Nitrogen-related local vibrational modes in ZnO:N

Abstract

We study the influence of nitrogen, a potential acceptor in ZnO, on the lattice dynamics of ZnO. A series of samples grown by chemical vapor deposition ͑CVD͒ containing different nitrogen concentrations, as determined by secondary ion mass spectroscopy ͑SIMS͒, was investigated. The Raman spectra revealed vibrational modes at 275, 510, 582, 643, and 856 cm Ϫ1 in addition to the host phonons of ZnO. The intensity of these additional modes correlates linearly with the nitrogen concentration and can be used as a quantitative measure of nitrogen in ZnO. These modes are interpreted as local vibrational modes. Furthermore, SIMS showed a correlation between the concentration of incorporated nitrogen and unintentional hydrogen, similar to the incorporation of the p-dopant magnesium and hydrogen in GaN during metalorganic CVD.There is increasing interest in investigating the properties of ZnO epitaxial films with a direct gap of 3.37 eV at room temperature. 1 The material is a potential competitor for GaN-based light-emitting devices in the ultraviolet and blue spectral range. There are reports of superior ZnO properties such as a high exciton binding energy combined with a low lasing threshold density 2 and a good resistance to bombardment with high-energy particles. 3,4 For other wide-band-gap semiconductors as GaN ͑Ref. 5͒ and ZnSe ͑Ref. 6͒ controlled p-type doping is problematic. As-grown ZnO typically has n-type conductivity with background concentrations between 10 16 and 10 17 cm Ϫ3 . However, there have been reports on the synthesis of p-conducting ZnO doped with As ͑Ref. 7͒ and a Ga/N codoping 8 as well as the fabrication of a p-n-junction by excimer-laser doping. 9 In this letter, we report on doping experiments with nitrogen as a potential acceptor and its influence on the lattice dynamics of ZnO.The ZnO thin films under investigation were grown by chemical vapor deposition ͑CVD͒ using a home built epitaxy system which consists of a horizontal quartz reactor and a resistance heating with different temperature zones. Metallic zinc was kept in one zone at a temperature of 470°C the growth temperature was 650°C. We used NO 2 as oxygen precursor and NH 3 as nitrogen source for the doping experiments. The epitaxial films were deposited on GaN/sapphire templates which offers the advantage of a lattice parameter similar to ZnO. We investigated samples containing different nitrogen concentrations. Secondary ion mass spectroscopy ͑SIMS͒ was applied to determine the concentration of nitrogen and unintentional dopants such as hydrogen. The primary ion species was cesium. Nitrogen was detected as 14 N 16 O Ϫ and hydrogen as 64 Zn 1 H Ϫ clusters. The given abso-lute concentrations are accurate to within half an order of magnitude. Despite this accuracy the relative error is less than 10%. The Raman-scattering experiments were carried out in backscattering geometry with a triple-grating spectrometer equipped with a cooled charge-coupled device detector. The lines at 488 and 514.5 nm of an Ar ϩ /Kr ϩ mixedgas laser were used...

Solution-based synthesis of cobalt-doped ZnO thin films

“…The band at 241 cm -1 is not readily identified, but we note that there is a peak in the calculated phonon density of states for ZnO at this wavenumber [45] although it appears that this is not normally observed. Indeed, there is no occurrence of a Raman peak at precisely this wavenumber in an extensive catalogue of (transition and other) metal compounds [46] but it is interesting to note that a strong peak has been observed at 240 cm -1 in nanophase TiO 2 [47] which was not clearly identified but was attributed to a long range order effect, occurring when bands known to be characteristic of the crystal structure were strong.…”

Isotopic study of the nitrogen-related modes in N+-implanted ZnO

“…As the intensity of those peaks increased linearly with N concentration, it was further suggested that their intensity could be used to determine the N concentration in N-doped ZnO samples. 5 However, Bundesmann et al disputed this interpretation, as they detected similar peaks in ZnO thin films doped with Fe, Sb, and Al, intentionally grown without N incorporation. 6 They attributed the additional Raman peaks to intrinsic host lattice defects.…”

“…3 Being nitrogen the primary acceptor candidate in ZnO, considerable effort has been devoted to characterize, by means of vibrational spectroscopy, the impurity incorporation and the formation of complexes that may reduce the doping efficiency of N in ZnO. [4][5][6][7] Kaschner et al 5 reported the observation of several additional Raman peaks between 275 and 643 cm −1 in N-doped GaN epitaxial films grown by chemical vapor deposition. They were attributed to local vibrational modes ͑LVMs͒ of N in substitutional O site in the ZnO lattice.…”

Electrical and Optical Properties of ZnO Irradiated with Hydrogen Plasma

“…Chen et al reported that this broad peak in the O + -implanted ZnO single crystal was much smaller than that in the B + -implanted one [21]. This broad Raman peak at around 580 cm −1 has been attributed to oxygen deficiency in ZnO [21][22][23]. The increase of the broad Raman peak indicates that VO was formed by the Zn-annealing.…”