L. S. Chuah

Porous Silicon as an Intermediate Buffer Layer for Zinc Oxide Nanorods

Zinc thin films were deposited onto porous silicon (PSi) substrates by dc sputtering using a Zn target. These films were then annealed under flowing (6 l/min) oxygen gas environment in the furnace at 600°C for 2 h. Porous silicon is used as an intermediate layer between silicon and ZnO films and it provides a large area composed of an array of voids. The PSi samples were prepared using photoelectrochemical method on n-type silicon wafer with (111) and (100) orientation. To prepare porous structures, the samples were dipped into a mixture of HF:ethanol (1:1) for 5 min with current densities of 50 mA/cm2, and subjected to external illumination with a 500 W UV lamp. The surface morphology and the nanorod structure of the ZnO films were characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD). We synthesized the ZnO nanorods with diameter of 80–100 nm without any catalysts or templates. The XRD pattern confirmed that the ZnO nanorods were of polycrystalline structure. The surface-related optical properties have been investigated by photoluminescence (PL) and Raman measurements at room temperature. Micro-Raman results showed that A1(LO) of hexagonal ZnO/Si(111) and ZnO/Si(100) have been observed at 522 cm–1 and 530 cm–1, respectively. PL spectra peaks are clearly visible at 366 cm–1 and 368 cm–1 for ZnO film grown on porous Si(111) and Si(100) substrates, respectively. The PL spectral peak position in ZnO nanorods on porous silicon is blue-shifted with respect to that in unstrained ZnO (381 nm).

Performance improvement of large area GaN MSM photodiode with thin AlGaN surface layer

Purpose – The purpose of this paper is to propose a new structure of GaN based metal‐semiconductor‐metal (MSM) photodiode, in which a thin unintentionally doped n‐type AlGaN layer is added on the conventional GaN on Si(111) device structure.Design/methodology/approach – A thin Al0.50Ga0.50N cap layer of 100 nm was incorporated in GaN MSM photodiode to enhance the effective Schottky barrier height and reduce the dark current. When the incident light with photon energy higher than the band edge of GaN but lower than the bandgap of AlGaN illuminates the front face of photodiode, the light can be transparent in the top AlGaN layer and is only absorbed by the GaN layer. As a result, the photogenerated carriers in the GaN layer would be influenced by the interface states of AlGaN/GaN. It is known that the density of the interface states is normally lower than that of surface states, so the recombination of photogenerated electron‐hole pairs will be reduced. A barrier height of 0.54 eV for normal GaN MSM photodi...

InN PHOTOCONDUCTORS ON DIFFERENT ORIENTATIONS OF Si SUBSTRATES

We have fabricated photoconductors of indium nitride (InN) grown by radio frequency (RF) sputtering. The InN thin films were deposited on Si(100), Si(110) and Si(111) substrates at room temperature. The Ag/Al contact has been deposited by thermal evaporation in vacuum (10-5Torr) and then annealed under the flowing of the nitrogen gas environment in order to relieve stress and also induce any favorable reactions between metals and the semiconductor. Current–voltage (I–V) measurements after heat treatment at 400°C were carried out for samples in dark and illumination conditions. It was found that Ag/Al formed a good ohmic contact on top of InN. In addition, the characteristics of the contacts were significantly affected by the orientation of substrates.

Study of electrical characteristics of ZnO Schottky photodiode on Si substrate

Purpose – The purpose of this paper is to demonstrate the n‐ZnO/p‐Si Schottky photodiodes.Design/methodology/approach – A Zn film was deposited on silicon substrate by dc sputtering deposition technology from high purity zinc (Zn) targets. Then, the Zn films were then annealed under flowing oxygen (O2) gas environment in the furnace. ZnO nanorods morphologies have been successfully prepared through a simple method. No catalyst is required.Findings – The structures and morphologies of the products were characterized in detail by using X‐ray diffraction, energy dispersive X‐ray, and scanning electron microscopy (SEM). According to experimental results, the current‐voltage characteristics of the device show the typical rectifying behaviour of Schottky diodes. The UV photocurrent measurement was performed using an UV lamp under a reverse bias.Originality/value – The paper demonstrates that the n‐ZnO/p‐Si diodes exhibit strong rectifying conduct described by the current‐voltage (I‐V) measurement under a dark a...

High‐quality In0.47Ga0.53N/GaN heterostructure on Si(111) and its application to MSM detector

Purpose – This paper aims to report on the use of radio frequency nitrogen plasma‐assisted molecular beam epitaxy (RF‐MBE) to grow high‐quality n‐type In0.47Ga0.53N/GaN on Si(111) substrate using AlN as a buffer layer.Design/methodology/approach – Structural analyses of the InGaN films were performed by using X‐ray diffraction, atomic force microscopy, and Hall measurement. Metal‐semiconductor‐metal (MSM) photodiode was fabricated on the In0.47Ga0.53N/Si(111) films. Electrical analysis of the MSM photodiodes was carried out by using current‐voltage (I‐V) measurements. Ideality factors and Schottky barrier heights for Ni/In0.47Ga0.53N, was deduced to be 1.01 and 0.60 eV, respectively.Findings – The In0.47Ga0.53N MSM photodiode shows a sharp cut‐off wavelength at 840 nm. A maximum responsivity of 0.28 A/W was achieved at 839 nm. The detector shows a little decrease in responsivity from 840 to 200 nm. The responsivity of the MSM drops by nearly two orders of magnitude across the cut‐off wavelength.Originalit...

Optical Analysis of Nanocrystalline ZnO Films Coated on Porous Silicon by Radio Frequency (RF) Magnetron Sputtering

Zinc oxide thin films have been deposited onto porous silicon (PSi) substrates at high growth rates by radio frequency (RF) sputtering using a ZnO target. The advantages of the porous Si template are economical and it provides a rigid structural material. Porous silicon is applied as an intermediate layer between silicon and ZnO films and it contributed a large area composed of an array of voids. The nanoporous silicon samples were adapted by photo electrochemical (PEC) etching technique on n-type silicon wafer with (111) and (100) orientation. Micro-Raman and photoluminescence (PL) spectroscopy are powerful and non-destructive optical tools to study vibrational and optical properties of ZnO nanostructures. Both the Raman and PL measurements were also operated at room temperature. Micro-Raman results showed that the A1(LO) of hexagonal ZnO/Si(111) and ZnO/Si(100) have been observed at around 522 and 530 cm–1, re- spectively. PL spectra peaks are distinctly apparent at 366 and 368 cm–1 for ZnO film grown on porous Si(111) and Si(100) substrates, respectively. The peak luminescence energy in nanocrystalline ZnO on porous silicon is blue-shifted with regard to that in bulk ZnO (381 nm). The Raman and PL spectra pointed to oxygen vacancies or Zn interstitials which are responsible for the green emission in the nanocrystalline ZnO.

PREFERENTIAL ORIENTATION GROWTH OF AlN THIN FILMS ON Si(111) SUBSTRATES BY PLASMA-ASSISTED MOLECULAR BEAM EPITAXY

This article reports the use of plasma-assisted molecular beam epitaxy (MBE) to grow AlN on Si(111) substrate at 850°C under UHV conditions for 15, 30, and 45 min. The films were characterized by high-resolution X-ray diffraction (HR-XRD) and micro-Raman spectroscopy. XRD measurement revealed that the AlN was epitaxially grown on Si(111). Micro-Raman result showed that all the allowed Raman modes of AlN and Si were clearly visible. Fourier transform infrared (FTIR) spectroscopy has been used to investigate the A1 (LO) and E1 (TO) modes with frequencies (890–899) cm-1 and (668–688) cm-1, respectively. The results are in good agreement with reported phonon frequencies of AlN grown on Si(111).

INFLUENCE OF Al MONOLAYERS ON THE PROPERTIES OF AlN LAYERS ON Si (111)

High-quality aluminum nitride (AlN) layers with full width at half maximum (FWHM) values of 11 arcmin were grown by plasma-assisted molecular-beam epitaxy on Si (111) substrates. AlN nucleation layers are being investigated for the growth of GaN on Si. Growth using AlN buffer layers leads to Al-polar films, with surfaces strongly dependent on the flux conditions used. Flat surfaces can be obtained by growing as Al-rich as possible, although Al droplets tend to form. Before starting the AlN growth, a few monolayers of Al are deposited on the substrate to avoid the formation of Si3N4. X-ray diffraction (XRD) techniques were employed to determine the surface and structural quality of the layers. XRD revealed that monocrystalline AlN was obtained. Best AlN films were obtained at high substrate temperatures (875°C) and III/V ratios close to stoichiometry.

Synthesis of ZnO Nanosheets by a Combined Electrodeposition and Illumination Method

To the best of our knowledge there are no reports about synthesizing ZnO nanosheet patterns on zinc foil substrates. ZnO nanosheets with the wurtzite structure have been successfully synthesized via electrochemical wet etching method. Zinc (Zn) foil was used as substrate in this technique. Without using any templates or catalysts, the ZnO nanosheet arrays on a zinc foil substrate were prepared. To adapt ZnO nanosheet structures, the Zn foil was immersed into an admixture of HNO3:ethanol (1:5), at current densities of 127 mA/cm2 at room temperature, and directed to outward light from a 100 W lamp. The constant etching duration is 30 min at room temperature. After etching, the as-prepared products were characterized by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD). The XRD pattern confirmed that the as-synthesized nanosheets are hexagonal wurtzite structure of polycrystalline structure. The growth direction of the nanosheets is parallel to the (0001) polar direction of ZnO. The optical properties of the ZnO nanosheet arrays were examined by Raman and photoluminescence spectroscopies at room temperature (RT). The photoluminescence spectrum of the as-synthesized nanosheet sample shows only a sharp and strong UV emission centered at 380 nm, which indicates that the ZnO nanosheets on Zn foil are of good optical property.

p-GaN/n-Si HETEROJUNCTION PHOTODIODES

PN photodiodes, as an alternative form of photodetectors, is based on carrier production in the high-field junction region, and it has a response time considerably faster than that of a photoconductor and is typically in the order of nanoseconds. Photodetectors operating in the short wavelength ultraviolet (UV) region are important devices that can be used in various commercial and military applications. In the present work, we fabricated the p-GaN/n-Si heterojunction photodiode to observe the photoelectric effects. From the results, the current–voltage characteristics of the device show the typical rectifying behavior of heterojunctions. The UV photocurrent measurement was performed using an Hg-lamp under a reverse bias.