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  • Polymer and Materials Science  (5)
  • 1
    ISSN: 1057-9257
    Keywords: MOVPE ; Quantum wells ; ZnSe/ZnSxSe1 - x heterostructures ; Gain ; Excitons ; Luminescence dynamics ; Transfer processes ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Optically pumped stimulated emission in ZnSe/ZnSxSe1 - x heterostructures grown by MOVPE has been observed up to 170 K. Gain measurements have been performed using the variable stripe length method. The underlying gain mechanism at 25 K is atributed to an excitonexciton scattering process. Photoluminescence excitation spectra and the temporal evolution of the luminescence indicate a transfer process from the ZnSxSe1 - x barrier into the ZnSe active layer.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1057-9257
    Keywords: MOVPE ; ZnSxSe1 - x ; MQW ; Optical properties ; Lasing Exciton ; Electron-hole plasma ; Dynamics ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Non-linear and stimulated emission of ZnSxSe1 - x/ZnSe multilayer structures grown by MOVPE on GaAs is investigated by means of high-density excitation spectroscopy using a high-resolution pulsed excimer-dye laser system as well as a picosecond titanium-sapphire laser for time-resolved measurements. Heterostructures with ZnSe layers or single and multiple quantum wells were grown in which the ZnSe layer widths were varied between 1 nm (strong quantum confinement) and 500 nm (quasi-bulk situation). The sulphur concentration in the ZnSxSe1 - x buffer, barrier and cap layers was chosen between x = 0.045 and x = 0.74 in order to find the most promising compromise with regard to maximum band offset to ZnSe for most efficient carrier confinement and suitable waveguiding properties depending on variation in the refractive index, and to highest structural quality of the actively emitting ZnSe layers. Stimulated emission is found to occur in all samples, being brightest in the sample with the lowest sulphur concentration in the cap and barrier layers. Its dynamical properties point to a probable interpretation in terms of electron-hole plasma creation, which, however, may not be the basic process in narrow-quantum wells or strongly structurally disturbed samples.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1057-9257
    Keywords: Scanning transmission electron microscopy (STEM) ; High-resolution ; Z-contrast ; ADF ; HAADF ; Electron energy loss spectroscopy (EELS) ; ZnSxSe1 - x/ZnSe quantum wells ; MOVPE ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Epitaxial growth techniques for ZnSxSe1 - x/ZnSe heterostructures have already achieved a high degree of development. Further improvements strongly require characterisation techniques with high compositional sensitivity and high spatial resolution. Therefore in this work high-resolution Z-contrast, which provides compositional information down to the atomic scale, has been used for the first time to characterise ZnSxSe1 - x/ZnSe quantum well structures. The influence of structural defects on Z-contrast is demonstrated by comparison of scanning transmission electron microscopy (STEM) bright field images and STEM Z-contrast micrographs of planar defects and dislocations. The compositional abruptness of ZnSxSe1 - x/ZnSe interfaces in MOVPE-grown quantum well (QW) structures is judged from high-resolution Z-contrast micrographs. Electron energy loss spectroscopy (EELS) measurements were performed for the first time in ZnSxSe1 - x/ZnSe QW structures in order to obtain quantitative compositional information with nanometre spatial resolution. From EELS line scans, which monitor the selenium concentration across ZnSe QWs, the obtainable spatial resolution is estimated to be about 1-2 nm. The problems that have prevented quantitative analysis of the selenium concentration up to now are discussed.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1057-9257
    Keywords: ZnSe ; ZnSSe ; GaAs substrate ; MOVPE ; Photoluminescence ; Quantum well ; Interface properties ; Growth interruption ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: In this work we report on ZnSxSe1 - x/ZnSe quantum wells grown by atmospheric pressure MOVPE on GaAs substrates. Diethylznc (DEZn), diethylselenium (DESe), diethylsulphur (DES) and H2S were used as source materials. Binary quantum wells with ZnSe or ZnS as barrier were usually grown in sets of 10 pairs on a 0.5 μm ZnSe or ZnS buffer, respectively. QWs with different well thicknesses (1-4 nm) show a typical shift to higher energies with decreasing well thickness and also a large decrease in PL intensity with increasing well thickness. Owing to the onset of relaxation above 5 nm well thickness, no PL emission could be observed. QWs of the same thickness but grown at different temperatures (420-520°C) show a broadened line shape with decreasing growth temperature. Growth interruption (3-50 s) also causes a broadening of the PL emission with decreasing interruption time. This is caused by interface reconstruction at higher growth temperatures and longer growth interruption. A ZnS buffer of 0.5 μm improves the PL line shape compared with a ZnSe buffer.ZnSxSe1 - x/ZnSe single and multiple QWs (Lz = 1-4 nm) were grown to reduce the strain in the structures. A typical quantum confinement energy shift of 160 meV for the 1 nm well can be observed in ZnS0.68Se0.32/ZnSe QWs. In comparison with the binary QWs, the FWHM could be reduced by about a factor of two to 34 meV. This improvement was obtained by an optimisation of the switching process to prevent the desorption of sulphur from the ZnSSe during the growth interruption.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1057-9257
    Keywords: ZnSe ; ZnSxSe1 - x ; MOVPE ; Photoluminescence ; Mapping ; Purified ; DESe source ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: In this paper we report new results concerning the cause of impurities responsible for the Ix, and I2-peaks in photoluminescence (PL) spectra of ZnSe grown by MOVPE. An improvement in ZnSe epilayer quality is obtained by using a metal organic selenium precursor with reduced chlorine concentration. The PL spectrum of such a layer shows typical excitonic transitions, but compared with samples grown with a more contaminated source, the intensity of the Ix, and the I2 peaks decreases relative to the free exciton transition. A Gaussian fitting of the donor-bound exciton peaks taking the background of other structures into account shows that the ratio between the Ix and I2 peaks does not differ significantly between two samples. Both the decrease in donor-bound exciton transitions and the unchangeability of the ratio Ix/I2lead to the conclusion that only chlorine impurities are responsible for Ixand Ix. In order to verify the homogeneity of impurity uptake across a 2 inch wafer, we performed PL mapping of ZnSxSe1 - x layers. Mapping of a 2 inch ZnSe wafer shows that the FWHM of Ix across a wafer does not vary significantly (1.55 ± 0.21 meV). On mapping a 2 inch ZnS0.3Se0.7 wafer fabricated with H2S as the sulphur source at TD = 480°C, we found a rotational symmetric dstribution of sulphur in the layer. The sulphur content x at the centre is nearly constant. The difference in x between the centre and the boundary of a bad surface region at the edge of the wafer is less than Δx = 0.045. The FWHM of the band edge luminescence follows the same tendency across the wafer. The dependence of homogeneity on the reactor design as well as the uptake of unintentional impurities from the precursor is discussed in detail.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
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