Silicon nanocrystals (nc-Si), have been shown to act as opto-electronic centers enabling light emission by carrier recombination, when precipitated in a silicon nitride (Si3N4) host. In this work, nc-Si and Germanium nanocrystals (nc-Ge) are studied in sputtered films of Si3N4 and SiGeN for application as tandem cell layers in a Si solar cell. The samples are annealed in a nitrogen gas and forming gas ambient, from 500 ºC to 900 ºC, to investigate the influence of temperature on photoluminescence and photoconductivity.

We demonstrate low-resistivity Ohmic contacts for n-Ge with ultra-shallow junction. Using the impurity δ-doping techniques with Ge homoepitaxy on Ge(111) below 400 ºC, we can achieve a very abrupt doping profile within a nanometer-scale width. By introducing the δ-doping to atomically controlled metal/Ge contacts, the current-voltage characteristics clearly show Ohmic conductions owing to the effective tunneling through the Schottky barrier. This approach is promising for a formation technology of ultra-shallow source/drain contacts for scaled Ge devices.

The formation of metal silicide plays important role in deciding the nature of the contact on silicon.Due to their chemically neautral nature,Au and Ag are used as contact metals in various devices.In particular the role of silicides is known to be crucial in defining the behavior of the electrical contact. The interaction of these metals with silicon at cluster level is still under the study.For naoscale devices, the nature at such interaction carries lot more inportance. Bulk Gold silicide(Cohesive energy ~ 3.81eV/atom) shows higher stability compared to silver silicide(Cohesive enrgy ~ 2.95 eV/atom). In the present work we show computational results based on Density Functional Theory(DFT) of Si cluster adsorption on Ag(111) surface and compare with the results of Si adsorption on Au(111). These results bring out the difference in Si cluster-metal surface interactions at the nanoscale. In particular the Si island-metal surface interaction shows island size dependence. We have presented results for most stable orientations only.

Spintronic devices generally require the spin of carriers to be utilized in the storage or manipulation of data. One theoretical model for ferromagnetism in dilute magnetic semiconductors (DMS) results from the percolation of ferromagnetic regions around dilute dopants such as Mn atoms in III-V or group IV materials through the interaction of Mn atoms with carriers. Our work employed Mn implantation in Ge with subsequent rapid thermal annealing or TEM in-situ annealing to study the correlation between structure and magnetic properties. The magnetic properties of 300-350 ºC implanted Ge:Mn (which produced crystalline Ge films) varied significantly with implantation dose and annealing condition due to precipitation and transformation of different MnxGe1-x secondary phases. It was found that Mn substitution of Ge and MnxGe1-x secondary phases can both result in ferromagnetic properties. By combining TEM in-situ annealing and ex-situ magnetic characterization, we have demonstrated detailed correlation of magnetic properties with nanoscale structures in Mn implanted Ge DMS materials.

Today’s image-based systems in civil security rely on sensors based either on CMOS or CCD technology, and they do not analyze color with sufficient equivalence to the human eye. The main reason for this is that the function of the fundamental color matching curves of the cones in the human eye can hardly be reproduced by technical systems [1]. Common color sensors are optimized for the three-area method in accordance with the standard DIN 5033-6. In civil security applications, for example, color sensors are used to identify the potential danger of whitish powder samples. Colorimetric classification based on the three-area method only provides suboptimal results [2], but not an unequivocal optical description. A series of tests performed at the IMT with more than 30 types of whitish powder samples were carried out with a spectrophotometer. These measurements showed that specimens with similar or different chemical compositions cannot be distinguished by the human eye, using the three-area method, exhibit characteristic differences in the CIE-L*a*b* measurement system under spectral method conditions. Until now, the realization of the spectral method on chip has been impossible with common color sensors. In this paper, we present a-Si:H-based bias-sensitive ni3p photodiodes, which have been fabricated successfully in a low temperature PECVD process. Present research focuses on the optimization of the continuous tunability of a-Si:H multispectral photodiodes, to increase the number of spectral stimulus specifications.

Conventional silicon sensors provide insufficient capability for color reproduction by producing three color signals (RGB) to fit their pixel sensitivities to that of the human eye. Photo detector devices based on a-SiC:H and a-SiGe:H offer the opportunity to shift spectral sensitivity continuously by varying external bias voltages. Based on the measurement results of Rieve et al. [2] we determined the absorption coefficients of a group of a-SiC:H and a-SiGe:H layers to simulate the penetration depth of photons at different energies into the device structure [3]. Knowing the indices of absorption, refraction and extinction, it is possible to engineer diodes in such a way that accumulations of charge carriers are generated at varying device depths. The use of common chromium (Cr) cathodes avoids a sharp falling edge of the sensitivity towards longer wavelengths. The use of low reflective and highly conductive aluminum-doped zinc oxide (ZnO:Al) as a material for the back contact of the diodes suppresses Fresnel reflections and has been verified experimentally. A dynamic range of more than 100 dB at 1000 lux could be obtained by I-/V- measurements of a-Si:H pin diodes fabricated between two ZnO:Al contacts. Optically the detectors show maxima between 440 nm and 630 nm with reduced Fresnel reflections and a very low leakage current of 10-8A/cm2 at -5 V bias voltage. Present research efforts concentrate on the verification of the optical properties and exact thicknesses of the chosen layers as well as on the development of a front and back illumination device structure which ensures a continuous spectral shift of the photosensitivity. This work requires extensive bandgap engineering and offers good prospects to improve security imaging tools as well as chemical analysis systems.

We made an amorphous-silicon (a-Si) solar cell with a nanowire-array structure on stainless steel(SS) by plasma enhanced chemical vapor (PECVD) deposition. This nanowire structure has an n-type Si nanowire array in which a-Si intrinsic layer and p type layer are sequentially grown on the surface of the nanowire. The highest open-circuit voltage (Voc) and short-circuit current density (Jsc) for AM 1.5 illumination were 620 mV and 13.4 mA/cm2, respectively at a maximum power conversion efficiency of 3.57%.

Strain engineering in composition-controlled Si-Si/Ge nanocluster multilayers with high germanium content (~ 50%) is achieved by varying thicknesses of Si/SiGe layers and studied by low temperature photoluminescence (PL) measurements. The PL spectra show reduction in strained silicon energy bandgap and a splitting presumably associated with partial removal of heavy hole-light hole degeneracy in SiGe valence band. Time-resolved PL measurements performed under different excitation wavelengths show dramatically different PL lifetimes, ranging from ~ 2 μs to 10 ns and an unusually high PL quantum efficiency. The results are explained by using the Si/SiGe interface recombination model, which is supported by ultra-high resolution transmission and analytical electron microscopy measurements.

We report on electrical properties in [(Ge+SiO2)/SiO2]×2 films deposited by magnetron sputtering on a periodically corrugated-rippled substrate and annealed in vacuum and forming gas. The rippled substrate caused a self-ordered growth of Ge quantum dots, while annealing in different environments enabled us to separate charge trapping in quantum dots from the trapping at the dot-matrix and matrix-substrate interfaces. We show that the charge trapping occurs mainly in Ge quantum dots in the films annealed in the forming gas, while Si–SiO2 interface trapping is dominant for the vacuum annealed films.

We propose a phototransistor geometry that incorporates silicon nanowires (SiNW) in the device channel. A set of two gates controls the charge flow inside the NW. This improves the device photo-response more than 10x when compared with a single gate phototransistor, leading to a photo-responsivity of greater than 104(A/W), while the dark properties of both devices are similar.

Interface state density profiling of the thermal oxide / n-type 4H-SiC interface which underwent post-oxidation nitric-oxide (NO) annealing showed that an interface state density of approximately 1×1011 cm−2eV−1 could be achieved at around 0.2 eV below the conduction band. It decreased exponentially by two orders to 1×109 cm-2eV-1 at around 0.9 eV from the conduction band. The values are comparable or better than other published work. The low interface state density achieved near the conduction band is important towards improved channel carrier mobility in SiC MOSFETs. A positive flat-band voltage shift of the SiC based MOS capacitor was also observed. The shift reduced under UV illumination. It could be attributed to slow acceptor-like (negatively-charged) traps, which may have contributed to the instabilities observed in drain current and threshold voltage suffered by SiC MOSFETs.

The Si nanocrystal-films are prepared by pulsed laser ablation of Si target in a mixture of helium and hydrogen gas. The total gas pressure and hydrogen partial gas pressure were varied to control structure of nanocrystal-film. The surface of Si nanocrystallite was hydrogenated and degree of hydrogenation increased with increasing hydrogen partial gas pressure. The aggregate structure of nanocrystal-film depended on both the total gas pressure and the hydrogen partial gas pressure. The former and the latter alter spatial confinement of Si species during deposition and the surface hydrogenation of individual nanocrystal, respectively. Spatial confinement increases probability of collision between nanocrystals in the plume. While, surface hydrogenation prevents coalescence of nanocrystals. The individual or aggregated nanocrystals formed in the plume reach the substrate and the nanocrystal-film is deposited on the substrate. The non-equilibrium growth processes during pulsed laser ablation are essential for the formation of the surface structure and the subsequent nanocrystal-film growth. Our results indicate that the structure of nanocrystal-film depends on the probabilities of collision and coalescence between nanocrystals in the plume. These probabilities can be varied by controlling the total gas pressure and the hydrogen partial gas pressure.

Most of actual photonic devices being sensitive in the visible (VIS) spectrum are based on crystalline silicon (x-Si). The production of x-Si requires an expensive high temperature process. The color reproduction with x-Si diodes additionally requires an integration of color filters [1] to realize a shift in spectral sensitivity. This work presents an amorphous silicon (a-Si:H) photodiode with an intermediate contact for color separation without color filters. Such detectors can be produced in a low cost and low temperature PECVD process, which allows their direct deposition on a custom specific ASIC [2]. Another advantage of a-Si:H is the up to 10 times higher light absorption compared to that of x-Si in the VIS spectrum [3]. The device consists of a metal (Cr) cathode, an amorphous NIP diode structure and a TCO (Al doped ZnO) anode. The I-layer includes an interior TCO contact buried between two P-layers. The thickness of this TCO layer is about 200 nm; the P-layers have a thickness of about 10 nm. The chromium cathode is sputtered on a glass substrate in a PVD process. The amorphous layers are deposited in a multi-chamber PECVD line; the buried and top TCO contacts are sputtered in the same line continuously under high-vacuum conditions. In a first photolithography step the top anode is patterned while the buried anode is uncovered. Afterwards, the diode must be patterned again, resulting in a final Cr, NIP-a-Si:H, TCO, PIP-a-Si:H and TCO multi-layer stack.

The spectral sensitivity of a common NIP diode can be shifted by external bias voltages. The spectral sensitivity at higher negative voltages overlays those of that at lower voltages and additionally shifts to longer wavelengths. The color reproduction is difficult; it can be improved by reducing the overlap of the spectral sensitivity [4]. The spectral response of the diodes presented in this work also can be shifted by the bias voltage. Furthermore, it can be split by substituting the disclosed anodes. The spectral response, using the cathode and top anode has a maximum at short wavelengths. If the diode between the interior anode and the cathode is used, the spectral sensitivity for longer wavelengths increases. Shorter wavelengths are blocked by the top part of the diode; it works like a filter. The presented device structure offers good prospects to improve color separation compared to currently existing detectors by using an additional intermediate contact.

The effect of laser-induced electronic excitations on the self-assembly of Ge quantum dots (QD) on Si(100)-(2x1) grown by pulsed laser deposition is studied. The experiment was conducted in ultrahigh vacuum. A Q-switched Nd:YAG laser (wavelength λ = 1064 nm, 10 Hz repetition rate) was split into two beams; one used to ablate a Ge target while the other to electronically excite the substrate. In situ reflection high-energy electron diffraction (RHEED) and ex situ atomic force microscopy (AFM) were used to study the morphology of the grown QDs. The dependence of the QD morphology on substrate temperature and ablation and excitation laser energy density was studied. Electronic excitation is shown to affect the surface morphology. For Ge coverage of 22 monolayer, it was observed that the excitation laser reduces the epitaxial growth temperature to 250 °C, a temperature at which no epitaxy is possible without excitation. Applying the excitation laser to the substrate during the growth changes the QD morphology and island density and improves the size uniformity of QDs at 390 °C. Surface diffusion measurement calculated from RHEED recovery curves show that the excitation-laser increases the surface diffusion of the Ge atoms. A purely electronic mechanism of enhanced surface diffusion of the Ge adatoms is involved.

We develop Si-based nano-photonic devices for the control of light at the nano-scales. We design high quality (Q) factor photonic crystal nanobeam cavities for a variety of Si compatible materials with low index, such as silicon rich oxide and silicon nitride, all with Q > 9,000 and small mode volumes. We apply these cavity designs to active materials such as Sinanocrystal doped silicon oxide and Er doped silicon nitride. By placing emitters in these cavities, we demonstrate that the cavity enhances emission processes. We show that the free carrier absorption process is greatly enhanced in the nanobeam cavities at both room and cryogenic temperatures. In addition, we demonstrate that nanobeam cavities made of Er-doped amorphous silicon nitride have enhanced absorption and gain characteristics compared to earlier designs that included silicon in the cavity. Because of the reduced losses, we observe linewidth narrowing and material transparency at both room temperature and cryogenic temperatures.

We present theoretical calculations for electron plasmon spectra at wurtzite Aluminum Gallium Nitride/ Silicon Carbide heterojunctions. Spontaneous and piezoelectric polarizations in wurtzite semiconductors give rise to polarization discontinuities at interfaces and to bound interface sheet charges. These charges are of the order of 1013 electrons per cm2 and give rise to two dimensional electron or hole gases near heterojunctions. Electron-electron interactions in the two-dimensional electron gases give rise to collective plasmon excitations. We calculate the dielectric function in these electron gases under the well-known and widely studied random phase approximation. Our calculations are relevant to the determination of the plasmon spectra at wurtzite Aluminum Gallium Nitride / Silicon Carbide heterojunctions and are of potential interest for determining the limits of mobility in two-dimensional electron gases. They are also of interest for terahertz electronics applications.

Group IV semiconductor nanowires are characterized by Raman spectroscopy. The results are analyzed in terms of the heating induced by the laser beam on the nanowires. By solving the heat transport equation one can simulate the temperature reached by the NWs under the exposure to a laser beam. The results are illustrated with Si and Si1-xGex nanowires. Both bundles of nanowires and individual nanowires are studied. The main experimental conditions contributing to the nanowire heating are discussed.

Metal-free and Au-catalyzed silicon nanowires (Si-NWs) grown at low temperatures have been analyzed through transmission electron microscopy (TEM) and scanning electron microscopy (SEM), and their crystalline phase studied. All the observed nanowires are crystalline, grow along two different directions, <110> or <112>, and contain high density of planar defects, such as stacking faults (SFs) and twins. The defect size is comparable to the wire diameter for the metal-free process whilst it is much larger than the wire diameter for the Aucatalyzed Si-NWs. In this latter case parallel SFs may re-arrange and transform in a metastable rhombohedral 9R polytype structure whose formation mechanism is discussed.

Silicon is by far the most successful material in the microelectronics industry enjoying a well-established fabrication and processing infrastructure. Two of the main challenges in traditional silicon electronic devices are (a) silicon’s relatively small and indirect fundamental energy band-gap, which severely limits optoelectronic applications, and (b) the absence of a suitable material to form a heterojunction barrier on silicon. Silicon based nanostructures are being explored as potential candidates to extent the applications of silicon in optoelectronics, provide for high-speed silicon quantum devices, increase the efficiency and reduce the cost in silicon photovoltaic solar cells, and facilitate cost-effective silicon sensors for biological, environmental, and other applications. Quantum size silicon nanolayers, nanowires, and nanodots embedded in oxide, nitride, and other amorphous matrices may provide an effective barrier for silicon, as well as band-gap engineering and enhanced optical transitions for solar cell and optoelectronic applications.