Vol. 06 INTERNATIONAL JOURNAL OF PHOTOENERGY 2004

Production of clusters and thin films of nitrides, oxidesand carbides by pulsed laser ablation and deposition

S. Orlando,1 A. Paladini,2 A. Santagata,1 V. Marotta,1 G. P. Parisi,1 M. Satta,1

D. Scuderi,2,3 D. Catone,2 A. Giardini,2 and A. Mele2,†

1 CNR-IMIP (sezione Istituto Materiali Speciali), I-85050 Tito Scalo (Pz), Italy2 Dipartimento di Chimica, Università di Roma “La Sapienza”, pl. A. Moro 5, I-00185 Roma, Italy

3 Facoltà di Farmacia, Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive,

Università di Roma “La Sapienza”, pl. A. Moro 5, I-00185 Roma, Italy

Abstract. A short introduction on the principles of laser-matter interaction, material evaporation, plumeformation, its reactivity with suitable gases and finally deposition are here illustrated. Experiments by massspectrometry of formation of clusters, precursors of thin films and nanoparticles of oxides, nitrides andcarbides by pulsed laser ablation (PLA) are reported. Pulsed laser ablation of targets combined with an intenseatomic source produced by radiofrequency (RF), are discussed in terms of generating chemical reactions orsupplying the loss of volatile components.

1. INTRODUCTION

Nanoparticles and thin films of various materials suchas metals, semi and superconductors, nitrides, oxidesand carbides have been extensively studied in the lastyears because their unique physical and chemical prop-erties [1–3]. The growing interest in producing thinfilms of these compounds arises due to the fact thatmany of them show novel properties as new electronicmaterials the including use as semiconductors, im-plants for quantum well and quantum dot devices, andin construction of solar cells and photovoltaic devices.The study of molecular clusters, which are herein re-ported, is an important feature of the production ofthin films or of systems of finite size such as nanos-tructure materials with tailored [4] properties.

A variety of methods have been used to fabricatethin films and nanoparticles of these materials, includ-ing Molecular Beam Epitaxy, Radiofrequency sputteringand Ion sputtering, Spray pyrolysis, Chemical VaporDeposition (CVD) and Pulsed Laser Ablation and Depo-sition (PLAD). In particular PLAD [5] has been shownto be a very powerful deposition method of all thesematerials. Compositional changes may occur duringpulsed laser ablation and thin film deposition. A veryimportant feature involved in thin film deposition byPLAD is the congruency of the vaporizing componentscompared to the solid and liquid films. The aim of var-ious studies was to maintain the stoichiometry of thecondensed phase. The loss of oxygen or nitrogen is veryimportant for a balance of the processes. The additionof these gases is advisable in order their loss. Activeoxygen or nitrogen could be more efficient with respectto the molecular elements [6]. A substrate of an atomic

†E-mail: aldo.mele@uniroma1.it

gas such as nitrogen or oxygen created by a radiofre-quency (RF) discharge may establish a new equilibriumto contrast the loss. This same technique may be usedto perform a chemical reaction between the atomicatmosphere and the ablated plume from a target madeof single components such as metals or graphite [7].

2. PRIMARY AND SECONDARY MECHANISMS

Laser matter interaction. Laser ablation refers toa process of particle injection, named “primary mech-anism” [8], similar to sputtering in which material re-moved rates typically exceed one tenth of monolayerper pulse. This is different in the characteristics withlaser induced desorption which occur without any de-tectable mesoscopic surface modification and struc-ture. The proportionality between yield and electronexcitation density, underlines the partially yield, whichis an “non-linear function” of the laser energy density(fluence) [9]. The expression of a “linear regime” ap-plies in the second case, at a fluence below the thresh-old. Thus the most relevant feature between the tworegimes is the change of the yield vs fluence. PLAD hasalso in most cases the unique capability in maintain-ing the stoichiometry of the thin film produced at highfluence.

A purely thermal model in the description of laserheating of the surface and subsequent material evap-oration in the nanosecond and femtosecond regimesis inadequate as shown by the velocity distribution ofthe emitted particles which exceed by far the predictedvalues also at 5000 K. Time resolved studies of laserdesorption in metals performed using pulsed laser op-erating in the femtosecond regime have shown that forvery fast processes, the metal may be considered as a

24 S. Orlando et al. Vol. 06

(a)

50 100 150 200 250 300100

110

120

130

140

n= 3 5 7 9

Al(NH3)n

λ = 347.5 nm

(c)

50 100 150 200 250 300100

110

120

130

140

n= 2 4 6

In(NH3)n

λ = 266.2 nm

(b)

50 100 150 200 250 300100

110

120

130

140 n= 3 5 7 9

GA(NH3)n

λ = 320 nm

(d)

50 100 150 200 250 300100

110

120

130

140

n= 2 4 6 8

In(NH3)n

λ = 266.2 nm

m/e

Inte

nsi

ty(a

rb.u

nit

s)

Figure 1. Set of mass spectra of Al(NH3)n, Ga(NH3)n, In(NH3)n and Cu(NH3)n clusters by reaction of Al, Ga, In and Cu

plume in a stream of NH3 and He (10%). Stagnation pressure 4 × 105 Pa. KrF eximer laser, wavelength 248 nm: Laser

fluence 1 J/cm2. Ionization wavelength: (a) 347.5 nm, (b) 320 nm, (c) 266 nm and (d) 290 nm.

two temperature system of the electron and the lat-tice [10]. This behavior can be described by two cou-pled non linear partial differential equations to takein account the initial vaporization of the surface, andto evaluate the depth of ablation at the damage flu-ence threshold. Characterization of composition, andvelocity distribution of the plume is performed throughvarious techniques, such as Intensified Time ResolvedOptical Imaging (ICCD), Optical Spectroscopy (OpticalMultichannel Analyzer) and Mass spectrometry (Timeof Fligth or Quadrupole Mass Spectrometers).

Laser ablation and clusters formation. At presentthere is a great interest in systems of finite and smallsize. Particularly interesting are systems with char-acteristics which may serve as the building blocksfor nanostructures and clusters assembled materials.Small systems often displays unique properties whichprovide a challenge of considerable scientific inter-est [11]. The analysis of the gas phase cloud, movingaway from the target, named “secondary mechanism”[8] is an important step in thin films fabrication. Thequestions to answer concern: chemical and structureidentity, and chemical reactivity. Studies on laser abla-tion and clusters formation have been carried out byseveral techniques. The LAMMA apparatus has beenfirst employed to analysis clusters by quadrupole massspectrometry [12]. Clusters ions have been produced

from several oxides of various elements. The use ofthis technique was pioneered to produce clusters of ni-trides, oxides and carbides starting from targets of thebare elements. The formation of clusters precursor ofthin film deposition was studied also by developing amore sophisticated laser ablation supersonic appara-tus [13]. Reaction of ammonia with bare metal targets,such as Alluminum, Indium, Gallium and copper (Fig-ure 1) have been examined in order to understand themechanism of nitrides formation [1].

3. EXPERIMENTAL METHODOLOGY

Pulsed laser ablation. The laser ablation experi-ments were carried out in a multiport stainless steelvacuum chamber equipped with a gas inlet, a rotat-ing multi-target and a heatable substrate holder [14].The vacuum pressure of the deposition chamber wasbelow 10–3 Pa whereas the reacting gas pressure dur-ing film depositions was 10 Pa. The deposition tem-perature could be varied from room temperature upto 1000 K. The fluence of the laser employed (Quan-tel Nd : YAG 581, λ = 532 nm, pulse duration = 7 ns,repetition rate = 10 Hz) has been kept nearly constantat 8 J/cm2. The laser impinges on the target surface atan angle of 45 ◦ with respect to the normal in a staticgas atmosphere. All the targets were rotated at 2 rpmduring depositions.

Vol. 06 Production of clusters and thin films of nitrides, oxides and carbides . . . 25

Cross Section

E.I. RF Negative Electrode

T.I.

T.I.

H Substrate

E.I. RF Negative Electrode

Ground

Front View

RF Negative Electrode

Figure 2. Schematic layout of the heatable sample holder (H) hooded by a stainless steel top-hat, with a 30 mm diameter

hole, in which it is inserted the output of the matching network coming from the RF power generator. (E.I. = electrical

insulator, T.I. = thermal insulator).

Ablation Laser

Pulsed Nozzle Metal Rotating TargetOSCILLOSCOPE

AMPLIFIER

CHANNELTRONPC

TOF

SKIMMER

Gas Inlet

AblationChamber

To Vacuum System

Dye Lasers

To Vacuum System

Pumped Chamber

Figure 3. Schematic of the supersonic beam experimental apparatus employed for the production of clusters.

Radiofrequency assisted PLAD apparatus. ThePLAD set up has been recently improved by employ-ing a RF plasma system placed just above the substrateholder (Figure 2), maintaining the older electricallyconnected to the ground [15]. The substrate holderis surrounded by an isolated stainless steel top-hatconnected to the RF generator, through a customizedmatching unit. A 3 cm diameter hole on the top-hat al-lows the deposition of the plume coming from the ab-lated target. The RF power generator is a 13.56 MHz ENIModel OEM-6, maximum power output 650 W.

Supersonic molecular beam apparatus. The ex-perimental apparatus we have employed for the pro-duction of clusters is an instruments constructed in our

laboratory and is reported in Figure 3 [13]. It consistsof two vacuum chambers separated by a conical skim-mer. In a small ablation chamber a mixture of reactivegases in He ranging from 0.1% to 100% at a stagnationpressure of (2–4)·105 Pa is introduced through apulsednozzle (200 s time duration and 1 mm diameter). Inthe same ablation chamber a rotating target is vapor-ized by a Nd : Yag laser doubled in frequency (532 nm,0.15 J/pulse). The jet formed, after being skimmed, en-ters in a second chamber equipped with time of flight(TOF) mass spectrometer. The clusters in the beam areionized by Nd : Yag pumped Dye laser or a KrF laser(=248 nm, 0.5 mJ/pulse) and detected by a channeltron.The mass selected signals are recorded by digital oscil-loscope, stored and processed on a PC.

26 S. Orlando et al. Vol. 06

6,1

16

,12

6,1

3 6,1

46

,15

7,1

4

9,1

8

10

,20

11

,22

12

,24

13

,26

14

,28

1000 1500 2000 m/e

1,0

1,2

1,3

1,4

1,5

1,6

1,8 2,1

2,2

2,3

2,4

2,5

2,6

2,7

2,8

3,5

3,6

3,7 3

,83

,93

,10

4,7

4,8

4,9

4,1

0 4,1

14

,124

,13

5,1

05

,11 5

,12

5,1

35

,14

LamCn

a.u.×

5a.

u.

Figure 4. Typical LAMMA mass spectra of lanthanium carbide clusters LamCn where m and n are (top) the index (bottom)of

a few cluster structures m = 7.

4. RESULTS AND DISCUSSION

Cluster production and analysis. Studies of clus-ter formation obtained by laser ablation of oxideshave been analyzed by LAMMA spectrometry and haveshown stoichiometry structure MnO+m with chemicalformula involving in integral values of n and m andnon stoichiometry formula associated with lattice de-fect [12]. It has been found that the same oxide maybe in various crystalline forms and present in differentyield for low and high molecular weight in the massspectra. More stable structures are usually formed ac-cording to their coordination number. However less sta-ble sequence of clusters may also be found. Assuming arole of clusters in the formation of thin film, the growthof a stable structure is thus important data to be takeninto account. The mass spectra of oxides indicate alsothe presence of fragmentation ions and particularly ofoxygen negation ions. This fact would justify the addi-tion of active oxygen to compensate this loss.

Other work on oxides concerns laser evaporationsystems such as Sc2O3, Y2O3 and La2O3 (Figure 4) in amixture with graphite, yielding ionized metal carbide offormula MemC+n [16]. The reaction takes place by a nearcomplete reduction of the oxide by means of graphite.Two sequences of clusters have been observed in themass spectra. The main sequence is (MeC2)X+ togetherwith carbide clusters of a more general formula MemC+nwith a ratio of carbon to metal greater than 2. This isprobably due to the effect of carbon the atom densityin the reaction region. This may find an interpretationin analogy with bare carbon .

Carbon clusters Cn have been found to be formedfrom graphite by pulsed laser ablation up n larger than60, as analyzed by LAMMA apparatus [16]. The originof the stability of neutral, positive and negative barecarbon cluster have been discussed and an even-oddeffect which was found in the cluster stability has beenexplained in terms of molecular orbital model. Neutraland positively charged odd species have a (molecularorbital completely filled and are more stable than evenspecies with only half filled π orbitals.

100 200 300 400 500

m/z

0

10

20

30

40

50

60

70

Inte

nsi

ta(u

.a.)

Zr+ 2

Zr 2

C+ Z

r 2C+ 2

Zr 2

C+ 3

Zr 2

C+ 4

Zr 3

C+ 3 Zr 3

C+ 4

Zr 3

C+ 5

Zr 4

C+ 5

Zr 4

C+ 6

ZrC in CH4

Figure 5. Non resonant one photon ionization TOF mass

spectra of ZrnCm clusters at KrF excimer laser ablation,

wavelength 248 nm: Laser fluence 0.5 mJ/pulse.

Vol. 06 Production of clusters and thin films of nitrides, oxides and carbides . . . 27

The photoionization mass spectra of zirconium car-bide thin film of ZrmCn obtained by ablation of a zir-conium carbide target in the presence of methane isreported in Figure 5; the cluster growth of n is up to 6.The relative intensity depends on the concentration ofmethane in the beam. The studies of the cluster systemswere chosen in order to ultimately determine the effectof the electron stability of the structure of the plasmacomponents. The growth mechanism may be connectedwith the process of selective chemistry of the speciesleading to thin film formation and deposition.

Recent studies have discovered the formation ofmetals carbides, nitrides and oxides by a method basedon tandem mass spectrometry [4]. These authors havediscovered stable structures of metal carbides, nitridesand oxides Me8C12 which make a new class of molec-ular clusters termed Met-Cars. The stability of thisspecies is based on a dodecahedron structure wherethe metal atom would occupied 8 unique positions.

Radiofrequency (RF) assisted thin film deposition.The present work is intended to show same results onPLAD thin film deposition from various materials andto established a relationship between the compositionof the films with what is expected by “normal vaporiza-tion” of the target. The congruency or incongruency ofthe process is defined by comparing the two counter-parts, films and targets, for the components changes.

A thermal treatment of the process considers thatevaporation-decomposition is what really take placeduring the laser pulse. In the range of the laser irra-diance of the experiments of about 108 W/cm2, it is as-sumed that the most removed from the target is dueto vaporization. The incongruency of the balance in theprocess is due in the case of multicomponents targetto the loss of one of the elements of the target. This ispreferably the most volatile [6]. Other possible changesmay take place in the gas phase, due to either the insta-bility of the species or in the plasma which may decom-pose or to ion-molecule reactions which may take placebetween the species present in the plasma or inducedby addition of ionized or neutral species to the vapor-ized materials. There is evidence that this may occuras shown by a comparison of the analysis of the plumeand the composition of the thin film. The analysis ofthe plume by mass spectrometry or by optical meth-ods have confirmed the view that the loss of compo-nent is quite large. Decomposition processes occur dueto the thermal spike induced by laser surface interac-tion. Herein, we report the synthesis of boron-nitrogenand metal- oxygen materials by using atomic nitrogenand atomic oxygen created by a radiofrequency (RF) dis-charge [15, 17].

Boron nitrides thin films have been deposited on Si(100) substrate by reactive pulsed laser ablation of pureboron target in the presence of N2 buffer and 13.56 MHzradiofrequency (RF) field apparatus shown in Figure 2.

The presence of the RF plasma allows the growth of thestable hexagonal boron (h BN) nitride from a boron tar-get in a nitrogen gas atmosphere. The IR spectroscopyshows that RF plasma enhances the formation of hexag-onal boron nitride (h BN) with respect to other phasesobtained by non-assisted pulsed laser ablation.

Metal oxides thin films have been fabricated by thesame apparatus. Target of tungsten trioxide and tita-nium oxide TiO2 have been irradiated [17] in the ap-paratus of Figure 2 by using 13.56 MHz radiofrequencypalsma, 10 Pa static atmosphere oxygen and a doubledfrequency Nd : Yag laser (λ = 532 nm). The depositionwas performed on a heatable Si (100) substrate up to1000 K. A comparison between a conventional pulsedlaser deposition system PLAD and the RF plasma as-sisted system displays the effect of the plasma. The ad-dition thermal effect due to RF on the surface of thesubstrate may improve the properties of the film. It hasbeen found that without the use of RF the adhesion israther poor. The influence of the plasma on a better ad-hesion is confirmed by the adhesive tape tear test. Fur-ther the geometry of the deposit is more homogeneousand there is not preferential angle target and substrate,possibly due to a more distribution of the componentsof the plasma which reaches the substrate.

It is worth mentioning the results of zinc oxide thinfilm deposition by RF assisted ablation [18]. This oxideis a wide-band semiconductor, which exhibits a x-raydiffraction pattern of ZnO samples without and with aRF discharge. From a zinc target in the presence of anoxygen atmospheres the pattern shows ordered spectraby increasing the temperature. Beyond the effect of thetemperature, which may produce films of different ori-entations, the (101) crystal growth orientation seemsto be preferred if the film is produced by RF plasma-assisted PLAD.

5. CONCLUSIONS

The study reported in the present article shows that theanalysis of the gaseous cloud formed above the targetby pulsed laser ablation and moving towards the sub-strate may provide an insight of the components intothe plasma which may be correlated with the thin filmdeposits. The cluster analysis is indicative of what arethe changes of components occurring on the way tothe target and their spatial and temporal distributionas measured by mass spectrometry. The experimentalconditions may strongly affect the laser-target interac-tions and the following gas phase reactions in a shorttime (∼ 10−5 s). The question is now what are the pa-rameters which have the largest effect on the process.In the assumption of a thermal process, we look forthe phase stability domains which are represented byT, P, V parameters and composition in gas phase dia-grams. The laser power density and the radiofrequencyatomic yield are both conditions which produce a large

28 S. Orlando et al. Vol. 06

variability. The analysis of the plume components andthe variability of the experimental set up of all systemsmay lead to a rather good inside of the phenomenologi-cal performance of the experiment, in order to establishthe best trend. The few examples reported have tried toanswer to basic questions of addressing the experimentto the right direction. Each system has to be examinedseparately in all variables, and a computational analysisapproch may be useful to establish the correct trend.

ACKNOWLEDGMENTS

This work was partially supported by Progetto Strate-gico MSTA II of Consiglio Nazionale delle Ricerche(CNR) of Italy.

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