Compensation effects in GaN:Mg probed by Raman spectroscopy and photoluminescence measurements

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in J. Appl. Phys. 113, 103504 (2013) and may be found at https://doi.org/10.1063/1.4794094.Compensation effects in metal organic chemical vapour deposition grown GaN doped with magnesium are investigated with Raman spectroscopy and photoluminescence measurements. Examining the strain sensitive E2(high) mode, an increasing compressive strain is revealed for samples with Mg-concentrations lower than 7 × 1018 cm−3. For higher Mg-concentrations, this strain is monotonically reduced. This relaxation is accompanied by a sudden decrease in crystal quality. Luminescence measurements reveal a well defined near band edge luminescence with free, donor bound, and acceptor bound excitons as well as a characteristic donor acceptor pair (DAP) luminescence. Following recent results, three acceptor bound excitons and donor acceptor pairs are identified. Along with the change of the strain, a strong modification in the luminescence of the dominating acceptor bound exciton and DAP luminescence is observed. The results from Raman spectroscopy and luminescence measurements are interpreted as fingerprints of compensation effects in GaN:Mg leading to the conclusion that compensation due to defect incorporation triggered by Mg-doping already affects the crystal properties at doping levels of around 7 × 1018 cm−3. Thereby, the generation of nitrogen vacancies is introduced as the driving force for the change of the strain state and the near band edge luminescence.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Temperature dependent photoluminescence of lateral polarity junctions of metal organic chemical vapor deposition grown GaN

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Journal of Applied Physics 110, 093503 (2011) and may be found at https://doi.org/10.1063/1.3656987.We report on fundamental structural and optical properties of lateral polarity junctions in GaN. GaN with Ga- to N-polar junctions was grown on sapphire using an AlN buffer layer. Results from scanning electron microscopy and Raman spectroscopy measurements indicate a superior quality of the Ga-polar GaN. An extremely strong luminescence signal is observed at the inversion domain boundary (IDB). Temperature dependent micro photoluminescence measurements are used to reveal the recombination processes underlying this strong emission. At 5 K the emission mainly arises from a stripe along the inversion domain boundary with a thickness of 4-5 μm. An increase of the temperature initially leads to a narrowing to below 2 μm emission area width followed by a broadening at temperatures above 70 K. The relatively broad emission area at low temperatures is explained by a diagonal IDB. It is shown that all further changes in the emission area width are related to thermalization effects of carriers and defects attracted to the IDB. The results are successfully used to confirm a theoretical model for GaN based lateral polarity junctions. Due to the strong and pronounced emission of IDBs even at elevated temperatures, it is demonstrated that lateral polarity junctions exhibit a strong potential for future high efficiency devices.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Polarity Control in Group-III Nitrides beyond Pragmatism

Controlling the polarity of polar semiconductors on nonpolar substrates offers a wealth of device concepts in the form of heteropolar junctions. A key to realize such structures is an appropriate buffer-layer design that, in the past, has been developed by empiricism. GaN or ZnO on sapphire are prominent examples for that. Understanding the basic processes that mediate polarity, however, is still an unsolved problem. In this work, we study the structure of buffer layers for group-III nitrides on sapphire by transmission electron microscopy as an example. We show that it is the conversion of the sapphire surface into a rhombohedral aluminum-oxynitride layer that converts the initial N-polar surface to Al polarity. With the various AlxOyNz phases of the pseudobinary Al2O3-AlN system and their tolerance against intrinsic defects, typical for oxides, a smooth transition between the octahedrally coordinated Al in the sapphire and the tetrahedrally coordinated Al in AlN becomes feasible. Based on these results, we discuss the consequences for achieving either polarity and shed light on widely applied concepts in the field of group-III nitrides like nitridation and low-temperature buffer layers

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

Quasi-phase-matched second harmonic generation of UV light using AlN waveguides

As an alternative to electrically injected diodes, UV light emission can be obtained via second harmonic generation (SHG). In weakly birefringent materials such as aluminum nitride (AlN), the phase matching of the driving and second harmonic waves can be achieved by the quasi-phase-matching (QPM) technique, where the polarity of the material is periodically changed commensurate with the coherence wavelength. QPM also allows the use of the highest nonlinear susceptibility, and therefore, higher conversion efficiencies are possible. In this work, the QPM SHG of UV light in AlN lateral polar structure-based waveguides is demonstrated. The peak intensity of the frequency doubled laser light was measured at 344nm and 472nm wavelengths, in agreement with dispersion-based theoretical predictions. These results confirm the potential of III-nitride-based lateral polar structures for quasi-phase-matched nonlinear optics and for frequency doubling media for UV light generation

Quasi-phase-matched second harmonic generation of UV light using AlN waveguides

As an alternative to electrically injected diodes, UV light emission can be obtained via second harmonic generation (SHG). In weakly birefringent materials such as aluminum nitride (AlN), the phase matching of the driving and second harmonic waves can be achieved by the quasi-phase-matching (QPM) technique, where the polarity of the material is periodically changed commensurate with the coherence wavelength. QPM also allows the use of the highest nonlinear susceptibility, and therefore, higher conversion efficiencies are possible. In this work, the QPM SHG of UV light in AlN lateral polar structure-based waveguides is demonstrated. The peak intensity of the frequency doubled laser light was measured at 344nm and 472nm wavelengths, in agreement with dispersion-based theoretical predictions. These results confirm the potential of III-nitride-based lateral polar structures for quasi-phase-matched nonlinear optics and for frequency doubling media for UV light generation

Aqueous Stability of Ga- and N‑Polar Gallium Nitride

The stability of III-nitride semiconductors in various solutions becomes important as researchers begin to integrate them into sensing platforms. . This study quantitatively compares the stability of GaN surfaces with different polarities. This type of quantification is important because it represents the first step toward designing semiconductor material interfaces compatible with solution conditions. A stability study of Ga- and N-polar GaN was conducted by immersion of the surfaces in deionized H₂O, pH 5, pH 9, and H₂O₂ solutions for 7 days. Inductively coupled plasma mass spectrometry of the solutions was conducted to determine the amount of gallium leached from the surface. X-ray photoelectron spectroscopy and atomic force microscopy were used to compare the treated surfaces to untreated surfaces. The results show that both gallium nitride surface types exhibit the greatest stability in acidic and neutral solutions. Gallium polar surfaces were found to exhibit superior stability to nitrogen polar surfaces in the solutions studied. Our findings highlight the need for further research on surface passivation and functionalization techniques for polar III-nitride semiconductors

Cathodoluminescence of silicon doped aluminum nitride with scanning transmission electron microscopy

Here, we apply cathodoluminescence in scanning transmission electron microscopy to infer the influence of dislocation strain fields on the formation of point defect complexes in Si doped AlN. In addition to identifying non-radiative recombination centers, tracking Si related defect emission energies reveals a red-shift at threading dislocations. We discuss these results in the context of multiple Si-vacancy defect complexes that can form and the influence of local strain on their formation energies. By correlating the electronic and structural properties at the nanoscale, cathodoluminescence elucidates the inhomogeneity of defect complexes in Si doped AlN and offers the potential for strain engineering to control the defect energy formation landscape

Second-Harmonic Generation of Blue Light in GaN Waveguides

Second-harmonic generation was studied in III-metal-polar GaN films grown on sapphire substrates by metalorganic chemical vapor deposition and formed into ridge waveguides. Broadband near-IR femtosecond pulses of an optical parametric amplifier system were injected by end-fire coupling and the nonlinear response was measured while tuning the central wavelength. A prominent peak was found at 450 nm for 1140 nm thick and 10 μm wide GaN waveguides. The measured second-harmonic peak was in agreement with the modal-dispersion phase matching condition calculated using the dispersion of the extraordinary refractive indices of GaN obtained by prism coupling