Disorder Induced IR Anomaly in Hexagonal AlGaN Short-Period Superlattices and Alloys
- PDF / 378,938 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 4 Downloads / 166 Views
*EE Department, University of Notre Dame, Notre Dame, IN 46556 **ECE Department, University of Boston, Boston,MA, 02215 ***APA Optics. 2950 N.E. 84th Lane, Blaine, MN, 55434 ****Institute for Silicate Chemistry, Odoevskogo 24/2, 199155 St.Petersburg, Russia
ABSTRACT We report an experimental (infrared reflectance spectroscopy) and theoretical study of the polar optical phonons in hexagonal ternary nitride compounds: AINm/GaNn (n=2-8, m=4, 8) superlattices (SL) and spontaneously ordered AlxGa,.-N (x=0.08-0.55) alloys. In infrared (IR) reflectivity spectra we revealed two modes having strong LO-TO splitting (20-150 cm-1), and several modes, having a small (1-3 cml1) LO-TO splitting. All modes have a very high damping parameter Ž20 cm- . The unusual observation is the negative value of the oscillator strength for the weak IR mode at -690 cm- , suggesting possible lattice instability, consistent with high damping observed. We found from lattice dynamical calculations that weak IR active modes correspond to modes localized at GaN-AIN interfaces. Our analysis has shown that an anomalous mode is induced by the disorder effects and arises due to strong overlapping of the LO-TO phonon branches of the bulk GaN and AIN. In SL samples the anomalous mode corresponds to phonons localized on interface inhomogenities. INTRODUCTION The study of the III-V nitrides GaN, AIN and their alloys has attracted considerable attention during the last several years due to their application in wide bandgap optoelectronics and microelectronics. The phonon properties of wurtzite GaN and AIN are intensively studied and well understood. Less is known about the phonon properties of ternary A1GaN. In the present paper we report the observation of an anomalous polar mode in IR reflectivity spectra of AIN/GaN short-period SLs and spontaneously ordered AIGaN alloys. This mode, observed at -690 cm- , has negative oscillator strength. To our knowledge this is the first observation of such modes in crystals. Our analysis and lattice dynamical calculations show that this anomalous mode induced by GaN-AIN interface inhomogenities and disorder effects. EXPERIMENTAL METHOD Short period superlattice films were fabricated using a switched atomic layer metalorganic vapor deposition process as described in [1]. The films were deposited onto 1.6 g.m GaN buffer layer, which was deposited on the basal plane of sapphire substrate. The superlattice (SL) consisted of a few hundred periods of (AINmGaNn) unit cell made up of m monolayers of AIN and n monolayers of GaN. In the present work we studied superlattices having m=4, 8 and n=2, 4, 6, 8. The SL period have been determined by X-ray diffraction and transmission electron spectroscopy measurements [1].
427 Mat. Res. Soc. Symp. Proc. Vol. 572 ©1999 Materials Research Society
AlxGal.xN alloys films having x=0.08, 0.22, 0.45, 0.55 and thickness 0.7, 1.4, 0.55, 0.45 g.tm, respectively, were grown by molecular beam epitaxy on the basal plane of sapphire substrates [2]. The X-ray data indicate SL ordering of the films [
Data Loading...
