Yongzhao Yao

Comparison of slicing-induced damage in hexagonal SiC by wire sawing with loose abrasive, wire sawing with fixed abrasive, and electric discharge machining

Crystal damage induced in hexagonal SiC by cutting was characterized by transmission electron microscopy and Raman scattering. Wiresawing with loose abrasive (WSLA) induces stacking faults (SFs), dispersive triangular crystal disordered areas, and dislocation half-loop bundles. Wiresawing with fixed abrasive (WSFA) induces SFs, crystal disordered layers, and dislocation half-loop bundles. Electric discharge machining (EDM) predominantly forms silicon, carbon, and 3C-SiC by 6H-SiC decomposition. The mechanisms inducing crystal damage by slicing were discussed on the basis of characterization results.

Molten KOH Etching with Na2O2 Additive for Dislocation Revelation in 4H-SiC Epilayers and Substrates

A novel etching solution using molten KOH with Na2O2 additive (KN etching) for dislocation revelation in 4H-SiC epilayers and substrates has been proposed. Threading screw and edge dislocations (TSDs and TEDs) have been clearly revealed as hexagonal etch pits differing in pit size, and basal plane dislocations (BPDs) as seashell-shaped pits. KN etching has provided a solution to the problem that KOH etching is not effective for dislocation identification in n+-4H-SiC. The influences of SiC off-axis angles, carrier concentrations, and growth techniques on the effectiveness of KN etching have also been investigated. It has been shown that KN etching is applicable to SiC epilayers and substrates with any off-axis angle from 0 to 8° and electron concentrations from 1015 to 1019 cm-3.

Photoluminescence and x-ray diffraction measurements of InN epifilms grown with varying In∕N ratio by plasma-assisted molecular-beam epitaxy

Photoluminescence (PL) and x-ray diffraction (XRD) have been performed to examine the In∕N ratio effect on the InN films. The PL of N-rich InN are more asymmetric, broadened, and have higher energy compared with In-rich ones. An important finding is that the N-rich and In-rich InN show a sharp difference in their PL peak energies. XRD shows that lattice constant a is affected more by the In∕N ratio rather than lattice constant c. The PL behavior of N-rich and In-rich InN films are explained in terms of two different types of native point defects that dominate the two growth regimes.

Dislocation Revelation from (0001) Carbon-face of 4H-SiC by Using Vaporized KOH at High Temperature

A novel etching technique using vaporized KOH to reveal various types of dislocations from the C-face of 4H-SiC has been proposed. Three different pit geometries have been observed, which can be attributed to three dislocation types commonly found in 4H-SiC. Pit positions on the Si-face and C-face have been compared to study the dislocation propagation behaviors across the sample thickness. Activation energy EA=49 kcal/mol has been obtained, indicating a surface-reaction-dominant process. This etching technique has provided an effective and inexpensive method of making inch-scale mapping of dislocation distribution for C-face epitaxial and bulky 4H-SiC.

Periodic supply of indium as surfactant for N-polar InN growth by plasma-assisted molecular-beam epitaxy

We have investigated the self-surfactant effect of In for N-polar InN growth by plasma-assisted molecular-beam epitaxy. We found that InN quality was significantly improved if a thin In coverage (about 1.8 ML) was introduced before InN growth. However, this In coverage was slowly consumed during subsequent InN growth under N-rich condition. Periodically restoring In coverage for thick InN growth was proposed to solve this consumption problem. We suggest that the effect of In surfactant is to terminate the surface N dangling bonds and form an In adlayer, under which an efficient diffusion channel for lateral N adatom transport is created.

A simultaneous observation of dislocations in 4H-SiC epilayer and n+-substrate by using electron beam induced current

With a new structure of Ni/n-SiC/n+-SiC/Al, we have achieved a simultaneous observation of the dislocations in n-SiC epilayer and n+-SiC substrate by electron beam induced current (EBIC). The EBIC images were compared to the results of a depth-controlled wet etching in KOH+Na2O2. It has been found that each type of dislocations has its own signature in EBIC images in terms of the darkness, shape and orientation of the dark contrast. By changing the accelerating voltage of the electron beam, we can also observe the depth dependent presence of each type of dislocations and where and how the dislocation conversion happens.

Expansion of a single Shockley stacking fault in a 4H-SiC (11 2 ¯0) epitaxial layer caused by electron beam irradiation

The expansion behavior of a single Shockley stacking fault (SSSF) originating from a basal plane dislocation in a 4H-SiC epitaxial layer on the (11 2 ¯0) a-plane under electron beam (EB) (//[11 2 ¯0]) irradiation was observed. The width of the SSSF was proportional to the EB current. EB irradiation at a fixed spot outside an SSSF can expand the SSSF as effectively as direct SSSF irradiation. It was found that the selective excitation of an SSSF and/or a Si-core partial dislocation (PD) is possible by appropriately setting the EB irradiation position because the cathodoluminescence spectrum varies with the irradiation position around an SSSF. The rate of SSSF expansion upon the indirect excitation of a Si-core PD is much larger than that upon direct SSSF excitation. However, the expansion rate under both indirect SSSF excitation and indirect Si-core PD excitation is smaller than that under indirect Si-core excitation. The C-core PD became mobile after supplying a threshold number of electron-hole pairs.

Correlation between dislocations and leakage current of p-n diodes on a free-standing GaN substrate

Dislocations that cause a reverse leakage current in vertical p-n diodes on a GaN free-standing substrate were investigated. Under a high reverse bias, dot-like leakage spots were observed using an emission microscope. Subsequent cathodoluminescence (CL) observations revealed that the leakage spots coincided with part of the CL dark spots, indicating that some types of dislocation cause reverse leakage. When etch pits were formed on the dislocations by KOH etching, three sizes of etch pits were obtained (large, medium, and small). Among these etch pits, only the medium pits coincided with leakage spots. Additionally, transmission electron microscopy observations revealed that pure screw dislocations are present under the leakage spots. The results revealed that 1c pure screw dislocations are related to the reverse leakage in vertical p-n diodes.

Cross-sectional observation of stacking faults in 4H-SiC by KOH etching on nonpolar

To evaluate the evolution of stacking faults (SFs) in 4H-SiC along the c-axis growth direction, techniques that can be used to determine the precise position of SFs and their fault types from cross-sectional nonpolar faces are urgently required. In this research, we have studied the feasibility of using cathodoluminescence (CL) imaging and face molten KOH etching to obtain information on the SF density and SF types. Particular attention has been paid to the possibility of determining the stacking sequence of SFs using their CL signatures or the geometrical properties of their KOH etch figures. Transmission electron microscopy (TEM) has been employed to clarify the atomic arrangement of SFs beneath the KOH etch figures, and a model is proposed to explain the formation of linear etch patterns during KOH etching due to the existence of SFs.