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Al x Ga 1 − x N/GaN high-electron-mobility transistor (HEMT) structures are key components in electronic devices operating at gigahertz or higher frequencies. In order to optimize such HEMT structures, understanding their electronic response at high frequencies and room temperature is required. Here, we present a study of the room temperature free charge carrier properties of the two-dimensional electron gas (2DEG) in HEMT structures with varying Al content in the Al x Ga 1 − x N barrier layers between x = 0.07 and x = 0.42. We discuss and compare 2DEG sheet density, mobility, effective mass, sheet resistance, and scattering times, which are determined by theoretical calculations, contactless Hall effect, capacitance-voltage, Eddy current, and cavity-enhanced terahertz optical Hall effect (THz-OHE) measurements using a low-field permanent magnet (0.6 T). From our THz-OHE results, we observe that the measured mobility reduction from x = 0.13 to x = 0.42 is driven by the decrease in 2DEG scattering time, and not the change in effective mass. For x < 0.42 , the 2DEG effective mass is found to be larger than for electrons in bulk GaN, which in turn, contributes to a decrease in the principally achievable mobility. From our theoretical calculations, we find that values close to 0.3 m 0 can be explained by the combined effects of conduction band nonparabolicity, polarons, and hybridization of the electron wavefunction through penetration into the barrier layer. [ABSTRACT FROM AUTHOR]

We investigate the interfaces and polarity domains at the atomic scale in epitaxial AlN and GaN/AlN grown by hot-wall metal organic chemical vapor epitaxy on the carbon face of SiC. X-ray diffraction, potassium hydroxide (KOH) wet chemical etching, and scanning transmission electron microscopy combined provide an in-depth understanding of polarity evolution with the film thickness, which is crucial to optimize growth. The AlN grown in a 3D mode is found to exhibit N-polar pyramid-type structures at the AlN–SiC interface. However, a mixed N-polar and Al-polar region with Al-polarity domination along with inverted pyramid-type structures evolve with increasing film thickness. We identify inclined inversion domain boundaries and propose that incorporation of oxygen on the ⟨ 40–41 ⟩ facets of the N-polar pyramids causes the polarity inversion. We find that mixed-polar AlN is common and easily etched and remains undetected by solely relying on KOH etching. Atomic scale electron microscopy is, therefore, needed to accurately determine the polarity. The polarity of GaN grown on mixed-polar AlN is further shown to undergo complex evolution with the film thickness, which is discussed in the light of growth mechanisms and polarity determination methods. [ABSTRACT FROM AUTHOR]

Owing to their very large piezoelectric coefficients and spontaneous polarizations, (Sc,Y) x Al1−xN alloys have emerged as a new class of III-nitride semiconductor materials with great potential for high-frequency electronic and acoustic devices. The thermal conductivity of constituent materials is a key parameter for design, optimization, and thermal management of such devices. In this study, transient thermoreflectance technique is applied to measure the thermal conductivity of ScxAl1−xN and YxAl1−xN (0 ≤ x ≤ 0.22) layers grown by magnetron sputter epitaxy in the temperature range of 100–400 K. The room-temperature thermal conductivity of both alloys is found to decrease significantly with increasing Sc(Y) composition compared to that of AlN. We also found that the thermal conductivity of YxAl1−xN is lower than that of ScxAl1−xN for all studied compositions. In both alloys, the thermal conductivity increases with the temperature up to 250 K and then saturates. The experimental data are analyzed using a model based on the solution of the phonon Boltzmann transport equation within the relaxation time approximation. The contributions of different phonon-scattering mechanisms to the lattice thermal conductivity of (Sc,Y) x Al1−xN alloys are identified and discussed. [ABSTRACT FROM AUTHOR]

We report on the free charge carrier properties of a two-dimensional electron gas (2DEG) in an AlN/AlxGa1–xN high electron mobility transistor structure with a high aluminum content (x = 0.78). The 2DEG sheet density N s = (7.3 ± 0.7) × 10 12 cm−2, sheet mobility μ s = (270 ± 40) cm2/(Vs), sheet resistance R s = (3200 ± 500) Ω / ◻ , and effective mass m eff = (0.63 ± 0.04) m 0 at low temperatures (T = 5 K) are determined by terahertz (THz) optical Hall effect measurements. The experimental 2DEG mobility in the channel is found within the expected range, and the sheet carrier density is in good agreement with self-consistent Poisson–Schrödinger calculations. However, a significant increase in the effective mass of 2DEG electrons at low temperatures is found in comparison with the respective value in bulk Al0.78Ga22N ( m eff = 0.334 m 0 ). Possible mechanisms for the enhanced 2DEG effective mass parameter are discussed and quantified using self-consistent Poisson–Schrödinger calculations. [ABSTRACT FROM AUTHOR]

The hot-wall metalorganic chemical vapor deposition (MOCVD) concept, previously shown to enable superior material quality and high performance devices based on wide bandgap semiconductors, such as Ga(Al)N and SiC, has been applied to the epitaxial growth of β-Ga2O3. Epitaxial β-Ga2O3 layers at high growth rates (above 1 μm/h), at low reagent flows, and at reduced growth temperatures (740 °C) are demonstrated. A high crystalline quality epitaxial material on a c-plane sapphire substrate is attained as corroborated by a combination of x-ray diffraction, high-resolution scanning transmission electron microscopy, and spectroscopic ellipsometry measurements. The hot-wall MOCVD process is transferred to homoepitaxy, and single-crystalline homoepitaxial β-Ga2O3 layers are demonstrated with a 2 ̄ 01 rocking curve width of 118 arc sec, which is comparable to those of the edge-defined film-fed grown ( 2 ̄ 01) β-Ga2O3 substrates, indicative of similar dislocation densities for epilayers and substrates. Hence, hot-wall MOCVD is proposed as a prospective growth method to be further explored for the fabrication of β-Ga2O3. [ABSTRACT FROM AUTHOR]

Use of an electron-hole plasma model with band-gap renormalization, band filling, free-carrier plasma effect and Coulomb interaction helps determine the carrier-induced variations in the refraction and absorption spectra of GaSb semiconductors. Band filling and screening of continuum-state Coulomb enhancement exert a major influence in the semiconductors. The model also helps determine the spectral features in doped semiconductors and develop new designs for GaAs optoelectronic devices.

The evolution of the optical properties of GaN nanowires (NWs) with respect to a sequence of surface functionalization processes is reported; from pristine hydroxylated to finally, 3-mercaptopropyltrimethoxysilane (MPTMS) functionalized GaN NWs. Photoluminescence, Raman, stationary, and time-resolved photoluminescence measurements were applied to investigate the GaN NWs with different surface conditions. A documented surface passivation effect of the GaN NWs induced by the MPTMS functionalization is determined based on our characterization results. A hypothesis associated with the surface band bending and the defect levels near the band edges is proposed to explain the observed experimental results. [ABSTRACT FROM AUTHOR]