Effects of Bilayer Thickness on the Morphological, Optical, and Electrical Properties of Al2O3/ZnO Nanolaminates

This report mainly focuses on the investigation of morphological, optical, and electrical properties of Al2O3/ZnO nanolaminates regulated by varying bilayer thicknesses. The growth mechanism of nanolaminates based on atomic layer deposition and Al penetration into ZnO layer are proposed. The surface roughness of Al2O3/ZnO nanolaminates can be controlled due to the smooth effect of interposed Al2O3 layers. The thickness, optical constants, and bandgap information of nanolaminates have been investigated by spectroscopic ellipsometry measurement. The band gap and absorption edge have a blue shift with decreasing the bilayer thickness on account of the Burstein-Moss effect, the quantum confinement effect and the characteristic evolution of nanolaminates. Also, the carrier concentrations and resistivities are found to be modified considerably among various bilayer thicknesses. The modulations of these properties are vital for Al2O3/ZnO nanolaminates to be used as transparent conductor and high resistance layer in optoelectronic applications.


Background
Nanolaminate is a composite structure formed by different stacking sequences of diverse materials, and the layer thickness is general at the nanometer scale [1][2][3][4]. This multilayered structure can endow the nanolaminate with unique properties, and these properties depend on or may be better than those of the constituent materials [5][6][7]. In recent years, a new kind of materials based on the structure of nanolaminate is began to be utilized for energy storage devices [8], innovative optical elements [9], and temperature sensitive substrates for biosensors [10]. Recently, Viter et al. explored on the tuning of structural properties and the enhancement of electronic and optical properties of 1D PAN (polyacrylonitrile) ZnO/Al 2 O 3 nanolaminates which will allow applications in different fields such as sensors and biosensors [11]. Baitimirova et al. also investigated the tuning of structural and optical properties of graphene/ZnO nanolaminates which may find applications in optical, bio-, and chemical sensors [12].
As one of the most promising candidates for transparent conductive oxide (TCO) materials, Al-doped ZnO (AZO) film has many advantages, such as abundant resource, low cost, nontoxicity, and good stability in hydrogen plasma. In general investigations, controlling the Al doping level is a common method to improve and modify the optical and electrical behaviors of AZO materials [13,14], which is crucial to achieve functionalization and tunability of TCObased devices [15,16]. However, few reports involve in the performance modulation of AZO by changing the structures of Al 2 O 3 /ZnO nanolaminates which is more simple and effective in semiconductor manufacturing process.
The atomic layer deposition (ALD) technique is suitable for fabricating nanolaminate structures for different purposes and applications [17][18][19]. This technique is based on selflimiting surface chemical reactions with excellent deposition effect, which can make the thicknesses of individual nanolayers well-controlled for the composite stack. Furthermore, between different sublayers, good nucleation and adhesion can be realized by designing the surface reactions. Therefore, high-quality nanolaminates with uniform and smooth surface can be realized by ALD technique, and the thickness can be controlled accurately as well.
In this work, Al 2 O 3 and ZnO materials were adopted to realize the nanolaminate structures in order to investigate the tunable characteristics of AZO by changing the bilayer thickness of Al 2 O 3 /ZnO nanolaminates. We investigate their morphological, optical, and electrical properties. The growth mechanism of nanolaminates and Al penetration into ZnO layer are proposed and discussed. With decreasing bilayer Al 2 O 3 /ZnO thickness in the nanolaminates, blue shift of the bandgap is observed and discussed on the basis of the Burstein-Moss (BM) effect, the quantum confinement effect, and the characteristic evolution of the nanolaminates. The tunable electrical properties are exposed by using a measurement system based on the Hall effect. It gives valuable references and ideas that transparent conductor and high-resistance layer can be achieved by varying the bilayer thickness in the nanolaminates.  3 ], and deionized water (H 2 O), respectively. The precursor carrier and purge gas was the high purity nitrogen (N 2 , flow rate 50 sccm). It was used to carry precursors into the chamber and bring the needless products out of the chamber.

Synthesis of Nanolaminates by ALD
In order to grow the Al 2 O 3 layers, the TMA and H 2 O were alternatively brought into the reactor chamber through TMA-H 2 O cycles (TMA/exposure/N 2 /H 2 O/exposure/N 2 ) with pulse time of 0.03/3/15/0.03/5/15 s. The surface reactions of ALD Al 2 O 3 layers can be defined by two self-limiting reactions as follows [20]: AlOAl where the asterisks indicate the surface species. As for ZnO layers, DEZ-H 2 O cycles of ZnO were the same as the TMA-H 2 O. The surface reactions of ALD ZnO layers are given by [20] ZnOH Ã þ Zn C 2 H 5 ð Þ 2 →ZnOZnC 2 H 5 where the asterisks also indicate the surface species. The structure diagram of ALD Al 2 O 3 /ZnO nanolaminates is shown in Fig. 1. For all nanolaminates, the interface with the substrate was Al 2 O 3 , while ZnO was the top layer at the surface of nanolaminates. The bilayer is constructed by two individual layers, i.e., Al 2 O 3 and ZnO, with the same thickness. In order to guarantee the same thicknesses of total nanolaminates, the number of bilayer was increased with the decrease of bilayer thickness. So five kinds of samples were prepared, named as 2 (25/25 nm), 5 (10/10 nm), 10 (5/5 nm), 25 (2/2 nm), and 50 (1/ 1 nm). The details can be found in Table 1. Note that the parameters in Table 1 are the empirical values, which are summarized from our preliminary experiments.

Characterization
The morphological characterization of Al 2 O 3 /ZnO nanolaminates was carried out by transmission electron microscope (TEM; FEI Tecnai G2 F20) and atomic force microscopy (AFM; Bruker Dimension Icon VT-1000, Santa Barbara, CA). The thickness, optical constants, and bandgap information were determined by spectroscopic ellipsometry (SE; J.A. Woollam, Inc., M2000X-FB-300XTF) measurements in the wavelength range of 200-1000 nm under an incident angle of 65°. The optical transmittance of nanolaminates has also been studied in the wavelength range of 200-1000 nm by using a dual beam spectrophotometer (Shimadzu UV-3600). A Hall effect measurement system (Ecopia HMS3000) was used to obtain electrical properties of the samples with a four-point probe.

Morphological Characteristics
Nanolaminates with different bilayer thicknesses grown on SiO 2 /Si substrates were measured on cross sections with TEM. Three illustrative examples of nanolaminates with a bilayer thickness of 50, 10, and 2 nm are shown in Fig. 2, including the high-magnification images of 50-and 10-nm samples. Clear boundaries can be observed between Al 2 O 3 and ZnO layers, and the thickness of total nanolaminates is indicated. With the help of X-ray diffraction (Bruker D8 As reported elsewhere [6,21], the Al 2 O 3 layer in Al 2 O 3 / ZnO nanolaminates prepared by ALD method is always in amorphous phase and it can block the ZnO crystal growth because the ZnO is forced to renucleate on the Al 2 O 3 surface. The crystallization process of ultrathin layers is very complex, and many factors should be taken into account, such as the interface energies, the thickness of the layers, the melting point of the system, and the bulk amorphous crystallization temperature [6,22]. Viter et al. discovered that Al 2 O 3 /ZnO nanolaminates with bilayer thickness of 20 nm (ratio 1:1) have amorphous nature and they attributed this result to the minimum thickness required to allow crystallization [22]. López et al. found similar phenomenon and they thought that the pulse as well as the purge duration of the growth procedure was too short to give their films enough time for generating some ordering and some crystalline phases [23]. Meanwhile, the Bohr radius of bulk ZnO is 23 Å [4]. Al 2 O 3 /ZnO 25 (2/2 nm) and Al 2 O 3 / ZnO 50 (1/1 nm) nanolaminates have ZnO thicknesses smaller than the Bohr radius; therefore, the quantum confinement effect should be taken into consideration. Especially for semiconducting sublayers ZnO, it is believed that this effect can cause dramatic change in the dielectric behavior [21], and we will discuss it in the following content.
To investigate the surface morphologies of the nanolaminates, AFM measurement is applied for the samples deposited on SiO 2 /Si substrates, and the 3D results are shown in Fig. 3. It can be observed that the hill-shaped features are dominated on the sample surface and surface height decreases with lower bilayer thickness. Samples with low bilayer thicknesses, i.e., Al 2 O 3 /ZnO 25 (2/2 nm) and Al 2 O 3 /ZnO 50 (1/1 nm), show smooth surface with insignificant surface roughness. The rootmean-square roughness R q of each nanolaminate is estimated from AFM data and approximately ranges from 0.81 to 1.30 nm. Moreover, the relationship between bilayer thickness and R q is revealed in Fig. 4. At first, the values of R q show linear behavior vs. the increase of bilayer thickness, then it remains stable when the bilayer thickness increases to a certain value, as is the case for other studies [23,24]. The Al 2 O 3 in this work is in amorphous phase under the above growth condition, which has been proved in our previous report as well [25]. The amorphous Al 2 O 3 layer is very smooth and conforms to the topography of underlying ZnO  [24]. Therefore, with the decrease of bilayer thickness, more Al 2 O 3 layers were interposed into the nanolaminates to smooth roughness, which leads to the nanolaminates more smooth. When the bilayer thickness increases to a certain value, this smooth effect is no longer obvious.

Optical Properties
By performing SE measurement [17,27,28] which is based on recording and calculating the change of a reflected linearly polarized light from the surface of samples, the optical constants and film thickness of nanolaminates can be deduced from the raw data. In order to get more accurate details, the nanolaminates grown on SiO 2 /Si substrates are chosen as the testing object because of its opaque to light during SE measurements. After raw data acquisition, a multilayer model is constructed containing semi-infinite Si substrate, SiO 2 layer, and AZO layer, as revealed in Fig. 5. The nanolaminates, i.e., the AZO layer in the model, are considered as a whole to be fitted. Oxidation layer of Si substrate is about 330 nm, which is directly substituted into the model without fitting. Moreover, no Bruggeman effective media approximation is introduced in this optical model  [29,30]. The final thickness and optical properties are fitted and evaluated to minimize the root-mean-square error (RMSE) which follows: Here, N, M, exp, and cal represent the number of data points in the spectra, the number of variable parameters in the model, the experimental data, and the calculated data, respectively.
The fitted thicknesses of nanolaminates are shown in Table 2. They are very close to the values obtained from TEM measurements, indicating the accuracy of fitting process. The fitting error RMSE is also revealed in Table 2, and the value is within permitted, demonstrating the reliability of fitting results. The thicknesses of sample 2 (25/25 nm), 5 (10/10 nm), and 10 (5/5 nm) show a smooth trend, and the small fluctuation results from varying degrees of ALD surface chemical reactions. In contrast, the thicknesses of sample 25 (2/2 nm) and 50 (1/1 nm) decline obviously with decreasing bilayer thickness. It follows that the Al 2 O 3 /ZnO nanolaminates show different thickness with low bilayer thickness, even if the cycle number stays the same in deposition process. Table 3 summarizes the growth rates of Al 2 O 3 and ZnO sublayers (thickness ratio 1:1) using the thicknesses listed in Table 2. The values increase at first and saturate finally when the cycles in sublayers increase. The variation in film thickness and growth rate may result from the interfacial reaction between Al 2 O 3 and ZnO layers which will be introduced in the following content, and samples with lower bilayer thickness will be more affected. Karvonen et al. gave similar explanation, and they attributed the variation in growth rate to the TMA etching of ZnO during the Al 2 O 3 growth [7]. Elam et al. found that the growth rates of Al 2 O 3 and ZnO increase with the number of ALD cycles [24]. They concluded that the reduced growth rate of early ALD cycles may result from the nucleation process occurring when making the transition from Al 2 O 3 to ZnO and from ZnO to Al 2 O 3 . Only when new crystals are formed does the growth rate achieve the steady state value.
The optical constants of Al 2 O 3 /ZnO nanolaminates are illustrated in Fig. 6. It shows various refractive index n and extinction coefficient k with different bilayer thicknesses. Figure 6a describes the refractive index dispersion spectra of the nanolaminates with different bilayer thicknesses. The values of n decrease gradually with decline of bilayer thickness in the range of 50 to 2 nm due to the growth change and the Al penetration [21,31]. The n(λ) characteristic of ZnO can be observed for nanolaminates with bilayer thicknesses of 50, 20, and 10 nm. And this line shape slowly degenerates and disappears when the bilayer thickness is below 4 nm. Consequently, the n(λ) characteristics tend to behaving like Al 2 O 3 as the sample 50 (1/1 nm) shown. The k dispersion spectra can be found in Fig. 6b. Different curves represent different samples with various bilayer thicknesses. In the region of 430-1000 nm, the extinction coefficients are approximately equal to 0, i.e., the nanolaminates are almost transparent in that wavelength region. Meanwhile, a blue shift occurs at the absorption edge with decreasing bilayer thickness. The shift distance of sample 25 (2/2 nm) and 50 (1/1 nm) is larger, so the absorption edge gradually moves out of the spectral region and presents the characteristics of Al 2 O 3 . As a whole, the characteristics of optical constants transfer from ZnO to Al 2 O 3 . The observed changes of n and k could be determined by two physical phenomena. On the one hand, they are affected by the quantum confinement effect. We can see that samples 25 (2/2 nm) and 50 (1/1 nm) have sublayer thicknesses smaller than the Bohr radius of bulk ZnO, so their dielectric behaviors change more dramatically than the other samples. On the other hand, it is based on the growth mechanism which leads to Al penetration into ZnO layers [22,24]. According to the growth mechanism, the substitution reaction of Zn with Al may occur in the interface between ZnO and Al 2 O 3 layers: where ZnO-OH and Al (C 2 H 5 ) 3 are the substance on the surface and gas phase, correspondently. Because of this interfacial reaction, Al doping into ZnO layers may  happen and ZnO ratio in nanolaminates can be reduced. Therefore, with the decrease of the bilayer thickness, the interface between ZnO and Al 2 O 3 layers increases, and the ratio of ZnO in the nanolaminates decreases accordingly. This can be verified by the high-magnification TEM images shown in Fig. 2d, e. When the bilayer thickness decreases, the boundaries between Al 2 O 3 and ZnO layers become wider and blurrier. It makes the characteristic of whole nanolaminates transfers to that of Al 2 O 3 . For better understanding of the blue shift of absorption edge, Tauc extrapolation is applied to evaluate the bandgap information of the nanolaminates with a bilayer thickness of 50, 20, and 10 nm. To evaluate the bandgap energies, extinction coefficients of the nanolaminates were used. Extinction, bandgap energy, and absorption coefficients are associated according to the following formulas [32]: where α is the optical absorption coefficient, A is a constant, and E g is the optical bandgap energy. On the basis of Eqs. (7) and (8), a plot of (αhν) 2 vs. hν has been made as demonstrated in Fig. 7. The value of bandgap energy E g can be graphically determined by x axis and the linear fitting in linear part of the absorption edge, which is provided in the inset figure of Fig. 7. The bandgap information of samples 25 (2/2 nm) and 50 (1/1 nm) is not revealed in Fig. 7, because the linear part of the absorption edge exceeds the spectral range evolved from the spectra of extinction coefficient, which could lead to inaccurate results. From Fig. 7, it can be seen that the bandgap energy of nanolaminate displays a growing trend with decreasing bilayer thickness, which could be interpreted by the BM effect [33][34][35]. Interposed Al 3+ takes the place of Zn 2+ in the interface of Al 2 O 3 /ZnO layers and provides an extra electron. So in nanolaminates, the concentration of free carriers increases, causing the bandgap energy moves to higher energy region. The following equation can describe this effect exactly [35]: where ΔE g BM and E g 0 represent the increment of bandgap caused by BM effect and intrinsic forbidden bandwidth, while h, m e * , and n e are the Plank's constant, effective electron mass in the conduction band, and electron carrier density, respectively. Figure 8 gives the spectra of transmittance and absorbance of the whole group. It can be found that sharp absorption edges are located in the region from 200 to 400 nm, namely ultraviolet region. Importantly, absorption edges move to the shorter wavelength (blue shift) with decreasing bilayer thickness, and this trend is exactly close to previous results calculated from SE measurement. This blue shift is due to the BM effect that makes the increase of bandgap. However, the blue shift is not successional, because in the nanolaminates of 25 (2/2 nm) and 50 (1/1 nm), the quantum confinement effect becomes dominant, and the interfacial reaction intensifies which makes the nanolaminates show the characteristics of Al 2 O 3 gradually. At this point, the blue shift is the total contribution of the BM effect, the quantum confinement effect, and the characteristic evolution of nanolaminates. That is to say, these three factors cause the enormous shift of the absorption edge. As a whole, the absorption edge can be modulated by the bilayer thickness in the ultraviolet region (200-400 nm). According to this, it can be applied as ultraviolet detector. Besides, all of the Al 2 O 3 /ZnO nanolaminates show a transmittance above 90% in visible and nearinfrared region, along with a sharp absorption band edge. The transmittance here shows nearly similar value and trend with that of many other TCO materials [36], which makes it possible to be applied as TCO material.

Electrical Properties
The Hall effect measurement is conducted to correlate the analyses with the electrical properties of the Al 2 O 3 / ZnO nanolaminates. Nanolaminates prepared on quartz substrates are selected as the test samples to remove the spatial resistivity distribution, and Fig. 9 displays the testing results. At the beginning, the carrier concentration and resistivity show little change and remain at around 10 19 cm −3 and 10 −2 Ω cm, respectively. With decreasing bilayer thickness, the carrier concentration sharply drops and the resistivity increases as well. It can be interpreted by the interfacial reaction of Al 2 O 3 /ZnO layers which results in the characteristic evolution of nanolaminates. The nanolaminates show insulation characteristic of Al 2 O 3 gradually and realize the tunableness of resistivity by changing their bilayer thickness. In addition, the values of the carrier concentration of nanolaminates 2 (25/25 nm), 5 (10/10 nm), and 10 (5/5 nm) are 4.99 × 10 19 , 5.26 × 10 19 , and 8.91 × 10 19 cm −3 , respectively. It shows a slow growth in accordance with the explanation of the bandgap results, and the values approximately equal to those of TCO materials from the results of other reports [25,37]. So these three kinds of nanolaminates possess not only favorable electrical conductivity but also excellent light transmittance in the visible and near-infrared region. It is vital for Al 2 O 3 /ZnO nanolaminates to play a role in the field of transparent conductor. The samples 25 (2/2 nm) and 50 (1/1 nm) present insulation characteristic and realize the tunableness of resistivity, which can be applied as highresistivity layer in semiconductor devices.

Conclusions
We have investigated the morphological, optical, and electrical properties of Al 2 O 3 /ZnO nanolaminates among various bilayer thicknesses ranging from 2 to 50 nm. The clear layer boundaries and low surface roughness show highquality morphologies of nanolaminates prepared by ALD method. With the decrease of the bilayer thickness, inserted Al 2 O 3 layers in nanolaminates begin to limit the roughness, which leads to the nanolaminates more smooth. When the bilayer thickness reaches a certain value, this roughness  limitation can be ignored. The thickness, optical constants, and bandgap information of nanolaminates have been extracted from SE analysis. With decreasing bilayer thickness, the absorption edge of extinction coefficient has a blue shift, and the optical band gap energies show a growing trend, because the BM effect, the quantum confinement effect, and the characteristic evolution of nanolaminates have significant influence on them. This blue shift also occurs in the transmission and absorbance spectra with high transmittance beyond 90% in the visible and near-infrared region. Moreover, by varying the bilayer thickness, the electrical properties also show two kinds of characteristics, and the modulation of characteristics is realized. The nanolaminates 2 (25/25 nm), 5 (10/10 nm), and 10 (5/5 nm) show high carrier concentration above 10 19 cm −3 , which can be applied as transparent conductive material. And also, the nanolaminates 25 (2/2 nm) and 50 (1/1 nm) possessing high resistivity can be used as high-resistivity layer in semiconductor manufacturing process.