A new recombination mechanism occuring in semiconductors containing extended defects is presented. The model is based on experimental data both from hydrogen plasma treated silicon, containing extended defects like platelets, and from oxygen precipitated silicon. The broad photoluminescence bands from these samples are attributed to the heavily damaged regions surrounding the extended defects, where electrons and holes can be localized in the strain-induced potential wells. From a theoretical calculation it is shown that the compressive strain field surrounding [111] and [100] platelets are sufficient to cause a local band gap reduction of as much as 0.3 eV, consistent with the experimental data.

The optical absorption of two new electronic transitions in silicon doped with tin has been investigated. At low temperatures two no-phonon lines are observed at 2755.3 and 4112.2 cm-1, each with strong coupling to a single quasi-localized vibration in the excited electronic state. These vibrations have quantum energies of 69.6 and 70.2 cm-1, respectively. At higher temperatures coupling to thermally excited vibrational states in the ground electronic states is observed for both lines. The transition with the no-phonon line at 4112.2 cm-1 has been studied in detail and is found to be well described using the adiabatic and Condon approximations. The optical properties of the two transitions are found to be quite similar. Moreover the relative intensities of the two lines are found to be dependent on the optical excitation conditions.

A new optical transition with a no-phonon energy of 0.615 eV discovered in electron-irradiated silicon grown by the Czochralski technique is investigated, revealing metastable properties of the related defect. The investigation is focused on the optical properties of the transition and its associated structure and on the mechanism governing the change of defect configuration. The transformation of the defect to the metastable state is suggested to be induced by excitonic Auger recombination. A pseudo-donor model is presented as an explanation of the optical spectrum.

Photoluminescence measurements on silicon implanted with boron, phosphorus, and dual implanted with phosphorus and boron are reported. A high intensity luminescence band between 0.9–1.03 eV is observed in the samples with dual implantation of boron and phosphorus. The luminescence band has similar spectral characteristics as that of the 1.018 eV W or II band which is observed in silicon samples irradiated with neutrons or ions. However the annealing behaviour of the luminescence band observed in our samples differs considerably from that of the W band, whose intensity increases on annealing at 525 K and is quenched on heating to 600 K. The luminescence band in our samples is not annealed out even at 900 C. The temperature dependence of the intensity of the luminescence band is also studied. It shows excitonic behaviour with an activation energy of 52 meV.

The photoluminescence from silicon lightly doped with copper and rapidly quenched from ~ 1100° C is dominated by a vibronic band with zero--phonon lines (ZPL) in the region of 943 meV. We have studied the effect of external fields on the electronic properties of this band and the results are consistent with the luminescence occurring at a defect with Td symmetry. The Zeeman measurements indicate that the luminescence is due to the recombination of excitons in J = 1, J = 2 states with the normal ordering of these levels reversed. This reversal is explained by the different response of these states to hydrostatic strains, and it is also argued that strain at the defect provides the binding potential for the exciton.

Radiation-induced defects (RD) in Si.:Al have been investigated by means of infrared absorption (IRA) and photoluminescence (PL) methods. It has been found that the main Al containing RD in electron-irradiated Si is an interstitial Al atom (Al1). In the process of annealing at about 225 °C the production of Al1-Al1 pairs takes place. The activatin energy of this RD production is 0.95 eV, which corresponds to Al1 migration energy along hexagonal interstitials. Al1 is shown to be a two - charge donor with the energy level E(+/++) = Ev + 0.20 eV. Besides the above mentioned defect as well as acceptor type defect with Ev + 0.21 eV level a number of other defects has been revealed in neutron-irradiated Si:Al. These centers give rise to a great number of IRA and PL lines in the spectral range from 0.1 to 1.2 eV. The classification of the observed defects has been done on the basis of the annealing results and uniaxial stress measurements.

The defect states introduced in P-, As- and Sb-doped silicon upon room-temperature electron-irradiation are studied by deep-level transient spectroscopy (DLTS). Evidence is provided for the involvement of the P-atom and the vacancy in the defect complex giving rise to the prominent electron trap commonly observed at ≈ EC-0.4 eV (EC being the edge of the conduction band). This electron trap together with another at EC-0.30 eV, apparently phosphorus related, exhibit configurationally metastable behaviour. Other electron traps observed at EC-0.27 eV and Ec-0.51 eV in Sb-doped material and EC-0.34 eV in As-doped material are attributed to complexes involving Sb and As atoms, respectively.

We present a systematic theoretical investigation on four vacancy-phosphorus impurity complexes in silicon, i.e., a vacancy with one through four phosphorus impurities on the nearest neighbour sites of the vacancy, using a semi-empirical self-consistent tight-binding theory. The calculations are based on the Lanczos-Haydock recursion Green’s function method. The predicted energy levels in the band gap for the five cases, the isolated Si vacancy and the four complexes, show a remarkable regularity. We shed light on this regularity by relating it to the localization of the wavefunctions on the Si and P atoms surrounding the vacancy. We compare our results with experimental work.

We have studied the effects of low-temperature preannealing and carbon on new donor formation at 650°C in phosphorus-doped Czochralski (CZ) silicon by deep-level transient spectroscopy (DLTS). In not preannealed carbon-lean samples, only a weak continuous DLTS spectrum often reported so far was observed. The intensity of this broad feature became significantly stronger in not preannealed carbon-rich samples. On the contrary, preannealed samples showed no such continuous spectra but two new DLTS peaks arising from shallow donor levels. Carbon enhanced the low-temperature peak, but retarded the high-temperature one. The latter peak is in strong correlation with the rodlike defect.

In addition to a new configuration of the previously reported multistable carbon-phosphorus pair, we report on two new multiconfìgurational defects in electron irradiated silicon doped, with arsenic and antimony. These defects are also identified as interstitial carbon-substitutional group V pairs, (). We identified two different types of metastability for . Type I is similar to the bistable carbon-carbon pair defect for which a bond switching mechanism has been reported [1]. Type II is similar to donor-acceptor pairs in silicon which show electrostatically driven metastability [2,3]. The three pairs have many similar features, but also show surprising donor related differences. In this paper we will discuss the general features of these defects.

A new photoluminescence system is reported for copper-doped silicon. The luminescence system consists of two doublet zero phonon lines at 1033.51±.05 meV, 1033.62±.05 meV, 1040.05±.05 meV and 1040.17±.05 meV. We establish from Zeeman studies that the lower energy lines are isotropic magnetic triplets, and the upper energy lines are singlets. From temperature dependence measurements the energy separation between triplet and singlet states is determined to be approximately 6 meV, in agreement with the spectroscopic separation. Also from temperature dependence PL we show that the transition probability ratio for the higher and lower states is 210. We conclude that the main features of the luminescence agree with radiative recombination of excitons bound to an axial isoelectronic defect. The chemical identity of the constituent(s) involved is uncertain although the participation of copper is likely.

We report an investigation on the electronic structure of two bound exciton (BE) systems from a complex defect in S-doped Si, by optical detection of magnetic resonance (ODMR). A spin-triplet (S=1) is identified to be the lowest electronic state of the BE's, which gives rise to deep photoluminescence (PL) emissions when recombining. A weak anisotropy of the magnetic interaction of the BE’s (not possible to resolve in Zeeman data) is revealed, which leads directly to the determination of the symmetry for the excited state of the defect. A S-related complex model is suggested as the identity of the defect. A critical test of two possible metastable configurations of the constituents of a single defect is undertaken.

Three different Electron Paramagnetic Resonance (EPR) signals, one trigonal and two orthorhombic, which originates from iron-indium pairs in silicon are investigated. It is shown that the two orthorhombic spectra can be explained as transitions within the two doublets of a S=3/2 system with a large zero-field splitting. The temperature dependence of-the intensities reveals that the newly discovered spectrum corresponds to the lower doublet and that the zero-field splitting is 9.8 ± 2.0 cm-1.

Excitons bound to neutral donors in AlxGa1-xAs/GaAs quantum wells were observed by high resolution resonant excitation photoluminescence, and temperature dependent photoluminescence measurements. Changes in the binding energy of excitons are observed when the donors are located in the center of the well, at the edge of the well, or in the center of the barrier. The variations in these binding energies are reported as a function of well size from 75–350Å. The binding energies increased as the well size was reduced to about 100Å, with further reductions in well size they decreased.

Light-hole free excitons bound to neutral donors were observed in AlxGa1-xAs/GaAs quantum wells. The transitions were observed, using selective excitation photoluminescence spectroscopy, in the energy region between the light-hole and heavy-hole free exciton transitions where no other intrinsic transitions exist. The neutral donor-bound heavy-hole free-exciton transitions were also observed when the light-hole bound exciton transitions were observed. Quantum well structures which showed no evidence of a heavy-hole donor bound exciton also showed no evidence of a light-hole donor bound exciton.

Free to bound transitions, free hole to bound electron, have also been observed in the AlxGa1-xAs/GaAs quantum wells. The diamagnetic shift of these transitions was used to distinguish them from excitonic transitions.

In this study we present photoluminescence decay measurements of free exciton (FE) and bound exciton (BE) recombination in doped GaAs/AIGaAs quantum wells (QW). It is found that the FE decay time is reduced in the doped QW’s compared to similar undoped samples. The low temperature decay time of the BE is slightly longer than for the FE, with BE decay times from 300 to 600 ps for well widths of 50 to 150 Å, respectively. It is also found that the observed decay of the BE is strongly influenced by the decay time of the FE. This is especially observed in the similarity of the temperature dependence for the FE and BE decay time.

Transitions from the 1s ground state to 2s excited states of the Be-acceptor confined in GaAs/AIGaAs quantum wells (QWs) have been observed via two independent spectroscopic techniques: Two-hole transitions of the bound exciton (BE) measured in selective photoluminescence and Resonant Raman Scattering. The dependence of the 1s - 2s transition energy on the QW thickness (50 Å < Lz < 140 Å) has been studied for the case of acceptors in the center of the QW. The experimentally determined 1s - 2s transition energies have then been added to recently calculated binding energies for the 2s excited state in order to obtain the total binding energies for the acceptor at different confinements. The derived binding energies are finally compared with theoretical predictions. The same kind of measurements have been performed for a QW with a given thickness, but in which the position of the acceptor has been varied from the center to the edge of the QW.

The dependencies of the binding energies of the exciton bound to the investigated acceptor on the QW thickness and the position of the acceptor in the QW have also been studied. For the case of varying QW thickness, an almost linear relationship between the binding energies of the BE and the acceptor binding the exciton is found. This fact implies that a correspondence to Haynes' rule in bulk material could be applied to these QW systems, but in this case for the same acceptor at different binding energies due to the effect of varying confinement.

Radiative recombination of the two dimensional electron gas (2DEG) in a modulation doped GaAs/AIGaAs heterojunction (the so called H-band luminescence) has been studied under transverse electric field perturbation (i.e. perpendicular to the layers) in special structures prepared by molecular beam epitaxy. Both positive and negative gate voltages have been applied to the GaAs/AIGaAs interface, and shifts of the H-band energy position depending on the gate voltage are induced by the corresponding changes in the notch potential and the potential across the GaAs layer. It is demonstrated that a transverse electric field allows a simple way to modify the width and shape of the potential, so that detailed spectroscopy can be done on the recombination of carriers localized in a interface potential.

We report the first definitive observation of photoluminescence from Si/Si1-xGex superlattice heterostructures. Excitons bound to a deep-level radiation damage center (I1) are observed in a series of low Ge content (x = 0.05) unrelaxed structures. We also present preliminary results on photoluminescence from as-grown narrow period (Si6Ge4) superlattices.

The physics of deep levels in semiconductors is reviewed, with emphasis on the fact that all substitutional impurities produce deep levels - some of which may not lie within the fundamental band gap. The character of a dopant changes when one of the deep levels moves into or out of the fundamental gap in response to a perturbation such as pressure or change of host composition. For example, Si on a Ga site in GaAs is a shallow donor, but becomes a deep trap for x>0.3 in AℓxGa1-xAs. Such shallow-deep transitions can be induced in superlattices by changing the period-widths and quantum confinement. A good rule of thumb for deep levels in superlattices is that the energy levels with respect to vacuum are relatively insensitive (on a >0.1 eV scale) to superlattice period-widths, but that the band edges of the superlattices are sensitive to changes of period. Hence the deep level positions relative to the band edges are sensitive to the period-widths, and shallow-deep transitions can be induced by band-gap engineering the superlattice periods.

The electronic properties of interface roughness in a quantum well are described. Interface roughness is shown to always produce localized bound states. Thus intrinsic roughness can explain the giant oscillator strength observed for “free” excitons in quantum wells.