氮化鎵缺陷能階為0.5 eV的電性研究

Abstract

本論文主要是藉由電性的量測，包括電流電壓 (I-V)、電容電壓 (C-V)、導納頻譜 (C-F&G-F)、深層能階暫態頻譜儀 (DLTS) 的量測來探討氮化鎵在電性上的表現。 本研究一開始主要在探討成核層上的腔體壓力條件，由x-ray diffraction得知以高壓的腔體環境磊晶氮化鎵成核層其樣品品質較好，但透過電性量測的結果得知未摻雜的氮化鎵本身是具有高阻值的材料特性，以至於電性量測受到高阻抗的影響而使得DLTS無法量測出缺陷訊號。雖然改善其成核層上的壓力升至高壓時，其阻值降低了一個數量級，在DLTS我們的確也量測到一個相同的缺陷能階 (樣品808 & 811)，但由於此訊號過於微弱，於是我們另外磊晶一層n型氮化鎵材料介於成核層與未摻雜的氮化鎵之中來改善其樣品特性。 研究結果得知此n型材料的形成將有助於降低未摻雜氮化鎵的阻值達3個數量級。另外，反應在蕭基二極體電流中，其電流的傳導機制也將以thermionic emission為主導。同樣的我們在此n型摻雜的樣品中量到了與樣品808及811相同的缺陷能階，文獻研究認定此缺陷與氮錯位有關，加上此樣品在電性表現上其結果都較其他樣品好，於是我們透過此樣品針對這一0.5 eV的缺陷能階作更進一步的研究討論。 在DLTS量測上我們發現此缺陷具有差不多的emission barrier及capture barrier。其emission barrier是在導電帶下方約0.5 eV，捕獲截面積為10-16 cm2。藉由改變filling pulse width (tp) ,其emission barrier不隨缺陷內載子濃度改變，但capture barrier將受到缺陷內載子濃度而大幅度變化。為了更進一步研究此缺陷的電性行為，我們引用Tadeusz Wosinski[1]所提出的缺陷理論分析，求得缺陷內無電子為empty能態時的能障高度為0.37 eV。文獻研究指出此缺陷為氮原子取代鎵空穴而成( )，於是我們大膽假設此缺陷會產成一個類似Schottky的intrinsic barrier height 為0.52 eV( )。換言之，缺陷的表現使得費米能階被pin在相同位能，而使得此缺陷同時具有emission等於capture 的相同能障，表現出dot-like defect的電性特質。

We have investigated the GaN films grown on sapphire substrates by metal-organic chemical vapor deposition (MOCVD) such as unintentionally GaN and intentionally Si-doped GaN above the nucleation layer. Deep level transient spectroscopy (DLTS) measurements on both samples reveal a same defect level. At~0.5 eV whose electron emission and capture mechanism in DLTS are investigated. For unintentionally doped sample, we modulated the growth pressure condition in the nucleation layer. The unintentional-doped low pressure nucleation layer exhibits highly resistive with 105 Ω, probably preventing the DLTS spectra from revealing trapping signals. On the other hand the DLTS measurement on the high pressure nucleation layer sample exhibits the defect level. For intentionally Si-doped sample grown between the nucleation layer and undoped GaN, the resistivity is reduced to 100 Ω, allowing us to study defect characteristics by DLTS measurement. The DLTS spectra exhibits a majority carrier trap with apparent energy close to 0.5 eV below conduction band, capture cross section of about 10-16 cm2 and the trap concentration of 1013 cm2. A comparison with previous literature suggests that this level is probably related to nitrogen antisite. By modulating filling pulse duration (tp), the DLTS peak temperature remains unchanged. Therefore, the electron emission barrier is not affected by trap concentration and is remained at 0.5 eV. The capture behaviors of the defect were investigated. We find that the defect has a capture barrier which is similar as the emission barrier for a similar time constant. The DLTS peak shifts toward a lower temperature with increasing tp. This result shows that the capture barrier is affected by the electron concentration. From extrapolation, the defect in an empty state has an intrinsic barrier height of 0.52 eV , suggesting that the defect behaves like a dot and the defect is clustered, which pins the Fermi level at a value that corresponds to the emission barrier height. From the result of our experiments, we have demonstrated that the intentionally Si doped sample exhibits good electrical characteristics. The 0.5 eV level exhibits a dot-liked defect and behaves like clusters, rather than an isolated point defect.