金屬-半導體-金屬光偵測器及零維量子結構

Abstract

本論文由兩個部分組成，金屬-半導體-金屬光偵測器與零維量子結構。在關於金屬-半導體-金屬光偵測器之第一部份，我們提出了改善元件特性的方法並以實驗證明﹔在另一部份中，自組式零維量子結構之成長與特性是討論的主題。 首先，我們利用一般金屬-半導體-金屬光偵測器之結構，研究在砷化鎵上之電洞蕭基能障高度。藉由量測金屬-半導體-金屬光偵測器中，由電洞電流所支配之暗電流，可獲得電洞之蕭基能障高度。利用高度摻雜之表面薄層，可控制電洞蕭基能障高度在0.48電子伏特至0.79電子伏特之間。經由這些在金屬-半導體-金屬光偵測器中經設計之蕭基接觸，可將其暗電流降低三個數量級以上。 為製作一簡易與可重製之850nm反射器，我們提出並成功製作了具有砷化鋁/砷化鎵超晶格高折射係數層之分散式布拉格反射器。該分散式布拉格反射器係由多層之純砷化鋁與砷化鎵構成，並由於量子侷限效應而不吸收850nm之放光。計算結果顯示該超晶格分散式布拉格反射器，相對於一相同層數之砷化鋁/砷化鋁0.2鎵0.8分散式布拉格反射器，具有較高之反射率。實驗結果中，所測得之反射率與頻譜，和計算結果一致。此外，由於其成長之變數較少，該超晶格反射器在分子束磊晶成長時相當容易控制。 基於以上研究，我們提出並製作了低暗電流，於850nm具高響應度之砷化鎵金屬-半導體-金屬光偵測器。利用調整後之蕭基接觸，達到最低之暗電流密度為4.5x10-7cm-2。此外，在同一元件中，藉由新設計附有超晶格反射器之共振腔加強結構，其850nm響應度約可達0.34A/W。指寬相等於指距之該元件，其等效外部量子效率約為48%。我們的設計在樣品成長與元件製程方面，均相當簡易並具可重製性。 接著，我們詳細地探討了在(100)砷化鎵基板上自組式砷化銦量子點的成長與特性。利用PL與AFM量測，我們個別探討了三個主要長晶參數，基板溫度，砷分子束流量與砷化銦成長速率。我們發現基板溫度係形成量子點之主導參數，而另外兩個參數呈現了較為複雜的行為。此外，在長波長量子點之”點在井中”(DWELL)的結構裡，我們分別探討了砷化銦量子點在砷化銦鎵量子井中的位置，與該砷化銦鎵量子井之銦含量等兩個變數。發現將砷化銦量子點成長於砷化銦0.23鎵0.77量子井之中央，在室溫下可獲得半高寬約28毫電子伏特之1.32微米放光。 為暸解量子點之傳輸特性，我們利用砷化銦鎵量子井射極，探討了透過自組式砷化銦/砷化鎵量子點之共振穿隧。由於其射極之能態較量子點中之基態能階為低，我們可以在所有元件中接近零伏特處清楚觀測到該共振穿隧。從PL與電流-電壓特性，我們無疑地得到了經過砷化銦量子點之共振穿隧，且該結果係具可控性與可重複性。 最後，利用Stranski-Krastanow成長模式，我們成功地在砷化銦介質中成長了砷化鎵反量子點。在AFM與TEM中皆觀察到該量子尺寸之三維島狀結構。由該觀測中，我們決定出其臨界厚度係介於2.25原子層與2.5原子層之間。由砷化鎵2.5原子層生成之反量子點，其基底直徑約為15-35nm，高度約為2-4nm，而密度約為3-4x1010cm-2。

This dissertation consists of two parts: metal-semiconductor-metal photodetectors (MSMPDs) and zero-dimensional (0-D) quantum structures. In the first part, methods to improve the device performance of MSMPDs are proposed and demonstrated experimentally. In the second part, the growth and characterization of self-assembled 0-D quantum structures are discussed. At first, hole Schottky barrier heights on GaAs have been studied experimentally by using a conventional metal-semiconductor-metal photodetector (MSMPD) structure. The Schottky barrier height for holes was obtained directly by the hole-current dominated dark current measurement of the MSM-PD. With a thin, highly doped surface layer, control of the Schottky barrier heights for holes from 0.48eV to 0.79eV was obtained. By using these engineered Schottky contacts in the MSM-PDs, over three orders of magnitude reduction in the dark currents of the MSM-PDs was achieved. To fabricate an easy and reproducible reflector at 850nm, we proposed and fabricated a distributed Bragg reflector (DBR) with AlAs/GaAs superlattice high index layers. The superlattice-DBR consists of multi-layers of pure AlAs and GaAs only, and does not absorb 850nm light emission due to quantum-confinement effects. Calculated results show that the superlattice-DBR has a higher reflectivity than a conventional AlAs/Al0.2Ga0.8As DBR at 850nm for a same number of periods of quarter-wave stacks. In the experiment, the measured reflectivity and spectrum are consistent with the calculated result. Besides, because of fewer growth variables, the SL-DBR is much easier to control during a MBE growth. Based on above works, we proposed and fabricated a GaAs MSMPD with both low dark current and high responsivity at 850nm. Using the modified Schottky contacts, a lowest dark current density of about 4.5x10-7cm-2 was achieved. Besides, in the same devices, the responsivity resulted from a newly designed resonant-cavity-enhanced (RCE) structure with a SL-DBR was about 0.34A/W at 850nm. The equivalent external quantum efficiency of the devices with equal finger spacing and finger width was about 48%. Our design is relatively easy and reproducible for both sample growth and device process. In the following, growth and characterization of self-assembled InAs quantum dots (QDs) on (100) GaAs substrate are discussed in detail. Using PL and AFM measurements, three main growth parameters, substrate temperature, arsenic beam flux and InAs growth rate, have been studied and discussed respectively. We found that the substrate temperature is the dominant parameter in the formation of QDs, and the other two parameters reveal more complicated behaviors. Besides, in the growth of long-wavelength QDs with the dot-in-well (DWELL) structure, the position of InAs QDs in InGaAs QWs and the indium composition of the InGaAs QWs have been studied. For the DWELL sample with InAs QDs in the middle of In0.23Ga0.77As QWs, 1.32□m emission with FWHM of about 28meV was obtained at room temperature. To investigate the transport properties of QDs, we experimentally studied the resonant tunneling through self-assembled InAs/GaAs quantum dots (QDs) using InGaAs quantum well emitters. Because the energy state of emitter was lower than the level of the ground state within InAs QDs, the resonant tunneling was observed clearly near zero bias in all devices. From the results of PL and current-voltage (I-V) characteristics, we can obtain unambiguously the resonant tunneling through the InAs QDs, both controllably and reproducibly. Finally, using Stranski-Krastanow growth mode, we have grown GaAs antidots in InAs matrix successfully. The quantum-sized 3-D islands were observed clearly in both AFM and TEM measurement. From these observations, the critical thickness is determined to be between 2.25ML and 2.5ML. For 2.5ML GaAs deposition, the grown antidots have a size about 15-35nm in base diameter and about 2-4nm in height with a density about 3-4x1010cm-2.