Ch. Kishan Singh

Knowledge of electronic band structure and its role on the electronic transport as well as optical properties of semiconducting as well as insulating materials are important to characterize/evaluate their potential for practical applications. In this work, the crystallographic, morphological, electronic transport, optical and electronic band structure properties of three Lanthanum (La) based double perovskite oxides, La₂ZnMnO₆, La₂NiTiO₆ and La₂NiRuO₆ were evaluated and compared. Polycrystalline samples were synthesized by following the solid-state reaction method. Analysis of powder x-ray diffraction (XRD) data indicates monoclinic (space group: P2₁/n) crystal structure. The unit cell size of these three systems correlates with their constituent transition metal ionic sizes. Analysis of Raman spectra and Fourier transform infrared (FTIR) spectra indicates that the octahedral vibrational modes correlate with the ionic mass of the constituent transition metals for these three systems. The temperature depended resistivity data (ρ(T)) indicate semiconducting behaviour for all the three systems. Among the samples, the La₂NiRuO₆ system shows lowest ρ(T) and the La₂NiTiO₆ system shows the highest ρ(T) values. The thermal activation energy is extracted from the analysis of ρ(T) data, following the thermally activated transport model. The estimated activation energy values are 134(8) meV for La₂NiRuO₆, 203(1) meV for La₂ZnMnO₆ and 280(5) meV for La₂NiTiO₆, respectively. Also, it is observed that the normalized resistivity and activation energy of the three systems follow a direct/simple scaling trend. Additionally, diffuse reflectance spectroscopy (DRS) measurements were carried out on these systems. The DRS results further supports the observed trend in ρ(T) data among the three systems. The experimental observations is further supported by the density functional theory calculations.

We present a methodology to mitigate the effect of the parasitic electrostatic contribution usually present in piezoresponse force microscopy (PFM) measurements for quantitative characterization of polycrystalline piezoelectric thin films using a case study on a set of Al1−xScxN thin films. It involves minimizing the voltage sensitivity of the measured piezoresponse by optimizing the optical lever sensitivity using the laser positioning of the beam-bounce system. Additionally, applying a dc-voltage offset (determined through Kelvin probe force microscopy) during PFM scans and positioning the probe over the interior/edge portion of the specimen are explored to minimize the local and nonlocal-electrostatic tip–sample interaction. The results show that the effective piezoelectric coefficient (d33-eff) of our c axis oriented wurtzite (wz)-Al1.0Sc0.0N thin film is ∼4.9 pm/V. The highest enhancement in the d33-eff value occurred in the wz-Al0.58Sc0.42N thin film. Above x ˃ 0.42, d33-eff reduces due to phase-mixing of the wz-Al1−xScxN phase with the cubic-Sc3AlN phase until piezoelectricity finally disappears at x ≈ 0.51.

We present a methodology to mitigate the effect of the parasitic electrostatic contribution usually present in piezoresponse force microscopy (PFM) measurement for quantitative characterization of polycrystalline piezoelectric thin films using a case study on a set of Al1-xScxN thin films. It involves minimizing the voltage sensitivity of the measured piezoresponse by optimizing the optical lever sensitivity using the laser positioning of the beam-bounce system. Additionally, applying a dc-voltage offset (determined through Kelvin probe force microscopy) during PFM scans and positioning the probe over the interior or edge portion of the specimen are explored to minimize the local and non-local electrostatic tip-sample interaction. The results shows that the effective piezoelectric coefficient (d33-eff) of our c-axis oriented wurtzite (wz)-Al1.0Sc0.0N thin film is 4.9 pm per Volt. The highest enhancement in the d33-eff value occurred in the wz-Al0.58Sc0.42N thin film. Above x = 0.42, the d33-eff reduces due to phase-mixing of the wz-Al1-xScxN phase with cubic-Sc3AlN phase till the piezoelectricity finally disappear at x = 0.51

Residual stress in polycrystalline-Ge thin film realized on glass substrate using Au-induced layer exchange crystallization process is evaluated using x-ray diffraction based technique. The measured stress is found to be tensile in nature, from which we delineate and discuss the extrinsic thermal and intrinsic growth stresses. An in-plane biaxial tensile strain ∼0.15% was estimated to be endured by the polycrystalline-Ge thin film. The narrowing effect that such strain and the crystallization or growth-related defects have on the optical energy band gap of the polycrystalline-Ge thin film is elucidated.

Raman and photoluminescence spectroscopic studies on oxygen vacancy‐induced structural disorder in Gd₂Ti₂O_7‐δ single crystals grown by optical floating zone technique under argon atmosphere are reported. The oxygen vacancies in Gd₂Ti₂O_7‐δ wafers decrease with thermal annealing in air atmosphere. The full width at half maximum of X‐ray diffraction rocking curve decreases from 245 to 157 arcsec and the optical transmittance increases from 23% to 87% (at 1000 nm) upon post growth thermal annealing. Raman spectroscopic studies reveal a monotonic increase in intensity of O–Gd–O (E_g) and Ti–O (A_1g) stretching modes with thermal annealing. Since these modes are associated with modulation of oxygen x parameter which is sensitive to Ti–O octahedron distortion, the increase in Raman intensity indicates an improvement in structural ordering of oxygen sub‐lattice in Gd₂Ti₂O_7‐δ. Moreover, the photoluminescence studies also corroborate the Raman analysis in terms of reduction of structural defects associated with oxygen vacancies as a function of thermal annealing. This study demonstrates the effectiveness of using Raman spectroscopy to probe the structural disorder in Gd₂Ti₂O_7‐δ crystals.

The present study reports Al induced crystallization of amorphous (a)-SiGe in the Al-Ge-Si ternary system with the a-SiGe/Al bilayer structure on glass at low temperature ∼350 °C. The origin of the Al induced layer exchange (ALILE) mechanism that occurs in the a-SiGe/Al system is investigated by studying the crystallization kinetics as well as the evolution of stress in the Al layer during the crystallization process. The growth kinetics was analyzed using Avrami's theory of phase transformation as the crystallization occurs under isothermal condition. It shows that initial growth of the polycrystalline (poly)-SiGe phase follows a 3D mode, characterized by n ∼ 3, where n is the Avrami constant. It then switched over to a 2D mode through an intermediate explosive growth as the crystallization fraction increases. The stress was evaluated using X-ray diffraction analysis based on multi-hkl sin²ψ formalism. A corroboration of the growth kinetics with stress analysis shows that nucleation and growth of the poly-SiGe phase inside the Al layer at 350 °C leads to buildup of compressive strain in the Al layer. The increase in strain energy due to compression in the Al layer at elevated temperature is the driving force that initiates the layer exchange process.

The role of deposition temperature in the formation of poly-SiGe alloy thin film in Si/Ge system is reported. For the set ofsamples deposited without any intentional heating, initiation of alloying starts upon post annealingat ∼ 500 °C leading to the formation of a-SiGe. Subsequently, poly-SiGe alloy phase could formonly at temperature ≥ 800 °C. Whereas, for the set of samples deposited at 500 °C, in-situ formation of poly-SiGe alloy thin film could be observed. The energetics of the incoming evaporated atoms and theirsubsequent diffusionsin the presence of the supplied thermal energy is discussed to understand possible reasons for lowering of formation temperature/energyof the poly-SiGe phase.