氮化鎵磊晶成長與元件特性研究

Abstract

本論文中，對氮化鎵材料之磊晶成長、製程研究、場效電晶體及發光二極體製作分別作深入的研究。在材料磊晶成長上，首次採用500埃高溫成長之單晶氮化鋁鎵薄膜當應力層在碳化矽基板上成長氮化鎵磊晶薄膜，取代原本低溫成長之氮化鎵緩衝層，且成功在有機金屬氣相磊晶法調變此應力層之鋁含量由氮化鎵到氮化鋁皆可成長高品質氮化鎵磊晶薄膜，且成長在碳化矽基板上氮化鎵磊晶薄膜之電性及結晶性皆優於成長於氧化鋁基板上。成長氮化鋁鎵與氮化鎵之異質接面於碳化矽基板上，採用500埃厚氮化鋁鎵/氮化鋁鎵當應力層，在77K霍爾效應量測中得到當時最高之電子遷移率，在SdH量測中確定為二維電子氣體現象且在電阻對高磁場量測中(□xx-H)得到兩個疊加之震盪曲線，可確定為應力層和上層氮化鋁鎵與氮化鎵之異質接面所產生之效應，直接證明應力層界面品質良好，並在其上成長出高品質氮化鎵磊晶薄膜之最佳佐證。 在氮化鎵元件製程上，材料蝕刻、N型及P型歐姆接點及蕭基金屬接點皆有詳細研究及成果。採用鎳金屬當作活性離子乾蝕刻之阻擋層，蝕刻出平整、高蝕刻選擇率及陡峭氮化鎵平台；利用矽及鋅原子擴散得到高濃度摻雜表面及鎢之金屬接點特性研究，採用完整製程研究來製作MODFET元件及發光二極體，提升元件操作特性。 採用有機金屬氣相磊晶法成長高品質氮化鎵磊晶薄膜，詳細比較成長於碳化矽基板和氧化鋁基板對氮化鎵磊晶薄膜特性影響。利用最佳磊晶成長成長條件，有效解決氮化鎵與氧化鋁及碳化矽基板因晶格差異所產生之應力。分別在兩種基板上成長AlGaN/GaN異質接面並得到高電子遷移率5128（7500）和5413（5750）cm2/Vs於氧化鋁和碳化矽基板上(目前發表最高值)。採用最佳AlGaN/GaN異質接面製作成MODFET元件。並對AlGaN/GaN及InGaN/GaN異質接面特性及場效電晶體直流操作之元件特性進行研究，在Ids-Vds特性曲線中皆可觀察到通道夾止(pinchoff)性質。比較兩種元件對照光環境下元件操作分析，發現GaN通道元件之Ids些微上升且互導值下降。首次成長InGaN/GaN異質接面並成功製作MODFET元件(Ids=90mA/mm；gm=48mS/mm)，在InGaN通道元件之Ids電流與互導值在照光環境下明顯上升，發現InGaN/GaN MODFET在光的靈敏性較AlGaN/GaN MODFET明顯。 在發光二極體研究上，比較鎂摻雜之氮化鎵薄膜在爐管及快速退火處理對鎂活化效率及材料特性影響，採用900oC、1min快速退火成功活化鎂摻雜氮化鎵薄膜，取代傳統爐管30min、700oC長時間活化處理。在發光二極體製程研究上：在提升發光區亮度上，採用電梳電極和薄金屬透明電極可提高發光亮度及降低操作電壓。成功研製PN界面、雙異質接面（DH）及量子井（MQW）發光二極體磊晶薄膜，並對材料及元件發光特性做詳細研究，在量子井發光二極體中得到最佳的發光特性（If=20mA:□＝470nm，FWHM=29nm，P=700□W）。利用摻雜電子電洞對及量子井結構，成功將發光波長由紫外光調變到藍光（470 nm）。首次採用等效電路中之虛部電容，定義雙異質接面（DH）及量子井（MQW）發光二極體發光層之總缺陷密度，與發光二極體之發光效率與功率有明顯關連性，簡化利用穿透式電子顯微鏡直接觀察發光層缺陷密度之方法。

The material growth, material characterization, device fabrication and device analysis of GaN-based material were the focus topic of this thesis. The influence of 500 Å AlGaN thin single films as buffer layers or strain layers for GaN depositions over 6H-SiC substrates were studied. The 500 Å AlGaN thin single films significantly improve the GaN material quality by relaxing the mismatch between GaN and SiC substrates. We found that a 3-period of GaN/Al0.08Ga0.92N thin film (100 Å/100 Å) will produce good quality GaN epitaxial layer. The mobility and carrier concentration are 612 cm2/V.s and 1.3×1017 cm-3 (at 300K) for the GaN epitaxial layer. We describ the growth and characterization of 2DEG mobility in Al0.08Ga0.92N/GaN heterostructures on 6H-SiC substrates. The high mobility of 5256 cm2/V.s at 4.2K is an indication of 2DEG phenomenon at the Al0.08Ga0.92N/GaN interface. The addition of two SdH oscillations in 2DEG-bulk structure was observed for fields as 10 T, this observation may occur as the 2DEG phenomenon at two 2DEG channels of AlGaN/GaN heterointerface. According to these SdH oscillations, we have high quality of AlGaN/GaN heterointerface at the interface of GaN/ AlGaN buffer layer. It also indicates that using period AlGaN-GaN strain layer as the buffer layer reduces the stress between GaN and SiC substrate and improved the material quality of GaN epitaxial layer. In the dry etching process, the Ni metal masks shown the good prevent property during the BCl3 etching process in RIE system with the selectivity etching ratios GaN to Ni of RIE mask is 23. The etching rate was increased with the increased plasma power and operation pressure. Characteristics of the sidewall of GaN mesa and etched GaN surface are as follows: the better etching condition of is 25 mtorr of chamber pressure, 5 sccm of BCl3 gas, and 200W that have the good etching surface morphology. These GaN:Mg films were activated with Rapid thermal annealing (RTA) treatment. The highest activated concentration is 3.6□1017 cm-3 and the bulk resistivity is 1.9 □.cm with 700oC furnace treatment. The activated concentration is 2.36□1017 cm-3 and the bulk resistivity is 1.63 □.cm with 900oC RTA treatment. Both of Mg activation efficiency are similar with these two thermal treatment systems. The acceptor mobility of P-type GaN films are higher when treated by the furnace than RTA. From PL spectra, the FWHM of Mg-relative peak by RTA treatment (42nm) is narrower than the furnace treatment (53nm), but the peak positions have the large fluctuation by using RTA thermal treatment. The RTA activation process is the fast way to activate the Mg atoms in GaN films The Si, as the n-type dopants, had successfully diffused into GaN films using SiOx/Si/GaN/Al2O3 structure annealing at high temperature. The large quantity of Si were to diffused into GaN to try to achieve a high concentration n+-type GaN. The activated carriers of Si-diffused sample annealed at 1000oC are increased 1.4 times. The Ti/Al contact on standard GaN without thermal treatment exhibit near linear I-V characteristics, and the specific contact resistivity □c values were reduced from 3.0□10-5 □.cm2 to 5.6□10-7 □.cm2 with 1000oC, 30 sec RTA treatment. This new technique to form the diffused n+-type GaN thin layer can be used to fabricate good ohmic contacts in GaN-based devices. The low ohmic contact resistivity and thermal stable W metal on n+ GaN with is achieved. Good ohmic characteristics are observed with carrier concentration higher than 8.4×1018 cm-3 without annealing, and the specific contact resistivity of 3.6×10-4 □cm2 is obtained without thermal annealing on n+ GaN (1.8×1019 cm-3). The barrier height of W on n-type GaN is calculated to be 0.058 eV, and the W metal barrier height shown the thermal stability as a value of 0.058eV for as-deposition and 300oC RTA treatment. In the GaN-based MODFET fabrication study, the high 2DEG mobility on AlGaN/GaN heterostructures are 5128 cm2/V.s and 5413 cm2/V.s at 77K on Al2O3 and SiC substrates. This improved stair structure reduced the diffusion of impurities into the 2DEG channel at AlGaN-GaN interface on SiC substrate because of the influence of interface scattering on 2EDG mobility. For the GaN-channel MODFET devices, the extrinsic transconductance was 109 mS/mm, full channel current 405 mA/mm and a pinch-off ability owing to transistor channel. The other Si lightly doped InGaN-channel MODFET was successfully fabricated with extrinsic transconductance was 56.4 mS/mm, full channel current 132 mA/mm. The InGaN/GaN heterostructures have the MODFETs’ performance (with pinch-off ability) in InGaN channel, but the 2DEG property was not observed from physical analysis may be due to the non-optimum InGaN growth of InGaN/GaN interface. The peak AlGaN/GaN MODFET’s gmL (extrinsic transconductance under light illumination=83mS/mm) is lower than the gmD (in dark=87mS/mm) with 2.7□m-gate width. However, the gmL (51mS/mm) of GaN/InGaN MODFET is higher than the gmD(48mS/mm). The Ids current both increased slightly under the microscopy lamp illumination, but both device performance show the gmD to be higher than gmL in GaN-channel MODFET and the gmD is lower than gmL in InGaN-channel MODFET. The photon induced photon current affected the Ids swing and increased the carrier scattering process in GaN channel. In addition, the photon induced photon current affect the better property Ids swing and increased the carrier in InGaN channel. The effect of photon induced Ids current is stronger in InGaN channel than in GaN channel. This is the first time the photoconductive properties were studied on InGaN and GaN channel MODFET’s. The material and device performance of InGaN/GaN DH and MQW LED structures were studied. As the pumping laser power increased, the InGaN D-A peak (466nm) exhibted the blue shift and the intensity increased, but the band-edge peaks of GaN and InGaN do not observe the blue shift phenomenon. The blue shift of donor to acceptor transition in InGaN is due to band filling. And the relative emission intensity ratios of the band-to-band peak to yellow peak in InGaN/GaN MQW structure increases and the band-edge GaN peak (369nm) was observed. The EL intensity of 420nm peak (band-edge emission of InGaN) increased and the 470nm peak (DA recombination) saturated with the high injecting current under DC operation in DH LED structure, and the ratio of 420nm/470nm peaks is increasing observably. In MQW LED structures, the 470nm EL peak showed high quality of optical confinement varied with injected current. The total defect densities in the active layers were identified by the imaginary capacitance from the admittance spectroscopy. The sheet defect density (Ddf) of the active layers are calculated as the values 4.5□109cm-2 and 6.4□108cm-2 for DH LED and MQW LED at 1Mz and zero bias occurred in the depletion regions. And the output power are 100□W and 700□W in DH LED and MQW LED measured at 20mA.