38th International Conference of IMAPS-CPMT Poland

Rzeszw - Czarna, 21-24 September 2014

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Composition analysis of epitaxial NbTiN films for superconductor photon detectors

Sylwia Przywska 1, Marek Guziewicz 2, Marcin Juchniewicz 2, Renata Kruszka 2,

Edyta Piskorska-Hommel 3, Jarosaw Domagaa 4, Alexandru Marin 5,

Petre Osiceanu5, Andriej Klimov 2, Jan Bar2, Wojciech Sysz2 ,

1 Faculty of Physics, Warsaw University of Technology, ul. Koszykowa 75, 00-662 Warsaw, Poland;

2Institute of Electron Technology, al. Lotnikow 32/46, 02-668 Warsaw, Poland; 3 Institute of Solid State Physics, University of Bremen, Otto-Hahn-Allee, 28359 Bremen,

4 Institute of Physics, PAS, al. Lotnikow 32/46, 02-668 Warsaw, Poland 5 Institute of Physical Chemistry, Spl. Independentei 202, Bucharest, Romania

Abstract: We report the characterization of the ultrathin NbTiN films for SSPDs. The higher quality of the ultrathin superconducting films in comparison to niobium nitride, as far as the fabrication technology of single photon detectors is concerned, was demonstrated. The films deposited on Al2O3 single crystals shown excellent both superconducting and structure properties. New results based on XPS studies of NbTiN films reveal presence of some contaminations like carbon and oxygen. The following XPS peaks were examined: Nb 3d, Ti 2p, O 1s, N 1s, C 1s and Al 2p. Compounds of NbN, NbTiN and some Nb-oxides have been revealed. The NbTiN films with thickness of 4 nm, grown on the Al2O3 and post-grown annealed at 1000

oC in Ar, reach critical temperature of 14K. Moreover, the films disclose the best superconducting properties - extremely high critical current density of 12106 A/cm2.

Key words: Superconductor, NbN, XPS, single-photon detector

1. INTRODUCTION

Superconducting single-photon detectors (SSPDs) are able to detect single optical photons, these have relatively high quantum efficiency and low dark counts rate and low jitter. The detectors are expected to play a leading role in such applications as optical quantum information processing, satellite communications and medical diagnostics, especially as detectors of singlet oxygen luminescence in photodynamic therapy. Construction of a detector includes 100 nm wide stripes patterned in an ultrathin NbN or NbTiN film. They are biased on a subcritical current. The absorption of a photon generates a hot spot that grows until a resistive region is formed across the nano-stripe, thus, produced a detectable voltage pulse. The higher quality of ultrathin superconducting NbTiN films in comparison to NbN films was demonstrated. High epitaxial quality of NbTiN films grown on the Al2O3 substrates was proved by HRXRD in our previous work [1,2], but film composition was not cleared because of problems regarding to composition study on so ultrathin films. New results for NbTiN and NbN films that based on X-ray photoelectron spectroscopy (XPS) studies reveal presence of contaminations such as carbon and oxygen.The quantitative composition analyses of NbTiN films as well as detection parameters of SSPD made with the NbTiN film are here presented.

2. TECHNOLOGY OF SUPERCONDUCTIONG FILMS

The NbTiN films were grown by high-temperature reactive radio-frequency magnetron sputtering using 1000C system from the Surrey Nanosystems Ltd. The films were deposited from a 3-inch diameter Nb and Ti targets at DC power of 220 W on Nb and of 80 W on Ti, respectively, in N2Ar plasma at temperature of 850oC. Al2O3(0001) and Si(001) are used as substrates. The thicknesses of

38th International Conference of IMAPS-CPMT Poland

Rzeszw - Czarna, 21-24 September 2014

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the studied layers was 4 nm. To improve superconducting properties of the films Rapid Thermal Annealing (RTA) was conducted at 1000oC in Ar for 10 min [1].

3. XPS STUDIES

3.1. Quantitative analysis of film composition XPS investigations of NbTiN films deposited on Si and Al2O3 were performed on samples assigned NbTiN 11Si and NbTiN 11sh, respectively. The XPS spectra were recorded by means PHI Quantera SXM which is the state of the art Scanning XPS Microprobe equipment in Bucharest. The studied films were cleaned in-situ by sputtering using Ar+ at 500 eV. Analysis of XPS spectra concerns following peaks: Nb 3d, Ti 2p, O 1s, N 1s, C 1s and Al 2p or Si 2p. The XPS quantification is performed by assigning quantification regions, subtracting the background from each region. We used two methods of background subtracting: Spline Shirley (used for titanium peaks), which requires setting up several nodes, necessary for the Shirley algorithm, and Shirley (used for other component peaks). Percentage atomic concentration parameter is computed from the raw peak area divided by the Relative Sensitivity Factor (RSF), which is extracted from the given library for every peak identified by a region. The atomic concentration Xi is computed using the formula:

==

m

i i

ii

A

AX

1

100 (1)

where Ai (i = 1, 2, ) are the adjusted intensities, which are determined from the measured intensity Ii, the transmission function Ti evaluated for electrons of recorded energy E, the relative sensitivity factor Ri for the transition i and the escape depth compensation exponent n. The adjusted intensities are defined by eq. 2 as follows:

( ) ini

i REET

IA 100= (2).

Multiplying Xi by the atomic mass of the components and normalising to 100% we can get mass % concentration. For quantitative analysis of Nb 3d and Ti 2p peaks we used the procedure given in papers [3, 4]. The intensity ratio of doublets 2p3/2 and 2p1/2 peaks are constrained to be at a ratio of 2:1, the intensity ratio of 3d3/2 and 3d5/2 doublets are constrained to be at a ratio of 2:3. FWHM of the peaks in doublets are forced to be alike, except for the Ti doublets in TiO2,because of Coster-Kroning effect, where FWHM of the 2p1/2 peak is broader than FWHM of the 2p3/2 peak.

The values of binding energy applied in our study are the average Binding Energy (BE) data from the NIST XPS database, or the values taken from the paper [3]. The binding energies, which are used in this work are shown in the table 1. The doublet splitting was constrained as we used the available literature data.

Tab. 1. Binding energies E(eV) of Nb and Ti components [3] applied in our XPS spectra simulations.

Peak NbN Nb2O5 NbN(1-x)O(x) NbNO/NbCO Nb2N(2-y)O(3-y) TiO TiN Ti2O3 TiO2

3d5/2 203.7 207.5 204.8 205.9 206.9 - - - -

3d3/2 206.5 209.9 206.8 208.8 209.5 - - - -

2p1/2 - - - - - 460.9 461.8 462.3 464.3

3.2. Composition of NbTiN films Samples NbTiN11sh and NbTiN11Si were taken under the investigation. Based on full XPS spectrum and using the formula (1) we computed atomic % concentrations for the samples and included in Tab.2. The concentrations of components change slowly with the sputtering time. As expected the concentration of carbon suddenly decreases after the first sputtering step, then the falling trend maintains slowly. Concentration of oxygen is gradually decreasing with the etching time, while Al

38th International Conference of IMAPS-CPMT Poland

Rzeszw - Czarna, 21-24 September 2014

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concentration increases slowly with the time, so we can suppose that oxygen is bonded with metals, more intensely on surface, while the Al signal relates to the sapphire substrate. Analyzing the Nb3d peak shown in Fig.1a., there are visible doublets 3d5/2, and 3d3/2 of the following components: NbN, Nb2O5, NbN1-xOx, Nb2N2-yO3-y, and NbNO with NbCO (which are not bonded together, but differences in their binding energy are too small to subtract their spectra). In the Ti2p peak there are visible doublets 2p3/2, and 2p1/2 which can be assigned to the following components: TiO, TiN, Ti2O3 and TiO2. The relative ratio of the noted components changes with the sputtering time. The obtained data for distribution of mass conc. of Nb and Ti compounds is presented in Tab. 3.

Tab. 2. At.% composition of the NbTiN11sh film before and after sputtering etching of the surface. Sputtering time Nb Ti O N C Al

0 25.0 1.8 20.9 17.2 33.2 1.9

0.1 min 39.1 2.0 21.5 26.4 8.5 2.4

0.3 min 43.7 2.4 17.2 28.6 5.1 2.8

0.5 min 45.5 3.0 14.0 30.00 3.3 3.9

a) b)

Fig. 1. An example of Nb3d (a) and Ti2p (b) peaks from XPS spectra measured on the NbTiN 11sh sample as-

deposited on sapphire substrate.

Tab. 3. Distribution of mass % concentration of compounds identified in the NbTiN 11sh film as-dep. Sputtering time

NbN Nb2O5 NbN(1-x)Ox NbNO/NbCO Nb2N(2-y) O(3-y) TiO TiN Ti2O

3

TiO2

As rec 21.7 15.5 34.6 10.0 18.1 17.5 11.8 41.9 28.8

0,2 min 27.9 20.4 43.5 8.2 - 16.6 30.1 30.5 22.8

0,5 min 33.1 17.3 47.1 2.5 - 7.2 29.6 28.9 34.3

Although, as expected, the NbN atomic concentration level is very high, our data shows that there is substantial amount of NbN(1-x)Ox. The concentration levels of NbN as well as NbN(1-x)Ox is increasing with the sputtering time. The concentration of Nb2O5 is mostly visible at the surface of the sample, then its level decreases with the time. The NbNO/NbCO concentration ratio, which indicates the level of contamination of our sample, is decreasing with the sputtering time. Undoubtedly, Nb2N (2-y)O(3-y) is present only on the surface of the sample. The titanium components on the surface are mostly oxides, but the TiN mass concentration increases with the sputtering time. In the case of NbTiN film annealed at 1000oC (NTN11shW) we calculated based on XPS measurements similar at. % concentration of Nb and Ti, but oxygen contamination is strongly present on the top. Moreover, N at. % concentration on the film surface is strongly reduced and in turn, it is increased to the level in the original NbTiN film

216 212 208 204 200

5

10

15

20

25

30

35

0

Binding energy [eV]

Co

unt

s x1

03

468 464 460 456 452

30

25

20

15

10

5

0

Binding energy [eV]

Co

unt

s x1

02

38th International Conference of IMAPS-CPMT Poland

Rzeszw - Czarna, 21-24 September 2014

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after surface cleaning by the Ar+ etching for 2 min. Above results confirm presence of oxidized surface on both the as-dep. - and annealed NbTiN film, and show differences in at.% concentrations.

4. PHOTODETECTOR QUANTUM EFFICIENCY CHARACTERIZATION

The system quantum efficiency (SQE) of the fabricated SSPD, based on the NbTiN film deposited on the sapphire substrate with post annealing , was estimated. For this, the dependence of the detector photon count rate on laser pulse intensity was measured. The SQE was determined as a ratio of the detector count rate to the number of photons, emitted by the laser within a linear part of observed dependence. The laser was operated at 10 MHz repetition rate. Linear dependence indicates that detector is in single photon absorption regime (Fig.3a). A plateau region indicates the background noise level which is due to so-called dark counts. The measurement of critical temperature TC, relied on measuring of the detector resistance as function of temperature. Fig. 3b shows the such dependence of the normalized resistance where TC =14K was registered. Measurements of current density on similar NbTiN film disclosed extremely high critical current density of 12106 A/cm2 which is the best value, up to now, known for superconducting films.

a) b)

Fig. 3. Photon count rate dependence on the laser intensity measured on the SSPD manufactured with the NbTiN film (a); measurement of critical temperature for the NbTiN film (NTN11shW after RTA) (b).

5. CONCLUSIONS Quantitative analysis of atomic concentrations on 4 nm thick NbTiN film was performed by XPS investigations. The film includes 3 at.% conc. of Ti. A deficiency of N concentration is observed in the film because of some oxides formed on the surface. Concentration levels of Nb-, Ti nitrides as well as contaminants like TiOx, NbN(1-x)Ox, and Nb-oxides were evaluated. TbTiN films deposited on Al2O3 and RTA annealed are the superior material for SSPD, as confirmed by measurements of SSPD SQE.

REFERENCES [1] Slysz, W., Guziewicz, M., Borysiewicz, M., , Domagala, J.Z., Pasternak, I., Hejduk, K., Rzodkiewicz, W.,

Ratajczak, J., Bar, J., Wegrzecki, M., Grabiec, P., Grodecki, R., Wegrzecka, I. and Sobolewski, R., Ultrathin NbN films for Superconducting Single-Photon Detectors, Acta Physica Polonica A120 (1), 200-204 (2011).

[2] Guziewicz, M., Slysz, W., Borysiewicz, M., Kruszka R., Sidor, Z., Juchniewicz, M., Golaszewska, K., Domagala, J.Z., Rzodkiewicz, W., Ratajczak, J., Bar, J., Wgrzecki, M. and Sobolewski R. Technology of ultrathin NbN and NbTiN films for superconducting photodetectors Acta Physica Polonica A 120 (no. 6-A), 76 -79 (2011).

[3] Biesinger, M. C., Lau, L.W.M., Gerson, A. R., Smart, R. St.C. Resolving chemical states in XPS analysis of first row transition metals, axides and hydroxides: Sc, Ti, V, Cu and Zn, Applied Surface Science (no. 257), 887-898 (2010).

[4] Darlinski, A., Halbritter, J., On the identification of interface oxides and interface serration by ARXPS, (no. 329), 266-271 (1987).