The Mechanism of Growth of GaN Films by the HVPE Method on SiC Synthesized by the Substitution of Atoms on Porous Si Substrates

The mechanisms of nucleation and growth of GaN films on porous silicon substrates with a buffer layer of silicon carbide grown by new method of substitution of atoms directly in the subsurface layer of the Si substrate were studied. P-type Si(111) plates with a deposited system of periodic mesopores of a diameter of 17 nm and a depth of 1.5 μm were used as the substrates for growth of SiC buffer layer. The buffer AlN layers of a thickness of the order of 500 nm followed by GaN layers of a thickness of the order of 5 μm were deposited by the HVPE method on SiC/Si substrates prepared in different ways. SEM micrographs, Raman and photoluminescence spectra of these samples are given. The influence of the Si substrate preparation method and the conditions of the SiC layer synthesis on the growth mechanisms of both AlN and GaN films has been experimentally revealed. The formation of two types of V-defects during the growth of GaN was observed. A theoretical model for the formation of defects of the first and second types is developed.

Source:IOPscience

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Metalorganic chemical vapour deposition of GaN layers on ZnO substrates using α-Al2O3 as a transition layer

This work addresses the instability of a ZnO substrate during metalorganic chemical vapour deposition (MOCVD) growth of GaN by using Al2O3 films deposited by atomic layer deposition (ALD) as a stabilizing transition layer on the Zn face of ZnO (0 0 0 1) substrates. A systematic study of Al2O3 films of different thicknesses (2–90 nm) under different ALDs and post-annealing conditions was carried out. However, this paper focuses on as-deposited 20 and 50 nm Al2O3 films that were transformed to polycrystalline α-Al2O3 phases after optimal annealing at 1100 °C for 10 min and 20 min, respectively. GaN layers were grown on ZnO substrates with these α-Al2O3transition layers by MOCVD using NH3 as a nitrogen source. Wurtzite GaN was observed by high resolution x-ray diffraction only on 20 nm Al2O3/ZnO substrates. Field-emission scanning electron microscopy showed a mirror-like surface, no etch pits and no film peeling in these samples. Room temperature photoluminescence showed a red-shift in the near band-edge emission of GaN, which may be related to oxygen incorporation forming a shallow donor-related level in GaN. Raman scattering also indicated the presence of a well-crystallized GaN layer on the 20 nm Al2O3/ZnO substrate.

Source:IOPscience

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Reuse of ScAlMgO4 substrates utilized for halide vapor phase epitaxy of GaN

ScAlMgO4 (SCAM) substrates with a small lattice-mismatch to GaN and c-plane cleavability are promising for fabricating high-quality free-standing GaN wafers. To reduce the cost in the fabrication of free-standing GaN wafers, the reuse of a SCAM substrate is demonstrated. By cleaving a SCAM substrate which has been already utilized for the growth of a thick GaN film by halide vapor phase epitaxy, the atomically flat surface can be obtained. The threading dislocation density of a 320 μm thick GaN film grown on this cleaved SCAM substrate is 2.4 × 107 cm−2, which is almost the same as that on a new SCAM substrate. This result indicates that a SCAM substrate can be reused for GaN growth.

Source:IOPscience

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On the stress relaxation mechanism of GaN thin films grown on Si(111) substrates

Gallium nitride (GaN) thin films grown on Si(111) substrates via plasma-assisted molecular beam epitaxy were investigated. The morphology of GaN films was studied by scanning electron microscopy. It was found that GaN films at nanoscale contain a large amount of cavities. The Raman measurements of GaN/Si(111) system were performed and showed that the position of the GaN E2 (TO) band is close to that of unstrained GaN. We proposed a stress relaxation mechanism to explain the low stress state of GaN films.

Source:IOPscience

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The substrate-induced effect of GaN MSM photodetectors on silicon substrate

GaN metal–semiconductor–metal photodetectors (MSM PDs) on silicon substrates and sapphire substrates were fabricated and characterized. We found that the current–voltage (I–V) characteristics of MSM PDs on the silicon substrate could be approximated by the Poole–Frenkel conduction behavior. This phenomenon was attributed to the presence of micro-grain structure in the silicon-substrate epitaxy layer. The voltage-dependent responsivity of GaN MSM PDs on the silicon substrate was also evidence of micro-grains inside the epitaxy layer. At a low frequency, the 1/f-form noise was a main contribution to both PDs. Moreover, the extremely low β (~0.7) extracted from GaN MSM PDs on the silicon substrate was first reported. Based on the current–voltage behavior, the extremely low β was believed to originate from the silicon-substrate-induced micro-grain.

Source:IOPscience

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Vertical-conducting InGaN/GaN multiple quantum wells LEDs with AlN/GaN distributed Bragg reflectors on Si(111) substrate

In this study, crack-free InGaN/GaN multiple quantum well (MQW) LEDs with five-pair AlN/GaN distributed Bragg reflectors (DBRs) were grown on a Si(111) substrate using a linear compositionally graded AlGaN layer to compensate for tensile stresses. DBR-based LEDs exhibited higher light efficiency and better crystalline quality than non-DBR-based LEDs. Vertical-conducting DBR-based LEDs were realized using through-hole structure. Through-holes were formed from the n-GaN layer to the Si substrate and filled with metals, which connected the n-GaN layer and the Si substrate. Insulating AlN, high-resistivity AlGaN layers, and the large band offset at the AlN/Si interface were all shorted by the metal filled into the through-holes. Compared to DBR-based LED without through-holes, DBR-based LEDs with through-holes exhibited significantly better electrical and optical properties.

Source:IOPscience

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Surface and optical characteristics of polycrystalline GaN layer with different pores profile of porous GaAs/GaAs substrate

This work investigated the influence of pores profile of a porous GaAs/GaAs substrate on surface and optical characteristics of an over-deposited GaN layer. Different pores profile of the porous GaAs/GaAs substrate was introduced by varying the DMF concentration of 50%, 75% and 90%. The pores distribution is more uniform, while the pores size is bigger with higher DMF concentration. In contrast, the pores depth is less deep when the DMF concentration was higher than 75%. Next, the GaN layer was deposited onto the porous GaAs/GaAs substrate using an e-beam evaporator system, followed by thermal annealing in ammonia ambient. It was found that the porous GaAs/GaAs substrate, etched by the DMF concentration above 75% gave lower surface roughness to the polycrystalline GaN layer although the surface morphology showed no significant changes. XRD measurement showed on non-porous substrate favoured hexagonal growth in the polycrystalline GaN layer. Instead, the porous GaAs/GaAs substrate favoured the cubic growth, especially the porous GaAs/GaAs substrate etched by 75% DMF concentration. Moreover, the GaN layer on the porous GaAs/GaAs substrate etched by 75% DMF concentration showed the smallest FWHM of NBE peak emission, while exhibited a relaxation level closer to a reported stress-free bulk GaN, as compared to other samples. After all, the porous GaAs/GaAs substrate, etched by 75% DMF concentration has improved the surface and optical characteristics of the layer due to its better porosity.

Source:IOPscience

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