Latha Nataraj

Direct-bandgap luminescence at room-temperature from highly-strained Germanium nanocrystals

Efficient room-temperature luminescence at optical telecommunication wavelengths and originating from direct band-to-band recombination has been observed in tensile-strained germanium nanocrystals synthesized by mechanical grinding techniques. Selected area electron diffraction, micro-Raman and optical-absorption spectroscopy measurements indicate high tensile-strains while combined photoluminescence spectroscopy, excitation-power evolution and time-resolved measurements suggest direct band-to-band recombination. Such band-engineered germanium nanocrystals offer great possibilities for silicon-photonics integration due to their superb light-emission properties, facile fabrication and compatibility with standard microelectronic processes.

Effects of boron and phosphorus doping on the photoluminescence of self-assembled germanium quantum dots

The doping of Ge quantum dots grown on Si (100) was investigated using atomic force microscopy and photoluminescence spectroscopy. The dots produced photoluminescence in the energy range from 0.75 to 0.95 eV. Compared to the undoped dots, the boron and phosphorus doped dots demonstrated a shift in peak emission toward lower energy by 60–80 meV, which is attributed to the decrease in density of the smaller sized dots. Increased photoluminescence intensity with temperature indicated higher activation energy with doping, suggesting a higher probability of radiative transitions at elevated temperatures, potentially important for various applications.

Structural and optoelectronic properties of germanium-rich islands grown on silicon using molecular beam epitaxy

We report on the structural and optoelectronic properties of self-assembled germanium-rich islands grown on silicon using molecular beam epitaxy. Raman, photocurrent, photoluminescence, and transient optical spectroscopy measurements suggest significant built-in strains and a well-defined interface with little intermixing between the islands and the silicon. The shape of these islands depends on the growth conditions and includes pyramid, dome, barn-shaped, and superdome islands. Most importantly, we demonstrate that these germanium-rich islands provide efficient light emission at telecommunication wavelengths on a complementary metal-oxide semiconductor-compatible platform.

Room temperature capacitance-voltage profile and photoluminescence for delta doped InGaAs single quantum well

Room temperature capacitance-voltage (C-V) profile and photoluminescence (PL) studies of δ-doped single InGaAs quantum well samples are reported. The purpose was to obtain the confined carrier occupancy in the conduction band offset and observe any relevant phenomena. The results show that the peak intensity of the C-V profiles was almost linearly proportional to sheet carrier concentration and the full width at half maximum of the C-V profiles became narrower with increasing doping level in the barrier layer. This is interpreted as being due to improved confinement of electrons as a result of band bending induced by the δ-doping layer. This explanation was further supported by PL data that show the transition corresponding to the dominant peak changed with different δ-doping levels and that all of the transitions were redshifted. Finally, theoretical calculations of the band structure based on a four band k⋅p method are presented to explain the observed results.

Doped group-IV semiconductor nanocrystals

Doped semiconductor nanocrystals offer great potential for microelectronics and integrated optoelectronics. Nanocrystal based technology is promising for several fields of technology such as thin conducting films, light emitting devices, tunable lasers, transistors, photovoltaics, and less harmful alternatives to toxic fluorescent dyes for application in bio-imaging, to name a few. While properties of nanocrystals are tunable through their size, considerable research is underway to explore the influence of dopants on the properties of semiconductor nanocrystals. The introduction of dopants has been elusive for these confined structures due to their nanoscale sizes as well as the possibility of making them degenerate even with the addition of very small quantities of the dopant. Here, we present a facile, low-cost procedure that we have developed for the synthesis of non-degenerate doped semiconductor nanocrystals and study their properties.

Direct-bandgap emission from hydrostatically tensile-strained Germanium nanocrystals

We report on the high room-temperature luminescence from Germanium nanocrystals synthesized by mechanical grinding. Transients and optical spectroscopy measurements are consistent with HRTEM and electron diffraction, suggesting high tensile strains favoring direct band-to-band transitions.

Enhanced Room-Temperature Light-Emission from Tensile-Strained Germanium Nanocrystals

We report on the high room-temperature luminescence from Germanium nanocrystals synthesized by mechanical grinding. Transients and optical spectroscopy measurements are consistent with HRTEM and electron diffraction, suggesting high tensile strains favoring direct band-to-band transitions.

Passivation of InAs quantum dots for novel photovoltaics

Novel photovoltaic device designs offer the possibility of limiting efficiencies well in excess of conventional limits [1,2]. Many of the suggested devices rely on the inclusion of nanostructures in order to improve conventional energy conversion mechanisms or allow new mechanisms to be exploited. Because nanostructures rely on quantum confinement realized by heterojunctions, strain induced defects at hetero-interfaces, and the non-ideal recombination pathways they bring, need to be passivated, in order for the novel devices to operate as intended [3,4]. We report attempts to passivate quantum dots in the much studied InAs/GaAs system using a Sb flux treatment immediately prior to capping of the quantum dots with GaAs. The photoluminescence results suggest an optimum exposure time to the Sb flux after which the performance degrades substantially. Temperature dependent photoluminescence results as well as X-ray diffraction results are also presented in order to explain the structure at the quantum dot - cap interface. The impact of these results in terms of the design of two novel photovoltaic devices, the intermediate band solar cell and the hot carrier solar cell is also discussed.

Structural and light-emission properties of bulk Germanium islands grown on Silicon using Molecular Beam Epitaxy

We report on the optoelectronic properties of bulk Germanium islands formed on Silicon by Molecular Beam Epitaxy. More specifically, we will discuss the role of strains and doping in favoring efficient light-emission at telecommunication wavelengths.

Optoelectronic Properties of Germanium Islands Formed on Silicon Using Stranski-Krastanov Growth by MBE

We report on the optoelectronic properties of bulk Germanium islands formed on Silicon by Molecular Beam Epitaxy. More specifically, we will discuss the role of strains and doping in favoring efficient light-emission at telecommunication wavelengths.