Growth of tetragonal PtO by molecular-beam epitaxy and its integration into β-Ga2O3 Schottky diodes

High-resistivity β-Ga2O3 thin films were grown on Si-doped n-type conductive β-Ga2O3 single crystals by molecular beam epitaxy (MBE). Vertical-type Schottky diodes were fabricated, and the electrical properties of the Schottky diodes were studied in this letter. The ideality factor and the series resistance of the Schottky diodes were estimated to be about 1.4 and 4.6× 106 Ω. The ionized donor concentration and the spreading voltage in the Schottky diodes region are about 4 × 1018 cm−3 and 7.6 V, respectively. The ultra-violet (UV) photo-sensitivity of the Schottky diodes was demonstrated by a low-pressure mercury lamp illumination. A photoresponsivity of 1.8 A/W and an external quantum efficiency of 8.7 × 102% were observed at forward bias voltage of 3.8 V, the proper driving voltage of read-out integrated circuit for UV camera. The gain of the Schottky diode was attributed to the existence of a potential barrier in the i–n junction between the MBE-grown highly resistive β-Ga2O3 thin films and the n-type conductive β-Ga2O3 single-crystal substrate.

Sedovite, U4+(Mo6+O4)2·nH2O, is reported as being one of the earliest supergene minerals formed of the secondary zone. The difficulty of isolating enough pure material limits studies to techniques that can access the nanoscale combined with theoretical analyses. The crystal structure of sedovite has been solved and refined using the dynamical approach from three-dimensional electron diffraction data collected on natural nanocrystals found among iriginite. At 100 K, sedovite is monoclinic a ≈ 6.96 Å, b ≈ 9.07 Å, c ≈ 12.27 Å, and V ≈ 775 Å3 with space group C2/c. The microporous structure presents a characteristic framework built from uranium polyhedra and disordered Mo pyramids creating pore hosting water molecules. To confirm the formula U4+(Mo6+O4)2·nH2O, the possible presence of a hydroxyl group that would promote Mo5+ was tested with density functional theory (DFT) computations at the ambient temperature. DFT predicts that sedovite is a ferromagnetic insulator with a fundamental bandgap of Eg ∼ 1.7 eV with its chemical and physical properties dominated by U4+ rather than Mo6+. The structural complexity, IG,tot, of sedovite was evaluated in order to get indirect information about the missing formation conditions.

Determination of conductivity anisotropy and the role of doping in single walled carbon nanotube thin films with THz spectroscopic ellipsometry

One of the most important research interests in the field of organic photovoltaic devices (OPVs) is the development of new materials which can serve as light-absorbing electron donors and hole-conducting (p-type) semiconductors. In this context, 1,3-polyazulenes were synthesized chemically and electrochemically. Their spectroscopic and electrochemical properties are compared with those of the 1,3-oligoazulenes Az1−Az6. The UV−vis spectra of the neutral azulenes Az1−Az6 show a linear correlation between the lowest absorption maximum and the inverse chain length 1/n leading to a band gap of Eg = 1.90 eV for infinite chain length. Derived from this correlation the effective conjugation length of chemically synthesized polyazulene is only about 10. By an alternative approach, a band gap of Eg = 1.46 eV was determined. Depending on the applied potential the oligomers Az2−Az6 undergo up to two reversible oxidation processes or further polymerization which results in the formation of polymer films at the electrode. The potentiodynamic oxidation of chemically synthesized polyazulene leads to electrocrystallization at the electrode, whereas films of polyazulenes are obtained directly upon oxidation of Az1−Az6. Chemically and electrochemically generated polyazulenes adsorbed on Pt show similar electrochemical behavior upon positive doping. The spectroelectrochemical investigations in combination with density functional theory (DFT) calculations lead to the conclusion that polyazulene can be oxidized up to a doping level of one charge per three or four azulene units. At this stage polarons or polaron pairs are formed (depending on the doping level) but not bipolarons. At higher doping levels the polymers start to decompose.

High-resolution photoemission spectroscopy (HRPES) measurements were collected and density functional theory (DFT) calculations were performed to track the exposure-dependent variation of the adsorption structure of 2-thiophenecarboxaldehyde (C4H3SCHO: TPCA) on the Ge(100) 2 × 1 reconstructed surface at room temperature. In an effort to identify the most probable adsorption structures on the Ge(100)-2 × 1 reconstructed surface, we deposited TPCA molecules at low exposure and at high exposure and compared the differences between the electronic features measured using HRPES. The HRPES data suggested three possible adsorption structures of TPCA on the Ge(100)-2 × 1 reconstructed surface, and DFT calculations were used to determine the plausibility of these structures. HRPES analysis corroborated by DFT calculations, indicated that an S-dative bonded structure is the most probable adsorption structure at relatively low exposure levels, the [4 + 2] cycloadduct structure is the second most probable structure, and the [2 + 2]-C=O cycloadduct structure is the least probable structure on the Ge(100)-2 × 1 reconstructed surface at relatively high exposure levels.

The use of a wide bandgap absorber layer in the top cell of a multi-junction silicon thin film solar cell is necessary to achieve a high-conversion efficiency. A higher bandgap of the absorber results in a higher open-circuit voltage (Voc) of the cell. In this work, intrinsic hydrogenated amorphous silicon oxide (i)a-SiO:H films have been prepared by using 13.56 MHz radio frequency plasma enhanced chemical vapour deposition (RF-PECVD) at a substrate temperature of 195 °C. The carbon dioxide (CO2) to silane (SiH4) ratio rc was varied and the influence of the ratio rc on the optoelectronic film properties was investigated. These thin films have been studied in detail in terms of their dark (σd) and photo (σph) conductivity and photoresponse PR (σph/σd). The defect density Nd and Urbach energy EU were determined by constant photocurrent method (CPM). The optical bandgap Eg,Tauc was derived from Tauc plots. Optical constants were determined by spectroscopic ellipsometry measurements in the range between 300 and 1000 nm. It was found that the increase in the CO2 to SiH4 ratio rc not only leads to a higher optical bandgap, but also higher defect density Nd and Urbach energy EU and on the other hand to a lower photoresponse PR. A suitable photoresponse PR = 8.64 × 105 at a high bandgap of Eg,Tauc = 1.91 eV and a defect density of Nd = 1.7 × 1016 cm−3 was achieved. The refraction index and extinction coefficient were lowered with increasing rc. The analysis of light-induced degradation in the (i)a-SiO:H layers showed a smaller increase of deep defects Nd and a higher increase of the Urbach energy EU in terms of the light soaking time with respect to the (i)a-Si:H reference layer.

Body: Beta-gallium oxide (β-Ga2O3) has emerged as an excellent candidate for transparent conducting oxides (TCO) applications such as ultraviolet photodetectors and optoelectronics due to its high electrical conductivity and transparency. To fulfill the extensive potential as an electronic material, β-Ga2O3 can be alloyed with Al2O3 for the tunability of a large bandgap up to 8.8 eV and modifiable photoresponse spectral range. The crystal structure of ground state Al2O3 is corundum phase (a), which is extensively different from the monoclinic phase of ground state Ga2O3. Density functional theory (DFT) calculations have denoted the monoclinic phase of (AlxGa1-x)2O3 is as an energetically preferred structure for up to 71% Al incorporation [1], despite the structural variations between these two materials. However, previous studies on the growth of (AlxGa1-x)2O3 heterostructures with high Al composition indicated impaired film quality or phase transformations [2], indicating the challenges in the controlling point and extended defects and achieving the theoretical maximum Al content while maintaining the monoclinic phase. These limitations have demonstrated the demand for direct characterization of Al incorporation and defects in beta-(AlxGa1-x)2O3 thin films. In this report, we investigated the atomic structure and defects in beta-(AlxGa1-x)2O3 thin films using scanning transmission electron microscopy (STEM). Our quantitative STEM analysis for beta-(AlxGa1-x)2O3 films grown by molecular beam epitaxy (MBE) and metal organic chemic vapor deposition (MOCVD) revealed two significant pieces of information for atomic-scale origins of growth characteristics and properties of the heterostructures. Our characterization indicated that 54% of the incorporated Al occupied the octahedrally coordinated Ga2 site, suggesting the non-equilibrium growth conditions lead to a higher energy structure. DFT calculations have shown that Al can be incorporated on the less thermally favorable tetrahedral Ga1 site due to its occupation in particular configurations on the surface and kinetic limitations. On the other hand, the formation of a planar defect perpendicular to the growth direction associated with a larger Al concentration corresponded to a local inclusion of the gamma phase. We have identified the local Al content reaches a critical value of about 50%, which could destabilize the monoclinic phase of (AlxGa1-x)2O3 films. DFT calculations have demonstrated these planar defects are thermally stable at higher Al compositions, and its formation reduces the in-plane stress of tensile-strained films. The projection of cation columns identically match with the recently observed divacancy-interstitial complex structures [3]. In summary, our STEM research offers essential atomic scale insights on exact control of impurity incorporation for semiconductor bandgap engineering, and eventually provides guidance to the future epitaxial growth of (AlxGa1-x)2O3 for high-performance power electronics applications. We acknowledge support by the Department of Defense, Air Force Office of Scientific Research GAME MURI Program (Grant No. FA9550-18-1-0479). [1] H. Peelaers, J. B. Varley, J. S. Speck, and C. G. Van De Walle, Structural and Electronic Properties of Ga2O3-Al2O3 Alloys, Appl. Phys. Lett. 112, 242101 (2018). [2] A. F. M. A. U. Bhuiyan, Z. Feng, J. M. Johnson, H. L. Huang, J. Sarker, M. Zhu, M. R. Karim, B. Mazumder, J. Hwang, and H. Zhao, Phase Transformation in MOCVD Growth of (AlxGa1-x)2O3 Thin Films, APL Mater. 8, 031104 (2020). [3] J. M. Johnson, Z. Chen, J. B. Varley, C. M. Jackson, E. Farzana, Z. Zhang, A. R. Arehart, H. L. Huang, A. Genc, S. A. Ringel, C. G. Van De Walle, D. A. Muller, and J. Hwang, Unusual Formation of Point-Defect Complexes in the Ultrawide-Band-Gap Semiconductor β-Ga2 O3, Phys. Rev. X 9, 041027 (2019).

Investigation of the structural anisotropy in a self-assembling glycinate layer on Cu(100) by scanning tunneling microscopy and density functional theory calculations

The construction of a SnO2 and ZnO thin film heterostructure is an efficient way to improve the photocatalytic properties of ZnO. However, the current techniques to produce it, for instance, chemical and physical vapor deposition are expensive and therefore not affordable for everyone. Several efforts have been made in order to obtain high quality thin films with lower cost using sol-gel base techniques. The aim of this work is to process high quality and low-cost SnO2 and ZnO thin film heterostructures supported on glass. The thin films were deposited by the spin coating method. The samples obtained were characterized by XRD, SEM, UV-Vis and FTIR. The thin film heterostructure exhibits homogeneous size nanoparticles (10~50 nm) that conform the surface and uniform submicronic thickness. High values of optical transmission >80% were measured for the heterostructure in the range (380-740 nm). The band gap of Eg 3.25 and 3.69 eV were obtained from Tauc’s plot.

ABSTRACTWe have studied Cu2S absorber layers prepared by physical vapor deposition (PVD) by calibrated spectral photoluminescence (PL) and by confocal PL as function of temperature T and excitation fluxes to obtain the absolute PL-yield at an excitation flux equivalent to the AM1.5 spectrum and to calculate the splitting of the quasi-Fermi levels (QFL) µ = Ef,n-Ef,p and the absorption coefficient α(E), both in the temperature range 20 K ≤ T ≤ 400 K. The PL-spectra reveal two peaks at E1 = 1.17 eV and E2 = 1.3 eV, of which the low energy peak is only detectable at temperatures T < 200 K. The samples show an impressive QFL-splitting of µ > 700 meV at 300 K associated with a pseudo band gap of Eg = 1.25 eV. The high energy peak shows an unexpected temperature behavior, namely an increase of the PL-yield with rising temperature at variance with the behavior of QFL-splitting that decreases with rising T from extrapolated T = 0K value of µ = 1.3 eV. The PL-yield versus temperature will be discussed in terms of different defect states in the band gap. Our observations indicate that, contrary to common believe, it is not the PL-yield, but rather the QFL-splitting that is the comprehensive indicator of the quality of the excited state in an illuminated semiconductor. A further examination of the lateral variation of the opto-electronic properties by confocal PL shows a strong correlation between the QFL-splitting, the Urbach energy EU and the optical band gap Eopt, respectively.

We have studied chalcocite (Cu2S) layers prepared by physical vapor deposition with varying deposition parameters by calibrated spectral photoluminescence (PL) and by confocal PL with lateral resolution of Δ x≈0.9 μm. Calibrated PL experiments as a function of temperature T and excitation fluxes were performed to obtain the absolute PL-yield and to calculate the splitting of the quasi-Fermi levels (QFLs) μ=Ef,n−Ef,p at an excitation flux equivalent to the AM 1.5 spectrum and the absorption coefficient α(ℏω), both in the temperature range of 20 K≤T≤400 K. The PL-spectra reveal two peaks at E#1=1.17 eV and E#2=1.3 eV. The samples show a QFL-splitting of μ>700 meV associated with a pseudo band gap of Eg=1.25 eV. The high-energy peak shows an unexpected temperature behavior, namely, an increase of PL-yield with rising temperature at variance with the behavior of QFL-splitting that decreases with rising T. Our observations indicate that, contrary to common believe, it is not the PL-yield, but rather the QFL-splitting that is the comprehensive indicator of the quality of the excited state in an illuminated semiconductor. A further examination of the lateral variation of opto-electronic properties by confocal PL and the surface contour shows no detectable correlation between Cu2S grains/grain boundaries and the PL-yield or QFL-splitting.

High-quality Sn-doped In2O3 (ITO) films were grown epitaxially on yttria stabilized zirconia (111) with oxygen-plasma assisted molecular beam epitaxy (MBE). The 12 nm thick films, containing 2–6% Sn, are fully oxidized. Angle-resolved x-ray photoelectron spectroscopy (ARXPS) confirms that the Sn dopant substitutes In atoms in the bixbyite lattice. From XPS peak shape analysis and spectroscopic ellipsometry measurements it is estimated that, in a film with 6 at.% Sn, ∼1/3 of the Sn atoms are electrically active. Reflection high energy electron diffraction (RHEED) shows a flat surface morphology and scanning tunneling microscopy (STM) shows terraces several hundred nanometers in width. The terraces consist of 10 nm wide orientational domains, which are attributed to the initial nucleation of the film. Low energy electron diffraction (LEED) and STM results show a bulk-terminated (1 × 1) surface, which is supported by first-principles density functional theory (DFT) calculations. Atomically resolved STM images are consistent with Tersoff–Hamann calculations that show that surface In atoms are imaged bright or dark, depending on the configuration of their O neighbors. The coordination of surface atoms on the In2O3(111)–1×1 surface is analyzed in terms of their possible role in surface chemical reactions.

Single-layer CrI3 grown by molecular beam epitaxy

Stoichiometric SnTe is theoretically a small gap semiconductor that undergoes\na ferroelectric distortion on cooling. In reality however, crystals are always\nnon-stoichiometric and metallic; the ferroelectric transition is therefore more\naccurately described as a polar structural transition. Here we study the Fermi\nsurface using quantum oscillations as a function of pressure. We find the\noscillation spectrum changes at high pressure, due to the suppression of the\npolar transition and less than 10 kbar is sufficient to stabilize the\nundistorted cubic lattice. This is accompanied by a large decrease in the Hall\nand electrical resistivity. Combined with our density functional theory (DFT)\ncalculations and angle resolved photoemission spectroscopy (ARPES) measurements\nthis suggests the Fermi surface $L$-pockets have lower mobility than the\ntubular Fermi surfaces that connect them. Also captured in our DFT calculations\nis a small widening of the band gap and shift in density of states for the\npolar phase. Additionally we find the unusual phenomenon of a linear\nmagnetoresistance that exists irrespective of the distortion that we attribute\nto regions of the Fermi surface with high curvature.\n

We conducted a detailed experimental investigation of the Ag(977) vicinal surface, a high Miller index surface derived from the (111) surface. The sample surface was prepared using standard methodology and its quality was examined by x-ray photoelectron spectroscopy, low energy electron diffraction (LEED) and scanning tunneling microscopy. I(V)-LEED analysis was used to determine the surface structure focusing the intricate relaxation dynamics expected for this surface. Our LEED analysis revealed an inward relaxation for the step chain (SC) atoms, whereas the corner atoms (CC) relaxed outwards. To gain more information on the obtained relaxations, we also performed density functional theory (DFT) calculations for the constructed structural model. Through charge distribution analysis, we found out that the step atoms interact weakly with their adjacent counterparts, resulting in terrace atoms presenting electronic environment similar to those found on flat surfaces. Furthermore, we conducted angle-resolved photoemission spectroscopy (ARPES) measurements to map the electronic structure of the surface. The DFT calculations and ARPES results have shown that the electronic bands observed arise from the hybridization between bulk and surface electronic states.