the PL spectra of GaAs1-XNX /GaAs heterostructures revealed a deep level emission at ~1.1 eV. This emission displays a similar size confinement effect as the quantum-well emission. Since the addition of N mainly shifts downward the conduction-band edge, this emission is attributed to a transition involving a VGa defect level at ~0.2 eV above the GaAsN valence-band edge. From the observed transition energy between this deep level and GaAs conduction band (CB), the GaAsN-GaAs band alignment is determined to be type-II with valence band offsets of 0.022 eV and 0.003 eV for x=0.6% and x=1.8%, respectively. As the composition of N increases, the band alignment likely transforms from type-II to type-I. The results of annealing support the assignment of VGa as the deep level. Annealing at a low temperature is found to increase N composition fluctuation. When the annealing temperature is increased to 800 ℃, the N composition fluctuation is decreased. Dilute N incorporation lowers the conduction-band edge but has a little affect on the valence band. Because the band structure of dilute N is impurity like, we use the band anticrossing (BAC) model to predict the band gap of GaAs1-xNx and the calculated results are found to close to our experimental data under strain.

Beside the PL analysis, we also investigate the N composition fluctuation of the GaAs1-XNX/GaAs quantum well by electric measurement. In the samples with a thick quantum well, the C-V measurement reveals significant carrier depletion near the sample surface due to EL2 trap whose presence is proved by photocapacitance measurement. This EL2 trap can deplete carriers in the quantum level. In thick GaAsN layer, DLTS and admittance measurement reveal two or three trapping signals. One of them is considered as the emission from the N composition fluctuation. From the temperature dependence of the emission time, the CB offset is obtained. By comparing with the PL data, the activation energy observed by the DLTS is very comparable. The N composition fluctuation is observed both by PL and DLTS measurement.

In summary, the band structure of the GaAs1-XNX/GaAs quantum well is studied. A type-II alignment is determined by analyzing a deep level emission probably induced by a low temperature growth. This deep level emission can be removed by high-temperature annealing. The N composition fluctuation reveals a small emission peak and increases after annealing. The N composition fluctuation is also investigated by electric measurements and the activation energy is close to PL data. The results of this work demonstrate the possibility of utilizing defect states for the experimental determination of the band structure of semiconductor nanostructure. The N composition fluctuation is also measured by the different measurement.