Bonna Newman

Reactivation of sub-bandgap absorption in chalcogen-hyperdoped silicon

Silicon doped with nonequilibrium concentrations of chalcogens using a femtosecond laser exhibits near-unity absorption of sub-bandgap photons to wavelengths of at least 2500 nm. Previous studies have shown that sub-bandgap absorptance decreases with thermal annealing up to 1175 K and that the absorption deactivation correlates with chalcogen diffusivity. In this work, we show that sub-bandgap absorptance can be reactivated by annealing at temperatures between 1350 and 1550 K followed by fast cooling (>50 K/s). Our results suggest that the defects responsible for sub-bandgap absorptance are in equilibrium at high temperatures in hyperdoped Si:chalcogen systems.

Soft x-ray emission spectroscopy studies of the electronic structure of silicon supersaturated with sulfur

We apply soft x-ray emission spectroscopy (XES) to measure the electronic structure of crystalline silicon supersaturated with sulfur (up to 0.7 at. %), a candidate intermediate-band solar cell material. Si L2,3 emission features are observed above the conventional Si valence band maximum, with intensity scaling linearly with S concentration. The lineshape of the S-induced features change across the insulator-to-metal transition, indicating a significant modification of the local electronic structure concurrent with the change in macroscopic electronic behavior. The relationship between the Si L2,3XESspectral features and the anomalously high sub-band gap infrared absorption is discussed.

Extended X-ray absorption fine structure spectroscopy of selenium-hyperdoped silicon

Silicon doped with an atomic percent of chalcogens exhibits strong, uniform sub-bandgap optical absorptance and is of interest for photovoltaic and infrared detector applications. This sub-bandgap absorptance is reduced with subsequent thermal annealing indicative of a diffusion mediated chemical change. However, the precise atomistic origin of absorptance and its deactivation is unclear. Herein, we apply Se K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy to probe the chemical states of selenium dopants in selenium-hyperdoped silicon annealed to varying degrees. We observe a smooth and continuous selenium chemical state change with increased annealing temperature, highly correlated to the decrease in sub-bandgap optical absorptance. In samples exhibiting strong sub-bandgap absorptance, EXAFS analysis reveals that the atoms nearest to the Se atom are Si at distances consistent with length scales in energetically favorable Se substitutional-type point defect complexes as calculated by d...

Retrograde Melting and Internal Liquid Gettering in Silicon

COMMUNICATION Retrograde Melting and Internal Liquid Gettering in Silicon By Steve Hudelson , Bonna K. Newman , Sarah Bernardis , David P. Fenning , Mariana I. Bertoni , Matthew A. Marcus , Sirine C. Fakra , Barry Lai , and Tonio Buonassisi * Control of metal impurities has proven essential for developing modern semiconductor-based materials and devices. The prop- erties of high-performance integrated circuit, photovoltaic, and thermoelectric devices are tailored by the intentional introduc- tion of dopant species, as well as the removal and passivation of detrimental impurities. [ 1 , 2 ] In addition, the speed and uni- formity of several common semiconductor growth methods, including bulk crystal and vapor-liquid-solid (VLS) growth, are regulated by impurity-semiconductor interactions. [ 3 , 4 ] Precise control over impurity chemical states and spatial distributions requires a deep fundamental understanding of the thermodynamics and kinetics regulating impurity phase and transport. Impurity engineering in semiconductors typi- cally involves thermal annealing, as impurity solubility and diffusivity increase exponentially with temperature. However, because of the lack of suitable analytical tools for studying sub-micron-scale distributions of fast-diffusing impurities at elevated temperatures, the vast majority of experimental inves- tigations so far have been conducted at room temperature. As a result, much remains to be explored concerning fundamental impurity-semiconductor reactions at realistic processing temperatures. It was recently proposed [ 5 ] that certain silicon-impurity sys- tems can undergo melting upon cooling, a phenomenon known as retrograde melting . The controlled creation of liquid metal- silicon droplets within or on the surface of a silicon matrix of arbitrary shape could provide novel opportunities to engineer semiconductor-based systems via solid-liquid and vapor-liquid segregation. The phenomenon of retrograde melting, whereby a liquid phase forms from a solid phase upon cooling , has been observed and studied in several organic and inorganic systems, including Fe-Zr [ 6 ] and Mg-Fe-Si-O. [ 7 ] One common pathway [ ∗ ] S. Hudelson, [+] Dr. B. K. Newman, S. Bernardis, D. P. Fenning, Dr. M. I. Bertoni, Prof. T. Buonassisi Massachusetts Institute of Technology Cambridge, Massachusetts, 02139 (USA) E-mail: buonassisi@mit.edu Dr. M. A. Marcus, S. C. Fakra Advanced Light Source Lawrence Berkeley National Laboratory Berkeley, California, 94720 (USA) Dr. B. Lai Advanced Photon Source Argonne National Laboratory Argonne, Illinois, 60439 (USA) [ + ] Present address: 1366 Technologies, Lexington, MA 02421, USA for this process to occur is via the catatectic reaction, occur- ring at an invariant point on a binary phase diagram involving transformation from Solid → Solid + Liquid. [ 8 ] Many binary sys- tems exhibit such an invariant point, [ 9 ] including Ag-In, Cu-Sn, Fe-Mn, and Fe-S, [ 10 ] but very few are semiconducting mate- rials. [ 11 ] Retrograde melting in most common silicon-impurity systems cannot occur by this pathway, as these systems do not possess a catatectic point. [ 11 ] A second pathway for retrograde melting has been observed in the ternary Sb-Bi-Te system, wherein decreasing solubility of Te in Sb 2 Te 3 with decreasing temperature can lead to supersat- uration of Te and formation of liquid droplets at temperatures above the eutectic temperature. [ 12 ] We propose that a similar pathway could also produce retrograde melting in binary semiconductor-impurity systems that exhibit retrograde solu- bility. Due to the high enthalpy of formation of point defects in certain semiconductors, the solid solubility of an impu- rity within the crystal structure increases with temperature, reaching a maximum well above the eutectic temperature. Many dissolved elements in silicon demonstrate this property, [ 13 ] including many of the 3d transition metals such as iron, copper, and nickel. [ 14 ] It is hypothesized that retrograde solubility can lead to retrograde melting, [ 5 ] if supersaturation occurs at a tem- perature above the eutectic temperature (as demonstrated in Figure 1 a ). To study temperature-dependent silicon-impurity reactions at the micro-scale, we carried out synchrotron-based hard X-ray microprobe experiments at high temperatures (up to 1500 ° C). We adapted an in situ microscope hot stage (Linkam TS1500) at beamlines 10.3.2 at the Advanced Light Source [ 15 ] and 2-ID-D at the Advanced Photon Source. [ 16 ] X-ray fluorescence microscopy ( μ -XRF) mapping was used to investigate the spatial distribu- tion of transition metal-rich particles as small as 50 nm [ 17 , 18 ] in silicon matrices. The chemical state of precipitated impurities detected by μ -XRF was determined by X-ray absorption micro- spectroscopy ( μ -XAS). [ 18 ] To verify that μ -XAS can distinguish between liquid and solid phases in metal-Si systems, we prepared a standard sample (see Experimental , sample 1) consisting of a thin layer ( ∼ 1 μ m) of e-beam evaporated Cu, Ni, and Fe sandwiched between a mc-Si wafer and a thin piece ( < 15 μ m) of monocrys- talline Czochralski Si (CZ-Si). The sample was then heated to 1045 ° C, well above the Cu-Si and Ni-Si eutectic temperatures, to ensure a liquid metal-silicon mixture. μ -XRF mapping of the standard at 1045 ° C revealed that the previously continuous film had dewetted, suggesting the presence of a high-temperature liquid state. After cooling the sample to room temperature, a visual inspection revealed that the Si cap layer was fused to the

Magnetic Trapping of Silver and Copper, and Anomalous Spin Relaxation in the Ag-He System

We have trapped large numbers of copper (Cu) and silver (Ag) atoms using buffer-gas cooling. Up to 3 x 10{12} Cu atoms and 4 x 10{13} Ag atoms are trapped. Lifetimes are as long as 5 s, limited by collisions with the buffer gas. Ratios of elastic to inelastic collision rates with He are >or=10{6}, suggesting Cu and Ag are favorable for use in ultracold applications. The temperature dependence of the Ag-3He collision rate varies as T;{5.8+/-0.4}. We find that this temperature dependence is inconsistent with the behavior predicted for relaxation arising from the spin-rotation interaction, and conclude that the Ag-3He system displays anomalous collisional behavior in the multiple-partial wave regime. Gold (Au) was ablated into 3He buffer gas, however, atomic Au lifetimes were observed to be too short to permit trapping.

Zeeman Relaxation of Cold Atomic Iron and Nickel in Collisions with \(^3\)He

We have measured the ratio γ of the diffusion cross section to the angular momentum reorientation cross section in the colliding Fe-\(^3\)He and Ni-\(^3\)He systems. Nickel (Ni) and iron (Fe) atoms are introduced via laser ablation into a cryogenically cooled experimental cell containing cold (<1 K) \(^3\)He buffer gas. Elastic collisions rapidly cool the translational temperature of the ablated atoms to the \(^3\)He temperature. γ is extracted by measuring the decays of the atomic Zeeman sublevels. For our experimental conditions, thermal energy is comparable to the Zeeman splitting. As a result, thermal excitations between Zeeman sublevels significantly impact the observed decay. To determine γ accurately, we introduce a model of Zeeman state dynamics that includes thermal excitations. We find γ\(_{Ni-^{3}He}=5×10^3\) and γ\(_{Fe-^{3}He}\leq3×10^3\) at 0.75 K in a 0.8 T magnetic field. These measurements are interpreted in the context of submerged shell suppression of spin relaxation, as studied previously in transition metals and rare earth atoms.

Detection of ZnS phases in CZTS thin-films by EXAFS

Copper zinc tin sulfide (CZTS) is a promising Earth-abundant thin-film solar cell material; it has an appropriate band gap of ∼1.45 eV and a high absorption coefficient. The most efficient CZTS cells tend to be slightly Zn-rich and Cu-poor. However, growing Zn-rich CZTS films can sometimes result in phase decomposition of CZTS into ZnS and Cu2SnS3, which is generally deleterious to solar cell performance. Cubic ZnS is difficult to detect by XRD, due to a similar diffraction pattern. We hypothesize that synchrotron-based extended X-ray absorption fine structure (EXAFS), which is sensitive to local chemical environment, may be able to determine the quantity of ZnS phase in CZTS films by detecting differences in the second-nearest neighbor shell of the Zn atoms. Films of varying stoichiometries, from Zn-rich to Cu-rich (Zn-poor) were examined using the EXAFS technique. Differences in the spectra as a function of Cu/Zn ratio are detected. Linear combination analysis suggests increasing ZnS signal as the CZTS films become more Zn-rich. We demonstrate that the sensitive technique of EXAFS could be used to quantify the amount of ZnS present and provide a guide to crystal growth of highly phase pure films.

Impact of defect type on hydrogen passivation effectiveness in multicrystalline silicon solar cells

In this work we examine the effectiveness of hydrogen passivation at grain boundaries as a function of defect type and microstructure in multicrystalline silicon. We analyze a specially prepared solar cell with alternating mm-wide bare and SiNx-coated stripes using laser beam-induced current (LBIC), electron backscatter diffraction (EBSD), synchrotron-based X-ray fluorescence microscopy (μ-XRF), and defect etching to correlate pre- and post-hydrogenation recombination activity with grain boundary character, density of iron-silicide nanoprecipitates, and dislocations. This study reveals that the microstructure of boundaries that passivate well and those that do not differ mostly in the character of the dislocations along the grain boundary, while iron silicide precipitates along the grain boundaries (above detection limits) were found to play a less significant role.

Synchrotron-Based Investigation of Metal Impurity Diffusion in Silicon Solar Cell Materials

This presentation details synchrotron-based diffusion experiments investigating the impact of high-temperature processing on the distribution of transition metal impurities in the bulk crystal and along grain boundaries in silicon solar cell materials. Article not available.

Synchrotron-Based X-Ray Absorption Spectroscopy applied to Novel Silicon Solar Cell Material

After doping silicon well above the solubility limit with selenium atoms using femto-second laser pulses, we observe strong absorption of photons below the band gap of silicon. This enhanced absorption allows the possibility of increasing the photocurrent and the efficiency of silicon solar cells. We use synchrotron-based X-ray absorption spectroscopy to probe the chemical state of selenium to understand the material’s unique absorption properties. Article not available.