Kok-Keong Lew

Recombination processes controlling the carrier lifetime in n−4H–SiC epilayers with low Z1/2 concentrations

The dominant recombination processes controlling the carrier lifetime in n-type 4H–SiC epitaxial layers grown with low concentrations of the Z1/2 defect (the dominant bulk lifetime killer), where Z1/2 no longer determines the lifetime, have been investigated by studying the variation in the carrier lifetime with temperature. The temperature dependent lifetimes were obtained primarily by low-injection photoluminescence decay for several low-Z1/2 epilayers over a wide temperature range. The results were fitted to simulations of the temperature dependent recombination rate, where bulk, surface and interface recombination was considered. No significant contribution from other bulk defects was observed, and upper limits to the bulk recombination rate were found to be consistent with the low Z1/2 concentrations measured in these materials. There was also no significant contribution from carrier capture at the epilayer/substrate interface, which is consistent with behavior expected at low injection for low-doped...

Effect of threading screw and edge dislocations on transport properties of 4H–SiC homoepitaxial layers

Local recombination properties of threading screw and edge dislocations in 4H–SiC epitaxial layers have been studied using electron beam induced current (EBIC). The minority carrier diffusion length in the vicinity of dislocations was found to vary with dislocation type. Screw dislocations had a more pronounced impact on diffusion length than the edge dislocations, evidencing stronger recombination activity. Temperature dependence of EBIC contrast of dislocations suggests that their recombination activity is controlled by deep energy levels in the vicinity of dislocation cores. This paper shows that the type of dislocation (screw or edge) can be identified from analysis of EBIC contrast.

4H-SiC Visible-Blind Single-Photon Avalanche Diode for Ultraviolet Detection at 280 and 350 nm

This paper reports on a 4H-SiC single-photon avalanche diode (SPAD) operating at UV wavelengths of 280 and 350 nm. The SPAD shows low dark currents of 20 and 57 fA at 80 V and 90% breakdown voltage, respectively. The quantum efficiency (QE) reaches its peak of 43% at 270 nm and is < 0.007% at 400 nm, indicating a high UV-to-visible rejection ratio of > 6100. The 4H-SiC SPAD shows a fast self-quenching and a high photon count rate of 1.44 MHz in the passive-quenching mode. At the wavelength of 280 nm, a single-photon detection efficiency (SPDE) of 2.83 % with a low dark count rate of 22 kHz is achieved at the reverse bias of 116.8 V. The SPDE at 350 nm is lower, which is 0.195%, owing to the correspondingly smaller QE. Optimization measurements were conducted on SPDE as a function of voltage bias and signal output threshold.

Nanocrystalline diamond films as UV-semitransparent Schottky contacts to 4H-SiC

A heterojunction between thin films of nanocrystalline diamond (NCD) and 4H-SiC has been developed. Undoped and B-doped NCDs were deposited on both n− and p− SiC epilayers. I-V measurements on p+ NCD∕n− SiC indicated Schottky rectifying behavior with a turn-on voltage of around 0.2V. The current increased over eight orders of magnitude with an ideality factor of 1.17 at 30°C. Ideal energy-band diagrams suggested a possible conduction mechanism for electron transport from the SiC conduction band to either the valence band or acceptor level of the NCD film. Applications as an UV semitransparent electrical contact to 4H-SiC are discussed.

On the high curvature coefficient rectifying behavior of nanocrystalline diamond heterojunctions to 4H-SiC

Heterojunctions of p+ B-doped nanocrystalline diamond (NCD) to n− 4H-SiC were studied by electrical and cathodoluminescence (CL) methods. Current rectification at 30 °C had a curvature coefficient γ0 of 42.1 V−1 at zero bias, γmax of 105.35 V−1 at 0.2 V, and a reverse current of <10 nA/cm2. The NCD sheet resistance decreased from 4.1×1011 to 403.56 Ω/sq. as the carrier density Ns was increased from 3.5×105 to 1.5×1016 cm−2 by B2H6 doping. The 348 cm2/V-s mobility of the B-free NCD films was comparable to that of single crystal diamond. CL data revealed traps 0.6–0.8 eV from the NCD EV edge.

High-performance 4H-SiC single photon avalanche diode operating at solar blind wavelength

A 4H-SiC SPAD with an off-mesa bonding pad operating at 280nm is presented in this paper, with a low dark count rate of 14kHz and 27kHz at single photon detection efficiency of 3.3% and 4.5%, respectively. The device has a low breakdown voltage of 117V and a low dark current of 17fA, 49fA and 147fA at 50%, 90%, and 95% of breakdown voltage, respectively. The quantum efficiency is measured to be 28% (32%) at 280nm (270nm) with a UV to visible rejection ratio >1400 (>1600).

X-ray diffraction study of crystal plane distortion in silicon carbide substrates

Bulk growth of 4H-SiC is challenging due to the required high growth temperatures and gradients used in sublimation physical vapor transport that are difficult to control, particularly over large diameter boules. We used x-ray diffraction mapping to show concave crystal plane curvature in substrates from five commercial suppliers with two suppliers producing wafers with ⩽2° curvature. The extent of curvature varied little for substrates from any particular supplier. Maps of peak position and full width at half maximum from symmetric and asymmetric reflections were used to identify defective regions in the crystal. Closer examination of the rocking curves in the defective regions found a low density of low angle grain boundaries only in substrates from one supplier.

Investigation of deep levels in nitrogen doped 4H–SiC epitaxial layers grown on 4° and 8° off-axis substrates

Intentionally doped n-type 4H–SiC films were grown on 4° and 8° off-axis substrates to investigate the influence of electron concentration on the incorporation of electron traps Z1/2 and EH6/7. No discernible change was seen in the Z1/2 and EH6/7 trap concentrations for films grown on both orientations with electron concentrations in the range of 1×1014 to 1×1016 cm−3, suggesting that the Z1/2 and EH6/7 traps are not associated with isolated carbon vacancies. The defect concentrations did not correlate with the measured carrier lifetimes, which is consistent with a carrier lifetime controlled by other recombination centers. Observed decreases in lifetime were related to increases in doping levels, with similar trends seen for both orientations. Carrier lifetimes in 8° material were slightly longer than in 4° films for similar doping concentrations, most likely being associated with surface recombination and/or extended defects.

H 2 etching and epitaxial growth on 4H-SiC boule domes

Silicon carbide (SiC) is a semiconductor material intended for high-temperature and high-power switching device applications. One of the main concerns in SiC is basal plane dislocations (BPD), which cause devices to fail. Many researchers have strived to reduce these dislocations with some progress by varying pre-growth treatments and growth parameters; however, more efforts are needed to overcome the issue. Conventional 4H-SiC growth takes place on 8deg off-axis substrate, which results in smooth morphology due to step flow growth. However, it is known that growth on off-axis substrates allows many BPDs to propagate into the subsequent epilayers which comprise of the active region of the device. One way to reduce propagation of these dislocations is to grow on low off-axis material. Unfortunately as the off-cut is lowered, the morphology worsens and step bunching dominates. This study aims at reducing step-bunching on low off-axis angles (< 4deg) during both etching and epitaxial growth, while simultaneously reducing basal plane dislocations by experimenting on boule domes with various off-axis angles.

Etch rates for Si-face 4H-SiC using H 2 and a C 3 H 8 partial pressure

In an effort to calibrate nitrogen incorporation into the growing layer, a nitrogen step-doped sample was grown. In an attempt to reduce or eliminate incidental etching during pre- and post- growth temperature ramps, additional samples were etched under different propane partial pressures. With increasing propane partial pressure the etch rate decreases.