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/Al2O3 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 TCO-based devices [15, 16]. However, few reports involve in the performance modulation of AZO by changing the structures of Al2O3/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 self-limiting 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, Al2O3 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 Al2O3/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 Al2O3/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.

Methods

Synthesis of Nanolaminates by ALD

Al2O3/ZnO nanolaminates based on Al2O3-ZnO bilayer stacks were deposited on SiO2/Si and quartz substrates by ALD technique. During deposition procedure, the reactor (PICOSUN) temperature was 150 °C. The precursors for Zn, Al, and O were diethylzinc [DEZ; Zn(C2H5)2], trimethylaluminum [TMA; Al (CH3)3], and deionized water (H2O), respectively. The precursor carrier and purge gas was the high purity nitrogen (N2, flow rate 50 sccm). It was used to carry precursors into the chamber and bring the needless products out of the chamber.

In order to grow the Al2O3 layers, the TMA and H2O were alternatively brought into the reactor chamber through TMA-H2O cycles (TMA/exposure/N2/H2O/exposure/N2) with pulse time of 0.03/3/15/0.03/5/15 s. The surface reactions of ALD Al2O3 layers can be defined by two self-limiting reactions as follows [20]:

$$ {\mathrm{AlOH}}^{\ast }+\kern0.5em \mathrm{Al}{\left({\mathrm{CH}}_3\right)}_3\to \mathrm{AlOAl}{{\left({\mathrm{CH}}_3\right)}_2}^{\ast }+\kern0.5em {\mathrm{CH}}_4 $$
(1)
$$ A\mathrm{lOAl}{{\left({\mathrm{CH}}_3\right)}_2}^{\ast }+\kern0.5em {\mathrm{H}}_2\mathrm{O}\to A{\mathrm{lOAlOH}}^{\kern0.5em \ast }+\kern0.5em {\mathrm{CH}}_4 $$
(2)

where the asterisks indicate the surface species. As for ZnO layers, DEZ-H2O cycles of ZnO were the same as the TMA-H2O. The surface reactions of ALD ZnO layers are given by [20]

$$ {\mathrm{ZnOH}}^{\kern0.5em \ast }+\kern0.5em \mathrm{Zn}{\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_2\to {\mathrm{ZnOZnC}}_2{{\mathrm{H}}_5}^{\ast }+\kern0.5em {\mathrm{C}}_2{\mathrm{H}}_6 $$
(3)
$$ {\mathrm{ZnOZnC}}_2{{\mathrm{H}}_5}^{\ast }+\kern0.5em {\mathrm{H}}_2\mathrm{O}\to {\mathrm{ZnOZnOH}}^{\ast }+\kern0.5em {\mathrm{C}}_2{\mathrm{H}}_6 $$
(4)

where the asterisks also indicate the surface species. The structure diagram of ALD Al2O3/ZnO nanolaminates is shown in Fig. 1. For all nanolaminates, the interface with the substrate was Al2O3, while ZnO was the top layer at the surface of nanolaminates. The bilayer is constructed by two individual layers, i.e., Al2O3 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.

Fig. 1
figure 1

The structure diagram of the Al2O3/ZnO nanolaminates

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Table 1 The parameters of samples with different bilayer thicknesses
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Characterization

The morphological characterization of Al2O3/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.

Results and Discussion

Morphological Characteristics

Nanolaminates with different bilayer thicknesses grown on SiO2/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 Al2O3 and ZnO layers, and the thickness of total nanolaminates is indicated. With the help of X-ray diffraction (Bruker D8 ADVANCE) measurements in advance (not given here), we find no characteristic peaks of Al2O3 and ZnO, and hence that all of the as-grown nanolaminates have amorphous state. This statement can be verified by the high-magnification TEM images. Even though Al2O3/ZnO 2 (25/25 nm) samples have the thickest bilayer in this work, the crystallization process does not exist in them.

Fig. 2
figure 2

TEM images of Al2O3/ZnO nanolaminates with different bilayer thicknesses: a 50 nm, b 10 nm, and c 2 nm. And high-magnification images: d 50 nm and e 10 nm

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As reported elsewhere [6, 21], the Al2O3 layer in Al2O3/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 Al2O3 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 Al2O3/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]. Al2O3/ZnO 25 (2/2 nm) and Al2O3/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 SiO2/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., Al2O3/ZnO 25 (2/2 nm) and Al2O3/ZnO 50 (1/1 nm), show smooth surface with insignificant surface roughness. The root-mean-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 Al2O3 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 Al2O3 layer is very smooth and conforms to the topography of underlying ZnO layers [26]. As mentioned above, due to the interposed Al2O3 layer, the crystal growth of ZnO is consequently interrupted. Through restricting the size of the ZnO nanocrystals, the interposed Al2O3 layers prevent the Al2O3/ZnO nanolaminates from roughening [24]. This smooth effect has been proven to have little to do with the Al2O3 layer thickness and only relates to the number of interposed Al2O3 layers [24]. Therefore, with the decrease of bilayer thickness, more Al2O3 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.

Fig. 3
figure 3

AFM 3D images of nanolaminates with different bilayer thicknesses: a 2 (25/25 nm), b 5 (10/10 nm), c 10 (5/5 nm), d 25 (2/2 nm), and e 50 (1/1 nm)

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Fig. 4
figure 4

Surface roughness of nanolaminates with different bilayer thicknesses

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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 SiO2/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, SiO2 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 because of the ignorable surface roughnesses of samples based on the AFM results. On account of this optical model, the Forouhi-Bloomer (FB) dispersion model is used to fit the ellipsometry spectra (Ψ and Δ in the range of 200–1000 nm) of the nanolaminates [29, 30]. The final thickness and optical properties are fitted and evaluated to minimize the root-mean-square error (RMSE) which follows:

$$ \mathrm{RMSE}=\sqrt{\frac{1}{2N-M-1}{\sum}_{i=1}^N\left[{\left({\psi}_i^{\mathrm{cal}}-{\psi}_i^{\mathrm{exp}}\right)}^2+{\left({\varDelta}_i^{\mathrm{cal}}-{\varDelta}_i^{\mathrm{exp}}\right)}^2\right]} $$
(5)
Fig. 5
figure 5

Optical model of samples grown on SiO2/Si substrate for SE analysis

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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 Al2O3/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 Al2O3 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 Al2O3 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 Al2O3 growth [7]. Elam et al. found that the growth rates of Al2O3 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 Al2O3 to ZnO and from ZnO to Al2O3. Only when new crystals are formed does the growth rate achieve the steady state value.

Table 2 The thicknesses of AZO layer obtained from TEM and fitted by SE, and the fitting error RMSE
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Table 3 The growth rates of samples with different bilayer thicknesses
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The optical constants of Al2O3/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 Al2O3 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 Al2O3. As a whole, the characteristics of optical constants transfer from ZnO to Al2O3. 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 Al2O3 layers:

$$ \mathrm{Zn}\hbox{-} {\mathrm{OH}}^S+\kern0.5em \mathrm{Al}{{\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_3}^g\uparrow \to \mathrm{AlOH}\hbox{-} {\mathrm{C}}_2{{\mathrm{H}}_5}^s+\kern0.5em \mathrm{Zn}{{\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_2}^g\uparrow $$
(6)

where ZnO-OH and Al (C2H5)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 Al2O3 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 Al2O3 and ZnO layers become wider and blurrier. It makes the characteristic of whole nanolaminates transfers to that of Al2O3.

Fig. 6
figure 6

The optical constants of nanolaminates grown on SiO2/Si substrate. a The refractive index n. b The extinction coefficient k

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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]:

$$ {\left(\alpha h\upsilon \right)}^2=A\left(E-{E}_g\right) $$
(7)
$$ \alpha =\frac{4 k\pi}{\lambda } $$
(8)

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 Al3+ takes the place of Zn2+ in the interface of Al2O3/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]:

$$ {E}_g={E}_g^0+\varDelta {E}_g^{\mathrm{BM}}={E}_g^0+\frac{h^2}{8{m}_e^{\ast }}{\left(\frac{3}{\pi}\right)}^{2/3}{n}_e^{2/3} $$
(9)

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.

Fig. 7
figure 7

Evaluated optical bandgap of nanolaminates with different bilayer thicknesses

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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 Al2O3 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 Al2O3/ZnO nanolaminates show a transmittance above 90% in visible and near-infrared 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.

Fig. 8
figure 8

a Transmittance and b absorbance spectra of nanolaminates grown on quartz substrates with different bilayer thicknesses

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Electrical Properties

The Hall effect measurement is conducted to correlate the analyses with the electrical properties of the Al2O3/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 1019 cm−3 and 10−2Ω 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 Al2O3/ZnO layers which results in the characteristic evolution of nanolaminates. The nanolaminates show insulation characteristic of Al2O3 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 × 1019, 5.26 × 1019, and 8.91 × 1019 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 Al2O3/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 high-resistivity layer in semiconductor devices.

Fig. 9
figure 9

Carrier concentration and resistivity of nanolaminates grown on quartz substrate with different bilayer thicknesses

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Conclusions

We have investigated the morphological, optical, and electrical properties of Al2O3/ZnO nanolaminates among various bilayer thicknesses ranging from 2 to 50 nm. The clear layer boundaries and low surface roughness show high-quality morphologies of nanolaminates prepared by ALD method. With the decrease of the bilayer thickness, inserted Al2O3 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 1019 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.