J. Borysiuk

Influence of substrate nitridation temperature on epitaxial alignment of GaN nanowires to Si(111) substrate

An arrangement of self-assembled GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy on a Si(111) substrate is studied as a function of the temperature at which the substrate is nitridized before GaN growth. We show that the NWs grow with the c-axis perpendicular to the substrate surface independently of nitridation temperature with only a slight improvement in tilt coherency for high nitridation temperatures. A much larger influence of the substrate nitridation process on the in-plane arrangement of NWs is found. For high (850 °C) and medium (450 °C) nitridation temperatures angular twist distributions are relatively narrow and NWs are epitaxially aligned to the substrate in the same way as commonly observed in GaN on Si(111) planar layers with an AlN buffer. However, if the substrate is nitridized at low temperature (~150 °C) the epitaxial relationship with the substrate is lost and an almost random in-plane orientation of GaN NWs is observed. These results are correlated with a microstructure of silicon nitride film created on the substrate as the result of the nitridation procedure.

Directional crystallization of GaN on high-pressure solution grown substrates by growth from solution and HVPE

Abstract High-pressure solution grown hexagonal GaN platelets have been used as substrates for subsequent solution growth of GaN in the 〈0xa00xa00xa01〉 directions and also for hydride vapor phase epitaxial (HVPE) growth of GaN. The solution growth was stabilized due to the application of large positive temperature gradient perpendicular to the surface of the GaN substrate. The growth by HVPE was carried out at atmospheric pressure in a horizontal quartz reactor. Relatively rapid growth rates of about 10xa0μm/h and about 50xa0μm/h have been achieved on the Ga-polar (0xa00xa00xa01) surface of the substrates for solution growth and HVPE, respectively, which constitutes an increase in the c -direction growth rate for both methods. Both methods produced nearly dislocation-free GaN.

Influence of AlN layer on electric field distribution in GaN/AlGaN/GaN transistor heterostructures

Distribution of built-in electric field in GaN/AlGaN/GaN transistor heterostructures without and with AlN layer was studied theoretically (solving the Schrodinger and Poisson equation) and experimentally (measuring contactless electroreflectance (CER) spectra and analyzing the AlGaN-related Franz-Keldysh oscillation). It is shown that the AlN layer changes very strongly the distribution of electric field in such heterostructures. This change can be very well predicted if the surface boundary conditions for self-consistent Schrodinger-Poisson calculations in GaN/AlGaN/GaN heterostructures are known. These conditions can be determined/verified by CER measurements of AlGaN-related Franz-Keldysh oscillation, which depends on the built-in electric field in AlGaN layer.

High pressure and time resolved studies of optical properties of n-type doped GaN/AlN multi-quantum wells: Experimental and theoretical analysis

High-pressure and time-resolved studies of the optical emission from n-type doped GaN/AlN multi-quantum-wells (MQWs) with various well thicknesses are analysed in comparison with ab initio calculations of the electronic (band structure, density of states) and optical (emission energies and their pressure derivatives, oscillator strength) properties. The optical properties of GaN/AlN MQWs are strongly affected by quantum confinement and polarization-induced electric fields. Thus, the photoluminescence (PL) peak energy decreases by over 1u2009eV with quantum well (QW) thicknesses increasing from 1 to 6u2009nm. Furthermore, the respective PL decay times increased from about 1u2009ns up to 10u2009μs, due to the strong built-in electric field. It was also shown that the band gap pressure coefficients are significantly reduced in MQWs as compared to bulk AlN and GaN crystals. Such coefficients are strongly dependent on the geometric factors such as the thickness of the wells and barriers. The transition energies, their oscilla...

HRTEM and LACBED of Zigzag Boundaries in GaN Epilayers

Defects recognition in GaN epilayers was performed using HRTEM and LACBED images. Edge type dislocations, basal plane and prismatic stacking faults were determined from HRTEM analysis. Stacking mismatch boundaries on zigzag steps were found and examined using LACBED patterns in bright and dark field. For stacking faults Bragg lines split into a main and a subsidiary line. The fault plane and displacement vector can be identified from trace analysis performed on LACBED patterns.

Structural and Chemical Characterization of Al(Ga)N/GaN Quantum Well Structures Grown by Plasma Assisted Molecular Beam Epitaxy

Growth of high quality GaN/AlN heterostructures by plasma assisted molecular beam epitaxy (PAMBE) is possible with excess of Ga on the surface. During growth of AlN this additional Ga acts as surfactant and improves mobility of the Al adatoms on the growing surface, at the possible cost of Ga segregation and creation of mixed AlGaN interlayer. Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) were used to determine chemical composition of high crystallographic quality GaN-AlN multilayer structure. It was shown that segregation occurs at AlN-GaN heterointerfaces, while GaN-AlN interfaces have abrupt stepwise change of the chemical composition. HRTEM results show creation of trench defects at the periphery of growing AlN islands in the case of nonoptimized growth.