We have fabricated PbS particles in glass of average size from 1.2 nm to 3 nm. This allows us to tune the energy of the lowest electron-hole pair transition from 1.7 eV to 0.8 eV respectively. Optical absorption spectra show a pronounced quantum peak, implying a narrow size distribution. We have performed pump-probe spectroscopy where the probe is cw and the pump is nanosecond; this allows us to investigate a wide time scale. We have observed bleaching of the lowest quantum peak with lifetimes of ∼2 microseconds. We interpret this in terms of trapped carriers. Phenomenological description of observed effect is also presented.

We have found that traps can dominate important relaxation processes in PbS nanoparticles prepared by precipitation in borosilicate glass. The primary photoluminescence peak energy falls about 50 meV below the lowest absorption peak energy and has a long decay time of ~4 µs. Photoinduced bleaching of the lowest absorption peak also has a long decay lifetime of ~2 µs. Photoinduced bleaching also exhibits a surprisingly long rise time of hundreds of nanoseconds. Such long-lifetime effects must be attributed to multiple traps.

We present a new analysis approach which combines photoconductivity and photother-mal deflection spectroscopies in a-Si:H thin films of various thicknesses to profile electronic defects.

Optical waveguides for three-dimensionally stacked chip fabrication technologies, in which optical connection between layers plays a central role, are described. CMOS-compatible approaches to optical via and waveguide fabrication are addressed. Detailed modeling is used for design optimization and also addresses how manufacturing variations from ideal design may affect device performance.

We report on a simple experiment in which the thickness of a hyperdoped silicon layer, supersaturated with sulfur by ion implantation followed by pulsed laser melting and rapid solidification, is systematically varied at constant average sulfur concentration, by varying the implantation energy, dose, and laser fluence. Contacts are deposited and the external quantum efficiency (EQE) is measured for visible wavelengths. We posit that the sulfur layer primarily absorbs light but contributes negligible photocurrent, and we seek to support this by analyzing the EQE data for the different layer thicknesses in two interlocking ways. In the first, we use the measured concentration depth profiles to obtain the approximate layer thicknesses, and, for each wavelength, fit the EQE vs. layer thickness curve to obtain the absorption coefficient of hyperdoped silicon for that wavelength. Comparison to literature values for the hyperdoped silicon absorption coefficients [S.H. Pan et al. Applied Physics Letters 98, 121913 (2011)] shows good agreement. Next, we essentially run this process in reverse; we fit with Beer’s law the curves of EQE vs. hyperdoped silicon absorption coefficient for those wavelengths that are primarily absorbed in the hyperdoped silicon layer, and find that the layer thicknesses obtained from the fit are in good agreement with the original values obtained from the depth profiles. We conclude that the data support our interpretation of the hyperdoped silicon layer as providing negligible photocurrent at high S concentrations. This work validates the absorption data of Pan et al. [Applied Physics Letters 98, 121913 (2011)], and is consistent with reports of short mobility-lifetime products in hyperdoped layers. It suggests that for optoelectronic devices containing hyperdoped layers, the most important contribution to the above band gap photoresponse may be due to photons absorbed below the hyperdoped layer.

Room-temperature infrared sub-band gap photoresponse in silicon is of interest for telecommunications, imaging and solid-state energy conversion. Attempts to induce infrared response in silicon largely centred on combining the modification of its electronic structure via controlled defect formation (for example, vacancies and dislocations) with waveguide coupling, or integration with foreign materials. Impurity-mediated sub-band gap photoresponse in silicon is an alternative to these methods but it has only been studied at low temperature. Here we demonstrate impurity-mediated room-temperature sub-band gap photoresponse in single-crystal silicon-based planar photodiodes. A rapid and repeatable laser-based hyperdoping method incorporates supersaturated gold dopant concentrations on the order of 1020 cm−3 into a single-crystal surface layer ~150 nm thin. We demonstrate room-temperature silicon spectral response extending to wavelengths as long as 2,200 nm, with response increasing monotonically with supersaturated gold dopant concentration. This hyperdoping approach offers a possible path to tunable, broadband infrared imaging using silicon at room temperature. Extending the optical response of silicon below the band gap towards infrared wavelengths is of interest for applications such as imaging. Here, Mailoa et al. achieve room-temperature infrared photoresponse from silicon doped with supersaturated concentrations of gold impurities.

Hyperdoping silicon with impurities is considered an attractive method to develop an intermediate band solar cell in silicon with the potential to increase the photovoltaic cell efficiency beyond that of the Shockley-Queisser limit by utilizing sub-band gap photons for energy generation. Unfortunately, to date sub-band gap photoresponse has not been observed in singlecrystal hyperdoped silicon at room temperature, which is crucial for the development of intermediate band solar cells. In this contribution, we report and analyze room-temperature sub-band gap photoresponse of single-crystal silicon hyperdoped with gold. We further discuss the potential of using gold-hyperdoped silicon for IBSC in silicon.

: This is a final report on an experimental research program to characterize the optoelectronic properties of very heavily sulfur doped silicon (also known as Black Silicon) in close collaboration with scientists at US Army Benet Labs, Harvard, and the Army Research Laboratories. Materials and devices will be prepared at Harvard. The thrusts of the proposed work were: i) to elucidate the origin of excess infrared absorption and to determine how this absorption contributes to photoresponse, and ii) to measure the transport properties of photoexcited charge carriers. By analyzing the photoconductivity and photoresponse of coplanar and diode devices we were able to set limits on the mobility-lifetime product of sulfur-hyperdoped material. We have proposed an excitation and charge separation mechanism that is consistent with all data.

Doping of silicon-on-insulator layers with sulfur to concentrations far above equilibrium by ion implantation and pulsed laser melting can result in large concentration gradients. Photocarriers generated in and near the impurity gradient can separate into different coplanar transport layers, leading to enhanced photocarrier lifetimes in thin silicon-on-insulator films. The depth from which holes escape the heavily doped region places a lower limit on the minority carrier mobility-lifetime product of 10(-8) cm(2)/V for heavily sulfur doped silicon. We conclude that the cross-section for recombination through S impurities at this concentration is significantly reduced relative to isolated impurities. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4746752]

Photoconductivity in silicon hyperdoped with sulfur and selenium above the insulator-to-metal transition was measured via photoinduced changes in the microwave reflectivity of hyperdoped layers formed on p-type silicon. Despite these materials' strong subgap optical absorption, exposing them to 1310 and 1550 nm light results in a change in conductivity per photon 10,000 times smaller than what is observed in untreated silicon exposed to 980 nm light. A similar bound applies for 405 nm light, which is absorbed entirely in the hyperdoped layer. We use these results to deduce that the photocarrier lifetime in the hyperdoped material is ≤100 ns.

The photomodulated transmission spectrum of a single Ge layer grown at 500°C on the Si(111) surface by molecular beam epitaxy is reported. The nominal Ge layer thickness was 50 nm. The modulation spectrum of thicker layers is dominated by a three-lobed structure centered 70–80 meV above the bulk direct band edge. This structure is ascribed to excitation-induced broadening of the lowest direct exciton.

We report pressure and laser tuned Raman scattering studies on CdSxSei1-x nanocrystals. The electron-phonon coupling strength was determined as a function of pressure beyond the bulk phase transition pressure point. The coupling strength at atmospheric pressure determined from the Stokes shifted photoluminescence and resonant Raman scattering is not drastically smaller than the bulk value as might be expected theoretically. As a function of pressure it also shows no abrupt changes at the bulk phase transition pressure point. These results indicate that deep traps play a critical part in the mechanism of the electron-phonon coupling.

The optoelectronic properties of multilayer a-Ge:H/a-Si:H superlattices with bandgaps between ∼ 1.4 and 1.1eV are presented. The dependence of the electronic properties on the band alignment and the layer thicknesses is established and quantified. Particular emphasis is given to properties relevant to practical device applications and which involve carrier transport perpendicular to the layers.

We report an experimental study of photocarrier lifetime, transport, and excitation spectra in silicon-on-insulator doped with sulfur far above thermodynamic saturation. The spectral dependence of photocurrent in coplanar structures is consistent with photocarrier generation throughout the hyperdoped and undoped sub-layers, limited by collection of holes transported along the undoped layer. Holes photoexcited in the hyperdoped layer are able to diffuse to the undoped layer, implying ( μτ ) h ∼ 5 × 10 −9 cm 2 /V. Although high absorptance of hyperdoped silicon is observed from 1200 to 2000 nm in transmission experiments, the number of collected electrons per absorbed photon is 10 −4 of the above-bandgap response of the device, consistent with ( μτ ) e < 1 × 10 −7 cm 2 /V.

Highly supersaturated solid solutions of selenium or sulfur in silicon were formed by ion implantation followed by nanosecond pulsed laser melting. n+p photodiodes fabricated from these materials exhibit gain (external quantum efficiency >3000%) at 12 V of reverse bias and substantial optoelectronic response to light of wavelengths as long as 1250 nm. The amount of gain and the strength of the extended response both decrease with decreasing magnitude of bias voltage, but >100% external quantum efficiency is observed even at 2 V of reverse bias. The behavior is inconsistent with our expectations for avalanche gain or photoconductive gain.