Session V46: Defects and Impurities in Si/Ge

Pathway connecting the four-interstitial ground state and chain in silicon

Dimer searches, minima hopping and nudged elastic band (NEB) calculations try to identify the lowest pathway connecting the two important silicon four-interstitial structures: $I_4$-chain and ground state. First, 400 tight-binding dimer searches reveal several local minima. However, the minima have high DFT formation energies and no pathway connecting chain and ground state is found. Second, we fit a classical modified embedded atom method potential to a database of interstitial clusters. Minima hopping using this potential finds several structures with DFT energies between chain and ground state but no pathway connecting them. Finally, we construct a pathway between chain and ground state with small displacements. The NEB energy barrier with DFT from chain to ground state is 0.6~eV. This indicates that during annealings the $I_4$-chain decays into the nearly immobile ground state, implying that chain structures predominate only for longer chains.   

Recombination rate annealing following transient neutron irradiation

Neutron irradiation of bipolar transistors generates defects by displacing silicon atoms. For each displacement collision, a pair of defects is created, the interstitial atom and the vacancy left behind. These defects reduce transistor gain by increasing the recombination rate of electrons and holes thereby increasing the base current of these devices. After the radiation ceases, these defects undergo reactions that produce composite defects that are less effective at carrier recombination. In this presentation, we discuss simulations of the temporal evolution of these defects and the carrier recombination rate. We find that the initial carrier recombination rate is dominated by recombination at interstitials in p-type silicon and vacancies in n-type silicon. For p-type silicon, the short-time annealing is controlled by athermal diffusion of interstitials, and this diffusion rate, which is governed by carrier recombination at the interstitial, can be controlled by injection of minority carrier electrons. Our results will be compared with experimental data.   

Theory of the Cyclotron Resonance in Si and Ge

A quantum theory is developed for the cyclotron resonance (CR) in silicon(Si) and germanium(Ge). The angular dependent CR peaks for heavy ``holes'' are analyzed, using the Dresselhaus-Kip- Kittel (DKK) formula: $\omega=(\omega_{t}^{2}cos^2\theta + \omega_{t}\omega_{l}sin^2 \theta)$, $\omega_{t}\equiv\frac{eB}{\m_{t}}$ ,$\omega_{l} \equiv\frac{eB}{\m_{l}}$.Their Fermi surfaces for Si(Ge) are spheroids oriented along $\left\langle 100\right\rangle$ axes with the transverse mass $m_{t}=0.46(0.29)m$ and the longitudinal mass $m_{l}=1.03(0.78)m$.The fluted energy surfaces represented by the quartic dispersion relations used by DKK were avoided.The CR should be observed only, when a substantial number of conduction electrons with a quadratic dispersion relation resonate at a single frequency.   

Stability of strained monohydride H:Si(105) and H:Ge(105) surfaces

We report atomic scale studies of the effect of applied strain and hydrogen environment on the reconstructions of the (105) Si and Ge surfaces. Surface energy calculations for monohydride-terminated (001) and (105) reconstructions reveal that the recently established single-height rebonded model is unstable not only with respect to (001), but also in comparison to other monohydride (105) structures. This finding persists for both Si and Ge, for applied biaxial strains from -4\% to 4\% , and for nearly the entire relevant domain of the chemical potential of hydrogen, thus providing a thermodynamics perspective on the recently observed H-induced destabilization of the Ge(105) surface.   

Using Si(100) - 2 x 1:H as a Platform for Patterned Silicon Growth

An ultra-high vacuum scanning tunneling microscope (UHV-STM) is used to create patterns at the atomic level by desorbing hydrogen atoms from the Si(100) -- 2 x 1:H surface thereby creating a clean silicon template for selective area chemical vapor deposition (CVD). Disilane (Si$_{2}$H$_{6})$ gas, when introduced, repassivates the clean silicon pattern with an inert mix of hydride and silicon-hydride species. Subsequent layers can be grown through repeated patterning allowing controlled silicon growth at the nanometer scale. Amorphous silicon growth has been demonstrated at room temperature with nanometer scale control over feature sizes. Results will also be presented on our attempts to grow single crystal silicon features by performing these experiments at elevated temperatures to promote silicon surface diffusion.   

Hole mobility in SiGe alloys from first principles.

First principles density functional theory is used to calculate the mobility of holes in Si$_{1-x}$Ge$_{x}$ alloys as a function of alloy composition. The alloy host is modelled within the virtual crystal approximation (VCA) using supercell techniques. The scattering matrix for carrier scattering in the presence of a Ge or Si substitutional atom in the VCA lattice is determined from the resultant energy splitting in the valence bands. The effect of the spin-orbit interaction is included in these calculations. The mobility is obtained from the scattering rate using the Boltzmann transport equation in the relaxation time approximation.   

First-principles calculation of phonon scattering of n-type carriers in SiGe alloys

We calculate the scattering matrix due to phonons in a Si$_{1- x}$Ge$_x$ random substitutional alloy using first principles density functional theory. The electron-phonon matrix elements are obtained from linear response theory in supercells containing up to 128 atoms. We calculate the n-type carrier mobility from the scattering rates of phonon and alloy scattering using the Boltzmann transport equation. Results are compared to experiments.   

MEIS study of As and Sb implantation in Si(001) with excess vacancy concentration and SIMOX.

We have studied the behavior of dopants (As and Sb) in Separation-by-IMplanted-OXygen (SIMOX) and in vacancy-rich layers in bulk Si(100). The vacancy layers were created by 240 keV O$_{2}^{+}$ or N$_{2}^{+}$ ion implantation at 400\r{ }C with a dose of 2.5x10$^{16}$ cm$^{-2}$. Each of the O or N pre-implanted samples was then implanted at 20 keV at room temperature with only one of the dopants to a dose of 5x10$^{14}$cm$^{-2}$. The samples were then annealed either using Rapid Thermal Annealing (RTA) or Furnace Annealing (FA) and characterized using Medium Energy Ion Scattering (MEIS) in both channeling and random directions. Bulk Si samples without O or N pre-implantation were also prepared for reference. The results showed marked differences in dopant diffusion between all of the samples, for instance, faster diffusion of As in the N pre-implanted compared to the O pre-implanted ones. Also, diffusion is faster for Sb in SIMOX than in Si without N or O pre-implantation. Another observed effect was that both dopants presented stronger Transient Enhanced Diffusion (TED) in the N pre-implanted samples. These differences might be associated with the structure of the vacancies (large clusters in SIMOX and point defects in the O or N pre-implanted samples) as well as with chemical effects of O or N in the pre-implanted samples.   

DFT calculations of formation energy and properties of Frenkel pairs in Si

The formation of vacancy-interstitial pairs (Frenkel pairs) underlies many materials processes, including the production of defects by irradiation or ion implantation. Here we present the results of density functional calculations of the formation and properties of Frenkel pairs in Silicon for various pair configurations. The detailed relaxations and overall stability of these pairs against recombination, even at large separations, are strongly dependent on the transfer of charge among the vacancy and interstitial states in the Si band gap, and are highly sensitive to the interstitial configuration. The results are discussed with respect to their implications for models of radiation-induced defect damage and related processes. This work was supported in part by AFOSR grant FA9550-05-1-0306.   

Effect of Hydrogen on the Migration Processes of Dislocations in Silicon

It was reported that the dislocation glide mobility in Si is enhanced by irradiation with H plasma. In this work, we perform the first-principles total-energy calculations, and investigate the effect of H on the migration processes of dislocations in Si. We found that the most stable site in the dislocation core is not located at the bond center (BC) site but at the back- bond (BB) site. By interacting with a H atom, the formation energy of a kink becomes negative, and the dislocation is spontaneously bent. As a result, the activation energy of dislocation motion decreases by 1.1eV. The migration processes of the kinks are discussed in the presentation. This study was supported by RSS21 project and Grant-in -Aid for Scientific Research (No.17064017) of MEXT of the Japanese government. The calculations in this work were carried out partly using the Numerical Materials Simulator in National Institute for Materials Science (NIMS), and partly using the NEC-SX5 at Cybermedia Center, Osaka University.   

Self-trapping-enhanced carrier recombination at light-induced boron-oxygen complexes in silicon

First-principles study of the BO$_{2}$ complex in B-doped Czochralski silicon reveals a self-trapping-enhanced carrier recombination mechanism that contrasts with the standard fixed-level Shockley-Read-Hall model for recombination. An O$_{2}$ dimer distant from the boron causes only a slow carrier recombination, which is, nevertheless, enough to drive the O$_{2}$ diffusion under light to form the BO$_{2}$ complex. We find that the BO$_{2}$ and O$_{2}$ produce nearly identical defect levels in the bandgap. Despite that, recombination at the BO$_{2}$ is substantially faster than that at the O$_{2}$, because the charge state of the latter inhibits a key step in the recombination, namely, the capture of the hole. This work was supported by the U. S. Department of Energy, BES and EERE, under Contract No. DE-AC39-98-GO10337.   

Energy gain by defect formation: a new tight binding annealed model for a-Si

We have obtained a new structure of a-Si by annealing with QM forces a Wooten CRN type 216 model. The annealing was done for 1.2 ns (1.2 million time steps) and the NRL tight binding method was employed. We obtain the total energy, electronic density of states, and force constants of the model. Whereas the starting model had no coordination defects several were present in the final model. As expected these defects were found to give rise to states in the gap as determined both by charge self-consistent TB calculations and DFT (pseudopotential) calculations, where the latter were performed on related ``relaxed'' models within DFT. The final model was of lower total energy than the starting model, both within the TB method and within the psedopotential method. The normal mode properties based on our force constant determination within TB will also be discussed.   

Effect of e-h Pairs on the Stability of Bond-Center Hydrogen in Silicon

Hydrogen injected into silicon at low temperature resides in the bond-centered site, H$^{+}$[BC]. This is the most fundamental hydrogen related defect in many covalent semiconductors. Thermal annealing shows H$^{+}$[BC] stability up to 150K, and correlates with Si vacancy migration. We report the first observation of the decay of H$^{+}$[BC] due to e-h pairs induced by energetic ions. Thin single crystal silicon films containing H$^{+}$[BC] ($\sim $35$\mu $m) were fabricated to permit subsequent transmission of energetic protons, creating high e-h pair concentrations and minimal displacement damage. In-situ infrared studies revealed a 90{\%} reduction of H$^{+}$[BC] sites after a 1.8 MeV proton dose of 50$\mu $C/cm$^{2}$ at 80K. Recombination of e-h pairs at defect sites is known to cause vacancy diffusion providing another mechanism for loss of H$^{+}$[BC]. Our results show the effect of e-h pairs on H$^{+}$[BC] stability. We interpret this in terms of the e-h pair/vacancy interaction.