Optical constants of Zn1−xLixO films prepared by chemical bath deposition technique

doi:10.1016/j.physb.2005.05.019

Physica B: Condensed Matter

Volume 366, Issues 1–4, 1 September 2005, Pages 44-54

https://doi.org/10.1016/j.physb.2005.05.019 Get rights and content

Abstract

Zn₁₋_xLi_xO films of different Li concentrations $x = 0.0 - 0.5$ were successfully deposited on glass substrates using chemical bath deposition (CBD) technique. Optical properties of the films were studied extensively in the wavelength range 190–800 nm from the measurements of the optical transmittance (T) and optical reflectance (R). The mechanism of the optical absorption follows the rule of direct allowed transition. Both the optical energy gap E_g and absorption coefficient ( $α$ ) were found to be Li concentration-dependent. The refractive index values have been fitted to the single-oscillator models. The values obtained for the single-oscillator energy E_o are consistent with the optical gap results. The dependence of the optical constants of ZnO on the photon energy ( $h ν$ ) at selected concentrations of Li were discussed. The exponential dependence of the absorption coefficient as a function of the incident photon energy suggests that the Urbach rule is obeyed and indicated the formation of a band tail. It was found that the optical energy gap decreases with increasing Li content; on the other hand the band tail width exhibits the opposite behaviour.

Introduction

ZnO films hold a unique position among materials because they are piezoelectric materials that have been the subject of extensive studies [1], [2], [3], [4]. Its piezoelectric properties are exploited in thin films applications, such as a surface acoustic wave (SAW) devices, ultrasonic transducers and sensors [5], [6], [7]. The crystal structure of ZnO is hexagonal (wurtzite type) which is suitable for fabrication of high-quality oriented or epitaxial thin film. Each Zn atom is tetrahedrally coordinated with four O atoms and the zinc d electrons hybridize with the oxygen p electrons [8]. Now, it is well-established that doping ZnO with Li ions increases its resistivity [9], [10] and induces a ferroelectric phase suitable for optical memory devices [9], [11], [12].

Accurate knowledge of the absorption coefficient, optical band gap and refractive index of semiconductors is indispensable for the design and analysis of various optical and optoelectronic devices. Usually ZnO films were fabricated with various deposition techniques [13], [14], [15], but high-quality films prepared at low temperatures are specially required in the film technology. In the last years, chemical bath deposition (CBD) has emerged as an excellent method for the deposition of polycrystalline thin film semiconductors. CBD has found out a special significance being a low temperature method as well as not highly expensive [16], [17].

In the present investigation, ZnO:Li films were deposited from a particular bath on glass substrates and their optical characteristics were measured. We report the optical absorption coefficient, optical energy gap, Urbach's tail of the films. The effect of Li concentrations on the behaviour of the measured optical parameters were presented.

Section snippets

Experimental details

Films of Zn₁₋_xLi_xO were prepared from bath containing sodium hydroxide, zinc sulfate and distilled water. They were stirred thoroughly using a glass rod at each stage to obtain a homogeneous mixture of the solution. The basic reaction involved is ${ZnSO}_{4} + 2 NaOH \overset{KCN}{⟶} Zn (OH)_{2} + {Na}_{2} {SO}_{4} \overset{Heating}{⟶} ZnO ↓ + {Na}_{2} {SO}_{4} + H_{2} O .$ The dopant source was lithium sulfate and Li atomic percentage in solution was 10%, 20%, 30%, 40% and 50%.

After the films were deposited on glass substrates, they were allowed to drip dry in air.

Structure

Fig. 1(a) shows that as-deposited ZnO films are of very poor crystallinity. The XRD patterns showed a broad hump indicating amorphous nature of ZnO films and no well-resolved peaks were observed leading to no definite conclusion about the structure of the ZnO films deposited from the bath. The ZnO films were heated in a furnace at 673 K for 5 h and the XRD pattern of such film is shown in Fig. 1(b). The Bragg's reflections were indexed and the obtained lattice parameters are listed in Table 1.

Conclusions

The direct-allowed transition is the most probable type of transition near the fundamental edge of pure ZnO and ZnO films doped with different concentrations of Li ions. The calculated values of E_g for undopd films are higher than that of the doped ones. The refractive index and real part of dielectric constant decrease with the increasing Li ions. The refractive index values have been fitted to the single-oscillator models. The values obtained for the single-oscillator energy E_o are consistent

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Optical constants of Zn1−xLixO films prepared by chemical bath deposition technique

Abstract

Introduction

Section snippets

Experimental details

Structure

Conclusions

Opt. Laser Technol.

Sol. Energ. Mater. Sol. Cells

J. Cryst. Growth

Sol. Energ.

J. Eur. Ceram. Soc.

J. Eur. Ceram. Soc.

Appl. Surf. Sci.

Physica B

Thin Solid Films

J. Phys. Chem. Solids

Appl. Surf. Sci.

J. Cryst. Growth

Superficies y Vacio

Proc. IEEE

IEEE Trans. Ultrason. Ferroelectr. Freq. Control

J. Korean Phys. Soc.

J. Am. Ceram. Soc.

Optical constants of Zn₁₋_xLi_xO films prepared by chemical bath deposition technique