InGaN LEDs

InGaN-based LEDs

InGaN-based light emitting diodes (LEDs) and laser diodes (LDs) have great commercial potential due to their ability of working in the short wavelength region, which has up to now been inaccessible for LED and LD technologies. Their applications vary from large area displays and efficient and long-lasting room lighting (LEDs) to high-density memory storage and high definition printing (LDs). The active region of these devices, in which the light is being generated, is an InGaN/GaN or InGaN/AlGaN quantum well. Even though devices based on this quantum well structure are already being mass-produced and are available in the market, the mechanisms by which this light is generated are poorly understood. By improving our understanding of these mechanisms, it should be possible to improve the structural design, and with it, the performance of the devices.

Basicd design of an InGaN LED

 

Polarization Field Effects versus Indium Fluctuations

The analysis of these devices by many researches has brought into focus two different recombination mechanisms. The first is dominated by indium fluctuations in the indium layer, the second is dominated by strong polarization fields induced by biaxial strain in the layer. There is much disagreement between the researches involved in this problem as to which recombination mechanism is responsible for light emission in these devices.

Schematic representation of the polarization field model.

The Indium fluctuation model.

Combined EL and TEM Study

Our approach combined High Resolution Transmission Electron Microscopy (HRTEM), Secondary Ions Mass Spectrometry (SIMS), Rutherford Back Scattering (RBS) and Electron Energy Loss Spectrometry (EELS) to determine the structural and chemical parameters of the quantum wells. Details of the determination of the Indium distribution using HRTEM as carried out by Chrisitan Kisielowski are described in a NCEM research highlight. Variable excitation power and temperature Photoluminescence (PL) and Electroluminescence (EL) were used to study the luminescence properties of the devices. Theoretical simulations were developed to relate the luminescence properties to the proposed recombination mechanism. We have found that both mechanisms were present in our samples. The indium fluctuation mechanism dominated in devices with narrow wells (L<3nm) and high indium concentration (x>0.20). The mechanism dominated by the polarization field dominated in a device with wide wells (L>3nm) and low indium concentration (x<0.15). We have also found that quantum wells with high indium concentrations showed greater fluctuation of the indium content across the well than quantum wells with low concentrations of indium. Our findings suggest that the two mechanisms compete with one another, and that the width of the well and the concentration of indium in the well play a crucial role in determining which of the two mechanisms will dominate.

Since in the recombination mechanisms based on indium fluctuations the electrons and holes are localized at regions in the layer where the indium concentration is highest, electrons and holes are less likely to find non-radiative recombination centers (e.g. dislocations). Therefore, devices based on this recombination mechanism will have better quantum efficiency than devices based on the recombination mechanism dominated by the polarization field. Using these results, we recommend that LEDs should be based on narrow quantum wells (L<3nm) with high indium concentration (x>0.20).

 

Biaxial Strain Characterization

We have developed a unique tool for the study of the effect of the polarization fields in InGaN QWs. A tensile biaxial strain is created in the epitaxial samples by means of a specially designed pressure cell. For a bulk (or even a thin film) semiconductor, this simply results in the shrinking of the energy-gap - a redshift of the light emitted. However, through the piezoelectric effect, the tensile strain also reduces the strength of the built-in polarization field. For an LED structure dominated by the polarization field effect, this results in a blueshift of the emitted light. Deviation from the model can be explained as screening due to doping, confinement effects, or localization at indium-rich nano-clusters.  Thus the direction and degree to which the color shifts teaches us about the mechanisms that dominate the radiative recombination in our structures.

 

Related Publications

Relation between Structural Parameters and the Effective Electron-Hole Separation in InGaN/GaN Quantum Wells
N.A. Shapiro, H. Feick, N.F. Gardner, W.K. Götz, P. Waltereit, J.S. Speck, E.R. Weber
to be published in phys. stat. sol.

Dependence of the luminescence energy in InGaN quantum-well structures on applied biaxial strain
N.A. Shapiro, Y. Kim, H. Feick, E.R. Weber, P. Perlin, J.W. Yang, I. Akasaki, and H. Amano
Phys. Rev. B. 62, R16318 (2000). ONLINE

The effects of indium concentration and well-thickness on the mechanisms of radiative recombination in In_xGa_1-xN quantum wells
N.A. Shapiro, P. Perlin, C. Kisielowski, L.S. Mattos, J.W. Yang, and E.R. Weber
MRS Internet J. Nitride Semicond. Res. 5,1 (2000). ONLINE

The magnitude of the piezoelectric effect in InGaN quantum wells
P. Perlin, C. Kisielowski, L. Mattos, N.A. Shapiro, J. Krüger, J. Yang, E.R. Weber
Mat. Res. Soc. Symp. Vol. , 187 (1998).

High-pressure investigation of InGaN quantum wells
P. Perlin, V. Iota, B.A. Weinstein, H. Teisseyre, T. Suski, S. Hersee, C. Kisielowski, E.R. Weber, J. Yang
Mat. Res. Soc. Symp. Vol. , 399 (1998).

InGaN/GaN quantum wells studied by high pressure, variable temperature, and excitation power spectroscopy
P. Perlin, C. Kisielowski, V. Iota, B.A. Weinstein, L. Mattos, N.A. Shapiro, J. Krüger, E.R. Weber, J. Yang
Appl. Phys. Lett. 73, 2778 (1998). ONLINE

An analysis of temperature dependent photoluminescence line shapes in InGaN
K.L. Teo, J.S. Colton, P.Y. Yu, E.R. Weber, M.F. Li, W. Liu, K. Uchida
Appl. Phys. Lett. 73, 1697 (1998). ONLINE

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