B. R. Tull, M. T. Winkler, and E. Mazur. 2009. “The role of diffusion in broadband infrared absorption in chalcogen-doped silicon.” Appl. Phys. A. Publisher's VersionAbstract
    Sulfur doping of silicon beyond the solubility limit by femtosecond laser irradiation leads to near-unity broadband absorption of visible and infrared light and the realization of silicon-based infrared photodetectors. The nature of the infrared absorption is not yet well understood. Here we present a study on the reduction of infrared absorptance after various anneals of different temperatures and durations for three chalcogens (sulfur, selenium, and tellurium) dissolved into silicon by femtosecond laser irradiation. For sulfur doping, we irradiate silicon in SF6 gas; for selenium and tellurium, we evaporate a film onto the silicon and irradiate in N2 gas; lastly, as a control, we irradiated untreated silicon in N2 gas. Our analysis shows that the deactivation of infrared absorption after thermal annealing is likely caused by dopant diffusion. We observe that a characteristic diffusion lengthcommon to all three dopantsleads to the reduction of infrared absorption. Using diffusion theory, we suggest a model in which grain size of the re-solidified surface layer can account for this characteristic diffusion length, indicating that deactivation of infrared absorptance may be caused by precipitation of the dopant at the grain boundaries.
    M. T. Winkler. 2009. “Non-Equilbrium Chalcogen Concentrations in Silicon: Physical Structure, Electronic Transport, and Photovoltaic Potential”. Publisher's VersionAbstract
    This thesis explores the structure and properties of silicon doped with chalcogens beyond the equilibrium solubility limit, with a focus on the potential presence of an impurity band and its relevance to photovoltaics. The investigations that we report here shed new light on the electronic role of sulfur dopants in particular, and also provide new evidence of a semiconductor-to-metal transition consistent with the formation of an electron-conducting impurity band. The thesis is divided into three primary studies. First, we describe doping silicon with a single fs-laser pulse. We find that irradiation above the melting threshold is sufficient for doping a thin layer of silicon to non-equilibrium sulfur concentrations. Next, we explore the interaction of many fs-laser pulses with a silicon substrate. Temperature-dependent electronic transport measurements indicate metallic conduction, while a form of Fermi level spectroscopy and optical absorption data indicate the presence of an impurity band located 200 − 300 meV below the conduction band edge. Third, we investigate silicon doped to non- equilibrium concentrations using a different technique: ion-implantation followed by pulsed laser melting and crystal regrowth. We determine one of the sulfur states present at low sulfur dose. Additional transport measurements point to the presence of a semiconductor-to- metal transition at sulfur doses corresponding to implanted sulfur concentrations just above 10^20 cm−3 . Finally, in the appendices of this thesis, we describe methods to laser-dope silicon while avoiding the development of significant surface roughness that typically characterizes such samples. Additionally, we present the status of investigations into laser-doping silicon with selenium to non- equilibrium concentrations.
    M. Shen, J. E. Carey, C. H. Crouch, M. Kandyla, H.A. Stone, and E. Mazur. 2008. “High-density regular arrays of nanometer-scale rods formed on silicon surfaces via femtosecond laser irradiation in water.” Nano Leters, 8, Pp. 2087–2091. Publisher's VersionAbstract
    We report on the formation of high-density regular arrays of nanometer-scale rods using femtosecond laser irradiation of a silicon surface immersed in water. The resulting surface exhibits both micrometer-scale and nanometer-scale structures. The micrometer-scale structure consists of spikes of 5-10 µm width, which are entirely covered by nanometer-scale rods that are roughly 50 nm wide and that protrude perpendicularly from the micrometer-scale spikes. The formation of the nanometer-scale rods involves several processes: refraction of laser light in highly excited silicon, interference of scattered and refracted light, rapid cooling in water, and capillary instabilities.
    A. Serpengüzel, A. Kurt, I. Inanc, J. E. Carey, and E. Mazur. 2008. “Luminescence of black silicon.” J. Nanophoton., 2, Pp. 021770–9. Publisher's VersionAbstract
    Room temperature visible and near-infrared photoluminescence from black silicon has been observed. The black silicon is manufactured by shining femtosecond laser pulses on silicon wafers in air, which were later annealed in vacuum. The photoluminescence is quenched above 120 K due to thermalization and competing nonradiative recombination of the carriers. The photoluminescence intensity at 10K depends sublinearly on the excitation laser intensity confirming band tail recombination at the defect sites.
    B. R. Tull. 2007. “Femtosecond Laser Ablation of Silicon: Nanoparticles, Doping and Photovoltaics”. Publisher's VersionAbstract
    In this thesis, we investigate the irradiation of silicon, in a background gas of near atmospheric pressure, with intense femtosecond laser pulses at energy densities exceeding the threshold for ablation (the macroscopic removal of material). We study the resulting structure and properties of the material ejected in the ablation plume as well as the laser irradiated surface itself. The material collected from the ablation plume is a mixture of single crystal silicon nanoparticles and a highly porous network of amorphous silicon. The crystalline nanoparticles form by nucleation and growth; the amorphous material has smaller features and forms at a higher cooling rate than the crystalline particles. The size distribution of the crystalline particles suggests that particle formation after ablation is fundamentally different in a background gas than in vacuum. We also observe interesting structures of coagulated particles such as straight lines and bridges. The laser irradiated surface exhibits enhanced visible and infrared absorption of light when laser ablation is performed in the presence of certain elements–-either in the background gas or in a film on the silicon surface. To determine the origin of this enhanced absorption, we perform a comprehensive annealing study of silicon samples irradiated in the presence of three different elements (sulfur, selenium and tellurium). Our results support that the enhanced infrared absorption is caused by a high concentration of dopants dissolved in the lattice. Thermal annealing reduces the infrared absorptance of each doped sample at the same rate that dopants diffuse from within the polycrystalline grains in the laser irradiated surface layer to the grain boundaries. Lastly, we measure the photovoltaic properties of the laser irradiated silicon as a function of several parameters: annealing temperature, laser fluence, background gas, surface morphology and chemical etching. We explore the concept of using thin silicon films as the irradiation substrate and successfully enhance the visible and infrared absorption of films < 2 micrometers thick. Our results suggest that the incorporation of a femtosecond laser modified region into a thin film silicon device could greatly enhance its photovoltaic efficiency.
    T. R. Polte, M. Shen, J. Karavitis, M. Montoya, J. Pendse, S. Xia, E. Mazur, and D. E. Ingber. 2007. “Nanostructured magnetizable materials that switch cells between life and death.” Biomaterials, 28, Pp. 2783–2790. Publisher's VersionAbstract
    Development of biochips containing living cells for biodetection, drug screening and tissue engineering applications is limited by a lack of reconfigurable material interfaces and actuators. Here we describe a new class of nanostructured magnetizable materials created with a femtosecond laser surface etching technique that function as multiplexed magnetic field gradient concentrators. When combined with magnetic microbeads coated with cell adhesion ligands, a microarray of virtual adhesive islands that can support cell attachment, resist cell traction forces and maintain cell viability. A cell death (apoptosis) response can then be actuated on command by removing the applied magnetic field, thereby causing cell retraction, rounding and detachment. This simple technology may be used to create reconfigurable interfaces that allow users to selectively discard contaminated or exhausted cellular sensor elements, and to replace them with new living cellular components for continued operation in future biomedical microdevices and biodetectors. Keywords: magnetic particles, magnetic gradient concentrator, culture substrate, apoptosis, mechanical force, cell shape
    R. A. Myers, R. Farrell, A. Karger, J. E. Carey, and E. Mazur. 2006. “Enhancing near-infrared avalanche photodiode performance by femtosecond laser microstructuring.” Appl. Opt., 45, Pp. 8825–8831. Publisher's VersionAbstract
    A processing technique using femtosecond laser pulses to microstructure the surface of a silicon ava- lanche photodiode (APD) has been used to enhance its near-infrared (near-IR) response. Experiments were performed on a series of APDs and APD arrays using various structuring parameters and post- structuring annealing sequences. Following thermal annealing, we were able to fabricate APD arrays with quantum efficiencies as high as 58% at 1064 nm without degradation of their noise or gain performance. Experimental results provided evidence to suggest that the improvement in charge collec- tion is a result of increased absorption in the near-IR.
    Z. Huang, J. E. Carey, M. Liu, X. Guo, E. Mazur, and J. C. Campbell. 2006. “Microstructured silicon photodetector.” Appl. Phys. Lett., 89, Pp. 033506–033508. Publisher's VersionAbstract
    Photodetectors fabricated on microstructured silicon are reported. The photodetectors exhibited high photoresponse; at 3 V bias, the responsivities were 92 A/W at 850 nm and 119 A/W at 960 nm. At wavelengths longer than 1.1 m, the photodetectors still showed strong photoresponse. A generation- recombination gain mechanism has been proposed to explain the photoresponse of these photodiodes. From measurements of the noise current density, the calculated gain was approximately 1200 at 3 V bias.
    T. Baldacchini, J. E. Carey, M. Zhou, and E. Mazur. 2006. “Superhydrophobic surfaces prepared by microstructuring of silicon using a femtosecond laser.” Langmuir, 22, Pp. 4917–4919. Publisher's VersionAbstract
    Superhydrophobic surfaces exhibit contact angles with water that are larger than 150 and negligible difference in contact angle between the advancing and receding contact angles, the so-called contact angle hysteresis. In this paper, we present a novel and simple structuring process that uses intense femtosecond- laser pulses to create microstructured superhydrophobic surfaces with remarkable wetting characteristics.
    B. R. Tull, J. E. Carey, E. Mazur, J. McDonald, and S. M. Yalisove. 2006. “Surface morphologies of silicon surfaces after femtosecond laser irradiation.” Mat. Res. Soc. Bull., 31, Pp. 626–633. Publisher's VersionAbstract
    In this article, we present summaries of the evolution of surface morphology resulting from the irradiation of single-crystal silicon with femtosecond laser pulses. In the first section, we discuss the development of micrometer-sized cones on a silicon surface irradiated with hundreds of femtosecond laser pulses in the presence of sulfur hexafluoride and other gases. We propose a general formation mechanism for the surface spikes. In the second section, we discuss the formation of blisters or bubbles at the interface between a thermal silicon oxide and a silicon surface after irradiation with one or more femtosecond laser pulses. We discuss the physical mechanism for blister formation and its potential use as channels in microfluidic devices.
    M. A. Sheehy, B. R. Tull, C. M. Friend, and E. Mazur. 2006. “Chalcogen doping of silicon via intense femtosecond-laser irradiation.” Mat. Sci. Eng. B, 137, Pp. 289–294. Publisher's VersionAbstract
    We have previously shown that doping silicon with sulfur via femtosecond- laser irradiation leads to near-unity absorption of radiation from ultraviolet wavelengths to below band gap short-wave infrared wavelengths. Here, we demonstrate that doping silicon with two other group VI elements (chalcogens), selenium and tellurium, also leads to near-unity broadband absorption. A powder of the chalcogen dopant is spread on the silicon substrate and irradiated with femtosecond-laser pulses. We examine and compare the resulting morphology, optical properties, and chemical composition for each chalcogen-doped substrate before and after thermal annealing. Thermal annealing reduces the absorption of below-band gap radiation by an amount that correlates with the diffusivity of the chalcogen dopant used to make the sample. We propose a mechanism for the absorption of below band gap radiation based on defects in the lattice brought about by the femtosecond laser irradiation and the presence of a supersaturated concentration of chalcogen dopant atoms. The selenium and tellurium doped samples show particular promise for use in infrared photodetectors as they retain most of their infrared absorptance even after thermal annealinga necessary step in many semiconductor device manufacturing processes.
    B. R. Tull, J. E. Carey, M. A. Sheehy, C. M. Friend, and E. Mazur. 2006. “Formation of silicon nanoparticles and web-like aggregates by femtosecond laser ablation in a background gas.” Appl. Phys. A, 83, Pp. 341–346. Publisher's VersionAbstract
    We show that the mechanism of nanoparticle formation during femtosecond laser ablation of silicon is affected by the presence of a background gas. Femtosecond laser ablation of silicon in a H2 or H2S background gas yields a mixture of crystalline and amorphous nanoparticles. The crystalline nanoparticles form via a thermal mechanism of nucleation and growth. The amorphous material has smaller features and forms at a higher cooling rate than the crystalline nanoparticles. The background gas also results in the suspension of plume material in the gas for extended periods, resulting in the formation (on a thin film carbon substrate) of unusual aggregated structures including nanoscale webs that span tears in the film. The presence of a background gas provides additional control of the structure and composition of the nanoparticles during short pulse laser ablation. PACS 81.16.-c
    M. A. Sheehy, L. Winston, J. E. Carey, C. M. Friend, and E. Mazur. 2005. “Role of the background gas in the morphology and optical properties of laser-microstructured silicon.” Chem. Mater., 17, Pp. 3582–3586. Publisher's VersionAbstract
    We irradiate silicon with a train of femtosecond pulses in the presence of SF6, H2S, H2, SiH4, and a mixture of Ar and SF6 in order to analyze the role of the background gas in determining the morphology and the optical properties of the resultant surfaces. We discuss factors that affect the surface morphology created during irradiation and show that the presence of sulfur in these gases is important in creating sharp microstructures. We also show that the presence of sulfur is necessary to create the near-unity absorptance for both above-band and below-band gap radiation (0.25 2.5 micrometer) by silicon; only samples with sulfur concentrations higher than 0.6% absorb 95% for above-band gap radiation and have a flat, featureless absorptance of 90% for below-band gap radiation. KEYWORDS silicon, infrared absorptance, laser materials processing, microstructures, sulfur doping, femtosecond laser irradiation, RBS, elemental semiconductors
    J. E. Carey, C. H. Crouch, M. Shen, and E. Mazur. 2005. “Visible and near-infrared responsivity of femtosecond-laser microstructured silicon photodiodes.” Opt. Lett., 30, Pp. 1773–1775. Publisher's VersionAbstract
    We investigated the current-voltage characteristics and responsivity of photodiodes fabricated with silicon that was microstructured using femtosecond-laser pulses in a sulfur-containing atmosphere. The photodiodes we fabricated have a broad spectral response ranging from the visible to the near-infrared (4001600 nm). The responsivity depends on substrate doping, microstructuring fluence, and annealing temperature. We obtained room- temperature responsivities as high as 100 A/W at 1064 nm, two orders of magnitude higher than standard silicon photodiodes. For wavelengths below the band gap, we obtained responsivities as high as 50 mA/W at 1330 nm and 35 mA/W at 1550 nm.
    J. E. Carey. 2004. “Femtosecond-laser Microstructuring of Silicon for Novel Optoelectronic Devices”. Publisher's VersionAbstract
    This dissertation comprehensively reviews the properties of femtosecond- laser microstructured silicon and reports on its first application in optoelectronic devices. Irradiation of a silicon surface with intense, short laser pulses in an atmosphere of sulfur hexafluoride leads to a dramatic change in the surface morphology and optical properties. Following irradiation, the silicon surface is covered with a quasi-ordered array of micrometer-sized, conical structures. In addition, the microstructured surface has near-unity absorptance from the near-ultraviolet (250 nm) to the near-infrared (2500 nm). This spectral range includes below-band gap wavelengths that normally pass through silicon unabsorbed. We thoroughly investigate the effect of experimental parameters on the morphology and chemical composition of microstructured silicon and propose a formation mechanism for the conical microstructures. We also investigate the effect of experimental parameters on the optical and electronic properties of microstructured silicon and speculate on the cause of below-band gap absorption. We find that sulfur incorporation into the silicon surface plays an important role in both the formation of sharp, conical microstructures and the near-unity absorptance at below-band gap wavelengths. Because of the novel optical properties, femtosecond-laser microstructured silicon has potential application in numerous optoelectronic devices. We use femtosecond-laser microstructured silicon to create silicon-based photodiodes that are one hundred times more sensitive than commercial silicon photodiodes in the visible, and five orders of magnitude more sensitive in the near-infrared. We also create femtosecond-laser microstructured silicon solar cells and field emission arrays.
    M. A. Sheehy. 2004. “Femtosecond-laser Microstructuring of Silicon: Dopants and Defects”. Publisher's VersionAbstract
    This dissertation deals with the incorporation of elements into silicon using a femtosecond laser in order to understand the source for below-band gap absorptance. Previous experimental results indicate that irradiation of silicon with a femtosecond laser in the presence of sulfur hexafluoride (SF6) leads to unique optical properties. The absorptance for above-band gap radiation is increased to 95%; the more interesting result is that the below-band gap absorptance goes from nearly 0% to 90%. In the first set of experiments performed, we irradiated silicon in the presence of H2S, SiH4, and H2. The absorptance for samples prepared in H2S is identical to that of samples prepared in SF6; the other samples have a trailing edge of absorptance for energies below the band gap. This result indicated that sulfur played a crucial role in the below-band gap absorptance. The next set of experiments involved incorporating selenium and tellurium from a powder source to investigate possible dependence of the optical properties on the size of the dopant (selenium and tellurium have the same valence, but are larger in atomic size than sulfur). Incorporation of these two elements also leads to near-unity absorptance for below-band gap radiation. A comparison of the composition and the optical properties before and after annealing showed that the source for below-band gap absorptance is likely due to both the incorporated chalcogen and defects. The final set of experiments deals with the incorporation of elements from other families. These studies bolster the results of the previous research and provide furtherdetails on the interaction of the dopant with the laser- modified surface. We speculate on some requirements the dopants must satisfy (i.e. atomic size and valence onfiguration) and propose further research that can be done in this area. These experiments provide significant insight into the optical absorption mechanism and show that this material has great potential for devices that operate in the infrared portion of the spectrum, such as infrared photodiodes and solar cells.
    C. H. Crouch, J. E. Carey, J. M. Warrender, M. J. Aziz, E. Mazur, and F. Y. Genin. 2004. “Comparison of structure and properties of femtosecond and nanosecond laser-structured silicon.” Appl. Phys. Lett., 84, Pp. 1850–1852. Publisher's VersionAbstract
    We compare the optical properties, chemical composition, and crystallinity of silicon microstructures formed in the presence of SF6 by femtosecond laser irradiation and by nanosecond laser irradiation. In spite of very different morphology and crystallinity, the optical properties and chemical composition of the two types of microstructures are very similar. The structures formed with femtosecond (fs) pulses are covered with a disordered nanocrystalline surface layer less than 1 m thick, while those formed with nanosecond (ns) pulses have very little disorder. Both ns-laser-formed and fs-laser-formed structures absorb near-infrared (1.12.5 m) radiation strongly and have roughly 0.5% sulfur impurities.
    M. Shen, C. H. Crouch, J. E. Carey, and E. Mazur. 2004. “Femtosecond laser-induced formation of submicrometer spikes on silicon in water.” Appl. Phys. Lett., 85, Pp. 5694–5696. Publisher's VersionAbstract
    We fabricate submicrometer silicon spikes by irradiating a silicon surface that is submerged in water with 400-nm, 100-fs laser pulses. These spikes are less than a micrometer tall and about 200 nm wide one to two orders of magnitude smaller than the microspikes formed by laser irradiation of silicon in gases or vacuum. Scanning electron micrographs of the surface show that the formation of the spikes involves a combination of capillary waves on the molten silicon surface and laser-induced etching of silicon. Chemical analysis and scanning electron microscopy of the spikes shows that they are composed of silicon with a 20-nm thick surface oxide layer.
    C. H. Crouch, J. E. Carey, M. Shen, E. Mazur, and F. Y. Genin. 2004. “Infrared absorption by sulfur-doped silicon formed by femtosecond laser irradiation.” Appl. Phys. A, 79, Pp. 1635–1641. Publisher's VersionAbstract
    We microstructured silicon surfaces with femtosecond laser irradiation in the presence of SF6. These surfaces display strong absorption of infrared radiation at energies below the band gap of crystalline silicon. We report the dependence of this below-band gap absorption on microstructuring conditions (laser fluence, number of laser pulses, and background pressure of SF6) along with structural and chemical characterization of the material. Significant amounts of sulfur are incorporated into the silicon over a wide range of microstructuring conditions; the sulfur is embedded in a disordered nanocrystalline layer less than 1 mm thick that covers the microstructures. The most likely mechanism for the below-band gap absorption is the formation of a band of sulfur impurity states overlapping the silicon band edge, reducing the band gap from 1.1 eV to approximately 0.4 eV.
    A. Serpengüzel, T. Bilici, I. Inanc, A. Kurt, J. E. Carey, and E. Mazur. 2004. “Temperature dependence of photoluminescence in non-crystalline silicon.” In . Photonics West. Publisher's VersionAbstract
    Crystalline silicon being ubiquitous throughout the microelectronics industry has an indirect bandgap, and therefore is incapable of light emission. However, strong room temperature visible and near-IR luminescence from non-crystalline silicon, e.g., amorphous silicon, porous silicon, and black silicon, has been observed. These silicon based materials are morphologically similar to each other, and have similar luminescence properties. We have studied the temperature dependence of the photoluminescence from these non-crystalline silicons to fully characterize and optimize these materials in the pursuit of obtaining novel optoelectronic devices.