Structural Properties of (GaIn)(AsN)/GaAs MQW Structures Grown by MOVPE
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GROWTH AND CHARACTERIZATION The (GaIn)(AsN)/GaAs multiple quantum well (MQW) heterostructures were grown by metal-organic vapor-phase epitaxy using an IR-heated horizontal reactor system (Aix 200). Triethylgallium (TEGa) and trimethylindium (TMIn) have been used as group-III sources and tetriarybutylarsine (TBAs) and 1,1-dimethylhydrazine (UDMHy) as group-V precursors. The growth was performed at a reactor pressure of 50 mbar under H2 carrier gas at a total flow of 6800 sccm. All the samples here investigated were grown at the same temperature (525°C) on [100]-oriented GaAs substrates. The growth rates of the quaternary (GaIn)(AsN) wells and the GaAs barrier layers were 0.25µm/h and 1.0µm/h , respectively. In order to avoid strain relaxation processes and the generation of associated structural defects, the total thickness of the MQW structures was chosen to be smaller than the critical thickness, i.e. the MQWs consist of only 5 periods. The nominal thickness of the GaAs barriers and the quaternary (GaIn)(AsN) wells are the same for all the samples investigated, i.e. db=90nm and dw=10nm, respectively. The growth parameters were optimized in order to have an In-content of x=0.33 in the quaternary layers. The N-content is constant in the individual well layers of one sample but it varies for the different samples of the investigated series in the range of y=0.01 and y=0.045. The x-ray diffraction experiments were performed by using a high-resolution multi-crystal x-ray diffractometer. A 4-crystal channel-cut monochromator-collimator arrangement has been used to reduce the wavelength and angular dispersion of the beam incident on the sample, which are for the CuKα-radiation used 2.5x10-5 and 60µrad, respectively. The experimental x-ray diffraction patterns were simulated by using a dynamical scattering model in the recursive formalism [9]. The second-order approximation of the angular deviation was used in order to obtain precise values of the lattice strain [10]. High quality specimens for TEM experiments were prepared in [011] cross-section geometry by using mechanical pre-thinning and subsequent ion-milling (3.5-4.5 keV Ar ions) at liquid nitrogen temperature in order to minimize ion-induced damage and artefacts [11]. In particular , the use of a cooled stage specimen holder prevents the eventual element interdiffusion due to the local increases of the temperature during ion bombardment. The TEM specimens were analyzed by using a Philips CM30 TEM/STEM electron microscope operating at 300 keV with an interpretable resolution limit of 0.23nm. The micrographs were acquired digitally by using a Gatan 1024x1024 slow scan CCD camera. SIMS analysis was carried out on a CAMECA ims 4f instrument using either a 2.5 keV O2+ or a 5.5 keV Cs+ ions, with off-normal impacting angles of 55° ( 2 keV ), and 42° (5.5 keV), respectively. The different conditions were chosen in order to improve the depth resolution, to better detect the indium and nitrogen signal and to reduce the matrix effects. The primary beam currents were ch
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