Microcrystalline Thin Films Research Articles

The Gaussian nature of the band-edge of ZnO microcrystalline thin films

A straightforward analytical approach based on the derivative of the absorption coefficient is presented, which enables probing the nature of the band edge (BE) of ZnO microcrystalline films. The study was conducted via transmission experiments at temperatures of 77–532 K and repeated for samples annealed up to 1073 K. It was found that the derivative of the natural log of the BE absorption coefficient resulted in a Gaussian function. The Gaussian linewidth is used in the electron–phonon (e–p) interaction model to characterize the defect-state of the films. The BE of the as-grown film was found to exhibit no thermal dependence and no e–p coupling, indicative of a disordered crystal. Upon annealing and improvement of the film quality, the thermal phonons became more activated, but only above room temperature with a phonon energy of ∼75 meV, while up to room temperature, the impact of phonons on the BE is insignificant. A disorder–order transition was determined to take place at an annealing temperature of ∼673 K. X-ray diffraction concurs with these results. The study indicates that the prevalent defects are of structural nature due to the inherent granular morphology of the films. This defect was found to dominate the behavior of the BE even at the elevated temperature regime rather than thermal phonons.

Transient absorption microscopy setup with multi-ten-kilohertz shot-to-shot subtraction and discrete Fourier analysis.

Recording of transient absorption microscopy images requires fast detection of minute optical density changes, which is typically achieved with high-repetition-rate laser sources and lock-in detection. Here, we present a highly flexible and cost-efficient detection scheme based on a conventional photodiode and an USB oscilloscope with MHz bandwidth, that deviates from the commonly used lock-in setup and achieves benchmark sensitivity. Our scheme combines shot-to-shot evaluation of pump-probe and probe-only measurements, a home-built photodetector circuit optimized for low pulse energies applying low-pass amplification, and a custom evaluation algorithm based on Fourier transformation. Advantages of this approach include abilities to simultaneously monitor multiple pulse modulation frequencies, implement the detection of additional pulse sequences (e.g., pump-only), and expand to multiple parallel detection channels for wavelength-dispersive probing. With a 40 kHz repetition-rate laser system powering two non-collinear optical parametric amplifiers for wide tuneability, we find that laser pulse fluctuations limit the sensitivity of the setup, while the detection scheme has negligible contribution. We demonstrate the 2-D imaging performance of our transient absorption microscope with studies on micro-crystalline molecular thin films.

LUMINESCENT MATERIALS FOR COMMUNICATION DEVICES: AN OVERVIEW

The luminescent material is microcrystalline powders or thin-films designed to provide visible colour emission on excitation by different sources. These are also called phosphors they can be developed in to a potential candidate for applications like Solar cell device, LEDs, Display devices.etc. Present paper is an Review of the phosphors materials for Light emitting devices (LED), their present status and challenges.

Low Threshold Single Mode ZnO Nanowire Nanolasers

With a wide bandgap of 3.36 eV at room temperature and large exciton binding energy of 60 meV (excitons being stable up to room temperature), ZnO holds promise for use in blue and ultraviolet optical devices [1], including ultraviolet microlasers. Lasing has been demonstrated with epitaxial and microcrystalline thin films [2], and in arrays of ZnO nanorods [3] and nanowires [4], for example. Usually, the feedback mechanism in nanorod and nanowire-based lasers is related to the formation of Fabry–Perot cavities for longitudinal modes [5], or to the structures ability to couple and facilitate guided modes [6]. Discriminating between the feedback mechanisms in ZnO nanostructures is not an easy issue, especially in microstructures consisting of nanorods with various orientations. Additionally, nanoscale systems also suffer from impure spectral characteristics and multimodal emission. Strategies to improve emission characteristics have seen limited success due to the sensitivity of nanoscale resonator cavities to optical properties dependant on structural shape and details of the crystallinity. Whispering gallery modes and complicated multimodal emission with a spread in gain factors compound the issue of spectrally pure emission in nanoscale systems. High crystallinity is a prerequisite for short-cavity ZnO nanorod lasers due to expected coexistence of heavy mirror losses and scattering losses caused by the intrinsic structure and surface defects typically affecting ZnO. Here, arrays of vertically aligned ZnO nanorods were fabricated by CVD, with very high structural purity and quality. Characterization of aluminum deposition as surrounding layers on nanorods of 2-5 μm in length and 150-200 nm in width will be presented, and we detail the effect of selectively coated individual nanorods on the waveguiding and emission quality of the nanorod laser. The aluminum deposition alters the hexagonal cavity properties by reducing the guided modes and lowering the gain of the higher order modes so that a better, more spectrally pure emission is possible. [1] U. Ozgur, Ya. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S.-J. Cho, M. Morkoc, J. Appl. Phys. 98, 041301 (2005). [2] Z. K. Tang, G. K. L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Appl. Phys. Lett. 72, 3270 (1998). [3] G. Visimberga, C. C. Faulkner, M. Boese and C. O'Dwyer, ECS Trans., 45, 51 (2012). [4] S. P. Lau, H. Y. Yang, S. F. Yu, H. D. Li, M. Tanemura, T. Okita, H. Hatano, H. H. Hng, Appl. Phys. Lett. 87, 013104 (2005). [5] Y. Zhang, R. E. Russo, S. S. Mao, Appl. Phys. Lett. 87, 043106 (2005). [6] H. Zhou, M. Wissinger, J. Fallert, R. Hauschild, F. Stelzl, C. Klingshirn, H. Kalt, Appl. Phys. Lett. 91, 181112 (2007).

Thinness- and Shape-Controlled Growth for Ultrathin Single-Crystalline Perovskite Wafers for Mass Production of Superior Photoelectronic Devices.

Thinness-controlled perovskite wafers are directly prepared using a geometry-regulated dynamic-flow reaction system. It is found that the wafers are a superior material for photodetectors with a photocurrent response ≈350 times higher than that made of microcrystalline thin films. Moreover, the wafers are compatible with mass production of integrated circuits.

A Bottom‐Up Approach toward All‐Solution‐Processed High‐Efficiency Cu(In,Ga)S2 Photocathodes for Solar Water Splitting

The development of solution‐processable routes to prepare efficient photoelectrodes for water splitting is highly desirable to reduce manufacturing costs. Recently, sulfide chalcopyrites (Cu(In,Ga)S2) have attracted attention as photocathodes for hydrogen evolution owing to their outstanding optoelectronic properties and their band gap—wider than their selenide counterparts—which can potentially increase the attainable photovoltage. A straightforward and all‐solution‐processable approach for the fabrication of highly efficient photocathodes based on Cu(In,Ga)S2 is reported for the first time. It is demonstrated that semiconductor nanocrystals can be successfully employed as building blocks to prepare phase‐pure microcrystalline thin films by incorporating different additives (Sb, Bi, Mg) that promote the coalescence of the nanocrystals during annealing. Importantly, the grain size is directly correlated to improved charge transport for Sb and Bi additives, but it is shown that secondary effects can be detrimental to performance even with large grains (for Mg). For optimized electrodes, the sequential deposition of thin layers of n‐type CdS and TiO2 by solution‐based methods, and platinum as an electrocatalyst, leads to stable photocurrents saturating at 8.0 mA cm–2 and onsetting at ≈0.6 V versus RHE under AM 1.5G illumination for CuInS2 films. Electrodes prepared by our method rival the state‐of‐the‐art performance for these materials.

Ammonia Oxidation at Electrochemically Platinum-Modified Microcrystalline and Polycrystalline Boron-Doped Diamond Electrodes

The electrochemical oxidation of ammonia was done at electrochemically Pt-modified microcrystalline and polycrystalline boron-doped diamond (BDD) thin films from sp3 Diamond Technologies and Element Six. The Pt electrodepositions were done by chronoamperometry and cyclic voltammetry using a 1 mM K2PtCl6 solution in 0.5 M H2SO4. The Pt-BDD electrodes were characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, X-ray diffraction, and cyclic voltammetry. The Pt-BDD electrodes were compared by SEM and ammonia oxidation. At BDD electrodes, cyclic voltammetry depositions favors the formation of cubic and cauliflower Pt nanostructures at microcrystalline and polycrystalline BDD electrodes, respectively. On the other hand, chronoamperometry electrodeposition favors dendrite formation structures in both BDD electrodes. Platinum electrodeposited by chronoamperometry favors the formation of platinum (100) facets, which enhances ammonia oxidation. Pt-microcrystalline BDD showed the highest ammonia oxidation peak current densities when compared to Pt-polycrystalline BDD.

Sensitivity of photoelectron energy loss spectroscopy to surface reconstruction of microcrystalline diamond films

In X-ray Photoelectron Spectroscopy (XPS), binding energies and intensities of core level peaks are commonly used for chemical analysis of solid surfaces, after subtraction of a background signal. This background due to photoelectron energy losses to electronic excitations in the solid (surface and bulk plasmon excitation, inter band transitions) contains valuable information related to the near surface dielectric function ɛ(ħω). In this work, the sensitivity of Photoelectron Energy Loss Spectroscopy (PEELS) is investigated using a model system, namely the well-controlled surface reconstruction of diamond. Boron-doped microcrystalline thin films with a mixture of (111) and (100) preferential orientations were characterized in the as-grown state, with a partially hydrogenated surface, and after annealing at 1150°C in ultra high vacuum. After annealing, the bulk (σ+π) plasmon of diamond at 34.5eV is weakly attenuated but no evidence for surface graphitization is observed near 6eV, as confirmed by electronic properties. Unexpected features which appear at 10±1eV and 19±1eV in the energy loss distribution are well described by simulation of surface plasmon excitations in graphite-like materials; alternatively, they also coincide with experimental inter band transition losses in some graphene layers. This comparative study shows that the PEELS technique gives a clear signature of weak effects in the diamond surface reconstruction, even in the absence of graphitization. It confirms the sensitivity of PEELS acquisition with standard XPS equipment as a complementary tool for surface analysis.

Thin film deposition of diamond using normal paraffins as source of diamond nucleation centers

We propose a process for diamond thin film (DTF) deposition using normal paraffins (nP) as source of diamond nucleation centers. We deposited micro-crystalline diamond thin films (MCDTF) on a Cu substrate using Hot Filament CVD (HFCVD) and Passive Pt/Pd Surface Catalysis (PPt/PdSC) methods. Beeswax and a 1:1 mixture of normal paraffins of the general formula CH3(CH2)nCH3 with n=22 and 26 were tested as nP starting material. The films obtained were characterized by scanning electronic microscopy (SEM), Raman scattering temperature dependent spectroscopy and X-ray diffraction (XRD) methods, all of which confirmed that the deposited material is MCDTF.

Processing of multilayer microcrystalline and nanocrystalline diamond thin films using Ar-rich microwave plasmas

Abstract

Comparative Study on the Properties of Galvanically Deposited Nano- and Microcrystalline Thin Films of PbSe

Thin films of PbSe having both nano- and microstructures have been deposited on transparent conducting oxide (TCO)-coated glass substrates electrochemically, from an aqueous solution of Pb(OAc)2, ethylenediamine tetraacetic acid (EDTA), and SeO2. A Pb strip acted as the sacrificial anode, while the TCO glass was the cathode. No external bias was applied. The formation of PbSe was pH sensitive, and pH ~3 was found to be optimum for film deposition. Films grown at room temperature (25°C) were nanocrystalline (~25 nm), while those deposited at 80°C were microcrystalline (~150 nm). Films were characterized by x-ray diffraction studies, field-emission scanning electron microscope image analysis, infrared spectral analysis, and by both alternating-current (a.c.) and direct-current (d.c.) electrical measurements. A blue-shift was observed for the nanocrystalline films. Film resistivity and junction properties were obtained from electrical measurements.

Effect of temperature, illumination and frequency on the electrical characteristics of Cu/p-Si Schottky diode prepared by liquid phase epitaxy

Microcrystalline thin Cu films were grown by liquid phase epitaxy, LPE on p-Si substrates. At the beginning of growth some of Cu atoms firstly reacted with Si forming a silicide layer (CuSi). After which a Cu film was epitaxially grown on the silicide seeds template which plays a key role in overcoming the lattice mismatch of Cu/p-Si. The topography of the grown Cu film on Si substrates was examined by scanning electron microscopy (SEM). The structural analysis by X-ray diffraction (XRD) confirmed a formation of Cu film as well as CuSi as an intermediate step. The current–voltage ( I– V) and capacitance–voltage ( C– V) characteristics of metal–semiconductor (Cu/p-Si) Schottky contacts were studied in the temperature range 300–375 K. The effect of the temperature on the series resistance R s , the ideality factor n and the barrier height Φ b were investigated. The Cu/p-Si diode shows non-ideal I– V behavior with an ideality factor greater than unity that can be ascribed to the interfacial layer, the interface states and the series resistance. The I– V characteristics of the investigated diode under high voltage region were found to be governed by space charge-limited currents. The extracted parameters were found to be strongly temperature dependent. The photocurrent properties of the device under illumination were also investigated. The photocurrent in the reverse direction is strongly increased by photo-illumination. In addition, capacitance–voltage–frequency ( C– V– f) characteristics were investigated at frequency range 10 kHz to 1 MHz. At each frequency, the measured capacitance decreases with increasing frequency due to a continuous distribution of the interface states.

Inductively coupled plasma-assisted RF magnetron sputtering deposition of boron-doped microcrystalline Si films

An innovative custom-designed inductively coupled plasma-assisted RF magnetron sputtering deposition system has been developed to synthesize B-doped microcrystalline silicon thin films using a pure boron sputtering target in a reactive silane and argon gas mixture. Films were deposited using different boron target powers ranging from 0 to 350 W at a substrate temperature of 250 °C. The effect of the boron target power on the structural and electrical properties of the synthesized films was extensively investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and Hall-effect system. It is shown that, with an initial increase of the boron target power from 0 to 300 W, the structural and electrical properties of the B-doped microcrystalline films are improved. However, when the target power is increased too much (e.g. to 350 W), these properties become slightly worse. The variation of the structural and electrical properties of the synthesized B-doped microcrystalline thin films is related to the incorporation of boron atoms during the crystallization and doping of silicon in the inductively coupled plasma-based process. This work is particularly relevant to the microcrystalline silicon-based p– i– n junction solar cells.

High rate growth and electronic property of μc-Si:H

The deposition parameters of hydrogenated microcrystalline silicon （μc-Si:H） films were optimized for two factors, the siliane concentration and total flow rate, under high deposition power density and high deposition pressure by very high frequency plasma enhanced chemical vapor deposition（VHF-PECVD）.The device-grade film with activation energy of 047 eV was finally prepared at a rate of 142 nm/s when the siliane concentration was 45% and the total flow rate was 100 sccm.Meanwhile,the deposition rate of microcrystalline thin films has reached 21 nm/s under 600 Pa. We have researched the electronic property and the conductivity of thin films by use of grain boundary trapping model and statistical shift of Fermi level model respectively. Moreover, we analyzed the effect of post-oxygenation on the electronic performance of thin films.

Low temperature plasma synthesis of silicon nanocrystals: a strategy for high deposition rate and efficient polymorphous and microcrystalline solar cells

It is generally accepted that increasing the deposition rate of amorphous (a-Si : H) and microcrystalline (μc-Si : H) thin films grown by plasma enhanced chemical vapour deposition leads to a deterioration in the films' properties. This ‘common sense’ rule places an upper limit on the deposition rate for a given film quality and thus on photovoltaic device efficiency. While this applies to a-Si : H and μc-Si : H films produced by the dissociation of silane and grown via the ‘radical path’, we have found that it is possible to increase the deposition rate and still improve or maintain film properties when the deposition process involves not only radicals but also silicon nanocrystals produced in the plasma. This will be illustrated in the case of polymorphous (pm-Si : H) and microcrystalline (μc-Si : H) materials and solar cells. The transfer of these processes from small area reactors to industrial ones is an exciting challenge for the low pressure plasma community, and one whose solution will open a route to lower cost photovoltaics in particular and large area electronics in general.

Microcrystalline/Amorphous Thin Si Films Deposition by a Newly Developed Dual Injection System Employing Hydrogen Plasma and Silicon Radicals at Low Temperature (300 °C) Chemical Vapor Deposition Process

Low temperature (300 °C) deposition of thin microcrystalline/amorphous silicon films by using newly developed single shower, dual injection, microwave excited high density plasma (1011 cm-3) system is described. This is an original setup based on an experimental modification of a dual shower system that was reported by us earlier. For the first time, for this system, experimental results are presented correlating between the deposition and plasma parameters with the resulting Si films atomic structure. The results consistent interdependence enables to pre-determine, in a wide range, by design, the growth of microcrystalline or amorphous thin Si films, by setting the proper deposition conditions for each film type.

Metal silicide-mediated microcrystalline silicon thin-film growth for photovoltaics

Microcrystalline thin Si films were grown by the metal-induced growth method. The metal catalyst (Co, Ni, or Co-coated Ni) first reacted to sputtered Si forming a silicide layer. Then a Si film was epitaxially grown above the silicide seed template. The crystallinity of Si films was investigated by X-ray diffraction (XRD) confirming Si film growth with CoSi 2 or NiSi 2 as an intermediate step. The grown Si films were fabricated into Schottky photodiodes. The Co-coated Ni modulated the silicide formation and gave a short-circuit current density ( J sc) of 10.6 mA/cm 2, which is one order higher than that for the single Co catalyst case.

Microcrystalline silicon deposited at high rate on large areas from pure silane with efficient gas utilization

Microcrystalline silicon thin film deposited by RF-PECVD and integrated in a tandem structure is a promising material for low cost photovoltaic solar cells compared to solar cells based on crystalline silicon. However, in order to allow a cost-effective mass production of solar cells based on this material, deposition processes should fulfill several conditions such as high deposition rate, good uniformity over large area and efficient gas utilization. In this work, it is shown that the atomic hydrogen density can be high enough to form microcrystalline thin films even from a pure silane RF discharge and that the pure silane regime is more efficient in terms of gas utilization. In situ Fourier transform infrared absorption and ex situ Raman spectroscopy measurements have been used to determine the fraction of dissociated silane in the discharge and the crystallinity of the deposited layers. Results have shown that microcrystalline silicon can be deposited uniformly on a large area substrate with a deposition rate of more than 10 Å / s with a low powder formation and an input power density of 0.28 W cm - 2 from a pure silane discharge.

Structural and optical investigation of plasma deposited silicon carbon alloys: Insights on Si-C bond configuration using spectroscopic ellipsometry

Amorphous (a–Si1−xCx:H) and microcrystalline (μc–Si1−xCx:H) thin films have been deposited by plasma-enhanced chemical-vapor deposition using SiF4–CH4−H2 rf plasmas. Gas flow rates have been varied to deposit films with different carbon content and microstructure. The microstructure and optical properties have been investigated by IR/Raman spectroscopy and by spectroscopic ellipsometry in the energy range of 1.5–5.0 eV. Ellipsometric spectra have been analyzed in terms of the tetrahedron model combined with the Bruggeman effective-medium approximation to determine the film microstructure and silicon–carbon-bond configurations. Correlation between Si–C bond configurations and optical properties of films has been studied as a function of carbon content and microstructure. It is found that the optical properties and the band gap value depend not only on the carbon content, but also on the Si–C bond configuration and microcrystallinity. The films consist of Si-centered Si–Si4−vCv(v=0–3) Si-centered tetrahedra with segregation of μc–Si. The amorphous matrix changes from Si–Si3C to Si–SiC3 with the increase of the μc–Si phase. The optical gap is found to increase with both carbon content and microcrystallinity.

High pressure regime of plasma enhanced deposition of microcrystalline silicon

An investigation of the effect of the total gas pressure on the deposition of microcrystalline thin films form highly diluted silane in hydrogen discharges was carried out at two different frequencies. The study was performed in conditions of constant power dissipation and constant silane partial pressure in the discharge while using a series of plasma diagnostics as electrical, optical, mass spectrometric, and in situ deposition rate measurements together with a simulator of the gas phase and the surface chemistry of SiH4∕H2 discharges. The results show that both the electron density and energy are affected by the change of the total pressure and the frequency. This in turn influences the rate of high energy electron–SiH4 dissociative processes and the total SiH4 consumption, which are favored by the frequency increase for most of the pressures. Furthermore, frequency was found to have the weakest effect on the deposition rate that was enhanced at 27.12MHz only for the lowest pressure of 1Torr. On the other hand, the increase of pressure from 1to10Torr has led to an optimum of the deposition rate recorded at 2.5Torr for both frequencies. This maximum is achieved when the rate of SiH4 dissociation to free radical is rather high; the flux of species is not significantly hindered by the increase of pressure and the secondary gas phase reactions of SiH4 act mainly as an additional source of film precursors.