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The crystalline quality of AlN epitaxial layers on sapphire substrates was improved by introducing trimethylgallium (TMGa) pulse flow into the growth of AlN nucleation layers. It was found that the density of both screw- and edge-type threading dislocations could be significantly reduced by introducing the TMGa pulse flow. With increasing TMGa pulse flow times, the lateral correlation length (i.e. the grain size) increases and the strain in the AlN epilayers changes from tensile state to compressive state. Unstrained AlN with the least dislocations and a smooth surface was obtained by introducing 2-times TMGa pulse flow. The crystalline improvement is attributed to enhanced lateral growth and improved crystalline orientation by the TMGa pulse flow.

We demonstrated the fabrication of GaN nanowire (NW) by selective etching of the GaN micro truncated-pyramid (μ-TP) in KOH solution. The GaN μ-TP, which consists of a (0 0 0 1) Ga-polar top surface and six {1 1 ¯ 0 1} N-polar sidewalls, were grown on the patterned AlN/Si template in metal organic chemical vapor deposition (MOCVD) system. KOH solution can selectively etch the N-polar sidewalls while leave the Ga-polar top surface intact. Hexagonal-prism-structured GaN NW with (0 0 0 1) top surface and {1 1 ¯ 0 0} sidewalls was obtained after adequate chemical etching. It was found that the three-dimensional geometry of the GaN NW is determined by the diameter of the (0 0 0 1) top surface and the height of the GaN μ-TP. And the chemical etching mechanism of GaN μ-TP towards GaN NW in hydroxide solution was explained.

We report the effect of periodic Si-delta-doping on the yellow luminescence (YL) and stress evolution in n-GaN epilayers. Photoluminescence measurements indicated that the YL component could be effectively suppressed with increasing dopant flow rate or decreasing period length. X-ray diffraction, transmission electron microscopy and secondary ion mass spectroscopy measurements revealed that the variations of dislocation density and carbon concentration are not the responsible mechanisms for the of YL emissions. The reduction of Ga vacancy related defects seems to be the probable reason for the YL suppression by Si-delta-doping. Moreover, Raman spectroscopy showed no obvious stress variation among these samples with different equivalent electron concentration. These observed features are entirely different from those reported for uniformly Si-doped GaN.

The arrays of edge threading dislocations (TD) have been observed in highly dislocated GaN film grown on Al 2 O 3 (0001) substrate. There are three kinds of arrays including straight-row, partial-circle and full-circle arrays. A theoretical model derived from the thermodynamics of grains evolution during the GaN recrystallization was used to explain the appearance of these arrays and the statistical distribution of edge TDs. Both the theoretical and experimental results show a Weibull distribution of edge TDs. It is concluded that in crystalline GaN epitaxial films, the edge TD arrays originate from grain coalescence after the recrystallization.

We investigated the influences of periodic Si δ-doping on the characteristics of n-GaN grown on Si (111) substrate by metal organic chemical vapor deposition (MOCVD). By using periodic δ-doping, the tensile stress in GaN film induced by Si-doping was reduced to 0.057 GPa/10 18 cm −3 electrons, which was 56% smaller than that in uniformly doped GaN. Moreover, superior electrical properties were achieved in periodic δ-doped GaN films without cracks by varying the Si doping level. X-ray diffraction measurements show similar 002 and 102 full widths at half maximum (FWHMs) for periodic δ-doped GaN and uniformly doped GaN with the same Si doping amount, implying equivalent crystalline qualities of GaN:Si through the two doping methods. A narrower FWHM of the near band edge emission of GaN was obtained in the photoluminescence spectrum with periodic δ-doping, indicating enhanced optical properties.

In the present paper, compositionally graded AlGaN buffer layer was employed to compensate the tensile stress and improve the crystalline quality of GaN grown on silicon substrate. During the growth of the graded buffer layer, the flow rate of trimethylaluminum (TMAl) was gradually decreased according to a series of functions, while the flow rate of trimethylgalliumm (TMGa) was increased with the same linear function. Therefore, these graded AlGaN layers showed different distributions of Al, which was confirmed by the high-resolution X-ray diffraction (HR-XRD) and the secondary ion mass spectroscopy (SIMS). A linear compositionally graded AlGaN layer was obtained by using the convex parabolic TMAl flow rate, which leads to a large compressive stress in GaN layer due to a linearly gradual change of lattice parameters and thermal expansion coefficient (TEC) from AlN seeding layer to GaN layer. Using the linear compositionally graded buffer layer, a 3-μm-thick crack-free GaN film with a low dislocation density was grown on silicon substrate. The dislocation evolution associated with stress was investigated by the cross-sectional micro-Raman scattering and transmission electron microscopy (TEM).

The influence of growth pressure on the properties of p-GaN grown by metal-organic chemical vapor deposition method was investigated. The p-GaN layers were grown at low temperature under growth pressures ranging from 100 to 400 mbar. We find that higher growth pressure leads to lower Mg incorporation in the p-GaN layer and results in high resistivity of p-GaN grown at 400 mbar. By optimizing the Cp2Mg/TMGa ratio, however, low sheet resistivity can be achieved for p-GaN grown at 100–300 mbar. The p-GaN grown at 300 mbar using optimal Cp2Mg/TMGa ratio of 1.3% shows the highest hole concentration of 5.0×10 17 cm −3 and the consequent minimum sheet resistivity of 2.6×10 4 Ω/sq. The superior electrical properties are ascribed to the reduction of compensation effect for the layer grown under the high growth pressure.

We have investigated the influence of the TEGa flow on the optical and structural properties of InGaN/GaN multiple quantum wells (MQWs) with an indium composition around 20%. The samples with five-pairs InGaN/GaN MQWs were grown on sapphire substrates by metalorganic chemical vapor deposition. Photoluminescence spectra at 8 K showed that the MQWs grown with a low amount of TEGa flow gave a strong single peak and a higher emission energy. High-resolution X-ray diffraction measurements showed a deterioration of the InGaN/GaN interfaces in the sample grown with the large TEGa flow. The luminescence thermal quenching characteristics suggested that more structural defects acting as non-radiative recombination centers formed in the MQWs when the TEGa flow increased. The results indicate that decreasing the TEGa flow help to build up a new growth balance during the growth of InGaN wells, leading to less structural defects, more homogeneous indium distribution and the abrupt MQWs interfaces.