Yunosuke Makita

Optical absorption and photoluminescence studies of β‐FeSi2 prepared by heavy implantation of Fe+ ions into Si

Mass‐separated 56Fe+ ions were implanted into Si(100) at 350 °C using three different energies and doses of 140 keV (1.32×1017 cm−2), 80 keV (6.20×1016 cm−2), and 50 keV (3.56×1016 cm−2). Their optical properties were investigated as a function of subsequent annealing temperature and its duration time. X‐ray diffraction analysis revealed that polycrystalline semiconducting β‐FeSi2 layers are formed in the as‐implanted and annealed samples. From Rutherford backscattering spectrometry analysis, the formation of β‐FeSi2 up to the surface was confirmed, and the average thickness and composition of the layers at peak concentration were estimated to be 70–75 nm and Fe:Si=1:2.0–2.2, respectively. The types of optical transition and the inverse logarithmic slope (E0) of the Urbach tail were investigated using room temperature optical absorption measurements. All the synthesized β‐FeSi2 layers exhibited a direct transition with direct band‐gap energies (Egdir) of 0.802–0.869 eV and with high optical absorption coe...

Molecular beam epitaxial growth of low‐resistivity ZnSe films

Single‐crystalline films of low‐resistivity ZnSe have been successfully grown by molecular beam epitaxy (MBE). The film shows good crystallinity, having a smooth and flat surface as revealed by RHEED and SEM investigations. The resistivity of as‐grown MBE ZnSe is low (typically ∼1 Ω cm) and the mobility of the film is relatively high (typically ∼300 cm2/V sec) at room temperature. The concentrations of the residual electrically active donor and acceptor centers are ND =8×1016 cm−3 and NA=6×1016 cm−3, respectively. The strong blue band‐edge photoluminescence peak is observed even at room temperature, and the luminescence intensity at longer wavelength is very weak.

Photoluminescence of very dilutely C+ ion‐implanted GaAs

Very dilute C+ (carbon) ion implantations were carried out for extremely pure GaAs wafers grown by molecular beam epitaxy (MBE). Low‐temperature photoluminescence measurements revealed that at least five sharp new emission lines are commonly formed near bound exciton emission region by the introduction of C for the dose range between 1015 and 1017 cm−3. It was for the first time demonstrated that the most dominant line among the above emission is identical to the g line in defect‐induced bound exciton emission series which are frequently observed in rather impure MBE‐grown or metalorganic chemical vapor deposition (MOCVD)‐grown GaAs samples.

Doping and electrical properties of Mg in LPE AlxGa1−xAs

Mg‐doped AlxGa1−xAs crystals with Al concentration 0⩽x⩽0.8 have been grown by the conventional LPE method. Doping characteristics of Mg and electrical properties of the crystals are studied using Hall measurements in the temperature range between 77 and 295 K. The results revealed are as follows. Magnesium works as an acceptor in AlxGa1−xAs. Its distribution coefficient is constant and about 0.6 for AlxGa1−xAs with x⩾0.1. Hole concentrations up to 1018 cm−3 are easily obtained for a high x of 0.65. The acceptor energy level Ei increases from 18 meV in GaAs to 40 meV in Al0.8Ga0.2As. The x dependence of Ei can be explained by the difference of effective hole masses and dielectric constants between GaAs and AlAs. Hole concentration, mobility, and resistivity, as well as their temperature dependences, are presented for various x values. These data show that Mg is a suitable acceptor in AlxGa1−xAs particularly with high x values.

Molecular beam epitaxy of ZnSexTe1−x (0 ≲ x ≲ 1)

Abstract Single-crystalline films of ZnSe x Te 1− x (0 ≲ x ≲ 1) have been successfully grown by molecular beam epitaxy. Evaluation of the epitaxial films shows the growth of good-crystallinity films with flat surfaces. The dependence of the composition ratio x on the molecular beam intensities of Zn, Se, and Te was studied and the following features became clear: (1) the incorporation rate of Se is much higher than that of Te; (2) the control of the composition ratio can be best achieved by maintaining the arrival rate ratio of Te/Se above unity; (3) the composition ratio depends mainly on the arrival rate ratio of Se/Zn.

Reduction of iron diffusion in silicon during the epitaxial growth of β-FeSi2 films by use of thin template buffer layers

We fabricated continuous highly (110)/(101)-oriented β-FeSi2 films on Si (111) substrates by the facing-target sputtering method. An epitaxial thin β-FeSi2 template buffer layer preformed on the silicon substrate was found to be essential in the epitaxial growth of thick β-FeSi2 films. It was proved that the template reduced the iron diffusion into the silicon substrate during thick β-FeSi2 film fabrication. Even though the annealing was performed at high temperature (880 °C) for a long duration (10 h), iron diffusion was effectively hindered by the template. By introducing this template buffer layer, an abrupt interface without appreciable defects between the β-FeSi2 film and the silicon substrate formed. The mechanism for the reduction of iron diffusion by the template buffer layer is discussed.

Photoluminescence of Mg‐doped GaAs grown by molecular beam epitaxy using Mg3As2 as a Mg source: A comparison with Mg+ ion implantation

Photoluminescence spectra of Mg‐doped GaAs grown by molecular beam epitaxy (MBE) are for the first time reported. Mg was introduced during MBE growth by using a synergic reaction of Mg3As2. Two near‐band‐edge emissions, g and [g‐g], were observed below bound exciton emissions which were originally obtained in Mg+ ion‐implanted GaAs. It is explicitly demonstrated that there is no essential difference of photoluminescence spectra between Mg+ ion‐implanted GaAs and the present material. These results conclusively indicate that the above two emissions are definitively related with the acceptor levels and exclusively not associated with crystalline defects produced by ion implantation. It is additionally presented that [g‐g] is principally established also in Mg‐doped GaAs prepared by liquid phase epitaxy, where [g‐g] is significantly suppressed due to the unintentional incorporation of Si donor atoms. For moderately Mg‐doped GaAs prepared by MBE a new fine emission with a doublet structure, temporarily denote...

Observation of new common emissions in GaAs produced by ion implantation of four acceptor impurities

Photoluminescence studies of C+, Mg+, Zn+, and Cd+ ion‐implanted GaAs layers were carried out at 2 K. Two conspicuous emissions denoted by g and [g‐g] were observed to be situated between bound exciton and band to acceptor emissions. It was found that these two are common emissions among the above acceptor impurities, and that they can be explicitly observed only when the background impurity concentration is extremely small.

Molecular Beam Epitaxy of ZnTe Single Crystal Thin Films

Thin films of ZnTe single crystal were grown successfully on germanium substrates by molecular beam epitaxy. The epitaxy condition and evaluation of the grown films were extensively investigated. The evaluation and the analysis were made with a scanning electron microscope, an ion microprobe analyser, an electron-probe microanalyser, and measurements of the current-voltage characteristics and photoluminescence spectra. Twin-free and stoichiometric ZnTe films with smooth and flat surface were grown at the growth rate less than 2 A/sec and for the substrate temperatures between 360°C and 400°C. Photoluminescence spectra of the films show sharp near-band-edge emission bands.

Effect of Multiple-Step Annealing on the Formation of Semiconducting β-FeSi2 and Metallic α-Fe2Si5 on Si (100) by Ion Beam Synthesis

Polycrystalline semiconducting β- FeSi2 layers on Si (100) have been formed by ion beam synthesis. Results from two different annealing processes, either two-step (2SA) annealing up to 900° C or three-step annealing (3SA) up to 1100° C, are discussed. β- FeSi2 grown by 3SA has shown a typical direct band-gap energy (E g dir) of 0.88 eV and a high localized defect density (N0) of 1.0×1018 cm-3, the latter being due to crystallographic mismatches or relevant defects at grain boundaries introduced during the transformation process from β to α. On the contrary, β- FeSi2 grown by 2 SA has shown a lower E g dir of 0.80 eV and a smaller N0 of 1.7×1017 cm-3, the former arising from a deviation of the stoichiometric composition to the Si-rich side. Broad PL bands near 0.8 eV have been observed at 2 K from both 2SA and 3SA samples, and we assign these PL bands to optical radiative transitions intrinsic to β- FeSi2.