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.

Gold-hyperdoped germanium extends photodetection into the infrared wavelengths. Pulsed-laser processing makes single crystal material with ultrahigh dopant concentrations. The laser parameters influence the dopant’s atomic position which leads to different light absorption and carrier transport properties. OCIS codes: 160.6990 Transition-metal-doped materials, 300.6495 Spectroscopy, terahertz, 140.3390 Laser materials processing Short wavelength infrared radiation detection is important in many applications and conventional detector materials are expensive, toxic or require low operation temperatures [1-3]. Developing novel semiconductor materials that overcome these limitations are important. Germanium (Ge) is a good material candidate due to its high absorption coefficients across a broad band in short wavelength infrared region (SWIR) and high carrier mobilities [4]. Bandgap energy of Ge is 0.67 eV (1.8 µm) and hence further engineering methods are of great interest to obtain photodetection beyond this cut-off wavelength of elemental Ge. Laser hyperdoping is a viable method to incorporate dopants in ultra-high non-equilibrium concentrations above the solid solubility limit, which enables the formation of an intermediate band between the conduction and valance bands [5]. Hyperdoped Ge has shown to be a promising candidate for efficient SWIR photodetection [6] . Laser hyperdoping can be achieved with ultrashort laser pulses or with nanosecond laser pulses but the resulting material is polycrystalline and has roughened surfaces. A better method to fabricate a hyperdoped material is the ion implantation followed by nanosecond pulsed laser melting which produces high quality materials that is single crystalline (Figure 1). High flux of dopant atoms implanted in the host semiconductor damages the lattice structure and amorphizes the material. Therefore, the pulsed laser process is carried out to melt the material deeper than the damaged layer. This process is followed by rapid solidification, where the material grows epitaxially from the crystalline substrate underneath, as the material cools down. During this rapid re-solidification, the dopants are trapped at concentrations above the thermodynamic solubility limit and hence ultra-high concentrations are achieved while preserving the material quality. The rapid solidification process must be slow enough to ensure the recrystallization and at the same time it must be fast enough to allow the incorporation of non-equilibrium dopant concentrations into the sample. Thus, the energy density (fluence) of the pulsed laser have significant effect on the material quality. This is a well-established method for chalcogen dopants (S, Se, Te) in silicon, but incorporating transition metals at supersaturating concentrations with better properties has more room for study [7]. Figure 01 – Hyperdoping by ion implantation followed by pulse laser melting. Figure adapted from ref. [5] In this work, we use ion implantation followed by pulsed laser melting to incorporate high concentrations of gold (Au) in Ge. Au hyperdoped Ge (Ge:Au) with different dopant doses and various pulsed laser melting fluences are studied. We evaluate the material quality using time resolved terahertz (THz) spectroscopy to investigate the charge carrier lifetime of the material. THz spectroscopy is a photoconductivity measurement with sub-picosecond time resolution. We use a 400-nm fs-laser pump pulse to excite the charge carriers in the material and probe with a broadband sub-picosecond THz pulse to map the photoconductivity decay dynamics [8]. Apart from the charge carrier lifetime analysis (Figure 2), the dopant concentration profile is analyzed by Rutherford Backscattering Spectrometry (RBS). Moreover, the sub band gap absorptance and DFT modelling of dopant distribution and their effect on band structure are being investigated. Figure 02 – Left: The change in photoconductivity of Ge:Au as a function of time. Charge carrier decay dynamics of different dopant concentrations and PLM fluences are investigated. The samples are LD_HF (low dose-high fluence), LD_LF (low dose-low fluence) HD_HF (high dose-high fluence) and HD_LF (high dose-low fluence) respectively. HD_LF has a significantly shorter lifetime than the rest. Right : Upper panel - The inverse of experimentally obtained half-lives. Triangles and circles represent LF and HF respectively. Sub-band gap absorptances are represented by unfilled triangles and circles. Lower panel - Total and substitutional dose of Au in hyperdoped Ge samples from RBS analysis. The unfilled bars and filled bars represent the total and substitutional doses, respectively. HD_LF has the most substitutional Au incorporation, while substitutional Au incorporation in HD_HF sample is much more similar to the two LD samples. In this study, we found that laser fluence determines the fraction of substitutional dopant (Figure 2). Material characterization shows that lifetime and absorption correlate with substitutional dose. Moreover, DFT modeling indicates that substitutional dopants are deep level defects which lead to high absorption and short lifetime. We found that the charge carrier lifetime is very long even after high concentration of Au is incorporated. We also found laser parameters that lead to highly substitutional dopant incorporation which enhances infrared light absorption. THz lifetime characterization allows optimizing processing parameters and shows hyperdoped germanium is a promising material for SWIR photodetectors.

Gold-hyperdoped germanium extends the photodetection beyond the cut-off wavelength of germanium into the infrared wavelengths. Ion implantation followed by pulsed-laser melting process makes a single crystal material with ultrahigh dopant concentrations. The laser parameters influence the dopant's atomic position which leads to different light absorption and carrier transport properties. Time resolved terahertz spectroscopy is used to evaluate the charge carrier lifetime which gives an insight into the defect energetics and atomic location.

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 (>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.

Au-hyperdoped Si produced by ion implantation and pulsed laser melting exhibits sub-band-gap absorption in the near infrared, a property that is interesting for Si photonics. However, the sub-band-gap absorption has previously been shown to be thermally metastable. In this work, we study the atomistic processes that occur during the thermal relaxation of Au-hyperdoped Si. We show that the first step in thermal relaxation is the release of substitutional Au from lattice sites. This process is characterized by an activation energy of around 1.6 eV, a value similar to that associated with Au diffusion in Si, suggesting that both processes could be rate limited by the exchange of substitutional and interstitial Au atoms. As the system further relaxes, Au is found to locally diffuse and become trapped at nearby lattice defects, notably vacancies and vacancy complexes. In fact, density-functional theory results suggest that the formation of Au dimers is energetically favourable after the Au becomes locally trapped. The dimers could subsequently evolve into trimers, etc., as other diffusing Au atoms become trapped at the dimer. At low Au concentrations, this clustering process does not form visible precipitation after annealing at 750 degrees C for 3 min. In contrast, spherical Au precipitates are found in samples with higher Au concentrations (>0.14 at.%), where the Au atoms and the associated lattice defect distributions are laterally inhomogeneous.

Gold hyperdoped silicon exhibits room temperature sub band gap optical absorption, with potential applications as infrared absorbers/detectors and impurity band photovoltaics. We use first-principles density functional theory to establish the origins of the sub band gap response. Substitutional gold AuSi and substitutional dimers AuSi − AuSi are found to be the energetically preferred defect configurations, and AuSi gives rise to partially filled mid-gap defect bands well offset from the band edges. AuSi is predicted to offer substantial sub-band gap absorption, exceeding that measured in prior experiments by two orders of magnitude for similar Au concentration. This suggests that in experimentally realized systems, in addition to AuSi, the implanted gold is accommodated by the lattice in other ways, including other defect complexes and gold precipitates. We further identify that it is energetically favorable for isolated AuSi to form AuSi − AuSi, which by contrast do not exhibit mid-gap states. The formation of dimers and other complexes could serve as nuclei in the earliest stages of Au precipitation, which may be responsible for the observed rapid deactivation of sub-band gap response upon annealing.

The epitaxial integration of functional oxides with wide band gap semiconductors offers the possibility of new material systems for electronics and energy conversion applications. We use first principles to consider an epitaxial interface between the correlated metal oxide SrRuO3 and the wide band gap semiconductor TiO2, and assess energy level alignment, interfacial chemistry, and interfacial dipole formation. Due to the ferromagnetic, half-metallic character of SrRuO3, according to which only one spin is present at the Fermi level, we demonstrate the existence of a spin dependent band alignment across the interface. For two different terminations of SrRuO3, the interface is found to be rectifying with a Schottky barrier of ≈1.3–1.6 eV, in good agreement with experiment. In the minority spin, SrRuO3 exhibits a Schottky barrier alignment with TiO2 and our calculated Schottky barrier height is in excellent agreement with previous experimental measurements. For majority spin carriers, we find that SrRuO3 recovers its exchange splitting gap and bulk-like properties within a few monolayers of the interface. These results demonstrate a possible approach to achieve spin-dependent transport across a heteroepitaxial interface between a functional oxide material and a conventional wide band gap semiconductor.

S. Lee, Dr. A. R. Damodaran, Dr. Z. Chen, Prof. L. W. Martin Department of Materials Science and Engineering University of California Berkeley, Berkeley , California 94720 , USA E-mail: lwmartin@berkeley.edu Dr. P. Gorai, N. Ferdous, Prof. E. Ertekin Department of Mechanical Science and Engineering University of Illinois Urbana-Champaign, Urbana , Illinois 61801 , USA N. Oh, Dr. J.-H. Kwon, Dr. A. Shah, Dr. E. Breckenfeld, Dr. R. V. K. Mangalam, Prof. P. V. Braun, Prof. M. Shim, Prof. J.-M. Zuo Department of Materials Science and Engineering and Materials Research Laboratory University of Illinois Urbana-Champaign, Urbana , Illinois 61801 , USA Dr. J. A. Moyer, Prof. P. Schiffer Department of Physics University of Illinois Urbana-Champaign, Urbana , Illinois 61801 , USA

Sr2Ti7O14, a new phase, is synthesized by leveraging the innate chemical and thermo-dynamic instabilities in the SrTiO3-TiO2 system and non-equilibrium growth techniques. The chemical composition, epitaxial relationships, and orientation play roles in the formation of this novel layered phase, which, in turn, possesses unusual charge ordering, anti-ferromagnetic ordering, and low, glass-like thermal conductivity.

Under the influence of an AC electric field, electrolytes on a planar microelectrodes exhibit steady fluid flow, termed as AC electro-osmosis. A numerical model using finite element scheme has been developed to solve the electrokinetic flow parameters of the AC electroosmosis in a slit microchannel. The solution was obtained using Poisson–Navier–Stokes–Nernst–Planck approach known as non equilibrium model. A thin-layer, low-frequency, nonlinear analysis of the system is performed including Faradaic currents from electrochemical reactions at the electrodes. Net flow velocity is observed for different geometric parameters of electrodes. Flow and field characteristic are observed for the maximum velocity attained in the investigated range of frequency. The interesting consequences of shape and field asymmetries, which generally lead to electro-osmotic pumping or electrophoretic motion in AC fields gives some basic issues for the microfluidic devices..