In order to reduce the material cost for silicon solar cells, several research groups are investigating methods to minimize the silicon consumption for making monocrystalline silicon wafers. One promising approach is deposition of an epitaxial layer on porous silicon, followed by detachment of the layer. This contribution discusses improvements in the epitaxial wafer fabrication by optimization of the porosification process. The introduction of a layered porous silicon structure allows to independently improve both epitaxial layer quality and detachment yield. In this way, we have managed to obtain 100µm thick silicon wafers with effective lifetimes up to 1.3ms, and 40µm thick wafers with effective lifetimes up to 700µs. We will also review the current status of the process development for solar cells made on thin wafers. Two approaches are presented. In the first approach, heterojunction solar cells are fabricated on freestanding epitaxial wafers of 40µm thickness. In the second approach, high efficiency (21%) heterojunction back-contacted cells are fabricated on wafers that are bonded to a glass superstrate. Challenges for device processing and limitations in cell performance are discussed.

We report on progress with PL imaging applications in silicon solar cell production, specifically focusing on the characterization of silicon bricks prior to wafer cutting. Silicon bricks represent an ideal opportunity to characterize and quantify the electronic material quality at an early stage of the PV value chain. Quantitative data on bulk lifetime can be obtained on bricks without any specific sample preparation, unlike unprocessed wafers. Spatially resolved bulk lifetime, interstitial iron concentration, and defect density measurements are demonstrated on bricks from different manufacturers including both high performance multicrystalline and older generation multicrystalline bricks. We find significant variability in bulk lifetime and iron concentration across the samples which is not related to its date of manufacture. However we do see a qualitative reduction in crystallographic defects in the newer high performance multicrystalline bricks. Data is parameterized in different ways to suggest possible paths to better predict solar cell efficiencies from an early stage of inspection.

Brick level PL measurements were previously performed using a conventional area scanning PL imaging system, which is associated with light spreading artefacts of weakly absorbed light. To overcome these artefacts, a new line scanning photoluminescence imaging system is used. We show a reduction in contrast smearing between high- and low lifetime regions in the new setup leading to image quality suitable for defect detection and quantitative measurements without deconvolution correction.

A number of thin silicon films are prepared through ultra-high-vacuum evaporation on optical quality fused quartz substrates with different growth temperatures. Through an analysis of grazing incidence X-ray diffraction results, a phase transition, from amorphous-to-crystalline, is found corresponding to increases in the growth temperature. The corresponding Raman spectra are also observed to change their form as the films go through this phase transition. Using a Raman peak decomposition process, this phase transition is then quantitatively characterized through the determination of the amount of intermediate-range order and the crystalline volume fraction for the various growth temperatures considered in this analysis. The possible device consequences of these results are then commented upon.

Nanostructures offer the opportunity to control the vibrational properties of via the scattering of phonons due to boundaries and mass disorder as well as through changes in the phonon dispersion due to spatial confinement. Advances in understanding these effects have the potential to lead to thermoelectrics with an improved figure of merit by lowering the thermal conductivity and to provide insight into electron-phonon scattering rates in nanoelectronics. Characterizing the phonon population in nanomaterials has been challenging because of their small volume and because optical techniques probe only a small fraction of reciprocal space. Recent developments in x-ray scattering now allow the phonon population to be evaluated across all of reciprocal space in samples with volumes as small as several cubic micrometers. We apply this approach, synchrotron x-ray thermal diffuse scattering (TDS), to probe the population of phonons within a Si/SiGe/Si trilayer nanomembrane. The distributions of scattered intensity from Si/SiGe/Si trilayer nanomembranes and Si nanomembranes with uniform composition are qualitatively similar, with features arising from the elastic anisotropy of the diamond structure. The TDS signal for the Si/SiGe/Si nanomembrane, however, has higher intensity than the Si membrane of the same total thickness by approximately 3.75%. Possible origins of the enhancement in scattering from SiGe in comparison with Si include the larger atomic scattering factor of Ge atoms within the SiGe layer or reduced phonon frequencies due to alloying.

There is significant interest to develop cheap CMOS compatible sensors that operate in the mid-infrared (MIR). To meet these requirements, Ge-on-Si is proving to be an exciting platform. There is the potential to realize waveguide integrated quantum well infrared photodetectors (QWIPs) based on Ge quantum wells (QWs). Intersubband absorption from p-Ge QWs has been demonstrated in the important atmospheric transmission window of 8-13 μm. An alternative strategy for sensing in the MIR is demonstrated through highly n-type doped Ge plasmonic antennas. These antennas demonstrate vibrational sensing of polydimethylsiloxane (PDMS) spin coated layers at 12.5 μm wavelength. These demonstrate enhanced sensing capabilities due to the localized hot spots of the antenna resonant modes.

Silicon Photonics is the technological to face the future challenges in data communications and processing. This technology follows the same paradigm as the technological revolution of the integrated circuit industry, that is, the miniaturization and the standardization. One of the most important building blocks in Silicon Photonics is the microresonator, a circular optical cavity, which enables many different passive and active optical functions. Here, we will describe the new physics of the intermodal coupling, which occurs when multi radial mode resonators are coupled to waveguides, and of the optical chaos, which develops in coupled sequence of resonators. In addition, an application of resonators in the label-free biosensing will be discussed.

In this study, we successfully demonstrate a carrier-injection InGaAsP variable optical attenuator (VOA) with a lateral P-I-N junction formed by Ni-InGaAsP alloy and Zn diffusion on a III-V on insulator (III-V-OI) wafer. The Ni-InGaAsP alloy for the n+ junction is formed by direct reaction between Ni and InGaAsP after annealing at 350°C. The p+ junction is formed by the Zn diffusion at 500°C using Zn doped spin-on glass (SOG). By both techniques, we successfully reduce the sheet and contact resistivity in the lateral P-I-N junction even with the relatively low-temperature process as compared with the P-I-N junction formed by conventional Si and Be ion implantation. By injecting carriers into the InGaAsP waveguide through the lateral P-I-N junction, we achieve the optical attenuation of -40 dB/mm with an injection current density of 40 mA/mm at a 1.55 μm wavelength.

We demonstrated that an InAs photodiode and a Quantum Dot Infrared Photodiode can be bonded to produce a hybrid broadband infrared photodetector. When cooled to 77 K the InAs photodiode can be used to detect wavelengths from visible to a cutoff wavelength of 3 μm while the Quantum Dot Infrared Photodiode detects wavelengths from 3 to 12 μm. The dark current and spectral response were measured on reference devices and bonded devices. Both sets of devices show similar dark current and spectral response, suggesting that no significant degradation of the devices after the bonding process.