In this work, we examine the structural and electronic properties of very thin suboxide layers embedded in silicon. These layers of about 2nm width are fabricated by exposing a silicon (001) surface to molecular oxygen up to background pressures of 2×10−6mbar inside the growth chamber of a molecular beam epitaxy system. This layer was then overgrown with silicon. At substrate temperatures between 550°C and 600°C in situ RHEED measurements reveal single crystalline overgrowth with the 2×1 surface reconstruction reappearing after about 60nm. Transmission electron microscopy reveals a continuous layer about 2nm wide with some inhomogenities. Electrical transport measurements across such a barrier exhibit current blocking behavior at low temperatures up to about 0.5V. Current at a given voltage increases with a thermal activation energy of about 50meV. Electron transport across such barrier layers generates hot electrons which are demonstrated to excite erbium ions, leading to light emission at 1.54μm wavelength.

Thin silicon suboxide (SiOx) layers were fabricated in situ by molecular beam epitaxy. The layers were grown by introducing molecular oxygen up to a background pressure of 2×10−6 mbar, while maintaining a silicon flux of 0.1 Å s−1. The layer thickness was varied between 2 and 200 nm. Ellipsometry and SIMS measurements indicate an oxygen content x between 1 and 1.6. The suboxide layers were overgrown with polycrystalline silicon. Transmission electron microscopy (TEM) measurements reveal a uniform amorphous suboxide layer with a sharp interface on the substrate side and some roughness on the surface side. Current–voltage characteristics of transport through such a layer show current blocking behavior up to ≈1.5 V for 10 nm layer thickness at low temperatures. At higher temperatures, the current at fixed voltage increases with an activation energy of ≈120 meV. It is demonstrated that the suboxide potential barrier can be used to enhance impact excitation of erbium ions in silicon by increasing the number of hot electrons with energies >0.8 eV.

We studied the electroluminescence of Er:O-doped Si pn diodes and unipolar structures with thin SiO1.6 suboxide barriers, which were deposited by molecular-beam epitaxy. These suboxide layers reveal a barrier height of about 320 meV in the conduction band and therefore raise the average kinetic energy of electrons injected through the barrier into the Er:O doped region. These electrons turn out to be advantageous for impact excitation processes with the erbium ion. Compared to conventional reverse biased pn diodes a ten-times higher στ product for impact excitation (1.2×10−19 cm2 s) can be achieved in pn diodes with a suboxide injector at 10 K. The saturation electroluminescence (EL) intensity is enlarged in reverse bias and suppressed in forward bias compared to a diode without a suboxide layer. These structures exhibit a reduction of the EL intensity by a factor of 3 for increasing temperature from 10 to 300 K and yield a two-times higher EL output at 1.54 μm and 300 K than an optimized reverse biased pn diode without a suboxide layer. At 300 K this results in an absolute output power of 250 nW and an external quantum efficiency of 1.3×10−4 at 1.54 μm. For the unipolar structure with an integrated suboxide barrier the EL output also depends on the current flow direction: Injecting the electrons hot through the suboxide barrier into the Er:O doped region results in a six times higher EL intensity at 1.54 μm than for the opposite biasing condition. The EL is detectable up to 300 K with a reduction of the intensity by a factor of 8 between 10 and 300 K. Monte Carlo simulations were performed on unipolar structures with an incorporated barrier to provide insight into the carrier density and carrier energy distribution after injection through the barrier.

We have investigated the doping and electroluminescence (EL) properties at 1.54 μm of erbium ions in Si:O and Si1−yCy:Er layers completely grown by MBE. Erbium in Si1−yCy:Er films shows a lower doping density (p=+1.6×1017 cm−3) than erbium in Si:O (n=−3×1018 cm−3). The energy levels responsible for doping and optical erbium activation are different because different activation energies are observed. To realise both maximum luminescence output and low doping concentration, it is important to incorporate the carbon on substitutional lattice sites. The efficiency for electron and hole excitation by impact processes is compared. Electrons are about 5000× more efficient for impact excitation of Er3+ than holes.

In this letter we report about the influence of the active region, the carrier type and their energy distribution on the impact excitation efficiency of erbium in silicon, By counterdoping the donors induced by erbium ions the active region, which is correlated to the space charge region width, can be enlarged to a maximum external quantum efficiency of 5 x 10(-5) at 300K, Electrons axe more efficient in impact exciting erbium ions than holes, The electron energy is tuned to higher energy by special suboxide injectors allowing to further increase the impact excitation cross section.

We report about optical experiments on single self-assembled quantum dots. In power-dependent low-temperature magneto-photoluminescence experiments we have analysed the emission spectra of single dots for increasing exciton occupation numbers. Decays from different configurations (up to 4 excitons) lead to a renormalization of the emission lines in the region of the s- and p-shell of the dot. Photoluminescence excitation spectroscopy further allows us to explore the absorption properties of a single-quantum dot. Both interband absorption and comparably strong phonon-assisted absorption via InGaAs and GaAs LO phonons are observed, as well as sequential phonon-assisted biexciton generation followed by sequential biexciton decay. Working towards applications of single quantum dots, local charge injection/extraction into/from dots is performed on special p–i–n structures via a STM-tip. STM-induced luminescence from a single-dot results in single-line emission from the quantum dot ground state under the condition of low injection currents. Reverse operation of such a single quantum dot LED allows in addition for spectrally resolved photocurrent experiments. Characteristic sharp peaks in the STM tip current vs. excitation energy are attributed to resonant quantum dot interband absorption processes.

In this letter, we report on the excitation efficiency of erbium ions by hot electrons and holes in Si:Er:O and Si1−yCy:Er pn diodes at 10 K. In forward bias, a higher electroluminescence efficiency at 1.54 μm is observed for incorporating the erbium ions in the p region of the diode, where enough holes are present to form bound excitons for erbium excitation. In reverse bias, electrons turn out to be 5000 times more efficient in impact exciting of Er3+ than holes at equal space-charge region widths. A dark region of 45 nm for electrons and about 70 nm for holes is present where no erbium excitation is possible. Impact excitation of Er codoped by C is much less efficient than for O codoping.

Summary form only given. Quantum dots, often referred to as artificial atoms, open the field of quantum resolved spectroscopy to semiconductor physics. Using near-field microscopy through electron beam written shadow masks, we have isolated single InGaAs quantum dots for advanced spectroscopic analysis. In power-dependent low-temperature photoluminescence studies, we have investigated the influence of few particle interactions on the emission spectra for increasing exciton occupation numbers. Decays from different configurations (up to 4 excitons) lead to the appearance of specifically renormalized emission lines in the region of the s- and p-shell. Contributions from different shells can thereby be assigned by magneto-photoluminescence experiments.

We report about spatially resolved magneto-optical experiments on a self-assembled InGaAs quantum dot. Using electron beam lithograpy for patterning a metal shadow mask we can isolate a single dot. This allows us to study the optical response of a single dot as a function of excitation power and magnetic field. We investigate the influence of many body interaction in the emission spectra for different exciton occupation numbers of the dot. The diamagnetic/orbital shift as well as Zeeman splitting in a magnetic field can be fully resolved and are used to identify the observed emission lines. Further we report on absorption properties of the quantum dot as a function of magnetic field. We analyse in detail the phonon-assisted absorption process connected with the GaAs LO-phonon 36 meV above the single-exciton ground state.

We have studied the influence of carbon codoping in Si1−yCy:Er layers on the photoluminescence efficiency of the 1.54 μm wavelength Er emission. All samples were prepared by molecular beam epitaxy (MBE) with carbon concentrations between y=0.08 and 0.4%. Maximum photoluminescence output at low temperature T=5 K could be realized for growth temperatures around 430°C and an erbium to carbon content ratio of about one ([Er]=4.5×1019 cm−3, y=0.1%). The efficiency could be further enhanced by annealing and is comparable to our best Si:Er:O samples. A decrease in photoluminescence intensity at 1.54 μm was observed for increasing sample temperature. It decreases stronger for carbon codoped Si1−yCy:Er layers than for Si:Er:O samples. High resolution photoluminescence spectra show both a difference in the spectral position of the main erbium line as well as in the fine structure for oxygen and carbon codoping.

Optical properties of single and coupled quantum dots are studied with high spatial resolution. Several techniques are used to resolve spectra of individual dots. Spatially isolated dots produced by double cleaved edge overgrowth are investigated by diffraction limited spectroscopy using a special microscope setup. Self-assembled InGaAs dots have been investigated by near-field optical techniques with metallic masks and by scanning tunneling microscopy cathodoluminescence. Photoluminescence and photoluminescence excitation spectra of selected quantum dots are discussed which reveal detailed information on ground and excited states as well as their coupling with neighboring dots.

Photoluminescence (PL) at 1.54 μm of erbium-doped Si1−yCy alloys grown by molecular beam epitaxy (MBE) has been analyzed depending on sample temperature, excitation density, and growth conditions. Erbium activation raises with increasing incorporation of substitutional carbon compared to interstitial carbon. For [Er]=4.5×1019 cm−3 and y=0.1% maximum PL output at 1.54 μm was achieved for growth temperatures at 430 °C. Additional annealing could further enhance PL intensity at 1.54 μm. For increasing sample temperature a decrease of PL intensity with two characteristic activation energies around 100 and 10–20 meV is observed, which results in quenching of PL intensity at lower temperatures for Si:Er:C layers compared to Si:Er:O layers. PL spectra show different fine structure for oxygen and carbon codoping by MBE or ion implantation, higher efficiency, and lower PL background for MBE-grown samples in contrast to ion-implanted layers.

We report about spatially resolved experiments on self-assembled InGaAs quantum dots. Single quantum dots can be investigated by using STM-induced luminescence spectroscopy. The quantum dot occupancy can be increased via the STM tip current, which results in state filling and therefore in the onset of discrete excited state luminescence. In the limit of low injection currents, a single emission line from the ground state of the dot is observed. Using near-field spectroscopy through shadow masks, we have investigated the optical properties of single self-assembled InGaAs quantum dots as a function of occupancy and magnetic field. This allows us to fully resolve diamagnetic/orbital effects, Zeeman splitting, and to determine manybody-corrections. Photoluminescence excitation spectra further reveal a strong contribution of phonon assisted processes in quantum dot absorption.

In this letter, we report on the influence of the space charge region width in erbium- and oxygen-doped silicon light-emitting diodes on the electroluminescence (EL) power at 1.54 μm under reverse bias conditions. The space charge region width was varied by codoping the Si:Er:O layer with boron, thereby compensating the Er–O donors. We observe a strong enhancement of the EL power with increasing width. The data indicate the existence of a dark region of approximately 45 nm in the pn junction, in which no light is generated due to a lack of hot carriers which are necessary for impact excitation of Er3+ ions.

We report about spatially resolved magneto-optical experiments on two different confined semiconductor systems. Using near-field spectroscopy through AFM-written shadow masks we have investigated the optical properties of single self-assembled InGaAs quantum dots as a function of excitation power and magnetic field. This allows us to identify and fully resolve diamagnetic/orbital effects and the Zeeman splitting in the ground state and excited state of a given quantum dot. Further we report on the transport properties of minority holes in a high mobility two-dimensional electron gas at finite electric fields and high magnetic fields. Making use of spatially resolved optical excitation and detection we study the propagation of optically excited minority holes within the sea of a drifting two-dimensional electron gas. At high magnetic fields we observe an E × B drift of holes which is dominated by the Hall-field of the electron gas. At the edge of the Hall-bar, we observe edge state transport of holes via skipping orbits opposite to the direction of the initial E × B drift.

We report about optical experiments on (i) natural quantum dots formed by well width fluctuations in narrow GaAs quantum wells and on (ii) self-assembled InGaAs quantum dots in GaAs. Contributions from a single natural quantum dot are isolated by spatially resolved spectroscopy and resonant charge injection in an electric-field-tunable coupled quantum well structure. In optical experiments the excitonic and bi-excitonic ground states are identified and growth related interface problems are analyzed. Spectral linewidths down to 50 meV allow thereby a complete separation of the individual levels and Zeeman sub-levels of the dot. The natural quantum dots with their fully resolved Zeeman levels are used as local probes to monitor the spin orientation and the creation of spin-polarized excitons in coupled quantum well structures. With cathodoluminescence excited by a scanning tunneling microscope tip and a special sample design we are able to excite selectively the ground state of a single self-assembled InGaAs quantum dot.

Low temperature STM-cathodoluminescence spectroscopy has been performed on single self-assembled InGaAs quantum dots. Specially designed samples with an electron injector have been grown by MBE on a GaAs substrate, which allow for controlled state filling in the self-assembled dots. Our samples have been analyzed by both spatially resolved photoluminescence and STM-cathodoluminescence. In contrast to the inhomogeneously broadened photoluminescence the STM-cathodoluminescence shows sharp excitonic lines corresponding to discrete levels of a single dot. The occupancy of the dot can be increased in a controlled way via the STM tip current, which results in state filling and therefore in the onset of discrete excited state luminescence. In the limit of low injection currents a single emission line from the ground state of a single dot is observed.

Fully confined excitons are observed in natural quantum dots, which are formed by well width fluctuations in narrow GaAs quantum wells. Contributions from a single quantum dot can be isolated by using spatially resolved spectroscopy and resonant charge injection in an electric-field-tunable coupled quantum well structure. In optical and magneto-optical experiments the excitonic and biexcitonic ground states are identified and analyzed.