Study of dispersion and carrier dynamics in semiconductors for optoelectronic applications

Abstract

A novel concept of a photoimpedance sensor is proposed and demonstrated. This sensor is an ac mode sensor as opposed to the conventional dc mode sensors and its complex impedance changes upon illumination. In the ac mode, the ac frequency becomes an additional parameter that determines the response of the sensor. The frequency dispersion of its capacitance (impedance) can tune its sensitivity and dynamic range: a lower ac frequency makes the sensor more sensitive to lower illumination levels and vice versa. This relationship between frequency and sensitivity has been shown to be due to the carrier generation-recombination and trapping-detrapping mechanisms. Such sensors are implemented in Cadmium Sulfide and fully depleted Silicon on Insulator (FDSOI) substrates.

The photoluminescence spectroscopy at high excitation intensities was used to study AlGaN and GaN reliability and lifetimes. The optical excitation caused damage to the AlGaN material at intensities which are much lower than the threshold for melting. Time resolved photoluminescence studies revealed the changes in carrier lifetimes indicating the formation of new defects in the material upon high intensity optical excitation.

Spanning the spectral range from IR to UV, AlInGaN alloys exhibit the band gaps ranging from IR (0.7-0.8 eV for InN) to deep UV (6.2 eV for AlN). These materials are suitable for both optical sensor and emitter applications. These nitride alloys have many unique properties, which affect the dynamics of charge carriers and determine the quantum efficiency. AlGaN alloys have been characterized to determine and describe their carrier dynamics using time resolved and integrated photoluminescence spectroscopy and light induced transient grating measurements. These studies reveal the different recombination paths (radiative and non-radiative) the carriers take to decay after optical excitation or electrical injection. The interplay of these mechanisms determines the emission efficiency of the material as well as the operating speed of the devices. Many features related to trapping, localization and efficiency droop emerge by comparing samples of different quality. A qualitative model to describe the various processes of carrier recombination is developed. These considerations are important in optimization of material for emitter applications.

In this dissertation, I present the investigation of the carrier dynamics and frequency dispersion in semiconductor materials for applications in light sensing and emitting devices. The results show that a frequency dispersion in the impedance of a sensor device arising due to carrier generation-recombination can lead to tunability of the sensitivity. The physical processes governing the carrier dynamics in AlGaN alloys are studied with optical techniques with the aim of understanding emission mechanisms and efficiency in these materials for UV LED applications.