We study magnetic circular dichroism of absorption (MCDA) of LT AlxGa1−xAs as a function of aluminum content. The MCDA spectrum of LT AlxGa1−xAs is distinctly different from the MCDA spectrum of LT GaAs, which has one paramagnetic and one diamagnetic peak at 0.95 eV and 1.18 eV, respectively. As the aluminum content increases, the spectrum of LT AlxGa1−xAs is dominated by a diamagnetic peak similar to the 1.18eV peak of the EL20-like defects in LT GaAs. However, the peak shifts to higher energies as x increases. The photoquenching and temperature dependence of this peak indicates an association with the EL2 defect. The paramagnetic peak observed in LT GaAs also shifts to higher energies but faster and eventually merges with the diamagnetic peak as the Al content increases. The study of the MCDA spectrum of LT AlxGa1−xAs as a function of aluminum content allows a better understanding of the MCDA phenomena of LT AlxGa1−xAs and LT GaAs, as well as the EL2-related transitions in bulk semi-insulating GaAs.

We report on the properties and applications of the epitaxial shadow mask (ESM) molecular beam epitaxy (MBE) technique. A summary of the results for various devices grown with this method is given as well as an outline for future applications in waveguide opto-electronic integrated circuits (OEICs). The ESM-MBE technique was first introduced for the in situ structuring of the lateral doping profile of n-i-p-i structures to provide selective ohmic contacts. The epitaxial mask, composed of a 9 μm AlGaAs layer with a 3 μm GaAs layer on top was grown on a semi-insulating GaAs substrate. Windows were etched into the mask by employing standard photolithographical techniques and wet chemical etching. For the regrowth the fact is exploited that, at a fixed position of the substrate, the molecular beams from the effusion cells are incident with different angles relative to the mask window. Therefore, the lateral doping profile can be structured in situ during the growth creating exclusive n- and p-doped regions which provide excellent selective and ohmic contacts to n-i-p-i structures. A continuous rotation of the substrate can be retained during the growth of undoped layers while the substrate has to be aligned for delta-doping the layers. With a simultaneous control of the shutter movement relative to the substrate rotation homogeneous doping layers can be achieved by the flash doping technique which means that the respective doping shutter is opened only for a short time when the substrate is in the optimum position. Numerous samples have been investigated and we conclude that employing this growth technique, a similar yield and crystal quality is achieved compared with standard growth techniques. In detail, this has been demonstrated by the growth of Bragg mirrors, which exhibit exactly the expected reflectance except for very narrow shadow mask windows (<20 μm) for which a 1% decrease in the growth rate was observed. Photoluminescence and absorption spectra from quantum well structures have confirmed that heterostructures have the same properties as samples grown on nonstructured substrates. The current–voltage characteristics of n-i-p-i modulator structures and light emitting devices exhibit an excellent p-n junction behavior. In a highly doped electro-optic n-i-p-i modulator device, for example, dark currents lower than 10 nA at Upn=−5 V have been achieved. Future applications for OEICs, like the growth of buried waveguide structures are in progress. Hereby, the ESM-MBE is employed to provide a lateral optical confinement by the in situ structuring of the lateral composition of the sample which results in regions with different refraction index.

We report on the ac switching behavior of surface-normal electro-optical modulators based on hetero-n-i-p-i structures. We have investigated systematically the way the transition times scale with device dimensions and present a straightforward theoretical model including sheet resistivities of n and p layers as well as contact resistances. The model is in excellent agreement with experimental data. With a voltage swing as low as 3.7 V, we achieved a single path contrast ratio in excess of 2:1 and maximum 3 dB frequencies larger than 250 MHz.

Through temperature-dependent conductivity measurements, we show evidence of a deep trap level in low-temperature (LT) Al0.3Ga0.7As layers with an activation energy of similar to 0.96 eV. This energy is near that of EL2-like defects found previously in ''normal'' epitaxial Al0.3Ga0.7As. It is also considerably larger than the 0.70 eV value typically associated with defects in LT GaAs, which may explain the observed large resistivity (> 10(11) Ohm cm) in LT Al0.3Ga0.7As. Current transient spectroscopy (CTS) of these samples yields a deep level activation energy of 1.01 eV, in close agreement with the value obtained from conductivity measurements. (C) 1996 American Institute of Physics.

We report on a comparative study of the electro-optical properties of a tunable two-dimensional electron plasma by transmission, photocurrent (PC), and photoluminescence (PL) measurements. The sheet electron density n(2) in the pseudomorphic strained InGaAs quantum wells of the investigated type-I hetero n-i-p-i structure can be tuned between zero and more than 5·1012 cm−2 by applying a voltage Upn between the selective and ohmic n- and p-contacts. A direct relation between n(2) and Upn can be determined by capacitance measurements. Many particle effects (bleaching of the exciton, shift of the absorption edge due to bandfilling, renormalization of the bandgap) have been studied as a function of the sheet electron density n(2) in the quantum wells. The carrier dynamics in the type-I hetero n-i-p-i structure was investigated by a detailed comparison of absorption, PL and PC measurements as a function of the applied voltage Upn. A photogenerated hole in the quantum well can either recombine in the quantum well or escape into the p-layers by tunnelling or thermally assisted hopping. The first process is monitored by PL, while the escape process, which varies strongly with the Upn bias dependent potential barrier height and width, contributes to the PC. Both recombination paths are compared with the e-h generation rate determined by the absorption coefficient, which is extracted from transmission experiments.

In n-i-p-i structures with selective ohmic contacts to the n- and the p-layers both, carrier densities and electric fields, can be tuned over a wide range by applying moderate external voltages. The resulting absorption changes due to phase space filling and Franz-Keldysh effect, respectively, can be superimposed constructively by a suitable sample design. In contrast to optically excited n-i-p-i structures whose dynamic behavior is governed by the internal electron-hole recombination lifetimes (well up to milliseconds), the time constants for structures provided with selective n- and p-contacts are RC times given by the resistance of the doped layers including contact resistances, and by the capacitance of the interdigitated n- and p-layers. Although the areal capacitance of such n-i-p-i structures is relatively large compared to p-i-n structures, very short RC times can be achieved for sufficiently small devices as RC time constants scale basically quadratically with the device width. We have investigated the dynamical response on a series of n-i-p-i modulators grown by epitaxial shadow mask MBE with a width of the n-i-p-i region ranging from 100 micrometer down to 5 micrometer. For the smallest devices time constants as low as 1.5 ns have been measured. Modulation at up to 250 MHz has been demonstrated with a decrease in switching contrast from 2.2 at dc operation to 1.75 at 250 MHz. The voltage swing used in these experiments as only 3.7 V. We stress that these devices were not optimized. The switching time for the 5 micrometer sample was increased by about a factor 10 due to high contact resistances. The switching contrast can be enhanced easily and with no penalty on the high frequency response by using n-i-p-i structures with a larger number of periods, as the RC time constant is independent of the number of periods. Thus, with a suitable design operation in the multi-GHz range as well as an improved switching contrast should be possible.

Vertical cavity surface emitting lasers (VCSELs) have been successfully integrated by fluidic self-assembly (FSA) into 10 mu m deep holes etched into an Si substrate. These 40 x 40 x 10 mu m(3) devices were fully planarised and metallised after integration. The VCSELs operated pulsed at 0.98 mu m with a threshold current as low as 75mA and a turn-on voltage of 2.9V.

Fluidic self-assembly is a new technique which makes possible the integration of devices fabricated using dissimilar materials and processes. The integration is accomplished by fluidically transporting trapezoidally shaped blocks made of one material into similarly shaped holes in a receptor substrate. In this paper, a systematic study of the FSA integration efficiency is presented. Blocks and holes were formed from silicon using anisotropic etching. Two different sizes were considered: large blocks of dimension 1.0 mm×1.2 mm, and small blocks of dimension 150 μm×150 μm. FSA was performed in either water or methanol using a bubble pump apparatus to recirculate blocks. FSA of large blocks resulted in 100% filling of a substrate containing 191 holes within 2.5 minutes. Similar experiments with small blocks and a substrate with a 64×64 array of holes yielded a fill ratio of 70%, due to undesirable adhesion of blocks to the substrate surface. Roughening the substrate resulted in a fill ratio of 90%. Also presented is a simple rate equation model of the FSA process, along with a discussion of which process parameters are important and how they can be optimized

This work investigates the temperature dependence of GaAs/AlAs thin-film structures. Based on an ac calorimetric method, the thermal diffusivity of a 700 Å/700 Å GaAs/AlAs periodic structure is measured from 190 to 450 K. Thermal conductivity of the structure is derived from the experiment. The results demonstrate that the thermal conductivity/diffusivity of the structure are lower than its corresponding bulk values. The temperature dependence of its thermophysical properties is weaker than that of typical bulk III–V materials. Interface scattering is believed as the major cause of the observed reduction in thermal conductivity.

In this paper three different versions of n-i-p-i based spatial light modulators will be discussed: 1) optically addressed modulators, based on the large non-linear absorption changes, 2) electrically addressed modulators with largely improved performance characteristics, and 3) optically addressed "smart modulators", consisting of a combination of fast detectors with gain with electro-optical modulators.

A molecular beam epitaxy growth monitoring method is developed for distributed Bragg reflectors and vertical-cavity surface-emitting laser (VCSEL) resonators. The wavelength of the substrate thermal emission that corresponds to the optical cavity resonant wavelength is selected by a monochromator and monitored during growth. This method allows VCSEL cavities of arbitrary design wavelength to be grown with a single control program. This letter also presents a theoretical model for the technique which is based on transmission matrices and simple thermal emission properties. Demonstrated reproducibility of the method is well within 0.1%.

Using high resolution x-ray diffraction techniques, we have studied the lattice parameter behavior of low-temperature (LT) AlxGa1−xAs as a function of annealing temperature and aluminum content. Similar to LT GaAs, the as-grown LT AlxGa1−xAs layers exhibit a dilated lattice constant which, upon annealing, contracts to that of ‘‘normal’’ material. The onset of this contraction in LT Al0.3Ga0.7As, however, is found to occur at an annealing temperature nearly 100 °C higher than that required for LT GaAs. In addition, the relative lattice expansion in the as-grown LT layer is found to be a decreasing function of Al content, ranging from 0.099% for LT GaAs to 0.059% for LT Al0.3Ga0.7As. This is attributed to lower than expected As incorporation in the LT AlxGa1−xAs during growth.

We report on photoluminescence and absorption measurements in type-I hetero n-i-p-i structures. The electron density in the pseudomorphic InGaAs/GaAs quantum wells is tunable between zero and more then 5 · 1012 cm-2. This electrical tuning of the subband filling is achieved by a variable voltage applied between selective n-and p-contacts fabricated by epitaxial shadow mask MBE. One of the advantages of having selective contacts to the n- and p-layers is to get reliable information about the electron density, independent of measured absorption and luminescence spectra. This allows a more rigorous analysis of the data on bandgap renormalization, bandfilling and k∥-conservation. Moreover, the experiments can be performed at low optical power and low carrier temperatures. In the present investigation a bandgap renormalization of -20 meV and a bandfilling induced shift of the absorption edge as large as +50 meV was observed for a sheet electron density of 5 · 1012 cm-2.

The cavity design of a surface-emitting laser (SEL) is crucial to device performance. In addition several properties of the laser and its constituent materials can be determined by probing the passive cavity itself. Specifically, the longitudinal mode behavior of passive SEL cavities can be used to measure the thermal dependence of the refractive index and the free-carrier absorption of the cavity materials.

We report on experimental results on the dynamical behaviour of n-i-p-i based smart pixels, composed of photoconductive switches and electroabsorptive n-i-p-i modulators. For the photoconductive switch we present switching times of 1.9 ns at an optical power of 880 mu W, corresponding to a switching energy of 1.7 pJ. The contrast of the electronic output signal is larger than 10(7) and a maximum dc gain exceeding 10(6) is achieved. For the opto-optical switching contrast ratios of 4:1 at 1.6 mW output power are shown with switching energies of 2.4 fJ/mu m(2) (1.7 pJ). The opto-optical gain is tunable from 10-10(6).