F. K. Reinhart

Characterization of GaAs/(GaAs)n(AlAs)m surface‐emitting laser structures through reflectivity and high‐resolution electron microscopy measurements

Careful investigation of the reflectivity of two very high finesse integrated Fabry–Perot interferometers is reported. These two structures, made of GaAs active layer (1.7 μm thick) surrounded by two superlattice/AlAs Bragg reflectors, exhibit vertical cw lasing action at and above room temperature when photopumped with thresholds of 16 mW at 300 K and 56 mW at 380 K. Reflectivity measurements together with theoretical calculations show that layer regularity, accurate thickness control, and low interface roughness are key parameters for high‐performance structures. Transmission electron microscopy on cleaved wedges and reflection electron microscopy are shown to be unique tools for measuring and characterizing these layers. Electron microscopy, optical reflection, and laser linewidth measurements are correlated and show that the layer flatness is dramatically increased by the introduction of six (2.5 A) GaAs wells in the AlAs growth of the integrated dielectric reflectors. Reflectives of 97%, Fabry–Perot ...

Indium Distribution in Ingaas Quantum Wires Observed with the Scanning Tunneling Microscope

The incorporation of In in the growth of crescent‐shaped In0.12Ga0.88As quantum wires embedded in (AlAs)4(GaAs)8 superlattice barriers is studied in atomic detail using cross‐sectional scanning tunneling microscopy. It is found that the In distribution in both the surface and the first subsurface layer can be atomically resolved in the empty‐ and filled‐state images, respectively. Strong In segregation is seen at the InGaAs/GaAs interfaces, but neither an expected enhancement of the In concentration at the center of the quantum wire compared to the planar quantum well nor In clustering beyond the statistical expectation is observed.

X‐ray diffraction reciprocal space mapping of a GaAs surface grating

A GaAs surface grating (period 574 nm) is analyzed by four crystal‐six reflection x‐ray diffraction. Two‐dimensional measurements in the vicinity of the 004 GaAs reciprocal lattice point show satellites in the transverse direction related to the periodicity of the grating. A cross pattern, centered on the 004 GaAs reciprocal lattice point, is formed by these satellites. An explanation is given by a model which includes the influence of transmission through the surface pattern on the substrate diffraction.

Phase modulation in GaAs/AlGaAs double heterostructures. I. Theory

This work aims at a systematic study of phase modulation in GaAs/AlGaAs double‐heterostructure waveguides with different doping profiles. Both theoretical (part I) and experimental (part II) aspects are investigated, leading to interesting new results. Phase modulation is the sum of a linear electro‐optic term, a quadratic electro‐optic term, and a free‐carrier term. The carrier term is shown to be the sum of a plasma term, an intervalence‐band term (for holes only), a band‐filling term, and a band‐shrinkage term, the latter being due to many‐body effects. A new analytic expression for the band‐filling term is derived which shows that the band‐filling effect does not depend on the carrier effective mass. We prove that the band‐shrinkage term is approximately half of the band‐filling term, but has opposite sign. The phase modulation is computed using an overlap integral between the optical intensity and the local refractive‐index difference. We also report an analytic expression for the modulation efficien...

Atomic structure and luminescence excitation of GaAs/(AlAs)n(GaAs)m quantum wires with the scanning tunneling microscope

We report on the imaging of a molecular beam epitaxially grown GaAs/(AlAs)n(GaAs)m quantum well‐wire array by means of cross‐sectional scanning tunneling microscopy (XSTM) and scanning tunneling‐induced luminescence (STL). XSTM provides atomically resolved cross‐sectional images of sets of quantum well wires with chemical sensitivity within the group III species and electrical sensitivity to single dopant atoms. This permits the precise observation of growth mechanisms and the identification of defects responsible for inhomogeneities in the growth morphology, as well as the determination of dopant incorporation throughout the structure. STL permits the relative quantum efficiency of individual quantum wires to be quantified.

Superconducting YBa2Cu3O7-δ thin films on GaAs with conducting indium-tin-oxide buffer layers

Superconducting YBa2Cu3O7−δ (YBCO) thin films have been grown in situ on GaAs with conducting indium‐tin‐oxide (ITO) buffer layers. Superconducting onset is about 92 K with zero resistance at 60 K. ITO buffer layers usually form Schottky‐like barriers on GaAs. The YBCO film and ITO buffer layer, grown by ion beam sputter codeposition, are textured and polycrystalline with a combined room‐temperature resistivity of about 1 mΩ cm.

Orientation dependence of the phase modulation in a p‐n junction GaAs/AlxGa1−xAs waveguide

Phase modulation has been measured in a reverse‐biased P‐i‐N GaAs/AlxGa1−xAs double heterostructure with heavily doped (N, P∼1018 cm−3) AlxGa1−xAs layers. These measurements were performed at two wavelengths (1.09 and 1.15 μm), for both TE and TM modes and with light propagating in the [110] and [110] directions. We found that the effective quadratic electro‐optic coefficients R11 and R12 have approximately the same magnitude and sign. The values are −(40±10)×10−17 cm2/V2 and −(54±10)×10−17 cm2/V2 for λ=1.15 μm and λ=1.09 μm, respectively. The contribution of the plasma effect due to the free carrier is comparable and has the same sign.

Capacitance-voltage profiling of quantum well structures

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.

Phase modulation in GaAs/AlGaAs double heterostructures. II, Experiment

Phase modulation in GaAs/AlxGa1−xAs double heterostructures with different doping profiles is systematically investigated. Very good agreement between the experimental measurements and the theory developed in Part I of this paper is reported. By measuring the phase modulation for both transverse electric and transverse magnetic modes along the [110] and [110] crystallographic directions, we are able to deduce accurate values for the linear electro‐optic coefficient. Values of r41=−1.68×10−10 cm/V at λ=1.15 μm and r41=−1.72×10−10 at λ=1.09 μm are obtained with an estimated accuracy of ±5%. An accurate estimation of the carrier effect permits us to deduce the quadratic electro‐optic coefficient for GaAs. The values are R11=−2.0×10−16 cm2/V2, R12=−1.7×10−16 cm2/V2 at λ=1.15 μm, and R11=−2.9×10−16 cm2/V2, R12=−2.4×10−16 cm2/V2 at λ=1.09 μm with an estimated uncertainty of ±25% for all values. Contrast measurement of Fabry–Perot fringes enables us to evaluate the modulator losses. A value of 4.8 cm−1 is repor...

Comparison of the lateral carrier transport between a GaAs single quantum well and the AlGaAs barrier during cathodoluminescence excitation

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%...