Transition of photoconductive and photovoltaic operation modes in amorphous Ga_2O_3-based solar-blind detectors tuned by oxygen vacancies

We report on the transition of photovoltaic and photoconductive operation modes of the amorphous Ga_2O_3-based solar-blind photodetectors in metal–semiconductor–metal(MSM) configurations. The conversion from Ohmic to Schottky contacts at Ti/Ga_2O_3 interface is realized by tuning the conductivity of amorphous Ga_2O_3 films with delicate control of oxygen flux in the sputtering process. The abundant donor-like oxygen vacancies distributed near the Ti/Ga_2O_3 interface fascinate the tunneling process across the barrier and result in the formation of Ohmic contacts. As a consequence, the serious sub-gap absorption and persistent photoconductivity(PPC) effect degrades the performance of the photoconductive detectors. In contrast, the photovoltaic device with a Schottky contact exhibits an ultra-low dark current less than 1 pA,a high detectivity of 9.82×10^(12) cm·Hz^(1/2)·W^(-1), a fast response time of 243.9 μs, and a high ultraviolet C(UVC)-toultraviolet A(UVA) rejection ratio of 103. The promoting performance is attributed primarily to the reduction of the subgap states and the resultant suppression of PPC effect. With simple architecture, low fabrication cost, and easy fusion with modern high-speed integrated circuitry, these results provide a cost-effective way to realize high performance solar-blind photodetectors towards versatile practical applications.

Responses of river bed evolution to flow-sediment process changes after Three Gorges Project in middle Yangtze River:A case study of Yaojian reach

The Three Gorges Project(TGP)has changed the flow-sediment process in the middle Yangtze River.For navigation purposes,there is an urgent need to study the changes of the river regime over a long-term period and the shoal-channel evolution over different seasons since the completion of the TGP.Based on analysis of the measured data and the results of a two-dimensional mathematical model,the changes of the river regime and river bed evolution in the Yaojian reach downstream of the TGP were studied.Results show that a high sediment transport flux helps to keep the main flow in the North Branch,while a low sediment transport flux helps to keep the main flow in the South Branch.Thus,the main branch will not change in the near future because of the low sediment transport load.In this study,the flow-sediment process adjusted by the TGP was restored to the conditions before the TGP,and the river bed evolution under the adjusted and non-adjusted flow-sediment conditions was calculated.After the completion of the TGP,the reservoir storage accelerated the flood recession process and decreased the erosion by 11.9%under the flow-sediment conditions in 2010,and the deposition in the flood season decreased by 56.4%.

Effect of a Single Threading Dislocation on Electrical and Single Photon Detection Characteristics of 4H-SiC Ultraviolet Avalanche Photodiodes

We fabricated 4H-SiC ultraviolet avalanche photodiode(APD)arrays and systematically investigated the effect of threading dislocations on electrical and single photon detection characteristics of 4H-SiC APDs.Based on a statistical correlation study of individual device performance and structural defect mapping revealed by molten KOH etching,it is determined with high confidence level that even a single threading dislocation within APD active region would lead to apparent device performance degradation,including increase of dark current near breakdown voltage,premature breakdown and reduction of single photon detection efficiency at fixed dark count rate.

High performance lateral Schottky diodes based on quasi-degenerated Ga_2O_3

Ni/β-Ga_2 O_3 lateral Schottky barrier diodes(SBDs) were fabricated on a Sn-doped quasi-degenerate n^+-Ga_2 O_3(201)bulk substrate. The resultant diodes with an area of 7.85 ×10^(-5) cm^2 exhibited excellent rectifying characteristics with an ideality factor of 1.21, a forward current density(J) of 127.4 A/cm2 at 1.4 V, a specific on-state resistance(R_(on,sp)) of1.54 mΩ·cm^2,and an ultra-high on/off ratio of 2.1 ×10^(11) at±1 V. Due to a small depletion region in the highly-doped substrate, a breakdown feature was observed at-23 V, which corresponded to a breakdown field of 2.1 MV/cm and a power figure-of-merit(VB2/R_(on)) of 3.4×10~5 W/cm^2. Forward current-voltage characteristics were described well by the thermionic emission theory while thermionic field emission and trap-assisted tunneling were the dominant transport mechanisms at low and high reverse biases, respectively, which was a result of the contribution of deep-level traps at the metal-semiconductor interface. The presence of interfacial traps also caused the difference in Schottky barrier heights of 1.31 eV and 1.64 eV respectively determined by current-voltage and capacitance-voltage characteristics. With reduced trapping effect and incorporation of drift layers, the β-Ga_2 O_3 SBDs could further provide promising materials for delivering both high current output and high breakdown voltage.

Alpha particle detector with planar double Schottky contacts directly fabricated on semi-insulating GaN:Fe template

Alpha particle radiation detectors with planar double Schottky contacts(DSC)are directly fabricated on 5-μm-thick epitaxial semi-insulating(SI)GaN:Fe film with resistivity higher than 1×10^(8)Ω·cm.Under 10 V bias,the detector exhibits a low dark current of less than 5.0×10^(-11) A at room-temperature,which increases at higher temperatures.Linear behavior in the semi-log reverse current-voltage plot suggests that Poole-Frenkel emission is the dominant carrier leakage mechanism at high bias.Distinct double-peak characteristics are observed in the energy spectrum of alpha particles regardless of bias voltage.The energy resolution of the SI-GaN based detector is determined to be8.6%at the deposited energy of 1.209 MeV with a charge collection efficiency of81.7%.At a higher temperature of 90℃,the measured full width at half maximum(FWHM)rises to 235 keV with no shift of energy peak position,which proves that the GaN detector has potential to work stably in high temperature environment.This study provides a possible route to fabricate the low cost GaN-based alpha particle detector with reasonable performance.

Enhanced single photon emission in silicon carbide with Bull's eye cavities

The authors demonstrate a Bull's eye cavity design that is composed of circular Bragg gratings and micropillar optical cavity in 4H silicon carbide(4H-SiC) for single photon emission. Numerical calculations are used to investigate and optimize the emission rate and directionality of emission. Thanks to the optical mode resonances and Bragg reflections,the radiative decay rates of a dipole embedded in the cavity center is enhanced by 12.8 times as compared to that from a bulk 4H-SiC. In particular, a convergent angular distribution of the emission in far field is simultaneously achieved, which remarkably boost the collection efficiency. The findings of this work provide an alternative architecture to manipulate light-matter interactions for achieving high-efficient SiC single photon sources towards applications in quantum information technologies.

NiO/Ga_(2)O_(3) p+-n异质结功率二极管中陷阱介导双极型电荷输运研究

构筑NiO/Ga_(2)O_(3)p+-n异质结是克服Ga_(2)O_(3)p型掺杂瓶颈从而实现双极型功率电子器件的有效途径,然而限制器件性能的缺陷行为与双极型电荷输运等物理机制尚不明晰.本论文研究了NiO/Ga_(2)O_(3)p+-n异质结中陷阱介导的载流子输运、俘获和复合动力学之间的内在关联特性.变温电流-电压特性的量化分析表明,在正偏亚阈值区,陷阱辅助隧穿占据主导地位,符合多数载流子陷阱介导的Shockley-Read-Hall复合模型,其陷阱激活能为0.64 eV,与深能级瞬态谱测试的陷阱能级位置(EC-0.67 eV)非常吻合;当正向偏压大于器件开启电压时,器件输运特性由少数载流子扩散所主导,器件理想因子接近于1.在反向偏置的高场作用下,器件漏电机制则由β-Ga_(2)O_(3)体材料中的陷阱引起的PooleFrenkel(PF)发射所导致.PF发射的势垒高度为0.75 eV,与等温变频深能级瞬态谱测得的陷阱能级位置(EC-0.75 eV)相一致.这一工作有助于建立NiO/Ga_(2)O_(3)p+-n异质结中双极型电荷输运和深能级缺陷行为间的内在关联,对理解和发展Ga_(2)O_(3)双极型功率整流器件具有重要的参考价值.

Band alignment and polarization engineering inκ-Ga_(2)O_(3)/GaN ferroelectric heterojunction

Ferroelectric-semiconductor heterostructures offer an alternative strategy to manipulate polarization towards advanced devices with engineered functionality and improved performance.In this work,we report on the heteroepitaxial construction,band structure alignment and polarization engineering of the single-phasedκ-Ga_(2)O_(3)/GaN ferroelectric/polar heterojunction.A type-II band alignment is determined at theκ-Ga_(2)O_(3)/GaN polar hetero-interface,with a valence band offset of(1.74±0.1)eV and a conduction band offset of(0.29■0.1)eV.Besides the band edge discontinuity,charge dipoles induced by spontaneous polarization lead to the observed band bending with built-in potentials of 0.9 and 0.33 eV,respectively,at theκ-Ga_(2)O_(3)surface andκ-Ga_(2)O_(3)/GaN interface.The polarization switching properties of ferroelectricκ-Ga_(2)O_(3)are identified with a remanent polarization of approximately 2.7μC/cm^(2)via the direct hysteresis remanent polarization/voltage(P-V)loop measurement.These findings allow the rational design ofκ-Ga_(2)O_(3)ferroelectric/polar heterojunction for the application of power electronic devices,advanced memories and even ultra-low loss negative capacitance transistors.

Hybrid plasmonic–dielectric metal-nanowire coupler for high-efficiency broadband nonlinear frequency conversion

Pursuing nanometer-scale nonlinear converters based on second harmonic generation(SHG)is a stimulating strategy for bio-sensing,on-chip optical circuits,and quantum information processing,but the light-conversion efficiency is still poor in such ultra-small dimensional nanostructures.Herein,we demonstrate a highly enhanced broadband frequency converter through a hybrid plasmonic–dielectric coupler,a ZnTe/ZnO single core–shell nanowire(NW)integrated with silver(Ag)nanoparticles(NPs).The NW dimension has been optimized to allow the engineering of dielectric resonances at both fundamental wave and second harmonic frequencies.Meanwhile,the localized surface plasmon resonances are excited in the regime between the Ag NPs and ZnTe/ZnO dielectric NW,as evidenced by plasmon-enhanced Raman scattering and resonant absorption.These two contributors remarkably enhance local fields and consequently support the strong broadband SHG outputs in this hybrid nanostructure by releasing stringent phase-matching conditions.The proposed nanoscale nonlinear optical converter enables the manipulation of nonlinear light–matter interactions toward the development of on-chip nanophotonic systems.

Boundedness of Marcinkiewicz integral with rough kernel on Triebel-Lizorkin spaces

This paper is a continuation of our previous work （Zhang and Chen, 2010b）. Following the same generalsteps of the proof there, we make essential improvement on our previous theorem by recalculating a key inequality. Our result shows that the Marcinkiewicz integral, with a bounded radial function in its kernel, is still bounded on the Triebel-Lizorkin space.