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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.

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 aluminum nitride (AlN) quasi-vertical Schottky barrier diode (SBD) was fabricated on an AlN bulk substrate. An undoped AlN layer, a Si-doped Al0.9Ga0.1N current spreading layer and an AlN buffer layer were grown by plasma-enhanced molecular beam epitaxy. The epitaxial AlN layer was etched down to the n-Al0.9Ga0.1N layer to form an Ohmic contact. Ni/Au and V/Al/Ni/Au were deposited on the top AlN layer as Schottky contacts and on the exposed n-Al0.9Ga0.1N layer as Ohmic contacts, respectively. The Ohmic characteristics on the n-Al0.9Ga0.1N layer, capacitance–voltage (C–V) and current–voltage (I–V) characteristics of the AlN SBD were investigated.

Single-crystal Aluminum Nitride (AlN) crystals enable the epitaxial growth of ultrawide bandgap Al(Ga)N alloys with drastically lower extended defect densities. Here, we report the plasma-MBE growth conditions for high Al-composition AlGaN alloys on single-crystal AlN substrates. An AlGaN growth guideline map is developed, leading to pseudomorphic AlxGa1−xN epitaxial layers with x ∼0.6–1.0 Al contents at a growth rate of ∼0.3 μm/h. These epitaxial layers exhibit atomic steps, indicating step flow epitaxial growth, and room-temperature band edge emission from ∼4.5 to 5.9 eV. Growth conditions are identified in which the background impurity concentrations of O, C, Si, and H in the MBE layers are found to be very near or below detection limits. An interesting Si segregation and gettering behavior is observed at the epitaxial AlGaN/AlN heterojunction with significant implications for the formation and transport of 2D electron or hole gases. Well-controlled intentional Si doping ranging from ∼2 × 1017 to 3 × 1019 atoms/cm3 is obtained, with sharp dopant density transition profiles. In Si-doped Al0.6Ga0.4N epilayers, a room-temperature free electron concentration of ∼3 × 1019/cm3, an electron mobility of ∼27 cm2/V s, and an n-type resistivity of ∼7.5 m Ω cm are obtained. The implications of these findings on electronic and photonic devices on single-crystal AlN substrates are discussed.

The evolution of surface morphology for single-crystal bulk Al-polar aluminum nitride substrates during ex situ cleaning, in situ cleaning, and subsequent homoepitaxy is investigated. Ex situ acid treatment is found to reveal atomic steps on the bulk AlN substrates. After in situ Al-assisted cleaning at high temperatures in a high vacuum environment monitored with reflection high-energy electron diffraction, cleaner atomic step edges are observed. Subsequent growth on the cleaned bulk AlN by molecular beam epitaxy is used to develop a phase-diagram for homoepitaxy on AlN single crystals. Secondary ion mass spectrometry profiles reveal high-purity epitaxial layers with undesired chemical impurity densities of Si, O, and C to be below detection limits. The grown homoepitaxial films are observed to oxidize in the ambient environment, but repeating the ex situ acid treatment again reveals atomic steps.

We compare the effectiveness of in situ thermal cleaning with that of Al-assisted cleaning of native surface oxides of bulk AlN for homoepitaxial growth by molecular beam epitaxy. Thermal deoxidation performed at 1450 ° C in vacuum results in voids in the AlN substrate. On the other hand, Al-assisted deoxidation at ≈ 900 ° C results in high-quality AlN homoepitaxy, evidenced by clean and wide atomic terraces on the surface and no extended defects at the growth interface. This study shows that Al-assisted in situ deoxidation is effective in removing native oxides on AlN, providing a clean surface to enable homoepitaxial growth of AlN and its heterostructures; furthermore, it is more attractive over thermal deoxidation, which needs to be conducted at much higher temperatures due to the strong bonding strength of native oxides on AlN.

Low-frequency noise in MOSFETs is considered to originate from two distinctive sources: Random Telegraph Signal caused by carrier traps at the border of the SiO 2/ Si interface and 1/f fluctuation due to inherent nature of lattice scattering in a Si crystal. It is very important to distinguish these two mechanisms. Relative amplitude of RTS and 1/f noise depends on the number of carriers under the gate electrode, which makes it channel size as well as gate-bias dependent. In this paper, we discuss the dependence of the amplitudes of RTS and 1/f noise in MOSFETs on sample geometry and gate bias condition. We discuss low-frequency noise reduction by utilizing low electron-temperature plasma for gate oxidation as well.

We studied the effects of an AlGaAsSb electron supply layer for InGaAs/InAlAs metamorphic high electron mobility transistors (mHEMTs) on GaAs substrate. By implementing an AlGaAsSb electron supply layer, we drastically improved the electron mobility of InGaAs/InAlAs heterostructures for mHEMTs. An AlGaAsSb electron supply layer for InGaAs/InAlAs heterostructures on GaAs substrates promises high-performance mHEMTs with low production cost.

Recent advances in mid-ultraviolet light-emitting diodes grown pseudomorphically on bulk AlN substrates have led to improved efficiencies and lifetimes. For a 266 nm device an output power of 66 mW at 300 mA has been achieved with an external quantum efficiency of 4.5%. More importantly, the lifetimes of these devices have been increased substantially. Testing of LEDs in both surface mount design (SMD) and TO-39 packages show L50 lifetimes well in excess of 1,000 hours under a variety of case temperatures and currents. Package-related catastrophic failures are eliminated through encapsulation and hermetic sealing, further reducing failure rates and extending the lifetime.

Random Telegraph Signal (RTS) noise was measured in submicron MOSFETs under various biasing conditions from sub threshold to inversion region and dependence of amplitude and mean time of capture and emission on electric field and electron concentration analyzed. Numerical model of charge carrier transport in the channel was used to estimate trap position between the source and drain electrodes from experimental characteristics.

Attempts were made to prepare a supported type of silica-alumina by depositing silica on an alumina surface by chemical vapour deposition (CVD). Silica was effectively and uniformly deposited on alumina using tetraethoxysilane at 240°C in the presence of oxygen. The resulting silica-alumina (CVD SiO 2 /Al 2 O 3 ) exhibited high catalytic activities for several types of organic reactions such as 2-butanol dehydration, m -xylene isomerization, cumene cracking and n -heptane cracking. The catalytic activities of CVD SiO 2 /Al 2 O 3 varied with silica loading, attaining maximum conversions for the respective reactions at different silica loadings, and were comparable to those of commercial silica-aluminas. The catalytic activities did not correlate with the total amount of acid measured by ammonia chemisorption, but could be associated with the amount of Bronsted acid. 31 P magic angle spinning NMR measurements on trimethylphosphine chemisorption elucidated that the Lewis acid sites on alumina were completely converted into Bronsted acid sites by the deposition of silica.

The capture and emission time constants dependence on drain current for constant gate voltage and variable drain voltage show that probability for charge carrier capture decreases with increasing lateral electric field while emission process is independent on lateral field intensity. We have performed analyses of SiO2 gate insulating layer from VA characteristics measured in wide temperature range (gate electrode area 1.5 μm2, insulating layer thickness 6 nm. Leakage current in reverse mode of n‐MOS sample (for gate electrode negative) is for applied voltage lower than 1 V exponential function of applied voltage with saturation current I0 = (1–3)×10−16 A and ideality factor near to 1. Saturation current value corresponds to Schottky barrier high about 1.2 eV. We suppose that in SiO2 gate insulating layer and on the interface Si‐SiO2 there are oxygen vacancies and interstitials. High density of overlapping energy localized states creates in SiO2 impurity conduction band about 1.2 eV above the Si conduction band. Between channel and interface Si‐SiO2 localized states exists g‐r stochastic exchange of electrons, which is a source of 1/f noise. Reducing the density of interface states by plasma oxidation leads to 1/f noise spectral density lowering. There exists electron exchange by tunnelling between interface Si‐SiO2 localized states and traps localized in gate insulating layer about lnm from channel. These quantum transitions are sources of RTS noise. Electron from channel trap can tunnel to interface Si/SiO2 localized state or it can enter through SiO2 impurity conductivity band to gate electrode by thermionic emission.

PMOS transistors with a strained SiGe channel have higher hole mobility compared with conventional pure‐silicon counterparts. In this paper, we report on the hole mobility and low‐frequency noise characteristics of the SiGe channel PMOS FETs. Epitaxial layers consist of 15nm bottom‐Si, lOnm Si0.7Ge0.3, 9nm cap‐Si are deposited selectively on the active region. Hole mobility in the SiGe channel PMOS FETs depends on the in‐plane channel direction. The maximum hole mobility was obtained for 〈100〉 direction device and the minimum was obtained for 〈110〉 direction device. Low frequency noise behaviour for SiGe channel PMOS FET was investigated and compared with pure Si PMOS FETs. The lowest input‐referred noise power (Svg) was obtained for the 〈100〉 direction SiGe PMOS FET. The highest trans‐conductance in the 〈100〉 channel results in lowest Svg. Hooge’s alpha (αH) for the SiGe channel PMOS FET was approximately 1/4 of the αH for the Si channel PMOS FET.

This paper investigates the emission and capture kinetics of random telegraph signals (RTS) in Quantum dots and submicron MOSFET structures. Emphasis is laid on the signals showing a capture process which deviates from the standard Shockley-Read-Hall kinetics. The proposed model distinguishes between primary processes consisting in quantum transitions of electrons between traps and the conduction or valence band and secondary processes, consisting in current modulation. If the RTS noise sources are quantum transitions of electrons between a shallow trap and the conduction band then the primary process is one-dimensional and it coincides with the secondary process — current modulation. For deep traps the primary process is a two dimensional g — r process. It is shown in this paper how to distinguish experimentally between the one or two-dimensional primary processes by measuring the probability density of the occupation time in both current states. These long-time measurements require a very stable power source and active shielding of low frequency magnetic fields.

The downscaling of electronic devices makes high field transport effects more important. In deep submicron technology high transversal and high lateral electric field exists. Application of drain voltage 1V results in electric field, which exceeds the silicon critical electric field. Electron temperature is then higher than lattice one and field dependent electron mobility must be considered. Due to small gate area we were able to activate one trap only and then in time domain two levels signal was observed. A systematic analysis of two level RTS signal was made to obtain information on capture and emission processes as a function of gate voltage, drain current and temperature for low and high lateral electric field. With increasing drain voltage capture time increases, while dependence on gate voltage is almost the same as for low drain voltage. For constant gate voltage and variable drain voltage emission process is independent on lateral field intensity, while capture time increases with lateral field ...

RTS noise of Si MOSFETs and GaN HFET was analysed by means of zero cross method. Noise spectral density of crossing events in 1 ms to 100s windows was flat over 10−5 to 103 frequency range without apparent 1/f noise. The fluctuation of crossing rate was same in every sample, although other noise components next to RTS noise are quite different in Si and GaN devices. Correlation analysis of pulse length in doublets and triplets of successive pulses didn’t reveal any mutual dependence.

In the present paper, emphasis is laid on those RTS showing a capture process, which deviates from the standard Shockley-Read-Hall kinetics. A modified two-step approach is proposed. In this case the charge carrier quantum transitions represent a primary process X(t), which involves two or three quantum states. The measurable quantity is the current modulation, which has discrete states, too. The current modulation is then represented by a secondary process Y(t). The proposed model can explain some of the complex switching phenomena being measured in nanoscale devices. The quadratic dependence of the capture rate on the current and the noise spectral density dependence on the current and temperature are analysed. It is shown that the occupation time probability density for emission is given by a superposition of two exponential dependencies, whereas the capture time constant distribution is purely exponential.

Crystallization behavior of 100nm amorphous silicon film with and without a 160nm poly-Ge layer on top was investigated. Ge was observed to increase the nucleation rate as well as to increase the incubation time for nucleation in a-Si. Activation energy for nucleation was 2.3eV for Ge-covered Si films as compared to 2.7eV for the control a-Si films with no poly-Ge. Activation energy for incubation is almost unchanged for both the films (-3.3eV). An alternative seeding technique for a-Si films using the increase in the incubation time for nucleation due to presence of poly-Ge is proposed.

Experiments were carried out for the n-channel devices, processed in a 0.3 µm spacer less complementary metal–oxide–semiconductor technology. Random-telegraph-signal measurements were performed for the constant gate voltage. It is supposed that electron concentration in the channel decreases from the source to the drain contact. Lateral component of the electric field is inhomogeneous in the channel and it has a minimum value near the source and reaching the maximum value near the drain electrode. Drain current is given by two components – diffusion and drift ones. Diffusion current component is independent on the x-coordinate and it is equal to the drift current component for the low electric field. The model explaining the experimentally observed capture time constant dependence on the lateral electric field and the trap position is given. From the dependence of the capture time constant τc on the drain current could be calculated longitudinal coordinate of the trap position.

Strong Broensted acid sites were generated on Al2O3 support by the chemical vapor deposition (CVD) of SiO2 using Si(OEt)4 under atmospheric pressure. The CVD silica-alumina had a maximum acid strength of Ho = −8.2 and exhibited high catalytic activities for both the dehydration of 2-butanol and the cracking of cumene.