High Gain AlGaN Solar Blind Avalanche Photodetector And Analysis Of Dark Current Mechanisms

The band gap of AlGaN semiconductor can be continuously adjustable from 3.4eV to 6.2eV,covering from 365 nm to 200 nm in the ultraviolet band,is the preparation of ultraviolet short wavelength light emitting and detector irreplaceable material system.The high Al component of AlGaN compounds are suitable for fabricating solid-state solar-blind avalanche photodiodes thanks to their intrinsic characteristic of solar blind that can relax system requirements for expensive optical filters to remove natural background radiation from sunlight.AlGaN solar-blind avalanche photodiodes are capable of detecting very weak solar-blind signals under intense background radiation and have outstanding advantages such as lower operation voltages,lower power consumption,smaller sizes,which are easy to integrate.So they could be a viable alternative to current bulky and fragile photomultiplier tubes.They also could show potential applications in early missile threat warning and tracking,environmental monitoring,flame detection and monitoring,non-line-of-sight communications,chemical and biological agent spectroscopy and threat detection,and covert space-to-space communication.High-Al-content AlGaN films are suffering from great difficulties,such as high dislocation density(typically 109 cm-2)and low p-type doping efficiency due to large Mg acceptor ionization energy;in addition,ionization coefficient of hole and electron in the nitride semiconductor is close,is not conducive to reducing noise,which hampers realizing high-gain APDs using conventional process.In order to improve the performance of AlGaN solar blind avalanche detector,in this paper we fabricate AlGaN ultraviolet solar-blind avalanche photodetectors with separate absorption and multiplication SAM-type structure,develope a series of special device technology and explore the polarization disposition of nitride semiconductor in the application of device structure design.Finally,we studied the effect of dislocation density on dark current mechanism of AlGaN solar blind APD.The main results are listed as follows:1.We made AlGaN solar blind avalanche detector with double mesa and SAM type structure.In order to reduce etching damage on the device,this paper developed a special kind of photoelectrochemical treatment process,compared with the traditional KOH solution treatment method,the step sidewall becomes smoother after photoelectric chemical processing,and dark current of devices decreases obviously,optical gain significantly increased.The devices were tested while the protection of current is set to 10-5 A,when the reverse bias voltage reaches 75V,the device begins to appear obvious avalanche,when the voltage is added to 84V,the gain of devices grow up to 1.2×104.At the same time,the temperature dependence of the I-V characteristic curves show,devices has a larger positive temperature coefficient,prove that the optical gain is caused by avalanche impact ionization,and not by the tunneling mechanism or other causes.2.The spontaneous and piezoelectric polarization in ?-nitrides can cause an internal electric field of up to several MV/cm,which is of the same order of magnitude as avalanche breakdown electric field in the multiplication region of AlGaN APDs.Therefore,we design innovation through a unique device structure exploitation nitride polarization temperament,develope a polarization enhanced SAM-type AlGaN solar blind avalanche photodetector.Those polarization charges result in an internal electric field in the multiplication region along the same direction as applied reverse bias electric field,which enhance carrier impact ionization rate and reduce the applied avalanche voltage,can reduce the avalanche point total dark current,easy to get higher gain.The maximum avalanche gain of SAM structure solar blind AlGaN avalanche detector is 2.1×104,is nearly 2 times that of conventional SAM-AlGaN structure;avalanche point dark current density also reduces one order of magnitude,down to 10-6A/cm2.3.Combined with the software simulation and micro structure analysis,research the different photoelectric properties of dark current mechanism of SAM type AlGaN solar blind avalanche detector.We found that dominating dark current mechanism in AlGaN APD structure under reverse bias is associated with the crystal quality of AlGaN materials.AlGaN APD structure with high density of dislocation exhibits a Zener-like tunneling phenomenon at relatively low reverse bias at 25V,having a negative temperature coefficient in the temperature-dependent I-V curves,and the simulation shows that trap-assisted nonlocal band to band tunneling current is a dominating dark current mechanism.For the APD structure with relatively low dislocation density,avalanche breakdown phenomenon can be observed at high reverse bias at 75.5V,showing a large positive temperature coefficient as shown in,and the simulation shows that the avalanche multiplication is the dominating dark current mechanism.