Nonpolar Quantum Wells Research Articles

Strong charge carrier localization interacting with extensive nonradiative recombination in heteroepitaxially grown m-plane GaInN quantum wells

The development of GaInN quantum well structures with nonpolar crystal orientation for light-emitting diodes and semiconductor lasers is currently one of the main foci of III-nitride-based optoelectronics research. One of the advantages of nonpolar orientations is the absence of polarization fields perpendicular to the quantum well plane. As a consequence, radiative recombination rates are higher compared to quantum wells on polar surfaces. However, due to high densities of threading dislocations and basal plane stacking faults in the case of heteroepitaxially grown nonpolar layers, and due to band gap inhomogeneities in the GaInN quantum wells, characterization of radiative and nonradiative recombination mechanisms is a complex challenge. So far, most published data about band gap fluctuations, charge carrier localization and internal quantum efficiency in nonpolar quantum wells are ambiguous. Here, we present temperature and excitation power density-dependent photoluminescence data featuring multiple characteristics related to strong charge carrier localization in m-plane (1–100) GaInN quantum wells. Thermally activated redistribution of charge carriers between localization sites in these quantum wells is weaker than in polar c-plane ones. The localization strength increases with higher indium concentration in the quantum wells. In the heteroepitaxially grown quantum well structures, the internal quantum efficiency is reduced even at low temperatures (T = 10 K) and especially for m-plane quantum wells with high indium mole fractions.

Impact of nonpolar AlGaN quantum wells on deep ultraviolet laser diodes

The radiation properties of nonpolar AlGaN quantum wells (QWs) were theoretically investigated by comparing them to those of c-plane AlGaN QWs with heavy holes as the top valence band (VB). First, the conditions to minimize the threshold carrier density of c-plane QW laser diodes were explored. A thin well width (∼1 nm) and reduction of the Al content in the well layer were important to reduce threshold carrier density because narrow wells suppressed the quantum confined Stark effect and AlGaN with a lower Al content had a lower density of states. Moreover, the emission wavelength was widely controlled by tuning the Al contents of both the well and barrier layers under the proposed conditions. Then the properties of nonpolar AlGaN QWs were investigated. Nonpolar AlGaN had several superior characteristics compared to c-plane QWs, including large overlap integrals, optical polarization suitable for both edge and surface emissions, an almost linearly polarized optical dipole between the conduction band and top VB due to the isolated VBs, and a reduced VB density of state. Finally, the threshold carrier densities of both nonpolar and optimized c-plane QWs were compared as functions of the transition wavelength. At a given wavelength, the threshold of nonpolar QWs was lower than that of c-plane ones. Particularly below 260 nm, nonpolar QWs had a low threshold, whereas that of c-plane QWs drastically increased due to the large VB mass of AlN and carrier population in the crystal-field splitting band.

Polar and Nonpolar ZnO Nanowire QWs Grown with PLD Using Nanowire Arrays with Tuning Density as Physical Templates

The encountered difficulties that prevent ZnO nanowires from being used as light-emitters are p-type doping and quantum well (QW) integration. The growth of homogenous nanowire quantum wells is usually influenced by the shadowing effect associated with nanowire growth density. In this paper, based on the growth density control of nanowire array, a new two-step pulsed laser deposition (PLD) strategy was demonstrated to grow two kinds of ZnO nanowire QWs, e.g. radial nonpolar QW and axial polar QW. The growth-density control of ZnO nanowires was realised by introducing a wetting layer and adjusting the substrate-target distances. The structural and optical characterizations of these two kinds of nanowire QWs prove that the radial nanowire QWs are more homogenous than axial QWs, which also show better optical properties.

Anisotropic strain effects on the photoluminescence emission from heteroepitaxial and homoepitaxial nonpolar (Zn,Mg)O/ZnO quantum wells

We report on the properties of nonpolar a-plane (Zn,Mg)O/ZnO quantum wells (QW) grown by molecular beam epitaxy on r plane sapphire and a plane ZnO substrates. For the QWs grown on sapphire, the anisotropy of the lattice parameters of the (Zn,Mg)O barrier gives rise to an unusual in-plane strain state in the ZnO QWs, which induces a strong blue-shift of the excitonic transitions, in addition to the confinement effects. We observe this blue-shift in photoluminescence excitation experiments. The photoluminescence excitation energies of the QWs are satisfactorily simulated when taking into account the variation of the exciton binding energy with the QW width and the residual anisotropic strain. Then we compare the photoluminescence properties of homoepitaxial QWs grown on ZnO bulk substrate and heteroepitaxial QWs grown on sapphire. We show that the reduction of structural defects and the improvement of surface morphology are correlated with a strong enhancement of the photoluminescence properties: reduction of full width at half maximum, strong increase of the luminescence intensities. The comparison convincingly demonstrates the interest of homoepitaxial nonpolar QWs for bright UV emission applications.

Contactless electroreflectance of polar and nonpolar GaN/AlGaN quantum wells

Contactless electroreflectance (CER) has been applied to study optical transitions between the ground and excited states in polar and a-plane nonpolar 2 nm wide GaN/Al0.12Ga0.88N quantum well (QW) structures. In addition to the fundamental transition, CER features related to optical transitions between excited states were clearly observed for the polar QW structure whereas such features were not observed for the nonpolar QW structure. This experimental result clearly shows that the polarization-related electric field leads to a quantum confinement of some extra states in the polar QW system. Such states are not confined in the nonpolar QW and, therefore, optical transitions between them are not detected, i.e., only the fundamental transition is observed in CER spectrum of the nonpolar QW.

Benefits of homoepitaxy on the properties of nonpolar (Zn,Mg)O/ZnO quantum wells on a-plane ZnO substrates

We report on the properties of nonpolar (Zn,Mg)O/ZnO quantum wells (QW) homoepitaxially grown by molecular beam epitaxy on a-plane ZnO substrates. We demonstrate a drastic improvement of the structural properties. We compare the photoluminescence properties of nonpolar homoepitaxial QWs and nonpolar heteroepitaxial QWs grown on sapphire and show that the reduction in structural defects and the improvement of surface morphology are correlated with a strong enhancement of the photoluminescence properties: reduction in full width at half maximum, strong increase in the luminescence intensities and their thermal stability. The comparison convincingly demonstrates the interest of homoepitaxial nonpolar QWs for bright UV emission applications.

Effects of low charge carrier wave function overlap on internal quantum efficiency in GaInN quantum wells

AbstractTo determine relevant processes affecting the internal quantum efficiency in GaInN quantum well structures, we have studied the temperature and excitation power dependent photoluminescence intensity for quantum wells with different well widths on (0001) c‐plane GaN and for quantum wells on nonpolar (11‐20) a‐plane GaN. In thick polar quantum wells, the quantum confined Stark effect (QCSE) causes a stronger intensity decrease with increasing temperature as long as the radiative recombination dominates. At higher temperatures, when the nonradiative recombination becomes more important, thick polar quantum wells feature a lower relative intensity decrease than thinner polar or nonpolar quantum wells. Excitation power dependent photoluminescence points to a transition from a recombination of excitons to a bimolecular recombination of uncorrelated charge carriers for thick polar quantum wells in the same temperature range. This transition might contribute to the limitation of nonradiative recombination by a reduced diffusivity of charge carriers. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Growth and characterization of green GaInN-based light emitting diodes on free-standing non-polar GaN templates

We demonstrate homoepitaxial growth of GaInN/GaN-based green (500–560 nm) light emitting diodes (LEDs) on a-plane and m-plane quasi-bulk GaN prepared by hydride vapor phase epitaxy (HVPE). We find that in order to achieve an emission peak wavelength beyond 500 nm, a minimum InN-fraction of ∼14% is needed for both, a- and m-plane quantum wells (QWs), while ∼8% are enough for c-plane-oriented QWs. Besides increasing the InN-fraction in these non-polar QWs, widening the QW also proves to effectively shift the emission to longer wavelengths without loosing efficiency with the benefit of maintaining a low InN-fraction.

(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.

Excitonic properties of polar, semipolar, and nonpolar InGaN∕GaN strained quantum wells with potential fluctuations

Excitonic properties of polar, semipolar, and nonpolar InGaN∕GaN strained quantum wells (QWs) were investigated in terms of exciton localization and polarization-induced electric fields. The spontaneous emission lifetimes measured at ∼10K for the (0001) polar QWs were 1.4ns at an emission wavelength of 400nm, but increased monotonically to 85ns at 520nm. On the other hand, those for {112¯2} and {11¯01} semipolar QWs and {112¯0} and {11¯00} nonpolar QWs were on the order of a few hundred picoseconds and independent of the emission wavelength. To quantitatively discuss these results, the crystalline orientation dependence of the spontaneous emission lifetime of 1s heavy hole excitons in InGaN∕GaN QWs at 0K was calculated, when lateral confinements were considered to express well-reported potential fluctuations. It is revealed that both the crystalline orientation and lateral confinement vary the spontaneous emission lifetime by orders of magnitude. Analyses of the experimental results suggest that excitons in the (0001) polar QWs are more strongly localized as the In composition increases, but the semipolar and nonpolar QWs exhibit the opposite tendency. These tendencies are attributed to differences in the growth characteristics.