Abstract
Amorphous materials have low mobility due to their nature of disorder. Surprisingly, some disordered materials showed photocurrent amplification not by conventional photoconductive gain. Recently, amorphous Silicon (a-Si) photodiodes with thin a-Si layer (∼40 nm) have shown a gain-bandwidth product of over 2 THz with very low excess noise and also have been used as a gain media in a cascaded system with single photon sensitivity. To unveil the true gain mechanism, we performed theoretical modeling and numerical analysis along with experimental data at different frequencies. We show evidence of highly effective carrier multiplication process within a-Si as the primary gain mechanism, especially at high frequency. We also show presence of trap-induced junction modulation at much lower frequency. We modeled the gain mechanism in a-Si by solving the transport equations including dynamics of defect states and carrier multiplication via the local field model. We further justified the application of local field model for thin a-Si, based on the property that in a-Si, the mean-free path for energy relaxation is orders of magnitude greater than the mean-free-path for momentum relaxation. The analysis further suggests that the carrier multiplication process in thin a-Si can be much more efficient than in thick a-Si, even stronger than single crystalline Si in some cases. Although seemingly counter intuitive, this is consistent with the proposed cycling excitation process where the localized states in the bandtails of disordered materials such as a-Si relax the k -selection rule and increase the rate of carrier multiplication.
Highlights
L OW noise amplification of photocurrent is desirable for high performance imaging, sensing, and optical communication [1]–[4]
We propose the physical picture of the gain mechanism in amorphous materials in general
In the presence of traps, photogenerated carriers can be captured in the defect states, which alters the potential profile inside the semiconductor. This results in additional band bending and affects the tunneling current at the electrode/semiconductor junction [33]–[36]. Applying such model to a device with an amorphous silicon (a-Si) layer sandwiched by two metal contacts, we find that there is an increase in the tunneling current due to photogenerated carrier trapping and modulation of the potential profile at the metal/a-Si junction, manifested as amplification of photoresponse
Summary
L OW noise amplification of photocurrent is desirable for high performance imaging, sensing, and optical communication [1]–[4]. Amorphous silicon (a-Si) detectors with gain of >1000 at less than 5 V bias have been reported [11] Such detectors showed a high gain-bandwidth product of over 2THz with an excess noise factor 17 times lower than Si APD at the gain of over 1000. Researchers observed photocurrent multiplication in a-Si p-i-n photodiodes [18], [19] In spite of these experimentally observed gain behaviors, the physics of the gain mechanism in disordered materials is not clear. Some attributed it to photoconductive gain from traps or band discontinuity [17], others attributed to avalanche type carrier multiplication [18]–[20]. The contributions of both mechanisms can be clearly separated with frequency dependent photocurrent gain because the junction modulation gain due to trap dynamics rolls off quickly at KHz range while the carrier multiplication mechanism has a much higher frequency response
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