CHAPTER 5. DEPOSITION AND CHARACTERIZATION OF ZINC STANNATE (Zn 2 SnO 4 ) THIN FILMS

Transcription

1 106 CHAPTER 5 DEPOSITION AND CHARACTERIZATION OF ZINC STANNATE (Zn 2 SnO 4 ) THIN FILMS 5.1 INTRODUCTION Post-transition-metal oxides and their alloys have unique physical properties. Despite their large band gap (>3eV), which makes them transparent under normal conditions; they can sustain a high concentration of carrier electrons with a high mobility. Although the binary oxides such as ZnO, SnO 2, In 2 O 3, etc., find numerous applications, recently, spinel type cadmium stannate (Cd 2 SnO 4 ) has been identified as one of the best candidates for TCO applications due its promising electrical and optical properties (Wu et al 1995). Even though Cd 2 SnO 4 shows excellent TCO properties, it is not suitable for the large volume industrial productions due to the toxicity of Cd. There exists many ternary oxides such as Zn 2 SnO 4, Cd 2 SnO 4, In 2 CdO 4, and In 4 Sn 3 O 12 etc. for modern TCO applications. In Figure 5.1, several combinations of semiconductor compounds for thin film TCO applications are illustrated. Among these ternary compounds, the spinel type zinc stannate (Zn 2 SnO 4 ) is being considered as an alternative for Cd 2 SnO 4 due to its low cost, nontoxicity, and similar spinel structure like Cd 2 SnO 4. Zinc stannate, Zn 2 SnO 4 (ZTO) possesses excellent properties like large band gap (E g = 3.6 ev), high electron mobility, high electrical conductivity and low absorption in the visible region (Stambolova et al 1997, Coutts et al 2000). It has demonstrated many potential applications such as (i) an interface layer in the CdS/CdTe world record polycrystalline solar cells (16.5% of total area

2 107 efficiency) (Wu et al 2001), (ii) detection of moisture and combustible gases, (iii) in photoelectrochemical applications, and (iv) together with silver as an electrical contact in flat panel displays (Ginely and Bright 2000, Yu and Choi 2002, Matsushima et al 1995). Figure 5.1 Practical TCO semiconductors for thin film transparent electrode applications (Minami 2005) The stoichiometric zinc stannate (ZnO-SnO 2 alloy system) exists in two different crystallographic phases like orthorhombic ZnSnO 3 and cubic spinel Zn 2 SnO 4. Among these two different stoichiometric phases, Zn 2 SnO 4 finds many applications. The zinc stannate (Zn 2 SnO 4 ) possesses cubic spinel AB 2 O 4 atomic structure as shown in Figure 5.2, where A is the first metal ion with 2+ valancy and B is the second metal ion with 3+ or 4+ valancy. This structure can be viewed as the combination of the rock salt and zinc blend structures. In the AB 2 O 4 structure, A and B ions occupy the tetrahedral and octahedral interstitial sites respectively and the oxygen ions occupy the face centered cubic close packing structure as shown in Figure 5.2. The spinel structure can be further classified in to normal and inverse spinel structures. In the normal spinel structure, the A 2+ ions occupy the tetrahedral sites and the B 3+ (or B 4+ ) ions occupy the octahedral sites. In the inverse spinel structure,

3 108 A 2+ ions and half of the B + ions occupy the octahedral sites together, and the other half of the B 3+ ions occupy the tetrahedral sites, which is governed by the generalized formula B (AB)O 4. Figure 5.2 The cubic spinel AB 2 O 4 structure. The interstitial position of Oxygen (blue shaded circles), B-atoms (red shaded circles) in the octahedral sites and A-atoms (green shaded circles) in the tetrahedral sites are shown in the red cubic structure (Wei and Zhang 2001, Segev and wei 2005). The zinc stannate films have the advantages of both ZnO and SnO 2, and are promising for TCO and sensor applications. In Table 5.1 some of the physical properties of zinc stannate thin films are presented. Only a few groups have studied the properties of zinc stannate films and most of the reported works focus on gas sensing application on Zn 2 SnO 4 and reports on Zn 2 SnO 4 for TCO applications are very limited. Hence, an attempt on the preparation of Zn 2 SnO 4 by spray pyrolysis method has been made and

4 109 systematic analysis on the structural, optical and electrical properties are discussed in this chapter along with its suitability for TCO applications. Table 5.1 Physical properties of zinc stannate thin films Property Zinc stannate Mineral name Zinc Tin Oxide (ZTO) Crystal structure Spinel Melting point[ C] 570 C Band gap (E g ) 3.6 Refractive index at 550nm wavelength 2.1 Electron effective mass 0.23m e Electron Hall mobility at 300K[cm 2 /Vs] LITERATURE REVIEW The physical characteristics of RF magnetron sputtered zinc stannate (ZTO) thin films have been reported by Wu et al (1997), where the authors have deposited the ZTO films at room temperature conditions and observed an amorphous structure. The surface roughness was studied by AFM and a mean square roughness (RMS) of 20 Å was reported. Similarly an amorphous structure has also been reported by Perkins et al (2002) for ZTO films deposited by RF magnetron sputtering and pulsed laser deposition methods. The properties of zinc stannate deposited by magnetron sputtering with different cathode power substrate temperatures was reported by Young et al (2002a, b). The authors have reported an amorphous structure for the films deposited at room temperature. However it has been observed that the films become crystalline with a substrate temperatures above 550 o C. The

5 110 AFM measurements performed for these samples revealed that the films deposited at 600 o C possess a maximum grain size of roughly 100 nm, and the observed surface roughness was 4.3 nm. The optical spectroscopy, spectroscopic ellipsometrty and FTIR methods were used to study the optical properties of the films. The refractive index and extinction coefficient values have been derived by modelling the ellipsometry data, and have reported the values (n=2 and k=0) in the visible region. The authors have observed the Brustein-Moss shift from the absorption edge of the transmission measurements. Electrical properties of the films have been studied and reported that: (i) the electrical resistivity vary between, and Ω.cm, (ii) the mobility is 12 to 26.1 cm 2 /Vs, and (iii) the carrier concentration is to cm -3. Minami et al (2005) have studied vacuum arc plasma evaporated (VAPE) zinc stannate thin films on hot substrates and reported the effect of deposition conditions on structure and composition. The authors have studied the film properties as a function of composition and substrate temperature variation. They have reported a deposition rate of 120 nm/s and the film thickness between 220 to 380 nm depending on deposition conditions, for films deposited on 300 o C substrates. Amorphous structure has been reported for the films deposited with at% Sn content. The optical transmission of films deposited at different substrate conditions did not change significantly. However, depending on the composition, the band edge shifts to lower wavelengths as the Sn content increases. The optimum electrical resistivity, hall mobility and the carrier concentration observed for the zinc stannate films deposited at 300 o C were ~10-2 Ωcm, ~10 cm 2 /Vs and ~10 19 cm -3, respectively. The structure of ZnO-SnO 2 thin films deposited using spray pyrolysis has been investigated by Bagheri-Mohagheghi and Shokooh-Saremi (2003). The authors have investigated the properties of the zinc stannate films

6 111 by varying the Zn and Sn concentrations in the films. A polycrystalline structure has been reported for the films deposited at 480 o C. For low zinc concentrations (for 1.7 at.% of Zn) a single phase SnO 2 structure with strong peaks at (1 0 1), (2 1 1) and additional weak peaks at (1 1 0), (2 0 0), (1 1 1), (0 0 2) have been reported by them. Whereas for zinc rich films (for 7.2 at.% of Zn) the authors have reported that the intensities of (1 0 1) and (2 1 1) peaks decreased with the simultaneous increase in the (2 0 0) peak intensity. However, for Zn concentrations exceeding 7.2 at.%, formation of a secondary ZnO phase has been reported. Electrical and optical properties of these films have also been studied. An average transmission between 90% and 80% for 12.5 and 20.7 at.% Zn concentrations, respectively in the visible range ( nm) have been reported. In this article, it has also been reported that the carrier concentration increases from to cm -3 upon increasing the Zn content in films from 12.5 to 20.7 at.%. Perkins et al (2002) have reported the properties of annealed Zn- Sn-O films deposited by sputtering and PLD methods. They reported that the zinc stannate films deposited by both methods are amorphous. The films annealed at 650 o C in N 2 atmosphere showed polycrystalline behaviour with X-ray diffraction peaks correspond to Zn 2 SnO 4. For sputter deposited films, the conductivity of the as-deposited films ranged from 10-2 to 10-1 Ω -1 cm -1, whereas, for the annealed films that the conductivity varied between 10-5 and 10 2 Ω -1 cm -1. Satoh et al (2005) have studied the effect of annealing on the structural and optical properties of the zinc stannate thin films deposited by rf magnetron sputtering. They have reported an amorphous structure for the asdeposited films prepared under different deposition conditions. However, a crystalline phase was observed after annealing at 650 and 750 o C in an oxygen

7 112 atmosphere for 1 h. The authors have reported a high transmission (~85%) values after annealing the films and simultaneously the absorption edge shifted to the lower wavelengths as a consequence of decreased disorder in the films. For the films annealed at 750 o C, they have reported a high band gap (~4.1 ev) values. They also reported that the films deposited in pure Ar possess a resistivity of Ω.cm, carrier concentration cm -3, and the Hall mobility 18 cm 2 /Vs, while the films deposited in an O 2 /Ar mixture had high resistivity values. The complex refractive index components (n) and (k) have been reported as ~2 and 0, respectively, at 500 nm wavelength. 5.3 DEPOSITION OF ZINC STANNATE (Zn 2 SnO 4 ) Figure 5.3 depicts the flow chart of preparation steps of zinc stannate (ZTO) films by chemical spray pyrolysis method. Zinc acetate [Zn(CH 3 COO) 2.2H 2 O] and tin chloride (SnCl 2.2H 2 O) chemicals were used as precursors to prepare the spray solution. At first, zinc acetate and tin chloride with concentrated HCl were dissolved using doubly distilled deionized water to form two transparent solutions respectively and were mixed together carefully to form zinc stannate spray solution. In the final spray solution, the concentration of [Zn] and [Sn] was fixed at 0.50 mol/lit. In order to form a homogeneous solution, the final mixture was mechanically stirred for 2h using a magnetic stirrer. The films were deposited on corning glass, Si(1 0 0) and quartz substrates. The compressed air was used as the carrier gas. Before the sample preparations, several trials were made to optimize the deposition conditions. Under the optimized conditions, during deposition; (i) the solution flow rate and carrier gas flow rate were maintained constant at 10 ml/min and 12 l/min, respectively; (ii) the spray nozzle to substrate distance was maintained at 30 cm, and (iii) the substrate temperature was maintained at 400 o C. In order to avoid the fast cooling of the substrate due to continuous spraying of the solutions, the solution was sprayed on the substrates for several spraying

8 113 cycles of 3 seconds followed by an interval of no spray for 2 minutes. The films have been sprayed for maximum up to 3h with the above said systematic steps, to prepare films of thickness approximately nm. In order to study the effect of post deposition annealing process, after deposition the samples were annealed at 600 C and 800 C in Ar atmosphere for 1h. Zinc acetate + 50ml Deionised H 2 O Stir for ½ h./room temp Tin(II)Chloride + 50ml Deionised H 2 O+ConcHCl Stir for ½ h./room temp Mixing of the two solution Stir for 2 h./room temp Spray solution Deposition/Substrate temperature. 400 o C Zinc Stannate(Zn:Sno 4 )Film Figure 5.3 Flow chart of zinc stannate (Zn 2 SnO 4 ) thin film deposition steps by spray pyrolysis method 5.4 CHARACTERIZATION STUDIES EDX Elemental Analysis The stoichiometry of as deposited and annealed zinc stannate films have been analyzed by EDX microanalysis. Figure 5.4 depicts the SEM-EDX spectrum recorded for the as deposited zinc stannate (ZTO) film. As mentioned in the experimental section, in order to avoid the strong Si peak from the substrates, films deposited on quartz substrates have been used for the elemental analysis. As shown in the Figure 5.4, the EDX analysis confirms the presence of Zn, Sn and O elements in the deposited films. In

9 114 Table 5.2, the amount of Zn and Sn in the (i) stoichiometric Zn 2 SnO 4 (Nikolic et al 2008), and (ii) the present experimental results of calculated amount of Zn and Sn in the film and are listed. The values obtained for the films are in good agreement with the standard values, which indicates the formation of the stoichiometric Zn 2 SnO 4. From the EDX analysis, it has also been observed that for the sample annealed at 800 o C, the ratio of Zn/Sn is slightly different from that of the as deposited films. The films annealed at 800 o C are slightly Zn deficient. It is well known that upon annealing at high temperatures, a small part of ZnO may escape through evaporation (Hashemi et al 1990) and this could be the reason for the observed zinc deficiency for the film annealed at 800 o C. The observed Zn/Sn ratio for the films measured from EDX measurements corresponds to the Zn 2 SnO 4 stoichiometry for both as deposited and annealed films. Si annealed at 600 o C Intensity (Cps) Intensity Cps O Zn (substrate peak) Sn Sn Sn Zn Zn Energy (kev) Figure 5.4 Typical SEM-EDX pattern of zinc stannate film annealed at 600 C

10 115 Table 5.2 Concentration of Zn and Sn in the zinc stannate films measured by EDX Amount of Zn and Sn in Sample the films [atomic %] Zn Sn Ideal Zn 2 SnO Deposited at 400 o C Annealed at 600 o C Annealed at 800 o C X-ray Diffraction Analysis The grazing angle X-ray diffraction spectra of zinc-stannate (ZTO) films deposited at 400 o C and the films annealed at different temperatures were shown in Figure 5.5. From this figure, it is clear that the film deposited at 400 o C Figure 5.5a shows diffraction peaks with amorphous background, which indicates that the as-deposited films are not completely crystallized. Annealing improved the degree of crystallization, and finally at 800 o C, completely crystallized films have been obtained as shown in Figure 5.5(b-c). The experimental peak positions have been compared to the theoretical peak positions of ZnO, SnO 2, ZnSnO 3 and Zn 2 SnO 4 and the miller indices have been indexed. Except two weak peaks of SnO 2 (Figure 5.5c), all other observed diffraction peaks were indexed to the face-centered spinel structured Zn 2 SnO 4, which is in good agreement with the reported standard values (JCPDS file No: ). The theoretical and experimental peak positions are listed in Table 5.3.

11 116 From the experimental peak positions (2 values) and their corresponding (h k l) values, the lattice parameters have been calculated. The calculated lattice parameter and cell volume are given in Table 5.4. For the films prepared at 400 C, with an initial composition close to Zn 2 SnO 4, a lattice parameter of a = b = c = Å is obtained, and the cell volume is calculated as Å 3. Due to annealing, the peaks became narrower indicating the grain growth (Table 5.4) in the films which leads to an increase in cell volume. Finally for the completely crystallized films (annealed at 800 o C), a lattice parameter of a = b = c = Å and a cell volume of Å 3 has been obtained. The calculated values are in good agreement with the reported values of lattice parameter a = Å and cell volume Å 3 (JCPDS file No: ), which confirmed the formation of cubic Zn 2 SnO 4 films. According to thermodynamical arguments, for solid state reactions at higher temperatures, it is quite normal that ZnSnO 3 is converted to Zn 2 SnO 4 and then to SnO 2 at very high temperatures (Shen and Zhang 1993). This indicates that Zn 2 SnO 4 is thermodynamically more stable than ZnSnO 3. However SnO 2 peaks are observed for the films annealed at 800 C (Figure 5.5c).

12 (311) (c) Annealed at 800 o C/1hr. (c) Annealed at 800 C/1h Intensity (Cps) SnO 2 (222) (400) (422) (511) (440) SnO 2 (b) Annealed at 600 o C/1hr. (b) Annealed at 600 C/1h (a) Deposited at 400 o C Grazing angle = 0.75 o (a) Deposited at 400 C Grazing angle = [ o ] Figure 5.5 Grazing angle X-ray diffraction patterns recorded for zinc stannate films deposited on Si (1 0 0) substrates, (a) film deposited at 400 C,(b) annealed at 600 C and (c) annealed at 800 C

13 118 Table 5.3 X-ray diffraction peak positions observed for zinc stannate films Theory Zn 2 SnO 4 2 Intensity hkl Deposited at 400 o C Experiment 2 position Annealed at 600 o C Annealed at 800 o C Table 5.4 Lattice parameter, cell volume, grain size and surface roughness of zinc stannate films Sample Deposited at 400 o C Annealed at 600 o C Annealed at 800 o C Lattice parameter a=b=c [Å] Cell volume [Å 3 ] Grain size [nm] Surface roughness by AFM [nm] ± ± ± ± ± ±

14 Surface Roughness The surface structures of both as-deposited and annealed films were studied by AFM and are shown in Figure 5.6. The AFM studies revealed that the films deposited at 400 o C possess smaller grains (Figure 5.6a) and showed a clear trend of grain growth while increasing the annealing temperature (Figure 5.6b,c), which is in good agreement with the results of XRD measurements. It has also been observed that while increasing the annealing temperature the surface roughness value also increased from 5.6 nm to 11.5 nm (Table 5.4). The increase in roughness value can be attributed to the grain growth observed by XRD and AFM measurements (a) (b) (c) Figure 5.6 AFM images of zinc stannate films (a) deposited at 400 o C, (b) annealed at 600 o C, and (c) annealed at 800 o C

15 Optical Properties The XRD analysis revealed that after annealing the films at high temperatures both the crystalline quality and grain size has been increased. The increase of crystalline quality is an indication for the increase of density of the films. The increase of film density should lead to films with a higher index of refraction, which would be an attractive feature for TCO applications. Hence in order to understand the optical properties of the films both optical transmittance and spectroscopic ellipsometry measurements have been carried out and discussed below Optical Transmittance Optical transmittance (T) measurements have been performed with a spectrophotometer in the spectral range from 900 nm to 200 nm. Figure 5.7 shows the optical transmittance measurements obtained for the zinc stannate (Zn 2 SnO 4 ) films deposited at 400 C and annealed at 600 and 800 C. The transmittance measurements revealed that the as-deposited films prepared with a substrate temperature of 400 C is highly transparent in the visible region. The average transmittance values of the films were found to be increased with the increasing annealing temperature. The absorption edges of the films have also been shifted towards the lower wavelength region (blue shift) upon annealing at high temperatures, which indicates the band gap broadening behaviour of the films.

16 121 Transmittance [%] Bandgap increase 30 As deposited at 400 o C 20 Annealed 600 o C/1 hr. 10 Annealed 800 o C/1 hr Wavelength [nm] Figure 5.7 Optical transmittance of the zinc stannate (Zn 2 SnO 4 ) films deposited at 400 C and annealed at 600 and 800 C Variable Angle Spectroscopic Ellipsometry (VASE) In order to derive the precise values of the optical constants of the films, spectroscopic ellipsometry measurements have been carried out for the both as -deposited and annealed films. The spectroscopic ellipsometry data in the form of Psi ( ) and delta ( ) were recorded for two different incident angles; 65 o and 70 o over a spectral range from 0.72 (5807 cm -1 ) to 5.2 ev (41940 cm -1 ). During measurements, the backside of the substrate was roughened mechanically in order to avoid the backside reflection.to obtain accurate values of the optical constants like refractive index (n), extinction coefficient (k) and band gap (E g ), the experimental ellipsometric data have been simulated theoretically using the Tauc - Lorentz oscillator model (Tauc et al 1966) In Figure 5.8, the ellipsometric data measured for the zinc stannate film deposited at 400 o C is depicted together with the theoretical simulations as a function of photon energy. From this figure, it is clear that there is a good

17 122 agreement between the experiment and the simulated curves, and hence the film properties can be described precisely. Similar measurements and simulations have also been performed for the annealed samples. In order to achieve very good fit parameters two Tauc Lorentz oscillator models have been used for the description of the experimental curves. The lower-energy oscillator has been used for the determination of the optical band gap E g values. [ o ] Depsoited at 400 o C Experiment Simulated data 65 o 70 o 0 [ o ] Energy [ev] Figure 5.8 Ellipsometric spectra of Zn 2 SnO 4 /glass deposited at 400 o C The refractive index n and extinction coefficient k are calculated from the modelled complex dielectric function and are plotted as a function of energy in Figure 5.9. From Figure 5.9, it is clear that the refractive index increases while the increasing the annealing temperature. The as-deposited

18 123 films prepared at 400 o C, showed a refractive index n = ~ 1.97 in the visible region and extinction coefficient k 0 with an optical band gap of ~ 3.3 ev. These values are in good agreement with the previously reported values (Young et al 2002a). Upon annealing, both refractive index and band gap of the films were increased. For the completely crystallized film annealed at 800 o C, a refractive index of n = ~ 2.07 and E g = ~ 3.87 ev has been obtained. The increase in refractive index due to annealing can be attributed to the crystallization (evidenced by XRD) and the densification of the films. The calculated n and E g values are listed in Table.5.5. For TCO applications, a material should possess a refractive index of n 2 and ZTO films prepared by spray pyrolysis almost satisfy these basic requirements and it can be used for TCO applications (Young et al 2002a,b) Sheet Resistance The sheet resistance values of Zn 2 SnO 4 films deposited on quartz substrates have been measured using four probe method and the values are given in Table 5.5. Initially for the as deposited films, the sheet resistance was observed to be ~ 8100 Ω/. Upon annealing, the sheet resistance value decreases (Table 5.5). The observed higher resistivity for the as deposited films is due to the fact that the films are not completely crystallized at low temperatures and showed only a slight improvement in crystallization at 600 o C. It has been reported that ZTO films begin to crystallize at above 575 o C (Mamazza et al 2005).

19 124 Refractive index (n) n value increase Extinction coeeficient (k) As deposited 400 o C Anealed at 600 o C Annealed at 800 o C Energy [ev] Figure 5.9 Variation of refractive index (n) and extinction coefficient (k) as a function of energy calculated using the dielectric function models for the zinc stannate films as measured by VASE Table 5.5 Variation of refractive index, band gap and sheet resistance of zinc stannate films Sample Refractive index (n) at 650 nm Band gap E g (ev) Sheet resistance [Ω/ ] Deposited at 400 o C Annealed at 600 o C Annealed at 800 o C

20 125 In our experiments the ZTO annealed at 600 C films become completely crystalline and showed a low sheet resistance value at 800 o C (Table 5.5). Compared to the reported values obtained for the films prepared by vacuum deposition methods, films prepared in the present study showed higher resistivity values. This may be due to the fact that spray pyrolysis is a non vacuum deposition method, and it requires further optimisation to achieve very low resistivity values. 5.5 CONCLUSION Zinc stannate (Zn 2 SnO 4 ) thin films have been deposited on Si (1 0 0) and quartz substrates by spray pyrolysis technique at a substrate temperature of 400 o C. The films were annealed at 600 o C and 800 o C in an Ar atmosphere and the physical properties of these films have been investigated in detail as a function of increasing annealing temperature. The elemental analysis of the films measured by energy dispersive X-ray analysis confirmed that the deposited films possess nearly Zn 2 SnO 4 stoichiometry and became slightly Zn deficient after annealing at 800 o C. The XRD analysis revealed the cubic structure of the films. The crystalline quality was found to be improved upon annealing at 800 o C. Due to annealing both grain size and cell volume are found to be increased. The observed trend of increasing grain size as a function of temperature has further been confirmed by AFM studies. Optical transmission measurements of the zinc stannate films showed a high transmission (>75%) in the visible region. While increasing the annealing temperature, the average transmission of the films increased, and the absorption edge shifted towards the shorter wavelength. The spectroscopic ellipsometry measurements showed the optical constants such as refractive index (n) and band gap (E g ) of the films, increased from 1.95 to 2.1 and 3.35 to 3.87 ev, respectively due to annealing. The electrical resistivity measurements showed that the as-deposited films possess relatively a higher sheet resistance

21 126 value and this value is found to be decreased due to annealing. The observed improvement both in optical and electrical properties are attributed to the improvement in the crystallization of the films due to annealing. For TCO applications, a material should possess a refractive index of n 2 and nearly zero extinction coefficient in the visible region together with low sheet resistance values. The ZTO films prepared in the present study satisfy certain basic requirements and it can be used for TCO applications.