We have studied the time-dependent charge transfer emission in a photovoltaic device made of a blend of poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclo-penta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3 benzothiadiazole)] and [6,6]-phenyl C-61-butyric acid methyl ester, a system showing a low band-gap suitable as absorber for bulk-heterojunction solar cells. We find a short-lived component (similar to 1 ns) and no evidence of a long-lived emission (mu s) expected if photo-generated free charge carriers would recombine radiatively via the charge transfer state. Our results explain, why the open circuit voltage loss in the investigated solar cell is much larger compared to GaAs-based or perovskite solar cells and highlight one of the key processes limiting the performance of bulk-heterojunction solar cells.

In this work, it is shown how different carrier recombination paths significantly broaden the photoluminescence (PL) emission bandwidth observed in type-II self-assembled SiGe/Si(001) quantum dots (QDs). QDs grown by molecular beam epitaxy with very homogeneous size distribution, onion-shaped composition profile, and Si capping layer thicknesses varying from 0 to 1100 nm are utilized to assess the optical carrier-recombination paths. By using high-energy photons for PL excitation, electron-hole pairs can be selectively generated either above or below the QD layer and, thus, clearly access two radiative carrier recombination channels. Fitting the charge carrier capture-, loss- and recombination-dynamics to PL time-decay curves measured for different experimental configurations allows to obtain quantitative information of carrier capture-, excitonic-emission-, and Auger-recombination rates in this type-II nano-system.

Efficient coupling to integrated high-quality-factor cavities is crucial for the employment of germanium quantum dot (QD) emitters in future monolithic silicon-based optoelectronic platforms. We report on strongly enhanced emission from single Ge QDs into L3 photonic crystal resonator (PCR) modes based on precise positioning of these dots at the maximum of the respective mode field energy density. Perfect site control of Ge QDs grown on prepatterned silicon-on-insulator substrates was exploited to fabricate in one processing run almost 300 PCRs containing single QDs in systematically varying positions within the cavities. Extensive photoluminescence studies on this cavity chip enable a direct evaluation of the position-dependent coupling efficiency between single dots and selected cavity modes. The experimental results demonstrate the great potential of the approach allowing CMOS-compatible parallel fabrication of arrays of spatially matched dot/cavity systems for group-IV-based data transfer or quantum optical systems in the telecom regime.

We investigate the evolution of the cross-plane thermal conductivity kappa of a SiGe superlattice (SL) as it is gradually converted into an alloy via post-growth thermal treatment at temperatures varying from 650 to 1000 degrees C. X-ray diffraction (XRD), transmission electron microscopy (TEM), secondary ion-mass-spectroscopy (SIMS), and photoluminescence (PL) spectroscopy measurements are used as complementary tools to gain insight into the structural properties of our SL and their evolution upon annealing. While the SL structure is preserved up to temperatures of similar to 850 degrees C, full alloying is observed for higher temperatures. The thermal conductivity data, collected with differential 3 omega method, show a monotonic increase of kappa from similar to 45 W/mK up to the values expected for a thin-film alloy. To understand the results, we compute the phonon mean-free-path (MFP) spectra using the experimentally determined composition profiles as input. The calculated thermal conductivity values are in good agreement with the experimental data and show that the increase of thermal conductivity is due to a gradual increase of MFP of low-to-mid-frequency phonons, i.e., to a weakening of interface scattering for alloyed SLs. The calculations also allow us to address finite-thickness effects on the measured thermal conductivity data. Although the used SL had a total thickness of only 250 nm, its thermal conductivity can be assumed to coincide with that of a "bulk" (infinitely thick) SL prior to annealing. As interdiffusion increases, boundary scattering at the film/substrate interface becomes relevant and leads to a slow increase of K. Besides its fundamental relevance, this work shows that the thermal stability of superlattices is limited, and that sizeable structural changes (including defect formation) occur already at operation temperatures of similar to 650 degrees C. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Recently, it was shown that lasing from epitaxial Ge quantum dots (QDs) on Si substrates can be obtained if they are partially amorphized by Ge ion bombardment (GIB). Here, we present a model for the microscopic origin of the radiative transitions leading to enhanced photoluminescence (PL) from such GIB-QDs. We provide an energy level scheme for GIB-QDs in a crystalline Si matrix that is based on atomistic modeling with Monte Carlo (MC) analysis and density functional theory (DFT). The level scheme is consistent with a broad variety of PL experiments performed on as-grown and annealed GIB-QDs. Our results show that an extended point defect consisting of a split-[110] self-interstitial surrounded by a distorted crystal lattice of about 45 atoms leads to electronic states at the Γ-point of the Brillouin zone well below the conduction band minimum of crystalline Ge. Such defects in Ge QDs allow direct transitions of electrons localized at the split-interstitial with holes confined in the Ge QD. We identify the relevant growth and annealing parameters that will let GIB-QDs be employed as an efficient laser active medium.

The evolution of the thermal conductivity of silicon-germanium superlattices upon thermal treatment was investigated experimentally and theoretically by Rastelli and co-workers (pp. 533–540). Several complementary techniques ranging from transmission-electron microscopy and X-ray diffraction to photoluminescence spectroscopy were employed to gain insight into the structural evolution. Silicon–germanium intermixing occurring during annealing produces a progressive smearing of the interfaces up to complete alloying. Using the structural information as input it was possible to reproduce theoretically the experimental results. The theoretical analysis shows that intermixing produces a significant reduction of scattering of phonons with mid–low frequency, accompanied by a modest increase of scattering of high-frequency phonons. The result is a gradual increase of thermal conductivity as the annealing temperature is increased. Effects of phonon scattering at the superlattice/substrate interface are also discussed. The work shows that while superlattices can display thermal conductivity values well below the alloy limit, their temperature stability is limited. This result is important also in view of potential applications of Si–Ge superlattices in thin-film thermoelectric devices. The cover image shows the evolution of the thermal conductivity of a superlattice annealed at increasing temperature. The calculations are able to reproduce the experimental results and show that phonon scattering at the thin film/substrate interface becomes important as annealing proceeds (see article for details). Examples of cross-sectional TEM images of the superlattice annealed at different temperatures are also shown.

Semiconductor light-emitters compatible with standard Si integration technology (SIT) are of particular interest for overcoming limitations in the operating speed of microelectronic devices. Light sources based on group IV elements would be SIT-compatible, but suffer from the poor optoelectronic properties of bulk Si and Ge. Here we demonstrate that epitaxially grown Ge quantum dots (QDs) in a defect-free Si matrix show extraordinary optical properties if partially amorphized by Ge-ion bombardment (GIB). In contrast to conventional SiGe nanostructures, these QDs exhibit dramatically shortened carrier lifetimes and negligible thermal quenching of the photoluminescence (PL) up to room temperature. Microdisk resonators with embedded GIB-QDs exhibit threshold behavior as well as a superlinear increase of the integrated PL intensity with concomitant line width narrowing as the pump power increases. These findings demonstrate light amplification by stimulated emission in a fully SIT-compatible group IV nanosystem.

We report on mapping of the local density of states in L3 photonic crystal resonators (PCR) via deterministically positioned single Ge quantum dots (QDs). Perfect site-control of Ge QDs on pre-patterned silicon-on-insulator substrates was exploited to fabricate in one processing run almost 300 L3 PCRs containing single QDs in systematically varying positions in the cavities. The alignment precision of the QD emitters was better than 20 nm. This type of parallel processing is essentially based on standard Si device technologies and is therefore scalable to any number and configuration of PCR structures. As a first demonstrator, we probed the coupling efficiency of a single Ge QD to the L3 cavity modes as a function of their spatial overlap. The results are in very good agreement with finite-difference time-domain simulations.

We fabricated photonic crystal nanocavities containing single SiGe QDs deterministically aligned to the antinodes of a cavity mode. Cavity coupling drastically enhances the vertical QD emission and allows, for the first time, the optical characterization of a single SiGe QD. For low excitation intensities, a single exponential PL decay with a time constant of 22ns at T=10K is observed for such a coupled QD-PhC-cavity system.

Highly photoconductive thin films of inorganic-capped PbS nanocrystal quantum dots (QDs) are reported. Stable colloidal dispersions of (NH4)3AsS3-capped PbS QDs were processed by a conventional dip-coating technique into a thin homogeneous film of electronically coupled PbS QDs. Upon drying at 130 °C, (NH4)3AsS3 capping ligands were converted into a thin layer of As2S3, acting as an infrared-transparent semiconducting glue. Photodetectors obtained by depositing such films onto glass substrates with interdigitate electrode structures feature extremely high light responsivity and detectivity with values of more than 200 A/W and 1.2×10(13) Jones, respectively, at infrared wavelengths up to 1400 nm. Importantly, these devices were fabricated and tested under ambient atmosphere. Using a set of time-resolved optoelectronic experiments, the important role played by the carrier trap states, presumably localized on the arsenic-sulfide surface coating, has been elucidated. Foremost, these traps enable a very high photoconductive gain of at least 200. The trap state density as a function of energy has been plotted from the frequency dependence of the photoinduced absorption (PIA), whereas the distribution of lifetimes of these traps was recovered from PIA and photoconductivity (PC) phase spectra. These trap states also have an important impact on carrier dynamics, which led us to propose a kinetic model for trap state filling that consistently describes the experimental photoconductivity transients at various intensities of excitation light. This model also provides realistic values for the photoconductive gain and thus may serve as a useful tool to describe photoconductivity in nanocrystal-based solids.

The main drawback of the rapidly evolving field of silicon photonics lies in the absence of efficient monolithically integrated radiation sources as a consequence of the indirect bandgap of Si and Ge. Relevant alternatives based on the hybrid combination of Si with optically active materials have to be technologically simple, temporally stable, and provide efficient coupling to the Si waveguides. Lead-sulfide nanocrystals (NCs) were blended into a polymer resist suitable for deep-UV- and electron-beam lithography and integrated into Si-based vertically slotted waveguides and ring resonators. The polymer both stabilizes the NC's photoluminescence emission against degradation under ambient conditions and allows lithographic patterning of this compound material. After integration into the optoelectronic structures and upon optical pumping, intense photoluminescence emission from ring resonators was recorded at the output of bus-waveguides. The resonator quality factors were investigated for polymer-NC compounds with NC concentrations in the range between 0.1 and 8 vol%. The spontaneous emission rate enhancement for vertically slotted resonators was estimated to be a factor of two higher as compared to unslotted ones. The stable integration of colloidal NCs as well as the improved light coupling to silicon circuits is an important step in the development of silicon-based hybrid photonics. c The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

We show that both the morphology and the optoelectronic properties of SiGe islands growing in the pits of periodically pre-patterned Si(001) substrates are determined by the amount of Ge deposited per unit cell of the pattern. Pit-periods (p) ranging from 300 to 900 nm were investigated, and Ge growth was performed by molecular beam epitaxy (MBE) at temperatures of 690 and 760 °C. The ordered SiGe islands show photoluminescence (PL) emission, which becomes almost completely quenched, once a critical island volume is exceeded. By atomic force and transmission electron microscope images we identify the transition from pyramid-shaped to dome-shaped islands with increasing p. Eventually, the nucleation of dislocations in the islands leads to PL quenching. Below a critical Ge coverage a narrowing and a blue shift of the PL emission is observed, as compared to islands grown on a planar reference area of the same sample.