Appl Phys A (2008) 93: 579–587DOI 10.1007/s00339-008-4696-7

Film growth mechanisms in pulsed laser deposition

Michael J. Aziz

Received: 12 October 2007 / Accepted: 9 April 2008 / Published online: 13 June 2008© Springer-Verlag 2008

Abstract This paper reviews our recent studies of the fun-damentals of growth morphology evolution in Pulsed LaserDeposition in two prototypical growth modes: metal-on-insulator island growth and semiconductor homoepitaxy. Bycomparing morphology evolution for pulsed laser deposi-tion and thermal deposition in the same dual-use chamberunder identical thermal, background, and surface prepara-tion conditions, and varying the kinetic energy by varyingthe laser fluence or using an inert background gas, we haveisolated the effect of kinetic energy from that of flux pulsingin determining the differences between morphology evolu-tion in these growth methods. In each growth mode analyti-cal growth models and Kinetic Monte Carlo simulations forthermal deposition, modified to include kinetic energy ef-fects, are successful at explaining much of what we observeexperimentally.

PACS 81.15.Fg · 68.55.-a · 81.07.Bc · 61.14.Hg · 81.15.Hi

1 Introduction

For the fabrication of new materials and assemblies of ma-terials, we often find ourselves saying “If I could only makethis particular structure, I bet it would have these wonderfulproperties”. But how can we place and hold the atoms wherewe want them to be? The revolution in materials processingthat has occurred over the past quarter century has ushered

M.J. Aziz (�)Harvard School of Engineering and Applied Sciences,Cambridge, MA 02138, USAe-mail: maziz@harvard.edu

in a host of new processing techniques, many of which ac-complish just this because, by design or accident, they con-trol the kinetics to produce materials that are permanentlystuck out of thermodynamic equilibrium. Our understand-ing of the kinetic laws dictating the final product that formsunder any particular processing conditions has, in general,lagged far behind the empiricism that has guided the devel-opment and use of processing techniques. This situation isto be contrasted with the current status of synthetic organicchemistry: because organic chemical reaction mechanismsand kinetics are so well understood, chemists are able tosynthesize, deliberately and rationally, an almost unlimitedvariety of organic structures. Empiricism can only get you sofar before diminishing returns make things difficult. Sooneror later a fundamental understanding of the phenomenologyand mechanisms involved is needed. This permits the intelli-gent generalization of the process to untested length scales,to untested materials, and to related processes, greatly en-hancing our capabilities for continued progress. The alter-native is the empirical exploration of a vast parameter spaceexperimentally—sometimes at great temporal and financialcost.

In this paper I review our recent studies of the funda-mentals of growth morphology evolution in Pulsed LaserDeposition (PLD), with an emphasis on a comparison withMolecular Beam Epitaxy (MBE) or Physical Vapor Deposi-tion (PVD). MBE/PVD is an ideal foundation upon whichto build because there is now a solid baseline of knowledgeabout surface structures; stress effects; atomistic mecha-nisms; growth modes; and the incorporation of dopants, im-purities, and alloying elements during growth. Two essentialdifferences between PLD and MBE are widely recognized:(1) in PLD the depositing species arrive in short bursts, onthe order of 10–100 µs, instead of in steady state; and (2) inPLD the depositing species have kinetic energy of order

580 M.J. Aziz

10–100 eV—some two orders of magnitude greater than inMBE. One of our goals has been to determine the relativecontributions of these features in determining the distinctiveaspects of PLD growth morphologies.

PLD uses a pulsed laser to ablate a target to produce thedepositing flux [1, 2], and has several distinct characteris-tics advantageous for the understanding of non-equilibriumgrowth from the vapor. One particularly dramatic differencebetween crystal growth in MBE and in PLD is the instanta-neous deposition rate. In MBE a typical growth rate mightbe only 1 monolayer (ML) per second, and near-equilibriumgrowth often occurs [3]. In PLD, one can grow films at theseaverage rates, but commonly the instantaneous rate is some3–5 orders of magnitude faster. The average growth speedis limited only by the repetition rate of the laser, which canbe changed abruptly without significant time lags. Hence,PLD growth is an area of opportunity for a variety of funda-mental kinetic studies that are difficult in MBE growth. Thecharacteristics that make it particularly interesting are:

1. PLD consists of periodic bursts of highly driven growthfollowed by relatively long periods of uninterrupted sur-face relaxation, permitting these two competing proc-esses to be isolated and studied separately.

2. In the proper ablation regime, ionized and neutral abla-tion products having kinetic energies in the range fromless than one to a few hundred eV can be produced [4–6].The variable kinetic energy can be used to study a varietyof phenomena such as enhanced low-temperature epitaxyand surface segregation/incorporation reactions.

3. The instantaneous deposition flux can be varied indepen-dently of either the kinetic energy of the ablated species,the average growth rate, or the average atomic mobilityon the surface.

Additionally, there are a number of practical advantagesof PLD, including “congruent transfer” from the target (un-der some circumstances [7]); layer-by-layer control by usingmultiple targets sequentially; the ability to ablate virtuallyany target; and the ability to deposit in reactive atmospheresfor doping, alloying, or compound formation.

The development of PLD technology began at about thesame time as that of MBE. However, although MBE hasmoved into production facilities, the PLD process remainedlargely in the laboratory. The reasons for this are tied tothe historical development of PLD [8]. The deposition ofinorganic materials by PLD began to be studied shortly af-ter the development of the pulsed ruby laser, and, by 1970,a full complement of III–V and II–VI semiconductor thinfilms had been grown. Although these films were uniformand had the same composition as the target material, theywere grown on glass and quartz substrates, thus were poly-crystalline and not suited for semiconductor devices. Duringthe same time period, the use of CW lasers (e.g., CO2) as a

heating source was also investigated, but it was found thatevaporation of multicomponent targets using the CW laserwas not congruent. The flexibility of the PLD technique wasdemonstrated as early as 1976 when multiple targets wereused to grow superlattices; however, once again the choiceof substrates was less than ideal, and polycrystalline andamorphous films resulted. During the early 1980s the de-velopment of pulsed UV excimer lasers had progressed tothe point that short pulsed (few tens of ns), high power (tensof MW) lasers became available commercially. During themid 1980s these lasers were “married” with the depositiontechnologies developed for MBE and CVD growth, with theresult being that PLD emerged as an alternative depositionprocess. PLD became popular when it was very successfullyused to grow thin, stoichiometric, epitaxial films of the hightemperature (HTc) superconducting oxides. Today, PLD isused widely for the deposition of HTc films, as well as forthe growth of dielectrics, ferroelectrics, and other materialswith complex composition [9].

There has been much valuable PLD research centered onquestions such as “what new material can be grown?” and“what processing conditions optimize the properties of thegrown film?”. The primary focus of such research clearlyhas been centered on the development of the PLD technol-ogy (e.g., elimination of particulates in deposition [10, 11]),to the extent that research on the fundamental issues offilm growth in PLD has remained relatively underdeveloped,especially experimentally. For example, consider item #1above. Simulation has been an important tool to isolate theeffects of deposition and relaxation [12, 13], but experimen-tal research involving modulating the deposition rate or tem-perature in MBE [14–20] is difficult. PLD readily permits usto study these two processes independently.

We have been studying two prototypical growth modes:metal-on-insulator island growth and semiconductor ho-moepitaxy. The particular emphasis has been the develop-ment of a fundamental understanding of the phenomenol-ogy and mechanisms underlying growth morphology evo-lution. Because we emphasize the phenomena, the most ef-fective progress is made in well-studied elemental materi-als with simple crystal structures—such as Ag and Ge—for which the “baseline” for new phenomena is well under-stood, including the morphology evolution in thermal depo-sition and the values of important materials parameters. Forboth growth modes, our efforts have been aimed at answer-ing the question, to what extent can the ‘MBE paradigm’ ofadatoms, islands, steps, and terraces be utilized to accountfor the growth morphologies observed in PLD? For eachgrowth mode, by comparing morphology evolution for PLDand thermal deposition in the same dual-use chamber underidentical thermal, background, and surface preparation con-ditions, we have isolated the effect of kinetic energy fromthat of flux pulsing in determining the differences between

Film growth mechanisms in pulsed laser deposition 581

morphology evolution in PLD and MBE/PVD. We have thenbeen able to adapt thermal deposition models based on theMBE paradigm by adding kinetic energy effects.

2 Semiconductor homoepitaxy

There have been reports of improved epitaxial growth char-acteristics [21] in PLD compared to MBE. Defect reductionfor Si has been attributed to improved layer-by-layer growthin PLD [22]. Fe and Co on Cu exhibit improved magneticproperties [23–28] when deposited by PLD. In the case ofFe on Cu(111), heteroepitaxial growth by PLD has been di-rectly observed to result in improved layer-by-layer growthover thermal deposition [23]. These studies were limited tothe first few monolayers of deposition and focused primar-ily on magnetic properties rather than on growth mecha-nisms. Kinetic Monte Carlo (KMC) simulations of pulsedflux [29] and pulsed flux with candidate energetic mecha-nisms [30] indicate that—under the conditions examined—pulsing a thermal flux is likely to lead to increased roughen-ing, and energetic mechanisms are necessary to obtain en-hanced smoothening except under conditions unlikely to beattainable experimentally.

As shown in Fig. 1, in semiconductor homoepitaxy forboth PLD and MBE the morphology evolution is charac-terized by increasing surface roughness as growth moundsdevelop with a well-characterized lateral separation, fol-lowed by the evolution of a pyramidal mound morphology,followed by epitaxial breakdown and the formation of anamorphous phase [31]. In Ge homoepitaxy we found thatfor PLD kinetic energies up to about 300 eV, the morphol-ogy in PLD and MBE goes through the same qualitativestages [32]. We quantitatively compared growth morphol-ogy and epitaxial breakdown in (a) PLD with peak kineticenergy ∼300 eV (PLD–KE); (b) PLD with suppressed ki-netic energy comparable to the thermal evaporation energy(PLD–TH); and (c) MBE. As is shown in Fig. 2, the thick-nesses at which epitaxial breakdown occurs are ranked inthe order PLD–KE > MBE > PLD–TH; also, the surface issmoother in PLD–KE than in MBE [31]. In fact, we foundno limit to the epitaxial thickness in PLD–KE. We foundthat the early occurrence of epitaxial breakdown in PLD–TH is consistent with the kinetics of MBE in pulses but withan instantaneous deposition rate accelerated by a factor of500, and separated by an inter-pulse period of negligible re-laxation. These and other results demonstrate that the en-hancement of epitaxial growth—the reduction in roughnessand the delay of epitaxial breakdown—are due to the highkinetic energy of depositing species in PLD.

We used quantitative Reflection High Energy ElectronDiffraction intensity oscillations to study the first few mono-layers of deposition, delineated the necessary conditions for

Fig. 1 AFM images of growth morphologies in Ge homoepitaxy.Left column: MBE; right column PLD with peak ion kinetic energy∼300 eV. Both cases are characterized by the development of rough-ness (a) and (d), growth mounds (b) and (e), and pyramidal shapes(c) and (f), before epitaxial breakdown and transition to amorphousphase (not shown). Scan edge length is 0.5 µm; vertical scale is 10 nm.Film thickness is shown in the right bottom corner of each image.Adapted from [32]

avoiding interference from Kikuchi features [33], and devel-oped a new diffraction model for the intensity during multi-monolayer deposition [34] that permitted us to determine theheight of the Ehrlich–Schwoebel step-edge attachment bar-rier which causes the growth instability leading to moundsin the topography [35].

The homoepitaxial breakdown mechanism of Ge (001)MBE [36] is that roughening during growth leads to suf-ficiently high slopes that eventually {111} stacking faultsreadily form, and their accumulation leads eventually togrowth of an amorphous phase. We can explain what weobserve in our comparison of PLD and MBE by startingwith this mechanism and adding energetic mechanisms inthe spirit of those invoked to explain enhanced smoothen-ing in sputter deposition [37]: the kinetic energy in the de-

582 M.J. Aziz

Fig. 2 RMS roughness vs. Ge film thickness and thickness of epitax-ial breakdown in Ge homoepitaxy by MBE, PLD with peak ion ki-netic energy ∼300 eV (PLD–KE), and PLD with thermal kinetic en-ergy (PLD–TH). From [31]

positing species facilitates the filling of the gap between thegrowth mounds. This occurs possibly by downhill momen-tum transfer [37], possibly by creating more mobile surfacespecies, and possibly by small island breakup [30] or tran-sient enhanced mobility [30, 38]—we have not been able todistinguish between these. More efficient gap filling delaysthe time at which the growth surface becomes sufficientlymisoriented that {111} stacking faults can form, thereby de-laying epitaxial breakdown.

3 Metal-on-insulator island growth

The Volmer–Weber growth mode of isolated 3D islands onan otherwise bare substrate is common in the depositionof dissimilar materials [39]. As shown in Fig. 3, in metal-on-insulator film growth the morphology evolution is char-acterized by a transition from isolated, equiaxed islands toelongated islands to multiply-connected non-percolating is-lands to a percolating metal film to the filling in of holes.Although the delay by kinetic processes of the morphologyevolution toward a uniform, pinhole-free film has often beenviewed as a nuisance, recent discoveries of surface plasmon-enhanced phenomena present opportunities for the exploita-tion of nanoparticulate and nanoporous metal films [40, 41].

The morphological progression is consistent with the fol-lowing picture. As isolated islands grow larger with furtherdeposition, they impinge upon each other and begin to co-alesce, driven by capillary forces toward a more equiaxedequilibrium shape—a process that delays the developmentof a contiguous film. The kinetics of this process have been

Fig. 3 Metal-on-insulator Volmer–Weber growth mode, illustratingtransition from equiaxed islands (top row) to extended, non-percolatingislands (middle row) to a percolating metal film with holes filling in(bottom row). Ag on mica; scan edge length is 5 µm. Inset is averagefilm thickness. Adapted from [49]

addressed for continuous deposition [42–44]. The time re-quired for coalescence increases with increasing island size,varying as the fourth power of island radius for surface dif-fusion mediated coalescence driven by classical capillar-ity [45]. For a given cluster of two or more coalescing is-lands, there is an island size above which the time requiredfor coalescence exceeds the average time interval before anadditional island impinges with one of the constituent is-lands in the cluster. It is beyond this point that clusters of co-alescing islands remain elongated on the surface: they haveundergone a kinetic freezing transition [42]. Further deposi-tion joins these elongated clusters, forming a tortuous net-work of island chains that eventually conducts electrically(the “percolation transition”). With further deposition, theintervening bare channels continue to fill in until no pinholesremain [46].

Film growth mechanisms in pulsed laser deposition 583

Fig. 4 (a) AFM images ofAg/mica morphology evolutionfor three different laser pulserepetition rates at constant laserfluence. Film thickness in nm isindicated in corner of eachimage. Films deposited at higherpulse rate advance through theprogression with lowertransition thickness. Scan edgelength is 3 µm. Bottom:Simulated film morphologies at0.025 ML/pulse; snapshots withhemispherical islands justreaching the elongationtransition: (b) 100 pulses,100 Hz; (c) 176 pulses, 10 Hz.Smaller islands reach elongationtransition at lower average filmthickness. Adapted from [51]

Pulsing of the deposition flux to manipulate island nu-cleation and growth has been investigated theoretically byJensen and coworkers [47, 48], who focused on the islandsize distribution prior to significant impingement, but nev-ertheless identified three broadly applicable growth regimeswhen the lifetime of an adatom on the substrate surface is:much shorter than the pulse duration; in between the pulseduration and the pulse period; and much longer than thepulse period. They found different scaling behavior for theisland density vs. pulse frequency in these three regimes. Wefind some of the same scaling behavior in our experiments,

despite the importance of impingement and coalescence inour experimental morphologies [49].

Experimentally, island and film morphologies were ob-served by ex situ Atomic Force Microscopy (AFM) af-ter growth and quenching to room temperature; addition-ally the percolation transition was monitored in situ bytime-resolved lateral electrical conductance measurements[49, 50].

We developed KMC simulations of island nucleation,growth, impingement and coalescence during flux pulsingthat neglected any effect of kinetic energy [49]. The ruleswere:

584 M.J. Aziz

Fig. 5 Film thickness atmorphology transition vs. pulsefrequency for constant amountdeposited per pulse. Top row:high temperature; middle row:intermediate temperature;bottom row: low temperature.From [49]. Experimentsmeasure percolation transitionwhereas simulations trackelongation transition.Simulations in (f) show samepower laws for two differentvalues of coalescence rateconstant

1. Irreversible adatom aggregation into islands, which areconstrained to be hemispherical;

2. Coalescence of island pairs: upon impingement, two is-lands are held for an interval proportional to the fourthpower of island radius, and then instantaneously fusedinto a single hemispherical island with the same total vol-ume.

The average number of islands per cluster of coalescing is-lands was monitored and found to increase exponentiallywith time over the period covered by the simulations. Whenthe average number of islands per coalescing cluster tra-versed 2.0, the film morphology was declared “elongated”with the implication that the percolation transition would oc-cur later by a fixed time factor.

The experimental morphology evolution for variouspulse repetition rates is reported in Fig. 4, and the measuredpercolation transition in the left-hand column of Fig. 5. Thesimulations reach only the elongation transition, and thoseresults are reported in the right-hand column of Fig. 5. Themeasurements of the percolation transition, the simulationsof the elongation transition, and analytical scaling argu-ments are all consistent with the transition scaling as pulsefrequency to the −1/3 power at high temperature (corre-

sponding to the “fast substrate diffusion” regime of Jensenand coworkers). At sufficiently low temperature, both exper-imentally and in the simulations, the scaling behavior showsdifferent power laws over different frequency regimes. Wehave found some correspondence but we have not found aone-to-one correspondence between the regimes observed inexperiment and simulation [49]. It is possible that at suffi-ciently low temperature, surface faceting and the associatedsingular surface energetics and kinetics invalidates the clas-sical coalescence kinetics on which the simulation model isbased.

These simulations were also used to compare pulsedand continuously deposited films, both with negligible ki-netic energy. In this case, the KMC simulations predict thatPLD films should advance to percolation with less depo-sition (i.e., they should “percolate faster”) than thermallydeposited films at the same average deposition rate. Thisoccurs because the higher instantaneous deposition rate inPLD creates a higher density of nucleated small islands, andsmaller diameter islands percolate at a smaller average filmthickness. At low substrate temperatures, the prediction of

Film growth mechanisms in pulsed laser deposition 585

Fig. 6 Film thickness at electrical percolation vs. temperature for PLD(open symbols) and thermal deposition (filled circles; same data shownin both panels for comparison). Average deposition flux was 0.06 nm/s.Peak ion kinetic energy in PLD was 55 eV in top panel and 110 eV inbottom panel. Adapted from [50]

faster percolation in PLD is confirmed experimentally, asshown in Fig. 6. However, in situ resistance measurementsand ex situ Atomic Force Microscopy topographs demon-strate that at high substrate temperatures, PLD films requiremore deposition to reach percolation (i.e., they “percolatemore slowly”) [50]. PLD experiments performed at vary-ing kinetic energy of the depositing Ag species suggest aregime in which increasing kinetic energy can delay the per-colation transition. Comparison was made with KMC sim-ulations (Fig. 7) of unconstrained two-island coalescence inthe presence of adatom–vacancy pair creation, which occurswith a greater-than-unity yield per incident ion at kinetic

Fig. 7 KMC simulation of evolution with time of coalescingtwo-island system in absence of deposition. Left column: purelythermal processes; right column superposes energetic mechanism ofadatom–vacancy pair creation. Energetic mechanism induces islandsto remain taller, and taller islands coalesce more slowly. Plot showschange in aspect ratio of island pair vs. time as two islands equilibratewith several different adatom–vacancy pair creating rates. From [50]

energy >50 eV. A surprising mechanism controlling thedelayed percolation of PLD films in the high-temperatureregime emerged: (1) the energetic deposition results in a netuphill atom flux from adatom–vacancy pair creation, induc-ing a vertical shape change; (2) taller-than-equilibrium is-lands coalesce more rapidly; (3) the result is an extendedtime period over which coalescence is efficient compared toisland–island impingement; (4) the percolation transition isdelayed.

586 M.J. Aziz

4 Summary

Comparing morphology evolution for PLD and thermal de-position in the same dual-use chamber under identical ther-mal, background, and surface preparation conditions, otherthan the differing nature of the deposition flux, has been es-sential to tease out some of the finer distinctions betweengrowth morphology evolution in PLD and thermal deposi-tion.

In semiconductor homoepitaxy, we found that the MBEpicture of roughening, mounding, pyramids, and extendeddefect accumulation to epitaxial breakdown also explainsour observations of PLD for kinetic energies up to about300 eV, and that kinetic energy effects promote smootheningand permit epitaxial growth to greater thicknesses withoutepitaxial breakdown.

In metal-on-insulator film growth, we found that the samemorphology progression occurred as in thermal deposition:equiaxed islands, elongated islands, percolating metal film,hole filling. The kinetic freezing model, involving the com-petition between island–island coalescence and deposition-driven island–island impingement, explains the morpholog-ical transitions in both thermal deposition and PLD. KMCsimulations based on the kinetic freezing model, with is-lands constrained to be hemispherical, predict that the rateof progression through the transition is higher with higherpulse repetition rate, which is consistent with the experi-ments. But in comparing PLD with steady state thermal de-position, the simulations predict a more rapid advancementthrough the progression in PLD, which is contrary to exper-iment at high temperature. For low temperatures, the highisland density of PLD dominates the morphology evolution,and PLD films reach percolation sooner than thermally de-posited films. As the temperature is increased, the PLD per-colation thickness approaches and then exceeds the thermalpercolation thickness, indicating the increasing importanceof an energetic effect. This effect appears to be kinetic en-ergy induced adatom–vacancy pair creation, which has thenet effect of moving atoms upward, resulting in a verticalshape transition of the islands. Taller islands coalesce morerapidly, thereby delaying the point of the elongation transi-tion, where coalescence is overwhelmed by impingement.

Acknowledgements This research was supported initially by DOEgrant DE-FG02-01ER45947 and subsequently by NSF grant DMR-0306997. The results reviewed here are the culmination of the com-bined efforts of Byungha Shin, Jeff Warrender, John Leonard, CraigArnold, Jonah Erlebacher, and Jim McCamy in my Rlaboratory, and Iconsider myself fortunate to have worked with them.

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