Finger Spacing Research Articles

A U-grooved metal-semiconductor-metal photodetector (UMSM-PD) with an i-a-Si:H overlayer on a [100] P-type Si wafer

A metal-semiconductor-metal photodetector (MSMPD) with U-shaped interdigitated electrodes and an intrinsic hydrogenated amorphous silicon (i-a-Si:H) heterojunction, fabricated on a p-type [100] Si wafer, was studied. This Si UMSM-PD with a 70 nm i-a-Si:H overlayer, 0.9-μm-deep recessed electrodes, and a finger width and spacing of 3 μm, had a full-width at half-maximum (FWHM) of 50.8 ps, and a fall-time of 140 ps for its temporal response, a responsivity of 0.18 A/W, as measured with a 830-nm incident laser, a dark current density of 1.5 pA/μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and an internal quantum efficiency of 27%. Its performance was much better than that of the conventional Si-based planar MSM-PD. The improved device characteristics were attributed to the U-shaped electrodes which resulted in a stronger lateral, electric field in the light absorption region of the photodetector.

Theoretical study of a GaAs lateral p-i-n photodetector

We present a numerical study of a high speed GaAs lateral p-i-n (LPIN) photodiode. The LPIN is a planar structure composed of interdigitated p+ and n+ wells. A metal–semiconductor–metal (MSM) photodiode with identical finger spacing and geometry is simulated for comparison. When pulsed with an 827 nm optical source at 0.68 mW/cm2, the lateral p-i-n exhibited improved frequency performance and responsivity compared to the MSM. The dark current and capacitance are similar in magnitude between the two devices. Based on these results, it is concluded that a lateral p-i-n can be an attractive alternative to standard MSM photodetectors.

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

U-grooved metal-semiconductor-metal photodetectors (UMSM-PD's) 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.

Frequency-domain demonstration of transit-time-limited, large-area InGaP-InP-InGaAs MSM photodetectors

We report transit-time-limited bandwidth (12 GHz at 12-V bias) for an InGaP-InP-InGaAs MSM photodetector with 2-μm finger spacing, (100-μm) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> active area, and abrupt heterojunctions. The same device exhibits a very gradual rolloff up to 20 GHz, with an on/off isolation of greater than 40 dB across the measurement band. This is one of the first reports of transit-time-limited performance measured in the frequency domain for InGaAs-based MSM photodetectors, and these are the largest such devices reported to date. A pseudomorphic In/sub 0.8/Ga/sub 0.2/P cap layer is found to have an electrically stable free surface, requiring no special pre-treatment or passivation.

High-speed polysilicon resonant-cavity photodiode with SiO/sub 2/-Si Bragg reflectors

Previously, it has been shown that the bandwidth of Si photodiodes can be increased by more than an order of magnitude, without sacrificing responsivity, by a resonant-cavity structure that utilized GeSi-Si asymmetric Bragg reflectors. We report an interdigitated p-i-n polysilicon resonant-cavity photodiode, which employs a Si-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Bragg reflector, that is more compatible with standard Si processing technology. For an absorbing region thickness of only 0.5 μm, a peak quantum efficiency of 40% was achieved and the dark current was <60 nA at 10 V. For 2 μm×2-μm finger width and spacing the bandwidth was 10 GHz.

Polarimetry of thin metal transmission gratings in the resonance region and its impact on the response of metal-semiconductor-metal photodetectors

The resonance behavior of metal transmission gratings and its impact on the response of metal-semiconductor-metal (MSM) photodetectors have been studied experimentally and theoretically. The metal gratings, with finger spacings in the subwavelength region of the visible light, were fabricated using e-beam lithography and lift-off. Strong resonances have been observed only in the S polarization. As a result, the light transmitted through a grating is primarily S polarized if the grating’s finger spacing is less than one-third of the wavelength of the incident light, but P polarized otherwise. Similar phenomenon has been observed in the response of MSM photodetectors since the fingers (electrodes) of an MSM photodetector basically form a grating. Theoretical simulations employing the rigorous modal-expansion theory fairly predict the observed phenomenon.

Overall performance improvement in GaAs MSM photodetectors by using recessed-cathode structure

We report the experimental work on the GaAs metal-semiconductor-metal photodetectors with recessed cathode and/or anode. The recessed-cathode detectors exhibit much superior dc and speed performance to the conventional one because of the enhancement of hole transport in this structure. Full-width at half-maximum and fall time of the temporal response were measured to be 21 and 13 ps at 5 V bias on a 50×50 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> detector with a finger width and spacing of 3 μm.

In/sub 0.53/Ga/sub 0.47/As MSM photodiodes with transparent CTO Schottky contacts and digital superlattice grading

Metal-semiconductor-metal (MSM) photodiodes with an In/sub 0.53/Ga/sub 0.47/As active region were investigated using a transparent cadmium tin oxide (CTO) layer for the interdigitated electrodes to improve the low responsivity of conventional MSM photodiodes with opaque electrodes. CTO is suitable as a Schottky contact, an optical window and an anti-reflection (AR) coating. The transparent contact prevents shadowing of the active layer by the top electrode, thus allowing greater collection of incident light. Responsivity of CTO-based MSM photodiodes with 1-/spl mu/m finger widths and 2-/spl mu/m finger spacings and without an AR coating between the electrodes was twice (0.62 A/W) that of a similar MSM photodiodes with Ti/Au electrodes (0.30 A/W). A thin 800 /spl Aring/ In/sub 0.52/Al/sub 0.48/As layer was inserted below the electrodes to elevate the electrode Schottky barrier height. A digitally graded superlattice region (660 A) was also employed to reduce carrier trapping at the In/sub 0.53/Ga/sub 0.47/As/In/sub 0.52/Al/sub 0.48/As heterointerface which acts to degrade photodiode bandwidth. Bandwidth of opaque electrode MSMs was elevated nearly an order of magnitude over a previous MSM photodiode design with an abrupt heterointerface, whereas the bandwidth of transparent electrode MSM's only improved about five times, indicating resistive effects may be intervening.

A comparative study of Si and GaAs metal-semiconductor-metal photodetectors

In this paper we present a comparative study of the optical DC and transient properties of the Si- and GaAs-metal-semiconductor-metal photodetectors (MSM-PD's). The interdigitated metal fingers for the detectors were fabricated on n-type Si and semi-insulating GaAs substrates by using optical lithography and lift-off technique. The width and spacing of the interdigitated metal fingers varied between 1 and 2 μm, and the detector area varied between 100 × 100 μm2 and 500 × 500 μm2. For Si-MSM-PD's we used 150 nm thick aluminum and for GaAs-MSM-PD's 280 nm thick Ti/Pt/Au multilayer metallization for the Schottky barrier formation. The Schottky diode capacitance vs. bias voltage as well as dark and illuminated I-V characteristics as a function of optical power have been measured. Also the photoresponsivity vs. wavelength has been measured for various bias voltages. The speed of the detectors was studied by using a colliding pulse mode-locked laser. The experimental results show that the leakage currents in GaAs-MSM detectors are much smaller than in Si detectors, but still a reasonable photoresponsivity can be achieved also in Si detectors. The speed of the Si detectors could be improved by fabricating them on an epitaxial n+ - n− -structure, which results in a fast impulse response 625 ps (FWHM), 292 ps (fall time).

Characterization and application of a low-profile metal–semiconductor–metal detector for low energy backscattered electrons

A metal–semiconductor–metal (MSM) detector for low energy backscattered electrons has been developed and tested. The effects of detector symmetry, finger spacing, and detection area on signal to dark current ratio were studied in order to determine an optimized detector geometry. The detector assembly consisting of Ni-GaAs MSM structures with low dark current densities was mounted in a scanning electron microscope on a special sample holder which enabled the test of the detector under conditions which can be expected in electron beam microcolumn applications. With this experimental setup, backscattered electron images of alignment marks of 50-nm-thick Au on a Si substrate were obtained. The signal to noise performance of the device has been compared to a commercial Everhart–Thornley detector. It is demonstrated that MSM detectors can be used as backscattered electron detectors in microcolumns.

Solar cells with mesh-structured emitter

The mesh-structured emitter solar cell (MESC) is introduced as a novel solar cell processing scheme. By the formation of inverted pyramids or microgrooves on a wafer with a homogeneous heavy phosphorus diffusion, a mesh of highly conducting emitter lines is formed. Using this technique, the lateral conductivity of the emitter can be increased, keeping the emitter dark saturation current at a low level. The high phosphorus surface concentration results in a low contact resistance even for screen-printed contacts. Thus, this technique is ideal for solar cells with screen-printed contacts, because the finger spacing of the front contact can be extended, resulting in smaller shadowing losses. Also the processing scheme of high-efficiency solar cells can be simplified, because the formation of the surface texturization and the locally deep diffused emitter can be combined in one step. The first cells with a mesh-structured emitter, evaporated front contacts and local ohmic rear contacts have shown efficien ies up to 21.1%. Lifetime test structures have been used to determine a low dark saturation current of 58 fA cm−2 for the mesh-structured emitter, although the structure is not yet optimized.

High-speed InGaAs metal-semiconductor-metal photodetectors with improved responsivity and process yield

High-performance In0.9Ga0.1P-InP-InGaAs metal-semiconductor-metal photodectors with semi-transparent Au Schottky contacts are fabricated and studied. The devices, with an active area of 50×50 μm2 and different finger spacings of 2, 3 and 4 μm, all exhibit high responsivities over 0.7 AW-1 and low dark currents below 10 nA. Extremely linear photoresponse without low frequency internal gain is also observed in these devices. The novel fabrication process used in this work is simple and has nearly 100% high yield. A device with a small finger spacing has also been demonstrated to have improved speed performance without sacrificing its responsivity.

High-responsivity InGaAs/InP-based MSM photodetector operating at 1.3-μm wavelength

A high-responsivity, low-capacitance and high-speed metal-semiconductor-metal photodetector (MSM-PD) with layer structure In0.85Ga0.15P/InP/InGaAs/InP has been fabricated and characterized. For a 50 μm × 50 μm device with 2-μm finger width and 2-μm finger spacing we obtained a front illumination at 1.3-μm wavelength responsivity of 0.71–0.73 A/W at 5-V bias. To the best of our knowledge this is the highest front illumination responsivity for MSM-PD. The mesa structure devices show a very low capacitance of less than 1 pF and a transient response bandwidth of 10 GHz at 3 dB. © 1996 John Wiley & Sons, Inc.

Wavelength detector using a pair of metal-semiconductor-metalphotodetectors with subwavelength finger spacings

A wavelength detector that employs two metal-semiconductor-metal photodetectors with different subwavelength finger spacings is proposed and demonstrated. The resonance effect of the subwavelength-spaced fingers results in a photocurrent ratio that has one-to-one correspondence with the wavelengths of the monochromatic incident light. The device has a spectral range of 450 – 800 nm with a resolution of 5 nm. This device makes automatic wavelength calibration in optical instrumentation feasible.

CAD models for multilayered substrate interdigital capacitors

Conformal mapping-based models are given for interdigital capacitors on substrates with a thin superstrate and/or covering dielectric film. The models are useful for a wide range of dielectric constants and layer thicknesses. Capacitors with finger numbers n/spl ges/2 are discussed. The finger widths and spacing between them may be different. The results are compared with the available data and some examples are given to demonstrate the potential of the models.

Very low dark current InGaP/GaAs MSM-photodetectorusing semi-transparent and opaque contacts

Metal-semiconductor-metal (MSM) photodetectors have been fabricated for the first time in the InGaP/GaAs materials system. Extremely low dark currents 86 (/spl plusmn/5)pA and 91 (/spl plusmn/5)pA were obtained at 5 V bias using opaque and semi-transparent metal (tungsten) contacts, respectively. A front illumination responsivity of 0.37 - 0.65 A/W at 5 V bias is obtained for a 50 /spl mu/m/spl times/50 /spl mu/m device with 2 /spl mu/m finger width and 2 /spl mu/m finger spacing. The devices, operating at 840 nm, have a 3 dB bandwidth of >9.5 GHz for semi-transparent contacts, and 12 GHz with opaque contacts.

Stability of thermoviscous Hele-Shaw flow

Viscous fingering can occur as a three-dimensional disturbance to plane flow of a hot thermoviscous liquid in a Hele-Shaw cell with cold isothermal walls. This work assumes the principle of exchange of stabilities, and uses a temporal stability analysis to find the critical viscosity ratio and finger spacing as functions of channel length, Lc. Viscous heating is taken as negligible, so the liquid cools with distance (x) downstream. Because the base flow is spatially developing, the disturbance equations are not fully separable. They admit, however, an exact solution for a liquid whose viscosity and specific heats are arbitrary functions of temperature. This solution describes the neutral disturbances in terms of the base flow and an amplitude, A(x). The stability of a given (computed) base flow is determined by solving an eigenvalue problem for A(x), and the critical finger spacing. The theory is illustrated by using it to map the instability for variable-viscosity flow with constant specific heat. Two fingering modes are predicted, one being a turning-point instability. The preferred mode depends on Lc. Finger spacing is comparable with the thermal entry length in a long channel, and is even larger in short channels. When applied to magmatic systems, the results suggest that fingering will occur on geological scales only if the system is about freeze.

Spin-valve effects in nickel/silicon/nickel junctions

We report on fabrication and characterization of a planar spin-valve device which has two sets of interdigited nanoscale Ni fingers as two electrodes in Schottky contact with Si. The finger width is 75 nm for one set and 150 nm for the other. A large length-to-width ratio of the fingers results in a single domain magnetization and a sharp magneto-resistance (MR) response. The switching field of the finger is determined by the finger width and spacing due to magnetostatic interaction. MR measurements reveal spin-valve effects in the Ni/Si/Ni junctions with MR changes of 0.3-0.6% at room temperature. The effects are discussed within a spin-valve model.

Epitaxial liftoff InGaAs/InP MSM photodetectorson Si

InGaAs:Fe/InP:Fe metal-semiconductor-metal (MSM) photodetectors for the long wavelength region were transferred by epitaxial liftoff (ELO) techniques onto an Si substrate. The transferred detectors, with a finger spacing and width of 1.5 and 1.0 µm, respectively, showed no deterioration in device performance. A fast impulse response with an FWHM of 23 ps and an external quantum efficiency of 48% was measured at a 7 V bias and 1.3 µm wavelength. The leakage current was 250 nA at a 7 V bias.

High-performance large-area InGaAs MSM photodetectors with a pseudomorphic InGaP cap layer

We report the fabrication and characteristics of high-performance InGaAs MSM photodetectors with a 300-/spl mu/m/spl times/300-/spl mu/m square and a 300-/spl mu/m-diameter circular detection area. With a pseudomorphic In/sub 0.9/Ga/sub 0.1/P cap layer, the detectors exhibit dark current densities less than 1 pA//spl mu/ on finger spacing. Bandwidth over 1 GHz has been obtained for these large-area detectors. Our results also show that the circular detector exhibits better speed performance as compared to the square one due to its smaller parasitic RC constant. >