Silicon Band Edge Research Articles

Role of Boron in Assisting the Super-Enhancement of Emissions from Carbon-Implanted Silicon.

The super enhancement of silicon band edge luminescence when co-implanted with boron and carbon is reported. The role of boron in the band edge emissions in silicon was investigated by deliberately introducing defects into the lattice structures. We aimed to increase the light emission intensity from silicon by boron implantation, leading to the formation of dislocation loops between the lattice structures. The silicon samples were doped with a high concentration of carbon before boron implantation and then annealed at a high temperature to activate the dopants into substitutional lattice sites. Photoluminescence (PL) measurements were performed to observe the emissions at the near-infrared region. The temperatures were varied from 10 K to 100 K to study the effect of temperature on the peak luminescence intensity. Two main peaks could be seen at ~1112 and 1170 nm by observing the PL spectra. The intensities shown by both peaks in the samples incorporated with boron are significantly higher than those in pristine silicon samples, and the highest intensity in the former was 600 times greater than that in the latter. Transmission electron microscopy (TEM) was used to study the structure of post-implant and post-anneal silicon sample. The dislocation loops were observed in the sample. Through a technique compatible with mature silicon processing technology, the results of this study will greatly contribute to the development of all Si-based photonic systems and quantum technologies.

Preparation and characterization of vanadium-implanted silicon for intermediate band solar cell applications

The research of intermediate band solar cells has become a hot spot in the field of photovoltaics. The preparation of intermediate band materials is a key for intermediate band solar cell applications. In this paper, we attempted to prepare the intermediate band photovoltaic materials with supersaturated vanadium-doped silicon by ion implantation and rapid thermal treatment (RTP). The results show that the RTP technique can recover the damage to the lattice and morphology caused by ion implantation and can promote carrier activation and mobility. Moreover, the near infrared absorption of the samples is significantly enhanced, which indicates that some subband absorptions have been implemented in the samples. Furthermore, the minority carrier lifetime increases with increasing the dose of vanadium ion. This agrees with the intermediate-band prediction, suggesting an intermediate band is formed in the vanadium-doped silicon. In addition, the Arrhenius analysis shows that the intermediate band is located at about 0.43 eV below the conduction band edge of silicon. Our results indicated that the IB photovoltaic materials can be effectively prepared by vanadium-implantation in silicon and RTP technique.

Turning Low-Nanoscale Intrinsic Silicon Highly Electron-Conductive by SiO2 Coating.

Impurity doping in silicon (Si) ultra-large-scale integration is one of the key challenges which prevent further device miniaturization. Using ultraviolet photoelectron spectroscopy and X-ray absorption spectroscopy in the total fluorescence yield mode, we show that the lowest unoccupied and highest occupied electronic states of ≤3 nm thick SiO2-coated Si nanowells shift by up to 0.2 eV below the conduction band and ca. 0.7 eV below the valence band edge of bulk silicon, respectively. This nanoscale electronic structure shift induced by anions at surfaces (NESSIAS) provides the means for low-nanoscale intrinsic Si (i-Si) to be flooded by electrons from an external (bigger, metallic) reservoir, thereby getting highly electron- (n-) conductive. While our findings deviate from the behavior commonly believed to govern the properties of silicon nanowells, they are further confirmed by the fundamental energy gap as per nanowell thickness when compared against published experimental data. Supporting our findings further with hybrid density functional theory calculations, we show that other group IV semiconductors (diamond, Ge) do respond to the NESSIAS effect in accord with Si. We predict adequate nanowire cross-sections (X-sections) from experimental nanowell data with a recently established crystallographic analysis, paving the way to undoped ultrasmall silicon electronic devices with significantly reduced gate lengths, using complementary metal-oxide-semiconductor-compatible materials.

Tuning p-Si(111) Photovoltage via Molecule|Semiconductor Electronic Coupling.

Photoelectrochemical (PEC) device efficiency depends heavily on the energetics and band alignment of the semiconductor|overlayer junction. Exerting energetic control over these junctions via molecular functionalization is an extremely attractive strategy. Herein we report a study of the structure-function relationship between chemically functionalized pSi(111) and the resulting solar fuels performance. Specifically, we highlight the interplay of chemical structure and electronic coupling between the attached molecule and the underlying semiconductor. Covalent attachment of aryl surface modifiers (phenyl, Ph; nitrophenyl, PhNO2; anthracene, Anth; and nitroanthracene, AnthNO2) resulted in high-fidelity surfaces with low defect densities (S < 50 cm/s). Electrochemical characterization of these surfaces in contact with methyl viologen resulted in systematically shifted band edges (up to 0.99 V barrier height) and correspondingly high photoelectrochemical performance (Voc up to 0.43 V vs MV2+) consistent with the introduction of a positive interfacial dipole. We extend this functionalization to HER conditions and demonstrate systematic tuning of the HER Voc using pSi(111)-R|TiO2|Pt architecture. Correlation of the shifts in barrier height with the photovoltage provides evidence for nonideality despite low surface recombination. Critically, DFT calculations of the electronic structure of the organic-functionalized interfaces show that the molecule-based electronic states effectively hybridized with the silicon band edges. A comparison of these interfacial states with their isolated molecular analogues further confirms electronic coupling between the attached molecule and the underlying semiconductor, providing an induced density of interfacial states (IDIS) which decreases the potential drop across the semiconductor. These results demonstrate the delicate interplay between interfacial chemical structure, interfacial dipole, and electronic structure.

Optical Dependence of Electrically Detected Magnetic Resonance in Lightly Doped Si:P Devices

Electrically-detected magnetic resonance (EDMR) provides a highly sensitive method for reading out the state of donor spins in silicon. The technique relies on a spin-dependent recombination (SDR) process involving dopant spins that are coupled to interfacial defect spins near the Si/SiO$_2$ interface. To prevent ionization of the donors, the experiments are performed at cryogenic temperatures and the mobile charge carriers needed are generated via optical excitation. The influence of this optical excitation on the SDR process and the resulting EDMR signal is still not well understood. Here, we use EDMR to characterize changes to both phosphorus and defect spin readout as a function of optical excitation using: a 980 nm laser with energy just above the silicon band edge at cryogenic temperatures; a 405 nm laser to generate hot surface-carriers; and a broadband white light source. EDMR signals are observed from the phosphorus donor and two distinct defect species in all the experiments. With near-infrared excitation, we find that the EDMR signal primarily arises from donor-defect pairs, while at higher photon energies there are significant additional contributions from defect-defect pairs. The optical penetration depth into silicon is also known to be strongly wavelength dependent at cryogenic temperatures. The energy of the optical excitation is observed to strongly modulate the kinetics of the SDR process. Careful tuning of the optical photon energy could therefore be used to control both the subset of spin pairs contributing to the EDMR signal as well as the dynamics of the SDR process.

Near-infrared quantum cutting and energy transfer mechanism in Lu2O3: Tm3+/Yb3+ phosphor for high-efficiency photovoltaics

Near-infrared downconversion phenomenon has been demonstrated in Lu2O3: Tm3+/Yb3+ phosphor upon direct excitation of Tm3+:1G4 level at 463 nm. The efficient energy transfer from Tm3+: 1G4 → Yb: 2F5/2 has been elucidated by the excitation spectra, the visible and NIR spectra as well as the decay curves of Tm: 1G4 state. The mechanism of downconversion in Lu2O3:Tm3+/Yb3+ has been discussed in detail. According to analysis of the dependence of the initial transfer rate over Yb3+ ion concentration, it could be included that energy transfer (ET) from Tm3+ to Yb3+ is a single-step ET process instead of a cooperative one. By varying the Yb3+ concentrations, we obtain the Lu2O3: 0.2%Tm3+/30%Yb3+ sample with theoretical quantum efficiency as high as 148.2%. Because the excited state of Yb3+ just above the band edge of crystalline silicon, it suggested that Lu2O3: Tm3+/Yb3+ sample will be beneficial to improve the conversion efficiency of c-Si solar cells.

Evidence of Silicon Band-Edge Emission Enhancement When Interfaced with SiO2:Er Films

Nearly two-orders of magnitude increase in room-temperature band-to-band (1.067 eV) infrared emission from crystalline silicon, coated with erbium-doped sol–gel films, have been achieved. Phonon-assisted band-to-band emission from coated and annealed p-Si is strongest for the sample annealed at 700°C. In this paper, evidence of the origin of the emission band from the band edge recombination activities is established. Enhancement of radiative recombination of free carriers is reasoned by stresses at the interface due to the annealed sol–gel-deposited silica. Comparative studies with other strained silicon samples are presented.

Improved photoelectrochemical hydrogen evolution using a defect-passivated Al2O3 thin film on p-Si

A large amount of external overpotential is normally required to split water using p-type silicon (p-Si) due to the insufficient driving force between the conduction band-edge and the hydrogen evolution level. We demonstrate how inserting an Al2O3 interlayer between p-Si and the electrolyte mitigates the requirement of overpotentials. Since the Al2O3 film decreased the number of interface defect states, electrons were observed to migrate into the Si surface so that negative charges accumulated at the band-edge of silicon. This resulted in band bending enhancement and a reduction of the overpotential requirement. In our result, the overpotential of ~150mV was reduced at a current density of 20mA/cm2, and the onset voltage of ~70mV was also reduced at the 1.4nm thickness of Al2O3 interlayer.

Maximizing Photoluminescence Extraction in Silicon Photonic Crystal Slabs.

Photonic crystal modes can be tailored for increasing light matter interactions and light extraction efficiencies. These PhC properties have been explored for improving the device performance of LEDs, solar cells and precision biosensors. Tuning the extended band structure of 2D PhC provides a means for increasing light extraction throughout a planar device. This requires careful design and fabrication of PhC with a desirable mode structure overlapping with the spectral region of emission. We show a method for predicting and maximizing light extraction from 2D photonic crystal slabs, exemplified by maximizing silicon photoluminescence (PL). Systematically varying the lattice constant and filling factor, we predict the increases in PL intensity from band structure calculations and confirm predictions in micro-PL experiments. With the near optimal design parameters of PhC, we demonstrate more than 500-fold increase in PL intensity, measured near band edge of silicon at room temperature, an enhancement by an order of magnitude more than what has been reported.

Silicon‐Modified Rare‐Earth Transitions—A New Route to Near‐ and Mid‐IR Photonics

Silicon underpins microelectronics but lacks the photonic capability needed for next‐generation systems and currently relies on a highly undesirable hybridization of separate discrete devices using direct band gap semiconductors. Rare‐earth (RE) implantation is a promising approach to bestow photonic capability to silicon but is limited to internal RE transition wavelengths. Reported here is the first observation of direct optical transitions from the silicon band edge to internal f‐levels of implanted REs (Ce, Eu, and Yb); this overturns previously held assumptions about the alignment of RE levels to the silicon band gap. The photoluminescence lines are massively redshifted to several technologically useful wavelengths and modeling of their splitting indicates that they must originate from the REs. Eu‐implanted silicon devices display a greatly enhanced electroluminescence efficiency of 8%. Also observed is the first crystal field splitting in Ce luminescence. Mid‐IR silicon photodetectors with specific detectivities comparable to existing state‐of‐the‐art mid‐IR detectors are demonstrated.

Directly patterned TiO2 nanostructures for efficient light harvesting in thin film solar cells

A novel, scalable, and low-cost strategy for fabricating sub-wavelength scale hierarchical nanostructures by direct patterning of TiO2 nanoparticles on glass substrates is reported. Two nanostructural designs of light-trapping back-surface reflectors (BSR) have been fabricated for increasing the photon-harvesting properties of thin-film solar cells: a quasi-periodic nano-crater design and a random nano-bump design. The efficient light-scattering properties of the nano-crater design over a broad wavelength range are demonstrated by the measured haze factor being larger than 40% at wavelengths (~700 nm) near the band edge of amorphous silicon (a-Si:H). The a-Si:H-based n-i-p solar cell fabricated with an only ~200 nm thick absorber layer on the nano-crater BSR shows a short-circuit current density (Jsc) of ~16.1 mA cm−2 representing a 28% enhancement compared to the cell deposited on a non-textured flat substrate. Measurements of the external quantum efficiency of the cell fabricated on the quasi-periodic nano-crater surface at long wavelengths, λ > 700 nm, demonstrate an increase of a factor of 5 relative to that of a flat reference solar cell. The theoretical modeling results of optical absorption corroborate well with the experimental findings and are used to identify the volumes of strong optical absorption in the a-Si:H active layer of the textured BSR devices.

Interface Effects on Total Energy Calculations for Radiation-Induced Defects

We present a new, approximate technique for estimating the polarization energy of point defects near interfaces in layered systems using semiconductor device simulation combined with a finite element quadrature technique. We show that we recapture the original, spherical Jost approximation in a homogeneous, infinite solid, as well as reproducing the exact result for a point charge near the interface of two dielectrics. We apply this technique to the silicon-silicon dioxide system for doped substrates, and for devices under bias. We show that the correction to calculated, bulk defect levels depends mildly on the distance from the interface. It depends more strongly on the substrate doping density. Finally, there is a significant dependence on gate bias. These results must be considered for proposed models for negative bias temperature instability (NBTI) that invoke tunneling from the silicon band edges into localized oxide traps.

Theory of Covalent Adsorbate Frontier Orbital Energies on Functionalized Light-Absorbing Semiconductor Surfaces.

Functional hybrid interfaces between organic molecules and semiconductors are central to many emerging information and solar energy conversion technologies. Here we demonstrate a general, empirical parameter-free approach for computing and understanding frontier orbital energies - or redox levels - of a broad class of covalently bonded organic-semiconductor surfaces. We develop this framework in the context of specific density functional theory (DFT) and many-body perturbation theory calculations, within the GW approximation, of an exemplar interface, thiophene-functionalized silicon (111). Through detailed calculations taking into account structural and binding energetics of mixed-monolayers consisting of both covalently attached thiophene and hydrogen, chlorine, methyl, and other passivating groups, we quantify the impact of coverage, nonlocal polarization, and interface dipole effects on the alignment of the thiophene frontier orbital energies with the silicon band edges. For thiophene adsorbate frontier orbital energies, we observe significant corrections to standard DFT (∼1 eV), including large nonlocal electrostatic polarization effects (∼1.6 eV). Importantly, both results can be rationalized from knowledge of the electronic structure of the isolated thiophene molecule and silicon substrate systems. Silicon band edge energies are predicted to vary by more than 2.5 eV, while molecular orbital energies stay similar, with the different functional groups studied, suggesting the prospect of tuning energy alignment over a wide range for photoelectrochemistry and other applications.

Passivation of Optically Black Silicon by Atomic Layer Deposited Al2O3

Optically black silicon nanostructures show excellent anti-reflection and light trapping properties minimizing reflection losses to less than 1.6% between 300 – 1100nm. Our light-trapping scheme enables an absorption enhancement factor of ∼10 at the band edge of silicon (1150nm) as compared to a simulated perfect ARC, where the Yablonovitch limit corresponds to a factor of 15. Just recently it was shown that similar wet-chemically black etched silicon surfaces can be exploited to fabricate high efficiency solar cells [1]. Towards the integration of our structures into a solar cell device, the passivation performance of atomic layer deposited thin Al2O3 films is investigated on a variety of black etched structures. The coatings lead to measured surface recombination velocities of less than 13cm/s on bifacially black structured as well as 12cm/s on polished 1 – 5Ωcm p-type Si CZ wafers. Thinner layers promise to be even more effective. This technology will enable high efficiencies on various solar cell concepts.

(Invited) Electroluminescence in Metal-Oxide-Semiconductor Tunnel Diodes with Nanometer-Thick Silicon

Electroluminescence in metal-oxide-semiconductor tunnel diodes with a transparent conductive oxide gate and a nanometer-thick silicon layer on silicon-on-insulator substrates has been studied. The electroluminescence spectra from the diodes are peaked at ~1120 nm (~1.11 eV). The peak shifts toward lower energy as the negative gate voltage value increases. The peak shift can be explained by a Stark effect. The peak tail intensity at the short-wavelength side of the peak at ~1120 nm increases as the silicon layer thickness decreases. This may indicate that the increase in the intensity is due to transitions between subbands near the silicon band edges. Small peaks have been observed at ~1050 nm (~1.18 eV) and ~1010 nm (~1.23 eV). The peak wavelengths are independent of the silicon layer thickness. This exhibits that the electroluminescence is due to defect-mediated transitions.

Conformal Al2O3 Coatings on Black Silicon by Thermal ALD for Surface Passivation

Upon inductive coupled plasma reactive ion etching (ICP-RIE) of Si surfaces needle-like nanostructures with aspect ratios up to 10 emerge showing excellent anti-reflection and light-trapping properties with absorption over 97% throughout the UV and VIS spectral range. In addition, the absorption at the band edge of silicon is enormously enhanced due to scattering. In this work we report on the feasibility to deposit conformal Al2O3 dielectric layers on these very rough silicon surfaces which enable adequate surface passivation. Lifetimes over 170μs have been measured on deep structured b-Si substrates. The optical properties of black silicon (b-Si) and the possible influence of applied alumina passivation layers as well as their passivation performance are discussed.

Enhancement of Light Absorption in Thin Film Silicon Solar Cells with Metallic Grating and One-Dimensional Photonic Crystals

We design an effective light trapping scheme through engineering metallic gratings and one-dimensional dielectric photonic crystals (PhCs) to increase the optical path length of light within the solar cells. This incorporation can result in broadband optical absorption enhancement not only for transverse magnetic polarized light but also for transverse-electric polarization. Even when no plasmonic mode can be excited, due to the high reflection of the PhCs, the absorption in the active region can still be enhanced. Rigorous coupled wave analysis results demonstrate that such a hybrid structure boosts the overall cell performance by increasing the light trapping capabilities and is especially effective at the silicon band edge. This kind of design can be used to increase the optical absorption over a wide spectral range and is relatively independent of the angle of incidence.

Integration of Self-Assembled Porous Alumina and Distributed Bragg Reflector for Light Trapping in Si Photovoltaic Devices

Light trapping is an important issue for thin film silicon photovoltaic cells due to the limited absorption coefficient for near infrared light. In this letter, we present a photonic structure that combines porous anodic aluminum oxide with a distributed Bragg reflector (DBR) on the backside of Si cells for light trapping. Simulation results show that this low-cost, self-assembled structure can provide more than 50% relative efficiency enhancement for a 2 μm thin film crystalline Si solar cell, as compared to a reference cell without any back structure. As a proof of concept, we incorporated the backside structure into thick silicon wafers. The enhancement of light absorption near the band edge of silicon is demonstrated for our proposed light-trapping structure, in agreement with the theoretical predictions.

Temperature dependent carrier lifetime studies of Mo in crystalline silicon

The capture cross sections of both electrons σn and holes σp were determined for interstitial molybdenum in crystalline silicon over the temperature range of −110 to 150 °C. Carrier lifetime measurements were performed on molybdenum-contaminated silicon using a temperature controlled photoconductance instrument. Injection dependent lifetime spectroscopy was applied at each temperature to calculate σp and σn. This analysis involved a novel approach that independently determined the capture cross sections at each temperature assuming a known defect density and thermal velocity. Since the energy state is in the lower half of the bandgap, the determination of σp is unaffected by the defect energy at all temperatures, and σp is found to decrease with temperature in a fashion consistent with excitonic Auger capture. At temperatures below 0 °C, the determination of σn is also unaffected by the defect energy due to the suppression of thermal emission, and σn decreases with temperature as well. It is shown that a projection of σn to higher temperature suggests the defect has an energy of 0.375 eV above the valance band edge of silicon.

Hybrid Photovoltaics Based on Semiconductor Nanocrystals and Amorphous Silicon

Semiconductor nanocrystals (NCs) are promising materials for applications in photovoltaic (PV) structures that could benefit from size-controlled tunability of absorption spectra, the ease of realization of various tandem architectures, and, perhaps, increased conversion efficiency in the ultraviolet region through carrier multiplication. The first practical step toward utilization of the unique properties of NCs in PV technologies could be through their integration into traditional silicon-based solar cells. Here, we demonstrate an example of such hybrid PV structures that combine colloidal NCs with amorphous silicon. In these structures, NCs and silicon are electronically coupled, and the regime of this coupling can be tuned by altering the alignment of NC energy states with regard to silicon band edges. For example, using wide-gap CdSe NCs we demonstrate a photoresponse which is exclusively due to the NCs. On the other hand, in devices comprising narrow-gap PbS NCs, both the NCs and silicon contribute to photocurrent, which results in PV response extending from the visible to the near-infrared region. The hybrid silicon/PbS NC solar cells show external quantum efficiencies of approximately 7% at infrared energies and 50% in the visible and a power conversion efficiency of up to 0.9%. This work demonstrates the feasibility of hybrid PV devices that combine advantages of mature silicon fabrication technologies with the unique electronic properties of semiconductor NCs.