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.
In this work, an AlGaN/GaN-HEMT heterostructure is exemplarily studied by a strict place-to-place correlational approach in order to help clarify some open questions in the wide field of reliability topics. Especially, vertical leakage current, its relation to dislocations in general, and specific types in particular are investigated on a highly defective material. With the aid of atomic force microscopy (AFM) in tapping mode, cathodoluminescence imaging, defect selective etching, and energy dispersive X-ray, the material’s defect content around the device relevant two dimensional electron gas is analyzed. The total dislocation density, as well as the density of threading screw, edge, and mixed type dislocations, is systematically investigated directly. The obtained result is statistically much more significant than is possible by conventional transmission electron microscopy studies and more precise than the results obtained by the indirect method of rocking curve analysis. The method of conductive AFM allowed mapping of variations in the vertical leakage current, which could be correlated with variations in barrier leakage or gate leakage. Spots of locally high leakage current could be observed and directly assigned to dislocations with a screw component, but with significant differences even within the same group of dislocation types. The electrical activity of dislocations is discussed in general, and a fundamental model for a potential dislocation driven vertical leakage is proposed.
was imaged by electron backscatter diffraction (EBSD) and scanning X-ray diffraction microscopy (SXRM). The crystal orientation at the surface, determined by EBSD, is correlated with the surface topography, which shows triangular pyramidal features with edges oriented in two different orientations rotated in the surface plane by 60. The bulk crystal orientation is mapped out using SXRM by measuring the diffracted X-ray intensity of an asymmetric Bragg peak using a nano-focused X-ray beam scanned over the sample. By comparing bulkand surface-sensitive measurements of the same area, buried twin domains not visible on the surface are identified. The lateral twin domain size is found to increase with the film thickness.
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.
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