Jyh-Wong Hong

Amorphous silicon/silicon carbide superlattice avalanche photodiodes

An a-Si/SiC:H superlattice avalanche photodiode (SAPD) has been successfully fabricated on an ITO/glass substrate by plasma-enhanced chemical vapor deposition. The room-temperature electron and hole impact ionization rates, alpha and beta , have been determined for the a-Si/SiC:H superlattice structure by photocurrent multiplication measurements. The ratio alpha / beta is 6.5 at a maximum electric field of 2.08*10/sup 5/ V/cm. Avalanche multiplications in the superlattice layer yields an optical gain of 184 at a reverse bias V/sub R/=20 V and an incident light power P/sub in/=5 mu W. An LED-SAPD photocouple exhibited a switching time of 4.5 mu s at a load resistance R-1.8 k Omega . >

Characteristics of MSM photodetectors with trench electrodes on p-type Si wafer

U-grooved metal-semiconductor-metal photodetectors (UMSM-PDs) having various trench depths of interdigitated electrodes and an intrinsic hydrogenated amorphous silicon (i-a-Si:H) to c-Si heterojunction have been fabricated successfully on a p-type [100] Si wafer. The U-grooved structures on c-Si were achieved with a simple orientation-dependent etching (ODE) process. Some important characteristics of the obtained UMSM-PDs are presented and discussed. An UMSM-PD with a 70 nm i-a-Si:H overlayer, 1.45 /spl mu/m-deep recessed electrodes, and 3 /spl mu/m finger width and spacing, had a full width at half maximum (FWHM) of 50.6 ps and a full-time of 132 ps for its temporal response under a bias of 15 V. The significant improvements of transient response for UMSM-PD, as compared to the conventional one, were attributed to the trench electrodes resulted in a stronger lateral electric field in the light absorption region of photodetector. At a bias of 20 V, this UMSM-PD had a responsivity of 0.25 A/W as measured with an 0.83-/spl mu/m incident semiconductor laser, a high photo/dark current ratio about 2000, and an internal quantum efficiency of 36%. This high photo/dark current ratio would be due to the additional i-a-Si:H overlayer on Si wafer. These mentioned performances were much better than those of the conventional Si-based planar MSM-PD.

Graded-gap a-SiC:H p-i-n thin-film light-emitting diodes

To improve the performance of hydrogenated amorphous-silicon carbide (a-SiC:H) p-i-n thin-film light-emitting diodes (TFLEDs), a p-i-n TFLED with a graded p-i junction was proposed and fabricated. The electroluminescence (EL) intensity of the proposed TFLED was more than 100 times higher than that of the basic p-i-n TFLED and about 35 times lower than that of the conventional green LED, at the same injection current density. This significant improvement is attributed to the better interface property and enhancement of hole injection efficiency by using the graded-gap p-i junction.<<ETX>>

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 a-SiC:H-based P-I-N thin-film light-emitting diodes with low-resistance and high-reflectance N/sup +/-a-SiCGe:H layer

The graded-gap a-SiC:H-based p-i-n thin-film light-emitting diodes (TFLEDs) with an additional low-resistance and high-reflectance n/sup +/-a-SiCGe:H layer were proposed and fabricated on indium-tin-oxide (ITO)-coated glass substrate in this paper. For a finished TFLED, a brightness of 720 cd/m/sup 2/ could be obtained at an injection current density of 600 mA/cm/sup 2/, and its EL (electroluminescence) threshold voltage was lowered to 8.6 V. In addition, the effects of reflectance and resistance of a-SiCGe:H film on the performance of TFLED were discussed. The optimum rapid thermal annealing (RTA) conditions for fabrication of TFLED after metallization were also studied and employed to improve the optoelectronic characteristics of TFLED.

Electrical and luminescent characteristics of a-SiC:H p-i-n thin-film LED'S with graded-gap junctions

a-SiC:H p-i-n thin-film LEDs (TFLEDs) containing a single graded-gap p-i-n junction (SG) or double graded-gap p-i-n and i-n junctions (DG) have been postulated and fabricated successfully on indium-tin-oxide (ITO)-coated glass substrates, with a plasma-enhanced chemical vapor deposition (PECVD) system. Some important characteristics and related physics of these two types of TFLEDs are presented and discussed. At an injection current density (J) of 600 mA/cm/sup 2/, the brightness (B) of the SG and DG TFLEDs obtained were 30 and 207 cd/m/sup 2/, respectively. This significant improvement of brightness, as compared to those of the previously reported TFLEDs with a highest brightness of 20 cd/m/sup 2/, could be ascribed to the reduced interface states with the graded-gap junctions, lower contact resistance between ITO and the p-layer due to plasma treatment of ITO prior to p-layer deposition, post metallization annealing of thermally evaporated Al on n-layer, and higher optical gaps (E/sub opt/s) of the doped layers employed. The slopes of the nearly linear B-J relationships show a diode factor very close to unity for the fabricated SG and DG TFLEDs. This implies that the electroluminescence (EL) mechanism of these TFLEDs might be a tail-to-tail-state recombination. In addition, the conduction currents of these TFLEDs are almost temperature dependent, and that of the DG TFLED might consist of an ohmic current and a space-charge-limited current (SCLC) within the lower and higher applied-bias regions, respectively.

A HYDROGENATED AMORPHOUS SI/SIC HETEROJUNCTION PHOTOTRANSISTOR

Abstract In order to improve performance of the a-Si:H homojunction phototransistor, a new hydrogenated amorphous Si/SiC heterojunction phototransistor was proposed and successfully fabricated on the ITO (indium-tin oxide)-coated glass substrate. The structure was glass/ITO/a-Si:H( n + − i − δp + )/a-SiC:H ( i − n + )/Al, in which the thin δp + a-Si:H layer of narrower band-gap was embedded as the base which formed the heterojunction with the a-SiC:H emitter. This heterojunction device revealed a significant improvement over the previously reported homojunction ones, with an optimal optical gain of 40 and a response speed of 10 μs. The physical phenomenon which causes this performance improvement is described qualitatively.

Optical and noise characteristics of amorphous Si/SiC superlattice reach-through avalanche photodiodes

In order to improve the performance of the a(amorphous)-Si:H/SiC:H superlattice avalanche photodiode (APD), a-Si:H/SiC:H superlattice reach-through APDs (SRAPDs) have been fabricated on ITO(indium tin oxide)/glass substrates by plasma-enhanced chemical vapor deposition (PECVD). For a typical electron-injection SRAPD, the ratio of room-temperature electron and hole impact ionization rates ( alpha / beta ) is 10.2 at an electric field 3.33*10/sup 6/ V/cm, the optical gain is 506 at an applied reverse-bias V/sub R/=18 V and an incident power P/sub in/=5 mu W emitted from a He-Ne laser, the rise time is 1 mu s at a load resistance R/sub L/ 1 k Omega , and the excess noise factor is 6.53 at a multiplication M=48. >

The hydrogenated amorphous silicon reach-through avalanche photodiodes (a-Si:H RAPDs)

The RAPD (reach-through avalanche photodiode) structure is adopted to improve the electrical and optical performance of photosensing devices made of a-Si:H. Both the electron-injection n/sup +/ -i- delta p-i-p/sup +/ and hole-injection p/sup +/-i- delta n-i-n/sup +/ a-Si:H RAPDs are fabricated on the indium-tin-oxide-coated glass substrates by plasma-enhanced chemical vapor deposition (PECVD). The photocurrent multiplication method is employed to study the multiplication factors and the impact ionization coefficients of the RAPDs. Since the electron-injection models have better performance, the relationships between the device dimensions and characteristics, such as I-V curves, optical gains, impact ionization rates, and excess noise factors, are further studied. The results indicate that the a-Si:H RAPD is a promising device for photosensing applications. >

Improved process of fabricating AC-coupled silicon micro-strip sensors

Abstract A single-sided silicon sensor with capacitors coupling and polysilicon bias resistors has been designed and fabricated. A proposed process with ONO (Oxide-Nitride-Oxide) replacing the usual SiO 2 layer as the dielectric of coupling capacitor, in conjunction with a reordering of sequence for layer formations, is to produce sensors with self-moisture-protection and free from the effect of pin holes. A comparison of presented data of IV, CV and RV measurements for the sensors with ONO and with SiO 2 dielectrics revealed that the ONO processes could lead to an excellent voltage-handling capability of the coupling capacitor. One sensor has been successfully tested twice in the beam at CERN in the past two years, yielding an S/N ratio of 20 and an efficiency above 95%, also demonstrating its excellent stability with respect to lengthy exposure to atmosphere.