Y.-Y. Chen

Surface passivation of Cu(In,Ga)Se2 using atomic layer deposited Al2O3

With Al2O3 passivation on the surface of Cu(In,Ga)Se2, the integrated photoluminescence intensity can achieve two orders of magnitude enhancement due to the reduction of surface recombination velocity. The photoluminescence intensity increases with increasing Al2O3 thickness from 5 nm to 50 nm. The capacitance-voltage measurement indicates negative fixed charges in the film. Based on the first principles calculations, the deposition of Al2O3 can only reduce about 35% of interface defect density as compared to the unpassivated Cu(In,Ga)Se2. Therefore, the passivation effect is mainly caused by field effect where the surface carrier concentration is reduced by Coulomb repulsion.

Flexible Ge-on-polyimide detectors

A flexible Ge-on-polyimide photodetector is demonstrated. The single crystalline thin film of Ge was transferred on the polyimide by adhesive wafer bonding and smart-cut techniques. A simple tunneling metal-oxide-semiconductor structure is fabricated for the detector applications. Due to the transparency of the polyimide, the responsivity of the detector is sensitive to the environments. At −2 V, the photodetector on the stainless steel has responsivities of 220, 200, and 140 mA/W at wavelength of 850, 1310, and 1550 nm, respectively.

On Design of Collaborative Mobile Sensor Networks for Deadline-Sensitive Mobile Target Detection

Mobile wireless sensor networks have been widely studied in the context of various applications, such as environmental monitoring, battlefield surveillance, and intrusion detection. One important problem in the context of such applications is detection of mobile targets (e.g., invaders) through the use of mobile sensor nodes. Consider a closed region, patrolled by several mobile sensors, attempting to capture targets which are in turn capable of sensing their environments in order to plan their movements and avoid detection. The focus of this paper is on an approach for solving this pursuit-evasion problem under a deadline constraint. Our central idea is the application of transient analysis of a discrete-time Markov model in (a) modeling and analyzing the above problem, and (b) investigating the trade-off between the number of mobile sensors and the quality of detection, as represented by the probability of detection by a given deadline. With the aid of the Markov model, two detection strategies, termed Single-hop Presence-aware Detection Strategy and Multi-hop Potentiality-aware Detection Strategy, are proposed with the aim of effectively capturing the mobile target. Furthermore, for practical purposes as well as for scalability, a distributed algorithm is developed such that each sensor is able to determine its movements by using local information only. Our results indicate that our models are able to (a) predict the expected detection quality accurately, and (b) determine the proper required number of sensors for satisfying the required quality of detection.

Narrow-band metal-oxide-semiconductor photodetector

Si-based photodetectors for narrow-band ultraviolet light (319 nm) and green light (500 nm) detection are demonstrated using a metal-oxide-semiconductor tunneling structure. By using appropriate selection of gate metal, the metal-oxide-semiconductor tunneling diode can detect specific range of light. Due to the spectral dependence of absorption and reflection of the Ag and Au as gate electrodes, the narrow-band detection of ultraviolet and green light can be achieved, respectively. The photodetectors with 130 nm thick Ag gate and 70 nm thick Au gate exhibit peak responsivities of 5.1 and 0.3 mA/W at 319 and 500 nm, respectively.

Germanium oxide passivation for Ge absorber

In photovoltaic industry, Ge material also plays an important role for III-V solar cells since Ge with smaller band-gap than Si is used as the bottom junction to absorb the infrared (IR) light, so the Ge layer passivation becomes more important. GeO<inf>2</inf> is a suitable material to passivate the surface due to the decrease of the interface defect density (D<inf>it</inf>) and the introduction of positive fixed charge. The excess carriers in the bulk Ge are generated without nonradiative recombination since the front surface recombination velocity (SRV) is reduced by GeO<inf>2</inf> layer passivation. The metal oxide semiconductor (MOS) capacitors show almost no frequency dispersion from 1 K to 1 MHz at room temperature for oxidation time of 1 and 5 hour. The minimum value of D<inf>it</inf> with 5 hour oxidation treatment is obtained around 1.8×10¹¹ cm⁻²eV⁻¹ by high-low frequency method. Furthermore, adding Al<inf>2</inf>O<inf>3</inf> capping layer on GeO<inf>2</inf> layer is used to avoid moisture in the air.

Thermal oxide, Al 2 O 3 and amorphous-Si passivation layers on silicon

The effective passivation needs (1) higher bandgap than Si with type 1 alignment, (2) low interface density at the interface between passivation layer and Si, and (3) ionized charges for field effect passivation. The thermal oxide (SiO2) with low interface defect density seems most effective but requires high growth temperature (900 °C). Al2O3 with trapped negative fixed charges can serve as the field effect passivation. Moreover, doped amorphous Si can also have the field effect passivation with the controlled ionized charge density. The effective life time is measured by quasi-steady-state photoconductance (QSSPC). Photoluminescence (PL) measurement is consistent with QSSPC, and can probe a local area with mapping ability on large samples. The dependence of PL intensity on surface recombination velocity is theoretically studied. The passivation of a-Si becomes less effective after crystallization at high temperature annealing, indicating the larger bandgap is necessary.

Low current cross-point memory using gadolinium-oxide switching material

Low current cross-point memory using gadolinium-oxide switching material in an IrOx/GdOx/W structure has been investigated for the first time. Memory device shows low current bipolar resistive switching phenomena and self-compliance phenomena as well, repeatable switching cycles, good uniformity, long program/erase endurance of >60k every cycles, and good data retention of >104 s at a low CC of 50 μA.

On detecting mobile target with deadline constraint in mobile sensor networks

Wireless sensor networks have been widely studied in various applications such as environmental monitoring, battlefield surveillance, and intrusion detection recently. One of the important applications is to detect mobile targets (e.g., invaders) by using mobile sensor nodes. Consider a closed region where there are several mobile sensors patrolling and trying to capture targets who are able to sense their environments and then plan their movements to avoid detection. The focuses of this paper are (a) to figure out a better movement strategy for a sensor for coping with the issue of detecting mobile targets with the deadline, and (b) to investigate the trade-off between the number of mobile sensors and the quality of detection represented by the detection probability before a given deadline. With the help of the proposed model, we design a detection strategy, called Single-hop Presence-aware Detection Strategy, to effectively capture the mobile target. The simulation results show that the model can predict the expected detection time precisely, and the number of sensors obtained from the model is adequate to satisfy the desired quality of detection.

Reabsorption Effects of Direct Band Emission of Ge

In this work, the reabsorption is systematically studied by thinning the Ge wafer from 500 μm to 1 μm. The lifetime of bulk Ge substrate can also be extracted using thickness dependent photoluminescence.

Single-crystalline Ge p-channel thin-film transistors with Schottky-barrier source/drain on flexible polyimide substrates

We fabricated the single-crystalline Ge (sc-Ge) p-channel thin-film transistors (TFTs) with Schottky-barrier source/drain (S/D) on flexible polyimide substrates by a simplified low-temperature process (≤ 250°C), which preserves the high mobility Ge channel. Adhesive wafer bonding and Smart-Cut techniques were utilized to transfer the sc-Ge thin film onto polyimide substrates. The device has a linear hole mobility of ~170 cm²V⁻¹s⁻¹, saturation hole mobility of ~120 cm²V⁻¹s⁻¹, and I<inf>on</inf> of ~1.6 μA/μm at V<inf>d</inf> = −1.5 V for the channel width/length = 280/15μm.