Photochemical Sciences Ph.D. Dissertations

Photocurrent Spectroscopy of CdS/Plastic, CdS/Glass, and ZnTe/GaAs Hetero-pairs Formed with Pulsed-laser Deposition

Date of Award

2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Bruno Ullrich

Second Advisor

Lewis Fulcher

Third Advisor

Deanne Snavely (Committee Member)

Fourth Advisor

John Cable (Committee Member)

Abstract

This dissertation presents photocurrent (PC) spectroscopy of thin-film cadmium sulfide (CdS) on plastic, CdS on glass, and zinc telluride (ZnTe) on gallium arsenide (GaAs) hetero-pairs. All samples have been prepared with pulsed-laser deposition (PLD) and the thesis is organized into three principal sections. The first section presents the PLD essentials and characterization of CdS thin films on transparent plastic substrates. The second part focuses on the exploitation of CdS films on glass to quench or modulate alternating photocurrent (APC) by additional constant blue light illumination. Finally, PC spectra modification of n-GaAs due to ZnTe PLD will be investigated.

First, the merger of a transparent plastic substrate with thin-film CdS for photonic application was realized using low-temperature PLD, where low-temperature PLD means the substrates were not externally heated. Although plastic is not considered to be a favored substrate material for semiconductor thin-film formation, the deposited CdS film possessed good adhesion to the plastic substrates and showed a blue-shifted photosensitivity with peak at 2.54 eV. The CdS deposition rate was monitored at different laser fluences and the maximum rate was found at 2.68 J/cm2. The visualization of the surface using an atomic force microscope (AFM) revealed its mosaic structure and electron probe microanalysis showed that target composition was maintained in the film. The study of thickness distribution revealed that the film deposition area is significantly increased with increase in laser fluence. The achieved results demonstrate the capability of PLD to form novel heterostructures with appealing and useful technological properties such as plasticity and low weight.

In the second part, APC control via blue light illumination employing thin-film PLD CdS on a glass is introduced. In fact, the APC driven through the CdS film in conjunction with bias was quenched when the sample was additionally illuminated with a blue light emitting diode (LED). It occurred that the quenching magnitude depends on the blue light intensity, chopped light intensity and its energy, and applied electric field. The quenching phenomenon is attributed to the shortening of available APC carriers because of the generation of direct current channels in the film and is described using a straightforward band diagram model.

Finally, the PC spectra modification of a n-GaAs substrate due to the PLD of thin-film ZnTe is presented. The intrinsic and extrinsic room temperature PC spectra of the n-GaAs and ZnTe/n-GaAs samples were investigated with lock-in technique by employing various optical chopping frequencies and biases. The PC magnitude of the bulk n-GaAs was increased with increasing chopper frequency, while PC of the ZnTe/n-GaAs sample showed an increase and decrease with frequency in the lower and higher energy range, respectively. Noteworthy, a frequency independent isosbestic point was observed at the crossover between these two behaviors at 1.88 eV. Additionally, a defect related PC peak at 1.37 eV was observed only for ZnTe/n-GaAs sample. The magnitude of the peak-and even its appearance-was found to be sensitively dependent on the sign of bias. This phenomenon caused by PLD created defect states on n-GaAs surface referred to “photonic-doping”.

Link to Full Text

Share

COinS