Effect of intrinsic stress on the optical properties of nanostructured ZnO thin films grown by rf magnetron sputtering

In this paper we report the effect of deposition temperature on the structural and optical properties of ZnO thin films prepared by rf magnetron sputtering. The films grown at lower deposition temperatures were in a state of large compressive stress, whereas the films grown at higher temperature (450 °C) were almost stress free. In the absorption spectra, the ZnO excitonic and the Zn surface plasmon resonance (SPR) peaks have been observed. A redshift in the optical band gap of ZnO films has also been observed with the increase in the deposition temperature. The shift in the band gap calculated from the size effect did not match with the observed shift values and the observed shift has been attributed to the compressive stress present in the films.     

Structural, Optical and Electrical Properties of Hydrogen-Doped Amorphous GaAs Thin Films

Amorphous CaAs films are deposited on substrates of quartz glass and sificon by rf magnetron sputtering technique in different gas ambient. First, the amorphous structure of the prepared samples is identified by x-ray diffraction. Second, analysis by radial distribution function and pair correlation function method is established to characterize the microstructure of the samples. Then, the content and bond type of hydrogen are analysed using Fourier transform infrared absorption spectroscopy. It is found that the bonded hydrogen content increases with increasing partial pressure PH of H2. However, the hydrogen content saturates at PH 〉 1 × 10^-1 Pa. Hydrogen addition shills the optical absorption edge to higher energy, decreases the dark conductivity and improves the photo-sensitivity. The optical gap, dark conductivity and photo-sensitivity of the films are dependent on the bonded hydrogen content. These results demonstrate that hydrogen has obvious passivation effects on rf sputtered amorphous GaAs thin films.     

Structural and optical properties of sputtered ZnO thin films

Zinc oxide (ZnO) and aluminium-doped zinc oxide (ZnO:Al) thin films were prepared by RF diode sputtering at varying deposition conditions. The effects of negative bias voltage and RF power on structural and optical properties were investigated. X-ray diffraction measurements (XRD) confirmed that both un-doped and Al-doped ZnO films are polycrystalline and have hexagonal wurtzite structure. The preferential 〈0 0 1〉 orientation and surface roughness evaluated by AFM measurements showed dependence on applied bias voltage and RF power. The sputtered ZnO and ZnO:Al films had high optical transmittance (>90%) in the wavelength range of 400-800 nm, which was not influenced by bias voltage and RF power. ZnO:Al were conductive and highly transparent. Optical band gap of un-doped and Al-doped ZnO thin films depended on negative bias and RF power and in both cases showed tendency to narrowing.     

The sputtering process and sputtered ion emission

In this review an attempt is made to delineate those physical characteristics of the sputtering event which are of importance for the discussion of sputtered ion emission. Emphasis is placed on the grossly disordered nature of the sputtering site as the sputtered atoms depart. The electronic nature of a disordered site is discussed and attention is drawn to the inapplicability of bulk density of states concepts to this region. In particular, it is argued that the concept of a band gap in the surface state density is inappropriate. Sputtered ion emission models are reviewed with respect to their consistency with the physics of the sputtering event and with the general features of sputtered ion emission. It is argued that a self-consistent model can be developed in the framework of which sputtered ion emission from oxidized surfaces, from clean alloy surfaces, and from the pure elements can be understood.     

Study on oxygen source and its effect on film properties of ZnO deposited by radio frequency magnetron sputtering

The structural and optical properties of ZnO thin films deposited at various oxygen partial pressure and rf-power of rf magnetron sputtering were investigated. The sputtered ZnO films are mainly formed with the oxygen which was supplied from a sputtering gas; therefore the film stoichiometry can be controlled by the oxygen partial pressure and rf-power. From photoluminescence study, it was found that the wide emission band above 550 nm was observed due to an increase of oxygen vacancies when the ZnO film changed from O-rich to Zn-rich. The chemical stoichiometry of the film will help us to understand the formation mechanism of intrinsic defects in ZnO films.     

Structural and optical properties of zinc nitride films prepared by rf magnetron sputtering

Zinc nitride films were prepared on quartz substrates by rf magnetron sputtering using pure zinc target in N₂-Ar plasma. X-ray diffraction (XRD) analysis indicates that the films just after deposition are polycrystalline with a cubic structure and a preferred orientation of (4 0 0). X-ray photoelectron spectroscopy (XPS) analysis also confirms the formation of N-Zn bonds and the substitution incorporation of oxygen for nitrogen on the surface of the films. The optical band gap is calculated from the transmittance spectra of films just after deposition, and a direct band gap of 1.01 ± 0.02 eV is obtained. Room temperature PL measurement is also performed to investigate the effect of defect on the band gap and quality of the zinc nitride films.     

Molybdenum thin films fabricated by rf and dc sputtering for Cu(In,Ga)Se_2 solar cell applications

Molybdenum(Mo) thin films, most commonly used as electrical back contacts in Cu(In?Ga)Se2(CIGS) solar cells, are deposited by rf and dc magnetron sputtering in identical systems to study the discrepancy and growth mechanisms of the two sputtering techniques. The results reveal that though different techniques generally de?posit films with different characteristic properties, Mo films with similar structural and physical properties can be obtained at respective suitable deposition conditions. Highly adhesive and conductive Mo films on soda lime glass are further optimized, and the as-fabricated solar cells reach efficiencies as high as 9.4% and 9.1% without an antireflective layer.     

Surface oxidation kinetics of sputtering targets

Films of compounds can be deposited by sputtering a metal or alloy target in an atmosphere containing a suitable reactive gas. Both Al and 90 : 10 In : Sn targets have been sputtered in argon/oxygen mixtures to obtain Al₂O₃ and indium tin oxide films. The experiments were carried out in a planar magnetron sputtering system with both dc and rf excitation. To investigate the kinetics of the reactive sputtering process, the time dependence of the total gas pressure was measured after a change in oxygen flow rate or sputtering power; a capacitance manometer gave accurate and reproducible results. There were simultaneous changes in the rf matching conditions when rf excitation was used. These changes can be attributed to the formation of an oxide on the target surface. The time dependence of the oxygen pressure measured for the Al and In : Sn targets have been used to compare various models of the reactive sputtering process. Fitting of the experimental values to these models yields values of the equilibrium oxide thickness on the target and these have been compared with measured values. For rf sputtering of an Al target at 500 W with flow rates of $\text{3}\text{ml}\text{min}$ and $\text{2.2}\text{ml}\text{min}$ for argon and oxygen respectively, both the calculated and the measured value of the oxide thickness is 100 nm.     

AgI nanostructure development in sputter-disordered and Al-doped Ag films probed by XRD,SEM, optical absorption and photoluminescence

By use of thermal evaporation and rf magnetron sputtering 300 and 500 Å thick Ag and Ag (Al) films were prepared. γ-AgInanoparticles were formed during (a) short time (2–5 min) iodization of undoped thermally evaporated Ag films, (b) longer time (12 h) iodization of undoped rf sputtered Ag films and (c) short time (2–15 min) iodization of thermally evaporated Ag_0.95Al_0.05 and Ag_0.90Al_0.10 films of 500 and 300 Å thickness respectively. Both rf sputtered and Al doped Ag films yielded ～20 to ～60 nm sized γ-AgI particles upon iodization. Optical absorption spectra reveal Z_1,2 and Z₃ exciton transitions with increased broadening of γ-AgI nanoparticles, suggesting the effect of disorder produced during film formation. Blue shift observed with increasing film thickness could be the effect of decreasing particle size, thereby increasing the quantum confinement effects. Photoluminescence studies show that the donor-acceptor recombination rate, enhanced by 25% for Ag_0.95Al_0.05I film relative to that of undoped AgI, is due to the tight binding of Al to surface defect sites.     

ZnO:Ag film growth on Si substrate with ZnO buffer layer by rf sputtering

ZnO buffer layers were deposited on n-Si (1 0 0) substrate by rf magnetron sputtering at a lower power of 40 W. Then Ag-doped ZnO (SZO) films were deposited on buffered and non-buffered Si at a higher sputtering power of 100 W. The effects of buffer layer on the structural, electrical and optical properties of SZO films were investigated. The three-dimensional island growth process of ZnO buffer layer was discussed. The energy band diagram of p-SZO/n-Si heterojunction was constructed based on Anderson's model. Results show the ZnO buffer layer leads to better properties of SZO film, including larger grain size, smoother surface, higher carrier mobility, better rectifying behavior, lower interface state density, and weaker deep-level emission. It is because the ZnO buffer layer effectively relaxes the partial stress induced by the large lattice mismatch between SZO and Si.     

Fabrication and Frequency Response Characteristics of AlN-Based Solidly Mounted Resonator

Film bulk acoustic resonator (FBAR) with solidly mounted resonator (SMR)-type is carried out by rf magnetic sputtering. To fabricate SMR-type FBAR, alternative high and low acoustic impedance layers, Mo/Ti multilayer, are adopted as Bragg reflector deposited by dc magnetron sputtering. The influences of sputtering pressure, substrate temperature and sputtering power on the surface roughness of Bragg reflector layer are discussed. From the atom force microscopy (AFM) analysis, the surface roughness of the Bragg reflector is improved remarkably by controlling deposition conditions. Under the appropriate sputtering condition, AlN thin films with highly c-axis-preferred orientation are deposited by rf magnetron sputtering. The performance of fabricated Mo/Ti SMR shows that the electromechanical coupling coefficient is 3.89%, the series and parallel resonant frequencies appear at 2.49 and 2.53GHz, with their quality factors 134.2 and 97.6, respectively.     

Optical band gap of zinc nitride films prepared on quartz substrates from a zinc nitride target by reactive rf magnetron sputtering

Polycrystalline zinc nitride films have been synthesized onto quartz substrates from the zinc nitride target and the nitrogen working gas by reactive rf magnetron sputtering at room temperature. X-ray diffraction study indicates that polycrystalline zinc nitride films are of cubic structure with the lattice constant a = 0.979(1) nm and have preferred orientations with (3 2 1) and (4 4 2). Its absorption coefficients as well as the film thickness are calculated from the transmission spectra, which are measured with a double beam spectrophotometer. The optical band gap has been determined from the photon energy dependence of absorption coefficient, an indirect transition optical band gap of 2.12(3) eV has been obtained.     

高电导a-Si:H:Y合金的电输运特性

本文报道由rf溅射技术将稀土元素Y掺入非晶硅,当掺Y浓度为20％左右时,获得了室温直流电导率为2×10¹Ω^-1·cm^-1的a-Si:H:Y合金膜。测量表明该合金膜是n型。变温电导测量指出,在测量温度范围内lnσ与l/T的关系可拟合于两条直线。对于衬底温度为260℃,290℃和330℃溅射的合金膜,其转折点分别出现在～70℃,～75℃和～90℃。这表明a-Si:H:Y合金膜存在两种电传导机制:在室温附近电子在Y施主杂质带内跳跃传导,在高温情况下电子在导带延展态内传导。并且得到Y施主杂质带中心处于导带E_c以下0.06—0.07eV。 关键词：     

The effect of low-energy ion bombardment on the density and crystal structure of thin films

The effect of low-energy ion bombardment on the growth and properties of thin films deposited by rf plasma sputtering at low substrate temperatures is studied. The dependences of the film thickness, density, crystal structure, and conductivity on the bias voltage applied to the substrate are obtained. At biases ranging from 0 to −30 V, nickel films are polycrystalline; at higher biases, they exhibit axial (111) texture. At the bias −60 V, the density of the Ni films is close to that of the bulk metal and the crystal structure of the films is the most ordered. With a further increase in the bias, the density of the films drops because of gas (argon and residual gases) atoms incorporated into the films. The same bias dependence of the density is observed for amorphous films of binary alloys of d and f metals. In this case, the films deposited at the substrate bias −40 V have the highest density.     

Deposition of nanostructured crystalline and corrosion resistant alumina film on bell metal at low temperature by rf magnetron sputtering

Aluminium oxide films deposited by rf magnetron sputtering for protective coatings have been investigated. The alumina films are found to exhibit grainy surface microstructure. The grain size, structure and density depend on different system parameters such as argon and/or oxygen flow rate and applied rf power etc. The effect of transition of the discharge from metallic to reactive mode on the surface characteristics of the alumina film is studied. X-ray diffractometry reveals that in poisoned mode of sputtering and under optimized power and pressure, crystalline alumina film can be grown. Different system conditions are optimized for corrosion resistant aluminium oxide films with good adhesion properties. Nanostructured alumina film is obtained at lower pressure (8 × 10⁻⁴ to 9 × 10⁻⁴ Torr) by rf reactive magnetron sputtering.     

Resistive switching in Au/TiO2/Pt thin film structures on silicon

The conditions and mechanisms of preliminary treatment in strong electric fields (forming) and subsequent resistive switching in Au/TiO₂/Pt thin-film structures on silicon were investigated. The thin TiO₂ films in these structures were prepared by different methods, namely, vacuum evaporation of metallic titanium followed by thermal oxidation in air and radio-frequency cathode sputtering of titanium dioxide from a powder target. The current-voltage and voltage-capacitance characteristics of the structures, as well as the dependences of their conductivity on the time of exposure to a dc voltage of different polarities and on the temperature were measured. The data obtained permitted the conclusion that the physical mechanism underlying the forming process consists in a sharp increase in the density of surface states in TiO₂ films due to the electric breakdown of the Schottky barrier at the contact with the platinum electrode, whereas the resistive switching of the structures is governed by the variation in the population of surface states in the TiO₂ band gap and/or in the defect concentration in the barrier region of the structures acted upon by voltage pulses of different polarities.     

Electrical and optical properties of reactive dc magnetron sputtered silver-doped indium oxide thin films: role of oxygen

Silver-doped indium oxide thin films have been prepared on glass and quartz substrates at room temperature (300 K) by a reactive dc magnetron sputtering technique using an alloy target of pure indium and silver (80:20 at. %). During sputtering, the oxygen flow rates are varied in the range 0.00–2.86 sccm keeping the magnetron power constant at 40 W. The resistivity of these films is in the range 10⁰–10^-3 Ω cm and they show a negative temperature coefficient of resistivity. The films exhibit p-type conductivity at an oxygen flow rate of 1.71 sccm. The work function of these silver–indium oxide films has been measured by a Kelvin probe technique. The refractive index of the films (at 632.8 nm) varies in the range 1.13–1.20. Silver doping in indium oxide narrows the band gap of indium oxide (3.75 eV). PACS 73.30.+y; 81.15.Cd; 78.20.Ci; 73.61.Le     

Influence of sputtering conditions on ionic conductivity of LiPON thin films

LiPON films were deposited using radio-frequency magnetron sputtering in a pure N₂ gas atmosphere. The influence of rf power, N₂ pressure, target–substrate distance and target density on thin film composition and ionic conductivity has been studied. Impedance measurements performed between 25 and 80 °C have indicated that ionic conductivity increases with nitrogen incorporation into the glass structure. An increase in the deposition rate with the target density has also been observed.     

Lattice deformation in thin BiFeO3 and Ba0.8Sr0.2TiO3 epitaxial films on (001)MgO single crystalline substrates

Bi_0.98Nd_0.02FeO₃ (BNFO) and Ba_0.8Sr_0.2TiO₃ (BST) epitaxial films produced by rf cathode sputtering on (001)MgO substrates are studied by means of XRD. The thickness dependences of uniform lattice deformations and linear dislocation density, generated due to epitaxial stress relaxation, are plotted.     

Sputtering in the high-voltage electron microscope

The 1.0 MeV electron microscope has been used to observe and analyse transmission sputtering caused by the electrons in the incident beam. The method, reviewed here, is particularly suitable for investigating low energy collision events. Total sputtering yields and angular distributions of sputtered atoms have been measured for (111) gold films to within a few eV of the sputtering threshold energy. It is shown that the results can be explained by the sputtering of surface atoms either directly by electrons, or indirectly by collision sequences generated down 〈110〉 directions. The necessity of using a many-body collision model to interpret the results is stressed. High resolution electron microscopy has been used to study the surface structure of (111) gold films during sputtering on a near-atomic level. It is shown how the results confirm a model where surface roughness develops due to the migration and agglomeration of surface vacancies produced during sputtering. The future scope of the 1.0 MeV electron microscope as an analytical tool for sputtering is also discussed. It is suggested that the rôle of long range focussed collision sequences in sputtering may be determined for materials of medium atomic number. A need for further high resolution studies of sputtered surfaces is identified; such studies are seen as complementary to those by other surface analysis techniques.