Laser Material Processing Division

Pulsed Laser Deposition

Semiconductor Nanostructures

Pulsed Laser Deposition technique is used to grow thin Films and Nanostructurs of Oxide semiconductor ZnO and related materials, Si, Mangenites, Field emitting material LaB₆ , ultra thin films and multilayers of high k-dielectric material ZrO₂, TiO₂, SiO₂ and Silicon Oxi-nitrides metal nanoparticles for plasmonic applications and nano metallic coatings.

ZnO and Related Materials

Currently ZnO is one of the most sought after semiconductors for developing blue and ultra-violet (UV) Photonic and Spin-Photonic devices such as light-emitting diodes (LED) and laser diodes, Spin-LED, solar blind UV photo detectors and transparent electronic devices etc. In bulk, ZnO has a wide and direct band gap of ~ 3.3 eV at room temperature with remarkably high excitonic binding energy which is ~ 60 meV and a rugged chemical and crystalline structure.

We have grown high crystalline and optical quality ZnO thin Films by using a novel in-house developed Buffer assisted PLD and studied its temperature dependent excitonic Photoluminescence in the range of 10K to Room Temperature (RT). We observed temperature dependent recombination's from free excitons, bound excitons and LO phonon assisted free and bound excitonic transitions. The transition energies of free excitonic transitions were close to the bulk values of ZnO. Varsani's coefficient for band gap variation with temperature was obtained from the PL spectra. Variation of line width of free excitonic transitions was studied as a function of temperature and a theoretical model was applied to interpret the temperature dependent broadening in terms of exciton-Phonon scattering.

Bandgap Engineering of ZnO

Band gap engineering of ZnO is essential to realize low dimensional structures such as Quantum wells and super-lattices. One of the ways to engineer the band gap of ZnO is to alloy it with desired concentrations of other binary oxides such as MgO and CdO. We have grown MgxZn1-xO films by PLD at different oxygen background pressures and observed that the ZnO band-gap can indeed be controlled only by changing the oxygen partial pressure in the ambient using a sintered pallet of ZnO containing a fixed 10% of MgO in the target. The monotonic increase in the band gap in the range of 3.45 to 3.78 eV was observed with varying oxygen partial pressure from 10-2 to 10-5 Torr during the growth as. The change in the band-gap of Mg_xZn_1-xO films grown at different oxygen partial pressures was attributed to change in the Mg concentration in the resulting films and was confirmed by EDAX measurements and Rutherford Back scattering analysis. We have adopted sequential ablation scheme to grow CdxZn1-xO alloy films using separate ZnO and CdO targets and achieved reduction in the ZnO band gap up to ~ 2.9 eV in predominantly single-phase CdxZn1-xO alloy films with Cd concentration up to ~8%. Sintered CdO and ZnO targets placed on a multi target carousal were ablated alternately with only one target being exposed to the laser beam at a time in a sequence. By controlling the total laser beam exposure time on the individual targets, films with different Cd compositions were grown.

ZnO Multiple Quantum Wells

We have studied temperature dependent photoluminescence (PL) from ZnO Multiple Quantum Wells (MQWs) with active layer thickness in the range of ~ 1 - 4 nm grown on (0001) Sapphire by pulsed laser deposition using an in-house developed buffer assisted growth scheme. To the best of our knowledge we have observed for the first time an efficient room temperature PL emanating from such MQWs. In the range of 10K to room temperature (RT), the spectral position of the PL peak shifted monotonically towards red with the increasing temperature in accordance with the empirical Varshni's relation due to the band-gap shrinkage. The spectral line width was found to increase with increasing temperature due to the scattering of excitons with acoustic and optical phonons in different temperature regimes. At 10K the PL peak shifted from ~ 3.4 to ~ 3.7 eV with decreasing thickness of the quantum well active layer from ~ 4 to 1 nm in good agreement with the calculated quantum confinement effects. From the observations of excitonic features at RT entwined with the band-edge in the absorption spectra of these MQWs these PL transitions are expected to be excitonic in nature and hence highly efficient

ZnO Quantum Dots

Although a significant wisdom on ZnO based quantum wells is created and reported in the literature the work on ZnO Quantum Dots (QDs) grown on solid substrates is quite scanty. A study on ZnO QDs grown on solid substrate is important not only to understand the three dimensional quantum confinement effects on the excitonic transitions but also to create new forms of this material which could open more frontiers for its applications. As an example, by growing QDs one can enhance the linear and nonlinear responses in ZnO significantly. To the best of our knowledge we have grown for the first time, multilayer of ZnO QDs of varying size by PLD embedded in alumina matrix on optically polished c-axis oriented alumina substrates.

ZnO based Nonvolatile Memories

(Transparent Resistive Random Access Memories)

Resistance switching characteristics, observed in metal oxide thin films, has recently attracted a great deal of attention to develop next generation low power, low cost, high speed, rugged and nonvolatile resistive random access memory (RRAM) devices. The memory effect in these materials is realized through the switching of the resistance of their thin films between two states of high and low resistances. We have developed a novel transparent RRAM devices based on ZnO and its variants. The schematic of the device is shown in figure (lower left). In these devices about 250 nm thick film of Zn1-xGaxO (x=0.0075) with resistivity ~ 1×10-4 ohm-cm was used as top and bottom electrodes. High resistive ZnO film of typical thickness ~ 90 nm was grown in between at room temperature using a novel in-house developed DC discharge assisted PLD. The as grown RRAM device appeared highly transparent and shiny to naked eyes with measured average transmission of ~ 80% in visible spectral region. The repeatable and reliable nonvolatile switching of the resistance of these device was obtained between LRS and HRS ( shown in lower right figure) at small and well defined switching voltages with a narrow dispersion.

Si Nanoparticles

We have grown a particulate free multilayer structure of Al2O3 capped Si quantum dots of different mean sizes ranging from 1 nm to 3 nm as confirmed by TEM analysis by using a off-axis deposition scheme. A monotonic blueshift in the band-gap of Si nanoparticles with decreasing particle size as observed in photoabsorption spectra of Si nanoparticles was in line with the putative quantum confinement effects. Room temperature photoluminescence from Si quantum dots grown for different times showed features without any apparent size dependent spectral shift which, albeit has earlier been explained by others originating from the defect levels at the interface of Si and SiO2 shells surrounding the nanoparticles but still have certain mysteries attached.

Laser Induced Oxidation of Si surface for the controlled growth of ultra thin SiO2 layer

Downscaling of device dimensions is essential for the development of new generation ultra large scale integrated (ULSI) circuits based on complementary metal oxide semiconductor field effect transistors (CMOSFET). However, because of continuous downscaling, the thickness of SiO2 gate dielectric has already reduced to a few mono-layers. Further thinning of SiO2 poses several challenges, including control of growth and uniformity of ultra-thin SiO2. We have very recently reported a novel technique based on pulsed laser heating to grow ultra-thin SiO2 with thickness less than 4 nm on the surface of Si substrate kept at room temperature [329]. Third harmonic laser pulse of a Q-switched Nd:YAG laser (355 nm) with a repetition rate of 10 Hz and pulse width of 6 ns was used to heat the silicon wafer in O2 ambient pressure to grow SiO2 in a very controlled manner. The laser fluence on the substrate was maintained at ~ 75 mJ/cm2. SiO2 was grown for different durations in the range of 30 to 180 sec in an oxygen pressure of ~ 10-2 Torr. The leakage current density against applied gate voltage (J-V) characteristics of the MOS devices using controlled pulsed laser heating revealed the low leakage current density and the breakdown field strength >10 MV/cm, signifying the excellent quality of the laser induced oxide.

High-K Dielectrics

Although Silicon dioxide (SiO2) has been used for more than 35 years as the primary gate-dielectric material in MOSFETs, probably due to the required combination of several desirable properties such as good electrical isolation, high quality Si- SiO2 interface and thermodynamic stability. However, current down scaling of CMOS technology requires that the thickness of the gate dielectric be reduced to only a few monolayers of SiO2. Further thinning of SiO2 poses a serious challenge because of large gate leakage current. In order to overcome the large gate leakage current mainly due to direct tunneling of electrons, introduction of new gate dielectric materials with high dielectric constant is essential. Using high-k dielectric, the physical thickness of the dielectric layer can be kept large, thereby reducing the gate leakage current, while maintaining the same value of capacitance. Titanium dioxide (TiO2)-based gate insulators for MOSFETs are of current interest in VLSI technology due to the high permittivity (k) of TiO2. We have used pulsed laser deposition technique for the deposition of thin TiO2 layers to fabricate Metal-TiO2-SiO2-Si (MTOS) capacitors. The deposition temperature and the post deposition annealing of high-k dielectric (TiO2) have been optimized to achieve low effective dielectric thickness (EDT) ~ in the range of 36 - 130 A as well as lower leakage current in MTOS capacitors. The best results were obtained by using a two-step deposition process of TiO2, where a thin buffer layer was deposited first at comparatively lower temperature (200 - 300ºC) followed by deposition and annealing at 750ºC. Lowering of the buffer layer deposition temperature helped to achieve lowest EDT ~1.6 nm during this study as well as lower leakage currents.

Diluted Magnetic Semiconductors

Conventionally semiconductor devices are based on the manipulation of the charge of electrons. The spin degree of freedom has recently been thought of an additional entity which can be manipulated to accomplish integration of functions of semiconductor and spin or magnetism based devices. This emerging branch of next generation devices will be based on spin electronics or spintronics. Transition Metal doped ZnO which is a well known wide bandgap diluted magnetic semiconductor is currently of immense interest for the development of spintronic and magneto-optical devices.

We have prepared bulk powders and grown thin films of MnxZn1-xO using Pulsed Laser Deposition and studied its structural, optical and magnetic properties. The MnxZn1-xO targets with Mn concentrations ranging from 1 to 20 mole% were prepared using MnO (99.99%) and ZnO (99.999%) powders. The mixture was calcined at 800°C, palletized and sintered at 1100°C. In particular cases of 2 & 5% MnO2, a low temperature processing was also performed where calcination and sintering were done at 400°C and 500°C respectively. Thin films were grown at a temperature of 600°C on (0001) sapphire substrates using third harmonic of a Q-switched Nd: YAG laser at a fluence of about 2 J/cm2. High Resolution XRD of the grown films revealed single Wurtzite phase and highly c-axis oriented growth of MnxZn1-xO on sapphire. The (0002) peak position was found to shift towards lower angles with increasing Mn concentration. Optical absorption measurements on these samples at room temperature revealed monotonic blue shift of the ZnO bandgap up to ~3.6 eV with increasing Mn concentration up to 20%. A significant midgap absorption at around 3eV due to 6A1 -4T2 transitions of Mn+2 ions was also observed. The temperature (T) dependent magnetization (M) measurement showed slight increase in M of Mn0.05Zn0.95O film with decrease in T from 300 to 50K and then a sudden rise in M below 50K indicating change in magnetic state of the film. However, from the Arrot-plot criterion at 5K we could not detect any evidence of spontaneous magnetization, which rules out any long-range ferromagnetic order. The temperature dependent magnetization measurement on the MnxZn1-xO targets (2 & 5% MnO) sintered both at low and high temperatures showed paramagnetic behavior up to 60K.

We also studied structural and optical properties of CoxZn1-xO alloy films grown by Pulsed Laser Deposition. The single wurtzite phase CoxZn1-xO targets with Co concentrations ranging from 1 to 20 mole % were prepared by mixing CoO (99.997%) and ZnO (99.999%) powders using standard ceramic processing. Thin films were grown at a temperature of 600°C on (0001) sapphire substrates using third harmonic of a Q-switched Nd: YAG laser (355 nm, 10 Hz, and 6 ns) at a fluence of ~ 2 J/cm2. The films were characterized using X-ray diffraction studies and optical transmission spectroscopy. The High Resolution XRD of the grown thin films revealed the highly crystalline and c-axis oriented growth without changing wurtzite structure. There were no impurity peaks corresponding to CoO related phase segregation, which indicated the homogeneous distribution of Co in the PLD grown films. The c-axis length and FWHM of (002) ZnO peak increased monotonically with increasing Co composition up to ~ 7%. The optical transmittance spectra measured at room temperature in the spectral range of 200 - 900 nm revealed highly transparent ~ 80% Co-ZnO thin films with a conspicuous mid gap absorption at ~659, 617 and 568 nm respectively due to intra-band Co+2 transitions. In order to determine the band gap (Eg) of the films, the absorption coefficient, a2 was plotted with respect to photon energy and linear portion of a 2 was extrapolated to a = 0. The band gap of Co doped ZnO blue shifted monotonically with increasing Co concentration. The similar trend of occurrence of mid-gap absorption due to Co doping was also reported by Tiwari et al. [3]. Further studies in this direction are underway.

Colossal Magneto-resistance Materials:

Epitaxial thin films of Colossal Magneto-resistance (CMR) material La0.5Pr0.2Ba0.3MnO3 were deposited on LaAlO3 single-crystal substrates using pulsed laser deposition (PLD) technique with different growth parameters. Structural, surface morphological, electrical, and magnetotransport measurements on these films revealed that unoptimized growth parameters during the deposition using the third harmonic of a Q-switched Nd: YAG laser yielded structurally inhomogeneous epitaxial films having a columnar morphology, while the optimized growth parameters using an excimer laser during the PLD resulted in homogeneous epitaxial films with a smooth morphology. Interestingly, at a temperature of 5 K, the films with unoptimized growth parameters showed a large high-field magnetoresistance (MR) of ~90% while the films with optimized growth parameters showed a high-field MR of only ~15%. It is contemplated that this exceptionally large MR in the unoptimized films might be due to the phase separation and coexistence of metallic and insulating phases.

Nano-metallic coating of Austenitic Stainless Steel

We have recently initiated experiments involving Pulsed Laser Deposition of nano-metallic coating of Austenitic Stainless Steel (SS) on similar substrates. Laser-deposited nano-metallic coating is intended to impart enhanced corrosion resistance to austenitic stainless steel parts.

Plasmonic Materials (metal nanoparticles)

Metal nanoparticles have generated immense interest in broad range of applications in the fields of photovoltaics, nano-optoelectronics and medicine. The unique properties of metal nanoparticles are due to the collective oscillation of conduction electrons termed as surface plasmons whose excitation resonance frequency depends on the shape, size of the metal nano particles and their dielectric environment. At our laboratory, we have recently initiated growth of copper, silver and gold nanoparticles by using pulsed laser deposition technique. Studies on the structural and optical properties of these nanoparticles are underway.