1.
Introduction
Transparent conductive oxides (TCO) have many practical applications in various fields, including photovoltaic cells, flat panel displays, sensors, organic light emitting diodes, etc[1–6]. One of the most conventionally used TCO materials is indium tin oxide (ITO), which shows high transparency and conductivity[5]. However, this kind of widely used material has some undesired limitations like toxicity, high cost of indium and chemical instability[6, 7]. These drawbacks have intrigued some researchers to find a candidate to replace ITO. Among all kinds of alternative materials, Al-doped ZnO (AZO) film has aroused intensive interest because of its low cost, abundant sources and easy processing as well as thermal stability[8–10]. In addition, AZO films also have exhibited excellent optical and electrical properties when compared with ITO films.
AZO thin films can be prepared by several thin film fabrication techniques, including sputtering[11, 12], sol–gel deposition[13], chemical vapor deposition (CVD)[14] and pulsed laser deposition (PLD)[15–17]. In recent years, atomic layer deposition (ALD) has been chosen as a promising technique to grow AZO thin films. In the ALD system, precursors are separated by purging processes according to its sequential chemistry surface[18–22]. Compared with other deposition techniques, this method exhibits several unique merits such as accurate control of thickness and composition, good conformality and high mass production at relatively low temperature. AZO thin films are not stable at high temperature because of Al diffusion, which deteriorates film stability[23]. Having considered these factors, we choose the ALD technique to deposit AZO films in this work. The microstructure, optical and electrical properties of AZO thin films are investigated with various growth temperatures and Zn : Al cycle ratios. These factors obviously influence the thin film performances[24–28].
2.
Experimental details
2.1
AZO films grown by atomic layer deposition
A commercial ALD system (Sentech) is used to deposit AZO thin films on glass substrates in this work. High purity nitrogen (N2) gas is introduced as a carrier and purging gas with inlets separated on the side of the chamber. Water and diethylzinc (Zn(C2H5)2, DEZ) vapor serve as oxygen and zinc precursors and they are alternatively injected into the reactor using nitrogen as a carrier gas with a flow rate of 80 sccm. Trimethyl aluminum (Al(CH3)3, TMA) and water are selected for Al2O3 growth and it is used as the dopant for ZnO. The substrate temperatures are changed from 100 to 250 °C with the chamber pressure set at 50 mTorr. A typical ALD growth process for the AZO thin films can be described as: [a × (DEZ + H2O) + b × (TMA + H2O)] × c. a × (DEZ + H2O) + b × (TMA + H2O) represents one supercycle of AZO thin film deposition. The supercycle is performed repeatedly to acquire the desired thickness. The Al composition in the AZO films is varied by changing the ratio of Zn : Al cycles, from 20 : 0 to 20 : 3. The relevant purge times and precursor pulse are kept constant for all deposited films.
2.2
Thin film characterization
The crystallographic orientation of the AZO films is determined by an X-ray diffractometer (XRD) with CuK radiation, and the thicknesses are measured through spectral ellipsometer. The electrical properties including resistivity, carrier concentration, and Hall mobility are measured by a Hall system[29]. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) are utilized to observe surface morphologies. X-ray photoelectron spectroscopy (XPS, VG Microtech ESCA2000) analysis is performed with monochromatic Al Ka radiation (1486.6 eV) in which it is calibrated with a C 1s peak (284.6 eV). The XPS spectra are fitted with the Gaussian-Lorentzian function for the quantitative analysis. Optical transmittance spectra are taken using a UV–Vis–NIR spectrophotometer.
3.
Results and discussions
3.1
Growth rates of ZnO and Al2O3 thin films
We firstly study the film growth rate of individual ZnO and Al2O3 deposited at 150 °C. Fig. 1 shows various thicknesses of the pure ZnO films and Al2O3 as a function of the ALD cycle. The growth rates of the ALD-grown ZnO and Al2O3 thin films are calculated to be around 0.15 ?/cycle and 0.06 ?/cycle, respectively. These are similar reports to those in Refs. [30–33], in which the growth rates vary from 0.13 to 0.20 nm per cycle for ZnO and 0.08 to 0.13 nm per cycle for Al2O3. It is obvious that the thicknesses of these two materials change linearly in terms of the ALD cycle, which demonstrates the self-limited property of the ALD growth process.
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Figure1.
(Color online) Thicknesses of ZnO and Al2O3 films as a function of ALD cycle.
3.2
Effects of growth temperatures on the AZO thin films
3.2.1
Growth rate of AZO thin films
Fig. 2 shows the growth rate of AZO thin films deposited by ALD at different growth temperatures ranging from 100 to 250 °C. In the low temperature region (below 150 °C), the growth rate of AZO film is relatively low because the reactants do not have sufficient energy for active chemical reaction, which may be a typical characteristic for oxides deposition through the ALD method from an alkyl precursor and water. The growth rate maintains almost constant in the relative high growth temperature regions (from 150 to 200 °C), indicating the existence of a process window. However, the growth rate begins to decrease with further increase of temperature, probably caused by precursor desorption/dissociation. The growth rate of AZO thin films is observed to be a little lower than that of the pure ZnO sample with the additional Al2O3 growth, which may be caused by the suppressed growth of ZnO film on Al2O3 film[28, 34].
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Figure2.
Growth rate of AZO films grown at various temperatures.
3.2.2
Surface morphology
To investigate the effect of temperature on thin film morphology, the surface morphology are evaluated through scanning electron microscopy (SEM). SEM images of AZO thin films grown at different temperatures are shown in Fig. 3. The scale size in each image is 200 nm. It is obvious that AZO thin films deposited at different temperatures show quite different surface morphology. Grains in the AZO thin films grown at 100, 200, and 250 °C are small and round, whereas those grains in the AZO thin films grown at 150 °C present textured morphology with a wedge-like shape[27]. The wedge shape originates from the a-axis preferred orientation and the round shape comes from the c-axis preferred orientation.
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Figure3.
Surface morphology of AZO films grown at different temperatures: (a) 100 °C, (b) 150 °C, (c) 200 °C, and (d) 250 °C.
3.2.3
Electrical properties
The electrical properties of the AZO thin films are investigated through the Hall measurement at room temperature. Table 1 shows the resistivity, Hall mobility, and carrier concentration of the AZO films deposited at various growth temperatures. It is obvious that AZO films exhibit natural n-type conduction behavior resulting from the zinc interstitial and oxygen vacancy. The electronic carrier concentration of these samples is in the range from 1.67 × 1017 to 3.5 × 1020 cm?3. The AZO sample deposited at 100 °C shows the poor electrical performances like low electron mobility, which could be attributed to the high impurity density coming from residual precursors such as O–H or O–C groups. It seems that Al3+ ions in ALD deposition are well incorporated in Zn2+ sites at 150 °C and also the AZO film exhibits good electrical performances. Above 250 °C, the AZO sample presents reduced Hall mobility, which is resulted from ionized impurity scattering induced by a high density of Al concentration[27]. Therefore, the growth temperature is maintained at 150 °C in the following experiments (i.e. changing the Zn : Al ratio).
| Temperature (°C) | Resistivity (Ω·cm) | Mobility (cm2/(V·s)) | Concentration (cm?3) | ||
| 100 | 8 × 101 | 0.26 | 1.67 × 1017 | ||
| 150 | 2.14 × 10?3 | 8.54 | 3.4 × 1020 | ||
| 200 | 3.86 × 10?3 | 15.5 | 8.2 × 1019 | ||
| 250 | 4.23 × 10?3 | 9.48 | 1.84 × 1020 |
Table1.
Electrical properties of AZO thin films deposited at different temperatures.
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| Temperature (°C) | Resistivity (Ω·cm) | Mobility (cm2/(V·s)) | Concentration (cm?3) | ||
| 100 | 8 × 101 | 0.26 | 1.67 × 1017 | ||
| 150 | 2.14 × 10?3 | 8.54 | 3.4 × 1020 | ||
| 200 | 3.86 × 10?3 | 15.5 | 8.2 × 1019 | ||
| 250 | 4.23 × 10?3 | 9.48 | 1.84 × 1020 |
3.3
Effects of Zn : Al ratio on the AZO thin films
3.3.1
Surface morphology and microstructure
The influence of the Zn : Al ratio is researched in this section. AFM images with the scan size 2 × 2 μm2 of the AZO thin films grown at different Zn : Al cycle ratios are depicted in Fig. 4. The surface of un-doped ZnO films is obviously different from AZO films. The film surface morphology is composed of nano-sized grains with a close-packed microstructure[28]. In addition, the surface roughness parameters of these AZO films are also derived from these images. RMS (root mean squared) roughness is used to describe the average deviation from a mean surface value[35]. The RMS values for these samples are 7.5, 4.3, 4.5, and 5.5 nm with the cycle ratio from 20 : 0 to 20 : 3. Low surface roughness is very desirable in device applications[36]. The surface roughness is closely related with Al doping. The roughness of un-doped ZnO film is 7.5 nm and then decreases with increasing Al doping, which is caused by the Al3+ ions substituting Zn2+ ions. In addition, a slight increase in RMS roughness value is observed with further increase of Zn : Al cycle ratio to 20 : 3 probably due to TMA etching of the ZnO surface[34].
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Figure4.
(Color online) 2 × 2 μm2 AFM scan images of AZO films as a function of Zn : Al cycle ratio. (a) 20 : 0. (b) 20 : 1. (c) 20 : 2. (d) 20 : 3.
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Figure5.
(Color online) XRD spectra of AZO films as a function of Zn : Al cycle ratio. (a) 20 : 0. (b) 20 : 2.
As shown in Fig. 5, XRD is performed to investigate the crystalline state and crystal orientation of ZnO and AZO thin films. The AZO thin film with a 20 : 2 ratio exhibits a polycrystalline state with (100) and (110) orientation and the intensity of the (100) peak is dominant, which indicates an a-axis preferred orientation. However, the un-doped ZnO thin film shows an amorphous state.
3.3.2
Electrical properties
Table 2 demonstrates the electrical properties of the AZO film deposited at various Zn : Al ratios. One can see that a certain Al-doping can lead to the decrease of film resistivity. In the cycle ratio range from 20 : 0 to 20 : 2, the resistivity varies from 7.07 × 101 Ω·cm to 2.14 × 10?3 Ω·cm. The lowest resistivity of 2.14 × 10?3 Ω·cm is obtained at the Zn : Al cycle ratio of 20 : 2 with the mobility of 8.54 cm2/(V·s). Un-doped AZO film exhibits relatively high resistivity. The electron concentration increases from 6.8 × 1015 cm?3 (for un-doped ZnO sample) to 3.4 × 1020 cm?3 (for the AZO film with 20 : 2 ratio). However, it decreases slightly for the sample with a higher Zn : Al cycle ratio, resulting from compensation effects, i.e. by native defect formation. Further increasing the Zn : Al cycle ratio (> 20 : 2) will lead to a high impurity scattering effect and thus decrease the electron mobility[16].
| Zn : Al ratio | Resistivity (Ω·cm) | Mobility (cm2/(V·s)) | Concentration (cm?3) | ||
| 20 : 0 | 7.07 × 101 | 13 | 6.8 × 1015 | ||
| 20 : 1 | 7.09 × 10?2 | 2.13 | 4.1 × 1019 | ||
| 20 : 2 | 2.14 × 10?3 | 8.54 | 3.4 × 1020 | ||
| 20 : 3 | 2.54 | 0.623 | 3.9 × 1019 |
Table2.
Electrical properties of AZO thin films deposited at different Zn:Al cycle ratios.
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| Zn : Al ratio | Resistivity (Ω·cm) | Mobility (cm2/(V·s)) | Concentration (cm?3) | ||
| 20 : 0 | 7.07 × 101 | 13 | 6.8 × 1015 | ||
| 20 : 1 | 7.09 × 10?2 | 2.13 | 4.1 × 1019 | ||
| 20 : 2 | 2.14 × 10?3 | 8.54 | 3.4 × 1020 | ||
| 20 : 3 | 2.54 | 0.623 | 3.9 × 1019 |
3.3.3
XPS analysis
XPS measurement is carried out to study chemical composition and bond states of AZO thin film with the cycle ratio of 20 : 2. The Al atomic composition percentage is calculated to be determined to 3.71 at.%. The Zn 2p2/3, Al 2p and O 1s core levels are shown in Figs. 6(a)–6(c). An Ar+ ion etching treatment is performed to remove surface contamination before XPS measurement. The Zn 2p3/2 peak is centered at around 1022.3 eV, which is the characteristic of ZnO structure. No metallic Zn peak located at around 1021.5 eV appears in the spectrum. The Al 2p energy peak is located at 73.93 eV, this binding energy value is attributed to Al2O3 and no metallic Al with a binding energy of 72.7 eV is observed. Compared with Zn 2p3/2 and Al 2p, the O 1s spectrum is composed of two component peaks which center at 530.4 and 532.3 eV by Gaussian fitting. The weak peak with a binding energy of 530.4 eV is associated with O2– ions in the ZnO structure. An additional higher binding energy peak at 531.9 eV results from the non-stoichiometric oxygen or chemisorbed oxygen species from AZO thin film due to exposure to the ambient environment.
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Figure6.
(Color online) XPS spectra of (a) Zn 2p3/2, (b) Al 2p, and (c) O 1s for AZO film with 20 : 2 Zn : Al cycle ratio.
3.4
Optical transmittance measurement
Optical transmittance of AZO films grown on glass substrates are measured by a UV–vis–NIR spectrophotometer. Fig. 7 reveals the transmittance spectrum for the AZO sample deposited at the optimized growth condition with the 20 : 2 Zn : Al cycle ratio and the growth temperature of 150 °C. In the visible region, the average transmittance of AZO film is above 80%. ALD-grown ZnO shows great potential as transparent conductive material in optoelectronic devices[35, 38, 39].
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Figure7.
Optical transmittance spectrum of AZO film with 20 : 2 Zn : Al cycle ratio.
4.
Conclusion
The microstructure, electrical and optical performances of ALD-grown AZO films are investigated through TMA, DEZn and H2O as precursors with various growth temperatures and Zn : Al cycle ratios. Growth rates for ZnO and Al2O3 thin film are determined to be around 0.15 and 0.06 ?/cycle, respectively. Both the surface roughness and conductivity can be improved by the incorporation of Al-doping. The lowest resistivity of 2.14 × 10?3 Ω·cm of AZO film is obtained at the Zn : Al cycle ratio of 20 : 2 and the temperature of 150 °C with the mobility of 8.54 cm2/(V·s). The transmittances of the AZO films are ~80 to 90% for all-level Al doping. ALD-grown AZO thin films exhibit great potential for optoelectronic device application.
