Photocurrent Level Research Articles

Characterization of High-Linearity Modified Uni-Traveling Carrier Photodiodes Using Three-Tone and Bias Modulation Techniques

The linearity performance of a back-illuminated InGaAs/InP modified uni-traveling carrier photodiode with a highly doped p-type absorber is characterized using three-tone and bias modulation measurement techniques. The bias modulation measurement is used to determine the voltage-dependent relative responsivity, R(V), for a range of photocurrent levels. The results confirm that the photocurrent dependence of the third-order output intercept point (OIP3) as determined by the three-tone method is directly related to the dependence of R(V) on photocurrent. The measured OIP3 show a weak dependence on frequency and a high value of 47.5 dBm at 20 GHz is achieved.

High-Power High-Responsivity Modified Uni-Traveling-Carrier Photodiode Used as V-Band Optoelectronic Mixers

A modified uni-traveling-carrier photodiode-based V-band optoelectronic mixer is demonstrated. The relative optical local oscillator (LO) and optical intermediate frequency (IF) signal levels and the reverse bias can be optimized for maximum up-converted power. It was found that the optimum optical IF percentage relative to the total incident optical power was approximately 1/3 for all photocurrent levels while the optimum reverse bias increased linearly with photocurrent. A simple analytical model that explains these observations is developed. A high up-converted power of -4.7 dBm was achieved at 60 GHz and photocurrent of 70 mA. The maximum up-converted power is primarily limited by the saturation characteristic of the photodiode.

A Method to Estimate the Junction Temperature of Photodetectors Operating at High Photocurrent

A method to estimate the junction temperature while operating at high photocurrent levels is presented. The relative responsivity change is measured at high operating current. Using a model for the temperature-dependence of the bandgap, a relation between the relative change in the output power and the internal junction temperature is derived. Good agreement between experimental data and simulations is achieved.

Photocurrent spectroscopy investigation of deep level defects in Mg-doped GaN and Mg-doped AlxGa1−xN (0.20<x<0.52)

Pulsed infrared photocurrent spectroscopy is used to investigate deep levels in highly resistive metal organic chemical vapor deposition-grown, magnesium-doped aluminum gallium nitride metal-semiconductor-metal test structures in the range of aluminum fraction from x=0.0 to x=0.52. Some background level of photocurrent is observed at all infrared pump wavelengths between 1.35 and 4.0 μm. The photocurrent decay time is a decreasing function of aluminum fraction. A peak photocurrent energy is observed for each aluminum fraction. With increasing aluminum fraction, the peak blueshifts and narrows.

Output Saturation and Linearity of Waveguide Unitraveling-Carrier Photodiodes

Waveguide unitraveling-carrier photodiodes (UTC-PDs) with different absorber and collector layer doping levels have been fabricated and characterized. These photodiode (PD) structures are fabricated on a platform that allows for the monolithic integration of multiquantum-well optical phase modulators and couplers for realizing novel coherent receivers. Compared to PD A, PD B has a lower and more graded p-doping profile in the absorber layer and also a higher n-doping level in the collector layer. For PD B a larger field is induced in the absorber layer at high photocurrent levels. Also the higher n-doping in the collector layer is adequate for providing charge compensation. For PD B, there is an enhancement in the RF response as the photocurrent level is increased. At a frequency of 1 GHz, the saturation current for PD A is around 65 mA and that for PD B is around 63 mA. For PD B, the third-order output intercept point at photocurrent levels of 30 and 40 mA is 37.2 and 34.9 dBm, respectively. That for PD A is 35.8 and 30.4 dBm. PD B is, therefore, favorable for linear operation at high current levels.

Frequency-selective absorbance detection: Refractive index and turbidity compensation with dual-wavelength measurement

A frequency-selective absorbance detection approach and its applications are described. First, a digital signal processor–lock-in amplifier (DSP–LIA)-based absorbance detector was evaluated. Compared to a simple operational amplifier (TL082CP)-based detector, the DSP–LIA-based detector showed lower noise levels, but the relative advantage was reduced under very low photocurrent levels (down to few nA). A 7 cm pathlength flow cell with this commercial LIA-based detector exhibited excellent Beer's law linearity ( r 2 = 0.9999) and a noise level of 7 micro absorbance units (μAU). The limit of detection (LOD, S/ N = 3) for methyl orange (MO) was 7 nM with this detector. Finally, as a more affordable alternative to an LIA, a balanced demodulator integrated circuit chip was used to fabricate a dual wavelength–frequency-selective LED-based absorbance detector. This device successfully compensated refractive index (RI) effect and turbidity effect in test flow systems. The LOD for MO with this system was 8 nM.

GaN UV detectors for synchrotron-based protein structure studies

AlGaN and GaN have been investigated as UV detector materials for applications in protein structure studies. I–V characteristics performed on the material using concentric contacts showed 4 orders of magnitude of greater dark current for the Al 0.1 Ga 0.9 N than for the GaN. Subsequently, interdigitated metal–semiconductor–metal (MSM) photodetectors were successfully fabricated on GaN. No changes in levels of dark surrent were recorded using varying metal electrodes with similar work functions (Pd and Au). The unbiased diodes showed a difference of 3 orders of magnitude between dark and photocurrent levels on exposure to UV. The responsivity for diodes with 25 and 100 μ m finger separation operated in unbiased mode was around 100 mA/W and was flat over the bandgap. These results show a responsivity in agreement with those from previous measurements for biased GaN photodetectors [Phys. Stat. Solid: 176 (1999) 157]. Using these results, a design for an unbiased GaN detector to be used for protein structure studies is proposed.

Optical quenching of photoconductivity in undoped n-GaN

The optical quenching of photoconductivity in undoped n-type GaN has been investigated. It was observed that for increasing 360 nm background intensity, at constant 632 nm quenching illumination intensity, the quenching increased for intensities up to 1012 photons cm−2 s−1 and decreased thereafter. At low background illumination intensities, the transient in photoconductivity induced by the quenching illumination exhibits a sharp rise followed by a slow decay; removal of the quenching illumination was also noted to induce a sharp photocurrent drop followed by a slow recovery to the photocurrent level induced by the background illumination alone. The quenching effect at constant background and quenching illumination wavelength and intensity is noted to keep nearly constant at low temperature and decrease rapidly when temperature is higher than 270 K. Furthermore, measurement of the spectral distribution of the quenching effect indicates the existence of a broadband hole trap centered around 1.5 eV above the valence band. To explain the observed experimental behavior, the conventional hole trap and recombination center model was extended to include an electron trap. This extended model was found to accurately predict the experimental results.

Optical characterization of the silicon photodiodes for the establishment of national radiometric standards

Optical properties of the silicon photodiodes are investigated in the visible spectral regime. Non-linearity measurement standard was established by using Hamamatsu S1337-11 type windowless silicon photodiode whose non-linearity value was found to be better than 6×10 −5 at photocurrent level of 10 −9 to 10 −4 A . Temperature effects on the spectral responsivity for S1337-11, S1337-1010BQ and S1227-1010BQ type photodiodes were analyzed between 20°C and 40°C at 488.1, 514.7 and 632.8 nm vacuum wavelengths. The spatial uniformities of the responsivity for three type photodiodes are performed with a laser beam having 1 mm diameter by using home made two-axis micro translation system. Results of the reflectance measurements for three elements of reflection-based trap detectors were compared with the predicted values obtained from Fresnel equations.

Thin anodic oxides on n-InP studied by photocurrent transients and surface analysis

Investigation of semiconductor/electrolyte interface modified by superficial oxides has been performed with photocurrent transients and surface analysis. Studies of the photocurrent transients before and after the modification of the semiconductor surface exhibit the electrical properties of the interface, whereas XPS analysis gives access to the chemical aspect. This work evidenced a correlation between the level of photocurrent, the surface oxidation and the texture (porosity, thickness) of the oxide layer.

Density of states distribution in AlN films measured by CPM and DLTS

Abstract Aluminium nitride films are prepared by radio frequency magnetron sputtering of pure aluminium target in argon–nitrogen–hydrogen mixture at the different value of the nitrogen–hydrogen flow rates. The argon flow rate and partial pressure are fixed. Nominal substrate temperature is equal to 275°C, glass and silicon wafers are used as substrates. Constant photocurrent method (CPM) and deep level transient spectroscopy (DLTS) are used for the density of electron states (DOS) distribution parameters investigations in the energy ranges which include nearly all AlN band gaps. The CPM is applied for experimental investigations of optical absorption coefficient and the DOS spectra in the 1.4–5.0 eV range. The DOS peaks are found at the energies of 2.2–2.7 and below 1.8 eV from the conduction band bottom. The second peak is attributed to the N Al antisite defects, whereas data on the first one has not been previously reported. Capacitance DLTS technique is used for the N(E) investigation in the energy range up to 0.85 eV below the conduction band bottom. Relatively narrow (15–150 meV halfwidth) peaks are found in the energy range. These peaks appearance is attributed to the nitrogen vacancy triplet. The conduction band tail slope characteristic energy is estimated as 25±10 meV.

GaAs devices with vertical and planar structures for optically activated high-voltage switching

A class of semi-insulating gallium arsenide diodes has been manifactured and tested, with a Schottky contact and a special ohmic contact on the back, which can be operated in reverse bias well above 5 V/μm were leakage currents are in the 1–10 μA range (for 3 mm diodes). Preliminary measurements show their feasibility as PhotoConductive Semiconductor Switches (PCSS). At fixed high-voltage bias, the current drawn by this device can be increased at least by two orders of magnitude, by illuminating the diode with low-intensity near-infrared light. The resulting change in resistance can be used in order to switch off (DC mode) the high voltage either in a series or a parallel load configuration. Measurements are reported for 3 mm diameter GaAs diodes, 0.2 mm thick, as for: current–voltage characteristics, current increase upon illumination with a 880 nm LED, linearity of photocurrent vs. illumination level. In addition to these diodes with vertical structure, special GaAs switches with a planar structure have been developed and tested at 670 nm with a focused laser diode beam; they are made of a 0.2 mm substrate and a series of two Schottky contacts facing each other on the same side of the substrate. The planar structures enable multiple independent switches on the same substrate, each one being able to withstand a voltage bias as high as 700 V with typical leakage currents below 20 nA, and in addition, they may provide a photocurrent/dark current gain as high as 10 6. Both types of devices can be activated either locally or remotely, e.g. with illumination via single or multiple optical fibers, thus providing a special type of PCSS for optically controlled high-voltage DC switching. We plan to use devices of this type in high-energy physics experiments, specifically, in order to control remotely the high-voltage bias of microstrip gas chambers.

Enhanced Quantum Efficiencies and Short-Circuit Photocurrents in Solid Porphyrin Thin Film Cells by Internal Electric Fields

Enhanced Quantum Efficiencies and Short-Circuit Photocurrents in Solid Porphyrin Thin Film Cells by Internal Electric Fields

Clocked-sense-amplifier-based smart-pixel optical receivers

We introduce the concept of synchronous smart-pixel optical receivers, and present the first use of a clocked-sense amplifier as a smart-pixel optical receiver. Such a receiver uses the controlled application of positive feedback to obtain low-power compact digital amplification. We describe the design and simulation of two types of optical receivers based on a clamped bit-line sense amplifier (CBLSA), and a conventional sense amplifier (CSA). Both of these circuits have been realized in 0.8 micron-linewidth foundry CMOS with hybrid-bonded GaAs-AlGaAs MQW detectors and modulators attached to the circuit. Operation in excess of 750 Mb/s is demonstrated, within a layout area of 44 μm×22 μm, with a bias-dependent estimated power dissipation of 1 to 2 mW. Operation with one or two input beams is possible, with approximate minimum detected photocurrent levels at 320 Mb/s of 8 μA (/spl sim/100 fJ) for single-beam operation and 2.5 μA/beam (/spl sim/30 fJ/beam) for two-beam operation, all in the CBLSA-based circuit.

X-ray-induced photoconductivity in YBa2Cu3O6 between 70 and 300 K.

The x-ray-induced photoconductivity in bulk ceramic samples of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$, with x\ensuremath{\approxeq}0, at temperatures between 70 and 300 K has been studied. In addition to a prompt current, which occurs at the beginning of the irradiation, the photocurrent exhibits a saturation growth stage up to values quite larger than the prompt. The amplitude of the saturating stage depends on dose and dose rate. The photocurrent level also increases with the irradiation temperature, but the fractional change in electric carriers decreases as the irradiation temperature increases. It has been found that after irradiation the samples show a persistent photoconductivity (PPC). The isothermal decay curves at the irradiation temperature of the PPC show that its thermal stability increases for an increase of either the irradiation dose or the irradiation temperature. Above 200 K the thermal stability decreases and the sample completely recovers after several hours at room temperature. The possible relation between this PPC and the well known PPC induced by laser illumination in thin films of this material is discussed. \textcopyright{} 1996 The American Physical Society.

Efficient harmonic generation using an electroabsorption modulator

A multiple quantum-well electroabsorption modulator has been assessed as a harmonic generator of millimeter-wave signals for radio-over-fiber systems. It is shown experimentally that the steep nonlinear absorption characteristics of electroabsorption modulators lead to efficient generation of high order harmonics when driven by a sinusoidal voltage. Photocurrent harmonic power levels generated using electroabsorption and Mach-Zehnder modulators are compared with theory.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optically controlled varactor diode

We describe the design and operating characteristics of an optically controlled varactor diode. Applications include optically controlled microwave elements and detection of optical signals in lightwave receivers. The structure consists of a p-n junction formed in an epitaxial layer of InP grown by metalorganic chemical vapor deposition on a semi-insulating InP substrate. A capacitance shift as high as 1.5 pF has been achieved for a photocurrent level as low as 9 μA.

The photoelectrochemical properties of Nb-doped TiO 2 semiconducting ceramic membrane

Photoelectrochemical properties of TiO 2 membranes were investigated for samples which were prepared from several colloidal suspensions and supported on F-doped SnO 2-coated glass. The effects of firing temperature, membrane thickness and Nb-doping level on quantum efficiency were examined in the range of 400-700°C, 0.1–1.1 μm and 0–10 mol % respectively. The conductivity, surface area and crystallinity of these membranes was also investigated as a part of this study. The colloidal suspensions were formed by hydrolysis of a titanium tetra-isopropoxide and niobium pentachloride solution. The photocurrent was found to be a function of firing temperature and membrane thickness. For undoped membranes, optimum photocurrent occurred at a thickness of ~0.3 μm for 600°C and 700°C, and 0.5 μm for 500°C. By doping Nb into the TiO 2 membrane, the conductivity and photocurrent of the membrane could be affected, and an optimum value in photocurrent versus Nb-doping level was determined. The surface area and crystallinity of TiO 2 were also influenced by Nb-doping level. The difference in crystallinity between supported and unsupported membranes was investigated as well.

Analytical electrical model for a Si liquid crystal light valve

An approximate analytical model has been developed which allows the voltage characteristics of the liquid crystal layer of a silicon liquid crystal light valve to be determined once the photocurrent level and device parameters have been specified. This model significantly decreases the computation time compared to current numerical techniques.

Preparation and Photocurrent Properties of Tin Dioxide Electrode Modified by Polyimide Lagmuir-Blodgett Films Possessing Tetraphenylporphyrin Unit

Polyimide Langmuir-Blodgett (LB) film containing tetraphenylporphyrin (TPP) unit was newly prepared on the surface of tin dioxide electrode by “Precursor method” (Electrode I), and the generation of photocurrent was measured by the wet system. The photocurrent spectrum obtained was very similar in shape to the absorption spectrum of the LB film. The quantum efficiency of the electrode modified by one layer of the polyimide LB film was 0.178, and the values decreased with increasing the number of layers. The long term stability of the photocurrent of Electrode I was compared with that of the electrode possessing the monomeric TPP LB film prepared from a 1:1 mixture of monoamino-TPP and stearic acid (Electrode II). These two electrodes were stable under the irradiation of monochromatic light of 430 nm, while in the case of the direct irradiation of the xenon lamp, Electrode I kept the same level of the initial photocurrent for a long time, but Electrode II decreased to 50% within one hour. The absorption spectra indicated that both LB films did not come off from the surface of the electrodes during the photocurrent measurements. The decrease of the photocurrent of Electrode II was explained by the hydrophobicity of the surface of the LB film possessing the long alkyl chain and by the diffusing effect of the reducing agent in the electrolyte solution.