2015 MRS Spring Meeting

Apr 6, 2015 ‐ Apr 10, 2015


The 2015 MRS Spring Meeting and Exhibit was held in San Francisco, California. Technical symposia were held within the following topical clusters: Energy, Nanomaterials, Electronics and Photonics, Soft and Biomaterials, and General – Fabrication and Characterization. Access to all of the content below is free – simply login with your MRS username and password for complete access. Don’t have a username and password? Create a free account.


Sessions

InGaN-Based Laser Diodes: Physical Origin of Gradual and Catastrophic Degradation

Apr 7, 2015 8:30am ‐ Apr 7, 2015 9:00am

Identification: CC1.02

Over the last few years, the research in the field of InGaN-based laser diodes has shown impressive advancements: these devices can currently cover the wavelength range between 375 nm and 530 nm, and are expected to find wide application in the next generation projectors, optical data storage systems, and biomedical devices. Moreover, it has been recently demonstrated that blue InGaN lasers can be used for the fabrication of high-intensity white lamps, for application in the automotive field. Most of these applications require high optical power levels (>0.25-1 W for the single laser diode): as a result, the devices are driven under extreme conditions; typical current densities can be in excess of 10 kA/cm2, corresponding to high levels of power dissipation (~50 kW/cm2) and self heating (Tj>100-150 �C). These factors may lead to the early degradation of the laser diodes, since temperature and current act as driving forces for the gradual degradation. This presentation describes the physical mechanisms responsible for the degradation of InGaN-based laser diodes submitted to high current/temperature stress; more specifically, we will discuss the following relevant topics: (i) the degradation of the efficiency of the quantum well region due to the generation of non-radiative centers, and the properties of the related defects; (ii) the changes of the electrical characteristics of the devices induced by the exposure to high temperatures; (iii) the sudden degradation of the laser diodes, due to catastrophic-optical damage and to electrostatic discharges. In addition, we will discuss the role of the various driving forces (temperature, current, optical power) in accelerating the degradation kinetics, and the relation between time-to-failure and material quality. The results described within the presentation will be critically compared to recent papers, to give an exhaustive description of the topic.

Speaker(s):

Designing Self-Powered Nanomotors and Pumps

Apr 7, 2015 8:30am ‐ Apr 7, 2015 9:00am

Identification: L1.01

Self-powered nano and microscale moving systems are currently the subject of intense interest due in part to their potential applications in nanomachinery, nanoscale assembly, robotics, fluidics, and chemical/biochemical sensing. We will demonstrate that one can build autonomous nanomotors over a wide range of length-scales �from scratch� that mimic biological motors by using catalytic reactions to create forces based on chemical gradients. These motors are autonomous in that they do not require external electric, magnetic, or optical fields as energy sources. Instead, the input energy is supplied locally and chemically. These "bots" can be directed by information in the form of chemical and light gradients. Further, we have developed systems in which chemical secretions from the translating nano/micromotors initiate long-range, collective interactions among themselves and neighboring inert particles. This behavior is reminiscent of quorum sensing organisms that swarm in response to a minimum threshold concentration of a signaling chemical. In addition, an object that moves by generating a continuous surface force in a fluid can, in principle, be used to pump the fluid by the same catalytic mechanism. Thus, by immobilizing the nano/micromotors, we have developed nano/microfluidic pumps that transduce energy catalytically. These non-mechanical pumps provide precise control over flow rate without the aid of an external power source and are capable of turning on in response to specific analytes in solution.

Speaker(s):

Large Area Formation of Self-Aligned Crystalline Domains of Organic Semiconductors on Transistor Channels using a Novel Crystallization Technique: CONNECT

Apr 7, 2015 8:30am ‐ Apr 7, 2015 8:45am

Identification: MM1.01

The electronic properties of solution-processable small molecule organic semiconductors (OSCs) have rapidly improved in recent years, rendering them highly promising for various low-cost, large area, flexible, and transparent electronic applications such as displays, RFID tags, and integrated logic circuits. In order for these applications to be realized, nucleation and crystallization of OSCs must be carefully controlled so that the OSCs are patterned and precisely registered to within the transistor channel with uniform device properties over a large area�a task that that remains a significant challenge. In this presentation, we introduce a novel nucleation and crystallization technique known as CONNECT (Controlled OSC NucleatioN and Extension for CircuiTs) that utilizes differential surface energy and solution shearing to induce nucleation and crystal growth at specific points, resulting in self-patterned and self-registered OSC film within the channel region with well-aligned crystalline domains. The well-aligned crystals with minimal grain boundaries over the channel region resulted in low variability in device-to-device characteristics over a large area with average on-current density of 0.4 ?A/?m and on/off ratio of 6.15 x 10(3). We have fabricated transistor density as high as 840 dpi, with a yield of 99%, previously unseen in literature. We have also built various logic gates and a 2-bit half adder circuit to demonstrate the feasibility of our technique in generating large-scale electronic circuits in a facile and economical manner.

Speaker(s):

Understanding the Mechanism of Plasma-Assisted Silicon Nanowire Growth Based on Low Surface Tension Metals

Apr 7, 2015 8:30am ‐ Apr 7, 2015 8:45am

Identification: S1.02

Radial junction thin film solar cells fabricated over silicon nanowire (SiNW) arrays benefit from an excellent light trapping and a reduced material consumption. A material efficient approach to grow the SiNWs on low cost substrates is a vapor-liquid-solid (VLS) method, where metal droplets assist the deposition of gaseous precursors and may play the role of catalysts depending on their nature. In order to optimize the device properties, it is very important to understand the growth mechanism of the NWs. Most of the studies available in the literature explain the mechanism for gold droplets, but gold is not a suitable candidate for making silicon electronic devices as it introduces deep band gap defects in silicon which act as recombination centres. On the other hand, low melting point metals, such as gallium (Ga), tin (Sn), indium (In), and bismuth (Bi) could be ideal choices for fabricating NWs at low temperature, but very few studies have been carried out to understand their exact role in the growth process. Moreover, they are not stable on the SiNW top and tend to wet the nanowire sidewall due to their lower surface energy than silicon. As a matter of fact, a thin metal layer on the nanowire sidewall could help to stabilize metal droplets on the top. We have carried out a series of experiments, studying the axial and radial growth of SiNWs during the plasma-assisted VLS process using Sn as a model system, and found that some of the experimental observations cannot be explained without the existence of the sidewall wetting layer. In this work, we will show how this wetting layer stabilizes the metal droplet on the top of the nanowire and even promotes the growth when the droplet from top is exhausted either due to the hydrogen radical assisted etching or the incorporation inside the nanowire.

Speaker(s):

Recent Progress in Understanding the Electrical Reliability of GaN High-Electron Mobility Transistors

Apr 7, 2015 9:00am ‐ Apr 7, 2015 9:30am

Identification: AA1.04

GaN High-Electron Mobility Transistors (HEMTs) are well on their way to revolutionizing RF, microwave and millimeter-wave communications and radar systems. GaN FETs are also uniquely poised to have a disruptive impact in electrical power management. In all these applications, device reliability remains a significant concern. As the field has expanded, great progress has recently taken place in understanding GaN transistor degradation, especially under high-voltage stress. Detailed electrical studies coupled with comprehensive failure analysis involving a variety of techniques have revealed a rich picture of degradation. Early studies showed that high voltage degradation of GaN HEMTs was characterized by a critical voltage (Vcrit) at which the device gate current abruptly increases. For stress voltage beyond Vcrit, prominent degradation was observed in the drain current and other electrical parameters of the device. More recently, it has been shown that degradation in the gate current can occur for voltages below the critical voltage suggesting that stress time is a key variable in degradation. Cross-section TEM and planar imaging techniques have shown that high-voltage stress induces prominent structural defects such as grooves, pits and cracks in the GaN cap and AlGaN barrier at the edge of the gate. The evolution of these defects correlates well with that of electrical degradation. Recently, a similar pattern of degradation has been observed under high-power DC and RF stress, although not in a consistent way. A significant recent finding is the role that moisture plays in the formation of these structural defects. This suggests a path for mitigation. Separately from device degradation, a significant anomaly affecting GaN transistors is electron trapping which can severely upset device operation on a wide time domain. This talk will review recent research on the electrical reliability and trapping of GaN HEMTs.

Speaker(s):

Combining Perovskites with Conventional Solar Cell Materials to Make Highly Efficient and Inexpensive Tandems

Apr 7, 2015 9:00am ‐ Apr 7, 2015 9:30am

Identification: C1.02

The efficiency of perovskite solar cells has soared from a few percent to over 17% in the last 2 years. They are very attractive for multijunction solar cell applications because the bandgap of perovskite semiconductors can be easily tuned in the range of 1.55 to 2.2 eV and the open circuit voltage of the cells is large. We have made highly efficient semitransparent perovskite solar cells using silver nanowire meshes as the top electrode. These cells can be used in combination with either silicon or copper indium gallium diselenide solar cells to make four-terminal tandems. We will also present detailed characterization of perovskite semiconductors made with different processing conditions to show what needs to be done to minimize recombination and make the solar cells stable.

Speaker(s):

About the Role of the Thermal Conductivity of the Laser Structure in the Catastrophic Optical Degradation of High Power Laser Diodes

Apr 7, 2015 9:00am ‐ Apr 7, 2015 9:15am

Identification: CC1.03

The degradation of laser diodes constitutes a challenge for laser manufacturers and end users. The catastrophic optical damage (COD) of laser diodes consists of the sudden drop off of the optical power. COD is generally associated with a thermal runaway mechanism in which the active zone of the laser is molten in a positive feedback process. Degraded devices present dark line defects (DLDs) along the laser cavity produced during the laser operation; these DLDs are regions of the active zone of the laser with very low or null light emission as revealed by cathodoluminescence (CL) studies of the degraded regions of the laser. These dark lines are locally generated, either at the front facet, or inside the cavity, propagating along the cavity driven by the optical field. The physical sequence leading to the formation of such lines and the associated loss of output optical power is described in the literature; however, there is a lack of consensus about the connection between the successive steps leading to COD. Understanding this is crucial to improve the technological factors that can strengthen the laser diodes. The full sequence of the degradation consists of different phases, in the first phase a weak zone of the laser is incubated, and the local temperature is increased in such a zone; when a critical temperature is reached the thermal runaway process takes place. Usually, the positive feedback leading to COD is circumscribed to the sequential enhancement of the optical absorption by the increase of the temperature. However, the meaning of the critical temperature has not been unambiguously established. Herein, we will discuss about the critical temperature, and the physical mechanisms involved in this phase; in particular, we will describe the defect morphology and the conditions under which such critical temperature can be reached, both in the front mirror face and the inner cavity. For this, we will analyze the meaning of the critical temperature and the influence of the progressive decrease of the thermal conductivity of the laser structure on the degradation during the laser operation. We will compare the critical temperature estimated by a thermomechanical model with the values usually reported, which range between 130�C and 200�C.

Speaker(s):

Advanced Functional Materials: Intrinsic and Doped Silicon Oxide

Apr 7, 2015 9:00am ‐ Apr 7, 2015 9:30am

Identification: E1.01

In comparison to other silicon materials, the particular two-phase structure of silicon oxide materials, in which hydrogenated microcrystalline silicon crystallites are surrounded by an oxygen-rich hydrogenated amorphous silicon phase, causes them present excellent photoelectrical material properties, such as a low-parasitic absorption in broadband spectral range, independent controllability of longitudinal and lateral conductivity, refractive indices (3.5-2.0), band gap (2.0-2.6 eV) and conductivity tenability (with orders of 1-10-9 S/cm) with oxygen doping, and so on. Various types of silicon oxide materials, including intrinsic, p- or n- type, applied in thin film solar cells have also played significant roles in improving the efficiency of various types of single-, dual-, and triple-junction thin-film solar cells from both the optical and electrical points of view. In this paper, we present our latest progress in studying the performance improvement role of intrinsic or doped silicon oxide materials in pin-type a-Si:H, a-SiGe:H, and ?c-Si:H single-junction solar cells. By effectively tuning the band gap values of intrinsic a-SiOx:H materials with oxygen doping and adopting the layers with a suitable band gap (1.86 eV) as the P/I buffer layers of a-Si:H solar cells fabricated on metal organic chemical vapor deposition boron-doped zinc oxide substrates, a significant Voc increase up to 909 mV and an excellent external quantum efficiency response of 75% at 400 nm typical wavelength can be achieved by matching the band gap discontinuity between the p-type a-SiOx:H window and a-Si:H intrinsic layers. The high leakage current characteristics of pin-type narrow-gap (Eg

Speaker(s):

Nanotailoring ZnO:X (X = Al, B) Films by Atomic Layer Deposition

Apr 7, 2015 9:15am ‐ Apr 7, 2015 9:30am

Identification: TT1.05

Transparent conductive oxides (TCOs) have become ubiquitous in many of today�s electronic devices and their importance will keep increasing with the growing role of solar energy in our society. However, TCOs will not just find more and wider applications but in many cases more precise control of their properties will also be required. In this contribution, we will focus on ZnO, which is an abundant material that can serve as a TCO when doped with group 13 elements. We will show that the method of atomic layer deposition (ALD) can be used to prepare high-quality ZnO:X (X = Al, B) films with a very precise control of their electronic properties even at the nanometer-level. ZnO:Al and ZnO:B films were prepared by ALD using the process based on diethylzinc (DEZ, Zn(C2H5)2) and H2O dosing and with either trimethylaluminum (TMA, Al(CH3)3); dimethylaluminum isoproproxide (DMAI, Al(CH3)2(OiPr)) or triisopropyl borate (TIB, B(OiPr)3) as precursors for the dopants. The doping was obtained using so-called supercycles in which the ratio of the DEZ and dopant cycles controls the dopant density in the films. By varying this ratio the dopant density can be precisely tuned and even easily graded throughout the film, which are important merits of the ALD technique in addition to its key features such as a unparalleled uniformity over large areas, an excellent conformality on 3D surface topologies and a relatively low thermal budget (typical substrate temperatures are 150 - 250 �C). From a detailed study involving Rutherford backscattering spectroscopy and transmission electron microscopy, it will be shown that by using DMAI and TIB as precursors, the lateral spacing of the dopants can be better controlled than with TMA due to fact that DMAI and TIB are larger molecules, leading to more steric hindrance at the surface. This better control correlates directly with the electronic properties of the films and it will be shown that the doping efficiency obtained for DMAI and TIB (up to 40-60%) is much higher than for TMA (5-10%). ZnO:Al and ZnO:B films with resistivities

Speaker(s):

The Research and Applications of the Si Base Thin Film Photovoltaic Modules

Apr 7, 2015 9:45am ‐ Apr 7, 2015 10:00am

Identification: E1.03

The high efficiency and stabilized properties of solar cell modules are very important for solar farm applications. In order to make high efficiency a-Si:H/uc-Si:H tandem solar cell modules, we optimized the property of each single layer such as amorphous intrinsic layer, intermediate reflective layer and microcrystalline intrinsic layer, n doped amorphous layer between microcrystalline i/n layer and employed the two-step growth method of low pressure chemical vapor deposition(LPCVD) process to fabricate the ZnO:B-TCO film etc. After optimization of the Si base solar module processes, the conversion efficiency of 11.87% can be achieved. The degradations of Si Base solar modules have been also investigated under light soaking and other various measurement conditions. The lower degradations of 5 - 7% drop in conversion efficiency have been obtained. On other hand, the Si base thin film modules for building integrated with photovoltaics (BIPV) applications have been developed. Such as the photovoltaic curtain wall, fence, sunroof, street lamps, pumps have been fabricated, and the solar farm with remote monitoring systems of solar farms have also been installed.

Speaker(s):
  • Tingkai Li, Hunan Gongchuang Photovoltaic Science and Technology Co
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