Free Access
Issue
Eur. Phys. J. Appl. Phys.
Volume 93, Number 2, February 2021
Article Number 21302
Number of page(s) 11
Section Surfaces and Interfaces
DOI https://doi.org/10.1051/epjap/2021200326
Published online 18 February 2021

© EDP Sciences, 2021

1 Introduction

Two dimensional electron gas (2DEG) is a system, where low-density electrons are limited to the free movement in two directions. For conventional GaAs-based 2DEGs, the conducting electrons are s or p electrons, only exhibiting the charge property due to the lack of interaction between electrons. However, since Ohtomo et al. [1] have reported oxide 2DEG at the interface between LaAlO3 (LAO) and SrTiO3 (STO), the oxide 2DEG has attracted many researchers' attention. Because of the strong correlation between 3d electrons, the oxide 2DEG displays magnetism, ferroelectric, spin polar or spin orbit coupling characteristics [28]. Moreover, owing to the strong interaction between the spin, charge, lattice and orbital etc., the STO-based two dimensional conductive interface has exhibited fascinating physical properties, including a high electron mobility, two dimensional superconductivity and ferromagnetism [9]. Many emerging properties have been extensively reviewed in other venues, for examples, Christensen et al. [9], Mannhart and Schlom [10], Hwang et al. [11], Pai et al. [12], Chen Yunzhong et al. [13,14], Huang et al. [15] and Yan et al. [16].

In oxide 2DEGs, the STO is the workhorse in oxide-electronics as Si in microelectronics. At room temperature, the STO crystal is a cubic Pm3m space group and its lattice parameter is 3.905 Å, which is similar to that of many other perovskite oxides and thus commonly used as their substrates. Especially, the lattice parameter of Si closely matches with the STO and the lattice mismatch of STO/Si is less than 2% [17]. It is hopeful that STO will become a candidate in transistor devices. Moreover, the STO with favorable stability hardly reacts with other materials, contributing to the formation of epitaxial heterointerfaces with various oxides (CaZrO3, EuO, GdTiO3, NdGaO3, and so on) [1824]. In addition, the STO has been extensively applied in the fields of ferroelectric memory, pyroelectric and microwave-controlled devices etc. [2528]. The applications of these devices are mostly based on their high permittivity (300 F/m), low dielectric loss and high critical breakdown field. Furthermore, the STO has the large bandgap (the indirect bandgap width is 3.25 eV and the direct bandgap width is 3.75 eV), meaning that it is a good insulator [29,30]. Because the bottom of conduction band is mainly composed of the Ti 3d electronic orbit [31], the STO-based conductive material can be formed by n type [3234] or p type [3537] doping. It can display d electronic characteristics by the filling of different d orbits. Thus, the STO-based 2DEG has a great potential in the application of oxide electronic devices. In this review, we summarized the recent progress about the fabrication methods of two dimensional conductive STO-based interfaces.

2 Fabrication methods of STO-based two dimensional conductive interfaces

As shown in Figure 1, the fabrication methods can be divided into two categories: (1) the epitaxy growth of heterointerface, (2) the surface treatment. The epitaxy growth includes the physical vapor deposition (PVD) and the chemical deposition (CD), such as PLD, the molecular beam epitaxy (MBE), the ALD and the spin coating et al. The surface treatments involve high vacuum annealing, Ar+ ion irradiation, ion doping and laser irradiation et al. In the following sections, we will discuss the different fabrication methods in details.

thumbnail Fig. 1

The diagram for the routine to obtain STO-based two dimensional conductive interface.

2.1 PLD method

PLD is one of the representative PVD technologies. With the invention of high-energy excimer laser, Dijkkamp et al. [38] firstly prepared the thin films of Y-Ba-Cu-O superconductors, thus causing the rapid development of PLD technology and gradually becoming one of the most widely applied deposition in the field. The basic process includes the bombardment of high-energy pulsed laser and the ablation of ceramic target. Ceramic target can be gasified into the plasma rapidly, and then the thin film is formed at the surface of single crystal substrates with the directional expansion of plume. The schematic diagram is shown in Figure 2.

In oxide heterointerfaces, Ohmoto and Hwang firstly reported a high-mobility electron gas at LAO/STO heterointerfaces by the PLD technology [1,39]. The PLD can atomically control the heteroepitaxial growth and form polarity discontinuities at interfaces. Then, the PLD technology has become a preferable method to prepare the oxide heterointerfaces. Many groups have fabricated a series of oxide heterointerfaces to reveal the two dimensional conductive behavior. For examples, Chen et al. have fabricated a high-mobility 2DEG at the γ-Al2O3/STO interface by PLD, and the mobility reaches to 1.4 × 105 cm2V−1s−1 [13,40,41]. Moreover, various (NdGaO3/STO, CaZrO3/STO, LaVO3/STO, Al2O3/STO and amorphous LAO/STO etc. [4246]) and multifunctional (ferromagnetism, superconductivity or their coexistence [4755]) oxide heterointerfaces have been prepared by the PLD technology. Meanwhile, Yan et al. have obtained the LAO/STO and (La0.3Sr0.7)(Al0.65Ta0.35)O3/SrTiO3 (LSAT/STO) heterointerfaces with different thicknesses and strains [56] (as shown in Fig. 3), and further found that the photoinduced resistance changes of LAO/STO interfaces presented a roughly greater value than that of strain-relaxed LSAT/STO due to the larger lattice mismatch. The PLD advantages are as following: 1. the PLD technology can prepare multicomponent complex films with the same stoichiometric ratio of target; 2. the high-energy pulsed laser can grow the film of high-melting-point materials; 3. it can precisely control the growing rate layer-by-layer; 4. it can flexibly adjust atmosphere, temperature, energy or other parameters.

thumbnail Fig. 2

Schematic diagram of PLD equipment.

thumbnail Fig. 3

(a–c) Temperature dependence of the sheet resistance, carrier density (n) and electron mobility (μ) for LAO/STO heterointerfaces with different thicknesses in dark, respectively. (d–f) Temperature dependence of sheet resistance, carrier density (n) and mobility (μ) for LSAT/STO heterointerfaces with different thicknesses in dark, respectively [56].

2.2 MBE method

MBE is another PVD technique to produce high-quality epitaxial films. For traditional MBE, firstly, the source material is heated in the ultra-high vacuum (∼10−10 Torr). Secondly, the evaporated atoms or molecules are ejected to the surface of substrate. Finally, the epitaxial growth of material is realized through the surface adsorption, migration and nucleation. The schematic diagram is shown in Figure 4. Compared with the PLD, the main difference is the heating method of source materials (thermal evaporation). The generated steady-state beam typically has the low energy of 1.0 eV or less [57], which successfully avoids the extrinsic defects in the substrate and is beneficial to the formation of a sharp heterointerface. Moreover, the ultra-high vacuum environment and low growing rate contribute to the growth of high-purity and high-crystallinity films. Furthermore, combined with the Reflection High Energy Electron Diffraction (RHEED), Mass Spectrometer (MS) and Auger Electron Spectrometer (AES), the MBE technique can not only monitor the molecular beam flux and the surface structure of film, but also fabricate epitaxial films with precise atomic control layer-by-layer.

Since Cho et al. [58,59] proposed the MBE technique in the 1960s, it has been widely applied to fabricate the various multilayer superlattice [60], quantum well [61], two dimensional conductive material [62,63], especially in the preparation of oxide two-dimensional electron liquid (2DEL). As shown in Figure 5, Warusawithana et al. [64] prepared perfect LAO/STO interfaces without extrinsic defects including the absence of oxygen deficiency. Moreover, by controlling the shutter-open times of La source and Al source, different stoichiometric LAO films are deposited on STO substrates. The results revealed that the cation stoichiometry of LAO film was a critical factor to form 2DEL. Through first principle calculations, they indicated that the origin of 2DEL was the intrinsic electronic reconstruction. Besides, due to the manageable, flawless, slow-growing advantages, Tsai et al. [65] also have prepared a series of high quality LAO films with different La/Al ratios and thicknesses by MBE technique. They showed that the control of La/Al ratio was an effective method to tune the carrier concentration of 2DEL.

thumbnail Fig. 4

Schematic diagram of MBE equipment.

thumbnail Fig. 5

Flux gradients and interfacial conductivity of mosaic samples. (a) Mosaic arrangement of substrates for each growth. (b) Temperature dependence of resistance of the 2DEL is plotted for a representative set of conducting samples from the mosaic growths. The samples are labelled with the mosaic number followed by substrate number—for example, 2–1D indicates mosaic 2, piece D of substrate 1. (c) A representative low-temperature resistance versus temperature plot shows the 2-DEL is superconducting. The scaling between the resistance and the sheet resistance is approximate [64].

2.3 ALD method

ALD technique is a special chemical deposition technique, which can achieve the sequential, self-limiting and self-saturating reaction at surface and is suitable for complex substrates [66,67]. The major process of ALD is a gas-solid chemisorption reaction, as shown in Figure 6, the reaction pathway consists of four primitive steps: 1. the pulsed adsorption reaction of precursor A (trimethylaluminum, Me3Al); 2. the purgation of redundant reactants and by-products via noble gas; 3. the pulsed adsorption reaction of precursor B (H2O); 4. the secondary purgation of redundant reactants and by-products via noble gas. Then, the film grows layer-by-layer on the substrate by the cycle of above reaction [68]. Based on the reaction mechanism, choosing a proper chemical reaction is the key issue in the ALD method, such as the need of precursor with excellent thermostability, reactivity and volatility etc. Therefore, only a small number of proper chemical reactions have been utilized in ALD process, limiting the application of ALD technique [69,70].

With the continual improvement of chip's integration level, the ALD technique exhibits considerable potential applications in the preparation of small-scale and complex devices. For examples, in the field of transistor, Bent et al. [71] have deposited the Al2O3 insulation layer via the particularity of chemisorption reaction. In the field of energy storage devices, Kozen et al. [72] have fabricated Al2O3 layer to protect the Li electrode due to the large-area uniformity property of ALD technique. Moreover, a few groups also have launched the profound and systematical research on the oxide 2DEG. For example, Lee et al. [7375] have prepared a series of amorphous oxide layers (LaAlO3, YAlO3, Al2O3, Y2O3, La2O3) at STO substrates by ALD. They found that only the LaAlO3, YAlO3 or Al2O3 layers could produce the 2DEG. The key factor is the oxidation of Me3Al precursor because the Me3Al can be oxidized by the TiO2 of STO termination in ALD process. The possible detailed reactions are the following:(1)or(2)

The reactions show that the change of Gibbs free energies (ΔG r) is negative at 573 K. The reaction pathway (1) or (2) is dependent on the stronger Al-O bond under the reduction of STO substrate by Me3Al. Furthermore, as shown in Figure 7, the amorphous Al2O3 layer has the critical thickness, meaning that the activation energy can be overcome by the enough thermal energy when the thickness of Al2O3 film is larger than the critical value at the appropriate thermodynamic condition.

thumbnail Fig. 6

Schematic diagram of ALD technique.

thumbnail Fig. 7

(a) A schematic of creation mechanism of 2DEG by the diffused-out oxygen atoms from STO surface through the grown amorphous Al2O3 layer by ALD. (b) The variation in the electron densities at Al2O3/STO heterostructures depending on the Al2O3 film thickness. (c) A transmission rate of oxygen atoms through the Al2O3 layer as a function of Al2O3 thickness using the value of 2.5 × 1010 oxygen atoms per cm2 per s at a 1 nm thickness (d) the number of total oxygen atoms transmitted during Al2O3 ALD was calculated using equation (1), and the experimental curve is obtained by dividing the number of electrons in (b) by 2 (assuming that one oxygen vacancy generates two free electrons) [74].

2.4 Spin coating method

As one of the different methods, the spin coating is an important and widely-used processing technique. The typical process is mainly divided into two steps: precursor solution dropping, high speed rotation and crystallization. Firstly, a drop of precursor solution is placed on a spinning flat substrate. Then the liquid spreads outward and forms a quite thin, uniform and multicomponent coating by the centrifugal force [76]. This scenario is depicted in Figure 8, taking YAlO3/STO heterointerfaces as an example [77]. The film is easily controlled by the spin speed, the viscosity of solution, the composition of precursor liquid and the annealing temperature et al. So, the spin coating method has been widely applied in the semiconductors, solar cells, diode and transistors devices [7882].

The spin coating technique has drawn more and more attentions due to its obvious advantages of mild process conditions, simple facilities, easy operation and control, etc. Specifically, the spin coating technique has been successfully applied to prepare the oxide 2DEG. Recently, the obtained 2DEGs at the (001), (011) and (111)-oriented LAO/STO interfaces not only exhibited excellent metallic conductive behavior but also had a high carrier mobility [8,83]. Meanwhile, we also have prepared a series of oxide 2DEGs by the spin coating method (such as γ-Al2O3, YAlO3 and Y2O3). As shown in Figure 9, the TEM results exhibited that the prepared γ-Al2O3 layer had high crystallinity and grew epitaxially along the (001) orientation of STO. The abrupt interface reflected no cation interdiffusion according to EDS analysis [84]. Moreover, through preparing YAlO3/STO heterointerfaces with different thicknesses and stoichiometry ratios, we revealed that the origin of metallicity was attributed to oxygen vacancies from the redox reaction between yttrium aluminum oxides film and STO substrates [77]. The results show that the spin coating technique has an enormous potential in the fabrication of oxide 2DEGs. Compared with the PLD, MBE and ALD, this technique needs no sophisticated facility and rigorous chemical reaction, avoiding the laser bombardment. However, the drawback is that it is hard to fabricate ultrathin film and precisely control the thickness by spin coating method.

thumbnail Fig. 8

Schematic diagram for preparing the YAlO3/STO heterointerfaces by the spin coating method [75].

thumbnail Fig. 9

(a) and (b) TEM images of Al2O3/STO heterointerfaces annealed at 800 °C. (c) and (d) Selected-area electron diffraction patterns of square in (b); weak spots (yellow spots) originate from γ-Al2O3. (e) The HAADF image of the Al2O3/STO heterointerface annealed at 800 °C. (f)–(i) EDS maps of the Al, Sr, Ti, and O concentration corresponding to (e) [84].

2.5 High-vacuum annealing

Generally, the fabrication methods of STO-based two dimensional conductive interface are focused on various epitaxial growth techniques. Based on the deposition of heterointerfaces between different oxides and STO substrates, the metallic-like conductive interfaces are formed. The main conductive mechanisms were attributed to the polar catastrophe, oxygen vacancy, and so on. However, there are many other ways to generate the two dimensional conductive surface at STO. For example, the high-vacuum annealing can form oxygen defects and electron doping at STO surface [85,86]. Thereby, the quasi-two dimensional conductive layer can be generated at STO by the surface treatment. Based on this technique, we have prepared self-doped STO by the thermal annealing in high vacuum (∼10−6 mbar) at 550, 650, 750, and 850 °C [87]. As shown in Figure 10, the results reveal that the samples with high annealing temperature (≥650 °C) exhibits an excellent metal-like conductive behavior, and the conductivities are monotonously increased with the increase of annealing temperature. The Hall effect measurement (20 K) shows that the electron density is 1.2 × 1017 cm−2 and the carrier mobility is 5700 cm2 · V−1 · s−1 for the sample annealed at 750 °C. Moreover, the annealing can effectively tune the electronic transport properties of STO and the resistance under irradiation is changed by the twelve orders of magnitude at 20 K. In addition, the oxygen-deficient oxides still exhibited many peculiar physical properties as same as the 2DEG at LAO/STO interfaces, such as persistent photoconductivity [88,89], magnetism [90] or their coexistence [91].

thumbnail Fig. 10

(a) Temperature dependent sheet resistance data measured on STO single crystals annealed in vacuum at temperatures of 550 °C (S55), 650 °C (S65), 750 °C (S75), and 850 °C (S85). The resistance of the as-received STO substrate exceeds the measurement limit. The corresponding optical images of the reduced STO substrates are shown in the insets. Also illustrated is the atomic perovskite structure of STO, where the green, blue and red balls represent Sr, Ti and O atoms, respectively. (b) Semi-logarithmic plots of the room temperature current-voltage characteristics measured in various reduced STO samples both in the dark (dashed lines) and under UV light illumination. Inset shows the schematic of the Pt/STO/Al device [87].

2.6 Ar+ ion irradiation

The low-energy Ar+ ion beam irradiation is another method to generate oxygen vacancy at STO surfaces. The conductive mechanism is consistent with that of the high vacuum annealing. When the Ar+ ion bombards the STO surface, the O atoms have the faster diffusion [92]. Then, a certain amount of oxygen vacancies is generated at STO surfaces, that is, the quasi-two dimensional conductive surface is formed by the Ar+ ion irradiation. Moreover, the irradiation time and energy of Ar+ ion can directly influence the conductive property and the thickness of conductive layer [9397]. As shown in Figure 11, Reagor et al. [96] have made a series of samples with different energy levels. Firstly, the metallic-like conductive behavior is found in the samples at the energy of above 200 eV. Secondly, the residual resistance ratio is up to about 100 for the sample formed by the beam energy of nearly 300 eV. The high residual resistance ratio indicated a highly ordered conduction channel was produced at the surface. Thirdly, the resistivity vs. temperature relationship implied that the conduction mechanism was a commonly electron-doped conduction behavior. In addition, the Ar+ ion-irradiated samples still show other physical phenomena, such as, anisotropic magnetotransport [98], spin-orbit coupling [94,99], blue-light emission [100], persistent photoconductivity [93,101] and so on. Overall, compared with high vacuum annealing, the Ar+ ion irradiation has two advantages: 1. the spatial distribution of oxygen vacancies is localized because the depth of oxygen vacancy strongly depends on the penetration depth of Ar+ ion. 2. the conductive domain can be easily controlled.

thumbnail Fig. 11

Four-probe resistivity versus temperature for several samples. The ion-bombarded samples at the lowest energies had a contact dresistance that diverged at low temperatures, and only the high-temperature data are shown [96].

2.7 Others

According to the results mentioned above, it is likely to find that the fabrication methods can significantly manipulate the conductive properties of STO-based interfaces. Therefore, a number of researchers are engaged in the exploration of novel preparation methods. And many other methods have also exhibited a potential to fabricate STO-based two dimensional conductive interfaces, such as laser irradiation [102], photoirradiation [103] and ion doping [104,105] etc. As shown in Figure 12, Zhang et al. [102] have realized the metal-like STO surface via KrF laser irradiation, which strongly depends on the vacuum and laser energy density. Like as Ar+ ion irradiation, the conductive mechanism was caused by oxygen vacancy due to the laser ablation. Besides, the UV light of 300 nm and the Cr, Nb, La ion doping can also induce insulator-metal phase transition of bare STO single crystals [33,103106].

thumbnail Fig. 12

(a) Temperature dependence of the sheet resistance in air and the partial pressure of ∼5, 5 × 10−2 and 5 × 10−4 Pa at the laser energy density of 500 mJ/cm2. (b) The sheet resistance of irradiated STO as a function of temperature at different energy densities in the partial pressure of 5 × 10−4 Pa [102].

2.8 Comparison of different methods

Finally, we summarize the characteristics of different methods, the merits and demerits are shown in Table 1. The results would provide a favorable reference for the preparation of STO-based two dimensional conductive interfaces. For examples, the chemical deposition of ALD can be used to grow a large-area film on the complex substrate and the conductive mechanism can be illustrated from an aspect of redox reaction. In addition, the high-melting-point materials can apply the PLD. And the MBE or spin coating is favorable to the fabrication of multicomponent and doped film, and so on.

Table 1

Comparison of different methods.

3 Conclusion and perspectives

Although the achievements are obtained over recent years, many challenges of all-oxide interfaces are still remained. For examples, the origin of oxide-2DEG is still controversial, and the defects (such as oxygen defects, nonstoichiometries, cation intermixing, and nonhomogeneities) are inevitable in the heterointerfaces [107,108]. In addition, the mobilities can be influenced by the quality of sample, the composition of capping layer and the altering of sample structure. And it is not even clear how these properties are limited. Therefore, in order to reveal these problems, the modified techniques and other novel fabrication methods need to be proposed to realize more perfect heterointerfaces.

Moreover, with the development of multifunctional and complicated oxide electronic devices, the combination of the situ monitoring RHEED and the advanced characterization techniques (such as MS, AES, X-ray photoelectron spectroscopy and nondestructive resonant X-ray reflectivity et al.) has more significance in improving the quality of heterointerfaces. Meanwhile, the assisting of external stimuli also can reduce the defects in the preparation. Inspired by these, the next challenge is to explore the proper oxides and fabrication methods, design an excellent microstructure, apply a single or multiple external stimuli to product multifunctional complex oxide devices (such as quantum devices, tunneling junctions or superconducting qubits). We hope that the review paves a way for the real application of oxide electronics and electronic devices.

Author contribution statement

Ming Li contributed to the data analysis, interpretation and writing of the manuscript. Prof. Shuanhu Wang and Prof. Yang Zhao contributed to the concept design. Prof. Kexin Jin contributed to the concept design and revising the manuscript. All authors discussed the results and commented on the manuscript. The authors declare no competing financial interests.

Acknowledgments

Project supported by the National Natural Science Foundation of China (Nos. 51572222 and 61471301) and the Fundamental Research Funds for the Central Universities (Grant No. 3102017jc01001).

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Cite this article as: Ming Li, Shuanhu Wang, Yang Zhao, Kexin Jin, Review on fabrication methods of SrTiO3-based two dimensional conductive interfaces, Eur. Phys. J. Appl. Phys. 93, 21302 (2021)

All Tables

Table 1

Comparison of different methods.

All Figures

thumbnail Fig. 1

The diagram for the routine to obtain STO-based two dimensional conductive interface.

In the text
thumbnail Fig. 2

Schematic diagram of PLD equipment.

In the text
thumbnail Fig. 3

(a–c) Temperature dependence of the sheet resistance, carrier density (n) and electron mobility (μ) for LAO/STO heterointerfaces with different thicknesses in dark, respectively. (d–f) Temperature dependence of sheet resistance, carrier density (n) and mobility (μ) for LSAT/STO heterointerfaces with different thicknesses in dark, respectively [56].

In the text
thumbnail Fig. 4

Schematic diagram of MBE equipment.

In the text
thumbnail Fig. 5

Flux gradients and interfacial conductivity of mosaic samples. (a) Mosaic arrangement of substrates for each growth. (b) Temperature dependence of resistance of the 2DEL is plotted for a representative set of conducting samples from the mosaic growths. The samples are labelled with the mosaic number followed by substrate number—for example, 2–1D indicates mosaic 2, piece D of substrate 1. (c) A representative low-temperature resistance versus temperature plot shows the 2-DEL is superconducting. The scaling between the resistance and the sheet resistance is approximate [64].

In the text
thumbnail Fig. 6

Schematic diagram of ALD technique.

In the text
thumbnail Fig. 7

(a) A schematic of creation mechanism of 2DEG by the diffused-out oxygen atoms from STO surface through the grown amorphous Al2O3 layer by ALD. (b) The variation in the electron densities at Al2O3/STO heterostructures depending on the Al2O3 film thickness. (c) A transmission rate of oxygen atoms through the Al2O3 layer as a function of Al2O3 thickness using the value of 2.5 × 1010 oxygen atoms per cm2 per s at a 1 nm thickness (d) the number of total oxygen atoms transmitted during Al2O3 ALD was calculated using equation (1), and the experimental curve is obtained by dividing the number of electrons in (b) by 2 (assuming that one oxygen vacancy generates two free electrons) [74].

In the text
thumbnail Fig. 8

Schematic diagram for preparing the YAlO3/STO heterointerfaces by the spin coating method [75].

In the text
thumbnail Fig. 9

(a) and (b) TEM images of Al2O3/STO heterointerfaces annealed at 800 °C. (c) and (d) Selected-area electron diffraction patterns of square in (b); weak spots (yellow spots) originate from γ-Al2O3. (e) The HAADF image of the Al2O3/STO heterointerface annealed at 800 °C. (f)–(i) EDS maps of the Al, Sr, Ti, and O concentration corresponding to (e) [84].

In the text
thumbnail Fig. 10

(a) Temperature dependent sheet resistance data measured on STO single crystals annealed in vacuum at temperatures of 550 °C (S55), 650 °C (S65), 750 °C (S75), and 850 °C (S85). The resistance of the as-received STO substrate exceeds the measurement limit. The corresponding optical images of the reduced STO substrates are shown in the insets. Also illustrated is the atomic perovskite structure of STO, where the green, blue and red balls represent Sr, Ti and O atoms, respectively. (b) Semi-logarithmic plots of the room temperature current-voltage characteristics measured in various reduced STO samples both in the dark (dashed lines) and under UV light illumination. Inset shows the schematic of the Pt/STO/Al device [87].

In the text
thumbnail Fig. 11

Four-probe resistivity versus temperature for several samples. The ion-bombarded samples at the lowest energies had a contact dresistance that diverged at low temperatures, and only the high-temperature data are shown [96].

In the text
thumbnail Fig. 12

(a) Temperature dependence of the sheet resistance in air and the partial pressure of ∼5, 5 × 10−2 and 5 × 10−4 Pa at the laser energy density of 500 mJ/cm2. (b) The sheet resistance of irradiated STO as a function of temperature at different energy densities in the partial pressure of 5 × 10−4 Pa [102].

In the text

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