Electron spin resonance of ion-implanted Si:P systems

The ESR of Si:P and (Si:P):Sb systems made by ion implantation has been observed. An anomalous line-broadening appears in the (Si:P):Sb system, and is considered to be due to the large spin-orbit interaction of Sb donor impurity. The effective spin-lattice relaxation time, T_1eff, of both the systems is found to be dominated by a thin layer with the shortest relaxation time T₁(χ).     

Compensation-induced optical transitions within the ground state of shallow donors in Ge and Si

An approximate formula for integral intensity of the compensation-induced parity-forbidden optical transition within the ground state of shallow donors in Ge and Si is derived in the limit of small compensations. The lineshapes for absorptions in Ge(Sb), Ge(P), Ge(As) and Si(As) are also calculated.     

Theory of adsorption: Ordered monolayers from Na to Cl on Si(001) and Ge(001)

First principles calculations of clean and adsorbate-covered surfaces of Si(001) and Ge(001) are reported. Chemical trends in the adsorption of ordered Na, K, Ge, As, Sb, S, Se and Cl overlayers are discussed. The calculations are based on the local-density approximation and employ non-local, norm-conserving pseudopotentials together with Gaussian orbital basis sets. The semi-infinite geometry of the substrate is properly taken into account by employing our scattering theoretical method. From total-energy minimization calculations we obtain optimal surface reconstructions which show asymmetric dimers for Si(001), Ge(001) and Ge:Si(001). For As:Si(001), Sb:Si(001) and Sb:Ge(001), we find symmetric adatom dimers in the equilibrium geometries. S or Se adlayers are found to be adsorbed in bridge positions forming a (1×1) unit cell with a geometry very close to the configuration of a terminated bulk lattice. Cl atoms adsorb on top of the dangling bonds of symmetric Si dimers residing in the first substrate-surface layer. Our calculations for Na:Si(001) and K:Si(001) confirm valley-bridge site adsorption for half monolayer coverage. For full monolayer alkali-metal coverage, adsorption in pedestal and valley-bridge positions is found to be energetically most favourable. The calculated optimal adsorption configurations are in excellent agreement with a whole body of recent experimental data on surface-structure determination. For these structural models, we obtain electronic surface band structures which agree very good with a wealth of data from angle-resolved photoemission spectroscopy investigations.     

Fine structure of shallow donor lines in germanium induced by magnetic field

Photoconductivity spectra of Ge doped with Sb and P were investigated in a magnetic field (H) at the temperature T = 4.2K. Measurements were performed in the Voigt configuration (H ∥ [111]). Fine structure of lines connected with optical transitions from the ground to the excited states of shallow donors due to spin splitting of impurity energy states were observed.     

Impurity doping into Mg2Sn: A first-principles study

We studied the formation energy and atomic structure of impurities in Mg₂Sn using first-principles plane-wave total energy calculations. Twenty elements, namely H, Li, Na, K, Rb, Sc, Y, La, Cu, Ag, Au, B, Al, Ga, In, N, P, As, Sb, and Bi, were selected as the impurity species. We considered structural relaxation of the atoms within the second nearest neighbors of the impurity atom in the 48-atom supercell. The results of the formation energy calculations suggested that Sc, Y, La, P, As, Sb, and Bi are good n-type dopants whereas Li and Na are good p-type dopants. The electrical properties of Li-, Na-, and Ga-doped Mg₂Sn and La-doped Mg₂(Si, Sn) composites reported previously can be explained by the low formation energies of Li, Na, Ga, and La in Mg₂Sn.     

Irradiation-induced compensation of n-type germanium: An optical study of vibrational absorption

n-type germanium crystals doped with As, Sb, P, P + In, P + Ga or As + B have been rendered transparent by high energy e^? or n⁰ irradiation. Localized vibrations of P, [P-In], [P-Ga], B and [B-As] have been observed. It is concluded that the compensation process must occur predominantly by the introduction of defects which act as acceptors and which do not involve the shallow donor impurity atom.     

Systematics of bound excitons and bound multiexciton complexes for shallow donors in silicon

We report luminescence measurements of transitions involving bound multiple exciton complexes in Si:Sb and Si:As. The systematics of the donor systems Si:P, Si:As and Si:Sb are examined. We find that the splittings between the two observed series of lines are independent of complex number and donor type.     

The electrical properties of doped silicon,grown by Molecular-Beam-Epitaxy (MBE)

Measurements of the Hall coefficients and of the resistivity of MBE-grown Si, doped with P, As, Sb, B, and Ga in the concentration range 10¹⁴ to 10²⁰ cm^–3, were carried out at 77 K and at 300 K. With the exception of Ga-doped Si, the measured mobilities were close to or higher than those of bulk materials at both temperatures. The Mott metal/non-metal transition has been observed in the present epitaxial materials and the measured values for the critical impurity concentration at which the transition occurs, agree with values reported by other workers for bulk silicon.     

Probing of the shallow donor and acceptor wave functions in silicon carbide and silicon through an EPR study of crystals with a modified isotopic composition

The spatial distributions of the unpaired-electron wave functions of shallow N donors in SiC crystals and of shallow P and As donors in silicon crystals were determined by studying crystals with a modified content of the ²⁹Si and ¹³C isotopes having a nonzero nuclear magnetic moment. As follows from the present EPR and available ENDOR data, the distribution of donor electrons in SiC depends substantially on the polytype and position in the lattice; indeed, in 4H-SiC, the unpaired electrons occupy primarily the Si s and p orbitals, whereas in 6H-SiC these electrons reside primarily in the s orbitals of C. The electron distributions for the N donor in the hexagonal position, which has a shallow level close to that obtained for this material in the effective-mass approximation, and for the donor occupying the quasi-cubic position differ substantially. The EPR spectrum of N in quasi-cubic positions was observed to have a hyperfine structure originating from a comparatively strong coupling with the first two coordination shells of Si and C, which were unambiguously identified. The effective-mass approximation breaks down close to the N donor occupying the quasi-cubic position, and the donor structure and the donor electron distribution become less symmetric. In silicon, reduction of the ²⁹Si content brought about a substantial narrowing of the EPR line of the shallow P and As donors and an increase in the EPR signal intensity, as well as a noticeable increase in the spin-lattice relaxation time T₁. This offers the possibility of selectively studying these spectra by optically exciting a region of the crystal in order to shorten T₁ and thereby precluding EPR signal saturation only in the illuminated part of the material. This method may be used to advantage in developing materials for quantum computers based on donors in silicon and SiC.     

Forbidden optical transitions between impurity levels in silicon and gallium phosphide

Experimental estimates are made of absorption cross sections for forbidden optical transitions from the ground state to long-lived excited states of P, As, Sb, In, and Ga impurities in silicon and Te impurities in gallium phosphide. The results can be used to predict the possibility of long-wavelength stimulated emission being excited as a result of the population inversion of long-lived impurity states in these materials.     

Determination of the deformation potential of shallow donors in silicon

The stress dependence of the energy of the ground state of group V impurities (P, Sb) in silicon was investigated by measurement of the Hall effect in a wide range of pressures. A conclusion was reached that the deformation potential of the lowest impurity state of shallow donors (Ξ¹_u) in silicon differs from the deformation potential of the conduction band (Ξ_u), the value of this difference being dependent on the type of the impurity. According to our data, the most probable values for ($\text{Ξ}{}_{\mathrm{u}}\text{ Ξ}{}^1{}_{\mathrm{u}}$) are 0.12 eV for phosphorous-doped silicon and 0.06 eV for antimony-doped silicon.     

Vacancy trapping and helium decoration in Sb ion-implanted tungsten studied by Mössbauer spectroscopy

Mössbauer effect measurements have been performed using sources of¹¹⁹Sb implanted in W without and with post-implanted helium. Each of the sources was subjected to an isochronal annealing sequence in order to study vacancy trapping, helium decoration and recovery of damage. Four sites have been identified for Sb implanted in tungsten; one of these corresponds with substitutional Sb atoms, two others are assigned to Sb atoms associated with vacancies, while the last one can be either vacancy or impurity associated. The development of site occupation as a function of annealing temperature is in accordance with the one-interstitial model. Injection of 2·10¹⁶ He/cm² leads to nucleation of helium bubbles. Helium atoms that are released from these bubbles at about 1300 K are retrapped by Sb atoms to form new bubbles.     

Mo study on charge distributions in octahedral PF−6, AsF−6 and SbF−6 anions

The charge distributions in octahedral PF^?₆, AsF^?₆, and SbF^?₆ anions are studied by means of the ab initio molecular orbital method. We find that the central P, As, and Sb atoms are all positively charged in these anions, while F atoms negatively charged, the order of the positive charge being Sb > P > As.     

Effect of hydrogenation on B/Si(0 0 1)-(1 × 2)

Ab initio calculations, based on pseudopotentials and density functional theory, have been performed to investigate the effect of hydrogenation on the atomic geometries and the energetics of substitutional boron on the generic Si(0 0 1)-(1 × 2) surface. For a single B atom substitution corresponding to 0.5 ML coverage, we have considered two different sites: (i) the mixed Si-B dimer structure and (ii) boron substituting for the second-layer Si to form Si-B back-bond structure, which is energetically more favorable than the mixed Si-B dimer by 0.1 eV/dimer. However, when both of these cases are passivated by hydrogen atoms, the situation is reversed and the Si-B back-bond case becomes 0.1 eV/dimer higher in energy than the mixed Si-B dimer case. For the B incorporation corresponding to 1 ML coverage, among the substitutional cases, 100% interdiffusion into the third layer of Si and 50% interdiffusion into the second layer of Si are energetically similar and more favorable than the other cases that are considered. However, when the surface is passivated with hydrogen, the B atoms energetically prefer to stay at the third layer of the Si substrate.     

Impurity centers in ZnSe:Na crystals

The influence of sodium impurity on photoluminescence (PL) spectra of ZnSe crystals doped in a growth process from a Se+Na melt is investigated. It is shown that the introduction of the impurity results in emergence of emission bands in the PL spectra due to the recombination of exciton impurity complexes associated with both donors and hydrogen-like acceptors. Apart from that, four bands generated by donor-acceptor pairs recombination and a band produced by electronic transitions from the conduction band to a shallow acceptor are discussed. As a result of the analysis it is concluded that Na impurity forms in ZnSe lattice Na_Zn hydrogen-like acceptors with activation energy of 105±3 meV, Na_i donor centers with activation energy of 18±3 meV, as well as Na_ZnV_Se and Na_iNa_Zn associative donors with activation energy of 35±3 and 52±9 meV, respectively.     

Impurity pairs and excitation relaxation in doped silicon

The kinetics of photoconductivity is studied in silicon doped with B, Al, Ga, In, P, As, and Sb with concentrations of 10₁₆–10₁₈cm_?3 at 4.2 and 10.5 K placed in an 8-mm microwave electric field under pulsed impurity excitation. It is found that infrared absorption by impurity pairs and a slow component of photoresponse relaxation arise at close impurity concentrations. It is shown that this component is due to an increase in the polarization hopping conductivity in the presence of the optical charge exchange of impurity states—isolated impurities and impurity pairs and dipoles (pairs of the major and compensating impurities). The hopping transfer processes of ion charges in the course of relaxation are analyzed. It is shown that the main contribution to polarization photoconductivity comes from hopping transitions in impurity pairs at relatively small concentrations and from hopping with the participation of isolated ions at increased concentrations.     

Indium-defect complexes in silicon studied by perturbed angular correlation spectroscopy

The formation of molecule-like complexes, consisting of a defect and a radioactive¹¹¹In atom, is studied using the perturbed angular correlation technique (PAC). The complexes are characterized by their defect specific electric field gradients which also contain information on the geometry of the formed complexes. Whereas the complex is formed with the¹¹¹In atom, its electric field gradient is measured after the decay of the radioactive¹¹¹In atom to¹¹¹Cd. Formation and dissolution of the molecule-like complexes is pursued for a variety of different conditions, such as sample temperature, dopant concentration and position of the Fermi level. In particular, the interaction of In atoms with the following defects in Si was investigated: Intrinsic defects, created by particle irradiation; substitutional donor atoms (P, As, Sb, Bi); and interstitial impurity atoms (Li, H, and an unidentified X defect); especially, the latter ones are known to passivate acceptor atoms in Si. Methodology and specific properties of the PAC technique will be illustrated with the help of these examples.     

Structure, electrical and magnetic properties, and the origin of the room temperature ferromagnetism in Mn-implanted Si

The structure and the electrical and magnetic properties of Mn-implanted Si, which exhibits ferromagnetic ordering at room temperature, are studied. Single-crystal n- and p-type Si wafers with high and low electrical resistivities are implanted by manganese ions to a dose of 5 × 10¹⁶ cm^?2. After implantation and subsequent vacuum annealing at 850°C, the implanted samples are examined by various methods. The Mn impurity that exhibits an electric activity and is incorporated into the Si lattice in interstitial sites is found to account for only a few percent of the total Mn content. The main part of Mn is fixed in Mn₁₅Si₂₆ nanoprecipitates in the Si matrix. The magnetization of implanted Si is found to be independent of the electrical resistivity and the conductivity type of silicon and the type of implanted impurity. The magnetization of implanted Si increases slightly upon short-term postimplantation annealing and disappears completely upon vacuum annealing at 1000°C for 5 h. The Mn impurity in Si is shown to have no significant magnetic moment at room temperature. These results indicate that the room temperature ferromagnetism in Mn-implanted Si is likely to be caused by implantation-induced defects in the silicon lattice rather than by a Mn impurity.     

An atomistic study of grain boundary segregation and cracking

A Monte Carlo model is applied to study a Σ = 5 (310) fcc tilt boundary structure and impurity segregation to this boundary. The Johnson-type potential functions are used to express atomic interactions for two binary alloys of NiCu and CuSb systems. Through atomic relaxation near absolute zero, the basic structural unit of the Σ = 5 CSL boundary is found to dissociate into two subunits. For the NiCu system, copper segregation to the boundary follows the Gibbsian equilibrium segregation behavior in which the segregation is localized on a few atomic layers, its amount increases with increase in the bulk concentration but decreases with increase in temperature. For the CuSb system, an assumption of the CuCu bond weakening due to the strong, adjacent CuSb bonds is also incorporated to realize the embrittling effect of Sb atoms. The preferential site for the large Sb atoms is the vertex of a pentagonal bipyramid in the Σ = 5 grain boundary. Without bond weakening, the Sb-segregated grain boundary maintains a structure combined with a distorted pentagonal bipyramid and a capped trigonal prism. However, with bond weakening some bonds of the trigonal prisms are disrupted in order to form new pentagonal bipyramids. When a uniaxial strain is applied in the direction perpendicular to the grain boundary plane, the weakened copper bonds become the source of microcracks thus enhancing brittle fracture along the Sb-segregated grain boundary.     

Local field corrections to binding energies of substitutional and interstitial donors in Si and Ge

Intervalley Umklapp matrix elements, with inclusion of local field screening effects, are computed for substitutional and interstitial point-charge impurity potentials in Si and Ge. The shallow character of substitutional donors is not affected in Ge and is even reinforced in Si, where a 20% reduction of the binding energy is obtained as a consequence of the local field effect. In both semiconductors the local field corrections strongly reinforce the non-effective-mass, deep-level character of interstitial point-charge donors.