臺灣博碩士論文加值系統

在本篇論文中，我們主要是探究鎳/銅在p型氮化鎵上經過熱退火處理所形成低歐姆接觸電阻的機制。
首先，我們利用X光光電子能譜術(XPS)對不同表面處理的p型氮化鎵試片作量測，藉此來比較試片表面氧和碳的含量，我們發現王水可以將表面髒污層(contamination layer)部分地去除，並證明表面處理對增加接面電流是有所幫助。
進一步，我們分別在p型氮化鎵上鍍上單層金屬(鉑、金、鎳、銅、鋁)，經過熱退火處理，發現鎳經過600℃、30秒之熱處理試片，可得到最低之特殊接觸電阻為3.20*10^-3 ohm-cm^2。為了再降低特殊接觸電阻，我們發展出雙層鎳/銅結構，先在p型氮化鎵試片鍍上20nm厚的鎳，然後再鍍一層20nm的銅，並經由通氮氣之熱退火處理400~700℃。如此經過600℃、30秒之熱處理試片，可得到最低之特殊接觸電阻為1.31*10^-4 ohm-cm^2。特殊接觸電阻的下降主要歸因於 (i) 增加接觸面積和局部電場；(ii) 氫從p型氮化鎵中提取出來。我們進一步推測在熱退火處理時，鎳會去除掉絕大部分的髒污層。在最終的接面結構，銅原子會穿越過p型氮化鎵界面導電度差的金屬層，以增加接面金屬的導電性。
基於上述的討論，我們也嘗試建構一個新穎的製程 ( X-方法)，並使用單一金屬層以形成歐姆接觸，但是使用這種製程所得到的特殊歐姆接觸非常的高。這個結果，我們猜測可能的原因是由於經由黃光製程時，光阻鋪蓋在p型氮化鎵的表面上，造成髒污層重新生長在表面，造成p型氮化鎵的表面粗糙度下降，導致增加特殊接觸電阻。

In this study, the formation mechanism of low-resistance ohmic contact to p-type GaN using an alloy of Ni/Cu was investigated.
The surface contamination layer on p-type GaN was partially removed by immersing the sample in the aqua regia solution. The Mg-doped samples were deposited with Ni(20nm)/Cu(20nm) and then annealed in nitrogen ambient at different annealing temperatures ranging from 400~700℃. A good ohmic contact with a specific contact resistance of 1.31*10^-4 ohm-cm^2 was obtained when the sample was annealed at 600℃ for 30s. The decrease of specific contact resistance is attributed to (i) the increased contact area and local electric field and (ii) the extraction of hydrogen from the p-type GaN. It is further suggested that the Ni overlayer removes most of the contamination layer during annealing. In the final contact structure, Cu atoms could penetrate throughout the entire conductivity of the interfacial metallic contact on p-type GaN film improves the conductivity of the contact.
Based on principles described above, we try to fabricate a novel process (X-method) to form ohmic contacts to p-type GaN using a single metallic layer. However, the high specific contact resistance can be obtained with “X-method”. A possible explanation is the re-growth of the contamination layer upon coating with photoresist on the p-type GaN surface during photolithography, which reduces the roughness of the p-type GaN surface and leads to the increase of the specific contact resistance.

Chap.1 Introduction
References ………………………………………………………… 4
Chap.2 Mechanism of ohmic contact to p-type GaN
2.1 Principles of formation of ohmic contact
2.1.1 Metal work function ……………………………………… 6
2.1.2 Contamination layer ……………………………………… 7
2.1.3 Surface state ……………………………………………… 7
2.1.4 Defect ...…………………………………………………… 8
2.1.5 Carrier concentration …………………………………… 9
2.2 Formula of current transport
2.2.1 Current-voltage relationship ………………………… 10
2.2.2 TE, TFE, FE ……………………………………………… 11
2.2.3 Contamination effect …………………………………… 12
2.3 Measurement tools
2.3.1 Two point probe method ……………………………… 14
2.3.2 Transmission line model (TLM) ……………………… 14
References …………………………………………………………… 16
Chap.3 Experiment
3.1 Surface treatment …………………………………………… 18
3.2 Single layer metal contacts …………………………… 20
3.3 Ni/Cu contacts ……………………………………………… 21
3.4 X-method ……………………………………………………… 22
References …………………………………………………………… 23
Chap.4 Results and discussion
4.1 Surface treatment
4.1.1 XPS measurement ……………………………………… 24
4.1.2 I-V characteristics …………………………………… 25
4.2 Single layer metal contacts
4.2.1 I-V characteristics
4.2.1.1 Ni contacts …………………………………… 27
4.2.1.2 Cu contacts …………………………………… 27
4.2.1.3 Pt contacts …………………………………… 28
4.2.1.4 Au contacts …………………………………… 28
4.2.1.5 Al contacts …………………………………… 28
4.2.2 Specific contact resistance measurement
4.2.2.1 Ni contacts …………………………………… 29
4.2.2.2 Cu contacts …………………………………… 29
4.2.3 Annealing effect upon Ni/p-GaN
4.2.3.1 AFM measurement ……………………………… 30
4.2.3.2 AES measurement ……………………………… 31
4.2.3.3 SIMS measurement ……………………………… 32
4.2.3.4 PL measurement ………………………………… 34
4.3 Ni/Cu contacts
4.3.1 I-V characteristics …………………………………… 35
4.3.2 Specific contact resistance measurement ………… 35
4.4 X-method
4.4.1 The process of X-method ……………………………… 37
4.4.2 I-V characteristics …………………………………… 37
4.4.3 Specific contact resistance measurement ………… 38
4.4.4 Problems analysis ……………………………………… 39
References ………………………………………………………… 41
Chap.5 Conclusions

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