Growth, Structural And Physical Properties Of Certain Antimony Based III-V Diluted Magnetic Semiconductors

Abstract

Semiconductor devices are the building blocks of electronics and communication technology in the modern world. The charge, mass and spin of charge carriers in the semiconductor devices lay the foundations of the technology developments in the modern age. But to date only the electronic charge of the semiconductors has been exploited for such applications. The significance of the spin of charge carriers is completely ignored because in a semiconductor the half of the carriers are in spin-up state and the remainder are in spin-down state. A new electronics termed as spintronics, spin-transport based electronics, is focused to utilise the spin degree of freedom of the charge carriers in addition to its electronic charge. The devices based on these have the potential for various technological advancements like non-volatility, increased data processing speed, decreased electronic power consumption and increased integration densities as compared to the conventional semiconductor devices. In this study, the author intended to study the growth and properties of magnetic impurity doped antimony based III-V compounds and compare these results with those of the films grown by MBE. This thesis is organised into seven chapters. The first introductory chapter gives a brief review of the work on spintronics, diluted magnetic semiconductors, Ferromagnetic / paramagnetic semiconductor hybrid structures with special emphasis on the properties of antimonides which have already been reported in the literature. This is followed by the scope of the thesis. The second chapter deals with technical details of various instruments used in the present research work. Third chapter describes the growth and structural properties of bulk crystals grown by Bridgman method and thin films grown by liquid phase epitaxy (LPE). Bulk crystals of InSb and GaSb doped with magnetic elements such as Mn and Fe are grown with different doping concentrations. Thin films of InSb and GaSb doped Mn with different doping concentrations are grown by LPE. The grown crystals are processed by slicing, lapping, polishing and chemical etching methods. X-ray diffraction studies are carried out to confirm alloy formation and to find the change in lattice parameter if any. From the powder diffraction patterns, the lattice parameter is refined with the help of Retvield refinement program. A systematic change of lattice parameter with the incorporation of magnetic impurities into InSb and GaSb is observed. Scanning electron microscopy and energy dispersive x-ray analysis are carried out to identify the secondary phases and their composition respectively. Chapter 4 gives the detailed magnetotransport studies carried out on InSb and GaSb crystals doped with Mn and Fe. Also, the magnetotransport studies carried out on thin films grown by liquid phase epitaxy are presented here. This chapter is divided into three sections of which the first section deals with Mn doped bulk crystals of InSb and GaSb, the second section deals with Fe doped bulk crystals of InSb and GaSb and the third section deals with Mn doped InSb and GaSb films grown on GaAs by Liquid Phase Epitaxy. Temperature dependence of zero field resistivity, magnetoresistance and Hall measurements are carried out from 1.4 to 300K. All the samples show p type conduction throughout the temperature range studied except for Fe doped InSb. Mn doped crystals show negative magnetoresistance and anomalous Hall effect below 10K. Anisotropy in magnetoresistance is also observed at low temperatures in InMnSb crystals. On the other hand, Fe doped samples exhibit positive magnetoresistance throughout the temperature range and no anomalous Hall effect is observed. Chapter 5 describes the magnetic properties of bulk InMnSb, GaMnSb, InFeSb and GaFeSb crystals so also the thin films of InMnSb/GaAs. DC magnetization measurements are carried out in the temperature range 2 - 300K. The Mn doped InSb and GaSb crystals as well as InMnSb/GaAs films, show a magnetic ordering below 10K which could arise from the InMnSb and GaMnSb alloy formation. Also, saturation in magnetization observed even at room temperature suggests the existence of ferromagnetic MnSb clusters in the crystals which has been verified by scanning electron microscopy studies. In Fe doped InSb crystals, the temperature dependent DC magnetization shows irreversibility under field cooled and zero field cooled conditions below 12K, suggesting a spin glass-like behaviour. Also, magnetization measurement shows the coexistence of ferromagnetic and paramagnetic phases throughout the temperature range studied. Existence of ferromagnetic phase could arise from secondary phases Fe1-xInx or FeSb2 present in the crystal as clusters and paramagnetic phase could arise from the randomly distributed Fe atoms in the InSb matrix. Fe doped GaSb crystals show interesting magnetic property that arises from the FeGa alloy (secondary phase) present in it. The EPR studies on Ga0.98Mn0.02Sb cluster-free (?) crystal suggest that the dominant Mn impurity in GaMnSb is Mn2+ (d5), described as ionized acceptor A−. This conclusion was derived from EPR experiments, which reveal a strong absorption line with an effective g factor very close to 2.00, the value typical for centre A−. The absence of EPR signal typical for neutral Mn acceptor A0 suggests that this center is absent in the crystal under investigation. The observed behavior is similar to that of Ga1-xMnxAs and In1-xMnxAs epilayers. EPR studies also reveal that the competition between antiferromagnetic and ferromagnetic phases exists in the studied crystal. Chapter 6 describes the optical measurements carried out on bulk Ga1-xMnxSb crystals and their films with different Mn doping concentrations. FTIR studies are carried out in the temperature range 4 - 300 K. From the FTIR studies, it is found that intra valance spin – orbit splitting band absorption is dominant compared to the fundamental absorption in doped crystals. In higher doped crystals (x > 0.01), fundamental band absorption merges with split-off band and could not be resolved. Free carrier absorption studies are also carried out in the energy range below the band gap. FTIR studies on GaMnSb/GaAs films suggest band gap narrowing effect due to Mn doping. Furthermore the Photoluminescence measurements are carried out in the temperature range 10 – 300 K for all the Mn doped GaSb crystals. PL studies also support the band gap narrowing and band filling effects. A comprehensive summary of this research investigation and scope for the further work are presented in the last chapter.