Nonpolar Quantum Research Articles

Modeling of injection characteristics of polar and nonpolar III-nitride multiple quantum well structures

Carrier confinement and injection characteristics of polar and nonpolar III-nitride quantum well (QW) light-emitting diode or laser diode structures are compared. We demonstrate that strongly inhomogeneous QW injection in multiple-QW (MQW) active region is one of the possible reasons holding back the advance of nonpolar laser structures. In polar structures, strong interface polarization charges induce the nonuniform carrier distribution among the active QWs so that the extreme p-side QW always dominates the optical emission. On the contrary, in nonpolar MQW structures, the inhomogeneity of QW populations is supported mainly by QW residual charges and the prevailing QW is the one closest to the n-side of the diode. For both polar and nonpolar structures, the QW injection inhomogeneity is strongly affected by the QW carrier confinement and becomes more pronounced in longer wavelength emitters with deeper active QWs. We show that in nonpolar structures indium incorporation into optical waveguide layers improves the uniformity of QW injection. On the contrary, QW injection in polar structures remains inhomogeneous even at high-indium waveguide layer compositions. We show, however, that polarization-matched design of the electron-blocking layer can noticeably improve the injection uniformity in polar MQW structure and enhance the structure internal quantum efficiency.

Recombination of free excitons in polar and nonpolar nitride quantum wells

We compare radiative and nonradiative recombination in polar and nonpolar GaInN/GaN quantum wells and demonstrate that purely radiative recombination can be achieved even at room temperature. The radiative recombination is due to free excitons, which partially dissociate at elevated temperatures. Nonradiative recombination is strongly inhibited in the polar case but not in the nonpolar case.

Electron-phonon interaction in non-polar quantum dots induced by the amorphous polar environment

We propose a Fröhlich-type electron-phonon interaction mechanism for carriers confined in a non-polar quantum dot surrounded by an amorphous polar environment. Carrier transitions under this mechanism are due to their interaction with the oscillating electric field induced by the local vibrations in the surrounding amorphous medium. We estimate the corresponding energy relaxation rate for electrons in Si nanocrystals embedded in a SiO2 matrix as an example. When the nanocrystal diameter is larger than 4 nm then the gaps between the electron energy levels of size quantization are narrow enough to allow for transitions accompanied by emission of a single local phonon having the energy about 140 meV. In such Si/SiO2 nanocrystals the relaxation time is in nanosecond range.

Polarization-induced charge carrier separation in polar and nonpolar grown GaN quantum dots

We have performed systematic studies of wurtzite GaN/AlN quantum dots grown on polar and nonpolar surfaces. For this purpose, experimentally observed quantum dot geometries have been employed within an eight-band k⋅p model. The spatial separation of electrons and holes due to polarization potentials is found to be much larger in nonpolar than in polar grown quantum dots. In order to improve the electron-hole overlap and thus the recombination rates, we have varied the shape, size, and the periodic arrangement of nonpolar quantum dots. We observed the strongest improvement of the charge carrier overlap in nonpolar quantum dots that have a reduced dimension. If the size is reduced below 60% of the dimensions reported recently in literature, this increase is clearly more pronounced than for the polar quantum dots, indicating much better recombination rates in smaller nonpolar quantum dots.

Towards a polariton-based light emitter based on non-polar GaN quantum wells

A polariton-based light emitter is a extraordinary concept as an alternative to a light-emitting diode (LED) or laser diode. The physics of a polariton laser is fundamentally different from the spontaneous emission process of an LED or the inversion and stimulated emission process of a laser diode. The rapid decay and emission from this polariton–exciton state bypasses the normal irreversible spontaneous emission and associated non-radiative decay mechanisms. An AlGaN/AlN nucleation bilayer was employed on r -plane sapphire to deposit non-polar GaN quantum wells embedded in an AlGaN-based cavity surrounded by top and bottom distributed Bragg reflectors (DBRs). The reflectance data show that the exciton and photon states can be tuned (by changing the angle of the sample) to the same energy. The characteristic strong coupling was observed in the reflectance data where the states of the exciton and photon do not overlap; rather they split into an upper polariton state and a lower polariton state. The photoluminescence (PL) showed a strong emission at a low stimulation level at a similar energy and angle.

Theory of GaN Quantum Dots for Optical Applications

The optical properties of III-N wurtzite heterostructures are dominated by the built-in polarization potential. We first review the dependence of III-N bulk valence band structure on strain and the key factors determining the polarization vector in polar and nonpolar quantum wells, including electromechanical effects. We then present a surface integral technique to determine the built-in potential in quantum dots (QDs) of arbitrary shape. We show for polar QDs how the polarization potential spatially separates electrons and holes vertically but confines them laterally, causing the radiative recombination rate to decrease rapidly with increasing dot height and a strong blueshift with increasing carrier density. Finally, we show that although the polarization potential can be much reduced in nonpolar GaN/AlN QDs, it is likely to remain significant in nonpolar InN/GaN QD structures.

Bipolaron States in the Quantum Wire in the Polar Environment

Formation of bipolaron states is investigated at interaction of two same carriers of the charge which are being non-polar quantum wire placed in polar environment. Alongside with effects of dimensional quantization of an electronic spectrum, quantization phonon spectrum is considered. Inoculating quasi-particle in a considered problem are electronic polarons at which allocation interaction with plasma vibrations of valent electrons (fast polarization) is accepted in attention. It is shown, that in case when the radius of a wire surpasses much more radius of electronic polaron, interaction of carriers considering only fast polarization is well described by the potential of interaction received within the limits of classical electrodynamics. The contribution into screening interactions of slow (ionic) polarization of the polar environment surrounding not polar wire, is defined by effective potential electron-phonon interactions of structure which is received by averaging on oscillator variables of electron-phonon interactions exact Hamiltonian. Using results of numerical calculations, structures of potential energy of interaction of the same carriers for various sets of parameters (high-frequency permittivity of a wire and its radius and high-frequency and low-frequency permittivities of environment) are constructed. It is shown, that for favourable combinations of parameters (small radius of a wire, a greater difference between high- and low-frequency transmittivities of environment) in the potential of interaction of the same charges it is probable formation of area of an effective attraction that leads to their binding in bipolaron pair with binding energy, on one-two orders of the corresponding pair exceeding energy in a volumetric crystal with the same values of material parameters. It can have defining value in an explanation, for example, of such effects, as high-temperature superconductivity, in structures with quantum wires.

Optical characteristics of III-nitride quantum wells with different crystallographic orientations

This article presents a direct comparison of calculated optical characteristics of polar, nonpolar, and semipolar III-nitride quantum wells. We show that the advantage of using wider quantum wells offered by nonpolar/semipolar technology is severely limited by narrower valence subband separation, thermal hole redistribution, and resulting optical gain degradation in wider wells. However, we emphasize the importance of using wider quantum wells to prevent electron leakage. We also show that gain characteristics of laser structures grown in nonpolar/semipolar orientations are less vulnerable to detrimental effect of nonradiative recombination.

(Zn, Mg)O/ZnO-based heterostructures grown by molecular beam epitaxy on sapphire: Polar vs. non-polar

Zinc oxide (ZnO) has recently attracted considerable attention because of its unique physical properties and its potential applications in the blue and UV spectral range. Up to now, ZnO-based heterostructures have mostly been grown in a c-orientation. The growth of non-polar layers along the a-direction [112¯0] has been proposed to avoid any built-in electric fields in the c-direction. Polar and non-polar quantum wells (QWs) embedded in (Zn, Mg)O barriers were grown on an optimized buffer. We compare the photoluminescence (PL) emission of a- and c-oriented QWs. From this comparison, we demonstrate that the QWs exhibit confinement but no indication of quantum confined Stark effect, contrary to what is observed in c-oriented structures. In the non-polar orientation, it is shown that the thermal quenching is not related to the thermal escape of excitons from the ZnO area, since the calculated activation energies are much lower.

Tight-binding study of the optical properties of GaN/AlN polar and nonpolar quantum wells

The electronic structure of wurtzite semiconductor superlattices (SLs) and quantum wells (QWs) is calculated by using the empirical tight-binding method. The basis used consists of four orbitals per atom (sp3 model), and the calculations include the spin–orbit coupling as well as the strain and electric polarization effects. We focus our study on GaN/AlN QWs wells grown both in polar (C) and nonpolar (A) directions. The band structure, wave functions and optical absorption spectrum are obtained and compared for both cases.

Electronic flexoelectricity in low-dimensional systems

Symmetry breaking at surfaces and interfaces and the capability to support large strain gradients in nanoscale systems enable unusual forms of electromechanical coupling. Here, we introduce the concept of electronic flexoelectricity, a phenomenon that is manifested when the mechanical deformation of nonpolar quantum systems results in the emergence of net dipole moments and hence linear electromechanical coupling proportional to local curvature. The concept is illustrated in carbon systems, including polyacetylene and nanographitic ribbons. Using density functional theory calculations for systems made of up to 400 atoms, we determine the flexoelectric coefficients to be of the order of $\ensuremath{\sim}0.1e$, in agreement with the prediction of linear theory. The implications of electronic flexoelectricity on electromechanical device applications and physics of carbon-based materials are discussed.

Rabi splitting at intersubband transition assisted by longitudinal optical phonon

Rabi splitting in absorption spectra of intersubband transition of a polar semiconductor quantum well is predicted if the intersubband frequency is near the longitudinal phonon frequency. The calculated value of the splitting for a GaAs quantum well changes from $1.82\phantom{\rule{0.3em}{0ex}}\mathrm{meV}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}6.86\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, when the electron concentration varies from ${10}^{16}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}{10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. It is also predicted that the absorptance is enhanced as compared to the one for a nonpolar quantum well with the same intersubband transition. The oscillator model of the dielectric function gives the same order of the splitting.

Optical properties of single non-polar GaN quantum dots

We_present a microphotoluminescence study of non polar GaN/AIN quantum dots (QDs) grown along the [1120] axis. Despite the high QD density, single exciton lines could be isolated on the high energy side of the spectral distribution of the QD array emission. Linewidths down to 0.5 meV are reported, which is one order of magnitude lower than previously reported linewidths for polar GaN/AIN QDs. This difference is attributed to the drastic reduction of the internal field in non-polar quantum dots. Temperature dependent measurements were performed up to 180 K.

Optical properties of GaN quantum dots grown on nonpolar (11-20) SiC by molecular-beam epitaxy

We report on nonpolar GaN quantum dots embedded in AlN, grown on (11-20) 6H–SiC by plasma-assisted molecular-beam epitaxy. These dots are aligned in the growth plane and present a constant aspect ratio of 10. Their optical properties were studied as a function of GaN coverage. Especially, the variation of their emission energy as compared to that of (0001) GaN quantum dots is a clear fingerprint of the reduced internal electric field present in these nonpolar nanostructures. Time-resolved spectroscopy confirmed this result by revealing lifetimes in the few 100 ps range in contrast to the much longer ones obtained for the (0001) GaN quantum dots.

Free and bound excitons in GaN∕AlGaN homoepitaxial quantum wells grown on bulk GaN substrate along the nonpolar (112¯0) direction

Nonpolar multiple quantum wells (MQWs) have been grown by plasma assisted molecular beam epitaxy on bulk GaN crystals oriented along the (112¯0) direction. The photoluminescence intensity of the nonpolar MQWs was significantly higher than that found for the polar samples, both at low (10 K) and room temperature. This is a consequence of the lack of built-in electric field in samples grown along the (112¯0) direction. Clearly resolved spectra of the excitons have been observed in the studied MQWs. Studies of these excitonic structures, by means of polarization and temperature measurements enabled us to assign the observed lines to free and bound excitons in GaN quantum wells.