Cha-Shin Lin

Reducing dark current in a high-speed Si-based interdigitated trench-electrode MSM photodetector

The authors have studied higher dark-current temperature dependence in a trench-electrode Si-based metal-semiconductor-metal (MSM) photodetector which has a hydrogenated intrinsic amorphous silicon (i-a-Si:H) dark-current suppression layer. The poor dark-current temperature-dependence performance could be improved significantly by reducing the number of trap states in the depletion region of the reverse-biased crystalline/amorphous Si heterojunction. To reduce the trap states, a modified plasma-enhanced chemical vapor deposition (PECVD) system, which reduced the ion bombardment on the Si substrate, was employed to deposit an i-a-Si:H layer. Moreover, since fewer trap states in a photodetector will result in a degradation of the fall time of the temporal response of the device, a Ti electrode, which has a lower Schottky barrier height (0.62 eV) than that (0.84 eV) of the previous Cr electrode used with i-a-Si:H, was employed for compensation. The device obtained exhibited very good dark-current stability and temporal response. The dark current only increased from 6 to 34 nA, when the operating temperature was increased from room temperature (R. T.) to 57/spl deg/C, much lower than that of the previously reported 3-V bias voltage one (from 22 to 209 nA). Device responsivity and quantum efficiency also showed obvious improvement, both at R. T. (0.192 A/W and 0.29) and 57/spl deg/C (0.213 A/W and 0.32, respectively) and were higher than those previously reported (0.174 A/W and 0.26, at 57/spl deg/C).

Phosphor-Free GaN-Based Transverse Junction White-Light Light-Emitting Diodes With Regrown n-Type Regions

In this study, we demonstrate a GaN-based phosphor-free white-light light-emitting diode (LED), which is composed of GaN-based dual-wavelength (blue and yellow-green) multiple-quantum-wells (MQWs) and a transverse p-n junction. The device was realized by the regrowth of n-type GaN layers on the sidewall of p-type GaN and undoped MQWs. The problems related to the bias-dependent shape of the electroluminescence spectra that occur in traditional phosphor-free white-light LEDs (with vertical p-n junctions) are greatly minimized. The current-voltage performance of our device is comparable to that of the commercially available phosphor white-light LEDs. In addition, the dynamic measurement results indicate that we can attain a much higher modulation bandwidth (22 versus 3 MHz) with this device than with the currently available commercial ones.

Electrical characteristics of a-SiGe:H thin-film transistors with Sb/Al binary alloy Schottky source/drain contact

Abstract The effects of composition of a-SiGe channel layer and annealing temperature on the electrical performances of the inverted-staggered thin-film transistors with Sb/Al binary alloy Schottky source/drain contact had been studied. The experimental results indicated that the device effective electron mobility could be enhanced significantly by reducing the Schottky barrier height between the source/drain contact metal and a-SiGe:H channel layer, and using an appropriate annealing temperature. Furthermore, by employing a composition-graded a-SiGe channel layer to form the source/drain Schottky contact with the top a-Ge:H channel segment, the device effective electron mobility could be further improved.

High-sensitivity planar Si-based MSM photodetector with very thin amorphous silicon-alloy quantum-well-like barrier layers

A high-sensitivity, low dark-current planar Si-based metal-semiconductor-metal photodetector (PD) has been successfully fabricated. Under a very weak 0.83-/spl mu/m incident light power (0.5 /spl mu/W) and a 4-V bias voltage, the device photocurrent-to-dark-current ratio (I/sub p//I/sub d/) could reach 10/sup 3/. Also, the average full-width at half-maximum and fall time of the device temporal response were 68.18 and 294.7 ps, respectively, as measured with a periodic 0.83-/spl mu/m 60-ps light pulse at a 10-V bias voltage. In contrast to the previously reported various Si-based PDs, this device exhibited significant improvements in sensitivity and temporal response due to the employed quantum-well-like amorphous silicon-alloy barrier layers.

High-speed Si-based metal-semiconductor-metal photodetectors with an additional composition-graded i-a-Si1−xGex:H layer

Abstract The response-speed of Si-based metal-semiconductor-metal (MSM) photodetectors was improved by depositing a composition-graded intrinsic hydrogenated amorphous silicon–germanium (i-a-Si 1− x Ge x :H) layer on crystalline silicon (c-Si). In contrast to the non-composition-graded one (using intrinsic hydrogenated amorphous silicon (i-a-Si:H) layer), the full width at half maximum (FWHM) and fall time of the photodetector transient response were improved from 145.2, 404.6 to 107.6, 223.4 ps respectively. The experimental results showed that the device responsivity and quantum efficiency were increased from 0.329 (A/W) and 0.492 to 0.414 and 0.619 respectively by the employed composition-graded technique. We propose that this enhancement is due to a smoother barrier that is formed at the c-Si and i-a-Si 1− x Ge x :H interface. A lower deposition temperature of i-a-Si 1− x Ge x :H layer could be used to further reduce the fall time of the device transient response from 315.6 (250 °C) to 97.6 (180 °C) ps. To improve the contact properties between Cr electrode and i-a-Si 1− x Ge x :H layer, an annealing technique in hydrogen ambient was employed. The device knee voltage, which is the applied voltage at which the device current start to enter the saturation region in its current (log-scale) versus applied voltage characteristics, could be reduced to around 3.5 V after annealing.

Optoelectronic characteristics of polymer light emitting diodes with poly(2-methoxy-5-(2′ethyl-hexoxy)-1,4-phenylene-vinylene) and hydrogenated amorphous silicon alloy heterointerfaces

In order to investigate the feasibility of combining polymer and inorganic films for light-emitting diode (LED) fabrication, the inorganic p-amorphous-Si:H/n-amorphous-SiCGe:H layer was employed as hole/electron injection layer (HIL/EIL) in the poly(2-methoxy-5-(2′ ethylhexoxy)-1,4-phenylene-vinylene) polymer LEDs (PLEDs). In contrast to the PLED without any amorphous HIL/EIL, which had an electroluminescence (EL) threshold voltage (Vth) of 10 V and a brightness of 1231 cd/m2 at an injected current density (J)=0.6 A/cm2, the EL Vth could be reduced to 6.9 V for PLED with a 6 nm p-a-Si:H HIL only, also, the brightness could be enhanced to 6450 cd/m2 (at J=0.3 A/cm2 only) for PLED with both p-amorphous-Si:H HIL and n-a-SiCGe:H EIL.

The bandwidth-efficiency product enhancement of GaN based photodiodes by launching a low-temperature-grown recombination center in photo-absorption region

We demonstrated a GaN-based p-i-n photodiode by inserting a thin low-temperature-grown-GaN layer between p-type Al0.2Ga0.8N window and intrinsic GaN layer. As compared with control device, our demonstrated one can achieve 3 fold bandwidth-efficiency-products improvement.