H. Munekata

New III‐V diluted magnetic semiconductors (invited)

A new class of diluted magnetic semiconductor (DMS) based on a III‐V semiconductor is reviewed. The new DMS, (In,Mn)As, was made possible by low temperature molecular beam epitaxial growth. Magnetic measurements and x‐ray diffraction showed homogeneous incorporation of Mn in the films under certain growth conditions, and inclusion of a MnAs‐like phase if the conditions are not optimized. The films can be made either p‐ or n‐type by choosing the growth conditions and/or doping. Homogeneous n‐type (In,Mn)As layers were paramagnetic and showed negative magnetoresistance. On the other hand, remanent magnetization was observed in p‐type samples at low temperature and an anomalous Hall effect associated with it. The presence of such effects was most readily explained in terms of formation of bound magnetic polarons. A first result of anomalous Hall effect in a heterojunction is also presented.

Structural quality and the growth mode in epitaxial ZnSe/GaAs(100)

We have investigated the effect of the initial growth mode on the structural quality and dislocation configuration in epitaxial ZnSe/GaAs(100). We find that a three‐dimensional initial growth mode strongly degrades the crystal quality and results in a high density of threading dislocations and short misfit dislocation segments. On the other hand a two‐dimensional growth mode, achieved by a Zn exposure treatment of GaAs surfaces, results in few threading dislocations and long misfit dislocations. These differences are explained by postulation of new dislocation generation sites created by island coalescence.

Growth mode and dislocation distribution in the ZnSe/GaAs (100) system

We have investigated the effect of the initial growth mode on the dislocation structure in ZnSe epilayers grown on GaAs(100) by molecular beam epitaxy. For the case where the initial growth occurred by the formation and coalescence of three‐dimensional islands, the threading dislocation density was found to be an order of magnitude higher and misfit dislocation lengths much shorter than that for the case where the initial growth proceeded by a two‐dimensional layer‐by‐layer mode. These differences are discussed in terms of dislocation formation at island coalescence boundaries for a three‐dimensional growth mode.

Effect of surface tension on the growth mode of highly strained InGaAs on GaAs(100)

We have investigated the molecular beam epitaxy growth of highly strained InGaAs on GaAs(100) as a function of the anion to cation flux ratio. Using reflection high energy electron diffraction the evolution of the film morphology is monitored and the surface lattice constant is measured. It is found that the cation to anion flux ratio dramatically affects the growth mode. Under arsenic‐rich conditions, growth is characterized by a two‐dimensional (2D) to three‐dimensional (3D) morphological transformation. However, for cation‐stabilized conditions, 3D islanding is completely suppressed, and 2D planar growth is observed. We associate these differences in the growth mode with corresponding changes in the surface tension of the overlayer. A high surface tension stabilizes 2D growth. An analysis which relates surface tension to a critical thickness for the onset of coherent island formation supports this view.

Microstructure and homogeneity in (In,Mn)As III-V-based diluted magnetic semiconductor epitaxial films

Microstructural evaluation of thick (In,Mn)As epitaxial films grown by molecular‐beam epitaxy on InAs/GaAs(100) substrates is carried out by transmission electron microscopy (TEM). Films grown at the substrate temperature of Ts=300 °C show the inclusion of MnAs crystallites in the zinc‐blende host matrix, in which two types of crystallite morphologies, rod and approximately round shapes, are identified. In contrast, the MnAs crystallites are not observed in films grown at Ts=200 °C, and TEM studies confirm that the films are primarily of zinc‐blende structure. Microstructural defects in the films are also discussed to assess the quality of epitaxy.

Interfacial roughness in InAs/GaAs heterostructures determined by soft x‐ray reflectivity

X‐ray reflectivity has been used to determine interfacial roughness of as‐grown 250‐A‐thick InAs overlayers deposited on GaAs substrates by molecular‐beam epitaxy under differing growth and substrate conditions. Results indicate that in each case the top surface was smooth, but that the buried interface separating these highly lattice‐mismatched materials exhibits root‐mean‐square roughness parameters in the range of 10–19 A. As‐stabilized growth produced the roughest InAs‐GaAs (buried) interface, while In‐stabilized growth on a slightly misoriented substrate produced the smoothest InAs‐GaAs interface.

Photoluminescence in a ZnSe/(Zn,Mn)Se heterostructure : magnetic-field induced transition from light to heavy excitons

Abstract Magneto-excitation spectroscopy experiments are performed up to 5.5 T at low temperature (4.2 K) in a ZnSe/Zn 0.73 Mn 0.27 Se double quantum well structure. The experimental results are compared to calculations of the excitonic transitions, taking into account the strain effects, the giant Zeeman effect in the (Zn,Mn)Se layers, the diamagnetic shift of the exciton. We show that a magnetic-field induced transition in the nature of the fundamental excitonic state (from light to heavy hole exciton) occurs for a certain critical magnetic field.