Reverse Bias Dark Current Research Articles

All-Silicon Photodetectors for Photonic Integrated Circuit Calibration

All-silicon highly-doped PN junction-based photodetectors, for photonic integrated circuit (PIC) calibration and power monitoring, are designed and fabricated in the C-band. The photodetector response is measured for different doping conditions. The photodetectors are integrated with an interferometric based phase-interrogator structure for a test calibration circuit. The measured devices show high responsivity (12 A/W) obtained under avalanche condition at 5.7 V reverse bias and reasonable dark current ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1~\mu \text{A}$ </tex-math></inline-formula> ) due to photon assisted tunneling effect and are therefore, proved to be an ideal candidate for power monitoring and phase calibration of PICs.

Three-dimensional radial-junction ZnO nanowire/a-Si:H core–shell infrared photodiodes

Hydrogenated amorphous silicon (a-Si:H)-based infrared photodiodes were fabricated by coating a-Si:H thin-film p-i-n layers over hydrothermally-synthesized disordered zinc oxide (ZnO) nanowire (NW) networks. Due to enhanced light scattering, the reversed biased three dimensional (3-D) radial-junction NW diodes showed an ∼10× increase in photocurrent under a broad spectrum (800–2000 nm) infrared (IR) illumination compared to planar devices. The diodes were optimized by using InGaZnO (IGZO) transparent top contacts that had 20% higher optical transmission in the IR compared to Al-doped ZnO electrodes. Reverse-bias dark current was minimized by optimizing the area of the NW sidewalls and the a-Si:H shell layer thickness. The former reduces the effects of carrier recombination along the NW core–shell interface and the latter minimizes the tunnelling current across the radial-junction device. An enhancement of ∼100× was achieved for these devices compared to non-optimized diodes.

On the Origin of Dark Current in Organic Photodiodes

AbstractMinimizing the reverse bias dark current while retaining external quantum efficiency is crucial if the light detection sensitivity of organic photodiodes (OPDs) is to compete with inorganic photodetectors. However, a quantitative relationship between the magnitude of the dark current density under reverse bias ( Jd) and the properties of the bulk heterojunction (BHJ) active layer has so far not been established. Here, a systematic analysis of Jd in state‐of‐the‐art BHJ OPDs using five polymers with a range of energy levels and charge transport characteristics is presented. The magnitude and activation energy of Jd are explained using a model that assumes charge injection from the metal contacts into an energetically disordered semiconductor. By relating Jd to material parameters, insights into the origin of Jd are obtained that enable the future selection of successful OPD materials.

Giant UV Photoresponse of GaN-Based Photodetectors by Surface Modification Using Phenol-Functionalized Porphyrin Organic Molecules.

Organic molecular monolayers (MoLs) have been used for improving the performance of various electronic device structures. In this work, the concept of organic molecular surface modification is applied for improving the performance of GaN-based metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors (PDs). Organic molecules of phenol-functionalized metallated porphyrin (hydroxyl-phenyl-zinc-tetra-phenyl-porphyrin (Zn-TPPOH)) were adsorbed on GaN, and Ni/Zn-TPPOH/GaN/Zn-TPPOH/Ni PD structures were fabricated. This process was beneficial in two ways: first, the reverse-bias dark current was reduced by 1000 times, and second, the photocurrent was enhanced by ∼100 times, in comparison to the dark and photocurrent values obtained for Ni/GaN/Ni MSM PDs, at high voltages of ±10 V. The responsivity of the devices was increased from 0.22 to 4.14 kA/W at 5 μW/cm2 optical power density at -10 V bias and at other voltages also. In addition to this, other PD parameters such as photo-to-dark current ratio and UV-to-visible rejection ratio were also enhanced. The spectral selectivity of the PDs was improved, which means that the molecularly modified devices became more responsive to UV spectral region and less responsive to visible spectral region, in comparison to bare GaN-based devices. Photoluminescence measurements, power-dependent photocurrent characteristics, and time-resolved photocurrent measurements revealed that the MoL was passivating the defect-related states on GaN. In addition, Kelvin probe force microscopy showed that the MoL was also playing with the surface charge (due to surface states) on GaN, leading to increased Schottky barrier height in dark conditions. Resultant to both these phenomena, the reverse-bias dark current was reduced for metal/MoL/GaN/MoL/metal PD structures. Further, the unusual photoconductive gain in the molecularly modified devices has been attributed to Schottky barrier lowering for UV-illuminated conditions, leading to enhanced photocurrent.

Direct gap Ge1-ySny alloys: Fabrication and design of mid-IR photodiodes

Chemical vapor deposition methods were developed, using stoichiometric reactions of specialty Ge3H8 and SnD4 hydrides, to fabricate Ge1-ySny photodiodes with very high Sn concentrations in the 12%–16% range. A unique aspect of this approach is the compatible reactivity of the compounds at ultra-low temperatures, allowing efficient control and systematic tuning of the alloy composition beyond the direct gap threshold. This crucial property allows the formation of thick supersaturated layers with device-quality material properties. Diodes with composition up to 14% Sn were initially produced on Ge-buffered Si(100) featuring previously optimized n-Ge/i-Ge1-ySny/p-Ge1-zSnz type structures with a single defected interface. The devices exhibited sizable electroluminescence and good rectifying behavior as evidenced by the low dark currents in the I-V measurements. The formation of working diodes with higher Sn content up to 16% Sn was implemented by using more advanced n-Ge1-xSnx/i-Ge1-ySny/p-Ge1-zSnz architectures incorporating Ge1-xSnx intermediate layers (x ∼ 12% Sn) that served to mitigate the lattice mismatch with the Ge platform. This yielded fully coherent diode interfaces devoid of strain relaxation defects. The electrical measurements in this case revealed a sharp increase in reverse-bias dark currents by almost two orders of magnitude, in spite of the comparable crystallinity of the active layers. This observation is attributed to the enhancement of band-to-band tunneling when all the diode layers consist of direct gap materials and thus has implications for the design of light emitting diodes and lasers operating at desirable mid-IR wavelengths. Possible ways to engineer these diode characteristics and improve carrier confinement involve the incorporation of new barrier materials, in particular, ternary Ge1-x-ySixSny alloys. The possibility of achieving type-I structures using binary and ternary alloy combinations is discussed in detail, taking into account the latest experimental and theoretical work on band offsets involving such materials.

All-plastic solar cells with a high photovoltaic dynamic range

We report on semitransparent air-processed all-plastic solar cells, fabricated from vacuum-free processes, comprising two polymer electrodes, a polymeric work-function modification layer and a polymer:fullerene photoactive layer. The active layer and the top PEDOT:PSS electrode were prepared by sequential film-transfer lamination on polyethylenimine-modified PEDOT:PSS bottom electrodes. The transferring of films offers ease of layer patterning and the misalignment of defects in the different layers resulting from the additive film transfer lamination process yields high shunt resistance values of 108 ohm cm2. Consequently, all-plastic solar cells fabricated with this process exhibit very low reverse bias dark current and can operate in the photovoltaic quadrant with light irradiance varying over five orders of magnitude. The analysis of the values of the open-circuit voltage as a function of light irradiance over that wide dynamic range points toward an ideality factor of n = 1.82 and a reverse saturation current density of 6.2 × 10−11 A cm−2 for solar cells with an active layer comprised of a blend of poly(3-hexylthiophene) and an indene fullerene bis-adduct.

Photovoltaic retinal prosthesis: implant fabrication and performance

The objective of this work is to develop and test a photovoltaic retinal prosthesis for restoring sight to patients blinded by degenerative retinal diseases. A silicon photodiode array for subretinal stimulation has been fabricated by a silicon-integrated-circuit/MEMS process. Each pixel in the two-dimensional array contains three series-connected photodiodes, which photovoltaically convert pulsed near-infrared light into bi-phasic current to stimulate nearby retinal neurons without wired power connections. The device thickness is chosen to be 30 µm to absorb a significant portion of light while still being thin enough for subretinal implantation. Active and return electrodes confine current near each pixel and are sputter coated with iridium oxide to enhance charge injection levels and provide a stable neural interface. Pixels are separated by 5 µm wide trenches to electrically isolate them and to allow nutrient diffusion through the device. Three sizes of pixels (280, 140 and 70 µm) with active electrodes of 80, 40 and 20 µm diameter were fabricated. The turn-on voltages of the one-diode, two-series-connected diode and three-series-connected diode structures are approximately 0.6, 1.2 and 1.8 V, respectively. The measured photo-responsivity per diode at 880 nm wavelength is ∼0.36 A W−1, at zero voltage bias and scales with the exposed silicon area. For all three pixel sizes, the reverse-bias dark current is sufficiently low (<100 pA) for our application. Pixels of all three sizes reliably elicit retinal responses at safe near-infrared light irradiances, with good acceptance of the photodiode array in the subretinal space. The fabricated device delivers efficient retinal stimulation at safe near-infrared light irradiances without any wired power connections, which greatly simplifies the implantation procedure. Presence of the return electrodes in each pixel helps to localize the current, and thereby improves resolution.

Dark current mechanisms in quantum dot laser structures

Current-voltage measurements have been performed on InAs/InGaAs/GaAs quantum dot structures with varying growth and design parameters. These measurements show that the forward and reverse bias dark currents decrease with increasing spacer growth temperature, however, they are relatively insensitive to the number of periods of the quantum dot layers. Temperature dependent current-voltage measurements show that the mechanism that governs the reverse bias leakage current is due to generation-recombination via mid-band traps assisted by the Frenkel-Poole emission of carriers from these traps.

GaN avalanche photodiodes grown on m-plane freestanding GaN substrate

M-plane GaN avalanche p-i-n photodiodes on low dislocation density freestanding m-plane GaN substrates were realized using metal-organic chemical vapor deposition. High quality homoepitaxial m-plane GaN layers were developed; the root-mean-square surface roughness was less than 1 Å and the full-width-at-half-maximum value of the x-ray rocking curve for (101¯0) diffraction of m-plane GaN epilayer was 32 arcsec. High quality material led to a low reverse-bias dark current of 8.11 pA for 225 μm2 mesa photodetectors prior to avalanche breakdown, with the maximum multiplication gain reaching about 8000.

Investigation of 1/f Noise Mechanisms in Midwave Infrared HgCdTe Gated Photodiodes

In this work, gated midwave infrared (MWIR) Hg1–xCdx Te photodiodes are used to investigate the physical origin of 1/f noise generation. Gated photodiodes were fabricated on liquid-phase epitaxy p-type HgCdTe MWIR material with a vacancy-doped concentration of 1.6 × 1016 cm−3 and x = 0.31. CdTe was thermally deposited and used as both a passivant and a mask for the plasma-based type conversion, and ZnS was used as an insulator. Fabricated devices show a R0A of 1–5 × 104 Ωcm2 with zero gate bias. Application of 2 V to the gate improves the R0A by more than two orders of magnitude to 6.0 × 106 Ωcm2, which corresponds to the p-type surface being at transition between depletion and weak inversion. Trap-assisted tunneling (TAT) current was observed at negative gate biases and reverse junction biases. For gate biases greater than 3 V, a field-induced junction breakdown was observed. An In = αIβf −0.5 trend was observed above 200 pA reverse bias dark current, with α = 3.5 × 10−5 and β = 0.82, which corresponds to the TAT dominated region. Below 200 pA, junction generation-recombination (GR) current starts to dominate and this previously mentioned trend is no longer observed. Junction GR current was not seen to be correlated with 1/f noise in these photodiodes.

GaN ultraviolet avalanche photodiodes with optical gain greater than 1000 grown on GaN substrates by metal-organic chemical vapor deposition

We report the performance of GaN p-i-n ultraviolet avalanche photodiodes grown on bulk GaN substrates by metal-organic chemical vapor deposition. The low dislocation density in the devices enables low reverse-bias dark currents prior to avalanche breakdown for ∼30μm diameter mesa photodetectors. The photoresponse is relatively independent of the bias voltage prior to the onset of avalanche gain which occurs at an electric field of ∼2.8MV∕cm. The magnitude of the reverse-bias breakdown voltage shows a positive temperature coefficient of ∼0.05V∕K, confirming that the avalanche breakdown mechanism dominates. With ultraviolet illumination at λ∼360nm, devices with mesa diameters of ∼50μm achieve stable maximum optical gains greater than 1000. To the best of our knowledge, this is the highest optical gain achieved for GaN-based avalanche photodiodes and the largest area III-N avalance photodetectors yet reported.

Fabrication and characterization of two-color midwavelength/long wavelength HgCdTe infrared detectors

High-performance 20-µm unit-cell two-color detectors using an n-p+-n HgCdTe triple-layer heterojunction (TLHJ) device architecture grown by molecular beam epitaxy (MBE) on (211)-oriented CdZnTe substrates with midwavelength (MW) infrared and long wavelength (LW) infrared spectral bands have been demonstrated. Detectors with nominal MW and LW cut-off wavelengths of 5.5 µm and 10.5 µm, respectively, exhibit 78 K LW performance with >70 % quantum efficiency, reverse bias dark currents below 300 pA, and RA products (zero field of view, 150-mV bias) in excess of 1×103 Ωcm2. Temperature-dependent current-voltage (I–V) detector measurements show diffusion-limited LW dark current performance extending to temperatures below 70 K with good operating bias stability (150 mV ± 50 mV). These results reflect the successful implementation of MBE-grown TLHJ detector designs and the introduction of advanced photolithography techniques with inductively coupled plasma (ICP) etching to achieve high aspect ratio mesa delineation of individual detector elements with benefits to detector performance. These detector improvements complement the development of high operability large format 640×480 and 1280×720 two-color HgCdTe infrared focal plane arrays (FPAs) to support third generation forward looking infrared (FLIR) systems.

Thermal cycling-induced changes in excess dark current in very long-wavelength HgCdTe photodiodes at low temperature

We have observed cooldown-to-cooldown changes in the reverse-bias dark current of some very long-wavelength (cutoff >14 µm) HgCdTe P-on-n heterojunction photodiodes operated at very low temperatures (40–45 K). Other photodiodes in the same arrays are stable between cooldowns. The unstable ones have high dark currents, in the tail of the dark current distribution. Current-voltage analysis indicates that the high dark current is caused by trap-assisted tunneling and that the number of traps changes from cooldown to cooldown. Devices with negligible trap-assisted tunneling current, limited by diffusion and band-to-band tunneling currents at reverse bias, are stable between cooldowns. Both types of devices are stable within a given cooldown over periods of at least 24 h.

Modeling of reverse bias dark currents in pin structures using amorphous and polymorphous silicon

The reverse bias dark currents in pin solar cells having standard amorphous silicon (a-Si:H) or polymorphous silicon (pm-Si:H) as the intrinsic layer have been studied using experiments and modeling. Our electrical–optical model, takes into account high field enhancement of thermal generation via the Poole–Frenkel effect. Polymorphous silicon films have a higher hole mobility and a lower bulk density of states (DOS) than standard a-Si:H. However this does not result in lower dark leakage currents, except in thick (∼2.5 μm) pin diodes. For the latter, we have measured dark leakage currents as low as ∼3 pA cm −2 at −3 V, which to our knowledge is the lowest reported for such devices. Modeling reveals that the dark leakage current is controlled by thermal generation through the mid-gap defects in the intrinsic layer for a-Si:H cells, while it is dominated by the p/i interface defects in pm-Si:H cells. We conclude by estimating from the model, the dark leakage current possible in pm-Si:H diodes, if their p/i interface DOS were to be as in standard a-Si:H ones.

Experimental study and modeling of reverse-bias dark currents in PIN structures using amorphous and polymorphous silicon

Polymorphous silicon (pm-Si:H) is a nanostructured silicon thin film, with a lower defect density of states and better electronic properties than standard amorphous silicon. We have studied the reverse-bias dark current in PIN structures using this material as the intrinsic layer and compared the results to amorphous silicon PIN devices. All the structures were grown using a standard plasma enhanced chemical vapor deposition process. For thick pm-Si:H devices, we have achieved reverse-bias dark current densities about ten times lower than those obtained using amorphous silicon as the intrinsic layer. This is consistent with the lower defect density of states in polymorphous silicon, which is about 7×1014 cm−3 against 5×1015 cm−3 for amorphous silicon. For a 2.5-μm-thick pm-Si:H diode, the current density obtained is as low as 3 pA cm−2 at −3 V. However, for thinner structures (∼0.5 μm), polymorphous and amorphous silicon show nearly the same reverse-bias leakage current. The experimental dark as well as illuminated characteristics of the diodes have been simulated using a model that incorporates field enhanced thermal generation under reverse-bias conditions (Poole–Frenkel effect). Results reveal that in pm-Si:H diodes, the P/I interface is much more defective than in standard a-Si:H PIN diodes. This fact is shown to completely mask the advantage of the lower defect density of pm-Si:H, in thin PIN diodes. However, in thick samples the electric field in the device and, therefore, also the Poole–Frenkel enhancement of thermal generation are smaller. The effect of the lower density of states in polymorphous silicon is then dominant, and we have achieved a dark current density of 3 pA cm−2 at −3 V for a 2.5-μm-thick diode, as already stated.

Passivation of type II InAs/GaSb superlattice photodiodes

Recently, excellent infrared detectors have been demonstrated using Type II InAs/GaSb superlattice materials sensitive at wavelengths from 3 μm to greater than 32 μm. These results indicate that Type II superlattice devices may challenge the preponderance of HgCdTe and other state-of-the-art infrared material systems. As such, surface passivation is becoming an increasingly important issue as progress is made towards the commercialization of Type II devices and focal plane array applications. This work focuses on initial attempts at surface passivation of Type II InAs/GaSb superlattice photodiodes using PECVD-grown thin layers of SiO 2. Our results indicate that silicon dioxide coatings deposited at various temperatures improve photodetector resistivity by several times. Furthermore, reverse-bias dark current has been reduced significantly in passivated devices.

Optical gain at low bias voltages in electrostatic-discharge-damaged silicon p-i-n photodiodes

Abstract The sensitivity of commercial p-i-n silicon photodiodes to electrostatic discharges was determined by monitoring continuously electronic and optoelectronic device properties during human body model step-stress tests. The device degradation threshold pulse amplitudes ranging from 175 to 300 V for reverse-bias stress and from 10000 to 13 500V for forward-bias stress were found, evidenced in all cases by an increase in the reverse-bias dark current. In the case of a forward-bias-stressed diode, however, we observed a substantial increase in the responsivity of the photodiode at low reverse-bias voltages after electrostatic-discharge-related degradation. The photocurrent gain, which increased with decreasing light intensity and increasing bias voltage, reached a saturation value exceeding 500 for low optical powers and a reverse-bias voltage of 10 V. Additionally it was observed than the open-circuit voltage of the photodiode at a given light power is a more sensitive parameter for device degradation than the reverse-bias dark current and the short-circuit current.

Optical gain at low bias voltages in electrostatic-discharge-damaged silicon p-i-n photodiodes

Abstract The sensitivity of commercial p-i-n silicon photodiodes to electrostatic discharges was determined by monitoring continuously electronic and optoelectronic device properties during human body model step-stress tests. The device degradation threshold pulse amplitudes ranging from 175 to 300 V for reverse-bias stress and from 10000 to 13 500V for forward-bias stress were found, evidenced in all cases by an increase in the reverse-bias dark current. In the case of a forward-bias-stressed diode, however, we observed a substantial increase in the responsivity of the photodiode at low reverse-bias voltages after electrostatic-discharge-related degradation. The photocurrent gain, which increased with decreasing light intensity and increasing bias voltage, reached a saturation value exceeding 500 for low optical powers and a reverse-bias voltage of 10 V. Additionally it was observed than the open-circuit voltage of the photodiode at a given light power is a more sensitive parameter for device degradation...

Effect of GaSb growth temperature on p-GaSb/n-GaAs diodes grown by MOVPE

Despite the 7% lattice mismatch, the reverse bias dark current of p-GaSb/n-GaAs junctions has been found to be unexpectedly low. MOVPE-grown diodes exhibited dark currents up to two orders of magnitude lower than comparable GaSb homojunctions. The initial nucleation of GaSb on GaAs was found to play an important role in determining the properties of the bulk layer. A larger number of small nucleation sites produced diodes with lower reverse bias leakage currents and growth temperature was in turn critical in determining the size of these nucleation islands. The authors present a study correlating growth temperature with nucleation site density, dark current and open circuit voltage under illumination. Diodes were grown at temperatures ranging from 490-550/spl deg/C using TMGa and TMSb. A reduction in reverse bias dark current of several orders of magnitude was observed across this narrow temperature range. The 'turn-on' voltage also increased with reduction in growth temperature and there was a corresponding increase in the average open voltage, V/sub oc/, under illumination. However, V/sub oc/ did exceed /spl sim/0.25 V. This is low compared to GaSb homojunctions (typically 0.45 V) and it may be limited by a low shunt resistance under illumination as well as low efficiency from the thin GaSb layers. Other alkyls [TIPGa and t-DMASb], which allow a further reduction in growth temperature, are being investigated.

Wide Band Gap a-Si:H Based Ihgh Gain Vidicon Devices Prepared by Chemical Anniealing

ABSTRACTPreviously it was shown that high quality wide gap hydrogenated amorphous silicon material could be prepared by a layer-by-layer technique involving hydrogen chemical annealing. Using this wide gap material, high electric field, n-i-p diode devices were fabricated. Reverse bias dark current was suppressed by optimization of the n-layer doping level (250ppm) and the thickness (2000Å). Working vidicon type device were prepared, tested, and optimized by further reduction in the high reverse bias leakage current. Vidicon devices showed very promising performance; however, at the present stage of development some point defects were observable at the highest reverse bias voltages probed (∼-6×105 V/cm).