Vishal Narang

Date of Graduation

2015

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Mohindar S Seehra

Committee Co-Chair

Wathiq Abdul-Razzaq

Committee Member

Alan D Bristow

Committee Member

Cheng Cen

Committee Member

Dimitris Korakakis

Abstract

This research was undertaken to determine the effect of Erbium in Erbium-alloyed AlN thin films on their structural, piezoelectric and magnetic properties. For this purpose, Erbium-alloyed AlN thin films with Er concentration of 0, 1, 3 and 4 atomic percent were deposited on (001) p-type Si substrates by reactive magnetron sputtering. The samples were characterized by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and Variable Angle Spectroscopic Ellipsometry (VASE). XRD measurements showed that Er alloying leads to preferential c-axis film deposition with a decrease in grain size and an increase in the lattice constant. XPS analysis was used to determine the Er concentration and its possible chemical state. VASE measurements were used to determine the thickness and refractive indices of the thin films. Refractive indices were used as metric to determine the structural order of polycrystalline thin films. Piezoelectric measurements showed that Er alloying results in the higher magnitude of the piezoelectric coefficient d33. Values of as high as 15.0 pm/V is measured for AlN:Er whereas the maximum value obtained for pure AlN thin films is 6.9 pm/V.;In order to determine the electronic state of Er in AlN:Er thin films, magnetization (M) of the samples was measured as a function of temperature (2 K to 300 K) and magnetic field (H) up to 90 kOe at select temperatures. In particular, the focus was to determine whether Er in AlN:Er is present as Er metal, Er2O3 or Er3+ substituting for Al3+. For this purpose, a detailed investigation of the magnetic properties of Er metal sample and a powder sample of Er2O 3 was also carried out for comparison purposes. Analysis of the results from these investigations showed that Er in AlN:Er is present as Er 3+ substituting for Al3+ in concentrations which are in good agreement with those determined from XPS. Lattice expansion of AlN:Er compared to AlN observed in XRD measurements is hence attributed to larger size Er3+ replacing Al3+.;New results obtained from the analysis of M vs. H data in Er2O 3 showed the presence of two magnetic-field induced transitions below its Neel temperature TN ∼ 3.3 K. Measurements of magnetization (M) vs. magnetic field (H) at 2 K show a transition at HSF ∼ 15 kOe which is interpreted in terms of spin flop transition and a second observed transition at Hsp ∼ 31 kOe represents field-aligned ferromagnetism in Er2O 3. This interpretation is confirmed by theoretical analysis. Magnetic field dependence of the Neel temperature TN is determined and it is observed to follow the equation TN(H) = TN(0) - D1H2 expected for antiferromagnets. From the temperature dependence of magnetic susceptibility of Er2O3 above TN and its fit to the Curie-Weiss law, the molecular field model is used to determine the nearest neighbor (nn) and next nearest neighbor (nnn) exchange constants J1 = -0.57 K and J2 = -3.38 K respectively. The determined values of J 1 and J2 are then used to calculate the magnetic field strength for field-induced ferromagnetic transition and it agrees very well with experimental value of 31 kOe.;The major conclusions from this research are that Erbium in Erbium alloyed AlN thin films is present as Er3+ substituting for Al 3+ thereby increasing its lattice constant, preferentially ordering the thin films and significantly enhancing the piezoelectric coefficient. For the related compound of Er2O3, two new magnetic-field induced transitions and associated magnitudes of exchange parameters are reported.

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