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The growth of reduced dislocation density GaAs/Si is performed by a novel two-step technique where the first epitaxy step takes place at 75° C and the second is performed at 580° C. The initial deposition is single crystal, continuous, and planar such that there is no contribution to the dislocation density from Volmer-Weber island coalescence and no trapping of dislocations in pinholes. Using this new growth technique, a reduced dislocation density the order of 106/cm2 was obtained. The improved crystallinity is indicated by the more narrow x-ray full-width-at-half-maximum (FWHM) value of 110 arcseconds. GaAs p-i-n diodes were grown on the reduced dislocation density GaAs/Si and it was found that the resistivity of the intrinsic region for the heteroepitaxial diodes was similar to homoepitaxial ones for small mesa sizes.
In-plane anisotropic strain can be employed in the design of a new class of optoelectronic devices, such as high contrast, polarization sensitive spatial light modulators. One of the key issues involved in realizing these devices is obtaining a controllable and uniform in-plane strain. We have studied the uniformity of thermally induced in-plane strain in MOCVD grown GaAs lift-off thin films mounted on LiTaO3 or CaCO3 substrates. The experiment exploits the straininduced splitting of the excitonic interband transition at low temperature through absorption measurements using a Ti-Sapphire laser focused to a spot size less than 100 μm. The polarization vector of the incident light was oriented along an axis which enhances both features. From the energy positions of these transitions, the magnitude as well as the type of the in-plane strain was determined. Topographic scans performed over a 1.4mm X 1.4mm area for the sample bonded to CaCO3, and along a 2 mm line for that bonded to LiTaO3 revealed variations in strain of less than 5%.
We present a process for creating in-plane anisotropic strain in (100) GaAs and GaAs/AlGaAs multiple quantum well (MQW) thin films. The host substrates used for bonding include (100) GaAs, (100) silicon, and lithium tantalate (LiTaO3) with a special crystalline orientation. A mutilayer metallization consisting of Au-Sn (Au: 80 wt% , Sn: 20 wt%, 0.95μm), Ti (500Å) adhesion layer and Pt (500Å) barrier layer is deposited on the thin films and the host substrates. By choosing a proper annealing temperature (380°C) and thickness of eutectic layer, the thin films and the substrates are bonded together. Photoluminescence measurements do not reveal any thermally induced strain in the thin films bonded to GaAs; however, they show the existence of in-plane biaxial strain in the films bonded on Si. Linearly polarized reflectance measurements reveal an optical anisotropy in the MQW bonded to LiTaO3, which possesses an orientation-dependent thermal expansion. This indicates that the in-plane strain in the thin films is induced by the different thermal expansions between the thin films and the substrates. This process can be used to develop a new class of devices with an artificially induced in-plane strain.
Thin film of GaAs/AlGaAs multiple quantum well (MQW) structure have been bonded to the lithium tantalate (LiTaO3) or calcium carbonate (CaCO3) substrates cut such that one of the linear thermal expansion coefficients almost matches that of the MQW while its orthogonal counterpart does not. By choosing the proper bonding and operating temperatures, in-plane anisotropic strain up to 0.3% has been achieved. The transmission spectrum shows an anisotropy in excitonic absorption which results in a polarization rotation of a light beam at normal incidence to the structure. The theoretical calculation is in agreement with the experimental results. Using the polarization rotation, we have demonstrated a novel MQW light modulator with an exceedingly high contrast ratio of 330:1.
Observation of Arsenic antisites (AsGa) in GaAs layers grown by molecular beam epitaxy (MBE) at low substrate temperatures (∼ 200°C) is reported, using electron paramagnetic resonance (EPR), magnetic circular dichroism in absorption (MCDA), and MCDA tagged by optically detected magnetic resonance (MCDA-ODMR). This experiment confirms that there is a MCD absorption band directly associated with AsGa in the GaAs layers. The AsGa concentration in the GaAs layers is found to decrease by about one order of magnitude after annealing at 600°C for two minutes.
Using a magnetic field to confine the plasma closer to the cathode has been shown to be advantageous in dry etching technology since this yields a high degree of ionization at low pressures. We report here the results of a study of magnetron reactive ion etching of GaAs using a freon discharge. Various characterization techniques have been employed to understand the etching process and identify the extent of surface damage. The results show that magnetron etching is capable of yielding high etch rates with low damage.
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