Yugang Zhou

Study of Photocurrent Properties of GaN Ultraviolet Photoconductor Grown on 6H-SiC Substrate

The properties of a photoconductive ultraviolet detector based on a GaN epilayer grown on a 6H-SiC substrate using metal-organic chemical vapor deposition were investigated. We obtained the detectable energy span of the device up to the ultraviolet region by photocurrent measurement. The spectral responsivity remained nearly constant for wavelengths ranging from 250 to 365 nm and dropped by three orders of magnitude within 15 nm of the band edge from 365 nm to 380 nm.The detector was measured to have a responsivity of 133 A/W at a wavelength of 360 nm under a 5 V bias, and the voltage-dependent responsivity was evatuated. Furthermore, a convenient method to determine the response time was developed. The relationship between response time and bias was obtained.

Investigation of the Polarization-Induced Charges in Modulation-Doped AlxGa1-xN/GaN Heterostructures through Capacitance-Voltage Profiling and Simulation.

Polarization-induced charges in modulation-doped Al0.22Ga0.78N/GaN heterostructures were investigated using the Capacitance–Voltage (C–V) method. The C–V profiling of the Pt/Al0.22Ga0.78N/GaN Schottky diodes with various Al0.22Ga0.78N thicknesses shows significant differences due to the change of the polarization field in the heterostructures. Numerical simulation based on the experimental results indicates that the sheet density of the polarization-induced charges at the heterointerface is 6.78×1012 cm-2 in the samples with the Al0.22Ga0.78N thickness of 30 nm or 45 nm. The charge density reduces to 1.30×1012 cm-2 in the sample with the Al0.22Ga0.78N thickness of 75 nm. It is thought that the reduction of the polarization-induced charges at the heterointerface is due to the partial relaxation of the Al0.22Ga0.78N layer on GaN. This work provides a technique for quantitative characterization of the polarization-induced charges in AlxGa1-xN/GaN heterostructures.

Microstructure and optical properties of GaN films on sapphire substrates

Abstract Transmission electron microscopy (TEM), double crystal X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurement were applied to study the correlation between the microstructure and material properties of the GaN films grown by light radiation heating metalorganic chemical vapor deposition (LRH-MOCVD), using GaN buffer layer on sapphire substrates. Corresponding to the density of the threading dislocation (TD) increasing approximately one order, the yellow luminescence (YL) intensity was strengthened from negligible to two orders higher than the band-edge emission intensity. The full width of half maximum (FWHM) of GaN (0002) peak of the XRD rocking curve was widened from 11 to 15 min, and in Raman spectra, the width of E 2 mode is broadened from 5 cm −1 to 7 cm −1 . A ‘zippers’ structure of GaN buffer layer was discovered by high-resolution electron microscope (HREM).

In-Situ Measurement of Junction Temperature and Light Intensity of Light Emitting Diodes With an Internal Sensor Unit

This study designed and tested an innovative light-emitting diode (LED) chip with a built-in sensor. Two electrically isolated units, the LED (for light emission) and the sensor (for monitoring junction temperature and light intensity), were integrated on a single chip. The sensor unit determines the junction temperature by measuring the forward voltage; the light output power of the LED unit can be precisely extrapolated with a polynomial function based on the photocurrent and junction temperature. This novel structure enables the in-situ real-time monitoring of the LED junction temperature and light output power, which allows a highly detailed and/or in-field LED reliability analysis and provides valuable feedback information for smart LED lighting systems.

Microstructures of GaN Buffer Layers Grown on Si(111) Using Rapic Thermal Process Low-Pressure Metalorganic Chemical Vapor Deposition

Microstructures of GaN buffer layers grown on Si (111) substrates using rapid thermal process low-pressure metalorganic chemical vapor deposition are investigated by an atomic force microscope (AFM) and a high-resolution transmission electron microscope (HRTEM). AFM images show that the islands appear in the GaN buffer layer after annealing at high temperature. Cross-sectional HRTEM micrographs of the buffer region of these samples indicate that there are bunched steps on the surface of the Si substrate and a lot of domains in GaN misorienting each other with small angles. The boundaries of those domains locate near the bunched steps, and the regions of the film on a terrace between steps have the same crystal orientation. An amorphous-like layer, about 3 nm thick, can also be observed between the GaN buffer layer and the Si substrate.

Study of LED Thermal Resistance and TIM Evaluation Using LEDs With Built-in Sensor

This study developed an LED thermal resistance and thermal interface material evaluation method using LEDs with an internal sensor unit by both steady-state and transient thermal measurements. The problem of the measurement delay time caused by the current switching, which exist in conventional methods can be avoided in both steady-state and transient thermal measurements. Slight error in junction temperature measurement caused by the self-illumination effect can be identified without the current switching issue. The error is compensated in steady-state thermal measurement and is insignificant in transient measurement. Compared with conventional methods, this method uses low-cost equipment, while realizing improved accuracy.

Characterization of Pb(Zr0.53Ti0.47)O3 Films on GaN

GaN/Pb(Zr0.53Ti0.47)O3 structures have been fabricated by light radiation heating low-pressure metal-organic chemical deposition and pulsed laser deposition. These structures show leakage current lower than 10-11 A at applied voltage of 5 V. X-ray diffraction shows that ferroelectric Pb(Zr0.53Ti0.47)O3 films directly on GaN are well crystallized with perovskite structure. Because of the high thermal stability and relatively smaller mismatch between GaN and ferroelectrics in comparison with that of Si/ferroelectric structures, GaN looks like more promising as semiconductor active layer for metal-ferroelectric-semiconductor field effect transistors.

Microstructure and physical properties of GaN films on sapphire substrates

Transmission electron microscopy (TEM), x-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurements were applied to study the correlation between the microstructure and physical properties of the GaN films grown by light radiation heating metalorganic chemical vapor deposition (LRH-MOCVD), using GaN buffer layer on sapphire substrates. When the density of the threading dislocation (TD) increases about one order of magnitude, the yellow luminescence (YL) intensity is strengthened from negligible to two orders of magnitude higher than the band edge emission intensity. The full width of half maximum (FWHM) of the GaN (0002) peak of the XRD rocking curve was widened from 11 min to 15 min, and in Raman spectra, the width of E{sub 2} mode is broadened from 5 cm{sup {minus}1} to 7 cm{sup {minus}1}. A zippers structure at the interface of GaN/sapphire was observed by high-resolution electron microscope (HREM). Furthermore the origins of TD and relationship between physical properties and microstructures combining the growth conditions are discussed.