Growth modes of InN (000-1) on GaN buffer layers on sapphire

In this work, using atomic force microscopy and scanning tunneling microscopy, we study the surface morphologies of epitaxial InN films grown by plasma-assisted molecular beam epitaxy with intervening GaN buffer layers on sapphire substrates. On smooth GaN buffer layers, nucleation and evolution of three-dimensional InN islands at various coverages and growth temperatures are investigated. The shapes of the InN islands are observed to be predominantly mesa-like with large flat (000-1) tops, which suggests a possible role of indium as a surfactant. Rough GaN buffer layers composed of dense small GaN islands are found to significantly improve uniform InN wetting of the substrates, on which atomically smooth InN films are obtained that show the characteristics of step-flow growth. Scanning tunneling microscopy imaging reveals the defect-mediated surface morphology of smooth InN films, including surface terminations of screw dislocations and a high density of shallow surface pits with depths less than 0.3 nm. The mechanisms of the three-dimensional island size and shape evolution and formation of defects on smooth surfaces are considered

Voltage modulated electro-luminescence spectroscopy and negative capacitance - the role of sub-bandgap states in light emitting devices

Voltage modulated electroluminescence spectra and low frequency ({\leq} 100 kHz) impedance characteristics of electroluminescent diodes are studied. Voltage modulated light emission tracks the onset of observed negative capacitance at a forward bias level for each modulation frequency. Active participation of sub-bandgap defect states in minority carrier recombination dynamics is sought to explain the results. Negative capacitance is understood as a necessary dielectric response to compensate any irreversible transient changes in the minority carrier reservoir due to radiative recombinations mediated by slowly responding sub-bandgap defects. Experimentally measured variations of the in-phase component of modulated electroluminescence spectra with forward bias levels and modulation frequencies support the dynamic influence of these states in the radiative recombination process. Predominant negative sign of the in-phase component of voltage modulated electroluminescence signal further confirms the bi-molecular nature of light emission. We also discuss how these states can actually affect the net density of minority carriers available for radiative recombination. Results indicate that these sub-bandgap states can suppress external quantum efficiency of such devices under high frequency operation commonly used in optical communication.Comment: 21 pages, 4 sets of figure

Ultrafast Hot Carrier Dynamics in GaN and its Impact on the Efficiency Droop

GaN is a key material for lighting technology. Yet, the carrier transport and ultrafast dynamics that are central in GaN light emitting devices are not completely understood. We present first-principles calculations of carrier dynamics in GaN, focusing on electron-phonon (e-ph) scattering and the cooling and nanoscale dynamics of hot carriers. We find that e-ph scattering is significantly faster for holes compared to electrons, and that for hot carriers with an initial 0.5$-$1 eV excess energy, holes take a significantly shorter time ($\sim$0.1 ps) to relax to the band edge compared to electrons, which take $\sim$1 ps. The asymmetry in the hot carrier dynamics is shown to originate from the valence band degeneracy, the heavier effective mass of holes compared to electrons, and the details of the coupling to different phonon modes in the valence and conduction bands. We show that the slow cooling of hot electrons and their long ballistic mean free paths (over 3 nm) are a possible cause of efficiency droop in GaN light emitting diodes. Taken together, our work sheds light on the ultrafast dynamics of hot carriers in GaN and the nanoscale origin of efficiency droop.Comment: Submitted, 10 pages, 4 figure

Possible surface plasmon polariton excitation under femtosecond laser irradiation of silicon

The mechanisms of ripple formation on silicon surface by femtosecond laser pulses are investigated. We demonstrate the transient evolution of the density of the excited free-carriers. As a result, the experimental conditions required for the excitation of surface plasmon polaritons are revealed. The periods of the resulting structures are then investigated as a function of laser parameters, such as the angle of incidence, laser fluence, and polarization. The obtained dependencies provide a way of better control over the properties of the periodic structures induced by femtosecond laser on the surface of a semiconductor material.Comment: 11 pages, 8 figures, accepted for publication in Journal of Applied Physic

Band gap bowing of binary alloys: Experimental results compared to theoretical tight-binding supercell calculations for CdZnSe

Compound semiconductor alloys of the type ABC find widespread applications as their electronic bulk band gap varies continuously with x, and therefore a tayloring of the energy gap is possible by variation of the concentration. We model the electronic properties of such semiconductor alloys by a multiband tight-binding model on a finite ensemble of supercells and determine the band gap of the alloy. This treatment allows for an intrinsic reproduction of band bowing effects as a function of the concentration x and is exact in the alloy-induced disorder. In the present paper, we concentrate on bulk CdZnSe as a well-defined model system and give a careful analysis on the proper choice of the basis set and supercell size, as well as on the necessary number of realizations. The results are compared to experimental results obtained from ellipsometric measurements of CdZnSe layers prepared by molecular beam epitaxy (MBE) and photoluminescence (PL) measurements on catalytically grown CdZnSe nanowires reported in the literature.Comment: 7 pages, 6 figure

Electroluminescence from single nanowires by tunnel injection: an experimental study

We present a hybrid light-emitting diode structure composed of an n-type gallium nitride nanowire on a p-type silicon substrate in which current is injected along the length of the nanowire. The device emits ultraviolet light under both bias polarities. Tunnel-injection of holes from the p-type substrate (under forward bias) and from the metal (under reverse bias) through thin native oxide barriers consistently explains the observed electroluminescence behaviour. This work shows that the standard p-n junction model is generally not applicable to this kind of device structure.Comment: 6 pages, 6 figure

Band anticrossing in GaNxSb1–x

Fourier transform infrared absorption measurements are presented from the dilute nitride semiconductor GaNSb with nitrogen incorporations between 0.2% and 1.0%. The divergence of transitions from the valence band to E– and E+ can be seen with increasing nitrogen incorporation, consistent with theoretical predictions. The GaNSb band structure has been modeled using a five-band k·p Hamiltonian and a band anticrossing fitting has been obtained using a nitrogen level of 0.78 eV above the valence band maximum and a coupling parameter of 2.6 eV

Optical properties of high quality Cu2ZnSnSe4 thin films

Cu2ZnSnSe4 thin films, fabricated on bare or molybdenum coated glass substrates by magnetron sputtering and selenisation, were studied by a range of techniques. Photoluminescence spectra reveal an excitonic peak and two phonon replicas of a donor-acceptor pair (DAP) recombination. Its acceptor and donor ionisation energies are 27 and 7 meV, respectively. This demonstrates that high-quality Cu2ZnSnSe4 thin films can be fabricated. An experimental value for the longitudinal optical phonon energy of 28 meV was estimated. The band gap energy of 1.01 eV at room temperature was determined using optical absorption spectr

Optical spin injection and spin lifetime in Ge heterostructures

We demonstrate optical orientation in Ge/SiGe quantum wells and study their spin properties. The ultrafast electron transfer from the center of the Brillouin zone to its edge allows us to achieve high spin-polarization efficiencies and to resolve the spin dynamics of holes and electrons. The circular polarization degree of the direct-gap photoluminescence exceeds the theoretical bulk limit, yielding ~37% and ~85% for transitions with heavy and light holes states, respectively. The spin lifetime of holes at the top of the valence band is found to be ~0.5 ps and it is governed by transitions between heavy and light hole states. Electrons at the bottom of the conduction band, on the other hand, have a spin lifetime that exceeds 5 ns below 150 K. Theoretical analysis of the electrons spin relaxation indicates that phonon-induced intervalley scattering dictates the spin lifetime.Comment: 5 pages, 3 figure