Structural, optical and electrical evolution of Al and Ga co-doped ZnO/SiO₂/glass thin film: role of laser power density

Abstract

This study investigates the characteristics of laser annealed thin films of Al–Ga co-doped zinc oxide (ZnO:Al–Ga) nanoparticles on top of SiO₂/glass. The samples are synthesized using simple sol–gel, spin coating and radio frequency magnetron sputtering methods. The studies on the structural, optical and electrical properties of the pre-annealed sample and samples annealed at different power densities are conducted using a variety of characterization techniques. The samples exhibit a hexagonal wurtzite structure. Spectroscopic and nano-imaging techniques confirm that by increasing the laser power density, the crystallinity of the samples is improved and the nanoparticle size is enhanced from ∼10 nm to ∼35 nm. Spectroellipsometry is employed to calculate the refractive index, extinction coefficient, and real and imaginary components of the dielectric constant. The resistivity exhibits a minimum value at 440 mJ cm⁻². Results demonstrate that the optical band gaps of the samples are between 3.29 and 3.41 eV, which are greater than that of pure bulk ZnO (band-gap of 3.21 eV). Several vibrational modes occur as a result of the dopant combination in the ZnO lattice. A discussion on the origins of modes and their intensity changes is provided. This work suggests that a laser annealing process can be an effective tool to fabricate various thin films with enhanced crystallinity. The optical and electrical properties can also be adjusted by varying the power density.

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1. Introduction

Due to several advantages compared to other semiconductors,¹ such as the large exciton binding energy of ∼60 meV at room temperature, high transparency, and low cost, ZnO is a promising material in optoelectronics, especially in excitonic devices.^2,3 Additionally, at a relatively low deposition temperature,⁴ ZnO films can be deposited via various techniques such as radio frequency (rf) or direct current sputtering,⁵ spray pyrolysis,⁶ spin coating,⁷ metal organic chemical vapor deposition⁸ and pulsed laser deposition.⁹

In the nano size range, ZnO is expected to possess interesting physical properties, and pronounced coupling which is quite different from that of its bulk counterpart.¹⁰ Although ZnO has many optoelectronic applications, it has some limitations such as its high catalytic activity that may degrade its functions. A composite material could be an efficient approach to overcome this issue. Among ZnO-based composite materials, ZnO–SiO₂ has attracted much attention due the compatibility and stability of the constituents.¹¹

The modification of ZnO to obtain better crystallization quality, and desirable optical, electrical, and ferromagnetic properties through impurity combination is currently an important approach for future applications. Among p-type doping, Li is a promising dopant for controlling the configuration between interstitial donor sites and substitutional acceptor sites of ZnO; such doped ZnO has been synthesized via thermal decomposition of Zn(Ac)₂·2H₂O followed by the hydrothermal method in the work of Lv and Liu.¹² However, a great number of ZnO-based applications consist of n-type ZnO thin films and they can be achieved by substitution of group-III impurities such as In, Ga, Al^13–15 in the Zn sites which act as shallow donors in ZnO.¹⁶ There are some issues in the case of single doped Al:ZnO and Ga:ZnO thin films. Al simply oxidizes and its thermal stability is lower than that of Ga. Moreover, the distortion in the Ga:ZnO crystal structure is less than that in Al:ZnO due to the lower approximate ionic radii of Ga³⁺ compared to Zn²⁺. However, Ga:ZnO has the drawback of low durability in a humid environment.¹⁷ Furthermore, the expense of Ga compounds is another disadvantage in Ga:ZnO especially for mass production. While co-doping of ZnO has been performed with physical methods,^16–19 co-doping based on the sol–gel method is beneficial in terms of homogeneity and a reduction in the synthesis cost of transparent conductive oxides.

Laser annealing could be another promising approach for changing material properties. Lasers have been used for more than 50 years in diverse fields of application.²⁰ Synthesis of high-quality ZnO thin films at relatively low temperatures facilitates the fabrication of devices based on low-melting-point substrates. The traditional post-annealing temperature is roughly 400 °C. However, exposing some substrate materials such as polymer and glass to a high temperature is not possible. Conversely, high quality Al–Ga:ZnO films are produced via laser annealing at low temperature which fulfils the prerequisite of low temperature for the production of solar cells, LCDs, etc. The wavelength of 1064 nm is enough to give a significant penetration depth in Al–Ga:ZnO films; a 1064 nm laser penetrates more inside the sample than is found when annealing with lasers of shorter wavelength. Hence, a laser annealing process is developed in this work through the Nd:YAG laser system with a wavelength of 1064 nm, in an air atmosphere at room temperature.

The first work on laser annealing of ZnO thin films deposited on Corning glass was published in 1984 by Bertolotti et al.²¹ They found that the treatment did not affect the refractive index but produced a marked decrease in guided mode losses. After this first significant study, many studies were done on ZnO thin films deposited by various techniques. Recently, Kim et al. studied the effect of KrF excimer laser annealing on sol–gel derived ZnO films by photoluminescence. They concluded that laser annealing removed or filled intrinsic defects such as O and Zn vacancies or interstitials.²² This result is in contradiction with the observation of Nagase et al.^23,24 Tsang et al. irradiated sol–gel derived ZnO:Al thin films by using an excimer laser leading to a significant improvement of structural, optical and electrical characteristics of the films. The resistivity of the laser-irradiated film was 4.4 × 10⁻² Ω cm and the optical transmittance was about 90%.²⁵

In our recent communication, we investigated different ZnO top layer thicknesses deposited on SiO₂/glass by rf magnetron sputtering for tuning optical and structural properties.²⁶ In this paper, Al–Ga, co-doped ZnO thin film synthesized using a sol–gel and spin coating method, was deposited on SiO₂/glass using rf magnetron sputtering. Subsequently, comprehensive fundamental studies on the effect of different laser power densities on the structural, optical and electrical properties of the samples were examined.

2. Experimental

ZnO:Al–Ga/SiO₂/glass heterostructure films are prepared as follows: initially, rf magnetron sputtering (HVC Penta Vacuum) is used to deposit a SiO₂ layer, 100 nm in thickness, on 2.5 cm × 2.5 cm quartz glass substrates using a high purity, 3 inch SiO₂ target. Before deposition, the substrates are cleaned by ethanol, acetone and deionized water followed by 10 min ultrasonication in a water bath. Spectroscopic reflectometry (filmetrics) is employed to measure the thicknesses of the films. The rf power and deposition time for SiO₂ deposition are 250 W and 100 min, respectively. The constant flow rate of Ar and the substrate temperature are 10 sccm and 50 °C, respectively. The running pressure of the growth chamber is 2.5 × 10⁻³ Torr. The substrates are stable at a distance of 12 cm over the targets. The ultimate pressure of 2 × 10⁻⁶ Torr for the sputtering chamber is reached using a mechanical rotary pump and turbo pump. After growth, the samples are cooled down to room temperature inside the chamber. Then, for the synthesis of 2 at% Al and Ga, 1-propanol (CH₃CH₂CH₂OH, nPrOH), diethanolamine (HN(CH₂CH₂OH)₂, DEA), and zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O, ZAD) are utilized as solvent, stabilizer, and precursor reagents, respectively. Equal molar amounts of 1-propanol and diethanolamine are used for the solvent. Zinc acetate dihydrate is dissolved in the solvent to a concentration of 0.75 mol L⁻¹.²⁷ Aluminum nonahydrate (Al(NO₃)₃·9H₂O) and gallium nitrate nonahydrate (Ga(NO₃)₃·9H₂O) are used as the dopant precursors, as the Al and Ga doping sources, respectively. All of the materials are purchased from Merck Chemicals and utilized as received. Finally, the Al–Ga co-doped ZnO films are deposited on the sputtered SiO₂/glass by the spin coating method. The spin coating speed is 3000 rpm and the deposition time is 1 min. Following the spin coating process, the solvent and organic residuals are evaporated by drying the samples at 100 °C for 5 min. In order to obtain the desired thickness, the processes of spin coating followed by pre-heating treatment are repeated. At last, the samples are totally annealed at 300 °C for an hour in air. The Nd:YAG laser having power densities of 119, 440 and 634 mJ cm⁻² (1064 nm, continuous-wave) is used for annealing. The laser beam area and the scanning velocity are 25 mm × 2 mm and 2 mm s⁻¹ respectively. The pulse width of the laser is 10 ps and the frequency is 5 Hz.

The structural analyses of the samples are carried out using X-ray diffraction (XRD) (D8 Advance Diffractometer, Bruker, USA) with CuKα radiation (0.154 nm) at 40 kV and 100 mA. A range from 20° to 80° is used for 2θ with a step size of 0.02° and a resolution of 0.011°. To observe the surface morphology, deposited layer formation and elemental analysis, field-emission scanning electron microscopy (FESEM, JEOLJSM 6380LA) with accompanying energy dispersive X-ray spectroscopy (EDX) as well as high resolution transmission electron microscopy (HR-TEM, JEOL ARM 200F) are used. A spectrophotometer (UV-3600) is used to record the transmittance. A luminescence spectrometer (LS 55, Perkin Elmer, USA) with 239 nm excitation wavelength is employed to carry out the room temperature photoluminescence (PL) spectroscopy. Raman spectroscopy using a Spectrum GX (NIR, FT-Raman, PerkinElmer) system with an Nd crystal laser source having a spot size of 1 mm is performed. The carrier concentration, mobility and resistivity are measured using a Nanometrics Hall Measurement system (HL5500). The J. A. Woollam RC2 spectroscopic ellipsometer with two rotating compensators is employed to obtain the refractive index and extinction coefficient dispersion curves of the films. The spectral range from 270 to 1100 nm and angle of incidence of 75° is applied for ellipsometric measurements.

3. Results and discussion

Fig. 1 illustrates the wide range XRD patterns of ZnO:Al–Ga/SiO₂/glass containing ZnO:Al–Ga nanoparticles annealed at different laser power densities. The XRD pattern of the samples can be indexed using the wurtzite crystal structure of ZnO [JCPDS36-1451] and no other crystalline phases are detected. The Ga and Al dopant atoms do not occupy zinc lattice sites and they have a tendency to occupy interstitial sites where they form neutral defects. Thus, they become ineffective as dopant impurities and do not give any signal in the XRD spectra. Substitution or interstitial combinations of the Al and Ga in the ZnO lattice contribute to considerable peak shifts to rather higher 2θ values in comparison with the pure ZnO, as reported in our recent work.²⁵ This positional shift is reasonable, as a lattice contraction is expected due to ionic size differences between Al³⁺ (0.75 Å), Ga³⁺ (0.62) and Zn²⁺ (0.74 Å) ions.²⁸

Fig. 1 (a) X-ray diffraction patterns of ZnO:Al–Ga thin film at different laser power densities. (b) Shift in the (101) diffraction peak due to changing laser power density.

Since the absorption of laser energy is controlled by the ZnO:Al–Ga thin film and the pulse repetition frequency, the crystallization of the thin films at different laser power densities is found to be varied. The absorption rate of laser energy and crystallization of ZnO:Al–Ga thin films are probably correlated with the laser repetition frequency. Thus, better crystallization occurs at higher laser power density. This improvement may be ascribed to a reduction of internal defects including inter-crystal gaps and voids or disorders in the ZnO:Al–Ga nanostructure by laser annealing.

For different laser power densities, the detected d values, lattice parameters, intensities, dislocation densities, tensile strains, grain sizes and diffraction angles (2θ) of the films are tabulated in Table 1. According to the intensity value for the two intense peaks of (100) and (101) depicted in Table 1, the higher level of crystallinity is achieved at the 634 mJ cm⁻² laser power density. Moreover, the grain sizes and tensile strains of the samples are estimated by Williamson–Hall (W–H) and Scherrer plots (see equations below).²⁹

K is a constant equal to 0.94 where β_experiment is the full width at half maximum (FWHM) of the XRD peaks; λ is the X-ray wavelength used; and θ is the Bragg's angle. The plots are not presented here, due to space constraints in this study, rather the strain and grain sizes are calculated (see Table 1). These methods are described in our recent study.³⁰ For the ZnO–SiO₂ interface, the Zn atoms are bound with two O atoms. However, the O-atoms of down sites are not able to bind with Zn atoms and instead defect sites can be created in the boundary between the SiO₂ and ZnO. These defects along with the lattice incongruity among the layers produce the greatest share of compressive strain for the pre-annealed sample. The laser annealing process has a slight effect on epitaxial strain. This contributes to the lattice relaxation process through which the peaks are shifted to positions closer to those of the bulk. This can be explained with respect to changes in the interatomic distances and lattice contraction. Moreover, the dislocation density (δ) is measured by the formula below, where D is the crystallite size.³¹ This gives more insight into the quantity of defects in the films.

Table 1 The power density dependence of XRD peak position, lattice parameters, intensity, strain, density of dislocation and grain size, obtained with different methods

Laser power density	hkl	2θ ±0.01	dhkl (nm) ±0.0002	a, c (nm) ±0.0002	δ ±0.0002	ε ±0.0002	Intensity (a.u.)	Grain size (nm)
								Scherrer ±2 nm	W–H ±2 nm	TEM ±2 nm	FESEM ±2 nm
Pre-annealed	(101)	36.10	0.2485	a = 0.3250	0.0065	0.0025	19	10	12	—	15
	(100)	31.75	0.2856	c = 0.5202			26
119	(101)	36.14	0.2482	a = 0.3246	0.0027	0.0018	23	16	19	—	22
	(100)	31.79	0.2852	c = 0.5198			35
440	(101)	36.20	0.2478	a = 0.3239	0.0010	0.0012	34	27	30	—	35
	(100)	31.82	0.2846	c = 0.5294			39
634	(101)	36.25	0.2475	a = 0.3232	0.0011	0.0004	38	40	43	55	46
	(100)	31.87	0.2838	c = 0.5192			50

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Therefore, a higher laser power produces a larger grain size. Thus, the β and δ values become smaller. The annealing process improves the crystallization of the films, and lowers the number of defects and dislocation density, which is clearly shown in Table 1.

The HR-TEM image of the ZnO:Al–Ga sample annealed at 634 mJ cm⁻², the corresponding selected area electron diffraction (SAED) pattern and the selected focusing area are shown in Fig. 2b, c and d, respectively. To confirm the presence of Al and Ga, the EDX mapping for the sample annealed at 634 mJ cm⁻² is depicted in Fig. 2e. The dopant nano-collection due to agglomeration of Al and/or Ga atoms caused by the annealing process appears in a dark color and is shown by the yellow arrows in Fig. 2a. The presence of lattice fringes obviously verifies that the material is crystalline. At the same time the crystals are placed inappropriately. The high resolution image focusing on a single particle shows the ZnO:Al–Ga lattice arrangement with a d-spacing of 1.44 Å (L₁) which corresponds to the (103) plane as shown Fig. 2d. The appearance of fringes in different directions confirms the poly-orientation of the crystal. Moreover, the d-spacing of 2.4 Å (L₃) is related either to pure ZnO (101), which is in good agreement with d-spacing extracted from XRD, and/or to Al₂O₃ (1120). The d-spacing of 3.65 Å (L₂) probably originates from Ga₂O₃. It is supposed that Ga migrates in the presence of laser treatment, forming hexagonal Ga₂O₃ nanocrystals with the incorporation of oxygen present during annealing, the synthesis process or in the ZnO site.³²

Fig. 2 TEM images of the ZnO:Al–Ga thin film. (a) Typical TEM micrograph of sample annealed at 634 mJ cm⁻² (b) HR-TEM and (c) SAED pattern of the corresponding sample. Sample exhibits a hexagonal spot pattern corresponding to the (100), (002), (101), (102), (110) and (103) planes. (d) High magnification of selected areas used to calculate d-spacings in different directions. (e) EDX mapping images of Zn, O, Al and Ga for the corresponding sample.

Furthermore, the presence of Al and Ga in the ZnO lattice structure confirms the presence of a Zn–Al or Zn–Ga semiconductor nano heterostructure responsible for the visible light harvesting and suitable for optoelectronic applications. However, it was not possible to focus on the dopant position in the HR-TEM of the sample.

Fig. 3 displays the top view FESEM images of all samples. The corresponding EDX point analysis and cross section image of the pre-annealed sample are presented as an inset to part (a). The segments comprise a top layer of ZnO:Al–Ga, an intermediate layer of SiO₂ and a bottom glass substrate surface. They are separated by yellow lines. The results show that the main elements presented within the inspection field include Zn, O, Al and Ga, with Zn being the most abundant element. A semi-quantitative view of the elemental composition in the inspection field in units of both weight percent and atomic percent is provided through the tabulated results.

Fig. 3 Top view FESEM images of (a) pre-annealed sample, and samples annealed at (b) 119, (c) 440 and (d) 634 mJ cm⁻². The inset of figure (a) shows a high magnification of the selected area cross section FESEM to confirm the formation of layers and the point EDX. The inset of figure (d) shows a high magnification of the selected area.

Both the macroscale temperature evolution history of the ZnO:Al–Ga film and the nanoscale atomic motion/mass transport of the ZnO:Al–Ga nanoparticle stack during laser annealing suggest that, through laser treatment, the closely packed ZnO:Al–Ga nanoparticle film sustains three processes. Initially, the ZnO:Al–Ga nanoparticle stack appears to be cubic close packed (ccp) right after coating (Fig. 3a). During annealing, the hot spots formed over contacts tend to drive the surface melting and merging together of small particles. This process tends to enlarge the grain size, modify the grain shape and compact internal gaps (Fig. 3b and c). Finally by further increasing the laser power, a much denser film stack is formed, where faceted large grains are achieved and impinge on each other (Fig. 3d).

Comparing the samples in-process to the finally treated area (Fig. 3d), it is found that the crystallized film is more compact and continuous, suggesting that the crystallinity of the ZnO:Al–Ga film has been considerably enhanced. The good film homogeneity and crystallinity result from grain growth, shape change and the removal of inter-grain defects like voids, gaps and discontinuities.

Fig. 4(a) shows the optical transmittance spectra of the ZnO:Al–Ga films. In all samples, acceptable optical transmittance and small reflectance in the visible wavelength area are found. The oscillations in the visible region are shown by transmittance and reflectance. The constructive and destructive interference among multiple bounce beams are responsible for these oscillations in the spectra. A sharp edge exists between 350 nm and 400 nm which is due to the inter-band absorption edge. The average optical transmittance in the visible range goes above 85% for samples annealed at 440 and 634 mJ cm⁻². A sharp absorption cut-off is obviously seen. The equation below clearly demonstrates the dependence of absorption index (α) on illumination wavelength (see Fig. 4(b)):

αhν ∝ (hν − E_g)^1/2

where E_g is the optical band-gap; α is the absorption coefficient; and hν is the incident photon energy. The E_g values of samples can be measured using a Tauc's plot as can be seen from Fig. 4c. We should bear in mind that E_g increases with laser power first, until the power equals 440 mJ cm⁻². This mainly occurs through an incremental increase in carrier concentration (the Burstein–Moss (BM) effect).³³ In the BM effect model, for an n-type semiconductor with a parabolic band, the band-gap broadening is specified by
where ΔE^BM is the blue-shift of the optical band; h is Planck's constant; n is the carrier concentration; and m* is the electron effective mass in the conduction band.³⁴ Hence, the increment in carrier concentration contributes to the increment in E_g. The E_g values of the samples are decreased sharply by increasing the laser power up to 634 mJ cm⁻². This is ascribed to the larger grain size, the consequence of electrostatic potentials and a decrease in the carrier concentration (shown later in Fig. 8b).³⁵

Fig. 4 (a) Transparency and (b) reflectance spectra of ZnO:Al–Ga at different laser power densities, and (c) optical band gap of ZnO:Al–Ga films.

The refractive index n and extinction coefficient k of the samples derived from model-fitting the experimental spectroscopic ellipsometric data are shown in Fig. 5. As can be seen from the figure, both n and k first decrease and then increase with the laser power density. This might be largely ascribed to the increment in the carrier concentration in the ZnO:Al–Ga films. The increase of n and k should be correlated to an increment in the density of the film as it is compacted.²¹ The essential electron excitation spectra of the films are labeled by the frequency dependence of the complex dielectric constant. The real and imaginary parts of the dielectric constant are determined in the following equations:

ε_real = n² − k²

ε_imaginary = 2nk

Fig. 5 Variations in (a) refractive index, (b) extinction coefficient, and (c) real and (d) imaginary parts of the dielectric constants of ZnO:Al–Ga thin films.

The real and imaginary parts of the dielectric constant (ε_real and ε_imaginary) are depicted in Fig. 5c and d respectively. From Fig. 5, it is observed that ε_real and ε_imaginary values reduce with wavelength. It is assumed that the dielectric constants of the films can be associated to the extinction coefficients and refractive indexes.

It is clear that vibrational properties of solids are related to the long and short-range orders in the spatial array of the constituent atoms. Raman spectroscopy is implemented on the pre-annealed and annealed samples in order to examine the influence of laser annealing power on the vibrational properties of the ZnO:Al–Ga nanostructures. The symmetry of wurtzite ZnO lies in the spatial group c_6v⁴, having two formula units in the primitive cell. Thus, the optical phonons at the center of the Brillouin zone (G) are described by the following irreducible relation:³⁶

Γ = 1A₁ + 2B₁ + 1E₁ + 2E₂

where the two polar branches are represented by A₁ and E₁. They are divided into longitudinal optical (LO) and transverse optical (TO) phonon components having diverse frequencies because of the macroscopic electric fields correlated with the LO phonons. The two E₂ branches are nonpolar; while the low frequency mode (E_2L) is associated with the heavy Zn sub-lattice, the high frequency mode (E_2H) contains only the oxygen atoms.³⁷ All these A₁, E₁ and E₂ branches are Raman and infrared active. In contrast, the two B₁ branches are both infrared and Raman inactive, and are so-called silent modes. Fig. 6 shows the Raman spectra of the pre-annealed and annealed samples. The spectra of the ZnO:Al–Ga films reveal four peaks, located at about 331, 377, 411 and 440 cm⁻¹. The peaks at 377, 411 and 440 cm⁻¹ are attributed to the A_1T, E_1T and E_2H modes, respectively. The random orientation of ZnO:Al–Ga (confirmed by XRD and TEM) nanocrystals creates the overlapping of the A_1L and E_1L modes, which results in the single broad LO peak at around 579 cm⁻¹ (ref. 38) (not shown).

Fig. 6 Raman spectra of samples at different power densities.

The peaks centered at about 331 cm⁻¹ have been previously examined in ZnO by other researchers^38–41 to describe the second order modes or multi-phonon processes.

The enhancement of the LO phonon mode with increasing laser power might occur because of the intra-band Frohlich interaction. Group III–V semiconductors are generally considered under (or close to) resonant conditions, where LO-phonon scattering is dipole-forbidden. The mode is strongly improved in nanostructures such as nanoparticles because of (partial) splitting of the electron and hole clouds (created by the incident laser photon) in the space.⁴² Moreover, LO phonons participating in the scattering are expected to be a mixture of A₁ and E₁ modes due to the symmetry breaking caused by the laser annealing. Thus, the slight increase of peak intensity by laser annealing can be explained. Furthermore, the enhancement in the intensity of Raman peaks may also be attributed to the modified crystallinity of the nanostructures because of the annealing process, as can be seen from the XRD patterns.

Fig. 7 illustrates the room-temperature PL spectra of the pre-annealed ZnO:Al–Ga thin film and the films annealed at various temperatures. The spectra can be divided into UV (∼370 nm) and visible light (450–650 nm) parts (see Fig. 7). The UV emission associated with free exciton–exciton collision processes is ascribed to the near band edge (NBE). The visible emission of ZnO thin films happens because of defects which form deep energy levels in the band gap. A schematic diagram of the energy levels of defects, e.g. oxide vacancy (V_O), oxide antisite defect (O_Zn), interstitial oxygen (O_i), interstitial zinc (Zn_i) and zinc vacancy (V_Zn) in ZnO thin films is shown in Fig. 7.⁴³

Fig. 7 Room temperature PL spectra of samples annealed at different laser power densities. The inset indicates the schematic band diagram based on PL data.

The spectra of ZnO:Al–Ga thin films in the pre-annealed state and when annealed at 119 mJ cm⁻² involve a weak UV emission at ∼375 nm as well as a strong broad green emission at ∼514 nm (2.40 eV). When the films are annealed at 440 or 634 mJ cm⁻², the peak intensity at 375 nm is gradually increased which is ascribed to the greater amount of photo-carrier recombination in the NBE due to the increasing laser power density. When the annealing power is increased to 634 mJ cm⁻², interstitial oxygen (O_i) defects become unstable and the amount of interstitial oxygen (O_i) diffusing into the oxygen vacancies (V_O) and oxygen interstitial (O_i) defects further decreases. This results in a reduction in the green emission intensity. The decrease in green emission happens because of decreasing nonradiative defects and increasing grain size.⁴⁴

Fig. 8 displays the room temperature dependence of resistivity, carrier concentration and mobility as a function of power density calculated by Hall measurement. Changing the laser power density has a measurable effect on the electrical properties of the films. It is well-known that the resistivity is proportional to the reciprocal of the product of carrier concentration and mobility. For power densities lower than 440 mJ cm⁻², the resistivity of the ZnO:Al–Ga film is decreased by power density, which is attributed to the increment in carrier concentration made by the laser-activated Al and Ga ions and decrement of oxygen vacancies.⁴⁵ By increasing the power density to 634 mJ cm⁻², the resistivity of the ZnO:Al–Ga films increases, which is largely due to the decrement of mobility shown in Fig. 8c. The mobility of thin films with carrier concentrations of ∼1.7 to 2.2 × 10¹⁹ cm⁻³ is defined using both ionized impurity scattering and grain boundary scattering. The decrement of mobility is ascribed to the scattering which happens due to the grain boundary shown in the FESEM images in Fig. 3.

Fig. 8 Electrical properties of ZnO:Al–Ga thin films as a function of laser power density.

Fig. 9 illustrates the wide scan X-ray photoelectron spectra (XPS) and their Gaussian-resolved components of Al 2p_3/2, O 1s, Zn 2p_3/2, Zn 2p_1/2 and Ga 2p_3/2 for the 634 mJ cm⁻² power density sample. The XPS spectra reveal valuable information about elemental compositions and bonding states for each constituent element of the deposited films. A wide scan spectrum of the film confirms the appearance of Zn 2p, Zn 3s, Zn 3p, Zn 3d, O 1s and Auger lines. The bonding energy correlated with the different components of O 1s, Zn 2p_3/2, Zn 2p_1/2, Al 2p_3/2 and Ga 2p_3/2, are shown in Fig. 9b, c, d, e and f respectively.^46,47 The symmetrical signal of Zn 2p which is depicted in Fig. 9c and d correlates to the covalent structure of ZnO. No apparent peak shifting or intensity variation can be observed as a result of the annealing process. The asymmetric Auger signal at an energy of ∼586 eV is ascribed to the bonding of Zn with oxygen in the ZnO lattice. A shoulder peak at ∼574 eV corresponds to the existence of Zn interstitials in the films that are one of the major sources of free electrons in doped ZnO films.⁴⁸ Similarly, there is no annealing power effect on the intensity of Auger signals. The green Gaussian fit in the O 1s peak is usually attributed to chemisorbed or dissociated oxygen or OH species on the film surface.⁴⁴ It is well known that oxygen vacancies can create free electrons, while chemisorbed oxygen or OH species can capture free electrons which are responsible for the existence of carriers.

Fig. 9 XPS spectra for the sample annealed at a power density of 634 mJ cm⁻². (a) Overall XPS spectra of the pre-annealed and annealed samples, and the Gaussian-resolved components of (b) O 1s, (c) Zn 2p_3/2, (d) Zn 2p_1/2, (e) Al 2p_3/2, and (f) Ga 2p_1/2.

4. Conclusion

The effect of laser annealing at different power densities on the structural, optical and electrical properties of ZnO:Al–Ga/SiO₂/glass synthesized by rf magnetron sputtering, sol–gel and spin coating has been studied systematically. While a combination of Al and Ga dopants encourages lattice defects in the ZnO nanostructures, laser annealing improves the crystalline quality due to a reduction in the number of internal defects, including inter-crystal gaps, voids and disorders. The carrier concentration, mobility and resistivity of ZnO:Al–Ga films vary considerably as the laser power density is increased. The transmittances of the ZnO:Al–Ga films remain at around 85% in the visible region. Additionally, the absorption cut-off wavelength of ZnO:Al–Ga films annealed at a lower power density shift to shorter wavelength due to the BM effect. However, after higher power density annealing of ZnO:Al–Ga films, red-shifts are found in the absorption cut-off wavelength. This could be ascribed to the smaller grain boundary and larger grain size. The present study examines the films through Raman spectroscopy to show the way in which laser annealing results in the manifestation of an intense and broad band in the spectral region close to the LO phonon frequencies due to the intra-band Frohlich interaction and the improvement in crystallinity. The photoluminescence results indicate that the laser power density can tune the luminescence efficiency of the samples. Both the refractive indexes and the extinction coefficients of the samples first decrease and then increase with the laser power density. This behavior can be explained by the change in the carrier concentration and film compaction.

Acknowledgements

The project was supported by the Universiti Teknologi Malaysia, AMTEC through Research Grant Vote number R.J130000.7609.4C112, and the Frontier material research alliance.

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