Kinetics of Si and Ge nanowires growth through electron beam evaporation

Si and Ge have the same crystalline structure, and although Si-Au and Ge-Au binary alloys are thermodynamically similar (same phase diagram, with the eutectic temperature of about 360°C), in this study, it is proved that Si and Ge nanowires (NWs) growth by electron beam evaporation occurs in very different temperature ranges and fluence regimes. In particular, it is demonstrated that Ge growth occurs just above the eutectic temperature, while Si NWs growth occurs at temperature higher than the eutectic temperature, at about 450°C. Moreover, Si NWs growth requires a higher evaporated fluence before the NWs become to be visible. These differences arise in the different kinetics behaviors of these systems. The authors investigate the microscopic growth mechanisms elucidating the contribution of the adatoms diffusion as a function of the evaporated atoms direct impingement, demonstrating that adatoms play a key role in physical vapor deposition (PVD) NWs growth. The concept of incubation fluence, which is necessary for an interpretation of NWs growth in PVD growth conditions, is highlighted

Heteroepitaxial Growth of Ge Nanowires on Si Substrates

Electron beam evaporation has been used to prepare Ge nanowires (NWs) on top of (111) Si substrates. Despite the non-UHV growth conditions, scanning and transmission electron microscopies demonstrate that NWs are single crystal with specific crystallographic growth directions ([111], [110], and [112]). NWs are faceted, exhibiting the lower energy plans on the surface. The faceting depends on the growth direction. Moreover, the detrimental effects for Ge NWs growth of O atoms contamination are discussed. Finally, we describe how a proper preparation of the Au catalyst is able to increase the Ge NW density by a factor of 4, while heteroepitaxy and faceting features are maintained

Kinetics of Si and Ge nanowires growth through electron beam evaporation

<p>Abstract</p> <p>Si and Ge have the same crystalline structure, and although Si-Au and Ge-Au binary alloys are thermodynamically similar (same phase diagram, with the eutectic temperature of about 360&#176;C), in this study, it is proved that Si and Ge nanowires (NWs) growth by electron beam evaporation occurs in very different temperature ranges and fluence regimes. In particular, it is demonstrated that Ge growth occurs just above the eutectic temperature, while Si NWs growth occurs at temperature higher than the eutectic temperature, at about 450&#176;C. Moreover, Si NWs growth requires a higher evaporated fluence before the NWs become to be visible. These differences arise in the different kinetics behaviors of these systems. The authors investigate the microscopic growth mechanisms elucidating the contribution of the adatoms diffusion as a function of the evaporated atoms direct impingement, demonstrating that adatoms play a key role in physical vapor deposition (PVD) NWs growth. The concept of incubation fluence, which is necessary for an interpretation of NWs growth in PVD growth conditions, is highlighted.</p

Generation of second harmonic radiation from sub-stoichiometric silicon nitride thin films

Enhancing second-order optical processes in Si-compatible materials is important for the demonstration of innovative functionalities and nonlinear optical devices integrated on a chip. Here, we demonstrate significantly enhanced Second-Harmonic Generation (SHG) by silicon-rich silicon nitride materials over a broad spectral range, and show a maximum conversion efficiency of 4.5 × 10-6 for sub-stoichiometric samples with 46 at. silicon. The SHG process in silicon nitride thin films is systematically investigated over a range of material stoichiometry and thermal annealing conditions. These findings can enable the engineering of innovative Si-based devices for nonlinear signal processing and sensing applications on a Si platform. © 2013 AIP Publishing LLC

Polarization Properties of Deep-Ultraviolet Optical Gain in Al-Rich AlGaN Structures

The polarization properties of the amplified spontaneous emission (ASE) in the deep-UV wavelengths from AlGaN/AlN structures with 70% Al content have been investigated. Samples are prepared through a novel liquid phase epitaxy mode leading to strong compositional fluctuations. Large net modal gain is quantified to 230 nm after femtosecond UV optical pumping by the variable-stripe length technique. A strong transverse electric (TE)-polarization mode of the amplified emission has been demonstrated, together with an unpolarized emission giving rise to absorption only. These measurements provide an important observation in order to understand the origin of the optical gain in AlGaN alloys with high Al content. # 2012 The Japan Society of Applied Physics I n recent years, the demand for compact deep-UV sources has increased due to potential applications in free-space communications, biochemical agent detection, disinfection, and medical diagnostics. Group-III nitrides, particularly AlGaN materials, are especially suited to cover the UV spectral range by varying the Al content. 1) Sub-300 nm AlGaN-based light emitting devices have been demonstrated with an internal quantum efficiency of 50% at 250 nm 2) and a power efficiency of 40% at 240 nm. 3) Currently, an intense investigation is targeting the deep-UV lasing from these materials. Recent theoretical and experimental works on AlGaN multi-quantum wells (MQW) suggest the possibility of achieving lasing at wavelengths as short as 220 nm. 9) One of the issues still under debate in the literature is the origin of the gain and its polarization properties. The transition between the conduction band and the top valence band is mainly transverse electric (TE) polarized for GaN, and transverse magnetic (TM) polarized for the c-planeoriented AlN heterostructure. 10) A turnover point is expected in the AlGaN material, depending on the Al content, material strain, and thickness of the well. Zhang et al. 11) calculated the band structure and material gain of 3 nm AlGaN QWs. Above 60% Al, they expect a large TM material gain, while, the TE spontaneous emission rate becomes negligible with increasing Al content. Yamaguchi found an abrupt polarization switch at an Al composition of 76% if the thickness of the well was 1.5 nm. 12) Moreover, considering a (0001)-oriented c-plane AlGaN/AlN QW structure (well thickness is 2.5 nm), Park 13) found a turnover point occurring above 80% Al content. Some experimental evidences confirm that for a wavelength as long as 267 nm, the laser emission at low temperature is still TE polarized. 8) Note that all the calculations consider the well as the active region. Compositional fluctuation introduces a different order, and the role of the fluctuations in the optical emission has received much less attention. 9) In this letter, we report on the polarization properties of the amplified spontaneous emission (ASE) of the AlGaN/ AlN MQW structure with strong compositional fluctuations under femtosecond optical pumping. We demonstrate that ASE is TE polarized, while the TM emission gives rise to absorption in the material. The investigated samples consist of a 0.5-m-thick AlN cladding layer, followed by 10 periods of Al 0:7 Ga 0:3 N (1.5 nm)/AlN (40 nm) MQWs and a 100 nm AlN cladding layer. A schematic picture of the sample is shown i

Indirect Diffusion Mechanism of Boron Atoms in Crystalline and Amorphous Silicon

The diffusion of B atoms in crystalline and amorphous Si has been experimentally investigated and modeled, evidencing the indirect mechanism of these mass transport phenomena. The migration of B occurs after interaction with self-interstitials in crystalline Si (c-Si) or with dangling bonds in amorphous Si (a-Si). In the first case, an accurate experimental design and a proper modeling allowed to determine the microscopic diffusion parameters as the B-defect interaction rate, the reaction paths leading to the diffusing species and its migration length. Moreover, by changing the Fermi level position, B atoms are shown to interact preferentially with neutral or doubly positively charged self-interstitials. As far as the amorphous case is concerned, B diffusion is revealed to have a marked transient character and to depend on the B concentration itself. In particular, boron atoms can move after the interaction with dangling bonds whose density is transiently increased after ion implantation or permanently enhanced by the presence of boron atoms themselves. Unexpectedly, B diffusivity in a-Si is seen to be orders of magnitude above than in c-Si and to depend on the thermal history, i.e. the relaxation status of the amorphous phase. These data are presented and their implications discussed

Enhanced second harmonic generation from InAs nano-wing structures on silicon

We demonstrate morphology-dependent second-harmonic generation (SHG) from InAs V-shaped nanomembranes. We show SHG correlation with the nano-wing shape and size, experimentally quantify the SHG efficiency, and demonstrate a maximum SHG enhancement of about 500 compared to the bulk. Experimental data are supported by rigorous calculations of local electromagnetic field spectra