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Polarization-induced carriers play an important role in achieving high electrical conductivity in ultrawide bandgap semiconductor AlGaN, which is essential for various applications ranging from radio frequency and power electronics to deep UV photonics. Despite significant scientific and technological interest, studies on polarization-induced carriers in N-polar AlGaN are rare. We report the observation and properties of polarization-induced two-dimensional electron gases (2DEGs) in N-polar AlGaN/AlN heterostructures on single-crystal AlN substrates by systematically varying the Al content in the 8 nm top layers from x = 0 to x = 0.6, spanning energy bandgaps from 3.56 to 4.77 eV. The 2DEG density drops monotonically with increasing Al content, from 3.8 × 1013/cm2 in the GaN channel, down to no measurable conductivity for x = 0.6. Alloy scattering limits the 2DEG mobility to below 50 cm2/V s for x = 0.49. These results provide valuable insights for designing N-polar AlGaN channel high electron mobility transistors on AlN for extreme electronics at high voltages and high temperatures, and for UV photonic devices.

Direct epitaxial integration of magnetic layers with wide bandgap nitride semiconductors will enable spin-controlled transport and photonic phenomena, seeding ideas for functional spintronic devices. Using plasma-assisted molecular beam epitaxy (MBE) in a previously unexplored window, significantly improved ferrimagnetic Mn4N layers are successfully grown on GaN with ~ 1 nm surface roughness. Distinct from earlier reports, the Mn4N layers grown on GaN are found to be 001 oriented with 12-fold in-plane symmetry in the diffraction pattern. This unique epitaxial registry originates from three equivalent rotational domains. The ferrimagnetic magnetotransport properties of low growth temperature Mn4N layers on GaN are comparable to those reported on cubic substrates such as MgO. However, a sign-flip of the Hall resistance is discovered for Mn4N layers grown above 300 °C.

N-polar aluminum nitride (AlN) is an important building block for next-generation high-power radio frequency electronics. We report successful homoepitaxial growth of N-polar AlN by molecular beam epitaxy (MBE) on large-area, cost-effective N-polar AlN templates. Direct growth without any in situ surface cleaning leads to films with inverted Al polarity. It is found that Al-assisted cleaning before growth enables the epitaxial film to maintain N-polarity. The grown N-polar AlN epilayer with its smooth, pit-free surface duplicates the structural quality of the substrate, as evidenced by a clean and smooth growth interface with no noticeable extended defects generation. Near band-edge photoluminescence peaks are observed at room temperature on samples with MBE-grown layers but not on the bare AlN templates, implying the suppression of nonradiative recombination centers in the epitaxial N-polar AlN.

Recent observation of high density polarization-induced two-dimensional electron gases in ultra-thin N-polar GaN layers grown on single-crystal AlN has enabled the development of N-polar high electron mobility transistors (HEMTs) on AlN. Such devices will take advantage of thermal and power handling capabilities of AlN, while simultaneously benefitting from the merits of N-polar structures, such as a strong back barrier. We report the experimental demonstration of N-polar GaN/AlGaN/AlN HEMTs on single-crystal AlN substrates, showing an on-current of 2.6 A/mm with a peak transconductance of 0.31 S/mm. Small-signal RF measurements revealed speeds exceeding ft/ fmax = 68/100 GHz. These results pave the way for developing RF electronics with excellent thermal management based on N-polar single-crystal AlN.

The polarization difference and band offset between Al(Ga)N and GaN induce two-dimensional (2D) free carriers in Al(Ga)N/GaN heterojunctions without any chemical doping. A high-density 2D electron gas (2DEG), analogous to the recently discovered 2D hole gas in a metal-polar structure, is predicted in a N-polar pseudomorphic GaN/Al(Ga)N heterostructure on unstrained AlN. We report the observation of such 2DEGs in N-polar undoped pseudomorphic GaN/AlGaN heterostructures on single-crystal AlN substrates by molecular beam epitaxy. With a high electron density of ∼4.3 [Formula: see text]/cm2 that maintains down to cryogenic temperatures and a room temperature electron mobility of ∼450 cm2/V s, a sheet resistance as low as ∼320 [Formula: see text] is achieved in a structure with an 8 nm GaN layer. These results indicate significant potential of AlN platform for future high-power RF electronics based on N-polar III-nitride high electron mobility transistors.

An analytical formula is derived for particle and energy densities of fermions and bosons, and their ballistic momentum and energy currents for anisotropic energy dispersions in generalized dimensions. The formulation considerably simplifies the comparison of the statistical properties and ballistic particle and energy transport currents of electrons, acoustic phonons, and photons in various dimensions in a unified manner. Assorted examples of its utility are discussed, ranging from blackbody radiation to Schottky diodes and ballistic transistors, quantized electrical and thermal conductance, generalized ballistic Seebeck and Peltier coefficients, their Onsager relations, the generalized Wiedemann-Franz law and the robustness of the Lorenz number, and ballistic thermoelectric power factors, all of which are obtained from the single formula. The new formulation predicts a thermoelectric power factor behaviour of 3D Dirac bands which has not been observed yet.

Spin–orbit torque (SOT) is a promising mechanism for writing magnetic memories, while field-effect transistors (FETs) are the gold-standard device for logic operation. The spin–orbit torque field-effect transistor (SOTFET) is a proposed device that couples an SOT-controlled ferromagnet to a semiconducting transistor channel via the transduction in a magnetoelectric multiferroic (MF). This allows the SOTFET to operate as both a memory and a logic device, but its realization depends on the choice of appropriate materials. In this report, we discuss and parametrize the types of materials that can lead to an SOTFET heterostructure.

Mn4N is a compound magnetic material that can be grown using MBE while exhibiting several desirable magnetic properties such as strong perpendicular magnetic anisotropy, low saturation magnetization, large domain size, and record high domain wall velocities. In addition to its potential for spintronic applications exploiting spin orbit torque with epitaxial topological insulator/ferromagnet bilayers, the possibility of integrating Mn4N seamlessly with the wide bandgap semiconductors GaN and SiC provides a pathway to merge logic, memory and communication components. We report a comparative study of MBE grown Mn4N thin films on four crystalline substrates: cubic MgO, and hexagonal GaN, SiC and sapphire. Under similar growth conditions, the Mn4N film is found to grow single crystalline on MgO and SiC, polycrystalline on GaN, and amorphous on sapphire. The magnetic properties vary on the substrates and correlate to the structural properties. Interestingly, the field dependent anomalous Hall resistance of Mn4N on GaN shows different behavior from other substrates such as a flipped sign of the anomalous Hall resistance.

This study investigates the optical and structural properties of multilayer InAs QDs heterostructures designed with varying levels of growth interruption. Samples were subject to growth interruptions of 0, 25, 50, and 75 s (samples A, B, C, and D, respectively). Low-temperature (9 K) photoluminescence (PL) experiments revealed an initial redshift in the ground-state emission peak (from 1131 to 1145 nm) as growth interruption (GI) was varied from 0 to 25 s. In addition, we observed a blueshift (from 1145 to 1100 nm) when the interruption lasted for more than 25 s. The thermal activation energies of samples A–D, calculated from temperature-dependent (8–300 K) PL experiments, were approximately 182.4, 241.5, 206.2 and 175.3 meV, respectively. The photoluminescence excitation (PLE) studies reveal the existence of two strong peaks at each detection energy, one at ~ 63 meV and other at ~ 140 meV. These two PLE peaks arise from the InAs QDs shifting toward the higher energy (meV) levels which have lower intensities with the increase of GI. Longer GI time increases the intensities of InAs wetting layer as well as GaAs matrix. This, in turn, worsens the optical property. In our structural study, high-resolution transmission electron microscopy (HRTEM) confirmed that a short interruption (25 s, Sample B) in the growth of QDs produced a better size homogeineity. We also observed that Sample B has a tensile stress along the growth direction (001), which increased the height of the InAs QDs to approximately 8 nm with the interplanar distance of 0.626 A along the 001 planes. In addition, the InAs QDs maintained their geometrical shape till 50 s and the shape of the dots started deterioting after 50 s as a result of the decrease in QDs height of 2 nm revealed from HRTEM. In contrast, high-resolution X-ray diffraction (HRXRD) found the strain energy drastically dropped to 0.4765 meV/atoms (75 s, Sample D) from 0.819 meV/atoms (25 s, Sample B), showing inhomegenous QDs size distribution. Higher strain energy of 0.819 meV in the stacked heterostructure improved QDs size homogeineity. The average indium content calculated from the out-plane HRXRD in samples A–D were 0.234%, 0.288%, 0.241%, and 0.218%, respectively. From the growth model, we noticed a significant change in the surface self-diffusion rate of indium adatoms. The indium adatoms migration to larger adjacent QDs is higher when the pre-matured QDs lie within the critical distance (X; ~ 30 nm). At X > 30 nm, the QDs start strinking, thereby reducing the height of QDs. Thus, this paper shows how our growth method helps in optimizing the QDs homogeineity and increasing the QDs dimension.

The nitride semiconductor materials GaN, AlN, and InN, and their alloys and heterostructures have been investigated extensively in the last 3 decades, leading to several technologically successful photonic and electronic devices. Just over the past few years, a number of new nitride materials have emerged with exciting photonic, electronic, and magnetic properties. Some examples are 2D and layered hBN and the III-V diamond analog cBN, the transition metal nitrides ScN, YN, and their alloys (e.g. ferroelectric ScAlN), piezomagnetic GaMnN, ferrimagnetic Mn4N, and epitaxial superconductor/semiconductor NbN/GaN heterojunctions. This article reviews the fascinating and emerging physics and science of these new nitride materials. It also discusses their potential applications in future generations of devices that take advantage of the photonic and electronic devices eco-system based on transistors, light-emitting diodes, and lasers that have already been created by the nitride semiconductors., 16 pages, 3 figures

In this paper, we discuss detailed strain effects on a bilayer InAs quantum dot with varying GaAs barrier thickness. The exploration of the range of GaAs barrier thickness effect on the InAs/GaAs quantum dots and detailed structure were characterized by transmission electron microscopy, atomic force microscopy, high-resolution X-Ray diffraction (HRXRD) and Raman spectroscopy to evaluate the impact of strained layer and also studied the optical properties by photoluminescence (PL) measurements. On varying the thickness of the GaAs barrier layer, the role of strain demonstrates a promising approach to tuning the quantum dot morphologies and structures and hence, optical properties. This can be easily observed from the HRXRD rocking curves which result in a shift of the zero order peak position. Both in-out-plane strain decrease as the thickness is increased. Even the Raman scattering peaks justify the decrease of strain on increasing the GaAs barrier thickness. Therefore, higher strain propagation indicates redshift in the emission wavelength and the dots are much more uniformly spread out. Structure with a range of 5.5nm-8.5nm GaAs barrier thickness interlayer reveals even high-quality crystallinity of the epilayers with the FWHM of 21.6 arcsecs for the (004) reflection. Uncoupled structure responses low crystalline quality with FWHM of 109 arcsecs. Dislocation density increases drastically with a decrease of strain which is an important aspect of lasers and other devices in increasing their efficiency. Activation energy also shows a positive correlation with coupling structure. Therefore, controlling diffusion length may be key to reducing defects in several strained structures.

The thermal stability of InAs/GaAs bilayer quantum dots structure has been investigated by photoluminescence (PL) measurements. The fabricated structure on thermal annealing PL shows no shift in peaks upto 650°C indicating a robustness till a certain temperature making it a suitable candidate for vertical cavity surface emitting lasers (VCSELs) and feedback lasers where ideally a fixed wavelength is required. Integrated Photoluminescence gave a high activation energy in the range of 200 meV for the ground state PL peak for all the coupled structures. Above 650°C there is a blue-shift in the PL peak. And at a very high temperature the dots start to diffuse into InAs wetting layer hence decreasing the quality of the crystal. The stability in the PL for temperatures below 650°C can be accounted by strain energy as it works against the interdiffusion of QD and the seed layer till a certain temperature hence it compensates for the temperature effect but after 650°C diffusion term becomes too strong and we observe a blue-shift in the peak. This can be justified theoretically by modifications in the Arrhenius diffusion equation. Due to this interdiffusion of In/Ga atom the dominance of the peak and the intensity of PL peak also changes as the QD composition changes [1-2]. Coupling the dots also leads to high activation energy which in-turn generates a stronger carrier confinement. But as the temperature increases, activation energy decreases weakening the carrier confinement potential because of interdiffusion between dot and seed layer.

In the present study, we have established a correlation between the photo-induced electronic phenomena and excited state lifetime of the photo generated carriers in double barrier Al0.3Ga0.7As\GaAs quantum well (QW) structures. The excited state lifetime was measured experimentally by picosecond time resolved photoluminescence spectroscopy for two samples with different well widths (5.3 nm and 16.5 nm). The faster nonradiative decay time of the narrower well can be attributed to the facile escape of electrons from well to barrier due to lower associated energy compared to that of the thicker well which resembles the simulated results of the energy level distribution. The proposed mechanism of carrier escape is further proven from the higher value of unconventional excitonic capacitance value in the thicker well, measured by impedance spectroscopy. The dependence of photo-induced capacitance on well thickness is explained by the lifetime of the excited carriers in this study. Dependence of the photo-generated capacitance (C) on externally applied bias voltage (V) was also studied to quantitatively establish a proportional relation between the carrier holding capacity of the well and the excitonic lifetime. The higher accumulation of charge and lower ground state energy of the thicker well is evident from the higher tunnelling current found for the same in the photocurrent (I) versus voltage (V) measurement. Thus the escape of electrons from the well to barrier is the key factor affecting the photo generated charge accumulation and its holding capacity which in turn influences the device performances.