The effect of dose rate and implant temperature on transient enhanced diffusion in boron implanted silicon

The effect of ion implantation dose rate and implant temperature on the transient enhanced diffusion (TED) of low energy boron implants into silicon was investigated. The implant temperature was varied between 5 and 40°C. The beam current was varied from 0.035 to 0.35 mA/cm². Three different defect regimes were investigated. The first regime was below the formation of any extended defects (5 keV B⁺ 2 × 10¹⁴/cm²) visible in the transmission electron microscope. The second regime was above the {311} formation threshold (2×10¹⁴/cm²) but below the subthreshold (type I) dislocation loop formation threshold. The final regime was above both the {311} and dislocation loop formation threshold (10 keV 5×10¹⁴/cm²). TED for these conditions is shown to be over after annealing at 750°C for 15–30 min. Secondary ion mass spectroscopy results for the three different damage regimes indicate that there is no measurable effect of dose rate or implant temperature on TED of boron implanted silicon for any of the damage regimes. It should be emphasized that the dose and energy of the boron implants is such that none of these implants approached the amorphization threshold. Above amorphization dose rate and implant temperature have dramatic effects on TED, but it appears that below the amorphization threshold there is little effect. These results suggest that for a given energy it is the ion dose not the extent of the implant damage that determines the extent of TED in boron implanted silicon.     

Porphyrins-Functionalized Single-Walled Carbon Nanotubes Chemiresistive Sensor Arrays for VOCs

Single-walled carbon nanotubes (SWNTs) have been used extensively for sensor fabrication due to its high surface to volume ratio, nanosized structure and interesting electronic property. Lack of selectivity is a major limitation for SWNTs-based sensors. However, surface modification of SWNTs with a suitable molecular recognition system can enhance the sensitivity. On the other hand, porphyrins have been widely investigated as functional materials for chemical sensor fabrication due to their several unique and interesting physico-chemical properties. Structural differences between free-base and metal substituted porphyrins make them suitable for improving selectivity of sensors. However, their poor conductivity is an impediment in fabrication of prophyrin-based chemiresistor sensors. The present attempt is to resolve these issues by combining freebase- and metallo-porphyrins with SWNTs to fabricate SWNTs-porphyrin hybrid chemiresistor sensor arrays for monitoring volatile organic carbons (VOCs) in air. Differences in sensing performance were noticed for porphyrin with different functional group and with different central metal atom. The mechanistic study for acetone sensing was done using field-effect transistor (FET) measurements and revealed that the sensing mechanism of ruthenium octaethyl porphyrin hybrid device was governed by electrostatic gating effect, whereas iron tetraphenyl porphyrin hybrid device was governed by electrostatic gating and Schottky barrier modulation in combination. Further, the recorded electronic responses for all hybrid sensors were analyzed using a pattern-recognition analysis tool. The pattern-recognition analysis confirmed a definite pattern in response for different hybrid material and could efficiently differentiate analytes from one another. This discriminating capability of the hybrid nanosensor devices open up the possibilities for further development of highly dense nanosensor array with suitable porphyrin for E-nose application.     

Electronic-nose for detecting environmental pollutants: signal processing and analog front-end design

Environmental monitoring relies on compact, portable sensor systems capable of detecting pollutants in real-time. An integrated chemical sensor array system is developed for detection and identification of environmental pollutants in diesel and gasoline exhaust fumes. The system consists of a low noise floor analog front-end (AFE) followed by a signal processing stage. In this paper, we present techniques to detect, digitize, denoise and classify a certain set of analytes. The proposed AFE reads out the output of eight conductometric sensors and eight amperometric electrochemical sensors and achieves 91 dB SNR at 23.4 mW quiescent power consumption for all channels. We demonstrate signal denoising using a discrete wavelet transform based technique. Appropriate features are extracted from sensor data, and pattern classification methods are used to identify the analytes. Several existing pattern classification algorithms are used for analyte detection and the comparative results are presented.     

Interdigitated Ge p-i-n photodetectors fabricated on a Si substrate using graded SiGe buffer layers

We report an interdigitated p-i-n photodetector fabricated on a 1-/spl mu/m-thick Ge epitaxial layer grown on a Si substrate using a 10-/spl mu/m-thick graded SiGe buffer layer. A growth rate of 45 /spl Aring//s/spl sim/60 /spl Aring//s was achieved using low-energy plasma enhanced chemical vapor deposition. The Ge epitaxial layer had a threading dislocation density of 10/sup 5/ cm/sup -2/ and a rms surface roughness of 3.28 nm. The 3-dB bandwidth and the external quantum efficiency were measured on a photodetector having 1-/spl mu/m finger width and 2-/spl mu/m spacing with a 25/spl times/28 /spl mu/m/sup 2/ active area. At a wavelength of 1.3 /spl mu/m, the bandwidth was 2.2, 3.5, and 3.8 GHz at bias voltages of -1, -3, and -5 V, respectively. The dark current was 3.2 and 5.0 /spl mu/A at -3 and -5 V, respectively. This photodetector exhibited an external quantum efficiency of 49% at a wavelength of 1.3 /spl mu/m.     

Structural characterization of thick, high-quality epitaxial Ge on Si substrates grown by low-energy plasma-enhanced chemical vapor deposition

In this paper, we report on the growth of epitaxial Ge on a Si substrate by means of low-energy plasma-enhanced chemical vapor deposition (LEPECVD). A Si_1?xGe_x graded buffer layer is used between the silicon substrate and the epitaxial Ge layer to reduce the threading dislocation density resulting from the lattice mismatch between Si and Ge. An advantage of the LEPECVD technique is the high growth rate achievable (on the order of 40 Å/sec), allowing thick SiGe graded buffer layers to be grown faster than by other epitaxial techniques and thereby increasing throughput in order to make such structures more manufacturable. We have achieved relaxed Ge on a silicon substrate with a threading dislocation density of 1 × 10⁵ cm^?2, which is 4?10x lower than previously reported results.