Determination of subcell I–V characteristics of multijunction solar cells using optical coupling

A method for the determination of the subcell I–V characteristics of multijunction solar cells in the presence of optical coupling is presented and applied to a Ga_0.50In_0.50P/Ga_0.99In_0.01As/Ge triple‐junction solar cell. Each of the subcells is described by a two‐diode model and can be illuminated by a narrowband light source externally. Optical coupling is then used explicitly to generate current in one subcell, which is not illuminated externally. This approach yields the magnitude of optical coupling and a relationship between the two diode parameters of each subcell. The remaining cell parameters are determined with the help of pulsed illumination. In this fashion, the open circuit voltage of individual subcells is accessible, despite the fact that not all junctions are illuminated. Copyright © 2015 John Wiley & Sons, Ltd.     

Effect of si—h and n—h bonds on electrical properties of plasma deposited silicon nitride and oxynitride films

Strong correlations were observed between the improvement in the metal-insulator-semiconductor (MIS) (aluminum-nitride-semiconductor) electrical properties of plasma deposited silicon nitride and oxynitride films and their (Si—H/N—H) bonding ratios in the film bulk. Total hydrogen concentration and spin density of all deposited films decreased with post-deposition annealing. Films with more Si—H bonds and stable (Si—H/ N—H) ratios generally have lowerV _fb shift, less positive trap charge and higher breakdown dielectric strength. Silicon oxynitride films with refractive indices of 1.75-1.80, as-deposited and after annealing in forming gas (10% H₂ + 90% N₂) at various temperatures, were found to have stable (Si—H/N—H) bonding ratios, lower silicon dangling bond density, and better MIS electrical properties compared to other plasma deposited nitride and oxynitride films.     

Waveguide photonic crystals with characteristics controlled with <Emphasis Type="Italic">p</Emphasis>-<Emphasis Type="Italic">i</Emphasis>-<Emphasis Type="Italic">n</Emphasis> diodes

A one-dimensional waveguide photonic structure—specifically, a photonic crystal with a controllable frequency characteristic—is designed. The central frequency of the spectral window of the photonic crystal can be tuned by choosing the parameters of disturbance of periodicity in the photonic crystal, whereas the transmission coefficient at a particular frequency can be controlled by varying the voltage at a p-i-n diode. It is shown that the possibility exists of using the waveguide photonic crystal to design a microwave device operating in the 3-cm-wavelength region, with a transmission band of 70 MHz at a level 3 dB and the transmission coefficient controllable in the range from −1.5 to −25 dB under variations in the forward voltage bias at the p-i-n diode from zero to 700 mV.     

1/f noise in very-long-wavelength infrared Hg1−xCdx Te detectors

This paper investigates 1/f noise performance of very-long-wavelength infrared (VLWIR) Hg_1−xCd_xTe (cutoff wavelengths λ_c=15 μm and λ_c=16 μm) photodiodes at 78 K, where detector current is varied by changing detector area, detector bias, and illumination conditions. Holding detector bias and temperature constant, the 1/f noise current is proportional to the detector current. Significant nonuniformity is observed in the noise data for each detector area because of the varying detector quality. Defects are presumed resident in the detectors to produce greater nonuniformity in 1/f noise as compared to dark current at 100-mV reverse bias. For λ_c=16 μm, 4-μ-radius, diffusion-limited diodes at 78 K and 100-mV reverse bias, the average dark current is I_d=9.76±1.59×10⁻⁸ A, while the average noise current measured at 1 Hz is i_n=1.01±0.63×10⁻¹² A/Hz^1/2. For all detector areas measured, the average ratio in 1-Hz bandwidth is α _D =i_n/I_d=1.39±1.09×10⁻⁵. The 1/f noise was also measured on one diode as a function of detector-dark current as the applied bias is varied. In the diffusion-limited portion of this detector’s current-voltage (I-V) curve, to about 130 mV, the 1/f noise was independent of bias. For this diode, the ratio α_D=i_n/I_d=1.51±0.12×10⁻⁵. The 1/f noise associated with tunneling currents is a factor of 3 greater than the 1/f noise associated with diffusion currents, α_T=i_n/I_T=5.21±0.83×10⁻⁵. In addition, 1/f noise was measured on detectors held at −100 mV and 78K under dark and illuminated conditions. The measured ratios α_P ∼α_D ∼1.5×10⁻⁵ were about the same for the dark and photon-induced diffusion currents. Therefore, the diffusion current appears to have a unique value of α as compared to the tunneling current. This may be indicative of unique noise-generation mechanisms associated with each current.     

Measurement of the absorption coefficient for light laterally propagating in light-emitting diode structures with In<Subscript>0.2</Subscript>Ga<Subscript>0.8</Subscript>N/GaN quantum wells

A procedure for measuring the absorption coefficient for light propagating parallel to the surface of a GaN-based light emitting diode chip on a sapphire substrate is suggested. The procedure implies the study of emission from one end face of the chip as the opposite end face is illuminated with a light emitting diode. The absorption coefficient is calculated from the ratio between the intensities of emission emerging from the end faces of the sapphire substrate and the epitaxial layer. From the measurements for chips based on p-GaN/In_0.2Ga_0.8N/n-GaN structures, the lateral absorption coefficient is determined at a level of (23 ± 3)cm^?1 at a wavelength of 465 nm. Possible causes for the discrepancy between the absorption coefficients determined in the study and those reported previously are analyzed.     

Responsivity of avalanche photodiodes in the presence of multiple reflections

It is shown theoretically, and verified experimentally, that a photodiode, illuminated by monochromatic light of varying wavelength, may show interference ripples in its responsivity, owing to multiple reflections between the diode front and back contact. The amplitude of the ripples is found to increase linearly with the avalanche multiplication factor.     

Infrared materials for thermophotovoltaic applications

Thermophotovoltaic generation of electricity is attracting renewed attention due to recent advances in low bandgap (0.5–0.7 eV) III-V semiconductors. The use of these devices in a number of applications has been reviewed in a number of publications.^1–4 Two potential low-bandgap diode materials are In_xGa_1−xAs_ySb_1−y and In_xGa_1−xAs. The performance of these devices are comparable (quantum efficiency, open circuit voltage, fill factor) despite the latter’s long-term development for optoelectronics. For an 1100°C blackbody, nominally 0.55 eV devices at 25°C exhibit average photon-weighted internal quantum efficiencies of 70–80%, open circuit voltage factors of 60–65%, and fill factors of 65–70%. Equally important as the energy conversion device is the spectral control filter that effectively transmits above bandgap radiation into the diode and reflects the below bandgap radiation back to the radiator. Recent developments in spectral control technology, including InGaAs plasma filters and nonabsorbing interference filters are presented. Current tandem filters exhibit spectral utilization factors of ∼65% for an 1100°C blackbody.     

Reduction of charge injection into PECVD SiNxHy by control of deposition chemistry

The high rate of charge trapping in thin-film silicon nitride causes its electrical properties to change with stressing level and time. The rate of shift of the high-frequency CV curves of Al/SiN_xH_y/cSi capacitors was used here to measure nitride charging rate and to compare PECVD nitrides deposited under various conditions of plasma power and gas mixture in the same parallel-plate reactor. By operating the plasma under high power to activate the NH₃ or N₂ and under low SiH₄ flow to ensure that all of the SiH4 reacts with N, it is possible to deposit N-rich nitride that has no detectable Si—H bonding, which bonding others have correlated with charge trapping. Nitride deposited under these conditions using NH₃ and 13 MHz rf power had charging rates for both gate polarities that were 20 times lower than those of nitride that had a “stoichiometric” N/Si ratio of 4/3 and that had its H distributed among Si—H and N—H bonds. MIS capacitors made with the latter nitride also had a high negative initial flat-bond voltage, indicating the presence of grown-in positive charge. This charge was large enough to invert the surface ofp-Si substrates. N-rich nitride free of Si—H that was deposited either using N₂ or using low-frequency rf power (≤400 kHz) had higher charging rates than did that deposited from NH₃ at 13 MHz. Also, the low-frequency material contained grown-in positive charge that is attributed to H⁺ implanted by the high ion bombardment energy of the low-frequency plasma.     

A comparison of gamma radiation effects on bromine- and hydrazine-treated HgCdTe photodiodes

In this study, we investigated the effects of gamma radiation on ZnS/CdTe-passivated HgCdTe photodiodes that were fabricated with one of two different surface treatments using bromine, Br₂, or hydrazine, N₂H₄. Unlike the ZnS-passivated HgCdTe photodiodes, the ZnS/CdTe-passivated HgCdTe photodiodes showed no degradation in resistance-area product at zero bias (R₀A) values after gamma irradiation of up to 1 Mrad. However, there is a significant difference between the bromine- and hydrazine-treated samples. Regardless of the dose of gamma radiation, there was little change in the forward current characteristics of the hydrazine-treated diode in comparison with the conventional bromine-treated diode. The hydrazine-treated diode showed fairly uniform R₀A values of >10⁷ Ω-cm² up to 1 Mrad of gamma irradiation, whereas the bromine-treated diode showed an abrupt change in R₀A values from ∼10⁶ Ω-cm² to ∼10⁷ Ω-cm² after gamma irradiation. Therefore, for use in a gamma radiation environment, the best combination for high-performance HgCdTe photodiodes is a ZnS/CdTe passivant that has been treated with hydrazine.     

Molecular beam epitaxial growth of P-ZnSe:N using a novel plasma source

We have studied the p-type doping in ZnSe molecular beam epitaxial growth using a novel high-power (5 kW) radio frequency (rf) plasma source. The effect of growth conditions such as the rf power, the Se/Zn flux ratio and the growth temperature on p-ZnSe:N was investigated. The net acceptor concentration (N_A—N_D) of around 1 × 10¹⁸ cm⁻³ was reproducibly achieved. The activation ratio ((N_A—N_D)/[N]) of p-ZnSe:N with N_A—N_D of 1.2 × 10¹⁸ cm⁻³ was found to be as high as 60%, which is the highest value so far obtained for N_A—N_D ∼ 10¹⁸ cm⁻³. The 4.2K photoluminescence spectra of p-ZnSe:N grown under the optimized growth condition showed well-resolved deep donor-acceptor pair emissions even with high N_A—N_D. On leave from Sumitomo Electric Industry Ltd. On leave from Sony Corp.     

Pressure dependence of intersubband transitions in HgTe/Hg<Subscript>0.3</Subscript>Cd<Subscript>0.7</Subscript>Te superlattices

The optical absorption coefficient of HgTe/Hg_0.3Cd_0.7Te superlattices (SLs) and its pressure dependence has been investigated at hydrostatic pressures up to 30 kbar at room temperature. The corresponding intersubband transition energies result from a comparison of experimental and theoretical absorption coefficients. The latter is based on the band structure, which is calculated using Kane’s four-band (8×8 k · p) model together with the envelope function approximation. The experimental linear pressure coefficients of the H1 — E1 and H1 — L1 intersubband transitions are in good agreement with the theoretical values, e.g., 7.15±0.3 meV/kbar and 6.2±0.3 meV/kbar compared to 7.4 and 6.4 meV/kbar, respectively. this is in stark contrast to the pressure dependence of ≤1 meV/kbar of the photoluminescence (PL) peaks of a similar SL reported in the literature. Consequently, we conclude that the reported PL peaks are not due to intersubband transitions and that the k · p model correctly reproduces the electronic band structure and its pressure dependence of HgTe/Hg_1−xCd_xTe SLs.     

Determination of the electron mobility and density in thin semiconductor films at microwave frequencies using the magnetoplasma resonance

The results of experimental and theoretical investigations of the magnetoplasma resonance at microwave frequencies (ω/2π=8 GHz) in AlGaAs/GaAs heterostructures and n-Cd_xHg_1−x Te films at liquid-nitrogen temperature are reported. To explain the experimental results, an expression is derived for the conductivity σ _xx (ω,B). It is shown that the parameters of the active layer in the samples — the electron mobility and density — can be determined by comparing the computational results with the experimental dependences. Fiz. Tekh. Poluprovodn. 33, 1224–1228 (October 1999)     

Efficient Selective Identity-Based Encryption Without?Random Oracles

We construct two efficient Identity-Based Encryption (IBE) systems that admit selective-identity security reductions without random oracles in groups equipped with a bilinear map. Selective-identity secure IBE is a slightly weaker security model than the standard security model for IBE. In this model the adversary must commit ahead of time to the identity that it intends to attack, whereas in an adaptive-identity attack the adversary is allowed to choose this identity adaptively. Our first system—BB₁—is based on the well studied decisional bilinear Diffie–Hellman assumption, and extends naturally to systems with hierarchical identities, or HIBE. Our second system—BB₂—is based on a stronger assumption which we call the Bilinear Diffie–Hellman Inversion assumption and provides another approach to building IBE systems.     

Wannier-Stark states in a superlattice of InAs/GaAs quantum dots

Electron and hole emission from states of a ten-layer system of tunneling-coupled vertically correlated InAs/GaAs quantum dots (QDs) is studied experimentally by capacitance—voltage measurements and deep-level transient spectroscopy. The thickness of GaAs interlayers separating sheets of InAs QDs was ≈3 nm, as determined from transmission electron microscope images. It is found that the periodic multimo-dal DLTS spectrum of this structure exhibits a pronounced linear shift as the reverse-bias voltage U _r applied to the structure is varied. The observed behavior is a manifestation of the Wannier—Stark effect in the InAs/GaAs superlattice, where the presence of an external electric field leads to the suppression of coupling between the wave functions of electron states forming the miniband and to the appearance of a series of discrete levels called Wannier—Stark ladder states.     

Interface properties of ZnO nanotips grown on Si substrates

ZnO nanotips have been grown on Si (100) using metalorganic chemical vapor deposition (MOCVD). The growth temperature is optimized for good crystallinity, morphology, and optical properties. ZnO nanotips exhibit a strong near band edge emission of ∼376 nm at room temperature with negligible green band emission. Pregrowth substrate treatment using diluted hydrofluoric acid (HF) and minimized oxygen exposure before the initial growth significantly reduces the interfacial SiO₂ thickness, while maintaining good morphology. An n-ZnO nanotips/p-Si diode is fabricated and its I–V characteristic is measured. The threshold voltage of the diode is found to be below 2.0 V with small reverse leakage current. The ZnO/p-Si diodes provide the possibility of integrating the ZnO nanotips with Si-based electronic devices.     

1/f noise in large-area Hg<Subscript>1−x</Subscript>Cd<Subscript>x</Subscript>Te photodiodes

The 1/f noise in photovoltaic (PV) molecular-beam epitaxy (MBE)-grown Hg_1−xCd_xTe double-layer planar heterostructure (DLPH) large-area detectors is a critical noise component with the potential to limit sensitivity of the cross-track infrared sounder (CrIS) instrument. Therefore, an understanding of the origins and mechanisms of noise currents in these PV detectors is of great importance. Excess low-frequency noise has been measured on a number of 1000-μm-diameter active-area detectors of varying “quality” (i.e., having a wide range of I-V characteristics at 78 K). The 1/f noise was measured as a function of cut-off wavelength under illuminated conditions. For short-wave infrared (SWIR) detectors at 98 K, minimal 1/f noise was measured when the total current was dominated by diffusion with white noise spectral density in the mid-10⁻¹⁵A/Hz^1/2 range. For SWIR detectors dominated by other than diffusion current, the ratio, α, of the noise current in unit bandwidth i_n(f = 1 Hz, V_d = −60 mV, and Δf = 1 Hz) to dark current I_d(V_d = −60 mV) was α_SW-d = i_n/I_d ∼ 1 × 10⁻³. The SWIR detectors measured at 0 mV under illuminated conditions had median α_SW-P = i_n/I_ph ∼ 7 × 10⁻⁶. For mid-wave infrared (MWIR) detectors, α_MW-d = i_n/I_d ∼ 2 × 10⁻⁴, due to tunneling current contributions to the 1/f noise. Measurements on forty-nine 1000-μm-diameter MWIR detectors under illuminated conditions at 98 K and −60 mV bias resulted in α_MW-P = i_n/I_ph = 4.16 ± 1.69 × 10⁻⁶. A significant point to note is that the photo-induced noise spectra are nearly identical at 0 mV and 100 mV reverse bias, with a noise-current-to-photocurrent ratio, α_MW-P, in the mid 10⁻⁶ range. For long-wave infrared (LWIR) detectors measured at 78 K, the ratio, α_LW-d = i_n/I_d ∼ 6 × 10⁻⁶, for the best performers. The majority of the LWIR detectors exhibited α_LW-d on the order of 2 × 10⁻⁵. The photo-induced 1/f noise had α_LW-P = i_n/I_ph ∼ 5 × 10⁻⁶. The value of the noise-current-to-dark-current ratio, α appears to increase with increasing bandgap. It is not clear if this is due to different current mechanisms impacting 1/f noise performance. Measurements on detectors of different bandgaps are needed at temperatures where diffusion current is the dominant current. Excess low-frequency noise measurements made as a function of detector reverse bias indicate 1/f noise may result primarily from the dominant current mechanism at each particular bias. The 1/f noise was not a direct function of the applied bias.     

Design of a Compact Ka-band Balanced Mixer Based on a Novel Wide-band Equivalent Circuit of the Schottky Diode

A compact Ka-band balanced mixer based on a novel Schottky diode model is presented in this paper. According to its physical structure, a novel 3D electromagnetic model of the Schottky diode is proposed. Meanwhile, a wide-band equivalent circuit is built, which takes all high frequency effects existing in the diode into account. All the parasitic reactances are extracted from the electromagnetic model-based S-parameters up to 110 GHz. Based on the proposed equivalent circuit, a Ka-band balanced mixer is designed and optimized, where bandpass filters with open stubs and a lowpass filter based on defected ground structure cells are used. The measured results show that the conversion loss is below 9 dB from 30 GHz to 40 GHz with the minimum of 6.7 dB at 35 GHz. The normalized size of the mixer is only 3.3λ _g  × 3.3λ _g , whereλ _g is the wavelength at 35 GHz.     

Effects of bias-temperature cycling on electrical characteristics of YBCO/YSZ/Si SulS capacitors

A bias-temperature cycling technique is developed in the electrical characterization of YBa₂Cu₃0_7−δ/yttria-stabilized zirconia (YSZ)/Si capacitors. This technique can be used to determine the electrical properties of the material components and their interfaces in the capacitor. Capacitance-voltage (C-V) measurements under no or weak illumination at temperatures ranging from 295 to 80K reveal that hysteresis due to mobile ions decreases with cooling and become vanishingly small at about 220K. Upon further cooling, a different mechanism due to traps in the YSZ/Si interface dominates the low-temperature hysteresis and stretchout of the C-V curves, which is evidenced by measurements for the illuminated device at 80K.     

UHF photoparametric amplifier

Measurements on a degenerate UHF photoparametric amplifier using an optical signal modulated at 690 MHz have confirmed the 3 dB noise figure predicted by Penfield and Sawyer. The silicon photoparametric diode served as both the photodetector and varactor and was novel in several respects: the conductivity sequence was p-v-n-n⁺in contrast to the usual p-v-n⁺conventionally used for microwave photodiodes; and the diode was designed to be illuminated parallel to the p-v junction. Such a structure has an inherently high Q and features are extended optical response (quantum efficiency ≈ 10 percent at 1.06 µ), coupled with good UHF photodetection behavior, when used either as a simple (unpumped) photodiode or as a photoparametric diode.     

Investigations into the source of 1/f noise in Hg<Subscript>x</Subscript>Cd<Subscript>1−x</Subscript>Te diodes

This work is aimed at elucidating the mechanism of 1/f noise in Hg_xCd_1−xTe diodes in order to reduce 1/f noise in high-performance thermal cameras. Currently, to reach the required signal-to-noise ratio, the operating temperature of thermal cameras is 77 K. If 1/f noise could be reduced, this operating temperature could be raised without reducing the signal-to-noise ratio, allowing cheaper, smaller, and lighter cooling systems to be used. There are still some fundamental aspects of 1/f noise that are not fully understood. One aspect in particular is whether the noise is caused by mobility or number density fluctuations because both could give the conductivity fluctuations observed. This paper explores the noise coupling and current coupling between diodes on an array. It is shown that 1/f noise is present on the photons emitted from a forward-biased diode, suggesting that the rate of radiative recombination fluctuates. The results suggest that these fluctuations are due to carrier number-density fluctuations. From correlation measurements, it is shown that at least 32% of the 1/f noise in the forward-biased diode is correlated with carrier density fluctuations in Hg_xCd_1−xTe diodes.