Performance of Lateral 4H-SiC Photoconductive Semiconductor Switches by Extrinsic Backside Trigger

Photoconductive semiconductor switch(PCSS)can be applied in pulsed high power systems and microwave techniques.However,reducing the damage and increasing the lifetime of silicon carbide(SiC)PCSS are still faced severe challenges.In this study,PCSSs with various structures were prepared on 4-inch diameter,500μm thick high-purity semi-insulating 4H-SiC substrates and their on-state resistance and damage mechanisms were investigated.It was found that the PCSS of an Au/TiW/Ni electrode system annealed at 950℃had a minimum on-state resistance of 6.0Ωat 1 kV bias voltage with a 532 nm and 170 mJ pulsed laser by backside illumination single trigger.The backside illumination single trigger could reduce on-state resistance and alleviate the damage of PCSS compared to the frontside trigger when the diameter of the laser spot was larger than the channel length of PCSS.For the 200 s trigger test by a 10 Hz laser,the black branch-like ablation on Au/TiW/Ni PCSS was mainly caused by thermal stress owing to hot carriers.Replacing metal Ni with boron gallium co-doped zinc oxide(BGZO)thin films annealed at 400℃,black branch-like ablation was alleviated while concentric arc damage was obvious at the anode.The major causes of concentric arc are both pulsed laser diffraction and thermal effect.

Fabrication of SiC fibres from yttrium-containing polycarbosilane

The yttrium as a sintering aid was introduced into polycarbosilane（PCS） to prepare yttrium-containing PCS（PYCS）.Two types of yttrium-containing SiC fibres,the SiC（OY） fibres and the SiC（Y） fibres,were fabricated with PYCS.The structural evolution and the associated properties on changing from SiC（OY） to SiC（Y） fibres during the sintering process were studied.The chemical composition of the SiC（OY） fibres is SiC1.53O0.22Y0.005 with an amorphous structure.The composition of SiC（Y） fibres is SiC1.23O0.05Y0.005.The fibres are composed of a large number of β-SiC crystallites with a size of 50 nm and a small amount of α-SiC crystalline.The tensile strength and fracture toughness of the SiC（OY） fibres are 2.25 GPa and 2.37 MPa·m1/2,respectively,and 1.61 GPa,1.91 MPa·m1/2,respectively for SiC（Y） fibres.The SiC（Y） fibres have a higher thermal stability than the SiC（OY） fibres.

S-Band 1mm SiC MESFET with 2W Output on Semi-Insulated SiC Substrate

A SiC MESFET structure is successfully prepared on a semi-insulated 50mm SiC substrate using a hotwall SiC reactor. The doping concentration for the channel layer is about 1.7 × 10^17 cm^-3 , and the thickness is about 0.35μm. An unintentionally n-doped buffer layer is employed between the substrate and the channel layer. A cap layer for Ohmic contact is doped to 10^19cm^-3. MESFET devices are fabricated using inductively coupled plasma etching and other conventional tools. Power devices with a 1mm gate width are measured and a 2W output at 2GHz is obtained.

Preparation and properties of porous silicon carbide ceramics through coat-mix and composite additives process

The core-shell structure silicon-resin precursor powders were synthesized through coat-mix process and addition of Al2O3-SiO2-Y2O3 composite additives.A series of porous silicon carbide ceramics were produced after molding,carbonization and sintering.The phase,morphology,porosity,thermal conductivity,thermal expansion coefficient,and thermal shock resistance were analyzed.The results show that porous silicon carbide ceramics can be produced at low temperature.The grain size of porous silicon carbide ceramic is small,and the thermal conductivity is enhanced significantly.Composite additives also improve the thermal shock resistance of porous ceramics.The bending strength loss rate after 30 times of thermal shock test of the porous ceramics which were added Al2O3-SiO2-Y2O3 and sintered at 1 650 ℃ is only 6.5%.Moreover,the pore inside of the sample is smooth,and the pore size distribution is uniform.Composite additives make little effect on the thermal expansion coefficient of the porous silicon carbide ceramics.

Fabrication of SiC MESFETs for Microwave Power Applications

4H-SiC MESFETs are fabricated on semi-insulating SiC substrates. Key processes are optimized to obtain better device performance. A microwave power amplifier is demonstrated from a 1mm SiC MESFET for S band operation. When operated at a drain voltage of 64V, the amplifier shows an output power of 4.09W, a gain of 9.3dB,and a power added efficiency of 31.3%.

MOCVD-Grown AlGaN/AlN/GaN HEMT Structure with High Mobility GaN Thin Layer as Channel on SiC

AlGaN/AlN/GaN high electron mobility transistor （HEMT） structures with a high-mobility GaN thin layer as a channel are grown on high resistive 6H-SiC substrates by metalorganic chemical vapor deposition. The HEMT structure exhibits a typical two-dimensional electron gas （2DEG） mobility of 1944cm^2/（V·s） at room temperature and 11588cm^2/（V ·s） at 80K with almost equal 2DEG concentrations of about 1.03 × 10^13 cm^-2. High crystal quality of the HEMT structures is confirmed by triple-crystal X-ray diffraction analysis. Atomic force microscopy measurements reveal a smooth AlGaN surface with a root-mean-square roughness of 0.27nm for a scan area of 10μm × 10μm. HEMT devices with 0.8μm gate length and 1.2mm gate width are fabricated using the structures. A maximum drain current density of 957mA/mm and an extrinsic transconductance of 267mS/mm are obtained.

Nanogrinding of SiC wafers with high flatness and low subsurface damage

Nanogrinding of SiC wafers with high flatness and low subsurface damage was proposed and nanogrinding experiments were carried out on an ultra precision grinding machine with fine diamond wheels. Experimental results show that nanogrinding can produce flatness less than 1.0μm and a surface roughness Ra of 0.42nm. It is found that nanogrinding is capable of producing much flatter SiC wafers with a lower damage than double side lapping and mechanical polishing in much less time and it can replace double side lapping and mechanical polishing and reduce the removal amount of chemical mechanical polishing.

Influence of pyrolysis temperature on structure and dielectric properties of polycarbosilane derived silicon carbide ceramic

β-SiC ceramic powders were obtained by pyrolyzing polycarbosilane in vacuum at 800-1200 °C. The β-SiC ceramic powders were characterized by TGA/DSC, XRD and Raman spectroscopy. The dielectric properties of β-SiC ceramic powders were investigated by measuring their complex permittivity by rectangle wave guide method in the frequency range of 8.2-18 GHz. The results show that both real part ε′ and imaginary part ε″ of complex permittivity increase with increasing pyrolysis temperature. The mechanism was proposed that order carbon formed at high temperature resulted in electron relaxation polarization and conductance loss, which contributes to the increase in complex permittivity.

Theoretical Investigation of Pinch off Voltage of Box-Like Ion-Implanted 4H-SiC MESFETs

A precise theoretical calculation off the pinch of voltage of the box-like ion implantation 4H-SiC MESFETs is investigated with the consideration of the effects of the ion-implanted channel and the depth of MESFETs channel.The implant depth profile is simulated using the Monte Carlo simulator TRIM.The effects of parameters such as temperature,acceptor density,and activation rate on channel depth a,pinch off voltage are studied.

Improved Epitaxy of 3C-SiC Layers on Si(100) by New CVD/LPCVD System

Single crystalline 3C-SiC epitaxial layers are grown on φ 50mm Si wafers by a new resistively heated CVD/LPCVD system,using SiH_4,C_2H_4 and H_2 as gas precursors.X-ray diffraction and Raman scattering measurements are used to investigate the crystallinity of the grown films.Electrical properties of the epitaxial 3C-SiC layers with thickness of 1～3μm are measured by Van der Pauw method.The improved Hall mobility reaches the highest value of 470cm 2/(V·s) at the carrier concentration of 7.7×10 17 cm -3 .

Effect of High Temperature Annealing on Characteristics of 4H Silicon Carbide MESFET

For very high temperature annealing (1620℃) after ion implantation for 4H silicon carbide (4H SiC),the residual components of Al and O in the alundum furnace impact seriously on the surface of material,which yields the derivation of SiOC.This causes a significant degradation of the 4H SiC surface characteristics according to the results of surface composition analysis.As validity,Ni/SiC ohmic contact measurement illustrates a higher specific contact resistance than the normal value by a factor of 2～3.Consequently the MESFET fabricated with this kind of 4H SiC material results in a degraded I V output performance compared with that of normal 4H SiC MESFET.

Determination of Structure and Polarity of Si C Single Crystal by X-Ray Diffraction Technique

Structure and polarity of the Si C single crystal have been analyzed with the four- circle X- ray diffraction method by a double- crystal diffractom eter.The hexagonal{ 10 15 } pole figure shows that this Si C sam ple has a6 H modification.The difference between the integrated intensities m easured byω scan in the triple- axis diffraction set- up finds some convincing evidence that the surface is either a Si- terminated face or C- terminated face.The experi- mental ratios of| F( 0 0 0 L) | 2 / | F( 0 0 0 L) | 2 are in good agreem entwith the calculated ones after the dispersion cor- rections to the atomic scattering factors( L=6 ,12 and18,respectively) .Thus,this m easurem ent technique is con- venient for the application of the materials with remarkable surface polarity.

Ti/4H-SiC Schottky Barrier Diodes with Field Plate and B^+ Implantation Edge Termination Technology

This paper describes the fabrication and electrical characteristics of Ti/4H-SiC Schottky barrier diodes （SBDs）. The ideality factor n = 1.08 and effective Schottky barrier heightφ= 1.05eV of the SBDs were measured with the method of forward current density-voltage （J-V）. A low reverse leakage current below 5.96 ×10^-3A/cm^2 at a bias voltage of - 1. 1kV was obtained. By using B^＋ implantation,an amorphous layer as the edge termination was formed. We used the PECVD SiO2 as the field plate dielectric. The SBDs have an on-state current density of 430A/cm^2 at a forward voltage drop of about 4V. The specific on-resistance Ro, was found to be 6. 77mΩ2 · cm^2 .

Fabrication of 4H-SiC Merged PN-Schottky Diodes

The design, fabrication and characteristics of 4H SiC merged PN Schottky (MPS) diodes with Ni Schottky contact and junction termination extension (JTE) edge termination are reported. A multiple energy implantation Al in the surface of the n - drift region below the face to face Schottky metal formed pn junctions, which screen the Schottky contact from high electrical, post implantation annealing has been done at 1500℃ for 30min in the ultra high purity Ar ambient. The devices can block more than 600V reverse voltage and the lowest leakage current at -600V is 1×10 -3 A/cm 2, while the forward current density at 3V is more than 200A/cm 2 for 1000μm devices, 1000A/cm 2 at 3 5V for 300μm devices.

Influence of Si on stability of TiC in Al melts

The influence of Si on the stability of TiC in Al melts was studied.It is found that TiC particles in Al melts become unstable with the addition of Si.When the melting temperature is below 890 °C,TiC will react with Al and Si to form TiAlxSiy and Al4C3 phases.But if the melt temperature is above 890 ℃,TiC will react with Al and Si to form to Ti3SiC2 and Al4C3.It is considered that the influence of Si on the stability of TiC in Al melts is due to its incursion into TiC crystal lattice during the holding,which will cause serious lattice distortion in TiC and then speed up the out-diffusion of surrounded C atoms.

Ohmic Contact Properties of Multi-Metal Films on n-Type 4H-SiC

An investigation of Au/Ti/Ni and Au/Ti/Pt ohmic contacts to n-type 4H-SiC and the behavior of metal films on SiC with thermal anneals is reported. Specific contact resistance as low as 2. 765 x 10^-6Ω·cm^2 was achieved after rapid thermal annealing in N2 for 2min at 950℃. SIMS analysis shows that the formation of Ni silicide after annealing supported a number of carbon atoms＇ outdiffusion from the SiC to form interstitial compound TiC. This process can create abundant C vacancies near the interface. It is the carbon defect layer that enhances the defect-assisted tunneling. The interface band structure within the defect level could make it clear why the metal-SiC contacts become ohmic during annealing.

High-Voltage Ti /6H-SiC Schottky Barrier Diodes

The fabrication and electrical characterization of Scho tt ky barrier diodes (SBD) on 6H-SiC,via thermal evaporation of Ni are reported.Th e Schottky barrier diodes are fabricated during the 6H-SiC epilayers grow n by using chemical vapor deposition on commercially available single-crystal 6 H-SiC wafers.The I-V characteristics of these diodes exhibit a sharp break down,with the breakdown voltage of 450V at room temperature.The diodes are demon strated to be of a low reverse leakage current of 5×10 -4 A·cm -2 at the bias voltage of -200V.The ideal factor and barrier height are 1 09 and 1 24-1 26eV,respectively.

A Simple Method of Surface Parameter Extraction for Gate Schottky Contact in 4H-SiC MESFETs

We investigate the effects of the surface states on the Schottky contacts in 4H-SiC MESFET. The Ti/Pt/Au gate metal contacts are deposited by electron beam evaporation and patterned by a lift-off process. Based on thermionic theory,a simple parameter extraction method is developed for determination of the surface states in metal/4H-SiC Schottky contacts. The interface state density and interface capacitance are calculated to be 4. 386 × 10^12 cm^-2 · eV^- 1 and 6. 394 × 10^-6 F/cm^2 ,which are consistent with the device＇s terminal characteristics.

Characteristics of N^+ Implanted Layer of 4H-SiC

The nitrogen ions implanted layer of p type 4H SiC epilayer is investigated.The fabrication processes and measurements of the implanted layer are given in details.The profile of implantation depth is simulated using the Monte Carlo simulator TRIM.Lateral Schottky barrier diodes and transfer length method (TLM) measurement structure are made on nitrogen implanted layers for the testing.The concentration of activated donors N d is about 3 0×10 16 cm -3 .The resulting value for the activation rate in this study is 2 percent.The sheet resistance R sh is 30kΩ/□ and the resistivity ρ(R sh × d ) of the implanted layer is 0 72Ω·cm.The electron mobility calculated is about 300cm 2/(V·s) in the N implanted layer.

Temperature Dependence of Performance of 6H-SiC Unipolar Power Devices

The temperature dependence of some performance of 6H SiC unipolar power devices is analyzed theoretically.By employing the temperature dependent ionization coefficient and mobility of a silicon carbide,the analytical expressions of the temperature dependent performance,such as breakdown characteristics and on resistance of 6H SiC unipolar power devices are derived in a closed form.The analytical results are compared with the experimental results,with good accordance found in the breakdown characteristics.