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Polarity-controlled ZnO films with an MgO buffer layer were grown on c-plane sapphire by plasma-assisted molecular-beam epitaxy. Convergent beam electron diffraction results showed that Zn-polarity (+c) growth occurred when the MgO layer was thicker than 3 nm, whereas O-polarity (−c) growth occurred when the layer was less than 2 nm. Reflection high-energy electron diffraction results revealed that MgO growth was Stranski–Krastanov mode, and that the growth mode transition from two- to three-dimensional occurred when the layer was thicker than 1 nm. In conclusion, polarity conversion apparently occurs due to the different atomic structure between the wetting layer and islands of MgO.

Hard x-ray valence band photoemission spectroscopy (PES) is realized using high-energy and high-brilliance synchrotron radiation. High-energy (∼6 keV) excitation results in larger probing depths of photoelectrons compared to conventional PES, and enables a study of intrinsic electronic property of materials in actual device structures much less influenced by surface condition. With this technique, requirements for surface preparation are greatly reduced, if not eliminated. It is a nondestructive tool to determine electronic structure from surface to genuine bulk as shown by a study on SiO2/Si(100). Electronic structure modification related to the ferromagnetism in the diluted magnetic semiconductor Ga0.96Mn0.04N is also observed.

Multi-quantum-well structures of ZnS:Mn-SiO2 were prepared by sequential rf sputtering. The thickness of ZnS:Mn layers differed from 2 to 10 nm, while that of SiO2 layers was 40 nm. It was shown that the efficiency of Mn luminescence is considerably larger in samples with ZnS:Mn thickness of 2 nm compared to samples with thicker layers. A possible mechanism to explain this effect is the formation of quantum-dot-like features in the low thickness layers, resulting in regions with higher local Mn concentration, hence higher luminescence efficiency.

A laser diode which includes II–VI device structure directly grown on III–V device structure is proposed. This idea makes possible one-chip-multiple-wavelength operation in the visible wavelength region by the vertical integration of individual light emitters, which is demonstrated by a ZnCdSe/ZnSe/ZnMgBeSe quantum-well structure for a blue-green light emitter grown by molecular beam epitaxy directly on a metalorganic chemical vapor deposition grown InGaP/InGaAlP device wafer for a red light emitting device. The feasibility of the II–VI/III–V complex light emitter is demonstrated by optical-pumping experiments. Optically pumped lasing at 504 and 664 nm is achieved from II–VI and III–VI laser structures on one chip simultaneously at room temperature with a threshold power of 115 and 84 kW/cm2, respectively. The present results clearly show the feasibility of epitaxial integration of II–VI and III–V laser structures, which will stimulate the investigation of multifunctional optical devices.

Effect of Ag photodoping into ZnO films grown by molecular beam epitaxy has been examined via the real-time measurement of photoluminescence (PL) spectra using a HeCd laser at room temperature. While an asymmetric deep-level (green-yellow) broadband (D) is observed in PL of as-grown ZnO, it appears to change to two distinct bands (at 2.29 and 2.50 eV) in the illuminated Ag/ZnO. From the result of a Gaussian fitting, the yellow band at 2.29 eV is evaluated to relate with photodoped Ag atoms. With increasing the laser illumination time (photodoping), the PL intensity of broadband (ID) decreases and contrarily, the intensity of the near band edge emission observed at 3.297 eV (INB) shows a tendency to increase. We believe that the Ag can be photodoped into ZnO at 300 K and it contributes to the suppression of deep-level band, which results in an increase of INB.

Omnidirectional (omni-) reflector and transmission filter optimized at a wavelength of 1.55 μm have been realized using Si/SiO2 one-dimensional photonic crystals (1D PCs). Photonic band structures in the PCs with and without a defect layer have been also analyzed. The 1D PCs fabricated have very large omnidirectional photonic band gaps (omni-PBGs), and their measured reflectance spectra are in very good agreement with the calculated results. The omni-PBG in a six-pair Si/SiO2 1D PC (no defect layer, a filling factor η=0.406) exists in a normalized frequency range of 0.223–0.297. Introducing a defect layer generates a defect state in the PBG, and it behaves like electronic impurity levels in the gap with a change in thickness of the defect layer (dD). A six-pair Si/SiO2 1D PC with the structure parameters of η=4.060, dH (Si thickness)=95.3 nm, dL (SiO2 thickness)=234.9 nm, and dD=2.336⋅dH could act as a 1.55 μm-transmission filter for normal incidence light.

Femtosecond three-pulse two-color photon echo and population grating (PG) techniques have been used to characterize CdTe quantum dots (QDs) grown on ZnSe by molecular beam epitaxy. The time evolution of the PG signal exhibits a fast decay (2.5–3 ps), which is attributed to migration and tunneling of photoexcited carriers to neighboring QDs, followed by a slow decay (∼20 ps for small dots and >100 ps for large dots), which is ascribed to the lifetime of excitons in the quantum dots. The three-pulse photon echo intensity versus population time can be used to deduce values for the homogeneous broadening (Γh=0.8–1.2 meV) and the exciton binding energy (∼13 meV). A weak dependence of these quantities on the detection wavelength is associated with a difference in dot sizes, which alters the coupling between the exciton and the acoustic phonons.

Photoluminescence (PL) and time-resolved PL spectra of zinc oxide (ZnO) films were investigated as a function of the grain size of the microcrystals. Correlation was found between the grain size and the optical properties—in the bound exciton states, both the PL intensity and PL decay time increased with increasing the grain size. This correlation can be well explained by the existence of nonradiative-surface and/or -interface states in the grain boundaries of ZnO microcrystals.

Room-temperature Ag photodoping into crystalline Al-doped ZnSe grown by molecular-beam epitaxy (MBE) has been achieved. A promising approach to apply to a patterned white light emitter has also been proposed. An Al–ZnSe with Ag islands was illuminated with a HeCd laser at 300 K and annealed in the MBE chamber. Then, their photoluminescence characteristics were measured in real time. The self-activated (SA) emission, observed in as-grown Al–ZnSe, undergoes a considerable change in intensity and band energy position through Ag photodoping and subsequent annealing. The resultant emission bands centered at 2.145 eV and 2.250 eV are assigned as a donor-to-acceptor pair transition related with Ag impurities rather than SA centers. In addition, chromaticity can be controlled by an adjustment of photoirradiated Ag-doped area.

Morphology evolution of ZnO(000 1) surface during plasma-assisted molecular-beam epitaxy was studied. In a step-flow growth mode, terraces bounded by anisotropic steps along 〈11 20〉 characterize the surface of ZnO epilayers. Adatoms favorably incorporate at upsteps, which translates hexagonal islands to regular terraces with equalized width. Surface morphology is sensitive to the Zn/O ratio. Oxygen stabilizes mobile Zn adatoms hence oxygen rich conditions result in rough step edges and irregular hexagonal terraces. Steps become smoother with increasing Zn/O ratio until the stoichiometric condition is achieved. As the Zn flux exceeds the stoichiometry, faceted hexagonal pits form on the ZnO(000 1) surface. Rather than the lack of surface mobility, the lack of stabilization of adatoms is responsible to the formation of pit.

We have investigated the possibility of Ag photodoping into crystalline (c-) ZnSe grown by molecular-beam epitaxy. A Ag/c-ZnSe sample was illuminated with a 325 nm HeCd laser at 10 K and its photoluminescence (PL) characteristics were measured in real time. Depending on the illumination time, there are considerable changes in the intensities of the deep-level peaks (2.059 and 2.345 eV) as well as the near-band-edge peak (2.798 eV). The emission band at 2.059 eV is almost independent of PL-excitation power, while the green luminescence band at 2.345 eV exhibits distinctly a power-dependent blueshift with increasing excitation power, which can be assigned as a donor-to-acceptor pair transition (AgiAgZn). In this letter, a model to explain Ag photodoping into c-ZnSe is proposed on the basis of the PL results. We believe that Ag, as an activator for luminescence, can be photodoped into c-ZnSe even though the extent and kinetics of the Ag doping are different from those of the amorphous chalcogenide.

We present the results of the formation of the InAs quantum dots (QDs) on the (100) In1−xAlxAs(InAlAs)/InP substrate by using relaxed InAlAs buffer layers with different compositions. Variations of surface morphology of InAs QDs as a function of InAs–InAlAs lattice mismatch have been evaluated by atomic force microscopy. When the lattice mismatch increases from 2.4% to 4.2%, the size of QDs decreases, and the density of QDs increases. Each of these dependences can be fitted to a power function of the misfit unless the Al diffusion, roughness of the buffer layer, and/or the ripening of small dots modify the size and density.

The composition dependence of the energy gap (Eg) of Zn1−x−yMgxBeySe quaternary alloys grown by molecular-beam epitaxy was investigated. The energy gap of Zn1−x−yMgxBeySe can be controlled in a range of 2.7 eV

N-type ZnTe layers with high electron concentration are grown by molecular-beam epitaxy using aluminum as the donor species. The ZnTe:Al layers show a high structural quality with a narrow x-ray diffraction linewidth (24 arcsec) and a high carrier concentration up to n=4×1018 cm−3 with low resistivity (ρ=0.017 Ω cm). The dependence of the electron mobility on the carrier concentration suggests that the dominant scattering mechanisms in the ZnTe:Al layers are ionized impurity scattering and polar optical phonon scattering. The photoluminescence spectrum of moderately doped ZnTe layers shows strong Al–donor-related bound exciton lines: I2 (2.378 eV) and donor–acceptor pair emission (zero phonon energy=2.324 eV) with a weak deep-level emission (2.19 eV). Highly Al-doped layers show an increase in the deep-level emission intensity and a decrease in carrier mobility, which are interpreted in terms of the increase in the carrier compensation.

We report on an experimental study of buffer mechanism in plasma-assisted molecular-beam epitaxy of ZnO on Al2O3(0001) with a MgO buffer. It lowers the surface energy and provides nucleation cores. As a result, lateral epitaxial growth of ZnO becomes favorable from the initial growth stage. The MgO buffer also affects the generation of dislocations in such a way that it reduces their density by introducing dislocation interactions. This study suggests that by employing an appropriate buffer to modify the initial nucleation environment, high quality heteroepitaxy is achievable even with large mismatch.

We report the experimental results of the valence band offset at a ZnO/GaN (0001) heterointerface. The ZnO/GaN (0001) heterointerface is prepared by growing a ZnO layer on (0001) GaN/Al2O3, in which the ZnO layer is epitaxially deposited by plasma-assisted molecular-beam epitaxy, while the GaN template is prepared by metalorganic chemical-vapor deposition. Ex situ ultraviolet and x-ray photoelectron spectroscopy have been used to measure the valence band offset ΔEV. The photoelectron spectroscopy measurements are done before and after Ar+ ion cleaning of the surfaces. Type-II band alignments with band offsets of ΔEV=1.0 eV (before cleaning) and 0.8 eV (after cleaning) with the valence band maximum of GaN being placed above that of ZnO are obtained.

We report the results of an experimental investigation on lasing mechanisms in optically pumped ZnO epilayers at room temperature. High-quality ZnO epilayers grown on sapphire by plasma-assisted molecular-beam epitaxy employing an MgO buffer were used. Free exciton emissions and their phonon replicas dominate the photoluminescence from low excited samples. Inelastic exciton–exciton scattering contributes to the mechanism of stimulated emission mainly at intermediate excitation. By using the variable stripe length method, we measured the near threshold optical gain spectrum of the ZnO epilayers. Different from the interband transition governed mechanisms, exciton–exciton scattering gives rise to a nearly symmetric gain spectrum with the peak at 3.17 eV. The electron-hole plasma emerges to contribute to the optical gain when excitation exceeds 220 kW/cm2.

A ZnCdTe/ZnTe/ZnMgSeTe quantum-well (QW) structure lattice matched to ZnTe is proposed for the light-emitting devices in the pure-green wavelength region. Thin ZnTe layers are inserted in between the ZnCdTe QW layer and ZnMgSeTe cladding layers, which improve the quality of the QW structure as demonstrated by its narrow photoluminescence line width (6.5 meV at 10 K). Optically pumped lasing at 552 nm at room temperature with a threshold optical power of 215 kW cm−2 is achieved. The present results clearly show the feasibility of ZnTe-based QW structures for the application to light-emitting devices in the pure-green wavelength region.

We report a remarkably low stacking-fault density in ZnSe epilayers directly grown on commercial epi-ready GaAs (001) substrates without GaAs buffer layer growth. It is found that proper pregrowth treatments on epi-ready GaAs (001) substrates to obtain clean surfaces are crucial for two-dimensional layer-by-layer growth and suppression of stacking fault generation. Chemical etching using a NH4OH-based solution is found to reduce not only the thickness of the oxide layers but also the ratio of Ga2O3 to As2O3 to about half of that before etching. A clean GaAs (001) surface characterized by a (4×1) reconstruction in the present case is obtained after thermal cleaning followed by Zn pre-exposure. Reflection high-energy electron diffraction intensity oscillations with more than 50 periods are observed even from the very beginning of ZnSe growth on GaAs substrates cleaned as such. The stacking fault density in such a ZnSe layer is in the low-105 cm−2 range.

We have investigated the optical and structural properties of high-quality ZnO films grown on epitaxial GaN (epi-GaN) by plasma-assisted molecular-beam epitaxy employing low-temperature buffer layers. High-resolution x-ray diffraction for both symmetric and asymmetric reflexes shows that crystalline defects in ZnO films have a similarity to epi-GaN used as a substrate. The quality of ZnO epilayers grown on epi-GaN is basically determined by epi-GaN. The photoluminescence (PL) spectrum at 10 K exhibits very sharp exciton emission with a linewidth of 1.5 meV, while deep-level emission is negligible, indicative of small residual strain. At 77 K, PL is dominated by a free-exciton emission line in the low-excitation regime, while it is overtaken by a new emission band due to biexcitons at its low-energy side as the excitation intensity increases. This biexciton emission band emerges even under the intermediate excitation regime of 100 W/cm2, which is 100 times smaller than the previously reported threshold for...

We report the origin of hexagonal-shaped etch pits generally observed by conventional wet etching processes on (0001) GaN-based films based on the investigation with transmission electron microscopy on GaN films after being etched with molten KOH. The origin of hexagonal-shaped etch pits is identified as nanopipes through careful characterization of abnormal contrast of nanopipe (open-core screw dislocation), “lobe contrast” of end-on edge and screw (full-core) dislocations, visible and invisible conditions of edge and screw dislocations. Consideration of energetics of these defects also suggests preferential etch pit formation at nanopipes because of much higher energy.

By introducing a thin MgO buffer, layer-by-layer growth of ZnO epilayers on Al2O3(0001) substrates is achieved by plasma-assisted molecular beam epitaxy. The MgO buffer is very effective on the improvement of surface morphology during the initial growth stage, which eventually leads to an atomically flat surface. As a result, (3×3) surface reconstruction of ZnO is observed and reflection high-energy electron diffraction intensity oscillations are recorded. Structural analysis indicates that the twin defect with a 30° in-plane crystal orientation misaligned is completely eliminated, while the total dislocation density is reduced. Free exciton emissions at 3.3774 eV (XA) and 3.383 eV (XB) are observed in photoluminescence at 4.2 K further indicating the high quality of the resulting ZnO epilayers.

Plasma-assisted molecular-beam epitaxy of ZnO epilayers on MgAl2O4(111) substrates is described. Acid mixed by H2SO4 and H3PO4 was used for the substrate etching, which provides atomically flat surfaces with a regular terrace array. The influence of the substrate on lateral growth and coalescence of ZnO islands during epitaxy is discussed with corresponding morphological and structural properties. The epitaxial relationship between the ZnO epilayer and MgAl2O4 substrate is determined as: ZnO[0110]∥MgAl2O4[112] and ZnO[2110]∥MgAl2O4[110]. X-ray diffraction, photoluminescence, and Hall-effect measurements are reported, which indicate that high-quality ZnO epilayers are obtained by using atomically flat MgAl2O4(111) substrates to suppress columnar growth.

We have achieved low-threshold optically pumped lasing at 444 nm at room temperature with high characteristic temperature (T0) from ZnSe/ZnMgBeSe single-quantum-well structures. The threshold intensity is as low as 15 kW cm−2, while T0 value is as high as 166 K. Lasing is observed up to 473 K. Lasing wavelength of 444 nm at room temperature is the shortest wavelength ever achieved in ZnSe-based laser diode structures. The laser structure includes a single ZnMgBeSe/ZnSe/ZnMgBeSe quantum well with a ZnSe well thickness of 4 nm. The (004) x-ray diffraction rocking curve of the ZnMgBeSe quaternary cladding layers shows a sharp diffraction peak with a full width at half maximum of 21 arcsec which is in contrast to that from a ZnMgSSe cladding layer showing much broader multiple peaks. The observed lasing features are partly ascribed to high crystal quality of the ZnMgBeSe layers and type-I band alignment, as has been supported by photoluminescence in addition to x-ray diffraction measurements.

Optical gain spectra of ZnO epitaxial thin films have been measured by using a pump–probe technique. The optical gain is thought to be due to electron–hole plasma. In the differential absorption spectra, we observed saturation of the exciton absorption, band-gap renormalization, as well as the optical gain. From the temporal changes of these structures, the dynamical properties of the photoexcited carriers are discussed.

We report that acoustic driving of ZnSe/ZnS quantum wells can yield ⩾30% shortening of the fast component (on the order of 102 ps) of exciton radiative decays and relatively enhanced tail emissions at greater instants. The shortening is attributed to the driving-induced relocalization of excitons to lower-energy states while the enhancement is indicative of the drift diffusion of mobile populations in the driving electric field. These results suggest that the radiative recombination predominantly occurs from localized excitons.

Adsorption of Zn on the GaAs(001)-(2×4) surface has been studied by reflection high-energy electron diffraction (RHEED) and total-reflection-angle x-ray spectroscopy in real time. Adsorption sites of Zn atoms on the (2×4) surface have been determined using rocking-curve analysis of RHEED. The exposure of the (2×4) surface to a Zn beam results in the preferred adsorption of Zn at Ga-vacancy sites in the missing dimer trenches of the (2×4) unit cell. The amount of adsorbed Zn atoms under a Zn flux has been estimated to be ∼0.50 and ∼0.20 monolayers at 200 and 250 °C, respectively.

The emission spectrum of high quality ZnO epilayers is studied from room temperature up to 550 K. At room temperature and low excitation power a single emission peak is observed which may be identified with the free exciton from its peak energy and dependence on temperature. However, when excitation intensities exceed 400 kW cm−2 a sharp peak emerges at lower energy which we attribute to exciton-exciton scattering. At higher excitation intensities (>800 kW cm−2) a second stimulated emission peak emerges at even lower energies: we attribute this peak to be stimulated emission of an electron hole plasma. Similar features are observed for all temperatures up to 550 K.

Zinc coverage and the structures of Zn-exposed As-stabilized GaAs(001)-(2×4) and -c(4×4) surfaces have been studied using x-ray photoelectron spectroscopy and scanning tunneling microscopy in order to clarify the role of the Zn pre-exposure process in ZnSe growth on GaAs(001). Since Zn atoms stick on the GaAs-(2×4) surface even though their interaction is very weak, Zn may act as a balancer to form a neutral ZnSe/GaAs interface. Zn can also remove excess As atoms and make a “pure” (2×4) structure that is the only possible starting surface for low-defect ZnSe heteroexpitaxy on a GaAs(001) surface.

The growth of very uniform Ge dots on Si(100) is achieved by using molecular beam epitaxy. The atomic force microscopy and the transmission electron microscopic observations illustrate that the size uniformity of the dots is not worse than ±3%, i.e., the base dimension is 100±3 nm. The Raman spectrum reveals a peak downward shift of Ge-Ge mode caused by the phonon confinement in the Ge dots. A very narrow photoluminescence peak with the width of 1.6 meV at the energy of 0.767 eV is observed at the temperature of 16 K. We attribute this peak to the free exciton longitudinal acoustic phonon replica originated from the Ge dots.

An almost 100% activation ratio {(NA-ND)/[N]} for a nitrogen-doped ZnSe molecular beam epitaxy (MBE) layer with the highest net acceptor concentration (NA-ND) of 1.2×1018 cm−3 was obtained using a high-power rf plasma source. Even at this high NA-ND value, a 4.2 K photoluminescence spectrum shows bound exciton emission and deep donor–acceptor pair emission with well-resolved phonon replicas. The high activation in nitrogen doping could be ascribed to the generation of the predominant atomic nitrogen and to the suppressed extraction of nitrogen ions and excited neutral nitrogen molecules due to the structure of the orifice placed between the MBE growth chamber and the plasma discharge tube of the high-power plasma source.

Excited neutral nitrogen species emitted from a rf plasma source were characterized by the laser-induced fluorescence (LIF) spectroscopy, while nitrogen ions were detected by the ion counting method. The LIF intensity for nitrogen molecules increases monotonously up to the rf power of 100 W and saturates over 100 W. On the contrary, ion count of nitrogen ions shows a gradual increase up to 100 W, then rapidly increases above 100 W. The correlation between the number of excited nitrogen species and the net acceptor concentration (NA−ND) of nitrogen doped ZnSe epitaxial layers for various rf powers has been studied. We confirm that the excited neutral nitrogen molecules are effective for acceptor doping, while nitrogen ions enhance carrier compensation presumably due to degradation of crystal quality. We show that the activation ratio {(NA−ND)/[N]} of p-ZnSe:N is greatly improved by removing ions from the nitrogen plasma.

We report the observation of optically pumped lasing in ZnO at room temperature. Thin films of ZnO were grown by plasma-enhanced molecular beam epitaxy on (0001) sapphire substrates. Laser cavities formed by cleaving were found to lase at a threshold excitation intensity of 240 kW cm−2. We believe these results demonstrate the high quality of ZnO epilayers grown by molecular beam epitaxy while clearly demonstrating the viability of ZnO based light emitting devices.

We have developed a high-power (5 kW) rf plasma source for nitrogen doping in ZnSe molecular beam epitaxy. Optical emission spectroscopy shows dominant atomiclike emissions around 800 nm due to excited neutral nitrogen atoms in the high power region and their intensities rapidly increase with increasing the rf power from 1 to 3 kW. The high net acceptor concentration (NA–ND) of 1.2×1018 cm−3 was achieved at the growth temperature of 220 °C and the activation ratio [(NA–ND)/N] as high as 60%, which is the highest value so far obtained for NA–ND∼1018 cm−3. Consequently, the PL spectrum showed well-resolved deep donor-acceptor pair emissions even with high NA–ND.

A novel scanning capacitance microscope (SCaM) is developed using a tungsten stylus as a cantilever. The deflection of the cantilever is detected by projecting the cantilever onto a photodetector. A combined SCaM, atomic force microscope (AFM), and scanning tunneling microscope has been constructed. The spatial resolution for SCaM measurements is estimated to be less than 100 nm. Local electrical properties of a SiO2/Si system are investigated using the SCaM and AFM functions of the combined microscope. A lateral p‐n junction is formed, by ion implantation of P into a lightly B‐doped Si wafer, the structure is then annealed and thermally oxidized. It is demonstrated that (1) the p‐n junction can be observed even through the oxide layer, (2) the local impurity concentration can be estimated from capacitance–voltage (C–V) characteristics, and (3) P diffuses into the B‐doped region during the thermal processes.

Existence of Zn‐As and Ga‐Se interfacial layers were suggested by transmission electron microscopy in Zn treated and Se treated or reacted ZnSe/GaAs interfaces, respectively. High densities of As precipitates and Shockley partials were introduced in films with Zn treatment on a c(4×4) As‐rich GaAs surface. In addition, high densities of vacancies and Shockley partials were obtained in samples with a Se‐reacted ZnSe/GaAs interface. Formation of the Shockley partials may originate from the stacking errors induced by disordering of Zn‐ or Ga‐interstitials on the GaAs surface.

We report low‐resistive Au ohmic contact for a molecular beam epitaxy grown nitrogen doped p‐type ZnTe (p‐ZnTe:N) layer with high thermal stability, where the p‐ZnTe surface is treated by oxygen plasma and HCl solution prior to Au film deposition. C contamination and native oxide layers of ZnTe are removed by oxygen plasma and HCl treatment, respectively, from the surface. As a result, a Te‐rich layer is formed on the surface. The annealing temperature dependence of specific contact resistance is investigated. The specific contact resistance of Au ohmic contact for p‐ZnTe:N with carrier concentration of 2×1018 cm−3 treated by oxygen plasma and HCl solution reaches as low as 5.8×10−6 Ω cm2 at an annealing temperature of 350 °C. This specific contact resistance value is the same as that of the reported Pd/(Pt)/Au contact for p‐ZnTe:N with carrier concentration of 3×1019 cm−3, and clearly shows higher thermal stability.

Deep hole traps in N‐doped p‐type ZnSe epilayer grown by molecular‐beam epitaxy have been studied using isothermal capacitance transient spectroscopy and deep level transient spectroscopy measurement (DLTS). Two hole traps with thermal activation energies of 0.68 and 0.85 eV were detected. These centers have large potential barrier heights in the carrier capture process. The thermal activation energies for the carrier capture were 0.23 eV on KT1 trap and 0.33 eV on KT2. These facts indicate that large lattice relaxation is induced in the hole capture/ emission processes. The deep electron trap related to active nitrogen doping was measured through comparison of the DLTS results from n‐type ZnSe co‐doped with chlorine and nitrogen with those from chlorine‐doped n‐type ZnSe. A deep electron trap associated with nitrogen doping with activation energy of 0.36 eV below the conduction band was found.

We report the growth of p‐type CdSe having zinc blende structure with the hole concentration of 1×1017 cm−3 by molecular beam epitaxy using a nitrogen plasma source. The electrical and optical properties of CdSe:N epilayers are characterized by Hall and photoluminescence (PL) measurements. PL spectra from p‐type CdSe:N epilayers at 14 K show dominant emission associated with a neutral donor‐bound excitation at 1.745 eV and two kinds of donor‐acceptor pair emission bands with zero phonon energies of 1.724 and 1.695 eV. An Au/p‐CdSe:N electrode is examined as ohmic contact for p‐ZnSe in terms of current‐voltage characteristic. It is shown that the Au p‐CdSe electrode works as an ohmic contact for p‐ZnSe.

This letter presents the growth and characterization of N‐doped ZnSe grown on (100)GaAs misoriented 4° off toward (110), and (010), and (211)GaAs‐A and ‐B. The effects of the surface steps on the formation of deep donors are investigated by comparing photoluminescence properties and net‐acceptor concentration of ZnSe:N epilayers grown on misoriented (100) surfaces with those grown on the exact (100)GaAs. It is shown that the use of the misoriented (100) surfaces suppresses the generation of deep donors and enhances the net acceptor concentration. The effects of bond natures at Zn sites on the N‐incorporation process are studied through the comparison of the net‐acceptor concentration of N‐doped ZnSe epilayers grown on the (211)A surface with those on the (211)B surface. It is found that the N incorporation is limited by a single‐dangling bond at the Zn sites.

This letter reports the photoluminescence (PL) properties of N‐doped ZnSe epilayers grown by molecular‐beam epitaxy using a microwave plasma source for N doping. The temperature dependence of PL spectra from N‐doped ZnSe epilayers with different net acceptor concentrations is described in detail. The emission due to transition from deep donors to free holes (DdF) is observed from highly N‐doped ZnSe epilayers at high temperatures, which gives a direct evidence for the formation of a deep N‐associated donor. The ionization energy for a deep N‐associated donor is estimated as 55±5 meV from comparison of the temperature dependence of the DdF emission energy with that of the free exciton energy.

The interface of a binary single quantum‐well (SQW) structure of ZnSe/CdSe, where CdSe less than one monolayer is sandwiched by ZnSe layers (submonolayer SQW), is characterized by photoluminescence spectroscopy. The dependence of the energy, linewidth, and intensity of excitonic emission from submonolayer SQWs on the well thickness of CdSe is extensively investigated. The characteristics of the excitonic emission are interpreted in terms of alloy formation at the interface.

Scanning tunneling microscopy (STM) is used to investigate the surface morphology of Ar+‐ion bombarded Si(001) surfaces and to elucidate the very beginning stages of solid phase epitaxy (SPE) processes of the Ar+‐ion bombarded Si surfaces. The Ar+‐ion bombarded Si surface consists of hillocks of 1–2 nm in diameter and 0.35–0.75 nm in height. The onset of SPE initiates at around 590 °C, at which a temperature (2×2) structure surrounded by amorphous regions is partially observed on terraces of the surface. During annealing at 590–620 °C, the areas of the (2×2) and c(4×4) reconstruction surrounded by amorphous regions develop. New defect models for the (2×2) and c(4×4) structures are proposed where alternating arrangements of the buckled dimers together with missing dimer defects are considered.

The n‐type ZnSe with electron concentration up to 3×1020 cm−3 and low resistivity down to 1×10−4 Ω cm has been grown using a planar‐doping technique of chlorine during molecular beam epitaxy. The photoluminescence evaluation shows that the planar‐doped ZnSe layers are superior to uniformly doped ones especially for the case of high Cl doping. The in‐depth profile of Cl concentration in a planar‐doped sample was measured with a secondary ion mass spectroscopy (SIMS). The SIMS analysis shows only slight diffusion of the incorporated Cl atoms even in highly doped samples.

The molecular beam epitaxial growth processes of Al single‐crystal layers deposited at room temperature on As‐stabilized and on Al‐stabilized AlAs surfaces are investigated in situ using reflection high‐energy electron diffraction (RHEED). The epitaxy relationship of Al with the underlying AlAs is (011)[100]Al//(001)[110]AlAs irrespective of the surface stoichiometry of AlAs. Detailed RHEED investigation suggests that the initially deposited two one‐monolayer‐thick Al layers form a sphalerite lattice on As‐(Al‐)stabilized AlAs with the same lattice spacing as AlAs, while the deposition of additional Al layers induces the structural phase transition from the sphalerite lattice to an fcc lattice with bulk Al lattice constant. Thick epitaxial Al layers show (4×1) surface reconstruction.

We found that m-plane GaN grown on m-plane sapphire nucleates in ambidirections at the initial growth stage, which seriously degrades the surface morphology and the crystallinity of m-GaN films. To avoid the ambidirectional islanding of m-plane GaN, off-cut m-plane sapphire (toward to the [112¯0] direction) substrates were introduced. When the off-angle was small as 1°–2°, the surface step determined the epitaxial orientation of m-GaN islands. Hence, an m-GaN film with a smooth surface and a low-dislocation density was obtained. However, the dislocation component of m-GaN film was increased with increasing of the off angle due to step bunches.

The photoluminescence and stimulated emission characteristics of two types of periodically polarity-inverted ZnO grating structures have been investigated in detail. Strong photoluminescence in the step regions is observed for all the samples. However, electron-hole plasma emission is observed only for one type of sample consisting of Zn-polar ZnO/CrN and O-polar ZnO/Cr2O3 layers. Moreover, strong stimulated emission occurs only in the Zn-polar layers. These optical phenomena have been discussed in terms of the diffusion of excitons and local crystal quality.

We report on the influence of heat treatments on the electrical properties of ZnO films grown by molecular-beam epitaxy. We note that the electrical resistance of the ZnO films is significantly changed by the heat treatments: the electrical resistance increases with the increase of ambient temperature, but above a critical temperature the resistance decreases with the increase of temperature, irrespective of ambient gases. On the other hand, it is found that the large amount of photocurrent is generated in the ZnO films, exposed to white sources: the photocurrent decreases with the increase of the obtained resistance, and the current increases with the decrease of the resistance. Also, it is shown that the x-ray diffraction linewidth of the ZnO films is significantly decreased by the heat treatments. These indicate that the increase/decrease of the electrical resistance is ascribed to the annihilation/formation of the residual donor-type defects in the ZnO films by the heat treatments. It is suggested tha...

We report the electrical characteristics of vertical and lateral type light emitting diodes(LEDs) grown with CrN buffer layer. The LED with CrN buffer showed lower reverse leakage current than the reference sample grown with conventional low-temperature GaN buffer. It was also observed that the density of open core screw dislocation was smaller by one order of magnitude, which was thought to relate to the leakage current of devices. The vertical type LED fabricated by chemical etching of CrN buffer showed lower series resistance, lower turn-on voltage, and larger light output power than those of the conventional LEDs.

Polarity-controlled ZnO thin films grown on different buffer layers were investigated as nonlinear optical materials for second harmonic generation. The effective nonlinear optical coefficient (deff) of ZnO grown on Cr-compound buffer layers showed a higher value than that of ZnO grown on MgO buffer layers. The correlations among the grain size and surface roughness with the values of deff were found to be strong. The deff of the ZnO film increased with decrease in the grain size and surface roughness. This relationship could be explained by the increase in reaction interface and the decrease in scattering probability at the surface as decrease in grain size and surface roughness, respectively.