InN quantum dots

InN has become a topic of interest for its band gap, being as small as 0.6-0.7 eV [66]. Potential application as active material for the important 1.3-1.55 pm wavelength region as well as in solar energy conversion using solar cells is expected if InN growth could be sufficiently mastered. Unfortunately, c-plane InN is highly lattice-mismatched by >10% to GaN(0001) and AlN(0001) lattice planes, making it very susceptible to plastic lattice relaxation. Otherwise the Stranski- Krastanow regime benefits from such strain values, and growth of coherently strained optically active quantum dots might be possible. The fairly low dissociation temperature for InN of around 550° C sets upper boundaries on growth temperature making growth of smooth heterostructures even more complicated.

Using MBE high-density (4-10¹¹ cm~²) arrays of InN quantum dots are grown for InN layer thicknesses >4 ML and growth temperatures around

Fig. 5.18. AFM images after InGaN growth for a) 60s, b) 120s, and c) 180s on top of SiO₂ treated GaN. (Reprinted with permission from [65], © 2006 American Institute of Physics.)

350-400°C [67]. Through an in vacuo surface characterization by STM a base length and dot height of 4-7 nm and 0.75-1.8 nm is measured, respectively, depending on the InN layer thickness (4-8 ML). Significantly larger dimensions (10 nm height, 30 nm base length) are observed when the material is grown using a plasma-assisted nitrogen source, indicating a sensitive dependence of the

Fig. 5.19. Dependence of InN QD density on growth temperature during MOVPE growth (AFM images). (Reprinted with permission from [70], © 2006 American Institute of Physics.)

growth chemistry on V/III ratio. Another study monitoring oscillations of the characteristic RHEED pattern indicates a critical layer thickness for the 2D/3D transition of about 2 MLs [68]. In the same study, however, plastic relaxation after the first InN monolayer is noticed, i.e., prior to the 2D/3D transition. The case for MOVPE-grown InN quantum dots is a little different, as the precursor pyrolysis of trimethylindium and in particular ammonia requires temperatures well above 400° C. Still, quantum dot structures of roughly 25 nm diameter and 3-6 nm height are realized at 550°C temperature and very high nominal V/III ratios of 15000 [69]. Note that the actual V/III ratio is much lower, as decomposition of ammonia at 550°C amounts to only about 0.01%. In the nitrogen carrier gas, quantum dot densities are of the order of 10⁷-10⁸ cm~². An argon carrier gas atmosphere allows reduction of the nominal V/III ratio to 5000, while at the same time the QD density is increased to 5-10¹⁰ cm~² [71]. Unfortunately, structural analysis by TEM reveal misfit dislocation networks at the interface between MOCVD-grown InN layers and the underlying GaN buffer layer [72]. Photoluminescence signals detected at energies around 950 meV, besides being weak, show no correlation to characteristic quantum dot properties such as size and barrier material, which underlines the still persisting difficulties to realize the potential of InN for optoelectronic applications [70].