Optoelectronic Properties Of GaN-based Ultraviolet Photodetectors

As the representative material of the third generation wide bandgap semiconductors,gallium nitride?GaN?has excellent properties such as direct bandgap,large light absorption coefficient,good thermal and chemical stability,making it very suitable for fabricating high-performance ultraviolet?UV?photodetectors?PDs?.GaN-based UV PDs have important applications in the fields of missile aircraft warning,combustion process detection,industrial flame detection and environmental ultraviolet detection.Due to the competitive advantage of price,most GaN-based UV PDs are prepared on standard sapphire substrates.However,there exists large lattice thermal mismatch between standard sapphire substrates and epitaxial GaN materials.Thus,the GaN film often contains a large dislocation density.The dislocation,which may lead to a large reverse leakage current density,increases the device's b ackground noise and reduces the photoelectric conversion efficiency.At present,two major methods for improving the quality of GaN crystals are to grow the epitaxial layer on a patterned sapphire substrate?PSS?,or to reduce the stress by growing a buffer layer before epitaxy.The main work of this thesis is summarized as follows.Firstly,relevant test systems for measuring electrical and optical characteristics of PDs are set up.A transient response test system with a high sensitivity is designed and b uilt.The system has a very short self-response time of about 8 ?s.A low-frequency noise test system is also designed and built to achieve accurate 1/f low-frequency noise measurements.In addition,by enhancing the light source power and reducing the attenuation,the spectral response system is optimized.Secondly,in order to improve the quality of GaN epitaxial wafers,GaN-based Schottky type UV PDs with graded doping,which can increase the efficiency of electrode collection of carriers,are fabricated on the PSS.At room temperature,they exhibit a very low dark current density of ¹.3×10^-8 A/cm² under a bias of -5 V.With the increase of temperature,the current transport mechanism changes from tunneling to diffusion and recombination at forward low bias.Under zero bias,they have a large ultraviolet/visible rejection ratio of ⁴.2×10³,a high photo-responsivity of ⁰.147 A/W and a high external quantum efficiency of ⁵0.7%.Even in the deep-UV region?360 nm-250 nm?,the average quantum efficiency is still greater than 40%.The average rise and fall time constants are estimated to be about 115 ?s and 120 ?s,respectively,which are almost unchanged with the increasing bias and temperature,showing good thermal stability.The specific detectivity limited by the thermal noise is determined ⁵.5×10¹³ cm×Hz^1/2/W at 0 V.The detectivity is determined ⁶.72×10¹⁰ cm×Hz^1/2 at-5 V,which is of the same order of magnitude as the highest detectivity performance reported for GaN-based Schottky UV PDs.Finally,AlGaN metal-semiconductor-metal type UV PDs are fabricated by growing AlN under high temperature as a buffer layer between the traditional sapphire substrate and the GaN epitaxial layer to improve the crystal quality.The devices exhibit a low dark current of ⁸ pA,benefitting from the presence of the high-resistance AlN buffer layer.The responsivity is about 0.07A/W at-10 V,corresponding to a quantum efficiency of ⁴1.5%.The average rise and fall time constants are estimated to be about 111 ?s and 77 ?s,respectively.The specific detectivity limited by the thermal noise is determined about 4.89×10¹² cm×Hz^1/2/W at 0 V.The detectivity is determined about 6.26×10⁹ cm×Hz^1/2 at-5 V.Finally,the degradation of the PDs is investigated.The dark current and the responsivity decreases are observed under long time electric stresses.After standing for 5 hours,the photoelectric properties of PDs are basically restored to the same level as before stress,which may be caused by the trap effect in materials.