Effect of substrate temperature on microstructure and optical properties of Ga doped ZnO films deposited by pulsed laser deposition

Introduction

The unique set of properties of zinc oxide (ZnO) film (e.g. wide band gap, large excition binding energy, high optical gain and radiation resistance) makes it an excellent candidate for optoelectronic devices applications.^1–3 Moreover, it is also a promising alternative material to indium tin oxide (ITO) because of its low material cost, non-toxicity, suitability for deposition at low temperature, besides the optical and electrical properties comparable to those of ITO. ⁴ Doping ZnO with various anions or cations is usually preferred to modulate the transmittance, band gap and electrical conductivity of ZnO film. The introduction of B, Al, Ga, In and F into ZnO can increase the conductivity by one order of magnitude or two.^5–10 High quality n-type ZnO films with high electrical conductivity and transparency in the visible region can be achieved by using Ga and Al as the dopants. So far, Al doped ZnO has been investigated widely. Nevertheless, Al exhibits a very high reactivity leading to oxidation during the growth of films. In comparison with Al, Ga is less reactive more resistive to oxidation due to its large electronegativity. Furthermore, the ionic size of Ga³⁺ is 0·62 Å and the corresponding bond lengths of Ga–O is 1·92 Å, which is comparable to the bond length of Zn–O (1·97 Å) and could result in small lattice deformation in the ZnO crystal structure. Excess electrons are released into the conduction band to obtain appropriate conductivity when Zn²⁺ ions are substituted by Ga³⁺ ions. Highly Ga doped ZnO also exhibits better electronic stability than that of Al doped ZnO when exposed to moisture, ¹¹ which may be critical for many device applications. This indicates that Ga seems to be the most appropriate and promising doping element in ZnO and is expected to produce less strain and local lattice distortion compared to Al doping.

Several chemical and physical deposition techniques have been used to deposit the Ga doped ZnO film (GZO), such as magnetron sputtering,^12,13 sol–gel process,^14,15 chemical vapour deposition,^16,17 molecular beam epitaxy ¹⁸ and pulsed laser deposition (PLD).^19–21 Among these methods, PLD method is regarded as a promising method for the deposition of GZO film and has drawn much attention due to the low deposition temperature, good adhesion to the substrate and accurate compositional control. In the case of PLD, transport and energy of the ablated particles is controlled by parameters such as substrate temperature and working gas pressure. Substrate temperature is one of the most important deposition parameters that plays a crucial role in the growth of GZO film, and influence the electrical and optical properties of GZO films. An appropriate substrate temperature can improve the crystalline quality and physical properties of GZO film. Many studies have addressed its effect on the electrical and optical properties of ZnO films doped with Al^22–24 and Ga.^25,26 The main purpose of the present study is to comprehensively study the influence of substrate temperature on the surface morphology, microstructure and optical properties of GZO films deposited by pulsed laser deposition.

Experimental

The Ga doped ZnO films were deposited in pulsed laser deposition system on Si (100) and fused silica substrates. A sintered ceramic target with a mixture of ZnO (99·99% purity) and Ga₂O₃ (99·999% purity) was employed as the source materials. High purity ZnO and Ga₂O₃ powders taken in their stoichiometric proportions were used for synthesising the 4 at.-%Ga doped ZnO pellets by the standard solid state reaction route. (100) silicon wafer and fused silica were used as substrates, and were cleaned ultrasonically in an acetone and methanol bath for 10 min before being loaded into the deposition system. The base pressure in the deposition chamber was 5×10⁻⁴ Pa. The target rotated continuously at 10 rev min⁻¹ was ablated with KrF excimer laser (the wavelength of 248 nm and repetition rate of 2 Hz), and the ceramic target was pre-ablated for 5 min to remove surface contamination before deposition. The work distance between the substrate and target was 7·0 cm. During deposition, the oxygen pressure was maintained at 13 Pa and the substrate temperature varied from 300 to 600°C. The deposition time was maintained at 1 h in all depositions.

The surface morphology of the deposited films was investigated by a scanning electron microscope (JEOL JSM 6330F) and an atomic force microscope (Nanoscope IIIa AFM). The microstructure and crystal quality of all the samples were analysed using X-ray diffraction (D8). The room temperature UV-visible transmittance spectra were measured using a UV-vis-NIR spectrophotometer. The refractive index (n) and extinction coefficient (k) of the GZO films were analysed by a spectroscopic ellipsometer (SE, J.A.Woollam Inc., M-2000DI) with a spectrum response ranging from 190 to 1700 nm.

Results and discussion

X-ray diffraction (XRD) was carried out to investigate the influence of substrate temperature on the crystal structure and orientation of the deposited GZO films, as shown in Fig. 1. It is observed that the structure of GZO films depends on the substrate temperature. Only a sharp (002) peak at 2θ = 34·67° is observed for the GZO films deposited at the substrate temperature below 500°C, which indicates that the film exhibits (002) plane preferential orientation due to the fact that the most densely packed (002) plane in wurtzite ZnO has the lowest surface free energy. When the substrate temperature increases to 600°C, a weak peak at 2θ = 36·45° corresponding to (101) plane in the displayed 2θ region is also observed besides the dominant (002) peak. Furthermore, the gallium characteristic peaks and gallium oxide peaks are not observed in the XRD patterns, indicating that the incorporation of Ga dopant does not affect the ZnO lattice structure by the formation of new phase. According to the XRD results, it shows that Ga atoms substituted Zn atoms in the hexagonal lattice and Ga ions may occupy the interstitial sites in the ZnO lattice or probably Ga segregates to the non-crystalline region in grain boundaries and forms Ga–O bond. These obtained results indicate that all the GZO films exhibit preferential orientation with c axis perpendicular to the substrate surface, and there is almost no effect of Ga atoms on the orientation.

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Patterns (XRD) of GZO films deposited at various substrate temperatures

Figure 2 shows the full width at half maximum (FWHM) and grain size of the GZO films deposited at various substrate temperatures. It can be observed that the FWHM corresponding to the (002) peak decreases as the substrate temperature increases from 300 to 400°C, and then almost has no change at the substrate temperature from 400 to 500°C. However, further increasing substrate temperature leads to the increase in FWHM. The large FWHM of GZO (002) peak indicates that the film deposited at low temperature is of inferior crystallinity. According to the FWHM, such variation tendency of FWHM means that the GZO film prepared at the substrate temperature of 400–500°C has the optimal crystal quality.

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Full width at half maximum (FWHM) and grain size of GZO films deposited at various substrate temperatures

The average grain size (L_hkl) of the deposited GZO films can be obtained from the FWHM of the (002) peak by using the well known Scherrer's equation ²⁷ (1) here λ = 0·15418 nm, B is the FWHM in radian of the diffraction peak. K is a constant that depends on crystallites shape (0·89 value is used in this work). The grain size along the c axis calculated versus the substrate temperature is depicted in Fig. 2. It can be seen that the grain size increases from 22·3 to 33·5 nm when substrate temperature increases from 300 to 400°C, and then decreases to be 24·8 nm at 600°C. The XRD pattern results demonstrate that the improvement of GZO film crystallinity with the increasing deposition temperature up to 500°C. In contrast, the crystallinity of GZO film deposited at 600°C is deteriorated. Similar behaviour was also observed in DC sputtered Ga doped ZnO films at the substrate temperature ranging from 150 to 400°C. ² The atomic kinetic energy is mainly determined by substrate temperature when the laser incident energy is fixed during the film deposition. High kinetic energy of the ionised and ejected species in the laser produced plasma along with the kinetic energy provided by the high substrate temperature enhances the adatom mobility on the substrate surface. Therefore, the surface diffusion of the adsorbed atoms increases with the increasing substrate temperature, which results in the improvement of the film quality and crystallinity. However, the adatoms are decomposed and re-evaporated from the surface when the substrate temperature is too high (for example, 600°C). Therefore, the GZO film becomes thermodynamically unstable, and the crystalline quality of the film is degraded. The growth temperature is predominately important for the crystalline of transparent conductive oxide films, which results in remarkable effect on the electrical and optical properties. The effect of substrate temperature on the microstructure of ITO films was also extensively investigated,^28–30 and similar results were also obtained. Beena et al. investigated the effect of substrate temperature on the structural and optical properties of ITO films deposited by pulsed laser ablation, and found that the ITO films show amorphous at room temperature and the crystalline nature of ITO films increases with the increasing substrate temperature. ²⁸

Figure 3 shows the surface morphology of the GZO films deposited at various substrate temperatures. It clearly shows that the surface morphologies of the GZO films vary significantly with the variation of substrate temperature when the other experimental conditions, such as oxygen pressure and laser incident energy were fixed. For the deposition at 300°C, a continuous, dense and smooth GZO film is observed. When the substrate temperature increasing to 500°C, the crystalline quality of film is improved and the grain size evidently becomes larger as compared with the sample deposited at 300°C. This might due to the fact that the ablated atoms with strong atomic mobility and diffusion migrate distantly and thus a denser film with larger grains can be obtained at higher temperature. However, the adatoms with too high kinetic energy collide quickly with each other and are re-evaporated at the same time when the substrate temperature is too high. Therefore, further increases in the substrate temperature to 600°C leads to the small grains interact with each other and coalesce together to form large particles composed of many small grains. This process of coalescence causes surface roughness.

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a 300°C; b 400°C; c 500°C; d 600°C

The effect of substrate temperature on the surface morphology of GZO films was also studied by AFM. The two-dimensional surface topography of the deposited GZO films at the scanning size of 2×2 μm are presented in Fig. 4. The AFM images reveal that the morphologies of films exhibit a dense and compact film structure, and no visible voids and defects over the film are observed. The average surface roughness calculated from AFM images slightly decreases from 7·2 to 5·0 nm as the substrate temperature increases from 300 to 500°C, and then increases to 15·6 nm when the substrate temperature further increases to 600°C. The ablated atoms reach the substrate surface with low energy and limited mobility at low temperature. Therefore consequently they cannot reach the appropriate sites, resulting in high surface roughness. With increasing substrate temperature the atoms gain enough energy and enhanced mobility.^31,32 Thus, atoms can reach more suitable sites and this lead to more dense and smooth films with improved crystalline quality. However, the migration of grain boundaries and coalescence of adjacent grains can lead to larger grains and rougher surface at too high temperature.

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Figure 5 shows the variations of the room temperature UV-vis optical transmittance spectra of the GZO films with the deposition temperature as a function of the wavelengths between 200 and 900 nm. It is observed that all the films show high transmittance of about 83% in the visible wavelength region. The high transparency of GZO films is associated with a good structural homogeneity and crystallinity. Apparent interference phenomenon can be noticed in every spectrum, implying a smooth and homogeneous surface of the film. All the transmittance spectra exhibit sharp absorption edges in the wavelength region between 360 and 400 nm. These absorption edges slightly shift to shorter wavelengths with the increasing substrate temperature. The obtained average transmittance in the visible region of the deposited GZO films is comparable to the reported transmittance of the ITO films.^28,33,34 The ITO films deposited at higher substrate temperatures showed higher percentage of transmittance, and average transmittance was about 82% for the ITO films deposited at the substrate temperature of 200–300°C. ²⁸ Marikkannu et al. ³⁴ observed that the transmittance of the ITO film in the visible range increased with the increasing substrate temperature and reached at about 80% for the substrate temperature of 450°C due to high crystalline nature of the as prepared ITO films. However, it is lower than that of about 92% for the ITO films annealed by rapid thermal annealing ³⁵ and about 87% reported for the inkjet printed ITO films annealed at temperature less than 400°C. ³⁶

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Room temperature UV-vis transmittance spectra of GZO films deposited at various substrate temperatures

The optical absorption coefficient (α) and the optical energy band gap (E_g) for the direct transition semiconductor are related by using the following equation: ³⁷ (αhv)² = C(hv−E_g), where α is the absorption coefficient, C is a constant that depends on the electron-hole mobility, hν is the photon energy and E_g is the optical band gap. Thus, the optical band gap of the GZO films deposited at various substrate temperatures can be obtained by plotting (αhv)² versus hv, and extrapolating the line portion of this plot to the energy axis is shown in Fig. 6. It can be seen that the E_g slightly increases from 3·28 to 3·33 eV as the substrate temperature increases from 300 to 600°C since the filling of electron in the conduction band can shift the Fermi level to a higher energy state. The change of the band gap may be due to the Burstein–Moss effect,^38,39 as well as polaron, ⁴⁰ strain, ⁴¹ and other types of imperfection.

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Relationship between (αhv)² and photo energy (hv) for GZO films deposited at various substrate temperatures

The spectroscopic ellipsometry has been employed to analyse the role of substrate temperature on the optical properties. Figure 7 shows the refractive index (n) and extinction coefficient (k) of the GZO films deposited at various substrate temperatures. It is found that the refractive index strongly depends on the substrate temperature, and the values of refractive index are in the range of 1·87–2·23 for all samples. As shown in Fig. 7a, the refractive index of the GZO film first increases as the substrate temperature increases from 300 to 400°C, and then decreases when further increases the substrate temperature to 600°C, which are similar to those previously reported for ZnO films prepared by DC magnetron sputtering technique. ⁴² The extinction coefficient is close to 0 in the wavelength region between 400 and 800 nm, as shown in Fig. 7b, indicating that the samples are highly transparent. It was considered that refractive index (n) is related to packing density of the film. The Drude equation^43,44 was usually used to confirm the above analysis (2) where p is the mean value of the porosity, n_f is refractive index of porous materials, n_b is the refractive index of bulk materials. The refractive index (n_b) of ZnO bulk is 2·008 at 633 nm. ⁴⁵ Based on equation (2), the porosity p of the samples at substrate temperature of 300, 400, 500 and 600°C are calculated to be 14·66, 3·32, 5·80 and 8·50%, respectively. This confirms that the bigger the refractive index, the smaller the porosity and the higher the pack density.

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a refractive index; b extinction coefficient

Conclusions

Highly c axis oriented and transparent Ga doped ZnO films have been prepared by pulsed laser deposition at substrate temperature ranging between 300 and 600°C. The effect of substrate temperature on the morphology, microstructure and properties of GZO films are investigated. The films are oriented along the c axis of the hexagonal structure whatever the substrate temperature. Grain size is found to increase from 22·3 to 33·5 nm then decreases to 24·8 nm as the substrate temperature increases to 600°C. The average surface roughness slightly decreases from 7·2 to 5·0 nm as the substrate temperature increases from 300 to 500°C, and then increases to 15·6 nm when the substrate temperature further increases to 600°C. The optical energy gap of the GZO films is observed to increase slightly from 3·28 to 3·33 eV as the substrate temperature increase from 300 to 600°C. The refractive index strongly depends on the substrate temperature, and is in the range of 1·87–2·23. The refractive index of the GZO film first increases as the substrate temperature increases from 300 to 400°C, and then decreases when further increases the substrate temperature to 600°C.