Showing papers in "Journal of Vacuum Science and Technology in 2012"

Status and prospects of Al2O3-based surface passivation schemes for silicon solar cells

Abstract: The reduction in electronic recombination losses by the passivation of silicon surfaces is a critical enabler for high-efficiency solar cells. In 2006, aluminum oxide (Al2O3) nanolayers synthesized by atomic layer deposition (ALD) emerged as a novel solution for the passivation of p- and n-type crystalline Si (c-Si) surfaces. Today, high efficiencies have been realized by the implementation of ultrathin Al2O3 films in laboratory-type and industrial solar cells. This article reviews and summarizes recent work concerning Al2O3 thin films in the context of Si photovoltaics. Topics range from fundamental aspects related to material, interface, and passivation properties to synthesis methods and the implementation of the films in solar cells. Al2O3 uniquely features a combination of field-effect passivation by negative fixed charges, a low interface defect density, an adequate stability during processing, and the ability to use ultrathin films down to a few nanometers in thickness. Although various methods can be used to synthesize Al2O3, this review focuses on ALD—a new technology in the field of c-Si photovoltaics. The authors discuss how the unique features of ALD can be exploited for interface engineering and tailoring the properties of nanolayer surface passivation schemes while also addressing its compatibility with high-throughput manufacturing. The recent progress achieved in the field of surface passivation allows for higher efficiencies of industrial solar cells, which is critical for realizing lower-cost solar electricity in the near future.

High power impulse magnetron sputtering discharge

Abstract: The high power impulse magnetron sputtering (HiPIMS) discharge is a recent addition to plasma based sputtering technology. In HiPIMS, high power is applied to the magnetron target in unipolar pulse ...

Spatial atomic layer deposition: A route towards further industrialization of atomic layer deposition

Abstract: Atomic layer deposition (ALD) is a technique capable of producing ultrathin conformal films with atomic level control over thickness. A major drawback of ALD is its low deposition rate, making ALD less attractive for applications that require high throughput processing. An approach to overcome this drawback is spatial ALD, i.e., an ALD mode where the half-reactions are separated spatially instead of through the use of purge steps. This allows for high deposition rate and high throughput ALD without compromising the typical ALD assets. This paper gives a perspective of past and current developments in spatial ALD. The technology is discussed and the main players are identified. Furthermore, this overview highlights current as well as new applications for spatial ALD, with a focus on photovoltaics and flexible electronics. 2012 American Vacuum Society.

Commercialization of dye sensitized solar cells: Present status and future research needs to improve efficiency, stability, and manufacturing

Abstract: Dye sensitized solar cells(DSSCs) have received a tremendous amount of attention since the first report of a 7% efficient cell in 1991. Confirmed record efficiencies are now 11.2% for small cells and 9.9% for submodules, and low-cost production methods are enabling manufacturing of DSSC products for a variety of markets. This review describes the present status of DSSCdevices and manufacturing as well as research challenges that must be addressed to continue the rapid commercialization of DSSC technology. These challenges fall into the categories of improving efficiency, stability, and manufacturability. Efficiency improvements will hinge on the development of new combinations of dyes, redox couples, and photoanodes. Best-case lifetimes are determined by the kinetics of various molecular-level processes, and realization of these lifetimes will require improved encapsulation of cells and modules. Low-cost and sustainable manufacturing of DSSC modules depends on use of high-throughput roll-to-roll processing and inexpensive, abundant materials. Prospects for simultaneous improvement of efficiency, stability, and manufacturing are discussed.

Pulsed high-density plasmas for advanced dry etching processes

Abstract: Plasma etching processes at the 22 nm technology node and below will have to satisfy multiple stringent scaling requirements of microelectronics fabrication. To satisfy these requirements simultaneously, significant improvements in controlling key plasma parameters are essential. Pulsed plasmas exhibit considerable potential to meet the majority of the scaling challenges, while leveraging the broad expertise developed over the years in conventional continuous wave plasma processing. Comprehending the underlying physics and etching mechanisms in pulsed plasma operation is, however, a complex undertaking; hence the full potential of this strategy has not yet been realized. In this review paper, we first address the general potential of pulsed plasmas for plasma etching processes followed by the dynamics of pulsed plasmas in conventional high-density plasma reactors. The authors reviewed more than 30 years of academic research on pulsed plasmas for microelectronics processing, primarily for silicon and conductor etch applications, highlighting the potential benefits to date and challenges in extending the technology for mass-production. Schemes such as source pulsing, bias pulsing, synchronous pulsing, and others in conventional high-density plasma reactors used in the semiconductor industry have demonstrated greater flexibility in controlling critical plasma parameters such as ion and radical densities, ion energies, and electron temperature. Specifically, plasma pulsing allows for independent control of ion flux and neutral radicals flux to the wafer, which is key to eliminating several feature profile distortions at the nanometer scale. However, such flexibility might also introduce some difficulty in developing new etching processes based on pulsed plasmas. Therefore, the main characteristics of continuous wave plasmas and different pulsing schemes are compared to provide guidelines for implementing different schemes in advanced plasma etching processes based on results from a particularly challenging etch process in an industrial reactor.

Atomic layer deposition for nanostructured Li-ion batteries

Abstract: Nanostructuring is targeted as a solution to achieve the improvements required for implementing Li-ion batteries in a wide range of applications. These applications range in size from electrical vehicles down to microsystems. Atomic layer deposition (ALD) could be an enabling technology for nanostructured Li-ion batteries as it is capable of depositing ultrathin films (1–100 nm) in complex structures with precise growth control. The potential of ALD is reviewed for three battery concepts that can be distinguished, i.e., particle-based electrodes, 3D-structured electrodes, and 3D all-solid-state microbatteries. It is discussed that a large range of materials can be deposited by ALD and recent demonstrations of battery improvements by ALD are used to exemplify its large potential.

Nanostructuring of molybdenum and tungsten surfaces by low-energy helium ions

Abstract: The formation of metallic nanostructures by exposure of molybdenum and tungsten surfaces to high fluxes of low energy helium ions is studied as a function of the ion energy, plasma exposure time, and surface temperature. Helium plasma exposure leads to the formation of nanoscopic filaments on the surface of both metals. The size of the helium-induced nanostructure increases with increasing surface temperature while the thickness of the modified layer increases with time. In addition, the growth rate of the nanostructured layer also depends on the surface temperature. The size of the nanostructure appears linked with the size of the near-surface voids induced by the low energy ions. The results presented here thus demonstrate that surface processing by low-energy helium ions provides an efficient route for the formation of porous metallic nanostructures.

Colloidal nanocrystal quantum dot assemblies as artificial solids

Abstract: The prospect of designing novel materials with electrical, optical, and magnetic properties by design has intrigued scientists and engineers for years. Building blocks for such “artificial solids” have emerged from recent advances in nanomaterial synthesis, characterization, and emerging understanding of their size-dependent properties. Colloidal semiconductor nanocrystal quantum dots (NQDs) stand out as an intellectually intriguing and experimentally advantageous system for the fundamental study of artificial solids and their technological development. The authors review the rapid evolution of artificial solids from an early theoretical concept towards the refined control of metamaterials with programmable electronic structure and their potential commercial applications, in particular, in next-generation energy technologies. The review is organized around the three independently adjustable parameters of artificial solids: (i) the electronic structure of NQD as artificial atom by tailoring the quantum con...

Metal versus rare-gas ion irradiation during Ti1−xAlxN film growth by hybrid high power pulsed magnetron/dc magnetron co-sputtering using synchronized pulsed substrate bias

Abstract: Metastable NaCl-structure Ti1-xAlxN is employed as a model system to probe the effects of metal versus rare-gas ion irradiation during film growth using reactive high-power pulsed magnetron sputter ...

Indium and impurity incorporation in InGaN films on polar, nonpolar, and semipolar GaN orientations grown by ammonia molecular beam epitaxy

Abstract: The effects of NH3 flow, group III flux, and substrate growth temperature on indium incorporation and surface morphology have been investigated for bulk InGaN films grown by ammonia molecular beam epitaxy. The incorporation of unintentional impurity elements (H, C, O) in InGaN films was studied as a function of growth temperature for growth on polar (0001) GaN on sapphire templates, nonpolar (101¯0) bulk GaN, and semipolar (112¯2), (202¯1) bulk GaN substrates. Enhanced indium incorporation was observed on both (101¯0) and (202¯1) surfaces relative to c-plane, while reduced indium incorporation was observed on (112¯2) for co-loaded conditions. Indium incorporation was observed to increase with decreasing growth temperature for all planes, while being relatively unaffected by the group III flux rates for a 1:1 Ga:In ratio. Indium incorporation was found to increase at the expense of a decreased growth rate for higher ammonia flows; however, smooth surface morphology was consistently observed for growth on s...

Chemical and structural study of electrically passivating Al2O3/Si interfaces prepared by atomic layer deposition

Abstract: Aluminum oxide (Al2O3) layers, prepared by atomic layer deposition (ALD), provide excellent surface passivation properties on crystalline Si surfaces, which are of major importance for photovoltaic applications. Beyond the chemical passivation by reduction of the electronic surface state density, a supportive field effect passivation mechanism emerges at the Al2O3/Si interface. The atomic origin of the fixed negative charges that are responsible for the field effect is currently under discussion. In this contribution, thin layers of Al2O3 with thicknesses ranging from the submonolayer region to several nanometers have been grown on Si substrates by means of thermal ALD. The principle elements of the samples have been quantified by x-ray photoelectron spectroscopy as a function of the film thickness. Changes at the interface upon thermal annealing have been investigated in detail. After the first few ALD cycles an imperfect Al2O3 layer is found together with the formation of an ultrathin SiOx interlayer. Continued deposition leads to stoichiometric Al2O3 growth. Within the first ∼1 nm from the Si interface, additional O (“excess O”), surpassing the Al2O3 and SiO2 stoichiometry, is observed. The excess O does not completely react with the Si surface to SiO2 during thermal annealing. Therefore, interstitial O in near-interface Al2O3 is suggested to provide the fixed negatively charged states.

Plasma-enhanced and thermal atomic layer deposition of Al2O3 using dimethylaluminum isopropoxide, [Al(CH3)2(μ-OiPr)]2, as an alternative aluminum precursor

Abstract: The authors have been investigating the use of [Al(CH3)2(μ-OiPr)]2 (DMAI) as an alternative Al precursor to [Al(CH3)3] (TMA) for remote plasma-enhanced and thermal ALD over wide temperature ranges of 25–400 and 100–400 °C, respectively. The growth per cycle (GPC) obtained using in situ spectroscopic ellipsometry for plasma-enhanced ALD was 0.7–0.9 A/cycle, generally lower than the >0.9 A/cycle afforded by TMA. In contrast, the thermal process gave a higher GPC than TMA above 250 °C, but below this temperature, the GPC decreased rapidly with decreasing temperature. Quadrupole mass spectrometry data confirmed that both CH4 and HOiPr were formed during the DMAI dose for both the plasma-enhanced and thermal processes. CH4 and HOiPr were also formed during the H2O dose but combustion-like products (CO2 and H2O) were observed during the O2 plasma dose. Rutherford backscattering spectrometry showed that, for temperatures >100 °C and >200 °C for plasma-enhanced and thermal ALD, respectively, films from DMAI had a...

Phase identification and control of thin films deposited by co-evaporation of elemental Cu, Zn, Sn, and Se

Abstract: Kesterite thin films [(i.e., Cu2ZnSn(S,Se)4 and related alloys] have been the subject of recent interest for use as an absorber layer for thin film photovoltaics due to their high absorption coefficient (>104 cm−1), their similarity to successful chalcopyrites (like CuInxGa1−xSe2) in structure, and their earth-abundance. The process window for growing a single-phase kesterite film is narrow. In this work, we have documented, for our 9.15%-efficient kesterite co-evaporation process, (1) how appearance of certain undesirable phases are controlled via choice of processing conditions, (2) several techniques for identification of phases in these films with resolution adequate to discern changes that are important to device performance, and (3) reference measurements for those performing such phase identification. Data from x-ray diffraction, x-ray fluorescence, Raman scattering, scanning electron microscopy, energy dispersive spectroscopy, and current-voltage characterization are presented.

Low temperature and roll-to-roll spatial atomic layer deposition for flexible electronics

Abstract: Spatial atomic layer deposition can be used as a high-throughput manufacturing technique in functional thin film deposition for applications such as flexible electronics. This; however, requires low-temperature processing and handling of flexible substrates. The authors investigate the process conditions under which low-temperature spatial atomic layer deposition of alumina from trimethyl aluminum and water is possible. The water partial pressure is the critical parameter in this case. Finally, our approach to roll-to-roll spatial atomic layer deposition is discussed.

Atomic layer deposition of GaN at low temperatures

Abstract: The authors report on the self-limiting growth of GaN thin films at low temperatures. Films were deposited on Si substrates by plasma-enhanced atomic layer deposition using trimethylgallium (TMG) and ammonia (NH3) as the group-III and -V precursors, respectively. GaN deposition rate saturated at 185 °C for NH3 doses starting from 90 s. Atomic layer deposition temperature window was observed from 185 to ∼385 °C. Deposition rate, which is constant at ∼0.51 A/cycle within the temperature range of 250 – 350 °C, increased slightly as the temperature decreased to 185 °C. In the bulk film, concentrations of Ga, N, and O were constant at ∼36.6, ∼43.9, and ∼19.5 at. %, respectively. C was detected only at the surface and no C impurities were found in the bulk film. High oxygen concentration in films was attributed to the oxygen impurities present in group-V precursor. High-resolution transmission electron microscopy studies revealed a microstructure consisting of small crystallites dispersed in an amorphous matrix.

Impact of electrode roughness on metal-insulator-metal tunnel diodes with atomic layer deposited Al2O3 tunnel barriers

Abstract: Metal-insulator-metal (MIM) tunnel diodes on a variety of high and low work function metals with various levels of root-mean-square roughness are fabricated using high quality atomic layer deposited Al2O3 as the insulating tunnel barrier. It is found that electrode surface roughness can dominate the current versus voltage characteristics of MIM diodes, even overwhelming the impact of metal work function. Devices with smoother bottom electrodes are found to produce current versus voltage behavior with higher asymmetry and better agreement with Fowler-Nordheim tunneling theory, as well as a greater percentage of functioning devices.

Effect of temperature and gas velocity on growth per cycle during Al2O3 and ZnO atomic layer deposition at atmospheric pressure

Abstract: The growth per cycle as a function of temperature during atomic layer deposition (ALD) of Al2O3 and ZnO at atmospheric pressure follows very closely the trend measured at typical (∼2 Torr) process pressure. However, the overall growth rate is found to be nearly 2 × larger at higher pressure and the magnitude of the growth increase can be adjusted by controlling the gas velocity near the growth surface. The growth increase at high pressure is approximately independent of process temperature at T 150 °C, especially for Al2O3. The relatively high growth/cycle measured at 760 Torr and T < 150 °C suggests that excess physisorbed water remains on the alumina or zinc oxide surface after the water purge step. Increasing the gas velocity in the growth zone reduces the growth rate, consistent with more efficient removal of excess water. To better understand the observed trends, we present analytical expressions for the boundary layer...

Reaction mechanisms for atomic layer deposition of aluminum oxide on semiconductor substrates

Abstract: In this work, we have studied the TMA/H2O (TMA = Al(CH3)3) atomic layer deposition (ALD) of Al2O3 on hydroxyl (OH) and thiol (SH) terminated semiconductor substrates. Total reflection x-ray fluorescence reveals a complex growth-per-cycle evolution during the early ALD reaction cycles. OH and SH terminated surfaces demonstrate growth inhibition from the second reaction cycle on. Theoretical calculations, based on density functional theory, are performed on cluster models to investigate the first TMA/H2O reaction cycle. Based on the theoretical results, we discuss possible mechanisms for the growth inhibition from the second reaction cycle on. In addition, our calculations show that AlCH3 groups are hydrolyzed by a H2O molecule adsorbed on a neighboring Al atom, independent of the type of backbonds (Si-O, Ge-O, or Ge-S) of AlCH3. The coordination of Al remains four-fold after the first TMA/H2O reaction cycle.

High rate roll to roll atomic layer deposition, and its application to moisture barriers on polymer films

Abstract: Atomic layer deposition has been shown to provide high quality single layer moisture barrier films on polymer substrates, but conventional pulse-based processes are too slow to be commercially feasible. One way to overcome this speed limitation is to avoid the need to pulse and purge precursors by moving the substrate between zones containing the precursors, passing through intermediate purge zones. Recently, several groups have reported various approaches to accomplishing this, including the approach discussed here in which the flexible web is passed between precursor zones in a serpentine pattern. Al2O3 and TiO2 barrier films 12 to 20 nm thick with water vapor transmission rates in the range of 10−4 g/m2/day have been demonstrated for web speeds in excess of 1 m/s on 100 mm wide polyethylene terephthalate web. Scale-up of this process to 300 mm wide web in a system capable of depositing 10–20 nm of film in a single pass is currently under way. This scale-up effort and the potential for very high volume, low cost moisture barrier production utilizing this technique are discussed.

Atomic layer deposition for electrochemical energy generation and storage systems

Abstract: Clean renewable energy sources (e.g., solar, wind, and hydro) offers the most promising solution to energy and environmental sustainability. On the other hand, owing to the spatial and temporal variations of renewable energy sources, and transportation and mobility needs, high density energy storage and efficient energy distribution to points of use is also critical. Moreover, it is challenging to scale up those processes in a cost-effective way. Electrochemical processes, including photoelectrochemical devices, batteries, fuel cells, super capacitors, and others, have shown promise for addressing many of the abovementioned challenges. Materials with designer properties, especially the interfacial properties, play critical role for the performance of those devices.Atomic layer deposition is capable of precise engineering material properties on atomic scale. In this review, we focus on the current state of knowledge of the applications, perspective and challenges of atomic layer deposition process on the electrochemical energy generation and storage devices and processes.

Substrate grain size and orientation of Cu and Cu–Ni foils used for the growth of graphene films

Abstract: Graphene growth on Cu foils by catalytic decomposition of methane forms predominantly single-layer graphene films due to the low solubility of carbon in Cu. On the other hand, graphene growth on Cu–Ni foils can result in the controlled growth of few-layer graphene films because of the higher solubility of carbon in Ni. One of the key issues for the use of graphene grown by chemical vapor deposition for device applications is the influence of defects on the transport properties of the graphene. For instance, growth on metal foil substrates is expected to result in multidomain graphene growth because of the presence of grains within the foil that exhibit a variety of surface terminations. Therefore, the size and orientation of the grains within the metal foil should influence the defect density of the graphene. For this reason, we have studied the effect of total anneal time and temperature on the orientation and size of grains within Cu foils and Cu–Ni alloy foils with a nominal concentration of 90/10 by w...

Mechanisms for hydrophilic/hydrophobic wetting transitions on cellulose cotton fibers coated using Al2O3 atomic layer deposition

Abstract: This report explores reactions that proceed during the first few cycles of inorganic film atomic layer deposition (ALD) on natural cellulose cotton fibers, and how surface reactions can explain the previously observed transitions in surface wetting upon ALD on cotton fibers. Atomic layer deposition of aluminum oxide and zinc oxide onto natural cotton cellulose produces a transition from hydrophilic to hydrophobic, then from hydrophobic back to hydrophilic, and we describe here the main factors that bring about. Interestingly, we show that air exposure and related adventitious carbon adsorption also affects the subsequent reactions and wetting properties obtained after subsequent ALD cycles. X-ray photoelectron spectroscopy and in situ Fourier transform infrared spectroscopy data indicate Al-(O-C-)3 bonding units form when trimethylaluminum interacts with surface –OH units during the first precursor doses, producing a hydrophobic finish on the cotton that remains for only a few ALD cycles. Also, field-emis...

Conformality of remote plasma-enhanced atomic layer deposition processes: An experimental study

Abstract: In total, five metal oxide and one metal plasma-enhanced atomic layer deposition (PEALD) processes were studied with respect to the conformality of the coatings. The study reveals that also high aspect ratio structures (up to 60:1) can be coated conformally with remote PEALD. Oxides could relatively easily be deposited into demanding 3D structures with rather short cycle times but not the silver metal. The key factor in achieving excellent conformality seems to be that sufficient radical density is required to overcome the loss of radicals by recombination. In the case of metals where hydrogen plasma is applied the recombination of hydrogen radicals causes major difficulties in obtaining perfect conformality.

Decomposition and phase transformation in TiCrAlN thin coatings

Abstract: Phase transformations and mechanisms that yield enhanced high temperature mechanical properties of metastable solid solutions of cubic (c)-(TixCryAlz)N coatings are discussed in this paper. Coating ...

Cathode encapsulation of organic light emitting diodes by atomic layer deposited Al2O3 films and Al2O3/a-SiNx:H stacks

Abstract: Al2O3 thin films synthesized by plasma-enhanced atomic layer deposition (ALD) at room temperature (25 °C) have been tested as water vapor permeation barriers for organic light emitting diode devices. Silicon nitride films (a-SiNx:H) deposited by plasma-enhanced chemical vapor deposition served as reference and were used to develop Al2O3/a-SiNx:H stacks. On the basis of Ca test measurements, a very low intrinsic water vapor transmission rate of ≤ 2 × 10−6 g m−2 day−1 and 4 × 10−6 g m−2 day−1 (20 oC/50% relative humidity) were found for 20–40 nm Al2O3 and 300 nm a-SiNx:H films, respectively. The cathode particle coverage was a factor of 4 better for the Al2O3 films compared to the a-SiNx:H films and an average of 0.12 defects per cm2 was obtained for a stack consisting of three barrier layers (Al2O3/a-SiNx:H/Al2O3).

Surprising importance of photo-assisted etching of silicon in chlorine-containing plasmas

Abstract: The authors report a new, important phenomenon: photo-assisted etching of p-type Si in chlorine-containing plasmas. This mechanism was discovered in mostly Ar plasmas with a few percent added Cl2, but was found to be even more important in pure Cl2 plasmas. Nearly monoenergetic ion energy distributions (IEDs) were obtained by applying a synchronous dc bias on a “boundary electrode” during the afterglow of a pulsed, inductively coupled, Faraday-shielded plasma. Such precisely controlled IEDs allowed the study of silicon etching as a function of ion energy, at near-threshold energies. Etching rates increased with the square root of the ion energy above the observed threshold of 16 eV, in agreement with published data. Surprisingly, a substantial etching rate was observed, independent of ion energy, when the ion energy was below the ion-assisted etching threshold. Experiments ruled out chemical etching by Cl atoms, etching assisted by Ar metastables, and etching mediated by holes and/or low energy electrons ...

Permeability and corrosion in ZrO2/Al2O3 nanolaminate and Al2O3 thin films grown by atomic layer deposition on polymers

Abstract: The authors studied moisture permeation and corrosion in Al2O3 and Al2O3/ZrO2 nanolaminate (NL) thin films grown by atomic layer deposition (ALD) at 100 °C on polyester substrates. A percolation model accurately described the dependence of permeation on the volume fraction of ZrO2 in the nanolaminates. As the fraction of ZrO2 was reduced to ∼0.5, moisture permeation in the NLs approached the measurement limit ∼1 × 10−4 g-H2O/m2-day, equivalent to Al2O3 with the same total thickness. However, resistance to corrosion by water was modestly better for Al2O3 than for the NL, and we proposed that corrosion in ALD Al2O3 films was associated with hydrogen incorporation and a consequent film chemical composition that is an oxy-hydroxide, AlOx(OH)3−2x. The authors present x-ray diffraction evidence for conversion of ALD Al2O3 to hydroxide corrosion products, AlO(OH) and Al(OH)3, after aging films in damp heat (85 °C/85% relative humidity) for two weeks.

Growth characteristics, material properties, and optical properties of zinc oxysulfide films deposited by atomic layer deposition

Abstract: Zinc oxysulfide—Zn(O,S)—is a wide bandgap semiconductor with tunable electronic and optical properties, making it of potential interest as a buffer layer for thin film photovoltaics. Atomic layer deposition (ALD) of ZnS, ZnO, and Zn(O,S) films from dimethylzinc, H2O, and H2S was performed, and the deposited films were characterized by means of x-ray diffraction, x-ray photoelectron spectroscopy, and spectroscopic ellipsometry. With focus on the investigation of Zn(O,S) film growth characteristics and material properties, the ZnO/(ZnO + ZnS) ALD cycle ratios were systematically varied from 0 (ZnS ALD) to 1 (ZnO ALD). Notably, a strong effect ofthematerial properties on the optical characteristics is confirmed for the ternary films. The Zn(O,S) ALD growth and crystal structure resemble those of ZnS up to a 0.6 cycle ratio, at whichpoint XPS indicates 10% oxygen is incorporated into the film. For higher cycle ratios thefilm structure becomes amorphous, which is confirmed with XRD patterns and also reflected inthe optical constants as determined by spectroscopic ellipsometry; in particular, the optical bandgap transforms from direct type for the (cubic) ZnS like phase to a more narrow bandgap withamorphous characteristics, causing bandgap bowing. A direct bandgap is recovered atyethigherZnO/(ZnO + ZnS) cycle ratios, whereproperties converge toward ZnO ALD in termsof film growth rate, crystallinity, and composition.Zinc oxysulfide—Zn(O,S)—is a wide bandgap semiconductor with tunable electronic and optical properties, making it of potential interest as a buffer layer for thin film photovoltaics. Atomic layer deposition (ALD) of ZnS, ZnO, and Zn(O,S) films from dimethylzinc, H2O, and H2S was performed, and the deposited films were characterized by means of x-ray diffraction, x-ray photoelectron spectroscopy, and spectroscopic ellipsometry. With focus on the investigation of Zn(O,S) film growth characteristics and material properties, the ZnO/(ZnO + ZnS) ALD cycle ratios were systematically varied from 0 (ZnS ALD) to 1 (ZnO ALD). Notably, a strong effect ofthematerial properties on the optical characteristics is confirmed for the ternary films. The Zn(O,S) ALD growth and crystal structure resemble those of ZnS up to a 0.6 cycle ratio, at whichpoint XPS indicates 10% oxygen is incorporated into the film. For higher cycle ratios thefilm structure becomes amorphous, which is confirmed with XRD patterns and also reflected ...

Growth morphology and electrical/optical properties of Al-doped ZnO thin films grown by atomic layer deposition

Abstract: The authors report electrical and optical characterization of zinc oxide (ZnO) and Al-doped zinc oxide (AZO) films grown by atomic layer deposition (ALD). A detailed analysis of ZnO growth morphology is presented with the help of atomic force microscopy imaging, roughness analysis, and x-ray photoelectron spectroscopy surface chemistry information. Initially the film grew as islands, which coalesced to complete the substrate coverage at 50 ALD cycles. The AZO films to be used as transparent conducting oxides for solar cell applications were grown on single crystalline Si (100) and float-glass substrates at temperatures from 150–325 °C. The amount of aluminum doping was varied from 2 to 8 %. The AZO film with 5% Al exhibited the highest conductivity in the film, which increased as the growth temperature increased. Hall effect measurements of an AZO film of thickness 575 nm doped at 5% on silicon and glass substrates showed a sheet resistance (Rs) of 100 Ω/□, which improved further to 25 Ω/□ after annealing...

Hot-wire-assisted atomic layer deposition of a high quality cobalt film using cobaltocene: Elementary reaction analysis on NHx radical formation

Abstract: Hot-wire-assisted atomic layer deposition (HW-ALD) has been identified as a successful method to form high quality metallic films using metallocene and NH3. A cobalt film formed by HW-ALD using cobaltocene and NH3 was successfully demonstrated. The authors have elucidated the mechanism of HW-ALD during the precursor feed period and the reducing period. In the case of cobalt, a deposition temperature above 300 °C is needed to avoid an inclusion of carbon impurities. This is because the physisorbed species are involved during the precursor feed period. NH2 radical promotes the dissociation of the carbon–metal bond during the reducing period. This is examined by elucidation of the gas-phase kinetics, estimation of the surface reactions by quantum chemical calculations, and analysis of the exhaust gas using a quadrupole mass spectrometer.