Molecular beam epitaxy growth of InGaN-GaN superlattices for optoelectronic devices

In the absence of native substrates for InGaN films, the achievement of thick InGaN films of high structural quality remains a challenge. The investigation of InGaN-GaN superlattice (SL) structures is one potential way to increase optical absorption at energies below the GaN bandgap while reducing the formation of detrimental defects. In this article the authors evaluate the structural and optical properties of InGaN-GaN superlattices grown by plasma assisted molecular beam epitaxy with indium compositions of up to 38% and periods from 8 to 20 nm. Of primary concern was the degree of film relaxation as determined by x-ray diffraction (XRD) reciprocal space mapping as a function of indium content and thickness of the InGaN layers. Indium well fractions of up to 0.15 were found to exhibit little or no relaxation for the structures tested by x-ray diffraction. For indium well fractions near ∼0.2, relaxations of the superlattices were in the range of 35% depending on total layer thickness. The samples with indium fractions of 0.33 and 0.38 had relaxations near 30%. For all of the superlattice layers, the onset of absorption began at significantly lower energy if one compares the average indium fraction in the SL to a uniform InGaN film of the same fraction. p-n photodiode structures based on superlattice layers were fabricated and tested for I-V characteristics and spectral response. The high indium content superlattice devices exhibited a substantial spectral response extension down to ∼2.3 eV. However, the I-V behavior was leaky at both forward and reverse biases, which is in a good agreement with the XRD analysis showing that material relaxation and defect generation occur in the SL layers with higher indium content.