This paper reports a study of the non-radiative processes competing with the excitation of the erbium ion in layers implanted with high concentrations of erbium and oxygen. These processes reduce the luminescence efficiency of the Si:Er system and dramatically increase the threshold current density calculated to be necessary for an ultimate goal, the Si/Ge:Er LASER. Using cross sectional TEM, photoluminescence as a function of temperature and DLTS, it is demonstrated that a two stage anneal procedure which avoids the formation of extended defects and removes specific deep states is necessary to obtain efficient Er3+ excitation at high erbium concentrations. Comparisons are made with damage resulting from germanium implantation into silicon. The role of multiple stage anneals is discussed in relation to the removal of Shockley-Hall-Read recombination centres

Using low temperature magnetotransport techniques we demonstrate that the electrical transport is phase coherent in a GaAs matrix containing self organising ErAs quantum dots and wires of the order of 3 nm diameter. Using telegraph noise spectroscopy we are able to measure the switching of the extremely small magnetic moment (~10-50 µB) of an isolated antiferromagnetic cluster of ErAs. Above a critical temperature (Tc~500 mK) the switching is thermally activated. Below this temperature the switching rate saturates and it is the tunnelling of the magnetisation which dominates. The magnetic field dependence of the cross-over temperature (Tc) from thermally activated to tunnelling regimes is in good agreement with simple theory for macroscopic quantum tunnelling.

We have studied the magnetic properties of self-organized ErAs aggregates embedded in a GaAs matrix using telegraph noise spectroscopy. The electrical transport through the samples is mesoscopic and exhibits universal conductance fluctuations and two-level fluctuations (TLF's). A statistical analysis of a TLF allows us to measure the extremely small magnetization of an isolated ErAs cluster. Rotating the sample in a magnetic field reveals the sixfold magnetic anisotropy expected for ErAs with a rocksalt structure. At high temperatures the switching is thermally activated, while at low temperatures it is the tunneling of the magnetization which dominates. The magnetic-field dependence of the crossover temperature between the thermally activated and tunneling regimes can be explained with a simple model.

Photoluminescence of silicon implanted with erbium and oxygen was measured in the time domain focussing on the temperature and excitation density dependence of the intra-4f-shell emission from Er3+. The decay of this luminescence is similar for the different optically active crystal field split Er-centres. At low temperatures the luminescence transients consist of a fast initial non-exponential component followed by slower exponential behaviour. An increase in excitation density results in a higher proportion of the luminescence decaying with the faster decay time. Our results indicate a relation of the fast component to nonradiative processes. Auger recombination is proposed as a possible mechanism.

The fabrication and characterization of planar, millimetre-wave diodes with in situ/ deposited epitaxial aluminium Schottky contacts is reported. A novel self-aligned technique developed to connect the in situ/ grown epitaxial Al Schottky contacts to the ex situ/ deposited bonding pads is described. The I-V/, noise and thermal characteristics of the epitaxial Al diodes are investigated and compared with those of conventionally made Ti/Pt/Au diodes. The epitaxial Al diodes have better ideality factors, higher barrier heights, higher series resistance and higher breakdown voltages than the Ti/Pt/Au diodes. The junction capacitances are almost the same. The noise figures of the diodes were measured using a single balanced mixer. The epitaxial Al diodes had higher noise than the Ti/Pt/Au diodes. The electrical characteristics of the epitaxial Al diodes were hardly changed (somehow improved) after a high-temperature annealing step, whereas the conventional Ti/Pt/Au diodes were catastrophic failures. The epitaxial Al diodes could have a higher burn-out resistance and a longer life than the conventionally made Ti/Pt/Au diodes.

As a preliminary to realising an Er-doped Si LED emitting at 1.54 mu m we have characterised silicon implanted with erbium over the dose range 5x10(13) cm(-2) to 1x10(15) cm(-2), coimplanted with oxygen or fluorine, and re-grown by a two stage anneal procedure. For the highest dose, damage accumulation results in a fully amorphous layer extending up to the surface, and transmission electron microscopy reveals that, after annealing, threading dislocations extend to the surface in these layers. Extended defects detected by TEM are shown to be optically active, and degrade the erbium-related luminescence.

Telegraph noise, i.e., two-level fluctuations (TLF), in the magnetoresistance of Er-doped GaAs has been used to probe the magnetic moment of the small ErAs clusters formed during the molecular beam epitaxy growth process. At high temperatures the TLF are thermally activated but below 350 mK tunneling of the magnetization dominates.

Sharp luminescence at 1.54 mu m from erbium doped porous silicon has been observed. The silicon was made porous after implantation of high doses of erbium and oxygen into p-type Czochralski silicon. The erbium related luminescence from porous silicon is an order of magnitude more intense than that from erbium doped single crystal silicon.

The electrical and thermal characteristics of three ohmic contact structures: (i) in situ deposited epitaxial Al on delta-doped GaAs, (ii) evaporated Au-Ge on delta-doped GaAs and (iii) evaporated Au-Ge on GaAs were compared with each other. The I-V characteristics of the non-alloyed Au-Ge and the epitaxial Al, as-deposited, on delta-doped GaAs were linear, whereas the non-alloyed Au-Ge contact on GaAs showed a nonlinear I-V characteristic, as expected. The specific contact resistivity (rho(c)) of the non-alloyed Au-Ge contacts on delta-doped GaAs was 1.4 x 10(-6) OMEGA cm2, whereas it was 9 x 10(-7) OMEGA cm2 for the epitaxial Al contacts on 6-doped GaAs, which is the lowest ever reported for a pure metal on delta-doped GaAs. No value of the specific contact resistivity (rho(c)) could be obtained for the Au-Ge contacts on GaAs because of the nonlinearity of the I-V characteristics. After an alloying process at 450-degrees-C all samples showed linear I-V characteristics. The specific contact resistivity of all samples varied inversely with the n+ doping concentration up to 5 x 10(17) cm-3. Above this concentration the specific contact resistivity of the Au-Ge and epitaxial Al contacts on delta-doped GaAs was constant and hardly changed after alloying. Our results confirm that the well known double-barrier (metal-n+-n) model is valid for the ohmic contacts on delta-doped GaAs.