Conductive atomic force microscopy study of InAs growth kinetics on vicinal GaAs (110)

Paloma Tejedor, Laura Díez-Merino, Igor Beinik, Christian Teichert
2009 Applied Physics Letters  
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
more » ... s 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. 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 kinetics. A new reaction pathway was proposed to account for the enhancement in 2D layer-by-layer growth observed by RHEED and the equalization of terrace sizes exhibited by nanostructured templates formed by H-assisted GaAs͑110͒ homoepitaxy, which involved the H-induced lowering of the Ehrlich-Schwoebel ͑E-S͒ kinetic barriers to atomic As incorporation 7 via dissociation of molecularly adsorbed AsH x ‫ء‬ precursor species, thus favoring the preferential attachment of adatoms to ascending step edges. 10 The presence of these AsH x intermediates on the H-terminated GaAs͑110͒ surface had been previously identified by photoemission yield spectroscopy and high resolution electron energy loss experiments. 11 Epitaxial growth of InAs on GaAs has been widely investigated for ͑001͒-oriented substrates, where coherent ͑i.e., dislocation-free͒ 3D islands, known as quantum dots, form by a modified Stranski-Krastanov growth mode. By contrast, growth of InAs on the GaAs ͑110͒ surface follows a layer-by-layer 2D mode irrespective of layer thickness and strain relaxation occurs only by the formation of misfit dislocations. 12,13 On GaAs ͑110͒ substrates misoriented toward ͑111͒A, growth proceeds by a particular type of step flow called step decoration, where nucleation occurs on the surface step edges, forming elongated strips along the main ͓110͔-step direction with effectively no nucleation centers on the terraces. 14 While this growth mode has been exploited to achieve ordered arrays of InAs nanowires selectively nucleated on ͓110͔-type bunched steps, 15 it has been the main obstacle to achieve self-organized InAs quantum dots on vicinal GaAs ͑110͒ substrates. In this work we investigate a different approach to modify the growth kinetics of InAs on GaAs ͑110͒ substrates misoriented toward ͑111͒A with the aim to induce the spontaneous formation of dotlike nanostructures for potential application to single electron devices. By using conductive AFM ͑C-AFM͒ we will show that the combination of the surfactant action of atomic hydrogen with the use of nanostructured buffer templates results in a kinetically driven enhancement in upward mass transport and a reduction in surface free energy, which alters the InAs a͒ Author to whom correspondence should be addressed. Electronic mail: ptejedor@icmm.csic.es. Tel.: ͑34͒ 91 3349088. FAX: ͑34͒ 91 3720623. APPLIED PHYSICS LETTERS 95, 123103 ͑2009͒
doi:10.1063/1.3232234 fatcat:e3weer274bdjnaupkgvcm44etm