M. Parlak

The effect of substrate and post-annealing temperature on the structural and optical properties of polycrystalline InSe thin films

Abstract X-ray diffraction, scanning electron microscopy, compositional analysis and transmission measurements are performed on InSe thin films grown by thermal evaporation at different substrate and annealing temperatures of 150–250°C and 100–200°C, respectively. An analysis of structural measurements indicates that there exist three phases of In–Se system like InSe, In2Se3 and In6Se7 in the same film at low-substrate temperature, but at high-substrate temperatures, In2Se3 phase was absent. The composition of InSe films changed significantly with substrate temperature and slightly with annealing temperature because of re-evaporation of selenium. The transmission measurements were carried out at room temperature over the spectral range 0.5 to 2.5 μm. As a result of optical energy gap studies, the direct energy band gap was found to be in between 1.21 and 1.38 eV depending on substrate and annealing temperatures.

Characterization of AgGa0.5In0.5Se2 thin films deposited by electron-beam technique

AgGa0.5In0.5Se2 thin films were deposited onto a quartz substrate by the electron-beam technique. For the investigation of the annealing effect on structural, optical and electrical properties of deposited films, samples were annealed in the temperature range 300–775 °C. The composition analyses of the deposited films carried out by energy dispersive x-ray analysis measurements have shown that the deposited AgGa0.5In0.5Se2 films were indium- and gallium-rich but selenium- and slightly silver-deficient and there was a remarkable change in composition with annealing. As a result of x-ray diffraction measurements, the as-deposited films were found to have an amorphous structure and after annealing at 300 °C a polycrystalline structure with different phases was observed. However, subsequent annealing resulted in the formation of single phase AgGa0.5In0.5Se2 thin film at about 775 °C. The absorption coefficient of the films was determined from the transmission spectra and the band gap values were calculated and found to vary between 1.57 and 2.43 eV following annealing in the temperature range 300–775 °C. The refractive index (n) and extinction coefficient (k) of the films were evaluated by applying the envelope method to the transmission spectra. The spectral distributions of these quantities for both as-deposited and annealed films were determined in detail and it was observed that there has been a remarkable influence of annealing on these quantities. The electrical properties of AgGa0.5In0.5Se2 thin films were also investigated by means of temperature dependent conductivity measurements in the temperature range 100–460 K. The resistivity of the samples depending on the annealing temperature varied between 6.5 × 105 and 16 Ω cm. As a result of the hot-probe method it was observed that the as-deposited films have indicated an n-type behaviour, while all the annealed AgGa0.5In0.5Se2 thin films have shown p-type conduction.

Growth and characterization of polycrystalline InSe thin films

Abstract Indium selenide films were obtained by the thermal evaporation of undoped crystals at substrate temperatures of 210 and 300 °C. Films were found to exhibit n-type conductivity. Scanning electron microscopy established that the films grown on glass substrates held at 210 °C had an atomic content of In 46.66 Se 53.34 , whereas the films obtained under the same conditions and annealed in vacuum at 150 °C had an atomic content of In 48.13 Se 51.87 . X-Ray diffraction indicated that the films were polycrystalline in nature and crystallized in the D 1 3h space group. From an analysis of Hall and conductivity measurements, performed in the temperature ranges 110–320 K and 10–320 K respectively, thermionic emission is considered to be the predominant transport mechanism above 200 K. The current transport below 75 K is due to the hopping of carriers between localized states. In the intermediate temperature region, both thermally assisted tunnelling and thermionic emission contribute to the conduction.

Current transport mechanisms in low resistive CdS thin films

The current transport mechanisms in polycrystalline CdS thin films have been studied as a function of temperature over the temperature range 20–230 K. Conductivity data for the high temperature region has been analysed using Seto’s model of thermionic emission. At intermediate temperatures it was found that thermionic emission and tunnelling of carriers through the potential barrier both contribute to the conductivity. Below 100 K Mott’s hopping process appears to be the predominant conduction mechanism.

Silicon nanowire-silver indium selenide heterojunction photodiodes

Structural and optoelectronic properties of silicon (Si) nanowire-silver indium selenide (AgInSe2) thin film heterojunctions were investigated. The metal-assisted etching method was employed to fabricate vertically aligned Si nanowire arrays. Stoichiometric AgInSe2 films were then deposited onto the nanowires using co-sputtering and sequential selenization techniques. It was demonstrated that the three-dimensional interface between the Si nanowire arrays and the AgInSe2 thin film significantly improved the photosensitivity of the heterojunction diode compared to the planar reference. The improvements in device performance are discussed in terms of interface state density, reflective losses and surface recombination of the photogenerated carriers, especially in the high-energy region of the spectrum.

Investigation of photovoltaic properties of amorphous InSe thin film based Schottky devices

In this study, device behavior of amorphous InSe thin films was investigated through I–V, C–V and spectral response measurements onto SnO2/p-InSe/metal Schottky diode structures. Various metal contacts such as Ag, Au, Al, In and C were deposited onto amorphous p-InSe films by the thermal evaporation technique. The best rectifying contact was obtained in a SnO2/p-InSe/Ag Schottky structure from I–V measurements, while the Au contact had poor rectification. Other metal contacts (Al, In and C) showed almost ohmic non-rectifying behaviors for all samples. The ideality factor and barrier height values with the Ag contact were found to be 2 and 0.7 eV, respectively.

Studies on device properties of an n-AgIn5Se8/p-Si heterojunction diode

In this study, polycrystalline thin films of ternary AgIn5Se8 compounds with n-type conductivity were deposited on p-type Si substrates from the powder of a Ag3In5Se9 single crystal by a thermal evaporation technique. Transport and photo-transport properties of the In/n-AgIn5Se8/p-Si/Al sandwich structure were investigated by analyzing temperature-dependent dc current–voltage (I–V), and photo-response measurements carried out in the temperature range of 200–360 K. In order to obtain the series resistance (Rs) and shunt resistance (Rsh) values, the parasitic resistance (Rp = ∂V/∂I) was analyzed for forward and reverse voltages. Devices showed very good diode behavior with the rectification factor of about 104 at 1.0 V in dark. The ideality factor n and the barrier height b values of the heterojunction diode were determined by performing different I–V plots. A space charge limited current method was used to determine the conduction mechanism at a high bias region. Spectral photo-response analyses showed a shifting of the band edge to the lower energies with increasing temperature.

Structural and electrical characterization of Ag3Ga5Te9 and Ag3In5Se9 crystals

X-ray powder diffraction studies revealed that Ag 3 Ga 5 Te 9 and Ag 3 In 5 Se 9 crystallize in orthorhombic and tetragonal systems, respectively. The temperature dependent conductivity and Hall effect measurements have been carried out between 65-480 K. Ag 3 Ga 5 Te 9 exhibits p-type conduction with a room temperature conductivity of 4.3 × 10 -4 (Ω. cm) -1 and mobility less than 1 cm 2 /V S. Ag 3 In 5 Se 9 was identified to be n-type with room temperature conductivity 7.2 × 10 -5 (Ω. cm) -1 and mobility 20 cm 2 /V. s. From temperature dependence of the conductivity three different impurity ionization energies were obtained for both compounds. The anomalous behavior observed in the temperature dependence of mobility was attributed to the different features of the microstructure.

Electrical, photo-electrical, optical and structural properties of CdSe thin films deposited by thermal and e-beam techniques

In this study, electrical, photo-electrical, optical and structural analyses of CdSe thin films deposited by thermal and e-beam evaporation techniques were carried out by measuring temperature dependent conductivity, mobility, photoconductivity under different illumination intensity, photoresponse, transmission and x-ray diffraction. As a result of these measurements, it was observed that the films deposited by the thermal evaporation technique have room temperature conductivity values approximately three orders of magnitude greater than the ones deposited by e-beam evaporation. Structural analysis showed that the films deposited by both methods with the same growth parameters have a mixed structure with the contribution of cubic and hexagonal structures. Two direct band gap values were obtained at 1.71 and 1.88 eV. Also, sublinear and supralinear photoconductivity behaviours related to sensitizing and imperfection levels lying at 0.12 and 0.28 eV below the conduction band were determined. The existence of these levels was clarified together with the valence band splitting by using photoresponse measurements.

Growth, electrical and structural characterization of β-GaSe thin films

GaSe thin films were deposited onto the glass substrates kept at 200° and 300°C by the thermal evaporation of GaSe crystals under the pressure of 10−5 Torr. X-ray analysis of the films revealed that films grown at 200°C are amorphous in nature while the films grown at 300°C are polycrystalline β-GaSe. The temperature dependent electrical conductivity measurements in the region of 320–100 K for the films grown at 300°C showed that the transport mechanisms are the thermionic emission of charged carriers and the variable range hopping above and below 180 K, respectively. Space charge limited current (SCLC) studies have also been performed on these films through the current-voltage measurements at different temperatures and a dominant hole trap at 0.233 eV from the top of the valance band with a trap density of ∼1.6 × 1011 cm−3 is identified.