Zm.CiSSEMICONDUCTOR NANOSTRUCTURES FOR OPTOELECTRONIC APPLICATIONS

Abstract

In recent past, the understanding and applications of quantum-confined structures in the development of LEDs and PV devices has spurt interest in the studies of such structures. However, the difficulties faced in the fabrication of appropriate quantumconfined structures have hampered their applications in photonics and energy generations technologies. The difficulties such as, the size distribution; in the prepared samples and the dominance of the surface non-radiative recombination have been identified as the major limitations in their use in LEDs. Dominance of the non-radiative contribution from the surface states in undoped nanostructures forbids their applications in photonics. By the introduction of an impurity in a nanostructure, the dominant recombination can be transferred from the surface states to the impurity states. The best light emitters for display and lighting industry are the II—VI group semiconductors doped with the transition or rare earth impurities which incur localized states in the band gap. The physics and technology of doped nanostructures is in infancy and little effort has been made in this direction. For the energy generation purposes, the nanostructured thin films provide more cost-effective solution and use a cheap support onto which the active component is applied as a thin coating. Solar cells, or photovoltaics (PV), convert the energy of the sun into electricity. In theory all parts of the visible spectrum from near-infrared to ultraviolet can be harnessed. The mainstay at present is use of CdS thin films as n partner of p-n junction of thin film solar cell structure. Nanotechnology ("nano") incorporation into the films shows special promise to both; the enhanced efficiency and lower total cost. The energy band gap of various layers can be tailored to the desired value by varying the size of nano-particles. This allows for more design flexibility in the absorber and window layers in the solar cells. Recent advances have demonstrated that it is an effective route to tune the energy band-gap and thus, emission wavelength by changing their constituent stoichiometrics. IIVI group Zni-xCdxS nanostructures are of considerable interest for a variety of optoelectronic applications. With increasing zinc content, a compositiontunable emission across the whole visible spectrum can be obtained by a systematic blue-shift in the emission wavelength of Zni^Cd^S ternary nanostructures. Zni-jCd^S semiconductor nanostructures are ubiquitous in photonic applications such as LED's because their properties may change when the luminescence transitions are modified by doping with external impurities. The energy of activator-related emission remains nearly unchanged while the excitation can be tuned by quantum confinement effects. On the other hand, an improvement in the photo-conversion efficiency of PV devices such as CdS/CdTe solar cell is expected if Zn^Cd^S solid solution films are used in place of the CdS alone because of their higher forbidden gap. In hetero-junction solar cells, the part of the cell which serves as a window should have the highest forbidden gap possible. This work includes preparation of uniform and luminescent Zni-jCd^S nanoparticles via low temperature (280 K) co-precipitation technique. A detailed particle size investigation and studies on the optical and emission properties with variation in mole fraction (x) is carried out, to obtain a well characterized system apt to be used in light emitting applications. The lattice constant, a of Zni-^Cd^S nanoparticles, is found to obey the Vegard's law and possesses a nearly linear relationship with x, indicates the successful formation of Zni-xCdxS ternary semiconductor. Combination of XRD, UV-Vis and TEM methods allows a realistic statement on the particle size and is found in the range of 3.0 - 4.0 nm with relative standard deviation of 12-15%. It is shown that the asprepared Zni-^CdjS nanoparticles possess both: the quantum confinement effect (QCE) and the composition effect. Energy band gap is found to increase with increased QCE, while it decreases with increase in V. Light emissions from Zni-;,CdxS nanoparticles are obtained in UV region via near band edge (NBE) transitions. Emissions in blue and green spectral regions are obtained via defect assisted transitions. QCE shifts the NBE emissions to higher energies compared to the absorption band of bulk counterpart, while it red-shits with increase in x. Blue and green emission wavelengths do not changes with QCE and composition. This study provides two-fold benefits: (i) as-prepared Zni-xCdxS nanoparticles can be used as the target material for the preparation of their nanocrystalline thin films and, (ii) provides a framework by the preparation of host nanoparticles for their use in light emitting applications. In next step, as-prepared nanoparticles are utilized as the target material for the synthesis of Zni.xCdxS nanocrystalline thin films. The major focus is on the growth and characterizations of the micro structural features and optical properties of Zni.^Cd^S thin films fabricated by pulsed laser deposition technique. In particular, effect of pulsed laser deposition parameters such as laser flux density, working pressure, deposition ii temperature and mole fraction on the structural, morphological and optical properties have been investigated. The variation in laser flux density provides regulation on surface roughness of deposited Zn^Cd^S nanocrystalline thin films. The laser flux densityat 3.33 J/cm results in minimum rms surface roughness of ~ 8.0 nm, and is attributed to moderate kinetic energies of the laser ablated plume species. The variation in working pressure provides a control on the particle size of Zni.xCdxS nanocrystalline thin films. The wide-ranging particle sizes from ~ 15 to 80 nm are obtained with working pressure variation from 0.015 to 200 mtorr. Structural phase transformation from zincblende to wrutzite structure is obtained in the as-deposited Zni.xCdxS nanocrystalline thin films. The structure transformation is found to depend on both: the deposition temperature and the mole fraction. It is established that the structure transformation temperature decreases with increase in mole fraction in Zni.xCdxS nanocrystalline thin films. Values in parentheses (0.1; > 400 °C), (0.3; > 300 °C), (0.5; > 200 °C) showsthe mole fractions and deposition temperatures at which zincblende to wrutzite structure transformation takes place. For mole fractions > 0.5, the structure transformation is independent of deposition temperature and remains in wrutzite structure. The above findings provide well regulated/controlled properties of the as-deposited nanocrystalline thin films and provide a process window for fabrication of the full device structure. Zni.jCdjS nanostructures for light emitting applications have been achieved by successful incorporation of manganese as dopant in the Zni.^Cd^S host. An approach has been devised and tested to enhance the emission intensity by reducing the lattice mismatch between the host (Zn^Cd^S) and dopant (Mn2+), thereby increasing the dopant concentration. Maximum of characteristic i.e. orange emission is obtained at mole fraction, x = 0.45, with 0.10% lattice mismatch between MnS and Zno.55Cdo.45S. The maximum orange emission intensity of the order ~ 105 (counts) is obtained at 5 mol% manganese concentration from Zno.55Cdo.45S nanoparticles. The defect assisted i.e. blue and green emissions are significantly reduced in Zno.55Cdo.45S nanoparticles, suggesting the efficient transfer of carriers from host states to dopant incurred excited states. It is shown that manganese is homogenously distributed in the lattice which rules out the possibility of concentration quenching at higher manganese concentration (10 mol%). It is therefore suggested that decline in the emission intensity at higher manganese concentration is due to the formation of separateMnSphase. 111 In further experiments an attempt has been made to increase the orange emission intensity via fabrication of core-shell nanostructures. An inorganic shell coating leads to an increase in emission intensity by effectively reducing the surface states. XRD of coreshell nanoparticles showed profound dependence of shell thickness characterized via systematic change in the peak shift from Zno.55Cdo.45S to ZnS phase with increase in shell to core (s/c) ratio from 0.05 to 1.0. Particle size differences between the core and the core-shell nanoparticles have been utilized to estimate the ZnS shell thickness. The shell thickness is estimated as ~ 0.55 nm at s/c ratio of 0.5. The enhancement in orange emission intensity (s/c ratio at 0.5) is ~ 15 times with respect to core nanoparticles. The Mn doped Zno.55Cdo.45S nanocrystalline thin films are deposited at previously optimized pulsed laser deposition parameters. Deposition parameters such as laser flux density, deposition temperature, and working pressure are kept constant at 3.33 J/cm2, 200 °C and 0.015 mtorr respectively. The orange emission intensity of the order ~104 (counts) is obtained from nanocrystalline thin films. It is found that orange emission from thin film structures decreases by a factor of ~ 5 when compared to the nanoparticles. In order to enhance the orange emission from Zno.5sCdo.45S:Mn nanocrystalline thin films, nanocomposites of Zno.55Cdo.4sS:Mn and ZnS have been fabricated. Thickness of ZnS top layer has been varied and its effect on orange emission is investigated. The orange emission intensity enhancement from nanocomposite Zno.5sCdo.45S:Mn/ZnS increases by a factor of ~2 only. It is suggested that the enhanced orange emission in nanostructures is due to passivation of surface states and is justified by the electron affinity model. The work on PV device includes the utilization of Zno.9Cdo.1S nanocrystalline thin films as the window layer of thin film solar cell structure. The Zno.9Cdo.1S nanocrystalline thin films were deposited at 3.33 J/cm laser flux density and 200 mtorr working pressure. so as to obtain minimal surface roughness and particle size of ~ 80 nm. The work on PV devices is divided into two parts. The first part established a test by which the uniform coverage by Zno.9Cdo.1S on the indium tin oxide (ITO) (front contact) is examined. This test is undertaken in order to probe the area for back contact deposition. Gold (Au) is used for this purpose because it makes a Schottky contact with Zno.9Cdo.1S. The test on the uniform coverage by Zno.9Cdo.1S on ITO is performed by gauging the I-V characteristics of Au/Zno.9Cdo.iS/ITO structure. The direct contact between Au and ITO results in Ohmic I-V characteristic and detects the presence of pin holes in Zno.9Cdo.1S films. The appearance of Schottky characteristic shows the uniform coverage of ITO by Zno.9Cdo.1S film. Use of this test allowed the optimization of cell preparation method, which led to the iv fabrication of devices having a low density of pinholes. The interface between Zn09Cdo iS/ITO forms Ohmic junction. The junction properties may change at higher processing temperatures steps involved in device fabrication. It is with this aim, the pre heat treatment of Zno.9Cdo.1S/ITO stack is carried out in both the oxidizing and the reducing atmosphere, for various durations. Indium (In) is chosen as contact to this stack as it makes an Ohmic junctionwith Zno.9Cdo.1S. All conditions tested in this study results in Ohmic I-Vcharacteristics. It has been ensured by the Au test method that the obtained Ohmic characteristics are not due to pin holes. It is therefore attributed to the Zno.9Cdo.1S/ITO interface junction properties. This study shows that Zno.9Cdo.1S/ITO interface preserves its characteristic during heat treatment steps involved in this study. The second part is a study of the photovoltaic conversionmechanism. It is a study of the photovoltaic response of the cells when the physical properties of the layer constituting thin films solar cell structure changes. This includes changes caused by varying the pulsed laser deposition parameters of Zni-*Cd*S films (window layer of the cell structure). It also includes the changes caused by cell activation step i.e. CdCl2 treatment. Total of 22 cells are fabricated and tested with variation in above mentioned device fabrication parameters. Two types of CdCl2 treatment: wet (chemical dipping in CdCl2) and dry (vapor deposition of CdCl2) have been given to the cells. CdCl2 application along with the heat treatment results in the grain growth and recrystallization in CdTe (absorber layer of the cell structure) which leads to the better cell performance. In this study, the dry CdCl2 treatment was found to be more effective than the wet CdCl2 treatment because the higher grain growth and recrystallization. The grain size of ~ 3.0 urn is obtained at heat treatment temperature of 425 °C while the grain size of- 0.8 urn is obtained at 450 °C. The recrystallization was gauged by the texture coefficient of the XRD peak intensities of CdTe films. Texture coefficient analysis also shows higher recrystallization for dry treated cells. The photo conversion efficiencies (77) of 5.2 and 3.1% are obtained for dry and wet CdCl2 treatment respectively. The variation in Zno.1Cdo.9S film deposition parameters includes the change in the thickness and deposition temperature. With a reduction of Zno.1Cdo.9S film thickness from 200 to 150 nm, photo conversion efficiency, r\ increases from 4.85 to 5.2%. This increase in photo conversion efficiency is attributed to the increased short circuit current density (Jsc). Further reduction in thickness results in lower efficiencies, and is attributed to the higher surface roughness of Zno.1Cdo.9S films. Higher films thickness(>150 nm)results in higher series resistance which degrades the overall cell performance. Deposition temperature of v the Zno.1Cdo.9S film significantly affects the cell parameters. Deposition of Zno.1Cdo.9S films at 400°C results in 5.2% efficiency. At lowertemperature (350°C) the efficiency is almost similar ~ 5.0%. It is attributed to the similar crystalline quality and surface roughness (~ 4.2 nm) of the Zno.1Cdo.9S films. Higher deposition temperature (500 °C) results in poor surface quality (i.e. increased pin holes accompanying non-uniformed grains), higher interdiffusion between Zno.1Cdo.9S and CdTe, and impurity diffusion from glass substrate to above lying layers. It eventually decreases the efficiencies close to ~ 2.2%. Best cell (rj is 5.2%) in this work is obtained at Zno.1Cdo.9S film deposition temperature and films thickness at 400 °C and 150nm respectively along with dry CdCl2 treatment at 425 °C. Comparatively lower efficiency (than previously achieved for CdS/CdTe solar cell) obtained in this work suggests that there are efficiency limiting factors such as (a) increased resistivity of Zno.1Cdo.9S films with respect to CdS ,(b) series resistance of the cell is contributing significantly, (c) there was not any special large area contact designing for increased light absorption and, (d) increased lattice mismatch between Zno.1Cdo.9S and CdTe films, (e) thermal and electrical stability issues of back contact, (f) solar concentrators were not used. These issues should be analyzed and presents the future scope of this work. Indeed, the above results showed that there is a great potential in the Zni-^CdxS nanostructures for both the light emitting, and PV applications. This work is a step forward towards applicability of this technology.