Cao Z. (Ed.) Thin Film Growth: Physics, materials science and applications

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178 Thin ﬁlm growth

shows a model of the transition 2HCP(100) Æ pre-3h Æ 3h. the groups of

microcrystallites articially selected with different shades of grey can help

in understanding the transitions between different particle arrangements.

Light grey microcrystallites rotate anticlockwise, while dark grey ones

rotate clockwise around the pivot row p. simultaneously, the spheres rows

d are displaced in the direction of the rotation axis, and they approach the

conning plates to maximize the lling fraction value. This results in the

alignment of all sphere rows in a square arrangement (Ramiro-manzano

et al., 2007). Figure 7.28 shows sEm images of pre-3h (a), pre-4h (b–c)

and pre-5h (d) supporting this model.

in summary, we have reported on the different phases appearing in hard

sphere systems. in the following we give the layering sequence in non-

charged colloidal crystals conned in wedge type cells.

7.4 Phase transition sequence in colloidal crystal

thin ﬁlms

in the following we give our contribution to clarify the new phases as well as

the transition appearing in the crystallization sequence between 1 and n mL

7.28 SEM images of pre-3h (a), pre-4h (b–c) and pre-5h (d). Inset in

(a) top view model of pre-3h. The spheres are in various shades of

grey in (a) to facilitate comparison with the proposed model. Image

(a) is taken from Ramiro-Manzano et al. (2007) with permission of the

Americal Physical Society (APS).

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179Phase transitions in colloidal crystal thin ﬁlms

with especial emphasis on the transition between 1 and 4 mL. We also give

dynamic models that will show some of the transitions.

7.4.1 From one to four monolayers

taking all the results reported so far by various research groups, it is possible

to present an overview of the phases appearing in the transition between 1

and 4 ML. Figure 7.29 shows the calculation of the colloidal crystal lling

fraction as a function of the normalized cell thickness value (h/F), with

h and F being the cell thickness and the particle diameter, respectively

(Ramiro-manzano et al., 2009). Figure 7.29 also shows sEm images of

the different particle arrangements appearing in colloidal lms between 1

and 4 mL. Continuous lines correspond to the most stable phases observed

experimentally. We present sEm images of well-known phases, D (Pieranski

et al., 1983), h (Pieranski et al., 1983), B (Pansu et al., 1984; Chou and

nelson, 1993; schmidt and Löwen, 1996), r (Pansu et al., 1984; schmidt

and Löwen, 1996; messina and Löwen, 2003) and P (neser et al., 1997)

corresponding to triangular, square, buckling, rhombic and prismatic particle

arrangements, respectively. it is worth discussing the transition 4h Æ 4D,

where we show sEm images of both the 4P phase as well as the 4HCP(011)

one. As we use dried samples, compact phases like the HCP(011) are more

favoured than other colloidal arrangements. However the 4r phase is slightly

present mixed with the HCP(011), and the HCP(011) extends over larger

sample regions than the 4P arrangement. Consequently, the transition could

be considered as an evolution of triangular gliding plane orderings 4h Æ

4P Æ 4HCP(011) Æ 4D.

to conclude this section, the order evolution in the transition is the

following:

1D Æ 1B Æ 2h Æ 2r Æ 2D Æ 2HCPL(100) Æ pre-3h

Æ 3h Æ 3r Æ 3D Æ 3HCPL(100) Æ pre-4h Æ 4h

Æ 4P Æ 4HCP(011) Æ 4D

HCPL(100) includes HCP(100) because it is a particular case of HCPL, as

detailed above.

7.4.2 From four to eight monolayers

the richest scenario of different colloidal arrangements corresponds to

orderings between four and eight monolayers. the reason for this diversity

is the inclusion of plenty of compact phases, i.e. the macled vicinals. Figure

7.15 shows sEm images of mVD

(a), mVh (b), and mVD

(c and d) phases.

Our experimental results conrm the existence of the pre-h in thin colloidal

ThinFilm-Zexian-07.indd 179 7/1/11 9:41:49 AM

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ff (%)

1.0 1.5 2.0 2.5 3.0 3.5

h/F

7.29 Calculation

of the ﬁlling

fraction between

1D and 4D as a

function of the

normalized cells

thickness value

(h/F), h and F

being the cell

thickness and the

particle diameter,

respectively. SEM

images illustrate

the transition

sequence. From

Ramiro-Manzano

et al. (2009) with

permission of the

Royal Society of

Chemistry (RSC).

ThinFilm-Zexian-07.indd 180 7/1/11 9:41:49 AM

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181Phase transitions in colloidal crystal thin ﬁlms

crystal from 2 to 5 mL (i.e., pre-3h, pre-4h and pre-5h phases, see Fig.

7.28). However, we have not found them in thicker colloidal thin lms. The

experimental sequence would be the following:

nD Æ nmVD

Æ pre-(n + 1)h Æ (n + 1)h Æ (n + 1) mVh

+ (n + 1) P Æ (n + 1) mVD

Æ (n + 1)D

where the mVD

includes the HCP(100) and the MVD

and mVh include

the HCP (011) because the HCP is a particular case of macled vicinals.

7.4.3 Beyond eight monolayers

as the interlayer distance of the D facet is larger than that of the h ordering,

one can check the total thickness value of the 7D arrangement is roughly

the same as the 8h colloidal crystal. therefore the D systems would be the

most probable since they show the highest ff value. this argument holds for

thicker thin lms and it leads to the extinction of the h facet and the rest of

orderings connected to it (r, P, mVh). only the mVh can be observed with

small terraces (2–3 sphere rows). Furthermore, all orderings with triangular

terraces survive (mVD

, mVD

), especially the mVD

and the transition:

nD Æ nmVD

Æ nHCPL(100) Æ (n + 1)D

For large thickness, the 12D overpasses 12HCP (100) and therefore for

thicker lms D ordering dominates.

7.5 Conclusions and future trends

We have shown a detailed description of the crystalline phases appearing

in thin lm colloidal crystals when the cell thickness increases gradually.

We have also shown the evolution dynamics from 1 mL to n mL in wedge

type cells with hard sphere particles. the evolution dynamics have been

exhaustively discussed and reported in the case of 1–4 mL. there are,

however, many unknown aspects to be unveiled. one of them concerns the

understanding of experimental ndings through theoretical models (at least

with hard sphere systems). Finally, the discovery of new particle orderings

induced either in at or patterned substrates could be of great interest in

fundamental science and could enable new potential applications.

7.6 Acknowledgements

the authors would like to thank a. moreno for providing wedge type cells

and the Electronic microscopy service of the uPV for technical support.

We would like to acknowledge the permission to include the gures of the

following authors: Pa. Pieranski, Pi. Pieranski, P. Pansu, L. strzelecki, E.

ThinFilm-Zexian-07.indd 181 7/1/11 9:41:50 AM

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182 Thin ﬁlm growth

Oguz, R. Messina and H. Löwen. This work has been partially supported by

the spanish projects, Consolider Csd2007-046 and Fis2009-07812.

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184 Thin ﬁlm growth

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185

Thin film growth for thermally unstable

noble-metal nitrides by reactive magnetron

sputtering

Z. Cao, Chinese academy of Sciences, P. R. China

Abstract: Thin lm growth for thermally unstable noble-metal nitrides

by reactive magnetron sputtering is exemplied by the deposition of

stoichiometric Cu

N and ternary Cu

NPd

structures, for which a proper

combination of applied voltage and working gas pressure is critical since

the nitriding reaction occurs at the target surface. Nitrogen re-emission from

deposits both inuences the growth process and leads to protruding features

via an orogenic motion mechanism of the nitride nanocrystals. Doping the

cubic Cu

N lattice gives rise to narrow band semiconductors which may

exhibit various intriguing properties – in Cu

NPd

0.238

a constant electrical

resistivity has been measured in a temperature range over 200 K. The results

summarized here are instructive for the deposition of other noble-metal

nitrides.

Key words: noble-metal nitride, Cu

N, reactive magnetron sputtering,

stoichiometry, morphology, intercalation, narrow band semiconductors.

8.1 Introduction

8.1.1 Reactive magnetron sputtering

Magnetron sputtering is an effective physical vapor deposition method

which has been widely employed in the enterprise of thin lm growth. This

technique is based on the sputtering of a solid target by energetic ionic

species from a magnetically enhanced glow discharge. One distinct feature

of the magnetron discharge is the application of a magnetic eld, often with

a eld strength in the range of 10

–10

Gauss, which serves to conne the

secondary electrons in a region near the cathode, thus the closed electron

drift in this small region can lead to a relatively high plasma density. Under

the condition of limited electron mobility across the magnetic eld, each

magnetic eld line is also a nearly equi-potential line of the electric eld. The

electric potential changes forcibly across the magnetic eld, hence a strong

electric eld develops along the direction perpendicular to the magnetic eld,

accelerating the positively charged species towards the target (Baranov et al.

2010). The ion sputtering on the target surface plays the role of vaporizing

the source materials – the sputtered atomic or molecular species generally

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186 Thin ﬁlm growth

have a kinetic energy of a few electron volts (depending on the plasma

condition and the nature of the target), and they y towards the substrate

where lm growth is taking place.

When a reactive gas is added into the working gas, this then turns into

the so-called reactive magnetron sputtering which has been widely applied

to obtain certain compound lms. However, reactive magnetron sputtering

as a lm deposition method is a quite vague concept, since when a reactive

gas is added, depending on the activity of the reactive gas and the conditions

of the plasma, the reaction can occur simultaneously on the substrate, on

route from the target to the substrate, and even on the target surface. This is

to say that reactive magnetron sputtering is characterized by its complexity,

that the sputter process in the target surface, the target process in the selvage

layer, the collisional transport of the sputtered species through the gas phase

and all the substrate processes are all strongly interrelated, being able to

inuence some specic aspects of the deposition results (Berg and Nyberg

2005, Musil et al. 2005).

Reactive magnetron sputtering has been very successfully applied to the

growth of various oxides, with the targets being either pure metals or bulk

oxides. Oxygen is a very active element that is ready to combine with most

metals, vaporized or in bulk, to form oxides even in conventional atmospheric

conditions. In the magnetron discharge, the oxygen can be easily ionized

and then accelerated towards the target to initiate sputtering. The sputtered

metal atoms and clusters have already incorporated some oxygen atoms.

Although the sputtered clusters may show deciency of oxygen with respect

to the anticipated compound stoichiometry, more oxygen will be captured

on the path of ight by the clusters and on the growing surface, bearing in

mind that often a considerably high substrate temperature is applied, which

effectively enhances the reaction of oxygen with the deposits. In fact, even

a neutral atom beam of oxygen sufces in the molecular beam epitaxy or

ablation growth of some oxide lms. A great number of reports on the

growth of thin lms for various oxides are available in literature (Chambers

2010).

Unlike oxygen, use of nitrogen in reactive magnetron sputtering is less

effective based on the following facts: nitrogen gas has a higher rst ionization

energy and a higher bonding dissociation energy than oxygen (1402.3 kJ

mol

–1

versus 1313.9 kJ mol

–1

for the former, and 945 kJ mol

–1

versus 495 kJ

mol

–1

for the latter), and nitrogen has a smaller dissociation energy from most

metal surfaces, or a smaller bonding energy with that metal, than oxygen.

Due to the large bonding dissociation energy of a nitrogen molecule, the

nitrogen gas is quite ‘inert’ – the nitrogen molecules in the growth chamber

have rarely any chance to incorporate into the deposits via reaction through

collisional transport or adsorption on to the growing surface. Furthermore,

this large bonding dissociation energy leads to a large fraction of molecular

ThinFilm-Zexian-08.indd 186 7/1/11 9:42:04 AM

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187Thin ﬁlm growth for thermally unstable noble-metal nitrides

ions in the discharge, which is obviously an unfavorable condition for the

deposition of nitride thin lms. It is even a detrimental factor in the case

of chemical vapor deposition of nitrides by using other kinds of plasma

discharges (Cao 2001, 2002, Cao and Oechsner 2004). The smaller bonding

energy with metals and the lower energy barrier for dissociation also imply

the re-emission of nitrogen atoms or molecules from the deposits, a frustrating

factor that is generally absent in the preparation of oxide lms. In the case of

preparing nitride lms for noble metals such as Cu, the inertness of nitrogen

ions plus the nobleness of the metals (Norskov and Hammer 1995) make

the deposition of thin lms of noble-metal nitrides, such as that of the cubic

copper nitride, a particular challenge (Hojabri et al. 2010).

8.1.2 Copper nitride (Cu

Copper nitride (Cu

N) is a binary compound in the anti-ReO

structure

which has a cubic unit cell of a lattice constant a = 0.382 nm, with 12 Cu

atoms occupying the middle points of the edges and eight N atoms sitting at

the vertices (Fig. 8.1) (Moreno-Armenta et al. 2004). The rst synthesis of

polycrystalline powder-like Cu

N was reported by Juza and Hahn in 1939.

N is an indirect band-gap semiconductor with a high electrical resistivity

at room temperature, and it begins readily to decompose at slightly elevated

temperatures, leaving behind highly conductive structures of Cu when

the decomposed regions are deliberately patterned. That Cu

N has drawn

much research effort towards the growth of high-quality thin lms and the

investigation of its physical properties from various aspects is simply due to

its decomposition without any complications or detrimental consequences,

8.1 Schematic illustration of a unit cell of cubic Cu

N: 8 Cu atoms

sit at the vertices while 12 N atoms occupy the middle points of the

edges.

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