Epitaxial SnO2 Research Articles

Tuning band gap and ferromagnetism in epitaxial Al-doped SnO2 films by defect engineering

The role of acceptor and donor defects in epitaxial Al-doped SnO2 films were systematically investigated. Al doping introduces acceptor defects (AlSn) at low doping concentration while donor ones (Ali) in a concentration range from 8 to 10at%. The band gap is firstly narrowed by hole-doping and then widened by electrons introduced by oxygen vacancies and Ali. Air-annealing absorbs oxygen and makes Al ions transformed from AlSn to Ali, corresponding to a decrease (increase) in the band gap when most Al ions occupy the substitutional (interstitial) sites. The saturation magnetization of the films is enhanced by AlSn doping, with the maximum value in the Sn0.98Al0.02O2 film. The magnetic moment is contributed by the localized holes introduced by AlSn. The existence of the ferromagnetism induced by holes in the film with oxygen vacancy gives a new insight into the behavior of defects in SnO2.

High electron mobility in epitaxial SnO2−x in semiconducting regime

We investigated the electronic transport properties of epitaxial SnO2−x thin films on r-plane sapphire substrates. The films were grown by pulsed laser deposition technique and its epitaxial growth direction was [101] and the in-plane alignment was of SnO2−x [010]//Al2O3[12̄10]. When the SnO2−x films were grown in the oxygen pressure of 30 mTorr, we have found the electron mobility of the 30 nm thick SnO2−x thin films strongly dependent on the thicknesses of the fully oxidized insulating SnO2 buffer layer. When the buffer layer thickness increased from 100 nm to 700 nm, the electron mobility of values increased from 23 cm2 V−1 s−1 to 106 cm2 V−1 s−1 and the carrier density increased from 9 × 1017 cm−3 to 3 × 1018 cm−3, which we attribute to reduction of large density of dislocations as the buffer layer thickness increases. In addition, we studied the doping dependence of the electron mobility of SnO2−x thin films grown on top of 500 nm thick insulating SnO2 buffer layers. The oxygen vacancy doping level was controlled by the oxygen pressure during deposition. As the oxygen pressure increased to 47.5 mTorr, the carrier density was found to decrease to 9.1 × 1016 cm−3 and the electron mobility values to 13 cm2 V−1 s−1, which is consistent with the dislocation limited transport properties. We also checked the conductance change of the SnO2−x during thermal annealing cycles, demonstrating unusual stability of its oxygen. The correlation between the electronic transport properties and microstructural defects investigated by the transmission electron microscopy was drawn. The excellent oxygen stability and high electron mobility of low carrier density SnO2−x films demonstrate its potential as a transparent oxide semiconductor.

Molecular beam epitaxy growth of SnO2 using a tin chemical precursor

The authors report on the development of a molecular beam epitaxy approach for atomic layer controlled growth of phase-pure, single-crystalline epitaxial SnO2 films with scalable growth rates using a highly volatile precursor (tetraethyltin) for tin and rf-oxygen plasma for oxygen. Smooth, epitaxial SnO2 (101) films on r-sapphire (101¯2) substrates were grown as a function of tin precursor flux and substrate temperatures between 300 and 900 °C. Three distinct growth regimes were identified where SnO2 films grew in a reaction-, flux-, and desorption-limited mode, respectively, with increasing substrate temperature. In particular, with increasing tin flux, the growth rates were found to increase and then saturate indicating any excess tin precursor desorbs above a critical beam equivalent pressure of tin precursor. Important implications of growth kinetic behaviors on the self-regulating stoichiometric growth of perovskite stannates are discussed.

Band gap widening and d 0 ferromagnetism in epitaxial Li-doped SnO 2 films

Abstract Epitaxial grown Li doped SnO2 films with room temperature d0 ferromagnetism were prepared by radio frequency magnetron sputtering. X-ray diffraction and X-ray photoelectron spectroscopy give clear evidence of Li presence at both substitutional and interstitial sites. A conversion of conductivity from n-type to p-type was observed as the Li concentration reaches 6 at.%. The band gap of films increased non-montonically with the Li concentration. All the films are ferromagnetic, and the largest saturation magnetization of 7.9 emu/cm3 is observed in the Sn0.88Li0.12O2 film which has the widest band gap. The consistency of the variation between the magnetic, structural, electrical and optical properties indicates that holes introduced by Li substitutions can enhance the ferromagnetism though p–p interaction.

Growth and surface properties of epitaxial SnO2

The surface potentials of SnO2 films grown epitaxially by magnetron sputtering on TiO2 and Al2O3 substrates with (110), (001), (101), and (100) SnO2 surface orientations are determined using in situ photoelectron spectroscopy. Epitaxial growth is verified using X-ray diffraction and low energy electron diffraction. The emphasis lies on the determination of work functions and ionization potentials of epitaxial SnO2 surfaces. SnO2 films prepared under chemically reducing conditions exhibit work functions φ of 4.25–4.48 eV and ionization potentials IP of 7.54–8.11 eV. It is furthermore demonstrated that a subsequent annealing in oxygen alters the surface dipole, visible through a large increase of ionization potential. This is due to a change of the surface termination from a reduced to a stoichiometric surface which exhibits Sn in the +IV oxidation state. The observed increase of IP varies from 0.48 eV for the (110) SnO2 surface to 1.05 eV for the (101) SnO2 surface.

Effect of heavy Ga doping on defect structure of SnO2 layers

Crystal defects in Ga‐doped SnO2 grown on SnO2 buffer layers on r‐plane sapphire by plasma‐assisted molecular beam epitaxy have been analysed by transmission electron microscopy (TEM). The (101)‐oriented epitaxial SnO2 buffer layers contain a high number of crystallographic shear plane (CSP) defects (∼1011 cm−2) and threading dislocations (TDs, ∼1010 cm−2). However, their density reduces with increasing layer thickness. Whereas a Ga atomic concentration of 3.2 × 1016 cm−3 does not lead to any change in the defect structure of the SnO2 layers, heavily doped SnO2 layers ([Ga] ∼ 6.1 × 1020 cm−3) contain a continuous network of coherent Ga‐rich precipitates appearing as platelets in crystallographically equivalent SnO2(100) and SnO2(010) planes. Electron conduction through this network might explain the reduced electrical resistivity compared to semi‐insulating SnO2 with lower Ga concentration

Effect of Sb doping on structural, electrical and optical properties of epitaxial SnO2 films grown on r-cut sapphire

Abstract Antimony-doped tin oxide (SnO 2 :Sb) films have been epitaxially grown on r-cut sapphire substrates by metalorganic chemical vapor deposition (MOCVD). Although the films were all (1 0 1) oriented with rutile structure, they showed different microstructure, electrical and optical properties as Sb doping varied from 0% to 7%. The doping of Sb could reduce the formation of (1 0 1) twins in SnO 2 films. The SnO 2 :Sb film with the lowest resistivity of 1.3 × 10 −3 Ω cm and the highest carrier concentration of 2.5 × 10 20 cm −3 was obtained at 5% Sb-doping. The undoped and Sb-doped SnO 2 films exhibited different electrical transport mechanism. The samples showed high transparency of ∼80% in the visible range. As Sb concentration increased, the absorption edge of the films shifted to shorter wavelength and the calculated optical band gaps were about 3.77–4.11 eV.

Electrical properties of Sb-doped epitaxial SnO2 thin films prepared using excimer-laser-assisted metal–organic deposition

Excimer-laser-assisted metal–organic deposition (ELAMOD) was used to prepare Sb-doped epitaxial (001) SnO2 thin films on (001) TiO2 substrates at room temperature. The effects of laser fluence, the number of shots with the laser, and Sb content on the electrical properties such as resistivity, carrier concentration, and carrier mobility of the films were investigated. The resistivity of the Sb-doped epitaxial (001) SnO2 thin film prepared using an ArF laser was lower than that of the film prepared using a KrF laser. The van der Pauw method was used to measure the resistivity, carrier concentration, and carrier mobility of the Sb-doped epitaxial (001) SnO2 thin films in order to determine the effect of Sb content on the electrical resistivity of the films. The lowest resistivity obtained for the Sb-doped epitaxial (001) SnO2 thin films prepared using ELAMOD with the ArF laser and 2 % Sb content was 2.5 × 10−3 Ω cm. The difference between the optimal Sb concentrations and resistivities of the films produced using either ELAMOD or conventional thermal MOD was discussed.

Room-temperature ferromagnetism in epitaxial Mg-doped SnO2 thin films

The magnetic behavior of epitaxial Mg-doped SnO2 thin films prepared by radio-frequency magnetron sputtering was investigated in this work. Room-temperature ferromagnetism with the saturation magnetization of about 6.9 emu/cm3 was observed in 6% Mg-doped SnO2 samples. And the saturation magnetization decreases when further doped to 8%, while the optical band-gap increases. The room-temperature ferromagnetism was induced by the holes created by Mg on the substitutional site. Additionally, Mg interstitials and oxygen vacancies play an important role in reducing the magnetic moments.

Twin structures of epitaxial SnO2 films grown on a-cut sapphire by metalorganic chemical vapor deposition

SnO2 films have been grown on a-cut (112¯0) sapphire substrates by metalorganic chemical vapor deposition. X-ray diffraction and transmission electron microscopy were employed to characterize the epitaxial relationship and film structure. The films were (101) oriented with pure rutile structure. The in-plane relationship was determined to be SnO2 [010]//Al2O3 [0001] and SnO2 [101¯]//Al2O3 [11¯00]. There are three kinds of {101} twins in the SnO2 film. These twins caused high density of planar defects in the film and slight misorientation of the growth plane. The film/substrate interface was flat, while the film surface was rough with steps and inclinations.

Synthesis and properties of epitaxial SnO2 films deposited on MgO (100) by MOCVD

Epitaxial tin oxide (SnO2) thin films have been prepared on MgO (100) substrates at 500–600 °C by metalorganic chemical vapor deposition method. Structural and optical properties of the films have been investigated in detail. The obtained films were pure SnO2 with the tetragonal rutile structure. An in-plane orientation relationship of SnO2 (110) [010]//MgO (200) [110] between the film and substrate was determined. Two variant structure of SnO2 were analyzed. The structure of the film deposited at 600 °C was investigated by high-resolution transmission electron microscopy, and an epitaxial structure was observed. The absolute average transmittance of the SnO2 film at 600 °C in the visible range exceeded 90%. The optical band gap of the film was about 3.93 eV.

Structural characteristics of epitaxial SnO 2 films deposited on a- and m-cut sapphire by ALD

Epitaxial SnO 2 thin films were grown on (1 1 0) (a-cut) and (1 0 0) (m-cut) sapphire substrates using plasma enhanced atomic layer deposition (PEALD) with dibutyltindiacetate (DBTDA) as a precursor. X-ray diffraction, X-ray pole figure, and high resolution TEM analyses revealed that SnO 2 film deposited on a-cut sapphire was (1 0 1) oriented with a small component of (2 0 0) orientation, and the (1 0 1) planes were slightly tilted due to the presence of (2 0 0) plane and/or twinning. SnO 2 film on m-cut sapphire was strongly (0 0 1) oriented with a small amount of the (3 0 1) orientation. The determined in-plane orientation relationships were [0 1 0]SnO 2//[0 0 1]Al 2O 3 and [1 0 1̄]SnO 2//[1̄ 1 0]Al 2O 3 (a-cut) and [0 1 0]SnO 2//[0 0 1̄]Al 2O 3 and [1 0 0]SnO 2//[0 1 0]Al 2O 3 (m-cut) consistent with the previous reports.

Structure and photoluminescence properties of epitaxial SnO2 films grown on α-Al2O3 (012) by MOCVD

SnO2 thin films have been successfully deposited on α-Al2O3 (0 1 2) substrates by metalorganic chemical vapor deposition (MOCVD) in the temperature range 500–700 °C. The films were epitaxially grown in the tetragonal SnO2 phase and were (1 0 1) oriented. In-plane orientation relationship [0 1 0]SnO2||[1 0 0]Al2O3 and [1 0 1̄]SnO2||[1̄ 2̄ 1]Al2O3 was determined between the film and substrate. Photoluminescence (PL) spectra measured at room temperature revealed that the film grown at 700 °C showed an intense ultra-violet (UV) PL peak at 333 nm, which was a band-edge emission peak in SnO2 films. At a temperature of 13 K, a new broad PL band centered at about 480 nm was observed. The corresponding PL mechanisms are discussed in detail.

The synthesis and photocatalytic properties of epitaxial SnO 2–ZnS nanocomb heterostructure

A growth of SnO 2–ZnS heterostructure was realized by a rational two-step thermal evaporation method. Starting with Sn powder, the SnO 2 bicrystalline nanoribbons were grown on the silicon wafers. The secondary growth of ZnS on the former SnO 2 nanostructures correspondingly resulted in the SnO 2–ZnS nanocomb structures. We investigated the morphology, detailed structure by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). It revealed that the ZnS nanobranches with [001] growth direction are epitaxial grown from the (− 101) crystal plane of the SnO 2 trunks with a good lattice match. The nanocomb exhibited an effective photocatalytic activity.

Growth of polar axis oriented tetragonal Pb(Zr,Ti)O 3 films on CaF 2 substrates with transparent (La 0.07Sr 0.93)SnO 3

Epitaxial (La 0.07Sr 0.93)SnO 3 [LSSO] films were deposited on CaF 2 substrates by pulse laser deposition. The (1 0 0) c orientation of LSSO films was observed only on (1 1 0)CaF 2, whereas (1 1 0) c orientation was found on (1 1 1)CaF 2 and (1 0 0)CaF 2. (0 0 1) polar axis oriented tetragonal Pb(Zr 0.35Ti 0.65)O 3 films were grown on the fabricated (1 0 0) c LSSO∥(1 1 0)CaF 2 by pulsed metal organic chemical vapor deposition. The (0 0 1)Pb(Zr 0.35Ti 0.65)O 3∥(1 0 0)cLSSO∥(1 1 0)CaF 2 stack structure exhibited about 70% transparency with an adsorption edge of approximately 330 nm.

Structural, electrical and optical properties of SnO 2 films deposited on Y-stabilized ZrO 2 (1 0 0) substrates by MOCVD

SnO 2 films have been deposited on Y-stabilized ZrO 2 (YSZ) (1 0 0) substrates at different substrate temperatures (500–800 °C) by metalorganic chemical vapor deposition (MOCVD). Structural, electrical and optical properties of the films have been investigated. The films deposited at 500 and 600 °C are epitaxial SnO 2 films with orthorhombic columbite structure, and the HRTEM analysis shows a clear epitaxial relationship of columbite SnO 2(1 0 0)||YSZ(1 0 0). The films deposited at 700 and 800 °C have mixed-phase structures of rutile and columbite SnO 2. The carrier concentration of the films is in the range from 1.15×10 19 to 2.68×10 19 cm −3, and the resistivity is from 2.48×10 −2 to 1.16×10 −2 Ω cm. The absolute average transmittance of the films in the visible range exceeds 90%. The band gap of the obtained SnO 2 films is about 3.75–3.87 eV.

Phase and Optical Characterizations of Annealed SnO Thin Films and Their p-Type TFT Application

Amorphous SnO thin films were prepared on quartz and 190 nm substrates by electron beam evaporation. X-ray diffraction results reveal that amorphous SnO transforms into polycrystalline (tetragonal litharge structure) after rapid thermal annealing in Ar ambient at and starts decomposing into (orthorhombic structure) with the expulsion of Sn atoms at . The optical properties were characterized via spectroscopic ellipsometry. The polycrystalline SnO thin films have a higher refractive index and a narrower bandgap than the amorphous ones, which is due to the polarizability enhancement in the crystallization process. Moreover, the relationship between and of the amorphous and polycrystalline SnO thin films can be explained by the “Moss rule” law, and a decreasing trend in was verified with the transformation from SnO to . Bottom-gate-type thin film transistors (TFTs) employing polycrystalline SnO channels on the substrates exhibit p-type field-effect transistor characteristics. The optimum field-effect mobilities and are 0.46 and , respectively, which are the same order of magnitude as those reported for epitaxial SnO TFTs.

Synthesis of orthorhombic structure epitaxial tin oxide film

Tin oxide (SnO 2) epitaxial film has been deposited on Y-stabilized ZrO 2 (YSZ) (100) substrate by metalorganic chemical vapor deposition (MOCVD). Structural and optical properties of the film as well as the epitaxial mechanism have been investigated in detail. The results of structure analyses show that the obtained film is epitaxial SnO 2 with orthorhombic columbite structure. A novel epitaxial relationship of orthorhombic SnO 2(100)||YSZ(100) with SnO 2 [001]||YSZ<001> was clearly observed from the interface area between the film and the substrate. The absolute average transmittance of the orthorhombic SnO 2 film in the visible range exceeds 90%, and the optical band gap of the film is about 3.79 eV.

Gas sensing properties in epitaxial SnO 2 films grown on TiO 2 single crystals with various orientations

Epitaxial SnO 2 films were deposited on TiO 2 single crystals with various orientations by plasma enhanced atomic layer deposition (PEALD), and their structural characteristics and gas sensing properties were investigated, particularly focusing on the crystallographic orientation dependence of H 2 gas response. Dibutyltindiacetate (DBTDA) was used as Sn source, and (1 0 0), (0 0 1), (1 1 0), and (1 0 1) TiO 2 were employed as substrates for SnO 2 deposition. All the SnO 2 films were ∼90 nm thick after 1000 ALD cycles and epitaxially grown on TiO 2 substrates, which were confirmed by X-ray pole figure and high resolution transmission electron microscopy (HRTEM). Differently oriented epitaxial SnO 2 films showed the different H 2 gas response and different temperature dependence of gas response. The (1 0 1) SnO 2 films grown on (1 0 1) TiO 2 exhibited the highest H 2 gas response of ∼380 toward 1000 ppm H 2/air at 400 °C, which was associated with the different temperature dependence of resistance in (1 0 1) film rather than the microstructural characteristics and chemical composition compared to the other films.

Epitaxial growth of SnO 2 film on Sn-doped TiO 2(110)

Using a simple vacuum deposition-and-annealing method, Sn-doped TiO 2 (110) single crystals were prepared. Compared with the case of a pure TiO 2 substrate, the lattice mismatch between SnO 2 and the Sn-doped TiO 2 was reduced to −2.85% for the a-axis from −3.25 to −2.85% and for the c-axis from −7.35 to −6.62%. Surface morphologies of deposited SnO 2 films were compared on the Sn-doped TiO 2 (110) and on the pure TiO 2 (110). Results showed that the use of Sn-doped TiO 2 (110) single crystal as a substrate was favorable for growing epitaxial SnO 2 (110) films.