3.7. PROCEDIMIENTO
4.1.5. Relación entre los Estilos de Aprendizaje y Rendimiento Académico en el
F ig.5.5.1.1 shows a schematic diagram of the near infrared spectrom eter assembled for the study of rare earth doped silicon.
L ock-in Am p M onochromator Spectrom eter controller Laser Personal C om puter Optical Chopper Ge diode F2 detector^ L3 Fluorescence L2 'Scattered light Temperature Controller Cryostat Sam ple
Figure 5.5.I.2. A block diagram o f the fluorescence m easurement system .
T h e solid line s h o w s the optical path from the so u rc e to the d e te c to r via the c h o p p e r disk, vario u s filters and lenses, the cryostat, the m o n o c h r o m a to r slit, m irrors and grating. T h e sa m p le s are p u m p e d using an argon ion laser with p o w e r o u tp u t o f up to 4 W at w a v e l e n g t h s b e tw e e n 4 5 7 an d 5 1 4 n m . T h e lig h t is c h o p p e d at a freq u en c y o f 33 0 H z by the c h o p p e r and filtered by FI (a b a n d pass filter allo w in g the ch o se n laser line th ro u g h ) befo re im p in g in g u p o n the s a m p le ( S I ) w h ic h is at the focal p o in t o f the lens (L I ). T his lens is also u sed to m o v e the b e a m aro u n d the area o f the sam ple. T h e sa m p le is m o u n te d in an O x fo rd In stru m e n t C F 1204 c o n tin u o u s flow cryosta t. T h e flu o re sc e n c e b e a m is then co lle c te d by lens L 2 a n d f o c u s e d by lens L3 th r o u g h a h ig h -p a s s filter (F 2 -filte rs o ut any p u m p la se r light) o n to the m o n o c h ro m a to r slits. T h e m o n o c h ro m a to r is a S pex 1704. T he re le v an t w a v e le n g th s are then se le c te d by the g ra ting (1 4 0 0 n m -1 7 5 0 n m ) an d pass th ro u g h a s e c o n d slit into the d e te c to r w h ic h is a liquid n itro g en c o o le d g e r m a n iu m P IN d io d e (7 0 0 n m - 1750nm). T his optical signal (the d ash e d line) is co n v e rte d to an electrical signal and a m plified by the lock-in amplifier. T h e pu rp o se o f the lock-in am p lifie r is to m easu re the signal w h ich is m o d u la te d in p h ase with the c h o p p e r re fere nce signal, e lim in a tin g any b a c k g r o u n d sig n a ls (w hic h o c c u r at o th e r fr e q u e n c ie s such as D C , 5 0 H z etc.). T h e o u tp u t o f the lock-in is recorded by the c o m p u te r (via an IE E E 488 bus), w hich is
Chapter 5: Material fabrication and analysis techniques
also used to control the tem perature of the cryostat and the w avelength of the spectrometer.
5.5.2 Fluorescence lifetimes
L ifetim e m easurem ents w ould allow the em ission cross-section to be calculated and this was attempted for the 1537nm transition using the spectrometer set-up illustrated in F ig .5.6.2.1. The output is recorded by a digital storage oscilloscope which is triggered from the mechanical optical-beam chopper and the fall and rise times were displayed on the screen. From the trace, direct measurements of the lifetime can be made Unfortunately, the samples yielded only feeble signals which were not sufficiently strong to provide meaningful measurements even after they had been amplified. This is a common problem with erbium doped silicon and the photoluminescence lifetime has only been successful for samples which have been heavily doped with o x y g e n a n d which exhibit significantly increased emission intensity.
Conclusion
In this chapter, the various fabrication and analytical methods employed throughout this project have been reviewed. Optimum conditions, as found in the literature, have been summarised and sample specification for experiment has been outlined. This is expanded upon in the next chapter in which the experimental results are presented.
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