Effect of Lanthanum Doping on the Structural, Morphological, and Optical Properties of Spray-Coated ZnO Thin Films ^†

Abstract

In recent years, transparent conducting oxide semiconductor materials have found applications in both science and technology, especially in the areas of semiconductors, optoelectronics, and a wide range of energy efficiency devices. These TCO materials are the building blocks of various optoelectronic devices, such as transparent thin-film transistors, solar cells, and light-emitting diodes. This work concentrates on the structure, morphology, and optical properties of ZnO and Zn_0.95La_0.05O thin films at 673 K using a chemical spray technique. The polycrystalline nature and wurtzite structure of ZnO were confirmed by using XRD analysis with preferred growth along the (1 0 1) plane. The Zn_0.95La_0.05O deposits showed maximum crystallinity of 15.4 nm and a strain value of 2.4 × 10⁻³. The lattice constants increased for lanthanum-doped ZnO thin films due to the ionic radii mismatch of the doping material, which causes lattice expansion. Fibrous morphology was observed for ZnO, and a mixed structure of grains and fibers was observed for Zn_0.95La_0.05O films, which confirms the insertion of La³⁺ into the Zn²⁺ position. The Zn_0.95La_0.05O deposits showed transmittance above 80% due to the increased crystalline quality and a bandgap of 3.32 eV. The photoluminescence spectra showed peaks corresponding to e-h recombination, zinc defects (Zn_i and O_zn), and oxygen vacancy (O_i and V_o). The lanthanum-doped ZnO films showed increased band-edge emission and decreased defect-related peaks due to the increased crystalline quality. Hence, the doping of La³⁺ ions into a ZnO lattice enhances the crystalline quality and increases the transparency of the host ZnO matrix, which is suitable for optoelectric device applications.

1. Introduction

The transparent conducting oxide (TCO) semiconductors are of great interest due to their better optical and electrical properties. Currently, a lot of research works are looking at TCO semiconductor material-based photodetectors [1], UV sensors [2], optical limiters [3], and solar cells [4]. The ZnO, ZnS, CdO, and CdS materials of the II–VI semiconducting groups are good examples of TCO materials. Compared to other materials, ZnO-based materials are of great interest due to their easy availability, bandgap tuning, nontoxic nature, high exciton binding energy (60 MeV), and low cost. ZnO-based semiconducting materials are very sensitive to ultraviolet light. Hence, they can be used in the fabrication of ultraviolet (UV) detectors. These properties of ZnO films can be enhanced by introducing suitable dopants into the host matrix. Lots of work has been carried out on the transition of metal ion-doped ZnO films. But the effect of rare-earth ion doping into the ZnO lattice is an emerging field and needs to be studied in detail. The insertion of rare earth ions into the ZnO lattice is a difficult task because of the difference in the ionic radii of La³⁺ and Zn²⁺ ions. Due to this mismatch, the dopant may occupy the interstitial positions and hinder the properties. However, the surface structure, bandgap, transparency, and electrical behavior of ZnO can be modified by the proper insertion of rare-earth ions into the matrix. These rare-earth ion-doped ZnO films can be synthesized using various spin coatings [5] sol–gel methods [6], SILAR [7], and spray techniques [8]. The spray pyrolysis technique is employed in the current study due to its simplicity, low cost, less chemical wastage, large area of deposition, and uniform coating. Here, the properties of the deposits mainly depend on the concentration of the precursor solution, substrate temperature, pressure of the carrier gas, nozzle to substrate distance, spray interval, and duration of the spray. Hence, optimizing all these parameters is very important to obtain better-quality thin films. Lanthanum is a less-studied topic, and due to the inconsistency in the available reports, this work focuses on lanthanum doping on the structure, morphological, and optical properties of spray-coated ZnO films.

2. Materials and Methods

2.1. Film Deposition

The ZnO and La:ZnO films were coated on glass substrates at 673 K using spray pyrolysis method. The chemicals used were zinc acetate dihydrate and lanthanum chloride heptahydrate dissolved in distilled water and constantly stirred for 30 min to obtain a uniform concentration of 0.05 M. The prepared precursor solution was filled in the burette and sprayed on the preheated glass substrates through a spray nozzle in optimized spray intervals. The films were maintained at the same temperature for 30 min to obtain better crystallinity.

2.2. Characterization Techniques

The structural properties of the thin films were determined using X-ray diffraction analysis. A scanning electron microscope was used to study the morphology of the deposited films. The optical properties, such as transmittance and bandgap of the material, were obtained using the UV–vis double-beam spectrometer. The defects in the films were determined using photoluminescence spectra analysis.

3. Result and Discussion

3.1. Structural Analysis

The XRD spectra of the deposited films are shown in Figure 1. Both the films showed a polycrystalline nature, and the noticed peaks matched the wurtzite structure. The spectra showed seven peaks at (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), and (1 1 2). The orientation of the peaks mainly depends on the atom configurations on the subsequent layers and primarily depends on the deposition parameters. The intensity of the (1 0 1) peak increased for Zn_0.95La_0.05O films. The shift of the diffraction peak toward the lower-angle side after lanthanum doping was observed. The estimated structural parameters are tabulated in

Table 1. Estimated structural parameters calculated using XRD data.

Sample	Deposition Temperature	Orientation	Crystallite Size (D) in nm	Lattice Constants (Å)		Microstrain × 10−3	Dislocation Density × 1015 (lines/m2)
				a	c
ZnO	673 K	(1 0 1)	9.8	3.168	5.135	2.9	10.4
Zn0.95La0.05O	673 K	(1 0 1)	15.4	3.222	5.161	2.4	4.2

.

The ‘D’ value of the deposited films was determined by using the Scherrer equation given below [9].

$$D=\frac{k\lambda }{\beta cos \theta }$$

(1)

The ‘D’ of the Zn_0.95La_0.05O films was found to be due to the insertion of La³⁺ ions into the Zn²⁺ lattice creating lattice expansion and increasing the crystallite size.

The strain ($\epsilon )$ values were determined by using the formula given below [10].

$$\epsilon =\frac{\beta cos \theta }{4}$$

(2)

The strain is due to the incorporation of La³⁺ ions into Zn²⁺ positions. The strain values were found to be decreased for Zn_0.95La_0.05O films.

The ‘a’ and ‘c’ were calculated using the below relation [10].

$$a=\frac{\lambda }{\sqrt{3}}sin {\theta }_{100}$$

(3)

$$c=\frac{\lambda }{sin {\theta }_{002}}$$

(4)

The lattice constant values ‘a’ and ‘c’ were found to be increased for Zn_0.95La_0.05O films due to the insertion of La³⁺ ions, which causes the lattice expansion in the doped films.

The dislocation density was calculated using the ‘D’ values by using the below formula [11].

$$\delta =\frac{1}{D^2}$$

(5)

The dislocation density refers to the number of dislocations per unit area. This is related to the crystallinity and purity of the deposits. The Zn_0.95La_0.05O films showed lower dislocation density values.

3.2. Scanning Electron Microscopy (SEM)

The morphology of ZnO and Zn_0.95La_0.05O films is shown in Figure 2. The pure ZnO films showed a fibrous morphology. The change in morphology from fibrous to a mixed granular–fibrous one was observed with the addition of lanthanum into the ZnO lattice. This change is due to the replacement of Zn²⁺ by La³⁺ ions. The lanthanum tends to change the morphology from fibrous to granular; however, compared to granular, fibrous was more observed due to the lower doping percentage of lanthanum. Hence, the lanthanum doping into the ZnO lattice changes the morphology of the deposited films.

3.3. Optical Studies

The transmittance and Tauc’s plot of the deposited films are shown in Figure 3a,b. Both ZnO and Zn_0.95La_0.05O films showed high transparency. Transparency in the range of 70% was noticed for ZnO films. However, the transparency increased after the lanthanum doping due to the increased crystallinity. As the crystallinity increases, the light travels easily through the films without being deflected. However, in the case of ZnO, the XRD results show a smaller crystallite size and higher strain and dislocation density values. Hence, due to the increased defects, the transparency of the film decreased. The shift in the absorption edge of Zn_0.95La_0.05O films confirms the better optical properties.

The Tauc’s plot relation [12] was used to determine the bandgap.

$$\alpha h\nu =A{\left(h\nu -E_g\right)}^n$$

(6)

The bandgap of the films showed a strong dependency on lanthanum doping. The ZnO films showed a bandgap of 3.1 eV, whereas the Zn_0.95La_0.05O films showed an increased bandgap of 3.32 eV. This may be due to the Burstein–Moss shift (B–M shift). Here, the bandgap of the material increases as the absorption edge shifts toward higher energies, and as a result, some states close to the conduction band are populated due to the increased carrier density of the deposited films due to the addition of La³⁺ ions, which leaves an extra electron in the valance band and helps to increase conductivity. Hence, the lanthanum doping into the ZnO lattice varies the optical properties of the deposited films.

3.4. Photoluminescence Study

The PL spectra of ZnO and Zn_0.95La_0.05O films are shown in Figure 4. The spectra were fitted using a Gaussian fit, which showed seven peaks corresponding to an e-h recombination at 385 nm, violet emission at 400 nm, blue emission peaks at 425 and 452 nm, green emission at 530 nm, and yellow-orange emissions at 580 and 605 nm. The band-edge peak at 385 nm corresponds to the crystalline quality and homogeneity of the deposited films [13]. As the crystal quality increases, the intensity of the band-edge emission increases. The Zn_0.95La_0.05O films showed increased band-edge emissions. Hence, the recombination of electron-hole pairs will be more in Zn_0.95La_0.05O films. The violet emission is due to the interface traps occurring inside the grain boundaries of ZnO [14]. The Zn_0.95La_0.05O films showed maximum violet emission. The zinc defects cause the blue emission. The intensity of the blue emission peaks increased for Zn_0.95La_0.05O films. The green emission is due to oxygen-related defects [15]. The intensity of the green emission peak decreased with the addition of La³⁺ into the ZnO lattice. The presence of singly ionized oxygen vacancies cause yellow-orange emission. The yellow-orange emission was found to be maximum for pure ZnO films due to increased oxygen-related defects.

4. Conclusions

The ZnO and Zn_0.95La_0.05O films were coated on chemically cleaned glass substrates at 673 K. The spectra confirmed the wurtzite structure of ZnO. The Zn_0.95La_0.05O films showed a maximum crystallite size of 15.4 nm and lower strain and dislocation density values. SEM images showed the change in morphology from fine fibrous to a mixed granular–fibrous morphology to Zn_0.95La_0.05O films. An increase in the optical transmittance (80%) and bandgap (3.32 eV) was observed for Zn_0.95La_0.05O films. The photoluminescence spectra showed peaks corresponding to band-edge emission, zinc-related defects, and oxygen defects. The band-edge emission increased, and defect-related peaks decreased with the inclusion of the lanthanum content in ZnO films. Hence, the lanthanum doping into the ZnO lattice will enhance the properties of ZnO films.