Lingyu Wan

Quality evaluation of homopetaxial 4H-SiC thin films by a Raman scattering study of forbidden modes

The crystal quality of a 4H-silicon carbide (4H-SiC) epitaxial layer is crucial to the development of high-performance 4H-SiC-based electronic power devices. However, the quality assessment of 4H-SiC homoepitaxial thin film is problematic because the same bulk material interferes with the probe of the epilayer. In this paper, we propose a simple and straightforward strategy to assess the quality of a homoepilayer using ultraviolet (UV) Raman spectroscopy (RS). Rather than focusing on the normally allowed modes, we shift our attention to the forbidden modes instead. We demonstrate that forbidden modes, which were usually ignored, are more sensitive to the crystalline imperfection and can be an effective quality probe. Our approach analyzes the crystal quality swiftly, without the need for the data fitting involved in the conventional method, and therefore makes the quality assessment much more efficient. The new method may also be applied to the other thin film materials.

Investigation of high indium-composition InGaN/GaN heterostructures on ZnO grown by metallic organic chemical vapor deposition

The optical properties and film quality for a series of high-In composition InGaN films grown on ZnO substrate by metal-organic chemical vapor deposition (MOCVD) are characterized by using high resolution X-ray diffraction (HRXRD), Rutherford backscattering (RBS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and Raman scattering spectroscopy (RSS). The In composition is evaluated by analyzing the RBS and PL emission spectra. The XPS measurements revealed the diffusion of Zn atoms from the substrate into InGaN films. All the analyses of experimental measurements have shown that the growth temperature played an important role in indium composition as well as of film quality. An optimum growth temperature is a necessary condition for obtaining high-quality films.

Synchrotron Radiation X-Ray Absorption Spectroscopy and Spectroscopic Ellipsometry Studies of InSb Thin Films on GaAs Grown by Metalorganic Chemical Vapor Deposition

A series of ultrathin InSb films grown on GaAs by low-pressure metalorganic chemical vapor deposition with different V/III ratios were investigated thoroughly using spectroscopic ellipsometry (SE), X-ray diffraction, and synchrotron radiation X-ray absorption spectroscopy. The results predicted that InSb films on GaAs grown under too high or too low V/III ratios are with poor quality, while those grown with proper V/III ratios of 4.20–4.78 possess the high crystalline quality. The temperature-dependent SE (20–300°C) and simulation showed smooth variations of SE spectra, optical constants (n, k, e1, and e2), and critical energy points ( , , , , and ) for InSb film when temperature increased from 20°C to 250°C, while at 300°C, large changes appeared. Our study revealed the oxidation of about two atomic layers and the formation of an indium-oxide (InO) layer of ∼5.4 nm. This indicates the high temperature limitation for the use of InSb/GaAs materials, up to 250°C.

Variation of phonon coupling factors in the photoluminescence of cadmium telluride by variable excitation power

Cadmium telluride (CdTe) thin films grown on indium antimonide (InSb) were examined by low temperature photoluminescence (PL), using an unfocused laser beam with variable excitation power over 300 times, to resolve the long standing issue of the huge variation of the resulting spectra in the deep region (1.40-1.52eV), where the expected phonon replicas alone were hardly observed. The phonon coupling strength, characterized by the Huang-Rhys parameter, or S-factor, as well as the peak width, were both found to increase with the excitation power. The puzzling coexistence of sharp peaks on top of a broad maximum was finally resolved by the awareness of beam intensity variation. Multiple phonon coupling processes can be present simultaneously, resulting in the superposition of narrow peaks on top of the broad maximum. Through the data fitting, three sets of donor-acceptor pair (DAP) recombination emissions with multiple values of Huang-Rhys parameter S can be identified.

X-ray absorption fine structure of ZnO thin film on Si and sapphire grown by MOCVD

X-ray absorption fine structure has been used to study the electronic structure, and bond length of ZnO thin films grown on sapphire and Si substrates by metalorganic chemical vapor deposition. X-ray absorption near edge structure (XANES) of O and Zn K-edge were shown, and a detailed analysis of extended x-ray absorption fine structure (EXAFS) of Zn K-edge indicates that difference substrates results in the contraction of Zn-O bond length.

Green light-emitting diodes with InGaN/GaN multiple quantum well structures: Time-resolved photoluminescence, emission dynamics and related studies

To explore the mechanism, breakthrough the current bottleneck and overcome the efficiency droop from green InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes (LEDs). The photoluminescence properties and carrier dynamics of green InGaN/GaN MQW LEDs are investigated by temperature-dependent photoluminescence (PL) from 10 K to 300 K and time-resolved PL (TRPL) measurements at 10 K in our new-built lab. With increasing temperature, a blue shift of PL behavior is attributed to band-tail states formed in local potential minima resembling In-rich clusters. The energy-dependent TRPL experiments are measured at 10 K to study the carrier dynamics in the MQWs. The results show that the PL slow decays for the low-energy side are much slower than the high-energy side. The depth of carrier localization is obtained by fitting the reduced slow decay time with the emission energy increasing. All the results indicate that the PL peak is related to localized radiative recombination.

Continuous-wave and Time-resolved Photoluminescence of GaN LED grown on amorphous SiC buffer

GaN LED grown upon amorphous SiC buffer is demonstrated. Its PL peak is red-shifted to 442 nm because of the decreased compressive strain induced by lattice mismatch when changing the SiC buffer to C-rich condition.