Haslan Abu Hassan

Far Infrared Optical Properties of Bulk Wurtzite Zinc Oxide Semiconductor

Polarized far infrared (FIR) reflectance technique was applied to study the optical properties of a bulk wurtzite zinc oxide (ZnO) single crystal. Room temperature polarized FIR reflectance spectra were taken at various angles of incidence, from 20° to 70°. The theoretical polarized FIR reflectance spectra were simulated based on the anisotropic dielectric function model. Good agreement was achieved between the experimental and the theoretical FIR reflectance spectra. Through this work, a complete set of reststrahlen parameters of a bulk wurtzite ZnO at the Brillouin zone centre was obtained. Additionally, other FIR optical properties such as the real and the imaginary parts of the complex dielectric function, real and imaginary parts of the refractive index, the absorption coefficient and the reciprocal of the absorption coefficient were also obtained by using numerical calculation.

Dispersion of Surface and Interface Phonon Polariton Modes in Wurtzite Based Multilayer System

Dispersion of the surface and interface phonon polariton modes in wurtzite based multilayer system is investigated theoretically. In this study, we present a new formulation of the surface polariton (SP) dispersion relation for wurtzite based multilayer system. The expression is derived through solving Maxwells electromagnetic equations directly by using algebraic manipulation method. The formulation is general and can be applied on any wurtzite layered structures in which the total number of layers in the system is equal or greater than two. In practice, the theory is employed to simulate the SP dispersion curves for several wurtzite layered structures, i.e., vacuum/ZnO, vacuum/ZnO/6H-SiC and vacuum/ZnO/GaN/6H-SiC. The features of these SP dispersion curves are analyzed and discussed in detail. It is shown that the characteristics of layers and the thicknesses of layers in the system have an important influence on the dispersion properties of the surface and interface phonon polariton modes.

Plasma-assisted MBE of GaN pn-junction Grown on Si(111) Substrate

We investigated growth of GaN pn-junction layers grown on silicon(111) by plasma-assisted molecular beam epitaxy system and its application for photo-devices. Si and Mg were used as n- and p-dopants, respectively. The reflection high energy electron diffraction images indicated a good surface morphology of GaN pn-junction layer. The thickness of GaN pn-junctions layers was about 0.705 nm. The absence of cubic phase GaN showed that this layer possessed hexagonal structure. According to XRD symmetric rocking curve ω/2θ scans of (0002) plane at room temperature, the full width at half-maximun of GaN pn-junction sample was calculated as 0.34°, indicating a high quality layer of GaN pn-junction. Surprisingly, there was no quenching of the A(LO) peak, with the presence of Si- and Mg-dopants in sample. The pn-junctions sample has a good optical quality which was measured by the photoluminescence system. For photo-devices applications, Ni and Al were used as front and back contacts, respectively. The current-voltage characteristics of the devices showed the typical rectifying behavior of heterojunction. The photo-current measurement was performed using a visible-lamp under forward and reverse biases. From the temperature-dependent measurements, the current at low bias exhibited much stronger temperature dependence and weaker field dependence. The effect of thermal annealing on front contact Ni was also carried out. The front contact Ni was annealed at 400 and 600 °C for 10 min in the nitrogen ambient. The results showed that 600 °C treated sample had a higher gain at 1.00 V/e than 400 °C treated and untreated samples.

Fabrication of porous ZnO thin films using wet chemical etching with 0.5% HNO3

Purpose – The purpose of this paper is to synthesize porous zinc oxide (ZnO) by means of strain etching/wet chemical etching method with the use of 0.5% of nitric acid (HNO3) etchant. The structural and surface morphological properties of the samples are accessed by using X‐ray diffraction (XRD) and scanning electron microscopy (SEM) characterization techniques.Design/methodology/approach – ZnO samples used in this work were deposited on the p‐Si (111) substrates by using radio frequency (RF) sputtering technique. Wet chemical etching processes with the use of 0.5% HNO3 etchant was applied on these samples in order to obtain porous structure. The porous ZnO samples are characterized by means of XRD and SEM to access their structural and surface morphological properties.Findings – The XRD and SEM cross‐sectional measurements revealed that the thickness of the etched ZnO thin films is proportional to the etching time. SEM micrographs show that the surface morphology of ZnO changes over etching time. On the ...

Sapphire surface polariton splitting due to resonance with aluminum nitride film phonon

Two thin aluminum nitride films have been prepared on sapphire substrates by molecular beam epitaxy technique. Then alkaline and acidic washing were used to remove the back-metal-coating of the sapphire substrate for one of the samples. (It caused also partial film dissolution). The surface polariton (SP) spectra have been measured by attenuated total reflection (ATR) technique. The measured SP dispersion is compared with one calculated using the literature film parameters. Due to the resonance interaction of sapphire substrate SP with the film transverse optical (TO) phonon the splitting of the dispersion curve of sapphire SP was found. The resonance takes place only for the frequency of the film TO phonon polarized along the surface of the anisotropic AlN film (perpendicular to the optical axis). The analysis of ATR and external reflectivity spectra shows the presence of some transition layer between the substrate and the film.

Surface phonon polariton responses of hexagonal sapphire crystals with non-polar and semi-polar crystallographic planes

The surface phonon polariton (SPhP) characteristics of hexagonal sapphire crystals with non-polar and semi-polar crystallographic planes are investigated. A formulation that considers the effects of crystal orientation is employed to calculate the SPhP dispersion curves of the samples. The SPhP dispersion curves indicate that the SPhP responses of sapphire crystals in non-polar and semi-polar orientations are directionally sensitive. Resonance frequencies and spectral strengths of the SPhP modes can be modulated simply by tuning the angular positions of the samples. The validity of the theoretical results is confirmed by the polarized infrared attenuated total reflection measurements.

Significant Influences of Co-Doping Ag and Sb on Electrical Properties and Thermoelectric Applications of AgPbmSbTem+2 Compounds Synthesized Using Solid State Microwave Technique

In this paper we reported the electrical conductivity and thermoelectric characterization of silver (Ag) and antimony (Sb) co-doped lead telluride bulk materials, which have been synthesized using solid state microwave technique. The doping level has performed first-principle calculations for the AgPbmSbTem+2 (LAST-m) (m = 0, 2, 4, 6, 8 and 10) to clarify the effect of simultaneous doping of Ag and Sb on PbTe. The Hall effect and thermoelectric measurements have shown n-type conductivity in AgPbmSbTem+2 samples. The samples show large and negative values of the Seebeck coefficient and moderate electrical conductivity. The Seebeck coefficient increased with doping levels increases at m=0 to 10. The value of the Seebeck coefficient is −419.69 μVK−1 for AgPb8SbTe10 at 338 K. It has been found that AgPb8SbTe10 sample has a higher thermoelectric power factor 1.87 mW K-2 m-1 at 310 K.

The investigation of Al0.29Ga0.71N/GaN/AlN and AlN/GaN/AlN thin films grown on Si (111) by RF plasma-assisted MBE

Recently, gallium nitride (GaN) and its related compounds involving Al and In have attracted much attention because of their potential to be used as high-efficiency UV light emitting devices, and as high frequency and high power electronic devices. Consequently, the growth and physics of GaN-based materials have attracted remarkable scientific attention. In this work, the growth and characterization of epitaxial Al0.29Ga0.71N and AlN layers grown on Si (111) by RF-plasma assisted molecular beam epitaxy (MBE) are described. The Al mole fraction was derived from the HR-XRD symmetric rocking curve (RC) ω/2θ scans of (0002) plane as x = 0.29. For AlN/GaN/AlN sample, the maximum Raman intensity at 521.53 cm−1 is attributed to crystalline silicon. It was found that the allowed Raman optical phonon mode of GaN, the E1 (high) is clearly visible, which is located at 570.74 cm−1. Photoluminscence (PL) spectrums of both samples have shown sharp and intense band edge emission of GaN without the existence of yellow em...

Porous Silicon Dioxide Synthesized using Photoelectrochemical (PEC) Wet Etching

Porous SiO2 can be used as a template to reduce substrate-induced stress, similar to porous GaN. Such a regrowth method may reduce the defect density in the epitaxial layer leading to high quality stress free layer on porous template. The samples were prepared on silicon (Si) wafers, (111)-oriented, with n- doping. After standard cleaning steps, SiO2 of 1200 Aring thickness was prepared by thermal oxidation of the Si at 1000deg C for 1.50 hours. The wafer was then cleaved into few pieces. To prepare porous structures by photoelectrochemical (PEC) method, the samples were dipped into a mixture of hydrofluoric acid (HF): water: ethanol under different etching durations. Structural properties of porous SiO2 have been investigated by scanning electron microscope (SEM). Elemental composition of the sample was identified using energy dispersive X-ray (EDX) analysis. Fourier transform infrared reflectance (FTIR) spectroscopy was used to characterize the chemical species and chemical bonding state.

The study of undoped ZnO/p-Si (111) thin films prepared by R.F. magnetron sputtering

We report on the investigation of zinc oxide (ZnO) thin films grown on p-type silicon (111) substrate by radio frequency (RF) magnetron sputtering tool. The structural and optical characteristics of the sample have been investigated by using scanning electron microscope (SEM), atomic force microscope (AFM), high resolution X-ray diffraction (HR-XRD), Raman spectroscopy, photoluminescence (PL) and Fourier transform infra-red (FTIR), respectively. The FTIR measurement reveals that ZnO retains its wurtzite structure due to the presence of vibrations in the Zn-O bonds. XRD analysis shows that there exists a slight impurity occurring in the sample with the presence of Al2O3 and the high and sharp peak of ZnO shows the good quality of crystallinity of ZnO. Raman spectroscopy measurements reveal that the ZnO film is a single crystal and has wurtzite structure and the presence of impurities such as oxygen deficiencies and interstitial defects. The PL spectra clearly prove that the Zn has high crystal quality due to the very high and sharp peak at wavelength of 390 nm. The deep level emissions also show that there is an oxygen defects in the ZnO film. The deposition of ZnO film onto Si substrate through RF sputtering results in the ZnO film having high quality crystal structure with the presence some impurities and defects.