Thermoelectric properties of nanostructured Al-substituted ZnO thin films

ZnO:Al nano-polycrystalline thin films were deposited by radio-frequency magnetron sputtering on glass substrates. The analysis of the morphology reveals well-connected whiskers with a preferred c-axis orientation perpendicular to the substrate and a dense columnar grain structure. The as-deposited films exhibited a low electrical resistivity of 1 × 10^− 3 Ω cm. Annealing in air produces an increase of the resistivity by more than three orders of magnitude and an increase in the absolute value of the Seebeck coefficient proportional to the resistivity. Annealing of the as-deposited sample in reducing Ar/H₂ atmosphere leads to a decrease in both the resistivity and the absolute value of the Seebeck coefficient. The change in the electrical transport properties is caused by the absorption and desorption of oxygen. Both resistivity and Seebeck coefficient recover to their initial values during annealing of the air-treated sample in reducing Ar/H₂ atmosphere, indicating a reversible process. The analysis by transmission electron microscopy after annealing reveals a stable columnar grain structure with an increase of the grain size. The increase in grain size is larger when the sample is annealed in reducing rather than in oxidising atmosphere. In summary, the reducing Ar/H₂ atmosphere was found to be advantageous for the thermoelectric properties resulting in a maximum power factor of 0.3 mW/K²m at 800 K.     

On the characterization of Al-0.11 wt.%Sc alloy thin film

The characterizations (i.e., film adhesion, optical reflectivity, illumination aging and anti-corrosion behavior) should be considered for the metallic thin film served as a candidate of autolamp materials. The characterizations of three candidates (i.e., sputtered Al-0.11 wt.% Sc, sputtered pure Al, and thermally evaporated pure Al) have been investigated and compared. The sputtered Al-0.11 wt.% Sc film performed much better than any of the other two regarding the characterizations concerned. Examining through AFM, the sputtered Al-0.11 wt.% Sc revealed better surface property (including flatness and smoothness) than the others. Excellence in surface properties and optical reflectivity for the Al-0.11 wt.% Sc may arise from grain refinement in the film. Under long-term illumination, the films led to grain growth and hillocks formation. This illumination aging degraded the surface properties and optical reflectivity of the films, especially for the pure Al films. The sputtered Al-0.11wt.%Sc revealed more corrosion-resistance than the other films in 5% NaCl solution regardless whether or not under illumination aging.     

Preparation and characterization of sputtered Cu films containing insoluble Nb

Copper films containing various amounts of insoluble Nb (up to 24.7 at.%) were prepared by r.f. magnetron sputtering. The crystallography and microstructure of the films were investigated for as-deposited and annealed Cu(Nb) thin films. Cu(Nb) thin films are found to consist of non-equilibrium supersaturated solid solution of Nb in Cu with a nanocrystalline microstructure. X-ray diffraction and scanning electron microscope analyses revealed a reduction in the grain sizes of the films with increasing Nb content in the films leading to a grain refinement. The electrical resistivity of as-deposited and annealed Cu(Nb) thin films is found to be low for an Nb content 2.7 at.%. Significant drops in the resistivity were observed for the high Nb contents after annealing at 530 °C which may be due to grain growth and formation of Nb-bearing phase in the film. Microhardness of the films was found to increase with the Nb concentration due to the combined effects of grain refinement and the solute strengthening of Nb.     

Resistivity and adhesive strength of thin film metallizations on single crystal quartz

Resistivity and adhesive strength were measured for the thin films 450 A Cr-1800 A Au, 450 A Cr-1000 A Mo-1800 A Au, 450 A Cr-1000 A Ni-1800 A Au, 450 A Mo-1800 A Au, 1800 A Au, and 2000 A Al on z-and AT-oriented single crystal quartz substrates in the as-deposited condition as well as after thermal annealing at 380 degrees C and 450 degrees C for 30 min in air or vacuum. The Cr-Au films exhibited significant resistivity increases after thermal annealing which were caused by the interdiffusion of Cr and Au. Barrier layers of Mo or Ni limited such increases after heat treatment. The Mo-Au, Au, and Al films exhibited resistivity decreases following thermal annealing. The mean adhesive strengths of the Cr-Au, Cr-Mo-Au, and Cr-Ni-Au films were excellent in the as-deposited and annealed conditions, ranging from 41 MPa to 70 MPa. The Mo-Au and Au films maintained relatively poor adhesion under all circumstances. Heat treatment improved the poor adhesive strength of the as-deposited Al films to values exceeding 63 MPa. Resistivity and adhesive strengths did not differ significantly between the z- and AT-oriented substrates.     

Low temperature synthesis of sol–gel derived Al-doped ZnO thin films with rapid thermal annealing process

A series of sol–gel derived Al-doped ZnO (AZO) thin films with rapid thermal annealing process at low temperature were studied to examine the influence of annealing temperature and the Al doping concentration on their microstructure, electrical and optical transport properties. Crystalline AZO thin films were obtained following an annealing process at temperatures between 400 and 600 °C for 10 min in argon gas ambient. AZO thin films with Al doping of 1 at% were oriented more preferentially along the (002) direction, and have larger grain size and lower electrical resistivity, while the highest average optical transmittances of 92% were observed in AZO films with Al doping of 2 at%. With the annealing temperature increasing from 400 to 600 °C, the grain size of AZO films increased, the optical transmittance became higher, and the electrical resistivity decreased to a lowest value of 1.2 × 10⁻⁴ Ω cm resulting from the increase of the carrier concentration and the mobility.     

Microstructure and properties of Al-doped ZnO thin films by nonreactive DC magnetron sputtering at room temperature following rapid thermal annealing

A series of Al-doped ZnO (AZO) thin films deposited by nonreactive DC magnetron sputtering at room temperature following rapid thermal annealing was studied to examine the influence of these Al doping concentration, sputtering power and annealing temperature on their microstructure, electrical and optical transport properties. AZO thin films with Al dopant of 3 wt% were oriented more preferentially along the (002) direction, bigger grain size and lower electrical resistivity The resistivity of AZO films decreases with the increase of Al content from 1 to 3 wt%, sputtering power from 60 to 100 W and the annealing temperature from 50 to 250 °C. Sputtering power and annealing had some effect on the average transmittance of AZO thin films. For AZO thin films with Al doping level of 3 wt%, the lowest electrical resistivity of 5.3 × 10⁻⁴ Ω cm and the highest optical transmittance of 88.7% could gain when the sputtering power was 100 W and the annealing temperature was 200 °C or above.     

The effect of rapid thermal annealing on structural and electrical properties of TiB2 thin films

This work reports on the effect of post-deposition rapid thermal annealing on the structural and electrical properties of deposited TiB₂ thin films. The TiB₂ thin films, thicknesses from 9 to 450 nm, were deposited by e-beam evaporation on high resistivity and thermally oxidized silicon wafers. The resistivity of as-deposited films varied from 1820 μΩ cm for the thinnest film to 267 μΩ cm for thicknesses greater than 100 nm. In the thickness range from 100 to 450 nm, the resistivity of TiB₂ films has a constant value of 267 μΩ cm.

A rapid thermal annealing (RTA) technique has been used to reduce the resistivity of deposited films. During vacuum annealing at 7 × 10⁻³ Pa, the film resistivity decreases from 267 μΩ cm at 200 °C to 16 μΩ cm at 1200 °C. Heating cycles during RTA were a sequence of 10 s. According to scanning tunneling microscopy analysis, the decrease in resistivity may be attributed to a grain growth through polycrystalline recrystallization, as well as to an increase in film density.

The grain size and mean surface roughness of annealed films increase with annealing temperature. At the same time, the conductivity of the annealed samples increases linearly with grain size. The obtained results show that RTA technique has a great potential for low resistivity TiB₂ formation.     

A study of hillock formation on Al---Ta alloy films for interconnections of TFT-LCDs

The relationship between hillock formation and microstructure was studied in Al---Ta alloy films for interconnections of thin-film transistor-liquid-crystal displays. In-situ scanning electron microscopy observation of hillock formation, transmission electron microscopy studies of microstructures of both the hillock and film, and in-situ stress measurements during isothermal annealing were carried out on Al-2at.%Ta alloy films deposited on glass substrates by d.c. magnetron sputtering. Hillock size increased with annealing temperature and time during variable temperature and isothermal annealing, respectively. Macroscopic hillock number density saturated soon after the appearance of hillocks. New hillocks were observed on the outer perimeter of old hillocks. The distance between hillocks ranged from 20 μm to 100 μm, an extremely large distance in comparison with the grain size. The relationship between hillock formation and microstructures on Al---Ta alloy films can be explained by a model in which hillock formation due to lateral diffusion, i.e. diffusion in the film plane, results in compressive stress relaxation in a large area around the hillock. The fine-grained film structure caused by the addition of Ta plays an important role in reducing hillock density.     

Corrosion behavior of Al, Sc-co-doped ZnO thin films in 3.5% NaCl solution

The corrosion behavior of Al, Sc-co-doped zinc oxide films (with Sc-dopant varying in 0, 0.01, 0.13, 0.24 and 1.07 wt.% Sc) in 3.5% NaCl solution has been investigated. It was compared to that of the commercial indium tin oxide (ITO) thin film. The films were prepared by sputtering on the ZnO (4 N) target with RF and on the targets of Al (4 N), Al-0.4 wt.% Sc, Al-0.8 wt.% Sc, Al-1.7 wt.% Sc alloy and pure Sc with DC. The electrochemical studies revealed that the corrosion resistance of the films increases in the order AZO < Sc-doped AZO < ITO < annealed AZO (at 300 °C for 1 h). The AZO doped with higher Sc-content is more resistant to corrosion. Examining the surface morphology through atomic force microscope (AFM) and scanning electron microscope (SEM), the film, which is more susceptible to corrosion depicted a rougher surface. The electrical resistivity of the films maintained almost unchanged in 120-h test. However, the optical transmittance varied with the concentration of Sc-dopant in the films. The higher Sc-dopant in the film the more stable it is to maintain higher transmittance. Higher corrosion resistance for the Sc-doped AZO in comparison with the monotonic AZO is attributed to a release of lattice strain by the Sc (III)-dopant which its size is similar to Zn (II). The most corrosion-resistant specimen (i.e., Sc-doped AZO annealed at 300 °C for 1 h) is considered to have the complete release of the lattice strain.     

Effect of heat treatment on characteristics of nanocrystalline ZnO films by electron beam evaporation

Zinc oxide thin films have been grown on glass substrate at room temperature by electron beam evaporation and then were annealed in annealing pressure 600 mbar at different temperatures ranging from 250 to 550 °C for 30 min. Electrical, optical and structural properties of thin films such as electrical resistivity, optical transmittance, band gap and grain size have been obtained as a function of annealing temperature. X-ray diffraction has shown that the maximum intensity peak corresponds to the (002) predominant orientation for ZnO films annealed at various temperatures. The full width at half maximum, decreases after annealing treatment which proves the crystal quality improvement. Scanning electron microscopy images show that the grain size becomes larger by increasing annealing temperature and this result agrees with the X-ray diffraction analysis.     

Post-annealing properties of aluminum-doped zinc oxide films fabricated by ion beam co-sputtering

Aluminum-doped zinc oxide (AZO) films were fabricated by using the ion beam sputter deposition (IBSD) method with dual metallic targets, Al and Zn, co-sputtered by argon ion beam in an oxygen ambient. Structural and electrical properties of AZO films before and after annealing were ex situ investigated by the X-ray diffractometer and Hall measurement with the Van der Pauw method, respectively. The intense (002) diffraction peak and simultaneously the low resistivity were observed in the as-deposited film. The resistivity of the film after 400 °C post-anneal increased more than two orders of magnitude than that of the as-deposited film resulting from the decrease of the donor concentration and mobility in the AZO film. The residual stress was derived from the results of the XRD patterns. Finally, it was found that the film resistivity increased as the annealing temperature increased and a corresponding shift of the energy band gap was observed.     

The effect of annealing on Al-doped ZnO films deposited by RF magnetron sputtering method for transparent electrodes

In this study, transparent conducting Al-doped zinc oxide (AZO) films with a thickness of 150 nm were prepared on Corning glass substrates by the RF magnetron sputtering with using a ZnO:Al (Al₂O₃: 2 wt.%) target at room temperature. This study investigated the effects of the post-annealing temperature and the annealing ambient on the structural, electrical and optical properties of the AZO films. The films were annealed at temperatures ranging from 300 to 500 °C in steps of 100 °C by using rapid thermal annealing equipment in oxygen. The thicknesses of the films were observed by field emission scanning electron microscopy (FE-SEM); their grain size was calculated from the X-ray diffraction (XRD) spectra using the Scherrer equation. XRD measurements showed the AZO films to be crystallized with strong (002) orientation as substrate temperature increases over 300 °C. Their electrical properties were investigated by using the Hall measurement and their transmittance was measured by UV-vis spectrometry. The AZO film annealed at the 500 °C in oxygen showed an electrical resistivity of 2.24 × 10^− 3 Ω cm and a very high transmittance of 93.5% which were decreased about one order and increased about 9.4%, respectively, compared with as-deposited AZO film.     

Thickness and temperature effects on electrical resistivity of (Bi0.5Sb0.5)2Te3 thin films

Thin films of (Bi_0.5Sb_0.5)₂Te₃ of different thickness were deposited on glass substrate by the flash evaporation method in a vacuum of 1 × 10^− 5 Torr. X-ray diffraction and transmission electron microscope analysis indicates that these films are polycrystalline even in the as-deposited state and the post-deposition annealing leads to grain growth. Electrical resistivity studies were carried out on these films as a function of temperature (300– 450 K) and film thickness (450–2000 Å). Temperature dependence of electrical resistivity shows that (Bi_0.5Sb_0.5)₂Te₃ films are semiconducting. It is found that electrical conduction activation energy decreases with increase of film thickness and this observation is explained based on the Slater model. Thickness dependence of electrical resistivity is analyzed using the effective mean free path model of size effect with perfect diffuse scattering. This analysis leads to the evaluation of the important physical parameters i.e., mean free path and bulk resistivity of hypothetical bulk.     

The microstructure of aluminium thin films on amorphous SiO2

The microstructure of aluminium thin films deposited onto amorphous SiO₂ by tungsten filament evaporation was studied by transmission and scanning electron microscopy and grazing-incidence X-ray diffraction. The early stages of film growth were characterized by an island or connected-network structure and a random grain orientation. The thickness at which complete coverage occurred ranged from 15 nm at 295 K to 100 nm at 625 K, and above this thickness a 〈111〉 fibre texture became apparent.The grain size distribution of films 1 μm thick was log-normal and the average grain size ranged from 500 nm for deposition at 295 K to 4 μm for deposition at 675 K. “Growth hillocks” were observed on the surface of films deposited at 295 K but were absent when higher substrate temperatures were used. Annealing caused grain growth and the formation of “annealing hillocks” which were of different structure from the growth hillocks.Examination of a small number of electron-beam-deposited aluminium films showed these to have similar microstructures to the filament-evaporated films, whereas sputter-deposited films were characterized by a smaller grain size and random orientation.     

Structural, electrical and optical studies of SILAR deposited cadmium oxide thin films: Annealing effect

Successive ionic layer adsorption and reaction (SILAR) method has been successfully employed for the deposition of cadmium oxide (CdO) thin films. The films were annealed at 623 K for 2 h in an air and changes in the structural, electrical and optical properties were studied. From the X-ray diffraction patterns, it was found that after annealing, H₂O vapors from as-deposited Cd(O₂)_0.88(OH)_0.24 were removed and pure cubic cadmium oxide was obtained. The as-deposited film consists of nanocrystalline grains of average diameter about 20-30 nm with uniform coverage of the substrate surface, whereas for the annealed film randomly oriented morphology with slight increase in the crystallite size has been observed. The electrical resistivity showed the semiconducting nature with room temperature electrical resistivity decreased from 10⁻² to 10⁻³ Ω cm after annealing. The decrease in the band gap energy from 3.3 to 2.7 eV was observed after the annealing.     

Rapid thermal annealed Al-doped ZnO film for a UV detector

A low-resistive Al-doped ZnO (AZO) film was achieved by rapid thermal annealing. A co-sputtering method was used in the initial growth of AZO films and a rapid annealing process was performed on the as-deposited AZO film under N₂ atmosphere for 3 min. An as-deposited AZO film had an optical transmittance of 84.78% at 550 nm and a resistivity of 7.8 × 10^− 3 Ω cm. A rapid annealing process significantly improved the optical transmittance and electrical resistivity of the AZO film to 99.67% and 1 × 10^− 3 Ω cm, respectively. The structural changes of the AZO films were investigated by X-ray diffraction and transmission electron microscopy. The high quality AZO film was used to fabricate a metal-semiconductor-metal (MSM) structure for a UV detector. The MSM device provided a stable current of 25 μA at a bias of 2 V in a dark condition. Under UV illumination, the MSM device was highly responsive to UV light uniformly and repeatedly, and it enhanced the current by 80% at 45 μA. This rapid thermal annealing process may provide a useful method to fabricate quality AZO films for photoelectric applications with a low thermal budget.     

Effect of thickness on structural,optical, electrical and morphological properties of nanocrystalline CdSe thin films for optoelectronic applications

This paper presents effect of thickness on the physical properties of thermally evaporated cadmium selenide thin films. The films of thickness 445 nm, 631 nm and 810 nm were deposited employing thermal evaporation technique on glass and ITO coated glass substrates followed by thermal annealing in air atmosphere at temperature 300 °C. The as-deposited and annealed films were subjected to the XRD, UV–Vis spectrophotometer, source meter, SEM and EDS to find the structural, optical, electrical, morphological and compositional analysis respectively. The structural analysis shows that the films have cubic phase with preferred orientation (1 1 1) and nanocrystalline nature. The structural parameters like inter-planner spacing, lattice constant, grain size, number of crystallites per unit area, internal strain, dislocation density and texture coefficient are calculated. The optical band gap is found in the range 1.69–1.84 eV and observed to decrease with thickness. The electrical resistivity is found to increase with thickness for as-deposited films and decrease for annealed films. The morphological studies show that the as-deposited and annealed films are homogeneous, smooth, fully covered and free from crystal defects like pin holes and voids. The grains in the as-deposited films are densely packed, well defined and found to be increased with thickness.     

Agglomeration behavior of Ag films suppressed by alloying with some elements

Agglomeration behavior of co-sputtered Ag-Au, Ag-Cu and Ag-Al thin films was investigated. The results showed that Ag-Au films agglomerated during annealing in a similar way to pure Ag films, but a degree of suppression of this effect was observed in Ag-Cu and Ag-Al films. In particular, no void formation was observed in Ag-Al films for annealing temperatures up to 600 °C. Different mechanisms are proposed to explain agglomeration suppression and the prevention of Ag atom migration, including the precipitation Cu atoms in Ag-Cu films, and the formation of very thin Al-oxide layers at the film surface and at the interface with the substrate. The resistivity of Ag-Au film increased by annealing due to the agglomeration, but that of Ag-Cu and Ag-Al films did not. However, the resistivity of as-deposited Ag-Al film was too high to be reduced enough by annealing.     

Correlation study between structural, magnetic and transport properties of annealed Co thin films

This paper presents structural, magnetization and transport properties measurements carried out on as-deposited Co (400 Å) thin film as well as samples annealed in the temperature range 100-500 °C in steps of 100 °C for 1 h. The samples used in this work were deposited on float glass substrates using ion beam sputtering technique. The magnetization measurements carried out using MOKE technique, clearly indicates that as-deposited as well as annealed samples up to 500 °C show well saturation magnetization with applied magnetic field. The as-deposited sample shows coercivity value (H_c) of 26 Oe, and it is increased to 94 Oe for 500 °C-annealed sample. A minimum coercivity value of 15 Oe is obtained for 200 °C annealed sample. The XRD measurements of as deposited films show microcrystalline nature of Co film, which becomes crystalline with increase in annealing temperature. The corresponding resistivity measurements show gradual decrease in resistivity. AFM technique was employed to study the surface morphology of as deposited film as well as annealed thin films. Observed magnetization, and resistivity behaviour is mainly attributed to the (i) change in crystal structure (ii) increase in grain size and (iii) stress relaxation due to the annealing treatment.     

Effect of deposition temperature and thermal annealing on the dry etch rate of a-C: H films for the dry etch hard process of semiconductor devices

The effect of deposition and thermal annealing temperatures on the dry etch rate of a-C:H films was investigated to increase our fundamental understanding of the relationship between thermal annealing and dry etch rate and to obtain a low dry etch rate hard mask. The hydrocarbon contents and hydrogen concentration were decreased with increasing deposition and annealing temperatures. The I(D)/I(G) intensity ratio and extinction coefficient of the a-C:H films were increased with increasing deposition and annealing temperatures because of the increase of sp² bonds in the a-C:H films. There was no relationship between the density of the unpaired electrons and the deposition temperature, or between the density of the unpaired electrons and the annealing temperature. However, the thermally annealed a-C:H films had fewer unpaired electrons compared with the as-deposited ones. Transmission electron microscopy analysis showed the absence of any crystallographic change after thermal annealing. The density of the as-deposited films was increased with increasing deposition temperature. The density of the 600 °C annealed a-C:H films deposited under 450 °C was decreased but at 550 °C was increased, and the density of all 800 °C annealed films was increased. The dry etch rate of the as-deposited a-C:H films was negatively correlated with the deposition temperature. The dry etch rate of the 600 °C annealed a-C:H films deposited at 350 °C and 450 °C was faster than that of the as-deposited film and that of the 800 °C annealed a-C:H films deposited at 350 °C and 450 °C was 17% faster than that of the as-deposited film. However, the dry etch rate of the 550 °C deposited a-C:H film was decreased after annealing at 600 °C and 800 °C. The dry etch rate of the as-deposited films was decreased with increasing density but that of the annealed a-C:H films was not. These results indicated that the dry etch rate of a-C:H films for dry etch hard masks can be further decreased by thermal annealing of the high density, as-deposited a-C:H films. Furthermore, not only the density itself but also the variation of density with thermal annealing need to be elucidated in order to understand the dry etch properties of annealed a-C:H films.