N型氮化鎵蕭基二極體之熱退火效應研究

Abstract

本論文主要是利用 I-V，AES，以及XPS量測方法對Ni/Ta/n-GaN Schottky contacts在熱退火的狀態下作有系統的研究。為了更容易了解它的載子傳導機制，我們也研究了純Ta及Ni Schottky diodes的特性來作為比較。我們的實驗結果指出在未退火的狀況下，我們所研究的diodes都具備了不錯的Schottky特性 。但是Ni diodes在退火溫度超過600 oC時開始出現了不穩定的狀態。當退火溫度從600 oC升到800 oC時，它的Schottky位障從0.85 eV降至0.7 eV。除此之外，Ta在低溫退火時維持著不良的整流特性；到了高溫>600 oC時它顯示出絕緣體的I-V特性。對Ni/Ta diodes來說，它在低溫的特性與Ta非常相似，我們歸咎於在低溫還是由Ta作為主導。當到了高溫>600 oC時，我們可以發現到非常不錯的Schottky特性。其中我們可發現到Schottky位障在0.9到1.25 eV之間且理想因子在1.08到1.2之間。這樣高的位障可以歸因於在Ni與GaN之間由Ta所形成的氧化物(Ta2O5)。它可以有效的防止Ni與Ta的相互擴散以及相分離這些會造成元件缺陷的現象。由於Ni/Ta Schottky contacts 不但貢獻了非常高的Schottky位障而且可在700 oC的高溫下保持一個小時的穩定。我們相信它具有作為高溫及高功率元件的潛力與理想Schottky diodes的極佳選擇。

We have carried out a systematic study of Ni/Ta/n-GaN bilayer Schottky contacts on various thermal annealing conditions by using current-voltage (I-V), Auger electron spectroscopy (AES), and X-ray photoemission spectroscopy (XPS) measurements. In order to get a better understanding of its carrier transport, pure Ta and pure Ni element Schottky diodes were also investigated for comparison. Our experimental results indicate that despite of different barrier height values, the as-deposited samples exhibit well-behaved Schottky properties, regardless of which type of GaN diodes. Nevertheless, upon thermal annealing the Ni diodes were found to be unstable at temperature ³600 oC. Its barrier height is decreased from 0.85 to 0.70 eV as the temperature is increased from 600 to 800oC. On the other hand, very different manners were observed for both Ta and Ni/Ta Schottky diodes under the thermal treatment. The Ta diode shows nonrectifying features at low annealing temperatures, and eventually become insulator-like I-V characteristics for temperatures above 700oC. For Ni/Ta diodes, basically its diode performance at low annealing temperatures is very similar to that of Ta diode, suggesting their carrier transports are governed predominantly by the interfacial status of Ta/GaN metal-semiconductor hetero interface. Nevertheless, when the annealing temperature is further increased to >600 oC, thermally stable excellent Schottky diodes were obtained. The corresponding barrier height and ideality factor lie in the range of 0.9-1.25 eV and 1.08-1.2, respectively. Such a high barrier height in Ni/Ta can be attributed to the formation of tantalum oxide intermediate layer between Ni metal film and GaN substrate, which prevents a sever interdiffusion of Ta and Ni and an evolution of their phase separation to deteriorate the device performance. Since the tantalum oxide is so stable, together with its unique properties that our high-temperature Ni/Ta/GaN diode not only possesses a highest GaN barrier height value, but also can endure hostile thermal treatment environment up to 700oC for one hour. All of the evidences show the Ni/Ta bilayer is nearly an ideal Schottky contact for GaN and has great potential for use in high temperature and high power electronic devices.