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The specific contact resistivity (ρC) and interfacial reaction between Au/Pt/Al/Ti metallic multi-layers and Si-doped n-type AlxGa1−xN (n-AlGaN) layers in modulation-doped Al0.22Ga0.78N/GaN heterostructures have been investigated. By means of the measurements based on the transmission line model, the ρC as low as 1.6 × 10−4 Ω cm2 is obtained. Based on the X-ray diffraction analysis, it is found that N atoms in n-AlGaN layer diffuse out and a much amounts of N-vacancies are formed in n-AlGaN layer near the interface after the sample is annealed at temperatures higher than 500 °C. It induces the heavy n-type doped region in n-AlGaN near the interface, and thus leads to the decrease of the ρC. With increasing the annealing temperature, more N atoms in n-AlGaN layer diffuse out and react with Ti atoms. Ti2N phase is formed at the interface after the sample is annealed at 800 °C. In this case, the ρC further decreases. [Copyright &y& Elsevier]

Pt/Au Schottky contacts were fabricated on modulation-doped Al0.22GaN0.78/GaN heterostructures. Some different surface treatments were used before the deposition of the metal contacts to prevent the formation of the native oxide layer on the samples’ surfaces. Comparing the barrier height and the ideal factor of the Schottky contacts, we found that the treatment of the boiling (NH4)2S solution can remove the native oxide layer effectively. Detailed X-ray photoelectron spectroscopy measurements were also made to investigate the chemical reactions on the surface of samples after different treatments. [Copyright &y& Elsevier]

Photoluminescence (PL), X-ray diffraction (XRD) and Raman spectra have been measured to characterize the effects of phosphorus on the optical and structural properties of GaN1−xPx alloys with x&les;15%, grown by means of light-radiation heating, low-pressure metal-organic chemical vapor deposition. The PL spectra of GaN1−xPx show the largest red shift of 100 meV from the band-edge emission of GaN. XRD rocking curves show that the (0 0 0 2) peak of GaN1−xPx shifts to smaller angles with increasing P content. The Raman spectra of GaN1−xPx, recorded in backscattering geometry, exhibit four new vibrational modes at 256, 314, 377 and 428 cm−1 compared with an undoped GaN sample. The modes at 377 and 428 cm−1 are attributed to gap modes related to the Ga–P bond vibrations, whereas the other two modes at 256 and 314 cm−1 are assigned to phosphorus clusters and disorder-activated scattering, respectively. The frequency of the A1 (LO) mode is found to decrease with increasing x at a much high rate of approximately 268 cm−1/x for x<0.03. This red shift is attributed to the effects of alloying and strain. Furthermore, we have not observed characterizations related to GaP resulted from the phase separation from PL, XRD and Raman spectra of all investigated GaN1−xPx samples. [Copyright &y& Elsevier]

Transport properties of the two-dimensional electron gas (2DEG) in AlxGa1−xN/GaN heterostructures were investigated by means of magnetotransport measurements. The mobilities corresponding to the 2DEG in the first and the second subbands in the trianglar quantum well at the heterointerface were obtained. When the AlxGa1−xN barrier is partially relaxed, the mobility of the 2DEG in the first subband decreases significantly, but there is an increase of mobility in the second subband. We suggest that the scattering from the piezoelectric polarization field modulated by the misfit dislocations at the heterointerface is responsible for the decrease of the 2DEG mobility. Meanwhile, the 2DEG in the second subband leaves farther from the heterointerface than that in the first subband, and thus suffers weaker scattering from the heterointerface roughness and the modulated polarization field. Therefore, the mobility of the 2DEG in the second subband is much higher than that in the first subband. [Copyright &y& Elsevier]