Fingerprints of carbon defects in vibrational spectra of gallium nitride (GaN) consider-ing the isotope effect

This work examines the carbon defects associated with recently reported and novel peaks of infrared (IR) absorption and Raman scattering appearing in GaN crystals at carbon ($^{12}C$) doping in the range of concentrations from $3.2*10^{17}$ to $3.5*10^{19} cm^{-3}$. 14 unique vibrational modes of defects are observed in GaN samples grown by hydride vapor phase epitaxy (HVPE) and then compared with defect properties predicted from first-principles calculations. The vibrational frequency shift in two $^{13}C$ enriched samples related to the effect of the isotope mass indicates six distinct configurations of the carbon-containing point defects. The effect of the isotope replacement is well reproduced by the density functional theory (DFT) calculations. Specific attention is paid to the most pronounced defects, namely tri-carbon complexes($C_N=C=C_N$) and carbon substituting for nitrogen $C_N$. The position of the transition level (+/0) in the bandgap found for $C_N=C=C_N$ defects by DFT at 1.1 eV above the valence band maximum, suggest that $(C_N=C=C_N)^+$ provides compensation of ${C_N}^-$. $C_N=C=C_N$ defects are observed to be prominent, yet have high formation energies in DFT calculations. Regarding ${C_N}$ defects, it is shown that the host Ga and N atoms are involved in the defect's delocalized vibrations and significantly affect the isotopic frequency shift. Much more faint vibrational modes are found from di-atomic carbon-carbon and carbon-hydrogen (C-H) complexes. Also, we note changes of vibrational mode intensities of $C_N$, $C_N=C=C_N$, C-H, and $C_N-C_i$ defects in the IR absorption spectra upon irradiation in the defect-related UV/visible absorption range. Finally, it is demonstrated that the resonant enhancement of the Raman process in the range of defect absorption above 2.5 eV enables the detection of defects at carbon doping concentrations as low as $3.2*10^{17} cm^{-3}$

Experimental Hall electron mobility of bulk single crystals of transparent semiconducting oxides

We provide a comparative study of basic electrical properties of bulk single crystals of transparent semiconducting oxides (TSOs) obtained directly from the melt (9 compounds) and from the gas phase (1 compound), including binary (β-Ga2O3, In2O3, ZnO, SnO2), ternary (ZnSnO3, BaSnO3, MgGa2O4, ZnGa2O4), and quaternary (Zn1−xMgxGa2O4, InGaZnO4) systems. Experimental outcome, covering over 200 samples measured at room temperature, revealed n-type conductivity of all TSOs with free electron concentrations (ne) between 5 × 1015 and 5 × 1020 cm−3 and Hall electron mobilities (μH) up to 240 cm2 V−1 s−1. The widest range of ne values was achieved for β-Ga2O3 and In2O3. The most electrically conducting bulk crystals are InGaZnO4 and ZnSnO3 with ne > 1020 cm−3 and μH > 100 cm2 V−1 s−1. The highest μH values > 200 cm2 V−1 s−1 were measured for SnO2, followed by BaSnO3 and In2O3 single crystals. In2O3, ZnO, ZnSnO3, and InGaZnO4 crystals were always conducting, while others could be turned into electrical insulators.Leibniz-Gemeinschaft http://dx.doi.org/10.13039/501100001664Leibniz-Institut für Kristallzüchtung (IKZ) im Forschungsverbund Berlin e.V. (3477)Peer Reviewe

Defects in AlN: High-frequency EPR and ENDOR studies

Compensation by deep-level defects and self-compensation of shallow donors in AlN are discussed in the light of results of a high-frequency pulse electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) study. It was suggested on the basis of the large mostly isotropic hyperfine interaction with A(27Al)=406 MHz that one of the deep-level defect is isolated interstitial Al2+ atom. Two types of effective-mass-like shallow donors with a delocalised wave function were shown to exist in unintentionally doped AlN. The experiments demonstrate how the transformation from a shallow donor to a deep (DX) centre takes place and how the deep DX centre can be reconverted into a shallow donor forming a spin triplet and singlet states with an exchange interaction of about 24 cm-1 and with a lowest triplet state. © 2009 Elsevier B.V. All rights reserved

Experimental Hall electron mobility of bulk single crystals of transparent semiconducting oxides

We provide a comparative study of basic electrical properties of bulk single crystals of transparent semiconducting oxides (TSOs) obtained directly from the melt (9 compounds) and from the gas phase (1 compound), including binary (β-Ga2O3, In2O3, ZnO, SnO2), ternary (ZnSnO3, BaSnO3, MgGa2O4, ZnGa2O4), and quaternary (Zn1−xMgxGa2O4, InGaZnO4) systems. Experimental outcome, covering over 200 samples measured at room temperature, revealed n-type conductivity of all TSOs with free electron concentrations (ne) between 5 × 1015 and 5 × 1020 cm−3 and Hall electron mobilities (μH) up to 240 cm2 V−1 s−1. The widest range of ne values was achieved for β-Ga2O3 and In2O3. The most electrically conducting bulk crystals are InGaZnO4 and ZnSnO3 with ne > 1020 cm−3 and μH > 100 cm2 V−1 s−1. The highest μH values > 200 cm2 V−1 s−1 were measured for SnO2, followed by BaSnO3 and In2O3 single crystals. In2O3, ZnO, ZnSnO3, and InGaZnO4 crystals were always conducting, while others could be turned into electrical insulators

Observation of the triplet metastable dtate of shallow donor pairs in AlN crystals with a negative-U behavior: A high-frequency EPR and ENDOR study

Theoretical predictions about the n-type conductivity in nitride semiconductors are discussed in the light of results of a high-frequency EPR an ENDOR study. It is shown that two types of effective-mass-like, shallow donors with a delocalized wave function exist in unintentionally doped AlN. The experiments demonstrate how the transformation from a shallow donor to a deep (DX) center takes place and how the deep DX center can be reconverted into a shallow donor forming a spin triplet and singlet states. © 2008 The American Physical Society

AlN overgrowth of nano-pillar-patterned sapphire with different offcut angle by metalorganic vapor phase epitaxy

We present overgrowth of nano-patterned sapphire with different offcut angles by metalorganic vapor phase epitaxy. Hexagonal arrays of nano-pillars were prepared via Displacement Talbot Lithography and dry-etching. 6.6 µm crack-free and fully coalesced AlN was grown on such substrates. Extended defect analysis comparing X-ray diffraction, electron channeling contrast imaging and selective defect etching revealed a threading dislocation density of about 109 cm-2. However, for c-plane sapphire offcut of 0.2° towards m direction the AlN surface shows step bunches with a height of 10 nm. The detrimental impact of these step bunches on subsequently grown AlGaN multi-quantum-wells is investigated by cathodoluminescence and transmission electron microscopy. By reducing the sapphire offcut to 0.1° the formation of step bunches is successfully suppressed. On top of such a sample an AlGaN-based UVC LED heterostructure is realized emitting at 265 nm and showing an emission power of 0.81 mW at 20 mA (corresponds to an external quantum efficiency of 0.86 %)

The 2020 UV emitter roadmap

Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm—due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments