Near-surface recrystallization of the amorphous implanted layer of ionimplanted 6H-SiC

R.J. Kuhudzai a,⇑, N.G. van der Berg a, J.B. Malherbe a, T.T. Hlatshwayo a, C.C. Theron a, A.V. Buys b,A.J. Botha b, E. Wendler c, W. Wesch c

a Physics Department, University of Pretoria, Pretoria, South Africab Laboratory for Microscopy and Microanalysis, University of Pretoria, South Africac Institut Für Festköperphysik, Friedrich-Schiller-Universität Jena, Jena, Germany

a r t i c l e i n f o

Article history:

Available online 14 March 2014

Keywords:

Ion implantation

SiC

Amorphisation

Recrystallization

Crystal growth

SEM

a b s t r a c t

The recrystallization and subsequent crystal growth during annealing of amorphous surface layers on 6H-

SiC produced by ion implantation is investigated. Amorphous surface layers were produced by ion

implantation of 360 keV ions of iodine, silver, xenon, cesium and strontium into single crystalline 6H-sil-

icon carbide samples. The ion fluence for all the implantations were in the order of 1016 cm�2. Vacuum

annealing of the damaged silicon carbide samples was then performed. The microstructure of SiC surfaces

before and after annealing was investigated using a high resolution field emission scanning electron

microscope (SEM). SEM analysis was complimented by Atomic Force Microscopy (AFM). SEM images

acquired by an in-lens detector using an accelerating voltage of 2 kV show nano-crystallites developed

for all implanted samples after annealing. Larger and more faceted crystallites along with elongated thin

crystallites were observed for iodine and xenon implanted 6H-SiC. Crystallites formed on surfaces

implanted with strontium and cesium were smaller and less faceted. Strontium, silver and cesium

implanted samples also exhibited more cavities on the surface. AFM was used to evaluate the effect of

annealing on the surface roughness. For all the amorphous surfaces which were essentially featureless,

the root mean square (rms) roughness was approximately 1 nm. The roughness increased to approxi-

mately 17 nm for the iodine implanted sample after annealing with the surface roughness below this

value for all the other samples. AFM also showed that the largest crystals grew to heights of about 17,

20, 45, 50 and 65 nm for Sr, Cs, Ag, Xe and I implanted samples after annealing at 1200 �C for 5 h respec-

tively. SEM images and AFM analysis suggest that iodine is more effective in promoting crystal growth

during the annealing of bombardment-induced amorphous SiC layers than the rest of the ions we

implanted. In samples of silicon carbide co-implanted with iodine and silver, few cavities were visible

on the surface indicating that iodine influenced the recrystallization. Surface crystallites that grew on

the iodine implanted surfaces were more resistant to thermal etching and decomposition than those that

grew on the silver implanted samples at temperatures of up to 1400 �C for 30 h.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

The extent of radiation damage retained after ion implantation

into single crystalline SiC and the annealing of that damage has

been an area of interest to researchers in the semiconductor and

nuclear materials field. Research has been undertaken in the quest

to understand the impact of damage retained on the fundamental

properties of SiC, given its promising future in a wide number of

applications ranging from, electronic and optical to nuclear

cladding in high temperature reactors [1–3]. The majority of the

work found in published papers focuses on the analysis of solid

phase epitaxy (SPE) emanating from the bulk crystalline and the

amorphous layer interface. Several Rutherford backscattering spec-

troscopy (RBS) in channelling mode studies [2–9] have shown that

heavy ion bombardment of single crystal SiC at room temperature

with ion energies in the order of hundreds of keV and to fluences

1 � 1015 cm�2 or higher results in an amorphous surface layer. In

these studies, they have shown that SPE commences after vacuum

annealing at 1000 �C – the minimum annealing temperature

reported. Studies by Pacaud et al. [10] showed that SPE starts at

temperatures even as low as 500 �C. Once SiC has been

http://dx.doi.org/10.1016/j.nimb.2014.02.072

0168-583X/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +27 124204777; fax: +27 123625288.

E-mail address: joeykuhu@yahoo.com (R.J. Kuhudzai).

Nuclear Instruments and Methods in Physics Research B 332 (2014) 251–256

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amorphized, very high annealing temperatures (typically above

1500 �C) are needed to completely recrystallize it into a single

crystal. A recent review [1] discusses the problems for full SPE

growth of the amorphous layer.

Very little information is available on a totally independent

recrystallization process taking place simultaneously on or near

the front surface of the samples as the annealing proceeds. For

amorphous surface layers produced by heavy ion implantation

with fluences of up to 1016 cm�2, the front surface is shielded from

the underlying crystalline bulk as the concentration of implanted

ions inhibits SPE and thus the recrystallization here does not follow

the pattern of the underlying crystalline bulk [11]. Snead et al. [12],

investigated the recrystallization of 6H-SiC amorphised by neutron

irradiation. In their case, neutron irradiation provided them the

opportunity to analyse the recrystallization behaviour of amor-

phous SiC without the influence of an impurity species. Snead

et al., observed star shaped crystallites forming on the surface

starting at 800 �C with the previously amorphous layer completely

crystallised at 1150 �C. For ion implanted SiC [5,13], surface

crystallites were also observed after recrystallization of silver and

iodine implanted SiC for annealing cycles at 1100 �C. The kinetics

of the recrystallization of amorphous SiC layers have been ex-

plained by using the classical nucleation and growth theory of

Avrami-Johnson-Mehl [14–16].

Our group has been characterising radiation damage produced

by ions of the significant fission products produced in fuel particles

of high temperature nuclear reactors. RBS and channelling studies

on 360 keV xenon ions to a fluence of 1 � 1016 cm�2 implanted at

room temperature, reveal an amorphous layer extending to a depth

of 200 nm from the surface [17]. The amorphous layers produced

by implantation of, iodine, strontium, silver and cesium ions, also

of energy 360 keV, into 6H-SiC wafers, were up to depths of 220,

240, 260 and 190 nm, respectively [6,7,9,18]. In this paper we

investigate the effect of these ions on the growth of crystals on

the surface of the damaged SiC as annealing proceeds. This growth

can reveal how these different ions interact with SiC and ultimately

the effect of any interaction on the release of fission products from

SiC coated fuel particles.

2. Experimental

6H-SiC wafers of thickness 0.3 mm and diameter 50 mm from

Intrinsic Semiconductors�, were individually implanted with

360 keV ions of 131Xe+, 127I+ and 133Cs+ up to a fluence of

1 � 1016 cm�2. Implantations for 109Ag+ and 88Sr + were up to a

fluence of 2 � 1016 cm�2. Two additional wafers of 6H-SiC were

co-implanted with 133Cs+ and 127I+ ions and 109Ag+ and 127I+ ions

respectively. The implantations were carried out at room temper-

ature. The flux was maintained at a rate below 1013 cm�2 s�1 to

minimise the increase in the substrate temperature, reducing the

probability of simultaneously annealing some of the radiation

damage produced during the room temperature implantations.

During the implantations the temperature of the samples rose to

about 55 �C. The angle of incidence of the ions was set at an angle

of 7� relative to the normal to avoid channelling effects during

implantation. For the annealing cycles, the samples were placed in-

side small graphite crucibles and annealed in a computer con-

trolled high temperature air cooled vacuum Webb� graphite

furnace. The annealing was performed at 1100 �C for 15 min and

at 1200 �C for 5 h. The pressure was maintained below 5 � 10�5 -

mbar throughout the annealing cycles.

The microstructure of SiC surfaces before and after annealing

was investigated using a high resolution Zeiss Ultra 55 field emis-

sion scanning electron microscope fitted with an in-lens detector.

A 2 kV accelerating voltage was employed during all SEM imaging.

The working distance was maintained at approximately 2 mm for

all the imaging experiments. SEM images were complimented by

AFM using Bruker’s Dimension Icon scanning probe microscope.

All AFM measurements were performed in contact mode using a

Si probe of tip radius 610 nm. Since the AFM images on their

own did not exhibit any additional features not already visible in

the SEM images, no AFM images are shown. However, the RMS

roughness values obtained from the AFM images were used to

quantify the increase in surface topography after annealing.

3. Results and discussion

Fig. 1(a) shows a typical SEM image of the as-received un-im-

planted 6H-SiC wafer. The surface had only some polishing marks

(P) observable on it and the surface roughness was found to be

approximately 0.5 nm. Fig. 1(b) shows the as-implanted surfaces

of the xenon implanted sample. The surfaces of all the other as-im-

planted samples (not shown) are similar to the xenon one. As the

sample surfaces were amorphous after implantation, the surfaces

were flat and featureless with the polishing marks less pronounced

after ion bombardment. The surface roughness was about 1 nm for

all the samples. The increase in RMS roughness, as measured by

AFM, after ion bombardment is due to sputter-induced roughening

[19].

The images showing the effect of annealing at 1100 �C for

15 min on implanted samples are shown in Fig. 2. In Fig. 2a, for

the strontium implanted sample, the surface is no longer feature-

less. The previously amorphous surface layer appears now to have

been recrystallized into a polycrystalline material as some crystal-

lites are observed on the surface although it is difficult to clearly

ascertain their size. Fig. 2b shows the SEM image of the cesium im-

planted sample; evidence of some recrystallization can be seen

from the contrast which seems to be depicting crystallites on the

surface. These crystallites are not as developed as those observed

on the strontium implanted sample. Elongated thin crystallites

(L) appear on the annealed surface of the silver implanted sample

(Fig. 2c), with some extending to a length of about 1 lm. There

seem to be no preferred orientation as these needle type crystal-

lites develop randomly on the surface. Star-like crystallites (S)

were observed on the surface of the xenon implanted sample

(Fig. 2d). These crystallites look similar to those observed by Snead

et al. [6] on annealing of neutron amorphised SiC. They appear to

nucleate from certain points or centres on the surface and grow

parallel to the surface. Again there appears to be no preferred ori-

entation for growth.

The surface of the iodine implanted sample (Fig. 3a) is charac-

terised by pockets of closely packed (densely packed regions) of

nano crystallites (C) as well as elongated thin crystallites (L). Some

of these long crystallites extend to lengths of about 1 lm. In this

case the thin crystallites seem to branch out from certain growth

centres (G) unlike those observed for the silver implantation. The

crystals are more developed than in the silver case, where needles

seem to grow in a sea of amorphous zones while for the iodine im-

planted sample, thin long crystallites grow along with pockets of

closely packed nano crystallites. The case of expedited growth in

the presence of iodine is supported by the image showing the sam-

ple co-implanted with silver and iodine (Fig. 3b). In this case, the

long thin crystallites are more pronounced than in the case of silver

implantation alone, and the seemingly amorphous zones observed

on silver implanted samples are now crystalized with close packed

nano crystallites now being visible. A similar trend is observed in

Fig. 3c which shows the annealed SiC sample co-implanted with

cesium and iodine ions. In this image the long thin crystallites

(size) are now visible which were not fully developed for the

sample only implanted with cesium. At 1100 �C for 15 min, all

252 R.J. Kuhudzai et al. / Nuclear Instruments and Methods in Physics Research B 332 (2014) 251–256

the surfaces look to have under gone recrystallization although io-

dine implanted sample seems to have the most developed crystal

structures.

Micrographs of the implanted samples annealed 1200 �C for 5 h

are shown in Figs. 4 and 5. For the strontium implanted sample

(Fig. 4a), the polycrystalline surface observed looks similar to the

one observed after the 15 min anneal at 1100 �C. However, the

nano crystallites on the surface are now more visible and larger.

Cavities (O) on the surface are observable at this annealing cycle.

The grain size of some of these crystallites is estimated to be about

100 nm with a few nano crystallites having grown to approxi-

mately to a diameter of 150 nm. AFM analysis shows the surface

roughening to approximately 5 ± 1 nm, with the height of some

of the larger crystals approximately 17 nm.

The recrystallization of the amorphous SiC into crystals occurs

according to a step-flow mechanism [20]. For the crystals to grow

materials must be provided from nearby areas resulting in the for-

mation of cavities. Crystal surfaces with higher surface energies

grow proportionally faster than surfaces with lower surface ener-

gies via surface diffusion, according to Wulf’s law [21], thereby

lowering the total energy of the system.

The cesium implanted sample is shown in Fig. 4b. In this case

the polycrystalline surface is composed of short thin crystallites

approximately 300 nm long, and irregular close packed nano crys-

tallites approximately 100 nm in diameter. The surface is also not

continuous as cavities are also prominent on this surface. Surface

roughness, as measured by AFM, was found to be approximately

6.3 ± 1 nm. The largest crystals on the surface grew to heights of

about 20 nm.

From the image of the silver implanted sample (Fig. 4c), the

recrystallized surface is characterised by a number of cavities

(O). The larger cavities on this annealed surface extend to diame-

ters beyond 200 nm and to depths of 45 nm. The top surface is

far from being a continuous layer. Although larger crystallites (of

diameter larger than 200 nm and height of 45 nm) develop on

the surface, their growth via surface diffusion leaves behind large

cavities or pits.

On annealing the xenon implanted 6H-SiC (Fig. 4d), the surface

is characterised by long, thin crystallites branching out from

growth centres. These elongated thin crystallites extend to a

lengths 700–900 nm on average. Large faceted crystals

(approximately 200–300 nm in diameter) characterised by

a b

L

cd

S

Fig. 2. SEM images of 6H-SiC wafers implanted with ions of (a) Sr, (b) Cs, (c) Ag and (d) Xe. All the samples were annealed at 1100 �C for 15 min. The ion energy was 360 keV

and all implantations were performed at room temperature. Ion fluences for all samples were 1 � 1016 cm�2 .

Pa b

Fig. 1. SEM images of (a) as received 6H-SiC, and (b) the as-implanted image of 6H-SiC implanted with Xe ions. The ion energy was 360 keV and the implantation was

performed at room temperature.

R.J. Kuhudzai et al. / Nuclear Instruments and Methods in Physics Research B 332 (2014) 251–256 253

multiple twinning (visible under higher magnification) have also

grown under annealing. These crystals grow to heights reaching

approximately 50 nm. The surface appears to be quite continuous,

with the large cavities observed for the silver implanted samples

not seen in this case. Although the growth mechanism appears

be the same, i.e. of larger crystals growing through a crystal surface

diffusion supply mechanism, the surface layer does not break up in

the xenon implanted sample. Large faceted crystals with a few cav-

ities can be seen on the iodine implanted sample, (Fig. 5a). In the

case of iodine these crystals easily grow to diameters of about

200 nm and in some cases 400 nm. These crystallites reach heights

of approximately 65 nm above the sample surface. Again the thin

and elongated crystals branching out from certain growth points

are visible. It is quite clear that the largest crystals grow on the

b

G

a

c

L

C

Fig. 3. SEM images of 6H-SiC wafers implanted with ions of (a) iodine, (b) co-implanted with ions of Ag and I, and (c) co-implanted with ions of Cs and I. All the samples were

annealed at 1100 �C for 15 min. The ion energy was 360 keV and all implantations were performed at room temperature. Ion fluencies for all samples were 1 � 1016 cm�2.

a

O

b

O

O

c d

G

Fig. 4. SEM images of 6H-SiC wafers implanted with ions of (a) Sr, (b) Cs, (c) Ag and (d) Xe. All the samples were annealed at 1200 �C for 5 h. The ion energy was 360 keV and

all implantations were performed at room temperature. Ion fluences for all samples were 1 � 1016 cm�2.

254 R.J. Kuhudzai et al. / Nuclear Instruments and Methods in Physics Research B 332 (2014) 251–256

iodine amorphised surfaces. Fewer cavities are also observed for

annealed samples implanted with iodine and xenon as compared

to the cesium, strontium and silver implanted samples.

Images of annealed samples of the co-implantation experiment

of iodine and silver and iodine and cesium are shown in Fig. 5b and

c respectively. Annealing results in the growth of large faceted

crystals, (larger than 200 nm in diameter). Crystals of this magni-

tude are not seen for the annealed single silver (Fig. 4c) and ce-

sium, (Fig. 4b) ion implanted samples. It appears iodine catalyses

crystal growth on the surface during annealing on amorphised

SiC surfaces.

The larger crystallites and fewer but larger cavities observed for

the iodine implanted surfaces compared to the other implanted

samples could suggest some form of stronger binding or chemical

interaction or catalytic action between the iodine and SiC. Mal-

herbe [1] discussed the suggestions previously made that iodine

and other halogen elements can aid crystal growth in SiC [22–

24]. According to Malherbe [1] this process is probably related to

compound formation in the amorphous SiC with free Si and C

due to the bombardment process. It is known that iodine reacts

with silicon above 600 �C to form SiI4 which reacts further with sil-

icon at high temperatures (above 800 �C) to form SiI2 [25,26]. The

SiI2 again decomposes above 1000 �C. These processes are pressure

dependent when performed in fluid conditions. Within a solid, the

above temperatures might be different. These processes might

explain the report by Ramesh et al. [20] that pure 3C-SiC is ob-

tained when heating Si and activated carbon powder in an iodine

atmosphere in a commercial microwave oven. These chemical

reactions between iodine and free Si, released in the SiC by the

ion bombardment process, might lead to the recrystallization

driven topography development.

Annealing for a longer time (30 h) at a higher temperature

(1400 �C) seems to support the case for some sort of stronger bind-

ing for in the presence of iodine. (Fig. 5d and e) show SEM images

of SiC implanted with Ag and I respectively after annealing for 30 h

at 1400 �C. From these images, the crystals on the Ag implanted

surface are not as faceted anymore. They appear to be not as de-

fined as in the images of the samples annealed at the lower tem-

perature (1200 �C for 5 h Fig. 4c). This may indicate that

decomposition due to thermal etching is already taking place. In

the case of the iodine implanted sample, the large crystals still ex-

hibit well defined facets indicating that the thermal etching is not

as advanced at this stage as in the case of silver. Similar results

have been observed after annealing at 1500 �C for 30 h using Ruth-

erford backscattering spectroscopy [27]. The authors reported an

increase in the detected surface carbon signal which was more

a b

c d

e

Fig. 5. SEM images of 6H-SiC wafers implanted with ions of (a) I, (b) co-implanted with ions of Ag and I, and (c) co-implanted with ions of Cs and I. All the samples were

annealed at 1200 �C for 5 h. (d) implanted with Ag and (e) implanted with I. The samples in (d) and (e) were annealed at 1400 ºC for 30 h. The ion energy was 360 keV and all

implantations were performed at room temperature. Ion fluences for all samples were 1 � 1016 cm�2.

R.J. Kuhudzai et al. / Nuclear Instruments and Methods in Physics Research B 332 (2014) 251–256 255

pronounced for the silver implanted sample than in the case of io-

dine implanted SiC. At these temperatures, the decomposition of

SiC is accompanied by the evaporation of silicon leaving carbon

on the surface [28].

The appearance of cavities on the surfaces after annealing might

have implications for the nuclear industry. Modern nuclear fuel

elements are coated with a SiC layer to retain radioactive fission

products within the fuel elements. Recently it was proposed that

these cavities might act as migration paths for implanted silver

in SiC [5]. It is therefore expected that iodine and xenon are more

likely to be retained in SiC while the large cavities observed on sil-

ver and cesium implanted samples could provide paths for their

release.

4. Conclusion

The recrystallization and the crystal growth on the surface after

annealing of the bombardment-induced amorphous SiC layers

were investigated using SEM and AFM. Growth of surface crystal-

lites was observed to be greatest on surfaces that had been

implanted with iodine ions. The faceted crystals on the surface of

the iodine implanted SiC grew to heights of about 65 nm and to

diameters as large as 400 nm. Large faceted crystals of diameters

extending to 300 nm and heights of 50 nm grew on surfaces im-

planted with xenon. The surfaces implanted with iodine and xenon

remained fairly continuous with few cavities observed after

annealing at 1200 �C for 5 h. After annealing the samples

implanted with silver, strontium and cesium, SEM showed some

cavities developed on their surfaces. Crystal growth was less pro-

nounced on surfaces that had been implanted with cesium and

strontium with crystals heights of 20 and 17 nm, respectively.

Although crystals on the silver implanted surfaces grew to heights

of about 45 nm, the surface however, was quite porous exhibiting

large cavities with diameters of about 200 nm distributed across

the surface. Large crystals similar to those that grew on the iodine

implanted samples were observed for the samples co-implanted

with cesium and iodine. These findings show that iodine plays a

major role in the growth of the surface crystals as the cesium single

element implanted surfaces did not exhibit crystals on this same

scale. A similar trend was observed on the surfaces co-implanted

with iodine and silver ions. Over all the surfaces were less porous

in the presence of iodine for the co-implanted samples as

compared to single element implantation with silver or cesium

indicating iodine enhanced the recrystallisation process. Surface

crystallites that grew on the iodine implanted surfaces were found

to be more stable and resistant to thermal etching and decomposi-

tion than those that grew on the silver implanted samples at tem-

peratures of up to 1400 �C for 30 h.

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