The effect of temperature on the efficiency of nitride-based multi-quantum well light-emitting diodes
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1229-LL09-06
The effect of temperature on the efficiency of nitride-based multi-quantum well lightemitting diodes. Oskari Heikkilä, Jani Oksanen and Jukka Tulkki Helsinki University of Technology, Department of Biomedical Engineering and Computational Science (BECS), P.O.Box 2200, FI-00150 TKK, Finland ABSTRACT We have recently developed a self consistent light-emitting diode (LED) model that accounts for the current transport and internal heating in AlGaAs-GaAs LEDs. In this paper we extend the model to describe multi-quantum well (MQW) active regions and III-N materials, within the limits of the currently known values and temperature dependencies of the recombination parameters in these materials. The MQW description accounts for the effect of the reduced wave function overlap to the recombination. We present simulation results obtained for an InGaN MQW LED with 4 wells at selected temperatures and discuss the factors limiting the efficiency and luminescent output of LEDs. INTRODUCTION The rapid development of light emitting diodes has enabled their use as a compact and efficient solid state light source in many applications ranging from telecommunications to general illumination [1, 2]. They are expected to become a ubiquitous and prevalently used light source in the near future, but to date their use in applications requiring large luminous flux is still seriously limited by the relatively low output power even in the most advanced LEDs [3]. In this paper we simulate the operation of GaN-InGaN MQW LEDs focusing on the efficiency of the light generation. We will also study the efficiency droop at high currents which has been vividly discussed in the recent LED research [4, 5]. We expand the previous work on simulating MQW LEDs [6, 7] by simulating the structures at elevated temperatures. Although the effect of temperature is often eliminated from scientific measurement setups, it has practical importance in the steady state operational characteristics of modern high power LEDs because they typically operate at substantially increased junction temperatures. THEORY Transport model and recombination We have modeled the current transport using the 1D drift-diffusion model for the nitride semiconductors introduced by Bulashevich et al. [6]. The drift-diffusion model describes the spatial distribution and flow of charge carriers and the related quasi-Fermi energies and the electrostatic potential in semiconductors. These quantities can be solved from a differential equation system consisting of the Poisson equation for the electrostatic potential φ , d dφ (1) − ε − P = − q ( p − n + N d+ − N a− ) dx dx and the continuity equations for the electron and hole current J n and J p ,
dJ n d dF = µn n n = qR (2) dx dx dx dJ p d dF = µ p p p = −qR (3) dx dx dx where ε is the permittivity, P is the combined spontaneous and piezoelectric polarization, n ( p ) is the electron (hole) density, q is the unit charge, N d+ ( N a− ) is the density of the ionized
donors (acceptors), Fn ( Fp ) is the quasi-Fermi energy of
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