Frank Habel

GaInN quantum wells grown on facets of selectively grown GaN stripes

Multiple GaInN quantum wells (QWs) were grown on facets with reduced piezoelectric fields (PFs) of selectively grown GaN stripes oriented along the ⟨11¯00⟩ and ⟨112¯0⟩ directions by metalorganic vapor phase epitaxy. We found a higher normalized growth rate for the GaInN QWs on the {11¯01} facets compared to the {112¯2} facets and the planar grown reference sample on unstructured template. The different luminescence wavelengths observed for the QWs on these different facets can partly be explained by the reduced PFs, but additionally indicate that the In incorporation efficiency depends on the facet type. On stripes with trapezoidal cross section, we found strong interfacet migration of In and Ga changing the local thickness and composition significantly.

Improved GaN layer morphology by hydride vapor phase epitaxy on misoriented Al2O3 wafers

Crack-free thick GaN layers have been grown by hydride vapor phase epitaxy on on-axis as well as on off-axis GaN-Al2O3 templates. A dramatic difference in surface quality could be traced back to the misorientation of the substrates: Mirror-like layers have been obtained for slightly off-oriented substrates, whereas pyramids and other surface structures were found on samples grown on exactly oriented wafers. Such excellent surfaces may make further surface treatment prior to a subsequent use of these wafers in further epitaxial processes obsolete.

Absorption and light scattering in InGaN-on-sapphire- and AlGaInP-based light-emitting diodes

Different experimental and simulation techniques aiming at a better understanding of lateral mode absorption in light-emitting diodes (LEDs) are presented in this paper. A measurement of transmitted power versus propagation distance allows us to derive the absorption losses of LED layer structures at their emission wavelength. Two models for the observed intensity distribution are presented: one is based on scattering, whereas the other relies on selective absorption. Both models were applied to InGaN-on-sapphire-based LED structures. Material absorption losses of 7 cm/sup -1/ for the scattering model and 4 cm/sup -1/ for the absorbing-layer model were obtained. Furthermore, these values are independent of the emission wavelength of the layer structure in the 403-433-nm range. The losses are most likely caused by a thin highly absorbing layer at the interface to the substrate. In a second step, interference of the modal field profile with the absorbing layer can be used to determine its thickness (d=75 nm) and its absorption coefficient (/spl alpha/ /spl ap/ 3900 cm/sup -1/). This method has also been tested and applied on AlGaInP-based layer structures emitting at 650 nm. In this case, the intensity decay of /spl alpha/=30 cm/sup -1/ includes a contribution from the absorbing substrate.

The pyroelectric coefficient of free standing GaN grown by HVPE

The present study reports on the temperature dependent pyroelectric coefficient of free-standing and strain-free gallium nitride (GaN) grown by hydride vapor phase epitaxy (HVPE). The Sharp-Garn method is applied to extract the pyroelectric coefficient from the electrical current response of the crystals subjected to a sinusoidal temperature excitation in a range of 0 °C to 160 °C. To avoid compensation of the pyroelectric response by an internal conductivity, insulating GaN crystals were used by applying C, Mn, and Fe doping during HVPE growth. The different pyroelectric coefficients observed at room temperature due to the doping correlate well with the change of the lattice parameter c. The obtained data are compared to previously published theoretical and experimental values of thin film GaN and discussed in terms of a strained lattice.

Substrates for wide bandgap nitrides

Different substrates for gallium nitride growth are discussed. The commercially relevant substrates, silicon carbide and sapphire, and the two most promising alternatives, silicon and gallium nitride, are compared in terms of suitability for epitaxial processes and in their effects on devices. An estimation on future market success is given.

Absorption of guided modes in light emitting diodes

The absorption of lateral guided modes in light emitting diodes is determined by the photocurrent measurement method. A theory for waveguide dispersion is presented and extended by ray-tracing simulations. Absorption coefficients of InGaN-on-sapphire and AlGaInP-based structures is evaluated by comparison with simulation curves. For nitride-based samples with emission wavelengths of 415 nm and 441 nm an absorption of 7 cm-1 is obtained. It is found that scattering is present in the buffer layer and influences the lateral intensity distribution. The investigated AlGaInP-based sample exhibits an absorption of α = 30 cm-1 at 650 nm emission wavelength.

Three-dimensional imaging of ELOG growth domains by scanning cathodoluminescence tomography

Columnar ELOG growth domains formed in a 65 μm thick HVPE GaN layer during overgrowth of hexagonal SiO 2 masks are three-dimensionally characterized using spatially and spectrally resolved scanning cathodoluminescence (CL) microscopy. In conjunction with cross-sectional imaging perpendicular to the c-plane, a direct visualization of the 3D domain formation is achieved by consecutive vertical series of CL mappings parallel to the c-plane. A perfect agreement between the local luminescence properties and the evolution of the local free carrier concentration during the different stages of overgrowth is confirmed by micro-Raman measurements. While mask-periodic domains with specific optical and electronic properties are observed for initial growth, homogeneously high crystal quality at the sample surface is evidenced.

High quality GaN layers grown on slightly miscut sapphire wafers

HVPE grown layers typically show a high density of pyramidal structures on the surface. We found that a slight off-orientation of the substrate totally suppresses the development of these structures. Further we found that a misorientation towards the M-plane of GaN features a smoother surface morphology as compared to an off-orientation towards the A-plane. After the improvement of the surface morphology and other properties of the HVPE grown layers, we studied self-separation processes. Our approaches to remove the thick GaN layer from the substrate were the introduction of a low-temperature intermediate layer and a structured dielectric mask.

Absorption in InGaN-on-sapphire LED structures: comparison between photocurrent measurement method (PMM) and photothermal deflection spectroscopy (PDS)

In this work, we investigate the absorption distribution in InGaN-on-sapphire based light-emitting diodes (LEDs). We observed by photothermal deflection spectroscopy (PDS) and transmission measurements that most of the absorption takes place in a thin layer close to the sapphire substrate. The lateral intensity distribution in the surrounding of LED emitters is determined by the photocurrent measurement method. Based on the observations by PDS and transmission, a model for the lateral light propagation in the LED-wafer containing also a thin, strong absorbing layer is presented. It is shown that interference of the mode profiles with the absorbing layer leads to different modal absorption which explains the non-exponential intensity distribution. We are able to estimate the optical thickness of the absorbing layer to be 75 nm. Furthermore, this layer can be identified as one of the major loss mechanism in InGaN-LEDs grown on sapphire substrate due to the large absorption coefficient which is effective at the emission wavelength.

Electroluminescence from GalnN Quantum Wells Grown on Non-(0001) Facets of Selectively Grown GaN Stripes

Non (0001) GalnN QWs have been grown by low pressure MOVPE on side facets of triangular shaped selectively grown GaN stripes. By analysing low temperature photo- and cathodoluminescence and room temperature electroluminescence, we found strong indications, that both, In and Mg are less efficiently incorporated on these side facets compared to the common (0001) plane with even lower efficiency for stripes running along (1–100) compared to (11–20). Nevertheless, we observed strong light emission from these quantum wells, supposed to be at least partly caused by the reduced piezo-electric field.