N. Izyumskaya

AlGaN/GaN MOS transistors using crystalline ZrO2 as gate dielectric

Epitaxial growth of ZrO2 has been achieved on MOCVD-grown GaN(0001) templates by oxides molecular beam epitaxy using reactive H2O2 for oxygen and organometallic source for Zr. Utilizing a low temperature buffer layer followed by high temperature insitu annealing and high-temperature growth, monoclinic (100)-oriented ZrO2 thin films were obtained. The full width at half maximum of ZrO2 (100) rocking curve was 0.4 arc degree for 30-nm-thick films and the rms roughness for a 5&mgr;m by 5 &mgr;m AFM scan was 4 Å. The employment of epitaxial ZrO2 layer in the AlGaN/GaN heterojunction field effect trasnsistor as a gate dielectric has resulted in the increase of the saturation-current density and pinch-off voltage as well as in near symmetrical gate-drain I-V behavior.

Structural and electrical properties of Pb(Zr,Ti)O3 grown on (0001) GaN using a double PbTiO3∕PbO bridge layer

Pb(Zr0.52Ti0.48)O3 films were deposited by rf magnetron sputtering on silicon-doped GaN(0001)∕c-sapphire with a PbTiO3∕PbO oxide bridge layer grown by molecular beam epitaxy. X-ray diffraction data showed the highly (111)-oriented perovskite phase in lead zirconate titanate (PZT) films with PbTiO3∕PbO bridge layers, compared to the pyrochlore phase grown directly on GaN. The in-plane epitaxial relationships were found from x-ray pole figures to be PZT[112¯]‖GaN[11¯00] and PZT[11¯0]‖GaN[112¯0]. The polarization-electric field measurements revealed the ferroelectric behavior with remanent polarization of 30–40μC∕cm2 and asymmetric hysteresis loops due to the depletion layer formed in GaN under reverse bias which resulted in a high negative coercive electric field (950kV∕cm).

Electron scattering mechanisms in GZO films grown on a-sapphire substrates by plasma-enhanced molecular beam epitaxy

We report on the mechanisms governing electron transport using a comprehensive set of ZnO layers heavily doped with Ga (GZO) grown by plasma-enhanced molecular-beam epitaxy on a-plane sapphire substrates with varying oxygen-to-metal ratios and Ga fluxes. The analyses were conducted by temperature dependent Hall measurements which were supported by microstructural investigations as well. Highly degenerate GZO layers with n > 5 × 1020 cm−3 grown under metal-rich conditions (reactive oxygen-to-metal ratio <1) show relatively larger grains (∼20–25 nm by x-ray diffraction) with low-angle boundaries parallel to the polar c-direction. For highly conductive GZO layers, ionized-impurity scattering with almost no compensation is the dominant mechanism limiting the mobility in the temperature range from 15 to 330 K and the grain-boundary scattering governed by quantum-mechanical tunnelling is negligible. However, due to the polar nature of ZnO having high crystalline quality, polar optical phonon scattering cannot b...

Ferromagnetism in ZnO- and GaN-Based Diluted Magnetic Semiconductors: Achievements and Challenges

Both GaN- and ZnO-based diluted semiconductors (DMSs) have recently attracted considerable interest fueled by theoretical predictions of ferromagnetic ordering in these materials above room temperature, making them especially attractive for spintronics. The intense experimental research that followed has revealed, however, a great deal of controversy. The local structure and magnetic behavior of GaN- and ZnO-based DMSs were found to depend strongly on a preparation technique and growth conditions for the materials of the same nominal composition and the reported results varied considerably from group to group. This problem highlighted clearly the lack of theoretical understanding of physical mechanisms underlying ferromagnetisms in these materials and the inadequacy of standard characterization techniques used to probe structural and magnetic properties of the DMSs. In this paper, we report on the recent progress in the theoretical and experimental studies of ZnO- and GaN-based DMSs and make special impact on critical discussion of experimental methods employed for investigation of their magnetic and optical properties.

Donor behavior of Sb in ZnO

Electrical behavior of Sb in ZnO:Sb layers doped in a wide concentration range was studied using temperature dependent Hall effect measurements. The layers were grown by plasma-enhanced molecular beam epitaxy, and the Sb concentration was changed by varying the Sb flux, resulting in electron concentrations in the range of 1016 to nearly 1020 cm−3. Upon annealing, the electron concentration increased slightly and more notable was that the electron mobility significantly improved, reaching a room-temperature value of 110 cm2/V s and a low-temperature value of 145 cm2/V s, close to the maximum of ∼155 cm2/V s set by ionized impurity scattering. Hall data and structural data suggest that Sb predominantly occupies Zn sublattice positions and acts as a shallow donor in the whole concentration range studied. In the layers with high Sb content (∼1 at. %), acceptor-type compensating defects (possibly Sb on oxygen sites and/or point-defect complexes involving SbO) are formed. The increase of electron concentration ...

About the Cu-related green luminescence band in ZnO

Cu-related green luminescence (GL) band in n-type ZnO layers grown by molecular beam epitaxy on sapphire and a bulk ZnO sample grown by hydrothermal method have been studied with above-bandgap excitation. Annealing of the samples in air at temperatures above 600°C resulted in an increase in the concentration of CuZn acceptors, followed by a dramatic enhancement of the Cu-related GL band peaking at 2.45eV, and a characteristic fine structure. The GL band quenched at temperatures between 250 and 500K due to escape of holes from the excited state of the CuZn acceptor to the valence band. Energy position of this state in the bandgap of ZnO and its capture cross section for holes were estimated as 0.38eV and 2×10−13cm2, respectively.

High quality epitaxial growth of PbTiO3 by molecular beam epitaxy using H2O2 as the oxygen source

Single crystalline PbTiO3 films have been epitaxially grown on SrTiO3 (001) substrates by molecular beam epitaxy using H2O2 as the source of active oxygen. The optimum growth conditions have been determined by analyzing a range of growth parameters affecting growth and used to attain single phase and stoichiometric PbTiO3 thin films. In situ reflection high-energy electron diffraction pattern indicated the PbTiO3 films to be grown under a two-dimensional growth mode. The full width at half maximum of the rocking curve of a relatively thin 65nm (001) PbTiO3 film is 6.2arcmin which is indicative of high crystal quality. The band gap of PbTiO3, as determined by ellipsometric measurement, is 3.778eV.

Structural and electrical properties of Pb(Zr,Ti)O3 films grown by molecular beam epitaxy

Single-crystal, single-phase Pb(ZrxTi1−x)O3 films (x=0–0.4) were grown on (001) SrTiO3 and SrTiO3:Nb substrates by molecular beam epitaxy. Layer-by-layer growth of the Pb(Zr,Ti)O3 films was achieved by using PbTiO3 buffer layers between the SrTiO3 substrates and the Pb(Zr,Ti)O3 films. The layers with low Zr content showed high crystallinity with full width at half maximum of ω-rocking curves as low as 4arcmin, whereas increase in Zr concentration led to pronounced angular broadening. The PbZr0.07Ti0.93O3 films exhibited remanent polarization as high as 83μC∕cm2, but local areas suffered from nonuniform leakage current.

Optical studies of strain and defect distribution in semipolar (11¯01) GaN on patterned Si substrates

Formation of defects in semipolar (11¯01)-oriented GaN layers grown by metal-organic chemical vapor deposition on patterned Si (001) substrates and their effects on optical properties were investigated by steady-state and time-resolved photoluminescence (PL) and spectrally and spatially resolved cathodoluminescence (CL). Near-band edge emission is found to be dominant in the c+-wings of semipolar (11¯01)GaN, which are mainly free from defect-related emission lines, while the c– wings contain a large number of basal stacking faults. When the advancing c+ and c— fronts meet to coalesce into a continuous film, the existing stacking faults contained in c— wings continue to propagate in the direction perpendicular to the c-axis and, as a result, the region dominated by stacking fault emission is extended to the film surface. Additional stacking faults are observed within the c+ wings, where the growing c+ wings of GaN are in contact with the SiO2 masking layer. Out-diffusion of oxygen/silicon species and conce...

Excitonic recombination dynamics in non-polar GaN/AlGaN quantum wells

The optical properties of GaN/Al0.15Ga0.85N multiple quantum wells are examined in 8 K–300 K temperature range. Both polarized CW and time resolved temperature-dependent photoluminescence experiment are performed so that we can deduce the relative contributions of the non-radiative and radiative recombination processes. From the calculation of the proportion of the excitonic population having wave vector in the light cone, we can deduce the variation of the radiative decay time with temperature. We find part of the excitonic population to be localized in concert with the report of Corfdir et al. (Jpn. J. Appl. Phys., Part 2 52, 08JC01 (2013)) in case of a-plane quantum wells.