一种减小GaN基肖特基结构紫外探测器暗电流的方法

提出了一种减小GaN肖特基结构紫外探测器暗电流的方法.该方法是在普通的GaN肖特基结构的表面增加一层薄的p-GaN.模拟计算结果表明,该层p-GaN能增加肖特基势垒高度,从而减小了器件的暗电流,提高了器件性能.进一步的计算还发现,对于p型载流子浓度较高的情况下,只需要很薄的一层p-GaN就能显著增加肖特基势垒高度,对于p型载流子浓度较低的情况下,则需要较厚的一层p-GaN才能有较好的肖特基势垒高度增加效果. 关键词： GaN 肖特基结构 紫外探测器 暗电流     

一种新型GaN基肖特基结构紫外探测器

提出了一种新型GaN基肖特基结构紫外探测器.该结构在常规的GaN肖特基结构紫外探测器上外加了一层禁带宽度更大的n型AlGaN层，模拟计算结果表明：与常规器件结构相比，该结构能有效地减小表面复合的影响，提高了器件的量子效率.进一步地研究结果还表明：采取较薄、载流子浓度较高的AlGaN层更有利于提高这种新结构器件的量子效率. 关键词： GaN 肖特基结构 紫外探测器 AlGaN     

以弱p型为有源区的新型p-n结构GaN紫外探测器

提出了以弱p型（p^－-GaN）为有源区的p-n结构GaN紫外探测器.由于弱p型层的载流子浓度较低,很容易增加耗尽区的宽度,从而可以增加器件的量子效率.通过模拟计算,研究了金属与p^－-GaN层的肖特基接触势垒高度、p^－-GaN层厚度等参数对器件性能的影响.研究结果表明,降低金属与p^－-GaN层的接触势垒高度、适当减小p^－-GaN层厚度能够实现有源层方向单一的内建电场,从而提高器件的量子效率.要制备出 关键词： 弱p型GaN 紫外探测器 量子效率     

结构参数对GaN肖特基紫外探测器性能的影响及器件设计

研究了GaN肖特基结构(n^--GaN /n⁺-GaN)紫外探测器的结构参数对器件性能的影响机理。模拟计算结果表明:提高肖特基势垒高度和减小表面复合速率,不仅可以增加器件的量子效率,而且可以极大地减小器件的暗电流;适当地增加n^--GaN层厚度和载流子浓度可以提高器件的量子效率,但减小n^--GaN层的载流子浓度却有利于减小器件的暗电流。我们针对实际应用的需要,提出了一个优化器件结构参数的设计方案,特别是如果实际应用中对器件的量子效率和暗电流都有较高的要求,肖特基势垒高度应该≥0.8 eV,n^--GaN层的厚度≥200 nm,载流子浓度1×10¹⁷ cm^-3 左右,表面复合速率＜1×10⁷ cm/s。     

器件参数对GaN基n~+-GaN/i-Al_x Ga_(1-x)N/n~+-GaN结构紫外和红外双色探测器中紫外响应的影响

研究了AlGaN层参数对GaN基n+-GaN/i-AlxGa1-xN/n+-GaN结构紫外和红外双色探测器中紫外响应的影响规律及物理机制.模拟计算发现:降低AlGaN层本底载流子浓度会增加器件的量子效率,当本底载流子浓度不能进一步降低时,可以通过减小AlGaN层的厚度以保证器件的量子效率.在材料生长和器件工艺过程中都应减少界面态.外加较小的反向偏压能大幅度提高紫外量子效率,分析表明,GaN/AlGaN/GaN形成的两个背靠背、方向相反的异质结电场是出现这些现象的根本原因.在实际器件设计中,应该根据需要调节各结构参数,以保证器件的量子效率.     

AlGaN-Based Solar-Blind Schottky Photodetectors Fabricated on AlN/Sapphire Template

We report AlGaN-based back-illuminated solar-blind Schottky-type ultraviolet photodetectors with the cutoff- wavelength from 280nm to 292nm without bias. The devices show low dark current of 2.1× 10^-6A/cm^2 at the reverse bias of 5 V. The specific detectivity D＊ is estimated to be 3.3 × 10^12cmHz^1/2 W^-1 . To guarantee the performance of the photodetectors, the optimization of AlGaN growth and annealing condition for Schottky contacts were performed. The results show that high-temperature annealing method for Ni/Pt Schottky contacts is effective for the reduction of leakage current.     

Ultraviolet Phototransistors on AlGaN/GaN Heterostructures

We report on the fabrication and characterization of phototransistors based on AIGaN/GaN heterostructure grown over 6H-SiC substrates. The device has two functions： as a high electron mobility transistor （HEMT） and an ultraviolet photodetector at the same time. As an HEMT, its maximum transconductance is 170mS/ram, while the minimum cutoff frequency fT and the maximum oscillation frequency fm are 19 and 35 GHz, respectively. As a photodetector, the device is visible blind, with an ultraviolet/green contrast of three orders of magnitude, and a responsivity as high as 1700 A/W at the wavelength of 362nm.     

Role of Ga vacancies in enhancing the leakage current of GaN Schottky barrier ultraviolet photodetectors

The leakage current of GaN Schottky barrier ultraviolet photodetectors is investigated. It is found that the photodetectors adopting undoped GaN instead of lightly Si-doped GaN as an active layer show a much lower leakage current even when they have a higher dislocation density. It is also found that the density of Ga vacancies in undoped GaN is much lower than in Si-doped GaN. The Ga vacancies may enhance tunneling and reduce effective Schottky barrier height, leading to an increase of leakage current. It suggests that when undoped GaN is used as the active layer, it is necessary to reduce the leakage current of GaN Schottky barrier ultraviolet photodetector.     

p型GaN/Al0.35Ga0.65N/GaN应变量子阱中二维空穴气的研究

对Ga面p型GaN/Al_0.35Ga_0.65N/GaN应变异质结构中形成的二维空穴气(2DHG)进行了研究.首先基于半导体-绝缘体-半导体异质结构模型确定了应变异质中的临界厚度，然后自洽求解薛定谔方程和泊松方程，计算了当中间势垒层AlGaN处于完全应变状态和半应变状态两种条件下，顶层GaN及中间层AlGaN厚度的变化对2DHG分布的影响.计算结果表明，势垒层AlGaN和顶层GaN的应变状态和厚度对极化引起的2DHG面密度及分布有重要影响.在此基础上制备了p型GaN/Al_0.35Ga_0.65N/GaN应变量子阱结构肖特基器件，并通过器件的C-V测试证实了异质结处2DHG的存在.器件响应光谱的测试结果表明，由于p型GaN/Al_0.35Ga_0.65N/GaN量子阱中强烈的极化作用和Stark效应使得器件零偏压和反向偏压时的响应光谱都向短波方向移动了10 nm，在零偏压下器件在280 nm处的峰值响应为0.022 A/W，在反向偏压为1 V时，峰值响应达到0.19 A/W，已经接近理论值. 关键词： AlGaN 二维空穴气 极化效应     

Comparison of electrical characteristic between AlN/GaN and AlGaN/GaN heterostructure Schottky diodes

Ni/Au Schottky contacts on AlN/GaN and AlGaN/GaN heterostructures are fabricated.Based on the measured current–voltage and capacitance–voltage curves,the electrical characteristics of AlN/GaN Schottky diode,such as Schottky barrier height,turn-on voltage,reverse breakdown voltage,ideal factor,and the current-transport mechanism,are analyzed and then compared with those of an AlGaN/GaN diode by self-consistently solving Schrdinger’s and Poisson’s equations.It is found that the dislocation-governed tunneling is dominant for both AlN/GaN and AlGaN/GaN Schottky diodes.However,more dislocation defects and a thinner barrier layer for AlN/GaN heterostructure results in a larger tunneling probability,and causes a larger leakage current and lower reverse breakdown voltage,even though the Schottky barrier height of AlN/GaN Schottky diode is calculated to be higher that of an AlGaN/GaN diode.     

Influence of Schottky drain contacts on the strained AlGaN barrier layer of AlGaN/AlN/GaN heterostructure field-effect transistors

Rectangular Schottky drain AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs) with different gate contact areas and conventional AlGaN/AlN/GaN HFETs as control were both fabricated with same size. It was found there is a significant difference between Schottky drain AlGaN/AlN/GaN HFETs and the control group both in drain series resistance and in two-dimensional electron gas (2DEG) electron mobility in the gate-drain channel. We attribute this to the different influence of Ohmic drain contacts and Schottky drain contacts on the strained AlGaN barrier layer. For conventional AlGaN/AlN/GaN HFETs, annealing drain Ohmic contacts gives rise to a strain variation in the AlGaN barrier layer between the gate contacts and the drain contacts, and results in strong polarization Coulomb field scattering in this region. In Schottky drain AlGaN/AlN/GaN HFETs, the strain in the AlGaN barrier layer is distributed more regularly.     

AlGaN/GaN肖特基结参数分析与电流运输机理研究

通过对AlGaN/GaN HEMT器件肖特基栅电流输运机理的研究,在变温下采用I-V法对AlGaN/GaN上的Ni/Au肖特基势垒高度和理想因子进行了计算. 通过对不同电流机理的分立研究,得到了更为准确的势垒高度值_b. 通过分析温度在300—550K之间肖特基反向泄漏电流的特性,得出结论：AlGaN材料的表面漏电不是HEMT器件反向泄漏电流的主要来源. 关键词： AlGaN/GaN异质结 肖特基结 理想因子     

Electrical Characteristics of InGaN/AlGaN and InGaN/GaN MQW Near UV-LEDs

Electrical characteristics of In0.05 Ga0.95N/Al0.07Ga0.9aN and In0.05 Ga0.95N/GaN multiple quantum well （MQW） ultraviolet light-emltting diodes （UV-LEDs） at 400hm wavelength are measured. It is found that for InGaN/AlGaN MQW LEDs, both ideality factor and parallel resistance are similar to those of InGaN/GaN MQW LEDs, while series resistance is two times larger. It is suggested that the Al0.07Ga0.93N barrier layer did not change crystal quality and electrical characteristic of p-n junction either, but brought larger series resistance. As a result, InGaN/AlGaN MQW LEDs suffer more serious thermal dissipation problem although they show higher light output efficiency.     

GaN基p-i-n和肖特基紫外探测器的响应光谱及暗电流特性

研究了p-i-n型和肖特基型Ga N基紫外探测器的响应光谱和暗电流特性。实验发现,随着p-Ga N层厚度的增加,p-i-n型紫外探测器的响应度下降,并且在短波处下降更加明显。肖特基探测器的响应度明显比pi-n结构高,主要是由于p-Ga N层吸收了大量的入射光所致。肖特基型紫外探测器的暗电流远远大于p-i-n型紫外探测器的暗电流,和模拟结果基本一致,主要是肖特基型探测器是多子器件,而p-i-n型探测器是少子器件。要制备响应度大、暗电流小的高性能Ga N紫外探测器,最好采用p-Ga N层较薄的p-i-n结构。     

Different influences of Schottky metal on the strain and relative permittivity of barrier layer between AlN/GaN and AlGaN/GaN heterostructure Schottky diodes

Ni/Au Schottky contacts on AlN/GaN and AlGaN/GaN heterostructures are fabricated.Based on the measured current–voltage and capacitance-voltage curves,the polarization sheet charge density and relative permittivity are analyzed and calculated by self-consistently solving Schrdinger’s and Poisson’s equations.It is found that the values of relative permittivity and polarization sheet charge density of AlN/GaN diode are both much smaller than the ones of AlGaN/GaN diode,and also much lower than the theoretical values.Moreover,by fitting the measured forward I–V curves,the extracted dislocations existing in the barrier layer of the AlN/GaN diode are found to be much more than those of the AlGaN/GaN diode.As a result,the conclusion can be made that compared with AlGaN/GaN diode the Schottky metal has an enhanced influence on the strain of the extremely thinner AlN barrier layer,which is attributed to the more dislocations.     

Effects of the strain relaxation of an AlGaN barrier layer induced by various cap layers on the transport properties in AlGaN/GaN heterostructures

The strain relaxation of an AlGaN barrier layer may be influenced by a thin cap layer above, and affects the transport properties of AlGaN/GaN heterostructures. Compared with the slight strain relaxation found in AlGaN barrier layer without cap layer, it is found that a thin cap layer can induce considerable changes of strain state in the AlGaN barrier layer. The degree of relaxation of the AlGaN layer significantly influences the transport properties of the two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures. It is observed that electron mobility decreases with the increasing degree of relaxation of the AlGaN barrier, which is believed to be the main cause of the deterioration of crystalline quality and morphology on the AlGaN/GaN interface. On the other hand, both GaN and AlN cap layers lead to a decrease in 2DEG density. The reduction of 2DEG caused by the GaN cap layer may be attributed to the additional negative polarization charges formed at the interface between GaN and AlGaN, while the reduction of the piezoelectric effect in the AlGaN layer results in the decrease of 2DEG density in the case of AlN cap layer.     

Charge accumulation in ultrathin Cs / n - GaN and Cs / n - InGaN interfaces

We report on the observation of new phenomena that arise under Cs adsorption on n-GaN(0001) and n-InGaN(0001) surfaces. First, an extremely highly quantum efficient photoemission has been found by excitation with visible light in the transparency region of GaN and InGaN. The photoemission is revealed to appear due to the formation of an electron accumulation layer in the vicinity of the surfaces. Second, a large variety of band bending and potential wells are provided by the Cs coverages. The accumulated charge density at the n-InGaN surface is much stronger than that at the n-GaN surface. Third, a new effect is revealed, namely, the appearance of an oscillation structure in the spectral dependences of the threshold photoemission. A model concept is proposed for photocurrent oscillations that takes into account the formation of an accumulation layer and the multiple-beam interference in parallel-sided GaN or InGaN samples.     

Control of surfaces and heterointerfaces of AlGaN/GaN system for sensor devices and their on-chip integration on nanostructures

For successful construction of sensor devices and their future on-chip integration on nanostructures, this paper discusses the present status of understanding and control of surfaces and heterointerfaces of the AlGaN/GaN material system by reviewing a series of works recently carried out by the authors’ group.Leakage currents in Schottky contacts are explained by the authors’ thin surface barrier (TSB) model. An important role is played by oxygen shallow donors in leakage in AlGaN Schottky diodes. A large leakage reduction has been achieved by a novel surface control process for oxygen gettering. An unprecedented high sensitivity has been obtained in AlGaN/GaN Schottky diode hydrogen sensor by applying the surface control process. Liquid-phase AlGaN/GaN sensors having an open-gate HFET structure show a very good pH sensing capability as well as a good sensing capability of polar liquids. Finally, the selective MBE growth of AlGaN/GaN nanowire network is discussed as a basic hardware structure for the on-chip integration of sensors, paying attention to the heterointerface control.     

Polarization effects simulation of AlGaN/GaN heterojunction by using a symbolistic δ-doping layer

Polarization effects in AlGaN/GaN heterojunction are simulated based on a traditional semiconductor device simulator. A δ doping layer is purposely inserted at the interface of the heterojunction in the simulation, so the ionized donors or acceptors can represent polarization-induced positive or negative fixed charges. The free electron distribution of single AlGaN/GaN heterostructures with Ga-face and N-face growth is compared, and the results of the simulation show that carrier confinement takes place only in the former structure. The dependence of sheet density of free electrons at the interface of Ga-face growth AlGaN/GaN on Al composition and the thickness of AlGaN is also investigated. The consistency of simulation results with the experiments and calculations reported by other researchers shows that this method can be effectively used to deal with the polarization effects in the simulation of GaN-based heterojunction devices.     

Influence of thermal stress on the characteristic parameters of AlGaN/GaN heterostructure Schottky contacts

Ni Schottky contacts on AlGaN/GaN heterostructures have been fabricated. The samples are then thermally treated in a furnace with N₂ ambient at 600 ^circC for different times (0.5, 4.5, 10.5, 18, 33, 48 and 72 h). Current-voltage (I-V) and capacitance-voltage (C-V) relationships are measured, and Schrödinger's and Poisson's equations are self-consistently solved to obtain the characteristic parameters related to AlGaN/GaN heterostructure Schottky contacts: the two-dimensional electron gas (2DEG) sheet density, the polarization sheet charge density, the 2DEG distribution in the triangle quantum well and the Schottky barrier height for each thermal stressing time. Most of the above parameters reduce with the increase of stressing time, only the parameter of the average distance of the 2DEG from the AlGaN/GaN interface increases with the increase of thermal stressing time. The changes of the characteristic parameters can be divided into two stages. In the first stage the strain in the AlGaN barrier layer is present. In this stage the characteristic parameters change rapidly compared with those in the second stage in which the AlGaN barrier layer is relaxed and no strain is present.