ZnO based LED: A Performance Optimization through TCAD Simulation

 

Abstract

In this paper, a two dimensional (2-D) generic model of quantum well light emitting diode (LED) based on ZnO, II-VI compound semiconductors has been reported. The structure has been designed and analyzed using ATLAS in conjuction with the BLAZE and LED modules of SILVACO software. A closed model has been developed for electrical and optical characterization of the device. The model includes all the relevant material physics and solutions of non-linear decoupled semiconductor transport and Poisson’s equations. All the radiative and non radiative recombination mechanisms have been given their due consideration in this analysis. The results obtained on the basis of numerical simulation exhibits the potential advantages of wide band gap ZnO material based LEDs over its competitive GaN material based LEDs.

Key words: ZnO, II-VI, Hetero-interface, MgZnO, CdZnO.

 

Introduction

In recent years, the liquid crystal display market has shifted to high-efficiency LEDs. Which have shown tremendous potential applications in areas of mobile appliances, automotive, biomedical, sensors and displays. The wide-bandgap semiconductor ZnO has excellent structural and physical properties compared to GaN, an III-V semiconductor. Other advantages of ZnO based materials are low cost, relative abundance, chemically stability, nontoxic nature and they are easy to prepare. On the other hand, TCAD tools provide the flexibility to design a number of device structures based on ZnO after the synthesis of the materials, for optimization of the physical parameters associated with the structure.

UV LEDs based on both III-V nitrides and II-VI oxides are facing the problems associated with p-type doping. Although significant progress has been made for III-V, p-type doping of high Al content AlGaN still presents many challenges. In the case of ZnO and its ternary alloys, the problem of p-type doping is still an open issue for researchers. The internal quantum efficiency and other physical parameters associated with ZnO-based light-emitting diodes can be analyzed and optimized through assessing a number of possible design approaches with the help of TCAD software tools. The advanced III-V nitrides & II-VI oxides are available in Silvaco’s material database.

In this paper, the Zno/MgxZn1-xO/CdyZn1-yO/MgxZn1-xO/ZnO structure has been designed and simulated to understand the technological developments for high efficiently LED applications. The results obtained on the basis on numerical simulation shows the dependency of the high efficiently LED on the important physical parameters.

 

Design and 2-D Simulation

Figure 1a shows the 2D cross-sectional view of a ZnO based LED. The n type ZnO substrate has been used to produce a multilayer structure. The four layer is device composed of two layers of intrinsic MgZnO semiconductor and a very thin layer of intrinsic CdZnO, which has been sandwiched between two MgZnO layers, forming the quantum well structure. The p-type ZnO has been used for a cladding layer as well as to define the contact due to its conductivity. The structure has been designed using the ATLAS framework interfaced with BLAZE electrical characterization. Figure 1b shows the energy band diagram of the simulated structure at zero bias condition. The discontinuity appears at each hetero-interface in the conduction band. It is clearly evidenced from the energy band diagram that energy offset in the conduction ∆EC and valance band ∆EV at each heterointerface is due to electron affinity differences and doping concentration gradient of each material, which justifies the classical theory of Anderson. The formation of a potential barrier within the heterojunction at the CdZnO quantum layer can be understood in terms of forces that arise due to gradient in electrostatic potential ∇V and electron affinity ∇χ. The same structure has been simulated for different mole fractions of Cd in CdZnO material to get the optimized internal quantum efficiency. [1]-[2]

Figure 1. Figure 1a (top) Schematic of ZnO/MgZnO/CdZnO/MgZnO/ZnO structure, Figure 1b (bottom) Band diagram.

 

The doping of the cladding region (ZnO) has been taken to be analytically uniform in the above simulation. In calculation of carrier recombination lifetimes, the non radiative SRH and Auger models have been taken into account while the radiative Chuang and spontaneous models have been used for optical recombination mechanisms. We have taken into account the Fermi-Dirac statistics by assuming a parabolic shape of he conduction band in all the calculations of carrier and doping densities. The K.P model has been employed to calculate the effective masses and band edge energies for drift diffusion simulation. The Lorentz model has been used for gain broadening due to intra-band scattering within the CdZnO quantum well. [3]

 

Results & Discussion

Simulation and optimization studies have been carried out for electrical and optical characterization of heterojunctions LED structure composed of p-ZnO/i-MgZnO/i-CdZnO/i-MgZnO/n-ZnO at room temperature. Figure 1 shows the schematic of the simulated structure along with the simulated energy band diagram at 0V biasing. The different mole fraction of cadmium has been taken for optimization of the internal quantum efficiency. The quantum well has been observed at MgZnO/CdZnO interface with band offset (0.4 eV in the conduction band), which allows the carriers to be confined in the active region. The analysis shows that more carriers can be confined by optimizing the band offset for a particular mole fraction of Cd.

Figure 2 shows the distribution of anode current due to radiative recombination of carriers under forward bias. The introduction of ZnMgO/ZnO heterinterfaces on each side of the active regions provides additional electrons injected into the CdZnO active region. The current is constant or negligible for low biasing conditions, as the voltage increases the current increases linearly after 3V as shown in curve. The design of the proposed LED based on II-VI material has been carried out by optimizing the device parameters, and the band gap engineering has been performed by altering the Cd mole fraction for operation at UV and blue wavelengths.

Figure 2. Simulated I-V Characteristics.

The spectral emission spectra as a function of wavelength are shown in Figure 3. The curves show that there is significant increase in the emission power density for different voltages. The curves indicate the relation between emission power density and applied voltage.

Figure 3. Simulated Emission spectra ZnO/Mg0.1Zn0.9O/Cd0.1Zn0.9O/Mg0.1Zn0.9O/ZnO at different voltages.

 

Figure 4. Simulated Internal Quantum efficiency ZnO/Mg0.1Zn0.9O/Cd0.1Zn0.9O/Mg0.1Zn0.9O/ZnO.

 

The spectral dependence of internal quantum efficiency, as a function of anode current, is estimated in Figure 5. The internal quantum efficiency of the LED structure ZnO/Mg0.1Zn0.9O/Cd0.1Zn0.9O/Mg0.1Zn0.9O/ZnO has been calculated with the ratio of radiative recombination to the total recombination rate. The quantum efficiency increases linearly with the anode current. The IQE shows linear dependence on radiative recombination mechanism as well as on the mole fraction of the Cd content of the active region where recombination takes place. The different IQE curves with different Cd content has been depicted in Figure 6. The simulated emission spectra, and emission intensity as a function of Cd composition (x=0.05, 0.07, 0.09) are also shown in Figure 7. There is a decrease in the output light intensity with the increasing Cd composition. The effect can be explained with the help of conduction band offset value. The band gap of CdZnO decreases as the Cd content increases. The expected shift of emmited light towards red wavelength with the increasing Cd content can be defined in terms of band gap engineering of CdZnO, which is dependent on Cd mole fraction. Basically, the analyses of optimization of the internal quantum efficiency show that the optical properties of LED are dependent on the physical parameters associated with the CdZnO, an active region. The optimization of internal quantum efficiency takes in to the account the emission properties as compared to electrical properties. The optimum value of internal quantum efficiency has obtained 71% for the Cd mole fraction 0.05. The optimum value of internal quantum efficiency at lower content of cadmium in ZnO can be explained in terms of the structure of CdO, which crystallizes in rock-salt structure and for moderate concentrations of Cd in CdZnO assumes the wurtzite structure of the parent ZnO compound.

Figure 5 (a) Power Spectral Density, (b) Luminous Power density and Internal Quantum efficiency as a function of different active layer thickness.

 

Conclusion

This work emphasises the capability of SILVACO software to simulate and optimize advance coumpound semiconductors based on II-VI group materials. The results shows that a small concentration of Cd in the CdZnO active layer can enhance the internal quantum efficiency and can dictate other figures of merit. The present study exhibits that ZnO/MgZnO/CdZnO based interfaces provides an alternative technology competitive to the III-V GaN based LED technologies. The simulation study through TCAD can establish better understandings of the unknown physical mechanisms, which in turn will help reduce the cost of these devices.

 

Reference:

  1. Sang Youn Han, Hyucksoo Yang, D. P. Norton, and S. J. Pearton, F. Ren, A. Osinsky, J. W. Dong, B. Hertog, and P. P. Chow, “Design and simulation of ZnO-based light-emitting diode structures,” J. Vac. Sci. Technol. B 23 (6),pp. 2504-2509, Nov/Dec 2005.
  2. Xin Dong, Hui Wang, Jin Wang, Wang Zhao, Long Zhao, Zhifeng Shi, “Fabrication of the p-MgZnO/ZnO/n-MgZnO double heterojunction by MOCVD,” Journal of Physics: Conference Series 276 (2011) 012093.
  3. ATLAS User’s Manual, Device Simulation Software, SILVACO International, Santa Clara, CA 95054.

 

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