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.     

Current conduction mechanisms of RF-Magnetron sputtered Y2O3 gate oxide on gallium nitride

Current conduction mechanisms through as-deposited and post-deposition annealed (200–800 °C) RF-magnetron sputtered Y₂O₃ gate oxides on n-type GaN have been systematically investigated with current–voltage measurements at temperature in the range of 25–175 °C. The possible current conduction mechanisms that govern the leakage current of Y₂O₃/GaN metal-oxide-semiconductor test structure are space-charge-limited conduction, Schottky emission, Poole–Frenkel emission, and Fowler-Nordheim tunneling. The dominance of these conduction mechanisms is depending on applied electric field and measurement temperatures.     

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.     

Performance comparison of Pt/Au and Ni/Au Schottky contacts on AlxGa1-x N/GaN heterostructures at high temperatures

In contrast with Au/Ni/Al 0.25 Ga 0.75 N/GaN Schottky contacts,this paper systematically investigates the effect of thermal annealing of Au/Pt/Al 0.25 Ga 0.75 N/GaN structures on electrical properties of the two-dimensional electron gas in Al 0.25 Ga 0.75 N/GaN heterostructures by means of temperature-dependent Hall and temperature-dependent current-voltage measurements.The two-dimensional electron gas density of the samples with Pt cap layer increases after annealing in N 2 ambience at 600℃ while the annealing treatment has little effect on the two-dimensional electron gas mobility in comparison with the samples with Ni cap layer.The experimental results indicate that the Au/Pt/Al 0.25 Ga 0.75 N/GaN Schottky contacts reduce the reverse leakage current density at high annealing temperatures of 400-600℃.As a conclusion,the better thermal stability of the Au/Pt/Al 0.25 Ga 0.75 N/GaN Schottky contacts than the Au/Ni/Al 0.25 Ga 0.75 N/GaN Schottky contacts at high temperatures can be attributed to the inertness of the interface between Pt and AlxGa1-xN.     

Analysis of electrical characteristics of Er/p-InP Schottky diode at high temperature range

We report on the temperature-dependent electrical characteristics of Er/p-InP Schottky barrier diodes. The current–voltage (I–V) and capacitance–voltage (C–V) measurements have been carried out in the temperature range of 300–400 K. Using thermionic emission (TE) theory, the zero-bias barrier height (Φ_bo) and ideality factor (n) are estimated from I–V characteristics. It is observed that there is a decrease in n and an increase in the Φ_bo with an increase in temperature. The barrier height inhomogenity at the metal/semiconductor (MS) interface resulted in Gaussian distribution of Φ_bo and n. The laterally homogeneous Schottky barrier height value of approximately 1.008 eV for the Er/p-InP Schottky barrier diodes is extracted from the linear relationship between the experimental zero-bias barrier heights and ideality factors. The series resistance (R_s) is calculated by Chenug's method and it is found that it increases with the decrease in temperature. The reverse-bias leakage current mechanism of Er/p-InP Schottky diode is investigated. Both Poole–Frenkel and Schottky emissions are described and discussed. Furthermore, capacitance–voltage (C–V) measurements of the Er/p-InP Schottky contacts are also carried out at room temperature in dark at different frequencies of 10, 100 and 1000 kHz. Using Terman's method, the interface state density is calculated for Er/p-InP Schottky diode at different temperatures.     

Fabrication and characterization of aluminum-doped zinc oxide Schottky diodes on n-GaN

Aluminum-doped zinc oxide (AZO) films were prepared using an electron-beam evaporation system to form Schottky contacts on n-type GaN at depositing temperatures varied from 100 to 400 °C. The current–voltage (I–V) measurements which showed a rectifying characteristic were carried out to deduce the Schottky barrier heights (SBHs) according to the thermionic emission theory. The SBHs were calculated by using a linear curve fit to forward characteristics of ln(I) against V, and have a small alteration around 0.7 eV. Hall-effect measurements were carried out to illuminate the alteration of the SBHs that were mainly affected by the carrier concentration of the AZO films. It has been found that the SBH ascends as the carrier concentration decrease, and the dislocations play an important role on the leakage current of the contacts.     

The effect of junction temperature on the optoelectrical properties of InGaN/GaN multiple quantum well light-emitting diodes

Thermal effects on the optoelectrical characteristics of green InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) have been investigated in detail for a broad temperature range, from 30 °C to 100 °C. The current-dependent electroluminescence (EL) spectra, current–voltage (I–V) curves and luminescence intensity–current (L–I) characteristics of green InGaN/GaN MQW LEDs have been measured to characterize the thermal-related effects on the optoelectrical properties of the InGaN/GaN MQW LEDs. The experimental results show that both the forward voltages decreased with a slope of ?3.7 mV/K and the emission peak wavelength increased with a slope of +0.02 nm/K with increasing temperature, indicating a change in the contact resistance between the metal and GaN layers and the existence of a band gap shrinkage effect. The junction temperature estimated from the forward voltage and the emission peak shift varied from 25.6 to 14.5 °C and from 22.4 to 35.6 °C, respectively. At the same time, the carrier temperature decreased from 371.2 to 348.1 °C as estimated from the slope of high-energy side of the emission spectra. With increasing injection current, there was found to be a strong current-dependent blueshift of ?0.15 nm/mA in the emission peak wavelength of the EL spectra. This could be attributed to not only the stronger band-filling effect but also the enhanced quantum confinement effect that resulted from the piezoelectric polarization and spontaneous polarization in InGaN/GaN heterostructures. We also demonstrate a helpful and easy way to measure and calculate the junction temperature of InGaN/GaN MQW LEDs.     

Micro-structural and temperature dependent electrical characterization of Ni/GaN Schottky barrier diodes

Micro-structural investigation of Ni/GaN Schottky barrier diodes has been carried out using high-resolution transmission electron microscopy and electron diffraction spectrum in order to emphasize the role of Ni/GaN interface in controlling the Schottky diode behavior. Variable temperature Hall effect measurement of GaN samples along with the current–voltage (I–V) characteristics of Ni/n-GaN Schottky barrier diodes have been measured in 100–380 K temperature range. Results are analyzed in terms of thermionic emission theory by incorporating the concept of barrier inhomogeneity at the metal/semiconductor interface. The observed anomaly of temperature dependence of Schottky barrier height and ideality factor are explained by invoking two sets of Gaussian distribution of SBH in the temperature ranges of 100–180 K and 220–380 K, respectively. The value of A** (effective Richardson constant) as determined from the modified Richardson plot is 29.2 A/(cm² K²), which shows an excellent agreement with the theoretical value (26.4 A/(cm² K²)) in the temperature range of 220–380 K.     

Analysis of the electrical properties of p–n GaAs homojunction under dc and ac fields

In this work, the n-type GaAs films were grown on p-type GaAs single crystalline substrate by metal organic chemical vapor deposition (MOCVD). The temperature dependence of the current density–voltage (J–V) characteristics of n-GaAs/p-GaAs homojunction contacts were measured in the temperature range 293–413 K. These characteristics showed a rectifying behavior consistent with a potential barrier formed at the interface. The forward current density–voltage characteristics under low voltage biasing were explained on the basis of thermionic emission mechanism. The high values of ideality factor (n) may be ascribed to the presence of an interfacial layer. Analysis of the experimental data under the reverse voltage biasing suggests a dominant mechanism was found to be a Schottky effect. The impedance properties and the alternating current (ac) conductivity of n-GaAs/p-GaAs homojunction were investigated as a function of frequency and temperature. The ac conductivity was found to obey the universal power law. The variation of the exponent s with the temperature suggested that the conduction mechanism is an overlapping large-polaron tunneling (OLPT) model associated with correlated barrier hopping (CBH) model at the higher temperature.     

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     

Numerical optimization of carrier confinement characteristics in (AlxGa1−xN/AlN)SLs/GaN heterostructures

We present numerical optimization of carrier confinement characteristics in (Al_xGa_1−xN/AlN)SLs/GaN heterostructures in the presence of spontaneous and piezoelectrically induced polarization effects. The calculations were made using a self-consistent solution of the Schrödinger, Poisson, potential and charge balance equations. It is found that the sheet carrier density in GaN channel increases nearly linearly with the thickness of AlN although the whole thickness and equivalent Al composition of Al_xGa_1−xN/AlN superlattices (SLs) barrier are kept constant. This result leads to the carrier confinement capability approaches saturation with thicknesses of AlN greater than 0.6 nm. Furthermore, the influence of carrier concentration distribution on carrier mobility was discussed. Theoretical calculations indicate that the achievement of high sheet carrier density is a trade-off with mobility.     

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.     

Pt/Au/n-InGaN肖特基接触的电流输运机理

基于热电子发射和热电子场发射模式,利用I-V方法研究了Pt/Au/n-InGaN肖特基接触的势垒特性和电流输运机理,结果表明,在不同背景载流子浓度下,Pt/Au/n-InGaN肖特基势垒特性差异明显.研究发现,较低生长温度制备的InGaN中存在的高密度施主态氮空位（V_N）缺陷导致背景载流子浓度增高,同时通过热电子发射模式拟合得到高背景载流子浓度的InGaN肖特基势垒高度和理想因子与热电子场发射模式下的结果差别很大,表明V_N缺陷诱发了隧穿机理并降低了肖特基势垒高度,相应的隧穿电流显著增大了肖特基势垒总的输运电流,证实热电子发射和缺陷辅助的隧穿机理共同构成了肖特基势垒的电流输运机理.低背景载流子浓度的InGaN肖特基势垒在热电子发射和热电子场发射模式下拟合的结果接近一致,表明热电子发射是其主导的电流输运机理.     

Evidence for the changes in hole injection mechanism with a CoPc hole injection layer

The hole injection in hole-only devices with the structures of Al/N,N′-bis(1-naphthyle)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/ITO and Al/NPB/cobalt phthalocyanine (CoPc)/ITO were analyzed. With the combined analysis of current density–voltage and impedance measurement, the charge injection mechanism based on the injection limited current model was investigated. The NPB single layer device shows Richardson–Schottky type thermionic emission in the entire applied bias range. On the other hand, the device with the CoPc hole injection layer shows thermionic emission until the applied bias reaches 3.7 V. Increasing the bias further, Fowler–Nordheim tunneling dominates the charge injection. The changes of hole injection mechanism were discussed by evaluating the energy level changes with internal field distributions.     

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.     

High-Power GaN Electronic Devices

Gallium Nitride (GaN) and related materials (especially AlGaN) recently have attracted a lot of interest for applications in high-power electronics capable of operation at elevated temperatures and high frequencies. The AlGaInN system offers numerous advantages. These include wide bandgaps, good transport properties, the availability of heterostructures (particularly AlGaN/GaN), the experience base gained by the commercialization of GaN-based laser and light-emitting diodes and the existence of a high growth rate epitaxial method (hydride vapor phase epitaxy, HVPE) for producing very thick layers or even quasisubstrates. These attributes have led to rapid progress in the realization of a broad range of GaN electronic devices. Al_xGa_1-xN (x=0 ～.25) Schottky rectifiers were fabricated in a lateral geometry employing p⁺-implanted guard rings and rectifying contact overlap onto an SiO₂ passivation layer. The reverse breakdown voltage (V_B) increased with the spacing between Schottky and ohmic metal contacts, reaching 9700 V for Al_0.25Ga_0.75N and 6350 V for GaN, respectively, for 100-µm gap spacing. Assuming lateral depletion, these values correspond to breakdown field strengths of <9.67×10⁵ Vcm^?2, which is roughly a factor of 5 lower than the theoretical maximum in bulk GaN. The figure of merit (V_B)²/R_ON, where RON is the on-state resistance, was in the range 94 to 268 MWcm^?2 for all the devices. Edge-terminated Schottky rectifiers were also fabricated on quasibulk GaN substrates grown by HVPE. For small-diameter (75?µm) Schottky contacts, Vs measured in the vertical geometry was ～700?V, with an on-state resistance (RON) of 3?mΩcm², producing a figure-of-merit V_B ²/R_ON of 162.8?MW-cm^?2. GaN p-i-n diodes were also fabricated. A direct comparison of GaN p-i-n and Schottky rectifiers fabricated on the same GaN wafer showed higher reverse breakdown voltage for the former (490?V vs. 347?V for the Schottky diodes), but lower forward turn-on voltages for the latter (～3.5?V vs. ～5?V for the p-i-n diodes). The forward I-V characteristics of the p-i-n rectifiers show behavior consistent with a multiple recombination center model. The reverse current in both types of rectifiers was dominated by surface perimeter leakage at moderate bias. Finally, all of the devices we fabricated showed negative temperature coefficients for reverse breakdown voltage, which is a clear disadvantage for elevated temperature operation. Bipolar devices are particularly interesting for high current applications such as microwave power amplifiers for radar, satellite, and communication in the l～5?GHz range, powers >l00?W, and operating temperatures >425°C. pnp Bipolar Junction Transistors and pnp Heterojunction Bipolar Transistors were demonstrated for the first time. For power microwave applications, small area self-aligned npn GaN/AlGaN HBTs were attempted. The devices showed very promising direct current characteristics.     

Preferential regrowth of indium–tin oxide (ITO) films deposited on GaN (0001) by rf-magnetron sputter

Effects of thermal treatments on the electrical properties and microstructures of indium–tin oxide (ITO)/GaN contacts have been investigated using a rf-magnetron sputter deposition followed by rapid thermal annealing. ITO films annealed at 800 °C revealed Schottky contact characteristics with a barrier height corresponding to ITO’s work function of 4.62 eV. The evolution of electrical properties of ITO/GaN contacts was attributed to the preferential regrowth of In₂O₃ (222)//GaN (0001) with an ideal metal–semiconductor Schottky contact. The feasible use of ITO/GaN as a transparent Schottky contact would be realized by the enhanced regrowth of In₂O₃ at high temperature. Received: 1 September 2000 / Accepted: 15 November 2000 / Published online: 28 February 2001     

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.     

In situ scanning electron microscope electrical characterization of GaN nanowire nanodiodes using tungsten and tungsten/gallium nanoprobes

A lithography-free technique for measuring the electrical properties of n-type GaN nanowires has been investigated using nanoprobes mounted in a scanning electron microscope (SEM). Schottky contacts were made to the nanowires using tungsten nanoprobes, while gallium droplets placed in situ at the end of tungsten nanoprobes were found to be capable of providing Ohmic contacts to GaN nanowires. Schottky nanodiodes were fabricated based on single n-type nanowires, and measured current–voltage (I–V) results suggest that the Schottky nanodiodes deviate from ideal diodes mainly due to their nanoscopic contact area. Additionally, the effect of the SEM electron beam on the I–V characteristics was investigated and was found to impact the transport properties of the Schottky nanodiodes, possibly due to an increase in carrier density in the nanodiodes.     

A simple method of extracting the polarization charge density in the A1GaN/GaN heterostructure from current-voltage and capacitance-voltage characteristics

An Ni Schottky contact on the AlGaN/GaN heterostructure is fabricated. The flat-band voltage for the Schottky contact on the AlGaN/GaN heterostructure is obtained from the forward current-voltage characteristics. With the measured capacitance-voltage curve and the flat-band voltage, the polarization charge density in the AlGaN/GaN heterostructure is investigated, and a simple formula for calculating the polarization charge density is obtained and analyzed. With the approach described in this paper, the obtained polarization charge density agrees well with the one calculated by self-consistently solving Schrodinger’s and Poisson’s equations.