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Simulator for Light Emitters based on Nitride Semiconductors (SiLENSe)—software tool for light emitting diode bandgap engineering.

1. Overview

Soft-Impact develops physical and chemical models for several software products designed by STR, Inc. Soft-Impact provided physical models for the software tool SiLENSe developed in STR for modeling band diagrams and characteristics of light emitting diodes (LEDs) based on group-III nitrides. These models allow the SiLENSe software to provide researchers with exhaustive information about complex physical processes responsible for the operation of group-III nitride-based LEDs.

The code provides the following properties of an LED heterostructure:

  • Band diagram of a nitride LED at various biases
  • Distribution of electron and hole concentrations in the device structure
  • Electric field distribution
  • Radiative and non-radiative recombination rates
  • Current-voltage (I-V) characteristic
  • Internal light emission efficiency as a function of current density
  • Wave functions of electrons and holes in a quantum-well active region
  • Light emission spectra of the LED

The above information forms a good basis for the LED structure optimization and for development of new light emitting devices.

2. Examples of simulations

Shown below are some results of simulations of a simple blue SQW LED structure consisting of n-GaN contact layer (Nd = 3E18 cm-3), an undoped 20%-InGaN SQW active region 3.5 nm thick, a p-type 10%-AlGaN emitter (Na = 7E19 cm-3), and a p-GaN contact layer (Na = 7E19 cm-3).

Fig. 1. Band diagram, carrier concentrations, SQW profile, and emission spectra variation with bias.

3. Underlying physics

The LED operation is considered within the 1D drift-diffusion model of carrier transport in the heterostructure that accounts for specific features of the nitride semiconductors - strong piezoeffect, existence of spontaneous electric polarization, low efficiency of acceptor activation, and high threading dislocation density (normally, ~1e7-1e9 cm-2) in the material. Additionally, the code is capable of modeling graded-composition heterostructures, which is important in view of the use of bandgap engineering principles. Along with bimolecular radiative electron and hole recombination, an original model of non-radiative carrier recombination on threading dislocation cores is incorporated into the SiLENSe code. The latter allows analyzing the interplay between the radiative and non-radiative recombination channels and predicting the internal emission efficiency of the LED structure as a function of threading dislocation density. The spectrum of light emission from a single- or multiple-quantum-well active region can be calculated with account of the complex valence band structure of nitride semiconductors by using the 8x8 Kane Hamiltonian. For this, a self-consistent solution of the Poisson and Schrodinger equations is found within the effective-mass approximation. Procedure of the quantum-well region identification and respective grid generation is totally automatized in the SiLENSe code. The SiLENSe code is supplied by a database of materials properties necessary for simulations.

The model implemented into the SiLENSe code used the following assumptions:

  • exact account of localized and distributed polarization charges in the LED structure induced by both spontaneous and piezo polarization in nitride semiconductors;
  • the Fermi statistics is used for electrons and holes covering the cases of both degenerate and non-degenerate semiconductors;
  • partial ionization of donors and acceptors depending on the respective quasi-Fermi level positions is allowed for;
  • strain in the LED structure is calculated assuming coherent growth of all epilayers on an underlying buffer layer;
  • bimolecular radiative electron and hole recombination is considered with neglect of quantum-confined effects on the recombination rate;
  • non-radiative carrier recombination considers the principal channel, recombination on threading dislocation cores;
  • I-V characteristic of an LED is computed with a given serial resistance that should account for both the lateral current spreading in the LED chip and ohmic contact resistances;
  • light emission spectra are computed with a post-processing module operating with the band profiles of the LED structure obtained and accounting for (i) the complex structure of the valence band of nitride materials and (ii) the contribution of the confined electronic states.