AL HUWAYZ, MARYAM
(2022)
Investigation of the electrical and optical properties of advanced semiconductors materials and devices.
PhD thesis, University of Nottingham.
Abstract
This thesis reports on an investigation of deep level defects in InAs quantum dots intermediate band solar cells (QD-IBSCs) and InAs/InGaAs quantum dot lasers structures that have applications in photovoltaics and optoelectronic technologies. In fact, it is of paramount importance to understand the characteristics of the defects and their influence on the quality of materials and the performance of devices. In this work, the effect of electrically active defects on the structural, optical and electrical properties of a set of intermediate band solar cells grown by Metal Organic Vapour Phase Epitaxy (MOVPE) and self-assembled InAs/InGaAs QDs based laser structures grown by Molecular Beam Epitaxy was studied using Atomic Force Microscopy, Transmission Electron Microscopy, Photoluminescence, current-voltage (I-V), capacitance-voltage (C-V), deep level transient spectroscopy (DLTS), and Laplace DLTS (LDLTS) techniques. In addition, the effect of Gamma irradiation on the properties of laser structures was also investigated.
Electrically active defects present in a set of p-i-n InAs/GaAs QD-IBSCs grown by MOVPE on GaAs substrates have been studied systematically. The device’s structures are almost identical, differing only in the growth temperature and thickness of the GaAs layers that cover each InAs QD layer. In order to distinguish between the roles played by the growth temperature and the insertion of the QDs in the active region of the devices, reference solar cells with the equivalent temperature growth sequence as the ones used for the fabrication of the QD-IBSCs were grown and their DLTS results were compared. These differences induce significant changes in the solar energy conversion efficiency of the photovoltaic cells. DLTS and Laplace DLTS measurements on control samples and the studied solar cell devices have clearly shown that electrically active traps play an important role in the device figures of merit, such as open circuit voltage, short circuit current, and shunt resistance. In particular, it was found that the well-known EL2 defect negatively affects both the open circuit voltage and shunt resistance, more in structures containing QDs, as a consequence of the temperature cycle required to deposit them. Other unidentified defects, that are absent in samples in which the QDs were annealed at 700 °C, contribute to a reduction of the short circuit current, as they increase the Shockley-Read-Hall recombination.
Extensive work on InAs QDs grown on GaAs substrates has been reported in the literature, however, research in the use of different substrate materials such as silicon to achieve an ideal and full integration of photonic and electronic systems is still under development. In this work we have investigated the effect of the substrate material (Si and GaAs) and strain reducing layer (SRL) on the electrical and optical properties of InAs QDs based laser structures grown by MBE. Two InAs QD laser structures with similar active regions grown on GaAs and Si substrates using SRL consisting of InAs QDs/6nm In0.15Ga0.85As SRL have been investigated. Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), photoluminescence (PL), I-V, C-V, DLTS and LDLTS have been used for the characterization of the grown samples. The formation of fairly similar QDs in both structures was evidenced, with higher strain for QDs in the sample grown on a Si substrate. A red shift of the InAs QD PL peak energy was observed for the sample grown on a Si substrate as compared to the sample grown on a GaAs substrate, which was associated with residual biaxial strain from the Si/GaAs heterointerface. This red-shift of the PL peak energy is accompanied by a broadening of the PL spectrum from ~31 meV to a value of ~46 meV. This broadening is attributed to the QD size inhomogeneity increase for samples grown on a Si substrate. DLTS and LDLTS showed that InAs QD laser devices grown on Si substrate showed more defects than the structures grown on GaAs substrates, suggesting that QDs/GaAs lasers should have better properties than QDs/Si lasers. However, the use of Si substrates to grow InAs QDs’ based lasers opens up new possibilities for controlling the density and size of QDs, and therefore the emission of InAs QDs, for photonic devices integration using Si substrates when using specific type of SRL/substrate.
A significant number of lattice defects are produced in semiconductors as a result of radiation in space, which reduce the devices’ performance. The effect of gamma irradiation on the electrical properties of InAs QD lasers grown on GaAs and Si substrates was studied. The number of traps either decreased, remained the same or new defects were created after irradiation. Moreover, the DLTS and Laplace DLTS have been able to reveal a close connection between the grown-in defects and those induced by radiation.
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