Core-level spectra and binding energies of transition metal nitrides by non-destructive x-ray photoelectron spectroscopy through capping layers

Abstract

We present the first measurements of x-ray photoelectron spectroscopy (XPS) core level binding energies (BE's) for the widely-applicable group IVb-VIb polycrystalline transition metal nitrides (TMN's) TiN, VN, CrN, ZrN, NbN, MoN, HfN, TaN, and WN as well as AlN and SiN, which are common components in the TMN-based alloy systems. Nitride thin film samples were grown at 400 °C by reactive dc magnetron sputtering from elemental targets in Ar/N2 atmosphere. For XPS measurements, layers are either (i) Ar + ion-etched to remove surface oxides resulting from the air exposure during sample transfer from the growth chamber into the XPS system, or (ii) in situ capped with a few nm thick Cr or W overlayers in the deposition system prior to air-exposure and loading into the XPS instrument. Film elemental composition and phase content is thoroughly characterized with time-of-flight elastic recoil detection analysis (ToF-E ERDA), Rutherford backscattering spectrometry (RBS), and x-ray diffraction. High energy resolution core level XPS spectra acquired with monochromatic Al Kα radiation on the ISO-calibrated instrument reveal that even mild etching conditions result in the formation of a nitrogen-deficient surface layer that substantially affects the extracted binding energy values. These spectra-modifying effects of Ar + ion bombardment increase with increasing the metal atom mass due to an increasing nitrogen-to-metal sputter yield ratio. The superior quality of the XPS spectra obtained in a non-destructive way from capped TMN films is evident from that numerous metal peaks, including Ti 2p, V 2p, Zr 3d, and Hf 4f, exhibit pronounced satellite features, in agreement with previously published spectra from layers grown and analyzed in situ. In addition, the N/metal concentration ratios are found to be 25-90% higher than those obtained from the corresponding ion-etched surfaces, and in most cases agree very well with the RBS and ToF-E ERDA values. The N 1s BE's extracted from capped TMN films, thus characteristic of a native surface, show a systematic trend, which contrasts with the large BE spread of literature "reference" values. Hence, non-destructive core level XPS employing capping layers provides an opportunity to obtain high-quality spectra, characteristic of virgin in situ grown and analyzed TMN films, although with larger versatility, and allows for extracting core level BE values that are more reliable than those obtained from sputter-cleaned N-deficient surfaces. Results presented here, recorded from a consistent set of binary TMN's grown under the same conditions and analyzed in the same instrument, provide a useful reference for future XPS studies of multinary materials systems allowing for true deconvolution of complex core level spectra.

Effect of ion-implantation-induced defects and Mg dopants on the thermoelectric properties of ScN

“…Compositional depth profiles were obtained by recording core level spectra after each sputtering step consisting of 3 minutes-long bombardment with 4 keV Ar+ ions followed by 10 min. irradiation at the reduced energy of 0.5 keV to minimize the surface damage and avoid forward implantation of surface species[37]. The Ar+ ion beam was incident at the 20° angle from the surface and rastered over the area of 3×3 mm 2 .…”

In-situ observation of self-cleansing phenomena during ultra-high vacuum anneal of transition metal nitride thin films: Prospects for non-destructive photoelectron spectroscopy

“…33 The latter provides a good barrier towards oxidation and allows nondestructive XPS with data quality characteristic of films grown and analyzed in-situ. 34 In addition, spectra recorded from poly-TiN films treated in a conventional way, i.e., with Ar þ ion etch, are included to illustrate the detrimental effects of ion bombardment. 35 Clearly, the Ti 2p spectrum from UHV-annealed poly-TiN is essentially identical to that obtained from epi-TiN samples and in agreement to Al-capped poly-TiN films.…”

“…The 3d spectrum of NbN ( Fig. 3(a)) is of particular interest as, in contrast to the Ti 2p spectra of poly-TiN, no satellite features are observed on the high BE side of the main peaks 34,38 hence, any residual oxide left on the surface after UHV anneal may be directly observed. Clearly, Nb 3d 5/2 and Nb 3d 3/2 oxide peaks, originally present at 207.3 and 210.2 eV in the spectrum from the as-deposited film, are completely gone after the 1000 C anneal.…”

Native target chemistry during reactive dc magnetron sputtering studied by ex-situ x-ray photoelectron spectroscopy

“…24 In this way, high quality core level spectra directly comparable to those acquired from films grown and analyzed in situ in a ultrahigh-vacuum (UHV) XPS system can be acquired. 25 Here, we report high-energy resolution core-level photoelectron spectra obtained ex situ from native Ti targets after magnetron sputtering in Ar/N2 atmospheres. Targets are in situ capped, immediately after the film growth, with a few nm thick metal overlayers prior to airexposure to prevent oxidation during transfer from the growth chamber into the XPS instrument.…”

“…The corresponding set of N 1s spectra (not shown), indicated an increasing signal intensity with increasing pN2, in agreement with the evolution of Ti 2p spectra discussed above. Spectra consisted of a main peak centered at 397.1 -397.4 eV, corresponding to TiNx, 25 and an asymmetric tail on the high BE side, more apparent in the spectrum obtained from targets sputtered with pN2 ≥ 0.27 mPa. Indeed, previously reported high-resolution TiN core level spectra of thin films contain satellite features also in the N 1s signal.…”