Plasma-Assisted MOCVD Growth of Superconducting NbN Thin Films Using Nb Dialkylamide and Nb Alkylimide Precursors** By Xiang Liu, Jason R. Babcock, Melissa A. Lane, John A. Belot, Andrew W. Ott, Matthew V. Metz, Carl R. Kannewurf , Robert P. H. Chang, and Tobin J. Marks* Niobium nitride (NbN) thin films are of considerable interest due to their excellent mechanical and chemical sta- bility, as well as having useful superconducting properties. Regarding the latter, NbN has a relatively high critical tem- perature (T c ), critical field (H c2 ), and critical current den- sity (J c ), and has a refractory nature. It is therefore a prom- ising candidate for superconducting microelectronics applications such as superconducting quantum interference devices (SQUIDs), radio frequency (rf) filters, antennae, and sensitive infrared (IR) sensors. [1±4] Thus far, NbN thin films have been grown primarily by physical vapor deposi- tion (PVD) techniques, including sputtering, [5±8] pulsed laser deposition (PLD), [9,10] and molecular beam epitaxy (MBE). [11] Among these, reactive sputtering has been most frequently employed, and NbN thin films with T c values approaching 16 K have been grown by optimized reactive sputtering. However, superconducting NbN film growth mechanisms have not been fully elucidated; moreover, optimal growth conditions are highly growth system spe- cific, and the growth reproducibility of high-quality films is not yet satisfactory. CVD is a widely utilized technique in film growth research as well as in industrial-scale production. Although the attractions of CVD include excellent step coverage, adaptability to large-scale growth processes, and relatively simple apparatus, relatively little work has been carried out on NbN x thin films. [12,13] Understanding and perfecting NbN CVD processes would better define the parameters required for large-scale NbN CVD, as well as for nitride CVD in general. Additional knowledge would also lead to better understanding of phase transformations as well as nitride CVD nucleation and growth processes. In this con- tribution, a pulsed source, plasma-assisted, low-pressure MOCVD process (POMBE), [14] using a hydrazine plasma, is implemented for the first time to grow high-quality, superconducting NbN thin films at low temperatures. Para- mount to efficient MOCVD processes is the availability of volatile metal±organic precursors, and an understanding of the structure/property relationships between precursors and the resultant films. In this report, two different types of metal±organic precursors (a previously described niobi- um(IV) dialkylamide and a new pentavalent alkylimide) are compared in the growth of highly oriented B1±NbN super- conducting thin films on MgO(001) (a = 4.213 ; 4 % lattice mismatch) substrates. Details of film growth, micro- structure, and superconducting properties, as well as the preparation of a new series of volatile Nb alkylimido pre- cursors are described. To establish the viability of POMBE for NbN film growth, experiments were first carried out with the known Nb dialkylamide, Nb(NEt 2 ) 4 (1), prepared and purified according to the literature, [15] using a hydrazine plasma as the nitrogen source. NbN films, grown at temperatures ranging from 350 C to 800 C, were analyzed by X-ray dif- fraction (XRD), which reveals that higher growth tempera- tures produce greater film crystallinity. This enhanced crys- tallinity can be correlated with higher superconducting critical temperatures, which are only distinct in films grown at substrate temperatures above 600 C. XRD results for a NbN film grown at 750 C reveal highly a-axis oriented film growth under these conditions (see Fig. 1A). For NbN films grown at temperatures above 700 C, the typical full width at half maximum (FWHM) of x rocking curve scans of the NbN(200) reflections (inset, Fig. 1A) are between 0.5 and 1.2, also suggesting that the films are highly textured. The rock salt superconducting d phase of NbN has a face-centered cubic lattice (B1 structure) with a = 4.392  (JCPDS-38-1155). Nitrogen atoms are located in the octa- hedral interstitial sites of the Nb atom sublattice. Accord- ing to the NbN phase diagram, [16] the superconducting d phase is metastable at 25 C, the presence of the c and e phases is possible, and thus the lattice of the d phase may be distorted. It has also been reported that the NbN lattice parameter is sensitive to the Nb/N ratio. [17] Our results show that critical temperatures can be correlated with the lattice parameters of the superconducting NbN films. Indeed, the film with the highest T c has a lattice parameter of 4.395 , which best corresponds to JCPDS data. These results suggest that the superconducting properties of POMBE-derived NbN films are sensitive to phase purity and stoichiometry, as has been reported for sputtered NbN thin films. [17] Impurities can have significant effects on the supercon- ducting properties of NbN films. From Auger electron spectroscopy (AES) studies, the principal impurities in the present NbN films are found to be C (probably from the Communications Chem. Vap. Deposition 2001, 7, No. 1 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim,2001 0948-1907/01/0101-0025 $ 17.50+.50/0 25 ± [*] Prof. T. J. Marks, Dr. J. R. Babcock, Dr. J. A. Belot, M. V. Metz The Materials Research Center, Department of Chemistry Northwestern University Evanston, IL 60208-3113 (USA) E-mail: tjmarks@casbah.acns.nwu.edu X. Liu, Dr. A. W. Ott, Prof. R. P. H. Chang The Materials Research Center, Department of Materials Science and Engineering Northwestern University Evanston, IL 60208-3107 (USA) M. A. Lane, Prof. C. R. Kannewurf The Materials Research Center Department of Electrical and Computer Engineering Northwestern University Evanston, IL 60208-3118 (USA) [**] This research was supported by the National Science Foundation through grant CHE-9807042. Characterization facilities were support- ed by NSF MRSEC grant DMR-9632472 to the Northwestern Materi- als Research Center.