Influence of hydrogen on the structural stability of annealed ultrathin Si/Ge amorphous layers

Semiconductor structures based on Si and Ge are generally submitted to hydrogenation because H passivates the dangling bonds of Si and Ge. By this way the devices prepared from those semiconductors, e.g., solar cells, have much better electrical properties. However, H stability is still a critical issue. In fact, there is wide evidence that H is very unstable against illumination as well as heat treatment. It has been seen that H out effuses from the samples under such treatments. As this causes unsaturation of the dangling bonds the electrical properties worsen significantly. In this work we will show that in the case of ultrathin Si/Ge amorphous layers the H thermal instability also affects the structural stability even up to the micrometric scale depending on the H content. Such type of structure can also be used to prepare SiGe alloys by mixing the layers with heat treatments. The samples were amorphous multilayers (MLs) of alternating ultrathin (3 nm) layers of Si and Ge deposited by sputtering on (100) oriented Si substrate. The total thickness of the MLs was 300 nm. The samples were hydrogenated by introducing H in the sputter chamber with flow rates varying from 0.8 to 6 ml/min. The MLs underwent different heat treatments, from the one at 350 ?C for 1 h up to the one at 250 ?C for 0.5 h + 450 ?C for 5 h. The samples were analysed by AFM, TEM, energy filtering TEM and Small-Angle X-Ray Diffraction (SAXRD). AFM showed that upon annealing the structure of the samples degrades with formation of surface bumps whose size increases by increasing the annealing temperature and/or time, for the same H content, or by increasing the H content for the same annealing conditions. For high H content and/or annealing conditions AFM showed that the bumps have blown up giving rise to craters. This suggests that H was released from its dangling bonds to Si and Ge and formed H bubbles in the MLs because of the energy supplied by the annealing. Additional energy for the break of the Si-H and Ge-H bonds could be the one supplied by the recombination of thermally generated carriers associated with the band gap fluctuations caused by the not uniform distribution of H in the MLs. The first sites of H accumulation are very likely nanocavities certainly present in the amorphous MLs. By TEM it has been seen that layer intermixing occurred which could be the first step of H bubbles formation. SAXRD measurements as well as TEM energy filtering maps for Si and Ge showed that Si and Ge interdiffusion took place in an asymmetric way as Si was seen to diffuse to the Ge layers whereas Ge did not diffuse to the Si layers. This might be due to the higher density of free dangling bonds in the Ge layers created by annealing because the binding energy of the Ge-H bond is smaller than the one of the Si-H bond

Structural Instability of Annealed a-Si/a-Ge Nanostructures

It is shown that heat treatments cause remarkable structural instability in nanostructures made of alternating 3 nm thick hydrogenated layers of a-Si and a-Ge deposited by sputtering. Upon annealing surface bumps form whose size and density increase with increasing H content. They are due to the presence of H bubbles inside the samples, which even blow up for the highest H content. The H bubbles form by accumulation of H2 molecules made possible by the break of the Si-H and Ge-H bonds driven by the energy supplied by the heat treatment and by the recombination of thermally generated carriers

Investigation of silicon oxynitride and amorphous silicon multilayers

Optical (refractive index) and structural properties of silicon oxynitride (SiON) and amorphous silicon (a-Si) multilayers grown by RF sputtering with thickness in the 10–30 nm range have been analysed by ellipsometry and TEM. Satisfactory agreement between the two techniques is obtained as regards the thickness determination of the SiON films. Disagreement with values obtained by the stylus method by extrapolation for the two types of layers is discussed. The interfaces of the SiON films are very good when n-type P doped Si is used as a target. They are wavy with average periodicity and amplitude on the order of 50 and 2 nm, respectively, when a semi-insulating Si target is used, despite the presence of a buffer layer. Hypothesis is made that P incorporation may improve the reconstruction of the SiON surface

Segmenting Workstation Screen Images

Introduction Recently, many organizations have been attempting to develop an inexpensive "Network Terminal." Such a device would have the same functionality as a PC without much of its hardware. In place of the RAM and hard drives that make current mulitmedia PCs so expensive, it would have a widebandwidth network connection and would use network servers to replicate functions previously implemented internally. One design scheme for the Network Terminal is to simply have it display downloaded screen images. This Generic Imaging Terminal (GIT) has many advantages: for one thing, it can be almost as simple as a television set. In addition, the interface is not operating system dependent, so the user could run MacOS, Windows, and UNIX in the same box. The GIT has one major disadvantage, however: uncompressed screen images can be as large as 8--10 MB. Sending these fast enough to provide full interactivity (10--20 fps) over today's networks is at best difficult and at worst imposs

AFM and TEM study of hydrogenated sputtered Si/Ge multilayers

Multilayers of hydrogenated ultrathin (3 nm) amorphous a- Si and a-Ge layers prepared by sputtering have been studied by atomic force microscopy (AFM) and transmission electron microscopy (TEM) to check the influence of annealing on their structural stability. The annealed multilayers exhibit surface and bulk degradation with formation of bumps and craters whose density and size increase with increasing hydrogen content and/or annealing temperature and time. Bumps are due to the formation of H2 bubbles in the multilayer. The craters are bumps blown up very likely because of too high a gas pressure inside. The release of H from its bonds to Si and Ge occurs within cavities very likely present in the samples. The necessary energy is supplied by the heat treatment and by the recombination of thermally generated carriers. Results by energy filtered TEM on the interdiffusion of Si and Ge upon annealing are also presented