Superatmospheric MOCVD Growth of Bulk InGaN for Template and Optoelectronic Applications

InGaN, a III-Nitride wide bandgap semiconductor with theoretical bandgaps ranging between 0.7 eV (InN) and 3.39 (GaN), covers the entire visible spectrum and the near IR and UV. This makes InGaN an important material for photonic applications, including LEDs, laser diodes, and possibly solar cells.

To obtain smaller bandgaps within the material, more indium must be incorporated into the crystal. However, the differing chemical properties of indium and gallium, along with the limitations presented by the precursors, make indium incorporation beyond ~20% difficult to achieve, while maintaining high crystal quality. Work by Cheng Li et.al. indicates that photoluminescent intensity improves at higher pressure growths (still subatmospheric). Basic thermodynamics indicate that the temperature can be increased during growth at increased growth pressures, allowing an increase in process space. Also, that material stability can be improved by increasing the pressure during growth of InGaN.

A superatmospheric metal-organic chemical vapor deposition (MOCVD) reactor was designed and fabricated at UNC Charlotte to investigate the superatmospheric growth of InGaN, especially for growing high indium content InGaN. This reactor was designed to minimize gas phase reactions and to sustain smooth precursor flow while under superatmospheric conditions, solving one of the main problems with superatmospheric growth.

Experiments were done to obtain recipes for initial growths. This included finding critical thicknesses for InGaN of a set composition on GaN templates, V:III ratio experiments, estimating nitrogen vacancies by comparing elemental percentages from the EDAX results, optimizing growth temperatures, and flows for epitaxial growth.

Bulk InGaN was grown on Al₂O₃ and on n-type GaN templates. Early growths of InGaN on Al₂O₃ showed absorption below the GaN bandgap, with absorption starting around 2.4eV (~516nm) [figure]. Indium deposition was confirmed through EDAX results. X-ray diffraction results will be included in full paper, indicating phase and composition results. Early results grown on Al₂O₃did not show high intensity room temperature photoluminescence, although it could be detected with a spectrometer. However, the early photoluminescence results corresponded with absorption results, indicating that they were accurate.

InGaN was also grown on n-type GaN substrates, which were also grown by MOCVD and donated by VEECO. Greater photoluminescence was measured from early growths on GaN templates with the same recipes, presumably because the crystalline quality was increased by growing on a smooth, relaxed substrate, with a closer lattice match. Growth conditions, rates, and results will be discussed, along with any conclusions about the effects and capabilities of superatmospheric growth on parameters of bulk InGaN.