Parametric study of AlGAN/GaN UV-Led Based on Quantum Confined Stark Effect (QCSE)

Shekhar Patra¹, Sanjay Tiwari¹, Umang Singh²

¹Photonics Research laboratory, SOS in Electronics and Photonics, Pt. Ravishankar Shukla University Raipur Chhattisgarh, India

²Research Scholar, Department of EE, III Bombay, Maharashtra, India

Abstract:

III- Nitride materials have radically changed the lighting industry allowing for the developments of high efficiency and brightness ultraviolet LED. In this paper, the characterised between electroluminescence spectra pick wavelength and QW thickness through simulations with presence on electric field. According the QCSE when electric field applied perpendicular to the QWs layers resulted in large read shifts in absorption. I have taken One Dimensional Drift Diffusion charge control solver (1D-DDCC) software for simulations that is solve Passion Schrodinger equation developed by Pro.Yuh-Renn WU of UM. This solver has many functions like- Tunnable parameters for all basic material, Hetrojunction simulation, Dopant activation energy, Eigen solve for Schrodinger equation.

Keywords: Aluminium Gallium Nitride(AlGaN), Quantum Confined Stark Effect(QCSE), Quantum Walls(QWs), Ultraviolet Light Emitting Diode (UV-LED), Electroluminescence, One Dimensional Drift Diffusion charge control solver(1D-DDCC).

Introduction

The fundamental breakthrough in the area of gallium nitride (GaN) semiconductor is LEDs, the first demonstration of high efficiency and high brightness blue LEDs [4,5]. GaN based blue and white LED achieves high efficiency surpassing that of any conventional light source and billions of LEDs are fabricated [1]. Gallium nitride, Aluminium nitride and indium nitride is the member of III-nitride family¹. In the branch of III-IV group has radically changes the lightning industry due to short wavelength brightness and high efficiency [1, 2]. In 2014 the Nobel Prize of Physics is given by “Royal Swedish Academy of Science Awarded to Isamu Akasaki, Hiroshi Amano, and Shuji Nkamura for “The invention of efficient blue light emitting diodes which has enabled bright and energy-saving white light sources”[11,12]. This experiment provide breakthrough to development of new semiconductor material& device technologies can lead to a prototype shift of a complete industry. Creating white light through blue LEDs, when adding Aluminium Nitride to Gallium Nitride alloy, the emission wavelength of AlGaN based LED can be turn over almost to UVA(400-320 nm), UVB(320-280 nm),UVC(280-200 nm) spectral range to emission wavelength[11].

Quantum Confined Stark Effect and Polarisation Effect in LED Device

The phenomena of splitting of spectral lines due to applied electric field are known as stark effect. First tine Quantum Confined Stark Effect is reported by Miller et al. for aluminium gallium arsenide(AlGaAs)  QWs based, he observed that electric field applied perpendicular to quantum wall layer resulted large red shift in absorption area [6,7,13]. QW change rectangular to swatooth shaped due to applied potential charge in QWs and then reduced overlap of the electron and hole wave functions are pushed in opposite direction [8]. The PL spectra of a subtract containing multiple in AlGaN QWs structure consisting of a single bulk AlGaN layer with increasing illumination power [16]. The effect was used by Im et al. to examine the reduction of oscillator strength in AlGaN/GaN (15), thicker wall ware first observed to have higher peak emission wavelength relative to the GaN band gap the luminescence life time of the red shifted emission from the thicker wall (10nn) was observed in 3μs that is 10⁴ times longer them the decay time from the thinner wall [15, 16].

Fig.1 Quantum well energy levels for unbiased and biased structure with carrier wave functions with different energy level. Applied field reduce overlap between carrier wave function [13].

UV LED and Applications

AlGaN ternary alloy is a tenable and direct band gap between 3.43eV to 6.11eV and it is very suitabled for fabricated optical devices [14]. Ultraviolet LEDs has a several advantages compare to conventional UV source such as mercury lamp, it is robust, compact, long stability and no required warm-up times, also used to water treatment system that required UV radiation strongly to volume the water treatment’s. The emission tuned to the wavelength of UVA (400-320), UVB (320-280) and UVC (280-200). The blow chart shows application of UV LED [11, 8, 10 ].

Fig.2 Application of UVA (400-320 nm), UVB (320-280 nm), UVC (280-200 nm) LEDs [11].

Simulation Methodology and Parameters

In this simulation One Dimensional Drift Diffusion charge control solver (1D-DDCC) software has been adopted for simulations that solve Poisson Schrodinger equation developed by University of Michigan, Ann Urbor. Pro.Yuh-Renn Wu added this solver. This solver has many functions like- Tunnable parameters for all basic material, Hetrojunction simulation, Dopant activation energy, Eigen solve for Schrodinger equation. I used this simulation software for study stark effect in AlGaN/GaN QWs. When i have varied QWs thickness and their effect on output EL Spectra with constant electric field [7, 8]. The structure is a pin diode and intrinsic region has multi quantum walls and barrier. The number of quantum is three with variable thickness. The doping layer thickness also changed according to QWs.

Parametric Table:-

The above table show the parameters use in this simulation. This is basically a PIN diode structure. Here there are 3quantum wells of 2 nm thickness each having an Aluminium composition of 50% and the 3^rd layer of the material is taken as electron Blocking Layer The simulation of this structure is done in One Dimensional Poisson, Drift-Diffusion, and Schrodinger Solver (1DDDCC). For parametric study effect of change of only one parameter is observed on electroluminescence spectra in terms of peak intensity and wavelength at that point.

Result and Discussion

 The third layer in the structure that I have provided is electron blocking layer. It is
present due to a difference in motilities of electron and hole in GaN/AlGaN semiconductors. AlGaN with a high band gap and thus a high Aluminium content of 75% is used. This layer is able to block electron flow out of active region (quantum well region) because the conduction band offset is 0.63 times more than valence band offset. Due to larger mobility of electrons as compared to holes an electron blocking layer is used so that the electron can combine in the active region rather than p type GaN region. An increase in EBL thickness off course leads to an increase but large EL thickness can also lead to increased voltage drop in EBL region. In this graph EBL is increased from 40 to 120 angstrom and study the increased intensity, thus an EBL thickness of 120 nm is chosen.

Fig: 3 Graph showing effects on EL intensity by increasing in EBL thickness in fixed bias voltage 4.7eV

The numbers of quantum wells are 3 and their thickness is 2 nm. The thickness can be varied to see the change in EL peak wavelength. So due to quantum confined stark effect (QCSE) there is going to be a red shift in EL spectra. This is shown in above graph. The doping layer thickness must also be changed according to quantum well thickness and also the region in which Schrodinger eqn. is to be solved.

Fig: 4 Graph plotted between El intensity and QWs thickness.

When have varied QWs thickness and their effect on output EL Spectra with constant electric field. The structure is a pin diode and intrinsic region has multi quantum walls and barrier. The number of quantum is three with variable thickness. The doping layer thickness also changed according to QWs.  I used a subtract Al_0.7Ga_0.3N and applied constant electric filled Vg=4.67eV and varied QWS thickness from 1nm to 4nm and observed changes in EL pic Spectra and also changes in wavelength. The target Wavelength 270nm is obtained in 2nm thickness. Also a graph plotted between peak intensity and thickness shown in below. The peak of EL intensity is obtained in 2nm Wall thickness.

Fig: 5 Graph showing the effect on El peak wavelength by variation on wall thickness.

Conclusions

III- nitride LEDs have already revolutionised the lightning industry and allow for the unprecedented energy saving of a global scale. But is has required some improvement to their performance, the impact of the polarization field inherent to this material must be calculated. In many case advantage in device development has been shift but a full understanding of the unique material challenges is controlling the polarization field for LEDs has been slower to material. The absorption band-gap Eg will be affected by QW thickness so band-gap is reduced when QWs thickness is reduced. Wavelength is also depends on QWs thickness respectively.

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