Growth of III-nitrides on Si(1 1 1) by molecular beam epitaxy Doping, optical, and electrical properties

Abstract

The growth of high-quality III-nitrides by plasma-assisted molecular beam epitaxy on Si(1 1 1) substrates is addressed. A combination of optimized AlN buffer layers and a two-step growth process leads to GaN layers of high crystal quality (8 arcmin X-ray diffraction full-width at half-maximum) and flat surfaces (57 Å rms). Low-temperature luminescence spectra, dominated by excitonic emissions at 3.465±0.002 eV, reveal the presence of a biaxial tensile strain of thermal origin. AlGaN layers, grown within the alloy range 0.10< x<0.76, have flat surfaces and exhibit strong excitonic luminescence. Si-doping of GaN and AlGaN produces n-type films reaching electron densities up to 2×10 19 and 8×10 19 cm −3, respectively. From photoluminescence and Hall data analysis a Si-donor ionization energy between 50 and 60 meV is derived in GaN. The exciton bound to Si neutral donors at 3.445 eV redshifts while the c-axis lattice parameter decreases as the Si-doping increases, indicating an enhancement of the biaxial tensile strain in the film. This strain increase is a consequence of a strong reduction of the density of dislocations reaching the free surface, due to a particular grain size and orientation governed by the presence of Si donors. Be-doping is also achieved on GaN giving the shallowest acceptor activation energy reported so far, around 90–100 meV. However, there is a severe limitation of the Be incorporation on substitutional sites, leading to the formation of complex, deep defects.