High carrier concentrations of n - and p -doped GaN on Si(111) by nitrogen plasma-assisted molecular-beam epitaxy
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High-quality doped GaN layers were grown on silicon substrates by radio frequency nitrogen plasma-assisted molecular-beam epitaxy. High-temperature-grown AlN (about 200 nm) was used as a buffer layer. In-growth doping was done using high-purity Si and Mg as n- and p-type dopants, respectively. X-ray diffraction revealed that monocrystalline GaN was obtained. This is in good agreement with the results of morphological study by atomic force microscopy. Micro-photoluminescence (PL) and micro-Raman spectroscopy were used to study the room-temperature optical properties of the doping films. No yellow-band emission was observed in the PL spectroscopy. From the Hall measurements, the resulting n-type doping concentration was measured to be (1–2) × 1019 cm−3. Fairly uniform hole concentration as high as (4–5) × 1020 cm−3 throughout the GaN crystal was achieved. In terms of the carrier concentration, it was found that the results determined from the Fourier transform infrared analysis are in good agreement with the results determined from the Hall measurements.
I. INTRODUCTION
GaN-based semiconductors have attracted interest due to their wide and direct band gap, and their potential application to blue–ultraviolet light emitting devices and high-power electrical devices.1 Silicon is increasingly being used as a substrate for GaN growth.2,3 The Si substrate for GaN growth has some advantages over other substrates: It can be obtained at low cost and the well developed Si growth technology ensures high quality p- and n-type Si wafers. Furthermore, the heteroepitaxial system of GaN on Si substrate can potentially combine the optoelectronic properties of GaN with those of highly advanced Si electronic devices. Direct growth of a GaN film on Si substrate results in either polycrystal growth or a substantial diffusion of Si into the GaN film. Thin AlN and SiC films have been used as buffer layers for GaN growth on Si substrate.3–5 Threading dislocations and inversion domain boundaries usually form at the early stage of growth and then propagate through the film surface.6 The initial growth mode and microstructure strongly depend on types of buffer layers, growth conditions, and growth methods. Until now, little effort has been made to study the initial growth of GaN under different growth conditions. N-type doping with Si is easily achieved in AlGaN layers grown by molecular beam epitaxy (MBE), which reach electron densities up to the mid 1019 cm−3 range for a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0336 J. Mater. Res., Vol. 22, No. 9, Sep 2007
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45% Al.7 In addition, Si-doping affects the layer morphology and structural defects present. P-type doping is still the bottleneck for a broad development of efficient devices. It is well known that, among the different acceptor species used, Mg yields the highest free hole densities in metalorganic vapor phase epitaxy (MOVPE) and MBE materials. However, the high activation energy of Mg
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