Creation of a Short-Range Ordered TwoDimensional Electron Gas Channel in Al2O3/ In2O3 Interfaces Sang Yeon Lee,† Jinseo Kim,† Ayoung Park,† Jucheol Park,‡ and Hyungtak Seo*,† †
Department of Energy Systems Research & Department of Materials Science and Engineering, Ajou University, Suwon 16499, Republic of Korea ‡ Gyeongbuk Science Technology Promotion Center, Gumi Electronics & Information Technology Research Institute, Gumi 39171, Republic of Korea S Supporting Information *
ABSTRACT: The tuning of electrical properties in oxides via surface and interfacial two-dimensional electron gas (2DEG) channels is of great interest, as they reveal the extraordinary transition from insulating or semiconducting characteristics to metallic conduction or superconductivity enabled by the ballistic transport of spatially confined electrons. However, realizing the practical aspects of this exotic phenomenon toward short-range ordered and airstable 2DEG channels remains a great challenge. At the heterointerface formed after deposition of an Al2O3 layer on a nanocrystalline In2O3 layer, a dramatic improvement in carrier conduction equivalent to metallic conduction is obtained. A conductivity increase by a factor of 1013 times that in raw In2O3, a sheet resistance of 850 Ω/cm2, and a room temperature Hall mobility of 20.5 cm2 V−1 s−1 are obtained, which are impossible to achieve by tuning each layer individually. The physicochemical origin of metallic conduction is mainly ascribed to the 2D interfacially confined O-vacancies and semimetallic nanocrystalline InOx (x < 2) phases by the clustered self-doping effect caused by O-extraction from In2O3 to the Al2O3 phase during ALD. Unlike other submetallic oxides, this 2D channel is air-stable by complete Al2O3 passivation and thereby promises applicability for implementation in devices. KEYWORDS: indium oxide, aluminum oxide, 2DEG, metallic conduction, atomic layer deposition ransparent oxide (TO) films, which are widely used as window electrodes and thin film transistor channels in various devices, exhibit high electrical conductivity and high optical transparency in the visible region of the spectrum.1 TO films are fabricated by tuning the charge transport of wide band gap oxides like indium gallium zinc oxide (IGZO), ZnO, and SnO2.2−5 When the TO film is tuned to have a very high level of conductivity as in metallic conduction, it is called a transparent conducting oxide (TCO). Among the various TCOs, the most widely used TCO is Sn-doped indium oxide (In2O3), which is called indium tin oxide (ITO) and can offer a very high carrier density.6,7 Thereby, In2O3, with an ultraviolet (UV) range bandgap at >3 eV, is itself an attractive TCO candidate that can be doped to have high carrier density without significant degradation of visible transparency.6 Amorphous or polycrystalline In2O3, among the prototype TCOs, is popularly used as a TCO because fabrication is relatively low and the growth mechanisms are simple.8 To fabricate high-μ [>80 cm2 V−1 s−1] In2O3 films instead of ITO, In2O3 films have been doped with metal dopants such as Ti, Zr,
Mo, and W.9 However, In2O3 has intrinsic n-type properties owing to its native n-type donor oxygen vacancy (O-vacancy) and indium interstitial (Ini).3 Ini, which is a native donor in undoped In2O3, generates a shallow donor level.10 The formation of an O-vacancy can provide two electrons to the lattice, which acts as a combination of electrical dipoles and lattice defects.11,12 In contrast to the low level of native O− vacancies that have a localized shallow donor state, a large amount of O-vacanciesintentionally created to increase conductivity at the metallic levelform deep donor levels or suboxide bands in single-crystals, and surface and grain boundaries in polycrystalline metal-oxide films.13 Upon increasing the O-vacancies or suboxide components to obtain high conductivity, the Fermi energy level (EF) is pinned at the O-vacancy states, which leads to a decrease in visible transparency by light absorption at the defect states.1,14 The
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© 2017 American Chemical Society
Received: March 21, 2017 Accepted: May 18, 2017 Published: May 18, 2017 6040
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Figure 1. (a) A schematic diagram of Al2O3/In2O3 heterostructure devices with Ti/Au electrodes through Al2O3. (b) Sheet resistance and (c) Hall mobility of Al2O3/In2O3 heterostructures as a function of temperature for different In2O3 thicknesses taken from Hall measurements. (d) Conductance depending on In2O3 thickness after time-dependent air-aging. Conductance maintained almost the same value after 1 year of aging since the Al2O3 layer provided the air and humid passivation. (e) Variation of the sheet carrier density and (f) activation energies as a function of temperature (T and 1000/T, respectively) for different In2O3 thicknesses taken from Hall measurements.
films are then essentially subject to air-instability due to water and O reactions at reactive O-vacancy sites. These limit elemental oxides including In2O3 from having conductivity at TCO levels. As a different approach to obtaining highly conductive oxides, it has been found that the 2DEG phenomenon can be applied to increase the electrical conductivity of oxide semiconductors up to the level of metallic conduction. 2DEG of oxide semiconductors has been studied using single crystalline heterojunctions. As a result, theoretical and experimental results have shown that superconductivity occurs in the interfaces at thicknesses 90% visible transparency. In Figure 4b, transmittance of the Al2O3/In2O3 heterostructures was increased to almost 90% at below 400 nm wavelength compared with the single In2O3 layer. Furthermore, Figure 4d shows that transmittance of the Al2O3/In2O3 heterostructures was further enhanced to almost 100% at 500 nm of wavelength by >10% increase of transmittance in the single In2O3 layer since the top Al2O3 offers the antireflection function. The antireflection function occurs from the threshold In2O3 thickness of 30 nm since In2O3 is crystallized beginning at ∼20 nm of film thickness (Figure S3), where the refractive index of In2O3 bixbyite (n ≈ 1.9985) is properly formed.31 Al2O3 is indeed known as the antireflection coating material32 and its proper thickness of maximum antireflection coating effect can be estimated by the quarter wavelength rule to invoke the destructive interference of the reflected beam expressed as dARC= λ/4nARC, where dARC is the thickness of the antireflection coating layer, λ is the wavelength of the incident beam, and nARC is the reflection index of the incident beam.33 At 500 nm (λ) and 1.69 of nARC for Al2O3, dARC is estimated at 74 nm, which is the middle of the actual Al2O3 thickness range of 50− 100 nm used in the heterostructure. This suggests that the 6044
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(Al2O3 region) to 2.70 eV (bulk In2O3 region) following Ar sputtering up to 315 s and the formation of O-deficient In2O3 resulted in downward band bending at the Al2O3/In2O3 interface region. Such a continuous change in VBM energy is correlated with the depth-resolved chemical composition change as well. This suggests that the 2D O-vacancy clusters for 2DEG channel creation are possible in nc-phase interfaces by chemically driven interfacial phase transition. Considering all findings of empty states from STEM-EELS and occupied states from VB edge and core level XPS analyses, the full band alignment of the Al2O3/In2O3 heterostructures was constructed (Figure 6d). It should be noted that the interfacial downward band bending creates a 2D potential well that draws and confines free In 3d electrons supplied from the interface and bulk O-vacancies. In contrast to the single crystalline 2DEG, which is very sensitive to the overlayer thickness controlled to less than 10 nm for optimized carrier conduction, 2DEG in Al2O3/In2O3 heterostructures in this study were obtained at the relatively thick 50−100 nm Al2O3, offering great stability of the 2DEG channel property under air-aging condition for 1 yr without reoxidation of the reduced In species or O-vacancy clusters in addition to the improved visible transparency by the antireflection effect of Al2O3. This result is comparable to recent reports for the significant conductivity improvement in different nanocrystalline heterointerfaces; Thimsen et al. reports ×108 increase of conductivity and mobility enhancement at the colloidal nanocrystal Al2O3/ZnO interface, the origin of which is ascribed to the removal of electron trap such as OH traps near metal-oxide surface instead of 2DEG channel formation.41 This is understandable as considering a high concentration of OH group at the surface of colloidal nanocrystals. However, in this study, the amount of OH surface group is very limited since In2O3 is deposited by the vacuum RF sputtering. In fact, O 1s XPS spectra in Figure 5 show small OH bond fraction at 5.5−6.2% constantly maintained in both as-dep In2O3 surface and Al2O3/In2O3 interface. Thereby, the role of OH group for conductivity change is excluded as the mechanism in this study. Rather, our previous result for embedded 2 DEG formation in InGaZnO thin film by photochemical H radical insertion indicates that the large amount of surface metal−OH phase formation is not relevant to embedded interfacial 2DEG creation at InGaZnO/ SiO2 but good for surface passivation.13 On the other hand, Faber et al. claimed the formation of 2DEG channel at the In2O3/ZnO heterointerface via the formation of seamless and dislocation-free crystalline metal oxide growth and sharp interfacial band bending.42 This study agrees with our finding that the interface band bending creates free electron localization at 2 DEG but chemical property of In2O3/ZnO heterointerface is quite sharp while we observed intermixed phase of Al2xIn2−2xO3 (0 < x < 1) at the heterointerface. We guess that this is due to the difference in film formation process, i.e., nonthermal solution synthesis for In2O3/ZnO or thermal ALD process for Al2O3.
metal (532 eV) binding states, where metal represents In or Al.34,35 For In ions, three types of binding states were observed in the In 3d spectrum: (1) O-deficient In0−2+ (443.03 eV), (2) In3+ as full oxidation states (444.15 eV), and (3) In−OH (445.35 eV)-related binding states.36,37 In contrast to the In 3d binding states in single In2O3, the Al2O3/In2O3 interface region showed remarkable changes in the metal oxidation states of the In 3d5/2 binding state. In 3d5/2 in Figure 5c reveals that the partial metallic and/or highly reduced In species (e.g., In2+, In1+, In0) acted as an embedded semimetallic 2D channel with O-vacancy clusters through InOx nanocrystallites. The reduced In oxidation states stem from O-extraction to Al ions upon the Al2O3 ALD process. Aluminum, one of the strongest oxidizing elements, caused reduction reaction of In2O3 by lattice Oextraction. This is analogous to the previous result regarding Al2O3 ALD on single crystalline SrTiO3;16 the surface of SrTiO3 was reduced by conversion of TiO2 to the Ti2O3 phase as a result of Al2O3 formation by extracting O from Ti ions with a stronger thermodynamic driving force (i.e., low Gibbs free energy). Generation/Mechanism of Short-Range Ordered 2DEG. To further investigate the physicochemical origin of 2DEG creation, cross-sectional high-resolution transmission electron microscopy (HR-TEM) analysis of the Al2O3/In2O3 heterostructure was performed (Figure 6a). In2O3 was nanocrystalline at the scale of ordering less than ∼10 nm under the columnar shape growth of domains. From Figure 3a highlighting the Al2O3/In2O3 interface regions, it was found that the heterostructure consisted of four distinctive types of morphology: (1) top intermixing with Al2O3, (2) second columnar structure, (3) third nanocrystalline In2O3 (nc-In2O3), and (4) bottom intermixing with glass layers. The top intermixing layer with Al2O3 was formed by chemical interaction between Al2O3 and In2O3 (see SI section 4 for TEM analysis of the single In2O3 layer). This top intermixing interfacial layer was responsible for the remarkable improvement in electrical properties of the Al2O3/In2O3 structure to metallic conduction levels. Figure 6b shows the O K1 edge scanning TEM (STEM)-electron energy loss spectroscopy (EELS) spectra for several selected local spots along the film depth in the Al2O3/In2O3 heterostructure. The O K1 onset energy of In2O3, Al2O3, and SiO2 appeared at 532.3, 536, and 537 eV, respectively.38−40 In the Al2O3/In2O3 interface of the top intermixing region, O K1 edge spectra show intermixed O K1 onset features between Al2O3 and In2O3 (at spots 3 and 4) and these interfacial spectral features definitively distinguish them from homogeneous Al2O3 and In2O3, suggesting the physicochemical differences of the interface from the pure phases. Thereby, STEM-EELS results confirm the existence of a local interfacial region consisting of an Al2O3−In2O3 mixed phase, which is considered to be differentiated from the stoichiometric Al2O3 and In2O3 phases. Comparing the top region (at spots 3 and 4) with the bottom region (at spots 12 and 13), we can see that the O K1 edge is positioned at energies much larger than those in the interface region, which indicates that oxygen is bonded to Al of the top intermixing layer (at 537 eV) and Si of the bottom intermixing layer (at 536 eV). This confirmed that the full oxidation state is maintained at these top and bottom regions but suggests that a partially reduced oxidation state might be present in the interface. The VB maximum energy (VBM) energy is determined against the Fermi energy (EF) level (i.e., 0 eV of binding energy). The VBM energies (Figure 6c) decreased gradually from 3.76 eV
CONCLUSION In conclusion, we demonstrate the creation of a 2D electron gas induced metallic channel amorphous-Al2O3/nc-In2O3 heterostructure. This nc-InOx at the 2D interface shows a transition in electrical conduction property from semiconductor to metal; conductivity increase by ×1013 from that in the raw In2O3, sheet resistance of 850 Ω/cm2, and room temperature Hall 6045
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AUTHOR INFORMATION
EXPERIMENTAL SECTION
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Corresponding Author
*
[email protected]. ORCID
Hyungtak Seo: 0000-0001-9485-6405 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the Nano-Material Technology Development Program (NRF-2014M3A7B4049368) through the National Research Foundation (NRF) funded by the Ministry of Science, ICT, and Future Planning. REFERENCES
Fabrication of Al2O3/In2O3 Heterostructures. In2O3 layers were deposited onto soda-lime glass via RF reactive sputtering at an RF power of 40 W at room temperature and a working pressure of 10 mTorr in a mixture of 10:1 Ar/O2. In order to fabricate the interface layer, 1000 Å thick Al2O3 was deposited on top of the In2O3 by atomic layer deposition (ALD) using trimethyl aluminum (TMA) precursor at the process temperature of 473 K. For the electrical characteristics of the Al2O3/In2O3 heterostructure, 50 nm-thick Au electrodes (diameter of 1 mm with a 10 nm-thick Ti paste metal layer) were deposited at the corner of 1 cm square samples by e-beam evaporator using a shadow mask on In2O3 surfaces. Electrical Measurement. Current−voltage (I−V) measurements (Keithley 4200-SCS) were conducted at room temperature. Sheet resistance and sheet carrier density were measured by the Hall measurement system (HMS-5000, Ecopia) using the Van der Pauw configuration. To investigate temperature effects, the temperature during Hall measurements was varied from 80 K to >300 K. Analysis of the Heterostructure Properties. The optical properties of Al2O3/In2O3 heterostructures and single In2O3 films were measured using ultraviolet−visible (UV/vis) spectrophotometer with Avantes spectroscopy system, AvaLight-DH-S-BAL balanced power source, and AvaSpec-ULS2048 Starline versatile fiber-optic spectrometer detector (Netherlands). The surface morphologies of In2O3 films were measured by an atomic force microscope (AFM, XE100, psia). The chemical composition of In 3d and O 1s were characterized by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Co., Theta probe) using an Al Kα X-ray source (1486.6 eV). The peak energy in XPS spectra was self-calibrated to the C 1s and O 1s reference peak states. Transmission electron spectroscopy (TEM) and scanning TEM (STEM) analysis were performed using a probe-corrected JEM ARM 200F (JEOL) equipped with a cold FEG source operating at 200 kV. Electron energy loss spectroscopy (EELS) spectra were measured across the In2O3 and Al2O3/In2O3 structures in the STEM mode by a high-resolution Gatan imaging filter (GIF Quantum ER). The energy dispersion was 0.1 eV/ ch and the entrance aperture of the GIF was set at 2.5 mm in diameter. TEM specimens were prepared using the focused ion beam (FIB) liftout method in a ZEISS Auriga.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b01964. SEM data; details of I−V results; summary on the hall measurement data; TEM and TEM-EELS analysis of In2O3 films with a thickness of 100 nm; TEM-EDS analysis; XPS depth data of In 3d, O 1s, and Al 2p against the Ar sputter time (PDF) 6046
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