Studies of 6H–SiC devices

Silicon carbide (SiC) is recently receiving increased attention due to its unique electrical and thermal properties. It has been regarded as the most appropriate semiconductor material for high power, high frequency, high temperature, and radiation hard microelectronic devices. The fabrication processes and characterization of basic device on 6H–SiC were systematically studied. The main works are summarized as follows:The homoepitaxial growth on the commercially available single-crystal 6H–SiC wafers was performed in a modified gas source molecular beam epitaxy system. The mesa structured p⁺n junction diodes on the material were fabricated and characterized. The diodes showed a high breakdown voltage of 800 V at room temperature. They operated with good rectification characteristics from room temperature to 673 K.Using thermal evaporation, Ti/6H–SiC Schottky barrier diodes were fabricated. They showed good rectification characteristics from room temperature to 473 K. Using neon implantation to form the edge termination, the breakdown voltage was improved to be 800 V.n-Type 6H–SiC MOS capacitors were fabricated and characterized. Under the same growing conditions, the quality of poly-silicon gate capacitors was better than Al. In addition, SiC MOS capacitors had good tolerance to γ rays.     

Effect of chemical and magnetic interactions on fractal growth under deposition–diffusion–aggregation in some binary metal systems

We have investigated fractal growth phenomena in three binary metal systems; the Sc-Co, Sc-Cr and Sc-Ni systems. For the Sc-Co and Sc-Cr systems, we describe the observed evolution process of the fractals based on the deposition–diffusion–aggregation model and find that the magnetic interaction can play an important role in influencing the fractal dimension, i.e. the stronger the magnetic moment of the aggregating particles, the smaller the dimension of the resultant fractal. Comparing the observations in the three systems studied, it was found that the chemical interaction between two dissimilar atoms might prevent the formation of the fractal pattern during deposition, for example in the Ni-Sc system . PACS 61.43. Hv; 68.55. -a; 66.30. Ny; 75.50. -y     

Wettability of Si and Al–12Si alloy on Pd-implanted 6H–SiC

Si C monocrystal substrates are implanted by Pd ions with different ion-beam energies and fluences,and the effects of Pd ion implantation on wettability of Si/Si C and Al–12 Si/Si C systems are investigated by the sessile drop technique.The decreases of contact angles of the two systems are disclosed after the ion implantation,which can be attributed to the increase of surface energy(σ_(SV)) of Si C substrate derived from high concentration of defects induced by the ionimplantation and to the decrease of solid–liquid surface energy(σ_(SL)) resulting from the increasing interfacial interactions.This study can provide guidance in improving the wettability of metals on Si C and the electronic packaging process of Si C substrate.     

Electrical properties and microstructural characterization of Ni/Ta contacts to n-type 6H–SiC

A Ni/Ta bilayer is deposited on n-type 6H–Si C and then annealed at different temperatures to form an ohmic contact. The electrical properties are characterized by I–V curve measurement and the specific contact resistance is extracted by the transmission line method. The phase formation and microstructure of the Ni/Ta bilayer are studied after thermal annealing. The crystalline and microstructure properties are analyzed by using glance incident x-ray diffraction(GIXRD),Raman spectroscopy, and transmission electron microscopy. It is found that the transformation from the Schottky to the Ohmic occurs at 1050?C and the GIXRD results show a distinct phase change from Ta2 C to Ta C at this temperature. A specific contact resistance of 6.5×10-5?·cm2is obtained for sample Ni(80 nm)/Ta(20 nm)/6H–Si C after being annealed at 1050?C. The formation of the Ta C phase is regarded as the main reason for the excellent Ohmic properties of the Ni/Ta contacts to 6H–Si C. Raman and TEM data reveal that the graphite carbon is drastically consumed by the Ta element, which can improve the contact thermal stability. A schematic diagram is proposed to illustrate the microstructural changes of Ni/Ta/6H–Si C when annealed at different temperatures.     

Application of an Al-doped zinc oxide subcontact layer on vanadium-compensated 6H–SiC photoconductive switches

Al-doped Zn O thin film(AZO) is used as a subcontact layer in 6H–Si C photoconductive semiconductor switches(PCSSs) to reduce the on-state resistance and optimize the device structure. Our photoconductive test shows that the onstate resistance of lateral PCSS with an n+-AZO subcontact layer is 14.7% lower than that of PCSS without an n+-AZO subcontact layer. This occurs because a heavy-doped AZO thin film can improve Ohmic contact properties, reduce contact resistance, and alleviate Joule heating. Combined with the high transparance characteristic at 532 nm of AZO film, vertical structural PCSS devices are designed and their structural superiority is discussed. This paper provides a feasible route for fabricating high performance Si C PCSS by using conductive and transparent Zn O-based materials.     

Effect of Au/Ni/4H–SiC Schottky junction thermal stability on performance of alpha particle detection

Au/Ni/n-type 4H–SiC Schottky alpha particle detectors are fabricated and annealed at temperatures between 400℃ and 700℃ to investigate the effects of thermal stability of the Schottky contact on the structural and electrical properties of the detectors. At the annealing temperature of 500?C, the two nickel silicides(i.e., Ni_(31)Si_(12) and Ni_2Si) are formed at the interface and result in the formation of an inhomogeneous Schottky barrier. By increasing the annealing temperature,the Ni_(31)Si_(12) transforms into the more stable Ni_2Si. The structural evolution of the Schottky contact directly affects the electrical properties and alpha particle energy resolutions of the detectors. A better energy resolution of 2.60% is obtained for 5.48-MeV alpha particles with the detector after being annealed at 600℃. As a result, the Au/Ni/n-type 4 H–SiC Schottky detector shows a good performance after thermal treatment at temperatures up to 700℃.     

Effect of surface groups on the luminescence property of ZnO nanoparticles synthesized by sol–gel route

Structural and optical properties of ZnO nanoparticles of diameter ~ 5 nm synthesized by a sol–gel route, have been studied using a variety of experimental techniques. The photoluminescence (PL) study carried out on these particles in the atmospheric and vacuum conditions shows a suppression of the defect related green luminescence (GL) band and a simultaneous enhancement of the near-band-edge ultra violet luminescence (UVL) when the surroundings of the nanoparticles are evacuated. This observation clearly suggests that GL is originating from certain groups that are physisorbed on the surface of the nanoparticles. Fourier transform infrared spectroscopy (FTIR) that has also been conducted at the vacuum and atmospheric conditions reveals the presence of the hydroxyl and the acetate groups in these nanoparticle samples. These groups are also found to be removed upon evacuation, suggesting that there is physical adsorption on the surface of the nanoparticles. When the PL spectrum is recorded again at the atmospheric condition, the GL intensity recovers almost up to its original value. Since there are substantial amount of water molecules present in air, which can source the hydroxyl groups, while the acetate groups are not expected to be abundant in air, this finding further suggests that the hydroxyl groups rather than the acetate groups are the likely cause for the GL emission observed in this system.     

Fabrication and characterization of the normally-off N-channel lateral 4H–SiC metal–oxide–semiconductor field-effect transistors

In this paper, the normally-off N-channel lateral 4H–Si C metal–oxide–semiconductor field-effect transistors(MOSFFETs) have been fabricated and characterized. A sandwich-(nitridation–oxidation–nitridation) type process was used to grow the gate dielectric film to obtain high channel mobility. The interface properties of 4H–Si C/SiO_2 were examined by the measurement of HF I–V, G–V, and C–V over a range of frequencies. The ideal C–V curve with little hysteresis and the frequency dispersion were observed. As a result, the interface state density near the conduction band edge of 4H–Si C was reduced to 2 × 10~(11) e V~(-1)·cm~(-2), the breakdown field of the grown oxides was about 9.8 MV/cm, the median peak fieldeffect mobility is about 32.5 cm~2·V~(-1)·s~(-1), and the maximum peak field-effect mobility of 38 cm~2·V~(-1)·s~(-1) was achieved in fabricated lateral 4H–Si C MOSFFETs.     

Effect of sintering temperature on luminescence properties of borosilicate matrix blue–green emitting color conversion glass ceramics

The color conversion glass ceramics which were made of borosilicate matrix co-doped(SrBaSm)Si_2O_2N_2:(Eu~(3+)Ce~(3+)) blue–green phosphors were prepared by two-step method in co-sintering. The change in luminescence properties and the drift of chromaticity coordinates(CIE) of the(SrBaSm)Si_2O_2N_2:(Eu~(3+)Ce~(3+)) blue–green phosphors and the color conversion glass ceramics were studied in the sintering temperature range from 600℃ to 800℃. The luminous intensity and internal quantum yield(QY) of the blue–green phosphors and glass ceramics decreased with the sintering temperature increasing. When the sintering temperature increased beyond 750℃, the phosphors and the color conversion glass ceramics almost had no peak in photoluminescence(PL) and excitation(PLE) spectra. The results showed that the blue–green phosphors had poor thermal stability at higher temperature. The lattice structure of the phosphors was destroyed by the glass matrix and the Ce~(3+) in the phosphors was oxidized to Ce~(4+), which further caused a decrease in luminescent properties of the color conversion glass ceramics.     

Effect of laser intensity on the semiconductor–metal transition in a doped Quantum Well

We demonstrate laser induced semiconductor–metal transition through an abrupt change in diamagnetic susceptibility of a donor at critical concentration in a GaAs/Al_xGa_1−xAs Quantum Well for finite barrier model in the effective mass approximation using variational principle. We have considered Anderson‘s localization due to the random distribution of impurities in our calculation. The nonparabolicity of the conduction band is also considered. Our results without laser field agree with the earlier theoretical results and also with the recent experimental results.     

Investigation of the structural and electrical properties of air-annealed ruthenium thin films on 6H–SiC at different temperatures

Ruthenium (Ru) Schottky contacts and thin films on n-type 6H–SiC were fabricated and characterised by physical and electrical methods. The characterisation was done after annealing the samples in air at various temperatures. Rutherford backscattering spectroscopy (RBS) analysis of the thin films indicated the oxidation of Ru after annealing at a temperature of 400 °C, and interdiffusion of Ru and Si at the Ru–6H–SiC interface at 500 °C. XRD analysis of the thin films indicated the formation of RuO₂ and RuSi in Ru–6H–SiC after annealing at a temperature of 600 °C. The formation of the oxide was also corroborated by Raman spectroscopy. The ideality factor of the Schottky barrier diodes (SBD) was seen to generally decrease with annealing temperature. The series resistance increased astronomically after annealing at 700 °C, which was an indication that the SBD had broken down. The failure mechanism of the SBD is attributed to deep inter-diffusions of Ru and Si at the Ru–6H–SiC interface as evidenced by the RBS of the thin films.     

Low frequency effects of surface states on 4H—SiC metal—semiconductor field effect transistor

The process-related surface state effect is investigated the fabrication of SiC devices, and a nonllnear model for 4H-SiC power metal-semiconductor field effect transistor (MESFET) is propose, which takes into account the surface related parameters. The frequency-and temperature-dependent transconductance dispersion is readily demonstrated in terms of the improved model. Simulation results show that larger dispersion and higher transition frequency occur in 4H-SiC MESFET than in GaAs MESFET. The advantage of this analytical model over the two-dimensional numerical simulation is the simplicity of calculations, therefore it is suitable for the processing improvement of SiC devices     

Effect of SiO2–metal–SiO2 plasmonic structures on InGaAs/GaAs quantum well intermixing

Dielectric–metal–dielectric sandwich structures have been fabricated on top of an InGaAs/GaAs single quantum well (QW) structure to enhance atomic interdiffusion across the QW interfaces at elevated temperature during rapid thermal annealing using a halogen lamp as the heating source. The QW intermixing enhancement is realized during rapid thermal annealing. By placing a properly designed SiO₂–Ag–SiO₂ structure on top of the QW sample, a blueshift in photoluminescence emission from 920 to 882 nm was observed, larger than that obtained in a SiO₂-capped QW annealed at the same condition. Finite-difference time-domain simulation and optical reflectance measurements showed that the enhanced QW intermixing is due to the plasmonic resonance-enhanced light absorption and suppressed light reflection from the SiO₂–Ag–SiO₂ structure.     

Effect of irradiation on microstructure and hardening of Cr–Fe–Ni–Mn high-entropy alloy and its strengthened version

ABSTRACT

A single-phase fcc high-entropy alloy (HEA) of 20%Cr–40%Fe–20%Mn–20%Ni composition and its strength with yttrium and zirconium oxides version was irradiated with 1.4?MeV Ar ions at room temperature and mid-range doses from 0.1 to 10 displacements per atom (dpa). Transmission electron microscopy (TEM), scanning transmission electron microscopy with energy dispersive X-ray spectrometry (STEM/EDS) and X-ray diffraction (XRD) were used to characterise the radiation defects and microstructural changes. Nanoindentation was used to measure the ion irradiation effect on hardening. In order to understand the irradiation effects in HEAs and to demonstrate their potential advantages, a comparison was performed with hardening behaviour of 316 austenitic stainless steel irradiated under an identical condition. It was shown that hardness increases with irradiation dose for all the materials studied, but this increase is lower in high-entropy alloys than in stainless steel.     

Effect of the low-temperature (20–60°C) heating of silicon on its microhardness

Changes in the microhardness of silicon samples exposed to temperatures of 20–60°C are studied. It is found that the microhardness increases; this effect is preserved at room temperature for 20 min and non-monotonically depends on the temperature and exposure time (the changes are maximal at ～40°C and ～100 s, respectively). The microhardness of samples with native oxide removed does not change. The results are discussed in terms of a model of processes in the silicon-oxide system, which was previously proposed for the case of irradiation of silicon with light. The practical importance of the effect is discussed.     

Effect of the annealing temperature on the long-term thermal stability of Pt/Si/Ta/Ti/4H–SiC contacts

The Pt/Si/Ta/Ti multilayer metal contacts on 4H–Si C are annealed in Ar atmosphere at 600°C–1100°C by a rapid thermal processor(RTP). The long-term thermal stability is evaluated by aging the annealed contact at 600°C in air. The contact's properties are determined by current–voltage measurement, and the specific contact resistance is calculated based on the transmission line model(TLM). Transmission electron microscope(TEM) and energy-dispersive x-ray spectrometry(EDX) are used to characterize the interface morphology, thickness, and composition. The results reveal that a higher annealing temperature is favorable for the formation of an Ohmic contact with a lower specific contact resistance, and causes the rapid degradation of the Ohmic contact in the aging process.     

Effect of an aggressive medium on discontinuous deformation of aluminum–magnesium alloy AlMg6

It is experimentally shown that the molecular (chemical) process of surface etching of deformed aluminum–magnesium alloy AlMg6 causes the development of a macroscopic plastic strain step with an amplitude of a few percent. Using numerical simulation of the polycrystalline solid etching process, it is shown that the corrosion front morphology varies during etching from Euclid (flat) to fractal (rough). The results obtained show the key role of the surface state on the development of macroscopic mechanical instability of a material exhibiting the Portevin–Le Chatelier effect.