Optical polarization in AlGaN - from materials characterization to prediction of emission characteristics of optoelectronic devices

The FBH currently develops AlGaN-based UV light emitting diodes (LED) and laser diodes for the wavelength range 200 – 350 nm in close cooperation with TU Berlin. Lately, we demonstrated LED operation down to 245 nm and stimulated emission from optically pumped laser structures down to 237 nm. Scientific interests in this field not only concern device optimization, but also more fundamental aspects. These include basic material properties, finding and refining of growth conditions in epitaxy, as well as the use of diverse substrates or differently oriented crystals.

Among the fundamental material properties with strong impact on optoelectronic devices rank the valence band ordering and the energy splitting between the valence bands. The valence band ordering defines the optical polarization of emitted light and thus device-relevant values like light extraction efficiency, external quantum efficiency, and optical confinement in laser waveguides. We studied the AlGaN valence band ordering by measuring the degree of optical polarization in photoluminescence experiments. During the investigations we varied several parameters: the aluminum content x in AlGaN, the crystal orientation, the crystal strain, the sample temperature, and the excited charge carrier density. As an example, Fig. 1 shows photoluminescence spectra from (11-22) AlGaN with varying aluminum content x measured with two orthogonal directions of light polarization. The degree of polarization r changes with x. For x ≈ 0.22 the valence bands are degenerated, resulting in similar emission intensities for both polarization directions. When the degree of optical polarization is known, the light extraction efficiency through crystal surfaces can be calculated. This is accomplished in Fig. 2 for a (0001)-oriented GaN crystal. The light extraction out of the (0001) surface declines with decreasing ρ and for ρ << 0 the maximum value of extracted light shifts to higher angles.

For the design of light emitting AlGaN quantum wells in optoelectronic devices, further effects on the valence band ordering have to be considered, like crystal strain and quantization effects. We have shown that compressive strain due to indium incorporation in AlGaN layers shifts the transition point for the polarization ρ = 0 to a higher aluminum content. Due to the compressive strain AlGaN quantum wells experience, a similar behavior is expected from higher aluminum content buffer or barrier layers. Additionally, quantization in quantum wells changes the light polarization by shifting energy levels for light and heavy hole bands differently. These effects can be utilized to force the optical polarization of AlGaN quantum wells to desired values by heterostructure strain management and proper selection of well thicknesses.

Publications:

C. Netzel, J. Stellmach, M. Feneberg, M. Frentrup, M. Winkler, F. Mehnke, T. Wernicke, R. Goldhahn, M. Kneissl, M. Weyers "Polarization of photoluminescence emission from semi-polar (11-22) AlGaN layers", Appl. Phys. Lett., vol. 104, no. 051906 (2014)

M. Martens, F. Mehnke, C. Kuhn, C. Reich, V. Kueller, A. Knauer, C. Netzel, C. Hartmann, J. Wollweber, J. Rass, T. Wernicke, M. Bickermann, M. Weyers, M. Kneissl "Performance Characteristics of UV-C AlGaN-Based Lasers Grown on Sapphire and Bulk AlN Substrates", IEEE Photonics Technol. Lett., vol. 26, no. 4, pp. 342-345 (2014).

C. Netzel, A. Knauer, M. Weyers "Quantum Efficiency Analysis of Near-Ultraviolet Emitting AlGaN and AlInGaN Structures", Jpn. J. Appl. Phys., vol. 52, no. 08JL14 (2013).

C. Netzel, A. Knauer, M. Weyers "Impact of light polarization on photoluminescence intensity and quantum efficiency in AlGaN and AlInGaN layers", Appl. Phys. Lett., vol. 101, no. 242102 (2012).

FBH research: 19.02.2014