B. Berghoff

Residual stress in Si nanocrystals embedded in a SiO2 matrix

Multiple quantum wells consisting of alternating Si and SiO2 layers were studied by means of Raman scattering. The structures were fabricated by the remote plasma enhanced chemical vapor deposition of amorphous Si and SiO2 layers on quartz substrate. The structures were subjected to a rapid thermal annealing procedure for Si crystallization. The obtained results suggest that the Si layers consist of nanocrystals embedded in an amorphous Si phase. It was found that the silicon nanocrystals inside 2nm thin layers are under high residual compressive stress. Moreover, the metastable Si III phase was detected in these samples supporting the presence of large compressive stresses in the structures. The compressive stress could be relaxed upon local laser annealing.

Lateral Si/SiO2 quantum well solar cells

The photovoltaic properties of Si∕SiO2 multiple quantum wells (QWs) embedded in lateral Schottky contacts are investigated. The QWs were fabricated by remote plasma enhanced chemical vapor deposition. By subsequent rapid thermal annealing, the two-dimensional Si layers are partially recrystallized, which gives rise to distinct quantum confinement effects. Although the current extraction along the quantum layers is hampered by the incomplete recrystallization, the data collected define the route to optimized Si based QW solar cells.

Improved charge transport through Si based multiple quantum wells with substoichiometric SiOx barrier layers

The vertical charge transport through Si/SiOx multiple quantum wells (QWs) is investigated. Upon thermal annealing, segregation of excess Si from the SiOx layers leads to the formation of highly conductive pathways between Si grains from adjacent QWs separated by ultrathin silicon oxide barriers with barrier heights of 0.53–0.65 eV. Compared to stoichiometric Si/SiO2 layer stacks, conductivity is increased by up to ten orders of magnitude, which opens the way to an efficient charge carrier extraction in photovoltaic systems with distinct quantum confinement.

Resonant and phonon-assisted tunneling transport through silicon quantum dots embedded in SiO2

Charge transport through SiO2∕Si∕SiO2 double-barrier structures (DBSs) and SiO2 single-barrier structures is investigated by low temperature I-V measurements. Resonant tunneling signatures accompanied by a negative differential conductance are observed if silicon quantum dots (Si QDs) are embedded in the amorphous SiO2 matrix. The I-V characteristics are correlated with the morphology of Si QDs extracted from transmission electron microscopy and photoluminescence. Evidence for phonon-assisted tunneling at low voltages has been found in the DBSs. These results show the potential but also the limitation for charge extraction from Si QDs embedded in SiO2.

Quasi-metallic behavior of ZnO grown by atomic layer deposition: The role of hydrogen

Zinc oxide (ZnO) fabricated by atomic layer deposition (ALD) is intrinsically well-conductive (∼5 mΩ cm), in contrast to the single-crystalline bulk material or sputtered ZnO thin films. There are generally three groups of candidates for the intrinsic n-type conductivity: intrinsic point defects, elemental impurities other than hydrogen, and incorporated hydrogen itself. In this study, we assess the different candidates concerning their impact on conductivity. In the presence of free electron densities of up to 5 × 1019 cm−3, impurities other than hydrogen are ruled out due to their ultra-low concentrations in the ppm range. Intrinsic point defects are also considered unlikely since the evolution of conductivity with deposition temperature is not reproduced in the Zn/O ratio as measured by Rutherford backscattering spectrometry. Hence, the most promising candidate is hydrogen with a concentration of ∼1 at. %, i.e., more than sufficient to account for the free electron density. In addition, we find a corre...

Confinement and transport in silicon based quantum structures

Crystallized Si QWs embedded between SiO<inf>2</inf> barrier layers were fabricated by remote plasma enhanced chemical vapor deposition of Si/SiO<inf>2</inf> and Si/SiO<inf>x</inf> stacks and subsequent high temperature annealing. Using SiO<inf>x</inf> instead of SiO<inf>2</inf> strongly enhances the overall crystallinity and the nanocrystallite size of the Si QWs. The phase separation of the SiO<inf>x</inf> into Si and SiO<inf>2</inf> increases the thickness of the Si QWs and yields thin SiO<inf>2</inf> barriers. Photoluminescence measurements on annealed stacks showed a quantum confinement of carriers within the Si QWs. The high degree of crystallization and the decrease of SiO<inf>2</inf> barrier height and thickness as well as possible shunting between adjacent Si layers improve the conductivity of annealed Si/SiO<inf>x</inf> stacks by up to seven orders of magnitude compared to Si/SiO<inf>2</inf> stacks.

Intrinsic ultrasmall nanoscale silicon turns n-/p-type with SiO2/Si3N4-coating

Impurity doping of ultrasmall nanoscale (usn) silicon (Si) currently used in ultralarge scale integration (ULSI) faces serious miniaturization challenges below the 14 nm technology node such as dopant out-diffusion and inactivation by clustering in Si-based field-effect transistors (FETs). Moreover, self-purification and massively increased ionization energy cause doping to fail for Si nano-crystals (NCs) showing quantum confinement. To introduce electron- (n-) or hole- (p-) type conductivity, usn-Si may not require doping, but an energy shift of electronic states with respect to the vacuum energy between different regions of usn-Si. We show in theory and experiment that usn-Si can experience a considerable energy offset of electronic states by embedding it in silicon dioxide (SiO2) or silicon nitride (Si3N4), whereby a few monolayers (MLs) of SiO2 or Si3N4 are enough to achieve these offsets. Our findings present an alternative to conventional impurity doping for ULSI, provide new opportunities for ultralow power electronics and open a whole new vista on the introduction of p- and n-type conductivity into usn-Si.

Investigation of dopant segregation induced surface-fields using THz pulse transmission

Doping layers like the emitter or a surface field in silicon solar cells have to fulfill two important functions: They have to allow ohmic contact formation and they should also suppress Auger and surface recombination effects. The latter can be supported by an electrical field, provided by the layer itself. In the case of an emitter this is provided by a peak doping concentration beneath the surface, like a segregation layer, formed during a silicidation process. Layers, like front surface fields applied in rear emitter concepts have a significant influence on the performance of the resulting solar cell, which is very sensitive for process fluctuations. For all these reasons, it is crucial to have an appropriate tool for analyzing these layers in terms of the resulting lifetime. Current established measurement methods are not able to provide the sufficient sensitivity, to carry out further investigations. In this work a method, based on THz transmission through the silicon material is presented to analyze these layers. Corresponding measurements were carried out with two kind of samples: a sample containing a front surface field and a sample with a dopant segregation layer. Depth dependent THz measurements were carried out, by etching the surface layers back. It is shown in this work, that the segregation layer has a large influence on the resulting effective excess carrier density or lifetime, respectively of the material.