Barbara Neubert

Bright semipolar GaInN∕GaN blue light emitting diode on side facets of selectively grown GaN stripes

The authors demonstrate the fabrication and evaluation of bright semipolar GaInN∕GaN blue light emitting diodes (LEDs). The structures are realized by growing five GaInN∕GaN quantum wells on the {11¯01} side facets of selectively grown n-GaN stripes with triangular shape running along the ⟨112¯0⟩ direction covered with a Mg-doped GaN top layer. The growth was done by metal organic vapor phase epitaxy using a conventional [0001] sapphire substrate. The devices have circular mesa structures with diameters between 70 and 140μm. Continuous wave on-wafer optical output powers as high as 700μW and 3mW could be achieved under dc conditions for 20 and 110mA, respectively. The current dependent blueshift of the peak emission wavelength caused by screening effects of the piezoelectric field was only 1.5nm for currents between 1 and 50mA. This is less than half the value measured on c-plane LEDs and confirms the reduced piezoelectric field in our LED structures.

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.

Piezoelectric fields in GaInN∕GaN quantum wells on different crystal facets

Direction and strength of piezoelectric built-in fields of GaInN quantum wells have been experimentally determined. The quantum wells have been grown either on the conventional {0001} crystal plane of GaN or on {11¯01} facets of selectively grown GaN stripes. The emission peak position of the electric-field-dependent photoluminescence can be modeled and yields value and sign of the piezoelectric field dependent on the strain of the quantum wells. On the semipolar {11¯01} facets, the quantum wells show a much weaker field (−0.1MV∕cm) compared to quantum wells grown on polar {0001} planes (−1.9MV∕cm), consistent with theoretic predictions.

Microbeam high angular resolution x-ray diffraction in InGaN∕GaN selective-area-grown ridge structures

GaN-based 6-μm-wide ridge waveguides with InGaN∕GaN multiple-quantum-wells (MQWs) produced by metal organic vapor-phase epitaxy in the regime of selective-area growth have been studied with microbeam high angular resolution x-ray diffraction and reciprocal-space mapping. Variation of the strain from 0.9% to 1.05% and a factor of 3 for the thickness enhancement of the MQW period have been measured for different widths of the oxide mask surrounding the GaN-based ridges. Only when the trapezoidal shape of the ridge cross section is taken into account can the difference between the experimentally measured thickness enhancement and predictions of the long-range gas-phase diffusion model be reconciled.

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.

High-efficiency red and infrared light-emitting diodes using radial outcoupling taper

In this paper, we give an overview of light-emitting diodes (LEDs) with radial tapers. Light is generated in the very center of a circularly symmetrical structure and is outcoupled at a tapered ring. Encapsulated devices with an emission wavelength of 980 nm achieve wallplug efficiencies of 48%. Non-encapsulated InGaAlP-based red-emitting LEDs show quantum efficiencies of 13%. A new device design combines the taper with a wafer-scale soldering technique promising a feasible fabrication method.

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.

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.

Efficient InAlGaP light-emitting diodes using radial outcoupling taper

We present results on efficient InGaAlP light-emitting diodes using lateral outcoupling taper. This concept is based on light generation in the very central area of a circularly symmetric structure and, after light propagation between two mirrors, outcoupling in a tapered mesa region. We have demonstrated the suitability of this concept on As-based Light-Emitting Diodes emitting at 980 nm. Since the idea is not limited to a certain material system, we fabricated InGaAlP-based LEDs emitting in the red wavelength regime. By adjusting the process flow to the new material system we were able to achieve external quantum efficiencies in the range of 13% for unencapsulated devices. Additionally we present a new concept combining the idea of outcoupling tapers with a waferscale soldering technique. First samples show external quantum efficiencies in the range of 11%.