Optical spectroscopy of impurity atoms in semiconducting Nanowires

Abstract

In a small and confined one dimensional geometry of nanowire, even few defects can influence the performance of the devices. Main objective of this thesis is to conduct photoluminescence study of such defect states in GaAs nanowires, which we will first introduce by diffusing copper impurity atoms, in very low concentrations.
In low concentrations, ideally single atom in a wire, the photoluminescence outcomes is expected to be influenced by the position of copper impurities in the wire. Therefore, these defect states can act as local probes in the material and provide useful information about the recombination kinetics. Single defects can be used to understand the effect of dielectric screening on the ionization energies of the impurities, within a nanowire.
As a part of this thesis, PL studies of high quality GaAs nanowires, which demonstrated benchmark solar cell performance, were conducted. Experiments were conducted to identify the conditions for diffusing copper in GaAs at low temperatures. As a part of this thesis, a dedicated PL setup, optimized for defect studies was assembled and a positioning scheme was developed to perform PL measurements on same nanowires before and after the diffusion.
Band gap luminescence peak for as grown GaAs nanowires was found ~10meV red shifted from theoretical values, and an attempt has been made to provide reason out the same. By comparing luminescence peak, before and after diffusion, associated with copper acceptor states in GaAs, it was confirmed that copper diffusion was successfully achieved at lower temperatures of 450oC. But, the possibility of creating single defect states was limited by background contamination of copper in our samples.

Popular Abstract

For the survival of the human body, food is very important; analogously for the survival of electronics, semiconductors are very important. Similar to how different spices add taste to the food, semiconductors become more functional when other elements are doped in it. Doping is generally measured in the amount of atoms added per unit volume of the semiconductor. If you are preparing big pot of food, adding small amount of extra salt would not create much difference in taste, but if the same amount of salt is added to a small portion, the taste would change dramatically. With progress in electronics industry, the volume of semiconductor material required to make it has considerably decreased. In this small volume addition of even small amount of doping can create significant changes in the properties of the semiconductor.
Now once the food has been prepared, your tongue detects different spices and their concentration in the food by finding which part of the tongue it interacts with. The signal detected by the tongue is then sent to the brain which processes it into information about the taste of the food. Similarly we try to detect the different signals from different doping and find their properties and concentration. To generate this signal, we give some energy to the semiconductor. The energy is first absorbed by the semiconductor and then released by different mechanisms related to the doping in the semiconductor. This releases different signals which are processed to find information about the doping in the semiconductor.
In this thesis, we will try to find a method to scarcely dope small volumes of semiconductor and then detect the different signals from them to find out information about both semiconductor and the doping material.

• author: Shroff, Shishir
• supervisor: Dan Hessman
• organization: Solid State Physics; Department of Physics
• course: FYSM60 20161
• year: 2017
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Photoluminescence, Nanowire, Defects, Diffusion, GaAs, Copper
• language: English
• id: 8904524
• date added to LUP: 2017-03-16 19:24:23
• date last changed: 2017-03-16 19:24:23