We report in situ observation of dislocation motion in the active area of forward biased 4H SiC PIN devices by employing an optical emission microscopy technique. Low current density and temperature elevation required to trigger this motion imply that a recombination-enhanced mechanism is involved. The glide of the partial dislocation loops is restrained by defects fixed in the epilayer, which then act as nucleation sites for the bright-line/dark-triangle formation. The stress-generated features are interpreted as multiple stacking faults spreading throughout the whole base region and nucleating in the vicinity of built-in defects and process-induced structural deficiencies. The detrimental impact of the planar defects on the carrier transport properties was directly confirmed by time-resolved electroluminescence measurements. Thus, the observed degradation of the SiC device performance is a direct consequence of gradually increasing density of electric stress induced defects in the active area.

In order to optimize and improve the design of power devices with improved surge current safe operating area it is necessary to obtain a good correlation between measured and simulated space and time resolved temperature distributions. Therefore, an IR microscope capable of measuring the space and time resolved surface temperature distributions in Si power diodes operating under self-heating conditions has been developed. The minimum detectable spot size is 15 mum, while the signal rise time is detector limited to about 1 mus. The lower temperature detectivity limit is about 10 degreesC over room temperature.Using this instrument dynamic thermal phenomena in fast recovery 3.3 kV Si power diodes having radiation-induced recombination centers [Proceedings of the 7th EPE, Trondheim, 1997] subjected to 1.2 ms 400-2000 A/cm(2) and 0.3-2 ms 2000 A/cm(2) current pulses have been studied. The experimental results have been compared to results from 2D device simulations including surface recombination and carrier lifetime temperature dependence. The agreement between experimental and device simulation results (i.e. dynamic IV characteristics and time and space resolved temperature distributions) is very good up to a peak current density of 1500 A/cm(2), and a reasonable good one for peak current densities up to 2000 A/cm(2) (1.2 ins current pulses). (C) 2001 Elsevier Science Ltd. All rights reserved.

An optical emission microscopy technique with spatial and spectral resolution capabilities is applied for stability studies of 4H–SiC material properties. From the example of a 4H–SiC p+/n−/n+ diode imaged at different stages of electrical overstress the mechanism of degrading performance is directly unveiled. We correlate this phenomenon with irreversible structural changes within the active region created by a nonuniform heating related stress. The stress-generated features are interpreted as multiple stacking faults spreading throughout the whole base region and nucleated in the vicinity of built-in defects and process-induced structural deficiencies.

Studies have been conducted on the carrier recombination radiation properties across operating 4H-SiC p(+)/n-n(+) structures. Spectral and spatial distributions of the electroluminescence (EL) emitted from the cross-sectional plane of forward biased diodes were investigated by combining optical emission microscopy and imaging spectroscopy techniques. The spectral content of the luminescence across the active region was analyzed as a function of injection in a temperature range of 300-500K. At low currents deep-boron related recombination is dominating within the p-n junction region and with rising forward bias is gradually taken over by emission from the injection regions via shallow-dopant transitions. Interestingly, this band-edge EL component is found to extend deep into the substrate, apparently resulting from excess carrier diffusion and recombination. The effective diffusion length of holes was estimated from the profiles of EL penetration into the 4H-SiC substrates. The observed dissimilarity of EL penetration in different diodes is discussed considering carrier trapping and processing-induced effects.

Diodes have been manufactured of 4H SiC and investigated under static forward and reverse bias. Current-voltage characteristics have been correlated with electroluminescence and Electron Beam Induced Current (EBIC) measurements. During reverse bias structural defects give rise to early microplasma breakdown, but in most cases the reverse current density saturates after a sudden increase of several orders of magnitude. After saturation of the activated defect, the device can withstand an increased bias up to the point where planar breakdown occurs. Electroluminescence from micropipe defects is shown to consist of 6 individual microplasmas, having turn-on voltages separated by less than 0.1 V.

The physics of fast recovery 3.3 kV Si power diodes having radiation induced recombination centers operating under forward bias at large current densities and high temperatures have been studied both experimentally and by means of computer simulations.In the experimental studies the dynamic IV characteristics, the surface temperature and the surface potential distribution in the n-base have been measured, while the diodes were being subjected to single 1.3 ms half-sine-wave current pulses having a density in the range of 100 to 7200 A/cm(2). The experimental dynamic TV characteristic curves obtained are rich in features and determined by the effects that temperature and carriers concentration have on the carriers mobility and lifetime, on the Fermi-Dirac distribution function and on the energy band gap.The experimental results have been used to check the validity of the physical models implemented in the simulation package AVANT! MEDICI.. Simulations performed using the standard physical models implemented in MEDICI give an excellent agreement with measurement results up to a peak current density of 1500 Amps/cm(2), and a reasonable good one up to a peak current density of 2000 Amps/cm(2). However, the agreement between measurements and simulations is very poor at peak current densities above 2000 Amps/cm(2).

High voltage Schottky-, Junction Barrier Schottky (JBS)- and PiN-diodes with an implanted JTE termination have been fabricated on the same 4H-SiC wafer. Blocking voltages of 2.5-2.8kV were reached for JBS and PiN diodes while the Schottky diodes reach about 2.0kV. It is shown that the JBS design increases the blocking voltage effectively compared to the Schottky device with less than 10% increase in on-state static lossses. Also, a comparison of static losses to a PIN diode gives a decrease of 40% for the JBS. The leakage current is also lowered by two decades compared to the Schottky device at its blocking voltage.

Time- and spectrally resolved electron-hole (e-h) recombination radiation from forward-biased kV-class 4H-SiC p(+)n(-)n(+) structures is studied by combining an intensified gated charge couple device (ICCD) camera with backside and cross-sectional emission microscopy techniques. We demonstrate the capability of this method to visualize, spectrally analyze and map intrinsic and processing-induced structural defects in the active region of devices. Moreover, carrier diffusivity and lifetime parameters can be readily obtained by analyzing the spatial distribution and dynamics of injected carriers.

A dynamically reduced breakdown voltage from more than 2 kV under static conditions to 300 V during reverse recovery was measured for 4H-SiC p(+)nn(+) diodes. Device simulation indicates that deep ...

High-voltage 4H silicon carbide diodes with breakdown voltages above 3500V were processed and characterized by using different techniques. Measurement results of the electrical properties of p(+)-nu -n and Schottky high-voltage diodes are presented. The achieved Schottky barrier height at room temperature was estimated to be Phi (b) = 1.17 eV from TV measurements. The p(+)-nu -n diodes showed negligible leakage current densities of J(r) less than or equal to 0.5 muA cm(-2) at 1000V reverse bias. We also present a simple model to quantitatively characterize excess currents observed under low forward biases in different diode structures.

The reverse recovery failure limit was measured with an optical technique for power diodes which sustain high levels of dynamic avalanche. Measurements and simulations indicate that these diodes withstand dynamic avalanche at the pn-junction and eventually fail as a result of a strongly inhomogeneous current distribution caused by the onset of impact ionisation at the diode nn+ junction — a mechanism similar to the reverse bias second breakdown of bipolar transistors.

The first direct measurements of electrically injected carrier distributions in the base of 4H-SiC power diodes are reported, A novel approach of combining micron-resolution free-carrier absorption technique with optical imaging of e-h recombination radiation was applied to study the spatial and temporal plasma build-up, The excess carrier distributions both along and across the base region were examined during forward biasing within a 50-250 A/cm(2) range. The revealed local deviations of carrier density and lifetimes are attributed to inherent structural imperfections of the epilayer, Numerical device simulation demonstrated a good agreement with experiment and validated the importance of actual lifetime, contact, and emitter injection parameters to the overall plasma formation.

Heterojunctions on SIC is an area in rapid development, especially GaN/SiC and AlGaN/SiC heterojunctions. The heterojunction can improve the performance considerably for BJTs and FETs. In this work heterojunction diodes have been manufactured and characterized. The structure was a GaN or AlGaN n-type region on top of a 6H-SiC p-type substrate. Two different approaches of growing the a-type region were tested. The GaN was grown with the MBE technique using a polycrystalline GaN buffer, whereas the AlGaN was grown with CVD and an AIN buffer. The AIGaN had an aluminum mole fraction of around 0.1. Mesa structures were formed using Cl-2 RIE of GaN/AlGaN, which showed good selectivity on 6H-SiC (about I:6). A Ti metallization with subsequent RTA was used as contact to GaN and AlGaN, and the contact to 6H-SiC was liquid InGa. Both I-V and C-V measurements were performed on the heterojunction diode. The ideality factor of the diodes. doping concentration of the SiC, and the band alignment of the heterojunction were extracted. (C) 1999 Elsevier Science S.A. All rights reserved.

SiC has several properties that makes it more suitable than silicon for high power devices. One problem with SiC bipolar devices is the short carrier life times, and this problem becomes more severe when designing devices for high voltage applications since the dimensions are larger. This work investigates how the Shockley-Read-Hall lifetime influences the on-state characteristics of a HBT or BJT switch in 4H-SiC. The on-state characteristics were simulated with varying SRH lifetimes in the base and drift region. Comparisons were made at 100 A/cm2 collector current density, JC, and at the base current density, JB, where the total on-state power loss of the design is at minimum. The SRH lifetime in the drift region is the dominant parameter for on-state performance, whereas the SRH lifetime in the base is of much less importance. The simulations showed that to reach an acceptable JC/JB-ratio of 100 at power minimum a SRH lifetime of at least 100 µs in the drift region was needed for the HBT design. This lifetime is far from the experimental values reported for 4H-SiC. The advantages of the heterojunction in comparison to ordinary BJTs decreases with shorter SRH lifetimes, but an improvement could always be seen.

A stable dynamic avalanche at a maximum power density of about 2.4 MW/cm2 was measured in small areas of 3.3 kV Si power diodes, using an optical measurement technique, and very good dynamic ruggedness was verified in a conventional turn-off measurement. Device simulations of a diode with a shallow n+ emitter indicate that impact ionization at the nn+ junction can result in negative differential resistance (NDR) and current filamentation, whereas a deep n+ emitter in the experimentally studied diode suppresses NDR. It is, therefore, proposed that the deep n+ emitter is important for the stable dynamic avalanche.

The time-resolved imaging of electron-hole recombination radiation from a forward-biased 4H–SiC p-i-n diode is reported. A novel approach of combining gated charge couple device technique with cross-sectional emission microscopy is demonstrated as a fast and informative method for characterization of both 4H–SiC material properties and the overall power device performance. We present the capability of the technique to visualize structural defects, to characterize spatial distribution and dynamics of injected carriers, and to provide effective carrier diffusion and lifetime parameters. From the results of lateral and in-depth imaging of the light emission we conclude that at low currents the injection of holes from the p+ emission is dominant. Furthermore, an effective carrier lifetime of 350 ns in the active n− region and a diffusion length of 15 μm in the substrate are readily obtained.

It is demonstrated by numerical simulation that the anisotropic material properties of 6H-SiC can have an important effect on the forward voltage drop of 10 kV 6H-SiC power diodes. A pronounced difference in the carrier distribution was seen for substrates with the surface normal parallel or orthogonal to the c-axis. In fact, it is found that current conduction along the low-mobility c-axis in 6H-SiC (i.e. when the normal to the substrate surface is parallel to the c-axis) can lead to a lower forward voltage drop than if the main current conduction is in the high-mobility direction (orthogonal to the c-axis). Depending on the device geometry the c-axis orientation can be used in the optimization of the forward voltage drop. The effect of anisotropy is expected to be of importance also for on-state losses in other bipolar 6H-SiC devices (e.g. IGBTs). For the design of 4H-SiC bipolar power devices the effect of anisotropic conduction properties is not likely to be critical.

Measurements of the safe reverse recovery limit were performed for 3.3-kV Si power diodes using a novel optical experimental technique. In this experiment, influence of the junction termination is effectively eliminated by optical generation of a laterally-localized carrier plasma. The turn-off failures observed in measurements at two temperatures showed no temperature dependence and could not be reproduced in ordinary one-dimensional (1-D) or two-dimensional (2-D) device simulations. To simulate the stability of the current density toward current filamentation, two 1-D diodes with an area ratio 1:19 and a 10% difference in initial carrier plasma level, were simulated in parallel. This resulted in a strongly inhomogeneous current distribution, and a rapid reverse voltage fall resembling the measured turn-off failures. Inhomogeneous current distribution in these simulations appears as the current decay ceases due to impact ionization, in qualitative agreement with a current instability condition proposed by Wachutka [1].