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III–V/III-nitride p–n junctions were realized via crystal heterogeneous integration, and the resulting diodes were characterized to analyze electrical behavior and junction quality. p-type In0.53Ga0.47As, which is a well-established base layer in InP heterojunction bipolar transistor (HBT) technology, was used in combination with a homoepitaxial n-type GaN. The latter offers low dislocation density, coupled with high critical electric field and saturation velocity, which are attractive for use in future HBT collector layers. Transmission electron microscopy confirms an abrupt interface in the fabricated heterogeneous diodes. Electrical characterization of the diodes reveals a near-unity ideality factor (n ∼ 1.07) up to 145 °C, a high rectification ratio of ∼108, and a low interface trap density of 3.7 × 1012 cm−2. [ABSTRACT FROM AUTHOR]

We demonstrate a pathway employing crystal polarity controlled asymmetric impurity incorporation in the wide bandgap nitride material system to enable 3D doping control during the crystal growth process. The pathway involves polarity specific supersaturation modulated growth of lateral polar structures of alternating Ga- and N-polar GaN domains. A STEM technique of integrated differential phase contrast is used to image the atomic structure of the different polar domains and their single atomic plane boundaries. As a demonstration, 1 μm wide alternating Ga- and N-polar GaN domains exhibiting charge balanced and periodic domains for superjunction technology were grown. The challenges in characterizing the resulting 3D doping profile were addressed with atom probe tomography with atomic scale compositional resolution corroborating capacitance measurements and secondary-ion mass spectroscopy analysis. [ABSTRACT FROM AUTHOR]

Contrary to the arsenides where donors undergo stable DX transition, we find that Ge in AlGaN does not suffer from the DX transition; instead, it undergoes a shallow donor (30 meV) to deep donor (150 meV) transition at ∼50% Al content in the alloy. This finding is of profound technological importance as it removes fundamental doping limitations in AlGaN and AlN imposed by the presumed DX−1 acceptor state. The charge state of Ge below and above the transition was determined by co-doping with Si, which remains a shallow donor in AlGaN for up to 80% Al. It was found that Ge occupied a donor state with a (0/+) thermodynamic transition for AlGaN alloys below and above the transition. Ge as a shallow donor was completely ionized at room temperature; however, the ionization of the deep donor required elevated temperatures, commensurate with its higher ionization energy. This behavior is not unique to Ge; preliminary findings show that Si and O in AlGaN may behave similarly. [ABSTRACT FROM AUTHOR]

In this work, we investigate the effect of different chemical treatments, such as solvents, bases, and acids, on the surface properties and electrical behavior of Schottky diodes fabricated on metalorganic chemical vapor deposition-grown, n-type, N-polar GaN. The I–V and C–V barrier heights of the as-grown Schottky diodes are found to be 0.40 eV and 0.60 eV, respectively, with an ideality factor n = 1.07. It is found that the solvent treatments neither change the surface nor the electrical performance of the Schottky diodes, as expected. However, the treatments by the alkaline photolithography developer and acid—the latter of which is often used to clean the surface of Ga-polar GaN films prior to metal contact deposition—degrade the performance of N-polar GaN. These base and acid treatments severely roughen the surface by creating triangular/hexagonal crystallographic facets. The I–V and C–V barrier heights of these base- and acid-treated diodes are increased to 0.63 eV and 1.00 eV, respectively, with ideality factor values n < 1.2. X-ray photoelectron spectroscopy studies indicate that base- and acid-treated surfaces have lower oxygen content as compared to the as-grown sample surface. It is understood that the increment in the barrier height for base- and acid-treated diodes could be due to the change in polarity, from N-polar to semi-polar/non-polar, on these crystallographic features. All these results demonstrate that, unlike for Ga-polar GaN, the N-polar GaN surface is highly reactive to bases and acids. [ABSTRACT FROM AUTHOR]

The growth of monolayer MoS2 crystals on chemomechanically polished (CMP) N-polar GaN using PTAS-assisted chemical vapor deposition is demonstrated. The formation of monolayer MoS2 was initially prevented by the as-grown GaN's large surface roughness. CMP reduces the roughness to 250 pm, enabling monolayer MoS2 triangles with edge lengths of 30 μm, a Raman peak separation of <20 cm–1, and an optical bandgap of 1.81 eV, which is on par with those obtained on smooth Ga-polar GaN. It is thus demonstrated that high-quality MoS2 monolayers can be obtained on N-polar GaN for future high-speed optoelectronic and quantum sensing applications. [ABSTRACT FROM AUTHOR]

Among group VI transition metal dichalcogenides, MoTe2 is predicted to have the smallest energy offset between semiconducting 2H and semimetallic 1T′ states. This makes it an attractive phase change material for both electronic and optoelectronic applications. Here, we report fast, nondestructive, and full phase change in Al2O3-encapsulated 2H-MoTe2 thin films to 1T′-MoTe2 using rapid thermal annealing at 900 °C. Phase change was confirmed using Raman spectroscopy after a short annealing duration of 10 s in both vacuum and nitrogen ambient. No thickness dependence of the transition temperatures was observed for flake thickness ranging from 1.5 to 8 nm. These results represent a major step forward in understanding the structural phase transition properties of MoTe2 thin films using external heating and underline the importance of surface encapsulation for avoiding thin film degradation. [ABSTRACT FROM AUTHOR]

The ultra-wide bandgap of Al-rich AlGaN is expected to support a significantly larger breakdown field compared to GaN, but the reported performance thus far has been limited by the use of foreign substrates. In this Letter, the material and electrical properties of Al0.85Ga0.15N/Al0.6Ga0.4N high electron mobility transistors (HEMT) grown on a 2-in. single crystal AlN substrate are investigated, and it is demonstrated that native AlN substrates unlock the potential for Al-rich AlGaN to sustain large fields in such devices. We further study how Ohmic contacts made directly to a Si-doped channel layer reduce the knee voltage and increase the output current density. High-quality AlGaN growth is confirmed via scanning transmission electron microscopy, which also reveals the absence of metal penetration at the Ohmic contact interface and is in contrast to established GaN HEMT technology. Two-terminal mesa breakdown characteristics with 1.3 μm separation possess a record-high breakdown field strength of ∼11.5 MV/cm for an undoped Al0.6Ga0.4N-channel layer. The breakdown voltages for three-terminal devices measured with gate-drain distances of 4 and 9 μm are 850 and 1500 V, respectively. [ABSTRACT FROM AUTHOR]

Neuropeptide Y (NPY) plays a central role in a variety of emotional and physiological functions in humans, such as forming a part of the body′s response to stress and anxiety. This work compares the impact of MCH and PEG spacer molecules on the performance of a potentiometric NPY sensor. An NPY‐specific DNA aptamer with thiol termination was immobilized onto a gold electrode surface. The performance of the sensor is compared when either an MCH‐ or PEG‐based self‐assembled monolayer is formed following aptamer immobilization. Backfilling the surface with alkanethiol spacer molecules like these is key for proper conformational folding of aptamer‐target binding. Non‐specific adhesion of NPY to the MCH‐based sensor surface was observed via surface plasmon resonance (SPR), and then confirmed via potentiometry. It is then shown that PEG improves the sensor′s sensitivity to NPY compared to the surfaces with an MCH‐based SAM. We achieve the detection of picomolar range NPY levels in buffer with a sensitivity of 36.1 mV/decade for the aptamer and PEG‐based sensor surface, thus demonstrating the promise of potentiometric sensing of NPY for future wearable deployment. The sensor′s selectivity was also studied via exposure to cortisol, a different stress marker, resulting in a 13x smaller differential voltage (aptamer‐specific) response compared to that of NPY. [ABSTRACT FROM AUTHOR]