T. Yasuda

MgxZn1−xO as a II–VI widegap semiconductor alloy

We propose a widegap II–VI semiconductor alloy, MgxZn1−xO, for the fabrication of heteroepitaxial ultraviolet light emitting devices based on ZnO. The c-axis oriented MgxZn1−xO films were epitaxially grown by pulsed laser deposition on ZnO epitaxial films and sapphire (0001) substrates using ceramic targets. Solid solution films were prepared with Mg content up to x=0.33, achieving a band gap of 3.99 eV at room temperature. MgO impurity phase segregated at x⩾0.36. Lattice constants of MgxZn1−xO films changed slightly (∼1%), increasing in a axis and decreasing in c-axis direction with increasing x. These films showed ultraviolet photoluminescence at energies from 3.36 (x=0) to 3.87 eV (x=0.33) at 4.2 K.

Band gap engineering based on MgxZn1−xO and CdyZn1−yO ternary alloy films

We describe the structural and optical properties of II–VI oxide alloys, MgxZn1−xO and CdyZn1−yO, grown by pulsed-laser deposition. Single-phase alloyed films of (Mg,Zn)O and (Cd,Zn)O with c-axis orientations were epitaxially grown on sapphire (0001) substrates. The maximum magnesium and cadmium concentrations (x=0.33 and y=0.07, respectively) were significantly larger than the thermodynamic solubility limits. The band gap energies systematically changed from 3.0 (y=0.07) to 4.0 eV (x=0.33) at room temperature. The photoluminescence peak energy deduced at 4.2 K could be tuned from 3.19 to 3.87 eV by using Cd0.07Zn0.93O and Mg0.33Zn0.67O at both ends, respectively. The lattice constants of the a axis were monotonically increasing functions of the concentrations of both alloys. The exciton–phonon coupling strength was determined in Cd0.01Zn0.99O grown on a lattice-matched ScAlMgO4 substrate.

Structure and optical properties of ZnO/Mg0.2Zn0.8O superlattices

ZnO/Mg0.2Zn0.8O superlattices with a band-gap offset of about 0.5 eV were epitaxially grown by laser molecular-beam epitaxy on a sapphire(0001) substrate using a ZnO buffer layer. The superlattice structure with a period ranging from 8 to 18 nm was clearly verified by cross-sectional transmission electron microscopy, Auger depth profile, and x-ray diffraction. As the well layer thickness decreased below 5 nm, the photoluminescence peak and absorption edge in the photoluminescence excitation spectra showed a blueshift, indicating a quantum-size effect.

Fabrication of alloys and superlattices based on ZnO towards ultraviolet laser

Abstract We have grown epitaxial Mg x Zn 1− x O alloy films and ZnO/Mg x Zn 1− x O ( x =0.20) superlattices on sapphire(0001) substrates by laser molecular beam epitaxy and characterized their structures by X-ray diffraction. Single phase Mg x Zn 1− x O could be obtained up to x =0.33, whereas MgO impurity phase with (111) orientation segregated at x >0.33. The bandgap of Mg x Zn 1− x O was successfully controlled as verified by the photoluminescence peaks shifting 3.36 eV ( x =0) to 3.87 eV ( x =0.33). It was found that the structure of the superlattices was greatly improved by the use of a ZnO buffer layer on sapphire substrate prior to the deposition of superlattice. Small angle X-ray diffraction peaks corresponding to the period of the superlattices ranging from 8 to 18 nm could be clearly observed.

Strain effects on exciton resonance energies of ZnO epitaxial layers

Magnitudes of strain in ZnO epitaxial layers grown on sapphire(0001) substrates under various growth conditions were experimentally determined by x-ray diffraction. We discuss the strain-induced energy shift on the exciton resonances, the results of which were analyzed theoretically using the Hamiltonian for the valence bands under in-plain biaxial strain. Comparative studies with GaN evidenced the advantages of ZnO in terms of sensitivity of the strain-induced energy shift and of piezoelectric effect in heterostructures.

Naturally formed ZnCdSe quantum dots on ZnSe (110) surfaces

We successfully realized ZnCdSe quantum dots on a cleavage-induced ZnSe (110) surface by depositing a ZnSe/ZnCdSe/ZnSe heterostructure under growth conditions that cannot lead to layer-by-layer growth of ZnSe. This growth mode introduces surface roughness to the newly deposited ZnSe layer, and ZnCdSe quantum dots are then formed. Cathodoluminescence and microphotoluminescence measurements demonstrate the formation of quantum dots.

Optical spectra in ZnO thin films on lattice-matched substrates grown with laser-MBE method

Optical properties in undoped ZnO thin films grown with laser-molecular beam epitaxy method on lattice-matched ScAlMgO 4 substrates have been investigated. The transmission spectrum on this substrate at 6 K has two sharp peaks both of which are attributed to resonances of A- and B-excitons, which reflects small nonradiative damping constant of excitons as well as high film crystallinity accomplished by virtue of lattice matching. This is, to our best knowledge, the first observation of exciton absorption and its direct estimation of parameters related to A- and B-excitons.

A new approach to ZnCdSe quantum dots

Abstract We propose a new approach of fabricating ZnCdSe quantum dots (QDs) on ZnSe and GaAs (110) surfaces by simply depositing a ZnSe/ZnCdSe/ZnSe heterostructure. The growth conditions are selected to introduce surface roughness on the over grown ZnSe which allows the formation of ZnCdSe QDs. Optical studies unambiguously demonstrate the formation of QDs. Resolution-limited sharp emission lines are observed by micro-photoluminescences and the linewidths are much less than the thermal energy. As time goes on, intermittent behaviors of the QD emissions are observed. A proper selection of growth conditions is essential in obtaining ZnCdSe QDs by this method, especially on GaAs (110) surfaces.

Intermittent photoluminescence and thermal ionization of ZnCdSe/ZnSe quantum dots grown by molecular beam epitaxy

We report the intermittent photoluminescence of ZnCdSe quantum dots (QDs) embedded in a ZnSe matrix grown by molecular beam epitaxy. The luminous time of the QD is strongly dependent on temperature but not on excitation intensity. This indicates that the ionization of the QDs is determined predominantly by thermal excitation of carriers into the ZnSe matrix.

Spontaneous formation and photoluminescence of ZnSe dot arrays

Highly luminescent ZnSe quantum dot arrays (QDAs) are spontaneously formed on cleavage-induced GaAs (110) surfaces. The QDAs are configured for their preferred growth on the step top. The confinement on carriers results from the difference in the band gaps of the strained ZnSe layer and the strain-relaxed ZnSe QDA. In contrast to other emissions from the ZnSe layer, the linewidth of the QDA emission is dependent neither on temperature nor on excitation intensity. Moreover, the energy position of the QDA emission is stable even at high excitations. These results reflect the δ functionlike density of states of the QDAs. This letter suggests a novel approach to semiconductor QDs and QDAs.