T. Takai

Aluminum-doped n-type ZnTe layers grown by molecular-beam epitaxy

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.

ZnTe-based light-emitting-diodes grown on ZnTe substrates by molecular beam epitaxy

We have investigated the ZnTe-based material system for the application to light-emitting devices. To this end, ZnTe homoepitaxy techniques have been developed to grow high-quality epitaxial layers. The conductivity control of ZnTe and ZnMgSeTe layers have been investigated. High structural quality n-type ZnTe layers with high carrier concentration are achieved by aluminum doping. Ambipolar conductivity control of quaternary layers is achieved. Aluminum doped ZnMgSeTe layers show a net carrier concentration of 5 × 10 16 cm -3 , while a high hole concentration of 2.5 x 10 19 cm -3 is achieved by p-type doping using a nitrogen plasma source. Based on those results, Zn 1-x Cd x Te/ZnMgSeTe triple-quantum-well(TQW) LED structures were fabricated. Bright electroluminescence was obtained at room temperature at the wavelength of 604 nm from Zn 0.7 Cd 0.3 Te and at 566 nm from Zn 0.85 Cd 0.15 Te TQW-LED.

Femtosecond three-pulse photon echo and population grating studies of the optical properties of CdTe/ZnSe quantum dots

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.

Molecular Beam Epitaxy of Al Doped n-ZnSe

Molecular beam epitaxy of Al doped n-type ZnSe is studied. Growth conditions such as Al cell temperature and substrate temperature are systematically examined. The electrical, optical and structural properties of ZnSe : Al layers are investigated. ZnSe : Al layers show high electron concentration of 8.37 × 10 18 cm -3 with low electrical conductivity of 6.92 x 10 -3 Ω cm along with high crystallinity.

Deep-level-transient spectroscopy of heavily Al-doped ZnSe layers grown by molecular-beam epitaxy

Using deep-level-transient spectroscopy, we have investigated deep levels in heavily Al-doped ZnSe layers grown by molecular-beam epitaxy. The Al concentration of the ZnSe layers lies in the range of 5×1018–9×1018cm−3. The ZnSe:Al layers exhibit two electron-trap centers with the thermal activation energies of 0.16eV (ND1) and 0.80eV (ND2). ND2 is a dominant trap center with a trap density of 3×1016cm−3, while the trap density of ND1 is estimated to be 2×1015cm−3. However, ND2 shows anomalous behaviors, different from isolated point defects, in the following points: (1) the emission peak of ND2 moves to the low temperature side with increasing filling pulse duration; (2) the emission peak of ND2 is broader than theoretically calculated one for an isolated point defect; and (3) the capacitance-transient curve is nonexponential. It is observed by high-resolution x-ray diffraction that heavy Al doping results in the relaxation and plastic deformation of the ZnSe lattice. These behaviors can be ascribed to ex...

Formation processes of CdTe quantum dots on ZnTe substrates studied by reflection high-energy electron diffraction and photoluminescence

Formation processes of self-organized CdTe quantum dots (QDs) grown on ZnTe (001) substrates by molecular beam epitaxy were studied. In situ reflection high-energy electron diffraction (RHEED) was used to study the initial growth processes and strain relaxation behaviors of CdTe QDs. The growth process of CdTe layer at the initial stage can be divided into three stages: (1) stable two-dimensional (2D) growth, (2) metastable 2D growth just before the QD formation, and (3) QD formation. It was found that the critical thickness for the 2D–3D transition is about 5.5 monolayers (ML) at the growth rate of 0.12 ML/s. The results of photoluminescence (PL) and μ-PL agree with the RHEED observations. In addition, by investigating the dependence of PL peak energy on the growth temperature, we found that interdiffusion between the cations (Cd and Zn) is activated at a higher growth temperature.

Silver photodoping into Al–ZnSe for application to white light emitters

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.

Strong luminescence due to localized excitons in CdTe/ZnSe fractional monolayer structures

Although the application of semiconductor quantum dots to light emitters has been expected, only a few successful achievements have been reported mainly due to the low luminescence efficiency. This paper will report on the strong luminescence of CdTe/ZnSe fractional-monolayer (FM) heterostructures. Various optical-spectroscopic methods are used to investigate the origin of the luminescence in this structure.

Band filling and thermal escape in CdTe/ZnTe quantum dots grown by molecular beam epitaxy

We have investigated the effects of band filling and thermal escape on the temperature dependence and excitation power dependence of photoluminescence of CdTe QDs grown by molecular beam epitaxy on ZnTe(100) substrate. The absorption spectrum indicates that the PL spectrum consists of two peaks. The experimental results of excitation power dependence were well simulated by the rate equation model taking into account the effect of band filling. The temperature dependence of PL showed significant difference for different excitation powers. It is suggested that either the wetting layer or excited states play an important role in band filling and thermal escape.

Photoluminescence Properties of Selectively Ag-Photodoped Al–ZnSe at Room Temperature

Selective Ag photodoping at room temperature (RT) has been succesfully performed on Al-doped n-type ZnSe grown by molecular beam epitaxy (MBE). Some 6-nm-thick Ag islands with a square shape (2 mm×2 mm) are deposited on the as-grown Al–ZnSe, which in turn is illuminated with a 325 nm HeCd laser at RT and the photoluminescence (PL) characteristics are measured in real time. The self-activated (SA) emission shown in Ag-undoped Al–ZnSe shifts to a shorter wavelength ΔE(I2)~+60 meV) via a series of processes of Ag photodoping and subsequent annealing in the MBE chamber. The emission bands centered at 2.245 eV and 2.146 eV distinctly show a blue shift with the increase of the PL-excitation power, which can be assigned to a donor-to-acceptor (DA) pair transition. That is, the SA bands in Al–ZnSe appear to be converted into DA bands by Ag photodoping. The coloring is controlled to some degree by adjustment of the percentage of the Ag-doped area included in the PL-excitation spot. For example, a 40% spectrum, which corresponds to one obtained from an excitation spot consisting of an Ag doped area of ~40% and an Ag undoped area of ~60%, appears yellowish-white to the naked eye. We believe that it is possible to realize a patterned ZnSe-based white-light emitter with Ag islands of micrometer size using the conventional UV-lithography and lift-off processes and RT–Ag photodoping.