Abstract
GaN nanorods, essentially free from crystal defects and exhibiting very sharp band-edge luminescence, have been grown by reactive direct-current magnetron sputter epitaxy onto Si (111) substrates at a low working pressure of 5 mTorr. Upon diluting the reactive N2 working gas with a small amount of Ar (0.5 mTorr), we observed an increase in the nanorod aspect ratio from 8 to ~35, a decrease in the average diameter from 74 to 35 nm, and a two-fold increase in nanorod density. With further dilution (Ar = 2.5 mTorr), the aspect ratio decreased to 14, while the diameter increased to 60 nm and the nanorod density increased to a maximum of 2.4 × 109 cm−2. Yet, lower N2 partial pressures eventually led to the growth of continuous GaN films. The observed morphological dependence on N2 partial pressure is explained by a change from N-rich to Ga-rich growth conditions, combined with reduced GaN-poisoning of the Ga-target as the N2 gas pressure is reduced. Nanorods grown at 2.5 mTorr N2 partial pressure exhibited a high intensity 4 K photoluminescence neutral donor bound exciton transitions (D0XA) peak at ~3.479 eV with a full-width-at-half-maximum of 1.7 meV. High-resolution transmission electron microscopy corroborated the excellent crystalline quality of the nanorods.
Highlights
Low-dimensional semiconductor nanostructures, such as nanorods (NR), have drawn an interest due to their great prospects in novel nanotechnology applications as well as in fundamental material physics, i.e., understanding the growth mechanisms of nanostructures, effects of different surface energies on the adatom mobility, etc
For the pressures of N2 (PN2) of 2.5 mTorr, the bases of some of the rods are slightly broader with a shoulder, after which the rods become narrower in diameter, similar to what frequently has been observed in molecular beam epitaxy (MBE) grown GaN-NRs on Si(111) [23,24]
The highest aspect ratio of nanorods was achieved for PAr = 0.5 mTorr while the best optical properties were achieved at PAr = 2.5 mTorr, which is correlated to a reduction in structural defects, as evidenced by high-resolution transmission electron microscopy (HRTEM) for lower N2 pressures where a continuous film was achieved
Summary
Low-dimensional semiconductor nanostructures, such as nanorods (NR), have drawn an interest due to their great prospects in novel nanotechnology applications as well as in fundamental material physics, i.e., understanding the growth mechanisms of nanostructures, effects of different surface energies on the adatom mobility, etc. DC-MSE has been demonstrated for the growth of high quality GaN continuous epilayers [16,17] as well as NRs. Besides the mass and energy of incident reactive species, selective control of metal–ion fluxes, substrate bias synchronized to probe gas–ion or metal–ion irradiation, is available in a more controlled manner with more complicated sputtering configurations being employed, such as high-power impulse magnetron sputtering (HiPIMS), hybrid HiPMS/DC-MSE co-sputtering using synchronized pulsed substrate bias, MSE with applying external magnetic field, etc. Besides the mass and energy of incident reactive species, selective control of metal–ion fluxes, substrate bias synchronized to probe gas–ion or metal–ion irradiation, is available in a more controlled manner with more complicated sputtering configurations being employed, such as high-power impulse magnetron sputtering (HiPIMS), hybrid HiPMS/DC-MSE co-sputtering using synchronized pulsed substrate bias, MSE with applying external magnetic field, etc. These methods have been applied to achieve single-phase multi-nary alloys grown at low temperature, which demonstrates a high capability to use MSE for the material growth of high-performance optoelectronics
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