Growth and Characterization of Metastable Wide Band-Gap Al1-xInxN Epilayers

Abstract: InN to 6.2 eV for AlN, which opens possibilities to engineer opto-electronic devices operating from infra-red to deep ultra-violet wavelengths. Al1-xInxN with the alloy composition x~0.2 can also be used as a lattice-matched electron confinement layer for GaN based electronic devices. However, the ternary Al1-xInxN system exhibits a miscibility gap for compositions in the range 0.1<0.9 where a stable alloy cannot be grown under thermodynamic equilibrium conditions, which is why low temperature growth techniques such as magnetron sputtering are of advantage for their syntheses.This thesis describes the growth and structural characterization of epitaxial 2h-Al1-xInxN(0001) [0<1] thin films synthesized by dual DC reactive Magnetron Sputter Epitaxy (MSE) in an ultra-high vacuum (UHV) deposition system. Growth parameters such as deposition temperature, substrate bias, and magnetron power settings were adjusted in order to control the film stoichiometry and crystallinity. The role of in-situ deposited Ti1-yZryN(111) [0£y£1] seed layers on the Al1-xInxN growth was also investigated. It was found that the ZrN(111) seed layers provide a wider stoichiometric composition region for the wide band-gap nitride at elevated temperatures due to its generally lower lattice mismatch as compared to TiN(111). Microstructural characterization of Al1-xInxN deposited at temperatures from room temperature to 900 °C was carried out by X-ray diffraction (XRD) techniques and transmission electron microscopy (TEM). TEM micrographs revealed a dense and columnar microstructure with column widths ranging from 10 to 200 nm depending on growth temperature and seed layer. In addition, a novel generic growth mode giving rise to extremely curved, though stress-free, crystal lattices was observed and investigated. It was found that these, so called, nano-grass structures arise due to specific kinetic and geometrical limitations during growth. Compositional differences are formed over the columns due to self-shadowing effects, which are partly preserved due to the low surface ad-atom mobility. Thus, extremely curved crystalline columns can be formed. XRD investigations showed thatsingle-phase wurtzite epitaxial Al1-xInxN was obtainable throughout the whole composition range for deposition temperatures of up to 600 °C onto ZrN(111) seed layers. At higher temperatures almost pure hexagonal AlN was formed. XRD and selected area electron diffraction also showed that the Al1-xInxN films were grown hetero-epitaxially onto Ti1-yZryN with the epitaxial relationship: Al1-xInxN(0001) // Ti1-yZryN(111) and Al1-xInxN[11-20] // Ti1-yZryN[110]. Based on the results, pseudo-binary phase diagrams for MSE deposition of 2h-Al1-xInxN, at temperatures up to 1000 °C, onto TiN(111) and ZrN(111) coated MgO(111) could be established.A study on the relationship between the Al1-xInxN mole fraction x, in the range 0.07<0.82, and the lattice parameters was carried out for epitaxial Al1-xInxN(0001)/ Ti1-yZryN/MgO(111) films using Rutherford Backscattering Spectrometry (RBS) and XRD. A non-linear relationship was found with a maximum deviation as large as 37 % from the commonly used linear Vegard’s rule. The highest relative deviations were found at low InN mole fractions, while the largest absolute deviation was found at x=0.63. This shows that Vegard’s rule is not directly applicable to determine the compositions in the Al1-xInxN system.Moreover, the post-growth thermal stability of the Al1-xInxN films was investigated by in-situ annealing in an X-ray diffractometer during extensive time periods at temperatures up to 1300 °C. It was found that the thermal stability of the Al1-xInxN films increased with a decreasing In content. Al0.87In0.13N deposited onto ZrN(111) at 300 °C showed to be stable up to 1050 °C, which is about the growth temperature used for GaN in HVPE, thus showing that Al1-xInxN can be used as a template for lattice-matched growth of GaN devices.An optimized growth process for Al0.8In0.2N(0001) onto lattice matched seed layers of Ti0.2Zr0.8N(111) was developed. A mild ion assistance together with a low deposition temperature was found to give the best over-all epi-layer quality. Full-width-at-halfmaximum (FWHM) values of ~40 arc min. was obtained in XRD rocking mode. Furthermore, luminescence at wavelengths as short as 248 nm was observed in these epilayers by cathodoluminescence (CL) measurements performed at 5 K. The corresponding energy of 5.0 eV is the highest reported to date for Al0.8In0.2N.In summary, these results point on the feasibility of metastable Al0.8In0.2N solid solutions as an active luminous material in opto-electronics. It also shows that MSE-grown Al1-xInxN has a high band gap, which make it an excellent choice as a strong charge carrier confinement epi-layer in lattice matched GaN or In1-zGazN hetero-structures.