Room Temperature Synthesized High Mobility Transparent

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Room Temperature Synthesized High Mobility Transparent Amorphous CdO-Ga2O3 Alloys with Widely Tunable Electronic Bands Chaoping Liu, Chun Yuen Ho, Roberto dos Reis, Yishu Foo, Pengfei Guo, Juan Antonio Zapien, Wladek Walukiewicz, and Kinman Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18254 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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ACS Applied Materials & Interfaces

Room Temperature Synthesized High Mobility Transparent Amorphous CdO-Ga2O3 Alloys with Widely Tunable Electronic Bands Chao Ping Liu†, Chun Yuen Ho†, Roberto dos. Reis‡, Yishu Foo†,§, Peng Fei Guo†,#, Juan Antonio Zapien§, Wladek Walukiewicz∆, and Kin Man Yu†* † Department of Physics, City University of Hong Kong, 83 Tat Chee Ave., Kowloon, Hong Kong ‡ National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA § Department of Materials Science and Engineering, City University of Hong Kong, 83 Tat Chee Ave., Kowloon, Hong Kong # Key Laboratory of Advanced Micro/Nano Functional Materials, School of Physics and Electronic Engineering, Xinyang Normal University, Xinyang 464000, China ∆ Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720 Keywords: Amorphous transparent conducting oxides, transparent conductors, flexible electronics, transparent electronics, oxide semiconductors Corresponding: [email protected] ORCID (Chao Ping Liu): 0000-0003-3802-9557 (Kin Man Yu) 0000-0003-1350-9642

ABSTRACT

In this work, we have synthesized Cd1-xGaxO1+δ alloy thin films at room temperature over the entire composition range by radio frequency magnetron sputtering. We found that alloy films with high Ga contents of x>0.3 are amorphous. Amorphous Cd1-xGaxO1+δ alloys in the composition range of 0.30.4 are thermally stable up to 500oC. Moreover, no 4 ACS Paragon Plus Environment

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degradation in the electrical properties of the amorphous alloys is observed after annealing. This suggests that amorphous Cd1-xGaxO1+δ alloys are thermally stable and are compatible with further processing at elevated temperature in device fabrication. Furthermore, AFM measurements on Cd1-xGaxO1+δ films show that for crystalline films with x0.34, the surface morphology becomes very smooth with RMS roughness 0.7, the resistivity is too high (>104 Ω-cm) to obtain reliable ohmic contacts for Hall Effect measurements. Nominally undoped RT deposited CdO sample has an N~2-3x1020 cm-3 with µ~40 cm2V-1s-1. This high N can be attributed to donors due to native defects such as oxygen vacancies and Cd interstitials.32 For 00.3, N shows a monotonic decrease and eventually becomes too low to measure (0.7. This reduction in N is consistent with the presence of Oi-related acceptors in these O-rich films.32 Moreover, it is also related to the lowering of the formation energy of acceptor native defects due to upward shift of the conduction band minimum (CBM). Such strong reduction in N has been observed by Chen et al. when CdO is alloyed with MgO for MgO content up to ~20% and was explained by the strong upward shift of the CBM due to the large difference in electron affinity between MgO and CdO (χ= -0.85 and -5.9 eV, respectively).34 In the present case, the electron affinity for amorphous Ga2O3 is ~ -4.5 eV 35 and the upward shift in CBM of the alloy is less drastic and therefore the reduction in N is much slower.

Figure 3. (a) Electrical properties of Cd1-xGaxO1+δ alloy thin films with 0≤x≤0.70, the shaded region represents amorphous alloys; (b) electrical properties of amorphous O-rich Cd0.64Ga0.36O1+δ as a function of % of O2 in sputtering gas (Ar/O2). For crystalline alloys the reduction in the carrier mobility µ as x increases is related to the increased scattering due to ionized impurities (for x0.442). The temperature-independent behavior of the mobility for the amorphous Cd1-xGaxO1+δ films suggests that defect scatterings are dominant in these samples. While a high conductivity is required for transparent conductor applications, for some devices (e.g., thin-film transistors or electron transporting layer in solar cells), it is desirable to be able to control the material conductivity over a wide range.38 Here, we demonstrate that the electrical properties of amorphous Cd1-xGaxO1+δ films can be effectively tuned by the oxygen stoichiometry. The oxygen content in the amorphous alloy film was controlled by introducing a few percent of O2 in the sputtering gas. As an example, the electrical properties of amorphous Cd0.64Ga0.36O1+δ films as a function of the % of O2 in the sputtering gas ([O2]/[Ar+O2]) were shown in Figure 3 (b). By increasing the O2 in the sputtering gas from 0 to 3% both N and the resistivity ρ can be tuned over 4 orders of magnitude. Specifically, N decreases from 5.2×1020 cm-3 to 1.5×1016 cm-3, while ρ increases from 7.3×10-4 Ω-cm to 77.4 Ω-cm. Such oxygen induced reduction in carrier concentration and mobility was also found for the crystalline undoped or doped CdO thin film32,39 and was attributed to the reduction of O-vacancy donors40 as well as the formation of Cd-vacancy and O-interstitial acceptors in an O2 rich deposition environment. These native acceptors compensate the native donors, resulting in reduced carrier concentration and mobility (when O2 > 1%). Note that the O-rich, highly compensated Cd0.64Ga0.36O1+δ film still maintains a decent mobility of >5 cm2/Vs (still much higher than that of amorphous silicon, 1100 nm. As shown in Figure 4 (a), the absorption edge of the amorphous films clearly blue shifts with increasing x. The optical properties of these alloys were further investigated by spectroscopic ellipsometry (SE). A three-layer (i.e., glass substrate, alloy film, and surface roughness) optical model was constructed for the SE modeling. The dielectric functions of alloy films were modeled by combining the Tauc-Lorentz model, Drude model and the Gaussian model, while the dielectric function of the surface roughness was modeled by using a 50/50% mixture of the alloy film and voids. Details of the SE fitting can be found in our previous work.41 In Figure S4(a) of supporting information, the SE spectra of a typical alloy film with x=0.155 at an incidence angle of 75 degree are shown with their best fits. As seen, excellent agreements were obtained between the measured SE spectra and the model fits over the entire spectral range of the SE data. Absorption coefficient for alloy films with different x obtained from SE fitting are plotted in Figure S4(b). For alloys with high electron concentration (low x), strong free carrier absorption in the low energy region (0.6, the electron concertation falls below 1019/cm3 and these free carrier effects become negligible. The intrinsic band gaps of Cd18 ACS Paragon Plus Environment

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xGaxO1+δ

films over the entire alloy composition range were calculated according to Equation (1) and the results are shown as square symbols in Figure 4 (b). When free-carrier effects are taken into account, the intrinsic band gaps of Cd1-xGaxO1+δ films exhibit a monotonic increase with increasing x. Specifically, band gaps of the amorphous alloys follow a rather linear trend while those of the crystalline alloys show some bowing. The band gap bowing effect of the crystalline materials was estimated using the expression, !

# !$

= "

%1 "&! '"%1 "&,

(2)

!

with b the bowing parameter. Assuming # $ = 4.8 *+ and ! = 2.2 *+, the bowing parameter of b is found to be 2.3 eV. It is worth noting that the calculated intrinsic gaps using this bowing shown in Figure 4(b) deviate significantly from the measured gaps of the amorphous alloys. 3.4. Electronic Band Offsets. Band offsets of different semiconductors are crucial in determining the performance of heterojunction devices. For instance, in a thin film solar cell (perovskite or CdTe), desired alignment for the conduction band minimum (CBM) of the electron transporting layer (ETL) and the CBM of the absorber layer is required to facilitate electron transport and minimize interface recombination. 47 , 48 Recently, transparent amorphous oxide alloys (e.g., In-Ga-O and Ga-Zn-Sn-O) were found to have great potential as ideal ETLs in organic photovoltaic devices owing to their widely tunable CBM by changing the alloy compositions. 49 We estimate the band edge positions of Cd1-xGaxO1+δ alloys from valance band maximum (VBM) positions measured by x-ray photoelectron spectroscopy (XPS) and intrinsic band gaps obtained by optical absorption. The VBM was determined by fitting the XPS valence band edge by convoluting a step function with a Gaussian function. Figure 5 (a) shows the XPS valence band spectra of representative samples spanning the complete composition range with the fitted spectra shown as solid lines. The binding energy of the VBM for Cd1-xGaxO1+δ alloys shows a monotonic increase from ~1.3 to 4.6 eV with increasing x. Note that the VBM of CdO is located at the L-point and is ~1 eV below the CBM at the Γ-point.50,51 It has been demonstrated that the Fermi level of semiconductors at the surface is pinned to a universal energy level, known as the Fermi level stabilization energy EFS, located ~4.9eV below the vacuum level.52,53,54,55,56 Since XPS is a surface sensitive technique, the VBM measured can be referenced to EFS. Figure 5 (b) shows the VBM (blue open circles) of the Cd1-xGaxO1+δ alloys estimated from the XPS measurements with respect to the vacuum level. The CBM data (red open squares) are obtained from the positions of the VBM and the intrinsic band gap shown in Figure 5 (b). The solid lines in the figure are the calculated CBM and VBM according to the bowing of the crystalline and amorphous alloys obtained in Figure 4(b) and the known band offsets of CdO and amorphous Ga2O3. With respect to the vacuum level, the CBM and the VBM for amorphous Ga2O3 are ~ -4.5 eV and -9.3 eV, respectively,35,48 while those for CdO are ~-5.8 and -8, respectively.56 Note that the calculated band edge energies agree with the experimental results rather well for alloys with x>0.2. The much higher VBM for crystalline alloys with x0.3. Also shown in Figure 5(b) are the Fermi levels EF positions of the highly conducting Cd1-xGaxO1+δ films (x≤0.61, N>2×1019 cm-3) estimated by assuming the simple parabolic dispersion relation,57 - =

.# /0∗

%33 / 4&//6 ,

(3)

with 7∗ being the reduced effective mass, N the carrier density from Hall measurement. Notice that the EF of the films lies at ~5 eV below vacuum. This is close to EFS (~4.9 eV) where the formation energies of native donors and acceptors are the same.53 As the alloy becomes more Ga rich with x>0.7 the CBM crosses EFS and the formation of native compensating acceptors becomes favorable. This reduces the electron concentration and eventually make the alloy insulating when x>0.7. This is very similar to the case mentioned earlier when CdO was alloyed with MgO where the strong upward shift of the CBM resulted in insulating CdMgO with MgO content >20%.34 XPS Cd 3d5/2, Ga 2p3/2 and O 1s core-level spectra were recorded from Cd1-xGaxO1+δ films with x=0, 0.06, 0.034, 0.57, 0.7 and 1 after sputtering 20 nm from the surface using an Ar gun to avoid surface contamination and the spectra were shown in Fig. S7 in supporting information. For pure CdO, the measured Cd 3d5/2 and O 1s binding energy of 403.4 eV and 528 eV, respectively are comparable to those obtained by others.58 For pure Ga2O3, the Ga 2p3/2 the binding energy of ~1118.2 eV lies within the range of 1117.6 to 1119.8 eV reported by Cui et al. for amorphous Ga2O3 deposited with different O2 partial pressure in the sputtering gas.59 Monotonic shifts in the binding energy are observed for all of the core levels as the Ga content x increases, suggesting that these Cd1-xGaxO1+δ films are indeed random alloys with increasing oxidation number as the Ga content increases. We note that the Cd 3d5/2 spectrum for the sample with x=0.7 shows a double peak feature. This may indicate that phase separation may occur in some of the samples with x>0.57.

4. CONCLUSIONS 10 ACS Paragon Plus Environment

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In this work, we have demonstrated the room temperature synthesis of Cd1-xGaxO1+δ alloy thin films over the whole composition range by magnetron sputtering methods. We found that as the Ga content x increases, the film undergoes a phase transition from polycrystalline to amorphous structure at x~0.3. In the amorphous phase the alloys are very smooth with a RMS roughness < 0.6 nm. The intrinsic band gap of amorphous alloys (x>0.3) spans a large range from 2.5 eV to 4.8 eV. Within the amorphous regime, in the alloy composition range of 0.310 cm2V-1s-1, resulting in a resistivity ρ≲10-3 Ω-cm. The conductivity of amorphous Cd1-xGaxO1+δ alloys can be controlled over 5 orders of magnitude by varying their O stoichiometry during sputtering. Furthermore, the position of the CBM of amorphous Cd1xGaxO1+δ alloys can also be tuned from 5.8 eV to 4.5 eV below the vacuum level. These amorphous films were also deposited on PET substrates with similar structures as well as optoelectronic properties. These results suggest that amorphous Cd1-xGaxO1+δ alloys with widely tunable optoelectronic properties can be engineered for various electronic applications such as transparent conducting layers and electron transport layers for photovoltaics as well as transparent field effect transistors. The room temperature deposition process is particularly appealing for organic devices and/or devices on flexible substrates. ASSOCIATED CONTENT Supporting Information Atomic force microscopy images; A comparison of transmission spectra and electrical properties of Cd0.644Ga0.356O1+δ on PET and glass substrates; temperature dependent electrical properties of Cd0.644Ga0.356O1+δ alloys with x=0.06 and 0.442; an example of the Spectroscopy Ellipsometry analysis and absorption coefficient plots of alloys with different compositions; free carrier effects on optical absorption; procedure for the determination of VBM by XPS measurements; XPS core level spectra and surface work function by Kelvin probe measurement on alloys with different composition.

ACKNOWLEDGEMENTS This work is supported by the General Research Fund of the Research Grants Council of Hong Kong SAR, China, under Project No. CityU 11267516. Materials characterization by RBS analysis (Electronic Materials Program) and TEM (National Center for Electron Microscopy/Molecular Foundry) at Lawrence Berkeley National Laboratory were supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE-AC0205CH11231. J. A. Z. acknowledges support by the Research Grants Council, University Grants Committee, Hong Kong (Project No. CityU 122812). Y. F. was supported by the Hong Kong Ph.D. Fellowship No. PF-15139, Research Grants Council, University Grants Committee, Hong Kong.

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Room Temperature Synthesized High Mobility Transparent

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