氮化鎵薄膜的結構缺陷及表面能態特性研究

Abstract

本論文運用包括霍爾、電壓電流特性、光電容、電容電壓特性、深層能階暫態能譜及x光吸收光譜等電學及光學量測方法，探討由有機金屬汽相磊晶成長氮化鎵薄膜及其蕭基二極體元件，(1)在有銦、砷或鎂元素摻入以及熱退火之下的結構缺陷；(2)其中光電容衰減效應的發現及研究；以及(3)其表面能態與在不同表面處理之下的表面鈍化效應。 新發現如下：(1)我們發現一個在導帶下方0.769電子伏特，與砷相關的新能階，極可能是砷佔據氮的位置所產生。我們發現一個呈現對數光電容衰減的暫態行為，是與錯位缺陷密切相關，我們並且發展出新的方法取得此行為之對數時間常數以及捕捉截面積。(2)我們發現熱退火對鎂摻雜薄膜產生的影響是非等向性的，並且對垂直薄膜表面方向的晶體微結構影響顯著，我們也發現晶相混合比例以及德拜溫度隨退火時間的增加而變化。(3)我們也成功發展出p型半導體的表面能態計算方法，並將之運用在分析不同表面處理下的p型氮化鎵蕭基二極體。 整體而論，在等價位銦元素摻雜方面，我們發現摻雜等價位銦元素可以抑制深層能階的生成，並改善了n型氮化鎵蕭基二極體的電學特性，顯示等價位銦元素摻雜是改善氮化鎵薄膜品質的可行方法。光電容量測亦可證實這個結果。關於熱退火效應，我們發現砷離子佈植氮化鎵薄膜的電學特性，可以經由熱退火恢復到未佈值樣品的狀態；我們還發現所有鎂摻雜氮化鎵薄膜的都顯示，在熱退火過程中，其混合晶相會變得包含更多更穩定的六方晶相結構，並且原子的排列更有秩序。最後在p型氮化鎵蕭基二極體的表面處理方面，透過硫化處理，我們成功的將p型氮化鎵蕭基二極體的理想因子降到1.14，據我們所知，此結果已稱得上世界級的水準。

Electrical and optical measurements, including Hall, current-voltage, photocapacitance, capacitance-voltage, deep level transient spectroscopy and x-ray absorption measurements, etc, has been employed to investigate the following effects, respectively. First, the various effects of structural defects and surface states on GaN films through the incorporation of In, As and Mg atoms into GaN with or without post-annealing have been intensively studied. Secondly, the novel observation and the detailed discussion of the long-term photocapacitance decay behavior will be reported. And at last, the influence of various surface treatments on surface states and the passivation of GaN surface will be taken up. Some salient features in our findings are listed as follows. (1) A trap-center related to arsenic is found located at 0.769 eV below the conduction band, which behaves most likely antisite in nature. (2) A decay phenomenon with logarithmic feature is found in the photocapacitance measurements and is relevant to the effect of traps associated with dislocations. By the way a technique is developed to retrieve the logarithmic time constants and capture cross sections as well. Moreover, (3) the effects of thermal annealing on Mg-doped GaN films are found to be anisotropic with respect to the crystal axes and have the greatest crystal microstructural change along the perpendicular of the epifilm surface. The effects also involved shifting of the structural mixing ratio between polytypes and increasing of Debye temperature of the films. In addition, we develop a method to characterize the surface states on p-GaN surface, and we use this method to analyze the p-GaN Schottky diodes with various surface treatments. As a whole, some important remarks on the practical viewpoints for operation and design of the possible modern electronic devices are worthy noted. On the isoelectronic In doping, we found In isodoping can effectively suppress the formation of deep levels and improve the electrical properties of n-GaN Schottky diodes, indicating that the isoelectronic In doping technique is a viable way to improve the n-GaN film quality. This has been confirmed by the results of photocapacitance measurements. As for the thermal annealing effects, we found the electrical properties of As-implanted GaN films can be recovered to that of the un-implanted ones, and all the findings on the Mg-doped GaN films indicated crystal structure relaxation towards a more stable hexagonal wurtzite configuration and atomic rearrangement to a more orderly condition after annealing. Finally on the surface treatment effects of p-GaN Schottky diodes, we realized the p-GaN Schottky diode with the ideality factor down to the value of 1.14 through the sulfide pretreatment, maybe nearly ideal optimization on the operation of p-GaN Schottky diodes currently.