Showing papers on "Responsivity published in 2011"

Hybrid graphene-quantum dot phototransistors with ultrahigh gain

Abstract: Graphene has emerged as a novel platform for opto-electronic applications and photodetector, but the inefficient conversion from light to current has so far been an important roadblock. The main challenge has been to increase the light absorption efficiency and to provide a gain mechanism where multiple charge carriers are created from one incident photon. Here, we take advantage of the strong light absorption in quantum dots and the two-dimensionality and high mobility of graphene to merge these materials into a hybrid system for photodetection with extremely high sensitivity. Exploiting charge transfer between the two materials, we realize for the first time, graphene-based phototransistors that show ultrahigh gain of 10^8 and ten orders of magnitude larger responsivity compared to pristine graphene photodetectors. These hybrid graphene-quantum dot phototransistors exhibit gate-tunable sensitivity, spectral selectivity from the shortwave infrared to the visible, and can be integrated with current circuit technologies.

Strong plasmonic enhancement of photovoltage in graphene

Abstract: From the wide spectrum of potential applications of graphene, ranging from transistors and chemical sensors to nanoelectromechanical devices and composites, the field of photonics and optoelectronics is believed to be one of the most promising. Indeed, graphene's suitability for high-speed photodetection was demonstrated in an optical communication link operating at 10 Gbit s(-1). However, the low responsivity of graphene-based photodetectors compared with traditional III-V-based ones is a potential drawback. Here we show that, by combining graphene with plasmonic nanostructures, the efficiency of graphene-based photodetectors can be increased by up to 20 times, because of efficient field concentration in the area of a p-n junction. Additionally, wavelength and polarization selectivity can be achieved by employing nanostructures of different geometries.

Broadband terahertz imaging with highly sensitive silicon CMOS detectors

Abstract: This paper investigates terahertz detectors fabricated in a low-cost 130 nm silicon CMOS technology. We show that the detectors consisting of a nMOS field effect transistor as rectifying element and an integrated bow-tie coupling antenna achieve a record responsivity above 5 kV/W and a noise equivalent power below 10 pW/Hz(0.5) in the important atmospheric window around 300 GHz and at room temperature. We demonstrate furthermore that the same detectors are efficient for imaging in a very wide frequency range from ~0.27 THz up to 1.05 THz. These results pave the way towards high sensitivity focal plane arrays in silicon for terahertz imaging.

Locally Oxidized Silicon Surface-Plasmon Schottky Detector for Telecom Regime

Abstract: We experimentally demonstrate an on-chip nanoscale silicon surface-plasmon Schottky photodetector based on internal photoemission process and operating at telecom wavelengths. The device is fabricated using a self-aligned approach of local-oxidation of silicon (LOCOS) on silicon on insulator substrate, which provides compatibility with standard complementary metal-oxide semiconductor technology and enables the realization of the photodetector and low-loss bus photonic waveguide at the same fabrication step. Additionally, LOCOS technique allows avoiding lateral misalignment between the silicon surface and the metal layer to form a nanoscale Schottky contact. The fabricated devices showed enhanced detection capability for shorter wavelengths that is attributed to increased probability of the internal photoemission process. We found the responsivity of the nanodetector to be 0.25 and 13.3 mA/W for incident optical wavelengths of 1.55 and 1.31 μm, respectively. The presented device can be integrated with other nanophotonic and nanoplasmonic structures for the realization of monolithic opto-electronic circuitry on-chip.

Infrared Photodetectors Based on Reduced Graphene Oxide and Graphene Nanoribbons

Abstract: The use of reduced graphene oxide (RGO) and graphene nanoribbons (GNRs) as infrared photodetectors is explored, based on recent results dealing with solar cells, light-emitting devices, photodetectors, and ultrafast lasers. IR detection is demonstrated by both RGO and GNRs (see image) in terms of the time-resolved photocurrent and photoresponse. The responsivity of the detectors and their functioning are presented.

Ultra compact 45 GHz CMOS compatible Germanium waveguide photodiode with low dark current

Abstract: We present a compact 13 × 4 μm2 Germanium waveguide photodiode, integrated in a CMOS compatible silicon photonics process flow This photodiode has a best-in-class 3 dB cutoff frequency of 45 GHz, responsivity of 08 A/W and dark current of 3 nA The low intrinsic capacitance of this device may enable the elimination of transimpedance amplifiers in future optical data communication receivers, creating ultra low power consumption optical communications

Applicability of Metal/Insulator/Metal (MIM) Diodes to Solar Rectennas

Abstract: The current-voltage (I-V) characteristics of metal/insulator/metal (MIM) diodes illuminated at optical frequencies are modeled using a semiclassical approach that accounts for the photon energy of the radiation. Instead of classical small-signal rectification, in which a continuous span of the dc I-V curve is sampled during rectification, at optical frequencies, the radiation samples the dc I-V curve at discrete voltage steps separated by the photon energy (divided by the electronic charge). As a result, the diode resistance and responsivity differ from their classical values. At optical frequencies, a diode with even a moderate forward-to-reverse current asymmetry exhibits high quantum efficiency. An analysis is carried out to determine the requirements imposed by the operating frequency on the circuit parameters of antenna-coupled diode rectifiers, which are also called rectennas. Diodes with low resistance and capacitance are required for the RC time constant of the rectenna to be smaller than the reciprocal of the operating frequency and to couple energy efficiently from the antenna. Existing MIM diodes do not meet the requirements to operate efficiently at visible-to-near-infrared wavelengths.

Terahertz responsivity of field effect transistors versus their static channel conductivity and loading effects

Abstract: We study the broadband photovoltaic response of field effect transistors on terahertz radiation. A simple physical analytical model of the response is developed. It is based on plasma density perturbation in the transistor channel by the incoming terahertz radiation. The model shows how the non-resonant detection signal is related to static (dc) transistor characteristics. We analyze loading effects related to capacitive, inductive, and resistive coupling of the detector to the read-out circuit as a function of modulation frequencies and loading resistors. As we show, the proposed physical model completed by loading effects fully describes the experimental results on the non-resonant sub-terahertz detection by all studied III-V (GaAs, GaN) and silicon based transistors. Field effect transistors were recently proposed as the best terahertz detecting pixels for fabrication of low cost focal plane arrays for terahertz imaging. This article gives prospects for electrical simulation of these transistors and their optimal integration in the focal plane arrays.

Germanium-tin p-i-n photodetectors integrated on silicon grown by molecular beam epitaxy

Abstract: GeSn heterojunction p-i-n diodes with a Sn content of 0.5% are grown with a special low temperature molecular beam epitaxy. The Sn incorporation in Ge is facilitated by a very low temperature growth step in order to suppress Sn surface segregation. Diodes with sharp doping transitions are realized as double mesa structures with a diameter from 1.5 up to 80 μm. An optical responsivity of these GeSn diodes of 0.1 A/W at a wavelength of λ=1.55 μm is measured. In comparison with a pure Ge detector the optical responsivity is increased by factor of 3 as a result of Sn caused band gap reduction.

Ultra-high-responsivity broadband detection of Si metal-semiconductor-metal Schottky photodetectors improved by ZnO nanorod arrays.

Abstract: This study describes a strategy for developing ultra-high-responsivity broadband Si-based photodetectors (PDs) using ZnO nanorod arrays (NRAs). The ZnO NRAs grown by a low-temperature hydrothermal method with large growth area and high growth rate absorb the photons effectively in the UV region and provide refractive index matching between Si and air for the long-wavelength region, leading to 3 and 2 orders of magnitude increase in the responsivity of Si metal-semiconductor-metal PDs in the UV and visible/NIR regions, respectively. Significantly enhanced performances agree with the theoretical analysis based on the finite-difference time-domain method. These results clearly demonstrate that Si PDs combined with ZnO NRAs hold high potential in next-generation broadband PDs.

A 280-GHz Schottky Diode Detector in 130-nm Digital CMOS

Abstract: A 2×2 array of 280-GHz Schottky-barrier diode detectors with an on-chip patch antenna (255 × 250 μm2) is fabricated in a 130-nm logic CMOS process. The series resistance of diode is minimized using poly-gate separation (PGS), and exhibits a cut-off frequency of 2 THz. Each detector unit can detect an incident carrier with 100-Hz ~ 2-MHz amplitude modulation. At 1-MHz modulation frequency, the estimated voltage responsivity and noise equivalent power (NEP) of the detector unit are 250 V/W and 33 pW/Hz1/2, respectively. An integrated low-noise amplifier further boosts the responsivity to 80 kV/W. At supply voltage of 1.2 V, the entire chip consumes 1.6 mW. The array occupies 1.5 × 0.8 mm2. A set of millimeter-wave images with a signal-noise ratio of 48 dB is formed using the detector. These suggest potential utility of Schottky diode detectors fabricated in CMOS for millimeter wave and sub-millimeter wave imaging.

Plasmonic terahertz detection by a double-grating-gate field-effect transistor structure with an asymmetric unit cell

Abstract: Plasmonic terahertz detection by a double-grating gate field-effect transistor structure with an asymmetric unit cell is studied theoretically. Detection responsivity exceeding 8 kV/W at room temperature in the photovoltaic response mode is predicted for strong asymmetry of the structure unit cell. This value of the responsivity is an order of magnitude greater than reported previously for the other types of uncooled plasmonic terahertz detectors. Such enormous responsivity can be obtained without using any supplementary antenna elements because the double-grating gate acts as an aerial matched antenna that effectively couples the incoming terahertz radiation to plasma oscillations in the structure channel.

Strained germanium thin film membrane on silicon substrate for optoelectronics.

Abstract: This work presents a novel method to introduce a sustainable biaxial tensile strain larger than 1% in a thin Ge membrane using a stressor layer integrated on a Si substrate. Raman spectroscopy confirms 1.13% strain and photoluminescence shows a direct band gap reduction of 100meV with enhanced light emission efficiency. Simulation results predict that a combination of 1.1% strain and heavy n(+) doping reduces the required injected carrier density for population inversion by over a factor of 60. We also present the first highly strained Ge photodetector, showing an excellent responsivity well beyond 1.6um.

Single-Photon Detectors in the Terahertz Range

Abstract: Semiconductor quantum dot detectors as well as semiconductor charge-sensitive infrared phototransistors are described. They are the only detectors that can count single photons in the terahertz region at present. In terms of the noise equivalent power (NEP), the detectors realize experimental values on the order of 10-21 W/Hz1/2, while theoretically expected values are even much lower, on the order of 10-24 W/Hz1/2. These NEP values are by several orders of magnitude lower than any other state-of-the-art highly sensitive detectors. In addition to the outstanding sensitivity, the detectors are featured by strong advantage of huge current responsivity (106-1010 A/W ) and extremely large dynamic range of response (106-108). The mechanism of detection as well as application of the detectors is discussed.

Mg0.55Zn0.45O solar-blind ultraviolet detector with high photoresponse performance and large internal gain

Abstract: A Schottky type metal-semiconductor-metal solar-blind ultraviolet detector was fabricated on high quality wurtzite Mg0.55Zn0.45O epitaxial film. Photoresponse spectra show a responsivity peak of 22 mA/W under 130 V bias. A sharp cutoff was recognized at a wavelength of 270 nm, and a temporal response measurement indicates a fast decay time of less than 500 ns. A large internal gain was observed and interpreted by a reduced Schottky barrier height model, which fits well with the experimental data.

Plasmonic terahertz detection by a double-grating-gate field-effect transistor structure with an asymmetric unit cell

Abstract: Plasmonic terahertz detection by a double-grating gate field-effect transistor structure with an asymmetric unit cell is studied theoretically. Detection responsivity exceeding 8 kV/W at room temperature in the photovoltaic response mode is predicted for strong asymmetry of the structure unit cell. This value of the responsivity is an order of magnitude greater than reported previously for the other types of uncooled plasmonic terahertz detectors. Such enormous responsivity can be obtained without using any supplementary antenna elements because the double-grating gate acts as an aerial matched antenna that effectively couples the incoming terahertz radiation to plasma oscillations in the structure channel.

36 GHz submicron silicon waveguide germanium photodetector

Abstract: We present two effective approaches to improve the responsivity of high speed waveguide-based Ge photodetectors integrated on a 0.25μm silicon-on-insulator (SOI) platform. The main cause of poor responsivity is identified as metal absorption from the top contact to Ge. By optimizing Ge thickness and offsetting the contact window, we have demonstrated that the responsivity can be improved from 0.6A/W to 0.95A/W at 1550nm with 36GHz 3dB bandwidth. We also demonstrate that a wider device with double offset contacts can achieve 1.05A/W responsivity at 1550nm and 20GHz 3dB bandwidth.

High-power high-linearity flip-chip bonded modified uni-traveling carrier photodiode

Abstract: We demonstrate a flip-chip bonded modified uni-traveling carrier (MUTC) photodiode with an RF output power of 0.75 W (28.8 dBm) at 15 GHz and OIP3 as high as 59 dBm. The photodiode has a responsivity of 0.7 A/W, 3-dB bandwidth > 15 GHz, and saturation photocurrent > 180 mA at 11 V reverse bias.

Surface plasmon-enhanced nanopillar photodetectors.

Abstract: We demonstrate nanopillar- (NP) based plasmon-enhanced photodetectors (NP-PEPDs) operating in the near-infrared spectral regime. A novel fabrication technique produces subwavelength elongated nanoholes in a metal surface self-aligned to patterned NP arrays that acts as a 2D plasmonic crystal. Surface plasmon Polariton Bloch waves (SPP-BWs) are excited by the metal nanohole array resulting in electric field intensity “hot spots” in the NP. The NP periodicity determines the peak responsivity wavelength while the nanohole asymmetry produces polarization-dependent coupling of the SPP-BW modes. Resulting photodetectors have 0.28 A/W responsivity peaked at 1100 nm at a reverse bias of −5 V. Designs for further increasing the optical coupling efficiency into the nanopillar are explored. This technology has potential applications for plasmonically enhanced focal plane arrays and plasmonic photovoltaics.

Solution processed reduced graphene oxide ultraviolet detector

Abstract: Electronic properties of graphene have been studied more extensively than its photonic applications, in spite of its exciting optical properties. Recent results on solar cells, light emitting diodes and photodetectors show its true potential in photonics and optoelectronics. Here, we have explored the use of reduced graphene oxide as a candidate for solution processed ultraviolet photodetectors. UV detection is demonstrated by reduced graphene oxide in terms of time resolved photocurrent as well as photoresponse. The responsivity of the detectors is found to be 0.12 A/W with an external quantum efficiency of 40%. (C) 2011 American Institute of Physics. [doi:10.1063/1.3640222]

Highly selective spectral response with enhanced responsivity of n-ZnO/p-Si radial heterojunction nanowire photodiodes

Abstract: A radial heterojunction nanowire diode (RND) array consisting of a ZnO (shell)/Si (core) structure was fabricated using conformal coating of a n-type ZnO layer that surrounded a p-type Si nanowire. In both ultraviolet (UV) and visible ranges, the photoresponsivity of the RND was larger than that of a planar thin film diode (PD) owing to the efficient carrier collection with improved light absorption. Compared to a PD, in the forward bias, a 6 μm long RND resulted in a ∼2.7 times enhancement of the UV responsivity at λ=365 nm, which could be explained based on the oxygen-related hole-trap mechanism. Under a reverse bias, UV-blind visible detection was observed while the UV response was suppressed.

A W-band CMOS Receiver Chipset for Millimeter-Wave Radiometer Systems

Abstract: This paper presents a W-band receiver chipset for passive millimeter-wave imaging in a 65 nm standard CMOS technology. The system comprises a direct-conversion receiver front-end with injection-locked tripler and a companion analog back-end for Dicke radiometer. The receiver design addresses the high 1/f noise issue in the advanced CMOS technology. An LO generation scheme using a frequency tripler is proposed to lower the PLL frequency, making it suitable for use in multi-pixel systems. In addition, the noise performance of the receiver is further improved by optimum biasing of transistors of the detector in moderate inversion region to achieve the highest responsivity and lowest NEP. The front-end chipset exhibits a measured peak gain of 35 dB, -3 dB BW of 12 GHz, NF of 8.9 dB, while consuming 94 mW. The baseband chipset has a measured peak responsivity (Rv) of 6 KV/W and a noise equivalent power (NEP) of 8.54 pW/Hz1/2. The two chipsets integrated on-board achieve a total responsivity of 16 MV/W and a calculated Dicke NETD of 1K with a 30 ms integration time.

High gain ultraviolet photodetectors based on AlGaN/GaN heterostructures for optical switching

Abstract: We report on the optoelectronic properties of Al0.25Ga0.75N/GaN-based ultraviolet (UV) photodetectors for the application as a high current, high gain optical switch. Due to an internal gain mechanism combined with the high conductivity of the two-dimensional electron gas at the heterostructure interface, photocurrents in the milliampere-range were obtained with UV illumination. By employing a mesa structure design with meander geometry very low dark currents below 50 nA up to a bias voltage of 100 V were achieved. Optical switching with an on/off-current-ratio of five orders of magnitude was demonstrated. The response time was determined to be 6 ms and persistent photoconductivity was observed. The photodetector is visible-blind with a cut-off wavelength of 365 nm according to the band gap energy of the GaN absorption layer. A high responsivity with a maximum of 70 A/mW at 312 nm and 100 V bias voltage was demonstrated.

Effect of asymmetric Schottky barrier on GaN-based metal-semiconductor-metal ultraviolet detector

Abstract: An asymmetric Schottky barrier metal-semiconductor-metal (MSM) ultraviolet (UV) detector with Ni/GaN/Au structure was designed and the effect of the asymmetric Schottky barrier on the detector response was investigated. This detector had response at 0 V bias and increased responsivity when a positive bias was applied to the Ni/GaN contact; however, the internal gain disappeared when a negative bias was applied to this point. This contrasts with a symmetric Ni/GaN/Ni Schottky barrier MSM UV detector which had no internal gain under positive/negative bias and almost no response at 0 V bias. The improved performance of the asymmetric Schottky barrier detector was because of the lower work function of Au causing reduction of Schottky barrier and hence enhancing a hole-accumulating and trapping process, which resulted in internal gain.

Highly sensitive fast-response UV photodetectors based on epitaxial TiO2 films

Abstract: Epitaxial TiO2 thin films were fabricated on LaAlO3 single crystal substrates by RF magnetron sputtering. Ag electrodes were then evaporated on the TiO2 thin films to form metal–semiconductor–metal photoconductive detectors. The TiO2 photodetector exhibited a maximum photoresponse of 3.63 A W−1 at 310 nm with a sharp cutoff wavelength at 380 nm. The ultraviolet (UV)–visible response rejection ratio (R310 nm/R390 nm) was about three orders of magnitude. The photocurrent response of the detectors scaled linearly with the applied bias and the incident light intensity. The dark current was only 0.14 nA at 10 V bias. A transient photovoltage with a rise time of ~8 ns and a full-width at half-maximum of ~90 ns was observed when the photodetector was under the irradiation of a 308 nm XeCl laser with 25 ns duration. The excellent performances of high responsivity and ultrahigh response speed suggest that the presented TiO2 detectors have promising potential in UV photodetection.

Influence of threading dislocations on GaN-based metal-semiconductor-metal ultraviolet photodetectors

Abstract: The influence of threading dislocations on the properties of GaN-based metal-semiconductor-metal (MSM) ultraviolet photodetectors was investigated. It was found that screw dislocations had a strong influence on the dark current of the photodetectors, while edge dislocations had the predominant effect on their responsivity. The dark current increased as the screw dislocation density increased due to their lowering of the Schottky barrier height. However, the responsivity of the photodetectors decreased with increasing edge dislocation density because of the dangling bonds along those edge dislocation lines which enhance the recombination of photogenerated electron-hole pairs. The results suggest that reducing both the screw and edge dislocation densities is an effective way to improve the photoelectric property of GaN-based MSM ultraviolet photodetectors.

A 10-Gb/s OEIC with Meshed Spatially-Modulated Photo Detector in 0.18- $\mu{\hbox {m}}$ CMOS Technology

Abstract: This paper describes the design of a 10-Gb/s fully integrated CMOS optical receiver, which consists of a novel spatially-modulated photo detector (SMPD), a low-noise trans-impedance amplifier (TIA), and a post-limiting amplifier on a single chip. The bandwidth of proposed meshed SMPD can be boosted up to 6.9 GHz under a reverse-biased voltage of 14.2 V. The measured responsivity of the meshed SMPD is 29 mA/W as illuminated by 850-nm light source. To compensate the relatively low responsivity of on-chip CMOS photo detector (PD), a high-gain TIA with nested feedback and shunt peaking is proposed to achieve low-noise operation. The optical receiver is capable of delivering 25-kΩ conversion gain when driving 50-Ω output loads. For a PRBS test pattern of 27- 1, the 10-Gb/s optoelectronic integrated circuit (OEIC) has optical sensitivity of - 6 dBm at a bit-error rate (BER) of 10 -11. Implemented in a generic 0.18-μm CMOS technology, the chip area is 0.95 mm by 0.8 mm. The trans-impedance amplifier, post amplifier, and output buffer respectively drain 38 mW, 80 mW, and 27 mW from the 1.8-V supply.

SWIR/MWIR InP-Based p-i-n Photodiodes With InGaAs/GaAsSb Type-II Quantum Wells

Abstract: This paper presents the performance characteristics of InP-based p-i-n photodiodes with strain-compensated and lattice-matched InGaAs/GaAsSb type-II multiple quantum well (MQW) absorption regions. The results show that photodiodes with strain-compensated and lattice-matched absorption regions have optical response out to 3.4 and 2.8 μm with dark current densities of 9.7 and 1.66 mA cm-2,respectively, at 290 K under -0.5 V reverse bias. The carrier transport mechanism responsible for the difference in responsivity and detectivity between strain-compensated and lattice-matched InGaAs/GaAsSb MQWs is discussed.

Performance and performance variations of sub-1 THz detectors fabricated with 0.15 μm CMOS foundry process

Abstract: Antenna-coupled field-effect transistors were integrated as multi-pixel (5×10) detector arrays for electromagnetic radiation between 550 and 600×GHz using commercial 0.15××m CMOS process technology. Reported is a minimum optical noise-equivalent-power (NEP) of 43 pW/ √Hz and a maximum (capacitive-loading-limited) optical responsivity of 970 V / W (both values averaged). An electrical NEP of 9 pW / √Hz is estimated. Inter-chip variations are analysed with a set of 15 samples showing a low standard deviation of less than 8× for both responsivity and NEP at the optimum operation point. Intra-chip variation is low for non-edge pixels. Both the very good NEP values and the low variations indicate that a cost-efficient CMOS process is well suitable for reliable fabrication of multi-pixel terahertz focal plane arrays.

Waveguide-integrated telecom-wavelength photodiode in deposited silicon.

Abstract: We demonstrate photodiodes in deposited polycrystalline silicon at 1550nm wavelength with 0.15A/W responsivity, 40nA dark current, and gigahertz time response. Subband absorption is mediated by defects that are naturally present in the polycrystalline material structure. The material exhibits a moderate absorption coefficient of 6dB/cm, which allows the same microring resonator device to act as both a demultiplexing filter and a photodetector. We discuss the use of deposited silicon-based complementary metal-oxide semiconductor materials for nanophotonic interconnects.