Results of photoluminescence and photoconductivity measurements in In x Ga 1−x N epitaxial films are presented. The photoluminescence peak energy and intensity show several anomalous behaviours. The peak energy changes with temperature exhibiting an inverted S-shape dependence, where it decreases, then increases with increasing temperature in the range 40-100 K and finally decreases with increasing temperature. The intensity shows a temperature dependence similar to that of amorphous semiconductors and disordered superlattices. A blue shift of the photoluminescence energy with increasing excitation intensity is observed. A large Stokes shift between the photoluminescence peak position and the band edge transition energy is found; it decreases with decreasing indium content. A persistent photoconductivity effect has been detected up to room temperature with a stretched-exponential function for its decay rate. All these observations can be explained in a consistent way by alloy potential fluctuations, and these clearly indicate the existence of compositional fluctuations. These two related effects thus appear to constitute the mechanism for the widely observed localized excitons in InGaN-based devices.
We report a detailed investigation of the photoluminescent properties of InN epifilms with free-electron concentrations ranging from 3.5 × 10 17 cm −3 to 5 × 10 19 cm −3 . It is found that the photoluminescence (PL) peak energy strongly depends on the electron concentration. We show that the broadening of the PL spectra with increasing free-electron concentration arises from the breaking of the k = 0 selection rule. The large asymmetric line shape of the photoluminescence spectra can be well described by the free-electron recombination band model. We establish an empirical relation between the full-width at half-maximum (FWHM) value of the PL spectra and the free-electron concentration, which provides a convenient formula to determine the free-electron concentration in InN epifilms by PL measurement. We point out that the peak energy of the PL spectra does not reflect the real band gap of InN epifilms. Calculations based on the effects of Burstein-Moss absorption, band tail and band renormalization were used to analyse the PL spectra, and the fundamental band gap of the intrinsic InN film was obtained. The corresponding expression for the band gap narrowing effect of the InN film is found to be E BGN = 1 × 10 −8 n 1/3 + 3.6 × 10 −7 n 1/4 + 2.3 × 10 −11 n 1/2 eV. The temperature-dependent band gap of the intrinsic InN was fitted by the Pässler equation. The Pässler parameters of the intrinsic InN are α = 0.55 meV K −1 , = 576 K and p = 2.2. It is found that the band gap energies at T = 0 K and room temperature are close to 0.68 eV and 0.62 eV, respectively. In addition, we show that the band gap obtained from the PL spectra is in excellent agreement with that obtained from infrared absorption.
We report solution-processed ZnO thin-film transistors (TFTs) on a flexible substrate, using polymethylmethacrylate (PMMA) as a dielectric layer. To improve the compatibility between the ZnO active layer and the PMMA dielectric, an O 2-plasma treatment has been applied to the PMMA dielectric. The structural and electrical characteristics of the ZnO-TFTs, which have different channel morphologies produced by various concentrations of the ZnO solution, were investigated. The ZnO trap centers of the ZnO-TFTs were decreased as the concentration of the ZnO solution increased. The ZnO-TFT with the optimized channel morphology exhibited a high field-effect mobility of 7.53 cm 2 V −1 s −1 .
We report bright white-light electroluminescence (EL) from a diode structure consisting of a ZnO nanorod (NR) and a p-type conducting polymer of poly(fluorine) (PF) fabricated using a hydrothermal method. ZnO NRs are successfully grown on an organic layer of PF using a modified seeding layer. The EL spectrum shows a broad emission band covering the entire visible range from 400 to 800 nm. White-light emission is possible because the ZnO-defect-related emission from the ZnO NR/PF heterostructure is enhanced to become over thousand times stronger than that from the usual ZnO NR structure. This strong green-yellow emission associated with the ZnO defects, combined with the blue PF-related emission, results in the white-light emission. Enhancement of the ZnO-defect emission is caused by the presence of Zn(OH)(2) at the interface between the ZnO NRs and PF. Fourier transform infrared spectroscopy reveals that the absorption peaks at 3441, 3502, and 3574 cm(-1) corresponding to the OH group are formed at the ZnO NR/PF heterostructure, which confirms the enhancement of defect emission from the ZnO NR/PF heterostructure. The processing procedure revealed in this work is a convenient and low-cost way to fabricate ZnO-based white-light-emitting devices.
The long-term relaxation and build-up transient of photoconductivily has been observed in Sic,Ge,/Si quantum wells. The long-term relaxation behaviour of photoconductivity can be described by a slretched-exponentid function, Ipc(t) = I w (0) e x p [-(t / r)~] (Oc @
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