We have found a new photoluminescence (PL) band with unusual properties in GaN. The blue band, termed as the BL C band, has a maximum at about 2.9 eV and an extremely short lifetime (shorter than 1 ns for a free electrons concentration of about 10 18 cm-3). The electron-and holecapture coefficients for this defect-related band are estimated as 10-9 and 10-10 cm 3 /s, respectively. The BL C band is observed only in GaN samples with relatively high concentration of carbon impurity, where the yellow luminescence (the YL1 band) with a maximum at 2.2 eV is the dominant defect-related PL. Both the YL1 and BL C bands likely originate from the C N defect, namely from electron transitions via the −/0 and 0/+ thermodynamic transition levels of the C N. BL C band appears only at high excitation intensities in n-type GaN samples co-doped with Si and C, and it can be found in wide range of excitation intensities in semi-insulating (presumably ptype) GaN samples doped with C. The properties and behavior of the YL1 and BL C bands can be explained using phenomenological models and first-principles calculations.
Carbon‐doping in the concentration range from [C] = 5 × 1017 to 1.2 × 1019 cm−3 is employed to achieve semi‐insulating properties of GaN layers as required for electronic power devices. Using propane as a carbon precursor, an independent analysis of the carbon incorporation during growth and its impact on electrical properties of the layers was obtained as growth parameters for optimum GaN quality could be applied. We observe that C is within precision of measurements fully incorporated in GaN as compensating deep acceptor. In a series of Si + C co‐doped samples, semi‐insulating properties were obtained for [C] > [Si] and the compensation efficiency for electrons is around unity. Through the extrinsic C‐doping technique previous ambiguous results on electrical and optical properties of GaN:C layers are clarified.
Carbon-doping is proposed to reduce the dislocation-mediated leakage currents in the GaN buffer layers. GaN:C grown by metalorganic vapor phase epitaxy using propane shows excellent quality up to [C] = 6.7 × 1018 cm−3. Locally probing dislocations by surface scanning potential microscopy reveal a transition from mostly neutral or weakly charged regions to dominantly negatively charged regions relative to the surrounding area at high doping levels. A relation between leakage currents and the relative dislocation charge state exists. Minimum leakage current is achieved if the dominant charge state of dislocation regions becomes negative against the surrounding.
The non-resonant carrier transfer in asymmetric double quantum wells is studied. Asymmetric cubic GaN/Al x Ga 1Àx N double quantum wells with Al content of x ¼ 0.26 AE 0.03 were grown on 3C-SiC (001) substrate by radio-frequency plasmaassisted molecular beam epitaxy. The barrier thickness d between a wide quantum well having 2.5 nm thickness and a narrow quantum well with width of 0.7 nm was varied from 1 to 15 nm. Furthermore, high resolution X-ray diffraction reciprocal space maps around the (113) direction provided the Al content and revealed a partially strain in the Al x Ga 1Àx N barriers and QWs. The coupling between the QWs was studied by interband photoluminescence spectroscopy at low temperatures. Four clearly distinguishable emission bands at 3.27 eV, 3.37 eV, 3.60 eV, and 3.74 eV are observed and could be assigned to the different layers. With decreasing barrier thickness d the photoluminescence intensity from the narrow QW is strongly reduced, indicating wave function redistribution from the narrow QW to the wide QW. The emission energies for the QWs are in good agreement with theoretical calculations using a Schrödinger-Poisson solver based on an effective mass model (nextnano 3 ). The PL intensity ratio of the narrow QW to the wide QW for varied barrier thicknesses was calculated by exploiting rate equations, revealing a good agreement between theory and experiment.
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