The generation of e-h pairs in silicon by incident light is the underlying principle behind most silicon-based photodetection, imaging, photometry, spectroscopy and photovoltaic technologies. In this work, we show that graphene provides voltage-tunable ways in which the photogenerated carriers in Si can be captured. Combining the photoexcitation in Si with the carrier capturing abilities of graphene results in photodetectors which not only respond with high quantum efficiency values, but, more interestingly, devices whose photocurrent responsivity can be completely tuned using an external voltage. Such tunability is quite useful for detection in variable light conditions, which makes it attractive for imaging and videographic applications, especially because they respond within milliseconds of the incidence. Layer thickening and doping can further enhance the absolute responsivity values, and devices with a few hundred mA/W and quantum efficiency up to ~65% could be fabricated. Most importantly though, another mode of photodetection, using photovoltage, is found to be incredibly sensitive to ultra-low intensity light, with photovoltage responsivity as high as 107 V/W and contrast sensitivity exceeding 106 V/W. This current-free method of detection is able to detect extremely low-absorbing materials, and coupled with scanning-photovoltage measurements, can give rise promising new ways of photodetection and imaging.
In this work we review the investigations of conductance fluctuations in doped silicon at low temperatures (2K < T < 20K) as it is tuned through the metal-insulator transition by changing the carrier concentration n. Spectral power, S(f), of the conductance fluctuation retains a generic 1/fα dependence. In the metallic regime (n>nc) the doped Si is like a weakly-localized electron system and the conductance fluctuation is governed by the mechanism of Universal conductance fluctuations. The relative variance of fluctuation follows the temperature dependence ∝ T-β, where β≈1/2. However, the noise diverges by orders of magnitude as n decreases through the critical concentration nc and the fluctuation also becomes strongly temperature dependent with β>> 1. At the transition (n/nc≈1) the fluctuation becomes strongly non-Gaussian below 20K as observed through the second spectrum S(2)(f). At T=4.2K, we find that after subtracting the Gaussian background , S(2)(f)∝ 1/fp where p is small (< 0.5) for metallic samples (n/nc≥ 1.5) and it grows to > 1 for samples close to the transition n/nc ≈1. The growth of non-Gaussianity is accompanied by a growth in low frequency spectral weight as seen through a significant enhancement of α from close to 1 (n>nc) to nearly 1.4 for n/nc ≈1. The growth of non-Gaussian fluctuation of extremely large magnitude with significant low frequency component points to a correlated low frequency dynamics of charge fluctuation near the insulator-metal transition. This has been interpreted as the onset of a glassy freezing of the electronic system across the transition.
We have investigated the resistance fluctuations in Si:P as a function of doping level n, across the Metal-Insulator transition at low temperatures. The fluctuation size increases sharply with decrease in the doping level, and shows indications of correlations (presented elsewhere in this conference). The measured jumps in voltage in a current biased sample due to resistance fluctuations were stored digitally and the fluctuation size statistics were estimated in the form of a Probabilty Density Function (PDF). On the metallic side, the PDF's were found to have more or less a Gaussian shape, as expected from an ensemble of small uncorrelated fluctuators. However, we find marked deviation of the PDF from a Gaussian behavior as the system crosses into the insulating side. The deviation starts to occur at the tail of the distribution, and grows in size with decreasing doping levels. The deviating part of the tail could be fitted with a log-normal expression. On the insulating side, this growth of a log-normal tail is also seen to occur as the temperature is lowered. The observations have been analysed using existing theories.