Miklós Serényi

Modification of Al/Si interface and Schottky barrier height with chemical treatment

The influence of different chemical treatments on the electrical behaviour of n- and p-type Al/Si Schottky junctions was studied. A Schottky barrier height of 0.91 eV was achieved on p-type Si probably due to the unpinning of the Fermi-level at the Al/Si interface. This is one of the highest barrier height values reported so far for a solid-state Schottky junction prepared to p-Si. A doping level reduction was observed in the vicinity of the Si surface for wafers with native oxide and for those boiled in acetone or annealed in forming gas. It was observed unexpectedly that the reactive plasma etch used for the formation of mesa structures decreases the apparent Schottky barrier height. The relation between the sum of n- and p-type Schottky barrier heights and forbidden gap is discussed.

Refractive index of sputtered silicon oxynitride layers for antireflection coating

Silicon nitride and silicon oxynitride dielectric layers were fabricated by reactive RF sputtering from an Si target in conventional equipment. Sputtering was done using a gas mixture of high-purity nitrogen and oxygen at a total pressure of 2}3 Pa. To investigate the sputtering process parameters silicon nitride was deposited on polished slices of Si. Multiple angle incidence ellipsometry was applied for the determination of the refractive index and thickness of the deposited layers. We found that the sputtered silicon oxynitride layers have excellent transparency in visible and near-infrared ranges and a wide refractive index range varying from 2.05 to 1.45. The refractive index of the layers decreases continuously with the increasing O 2 partial pressure that did not exceed the value of 0.01 Pa. Being a low-temperature process, reactive sputtering is well appropriate for the compound semiconductor devices. ( 2001 Elsevier Science Ltd. All rights reserved.

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.

Relationship between structural changes, hydrogen content and annealing in stacks of ultrathin Si/Ge amorphous layers

Hydrogenated multilayers (MLs) of a-Si/a-Ge have been analysed to establish the reasons of H release during annealing that has been seen to bring about structural modifications even up to well-detectable surface degradation. Analyses carried out on single layers of a-Si and a-Ge show that H is released from its bond to the host lattice atom and that it escapes from the layer much more efficiently in a-Ge than in a-Si because of the smaller binding energy of the H-Ge bond and probably of a greater weakness of the Ge lattice. This should support the previous hypothesis that the structural degradation of a-Si/a-Ge MLs primary starts with the formation of H bubbles in the Ge layers.

On the formation of blisters in annealed hydrogenated a-Si layers

Differently hydrogenated radio frequency-sputtered a-Si layers have been studied by infrared (IR) spectroscopy as a function of the annealing time at 350°C with the aim to get a deeper understanding of the origin of blisters previously observed by us in a-Si/a-Ge multilayers prepared under the same conditions as the ones applied to the present a-Si layers. The H content varied between 10.8 and 17.6 at.% as measured by elastic recoil detection analysis. IR spectroscopy showed that the concentration of the clustered (Si-H)n groups and of the (Si-H2)n (n ≥ 1) polymers increased at the expense of the Si-H mono-hydrides with increasing annealing time, suggesting that there is a corresponding increase of the volume of micro-voids whose walls are assumed from literature to be decorated by the clustered mono-hydride groups and polymers. At the same time, an increase in the size of surface blisters was observed. Also, with increasing annealing time, the total concentration of bonded H of any type decreases, indicating that H is partially released from its bonds to Si. It is argued that the H released from the (Si-H)n complexes and polymers at the microvoid surfaces form molecular H2 inside the voids, whose size increases upon annealing because of the thermal expansion of the H2 gas, eventually producing plastic surface deformation in the shape of blisters.

Study on the RF Sputtered hydrogenated amorphous silicon–germanium thin films

In this study mechanical and electrical properties of the RF sputtered hydrogenated amorphous silicon germanium thin films deposited at room temperature have been discussed. Interesting correlation between the resistance and the flow of hydrogen is observed. Further, both band and hopping conduction mechanisms are found to exist simultaneously for the studied amorphous silicon germanium films.

On the mechanisms of hydrogen-induced blistering in RF-sputtered amorphous Ge

Hydrogenated amorphous germanium, a-Ge:H, is a material of interest for optoelectronic applications such as solar cells and radiation detectors because of the materials potential to extend the wavelength sensitivity of hydrogenated amorphous silicon. For such applications, the best structural quality is required. Here, we investigate the mechanisms of blister formation in a-Ge:H films obtained by RF (radio frequency) sputtering when submitted to annealing. By a Fourier transform IR spectroscopy study of the Ge–H stretching vibrations, it is found that annealing increases the density of GeH2 dihydrides residing on the internal surfaces of nanovoids which increase their size because of that. The thermal energy supplied by annealing also favours the breakage of the Ge–H bonds with consequent release inside the cavities of atomic H which then reacts to produce molecular H2. The expansion of the H2 gas causes the nanovoids to enhance their volume up to the formation of surface blisters. The presence of H2 in the blisters is confirmed by the activation energy for the onset of blistering as measured by Arrhenius plots. The reduced H content observed by elastic recoil detection analysis and secondary neutral mass spectrometry in the annealed samples where blister bursting took place further supports the hypothesis of H2 filling the blisters before they explode.

Studies on the RF sputtered amorphous SiGe thin films

In this study, rf sputtered hydrogenated amorphous silicon-germanium thin films deposited at room temperature have been investigated by spectroscopic ellipsometry and Rutherford backscattering. Technological parameters were determined for good layer quality of amorphous material. The layer thicknesses were first evaluated from the Rutherford backscattering and spectroscopic ellipsometry measurements, then measured directly by step-profiler, and compared to each other. The inherence of technological parameters and composition of the layers is discussed.

Role of metallization type in semi-insulating GaAs-based optoelectronic switches

Interdigitated planar photodetector structures were fabricated on semi-insulating GaAs material using ohmic-ohmic and Schottky-Schottky contacts. The dc and pulse response performance of the devices indicate that the devices with Schottky contacts are more suitable for high-speed photodetection and pulse generation. The differences between the two types of devices are interpreted with the difference in contact type, trapping mechanisms, and contact resistance effects.

Electron irradiation induced amorphous SiO2 formation at metal oxide/Si interface at room temperature; Electron beam writing on interfaces

Al2O3 (5 nm)/Si (bulk) sample was subjected to irradiation of 5 keV electrons at room temperature, in a vacuum chamber (pressure 1 × 10−9 mbar) and formation of amorphous SiO2 around the interface was observed. The oxygen for the silicon dioxide growth was provided by the electron bombardment induced bond breaking in Al2O3 and the subsequent production of neutral and/or charged oxygen. The amorphous SiO2 rich layer has grown into the Al2O3 layer showing that oxygen as well as silicon transport occurred during irradiation at room temperature. We propose that both transports are mediated by local electric field and charged and/or uncharged defects created by the electron irradiation. The direct modification of metal oxide/silicon interface by electron-beam irradiation is a promising method of accomplishing direct write electron-beam lithography at buried interfaces.