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