Rong-Hwei Yeh

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).

Optoelectronic Characteristics of Direct-Current and Alternating-Current White Thin-Film Light-Emitting Diodes Based on Hydrogenated Amorphous Silicon Nitride Film

Direct-current and alternating-current white thin- film light-emitting diodes (DCW and ACW TFLEDs) have been fabricated and demonstrated with the intrinsic hydrogenated amorphous silicon nitride (i-a-SiN:H) film as the luminescent layer. The achievable brightness of the representative DCW and ACW TFLEDs were 200 and 170 cd/m2 at an injection-current density of 600 and 100 mA/cm2, respectively. The electroluminescence (EL) threshold voltage of the DCW TFLED was 10.9 V, and its peak wavelength and full-width at half-maximum (FWHM) of the EL spectrum were about 455 and 230 nm, respectively. For the ACW TFLED, the EL threshold voltage was 8.4 V, and its peak wavelength and FWHM of the EL spectrum were about 535 and 260 nm, respectively. In addition, their current-conduction mechanism was also investigated. Within the lower applied-voltage region, they showed an ohmic current, while for the higher applied-voltage region, the Frenkel-Poole emission was the main mechanism. It was also found that the H2 -plasma treatment of luminescent i-a-SiN:H layer of an ACW TFLED played an important role in improving device performances, such as decreased EL threshold voltage, increased brightness, and broadened and blue-shifted EL spectrum. The EL spectra of an ACW TFLED under either DC forward or reverse bias or the sinusoidal AC voltage were qualitatively very similar, with a peak wavelength at about 535 nm and a broad FWHM about 260 nm. Moreover, the EL intensity of an ACW TFLED increased with an AC frequency of up to 180 kHz and, then, decreased rapidly and became very weak as the frequency was up to about 500 kHz.

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.

Alternating-Current White Thin-Film Light-Emitting Diodes Based on Hydrogenated Amorphous Carbon Layer

Alternating-current white thin-film light-emitting diodes (ACW-TFLEDs) have been fabricated and demonstrated with composition-graded hydrogenated intrinsic amorphous silicon carbide (i-a-SiC : H) layers. It was found that H2-plasma treatment of luminescent i-a-C : H layer played an important role in decreasing the ACW-TFLED electroluminescence (EL) threshold voltage, increasing the brightness, and broadening the EL spectrum. The EL spectra of the ACW-TFLED under either dc forward or reverse bias, or the sinusoidal alternating-current voltage were qualitatively very similar, with a peak wavelength at about 505 nm and a broad full-width at half maximum (FWHM) about 240 nm. This device revealed a brightness about 800(500)cd/m2 under dc forward (reverse) bias at an injection current density of 600 mA/cm2

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.

Effects of Hydrogenation on Optoelectronic Properties of a-C:H Thin-Film White-Light-Emitting Diodes With Composition-Graded Carrier-Injection Layers

In this paper, thin-film white-light-emitting diodes (TFWLEDs) were fabricated with a hydrogenated intrinsic amorphous carbon (i-a-C:H) film as the luminescent layer and a composition-graded (CG) hydrogenated intrinsic amorphous silicon carbide (i-a-SiC:H) film as the carrier-injection layers. The demonstrated TFWLEDs could be operated under direct-current (dc) forward or reverse bias or sinusoidal alternating-current (ac) voltage. The hydrogenation process for the luminescent or CG carrier-injection layer has been investigated to greatly enhance the optoelectronic properties of the obtained TFWLEDs. For the hydrogenated TFWLEDs, the highest obtainable brightnesses were 813 and 507 cd/m2 at an injection-current density of 0.6 A/cm2, and the lowest electroluminescence (EL) threshold voltages were 9.1 and 8.9 V, under dc forward and reverse biases, respectively. These enhanced optoelectronic properties were attributed to the passivation of dangling bonds and the forming of more H2-compensated amorphous film by the employed hydrogenation process. In addition, the electrical transport mechanisms of the TFWLEDs were studied. In the low-applied-bias range, the ohmic current was the dominated one. In the high-applied-bias range, a Poole-Frenkel emission current resulted from the field-assisted hopping along the traps in amorphous film was observed. Moreover, a significant red-shift in EL spectra has been observed while the applied ac frequencies were higher than 1 kHz, and its origin has been attributed to the lower mobilities of charge carriers.

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.

Hydrogenated Amorphous Silicon Carbide Alternating-Current Thin-Film Light-Emitting Diodes

Hydrogenated amorphous silicon carbide alternating-current thin-film light-emitting diodes (a-SiC:H ACTFLEDs) have been successfully fabricated on indium-tin-oxide-coated glass substrates and characterized. The electroluminescence (EL) spectra of an obtained ACTFLED under either a dc (positive or negative) bias or a sinusoidal ac voltage were qualitatively very similar, with a peak wavelength at about 655 nm and a shoulder at about 620 nm. The EL threshold voltage and brightness of the ACTFLEDs were sensitive to the contact behavior between the metal electrode employed and amorphous film. Moreover, the EL intensity of an ACTFLED increased with the frequency up to 500 kHz and then decreased rapidly and became very weak as the frequency increased to about 1 MHz. A model based on the equivalent circuit has also been proposed to explain this frequency-dependent EL behavior.