Plan-view Cathodoluminescence Research Articles

Combined SEM-CL and STEM investigation of green InGaN quantum wells

The microstructure of green-emitting InGaN/GaN quantum well (QW) samples grown at different temperatures was studied using cross-section scanning transmission electron microscopy (STEM) and plan-view cathodoluminescence (CL). The sample with the lowest InGaN growth temperature exhibits microscale variations in the CL intensity across the sample surface. Using STEM analysis of such areas, the observed darker patches do not correspond to any observable extended defect. Instead, they are related to changes in the extent of gross-well width fluctuations in the QWs, with more brightly emitting regions exhibiting a high density of such fluctuations, whilst dimmer regions were seen to have InGaN QWs with a more uniform thickness.

Spatial inhomogeneities in AlxGa1−xN quantum wells induced by the surface morphology of AlN/sapphire templates

The effects of the template on the optical and structural properties of Al0.75Ga0.25N/Al0.8Ga0.2N multiple quantum well (MQWs) laser active regions have been investigated. The laser structures for optical pumping were grown on planar c-plane AlN/sapphire as well as on thick epitaxially laterally overgrown (ELO) AlN layers on patterned AlN/sapphire. Two ELO AlN/sapphire templates were investigated, one with a miscut of the sapphire surface to the m-direction with an angle of 0.25°, the other with a miscut angle of 0.25° to the sapphire a-direction. The MQWs are studied by atomic force microscopy, plan-view cathodoluminescence (CL) at room temperature and 83 K as well as transmission electron microscopy using high-angle annular dark-field imaging and energy-dispersive x-ray spectroscopy. The results are compared to optical pumping measurements. It was found that the surface morphology of the templates determines the lateral wavelength distribution in the MQWs observed by spectral CL mappings. The lateral wavelength spread is largest for the laser structures grown on ELO AlN with miscut to sapphire a-direction caused by the local variation of the MQW thicknesses and the Ga incorporation at macrosteps on the ELO-AlN. A CL peak wavelength spread of up to 7 nm has been found. The MQWs grown on planar AlN/sapphire templates show a homogeneous wavelength distribution. However, due to the high threading dislocation density and the resulting strong nonradiative recombination, laser operation could not be achieved. The laser structures grown on ELO AlN/sapphire show optically pumped lasing with a record short wavelength of 237 nm.

Reduction of threading dislocations in N-polar GaN using a pseudomorphicaly grown graded-Al-fraction AlGaN interlayer

We report on an improvement for the crystal quality of N-polar GaN with a pseudomorphicaly grown graded-Al-fraction AlGaN interlayer which introduces strain management in heterostructure and brings in the inclination and annihilation of threading dislocations (TDs). Significant blocking of screw and edge component TDs are observed by transmission electron microscopy, and the reduction of TDs densities are estimated by high resolution X-ray diffraction and plan-view cathodoluminescence measurement. Photoluminescence measurement shows a good optical property associated with the significant reduction of edge TDs. All the results show that insertion of graded AlGaN interlayer is a convenient method to achieve excellent crystal quality in N-polar GaN growth.

Effects of Nano- and Microscale SiO2 Masks on the Growth of a-Plane GaN Layers on r-Plane Sapphire

We report on the combined effects of a-plane GaN layers on a nanoscale patterned insulator on an r-plane sapphire substrate and epitaxial lateral overgrowth (ELOG) techniques. The fully coalescent a-plane GaN layer using nano- and microscale SiO2 masks showed the formation of nano- and microscale voids on the masks, respectively. Atomic force microscopy (AFM) measurements revealed a surface roughness of 0.63 nm and a submicron pit density of ∼7.8 ×107 cm-2. Photoluminescence (PL) intensity was enhanced by a factor of 9.0 in comparison with that of a planar sample. Omega full-width at half-maximum (FWHM) values of the (112̄0) X-ray rocking curve along the c- and m-axes were 553 and 788 arcsec, respectively. A plan-view cathodoluminescence (CL) mapping image showed high luminescence intensity on the SiO2 masks.

Cross-sectional and plan-view cathodoluminescence of GaN partially coalesced above a nanocolumn array

The optical properties of GaN layers coalesced above an array of nanocolumns have important consequences for advanced optoelectronic devices. GaN nanocolumns coalesced using a nanoscale epitaxial overgrowth technique have been investigated by high resolution cathodoluminescence (CL) hyperspectral imaging. Plan-view microscopy reveals partially coalesced GaN layers with a sub-μm scale domain structure and distinct grain boundaries, which is mapped using CL spectroscopy showing high strain at the grain boundaries. Cross-sectional areas spanning the partially coalesced GaN and underlying nanocolumns are mapped using CL, revealing that the GaN bandedge peak shifts by about 25 meV across the partially coalesced layer of ∼2 μm thick. The GaN above the nanocolumns remains under tensile strain, probably due to Si out-diffusion from the mask or substrate. The cross-sectional data show how this strain is reduced towards the surface of the partially coalesced layer, possibly due to misalignment between adjacent partially coalesced regions.

Inclined dislocation arrays in AlGaN/AlGaN quantum well structures emitting at 290 nm

We report on the structural and optical properties of deep ultraviolet emitting AlGaN/AlGaN multiple quantum wells (MQWs) grown on (0001) sapphire by metal-organic vapor phase epitaxy using two different buffer layer structures, one containing a thin (1 μm) AlN layer combined with a GaN interlayer and the other a thick (4 μm) AlN layer. Transmission electron microscopy analysis of both structures showed inclined arrays of dislocations running through the AlGaN layers at an angle of ∼30°, originating at bunched steps at the AlN surface and terminating at bunched steps at the surface of the MQW structure. In all layers, these inclined dislocation arrays are surrounded by AlGaN with a relatively higher Ga content, consistent with plan-view cathodoluminescence maps in which the bunched surface steps are associated with longer emission wavelengths. The structure with the 4 μm-thick AlN buffer layer had a dislocation density lower by a factor of 2 (at (1.7±0.1)×109 cm−2) compared to the structure with the 1 μm thick AlN buffer layer, despite the presence of the inclined dislocation arrays.

Cathodoluminescence Study on Spatial Luminescence Properties of InN/GaN Multiple Quantum Wells Consisting of 1-Monolayer-Thick InN Wells/GaN Matrix

Spatially resolved luminescence properties of InN/GaN multiple quantum wells (MQWs) consisting of nominally one monolayer (1-ML)-thick InN QWs embedded in a GaN matrix are studied by cross-sectional and plan-view cathodoluminescence measurements. First it is confirmed that the dominant emission peaks observed at around 390 nm to 430 nm in the MQWs samples are attributed to the effects of inserting ∼1-ML-thick InN wells in the GaN matrix, resulting in efficient localization of GaN excitons at InN QWs. Furthermore, it is revealed that the detailed structure of the MQWs, such as the thickness distribution and interface sharpness, is very sensitive to the presence of surface defects such as hillocks around screw-component threading dislocations, resulting in different emission wavelengths/energies. This is because the epitaxy process for depositing such thin InN wells is seriously affected by the atomic-level surface structures/properties of the growth front. It will be concluded that it is necessary to use lower dislocation density GaN bulk templates to obtain much higher structural quality InN/GaN MQWs good enough for characterizing their optical properties.

Defect characterization of Si-doped GaN films by a scanning near-field optical microscope-induced photoluminescence

We developed a tapping-mode-scanning near-field optical microscope to measure near-field photoluminescence (SNOM-PL) with nanometer spatial resolution using an ultraviolet laser, and we measured the defect distribution of a Si-doped GaN film. The obtained result was compared with one measured by high spatial resolution cathodoluminescence (CL) spectroscopy. Some dark spots with an average period of 100–300nm were observed in the image of the peak intensity of near-field PL at about 362nm, measured with a pyramidical cantilever, and in a plan-view CL image at the same wavelength. The near-field PL image agreed well with the plan-view CL image. The threading dislocations in GaN films were found to act as nonradiative recombination centers not only for band-edge emission but also for yellow luminescence around 580nm. Furthermore, it was found that free carrier concentration decreased near the threading dislocations. The SNOM-PL we developed has at least a spatial resolution of about 100nm. The SNOM-PL could potentially be applied to the estimation at a nanometer scale of defects in semiconductor films without requiring a vacuum environment.

Band-to-band and sub-band gap cathodoluminescence from GaAsP/GaInP epistructures grown on GaAs substrates

Lattice-matched GaAs x P 1− x /GaInP double heterostructures (DHs) are grown on GaAs substrates by step grading the intervening lattice mismatch when x < 1 . Cathodoluminescence (CL) measurements on these structures reveal a broad peak approximately 0.15 eV below the band-to-band (BB) emission. While the intensity of the BB peak is strong at room temperature, the sub-band-gap (SBG) feature quenches rapidly between 77 and 300 K. Beam energy-dependent and cross-sectional CL demonstrate that the SBG emission originates in the active layer near the lower interface of the DH. At 77 K, spectrally resolved plan-view CL images show little spatial correlation between the BB and SBG peaks, but when the SBG peak is observed at room temperature, the BB and SBG emission are highly correlated. The temperature-dependent spatial correlation is attributed to differences in thermal quenching of the radiative SBG recombination mechanism.

Optical Micro-Characterization of Complex GaN Structures

Various techniques of pattern-controlled epitaxy like epitaxial lateral overgrowth (ELO) and lateral seeding epitaxy have successfully been used to overcome the lattice mismatch problem, however, resulting in a complex system of self-organized growth domains. For a detailed understanding a correlation of the structural, electronic, and optical properties on a micro-scale is mandatory. Scanning cathodoluminescence (CL) microscopy provides a powerful tool compiling low temperatures, spatial resolution Δx < 45 nm and high spectral resolution. ELO GaN on stripe patterned SiO 2 and W masks were characterized by CL microscopy, directly visualizing the formation of different growth domains: 1. the coherently grown regions between the stripes; 2. the ELO areas coalescing on the center of the masks and showing a blue shifted and strongly broadened CL where μ-Raman evidences the reduction of local strain and probes the free carrier concentration which is dramatically changed over the masks; 3. the coalescence regions with poor epitaxial perfection including voids and strong incorporation of impurities; 4. the transition regions between 1. and 2. at the very edges of the masks characterized by high defect density and strongly blue shifted CL. Plan view CL directly images the local areas of improved crystal perfection only involving part of the total ELO regions. The formation of specific micro-domains in 5 μm thick crack-free Al 0.19 Ga 0.81 N, exhibiting different Al concentrations, during the initial stage of MOVPE grown on patterned GaN substrates and the transition to finally homogeneous growth is directly visualized by cross-sectional CL.

Distribution of Cathodoluminescence in Triple Quantum Well of ZnCdSe/ZnSe

The spatial distributions of cathodoluminescence (CL) in a triple quantum well (TQW) of ZnCdSe/ZnSe grown by metalorganic vapor phase epitaxy (MOVPE) method on GaAs(001) have been examined using a low-temperature CL measurement system combined with a transmission electron microscope (TEM). The CL intensity distribution appearing in the CL images of cross-sectional samples of the TQW can be attributed to the well thickness fluctuation in each QW layer and to competition among channels of recombination such as radiative recombination due to impurities and non-radiative recombination at defects. Moreover, plan-view CL images of the TQW samples show the distribution of the misfit dislocations at the interfaces of each QW layer individually, and reveal the thickness fluctuation in the individual QW layers.