T he quality o f the film grow n is extrem ely sensitive to sample preparation and the laser param eters w hich lead to variations in m icrostructure, texture and com position o f the film.
2.3.1. Choice o f background pressure
A lm ost all film s w ere grow n using PLD with a background gas pressure. Briefly, the use o f an am bient gas during pulsed laser deposition can be characterised as either passive or active. The passive use o f an am bient gas is m ainly to com pensate for some loss o f a constituent elem ent such as oxygen in m etallic oxides. For exam ple, the deposited L a-C a-M n-O oxides in this w ork tend to be deficient in oxygen w hen the ablation experim ent is perform ed in vacuum . In situ processing o f these m anganite oxides typically requires 0.2 -1 m bar o f background oxygen in the cham ber during deposition. W hen the gas is adm itted in-situ during PLD , how ever, the typical p articu late size changes as the am bient gas pressure varies. In fact, trem endous potential could be derived from the active use o f an am bient gas during PLD, in which either inert or reactive gas can be introduced deliberately to form particulate w ith a desired size or com position. For exam ple, by incorporating an am bient gas during laser deposition, ultrafine particles can be fabricated with particle diam eters ranging from a few nanom eters to a few tens o f nanometers.
T he origins o f the form ation o f particulate and the m echanism s w hereby a specific elem ent is enriched in the particulate are clearly different for PLD processes in vacuum versus those in an inert am bient gas. The effect of ambient gas pressure on the nature of particulate is m ost likely related to the increased collisions betw een the ejected species and the am bient gas as the pressure increases. The mean free path o f ejected species decreases w ith increasing gas pressure. W hen laser ablation is perform ed in ultra-high vacuum , there are virtually no collisions betw een the ejected species before they reach the nearby substrate. Therefore, particulates are predom inantly form ed from solidified liquid droplets that are expelled from the target by the recoil pressure. A t the same time, the vapour species are deposited as a uniform background film. W hen the am bient gas pressure increases, how ever, the vapour species can undergo enough collisions that nucléation and growth o f these vapour species to form larger size can occur before their arrival at the substrate. The fact that the size increases as the am bient gas pressure increases strongly suggests that the ultrafine particulate are form ed from the vapour
C hapter Two G rowth and D eposition o f M agnetic Thin Films
species instead o f liquid droplets. Since the grow th m echanism is con trolled by diffusion, the residence tim e o f the particulate in the vapour controls the size o f the particulate. The longer the residence tim e, as is the case w ith increased am bient gas pressure, the larger the particulate [2.22].
2.3.2. Effects of the substrate tem perature
The substrate tem perature is an im portant param eter for optim ising the film grow th, influencing nucléation processes and the m obility o f the atoms across the substrate. A decrease o f substrate tem perature has the additional effect o f decreasing the surface diffusion coefficient o f the absorbed vap ou r atom s. In the film -d ep ositio n case, how ever, film atom s continue to be added to the system until som e type o f film nucléation occurs. Thus film form ation is not slow ed at low tem perature. A decreased tem perature can, how ever, slow the form ation o f an equilibrium com pound or crystal structure, so that a m etastable m icrostructure is produced. A com m on exam ple o f this phenom enon is the change o f a single perovskite phase to an im purity perovskite at low substrate temperatures, as discussed in chapter four o f this thesis.
2.3.3. Effects of target-substrate distance
The effect o f target-to-substrate distance is m ainly reflected in the angular spread of the ejected flux. In general, the particulate trajectories are more divergent when a defocused laser beam is used, as opposed to em erging as a collim ated je t for a tightly focused beam. H owever, when PLD is perform ed in a poor vacuum with an am bient gas, or at a substantially large target-to-substrate distance in w hich coalescence o f particulate can take place, m arkedly different particulate characteristics m ay occur, depending on the position o f the substrate.
The specific effects of target-to-substrate distance and am bient pressure are interrelated. D ue to the increased collisions betw een the laser-produced plum e and the background gas, the plum e dim ension decreases as the background gas pressure increases. W hen the target-to-substrate distance is m uch sm aller than the length o f the visible plum e L, there is no m arked difference in particulate size and the density. As the target-to- substrate distance increases, the proportion of sm aller particulate decreases, and a few larger particulate appear. Once the substrate is located far beyond L, the adhesion to the substrate o f the ejected matter, including the particulate and atomic species, is poor.
Chapter Two Growth and D eposition o f M agnetic Thin Films
2.3.4. The effects of laser wavelength
The laser wavelength X com es into play prim arily in the effectiveness of the absorption of the laser power in to the target [2.18]. The variation of absorption coefficient with w avelength is more com plex since various absorption m echanism s, such as lattice vibration, free carrier absorption, im purity centres, or bandgap transition, can take place. For perovskite-type m etallic oxides used in this w ork, the penetration depth appears to be larger in the near IR than in the U V, som ew hat reverse to that o f the metals.
Figure 2.3 shows an exam ple of the laser w avelength effect on YBCO films deposited at 1054, 532, and 355 nm and examined by Scanning Electron M icroscopy (SEM). The
Fig.2.3. SEM photom icrographs o f YBCO film s deposited by PLD using different laser wavelengths:(a) 1064 nm; (b) 532 nm; [c] 355 nm;
(d) the window in (c), m agnified by a fa c to r o f 10 [2.2].
films deposited using 1064 nm radiation were relatively thick in contrast the films deposited using visible or UV radiation, indicating that more material was ablated with an infrared (IR) laser pulse. The films deposited with IR radiation were extremely
C hapter Two G rowth an d D eposition o f M agnetic Thin Films
rough, (b) appeared sm oother and the m ean size o f the surface features decreased as the laser wavelength decreased. A further reduction in the particulate size and the density was also observed w hen com paring to the film s deposited using 248 and 193 nm laser respectively. The observed results can be prim arily interpreted as the effect o f laser- target interaction, i.e., the laser w avelength affects the nature o f vapour species. As a result o f a change in the photo penetration depth o f the laser (w hich decreases with decreasing wavelength) and an alteration in the interaction o f the radiation field with the ejected m aterial (the photofragm ents have a larger absorption cross section at shorter wavelength) the plasm a flux com position varies from large clusters and particles (for IR radiation) to atoms (for U V excitation). These results confirm the hypothesis generated from the film m orphology studies. The increased photofragm entation by the U V laser results in a sm oother morphology.
2.3.5. Effects o f laser fluence
For a chosen m aterial and a fixed w avelength, the laser fluence on the target has the m ost significant effect on the particulate size and density. The laser pow er density is controlled by varying the pulse energy and the size o f the laser spot on the target. In general, there exists a threshold laser fluence, below w hich the particulate are barely observable. Above the threshold laser fluence, the particulate num ber density increases rapidly with increasing fluence. H ow ever, the rate will reduce at a higher fluence and find towards saturation. The saturation of the particulate deposition and film deposition, in general, is largely due to the saturation in the ablation process. For exam ple, plasm a shielding o f the target is one o f the m echanism s that reduces the ablation rate, and is more often encountered in laser-ablation deposition using longer wavelengths.
2.3.6. Effects of laser repetition rate
The effect o f laser repetition rate mainly influences the deposition rate and the structure o f the film. The surface morphology and pinhole density o f the Y BCO film deposited at a fixed laser w avelength and fluence and different repetition rates from 1 Hz to 100 Hz was exam ined by SEM [2.2]. Pinholes w ere observed in the film s deposited at a low repetition rate, and surface outgrow ths o f a-axis-oriented grains w ere seen in film deposited at high repetition rate. The surface roughness o f the deposited film s also is determined by an optimum repetition rate.
C hapter Two G rowth an d D eposition o f M agnetic Thin Films
2.3.7. Effects o f ion bom bardm ent during film growth
Low energy ion irradiation (<100 eV) during crystal film growth can be used to provide new chem ical reaction pathw ay, modify film -grow th kinetics and controllably alter the physical properties o f film s deposited by a variety o f techniques. The effects o f ion bom bardm ent during film grow th, w hile often being o f great benefit, are com plex. Possible effects include collisionally induced dissociative chem isorption, preferential sputtering, trapping, enhanced diffusion, stim ulated desorption, collisional m ixing, recoil implantation and alteration in segregation behaviour.