Randolph E. Treece

THE EFFECT OF ANNEALING ON THE STRUCTURE AND DIELECTRIC PROPERTIES OF BAXSR1-XTIO3 FERROELECTRIC THIN FILMS

The effect of a postdeposition anneal on the structure and dielectric properties of epitaxial BaxSr1−xTiO3 (BST) thin films (x=0.35–0.65) have been measured. The films were grown by pulsed laser deposition on LaAlO3 (001) substrates. The films were single phase and (001) oriented with a lattice parameter larger than the bulk. The dielectric properties of the x=0.35 film exhibited a broad temperature dependence and a peak at 168 K, which is 36 K below the peak observed in bulk BST (x=0.35). Annealing films for 8 h in flowing oxygen at 900 °C caused the lattice parameter to decrease and dielectric properties to become more like the bulk. Annealing also resulted in an increased electric field dependent dielectric tuning without increased dielectric loss.

High-temperature thermopower in La2/3Ca1/3MnO3 films: Evidence for polaronic transport.

Thermoelectric power, electrical resistivity, and magnetization experiments, performed in the paramagnetic phase of La{sub 2/3}Ca{sub 1/3}MnO{sub 3}, provide evidence for polaron-dominated conduction in colossal magnetoresistance materials. At high temperatures, a large, nearly-field-independent difference between the activation energies for resistivity {rho} and thermopower {ital S}, a characteristic of Holstein polarons, is observed, and ln{rho} ceases to scale with the magnetization. On approaching {ital T}{sub {ital c}}, both energies become field dependent, indicating that the polarons are magnetically polarized. Below {ital T}{sub {ital c}}, the thermopower follows a law {ital S}({ital H}){approximately}1/{rho}({ital H}) as in nonsaturated ferromagnetic metals. {copyright} {ital 1996 The American Physical Society.}

High-temperature thermopower in La{sub {bold 2/3}}Ca{sub {bold 1/3}}MnO{sub {bold 3}} films: Evidence for polaronic transport

Thermoelectric power, electrical resistivity, and magnetization experiments, performed in the paramagnetic phase of La{sub 2/3}Ca{sub 1/3}MnO{sub 3}, provide evidence for polaron-dominated conduction in colossal magnetoresistance materials. At high temperatures, a large, nearly-field-independent difference between the activation energies for resistivity {rho} and thermopower {ital S}, a characteristic of Holstein polarons, is observed, and ln{rho} ceases to scale with the magnetization. On approaching {ital T}{sub {ital c}}, both energies become field dependent, indicating that the polarons are magnetically polarized. Below {ital T}{sub {ital c}}, the thermopower follows a law {ital S}({ital H}){approximately}1/{rho}({ital H}) as in nonsaturated ferromagnetic metals. {copyright} {ital 1996 The American Physical Society.}

Magnetothermopower in La0.67Ca0.33MnO3 thin films

We have measured the thermopower S(T,H) and resistivity ρ(T,H) of laser ablated La0.67Ca0.33MnO3 films, as a function of temperature and external magnetic field. On heating, a metal–insulator transition occurs at temperatures below the resistivity peak, observed via a jump between a low T regime, where S∝T, and a high T regime, where S∝1/T. An applied magnetic field shifts the transition in a postannealed sample to higher temperatures, causing a giant magnetothermopower effect 100×(S8T−S0T)/S8T=−1400%. Both S(T) and ρ(T) are activated at high temperatures, but with significantly different activation energies. We interpret this as an evidence of small polarons at high temperatures.

Pulsed laser deposition of high‐quality NbN thin films

Pulsed laser deposition has been used to grow superconducting NbN thin films from niobium targets in a reactive gas atmosphere of N2 (10% H2). The structural and electrical properties of the deposited films have been determined as a function of substrate temperature and crystallographic orientation. Highly textured NbN was deposited on MgO (100). Films deposited on MgO at 600 °C in 60 mTorr gas pressure were characterized by Tc=16.6 K, Jc (4.2 K)=7.1 MA/cm2, and λ(0)=3200 A. Films grown on amorphous fused silica, under the same conditions, were polycrystalline and characterized by Tc=11.3 K and Jc (4.2 K)=1.8 MA/cm2.

Non-Gaussian noise in a colossal magnetoresistive film

Anomalous non-Gaussian effects appear in the resistance fluctuations of a macroscopic sample of the colossal magnetoresistive material La2/3Ca1/3MnO3. Individual resistance switches are frequently resolved. The results indicate the presence of highly inhomogeneous hopping resistance among magnetic domains in this partially disordered material

Electron emission from GaN n–p junctions

We report on electron emission from cesiated GaN n–p junctions in forward bias. Surface electric fields ∼3 V/μm caused a fivefold increase in emission current. Initial maximum currents in excess of 200 nA degrade to 50 nA due to charge trapping but are quickly recovered at zero bias. Energy spectra confirm negative electron affinity 80 h after cesiation, indicate resistive losses limit the emission current, and reveal significant emission at energies above the Fermi level of the injecting contact.

Thin Films of Semiconducting SnSi Alloys Grown by Pulsed Laser Deposition

Semiconducting Sn x Si 1−x (0≤x≤0.6) thin-film alloys have been grown by pulsed laser deposition (PLD). These new materials are amorphous to X-rays and display small positive optical band gaps, suggesting potential applications in solar cells. The tin silicide films were grown by depositing very thin (1–30 A) alternating atomic layers from individual Sn and Si targets utilizing an automated multi-target holder coupled to a conventional PLD system. The value of x was selected by controlling the thickness of the atomic layers. The films were characterized by X-ray diffraction, optical absorption, Rutherford backscattering spectroscopy, temperature-dependent resistivity, and X-ray photoelectron spectroscopy. Tin segregation is prevented by keeping the Sn layer thickness below a critical value. Compositions beyond x > 0.6 led to semimetallic Sn x Si 1−x films with tin crystallites.