F. Flack

Zero-dimensional excitonic confinement in locally strained Zn1−xCdxSe quantum wells

We report on the observation of zero-dimensional (0D) excitonic confinement in locally strained Zn1−xCdxSe quantum wells. Strain fields from self-organized CdSe quantum dots are used to locally modulate the band structure of a nearby quantum well in a heterostructure, resulting in confinement in all three dimensions. The 0D nature of excitonic confinement is verified by the observation of sharp lines in photoluminescence (PL) spectra. The temperature dependence of the PL lifetime is markedly different than that of the CdSe quantum dots. High-resolution spectra show that the PL lines from the localized states are split into linearly polarized doublets.

Magneto-optical spin spectroscopy in hybrid ferromagnetic semiconductor heterostructures

We report on magneto-optical measurements of diluted magnetic semiconductor quantum well structures in which an iron film is epitaxially grown and subsequently patterned on top of the heterostructure. The magnetic field due to the iron penetrates the quantum well near the edges of the patterned regions. Photoluminescence, Hanle effect, and time-resolved Faraday rotation measurements demonstrate that the films can be processed without significantly altering the electronic properties of the quantum well. The smallest length scale probed in these far-field optical measurements is of order 10 μm, so that no effects of the local magnetic fields at the film edges are observed for structures with 50-nm-thick ferromagnetic layers.

Spectroellipsometry for characterization of Zn1−xCdxSe multilayered structures on GaAs

The dielectric functions of 0.5–1.5‐μm‐thick Zn1−xCdxSe (0≤x≤0.34) epilayers on (100) GaAs were measured by spectroellipsometry (SE) over the photon energy range 1.5≤E≤5.3 eV. These spectra were parameterized using the Sellmeier and Lorentz equations for photon energies below and above the fundamental gap region, respectively. We have demonstrated the usefulness of this parameterization in analyses of SE data collected on a ZnSe heterostructure and a Zn1−xCdxSe quantum well that provide accurate layer thicknesses and compositions.

Optical spectroscopy of II–VI (magnetic) semiconductor quantum dots

We have probed the magnetooptical properties of II–VI semiconductor quantum dots at low temperatures (T=5K) in magnetic fields up to 6T. A layer of coherently strained CdSe dots of diameters 5–20nm is introduced into heterostructures containing either a Zn0.8Cd0.2Se quantum well or a barrier layer of MnSe. In the quantum well structure, carriers are localized in strain fields due to the nearby quantum dot layer. The effective g-factors g∗ of excitons in the localized states are ∼−1.5. In contrast, g∗∼6–7 for carriers localized in CdSe islands coupled to an adjacent MnSe barrier. The enhancement of the g-factor in this case is due to the overlap of the tails of the localized wave functions with Mn++ ions in the barrier.

Iodine use in solid‐source III–V molecular‐beam epitaxy

Use of iodine for in situ etching of GaAs, AlAs, and InAs in solid‐source molecular‐beam epitaxy has been explored. Reflectance high‐energy electron‐diffraction intensity oscillations have been observed during iodine etching of GaAs and InAs, indicating a molecular layer‐by‐layer nature of material removal. Etch rates of GaAs and InAs determined from the period of the oscillations are comparable for a given iodine flux. Secondary‐ion‐mass spectroscopy depth profiles indicate that the etching of AlAs is negligible, and that iodine can be used as a selective etch which stops on AlAs or AlGaAs layers. Electrical properties of GaAs layers grown with an iodine beam impinging on the surface are comparable to those of the layers grown without iodine. Use of iodine for the surface cleaning of GaAs has also been examined. Our results show that iodine etching prior to growth reduces the level of carbon contamination at the substrate epilayer interface.

Spin transport and optically-probed coherence in magnetic semiconductor heterostructures

Molecular beam epitaxy is used to “spin engineer” an environment wherein quantum-confined electronic states in a wide band gap II–VI semiconductor quantum well (Zn1−xCdx Se) are strongly exchange-coupled to systematic 2D distributions of localized spins (Mn2+ ions). Magneto-optical spectroscopy of undoped structures demonstrates that such a scheme successfully produces well-confined excitonic states whose Zeeman splitting in modest magnetic fields greatly exceeds the inhomogeneous line widths. In modulation-doped structures, a combination of magneto-transport and magneto-optical measurements shows the formation of a “magnetic” two-dimensional electron gas characterized by spin gaps which are much larger than Landau level gaps. This results in a novel quantum Hall system which can be highly spin polarized even at large filling factors. Time-resolved Faraday/Kerr effect measurements in the Voigt geometry probe the electronic spin dynamics of the exciton/electron gas, revealing terahertz and gigahertz oscillations that originate from the coherent spin precession of electrons and local moments, respectively.

Composition and temperature dependence of the optical properties of Zn 1-x Cd x Se (0 ≤ x ≤ 0.34) below the fundamental bandgap

The II-VI ternary semiconductor alloy system Zn(1-x) Cd(x) Se with 0 < or = x < or = 0.2 has important applications as the active material in blue-green light-emitting diodes and lasers. For the wavelength and temperature ranges over which these devices are designed to operate, a knowledge of the optical properties of the alloys is important. We report the results of spectroscopic ellipsometry measurements of the real part of the dielectric function epsilon1 for Zn-rich Zn(1-x) Cd(x) Se layers deposited epitaxially on (100) GaAs. We derive compact expressions that allow one to calculate accurate epsilon1 spectra from 1.5 eV, the low-energy limit of our ellipsometer, to E0-0.05 eV, where E0 is the fundamental bandgap energy, for any composition and temperature within the ranges 0 < or = x < or = 0.34 and 25 < or = T < 260 degrees C. Furthermore, we expect that the results can also be extrapolated to cover the substrate temperature range typically used for the growth of these films (250-300 degrees C). Hence the results presented here are also useful in future real-time spectroscopic ellipsometry studies of Zn(1-x) Cd(x) Se film growth.

Iodine-assisted molecular beam epitaxy

Abstract Iodine was introduced into our solid source molecular beam epitaxy chamber during the growth of GaAs and AlGaAs layers and strained-layer InGaAs quantum wells. Photoluminescence spectra of these samples taken at 4.2 K were compared to spectra from a series of test samples which were grown in the absence of iodine flux. We have observed a general improvement of the material quality of AlGaAs layers grown at substrate temperatures around and below 600°C since the introduction of iodine into our chamber regardless of whether an iodine flux impinged on the substrate during the growth process or not. Strong excitonic peaks with full width at half maximum of only 8 meV were observed in 4.2 K photoluminescence spectra of undoped Al 0.2 Ga 0.8 As layers grown at substrate temperatures as low as 550°C using As 4 . The epitaxial layers grown since the introduction of iodine are the brightest Al 0.2 Ga 0.8 As samples that we have obtained from our molecular beam epitaxy system. These results suggest that the presence of iodine in a molecular beam epitaxy system can result in improved optical properties for AlGaAs. The 4.2 K photoluminescence spectra and secondary ion mass spectroscopy depth profiles also show that iodine preferentially removes Ga from AlGaAs layers during growth, resulting in layers with higher Al content.