C-AFM and KPFM techniques have been applied to investigate advanced junctions that are currently involved in highly efficient silicon solar cells. Our first study focuses on silicon heterojunctions and notably hydrogenated amorphous silicon (a-Si:H)/crystalline silicon (c-Si) P/n or N/p heterostructures which band bending at the interface forms a 2D channel. This conductive channel was indeed evidenced for the first time by cross-sectional investigations by C-AFM confirming the analysis of macroscopic planar conductance measurements. A second example of nanoscale characterization concerns the passivating selective contacts consisting in a thin silicon oxide (SiOx) layer between the c-Si and a highly doped polysilicon (poly-Si) layer. The electrical carrier transport is here not limited by the oxide layer and it is assumed that tunnelling through the oxide and/or the presence of pinholes are the main competitive mechanisms. For this specific heterostructure KPFM reveals local surface potential drops of 15- 30 mV, which do not exist on samples without SiOx. These potential drops suggest the presence of pinholes that are formed during the poly-Si annealing process performed in the range of 700-900°C. Finally, in a third study, we concentrate on p-i-n radial junction (RJ) silicon nanowire (SiNW) devices that are investigated under illumination by KPFM, in the so-called surface photovoltage (SPV) technique. This work focuses on the possibility of extracting the open-circuit voltage (VOC) on single isolated SiNW RJ by local SPV measurements using different AFM tip shapes and illumination directions in order to minimize shadowing effects.
We present an innovative approach for the growth of crystalline silicon on GaAs using plasma-enhanced chemical vapor deposition (PECVD). In this process the substrate is kept at low temperature (175 °C) and epitaxial growth is obtained via the impact of charged silicon clusters which are accelerated towards the substrate by the plasma-potential and melt upon impact. Therefore, this is a nanometer size epitaxial process where the local temperature (nm scale) rises above the melting temperature of silicon for extremely short times (in the range from ps to ns). This allows obtaining epitaxial growth even on relatively rough GaAs films, which have been cleaned in-situ using a SiF4 plasma etching. We present in-plane X-Ray Diffraction (XRD) measurements which are consistent with the hypothesis that the epitaxial growth happens at a local high temperature. Indeed, the tetragonal structure observed and the low in-plane lattice parameter determined from XRD can only be explained by the thermal mismatch induced by a high growth temperature. The effect of the plasma on the underlying GaAs properties, in particular the formation of hydrogen complexes with GaAs dopants (C, Si, Te) is studied in view of the integration of the c-Si epi-layers into devices.
We fabricated (n) c-Si/ (p) GaAs heterojunctions, by combining low temperature (∼175°C) RF-PECVD for Si and metal organic vapor phase epitaxy for GaAs, aiming at producing hybrid tunnel junctions for Si/III-V tandem solar cells. The electrical properties of these heterojunctions were measured and compared to that of a reference III-V tunnel junction. Several challenges in the fabrication of such heterostructures were identified and we especially focused in this study on the impact of atomic hydrogen present in the plasma used for the deposition of silicon on p-doped GaAs doping level. The obtained results show that hydrogenation by H2 plasma strongly reduces the doping level at the surface of the GaAs:C grown film. Thirty seconds of H2 plasma exposition at 175°C are sufficient to reduce the GaAs film doping level from 1×1020 cm−3 to <1×1019 cm−3 at the surface and over a depth of about 20 nm. Such strong reduction of the doping level is critical for the performance of the tunnel junction. However, the doping level can be fully recovered after annealing at 350°C.
This paper deals with the study of thin silicon films deposited by plasma-enhanced chemical vapor deposition on the industrial iron-nickel alloy substrate. This approach is promising for fabrication of low-cost high-efficiency solar cells. The main aim is to characterize the intrinsic hydrogenated microcrystalline silicon layer which fulfills its role of the absorber and has a direct impact on the solar cell performance. The real-time ellipsometric data obtained during the material deposition in the reactor are used to study the composition of the grown material. Based on the designed optical model, the evolution of the material crystallinity as well as the thickness and composition of the surface roughness layer are established in addition to an estimation of the average growth rate. Transmission electron microscopy was used to obtain the images of material structure and to verify conclusions of optical modeling.
In this work we introduce the main principles behind efficient light trapping in silicon nanowire structures. The ultimate performance of vertical periodic crystalline silicon nanowire arrays has been determined and optimized values have been presented as a function of the nanowire length. The further improvement of the performance has been demonstrated using dual-diameter periodic silicon nanowire arrays where the already optimized nanowire structure has been used as the starting point. The improved efficiency of this densely packed structure has been compared with the reference flat films in order to evaluate theoretical improvement of the light trapping efficiency. In the last part of our work we present the efficient light trapping inside amorphous silicon nanowire based radial junction solar cells fabricated using plasma enhanced vapor-liquid-solid process.
Optodielectrophoresis has been shown to be an interesting tool for massive manipulation of microparticles using external
electric fields. Here, no electrode fabrication is need it since they are created by the light distribution incident onto a
photoconducting material. We propose the use of this device for the recording of dynamic holograms in hydrogenated
amorphous silicon (a:Si-H)-liquid crystals hybrid devices. The device consists of 5CB liquid crystal sandwiched between
a photoconductive a:Si-H substrate and an ITO covered glass-plate. Diffraction efficiency of 3.3% is obtained when
holograms are recorded with a low power He-Ne laser.
Low temperature plasma processes provide a toolbox for etching, texturing and deposition of a wide range of materials. Here we present a bottom up approach to grow epitaxial crystalline silicon films (epi-Si) by standard RFPECVD at temperatures below 200°C. Booth structural and electronic properties of the epitaxial layers are investigated. Proof of high crystalline quality is deduced from spectroscopic ellipsometry and HRTEM measurements. Moreover, we build heterojunction solar cells with intrinsic epitaxial absorber thickness in the range of a few microns, grown at 175°C on highly doped (100) substrates, in the wafer equivalent approach. Achievement of a fill factor as high as 80 % is a proof that excellent quality of epitaxial layers can be produced at such low temperatures. While 8.5 % conversion efficiency has already been achieved for a 3.4 μm epitaxial silicon absorber, the possibility of reaching 15 % conversion efficiency with few microns epi-Si is discussed based on a detailed opto-electrical modeling of current devices.
Photovoltaic research has moved from popular solar cells, based on crystalline silicon substrates with thicknesses of
around 250 μm, to the thin film structures saving large amount of the active material. The next generation of solar cells
requires substantial increase of the energy conversion efficiency, which can be achieved by enhancing of the optical
trapping inside the cell. In this work we study the efficiency of light trapping inside vertical silicon nanowire solar cells.
The main focus is on the optical trapping inside single vertical nanowires, which can enhance optical absorption far
beyond capabilities of a thin film. Spectral optical absorption modeling based on RCWA together with the electromagnetic
field distribution analysis gave insight into the light trapping inside the nanowires. Results provide a guide for
the optimization of nanowires diameters, density and length for maximal short circuit currents with minimal material
demands.
We present the integration of an absorbing planar photonic crystal within a thin film photovoltaic cell. The devices are
based on a stack including a hydrogenated amorphous silicon P-i-N junction surrounded by TCO layers, with a back
metallic contact. Optical simulations exhibit a significant increase of the integrated absorption in the 300-720nm
wavelength range. The global electro-optical characteristics of such a new solar cell, and the impact of surface
passivation, are also discussed. Carrier generation rate maps calculated by optical simulations are introduced as input
data in a commercial electrical simulation software. The fabrication of such a device is finally addressed, with a specific
focus on the use of low cost nanopatterning processes compatible with large areas.
The absorption of thin hydrogenated amorphous silicon layers can be efficiently enhanced through a controlled periodic
patterning. Light is trapped through coupling with photonic Bloch modes of the periodic structures, which act as an
absorbing planar photonic crystal. We theoretically demonstrate this absorption enhancement through one or two
dimensional patterning, and show the experimental feasibility through large area holographic patterning. Numerical
simulations show over 50% absorption enhancement over the part of the solar spectrum comprised between 380 and
750nm. It is experimentally confirmed by optical measurements performed on planar photonic crystals fabricated by
laser holography and reactive ion etching.
UV-based imaging systems can be used for low-altitude rockets detection or biological agents identification (for instance weapons containing ANTHRAX). Compared to conventional CCD technology, CMOS-based active pixel sensors provide several advantages, including excellent electro-optical performances, high integration, low voltage operation, low power consumption, low cost, long lifetime, and robustness against environment. The monolithic integration of UV, visible and infrared detectors on the same uncooled CMOS smart system would therefore represent a major advance in the combat field, for characterization and representation of targets and backgrounds. In this approach, we have recently developped a novel technology using polymorphous silicon. This new material, fully compatible with above-IC silicon technology, is made of nanometric size ordered domains embedded in an amorphous matrix. The typical quantum efficiency of detectors made of this nano-material reach up to 80 % at 550 nm and 30 % in the UV range, depending of the design and the growth parameters. Furthermore, a record dark current of 20 pA/cm2 at −3 V has been reached. In addition, this new generation of sensors is significantly faster and more stable than their amorphous silicon counterparts. In this paper, we will present the relationship between the sensor technology and the overall performances.
Top gate and bottom gate microcrystalline silicon thin film transistors (TFTs) have been produced by the radio frequency glow discharge technique using three preparation methods: the standard hydrogen dilution of silane in hydrogen, the use of the layer-by-layer technique, and the use of SiF4-Ar-H2 feedstock. In all cases, stable top gate TFT with mobility values around 1 cm2/V.s have been achieved, making them suitable for circuit on glass applications. Moreover, the use of SiF4 gas combined with specific treatments of the a-SiN:H dielectric in bottom gate TFTs, fully compatible with today's a-Si:H process, lead to lateral growth of the silicon crystallites and an enhancement of the mobility to reach stable values of around 3 cm2/V.s.
Jacques Boulmer, A. Desmur-Larre, C. Guedj, D. Debarre, Philippe Boucaud, Francois Julien, Elieser Finkman, Keith Nugent, R. Laval, J.-B. Ozenne, H. Yang, D. Bouchier, Christian Godet, Pere Roca i Cabarrocas, G. Calvarin, C. Clerc
Si1-x-yGexCy / Si heterostructures are realized by pulsed laser induced epitaxy (PLIE) from C+ implanted pseudomorphic Si0.84/Ge(subscript 0.16 films and from hydrogenated amorphous SiGeC films deposited on Si(100). The laser treated samples are examined by electron channeling, energy dispersive X-ray analysis, Rutherford backscattering spectroscopy, X-ray diffraction and Raman spectroscopy. First results show that laser induced epitaxy is effective, provided that laser fluence exceeds a threshold for which the melted depth is larger than the initial SiGeC layer. In addition, carbon incorporation in substitutional sites is demonstrated.
We have applied the dual-beam photocurrent spectroscopy to study the excited defect states in a-Si:H. The pump beam is used to create the excited state while the second beam is used as a probe. It is shown that the anomalous band in dual-beam photocurrent spectra of a-Si:H results from a combination of two processes: photocurrent enhancement due to excitations by the probe light for the D0 states to the conduction band. Using the dual-beam photocurrent spectra, we measured the optical transition energy (0.77 eV) and the electron correlation energy (0.16 eV) for the filled D- defects (i.e., excited D0 defects). The dual-beam photocurrent spectroscopy may also be used for study of deep gap states in other thin film semiconductors.
Application of spectroscopic phase modulated ellipsometry (PME) to study both ultrafast and slow processes of interaction of silane (SiH4) with thin film Pd, and to the investigation of the growth kinetics of a-Si:H films deposited by rf glow discharge under UV light irradiation are presented. As compared to other ellipsometric techniques like rotating analyzer ellipsometry (RAE), the phase modulation uses a high frequency of about 50 kHz provided by a photoelastic modulator. Thus, PME allows one to reach 1 - 5 ms time resolution which permits faster real-time measurements than RAE. This remarkable feature of PME makes it particularly suitable for in-situ applications. Changes of optical properties of Pd thin films exposed to SiH4 at different fluxes are monitored by in situ single wavelength ellipsometry in the case of high fluxes which lead to ultrafast process and by in situ spectroscopic ellipsometry at small fluxes and slow kinetics. The study reveals a complicated character of the process which depends on initial flux of silane and leads to formation of Pd disilicide, Pd hydride, and an intrinsic porosity. A qualitative model of the process is proposed.
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