Surface Barrier Model Research Articles

Surface related tunneling leakage in β-Ga2O3 (001) vertical Schottky barrier diodes

β-Ga2O3 (001) Schottky barrier diodes (SBDs) fabricated on a halide vapor phase epitaxy-grown epilayer showed anomalous reverse leakage characteristics, which could not be explained through thermionic field emission theory. A systematic investigation through the measurements and simulations of capacitance-voltage and current density-voltage characteristics suggested the presence of a thin surface layer on the epilayer with high density of oxygen vacancy states. This thin surface layer allowed the tunneling of electrons and caused anomalous reverse leakage properties (Thin surface barrier model). Annealing of the epilayer in an oxidative environment passivated the surface oxygen vacancy states and reduced the reverse leakage current enormously.

Using a volumetric apparatus to identify and measure the mass transfer resistance in commercial adsorbents

Abstract The mass transfer coefficient is a fundamental property needed to design adsorption gas separations. A collaborative study is presented where commercial LiLSX beads used in air vacuum swing adsorption for the production of oxygen are tested in two volumetric apparatuses. The initial results based on the software available in the commercial system seemed to point to a surface barrier model for the adsorption kinetics of nitrogen, but this system is known to be macropore diffusion controlled. A detailed model of the system and a new way of representing the experimental data are used to show that the mass transfer kinetics is clearly a diffusion process. Guidelines and recommendations on which tests are needed to ensure the correct use of a volumetric system in this case are presented. Through the correct interpretation of the flow through the valve in the two volumetric apparatuses, consistency in the mass transfer time constant is achieved. The effect of using the correct diffusion time constant vs the one obtained using the traditional approach is demonstrated comparing a typical oxygen vacuum swing adsorption process. A drop in performance of nearly 15% in both productivity and energy consumption is predicted if the incorrect diffusion time constant is used.

Mechanism Study of Gate Leakage Current for AlGaN/GaN High Electron Mobility Transistor Structure Under High Reverse Bias by Thin Surface Barrier Model and Technology Computer Aided Design Simulation

Gate leakage current mechanism in GaN high electron mobility transistors (HEMTs) has been studied using a two-dimensional thin surface barrier (TSB) model to represent two unintentional donor thin layers that exit under and outside the gate electrode due to the existence of surface defects. The donor thin layer outside the gate affects the reverse gate current at the high gate voltage above the pinch-off voltage. Higher donor concentration of thin layer outside the gate results in larger ratio of lateral to vertical components of the electric field at the gate edge. On the other hand, the electric field at the center of the gate has only the vertical electric field component. As a result, the two-dimensional effects are only important for the reverse gate current above the pinch-off voltage. We have confirmed in this paper that the simulation results provided by our model correlate very well with the experimental reverse gate current characteristics of the device for a very wide range of reverse gate voltage from 0.1 to 90 V.

Momentum-resolved photoelectron interference in crystal surface barrier scattering

The quantum-mechanical transmission process of a photoelectron through a surface leaves distinctive marks in the electron momentum distribution emitted from a crystal. By energy-resolved measurements of the complete photoemission half-space above a Cu(001) surface using a momentum microscope, we identify the two-path interference of directly emitted photoelectrons and electrons, which have been scattered through the surface quantum well via a diffraction process. The basic principle of photoelectron surface barrier interference is revealed by an analytical model. A quantitative comparison to the experiment is obtained by one-step photoemission calculations for different surface barrier models.

The transport mechanism of gate leakage current in AlGaN/GaN high electron mobility transistors

The temperature dependence of the I-V characteristics on Au/Ni-HEMT Schottky contacts was measured and analyzed. Large deviations from the thermionic emission and thermionic-field emission model were observed in the I-V-T characteristics. The thin surface barrier model only fits the measured curves in the high bias region, but deviates drastically in the low bias region. Using a revised thin surface barrier model, the calculated curves match well with the measured curves. It is also found that tunneling emission model is the dominant current transport mechanism at low temperature, yet thermionic-field emission model is the dominant current transport mechanism at high temperature.

Current Transport, Fermi Level Pinning, and Transient Behavior ofGroup-III Nitride Schottky Barriers

The current transport, Fermi level pinning and transient behavior of Group-III nitride Schottky barriers are reviewed. First, an overview of interface models is given. Then, the current transport mechanism in GaN, AlGaN, and InGaN Schottky barriers is discussed. We show that discrepancy in barrier height measurements in the I-V and the C-V methods, as well as large reverse leakage currents, can be explained by using the thin surface barrier (TSB) model. This understanding has led to a large leakage reduction by using an oxygen gettering process. Finally, the transient behavior of AlGaN/ GaN planar Schottky diodes is discussed to get insight into the surface-related current collapse phenomenon in AlGaN/GaN high electron mobility transistors (HEMTs). The responses are explained in terms of the dispersive transport caused by a time-continual random walk with hopping through surface states. This provides a new understanding of the current collapse.

Current collapse transient behavior and its mechanism in submicron-gate AlGaN∕GaN heterostructure transistors

The current collapse transient behavior of a practical submicron AlGaN∕GaN heterostructure field effect transistor (HFET) is investigated, and its mechanism is proposed. First, the steady-state and transient characteristics of the Schottky diode obtained by connecting the source and drain electrodes of the transistor have been investigated. The steady-state characteristics can be explained by the thin surface barrier model, indicating the presence of tunneling injection of electrons. Turn-on and turn-off transient characteristics of the reverse current of Schottky diode showed very slow nonexponential transients covering six orders of magnitude of time scale from milliseconds to thousands of seconds. They are very similar to those of a large planar Schottky diode studied recently by the authors. The HFET device showed a clear current collapse behavior after a gate stress beyond pinch off. Pulsed gate stress visualized drain current transients which again included very slow nonexponential transients covering six orders of magnitude of time scale. The whole experimental results are explained consistently by a model in which the current collapse is due to surface state charging near the source side and drain side of the gate edge where its rate limiting process is not the usual Shockley–Read–Hall capture-emission process but the dispersive electron transport through the surface states by time-continual hopping, which is triggered by the tunneling injection process at the gate edge.

Mechanism and control of current transport in GaN and AlGaN Schottky barriers for chemical sensor applications

Strong interests are recently emerging for development of integrated high-performance chemical sensor chips. In this paper, the present status of understanding and controlling the current transport in the GaN and AlGaN Schottky diodes is discussed from the viewpoint of chemical sensor applications. For this purpose, a series of works recently carried out by our group are reviewed in addition to a general discussion. First, current transport in GaN and AlGaN Schottky barriers is discussed, introducing the thin surface barrier (TSB) model to explain the anomalously large leakage currents. Following this, attempts to reduce the leakage currents are presented and discussed. Then, as an example of gas-phase sensors using Schottky barriers, a Pd/AlGaN/GaN Schottky diode hydrogen sensor developed recently by our group is presented with a discussion on the sensing mechanism and related current transport. On the other hand, in liquid-phase sensors, contact is made between liquid and semiconductor which is regarded as a kind of Schottky barrier by electrochemists. As one of such liquid-phase sensors, open-gate AlGaN/GaN heterostructure field effect transistor (HFET) pH sensor developed recently by our group is presented. Finally, a brief summary is given together with some remarks for future research.

Characterization of Ni/Au GaN Schottky contact base on I-V-T and C-V-T measurements

Based on the temperature-dependent current-voltage (I-V-T) measurements and the temperature-dependent capacitance-voltage (C-V-T) measurements of Schottky diodes fabricated on n-type GaN，the mechanism of the electrical current transport was discussed using thin surface barrier (TSB) model. The experiment results indicated that there are different mechanisms at different temperatures and bias. Based on this assumption we give a modified I-V characteristic formula which gives excellent fit to the experiment data. The SBHs determined from high-temperature I-V curves，low-temperature C-V curves，and the metal work function agree well each other.

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

Abstract 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.

Large reduction of leakage currents in AlGaN Schottky diodes by a surface control process and its mechanism

Leakage currents in AlGaN Schottky diodes were investigated systematically by using a rigorous computer simulation based on the thin surface barrier model taking account of unintentionally doped surface donors. The leakage currents in AlGaN Schottky diodes have stronger bias dependence and smaller temperature dependences as compared with those of GaN diodes. It was shown that these features were associated with shallow oxygen donors located near the AlGaN surface. Then, an attempt was made to remove oxygen and suppress leakage currents by a surface control process using an ultrathin Al layer and subsequent annealing. An in situ x-ray photoelectron spectroscopy analysis indicated the formation of Al2O3 layer during the surface control process, suggesting efficient gettering of oxygen from the surface. C-V analysis directly indicated the reduction of shallow donors by the surface control process. A remarkable reduction of reverse leakage currents of four to five orders of magnitude took place in large area AlGaN Schottky diodes after the application of the surface control process. This process also reduced leakage currents of the gate of the heterostructure field effect transistor device by more than one order of magnitude and increased temperature dependences of current.

Gate control, surface leakage currents, and peripheral charging in AlGaN/GaN heterostructure field effect transistors having nanometer-scale schottky gates

Gate control properties together with gate leakage currents in AlGaN/GaN heterostructure field effect transistors (HFETs) with nanometer-scale Schottky gates were investigated, focusing on the effects of AlGaN surfaces at the gate periphery. Fabricated AlGaN/GaN HFETs showed unexpectedly small gate length (LG) dependence of transconductance, gm. Comparing the transfer characteristics from theory and experiment, effective LG values in the fabricated devices were found to be much longer than the geometrical size on the order of 100 nm, indicating the formation of virtual gates. Detailed analysis of the gate leakage current behaviors based on a thin surface barrier model showed the presence of a strong electric field at the gate periphery. The mechanism of the virtual gate formation was discussed based on the obtained nanometer-scale Schottky gate behaviors.

Lateral tunneling injection and peripheral dynamic charging in nanometer-scale Schottky gates on AlGaN/GaN hetrosturucture transistors

The gate leakage and gate control characteristics of AlGaN/GaN heterostructure field effect transistors (HFETs) were systematically investigated in an attempt to clarify possible effects of surface states. The experiments were compared to rigorous computer simulations. We observed large amounts of leakage currents in the Schottky diodes fabricated on the AlGaN epitaxial layers. By the calculation based on a thin surface barrier model in which the effects of surface defect donor were taken into account, this large leakage was well explained by enhancement of tunneling transport processes due to the barrier thinning associated with ionization of surface-defect donor. On the other hand, the analysis on the current-voltage characteristics for the nanometer-scale Schottky contacts on AlGaN/GaN HFETs, indicated additional lateral leakage components. The comparison of the gate control characteristics between experiment and calculation clearly showed that the effective lateral expansion of gate length significantly impeded the gm enhancement by the reduction of geometrical gate length. This can be explained by the lateral electron tunneling process at the AlGaN surface stimulated by the pronounced gate leakage currents. Due to frequent tunneling transfer at the gate periphery, surface state occupancy near the gate becomes governed by the metal Fermi level, causing the dynamic surface state charging effects. This resulted in effective widening of the gate length, leading to degradation of gate control performance in AlGaN/GaN HFETs.

Gate leakage in AlGaN/GaN HEMTs and its suppression by optimization of MOCVD growth

Gate-drain current-voltage characteristics in unpassivated AlGaN/GaN high electron mobility transistors (HEMTs) grown by metal-organic chemical vapor deposition (MOCVD) on sapphire substrates were investigated. Under a fixed V/III ratio for AlGaN layer growth, the growth window for getting low gate leakage and good two-dimensional electron gas mobility is narrow. We designed a multi-step growth of the AlGaN for HEMTs, i.e., high-V/III-ratio AlGaN layer starting from the AlGaN/GaN interface, then low-V/III-ratio AlGaN layer, which yielded the best 2DEG mobility and also reduced gate leakage. It was also found that the forward current and reverse current before pinch off can be explained by the thin surface barrier (TSB) model, and the AlGaN layer grown under lower effective V/III ratio shows a larger surface donor density but smaller leaky area. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Tunneling Injection of Electrons at Nanometer-Scale Schottky Gate Edge of AlGaN/GaN Heterostructure Transistors and Its Computer Simulation

Gate leakage currents in AlGaN/GaN HFETs were investigated by comparing experiments with computer simulations based on the thin surface barrier (TSB) model involving unintentional surface donors. Leakage currents in large area Schottky diodes were explained by the TSB model involving nitrogen vacancy related deep donors and oxygen shallow donors. On the other hand, in AlGaN/GaN HFETs with nanometer scale Schottky gates, gate leakage currents include an additional leakage component due to lateral electron injection through tunneling at the gate edge where the barrier thinning is mainly controlled by oxygen donors. By combining vertical and lateral tunneling components, experiments could be reproduced on computer. Lateral components may be responsible for current collapse. [DOI: 10.1380/ejssnt.2005.433]

Computer simulation of current transport in GaN and AlGaN Schottky diodes based on thin surface barrier model

This paper attempts a rigorous computer simulation of the current transport in GaN and AlGaN Schottky diodes on the basis of the thin surface barrier (TSB) model recently proposed by the authors’ group. First, a computer program was developed which can calculate current transport through an arbitrary potential profile of Schottky barrier by a combined mechanism of thermionic emission (TE), thermionic-field emission (TFE) and field emission (FE). Then, from the view point of the TSB model, attempts were made to fit the theoretical temperature dependent current voltage (I–V–T) curves to the measured I–V–T data taken on Ni/n-GaN and Ni/n-AlGaN Schottky diodes changing the barrier profiles and the energy depth of the surface donor. As compared with the previous poor fitting using approximate analytic formulas, excellent fitting was obtained for both forward and reverse current, confirming the validity of the TSB model as the mechanism for anomalously large leakage currents in GaN and AlGaN Schottky diodes. Best fittings for GaN and Al0.26Ga0.74N were obtained for exponentially decaying distributions of surface defect donors with the peak density of 5 × 1018cm−3 and 1 × 1019cm−3, the characteristic decay depth of 11nm and 11.5nm and the energy depth of 0.25eV and 0.37eV, respectively.

Analysis and control of excess leakage currents in nitride-based Schottky diodes based on thin surface barrier model

Using a rigorous computer simulation program for current transport through a Schottky barrier with an arbitrary potential profile, the leakage current mechanism in GaN and AlGaN Schottky diodes was investigated on the basis of the thin surface barrier (TSB) model recently proposed by the authors’ group. Computer simulation assuming various possible defect density distributions was carried out to reproduce the measured temperature dependent current voltage (I–V)-temperature characteristics of the GaN and AlGaN Schottky diodes which showed excessive reverse leakage. By assuming exponentially decaying distributions from surface for defect donors with energy depth of 0.25 eV for GaN and 0.37 eV for Al0.15Ga0.85N, I–V curves measured by our group as well as reported in the literatures were almost completely reproduced both in forward and reverse direction over a wide temperature range. The defect donors are proposed to be N vacancies or their related complexes that are formed during metal deposition. The result confirms the validity of the TSB model. From the viewpoint of the TSB model, attempts were also made to suppress leakage currents. It was found that a low-energy electrochemical metal deposition process and a metal–insulator–semiconductor Schottky structure using an ultrathin Al2O3 film by electron cyclotron resonance oxidation of Al film were remarkably effective in reducing excess leakage currents due to reduction of defect deep donors.

Leakage mechanism in GaN and AlGaN Schottky interfaces

Based on detailed temperature-dependent current–voltage (I–V–T) measurements the mechanism of leakage currents through GaN and AlGaN Schottky interfaces is discussed. The experiments were compared to calculations based on thin surface barrier model in which the effects of surface defects were taken into account. Our simulation method reproduced the experimental I–V–T characteristics of the GaN and AlGaN Schottky diodes, and gave excellent fitting results to the reported Schottky I–V curves in GaN for both forward and reverse biases at different temperatures. The present results indicate that the barrier thinning caused by unintentional surface-defect donors enhances the tunneling transport processes, leading to large leakage currents through GaN and AlGaN Schottky interfaces.

Excited Surface Band Structures and Surface Barrier Features in VLEED

We have tested a surface barrier model for Cu(001) for consistency with below vacuum level data from inverse photoemission and above vacuum level data from very low energy electron diffraction (VLEED). The model has an exponential-type saturation which extends to the center of the first row of atoms in the crystal. The barrier model can account for the limited data available for the surface barrier band structure below the vacuum level but not for the strong surface barrier features which occur in VLEED reflectivity spectra for θ = 80° and ϕ = 45°. This is the case even when allowance is made for dynamical variation of the key barrier structural features. It is concluded that this form of the barrier saturation is not correct for real crystals.

Mechanism of anomalous current transport in n-type GaN Schottky contacts

Temperature T dependences of the current–voltage (I–V) characteristics of Ni and Pt/n-GaN Schottky contacts were measured in detail, and the results were analyzed from various viewpoints. Large deviations from the thermionic emission transport were observed in the I–V–T behavior with anomalously large reverse leakage currents. Forward characteristics could be fitted into the classical thermionic-field emission (TFE)/field emission (FE) model. However, an unusually high doping density had to be assumed, and the reverse characteristics were far away from measured data. A new thin surface barrier (TSB) model was proposed in which the width of the Schottky barrier is reduced due to the presence of unintentional surface donors. Analysis of TFE/TE process through the TSB region has led to sets of I–V–T curves that reproduce almost perfectly the observed forward and reverse I–V–T behavior with correct orders of magnitude of currents. Deep donors related to nitrogen vacancy are suggested to be the origin of surface donors producing TSBs.