Synthesis and properties of novel semiconductors and semiconductor nanostructures using energetic beams

Pulsed laser melting for new materials synthesis

We use Pulsed Laser Melting (PLM) processes, illustrated in Fig. 1, in the fabrication of supersaturated semiconductors, some of which cannot be fabricated by any other method.  The solid solubility limit determines the maximum impurity concentration that can be present in a semiconductor grown near equilibrium. PLM of layers of several hundred of nanometers of thickness creates temperature gradients so steep that, once the laser pulse ends, the resolidifying crystal grows at speeds of several meters per second! Under these conditions, elements that normally refuse to join the crystal cannot run away fast enough:  solute trapping can result in a single crystal with dopant concentrations several orders above the solid solubility limit. Our earlier work established the laws governing composition selection by a growing solid [Refs. mja026, mja125]. Now we are focusing on using this understanding to attempt develop new semiconductors with potential applications in light emission, detection, photovoltaics, and spintronics.

Fig 1. Paradigm for forming novel alloys, doping levels, and patterns by ion implantation and Pulsed Laser Melting induced rapid solidification

Intermediate band materials based on supersaturated semiconductors

The intermediate band solar cell (IBSC) is a promising device based on the intermediate band (IB) concept that has been proposed within the third generation of photovoltaics to exceed the efficiency of conventional solar cells. An IB material is characterized by the existence of an electronic energy band of allowed states within the conventional band gap that enables the optoelectronic conversion of sub-band gap photons. In this process, one photon pumps an electron from the valence band (VB) to the IB and a second photon pumps an electron from the IB to the conduction band (CB) (Fig. 2). To permit the occurrence of both processes, the IB must be a partially-filled, or metallic, band. Because the process described occurs in addition to the ordinary pumping of electrons from the VB to the CB by photons with energy greater than that of the band gap, the solar cell photocurrent could be higher than in the case of a conventional design.

We are interested in the potential of supersaturated semiconductors with deep impurities to form intermediate band semiconductors. 

Fig 2. Diagram of the different optical transitions in a semiconductor with an intermediate band.

Insulator-to-metal transition in hyperdoped silicon

We have shown that "hyperdoping" silicon with the "chalcogens" sulfur or selenium at the level of about 1 atomic percent causes an insulator-to-metal transition [Ref. mja211]. Comparison of theory to experiment suggests that this is caused by the formation of an impurity band from the chalcogen impurity states in the middle of the gap, when the orbitals overlap sufficiently for delocalization. However, the band appears to be completely occupied, and the insulator-to-metal transition appears to occur when the band gets so broad that it crosses the conduction band, as shown in Fig. 3 [Ref. mja219]. 

Fig 3.Intermediate band width vs dopant concentration. If the band is completely filled, the insulator-to-metal transition occurs when the top of the intermediate band crosses the conduction band edge. Maximum theoretical efficiency of a photovoltaic device is tabulated for normal, "single junction" cells and for ideal intermediate band solar cells (IBSCs).

We are pursuing several approaches to forming a partially-filled intermediate band in the band gap to better enable optoelectronic conversion. For the IBPV effect, we have developed a procedure for vetting heavily doped semiconductors against a figure of merit for IBPV performance: the ratio of the recombination lifetime of the photogenerated carriers to the transit time for the carriers to exit the IB material in the p-IB-n device structure shown in the lower-right of Fig. 3 [Ref. mja239]. We determined that this ratio is an order of magnitude too small for sulfur in silicon to be promising for IBPV but this points the way in the search for better materials [Ref. mja239].

It turns out to be easier to obtain sub-bandgap photodetection, i.e. conversion of photons to an electrical signal when you also put in electrical energy, as discussed below. Once we can do that readily, we may be able to find ways of modifying the devices so we can be taking energy out, i.e. photovoltaics.   

Infrared photodetectors based on hyperdoped silicon

We have been developing hyperdoped silicon alloys with optoelectronic conversion and photodetection in the near and mid infrared region. This could permit silicon to replace those much more expensive compound semiconductors that are currently used in night vision imaging arrays and in fiberoptic communications. It would also permit the much more ready integration of these functions with microprocessors made of silicon microelectronics.

With chalcogen hyperdoping we have demonstrated a slightly extended amount of infrared response (i.e. slightly farther out into the IR than normal silicon), along with substantial gain, in silicon photodetectors [Ref. mja215]. In soon-to-be published work we have shown that other impurities permit photodetection all the way out to wavelengths of about 1.8 microns, which transcends the 1.55 micron "sweet spot" for fiberoptic communications. [Ref. mja241]

Lateral patterning of the band structure with PLM

Lateral Patterning of conduction band edge in Ga(As,N) enables new device geometries [Ref. mja189]. 

BEEM image and I-V of GaNAs dot in GaAs

Point defect engineered Si sub-bandgap light-emitting diode

We can get a sharp sub-bandgap emission line out of native point defects manipulated in silicon through ion implantation and PLM [Ref. mja175]. The emission is quenched above liquid nitrogen temperature because of the position of the defect within the band gap, but other defects may enable room-temperature emission [Ref. mja196]. 

Point defect engineered sub-bandgap Si LED

Relevant publications

mja026. M.J. Aziz and T. Kaplan, “Continuous growth model for alloy solidification”, Acta Metallurgica et Materialia 36, 2335-2347 (1988).  This paper develops the Continuous Growth Model, in its final, successful, form, for the kinetics of rapid solidification. It answers two questions: (a) given a liquid composition and a solidification speed, what solid composition grows from it?; (b) given a liquid composition and interface temperature, how fast does the solid grow?  Click here to see the paper.

mja075. R. Reitano, P.M. Smith and M.J. Aziz, “Solute Trapping of Group III, IV and V Elements in Silicon by Aperiodic Stepwise Growth Mechanism”, Journal of Applied Physics 76, 1518-1529 (1994). In this paper we show that the dependence of solute trapping on the crystallographic orientation of the crystal-melt interface is sharply peaked near {111}, and is explained by the rapid lateral growth of {111} terraces through an extension of the Continuous Growth Model.  Click here to see the paper.

mja125. J.A. Kittl, P.G. Sanders, M.J. Aziz, D.P. Brunco and M.O. Thompson, “Complete Experimental Test for Kinetic Models of Rapid Alloy Solidification”, Acta Materialia 48, 4797-4811 (2000). This paper reported the first quantitative test of rapid solidification models in the case of non-dilute solutions, where the predictions of the models can be distinguished. The Continuous Growth Model Without Solute Drag is the winner.  Click here to see the paper.

mja211. M.T. Winkler, D. Recht, M.-J. Sher, A.J. Said, E. Mazur and M.J. Aziz, “Insulator to Metal Transition in Sulfur-Doped Silicon”, Phys. Rev. Lett. 106, 178701 (2011). In this paper we demonstrated an insulator-to-metal transition in silicon hyperdoped with sulfur by PLM. Above a sulfur concentration of about 0.7 atomic percent the resistance remains finite as absolute zero is approached -- the signature of a metal.  Click here to see the paper.

mja219. E. Ertekin, M.T. Winkler, D. Recht, A.J. Said, M.J. Aziz, T. Buonassisi, and J.C. Grossman, “Insulator-to-Metal Transition in Selenium-Hyperdoped Silicon: Observation and Origin”, Phys. Rev. Lett. 108, 026401 (2012). In this paper we demonstrated experimentally an insulator-to-metal transition in silicon hyperdoped with selenium, and compared to calculations of the band structure using Density Functional Theory. The comparison indicated a band of delocalized states in the gap was broadening to cross the conduction band edge at the insulator-to-metal transition. The agreement of theoretical and experimental optical absorption spectrum corroborated the interpretation.  Click here to see the paper.

mja239. J.T. Sullivan, C.B. Simmons, J.J. Krich, A.J. Akey, D. Recht, M.J. Aziz and T. Buonassisi, "Methodology for vetting heavily doped semiconductors for intermediate band photovoltaics: A case study in sulfur hyperdoped silicon", J. Appl. Phys. 114, 103701 (2013). In this paper we developed a methodology for vetting heavily doped semiconductors for their utility in intermediate band photovoltaic devices. We made the appropriate measurements for sulfur in silicon and determined that the carrier lifetime is at least an order of magnitude too small to be useful for IBPV.  

mja215. A.J. Said, D. Recht, J.T. Sullivan, J.M. Warrender, T. Buonassisi, P.D. Persans, and M.J. Aziz, “Extended Infrared Photoresponse and Gain in Chalcogen-Supersaturated Silicon Photodiodes”, Appl. Phys. Lett. 99, 073503 (2011). In this paper we reported the first extension of silicon photodetection into the sub-bandgap regime by hyperdoping with sulfur or selenium, and showed that the observed gain is inconsistent with the most common gain mechanisms.  Click here to see the paper.

mja241. J.P. Mailoa, A.J. Akey, C.B. Simmons, D. Hutchinson, J. Mathews, J.T. Sullivan, D. Recht, M.T. Winkler, J.S. Williams, J.M. Warrender, P.D. Persans, M.J. Aziz and T. Buonassisi, "Room-temperature sub-band gap optoelectronic response of hyperdoped silicon", Nature Communications, in press (2013). In this paper we demonstrated that a suitable choice of impurity gives us mid-gap states that enable room-temperature photodetection well out into the IR, past the fiberoptic communications window.  

mja189. T. Kim, K. Alberi, O.D. Dubon, M.J. Aziz, and V. Narayanamurti “Composition Dependence of Schottky Barrier Heights and Band Gap Energies of GaNxAs1-x synthesized by ion implantation and pulsed laser melting”, Journal of Applied Physics 104, 113722 (2008). In this paper we show that we can pattern dots of Ga(N,As) in a layer of GaAs. The dots have a lower conduction band edge and so they can be imaged by Ballistic Electron Emission Microscopy. This opens up the possibility of arbitrarily patterning the conduction band, where free electrons will be confined.  Click here to see the paper.

mja175. J. Bao, M. Tabbal, T. Kim, S. Charnvanichborikarn, J.S. Williams, M.J. Aziz and F. Capasso, “Point Defect Engineered Si Sub-Bandgap Light-emitting Diode”, Optics Express 15, 6727 (2007).  In this paper we manipulated point defects injected by ion implantation and not annihilated by PLM to make a sub-bandgap LED.  Click here to see the paper.

mja196. D. Recht, F. Capasso, and M.J. Aziz, “On the Temperature Dependence of Point-Defect-Mediated Luminescence in Silicon”, Applied Physics Letters 94, 251113 (2009).  In this paper we developed a model of the temperature-dependence of the point defect luminescence in mja175, based on the kinetics of carrier emission and capture. It points the direction for extending the temperature for measurable optical emission up to and beyond room temperature.  Click here to see the paper.