Vertical Quantum Wells Research Articles

Group IV Light Sources to Enable the Convergence of Photonics and Electronics

Group IV lasers are expected to revolutionize chip-to-chip optical communications in terms of cost, scalability, yield, and compatibility to the existing infrastructure of silicon industries for mass production. Here, we review the current state-of-the-art developments of silicon and germanium light sources towards monolithic integration. Quantum confinement of electrons and holes in nano-structures has been the primary route for light emission from silicon, and we can use advanced silicon technologies using top-down patterning processes to fabricate these nano-structures, including fin-type vertical multiple quantum wells. Moreover, the electromagnetic environment can also be manipulated in a photonic crystal nano-cavity to enhance the efficiency of light extraction and emission by the Purcell effect. Germanium is also widely investigated as an active material in Group IV photonics, and novel epitaxial growth technologies are being developed to make a high quality germanium layer on a silicon substrate. To develop a practical germanium laser, various technologies are employed for tensile-stress engineering and high electron doping to compensate the indirect valleys in the conduction band. These challenges are aiming to contribute towards the convergence of electronics and photonics on a silicon chip.

Vertical InGaN Multiple Quantum Wells Light-Emitting Diodes Structures Transferred from Si(111) Substrate onto Electroplating Copper Submount with Through-Holes

Vertical InGaN Multiple Quantum Wells Light-Emitting Diodes Structures Transferred from Si(111) Substrate onto Electroplating Copper Submount with Through-Holes

Vertical InGaN Multiple Quantum Wells Light-Emitting Diodes Structures Transferred from Si(111) Substrate onto Electroplating Copper Submount with Through-Holes

Vertical InGaN multiple quantum wells light-emitting diodes (LEDs) with through-holes structure were transferred from Si(111) substrate onto the electroplating copper submount successfully. The additional series resistances induced by the AlN buffer layer and other interlayer were shorted by the metals filled through-holes. The LED with through-hole structure shows a low vertical conducting operating voltage and a small series resistance. Combining with substrate removal and copper electroplating technique, the operating voltage at 350 mA and series resistances of the LED were reduce from 5.6 to 5.1 V and 7 to 4 Ω, in comparison with through-hole LED before substrate removal. At the same time, the light output intensity was improved by 75%, which was mainly attributed to both the removal of light absorptive substrate and the substitution for highly thermal conductive copper submount with metal reflector.

Ultrashort-period lateral composition modulation in TlInGaAsN/TlInP structures

We prepared TlInGaAsN/TlInP quantum well structures using gas source molecular-beam epitaxy and characterized them by means of transmission electron microscopy and scanning transmission electron microscopy. It was found that naturally formed vertical quantum wells, so-called lateral composition modulation (LCM), with a periodicity of ∼1 nm are formed in TlInGaAsN layers. We discuss their formation process using a simple kinetic Ising model for layer-by-layer growth, and point out that the formation of ultrashort-period LCM is a universal phenomenon in most of epitaxially grown III-V semiconductor alloys.

Time-resolved cathodoluminescence of InGaAs/AlGaAs tetrahedral pyramidal quantum structures

An original time resolved cathodoluminescence set up has been used to investigate the optical properties and the carrier transport in quantum structures located in InGaAs/AlGaAs tetrahedral pyramids. An InGaAs quantum dot formed just below the top of the pyramid is connected to four types of low-dimensional barriers: InGaAs quantum wires on the edges of the pyramid, InGaAs quantum wells on the (111)A facets and segregated AlGaAs vertical quantum wire and AlGaAs vertical quantum wells formed at the centre and at the pyramid edges. Experiments were performed at a temperature of 92 K, an accelerating voltage of 10 kV and a beam probe current of 10 pA. The cathodoluminescence spectrum shows five luminescence peaks. Rise and decay times for the different emission wavelengths provide a clear confirmation of the peak attribution (previously done with other techniques) to the different nanostructures grown in a pyramid. Moreover, experimental results suggest a scenario where carriers diffuse from the lateral quantum structures towards the central structures (the InGaAs quantum dot and the segregated AlGaAs vertical quantum wire) via the InGaAs quantum wires on the edges of the pyramid. According to this hypothesis, we have modeled the carrier diffusion along these quantum wires. An ambipolar carrier mobility of 1400 cm2/V s allows to obtain a good fit to all temporal dependences.

Alloy Segregation, Quantum Confinement, and Carrier Capture in Self-Ordered Pyramidal Quantum Wires

The structure and photoluminescence (PL) characteristics of AlGaAs quantum wires self-formed by metallorganic vapor-phase epitaxy in inverted tetrahedral pyramids are reported. Capillarity-driven Ga-Al segregation yields vertical quantum wires (VQWRs) at the center of the pyramid, connected to more-weakly segregated vertical quantum wells (VQWs) formed along their wedges. The segregation is evidenced in transmission electron microscope images and in the PL spectra of these structures. Transitions between quantum-confined electron and hole states in the VQWR are identified in the micro-PL spectra with energies in good agreement with model calculations. The temperature dependence of the micro-PL spectra clearly reveals efficient carrier capture into the VQWR from the VQWs, particularly at an intermediate temperature range ( [similar to] 100 K) where carrier mobility is enhanced. These wires offer new possibilities for tailoring the confinement potential in one-dimensional systems.

Carrier transfer and magneto-transport in single modulation-doped V-grooved quantum wire modified by ion implantation

A single Al 0.5Ga 0.5As/GaAs V-grooved quantum wire modified by selective ion implantation and rapid thermal annealing was investigated by using spatially resolved micro-photoluminescence spectroscopy and magneto-resistance measurements. The results of spatially resolved photoluminescence indicate that the ion-implantation-induced quantum well intermixing significantly raises the electronic sub-band energies in the side quantum wells (SQWs) and vertical quantum wells, and a more efficient accumulation of electrons in the quantum wires is achieved. Processes of real space carrier transfer from the SQW to the quantum wire was experimentally observed, and showed the blocking effect of carrier transfer due to the existence of the necking quantum well region. Furthermore, magneto-transport investigation on the ion-implanted quantum wire samples shows the quasi-one-dimensional intrinsic motion of electrons, which is important for the design and the optimization of one-dimensional electronic devices.

Optical and electronic properties of single modulation doped GaAs/AlGaAs V-grooved quantum wire modified by ion implantation

Single GaAs/Al0.5Ga0.5As V-grooved quantum wire modified by selective ion-implantation and rapid thermally annealing was investigated by spatially-resolved microphotoluminescence and magneto-resistance measurement. Spatially-resolved photoluminescence results indicate that the ion-implantation induced quantum well intermixing raises significantly the electron subband energies of the side quantum wells and vertical quantum wells, and more efficient accumulation of electrons in the quantum wires is achieved. Furthermore, the polarization properties of the photoluminescence from the quantum wires show large linear polarization degree up to 63%. Magnetotransport investigation on the ion implanted quantum wire samples presents the quasi-one dimensional intrinsic motion of electrons, which is important for the design and optimization of one dimensional electronic devices.

Impact of nucleation conditions on the structural and optical properties of M-plane GaN(11̄00) grown on γ-LiAlO2

We investigate the structural and optical properties of M-plane GaN(11̄00) films grown on LiAlO2(100) with nucleation layers grown at high and low temperatures. Samples with a high temperature nucleation layer are found to exhibit a highly anisotropic surface morphology with pronounced corrugation, which basically replicates the surface morphology of the substrate. Photoluminescence spectra of these layers are dominated by a transition at 3.356 eV, which is absent for samples with a low-temperature nucleation layer. In conjunction with scanning electron microscopy, cathodoluminescence maps reveal that this transition predominantly stems from regions below the trenches of the surface corrugation. Transmission electron microscopy shows an abundance of stacking faults within these regions. Excitation-dependent and time-resolved photoluminescence demonstrates the intrinsic character of the 3.356 eV emission, which is thus attributed to excitons bound to stacking faults acting as ultrathin vertical quantum wells in these samples. Low-temperature nucleation is imperative to avoid thermal roughening of the substrate and thus the formation of a high density of stacking faults.

High internal quantum efficiency, narrow linewidth InGaAs/GaAs/AlGaAs quantum wire light-emitting diodes

High internal quantum efficiency (∼60%), narrow linewidth (as narrow as 14 meV) exciton emission at room temperature has been obtained using strained InGaAs V-groove quantum wire (QWR) light-emitting diodes (LEDs). The high efficiency is achieved with the aid of selective carrier injection through self-ordered AlGaAs vertical quantum wells (VQWs), where the VQWs are separated from the InGaAs QWRs by thin GaAs spacer layers in order to reduce nonradiative recombination and inhomogeneous alloy broadening. Evidence for excitonic recombination in these LEDs up to RT is provided by measurements of the emission energy shifts at high magnetic fields.

Carrier transfer between V-grooved quantum wire and vertical quantum well

We report the temperature dependence of the photoluminescence from our V-grooved GaAs/AlGaAs quantum wires. Based on the specially resolved luminescence results, our results show that in the processes of real space carrier transfer from the vertical quantum well region to quantum wire region, a 3.5 meV thermal activation energy has been experimentally observed in our sample. The thermal activation is attributed as the scattering process of carriers from two-dimensional states in the vertical quantum well to the one-dimensional quantum wire states. Based on numerical energy band structure analysis, a thermal activation energy of 2–9 meV is obtained theoretically, in good agreement with the experimental data.

Strain Induced Compositional Modulations in AlGaAs Overlayers Induced by Lateral Surface Gratings

ABSTRACTStrain and compositional modulation in AlxGa1−xAs layers grown by metalorganic vapour phase epitaxy (MOVPE) over a sinusoidally shaped GaAs (001) surface grating were studied by scanning electron microscopy (SEM), X-ray grazing-incidence diffraction (GID) and photoluminescence (PL). Two growth temperatures and two compositions were chosen to realize planar overlayers. By SEM a periodic reduction in Al-content was found at the valley positions of the GaAs grating. The appearance of such vertical quantum wells (VQWs) has been explained by the growth rate anisotropy between high-index and (001) planes and a curvature-induced capillarity flow of Ga. Estimated from PL energies a larger reduction of the Al-concentration in the VQW and also at the high-index sidewall facets was found than compared to predictions from the capillarity flow theory. Using depth-resolved GID we show that the formation of VQWs is accompanied by a periodic lateral strain field. Therefore we assume, that the formation of the VQWs is influenced by strain induced diffusion due to the interaction of opposite sidewall facets.

THE MICRO-PHOTOLUMINESCENCE OF A SINGLE V-GROOVE GaAs/AlGaAs QUANTUM WIRE

The micro-photoluminescence(μ-PL) scan has been performed on a single V-groove GaAs/AlGaAs quantum wire in the direction perpendicular to the wire.The variatio n of μ-PL spectra from various quantum structures with different spatial positi ons has been observed.In the region of quantum wire (QWR) the PL spectra contain the peaks from QWR,necking quantum well (NQWL) and vertical quantum well (VQWL) ,while in the area about 1μm and farther away from the QWR the PL spectra show the peaks from side-wall quantum well (SQWL) and top quantum well (TQWL).All the scanned PL spectra have been fitted by Gauss line shape.The asymmetric PL peaks of QWR and SQWL were decomposed into two components which were ascribed to the optical transitions between the electron ground state in the conduction band and the heavy hole and the light hole ground states in the valence band.The variati on of PL intensity from luminescent components with the spatial positions direct ly demonstrates the origin of the photoluminescence relating to different quantu m structures,and reflects the carrier transfer from SQWL to QWR,resulting in th e PL quenching of SQWL.

Energy sublevels in an Al0.5Ga0.5As/GaAs/Al0.5Ga0.5As quantum wire

We report the successful fabrication of a V-grooveAl0.5Ga0.5As/GaAs/Al0.5Ga0.5As quantum wire system and the temperature-dependent photoluminescence (PL) measurement. The PL spectra are dominated by four features at 681, 642, 635 and 621 nm attributed to the luminescences from quantum wire, top, vertical and side-wall well regions by micro-PL measurements. By the calculations of the energy structure, discrete states (localized sublevels) in the quantum wire region and continuum states (extended along the side-wall and vertical quantum wells) in side-wall and vertical quantum wells have been obtained in both the conduction and valence bands. The calculated excitation energies explain very well the peak positions and their temperature dependence in the photoluminescence measurements.

Selective carrier injection into V-groove quantum wires

We have observed selective carrier injection into GaAs/AlGaAs V-groove quantum wires (QWRs) via self-ordered vertical quantum wells (VQWs). Room-temperature I–V characteristics of QWR diodes show a turn-on voltage lower by 0.2 V as compared with planar QW diodes, consistent with the band-gap reduction of 0.2 eV at the vertical QW. This selective injection results in narrow linewidth electroluminescence (∼5 nm at 300 K) emanating exclusively from the QWR from 10 K up to 300 K.

Exciton diffusion dynamics in quantum nanostructures on V-groove patterned substrates

Time-resolved photoluminescence (PL) measurements were performed for two different quantum nanostructures on V-groove patterned substrates; SiGe/Si quantum wells (QWs) on V-grooved Si substrates and AlGaAs spontaneous vertical quantum wells on V-grooved GaAs substrates. Anomalous behaviours of the PL, such as the decrease of the decay time of the SiGe (111) QWs and the (111)A AlGaAs layer, were fully explained by taking the exciton diffusion towards the bottom of the V-groove into account, showing that the exciton diffusion driven by the spatial nonuniformity of the alloy compositions and/or geometry of the substrates is a key to controlling the PL properties of the nanostructures.

Carrier quantum confinement in self-orderedAlxGa1−xAsV-groove quantum wells

Observation of quantum confined states and optical anisotropy of the interband absorption in self-ordered ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ vertical quantum wells (VQW's) grown on V-grooved substrates is reported. The variation in Al composition across the VQW's was determined by parallel electron energy-loss spectroscopy. It was then employed to calculate the eigenstates of the confined carriers in these structures, with valence-band mixing included using a $4\ifmmode\times\else\texttimes\fi{}4$ $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Luttinger model. Polarized photoluminescence excitation spectra of the structure show features attributed to transitions between electron and heavy-hole or light-hole confined eigenstates, with measured energies in good agreement with the calculated ones. Polarization anisotropy associated with confined electrons and holes is evidenced by the measured spectra and explained by valence-band mixing in the (011) self-ordered quantum well.

Self-limiting growth of quantum dot heterostructures on nonplanar {111}B substrates

We study the evolution of AlGaAs/GaAs growth during organometallic chemical vapor deposition on pyramidal recess patterns etched into GaAs {111}B substrates. Cross-sectional atomic force microscopy clearly demonstrates the self-organized growth behavior in the inverted pyramid structures. During AlGaAs deposition, the side corners and the tip of the pyramid sharpen up to a self-limited radius of curvature of less than 10 nm. In addition, vertical Ga-rich AlGaAs quantum wells are formed at these corners. Subsequent GaAs growth results in the formation of GaAs quantum wires along the corners of the pyramid. These wires meet at the tip of the pyramid, forming a quantum dot structure at this point.

Low-pressure OMCVD growth of AlGaAs vertical quantum wells on non-planar substrates

AlGaAs vertical quantum well structure were formed by low-pressure (20 mbar) organometallic chemical vapor deposition on V-grooved substrates. Ga segregation in the wells of these structures was evidenced by means of atomic force microscopy and photoluminescence experiments. Transmission electron microscopy cross sections resolve different branches in the well. This substructure, whose definition is enhanced in low-pressure growth, is associated with the formation of specific facets at the bottom of the grooves. The dependence of the growth rate on the crystallographic facet and the Al mole fraction is the key mechanism for the establishment of the self-limiting profile in the V-grooves.

Structure and formation mechanisms of AlGaAs V-groove vertical quantum wells grown by low pressure organometallic chemical vapor deposition

The structure of AlGaAs vertical quantum well (VQW) structures grown by low-pressure organometallic chemical vapor deposition on V-grooved GaAs substrates was analyzed as a function of growth temperature and Al mole fraction using transmission electron microscopy and atomic force microscopy (AFM). The low-pressure growth yields several, extremely narrow (a few nm wide) branches of Ga-enriched VQWs at the bottom of the grooves. The variation in Al content across the VQW was evaluated by measuring the AlGaAs oxide thickness on a cleaved edge of the structure using AFM in air. The transmission electron microscopy analysis demonstrates that the different VQW branches originate from distinct nanofacets that self-order at the bottom of the V-groove, probably due to facet-induced segregation of group III species.