三族氮化物奈米線之光電導與電傳導特性之研究

Abstract

在此論文中，我們成功地製作出氮化鎵、氮化銦以及氮化鋁(GaN、InN、AlN) 的單根奈米線元件來作為光偵測器(photoconductor)，並分別探討它們在不同波長的光源照射下之光電導(photoconductivity)及電傳導(electronic transport)特性。在材料成長方面，氮化鎵及氮化鋁奈米線是以化學氣相沈積法(CVD, chemical vapor deposition)成長，而氮化銦奈米線是由金氧化學氣相沈積法(MOCVD, metal organic chemical vapor deposition) 來成長。在元件製作方面，我們分別利用電子束顯影(e-beam lithography) 製作氮化鎵奈米線元件、以及聚焦離子束(focus ion-beam)技術來製作氮化銦及氮化鋁奈米線元件，並使用不同的實驗條件來達成奈米線和接觸金屬的歐姆接觸(ohmic contact)。 在氮化鎵奈米線方面，我們首先製作出氮化鎵橋式光偵測器(GaN bridging photoconductor) 並發現元件具有非常高的光電流反應度(photocurrent responsivity)。我們更一進步研究發現單根氮化鎵奈米線的光電流增益(photocurrent gain)比其塊材(bulk) 材料或薄膜(thin film) 材料高出三到四個數量級(103-104)，可作為高光偵測效率的光偵測器。而從光電流增益對不同照光功率的實驗我們得知造成這種高增益的機制為表面能帶彎曲(surface band bending)引起的電子電洞對復合時間變長，而非一般的缺陷(bulk trap) 捕捉光電子或光電洞所造成。另外，我們從不同尺寸的奈米線光導和電導量測結果發現當奈米線的直徑小於其臨界直徑(critical diameter, 30-40nm) 時，會發生尺寸效應(size effect)。我們發現此臨界半徑遠小於之前文獻報導的c軸向<001>氮化鎵奈米線的臨界直徑(80-100nm)，這代表我們可利用較多尺寸的m軸奈米線來作為高增益光偵測器。我們亦提出不同尺寸下的能帶圖來解釋其造成尺寸效應的原因。 在氮化銦奈米線方面，由不同溫度下的電流-電壓(I-V curve)實驗結果中，我們發現其暗電導會隨溫度增加而增加，顯示奈米線具有半導體的傳導行為(semiconducting transport)，不同於早期文獻報導氮化銦奈米線電性呈現類似金屬(metal-like)的特性。而室溫測到的電導值(conductivity) 約為10 (Ω-1cm-1)，亦遠小於之前文獻的報導值(>1000 Ω-1cm-1)。我們亦計算元件的光電流靈敏度(sensitivity) 發現其值在室溫時約為0.3，其數值和一般紅外光偵測器(IR detector)接近。但是光電流增益值(~107)卻遠高於目前一般常用的IR detector (通常<104)，這表示銦化鎵奈米線可作為高光電流增益的紅外光偵測器。我們的實驗結果顯示光電流在空氣及真空環境中有明顯的差異，推測可能原因為奈米線表面所吸附的氧分子會捕捉表面電子形成氧離子，此氧離子會和光電洞復合而造成光電子無法與光電洞復合，因此會增加復合時間而造成高光電流增益。 而在氮化鋁奈米線方面，我們則發現在不同波長光源的照射下，同一奈米線會呈現正光導和負光導不同的光反應現象。其中在光能量為1.43 和2.33電子伏特(eV)的光波長照射下會產生正光導，而在光能量3.06和3.81電子伏特的光波長照射下會產生負光導現象。對此我們也提出一個模型來解釋這樣的現象，其中負光導的原因可能來自於氮化鋁本體(bulk)的捕捉電子陷阱(electron trap)和復合中心(recombination center) 在照光時跟電子電洞對作用產生的一連串反應所造成，而當量測環境由一般大氣環境轉換到真空環境時，負光導會轉變成為正光導。因此推測在照光時氧氣和奈米線表面的作用是造成正光導的主要原因。我們推測氮化鋁奈米線光導的來源可以分為兩部分，正光導的來源是奈米線的表面氧分子敏化作用(oxygen sensitization effect)所造成，而負光導則是是由本體的捕捉電子陷阱所造成。我們以此來解釋造成氮化鋁奈米線不同光導反應的原因。

In this thesis, we have successfully fabricated three different kinds (GaN、InN、AlN) of single nanowire device as phootodetectors. We measured the photocurrent response of nanowires to discuss the photoconductivity and electronic transport of nanowires under the photo excitation at different wavelength. For material growth, the GaN and AlN nanowires were grown by chemical vapor deposition and InN nanowires were grown by metal organic chemical vapor deposition (MOCVD). For device fabrication, e-beam lithography was utilized to make single GaN nanowire device and focus ion-beam technique was applied to fabricate single InN nanowire device. We made use of different experimental parameters to reach ohmic contact between nanowires and contact metal. For GaN nanowires, we first fabricated the GaN bridge structure as photodetector and found the devices have very high photocurrent responsivity. Meanwhile, we discover the calculated photocurrent gain of single GaN nanowire was three to four orders magnitude higher than its bulk or thin film counterparts. It suggests that single GaN nanowire has potential for being high photo-detection efficiency detector. The intensity-dependent gain study showed that the gain value is very sensitive to the excitation intensity following an inverse power law and no gain saturation was observed in this investigated intensity range from 0.75 to 250 W/m2. This behavior strongly suggested a surface-dominant rather than trap-dominant high gain mechanism in this one-dimensional nanostructure. In addition, we measured the photo response and conductivity in different size of nanowires.The result shows an obvious size effect while the nanowire diameter is smaller than a critical diameter of 30-40nm. The critical diameter of m-axial CVD-grown nanowire is much smaller than the reported value of c-axial MBE-grown nanowire (80-100nm). It indicates that we can make use of more m-axial CVD-grown nanowire as high-gain photodetector. We also propose the band diagram of different size to elucidate the sizedependent effect of GaN nanowire. For InN nanowires, we report on the photoconductivity study of the individual infraredabsorbing InN nanowires. Temperature-dependent dark conductivity result indicates the semiconducting transport behavior of these InN nanowires. A measured conductivity of 10 Ω-1cm-1 is much lower than the previous reported value (>1000 Ω-1cm-1). The photo sensitivity of 0.3 and calculated ultrahigh photoconductive gain of around 107 at room temperature are obtained under 808 nm excitation. Furthermore, our studies suggest that the photocurrent in InN NWs is sensitive to the oxygen environment and its PC could be surface dominant and follows a similar mechanism of molecular sensitization. The excitation of electron from surface state created by foreign oxygen molecule could give rise to a similar effect as interband excitation since the lifetime of photoelectron is also determined by the readsorption rate of oxygen. For AlN nanowires, photoconductivity of individual AlN nanowires has been characterized using different subband gap excitation sources. It is interesting that both positive photocurrent under 1.53 and 2.33 eV excitations and negative photocurrent under 3.06 and 3.81 eV excitations are observed from the wide band gap nitride nanowires. The negative photoconductivity, which is attributed to the presence of electron trap and recombination center in the bulk of AlN, is capable to be inversed by a strong positive photoconductive mechanism of surface while changes the ambience from the atmosphere to the vacuum. An oxygen molecular sensitization effect is proposed to explain the enhancement of positive photocurrent and the inversion of negative photoresponse in the vacuum.