T. Stoica

Mechanism of molecular beam epitaxy growth of GaN nanowires on Si(111)

GaN nanowires have been grown without external catalyst on Si(111) substrates by plasma-assisted molecular beam epitaxy. Nanowire aspect ratios (length/diameter) of about 250 have been achieved. During the initial stage of the growth, there is a nucleation process in which the number of wires increases and the most probable nucleation diameter of about 10nm has been observed, which slowly increases with deposition time. For deposition time longer than the nucleation stage, the nanowire length as a function of diameter monotonically decreases. This phenomenon can be explained by adatom diffusion on the nanowire lateral surface towards the tip.

Band engineering and growth of tensile strained Ge/(Si)GeSn heterostructures for tunnel field effect transistors

In this letter, we propose a heterostructure design for tunnel field effect transistors with two low direct bandgap group IV compounds, GeSn and highly tensely strained Ge in combination with ternary SiGeSn alloy. Electronic band calculations show that strained Ge, used as channel, grown on Ge 1−xSnx (x > 9%) buffer, as source, becomes a direct bandgap which significantly increases the tunneling probability. The SiGeSn ternaries are well suitable as drain since they offer a large indirect bandgap. The growth of such heterostructures with the desired band alignment is presented. The crystalline quality of the (Si)Ge(Sn) layers is similar to state-of-the-art SiGe layers.

Interface and Wetting Layer Effect on the Catalyst-Free Nucleation and Growth of GaN Nanowires†

To avoid catalyst-induced contaminations that might alter the electronic properties of the material, catalyst-free growth is preferable. However, the nucleation and growth mechanisms of GaN wires in the catalyst-free procedure are still under debate. Two mechanisms are usually invoked for the nucleation and the growth of NWs. One is based on the Ga-droplet formation followed by the well-established vapor–liquid–solid mechanism, [13] and the other is based on small GaN clusters as nucleation seeds and a vapor–solid growth process. [12] In the present work the formation of crystalline GaN nanoclusters as possible NW precursors in catalyst-free plasma-assisted MBE (PAMBE) growth is studied by high-resolution transmission electron microscopy (HRTEM) imaging. Details of the interface between the GaN layer and the substrate are investigated and discussed in connection with the mechanism of catalyst-free NW growth. GaN NWs were grown at 7808C by PAMBE without the use of catalysts, on clean Si(111) and oxidized Si(100) substrates, according to the procedure described elsewhere. [6] The GaN NWs were investigated by HRTEM using a JEOL 3000F

Selective-area catalyst-free MBE growth of GaN nanowires using a patterned oxide layer

GaN nanowires (NWs) were grown selectively in holes of a patterned silicon oxide mask, by rf-plasma-assisted molecular beam epitaxy (PAMBE), without any metal catalyst. The oxide was deposited on a thin AlN buffer layer previously grown on a Si(111) substrate. Regular arrays of holes in the oxide layer were obtained using standard e-beam lithography. The selectivity of growth has been studied varying the substrate temperature, gallium beam equivalent pressure and patterning layout. Adjusting the growth parameters, GaN NWs can be selectively grown in the holes of the patterned oxide with complete suppression of the parasitic growth in between the holes. The occupation probability of a hole with a single or multiple NWs depends strongly on its diameter. The selectively grown GaN NWs have one common crystallographic orientation with respect to the Si(111) substrate via the AlN buffer layer, as proven by x-ray diffraction (XRD) measurements. Based on the experimental data, we present a schematic model of the GaN NW formation in which a GaN pedestal is initially grown in the hole.

Tensely strained GeSn alloys as optical gain media

This letter presents the epitaxial growth and characterization of a heterostructure for an electrically injected laser, based on a strained GeSn active well. The elastic strain within the GeSn well can be tuned from compressive to tensile by high quality large Sn content (Si)GeSn buffers. The optimum combination of tensile strain and Sn alloying softens the requirements upon indirect to direct bandgap transition. We theoretically discuss the strain-doping relation for maximum net gain in the GeSn active layer. Employing tensile strain of 0.5% enables reasonable high optical gain values for Ge0.94Sn0.06 and even without any n-type doping for Ge0.92Sn0.08.

Misfit dislocations in finite lateral size Si1-xGex films grown by selective epitaxy

Abstract In this paper, the reduction of misfit dislocation density on small pads of selectively grown Si 1- x Ge x films is studied experimentally and theoretically. The experiments were performed mainly on Si 1- x Ge x layers with x =0.12 grown on patterned substrates with pad sizes of 10×10 to 10 4 ×10 4 μm 2 . The misfit dislocations were detected by optical and transmission electron microscopy. It was observed that on small pads, misfit dislocations are generated at a significantly higher critical thickness than on extended areas, while pads of size 10×10 μm 2 or smaller showed no evidence of misfit dislocations at all. The theoretical analysis was performed in two steps. First, an elastic strain model was used to calculate the pad size dependence of the critical thickness. The main hypothesis of the model is that the density of misfit dislocations is solely affected by the elastic relaxation at the edges of small epitaxial areas. This equilibrium model can explain only the experimentally observed absence of misfit dislocations on small pads; however, it predicts a critical thickness for finite sizes much lower than the observed one. Second, a kinetic approach was further performed, in which the relaxation is supposed to be due to nucleation of misfit dislocations at defects and self-multiplication. The best fit with the experimental results was obtained for time constants for generation from defects of 600 min and for self-multiplication of 7 min and a gliding velocity of 12 μm/min. While the onset of relaxation seems to be due generation at defects, the self-multiplication process determines to a great extend the density of misfit dislocations.

Investigation on Localized States in GaN Nanowires

GaN nanowires with diameters ranging between 50 and 500 nm were investigated by electrical and photoinduced current techniques to determine the influence of their size on the opto-electronic behavior of nanodevices. The conductivity, photoconductivity, and persistent photoconductivity behavior of GaN nanowires are observed to strongly depend on the wire diameter. In particular, by spectral photoconductivity measurements, three main sub-band-gap optoelectronic transitions were detected, ascribed to the localized states giving rise to the characteristic blue, green, and yellow bands of GaN. Photoconductivity with below-band-gap excitation varies orders of magnitude with the wire diameter, similarly to that observed for near-band-edge excitation. Moreover, yellow-band-related signal shows a superlinear behavior with respect to the band-edge signal, offering new information for the modeling of the carrier recombination mechanism along the nanowires. The photoconductivity results agree well with a model which takes into account a uniform distribution of the localized states inside the wire and their direct recombination with the electrons in the conduction band.

Lateral confinement by low pressure chemical vapor deposition-based selective epitaxial growth of Si1−xGex/Si nanostructures

Among the growth approaches being considered currently to realize quantum dots and quantum wires is the selective epitaxial growth on patterned substrates. With this technique the feature size and geometry are mainly limited by the lithographic process. With optical lithography we achieved a lateral dimension of ⩾0.4 μm. Therefore, to further reduce the lateral dimension, but still using optical lithography, the tendency toward facet formation during selective epitaxial growth was investigated. Si0.70Ge0.30 multiple quantum well structures with Si0.935Ge0.065 spacers and buffers were deposited on (001) Si. The buffer thickness was varied so as to achieve facet junction. While on large areas the Si0.935Ge0.065 buffer was relaxed, for dots ⩽300 μm or narrower the structures remained strained even for buffer thicknesses exceeding by a factor of two–three the critical thickness of large area. In dots and wires where facet junctioning has taken place a rounded region between facets (approximately 50 nm broad) ...

Size distribution and electroluminescence of self-assembled Ge dots

In this article we study the electroluminescence of p-i-n diode structures with Ge dots consisting of coherent three-dimensional small (pyramids) and larger (dome) islands. The Ge dots are formed through strain-induced islanding. The diode structures, including one layer with Ge dots, were deposited on Si mesas with variable areas in order to study the influence of limited area deposition on self-assembling. It was observed that the reduction of deposited area improves island uniformity. The combined analysis of island distribution and electroluminescence spectra has lead to the conclusion that domes in small diodes have a smaller Si content or are less relaxed than domes in larger diodes. The diodes are found to emit up to room temperature near the optical communication wavelength of 1.3 microns.

Influence of the adatom diffusion on selective growth of GaN nanowire regular arrays

Molecular beam epitaxy (MBE) on patterned Si/AlN/Si(111) substrates was used to obtain regular arrays of uniform-size GaN nanowires (NWs). The silicon top layer has been patterned with e-beam lithography, resulting in uniform arrays of holes with different diameters (dh) and periods (P). While the NW length is almost insensitive to the array parameters, the diameter increases significantly with dh and P till it saturates at P values higher than 800 nm. A diffusion induced model was used to explain the experimental results with an effective diffusion length of the adatoms on the Si, estimated to be about 400 nm.