Cavity polaritons in ZnO-based hybrid microcavities

Among wide-bandgap semiconductors, ZnO is a very attractive candidate for blue-ultraviolet lasers operating at room temperature owing to its large exciton binding energy and oscillator strength. Especially, ZnO-based microcavity structures are most conducive for polariton lasing at room temperature. We report the observation of cavitypolaritons in bulk ZnO-based hybrid microcavities at room temperature. The bulk ZnO-based hybrid microcavities are composed of 29 pairs of Al0.5Ga0.5N∕GaNdistributed Bragg reflector (DBR) at the bottom of the λ-thick cavity layer and eight pairs of SiO2∕Si3N4 DBR as the top mirror, which provided cavityQvalues of ∼100. Anticrossing behavior between the lower and upper polariton branches was observed at room temperature. From the polariton dispersion curve, the vacuum Rabi splitting was estimated to be ∼50meV. These results are promising toward the realization of ZnO-based microcavity polariton devices

Epitaxial growth of ZrO2 on GaN templates by oxide molecular beam epitaxy

Molecular beam epitaxial growth of ZrO2 has been achieved on GaN(0001)∕c-Al2O3substrates employing a reactive H2O2 oxygen source. A low temperature buffer followed by in situ annealing and high temperature growth has been employed to attain monoclinic, (100)-oriented ZrO2thin films. The typical full width at half maximum of a 30-nm-thick ZrO2 (100) film rocking curves is 0.4arcdeg and the root-mean-square surface roughness is ∼4Å. ω−2θand pole figure x-ray diffraction patterns confirm the monoclinic structure of ZrO2. Data support an in-plane epitaxial relationship of ZrO2 [010]∥∥GaN[112¯] and ZrO2 [001]∥∥GaN[11¯00]. X-ray diffraction and reflection high-energy electron diffraction analyses reveal in-plane compressive strain, which is mainly due to the lattice mismatch

Large pyroelectric effect in undoped epitaxial Pb(Zr,Ti)O3 thin films on SrTiO3 substrates

We have studied pyroelectric and ferroelectric properties of Pb(Zr,Ti)O3thin filmsgrown epitaxially on SrTiO3(001) substrates by rf magnetron sputtering. The pyroelectric coefficient was measured in the temperature range from 280 to 370 K using the Byer–Roundy method. Values as high as 48 nC/cm2 K have been obtained at 300 K. The PZTthin films exhibited a remanent polarization of 45–58 μC/cm2. The improved pyroelectric coefficient was attributed to a high crystalline quality of the films, as revealed by x-ray diffraction that showed only (001)-oriented perovskitePZT phase and a ω-rocking curve full width at half maximum value as low as 4.2 arc min for 300 nm thick films

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

Single crystalline PbTiO3films 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 PbTiO3thin films.In situ reflection high-energy electron diffraction pattern indicated the PbTiO3films to be grown under a two-dimensional growth mode. The full width at half maximum of the rocking curve of a relatively thin65nm (001) PbTiO3film is 6.2arcmin which is indicative of high crystal quality. The band gap of PbTiO3, as determined by ellipsometric measurement, is 3.778eV

Enhanced microwave dielectric tunability of Ba0.5Sr0.5TiO3 thin films grown with reduced strain on DyScO3 substrates by three-step technique

Tunable dielectric properties of epitaxial ferroelectric Ba0.5Sr0.5TiO3 (BST) thin films deposited on nearly lattice-matched DyScO3 substrates by radio frequency magnetron sputtering have been investigated at microwave frequencies and correlated with residual compressive strain. To reduce the residual strain of the BST films caused by substrate clamping and improve their microwave properties, a three-step deposition method was devised and employed. A high-temperature deposition at 1068 K of the nucleation layer was followed by a relatively low-temperature deposition (varied in the range of 673–873 K) of the BST interlayer and a high-temperature deposition at 1068 K of the top layer. Upon post-growth thermal treatment at 1298 K the films grown by the three-step method with the optimized interlayer deposition temperature of 873 K exhibited lower compressive strain compared to the control layer (−0.002 vs. −0.006). At 10 GHz, a high dielectric tunability of 47.9% at an applied electric field of 60 kV/cm was achieved for the optimized films. A large differential phase shift of 145°/cm and a figure of merit of 23°/dB were obtained using a simple coplanar waveguide phase shifter at 10 GHz. The low residual strain and improved dielectric properties of the films fabricated using the three-step deposition technique were attributed to reduced clamping of the BST films by the nearly lattice-matched substrate

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)

Strain induced variations in band offsets and built-in electric fields in InGaN/GaN multiple quantum wells

The band structure, quantum confinement of charge carriers, and their localization affect the optoelectronic properties of compound semiconductor heterostructures and multiple quantum wells (MQWs). We present here the results of a systematic first-principles based density functional theory (DFT) investigation of the dependence of the valence band offsets and band bending in polar and non-polar strain-free and in-plane strained heteroepitaxial In x Ga1- xN(InGaN)/GaN multilayers on the In composition and misfit strain. The results indicate that for non-polar m-plane configurations with [12¯10]InGaN // [12¯10]GaN and [0001]InGaN // [0001]GaN epitaxial alignments, the valence band offset changes linearly from 0 to 0.57 eV as the In composition is varied from 0 (GaN) to 1 (InN). These offsets are relatively insensitive to the misfit strain between InGaN and GaN. On the other hand, for polar c-plane strain-free heterostructures with [101¯0]InGaN // [101¯0]GaN and [12¯10]InGaN // [12¯10]GaN epitaxial alignments, the valence band offset increases nonlinearly from 0 eV (GaN) to 0.90 eV (InN). This is significantly reduced beyond x ≥ 0.5 by the effect of the equi-biaxial misfit strain. Thus, our results affirm that a combination of mechanical boundary conditions, epitaxial orientation, and variation in In concentration can be used as design parameters to rapidly tailor the band offsets in InGaN/GaN MQWs. Typically, calculations of the built-in electric field in complex semiconductor structures often must rely upon sequential optimization via repeated ab initio simulations. Here, we develop a formalism that augments such first-principles computations by including an electrostatic analysis (ESA) using Maxwell and Poisson\u27s relations, thereby converting laborious DFT calculations into finite difference equations that can be rapidly solved. We use these tools to determine the bound sheet charges and built-in electric fields in polar epitaxial InGaN/GaN MQWs on c-plane GaN substrates for In compositions x = 0.125, 0.25,…, and 0.875. The results of the continuum level ESA are in excellent agreement with those from the atomistic level DFT computations, and are, therefore, extendable to such InGaN/GaN MQWs with an arbitrary In composition

Dielectric functions and critical points of PbTiO3, PbZrO3, and PbZr0.57Ti0.43O3 grown on SrTiO3 substrate

Single crystalline PbTiO3, PbZrO3, and PbZr0.57Ti0.43O3 thin films on SrTiO3 (001) substrates were grown by a combination of molecular beam epitaxy and rf sputtering methods. The authors measured the dielectric functions of the thin films using spectroscopic ellipsometry and determined the interband critical point energies using standard critical point model. They compared the critical point energies to the band structure calculations in the literature. The data suggest that anticrossing behavior occurs between Ea and Eb near Zr=0.17. This phenomenon is attributed to a coupling between X1c and X3c bands caused by intrinsic alloy disorder

Impurity complexes and conductivity of Ga-doped ZnO

Using hybrid functional theory together with experimental measurements, we investigate the influence of gallium impurities and their complexes on electrical properties of ZnO. In contrast to the behavior of isolated Ga impurities and native defects, the calculated formation energies of Ga complexes are consistent with our experimental data. We show that for high levels of Ga doping the acceptor behavior of GaZn-VZn and GaZn-Oi complexes explains the conductivity measurements and compensation levels in ZnO. The computed binding energies of these complexes are also consistent with the binding energies obtained from the measurements of the temperature dependence of carrier mobility. Our results show that the formation of defect complexes, often overlooked by theory, can be indispensable in capturing the defect physics