Blue-violet photoluminescence from large-scale highly aligned boron carbonitride nanofibers

We report on the strong blue-violet photoluminescence (PL) at room temperature from the large-scale highly aligned boron carbonitride (BCN) nanofibers synthesized by bias-assisted hot filament chemical vapor deposition. The photoluminescence peak wavelength shifts in the range of 470-390 nm by changing the chemical composition of the BCN nanofibers, which shows an interesting blue and violet-light-emitting material with adjustable optical properties. The mechanism for the shift of the PL peaks at room temperature is also discussed. (C) 2000 American Institute of Physics. [S0003-6951(00)04427-2].

Self-organized type-II In0.55Al0.45As/Al0.50Ga0.50As quantum dots realized on GaAs(311)A

Self-organized In0.55Al0.45As/Al0.50Ga0.50As quantum dots are grown by the Stranski-Krastanow growth mode using molecular beam epitaxy on the GaAs(311)A substrate. The optical properties of type-II InAlAs/AlGaAs quantum dots have been demonstrated by the excitation power and temperature dependence of photoluminescence spectra. A simple model accounting for the size-dependent band gap of quantum dots is given to qualitatively understand the formation of type-II In0.55Al0.45As/Al0.50Ga0.50As quantum dots driven by the quantum-confinement-induced Gamma --> X transition. The results provide new insights into the band structure of InAlAs/AlGaAs quantum dots. (C) 2000 American Institute of Physics. [S0003-6951(00)00725-7].

Epitaxial growth of GaNAs/GaAs heterostructure materials

A series of systematic experiments on the growth of high quality GaNAs strained layers on GaAs (001) substrate have been carried out by using DC active Nz plasma, assisted molecular beam epitaxy. The samples of GaNAs between 3 and 200 nm thick were evaluated by double crystal X-ray diffraction (XRD) and photoluminescence (PL) measurements. PL and XRD measurements for these samples are in good agreement. Some material growth and structure parameters affecting the properties of GaNAs/GaAs heterostructure were studied; they were: (1) growth temperature of GaNAs epilayer; (2) electrical current of active N-2 plasma; (3) Nz flow rate; (4) GaNAs growth rate; (5) the thickness of GaNAs strained layer. XRD and PL measurements showed that superlattice with distinct satellite peaks up to two orders and quantum well structure with intensity at 22 meV Fourier transform infrared spectroscopy (FWHM) can be achieved in molecular beam epitaxy (MBE) system. (C) 2000 Published by Elsevier Science S.A. All rights reserved.

Energy structure and fluorescence of Eu2+ in ZnS : Eu nanoparticles

Eu2+-doped ZnS nanoparticles with an average size of around 3 nm were prepared, and an emission band around 530 nm was observed. By heating in air at 150 degrees C, this emission decreased, while the typical sharp line emission of Eu3+ increased. This suggests that the emission around 530 nm is from intraion transition of Eu2+: In bulk ZnS:Eu2+, no intraion transition of Eu2+ was observed because the excited states of Eu2+ are degenerate with the continuum of the ZnS conduction band. We show that the band gap in ZnS:Eu2+ nanoparticles opens up due to quantum confinement, such that the conduction band of ZnS is higher than the first excited state of Eu2+, thus enabling the intraion transition of Eu2+ to occur.

Effect of buffer layer growth conditions on the secondary hexagonal phase content in cubic GaN films on GaAs(001) substrates

In this letter, we investigated the effect of the buffer layer growth conditions on the secondary hexagonal phase content in cubic GaN films on GaAs(0 0 1) substrate. The reflection high-energy electron diffraction (RHEED) pattern of the low-temperature GaN buffer layers shows that both the deposition temperature and time are important in obtaining a smooth surface. Four-circle X-ray double-crystal diffraction (XRDCD) reciprocal space mapping was used to study the hexagonal phase inclusions in the cubic GaN (c-GaN) films grown on the buffer layers. The calculation of the volume contents of the hexagonal phase shows that higher temperature and longer time deposition of the buffer layer is not preferable for growing pure c-GaN film. Under optimized condition, 47 meV FWHM of near band gap emission of the c-GaN film was achieved. (C) 2000 Elsevier Science B.V. All rights reserved.

Growth and characterization of strained superlattices delta-GaNxAs1-x/GaAs by molecular beam epitaxy

A series of superlattices delta-GaNxAs1-x/GaAs were grown by a DC plasma-N-2-assisted molecular beam epitaxy. The evolution of the surface reconstruction during the growth has been studied with the use of in situ reflection high-energy electron diffraction. The superlattices have been characterized by high-resolution X-ray diffraction measurements. Distinct satellite peaks indicate that the superlattices are of good quality. The N compositions in strained GaNxAs1-x monolayers are obtained from the dynamical simulations of the measured X-ray diffraction patterns. The periodicity fluctuations of N composition are obtained from a kinematical method dependent on the broadening of the satellite peaks of the X-ray diffraction. (C) 2000 Elsevier Science B.V. All rights reserved.

Effects of rapid thermal annealing on the optical properties of GaNxAs1-x/GaAs single quantum well structure grown by molecular beam epitaxy

The effect of rapid thermal annealing (RTA) on the optical properties of GaNxAs1-x/GaAs strained single quantum well (SQW) was studied by low-temperature photoluminescence (PL). The GaNxAs1-x/GaAs SQW structures were prepared by dc active nitrogen plasma assisted molecular beam epitaxy. PL measurements on a series of samples with different well widths and nitrogen compositions were used to evaluate the effects of RTA. The annealing temperature and time were varied from 650 to 850 degrees C and 30 s to 15 min, respectively. Remarkable improvements of the optical properties of the samples were observed after RTA under optimum conditions. The interdiffusion constants have been calculated by taking into account error function diffusion and solving the Schrodinger equation. The estimated interdiffusion constants D are 10(-17)-10(-16) cm(2)/s for the earlier annealing conditions. Activation energies of 6-7 eV are obtained by fitting the temperature dependence of the interdiffusion constants. (C) 2000 American Institute of Physics. [S0021-8979(00)10401-3].

Observation of field-driven blue shift in delta-doped Si n-i-p-i multiple quantum wells

We report the observation of the field-driven blue shift at near absorption edge in the photo-current response spectra of delta-doped Si n-i-p-i multiple quantum wells due to the widening of the effective energy gap. This phenomenon differs from the observed results in GaAs/AlGaAs and GeSi/Si superlattices, because the physical mechanisms of forming energy band in these superlattice samples are different. Our experimental results are interpreted satisfactorily by the theoretical calculation. (C) 1999 Elsevier Science Ltd. All rights reserved.

Temperature-induced turnover of the well and barrier layers in ZnSe/Zn0.84Mn0.16Se superlattices

Photoluminescence of ZnSe, Zn0.84Mn0.16Se alloy, and ZnSe/Zn0.84Mn0.16Se superlattice (SL) have been measured in the temperature range from 10 to 300 K. It is found that the band gap of the ZnSe was smaller than that of the Zn0.84Mn0.16Se alloy at 10 K, but larger than that of the alloy at 300 K. Then the well and barrier layers of the ZnSe/Zn0.84Mn0.16Se SL would be expected to turn over at about 180 K. This type of turn over was observed in the SL sample. The turn over took place at 80 K, somewhat lower than the expected temperature. A calculation including the strain in the ZnSe/Zn0.84Mn0.16Se SL indicates that the heavy-hole bands begin crossing at 75 K, which agrees well with experimental results. [S0163-1829(99)13127-8].

A comparison of photoluminescence properties of InGaAs GaAs quantum dots with a single quantum well

Variable temperature photoluminescence (PL) measurements for In0.3Ga0.7As(6 nm)/GaAs(34 nm) quantum dot superlattices with a period of 20 and an In0.3Ga0.7As(6 nm)/GaAs(34 nm) reference single quantum well have been conducted. It is found that the temperature dependence is different between the quantum dots and the reference single quantum well. The PL peak energy of the single quantum well decreases faster than that of the quantum dots with increasing temperature. The PL peak energy for the InGaAs/GaAs quantum dots closely follows the InAs band gap in the temperature range from 11 to 170 K, while the PL peak energy for the InGaAs/GaAs quantum well closely follows the GaAs band gap. In comparison with InAs/GaAs quantum dots, the InGaAs/GaAs quantum dots are more typical as a zero-dimensional system since the unusual PL results, which appear in the former, are not obvious for the latter. (C) 1999 American Institute of Physics. [S0021-8979(99)08615-6].

Wavelength tuning in GaAsAlGaAs quantum wells by InAs submonolayer insertion

Wavelength tuning of exciton emissions has been achieved simply by inserting an InAs submonolayer at the centre of GaAs quantum wells during molecular beam epitaxy growth. Photoluminescence measurements show that the emission energy can be effectively tuned from the quantum-well-determined energy down to less than the band gap of GaAs, depending on the well width as well as the InAs layer thickness. Using the effective-mass approximation, the tuning effect can be well predicted theoretically The results reported here may provide an alternative way to tune the wavelength in optoelectronic devices.

Radiative transition in delta-doped GaAs superlattices

Radiative transition in delta-doped GaAs superlattices with a weak coupling was investigted at low temperature, The experimental results show that the transitions from both electron ground state and excited state to hole state have been observed, Based on the effective mass approximation theory, the structures of energy band and photoluminescence spectra for the samples used were calculated. Comparing the experiment with theory, a good agreement was abtained.

Pressure behavior of deep centers in ZnSxTe1-x alloys

We have measured photoluminescence of ZnSxTe1-x alloys (x > 0.7) at 300 K and under hydrostatic pressure up to 7 GPa. The spectra contain only a broad emission band under excitation of the 406.7 nm line. Its pressure coefficients are 47, 62 and 45 meV/GPa for x = 0.98, 0.92 and 0.79 samples, which are about 26%, 7% and 38% smaller than that of the band gap in the corresponding alloys. The Stokes shifts between emission and absorption of the bands were calculated by fitting the pressure dependence of the emission intensity, being 0.29, 0.48 and 0.13 eV for the three samples, respectively. The small pressure coefficient and large Stokes shift indicate that the emission band observed in our samples may correspond to the Te isoelectronic center in the ZnSxTe1-x alloy.

Interference effects in differential reflectance spectra of the GaAs epilayers grown on Si substrate

We report the observation of oscillating features in differential reflectance spectra from the GaAs epilayer grown on Si substrate in the energy range both below and above the fundamental band gap. It is demonstrated that the oscillating features are due to the difference in the interference between two neighboring areas of the sample. The interference arises from two light beams reflected from different interfaces of the sample. The calculated spectra in the nonabsorption region are in good agreement with measured data. It is shown that the interference effect can be used as a sensitive method to characterize the inhomogeneity of the semiconductor heterostructures. (C) 1998 American Institute of Physics. [S0021-8979(98)08723-4].

Optical properties of ZnSxTe1-x mixed crystals

The Raman scattering and the photoluminescence (PL) of ZnSxTe1-x mixed crystals grown by MBE, covering the entire composition range (0 less than or equal to x<1), were investigated. The results of Raman studies show that the phonons in ZnSxTe1-x mixed crystals display two-mode behavior. In addition, photoluminescence spectra obtained in backscattering and edge-emission geometries, reflection spectra and the temperature dependence of the photoluminescence of ZnSxTe1-x from 10 to 300K were employed to find out the origins of PL emissions in ZnSxTe1-x with different x values, The results indicate that the emission bands, for the samples with small x values, can be related to the band gap transitions or a shallow level emission center, while for the samples with large x values, they are designated to strong radiative recombinations of Te isoelectronic centers.