Growth and oxidization stability of cubic Zr1-

xGdxN solid solution thin films

Carina Höglund, Björn Alling, Jens Jensen, Lars Hultman, Jens Birch and R. Hall-Wilton

Linköping University Post Print

N.B.: When citing this work, cite the original article.

Original Publication:

Carina Höglund, Björn Alling, Jens Jensen, Lars Hultman, Jens Birch and R. Hall-Wilton,

Growth and oxidization stability of cubic Zr1-xGdxN solid solution thin films, 2015, Journal

of Applied Physics, (117), 19, 195301.

http://dx.doi.org/10.1063/1.4921167

Copyright: American Institute of Physics (AIP)

http://www.aip.org/

Postprint available at: Linköping University Electronic Press

http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-119249

Growth and oxidization stability of cubic Zr1−xGdxN solid solution thin filmsC. Höglund, B. Alling, J. Jensen, L. Hultman, J. Birch, and R. Hall-Wilton Citation: Journal of Applied Physics 117, 195301 (2015); doi: 10.1063/1.4921167 View online: http://dx.doi.org/10.1063/1.4921167 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/117/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Phase stability of ScN-based solid solutions for thermoelectric applications from first-principles calculations J. Appl. Phys. 114, 073512 (2013); 10.1063/1.4818415 Strong electron correlations stabilize paramagnetic cubic Cr1−xAlxN solid solutions Appl. Phys. Lett. 102, 031910 (2013); 10.1063/1.4788747 Mixing thermodynamics of TM 1 − x Gd x N ( TM = Ti , Zr , Hf ) from first principles Appl. Phys. Lett. 98, 241911 (2011); 10.1063/1.3600059 Wurtzite structure Sc 1 − x Al x N solid solution films grown by reactive magnetron sputter epitaxy: Structuralcharacterization and first-principles calculations J. Appl. Phys. 107, 123515 (2010); 10.1063/1.3448235 Degradation of ZrN films at high temperature under controlled atmosphere J. Vac. Sci. Technol. A 22, 2071 (2004); 10.1116/1.1786308

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

130.236.83.172 On: Thu, 18 Jun 2015 08:45:00

Growth and oxidization stability of cubic Zr12xGdxN solid solution thin films

C. H€oglund,1,2,a) B. Alling,2,3 J. Jensen,2 L. Hultman,2 J. Birch,2 and R. Hall-Wilton1,4

1European Spallation Source ESS AB, P.O. Box 176, SE-221 00 Lund, Sweden2Department of Physics, Chemistry and Biology (IFM), Thin Film Physics Division, Link€oping University,SE-581 83 Link€oping, Sweden3Max-Planck-Institut f€ur Eisenforschung GmbH, D-402 37 D€usseldorf, Germany4Mid-Sweden University, SE-851 70 Sundsvall, Sweden

(Received 27 March 2015; accepted 28 April 2015; published online 15 May 2015)

We report Zr1�xGdxN thin films deposited by magnetron sputter deposition. We show a solid

solubility of the highly neutron absorbing GdN into ZrN along the whole compositional range,

which is in excellent agreement with our recent predictions by first-principles calculations. An oxi-

dization study in air shows that Zr1�xGdxN with x reaching from 1 to close to 0 fully oxidizes, but

that the oxidization is slowed down by an increased amount of ZrN or stopped by applying a cap-

ping layer of ZrN. The crystalline quality of Zr0.5Gd0.5N films increases with substrate tempera-

tures increasing from 100 �C to 900 �C. VC 2015 Author(s). All article content, except whereotherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

[http://dx.doi.org/10.1063/1.4921167]

INTRODUCTION

Gadolinium compounds and their alloys are of interest

due to the high thermal neutron absorption cross-section of

Gd and due to its magnetic properties. Among all naturally

occurring elements, Gd has the highest thermal neutron

absorption cross-section of 49 700 b. One of its stable iso-

topes, 157Gd, with a natural abundance of 15.65% has an

absorption cross-section as high as 259 000 b and is commer-

cially available. Gd is ferromagnetic at temperatures below

289 K, which makes it a common element in ferromagnetic

superconductors. At higher temperature it is strongly para-

magnetic, resulting in Gd(III) chelates being the leading con-

trast agents in magnetic resonance imaging.1

The high probability of Gd to absorb thermal neutrons

enables new types of neutron detectors that could raise detec-

tor resolution and detection efficiency to a completely new

level. A second application is found within thermal neutron

shielding, where the nowadays very large and heavy materi-

als could be replaced by micro-engineered and miniaturized

solutions.2 New large-scale neutron scattering facilities like

the European Spallation Source (ESS)3 and target station 2

at the Spallation Neutron Source (SNS)4 are striving to push

the limits within these fields with requirements that are not

yet fulfilled by state of the art technologies. In addition to

the enhanced requirements there is a severe shortage in the

supply of 3He gas,5,6 the nowadays primarily used neutron

absorbing and converting material in neutron detectors. This

has forced the community to search for alternative solu-

tions7,8 and the boron isotope 10B is seen as one of the most

promising replacements.9–15 The main options are technolo-

gies based on 10B4C thin films and this is a field in which the

authors’16–18 and Nowak and co-workers’19 research and de-

velopment work have enabled rapid progress. These types of

detectors are mainly intended for use in large area gaseous

neutron detectors and a significant fraction of the detectors at

the ESS will be based on this technology.20,21 A limitation of

the 10B4C technology is the relatively low efficiency of a

few percent for one layer of 10B4C to absorb and convert a

neutron into the two reaction particles 7Li and 4He (10Bþ n

! 7Liþ 4Heþ c) and to detect those.22,23 The solution has

been to collect the neutrons over several layers,9,24–27 result-

ing in the detector mechanics being the limiting factor for

the spatial resolution. Instead, a detector containing only

one, several micrometer thick, layer of Gd could provide an

efficiency of approximately 20%, or around 50% if the iso-

tope 155Gd or 157Gd is used.28,29 Such a detector would fulfill

the efficiency and resolution requirements of 20% and

<0.2 mm, respectively, and allow for parallax corrections in

the varying sample-detector distance setup in the Neutron

Macromolecular Crystallography (NMX) instrument that

will be built at the ESS.3 Initial studies on the potential of

solid neutron converters in combination with micro pattern

gaseous detectors have been reported.30 Schulz and

co-workers developed micro-strip gas chambers (MSGC)31

that contain neutron converting layers of Gd29,32 and in par-

allel Sauli developed gas electron multiplication (GEM) neu-

tron detectors, which can be an alternative to MSGCs.33

Both technologies have the potential to be used in an instru-

ment like NMX, but require long-term stable thin films con-

taining high amounts of Gd. GdN, which has a higher Gd

content per unit volume than Gd2O334 or even pure Gd could

be a suitable solution.

The reduction of experimental backgrounds for neutron

spallation sources is a very topical issue35 and instrument

performance is typically defined by signal-to-background

ratios. While the signal is defined by the intensity of the

source, the background can be improved with more advanced

instrument designs, resulting in improved performances.

Additionally, large volumes of radiation shielding are

required and, therefore, significant cost savings are expected

for advances in the search for new effective materials.2,36a)Email: carina.hoglund@esss.se. Tel.: þ46 72 179 2023.

0021-8979/2015/117(19)/195301/6 VC Author(s) 2015117, 195301-1

JOURNAL OF APPLIED PHYSICS 117, 195301 (2015)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

130.236.83.172 On: Thu, 18 Jun 2015 08:45:00

Whilst the work this far has concentrated on reducing fast

neutron and gamma ray background, there is also an effort

needed in reducing the background from thermal neutrons

preventing “cross-talk” between local neutron detector ele-

ments. Using Gd compounds, neutron shielding can be engi-

neered to be both compact and very efficient. At the

moment, this is typically solved by using Gd in the form of

Gd2O3 that is mixed with an epoxy and painted onto surfaces

to be shielded, but epoxy is not ideal due to outgassing and

does not allow for something mechanically precise.

Therefore, stable Gd-rich compounds that are precisely de-

posited as thin films present a potential application niche.

During the past ten years, it has been shown that

rocksalt-structured GdN thin films can be grown both with

physical vapor deposition techniques like reactive sputter-

ing,37–40 reactive thermal evaporation,41–43 and molecular

beam epitaxy,44,45 but also with metal-organic chemical

vapor deposition processes.46,47 The intention has mainly

been to deposit high quality films for investigations of their

optical and magnetic properties. The foremost issue related

to growth has been the high tendency to oxidization, which

(when needed) has been solved with buffer and/or capping

layers.37,43,44,48 GdN single crystals have been shown to

have a metallic electrical conductivity.49 For use in a neutron

detector, we are seeking a solution for a stable enough com-

pound that is electrically conducting and contains high

amounts of Gd. The stable oxide Gd2O3, therefore, has the

drawback that it is an insulator, which hinders the transport

of built up charges in a detector, and that the atomic concen-

tration of Gd is only 40% of the compound, which lowers

the neutron absorption efficiency significantly.

Recently, the authors presented a first-principles study on

the mixing thermodynamics of GdN with the transition metal

nitrides TiN, ZrN, and HfN.50 These binary compounds were

chosen because they are thoroughly studied, chemically quite

inert, thermally stable, and electrically conducting.51,52 They

are also known to be good oxidization barriers and are industri-

ally used as such.53 To alloy GdN with one of these com-

pounds could result in a solid solution that is straightforward to

deposit as a thin film, that is both conducting and oxidization

resistant, and contains a high amount of Gd.

In this experimental study, we have chosen ZrN as the

alloying compound to explore the solid solubility of ZrN and

GdN and the resulting thin film properties. ZrN was pre-

ferred over TiN because the mixing enthalpy calculations in

Ref. 50 stipulates phase separation for the latter, while HfN

was disregarded because it is more exotic, less explored, and

considerably more expensive than ZrN. We have deposited

solid solution films over the full compositional range of

Zr1�xGdxN from ZrN to GdN, thus verifying theoretical pre-

dictions of the mixing tendency of the alloy. Results for oxi-

dization resistance and crystalline quality are also presented.

EXPERIMENTAL PROCEDURES

Thin film deposition

Deposition experiments were performed in an ultrahigh

vacuum chamber at a base pressure of 4 � 10�6 Pa. Reactive

magnetron sputter deposition using unbalanced type II

magnetrons with 75 mm diameter Zr and Gd elemental targets

was used to grow Zr1�xGdxN films, with x ranging from 0 to

1, onto polished Al2O3(0001) and Si(001) substrates. The Ar

and N2 partial pressures were set to 0.53 and 0.13 Pa, respec-

tively. As references for samples included in the oxidization

study and as diffusion barriers at high deposition tempera-

tures, seed and/or capping layers of ZrN(111) or ZrN(001)

were deposited additionally. The deposition system is

described in detail elsewhere.54 The ZrN seed and capping

layers were chosen because they are known to be temperature

stable, efficient diffusion barriers,51,53 and serve as lattice-

matched templates for epitaxial film growth, especially for the

ones with the lowest GdN contents. Al2O3(0001) substrates

were chosen as the base substrates due to their temperature

stability and to avoid overlap of film peaks with substrate

peaks in X-ray diffraction (XRD).

Prior to deposition, the substrates were cleaned in ultra-

sonic baths of trichloroethylene, acetone, and 2-propanol and

blown dry in dry N2. The substrate heater was slowly ramped

up to the chosen deposition temperature, which was con-

trolled by a thermocouple positioned behind the substrate

and calibrated by pyrometry. For seed and capping layers,

the same substrate temperatures were used as during the film

depositions.

The Zr and Gd magnetron powers were set to a total

power of 300 W, with relative adjustments of the powers to

obtain various compositions, x between 0 and 1, in

Zr1�xGdxN. Rutherford Backscattering Spectrometry (RBS)

results show that for a molar fraction of x¼ 0.5, the Zr and

Gd magnetron powers need to be 200 W and 100 W,

respectively.

Thin film analysis techniques

Compositional analysis was mainly performed with

RBS using a 2.0 MeV Heþ beam at 6� incidence and 172�

scattering angle. The advantage with RBS for Zr1�xGdxN

thin films is the possibility to obtain very accurate composi-

tional ratios between the metal atoms. The sensitivity for

contaminants such as H (Heþ can not be backscattered on

H), C, and O is low. However, C and N can be distinguished

if the films contain several atomic percent of each kind. For

this study, RBS was used to determine the ratio between Zr

and Gd, meaning x. It was also a key technique to judge

whether the film was oxidized or not, since that can be

extracted from the data both by looking at change in the areal

density of the film when oxidized and/or by determining the

amount of O relative to N.

As a complementary technique to determine the compo-

sition of the light elements in the Zr1�xGdxN thin films, we

have used time-of-flight Elastic Recoil Detection Analysis

(ToF-ERDA). This compositional analysis technique was

performed using a 31.5 MeV 127I8þ beam at 66� incidence

and 45� recoil scattering angle. The recoil energy of each

element was converted to relative elemental depth profiles

using the CONTES code.55 As the sensitivity is good for

light elements, we have used ToF-ERDA to quantify the

amounts of N and impurities like Ar (from sputter gas), O,

C, and H in the film.

195301-2 H€oglund et al. J. Appl. Phys. 117, 195301 (2015)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

130.236.83.172 On: Thu, 18 Jun 2015 08:45:00

The Gd isotope distribution in the films was measured

by time-of-flight Secondary Ion Mass Spectrometry (ToF-

SIMS) using a TOF.SIMS V instrument (ION-TOF GmbH,

Germany). A dual-beam depth profiling procedure was

applied with a 1.0 keV O2þ sputter beam, having a current of

300 nA and scanned over 400 � 400 m2. A pulsed 30 keV

Biþ beam was used as the analysis beam. The current was

typically 1.5 pA with an analysis field of view of 100

� 100 lm2 at the centre of the sputter crater. Positive

ToF-SIMS spectra were acquired between the sputter cycles

in the so-called spectroscopy mode (mass resolution

m/Dm� 8000, beam spot� 5 lm).

The crystal structure was characterized by Cu Ka XRD

using a Philips Bragg-Brentano diffractometer. The film

thickness was measured with cross-sectional scanning elec-

tron microscopy (SEM) for films with high ZrN content

using a LEO 1550 instrument, equipped with an in-lens de-

tector operated at 5 kV at a working distance of �3 mm.

RESULTS AND DISCUSSION

The solid solubility of GdN into ZrN was explored by

depositing a series of Zr1�xGdxN films with 0� x� 1 at a

substrate temperature of 700 �C. The films were sandwiched

between seed and capping layers of ZrN to avoid any reac-

tions with the substrate or oxidization due to exposure to air

before the characterization was done. It has been shown pre-

viously for the chemically similar Sc1�xAlxN system that a

comparison between calculated lattice spacings over the

whole composition range with experimental measurements is

a reliable tool to reveal secondary phase formation during

film growth.56–58 Applying this comparison to the present

system, Figure 1 shows the calculated lattice parameters for

the cubic disordered solid solutions of Zr1�xGdxN from Ref.

50 together with the experimental curve, which is a combina-

tion of the relations between the Zr and Gd metals obtained

from RBS and the lattice parameters calculated from XRD

data. The measured lattice parameters follow the same

increasing trend with increasing GdN content as the calcula-

tions. The calculated lattice parameters slightly overestimate

our measured values, which is the usual condition found for

calculations employing the generalized gradient approxima-

tion for exchange-correlation effects in nitrides.57,59

However, this overestimation vanishes for the GdN rich

compositions. In Ref. 50, it was shown that the theoretical

overestimation for pure GdN with respect to experiments is

smaller compared to the case of pure ZrN. One should also

keep in mind that the calculations were done for the ideal

nitrogen stoichiometry while a slight understoichiometry is

found in our measurements (see below). The effect of such

understoichiometry on the lattice spacing can be different in

different nitrides, possibly adding to the slight differences in

slope between theory and experimental curves in Figure 1.

However, the gradual increase in lattice spacing with compo-

sition almost follows Vegard’s rule60 and the theoretical pre-

dictions and therefore it is a strong evidence of the formation

of solid solutions over the full compositional range.

To determine the most favorable substrate temperature

for high quality epitaxial Zr1�xGdxN films, Zr0.5Gd0.5N was

deposited with a ZrN seed and capping layer onto

Al2O3(0001) at substrate temperatures between 100 and

900 �C. As seen in the different XRD scans in Figure 2, it is

mainly the orientation of the ZrN that determines the orienta-

tion of the Zr0.5Gd0.5N. The preferred growth orientation for

the highest (900 �C) and lowest (100 �C) applied tempera-

tures is h111i, while temperatures between 300 and 700 �Callow for a mixture of h111i and h001i oriented growth. No

other growth orientations were seen in longer overview

scans. While the hexagonal symmetry of the Al2O3(0001)

substrate surface will favor h111i oriented nucleation at high

temperatures, the in-plane lattice mismatch of Al2O3(0001)

compared to ZrN(111) is as large as 19.4%, which reduces

the possibility of forming continuous epitaxial layers, in par-

ticular, at limited growth temperatures. On the other hand,

Petrov and co-workers have previously shown that increased

adatom mobility, as induced by increased substrate tempera-

ture, generally promotes a h001i preferred growth orienta-

tion.61 Thus, the mixture of ZrN h111i and h001i oriented

growth at intermediate growth temperatures may be

explained by a competition between the driving forces for

h111i oriented epitaxy and for h001i oriented growth due to

FIG. 1. Measured (this work) and calculated (Ref. 50) lattice parameters for

the cubic rocksalt solid solution of Zr1�xGdxN with 0� x� 1.

FIG. 2. XRD data from (111) and (001) oriented Zr0.5Gd0.5N films on

Al2O3(0001) and (111) and (001) oriented ZrN seed and capping layers

grown at substrate temperatures between 100 and 900 �C.

195301-3 H€oglund et al. J. Appl. Phys. 117, 195301 (2015)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

130.236.83.172 On: Thu, 18 Jun 2015 08:45:00

increased adatom mobility. At the highest temperature

though, the h111i oriented preferred growth indicates that

the interaction with the substrate becomes the determining

factor. An additional observation is the shift of the

Zr0.5Gd0.5N 111 peak towards higher angles, meaning

smaller out-of-plane lattice parameter, with increasing tem-

perature. This behavior is consistent with an increasing influ-

ence by the substrate surface lattice as the temperature is

increases, as evidenced by the trend towards h111i oriented

growth. Also a nitrogen under-stoichiometry, as is often

observed for reactively grown transition metal nitrides, may

have a similar influence on the lattice parameter evolution.

While it is beyond the scope of this work to clarify the rea-

son for this temperature effect, we note that the coefficients

of thermal expansion (CTE) of most transition metal nitrides

(e.g., CTEZrN¼ 7–9 �10�6 K�1) and sapphire perpendicular

to the C-plane (CTEAl2O3¼ 7–10 �10�6 K�1) are very simi-

lar and cannot explain the large observed peak shifts.

The XRD data for the intermediate growth temperature

500 �C, which exhibits the highest 002 peak intensity,

exhibits an inconsistency in the Zr0.5Gd0.5N 002 peak shift

direction. We find this interesting, as it is an indication of a

material with anisotropic mechanical and/or structural prop-

erties, as might be for the case in Ref. 50 predicted ordered

structure of ZrGdN2. Nevertheless, the formation of that

phase could not be found in any of the measured XRD

scans or X-ray pole figures (not shown), even though the

compositional ratios were correct and the in-plane lattice

mismatch was expected to be only 3.9% according to the

calculated lattice parameters in Ref. 50. Instead, the substi-

tutionally disordered Zr0.5Gd0.5N solid solution was

observed for all applied substrate temperatures. It was noted

in Ref. 50 that the predicted critical ordering temperature,

Tc¼ 1020 K (747 �C), is in the same range as where bulk

diffusion becomes extremely slow in nitrides.55 Since sev-

eral of our samples were grown at lower temperatures, the

absence of an ordered phase illustrates that surface diffusion

alone, which is active at all the considered temperatures, is

insufficient to create a long-range metal-site ordering in this

nitride system.

ToF-ERDA depth profiles were recorded for films that,

according to RBS, contain x¼ 0.30 in Zr1�xGdxN, both with

and without ZrN capping layers. Both films show composi-

tional levels of H, C, F, and Ar below the ToF-ERDA detec-

tion limit of 0.03 at. %. The level of O is also close to the

detection limit in the film that was covered with a capping

layer, while the film without a capping layer shows higher O

levels close to the surface. This indicates that the films do

not contain any O before they are exposed to air and start to

oxidize. It also shows that the sputtering targets are pure and

the vacuum conditions good enough to provide films without

contaminations. We further conclude that no Ar has been

incorporated during the sputter deposition. The N content in

the as-deposited films has been determined by combining

ToF-ERDA and RBS results and is found to be in the region

of 45–50 at. %.

A ToF-SIMS depth profile of a typical GdN film sand-

wiched between seed and capping layer of ZrN was recorded

and an important result for future neutron detector applications

is that the natural abundance of the five most common isotopes

in Gd, 155Gd¼ 14.8%, 156Gd¼ 20.6%, 157Gd¼ 15.7%, 158Gd

¼ 24.8%, and 160Gd¼ 21.8% could be confirmed.

To follow the oxidization of pure GdN without capping

layer, �400 nm of GdN was deposited onto an Al2O3 sub-

strate and then exposed to air at room temperature. The oxi-

dation was followed by the surface color change, recorded

every 4 s with a digital camera. The continuous oxidation

process is illustrated by a selection of these images of GdN

films on transparent Al2O3, exposed to air up to 19 h as

shown in Figure 3. The as-deposited sample had a silver-

metallic color that within the first minutes changed to be

more golden and then brownish. The rapid color change indi-

cates that the oxidization starts immediately. After the brown

appearance and within the first hour, all the colors of the

rainbow brightly appeared after each other. During the fol-

lowing few hours, the film appeared to be purple and green

with some interference color gradients indicating a slight

thickness gradient of the oxide and remaining film. After less

than 19 h, the film has become fully transparent and is com-

pletely oxidized. RBS measurements on a completely oxi-

dized film show that the majority of N has been replaced by

O. A similar but much faster color change, probably due to a

GdN film thicknesses of only �100 nm, has been reported in

Ref. 41.

To investigate the oxidization stability of Zr1�xGdxN, a

series of films with 0� x� 1 were deposited with and with-

out ZrN capping layers onto Si and Al2O3 substrates at

400 �C. These films were measured with RBS within two

weeks after they were deposited. A second measurement was

done 3–4 months after deposition, for those films that were

not fully oxidized already in the first measurement. All sam-

ples were stored in room temperature and in air. The results

are presented in Figure 4 and show that films with a high

amount of GdN oxidize within a very short time. Increasing

the amount of ZrN significantly slows down the oxidization

and films with x� 0.3 were just slightly oxidized even after

several months. The idea to alloy GdN with a chemically

more stable transition metal nitride with the purpose to pre-

vent it from oxidization is thus working, although oxidation

can not fully be avoided even with GdN contents as low as

x¼ 0.2.

FIG. 3. Color change for a GdN thin film on Al2O3 substrate during expo-

sure to air for up to 19 h.

195301-4 H€oglund et al. J. Appl. Phys. 117, 195301 (2015)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

130.236.83.172 On: Thu, 18 Jun 2015 08:45:00

Figure 5 shows typical RBS spectra of Zr1�xGdxN,

recorded from the same film, both as-deposited and after 4

months. Here, we show Zr0.6Gd0.4N on Si. Comparing the Zr

and/or Gd spectra, respectively, clearly shows the increase in

the areal density when O replaces N during oxidization. The

complete transition is also seen in that the as-deposited film

contains no oxygen (within the detection limit) and the oxi-

dized film contains no nitrogen. The ToF-ERDA depth profile

of the as-deposited film, mentioned above, confirmed that

those films were free from oxygen. Even though the nitrogen

completely escapes from the film and is replaced by oxygen,

the ratios between the Gd and Zr content stay constant for all

measured samples before and after oxidization, respectively.

For films that were oxidized half way through in the oxi-

dation study in Figure 4, an abrupt interface between the oxi-

dized layer on top and the not oxidized layer below can be

identified in the analyses of the RBS-spectra and their corre-

sponding simulations. We therefore conclude that within the

�2 mm � 2 mm RBS beam spot, the thickness of the formed

oxide is the same within the measurement errors. We find it

an interesting observation that none of all the samples in the

oxidization study showed a gradient at the interface between

the oxidized layer and the intact nitride layer, but always a

sharp transition. This is in agreement with the observations

from the surface color change in Figure 3, where a uniform

interference color over the whole film surface, �8 mm

� 10 mm, within the first 30 min corresponds to a uniformly

thick oxide layer in the film.

For several applications, including highly efficient neu-

tron detectors, it is possibly more beneficial to protect a film

that contains a high amount of Gd with a thin (preferably

conducting) oxidization barrier, than to use a Gd compound

that degrades over time. We therefore tested a 260 nm

Zr0.5Gd0.5N film deposited with both 35 nm ZrN capping and

seed layers. The film was measured with RBS soon after it

was deposited and again after storing it in air and at room

temperature for one year. Within the measurement accuracy,

the compositions of the Zr0.5Gd0.5N film and ZrN protective

layers were not changed during that time. This experiment

shows that applying a capping and seed layer prevents the

film from oxidizing when it is exposed to air and promises a

long lifetime of the nitride structure, even with higher

amounts of GdN. The minimum required thickness of the

protective layers remains to be determined for the different

compositions. The final decision, whether to use capping

layers or films with lower Gd-content for neutron detector

applications depends on the detector requirements and if the

films can be kept in oxygen free atmosphere.

It was also observed by RBS analysis that the inter-

face between a Zr0.55Gd0.45N film and a Si substrate after

one year still exhibited an abrupt transition, when only a

ZrN capping layer but no seed layer was used. This shows

that no significant amount of substrate elements have dif-

fused into the film, even though there is a native oxide on

the Si substrate, but rather that gaseous oxygen is responsi-

ble for the oxidation. The same observation was made for

all measurements included in Figure 4 that showed partly

oxidized films, where the oxygen only was found in the

top layers.

CONCLUSIONS

We have shown that (001) and (111)-oriented solid

solution Zr1�xGdxN alloy thin films can be grown by

magnetron sputter deposition over the whole composi-

tional range from ZrN to GdN, in excellent agreement

with theoretical predictions. The alloys were substitution-

ally disordered and no ordered alloys could be observed

for deposition temperatures up to 900 �C. The oxidization

of the Zr1�xGdxN films is substantially reduced when

increasing the amount of ZrN and can even be arrested

by applying a capping layer of ZrN. Even though this

study is an initial experimental work on Gd containing

compounds with applications within neutron detection and

shielding in mind, it adds a significant amount of infor-

mation to the judgment about the potential of these mate-

rials for those applications.

ACKNOWLEDGMENTS

The authors would like to acknowledge the Tandem

Laboratory at Uppsala University for giving access to their

ion beam facilities, and Dr. Muhammad Junaid for his

laboratory assistance. B.A. acknowledges the financial

FIG. 4. Level of oxidization in Zr1�xGdxN (0� x� 1) thin films from RBS,

obtained within 2 weeks or 3–4 months after deposition.

FIG. 5. Measured data from RBS of an as-deposited and later fully oxidized

Zr0.6Gd0.4N thin film, together with simulated elemental spectra for Gd, Zr,

N (as-deposited state only), and O (oxidized state only), respectively.

195301-5 H€oglund et al. J. Appl. Phys. 117, 195301 (2015)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

130.236.83.172 On: Thu, 18 Jun 2015 08:45:00

support from the Swedish Research Council (VR) through

Grant Nos. 621-2011-4417 and 330-2014-6336.

1P. Caravan, J. J. Ellison, T. J. McMurry, and R. B. Lauffer, Chem. Rev.

99, 2293 (1999).2N. Cherkashyna et al., “Overcoming high energy backgrounds at pulsed

spallation sources,” in Proceedings of International Conference on

Advanced Neutron Sources ICANS XXI [J. Neutron Res. (to be pub-

lished)]; e-print arXiv:1501.02364.3S. Peggs et al., ESS Technical Design Report, ESS-2013-0001 (2013).4R. K. Crawford, R. Dean, P. Ferguson, J. Galambos, F. Gallmeier, T.

McManamy, and M. Rennich, J. Phys.: Conf. Ser. 251, 012054 (2010).5A. Cho, Science 326, 778 (2009).6D. Kramer, Phys. Today 64(5), 20 (2011).7K. Zeitelhack, Neutron News 23(4), 10–13 (2012).8See http://www.icnd.org for International collaboration on the develop-

ment of neutron detectors.9T. Bigault, J. Birch, J. C. Buffet, J. Correa, R. Hall-Wilton, L. Hultman, C.

H€oglund, B. Gu�erard, A. Khaplanov, F. Piscitelli, and P. van Esch,

Neutron News 23(4), 20 (2012).10M. O. Klein, Ph.D. thesis, Ruprecht-Karls-Universit€at Heidelberg, 2000.11J. L. Lacy, A. Athanasiades, L. Sun, C. S. Martin, T. D. Lyons, M. A.

Foss, and H. B. Haygood, Nucl. Instrum. Methods Phys. Res., Sect. A 652,

359 (2011).12I. Stefanescu, Y. Abdullahi, J. Birch, I. Defendi, R. Hall-Wilton, C.

H€oglund, L. Hultman, D. Seiler, and K. Zeitelhack, Nucl. Instrum.

Methods Phys. Res., Sect. A 727, 109 (2013).13I. Stefanescu, Y. Abdullahi, J. Birch, I. Defendi, R. Hall-Wilton, C.

H€oglund, L. Hultman, M. Zee, and K. Zeitelhack, J. Instrum. 8, P12003

(2013).14M. Henske, M. Klein, M. K€ohli, P. Lennert, G. Modzel, C. J. Schmidt, and

U. Schmidt, Nucl. Instrum. Methods Phys. Res., Sect. A 686, 151 (2012).15F. Piscitelli, J. C. Buffet, J. F. Clergeau, S. Cuccaro, B. Gu�erard, A.

Khaplanov, Q. L. Manna, J. M. Rigal, and P. V. Esch, J. Instrum. 9,

P03007 (2014).16C. H€oglund, J. Birch, K. Andersen, T. Bigault, J. C. Buffet, J. Correa, P.

Van Esch, B. Guerard, R. Hall-Wilton, J. Jensen, A. Khaplanov, F.

Piscitelli, C. Vettier, W. Vollenberg, and L. Hultman, J. Appl. Phys. 111,

104908 (2012).17L. Hultman, J. Birch, and C. H€oglund, Patent SE 535 805 C2 (30 June 2011).18C. H€oglund, K. Zeitelhack, P. Kudejova, J. Jensen, G. Greczynski, J. Lu,

L. Hultman, J. Birch, and R. Hall-Wilton, J. Rad. Phys. Chem. 113, 14

(2015).19G. Nowak, M. St€ormer, H.-W. Becker, C. Horstman, R. Kampmann, D.

H€oche, M. Haese-Seiller, J.-F. Moulin, M. Pomm, C. Randau, U. Lorenz,

R. Hall-Wilton, M. M€uller, and A. Schreyer, J. Appl. Phys. 117, 034901

(2015).20O. Kirstein et al., “Neutron position sensitive detectors for the ESS,”

Proceedings of Science, VERTEX’14 (2014), p. 029.21R. Hall-Wilton, C. H€oglund, M. Imam, K. Kanaki, A. Khaplanov, O.

Kirstein, T. Kittelmann, B. Nilsson, and J. Scherzinger, IEEE Nucl. Sci.

Symp. Med. Imaging Conf. Rec. 2012, 4283.22F. Piscitelli, Ph.D. thesis, University of Perugia, 2014.23F. Piscitelli and P. Van Esch, J. Instrum. 8, P04020 (2013).24J. Correa, Ph.D. thesis, University of Zaragoza, 2012.25K. Andersen, T. Bigault, J. Birch, J.-C. Buffet, J. Correa, P. vanEsch, B.

Guerard, R. Hall-Wilton, L. Hultman, C. H€oglund, J. Jensen, A.

Khaplanov, O. Kirstein, F. Piscitelli, and C. Vettier, “Multi-grid boron-10

detector for large area applications in neutron scattering science,” in

Proceedings of ICANS XX (2012).26K. Andersen, T. Bigault, J. Birch, J. C. Buffet, J. Correa, R. Hall-Wilton,

L. Hultman, C. H€oglund, B. Gu�erard, J. Jensen, A. Khaplanov, O.

Kirstein, F. Piscitelli, P. Van Esch, and C. Vettier, Nucl. Instrum. Methods

Phys. Res., Sect. A 720, 116 (2013).27J. Birch, J. C. Buffet, J. Correa, P. van Esch, B. Gu�erard, R. Hall-Wilton,

C. H€oglund, L. Hultman, A. Khaplanov, and F. Piscitelli, Trans. Nucl. Sci.

60(2), 871 (2013).

28D. Pfeiffer, IEEE Nucl. Sci. Symp. Med. Imaging Conf. 2014, N46-5.29C. Schulz, “Entwicklung eines neuartigen Niederdruck-Detektors mit

hoher Orts- und Flugzeitaufl€osung zum Nachweis thermischer

Neutronen,” Ph.D. dissertation (Freie Universit€at Berlin, Berlin, Germany,

1999).30M. Titov and L. Ropelewski, “Micro-pattern gaseous detector technologies

and RD51 collaboration,” Mod. Phys. Lett. A 28, 1340022 (2013).31A. Oed, Nucl. Instrum. Methods Phys. Res., Sect. A 263, 351 (1988).32B. Mindur, S. Alimov, T. Fiutowski, C. Schulz, and T. Wilpert, J. Instrum.

9, P12004 (2014).33F. Sauli, “GEM: A new concept for electron amplification in gas

detectors,” Nucl. Instrum. Methods Phys. Res., Sect. A 386, 531 (1997).34G. Adachi and N. Imanaka, Chem. Rev. 98, 1479 (1998).35International Conference Advanced Neutron Sources XXI Mito, Japan

(2014), Abstracts S-01–S-07, session 9, shielding.36N. Cherkashyna et al., “High energy particle background at neutron spalla-

tion sources and possible solutions,” J. Phys.: Conf. Ser. 528, 012013

(2014).37F. Leuenberger, A. Parge, W. Felsch, K. Fauth, and M. Hessler, Phys.

Rev. B 72, 014427 (2005).38E. Shalaan and H. Schmitt, Opt. Commun. 260, 588 (2006).39K. Senapati, F. Thomas, M. E. Vickers, M. G. Blamire, and Z. H. Barber,

Phys. Rev. B 83, 014403 (2011).40H. Yoshitomi, S. Kitayama, T. Kita, O. Wada, M. Fujisawa, H. Ohta, and

T. Sakurai, Phys. Rev. B 83, 155202 (2011).41W. R. McKenzie, P. R. Munroe, F. Budde, B. J. Ruck, S. Granville, and H.

J. Trodahl, Curr. Appl. Phys. 6, 407 (2006).42J. Kennedy, S. Granville, A. Markwitz, B. J. Ruck, and H. J. Trodahl,

Nucl. Instrum. Methods Phys. Res., Sect. B 266, 1558 (2008).43S. Granville, B. J. Ruck, F. Budde, A. Koo, D. J. Pringle, F. Kuchler, A. R.

H. Preston, D. H. Housden, N. Lund, A. Bittar, G. V. M. Williams, and H.

J. Trodahl, Phys. Rev. B 73, 235335 (2006).44J. W. Gerlach, J. Mennig, and B. Rauschenbach, Appl. Phys. Lett. 90,

061919 (2007).45F. Natali, N. O. V. Plank, J. Galipaud, B. J. Ruck, H. J. Trodahl, F.

Semond, S. Sorieul, and L. Hirsch, J. Cryst. Growth 312, 3583 (2010).46A. P. Milanov, T. B. Thiede, A. Devi, and R. A. Fischer, J. Am. Chem.

Soc. 131, 17062 (2009).47J. R. Brewer, Z. Gernhart, H.-Y. Liu, and C. L. Cheung, Chem. Vap.

Deposition 16, 216 (2010).48H. J. Trodahl, A. R. H. Preston, J. Zhong, B. J. Ruck, N. M. Strickland, C.

Mitra, and W. R. L. Lambrecht, Phys. Rev. B 76, 085211 (2007).49P. Wachter, Results Phys. 2, 90 (2012).50B. Alling, C. H€oglund, R. Hall-Wilton, and L. Hultman, Appl. Phys. Lett.

98, 241911 (2011).51L. Hultman, Vacuum 57, 1 (2000).52L. E. Toth, Transition Metal Carbides and Nitrides (Academic, New

York, 1971), p. 188.53J. L. Ruan, D. F. Lii, J. S. Chen, and J. L. Huang, Ceram. Int. 35, 1999

(2009).54J. Birch, S. Tungasmita, and V. Darakchieva, Vacuum Science and

Technology: Nitrides as Seen by Technology (Research Signpost,

Trivandrum, 2002), pp. 421–456.55M. S. Janson, “CONTES conversion of time-energy spectra—A program

for ERDA data analysis,” Internal Report, Uppsala University, 2004.56C. H€oglund, B. Alling, J. Birch, M. Beckers, P. O. A. Persson, C. Baehtz,

Z. Czig�any, J. Jensen, and L. Hultman, Phys. Rev. B 81, 224101 (2010).57C. H€oglund, J. Bareno, J. Birch, B. Alling, Z. Czig�any, and L. Hultman,

J. Appl. Phys. 105, 113517 (2009).58C. H€oglund, J. Birch, B. Alling, J. Bare~no, Z. Czig�any, P. O. A. Persson,

G. Wingqvist, A. Zukauskaite, and L. Hultman, J. Appl. Phys. 107,

123515 (2010).59B. Alling, A. Karimi, and I. Abrikosov, Surf. Coat. Technol. 203, 883

(2008).60L. Vegard, Z. Phys. 5, 17 (1921).61I. Petrov, P. B. Barna, L. Hultman, and J. E. Greene, J. Vac. Sci. Technol., A

21(5), S117 (2003).

195301-6 H€oglund et al. J. Appl. Phys. 117, 195301 (2015)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

130.236.83.172 On: Thu, 18 Jun 2015 08:45:00