Correlation between morphology and electro-optical properties of nanostructured CdO thin films: Influence of Al doping

doi:10.1016/j.surfcoat.2012.10.003

Surface and Coatings Technology

Volume 213, December 2012, Pages 15-20

https://doi.org/10.1016/j.surfcoat.2012.10.003 Get rights and content

Abstract

Al doped cadmium oxide thin films have been deposited on glass substrates by sol–gel dip coating method with precursor solutions having different Al concentrations. The content of aluminum in the sol was varied from 0 to 5 at.%. X-ray diffraction, Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), UV–Visible spectroscopy and Hall effect analyses have been used to determine correlations between the film microstructure and the optoelectronic properties. X-ray diffraction analyses, clearly established that the crystallinity deteriorates by Al doping. Crystallite size calculations based on Debye Scherrer formula indicated that the CdO crystallite size decreases with Al content (from 37 to about 11 nm). SEM and AFM were applied to study the morphology and to estimate the surface roughness of the obtained films. All films were very smooth, with a characteristic spherical grain size depending on Al content. The Root Mean Square (RMS) roughness of the films varied from 4.05 to 4.35 nm with increasing Al concentration. The AFM images were also used for the analysis of the fractal behavior of the films. Doping with Al increased the films' optical transparency in the visible region and their conductivity.

Highlights

► Physical properties of the CdO:Al nanostructured films at different Al% are studied. ► Maximum carrier concentration has been achieved for 1 at.% Al doped film. ► The undoped sample has maximum carrier mobility. ► The direct optical band gap shifted to the higher energy. ► The samples have relatively smooth surface with RMS roughness of about 4 nm.

Introduction

In recent years, metal oxide semiconductor materials have attracted much attraction due to their application in various fields ranging from electronic devices such as displays [1] solar cells [2] and thin film transistors [3] to gas sensors [4], [5]. Pure cadmium oxide (CdO) is an n-type degenerate semiconductor with high electrical conductivity (10²–10⁴ S/cm) [6]. With respect to its optical properties, however, CdO has an indirect band gap of 2.06 eV and a direct one of 2.59 eV, which are lower than that of ZnO, a fact that reduces the transmittance range of this material in the visible region [7]. Doping of CdO with metallic ions of smaller ionic radius than that of Cd^2 +, like In, Sn and Al improves its electrical conduction and increases its optical energy gap [6]. Although Al is a popular dopant in ZnO thin films, reports on CdO:Al thin films are very limited [8], [9], [10], [11].

In this study, we have prepared CdO:Al thin films by the less reported sol–gel dip-coating process applying a precursor based on cadmium acetate along with methanol as the solvent, the rare-reported monoethanolamine as the stabilizer, glycerol, 2-methoxyethanol and aluminum nitrate. Maity and Chattopadhyay [11] have used cadmium acetate, isopropyl alcohol and acetic acid as the precursor material, solvent and pH controller respectively. Murali et al. [9] have used an acrylamide precursor based on cadmium chloride and Ilican et al. [10] have reported a precursor solution very similar to ours, but with higher molarities and triethylamine as the stabilizer. Here, we have utilized monoethanolamine as the stabilizer. It should be noted that while most of the existing works focus on the study of the structural, optical and/or electrical properties of the prepared films the present study mainly focuses on the correlation of structural, optical and electrical properties of Al-doped CdO films with surface morphology. SEM and AFM imaging have been applied to investigate the morphological features of CdO:Al thin films. The structural properties and surface topographies of the thin films were studied as a function of film composition for various Al concentrations.

Section snippets

Sample preparation

All samples considered in this study were deposited on soda-lime glass substrates by sol–gel dip-coating method at room temperature. Prior to dip coating, the glass substrates were first degreased by detergent, rinsed thoroughly by deionized water and then in boiling water. In order to remove macroscopic contaminations, the substrates were cleaned ultrasonically in a mixture of ethanol and acetone (each of 50% in volume) for 15 min. The latter procedure was repeated in deionized water. Finally,

Results and discussion

XRD patterns of CdO thin films doped with aluminum, whose concentration changes from 0 to 5 at.% are shown in Fig. 1. For undoped films the peaks associated to planes (1 1 1), (2 0 0), (2 2 0) and (3 1 1) of the cubic face-centered structure (JCPDS No. 5-0640 [13]) are observed. It can be seen that the films have the (1 1 1) preferred orientation. The undoped thin film has shown the highest (111) diffraction peak intensity while the peak intensities of the aluminum doped films decreased with

Conclusions

Smooth and crack-free CdO:Al thin films with nano-grain size were successfully grown on glass substrates utilizing a new precursor, by sol–gel dip-coating method. XRD results showed that the films have (111) preferred orientation and the crystallite size decreased with Al doping. SEM images revealed a flat-grain surface and single mode size distribution of grains. AFM imaging indicated high smooth surfaces with small RMS roughness values. The fractal analysis based on AFM imaging data was

References (49)

K. Matsubara et al.
Thin Solid Films
(2003)
A.A. Dakhel
Opt. Mater.
(2009)
D.M. Carballeda-Galicia et al.
Thin Solid Films
(2000)
R.K. Gupta et al.
Curr. Appl. Phys.
(2009)
K.R. Murali et al.
J. Alloys Compd.
(2010)
E.G. Birgin et al.
J. Comput. Phys.
(1999)
Z.Q. Xu et al.
Mater. Res. Bull.
(2006)
R.J. Deokate et al.
Appl. Surf. Sci.
(2008)
I. Akyuz et al.
J. Alloys Compd.
(2011)
D. Lamb et al.
Thin Solid Films
(2009)

F. Somma et al.

Thin Solid Films

(2000)

R. Kumaravel et al.

Mater. Chem. Phys.

(2010)

P.K. Ghosh et al.

Sol. Energy Mater. Sol. Cells

(2004)

B. Saha et al.

Sol. Energy Mater. Sol. Cells

(2007)

F.E. Ghodsi et al.

Sol. Energy Mater. Sol. Cells

(2008)

B. Oh et al.

J. Appl. Phys.

(2006)

P.F. Garcia et al.

J. Appl. Phys. Lett.

(2003)

G. Kiriakidis et al.

Sensor Lett.

(2008)

G. Kiriakidis et al.

Rev. Adv. Mater. Sci.

(2005)

S. Ilican et al.

Optoelectron. Adv. Mater. Rapid Commun.

(2009)

R. Mitay et al.

Sol. Energy Mater. Sol. Cells

(2006)

Powder Diffraction File, JCPDS-International Center for Diffraction Data, Pennsylvania

(1972)

A. Maiti et al.

Phys. Rev. Lett.

(1996)

C.R.M. Grovenor

J. Phys. C

(1985)

Cited by (66)

Studying of cadmium oxide/ Aluminium films for optoelectronics: structural, optical and electrical characteristics
2024, Inorganic Chemistry Communications
In the results of this research, thin films of CdO-Al were produced using the spray pyrolysis (SP) technique. The primary aim was to explore how varying concentrations of aluminum (Al) influence the physical properties of the films. The X-ray diffraction (XRD) analysis confirmed the presence of a cubic crystal structure in the pure CdO films. Additionally, it revealed that the introduction of increased Al doping led to lattice shrinkage and a reduction in lattice parameters. Elevated levels of aluminum (Al) content were correlated with a decrease in crystallite size and an augmentation in lattice strain, as indicated by both scanning electron microscopy (SEM) and X-ray diffraction (XRD) data. Additionally, an X-ray photoelectron spectroscopy (XPS) examination revealed the presence of Al3+ ions within the films. The optical energy gap of CdO-Al films exhibited a slight increase, reaching up to 3% aluminum doping. The analysis of the refractive index dispersion and non-linear refractive index revealed a decreasing trend up to 3% aluminum content, with subsequent fluctuations at higher concentrations. The measurements of sheet resistance demonstrated a noticeable decrease with an increase in aluminum content, specifically up to 3%.
Effect of oxygen and nitrogen sensitization on structural, optical and electrical properties of PbSe thin films
2023, Thin Solid Films
An effective two-step sensitization technique was implemented in PbSe thin films, leading to a remarkable improvement in the optoelectronic performance of PbSe photodetectors. The phase composition, surface and cross-sectional morphology, temperature-dependent carrier concentration and mobility as well as the detectability of PbSe films after sensitization were systematically investigated using characteristic techniques. The optimal PbSe film oxygen sensitized at 450 °C exhibited a room-temperature photodetection with a carrier concentration of 5.31 × 10¹⁶ cm⁻³, carrier mobility of 16.52 cm²/V⁻¹s⁻¹, and detectivity of 2.25 × 10⁹ Jones. High-temperature argon heat treatment and oxygen sensitization of PbSe films are expected to suppress recombination losses and significantly improve the optoelectronic properties of sensitized PbSe films due to enhanced carrier separation and transport. The enhanced photoresponse mechanism of the PbSe films through two-step sensitization method was also discussed. These results suggest that the proposed method can be applied to high-performance lead selenide photodetectors.
Integrated hydrothermal-green approach to synthesize Fe, Ag doped copper sulfide nanopowders and investigations of their thermoelectric properties
2023, Physica B: Condensed Matter
Copper sulfide and Fe, Ag doped copper sulfide nanopowders were synthesized by integrated hydrothermal-green approach, using Urtica dioica leaf extract as a solvent. With Fe and Ag doping, covellite as the major phase was sustained but reduction in crystallite size and change in morphology was observed. Decrease in electrical conductivity from ∼186.24 Scm⁻¹ for undoped copper sulfide to ∼34.97 Scm⁻¹ and ∼23.26 Scm⁻¹ on doping Fe and Ag, respectively, at 300 K, was witnessed due to formation of different phases of copper sulfide and incorporation/segregation of Ag/Fe into copper sulfide lattice. Conversely, Seebeck coefficient showed enhanced values on doping Fe and Ag. The maximum power factor of ∼22.22 μW/mK² was found for undoped copper sulfide. Thus, the incorporation/segregation of Fe/Ag ions in copper sulfide lattice and grain boundaries was found to alter the thermoelectric performance of copper sulfide.
A review of various single layer, bilayer, and multilayer TCO materials and their applications
2022, Materials Chemistry and Physics
Since fabricating the first transparent conductive oxide (TCO) material, several investigations and applications have been reported in such fields as portable and flexible electronics, solar cells and sensors about this material. This wide range of applications is because of the immense potential in this area and the demands for new devices and technologies. TCOs are used in many areas, such as energy generation, energy-saving, and sensors. In this review, various types of single-layer, bilayer and multilayer TCO materials and their optical and electrical properties, various deposition techniques and applications are investigated. It is shown how manipulation of effective parameters such as doping, growth method and temperature can change the electrical and optical properties. This can lead researchers to proper control on TCO fabricated layers to improve them in their desired context.
Growth and characterization of electron beam evaporated NiO thin films for room temperature formaldehyde sensing
2022, Sensors and Actuators A: Physical
Herein, we report the formaldehyde gas sensing performance of NiO thin films deposited by the electron beam evaporation technique. NiO thin films were grown over glass substrates by the variation of deposition temperature from room temperature (RT) to 500⁰C. The structure, chemical composition and morphological studies were conducted by XRD, FESEM, EDS, AFM, XPS, RBS and HRTEM techniques. The XRD characterization revealed the formation of NiO in the polycrystalline phase with the face-centered cubic structure without impurity phases. The surface and morphological characterization confirmed the growth of the NiO thin layer and the homogeneous distribution of Ni and O elements on the surface of the films. The optical transmittance increased with the rise in deposition temperature. The optical band gap was estimated using Tauc relation and it was shown a decreasing trend with the rise in deposition temperature. The electrical characteristics of thin films were carried out using Hall measurements and presented. The developed gas sensors tested at room temperature have displayed good selectivity towards formaldehyde with a low detection limit of 10 ppm. The NiO thin film sensor prepared at a deposition temperature of 400⁰C has exhibited a short response and recovery time of 25 s and 9 s respectively towards 50 ppm concentration of formaldehyde. The detailed formaldehyde sensing mechanism for p-type NiO thin films was discussed.
Chromium – An effective dopant for engineering the structural and the optical properties of CdO nanostructures grown by SILAR method
2021, Optical Materials
Citation Excerpt :
Thus, these doped materials can display different characteristics which usually cannot be found in that host metal oxide. Previous studies report that the optoelectronic properties of CdO nanostructures are likely to be tuned by introducing different metallic ions to the prepared CdO samples [3,7–10]. Therefore, one can hope that Cr doping may generate some promising physical properties of CdO nanostructures.
Cadmium oxide thin films were prepared with various levels (1%, 3% and 5%) of chromium dopants. Successive ionic layer adsorption and reaction (SILAR) method has been employed to prepare these thin films on glass substrates at room temperature. The changes in morphological, structural, and optical properties of the produced CdO thin films were explored. The effects of Cr doping were studied thoroughly by performing X-ray diffractometer (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy analysis (EDS), ultraviolet–visible spectrometer (Uv–vis), and photoluminescence (PL) spectrometer measurements of the thin films. The XRD measurements show that the samples have the cubic crystal structure. Surface morphologies of the samples are clearly affected by different concentrations of Cr doping. The EDS analysis affirmed the presence of Cr elements in the structure of CdO nanostructures after doping. It was observed that the band gaps calculated using absorption measurements can be manipulated from 2.51 eV to 2.87 eV with varying Cr doping rates in the CdO nanostructures. The PL spectrum displayed that the PL intensity of the CdO nanostructures may also be tuned depending on different concentrations of Cr dopings.

View all citing articles on Scopus

View full text

Correlation between morphology and electro-optical properties of nanostructured CdO thin films: Influence of Al doping

Abstract

Highlights

Introduction

Section snippets

Sample preparation

Results and discussion

Conclusions

Thin Solid Films

Opt. Mater.

Thin Solid Films

Curr. Appl. Phys.

J. Alloys Compd.

J. Comput. Phys.

Mater. Res. Bull.

Appl. Surf. Sci.

J. Alloys Compd.

Thin Solid Films

Thin Solid Films

Mater. Chem. Phys.

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

J. Appl. Phys.

J. Appl. Phys. Lett.

Sensor Lett.

Rev. Adv. Mater. Sci.

Optoelectron. Adv. Mater. Rapid Commun.

Sol. Energy Mater. Sol. Cells

Powder Diffraction File, JCPDS-International Center for Diffraction Data, Pennsylvania

Phys. Rev. Lett.

J. Phys. C