Temperature dependence of current-and capacitance–voltage characteristics of an Au/4H-SiC Schottky diode

Highlights

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I have fabricated an Au/4H-SiC Schottky diode in optimum conditions.

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The abnormal behaviors observed at low temperatures are related to Schottky barrier inhomogeneities.

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The barrier abnormality can be successfully commented by both multi-Gaussian distribution model and pinch-off model.

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Thus, it is noted that multi-Gaussian distribution model is good agreement with pinch-off model to interpret abnormalities.

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In this study, the current–voltage (I–V) and capacitance–voltage (C–V) measurements of an Au/4H-SiC Schottky diode are characterized as a function of the temperature in 50–300 K temperature range. The experimental parameters such as ideality factor and apparent barrier height presents to be strongly temperature dependent, that is, the ideality factor increases and the apparent barrier height decreases with decreasing temperature, whereas the barrier height values increase with the temperature for C–V data. Likewise, the Richardson plot deviates at low temperatures. These anomaly behaviors observed for Au/4H-SiC are attributed to Schottky barrier inhomogeneities. The barrier anomaly which relates to interface of Au/4H-SiC is also confirmed by the C–V measurements versus the frequency measured in 300 K and it is interpreted by both Tung’s lateral inhomogeneity model and multi-Gaussian distribution approach. The values of the weighting coefficients, standard deviations and mean barrier height are calculated for each distribution region of Au/4H-SiC using the multi-Gaussian distribution approach. In addition, the total effective area of the patches NA_e is obtained at separate temperatures and as a result, it is expressed that the low barrier regions influence meaningfully to the current transport at the junction. The homogeneous barrier height value is calculated from the correlation between the ideality factor and barrier height and it is noted that the values of standard deviation from ideality factor versus q/3kT curve are in close agreement with the values obtained from the barrier height versus q/2kT variation. As a result, it can be concluded that the temperature dependent electrical characteristics of Au/4H-SiC can be successfully commented on the basis of the thermionic emission theory with both models.

Keywords

• 4H-SiC; 
• Schottky diodes; 
• Schottky barrier anomalies
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• SOURCE:SCIENCEDIRECT
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Quality improvement of single crystal 4H SiC grown with a purified β-SiC powder source

In the processing of single crystal SiC using the PVT method, defects such as micropipes and dislocations occur due to various reasons, including growth rate, temperature gradient, seed quality, pressure change and the SiC source powder. Among these factors, the SiC source powder was investigated to reduce defects in single crystal SiC. β-SiC powder was used to reduce the growth temperature and change basic properties of the particle, including microstructure, particle size and chemical composition, through the purification process. The structure of the purified β-SiC particle was changed into a spherical structure and its particle size expanded. Chemical analysis revealed reduced free carbon, oxide phases such as silica (SiO₂), silicon oxycarbide and metallic impurities. Purified β-SiC powder showed increased particle size of 37 µm and showed improved purity. With this, we grew single crystal 4H SiC and compared the micropipe and dislocation density to that of single crystal 4H SiC grown with non-purified β-SiC powder. The experimental results confirmed that the 4H SiC wafer grown by purified β-SiC powder exhibited improved quality.

Keywords

• Purified β-SiC powder; 
• Chemical composition; 
• Single crystal 4H SiC; 
• Micropipe and dislocation density
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    • SOURCE:SCIENCEDIRECT
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Study of barrier inhomogeneities using I–V–T characteristics of Mo/4H–SiC Schottky diode

In the present work we investigate the forward current–voltage (I–V  ) characteristics, over a wide temperature range 298–498 K, of Mo/4H–SiC Schottky diode for which aluminum ion implantation was used to create the high resistivity layer forming the guard ring. The (I–V  ) analysis based on Thermionic Emission (TE) theory shows a decrease of the barrier height ϕ  _B and an increase of the ideality factor n   when the temperature decreases. These anomalies are mainly due to the barrier height inhomogeneities at the metal/semiconductor interface as we get a Gaussian distribution of the barrier heights when we plot the apparent barrier height ϕ  _ap versus q  /2kT  . The mean barrier height and the standard deviation obtained values are $\stackrel{}{}$_B0=1.160 eV and σ₀=88.049 mV, respectively.

However, by means of the modified Richardson plot ${}_{}{}^{\mathrm{}}{}^{\mathrm{}}{}_{\mathrm{}}^{\mathrm{}}\mathrm{}{}^{\mathrm{}}{}^{\mathrm{}}$ versus q  /kT  , the mean barrier height and the Richardson constant values obtained are $\stackrel{}{}$_B0=1.139 eV and A  ^*=129.425 A/cm² K², respectively. The latter value of $\stackrel{}{}$_B0 matches very well with the mean barrier height obtained from the plot ofϕ_ap versus q/2kT. The Richardson constant is much closer to the theoretical value of 146 A/cm² K².

The series resistance R_s is also estimated from the forward current–voltage characteristics of Mo/4H–SiC Schottky contact. This parameter shows strong temperature dependence. The T₀ effect is validated for the 298–498 K temperature range for the used Schottky diode and provides a clear evidence for the barrier inhomogeneity at the Mo/4H–SiC interface. Finally, we note the impact of the implantation process as well as the choice of the used ion on the characterized parameters of the Schottky contact.

Keywords

• Mo/4H–SiC Schottky diodes; 
• Metal/semiconductor interface; 
• Barrier height inhomogeniety; 
• T₀ effect;
• Aluminum ion implantation
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• SOURCE:SCIENCEDIRECT
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Fabrication and characterization of single-crystal 4H-SiC microactuators for MHz frequency operation and determination of Young’s modulus

Highlights

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Single crystal 4H-SiC MEMS actuators with MHz resonant frequency were demonstrated.

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Young’s modulus of 4H-SiC were determined.

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The device truly exploits SiC superior material properties for harsh environments.

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The high frequency is desirable for ultrafast and high resolution sensors/actuators.

Silicon carbide (SiC), a wide bandgap semiconductor, is more desirable over conventional silicon (Si) to satisfy the increasing demands for microelectromechanical system (MEMS) to operate in harsh environments due to the excellent physical, mechanical and chemical inertness. Research in MEMS devices based on single or polycrystalline SiC on Si or SOI substrate, such as 3C-SiC polytype, have been widely carried out. Another promising candidate is “all”-SiC, i.e., homoepitaxial single crystal SiC layer on single-crystal SiC substrate, such as 4H and 6H-SiC polytypes. They truly exploit the superior material properties of SiC and provide advantages for MEMS devices to operate in hostile conditions. In this work, 4H-SiC MEMS actuators in cantilever and bridge configurations were fabricated by a surface micromachining process, and their dynamic responses were characterized to determine Young’s modulus of 4H-SiC and frequency performance. Resonant frequencies of 1.208 MHz from cantilever and 1.338 MHz from bridge actuators were achieved. These high frequency operation capabilities are particular interesting for ultrafast and high resolution sensors and actuators.

Graphical abstract

Keywords

• 4H-SiC; 
• MEMS; 
• Microactuator; 
• MHz frequency; 
• Young’s modulus

Source:Sciencedirect

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Effects of surface/interface states on Schottky contacts for 4H-SiC

Electrical characteristics of sputter deposited Ni, Cr, Au, Pt, and Ti contacts on SiC-4H were studied. The 4H-SiC substrates used were test grade, single crystalline, and n-type. These contacts were found to be rectifying. Saturation current densities, ideality factors and Schottky barrier heights were determined from these measurements. Argon plasma sputtering degraded the contact by increasing the reverse current and lowering the turn-on voltage. The calculated Schottky barrier heights were ranged from /spl sim/ 0.8 eV to /spl sim/ 13 eV and reverse current was found to be as low as 136.5 nA/cm/sup 2/ at -5 V. For RCA cleaned samples, the barrier heights are weakly dependent on metal work function and were largely determined by metal induced gap states (MIGS) and electronegativity difference between metal and semiconductor. However, in RCA + Ar plasma sputter cleaned samples, the barrier heights were independent of the metal work function determined entirely by MIGS and surface/interface states.

Index Terms:
Ar, Schottky contacts, surface/interface states, sputter deposition, single crystalline, saturation current densities, ideality factors, argon plasma sputtering, reverse current, calculated Schottky barrier heights, metal work function, metal induced gap states, SiC:H, Ni, Cr, Au, Pt, Ti

Citation:
Md.M. Islam, K. Das, "Effects of surface/interface states on Schottky contacts for 4H-SiC," ssst, pp.378-382, Proceedings of the Thirty-Seventh Southeastern Symposium on System Theory.

source:IEEE

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Neutron diffraction measurement of the internal stresses following heat treatment of a plastically deformed Al/SiC particulate metal–matrix composite

Abstract
The generation of internal misfit stresses between matrix and reinforcement has been studied in an Al/SiC particulate-reinforced metal–matrix composite. Bars of the composite were deformed in bending to introduce a varying residual stress field, and subsequently heat treated at different temperatures and for different times to study the evolution of the macroscopic stress field and the interphase stresses. The results show that the shape misfit stresses between the matrix and reinforcement, which arise from the difference in thermal expansion coefficient between the two phases, are reduced by the plastic deformation, but is re-generated by heat treatment.

Higher temperatures and longer times increase the degree to which the shape misfit stresses return to their initial values. The re-generation of the shape misfit stresses is accompanied by a reduction in the variation of the macrostress field induced by the plastic bend.

Keywords:Metal–matrix composite; Internal stress; Residual stress; Neutron diffraction

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Kinetics of heterogeneous catalytic reactions

A wide variety of techniques are usually employed in investigations of heterogeneous catalysis. These investigations typically involve one or more of the following experimental approaches: (i) synthesis and testing of catalytic materials, (ii) characterization of bulk and surface properties, (iii) evaluation of surface adsorptive properties and chemical reactivity, and (iv) assessment of catalyst performance. The recent advances in quantum chemical techniques, combined with improved computer performance, make it possible to conduct quantum chemical calculations to represent more realistic models of active sites and more complex reaction schemes. Since these experimental and theoretical investigations are conducted under different conditions and on a variety of related materials, it is useful to conduct analyses of appropriate reaction schemes to consolidate the results. An important consequence of conducting reaction kinetics analysis in conjunction with results from quantum chemical calculations and experimental studies is that quantitative knowledge about the catalytic process is built at the molecular level into the kinetic model. This process of extracting fundamental knowledge provides a molecular-level basis for comparisons between catalyst systems and provides unifying principles for the design of new catalyst systems. This review provides an introduction to methods for analyses of reaction schemes that describes the results from both experimental and theoretical investigations. Several case studies are provided to illustrate different analysis methods.

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Kinetics of heterogeneous catalytic reactions: Analysis of reaction schemes

Abbreviations
ai, thermodynamic activity of component i; Ai, preexponential factor for rate constant i; AI, chemical affinity for the step I; C°, concentration at standard state conditions; Csites, surface concentration of sites; CSTR, continuous-flow stirred tank reactor; Ei, activation energy; F, molecular flow rate of feed to reactor; Fs,i, molecular site velocity for species i in a flow reactor; Fi′′, number of gas-phase molecule colliding with a surface per unit area per unit time; Floc, fraction of gaseous local entropy; ΔG, change of Gibbs free energy; h, Plancks constant; ΔH, change of enthalpy; I1, I2, I3, moments of inertia about the principal axes; kB, Boltzmann constant; ki,for, ki, forward rate constant for reaction i; ki,rev, k−i, reverse rate constant for reaction i; k, Ki,eq, equilibrium constant for reaction i; m, mass of a molecule; NC, number of carbon atoms in surface species; Ni, number of molecules of species i in the reactor; Ng, number of gas-phase molecules; Nsat, number of adsorbed molecules at saturation; Nsite, number of catalytic sites; Ns,i, number of gaseous molecules for species i per site in a batch reactor; Pi, partial pressure of species i; PFR, plug flow reactor; Ptot, total pressure; ri, rate of the chemical reaction i or elementary step i; ri′′′, rate per unit reactor volume; , forward rate of elementary step i; Ri, rate of production of species i; R, gas constant; SA, surface area of active sites; ΔS, change of entropy; Strans,3D°,translational entropy of species with three degrees of translation freedom; Strans,2D°, translational entropy of species with two degrees of translation freedom;

Srot, rotational entropy; Svib, vibrational entropy; SR, number of catalytic sites in the reactor; XRC,i, degree of rate control for step i; V, volume; VR, volume of reactor; Zi, reversibility of step i; αH, linear variation of the adsorption heat with carbon number; φi, dimensionless sensitivity of the overall rate with respects to ki; ϱi,tot, total sensitivity with respect to ki; γ, activity coefficient; νi, vibrational frequency; ν‡, frequency factor; νij, stoichiometric coefficient for the j reactant and i elementary step; σr, rotational symmetry number; σ, sticking coefficient, symmetry number of a molecule; σi, stoichiometric coefficient of the linear combination of step i that leads to an overall stoichiometric reaction; τ, space time; , turnover frequency of reaction i; Ω, number of distinguishable configurations of a compound; °, standard state, degrees, inlet condition; ‡, activated complex.


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Structures and electron affinities of silicon hydrides Si4Hn (n=2–10)

The silicon hydrides clusters structures and electron affinities of the Si4Hn/Si4Hn− (n=2–10) species have been examined using three density functional theory (DFT) methods. The basis set used in this work is of double-ζ plus polarization quality with additional diffuse s- and p-type functions, denoted DZP++. The geometries are fully optimized with each DFT method independently. Three different types of energy separations presented in this work are the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). The most reliable EAad, obtained at the B3LYP and BPW91 levels of theory, are 1.53 or 1.54 eV (Si4H2), 2.45 or 2.47 eV (Si4H3 for Cs←C1) and 2.50 eV for C1←C1 of Si4H3, 1.73 or 1.79 eV (Si4H4), 2.43 or 2.38 eV (Si4H5), 1.61 or 1.55 eV (Si4H6), 2.20 or 2.23 eV (Si4H7), and 2.34 or 2.37 eV (Si4H9). For Si4H8 and Si4H10, there are no reliable EAad but there are reliable VDE. The values of VDE are from 1.29 eV (BPW91) to 1.63 eV (B3LYP) for Si4H8, and from 0.55 eV (B3LYP) to 0.69  eV (BPW91) for Si4H10.

Conclusion and Future look about sic4h

1.Good C‐V characteristics characteristics in SiC MOS capacitors capacitors have been demonstrated demonstratedsimply by oxidation in dryO2 at 800oC, on the basis of hermodynamic and kinetic consideration.

2.High‐k dielectric films will be applicable for SiC gate stacks by using stable
interfacial SiO2 layer.

3.SiC interface research is old but will bea hot topic.

4.Si-face is much better than C-face due to a considerably lower oxidationrate in the present method.

5.MOSFET fabrication and characterization will be the next challenge.

Fabrication and characterization of n-ZnO on p-SiC heterojunction diodes on 4H-SiC substrates

We report on the growth and characterization of n-ZnO/p-4H-SiC heterojunction diodes. Our n-ZnO layers were grown with radical-source molecular beam epitaxy (RS-MBE) on p-4H-SiC epilayers, which was previously prepared in a horizontal hot-wall reactor by chemical vapour deposition (CVD) on the n-type 4H-SiC wafers. Details on the n-ZnO growth on 8∘-off 4H-SiC wafers, the quality of the layers and the nature of realized p–n structures are discussed. Mesa diode structures were fabricated. Al was sputtered through a circle mask with diameter 1 mm and annealed to form Ohmic contacts to p-SiC. Ohmic contacts to the n-ZnO were formed by 30 nm/300 nm Ti/Au sputtered by electron beam evaporation. Electrical properties of the structures obtained have been studied with Hall measurements, and current–voltage measurements (I–V).I–V measurements of the device showed good rectifying behavior, from which a turn-on voltage of about 2 V was obtained.

Source: Superlattices and Microstructures

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Progress in the use of 4H-SiC semi-insulating wafers for microwave power MESFETs

The use of Silicon Carbide semi-insulating wafers has to be mastered in order to reach output power densities up to 3–4 W mm⁻¹ in the 1–2 GHz frequency range (values that are currently targeted). This study shows that the buffer layer doping level and thickness have a great influence on the MESFET behavior. We have studied three epilayer configuration on SI substrates that differ only by the buffer layer. On two of them, a slow drain current decrease in d.c. mode was observed. On the third one, no d.c. current drift was observed without rf input power, but drain current decreases instantaneously when rf input power is switched on. Traps, located either in the buffer layer or in the substrate are supposed to be responsible for these drift phenomena. Load–pull measurements were performed at 2 GHz on transistors fabricated on the three different structures. One of them, with a 2 mm gate periphery, has been measured under 72 V drain–source bias voltage and 2.1 W mm⁻¹ power density was obtained at 2 GHz. We believe these results are the first to be published on a SiC MESFET with d.c. bias voltage over 70 V.

Source: Materials Science and Engineering: B

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Electrical activation of implanted phosphorus ions in [0001]- and [11–20]-oriented 4H-SiC

Aluminum-doped 4H-SiC epilayers with [0001]- or [11–20]-oriented faces were implanted with phosphorus and subsequently annealed in a temperature range of 1550–1700 °C. The electrical activation of phosphorus ions was studied by Hall effect investigations. Identical free electron concentrations are observed at high temperatures in both types of SiC samples indicating that the electrical activation of implanted phosphorus ions is independent of the orientation of the wafers. The compensation generated by the phosphorus implantation is greater in 4H-SiC samples with (0001) face. Phosphorus donor concentrations above 1020 cm-3 could be activated and an extremely low sheet resistance of 29 Ω/◻ was determined in the implanted 4H-SiC layer. 

Source:IEEE

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Single-crystalline 4H-SiC micro cantilevers with a high quality factor

Single-crystalline 4H-SiC micro cantilevers were fabricated by doping-type selective electrochemical etching of 4H-SiC. Using this method, n-type 4H-SiC cantilevers were fabricated on a p-type 4H-SiC substrate, and resonance characteristics of the fabricated 4H-SiC cantilevers were investigated under a vacuum condition. The resonant frequencies agreed very well with the results of numerical simulations. The maximum quality factor in first-mode resonance of the 4H-SiC cantilevers was 230,000. This is 10 times higher than the quality factor of conventional 3C-SiC cantilevers fabricated on an Si substrate.

Highlights

• We fabricated single-crystalline 4H-SiC cantilevers by electrochemical etching.

• Very sharp resonance was observed for the fabricated 4H-SiC cantilevers.

• The maximum quality factor of 4H-SiC cantilevers is 230,000.

• The quality factor is 10 times higher than that of 3C-SiC cantilevers.

Source: Sensors and Actuators A: Physical

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The growth of low resistivity, heavily Al-doped 4H–SiC thick epilayers by hot-wall chemical vapor deposition

Heavily Al-doped 4H–SiC thick epilayers (∼90 μm) were grown on 3-in n⁺ 4H–SiC wafers by using the hot-wall CVD method. On the purpose of enhancing incorporated Al-dopant concentration, the growth condition dependence of the Al incorporation behavior in the heavy doping range near Al solubility limit in 4H–SiC was investigated by varying the growth parameters, i.e., growth rate, pressure, temperature and Al-dopant source flow rate. A series of thick epilayers possessing Al-dopant concentration from 9.6×10¹⁹ to 4.7×10²⁰ cm⁻³ were obtained. Among them, the epilayer with Al-dopant concentration of 3.5×10²⁰ cm⁻³ demonstrates a comparably low resistivity of 16.5 mΩ cm as that of commercial n⁺ 4H–SiC wafer. The incorporated Al-dopant concentration dependences on surface morphology, crystalline quality and crystal structures of the heavily Al-doped thick epilayers on n⁺ 4H–SiC substrates were characterized and discussed.

Highlights

• We report the growth of 90 μm thick, heavily Al-doped 4H–SiC epilayers by HW-CVD.

• A relatively high growth rate of about 22 μm/h is verified.

• The resistivity of 16.5 mΩ cm and Al concentration of 3.5×10²⁰ cm⁻³ are achieved.

• The morphology, resistivity, quality and lattice constant change are reported.

Source: Journal of Crystal Growth

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Analysis of Anomalous Charge-Pumping Characteristics on 4H-SiC MOSFETs

Anomalous charge-pumping characteristics of 4H-silicon carbide (SiC) MOSFETs were analyzed. Charge-pumping measurements of n- and p-channel 4H-SiC MOSFETs with and without NO annealing were performed. Measurements using various pulse fall times revealed that the geometric component exists in the n-channel 4H-SiC MOSFETs and is particularly large in the unannealed n-channel 4H-SiC MOSFETs with low channel mobility. In addition, influence of interface states on the charge-pumping curves is significant in the unannealed 4H-SiC MOSFETs. The charge-pumping curves are distorted by these two nonideal effects, making the analysis of the charge-pumping curves difficult. A sufficiently long pulse fall time, which is on the order of 1-10 mus for the n-channel 4H-SiC MOSFETs with a 10-mum gate length, is required to minimize the effect of the geometric component.

Source:IEEE

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Demonstration of a 140 A, 800 V, fast recover 4H-SiC P-i-N/Schottky barrier (MPS) diode

DC and transient test results of high power, fast recovery 4H-SiC MPS diodes using multi-step junction termination (MJTE) designs are presented. The MJTE design allows full utilization of the superior breakdown properties of SiC. 4H-SiC MPS diode DC properties were studied and the transient properties were obtained by using an inductively-loaded half-bridge inverter circuit at high current and high temperatures (high-T). Results show that the replacement of Si freewheeling diodes by SiC diodes results in far less storage charge in the diodes and substantial reduction in diode turn-off energy loss, especially at high-T.

Source:IEEE

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20 kV, 2 cm2, 4H-SiC gate turn-off thyristors for advanced pulsed power applications

The development of high-voltage power devices based on wide bandgap semiconductor such as silicon carbide (SiC) has attracted great attention due to its superior material properties over silicon for high-temperature applications. Among the high-voltage SiC power devices, the 4H-SiC gate turn-off thyristor (GTO) offers excellent current handling, very high voltage blocking, and fast turn-off capabilities. The 4H-SiC GTO also exhibits lower forward voltage drop than the IGBT-based switches, resulting in lower losses during normal operation. It is an ideal switch for pulsed power applications that require high turn-on di/dt. In order to achieve a blocking capability of or greater than 20 kV in SiC, a thick drift epi-layer (> 160 μm) with an improved carrier lifetime (5 ~ 10 μs) is necessary to obtain a full conductivity modulation. In this paper, for the first time to our knowledge, we report our recently developed 1×2 cm², 20 kV, 4H-SiC p-GTO using a 160 μm, 2×10¹⁴/cm³ doped, p-type drift layer. The active conducting area of the device is 0.53 cm². Due to the limitations of the high-voltage test set-up, the 4H-SiC p-GTO showed an on-wafer gate-to-anode blocking voltage of 19.9 kV at a leakage current of 1 μA, which corresponds to a one-dimensional (1D) maximum electrical field of ~ 1.5 MV/cm at room-temperature. To measure this large area, 4H-SiC, p-GTO at high current levels (> 100 A/cm²), the forward characteristics of the device were evaluated using a Tektronix 371 curve tracer in pulse mode. A differential specific on-resistance of 11 MΩ-cm² was obtained at a gate current of 0.35 A and a high current of 300 A/cm2 ~ 400 A/cm². More results and discussion will be presented at the conference.

Source:IEEE

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MOS interface properties and MOSFET performance on 4H-SiC{0001} and (11-20) processed by N2O oxidation

Silicon carbide (SiC) has attracted increasing attention as a promising wide bandgap semiconductor projected to high-power and high-temperature electronics. Although SiC MOSFETs are recognized as ideal power switches, SiC MOSFETs have still suffered from low effective channel mobility. In recent years, post-oxidation nitridation in an NO ambience is widely used to improve SiO₂/4H-SiC(0001) interface properties and thereby to increase effective channel mobility of MOSFETs as presented in S. Dimitrijev et al. (1997) and G. Y. Chung et al. (2001). Direct oxidation with N₂O has been also proposed as an alternative to form the "nitrided" MOS interface for the safety reason according to L. A. Lipkin et al. (2002). In this study, the interface state density and MOSFET performance have been investigated on4H-SiC(0001), (000-1), and (11-20) stated in H. Yano et al. (1999) by using N₂O oxidation. Effects of doping concentration in the p-body on MOSFET performance are discussed.

Source:IEEE

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Characterization of 4H-SiC Epitaxial Layers and High-Resistivity Bulk Crystals for Radiation Detectors

Defect and electrical characterization of bulk semi-insulating (SI) 4H-SiC crystals and SI and n-type 4H-SiC epitaxial layers grown by chemical vapor deposition (CVD) on highly doped (0001) 4H-SiCsubstrates is reported. Optical microscopy, electron beam induced current (EBIC) imaging, current-voltage ($I$–$V$) measurements, thermally stimulated current (TSC) spectroscopy (94 K–620 K), Hall effect, and van der Pauw measurements have been conducted for characterization and defect correlation studies. Both epitaxial layers exhibited relatively shallow levels related to Al, B, $L$ - and D-centers. Deep level centers in the n-type epitaxial layer peaked at ${sim} 400$ K ($E_{a} sim 1.1$ eV), and ${sim} 470$ K were correlated with $IL_{2}$ defect and 1.1 eV center in high-purity bulk SI 4H-SiC. The SI epitaxial layer exhibited peak at ${sim} 290$ K ( $E_{a} = 0.82hbox{--}0.87$ eV) that was attributed to $IL_{1}$ and HK2 centers, and at ${sim} 525$ K that was related to intrinsic defects and their complexes with energy levels close to the middle of the band-gap. Results of EBIC and optical microscopy showed segregation of threading dislocations around comet tail defects in the n-type epitaxial layer. The $I$– $V$ characteristics of the devices on SI epitaxial layer exhibited steps corresponding to the ultimate trap filling of deep centers. The high-temperature resistivity measurements of bulk SI 4H-SiC sample revealed resistivity hysteresis that was attributed to the filling of the deep-level electron trap centers. The responsivity of the n-type epitaxial 4H-SiC detector in the soft X-ray energy range is reported for the first time.

Source:IEEE

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Defect correlation studies on 4H-SiC crystals and epitaxial layers for radiation detector applications

Nuclear radiation detectors in the energy range of soft x-rays have been fabricated using bulk semi-insulating (SI) 4H-SiC crystals and SI and n-type 4H-SiC epitaxial layers grown by chemical vapor deposition (CVD) on highly doped (0001) 4H-SiC substrates. The devices have been characterized by optical microscopy, current-voltage (I-V) measurements, thermally stimulated current (TSC) spectroscopy (94K - 650 K), Hall effect, van der Pauw measurements, and electron beam induced current (EBIC) technique. Both epitaxial layers exhibited relatively shallow levels related to Al, B, L- and D- centers. Deep level centers in the n-type epitaxial layer peaked at ~ 400 K (E_a ~ 1.1 eV) and ~ 470 K were correlated with IL2 defect and 1.1 eV center in high purity bulk SI 4H-SiC. The SI epitaxial layer exhibited peak at ~ 290 K (E_a = 0.82 - 0.87 eV) that was attributed to IL1 and HK2 centers, and at ~ 525 K that was related to intrinsic defects and their complexes with energy levels close to the middle of the band gap. Results of EBIC and optical microscopy characterization showed segregation of threading dislocations around comet tail defects in the n-type epitaxial layers. The I-V characteristics of the devices on SI epitaxial layers obtained in wide temperature range (94K - 650 K) exhibited steps at ~ 1 V and ~ 70 V corresponding to the ultimate trap filling of deep centers peaked at >; 500 K and at ~ 250 K (E_a ~ 0.57 eV), & ~ 300 K (E_a ~ 0.85 eV) respectively. The high temperature resistivity measurements of bulk SI 4H-SiC sample revealed resistivity hysteresis that was attributed to the filling of the deep level electron trap centers. The responsivity of the n-type epitaxial SiC sensors to low energy x-rays is reported for the first time.

Source:IEEE

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Performance of thin 4H-SiC UV avalanche photodiodes

The large bandgap of 4H-SiC (3.25 eV) makes it a suitable material for visible-blind UV detection. In the paper, the performance of 4H-SiC avalanche photodiodes (APDs) with a thin avalanche width of 0.1 μm is evaluated. Avalanche photodiodes with thin multiplication regions can greatly improve the signal-to-noise ratio of photoreceiver systems by providing internal gain while maintaining a high operating speed and low operating voltage. The diodes exhibit a peak unity-gain responsivity of 144 mA/W at a wavelength of 265 nm. Photomultiplication measurements carried out on these diodes showed that β>α in 4H-SiC, where β and α are the hole and electron ionisation coefficients, respectively. The 4H-SiCAPDs also exhibit very low excess noise corresponding to k=0.1 (where k=α/β for hole multiplication) in the local model when illuminated by 325 nm light. This is much lower than that of commonly used Si APDs with identical thickness and indicates that 4H-SiC is well suited for high gain, low noise UV detection. In view of the large β/α ratio measured in these thin 4H-SiC APDs, multiplication must be initiated by hole injection to ensure a low excess-noise performance.

Source:IEEE

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Optical assessment of purity improvement effects in bulk 6H and 4H-SiC wafers grown by physical vapor transport

We report on purity improvement effects which recently have been observed when comparing different series of wafers produced in two separate, but basically similar, home-made physical vapor transport (PVT) reactors. Looking in detail for the origin of this phenomenon, we have found a strong influence of the residual purity of the graphite material used to manufacture the crucibles. After proper optimization, a second effect has been found. It manifests when the residual level of impurities in the seed material is high and provides evidence for in situ auto-doping. Finally, quantitative analyses of C(V) characteristics and Raman spectra have been done. In this way we follow the trend in residual carrier concentration and mobility.

Source: Materials Science and Engineering: B

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Wafer warpage, crystal bending and interface properties of 4H-SiC epi-wafers

The relationship between the warpage of 4H-SiC CVD grown epi-wafers with crystal bending and substrate properties is investigated. The wafer surface preparation before and after epitaxy is found to affect both long range properties such as the wafer flatness and to some extent local properties such as the epi-substrate interface. Structural characterisation is carried out using X-ray diffraction techniques and KOH etching.

Source: Diamond and Related Materials

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Schottky barrier detectors on 4H-SiC n-type epitaxial layer for alpha particles

Schottky barrier detectors have been fabricated on 50 μm n-type 4H-SiC epitaxial layers grown on 360 μm SiC substrates by depositing ∼10 nm nickel contact. Current–voltage (I–V) and capacitance–voltage (C–V) measurements were carried out to investigate the Schottky barrier properties. The detectors were evaluated for alpha particle detection using a ²⁴¹Am alpha source. An energy resolution of ∼2.7% was obtained with a reverse bias of 100 V for 5.48 MeV alpha particles. The measured charge collection efficiency (CCE) was seen to vary as a function of bias voltage following a minority carrier diffusion model. Using this model, a diffusion length of∼3.5 μm for holes was numerically calculated from the CCE vs. bias voltage plot. Rise-time measurements of digitally recorded charge pulses for the 5.48 MeV alpha particles showed a presence of two sets of events having different rise-times at a higher bias of 200 V. A biparametric correlation scheme was successfully implemented for the first time to visualize the correlated pulse-height distribution of the events with different rise-times. Using the rise-time measurements and the biparametric plots, the observed variation of energy resolution with applied bias was explained.

Source:sciencedirect

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Ab initio theoretical and photoemission studies on formation of 4H-SiC(0 0 0 1)/SiO2 interface

Formation mechanisms of initial 4H-SiC (0 0 0 1)/SiO₂ interface were analyzed by density functional theory (DFT) and angle-dependent X-ray photoelectron spectroscopy (ADXPS). Through the theoretical model calculations, either C or O interstitial is likely to exist in the oxidation process of 4H-SiC. Besides, there is one suboxide theoretically more easily to form and more stable than any others. The results of the ADXPS experiment revealed only one suboxide with shift of +0.94 eV relative to the 4H-SiC bulk component rather than three ones, which verified the theoretical results. These calculation and experimental results demonstrated there is only one rather than three silicon suboxides that induced the high density of states in the 4H-SiC/SiO₂ interface. Besides, we did some speculations about the formation mechanism of the initial 4H-SiC/SiO₂ interface according to the theoretical and experimental results.

Highlights

• Either C or O interstitial is likely to exist in the oxidation process of 4H-SiC.

• One suboxide is more easily to form and more stable than any others.

• The experimental results of ADXPS support our theoretical calculations.

• We do speculations about the formation mechanism of initial 4H-SiC/SiO₂ interface.

Source: Applied Surface Science

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Effect of graphene/4H-SiC(0 0 0 1) interface on electrostatic properties in graphene

Electrostatic properties, quantum capacitance (Cq) and local density of states (LDOS) are evaluated for graphene on 4H-SiC(0 0 0 1) by measuring the local capacitance with Scanning Capacitance Spectroscopy (SCS). Two distinct samples were used for comparative study, viz., graphene exfoliated and deposited on 4H-SiC(0 0 0 1)—DG, and graphene grown epitaxially on 4H-SiC(0 0 0 1)—EG. We observed a distinctly lower screening length (rscr) and Cq while wider variations in the LDOS for EG. Such differences are attributed to the peculiar interface between EG/4H-SiC(0 0 0 1), which is known to be more or less defective having the presence of positive charges.

Research highlights

► Local electrostatic properties are evaluated for graphene on 4H-SiC(0 0 0 1).

► Lower screening length andCq are observed for epitaxial graphene on 4H-SiC(0 0 0 1).

► Such differences are attributed to the peculiar interface between EG/4H-SiC(0 0 0 1).

► The results are compared with graphene deposited on 4H-SiC(0 0 0 1).

Source: Physica E: Low-dimensional Systems and Nanostructures

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High resolution alpha particle detection using 4H–SiC epitaxial layers: Fabrication, characterization, and noise analysis

In this article we report the fabrication and characterization of large area, room-temperature operable and very high resolution Schottky barrier detectors for alpha particles using 20 μm thick n-type 4H–SiC epitaxial layers. Schottky barriers were fabricated by depositing circular nickel contacts of ~11 mm² area on the 4H–SiC epitaxial layers. Room temperature current–voltage measurements revealed very high Schottky barrier height of 1.6 eV and extremely low leakage current of 3.5 pA at an operating reverse bias of −90 V. We also report an energy resolution of 0.29%, which is the best resolution obtained so far for uncollimated 5.48 MeV alpha particles in 4H–SiC epitaxial detectors with such a large area. Very low micropipe density (<1 cm⁻²) and low effective doping concentration (2.4×10¹⁴ cm⁻³) in the epilayer helped to achieve a high resolution even with the large detector area and a broad source. A diffusion length of ~18.6 μm for holes has been determined in these detectors following a calculation based on a drift-diffusion model. A noise analysis in terms of equivalent noise charge revealed that the white series noise due to the detector capacitance has substantial effect on their spectroscopic performance.

Source:sciencedirect

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