Time-Resolved Photoluminescence Studies of InGaP Nanowires for Improving the Internal Quantum Efficiency

Abstract

Semiconductor Nanowires are promising building blocks for advanced optoelectronic devices since their small diameter give rise to quantization effects. The small diameter also makes them susceptible to non-radiative recombination due to surface states. A consequence of non-radiative surface recombination is a reduction of a total recombination lifetime. This is in turn limits an internal quantum efficiency (IQE) of optoelectronic devices because the IQE is defined as a ratio between the radiative recombination lifetime and the total recombination lifetime. It is therefore important to produce the nanowires with as long total lifetime as possible in order to achieve a significant IQE.
This thesis aims to improve the IQE of the nanowires by using the surface passivating layer with a larger bandgap material on ternary alloy (InGaP) nanowires. The larger bandgap of the passivating layer will allocate the charge carriers into the center nanowire which, therefore, decrease the chance of the recombination at the surface. The passivating layer covers the side facet of the nanowires as a shell. The nanowires with this shell layer are called core-shell nanowires.
Photoluminescence (PL) spectroscopy was used to evaluate the material composition and the bandgap of the core nanowire as well as the shell layer. Various compositions of shell materials give different band offsets between the core and the shell. Time-resolved photoluminescence (TRPL) was performed on the nanowires with and without the shell to measure the recombination lifetime. Finally, the energy structure of the non-radiative recombination center was studied by a temperature dependent PL (TDPL) and a power dependent TRPL.
For the plain nanowires, the result shows that the non-radiative recombination is indeed related to surface states as expected. The total lifetime of the thinner wires is shorter than the thicker wires. Therefore, the IQE will degrade for the devices based on the thin nanowires. The TDPL reveals that the intensity is two orders of magnitude lower at room temperature (300 K) compared with the lowest measured temperature (4 K). The decreasing intensity is a result of two quenching processes. As the temperature increases, charge carriers gain higher thermal energies. The first quenching process occurs as the charge carriers escape the potential confinement with their higher thermal energies. Another process corresponds to a non-radiative recombination center with an activation energy of 17 meV. Power dependent TRPL at low (5 K) and high (100 K) temperature confirmed that the non-radiative recombination center is thermally activated.

Popular Abstract

The performance of the computer processor has improved drastically over the past decade through an increasing number of transistors employed on a single processor chip. A simple approach for having a larger amount of transistors on a single circuit is to scale down the device. The smallest feature of a transistor is for example only a few nanometers (nm=0.000000001m) large which is about ten thousand times smaller than human’s hair. However, electrical and optical properties of miniaturized devices are impacted by quantum effects, such as energy quantization, and tunneling. These effects can either limit or improve the performance of the devices.
A new nanostructure, Semiconductor Nanowire, is introduced as a building block of advanced electrical devices. It is a semiconductor material with a cylindrical shape, in which the diameter can be on a scale of nanometers and the length is about a few micrometers. The configuration of the nanowire provides a large surface comparing to its volume. With its large surface, it is therefore suitable for sensing applications, especially as an optoelectronic device. The purpose of the optoelectronic devices is to be either a light detector or a light source. For the nanowire as the light detector, the absorbed light excites electrons from a stationary state to move freely within the nanowire. Those free electrons make the device electrically conductive. For the light source, the electrons can be excited by an external energy; such as electricity or light. The excited electrons would then relax back to their stationary state whereupon release the excess energy in the form of light.
The efficiency of those devices is determined by how well the energy is transferred between the free electrons and the light. An ideal condition is that every electron should emit the light as it relaxes for the light source. In contrast for the light detector, all the absorbed light should give rise to the free electrons. However, defects in the material and the surfaces of the devices can create “dark” pathways for the electrons to relax without the light emission. This makes the device less efficient. This is problematic since the nanowires are so thin that electrons are always close to the surface. There is a method called surface passivation which most surface of the wire is covered by another semiconductor material. With the proper passivation, the free electrons will move away from the surface. Consequently, the “dark” pathways can be avoided.
In this study, there are several measurements performed on the nanowires with and without surface passivation in order to compare the device performance. The performance of the nanowires is expected to be improved when employing the surface passivation. Also, the study is to better understand the “dark” pathways involving the surface condition of the nanowires.

• author: Chayanun, Lert
• supervisor: Niklas Sköld
• organization: Department of Physics; Solid State Physics
• course: FYSM30 20161
• year: 2017
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• language: English
• id: 8923208
• date added to LUP: 2017-08-23 12:37:04
• date last changed: 2017-08-23 12:37:04