AlN layers annealed at high temperatures offer low threading dislocation densities of mid 108 cm−2 and are therefore increasingly used as base layers in ultraviolet (UV) light emitting diode (LED) heterostructure growth. These LEDs, just like those grown on conventional metalorganic vapor phase epitaxy (MOVPE) AlN templates, often suffer from long-wavelength parasitic luminescence. In this work, luminescence properties of far-UVC LED heterostructures grown on MOVPE-AlN/sapphire templates and high-temperature annealed AlN/sapphire templates are compared. To investigate the origin of parasitic emission with high spatial resolution, cross section scanning transmission electron microscopy was combined with cathodoluminescence measurements. As a result, the main origin of the parasitic luminescence band centered at 3.5 eV (354 nm) for the heterostructure grown on annealed AlN is assigned to point defects related to oxygen in the AlN template layer. The defect band centered at 3.0 eV (413 nm) for the heterostructure grown on MOVPE-AlN was found to be related to self-compensating VAl-Si point defect complexes in the n-AlGaN layer and oxygen incorporation close to the AlN/sapphire interface. The results also suggest that the type of AlN template determines the kind of parasitic luminescence from the n-AlGaN layer.

Herein, integrating GaN quantum dots (QDs) within a resonant cavity is focused on. Utilizing metal-organic vapor phase epitaxy, controlled growth of GaN QDs on AlN is achieved. A deep-UV distributed Bragg reflector (DBR) with high reflectivity in the 250-300 nm range, using AlN and Al0.7Ga0.3N layers to maximize refractive index contrast, is developed. A 50-period DBR achieves 98% reflectivity at a wavelength of 272 nm. Scanning transmission electron microscopy and electron energy loss spectroscopy analyses reveal a trisection of DBR periods, attributed to a Ga composition pulling effect during growth. The real structure's reflectivity is simulated and matched well with measured data, though actual reflectivity is lower than the ideal. Cathodoluminescence studies at T = 17 K show emission peaks from both the DBR and the GaN QDs. Further, single-photon emission is demonstrated with a g(2)(t = 0) value of 0.41 at 272.7 nm, confirming the potential for deep-UV single-photon sources. Additionally, the creation of a planar resonant cavity with enhanced emission intensity and vertical nanopillar structures with an aspect ratio of 11 and a diameter of 400 nm confirm the successful integration of GaN QDs in advanced UV photonic structures.

Using nano-cathodoluminescence performed in scanning transmission electron microscope (STEM-CL), we have investigated a photonic-bandgap-crystal (PBC) laser structure at T = 17 K. In cross-sectional STEM images the full device structure is clearly resolved. The most dominant luminescence originates from the 3-fold MQW of the active region. The MQW shows a distinct peak wavelength change in growth direction indicating different structural and/or chemical properties of the individual quantum wells. In detail, a clear shift from 427 nm to 438 nm from the first to the top QW is observed, respectively.

AlN films, a-plane and m-plane oriented, were obtained by molecular beam epitaxy overgrowth on GaN isolated nanopillars following a three-step process involving: dry etching of a GaN buffer layer to obtain nanopillars; GaN overgrowth on nanopillars and finally AlN overgrowth until nanocrystals coalescence. The resulting a-plane AlN layer shows a roughness, with a RMS of 90 nm over a 5 x 5 mu m2, higher than the m-plane AlN one with a RMS of 35 nm over the same area, which shows much better morphological quality. Plan-view transmission electron microscopy images of the m-plane AlN layer reveal stacking faults, but threading dislocations are barely seen. Cross-sectional transmission electron microscopy image shows stacking faults in bunches running through the sample volume, separated by areas (50 to 100 nm wide) which seem free of them.

We present a nanoscopic investigation of the carrier transport into individual single InP quantum dots (QDs) of a membrane external-cavity surface-emitting laser structure (MECSEL) by means of highly spatially resolved cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope (STEM-CL). The lateral STEM-CL spectrum linescans across a single InP QD exhibit a characteristic change of excitonic transitions during this linescan. This gives direct access to the QD population by the generated excess carriers and the renormalization of the QD ground state while the electron beam approaches and subsequently recedes the QD position.

We report on the formation process of GaN/AlN quantum dots (QDs) which arises after the deposition of 1–2 monolayers of GaN on an AlN/sapphire template followed by a distinct growth interruption (GRI). The influence of the duration of a GRI on the structural and optical properties of the GaN layer has been systematically investigated. QDs develop from initially bulky GaN islands, which nucleate in close vicinity to bundles of threading dislocations (TDs). For prolonged GRIs, a decreasing island size is observed which is consistent with a systematic blue shift of the emission wavelength. In addition, fragmentation of the bulky GaN islands into several smaller islands occurs, strongly dependent on local strain fields caused by TDs as well as on the different facet orientation of the islands. This morphological transition during GRI eventually leads to GaN QD formation, which assemble as clusters with a density of 10 8 cm −2 . Desorption of GaN is identified as the major source for this morphological transition. The GRI time allows for tuning of the QD emission wavelength in the ultraviolet spectral range.

Selective area epitaxial growth is an important technique, both for monolithic device integration as well as for defect reduction in heteroepitaxy of crystalline materials on foreign substrates. While surface engineering with masking materials or by surface structuring is an effective means for controlling the location of material growth, as well as for improving crystalline properties of epitaxial layers, the commonly involved integral substrate heating presents a limitation, e.g., due to constraints ofr the thermal budget applicable to existing device structures. As a solution, an epitaxial growth approach using a laser source only locally heating the selected growth area, in combination with metal-organic precursors to feed a pyrolithic chemical reaction (also known as metal-organic vapor phase epitaxy, MOVPE), is presented. Without masking or surface structuring, local epitaxial growth of III-V compound semiconductor layers on a 50-1500 µm length-scale, with high structural and optical quality, is demonstrated. We discuss general design rules for reactor chamber, laser heating, temperature measurement, sample manipulation, gas mixing, and distinguish laser-assisted local MOVPE from conventional planar growth for the important compound semiconductor GaAs. Surface de-oxidation prior to growth is mandatory to realize smooth island surfaces. Linear growth rates in the range 0.5-9 µm/h are demonstrated. With increasing island diameter, the probability for plastic deformation within the island increases, depending on reactor pressure. A step-flow mode on the island surface can be achieved by establishing a sufficiently small temperature gradient across the island.

We report on nanoscopic exploration of the luminescence from individual InP quantum dots (QDs) by means of highly spatially resolved cathodoluminescence (CL) spectroscopy directly performed in a scanning transmission electron microscope (STEM). A 7-fold layer stack with high-density InP quantum dots is embedded as an active medium membrane in an external-cavity surface-emitting laser. We characterize the vertical transfer of carriers within the periodic separate confinement heterostructure and determine the capture efficiency of carriers from the cladding layer into the quantum dot layers. Benefiting from the nanoscale resolution of our STEM-CL, we perform single-dot spectroscopy on single isolated QDs in the STEM lamella resolving the details of the excitonic structure of individual quantum dots. Executing highly spatially resolved spectrum line scans within the QD layers, we directly visualize the lateral transport, i.e., the efficient lateral capture of carriers into an individual QD. We observe a characteristic change of the spectral fingerprint during this line scan, while the electron beam is approaching and subsequently receding from the quantum dot position. This directly correlates to the increase and decrease of the numbers of excess carriers reaching the dot, i.e., altering the quantum dot population. The characteristic shift of emission energies visualize the renormalization of the ground-state energy of the single dot, and the intensity ratio of the excitonic recombinations verifies this change of the occupation and the state-filling.

Blue and yellow emission bands in carbon-doped GaN grown by MBE were investigated in low-temperature cathodoluminescence measurements performed in a scanning transmission electron microscope (STEM-CL) with high spatial resolution. Blue luminescence at 2.85 eV and two contributions in the spectral range of the yellow emission band around 2.2 eV separated by 120 meV are observed in carbon-doped material, whereas only one distinctive yellow luminescence contribution was found in unintentionally-doped GaN.

Freshly introduced a-screw dislocations in gallium nitride are an effective source of ultraviolet radiation, characterized by intense emission of narrow luminescence doublet lines in the spectral range of 3.1-3.2 eV. Furthermore, an additional narrow spectral line with an energy of 3.3 eV has been found at the points of intersection of such dislocations, where extended dislocation nodes were formed. In this communication, we report on the spectral properties of the characteristic luminescence of such nodes, which were obtained for the (0001) gallium nitride samples with dislocations introduced by nanoindentation. The spectral position of the dislocation-related luminescence doublet experiences a redshift with increasing distance from the indentation site. It follows the spectral shift of the excitonic near-bandgap emission, associated with stress relaxation. The luminescence of the intersection points exhibits a similar tendency. At certain local positions, its doublet fine structure is observed, which has a spectral linewidth of the order of or even less than that of the exciton. In this case, the spectral splitting between components of the doublet varies irregularly depending on the position of the exciton (i.e., on the mechanical stress). We see a clear indication of quantum dot-like emission. The fine structure of the luminescence of the intersection points can be easily explained by the energy dependence of emission on their size, as well as on their density, in particular, by the formation of paired nodes, which were previously observed in experiments in a transmission electron microscope.

•Epitaxial growth of AlN on Si (111) by pulsed reactive sputtering.•Change of N-precursor change growth mode from three-dimensionel to two-dimensional.•Change in growth mode approved by scanning transmission electron microscopy images.•Increase of ad-atom mobililty by use of NH3.•Smooth surface morphology for growth with NH3 with low roughness as 0.14 nm.

We present a pulsed reactive magnetron sputter process for high quality AlN on Si (111) beneficially avoiding any high-temperature growth. Initially, metallic aluminium with a nominal thickness of about one monolayer is deposited at a substrate temperature around 850 degrees C in an Ar plasma followed by sputtering in an Ar/N plasma. For 250 nm thick AlN layers a surface roughness below 0.2 nm rms is obtained as determined by atomic force microscopy (AFM). Using an Al nucleation step prior to AlN growth substantially improves the crystalline properties of AlN. The FWHM values of the AlN (0 0 0 2) and the AlN (1 0 1 0) diffraction peaks with 0.45 degrees and 0.86 degrees, respectively, are comparable to state-of-the-art AlN on Si layers grown by metalorganic vapour phase epitaxy (MOVPE). Two different N-precursor gases, namely N2 and NH3, lead to distinct layer qualities as revealed by atomic force microscopy and transmission electron microscopy. Only with NH3 substantial lateral growth can be achieved at T = 850 degrees C which is mandatory to obtain smooth surface morphologies. In MOVPE such lateral AlN growth is typically only achieved at high growth temperatures (T > 1000 degrees C).

Color-tunable InGaN/GaN multi-quantum-well (MQW) light-emitting diodes (LEDs) are reported based on GaN microfacet structure directly grown on c-plane patterned sapphire substrate by metal organic vapor phase epitaxy (MOVPE) through promoting 3D growth. By adjusting GaN growth temperature and pattern arrangement, a GaN microfacet with almost pure {101 over bar 1} semipolar facets is obtained. The multifacetted InGaN/GaN MQW LED chip evolves three distinct emission peaks around 630, 530, and 450 nm in electroluminescence (EL) as injection current increases from 1 to 100 mA. The EL behavior originates from locally different facets of the complex 3D structure: MQWs grown on c-planes and semipolar facets, respectively, which is confirmed by cathodoluminescence characterization in a scanning transmission electron microscope (STEM-CL). Considering the dependence of emission wavelength and intensity on injection currents, a programmable power supply is designed to drive the LED. The specific color of the LED is tuned by time-shared driving of the currents based on three channels with controllable magnitudes and duty cycle from the power supply, covering red, yellow, green, cyan, blue, and purple. Furthermore, white LEDs with high color rendering index (CRI) up to 96.1 and correlated color temperature (CCT) between 4000 and 10 000 K are achieved.

Advanced characterization methods with nanoscale resolution are powerful tools in order to overcome the continuing challenges in the optimization of nitride semiconductor nanostructures for more efficient nanophotonic devices in the UV and green spectral range. This chapter is devoted to the study of optical, electronic, and structural properties of these nitride based nanostructures. In the first part, we discuss several state-of-the-art nanoscale characterization techniques including scanning transmission electron microscopy cathodoluminescence (STEM-CL), tip-enhanced Raman spectroscopy (TERS), micro-photoluminescence (µPL), X-ray diffraction (XRD), and scanning tunneling microscopy and spectroscopy (STM/STS). This selection of complementary microscopic and spectroscopic techniques provides unique insights into a multitude of nanostructure properties such as charge carrier excitation, relaxation, diffusion, and recombination dynamics, vibrational and structural properties including strain, segregation, as well as clustering, and surface and interface morphology. In the second part, we apply and combine these techniques to obtain detailed information on nanoscale properties of nitride based micro-columns, quantum wires, and heterostructures. The study of these nitride nanostructures provides not only insight into device limitations, but also contributes to the fundamental understanding of structural and optical properties of III-nitride nanostructures.

AlN/GaN/AlN quantum disks (Q-disks) embedded in self-assembled hexagonally shaped GaN nanowires have been grown on the Si (111) substrate by plasma-assisted molecular beam epitaxy. To correlate the structural and optical properties of individual Q-disks inside the nanowire, highly spatially resolved cathodoluminescence (CL) spectroscopy in a scanning transmission electron microscope has been performed at 18 K. CL spectrum linescans along a single nanowire clearly identify the emission from the GaN base around 354 nm as well as two recombination channels at 347 nm and 330 nm directly correlated with the AlN/GaN/AlN Q-disk. A detailed characterization of these individual quantum objects is presented.

Microcavities with InGaN quantum wells or GaN-based quantum dots as active medium are building blocks of electrically-driven, low-threshold surface-emitting lasers or single photon emitters in the visible-to-UV spectral range. In this chapter, we highlight essential developments in epitaxial growth techniques of such nitride-based microcavities and their active regions. Modern analytical techniques for structural and optical characterization of these complex heterostructures as presented in this chapter are essential to solve remaining challenges.

Investigating closely stacked GaN/AlN multiple quantum wells (MQWs) by means of cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope, we have reached an ultimate spatial resolution of σ CL = 1.8 nm . The pseudomorphically grown MQWs with high interface quality emit in the deep ultraviolet spectral range. Demonstrating the capability of resolving the 10.8 nm separated, ultra-thin quantum wells, a cathodoluminescence profile was taken across individual ones. Applying a diffusion model of excitons generated by a Gaussian-broadened electron probe, the spatial resolution of cathodoluminescence down to the free exciton Bohr radius scale has been determined.

The effect of high temperature annealing (HTA) on crystalline quality improvement of h-BN films grown on sapphire substrates has been investigated. It is found that BN grown using conventional molecular beam epitaxy is disordered due to the growth temperature below 1000 °C. By annealing at a temperature of 1700 °C, thermodynamically stable crystalline h-BN could be obtained at wafer scale, where diffusion of atoms in the as-grown BN film is enhanced and the structural defect density decreases. The crystalline h-BN has been confirmed by x-ray diffraction, Raman scattering, and atomic force microscopy measurements. This work demonstrates that HTA is a simple and effective way to achieve wafer-scale crystalline h-BN films, which have numerous potential applications in next-generation two-dimensional devices and flexible III-nitride optoelectronic devices.