We investigated the n-type doping of the wide-gap II-VI semiconductor (CdMg)Te. The n-type doping of (CdMg)Te has previously been achieved in only a small range of magnesium concentration. By the use of zinc iodine as dopant source material, we obtained highly doped (CdMg)Te layers up to a magnesium concentration of 40%. The limiting factor for the free carrier concentration at room temperature is the occurrence of a deep level, which dominates the electrical properties at room temperature of layers with more than 30% magnesium. Compensating defects or defect complexes are considered, to explain the observed properties of the deep level, which do not seem to be characteristic of an isolated donor state.
Magnetization measurements performed on molecular beam epitaxy grown Cd_{1-x}Mn_{x}Te structures revealed basically similar magnetic properties of thick epilayers to their bulk counterparts. However, remarkably different properties were detected for a superlattice. These are attributed to a smearing out of the Mn profile in the superlattice.
Upon deposition of Cu on a 1 × 1 α-Al_{2}O_{3} (0001) surface at room temperature the surface structure was found to change with increasing amounts of Cu deposited, from two-dimensional monolayer islands through three-dimensional nuclei until reaching a thin film. Subsequent surface heat-treatment to 650°C produced a stable Cu(111)-R30° superstructure as observed by LEED.
The nonequilibrium growth technique of molecular beam epitaxy (MBE) has provided for the fabrication and investigation of a multitude of novel layered heterostructures based on II-VI compound semiconductors. The ability to grow epitaxial metastable magnetic and semimagnetic semiconductors layered with conventional II-VI semiconductors has resulted in structures which, for example, exhibit frustrated antiferromagnetism, and a wide wavelength tunability due to selftrapping of excitons in ZnTe-containing layered structures and due to extremely large (≈ 1 eV) quantum shifts of light emission from MnTe/CdTe superlattice structures. In addition, the control in the stoichiometry of surfaces and the composition of molecular beams used in the MBE growth technique has allowed for the fabrication of very advanced heterostructures which have combined the II-VI and III-V families of compound semiconductors. The work which will be described in the following review represents a very small sampling of the many important results achieved in the field of II-VI based heterostructures. The topics have been selected to illustrate and provide an example of the utility of MBE and the potential of "engineered" II-VI heterostructures and quantum wells.
The heterovalent interface ZnSe/GaAs, despite the small lattice misfit, still poses certain problems. The condition of the substrate surface prior to growth start determines the initial growth conditions, which on the other hand are assumed to be responsible for defect densities. Since Zn, in contrast to Se, hardly binds to GaAs the initial surface during growth start is essentially Se terminated. Therefore the binding of Mg to Se terminated GaAs was investigated. The structural quality of 140 nm thick ZnSe layers on different MgSe coverages were compared to conventionally grown and Te initiated ZnSe epilayers of the same thickness.
We describe the main problems encountered in MBE growth of GaAs/AlAs superlattices and heterostructures. Then, basic features for the understanding of their electronic properties are given, in the envelope-function formalism, and some related optical experiments are reviewed.
We report on growth by molecular beam epitaxy of thick layers of MnTe with zinc blende structure. Films as thick as 5.6 µm were obtained. Characterization by X-ray diffraction proved their good structural quality. We determined the lattice constant and its temperature dependence. Broad luminescence due to internal Mn^{2+}- transitions was observed. It showed an unexpected temperature dependence.
The strain relaxation kinetics of ZnTe/CdTe and CdTe/ZnTe heterostructures grown on GaAs substrates by molecular beam epitaxy are studied by in situ reflection high-energy electron diffraction. The observed critical layer thickness is 5 monolayers for ZnTe/CdTe and less than 1 monolayer for CdTe/ZnTe. The relaxation is anisotropic. Dislocation core parameters and relaxation rate constants were determined using a kinetic model and assuming strain-dependent activation energy of dislocation movement.
This paper discusses molecular beam epitaxy with particular emphasis on the production of state of the art electronic and optoelectronic low dimensional structures and devices. The molecular beam epitaxy process is outlined briefly and the practical problems associated with producing "state of the art" (Al,Ga)As/GaAs structures are considered. Examples include high mobility electron and hole gases, low threshold current lasers and the multi-quantum well solar cells.
Samples with InGaAs/GaAs quantum wells were grown by metallo-organic chemical vapour deposition in order to detect and analyze GaSb islands deposited on the surface. Results of photoreflectance measurements of quantum wells are reported. The correspondence between broadening of quantum well transition lines and GaSb structures has been observed.
The first results obtained with the use of Ga_{2}S_{3} and Ga_{2}Se_{3} compounds as sources of donor elements for molecular beam epitaxy of Al_{x}Ga_{1-x}Sb (0 ≤ x ≤ 1) and Al_{x}Ga_{1-x}As (0 ≤ x ≤ 0.4) are reported. In GaAs free electron concentrations obtained when incorporating the donors from these sources can be easily controlled in the range of three orders of magnitude. For Al_{x}Ga_{1-x}Sb it was possible to compensate the high concentration of native acceptors and to obtain n-type of conductivity.
Photoluminescence studies of molecular beam epitaxy grown ZnSe-on-GaAs layers are presented. The high sensitivity of the PL technique allowed for identification unintentional dopants in pure ZnSe sample. Characteristic photoluminescence lines due to extended defects were observed. The experimental results obtained show a correlation between intentional doping level and extended defects concentration. We conclude also that even though molecular beam epitaxy layers are grown at low temperature, the self-compensation mechanism may still be important. For heavily doped sample edge emission is deactivated likely due to efficient energy transfer link with deep donor-acceptor pair bands.
We report on growth by molecular beam epitaxy of cubic MnTe(111) layers on BaF_{2} (111) substrates. Layers as thick as 0.2-1.0 μm were grown. Basic characterization by X-ray diffraction shows that the cubic crystal structure is deformed to orthorhombic symmetry.
Conduction and valence band edges in diluted magnetic semiconductors undergo enormous Zeeman shifts when a magnetic field is applied, reaching values in excess of 100 meV at low temperatures. These Zeeman shifts can thus have profound consequences on the properties of DMS/non-DMS heterostructures, since they provide the opportunity of tuning their band alignment by varying an applied field. This leads to a variety of entirely new effects, and also provides a powerful tool for probing the effect of band alignment on the properties of semiconductor heterostructures in general. We illustrate this with several examples. First, using the ZnSe/ZnMnSe system, we discuss the creation of a spatial spin modulation (spin superlattice). Second, we use the drastic differences in the Zeeman splitting occurring in different layers of a DMS/non-DMS superlattice in order to pinpoint the localization in space of the specific electronic states involved in optical transitions. We illustrate this by investigating the localization of above-barrier states in type-I ZnSe/ZnMnSe superlattices, and of spatially-direct (type-I) excitons which occur in ZnTe/CdMnSe and ZnMnTe/CdSe type-II super-lattices. Finally, we exploit Zeeman tuning to demonstrate the confinement effects which occur in a single quantum barrier.
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