[show abstract][hide abstract] ABSTRACT: The composition dependence of the Fermi-level pinning at the oxidized (0001) surfaces of n-type InxGa1−xN films (0 ⩽ x ⩽ 1) is investigated using x-ray photoemission spectroscopy. The surface Fermi-level position varies from high above the conduction band minimum (CBM) at InN surfaces to significantly below the CBM at GaN surfaces, with the transition from electron accumulation to depletion occurring at approximately x = 0.3. The results are consistent with the composition dependence of the band edges with respect to the charge neutrality level.
[show abstract][hide abstract] ABSTRACT: Indium nitride (InN) is identified as a promising terahertz (THz) emitter based on the optical and electronic properties of high quality In- and N-face samples. Time domain THz spectroscopy has been employed to measure the pump wavelength and background carrier concentration dependence of THz emission from InN. There is no discernable difference between the In- and Nface InN samples, as expected for the improved crystalline quality and concomitant low background electron density and high mobility for both polarities. While there is only a weak dependence of THz signal on pump wavelength from 800 nm to 1500 nm, there is a strong dependence on background electron density. Modeling shows that the dominant mechanism for THz generation in bulk InN is the current associated with the diffusion of the photo-generated electrons at elevated electron temperature (photo-Dember effect) and the redistribution of the background electrons under drift, with larger screening from the higher mobility electrons as compared to holes. Compensation or p-type doping in conjunction with manipulation of the large internal electric fields in InN/InGaN nanostructures should lead to significant improvements in THz emitters.
[show abstract][hide abstract] ABSTRACT: The authors report the observation of strong polarization anisotropy in the photoluminescence (PL) and the absorption spectra of  oriented A-plane wurtzite InN films grown on R-plane (102) sapphire substrates using molecular beam epitaxy. For A-plane films the c axis lies in the film plane. The PL signal collected along  with electric vector E⊥c is more than three times larger than for E‖c. Both PL signals peak around 0.67 eV at 10 K. The absorption edge for E‖c is shifted to higher energy by 20 meV relative to E⊥c. Optical polarization anisotropy in wurtzite nitrides originates from their valence band structure which can be significantly modified by strain in the film. The authors explain the observed polarization anisotropy by comparison with electronic band structure calculations that take into account anisotropic in-plane strain in the films. The results suggest that wurtzite InN has a narrow band gap close to 0.7 eV at 10 K.
[show abstract][hide abstract] ABSTRACT: Femtosecond two-color pump-probe experiments on intersubband absorption spectra of electrons in a GaN/AlN superlattice show distinct spectral holes unraveling both the homogeneous broadening contribution and the underlying optical phonon progression.
[show abstract][hide abstract] ABSTRACT: Transmission Electron microscopy was applied to determine similarities and difference in structural perfection between InN epi-layers grown by MBE on c-and r-plane Al2O3 and InN nanocrystals grown on similar substrates using non catalytic, template-free hydride metal-organic vapor phase epitaxy. The study showed that nanocrystals had more perfect crystallinity compared to the layers. While the InN layer growth direction followed the crystallographically required epitaxial growth orientation, the nanorod growth was randomly oriented. The opposite growth polarity has been determined for the InN layers grown along c-direction (In-polarity) in comparison to nanocrystals grown along same direction (N-polarity).
[show abstract][hide abstract] ABSTRACT: With current advances in sub-angstrom resolution scanning transmission electron microscopy (STEM), it is now possible to image directly local crystal structures of materials where dramatically different atoms are separated from each other at distances about or less than 1 angstrom. We achieved direct imaging of atomic columns of nitrogen in close proximity to columns of aluminum in wurtzite aluminum nitride by using annular dark field imaging in an aberration-corrected STEM. This ability allows direct determination of the local polarity in nanoscale crystals and crystal defects.
[show abstract][hide abstract] ABSTRACT: High-quality epitaxial InN thin films grown on (0001) sapphire with GaN buffer were characterized using transmission electron microscopy. It was found that the GaN buffer layer exhibits the (0001) Ga polarity and the InN film has In-terminated polarity. At the InN/GaN interface, there exists a high density of misfit dislocation (MD) array. Perfect edge threading dislocations (TDs) with (1/3)〈1120〉 Burgers vectors are predominant defects that penetrate the GaN and InN layers. Pure screw and mixed TDs were also observed. Overall, the TD density decreases during film growth due to annihilation and fusion. The TD density in GaN is as high as ∼1.5 × 1011 cm−2, and it drops rapidly to ∼2.2 × 1010 cm−2 in InN films. Most half-loops in GaN are connected with MD segments at the InN/GaN interface to form loops, while some TD segments threaded the interface. Half-loops were also generated during the initial stages of InN growth.
Journal of Materials Research. 06/2006; 21(07):1693 - 1699.
[show abstract][hide abstract] ABSTRACT: As-grown InN is known to exhibit high unintentional n-type conductivity. Hall measurements from a range of high-quality single-crystalline epitaxially grown InN films reveal a dramatic reduction in the electron density (from low 1019 to low 1017 cm−3) with increasing film thickness (from 50 to 12 000 nm). The combination of background donors from impurities and the extreme electron accumulation at InN surfaces is shown to be insufficient to reproduce the measured film thickness dependence of the free-electron density. When positively charged nitrogen vacancies (VN+) along dislocations are also included, agreement is obtained between the calculated and experimental thickness dependence of the free-electron concentration.
[show abstract][hide abstract] ABSTRACT: Large optical bleaching effect was found in epitaxial InN layers from Z-scan measurements at 1.054 μm laser wavelength. Optical transmittance increases nearly five times at the largest light intensities used in experiment. The bleaching recovery time shorter than 3 ps was evidenced from time-resolved measurements at this wavelength, which is much faster than the electron lifetimes of 40 and 240 ps determined on two epitaxial layers by visible pump-terahertz probe technique. Spectral investigations of the bleaching effect performed in the wavelength range from 1 to 1.55 μm support the conclusions on a narrow band gap of InN.
[show abstract][hide abstract] ABSTRACT: A study of the photoluminescence characteristics of InxGa1−xN alloys in which the Fermi level is controlled by energetic particle irradiation is reported. In In-rich InxGa1−xN the photoluminescence intensity initially increases with irradiation dose before falling rapidly at high doses. This unusual trend is attributed to the high location of the average energy of the dangling-bond-type native defects (the Fermi level stabilization energy). Our calculations of the photoluminescence intensity based on the effect of the electron concentration and the minority carrier lifetime show good agreement with the experimental data. Finally the blueshift of the photoluminescence signal with increasing electron concentration is explained by the breakdown of momentum conservation due to the irradiation damage.
[show abstract][hide abstract] ABSTRACT: The first evidence of successful p-type doping of InN is presented. It is shown that InN:Mg films consist of a p-type bulk region with a thin n-type inversion layer at the surface that prevents electrical contact to the bulk. Capacitance-voltage measurements indicate a net concentration of ionized acceptors below the -type surface. Irradiation with 2 MeV He+ ions is used to convert the bulk of InN:Mg from p to n-type, at which point photoluminescence is recovered. The conversion is well explained by a model assuming two parallel conducting layers (the surface and the bulk) in the films.
[show abstract][hide abstract] ABSTRACT: We report femtosecond near-infrared transient photoreflection measurements of native n-type indium nitride and silicon-doped indium nitride thin films. The overall time dependence of the ultrafast reflectivity transient is characterized by the different time scales of carrier cooling and carrier recombination. Experimental analysis demonstrates nonradiative recombination in the picosecond and subpicosecond range as the dominant recombination mechanism at room temperature even at very high carrier concentrations. Silicon-doped InN films exhibit carrier lifetimes as short as 680 fs.
[show abstract][hide abstract] ABSTRACT: GaN, AlN, InN and their alloys have long been considered as very promising materials for device applications. Semiconductor alloys such as InxGa1-xN have been successfully used in the fabrication of blue-green light emitting diodes and laser diodes. Recently, growth of high quality InN as well as InxGa1-xN have been demonstrated. In Particular, progress in the manufacturing of very high quality, single-crystal InN thin films has opened up a new challenging research avenue in the III-nitride semiconductors. InN together with its alloys of GaN and AlN enable the operation of light emitting diodes and diode lasers ranging in spectral wavelength from infrared all the way down to deep ultraviolet. It has also been predicted that InN has the lowest electron effective mass among all the III-nitride semiconductors. As a result, very high electron mobility and very large saturation velocity are expected. In this paper, we report experimental results of electron transient transport on InN thick film grown on GaN. Electron drift velocity as large as 7.5x10^7cm/ has been found when the sample is excited by an ultrafast laser pulse with pulse width about 600 femtoseconds. Our findings demonstrate that InN has great potential for use in the ultrafast electronic devices.
[show abstract][hide abstract] ABSTRACT: We have investigated temperature dependence of Hall mobility and carrier density for thin InN films with low and high carrier density grown by Molecular Beam Epitaxy (MBE) and Plasma Source Molecular Beam Epitaxy (PSMBE), respectively. At very low temperatures, a large concentration of carriers which are temperature independent is observed in both the low and high density films. However, the behavior of mobility for the low density film is different from that of the high density film particular for temperatures less than 300K. For the low density film, mobility increases with temperature and passes through a maximum around 250 K in contrast to temperature independent mobility observed for the high density film for T
[show abstract][hide abstract] ABSTRACT: We investigate the temperature dependence of Hall mobility μ and carrier density Ne for thin InN films grown by molecular-beam epitaxy and plasma source molecular-beam epitaxy over three orders-of-magnitude difference in their carrier density: for the low-density film Ne = 5.8×1017/cm3 and for the high-density film Ne = 3.2×1020/cm3. In both the films, for temperature up to 300 K, a large temperature-independent concentration of carriers is observed. For higher temperatures, however, carrier density increases with temperature. The characteristic behavior of the mobility for the low-density film is different from that of the high-density film, particularly for temperatures less than 300 K. The low-density film shows a peak behavior in the mobility around 250 K in contrast to the temperature-independent mobility observed for the high-density film for T<300 K. We investigate theoretically the concentrations of donor, acceptor, and threading dislocations for both the films and also discussed various electron-scattering mechanisms which contribute to the mobility in these films.
Journal of Applied Physics 01/2006; 99(2):023504-023504-5. · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: A new approach to studying Schottky barrier formation on a nanometer scale is demonstrated using both Auger electron spectroscopy
core level shift and secondary electron threshold work function measurements on cleaved epilayers. Band bending induced by
metallization of cleaved epilayer surfaces can be investigated without introducing defects due to chemical or ion beam surface
cleaning. For GaN epilayers, this approach also avoids complications due to piezoelectric effects on polar-axis growth surfaces.
Initial investigations of Au and Ag Schottky contact formation on GaN in ultrahigh vacuum reveal the presence of a pinning
level ∼1.7 eV above the valence band edge.
Journal of Electronic Materials 01/2006; 35(4):581-586. · 1.64 Impact Factor