Showing papers in "Semiconductor Science and Technology in 2019"

Wide-bandgap, low-bandgap, and tandem perovskite solar cells

Abstract: Organic-inorganic metal halide perovskite single-junction solar cells have attracted great attention in the past few years due to a high record power conversion efficiency (PCE) of 23.7% and low-cost fabrication processes. Beyond single-junction devices, low-temperature solution processability, and bandgap tunability make the metal halide perovskites ideal candidates for fabricating tandem solar cells. Tandem solar cells combining a wide-bandgap perovskite top cell and a low-bandgap bottom cell based on mixed tin (Sn)-lead (Pb) perovskite or a dissimilar material such as silicon (Si) or copper indium gallium selenide (CIGS) offer an extraordinary opportunity to achieve PCEs higher than Shockley-Queisser (SQ) radiative efficiency limits (~33%) for single-junction cells. In this review, we will summarize recent research progress on the fabrication of wide- (1.7 to 1.9 eV) and low-bandgap (1.1 to 1.3 eV) perovskite single-junction cells and their applications in tandem cells. Key challenges and issues in wide- and low-bandgap single-junction cells will be discussed. We will survey current state-of-the-art perovskite tandem cells and discuss the limitations and challenges for perovskite tandem cells. Lastly, we conclude with an outlook for the future development of perovskite tandem solar cells.

The Quantum Hall Effect in the Era of the New SI.

Abstract: The quantum Hall effect (QHE), and devices reliant on it, will continue to serve as the foundation of the ohm while also expanding its territory into other SI derived units. The foundation, evolution, and significance of all of these devices exhibiting some form of the QHE will be described in the context of optimizing future electrical resistance standards. As the world adapts to using the quantum SI, it remains essential that the global metrology community pushes forth and continues to innovate and produce new technologies for disseminating the ohm and other electrical units.

InAs quantum dots grown on metamorphic buffers as non-classical light sources at telecom C-band: a review

Abstract: Long-distance quantum communication and computation is based on the exchange of information via photons as flying qubits. In all foreseen implementations, from quantum relays and networks to remote quantum computing, the photons must be able to propagate over long distances, in silica fibers, with limited absorption and wave packet dispersion. When propagating into silica fibers, photons in the so-called telecom C-band (i.e. wavelength around 1550 nm) will experience the absolute minimum of absorption, together with a limited photon wave packet dispersion. This implies that losses of information will be minimized and additionally, the capability of the photons to quantum interfere (the so-called Hong–Ou–Mandel effect) will be negligibly affected. This motivated the search for efficient non-classical light sources in this wavelength range. In the present review, we discuss the approaches followed to red-shift the emission wavelength from the near-infrared (NIR) to telecom wavelengths. In particular, the use of metamorphic buffers (MMBs) enabled the use of highly developed InAs/GaAs systems, engineering the dots to emit single and entangled photons in the telecom C-band. The main advantage of this approach is set by the choice of the material system: being the same as state-of-the-art structures emitting in the NIR range, it opens the possibility of achieving comparable performances even at telecommunication wavelengths. Here we will discuss the current state-of-the-art in the generation of non-classical photons, comparing the properties and performances of MMB-based results with other competing quantum dot (QD) platforms. We report in particular that very low fine-structure splitting can be observed with conservative values, on average well below 10 μeV. This allowed for the observation of post-selected entanglement fidelity higher than 0.6. Additionally, on-demand single-photon emission with was observed in addition to a very low emitter density of and a decay time of τ ≈ 1.2 ns comparable to standard InAs QDs emitting in the NIR range. A final highlight on approaches to further improve the current device performances will also be reported.

A Comprehensive Simulation Study of Hybrid Halide Perovskite Solar Cell with Copper Oxide as HTM

Abstract: Perovskite solar cells (PSCs) have attracted considerable attention as a competitor technology in solar cells due to the rapid enhancement in their power conversion efficiency (PCE) in recent years. PSCs have several advantages such as their bandgap tunability, lower cost, tolerance of high impurities, protracted diffusion length and wide optical absorption. In this paper, simulation of PSCs with copper oxide as a hole transport material and different electron transport materials has been presented. The proposed materials are a replacement to the ordinary hole and electron transport materials; such as the titanium dioxide and the expensive spiro-OMeTAD. In addition, a comprehensive study for optimizing the features and parameters of the PSCs, such as the thickness and defect density of the perovskite layer, the doping concentrations, and the bandgap energy, has been introduced. The simulation and the performance evaluation of the designed PSCs have been carried out using SCAPS-1D. The results show that mixed halide PSC with zinc oxysulfide as electron transport material and copper oxide as hole transport material has an enhanced performance with a power conversion efficiency of up to 30.82%.

Nanowire photodetectors based on wurtzite semiconductor heterostructures

Abstract: Using nanowires for photodetection constitutes an opportunity to enhance the absorption efficiency while reducing the electrical cross-section of the device. They present interesting features like compatibility with silicon substrates, which offers the possibility of integrating detector and readout circuitry, and facilitates their transfer to flexible substrates. Within a nanowire, it is possible to implement axial and radial (core-shell) heterostructures. These two types can be combined to obtain three-dimensional carrier confinement (dot-in-a-wire). The incorporation of heterostructures in nanowire photodetectors opens interesting opportunities of application and performance improvement. Heterojunctions or type-II heterostructures favor the separation of the photogenerated electrons and holes, and the implementation of quantum dots in a nanowire can be applied to the development of quantum photodetectors. This paper provides a general review of latest progresses in nanowire photodetectors, including single nanowires and heterostructured nanowires.

In search of a true hot carrier solar cell

Abstract: Several decades ago, it was proposed that one could improve the single junction solar cell by the concentration of effort onto the hot carriers. In the intervening years, no significant progress has been made in this direction, although the effort continues to involve a large body of workers. While this community is focused on the role of phonons and selectively measuring only a fraction of the energy spectrum, it has not generally addressed the true usage of the hot carriers that can be supported in the system. Here, we discuss a different approach relying upon the use of 'metastable' levels provided by the satellite valleys of the conduction band. Such usage retains the hot carriers at higher energy levels than normal, and promises to provide the long-sought improvement with hot carrier solar cells.

Fabrication technology for high light-extraction ultraviolet thin-film flip-chip (UV TFFC) LEDs grown on SiC

Abstract: Author(s): SaifAddin, Burhan K; Almogbel, Abdullah; Zollner, Christian J; Foronda, Humberto; Alyamani, Ahmed; Albadri, Abdulrahman; Iza, Michael; Nakamura, Shuji; DenBaars, Steven P; Speck, James S | Abstract: The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to their poor light extraction efficiency (LEE). To improve the LEE of AlGaN LEDs, we developed a fabrication technology to process AlGaN LEDs grown on SiC into thin-film flip-chip LEDs (TFFC LEDs) with high LEE. This process transfers the AlGaN LED epi onto a new substrate by wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a highly selective SF6 plasma etch that stops at the AlN buffer layer. We optimized the inductively coupled plasma (ICP) SF6 etch parameters to develop a substrate-removal process with high reliability and precise epitaxial control, without creating micromasking defects or degrading the health of the plasma etching system. The SiC etch rate by SF6 plasma was ~46 \mu m/hr at a high RF bias (400 W), and ~7 \mu m/hr at a low RF bias (49 W) with very high etch selectivity between SiC and AlN. The high SF6 etch selectivity between SiC and AlN was essential for removing the SiC substrate and exposing a pristine, smooth AlN surface. We demonstrated the epi-transfer process by fabricating high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further enhance the light extraction, the exposed N-face AlN was anisotropically etched in dilute KOH. The LEE of the AlGaN LED improved by ~3X after KOH roughening at room temperature. This AlGaN TFFC LED process establishes a viable path to high external quantum efficiency (EQE) and power conversion efficiency (PCE) UV-C LEDs.

Performance enhancement of a proposed solar cell microstructure based on heavily doped silicon wafers

Abstract: This paper aims to present a proposed npn solar cell microstructure based on low cost heavily doped Silicon wafers. The physical perception of the proposed structure is based on the idea of vertical generation and lateral collection of light generated carriers. It should be mentioned that our structure can be utilized whenever the diffusion length of photogenerated electron hole pairs is smaller than the penetration depth of the solar radiation. The enhancement in the structure performance is attained by the optimization of the structure technological and geometrical parameters and based on practical considerations. This enhancement enables achieving the maximum possible structure conversion efficiency. Moreover, the optical performance, in terms of the spectral response and external quantum efficiency, is presented. The optimization is carried out using SILVACO TCAD process and device simulators. The main parameters used in optimization include the thickness and doping of the top n + layer as well as the sidewall emitter. Additionally, the structure base width along with the notch depth are considered. Finally, back surface treatment is introduced. The structure conversion efficiency in the initial step before optimization was 10.7%. As a result of the optimization process, the structure conversion efficiency is improved to about 15% above the initial case study by 4%.

Exploring the magnetic ground state of Vanadium doped Zinc Sulphide

Abstract: Vanadium doped ZnS quantum dots (QDs) were synthesized with the generic formula Zn1-xVxS (where x = 0, 0.05, 0.10 and 0.15). Polyvinylpyrrolidone capped QDs were prepared by chemical co-precipitation technique. The phase purity of the samples were confirmed by X-ray diffraction. Crystallite size of 1.7-2.2 nm was obtained from the Scherrer's formula. Optical absorption studies revealed a band gap varying from 3.9-4.1 eV. Photoluminescence studies revealed a strong blue shift due to the strong quantum confinement in the particles. Magnetic studies indicated a ferromagnetic ground state for the doped samples

An efficient wide range photodetector fabricated using a bilayer Bi2S3/SnS heterojunction thin film

Abstract: This study presents a detailed optoelectronic investigation on the bilayer Bi2S3/SnS heterojunction photodetector. The Bi2S3 and SnS nanostructures were synthesized by an ultrasonication method and deposited in the form of thin films by a thermal evaporation system to fabricate the photodetectors. The performed optoelectrical measurements showed that the Bi2S3 and Bi2S3/SnS photodetectors are respectively n- and p-type electrical conductivity. This finding was further confirmed by current–voltage (I–V) analysis, Mott–Schottky plots and photoresponse measurements. According to the obtained gain and sensitivity values, the Bi2S3/SnS heterojunction provides a higher photon-to-electrical carrier conversion relative to the Bi2S3 photodetectord due to the enhanced photogeneration of electron–hole pairs and separation of the photoinduced charge carriers under the influence of the electric field build-in at the heterojunction's interface. The heterojunction exhibited a remarkable photoresponse from 400 to 800 nm by giving the photocurrent gain (G), sensitivity (S), and external quantum efficiency (EQE) values of 8460%, 450% and 45%, respectively. The obtained results represent the Bi2S3/SnS heterojunction photodetector as a potential candidate for devising high-performance optoelectronic devices.

Low voltage & controlled switching of MoS2-GO resistive layers based ReRAM for non-volatile memory applications

Abstract: Emerging information technology and data deluge foster the unprecedented demands of higher chip density, clocking speed, data storage and lower power dissipation for on-chip non-volatile memories (NVMs). Here, two types of metal-insulator-metal (MIM) based NVM structures were fabricated and demonstrated involving controlled functionalization of molybdenum disulfide (MoS2) and graphene oxide (GO) nanocomposite as a resistive switching layer. The first type of device constitutes Aluminum (Al) top and bottom electrode resulting in the Al/MoS2-GO/Al structure. While the second type of device uses Al top electrode and Indium Tin Oxide (ITO) bottom electrode resulting in Al/MoS2-GO/ITO. The current-voltage (I-V) characteristics for fabricated Al/MoS2-GO/Al and Al/MoS2-GO/ITO MIM structures exhibited considerable I-ON/I-OFF ratio of similar to 10(2) (SET and RESET state at 0.5 V and -0.4 V) and similar to 10(1) (SET and RESET state at 0.3 V and -1V), respectively. The I-V characteristics for Al/MoS2-GO/Al MIM structure showed low voltage switching, substantial memory retention similar to 10(4) s and endurance for up to 25 cycles. The low voltage and controlled switching operation for Al/MoS2-GO/Al MIM structures may be attributed to the presence of a large number of oxygen vacancies, defects in MoS2-GO, promoting enhanced charge hopping via interfacial oxide at MoS2-GO/Al interface as compared to MoS2-GO/ITO.

Gradual reset and set characteristics in yttrium oxide based resistive random access memory

Abstract: This paper addresses the resistive switching behavior in yttrium oxide based resistive random access memory (RRAM) (TiN/yttrium oxide/Pt) devices. We report the coexistence of bipolar and unipolar resistive switching within a single device stack. For bipolar DC operation, the devices show gradual set and reset behavior with resistance ratio up to two orders of magnitude. By using nanosecond regime pulses (20 to 100 ns pulse width) of constant voltage amplitude, this gradual switching behavior could be utilized in tuning the resistance during set and reset spanning up to two orders of magnitude. This demonstrates that yttrium oxide based RRAM devices are alternative candidates for multibit operations and neuromorphic applications.

Ga2O3 metal-oxide-semiconductor field effect transistors on sapphire substrate by MOCVD

Abstract: Si-doped gallium oxide (Ga2O3) thin films were grown on a c-plane sapphire substrate by metalorganic chemical vapor deposition (MOCVD) and fabricated into metal oxide semiconductor field effect transistors (MOSFETs). The Ga2O3 MOSFETs exhibited effective gate modulation of the drain current with a complete channel pinch-off for VG < −25 V, and the three-terminal off-state breakdown voltage was 390 V. The device shows a very low gate leakage current (~50 pA mm−1), which led to a high on/off ratio of ~108. These transistor characteristics were stable from room temperature to 250 °C.

Shaping the spectrum of terahertz photoconductive antenna by frequency-dependent impedance modulation

Abstract: In this paper, we report on an approach for shaping the spectra of THz pulse generation in photoconductive antennas (PCAs) by frequency-dependent impedance modulation. We introduce a theoretical model describing the THz pulse generation in PCAs and accounting for impedances of the photoconductor and of the antenna. In order to showcase an impact of frequency-dependent impedance modulation on the spectra of THz pulse generation, we applied this model to simulating broadband PCAs with log-spiral topology. Finally, we fabricated two different log-spiral PCAs and characterized them experimentally using the THz pulsed spectroscopy. The observed results demonstrate excellent agreement between the theoretical model and experiment, justifying a potential of shaping the spectra of THz pulse generation in PCA by modulation of frequency-dependent impedances. This approach makes possible optimizing the PCA performance and thus accommodating the needs of THz pulsed spectroscopy and imaging in fundamental and applied branches of THz science and technologies.

Lateral source field-plated β-Ga2O3 MOSFET with recorded breakdown voltage of 2360 V and low specific on-resistance of 560 mΩ cm2

Abstract: In this letter, lateral β-Ga2O3 MOSFETs with source field plate are fabricated on Si-doped homoepitaxial film on (010) Fe-doped semi-insulating β-Ga2O3 substrate. The drain extension in the source field plate effectively suppresses the peak electric field in the Ga2O3 channel, and improves the breakdown voltage greatly. Moreover, fluorinert FC-770 is used to reduce air breakdown potential during the breakdown testing. The breakdown voltage of the device with Lsd of 28 μm is measured as high as 2360 V, which is the highest value in reported lateral Ga2O3 MOSFET. Besides, Si-ion implantation is adopted to reduce the ohmic contact resistance (Rc). The value of specific on-resistance (Ron,sp) is calculated to be 560 mΩ cm2, which is a recorded value under such high breakdown voltage, and also lower than the theoretical limit of Si-based power devices under the same breakdown voltage.

Emergence of high quality sputtered III-nitride semiconductors and devices

Abstract: This article provides an overview of recent development of sputtering method for high-quality III-nitride semiconductor materials and devices. Being a mature deposition technique widely employed in semiconductor industry, sputtering offers many advantages such as low cost, relatively simple equipment, non-toxic raw materials, low process temperatures, high deposition rates, sharp interfaces, and possibility of deposition on large-size substrates, including amorphous and flexible varieties. This review covers two major research directions: (1) ex situ sputtered AlN buffers to be used for subsequent growth of GaN-based structures by conventional techniques, such as metal-organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), or molecular beam epitaxy (MBE), and (2) deposition of the entire III-nitride layered stacks and device structures by sputtering. Replacing conventional in situ GaN or AlN buffer layers with ex situ sputtered AlN buffers for MOCVD, HVPE, or MBE growth of III-nitride films on sapphire and silicon substrates results in the improved crystal quality through reduction in dislocation density and residual strain. Extensive efforts in the field of sputter deposition of III-nitrides resulted in crystalline quality of sputtered III-nitride films compatible with that of MOCVD and MBE grown layers despite the lower temperatures used in sputtering. For example, sputtering techniques made it possible to achieve GaN layers heavily doped with Si and Ge to electron concentrations in mid-10alt;supagt;20alt;/supagt; cmalt;supagt;-3alt;/supagt; range with mobilities exceeding 100 cmalt;supagt;2alt;/supagt; Valt;supagt;-1alt;/supagt; salt;supagt;-1alt;/supagt;, resulting in conductivities as high as those of benchmark transparent conducting oxides such as indium tin oxide (ITO). For moderate levels of doping with Si, mobilities comparable to state-of-the-art MOCVD-grown material have been demonstrated (up to ~1000 cmalt;supagt;2alt;/supagt; Valt;supagt;-1alt;/supagt; salt;supagt;-1alt;/supagt;). The first promising results have been reported for devices (light emitters and field effect transistors) entirely produced by sputtering.

Valence band states in an InAs/AlAsSb multi-quantum well hot carrier absorber

Abstract: In this study, detailed temperature dependent simulations for absorption and photogenerated recombination of hot electrons are compared with experimental data for an InAs/AlAsSb multi-quantum well. The simulations describe the actual photoluminescence (PL) observations accurately; in particular, the room temperature e1-hh1 simulated transition energy of 805 meV closely matches the 798 meV transition energy of the experimental PL spectra, a difference of only 7 meV. Likewise, the expected energy separations between local maxima (p1-p2) in the simulated/experimental spectra have a difference of just 2 meV: a simulated energy separation of 31 meV compared to the experimental value of 33 meV. Utilizing a non equilibrium generalized Planck relation, a full spectrum fit enables individual carrier temperatures for both holes and electrons. This results in two very different carrier temperatures for holes and electrons: where the hole temperature, T-h, is nearly equal to the lattice temperature, T-L; while, the electron temperature, T-e, is 'hot' (i.e., T-e > T-L). Also, by fitting the experimental spectra via three different methods a 'hot' carrier temperature is associated with electrons only; all three methods yield similar 'hot' carrier temperatures.

Observation of impurity band conduction and variable range hopping in heavily doped (010) β-Ga2O3

Abstract: Temperature-dependent resistivity and Hall measurements were performed on a heavily Sn-doped (010) β-Ga2O3 substrate grown by edge-defined film-fed method. At room temperature, the Hall electron mobility was 77.7 cm2 V−1 s−1. A maximum mobility of 95.2 cm2 V−1 s−1 was achieved at T = 185 K. Because the doping concentration is near the Mott critical value of 2.48 × 1018 cm−3, Hall electron concentration and mobility data were analyzed using the two-band model, which considers transport through both the conduction band and an impurity band. At T > 200 K, charge transport is limited by free carriers in the conduction band. The donor ionization energy was extracted to be E CD = 4 meV. At T ≤ 200 K, the net conductivity is due to electrons in the impurity band, with a concentration 7.6 × 1018 cm−3 and a low mobility of ~0.09–2.27 cm2 V−1 s−1. Over a very wide temperature range, 1.7–200 K, transport in the impurity band was found to occur via variable range hopping, where the average hopping length increases with decreasing temperature.

Quantum computing with quantum-Hall edge state interferometry

Abstract: Electron interferometers based on Hall edge states proved to be robust demonstrators of the coherent quantum dynamics of carriers. Several proposals to expose their capability to build and control quantum entanglement and to exploit them as building block for quantum computing devices has been presented. Here, we review the time-dependent numerical modeling of Hall interferometers operating at the single-carrier level at integer filling factor (FF). By defining the qubit state either as the spatial localization (at FF 1) or the Landau index (at FF 2) of a single carrier propagating in the edge state, we show how a generic one-qubit rotation can be realized. By a proper design of the 2DEG potential landscape, an entangling two-qubit gate can be implemented by exploiting Coulomb interaction, thus realizing a universal set of quantum gates. We also assess how the shape of the edge confining potential affects the visibility of the quantum transformations.