High-performance In/sub 0.53/Ga/sub 0.47/As thermophotovoltaic devices grown by solid source molecular beam epitaxy

In/sub 0.53/Ga/sub 0.47/As-based monolithic interconnected modules (MIMs) of thermophotovoltaic (TPV) devices lattice-matched to InP were grown by solid source molecular beam epitaxy. The MIM device consisted of ten individual In/sub 0.53/Ga/sub 0.47/As TPV cells connected in series on an InP substrate. An open-circuit voltage (V/sub oc/) of 4.82 V, short-circuit current density (J/sub sc/) of 1.03 A/cm/sup 2/ and fill factor of /spl sim/73% were achieved for a ten-junction MIM with a bandgap of 0.74 eV under high intensity white light illumination. Device performance uniformity was better than 1.5% across a full 2-in InP wafer. The V/sub oc/ and J/sub sc/ values are the highest yet reported for 0.74-eV band gap n-p-n MIM devices.     

n+-SnO2/a-SiC:H/metal thin-film photodiodeswith voltage-controlled spectral sensitivity

n⁺-SnO₂/a-SiC/metal photodiodes with voltage-controlled photosensitivity have been realized by using both carbon-rich and silicon-rich a-SiC alloys. Carbon-rich devices show a response peak located at 530 nm independent of the applied voltage, which in turn only affects the peak height. At variance, in silicon-rich structures the response peak is located at 480, 510, and 570 nm when the applied voltage is -4, 0, and +4 V, respectively, with corresponding quantum yield values of 17, 3, and 25%. For explaining the observed behavior we present a simple model of n⁺-SnO₂/a-SiC/metal diodes, which takes into account light-induced modulation of n⁺-SnO₂/a-SiC barrier height, primary photocurrent generation and photoconductivity effects     

Development of high-quantum-efficiency,lattice-mismatched, 1.0-eV GaInAs solar cells

High-quantum-efficiency, lattice-mismatched, 1.0-eV GaInAs solar cells grown by organometallic vapor phase epitaxy have been developed for ultimate integration into AlGaAs/GaAs/GaInAs 3-junction, 2-terminal monolithic devices. The more standard n/p junction was replaced with an n-i-p structure in the GaInAs cell in order to increase the short-circuit current by overcoming the material deficiencies which arise as a result of accommodating the lattice mismatch. This led to single junction 1.0-eV GaInAs cells with internal quantum efficiencies >90% and short-circuit-current densities that match or closely approach those needed to current match the upper AlGaAs and GaAs cells. A 4.1% (1-sun, air mass 0,25°C) power conversion efficiency was achieved with a developmental structure, indicating the potential of the lattice-mismatched n-i-p 1.0-eV GaInAs cell. An analogous device designed to allow direct monolithic integration with the upper AlGaAs and GaAs cells, with a modified grading layer of AlGaInAs in place of the usual GaInAs, achieved an efficiency of 2.2%, primarily due to a lower open-circuit voltage. The open-circuit voltage is perhaps limited by structural defects revealed in transmission electron micrographs.     

An a-Si:H/a-Si,Ge:H bulk barrier phototransistor with a-SiC:Hbarrier enhancement layer for high-gain IR optical detector

The design and fabrication of a high-gain amorphous silicon/amorphous silicon germanium (a-Si:H/a-Si,Ge:H) bulk barrier phototransistor for infrared light detection applications are reported. The a-Si,Ge:H material featured a lower energy gap and is suitable for the absorption of longer wave light, but it also leads to a low breakdown voltage and high dark current. An additional a-SiC:H thin-film layer was used at the collector/base interface in the conventional amorphous bulk barrier phototransistor to enhance the function of the bulk barrier and obtain high optical gain     

Computer simulation and modeling of graded bandgap CuInSe2/CdS based solar cells

This paper proposes the graded bandgap absorber material, Cu_1-xAg_xIn_1-y-zGa_yAl_z Se/sub 2(1-u$/ -/sub w/)S_2uTe_2w (CIS*) multinary system, to improve the low open-circuit voltage (V_OC) seen in CuInSe₂/CdS solar cells, without sacrificing the short-circuit current density (J_sc). It also proposes a p-i-n model for the CuInSe₂/CdS solar cell, where the intrinsic region is the graded bandgap CIS*. Reflecting surfaces are provided at the p-i and n-i interfaces to trap the light in the narrow intrinsic region for maximum generation of electron and hole pairs (EHP's). This optical confinement results in a 25-40% increase in the number of photons absorbed. An extensive numerical simulator was developed, which provides a 1-D self-consistent solution for Poisson's equation and the two continuity equations for electrons and holes. This simulator was used to generate J-V curves to delineate the effect of different grading profiles on cell performance. The effects of a uniform bandgap, normal grading, reverse grading, and a low bandgap notch have been considered. Having established the inherent advantages to these grading profiles an optimal doubly graded structure is proposed with grading between 1.5 eV and 1.3 eV regions which has V_OC=0.86 V, η=17.9%, FF=0.79 and J_sc=26.3 mA/cm² compared to 0.84 V, 14.9%, 0.76, and 23.3 mA/cm², respectively, for the highest efficiency 1.4-eV uniform bandgap cell. Replacing the thick CdS(2.42 ev) layer assumed in our simulations with a wide gap semiconductor such as ZnO(3.35 ev) increases all current densities by about 5 mA/cm², and increases the optimal calculated efficiency from 17.9% to roughly 21% for a doubly graded structure with a thickness of 1 μm and bandgaps ranging from 1.3 eV to 1.5 eV     

Photoluminescence of quasi-direct transitions in disorderedIn1-xGaxP/graded GaP alloys

We have examined the photoluminescence and photoluminescence kinetics of a series of In_1-xGa_xP alloys in an effort: 1) to elucidate the electronic structure of the conduction band versus alloy composition, especially near the direct-indirect crossover; 2) to determine precisely the composition of the direct-indirect crossover, and its temperature dependence; and 3) to understand the nonradiative decay mechanism and its temperature dependence. We find that the fundamental bandgap is only determined by the Γ_1c and X_1c states in samples with Ga-compositions ranging from 0.58 to 0.75, and that the 2-K direct-indirect crossover from Γ_1c, to X_1c occurs at x=0.69 and is not strongly temperature-dependent. Further, we find, in agreement with our spectroscopic ellipsometry measurements at room temperature, that the mixing near crossover is rather complicated and leads to the previous observation of quasi-direct transitions. Our combined photoluminescence and spectroscopic ellipsometry measurements have therefore clearly resolved the controversy regarding the bandgap crossover. This has strong implications for the realization of InGaP-based efficient light-emitting devices with emission at higher energies     

Small angle X-ray scattering from microvoids in the a-SiC:H alloy

Small-angle X-ray scattering (SAXS) measurements on a series of a-SiC:H alloy films deposited in a glow discharge reactor using SiH₄/CH₄ gas mixtures are discussed. It was found that the introduction of C into the a-Si matrix produces a large SAXS signal, which suggests a sizable microvoid density, and which further increases with increasing C content. A Guinier analysis shows that these microvoids are spherical and have an average radius of approximately 6 Å, which increases only slightly with C content. Other measurements (infrared, photoconductivity, film density, optical) made on identically prepared material are included to identify the possible origins of these microvoids and to determine how they affect material quality     

Current gain-Early voltage products in heterojunction bipolartransistors with nonuniform base bandgaps

The tradeoff between common-emitter current gain (β) and Early voltage (V_A) in heterojunction bipolar transistors (HBTs) where the bandgap varies across the base has been studied. The Early voltage depends exponentially on the difference between the bandgap at the collector side of the base and the largest bandgap in the base, allowing very high Early voltages with only very thin narrow bandgap regions. Using Si/Si_1-xGe_x/Si HBTs with a two-layer stepped base, βV_A products of over 100000 V have been achieved for devices with a cutoff frequency expected to be about 30 GHz     

Optimization of Al/a-SiC:H optical sensor device by means of thermal annealing

The optimization of optoelectronic properties of Al/a-SiC:H Schottky diodes grown as Al/a-SiC:H/c-Si(n) structures is studied by means of thermal annealing of a-SiC:H thin films. According to the spectral response of the Schottky diodes the measured quantum efficiency, η_measured, increases with increasing annealing temperature (400–600 °C), whereas η_measured decreases for T_a>600 °C. For T_a=600 °C, optimum material quality of a-SiC:H films is achieved and the spectral response of the Al/a-SiC:H/c-S(n) structures present very high and almost constant values (η_measured80%) for the whole range of wavelengths from 500 up to 850 nm. These results show that our Al/a-SiC:H/c-S(n) structures can be very attractive as optical sensors. Diffusion length calculations as well as the mobility by lifetime product (μτ)_p of the minority carriers (holes) of a-SiC:H films present a dependence on T_a similar to that of the measured quantum efficiency. Finally, the quantum efficiency of films processed with T_a=675 °C is found to increase when the Al/a-SiC:H/c-S(n) structures are exposed to hydrogen, a result that could be promising for the construction of a hydrogen detection sensor.     

A novel high open-circuit voltage p-n InP solar cell design

The performance of InP solar cells has been limited by low open-circuit voltages. While the reported short-circuit current densities are approaching the theoretical limit, the open-circuit voltages have yet to obtain what is expected from a semiconductor with a direct band gap of 1.35 eV. This work investigates the factors that determine the open-circuit voltage and presents the design and fabrication of a novel high open-circuit voltage p-n InP solar cell. the key aspect of the novel design is a complete analysis of the top contact metallization effects on the reverse saturation current density and the open-circuit voltage. the features of the design are not specific to InP solar cells but are applicable to other advanced material solar cells that require a thin emitter for an optimal design (those materials with a high absorption coefficient). By minimizing the reverse saturation current density, a high open-circuit voltage and high efficiency may be obtained. In addition, a complete analysis of the solar cell modelling is provided, with comparisons to other published InP solar cell models and device results to juxtapose the key material and design parameter effects.     

Electroluminescence characteristics and current-conductionmechanism of a-SiC:H p-i-n thin-film light-emitting diodes with barrierlayer inserted at p-i interface

In order to improve the electroluminescence (EL) characteristics of the hydrogenated amorphous silicon carbide (a-SiC:H) p-i-n thin-film light-emitting diode (TFLED), a barrier layer (BL) was inserted at its p-i interface to enhance the hole injection efficiency under forward-bias operation. The a-SiC:H TFLED's with various optical gaps of BL had been fabricated and characterized. In addition, a composition-graded n⁺-layer was used to reduce its series and contact resistances to the Al electrode and hence the EL threshold voltage (V_th) of an a-SiC:H BL TFLED. The highest obtainable brightness of an a-SiC:H BL TFLED was 342 cd/m² at an injection current density of 600 mA/cm² and the lowest EL V _th achievable was 6.0 V. The current-conduction mechanism of an a-SiC:H BL TFLED had also been investigated. Within the lower applied-bias region, it showed an ohmic current, while within the higher applied-bias region, a space-charge-limited current (SCLC) was observed     

Monolithic perovskite/silicon tandem solar cells offer an efficiency over 29％

Organic-inorganic hybrid perovskite semiconductors pos-sessing superior optoelectronic properties (e.g.long carrier dif-fusion lengths,high optical absorption coefficient,low ex-citon binding energy,and high defect tolerance) are attract-ing serious attention.The certified power conversion effi-ciency (PCE) for single-junction perovskite solar cells have ex-ceeded 25％[1,2].As a very promising PCE-enhancement strategy,tandem structure made by stacking a perovskite cell on a market-dominant silicon cell can yield much higher PCEs beyond the Shockley-Queisser limit of single-junction devices without adding substantial cost[3].To satisfy current-match-ing in tandem configuration,the top perovskite cell requires an ideal bandgap of ～1.7 eV rather than the ones (～1.5-1.6 eV)typically used for highly efficient single-junction perovskite devices given that the bottom silicon cell holds a bandgap of 1.12 eV[4].Such wide-bandgap perovskites achieved through I/Br alloying usually suffer from photoinduced phase segrega-tion and relatively low radiative efficiency,which inevitably result in large open-circuit voltage (Voc) deficits[5,6].Several strategies like adjusting perovskite composition[7,8],additive engineering[9,10],and upper surface passivation[11,12] have been utilized to stabilize these wide-bandgap perovskites and improve film quality to reduce Voc losses.The reported pe-rovskite/silicon tandem devices suffer from low Voc (＜1.9 V)and PCEs (≤28％)[13].There is still a large room for enhancing PCEs given that the predicted PCE limit is beyond 30％ for this tandem technology[14].     

Experimental investigation of the excess charge

The open-circuit voltage of about 600 mV developed by 0.1 ohm-cm silicon solar cells under air mass zero illumination is about 100 mV less than voltages predicted from simple diffusion theory. The lower measured voltages appear to be controlled by junction current transport processes associated with the thin top diffused layer. Mechanisms such as low n⁺ layer minority carrier lifetime and bandgap narrowing due to heavy doping effects (HDE) have been suggested to explain these results. Experimental determinations of the properties of the diffused layer are required to assess which of these mechanisms predominate. While direct measurement is difficult, an indirect measurement methodology exists by which the lifetime or transit time in the diffused layer can be obtained. Nine p-type, 1×2 cm, 〈111〉 orientation silicon wafers were phosphorus diffused at 880°C for 45 minutes using P0Cl₃. Open-circuit voltages of 595-612 mV, typical of all 0.1 ohm-cm cell voltages, were obtained. From the open-circuit voltage and short-circuit current, the diffusion controlled I₀ was obtained. In addition to illuminated I-V characteristics, the time constants from the Open-Circuit Voltage Decay method, and the minority carrier diffusion lengths in the base region were measured. The base region charge was determined using the base region diffusion length measured by an X-ray method. The data from these experiments combined with simple theory can imply the minority carrier time constant and the excess charge in the diffused layer. From this, certain conclusions are drawn about the relative roles of bandgap shrinkage and recombination rates in the diffused layer.     

Nitride based high power devices: design and fabrication issues

We have modeled the breakdown voltage, critical current density and maximum operating frequency of several nitride based high power and high temperature electronic devices. It is found that the minority carrier recombination lifetime and the critical field for electric breakdown are important model parameters which influence device design and performance. Planar geometry GaN Schottky devices were fabricated and used to experimentally estimate these important parameters. Current–voltage measurements have indicated the importance of the non-planar geometries for achieving large breakdown voltages. The minority carrier (hole) diffusion length and recombination lifetime have been measured using the electron beam induced current technique. The measured hole lifetime of 7 ns and estimate for the critical field indicate the possibility of AlGaN based thyristor switch devices operating at 5 kV with current densities up to 200 A/cm² and at frequency above 2 MHz. The GaN structural and optical material quality as well as processing requirements for etching are also discussed.     

Direct tunneling at the front contact of amorphous silicon p-i-ndevices

The device physics behind hole direct tunneling currents at the front contact of a-Si:H p-i-n homojunction have been explored. In this paper, the dark I-V, the light I-V, and the QE characteristics of this structure with and without hole direct hole tunneling currents are evaluated and compared. The three differential equation systems of the Poisson's equation, the continuity equation for free electrons, and the continuity equation for free holes have been solved with allowances for direct tunneling currents. Hole direct tunneling currents at the front contact of a-Si:H p-i-n homojunctions give rise to a significant increase in the dark current level at high forward voltages and to an increase in the open-circuit voltage of the light I-V characteristic when the front electron barrier is low. The hole thermionic emission current and the hole direct tunneling current have been carefully compared to the front contact. Hole tunneling currents introduce important modifications to the carrier transport physics not only to the front contact but also in the bulk of the a-Si:H p-i-n homojunction     

a-SiC:H/pin势阱结构可见光注入式电致发光(英文)

a-SiC:H/pin势阱结构可见光注入式电致发光(英文)@朱冰$中国科技大学 @陈培力$宁波大学 @白贵儒$宁波大学     

Microscopic properties of H2 diluted HWCVD deposited a-SiC:H film

Hydrogenated amorphous silicon carbon (a-SiC:H) films were deposited by hot wire chemical vapor deposition. The evolution of microscopic properties like network bonding, disorder, density and chemical composition are studied as a function of H₂ dilution. The sp² and sp³ carbon clusters, hydrogen content and the density of the material has significant effect on the dielectric properties like the leakage current of M/a-SiC:H/〈Si〉 MIS structures made with both Cu and Al metal electrode. A higher leakage current is observed in the case of Cu electrode. These changes are important for its applicability as a low dielectric constant barrier material in microelectronic devices.     

Quaternary InGaAsSb Thermophotovoltaic Diodes

In_xGa_1-xAs_ySb_1-y thermophotovoltaic (TPV) diodes were grown lattice matched to GaSb substrates by metal-organic vapor phase epitaxy in the bandgap range of EG = 0.5 to 0.6 eV. InGaAsSb TPV diodes, utilizing front-surface spectral control filters, are measured with thermal-to-electric conversion efficiency and power density (PD) of n_TPV = 19.7% and PD = 0.58 W/cm², respectively, for a radiator temperature of T_radiator = 950 degC, diode temperature of T_diode = 27 degC, and diode bandgap of EG = 0.53 eV. Practical limits to TPV energy conversion efficiency are established using measured recombination coefficients and optical properties of front surface spectral control filters which for 0.53-eV InGaAsSb TPV energy conversion are n_TPV = 28% and PD = 0.85 W/cm² at the above operating temperatures. The most severe performance limits are imposed by 1) diode open-circuit voltage (V_OC) limits due to intrinsic Auger recombination and 2) parasitic photon absorption in the inactive regions of the module. Experimentally, the diode V_OC is 15% below the practical limit imposed by intrinsic Auger recombination processes. Analysis of InGaAsSb diode electrical performance versus diode architecture indicates that V_OC and thus efficiency are limited by extrinsic recombination processes such as through bulk defects     

Substrates and interlayer coupling effects on Mo1-xWxSe2 alloys

Two-dimensional(2D) transition metal dichalcogenides alloys are potential materials in the application of photodetectors over a wide spectral range due to their composition-dependent bandgaps. The study of bandgap engineering is important for the application of 2D materials in devices. Here, we grow the Mo1-xWxSe2 alloys on mica, sapphire and SiO2/Si substrates by chemical vapor deposition(CVD) method. Mo1-x Wx Se2 alloys are grown on the mica substrates by CVD method for the first time. Photoluminescence(PL) spectroscopy is used to investigate the effects of substrates and interlayer coupling force on the optical bandgaps of as-grown Mo1-xWxSe2 alloys. We find that the substrates used in this work have an ignorable effect on the optical bandgaps of as-grown Mo1-xWxSe2. The interlayer coupling effect on the optical bandgaps of as-grown Mo1-xWxSe2 is larger than the substrates effect. These findings provide a new way for the future study of the growth and physical properties of 2D alloy materials.