Xiaoli Ji

Deep-level traps induced dark currents in extended wavelength InxGa1−xAs/InP photodetector

The dark current mechanism of extended wavelength InxGa1−xAs photo-detectors is still a debated issue. In this paper, the deep-level transient spectroscopy (DLTS) and dark current characteristics of InxGa1−xAs/InP detectors are investigated. Using trap parameters obtained from DLTS measurement, the device simulations of current-voltage characteristics are carried out by Silvaco Altas. The results reveal that the dark current at the low reverse bias voltage is associated with deep level trap induced trap assisted tunneling and Shockley-Read-Hall generation mechanism. The reduction of the deep level trap concentration in InxGa1−xAs absorption layer could dramatically suppress the dark current near zero bias in extended wavelength InxGa1−xAs/InP detectors.

2.6 μm MBE grown InGaAs detectors with dark current of SRH and TAT

We fabricate 2.6 μm InGaAs photodetectors by MBE technology and study its dark current mechanisms. Deep-level transient spectroscopy (DLTS) demonstrates a deep-level trap located at E c - 0.25 eV in the absorption layer. Using the trap parameters, a dark current model is constructed and the device simulation generates the dark current characteristic which agrees well with the experimental data. The model suggests that the dark current at low reverse voltage is dominated by the Shockley-Read-Hall (SRH) and trap-assisted tunneling (TAT). Furthermore, it predicts some basic rules for suppressing the dark current in 2.6 μm InGaAs detectors.

The energy distribution of NBTI-induced hole traps in the Si band gap in PNO pMOSFETs

By applying a low temperature sweeping technique, we indentify the energy profile of recoverable hole traps for nitrided oxide and SiO2 pMOSFETs subject to Negative Bias Temperature Stress (NBTS). It is found that the energy distribution of hole traps for nitrided oxide devices has two obvious peaks, one in the lower and one in the upper half of the silicon band gap. Both peaks gradually develop with increasing the stress time and temperature. We attempt to compare the energy profile for nitrided oxide and SiO2 devices to identify the nitrogen effect on the hole traps generated under NBTS.

Influence of AlGaN/GaN interface polarization fields on the properties of photoconductive detectors

The Al0.22Ga0.78N/GaN/Al0.22Ga0.78N multilayer heterostructure photoconductive detectors were designed and fabricated. The influence of AlGaN/GaN interface polarization fields on the properties of the photodetector was investigated. The energy band profile of the heterostructure was approximately calculated. Results indicate that electron and hole wells exist on the GaN sides of the Al0.22Ga0.78N/GaN/Al0.22Ga0.78N interfaces. The existence of two-dimensional electron gas was proved by variable temperature Hall measurements. The spectral response shows that the responsivity of the heterostructure photodetector was as high as 4300 A/W at 355 nm under 3 V bias, which is nearly ten times higher than that of a GaN monolayer structure photodetector. The response time and dark resistance of the photodetectors were also discussed.

Impact of SiNx passivation on the surface properties of InGaAs photo-detectors

We investigate surface passivation effects of SiNx films deposited by inductive coupled plasma chemical vapor deposition (ICPCVD) and plasma enhanced chemical vapor deposition (PECVD) technologies for InAlAs/InGaAs/InP photo-detectors. It is found that ICPCVD deposited SiNx film effectively reduces the densities of the interface states and slow traps near SiNx/InAlAs interface, which realize the small surface recombination velocity and low surface current for InAlAs/InGaAs/InP photo-detectors. By comparing C-V and XPS results, it is suggested that the trap density reduction by ICPCVD technology could be attributed to the disorder suppression on InAlAs surface due to the high density of SiNx film and less processing energy to the InAlAs surface.

Background limited ultraviolet photodetectors of solar-blind ultraviolet detection

The background noise in solar-blind ultraviolet (UV) detectors from the solar-irradiance leakage photons has been compared with the detector noise from the dark current. It has been found that the background noise is the deterministic limiting factor in solar-blind UV photodetection. The detector performance of background limited ultraviolet photodetector limit and the ultimate signal fluctuation limit are therefore proposed. It proves to be an effective method for device optimization by suppressing the declining tails above 285u2009nm in the response curves. The absorptive spectral filter requirement has been discussed for common solar-blind UV detectors to achieve the detectivity improvement.

The physical mechanisms of I G Random Telegraph Noise in deeply scaled pMOSFETs

The physical mechanism of I_G Random Telegraph Noise (I_G-RTN) has been studied in deeply scaled pMOSFETs subject to Negative Bias Temperature Stress (NBTS). Using carrier separation technique, we identify the majority carriers in I_G-RTN are channel holes. By investigating the electric field and temperature dependence of the capture time τ_c and emission time τ_e in I_G-RTN, it is found that the physical origin of I_G-RTN are NBTS-induced switching traps; Further quantitative analysis of I_G-V_G reveal that I_G-RTN is related to a tunneling process. Based on these results, we propose a tunneling model through NBTS-induced switching traps to explain the discrete gate leakage. The model provides a good agreement between the predicted and experimental data.

Improvement of surface leakage current of 2.6 µm InGaAs photodetectors by using inductive coupled plasma chemical vapor deposition technology

Low surface leakage current is one of the prerequistites to reach the low leakage and high efficiencies of mesa type photodiodes. In this paper, we have studied the surface leakage of 2.6 µm mesa InGaAs p–i–n photodetectors by using two different passivation technologies: inductive coupled plasma chemical vapor deposition (ICPCVD) and plasma enhanced chemical vapor deposition (PECVD). It is found that the total leakage current of the detector with ICPCVD technology is significantly reduced compared to that with PECVD technology due to the decrease of the devices surface leakage current. A TCAD-based dark current model further reveals that the reduction of the surface leakage current at the low reverse voltage is about two orders of magnitude. As a result, ICPCVD is the promising deposition technology for SiNx passivation layer on mesa InGaAs photodetectors.

Revised depth map estimation for multi-view stereo

Optical flow estimation is one of the popular methods to obtain the depth maps in multi-view stereo due to its high accuracy and robustness. In traditional optical flow estimation, the energy function contains three assumptions: intensity constancy assumption, gradient constancy assumption, and global smoothness assumption. In this work, we propose a local smoothness assumption to constrain the optical flow disparity in neighboring pixels. We first study the new smoothness term and its corresponding energy function, and present a practical iteration approach to minimize the energy function. Later we apply this new estimation method to the multi-view stereo system and obtain the depth maps of different image pairs. Our results demonstrate the good performance of the algorithm in acquiring smoothing surface when comparing to the traditional methods.

Physical understanding of negative bias temperature instability below room temperature

The physical mechanism of VT degradations under negative bias temperature stress below room temperature has been studied for SiO2 and plasma nitrided oxide (PNO-based) pMOSFETs. It is found that VT degradations in both devices exhibit strong dependence on the electric field and temperature. The analysis shows that this strong dependence follows multi-phonon field-assisted tunneling theory, which indicates the inelastic hole trapping mechanism in the low temperature negative bias temperature instability (NBTI). On the other hand, by applying a low temperature sweeping technique, the energy distribution of these NBTI-induced hole traps below room temperature is indentified. The energy distribution of hole traps has two obvious peaks, one in the lower and one in the upper half of the silicon band gap. Both peaks gradually develop with increasing the stress time and temperature. We attempt to compare the energy profile for SiO2 and PNO devices to identify the trap precursors in NBTI below room temperature.