P. A. Anderson

Buried p-type layers in Mg-doped InN

Variable magnetic field Hall effect, photoluminescence, and capacitance-voltage (CV) analysis have been used to study InN layers grown by plasma assisted molecular beam epitaxy. All three techniques reveal evidence of a buried p-type layer beneath a surface electron accumulation layer in heavily Mg-doped samples. Early indications suggest the Mg acceptor level in InN may lie near 110meV above the valence band maximum. The development of p-type doping techniques offers great promise for future InN based devices.

Magnetic and optical properties of the InCrN system

Room-temperature ferromagnetic In1−xCrxN films with x ranging from 0.0005 to 0.04 and antiferromagnetic CrN films have been grown by plasma-assisted molecular-beam epitaxy. Electron and x-ray-diffraction techniques could find no evidence for precipitates or phase segregation within the films. Ferromagnetism was observed in the In1−xCrxN layers over a wide range of Cr concentrations, with the magnitude of the ferromagnetism found to correlate with the background carrier concentration. Higher n-type carrier concentrations were found to lead enhanced ferromagnetism, with maximum saturation and remnant moments of 7 and 0.7emu∕cm3, respectively. The addition of Cr to the InN matrix led to reduced photoluminescence intensity and a shift of the peak to higher energy. These observations along with a band-gap-like optical transmission feature at 0.7 eV suggest that CrN has an indirect gap of approximately 0.7 eV and a direct Γ-valley gap greater than 1.2 eV.

Optical and compositional properties of indium nitride grown by plasma assisted molecular beam epitaxy

Single-crystalline and polycrystalline indium nitride films have been grown on (0001) sapphire and silica glass using plasma assisted molecular beam epitaxy (PAMBE). Optical measurements on the films revealed a luminescence feature in the vicinity of 0.8 eV for all films, both on sapphire and glass. No feature around 1.9 eV could be identified above the background noise. To our knowledge this is the first report of polycrystalline InN exhibiting the 0.8 eV feature. Ion beam analysis of the material could find no measurable oxygen contamination in the bulk of the films. These results, along with recent reports of blue shifting of the absorption onset of InN films with increasing oxygen content, appear to point towards oxygen contamination as being the source of the previously reported higher bandgap.

Structural and optical properties of indium nitride grown by plasma-assisted molecular beam epitaxy

The bandgap of indium nitride has long been accepted to be 1.9 eV. However, recent results have cast doubt over this as modern epitaxy techniques have allowed experimental studies of high quality material. Single crystalline and polycrystalline indium nitride films have been grown on (0001) sapphire and silica glass using plasma assisted molecular beam epitaxy (PAMBE). Optical measurements on the films revealed a luminescence feature in the vicinity of 0.8 eV for all films, both on sapphire and glass. No feature around 1.9 eV could be identified above the background noise. To our knowledge this is the first report of polycrystalline InN exhibiting the 0.8 eV feature. Ion beam analysis of the material could find no measurable oxygen contamination in the bulk of the films. These results along with recent reports of blue shifting of the absorption onset of InN films with increasing oxygen content appear to point towards oxygen contamination as being the source of the previously reported higher bandgap. Like other groups we observed a small anomalous blue shifting of the luminescence with increasing temperature when using a germanium detector. We have verified that this is a real feature by measuring the temperature dependent PL with a lead sulphide detector. Two distinct growth regimes were identified. High In:N flux ratios lead to spotty RHEED accompanied by a morphology of flat plateaus separated by narrow valleys. Low In:N flux ratios lead to rough films consisting of facets largely disjoint from each other. Surprisingly, this regime gave streaky RHEED, suggesting high levels of crystal alignment between facets and high crystal quality within facets.

Influence of Nitrogen Species on InN Grown by PAMBE

Active nitrogen species produced by an Oxford Applied Research HD-25 plasma source have been monitored by optical emission spectroscopy and quadrapole mass spectroscopy. Both techniques confirmed that at higher RF powers and lower flow rates the efficiency of atomic nitrogen production increased; emission spectroscopy confirmed that this was at the expense of active molecular nitrogen (N2 * ). InN films grown on (0001) sapphire/GaN with higher relative molecular content were found to have lower carrier concentrations than the corresponding films grown with higher atomic content. However, electrical properties of films grown on (111) YSZ showed insensitivity to the active nitrogen content. Etching experiments revealed that films grown on sapphire/GaN were nitrogen-polar, while films grown on YSZ were In-polar, suggesting that film polarity can greatly influence the effect active species have on growth. Lattice relaxation, as measured by reflection high-energy electron diffraction, revealed that the N-polar films grown under high relative molecular flux relaxed fully after ~60 nm of growth, while the corresponding In-polar film relaxed fully within the first several nm of growth.

Towards Quantifying the Bandgap Energy of Indium Nitride

InN grown on sapphire and silica glass, and InGaN alloys grown on silica glass have been studied by TEM, optical absorption, photoluminescence (PL) and photoconductivity (PC). The peak PL location from InGaN films was found to reduce steadily in energy with increasing indium mole fraction from 3.4 eV to 0.7 eV. The 0.7 eV PL feature was observed despite n-type carrier concentrations as high as 1020 cm-3. PL and PC studies on InN grown on sapphire showed a PC onset near 0.73 eV and strong PL signal around 0.65 eV. TEM could find no evidence for indium clustering within the InN films. These observations strongly suggest that the 0.7 eV feature is related to band to band transitions and not a deep level state