臺灣博碩士論文加值系統

　　本論文中將會先從探討Ni及Pt的厚度在 Ni/Pt/Ag/Au以及Ni/Ag/Au沉積在正型砷化鎵當作歐姆接觸金屬結構之影響，為了得到最低的特徵阻值，我們尋求最佳的Ni, Pt的金屬厚度還有最佳的回火條件。首先利用傳輸線模型(TLM, transmission line model)的方法求得Ni/Pt/Ag/Au金屬材料在正型砷化鎵的特徵阻值，其中發現當Ni、Pt、Ag以及Au的厚度分別是25nm、50nm、300nm及20nm時，其特徵阻值會降到最低( ρc =1.76×10-6Ω-cm2 )，回火溫度為300℃而回火時間為1分鐘。接著，利用相同方式找尋出Ni/Ag/Au金屬材料在正型砷化鎵的特徵阻值，發現當Ni、Ag以及Au的厚度分別是25nm、300nm及20nm時，其特徵阻值會降到最低( ρc=1.2×10-6Ω-cm2 )，回火溫度為300℃而回火時間為1分鐘。
　　此外，我們也研究此歐姆接觸金屬蒸鍍在正型砷化鎵基板上，經過長時間的回火(4 ~ 60小時,回火溫度為200℃)，歐姆接觸的特徵阻值的變化，我們可以發現Ni(25nm)/Ag(300nm)/Au(20nm)這個冶金結構在正型砷化鎵上，特徵阻值會隨著回火時間的增加而增加，大約是增加五到十倍。
　　緊接著，將具有最低特徵阻值的歐姆接觸金屬組成結構(亦即Ni (25nm)/Ag(300nm)/ Au(20nm)) 蒸鍍在雙接面太陽能電池的正電極，來探討Ni/Ag/Au姆接觸金屬材料應用於元件上之後的元件特性，如同歐姆接觸金屬蒸鍍在正型砷化鎵基板上的結果，太陽能電池的效率(Eff)、填充因子(FF)、短路電流(Isc)、開路電壓(Voc)皆在最佳歐姆接觸金屬組成的最佳條件下有最好的表現。

　　In this dissertation, we discussed the effects of the thickness of the Ni and Pt on the Ni/Pt/Ag/Au ohmic contact metallurgical structure on p-GaAs in the beginning. We sought for the optimum thickness of the Ni and Pt layers and the suitable annealing temperature and time for the lowest specific contact resistance ρc by transmission line model method (TLM).We found out that the optimum metallurgical structure is Ni(25nm)/Pt(50nm)/Ag(300nm)/Au(20nm) which annealed at 300℃ for one minute. The lowest value (ρc=1.76×10-6 Ω-cm2) of specific contact resistance could be attained. And then, we used the same method to seek for the optimum thickness of the Ni layer and the suitable annealing temperature and time for the lowest specific contact resistance ρc. We found out that the optimum metallurgical structure is Ni(25nm)/ Ag(300nm)/Au(20nm). The lowest specific contact resistance value (ρc=1.2×10-6 Ω-cm2) could be attained. The sample annealed at 300℃ for one minute.
　　In addition, we studied the thermal stability of these ohmic contact metallurgical structures on the bulk GaAs. The ρc of these ohmic contact metallurgical structures were increased five to ten times after annealing for 4 to 60 hours when held annealing temperature at 200℃ in H2 ambient environment.
　　Sequentially, we applied the optimum ohmic contact metallurgical structure Ni (25nm) / Ag (300nm) / Au (20nm) as the positive electrode of the dual-junction (DJ) solar cells. Then, we measured the efficiency (Eff), fill factor (FF), open circuit voltage (Voc) and short circuit current (Isc) of solar cells. As the result of the optimum ohmic contact condition, all the parameters of solar cell had the best performance at the optimum ohmic contact condition.

摘要…………………………………………………………………………………I
Abstract…………………………………………………………………………III
Acknowledgment…………………………………………………………………V
Content……………………………………………………………………………VI
Figure Captions…………………………………………………………………………VIII
List of Tables……………………………………………………………………………XIV
Chapter 1 Introductions…………………………………………………………1
Chapter 2 Theoretic foundations…………………………………………4
2.1 Metal and Semiconductor contact theory………………………………4
2.2 Transmission Line Model (TLM)……………………………………11
2.3 Measurement method………………………………………………15
2.4 Basic concepts of the solar cell………………………………………17
2.5 The equivalent circuit analysis of a solar cell…………………………18
2.6 Fundamental solar cell parameters……………………………………19
Chapter 3 Fabrication process and the electric characteristic of
ohmic contact metals…………………………………………………22
3.1 GaAs TLM Fabrication Process………………………………………22
3.1.1 Wafer cleaning process………………………………………………22
3.1.2 Photolithography process……………………………………………23
3.1.3 Metal deposition…………………………………………………23
3.1.4 Mesa etching process……………………………………………24
3.2 Specific contact resistance analysis……………………………………24
3.2.1 Ni/Ag/Au……………………………………………………………25
3.2.2 Ni/Pt/Ag/Au…………………………………………………………28
3.2.3 Ni/Pt/Au……………………………………………………………32
3.3 Thermal stability of ohmic contacts…………………………………33

Chapter 4 Characteristics of the InGaP on GaAs dual -junction solar
cell………………………………………………………………………37
4.1 InGaP/GaAs dual-junction solar cell structure……………………37
4.2 Dual-junction solar cell fabrication process…………………………38
4.3Current-Voltage measurement system (I -V) …………………………40
4.4 The best condition on the DJ solar celll………………………………41
4.5 The best condition on the high concentration DJ solar cell……………46
Chapter 5 Conclusions…………………………………………………52
References…………………………………………………………………54

Figure Captions
Figure 2.1 A Schottky barrier formed by contacting an n -type
semiconductor with a metal having a larger work function: b and diagram
for the metal and semiconductor before joining…………………………5
Figure 2.2 Band diagram for the junction at equilibrium.………………6
Figure 2.3 Ohmic metal-semiconductor contact：Φm＜Φs for an n-type
semiconductor at the equilibrium band diagram for t he junction. ………8
Figure 2.4 Ohmic metal-semiconductor contact：Φm＞Φs for an p-type
semiconductor at the equilibrium band diagram for the junction. ………9
Figure 2.5 Ohmic metal-semiconductor contact：Heavy doped for an
n-type semiconductor at the equilibrium band diagram for the
junction. ………………………………………………………………10
Figure 2.6 Ohmic metal-semiconductor contact：Heavy doped for an
p-type semiconductor at the equilibrium band diagram for the
junction…………………………………………………………………11
Figure 2.7 A slab of material with ohmic contacts on the two ends
exhibits a resistance composed of the end -to-end resistance of the
material, plus the two contact resistance. ………………………………14
Figure 2.8 The approaches used to model the ohmic contact of a transmission line…………………………………………………………15
Figure 2.9 Basic pattern used to experimentally determine contact resistance parameters. Ohmic contacts are separated by increasing distance……………………………………………………………………16
Figure 2.10 Plot of measured resistance as a function of contact separation yields sheet resistance, contact resistance, and other parameters…………17

Figure 2.11 Equivalent circuit of a solar cell, including series and shunt
resistances………………………………………………………………19
Figure 2.12 Terminal I-V properties of a p-n junction diode in the dark
and in illumination………………………………………………………..21
Figure 3.1 Fig. 3.1 Variations of specific contact resistance for Ni (50nm) / Ag (300nm) / Au (20nm) metal structure deposited on p-type GaAs after annealing at different temperatures in H2 ambient for 1 minute…………26
Figure 3.2 Variations of the lowest specific contact resistance for Ni (x) / Ag (300nm) / Au (20nm) metal structures deposited on p-type GaAs after annealing at different temperatures in H2 ambient for 1 minute…………27
Figure 3.3 Various of specific contact resistance for Ni (50nm) / Ag (300nm) / Au (20nm) metal structure deposited on p-type GaAs after annealing 300℃ for various time in H2 ambient….……………………………………28
Figure 3.4 Variations of the lowest specific contact resistance for Ni (x) / Pt(50nm) / Ag (300nm) / Au (20nm) metal structures deposited on p-type GaAs after annealed at different temperatures in H2 ambient for 1 minute.
……………………………………………………………………………30
Figure 3.5 Various of specific contact resistance for Ni (25nm) / Pt(50nm) / Ag (300nm) / Au (20nm) metal structure deposited on p-type GaAs after annealing 300℃ for various time in H2 ambient.………………………31
Figure 3.6 Variations of the lowest specific contact resistance for Ni (25nm) / Pt(y) / Ag (300nm) / Au (20nm) metal structures deposited on p-type GaAs after annealing at different temperatures in H2 ambient for 1 minute. ……32
Figure 3.7 Variations of specific contact resistance for Ni (50nm) / Pt (80nm)/ Au (200nm) metal structure deposited on p -type GaAs after annealing at different temperatures in H2 ambient for 1 minute. …………33

Figure 3.8 Variations of specific contact resistance for Ni (10, 25, 50nm) / Ag (300nm) / Au (20nm) metal structures deposited on p-type GaAs and annealed at 200℃ for a longtime in H2 ambient.……………………………34
Figure 3.9 V Variations of specific contact resistance for Ni (10, 25, 50nm) / Pt(50nm) / Ag (300nm) / Au (20nm) metal structures deposited on p-type GaAs and annealed at 200℃ for a longtime in H2 ambient…………………35
Figure 3.10 Variations of specific contact resistance for Ni ( 25nm) / Pt(10, 25, 50,80nm) / Ag (300nm) / Au (20nm) metal structures deposited on p-type GaAs and annealed at 200℃ for a longtime in H2 ambient.………36
Figure 4.1 Schematic cross-sections of the InGaP/GaAs Dual -junction
solar cell.……………………………………………………………………38
Figure 4.2 Schematic of the current-voltage characteristic measurement
system. (solar simulator). …………………………………………………41
Figure 4.3 The Jsc versus annealing temperature of Ni (25nm) / Ag (300nm) / Au (20nm) ………………………………………………………………43
Figure 4.4 The Voc versus annealing temperature of Ni (25nm) / Ag (300nm) / Au (20nm)…………………………………………………………………44
Figure 4.5 The fill factor versus annealing temperature of Ni (25nm) / Ag (300nm) / Au (20nm)………………………………………………………45
Figure 4.6 The efficiency versus annealing temperature of Ni (25nm) / Ag (300nm) / Au (20nm)………………………………………………………46
Figure 4.7 The Jsc versus annealing temperature of Ni (25nm) / Ag (300nm) / Au (20nm) on the high concentration DJ solar cell.………………………48
Figure 4.8 The Voc versus annealing temperature of Ni (25nm) / Ag (300nm) / Au (20nm) on the high concentration DJ solar cell………………………49
Figure 4.9 The fill factor versus annealing temperature of Ni (25nm) / Ag (300nm) / Au (20nm) on the high concentration DJ solar cell.……………50
Figure 4.10 The efficiency versus annealing temperature of Ni (25nm) / Ag (300nm) / Au (20nm) on the high concentration DJ solar cell.……………51

List of tables
Table 2.1 Work functions of some metals. …………………………………7
Table 2.2 Electron affinity of some semiconductors. ………………………7
Table 3.1 The composite structure of the Ni/Ag/Au and Ni/Pt/Ag/Au……23
Table 4.1 Compare the output parameters with different annealing temperature…………………………………………………………42
Table 4.2 Compare the output parameters with different annealing temperature on the high concentration DJ solar cell……………………47

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