自組式有機金屬汽相磊晶系統(MOCVD) LabVIEW自動控制程式設計和低溫氮化鎵磊晶成長及其光電特性研究

Abstract

本論文主要分為2大部分，一為利用LabVIEW人機介面平台整合MOCVD磊晶系統軟硬體通訊介面，設計一套符合自組式MOCVD系統硬體設計的全自動磊晶控制程式；另一大項則為低溫氮化鎵磊晶成長研究。 在該控制程式設計上除了整合各項硬體時序監控(Time sequence control and monitoring)外，尚有另一獨立狀態判定重複迴圈設計。該重複迴圈提供持續不間斷的特定緊急狀態觸發判定，可供強制系統動作以確保人機安全；程式尚保留了另一獨立"中斷程式"控制按鍵，提供使用者強迫中斷使用。該LabVIEW控制程式實際運行時間已逾一年(24hrs不中斷)，完成近200次的磊晶程序並無任何差錯，可見該程式擁有絕佳穩定性及可靠度。在程式人機介面設計亦相當美觀，在整體操作功能上已不遜於商用機台。 在研究部分，利用該自組式NH3預熱MOCVD系統(TP-MOCVD)在低溫環境(<1150°C)下成長一系列(650~1130°C)氮化鎵薄膜並藉由量測低溫光激螢光光譜來初步判定低溫成長GaN磊晶層品質。在我們的研究中，TP-MOCVD系統GaN成長溫度700°C的13K PL譜線仍保有h-GaN的近能帶邊緣(NBE, 3.47eV)發光，且沒有發現與雜質、缺陷相關的2.2eV黃光訊號。就我們所知，這是在MOCVD系統中目前擁有如此近能帶邊緣發光的最低成長溫度。 此外，在中間成長溫度區域(800~850°C)所成長的GaN其峰值PL強度和整體積分強度均驟降，還有1130°C成長GaN光性反而嚴重變差的可能原因與NH3 所裂解的(N-H)x radicals濃度變化有關。再者，GaN 650°C成長樣品譜線相較於其他高溫成長樣品其譜線轉由3.27eV的低能量波峰所主導。藉由θ/2θ XRD繞射實驗已初步排除產生GaN Cubic/hexagonal的混合晶相結構，且由於低能量位置呈現3個近似等能量間距的峰值譜線，間距約為70~90meV；初步認定其峰值為淺層施子受子對(DAP)和其1，2階phonon replicas(Shallow donor and acceptor pair and its 1st、2nd order phonon replicas )復合發光。

This thesis is divided into two major parts. One is focused on automatic control epitaxy program design for the home-made MOCVD system based on LabVIEW platform. The other one is focus on the study on the optical and electrical properties of low-temperature grown gallium nitride(GaN). The main function of LabVIEW control program is to integrate all the apparatus installed on the home-made MOCVD system, and its most featured function is that data flows can be transmitted and received simultaneously within 0.1sec by using "#8 independent RS-232 ports" parallel protocol flow-bus microprocessor cards. The LabVIEW epitaxial control program also have embedded sub-trigger routine program that designed for continuing detecting the various default emergency states, if any of the emergency state triggers, the LabVIEW program will automatically forced itself to load into the user-defined safety state mode to make the first aid of crews and system. The WK2008 MOCVD epitaxy program had been executed over 185 runs, well proving its stability and reliability. The epitaxy control program on LabVIEW platform is comparable to commercial MOCVD system in various aspects. The second part of this thesis is study on the optical and electrical properties of low-temperature grown gallium nitride(LT-GaN) by our home-made MOCVD. We use our home-made MOCVD called WK2008 TP-MOCVD system to grow a series of Low-temperature GaN ranging from 650 to 1130°C. And we use 13K PL measurement to characterize the epitaxial quality of LT-grown GaN films. According to our data, we show the lowest temperature grown GaN at 700°C that the 13K PL spectrum is dominated by near-band edge emission (NBE, 3.47eV) without defects or impurities related yellow band luminescence (YL, 2.2eV). To our knowledge, no GaN film with this dominant NBE within temperature range that less than 900°C have been grown before by any technique using NH3 as V source. Besides, in our 13K PL data, an anomalous behavior of NBE peak intensity and integral intensity of full spectrum had been observed. In the medium temperature range (800-850°C), the NBE peak intensity and integral intensity of full spectrum is greatly reduced, and the exact reason for this phenomenon is still unclear. We may possibly attribute it to relating to the concentration of (N-H)x radicals that is produced by NH3 thermal cracking. The 13K PL main emission peak of 650°C grown GaN is "red-shifted" to 3.27eV. The postulation of cubic/hexagonal mixed phases in 650°C grown GaN is preliminary precluded by the single diffraction peak (34.52°) of θ/2θ XRD experiment. The possible reason of red-shift may be attributed to the dominance of donor acceptor pair (DAP) and its 1st and 2nd phonon replicas of hexagonal phase GaN, because of its approximately equallivant energy separation (about 70-90meV) between the sequential emission peaks.