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研究生： 王偉庭
Wang, Wei-Ting
論文名稱： 氮化鈦薄膜應用於發光二極體透明電極之研究
The Study of TiN as Transparent Contact for Visible Light Emitting Diodes
指導教授： 王永和
Wang, Yeong-Her
洪茂峰
Houng, Mau-Phon
學位類別： 碩士
Master
系所名稱： 電機資訊學院 - 微電子工程研究所
Institute of Microelectronics
論文出版年： 2002
畢業學年度： 90
語文別： 英文
論文頁數： 90
中文關鍵詞： 透明電極 、氮化鈦 、發光二極體
外文關鍵詞： Transparent Contact, Light Emitting Diodes (LEDs), Titanium Nitride (TiN)
相關次數： 點閱：94  下載：5 
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    • 一般的AlGaInP發光二極體都是成長在n-型GaAs基板上，電流自上面流下經過p層然後至活性層以產生光，為了要得到高發光效率，p層的電流分佈非常重要。為了促進電流分佈以提高發光二極體的發光效率，一法為成長一層厚的GaP窗口層在最上面，但因為以有機金屬氣相沈積法不易成長足夠厚度且高品質之GaP窗口層，因此另一法為成長透明導電薄層，即本論文提出之氮化鈦透明導電膜。
    本論文利用真空濺鍍系統在AlGaInP發光二極體上沈積氮化鈦薄膜(約50nm)來作為透明電極。同時也將氮化鈦沈積在透明基材上,以研究薄膜之光穿透率。回火前氮化鈦薄膜沈積在透明基材上於可見光範圍透光率可至60%，回火後透光率下降；回火後氮化鈦薄膜沈積於GaP上電阻率可至850μΩ-cm，較回火前電阻率為低。
    氮化鈦沈積在p型磷化鎵上呈現整流特性。為了同時兼顧氮化鈦的光特性及電特性，乃嘗試先沈積一層鈦當作緩衝層以提升氮化鈦的薄膜品質。如預期的電流有明顯的增加，發光二極體的臨界電壓(在20mA時)也明顯下降。

    AlGaInP light emitting diode was grown on n-GaAs substrate, the drive current was though the p-type material to active layer and generate light. A very important factor for high efficiency of the device is the current spreading effect of p-type layer. There are two methods for high efficiency LEDs to improve the current spreading effect, one is the growth of a thick lattice-matched window layer (with a bandgap higher than that of the active region) which maximizes the light extraction from the surface emitting device. An associated problem with the AlGaInP system is the difficulty of growing defect-free p-type material with sufficient thickness and a high conductivity to act as a spreading layer under the top metal contact. The other is to deposit a transparent conductor. TiN is evaluated as a potential low-cost current spreading layer (CSL); hence the device structures describe in this paper have been designed to estimate the effectiveness of TiN.
    The work reported in this paper TiN was deposited on AlGaInP LED surfaces to form transparent contacts by sputtering system. In order to investigate the light transparency, TiN was deposited on transparent substrate also. The light transparency of the as-deposited TiN thin films (~50nm) by RF sputtering system was about 60% at visible region, and decreased after annealing. The resistivity of TiN deposited on GaP was 850μΩ-cm after annealing.
    The TiN deposited on p-GaP exhibits rectifying properties. To look after both light output and electrical properties, we deposited a thin Ti to improve the quality of TiN. As expectancy we want the current increase obviously, and the turn-on voltage decreased obviously also.

    Abstract I Contents III List of Figures V List of Tables VIII Contents Chapter 1 Introduction 1 1.1 Motivation 1 1.2 Organization of the Thesis 3 Chapter 2 Background Theory 5 2.1 A Short View of Light Emitting Diodes 5 2.1.1 Introduction 5 2.1.2 Epitaxial Growth 5 2.1.3 Current Spreading and Light Extraction 8 2.2 Efficiency of LED 13 2.2.1 Fundamental concept 13 2.2.2 Theory of Fresnel Loss 14 2.2.3 Theory of Snell’s Law 16 2.3 Fundamental Characteristics of GaP 17 2.4 Fundamental Characteristics of TiN 17 2.5 Ohmic Contacts 19 2.5.1 Theory of Ohmic Contacts 21 2.5.2 Practical Ohmic Contacts 21 Chapter 3 Material and Dervice Fabrication 23 3.1 Sputtering Techniques 23 3.1.1 Principle of Sputtering 24 3.1.2 Glow Discharge DC Sputtering 24 3.1.3 High Rate Sputteing Deposition 25 3.1.4 Reactive Sputter Deposition 26 3.2 Preparation and Process Concepts 27 3.2.1 Pre-Processing Cleaning 27 3.2.2 Sputtering Materials 28 3.2.3 Mask Design and Fabrication of the Transparent Contact 28 3.3 Characterization of Films 30 3.4 Electro-Optical Measurement of LEDs 31 Chapter 4 Results abd Discussion 34 4.1 Introduction 34 4.2 Growth of TiN Films 34 4.2.1 Resistivity and Thickness of Deposition Thin Films 34 4.2.2 Light Transparency of Deposition Thin Films 35 4.2.3 Sample annealing 36 4.2.4 Summary of this section 37 4.3 LED Performance 37 4.3.1 Electrical Properties 37 4.3.2 Optical Properties 38 4.3.3 Lossy Transmission Line 42 4.4 Discussions on Depth Profile Analysis of TiN/GaP Interface 43 Chapter 5 Conclusions and Future Works 45 5.1 Conclusions 45 5.2 Future Works 45 References 47 List of Figures Fig.1.1 The energy gap for (AlxGa1-x)yIn1-yP as a function of the lattice constant. A direct bandgap ranging 1.9–2.3 eV (red to green spectral range) is obtained while maintaining lattice matching with GaAs. 52 Fig.1.2 Chip structure and typical current flow paths through double heterojunction AlGaInP LED chips. (a) Conventional LED structure. (b) LED chip with an n-type current-blocking layer below the top contact 52 Fig.1.3 A simple LED operation circuit diagram. 53 Fig.1.4 LED emission pattern (a) LED suffer from current crowing (b) LED has better current spreading. 53 Fig.1.5 The device structures: (a) consist of a p-type GaP spreading layer 6 μm (b) and (c) were both fabricated with TiN 54 Fig.1.6 The organization flow chart of this thesis. 55 Fig 2.1 Surface light emission intensity profiles for square-shaped (AlxGa1-x)0.5In0.5P LED chips with various p-type GaP upper window thicknesses. A dip in the intensity profile at the center of the chip is due to absorption by the ohmic contact on the top surface of the chip. (Reprinted with permission from Journal of Electronic Materials, vol. 20, pp. 1125–1130, 1991, a Publication of The Materials, Metals, & Materials Society, Warrendale, PA 15086). 59 Fig.2.2 Simplified schematic illustration of light extraction from various LED structures with (a) an absorbing substrate and a “thin” window layer, (b) an absorbing substrate and a “thick” window layer, (c) a DBR below the active layer, and (d) a transparent substrate. (After F. A. Kish, “Light-Emitting Diodes,” in Encyclopedia of Chemical Technology, 4th ed., vol. 15, J. I. Kroschwitz, Ed. New York: Wiley, 1995, pp. 217–246. Reprinted with permission of John Wiley & Sons, Inc.) 60 Fig.2.3 Some fundamental concepts of light source devices (a) Luminous Flux (b) Luminous Intensity (c) Illuminance (d) Luminance (e) relationship of all 61 Fig.2.4 Fresnel Loss diagram 62 Fig.2.5 Snell's law diagram 62 Fig.2.6 Transmission diagram with a thin film coating 63 Fig.2.7 Epoxy of LED 63 Fig.2.8 Titanium Nitride (TiN) lattice structure 64 Fig.2.9 Structure model of titanium nitride thin films: (a)as-sputtered (b)after annealing 300-450k (c) after annealing above 450k 64 Fig.2.10 Schematic band energy diagram of a metal/n-semiconductor contact showing the three major current transport mechanisms: thermionic emission (TE), thermionic-field emission (TFE) and field-emission (FE). 65 Fig.2.11 metal-semiconductor(p or n type) energy bandgap at thermal equipment (a) for n-type semiconductor (b) for p-type semiconductor 66 Fig. 3.1 (a) The sputtering process(Chapman 1980, p 178) (b) A simple DC sputtering system(Chapman 1980, p 178) 67 Fig. 3.2 The sputtering yield of argon ions on copper ( Carter & Colligon 1968, p 182 ). 68 Fig. 3.3 The influence of a magnetic field on electron motion ( Chapman 1980, p 263 ). 69 Fig. 3.4 Schematic of the experimental setup used for multilayers sputtering system 70 Fig.3.5 A commercial LED process diagram 71 Fig.3.6 The two masks are needed for the LED p-side electrode 72 Fig. 3.7 The gist of processing chart. 73 Fig. 4.1 The dependence of film thickness and refractive index vs. deposition time at working pressure of 5m~7m torr in TiN with various RF power 50W~150W 74 Fig.4.2 (a) The diagram of resistivity vs. gas flow ratio (Ar/N2 sccm) (b) The diagram of resistivity vs. substrate heating temperature under different RF power. 75 Fig.4.3 The light transparency of the TiN deposited on Arton film 76 Fig.4.4 Different Gas Flow Ratio condition at the same thickness (a)Without nitride (b) With nitride 1 sccm 77 Fig.4.5 TiN film with different substrate heating and (a) Before annealing (b)After annealing 480 0 C with N2 10min 78 Fig. 4.6 Summary of resistivity and light transparency with different R.F power and deposition time (a) deposit TiN on GaP/LED (b)deposition time 5 minutes (c) deposition time 8 minutes 79 Fig.4.7 The structures of LED without TiN were shown in different p-side metal contact (a)100×100μm (b) 300×300μm (c)all cover,(d)I-V cure of (a )~(c)structures (e) I-V cure (b)structure with different LED chip size at 3×3mm2, 1×1mm2, respectively 80 Fig.4.8 The structures of LED with TiN were shown in different p-side metal contact (a)100×100μm (b) 300×300μm (c)all cover, (d) I-V cure of (a)~(c)structures (e) I-V cure (b)structure with different LED chip size at 3×3mm2, 1×1mm2, respectively 81 Fig.4.9 Summary of the electrical and optical properties of LED device with and without TiN coating (a)LED without TiN (b) )LED with TiN (c) LED electrical properties (d) LED optical properties 82 Fig.4.10 LED emission pattern with p-metal 300μm×300μm at 1mm×1mm square chip size.(a)~ (c) with TiN (d)~ (f)without TiN (a) apply I =100mA (b)apply I=5mA (c) apply I=5mA on TiN film (d) apply I =100mA (e) apply I =50 mA (f)apply I =20mA 83 Fig.4.11 Adobe Photoship image software to analysis the emission pattern pictures (a)LED with TiN (b)LED without TiN (c) correspond to picture (a) (d) correspond to (b) 84 Fig.4.12 LED emission pattern at bigger chip size (with p-metal 300μm×300μm at 3mm×3mm square chip size) (a) without TiN apply I=20 mA (b) without TiN apply I=100 mA (c) with TiN apply I=20 mA (d) with TiN apply I=100 mA 85 Fig.4.13Summarizes the results of the light output versus chip emission area at a drive current of 50 mA and 100 mA 86 Fig.4.14 Lossy transmission line model corresponding to the spreading resistance calculation, where R is derived from the resistivity of TiN layer and G is the conductance of the semiconductor layers (the diode resistance contribution to G is ignored in the calculations) 88 Fig.4.15 SIMS depth profile analysis of bare LED chip 88 Fig.4.16 SIMS depth profile analysis of LED chip with TiN (a) before annealing (b) after annealing 89 Fig.4.17 SIMS depth profile analysis of LED chip with TiN whin Ti thin film (a) before annealing (b) after annealing 90 List of Tables Table 2.1 Properties of GaxIn1-xP 59 Table 2.2 Physical Properties of Titanium Nitride (TiN) Coatings. 60 Table 2.3 Physical Properties of Indium Tin Oxide (ITO) . 61

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