 In this module on thin film fabrication in the last lecture we looked at one of the most sophisticated approach to make thin films that too to confine in monolayer thicknesses that is in atomic scale which we call it as molecular beam epitaxy. Molecular beam epitaxy is the one of the most refined thin film fabrication unit that is available both for the industry as well as for basic research. As we saw in the last lecture the conditions that are used in MBE is not something that we can use practically this research conditions because we work with very ultra high value but normally for us to evaluate any system and to look for new possibilities, new avenues it is very difficult to maintain a MBE chamber and to conduct a research and specially in our country. So, the best way to look for opportunities in research is to go for much more faster and more friendly techniques one of the technique is pulse laser deposition it is actually abbreviated or people popularly refer to as PLD technique. PLD technique is one of the most used throughout the world for making thin films of any material that is of interest in this talk I am going to tell you about what this PLD is all about and as you would see it is interaction of laser with matter and that will actually ablate a material and will help us in making films with proper substrates. So, in terms of a definition pulse laser deposition is a thin film deposition specifically a physical vapor deposition because there is not much chemistry mode and it is also called a PVD because a complementary chemical technique called CVD is there chemical vapor so this is a complementary technique to CVD technique where a high power pulse laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. So this is a high vacuum chamber usually for normal practices we use 10 power minus 7 but you can also use 10 power minus 11 that is ultra high vacuum chambers. So in this whole episode of deposition using pulse laser the ejected species they expand into the surrounding vacuum and in the in the form of a plume which we call it as laser plume and this will actually bring about energetic species which carries atoms molecules fragmented molecule molecular species electrons ions clusters all this are they come in essence and reach the surface of the substrate and at the substrate there is another reaction that is going on which will decide whether you are getting the required film or not. So, this is the simple dynamics of a pulse laser deposition we will see some animation of it in the next few slides. Just to tell you what a how simple this could be you shine a laser and through optics you try to converge it usually this laser is incident of 45 degrees to a rotating target this target is actually keeps rotating and as it rotates laser comes and strikes and this is kept perpendicular the substrate is kept perpendicular to the rotating target. So, whatever that is removed from the surface of the target is now going to come and reach the surface of the substrate. So, once the substrate is kept at high temperature then you can actually make that decide film whatever you want. So, this is the simple protocol what it involves is a vacuum chamber and some optics and then whatever is happening inside is to do with the vacuum chamber. So, it is a very simple protocol compared to molecular beam epitaxy in terms of achieving vacuum and in terms of handling the instrument also. The removal of atoms from the bulk material is done by vaporization of the bulk at the surface region in a state of non equilibrium and is caused by a coulomb explosion. So, we will look into some of the requirements of what really happens at this plume and target surface. So, what exactly comes out in the form of ions and electrons and molecular species as a laser plume is nothing, but a coulomb explosion. In this the incident laser pulse actually penetrates into the surface of the material within the penetration depth. So, when it is the interaction of a laser plume or a optical beam with the solid material therefore, how much of this light or the photon can enter into the solid determines what will come out. So, if it does not penetrate high enough then you would hardly see anything coming out, but if the penetration depth is quite deep then you will get almost all the species that is required out from the surface of the target. So, this dimension is actually dependent the dimension meaning the penetration depth is dependent on the laser wavelength and the index of refraction of the target material at the applied laser wavelength. So, in this caution to be taken that it should be a material which will absorb your laser plume if it does not absorb your laser light then it will not emit. Therefore, your optical coefficient of your material is important in essence you cannot ablate a insulator that easily as much as you ablate a metallic material. So, this becomes a inherent problem especially because the penetration of the laser light is the governing factor here. So, therefore, it puts some quantum restrictions on to the sort of materials that you can use with these. So, it is typically in the region of 10 nanometer for most materials meaning the penetration depth. So, if you can achieve this much then you can actually bring about the surface of the material in the form of a plume. Typically, this is our laser PLD will look like this is the vacuum chamber as you see here and at the rear side you see excimer laser that is housed and in between you see this glass windows mainly to shield from the radiation because what you are pumping is actually a UV radiation from excited krypton fluoride and this is actually giving a UV radiation of 248 nanometer wavelength because you are using ultraviolet radiation it is important that you shield it. Therefore, this is not a very friendly one and care should be taken because you would not see this UV radiation coming out of the laser. Therefore, out of curiosity one should not even peek into the pathway because that can easily burn your eye and the person can even go blind if he takes one shot of this UV light because it is highly monochromatic and high power laser. Therefore, it is always important to have some protocol where this radiation effects are minimized so that the handling becomes easy. So, this is typically a PLD setup in any PLD lab you would see the vacuum system then the optical arrangement bringing the laser laser light into the vacuum chamber and then of course the laser. Typically, when you are trying to do reaction what you would see during the PLD deposition is some sort of a flash like this which means the plume is actually coming with very high intense colors and the color can actually tell you whether you are really depositing the right material and the right conditions are not. So, it can be as friendly as it would be if you get used to the laser plume. So, typically during the process you would see some sort of flashes like this that is how it goes. Now, the next question is why we need to take PLD so seriously when you have a more refined setup like MBE, but we should also know that research is not just limited to the efficiency, but also to the economic considerations. Any lab can easily afford vacuum deposition chambers where you can make moly crystalline and amorphous materials. If you look at the cost, this is actually in Hong Kong dollars, but nevertheless it just gives you how simple a vacuum system can be which involves thermal evaporation. Whereas, if you go for sputtering it is almost four times costlier than a thermal evaporation technique, but again you can play around with polycrystalline films. The next to transcend is laser PLD. Laser PLD is almost of the same sputtering cost, but then you can actually play around with single and multiphase compounds or tin films, but there is a difference here beyond this whatever you see those are very very costly equipments. CVD for example, chemical vapor deposition and molecular epitaxy they are of a higher value, but they usually guarantee epitaxial growths meaning the film will be always single crystalline. You will not see any polycrystalline faces therefore, to get a single or a multiphase product you need to choose what setup deposition technique that you would look for. For depositions involving very high areas large areas usually we go for CVD that is one of the reason why chemical vapor deposition method still stands as a special case amidst all the other physical vapor deposition techniques. So, what really brings the divide is not just the cost it is also the sort of final product that you are looking for either it is a semi epitaxial growth or a epitaxial growth. So, epitaxy is the angle line word therefore, certainly this is different from MB. As I told you in a MBE chamber in the last lecture MB chamber is usually tagged to other characterization techniques like read magnet optical care effect lead and atomic energy spectroscopy and also to a STM, but PLD instrument is not necessarily interface with all these other techniques. In the sense you can have the luxury of breaking the vacuum and taking the material out for characterization. So, that way it is more friendly you can play around with a variety of materials, but in MB you never get to break the vacuum. So, it is always inside the vacuum chamber therefore, you need to circulate your sample from the processing chamber to characterization chambers again you clean the crystal again you go into this. So, it is a loop, but in PLD you can actually after this you can come out and do excite to measurement in a AFM or a STM chamber you can do a excite to measurement on a lead chamber. So, this gives a flexibility to play around with different systems. One of the pioneers in this field is a professor Venkatesan who actually started the application of oxide thin films using PLD especially he was the first one to demonstrate how a high temperature superconductor like yttrium barium copper can be made as a thin film for device applications using PLD as a convenient route. Therefore, I just wanted to show this setup and this is a good article that has come out in APS magazine. So, pulse laser deposition offers a very fast route to prototyping any thin film coating. So, if you want a quick study on any complex material the best thing to adopt is PLD. Just to give you a clue why you can be versatile is that this is a bismuth strontium calcium cuprate and this is a material which shows high temperature superconductor above 130 Kelvin. And if you look at the crystal structure you would see the copper oxygen sheets and then the barium bismuth and strontium oxide layers which are sandwiched by copper oxygen planes. So, this is the one single unit cell of this bismuth copper oxide cuprate. This is one unit cell with so many layers and it can be effectively fabricated using pulse laser deposition. So, a complex unit cell, but PLD will exactly bring about the same arrangement. So, therefore, PLD has lot of attraction. Other commercial instruments like PVD is one company which brings about and this is one of the PLD chamber that they market. You can see the complexity of this and typically the target the target of the material which you would like to deposit as a thin film is mounted on a sample holder like this. What you see here is nothing but a used target of a particular material which has gone through several deposition. So, you would see the influence of the laser beam interacting with the material and several times it has been used for operation. But not only that you can also use multi target holder. So, if you want to make a complex material like what I showed in the previous slide, bismuth, strontium copper oxide, you can even start with individual oxides and try to judiciously make such a layered copper and that is why you have different target holders of with different fashions. So, you can actually mount more than 6 or 7 at a time and with computer monitoring you can switch between these targets and try to make complex oxides. All these are possible in a PLD chamber. So, the inside part of the chamber essentially will look like this, you can see here these are the rotating wheels depending on the choice material if you can keep switching between two targets and therefore, this is how the deposition chamber will look like. This is another example of how PLD can be made can be used for a superconductor and you can see here several stockings of this superconductors can be made. This is Yipco which is coated and then capped with the Siria, then Presulium cuprate and then LA Elvorty substrate. So, this is a device configuration. All this can be achieved with the same chamber with different target carousel. So, if you have 6 or 7 targets mounted you can vary such a complicated device stacking. So, in terms of versatility of PLD they have non-volatile targets and multi component targets are possible and multi targets for multi layer or alloy films we can try to achieve using PLD and this is also operated under any ambient gas. For example, you can use argon, you can use oxygen, you can use nitrogen, you can use any other helium for that matter any gas can be used depending on the sort of system that you are going to use. And another thing with the number of pulses that you are generating you can try to control the thickness of the tin film and we can also control the substrate temperature. For example, if you want to prepare polymer films then you can prepare at room temperature, if you want oxide films you can prepare at 800 degree C. So, you have flexibility to play around with the substrate temperature and the position rate can also be fine tuned. Disadvantages are if you are trying to transcend this to larger levels for large area depositions it is not possible. So, even to make a 6 inch wave it is very difficult, but people are aiming at such large area depositions. Because of the energetics that is involved in this reaction usually you end up with some sort of particulates sticking meaning some chunks of material can also be thrown out along with a very good layer growth. Therefore, this can become a problem for this off axis PLD has been suggested and target surface modification can happen especially when your laser is hitting the target with such a very high energy there may be congruent melting at the surface region. Therefore, if your material is not sensitive or if it is very sensitive then it can go into a liquidous space and it can change the property. So, such problems are inherent when using this some applications we can make ceramic films when I say ceramic films it is not about clay and pottery we mean the superconductor as a ceramic we can make piezoelectric as a ceramic nitrates as ceramic. So, several electronic materials termed as ceramics can be made hard coatings like diamond like films carbides nitrates can be made alloys and multi component systems can be made including manganades for magnetic application multi layers can be made super lattices or hetero junctions can be made using this and graded layers can be also be achieved by this way you are almost covering entire spectrum of compounds whatever you want you can actually translate this into a thin film. So, that way it is a very friendly technique one can look at coming to the energetics and what exactly happens inside the vacuum chamber there are three regimes that we need to take into consideration and each one is a dynamic in itself. Therefore, to handle this dynamics is the proficiency that is required. So, if we start as a beginner we need to understand that we should become more friendly with all these three regimes and one should have a feel for what this regime one will do on regime two and then will affect on regime three. So, a comprehensive understanding of what is this dynamics is what is needed to exploit this deposition process regime one is laser target interaction. So, laser is coming interacting with the target what are all the things that we need to know regime two gas phase transformation. So, this is nothing, but the plume dynamics. So, what really happens with the plume what happens suppose the plume is going like this what will happen if the plume is actually narrow down like this like a bullet. So, the way the shape of the plume and the color of the plume also will control the fine stociometry regime three is deposition and film growth now something is coming from the plume and I am depositing it what should be the control how I can get the best film that I want. So, all these three are important in the PLD process specially when we talk about laser target interaction there are two things that we need to understand the thermal process and the non-thermal process. Thermal process where you have the energy of your laser light that is EH nu the energy is less than the binding energy of your substrate material. For example, you are actually trying to break the bond and bring out the solid material in form of ion molecule or atom or as electron. So, so many things are actually going to come out in the form of a plume and when it is happening we need to know whether the EH nu that is the laser light is sufficient enough to overcome the binding energy. If it is not then if it is less than EB then there is thermal equilibrium and there will be a maximum velocity distribution and in that case the whole growth process can be very different. The other form is non-thermal process in non-thermal process you actually have two situations one is a electronic process and another one is a bond breaking process. Usually in the material science we give more attention to bond breaking process because you know the material and you know whether it is a ceramic whether it is a polymer or whether it is a metallic material. So, you know how much of the binding energy each material is associated with therefore, you can try to transcend that by increasing the laser energy. So, if your laser light is EH nu which is going to be for greater than or equal to EB then you can think of desorption of surface atoms through bond breaking. For polymers and large molecules bond breaking changes the molecular structure resulting in a sudden change in volume or micro explosion and blast wave ejecting surface atoms. So, you need to have some amount of idea about the binding energy of the atoms and molecules in the in the target so that you can essentially maneuver or you can alter the laser light. So, congruent evaporation and subsurface super boiling these are parameters that we need to keep in mind because your laser light essentially will bring about knocking of this material and because of the continuous heating on the target material there will be some amount of melting and boiling that will be happening at the junction. So, we need to know what sort of energy that is coming out when laser light is interacting with the surface. For example, if you use carbon dioxide laser the wavelength actually will generate only 0.1 EB and that is not good enough for updating any of your inorganic material. So, what you do you try to use NDIT, NDIT is another laser material and it actually gives in more than 1000 nanometer wavelength and corresponding to 1.17 EB, but you can actually use filters to generate second, third or fourth harmonic once thereby you can increase the energy of your laser light. So, that way you can go up to 4.66 and this goes into UV. So, when you are using fourth harmonic you are actually getting a UV light from NDIT. So, this is a high power laser when you are operating at 266 nanometer and then you have eximer lasers. Eximer lasers xenon fluoride is well known, but mostly it is a krypton fluoride which is used which is 240 nanometer and then you can also use argon fluoride and fluorine, but fluorine because of the handling hazard it is not usually recommended in the commercial lasers, but argon fluoride and krypton fluoride are used where you can see very high power laser efficiency searching. So, if this is the energy that I get from the laser source then I should also know what is the EB governing this material which I am planning to oblique. For example, if it is a organic material involving carbon-carbon single bond triple bond or double bond that those are actually pretty strong the binding energy or EB is of the order of 8.7 for carbon-carbon triple bond and carbon-carbon double bond it is 6.34 EB. So, to break this bond and bring it out you need sufficient energy. So, you need to have this idea and also you would find out that it is the binding energy is more for ionic materials than covalent then even metallic materials, but those which are governed by van der Waals force and hydrogen bond those can be easily broken. So, from this numbers we can see that depending on the choice of the laser you can actually try to extend the ablation of material from ionic material down to organic depending on the choice of the laser source. So, when you are thinking about a target to substrate there is a gas phase transportation here is your target material like laser has come it has hit now everything is going into a gas phase transfer transportation. So, everything is being moved and typically the translational energies of the order of 10 to 100 electron volt and that is what makes this whole process very rich because the ions and the molecules and the electrons atoms everything is carrying very high kinetic energy as they are rushing to the substrate. So, in this there are two different conditions one is thermal and non-thermal conditions where you can see the gas phase transfer transportation can actually vary quite a bit. Look at the shape of this plume and look at the shape of this plume. So, depending on the shape the energies of this transportation will change remarkably. So, in the thermal case it is actually a cos theta dependent whereas, in the non-thermal case you will see its power factor is associated with it is cos n cos to the power n theta where n is always greater than 1. So, in typically it is of the order of 10 to 25. Therefore, depending on the plume you can actually modify the whole growth process. So, one should have an idea just by looking at it whether your plume is really good or not otherwise you are heating somewhere and nothing is arriving at this substrate. So, you have to have an idea how to maneuver this gas phase transportation. So, once this plume is directed and it is reaching the substrate then the next thing that we need to understand is the nucleation and growth of the film on the substrate surface. The nucleation process and growth kinetics depends on several parameters growth parameters one laser parameter I have touched upon it the laser fluence or the laser energy is very important. Therefore, you need to know what is the optimum laser energy that you need to use in fact as a there is a calibrant which can be used a calibrating instrument which will tell whether you are getting the required laser energy before every defunction because continuously since the coating is done the optics can also take this oblation and it can filter the laser light from reaching the source to the full section. Therefore, it has to be calibrated then surface temperature suppose you are making a high temperature superconductor film the substrate also has to be elevated to a particular temperature for the film to go grow and show a crystalline phase otherwise it will be amorphous phase and it may not show even superconductor conductivity and the substrate surface also is very important we cannot simply mount anything even if you are making a mounting a silicon or any other single crystal pre-treatment has to be made it should be highly uniform and it should be treated properly before the deposition is made and the background pressure as I told you the plume dynamics depends on the pressure. Suppose I am using oxygen whether I use this deposition at 10 millitor or 100 millitor or 400 millitor oxygen pressure that will decide whether it the film is going to be fully stoichiometric or not if you are aiming for a oxygen. So, background pressure is also important. In PLD just like the way I emphasized in the earlier lecture on MBE the growth mode also proceeds similar to molecular beam epitaxy in that it can be a step flow method it can be a layer by layer growth or it can be three-dimensional growth as I told you false laser deposition because of the high kinetic energy that is involved in these plume dynamics usually you get the layer by layer growth which is the most favored deposition growth mode. But 3D growth mode brings about lot of roughness in the film and therefore that is not decided. So, we need to have an idea about how to grow a layer by layer growth the next animation slide you can see we can monitor the film growth using read oscillation and as you would see here every maxima in the read oscillation corresponds to one full coverage of the layer and every minima corresponds to almost half the coverage of the thin film. So, this is what we mean by layer by layer coverage therefore, if you aim for such deposition then you get a flat terrace and that is more desired for making even heterostructures. So, one should always look for a situation where you can actually get this maximas corresponding to coverages typically you would not expect this sort of coverages, but what could happen is sometimes in the initial phase there could be a three-dimensional growth and then you can proceed to a two-dimensional coverage therefore, we need to have some optimization before we try to go for several layer of stackings. So, we need to do a individual calibration if we are going for several multi target systems. So, in such cases some calibration is needed as to evaluate whether a two-dimensional growth is happening. This is another view graph which tells us the distance between the target and the substrate can also play an important role in getting the right type of material. For example, if you allow the laser plume to converge like this and then it hits the substrate then you would see if we adjust this distance between the target and the heater where the substrate is mounted you would see that the microstructure of the films changes systematically and the microstructure of the film can be greatly influenced if you are going to introduce a mask between the plume and the substrate. So, the distance between target and the substrate can also play an important role because in this case you are actually cutting half the plume in this case nearly three-quarter the plume in this case you do not cut it at all in this case you cut it half, but then let only the mask do the job therefore, the microstructure of the film can be manoeuvred by this distance. So, it is very sensitive that way and we should also understand when the plume has reached the substrate again the sputter flux will go back and it will also create a thermalized region in between. So, the incoming plasma will be actually encountering a thermalized region. So, the gas pressure that you use during the deposition will actually try to clear up this thermalized region therefore, care should be taken to get the right type of plume that you desire. In fact, those who are familiar with this art will easily make out whether you are really doing the right deposition for example, the oxide plume for manganate has to be blue whereas, for high temperature superconductor it has to have a crimson red tinge to it and if you are looking for titanium nitrate then it has to have a bluish tinge. So, just by looking at the plume itself one can decide whether you are doing the right deposition with right parameters or not. So, it can become that friendly provided we know how to control the plume dynamics. Now, I will take you through some examples briefly if possible on a greater detail in the subsequent lectures. First let me start with the animation of how various layers can be coated using PLD. You can see here we are actually switching between two targets and this can be done and what you see as a laser light is nothing but the reed incident beam which is going to monitor whether you are hitting it right. So, this reed oscillations as you can see in this cartoon the reed diffraction spots are there and those spots will tell you whether you are really doing a layer by layer oscillation. As a result you can see here this sort of oscillations are coming depending on the way you are trying to coat the material. So, you can make these tackings alternately using reed oscillation and that way you can control and make a periodic deposition of superconductor. In this case this is your S R T I O 3 then you can put this green patch is nothing but your barium cuprate and then you can put etrium cuprate and barium cuprate. You are essentially imitating a unit cell and you can do that by taking a separate target of B A C O 2 and a separate target of etrium C O 2 and keep on making this repeat deposition and this can be monitored using the reed oscillation. Such sophistications are already available in today's commercial PLD instrument. Now, if you look at the range of materials that you can make out of this deposition process you have etrium barium copper these are all the superconductors people have used as early as 1987. So, PLD became very prominent only after the discovery of the superconductor then you have the oxides silica can be made carbides nitrides even carbon and C 60 can be made diamond like carbons can be made polymers like polyethylene PMMA can be made metallic systems alloys multi layers borides can be made out of this. So, if you look at the spectrum almost you can achieve any sort of material as a thin fact. One of the example that I want to show here is L U 2 O 3 which is done in our own group L U 2 O 3 grown on several substrates why L U 2 O 3 because this is a very important molecule for tunable laser materials if you can dope with for example cerium then this can be used for tunable as a tunable laser material or as new phosphorus. So, there is lot of challenging work going on in L U 2 O 3, but the point is to make a single crystalline film it is very very difficult and as you see here using PLD we can make a single crystalline film of lutetium oxide and this is the optical measurement in transmit transmittance mode and in see here you can make a very nice quality L U 2 O 3 film and look at the transmittance level it is nearly 90 percent transmittance which means it is a it is almost a transparent electrode sort of thing. So, you can make such clean material on a variety of substrates for example, here we have used yttrium stabilized zirconium why we use yttrium stabilized zirconium because the lutetium oxide cell constant is exactly twice that of yttrium yttrium stabilized zirconium which is a cubic material. So, in the next slide I will show you how we how good quality film we can make, but it is also surprising that you could see lutetium oxide grown on silicon showing such high laser action. This is the laser that is incident on the film and this is the lasing light that is coming out of L U 2 O 3 which is grown by PLD. Incidentally that full with that half maximum of this emission peak is only 1.2 nanometer. So, if it is showing such sharp emission then you can you can see the lasing action of such emitting stuff this can be made out of PLD and this is a good dm image that tells how PLD can clearly help you in growing epitaxial films. For example, if you see the electron diffraction pattern of YSZ this is the substrate region and this is the interface region along this line and this is the L U 2 O 3 which is cerium dough and these are the lattice fringes what you see as stripes are the lattice fringes in other words those are the atoms. So, if you look at the interface here there is no distortion between this lattice and this lattice in other words if you look at the diffraction spots between two diffraction spots you see repeats. So, they are exactly matching and therefore, you can easily grow L U 2 O 3 on ythria steplase zirconia because they have exact lattice matching. One lattice plane is equal to half of the lattice plane of YSZ therefore, it is exactly able to sit on YSZ and grow as a epitaxial layer. So, this is one convenient way where you can realize highly oriented epitaxial films of such complex oxides. Another example is that of manganese thin films I have discussed about this manganese extensively in the next module, but I will show you only the study relate to thin films. As you see here if you keep on doping ruthenium in this LAPB MNO3 a strong ferromagnetic loop is shown with 10 percent ruthenium 20 percent ruthenium 30 percent and 40 percent still you can see a very strong ferromagnetic signal. However, if you look at the conductivity data as you increase the ruthenium concentration you can see there is a upward in resistivity that we do not know whether it comes from the bulk phenomena or whether it is due to the antiferromagnetic interactions that are competing in this set of molecules. So, there is no way you can clear this doubt other than making thin films because in thin films you do not bring in the grain boundary issue. Therefore, if there is any upturn in resistivity you can directly correlate that to magnetic phenomena. So, that way if you are only studying bulk then you cannot resolve some of the issues for which PLD can be a very good system to work with you can make films of very high quality and this is again HRTEM image high resolution transmission electron microscopy which gives you idea about the interface as you see here every dot here is nothing but a unit cell or the atomic position of lanthanum oxide. This is the lanthanum oxide picture and this is the magnet film that is grown on the substrate as you can see here very clearly there are there are no two different spots if there are two different spots then that would correspond that it is not growing effort actually, but if you if you magnify these spots and see here you can see two spots together in each spot whatever you see here is nothing but two spots and that one coming from LAO and one coming from magnet film what it means is they have very close matching and therefore, they are epitaxially able to grow and if you look at the interface here this region seems to have little bit of interfacial problem roughness, but you can see here there is no change in the interface ordering because of the lattice mesh they are able to grow epitaxially. So, highly oriented manganese film can be made using PLD and this is another example by Krebs group in Germany where they have used PLD for a for Thilpen deposition of PMMA. This is poly methyl methacrylate as you can see the IR is matching TEM of a polycarbonate film can also be made embedding nano crystalline silver grains and you can also see that you can get predominantly oligomer based PMMA using this PLD. So, you can extend this to that, but there are some issues that we can try to address as we use this process extensively one is droplet effect another one is the issue of large area deposition. There are several refinements that have come on the way just to help us to increase the utilization of this method. For example, to avoid the droplet effect or to minimize on the grain boundary issue or roughness there are several schemes that are proposed. One is you can use the this is the typical setup where laser comes ablation and then that goes and deposits on the substrate. But there are some refinements made for example, we can have a gas pulse which is actually crossing the ablated bloom which can bring down on the heavier atoms or chunks arriving at the substrate. Therefore, you can reduce on the roughness of the film. There is another way to do that you can use RF plasma here. So, one can use RF plasma which will also interfere with your laser bloom as a result you can try to minimize on the roughness. So, this set of refinements can be made and there is another way one can do it. For example, look at the growth of germanium on germanium 001 assist group in the Cambridge have attempted this. This we call it as homoepitaxy because you are growing the same material on the same single crystal. Therefore, it is called homoepitaxy and look at the dynamics here. If I use MBE, if I use PLD with high kinetic energy that is 300 electron volt and if I use PLD, but with a very less energy then you can see the these are the thickness of the films 28 nanometer, 30 nanometer, 27 nanometer almost same thickness, but you can see the microstructure vividly changing. What does it mean? This can greatly affect the epitaxial growth of your film and this cartoon tells us that PLD with low threshold or with very less kinetic energy comparable to thermal evaporation the epitaxy epitaxy actually breaks down even with just 20 nanometer thick film. Whereas the next one to break down is molecular beam MBE based film whereas the PLD with very high kinetic energy can show a very high epitaxy even up to 100 nanometers. So, that is the advantage of the PLD process and another refinement has come this is called as Aurora PLD method. What they are doing here is they are trying this is this is the typical PLD process, but what they have inserted here is near to the substrate they have placed a magnet and you can see once you do that the zinc oxide plume that is reaching the substrate the plume dynamic changes here. You can see plume is actually traveling, but it is not clearly reaching the substrate here whereas in the case of the Aurora method where they have kept this magnet near to the substrate the plume is actually reaching up to the substrate. Therefore, the plume dynamics changes so in effect this magnet is actually kept like a reflecting mirror. So, the electrons which are also going they get reflected and they transfer the kinetic energy to the atoms and species as a result you have a activated deposition process. So, if you actually look at the quality of films from a conventional PLD where you have the PL efficiency is noted here it is twice the PL efficiency for a Aurora PLD approach and if it is done in argon you still feel 3 to 4 times enhancement in the PL intensity as you know the band to band emission of zinc oxide actually is pronounced with a 380 nanometer peak and so even though you are getting a good quality film yet the PL efficiency suffers when you are using a conventional PLD. So, you can bring about lot of modifications and that is also reflected in the carrier density and mobility therefore, we can try to fine tune on those issues also. There are some links which can give you more idea on the application of PLD, but I stop here and we will try to give you some more examples in the bibliography so that you can refer to other processing parameters which can be taken into account to get a improved film growth and also to extend this process to variety of other materials.