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In summary, this Special Topic provides an excellent overview and cross section of key challenges in the development of high-efficiency, high power, and long lifetime UV/DUV LEDs as well as the rapidly emerging area of UVB and UVC laser diodes. Recently, several groups have reported current-injection laser diodes in the UVB and UVC spectral region with emission wavelength as short as 272 nm.[[17]] Even though the demonstrated devices are still a long way from what is needed for practical systems, these breakthroughs illustrate that semiconductor UV/DUV laser diode technologies show great potential. Finally, Kondo I et al. i demonstrate high-power operation of AlGaN-based UVB laser diodes.[41] By improving the injection efficiency through modulation of the polarization charge at the interface between the p-side waveguide layer and the EBL, they were able to show pulsed laser diodes with a peak light output power of 150 mW at a lasing wavelength of 300 nm. Group III-nitride based light emitters in the ultraviolet (UV) and deep ultraviolet (DUV) spectral range are the fundamental building block for several key applications. [Extracted from the article]

This Letter investigates the cryogenic behavior of NbO2 threshold switching devices. Pt/NbO2/Pt devices are demonstrated to be well functional as threshold switching devices at ultra-low temperature (4 K). When the temperature decreases, the OFF-state resistance of NbO2 increases and the switching voltage increases. With the extracted characteristics of NbO2 ranging from 4 K to 300 K, we continue to study the neuromorphic system using the crossbar array with resistive memories as resistive synapses and NbO2 as oscillation neurons at different temperatures through SPICE simulation. The simulation results show that the oscillation systems could still work properly at 4 K. The oscillation amplitude decreases as temperature increases. The oscillation frequency depends on both the temperature and the input voltage. [ABSTRACT FROM AUTHOR]

We demonstrate that a ferroelectric can cause a differential voltage amplification without needing an external energy source. As the ferroelectric switches from one polarization state to the other, a transfer of energy takes place from the ferroelectric to the dielectric, determined by the ratio of their capacitances, which, in turn, leads to the differential amplification. This amplification is very different in nature from conventional inductor-capacitor based circuits where an oscillatory amplification can be observed. The demonstration of differential voltage amplification from completely passive capacitor elements only has fundamental ramifications for next generation electronics. [ABSTRACT FROM AUTHOR]

We study the effects of strain relaxation on the dielectric properties of epitaxial 40 nm Pb(Zr0.2Ti0.8)TiO3 (PZT) films. A significant increase in the defect and dislocation density due to strain relaxation is observed in PZT films with tetragonality c/a<1.07 grown on SrTiO3 (001) substrates, which results in significant frequency dispersion of the dielectric constant and strong Rayleigh type behavior in those samples. This combined structural-electrical study provides a framework for investigating strain relaxation in thin films and can provide useful insights into the mechanisms of fatigue in ferroelectric materials. [ABSTRACT FROM AUTHOR]

A spintronic device, called the "strain assisted spin transfer torque (STT) random access memory (RAM)," is proposed by combining the magnetostriction effect and the spin transfer torque effect which can result in a dramatic improvement in the energy dissipation relative to a conventional STT-RAM. Magnetization switching in the device which is a piezoelectric-ferromagnetic heterostructure via the combined magnetostriction and STT effect is simulated by solving the Landau-Lifshitz-Gilbert equation incorporating the influence of thermal noise. The simulations show that, in such a device, each of these two mechanisms (magnetostriction and spin transfer torque) provides in a 90± rotation of the magnetization leading a deterministic 180± switching with a critical current significantly smaller than that required for spin torque alone. Such a scheme is an attractive option for writing magnetic RAM cells. [ABSTRACT FROM AUTHOR]

We report a proof-of-concept demonstration of negative capacitance effect in a nanoscale ferroelectric-dielectric heterostructure. In a bilayer of ferroelectric Pb(Zr0.2Ti0.8)O3 and dielectric SrTiO3, the composite capacitance was observed to be larger than the constituent SrTiO3 capacitance, indicating an effective negative capacitance of the constituent Pb(Zr0.2Ti0.8)O3 layer. Temperature is shown to be an effective tuning parameter for the ferroelectric negative capacitance and the degree of capacitance enhancement in the heterostructure. Landau's mean field theory based calculations show qualitative agreement with observed effects. This work underpins the possibility that by replacing gate oxides by ferroelectrics in nanoscale transistors, the sub threshold slope can be lowered below the classical limit (60 mV/decade). [ABSTRACT FROM AUTHOR]

State-of-the-art semiconducting aluminum nitride (AlN) films were characterized by cathodoluminescence (CL) spectroscopy in the range of 200–500 nm in an attempt to identify the energy levels within the bandgap and their associated defects. Near-band edge emission (around 206 nm) and high-intensity peaks centered in the near UV range (around 325 nm) are observed for both n- and p-type AlN films. The near UV peaks are potentially associated with oxygen contamination in the films. The p-type AlN films contain at least two unidentified peaks above 400 nm. Assuming that the dopant concentration is independent of compensation (i.e., in the perfect doping limit), three effective donor states are found from Fermi–Dirac statistics for Si-doped AlN, at ∼0.035, ∼0.05, and ∼0.11 eV. Similarly, a single effective acceptor energy of ∼0.03–0.05 eV (depending on the degeneracy factory considered) was found for Be doped AlN. CL investigation of doped AlN films supports claims that AlN may be a promising optoelectronic material, but also points to contaminant mitigation and defect theory as major areas for future study. [ABSTRACT FROM AUTHOR]

A correction is presented to the article "Differential voltage amplification from ferroelectric negative capacitance" which was published in the perodical.

We demonstrate an electrically induced, non-volatile, metal-insulator phase transition in a MoS2 transistor. A ferroelectric capacitor made of single crystalline, epitaxially grown PbZr0.2Ti0.8O3 was connected to the gate of a field effect thin film MoS2 transistor. When a voltage is applied to this ferroelectric capacitor, a clear transition from an insulator to a metal and vice versa is observed in the transistor. Importantly, when the biased voltage is turned off, the remnant polarization in the ferroelectric can keep the MoS2 in its original phase, thereby providing a non-volatile state. Thus, a metallic or insulating phase can be written, erased, or retained simply by biasing the externally connected ferroelectric capacitor. [ABSTRACT FROM AUTHOR]

We demonstrate non-volatile, n-type, back-gated, MoS2 transistors, placed directly on an epitaxial grown, single crystalline, PbZr0.2Ti0.8O3 (PZT) ferroelectric. The transistors show decent ON current (19μA/lm), high on-off ratio (107), and a subthreshold swing of (SS~92 mV/dec) with a 100 nm thick PZT layer as the back gate oxide. Importantly, the ferroelectric polarization can directly control the channel charge, showing a clear anti-clockwise hysteresis. We have selfconsistently confirmed the switching of the ferroelectric and corresponding change in channel current from a direct time-dependent measurement. Our results demonstrate that it is possible to obtain transistor operation directly on polar surfaces, and therefore, it should be possible to integrate 2D electronics with single crystalline functional oxides. [ABSTRACT FROM AUTHOR]

A correction to the article "Experimental evidence of ferroelectric negative capacitance in nanoscale heterostructures" that was published in the September 12, 2011 issue is presented.

Ultra-wide bandgap AlGaN has attracted recent attention as a promising channel material for next-generation high electron mobility transistors (HEMTs) for RF power due to its high critical field, excellent transport properties, and potential for operation in extreme environments. However, the effects of temperature on the transport properties are not fully understood. Here, Al0.62Ga0.38N/Al0.45Ga0.55N HEMTs have been fabricated and characterized up to 150 °C at DC and RF to evaluate the effect of temperature on electron mobility and carrier velocity. Measured results indicate that both mobility and carrier velocity exhibit modest dependence on temperature, suggesting that AlGaN channel HEMTs are promising for future RF power applications. [ABSTRACT FROM AUTHOR]

We report on 193 nm excimer laser-based liftoff (LLO) of Al0.26Ga0.74N/GaN high electron mobility transistors (HEMTs) with thick (t > 10 μm) AlN heat spreading buffer layers grown over sapphire substrates. The use of the thick AlN heat spreading layer resulted in thermal resistance ( R t h ) of 16 K mm/W for as-fabricated devices on sapphire, which is lower than the value of ∼25–50 K mm/W for standard HEMT structures on sapphire without the heat-spreaders. Soldering the LLO devices onto a copper heat sink led to a further reduction of R t h to 8 K mm/W, a value comparable to published measurements on bulk SiC substrates. The reduction in R t h by LLO and bonding to copper led to significantly reduced self-heating and drain current droop. A drain current density as high as 0.9 A/mm was observed despite a marginal reduction of the carrier mobility (∼1800 to ∼1500 cm2/V s). This is the highest drain current density and mobility reported to-date for LLO AlGaN/GaN HEMTs. [ABSTRACT FROM AUTHOR]

We demonstrate a technique to quickly build and spatially map the frequency response of optoelectronic devices. The transfer function of a linear system is the Fourier transform of its impulse response. Such an impulse response is obtained from transient photocurrent measurements of devices such as photodetectors and solar cells. We introduce and apply Fourier transform impedance spectroscopy (FTIS) to a PbS colloidal quantum dot SiC heterojunction photodiode and validate the results using intensity-modulated photocurrent spectroscopy. Cutoff frequencies in the devices were as high as ∼10 kHz, showing their utility in advanced thin film and flexible electronics. The practical frequencies for FTIS lie in the mHz–kHz range, ideal for composite materials such as quantum dot films that are dominated by interfacial trap states. These can lead to characteristic lengths for charge collection ∼20–500 μm dominated by transmission line effects, rather than intrinsic diffusion and drift length scales, enabling extraction of interfacial capacitances and series/parallel resistances. [ABSTRACT FROM AUTHOR]

A ferroelectric material in a ferroelectric–dielectric heterostructure can provide a charge boost, as often discussed in the context of negative capacitance, and, in doing so, can reduce the power dissipation in field-effect transistor technology. However, there is an ongoing debate on whether the charge boost in such a heterostructure is a transient phenomenon or a steady state one. In this Letter, we use the positive-up-negative-down (PUND) measurement technique on a ferroelectric–dielectric (FE–DE) capacitor to show that the charge boost is a steady-state effect—i.e., the charge boost remains intact after the initial transient effects subside and all the voltages in the system reach constant values and steady states. We also demonstrate differential charge boost in steady state using a staircase voltage pulse measurement technique. An experimentally calibrated multi-domain SPICE model of an FE–DE stack is used to accurately simulate the PUND and staircase voltage hopping methods. [ABSTRACT FROM AUTHOR]

We present the temperature-dependent electrical characteristics of high-current depletion (D-mode) and barrier-recessed enhancement-mode (E-mode) ultrawide bandgap (UWBG) AlxGa1−xN channel insulated gate heterojunction field-effect transistors fabricated on the same wafer. The key motivation is the higher Baliga figure of merit for devices with the UWBG AlGaN channel and their strong potential for use in high-power, high-temperature harsh environmental applications. Over a temperature range of 125 °C, the VTH shifted in the opposite direction for D- and E-mode devices with a rate of +13.5 mV/K and −23 mV/K, respectively, giving an overall shift of +1.7 V and −2.9 V. This was attributed to changes in the fixed and trapped charge densities in the dielectric and at the dielectric–AlGaN barrier interface. A single deep sub-bandgap trap level was sufficient to explain the threshold shifts in both devices. The effective channel mobility in the E-mode devices was argued to be limited by charge scattering, arising from the same charges introduced during barrier recessing that shifted VTH. [ABSTRACT FROM AUTHOR]

Tunable, non-volatile, small-signal capacitance is observed and characterized in a TiN/ferroelectric Hf0.5Zr0.5O2 (HZO)/TiN stack. The non-volatility of the small-signal capacitance originates from the non-uniform distribution of oxygen vacancies near/at the bottom electrode, resulting in polarity-dependent responses of the domain-wall vibration. Our hypothesis also agrees with the observed frequency dispersion, which could be explained with the Maxwell Wagner effect. Furthermore, we have shown more than 104 endurance and 10-year retention at 85 °C for this non-volatile capacitance. With tunable capacitance at a fixed bias, HZO capacitors can potentially be applied to reconfigurable analog or mixed-signal circuits. [ABSTRACT FROM AUTHOR]

A ferroelectric field-effect transistor (FeFET) with scaled dimensions (170 nm and 24 nm of gate width and length, respectively) and a 10 nm thick Si doped HfO 2 ferroelectric in the gate oxide stack are characterized at cryogenic temperatures down to 6.9 K. We observe that a decrease in temperature leads to an increase in the memory window at the expense of an increased program/erase voltage. This is consistent with the increase in the ferroelectric coercive field due to the suppression of thermally activated domain wall creep motion at cryogenic temperatures. However, the observed insensitivity of the location of the memory window with respect to temperature cannot be explained by the current understanding of the device physics of FeFETs. Such temperature dependent studies of scaled FeFETs can lead to useful insights into their underlying device physics, while providing an assessment of the potential of this emerging technology for cryogenic memory applications. [ABSTRACT FROM AUTHOR]

In this Letter, we demonstrate a Pt/BaTiO3/Al0.58Ga0.42N lateral heterojunction diode with enhanced breakdown characteristics. By using BaTiO3, a high-k material, as a dielectric material between the anode and semiconductor, the peak electric field at the anode edge near the cathode was significantly reduced and an average breakdown field exceeding 8 MV/cm was achieved for devices with an anode to cathode spacing of <0.2 μm. In contrast, Pt/Al0.58Ga0.42N control Schottky diodes displayed an average breakdown field of ∼4 MV/cm for devices with similar dimensions. The use of a high-k dielectric can more effectively utilize the high breakdown fields in ultra-wide bandgap materials by proper management of the electric field. This demonstration thus provides a framework to realize ultra-scaled lateral devices with improved breakdown characteristics. [ABSTRACT FROM AUTHOR]

Ultrawide bandgap (UWBG) AlGaN-channel metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs) with a ZrO2 gate dielectric achieve peak current in excess of 0.4 A/mm and current ON/OFF ratios >106 with subthreshold swings as low as 110 mV/decade. These devices have strong potential for use in power and radio frequency electronics or as true solar-blind photodetectors. In this work, we present the photoresponse analysis in UWBG AlGaN MOSHFETs. Persistent photoconductivity with the decay time above 10 minutes can be quenched by illuminating with strong UV light at 365 nm and 254 nm, suggesting deep traps to be responsible for this behavior. Upon correlating the optical response under various illumination conditions with cathodoluminescence of these devices, we identified two key trap levels at ∼2.48 ± 0.14 eV and 3.76 ± 0.06 eV, controlling the slow response time. By depth-profiling using cathodoluminescence, these traps are identified to be at the AlN/AlGaN interface at the back of the device, due to partial relaxation from the lattice mismatch between AlN and Al0.4Ga0.6N. [ABSTRACT FROM AUTHOR]

In this letter, we design and demonstrate an improved metalorganic chemical vapor deposition (MOCVD) grown reverse Al-composition graded contact layer, whereby the Al-composition of AlxGa1−xN in the contact layer is graded from the higher Al-composition as in the channel to lower Al-composition, to achieve a low resistance contact to MOCVD grown ultrawide bandgap Al0.70Ga0.30N channel metal-semiconductor field-effect transistors. Increasing the thickness of the reverse graded layer was found to improve contact layer resistance significantly, leading to a contact resistivity of 3.3 × 10−5 Ω cm2. Devices with a gate length, LG, of 0.6 μm and a source-drain spacing, LSD, of 1.5 μm displayed a maximum current density, IDS,MAX, of 635 mA/mm with an applied gate voltage, VGS, of +2 V. Breakdown measurements on transistors with a gate to drain spacing, LGD, of 770 nm had breakdown voltage greater than 220 V, corresponding to a minimum breakdown field of 2.86 MV/cm—almost 3× higher than that exhibited by lateral GaN channel devices with similar dimensions. This work provides a framework for the design of low resistance contacts to MOCVD grown high Al-content AlxGa1−xN channel transistors. [ABSTRACT FROM AUTHOR]

We report the effects of lanthanum doping/alloying on antiferroelectric (AFE) properties of ZrO2. Starting with pure ZrO2, an increase in La doping leads to the narrowing of the AFE double hysteresis loops and an increase in the critical voltage/electric field for AFE → ferroelectric transition. At higher La contents, the polarization-voltage characteristics of doped/alloyed ZrO2 resemble that of a non-linear dielectric without any discernible AFE-type hysteresis. X-ray diffraction based analysis indicates that the increased La content while preserving the non-polar, parent AFE, tetragonal P42/nmc phase leads to a decrease in tetragonality and the (nano-)crystallite size and an increase in the unit cell volume. Furthermore, antiferroelectric behavior is obtained in the as-deposited thin films without requiring any capping metallic layers and post-deposition/-metallization anneals due to which our specific atomic layer deposition system configuration crystallizes and stabilizes the AFE tetragonal phase during growth. [ABSTRACT FROM AUTHOR]

We report the performance of a bipolar epitaxial graphene (EG)/p-SiC/n+-SiC UV phototransistor fabricated with a Schottky (EG)/SiC junction grown using a SiF4 precursor. The phototransistor showed responsivity as high as 25 A/W at 250 nm in the Schottky emitter (SE) mode. The Schottky collector (SC) mode showed a responsivity of 17 A/W at 270 nm with a visible rejection (270 nm:400 nm)>103. The fastest response was seen in the SC-mode, with 10ms turn-on and 47 ms turn-off, with a noise equivalent power of 2.3 fW at 20 Hz and a specific detectivity of 4.4 x 1013 Jones. The high responsivity is due to internal gain from bipolar action. We observe additional avalanche gain from the device periphery in the SC-mode by scanning photocurrent microscopy but not in the SE-mode. This high-performance visible-blind photodetector is attractive for advanced applications such as flame detection. [ABSTRACT FROM AUTHOR]

The authors report on pulsed lateral epitaxial overgrowth of aluminum nitride films on basal plane sapphire substrates. This approach, at temperatures in excess of 1150 °C, enhanced the adatom migration, thereby significantly increasing the lateral growth rates. This enabled a full coalescence in wing regions as wide as 4–10 μm. Atomic force microscopy and cross-section transmission electron microscopy were used to establish the reduction of threading dislocations in the lateral growth. Cross-sectional monochromatic cathodoluminescence and photoluminescence measurements confirmed the improved optical properties of the laterally overgrown aluminum nitride films. [ABSTRACT FROM AUTHOR]

Publisher's Note: "Trap characterization in ultra-wide bandgap Al0.65Ga0.4N/Al0.4Ga0.6N MOSHFET's with ZrO2 gate dielectric using optical response and cathodoluminescence" [Appl. Phys. This article was originally published online on 19 November 2019 with author MVS Chandrashekhar incorrectly spelled. All online versions of the article were corrected on 20 November 2019. [Extracted from the article]