ALBERT

Notes: Summary We present a comprehensive discussion of the excitonic properties of V-shaped GaAs and InGaAs quantum wires grown on patterned substrates. Systematic linear and non-linear spectroscopic studies have been performed in order to clarify the impact of lateral confinement on the exciton wave function, namely: enhanced exciton binding energy, localization in magnetic field, recombination from excited states and multiphoton absorption. The careful evaluation of the electron confinement energies, based on the actual quantum wire profile obtained by TEM micrographs, including the internal piezoelectric field induced by off-diagnoal terms of strain tensor, reproduces quite well the measured one-dimensional states. Finally, application of quantum wires in a p-i-n wave guide for bistable operation is demonstrated.

Notes: Summary Photoluminescence (PL) and electroluminescence (EL) measurement of GaAs/AlGaAs quantum wires (QWR) located in the active region of a p-i-n junction are reported. The samples are fabricated by molecular-beam epitaxial growth on V-grooved substrates. Good control of the interface, defect density and doping profile have been achieved. Homogeneous current injection into the quantum wires is achieved with efficiencies comparable to current injection into a quantum well control sample. PL with and without an applied voltage across the junction was measured at 86 K and 300 K for different excitation densities. Peaks appearing with an applied voltage correspond to the active-region QWR transitions and are also observed on the EL spectra measured at 120 K and at 300 K. Clear evidence of 1D confinement is observed in both PL and EL spectra. They show a one-dimensional splitting of about 24 meV and a saturation of the ground state at high excitation density. The polarisation of the PL and EL is in good agreement with the expected anisotropy of the 1D matrix elements.

Notes: Low-temperature photoluminescence (PL) measurements of GaAs, AlxGa1−xAs, and AlAs samples grown by molecular beam epitaxy have been carried out to study the effects of hydrogen diffusion. Following exposure to a hydrogen plasma, the PL spectra of AlxGa1−xAs change. In particular, direct gap AlxGa1−xAs shows a strong increase in the total PL intensity whereas the PL spectra of indirect gap AlxGa1−xAs show an increase in the excitonic-related recombinations after hydrogenation; the binary compounds present less dramatic changes. We interpret our results in terms of hydrogen passivation of deep and shallow centers (DX), whose densities are higher for aluminum concentration near the direct to the indirect gap crossover.

Notes: Capacitance-voltage (C-V) analysis of high quality MBE grown quantum well samples shows that carrier distributions are averaged over the scale of the Debye length. This averaging process results in strongly temperature dependent C-V-deduced doping distributions that can be very different from the actual ones. The doping distribution of the structure is obtained by fitting numerically simulated curves to the measured C-V curves and doping profiles, respectively. Although the calculations do not take quantum size effects into account they show good agreement with the measured data. © 1996 American Institute of Physics.

Notes: Identical GaAs/Al0.2Ga0.8As multiple-quantum-well (MQW) structures uniformly doped with Si at various concentrations ranging from 1×1017 to 1×1019 cm−3 are grown by molecular-beam epitaxy to study the effects of the background Si-doping level on the Zn diffusion-induced disordering process. After Zn diffusions at 575 °C for 4 and 16 h, the structures are investigated by secondary-ion-mass spectrometry, and by transmission electron microscopy on cleaved wedges of the sample. The results show that the totally and partially disordered regions are always behind the Zn diffusion front. A dependence of the effective Zn diffusivity and of the disordering rate of the structures on the background Si-doping level is observed. The effective Zn diffusivity and the disordering rate are significantly reduced with increasing background Si concentration. Before Zn diffusion, photoluminescence spectra of the Si-doped MQW structures exhibit an increase in intensity of the Si donor–column-III vacancy complex emission band with increasing Si-doping level. This indicates that the concentration of column-III vacancies in the MQW structures increases as the background Si concentration increases. After Zn diffusion, an important decrease in intensity of the column-III vacancy related emission band is observed on the photoluminescence spectra taken in the Zn-diffused regions.The systematical analysis of the photoluminescence spectra of the Zn-diffused MQW structures as a function of diffusion time and as a function of etching depth below the sample surface makes it possible to describe the physical processes occurring during Zn diffusion. A model based on the "kick-out'' mechanism of Zn diffusion is proposed to explain the effect of the background Si-doping level on the effective Zn diffusivity. The model shows that the effective Zn diffusivity is controlled by the concentration of column-III interstitials behind the Zn diffusion front and by the donor concentration in the sample. During the incorporation of Zn into the crystal lattice, column-III interstitials are generated. The supersaturation of these interstitials behind the Zn diffusion front is responsible for the enhancement of Al–Ga interdiffusion. Since column-III interstitials and column-III vacancies can mutually annihilate, the concentration of column-III interstitial and column-III vacancy in the Zn-diffused region is reduced with increasing Si-doping level, leading to a retardation of Zn diffusion into the MQW structure. On the other hand, a decrease of the effective Zn diffusivity caused by an increase in donor concentration in the samples is also demonstrated.Our results give evidence for the Fermi-level effect and the interactions between different point defects during Zn diffusion-induced disordering of GaAs/AlGaAs multilayered structures. © 1996 American Institute of Physics.

Notes: Modulated reflectance spectroscopy and resonant Raman scattering have been used to study the quantized states of crescent-shaped GaAs quantum wires. Distinct one-dimensional excitonic transitions originating from the quantum wires together with the expected resonances from the bent quantum wells are observed. The quantum wire transition energies compare very well with those calculated using a V-shaped potential derived from transmission electron microscopy measurements of the lateral variation of the GaAs well width. The spectroscopic techniques employed here provide alternative methods of probing the confined states in quantum wires of low luminescence efficiency.

Notes: Molecular-beam epitaxy grown decoupled nominally square GaAs/AlAs multiquantum wells, producing levels deep in the well, have been studied by x-ray diffraction, photoluminescence excitation, and emission. The well width and period fluctuation (AlAs/GaAs/AlAs interface roughness) of the multiquantum wells were obtained by x-ray diffraction investigations. Using a smoothed profile of the interface as suggested by D. F. Nelson, R. C. Miller, C. W. Tu, and S. K. Sputz, Phys. Rev. B 36, 8063 (1987), the earlier verified theoretical approach [see Oelgart et al. Phys. Rev. B 49 (March 1994)] excellently predicts the experimentally observed transition energies.

Notes: In a recent paper we measured the lateral hole diffusion in a GaAs/AlGaAs single quantum well (SQW) by a novel method. At helium temperature, we estimate a lateral hole diffusion length in the QW of 1.5 μm. However, the assumption that diffusion takes place mainly in the SQW needs to be checked, as the measured diffusion length is the result of two competing processes: (i) hole diffusion in the SQW plane itself and (ii) hole diffusion in the barrier followed by recombination in the SQW. We present here a comparison between the lateral hole distribution in the SQW and in the AlGaAs barrier. First, we estimate the hole diffusion length in the barrier fitting experimental cathodoluminescence linescans on simulated ones. Second, using the measured diffusion lengths in the QW plane and in the bulk barrier and modeling the carrier transport, we deduce the lateral hole distribution in both layers. It is found that even for very large barriers (1.2 μm), the hole diffusion in the barrier contributes less than 0.1% of the total lateral hole diffusion. The lateral transport is mainly carried by holes in the QW (2D diffusion) due to their confinement in the well.

Notes: The optical properties of low pressure metal organic vapor deposition grown GaAs/AlxGa1−xAs (x=0.5) single quantum well structures (SQW) with grown-in dislocations (GD) were studied by low temperature cathodoluminescence (CL) and photoluminescence (PL). High luminescence efficiency around the GD was observed and attributed to impurity decoration. CL spectra show a region surrounding the GD that consists of Si impurities and native defects in the SQW and barrier layers. The diameter of this region was found to be in the order of 1 μm using spectrally resolved CL micrographs.

Notes: GaAs/AlxGa1−xAs multiple-quantum-well (MQW) structures with identical well thicknesses but with different Al contents x in the barrier (x≈0.1, 0.2, 0.45, and 1) were grown by molecular-beam epitaxy to study the impurity-induced disordering mechanism. The disordering of the structures is observed directly by transmission electron microscopy on cleaved wedges of the sample, by the secondary electron imaging mode of scanning electron microscopy, and by secondary-ion-mass spectroscopy after Zn diffusions at 575 °C during different times (1, 4, 9, and 16 h). The results show that the totally and partially disordered regions are always behind the Zn diffusion front. The partially disordered extent depends on x. As x increases, the disordering rate increases due to the increase in Zn diffusivity. The effect of high Zn concentration is investigated by photoluminescence and by Raman scattering measurements. The systematical analysis of the photoluminescence spectra of the MQW structures diffused for different times and of the photoluminescence spectra taken on different depths below the sample surface makes it possible to describe the physical processes occurring during Zn diffusion. The column-III vacancies are created at the sample surface. They diffuse into the bulk of the sample where they are filled by other defects. Using the x-ray-diffraction technique, an expansion of the lattice constant in the region behind the Zn diffusion front was observed. This is due to a supersaturation of column-III interstitials. During the incorporation of Zn into the crystal lattice, column-III interstitials are generated. These interstitials could be responsible for the enhancement of the Al-Ga interdiffusion. The important role of the electric field at the p-n junction formed by Zn diffusion is discussed. The negatively charged column-III vacancies and the positively charged column-III interstitials are confined, respectively, on the n and p sides of the p-n junction. The results give evidence for the self-interstitial mechanism of Zn diffusion-induced disordering in GaAs/AlGaAs MQW structures.