M. Grün

Electrical spin injection from ZnMnSe into InGaAs quantum wells and quantum dots

We report on efficient injection of electron spins into InGaAs-based nanostructures. The spin light-emitting diodes incorporate an InGaAs quantum well or quantum dots, respectively, as well as a semimagnetic ZnMnSe spin-aligner layer. We show a circular polarization degree of up to 35% for the electroluminescence from InGaAs quantum wells and up to 21% for InGaAs quantum dots. We can clearly attribute the polarization of the emitted photons to the spin alignment in the semimagnetic layer by comparison to results from reference devices (where the ZnMnSe is replaced by ZnSe) and from all-optical measurements.

Polarized luminescence in CdS/ZnSe quantum-well structures

The photoluminescence from type II CdS/ZnSe quantum-well structures is found to be polarized with respect to the 〈110〉 directions with polarization degrees up to 20%. The absolute polarization direction is related to the interface bond directions in samples with differently prepared interfaces. The observations are explained by the detailed analysis of the epitaxial growth process and polarization sensitive luminescence experiments.

Stimulated emission of II–VI epitaxial layers

Abstract After a short review of the typical gain processes in II–VI semiconductors, we present examples for various compounds considering rather thick epilayers and superlattices. The observed gain processes include inelastic scattering processes, biexcitons and plasma formation.

Properties and self-organization of CdSe:S quantum islands grown with a cadmium sulfide compound source

We demonstrate a new technique to grow high-quality CdSe quantum films and islands with a very small sulfur contamination by using a cadmium sulfide compound source as Cd supply and additional Se flux. By monitoring the lattice constant with reflection high-energy electron diffraction, it is shown that the sulfur is almost completely substituted by Se and CdSe with a contamination below 5% sulfur is formed. The quantum structures obtained by the new method are generally of higher quality than those obtained by more conventional growth methods using elemental sources, even if migration enhanced methods were employed. With a brief growth interruption or post-growth annealing step the initially smooth CdSe layer can be reorganized into islands. The duration of this step as well as the initial amount of deposition allows a rather good control over the island formation. A strongly enhanced growth rate is observed for the first few monolayers of the ZnSe capping layer, which indicates a partial dissolution of the islands in the ZnSe growth front and Cd segregation.

On the nature of nanometer-scale islands formed by cadmium selenide deposition on hexagonal cadmium sulfide (0001)A

Using ex situ atomic force microscopy, nanometer-scale islands were observed on CdS(0001)A surfaces to appear after deposition of small amounts of CdSe. The sizes of these islands are similar to those reported in atomic force microscopy (AFM) studies of uncapped CdSe/ZnSe structures. Investigations by force modulation microscopy and photoluminescence spectroscopy show, however, that they are difficult to assign to a CdSe Stranski–Krastanow growth. Instead, AFM points to an onset of the island formation with the exposure of the samples to atmosphere. By use of electron microprobe analysis, it is suggested that these islands consist of selenium.

Wurtzite-type CdS and CdSe epitaxial layers I. Growth and characterization

Abstract CdS and CdSe layers have been grown by hot-wall (-beam) epitaxy on BaF2(111) and GaAs( 1 )B ( 1 ) ( 1 )BBsubstrates. Both CdS and CdSe adopted the wurtzite crystal structure, as found by electron diffraction. For characterization, photoluminescence and reflection spectra were measured at 4.2 K. The luminescence spectra revealed dominant bound-exciton features and a better quality of these hexagonal layers than usually obtained for zinc-blende-type ones.

Picosecond luminescence dynamics in CdS/CdSe Stark superlattices

Screening of the intrinsic piezoelectric fields in hexagonal, strained layer CdS/CdSe type II superlattices by optically generated electron‐hole pairs leads to a shift of the optical transitions of up to 400meV. After intense picosecond excitation, the transition energy follows a logarithmic law as a function of time. This temporal behavior is consistent with model calculations of the density dependent transition‐matrix elements in the tilted bandstructure.

Suppression of lateral fluctuations in CdSe-based quantum wells

We report a reduction of inhomogeneous broadening in CdSe-related quantum wells in ZnSe by employing a growth technique that uses a CdS-compound source instead of the standard Cd elemental source for molecular-beam epitaxy. Assisted by the low sticking coefficient of sulfur and possibly an exchange reaction between S and Se, only a small S contamination is observed. A comparison with standard layers reveals an increase in quality and homogeneity by a strong reduction of the photoluminescence (PL) linewidth. Samples obtained by our method show extremely little lateral confinement as indicated by a lack of sharp single dot emission lines in micro PL and the absence of the extensive redshift observed in temperature dependent PL of fluctuating well potentials.

Carrier-density-dependent electron effective mass in Zn1−xMnxSe for 0⩽x⩽0.13

We used room-temperature infrared reflectivity measurements to investigate n-type chlorine-doped Zn1−xMnxSe epilayers (0⩽x⩽0.13). By making Drude-Lorentz-type multioscillator fits to our data, we extracted the optical electron effective mass (m*) in doped Zn(Mn)Se:Cl samples with different Mn content and doping concentrations. Our results indicate that m* in Zn1−xMnxSe is lower than that for ZnSe. In n-type chlorine-doped ZnSe samples with different doping concentrations, m* varied from 0.133m0 to 0.152m0, while in Zn0.87Mn0.13Se:Cl samples, we found a variation from 0.095m0 to 0.115m0 within ±9% experimental accuracy. From theoretical calculations, we estimate that the band-edge electron masses in ZnSe:Cl and Zn0.87Mn0.13Se:Cl should be about 0.132m0 and 0.093m0, respectively.

Electrical and optical properties of the CdS quantum wells of CdS/ZnSe heterostructures

Earlier we reported the investigation of the electrical properties of selectively doped and degenerate CdS/ZnSe quantum heterostructures grown by molecular beam epitaxy [V. Kažukauskas, M. Grun, St. Petillon, A. Storzum, and C. Klingshirn, Appl. Phys. Lett. 74, 395 (1999)]. The maximum Hall mobilities in these heterostructures were found to be less than 400 cm2/Vs. In the present work we analyze in detail the scattering mechanisms in order to increase the carrier mobility and to optimize these quantum structures. We demonstrate that the Hall mobility can reach in the CdS quantum wells at low temperatures 2800 cm2/V s for slightly doped structures, having an effective sheet carrier density 2.6×1011 cm−2. In these structures the mobility is mostly limited by interface alloying scattering. At high doping levels carriers become redistributed between the quantum well and the ZnSe doped layer. This causes the parallel conductivity phenomena, which diminishes the effective mobility. Near room temperature the sca...