The behavior of O atoms in Ti film is investigated under high-flux, low-energy molecular water ion implantation. After 10 min of irradiation at room temperature, the anomalously deep penetration of oxygen without formation of new chemical compounds
observable by XRD has been registered in Ti films using Auger spectroscopy analysis. It is shown that the surface energy increases under ion irradiation, and the relaxation processes minimizing the surface energy initiate the redistribution of atoms. Two surface energy relaxation processes are considered: (i) the mixing of atoms on the surface resulting in annihilation of surface vacancies; and (ii) the annihilation of surface
vacancies by atoms transported from the bulk. The theoretical considerations are in agreement with the experimental results if to assume that the mass-transport in the bulk is controlled by the processes on the surface and the adsorption of reactive atoms or molecules leads to local and long-range restructuring and adatom relocation at the surface.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/2081

Pranevicius, L.

Tuckute, S.

Urbonavicius, M.

Electronic Sumy State University Institutional Repository

390                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 1, Part II              OXYGEN IMPLANTATION AND BEHAVIOUR INTO TI THIN FILMS FROM WATER VAPOUR PLASMA Simona Tuþkutơ1,2, Liudas Praneviþius1,2, Liudvikas Praneviþius1,2,  Marius Urbonaviþius1  1 Vytautas Magnus University, 8 Vileikos Str., 44404, Kaunas, Lithuania 2 Lithuanian Energy Institute, 3 Breslaujos Str., 44403, Kaunas, Lithuania  ABSTRACT The behavior of O atoms in Ti film is investigated under high-flux, low-energy molecular water ion implantation. After 10 min of irradiation at room temperature, the anomalously deep penetration of oxygen without formation of new chemical compounds observable by XRD has been registered in Ti films using Auger spectroscopy analysis.  It is shown that the surface energy increases under ion irradiation, and the relaxation processes minimizing the surface energy initiate the redistribution of atoms. Two surface energy relaxation processes are considered: (i) the mixing of atoms on the surface resulting in annihilation of surface vacancies; and (ii) the annihilation of surface vacancies by atoms transported from the bulk. The theoretical considerations are in agreement with the experimental results if to assume that the mass-transport in the bulk is controlled by the processes on the surface and the adsorption of reactive atoms or molecules leads to local and long-range restructuring and adatom relocation at the surface.  Key words: hydrogen, water vapour, plasma, ion implantation  INTRODUCTION  Titanium and its alloys have many industrial applications thanks to their excellent corrosion resistance and high specific strength. Generally, titanium is covered by a passive titanium oxide (TiO2)  film.  Though very  thin  (usually  a  few nanometers), this compact film acts as a protective layer against corrosion.  Oxygen permeation through TiO2 films  and its  transport  in  the  bulk  is  a  complex process involving interfacial charge transfer, adsorption, absorption, trapping, and transport, and, thus, is inherently influenced by properties of the oxide, such as the chemical composition and structure, the presence of Tin+ interstitials and/or oxygen vacancies, the type and concentration of impurities, hydrogen solubility, the adsorption characteristics of the surface, and the oxide thickness, porosity and uniformity [1,2]. The mechanism by which the oxide barrier layer influences oxygen permeation into Ti is still not well established.  The retardation of oxygen permeation by TiO2 films  may  be  due  to  a                                                  e-mail: simona@hydrogen.lt  Nanomaterials: Applications and Properties (NAP-2011). Vol. 1, Part II              391  combination of low oxygen absorption at the oxide surface and/or slow oxygen transport  in  the  oxide.  In  general,  it  is  difficult  to  predict  the  rate  of  oxygen  transport through a TiO2 film and how many oxygen atoms, generated on the TiO2 surface, reach the Ti substrate. In this work, we study feasible physical models explaining oxygen permeation through a thin Ti film. EXPERIMENTAL TECHNIQUE The 0.5 - 1 Pm thick Ti films were deposited on 2 mm thick stainless steel and Si(111) substrates using magnetron sputter deposition technique. Samples were  located  at  different  sites  of  PVD  –  75  vacuum  chamber.  Details  of  experimental technique are published in [3, 4]. Three cases were distinguished: (i)  case  A,  the  ion  current  density  from  plasma  to  the  sample  was  about  5  PA/cm2, (ii) case B - 1.5 mA/cm2, and (iii) case C - 10 mA/cm2 . A vacuum system including rotary and cryogenic pumps enabled a base pressure of 2·10-4 Pa.  The  water  vapour  was  injected  into  vacuum  chamber  using heated water container (Fig. 1).   Fig. 1 - The scheme of the experimental technique  Plasma  implantation  was  performed  at  10  Pa  pressure  of  water  vapour.  Flow of water vapour was continously controlled by mass flow controller. The plasma was generated using DC and RF energy sources. The dissipated power in plasma was verified in the range 100 - 300 W. The microstructure of the samples was characterized by X - ray diffraction (XRD) method using Bruker diffractometer (Bruker D8). The measurements were performed with 2ș angle in the range 20ż – 70ż using Cu KĮ radiation in steps of 0.01ż. The identification of peaks has been done using Search – Match software. The thickness and surface topography of Ti films were measured using the nanoprofilometer (AMBIOS XP 200). The surface views before and after hydrogenation were investigated by a scanning electron microscopy (SEM, JEOL JSM – 5600). The elemental composition of plasma treated films was analyzed by energy dispersive X–ray spectroscopy (EDX, Bruker Quad 5040). The distribution profiles of oxygen in titanium films after hydrogenation were measured by Auger Electron Emission spectroscopy (AES, PHI 700XI AUGER NANOPROBE). 392                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 1, Part II              EXPERIMENTAL RESULTS: ANALYSIS OF SURFACE TOPOGRAPHY The height of a step on the boundary between the non-irradiated and water plasma irradiated areas has been investigated in [3]. It has been shown that under water plasma irradiation the elevation height between irradiated and non-irradiated areas does not exceed 2 nm. On the basis of these experimental results it has been concluded that for irradiation parameters used in the present work the steady state surface erosion and surface growth due to accommodation of implanted species rates in the near-surface region are approximately equal. The studies of surface roughness on a nanometric scale revealed that surfaces of Ti film after water plasma treatment become rough. Fig. 2 includes the surface topography profiles of the as-deposited Ti film - Fig. 2a, and after plasma treatment during 5 min - Fig. 2b. The surface roughness increases from ~10 nm up to ~75 nm.  Fig. 2 - The profiles of surface topography: a) - profile of as-deposited Ti film, and b) – profile of Ti after plasma treatment during 5 min  EXPERIMENTAL RESULTS: XRD ANALYSIS Fig.  3 includes XRD patterns of Ti film after different durations of irradiation: curve 1 - 2 min, curve 2 - 5 min, and curve 3 – 10 min. It is seen that structural reconstructions and formation of a new Ti4H209 phase begins after 10 min of irradiation. Characteristic peak of Ti shows that  Fig. 3 - XRD patterns after different durations of hydrogenation Nanomaterials: Applications and Properties (NAP-2011). Vol. 1, Part II              393  initially amorphous film becomes nanocrystalline with the mean size of crystallites estimated using Scherrer equation is equal to about 60 nm.  Distribution of o atoms The Auger depth distribution profiles of O and Ti atoms in Ti film after irradiation during 10 min by ions extracted from the water plasma are shown in Fig. 5a and Fig. 5b. Two regions may be distinguished: (i) in the near surface layer with thickness less than 100 nm, the oxygen concentration decreases from 75 at.% to 40 at.% (Fig. 5a), and (ii) in the bulk in the range from 100 nm across entire film thickness, the concentration of oxygen decreases from 40 at.% to 25.at.% (Fig. 5b).  Fig. 5 - The Auger distribution profiles of O and Ti atoms in Ti film after irradiation during 10 min by ions extracted from the water plasma  Discussions According to the new concept of molecular surface science the surface may be treated as a separate phase with a different structure, composition and electronic properties. It reveals that processes of relaxation and reconstruction relocate surface atoms from their bulk-like positions and indicates to the unique importance of surface defects (roughness) in controlling surface phenomena. This phenomenon is consistent with the latest discoveries in molecular surface science [5]. At rough edges, such as at stepped surfaces, the atoms at the step relax by a large amount in order to smooth the surface irregularities. In this work, the main emphasis is made on analysis of the role of surface topography instabilities on the mechanism of oxygen transport in Ti film under molecular water ion irradiation. Two dominant surface topography relaxation mechanisms are distinguished: (i) the dynamic relocation of surface adatoms and annihiliation of surface vacancies (Fig. 6a),  and  (ii)  the  transport  of  O  atoms  from  the  surface into to the bulk and Ti atoms from the bulk on the surface (Fig. 6b). For modeling of oxygen behavior in Ti films some assumptions are need-ed. Let us assume that at elevated temperatures surface adatoms under the thermal or ballistic displacement effects are mobile. 394                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 1, Part II              This assumption is verified in publications [6]. The surface dy-namic state is prerequisite for sto-chastic mixing of atoms between neighboring monolayers. The ada-tom on the surface is located in some adsorption site determined to first order by the local crystal struc-ture. It oscillates in the potential well at a frequency210 2 ¹¸·©¨§ amEw di, where Ed is  the  height  of  the  po-tential well, or the activation ener-gy, for surface diffusion; m is the adatom mass, and a is the width of the well. The effect of the ion irradiation is to increase the relocation frequency above the thermal levels. The total frequency relocation probability is the sum of thermally activated jumps plus the ion impact induced jumps              TkEiidwCIDw e00 ,                      (1) where D is the number of displaced atoms on the surface by one incident ion, I0 is the flux of ions and C is the surface atom concentration. The Eq. (1) is valid if thermal and “ballistic” relocations are independent. Mathematically the flux of displaced i atoms is equal to wici, where wi is the frequency probability of surface displacements defined by the Eq.(1) and ci is the partial surface concentration of i atoms. The flux of readsorbed atoms is equal to DijEiwici, where Ei is the probability of readsorption and Dij is the sticking probability of i atom to j atom on the surface. At steady state, when all displaced atoms are readsorbed, DijEi = 1. For simplicity and in accordance with experimental observations, the behavior of broken oxygen atoms in Ti film will be considered in the following way. The affinity of Ti atoms to oxygen is high and they tend to form Ti-O bonds. The surface atoms are buried by the displaced and readsorbed O and Ti atoms.  In this way, the oxidation kinetics may be limited by the transport of Ti atoms from the bulk to the surface or O atoms from the surface into the bulk. The conservation of continuity of material requires that these fluxes would be balanced.  The rate equations including processes of atom displacement and readsorption have been analyzed in [7]. It has been shown that the stochastic mixing of atoms on the surface may be described by the diffusion equation written in finite increments with step one monolayer h0 and the effective diffusion coefficient equal to )(20 ad VVhD  and surface movement velocity equal to V= h0(Vd – Va), where  Fig. 6 - Schematic presentation of atom transport mechanisms Nanomaterials: Applications and Properties (NAP-2011). Vol. 1, Part II              395  ¦ jjjd cwV   and  ¦¦ i jjija cV N .                    (2) The balanced dynamic state on the surface is realized if atom displacement and readsorbtion rates are equal, Vd | Va. This regime may be realized in two ways: 1) if the generation rate of surface vacancies is equal to the annihilation rate of surface vacancies by mobile adatoms, and 2) if the annihilation of surface vacancies takes place by atoms arriving from the bulk in the balanced way. Atomic motion minimizes surface energy and stabilizes the surface roughness. The wells of the surface are filled by atoms which are transported from the bulk, and atoms from the surface elevations are moved into the bulk. Above mentioned processes may be interpreted in the following way. The stochastic mixing of atoms on the surface initiates surface roughening. The roughness depth increases in time as (Dt)1/2. In many cases the increase in roughness is correlated with the increase of surface energy. The increase of surface roughness, or surface energy (i.e. the number of broken links multiplied by the pair-interaction (bond) energy), can not be infinite. When surface energy reaches critical, the stabilization of the surface energy starts. The stabilization of surface energy by diffusion of adatoms and annihilation of surface vacancies and occupation of empty sites on the surface is dominant process in the range of  low  temperatures  (Fig. 6a). The transport of atoms from the bulk into the empty sites on the surface and annihilation of surface vacancies becomes dominant at elevated temperatures when point defects are mobile in the matrix of bulk material (Fig. 6b). The more open (rougher) surface topography, the larger relaxation fluxes are. The above presented model considers two balanced fluxes. Both flux are equal and have opposite directions. They follow to the mass-conservation law and continuity of material. Transport of oxygen atoms into the bulk becomes inefficient if one or both of these fluxes are physically non-realizable. Let us consider some examples, which are in consistence with the experimental re-sults: 1) the efficient oxidation starts at temperature when Ti atoms become mobile and take part in the relaxation mechanism of surface topography; 2) the presence of the thin TiO2 layer  on  the  surface  of  Ti  acts  as  barrier  for  atom  transport from the surface into the bulk, and in this way hinders efficiency of oxygen transport from the surface into the bulk and stops it when continuous dielectric film is formed. On the contrary, the pretreatment processes removing barrier layers on the surface and related with the generation of defects on mate-rials increases the efficiency of the transport of oxygen. The prebombardment of materials by non-reactive Ar ions increases efficiency of oxygen transport. In this way, two O transportation mechanisms from the surface into the bulk of Ti film are in action under ion irradiation: (i) the penetration of O at-oms as result of stochastic mixing of atoms on the surface, and (ii) the trans-portation of O atoms by fluxes driven by surface energy relaxation processes. The first mechanism forms near-surface region highly saturated by O atoms and  the  second  one  explains  distribution  of  O  atoms  along  the  entire  Ti  film  thickness. 396                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 1, Part II              CONCLUSIONS It  is  shown  that  the  oxygen  transport  from  the  surface  into  the  bulk  is  controlled by the processes on the surface, and the adsorption of reactive atoms or molecules leads to local and long-range restructuring and adatom relocation at the surface. Experimentally, the anomalously deep penetration of oxygen has been registered in Ti films without formation of chemical compounds observable by XRD after molecular water ion irradiation during 10 min at room temperature (Fig. 3). Surface topography analysis results showed that after irradiation surfaces become rough and for considered irradiation parameters the balance between the surface erosion and surface growth due to accommodation of implanted species in the near-surface region exists. The experimental observations are explained assuming that processes on the surface (surface roughening) initiate the transport of oxygen atoms from the surface into the bulk. The relaxation of surface energy and reconstruction of surface topography relocate surface atoms from their bulk-like positions and drive into the underlying monolayers.  Acknowledgement This research was funded by a grant (Nr. ATE-02/2010) from the Research Council of Lithuania. The authors are grateful to professors V. Girdauskas and A. Kanapickas for their useful discussions, doctoral student K. Gedvilas for his contribution in experimental work. Authors highly appreciate prof. D. Milcius, the head of the Center for Hydrogen Energy Technologies, for the access to the experimental technique. REFERENCES [1] Pound B. Encyclopedia of Electrochemistry, Corrosion and Oxide Films, 2003, Vol. 4,  p. 118-155,  WILEY-VCH. [2] Anpo M.javascript:void(0);, Dohshi S.javascript:void(0);, Kitano M.javascript:void(0); Et  al.  Annual  review of  materials  research,  2005,  Vol.  35, p. 1-27. [3] Praneviþius L.L., Urbonaviþius M. Experimental study of hydrogenation of Ti films using magnetron water vapour plasma. Submitted for publication in Conference of Young Scientists on Energy Issues, 2011. [4] TeleviþLǌWơ M., Švaikovskij A., Tuþkutơ S. Modeling of water molecular ion implantation. Submitted for publication in Conference of Young Scientists on Energy Issues, 2011. [5] Kang Y.C., and Ramsier R.D. Applied Surface Science, 2002, Vol. 195 p. 196–201. [6] Somorjai G.A. MRS Bull, 1998, Vol. 23 p. 11-29. [7] L. Praneviþius, D. Milþius, L.L. Praneviþius et al. Central European Journal of Physics, 2004, 2(1) p. 67-89.    

Oxygen implantation and behaviour into Ті thin films from water vapour plasma

SocioEconomic Challenges Journal (Sumy State University)

390                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 1, Part II             
 
OXYGEN IMPLANTATION AND BEHAVIOUR INTO TI 
THIN FILMS FROM WATER VAPOUR PLASMA 
Simona Tuþkutơ1,2, Liudas Praneviþius1,2, Liudvikas Praneviþius1,2,  
Marius Urbonaviþius1 
 
1 Vytautas Magnus University, 8 Vileikos Str., 44404, Kaunas, Lithuania 
2 Lithuanian Energy Institute, 3 Breslaujos Str., 44403, Kaunas, Lithuania 
 
ABSTRACT 
The behavior of O atoms in Ti film is investigated under high-flux, low-energy 
molecular water ion implantation. After 10 min of irradiation at room temperature, the 
anomalously deep penetration of oxygen without formation of new chemical compounds 
observable by XRD has been registered in Ti films using Auger spectroscopy analysis.  
It is shown that the surface energy increases under ion irradiation, and the 
relaxation processes minimizing the surface energy initiate the redistribution of atoms. 
Two surface energy relaxation processes are considered: (i) the mixing of atoms on the 
surface resulting in annihilation of surface vacancies; and (ii) the annihilation of surface 
vacancies by atoms transported from the bulk. The theoretical considerations are in 
agreement with the experimental results if to assume that the mass-transport in the bulk 
is controlled by the processes on the surface and the adsorption of reactive atoms or 
molecules leads to local and long-range restructuring and adatom relocation at the 
surface. 
 
Key words: hydrogen, water vapour, plasma, ion implantation 
 
INTRODUCTION  
Titanium and its alloys have many industrial applications thanks to their 
excellent corrosion resistance and high specific strength. Generally, titanium is 
covered by a passive titanium oxide (TiO2)  film.  Though very  thin  (usually  a  
few nanometers), this compact film acts as a protective layer against corrosion.  
Oxygen permeation through TiO2 films  and its  transport  in  the  bulk  is  a  
complex process involving interfacial charge transfer, adsorption, absorption, 
trapping, and transport, and, thus, is inherently influenced by properties of the 
oxide, such as the chemical composition and structure, the presence of Tin+ 
interstitials and/or oxygen vacancies, the type and concentration of impurities, 
hydrogen solubility, the adsorption characteristics of the surface, and the oxide 
thickness, porosity and uniformity [1,2]. The mechanism by which the oxide 
barrier layer influences oxygen permeation into Ti is still not well established.  
The retardation of oxygen permeation by TiO2 films  may  be  due  to  a  
                                               
 e-mail: simona@hydrogen.lt  
Nanomaterials: Applications and Properties (NAP-2011). Vol. 1, Part II              391 
 
combination of low oxygen absorption at the oxide surface and/or slow oxygen 
transport  in  the  oxide.  In  general,  it  is  difficult  to  predict  the  rate  of  oxygen  
transport through a TiO2 film and how many oxygen atoms, generated on the 
TiO2 surface, reach the Ti substrate. In this work, we study feasible physical 
models explaining oxygen permeation through a thin Ti film. 
EXPERIMENTAL TECHNIQUE 
The 0.5 - 1 Pm thick Ti films were deposited on 2 mm thick stainless steel 
and Si(111) substrates using magnetron sputter deposition technique. Samples 
were  located  at  different  sites  of  PVD  –  75  vacuum  chamber.  Details  of  
experimental technique are published in [3, 4]. Three cases were distinguished: 
(i)  case  A,  the  ion  current  density  from  plasma  to  the  sample  was  about  5  
PA/cm2, (ii) case B - 1.5 mA/cm2, and (iii) case C - 10 mA/cm2 . 
A vacuum system including rotary and cryogenic pumps enabled a base 
pressure of 2·10-4 Pa.  The  water  vapour  was  injected  into  vacuum  chamber  
using heated water container (Fig. 1). 
 
 
Fig. 1 - The scheme of the experimental technique 
 
Plasma  implantation  was  performed  at  10  Pa  pressure  of  water  vapour.  
Flow of water vapour was continously controlled by mass flow controller. The 
plasma was generated using DC and RF energy sources. The dissipated power 
in plasma was verified in the range 100 - 300 W. 
The microstructure of the samples was characterized by X - ray diffraction 
(XRD) method using Bruker diffractometer (Bruker D8). The measurements 
were performed with 2ș angle in the range 20ż – 70ż using Cu KĮ radiation in 
steps of 0.01ż. The identification of peaks has been done using Search – Match 
software. The thickness and surface topography of Ti films were measured 
using the nanoprofilometer (AMBIOS XP 200). The surface views before and 
after hydrogenation were investigated by a scanning electron microscopy 
(SEM, JEOL JSM – 5600). The elemental composition of plasma treated films 
was analyzed by energy dispersive X–ray spectroscopy (EDX, Bruker Quad 
5040). The distribution profiles of oxygen in titanium films after hydrogenation 
were measured by Auger Electron Emission spectroscopy (AES, PHI 700XI 
AUGER NANOPROBE). 
392                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 1, Part II             
 
EXPERIMENTAL RESULTS: ANALYSIS OF SURFACE TOPOGRAPHY 
The height of a step on the boundary between the non-irradiated and water 
plasma irradiated areas has been investigated in [3]. It has been shown that 
under water plasma irradiation the elevation height between irradiated and non-
irradiated areas does not exceed 2 nm. On the basis of these experimental 
results it has been concluded that for irradiation parameters used in the present 
work the steady state surface erosion and surface growth due to accommodation 
of implanted species rates in the near-surface region are approximately equal. 
The studies of surface roughness on a nanometric scale revealed that 
surfaces of Ti film after water plasma treatment become rough. Fig. 2 includes 
the surface topography profiles of the as-deposited Ti film - Fig. 2a, and after 
plasma treatment during 5 min - Fig. 2b. The surface roughness increases from 
~10 nm up to ~75 nm. 
 
Fig. 2 - The profiles of surface topography: a) - profile of as-deposited Ti film, 
and b) – profile of Ti after plasma treatment during 5 min 
 
EXPERIMENTAL RESULTS: XRD ANALYSIS 
Fig.  3 
includes XRD 
patterns of Ti film 
after different 
durations of 
irradiation: curve 1 
- 2 min, curve 2 - 
5 min, and curve 3 
– 10 min. It is seen 
that structural 
reconstructions 
and formation of a 
new Ti4H209 
phase begins after 10 min of irradiation. Characteristic peak of Ti shows that 
 
Fig. 3 - XRD patterns after different durations of 
hydrogenation 
Nanomaterials: Applications and Properties (NAP-2011). Vol. 1, Part II              393 
 
initially amorphous film becomes nanocrystalline with the mean size of 
crystallites estimated using Scherrer equation is equal to about 60 nm. 
 
Distribution of o atoms 
The Auger depth distribution profiles of O and Ti atoms in Ti film after 
irradiation during 10 min by ions extracted from the water plasma are shown in 
Fig. 5a and Fig. 5b. Two regions may be distinguished: (i) in the near surface 
layer with thickness less than 100 nm, the oxygen concentration decreases from 
75 at.% to 40 at.% (Fig. 5a), and (ii) in the bulk in the range from 100 nm 
across entire film thickness, the concentration of oxygen decreases from 40 
at.% to 25.at.% (Fig. 5b). 
 
Fig. 5 - The Auger distribution profiles of O and Ti atoms in Ti film after 
irradiation during 10 min by ions extracted from the water plasma 
 
Discussions 
According to the new concept of molecular surface science the surface 
may be treated as a separate phase with a different structure, composition and 
electronic properties. It reveals that processes of relaxation and reconstruction 
relocate surface atoms from their bulk-like positions and indicates to the unique 
importance of surface defects (roughness) in controlling surface phenomena. 
This phenomenon is consistent with the latest discoveries in molecular surface 
science [5]. At rough edges, such as at stepped surfaces, the atoms at the step 
relax by a large amount in order to smooth the surface irregularities. In this 
work, the main emphasis is made on analysis of the role of surface topography 
instabilities on the mechanism of oxygen transport in Ti film under molecular 
water ion irradiation. 
Two dominant surface topography relaxation mechanisms are 
distinguished: (i) the dynamic relocation of surface adatoms and annihiliation 
of surface vacancies (Fig. 6a),  and  (ii)  the  transport  of  O  atoms  from  the  
surface into to the bulk and Ti atoms from the bulk on the surface (Fig. 6b). 
For modeling of oxygen behavior in Ti films some assumptions are need-
ed. Let us assume that at elevated temperatures surface adatoms under the 
thermal or ballistic displacement effects are mobile. 
394                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 1, Part II             
 
This assumption is verified in 
publications [6]. The surface dy-
namic state is prerequisite for sto-
chastic mixing of atoms between 
neighboring monolayers. The ada-
tom on the surface is located in 
some adsorption site determined to 
first order by the local crystal struc-
ture. It oscillates in the potential 
well at a frequency
21
0 2 ¹¸
·
©¨
§ amEw di
, where Ed is  the  height  of  the  po-
tential well, or the activation ener-
gy, for surface diffusion; m is the adatom mass, and a is the width of the well. 
The effect of the ion irradiation is to increase the relocation frequency 
above the thermal levels. The total frequency relocation probability is the sum 
of thermally activated jumps plus the ion impact induced jumps 
             
Tk
E
ii
d
w
C
ID
w

 e00 ,                      (1) 
where D is the number of displaced atoms on the surface by one incident 
ion, I0 is the flux of ions and C is the surface atom concentration. The Eq. (1) is 
valid if thermal and “ballistic” relocations are independent. 
Mathematically the flux of displaced i atoms is equal to wici, where wi is 
the frequency probability of surface displacements defined by the Eq.(1) and ci 
is the partial surface concentration of i atoms. The flux of readsorbed atoms is 
equal to DijEiwici, where Ei is the probability of readsorption and Dij is the 
sticking probability of i atom to j atom on the surface. At steady state, when all 
displaced atoms are readsorbed, DijEi = 1. 
For simplicity and in accordance with experimental observations, the 
behavior of broken oxygen atoms in Ti film will be considered in the following 
way. The affinity of Ti atoms to oxygen is high and they tend to form Ti-O 
bonds. The surface atoms are buried by the displaced and readsorbed O and Ti 
atoms.  In this way, the oxidation kinetics may be limited by the transport of Ti 
atoms from the bulk to the surface or O atoms from the surface into the bulk. 
The conservation of continuity of material requires that these fluxes would be 
balanced.  
The rate equations including processes of atom displacement and 
readsorption have been analyzed in [7]. It has been shown that the stochastic 
mixing of atoms on the surface may be described by the diffusion equation 
written in finite increments with step one monolayer h0 and the effective 
diffusion coefficient equal to )(20 ad VVhD  and surface movement velocity 
equal to V= h0(Vd – Va), where 
 
Fig. 6 - Schematic presentation of atom 
transport mechanisms 
Nanomaterials: Applications and Properties (NAP-2011). Vol. 1, Part II              395 
 
¦ 
j
jjd cwV   and  ¦¦ 
i j
jija cV N .                    (2) 
The balanced dynamic state on the surface is realized if atom 
displacement and readsorbtion rates are equal, Vd | Va. This regime may be 
realized in two ways: 1) if the generation rate of surface vacancies is equal to 
the annihilation rate of surface vacancies by mobile adatoms, and 2) if the 
annihilation of surface vacancies takes place by atoms arriving from the bulk in 
the balanced way. Atomic motion minimizes surface energy and stabilizes the 
surface roughness. The wells of the surface are filled by atoms which are 
transported from the bulk, and atoms from the surface elevations are moved 
into the bulk. 
Above mentioned processes may be interpreted in the following way. The 
stochastic mixing of atoms on the surface initiates surface roughening. The 
roughness depth increases in time as (Dt)1/2. In many cases the increase in 
roughness is correlated with the increase of surface energy. The increase of 
surface roughness, or surface energy (i.e. the number of broken links multiplied 
by the pair-interaction (bond) energy), can not be infinite. When surface energy 
reaches critical, the stabilization of the surface energy starts. The stabilization 
of surface energy by diffusion of adatoms and annihilation of surface vacancies 
and occupation of empty sites on the surface is dominant process in the range 
of  low  temperatures  (Fig. 6a). The transport of atoms from the bulk into the 
empty sites on the surface and annihilation of surface vacancies becomes 
dominant at elevated temperatures when point defects are mobile in the matrix 
of bulk material (Fig. 6b). The more open (rougher) surface topography, the 
larger relaxation fluxes are. 
The above presented model considers two balanced fluxes. Both flux are 
equal and have opposite directions. They follow to the mass-conservation law 
and continuity of material. Transport of oxygen atoms into the bulk becomes 
inefficient if one or both of these fluxes are physically non-realizable. Let us 
consider some examples, which are in consistence with the experimental re-
sults: 1) the efficient oxidation starts at temperature when Ti atoms become 
mobile and take part in the relaxation mechanism of surface topography; 2) the 
presence of the thin TiO2 layer  on  the  surface  of  Ti  acts  as  barrier  for  atom  
transport from the surface into the bulk, and in this way hinders efficiency of 
oxygen transport from the surface into the bulk and stops it when continuous 
dielectric film is formed. On the contrary, the pretreatment processes removing 
barrier layers on the surface and related with the generation of defects on mate-
rials increases the efficiency of the transport of oxygen. The prebombardment 
of materials by non-reactive Ar ions increases efficiency of oxygen transport. 
In this way, two O transportation mechanisms from the surface into the 
bulk of Ti film are in action under ion irradiation: (i) the penetration of O at-
oms as result of stochastic mixing of atoms on the surface, and (ii) the trans-
portation of O atoms by fluxes driven by surface energy relaxation processes. 
The first mechanism forms near-surface region highly saturated by O atoms 
and  the  second  one  explains  distribution  of  O  atoms  along  the  entire  Ti  film  
thickness. 
396                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 1, Part II             
 
CONCLUSIONS 
It  is  shown  that  the  oxygen  transport  from  the  surface  into  the  bulk  is  
controlled by the processes on the surface, and the adsorption of reactive atoms 
or molecules leads to local and long-range restructuring and adatom relocation 
at the surface. Experimentally, the anomalously deep penetration of oxygen has 
been registered in Ti films without formation of chemical compounds 
observable by XRD after molecular water ion irradiation during 10 min at room 
temperature (Fig. 3). 
Surface topography analysis results showed that after irradiation surfaces 
become rough and for considered irradiation parameters the balance between 
the surface erosion and surface growth due to accommodation of implanted 
species in the near-surface region exists. The experimental observations are 
explained assuming that processes on the surface (surface roughening) initiate 
the transport of oxygen atoms from the surface into the bulk. The relaxation of 
surface energy and reconstruction of surface topography relocate surface atoms 
from their bulk-like positions and drive into the underlying monolayers. 
 
Acknowledgement 
This research was funded by a grant (Nr. ATE-02/2010) from the 
Research Council of Lithuania. The authors are grateful to professors V. 
Girdauskas and A. Kanapickas for their useful discussions, doctoral student K. 
Gedvilas for his contribution in experimental work. Authors highly appreciate 
prof. D. Milcius, the head of the Center for Hydrogen Energy Technologies, for 
the access to the experimental technique. 
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Oxygen implantation and behaviour into Ті thin films from water vapour plasma

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