Pronounced Photoluminescence Research Articles

Optical and structural analysis of Ge quantum dots embedded in strained Si quantum wells grown on patterned substrates

AbstractGermanium quantum dots embedded in silicon have been used in the past to improve the opto-electronic properties of Si based materials. The idea is to overcome the limitation of the indirect band gap of Si by a strong localization of the carriers in quantum dots. However, the Ge quantum dots provide a strong carrier confinement only for the holes, the electrons are only weakly confined in the Si. In this study we embedded the Ge quantum dots in strained Si quantum wells grown on relaxed SiGe buffer layers. The strained Si quantum wells provide a confinement of the electrons in the vicinity of the Ge dots.The structures were deposited on planar as well as on patterned substrates by molecular beam epitaxy. The structural and optical properties of the samples were analyzed using high resolution cross sectional transmission electron microscopy (TEM) as well as low temperature photoluminescence. The size of the mesa structures have been used as experimental parameter. Relaxed buffer layers grown on line shaped mesa structures show a strongly reduced dislocation density. Consequently the deep luminescence attributed to dislocations in the buffer layers is strongly reduced and pronounced photoluminescence of the quantum structures grown on top of the buffer layers can be observed.

Effect of Melt Stoichiometry on Shallow Acceptor Formation in Heavily Doped GaAs

The behaviour of group III, IV and VI dopants in vertical Bridgman grown GaAs was studied as a function of the stoichiometry of the melt. The melt composition was abruptly changed from stoichiometric to Ga-rich by intentional injection of gallium during the growth. In GaAs co-doped with Si and B, the Ga injection causes a drastic decrease of the free electron concentration which is explained by an increased concentration of As-sublattice acceptors. Spatially resolved photoluminescence (PL) results confirm this interpretation qualitatively as (SiAs)- and (BAs)-related modes dominate the spectra from the Ga-rich area of the sample. Pronounced PL peaks related to As-sublattice acceptors also appear in samples doped with B and Ge after Ga injection. The assignment of the PL modes was possible by taking account of the data in the literature and with respect to the results of Hall, Raman and mass spectrometric measurements. In GaAs:Sn and in group VI-doped samples, however, the Ga injection causes PL modes not previously reported.

Photoluminescence Enhancement of Porous Silicon by Organic Cyano Compounds

A pronounced photoluminescence enhancement on chemically oxidized porous silicon was induced by a series of organic cyano compounds including 1,2-dicyanoethylene (CE), 1,3-dicyanobenzene (1,3-CB), 1,4-dicyanobenzene (1,4-CB), 1-cyanonaphthalene (1-CN), and 9-cyanoanthracene (9-CA). Photoluminescence enhancement effects were reversible for all compounds studies in this work. A dependence of photoluminescence enhancement on the steric effect and the electronic characteristics of these compounds and the structure of the porous silicon substrates were analyzed in terms of the photoluminescence enhancing factors. Surface chemical composition examined by Fourier transform infrared (FTIR) spectra demonstrated that the surface Si−H bonds were not changed and no new luminescent compounds were formed on porous silicon surface during adsorption of cyano compounds. A mechanism based on induced surface states acting as radiative recombination centers by cyano compounds adsorption was suggested.

Exciton dynamics within growth islands of GaAs/Al x Ga1−x As single quantum wells

The localization dynamics of excitons within growth islands of GaAs/Al x Ga1−x As single quantum wells (SQW) have been investigated by time-resolved photoluminescence spectroscopy (TRPL). Several samples with different substrate misorientation and growth interruption times are compared with regard to the photoluminescence (PL) emission dynamics. For monolayer (ML) islands larger than the exciton radius,i.e. long growth interruption, the time evolution of the different PL peaks reflects the transfer of free excitons between growth islands as well as the localization within a single growth island. The samples with shorter growth interruption reveal a much less pronounced PL splitting due to ML islands. The dynamics appears to be mainly determined by exciton localization to thicker ML islands. The density of localization centres is larger for the sample grown on misoriented substrates.

Exciton dynamics within growth islands of GaAs/Al0.17Ga0.83As single quantum wells.

The localization dynamics of excitons within growth islands of GaAs/AlxGa1−xAs single quantum wells (SQW) have been investigated by time-resolved photoluminescence spectroscopy (TRPL). Several samples with different substrate misorientation and growth interruption times are compared with regard to the photoluminescence (PL) emission dynamics. For monolayer (ML) islands larger than the exciton radius,i.e. long growth interruption, the time evolution of the different PL peaks reflects the transfer of free excitons between growth islands as well as the localization within a single growth island. The samples with shorter growth interruption reveal a much less pronounced PL splitting due to ML islands. The dynamics appears to be mainly determined by exciton localization to thicker ML islands. The density of localization centres is larger for the sample grown on misoriented substrates.

Comparison of band structure calculations and photoluminescence experiments on HgTe/CdTe superlattices grown by molecular beam epitaxy

We have grown HgTe/CdTe superlattices by molecular beam epitaxy; barrier thicknesses were in the range from 15 to 91 Å and the well thickness was maintained at a constant value of 30 Å. The infrared photoluminescence was investigated by means of Fourier transform infrared spectroscopy in the temperature range from 4.2 to 300 K. All superlattices showed pronounced photoluminescence at temperatures up to 300 K. To gain more detailed insight into the band structure of the HgTe/CdTe superlattices, band structure calculations were performed. The concept of the envelope function approximation was followed. Employing the transfer matrix method, the calculations were completed taking into account an eight band k⋅p model. An important parameter in these calculations is the natural valence band offset (VBO) between the well and barrier materials. As a general trend, the value for the direct gap decreases with increasing VBO. The experimentally determined energies of the band gap are in reasonable agreement with the values obtained by the theoretical calculations. A comparison between theory and experiment shows that the observed transition energies are closer to calculations employing a large offset (350 meV) as opposed to a small VBO (40 meV).

Visible luminescence from porous silicon and siloxene

Infrared transmission and Raman spectra of siloxene (Si6O3H6) and porous Si have been analyzed in terms of the structural composition. Siloxene consists of Si6-rings interconnected by oxygen and terminated by hydrogen, and corrugated Si(111)-planes saturated by OH and H. The pronounced photoluminescence of siloxene is mainly related to the Si6-rings. Porous Si shows similar luminescence and almost identical vibrational properties as siloxene, which makes this class of materials a possible candidate for the origin of the luminescence in p-Si. Annealing at temperatures below 300 °C and moderate illumination of siloxene induces the transformation from the Si-plane-like structure to the Si-ring-like structure. Anneal temperatures above 300 °C and intense illumination of both siloxene and p-Si leads to an effusion of hydrogen and an irreversible oxidation of the samples.

Molecular beam epitaxial growth of (100) oriented CdTe on Si (100) using BaF2-CaF2 as a buffer

Epitaxial CdTe (100) has been grown on (100) oriented Si by molecular beam epitaxy using BaF2-CaF2 as a buffer. Two-dimensional (2-D) growth of BaF2(100) is obtained using low-temperature thermal cycles during growth. CdTe growth is also 2-D above 270 °C substrate temperature and a 2×1 surface reconstruction indicating a Te-stabilized surface is obtained. The growth is 3-D at lower substrate temperatures. Good structural quality films showing sharp electron channeling patterns and pronounced photoluminescence at 77 K are obtained. The full width at half maximum of the band-edge peak is 12 meV at 77 K.

Rapid Thermal Annealing and Ion Implantation of Heteroepitaxial ZnSe/GaAs

ABSTRACTWe describe the effects of rapid thermal annealing on the photoluminescence (PL) and electrical properties of heteroepitaxial ZnSe grown by molecular beam epitaxy on GaAs, using either no cap or plasma-deposited SiO2, Si3N4, or diamond-like C caps, and annealing temperatures from 500 to 800°C. Capless anneals (in contact with GaAs) produce badly degraded PL properties, while capped anneals can prevent this degradation. We show that Si3N4 is significantly more effective in preventing Zn out-diffusion through t e cap than previously employed SiO2 films, as evidenced by less pronounced PL features related to the creation of Zn vacancies during the anneal. Implant damage tends to enhance the Zn vacancy formation. Rapid thermal annealing with Si3N4 caps is shown to optically activate shallow N acceptor implants.