Showing papers in "Physica Status Solidi (a) in 2017"

Electronic Properties of Bulk and Monolayer TMDs: Theoretical Study Within DFT Framework (GVJ-2e Method)

Abstract: Accurate prediction of band gap for new emerging materials is highly desirable for the exploration of potential applications. The band gaps of bulk and monolayer TMDs (MoS2, MoSe2, WS2, and WSe2) are calculated with the recently proposed by us GVJ-2e method, which is implemented within DFT framework without adjustable parameters and is based on the total energies only. The calculated band gaps are in very good agreement with experimental ones for both bulk and monolayer TMDs. For monolayer MoS2, MoSe2, WS2, and WSe2, direct band gaps are predicted to be 1.88 eV, 1.57 eV, 2.03 eV, 1.67 eV correspondingly, and for bulk TMDs, indirect band gaps of 1.23 eV (MoS2), 1.09 eV (MoSe2), 1.32 eV (WS2), 1.21 eV (WSe2) are predicted. The GVJ-2e method demonstrates good accuracy with mean absolute error (MAE) of about 0.03 eV for TMDs PL gaps (and 0.06 eV for QP gaps). GVJ-2e method allows to equally accurately obtain band gaps for 3D and 2D materials. The errors of GVJ-2e method are significantly smaller than errors of other widely used methods such as GW (MAE 0.23 eV), hybrid functional HSE (MAE 0.17 eV), TB-mBJ functional (MAE 0.14 eV).

Dielectric surface passivation for silicon solar cells: A review

Abstract: Silicon wafer solar cells continue to be the leading photovoltaic technology, and in many places are now providing a substantial portion of electricity generation. Further adoption of this technology will require processing that minimises losses in device performance. A fundamental mechanism for efficiency loss is the recombination of photo-generated charge carriers at the unavoidable cell surfaces. Dielectric coatings have been shown to largely prevent these losses through a combination of different passivation mechanisms. This review aims to provide an overview of the dielectric passivation coatings developed in the past two decades using a standardised methodology to characterise the metrics of surface recombination across all techniques and materials. The efficacy of a large set of materials and methods has been evaluated using such metrics and a discussion on the current state and prospects for further surface passivation improvements is provided.

Solar blind photodetector based on epitaxial zinc doped Ga2O3 thin film

Abstract: We report on the fabrication and characterization of solar blind photodetectors (SBPs) based on undoped β-Ga2O3 and Zn doped (∼5 × 1020 cm−3) β-Ga2O3 (ZnGaO) epitaxial films with cutoff wavelength of ∼260 nm. The epilayers were grown on c-sapphire by the metal organic chemical vapor deposition technique and their structural, electrical and optical properties were characterized using various methods. As grown films have a large number of defects, resulting in detectors with enhanced internal gain, hence, high spectral responsivity >103 A/W. Post growth annealing in oxygen improved the quality of the epilayers, leading to detectors with reduced dark current (∼nA to ∼pA) and increased out of band rejection ratio. At 20 V bias, a ZnGaO detector showed a peak responsivity of 210 A/W (at 232 nm) and an out of band rejection ratio (i.e., R232 nm/R320 nm) of 5 × 104. Alternatively, for a β-Ga2O3 detector these parameters were found to be five times and three times lower, respectively, suggesting that ZnGaO detectors have superior performance characteristics. These results provide a roadmap toward achieving high responsivity SBPs based on epitaxial ZnGaO films, laying a solid foundation for future applications.

The role of hydrogenation and gettering in enhancing the efficiency of next-generation Si solar cells: An industrial perspective

Abstract: We discuss the importance of gettering and hydrogenation for next-generation silicon solar cells in the context of industrial cell fabrication. Gettering and hydrogenation play a vital role for p-type cell technologies in improving the silicon material's minority charge carrier lifetime. These mechanisms are naturally incorporated during screen-printed cell fabrication through the phosphorus emitter diffusion, silicon nitride deposition and subsequent metallisation firing processes. While the transition towards emitters with lower dopant concentrations and/or thermal oxide passivation can reduce surface recombination, it can negatively impact the ability to getter common impurities such as iron. For cell technologies with alternative low-temperature metallisation approaches, the ability to hydrogenate bulk defects is greatly reduced. Ultra-high efficiency n-type technologies tend to use heterojunction structures rather than diffused layers, but in doing so, do not benefit from phosphorus gettering. Also, particularly for amorphous silicon-based heterojunction structures, the imposed temperature constraints strongly limit the ability to passivate bulk defects. As a result, high-efficiency n-type technologies rely on the use of ‘high-quality’ wafers or would require the deliberate addition of gettering and hydrogenation processes before cell fabrication. A potential high-efficiency hybrid homojunction/heterojunction structure is then discussed that could naturally enable gettering and bulk hydrogenation throughout cell fabrication. Calibrated implied open circuit voltage (Voc) map of a p-type mono-crystalline wafer highlighting the impact of pre-hydrogenating the top half of the wafer.

Oxygen vacancies effects in a-IGZO: Formation mechanisms, hysteresis, and negative bias stress effects

Abstract: The amorphous oxide semiconductor Indium-Gallium-Zinc-Oxide (a-IGZO) has gained a large technological relevance as a semiconductor for thin-film transistors in active-matrix displays. Yet, major questions remain unanswered regarding the atomic origin of threshold voltage control, doping level, hysteresis, negative bias stress (NBS), and negative bias illumination stress (NBIS). We undertake a systematic study of the effects of oxygen vacancies on the properties of a-IGZO by relating experimental observations to microscopic insights gained from first-principle simulations. It is found that the amorphous nature of the semiconductor allows unusually large atomic relaxations. In some cases, oxygen vacancies are found to behave as perfect shallow donors without the formation of structural defects. Once structural defects are formed, their transition states can vary upon charge and discharge cycles. We associate this phenomenon to a possible presence of hysteresis in the transfer curve of the devices. Under NBS, the creation of oxygen vacancies becomes energetically very stable, hence thermodynamically very likely. This generation process is correlated with the occurrence of the negative bias stress instabilities observed in a-IGZO transistors. While oxygen vacancies can therefore be related to NBS and hysteresis, it appears unlikely from our results that they are direct causes of NBIS, contrary to common belief.

Selective emitters for thermophotovoltaic applications

Abstract: Applying thermophotovoltaic (TPV) technologies to existing energy generators allows us to increase energy output while utilizing present infrastructure by reclaiming the heat lost during the production process. In order to maximize the efficiency of these sources, the conversion efficiency of the TPV system needs to be optimized. Selective emitters are often used to tailor the spectrum of incident light on the diode, blocking any undesirable light that may lead to device heating or recombination. Over the years, many different technologies have been researched to create an ideal selective emitter. Plasmas and rare-earth emitters provided highly selective spectra early on, but their fixed peaks required tailoring the diode's band gap to the emitter's characteristic wavelength. Recent advances in engineerable materials, such as photonic crystals and metamaterials, allow the opposite to take place; an appropriate selective emitter can be designed to match the TPV diode, allowing the diode structure to be optimized independently from the emitter.

On the recombination behavior of p+‐type polysilicon on oxide junctions deposited by different methods on textured and planar surfaces

Abstract: We investigate the passivation quality of hole-collecting junctions consisting of thermally or wet-chemically grown interfacial oxides, sandwiched between a monocrystalline-Si substrate and a p-type polycrystalline-silicon (Si) layer. The three different approaches for polycrystalline-Si preparation are compared: the plasma-enhanced chemical vapor deposition (PECVD) of in situ p+-type boron-doped amorphous Si layers, the low pressure chemical vapor deposition (LPCVD) of in situ p+-type B-doped polycrystalline Si layers, and the LPCVD of intrinsic amorphous Si, subsequently ion-implanted with boron. We observe the lowest J0e values of 3.8 fA cm−2 on thermally grown interfacial oxide on planar surfaces for the case of intrinsic amorphous Si deposited by LPCVD and subsequently implanted with boron. Also, we obtain a similar high passivation of p+-type poly-Si junctions on wet-chemically grown oxides as well as for all the investigated polycrystalline-Si deposition approaches. Conversely, on alkaline-textured surfaces, J0e is at least 4 times higher compared to planar surfaces. This finding holds for all the junction preparation methods investigated. We show that the higher J0e on textured surfaces can be attributed to a poorer passivation of the p+ poly/c-Si stacks on (111) when compared to (100) surfaces.

Nanocrystalline silicon emitter optimization for Si-HJ solar cells: Substrate selectivity and CO2 plasma treatment effect

Abstract: We investigated hydrogenated nanocrystalline silicon (nc-Si:H) films as doped emitter layers for silicon heterojunction solar cells. Firstly, we focused on the effect of the nc-Si:H deposition conditions and film growth on the intrinsic hydrogenated amorphous silicon passivation layer ((i)a-Si:H) underneath. Three different p-doped emitters were compared: nc-Si:H, nc-SiOx:H, and a-Si:H. We found that the nc-Si:H and nc-SiOx:H growth enhances the passivation of the epitaxy-free (i)a-Si:H layer, yielding implied open circuit voltages above 730 mV. Secondly, for (p)nc-Si:H emitters, we observed a trade-off between fill factor (FF) and open circuit voltage (Voc) by using two types of (i)a-Si:H films. A slight epitaxy of the (i)layer seems to promote the rapid nucleation of nc-Si:H, thereby positively affecting the FF (79.5%) and series resistance but reducing Voc (670 mV). Contrarily, on well-passivating (i)a-Si:H the nc-Si:H nucleation is more difficult resulting in S-shaped I–V curves, presumably due to low built-in voltage and a poor emitter/TCO contact. To circumvent this dilemma, a CO2 plasma treatment is used to oxidize the a-Si:H surface before the nc-Si:H emitter deposition thereby enhancing nucleation. Accordingly, a FF of 74.5% with Voc of 727 mV is reached in the best device, yielding a conversion efficiency of 21%. HR-TEM micrograph of the front layer stack of the solar cell. The image shows a region close to the valley between two pyramids. From bottom to top: c-Si substrate, (i)a-Si:H passivation layer showing epitaxially grown regions, (p)nc-Si:H emitter layer, and In2O3:Sn (ITO). Yellow lines highlight layers and individual crystals. Silicon zone axis orientation is .

Thermal annealing effect on β-Ga2O3 thin film solar blind photodetector heteroepitaxially grown on sapphire substrate

Abstract: This paper presents the effect of thermal annealing on β-Ga2O3 thin film solar-blind (SB) photodetector (PD) synthesized on c-plane sapphire substrates by a low pressure chemical vapor deposition (LPCVD). The thin films were synthesized using high purity gallium (Ga) and oxygen (O2) as source precursors. The annealing was performed ex situ the under the oxygen atmosphere, which helped to reduce oxygen or oxygen-related vacancies in the thin film. Metal/semiconductor/metal (MSM) type photodetectors were fabricated using both the as-grown and annealed films. The PDs fabricated on the annealed films had lower dark current, higher photoresponse and improved rejection ratio (R250/R370 and R250/R405) compared to the ones fabricated on the as-grown films. These improved PD performances are due to the significant reduction of the photo-generated carriers trapped by oxygen or oxygen-related vacancies.

Coherence Properties and Quantum Control of Silicon Vacancy Color Centers in Diamond

Abstract: Atomic-scale impurity spins, also called color centers, in an otherwise spin-free diamond host lattice have proven to be versatile tools for applications in solid-state-based quantum technologies ranging from quantum information processing (QIP) to quantum-enhanced sensing and metrology Due to its wide band gap, diamond can host hundreds of different color centers However, their suitability for QIP or sensing applications has only been tested for a handful of these, with the nitrogen vacancy (NV) strongly dominating this field of research Due to its limited optical properties, the success of the NV for QIP applications however strongly depends on the development of efficient photonic interfaces In the past years the negatively charged silicon vacancy (SiV−) center received significant attention due to its highly favourable spectral properties such as narrow zero phonon line transitions and weak phonon sidebands Here, the recent work investigating the SiV center's orbital and electron spin coherence properties is reviewed as well as techniques to coherently control its quantum state using microwave as well as optical fields Also, potential future experimental directions to improve the SiV's coherence time scale and to develop it into a valuable tool for QIP applications are outlined

Analysis of the losses of industrial-type PERC solar cells

Abstract: The loss analysis of state-of-the-art p-type Czochralski-grown silicon passivated emitter and rear solar cells (PERC) fabricated in a manner close to industrial production is presented in this paper. The 6-inch solar cells are featuring a homogeneous emitter on the front side, an Al2O3 passivation layer and local contacts on the rear side. The peak energy conversion efficiencies obtained are 21.1% for a standard antireflection coating (ARC) and 21.4% for a double-layer ARC. The loss analysis is based on an extended characterization of the solar cells and of special samples, which allow the separation of the contributions of each region of the solar cell including metallization. Their impact is determined experimentally on the open-circuit voltage, the short-circuit current density, and the fill factor. Based on the measurements, the devices are numerically simulated. The free energy losses are analyzed using the simulation model. Based on the results from the loss analysis, it is found that the integration of a selective emitter structure and the further improvement of the rear surface would allow for efficiencies above 22% in the short term.

Graphene integration with nitride semiconductors for high power and high frequency electronics

Abstract: Group III nitride semiconductors (III-N), including GaN, AlN, InN, and their alloys, are currently the materials of choice for many applications in optoelectronics (light-emitting diodes, laser diodes), and high-power and high-frequency transistors. Due to its attractive electrical, optical, mechanical, and thermal properties, graphene (Gr) integration with III-N technology has been considered in the last few years, in order to address some of the major issues which still limit the performances of GaN-based devices. To date, most of the studies have been focused on the use of Gr as transparent conductive electrode (TCE) to improve current spreading from top electrodes and light extraction in GaN-LEDs. This paper will review recent works evaluating the benefits of Gr integration with III-N for high power and high frequency electronics. From the materials side, recent progresses in the growth of high quality GaN layers on Gr templates and in the deposition of Gr on III-N substrates and templates will be presented. From the applications side, strategies to use Gr for thermal management in high-power AlGaN/GaN transistors will be discussed. Finally, recent proposals of implementing new ultra-high-frequency (THz) transistors, such as the Gr base hot electron transistor (GBHET), by Gr integration with III-N will be highlighted.

Infrared, transient thermal, and electrical properties of silver nanowire thin films for transparent heaters and energy-efficient coatings

Abstract: In this study, we investigate the infrared and electrical properties as well as the thermal response of transparent silver nanowire (AgNW) based thin-film heaters, when subjected to Joule heating. Controlling the number of layers and hence the deposition time, our spray-coating technique allows to modulate the thermal and electrical properties of the thin films in a precise manner. In addition, this technique enables the fabrication of homogeneous and large-area heaters, which, in terms of their electro-optical properties, nicely compare to the performances of state-of-the-art AgNW transparent electrodes. The thermal response and the electrical properties are accurately reproduced by a purposely developed physical model, which shows that the temperature dependence of the AgNW film resistance is lowered by a factor of 2 compared to bulk silver, independently of the number of deposited layers. Compared to uncoated glass, the emissivity decreases by 58% at a coverage rate of 58%. At the same time, the AgNW film can sustain a transparency as high as 81.3%. Therefore, AgNW-based thin films can be used as a low-emissivity coating, for e.g., energy-efficient window glazing applications. Finally, we accurately determine the fragmentation temperature of AgNWs, which sets the ultimate limitation of use for heating applications.

Investigation of energy band gap and conduction mechanism of magnesium substituted nickel ferrite nanoparticles

Abstract: Polycrystalline ferrite nanoparticles having the general formula Ni1−xMgxFe2O4 (in which magnesium content x = 00, 01, 02, 03, 04, and 05) were synthesized by auto combustion method The formation of cubic spinel structure of Mg substituted nickel ferrites were confirmed by X-ray diffraction (XRD) measurement with the space group of Fd3m-Oh7 The average particle size of the ferrites was determined by Scherrer formula; ranging from 4141 to 4593 nm The lattice constant increases with increase in Mg concentration The porosity of the ferrite samples was estimated using Hendricks and Adam's method; the porosity found to increase with an increase in Mg concentration The decrease of DC electrical resistivity with an increase in temperature indicates the semiconducting nature of the ferrites The decrease of dielectric constant with an increase in frequency was due to the hopping of free and localized electric charge carriers The energy band gap of the ferrite nanoparticles were determined by Tauc plot and it was found to increase with an increase in Mg concentration (284 to 294 eV)

High-resolution patterning of organohalide lead perovskite pixels for photodetectors using orthogonal photolithography

Abstract: Organohalide lead perovskite (CH3NH3PbI3) is a novel material with promising applicability for visible light photo-detectors. The ability to develop perovskite photo-detector devices using a low temperature solution based process allows straightforward combinations with other materials, including traditional crystalline semiconductors, with minimal contributions to cost and process complexity. There is, however, a need for high-resolution structuring of the perovskite film to minimize cross-talk between neighboring detectors (pixels) for imaging purposes. This work presents a method to develop Ch3NH3PbI3 thin films possessing high-resolution patterning, using lithography processing with hydrofluoroether solvents. The results presented herein confirm that, unlike the majority of traditional solvents utilized in conventional photolithography, hydrofluoroethers do not adversely affect CH3NH3PbI3 films, enabling photolithographic processing. Transfer of the resist pattern is achieved using a SF6 plasma functionalization process which extracts iodine and organic components from the film, converting the perovskite into PbF2. This work also demonstrates that isolation of perovskite photodetecting pixels with a 20 μm-wide stripe of PbF2 leads to a 4.5-fold reduction in the cross-talk between neighboring pixels. It is believed that our method will facilitate simple monolithic integration of perovskite photodiodes to the silicon backplane chip utilized in active-pixel sensor and charge-coupled device applications.

The detailed balance limit of perovskite/silicon and perovskite/CdTe tandem solar cells

Abstract: Here, the detailed balance limit of the light to electric power conversion efficiency η of the tandem solar cells with a solution processible perovskite top cell is presented, investigating a bandgap range from 1.4 to 2.6 eV for the top cell. As the bottom cell, Si and CdTe are considered. On the one hand, Si is attractive due to its ability to convert photons down to the near infrared into excited charger carriers. On the other hand, it is shown that CdTe is a more suitable bottom cell for wide band perovskite top cells, when both cells are connected in series. Here, a four terminal (4T) tandem device is compared to several two terminal (2T) tandem configurations: (i) a tandem cell that is optimized for maximum efficiency under current matching conditions; (ii) a tandem where emitted photons from the top cell are reabsorbed in the bottom cell (photon recycling); and (iii) a tandem cell with current matching over a broad spectral range due to an incomplete light absorption in the top cell.

Progress in high‐luminance LED technology for solid‐state lighting

Abstract: Increasing the luminance of white LEDs to the 200 Mnit level and beyond, opens a completely new design space for a wide range of lighting applications, by allowing significant reductions in optics and luminaire size as well as costs. Moreover, new applications, such as dynamic beam steering, are enabled by the ability to create arrays of densely packed, individually addressable high-luminance emitters. The development of such high-luminance LEDs requires improvements in all LED technology elements. In this paper, we discuss recent advances in epitaxy, die, phosphor, and package technology that are critical to achieving these benefits.

Improvement of kesterite solar cell performance by solution synthesized MoO3 interfacial layer

Abstract: In this study, an ultra-thin MoO3 layer synthesized by a solution-based technique is introduced as a promising interfacial layer to improve the performance of kesterite Cu2ZnSnSe4 (CZTSe) solar cell. Solar cells with 10 nm of MoO3 between Mo rear contact and CZTSe had larger minority carrier life time and open-circuit voltage compared to the reference solar cells. Temperature dependent current density–voltage measurement indicated that the activation energy (EA) of the main recombination is higher (∼ 837 meV) in solar cells with MoO3 layer, as compared to conventional solar cells where EA ∼ 770 meV, indicating reduced interface recombination. A best efficiency of 7.1% was achieved for a SLG/Mo/MoO3/CZTSe/CdS/TCO solar cell compared to the reference solar cell SLG/Mo/CZTSe/CdS/TCO for which 5.9% efficiency was achieved.

Amorphous and crystalline In2O3-based transparent conducting films for photovoltaics

Abstract: We reported solar cells with reduced electrical and optical losses using hydrogen-doped In2O3 (In2O3:H) transparent conducting layers with low sheet resistance and high transparence characteristics. The transparent conducting oxide (TCO) films were prepared by solid-phase crystallization of amorphous (a-) In2O3:H films grown by magnetron sputtering. The polycrystalline (poly-) In2O3:H films exhibited electron mobilities (over 100 cm2V−1 s−1) 2 and 3 times greater than those of conventional TCO films. This paper describes (i) the current status of the electrical properties of In2O3-based TCO; (ii) the structural and optoelectrical properties of the a-In2O3:H and poly-In2O3:H films, focusing on the inhomogeneity and stability characteristics of the films; and (iii) the electrical properties of bilayer TCO. The potential of these high mobility TCO films for solar cells was also described.

Low cost high voltage GaN polarization superjunction field effect transistors

Abstract: A comprehensive overview of novel high voltage GaN field effect transistors (FETs) based on the Polarization Superjunction (PSJ) concept and a cost-effective approach towards manufacturing these high performance devices are presented. Current challenges impeding wider adoption of GaN power switching transistors in applications, and latest results of scaled-up PSJ-FETs from POWDEC KK, have also been discussed. The article also presents hard-switching characteristics of 400V-to-800V boost converter constructed using a PSJ-FET grown on sapphire substrate and the future direction of GaN power semiconductor technology based on monolithic integration for advanced power electronics.

Proximity gettering technology for advanced CMOS image sensors using carbon cluster ion-implantation technique: A review

Abstract: A new technique is described for manufacturing advanced silicon wafers with the highest capability yet reported for gettering transition metallic, oxygen, and hydrogen impurities in CMOS image sensor fabrication processes. Carbon and hydrogen elements are localized in the projection range of the silicon wafer by implantation of ion clusters from a hydrocarbon molecular gas source. Furthermore, these wafers can getter oxygen impurities out-diffused to device active regions from a Czochralski grown silicon wafer substrate to the carbon cluster ion projection range during heat treatment. Therefore, they can reduce the formation of transition metals and oxygen-related defects in the device active regions and improve electrical performance characteristics, such as the dark current, white spot defects, pn-junction leakage current, and image lag characteristics. The new technique enables the formation of high-gettering-capability sinks for transition metals, oxygen, and hydrogen impurities under device active regions of CMOS image sensors. The wafers formed by this technique have the potential to significantly improve electrical devices performance characteristics in advanced CMOS image sensors.

Impurity control in high performance multicrystalline silicon

Abstract: We report results from a national project about impurities in high performance multicrystalline silicon: Contamination sources, transport routes, interaction with crystal defects and impact on solar cell efficiency parameters. Several ingots were produced in a lab scale furnace. Growth parameters and crucible types were varied, and high purity quartz crucibles were compared to novel silicon nitride crucibles. The material was characterized by a range of methods including FTIR, GDMS, NAA, EBSD, dislocation etching, lifetime analysis. Wafers were processed, and lifetime was measured during the cell processing to analyse the effect of separate processing steps. Models were constructed to simulate the impurity transport during the crystallization. The diffusivity in quartz and silicon nitride crucibles required in the modelling is not generally known, and experiments were performed to evaluate them. Variations in dislocation density have the strongest impact on lifetime, even for this high quality material. The quartz crucible has the highest contamination potential, but the uncertainty in diffusion parameters makes it difficult to conclude if this potential is reached, and for which elements. We therefore make an overview showing which value of the diffusion coefficient shift the contamination dominance between feedstock/crucible/coating. The use of high purity crucibles enables comparable or even higher lifetime material to be made in lab scale compared to industrial settings. Our results indicate that a thin layer of higher purity, lower porosity quartz on the inside of the crucible can be an effective method to transfer this improvement to industrial conditions. The cell processing indicates that the diffusion/gettering sequence profoundly decreases the lifetime by rendering the random high angle grain boundaries electrically active. However, this effect is reversed after firing of the anti-reflective coating, when the remaining active defects are dislocation clusters and CSL boundaries. Silicon nitride crucibles appear to be a promising low oxygen alternative to traditional quartz crucibles in terms of purity.

Radiation resistance of wide-bandgap semiconductor power transistors

Abstract: Radiation resistance of state-of-the-art commercial wide-bandgap power transistors, 1700 V 4H-SiC power MOSFETs and 200 V GaN HEMTs, to the total ionization dose was investigated Transistors were irradiated with 45 MeV electrons with doses up to 2000 kGy Electrical characteristics and introduced defects were characterized by current–voltage (I–V), capacitance–voltage (C–V), and deep level transient spectroscopy (DLTS) measurements Results show that already low doses of 45 MeV electrons (>1 kGy) cause a significant decrease in threshold voltage of SiC MOSFETs due to embedding of the positive charge into the gate oxide On the other hand, other parameters like the ON-state resistance are nearly unchanged up to the dose of 20 kGy At 200 kGy, the threshold voltage returns back close to its original value, however, the ON-state resistance increases and transconductance is lowered This effect is caused by radiation defects introduced into the low-doped drift region which decrease electron concentration and mobility GaN HEMTs exhibit significantly higher radiation resistance They keep within the datasheet specification up to doses of 2000 kGy Absence of dielectric layer beneath the gate and high concentration of carriers in the two dimensional electron gas channel are the reasons of higher radiation resistance of GaN HEMTs Their degradation then occurs at much higher doses due to electron mobility degradation

Impacts of gate bias and its variation on gamma-ray irradiation resistance of SiC MOSFETs

Abstract: Gamma-ray irradiation into vertical type n-channel hexagonal (4H)-silicon carbide (SiC) metal-oxide-semiconductor field effect transistors (MOSFETs) was performed under various gate biases. The threshold voltage for the MOSFETs irradiated with a constant positive gate bias showed a large negative shift, and the shift slightly recovered above 100 kGy. For MOSFETs with non- and a negative constant biases, no significant change in threshold voltage, Vth, was observed up to 400 kGy. By changing the gate bias from positive bias to either negative or non-bias, the Vth significantly recovered from the large negative voltage shift induced by 50 kGy irradiation with positive gate bias after only 10 kGy irradiation with either negative or zero bias. It indicates that the positive charges generated in the gate oxide near the oxide–SiC interface due to irradiation were removed or recombined instantly by the irradiation under zero or negative biases.

Effect of V-pits embedded InGaN/GaN superlattices on optical and electrical properties of GaN-based green light-emitting diodes

Abstract: We investigated effects of V-pits embedded InGaN/GaN superlattices (SL) on optical and electrical properties of high power green LEDs by changing the number of SL period and SL growth temperature. Surface morphology of V-pits embedded InGaN/GaN SL with various periods and growth temperatures was evaluated by using atomic force microscopy (AFM). It was found that density and size of V-pit increase with decreasing SL growth temperature and increasing SL periods. Experimental studies using scanning electron microscopy (SEM) equipped with cathodoluminescence (CL) indicated that SL with larger V-pits appear to be more effective in suppressing the lateral diffusion of carriers into threading dislocations (TD). Compared to c-plane quantum wells, narrower quantum wells on the V-pit sidewall were clearly observed by performing high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The external quantum efficiency (EQE) and the efficiency droop of green LEDs grown on underlying SL with larger V-pits are improved at high injection current regime, which is attributed to a more efficient hole injection into multiple quantum well, and also to a higher V-pit potential barrier height that could more effectively suppress the lateral diffusion of carriers into non-radiative recombination centers of TDs.

Effect of growth temperature on the synthesis of carbon nanotube arrays and amorphous carbon for thermal applications

Abstract: 1 Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA 22904-4746, USA Department of Mechanical and Aerospace Engineering, University of California-Irvine, Irvine, CA 92697, USA Centro Tecnol ogico da Marinha em S~ao Paulo, Av. Prof. Lineu Prestes, 2468, Cidade Universit aria, S~ao Paulo 05508-000, SP, Brazil 4 Sandia National Laboratories, P.O. Box 5800 Box 5800, Albuquerque, New Mexico 87185, USA 5 School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China

Etching Kinetics of (100) Single Crystal Diamond Surfaces in a Hydrogen Microwave Plasma, Studied with In Situ Low-Coherence Interferometry

Abstract: A low-coherence interferometry (LCI) was used to measure in situ the etch rate (ER) of synthetic single crystal (SC) diamonds in H2 microwave plasma, at substrate temperatures in the broad range of 800–1370 °C. The method allows the collection of the kinetic data on a single sample without switching off the plasma. (100)-orientated SC plates of CVD and IIa type HPHT diamond were systematically etched in pure hydrogen at pressure p = 130 Torr and microwave power density of ≈300 W cm−3. The activation energies Ea of 42 ± 5 and 32 ± 4 kCal mol−1 have been determined for the CVD and HPHT substrates. An enhanced etching rate of a subsurface defected layer with thickness of ∼1 μm or less, formed upon polishing of the samples, is revealed. Surface morphology, roughness, and the shape of etch pits produced by a selective etching of defects, were characterized with optical profilometry. CH and dimer C2 radicals were detected in the H2 plasma with optical emission spectroscopy, as a result of the diamond etching, and the emission intensity of these species was linked to the substrate etch rate.

Ohmic contacts to Al-rich AlGaN heterostructures

Abstract: Due to the ultra-wide bandgap of Al-rich AlGaN, up to 5.8 eV for the structures in this study, obtaining low resistance ohmic contacts is inherently difficult to achieve. A comparative study of three different fabrication schemes is presented for obtaining ohmic contacts to an Al-rich AlGaN channel. Schottky-like behavior was observed for several different planar metallization stacks (and anneal temperatures), in addition to a dry-etch recess metallization contact scheme on Al0.85Ga0.15N/Al0.66Ga0.34N. However, a dry etch recess followed by n+-GaN regrowth fabrication process is reported as a means to obtain lower contact resistivity ohmic contacts on a Al0.85Ga0.15N/Al0.66Ga0.34N heterostructure. Specific contact resistivity of 5 × 10−3 Ω cm2 was achieved after annealing Ti/Al/Ni/Au metallization.