X iv :1 51 2. 01 42 7v 1 [ co nd -m at .m tr lsc i] 4 D ec 2 01 5 Infrared dielectric functions, phonon modes and free-charge carrier properties of high-Al-content AlxGa1−xN alloys determined by mid-infrared spectroscopic ellipsometry and optical Hall effect S. Schöche, T. Hofmann, 2 D. Nilsson, A. Kakanakova-Georgieva, E. Janzén, P. Kühne, K. Lorenz, M. Schubert, and V. Darakchieva a) Department of Electrical, and Computer Engineering, and Center for Nanohybrid Functional Materials, University of Nebraska-Lincoln, Lincoln, 68588-0511, U.S.A. Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-58183, Sweden IPFN, Instituto Superior Técnico, Universidade de Lisboa, Estrada Nacional 10, 2695-066 Bobadela LRS, Portugal

We report on the analysis of a combined mid-infrared spectroscopic ellipsometry and mid-infrared optical Hall effect investigation of wurtzite structure c-plane oriented, crack-free, single crystalline, and high-Al-content AlxGa1−xN layers on 4H-SiC. For high-Al-content AlxGa1−xN, a two mode behavior is observed for both transverse and longitudinal branches of the infrared-active modes with E1 symmetry, while a single mode behavior is found for the longitudinal modes with A1(LO) symmetry. We report their mode dependencies on the Al content. We determine and discuss static and high frequency dielectric constants depending on x. From the analysis of the optical Hall effect data, we determine the effective mass parameter in high-Al-content AlxGa1−xN alloys and its composition dependence. Within the experimental uncertainty limits, the effective mass parameters are found isotropic, which depend linearly on the Al content. The combination of all data permits the quantification of the free electron density N and mobility parameters μ.

In this work we have studied the carbonization of 3C-SiC on misoriented Si substrates, using different thermal ramp rates and shapes. We observed that the heating rate (°C/sec) from carbonization temperature to film growth temperature plays a major role in controlling the void density. Moreover, void formation can be eliminated by the introduction of silane at different temperatures during the heating ramp. The studies were performed on a small research reactor and the results were successfully transferred to a production scale reactor, aimed to the production of 3C-SiC power devices manufactured on 100 and 150 mm Si substrates.

The luminescence properties of cubic GaN films grown upon 3C-SiC/Si (001) substrates by MOCVD were investigated. The spectra show luminescence peaks which are associated with donor bound exciton recombination and donor acceptor pair recombination. A reduced peak energy for the (DX)-X-0 emission compared with values reported in the literature suggests a tensile-strain-reduced bandgap of approximately 3.27eV, which is consistent with the absorption edge in photoluminescence-excitation spectroscopy. The presence of hexagonal material introduces a broad emission band at 3.40eV with a FWHM of 190meV, extending to energies up to 3.60eV. The intensity of this emission scales linearly with excitation power, its peak energy and width remaining unchanged. This band is associated with an absorption edge below 3.70eV and therefore is not caused by absorption into phase-pure cubic or hexagonal GaN. The photoluminescence lifetimes measured across this band reduce from 0.40 to 0.20ns with increasing emission energy. All these observations can be explained by considering a type-II-band alignment adjacent to stacking faults within the cubic GaN. (C) 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In this work, we studied how to optimize the carbonization process in order to deposit cubic SiC (3C-SiC) on misoriented Si substrates. The carbonization process is a key step to obtain device-quality layers and to partially release the strain due to the lattice mismatch between the substrate and the thin 3C-SiC film. One of the common problems in SiC/Si epitaxy is the presence of interfacial voids that can degrade the structural quality of the deposited film and the electrical properties in vertical devices. Carbonization optimization requires careful control of several parameters, in particular temperature, precursor flows, and heating ramps. Precise tailoring of this process on a production reactor with the understanding of all the involved variables would require huge effort in terms of time and money, so it is desirable to perform part of this work on a smaller scale research reactor, which is more versatile and economically viable. Starting from a baseline process for 3C-SiC power devices on 100 mm Si already developed on a production reactor, we used a small research reactor to investigate how the thermal heating profile during the carbonization ramp influences the void density on the Si substrate. We have also studied how the introduction of silane during the carbonization ramp can effectively suppress void formation. After having obtained a comprehensive understanding of the variables involved in the process, the key ideas were successfully implemented in the production reactor, resulting in a significant reduction of interfacial voids.

In this work we employ terahertz (THz) ellipsometry to determine two-dimensional electron gas (2DEG) density, mobility and effective mass in AlGaN/GaN high electron mobility transistor structures grown on 4H-SiC substrates. The effect of the GaN interface exposure to low-flow-rate trimethylaluminum (TMA) on the 2DEG properties is studied. The 2DEG effective mass and sheet density are determined to be in the range of 0.30-0.32m(o) and 4.3-5.5x10(12) cm(-2), respectively. The 2DEG effective mass parameters are found to be higher than the bulk effective mass of GaN, which is discussed in view of 2DEG confinement. It is shown that exposure to TMA flow improves the 2DEG mobility from 2000 cm(2)/Vs to values above 2200 cm(2)/Vs. A record mobility of 2332 +/- 61 cm(2)/Vs is determined for the sample with GaN interface exposed to TMA for 30 s. This improvement in mobility is suggested to be due to AlGaN/GaN interface sharpening causing the reduction of interface roughness scattering of electrons in the 2DEG. (C) 2016 WILEY-VCH Verlag GmbH & Co.

While increasing temperature is essential for reducing carbon and oxygen incorporation, it reduces the incorporation of silicon as a donor.

AlGaN/GaN high electron mobility transistors (HEMTs) fabricated on a heterostructure grown by metalorganic chemical vapor deposition using an alternative method of carbon (C) doping the buffer are characterized. C-doping is achieved by using propane as precursor, as compared to tuning the growth process parameters to control C-incorporation from the gallium precursor. This approach allows for optimization of the GaN growth conditions without compromising material quality to achieve semi-insulating properties. The HEMTs are evaluated in terms of isolation and dispersion. Good isolation with OFF-state currents of 2 × 10 −6 A/mm, breakdown fields of 70 V/µm, and low drain induced barrier lowering of 0.13 mV/V are found. Dispersive effects are examined using pulsed current–voltage measurements. Current collapse and knee walkout effects limit the maximum output power to 1.3 W/mm. With further optimization of the C-doping profile and GaN material quality this method should offer a versatile approach to decrease dispersive effects in GaN HEMTs.

Homoepitaxial layers of AlN and heteroepitaxial layers of AlN on 4H-SiC substrates were grown by metalorganic chemical vapor deposition at high temperatures up to 1400 degrees C, and with various thicknesses. From high resolution x-ray diffraction measurements of a set of homoepitaxial layers of high structural quality, the following mean value of the lattice constants was determined: a = 3.111 31 +/- 0.000 16 angstrom, and c = 4.980 79 +/- 0.000 11 angstrom. The structural quality of the heteroepitaxial layers was demonstrated, for example, by extracting the full width at a half maximum of the (0002) and (10-12) rocking curves of 25 arcsec and 372 arcsec, respectively, associated with a crack-free heteroepitaxial layer with a thickness of 1.3 mu m grown at 1240 degrees C. From measuring a set of AlN heteroepitaxial layers, the following value of the biaxial strain relaxation coefficient was determined: R-B = -0.556.

The creation of a semi insulating (SI) buffer layer in AlGaN/GaN High Electron Mobility Transistor (HEMT) devices is crucial for preventing a current path beneath the two-dimensional electron gas (2DEG). In this investigation, we evaluate the use of a gaseous carbon gas precursor, propane, for creating a SI GaN buffer layer in a HEMT structure. The carbon doped profile, using propane gas, is a two stepped profile with a high carbon doping (1.5 × 1018 cm−3) epitaxial layer closest to the substrate and a lower doped layer (3 × 1016 cm−3) closest to the 2DEG channel. Secondary Ion Mass Spectrometry measurement shows a uniform incorporation versus depth, and no memory effect from carbon doping can be seen. The high carbon doping (1.5 × 1018 cm−3) does not influence the surface morphology, and a roughness root-mean-square value of 0.43 nm is obtained from Atomic Force Microscopy. High resolution X-ray diffraction measurements show very sharp peaks and no structural degradation can be seen related to the heavy carbon doped layer. HEMTs are fabricated and show an extremely low drain induced barrier lowering value of 0.1 mV/V, demonstrating an excellent buffer isolation. The carbon doped GaN buffer layer using propane gas is compared to samples using carbon from the trimethylgallium molecule, showing equally low leakage currents, demonstrating the capability of growing highly resistive buffer layers using a gaseous carbon source.

•Review of papers examining options for increasing residential PV self-consumption.•Two main options: battery energy storage and demand side management (DSM).•Higher potential for increased self-consumption with battery storage than DSM.•Further research needed for a comprehensive view of technologies and potential.

The phonon mode parameters and anisotropic mid-infrared dielectric function tensor components of high- Al-content Al_xGa_1-xN alloys in dependence of the Al content x are precisely determined from mid-infrared spectroscopic ellipsometry measurements for a set of high-quality Si-doped Al_xGa_1-xN epitaxial layers on 4H-SiC substrates. Two-mode behavior of the E_1(TO) modes and one-mode behavior of the A_1(LO) mode are found in agreement with previous Raman scattering spectroscopy reports. The composition dependencies of the IR active phonon frequency parameters are established and a discussion on the silent B_1 mode that may be disorder activated is provided. The static dielectric constants in dependence of x are determined by using the best-match model derived phonon mode frequency and high-frequency dielectric constant parameters and applying the Lydanne-Sachs-Teller relation. The effective mass parameter in high-Al-content Al_xGa_1-xN alloys and its composition dependence are determined from mid-infrared optical Hall effect measurements. Furtheremore, the free electron concentration N and mobility parameters μ of Al_xGa_1-xN films with similar Si doping levels are investigated as function of the Al content, x and discussed.

Chlorinated chemical vapor deposition (CVD) chemistry for growth of homoepitaxial layers of silicon carbide (SiC) has paved the way for very thick epitaxial layers in short deposition time as well as novel crystal growth processes for SiC. Here, we explore the possibility to use a brominated chemistry for SiC CVD by using HBr as additive to the standard SiC CVD precursors. We find that brominated chemistry leads to the same high material quality and control of material properties during deposition as chlorinated chemistry and that the growth rate is on average 10% higher for a brominated chemistry compared to chlorinated chemistry. Brominated and chlorinated SiC CVD also show very similar gas-phase chemistries in thermochemical modeling. This study thus argues that brominated chemistry is a strong alternative for SiC CVD because the deposition rate can be increased with preserved material quality. The thermochemical modeling also suggest that the currently used chemical mechanism for halogenated SiC CVD might need to be revised.

Al x Ga 1− x N (0.63≤ x ≤1) layers grown by hot‐wall metal‐organic chemical vapor deposition were intentionally doped with silicon at the atomic concentration of [Si] ∼ 2 × 10 18 cm −3 . Efficient silicon incorporation into the Al x Ga 1− x N layers was obtained for all Al contents, x , whereas the resistivity drastically increases for x > 0.84. Degradation of the structural quality and compensation by residual oxygen and carbon impurity were ruled out as possible explanations for the increased resistivity. Frequency dependent capacitance‐voltage measurements indicate that the Si donor is electrically active. Complementary electron paramagnetic resonance measurements suggest formation of stable Si‐related DX centers and increase in the activation energy of the silicon donor for x > 0.84.

A high mobility of 2250 cm2/V·s of a two-dimensional electron gas (2DEG) in a metalorganic chemical vapor deposition-grown AlGaN/GaN heterostructure was demonstrated. The mobility enhancement was a result of better electron confinement due to a sharp AlGaN/GaN interface, as confirmed by scanning transmission electron microscopy analysis, not owing to the formation of a traditional thin AlN exclusion layer. Moreover, we found that the electron mobility in the sharp-interface heterostructures can sustain above 2000 cm2/V·s for a wide range of 2DEG densities. Finally, it is promising that the sharp-interface AlGaN/GaN heterostructure would enable low contact resistance fabrication, less impurity-related scattering, and trapping than the AlGaN/AlN/GaN heterostructure, as the high-impurity-contained AlN is removed.

Establishing n- and p- type conductivity via intentional doping in epitaxial layers is fundamental to any semiconductor material system and its relevant device applications. Process parameters such ...

The high-Al-content Al x Ga 1-x N alloys, x>0.70, and AlN is the fundamental wide-band-gap material system associated with the technology development of solid-state LEDs operating at the short wavelengths in the deep-UV (λ < 280 nm). Yet, their properties are insufficiently understood. The present study is intended to bring elucidation on the long-time debated and much speculated Si transition from shallow donor in GaN to a localized deep DX defect in Al x Ga 1-x N alloys with increasing Al content. For that purpose electron paramagnetic resonance is performed on a particular selection of high-Al-content epitaxial layers of Al 0.77 Ga 0.23 N, alternatively Al 0.72 Ga 0.28 N, alloy composition.