The technique of facet modulation selective epitaxy and its application to quantum-well wire doublet fabrication are described. Successful fabrication of wire doublets in the AIXGal+As material system is achieved. The smallest wire fabricated has a crescent cross section less than 140 A thick and less than 1400 A wide. Backscattered electron images, transmission electron micrographs, cathodoluminescence spectra, and spectrally resolved cathodoluminescence images of the wire doublets are presented.Semiconductor structures exhibiting quantum contlnement in two or three dimensions have attracted considerable attention for their potential in improving optoelectronic devices'*2 and in revealing new phenomena in solidstate physics, such as polarization anisotropy in quantum wires.3 Many approaches to fabricate these quantum-confined structures have been studied. Already grown quantum-well material has been physically patterned by a combination of lithography and etchingk6 or has been selectively disordered by ion implantation7 to achieve lateral confinement. In situ formation of nanostructures during epitaxial growths has also been studied. Migration-enhanced epitaxy on tilted substrates has exhibited quantumconfinement effects,8 and stimulated emission from quantum wires9 has been demonstrated by performing metalorganic vapor-phase epitaxy (MOVPE) growths on etched substrates. Recently, wire and dot structures have been selectively grown on substrates covered with patterned dielectric masks.10-'2 The formation of crystal facets in single precursor chemistry (trimethyl) selective growth has also been studied as a potential method for quantumwell wire fabrication.13In this letter, we describe a new technique, facet modulation selective epitaxy, and present its application to quantum-well wire doublet fabrication in the Al,Ga, _ As material system. Facet modulation selective epitaxy, as it is defined here, is the application of different precursor chemistries to layers within a single growth to alter sequentially the appearance of facets on a growing structure and to thereby form heterostructures of novel geometry. Two precursor chemistries are used here: a combination of diethylgallium chloride (DEGaCl) and arsine (ASH,) for GaAs growth and a combination of trimethylaluminum (TMAl), trimethylgallium (TMGa), and arsine (ASH,) for Al,Ga, _ As growth. The morphology of GaAs selective growth using DEGaCl and AsH3 is dominated by the appearanceofthe [ill] and ( 110 1 dent on the growth temperature and the mask opening orientation.'07"7'4 he T morphology of A1,Gal _ As selective growth using TMAl, TMGa, and ASH, is similar, but the bounding planes include higher-index-number planes (one or more indices greater than one) in addition to the { 111) and [ 110) families of slow growth planes.One application of facet modulation selective epitaxy is the fabrication of quantum-well wire doublets in one single growth as illustrated in Fig. 1. Using DEGaCl and ASH,, a GaAs buffer bounded by the low-index-number facets is grown on...
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