Research Skills :

Porous Silicon

Although Silicon (Si) is the key material in the electronics industry, but it does not fulfill of optical communications because of its small and indirect energy gap. Recently Porous Silicon (PS) showed promising future for optoelectronics. PS is one of my main research since 1990. I prepared PS from n-type and p-type Si with different resistivities and performed the following measurements on them:

Composition

The composition of PS is not straightforward, since the nature of the material is affected by the doping of the Si, its orientation, strength of HF during the anodizing, current density, and time. Our measurements showed the relative concentrations of Si, O, C, and H found 2.3, 1.2, 1.4, and 1.2 respectively in PS layer. The theoretical curve showed a close fit. By slightly incresing the Si to 2.5, and using 1.3 for C. The composition data were taken by Elastic Recoil Detection and Nuclear Reaction Analysis methods and confirmed by Rutherford Back Scattering measurement.

• Photoluminescence

PL is a contactless nondestructive method of probing the electronic structure of materials. The PL of the PS films, were taken with an Ar laser operating at 488 nm, showed peak emission at about 740-780 nm and full width half maximum (FWHM) of 5
about 90 nm. The PL spectra and FWHM remained essentially the same up to 300º C. After higher temperature annealing, the intensity dropped considerably and there was a pronounced red shift in the wavelength of peak emission, from 740 to 910 nm.
The PL data also showed that the peak shift is not large from 10 to 150 K and there is no significant change in FWHM. This is probably due to the fact that PL is composed of a range of differing individual sources.

• Electroluminescence

EL is the conversion of electrical energy into light. A typical EL spectrum shows the wavelength peak occurs at about 540 nm with a FWHM of 86 nm. We have shown that EL in PS is a phenonemnon whose origins are distinctly different from that of PL. A possible origin is recombination radiation in current-carrying Si monohydrides which is likely to be present in the material.
• Photocurrent
• Chemiluminescence
CL is a process that a chemical reaction releases its energy in the form of light. CL occurs when a PS film is anodically oxidized in an aqueous solution of 1M HCl (or other chemicals) at a constant current density. A clear red light was observed even in daylight. This emission, which ceases within seconds due to formation of a continuous oxide layer, was much more intense than the EL, but much poorer durability.
• Current-Voltage
The current-voltage characteristics of PS show Schottky junction like behaviour. After annealing up to 500 ºC, the I-V curves show relatively small changes involving an increase in current at a given voltage of about a factor of 2. From this we conclude that the annealing does not cause large changes in the specimen conductance.
• Soft X-ray Emission
From Si L23 soft X-ray emission we find that oxide grows with time on the surface of PS samples. Most of the oxygen is not present as oxide. However, after about 30 days exposure, the valance band shows clear oxide peaks. After several months, it closely resembles that of furnace-oxidised Si. Since the amount of oxide present on fresh samples shows varation, despite similar preparation conditions, it indicates the sensitivity to slight details in the preparation process. This variability may account for some of the difficulty in obtaining consensus on some PS properties.
• Photoconductivity (PC)
PC is an optical and electrical phenomenon in which a material becomes more conductive due to the absroption of light. We found that inceasing etching time, current density, or electrolyte concentration, created smaller Si pore size and layer porosity, shifting the PC peak from 600 to 520 nm. This shift in the spectral distribution of the PC of the device is indirect evidence of the change in PS microstructure, as opposed to direct observations from others with, for example, scanning electron microscopy. In general, we can say that for PS, the study of PC and transport is still in its infancy.
• Scattering
• Optoelectronics
We have found that although the total porosity of PS can be determined relatively simply by a gravimetric method and electron microscopy, this parameter can only
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partially explain electrical current transport. A apparently cyclic variation in porosity with anodization time reveals oscillatory creating and etching of pores in silicon, with potential applications in finely controlling the atomic-scale structure in this promising optoelectronics material.
• Gas Sensor
Besides its exciting electro-optical properties, PS shows some interesting features for sensor devices. Our PS samples under different anodization parameters appear that they have best response to oxygen gas. The sensing mechanism is based on a gas-induced modulation effect of the conductivity due to the adsorbed molecules in the porous film. The observed behaviour suggests that the application of PS in future chemical gas and biological sensors is feasible.
• Photovoltaic
PV is the basic physical process that converts light to electricity. It can be suggested that the PV properties are mainly dominated by the absorption of light in PS layer and PS/Si heterojunction. Our devices have shown a maximum external quantum efficiency of 0.14%. All the PS samples found to be very sensitive to optical power with various efficiencies. The main task to be achieved in such devices is the enhancement of the external quantum efficiency.
• DLA Simulation
• Refractive Index
• Solar Cells
• Waveguide

CdS

• Preparation of Wurtzite CdS
• Photoconductivity
• Impurities Doping
Calculation of Band Strutures
• Lithium Atom by Hartree method
• Beryllium Atom by K K R method
• Boron Atom by Hartree Fock method

Rough Surfaces

• Two-scale Kirchhoff Theory
Glancing Angle Deposition
• Preparation of Pillar sample
• Preparation of Zig Zag sample
• Preparation of Helix sample
• Photoluminescence
• Photoconductivity
• Birefringences
• Chirallity
• Scattering

 

Special Skills :

Microscopy

• Atomic Force Microscope (Digital Instruments Multimode SPM, NS4)
• Scanning Electron Microscope (LEO 1530 Field Emission)
Spectroscopy
• Spectroscopic Ellipsometry (WVASE32 M-2000VI, J. A. Woollam Co.)
Experimental Techniques
• Four point probe resistivity measurements
• Van der Pauw method for resistivity measurements
• Familiarity with Ar laser system used for Photoluminescence

Computers

• Use of Macintosh and IBM compatible computers
• Software: Excel, Origin, Fortran, Maple, HTML familiarity with MATLAB
• Interfacing computer, data acquisition, control and equipment

Electronics

• Experience with both analog and digital circuits used in experiments
• High voltage power supply (7000 V)
• Use of spectrum analyzer, digital oscilloscope, lock-in amplifier, digital multimeter, etc.
Vacuum Systems
• Built a glass diffusion pump with three jet chamber
• Use and maintenance of mechanical and turbo pump stations, leak detector systems
• Vacuum Evaporator
• Electron Gun Beam

Machine Shop

• Experience with lathe, vertical mill, drill press, soft soldering, and other machine shop equipment
• Understanding and designing technical drawings