Transport mechanisms in structurally ordered piezoelectric Ca3TaGa3Si2O14 (CTGS) single crystals are studied in the temperature range of 1000-1300 °C by application of the isotope 18O as a tracer and subsequent analysis of diffusion profiles of this isotope using secondary ion mass spectrometry (SIMS). Determined oxygen self-diffusion coefficients enable calculation of oxygen ion contribution to the total conductivity, which is shown to be small. Since very low contributions of the cations have to be expected, the total conductivity must be dominated by electron transport. Ion and electron conductivities are governed by different mechanisms with activation energies (1.9±0.1) eV and (1.2±0.07) eV, respectively. Further, the electromechanical losses are studied as a function of temperature by means of impedance spectroscopy on samples with electrodes and a contactless tone-burst excitation technique. At temperatures above 650 °C the conductivity-related losses are dominant. Finally, the operation of CTGS resonators is demonstrated at cryogenic temperatures and materials piezoelectric strain constants are determined from 4.2 K to room temperature.

The homo- and heteroepitaxial deposition of LGS (langasite, La3Ga5SiO14) thin films on LGS single crystals, Si and SiO2 substrates by pulsed laser deposition (PLD) is demonstrated. PLD is performed at substrate temperatures up to about 700 °C and results initially in Ga deficient films. Two strategies of counterbalancing the Ga deficit are realized. First, off-stoichiometric targets with an enhanced Ga content are applied. Secondly, an increased oxygen partial pressure up to about 6 Pa is used during deposition to suppress evaporation of Ga suboxides. Combining these adaptions results in the growth of stoichiometric LGS thin films. Films deposited on LGS substrates do not show any additional X-ray diffraction reflexes nor broadening of the peaks with respect to the single crystalline substrates. Therefore, the homoepitaxial approach can be considered successful. The deposition on Si and SiO2 substrates under the same conditions leads to the formation of polycrystalline films. However, post-annealing at 800 °C increases crystallinity. Stoichiometry and homogeneous distribution of La, Ga and Si cations are confirmed by secondary neutral mass spectrometry (SNMS). The composition remains constant within the film, implying stable process parameters.

By using systematic first-principles calculation, we found that AlTlO3 compound of LiNbO3 structure shows large piezoelectric constants e33 of 10.7 C/m2 and d33 of 56.7 pC/N. These piezoelectric constants are approximately six times larger than those of LiNbO3. AlTlO3 is predicted to be stabilized above 7 GPa. On the other hand, the calculated dielectric constant ε33 shows diverged behavior around 2 GPa. This result indicates that AlTlO3 can be quenchable. Decomposition of the predicted piezoelectric constant revealed that the large piezoelectricity of AlTlO3 originates from the Tl displacement in accordance with external perturbation, which drives the ferroelectric soft mode of the corresponding paraelectric phase. However, the energy difference between the ferroelectric and paraelectric phases was small, approximately 1 meV/f.u. These insights suggest that fluctuation between ferroelectric and paraelectric phases causes large piezoelectricity in AlTlO3.

Single-phase multiferroic materials have attracted considerable attention among scientists, due to the strong drive in industry towards device miniaturization, addition of new functionalities, etc. Currently, most of the discovered materials have at-least one ferroic order active only at low temperatures, thereby hindering their induction into practical devices. κ-Al2O3-type AlxFe2-xO3 (x-AFO) oxides belong to a new class of metastable multiferroic compounds (space group: Pna21), with relatively high Curie temperatures. The current work investigates the effect of thin film deposition conditions on the ferroelectric and ferrimagnetic properties of Al0.5Fe1.5O3 (0.5-AFO). Substrate temperature and oxygen partial pressure during deposition were found to be the critical parameters in obtaining high quality films. Optimizing the deposition conditions of 0.5-AFO enabled observation of both ferroelectricity and ferrimagnetism at room temperature.

The recent discovery of ferroelectric behavior in doped hafnia-based dielectrics, attributed to a non-centrosymmetric orthorhombic phase, has potential for use in attractive applications such as negative differential capacitance field-effect-transistors (NCFET) and ferroelectric random access memory devices (FeRAM). Alloying with similar oxides like ZrO2, doping with specific elements such as Si, novel processing methods, encapsulation and annealing schemes are also some of the techniques that are being explored to target structural modifications and stabilization of the non-centrosymmetric phase. In this study, we utilized synchrotron-based x-ray diffraction in the grazing incidence in plane geometry (GIIXRD) to determine the crystalline phases in hafnia-zirconia (HZO) compositional alloys deposited by atomic layer deposition (ALD). Here we compare and contrast the structural phases and ferroelectric properties of mechanically confined HZO films in metal-insulator-metal (MIM) and metal-insulator-semiconductor (MIS) structures. Both MIM and MIS structures reveals a host of reflections due to non-monoclinic phases in the d-spacing region between 1.75Å to 4Å. The non-monoclinic phases are believed to consist of tetragonal and orthorhombic phases. Compared to the MIS structures a suppression of the monoclinic phase in MIM structures with 50% zirconia or less was observed. The correlation of the electrical properties with the structural analysis obtained by GIIXRD highlights the importance of understanding the effects of the underlying substrate (metal vs. Si) for different target applications.

La3+ doped yttrium iron garnet films have been grown on (111) oriented gadolinium gallium garnet substrates via Liquid phase epitaxy technique as a basic material for ISHE device fabrication. Pt as a material with a large spin hall angle was used as a spin detection layer. We investigated the dependence of the spin pumping effect on the power and frequency of the excitation microwaves in La:YIG/Pt bilayers by measuring the ISHE voltage. We demonstrated that the area under the ISHE curve(SISHE) across a wide power range had a nearly linear correlation with the input microwave power (Pin). The parameter SISHE can be used to describe the spin current energy in a Pt layer which can be a useful parameter for a microwave rectifier.

We report on photo-induced magnetic effect and infrared (IR) spectroscopic behavior of colossal magnetorestive manganite La0.7Sr0.3MnO3 (LSMO) with rare earth co-doping (Er3+ and Yb3) at room temperature. X-ray diffraction shows that a structural phase transformation from rhombohedral to hexagonal with increasing Er and Yb concentrations. Photo-induced magnetoresistance showed that Er3+ and Yb+3 ions have significant influence on the magnetic and electrical ordering of LSMO under the influence of UV light. The optical excitation also shows an enhancement in the magnetotransport properties of LSMO.

Cubic gallium nitride (GaN) is a wide bandgap semiconductor that exhibits a high crystallographic symmetry resulting in a lower inbuilt polarization which is useful for more efficient phosphor-free green light-emitting diodes. It has been grown using molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD), which produce highly ordered thin films on compatible substrates. In this work, we report the chemical synthesis of GaN using chemical metathesis reaction in diethyl ether with lithium nitride and anhydrous gallium chloride as precursors, inside a nitrogen glove box at the room temperature. The resulting product was subsequently washed to remove lithium chloride and dried before vacuum annealing in a furnace at 850°C. Powder X-ray diffraction (XRD) scans of the as-prepared and annealed product reveal a mixed phase of GaN along with Ga2O3. Energy dispersive X-ray spectroscopy (EDAX) measurements show a nitrogen-poor product, which correlates well with the nearly black color of the powder. Diffuse reflectance spectroscopy (DRS) measurements were carried out with the obtained product on a barium sulfate substrate in a Perkin-Elmer Lambda 1050-UV-Vis-NIR spectrophotometer showing a strong absorbance below 400 nm. The energy band gap is bounded by values extracted from the Tauc plot and DRS measurements in the range of 3.2-3.5 eV, which is in good agreement with the known excitonic bandgap of cubic GaN (∼ 3.3 eV). Initial photoluminescence (PL) measurements using a Perkin-Elmer LS-55 spectrophotometer with an excitation wavelength of 310 nm reveal a weak emission centered around 440 nm corresponding to the known defect centers (D0X) in GaN. Further development of this process to form inks is expected to provide an alternate pathway to producing flexible phosphor-free lighting devices.

The current transient was studied on AlGaN/GaN HEMTs for RF power amplifiers under different temperatures. The current transient measurements include two different approaches. One is to measure the current transient from off-state (without bias) to a quiescent point (Q-point). Different transient behaviors were observed while switching to different Q-points. Another one is to measure the current transient from different currents to the Q-point of VDS = 28 V and ID = 100 mA/mm. The different currents before switching to Q-point of VDS = 28 V and ID = 100 mA/mm show the different transient characteristics. Most of the current transient demonstrates temperature independence in this study.