Rare Earth Doped III-Nitride Nanostructures for Device Applications.
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Rare earth(RE)-doped III-nitride nanostructures (thin films, columnar structures, and nanorods) are interesting materials for the optoelectronics, lasers, spintronics, light emitting diodes, solar cells, and display devices due to their interesting properties like room temperature dilute magnetic semiconductor (DMS), sharp emission wavelengths in a broader range from the near infrared to ultraviolet region. The advancement of growth techniques like molecular beam epitaxy (MBE) is being used as an effective tool for introducing RE-ions into GaN material for the enhanced structural, optical, magnetic, and electronic properties. In this present thesis, I tried to explore the issues and concerns related to these aforementioned materials by growing the undoped and RE-doped GaN, and In- GaN nanostructures by using MBE. In the beginning of this study, the growth, surface, luminescence, and magnetic properties are studied from the Er-doped GaN thin films of 180 nm thickness grown by MBE on c-sapphire substrates without any buffer layer and with different Er-concentrations. In situ reflection high energy electron diffraction (RHEED) patterns revealed the crystalline and uniform growth of the films. X-ray Diffraction (XRD) pattern of thin films showed c-axis oriented growth. The atomic force microscopy (AFM) analysis showed an enhancement of surface morphology and smoothness with increasing Er-doping, which could be due to the minimization of surface defects because of the gettering effect of rare earth. The scanning area dependent surface morphology analysis shows a power law dependence indicating the fractal nature of the surface, which is confirmed by the observation of non-integer D (fractal dimension) value. X-ray photoelectron spectroscopy (XPS) reveals the formation of GaN:Er phase and rules out the presence of Ga and Er metallic and native oxide phases. Semi-quantitative elemental composition of the films were carried out using N 1s, Ga 2p3/2, and Er 4d photoemission lines. The Er concentration was estimated from XPS speci tra and was found to be between 3.0 - 9.0 at. % (~1021 at./cm3). Photoluminescence (PL) and cathodoluminescence (CL) studies show visible emission and concentration quenching of Er3+ ion in agreement with reported results. The excitation processes of Er3+ ion might be affected by the charge trapping due to Er-doping induced defect complexes. The magnetic measurements carried out by superconducting quantum interference device (SQUID) showed a ferromagnetic-paramagnetic phase transition at low temperature, contrary to the reported room temperature ferromagnetism in metal-organic chemical vapor deposition (MOCVD) grown GaN:Er thick films of 550 nm. In the second part of this study, the green-gap issue has been addressed with the high In-content In1−xGaxN thin films with indium content of x = 14 - 18 at. % grown by using MBE at high growth temperatures from 650 ◦C to 800 ◦C. In situ RHEED of the In1−xGaxN films confirmed the Stranski-Krastanov growth mode. XRD of the films confirmed their high crystalline nature having c-axis orientation with a small fraction of secondary InN phase admixture. High resolution crosssectional scanning electron microscopy (SEM) images showed two-dimensional epilayers growth with thickness of about 260 nm. The high growth temperature of In1−xGaxN epilayers is found to be favorable to facilitate more GaN phase than InN phase. All the fundamental electronic states of In, Ga, and N were identified by XPS and the indiumIn-composition has been calculated and found to be 14- 18 at.%. The composition calculations from XRD, XPS, and PL are closely matching with each other. The biaxial strain has been calculated and found to be increasing with the In content. Growing In1−xGaxN at high temperatures resulted in the reduction in stress/strain which affects the radiative electron-hole pair recombination. The In1−xGaxN film with lesser strain showed brighter and stronger green emission than films with larger built-in strain. A weak S-shaped near band edge emission profile confirms the relatively homogeneous distribution of In. The near band edge (NBE) emission studies at different places on the substrate showed highly homogeneous distribution of In in all over the sample. The optical emission properties of undoped In1−xGaxN thin films were compared with the RE-doped In1−xGaxN thin films and the enhanced optical emission has been demonstrated from the Yb-doped In1−xGaxN thin films. In the third part of this study, the growth of high In content InGaN:Yb nanorods grown on c-plane sapphire (0001) substrates using plasma assisted MBE were reported. The in situ RHEED patterns recorded during and after the growth revealed crystalline nature of the nanorods. The nanorods were examined using high resolution SEM and AFM. The PL studies of the nanorods showed the visible emissions. The In composition was calculated from XRD, XPS, and the PL spectroscopy. The In-concentration was obtained from PL using modified Vegard’s law and found to be around 37 % for InGaN and 38 % for Yb (3.2 ± 1%)-doped InGaN with a bowing parameter b = 1.01 eV. The Ybdoped In1−xGaxN showed significant enhancement in PL properties compared to the undoped InGaN. The Yb-doped InGaN nanorods demonstrated the shifting of the PL band at room temperature, reducing luminescence amplitude temperature dependent fluctuation, and significant narrowing of excitonic emission band as compared to the undoped InGaN. A strong nanorod size dependent PL position has been revealed from the PL studies at different places on the substrate. The magnetic properties measured by SQUID reveal the above room temperature ferromagnetism, which can be explained by the double exchange mechanism and magnetostriction.