Analytical Transient Responses and Gain-Bandwidth Products of Low-Dimensional High-Gain Photodetectors.

Low-dimensional photodetectors, in particular those in photoconductive mode, often have extraordinarily high photogain. However, high gain always comes along with a slow frequency response. The gain-bandwidth product (GBP) is a figure of merit to evaluate the performance of a photodetector. Whether the high-gain photoconductors can outperform standard PIN photodiodes in terms of GBP remains an open question. In this article, we derived the analytical transient photoresponses of nanowire photoconductors which were validated with the simulations and experiments. Surprisingly, the fall transients do not follow a simple time-dependent exponential function except for some special cases. Given the analytical photogains that were established previously, we derived the theoretical GBP of high-gain nanowire photoconductors. Analysis of the analytical GBP indicates that nanoscale photoconductors, although having extremely high gain, will never outperform typical PIN photodiodes in terms of GBP.

Toward Defect-Free Doping by Self-Assembled Molecular Monolayers: The Evolution of Interstitial Carbon-Related Defects in Phosphorus-Doped Silicon.

Self-assembled molecular monolayer (SAMM) doping on semiconductors has been widely appraised for its advantages of doping nanoelectronic devices for applications in the complementary metal-oxide-semiconductor transistor (CMOS) industry. However, defects introduced by SAMM-doping will limit the performance of the devices. Previously, we have found that SAMM-doping can bring carbon impurities into the silicon substrate and these unwanted carbon impurities can deactivate phosphorus dopants by forming an interstitial carbon (Ci)-substitutional phosphorus (Ci-Ps) complex. Herein, to develop a defect-free SAMM-doping process, the generation and annihilation of Ci-related defects are investigated by extending the thermal annealing time from 2 to 10 min using secondary ion mass spectrometry and deep-level transient spectroscopy. The results show that the concentration of Ci-related carbon defects is lower after a longer time of thermal annealing, although a longer annealing time actually introduces a higher concentration of carbon impurities into Si. This observation indicates that interstitial carbon evolves into substitutional carbon (Cs) that is electrically inactive during the thermal annealing process. A defect-free SAMM-doping process may be developed by an appropriate post-annealing process.

Dynamics of Charge Carriers in Silicon Nanowire Photoconductors Revealed by Photo Hall Effect Measurements.

Photoconductors have extraordinarily high gain in quantum efficiency, but the origin of the gain has remained in dispute for decades. In this work, we employ photo Hall effect to reveal the gain mechanisms by probing the dynamics of photogenerated charge carriers in silicon nanowire photoconductors. The results reveal that a large number of photogenerated minority electrons are localized in the surface depletion region and surface trap states. The same number of excess hole counterparts is left in the nanowire conduction channel, resulting in the fact that excess holes outnumber the excess electrons in the nanowire conduction channel by orders of magnitude. The accumulation of the excess holes broadens the conduction channel by narrowing down the depletion region, which leads to the experimentally observed high photo gain.

Deep level transient spectroscopic investigation of phosphorus-doped silicon by self-assembled molecular monolayers.

It is known that self-assembled molecular monolayer doping technique has the advantages of forming ultra-shallow junctions and introducing minimal defects in semiconductors. In this paper, we report however the formation of carbon-related defects in the molecular monolayer-doped silicon as detected by deep-level transient spectroscopy and low-temperature Hall measurements. The molecular monolayer doping process is performed by modifying silicon substrate with phosphorus-containing molecules and annealing at high temperature. The subsequent rapid thermal annealing drives phosphorus dopants along with carbon contaminants into the silicon substrate, resulting in a dramatic decrease of sheet resistance for the intrinsic silicon substrate. Low-temperature Hall measurements and secondary ion mass spectrometry indicate that phosphorus is the only electrically active dopant after the molecular monolayer doping. However, during this process, at least 20% of the phosphorus dopants are electrically deactivated. The deep-level transient spectroscopy shows that carbon-related defects are responsible for such deactivation.

Nanoscale Nitrogen Doping in Silicon by Self-Assembled Monolayers.

This Report presents a nitrogen-doping method by chemically forming self-assembled monolayers on silicon. Van der Pauw technique, secondary-ion mass spectroscopy and low temperature Hall effect measurements are employed to characterize the nitrogen dopants. The experimental data show that the diffusion coefficient of nitrogen dopants is 3.66 × 10(-15) cm(2) s(-1), 2 orders magnitude lower than that of phosphorus dopants in silicon. It is found that less than 1% of nitrogen dopants exhibit electrical activity. The analysis of Hall effect data at low temperatures indicates that the donor energy level for nitrogen dopants is located at 189 meV below the conduction band, consistent with the literature value.