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.     

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     

Hydrogen Sensors Based on AlGaN/AlN/GaN Schottky Diodes

Pt/AlGaN/AlN/GaN Schottky diodes are fabricated and characterized for hydrogen sensing. The Pt Schottky contact and the Ti/Al/Ni/Au ohmic contact are formed by evaporation. Both the forward and reverse currents of the device increase greatly when exposed to hydrogen gas. A shift of 0.3 V at 300 K is obtained at a fixed forward current after switching from N2 to 10%H2＋N2. The sensor responses under different concentrations from 50ppm H2 to 10%H2＋N2 at 373K are investigated. Time dependences of the device forward current at 0.5 V forward bias in N2 and air atmosphere at 300 and 373K are compared. Oxygen in air azcelerates the desorption of the hydrogen and the recovery of the sensor. Finally, the decrease of the Schottky barrier height and sensitivity Of the sensor are calculated.     

Effect of CO on Characteristics of AlGaN/GaN Schottky Diode

Pt Schottky diode gas sensors for CO are fabricated using A1GaN/GaN high electron mobility transistor（HEMTs）structure. The diodes show a remarkable sensor signal （3 mA, in N2, 2mA in air ambient） biased 2 V after 1% CO is introduced at 50℃. The Schottky barrier heights decrease for 36meV and 27meV in the two cases respectively. The devices exhibit a slow recovery characteristic in air ambient but almost none in the background of pure N2, which reveals that oxygen molecules could accelerate the desorption of CO and offer restrictions to CO detection.     

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.     

Evaluation of Schottky junction parameters from current-voltage characteristics exhibiting large excess currents

A new method is presented to extract the junction parameters from current-voltage characteristics of Schottky junctions exhibiting high leakage or other excess currents at low bias and current levels, for reduction of errors and uncertainity of the evaluation. In contrary to the Richardson plot, using the new method presented here, the Schottky junction parameters are evaluated from the measured experimental points directly instead of the extrapolated values of saturation current. The temperature dependence of forward bias, which is necessary for a given thermionic emission current level, is used. After transformation of the data, a linear plot is obtained, which yields both the barrier height and effective Richardson constant.     

A Physics-Based Compact Direct-Current and Alternating-Current Model for AlGaN/GaN High Electron Mobility Transistors

A set of analytical models for the dc and small signal characteristics of AIGaN/GaN high electron mobility transis- tors （HEMTs） are presented. A modified transferred-electron mobility model is adapted and a phenomenological low-field mobility model is developed. We calculate the channel charge considering the neutralization of donors and the contribution of free electrons in the AlGaN layer. The gate-to-source and gate-to-drain capacitances are obtained analytically, and the cut-off frequency is predicted. The models are implemented into the HSPICE simulator for the dc, ac and transient simulations and verified by experimental data for the first time. A high efficiency class-E GaN HEMT power amplifier is designed and simulated by the HSPICE to verify the applicability of our models.     

Theoretical Analysis of Current Crowding Effect in Metal/AIGaN/GaN Schottky Diodes and Its Reduction by Using Polysilicon in Anode

There exists a current crowding effect in the anode of AIGaN/GaN heterojunction Schottky diodes, causing local overheating when working at high power density, and undermining their performance. The seriousness of this effect is illustrated by theoretical analysis. A method of reducing this effect is proposed by depositing a polysilicon layer on the Schottky barrier metal. The effectiveness of this method is provided through computer simulation. Power consumption of the polysilicon layer is also calculated and compared to that of the Schottky junction to ensure the applicability of this method.     

AlGaN/GaN高电子迁移率晶体管肖特基高温退火机理研究

在不同应力条件下,研究了AlGaN/GaN高电子迁移率晶体管高温退火前后的电流崩塌、栅泄漏电流以及击穿电压的变化.结果表明,AlGaN/GaN高电子迁移率晶体管通过肖特基高温退火以后,器件的特性得到很大的改善.利用电镜扫描(SEM)和X射线光电子能谱(XPS)对高温退火前、后的肖特基接触界面进行深入分析,发现器件经过高温退火后,Ni和AlGaN层之间介质的去除,并且AlGaN材料表面附近的陷阱减少,使得肖特基有效势垒提高,从而提高器件的电学特性. 关键词： AlGaN/GaN高电子迁移率晶体管 肖特基接触 界面陷阱     

Schottky diode based on porous GaN for hydrogen gas sensing application

This article reports the study of Pd Schottky contact on porous n-GaN for hydrogen gas sensing. Upon exposure to 2% H₂ in N₂, porous GaN sensor exhibited significant change of current. Morphological studies revealed that Pd contact deposited on porous GaN has ridge-trench-like morphology, a dense porous network was found in between the ridges. The dramatic change of current was attributed to the unique microstructure at Pd/porous GaN interface, which allowed higher accumulation of hydrogen; this resulted in a stronger effect of H-induced dipole layer and led to a significant change in the electrical characteristics of the porous sensor.     

High-temperature current conduction through three kinds of Schottky diodes

Fundamentals of the Schottky contacts and the high-temperature current conduction through three kinds of Schottky diodes are studied. N-Si Schottky diodes, GaN Schottky diodes and AlGaN/GaN Schottky diodes are investigated by I--V--T measurements ranging from 300 to 523~K. For these Schottky diodes, a rise in temperature is accompanied with an increase in barrier height and a reduction in ideality factor. Mechanisms are suggested, including thermionic emission, field emission, trap-assisted tunnelling and so on. The most remarkable finding in the present paper is that these three kinds of Schottky diodes are revealed to have different behaviours of high-temperature reverse currents. For the n-Si Schottky diode, a rise in temperature is accompanied by an increase in reverse current. The reverse current of the GaN Schottky diode decreases first and then increases with rising temperature. The AlGaN/GaN Schottky diode has a trend opposite to that of the GaN Schottky diode, and the dominant mechanisms are the effects of the piezoelectric polarization field and variation of two-dimensional electron gas charge density.     

An Anomalous Gain Mechanism in GaN Schottky Barrier Ultraviolet Photodetectors

The gain mechanism in GaN Schottky barrier ultraviolet photodetectors is investigated by focused light beam. When the incident light illuminates the central region of the Schottky contact electrode, the responsiyity changes very little with the increase of reverse bias voltage. However, when the incident light illuminates the edge region of the electrode, the responsivity increases remarkably with the increase of reverse bias voltage, and the corresponding quantum efficiency could be even higher than 100%. It is proposed that the surface states near the edge of the electrode may lead to a reduction of effective Schottky barrier height and an enhancement of electron injection, resulting in the anomalous gain.     

Modeling of trap-assisted tunneling in AlGaN/GaN heterostructure field effect transistors with different Al mole fractions

Although significant progress has been achieved in GaN based high power/high frequency electronic devices, surface-related problems still need an immediate solution. In particular, leakage currents through Schottky contacts not only impede device reliability but also degrade power efficiency and noise performance in such devices. This article discusses the mechanism of leakage currents through GaN Schottky and AlGaN/GaN Schottky interfaces for both forward and reverse biases. A theoretical model for the calculation of currents based on trap-assisted tunneling is discussed. In the calculation the trap energy has been assumed as a fitting parameter which is about 0.48 eV for different Al mole fractions. The comparison of the results obtained with the existing experimental data in the literature shows a good agreement.     

Surface and interface issues in wide band gap semiconductor electronics

Wide band gap (WGB) materials are the most promising semiconductors for future electronic devices, and are candidates to replace the conventional materials (Si, GaAs, …) that are approaching their physical limits. Among WBG materials, silicon carbide (SiC) and gallium nitride (GaN) have achieved the largest advancements with respect to their material quality and device processing. Clearly, the devices performances depend on several surface and interface properties, which in turn are often crucially determined by the quality of the available material, as well as by the device processing maturity. In this paper, some surface and interface issues related to SiC and GaN devices processing are reviewed. First, the control of metal/SiC barrier uniformity and surface preparation will be discussed with respect to the performance of Schottky-based devices. Moreover, the impact of high-temperature annealing required for high-voltage Schottky diodes and MOSFETs fabrication, on the surface morphology and device performances will also be briefly presented. In the second part, it will be shown that for GaN the material quality is still the main concern, since dislocations have a severe influence on the current transport and barrier homogeneity of metal/GaN interfaces. Other practical implications of thermal annealing and surface passivation during GaN-based devices fabrication will also be addressed.     

Performance Improvement of GaN Based Schottky Barrier Ultraviolet Photodetector by Adding a Thin A1GaN Window Layer

We propose a new structure of GaN based Schottky barrier ultraviolet photodetector, in which a thin n-type A1GaN window layer is added on the conventional n^--GaN/n^＋-GaN device structure. The performance of the Schottky barrier ultraviolet photodetector is found to be improved by the new structure. The simulation result shows that the new structure can reduce the negative effect of surface states on the performance of Schottky barrier GaN photodetectors, improving the quantum efficiency and decreasing the dark current. The investigations suggest that the new photodetector can exhibit a better responsivity by choosing a suitably high carrier concentration and thin thickness for the A1GaN window layer.     

Barrier Enhancement Effect of Postannealing in Oxygen Ambient on Ni/AIGaN Schottky Contacts

Al0.2 Ga0.8N/GaN samples are grown by metalorganic chemical vapour deposition （MOCVD） method on （0001） sapphire substrates. A 10nm-thick Ni layer is deposited on AlGaN as the transparent Schottky contact. The effect of postannealing in oxygen ambient on the electrical properties of Ni/AlGaN is studied by current-voltage- temperature （I-V-T） measurement. The annealing at a relatively low temperature of 300℃ for 90 s results in a decrease of the ideality factor from 2.03 to 1.30 and an increase of the Schottky barrier height from 0.77eV to 0.954 e V. The I-V-T analysis confirms the improvement originated from the formation of NiO, a layer with higher resistance, which could passivate the surface states of AlGaN and suppress the tunnelling current. Furthermore, the annealing also leads to an increase of the transmittance of the contacts from 57.5% to 78.2%, which would be favourable for A1GaN-based photodetectors.     

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.     

Transport mechanism of reverse surface leakage current in AlGaN/GaN high-electron mobility transistor with SiN passivation

The transport mechanism of reverse surface leakage current in the AlGaN/GaN high-electron mobility transistor(HEMT) becomes one of the most important reliability issues with the downscaling of feature size.In this paper,the research results show that the reverse surface leakage current in AlGaN/GaN HEMT with SiN passivation increases with the enhancement of temperature in the range from 298 K to 423 K.Three possible transport mechanisms are proposed and examined to explain the generation of reverse surface leakage current.By comparing the experimental data with the numerical transport models,it is found that neither Fowler-Nordheim tunneling nor Frenkel-Poole emission can describe the transport of reverse surface leakage current.However,good agreement is found between the experimental data and the two-dimensional variable range hopping(2D-VRH) model.Therefore,it is concluded that the reverse surface leakage current is dominated by the electron hopping through the surface states at the barrier layer.Moreover,the activation energy of surface leakage current is extracted,which is around 0.083 eV.Finally,the SiN passivated HEMT with a high Al composition and a thin AlGaN barrier layer is also studied.It is observed that 2D-VRH still dominates the reverse surface leakage current and the activation energy is around 0.10 eV,which demonstrates that the alteration of the AlGaN barrier layer does not affect the transport mechanism of reverse surface leakage current in this paper.     

Impact of dopants in GaN on the formation of two-dimensional electron gas in AlGaN/GaN heterostructure field-effect transistors

The two-dimensional electron gas distribution in AlGaN/GaN high electron mobility transistors is determined from the solution of the coupled Schr?dinger’s and Poisson’s equations. Considering the piezoelectric effect, the two-dimensional electron gas concentration is calculated to be as high as 7.7×10¹⁹ cm^-3. In order to obtain an understanding of how the two-dimensional electron gas distribution is influenced by dopants in GaN, we observed the two-dimensional electron gas concentration and occupation of sub-bands versus dopant concentration in the GaN layer of an AlGaN/GaN heterostructure. Our results show that the two-dimensional electron gas concentration is slightly increased at higher doping levels in GaN, while the quantum confinement in the AlGaN/GaN heterostructure is weakened with the increase of donor concentration in the GaN layer. Received: 26 May 2001 / Accepted: 23 July 2001 / Published online: 23 January 2002     

Investigation on the barrier height and inhomogeneity of nickel silicide Schottky

The apparent Schottky barrier height (SBH) of the nickel silicide Schottky contacts annealed at different temperatures was investigated based on temperature dependence of I-V characteristic. Thermionic emission-diffusion (TED) theory, single Gaussian and double Gaussian models were employed to fit I-V experimental data. It is found the single Gaussian and double Gaussian SB distribution model can give a very good fit to the I-V characteristic of apparent SBH for different annealing temperatures. Also, the apparent SBH and the leakage current increase with annealing temperatures under reverse voltage. In addition, the homogeneity of interfaces for the samples annealed at temperatures of 500 and 600 °C is much better than that of the samples annealed at temperatures of 400, 700, and 800 °C. This may result from the phase transformation of nickel silicide due to the different annealing temperatures and from the low Schottky barrier (SB) patches.