Time-Resolved Spectroscopy of InGaN
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ABSTRACT
We have used time-resolved photoluminescence (PL), with 400 nm (3.1 eV) excitation, to examine InxGa1-xN/GaN light-emitting diodes (LEDs) before the final stages of processing at room temperature. We have found dramatic differences in the time-resolved kinetics between dim, bright and super bright LED devices. The lifetime of the emission for dim LEDs is quite short, 110 ± 20 ps at photoluminescence (PL) maximum, and the kinetics are not dependent upon wavelength. This lifetime is short compared to bright and super bright LEDs, which we have examined under similar conditions. The kinetics of bright and super bright LEDs are clearly wavelength dependent, highly non-exponential, and are on the nanosecond time scale (lifetimes are in order of 1 ns for bright and 10 ns for super bright LED at the PL max). The nonexponential PL kinetics can be described by a stretched exponential function, indicating significant disorder in the material. Typical values for β, the stretching coefficient, are 0.45 – 0.6 for bright LEDs, at the PL maxima at room temperature. We attribute this disorder to indium alloy fluctuations. From analysis of the stretched exponential kinetics we estimate the potential fluctuations to be approximately 75 meV in the super bright LED. Assuming a hopping mechanism, the average distance between indium quantum dots in the super bright LED is estimated to be 20 Å.
INTRODUCTION
Recently there has been world-wide interest in the use of nitride semiconductors (e.g., GaN, InN, and AlN) for opto-electronic devices such as lasers and light-emitting diodes. The large changes in physical properties such as band gap, crystal structure, phonon energy, and electronegativity difference between GaN and GaAs, demonstrate that nitride semiconductors are fundamentally distinct from traditional III-V semiconductors. In spite of the impressive progress made in recent years [1] in the development of LEDs and lasers, significant work needs to be done in terms of the optimization of device performance. In order to achieve this goal, the physics underlying the operation of these devices must be better understood. Furthermore, new diagnostic techniques for the characterization of materials and devices will greatly aid in the longterm commercialization of this technology.
F99W11.58
It has been recognized that under typical growth conditions there is a positive enthalpy for indium mixing in GaN. Electron microscopy and cathodoluminescence of InGaN has demonstrated the existence of nanometer and micron scale regions of high indium concentration [2]. It has been hypothesized that the nanoscale regions of high indium concentration are critical to LED operation [1]. We have previously used timeresolved photoluminescence to investigate indium concentration fluctuations, in InGaN/GaN multiple quantum wells and LEDs.[3] In this paper we discuss the result of time-resolved photoluminescence obtained from a set of LEDs with different quantum efficiency.
EXPERIMENT
The light emitting diodes were grown by metal organic chemic
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