High-speed InGaAs on Si metal-semiconductor-metal photodetectors

High-speed long-wavelength metal-semiconductor-metal (MSM) photodetectors were fabricated on the Fe-doped InP/GaAs material system on Si. The detector layers were grown by MOCVD on exactly oriented Si(001) patterned with submicrometre pitch V-grooves. The devices show a fast impulse response (44 ps FWHM, 83 ps fall time) and a large bandwidth of 3.4 GHz for illumination with 1.31 μm light pulses at 5 V bias     

ZnSTeSe metal-semiconductor-metal photodetectors

High-quality quaternary ZnSTeSe epitaxial layers were successfully grown by molecular beam epitaxy (MBE). It was found that a ZnS_0.18 Se_0.82 layer was automatically formed in between the ZnSe buffer layer, and the ZnS_0.17Te_0.08Se_0.75 epitaxial layer, when we increased the ZnS cell temperature. ZnSTeSe metal-semiconductor-metal (MSM) photodetectors were also fabricated for the first time. It was found that we could achieve a photocurrent to dark current contrast higher than five orders of magnitude by applying a 10-V reverse bias. We also found that the maximum photoresponsivity is about 0.4 A/W under a 10-V reverse bias. Such a value suggests that the ZnSTeSe MSM photodetector is potentially useful in the blue-UV spectral region     

Optimization of high-speed metal-semiconductor-metal photodetectors

Circular-aperture Metal-Semiconductor-Metal (MSM) photodetectors have been fabricated in order to decrease the device capacitance. The frequency-response of the MSM photodetectors has been calculated using a first-order approximation. The design of the circular-aperture detectors and conventional square-aperture detectors has been optimized using this approximation. The largest 3 dB bandwidths are obtained with 33 fingers for the circular detectors and 29 for the square detectors. These numbers do not change regardless of the size of the MSM photodetectors     

Doping-induced bandwidth enhancement in metal-semiconductor-metal photodetectors

A novel technique is demonstrated for enhancing metal-semiconductor-metal photodetector bandwidths without reducing responsivity. The authors have observed bandwidth increases of up to 10 GHz from control to enhanced photodetectors which corresponds to about 70% enhancement. This technique involves modifying the internal electric field structure, by introducing a buried n-type doped layer that is completely depleted, to reduce the relatively long hole transit time.<>     

Novel metal-semiconductor-metal photodetectors on bulk semi-insulating indium phosphide

Depositing Pd or An on InP at substrate temperatures near 77 K (LT) has previously been found to significantly reduce the interaction between the metal and semiconductor upon formation of the interface. In this letter, this technique was used to fabricate metal-semiconductor-metal photodetectors (MSMPDs) on semi-insulating (SI) InP substrates with superior characteristics compared to detectors formed using standard room temperature (RT) metal deposition. Detectors having a LT-Ps-SI-InP structure had a dark current of 80 nA at 5 V, which was a factor of 4 lower than the dark current of conventional MSMPDs. Additionally, LT-Pd-SI-InP MSMPDs exhibited excellent saturation characteristics and a responsivity of 0.75 A/W. Detectors with an indium-tin-oxide (ITO)-LT-Au (200 /spl Aring/)-SI-InP structure had a higher responsivity of 1.0 A/W, due to the relative transparency of this metallization. In contrast, MSMPDs with RT metallizations had poor saturation characteristics, consistent with the results of others. The difference in the illuminated characteristics of MSMPDs with RT and LT metallizations was due to a change in the internal photoconductive gain mechanism. In RT detectors, hole trapping at interface states near the cathode dominated the gain mechanism. In LT detectors, the difference in carrier transit-times dominated.     

1.3 μm GaSb metal-semiconductor-metal photodetectors

Metal-semiconductor-metal photodetectors employing GaSb active regions and Al_0.5Ga_0.5Sb barrier-enhancing abrupt regions have been fabricated on InP substrates to assess the role of hole velocity and to compare with similar photodetectors made using Ga _0.47In_0.53As active region. Devices exhibit photoresponse in the 0.2-0.65 A/W range, dark currents of ≈10^-6 A at 300 K and ≈10^-10 A at 77K for 25×25 μm² area, and have a voltage-sensitive 3-dB bandwidth exceeding ≈1 GHz at 300 K and 10 GHz at 77 K. The enhanced barrier heights are estimated to be ≈0.30 eV. The fall time continues to be the significant component of time delay; its temperature dependence indicates that the hole velocities do improve significantly at lower temperatures. The 300 K behavior appears to be dominated by defect and impurity densities, and the effects of abrupt barriers. The larger than expected dark currents are believed to result from defects associated with lattice mismatch     

High-speed photodetectors

This paper is intended as a status report on high-speed detectors for the visible and near-infrared portion of the optical spectrum. Both vacuum and solid-state detectors are discussed, with the emphasis on those devices which can be used as direct (noncoherent) detectors of weak optical signals modulated at microwave frequencies. The best detectors for this application have internal current gain and in this regard the relevant properties and limitations of high-frequency secondary emission multiplication in vacuum tube devices and avalanche multiplication in p-n junctions are summarized.     

High-speed metal-semiconductor-metal photodiodes with Er-doped GaAs

Very high-speed MSM photodiodes have been fabricated on Er-doped GaAs over a doping range of 10¹⁸-10²⁰ cm^-3 . The impulse response (characterized by photoconductive sampling) of these diodes, with finger widths/spacings of 2 μm, has been found to be tunable over a range of about 3 ps-22 ps. Electro-optic sampling was used to characterize MSM diodes with finger widths/spacings of 0.5 μm and 1 μm on a sample with [Er]=10¹⁹ cm^-3, resulting in 3-dB bandwidths of 160 GHz and 140 GHz, respectively, corresponding to pulse widths of 2.7 ps and 3.3 ps. Correlation measurements were also done on the GaAs:Er samples, using an all-electronic Sampling Optical Temporal Analyzer (SOTA) structure     

Low-noise solar-blind AlxGa1-xN-based metal-semiconductor-metal ultraviolet photodetectors

We report the growth, fabrication, and characterization of high performance Schottky metal-semiconductor-metal solar-blind photodetectors fabricated on epitaxial Al_0.4Ga_0.6N layers grown by metalorganic chemical vapor deposition. The devices exhibited low dark current (<2 pA at 30 V) and a gain-enhanced ultraviolet (UV) photocurrent for bias voltages >40 V. The gain was corroborated by external quantum efficiency measurements reflecting a quantum efficiency as high as 49% (at=272 nm) at 90 V bias, with a corresponding responsivity R=107 mA/W. A visible-to-UV rejection factor of more than three orders of magnitude was demonstrated. Time-domain and frequency-domain speed measurements show a 3-dB bandwidth of ∼100 MHz. Low-frequency noise measurements have determined a detectivity (D^*) as high as 3.3 10¹⁰ cm·Hz^1/2/W for a 500 Hz bandwidth at 37 V bias.     

Nitride-Based ultraviolet metal-semiconductor-metal photodetectors with a low-temperature GaN layer

The GaN metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors with a low-temperature (LT)-GaN layer have been demonstrated. It was found that we could achieve a two orders of magnitude smaller, photodetector-dark current by introducing a LT-GaN layer, which could be attributed to the larger Schottky-barrier height between the Ni/Au metal contact and the LT-GaN layer. It was also found that photodetectors with the LT-GaN layer could provide a larger photocurrent to dark-current contrast ratio and a larger UV-to-visible rejection ratio. The maximum responsivity was found to be 3.3 A/W and 0.13 A/W when the photodetector with a LT-GaN layer was biased at 5 V and 1 V, respectively.     

Fast AlGaN metal-semiconductor-metal photodetectors grown onSi(111)

Metal-semiconductor-metal photodetectors have been fabricated on AlGaN grown on Si(111) by molecular beam epitaxy for solar-blind applications. A cutoff wavelength of 290 nm and an ultraviolet/visible contrast of more than three orders of magnitude are achieved. Time response measurements show fast exponential decays. With a minimum decay time of 150 ns. The photodetectors present responsivities that increase with bias, reaching 15 mA/W at 4 V bias     

High-speed zero-bias waveguide photodetectors

We describe an InGaAsP waveguide PIN photodetector that (unlike conventional PINs) provides for light absorption perpendicular to current collection, a feature that results in high-speed, high-efficiency and relatively bias-insensitive operation. Our uncoated, packaged device has displayed impulse response of 40 ps (Tektronix S-4 sampling head) and efficiency of 25% at zero bias.     

Dark current characteristics of GaAs metal-semiconductor-metal(MSM) photodetectors

Calculations of the electron and hole components of dark current in a GaAs metal-semiconductor-metal (MSM) photodetector are presented. A quantum-mechanical model for the electron and hole transport behavior in the contact regions which is used to determine dark current as a function of electric field is developed. The model reduces to a conventional thermionic emission model if an ideal barrier transmission coefficient is assumed. In order to assess the accuracy of the model, photodetectors have been fabricated and tested. Theoretical calculations and experimental data are compared, and good agreement is obtained. Possible modifications to enhance the usefulness of the model are discussed     

Schottky metal-semiconductor-metal photodetectors on GaN filmsgrown on sapphire by molecular beam epitaxy

We report high-performance back-to-back Schottky metal-semiconductor-metal photodetectors fabricated on GaN epitaxial layers grown by molecular beam epitaxy. The photodetectors exhibit very low dark current (<1 pA at 30 V) and high external quantum efficiency (>70%). Medici simulation of the depletion region width indicated an absence of photoconductive gain. The temporal response has also been characterized     

Voltage-tunable dual-band infrared photodetectors with Si/SiGe metal-semiconductor-metal heterostructure

A simple and low-cost structure of voltage-tunable dual-band near-infrared photodetector (PD) has been proposed, in which the PDs were developed by using Si_0.8Ge_0.2/Si metal-semiconductor-metal (MSM) heterostructure. The Si_0.8Ge_0.2/Si layers were deposited by ultrahigh-vacuum chemical vapor deposition system and a transparent layer of indium-tin oxide (ITO) was used as a metal layer to enhance the entrance of photons. In this study, we found that only one band was detected with a peak wavelength located at 950 nm at zero applied bias. When bias was increased to 1 V, in contrast a dual-band was achieved, where two peak wavelengths were centered at 950- and 1150-nm. It is suggested that the two bands are the absorption of top-Si and bottom-Si_0.8Ge_0.2 layers, respectively. The spectra of Si bulk and Si_0.8Ge_0.2 layer were also measured to verify our results and relating mechanisms are explained here.     

Double-heterostructure InGaAs/InP PIN photodetectors

High-quality n⁺InP/InGaAs/p⁺InP double-heterojunction PIN photodetectors have been grown by special liquid-phase-epitaxial techniques. This development allows front illumination which substantially facilitates device fabrication and operation. Furthermore, good control of pn junction location and layer thickness in these structures permits optimization of high-speed performance. For convenient device areas, low dark-current densities (2.5×10⁻⁵ A/cm²) and short response-times (50 ps) are obtained. High-speed design parameters are discussed, and the reduction of response times to 30 ps is predicted.     

GaInAs metal/semiconductor/metal photodetectors with Fe:InP barrier layers grown by chemical beam epitaxy

A GaInAs metal/semiconductor/metal (MSM) photodetector with a dark current less than 1 mu A is described. An Fe-doped InP layer was introduced between the metal and the GaInAs absorbing layer to improve the Schottky barrier height. A breakdown voltage of 30 V was achieved. A DC quantum efficiency of 64% and an impulse response, 1/e fall time of 190 ps were measured for a 20 mu m*100 mu m device. The layer structure is very attractive for integration with high-performance GaInAs/InP FETs.<>     

GaN metal-semiconductor-metal photodetectors with low-temperature-GaN cap layers and ITO metal contacts

Nitride-based metal-semiconductor-metal (MSM) photodetectors (PDs) with low-temperature (LT) gallium nitride (GaN) cap layers and indium-tin-oxide (ITO) metal contacts were successfully fabricated. It was found that we could achieve three orders of magnitude smaller dark current by the introduction of the LT-GaN layer. For the PDs with LT-GaN cap layers, the maximum responsivity at 350 nm was found to be 0.1 and 0.9 A/W when the device was biased at 1 and 5 V, respectively. Operation speed of PDs with LT-GaN cap layers was also found to be faster than that of conventional PDs without LT-GaN cap layers.     

Two-dimensional ensemble Monte Carlo calculation of pulse responsesof submicrometer GaAs metal-semiconductor-metal photodetectors

Pulse responses of top-illuminated GaAs metal-semiconductor-metal photodetectors (MSM PDs) are evaluated by using a two-dimensional ensemble Monte Carlo technique. Fundamental assumptions and the model used for the evaluation are detailed. Pulsewidths for MSM PDs are presented as functions of the gap length between metal electrodes and the photon energy of optical pulses. It is also shown that reducing the thickness of the absorption region is very effective for shortening the pulsewidth of MSM PDs     

High-speed photodetectors in optical communication system

This paper presents a review and discussion for high-speed photodetectors and their applications on optical communications and microwave photonics.A detailed and comprehensive demonstration of high-speed photodetectors from development history,research hotspots to packaging technologies is provided to the best of our knowledge.A few typical applications based on photodetectors are also illustrated,such as free-space optical communications,radio over fiber and millimeter terahertz signal generation systems.