Study of the temperature distribution in Si nanowires under microscopic laser beam excitation

The use of laser beams as excitation sources for the characterization of semiconductor nanowires (NWs) is largely extended. Raman spectroscopy and photoluminescence (PL) are currently applied to the study of NWs. However, NWs are systems with poor thermal conductivity and poor heat dissipation, which result in unintentional heating under the excitation with a focused laser beam with microscopic size, as those usually used in microRaman and microPL experiments. On the other hand, the NWs have subwavelength diameter, which changes the optical absorption with respect to the absorption in bulk materials. Furthermore, the NW diameter is smaller than the laser beam spot, which means that the optical power absorbed by the NW depends on its position inside the laser beam spot. A detailed analysis of the interaction between a microscopic focused laser beam and semiconductor NWs is necessary for the understanding of the experiments involving laser beam excitation of NWs. We present in this work a numerical analysis of the thermal transport in Si NWs, where the heat source is the laser energy locally absorbed by the NW. This analysis takes account of the optical absorption, the thermal conductivity, the dimensions, diameter and length of the NWs, and the immersion medium. Both free standing and heat-sunk NWs are considered. Also, the temperature distribution in ensembles of NWs is discussed. This analysis intends to constitute a tool for the understanding of the thermal phenomena induced by laser beams in semiconductor NWs

ULTRASONIC PROPERTIES OF Se - Ge GLASSES, BETWEEN 1 K AND 100 K

Nous avons étudié quelques propriétés élastiques et anélastiques de verres de sélénium-germanium, entre 1 K et 100 K et vers 100 MHz, grâce à des méthodes de propagation d'ondes ultrasonores. Nous constatons que ces propriétés sont sensibles à la rigidité du réseau et nous établissons un parallèle avec les calculs théoriques basés sur le concept de mode de fréquence nulle.We have studied some elastic and anelastic properties of selenium-germanium glasses, between 1 K and 100 K and around 100 MHz, with ultrasonic propagative methods. We establish that those properties are sensitive to the rigidity of the network and we compare them with theoretical computation based upon the zero frequency mode concept

ULTRASONIC STUDY OF TUNNELLING DEFECTS IN SELENIUM-GERMANIUM GLASSES

Nous mesurons les variations de la vitesse acoustique dans des verres de Selenium-Germanium, au-dessous de 1 K et vers 100 MHz. Nos résultats s'expliquent bien par l'interaction résonante entre l'onde ultrasonore et des défauts tunnel. Ils montrent que les grandeurs caractéristiques des défauts tunnel sont sensibles la rigidité du réseau.We measure the variations of the acoustic velocity in Selenium-Germanium glasses, below 1 K and around 100 MHz. Our results are well explained with the resonant interaction between the ultrasonic wave and the tunnelling defects. They show that the characteristic parameters of the tunnelling defects are sensitive to the rigidity of the network

Internal friction in amorphous polystyrene at low temperature and high frequency

The existence in polystyrene of an internal-friction peak around 80 K at 50 MHz is reported. This peak displays many similarities with the peak observed in amorphous selenium. It is interpreted in the frame of the same model. We explain our results with thermal activated relaxation processes displaying a constant distribution of barrier heights between 0 and 600 K. We suggest that the observed peak is the resolution at high frequency of the ε-shoulder which has been reported previously at much lower frequency.Nous présentons l'observation, dans le polystyrène, d'un pic de frottement interne localisé autour de 80 K à 50 MHz. Ce pic présente beaucoup de similitudes avec celui observé dans le sélénium amorphe et est interprété dans le cadre du même modèle. Nous expliquons nos résultats à l'aide de processus de relaxation par activation thermique dont les énergies sont distribuées de façon constante entre 0 et 600 K. Nous suggérons que le pic observé est la résolution à haute fréquence de l'épaulement ε qui a été observé précédemment à beaucoup plus basse fréquence