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分离吸收层与倍增层结构的低压4H-SiC雪崩光电探测器及其p型欧姆接触的研究
Research and Fabrication of UV SAM 4H-SiC APDs with Low Breakdown Voltage and Its p-type Ohmic Contacts
【作者】 朱会丽；
【导师】 吴正云；
【作者基本信息】 厦门大学 ， 凝聚态物理， 2007， 博士
【摘要】 紫外微弱光信号和单光子信号的探测主要应用于激光诱导荧光性生物报警系统、非线性光线隐蔽通讯、非破坏性物质分析、高能物理、光时域反射和空气污染超高灵敏度探测等领域,它要求探测器具有高量子效率、低暗电流、低的过剩噪声和可见盲等特性,4H-SiC雪崩光电探测器（APD）是惟一能够满足这些要求的器件。近年来,国际上已有研究小组对4H-SiC APDs进行制备和研究,但所设计的APD结构较为简单,一般由PN结或者PIN结构成,不能有效地解决吸收层厚度对高量子效率、快响应速率和低击穿电压之间相互限制的矛盾;而已报道的分离吸收层与倍增层（SAM）结构4H-SiC APDs的击穿电压过大;另外,对于金属与p型4H-SiC接触,其一般形成大的势垒高度,因此制备具有低欧姆接触电阻率的4H-SiC p型欧姆接触比较困难。据了解,目前国内还未发现4H-SiC APDs的相关报道。针对以上问题,本文主要开展了以下几方面的工作,并取得了较好的结果。1.根据APD的结构特性和4H-SiC的材料特性设计了高响应度、低击穿电压的SAM结构4H-SiC APD。从理论上分析了p+层和耗尽区的不同厚度对器件光谱响应和时间响应的影响,综合考虑光谱响应、时间响应和击穿电压等因素对4H-SiC APD外延片参数进行优化设计。2.成功制备了工作在低击穿电压下的SAM结构4H-SiC APDs,并采用自行设计的紫外光电测试系统对所制备器件的光电流、暗电流和光谱响应等进行测试与分析。从APDs的反向I-V特性可以看出,器件的击穿电压（Vb）和穿通电压分别为-55 V和-27.5 V。在穿通电压前,器件的暗电流基本保持在十几pA,当反向偏压增大到50 V（约90% Vb）时,其暗电流约为60 nA,此时器件的倍增因子达到1.8×104。对器件在0～-35 V偏压下的光谱响应和量子效率的测试结果分析表明,在零偏压下,在270 nm峰值波长下获得最大光谱响应度约为0.070 A/W,相应的量子效率为32.6%,据我们所知,此结果是目前所报道的4H-SiC APDs低压下获得的最大量子效率。并且,在零偏压下器件的紫外可见比约为三个数量级,其归一化探测率最大值约为6.0×1013 cmHz1/2W-1,相应的噪声等效功率约为3.75×10-16 W。当反向偏压从0 V增大到35 V时,器件光谱响应的峰值波长由270 nm移动到280 nm,且响应度增强为0.077 A/W,对应的外量子效率为35%,同时在短波长处（220～260 nm）器件的响应度增强比较明显,出现该现象的原因可能是在p+层吸收短波长产生的光生载流子复合减小所致。总之,本文制备的SAM结构4H-SiC APDs在较低的击穿电压下获得了较好的紫外光探测性能。3.为了提高4H-SiC p型欧姆接触的性能,对一组Al基多层金属制备的4H-SiC p型欧姆接触进行了系统地研究。采用标准的半导体工艺制备完成了四种Al基4H-SiC p型欧姆接触,分别为:Ti（1200 （?））/Al（600 （?））/Au（850 （?））、Al（1200 （?））/Ti （600 （?））/Au（850 （?））、Ti（600 （?））/Al（1200 （?））/Au（850 （?））和Al（600 （?））/Ni（600 （?）） /Al（400 （?） ）/Au（850 （?））,并通过线性传输线方法（LTLM）对其比接触电阻进行测量。结果发现,金属构成和厚度分别为Ti（600 （?））/Al（1200 （?））/Au（850 （?））的最佳条件下,获得最低的比接触电阻率约为3.6×10-5Ωcm2,同时测得4H-SiC/Ti（1200 （?））/Al（600 （?））/Au（850 （?））经930℃退火后形成欧姆接触的比接触电阻率约为4.2×10-4Ωcm2。为了更好地了解欧姆接触形成的机理,找出影响欧姆接触性能的主要因素,选择了Ti（1200 （?））/Al（600 （?））/Au（850 （?））结构的欧姆接触,分别采用扫描电子显微镜（SEM）、俄歇电子能谱（AES）、X射线光电子能谱（XPS）和X射线衍射谱（XRD）等进行测试分析。结果表明,在高温退火过程中金属之间以及金属/4H-SiC之间反应生成的TiC、TiAl3和Au（35 at%）+Ti（42 at%）等物质对提高4H-SiC p型欧姆接触的性能有重要作用。
【Abstract】 Low-level and single photon ultraviolet （UV） signal detections are mainly used in laser-induced fluorescence biological-agent detection, non-line-of-sight covert communica- tions, non-destructive material analysis, high energy physics, optical time domain reflectometer, ultra-high sensitivity for air contamination detection, and so on. 4H-SiC APD is the only promising UV detector that satisfies low-level and single photon detection requirements, e.g. high quantum efficiency, low dark current, low excess noise, high speed and visible-blind operation.By far, 4H-SiC APDs based on PN or PIN structure have been reported, which can not effectively resolve the trade-off in the maximum achievable photoresponse, the faster response time and the operating voltage. The separated absorption and multiplication （SAM） 4H-SiC APDs have been reported, which showed a relative high breakdown voltage. In addition, on p-type 4H-SiC, metals usually form a Schottky contact with a large barrier height. Therefore, it’s difficult to attain 4H-SiC p-type Ohmic contacts with low contact resistivity. Up to now, the relevant reports on 4H-SiC APDs have not been found in China.The purposes of this work are to fabricate a high-performance 4H-SiC APD and to achieve a good 4H-SiC p-type Ohmic contact. The achieved results are as follows:Firstly, according to the structure properties of APD and the 4H-SiC material properties, a SAM 4H-SiC APD with high spectral responsivity and low breakdown voltage was designed. The influences of various thicknesses of p+ epilayer and the depletion region on spectral response and time response have been theoretically analysed. Considering the factors of spectral response, time response, and breakdown voltage, the optimal parameters of SAM 4H-SiC APDs were determined.Secondly, the SAM 4H-SiC UV APDs with low breakdown voltage were fabricated and characterized successfully. The low breakdown voltage （Vb） of -55 V and the reach through voltage of -27.5 V were obtained from the reverse I-V characteristic. The dark current kept at 10 pA level from 0 V to the reach through voltage. At -50 V （90% Vb）, the dark current was 60 nA and a high gain about 1.8×104 was achieved. The typical spectral responsivitives and quantum efficiencies of APDs were measured at different reverse bias voltages ranging from 0 V to -35 V. At 0 V, the peak responsivity was calculated to be about 0.070 A/W at 270 nm, corresponding to a peak external quantum efficiency of 32.6%. To the best of our knowledge, it was the highest quantum efficiency achieved at low reverse bias at present. The UV-to-visible rejection ratio around three orders of magnitude was also extracted from the spectral response. Furthermore, at 0 V, the maximum spectral detectivity about 6.0×1013 cm Hz1/2W-1 and the corresponding noise equivalent power 3.75×10-16 W were obtained. With the increasing reverse bias, the peaks of response wavelength shifted from 270 nm to 280 nm and the peak responsivity increased to 0.077 A/W, corresponding to the maximum external quantum efficiency of 35%. The results of spectral response also showed the spectral responsivities were enhanced within the whole UV range, especially in the range of 220 nm to 260 nm which might be due to the decrease of photogenerated carrier recombination in the p+ layer. In conclusion, the fabricated 4H-SiC APDs with low breakdown voltage have excellent performance for UV signal detection.Thirdly, in order to improve the performance of 4H-SiC p-type Ohmic contacts, four kinds of 4H-SiC p-type Ohmic contacts used Al-based multiple metals, including Ti（1200 （?））/Al（600 （?））/Au（850 （?））, Al（1200 （?））/Ti （600 （?））/Au（850 （?））,Ti（600 （?））/Al（1200 （?））/Au（850 （?）） and Al（600 （?））/Ni（600 （?）） /Al（400 （?） ）/Au（850 （?））, were fabricated and characterized. The lowest specific contact resistance of 3.6×10-5 （?）cm2 was achieved by the linear transmission line method （LTLM） after Ti（600 （?））/Al（1200 （?））/Au（850 （?）） annealed at 930℃for 12 minutes, and the specific contact resistance of 4H-SiC/Ti（1200 （?））/Al（600 （?））/Au（850 （?）） Ohmic contact was about 4.2×10-4 （?）cm2. Furthermore, in order to reveal the mechanism of Ohmic behavior, the scanning electron microscope （SEM）, Auger electron spectroscopy （AES）, X-ray photoelectron spectroscopy （XPS） and Panalytical X’pert PRO x-ray diffraction （XRD） were used to analyse Ohmic contact morphology, chemical composition and the phase formation of 4H-SiC/Ti（1200 （?））/Al（600 （?））/Au（850 （?）） sample before and after annealing. The results showed that the formation of TiC、TiAl3 and Au（35 at%）+Ti（42 at%） occurred by annealing at 930℃improved the Ohmic behavior of the contacts.