Nanosecond Pulsed Laser Melting Research Articles

Optical, Electrical, and Optoelectronic Characterization of Ti‐Supersaturated Gallium Arsenide

Herein, a detailed investigation on the properties of supersaturated gallium arsenide (GaAs) using Ti+ implantation followed by nanosecond pulsed laser melting (PLM) is presented. The supersaturated samples are analyzed by means of electrical, optical, and optoelectronic characterization. The sheet resistance results obtained using van der Pauw configuration measurements do not show activation of the implanted Ti+ in semi‐insulating GaAs after PLM. Absorptance measurements show a sub‐bandgap absorption (up to 6.5% for λ = 1000 nm) of the supersaturated GaAs:Ti and the just PLM‐processed GaAs, with the same laser melting fluence used (0.50 J/cm−2). The origin of this sub‐bandgap absorption is analyzed. Optoelectronic measurements show a similar sub‐bandgap photo‐response related to the absorption analyzed. The photo‐response measured below the bandgap originates from point defects introduced by the PLM process.

Band-gap and strain engineering in GeSn alloys using post-growth pulsed laser melting

The pseudomorphic growth of Ge1−x Sn x on Ge causes in-plane compressive strain, which degrades the superior properties of the Ge1−x Sn x alloys. Therefore, efficient strain engineering is required. In this article, we present strain and band-gap engineering in Ge1−x Sn x alloys grown on Ge a virtual substrate using post-growth nanosecond pulsed laser melting (PLM). Micro-Raman and x-ray diffraction (XRD) show that the initial in-plane compressive strain is removed. Moreover, for PLM energy densities higher than 0.5 J cm−2, the Ge0.89Sn0.11 layer becomes tensile strained. Simultaneously, as revealed by Rutherford Backscattering spectrometry, cross-sectional transmission electron microscopy investigations and XRD the crystalline quality and Sn-distribution in PLM-treated Ge0.89Sn0.11 layers are only slightly affected. Additionally, the change of the band structure after PLM is confirmed by low-temperature photoreflectance measurements. The presented results prove that post-growth ns-range PLM is an effective way for band-gap and strain engineering in highly-mismatched alloys.

Synthesis of relaxed Ge0.9Sn0.1/Ge by nanosecond pulsed laser melting

We present a new approach to the fabrication of fully relaxed Ge1-ySny layers on Ge with Sn concentration y up to 13 at. % The incorporation of Sn in Ge was obtained by sputtering of thin Sn films (<20 nm) directly on Ge wafers followed by pulsed laser melting in air. Microstructural analyses combining high-resolution transmission electron microscopy, atom probe tomography and nanobeam precession electron diffraction were performed to investigate the Sn distribution and the strain state down to the nanoscale. If y < 6 at. %, Ge1-ySny layers are fully substitutional and fully strained with no or very few extended defects. The formation of Sn-rich regions in correspondence of dislocations is instead observed for y > 6 at. %. However, outside these regions, Ge1-ySny layers present a very homogeneous Sn distribution, full Sn substitutionality, full strain relaxation, and excellent crystalline quality. This new approach could offer, if properly optimized, an alternative to epitaxy or ion implantation to fabricate high quality Ge1-ySny alloys, with the important add-on of the ability of the pulsed laser melting to perform spatially confined thermal processes (both laterally and in depth).

High-quality single-crystalline epitaxial regrowth on pulsed laser melting of Ti implanted GaAs

We present a detailed investigation on the formation of supersaturated GaAs using Ti+ implantation followed by nanosecond Pulsed Laser Melting (PLM). We have synthesized high-crystal quality supersaturated GaAs layers with concentrations of Ti above the insulator to metal transition (Mott limit). The Ti-implanted concentration depth profiles after PLM obtained by Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) show a redistribution of Ti impurities within the first hundred nanometers and superficial concentration up to 1 × 1021 cm−3. Raman spectroscopy of these Ti supersaturated, and regrown GaAs samples shows a sharp crystalline peak and tensile strain due to the Ti lattice incorporation. Scanning Transmission Electron Microscopy (STEM) and high-resolution Transmission Electron Microscopy (TEM) images show a good GaAs crystallinity after the PLM process. Energy-Dispersive X-ray Spectroscopy (EDS) reveals an enhanced Ti signal inside bubble-like structures and an appearance of interface oxide layer with all processed samples.

Carrier Dynamics and Absorption Properties of Gold-Hyperdoped Germanium: Insight Into Tailoring Defect Energetics

Hyperdoping germanium with gold is a potential method to produce room-temperature short-wavelength-infrared radiation (SWIR; $1.4--3.0\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$) photodetection. We investigate the charge carrier dynamics, light absorption, and structural properties of gold-hyperdoped germanium ($\mathrm{Ge}$:$\mathrm{Au}$) fabricated with varying ion implantation and nanosecond pulsed laser melting conditions. Time-resolved terahertz spectroscopy (TRTS) measurements show that $\mathrm{Ge}$:$\mathrm{Au}$ carrier lifetime is significantly higher than that in previously studied hyperdoped silicon systems. Furthermore, we find that lattice composition, sub-band-gap optical absorption, and carrier dynamics depend greatly on hyperdoping conditions. We use density functional theory (DFT) to model dopant distribution, electronic band structure, and optical absorption. These simulations help explain experimentally observed differences in optical and optoelectronic behavior across different samples. DFT modeling reveals that substitutional dopant incorporation has the lowest formation energy and leads to deep energy levels. In contrast, interstitial or dopant-vacancy complex incorporation yields shallower energy levels that do not contribute to sub-band-gap light absorption and have a small effect on charge carrier lifetimes. These results suggest that it is promising to tailor dopant incorporation sites of $\mathrm{Ge}$:$\mathrm{Au}$ for SWIR photodetection applications.

A critical evaluation of Ag- and Ti-hyperdoped Si for Si-based infrared light detection

Following recent successful demonstrations of enhanced infrared absorption in Au-hyperdoped Si, there has been strong interest in fabricating other metal-hyperdoped Si systems as a highly attractive approach for Si-based infrared photodetection. In this work, we address the somewhat contentious issue in the literature as to whether it is possible, using ion implantation and nanosecond pulsed-laser melting, to achieve hyperdoping of Si with Ag and Ti at concentrations exceeding that required to form an intermediate impurity band within the Si bandgap (NIB∼6×1019cm−3). A wide range of characterization techniques were used to investigate these material systems, especially the quality of liquid-phase epitaxy, impurity concentration distribution both in depth and laterally, and impurity lattice location. Our results indicate that the high concentrations of opto-electrically active Ag or Ti in monocrystalline Si required to form an impurity band are not achieved. In particular, the usual behavior during rapid solidification is for near-complete surface segregation of the impurity, or for it to be trapped within a highly defective subsurface layer due to filamentary breakdown. Although our measurements showed that the maximum concentration of impurities outside metal-rich filaments is comparable to NIB for both Ag and Ti, there is no preferential Ag or Ti lattice location after pulsed-laser melting anywhere in the material. Thus, the concentration of opto-electrically active Ag and Ti that can be homogeneously incorporated into Si is expected to be well below NIB, leaving Au as the only viable impurity to date for achieving the required level of hyperdoping in Si.

Gold-Hyperdoped Germanium with Room-Temperature Sub-Band-Gap Optoelectronic Response

Short-wavelength-infrared (SWIR; 1.4--3.0 \textmu{}m) photodetection is important for various applications. Inducing a low-cost silicon-compatible material, such as germanium, to detect SWIR light would be advantageous for SWIR applications compared with using conventional (III-V or II-VI) SWIR materials. Here, we present a scalable nonequilibrium method for hyperdoping germanium with gold for dopant-mediated SWIR photodetection. Using ion implantation followed by nanosecond pulsed laser melting, we obtain a single-crystal material with a peak gold concentration of 3 \ifmmode\times\else\texttimes\fi{} ${10}^{19}\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ $({10}^{3}$ times the solubility limit). This hyperdoped germanium has fundamentally different optoelectronic properties from those of intrinsic and conventionally doped germanium. This material exhibits sub-band-gap absorption of light up to wavelengths of at least 3 \textmu{}m, with a sub-band-gap optical absorption coefficient comparable to that of commercial SWIR photodetection materials. We show that germanium hyperdoped with gold exhibits sub-band-gap SWIR photodetection at room temperature, in contrast with previous doped-germanium photodetector studies, which only show a low-temperature response. This material is a potential pathway to low-cost room-temperature silicon-compatible SWIR photodetection.

Critical behavior of the insulator-to-metal transition in Te-hyperdoped Si

Hyperdoping Si with chalcogens is a topic of great interest due to the strong sub-band-gap absorption exhibited by the resulting material, which can be exploited to develop broadband room-temperature infrared photodetectors using fully Si-compatible technology. Here, we report on the critical behavior of the impurity-driven insulator-to-metal transition in Te-hyperdoped Si layers fabricated via ion implantation followed by nanosecond pulsed-laser melting. Electrical transport measurements reveal an insulator-to-metal transition, which is also confirmed and understood by density functional theory calculations. We demonstrate that the metallic phase is governed by a power-law dependence of the conductivity at temperatures below 25 K, whereas the conductivity in the insulating phase is well described by a variable-range hopping mechanism with a Coulomb gap at temperatures in the range of 2--50 K. These results show that the electron wave function in the vicinity of the transition is strongly affected by the disorder and the electron-electron interaction.

Evidence for vacancy trapping in Au-hyperdoped Si following pulsed laser melting

Nanosecond pulsed laser melting can be used to rapidly recrystallize ion-implanted Si through liquid phase epitaxy. The rapid resolidification that follows the melting results in a supersaturation of impurities and hyperdopes the Si, inducing novel optoelectronic properties with a wide range of applications. In this work, structural changes in the Si lattice in Au-hyperdoped Si are studied in detail. Specifically, we show that the local skewing of the lattice observed previously in regions of extremely high Au concentrations (&amp;gt;1.4 at. %) can be related to the displacement of Au from perfect lattice positions. Surprisingly, although the incorporation of the larger Au atoms into Si is expected to cause swelling of the lattice, reciprocal space mapping shows that a small amount (0.3 at. %) of lattice contraction (decrease in lattice parameter) is present in the hyperdoped layer. Furthermore, positron annihilation spectroscopy shows an elevated concentration of vacancies in the hyperdoped layer. Based on these observations and with the aid of density functional theory, we propose a phenomenological model in which vacancies are kinetically trapped into lattice sites around substitutional Au atoms during resolidification. This vacancy trapping process is hypothesized to occur as a means to minimize lattice strain and may be universal in pulsed laser melted Si systems.

Formation of Q-carbon and diamond coatings on WC and steel substrates

Recently we reported the formation of diamond and a new phase of Q-carbon, harder than diamond, on sapphire (0001) substrates by direct conversion of amorphous carbon layers by nanosecond pulsed laser melting or annealing. Here we show that this process can be extended to other substrates, specifically WC (Co-bonded) and steels that are needed for high-speed machining and oil and gas exploration and biomedical applications. Since diamond and Q-carbon layers are formed via laser melting and quenching of carbon, these layers are quite adherent to the substrates, thus solving a critical adhesion problem associated with initial graphitic layers in CVD diamond coatings. We have used these Q-carbon, diamond and Q-carbon-diamond composites as seed layers for HFCVD (hot filament chemical vapor deposition) to grow thicker and more adherent layers of diamond.

Pulsed laser melting of bismuth telluride thermoelectric materials

While laser melting, deposition, and annealing of semiconductor thin films are well-established, pulsed laser processing of bulk semiconductor material powders has been minimally investigated. This work investigates nanosecond pulsed laser melting of bismuth telluride, a common thermoelectric material. Using both experimental and computational modeling methods, a process window was determined in which melting is achieved before ablation occurs. The process window was determined for Bi2Te3 powder compacts, with melt depths on the order of 20–100 μm. The melted region had a fine-grained, columnar microstructure and showed typical selective laser melting defects such as microcracking. Stroboscopic imaging was used as an in situ imaging technique to observe molten pool features, revealing a teardrop-shaped molten pool with little resolidification between pulses. A simplified thermal model of the nanosecond pulsed laser melting process was developed, and two models for the solid phase thermal conductivity were used to investigate the impact of thermal conductivity on the size, shape, and duration of the molten pool. The results demonstrate the narrow process window available for pulsed laser processing of bulk bismuth telluride powders, and the discussion includes thermal management approaches possible for reducing defects such as microcracking.

Synthesis of diamond nanostructures from carbon nanotube and formation of diamond-CNT hybrid structures

We report direct conversion of multiwall carbon nanotubes (CNTs), synthesized by chemical vapor deposition, into diamond by nanosecond pulsed laser melting process at ambient temperature and pressure in air without any catalysts. The Raman spectroscopy of the CNTs after the laser irradiation showed the characteristic diamond peak at around 1324-1325 cm−1. The downshift of this peak from its theoretical position (at 1332 cm−1) is explained by phonon confinement in nanostructured diamond. The SEM and TEM images show the formation of diamond mostly at the tip and bends of the CNTs. The grain size distribution and the shape of the converted nanodiamonds suggest that the transformation takes place by melting of the CNTs in a super undercooled state by nanosecond laser pulses, and subsequent rapid quenching to convert it into phase-pure diamond. The EBSD analysis illustrates the phase-pure single crystal diamond formation at the tips and bends of the CNTs. The high-resolution electron energy-loss spectrum in the STEM contains characteristic σ* peak at 292 eV for sp3 bonding of diamond. This study on the laser-induced direct conversion of CNTs to diamond marks a major breakthrough in the formation of diamond nanostructures and diamond-CNT hybrid for a variety of potential applications.

High level active n+ doping of strained germanium through co-implantation and nanosecond pulsed laser melting

Obtaining high level active n+ carrier concentrations in germanium (Ge) has been a significant challenge for further development of Ge devices. By ion implanting phosphorus (P) and fluorine (F) into Ge and restoring crystallinity using Nd:YAG nanosecond pulsed laser melting (PLM), we demonstrate 1020 cm−3 n+ carrier concentration in tensile-strained epitaxial germanium-on-silicon. Scanning electron microscopy shows that after laser treatment, samples implanted with P have an ablated surface, whereas P + F co-implanted samples have good crystallinity and a smooth surface topography. We characterize P and F concentration depth profiles using secondary ion mass spectrometry and spreading resistance profiling. The peak carrier concentration, 1020 cm−3 at 80 nm below the surface, coincides with the peak F concentration, illustrating the key role of F in increasing donor activation. Cross-sectional transmission electron microscopy of the co-implanted sample shows that the Ge epilayer region damaged during implantation is a single crystal after PLM. High-resolution X-ray diffraction and Raman spectroscopy measurements both indicate that the as-grown epitaxial layer strain is preserved after PLM. These results demonstrate that co-implantation and PLM can achieve the combination of n+ carrier concentration and strain in Ge epilayers necessary for next-generation, high-performance Ge-on-Si devices.

Room-Temperature Ferromagnetism and Extraordinary Hall Effect in Nanostructured Q-Carbon: Implications for Potential Spintronic Devices

We report extraordinary Hall effect and room-temperature ferromagnetism in undoped Q-carbon, which is formed by nanosecond pulsed laser melting and subsequent quenching process. Through detailed structure–property correlations in Q-carbon thin films, we show the excess amount of unpaired electrons near the Fermi energy level give rise to interesting magnetic and electrical properties. The analysis of the extraordinary Hall effect in Q-carbon follows nonclassical “side-jump” electronic scattering mechanism. The isothermal field-dependent magnetization plots confirm room-temperature ferromagnetism in Q-carbon with a finite coercivity at 300 K and a Curie temperature of 570 K, obtained by the extrapolation of the fits to experimental data using modified Bloch’s law. High-resolution scanning electron microscopy and transmission electron microscopy clearly illustrate the formation of Q-carbon and its subsequent conversion to single-crystalline diamond. Further, we found n-type conductivity in Q-carbon in the e...

Direct conversion of h-BN into c-BN and formation of epitaxial c-BN/diamond heterostructures

We have created a new state of BN (named Q-BN) through rapid melting and super undercooling and quenching by using nanosecond laser pulses. Phase pure c-BN is formed either by direct quenching of super undercooled liquid or by nucleation and growth from Q-BN. Thus, a direct conversion of hexagonal boron nitride (h-BN) into phase-pure cubic boron nitride (c-BN) is achieved by nanosecond pulsed laser melting at ambient temperatures and atmospheric pressure in air. According to the P-T phase diagram, the transformation from h-BN into c-BN under equilibrium processing can occur only at high temperatures and pressures, as the hBN-cBN-Liquid triple point is at 3500 K/9.5 GPa or 3700 K/7.0 GPa with a recent theoretical refinement. Using nonequilibrium nanosecond laser melting, we have created super undercooled state and shifted this triple point to as low as 2800 K and atmospheric pressure. The rapid quenching from super undercooled state leads to the formation of a new phase, named as Q-BN. We present detailed characterization of Q-BN and c-BN layers by using Raman spectroscopy, high-resolution scanning electron microscopy, electron-back-scatter diffraction, high-resolution TEM, and electron energy loss spectroscopy, and discuss the mechanism of formation of nanodots, nanoneedles, microneedles, and single-crystal c-BN on sapphire substrate. We have also deposited diamond by pulsed laser deposition of carbon on c-BN and created c-BN/diamond heterostructures, where c-BN acts as a template for epitaxial diamond growth. We discuss the mechanism of epitaxial c-BN and diamond growth on lattice matching c-BN template under pulsed laser evaporation of amorphous carbon, and the impact of this discovery on a variety of applications.

Enhancing the Infrared Photoresponse of Silicon by Controlling the Fermi Level Location within an Impurity Band

Strong absorption of sub‐band gap radiation by an impurity band has recently been demonstrated in silicon supersaturated with chalcogen impurities. However, despite the enhanced absorption in this material, the transformation of infrared radiation into an electrical signal via extrinsic photoconductivity—the critical performance requirement for many optoelectronic applications—has only been reported at low temperature because thermal impurity ionization overwhelms photoionization at room temperature. Here, dopant compensation is used to manipulate the optical and electronic properties and thereby improve the room‐temperature infrared photoresponse. Silicon co‐doped with boron and sulfur is fabricated using ion implantation and nanosecond pulsed laser melting to achieve supersaturated sulfur concentrations and a matched boron distribution. The location of the Fermi level within the sulfur‐induced impurity band is controlled by tuning the acceptor‐to‐donor ratio, and through this dopant compensation, three orders of magnitude improvement in infrared detection at 1550 nm is demonstrated.

Pulsed laser dewetting of Au films: Experiments and modeling of nanoscale behavior

Abstract

High Response in a Tellurium-Supersaturated Silicon Photodiode

Single crystalline silicon supersaturated with tellurium are formed by ion implantation followed by excimer nanosecond pulsed laser melting (PLM). The lattice damaged by ion implantation is restored during the PLM process, and dopants are effectively activated. The hyperdoped layer exhibits high and broad optical absorption from 400 to 2500nm. The n+ p photodiodes fabricated from these materials show high response (6.9A/W at 1000nm) with reverse bias 12 V at room temperature. The corresponding cut-off wavelength is 1258nm. The amount of gain and extended cut-off wavelength both increase with increasing reverse bias voltage; above 100% external quantum efficiency is observed even at a reverse bias of 1 V. The cut-off wavelength with 0 V bias is shorter than the commercial silicon detector. This implies that the Burstein-Moss shift is due to hyperdoping. The amount of the extended cut-off wavelength increases with increasing reverse bias voltage, suggesting existence of the Franz—Keldysh effect.

Morphology transitions in bilayer spinodal dewetting systems

In spontaneous pattern formation by spinodal dewetting, attractive intermolecular forces overcome surface tension and cause an ultrathin liquid film on a low energy substrate to produce ordered structures. Spinodal dewetting in single-layer film on a substrate is usually manifested by an early stage surface deformation and a highly nonlinear ripening stage that results in characteristic morphologies, typically bicontinuous- or holelike states. Here we have experimentally constructed the dewetting morphology phase diagrams for a bilayer (Ag, Co) liquid film system on SiO${}_{2}$. Nanosecond pulsed laser melting was used to initiate and foster the dewetting as a function of film thickness and arrangement. The early stage ripening morphology was observed by scanning electron microscopy from which the phase diagrams were constructed. Unlike single-layer films, which only show one morphology transition between the bicontinuous to hole states as the film thickness is increased, the bilayer system can have multiple transitions. We have utilized the thickness-dependent free energy curvature approach [Sharma and Khanna, Phys. Rev. Lett. 81, 3463 (1998)] to analyze the phase diagram. The location of the multiple transitions cannot be predicted from the curvature minima, as was the case for single-layer films. Nevertheless, despite the complexity from multiple interacting forces and different surface deformation mode in bilayer systems, the phase diagram can be completely generated by knowledge of the free energy curvature of the respective single-layer films. These results can permit improved modeling of the nonlinear dynamics in naturally driven self-organized phenomenon and help design nanomaterials for advanced applications.

Controlling dopant profiles in hyperdoped silicon by modifying dopant evaporation rates during pulsed laser melting

We describe a method to control the sub-surface dopant profile in “hyperdoped” silicon fabricated by ion implantation and pulsed laser melting. Dipping silicon ion implanted with sulfur into hydrofluoric acid prior to nanosecond pulsed laser melting leads to a tenfold increase in the rate of sulfur evaporation from the surface of the melt. This results in an 80% reduction of the near-surface dopant concentration, effectively embedding the hyperdoped region in a layer up to 180 nm beneath the surface. This method should facilitate the development of blocked impurity band devices.