Session H28: Focus Session: Dopants and Defects in Semiconductors - Si

Reliability of III-V electronic devices -- the defects that cause the trouble

Degradation of electronic devices by hot electrons is universally attributed to the generation of defects, but the mechanisms for defect generation and the specific nature of the pertinent defects are not known for most systems. Here we describe three recent case studies [1] in III-V high-electron-mobility transistors that illustrate the power of combining density functional calculations and experimental data to identify the pertinent defects and associated degradation mechanisms. In all cases, benign pre-existing defects are either depassivated (irreversible degradation) or transformed to a metastable state (reversible degradation). This work was done in collaboration with R.D. Schrimpf, D.M. Fleetwood, Y. Puzyrev, X. Shen, T. Roy, S. DasGupta, and B.R. Tuttle. Devices were provided by D.F. Brown, J. Speck and U. Mishra, and by J. Bergman and B. Brar. \\[4pt] [1] Y. S. Puzyrev et al., Appl. Phys. Lett. \textbf{96}, 053505 (2010); T. Roy et al., Appl. Phys. Lett. \textbf{96}, 133503 (2010); X. Shen et al., J. Appl. Phys. \textbf{108}, 114505 (2010).   

Near-Interface Defects in SiO$_{2}$/SiC MOS Devices

The implementation of SiO$_{2}$/SiC MOSFETS for high power applications has been hindered by the high density of near-interface states. We have developed a method to distinguish both the energy and spatial distribution of defect states near insulator-semiconductor interfaces through a comparison of the thermal emission energy extracted from constant capacitance transient spectroscopy (CCDLTS) measurements and the interface Fermi energy (F$_{P})$. The dependence of F$_{P}$ on trap filling voltage at the CCDLTS peak temperature is determined from temperature-dependent 1MHz C-V curves. Capture by tunneling into oxide traps is detected in 4H- and 6H-SiC capacitors fabricated by oxidation followed by NO-annealing, with the difference in thermal emission energies consistent with the conduction band offsets of the two polytypes at the SiO$_{2}$/SiC interface. Comparison with results from first principles calculations suggests that the observed oxide traps are C$_{O}$=C$_{O}$ and interstitial Si [1]. SiC defects having energies close to the SiC conduction band are suggested to be carbon di-interstitial defects, (C$_{2})_{i}$, introduced during standard oxidation [1]. Well-known traps introduced in SiC by ion-implantation are observed in 4H-SiC MOS capacitors fabricated by N-implantation followed by standard oxidation, thus validating this new method [2]. \begin{enumerate} \item A.F. Basile, \textit{et al}., J. Appl. Phys. \textbf{109}, 064514 (2011) \item A.F. Basile, \textit{et al}., J. Appl. Phys. \textbf{109}, 114505 (2011). \end{enumerate}   

Doping in Si/SiO$_{2}$ Structures: A first-principles metadynamics study

Dopant diffusion in semiconductor devices is a field of study with tremendous technological importance. We have performed first-principles metadynamics simulations of the diffusion of n-type dopants at the Si/SiO$_{2}$ interface using the ab-initio MD method. After the generation of a vacancy in the Si region, arsenic, phosphorus and silicon atoms show varying mechanisms of diffusion, including both substitutional and interstitial. Although at the near-interface silicon region arsenic is the first to diffuse interstitially, its interstitial position is more stable and thus less likely to diffuse across the interface relative to phosphorus and silicon. As arsenic crosses the interface, however, its relative stability decreases with respect to phosphorus and silicon and diffusion into the oxide becomes unfavorable. This is in agreement with experimentally observed arsenic pile-up at the Si/SiO$_{2}$ interface. We quantify the diffusion mechanisms by comparing free energy barriers in the silicon region as well as at the interface. We find the largest barriers exist for silicon, while the smallest barriers exist for arsenic.   

The lifetime recovery puzzle in intermediate band materials: A new experimental approach

We have recently observed that deep-level impurities in silicon -- such as the chalcogens S and Se -- can drive an insulator-to-metal transition. The existence of this transition has potential ramifications for the development of intermediate band (IB) solar cells, and significant progress has recently been made toward explaining the origin of this transition. Recently, however, theoretical disagreement has arisen regarding whether impurity concentrations beyond the insulator-to-metal transition should result in increased non-radiative recombination rates or instead yield ``lifetime recovery.'' Very few measurements of carrier lifetime in IB-candidates have been reported that could help clarify this issue, which has important impacts on the selection of IB-materials and the design of IB solar cells. We have developed a technique based on transient optical absorption to measure the trapping rate of the intermediate states and will discuss the results of these measurements in the hyperdoped silicon system (Si doped with chalcogens to concentration $>$10$^{20}_{ }$cm$^{-3})$. We will also discuss the impact of these measurements on the current theoretical disagreement.   

Single-Electron Capacitance Spectroscopy of Individual Dopants in Silicon

Motivated by recent transport experiments and proposed atomic-scale semiconductor devices, we present measurements that extend the reach of scanned-probe methods to discern the properties of individual dopants tens of nanometers below the surface of a silicon sample. Using a capacitance-based approach, we have both spatially resolved individual subsurface boron acceptors and spectroscopically detected single holes entering and leaving these minute systems of atoms. A resonance identified as the B $^{+}$ state is shown to shift in energy from acceptor to acceptor. The resonance is absent in a control sample that does not contain the boron acceptors. By directly measuring the quantum levels and testing the effect of dopant-dopant interactions, this method represents a valuable tool for the development of future atomic-scale semiconductor devices.   

Charge States of Individual Group V Donors on n-doped Si(111)-(2x1) Surface

Functionality of semiconductor devices now relies upon only a few atoms and study of individual dopants in silicon has thus been rapidly growing in importance. Group V donors are especially interesting due to their potential applications in quantum computing and spintronics. The charge state of the dopants is of fundamental importance for conventional semiconductor devices as well as in concept QIP and spintronic devices. We combine density functional theory simulations and ion implantation and cross-sectional scanning tunneling microscopy (XSTM) to study individual Group V donors in cleaved n-doped Si(111)-(2x1) surface. We present a detailed analysis of the dopant-induced charging effects and discuss the surface charge dependence on the local reconstruction induced by the individual dopants.   

First-principles calculations for Er impurities in Si

Erbium-doped Si is a promising material for the development of silicon-based light sources that can interface with CMOS technology, optical fiber, and spin centers for quantum computing. Using density functional theory with a screened hybrid functional we examine the structural and electronic properties of Er(III) impurities in Si, focusing on the site preference and the Er effects on the electrical properties of the Si host. We find that Er is stable either at the tetrahedral intersitital site or at the substitutional site, depending on the Fermi-level position; Er sitting at the hexagonal interstitial site is higher in energy at all Fermi levels, in agreement with experimental observations. In p-type Si, i.e., for Fermi levels near the valence band, Er prefers the tetrahedral interstitial site and acts as a donor. In n-type Si, i.e., for Fermi levels near the conduction band, Er prefers the substitutional site and acts as an acceptor. We will also discuss the impurity-to-band optical transitions determined from calculated configuration coordinate diagrams.   

Deactivation of deep level impurities in hyperdoped silicon

Extremely high concentrations of deep level impurities in silicon have exhibited unique properties of interest for optoelectronic and photovoltaic applications. For example, silicon hyperdoped with chalcogens demonstrates significant infrared absorption at wavelengths longer than the band edge of silicon. Hyperdoped silicon is fabricated by high-dose ion implantation followed by pulsed laser melting and rapid re-solidification. The result is a metastable supersaturated solid solution with doping concentrations orders of magnitude above the room temperature solubility limit. Thermal annealing results in a deactivation of the sub-gap absorption in this material, suggesting that the precise chemical state of the deep level impurities is a critical component of the enhanced absorption. To gain further insight to the absorption mechanism and the stability this material, we present a detailed investigation of the deactivation induced by rapid thermal annealing of silicon hyperdoped with sulfur.   

Single crystal silicon hyperdoped with transition metals

Silicon hyperdoped with sulfur and selenium by ion implantation and pulsed laser melting has recently been shown to undergo an insulator to metal transition. While experimental and theoretical investigations have begun to unravel the nature of this transition, little has been done to generalize this work to other dopants. This talk will discuss recent progress in hyperdoping silicon with transition metal dopants focusing on challenges not present in the silicon-chalcogen system. In particular, experimental results (e.g., SIMS, RBS, TEM) on the role of dopant selection and resolidification velocity in preventing segregeation and cellular breakdown of the solidification front will be addressed in detail. In addition, measurements of the optoelectronic properties of silicon hyperdoped with transition metal dopants will be reported.   

Theoretical and Experimental Framework of an Insulator-to-Metal Transition in Selenium-Hyperdoped Silicon

Following the discovery of black silicon in 1998, hyperdoping - doping to concentrations orders of magnitude larger than the solubility limit - has emerged as a promising method for designing semiconductors with unique optical and electronic properties. Black silicon (silicon hyperdoped with chalcogens), synthesized by pulsed laser techniques, has recently received substantial interest owing to its broad, sub-band gap absorption down to photon energies as low as 0.5 eV, suggesting applications towards infrared detection and intermediate band photovoltaics. Until now, there has not been a clear explanation of these properties. In this presentation, we combine computational and experimental evidence to probe the origin of sub-band gap optical absorption and metallicity in black silicon. Temperature-dependent conductivity measurements show that black silicon undergoes an insulator-to-metal transition at a critical dopant concentration. Our computational analysis based on density functional theory and quantum Monte Carlo methods suggest that the enhanced optical properties result from this insulator-to-metal transition that appears to be a classic impurity-driven Mott transition, driven largely by many-body effects.   

Optical Absorption Mechanisms in Sulfur Hyper-doped Silicon

Silicon that is doped with sulfur, a deep-level donor, to concentrations approaching 1{\%} at. demonstrates sub-band gap optical absorption, and has potential applications as an intermediate band solar cell material and a short-wavelength infrared (SWIR) photodetector. Understanding the nature of the absorption mechanism will aid in creating future devices with this exciting material. To elucidate the absorption mechanism, the reflectivity and absorption coefficient have been measured to photon energies down to 75 meV. We report on these new measurements as well as data fitting that give insight into absorption mechanism within these materials.   

Atomistic study of heavy doping in Si nanowires

Dopant atoms are becoming increasingly important in the nanoscaled field-effect transistors (FET) because of their tendency to influence device parameters such as sub-threshold current-voltage characteristics and gate-to-channel electrostatic coupling. Achieving high doping concentrations is essential for the realization of Si nanowire FET where low resistance contacts or tunnel junctions and narrow depletion widths are needed. In an effort to understand the dopants effect on these devices as a function of scaling parameters, we use self-consistent field (SCF) tight-binding (TB) method as implemented in NEMO3D to obtain an accurate quantitative description of the band structure, confinement geometries and valley-orbit interaction from a full band-structure technique as a function of dopant location, concentration and applied electrical field. Our method solves the Poisson equation iteratively coupled with the atomistic TB Hamiltonian for charge self-consistency to provide an accurate description of the electrostatics. Our simulations show how the band structure of the nanowire is affected by the presence of few impurities.   

Femtosecond-laser hyperdoping: controlling sulfur concentrations in silicon for band gap engineering

Doping silicon to concentrations above the metal-insulator transition threshold yields a novel material that has potential for photovoltaic applications. By focusing femtosecond laser pulses on the surface of a silicon wafer in a sulfur hexafluoride (SF6) environment, silicon is doped with 1\% atomic sulfur. This material exhibits near-unity, broadband absorption from the visible to the near infrared ($<$ 0.5 eV, deep below the silicon band gap), and metallic-like conduction. These unusual optical and electronic properties suggest the formation of an intermediate band. We report on the femtosecond laser doping techniques we employ and the resulting material properties. By changing the laser parameters and ambient environment we can control the dopant profiles, crystallinity, and surface morphology. We perform optical absorption and temperature-dependent Hall measurements to investigate electron transport and to identify the energy states of the sulfur donors.