SELES 1.0 — Segregation Effect on Light Emission Spectrum

• Download the evaluation version of SELES

1. Physical model

Fig. 1. Calculated composition profile in MQW structure

The SELES software is specially designed for calculation of characteristics of a LED heterostructure with account for surface segregation effects. You can download SELES documentation, demo version, and application examples.

The main goal of the one-dimensional SELES code is to make a bridge from MOVPE growth recipes to the characteristics of light-emitting diodes (LEDs) based on a group-III nitride heterostructures. Basically, the LED heterostructure consists of the n- and p-emitters, separated by a single or multiple quantum well active region. But the real composition profile can be significantly different from the "ideal" one, i.e. expected from the precursor switching schedules and thick layers growth results.

Fig. 2. Band diagram and wave functions of electron and hole ground states

The indium surface segregation is known to be the most important effect leading to such difference. The segregation is known to be a factor affecting the composition profile in the ternary and quaternary compound heterostructures with very different volatility of the atomic species. Normally, segregation results in a delayed incorporation of a volatile species into the crystal, in appearance of the ternary compound tail in a binary "cap" layer, and, eventually, in formation of liquid droplets. The strain in the epilayer due to lattice mismatch is found to be a major factor affecting segregation.

Fig. 3. Emission spectrum

The role of segregation in limitation of In incorporation into the solid can be analyzed with the software. With the SELES software, one can also estimate the effect of composition profile variation on the LED characteristics. The known composition profile is used to calculate the band diagram of the LED heterostructure and light emission spectrum. An accurate quantum-mechanical description of the carriers in the heterostructure, based on the self-consistent solution of the Poisson and Schrodinger equations, is provided within the SELES code.

The electroluminescence (EL) spectra computed with the package show that In surface segregation normally results in a systematic blue shift of the EL spectra (~50-100 meV) due to incomplete In incorporation into the crystal in unsteady MOVPE growth [4]. The bias variation produces the transformation of the QW band diagram in such a way as new electron and hole states are formed at high biases. The latter phenomenon results in a multi-peak structure of the EL spectra and, consequently, in additional blue-shift of the emission wavelength. Besides, the secondary peaks provide a higher broadening of the spectra, which is undesirable for practical applications.

2. Capabilities of SELES 1.0

Fig. 4. Distribution of indium content across an GaN/InGaN/AlGaN quantum well structure [3]

The SELES 1.0 software includes the following options:

• Calculation of the composition profile in a LED heterostructure with account for indium surface segregation.
• Calculation of the band diagram of a LED heterostructure at a given bias.
• Calculation of light emission spectrum at given bias.

3. User interface

The user interface includes easy stage-by-stage input of the growth process in terms of real MOVPE reactor: temperature, pressure, and precursor and carrier gas flow rates, as well as visualization of the simulation results.

Fig. 5. User Interface. Input of growth regime

4. System requirements

• Operation System—Windows 98/2000/ME/XP
• RAM—256 Mb
• Disk Space—2 Mb for the SELES program files and about 1Mb per typical simulation to save results.
• Display and video card with the support of 1024x768 resolution in the High Color mode
• Mouse

5. References

[1] Surface Segregation and Composition Fluctuations in ammonia MBE and MOVPE of InGaN. S.Yu.Karpov, R.A.Talalaev, E.V.Yakovlev and Yu.N.Makarov. Mat.Res.Soc.Proc., Vol.639, p.G3.18.2-G3.18.6, (2001).

[2] Indium segregation kinetics in MOVPE of InGaN-based heterostructures. S.Yu. Karpov, R.A. Talalaev, I.Yu. Evstratov, and Yu.N. Makarov. Physica Status Solidi (a), Vol.192, N 2, p.417-423, (2002).

[3] Indium segregation in MOVPE grown InGaN based heterostructures. R.A. Talalaev, S.Yu. Karpov, I.Yu. Evstratov, Yu.N. Makarov. Physica Status Solidi (c), Vol.01, p.311-314 , (2002).

[4] Segregation effects and bandgap engineering in InGaN quantum-well heterostructures. K.A. Bulashevich, R.A. Talalaev, S.Yu. Karpov, I.Yu. Evstratov, and Y.N. Makarov. Materials Research Society Symposium Proceedings, Vol.743, p.L6.5.1-L6.5.6, (2003).