We demonstrate the application of low-temperature cathodoluminescence (CL) with high lateral, depth, and spectral resolution to determine both the lateral (i.e., perpendicular to the incident primary electron beam) and axial (i.e., parallel to the electron beam) diffusion length of excitons in semiconductor materials. The lateral diffusion length in GaN is investigated by the decrease of the GaN-related luminescence signal when approaching an interface to Ga(In)N based quantum well stripes. The axial diffusion length in GaN is evaluated from a comparison of the results of depth-resolved CL spectroscopy (DRCLS) measurements with predictions from Monte Carlo simulations on the size and shape of the excitation volume. The lateral diffusion length was found to be (95 ± 40) nm for nominally undoped GaN, and the axial exciton diffusion length was determined to be (150 ± 25) nm. The application of the DRCLS method is also presented on a semipolar (112¯2) sample, resulting in a value of (70 ± 10) nm in p-type GaN.
Herein, the optimization of (In)GaN heterostructures for chemical sensing is presented. The metalorganic vapor phase epitaxy (MOVPE)‐grown sensor consists of an InxGa1−xN quantum well (QW) placed close to the surface of a GaN substrate with a thin GaN cap layer on top. The photoluminescence (PL) wavelength of this QW is sensitive to surface potential changes and thus its optical signal is used as sensor response. Simulations are performed with nextnano to improve its sensitivity. Sensor parameters such as the cap layer thickness d, QW thickness Lz, background buffer layer doping concentration N, and indium concentration x of the QW are varied. It is found that a thin cap layer, together with high background doping and medium QW thickness, is ideal. The indium content does not show a strong influence on sensitivity. The trends found in the simulations are mostly confirmed in real‐world experiments performed in a chemical sensing setup, yet quantitative deviations exist.
GaN based laser diodes with semipolar quantum wells are typically grown on free‐standing pseudo‐substrates of small size. We present an approach to create a distributed‐feedback (DFB) laser with semipolar quantum wells (QWs) on c‐oriented templates. The templates are based on 2‐inch sapphire wafers, the method could easily be adapted to larger diameters which are available commercially. GaN nanostripes with triangular cross‐section are grown by selective area epitaxy (SAE) and QWs are grown on their semipolar side facets. The nanostripes are completely embedded and can be sandwiched inside a waveguide. For optical pumping, open waveguide structures with only a bottom cladding are used. Using nanoimprint lithography, stripe masks with 250 nm periodicity were fabricated over the whole wafer area. The periodicity corresponds to a 3rd order DFB structure for a laser emitting in the blue wavelength regime. These samples were analyzed structurally by high‐resolution transmission electron microscopy (HRTEM), and spatio‐spectrally by cathodoluminescence (CL) inside a scanning transmission electron microscope (STEM). Samples with an undoped cap are pumped optically for stimulated emission. To prove the feasibility of realizing a 2nd order DFB structure with this approach, stripes with a 170 nm periodicity are fabricated by electron beam lithography and SAE.
In this work we investigate the epitaxial growth of boron containing (Al,Ga)N layers and superlattices for applications in the active region of UV‐LEDs. For AlBN layers containing 5% of boron as quantified by secondary ion mass spectrometry, columnar growth has been observed. Transmission electron microscopy (TEM) studies revealed B‐lean, crystalline columns, separated by highly boron containing amorphous intermediate regions. However, in the first few nanometers of AlBN growth, no evidence of columnar growth can be found. Indeed, first layers of AlBGaN/AlN superlattice stacks do not show any sign of columnar growth. However, for increasing thickness, 3D‐like growth develops through formation of pyramidal structures. TEM‐based selective area electron diffraction analysis does not show evidence of lattice tilting, proving that the observed inclined facets are not a result of lattice twinning. Thin AlBGaN layers with different thickness containing 1% boron show similar photoluminescence (PL) spectra with slightly reduced intensity as compared to similar B‐free AlGaN layers. Although small grainy structures develop and increase in density for AlBGaN layers with a thickness of 10 nm and more, the PL intensity increases steadily with layer thickness, showing that these grains do not drastically deteriorate their luminescence properties.
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