Investigation of growth characteristics, compositions, and properties of atomic layer deposited amorphous Zn-doped Ga2O3 films

Applied Surface Science 476 (2019) 733–740

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Investigation of growth characteristics, compositions, and properties of atomic layer deposited amorphous Zn-doped Ga2O3 films

T

⁎

Jiajia Taoa, Hong-Liang Lua, , Yang Gua, Hong-Ping Maa, Xing Lia, Jin-Xin Chena, Wen-Jun Liua, Hao Zhangb, Ji-Jun Fengc a

State Key Laboratory of ASIC and System, Shanghai Institute of Intelligent Electronics & Systems, School of Microelectronics, Fudan University, Shanghai 200433, China Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China c Shanghai Key Laboratory of Modern Optical System, School of Optical-Electrical and Computer Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Ga2O3 ZnO ZGO Atomic layer deposition

Diethylzinc and H2O were used as the precursors for the thermal atomic layer deposition (TH-ALD) of ZnO deposition while the trimethylgallium and O2 plasma were used as a reactant for the plasma-enhanced atomic layer deposition (PE-ALD) of Ga2O3, respectively. The Zn-doped Ga2O3 (ZGO) films were fabricated by a combination of PE-ALD of Ga2O3 and TH-ALD of ZnO at a low temperature of 200 °C. The results show that asdeposited ZGO films were amorphous while ZnO with a crystalline structure. XPS results indicate that the Zn content in ZGO films increased from 9.70 to 24.65 at.% with the cycle ration of Ga2O3 with respect to ZnO decreasing from 7:1 to 3:1 while the oxygen vacancy increased from 27.65% to 37.93%. The rise in Zn doping contents is also accompanied by significant variations in the morphological, electrical, and optical properties of the ZGO films, including a decrease of film density and resistivity, an increase of RMS roughness, a strong transmittance in the ultraviolet-visible (UV–vis) area, and a widening of the band gap from 4.64 to 5.25 eV. These findings help deposit ZGO films with desired structure and properties for electronic device applications.

1. Introduction Semiconducting oxides have become important materials for novel devices with new functionality due to their wide bandgap and chemically and thermally stable characteristics. Recently, different phases of Ga2O3 have attracted much attention in fields such as catalysis [1], gas sensing [2], photovoltaics [3], photodetectors [4], and high power and high voltage electronic devices [5] due to their high thermal and chemical stability, temperature dependent conducting behavior, wide bandgap energy, and high breakdown voltage. However, these applications of amorphous Ga2O3 should be followed by the doping of Ti, Al, and Zn elements, which allows for great flexibility in designing and optimizing the devices [6–8]. As well-known fact, the doping of semiconductors with selective elements offers an effective approach towards promoting their electrical and optical properties. We can get the tunable band gap of Ga2O3 through doping, which allows design devices, such as high sensitive wavelength-tunable photodetectors, to have a broader range of cutoff wavelength tunable optical filters. Additionally, the carrier density and conductivity of the Ga2O3 film can be tuned by depositions and doping. This is important for Ga2O3 based thin film ⁎

transistors and other Ga2O3 based microelectronic devices. Among the doping elements, Zn and Ga have a similar atomic radius, and the covalent bond lengths of GeO and ZneO are estimated to be 0.19 and 0.20 nm, respectively. The bond length of ZneO, which is slightly longer than that of GaeO, minimizes the deformation of the Ga2O3 lattice, even at high Zn concentrations [9]. To the best of our knowledge, Ga-doped ZnO (GZO) has been widely studied while Zn-doped Ga2O3 (ZGO) is rarely reported. It is believed that the doping effects of Zn will provide the high quality, transparent, tunable band gap, and conductive Ga2O3 films. In fact, the ZGO or GZO thin films have been prepared by various techniques, such as spray pyrolysis [10], thermal evaporation [11], sputtering [12], pulsed laser deposition [13], chemical vapor deposition (CVD) [14], and atomic layer deposition (ALD) [15]. Among these growth techniques, ALD provides a number of advantages for preparing this kind of films. A monolayer-by-monolayer film growth fashion can be achieved in the ALD method through iterative and self-saturating precursor exposures [16]. As a result, it enables precise film thickness control, high uniformity in both thickness and composition over large areas, and excellent step coverage in nano-3D structures. In addition,

Corresponding author. E-mail address: [email protected] (H.-L. Lu).

https://doi.org/10.1016/j.apsusc.2019.01.177 Received 19 September 2018; Received in revised form 18 January 2019; Accepted 21 January 2019 Available online 22 January 2019 0169-4332/ © 2019 Published by Elsevier B.V.

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Table 1 Summary of the details of the parameters for one growth supercycle during the reaction process, estimated and measured thickness of ZGO films. Sample

Number of Ga2O3

Number of ZnO

Number of supercycle

Estimated thickness (nm)

Measured thickness (SE, nm)

Measured thickness (XRR, nm)

Ga2O3 Ga/Zn = 7:1 Ga/Zn = 5:1 Ga/Zn = 3:1 ZnO

300 210 185 150 0

0 30 37 50 100

300Ga2O3 30(7Ga2O3 + 1ZnO) 37(5Ga2O3 + 1ZnO) 50(3Ga2O3 + 1ZnO) 100ZnO

21 20.70 20.35 20.50 20.45

20.18 22.61 33.42 40.23 21.13

17.80 25.27 31.70 41.74 17.95

modulated by a mass flow controller (MFC). For the remote plasma process, the O2 flow was set at 50 sccm, a pressure of 15 mTorr, and a RF power of 200 W. The PE-ALD Ga2O3 growth sequence is composed of 20 ms of TMGa exposure, 2 s of Ar purging, 5 s of O2 plasma exposure, and 2 s of Ar purging. Moreover, the typical TH-ALD sequence of ZnO consists of several sequential steps including 0.2 s of DEZn exposure, 2 s of Ar purging, 0.2 s of reactant exposure, and 2 s of Ar purging. For one supercycle of Zn-doped Ga2O3 (ZGO) deposition, 1 cycle of ZnO was performed after m cycles of Ga2O3, as shown in Fig. S1 and Table S1. Finally, the ZGO thin films were constituted as a sandwich structure by stacking Ga2O3 and ZnO. The details of the parameters for one growth supercycle during the reaction process are provided in Table 1. The total thickness of Zn-doped Ga2O3 (ZGO) films was projected to be 20 nm. The relevant reactions of the ALD process can be written as follows [20,21]:

the growth temperature of ALD is typically lower than sputtering and CVD, which is useful to prepare thin layers on polymer substrates for flexible devices. To date, the thermal atomic layer deposition (TH-ALD) and the plasma-enhanced atomic layer deposition (PE-ALD) of GZO have been reported by a couple of research groups [17,18]. However, the growth characteristics or film properties of ZGO that deposited by a combination of PE-ALD of Ga2O3 and TH-ALD of ZnO at low temperatures have been rare reported. Additionally, a comparison between ZGO deposited by a combination of PE-ALD of Ga2O3 and TH-ALD of pure ZnO or PEALD of pure Ga2O3 has not been reported yet. Furthermore, low temperature growth is essential for application in future flexible electronics and display devices. The temperature plays an important role in the reaction process of different precursors in ALD deposition. For example, a diethyl zinc precursor is limited by a temperature of less than 50 °C during the PE-ALD deposition of ZnO [19] while the TH-ALD of Ga2O3 film cannot be deposited at temperatures less than 250 °C when using trimethylgallium (TMGa) and H2O precursors [15]. Instead, it is easy to deposit ZGO films using a combination of TH-ALD of ZnO and PE-ALD of Ga2O3 at a low temperature of 200 °C. By introducing combinations of different types of ALD technology, we can overcome the limitations of reaction conditions for the deposition of composite film materials, which can extended to other novel composite films for electronic devices. In the present study, diethylzinc (DEZn) and H2O were used as the precursors for TH-ALD of ZnO deposition while TMGa and O2 plasma were used as a reactant for PE-ALD of Ga2O3. Zn-doped Ga2O3 (ZGO) films were successfully fabricated by a combination of PE-ALD of Ga2O3 and TH-ALD of ZnO at a low temperature of 200 °C. Furthermore, we investigated the ZGO films as a function of the Zn doping concentrations via electrical, structural, morphological, and optical characterization.

Ga − OH∗ + Ga(CH3 )3 → Ga − O − Ga − (CH3 )∗2 + CH 4

(1)

Ga − (CH3 )∗2 + 15/4O2 → Ga − OH∗ + 2CO2 + 5/2H2 O

(2)

Ga − OH∗ + Zn(C2 H5 )2 → Ga − O − Zn − C2 H∗5 + C2 H6

(3)

Zn − C2 H∗5 + H2 O → Zn − OH∗ + C2 H6

(4)

where “*” refers to surface species. 2.3. Thin films characterization The thickness of the deposited Zn-doped Ga2O3 films was monitored using spectroscopic ellipsometry (SE, Sopra, GES5E) and fitted with a Cauchy-model. In order to determine the results of film thickness by SE, the thickness of the films was also measured using X-ray reflectivity (XRR). XRR was carried out using a Bruker Advance Discover system with Cu Kα radiation. Moreover, the density was characterized by XRR data measured on a PANalytical Empyrean diffractometer with Cu Kα radiation. To reveal the morphologies of the samples, the samples were examined using a Philips XL 30 FEG field-emission scanning electron microscope (SEM) and a Veeco Dimension 3100 atomic force microscopy (AFM) under a tapping mode. X-ray diffraction (XRD, D8 Advance, Bruker, Germany) with CuKα radiation (40 kV, 40 mA, λ = 1.54056 Å) from 20° to 80° at a 2θ scale was employed to determine the microstructures and the crystallinity of the samples. The carrier concentration, the carrier mobility, and the conductivity of the samples were measured via the Hall-effect measurement (Ecopia, HMS3000) at room temperature under a magnetic field of 0.58 T. Ultraviolet-visible spectroscopy (UV–vis) absorption spectra was measured on a UV–vis spectrophotometer (Shimadzu, UV-2550) over a range of 200–800 nm. Tauc plots based on the UV–Vis data were used to determine the optical band gap. The chemical compositions and the bonding states of deposited films were investigated through X-ray photoelectron spectroscopy (XPS). The XPS date was obtained using Thermo Fisher ESCALAB 250X with monochromatic Al Kα radiation. Considering the sample-charging effect, the binding energy values (BE) were corrected by referencing the C 1s peak at 284.6 eV.

2. Experimental 2.1. Substrates and precursors All the depositions were performed on p-type Si (100) wafers and quartzs in a type ALD (BENEQ TFS200) reactor. Before the reaction, the wafers were cleaned with the standard RCA process while the quartzs were rinsed in an ultrasonicator sequentially with a washing powder solution, acetone, ethylalcohol, and deionized (DI) water for 15 min. All substrates were dried using an N2 gun and then transferred into the reaction chamber immediately. Ga2O3 was conducted using TMGa and O2 since the Ga and O sources from PE-ALD. Accordingly, ZnO was grown from the precursor of DEZn and DI water through TH-ALD. 2.2. Thin films deposition The TH-ALD and PE-ALD were employed for the growth of ZnO and Ga2O3 films, respectively. The susceptor temperature Ts was fixed at 200 °C for the deposition of all films unless stated otherwise. The precursors of TMGa were maintained at 13 °C whereas the DEZn and water were kept at room temperature (20 °C). Ar was employed as both the carrier and purge gas during the experiment. The flow rate of Ar was 734

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Fig. 1. (a) XRR patterns of ZGO films with different ZnO concentrations, (b) representative SE data of an as deposited Ga(3)Zn(1) thin film. The dashed red lines are the fittings produced by the Cauchy Model. (c) The variation of the refractive index (n) with the wavelength of representative Ga(3)Zn(1) thin film. (d) Thickness of samples measured by XRR and SE. (e) Density of samples measured by XRR. (f) Roughness of samples measured by XRR and AFM. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

and ZnO films, which indicates that more defects exist in the GZO films [25]. The roughness of samples is also given in Fig. 1(f) from XRR and AFM results. When compared to ZnO and Ga2O3 films, the ZGO films exhibited relatively higher surface roughness. The increase in RMS roughness value may be attributed to the coalescence or migration of the grains at the interface between amorphous Ga2O3 and crystalline ZnO.

3. Results and discussion 3.1. Growth characteristics To gather great insight into the growth of GZO film, XRR and SE measurements were carried out to analyze the parameters and the growth rate of as-deposited films. The results are shown in Fig. 1 and listed in Table 1. Fig. 1(a) shows the reflectance spectra of samples from XRR results. A representative image of SE results of Ga(3)Zn(1) film is shown in Fig. 1(b). Accordingly, Fig. 1(c) shows a typical variation of the refractive index (n) with the wavelength of Ga(3)Zn(1). The other samples were subjected to the same tests and simulations. Fig. 1(d) shows the measured thickness of ZGO films as a functions of a decreasing cycle ration of Ga2O3 with respect to ZnO. The thickness measured from SE and XRR are consistent with each other with experimental error. The thickness of 300 cycles PE-ALD of Ga2O3 and 100 cycles of TH-ALD ZnO were 21 and 20 nm, respectively. The growth rate of PE-ALD Ga2O3 and TH-ALD ZnO were 0.7 and 2.0 Å/cycle, which coincides with previous reports [22]. The thickness of ZGO films increased with a decreasing the cycle ratio of Ga2O3 with respect to ZnO. In other words, the growth rate of GZO films drastically increased when the cycle ratio of Ga2O3 decreased with respect to ZnO. For example, Ga(3)Zn(1) has a growth rate of 1.64 nm/supercycle, which is much higher than that superposition rate of pure Ga2O3 and ZnO. The growth tendency is different from that of the sole PE and TH-ALD of GZO films in previous studies [23]. In this work, one supercycle contains m cycles of PE-ALD of Ga2O3 and 1 cycle of TH-ALD of ZnO. To date, there have been no reports of such growth patterns. The increases in the growth rate of ZGO films deposited by a combination of PE and TH-ALD may be attributed to interaction and CVD reactions during the ALD process. As shown in Fig. 1(e), the determined densities measured by XRR for Ga2O3, Ga(7)Zn(1), Ga(5)Zn(1), Ga(3)Zn(1), and ZnO films grown on Si (100) were 5.3, 5.1, 3.9, 3.4, 2.8, and 5.6 g cm−3, respectively. The density of these films is lower than that of reported bulk films because the amorphous structure is less dense than crystal material [24]. The ZGO films show low density compared to pure Ga2O3

3.2. Crystallinity, structure and morphology XRD patterns of the ZGO thin films deposited on the Si (100) as a function of the cycles of Ga2O3 are shown in Fig. 2(a). As a reference for the ZGO films, the patterns of pristine ZnO and Ga2O3 thin films are also given in Fig. 2(a). The diffraction peak appears at 2θ = 34.45°, corresponding to the (002) plane of ZnO (PDF#36–1541) [26], indicating a hexagonal wurtzite-type polycrystalline structure. On the contrary, there are no any noticeable peaks in the Ga2O3 and Ga(3)Zn(1) samples, which indicates that the deposited Ga2O3 and ZGO thin films are amorphous. It would appear that Zn doping has no effect on the crystallinity of ZGO films. This is due to the amorphous phase of as-deposited Ga2O3 films, which induces poor crystallinity. It is noted that the crystallinity changes of ZGO is different from other ALD doping films. For example, ALD-Al doped ZnO shows better crystallinity at 4% Al doping concentrations than other Al concentrations [27–29]. Fig. 2(b) shows the representative SEM image of the top view of Ga(3) Zn(1) thin film, while Fig. 2(d) shows a cross-sectional SEM image. The images show that the surface of the film is very flat. The average thickness of Ga(3)Zn(1) thin film is about 41.41 nm, which is consistent with the results of XRR and SE tests. The roughness of Ga(3)Zn(1) thin film was also estimated by AFM measurements, as shown in Fig. 2(c). The AFM results show the same variation tendency of roughness as the XRR analysis. The RMS roughness values (Fig. S2) of Ga2O3, Ga(7)Zn (1), Ga(5)Zn(1), Ga(3)Zn(1), and ZnO films were 0.85, 0.94, 0.98, 0.93, and 0.77 nm, respectively.

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Fig. 2. (a) XRD patterns of the grown Ga2O3, ZnO, and Ga(3)Zn(1) films grown on Si (100) substrate, (b) representative SEM images of the top view and (d) cross view of as deposited Ga(3)Zn(1) thin film, (c) AFM surface morphologies of the deposited Ga(3)Zn(1) sample.

it can be concluded that the deposited films are Ga2O3. In addition, the Si 2p signal from the silicon substrate cannot be seen from the XPS spectra, which indicates that the thickness of Ga2O3 exceeded the detection depth of XPS. Additionally, the XPS spectra for the ZnO film deposited by TH-ALD are shown in Fig. 4. XPS scans of Zn 2p, Zn 3s, Zn 3p, Zn 3d, and O 1s are shown in Fig. 4(b) after considering the binding energy values and taking the C 1s peak (284.6 eV) as reference. This suggests that there were no other impurities in the TH-ALD of ZnO. Four peaks of the O1s core level spectra were deconvoluted three peaks, as shown in Fig. 4(c). The lower energy peak located at 529.8 eV should a result of ZneO bonding [35] while the peak at 531.4 eV is associated with oxygen vacancy [36]. The small higher energy peak at 532.6 eV implies the presence of hydrated oxide species (OeH) on the film surface [37,38]. The Zn 2p spectrum in Fig. 4(d) shows a doublet whose binding energies are 1021.1 and 1044.2 eV and can be identified as Zn 2p3/2 and Zn 2p1/2 lines, respectively. The energy difference between these two peaks is 23.1 eV, which agrees well with the stand values of 23.0 eV [39–41]. The spectra of survey, Zn 2p, Ga 2p, and O 1s of different samples were studied to understand the effects of doping concentrations on the changing of chemical states in ZGO films. The Zn concentrations in ZGO films were determined as 24.65 at.%, 16.36 at.%, and 9.7 at.% for Ga(7) Zn(1), Ga(5)Zn(1), and Ga(3)Zn(1), respectively. As shown in Fig. 5(a), the peaks of three main elements, Ga, Zn, and O were obtained clearly in the Ga(3)Zn(1) film. No other impurity peaks were observed, indicating that no other impurities were involved in the preparation of the samples and that the samples were extremely pure. The XPS spectra for Ga 2p and Zn 2p are shown in Fig. 5(b) and (c) for different ZGO

3.3. Thin film composition Further surface composition of ALD Ga2O3, ZnO, and ZGO samples deposited on Si substrate at 200 °C were determined by using XPS. The core level spectra are shown in Figs. 3, 4, and 5 for Ga2O3, ZnO, and ZGO, respectively. Fig.3 shows the XPS spectra of PE-ALD Ga2O3 films. In survey spectra (Fig. 3b), the signals come from gallium (Ga 3d, Ga 3p, Ga 3 s, and Ga 2p), oxygen (O 1s), and carbon (C 1s, as introduced); as well as the respective auger peaks from gallium (Ga LMM) and oxygen (O KVL) were observed clearly. Some small peaks with intensities lower than each main photoelectron peak were also shown Fig. 3b. Contamination of carbon in pristine Ga2O3 was found to be relatively low. As indicated in Fig. 3(a), the high resolution C 1s spectra were dominated by three species, as previous reported [30]. The C 1s peak at 284.3 eV corresponds to binding energy of the aliphatic and adventitious carbon whereas the peak at 284.8 eV is associated with hydrocarbon (CeH). The carbon associated with double bonds and oxygen, such as carboxylic acid signals, is at 288.7 eV. The O 1s spectra (Fig. 3(c)) recorded for Ga2O3 included three peaks. The main peak at 530.5 eV match well to the lattice oxide in Ga2O3. The oxygen vacancy associated peak positioned at 531.1 eV is observed in this work. Another peak at 532.3 eV can be contributed to the HeO bond, which is typically produced in each half PE-ALD cycle [31]. A similar bond was also reported in the PE-ALD of other metal oxides such as Al2O3 [32]. As shown in Fig. 3d, two peaks at the binding energy of 1144.8 and 1117.7 eV were assigned to the 2p1/2 and 2p3/2 core level of Ga3+ [33]. The energy spacing between two lines was 27.1 eV, which is consistent with previous reports for Ga 2p [34]. The Ga/O ratio (0.63) coincides with the stoichiometry expected for Ga2O3. Through the analysis above,

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Fig. 3. XPS spectra of a Ga2O3 film: (a) C 1s, (b) Survey, (c) O 1 s, and (d) Ga 2p spectra.

Fig. 4. XPS spectra of a ZnO film: (a) C 1 s, (b) Survey, (c) O 1s, and (d) Zn 2p spectra. 737

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Fig. 5. XPS spectra of ZGO films: (a) survey spectra of Ga(3)Zn(1), (b) Ga 2p spectra, (c) Zn 2p spectra, and O 1s spectra of Ga(7)Zn(1) (d), Ga(5)Zn(1) (e), and Ga(3) Zn(1) (f).

the ZGO films also displayed good transparency in the UV spectra regions (200–380 nm) with the average transmittance values at about 80%, which is an important property for applications of this material when attempting to achieve solar-blind photodetection [45]. It can be seen that the transmittance of films increases when the cycle ratio of Ga2O3 and ZnO decreases. The Zn doping clearly improved the transmittance of Ga2O3. The increase in average transmission is attributed to the increase in carrier density and the larger bandgap of amorphous ZGO films [46]. It is well known that O2 plasma treatment can improve film transmittances [47]. The O2 plasma plays an important role in the preparation and surface treatment of films during the deposition of the PEALD-Ga2O3 process. Except for this case, Zn doping and O2 plasma in the PE-ALD process of Ga2O3 greatly affect the transmittance of ZGO films. As a direct bandgap material, the absorption coefficient (α) and the optical band gap (Eg) of all deposited films are evaluated using the following equation [48]:

films. The Ga 2p and Zn 2p of Ga2O3 and ZnO film are also given for comparison. As seen from the spectra, the Ga 2p peak decreased while the Zn 2p peak increased with increasing cycles of ZnO/Ga2O3 during the supercycle, which suggests that the ZnO doped successfully into Ga2O3, as expected. Furthermore, the peak position of Ga 2p3/2 shifted towards lower binding energy while the peak position of Zn 2p3/2 moved towards higher binding energy when the cycles of ZnO/Ga2O3 increased during the supercycle. This is due to the formation of ZneO bonds and the enlargement of the hole near the surface of Ga2O3 [42]. The formation of ZneO bonds will help reduce the resistance at the interface because ALD of ZnO has a low resistivity. As shown in Fig. 5(d) to (f), O 1s XPS spectra of ZGO films were resolved into three components centered at 530.5, 531.6, and 532.3 eV for lattice oxygen (GaeO and ZneO bonds), oxygen vacancy, and OeH bonds, respectively. It shows that the relative area percentage of oxygen vacancy increased from 27.65% to 37.93% when more Zn was doped into ZGO films. This means that the concentrations of oxygen vacancy increased, which could lead to an increase in carrier concentrations. It is well known that the more oxygen vacancies in the film, the lower resistivity [43]. These findings indicates that the Ga(3)Zn(1) has the lowest resistivity and the highest carrier concentrations among all ZGO films, which is beneficial for the application of transparent conducting oxide substrates and related micronanometer electronic devices.

α = −ln(T)/t

(5)

where t is the thickness of film and T is the film transmittance.

(αh ν)2 = A(h ν − Eg)

(6)

where α is the absorption coefficient, h is the Planck's constant, ν is the frequency of the incident photon, and A is a constant depending on the electron or hole mobility. As indicated in Fig. 6(b), the band gap of all films can be obtained by using the Tauc plot, the plot of (αhν)2 versus the (hν), and extrapolating the linear part of this plot to the horizontal axis. The values of 3.20 and 4.64 eV were in accordance with previous reported values for the band gap of the ZnO and Ga2O3 thin films [49,50]. The calculated bandgap of ALD grown Ga(7)Zn(1), Ga(5)Zn (1), and Ga(3)Zn(1) are 4.90, 5.17, and 5.25 eV, respectively. The values becoming larger with decreasing cycles of Ga2O3 in a supercycle. Compared with the bandgap of Ga2O3 and ZnO films, the high-energy shift of the optical band gap of ZGO films is typically believed to be due to be a result of the Burnstein-Moss effect widening the bandgap [51]. The high carrier concentration induced by Zn doping would fill the electronic states of the conduction band in the ZGO films. Similar behavior of the optical bandgap was observed in previous reports [52].

3.4. Optical and electrical properties To assess the transparency and determine the optical band gap of deposited films, optical transmission measurements were carried out in the wavelength region ranging from 200 to 800 nm. Fig. 6(a) shows the transmittance of ZnO, Ga2O3, and ZGO thin films, as deposited by the ALD process at 200 °C on quartz, relative to the wavelength of light as a function of changing the cycles of Ga2O3 and ZnO. As shown in Fig. 6(a), the average transmittance value of all ZGO films is around 85–92% over the visible wavelength range (from 380 to 780 nm), which shows excellent transparency properties compared to ZnO and Ga2O3 films. In particular, the Ga(3)Zn(1) exhibits transparency comparable to bare quartz, which implies good use of transparent electrodes, anti-reflective coating materials, and light-emitting diodes [44]. Importantly, 738

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Fig. 6. (a) Optical transmittance of ZnO, Ga2O3, and ZGO thin films, as deposited by the ALD process at 200 °C on quartz, versus the wavelength of light as a function of the changing cycles of Ga2O3 and ZnO. The figure also shows the transmittance spectrum of the bare quartz substrate. (b) (αhν)2 versus (hν) plot for different films deposited on a quartz substrate. The band gap energy is determined from the linear fit (dark gray line) to the absorption edge as indicated.

The electrical properties of deposited films were determined by Hall measurements. The resistivity of ALD grown Ga2O3, Ga(7)Zn(1), Ga(5) Zn(1), Ga(3)Zn(1), and ZnO is 1.80 × 104, 8.35 × 103, 3.54 × 103, 1.52 × 103, and 1.25 × 10−2 Ω·cm, respectively. The 1.80 × 104 Ω·cm resistivity of Ga2O3 decreased to 1.52 × 103 Ω·cm after being doped with Zn. The carrier concentration and the mobility of the studied films are difficult to measure in the known ZGO because the samples show high resistivity in room temperature. Nevertheless, the carrier concentration seems to increase with increasing Zn doping concentrations according to the XPS and optical analysis.

[2] [3]

[4]

[5]

4. Conclusions

[6]

We have introduced a new technology pertaining ALD processing of ZGO films by combining the PE-ALD of Ga2O3 and the TH-ALD of ZnO. The ZGO films were constituted as a sandwich structure by stacking different cycles (7, 5, and 3) of Ga2O3 and 1 cycle of ZnO. A higher growth rate of ZGO films was obtained at 200 °C compared with that of pure Ga2O3 and ZnO. As-deposited ZGO films were amorphous while ZnO was formed with polycrystalline structure. XPS results indicate that the Zn content in ZGO films increased from 9.70 at.% to 24.65 at.% when the cycle ration of Ga2O3 increased, with respect to ZnO, from 7:1 to 3:1 whereas the oxygen vacancy increased from 27.65% to 37.93% with increased Zn doping of ZGO films. Along with increased Zn doping concentration, the band gap increased from 4.64 to 5.25 eV. Surface morphology studies of the films showed smooth films (a RMS roughness of less than 1 nm) with a slight increase in roughness upon increasing the Zn concentration. While the growth mechanism needs to be explored further, this new ALD approach may be extended to other material systems for improved property control.

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

Acknowledgements [15]

This work is supported by the National Natural Science Foundation of China (Nos. U1632121, 11804055, 51861135105 and 61874034), National Key R&D Program of China (No. 2016YFE0110700), and Natural Science Foundation of Shanghai (No. 18ZR1405000).

[16]

Appendix A. Supplementary data

[18]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.01.177.

[19]

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