Vertical Quantum Wells Research Articles

Self organized growth of doped vertical quantum wells for normal incidence intersubband transitions

The self-organized growth of N-doped vertical AlGaAs quantum wells by metalorganic vapor phase epitaxy of a single doped AlGaAs layer on a submicron grating is described. Intersubband absorption at normal incidence is demonstrated in those vertical quantum wells. This opens new possibilities for infrared quantum well devices using intersubband transitions, including normal incidence infrared modulators.

Rectangular AlGaAs/AlAs Quantum Wires Using Spontaneous Vertical Quantum Wells

We propose a lateral confinement enhanced rectangular AlGaAs/AlAs quantum wire (QWR) structure grown in situ, which utilizes the Ga-rich AlGaAs spontaneous vertical quantum wells formed during the growth of the AlGaAs epilayer in the V-grooves. AlGaAs/AlAs QWR structures have been grown on V-grooved substrates by low-pressure metalorganic vapor phase epitaxy (MOVPE), and been investigated by transmission electron microscopy, photoluminescence and cathodoluminescence measurements.

Growth temperature dependence of AlGaAs spontaneous vertical quantum wells on V-grooved substrates by low-pressure metalorganic vapor phase epitaxy

We have studied the growth of AlGaAs on V-grooved GaAs substrates by low-pressure (100 Torr) metalorganic vapor phase epitaxy (MOVPE) in the temperature range of 600 to 700°C. The growth temperature dependence of facet formation, the facet dependence of AlGaAs composition on the V-grooves, and the growth temperature dependence of spontaneous vertical quantum well (SVQW) widths have been investigated by low temperature photoluminescence (PL), transmission electron micrograph (TEM) and spectrally and spatially resolved low temperature cathodoluminescence (CL). The width of the SVQW varies from 140 to 250 Å with increasing the growth temperature from 600 to 700°C.

Carrier transfer and capture into quantum wire arrays grown on V-groove substrates

The mechanisms of carrier transfer from barrier regions (superlattices, side-wall and vertical quantum wells) into arrays of quantum wires are investigated by time-resolved photoluminescence. The wires are grown by different techniques on V-groove substrates. The transfer times range from 25 to 900 ps, depending on the details of the whole structure. The fast carrier transfer is modeled by a diffusion equation which allows one to estimate the quantum mechanical capture time from a two-dimensional vertical QW into a one-dimensional QWR to be <2 ps.

Cavities for Intersubband Transitions

ABSTRACTThe different possible geometries for intersubband transitions in microcavities are discussed. A consequence of the selection rule governing intersubband transition is the vanishing interaction with the electromagnetic field inside a usual vertical cavity. The geometry of vertical planar cavities which has been used extensively with interband transitions is therefore useless in the case of intersubband transitions. Different solutions are reviewed to overcome this problem. The breakdown of the selection rule in a vertical cavity is first discussed. This can be done with the use of vertical quantum wells, or thanks to intracavity diffraction gratings. Second, the use of in plane cavities is discussed. Two solutions are here envisaged: Whispering gallery modes in microdisk cavities, and efficient etched air/GaAs Bragg mirrors. Concerning the latter attractive solution, the losses by diffraction into the substrate are evaluated theoretically and experimentally. The solution of the Maxwell equations by a finite element method in this three dimensional system shows the great importance of diffraction. These results are confirmed by waveguided Fourier transform spectroscopy. To overcome this difficulty, we propose the use of lower refractive index substrates, such as oxidized AlAs.

Low-pressure organometallic chemical vapor deposition of quantum wires on V-grooved substrates

The structure of GaAs/AlGaAs quantum wires (QWRs) and vertical quantum wells (VQWs) grown by low-pressure organometallic chemical vapor deposition was investigated by conventional and high resolution transmission electron microscopy, and by low-temperature cathodoluminescence. The lower wire boundaries show a much smaller radius of curvature, as compared with atmospheric pressure growth of similar structures, and the upper boundaries show distinct faceting. More abrupt interfaces are obtained due to the kinetically limited growth, with measured interface grading as small as one or two monolayers. The VQW structures formed in the AlGaAs barrier exhibit several branches related to the faceting of the QWR boundaries. These characteristics of the low-pressure nonplanar growth should allow the fabrication of quasi-one-dimensional QWRs with size and shape controlled on the monolayer level.

Photoluminescence properties of 13×13 nm GaAs quantum wires buried in trench structures reduced by growing GaAs/AlAs superlattice layers

We report lateral-size control of GaAs/AlAs trench-buried quantum wires (QWRs) on a scale of 10 nm by metalorganic chemical vapor deposition using nonplanar substrates. The lateral width is reduced to 12–13 nm by growing GaAs/AlAs superlattice layers (SLs) on the (110) sidewall facets of the trenches, where the roughness of the SL sidewalls is approximately several monolayers. Low-temperature photoluminescence (PL) properties of nearly square 13×13 nm QWRs buried in the trenches exhibit a strong PL blue shift of 85 meV with respect to the band-gap energy of GaAs bulk and PL polarization anisotropy of 25% due to two-dimensional quantum confinement effects. The emission from Ga-rich AlGaAs regions in the trenches constituting AlGaAs vertical quantum wells was also observed. We demonstrate that substituting GaAs/AlAs SLs for the AlGaAs layer effectively eliminates the undesired emission levels caused by the inevitable Al content fluctuation in the AlGaAs layer grown on nonplanar structures.

Composition profile of an AlGaAs epilayer on a V-grooved substrate grown by low-pressure metalorganic vapor phase epitaxy

A composition distribution of an AlxGa1−xAs epilayer on a V-grooved substrate grown by low-pressure metalorganic vapor phase epitaxy is described. The aluminum (Al) content of an AlxGa1−xAs epilayer on a V-grooved substrate is found to be on the order of x(111)A≳xtop(100)≳xedge-facet ≳xbottom-intersection by using spectrally and spatially resolved low temperature cathodoluminescence (CL) measurements. The composition difference between (111)A surfaces and bottom intersections is as much as 30%. The resulting spontaneous vertical quantum wells and spontaneous waveguides in the bottom intersections are expected to be applicable in low-threshold current laser devices.

Characterization of a single-layer quantum wire structure grown directly on a submicron grating

A single AlGaAs/GaAs quantum well (QW)/quantum wire (QWR) structure was grown by metalorganic vapor phase epitaxy on a submicron period grating of V grooves. High resolution transmission electron microscopy studies of the sample identifies three regions of the QW; between the grooves, approximately 3.5 nm thick and oriented along (100), on the sidewalls of the V groove slightly thinner and oriented along {111}. At the bottom of the groove an approximately 70 nm wide crescent shaped region forms a QWR. In addition, a vertical quantum well (VQW) extends from the bottom of each V groove in the GaAs substrate to the surface. The luminescence spectra of the sample are dominated by a peak originating in the QW, with additional peaks of the QWR, the VQW and the AlGaAs barrier. The striped nature of the sample is revealed in the top view cathodoluminescence (CL) images of all four peaks. In side view CL images, the QWR emission appears spot like, whereas the emission of the VQW is elongated in the direction perpendicular to the surface. Photoluminescence excitation spectroscopy reveals that the main source for excitation in the QWR comes from the VQW, even though a small contribution comes from the QW.

Normal incidence intersubband absorption in vertical quantum wells

N-doped vertical AlGaAs quantum wells have been fabricated by metalorganic vapor phase epitaxial growth of a single-doped AlGaAs layer on a submicron grating. Intersubband absorption at normal incidence is demonstrated in those quantum wells. This opens new possibilities for infrared quantum well devices using intersubband transitions.

Cathodoluminescence of single quantum wires and vertical quantum wells grown on a submicron grating

We present cathodoluminescence (CL) investigations of a corrugated GaAs/AlGaAs single quantum well (QW) structure grown on a submicron grating. The CL spectra have four distinct emission peaks. Using plan-view and cross-sectional CL imaging together with cross-sectional transmission electron microscope imaging, we have assigned the four peaks: They originate in the nominal QW, a quantum wire (QWR), a vertical quantum well (VQW), and the barrier, respectively. We have CL-imaged and -characterized single QWRs and VQWs.

Atmospheric and low pressure metalorganic vapor phase epitaxial growth of vertical quantum wells and quantum well wires on submicron gratings

Nonplanar metalorganic vapor phase epitaxial growth on submicron gratings has been studied. Growth conditions have been determined to preserve the grating structure and also to enhance the formation of crescent shaped quantum well wire-like GaAs layers. These growth parameters have been used to grow the layer structure of a quantum well wire (QWW) laser, only needing one growth run. Although there is not yet clear evidence for two-dimensional quantum confinement, this technique offers some interesting perspectives for the realization of QWW lasers.

Fabrication of quantum well wires and vertical quantum wells on submicron gratings by MOVPE

The fabrication of quantum well wires (QWWs) by overgrowing submicron gratings is reviewed. The advantages of one-dimensional semiconductor structures have already been understood for some time and are expected to alter laser performances, such as extremely low threshold currents, very high modulation bandwidths, very low temperature dependence. The realization of the nanostructures, however, has set some major demands on crystal growth. The MOVPE growth on submicron gratings (etched in a GaAs substrate) has become one of the most successful techniques and is studied in this paper. Vertical ‘quantum wells’ (VQWS) (formed during growth of bulk Al x Ga 1- x As on gratings) are also studied and a new fabrication technique for QWW lasers is proposed. Finally the application of QWWs and VQWs is discussed for the realization of Fabry-Perot and distributed feedback laser diodes.

Anisotropic photoluminescence behaviour of vertical AlGaAs structures grown on gratings

The formation of vertical AlGaAs quantum wells during MOVPE growth of AlGaAs layers on submicron gratings has been investigated. The influence of the growth temperature, growth velocity, V/III ratio and the overall Al content has been examined and explained using surface diffusion effects of the group III atoms (or reactant species). 77 K PL measurements show a polarization anisotropy and indicate lateral quantum confinement. Also the growth of QWWs on submicron gratings is reported, showing very large PL intensities and polarization anisotropy. Finally the realization of QWWs has been proposed by growing Al x Ga 1- x As/Al y Ga 1- y As QWs on gratings and based on the different surface mobilities of Ga and Al.