J. L. Pura

Electromagnetic field enhancement effects in group IV semiconductor nanowires. A Raman spectroscopy approach

Semiconductor nanowires (NWs) are the building blocks of future nanoelectronic devices. Furthermore, their large refractive index and reduced dimension make them suitable for nanophotonics. The study of the interaction between nanowires and visible light reveals resonances that promise light absorption/scattering engineering for photonic applications. Micro-Raman spectroscopy has been used as a characterization tool for semiconductor nanowires. The light/nanowire interaction can be experimentally assessed through the micro-Raman spectra of individual nanowires. As compared to both metallic and dielectric nanowires, semiconductor nanowires add additional tools for photon engineering. In particular, one can grow heterostructured nanowires, both axial and radial, and also one could modulate the doping level and the surface condition among other factors than can affect the light/NW interaction. We present herein a study of the optical response of group IV semiconductor nanowires to visible photons. The study ...

Sequential description of the catastrophic optical damage of high power laser diodes

Cathodoluminescence (CL) analysis of high power laser diodes permits to reveal the main defects issued from the catastrophic optical degradation (COD). These defects are revealed as discontinuous dark lines along the ridge. The different levels of damage are analysed, and a thermomechanical model taking account of the thermal and mechanical properties of the laser structure is settled up. In this model the COD is described as a local temperature enhancement, which generates thermal stresses leading to the generation of dislocations, which are responsible for the degradation of the thermal conductivity of the of the active zone of the laser.

Growth dynamics of SiGe nanowires by the vapour–liquid–solid method and its impact on SiGe/Si axial heterojunction abruptness

The vapour-liquid-solid (VLS) method is by far the most extended procedure for bottom-up nanowire growth. This method also allows for the manufacture of nanowire axial heterojunctions in a straightforward way. To do this, during the growth process, precursor gases are switched on/off to obtain the desired change in the nanowire composition. Using this technique, axially heterostructured nanowires can be grown, which are crucial for the fabrication of electronic and optoelectronic devices. SiGe/Si nanowires are compatible with complementary metal oxide semiconductor (CMOS) technology, which improves their versatility and the possibility of integration with current electronic technologies. Abrupt heterointerfaces are fundamental for the development and correct operation of electronic and optoelectronic devices. Unfortunately, the VLS growth of SiGe/Si heterojunctions does not provide abrupt transitions because of the high solubility of group IV semiconductors in Au, with the corresponding reservoir effect that precludes the growth of sharp interfaces. In this work, we studied the growth dynamics of SiGe/Si heterojunctions based on already developed models for VLS growth. A composition map of the Si-Ge-Au liquid alloy is proposed to better understand the impact of the growing conditions on the nanowire growth process and the heterojunction formation. The solution of our model provides heterojunction profiles that are in good agreement with the experimental measurements. Finally, an in-depth study of the composition map provides a practical approach to the drastic reduction of heterojunction abruptness by reducing the Si and Ge concentrations in the catalyst droplet. This converges with previous approaches, which use catalysts aiming to reduce the solubility of the atomic species. This analysis opens new paths to the reduction of heterojunction abruptness using Au catalysts, but the model can be naturally extended to other catalysts and semiconductors.

Mechanisms driving the catastrophic optical damage in high-power laser diodes

The catastrophic optical damage (COD) of laser diodes consists of the sudden drop off of the optical power. COD is generally associated with a thermal runaway mechanism in which the active zone of the laser is molten in a positive feedback process. The full sequence of the degradation follows different phases: in the first phase, a weak zone of the laser is incubated and the temperature is locally increased there; when a critical temperature is reached the thermal runaway process takes place. Usually, the positive feedback leading to COD is circumscribed to the sequential enhancement of the optical absorption in a process driven by the increase of the temperature. However, the meaning of the critical temperature has not been unambiguously established. Herein, we will discuss about the critical temperature, and the physical mechanisms involved in this process. The influence of the progressive deterioration of the thermal conductivity of the laser structure as a result of the degradation during the laser operation will be addressed.

Electromagnetic Field Enhancement on Axially Heterostructured NWs: The Role of the Heterojunctions

Semiconductor nanowires are the building blocks of future nanoelectronic devices. The study of the interaction between nanowires and visible light reveals resonances that promise light absorption/scattering engineering for photonic applications. We carried out experimental measurements through the micro-Raman spectroscopy of different group IV nanowires, both homogeneous Si nanowires and axially heterostructured SiGe/Si nanowires. These experimental measurements show an enhancement of the Raman signal in the vicinity of the heterojunction of SiGe/Si nanowires. The results are analysed in terms of the electromagnetic modelling of the light/nanowire interaction using finite element methods. The presence of axial heterostructures is shown to produce electromagnetic resonances, and the results are understood as a consequence of a finite change in the relative permittivity of the material at the SiGe/Si heterojunction. This effect opens a path to controlling interactions between light and matter at the nanoscale with direct applications in photonic nanodevices.

About the impact of the materials properties in the catastrophic degradation of high power GaAs based laser diodes

The catastrophic degradation of high power lasers depends on both external factors, associated with the technological processes followed to fabricate the laser, and also on intrinsic aspects related to the materials forming the laser structure, more specifically the active zone composed by the QW, guide layers and claddings. The materials properties: optical, thermal and mechanical, play a paramount role in the degradation of the laser. We analyse here how these properties have an impact on the mechanisms responsible for the catastrophic degradation.

Thermomechanical degradation of single and multiple quantum well AlGaAs/GaAs laser diodes

Abstract The catastrophic degradation of laser diodes with active zones comprising either single (SQW) or multiple quantum wells (MQW) has been analysed via finite element methods. This analysis is based on a physical model that explicitly considers the thermal and mechanical properties of the diode laser structure and the relevant size effects associated with the small thickness of the active layers of the device. The reduced thermal conductivities and the thermal barriers at the interfaces result in a significant local heating process which is accentuated as more quantum wells form the active part of the device. Therefore, in the design of high power devices, the SQW configuration would be more appropriate than the MQW alternative.

Catastrophic optical damage of high power InGaAs/AlGaAs laser diodes

Abstract The defects generated by the catastrophic optical degradation (COD) of high power laser diodes have been examined using cathodoluminescence (CL). Discontinuous dark lines that correspond to different levels of damage have been observed along the ridge. Finite element methods have been applied to solve a physical model for the degradation of the diodes that explicitly considers the thermal and mechanical properties of the laser structure. According to this model, the COD is triggered by a local temperature enhancement that gives rise to thermal stresses leading to the generation of dislocations. Damage is initially localized in the QW, and when it propagates to the waveguide layers the laser ends its life.