The authors report on the growth, structure, and emission properties of AlGaN double heterostructures having a graded-index-separate-confinement-heterostructure design. These devices were grown on the Si-face of 6H-SiC substrates by plasma-assisted molecular-beam epitaxy. The active region of the device consists of 75-nm thick Al 0.72 Ga 0.28 N film, confined by two 50-nm thick compositionally graded Al x Ga 1Àx N films (x ¼ 1-0.8 and x ¼ 0.8-1) and two AlN cladding layers. X-ray diffraction and transmission electron microscopy provide evidence that the compositionally graded AlGaN layer may also be serving as a strain transition buffer, by blocking threading defects in the vicinity of the AlN/AlGaN heterointerface. Polarization dependent photoluminescence studies indicate that the emission from these structures at 257 nm is transverse magnetic polarized. Simulation studies indicate that the vertical confinement of the optical mode in these structures is 32.5% and simulations of the band structure indicate the formation of a p-n junction resulting from polarization-induced doping. Electron-beam pumping of these structures provides evidence of the onset of stimulated emission at room temperature. V

No abstract

The authors report the development of high internal quantum efficiency AlN/AlGaN/AlN double heterostructures and AlGaN/AlN multiple quantum wells (MQWs) grown on 6H-SiC and 4H-SiC substrates of various miscuts by plasma-assisted molecular-beam epitaxy. The authors find that the luminescence spectra for identical MQWs show a single peak across the gap, with a wavelength that is redshifted by $20 nm as the excess Ga during growth of the wells increases. The internal quantum efficiency of the double heterostructures emitting at 250 nm is found to be 43%, and that of the multiple quantum wells emitting at 245 nm is 68%. These results suggest that AlGaN alloys on SiC substrates are capable of producing deep-ultraviolet emitters with high efficiency. The authors propose that these results can be accounted for by the introduction of lateral band structure potential fluctuations due to the changing of the growth mode from physical vapor phase epitaxy to liquid phase epitaxy (LPE) as the excess gallium increases. In this LPE mode the arriving active nitrogen species from the plasma source and aluminum atoms from the aluminum effusion cells dissolve in the excess liquid gallium and incorporate into the film from the liquid phase.

Plasma-enhanced atomic layer deposition (PEALD) enables the epitaxial growth of ultrathin indium nitride (InN) films at significantly reduced process temperatures and with greater control of layer thickness compared to other growth methods. However, the reliance on plasma-surface interactions increases the complexity of the growth process. A detailed understanding of the relationship between the plasma properties and the growth kinetics is therefore required to guide the tuning of growth parameters. We present an in situ investigation of the early-stage PEALD growth kinetics of epitaxial InN within three different plasma regimes using grazing incidence small-angle x-ray scattering (GISAXS). The GISAXS data are supported by diagnostic studies of the plasma species generation in the inductively coupled plasma source as a function of the relative concentrations of the nitrogen/argon gas mixture used in the growth process. The growth mode is found to be correlated to the production of nitrogen species in the plasma, with high concentrations of the atomic N species promoting Volmer–Weber growth (i.e., island growth) and low concentrations promoting Stranski–Krastanov growth (i.e., layer-plus-island growth). The critical thickness for island formation, island center-to-center distance, and island radius are found to increase with ion flux. Furthermore, the island center-to-center distance and areal density are observed to change only during plasma exposure and to continue changing with exposure even after the methylindium adlayer is believed to have fully reacted with the plasma. Our results demonstrate the potential to control the growth kinetics during PEALD of epitaxial films by intentionally accessing specific regimes of plasma species generation.

The surface topological evolution during the growth of indium nitride (InN) by plasma-assisted atomic layer epitaxy (ALEp) on gallium nitride (GaN) (0001) substrates was studied using in situ real-time grazing incidence small-angle x-ray scattering (GISAXS) for 180, 250, and 320 °C growth temperatures. The GISAXS data reveal that the ALEp growth of InN on GaN in this temperature range proceeds in a Stranski–Krastanov mode, in which the 2D–3D transition occurred after 2.3 monolayers for 180 °C, 1 monolayer for 250 °C, and 1.5 monolayers for 320 °C. The corresponding initial island center-to-center distances were 7.4, 11.6, and 11.7 nm. Additionally, island coarsening was observed to increase with temperature. After 200 growth cycles, the mean island diameters were 3.9, 5.6, and 7.0 nm, and the mean island center-to-center distances were 8.6, 13.7, and 17.1 nm for 180, 250, and 320 °C growth temperatures, respectively. For the 320 °C growth, the mean island shape was observed to gradually evolve from relatively mounded to cylindrical. These results are supported by atomic force microscopy and specular x-ray reflectivity.