Wai-keung Patrick Fong

Molecular beam epitaxy growth of high quality p-doped SnS van der Waals epitaxy on a graphene buffer layer

We report on the systematic investigation of optoelectronic properties of tin (IV) sulfide (SnS) van der Waals epitaxies (vdWEs) grown by molecular beam epitaxy (MBE) technique. Energy band simulation using commercial CASTEP code indicates that SnS has an indirect bandgap of size 0.982 eV. Furthermore, our simulation shows that elemental Cu can be used as a p-type dopant for the material. Growth of high quality SnS thin films is accomplished by MBE technique using graphene as the buffer layer. We observed significant reduction in the rocking curve FWHM over the existing published values. Crystallite size in the range of 2–3 μm is observed which is also significantly better than the existing results. Measurement of the absorption coefficient, α, is performed using a Hitachi U-4100 Spectrophotometer system which demonstrate large values of α of the order of 104 cm−1. Sharp cutoff in the values of α, as a function of energy, is observed for the films grown using a graphene buffer layer indicating low concent...

Physical mechanisms for hot-electron degradation in GaN light-emitting diodes

We report investigations on the degradation of GaN-based light-emitting diodes due to high dc current stress by examining two types of devices with the same fabrication procedures except for the growth conditions for the InGaN quantum wells (QWs). Higher trimethylindium and triethylgallium fluxes are used for type A devices resulting in a threefold increase in the InGaN QWs growth rate compared to type B devices. Detailed structural and optoelectronic properties of the devices are investigated by transmission electron microscopy, atomic force microscopy, thermal imaging, I-V measurements, and the low-frequency noise properties of the devices as a function of the stress time, tS. The experimental data show that the QWs in type B devices are dominated by spiral growth and they have substantially higher strain nonuniformity than type A devices. The highly strained GaN/InGaN interfaces in device B are also responsible for the faster increase in the defect density due to hot-electron injection. The defects enhance the trap-assisted tunneling in the multiple quantum wells (MQWs) resulting in the development of hot spots among type B devices after high current stressing of the MQWs. This in turn leads to an increase in the defect generation rate resulting in a thermal run-away condition that ultimately resulted in the failure of the device. The data show that an increase in the growth rate in the InGaN layer led to the domination by the step flow growth mode over the spiral growth mode in the MQWs. This is the main reason for the reduction in the dislocation density in type A devices and hence their increase in device reliability.We report investigations on the degradation of GaN-based light-emitting diodes due to high dc current stress by examining two types of devices with the same fabrication procedures except for the growth conditions for the InGaN quantum wells (QWs). Higher trimethylindium and triethylgallium fluxes are used for type A devices resulting in a threefold increase in the InGaN QWs growth rate compared to type B devices. Detailed structural and optoelectronic properties of the devices are investigated by transmission electron microscopy, atomic force microscopy, thermal imaging, I-V measurements, and the low-frequency noise properties of the devices as a function of the stress time, tS. The experimental data show that the QWs in type B devices are dominated by spiral growth and they have substantially higher strain nonuniformity than type A devices. The highly strained GaN/InGaN interfaces in device B are also responsible for the faster increase in the defect density due to hot-electron injection. The defects enh...

Study of low-frequency excess noise in GaN thin films deposited by RF-MBE on intermediate-temperature buffer layers

Low-frequency excess noise was measured in a series of GaN epitaxial films deposited by RF-plasma assisted molecular beam epitaxy (MBE). The GaN epitaxial layers were grown on double buffer layers, each consisting of an intermediate-temperature buffer layer (ITBL) deposited at 690/spl deg/C and a conventional low-temperature buffer layer grown at 500/spl deg/C. The Hooge parameters for the as-grown films were found to depend on the thickness of ITBL with a minimum value of 7.34/spl times/10/sup -2/ for an optimal ITBL thickness of 800 nm. The observed improvements in the noise properties are attributed to the relaxation of residual strain within the material, leading to a corresponding reduction in crystalline defects.

Effects of rapid thermal annealing on the structural properties of GaN thin films

Effects of rapid thermal annealing (RTA) on the structural properties were investigated in undoped GaN film grown by rf-plasma-assisted molecular beam epitaxy (MBE), Detailed characterizations of the photoluminescence (PL), high-resolution X-ray diffraction and low-frequency noise were conducted on both the as-grown and annealed films, PL and X-ray diffraction measurements showed that the crystallinity of the films improved with RTA at 800/spl deg/C with significant reduction in the yellow emission. Annealing at 900/spl deg/C and 1000/spl deg/C resulted in an increase in the FWHHM of the X-ray diffraction, indicative of thermal decomposition of the materials, The results are in excellent agreement with our study of low-frequency noise, which demonstrates similar trends in the magnitudes of the Hooge parameters as a function of the annealing temperature. The temperature dependence of the voltage noise power spectra S/sub u/(f) was examined from 400 K to 80 K in the frequency range between 30 Hz and 100 kHz. At the low-frequency range the fluctuation is dominated by 1/f/sup /spl gamma// noise, and for f>1 kHz the noise is dominated by G-R noise processes. Our experimental results show that 800/spl deg/C is the optimal temperature for RTA, which results in substantial improvements in both the optical, structural and noise properties for the material,,whereas annealing at 1000/spl deg/C is found to result in significant material degradation.

Low-frequency noise in GaN thin films deposited by rf-plasma assisted molecular-beam epitaxy

We report detailed investigations of low-frequency excess noise in GaN thin-film cross-bridge structures deposited by rf-plasma assisted molecular-beam epitaxy on top of an intermediate-temperature buffer layer (ITBL) grown at 690 °C. The experimental data indicates strong dependence of the voltage noise power spectra on the thickness of the ITBL with an optimal thickness of 800 nm. A model has been presented to account for the observed noise, which stipulates that the phenomenon arises from the thermally activated trapping and detrapping of carriers. The process results in the correlated fluctuations in both the carrier number and the Coulombic scattering rate. Detailed computation shows that number fluctuation dominates in our samples. Our numerical evaluation indicates a reduction in the trap density by over an order of magnitude with the use of an ITBL in the growth of GaN thin films.

Ion channeling studies on mixed phases formed in metalorganic chemical vapor deposition grown Mg-doped GaN on Al₂O₃(0001)

Rutherford backscattering spectrometry and ion channeling were used to determine the relative quantities of wurtzite and zinc-blende phases in metalorganic chemical vapor deposition grown Mg-doped GaN(0001) on an Al2O3(0001) substrate with a GaN buffer layer. Offnormal axial channeling scans were used. High-resolution x-ray diffraction measurements also confirmed the presence of mixed phases. The in-plane orientation was found to be GaN[110]‖GaN[1120]‖Al2O3[1120]. The effects of rapid thermal annealing on the relative phase content, thickness and crystalline quality of the GaN epilayer were also studied.

Characterization of low-frequency noise in molecular beam epitaxy-grown GaN epilayers deposited on double buffer layers

We report the growth of high-mobility Si-doped GaN epilayers utilizing unique double buffer layer (DBL) structures, which consist of a thin buffer layer and a thick GaN intermediate-temperature buffer layer (ITBL). In this study, three types of DBL were investigated: (i) thin GaN low-temperature buffer layer /GaN ITBL (type I); (ii) nitridated Ga metal film/GaN ITBL (type II); and (iii) thin AlN high-temperature buffer layer /GaN ITBL (type III). Systematic measurements were conducted on the electron mobilities and the low-frequency noise over a wide range of temperatures. It is found that the electron mobilities of the GaN films are substantially improved with the use of DBLs, with the sample using type III DBL which exhibits the highest low-temperature mobility. Furthermore, the same sample also demonstrates the elimination of deep levels at 91 and 255 meV below the conduction band. This is believed to result from the relaxation of tensile stress during growth with the use of type III DBLs.

Characteristics of MBE-Grown GaN Detectors on Double Buffer Layers Under High-Power Ultraviolet Optical Irradiation

In this paper, present experimental investigations on the radiation hardness of GaN-based Schottky diode photodetectors. High-power ultraviolet (UV) radiation obtained from a Xenon lamp is used as the light source for the optical-stressing experiment. Two types of devices are being investigated. One has a double-buffer-layer structure that consists of a conventional high-temperature AlN buffer layer and an intermediate temperature buffer layer (type I), and the control device was fabricated with only a conventional AlN buffer layer (type II). Detailed current-voltage, capacitance-voltage, flicker noise, and responsivity measurements performed on the detectors show that the degradations of the devices arose from the defects present at the Schottky junctions due to the exposure of the devices to the high-power UV radiation. Both types of devices exhibit degradation in their optoelectronic properties. However, type-I devices, in general, exhibit gradual and slow degradation, whereas type-II devices exhibit catastrophic breakdowns in the device characteristics. The experimental data indicate significant improvement in the radiation hardness for type-I devices

Realization of Erythemal UV Detectors Using Ni/GaN Schottky Junctions

In this paper, we present the design, fabrication, and characterization of a novel UV photodetector with a cutoff wavelength of 300 nm without utilizing AlGaN-based junctions or multilayer optical filters. The active region of the device consists of a pair of Ni/GaN Schottky junctions connected in antiparallel configuration. Each junction, by itself, exhibits a cutoff wavelength of 360 nm-characteristic of band-to-band absorption in GaN. A polymer film, which exhibits strong absorption of photons at about 320 nm and below, is deposited on top of one of the Schottky junctions. Due to the antiparallel connection of the two junctions, the overall photocurrent is the difference between the two individual junctions. Our experimental results clearly demonstrate the photocurrent cancellation effect. Using this novel design, we have successfully pushed the cutoff wavelength of the complete device down to approximately 300 nm.

Study of GaN thin films grown on intermediate-temperature buffer layers by molecular beam epitaxy

A detailed characterisation study of GaN thin films grown by rf-plasma molecular beam epitaxy on intermediate-temperature buffer layers (ITBL) was carried out with Hall, photoluminescence (PL) and deep-level transient Fourier spectroscopy (DLTFS) techniques. The unique feature of our GaN thin films is that the GaN epitaxial layers are grown on top of a double layer that consists of an ITBL, which is grown at 690 degreesC, and a conventional low-temperature buffer layer deposited at 500 degreesC. It is observed that the electron mobility increases steadily with the thickness of the ITBL, which peaks at 377 cm(2)V(-1)S(-1) for an ITBL thickness of 800 nm. The PL also demonstrated systematic improvements with the thickness of the ITBL. The DLTFS results suggest a three-order-of-magnitude reduction in the deep level at E-c-0.40 eV in the device fabricated with the GaN films grown on an ITBL thickness of 1.25 mum in comparison with the control device without an ITBL. Our analyses indicate that the utilization of an ITBL in addition to the conventional low-temperature buffer layer leads to the relaxation of residual strain within the material, resulting in an improvement in the optoelectronic properties of the films