臺灣博碩士論文加值系統

本論文中將會先從探討Cu的厚度在 Ni/Pt/Cu/Au 以及Ni , Pt及Cu的厚度對於 Ni/Pt/Cu/Ni/Au 沉積在正型砷化鎵當作歐姆接觸之影響，為了得到最低的特徵阻值，我們尋求最佳的Ni, Pt及Cu的金屬厚度還有最佳的回火條件。首先利用TLM (transmission line model)的方法求得Ni/Pt/Cu/Ni/Au金屬材料在正型砷化鎵的特徵阻值，其中發現當Ni, Pt及Cu的厚度分別由20nm, 20nm及20nm增加到50nm, 50nm及160nm時，其特徵阻值會降到最低 ( ρc �� 2.22×10-6 Ω-cm2 )，回火溫度為280℃而回火時間為1分鐘。緊接著，將具有最低特徵阻值的歐姆接觸金屬組成(亦即Ni (50nm)/ Pt (50nm)/ Cu (160nm)/ Ni (40nm)/ Au (50nm))蒸鍍在雙接面太陽能電池的正電極，並且與未含Cu的歐姆接觸金屬組成(亦即Ni (50nm)/ Pt (80nm)/ Au(200nm))做比較，來探討Ni/Pt/Cu/Ni/Au材料應用於元件上之後的元件特性。
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及雙接面太陽能電池上，經過長時間的回火(0分鐘 到 12小時)，歐姆接觸的特徵阻值及效率的變化，我們可以發現Ni (50nm)/ Pt (50nm)/ Cu (160nm)/ Ni (40nm)/ Au (50nm) 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，而此冶金結構在太陽能電池上當作正電極時，發電的效率也會隨著回火時間的增加而降低。

In this dissertation, we discussed the effects of the thickness of the Cu on the Ni/Pt/Cu/Au contact and of the individual Ni, Pt and Cu on the Ni/Pt/Cu/Ni/Au contact to p-GaAs in the beginning. Then, we sought for the optimum thickness of the Ni, Pt and Cu layers and the suitable annealing temperature and time for the lowest specific contact resistance ρc by transmission line method (TLM). As increasing the thickness of the Ni, Pt and Cu layers to 50nm, 50nm and 160nm, respectively, the lowest value (ρc �� 2.22×10-6 Ω-cm2) of specific contact resistance could be attained.
Sequentially, we applied the optimum metallurgical structure of Ni (50nm) / Pt (50nm) / Cu (160nm) / Ni (40nm) / Au (50nm) layers as the positive electrode of the dual-junction (DJ) solar cells. Then, we measured these solar cells efficiency and compared the results with those of DJ solar cells having Ni (50nm) / Pt (80nm) / Au (200nm) p-type ohmic contact but without the copper metallurgical structure. We obtained the conversion efficiency about 17.19% for DJ solar cell with Ni (50nm) / Pt (50nm) / Cu (160nm) / Ni (40nm) / Au (50nm) and 17.39% for that with Ni (50nm) / Pt (80nm) / Au (200nm).
In addition, we studied the thermal stability of these contact metals on the bulk GaAs and DJ solar cells. The ρc of Ni (50nm)/Pt (50nm)/Cu (160nm)/Ni (40nm)/Au (50nm) metallurgical structure is increased from 2.22×10-6 Ω-cm2 to 2.26×10-5 Ω-cm2 and the efficiency of DJ solar cell is decreased from 17.19% to 15.89% after annealing for 12 hours when held annealing temperature at 200oC in H2 ambient environment.

Content
Abstract(Chinese)……………………………………………………….I
Abstract(English)……………………………………………………...III
Acknowledgment…………………………………………………….....V
Content……………………………………………………………........VI
Figure Captions……………………………………………………...VIII
List of Tables………………………………………………………....XIV

Chapter 1 Introductions………………………………………………..1
Chapter 2 Theoretic foundations………………………………………2
2.1 Metal and Semiconductor contact theory………………….2
2.2 Transmission Line Model (TLM)………………………….6
2.3 Measurement method……………………………………...8
2.4 Basic concepts of the solar cell……………………………9
2.5 The equivalent circuit analysis of a solar cell……………10
2.6 Fundamental solar cell parameters……………………….10
Chapter 3 Fabrication process and the electric characteristic of
ohmic contact metals………………….…………………...12
3.1 GaAs TLM Fabrication Process………………………….12
3.1.1 Wafer cleaning process………………………………12
3.1.2 Photolithography process……………………………12
3.1.3 Metal deposition……………………………………..13
3.1.4 Mesa etching process………………………………...13
3.2 Specific contact resistance analysis………………………14
3.2.1 Ni/Pt/Cu/Au………………………………………….15
3.2.2 Ni/Pt/Cu/Ni/Au………………………………………15
3.2.3 Ni/Pt/Au……………………………………………...17
3.3 Thermal stability of ohmic contacts……………………...18
3.4 Surface morphology……………………………………...18
Chapter 4 Characteristics of the InGaP on GaAs dual-junction solar cell…………………………………………………………..20
4.1 InGaP/GaAs dual-junction solar cell……………………..20
4.1.1 Dual-junction solar cell structure……………………20
4.1.2 Current-Voltage measurement system (I-V) ………...20
4.2 Characteristics of the dual-junction solar cell……………21
4.2.1 The effect of cap layer……………………………….21
4.2.2 The best annealing temperature of contact metal on the
DJ solar cell…………………………………………22
4.2.3 The thermal stability of DJ solar cells……………….22
Chapter 5 Conclusions………………………………………………...25
References………………………………………………………………27










Figure Captions
Figure 2.1 A Schottky barrier formed by contacting an n-type semiconductor with a metal having a larger work function: band diagram for the metal and semiconductor before joining. ……………………….30
Figure 2.2 Band diagram for the junction at equilibrium. ……………..30
Figure 2.3 Ohmic metal-semiconductor contact：Φm＜Φs for an n-type semiconductor at the equilibrium band diagram for the junction. ……...31
Figure 2.4 Ohmic metal-semiconductor contact：Φm＞Φs for an p-type semiconductor at the equilibrium band diagram for the junction. ……...31
Figure 2.5 Ohmic metal-semiconductor contact：Heavy doped for an n-type semiconductor at the equilibrium band diagram for the junction. ………………………………………………………………...32
Figure 2.6 Ohmic metal-semiconductor contact：Heavy doped for an p-type semiconductor at the equilibrium band diagram for the junction. ………………………………………………………………...32
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. ………………………………33
Figure 2.8 The approaches used to model the ohmic contact of a transmission line. ……………………………………………………….33
Figure 2.9 Basic pattern used to experimentally determine contact resistance parameters. Ohmic contacts are separated by increasing distance. ………………………………………………………………...34
Figure 2.10 Plot of measured resistance as a function of contact separation yields sheet resistance, contact resistance, and other parameters. ……………………………………………………………...34
Figure 2.11 Equivalent circuit of a solar cell, including series and shunt resistances. ……………………………………………………………...35
Figure 2.12 Terminal I-V properties of a p-n junction diode in the dark and in illumination. ……………………………………………………..35
Figure 3.1 Variations of specific contact resistance for Ni (50nm) / Pt (20nm) / Cu (80nm) / Au (50nm) metal structure deposited on p-type GaAs after annealed at different temperatures in H2 ambient for 1 minute. ………………………………………………………………….37
Figure 3.2 Various of specific contact resistance for Ni (50nm) / Pt (20nm) / Cu (80nm) / Au (50nm) metal structure deposited on p-type GaAs after annealing 280oC for various time in H2 ambient. …………..37
Figure 3.3 Variations of the lowest specific contact resistance and the associated annealing temperature as a function of Cu thickness for Ni (50nm) / Pt (20nm) / Cu / Au (50nm) metal structure sequentially deposited on p-type GaAs substrate where the annealing time is 1 minute. ………………………………………………………………….38
Figure 3.4 Variations of specific contact resistance for Ni (50nm) / Pt (50nm) / Cu (40nm) / Ni (40nm) / Au (50nm) metal structure deposited on p-type GaAs after annealed at different temperatures in H2 ambient for 1 minute. ………………………………………………………………..38
Figure 3.5 Various of specific contact resistance for Ni (50nm) / Pt (50nm) / Cu (40nm) / Ni (40nm) / Au (50nm) metal structure deposited on p-type GaAs after annealing 280oC for various time in H2 ambient. ………………………………………………………………...39
Figure 3.6 Variations of the lowest specific contact resistance and the associated annealing temperature as a function of Cu thickness for Ni (50nm) / Pt / Cu (40nm) / Ni (40nm) / Au (50nm) metal structure sequentially deposited on p-type GaAs substrate where the annealing time is 1 minute. ……………………………………………………………..39
Figure 3.7 Variations of the lowest specific contact resistance and the associated annealing temperature as a function of Cu thickness for Ni (50nm) / Pt (50nm) / Cu / Ni (40nm) / Au (50nm) metal structure sequentially deposited on p-type GaAs substrate where the annealing time is 1 minute. ……………………………………………………………..40
Figure 3.8 Variations of the lowest specific contact resistance and the associated annealing temperature as a function of Cu thickness for Ni / Pt (50nm) / Cu (160nm) / Ni (40nm) / Au (50nm) metal structure sequentially deposited on p-type GaAs substrate where the annealing time is 1 minute. ……………………………………………………………..40
Figure 3.9 Variations of specific contact resistance for Ni (50nm) / Pt (80nm) / Au (200nm) metal structure deposited on p-type GaAs after annealed at different temperatures in H2 ambient for 1 minute. ………..41
Figure 3.10 Variations of specific contact resistance for Ni (50nm) / Pt (50nm) / Cu (20, 80, 160nm) / Ni (40nm) / Au (50nm) metal structure deposited on p-type GaAs and held at 200oC in H2 ambient for a long-term annealing. ……………………………………………………41
Figure 3.11 Surface morphology of Ni (50nm) / Pt(20nm) / Cu(40nm) / Au(50nm) annealed at 250oC for 1 minute. …………………………….42
Figure 3.12 Surface morphology of Ni (50nm) / Pt(20nm) / Cu(40nm) / Au(50nm) annealed at 350oC for 1 minute. …………………………….42
Figure 3.13 Surface morphology of Ni (50nm) / Pt(20nm) / Cu(40nm) / Au(50nm) annealed at 400oC for 1 minute. …………………………….43
Figure 3.14 Surface morphology of Ni (50nm) / Pt(20nm) / Cu(40nm) / Ni(40nm) / Au(50nm) annealed at 350oC for 1 minute. ………………..43
Figure 3.15 Surface morphology of Ni (50nm) / Pt(20nm) / Cu(40nm) / Ni(40nm) / Au(50nm) annealed at 400oC for 1 minute. ………………..44
Figure 4.1 Schematic cross-sections of the InGaP/GaAs Dual-junction solar cell.………………………………………………………………...44
Figure 4.2 Schematic of the current-voltage characteristic measurement
system. (solar simulator). ………………………………………………45
Figure 4.3 The efficiency versus annealing temperature of Ni (50nm) / Pt (50nm) / Cu (80nm) / Ni (40nm) / Au (50nm). ………………………...46
Figure 4.4 The Isc versus annealing temperature of Ni (50nm) / Pt (50nm) / Cu (80nm) / Ni (40nm) / Au (50nm). …………………………………47
Figure 4.5 The Voc versus annealing temperature of Ni (50nm) / Pt (50nm) / Cu (80nm) / Ni (40nm) / Au (50nm). …………………………………47
Figure 4.6 The fill factor versus annealing temperature of Ni (50nm) / Pt (50nm) / Cu (80nm) / Ni (40nm) / Au (50nm). ………………………...48
Figure 4.7 Compare the efficient variation of different Cu thickness deposited on the solar cells during long-term annealing. ………………50
Figure 4.8 Compare the efficient variation of including and excluding copper layer deposited on the solar cells during long-term annealing. ...50
Figure 4.9 Variation of efficiency for DJ solar cell with Ni (50nm) / Pt (50nm) / Cu (20nm) / Ni (40nm) / Au (50nm) contact structure before and after long-term annealing and held annealing temperature at 200oC in H2 ambient. ………………………………………………………………...51
Figure 4.10 Variation of efficiency for DJ solar cell with Ni (50nm) / Pt (50nm) / Cu (80nm) / Ni (40nm) / Au (50nm) contact structure before and after long-term annealing and held annealing temperature at 200oC in H2 ambient. ………………………………………………………………...51
Figure 4.11 Variation of efficiency for DJ solar cell with Ni (50nm) / Pt (50nm) / Cu (160nm) / Ni (40nm) / Au (50nm) contact structure before and after long-term annealing and held annealing temperature at 200oC in H2 ambient. ……………………………………………………………..52
Figure 4.12 Variation of efficiency for DJ solar cell with Ni (50nm) / Pt (80nm) / Au (200nm) contact structure before and after long-term annealing and held at 200oC. …………………………………………...52
Figure 4.13 Variation of Isc for DJ solar cell with Ni (50nm) / Pt (50nm) / Cu (20nm) / Ni (40nm) / Au (50nm) contact structure after long-term annealing and held at 200oC. …………………………………………...53
Figure 4.14 Variation of Isc for DJ solar cell with Ni (50nm) / Pt (50nm) / Cu (80nm) / Ni (40nm) / Au (50nm) contact structure after long-term annealing and held at 200oC. …………………………………………...53
Figure 4.15 Variation of Isc for DJ solar cell with Ni (50nm) / Pt (50nm) / Cu (160nm) / Ni (40nm) / Au (50nm) contact structure after long-term annealing and held at 200oC. …………………………………………...54
Figure 4.16 Variation of Isc for DJ solar cell with Ni (50nm) / Pt (80nm) / Au (200nm) contact structure after long-term annealing and held at 200oC. …………………………………………………………………..54
Figure 4.17 Variation of Voc for DJ solar cell with Ni (50nm) / Pt (50nm) / Cu (20nm) / Ni (40nm) / Au (50nm) contact structure after long-term annealing and held at 200oC. …………………………………………...55
Figure 4.18 Variation of Voc for DJ solar cell with Ni (50nm) / Pt (50nm) / Cu (80nm) / Ni (40nm) / Au (50nm) contact structure after long-term annealing and held at 200oC. …………………………………………...55
Figure 4.19 Variation of Voc for DJ solar cell with Ni (50nm) / Pt (50nm) / Cu (160nm) / Ni (40nm) / A u(50nm) contact structure after long-term annealing and held at 200oC. …………………………………………...56
Figure 4.20 Variation of Voc for DJ solar cell with Ni (50nm) / Pt (80nm) / Au (200nm) contact structure after long-term annealing and held at 200oC. …………………………………………………………………..56
Figure 4.21 Variation of fill factor for DJ solar cell with Ni (50nm) / Pt (50nm) / Cu (20nm) / Ni (40nm) / Au (50nm) contact structure after long-term annealing and held at 200oC. ………………………………..57
Figure 4.22 Variation of fill factor for DJ solar cell with Ni (50nm) / Pt (50nm) / Cu (80nm) / Ni (40nm) / Au (50nm) contact structure after long-term annealing and held at 200oC. ………………………………..57
Figure 4.23 Variation of fill factor for DJ solar cell with Ni (50nm) / Pt (50nm) / Cu (160nm) / Ni (40nm) / Au (50nm) contact structure after long- term annealing and held at 200oC. ……………………………….58
Figure 4.24 Variation of fill factor for DJ solar cell with Ni (50nm) / Pt (80nm) / Au (200nm) contact structure after long-term annealing and held at 200oC. ………………………………………………………………..58



List of tables
Table 2.1 Work functions of some metals. ……………………………..29
Table 2.2 Electron affinity of some semiconductors. …………………..29
Table 3.1 The composite structure of the Ni/Pt/Cu/Au and Ni/Pt/Cu/Ni/Au. …………………………………………….36
Table 4.1 Compare the efficiency before and after cap etching. ……….45
Table 4.2 Compare the efficiency with different annealing
temperature. ………………………………………………...46
Table 4.3 The measurement results of each sample before and after long-time annealing.………………………………………...49

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