Zahid Hasan Mahmood

A review on electronic and optical properties of silicon nanowire and its different growth techniques

Electronic and optical properties of Silicon Nanowire (SiNW) obtained from theoretical studies and experimental approaches have been reviewed. The diameter dependency of bandgap and effective mass of SiNW for various terminations have been presented. Optical absorption of SiNW and nanocone has been compared for different angle of incidences. SiNW shows greater absorption with large range of wavelength and higher range of angle of incidence. Reflectance of SiNW is less than 5% over majority of the spectrum from the UV to near IR region. Thereafter, a brief description of the different growth techniques of SiNW is given. The advantages and disadvantages of the different catalyst materials for SiNW growth are discussed at length. Furthermore, three thermodynamic aspects of SiNW growth via the vapor–liquid–solid mechanism are presented and discussed.

Determination of InN–GaN heterostructure band offsets from internal photoemission measurements

Band discontinuities at the InN–GaN heterointerface are experimentally determined from internal photoemission spectroscopy measurements on n+ InN on GaN epilayers. The photocurrent shows two threshold energies, one at 1.624eV and the other at 2.527eV. From these, we obtain the band offsets ΔEv=0.85eV and ΔEc=1.82eV.

Design and simulation of an optical wavelength division demultiplexer based on the photonic crystal architecture

A Wavelength Division De-multiplexer has been proposed based on a hexagonal lattice 2D photonic crystal structure. The structure has been designed using GaAs rods, dielectric constant 11.4, which are suspended in air. The device has been proposed for De-multiplexing two wavelengths of 1.31 μm and 1.55 μm. The Plane Wave Expansion method and the Finite Difference Time Domain method have been utilized for simulating the photonic band-gap and results respectively. The localization property of photonic crystal has been exploited to guide the wavelengths in two different output paths thereby obtaining low crosstalk and more than 70% of transmission.

High frequency compatibility of doped multilayer Graphene Nanoribbon in VLSI interconnect with respect to skin depth effect and layer width variation

In this research work, variation of skin depth in high frequency for undoped and doped multilayer Graphene Nanoribbon (GNR) is analyzed and compared with Cu as an interconnect material in VLSI fabrication process. Another acknowledgement of this skin depth for doped multilayer GNR is examined by varying the width of each GNR layer. After producing these simulations, homogeneity of variation of skin depth of doped multilayer GNR to Cu at high frequency has been introduced. Finally, doped multilayer GNR with relatively thicker layer width and maximum possible layers have been proposed as a better interconnect material.

Broadband optical absorption measurement of silicon nanowires for photovoltaic solar cell applications

The broadband optical absorption properties of silicon nanowire films fabricated by electroless metal deposition technique followed by HF/Fe(NO3)3 solution-based chemical etching at room temperature on p-type silicon substrates have been measured and we have found higher absorption than that of the solid thin films of equivalent thickness. The observed behavior is effectively explained by light scattering and light trapping, though some of the observed absorption is due to a high density of surface states in the nanowire films. The synthesized structures absorbed more than 82% of incident radiation in case of Cu-deposited silicon nanowires, whereas for Ag it was a maximum of 83%, which is much greater than that of the bulk silicon as they absorbed a maximum of 43% of the radiation.

Studies on optical characteristics of CuInSe2 thin films

CuInSe2 is considered as a striking semiconductor for second generation solar cells. An investigation of optical properties of CuInSe2 thin films is essential to evaluate its perfectibility as high efficiency solar cells. The films were fabricated by thermal co-evaporation technique. For this experiment, a shimadzu spectrophotometer of model number 1201 is used. The optical properties of these films are determined for the wavelength range 350 nm–1100 nm. From the experiment it is evident that the reflectance and transmittance of the films are negligible in comparison to the absorption of these films. The high absorption coefficient of the order of 104/cm of the film material also supports this. The band gap of the CuInSe2 films was evaluated to be 1.1 eV. From XRD and EDAX analysis it is evident that CuInSe2 films are polycrystalline in nature having ideal stoichiometric composition.

Study of charge density at InxGa1-xN/GaN heterostructure interface

Energy-bands in undoped InxGa1-xN/GaN heterostructures have been simulated using 1D Poisson/Schrödinger solver: A Band Diagram Calculator, by self-consistent solution of Schrödinger and Poisson equations. The formation of two-dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG) were observed at the interface of undoped InxGa1-xN/GaN based heterostructures. Charge concentrations throughout the structures were analyzed which show the confinement of charge in a quantum well at the heterointerface. Charge density as a function of depth from surface to substrate has also been presented in this paper. The formation of 2DEG and 2DHG and the dependence of their densities on layer thickness, alloy composition and temperature have been investigated.

Etching of p-type metal contact deposited on GaAs

In this paper, both wet etching and dry etching of metal contacts deposited on GaAs wafer have been described. A /spl sim/240 nm thin layer of p-type contact was formed on GaAs bulk wafer using Au-Zn-Au layer. Wet etching of the metal contact was done using KI-I/sub 2/ and HNO/sub 3/-HCl etch solutions and the dry etching was performed in an argon plasma reactive ion etcher chamber. Results are discussed for both the etching processes.

Electrical, optical and structural properties of transparent conducting Al doped ZnO (AZO) deposited by sol-gel spin coating

Al doped ZnO (AZO) films are fabricated by using sol-gel spin coating method and changes in electrical, optical and structural properties due to variation in film thickness is studied. AZO films provide c-axis orientation along the (002) plane and peak sharpness increased with film thickness is evident from XRD analysis. Conductivity (σ) of AZO films has increased from 2.34 (Siemens/cm) to 20156.27 (Siemens/cm) whereas sheet resistance (Rsh) decreases from 606300 (ohms/sq.) to 2.08 (ohm/sq.) with increase of film thickness from 296 nm to 1030 nm. Optical transmittance (T%) of AZO films is decreased from around 82% to 62% in the visible region. And grain size (D) of AZO thin films has been found to increase from 19.59 nm to 25.25 nm with increase of film thickness. Figure of Merit is also calculated for prepared sample of AZO. Among these four sample of AZO thin films, L-15 sample (having thickness in 895 nm) has provided highest figure of merit which is 5.49*10^-4 (Ω-1).Al doped ZnO (AZO) films are fabricated by using sol-gel spin coating method and changes in electrical, optical and structural properties due to variation in film thickness is studied. AZO films provide c-axis orientation along the (002) plane and peak sharpness increased with film thickness is evident from XRD analysis. Conductivity (σ) of AZO films has increased from 2.34 (Siemens/cm) to 20156.27 (Siemens/cm) whereas sheet resistance (Rsh) decreases from 606300 (ohms/sq.) to 2.08 (ohm/sq.) with increase of film thickness from 296 nm to 1030 nm. Optical transmittance (T%) of AZO films is decreased from around 82% to 62% in the visible region. And grain size (D) of AZO thin films has been found to increase from 19.59 nm to 25.25 nm with increase of film thickness. Figure of Merit is also calculated for prepared sample of AZO. Among these four sample of AZO thin films, L-15 sample (having thickness in 895 nm) has provided highest figure of merit which is 5.49*10^-4 (Ω-1).

Experimental Study on Structural, Optical and Electrical Properties of Chemical Bath Deposited CdZnS Thin Films

CdZnS thin films have been prepared from an aqueous solution containing cadmium chloride, zinc acetate and thiourea by chemical bath deposition (CBD) method on a glass substrate at 82°C. Amount of cadmium chloride and zinc acetate were systematically varied as y=0.2, 0.4, 0.6, 0.8 (y=a/(a+b), a=volume of zinc acetate, b=volume of cadmium chloride) and subsequent effects on the films property were studied by diffractometer, scanning electron microscope, spectrophotometer, four point probe and hall effect measurement. X-ray diffraction result shows that an increase in the Zn-content decreases the crystalline quality of the films. Transmittance and absorbance data was obtained by using UV-Vis spectrometer. Transmittance of 85-90% is found beyond the 700 nm wavelength. Optical bandgaps calculated from these data show a gradual increase in value with a gradual increase in Zn content. The electrical resistance of the so prepared films were measured by four point probe method. The result indicates that the resistance increases with increasing Zn content.