Carrier transport mechanism of strained AlGaN/GaN Schottky contacts

Using polarization field effect-based thermionic field emission (PFE-TFE) model based on current–voltage–temperature data, possible carrier transport mechanisms for Pt/Au and Cr/Pd Schottky contacts to Al_0.25Ga_0.75N/GaN layers were investigated. Thermionic emission (TE) model was also investigated to compare to the PFE-TFE. It was shown that Schottky barrier heights (SBHs) are significantly affected by a polarization field-induced carrier density of the AlGaN layer. In addition, relatively little temperature dependence on the leakage current density of both contacts was found, which is in good agreement with the PFE-TFE model. The results indicate that the TFE is responsible for the current flow across the metal/AlGaN–GaN interface at T ≥ 293 K.     

Different temperature dependence of carrier transport properties between AlxGa1-xN/InyGa1-yN/GaN and AlxGa1-xN/GaN heterostructures

The temperature dependence of carrier transport properties of AlxGa1-xN/InyGa1-yN/GaN and AlxGa1-xN/GaN heterostructures has been investigated.It is shown that the Hall mobility in Al0.25Ga0.75N/In0.03Ga0.97N/GaN heterostructures is higher than that in Al0.25Ga0.75N/GaN heterostructures at temperatures above 500 K,even the mobility in the former is much lower than that in the latter at 300 K.More importantly,the electron sheet density in Al0.25Ga0.75N/In0.03Ga0.97N/GaN heterostructures decreases slightly,whereas the electron sheet density in Al0.25Ga0.75N/GaN heterostructures gradually increases with increasing temperature above 500 K.It is believed that an electron depletion layer is formed due to the negative polarization charges at the InyGa1-yN/GaN heterointerface induced by the compressive strain in the InyGa1-yN channel,which e-ectively suppresses the parallel conductivity originating from the thermal excitation in the underlying GaN layer at high temperatures.     

Ni/Ag Schottky contacts on Al0.11Ga0.89N grown on Si (1 1 1) substrate by plasma-assisted MBE

Al_0.11Ga_0.89N/GaN samples are grown by plasma-assisted molecular beam epitaxy method on (1 1 1) silicon substrates. High purity gallium (7N) and aluminum (6N5) were used to grow Al_0.11Ga_0.89N, GaN, and AlN, respectively. The surface morphology, structural and optical properties of the sample has been investigated by scanning electron microscope (SEM), and high-resolution X-ray diffraction (HR-XRD), respectively. HR-XRD measurement showed that the sample has a typical diffraction pattern of hexagonal Al_0.11Ga_0.89N/GaN heterostructures. Ni/Ag bilayers are deposited on Al_0.11Ga_0.89N as the Schottky contacts. The effect of annealing in oxygen ambient on the electrical properties of Ni/Ag/Al_0.11Ga_0.89N is studied by current–voltage measurement. The annealing at a temperature of 700 °C for 10?min results in an increase in the ideality factor from 1.033 to 1.042 and an increase in the Schottky barrier height from 0.708 to 0.811 eV. Furthermore, the annealing in oxygen ambient also leads to an increase in the surface roughness of the contacts from 0.0098 to 0.1360 μm which is in agreement with the SEM results.     

用肖特基电容电压特性数值模拟法确定调制掺杂AlxGa1-xN/GaN异质结中的极化电荷

研究发展了用肖特基电容电压特性数值模拟确定调制掺杂Al_xGa_1-xN/GaN异质结中极化电荷的方法.在调制掺杂的Al_0.22Ga_0.78N/GaN异质结上制备了Pt肖特基接触,并对其进行了C-V测量.采用三维费米模型对调制掺杂的Al_0.22Ga_0.78N/GaN异质结上肖特基接触的C-V特性进行了数值模拟,分析了改变样品参数对C-V特性的影响.利用改变极化电荷、n-AlGaN 关键词： xGa_1-xN/GaN异质结')" href="#">Al_xGa_1-xN/GaN异质结 极化电荷 电容电压特性 数值模拟     

AlGaN/GaN异质结Ni/Au肖特基表面处理及退火研究

采用O₂等离子体及HF溶液对AlGaN/GaN异质结材料进行表面处理后，Ni/Au肖特基接触特性比未处理有了明显改善，反向泄漏电流减小3个数量级.对制备的肖特基接触进行200—600℃ 5min的N₂气氛退火，发现退火冷却后肖特基反向泄漏电流随退火温度增大进一步减小.N₂气中600℃退火后肖特基二极管C-V特性曲线在不同频率下一致性变好，这表明退火中Ni向材料表面扩散减小了表面陷阱密度；C-V特性曲线随退火温度增大向右移动，从二维电子气耗尽电压绝对值减小反映了肖特基势垒的提高. 关键词： AlGaN/GaN 肖特基接触 表面处理 退火     

The leakage current mechanisms in the Schottky diode with a thin Al layer insertion between Al0.245Ga0.755N/GaN heterostructure and Ni/Au Schottky contact

This paper investigates the behaviour of the reverse-bias leakage current of the Schottky diode with a thin Al inserting layer inserted between Al0.245Ga0.755 N/GaN heterostructure and Ni/Au Schottky contact in the temperature range of 25-350°C. It compares with the Schottky diode without Aluminium inserting layer. The experimental results show that in the Schottky diode with Al layer the minimum point of I-V curve drifts to the minus voltage, and with the increase of temperature increasing, the minimum point of I-V curve returns the 0 point. The temperature dependence of gate-leakage currents in the novelty diode and the traditional diode are studied. The results show that the Al inserting layer introduces interface states between metal and Al0.245Ga0.755N. Aluminium reacted with oxygen formed Al2O3 insulator layer which suppresses the trap tunnelling current and the trend of thermionic field emission current. The reliability of the diode at the high temperature is improved by inserting a thin Al layer.     

Electrical characteristics of Pt/Au Schottky contacts to plasma-etched Al0.45Ga0.55N

The effects of inductively coupled plasma (ICP) etching on electrical properties of Pt/Au–Al_0.45Ga_0.55N Schottky contacts are investigated. There are two linear parts in the ln I–V curves of ICP-etched Schottky contacts at small forward currents at 198–298 K. Thermionic field emission (TFE) theory analysis shows that Schottky contact with ICP etching has much lower barrier height and higher tunnel transmission probability than that without ICP etching, which could be attributed to plasma damage introduced on the ICP-etched surface. The down linear part is probably connected to surface tunneling component originated from plasma-etched surface which joins Schottky area to Ohmic area.     

Temperature dependent energy relaxation time in AlGaN/AlN/GaN heterostructures

The two-dimensional (2D) electron energy relaxation in Al_0.25Ga_0.75N/AlN/GaN heterostructures was investigated experimentally by using two experimental techniques; Shubnikov-de Haas (SdH) effect and classical Hall Effect. The electron temperature (T_e) of hot electrons was obtained from the lattice temperature (T_L) and the applied electric field dependencies of the amplitude of SdH oscillations and Hall mobility. The experimental results for the electron temperature dependence of power loss are also compared with the current theoretical models for power loss in 2D semiconductors. The power loss that was determined from the SdH measurements indicates that the energy relaxation of electrons is due to acoustic phonon emission via unscreened piezoelectric interaction. In addition, the power loss from the electrons obtained from Hall mobility for electron temperatures in the range T_e > 100 K is associated with optical phonon emission. The temperature dependent energy relaxation time in Al_0.25Ga_0.75N/AlN/GaN heterostructures that was determined from the power loss data indicates that hot electrons relax spontaneously with MHz to THz emission with increasing temperatures.     

Identification and elimination of inductively coupled plasma-induced defects in AlxGa1 - xN/GaN heterostructures

By using temperature-dependent Hall,variable-frequency capacitance-voltage and cathodoluminescence (CL) measurements,the identification of inductively coupled plasma (ICP)-induced defect states around the Al x Ga 1-x N/GaN heterointerface and their elimination by subsequent annealing in Al x Ga 1-x N/GaN heterostructures are systematically investigated.The energy levels of interface states with activation energies in a range from 0.211 to 0.253 eV below the conduction band of GaN are observed.The interface state density after the ICP-etching process is as high as 2.75×10 12 cm 2 ·eV 1.The ICP-induced interface states could be reduced by two orders of magnitude by subsequent annealing in N 2 ambient.The CL studies indicate that the ICP-induced defects should be Ga-vacancy related.     

Quantum dots-templated growth of strain-relaxed GaN on a c-plane sapphire by radio-frequency molecular beam epitaxy

We investigated the quantum dots-templated growth of a(0001) GaN film on a c-plane sapphire substrate.The growth was carried out in a radio-frequency molecular beam epitaxy system.The enlargement and coalescence of grains on the GaN quantum dots template was observed in the atom force microscopy images,as well as the more ideal surface morphology of the GaN epitaxial film on the quantum dots template compared with the one on the AlN buffer.The Ga polarity was confirmed by the reflected high energy electron diffraction patterns and the Raman spectra.The significant strain relaxation in the quantum dots-templated GaN film was calculated based on the Raman spectra and the X-ray rocking curves.Meanwhile,the threading dislocation density in the quantum dots-templated film was estimated to be 7.1×107cm-2,which was significantly suppressed compared with that of the AlN-buffered GaN film.The roomtemperature Hall measurement showed an electron mobility of up to 1860cm2 /V·s in the two-dimensional electron gas at the interface of the Al 0.25Ga0.75 N/GaN heterojunction.     

Determination of the series resistance under the Schottky contacts of AlGaN/AlN/GaN Schottky barrier diodes

Rectangular AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs) were fabricated, and the gate and the source of the HFETs consisted of AlGaN/AlN/GaN Schottky barrier diodes (SBDs). Based on the measured forward current-voltage and the capacitance-voltage characteristics of the AlGaN/AlN/GaN SBDs, the series resistance under the Schottky contacts (R_S) was calculated using the method of power consumption, which has been proved to be valid. Finally, the method of power consumption for calculating R_S was successfully used to study the two-dimensional electron gas electron mobility for a series of circular AlGaN/AlN/GaN SBDs. It is shown that the series resistance under the Schottky contacts cannot be neglected and is important for analysing and characterizing the AlGaN/AlN/GaN SBDs and the AlGaN/AlN/GaN HFETs.     

Improved device performance in nonpolar a ‐plane GaN LEDs using a Ni/Al/Ni/Au n‐type ohmic contact

The authors report upon the increased light‐output power (P_out) via a reduction in the forward voltage (V_f) for nonpolar a ‐plane GaN LEDs using Ni/Al/Ni/Au n‐type ohmic contacts. The specific contact resistivity of the Ni/Al/Ni/Au contact is found to be as low as 5.6 × 10^–5 whereas that of a typical Ti/Al/Ni/Au contact is 6.8 × 10^–4 Ω cm², after annealing at 700 °C. The X‐ray photoelectron spectroscopy results show that the upward surface band bending is less pronounced for the Ni/Al contact compared to the Ti/Al contact, leading to a decrease in the effective Schottky barrier height (SBH). The V_f of the nonpolar LEDs decreases by 10% and P_out increases by 15% when the Ni/Al/Ni/Au scheme is used instead of the typical Ti/Al/Ni/Au metal scheme. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Tunneling induced electron transfer in SiNx/AlGaN/GaN based metal-insulator-semiconductor structures

Tunneling induced electron transfer in SiN_x/Al_0.22Ga_0.78N/GaN based metal-insulator-semiconductor (MIS) structures has been investigated by means of capacitance-voltage (C-V) measurements at various temperatures. Large clock-wise hysteresis window in C-V profiles indicates the injection of electrons from the two-dimensional electron gas (2DEG) channel to the SiN_x layer. Depletion of the 2DEG at positive bias in the negative sweeping direction indicates that the charges injected have a long decay time, which was also observed in the recovery process of the capacitance after injection. The tunneling induced electron transfer effect in SiN_x/Al_0.22Ga_0.78N/GaN based MIS structure opens up a way to design Al_xGa_1−xN/GaN based variable capacitors and memory devices.     

High temperature electron transport properties of AlGaN/GaN heterostructures with different Al-contents

Electron transport properties in AlGaN/GaN heterostructures with different Al-contents have been investigated from room temperature up to 680 K. The temperature dependencies of electron mobility have been systematically measured for the samples. The electron mobility at 680 K were measured as 154 and 182 cm²/V·s for Al_0.15Ga_0.85N/GaN and Al_0.40Ga_0.60N/GaN heterostructures, respectively. It was found that the electron mobility of low Al-content Al_0.15Ga_0.85N/GaN heterostructure was less than that of high Al-content Al_0.40Ga_0.60N/GaN heterostructure at high temperature of 680 K, which is different from that at room temperature. Detailed analysis showed that electron occupations in the first subband were 75% and 82% at 700 K for Al_0.15Ga_0.85N/GaN and Al_0.40Ga_0.60N/GaN heterostructures, respectively, and the two dimensional gas (2DEG) ratios in the whole electron system were 30% and near 60%, respectively. That indicated the 2DEG was better confined in the well, and was still dominant in the whole electron system for higher Al-content AlGaN/GaN heterostructure at 700 K, while lower one was not. Thus it had a higher electron mobility. So a higher Al-content AlGaN/GaN heterostructure is more suitable for high-temperature applications.     

Comparison of optical properties between Wurtzite and zinc-blende Ga0.75Al0.25N

In order to compare optical properties of Wurtzite and zinc-blende Ga_0.75Al_0.25N and lay a foundation for preparation of Ga_0.75Al_0.25N photocathodes, absorption coefficient, complex refractive index, dielectric function, reflectivity and loss function of Ga_0.75Al_0.25N in two forms are calculated using first principle based on density functional theory. Results show that zinc-blende Ga_0.75Al_0.25N owns smaller band gap than Wurtzite Ga_0.75Al_0.25N, and its conduction band is more broad. The optical properties differences of two structures mainly occur at the range of 8.6–26.0 eV. The peaks of imaginary dielectric function and absorption curves are at higher energy point for zinc-blende Ga_0.75Al_0.25N. The highest absorption of Wurtzite Ga_0.75Al_0.25N is 351386.171/cm at 11.05 eV, which is smaller than zinc-blende of 437809.895/cm at 13.84 eV. The average reflectivity of Wurtzite Ga_0.75Al_0.25N is lower than that of zinc-blende Ga_0.75Al_0.25N. The results are conductive for designing component structures of Ga_0.75Al_0.25N photocathodes.     

Thermal annealing behaviour of Al/Ni/Au multilayer on n-GaN Schottky contacts

Recently GaN-based high electron mobility transistors (HEMTs) have revealed the superior properties of a high breakdown field and high electron saturation velocity. Reduction of the gate leakage current is one of the key issues to be solved for their further improvement. This paper reports that an Al layer as thin as 3 nm was inserted between the conventional Ni/Au Schottky contact and n-GaN epilayers, and the Schottky behaviour of Al/Ni/Au contact was investigated under various annealing conditions by current--voltage (I--V) measurements. A non-linear fitting method was used to extract the contact parameters from the I--V characteristic curves. Experimental results indicate that reduction of the gate leakage current by as much as four orders of magnitude was successfully recorded by thermal annealing. And high quality Schottky contact with a barrier height of 0.875 eV and the lowest reverse-bias leakage current, respectively, can be obtained under 12 min annealing at 450°C in N₂ ambience.     

The effects of GaN capping layer thickness on two-dimensional electron mobility in GaN/AlGaN/GaN heterostructures

In this paper we present a study of the effect of GaN capping layer thickness on the two-dimensional (2D)-electron mobility and the two-dimensional electron gas (2DEG) sheet density which is formed near the AlGaN barrier/buffer GaN layer. This study is undertaken using a fully numerical calculation for GaN/Al_xGa_1−xN/GaN heterostructures with different Al mole fraction in the Al_xGa_1−xN barrier, and for various values of barrier layer thickness. The results of our analysis clearly indicate that increasing the GaN capping layer thickness leads to a decrease in the 2DEG density. Furthermore, it is found that the room-temperature 2D-electron mobility reaches a maximum value of approximately 1.8×10³ cm² /Vs⁻¹ for GaN capping layer thickness grater than 100 Å with an Al_0.32Ga_0.68N barrier layer of 200 Å thick. In contrast, for same structure, the 2DEG density decreases monotonically with GaN capping layer thickness, and eventually saturates at approximately 6×10¹² cm⁻² for capping layer thickness greater than 500 Å. A comparison between our calculated results with published experimental data is shown to be in good agreement for GaN capping layers up to 500 Å thickness.     

Thermal stability of Ti/Al/Pt/Au and Ti/Au Ohmic contacts on n-type ZnCdO

The specific contact resistivity and chemical intermixing of Ti/Au and Ti/Al/Pt/Au Ohmic contacts on n-type Zn_0.05Cd_0.95O layers grown on ZnO buffer layers on GaN/sapphire templates is reported as a function of annealing temperature in the range 200-600 °C. A minimum contact resistivity of 2.3 × 10⁻⁴ Ω cm² was obtained at 500 °C for Ti/Al/Pt/Au and 1.6 × 10⁻⁴ Ω cm² was obtained at 450 °C for Ti/Al. These values also correspond to the minima in transfer resistance for the contacts. The Ti/Al/Pt/Au contacts show far smoother morphologies after annealing even at 600 °C, whereas the Ti/Au contacts show a reacted appearance after 350 °C anneals. In the former case, Pt and Al outdiffusion is significant at 450 °C, whereas in the latter case the onset of Ti and Zn outdiffusion is evident at the same temperature. The improvement in contact resistance with annealing is suggested to occur through formation of TiO_x phases that induce oxygen vacancies in the ZnCdO.     

Ni/Au Schottky gate oxidation and BCB passivation for high-breakdown-voltage AlGaN/GaN HEMT

A new AlGaN/GaN high electron mobility transistor (HEMT) employing Ni/Au Schottky gate oxidation and benzocyclobutene (BCB) passivation is fabricated in order to increase a breakdown voltage and forward drain current. The Ni/Au Schottky gate metal with a thickness of 50/300 nm is oxidized under oxygen ambient at 500 C and the highly resistive NiO is formed at the gate edge. The leakage current of AlGaN/GaN HEMTs is decreased from 4.94 μA to 3.34 nA due to the formation of NiO. The BCB, which has a low dielectric constant, successfully passivates AlGaN/GaN HEMTs by suppressing electron injection into surface states. The BCB passivation layer has a low capacitance, so BCB passivation increases the switching speed of AlGaN/GaN HEMTs compared with silicon nitride passivation, which has a high dielectric constant. The forward drain current of a BCB-passivated device is 199 mA /mm, while that of an unpassivated device is 172 mA /mm due to the increase in two-dimensional electron gas (2DEG) charge.     

AlxGa1-x N/GaN调制掺杂异质结构的子带性质研究

通过低温和强磁场下的磁输运测量研究了Al_0.22Ga_0.78N/GaN调制掺杂异质结构中2DEG的子带占据性质和子带输运性质.在该异质结构的磁阻振荡中观察到了双子带占据现象，并发现2DEG的总浓度随第二子带浓度的变化呈线性关系.得到了该异质结构中第二子带被2DEG占据的阈值电子浓度为7.3×10¹²cm^-2.采用迁移率谱技术得到了不同样品的分别对应于第一和第二子带的输运迁移率.发现当样品产生应变弛豫时第一子带的电子迁移 关键词： AlGaN/GaN异质结 二维电子气 子带占据 输运迁移率