We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

In the past several years, research in each of the wide‐band‐gap semiconductors, SiC, GaN, and ZnSe, has led to major advances which now make them viable for device applications. The merits of each contender for high‐temperature electronics and short‐wavelength optical applications are compared. The outstanding thermal and chemical stability of SiC and GaN should enable them to operate at high temperatures and in hostile environments, and also make them attractive for high‐power operation. The present advanced stage of development of SiC substrates and metal‐oxide‐semiconductor technology makes SiC the leading contender for high‐temperature and high‐power applications if ohmic contacts and interface‐state densities can be further improved. GaN, despite fundamentally superior electronic properties and better ohmic contact resistances, must overcome the lack of an ideal substrate material and a relatively advanced SiC infrastructure in order to compete in electronics applications. Prototype transistors have been fabricated from both SiC and GaN, and the microwave characteristics and high‐temperature performance of SiC transistors have been studied. For optical emitters and detectors, ZnSe, SiC, and GaN all have demonstrated operation in the green, blue, or ultraviolet (UV) spectra. Blue SiC light‐emitting diodes (LEDs) have been on the market for several years, joined recently by UV and blue GaN‐based LEDs. These products should find wide use in full color display and other technologies. Promising prototype UV photodetectors have been fabricated from both SiC and GaN. In laser development, ZnSe leads the way with more sophisticated designs having further improved performance being rapidly demonstrated. If the low damage threshold of ZnSe continues to limit practical laser applications, GaN appears poised to become the semiconductor of choice for short‐wavelength lasers in optical memory and other applications. For further development of these materials to be realized, doping densities (especially p type) and ohmic contact technologies have to be improved. Economies of scale need to be realized through the development of larger SiC substrates. Improved substrate materials, ideally GaN itself, need to be aggressively pursued to further develop the GaN‐based material system and enable the fabrication of lasers. ZnSe material quality is already outstanding and now researchers must focus their attention on addressing the short lifetimes of ZnSe‐based lasers to determine whether the material is sufficiently durable for practical laser applications. The problems related to these three wide‐band‐gap semiconductor systems have moved away from materials science toward the device arena, where their technological development can rapidly be brought to maturity.

Large‐band‐gap SiC, III‐V nitride, and II‐VI ZnSe‐based semiconductor device technologies

Wide bandgap semiconductors are extremely attractive for the gamut of power electronics applications from power conditioning to microwave transmitters for communications and radar. Of the various materials and device technologies, the AlGaN/GaN high-electron mobility transistor seems the most promising. This paper attempts to present the status of the technology and the market with a view of highlighting both the progress and the remaining problems.

AlGaN/GaN HEMTs-an overview of device operation and applications

Wide bandgap semiconductors show superior material properties enabling potential power device operation at higher temperatures, voltages, and switching speeds than current Si technology. As a result, a new generation of power devices is being developed for power converter applications in which traditional Si power devices show limited operation. The use of these new power semiconductor devices will allow both an important improvement in the performance of existing power converters and the development of new power converters, accounting for an increase in the efficiency of the electric energy transformations and a more rational use of the electric energy. At present, SiC and GaN are the more promising semiconductor materials for these new power devices as a consequence of their outstanding properties, commercial availability of starting material, and maturity of their technological processes. This paper presents a review of recent progresses in the development of SiC- and GaN-based power semiconductor devices together with an overall view of the state of the art of this new device generation.

A Survey of Wide Bandgap Power Semiconductor Devices

An object is to provide a semiconductor device of which a manufacturing process is not complicated and by which cost can be suppressed, by forming a thin film transistor using an oxide semiconductor film typified by zinc oxide, and a manufacturing method thereof. For the semiconductor device, a gate electrode is formed over a substrate; a gate insulating film is formed covering the gate electrode; an oxide semiconductor film is formed over the gate insulating film; and a first conductive film and a second conductive film are formed over the oxide semiconductor film. The oxide semiconductor film has at least a crystallized region in a channel region.

Semiconductor device, and manufacturing method thereof

Due to the high critical field in 4H-SiC, the drain charge and switching loss densities in a SiC power device are approximately 10X higher than that of a silicon device. However, for the same voltage and resistance ratings, the device area is much smaller for the 4H-SiC device. Therefore, the total drain charge and switching losses are much lower for the 4H-SiC power device. A 2.3 kV, 13.5 mW-cm2 4H-SiC power DMOSFET with a device area of 2.1 mm x 2.1 mm has been demonstrated. The device showed a stable avalanche at a drain bias of 2.3 kV, and an on-current of 5 A with a VGS of 20 V and a VDS of 2.6 V. Approximately an order of magnitude lower parasitic capacitance values, as compared to those of commercially available silicon power MOSFETs, were measured for the 4H-SiC power DMOSFET. This suggests that the 4H-SiC DMOSFET can provide an order of magnitude improvement in switching performance in high speed switching applications.

4H-SiC DMOSFETs for High Speed Switching Applications

A 4H-SiC pin diode with improved termination named mesa JTE has been developed and a high blocking voltage of 6.2kV and a low VF of 4.7V at 100A/cm2 have been achieved. A developed diode has a short reverse recovery time of 28.5ns at room temperature. By evaluation of reverse recovery characteristics, a developed diode has a minority carrier lifetime of longer than 1μs at 350°C in spite of a minority carrier lifetime of 64ns at room temperature. Furthermore, a developed diode has 1/29th lower recovery loss than those of a commercialized 4.5kV Si diode.

Dynamic Characteristics of 6.2 kV High Voltage 4H-SiC pn Diode with Low Loss

High-energy neutrons produced by cosmic ray interactions with our atmosphere are known to cause single-event burnout (SEB) failure in power devices operating at high fields. We have performed accelerated high-energy neutron SEB testing of SiC and Si power devices at the Los Alamos Neutron Science Center (LANCSE). Comparing Wolfspeed SiC MOSFETs having different voltage (900V – 3300V) and current (3.5A – 72A) ratings, we find a universal behavior when scaling failure rates by active area, and scaling drain bias by avalanche voltage. Moreover, diodes and MOSFETs behave similarly, revealing that the SiC drift dominates the failure characteristics for both device types. This universal scaling holds for SiC MOSFETs from other manufacturers as well. The SEB characteristics of Si power IGBT and MOSFET devices show that near their rated voltages failure rates of Si devices can be 10X higher than that of comparable SiC MOSFET devices. Thus, Si devices are more susceptible to SEB failure from voltage overshoot conditions.

Reliability of SiC Power Devices against Cosmic Ray Neutron Single-Event Burnout

This work explores the effects of extended epitaxial defects on 4H-SiC power devices. Advanced defect mapping techniques were used on large quantities of power device wafers, and data was aggregated to correlate device electrical characteristics to defect content. 1200 V class Junction Barrier Schottky (JBS) diodes and MOSFETs were examined in this manner; higher voltage 3.3 kV class devices were examined as well. 3C inclusions and triangular defects, as well as heavily decorated substrate scratches, were found to be device killing defects. Other defects were found to have negligible impacts on device yield, even in the case of extremely high threading dislocation content. Defect impacts on device reliability was explored on MOS-gate structures, as well as long-term device blocking tests on both MOSFETs and JBS diodes. Devices that passed on-wafer electrical parametric tests were found to operate reliably in these tests, regardless of defect content.

Performance and Reliability Impacts of Extended Epitaxial Defects on 4H-SiC Power Devices

A novel high voltage SiC MOS device named SIAFET (Static induction Injected Accumulated FET) is proposed, which has no pn junction in its on-current flow path and has a conductivity modulation by carriers injected from a p+ buried gate. SIAFET with blocking voltage (BV) of 5500 V and specific on-resistance RonS (without the conductivity modulation) of 57 m/spl Omega/cm/sup 2/ was designed by using the 6200 V mesa JTE and was fabricated using 4H-SiC substrates. Its basic operation has been confirmed for the first time and it has been demonstrated that its RonS has been shown to reduce to less than 1/6 by SIAFET action. Although its achieved BV is relatively low (2030 V and 4580 V) and its RonS is relatively high (172 m/spl Omega/cm/sup 2/ and 387 m/spl Omega/cm/sup 2/), RonS of 4580 V SiC-SIAFET is less than 1/25 that of the theoretical RonS limit of Si-MOSFET for this BV.

John W. Palmour

Papers

4H-SiC DMOSFETs for High Speed Switching Applications

Dynamic Characteristics of 6.2 kV High Voltage 4H-SiC pn Diode with Low Loss

Reliability of SiC Power Devices against Cosmic Ray Neutron Single-Event Burnout

Performance and Reliability Impacts of Extended Epitaxial Defects on 4H-SiC Power Devices

4.5 kV novel high voltage high performance SiC-FET "SIAFET"