Study of trap states in zinc oxide (ZnO) thin films for electronic applications

The electrical properties of ZnO thin films grown by pulsed laser deposition were studied. Field-effect devices with a mobility reaching 1 cm2/V s show non-linearities both in the current–voltage and in the transfer characteristics which are explained as due to the presence of trap states. These traps cause a reversible threshold voltage shift as revealed by low-frequency capacitance–voltage measurements in metal insulator semiconductor (MIS) capacitors. Thermal detrapping experiments in heterojunctions confirm the presence of a trap state located at 0.32 eV

PII: S0378-7753(99)00435-8

Abstract Amorphous tin-oxide films were prepared by spray pyrolysis of SnCl P 2H O mixed with CH -COOH and deposited onto a stainless 2 2 3 Ž . Ž . steel substrate at mild temperatures 3508C . The films grown were characterized by X-ray photoelectron spectroscopy XPS and Ž . scanning electron microscopy with energy-dispersive X-ray analysis SEM-EDX . Also, they were tested as electrodes in lithium rechargeable batteries. The XPS results suggest that the substrate is thoroughly coated and that the films are composed mainly of SnO and SnO . These films exhibit good charge-discharge properties over more than 100 cycles. Heating at 6008C causes significant changes in 2 their surface composition, in the virtual disappearance of the tin component and in the presence of oxygen-bound Fe. Under these conditions, the reversible capacity dramatically fades and the cell behaves similarly to that made from uncoated substrate. q 2000 Elsevier Science S.A. All rights reserved

Photoconductivity Study of Sputter-Deposited Cu2OCu_2O Films

Cuprous oxide $(Cu_2O)$ thin films were deposited by radio frequency sputtering technique on fused silica substrates. The X-ray diffraction study of the $Cu_2O$ samples showed reflections from (111) and (200) planes of cubic $Cu_2O$. The samples were then annealed at 1008 K in nitrogen $(N_2)$ atmosphere. Annealed samples indicated strain relaxation. The samples were then characterized optically by measuring the transmittance using an ultraviolet-visible-near infrared photospectrometer. The band gap of the as-deposited samples were found to be ≈ 2.1 eV, whereas the annealed samples had a band gap of ≈ 2.6 eV. The transient photocurrent decay measurements of the annealed films indicated slow non-exponential power law decays in several time windows, indicating multiple trapping of the carriers in the deep defects within the band gap. The steady-state photo and dark current measurement and persistent photocurrent (PPC) was carried out on the annealed samples. In general, the photocurrent was found to be much smaller than the dark current. The steady-state and transient photocurrent measurements were utilized to determine the carrier lifetime-mobility product, 〈μτ 〉 of the samples and to determine the carrier mobility, 〈μ 〉

Optical Properties of Lead-Free NKN Films from Transmission and Spectral Ellipsometry

Thin films of lead-free NaxK1-xNbO3 (NKN) were prepared by pulsed laser deposition (PLD) from ceramic targets of stoichiometric NKN or doped with 6% of lithium niobate (LN). At a deposition temperature of 600 degrees C a strong dependence of film morphology on laser fluence is revealed by SEM microscopy. Optical properties like band gap and index of refraction were obtained through transmission measurements and analysis based on the Tauc model. The results were compared to spectral ellipsometry using a fit to the Tauc-Lorentz model, and slightly smaller band gap values were obtained. C-V characteristics of sandwich structures yielded high permittivity of 380 in pure NKN films and tunability at 1kHz of 3.2% at 1.5 MV/m applied electric field

Photoinduced excess carrier dynamics in PLD grown ZnO

ZnO shows a number of similarities with other wide gap semiconductor materials as, e.g., GaN as far as photoluminescence and photoconductivity are concerned. Depending on film quality a broad luminescence band is found in the yellow and or green spectral region apart from a narrow excitonic line. This study focuses on the observation of non exponential photoinduced carrier density decay in ZnO. We have deposited thin polycrystalline ZnO films on sapphire by a cyclic pulsed laser deposition process. We extracted a room temperature band gap of 3.31 eV from absorption spectroscopy measurements, and found evidence for strong sub gap Urbach tails. Photocurrent transients were measured upon pulsed laser excitation at 532 and 266 nm and compared with transient microwave conductivity decay upon excitation at 355 nm. Both measurements yield power law decay with an exponent from amp; 8722;0.3 to amp; 8722;0.4. In addition, ps pulses were used to monitor the initial photoluminescence decay near the bandgap. We have already observed similar power law behaviour in polycrystalline GaN films prepared by the same PLD reactor. The interpretation will consider the hypothesis of minority carrier capture and a model invoking thermalization in broad band tail distribution with delayed subsequent recombination during the deca

ZnO films grown by laser ablation with and without oxygen CVD

In the present work we have studied the properties of zinc oxide (ZnO) thin films grown by laser ablation of ZnO targets under different substrate temperature and background oxygen conditions. The ZnO layers were deposited with a Pulsed Laser Deposition (PLD) system on pre-nitrided (0001) sapphire (Al2O3), using the base line of a Nd:YAG laser at 1064 nm. The films were characterized by different structural and optical methods, including X-ray diffraction (XRD), scanning electron microscopy (SEM), optical transmission spectroscopy, and steady-state photoluminescence (PL). XRD analysis with rocking curves and θ–2θ scans indicates preferential growth along the c-axis direction with a full width at half maximum (FWHM) smaller than 1.5∘. Low-temperature photoluminescence (PL) showed strong excitonic emission near 3.36 eV between 9 and 65 K.FCT-POCTI/FAT/42185/2001 (LAPLAS)FCT-POCTI/FAT/48822/2002 (OPTAR