Optical Properties of Undoped, n-Doped and p-Doped GaN/AlN Superlattices
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Optical Properties of Undoped, n-Doped and p-Doped GaN/AlN Superlattices Plamen Paskov1, Bo Monemar1, Satoshi Kamyiama2, Hiroshi Amano2, and Isamu Akasaki2 1 Department of Physics, Chemistry and Biology, Linkoping University, Linkoping, SE-58183, Sweden 2 Department of Electrical and Electronic Engineering and High-Tech Reserch Center, Meijo University, Nagoya, 468, Japan
ABSTRACT We report a photoluminescence study of near-surface GaN/AlN superlattices grown by metalorganic chemical vapor deposition (MOCVD) on a thick GaN layer. Undoped, Si-doped and Mg-doped structures with the well/barrier thickness ratio 3:1 and different periods are investigated. It is found that the energy position, intensity and linewidth of the emission are determined by the interplay of the built-in polarization field, depletion field and screening by the free carriers. In n-type structures an electron accumulation at the bottom interface is evidenced by the observed recombination of the two-electron gas with the photo-excited holes. INTRODUCTION GaN/AlGaN superlattices (SL) are presently explored for a number of device applications such as strain management and dislocation filter layers, cladding layers in laser diodes, barriers in high electron mobility transistors (HEMTs), and active layers in infrared detectors based on the intersubband (ISB) absorption. For advanced device performances, e.g. higher sheet carrier density in HEMTs, shorter emission wavelength of UV emitters and tuning the ISB absorption in the optical telecommunication spectral region, a high Al mole fraction in AlGaN layers or even pure AlN barriers is desirable. However, the epitaxial growth of GaN/AlN SLs is challenging due the large lattice mismatch between the GaN and AlN. To prevent the strain relaxation and the consequent cracking and defect formation in structures grown on an AlN (GaN) template the thickness of the wells (barriers) has to be kept below a certain critical value. Molecular beam epitaxy (MBE) providing a precise control of the layer thicknesses and smooth interfaces, is the most commonly used for SL growth, especially for structures intended for ISB device applications. Recently, with the development of strain controlling techniques, namely, the indium surfactant assisted growth [1] and the use of combination of N2 and H2 carrier gases [2], metalorganic chemical vapor deposition (MOCVD) have also been employed successfully for growth of GaN/AlN SLs. In this work, we investigate the effect of the SL period and doping on the emission properties of MOCVD grown GaN/AlN SLs with very thin barriers. SAMPLES The samples discussed here are 10 period GaN/AlN layers pseudomorphically grown on a 2-µm-thick GaN buffer layer on a sapphire substrate. The GaN layer was grown with H2 carrier
gas, while for the SL structure N2 gas was used [2]. For all samples the GaN/AlN thickness ratio was kept 3:1. No additional capping layer was present, i.e. the outmost barrier faced the surface. Three sets of SLs with almost identical period were studied: one c
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