Temperature-Dependent Characteristics of GaN Schottky Barrier Diodes with TiN and Ni Anodes

The effect of temperature on the characteristics of gallium nitride (GaN) Schottky barrier diodes (SBDs) with TiN and Ni anodes is evaluated. With increasing the temperature from 25 to 175℃, reduction of the turn-on voltage and increase of the leakage current are observed for both GaN SBDs with TiN and Ni anodes. The performance after thermal treatment shows much better stability for SBDs with Ti N anode, while those with Ni anode change due to more interface states. It is found that the leakage currents of the GaN SBDs with TiN anode are in accord with the thermionic emission model whereas those of the GaN SBDs with Ni anode are much higher than the model. The Silvaco TCAD simulation results show that phonon-assisted tunneling caused by interface states may lead to the instability of electrical properties after thermal treatment, which dominates the leakage currents for GaN SBDs with Ni anode. Compared with GaN SBDs with Ni anode, GaN SBDs with TiN anode are beneficial to the application in microwave power rectification fields due to lower turn-on voltage and better thermal stability.     

Characterization of vertical Au/β-Ga_2O_3 single-crystal Schottky photodiodes with MBE-grown high-resistivity epitaxial layer

High-resistivity β-Ga_2O_3 thin films were grown on Si-doped n-type conductive β-Ga_2O_3 single crystals by molecular beam epitaxy(MBE).Vertical-type Schottky diodes were fabricated,and the electrical properties of the Schottky diodes were studied in this letter.The ideality factor and the series resistance of the Schottky diodes were estimated to be about1.4 and 4.6×10~6 Ω.The ionized donor concentration and the spreading voltage in the Schottky diodes region are about4×10~(18)cm~(-3) and 7.6 V,respectively.The ultra-violet(UV) photo-sensitivity of the Schottky diodes was demonstrated by a low-pressure mercury lamp illumination.A photoresponsivity of 1.8 A/W and an external quantum efficiency of8.7 ×10~2%were observed at forward bias voltage of 3.8 V,the proper driving voltage of read-out integrated circuit for UV camera.The gain of the Schottky diode was attributed to the existence of a potential barrier in the i-n junction between the MBE-grown highly resistive β-Ga_2O_3 thin films and the n-type conductive β-Ga_2O_3 single-crystal substrate.     

Electronic structures and optical properties of Si-and Sn-doped β-Ga_2O_3: A GGA+U study

The electronic structures and optical properties of β-Ga_2O_3 and Si-and Sn-doped β-Ga_2O_3 are studied using the GGA + U method based on density functional theory. The calculated bandgap and Ga 3d-state peak of β-Ga_2O_3 are in good agreement with experimental results. Si-and Sn-doped β-Ga_2O_3 tend to form under O-poor conditions, and the formation energy of Si-doped β-Ga_2O_3 is larger than that of Sn-doped β-Ga_2O_3 because of the large bond length variation between Ga–O and Si–O. Si-and Sn-doped β-Ga_2O_3 have wider optical gaps than β-Ga_2O_3, due to the Burstein–Moss effect and the bandgap renormalization effect. Si-doped β-Ga_2O_3 shows better electron conductivity and a higher optical absorption edge than Sn-doped β-Ga_2O_3, so Si is more suitable as a dopant of n-type β-Ga_2O_3, which can be applied in deep-UV photoelectric devices.     

High performance lateral Schottky diodes based on quasi-degenerated Ga_2O_3

Ni/β-Ga_2 O_3 lateral Schottky barrier diodes(SBDs) were fabricated on a Sn-doped quasi-degenerate n~+-Ga_2 O_3(201)bulk substrate. The resultant diodes with an area of 7.85 ×10~(-5) cm~2 exhibited excellent rectifying characteristics with an ideality factor of 1.21, a forward current density(J) of 127.4 A/cm2 at 1.4 V, a specific on-state resistance(R_(on,sp)) of1.54 mΩ·cm~2,and an ultra-high on/off ratio of 2.1 ×10~(11) at±1 V. Due to a small depletion region in the highly-doped substrate, a breakdown feature was observed at-23 V, which corresponded to a breakdown field of 2.1 MV/cm and a power figure-of-merit(VB2/R_(on)) of 3.4×10~5 W/cm~2. Forward current-voltage characteristics were described well by the thermionic emission theory while thermionic field emission and trap-assisted tunneling were the dominant transport mechanisms at low and high reverse biases, respectively, which was a result of the contribution of deep-level traps at the metal-semiconductor interface. The presence of interfacial traps also caused the difference in Schottky barrier heights of 1.31 eV and 1.64 eV respectively determined by current-voltage and capacitance-voltage characteristics. With reduced trapping effect and incorporation of drift layers, the β-Ga_2 O_3 SBDs could further provide promising materials for delivering both high current output and high breakdown voltage.     

Study on the nitridation of β-Ga_2O_3 films

Single-crystal GaN layers have been obtained by nitriding β-Ga_2O_3 films in NH_3 atmosphere. The effect of the temperature and time on the nitridation and conversion of Ga_2O_3 films have been investigated. The nitridation process results in lots of holes in the surface of films. The higher nitridation temperature and longer time can promote the nitridation and improve the crystal quality of GaN films. The converted Ga N porous films show the single-crystal structures and lowstress, which can be used as templates for the epitaxial growth of high-quality GaN.     

Degradation of β-Ga_2O_3 Schottky barrier diode under swift heavy ion irradiation

The electrical characteristics and microstructures of β-Ga_2 O_3 Schottky barrier diode(SBD) devices irradiated with swift heavy ions(2096 Me V Ta ions) have been studied. It was found that β-Ga_2 O_3 SBD devices showed the reliability degradation after irradiation, including turn-on voltage Von, on-resistance Ron, ideality factor n, and the reverse leakage current density Jr. In addition, the carrier concentration of the drift layer was decreased significantly and the calculated carrier removal rates were 5 × 10~6–1.3 × 10~7 cm~(-1). Latent tracks induced by swift heavy ions were observed visually in the whole β-Ga_2 O_3 matrix. Furthermore, crystal structure of tracks was amorphized completely. The latent tracks induced by Ta ions bombardments were found to be the reason for the decrease in carrier mobility and carrier concentration. Eventually,these defects caused the degradation of electrical characteristics of the devices. In terms of the carrier removal rates, theβ-Ga_2 O_3 SBD devices were more sensitive to swift heavy ions irradiation than Si C and Ga N devices.     

Synthesis and characterization of β-Ga_2O_3@GaN nanowires

In this work, we prepared the β-Ga_2O_3@GaN nanowires(NWs) by oxidizing GaN NWs. High-quality hexagonal wurtzite GaN NWs were achieved and the conversion from GaN to β-Ga_2O_3 was confirmed by x-ray diffraction, Raman spectroscopy and transmission electron microscopy. The effect of the oxidation temperature and time on the oxidation degree of GaN NWs was investigated systematically. The oxidation rate of GaN NWs was estimated at different temperatures.     

Investigation of β-Ga_2O_3 films and β-Ga_2O_3/GaN heterostructures grown by metal organic chemical vapor deposition

In this work,(-201) β-Ga_2O_3 films are grown on GaN substrate by metal organic chemical vapor deposition(MOCVD). It is revealed that the β-Ga_2O_3 film grown on GaN possesses superior crystal quality, material homogeneity and surface morphology than the results of common heteroepitaxial β-Ga_2O_3 film based on sapphire substrate. Further, the relevance between the crystal quality of epitaxial β-Ga_2O_3 film and the β-Ga_2O_3/GaN interface behavior is investigated. Transmission electron microscopy result indicates that the interface atom refactoring phenomenon is beneficial to relieve the mismatch strain and improve the crystal quality of subsequent β-Ga_2O_3 film. Moreover, the energy band structure of β-Ga_2O_3/GaN heterostructure grown by MOCVD is investigated by X-ray photoelectron spectroscopy and a large conduction band offset of 0.89 eV is obtained. The results in this work not only convincingly demonstrate the advantages of β-Ga_2O_3 films grown on GaN substrate, but also show the great application potential of MOCVD β-Ga_2O_3/GaN heterostructures in microelectronic applications.     

Preparation and NO_2-gas sensing property of individual β-Ga_2O_3 nanobelt

Monoclinic gallium oxide (β-Ga_2O_3) nanobelts are synthesized from gallium and oxygen by thermal evaporation in an argon atmosphere and their NO_2 sensing properties are studied at room temperature.Electron microscopy studies show that the nanobelts have breadths ranging from 30 to 50 nm and lengths up to tens of micrometers.Both the x-ray diffraction (XRD) and the selected are electron diffraction (SAED) examinations indicate that β-Ga_2O_3 nanobelts have grown into single crystals.Room temperature NO_2 sensing tests show that the current of individual β-Ga_2O_3 nanobelt decreases quickly,and then gently when the NO_2 concentration increases from low to high.It is caused by the NO_2 molecule chemisorption and desorption processes in the surface of β-Ga_2O_3 nanobelt.     

Fabrications and characterizations of high performance 1.2 kV,3.3 kV,and 5.0 kV class 4H–SiC power SBDs

In this paper, 1.2 kV, 3.3 kV, and 5.0 kV class 4H–SiC power Schottky barrier diodes(SBDs)are fabricated with three N-type drift layer thickness values of 10 μm, 30 μm, and 50 μm, respectively. The avalanche breakdown capabilities,static and transient characteristics of the fabricated devices are measured in detail and compared with the theoretical predictions. It is found that the experimental results match well with the theoretical calculation results and are very close to the 4H–SiC theoretical limit line. The best achieved breakdown voltages(BVs) of the diodes on the 10 μm, 30 μm, and 50 μm epilayers are 1400 V, 3320 V, and 5200 V, respectively. Differential specific-on resistances(R_(on-sp)) are 2.1 m?·cm~2,7.34m?·cm~2, and 30.3 m?·cm~2, respectively.     

Study and optimal simulation of 4H-SiC floating junction Schottky barrier diodes’ structures and electric properties

This paper stuides the structures of 4H-SiC floating junction Schottky barrier diodes. Some structure parameters of devices are optimized with commercial simulator based on forward and reverse electrical characteristics. Compared with conventional power Schottky barrier diodes, the devices are featured by highly doped drift region and embedded floating junction layers, which can ensure high breakdown voltage while keeping lower specific on-state resistance, and solve the contradiction between forward voltage drop and breakdown voltage. The simulation results show that with optimized structure parameter, the breakdown voltage can reach 4.36 kV and the specific on-resistance is 5.8 mΩ·cm2 when the Baliga figure of merit value of 13.1 GW/cm2 is achieved.     

SiC epitaxial layers grown by chemical vapour deposition and the fabrication of Schottky barrier diodes

This paper presents the results of unintentionally doped 4H-SiC epilayers grown on n-type Si-faced 4H-SiC substrates with 8° off-axis toward the [11\overline 2 0] direction by low pressure horizontal hot-wall chemical vapour deposition. Growth temperature and pressure are 1580~°C and 10⁴~Pa, respectively. Good surface morphology of the sample is observed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). Fourier transform infrared spectroscopy (FTIR) and x-ray diffraction (XRD) are used to characterize epitaxial layer thickness and the structural quality of the films respectively. The carrier concentration in the unintentional 4H-SiC homoepitaxial layer is about 6.4×10¹⁴~cm^-3 obtained by c--V measurements. Schottky barrier diodes (SBDs) are fabricated on the epitaxial wafer in order to verify the quality of the wafer and to obtain information about the correlation between background impurity and electrical properties of the devices. Ni and Ti/4H-SiC Schottky barrier diodes with very good performances were obtained and their ideality factors are 1.10 and 1.05 respectively.     

Modeling capacitance voltage characteristic of TiW/p-InP Schottky barrier diode

The capacitance-voltage(C-V) characteristic of the TiW/p-InP Schottky barrier diodes(SBDs) is analyzed considering the effects of the interface state(N_(ss)), series resistance(R_s), and deep level defects. The C-V of the Schottky contact is modeled based on the physical mechanism of the interfacial state and series resistance effect. The fitting coefficients α andβ are used to reflect the N_(ss) and R_s on the C-V characteristics, respectively. The α decreases with the increase of frequency,while β increases with the increase of frequency. The capacitance increases with the increase of α and the decrease of β.From our model, the peak capacitance and its position can be estimated. The experimental value is found to be larger than the calculated one at the lower voltage. This phenomenon can be explained by the effect of deep level defects.     

Simulation of near-infrared photodiode detectors based on β-FeSi_2/4H-SiC heterojunctions

In this paper,we propose a near-infrared p-type β-FeSi2/n-type 4H-SiC heterojunction photodetector with semiconducting silicide(β-FeSi2) as the active region for the first time.The optoelectronic characteristics of the photodetector are simulated using a commercial simulator at room temperature.The results show that the photodetector has a good rectifying character and a good response to near-infrared light.Interface states should be minimized to obtain a lower reverse leakage current.The response spectrum of the β-FeSi2/4H-SiC detector,which consists of a p-type β-FeSi2 absorption layer with a doping concentration of 1×1015cm-3 and a thickness of 2.5 μm,has a peak of 755 mA/W at 1.42 μm.The illumination of the SiC side obtains a higher responsivity than that of the β-FeSi2 side.The results illustrate that the β-FeSi2/4H-SiC heterojunction can be used as a near-infrared photodetector compatible with near-infrared optically-activated SiC-based power switching devices.     

Bidirectional and Unidirectional Negative Differential Thermal Resistance Effect in a Modified Lorentz Gas Model

Recently,the negative differential thermal resistance effect was discovered in a homojunction made of a negative thermal expansion material,which is very promising for realizing macroscopic thermal transistors.Similar to the Monte Carlo phonon simulation to deal with grain boundaries,we introduce positive temperature-dependent interface thermal resistance in the modified Lorentz gas model and find negative differential thermal resistance effect.In the homojunction,we reproduce a pair of equivalent negative differential thermal resistance effects in different temperature gradient directions.In the heterojunction,we realize the unidirectional negative differential thermal resistance effect,and it is accompanied by the super thermal rectification effect.Using this new way to achieve high-performance thermal devices is a new direction,and will provide extensive reference and guidance for designing thermal devices.     

The pulsed microwave damage trend of a bipolar transistor as a function of pulse parameters

In the present paper we conduct a theoretical study of the thermal accumulation effect of a typical bipolar transistor caused by high power pulsed microwaves(HPMs),and investigate the thermal accumulation effect as a function of pulse repetition frequency(PRF) and duty cycle.A study of the damage mechanism of the device is carried out from the variation analysis of the distribution of the electric field and the current density.The result shows that the accumulation temperature increases with PRF increasing and the threshold for the transistor is about 2 kHz.The response of the peak temperature induced by the injected single pulses indicates that the falling time is much longer than the rising time.Adopting the fitting method,the relationship between the peak temperature and the time during the rising edge and that between the peak temperature and the time during the falling edge are obtained.Moreover,the accumulation temperature decreases with duty cycle increasing for a certain mean power.     

Influence of annealing treatment on the luminescent properties of Ta:β-Ga_2O_3 single crystal

Ta~(5+)doped β-Ga_2O_3 single crystals were grown by using the optical floating zone method, and then annealed in the air and nitrogen gas at 1400℃ for 20 hours.The transmittance spectra, photoluminescence(PL), x-ray irradiation spectra, and PL decay profiles of the samples were measured at room temperature.The relevant results show that the optical transmittance of the samples annealed in the air or nitrogen gas was improved.By drawing the(ahv)~2–hv graph,it can be seen that the band gap decreased after being annealed in the air, but increased in nitrogen gas.The PL spectra and x-ray irradiation spectra show that the luminescent intensity of the sample annealed in the air increased substantially,while decreased for the sample annealed in nitrogen.The PL decay time of the Ta:β-Ga_2O_3 annealed in the air increased significantly compared with that of the Ta:β-Ga_2O_3 sample without annealing, but the tendency after annealing in nitrogen gas was opposite.     

Thermal Behaviour for InGaAsP／InP Multi-Quantum-Well Superluminescent Diodes

Using a two-dimensional thermal flow model, we calculate the thermal resistance and the temperature distribution of InGaAsP/InP multi-quantum-well superluminescent diodes. The influence of lateral chip size and composition are evaluated. The results reveal that when the injection power reaches 1 W, temperatures in the active region rises up to almost 5OK. The width and length of the chip also have strong influence on the thermal resistance that can reach two orders of magnitude. The thermal resistance will change from 290 K/W to 68 K/W when the chip width increases from 500μm to 2500μm, and a similar result exists for the length. There is small effect on thermal resistance for active width. In view of the characteristics of output power versus the input current under pulsed and continues currents, the fitted experimental thermal resistance matches well with the measured results.     

Analysis and compensation of thermal lens effects in Tm：YAP lasers

The thermal lens effects in Tm：YAP laser are analyzed by solving the Poisson equation with finite difference method. The thermal focal lengths measured are in the range of 40-90 mm at the pump power of 16-34 W, consistent with the simulation results. The temperature contribution coefficient （the linear coefficient between the maximum temperature in the laser crystal and the pump power） of 1.19 K/W is also obtained. The convex lens and plano-concave mirror thermal lens compensation methods are proposed and applied to a high power pumped Tm：YAP laser.     

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.