Growth and Scanning Tunneling Microscopy Studies of Manganese Induced Structures on w-Gallium Nitride (0001¯)

A great deal of research work has been put into GaN based diluted magnetic semiconductors with the desire of fabricating applicable spintronic devices. Dietl et al. (1) (2) proposed that Mn-doped GaN would be ferromagnetic with TC above room temperature on the basis of mean-field Zener model calculations. (3) (4) (5) The diluted III-V magnetic semiconductors ∼DMS are materials which exhibit spontaneous ferromagnetism mediated by holes in the valence band of the host semiconductor and thus represent new materials with promising applications in spintronics. DMS have been a point of constant debate over the past decade due to various contradicting reports published, both experimental and theoretical, regarding the properties of these materials. Many groups reported growth of Mn-doped GaN by molecular beam epitaxy (MBE), but with varying results, (6) (7) (8) (9) (10) (11) (12) (13) (14) some experimental reports propose a ferromagnetic behavior over a wide range of Curie temperatures from 10K to 940K. Few reports state that the lattice constant of GaN increases after Mn doping and there are reports stating that the lattice constant of GaN actually decreases on Mn incorporation. The position of Mn atoms in the GaN bulk has never been experimentally determined and the role it plays, either, as substitutional Mn or interstitial Mn has never been experimentally verified. Efforts to introduce transition metal atoms into III-V semiconductor host materials have been difficult due particularly to solubility issues. This low solubility could be exploited in order to develop an ideal magnetic/semiconductor bi-layer. (15) Magnetic MnGa was found to grow with an abrupt interface and well-defined epitaxial orientation on top of wurtzite (w)-GaN. In that study, thick layers of MnGa were deposited. Reflection high energy electron diffraction (RHEED) patterns suggested a well-ordered structure at the initial stage of growth. Certainly, a thin ordered layer of antiferromagnetic element like Mn atoms on GaN could be of great interest for not only fundamental investigations but also for ultimately thin (single monolayer) magnetic devices like spin current carriers etc. We conducted a series of experiments aimed at understanding the precise position and electronic properties of Mn atoms on/in to GaN surface.;GaN samples were prepared on a pre-cleaned Sapphire (0001) substrate. The as grown samples show the standard GaN reconstructions well studied earlier. (16) (17) The samples were annealed to evaporate the excess Ga and to create a clear 1x1 structure. Mn was then deposited on to this clear 1x1 surface at different temperatures. The growth was constantly monitored with Reflection High Energy Electron Diffraction. The samples were transferred in situ in to the analysis chamber, where Scanning Tunneling Microscopy (STM) and Auger Electron Spectroscopy (AES) experiments were performed. Results indicate that the Mn, when deposited on to a bare GaN surface at low temperature forms a 3 x 3 structure, which is very unstable and quickly transforms to highly stable structure when small amount of heat is provided. Mn when deposited at higher temperatures prefers forming a very stable ✓3 x ✓3 R30° structure. Theoretical calculations show that the Mn atoms prefer to replace the Ga atoms in the surface ad layer, pushing the Ga atoms in the vicinity and forming a high density 2-D Mn-Ga structure as an adlayer. The ✓3 x ✓3 R30° structure has very interesting spintronic properties as evident in spectroscopy performed on this surface.;To test the behavior of Mn on to GaN with excess Ga ad-atoms, GaN was grown under Ga rich conditions resulting in a final surface with excess Ga accumulation of ∼ 0.1-0.4 ML (not including Ga droplets). Mn was then deposited on to this Ga rich N-polar GaN surface at a low temperature. The presence of Mn on to the Ga rich 3x3 reconstructed surface has a dramatic effect of splitting the reconstructed surface in to striped structure, with trenches separating these bands, referred to as the wetting layer. The trenches can be attributed, either to the sinking of the Mn atoms in to the adlayer, causing stress in the 3x3 reconstructed Ga layer or to the presence of Mn generated vacancies in the adlayer. The excess Mn forms three dimensional islands along with the surface Ga ad-atoms. The islands have distinct quantum heights namely five monolayers and six monolayers, thus pointing to an electronic growth mode preferred by the islands. The islands are several square nm in area and have a well-defined rectangular unit cell. Spectroscopy data shows a very metallic nature of the islands.;The field of nano-engineering is diversifying rapidly bringing in more demands for different material growth techniques, resulting in to increased interest in Pulsed Laser Deposition. Pulsed Laser Deposition is a very diverse technique which can be used for a wide range of material growths. We have engineered a pulsed laser deposition system that has a vertical target wheel assembly, completely shielded from the surrounding molecular beam epitaxy environment and can be used alongside of MBE. The system has a capability of storing 8 elements. The Excimer Laser is mounted on the system frame, which supports the distribution, molecular beam epitaxy and scanning tunneling microscopy chambers. The laser points directly on the target selected. The power and pulse-rate of the laser are tunable, up to a maximum of 150mJ depending on gas mixture; this allows a lot of experimental freedom with the setup. The laser beam is rastered on the surface of the target using a focusing lens. The laser operation, the lens movement, and the sample rotation are automated for improving precision and for the ease of operation. Combined with the molecular beam epitaxy, the pulsed laser deposition system would open up infinite possibilities for growing new materials using different material growth techniques.