The 25th International Conference on Amorphous and Nano-crystalline Semiconductors
August 18–23, 2013 Toronto, Ontario Canada
Nano- and Microcrystalline Silicon: Growth and Characterization III
Chair: Ruud Schropp, Energy Research Center of the Netherlands and Eindhoven University of Technology
Co Heavily Doped Silicon: The Possible Intermediate Band Material for PV Application
University of Chinese Academy of Sciences, Beijing 100049, China
The intermediate bands (IB) within the band gap of a semiconductor have been proposed to enhance the solar cell efficiency up to 63% in ideal conditions by additional absorption of the low energy photons related to the VB-IB and IB-CB transitions. Heavily doping of deep level impurities into a semiconductor with the concentration exceeding the Mott limit is one approach to achieve the IB material.
In this paper, the IB of deep impurity Co in silicon is reported. Co supersaturated Si samples with the concentration above the Mott limit (5.9x1019 cm–3) were prepared by ion implantation and pulse laser melting. The Raman scattering and the Grazing angle XRD measurements indicate that samples were well crystallized without any Co-Si alloy peaks.
The current-voltage curves of the heavily Co-implanted samples present a rectifying behavior when measured with sandwich structure, which is related to the contact between the Co implanted layer (CIL) and the Si substrate. The temperature dependences of the resistance for the CIL/Si structures show that the resistivity of the CIL is two orders of magnitude lower than that of the substrate at low temperature for the samples with high Co concentration. Moreover, the CIL resistivity almost does not change with the temperature which means an insulator-to-metal transition probably happens in the low temperature range. The transition occurs in the Co concentration corresponding to the Mott limit. Based on a simplified two-layer structure consisting of an intermediate band layer and a substrate layer and a simple Schottky junction model, the temperature dependences of the resistance are well fitted which proves the existence of IB. The fitting IB location is at about 0.5 eV below the conduction band edge of Si. The temperature dependences of the Hall mobility also show a two-layer characteristic, which is coincident with the resistance results.
n-type amorphous silicon thin film was deposited onto the Co implanted p-type Si substrate to form a solar cell. A shoulder appears in the quantum efficiency spectrum in the wavelength range of 1100–1500 nm for the solar cell, which indicates the contribution of IB in the implanted layer.
Keywords: temperature dependence of resistivity, intermediate band, Co heavily doping, silicon
Surface-Doping and Quantum Confinement Effects in Si Nanocrystals Observed by Scanning Tunneling and Photocurrent Spectroscopy
1. Racah Institute of Physics and the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
2. Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada
3. NRC-National Institute for Nanotechnology, Edmonton, Alberta, T6G 2M9, Canada
Semiconductor nanocrystals (NCs) have a very promising role in future opto-electronic and photonics applications. This is partly due to the effect of quantum confinement on the semiconductor band-gap, allowing tuning of the optical absorption and emission at will. There is, however, a longstanding controversy regarding the applicability of optical methods for an accurate determination of the band-gap in Si NCs, due to, e.g. the effect of surface defects. In this work we present a systematic study utilizing scanning tunneling spectroscopy, photoluminescence (PL), and photocurrent spectroscopy on single Si-NCs and their arrays. The energy gaps extracted from the tunneling spectra acquired from single colloidal Si-NCs increase with decreasing NC size, manifesting the effect of quantum confinement, irrespective of the functionalizing surface group. This is consistent with the blue-shift revealed by PL from dodecyl functionalized colloidal Si-NCs. The tunneling spectra measured on NCs functionalized upon exposure to NH4Br and allylamine reveal band-edge shifts toward higher energies, akin to p-type doping effect. This behavior is accounted for by the combined contributions of the ligands dipole moments and charge transfer between a Si-NC and its surface groups. Concomitantly, size-independent PL spectra, which cannot be associated with the NC band-gaps, were observed for the latter Si-NCs. The effect of quantum confinement was observed also for ensembles of Si-NCs embedded in a SiO2 matrix, even above the percolation threshold of the Si-NC phase, a regime where the PL is quenched due to electron delocalization. Here, quantum confinement manifested itself by a blue-shift of the onset of the photocurrent spectra with decreasing NC size. In addition, these spectra revealed a strong dependence of surface-recombination induced photocurrent-quenching on Si-NC size.
Keywords: nanocrystalline silicon, scanning tunneling spectroscopy, photocurrent spectroscopy, quantum confienement
Charge Injection and Retention in SiC/Si-Nanocrystals/SiC Sandwiched Structures Prepared by Laser Crystallization Technique
National Laboratory of Solid State Microstructures and School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China
Silicon nanocrystals (Si-NCs) have been attracted much attention since they can be used in many kinds of devices such as non-volatile memories and light emitting diodes. In order to further improve the device performance, it is important to give a deep insight on the charge injection and transportation process in Si-NCs based materials.
In our previous work, we prepared Si-NCs/amorphous SiC multilayers by thermally annealing amorphous Si/SiC stacked structures and studied their optical properties. The size-dependent luminescence behaviors have been observed. Compared with SiO2, amorphous SiC has a low band gap as well as the low barrier offset between Si and SiC which can make the devices based on Si NCs/SiC materials work at relatively low voltage. In this work, we use KrF pulsed excimer laser with wavelength of 248 nm to crystallize an ultrathin amorphous Si (4 nm) film and get SiC/Si-NCs/SiC sandwiched structures on p-type Si substrates. In order to study the carrier injection process, Atomic Force Microscopy (AFM)/Kelvin Force Microscopy (KFM) is used to characterize the surface morphologies and surface potential of the samples. AFM topography demonstrates the formation of Si-NCs with area density of 5x1010 cm–2 induced by laser crystallization. Furthermore, the charge injection process is realized by exerting a bias on the conductive AFM tip (Pt-Ir coated). A MIS capacity model is employed to extract the information of charging density from the potential shift data. More holes are injected into Si-NCs than electrons under a tip-sample bias of +3 V and –3 V respectively. Unusually, holes instead of electrons are injected when the tip-sample bias is –5 V. This phenomenon strongly implies a bipolar injection behavior where opposite carriers can be injected from the tip and substrate separately. Therefore the KFM measurements actually detect a 'net' injection quantity. The result is also confirmed by the CAFM current-voltage curve. Under high voltage condition, a stable leakage current with FN tunneling mechanism is detected. The temporal decay of injected carriers shows that holes stay much longer than electrons in Si-NCs, which can be attributed to the larger band offset in valence band than that in conduction band of Si-NCs/SiC interface. No lateral charge dispersion is detected which suggests that the Si nanocrystals are well isolated and the injected charges escape to the substrate through the tunneling SiC layer. And the carrier retention time is indeed reduced with a thinner tunneling SiC layer, which confirms the charge retention mechanism.
Keywords: Si nanocrystals, SiC, KFM, charge injection
Structural Fingerprints in Temperature-dependent Hall Measurements after Ion Implantation Amorphization and Recrystallization of InGaAsP/InP
1. Institut Interdisciplinaire d'Innovation Technologique (3IT), Université de Sherbrooke, Québec, Canada, J1K 2R1
2. Département de physique, Université de Montréal, Montréal, Québec, Canada, H3C 3J7
3. Regroupement québécois sur les matériaux de pointe (RQMP), Québec, Canada
Novel photoconductive materials were produced after solid phase recrystallization of InGaAsP/InP heterostructures amorphized by ion implantation. High fluence, multiple-energy (up to 2.5 MeV) Fe implantations were carried out in order to uniformly damage and induce amorphization throughout the 1.5 μm thick InGaAsP epilayers. The band gap of InGaAsP was 0.79 eV. Recrystallization was obtained by rapid thermal processing at various temperatures between 300°C and 750°C. These materials have shown ultrafast photocarrier recombination dynamics and have been used for pulsed terahertz spectroscopy applications at 1550 nm. Here, we report on their temperature-dependent Hall transport properties, measured between 85K and 400K. Recrystallized InGaAsP layers were well compensated when thermal processing was done at 500°C. Their effective carrier density was 1013 cm–3 at room temperature, with a thermal activation energy of 0.33 eV. We assigned this insulating behavior to the presence of Fe and to the recrystallized microstructure itself. Their temperature-dependent conductivity and Hall data showed a transport mechanism involving mixed conductivity. A parallel conduction model, combining extended band conduction and hopping-like transport was able to fit the data. Furthermore, signs of barrier-controlled conduction were found when analyzing Hall mobility for the whole set of recrystallization temperature. These fingerprints suggest that optical and electrical properties of these materials have a microcrystalline origin.
Keywords: InGaAsP, Fe ion implantation, amorphization, recrystallization, Hall effect
Crystallized Silicon Quantum dots and Nanocrystalline Structures: Experimental Characterization and Atomistic Simulations
1. New Technology Research Centre, University of West Bohemia, Univerzitni 8, 306 14, Pilsen, Czech Republic
2. Department of Physics & Astronomy, Ames Laboratory, Microelectronics Research Center & Department of Electrical & Computer Engineering, Iowa State University, Ames, IA 50011
3. Department of Physics, University of Science and Technology of China, Hefei, China
We have synthesized silicon quantum dot structures from thin film silicon multi-layers with an annealing procedure that is integrated in a x-ray diffraction (XRD) set-up. This set-up makes it possible for real-time monitoring of the formation phases of the quantum dots. We study the microstructure of crystallized amorphous silicon layers through experimental measurements combined with atomistic simulations of realistic nanocrystalline silicon (nc-Si) models. We have deposited multi-layers of hydrogenated amorphous silicon (a-Si:H) with stoichiometric dielectric layers of SiO2 using plasma enhanced chemical vapor deposition on c-Si and Corning glass substrates, with thicknesses ranging from 600 nm to 900 nm. The a-Si:H and SiO2 sub-layers had uniformly alternating thicknesses in each multi-layer stack and were investigated for thicknesses of 20, 15, 10 and 5 nm. The multi-layer stack was recrystallized in vacuum at high temperature annealing, up to 1100°C. The crystallized structure consists of silicon quantum dots and nanocrystalline structures embedded in an amorphous matrix. The structure of the quantum dot/nanocrystalline region were characterized through XRD, Raman and Fourier transform infra-red measurements. Electron microscopy was utilized to study the real space structure of the amorphous and recrystallized regions and the quantum dot sizes. We have achieved quantum dot diameters of up to 5 nm and we will discuss the different stages of the dot formation and how the dot sizes can be controlled in our experimental synthesis.
We compare our experimental nanocrystalline and quantum dot structures with classical molecular dynamics simulations. The crystallized silicon structures with large crystalline filling fractions exceeding 40% have been simulated with a robust classical molecular dynamics technique. In this technique an inhomogeneous temperature distribution was utilized to disorder the boundaries of the cell, and the crystallite size and volume fraction varied by controlling the time over which the simulation was performed. The simulated nc-Si cells have in excess of 10,000 atoms. nc-Si with both three-dimensional crystallites and two-dimensional conical crystallites embedded in an amorphous silicon background, have been generated. Hydrogen atoms have been introduced into these structures to passivate dangling bond defects. The crystalline filling fractions and structural order of nc-Si with varying crystallite fractions will be compared with our Raman and XRD measurements. The interfacial structure between crystalline and amorphous regions will be described and compared with measurement.
We will discuss how such recrystallized structures can be utilized in quantum dot solar cells.
Keywords: multi-layers, quantum dots, nanocrystalline silicon, x-ray diffraction, atomistic simulations