Properties and applications of the ‘‘epitaxial shadow mask molecular beam epitaxy technique’’

Abstract

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.