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ZnSe/GaAs heterojunctions were investigated by contactless electroreflectance and photoreflectance techniques. Negative surface charge densities on the order of 1012 cm-2 were observed for films grown on n-type GaAs indicating a large contribution to the conduction band barrier between the materials due to band bending. The conduction band offset was also measured using a new photoreflectance technique involving a tunable pump laser.
Epilayers of metastable zincblende MnSe and MnTe have been grown by molecular beam epitaxy. The MnSe structures have been used to study magnetic ordering in thin layers by means of optical and magnetic measurements. Preliminary optical reflectance measurements performed on samples of zincblende MnTe indicate a bandgap of approximately 3.15eV, while double barrier quantum structures demonstrate optical transitions corresponding to 2D electron and hole confinement. Optical measurements further provide insight into the band offsets existing between MnTe and CdTe, as well as between MnTe and ZnTe epilayers.
Structures of CdTe-Cd0.6Mn0.4Te superlattices which are caused by the lattice mismatch between suterlattice layers have been studied by high resolution electron microscopy (HREM). In thin-layer superlattices, the crystal lattice in each layeris elastically distorted, resulting in the change of the crystal symmetry from cubic to rhombohedral. The presence of the small rhombohedral distrotion has been confirmed through a phase contrast effect in HREM images. In a thick-layer superlattice, the lattice mismatch is accommodated by dissociated misfit dislocations. Burgers vectors of partial misfit dislocations have been identified from the shift of lattice fringes in HREM images.
Two epitaxial orientations [(111) and (100)] of CdTe are grown on (100) GaAs in the presence of a 14.6% lattice mismatch. Consistent nucleation of a selected orientation is achieved by employing specific growth techniques. The growth techniques for selection of both orientations are described. High resolution electron microscopy has been used to investigate the interface between the CdTe epilayer and the GaAs substrate. For the (111) orientation strong interaction exists between the epitaxial deposit and the substrate, whereas a weakened interaction between deposit and substrate induces the (100) orientation.
Recent successes in molecular beam (MBE) epitaxial growth of diluted magnetic semiconductor (DMS) artificial microstructures are now generating structures in which optically excited lower dimensional electronic and magnetic phenomena can be investigated. We consider an example which illustrates the present early state of affairs: transient magnetic polarons in connection with localized quasi 2-dimensional (2D) electronic excitations. We also comment on recent measurements and antiferromagnetic ordering in ultrathin layers in magnetic semiconductors.
Application of moderate external electric fields to ZnSe/(Zn,Mn)Se superlattices is shown to yield readily measurable spectral shifts of the exciton ground state resonance in the quantum well limit. This shows that confinement effects increase the classical field ionization threshold. At high applied fields and low temperatures, externally injected hot electron excited luminescence from internal excitations of the Mn-ion is observed.
The growth by molecular beam epitaxy (MBE) of ultrathin layer II-VI compound semiconductors with magnetic constituents has opened new possibilities in the study of both lower dimensional electronic and magnetic phenomena. Some aspects of the latter are presented here with the zincblende MnSe/ZnSe superlattice as the test system. Initial indicators suggest that interfaces play a key role in determining the magnetic character of the heterostructure.
The integration of several optoelectronic device functions onto a common substrate material is an area which is currently being actively pursued. In an effort to achieve this objective, experiments are under way to examine the epitaxial growth and material properties of a variety of both II–VI and III–V compounds grown on a substrate where the II–VI/III–V heterostructure can be utilized. This paper describes some recent developments involving the molecular beam epitaxial (MBE) growth and characterization of two important II–VI/III–V heterostructures: ZnSe/GaAs and InSb/CdTe;. a comparison is made between epitaxial layer/substrate interfaces and epilayer/epilayer interfaces for both heterostructures. The ZnSe/GaAs heterointerface, having a 0.25% lattice constant mismatch, has potential for use in passivation of GaAs devices. The InSb/CdTe heterointerface possesses an even closer lattice match, ∼0.05% (comparable to the (Al,Ga)As/GaAs material system), and is motivated by possible device applications provided by InSb/CdTe quantum wells.
A Monte Carlo study of the growth of ZnSe by Molecular beam epitaxy is presented. The study is focused on the role of surface kinetic reactions on the structural quality of the epilayers. Two different models for the incorporation of Se molecules, one with a highly reactive physisorbed state and the other with a relatively nonreactive physisorbed state are employed for simulations. It is shown that the structural quality of the epilayers is very sensitive to the flux ratio if the physisorbed state is relatively nonreactive. It is also shown that if the physisorbed state is highly reactive, good quality epilayers are obtained over a wide range of flux ratio.
CdTe films have been grown on GaAs substrates with two types of interfaces - one with the epitaxial relation (111)CdTe║ (100)GaAs and the other with (100)CdTe║ (100)GaAs,. High resolution electron microscope observation of the two types of interfaces was carried out in order to determine the role of the substrate surface microstructure in determining the epitaxy. The interface of the former type shows a direct contact between the CdTe and GaAs crystals, while the interface of the latter type has a very thin oxide layer (∼10 Å in thickness) between the two crystals. These observations suggest that details of the substrate preheating cycle prior to film growth is the principle factor in determining which epitaxial relation occurs in this system. The relation between interfacial structures and the origin of the two epitaxial relations is discussed.
The successful thin film growth of diluted magnetic semiconductors (DMS) by molecular beam epitaxy has “nucleated” a new field of research in which the DMS material is incorporated in a variety of novel superlattice and quantum well structures. The observation of reflection high energy electron diffraction intensity oscillations in ZnSe and MnSe has enabled the fabrication of ultrathin layered structures involving the “hypothetical” zincblende magnetic semiconductor MnSe. The expected antiferromagnetic ordering of MnSe is increasingly inhibited as the MnSe layer thickness is reduced from ten monolayers to the quasi-2D limit of one monolayer. Further developments include new observations of the epitaxial growth and nucleation of ZnSe, utilizing GaAs epilayers as the substrate material.
The effect of partial coherency on electron diffraction patterns of Cd1−xMnxTe – Cd1−yMny Te superlattices has been investigated. Observed diffraction patterns are compared with intensity calculations performed using dynamical diffraction theory with a model of an extended incoherent monochromatic source. From this study, a new method of electron diffraction for characterization of multilayer structures can be developed. Under the condition that the lateral coherent distance of the incident beam covers two adjacent layers, diffraction beams arising from the two layers give rise to an interference fringe in a diffraction spot. With this type of diffraction pattern, one can determine the refractive index of a crystal in the multilayer structure.
By employing metalorganic molecular beam epitaxy (MOMBE), the heteroepitaxy of ZnSe on GaAs has been achieved using diethylselenium and diethylzinc. Significant (10x ∼ 15x) growth rate enhancement has been observed when radiation from an argon ion laser is incident to the surface; photons with energies greater than the bandgap at the growth temperature contribute to the enhancement. Photo-thermal effects are ruled out due to the low power densities used (∼200 mW/cm2). Growth rate enhancementis found to be a function of substrate temperature, VI/II gas flow ratio, laser wavelength and intensity. To further understand the effect of the laser on ZnSe growth, solid sources of Zn and Se are used in conjunction with metalorganic gas sources. The effect of laser illumination is found to depend on the combination of precursors employed: both growth rate enhancement and growth rate suppression are observed. Laser-assisted growth has application for achieving. selective area epitaxy and for tuning the surface stoichiometry.
Photo-assisted epitaxy is a versatile growth technique which allows in situ modification of surface chemical reactions. Under appropriate growth conditions the surface stoichiometry can be tuned by selectively desorbing surface species, or by decomposing particular molecular species, or by affecting the reaction rate constant of a chemical process. A potential application of laser-assisted growth rate enhancement or growth rate retardation is in the area of maskless selective area epitaxy. We have investigated the effect of photons on the growth of ZnSe by solid and gaseous source molecular beam epitaxy using various combination of sources. Significant growth rate enhancement (up to 20x), as well as growth rate suppression (as much as 70%), have been observed depending on the sources employed. In all cases, the laser power density remained low (∼200 mW/cm2), and the creation of photo-generated carriers was found to be required. An electron beam incident to the surface has a similar effect and increased the growth rate.
Multilayer InSb/CdTe and InSb/MnTe heterostructures are grown by molecular beam epitaxy. The analysis of multilayer structures by transmission electron microscopy (TEM) and x-ray rocking curves confirms the presence of periodic heterointerfaces. TEM, x-ray, and x-ray photoelectron spectroscopy (XPS) measurements are found to provide a sensitive indicator for the presence of ultra-thin interfacial layers. Double barrier structures in the MnTe/InSb/MnTe configuration exhibit the negative differential resistance which is characteristic of resonant tunneling diodes.
InxGa1-xAs epilayers (x < 0.2) have been grown on GaAs substrates by atmospheric pressure OMVPE. The effects of varying the substrate temperature and the gas composition on the properties of the epilayers were investigated. The investigations have shown that higher mobilities were obtained at low growth temperatures (610°C), while optimum optical properties were obtained at higher growth temperatures (690°C). Variation of the AsH3 overpressure yielded optimum electrical and optical properties at a V/III ratio of 50. 77 K mobilities higher than 42 000 cm2/V.s and photoluminescence linewidths as low as 4.4 meV were obtained for x = 0.087 and x = 0.137 epilayers, respectively.
Photoluminescence (PL) experiments at 2K are performed on MOCVD grown MnSe. The precursors used in the growth stage are methylpentacarbonylmanganese and diethylselenide. Pyrolysis of the percursors is realized inside a gradient reactor under a constant H2 flux, between 280-55°TC. The compound is epitaxially grown on various substrates (Si, InP, GaSb, GaAs, ZnTe/GaAs, etc.). On some of these samples the compound presents a zinc blende structure, while in the other samples rock salt formation has been identified. The first substrate is used because of its interest in Si technology, while the others are used because MnSe can be grown in the zinc blende phase for very thin layers. For the first time x-ray diffraction data has allowed us to determine the lattice constant of zincblende MnSe (aMnse (oct)=5.818Å), confirming the close approximation (a ∼ 5.9Å) used from the Zn1-xMnxSe alloy. These compounds have visible Mn++ transitions at 2.12-5eV; other features are also visible at 2.3-4, 2.7, and 3.0eV. The energy gap transition of tetrahedral thin film layers of MnSe is seen for the first time in PL spectra. A temperature dependant PL study is performed on MnSe in the 2-200K range. Reflectivity experiments are used to attempt to identify the internal manganese transitions. A qualitative PL analysis of the samples grown at different temperatures and on different substrates is provided. A Stokes shift is encountered when the results are compared.
High quality InSb epilayers were grown on GaAs substrates by metal organic chemical vapor deposition using a two-step growth procedure involving trimethal indium (TMIn) predeposition. From transmission electron microscopy studies, we found that an interdiffusion layer of thickness of 10 Å forms at the interface when the substrate is exposed to TMIn for approximately 6 secs prior to the growth of the InSb filns. Hall mobilities up to σ 52,000 cm2/V-s were obtained at 300 K on a 2.1-μm-thick InSb heteroepitaxial film. In contrast, samples without TMIn predeposition showed polycrystallinity of the InSb films grown on single crystalline GaAs substrates. The effect. of TMNIn predeposition is to minimize the misorientation of the grains, suppress the polycrystallinity, decrease the density of threading dislocations, and increase the electron mobilities in the films. However, we found that too much TMIn predeposition gives rise t.o an intermixing layer at the InSb/GaAs interface which deteriorates the film quality. Details of the effect of the TMIn predeposition on the microstructure of InSb/GaAs with different predeposition times (zero, 6, and 12 secs) are discussed.
A novel zinc precursor adduct tetramethylmethylene diamine: dimethylzinc (TMMD:DMZn) has been used to grow ZnSe-ZnS strained layer superlattices by low pressure MOVPE at 300'C. The use of this new precursor leads to improved material purity and lower homogeneous gas phase premature reaction than in the case of dimethylzinc. Optical characterization has been made between liquid helium and room temperature. The 2K photoluminescence lines range between 2.85 eV and 3.15 eV. The 2K photoluminescence is characterized by a linewidth at half maximum ranging from 13-45 meV. Additional reflectivity and photoreflectance experiments have been carried out to measure light-hole excitons. A variational calculation of the Rydberg energies has been performed using a sophisticated trial function. The experimental data is in agreement with the theoretical value when a small conduction band offset is used.
In the design of GaAs devices, material selection for heteroepitaxy is limited due to the large amount of strain produced by lattice mismatch. To overcome this problem, a new growth method has been proposed called van der Waals Epitaxy (VDWE). This process attempts to decouple the interface through the growth of a van der Waals bonded crystal layer in-between the two materials with dissimilar lattice constants, thereby accommodating the lattice mismatch. Typical van der Waals materials such as GaSe, have hexagonal crystal structures and are grown with the c-axis perpendicular to the surface due to the nature of the bonding. These materials have a low surface energy and are not very reactive. In addition, they are only stable at relatively low temperatures. This poses two constraints to the growth of GaAs on GaSe. Initial experiments were done to investigate the possibility of growing GaAs (111) layers homoepitaxially at temperatures compatible with van der Waals crystals, with high crystallinity and a smooth surface. Twinned growth layers 1.7μm thick, were obtained at 350° C on a GaAs (111)B substrate using MBE. Further experiments looked at homoepitaxial growth on (111)A substrates and at slightly higher temperatures. These results were then compared to GaAs layers grown on GaSe layers in which nucleation occurs on a van der Waals surface as opposed to the covalently bonded GaAs substrate. The relative quality of the layers was analyzed using RHEED and TEM.