The selective growth of nanometer scale GaAs wire and dot structures using metalorganic vapor phase epitaxy is demonstrated. Spectrally resolved cathodoluminescence images as well as spectra from single dots and wires are presented. A blue shifting of the GaAs peak is observed as the size scale of the wires and dots decreases.
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...
Formation of highly-uniform and densely-packed arrays of GaAs dots by selective epitaxy using diethylgallium-chloride and arsine is reported. The arrays of GaAs dots are imaged using atomic force microscopy (AFM). Accounting for the AFM tip radius of curvature, the smallest GaAs dots formed are 15-20 nm in base diameter and 8-10 nm in height with slow-growth crystal planes limiting individual dot growth. Completely selective GaAs growth within dielectric-mask openings at these small size-scales is also demonstrated. The uniformity of the dots within each array ranged from 6% for the larger dots to 16% for the smallest dots (normalized standard deviations of the areas of individual dots within each array).
There is currently great interest in fabrication of structures that are two and three dimensional analogs of the conventional quantum well. We review here the physics behind the use of arrays of such lower dimensional structures in semiconductor laser active layers.Methods which are currently under investigation for producing such structures will be discussed.Q uantum wires and quantum dots are two and three dimensional analogs of the conventional quantum well (see Figure 1). A quantum dot, for example, would confine an electron in three dimensions to a size comparable to the de Broglie wavelength of the electron in the crystal. To be useful in a device, arrays of quantum wires and quantum dots must be considered as illustrated in Fig. 1. In such arrays quantum dots and wires composed of a low bandgap material would be imbedded in a higher bandgap host (e.g., GaAs dots in an AlGaAs host). A potentially important application of these structures is to laser diodes.' In such devices, the conventional bulk active layer is replaced with an array of quantum dots or quantum wires. As discussed below, these devices would have many advantages over conventional devices.The most important property of quantum wires and quantum dots as concerns their application to laser diode active layers is their density of states functions. Idealized versions of these functions are presented in Fig. 2 for bulk, conventional quantum well, quantum wire, and quantum dot active layers. In reducing dimension it is apparent that these functions take on more singular behavior. For the quantum dot, the behavior is analogous to the density of states of an atomic system. This narrowing of the density of states function carries over directly into the optical gain spectrum, which, for k-conserving transitions, is proportional to the effective density of states of the electronic system and the Fermi t
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