Physical properties of transparent perovskite oxides (Ba,La)SnO3 with high electrical mobility at room temperature

Transparent electronic materials are increasingly in demand for a variety of optoelectronic applications. BaSnO3 is a semiconducting oxide with a large band gap of more than 3.1 eV. Recently, we discovered that La doped BaSnO3 exhibits unusually high electrical mobility of 320 cm^2(Vs)^-1 at room temperature and superior thermal stability at high temperatures [H. J. Kim et al. Appl. Phys. Express. 5, 061102 (2012)]. Following that work, we report various physical properties of (Ba,La)SnO3 single crystals and films including temperature-dependent transport and phonon properties, optical properties and first-principles calculations. We find that almost doping-independent mobility of 200-300 cm^2(Vs)^-1 is realized in the single crystals in a broad doping range from 1.0x10^19 to 4.0x10^20 cm^-3. Moreover, the conductivity of ~10^4 ohm^-1cm^-1 reached at the latter carrier density is comparable to the highest value. We attribute the high mobility to several physical properties of (Ba,La)SnO3: a small effective mass coming from the ideal Sn-O-Sn bonding, small disorder effects due to the doping away from the SnO2 conduction channel, and reduced carrier scattering due to the high dielectric constant. The observation of a reduced mobility of ~70 cm^2(Vs)^-1 in the film is mainly attributed to additional carrier-scatterings which are presumably created by the lattice mismatch between the substrate SrTiO3 and (Ba,La)SnO3. The main optical gap of (Ba,La)SnO3 single crystals remained at about 3.33 eV and the in-gap states only slightly increased, thus maintaining optical transparency in the visible region. Based on these, we suggest that the doped BaSnO3 system holds great potential for realizing all perovskite-based, transparent high-frequency high-power functional devices as well as highly mobile two-dimensional electron gas via interface control of heterostructured films.Comment: 31 pages, 7 figure

High Mobility in a Stable Transparent Perovskite Oxide

We discovered that La-doped BaSnO3 with the perovskite structure has an unprecedentedly high mobility at room temperature while retaining its optical transparency. In single crystals, the mobility reached 320 cm^2(Vs)^-1 at a doping level of 8x10^19 cm^-3, constituting the highest value among wide-band-gap semiconductors. In epitaxial films, the maximum mobility was 70 cm^2(Vs)^-1 at a doping level of 4.4x10^20 cm^-3. We also show that resistance of (Ba,La)SnO3 changes little even after a thermal cycle to 530 Deg. C in air, pointing to an unusual stability of oxygen atoms and great potential for realizing transparent high-frequency, high-power functional devices.Comment: 15 pages, 3 figure

BaSnO3: thin film growth, transport properties, devices, and interfaces

학위논문(박사)--서울대학교 대학원 :자연과학대학 물리·천문학부,2015. 8. 차국린.Of great signi cance to the eld of oxide electronics is the discovery of an oxide material possessing all the three important physical properties: the perovskite structure, high oxygen stability, and high electron mobility. The perovskite structure implies novel physical properties typically exempli ed by superconductivity or colossal magentoresistance; the high oxygen stability ensures the bipolar dopability of oxide materials and the reliability of devices made of them; the high electron mobility enables the triumph of oxide materials over nitrides in the competition for high speed device applications. It is BaSnO3 that ts for the description by judging from the cubic perovskite structure, the thermal stability up to 1,000 C and the electron mobility as high as 320 cm2 V??1 s??1 in the metallic state. Discussed in this dissertation will be the endeavors to understand and utilize the phyical proprerties of BaSnO3 such as the growth of BaSnO3 epitaxial lms, the analyses of the transport properties of the lms depending on the choices of dopants, the fabrication of eld e ect devices based on BaSnO3, and the investigation of interfaces formed between BaSnO3 and polar perovskite oxides. The BaSnO3 epitaxial lm growth on SrTiO3 substrates has been successfully carried out by using the pulsed laser deposition technique. The crystallinity of the lms has been investigated by X-ray di raction analyses; the full width at half maximum of the !-rocking curves as narrow as 0.084 has proven that high-crystalline BaSnO3 lms can be grown on the lattice-mismatched SrTiO3 substrates. Cross-sectional transmission electron spectroscopy and etch-pit developing technique have been employed to inspect crystallographic defects or disorders in the BaSnO3 lms and have revealed the density of threading dislocations to the amount of 6 1010 cm??2. The transport properties of electron-doped BaSnO3 epitaxial lms on the SrTiO3 substrates have been studied in details with the relaxation time approximation in the electron scattering theory. The semi-empirical analyses evince the dominance of the threading dislocations among the sources of the conduction electron scattering and elucidate the hampered electron mobility in the epitaxial lms ( 70 cm2 V??1 s??1). Lanthanum is of an advantage that high electron mobility can be achieved in the BaSnO3 system doped by it. But, it has been learned that lanthanum is likely to create the antisite defects substituting the Sn-sites in lieu of the Ba-sites and trap up to about 3.7 1019 conduction electrons per cubic centimeters. Antimony, another dopant tried in order to avoid the antisite problem, has provided the BaSnO3 epitaxial lms with even poorer electron mobility ( 10 cm2 V??1 s??1). The electon a nity of antimony, the propensity of antimony to migrate, and the charged cores of the threading dislocations work together and create highly e ective defect clusters in scattering the conduction electrons. The device fabrication has employed the metal-insulator-semiconductor structure where a slightly doped BaSnO3 layer has been used as the semiconductor channel. Two insulator materials, Al2O3 and LaInO3, have been tried to form interfaces with the channel. At rst, an Al2O3/BaSnO3 transistor has been demonstrated with the device performances far better than those of Al2O3/SrTiO3 and Al2O3/KTaO3 transistors. Especially, the eld e ect mobility is two orders of higher in the Al2O3/BaSnO3 transistor. Next, an LaInO3/BaSnO3 transistor has been fabricated with a heavily doped metallic BaSnO3 layers used as the metal gate and contact terminals. The employment of metallic BaSnO3 layers lets the transistor be composed exclusively of perovskite oxides. The \all-perovskite transistor" has shown remarkable device performances: 90 cm2 V??1 s??1 of the eld e ect mobility, 107 of the current on/o ratio, and 0.65 V dec??1 of the subthreshold swing. The eld e ect mobility of the LaInO3/BaSnO3 transistor is 18 times higher than that of the famous LaAlO3/SrTiO3 transistor at room temperature. Last but not least, the conductive interface formation between LaInO3 and BaSnO3 has been discussed in the context of polarity discontinuity at the interface. The 103 times of enhancement in sheet conductance of La-doped BaSnO3 layer has been discovered, which implies formation of two-dimensional electron gas by the interface formation only. The enhanced sheet conductance reaches about the order of 10??4 ??1 sq. The robustness of the two-dimensional electron gas after oxygen annealing process and the lack of it in the non-polar interface made of BaHfO3 or SrZrO3 and La-doped BaSnO3 layers, which show the two-dimensional electron gas is formed exclusively by the polar interface, have been con rmed. The investigation of the e ect of La concentration in La-doped BaSnO3 layer on the enhanced sheet conductance at the interface leads to the conclusion that the origin of the two-dimensional electron gas at the interface is the accumulation of electrons induced by the intrinsic polarization in the LaInO3 layer.Contents Abstract ii Acknowledgements iv Contents x List of Figures xii List of Tables xvi 1 Introduction 1 1.1 A brief history of BaSnO3 research . . . . . . . . . . . . . . . . . . . . . . 2 1.2 BaSnO3 as a wide band gap semiconductor . . . . . . . . . . . . . . . . . 6 1.2.1 Basic material properties of BaSnO3 . . . . . . . . . . . . . . . . . 7 1.2.2 Thermal stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1.2.3 High electron mobility . . . . . . . . . . . . . . . . . . . . . . . . . 14 2 Epitaxial growth of BaSnO3 thin lms 18 2.1 Pulsed laser deposition of BaSnO3 epitaxial lms . . . . . . . . . . . . . . 19 2.2 Structural properties of BaSnO3 epitaxial thin lms on SrTiO3 substrates 25 2.2.1 X-ray di raction analysis . . . . . . . . . . . . . . . . . . . . . . . 27 2.2.2 Threading dislocations . . . . . . . . . . . . . . . . . . . . . . . . . 38 3 Transport properties of electron-doped BaSnO3 epitaxial lms 43 3.1 Carrier transport in degenerate BaSnO3 lms . . . . . . . . . . . . . . . . 44 3.1.1 Screening by conduction electrons . . . . . . . . . . . . . . . . . . 48 3.1.2 Ionized impurities scattering . . . . . . . . . . . . . . . . . . . . . 51 3.1.3 Threading dislocations scattering . . . . . . . . . . . . . . . . . . . 54 3.1.4 Grain boundaries scattering . . . . . . . . . . . . . . . . . . . . . . 56 3.2 Semi-empirical analysis of carrier transport in electron-doped BaSnO3 epitaxial lms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 3.2.1 La-doped BaSnO3 epitaxial lms . . . . . . . . . . . . . . . . . . . 58 3.2.2 Sb-doped BaSnO3 epitaxial lms . . . . . . . . . . . . . . . . . . . 66 4 Field e ect devices based on BaSnO3 thin lms 75 4.1 Metal-insulator-semiconductor eld e ect transistors . . . . . . . . . . . . 75 4.2 Devices utilizing Al2O3 dielectrics . . . . . . . . . . . . . . . . . . . . . . 80 4.3 LaInO3 as a dielectric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 x Contents xi 4.3.1 Electronic structure . . . . . . . . . . . . . . . . . . . . . . . . . . 84 4.3.2 Thin lm growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 4.3.3 Dielectric properties . . . . . . . . . . . . . . . . . . . . . . . . . . 86 4.4 Devices utilizing LaInO3 dielectrics . . . . . . . . . . . . . . . . . . . . . . 92 4.4.1 Fabrication process and device structure . . . . . . . . . . . . . . . 93 4.4.2 Device performances . . . . . . . . . . . . . . . . . . . . . . . . . . 96 5 Conductive interfaces between LaInO3 and BaSnO3 layers 99 5.1 Two-dimensional electron gases in polar interfaces . . . . . . . . . . . . . 102 5.1.1 Wurtzite interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 5.1.2 Perovskite oxides interfaces . . . . . . . . . . . . . . . . . . . . . . 105 5.2 LaInO3/BaSnO3 interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 5.2.1 Discovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 5.2.2 Annealing in O2 environment . . . . . . . . . . . . . . . . . . . . . 110 5.2.3 Contribution of oxygen vacancies. . . . . . . . . . . . . . . . . . . 112 5.2.4 Polar vs. non-polar interfaces . . . . . . . . . . . . . . . . . . . . . 113 5.2.5 LaInO3 thickness dependence . . . . . . . . . . . . . . . . . . . . . 115 5.2.6 Preferential La di usion . . . . . . . . . . . . . . . . . . . . . . . . 116 5.2.7 La concentration dependence . . . . . . . . . . . . . . . . . . . . . 117 A Curriculum Vitae 120 Bibliography 125Docto

Thermally stable pn-junctions based on a single transparent perovskite semiconductor BaSnO3

We report p-doping of the BaSnO3 (BSO) by replacing Ba with K. The activation energy of K-dopants is estimated to be about 0.5 eV. We have fabricated pn junctions by using K-doped BSO as a p-type and La-doped BSO as an n-type semiconductor. I-V characteristics of these devices exhibit an ideal rectifying behavior of pn junctions with the ideality factor between 1 and 2, implying high integrity of the BSO materials. Moreover, the junction properties are found to be very stable after repeated high-bias and high-temperature thermal cycling, demonstrating a large potential for optoelectronic functions

High-k perovskite gate oxide BaHfO3

We have investigated epitaxial BaHfO3 as a high-k perovskite dielectric. From x-ray diffraction measurement, we confirmed the epitaxial growth of BaHfO3 on BaSnO3 and MgO. We measured optical and dielectric properties of the BaHfO3 gate insulator; the optical bandgap, the dielectric constant, and the breakdown field. Furthermore, we fabricated a perovskite heterostructure field effect transistor using epitaxial BaHfO3 as a gate insulator and La-doped BaSnO3 as a channel layer on SrTiO3 substrate. To reduce the threading dislocations and enhance the electrical properties of the channel, an undoped BaSnO3 buffer layer was grown on SrTiO3 substrates before the channel layer deposition. The device exhibited a field effect mobility value of 52.7 cm2 V−1 s−1, a Ion/Ioff ratio higher than 107, and a subthreshold swing value of 0.80 V dec−1. We compare the device performances with those of other field effect transistors based on BaSnO3 channels and different gate oxides

All-perovskite transparent high mobility field effect using epitaxial BaSnO3 and LaInO3

We demonstrate an all-perovskite transparent heterojunction field effect transistor made of two lattice-matched perovskite oxides: BaSnO3 and LaInO3. We have developed epitaxial LaInO3 as the gate oxide on top of BaSnO3, which were recently reported to possess high thermal stability and electron mobility when doped with La. We measured the dielectric properties of the epitaxial LaInO3 films, such as the band gap, dielectric constant, and the dielectric breakdown field. Using the LaInO3 as a gate dielectric and the La-doped BaSnO3 as a channel layer, we fabricated field effect device structure. The field effect mobility of such device was higher than 90 cm2 V−1 s−1, the on/off ratio was larger than 107, and the subthreshold swing was 0.65 V dec−1. We discuss the possible origins for such device performance and the future directions for further improvement