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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Hydrogen passivation of electron trap in amorphous In-Ga-Zn-O thin-film transistors

We report an experimental evidence that some hydrogens passivate electron traps in an amorphous oxide semiconductor, a-In-Ga-Zn-O (a-IGZO). The a-IGZO thin-film transistors (TFTs) annealed at 300 °C exhibit good operation characteristics; while those annealed at ≥400 °C show deteriorated ones. Thermal desorption spectra (TDS) of H{sub 2}O indicate that this threshold annealing temperature corresponds to depletion of H{sub 2}O desorption from the a-IGZO layer. Hydrogen re-doping by wet oxygen annealing recovers the good TFT characteristic. The hydrogens responsible for this passivation have specific binding energies corresponding to the desorption temperatures of 300–430 °C. A plausible structural model is suggested.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Barrier height enhancement of Ni/GaN Schottky diode using Ru based passivation scheme

Wet chemical passivation of n-GaN surface using Ru based solution has been reported. X-ray photoelectron spectroscopy characterization of the GaN surface revealed removal of surface oxides by the introduction of Ru complex species. Ni/n-GaN Schottky barrier diodes were fabricated on passivated GaN and a remarkable improvement in Schottky barrier height from 0.76 eV to 0.92 eV was observed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Near surface inversion layer recombination in Al{sub 2}O{sub 3} passivated n-type silicon

On n-type silicon, negatively charged surface passivation layers create a near surface recombination channel, which could significantly reduce the effective carrier lifetime at low injection levels (Δn < 10{sup 14 }cm{sup −3}). This effect is described by Shockley Read Hall recombination at homogeneously distributed defects in the silicon wafer. In the near surface region, fixed charges in the dielectric layer significantly change the carrier concentrations and the recombination rate of defects. Sentaurus device simulations show that the contribution of the near surface recombination to the effective carrier lifetime depends on the properties of the involved defects. The lifetime reduction is strongest when the involved defects have an energy level in the lower half of the band gap and a very high electron to hole capture cross section ratio. For the simulation, a very low defect density in the order of 10{sup 8 }cm{sup −3} is assumed, which is a realistic value in highly pure float zone silicon. Quasi-steady state photoconductance measurements on n-type silicon with Al{sub 2}O{sub 3} passivation are done and fitted with the recombination model. Very good correlation between simulation and experiment is achieved when the involved recombination centers have an electron to hole capture cross section ratio of 10{sup 7} and an energy level of −0.2 eV w.r.t. the intrinsic level. The simulated defect properties are discussed in respect of transition metal and doping related defects reported in literature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Solution-processed amorphous silicon surface passivation layers

Amorphous silicon thin films, fabricated by thermal conversion of neopentasilane, were used to passivate crystalline silicon surfaces. The conversion is investigated using X-ray and constant-final-state-yield photoelectron spectroscopy, and minority charge carrier lifetime spectroscopy. Liquid processed amorphous silicon exhibits high Urbach energies from 90 to 120 meV and 200 meV lower optical band gaps than material prepared by plasma enhanced chemical vapor deposition. Applying a hydrogen plasma treatment, a minority charge carrier lifetime of 1.37 ms at an injection level of 10{sup 15}/cm{sup 3} enabling an implied open circuit voltage of 724 mV was achieved, demonstrating excellent silicon surface passivation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Impacts of SiN passivation on the degradation modes of AlGaN/GaN high electron mobility transistors under reverse-bias stress

Impacts of SiN passivation on the degradation modes of AlGaN/GaN high electron mobility transistors are investigated. The gate leakage current decreases significantly upon removing the SiN layer and no clear critical voltage for the sudden degradation of the gate leakage current can be observed in the reverse-bias step-stress experiments. Gate-lag measurements reveal the decrease of the fast-state surface traps and the increase of slow-state traps after the passivation layer removal. It is postulated that consistent surface charging relieves the electric field peak on the gate edge, thus the inverse piezoelectric effect is shielded.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Properties of the c-Si/Al{sub 2}O{sub 3} interface of ultrathin atomic layer deposited Al{sub 2}O{sub 3} layers capped by SiN{sub x} for c-Si surface passivation

This work presents a detailed study of c-Si/Al{sub 2}O{sub 3} interfaces of ultrathin Al{sub 2}O{sub 3} layers deposited with atomic layer deposition (ALD), and capped with SiN{sub x} layers deposited with plasma-enhanced chemical vapor deposition. A special focus was the characterization of the fixed charge density of these dielectric stacks and the interface defect density as a function of the Al{sub 2}O{sub 3} layer thickness for different ALD Al{sub 2}O{sub 3} deposition processes (plasma-assisted ALD and thermal ALD) and different thermal post-deposition treatments. Based on theoretical calculations with the extended Shockley–Read–Hall model for surface recombination, these interface properties were found to explain well the experimentally determined surface recombination. Thus, these interface properties provide fundamental insights into to the passivation mechanisms of these Al{sub 2}O{sub 3}/SiN{sub x} stacks, a stack system highly relevant, particularly for high efficiency silicon solar cells. Based on these findings, it was also possible to improve the surface passivation quality of stacks with thermal ALD Al{sub 2}O{sub 3} by oxidizing the c-Si surface prior to the Al{sub 2}O{sub 3} deposition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Recombination and thin film properties of silicon nitride and amorphous silicon passivated c-Si following ammonia plasma exposure

Recombination at silicon nitride (SiN{sub x}) and amorphous silicon (a-Si) passivated crystalline silicon (c-Si) surfaces is shown to increase significantly following an ammonia (NH{sub 3}) plasma exposure at room temperature. The effect of plasma exposure on chemical structure, refractive index, permittivity, and electronic properties of the thin films is also investigated. It is found that the NH{sub 3} plasma exposure causes (i) an increase in the density of Si≡N{sub 3} groups in both SiN{sub x} and a-Si films, (ii) a reduction in refractive index and permittivity, (iii) an increase in the density of defects at the SiN{sub x}/c-Si interface, and (iv) a reduction in the density of positive charge in SiN{sub x}. The changes in recombination and thin film properties are likely due to an insertion of N–H radicals into the bulk of SiN{sub x} or a-Si. It is therefore important for device performance to minimize NH{sub 3} plasma exposure of SiN{sub x} or a-Si passivating films during subsequent fabrication steps.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Direct evidence of void passivation in Cu(InGa)(SSe){sub 2} absorber layers

We have investigated the charge collection condition around voids in copper indium gallium sulfur selenide (CIGSSe) solar cells fabricated by sputter and a sequential process of selenization/sulfurization. In this study, we found direct evidence of void passivation by using the junction electron beam induced current method, transmission electron microscopy, and energy dispersive X-ray spectroscopy. The high sulfur concentration at the void surface plays an important role in the performance enhancement of the device. The recombination around voids is effectively suppressed by field-assisted void passivation. Hence, the generated carriers are easily collected by the electrodes. Therefore, when the S/(S + Se) ratio at the void surface is over 8% at room temperature, the device performance degradation caused by the recombination at the voids is negligible at the CIGSSe layer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Tantalum oxide/silicon nitride: A negatively charged surface passivation stack for silicon solar cells

This letter reports effective passivation of crystalline silicon (c-Si) surfaces by thermal atomic layer deposited tantalum oxide (Ta{sub 2}O{sub 5}) underneath plasma enhanced chemical vapour deposited silicon nitride (SiN{sub x}). Cross-sectional transmission electron microscopy imaging shows an approximately 2 nm thick interfacial layer between Ta{sub 2}O{sub 5} and c-Si. Surface recombination velocities as low as 5.0 cm/s and 3.2 cm/s are attained on p-type 0.8 Ω·cm and n-type 1.0 Ω·cm c-Si wafers, respectively. Recombination current densities of 25 fA/cm{sup 2} and 68 fA/cm{sup 2} are measured on 150 Ω/sq boron-diffused p{sup +} and 120 Ω/sq phosphorus-diffused n{sup +} c-Si, respectively. Capacitance–voltage measurements reveal a negative fixed insulator charge density of −1.8 × 10{sup 12 }cm{sup −2} for the Ta{sub 2}O{sub 5} film and −1.0 × 10{sup 12 }cm{sup −2} for the Ta{sub 2}O{sub 5}/SiN{sub x} stack. The Ta{sub 2}O{sub 5}/SiN{sub x} stack is demonstrated to be an excellent candidate for surface passivation of high efficiency silicon solar cells.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Surface passivation of (100) GaSb using self-assembled monolayers of long-chain octadecanethiol

The passivation of (100) GaSb surface was investigated by means of the long-chain octadecanethiol (ODT) self-assembled monolayer (SAM). The properties of ODT SAM on (100) GaSb were characterized by the atomic force microscopy using Kelvin probe force microscopy mode and X-ray photoelectron spectroscopy. The chemical treatment of 10 mM ODT-C{sub 2}H{sub 5}OH has been applied to the passivation of a type-II superlattice InAs/GaSb photodetector. The electrical measurements indicate that the current density was reduced by one order of magnitude as compared to an unpassivated photodetector.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Ionic liquid gating on atomic layer deposition passivated GaN: Ultra-high electron density induced high drain current and low contact resistance

Herein, we report on achieving ultra-high electron density (exceeding 10{sup 14 }cm{sup −2}) in a GaN bulk material device by ionic liquid gating, through the application of atomic layer deposition (ALD) of Al{sub 2}O{sub 3} to passivate the GaN surface. Output characteristics demonstrate a maximum drain current of 1.47 A/mm, the highest reported among all bulk GaN field-effect transistors, with an on/off ratio of 10{sup 5} at room temperature. An ultra-high electron density exceeding 10{sup 14 }cm{sup −2} accumulated at the surface is confirmed via Hall-effect measurement and transfer length measurement. In addition to the ultra-high electron density, we also observe a reduction of the contact resistance due to the narrowing of the Schottky barrier width on the contacts. Taking advantage of the ALD surface passivation and ionic liquid gating technique, this work provides a route to study the field-effect and carrier transport properties of conventional semiconductors in unprecedented ultra-high charge density regions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Trapped charge densities in Al{sub 2}O{sub 3}-based silicon surface passivation layers

In Al{sub 2}O{sub 3}-based passivation layers, the formation of fixed charges and trap sites can be strongly influenced by small modifications in the stack layout. Fixed and trapped charge densities are characterized with capacitance voltage profiling and trap spectroscopy by charge injection and sensing, respectively. Al{sub 2}O{sub 3} layers are grown by atomic layer deposition with very thin (∼1 nm) SiO{sub 2} or HfO{sub 2} interlayers or interface layers. In SiO{sub 2}/Al{sub 2}O{sub 3} and HfO{sub 2}/Al{sub 2}O{sub 3} stacks, both fixed charges and trap sites are reduced by at least a factor of 5 compared with the value measured in pure Al{sub 2}O{sub 3}. In Al{sub 2}O{sub 3}/SiO{sub 2}/Al{sub 2}O{sub 3} or Al{sub 2}O{sub 3}/HfO{sub 2}/Al{sub 2}O{sub 3} stacks, very high total charge densities of up to 9 × 10{sup 12} cm{sup −2} are achieved. These charge densities are described as functions of electrical stress voltage, time, and the Al{sub 2}O{sub 3} layer thickness between silicon and the HfO{sub 2} or the SiO{sub 2} interlayer. Despite the strong variation of trap sites, all stacks reach very good effective carrier lifetimes of up to 8 and 20 ms on p- and n-type silicon substrates, respectively. Controlling the trap sites in Al{sub 2}O{sub 3} layers opens the possibility to engineer the field-effect passivation in the solar cells.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Enhanced thermoelectric efficiency in topological insulator Bi 2 Te 3 nanoplates via atomic layer deposition-based surface passivation

We report in-plane thermoelectric measurements of Bi 2 Te 3 nanoplates, a typical topological insulator with Dirac-like metallic surface states, grown by chemical vapor deposition. The as-grown flakes exposed to ambient conditions exhibit relatively small thermopowers around -34 μV/K due to unintentional surface doping (e.g., gas adsorption and surface oxidation). After removal of the unintentional surface doping and surface passivation by deposition of 30 nm of Al 2 O 3 using atomic layer deposition (ALD), the Seebeck coefficient of these flakes increases by a factor of 5× to -169 μV/K. We believe that the ALD-based surface passivation can prevent the degradation of the thermoelectric properties caused by gas adsorption and surface oxidation processes, thus, reducing the unintentional doping in the Bi 2 Te 3 and increasing the Seebeck coefficient. The high surface-to-volume ratio of these thin (~10 nm thick) nanoplates make them especially sensitive to surface doping, which is a common problem among nanomaterials in general. An increase in the sample resistance is also observed after the ALD process, which is consistent with the decrease in doping.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Hydrogen passivation of poly-Si/SiO x contacts for Si solar cells using Al 2 O 3 studied with deuterium

Here, the interplay between hydrogenation and passivation of poly-Si/SiO x contacts to n-type Si wafers is studied using atomic layer deposited Al 2 O 3 and anneals in forming gas and nitrogen. The poly-Si/SiO x stacks are prepared by thermal oxidation followed by thermal crystallization of a-Si:H films deposited by plasma-enhanced chemical vapor deposition. Implied open-circuit voltages as high as 710 mV are achieved for p-type poly-Si/SiO x contacts to n-type Si after hydrogenation. Correlating minority carrier lifetime data and secondary ion mass spectrometry profiles reveals that the main benefit of Al 2 O 3 is derived from its role as a hydrogen source for chemically passivating defects at SiO x ; Al 2 O 3 layers are found to hydrogenate poly-Si/SiO x much better than a forming gas anneal. By labelling Al 2 O 3 and the subsequent anneal with different hydrogen isotopes, it is found that Al 2 O 3 exchanges most of its hydrogen with the ambient upon annealing at 400 °C for 1 h even though there is no significant net change in its total hydrogen content.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Plasma charge injection technology and its application to c-Si solar cells for field-effect passivation

A plasma charge injection technology applicable for field-effect passivation in crystalline silicon solar cells is discussed. The technology uses an inert-gas plasma (helium, argon, N 2 , etc.) as a charge source and a DC bias to extract desired charges from a remote plasma source. A charging model is proposed, and it gives a good guideline to determine a proper charging operation condition for a desired injection charge density. The technology can introduce electric charges into a nitride–oxide or oxide–nitride–oxide passivation stack in a few hundred milliseconds with a charge density equivalent to or higher than that of A l 2 O 3 in a range of 5 × 10 12 to 8 × 10 12 cm -2 . Most-like charge injection mechanisms are discussed for both negative and positive charging. This technology uses a cheap inert-gas plasma which does not cause any parasitic film deposition nor any corrosion inside the chamber during the charging operation and, thus, does not require regular maintenance for chamber cleaning, which leads to a very low cost of ownership. This charging technology is promising for a low-cost alternative to the complex A l 2 O 3 technology. This plasma charge injection can be another important application of the plasma technology among other well-known applications such as dry etching, thin-film deposition, sputtering, etc.

14 SOLAR ENERGY↗
DOE OSTI2022

Non-Equilibrium Doping of Poly-Si:Ga/SiOx Passivating Contacts Solar Cells

In this contribution, we show that Ga-hyperdoped poly-Si/SiOx contacts are realized by pulsed laser annealing and post-hydrogenation. The motivation behind this work is to improve the inferior performance of boron-doped poly- Si/SiOx contacts when compared to the phosphorus-doped contacts. Here we used Ga as a novel p-type dopant to replace B, as B is known to accumulate at the interface between the oxide and the crystalline silicon and cause recombination. Due to a high diffusivity of Ga in oxide and a much higher segregation coefficient (Si:SiOx), Ga is shown to be immune to the oxide pileup phenomenon. However, Ga has a low solid solubility limit in Si (5x10^19 cm-3), which is not enough to form a low resistivity contact. To overcome this problem, we explored pulsed laser annealing via excimer laser by varying the laser energy densities and pulses. The passivation and electrical properties are studied systematically using photoluminescence (PL), secondary ion mass spectrometry (SIMS), and Van der Pauw-Hall measurements. We show an active Ga doping concentration above 1x10^20 cm^-3, which is above the solid solubility limit of Ga in Si. Good passivation quality is demonstrated with the highest implied Voc (iVoc) of 721 mV on single side polished samples. Furthermore, a low diode contact resistivity of ~22 m..omega.. cm^2 is obtained using Ti/Ag contacts, with minimal metal-to-semiconductor contact resistivity of 0.9 m..omega.. cm^2.

non-equilibrium doping↗
DOE OSTI2023

Design of passive and structural conductors for tokamaks using thin-wall eddy current modeling

Abstract A new three-dimensional electromagnetic modeling tool ( ThinCurr ) has been developed using the existing PSI-Tet finite-element code in support of conducting structure design work for both the SPARC and DIII-D tokamaks. Within this framework a 3D conducting structure model was created for both the SPARC and DIII-D tokamaks in the thin-wall limit. This model includes accurate details of the vacuum vessel and other conducting structural elements with realistic material resistivities. This model was leveraged to support the design of a passive runaway electron mitigation coil (REMC), studying the effect of various design parameters, including coil resistivity, current quench duration, and plasma vertical position, on the effectiveness of the coil. The REMC is a non-axisymmetric coil designed to passively drive large non-axisymmetric fields during the plasma disruption thereby destroying flux surfaces and deconfining RE seed populations. These studies indicate that current designs should apply substantial 3D fields at the plasma surface during future plasma current disruptions as well as highlight the importance of having the REMC conductors away from the machine midplane in order to ensure they are robust to off-normal disruption scenarios.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Defect passivation of perovskites in high efficiency solar cells

Abstract Metal halide perovskite solar cells (PSCs) have enormous potential as the next-generation photovoltaic technology. Being solution-processed at relatively low temperatures, it is inevitable that crystallographic defects are formed in abundance during fabrication. Such defects may cause undesirable energy losses by non-radiative recombination to limit the performance of PSCs. More importantly, it has become apparent that defect activity is fundamentally responsible for the operational instability issues hindering the commercialization readiness of PSCs. It is therefore necessary to develop strategies to minimize defect formation and to passivate formed defects. Here, we discuss recent advances on such defect mitigation and passivation strategies. We especially emphasize on methodologies that are incorporated into state-of-the-art PSCs that have demonstrated world record efficiencies and long-term stability.

14 SOLAR ENERGY↗