Papers by Author: F.K. Yam

Abstract: Heterostructures consisting of ZnO and diamond appear to have an elusive nature. A rectifying behaviour was previously observed only for heterojunctions with very lightly doped p-type diamond using residual boron gas during the chemical vapour deposition process or type IIb diamond. Other studies, however, claimed to obtain a rectifying behaviour for heterojunctions with p-type diamond with higher carrier densities between 10¹⁸ 10¹⁹ cm^-3. In this work we investigate the behaviour of n-type ZnO on heavily boron-doped p-type diamond. This heterostructure that is sensitive to UV light has been fabricated using ZnO nanorods grown on heavily boron-doped chemical vapour deposition diamond substrates. The I - V measurements show a rectifying characteristic. The threshold voltages under dark and UV conditions are 3.66 and 2.52 V, respectively. The UV illumination also results in an increased current flow. The electrical behaviour due to the UV illumination will be discussed.

Abstract: In this study, an efficient method to achieve a wide range of high-quality zinc oxide (ZnO) nanostructures through zinc powder evaporation at different temperatures is developed. ZnO nanostructures could be synthesized on n-type silicon substrates by a simple thermal evaporation technique without a catalyst at 600°C, 700°C, 800°C, and 900°C. Samples are annealed in wet oxygen and ambient argon gases. Surface morphology, crystallinity, and optical properties of the ZnO nanostructures are examined by scanning electron microscopy and transmission electron microscope measurements, X-ray diffraction, and photoluminescence measurement.

Abstract: A comparative study of the structural and electrical properties of GaN films grown by plasma-enhanced metalorganic chemical vapor deposition (PE-MOCVD) at 700°C, with and without AlN buffer layer is presented . Hydrogen plasma was used in addition to nitrogen plasma to produce GaN. The introduction of H-plasma is found to influence the properties of the films. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and Hall Effect measurements show that the sample with AlN buffer layer possesses a smoother and more homogenous morphological characteristics as well as a lower background electron and higher Hall mobility as compared to the sample without buffer layer. X-ray diffraction (XRD) reveals that hydrogenation is capable of producing the epitaxial GaN films at reduced temperatures with the full width at half maximum (FWHM) of the x-ray rocking (XRC) of GaN (0002) reflection was found to be 54.8 and 256 arcmin for samples with buffer layer and without buffer layer, respectively.

Abstract: This paper presents an analysis of the characteristics of two gallium nitride (GaN) films grown on (0001) plane sapphire substrates by low-pressure metalorganic chemical vapor deposition (MOCVD) is presented. The GaN films were characterized by a variety of methods, including scanning electron microscopy (SEM), x-ray diffraction (XRD), photoluminescence (PL), and Raman scattering. SEM micrographs revealed that different growth conditions will lead to different surface morphology of the films. XRD measurements indicated that both films were highly oriented and mono crystalline. PL spectra for both samples exhibited an intense and sharp band edge peak at 3.42 eV with full width at half maximum (FWHM) of 15 and 35 meV respectively. Raman scattering showed that the peaks of E2(high) phonon mode were observed at 568.1 and 570.1 cm-1 respectively. The different growth mode of these films were linked to the growth conditions, in which the growth mechanism could be correlated with the shift of E2(high) phonon mode in Raman scattering.

Abstract: The Current-Voltage-Temperature (I-V-T) characteristics of single layer deposition, consisting of Zr, Ti, or Cr/p-GaN Schottky diodes were determined in the temperature range 27- 100oC. Sputtering method was used for deposition of these metals on p-GaN. Analysis of the measured characteristics at room temperature allows the determination of the electrical parameters, the saturation current Io and the ideality factorη. The barrier heights and effective Richardson coefficients were determined through activation energy plot. It was found that pinning of Fermi level occurred for these metal contacts on p-GaN with the carrier concentration of 5.6x 1017 cm-3, where the Schottky barrier heights of Zr, Ti, or Cr/p-GaN are determined to be in the same range (~0.87eV).

Abstract: The barrier height of as-deposited Cr contacts was found to be ΦB= 0.87eV with the ideality of η=1.51 and remained almost unchanged after further annealing at 500 oC for 5 minutes. The barrier height of diodes were increased drastically after annealed at 600 oC where ΦB=1.01eV with η=1.69. Upon annealing at 700 oC for 5 minutes, the ΦB decreased to 0.61eV and the Cr diodes were degraded for higher temperature in this annealing duration. The ΦB remained in 0.80- 0.90eV respectively at 800 oC for 2 minutes and 900-1000 oC for 1 minute.

Abstract: Pd Schottky diode exhibited stable rectifying behavior up to 500°C for 35 minutes in sequential annealing; with the Schottky barrier heights (SBHs), ΦB (I-V) of 0.6-0.7eV with the leakage current (LC) of 20 A at -5V. With the same range of SBHs, PdSi diodes were stable up to 500°C for 5 minutes with the LC of 0.182mA at -5V. The electrical characteristics obtained in this study are also compared with those obtained for Pd and PdSi Schottky diodes on p-GaN.

Abstract: In this study, we investigated the contact characteristics of bi-layer thin films, Ag (200nm)/Ti (100nm) on Si-doped n-type Al0.27Ga0.73N film grown on sapphire substrate. The contacts were annealed at different temperatures (400°C-800°C) for 10 min in N2 ambient. The effects of cryogenic cooling immediately after heat treatment for improving ohmic behavior (I-V linearity) of the samples were reported. Specific contact resistivity, ρc, was determined using transmission line method (TLM) via current-voltage (I-V) measurements. Scanning electron microscopy (SEM) measurements were carried out on the as-deposited, annealed (N), and annealed-and-cryogenically (N+C) treated contacts where the surface morphology of each of these conditions were compared. In addition, measurement of the samples by atomic force microscopy (AFM) was taken in order to characterize the surface morphology.

Abstract: Gallium nitride (GaN) is a highly promising wide band gap semiconductor with applications in high power electronic and optoelectronic devices. Thin films of GaN are most commonly grown in the hexagonal wurtzite structure on sapphire substrates. Growth of GaN onto silicon substrates offers a very attractive opportunity to incorporate GaN devices onto silicon based integrated circuits. Although direct epitaxial growth of GaN films on Si substrates is a difficult task (mainly due to the 17% lattice mismatch present), substantial progress in the crystal quality can be achieved using a buffer layer. A full characterization of the quality of the material needs to be assessed by a combination of different techniques. In this work, a detailed characterization study of GaN thin film grown on Si(111) with AlN buffer layer by low pressure metalorganic chemical vapor deposition (LP-MOCVD) was carried out. Post deposition analysis includes scanning electron microscopy (SEM), x-ray diffraction (XRD), Hall and infrared (IR) spectroscopy techniques. The IR spectra were compared to the calculated spectra generated with a damped single harmonic oscillator model. Through this method, a complete set of reststrahlen parameters (such as ε∞, S, wTO, γ) of the GaN epilayer were obtained. Our results show that the GaN film has a single crystalline structure. Current-voltage characteristics (I-V) of this GaN/Si heterojunction were measured at room temperature. Rectification behavior was observed for this anisotype heterojunction. The electrical characteristics of Ni Schottky barriers on this unintentionally doped n-type film were also investigated. The barrier height of Ni/GaN Schottky barriers has been determined to be 0.93 eV by I-V measurement.

Abstract: GaN-based materials have been the subject of intensive research recently for blue and ultraviolet light emission and high temperature/high power electronic devices. Ohmic contacts with low contact resistance are essential in improving the electrical and optical performances of the devices. A wide variety of contact metallizations have been reported for p-GaN, including the standard Ni/Au as well as Ni. Different surface pretreatments have been investigated to lower the contact resistivity. To employ metal layers as a reliable ohmic contact on GaN, it is essential to understand the thermal stability of metal-GaN contact in addition to developing low resistance ohmic system. In this paper, we report on the characteristics of Ni/Ag bi-layer contacts on p-type GaN. The structural and electrical stability of the contacts at various annealing temperatures (480°C – 780°C)were investigated. Changes in the surface morphology of the contacts on annealing were examined using scanning electron microscopy (SEM). Specific contact resistivity and barrier height, determined using transmission line method (TLM) and current-voltage (I-V) measurements were calculated.