三族氮化物微結構之成長與光電特性量測

Abstract

本論文初期我們利用巨觀及微觀角度的電性量測設備探討對於氮化鎵表面微結構對於薄膜電性的影響。首先利用矽原子調制摻雜技術，我們已能將氮化鎵表面延伸錯位(threading dislocation)密度降低至108 cm-2的水準，在低錯位密度情況下顯示室溫電子遷移率可以從57cm-2/V-s提升至322 cm2/V-s。藉由分析變溫霍爾量測所顯示電子遷移率隨溫度變化的趨勢，我們發現高錯位密度情況電子遷移率主要受限於帶電的延伸錯位。而當錯位密度降低時，電子遷移則主要受限於點缺陷所形成的短距散射中心(shirt-range scattering center)。除此之外，利用導電原子力顯微鏡(C-AFM)分析氮化銦表面之V型缺陷(V-defect)及平坦處的電流分布，我們發現V型缺陷側壁的電流強度遠較平坦處高約三個數量級以上，電流-電壓關係顯示在平坦處的電子傳導機制主要受限於表面氧化層阻擋電流，而在V型缺陷側壁由於表面氧化層較薄，電子傳導則是遵照蕭特基(Schottky)傳導機制。 本論文另外提出「流量調制磊晶法」(FME)及「成長中斷模式」(growth interruptions)用以成長氮化銦奈米結構。在600℃利用FME模式成長氮化銦時，三族和五族交替通入反應腔，而當通入TMIn反應氣體時加入250sccm的NH3背景流量。實驗顯示當五族通入階段的NH3流量大於1500sccm後，PL發光峰值位於0.75eV 而譜峰寬度約75 meV。除此之外，隨著增加五三比由10000增加至60000氮化銦等效厚度固定於大約25奈米，有別於於傳統磊晶模式當五三比超過30000時成長速度下降至原先的1/4。我們認為利用FME成長模式在高流量的NH3環境下可以抑制氮原子形成錯位(stacking fault)，因此不但不會抑制氮化銦成長速度，發光強度亦不受NH3流量增加而降低。 最後，我們利用重複次數的「成長中斷模式」(growth interruptions)在700℃成長出無金屬銦顆粒(In droplet)的氮化銦微結構。可以成功地解決氮化銦難以在高溫成長的困難。實驗結果顯示，當每次截斷TMIn供應而保持NH3通入的時間超過15秒，重複45次之後表面不但沒有金屬銦顆粒存在，PL發光譜峰也由0.75eV降低至0.7 eV。這個結果據我們所知已不遜於世界級的水準。利用時間解析光譜量測所顯示低溫載子侷限能量大約12meV，超過一般量測約5至10 meV的侷限能量，同時我們亦觀測到有少量金屬銦(In clusters)被包覆於於氮化銦微結構內。進而推測利用成長中斷模式於高溫製備氮化銦可能在被包覆的銦周圍形成的受體銦空缺(In vacancy)，此能階極有可能主導0.70eV的發光行為。

This thesis elucidates the macroscopic and microscopic electrical properties of the GaN epilayer. First, Si-modulation doping layers (Si-MDLs)are used to reduce the dislocation density to less than 108 cm-2 and improve electron mobility to 322 cm2/V-s Analysis of temperature-dependent mobilities indicates in the high dislocation regime, the electron transport is limited by charged threading dislocations. In the low-dislocation regime, electrons more easily collide with point defects as short-range scattering centers. The consistency between the estimated density and that determined by deep level transient spectroscopy (DLTS) reveals that the short-range scattering centers may be nitrogen interstitial with an energy level at EC-1.01□0.09 eV. On the other hand, conductive atomic force microscopy reveals the spatially resolved current distribution around a V-defect. The current intensity in the V-defect is three orders of magnitude higher than in the surrounding regions. Further static current-voltage measurement suggests that the current flow is governed by Schottky emission and Fowler-Nordheim tunneling in the V-defect region and in the surrounding area, respectively. Flow-rate modulation epitaxy (FME) is utilized herein to fabricate InN nanostructures. At 600℃ with low background NH3 flows of 250 sccm during the In step with an NH3 flow rate that exceeds 1500 sccm in the N-step prevents the generation of droplets and optimize quality. The FME growth mode has the advantage that the growth efficiency is not suppressed, even for an effective V/III of 60000, unlike the situation in the conventional mode, in which the growth rate is reduced by 75% when V/III exceeds 30000. Together with the sustained photoluminescence efficiency, which peaks at 0.75 eV, this result reveals that FME suppresses the formation of stacking faults of nitrogen atoms in the high-V/III-ratio regime. Finally, a series of InN dots was fabricated at 700℃ by metalorganic chemical vapor deposition (MOCVD) with repetitive interruptions of group-III precursor. Interruption time of each cycle exceeds 15s under NH3 ambient result in a successful removal of indium droplets at elevated growth temperature and is probably explained by a converting into InN. As for droplet-free InN samples, photoluminescence (PL) spectra revealed ~0.70 eV emissions with linewidth of ~60 meV. Based on the time-resolved PL measurements (TRPL), the 0.70 eV emissions are probably correlated with holes localized at deep level states near valance band, in which is in coincident with In vacancies nearby embedded indium clusters.