B. Łucznik

Correlation between luminescence and compositional striations in InGaN layers grown on miscut GaN substrates

The influence of the miscut angle of GaN substrate on compositional and optical properties of InxGa1−xN epilayers (0.05<x<0.1) was examined using x-ray diffraction, photoluminescence (PL), cathodoluminescence, and Z-contrast scanning electron microscopy. We show that single atomic steps bunch during growth of InGaN and form macrosteps. Indium is incorporated differently at treads and risers of these macrosteps, which causes the layer to decompose and induces the formation of compositional growth striations. Since the growth step density increases with growing miscut angle of the substrate, the average indium concentration decreases and the average PL peak energy blueshifts and broadens with increasing miscut angle. The presented work enables understanding on microscopic scale effects related to the inhomogeneous distribution of indium in InGaN layers on miscut substrates, which is significant from the point of view of optoelectronic applications.

Complete in-plane polarization anisotropy of the A exciton in unstrained A-plane GaN films

Using reflectance spectroscopy, the in-plane polarization behavior of unstrained C- and A-plane GaN films is experimentally investigated. While no in-plane polarization anisotropy is observed for all three band-gap related excitons (A, B, and C) in unstrained C-plane GaN films, the A exciton is completely linearly polarized perpendicular to the c axis in unstrained A-plane GaN films. However, the B and C excitons are only partially polarized. This observation is in excellent agreement with results based on band-structure calculations using the Bir-Pikus Hamiltonian for the wurtzite crystal structure.

Free and bound excitons in GaN∕AlGaN homoepitaxial quantum wells grown on bulk GaN substrate along the nonpolar (112¯0) direction

Nonpolar multiple quantum wells (MQWs) have been grown by plasma assisted molecular beam epitaxy on bulk GaN crystals oriented along the (112¯0) direction. The photoluminescence intensity of the nonpolar MQWs was significantly higher than that found for the polar samples, both at low (10 K) and room temperature. This is a consequence of the lack of built-in electric field in samples grown along the (112¯0) direction. Clearly resolved spectra of the excitons have been observed in the studied MQWs. Studies of these excitonic structures, by means of polarization and temperature measurements enabled us to assign the observed lines to free and bound excitons in GaN quantum wells.

Effect of efficiency “droop” in violet and blue InGaN laser diodes

We have studied two types of InGaN laser diodes emitting at 410 and 440 nm. Each device was characterized by measuring light-current characteristics in two geometries for which the light was collected: along the resonator and perpendicularly to the cavity. In the first configuration, the 410 nm device displays no reduction of differential efficiency while 440 nm laser shows evidence of droop. In the perpendicular configuration both devices show the pronounced droop. We associate the suppression of the droop for 410 nm laser in the “along cavity” configuration with the appearance of the stimulated recombination.

Substrate misorientation induced strong increase in the hole concentration in Mg doped GaN grown by metalorganic vapor phase epitaxy

We demonstrate that relatively small GaN substrate misorientation can strongly change hole carrier concentration in Mg doped GaN layers grown by metalorganic vapor phase epitaxy. In this work intentionally misoriented GaN substrates (up to 2° with respect to ideal ⟨0001⟩ plane) were employed. An increase in the hole carrier concentration to the level above 1018 cm−3 and a decrease in GaN:Mg resistivity below 1 Ω cm were achieved. Using secondary ion mass spectroscopy we found that Mg incorporation does not change with varying misorientation angle. This finding suggests that the compensation rate, i.e., a decrease in unintentional donor density, is responsible for the observed increase in the hole concentration. Analysis of the temperature dependence of electrical transport confirms this interpretation.

Growth of Bulk GaN Crystals by HVPE on Single Crystalline GaN Seeds

In this chapter, the problems related to bulk crystallization of GaN by HVPE are discussed. High quality small single crystalline GaN seeds grown under high pressure and large, flat, free standing GaN substrates grown by HVPE were used in long duration HVPE experiments. The characterization results of the obtained bulk crystals suggest that strong dependence of physical properties (mostly oxygen content) on the orientation of the crystallization front is the main reason for strain and structural defects generation in the new grown material.

Step-flow growth mode instability of N-polar GaN under N-excess

GaN layers were grown on N-polar GaN substrates by plasma-assisted molecular beam epitaxy under different III/V ratios. Ga-rich conditions assure step-flow growth with atomically flat surface covered by doubly-bunched steps, as for Ga-polar GaN. Growth under N-excess however leads to an unstable step-flow morphology. Particularly, for substrates slightly miscut towards ⟨101¯0⟩, interlacing fingers are covered by atomic steps pinned on both sides by small hexagonal pits. In contrast, a three-dimensional island morphology is observed on the Ga-polar equivalent sample. We attribute this result to lower diffusion barriers on N-polar compared to Ga-polar GaN under N-rich conditions.

Doping-Induced Contrast in the Refractive Index for GaInN/GaN Structures at Telecommunication Wavelengths

The authors report on the doping-induced contrast in the refractive index in GaInN/GaN-structures at telecommunication wavelengths. This contrast appears because of the plasma edge effect which has been directly observed in reflectance spectrum of GaInN:Si-layer. The refractive index has been calculated from the reflectance by using Kramers–Kronig relations and measured by ellipsometry. It has been found that the contrast between the refractive index of GaInN:Si and GaN layers equals ~15% at the wavelength of 1.55-µm and the absorption coefficient is still close to zero at this wavelength. It means that GaN-based waveguides operating at 1.55-µm can be obtained by Si-doping.

High-pressure direct synthesis of aluminium nitride

We report the results of direct synthesis of aluminium nitride (AlN) under high nitrogen pressure up to 1 GPa and temperatures up to 2000 K. At pressure from 10 to 650 MPa we observe the combustion synthesis of AlN. As the result of the combustion process one can obtain the AlN sintered powder or AlN/Al metal matrix composites. For N2 pressure higher than 650 MPa the crystal growth of AlN from the solution of atomic nitrogen in aluminium is possible. Both needle-like and bulk AlN single crystals, up to 1 cm and 1 mm, respectively, have been obtained.

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.