Materials Today Advances

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Chemical mechanism of formation of two-dimensional electron gas at the Al ₂ O ₃ /TiO ₂ interface by atomic layer deposition

Jeongwoo Park

^a

, Hyobin Eom

^a

, Seong Hwan Kim

^b

, Tae Jun Seok

^c

, Tae Joo Park

^c

, Sang Woon Lee

^b

, Bonggeun Shong

^a^,^*

aDepartment of Chemical Engineering, Hongik University, Seoul, 04066, South Korea

bDepartment of Energy Systems Research and Department of Physics, Ajou University, Suwon, 16499, Republic of Korea

cDepartment of Materials Science and Chemical Engineering, Hanyang University, Ansan, 15588, Republic of Korea

a r t i c l e i n f o

Article history:

Received 2 September 2021 Received in revised form 27 November 2021 Accepted 30 November 2021 Available online 7 December 2021

Keywords:

2DEG Adsorption Surface chemistry ALD

Anatase Alumina

a b s t r a c t

Two-dimensional electron gases (2DEGs) localized at oxide heterointerfaces can potentially be used in applications associated with the design of novel electronic device architectures. Recent studies have reported that atomic layer deposition (ALD) of Al2O3on TiO2substrates can generate 2DEG states, owing toin situformation of interfacial oxygen vacancies (VO). However, the chemical mechanism governing the adsorption of the trimethylaluminum (TMA) precursor on the TiO2surface remains unclear. In this work, an investigation aimed at elucidating the formation of the 2DEG state through the reaction of TMA on TiO2was performed using periodic dispersion-corrected DFTþU calculations. Dimethylether, whose desorption leaves VOsurrounded by Ti^3þ, can be formed via direct methylation of the lattice oxygen on the TiO2surface. The experimentally observed dependence of the carrier density on the process temperature of Al2O3ALD confirmed the endothermic nature of VOformation. Furthermore, the emergence of geometrically confined n-type electronic states corresponding to interfacial VOconfirmed the formation of 2DEGs at the heterointerface. Our study provides a fundamental understanding of 2DEG formation in the Al2O3/TiO2heterojunction interface.

1. Introduction

Two-dimensional electron gases (2DEGs) are a group of metallic electrons formed at the interface with high charge density and mobility. These electrons can move freely in a direction parallel to the interface but are geometrically conﬁned in the vertical direction. The advantages of 2DEGs have led to research aimed at the development of various electronic devices such as power electronics based on high-electron mobility transistors, often based on group IIIeV compound semiconductor interfaces [1]. Moreover, 2DEGs formed at interfaces among different oxide materials provide novel possibilities for innovative electronic device structures because both materials comprising the interface can be insulating [2e4]. However, 2DEGs at ferroelectric perovskite oxide heterointerfaces, such as LaAlO3/SrTiO3, often require epitaxial deposition conditions on single crystalline substrates [5].

The formation of 2DEGs at oxide heterointerfaces via the atomic

layer deposition (ALD) technique has recently been reported [6e9].

Through ALD, uniform and conformal thinfilms withfine thickness control can be deposited, and this process is compatible with mass- production [10]. Moreover, nanolaminated multilayers including heterointerfaces can be easily formed by alternating two different ALD processes [11e13]. Several types of electronic devices, such as field-effect transistors, conductive bridge random access memory, and sensors, are realized by applying ALD of Al2O3using trimethylaluminum (TMA) and water on ALD-deposited TiO2substrates.

Similar revelation of 2DEG states by ALD of an Al₂O₃overlayer has been reported for other oxide substrates such as SrTiO3, In2O3, WO3, and ZnO [14e17]. These substrate oxides exhibit relatively high reducibility, and hence, oxygen vacancies (V_O) are easily created [18]. Owing to the reducing power of TMA during deposition on various substrates, partial reduction of the interface is often observed for a wide range of substrate materials [19e23]. For example, the frequently observedin situremoval or“clean-up”of the native oxides on compound semiconductor substrates during oxide ALD results in shifts in the Fermi level [24]. Interestingly, unlike typical proton-transfer-type reaction that is often assumed

*Corresponding author.

E-mail address:[email protected](B. Shong).

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https://doi.org/10.1016/j.mtadv.2021.100195

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to occur during adsorption of TMA given enough coverage of surface hydroxyl groups (*OH, where*denotes surface-bound species) [25], direct transfer of CH3groups to the surface has been suggested on oxide surfaces with low density of hydroxyl groups [21,26e28].

As ALD is entirely based on surface reactions, analyzing the reaction mechanism between the substrate and precursor can provide fundamental and crucial information [29]. For example, the reaction mechanism governing the “clean-up” of native oxide compound semiconductors by ALD precursors are investigated using computational chemistry approaches [30e32]. The surface chemistry of ALD Al₂O₃ on TiO₂ also have been considered by previous experimentalin situstudies [33e35]. These studies have shown that exposure of TMA creates VO(equivalent to Ti^3þ) on TiO2, part of which is preserved even after the successive H₂O exposures, and is then protected by formation of Al2O3 encapsulation overlayer. A signiﬁcant concentration of VOdefects can be generated through the easy removal of TiO2 lattice oxygen by thermal annealing, electron beam exposure, or ultraviolet irradiation [36].

Then, VOlocalized on the surfaces of TiO2corresponds to the 2DEG states [37e39]. Lattice oxygen on these surfaces can also be removed via chemical reactions with various compounds [40,41].

Recently for other ALD systems, theoretical methods such as density functional theory (DFT) calculations [42,43], Monte Carlo and kinetic Monte Carlo simulations [44e46], and microkinetic ana- lyses [47e49] are successfully applied to analyze the surface chemical reactions, elucidating mechanistic details to provide clues for efﬁcient process development.

In this study, the chemical mechanism governing the TMA- induced formation of 2DEG at the Al2O3/TiO2 interface during ALD is investigated through dispersion-corrected DFTþU calculations. The results revealed that the lattice oxygen of TiO₂can be removed as dimethylether via the transfer of CH3ligands from the TMA precursor. The overall reaction involves kinetic and thermo- dynamic activation, which is conﬁrmed by experimental observation of the temperature dependence exhibited by the 2DEG concentration. Electronic analysis of the resulting surface structure upon TMA adsorption showed that the midgap n-type state is formed by the interface-localized VO.

2. Methods

The electronic structure calculations at the DFT þU level of theory were performed using the Vienna ab initio simulation package (VASP) version 5.4.4 [50]. The PBE (Per- deweBurkeeErnzerhof) exchange-correlation functional [51] and D3 dispersion correction with BeckeeJohnson damping [52] were applied using projector-augmented wave methods. A Hubbard U parameter [53] of Ueff¼3 eV was adopted based on the structural and electronic calibration of bulk TiO2(see below). A plane-wave cutoff energy of 500 eV and a 551 Monkhorst-Pack k-point mesh were used. Anatase, whose lowest energy direction is (101) [54], was the observed phase of TiO2 under the current experimental conditions [6]. At the temperature range of the Al₂O₃ALD process (150C-300C), almost no OH groups would persist on anatase surfaces under vacuum [55]. Therefore, bare anatase (101) surface models without any OH were built. A 10.463 (x)11.408 (y)9.558 (thickness) Å³6-TiO₂layer slab containing 36 Ti and 72 O atoms was used for adsorption energy calculations. In addition, a 12-TiO2layer slab (containing 72 Ti and 144 O atoms) with the same lateral dimensions and thickness of 14.414 Å was used for electronic structure calculations. Vacuum spaces of ca. 18 Å between slabs were assigned to prevent nonphysical interactions. Only the upper two Ti layers of the slab and the adsorbates were allowed to relax during optimization. All geometries were optimized until the net force between all atoms was 0.02 eV/Å. For calculations of

transition states, the climbing image nudged elastic band (Cl-NEB) method was employed [56] with 4 intermediate images between the neighboring local minima. The adsorption energy is deﬁned as E¼ E_adþ E_by ðE_slabþ E_TMAÞ, where E_ad; E_by, E_slab, and E_TMA correspond to the energy of (slab with the adduct), (gaseous byproduct), (clean slab), and (gaseous TMA molecule), respectively.

The projected density of state (PDOS) data was obtained with a 991 Monkhorst-Pack k-point mesh.

Experimentally, the TiO2thinfilms (ca. 20 nm) were grown via ALD on a thermally grown SiO2(300 nm)/Si substrate at 300C in a 4-inch traveling-wave ALD reactor (CN-1 Co., Atomic Classic). The purge/carrier gas was high-purity Ar (99.9999%) with aflow rate of 200 sccm (standard cubic centimeter per minute). Titanium tet- raisopropoxide (TTIP) and H₂O were used as the Ti precursor and oxygen source, respectively. TTIP was heated to 70C in a bubbler type canister. The pulse times of TTIP and H2O were 1 and 2 s, respectively, with a purge time of 10 s in each case. For the formation of the Al₂O₃/TiO₂heterostructure, Al₂O₃films (6 nm) were grown by means of ALD at temperatures ranging from 150C to 300C on the pre-grown TiO2films. Trimethylaluminum (TMA) and H₂O were used as the Al precursor (pulse time: 0.7 s) and oxygen source (pulse time: 1 s), respectively. The precursor and reactant purge times were 10 s for both purge steps. The growth rates of Al₂O₃and TiO₂films were ~1.1 and 0.38 Å/cycle, respectively. The thickness of eachfilm was measured via spectroscopic ellipsometry (J.A. Woolam Co., EC110). For the contact with the Al2O3/TiO2het- erostructure, in the electrical measurements, indium electrodes were formed by soldering at each corner of square samples (1515 mm) obtained from the structure. The sheet resistance and carrier density were measured with a Hall measurement system (Ecopia Co., HMS-3000) using the standard van der Pauw config- uration at 25C.

3. Results and discussion

The effective Hubbard U parameter Ueff, deﬁned as the differ- ence between on-site Coulomb term (U) and exchange term (J), has a strong effect on geometrical structures and electronic properties.

In previous studies, Ueffvalues of ca. 3e5 eV were found to be optimal for GGAþU calculations of TiO₂[57e59]. The parameters for optimal calculation in this study were determined as follows.

First, the geometry of the tetragonal 4/mmm bulk unit cell of anatase TiO₂was obtained as a function of U_eff(Fig. S1[aec], Sup- porting Information). In terms of the Poisson ratio (c/a) and TieO bond lengths, small Ueffvalues close to 2 eV yielded values close to the experimentally known values [60]. The bulk band gap (Eg) was then evaluated (Fig. S1[d], Supporting Information), and values approaching Ueff~ 10 eV yielded values close to the experimental value (Eg¼3.2 eV [61]). Therefore, to accurately model both the surface chemistry (which would be strongly dependent on the geometry of the substrate) and electronic structures, an intermediate value of Ueff¼3 eV was chosen.

The mechanism for the chemical formation of V_Oon the TiO₂ surface by the TMA precursor was investigated, as shown inFig. 1 and Fig. S2, Supporting Information. Initially, molecular adsorption of TMA would occur on the bridging oxygen (O_2c), forming OeAl coordinate covalent bonds with signiﬁcant exothermicity (1.28 eV). In a previous study regarding ALD of Al2O3on TiO2[62], hydration of the surface was assumed, and therefore, H-transfer from surface OH to CH₃ ligands was considered as the step following molecular adsorption. However, the expectation was that the small concentration of OH groups would be depleted during initial exposure to TMA, thereby allowing the occurrence of reductive reactions [63]. Therefore, in this study, the surface- binding reaction of the CH3 ligands on the bare TiO2 surface

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without OH was investigated.

Among the possible adsorption sites for CH₃, the lattice oxygen atoms on the anatase (101) surface exist as either in-plane oxygen (O3c) or bridging oxygen (O2c). The removal of O2cis easier than the removal of O_3c[64], and hence, O_2cwas considered the binding site for CH3. The ﬁrst CH3 ligand dissociation step from Al onto O2c

involved a transition state of 0.38 eV (activation energy, Ea ¼ 1.66 eV). Afterward, a stable adsorption conﬁguration of surface-bound methoxy, *OCH3 occurred at 1.79 eV. For this conﬁguration, the Al of the *Al(CH3)2 moiety becomes bridging between two neighboring O_2catoms, thereby providing additional stabilization. The direct desorption of OCH3from*OCH3required a fairly high energy of 4.29 eV (2.50 eV above vacuum). Moreover, dissociation of*OCH3into surface*H and removal of formaldehyde (CH₂O) including an O_2catom require exothermicity of more than 3 eV above vacuum. Therefore, occurrence of these pathways can be expected quite unlikely. Instead, the second CH3 ligand on

*Al(CH₃)₂could also migrate to the same O_2cthrough a transition state of 0.75 eV (Ea¼2.54 eV), leading to the formation of adsorbed dimethylether,*CH3OCH3. This second ligand dissociation step can be considered rate-determining. The conﬁguration of (*CH3OCH3 þ *AlCH3) was also reasonably stable with E ¼ 0.62 eV. Afterward, CH3OCH3 may desorb molecularly, requiring a desorption energy of 1.35 eV (0.73 eV above vacuum), leaving*Al(CH3) and a VOon the surface. An alternative mechanism involved adsorption of CH3on Ti5c(Fig. S2, Supporting Informa- tion); however, considerably greater Ea and endothermicity than those of the desorption of dimethylether process were found for such reactions. Compared with the formation of VO (formation

energy: ca. 4 eV) on the bare TiO2 surfaces [65], the currently identiﬁed chemical mechanism is signiﬁcantly more facile. It is noted that current study assumed reaction of the defect-free lowest-energy surface of single crystalline anatase (101); since desorption of surface species would occur more easily from defective surfaces [66,67], actual reaction of TMA from realistic ALD-TiO2 surfaces with defects can be expected to involve less barrier and endothermicity. Therefore, dimethylether production via direct methylation of O2c can be considered the preferred pathway for removal of the lattice oxygen by TMA.

The properties of the 2DEG formed at the Al₂O₃/TiO₂interface exhibited a strong dependence on the Al2O3 ALD temperature (Fig. 2). The sheet resistance of the 2DEG showed a logarithmic decrease with the increasing process temperature of Al2O3 ALD (Fig. 2a). Therefore, unlike those of perovskite heterointerface 2DEGs, the properties of the currently developed ALD oxide 2DEG can be easily modulated by simply changing the process temperature. Applying linear regression to the pseudo-Arrhenius plot of the electronic carrier density (ns) against 1/T (Fig. 2b; T: absolute temperature for the ALD process) yielded an apparent activation energy of ca. 0.74 eV (R²¼0.99). With the increasing temperature, the above-described reaction for the formation of CH3OCH3and VO

by TMA via an endothermic process involving activation barriers was facilitated. Assuming that n_sof 2DEG is proportional to the concentration of VO, the proposed reaction mechanism for VOfor- mation (as determined through the DFT calculation) seems reasonable.

The projected density of states (PDOS) plots and partial charge density isosurfaces associated with the formation of VOon the Fig. 1.The proposed energy diagram for adsorption of TMA on TiO2when methyl ligand is directly bound to the bridging oxygen (O2c). The red, green, and blue pathways represent mechanisms that generate dimethylether (CH3OCH3), methoxy (OCH3), and formaldehyde (CH2O) as gaseous byproducts, respectively. In the chemical structures, skyblue¼Ti, gray¼Al, red¼O, gray¼C, and white¼H atoms, respectively.

Fig. 2.Experimentally observed dependence of the interfacial-2DEG properties on the ALD process temperature of Al2O3: a) sheet resistance and b) pseudo-Arrhenius plot of carrier density (ns).

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surface are shown inFig. 3; the positions of the energy bands and defect states are summarized inTable 1. From the PDOS results, a band gap of ~2.25 eV was determined for the pure 12-layer TiO2

slab. This value is consistent with that reported in previous DFTþU studies [57e59], but it is lower than the reported experimental value (ca. 3.2 eV) [61]. The valence and conduction bands are mainly composed of O 2p and Ti 3d orbitals, respectively. The intrinsic Fermi level is located near the valence band edge (VBM,0.31 eV). However, with the formation of VOand*Al(CH3), the Fermi level shifted toward the conduction band edge. Moreover, a new n-type Ti 3d state occurred below the Fermi level at0.03 eV. Such midgap defect states can be associated with the presence of VOand corresponding Ti^3þ [68]. Geometrically, this state is localized at the location of the surface V_O. An increase in the areal density of [VOþ*Al(CH3)] to two per computational unit cell (corresponding to ca. 1.710¹⁴cm²) led to a more pronounced expression of the shallow defect state (0.01 eV). A two- dimensional-like localization of this state was also observed.

Theseﬁndings suggest that the [VOþ*Al(CH3)] structure formed by TMA can provide a shallow-donor level localized at the Al₂O₃/TiO₂ interface, conﬁrming the electronic observations reported for recent experiments [6,33e35].

4. Conclusions

In this study, the chemical mechanism for the formation of 2DEGs localized at the Al₂O₃/TiO₂ heterostructures via ALD was investigated through quantum mechanical DFTþU calculations.

Consecutive migration of the CH3ligands from the TMA precursor to the lattice oxygen of the TiO₂ surface can form dimethyle- therdthe desorption of which leaves oxygen vacancies on the surface. The endothermicity of such a reaction was experimentally conﬁrmed by the dependence of the carrier concentration on the ALD process temperature of Al₂O₃. The electronic structure of the TiO2 slab model conﬁrmed VO-induced formation of a surface- localized shallow n-type dopant state. This study provides theoretical understanding of mass-producible ALD-based oxide heterostructure 2DEGs.

Credit author statement

Jeongwoo Park: Methodology, Validation, Formal analysis, Investigation, Data curation, Writingeoriginal draft, Visualization, Hyobin Eom: Software, Formal analysis, Investigation, Data curation, Writing e original draft, Visualization, Seong Hwan Kim:

Fig. 3.Projected density of states plots (left) and partial charge density isosurfaces (right, yellow) of the clean and TMA-modiﬁed TiO2surfaces.

Table 1

Valence band maximum, conduction band minimum, and band gap energy (Eg) associated with the formation of pure TiO2and oxygen vacancies (VO). The state between the band gaps represents the defect level (Ti^3þ) induced by Vo.

Species VBM (eV) CBM (eV) Band gap (eV) Defect level energy range (eV)

Pure TiO2 0.31 1.94 2.25 N/A

1VO 1.59 0.70 2.29 0.43e0.03

2VO 1.93 0.35 2.28 0.95e0.01

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Methodology, Validation, Investigation, Data curation, Tae Jun Seok: Resources, Data curation, Writing eoriginal draft, Visuali- zation, Tae Joo Park: Resources, Writing e review & editing, Funding acquisition, Sang Woon Lee: Resources, Formal analysis, Writingereview&editing, Funding acquisition, Bonggeun Shong:

Conceptualization, Writingereview&editing, Supervision, Project administration, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing ﬁnancial interests or personal relationships that could have appeared to inﬂuence the work reported in this paper.

Acknowledgement

This work was supported by the Samsung Research Funding&

Incubation Center of Samsung Electronics (SRFC-TA 1903-03), by Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Government (MOTIE) (P0012451, The Competency Development Program for Industry Specialist), and by the National Supercomputing Center including supercomputing resources and technical support (KSC-2020-CRE-0082).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtadv.2021.100195.

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