量子點紅外線偵測器及銻化鎵材料之研究

Abstract

本論文分成兩個部分：第一部份為砷化銦量子點紅外線偵測器（第三∼六章），另一部份為銻化鎵材料之研究（第七、八章）。 在第一個部分我們利用分子束磊晶系統成長高品質之砷化銦量子點製作紅外線偵測器，研究包括：偵測器其響應值對於溫度及偏壓的相依性探討、垂直偶和的量子點紅外線偵測器的特性分析、以及偵測器中的光電流頻譜調整及躍遷能階之研究。 量子點紅外線偵測器的響應對於溫度及偏壓的相依性與量子井紅外線偵測器，具有相當的差異性。對於量子井而言，響應的大小對於溫度變化溫度並沒有太大的改變，對於偏壓則呈現線性的相依性。但是，對於量子點而言：響應對於溫度及偏壓都是呈現指數關係的變化。經由詳細的元件電流增益行為的量測，對於此響應行為我們有深入的分析。在100度K的溫度變化中，元件的電流增益約有兩個數量級的改變，如此巨大的改變是由於量子點中額外注入的載子所產生的庫輪排斥力作用，此外我們亦計算了額外注入的載子數目。此額外注入的載子會改變量子點中的費米能階並且改變元件的量子效率。 對於垂直耦合的的量子點紅外線偵測器的特性，我們做了詳細的分析研究。對於垂直耦合的量子點中，會形成一個微能帶，此微能帶會使得載子在量子點中自由的移動帶，會使得量子點中的能階分佈更為均勻，進而縮小光電流頻譜的半高寬並提高元件之量子效率。此外，此垂直耦合的量子點亦可以提高自由載子的捕捉機率，因此，此結構亦可提高元件的頻率響應頻寬。 我們在標準的量子點紅外線偵測器中的量子點下方加入一層高能障之砷化鋁鎵，藉此調整元件中的載子躍遷行為。標準之偵測器其光電流響應頻譜會隨著加入之高能障層而分離成兩個訊號，其中一個光訊號並不會隨著此高能障層與量子點的距離改變而有所變化，但是另一個光訊號會慢慢的往短波長移動並且響應強度也會慢慢減弱。當此高能障層與量子點的距離只有五奈米的時候，只有一個光響應訊號會存在。因此，此光電流頻譜的半高寬與峰值波長的比例會從25％降低至10％，且元件之量子效率亦可提高。此外，此加入之高能障層亦會降低元件之暗電流，進而提高元件之偵測度表現。 第二部分為銻化鎵材料的研究，分成兩個章節：一部份為銻化鎵在砷化鎵基材中的量子點成長，另一部份為銻化鎵的矽基板上的磊晶研究。 對於銻化鎵量子點的成長我們有系統的做了一系列的實驗，包括：磊晶材料的厚度、磊晶溫度及五族三族的通量比例等。我們藉由原子力顯微鏡分析成長之量子點的表面型態，包括量子點密度及大小。此外，我們亦研究了此第二類超晶格量子點的光激光譜行為，分別分析其光譜在不同的激發光密度及不同的溫度下的表現，有別於第一類超晶格量子點，此種類的量子點光激光譜有極不同的行為。另外，我們亦發現了一層、兩層及三層的銻化鎵原子層的光譜訊號，藉由理論的分析，我們可以研究推估銻化鎵及砷化鎵兩種異質材料的價帶不連續的能量大小。 我們對於銻化鎵材料在矽基板上成長的緩衝層材料做了研究探討。如果將銻化鎵直接成長於矽基板上，銻化鎵材料的表面會非常的粗糙，藉由銻化鋁緩衝層的加入，我們可以成功的提高銻化鎵的磊晶品質，此銻化鋁一開始會在矽基板表面形成量子點，並隨著更多銻化鋁的成長此量子點會連結起來，此過程會吸收因為晶格常數不匹配而產生的應力，進而使得銻化鎵的磊晶品質提升。此外，我們亦使用的銻化鋁/銻化鎵間格的超晶格結構去阻擋晶格缺陷的延展，此種緩衝層結構會更進一步的提升銻化鎵的磊晶品質。

The thesis was separated into two parts: one is the quantum dot infrared photodetectors (QDIPs) based on the InAs/GaAs QDs (chapter 3, 4, 5 and 6), and the other is the GaSb material study (chapter 7 and 8). In the first parts, we studied the temperature dependent responsivity behavior of QDIPs, vertically coupled QDIPs, and photocurrent spectra tuning of QDIPs. Temperature dependent behavior of the responsivity of InAs/GaAs quantum dot infrared photodetectors was investigated with detailed measurement of the current gain. The current gain varied about two orders of magnitude with 100K temperature change. The dramatic change of the current gain is explained by the repulsive coulomb potential of the extra carriers in the QDs. With the measured current gain, the extra carrier number in QDs was calculated. The extra electrons in the QDs elevated the Fermi level and changed the quantum efficiency of the QDIPs. The temperature dependence of the responsivity was qualitatively explained with the extra electrons. Vertically coupled InAs/GaAs quantum dot infrared photodetectors (QDIPs) were studied. With vertically coupled quantum dots, the formation of the mini-bands among quantum dot (QD) layers enhances the uniformity of QD states and results in a narrow response spectrum and higher peak quantum efficiency. The mini-bands increase the capture probability and also facilitate the carrier flow among QD layers and leads to more uniform carrier distribution. Because these, the frequency response of vertically coupled quantum dot infrared photodetectors were much faster than that of the conventional ones. The quantum dot infrared photodetectors (QDIPs) with an additional thin Al0.2Ga0.8As layer near the quantum dot (QD) layers were studied. With the thin Al0.2Ga0.8As layer, the carrier transitions of the QDIPs can be refined. The board absorption peak of the InAs/GaAs QDIPs splits into two response peaks with the additional Al0.2Ga0.8As layer. These two signals have different behaviors as the spacing between the Al0.2Ga0.8As layer and QDs is changing. One of the peaks remains fixed at the same wavelength, and the other peak shifts to higher energy and the intensity becomes weaker as the Al0.2Ga0.8As layer is closer to the QD layers. A much narrow photocurrent spectrum was observed when the Al0.2Ga0.8As layer is 5 nm to the QDs. The fractional spectra width is reduced from 25% to 10% and the quantum efficiency is enhanced. Combining with the reduced dark current due to the higher barrier, the detectivity increases for about 5 times. In the second part, the GaSb material was studied, including the GaSb/GaAs quantum dots growth and GaSb growth on silicon substrate. The growth conditions of GaSb/GaAs quantum dots were studied systematically, including the GaSb film thickness, substrate temperature and the V/III beam equivalent flux ratio. The morphology of quantum dots is studied by the atomic force microscope. And, the excitation power density and temperature dependent photoluminescence is also studied. Due to the type-II band alignment, the result is different from the type-I band alignment system. Also, the distinct light emission peaks from monolayers of GaSb quantum wells in GaAs were observed. Discrete atomic layers of GaSb for the wetting layer prior to quantum dot formation give rise to transition peaks corresponding to quantum wells with one, two and three monolayers. From the transition energies we were able to deduce the band offset parameter between GaSb and GaAs. By fitting the experimental data with the theoretical calculated result using an k•p Burt’s Hamiltonian along with the Bir-Picus deformation potentials, the valence band discontinuity for this type II heterojunstion was determined to be 0.45 eV. The heterojunction growth of GaSb on Silicon (001) substrate with different buffer layer structure was studied. When the GaSb deposited directly on the silicon surface, the epitaxial GaSb shows a non-mirror surface. It is necessary to use the AlSb as the buffer layer in the heterostructure growth. The AlSb forms QDs on silicon surface at first few monolayers. When more AlSb deposited, the QDs would coalesce. The process is strain relief mechanism and results in better GaSb crystal quality. Furthermore, the GaSb/AlSb superlattice interface would merge and stop the dislocation propagation. Therefore, the superlattice buffer layer further increases the GaSb crystal quality.