Table of contents

This special issue contains a collection of papers presented at the workshop entitled `The Physics of Group IV Materials' held in Exeter on 7–10 April 2003. The workshop was sponsored by the two EPSRC-funded Research Networks on `Defects in Silicon and Si/Ge' and `Diamond and Diamond-Like Carbon'. It was attended by about 100 delegates, most of them students and postdoctoral researchers from the UK, who heard 17 invited talks from speakers prominent in their fields from all over the world. Eight additional oral presentations selected from the contributed papers and 36 posters were presented at the workshop. The discussions were very illuminating, and we hope that the workshop fulfilled its objective in informing quite a wide, and generally young, audience about the very diverse and excellent current research work on Group IV materials. We would like to thank the local organizers, Professor Bob Jones and his students James Adey and Stephen Sque, for the smooth running of the workshop. We would like to thank the editorial committee: Dr David Carey, Professor Paul Coleman, Professor Phillip John, Dr Mark Newton and Professor Arthur Willoughby for their help with the selection of papers for the workshop and with the proceedings.

Radiation-induced divacancy-related levels in high-purity oxygen-enriched n-type silicon have been studied with the use of deep level transient spectroscopy (DLTS) and Laplace-DLTS. It has been shown that heat treatment at 250 °C results in a shift of the divacancy (V₂)-related peaks observed by 'standard' DLTS. Using Laplace-DLTS it is demonstrated that the shift is due to annealing of V₂ and formation of a new acceptor centre. The new centre has presumably two negative charge states: singly and doubly negative. The formation of the new centre holds a close one-to-one correlation with the annealing of V₂, indicating that the new centre is a result of divacancy interaction with an impurity or a defect. The close position of the electronic levels of the new centre to that of V₂ suggests a similar electronic and microscopic structure of the new centre to V₂, and a tentative identification is a divacancy–oxygen centre.

Defect reactions associated with the elimination of divacancies (V₂) have been studied in n-type Czochralski (Cz) grown and float-zone (FZ) grown Si crystals by means of conventional deep-level transient spectroscopy and high-resolution Laplace deep-level transient spectroscopy (LDLTS). Divacancies were introduced into the crystals by irradiation with 4 MeV electrons. Temperature ranges of the divacancy disappearance were found to be 225–275 °C in Cz Si crystals and 300–350 °C in FZ Si crystals upon 30 min isochronal annealing. Simultaneously with the V₂ disappearance in Cz Si crystals a correlated appearance of two electron traps with activation energies for electron emission 0.23 eV {E(0.23)} and 0.47 eV {E(0.47)} was observed. It is argued that the main mechanism of the V₂ disappearance in Cz Si crystals is related to the interaction of mobile divacancies with interstitial oxygen atoms. This interaction results in the formation of V₂O centres, which are responsible for the E(0.23) and E(0.47) traps. Electronic properties of the V₂O complex were found to be very similar to those of V₂ but energy levels of the two defects could easily be separated using LDLTS.

In FZ Si crystals, a few electron traps appeared simultaneously with the V₂ annihilation. The small concentration of these traps compared with the V₂ concentration before annealing prevented their reliable identification.

We show that the detailed atomic structure of vacancy complexes in Si can be experimentally determined by combining positron lifetime and electron momentum distribution measurements. The divacancies, vacancy–oxygen complexes (A-centres) and vacancies paired with a donor impurity, V–P and V–As (E-centres), are identified in electron-irradiated Si. The formation of native vacancy defects is observed in highly As-doped Si at the doping level of 10²⁰ cm⁻³. The defects are identified as monovacancies surrounded by three As atoms. We verify the formation mechanism of these defects by annealing experiments in irradiated Si. We show that the migration of V–As pairs at 450 K leads to the formation of V–As₂ complexes, which convert to V–As₃ defects at 700 K. These results explain the electrical deactivation and clustering of As in epitaxial or ion-implanted Si during postgrowth heat treatment at 700 K.

Hydrogen in a dilute SiGe alloy is considered theoretically in comparison with hydrogen in Si and Ge. Structural, vibrational and electronic properties of crystalline alloys containing  at.% Ge complexed with single hydrogen atoms are considered. The behaviour of bond-centred hydrogen is found to be weakly perturbed, when compared to hydrogen in pure Si.

Divacancy–hydrogen complexes (V₂H and V₂H₂) in Si are studied by ab initio modelling using large supercells. Here we pay special attention to their electronic structure, showing that these defects produce deep carrier traps. Calculated electrical gap levels indicate that V₂H₂ is an acceptor, whereas V₂H is amphoteric, with levels close to those of the well known divacancy. Finally our results are compared with the available data from deep level transient spectroscopy and electron paramagnetic resonance experiments.

We report the results of first-principles calculations on the early stages of hydrogen aggregation in silicon and diamond. We demonstrate that the hydrogenated glide dislocation dipole is the preferred structure for small numbers of H atoms in silicon and that it expands by dislocation glide, with hydrogen condensing in the shuffle plane between the dislocations. This structure is a good candidate for the initial stage in the development of hydrogen-induced platelets. We investigate the effect of shear and dilation on the energies of hydrogenated structures and compare the relative stabilities of these structures in silicon and diamond. We describe the method of determination of the Burgers vectors of dilation and shear for the dislocation dipoles by varying the lattice vectors of their supercells.

Czochralski silicon samples implanted to a dose of 5 × 10¹⁵ cm⁻² with 0.5 MeV O and to a dose of 10¹⁶ cm⁻² with 1 MeV Si, respectively, have been studied by positron annihilation spectroscopy. The evolution of divacancies to vacancy (V)–O complexes is out-competed by V–interstitial (I) recombination at 400 and 500 °C in the Si- and O-implanted samples; the higher oxygen concentration makes the latter temperature higher. The defective region shrinks as the annealing temperature increases as interstitials are injected from the end of the implantation range (R_p). V_mO_n (m> n) are formed in the shallow region most effectively at 700 °C for both Si and O implantation. V_xO_y (x< y) are produced near R_p by the annealing. At 800 °C, implanted Si ions diffuse and reduce m and implanted O ions diffuse and increase n in V_mO_n. All oxygen-related vacancy-type defects appear to begin to dissociate at 950 °C, with the probable formation of oxygen clusters. At 1100 °C, oxygen precipitates appear to form just before R_p in O-implanted silicon.

Electronic properties of the vacancy–oxygen (V–O) complex and the interstitial oxygen and carbon atom pair (C_i–O_i) in unstrained Si_1−xGe_x crystals have been studied by means of capacitance transient techniques. The Si_1−xGe_x crystals were grown by the Czochralski method and were doped with either phosphorus or boron during growth. The V–O and C_i–O_i centres were introduced into the crystals by room temperature irradiation with 4 MeV electrons or with γ-rays from a ⁶⁰Co source.

The enthalpy of electron ionization for the single acceptor level of the V–O centre relative to the conduction band edge, ΔH_n, was found to increase with a rate d (ΔH_n)/d x = 0.56 eV upon increase in Ge content. The enthalpy of hole ionization for the single donor level of the C_i–O_i centre relative to the valence band edge, ΔH_p, was found to decrease with a rate d (ΔH_p)/d x = −0.96 eV. For both the V–O- and C_i–O_i-related levels no significant changes in the values of the electron (hole) capture cross section or entropy of ionization with the changes in Ge content were observed. The levels are not pinned to the conduction band edge or to the valence band edge. It is argued that the value of the enthalpy of V–O ionization can be correlated with the lattice parameter or the Si–Si bond length: the larger this parameter, the bigger the enthalpy of the ionization.

We report the results of a study into the influence of implanted impurities on luminescence in the region of the well-known D1 luminescence band that is associated with dislocations in silicon. A photoluminescence band at around 0.78 eV, which is sometimes seen in silicon containing a high density of dislocations, has been attributed to the presence of oxygen complexes. In this study we have deposited layers of Si_0.9Ge_0.1 onto single-crystal Si substrates by MBE in order to induce dislocations in the silicon substrate. The samples have subsequently been implanted with iron, erbium or oxygen in order to study the effect of implanted impurities on D-band photoluminescence at around 800 meV. Following implantation with oxygen, two luminescence bands appear at around 0.85 and 0.78 eV, respectively. These bands are not present in either the unimplanted sample or those subject to Er or Fe implantation. The correlation between oxygen doping and the appearance of these bands supports the conjecture that they are associated with oxygen complexes.

Density functional theory is used to investigate boron interstitial clusters and defects formed with carbon and oxygen. Using data from experimental techniques such as deep level transient spectroscopy, and electron paramagnetic resonance, photoluminescence and infrared studies, we are able to assign structures to many observed centres and begin to develop a series of reaction paths for the evolution of boron with annealing temperature depending on the relative concentrations of impurities.

Among other results we demonstrate that a metastable defect composed of two boron interstitials and a self-interstitial has symmetry, vibrational modes and an electronic structure consistent with the I2 photoluminescence centre, also known as the Y centre.

Ion implantation and plasma processing techniques are routinely used for the fabrication of semiconductor devices. In particular, these techniques employ low energy ions, which modify the electrical and optical properties of the semiconductor material, and, consequently, of the devices that are fabricated thereon, by creating defects in the semiconductor lattice. In this paper, we review our results on the electrical characterization of defects created in Si by low energy noble gas ions (He, Ne, and Ar) and hydrogen ions using deep level transient spectroscopy. The properties of defects introduced in Si_1−xGe_x during ion etching and electron beam evaporation of metal contacts are also reviewed.

The preparation of ordered and arranged Si quantum dots using a SiO/SiO₂ superlattice approach is presented. The different processes of phase separation and crystallization are studied in detail by infrared (IR) absorption and photoluminescence (PL) spectroscopy for different annealing temperatures from 300 to 1100 °C. IR spectra show a continuous shift of the Si–O–Si asymmetric stretching mode to higher energies with increasing annealing temperature, which is a sign of phase separation to Si and SiO₂. Three PL bands are distinguished and correspond to the three processes of phase separation. A band centred at 2.2 eV is present in as-prepared samples and vanishes for annealing above 800 °C which is clearly correlated with defects. The second band shifting from 1.7 to 1.4 eV is detected for annealing temperatures between 300 and 900 °C. A strong red luminescence due to quantum confinement is observed for annealing above 900 °C. Our results indicate that the different and seemingly contradictory PL observations in the literature could originate from different states of network reorganization during the phase separation and crystallization processes. The origins of the different IR and PL bands are discussed in comparison with those of bulk crystalline SiO and SiO₂.

We present the results of theoretical calculations of the structure, diffusion and rotational energies, and the ro-vibrational modes, of the hydrogen molecule in the hexagonal part of 4H-SiC and in 2H-GaN. In both materials, the molecule is stable and aligned along the c-axis. Its rotational barrier is sufficiently large that ortho- and para-forms have almost degenerate ro-vibrational modes. The origin of two modes at 4090 and 4110 cm⁻¹ attributed to molecules observed in multi-transmission FTIR experiments on Mg doped insulating GaN grown by OMVPE is discussed.

Following recent experiments leading to the assignment of a trigonal vacancy–nitrogen–hydrogen complex in single-crystal chemical vapour deposited diamond, we have employed first-principles density functional supercell and cluster techniques to examine the structure and properties of such a defect. We find that the trigonal defect where H bonds directly to the nitrogen atom is unstable and instead a planar structure with H bonding to carbon is the ground state. The complex is expected to be thermally stable as well as optically, magnetically and electrically active. We also discuss the dynamic properties of the complex, and tentatively propose a trigonal form of the defect made up from a superposition of the three equivalent C_1h configurations.

Transition metals (TM) from the first transition series are commonly used as solvent catalysts in the synthesis of diamond by high pressure, high temperature processes. Ab initio calculations on these metals, in finite clusters of tetrahedrally coordinated carbon, enable us to investigate trends in their stability and properties. By carrying out systematic studies of interstitial, substitutional and semi-vacancy TM defects, we show that the electronic structure of the TMs is complicated by the presence of 'dangling bonds' when the TM disrupts the crystal lattice: interstitial defects conform to the Ludwig–Woodbury (LW) model, whilst substitutional and semi-vacancy defects move from approximating the LW model early in the transition series to approaching the vacancy model for the heavier metals. Multi-configurational self-consistent field methods allow genuine many-electron states to be modelled; for neutral interstitial, and all substitutional TMs, the crystal fields are found to exceed the exchange energies in strength. Consequently, low spin states are found for these defects. We find substitutional defects to be the most stable, but that semi-vacancy TMs are very similar in energy to the substitutional defects late in the transition series; interstitial defects are only metastable in diamond. Given appropriate charge compensators neutral and positively charged interstitial TM defects were stable, while negatively charged species appeared to be strongly disfavoured.

All of the electron paramagnetic resonance (EPR) and optical data relating to some of the basic nickel-related defects in diamond have been compared, as well as previously little considered information about the effects of spin–orbit coupling and the magnitude of the crystal field. New models are proposed for the EPR defects, which are found in synthetic diamond grown at high pressure and high temperature with getters to reduce the content of nitrogen impurity: NIRIM-1 and NIRIM-2, which some authors have attributed to isolated interstitial nickel at an undistorted, or distorted, T_d site, respectively. It is suggested that NIRIM-1 is more likely to be substitutional Ni_s⁺, 3d⁵, at a T_d site, than the previously suggested interstitial Ni_i⁺, 3d⁹, and that NIRIM-2 is Ni_i⁺, 3d⁹, pinned at a site 0.308 nm along from a B_s⁻ impurity, beyond one of its nearest C neighbours. This supports the suggestion that isolated interstitial Ni_i is mobile.

The formation and annealing out of paramagnetic defects in high-pressure and high-temperature diamond are investigated through a comprehensive heat treatment study of as-grown crystals. Diamond crystals are subjected to a sequence of isochronal anneals in the temperature range between 1550 and 2000 °C and the concentrations of paramagnetic defects are determined by electron paramagnetic resonance measurements after each annealing step. The quantitative data are analysed in the light of recently published results on the behaviour of nickel and/or nitrogen defects upon annealing. Like the P1 and W8 centres, the NIRIM1 nickel-related defects show a strong decrease in their concentration upon annealing at 1900 °C, whereas the NIRIM2 centres seem to be rather stable. The evolution of the nickel-related centre concentration through the annealing sequence shows that not all W8 defects transform into paramagnetic nickel–nitrogen complexes. Indeed, most of the W8 defects are transformed in other non-paramagnetic centres, with the AB1, AB3, and AB6 defects being probably by-products of this process. The relatively low concentration of nickel–nitrogen paramagnetic complexes formed upon annealing shows that their formation does not play any fundamental role in the kinetics of the nitrogen aggregation during heat treatments.

The perfect 60° glide dislocation in diamond serves as an example of how different aspects of dislocations can be modelled in an approach combining continuum elasticity theory with atomistic density-functional-based tight-binding calculations. After investigating the perfect 60° dislocation and its isolated Shockley partials, the energetics of dissociation are discussed. The 60° dislocation is found to dissociate with a substantial lowering of its line energy. However, an energy barrier to dissociation is found. The glide motion of the 30° Shockley partial involved is modelled in a process involving the thermal formation and subsequent migration of kinks along the dislocation line.

Diamond coatings on Co cemented tungsten carbide (WC-Co) hard metal tools are widely used for cutting non-ferrous metals. It is difficult to deposit diamond onto cutting tools, which generally have a complex geometry, using a single step growth process. This paper focuses on the deposition of polycrystalline diamond films onto dental tools, which possess 3D complex or cylindrical shape, employing a novel single step chemical vapour deposition (CVD) growth process. The diamond deposition is carried out in a hot filament chemical vapour deposition (HFCVD) reactor with a modified filament arrangement. The filament is mounted vertically with the drill held concentrically in between the filament coils, as opposed to the commonly used horizontal arrangement. This is a simple and inexpensive filament arrangement. In addition, the problems associated with adhesion of diamond films on WC-Co substrates are amplified in dental tools due to the very sharp edges and unpredictable cutting forces. The presence of Co, used as a binder in hard metals, generally causes poor adhesion. The amount of metallic Co on the surface can be reduced using a two step pre-treatment employing Murakami etching followed by an acid treatment. Diamond films are examined in terms of their growth rate, morphology, adhesion and cutting efficiency. We found that in the diamond coated dental tool the wear rate was reduced by a factor of three as compared to the uncoated tool.

For biomedical and electronic applications, it is highly desirable to deposit smooth diamond films with crystal sizes in the nanoscale range. We present experimental results of chemical vapour deposition diamond growth from CH₄ with incremental substitution of H₂ with He or Ar gases; the concentrations of the inert gases were varied between 0 and 98 vol%. Results show that initially the addition of either argon or helium increases the growth rate and significantly alters the film structure and crystallinity up to 60 vol%. With additions of argon or helium greater than 60 vol% in the gas phase the growth decreases and there is degradation of the crystal structure. In general, nanocrystalline diamond has been deposited at dilutions in excess of 90 vol% helium or argon.

The growth of polycrystalline diamond films onto molybdenum rods and dental burrs by using a new hot filament chemical vapour deposition (CVD) system has been investigated. Negative dc bias voltage relative to the filament was applied to the molybdenum substrate prior to deposition. This led to much improved film adhesion and increased nucleation density. There was a factor of four improvement in the adhesive force from 20 to 80 N when a bias voltage of −300 V was employed to the substrate.

The CVD coated molybdenum dental burr was found to give much improved performance and lifetime compared to the conventional sintered diamond burr. The CVD diamond burr showed no signs of deterioration even after 1000 operations whereas the conventional sintered diamond burrs were ineffective after between 30 and 60 operations. This represents a 30-fold improvement when CVD is applied. CVD diamond growth onto dental burrs has the potential for replacing exciting technology by achieving better performance and lifetime in a cost-effective manner.