The properties and deposition process of GaN films grown by reactive sputtering at low temperatures

Abstract

Polycrystalline gallium nitride films, 100 nm to 1 m thick, were deposited under a range of conditions. Substrate electrode temperatures during sputtering were varied from room temperature to 450°C, the pressure from 0.15 to 6.0 Pa, the nitrogen fraction of the deposition atmosphere from 10% to 100% and the target bias from −400 to − 1800 V. The deposition rates as functions of these conditions are in the range 0.5-25 nm/ min. The growth rate is considered to be controlled respectively by the thermally activated desorption from the substrate, changes in the mean free path and concentration of gas particles, differences between the sputter yields of Ga and GaN in Ar and N 2 , and changes in the ion current and sputter yields. The films are generally columnar, with the grain size increasing with film thickness. The most crystalline films were grown at mid range temperatures, low N 2 concentrations, and low target biases, and the most disordered were grown at low pressures. The latter two cases suggest that decreasing the energy of particles incident on the film during deposition results in a more ordered film. The biaxial stress is compressive and shows an increasing trend with the target bias and N 2 concentration, reaching 4.7 GPa at 75% N 2 . Oxygen contamination of 3 -30 at. % has a major effect on the optical properties of the films, increasing the band gap values from 3.02 to Ͼ 4.0 eV and the Urbach tail energies from around 150 to 840 meV and decreasing the refractive index from 2.46 to 2.03. At a 40% N 2 deposition fraction, the N:Ga ratio is more or less constant at 1:1. Since the absolute oxygen incorporation rate changes very little, it is the relative film deposition rate which determines the final oxygen concentration. Excess Ga at low N 2 concentrations causes a decrease in the band gap and an increase in the Urbach tail energy.

Effects of substrate temperature, substrate orientation, and energetic atomic collisions on the structure of GaN films grown by reactive sputtering

“…It is worth noting that unlike several results reported in the literature, 17,25,26 no cracking or peeling from substrate was observed in SEM, AFM, or the optical confocal microscopies in films at all temperatures, despite the large stress observed in the growth at high temperatures.…”

“…The values estimated for the strain and the resulting tensile stress are very large, compared to most reports. 17,25,26,41,62,63 This can be caused by the high adherence of the films to the substrate provided by the energetic deposition, in association with the tendency of the grain interfaces to merge. In SEM images of the cross section of the film deposited at 1000 C (and a-plane sapphire), the grain contours are still apparent (Fig.…”

“…[13][14][15][16] Differently, in reactive sputtering, the substrate temperature has a minor effect on the activation of the impinging species. Because of the relatively low working pressures of reactive sputtering, the activation of precursors is almost exclusively caused by the plasma, [17][18][19][20][21][22][23][24][25][26][27] and thus occurs independently of the substrate temperature. In the growth of GaN by reactive sputtering, the increase in the substrate temperature produces an increase in the vapor pressure of the film, which then produces the sublimation of N and Ga far from equilibrium, while the incidence of the depositing species is directional and almost exclusively dependent on the parameters of the plasma.…”

“…The detailed understanding of the complex kinetics of the growth in association with the production of defects in order to produce high quality material is a main challenge to the development of this technique. [17][18][19][20][21][22][23][24][25][26][27]29,30,36 Molecular dynamics simulations 37,38 showed that much less damage by the undesired effects of ion bombardment is produced in GaN than in Si, Ge, or GaAs during the film growth. 37,38 This remarkable difference (about a factor of 5) is explained by the higher threshold displacement energy of atoms in GaN and favors tests on the sputtering deposition of GaN in relation to other common functional semiconductors.…”

“…[13][14][15][16] However, because these techniques are complex and expensive, the search for simpler and more versatile techniques capable of producing high quality homogeneous GaN films over large areas has attracted interest. 7,[17][18][19][20][21][22][23][24][25][26][27] Among the techniques under development for GaN growth based on physical processes are pulsed laser deposition (PLD) 28 and sputtering. [17][18][19][20][21][22][23][24][25][26][29][30][31] In the case of reactive sputtering, significant improvement has been achieved, [17][18][19][20][21][22][23][24][25][26][27]31 including the growth of epitaxial single crystal GaN, which was obtained by using magnetron-sputtering epitaxy (MSE).…”