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In this paper a -plane GaN-layers, grown by two different techniques are compared, mainly focusing on the surface morphology and microstructure of the samples: Layers grown by metal organic vapor phase epitaxy (MOVPE) exhibit a flat surface interrupted by a certain number of pits with triangular shape. In contrast, specimens processed by LPSG developed a rough surface with a roof like morphology. The morphology is discussed in view to the facetting behaviour of the GaN. The observed thickness of the layers implies, that growth in this direction is slightly faster compared to the regular c -plane growth. TEM investigations showed that the microstructure of the a -plane samples is dominated by a high density of dislocations and stacking faults. In photoluminescence measurements a slightly different characteristic was seen for the materials grown by the two techniques. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on first studies of the phonon mode properties of hexagonal AlxInyGa1—x—yN films. The 200–330 nm thick quaternary films (0 ≤ x ≤ 0.40, y ≈ 0.12), grown by MOCVD on 6H-SiC substrates, are pseudomorphically strained to the GaN buffer layers. We observe and differentiate the influence of strain and alloying on the frequency of the GaN-like E1(TO) phonon mode of AlxInyGa1—x—yN using infrared spectroscopic ellipsometry. Assuming the effective electron mass value of GaN, the free-electron concentrations are estimated from the A1-LO-phonon–plasmon coupled mode frequencies and increase by about two orders of magnitude in going from x = 0 to x = 0.40. We further obtain the composition dependence of the polarity for the AlN-like phonon mode with E1 symmetry. A possibly disorder-induced absorption band occurs above the GaN-like E1(TO) mode.

A GaInN quantum-well structure with an average In-concentration of 16%, grown by metal organic chemical vapor deposition (MOCVD) on a SiC substrate, was studied by transmission electron microscopy (TEM) and photoluminescence spectroscopy (PL) before and after an one-hour annealing treatment at 980 °C. Due to the heat treatment, the formation of In-rich clusters with extensions between 10 nm and 100 nm and In-concentrations above 80% was observed. Nevertheless, the PL exhibits a blue shift in the PL-peak energy of about 90 meV compared to the as-grown sample and do not show a significant influence of the In-rich clusters on the emission wavelength.

We studied the development of V-shaped defects in GaInN–GaN quantum well superlattices. We observed that these defects could not be suppressed by varying growth parameters like strain, In content, GaInN growth temperature etc. However, perfect superlattices without such defects could be grown by cycling the temperature between low (for the GaInN wells) and high temperatures (for the GaN barriers). Although a large hydrogen/nitrogen ratio in the carrier gas seems to hinder the defect formation in GaN–AlGaN superlattices, it was not possible to suppress the defect formation in a GaInN–GaN superlattice by decreasing the total nitrogen flow.

In this paper, we present our investigations of the waveguiding properties of various GaInN/GaN-DH and laser structures; particularly with regard to the influence of the substrate. Due to the small refractive index of sapphire and the neccessity of a thick GaN buffer, several higher order modes (up to 30) are guided in a complete laser structure on sapphire. This and the fact, that the nitrides are birefringent, lead to mode coupling phenomena. To support our experimental results we performed calculations with a 4 × 4 transfer-matrix method, taking into account the birefringence of the material. The results are in good qualitative agreement with our experimental data.

We report on the application of infrared spectroscopic ellipsometry (IR-SE) for wavelengths from 3 to 30 μm as a novel approach for nondestructive optical characterization of free-carrier and optical-phonon properties of group III-nitride heterostructures grown by metal-organic vapor phase epitaxy on sapphire. Model calculations for the ordinary (ε⊥) and extraordinary (ε∥) dielectric functions of the heterostructure components provide sensitivity to the IR-active phonon frequencies and anisotropy of the free-carrier response.

We investigate the relations between thickness and strain of GaInN–GaN MQW layers and the formation of defects. The samples were grown by MOPVE on sapphire or SiC substrates. We have deposited several GaInN–GaN MQW structures differing in thicknesses of both, GaInN well and GaN barrier material. We also produced MQWs grown at different temperatures and with different indium content. Scanning transmission electron microscopy images of such structures revealed that the formation of V-defects is initiated by threading dislocations. The generation of the defects always started at about the third quantum well in the structure, irrespective of the average strain of the structures, which was changed through the mentioned variation of the well and barrier thicknesses. Moreover, we could not observe any significant variations in size or density of the V-defects with varying indium content or growth temperature.

Both the large linewidth of optical transitions as well as the sizeable “Stokes” shift seem to suggest strong localization effects in nitride quantum wells. In contrast, we clearly show that the “Stokes” shift can be easily understood in terms of the huge piezoelectric fields present in such structures. At the same time, those fields lead to a characteristic dependence of the linewidth on well width, which we use to quantitatively separate fluctuation and field effects. We find that in high-quality quantum wells, the relative compositional fluctuation is less than 10% and the well width fluctuation is of the order 1 monolayer.

In this paper we investigate the influence of spontaneous polarization on the luminescence behavior of nitride heterostructures. Spontaneous polarization is normally screened by charged surface adsorbates. Using electron stimulated desorption we can remove this coverage and spontaneous polarization comes into effect. For GaN/Al0.15GaN and In0.11GaN quantum wells this results in an enormous blue-shift of the emission, in our case of up to 500 meV. In bulk-like layers the Franz-Keldysh effect slightly lowers the emission energies and the lines become broadened.

Abstract: The low pressure metalorganic vapor phase epitaxy growth of wurzite (Al, In, Ga)N heterostructures on sapphire substrates is investigated by quantitative analytical scanning transmission electron microscopy techniques like atomic number (Z-) contrast imaging and convergent beam electron diffraction (CBED). Especially (In, Ga)N quantum wells of different thicknesses as well as superlattices were analyzed with respect to defects, chemical composition variations, interface abruptness and strain (relaxation) effects. The interfaces in In0.12Ga0.88N/GaN quantum wells appear to be asymmetric. Additionally, we found composition variations of ΔxIn≥0.03 within the InGaN quantum wells. The application of electron diffraction techniques (CBED) yields quantitative information on strain and relaxation effects. For the case of 17 nm thick InGaN quantum wells, we observed relaxation effects which are not present in the investigated thin quantum wells of 2 nm thickness. The experimentally obtained diffraction patterns were compared to simulations in order to get values for strain within the quantum wells. Additionally, the influence of dislocations on the digression of superlattices is investigated.

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