掃描式探針顯微術於氮化鎵薄膜表面微結構之電性研究

Abstract

在本論文中，我們針對n型氮化鎵表面的V型缺陷進行一系列電性行為的探討。實驗結果發現，隨著V型缺陷密度由4.1×105cm-2增加至1.03×107cm-2，在氮化鎵表面蒸鍍鎳金屬所製成直徑為100μm的蕭基元件之所屬能障從1.44eV下降至1.19eV，而且在-1V的操作下反向偏壓漏電流從10-10A急遽的攀升至10-7A；為了瞭解V型缺陷對釐米尺寸n-GaN蕭基元件電流傳導機制之影響，我們進一步的利用導電原子力顯微鏡(Conductive Atomic Force Microscopy, C-AFM)，特別針對單一個V型缺陷進行微觀電性之量測。我們發現到，在順偏8V時，V型缺陷側壁的電流值約為平坦區域的10倍以上，證明V型缺陷較平坦處具有較佳的傳導性質；另外在V型缺陷與平坦處相交的邊界，以及六個側壁彼此相交的邊線均存在嚴重的反向偏壓漏電流，而其他區域則無明顯的電壓崩潰發生。上述V型缺陷電流-電壓之特性行為應可歸因於V型缺陷內部表面能態密度的提高，產生缺陷增強穿隧電流（defect assisted tunneling current）效應，進而主導了整個V型缺陷正反偏壓之電流傳導機制。所以在此論文中，我們確立V型缺陷應是主導了整個蕭基元件的整流特性主要的電流傳導機制。

We have studied the electrical properties of n-GaN (Ni/GaN) Schottky diode covered with variety of densities of V-defects. The I-V characteristics of the diodes with 100μm diameter indicated that as the densities of V-defects on GaN films increased from 4.1×105cm-2 to 1.03×107cm-2, the Schottky barrier height decreased from 1.44eV to 1.19eV and the reverse bias leakage current at -1V is increased sharply from 10-10A to 10-7A . Further studies on a single V-defect using conductive atomic force microscope (C-AFM) demonstrated that the value of forward current biased at 8V at V-defect side-walls appears to be one order higher than that of plain region, indicating that the conductivity mainly occurred at the side-walls of V-defect. Moreover, rather interesting I-V behavior was observed at reverse bias. Large reverse leakage current was observed on both the perimeter and crest-line of hexagonal V-defects, relatively few current leakage is observed on the facet of side-walls. Such I-V characteristics can be attributed to the appearance of large density of surface states in V-defects, which reduced the Schottky barrier height and provided alternating current leakage path at reverse bias. We believe that the appearance of large density of V-defect in GaN film is the major factor responsible for the current transports in a large-area Schottky diodes made from n-type GaN film.