InGaAs (110) semiconductor quantum dots (QDs) offer very promising prospects as a material base for a new generation of high-speed spintronic devices, such as single electron transistors for quantum computing. However, the spontaneous formation of InGaAs QDs is prevented by two-dimensional (2D) layer-by-layer growth on singular GaAs (110) substrates. In this work we have studied, by using atomic force microscopy and photoluminescence spectroscopy (PL), the growth of InGaAs/GaAs QDs on GaAs (110) stepped substrates by molecular beam epitaxy (MBE), and the modification of the adatom incorporation kinetics to surface steps in the presence of chemisorbed atomic hydrogen. The as-grown QDs exhibit lateral dimensions below 100 nm and emission peaks in the 1.35–1.37 eV range. It has been found that a step meandering instability derived from the preferential attachment of In adatoms to [11−0]-step edges relative to [11−n]-type steps plays a key role in the destabilization of 2D growth that leads to 3D mound formation on both conventional and H-terminated vicinal substrates. In the latter case, the driving force for 3D growth via step meandering is enhanced by H-induced upward mass transport in addition to the lower energy cost associated with island formation on H-terminated substrates, which results in a high density array of InGaAs/GaAs dots selectively nucleated on the terrace apices with reduced lateral dimensions and improved PL efficiency relative to those of conventional MBE-grown samples.
Conductive atomic force microscopy has been used to investigate the effect of atomic hydrogen and step orientation on the growth behavior of InAs on GaAs ͑110͒ misoriented substrates. Samples grown by conventional molecular beam epitaxy exhibit higher conductivity on ͓110͔-multiatomic step edges, where preferential nucleation of InAs nanowires takes place by step decoration. On H-terminated substrates with triangular terraces bounded by ͓115͔-type steps, three-dimensional InAs clusters grow selectively at the terrace apices as a result of a kinetically driven enhancement in upward mass transport via AsH x intermediate species and a reduction in the surface free energy. © 2009 American Institute of Physics. ͓doi:10.1063/1.3232234͔ Molecular beam epitaxial ͑MBE͒ growth of III-V compounds on ͑110͒-oriented substrates has recently acquired special relevance in the growing field of semiconductor spintronics following the experimental demonstration of a predicted enhancement in the electron spin-relaxation time by more than an order of magnitude in ͑110͒-InAs superlattices relative to the corresponding ͑001͒ structures.1-3 This improvement in spin dynamics together with the unique characteristics of InAs, which include narrow bandgap, high electron mobility, and strong spin-orbit interaction, make ͑110͒ InAs the prime candidate for nonmagnetic, low-power, highspeed spintronic devices, i.e., spin field effect transistors. For high-performance transistors to be exploited in quantum computing or quantum cryptography applications, the ability to grow ͑110͒ InAs low-dimensional structures, i.e., nanodots and nanowires, in predefined locations and alignment has to be demonstrated. 4,5 Self-organized growth on GaAs͑110͒ substrates misoriented toward ͑111͒A is an attractive method to create nanostructured templates 6,7 for subsequent nucleation of twodimensional ͑2D͒ and three-dimensional ͑3D͒ arrays of dots and wires having nanoscale dimensions. In previous studies we used a combination of atomic force microscopy ͑AFM͒ and in situ reflection high-energy electron diffraction ͑RHEED͒ measurements to investigate the variations in growth kinetics and morphology that take place when the ͑110͒ vicinal surface is exposed to a beam of atomic hydrogen prior to or during GaAs homoepitaxy from molecular beams of Ga and As 4 . 8 RHEED intensity oscillations, which indicate 2D layer-by-layer growth, were recorded in a much wider range of growth conditions in the presence of chemisorbed H compared to conventional MBE growth from As 4 or As 2 .9 The arsenic incorporation coefficient determined from As-induced oscillations during growth on the H-terminated surface was approximately twice that of As 4 on the nonexposed surface, i.e., 0.3 at 500°C, and its value exhibited a temperature independent behavior. In the absence of any molecular beam mass spectrometry data relative to the sticking coefficient of As 4 on the ͑110͒ surface, these measurements were semiquantitative, but they pointed to a H-mediated enhancement in the As incorporation k...
Atomic hydrogen modification of the surface energy of GaAs (110) epilayers, grown at high temperatures from molecular beams of Ga and As4, has been investigated by friction force microscopy (FFM). The reduction of the friction force observed with longer exposures to the H beam has been correlated with the lowering of the surface energy originated by the progressive de-relaxation of the GaAs (110) surface occurring upon H chemisorption. Our results indicate that the H-terminated GaAs (110) epilayers are more stable than the As-stabilized ones, with the minimum surface energy value of 31 meV∕Å(2) measured for the fully hydrogenated surface. A significant reduction of the Ga diffusion length on the H-terminated surface irrespective of H coverage has been calculated from the FFM data, consistent with the layer-by-layer growth mode and the greater As incorporation coefficient determined from real-time reflection high-energy electron diffraction studies. Arsenic incorporation through direct dissociative chemisorption of single As4 molecules mediated by H on the GaAs (110) surface has been proposed as the most likely explanation for the changes in surface kinetics observed.
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