X-ray absorption study of defects in reactively sputtered GaN films displaying large variation of conductivity

Abstract

Undoped GaN films were epitaxially grown on c-sapphire by rf magnetron reactive sputtering of GaAs at different N 2 percentages in Ar-N 2 sputtering atmosphere. These films display a large increase in conductivity from ∼10 −6 Ω −1 cm −1 to ∼10 2 Ω −1 cm −1 with decrease of N 2 in sputtering atmosphere from 100% to 10%. X-ray photoelectron spectroscopy reveals a monotonous decrease of N/Ga ratio, concurrently with increase of uncoordinated/interstitial Ga in the films, with decrease of N 2 percentage in sputtering atmosphere. Extended x-ray absorption fine structure measurements and x-ray absorption near edge structure (XANES) simulation at Ga K-edge demonstrate the absence of N vacancies in sputtered GaN films and point towards the presence of Ga vacancies, which increase with decrease of N 2 percentage in sputtering atmosphere. XANES measurements at O K-edge reveal that O interstitials are present in most of the films, except for the film grown at low N 2 percentages (∼10%), in which case, O substitution on N-site may also take place. A monotonous increase of n-type conductivity is observed with increase of Ga interstitials, which suggests that Ga interstitials are the main source of n-type conductivity in sputtered GaN films. The low conductivity of the films grown at high N 2 percentages (≳75%) is attributed to the limited presence of Ga interstitials and a strong compensation, dominated by N and O interstitials. The high conductivity GaN films grown at low N 2 percentages may be partly contributed by O substituted on N site, along with Ga vacancies and possibly V Ga -O N complexes, as the compensating defects.

Microstructural dependence of residual stress in reactively sputtered epitaxial GaN films

“…The EDX data also confirm the absence of arsenic and the presence of oxygen impurity (~3 at.%) in all the films. These results are in agreement with earlier XPS and SIMS studies of undoped GaN films [18,44], which also indicated the possibility of excess nitrogen in the films grown at higher N2 percentages, as in the case of Si-doped films. All the films reveal the presence of ~2 at.% Si, irrespective of N2 percentage in sputtering atmosphere.…”

Electrical transport in epitaxially grown undoped and Si-doped degenerate GaN films

“…In the case of Si-doped GaN films, EDX measurements have shown the presence of ∼2 at.% Si in all the films along with oxygen impurity (∼3 at.%) and confirmed the absence of arsenic [42]. It was also found that the N/Ga ratio in Si-doped films decreased with decrease of N 2 percentage in sputtering atmosphere, similar to the observations made for undoped GaN films [42,44].…”

“…It has earlier been shown [20,42] that both undoped and Si-doped GaN films grown at 100% N 2 possess large hydrostatic strain due to the presence of N interstitials (known to be acceptors in GaN [45,46]), which decreased drastically with decrease of N 2 percentage in sputtering atmosphere. The high resistivity of undoped GaN films grown at 100% N 2 and its decrease with N 2 percentage was hence primarily attributed to the strong compensation by N interstitials and their subsequent decrease [44]. Thus, the low carrier concentration of Sidoped GaN films grown at 100% N 2 is also attributed to the compensation by N interstitials and its subsequent increase in the film grown at 75% N 2 to the substantial decrease of N interstitials, as shown earlier [42].…”

“…The phi (f) scans of (10 11) reflection in both cases [20,42] have confirmed the epitaxial character of the films, conforming to the in-plane relationship of GaN [11 2 0]║α-Al 2 O 3 [10 10] [43] and alignment with the oxygen sub-lattice of sapphire. XPS measurements of undoped GaN films [44] have shown that the N/Ga ratio decreased substantially with decrease of N 2 percentage in sputtering atmosphere. Furthermore, XPS and secondary ion mass spectroscopy (SIMS) measurements of sputtered undoped GaN films [20,44] revealed the absence of arsenic but the presence of oxygen impurity in the films.…”

“…XPS measurements of undoped GaN films [44] have shown that the N/Ga ratio decreased substantially with decrease of N 2 percentage in sputtering atmosphere. Furthermore, XPS and secondary ion mass spectroscopy (SIMS) measurements of sputtered undoped GaN films [20,44] revealed the absence of arsenic but the presence of oxygen impurity in the films. In the case of Si-doped GaN films, EDX measurements have shown the presence of ∼2 at.% Si in all the films along with oxygen impurity (∼3 at.%) and confirmed the absence of arsenic [42].…”