人體呼氣中超過兩百種揮發性氣體(VOC)，有些可作為偵測特定疾病的標記氣體，由於這些氣體只以微量濃度(ppt-ppb)存在於呼氣中，且以現今的半導體技術，非常難以偵測微量濃度的VOC 氣體。為了偵測如此低濃度的氣體，必須優化本感測器在氣體偵測上的響應表現。本篇研究著重於優化超薄型電阻式氮化銦感測器於偵測低濃度(sub-ppm)氨氣的響應表現，在眾多三五族半導體材料中，由於氮化銦有著最小有效質量(effective mass)及高電子飄移速率(electron drift velocity)，故較常被選用為開發高速及高頻元件的材料之一。除了
上述的優點之外，氮化銦擁有一層二維電子累積層(2-dimensional electron gas)於表面，使其在電阻式感測器的領域裡，有著不可或缺的地位。然而超薄型氮化銦的製作過程需考慮氮化銦的熱穩定性及表面型態的連續性，這在氮化銦薄膜的製作上提升不少難度。故製作過程中，先在氧化鋁(sapphire)基板上沉積一層氮化鋁做為緩衝層(buffer layer)，再使用射頻有機金屬分子束磊晶(RF-MOMBE)長成氮化銦薄膜。RF-MOMBE 結合了有機金屬氣相磊晶(MOVPE)及分子束磊晶(MBE)兩者的優點，使製程上操作的溫度更低，工業應用上也有諸
多優點，這使得RF-MOMBE 對於製作高品質氮化銦薄膜的製程備受挑戰。
本篇論文使用電阻式的超薄型氮化銦感測器偵測低濃度(sub-ppm)的氨氣，主要目標為找出優化氮化銦感測器響應的方法。簡而言之，優化響應有三個步驟：首先確認超薄型氮化銦薄膜品質，第二步實驗分析退火時間、操作溫度、薄膜厚度及薄膜上的電極設計，最後使用此最佳化的氮化銦感測器偵測低濃度氨氣，將可得到最低的訊號雜訊比(SNR)。再者，透過spectrum 得知主要電路雜訊所在的頻率位置，使用濾波器加強SNR 的比例；由退火實驗結果得知，控制退火時間大約40 小時即可得到五倍大的訊號響應；至於薄膜厚度調變的結果顯示大約40nm 的薄膜厚度可以得到最佳化的響應；又將平行版電極設計改為指叉式(interdigital)電極可提升更低濃度的偵測極限。上述所提到的電路雜訊大多來自於電源供應器的雜訊與些許高頻雜訊，故在此利用二階RC 低通濾波器與電源供應器串接即可有
效地抑制電路雜訊，使得本感測器在0.2ppm 的氨氣濃度SNR 測試得到改善。最後，合併所有優化的參數(200 小時熱退火、40nm 薄膜厚度、100um 指叉式間隙、250℃操作溫度及低通濾波器)可得到大約24 倍的SNR 比，成為一個最佳化的氮化銦氣體感測器。

Human breath has more than 200 VOC gases each of which could be a possible bio-marker for certain diseases, but due to very low concentration (ppt-ppb range) the detection of these gases seems impossible by currently available semiconductor materials. In order to detect such low range concentration we need to optimize the performance of our sensors. This study reports on optimizing the response of ultra-thin InN based resistive type gas sensor for sub-ppm range ammonia sensing. Among various III-V compounds, InN has recently gained a lot of interest for developing high speed and high frequency devices owing to its smallest effective mass and high electron drift velocity. Apart from these properties, InN has been found to have two dimensional electron gas accumulations which make it a promising material for ultra-thin resistive type gas sensor. Fabrication of ultra-thin InN is restricted by certain growth difficulties such as InN thermal instability and inconsistent surface morphology. In this work RF-radical source metalorganic molecular beam epitaxy (RF-MOMBE) was used to fabricate ultra-thin InN on Sapphire substrate with AlN as buffer layer. RF-MOMBE combines the advantages of both MBE and MOVPE enabling relatively lower temperature growth. However it is challenging to grow ultra-thin InN with good electrical performance by RF-MOMBE when compared to molecular beam epitaxy (MBE).
The fabricated ultra-thin InN on sapphire was used as resistive type gas sensor for detecting sub-ppm range ammonia gas. Here the target is to find ways to optimize the response of InN film grown by RF-MOMBE for ammonia sensing even with low electrical mobility and low sheet carrier density. The optimization of sensor response is done in 3 steps. Firstly the quality of InN ultra-thin film grown by RF-MOMBE is observed. Secondly the dependence of sensor response on annealing time, temperature, thickness and electrode design was experimentally studied. Finally for sub-ppm detection, signal to noise ratio (SNR) was found to be very low. To filter the noise, firstly the dominant noise source was found and then frequency spectrum was plotted to see the noise frequency range. Later filter circuit was implemented to enhance the SNR. From the annealing result, a fivefold increase in gas response was found for 40 hours of annealing. Thickness modulation studied revealed that the optimal thickness for ammonia gas sensing for InN grown by RF-MOMBE is around 40nm. Also change in electrode design from parallel plate design to interdigitated electrodes had a significant effect on enhancing the detection limit. From the noise spectrum it was found that most of the noise are from power source and are high frequency noises. So a 2nd order low pass RC filter was cascaded with power supply to suppress the noise. Even for 0.2 ppm we could significantly enhance the SNR. Finally all the optimal parameters (200hours annealing time, thickness of 40nm, IDE with gap of 100μm and operating temperature of 2500C along with low pass filter) were integrated and an enhancement of about 24 folds in sensing response was observed.

ACKNOWLEDGEMENT…………………………………………………………………….....i
ABSTRACT…………….………………………………………………………………………..ii
CONTENT...………..………………………………………………………………………........iv
LIST OF FIGURES .……...……………………………………….………….…….…..……...vi
LIST OF TABLES...…...………………………………………………………………...……....ix
CHAPTER : 1 INTRODUCTION 1
1.1 BACKGROUND 1
1.2 SEMICONDUCTOR BASED GAS SENSOR 2
1.3 RESISTIVE TYPE SEMICONDUCTOR GAS SENSOR 4
1.4 PROPERTIES OF InN 5
1.4.1 ELECTRONIC PROPERTIES 5
1.4.2 CHEMICAL SENSING PROPERTIES 7
1.5 GROWTH AND EVOLUTION OF InN 8
1.5.1 METAL ORGANIC VAPOR PHASE EPITAXY FUNDAMENTALS 9
1.5.2 MOLECULAR BEAM EPITAXY (MBE) FUNDAMENTALS 13
1.5.3 METAL ORGANIC MOLECULAR BEAM EPITAXY FUNDAMENTALS 16
1.5.4 SUBSTRATE AND BUFFER LAYER 19
CHAPTER : 2 THEORY AND MECHANISM 21
2.1 DEPENDENCE OF InN FILM QUALITY ON GROWTH PARAMETERS 21
2.2 RESPONSE OF SEMICONDUCTOR BASED GAS SENSORS TO VOC FOR BIO-MEDICAL APPLICATION 22
2.3 DEPENDENCE OF GAS SENSOR RESPONSE ON InN THICKNESS 23
2.4 DEPENDENCE OF ELECTRODE GAP AND ELECTRODE DESIGN 28
2.5 MOTIVATION AND OBJECTIVES 30
CHAPTER : 3 SENSOR DESIGN AND FABRICATION 32
3.1 SENSOR DESIGN 32
3.2 GROWTH OF ULTRA-THIN InN FILM BY RF-MOMBE 34
3.3 SENSOR FABRICATION 36
3.3.1 SENSING DEVICE FABRICATION 37
3.3.2 HEATER FABRICATION 38
3.3.3 ASSEMBLY AND BONDING 39
3.4 EXPERIMENTAL SETUP FOR GAS SENSING 40
CHAPTER : 4 RESULTS AND DISCUSSION 43
4.1 CHARACTERIZATION OF InN FILM GROWN BY RF-MOMBE 43
4.2 GAS SENSING RESPONSE OF InN FILM 45
4.2.1 RESPONSE FOR DIFFERENT CONCENTRATION 46
4.2.2 RESPONSE FOR DIFFERENT ANNEALING TIME 48
4.2.3 RESPONSE FOR DIFFERENT TEMPERATURE. 50
4.2.4 RESPONSE FOR DIFFERENT THICKNESS 51
4.2.5 RESPONSE FOR DIFFERENT ELECTRODE DESIGN 55
4.2.6 RESPONSE AFTER NOISE FILTERATION 59
4.3 RESPONSE AFTER INTERGRATING ALL THE OPTIMAL PARAMETERS 62
CHAPTER : 5 CONCLUSION 65
CHAPTER : 6 FUTURE WORK 67
REFERENCES 67

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