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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

On the reliability of pulse power saturation models for broad-area GaAs-based lasers

With short current pulses, GaAs-based lasers can achieve high output powers if self-heating and catastrophic optical damage are suppressed. However, the pulse power is still severely limited by internal saturation mechanisms. Over the past decade, various power loss mechanisms have been identified by numerical laser simulation but published conclusions differ even for the same laser diode. We here investigate the reliability of such simulations and find that the error range remains relatively small if all saturation mechanisms are considered simultaneously in a self-consistent model, including a realistic hole mobility. Accurate pulse power predictions are demonstrated by simulating measurements on two different laser structures without making material parameter adjustments.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1993

Charge-carrier mobilities in Cd(0.8)Zn(0.2)Te single crystals used as nuclear radiation detectors

Charge-carrier mobilities were measured for the first time in Cd(0.8)Zn(0.2)Te single crystals using time-of-flight measurements of charge carriers produced by short (10 ns) light pulses from a frequency-doubled Nd:YAG laser (532 nm). The electron mobility displayed a T exp -1.1 dependence on the absolute temperature T in the range 200-320 K, with a room-temperature mobility of 1350 sq cm/V s. The hole mobility displayed a T exp -2.0 dependence in the same temperature range, with a room-temperature mobility of 120 sq cm/V s. Cd(0.8)Zn(0.2)Te appears to be a very favorable material for a room-temperature electronic nuclear radiation detector.

Burshtein, Z.↗
DOE OSTI2014

A charge transport study in diamond, surface passivated by high-k dielectric oxides

The recent progress in the growth of high-quality single-crystalline diamond films has sparked interest in the realization of efficient diamond power electronic devices. However, finding a suitable passivation is essential to improve the reliability and electrical performance of devices. In the current work, high-k dielectric materials such as aluminum oxide and hafnium oxide were deposited by atomic layer deposition on intrinsic diamond as a surface passivation layer. The hole transport properties in the diamond films were evaluated and compared to unpassivated films using the lateral time-of-flight technique. An enhancement of the near surface hole mobility in diamond films of up to 27% is observed when using aluminum oxide passivation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Carrier transport properties of Be-doped InAs/InAsSb type-II infrared superlattices

The InAs/InAsSb type-II superlattice materials studied to date for infrared detector applications have been residually n-type, but p-type absorber regions with minority carrier electrons can result in increased photodiode quantum efficiency, R{sub o}A, and detectivity. Therefore, Be-doped InAs/InAsSb superlattices were investigated to determine the p-type InAs/InAsSb superlattice material transport properties essential to developing high quality photodiode absorber materials. Hall measurements performed at 10 K revealed that the superlattice converted to p-type with Be-doping of 3 × 10{sup 16} cm{sup −3} and the hole mobility reached 24 400 cm{sup 2}/Vs. Photoresponse measurements at 10 K confirmed the 175 meV bandgap and material optical quality.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Effects of sodium on electrical properties in Cu{sub 2}ZnSnS{sub 4} single crystal

We have studied the effect of sodium on the electrical properties of Cu{sub 2}ZnSnS{sub 4} (CZTS) single crystal by using temperature dependence of Hall effect measurement. The sodium substitution on the cation site in CZTS is observed from the increasing of unit-cell size by powder X-ray diffraction. Sodium increases the effective hole concentration and makes the thermal activation energy smaller. The degree of compensation decreases with sodium incorporation, thus the hole mobility is enhanced. We revealed that sodium is important dopant in CZTS to control the electrical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Characterization of N-doped multilayer graphene grown on 4H-SiC (0001)

Large-area graphene film doped with hetero-atoms is of great interest for a wide spectrum of nanoelectronics applications, such as field effect devices, super capacitors, fuel cells among many others. Here, we report the structural and electronic properties of nitrogen doped multilayer graphene on 4H-SiC (0001). The incorporation of nitrogen during the growth causes an increase in the D band on the Raman signature indicating that the nitrogen is creating defects. The analysis of micro-Raman mapping of G, D, 2D bands shows a predominantly trilayer graphene with a D band inherent to doping and inhomogeneous dopant distribution at the step edges. Ultraviolet photoelectron spectroscopy (UPS) indicates an n type work function (WF) of 4.1 eV. In addition, a top gate FET device was fabricated showing n-type I-V characteristic after the desorption of oxygen with high electron and holes mobilities.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Energy band alignment of atomic layer deposited HfO{sub 2} on epitaxial (110)Ge grown by molecular beam epitaxy

The band alignment properties of atomic layer HfO{sub 2} film deposited on epitaxial (110)Ge, grown by molecular beam epitaxy, was investigated using x-ray photoelectron spectroscopy. The cross-sectional transmission electron microscopy exhibited a sharp interface between the (110)Ge epilayer and the HfO{sub 2} film. The measured valence band offset value of HfO{sub 2} relative to (110)Ge was 2.28 {+-} 0.05 eV. The extracted conduction band offset value was 2.66 {+-} 0.1 eV using the bandgaps of HfO{sub 2} of 5.61 eV and Ge bandgap of 0.67 eV. These band offset parameters and the interface chemical properties of HfO{sub 2}/(110)Ge system are of tremendous importance for the design of future high hole mobility and low-power Ge-based metal-oxide transistor devices.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Mg dopant in Cu{sub 2}ZnSnSe{sub 4}: An n-type former and a promoter of electrical mobility up to 120 cm{sup 2} V{sup −1} s{sup −1}

Mg-doped Cu{sub 2}ZnSnSe{sub 4} (CZTSe) bulk materials with the (Cu{sub 2−x}Mg{sub x})ZnSnSe{sub 4} formula at x=0, 0.1, 0.2, 0.3, and 0.4 were prepared at 600 °C for 2 h with soluble sintering aids of Sb{sub 2}S{sub 3} and Te. Defect chemistry was studied by measuring structural and electrical properties of Mg-doped CZTSe as a function of dopant concentration. Except at x=0, all Mg-doped CZTSe pellets showed an n-type behavior. The Mg-doped CZTSe pellets showed an n-type behavior. n-Type Mg-CZTSe pellets at x=0.1 showed the highest electrical conductivity of 24.6 S cm{sup −1} and the net hole mobility of 120 cm{sup 2} V{sup −1} s{sup −1}, while they were 11.8 S cm{sup −1} and 36.5 cm{sup 2} V{sup −1} s{sup −1} for the undoped p-type CZTSe. Mg dopant is a strong promoter of electrical mobility. Mg dopant behaves as a donor defect in CZTSe at a 5% doping content, but is also used as an acceptor at a high content above 5%. Mg doping has further developed CZTSe into a promising semiconductor. - Graphical abstract: The effects of extrinsic doping of Mg{sup 2+} on the electrical properties of Cu{sub 2}ZnSnSe{sub 4} bulks. - Highlights: • (Cu{sub 2−x}Mg{sub x})ZnSnSe{sub 4} bulks were fabricated by liquid-phase sintering at 600 °C. • All Mg-x-CZTSe pellets except at x=0 exhibited n-type conductivity. • Electrical properties of CZTSe pellets changed with the Cu and Mg ratios. • Mg{sup 2+} goes to the Cu{sup 1+} site to form the Mg{sub Cu}{sup 1+} donor defect for the n-type CZTSe. • n-Type Mg-0.1-CZTSe bulk with 5% Mg showed the highest mobility of 120 cm{sup 2} V{sup −1} s{sup −1}.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Positive Charge in SOS Heterostructures with Interlayer Silicon Oxide

The continuous transfer of (001)Si layers 0.2–1.7 μm thick by implanted hydrogen to the c-sapphire surface during direct bonding at high temperatures of 300–500°C is demonstrated for the first time. The formation of an intermediate silicon-oxide layer SiO{sub x} during subsequent heat treatments at 800–1100°C, whose increase in thickness (up to 3 nm) correlates with an increase in the positive charge Q{sub i} at the heterointerface to ~1.5 × 10{sup 12} cm{sup –2} in contrast to the negative charge at the SiO{sub x}/Al{sub 2}O{sub 3} ALD heterointerface. During silicon-layer transfer to sapphire with a thermal silicon-dioxide layer, Q{sub i} decreases by more than an order of magnitude to 5 × 10{sup 10} cm{sup –2} with an increase in the SiO{sub 2} thickness from 50 to 400 nm, while the electron and hole mobilities barely differ from the values in bulk silicon. Based on these results, a qualitative model of the formation of positively charged oxygen vacancies in a 5-nm sapphire layer near the bonding interface is proposed.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Low-voltage polymer/small-molecule blend organic thin-film transistors and circuits fabricated via spray deposition

Organic thin-film electronics have long been considered an enticing candidate in achieving high-throughput manufacturing of low-power ubiquitous electronics. However, to achieve this goal, more work is required to reduce operating voltages and develop suitable mass-manufacture techniques. Here, we demonstrate low-voltage spray-cast organic thin-film transistors based on a semiconductor blend of 2,8-difluoro- 5,11-bis (triethylsilylethynyl) anthradithiophene and poly(triarylamine). Both semiconductor and dielectric films are deposited via successive spray deposition in ambient conditions (air with 40%–60% relative humidity) without any special precautions. Despite the simplicity of the deposition method, p-channel transistors with hole mobilities of >1 cm{sup 2}/Vs are realized at −4 V operation, and unipolar inverters operating at −6 V are demonstrated.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Semiconductor- to metallic-like behavior in Bi thin films on KCl substrate

Bi thin films, with a thickness of 100 nm, are deposited by electron-beam evaporation on a freshly cleaved (100) KCl substrate. The substrate temperature during film growth (T{sub dep}) ranges from room temperature up to 170 °C. Films deposited at room temperature exhibit a maze-like microstructure typical of the rhombohedral (110) texture, as confirmed by X-ray diffraction. For T{sub dep} above 80 °C, a different microstructure appears, characterized by concentric triangular shapes corresponding to the trigonal (111) texture. Temperature dependence of the resistivity shows a transition from a semiconductor-like behavior for films deposited at room temperature to a metallic-like behavior for T{sub dep} above 80 °C. From magnetoresistance measurements between room temperature and 1.6 K, we extract the electron and hole mobilities, concentrations, and mean free paths, which allow to draw a complete picture of the transport properties of both types of films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

A polarity-driven nanometric luminescence asymmetry in AlN/GaN heterostructures

Group III Nitrides nanowires are well suited materials for the design of light emitting devices. The internal electric field created by spontaneaous and piezoelectric polarizations in these materials poses some difficulties, but also possible solutions, towards this goal. Here, we report on the high spatial asymmetry of the cathodoluminescence intensity across a GaN quantum well embedded in an AlN nanowire, when a 60 keV, 1 nm wide electron beam is scanned over this heterostructure. This asymmetry is remarkable between positions at different sides of the quantum well. We interpret this asymmetry as originating from the different drift directions of carriers due to the internal electric field. This interpretation is corroborated by the direct determination of the polarity with convergent beam electron diffraction. A precise knowledge of hole mobility and diffusion coefficients would allow an estimate of the electric field in the AlN segment of the nanowire.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2012

Alkyl Surface Treatments of Planar Zinc Oxide in Hybrid Organic/Inorganic Solar Cells

Hybrid organic/inorganic solar cells have not lived up to their potential because of poor interface properties. Interfacial molecular layers provide a way of adjusting these devices to improve their performance. We have studied a prototypical system involving poly(3-hexylthiophene) (P3HT) on planar zinc oxide (ZnO) films that have been modified with two types of molecules having identical 18-carbon alkyl chain termination and different surface attachments: octadecanethiol (ODT) and octadecyltriethoxysilane (OTES). We examined the functionalized surfaces using water contact angle measurements, Kelvin probe measurements, infrared absorbance spectroscopy, and atomic force microscopy. These have shown that OTES forms disordered incomplete monolayers, while ODT is prone to develop multilayered islands. Both treatments enhance polymer ordering. However, inverted solar cell devices fabricated with these treated interfaces performed very differently. ODT improves the short circuit current (J{sub SC}), open circuit voltage (V{sub OC}), and power conversion efficiency ({eta}), while these parameters all decrease in devices constructed from OTES-treated ZnO. The differences in V{sub OC} are related to modifications of the surface dipole associated with deposition of the two types of alkyl molecules, while changes in J{sub SC} are attributed to a balance between charge transfer blocking caused by the saturated hydrocarbon and the improved hole mobility in the polymer.

14 SOLAR ENERGY↗
DOE OSTI2015

Ultrasensitive organic phototransistors with multispectral response based on thin-film/single-crystal bilayer structures

We report on highly efficient organic phototransistors (OPTs) based on thin-film/single-crystal planar bilayer junctions between 5,6,11,12-tetraphenyltetracene (rubrene) and [6,6]-phenyl-C{sub 61}-butyric acid methyl ester (PC{sub 61}BM). The OPTs show good field-effect characteristics in the dark, with high hole-mobility (4–5 cm{sup 2} V{sup −1} s{sup −1}), low-contact resistance (20 kΩ cm), and low-operating voltage (≤5 V). Excellent sensing capabilities allow for light detection in the 400–750 nm range, with photocurrent/dark current ratio as high as 4 × 10{sup 4}, responsivity on the order of 20 AW{sup −1} at 27 μW cm{sup −2}, and an external quantum efficiency of 52 000%. Photocurrent generation is attributed to enhanced electron and hole transfer at the interface between rubrene and PC{sub 61}BM, and fast response times are observed as a consequence of the high-mobility of the interfaces. The optoelectronic properties exhibited in these OPTs outperform those typically provided by a-Si based devices, enabling future applications where multifunctionality in a single-device is sought.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Optical and carrier transport properties of graphene oxide based crystalline-Si/organic Schottky junction solar cells

We investigated the graphene oxide (GO) based n-type crystalline silicon (c-Si)/conductive poly(ethylene dioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) Schottky junction devices with optical characterization and carrier transport measurement techniques. The optical transmittance in the UV region decreased markedly for the films with increasing the concentration of GO whereas it increased markedly in the visible-infrared regions. Spectroscopic ellipsometry revealed that the ordinary and extraordinary index of refraction increased with increasing the concentration of GO. The hole mobility also increased from 1.14 for pristine film to 1.85 cm{sup 2}/V s for the 12–15 wt. % GO modified film with no significant increases of carrier concentration. The highest conductivity was found for a 15 wt. % GO modified PEDOT:PSS film: the c-Si/PEDOT:PSS:GO device using this sample exhibited a relatively high power conversion efficiency of 11.04%. In addition, the insertion of a 2–3 nm-thick GO thin layer at the c-Si/PEDOT:PSS interface suppressed the carrier recombination efficiency of dark electron and photo-generated hole at the anode, resulting in the increased photovoltaic performance. This study indicates that the GO can be good candidates for hole transporting layer of c-Si/PEDOT:PSS Schottky junction solar cell.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Growth and characterization of insulating ferromagnetic semiconductor (Al,Fe)Sb

We investigate the crystal structure, transport, and magnetic properties of Fe-doped ferromagnetic semiconductor (Al{sub 1−x},Fe{sub x})Sb thin films up to x = 14% grown by molecular beam epitaxy. All the samples show p-type conduction at room temperature and insulating behavior at low temperature. The (Al{sub 1−x},Fe{sub x})Sb thin films with x ≤ 10% maintain the zinc blende crystal structure of the host material AlSb. The (Al{sub 1−x},Fe{sub x})Sb thin film with x = 10% shows intrinsic ferromagnetism with a Curie temperature (T{sub C}) of 40 K. In the (Al{sub 1−x},Fe{sub x})Sb thin film with x = 14%, a sudden drop of the hole mobility and T{sub C} was observed, which may be due to the microscopic phase separation. The observation of ferromagnetism in (Al,Fe)Sb paves the way to realize a spin-filtering tunnel barrier that is compatible with well-established III-V semiconductor devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

First-principles investigation of metal-doped cubic BaTiO{sub 3}

Highlights: • Electronic properties of metal-doped cubic BaTiO{sub 3} are studied by DFT calculations. • Most transition-metal doping can effectively reduce the band gap of BaTiO{sub 3}. • The effective mass can be reduced by doping BaTiO{sub 3} with metals. • Substituting Ti with Fe or Mo could improve the photoelectric properties of BaTiO{sub 3}. - Abstract: The electronic properties of metal-doped cubic BaTiO{sub 3} have been investigated using first-principles calculations. Through systematic analysis on effects of various metals doping through substituting Ti with the main group, 3d and 4d transitions metal atoms, we show that most transition metal doping can effectively reduce the band gap of BaTiO{sub 3}. The partial density of states analysis indicates that different doping elements have different effects on the band-gap engineering. The effective mass of BaTiO{sub 3} before and after doping was further analyzed. And we found that the hole effective mass apparently decrease after Fe, Cu, Zn, Mo, and Ag doping, which could lead to increase of hole mobility and charge transfer enhancement. The calculation results showed that substituting Ti with Fe or Mo would be more effective for improving the photoelectric property of BaTiO{sub 3}. Our theoretical calculations provide useful insights and beneficial guidance into experimental study of BaTiO{sub 3} in the photoelectric applications.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Benzimidazole Based Hole‐Transporting Materials for High‐performance Inverted Perovskite Solar Cells

Abstract Interfaces play a decisive role in perovskite solar cells’ power conversion efficiency and their long‐term durability. Small‐molecule hole‐transporting materials (HTMs) have grabbed enormous attention due to their structural flexibility, material properties, and stabilities, allowing for improved operational durability in perovskite photovoltaics. This study synthesizes and investigates a new class of benzimidazole‐based small molecules, named YJS001 and YJS003 , serving as the HTMs to enable high‐efficiency mixed‐cation mixed‐halide perovskite solar cells. The benzimidazole‐based materials are dopant‐free HTMs composed of donor and acceptor building blocks that are designed to engineer the energy level alignment near the HTM/perovskite interface. Mixed‐cation mixed‐halide perovskites can be grown uniformly on both HTMs with large crystalline grains. It is discovered that the donor‐rich YJS003 ‐based solar cell exhibits a high open‐circuit voltage of 1.09 V with a champion power conversion efficiency of over 20%. Power‐dependent current–voltage characteristics of the solar cells are analyzed, from which the high performance of YJS003's excellent hole mobility and well‐aligned energy level is attributed. This work introduces a new class of benzimidazole‐based small molecules as HTMs, that paves the path for dopant free interface material development for commercialization of perovskite solar cells.

14 SOLAR ENERGY↗