Stanislav I. Soloviev

Time-Dependent Dielectric Breakdown of 4H-SiC MOS Capacitors and DMOSFETs

Time-dependent dielectric breakdown measurements were performed at 200 degC on 4H-SiC MOS capacitors and vertical DMOSFETs with 50-nm-thick nitrided oxides in order to better understand the physical mechanisms of failure and to predict the component reliability. Oxide breakdown locations are shown to have no correlation to defects in the SiC epitaxial layer. Characterization of the electric-field acceleration of failures indicates that failure modes differ at low and high electric fields. Specifically, extrapolations from measurements at electric fields greater than 8.5 MV/cm predict anomalously high reliability at normal operating fields. Thus, we have shown that SiC MOS reliability characterization must ensure that electric field stresses be performed at low electric fields in order to accurately predict failure times.

Impact Ionization Coefficients in 4H-SiC

Photomultiplication measurements using 244- and 325-nm excitation have been undertaken on a series of thick 4H-SiC avalanche diodes. With avalanche widths of between 2.7 and 6 mum and the ability to measure multiplication as low as 1.001, a much wider electric field range has been covered than reported to date. The results show that the hole ionization coefficient (beta) can be obtained with a high degree of accuracy down to electric fields as low as ~0.9 MV/cm. The value of electron ionization coefficient (alpha) has been determined from mixed carrier multiplication characteristics, and the beta/alpha ratio is found to increase significantly with decreasing electric fields. Ionization coefficients are parameterized over the electric field range from 0.9 to 5 MV/cm, enabling the multiplication and breakdown characteristics of 4H-SiC to be predicted accurately.

Solar-Blind 4H-SiC Single-Photon Avalanche Diode Operating in Geiger Mode

A solar blind 4H-SiC single photon avalanche diode (SPAD) with a sharp cutoff at a wavelength of 280 nm is reported. The SPAD with separate absorption and multiplication layers was designed for operation in Geiger mode. A thin film optical filter deposited on a sapphire window of the device package provided sensitivity in the wavelength range between 240 and 280 nm with a very high solar photon rejection ratio. An estimated dark current of 0.4 pA (0.75 nA/cm2) at a gain of 1000 was measured on a device with an effective mesa diameter of 260 mum. A single photon detection efficiency of 9.4% and a dark count probability of 4 times 10-4 were demonstrated at a wavelength of 266 nm for the same device.

Temperature Dependent Characteristics of Nonreach-Through 4H-SiC Separate Absorption and Multiplication APDs for UV Detection

Silicon carbide (SiC) separate absorption multiplication region avalanche photodiodes (SAM-APDs) for UV detection in harsh environment applications were designed and fabricated. The devices were intentionally designed to operate under nonreach-through conditions in order to eliminate field-induced leakage current. The gain of 2500 and quantum efficiency of ~45% at room temperature were achieved at the wavelength of 290-300 nm for a packaged device with an active area of 1 x 1 mm2. The temperature dependency of the current-voltage characteristics and responsivity was examined in the temperature range from room temperature to 230degC.

Solar-Blind 4H-SiC Avalanche Photodiodes

In this work, solar-blind UV 4H-SiC avalanche photodetectors were fabricated and tested in linear and Geiger modes. APDs with both PIN and separate absorption and multiplication (SAM) structures were investigated. PIN structures demonstrated higher quantum efficiencies while the SAM structure exhibit lower leakage currents. Deposition of a thin film optical filter on top of the devices was used to provide a high photon rejection ratio of (Stas add value here). However, an external filter showed a better photon rejection ratio compared to the deposited one by about one order of magnitude.

Silicon carbide photomultipliers and avalanche photodiode arrays for ultraviolet and solar-blind light detection

Silicon carbide is known for its large bandgap and suitability to make highly sensitive ultraviolet photo-detectors. These devices show appreciable quantum efficiencies in the 240 nm – 350 nm wavelength range in combination with low dark currents. We present recent results on 4H-SiC avalanche photodiode arrays and SiC-based solid-state photomultiplier arrays suitable for ultraviolet and solar-blind light detection. A novel SiC-based photomultiplier array was demonstrated. Additional solar-blind optical filter allowed achieving a solar photon rejection ratio of more than 106 in combination with 40% quantum efficiency at 280 nm. More than 50% of pixels of the array have demonstrated low dark count rates in the range of several kHz and single photon detection efficiencies of more than 30% at 266 nm in the solar-blind wavelengths range. The photomultiplier array operating in Geiger mode has demonstrated a linearly increasing response with an increase in number of incident photons. We report on the electrical and optical characteristics of solar-blind 4H-SiC avalanche photodiode arrays and photomultipliers.

Influence of Defects in 4H-SiC Avalanche Photodiodes on Geiger-Mode Dark Count Probability

4H-SiC single photon avalanche diodes are reported. A separate absorption and multiplication non-reach through device structure was optimized for operation in Geiger mode. An estimated dark current at a gain of 1000 was ranging between 0.4 pA (0.75 nA/cm2) and 20nA (38 A/cm2) on devices with an effective mesa diameter of 260 m. The electron beam induced current technique was used to image defects in the active region of studied devices. Increased reverse bias leakage current and increased Geiger mode dark count probability were correlated with the presence of large number of defects. Single photon detection efficiencies of up to 11% were measured at a wavelength of 266 nm in Geiger mode.

Temperature Dependence of Hole Impact Ionization Coefficient in 4H-SiC Photodiodes

Hole initiated multiplication characteristics of 4H-SiC Separate Absorption and Multiplication Avalanche Photodiodes (SAM-APDs) with a n- multiplication layer of 2.7 µm were obtained using 325nm excitation at temperatures ranging from 300 to 450K. The breakdown voltages increased by 200mV/K over the investigated temperature range, which indicates a positive temperature coefficient. Local ionization coefficients, including the extracted temperature dependencies, were derived in the form of the Chynoweth expression and were used to predict the hole multiplication characteristics at different temperatures. Good agreement was obtained between the measured and the modeled multiplication using these ionization coefficients. The impact ionization coefficients decreased with increasing temperature, corresponding to an increase in breakdown voltage. This result agrees well with the multiplication characteristics and can be attributed to phonon scattering enhanced carrier cooling which has suppressed the ionization process at high temperatures. Hence, a much higher electric field is required to achieve the same ionization rates.

Comparison of 4H-SiC Separate Absorption and Multiplication Region Avalanche Photodiodes Structures for UV Detection

We designed and fabricated silicon carbide (SiC) separate absorption multiplication region avalanche photodiodes (SAM-APDs) for UV detection in harsh environment applications. Two variations of device types were compared. Type I was designed to achieve reach-through (i.e. multiplication, charge, and absorption layers are depleted) prior to reaching high gain while Type II was designed not to achieve reach-through (i.e. only the multiplication region is depleted). It was found that both the dark current behavior and the responsivity were improved significantly by employing a nonreach-through design. According to preliminary measurements, the maximum quantum efficiency of the type II was ~70 % at the wavelength of 300 nm.

Impact Ionization Coefficients in 4H-SiC by Ultralow Excess Noise Measurement

Photomultiplication and excess noise measurements have been undertaken on two 4H-SiC avalanche photodiodes (APDs) using 244-nm light and 325-nm light. The structures are APDs with separate absorption and multiplication regions having multiplication regions of 2.74 and 0.58 μm , respectively. Pure injection conditions in the thicker device permit the measurement of pure-hole-initiated photomultiplication and an excess noise factor. Ionization coefficients for both carrier types have been extracted from these data using a local model. The use of the excess noise factor to infer the value of the less readily ionizing coefficient α from pure hole injection measurements is more robust than direct extraction from mixed injection measurements. This is because mixed injection introduces uncertainty in the generation profile. We report a significant reduction of the electron ionization coefficient α at low fields. The more readily ionizing hole coefficient β remains very similar to prior work.