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1.
The designs of two- and three-junction solar cells based on GaPNAs/Si lattice-matched hetero-structures are calculated. It is shown that the efficiency of two-junction solar cells constituted by a junction based on a GaPNAs solid solution with a band gap E g of 1.78 eV and a junction based on Si may reach a value of 30.3% under AM1.5 D, 100 mW/cm 2, and 35.4% under AM1.5D, 20 W/cm 2. The maximum values of the efficiency of the three-junction solar cell constituted by top and middle junctions based on GaPNAs with E g of 2 and 1.5 eV, respectively, and a Si-based bottom junction are 39.2% under AM1.5 D, 100 mW/cm 2, and 44.5% under AM1.5D, 20 W/cm 2. It is shown that the thickness and minority carrier lifetime of the photoactive layers affect the efficiency of solar-light conversion by the heterostructures being developed. 相似文献
2.
The effect of Ti, Cu and Fe on silicon solar cells has been investigated. Ti severely degrades cell performance above a concentration of 10 11 cm ?3. The presence of 2 × 10 14 cm ?3 Ti results in a 63% loss in cell performance and more than an order of magnitude reduction in carrier lifetime. Ti gives rise to two deep levels in Si at Ev + 0.30 eV and Ec ? 0.27 eV. Copper, at concentrations below 10 16 cm ?3, has negligible effect on cell performance and carrier lifetime. Above 10 16 cm ?3 copper occasionally produces a 10–15% loss in cell performance with a noticeable increase in junction excess current. No recombination centers were found due to Cu, instead considerable precipitation in the starting material was observed. Fe begins to hurt the cell performance above a concentration of 2 × 10 14 cm ?3. Iron at 1.7 × 10 15 cm ?3 results in 46% loss in cell efficiency and about an order of magnitude reduction in lifetime. Fe induces a deep level in silicon at Ev + 0.4 eV. The active center density, for both Ti and Fe, is only a very small fraction of the total impurity content in the starting silicon wafer. 相似文献
3.
Using metalorganic chemical vapor deposition, we heteroepitaxially grew undoped gallium nitride epilayers on sapphire. Assessing the epilayers at different growth stages, we investigated changes in epilayer strain and the lifetime of minority nonequilibrium charge carriers. The in-plane compressive strain was evaluated by x-ray diffraction and bandgap photoluminescence. The epilayer thickness ranged from 200 nm (islets) to 3.5 μm (continuous structure). The carrier lifetimes, measured using a light-induced transient grating technique, revealed a correlation between strain and the density of edge-type threading dislocations. This dislocation density was 10 9 cm ?2 to 10 11 cm ?2, corresponding to the dominant mechanism for nonradiative carrier recombination. How the carrier lifetime depended on the growth stage differed between the surface and interfacial measurements. On the surface side, the carrier lifetime increased monotonically up to ~500 ps with thickness; on the interface side, the lifetime changed little with thickness, except in the thickest sample, where the carrier lifetime increased with thickness. We attributed this behavior to defect healing aided by long-term annealing, leading to mutual lateral motion and annihilation of mixed threading dislocations. 相似文献
4.
We investigated the electrical properties of Ti(30?nm)/Al(200?nm) contacts to molecular beam epitaxy-grown N-polar n-GaN with different carrier concentrations. Samples with carrier concentration of 1.2?×?10 18?cm ?3 showed nonohmic behaviors when annealed at 300°C, but ohmic at 500°C and 700°C. All samples with carrier concentration of 2.0?×?10 19?cm ?3 exhibited ohmic behavior. x-Ray photoemission spectroscopy (XPS) results showed that, for samples with carrier concentration of 1.2?×?10 18?cm ?3, the Ga 2 p core levels shift to lower or higher binding energy upon annealing at 300°C or above 500°C, respectively. Scanning transmission electron microscopy (STEM) results showed that, for samples with carrier concentration of 1.2?×?10 18?cm ?3, a wurtzite AlN layer (??2?nm thick) formed at the metal/GaN interface when the samples were annealed at 500°C. An interfacial wurtzite AlN layer also formed upon annealing at 700°C, but its thickness was ??4?nm. Based on the XPS and STEM results, the ohmic contact formation and degradation mechanisms are described and discussed. 相似文献
5.
This paper will describe: (1) the first comparative study of recombination mechanisms between doped and undoped p-type Hg 1-xCd xTe liquid phase epitaxy films with an x value of about 0.22, and (2) the first determination of τ A7
i/τ A1
i ratio by lifetime’s dependence on both carrier concentration and temperature. The doped films were either copper- or gold-doped
with the carrier concentration ranging from 2 x 10 15 to 1.5 x 10 17 cm -3, and the lifetime varied from 2 μs to 8 ns. The undoped (Hg-vacancy) films had a carrier concentration range between 3 x
10 15 and 8 x 10 16 cm -3, and the lifetime changed from 150 to 3 ns. It was found that for the same carrier concentration, the doped films had lifetimes
several times longer than those of the undoped films, limited mostly by Auger 7 and radiative recombination processes. The
ineffectiveness of Shockley-Read-Hall (SRH) recombination process in the doped films was also demonstrated in lifetime vs
temperature curves. The important ratio of intrinsic Auger 7 lifetime to intrinsic Auger 1 lifetime, τ A7
i/τ A1
i, was determined to be about 20 from fitting both concentration and temperature curves. The reduction of minority carrier
lifetime in undoped films can be explained by an effective SRH recombination center associated with the Hg vacancy. Indeed,
a donor-like SRH recombination center located at midgap (E v+60 meV) with a capture cross section for minority carriers much larger than that for majority carriers was deduced from fitting
lifetime vs temperature curves of undoped films. 相似文献
6.
Measurements of the dislocation density are compared with locally resolved measurements of carrier lifetime for p‐type multicrystalline silicon. A correlation between dislocation density and carrier recombination was found: high carrier lifetimes (>100 µs) were only measured in areas with low dislocation density (<10 5 cm −2), in areas of high dislocation density (>10 6 cm −2) relatively low lifetimes (<20 µs) were observed. In order to remove mobile impurities from the silicon, a phosphorus diffusion gettering process was applied. An increase of the carrier lifetime by about a factor of three was observed in lowly dislocated regions whereas in highly dislocated areas no gettering efficiency was observed. To test the effectiveness of the gettering in a solar cell manufacturing process, five different multicrystalline silicon materials from four manufacturers were phosphorus gettered. Base resistivity varied between 0·5 and 5 Ω cm for the boron‐ and gallium‐doped p‐type wafers which were used in this study. The high‐efficiency solar cell structure, which has led to the highest conversion efficiencies of multicrystalline silicon solar cells to date, was used to fabricate numerous solar cells with aperture areas of 1 and 4 cm 2. Efficiencies in the 20% range were achieved for all materials with an average value of 18%. Best efficiencies for 1 cm 2 (20·3%) and 4 cm 2 (19·8%) cells were achieved on 0·6 and 1·5 Ω cm, respectively. This proves that multicrystalline silicon of very different material specification can yield very high efficiencies if an appropriate cell process is applied. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
7.
Results are presented for minority carrier lifetime in n-type molecular beam epitaxy Hg 1−xCd xTe with x ranging from 0.2 to 0.6. It was found that the lifetime was unintentionally degraded by post-growth annealing under
Hg saturated conditions in a H 2 atmosphere that was both time and temperature dependent. This effect was minimal or non-existent for x∼0.2 material, but
very strong for x ≥ 0.3. Hydrogen was identified as responsible for this degradation. Identical annealing in a He atmosphere
avoids this degradation and results in neartheoretical lifetime values for carrier concentrations as low 1 × 10 15 cm −3 in ≥0.3 material. Modeling was carried out for x∼0.2 and x∼0.4 material that shows the extent to which lifetime is reduced
by Shockley-Real-Hall recombination for carrier concentrations below 1 × 10 15 cm −3, as well as for layers annealed in H 2. It appears that annealing in H 2 results in a deep recombination center in wider bandgap HgCdTe that lowers the lifetime without affecting the majority carrier
concentration and mobility. 相似文献
8.
Iodine-doped CdMgTe/CdSeTe double heterostructures (DHs) have been grown by molecular beam epitaxy and studied using time-resolved photoluminescence (PL), focusing on absorber layer thickness of 2 μm. The n-type free carrier concentration was varied to ~7 × 10 15 cm ?3, 8.4 × 10 16 cm ?3, and 8.4 × 10 17 cm ?3 using iodine as dopant in DHs. Optical injection at 1 × 10 10 photons/pulse/cm 2 to 3 × 10 11 photons/pulse/cm 2, corresponding to initial injection of photocarriers up to ~8 × 10 15 cm ?3, was applied to examine the effects of excess carrier concentration on the PL lifetimes. Iodine-doped DHs exhibited an initial rapid decay followed by a slower decay at free carrier concentration of 7 × 10 15 cm ?3 and 8.4 × 10 16 cm ?3. The optical injection dependence of the carrier lifetimes for DHs was interpreted based on the Shockley–Read–Hall model. The observed decrease in lifetime with increasing n is consistent with growing importance of radiative recombination. 相似文献
9.
The epitaxial growth of HgCdTe on alternative substrates has emerged as an enabling technology for the fabrication of large-area
infrared (IR) focal plane arrays (FPAs). One key technical issue is high dislocation densities in HgCdTe epilayers grown on
alternative substrates. This is particularly important with regards to the growth of HgCdTe on heteroepitaxial Si-based substrates,
which have a higher dislocation density than the bulk CdZnTe substrates typically used for epitaxial HgCdTe material growth.
In the paper a simple model of dislocations as cylindrical regions confined by surfaces with definite surface recombination
is proposed. Both radius of dislocations and its surface recombination velocity are determined by comparison of theoretical
predictions with carrier lifetime experimental data described by other authors. It is observed that the carrier lifetime depends
strongly on recombination velocity; whereas the dependence of the carrier lifetime on dislocation core radius is weaker. The
minority carrier lifetime is approximately inversely proportional to the dislocation density for densities higher than 10 5 cm −2. Below this value, the minority carrier lifetime does not change with dislocation density. The influence of dislocation density
on the R 0A product of long wavelength infrared (LWIR) HgCdTe photodiodes is also discussed. It is also shown that parameters of dislocations
have a strong effect on the R 0A product at temperature around 77 K in the range of dislocation density above 10 6 cm −2. The quantum efficiency is not a strong function of dislocation density. 相似文献
10.
Sb 2Te 3 and Bi 2Te 3 thin films were grown on SiO 2 and BaF 2 substrates at room temperature using molecular beam epitaxy. Metallic layers with thicknesses of 0.2?nm were alternately deposited at room temperature, and the films were subsequently annealed at 250°C for 2?h. x-Ray diffraction and energy-filtered transmission electron microscopy (TEM) combined with high-accuracy energy-dispersive x-ray spectrometry revealed stoichiometric films, grain sizes of less than 500?nm, and a texture. High-quality in-plane thermoelectric properties were obtained for Sb 2Te 3 films at room temperature, i.e., low charge carrier density (2.6?×?10 19?cm ?3), large thermopower (130? ??V?K ?1), large charge carrier mobility (402?cm 2?V ?1?s ?1), and resulting large power factor (29? ??W?cm ?1?K ?2). Bi 2Te 3 films also showed low charge carrier density (2.7?×?10 19?cm ?3), moderate thermopower (?153? ??V?K ?1), but very low charge carrier mobility (80?cm 2?V ?1?s ?1), yielding low power factor (8? ??W?cm ?1?K ?2). The low mobilities were attributed to Bi-rich grain boundary phases identified by analytical energy-filtered TEM. 相似文献
11.
Rapid thermal processing of semiconductors involves significant photonic and subsequent thermal excitation. In the past, photonic excitation during rapid thermal annealing had been speculated to lead to significant enhancement of dopant diffusion or activation. In this work we present some experimental results indicating the absence of any such enhancement at high temperatures (1000–1050°C) which most often are employed during the metal-oxide–semiconductor device processing. The implanted dopant (boron, arsenic or phosphorus) movement in silicon during different rapid thermal annealing conditions was studied using secondary ion mass spectroscopy (SIMS) technique. To understand the effect of point defects in controlling the diffusion process, the concentrations of charged and neutral point defects were calculated as a function of carrier concentration using previously published defect-carrier relations. The dependence of free carrier concentration on lattice perturbation parameters such as impurities and temperature was formulated and used in calculating carrier lifetimes ( τ) in silicon. We qualitatively analyze two competing reactions, (i) the phonon release at the defect sites and (ii) the Auger electron process due to many electron interactions, to explain the apparent absence of any enhanced dopant diffusion. In our analyses, we obtain a highest free carrier lifetime of about 442 ns in the case of low dose (1e13/cm 2) implanted sample during the transient stage (700°C) of the dopant activation cycle. The corresponding smallest (17 fs) free carrier lifetime was obtained for the high dose implanted sample (dopants already activated) at 1000°C, the steady state part of an extended anneal cycle. Based on the detailed free carrier lifetime analyses, we suggest that any enhanced dopant activation or diffusion, at the best, may occur only at very low temperatures in the samples implanted with low doses of dopant atoms. 相似文献
12.
High purity GaAs buffer layers of carrier concentration in the low (l-5)×l0 l4/cm 3 range with 77K electron mobility over 100,000 cm 2/V-sec and 300K mobility around 8000 cm?/ V-sec have been grown by liquid phase epitaxy on Cr-doped GaAs substrates using the graphite sliding boat method. The high purity has been achieved with systematic and concurrent long term bake-outs (24 hrs) of both LPE melt and substrate, both exposed to the H 2 ambient gas stream at 775?C, prior to epitaxial growth at 700?C. Substrate surface degradation was reduced by using Ga:GaAs etch melts that were undersaturated at 700?C by 5? to 40?C. Best buffer layer morphologies with regard to surface planarity were obtained using etch melts that were saturated by near 85% of weight of GaAs at 700°C. The importance of substrate preconditioning in order to achieve the low ( 1 -2)×l0 14 was examined and found to be critical. Melt and substrate bake outs at 800?C, and use of a 40?C undersaturated etch melt prior to epitaxial growth at 800?C resulted in a p-type layer of carrier concentration, 1 .9×l0 l2/cm 3 and resistivity 1×10 5 ohm-cm. Chromium doping at 700?C resulted in buffer layers with sheet resistivities greater than 10 ohms/sq and low pinhole densities. 相似文献
13.
Extracting energy from low vibration frequencies (<10 Hz) using piezoelectric energy harvester promises continuous self‐powering for sensors and wearables. The piezoelectric compliant mechanism (PCM) design provides a significantly higher efficiency by fostering a uniform strain for its 1st mode shape, and so is interesting for this application. In this paper, a PCM energy harvester with bimorph Pb(Zr,Ti)O 3 (PZT) films on Ni foil deposited by rf magnetron sputtering is shown to have high efficiency and large power for low frequency mechanical vibration. In particular, {001} textured PZT films are deposited on both sides of polished Ni foils with (100) oriented LaNiO 3 seed layers on HfO 2 buffer layers. The performance of PCM with an active area of 5.2 cm 2 is explored for various excitation accelerations (0.02–0.16 g [g = 9.8 m s ?2]) around 6 Hz. The PCM device provides a power level of 3.9 mW cm ?2 g 2 and 65% mode shape efficiencies. 相似文献
14.
In recent years, hybrid perovskite solar cells (HPSCs) have received considerable research attention due to their impressive photovoltaic performance and low‐temperature solution processing capability. However, there remain challenges related to defect passivation and enhancing the charge carrier dynamics of the perovskites, to further increase the power conversion efficiency of HPSCs. In this work, the use of a novel material, phenylhydrazinium iodide (PHAI), as an additive in MAPbI 3 perovskite for defect minimization and enhancement of the charge carrier dynamics of inverted HPSCs is reported. Incorporation of the PHAI in perovskite precursor solution facilitates controlled crystallization, higher carrier lifetime, as well as less recombination. In addition, PHAI additive treated HPSCs exhibit lower density of filled trap states (10 10 cm ?2) in perovskite grain boundaries, higher charge carrier mobility (≈11 × 10 ?4 cm 2 V ?1 s), and enhanced power conversion efficiency (≈18%) that corresponds to a ≈20% improvement in comparison to the pristine devices. 相似文献
15.
The relationship between the structural quality of low-temperature GaAs layers and the photoexcited carrier lifetime has been
studied. Transmission electron microscopy, x-ray rocking curves, time-resolved reflectance methods, and photoconductive-switch-response
measurements were used for this study. For a variety of samples grown at temperatures in the vicinity of 200°C, subpicosecond
carrier lifetimes were observed both in as-grown layers, as well as in the same layers after post-annealing and formation
of As precipitates. These results suggest that the carrier lifetime, which was found to be shorter in the as-grown layers
than in the annealed ones, might be related to the density of As Ga antisite defects present in the layers. The annealed layers which contained structural defects before annealing appeared
to exhibit the longest carrier lifetime due to gettering of As on these defects (and formation of relatively large As precipitates)
and depletion of extra As (As Ga) defects from the layer. It was found as well that the responsivity of detectors fabricated on these layers depended strongly
on the structural quality of the layers, with the greatest response obtained not for the layers with the fewest defects, but
for the layers with 10 7–10 8/cm 2 of pyramidal defects. 相似文献
16.
The effect of thermal and radiation defects on the minority-charge-carrier recombination in the base region of a diffused silicon p- n structures with doping levels of 10 14–10 18 cm −3 was studied. The parameters of thermal defects responsible for the change of the carrier lifetime in Si following its thermal treatment during the production of p- n structures are determined. The effective minority-charge-carrier trapping is observed in heavily-doped structures at T ⩽ 100 K. The dependence of the coefficient of the radiation-induced change in the carrier lifetime on the base-region doping level was found. Using this data and the results obtained by the capacitance spectroscopy technique the analysis of recombination properties of defects has been done. At carrier concentrations n0, p0 ⪢ 10 16 cm −3 the coefficient of the radiation-induced change in the carrier lifetime ( Kτ) is shown to be determined by the introduction of the E-center in n-Si and the defect level Ec − 0.27 eV in p-Si. At a low excitation level in heavily-doped p- n structures a significant decrease in Kτ is observed at T = 78 K as compared to the value at 300 K. 相似文献
17.
Si:(Er,O)-based tunnel light-emitting diodes were fabricated and exhibited the shortest ever recorded characteristic rise
time for erbium electroluminescence. This is due to the formation of Er-related centers with an effective excitation cross
section for erbium ions of ∼7×10 −16 cm 2 and an excited-state lifetime of ∼17 μs. The lifetime of the first excited state of erbium ions after turning off the reverse
current was measured for the first time; this lifetime is associated with Auger energy transfer to free electrons in the electrically
neutral region of the diode.
__________
Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 34, No. 8, 2000, pp. 965–969.
Original Russian Text Copyright ? 2000 by Emel’yanov, Sobolev, Trishenkov, Khakuashev. 相似文献
18.
This paper reports the effects of electron cyclotron resonance (ECR)H 2 plasma hydrogenation on the characteristics of HgCdTe devices for the first time. We compared the characteristics of photodiodes
and n-channel enhancement type field effect transistors (FETs) in the hydrogenated regions with those in the unhydrogenated
regions on the same wafer. From the measurement of the photodiodes, it was found that the steady-state minority carrier diffusion
length was increased from 19 to 28 urn by the hydrogenation. The surface mobility of the n-channel FET was about 5800 cm 2Vs and was not varied by hydrogénation. From these facts, the steady-state minority carrier lifetime is increased about two
times by the ECR H 2 plasma hydrogenation. We believe that the ECR hydrogenation can effectively reduce the surface trap-states which results
in increasing the minority carrier lifetime and improving the characteristics of HgCdTe devices. 相似文献
19.
Here, we present the performance of a thermoelectric (TE) module consisting of n-type (La0.12Sr0.88)0.95TiO3 and p-type Ca3Co4?xO9+δ materials. The main challenge in this investigation was operating the TE module in different atmospheric conditions, since n-type has optimum TE performance at reducing conditions, while p-type has optimum at oxidizing conditions. The TE module was exposed to two different atmospheres and demonstrated higher stability in N2 atmosphere than in air. The maximum electrical power output decreased after 40 h when the hot side was exposed to N2 at 600°C, while only 1 h at 400°C in ambient air was enough to oxidize (La0.12Sr0.88)0.95TiO3 followed by a reduced electrical power output. The module generated maximum electrical power of 0.9 mW (~?4.7 mW/cm2) at 600°C hot side and δT?~?570 K in N2, and 0.15 mW (~?0.8 mW/cm2) at 400°C hot side and δT?~?370 K in air. A stability limit of Ca3Co3.93O9+δ at ~?700°C in N2 was determined by in situ high-temperature x-ray diffraction. 相似文献
20.
The basic mechanisms related to the photoexcitation of electron-hole pairs in semiconductors under conditions of low excitation density, low temperature and high crystal purity are reviewed. The use of high-resolution emission spectroscopy of band-to-impurity optical transitions in GaAs to measure the energy distribution functions of electrons and holes in optically excited carrier plasmas of well defined densities (10 10 cm ?3≤ n≤ 10 13 cm ?3) is described. With this experimental method (i) the energy relaxation of initially hot carrier distributions after pulsed photoexcitation ( ), (ii) stationary non-equilibrium distributions of electrons in the conduction band under cw photoexcitation ( ) and (iii) the transport properties of resonantly excited carrier plasmas in low electric fields ( ) are investigated. The observed distribution functions are compared with theoretical results on the basis of the known band structure data of GaAs, taking into account polar optic and acoustic phonon scattering, the interaction among the carriers, ionized impurity scattering, and using approximate solutions of the appropriate transport equation. 相似文献
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