| Abstract |
Among the group-III nitride (III-N) semiconductors, InN has been the least studied and also the most complex. However, InN is a promising material for sub-THz electronic devices due to the very high values of its electron low-field mobility (14,000 cm2/V.s) and maximum drift velocity (5.2 x 107 cm/s). InN and InN-rich alloys are also very interesting for optoelectronic devices in the IR wavelength region of telecommunications, as well as tandem solar cell applications, due to its 0.65 eV bandgap. This PhD dissertation is based on the study of plasma assisted molecular beam epitaxy (PAMBE) of InN on Si (111) and r-plane (0211) sapphire substrates. Epitaxial growth on silicon is interesting for low cost production and/or monolithic integration with Si integrated circuits (ICs). Growth of a-plane InN on r-plane ( 1102) sapphire substrates can be used for realizing quantum well heterostructures, free from polarization induced electric fields. Also, it has been theoretically predicted that nitrogen stabilized non-polar surfaces could be free from electron accumulation.
Direct InN growth on Si (111), using the optimum conditions for InN growth on GaN (0001) – substrate temperature 400-450oC and stoichiometric III/V flux ratio – results to 3D growth mode and porous columnar InN epilayers with bad adhesion at the InN/Si interface. A two-step growth process was developed, consisting of nucleating a very thin InN layer on Si at low temperature under N-rich growth conditions, and the growth of the main epilayer at the optimum InN (0001) growth conditions. The fast coalescence of the initial 3D islands of InN results to a continuous 20 nm InN film on the Si (111) surface with low 10 x 10 μm2 AFM rms surface roughness of 0.4 nm, which allows the main epilayer to be overgrown in step-flow growth mode, achieving an atomically smooth surface. The fast coalescence also assists defects annihilation near the InN/Si interface and 0.5 μm films exhibited threading dislocation (TD) density of 4.0x109 cm-2 for the edge-type and 1.7x109 cm-2 for the screw-type TDs. Similar defect densities were determined by TEM for InN films grown after initial deposition of an AlN/GaN nucleation layer on Si. However, those films exhibited significantly better electron mobility and lower crystal mosaicity according to XRD rocking curves.
The experiments of InN growth on r-plane ( 1102) Al2O3 substrates revealed that different InN crystallographic orientations could be realized depending on the InN nucleation conditions. Single crystal cubic (001) InN was grown on r-plane sapphire by using one-step growth at ~ 400oC, while polar c-plane (0001) or semipolar s-plane (1110) InN were observed by using a two-step growth process with InN nucleation at low temperature under N-rich or near stoichiometric III/V flux ratio conditions, respectively. Pure a-plane (0211) InN films were realized only when a-plane GaN or AlN nucleation-buffer layers were initially grown on r-plane sapphire. The structural quality of the a-plane InN films improved with increasing epilayer thickness, which is attributed to interaction and annihilation of defects. However, the growth of a-plane InN proceeds in 3D growth mode resulting to increasing surface roughness with increasing film thickness. A comparative study of the thickness dependent electrical properties of a-plane InN films grown on r-plane Al2O3 and c-plane films grown on GaN/Al2O3 (0001) templates was carried out by room temperature Hall-effect measurements. For both InN orientations, a rather linear increase of the electron sheet density (NS) with increasing thickness, consistent with a constant bulk concentration around 1 x 1019 cm-3 was observed. However, the electron mobilities of the c-plane InN films were more than three times those of the a-plane films, attributed to the presence of higher dislocation density (1.4 x 1011 cm-2) in the a-plane InN films. The analysis of the Hall-effect measurements, by considering the contribution of two conducting layers, indicates a similar accumulation of low mobility electrons with NS > 1014cm-2 at the films’ surface/interfacial region for both the a- and c-plane InN films. In general, similar electron concentrations were measured for all the different orientation InN films (polar c-plane, non-polar a-plane, semi-polar s-plane and cubic (001) InN). This suggests that similar surface/interfacial electron accumulation occurs independently of the InN crystallographic orientation, and the bulk donors are not related to the threading dislocations, since significant variations of defect densities occur for the different InN orientations. A SIMS investigation of a c-plane InN film exhibiting electron concentration of 1.09 x 1020 cm-3excludes hydrogen as the possible donor since its concentrations was 6.5x1018 cm-3. Only oxygen approached a concentration level near 1020 cm-3and this might be the unintentionally incorporated donor.
Finally, the spontaneous growth of InN nanopillars (NPs) on Si (111) and r-plane sapphire substrates was investigated. Optimization of the different growth parameters resulted to well-separated (0001) InN NPs on Si (111) that exhibited photoluminescence. Almost in all cases, the growth rate of the InN NPs along the c-axis is multiple of the In-limited growth rate. A non-uniform amorphous SixNy layer was inevitable under unoptimised growth conditions, leading to frequently observed NP misorientation (tilt) on Si substrates. Only c-axis oriented InN NPs were formed on the r-plane sapphire substrates.
In conclusion, the thesis has created new scientific knowledge for the heteroepitaxy of InN on Si (111) and ( 02 1 1 ) sapphire. Comparison with c-plane InN grown on GaN (0001) allowed the generic characteristics of InN to be extracted from the orientation-dependent ones.
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Heteroepitaxy of InN on silicon (111) and R-Plane sapphire substrates
