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We demonstrated low-resistive GaInN-based tunnel junctions with high In mole fractions (over 0.35) grown by metalorganic vapor phase epitaxy (MOVPE). A 2 nm heavily Mg-doped Ga0.6In0.4N/15 nm heavily Si-doped GaN tunnel junction was grown on a standard-sized (310 × 310 µm2) LED. The tunnel junction showed 0.06 V drop at 20 mA (26 A/cm2), corresponding to 4 × 10−3 Ωcm2 as the resistivity. This result indicates that MOVPE-grown tunnel junctions showed an almost comparable resistivity as conventional p-contact. We also found that the tunnel junction resistivity is decreased to 4 × 10−4 Ω cm2 with an increase of current density up to 5 kA/cm2. A tunneling through midgap states was suggested from forward-biased I–V characteristics of transmission line model test structures with the tunnel junctions.

A newly developed moth-eye patterned sapphire substrate (MPSS) technology increased light extraction efficiency (LEE) in the vertical direction in nitride-based blue light emitting diodes (LEDs). MPSS is characterized by a submicron-scale periodical structure that exerts the Bragg diffraction effect at the interface between gallium nitride (GaN) and sapphire. Through simulations and experiments, it was revealed that emphasis of the diffraction effect by optimizing the MPSS structure enhanced vertical emission from LEDs. As a result, resin-encapsulated, large-sized LEDs on MPSS with a large diffraction effect achieved a high LEE of 0.76 that was comparable to that of commercially available thin-film LEDs. Bird's-eye view of scanning electron microscope (SEM) image of MPSS.

We investigated the influence of polarization charges in nitride-based semiconductors. The influence due to polarization charges was calculated excluding the influences of the band offset. We found that the polarization charges (1 × 1013 cm−2) resulted in an energy spike of more than 100 meV at the location of the charges, which is a similar value to the band offset. We then proposed the concept of polarization dilution to suppress the energy spike for better hole transport by using a graded Mg-doped AlGaN layer in UV-LEDs. Device simulation results indicate lower operating voltage and higher injection efficiency by using the polarization dilution. So far, our actual 350 nm LED with the polarization dilution showed lower operating voltage. These results suggested such polarization-charge management is important in the design of the nitride semiconductors.

In this study, we have aimed to improve the light extraction efficiency by using p- and n- high-reflectivity indium tin oxide (ITO)/Al electrodes with a reflectivity of ∼77% at 350 nm, laser lift-off (LLO) method, and silicone sealing. By a combination of these three methods, the light output power and operating voltage of UV LEDs were improved to 23.8 mW and 6.6 V at 100 mA. Furthermore, the external quantum efficiency and light extraction efficiency were improved to 6.6 and 25.3% at 100 mA. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We propose moth-eye patterned sapphire substrate (MPSS) as a solution to improve the light extraction efficiency and to reduce the production cost of LEDs. The MPSS samples' surfaces consisted of a triangular grid of about 375 nm high truncated cones with different pitches of 460 nm, 500 nm, 600 nm, 700 nm and 800 nm. A commercially available patterned sapphire substrate (PSS) and a flat sapphire substrate (FSS) was also used in the experiments for comparison. According to the cathode luminescence observation, the GaN template on MPSS with a thickness of 3 µm showed a threading dislocation density (TDD) of around 1.9 × 108 cm–2. Transmission electron microscope observation revealed that many of the dislocations were bent and disappeared as loops formed in the vicinity of MPSS cones. On the other hand, PSS required a GaN template thicker than 5µm to achieve a level of TDD equal to MPSS. The LED on MPSS with a pitch of 600 nm showed the highest light output power among the evaluated LEDs as it was 1.89 times higher than that on FSS and 1.05 times higher than that on PSS. The pitch dependence of the light output improvement agrees with the pitch dependence of the simulated transmissivity at the GaN/sapphire interface. As a result of our comparison, we concluded that MPSS provides the most cost effective solution for high performance LEDs.

We found out the reduction of contact resistance in V-based electrode for high AlN molar fraction n-type AlGaN by using thin SiNx intermediate layer. The contact resistivity for n -type Al0.70Ga0.30N with the V/Al/Ni/Au electrode using SiNx intermediated layer reached 1.13 × 10-6 Ω cm2. Moreover, contact resistivity using SiNx intermediated layer had been reduced more than one order of magnitude in all AlN molar fractions from 0.62 to 0.87. Using this electrode, we also demonstrated UV light-emitting diodes (LEDs) on n -type Al0.70Ga0.30N underlying layer with an emission wavelength of approximately 283 nm. An operating LED voltage using a V/Al/Ni/Au electrode with SiNx intermediated layer was 3.3 V lower at 100 mA current injection than that without SiNx intermediated layer. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigated dislocation density dependence on stimulated emission characteristics of AlGaN/AlN multiquantum wells (MQWs: emission wavelength of approximately 285 nm) on AlN templates with various dislocation densities. We found that the stimulated emission characteristics of the deep UV MQWs were strongly dependent on the dislocation densities. A reduction of the dislocation densities is very important in order to realize good stimulated emission characteristics of deep UV MQWs. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Strain relaxation in a GaInN/GaN heterostructure is analyzed by combining in situ X-ray diffraction (XRD) monitoring and ex situ observations. Two different characteristic thicknesses of GaInN films are defined by the evolution of in situ XRD from the full width at half-maximum of symmetric (0002) diffraction as a function of GaInN thickness. This in situ XRD measurement enables to clearly observe the critical thicknesses corresponding to strain relaxation in the GaInN/GaN heterostructure caused by the formation of surface pits with bent threading dislocations and the generation of misfit dislocations on GaInN during growth. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have fabricated stacked InAs quantum dots (QDs)/GaAs LED for a wideband near-infrared light source. The 15-stacked InAs QDs/GaAs structure and the indium tin oxide (ITO) electrode were used for the LED. Good current-voltage characteristics were achieved. The built-in voltage was 1.0 V. Gaussian-shape luminescence with a central wavelength of 1144 nm and a full width at half maximum (FWHM) of 99 nm was observed. We achieved a maximum output power of 2.9 mW. These LED properties should be suitable for practical applications. The transmission image of a finger was taken by using the 15-stacked InAs QDs/GaAs LED and the InGaAs camera without special optical settings or the image processing. As a result, the blood vessel under the skin was observed. These results indicate that the 15-stacked InAs QDs/GaAs LED can be equipped with practical measuring instruments (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on laser lift-off (LLO) of AlN/sapphire for UV light-emitting diodes (LEDs). Underfill between chip and submount is a key factor for the successful LLO of AlN/sapphire. We fabricated thin-film-flip-chip UV LEDs with a peak wavelength of 343 nm using the LLO. Moreover surface texturing was carried out on the exfoliated AlN surface. The light output power from the UV LED after the LLO and the surface texture at exfoliated surface is 1.7 times higher than that before the LLO at 20 mA (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The crystalline quality of GaN and Al0.08Ga0.92N epitaxial layers on (100) β-Ga2O3 substrates was significantly improved by the facet-controlled growth method. The facets were controlled by changing the nitrogen ambient thermal annealing temperature. We demonstrated the high-crystalline-quality GaN and Al0.08Ga0.92N on β-Ga2O3 substrates, which were comparable to GaN and AlGaN on sapphire substrates using low-temperature buffer layers. This method is useful for the fabrication of vertical-type ultraviolet (UV) light-emitting diodes (LEDs) on β-Ga2O3 substrates. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

AlGaN/GaN heterostructure field-effect transistors were grown by metalorganic vapor phase epitaxy on Fe-doped GaN substrates with a preflow of the Mg source. We realized a low drain leakage current, 1.05 μA/mm, at V DS = 20 V and V GS = ―5 V with L GD = 3 μm. This leakage current was one order of magnitude lower than that obtained without a preflow of the Mg source. The breakdown voltage was over 250 V with L GD = 10 μm. The on resistance was estimated to be 3.6 mΩ cm 2 . No significant redistribution or memory effect of Mg was observed by secondary ion mass spectroscopy measurement.

We fabricated and characterized AlGaN/GaN heterostructure growth by MOVPE on vicinal m-plane free-standing GaN substrates prepared by the Na flux method. The miscut angle in the LPE-GaN substrate has a great influence on the surface morphology and crystalline quality of epitaxial GaN and AlGaN films. In particular, AlGaN/GaN on a 4° miscut-angle LPE-GaN substrate shows large step bunching, small-miscut-angle LPE GaN substrate is essential for fabricating on FET structure with a flat surface.

GaInN/GaN p-i-n double-heterojunction structures were grown by metal-organic vapor phase epitaxy on c-plane sapphire substrates, and light-emitting solar cells with different GaInN active layer thicknesses were fabricated. The thickness of the GaInN active layer was varied from 100 to 400 nm, while the thickness of both n-type and p-type GaN layers was kept constant. A semitransparent ohmic contact to p-type GaN was formed by electron-beam evaporation of Ni/Au (5 nm/5 nm). The film thickness of p-GaN was 50 nm. The external quantum efficiency (EQE) of the device with a 400 nm GaInN layer exceeded 60% at a wavelength of approximately 380 nm. The transparency of the Ni/Au electrode was 68%. Therefore, the internal quantum efficiency of this device exceeded 95%. Note that this device emitted green light when a forward voltage was applied. The electroluminescence peak wavelength was approximately 525 nm, which was much longer than the EQE peak wavelength. The origin of this large emission shift is discussed. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We demonstrated high-temperature operation of normally off-mode junction-heterostructure-field-effect transistors (JHFETs) with a p-GaN gate that shows a very small shift of the threshold voltage against ambient temperature. Distinct normally off-mode operation with a maximum drain current of 93.2 mA/mm at 300 °C was realized. Therefore, normally off-mode GaN-based JFETs are greatly superior to Si-based devices as high-temperature switching devices. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We demonstrated activation annealing of Mg-doped p-type Al 0.17 Ga 0.83 N in different gases. The hole concentration of Al 0.17 Ga 0.83 N annealed in oxygen is higher than that annealed in nitrogen or air. A hole concentration of 1.3 × 10 16 cm -3 at room temperature was achieved by annealing in oxygen flow at 900 °C. Secondary ion mass spectroscopy shows that hydrogen dissociation from Mg-doped Al 0.17 Ga 0.83 N is found to be enhanced by annealing in a flow of oxygen, compared with annealing in a flow of nitrogen. We confirmed the effect of activation annealing in oxygen flow on the performance of UV light-emitting diode (LED). At a DC current of 100 mA, the output power of the LED annealed in oxygen flow at 900 °C is four times higher than that of the LED annealed in nitrogen flow at 800 °C.

Flip-chip 1×1 mm2 GaInN-based light-emitting diodes (LEDs) with an interdigitated electrode design were investigated. Two types of GaInN-based LED having a 4-line p-electrode with a width of 160 μm and an 8-line p-electrode with a width of 50 μm were fabricated. The 8-line device with a 50-μm-wide p-electrode shows 20% higher output power at a forward current of 350 mA than the 4-line device with a 160-μm-wide p-electrode. The three dimensional simulation of the LED operation was conducted to clarify the origin of the difference in output power. From simulation results, the output power of the device with a 50-μm-wide 8-line p-electrode was estimated to be 10% higher than that of the device with a 160-μm-wide 4-line p-electrode. A large difference in current density between the middle and edge of each 160-μm-wide electrode is suggested. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A major obstacle to achieving high-performance devices using nonpolar a-plane and m-plane GaN is the existence of high-density threading dislocations and stacking faults. Low-defect-density nonpolar plane GaN films were previously grown by sidewall epitaxial overgrowth using metalorganic vapor phase epitaxy [1, 2]. In this study, we control the growth-rate ratio of Ga-polar GaN to N-polar GaN by adjusting the V/III ratio. It is possible to grow GaN only from the N-face sidewall of grooves by maintaining a high V/III ratio, which reduces the number of coalescence regions on grooves and decreases the threading-dislocation density and stacking-fault density. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Bridge layers of AlN and Al0.25Ga0.75N have been formed on linearly grooved substrates of 6H-SiC by high temperature metal organic vapor phase epitaxy. Void assisted dislocation reduction was achieved in the formed bridge layers. Distinct difference in growth and coalescence behaviors of the Al0.25Ga0.75N layers was observed when the layers were grown on the substrates with grooves formed along 〈100〉 and 〈110〉 directions. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The growth of a-plane AlN (a-AlN) on r-plane sapphire substrate and the epitaxial lateral overgrowth (ELO) of a-AlN on patterned a-AlN has been performed by high-temperature metalorganic vapor phase epitaxy. The ELO a-AlN layers coalesced well on the a-AlN template with trenches formed along 〈100〉. A detailed study of the microstructure by transmission electron microscopy revealed that the stacking faults due to facet slip are completely annihilated above the trenches of the patterned AlN. Furthermore, a dislocation density as low as 9.1×107 cm–2 was achieved by ELO. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Crack-free and low-dislocation-density AlGaN was successfully grown on grooved AlN layer. UV light-emitting diodes (LEDs) with a peak wavelength of 330 nm on these AlGaN films were fabricated. The efficiency of the UV LEDs is found to be strongly affected by the crystalline quality of the underlying AlN.

Radiative recombination mechanism in highly modulation doped GaN/AlGaN multiple quantum wells

In this study, the anisotropic strain in a-plane AlGaN on GaN was investigated by X-ray diffraction (XRD) analysis using AlGaN/GaN heterostructure grown on r-plane sapphire. An a-plane GaN layer is compressively strained, particularly in the m-axis direction. According to XRD reciprocal lattice space mapping, the AlGaN layer was strained under tensile stress and grown almost coherently to the underlying GaN layer. The tensile stress in the AlGaN layer in the c-axis direction is approximately 1.7 times larger than that in the m-axis direction.

AlN layers have been grown on 6H-SiC substrates at high temperatures. Single step growth of AlN did not yield high quality layers. A novel two step method involving the growth of AlN layer on already grown and relaxed AlN was found to be effective in getting better quality layers as evidenced by structural and morphological analyses. Microcracks and nano-island surfaces of the first grown AlN acted as naturally patterned trenches and were found to help in achieving high quality AlN layers. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thick AlN layers with atomically flat surfaces were successfully grown directly on sapphire substrates by metal-organic vapor phase epitaxy at high temperatures between 1300 °C and 1600 °C. Dislocations such as those of the edge, screw and mixed types exhibited different behaviors in the AlN epilayers. The dislocation density of AlN was less than 2 x 10 9 cm -2 . Free and bound excitons of AlN were observed with peak energies of 6.084 eV and 6.063 eV, respectively.

Epitaxial lateral overgrowth (ELO) of low-dislocation-density AlN layers on trench-patterned sapphire substrates is demonstrated for the first time. Sapphire (0001) substrates with trench patterns formed along two different directions, (1010) and (1120), were used. We can obtain fully coalesced AlN only on the sapphire substrate having (1120) trenches. The dislocation density of ELO-AlN is as low as 6.7 × 10 8 cm -2 .

AlGaN/GaN hetero-field-effect-transistor-type (HFET-type) photosensors are fabricated with a p-GaInN optical gate for the detection of visible light. These photosensors employ a two-dimensional electron gas at the heterointerface between AlGaN and GaN as a highly conductive channel with a high electron mobility. By changing the InN molar fraction in the p-GaInN optical gate, the wavelength range of the photosensitivity of the HFET-type photosensors can be controlled. The photosensitivity of the AlGaN/GaN HFET-type photosensors with a p-GaInN optical gate greatly surpassed those of commercially available Si pin and Si avalanche photodiodes, and was comparable to those of photomultiplier tubes. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper reviews the recent development of group III AlGaN-based UV light emitters. Critical issues with respect to the development of high-performance devices are discussed.

Using the two-step metalorganic vapor phase epitaxial growth on ZrB2 substrate, we demonstrate violet and UV light-emitting diodes (LEDs). The violet LED shows extremely linear L–I characteristics and a sharp single spectrum with a peak wavelength of 410 nm. In the UV-LED on ZrB2, we confirmed that the vertical conduction through the ZrB2/AlGaN interface is superior to the conventional lateral conduction. The use of ZrB2 substrate is promising for UV light emitting devices with low operating voltage. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We demonstrated a UV-light-emitting diode on low-dislocation-density AlGaN. To improve the performance of the UV-LED, we modified the Ti/Au pad form of the UV-LED. The output power of the new LED is approximately 25% higher than that of the conventional LED. Moreover, the decrease in operating voltage is attributed to the spread injection current. This new UV-light-emitting diode shows a peak wavelength of 363 nm and an output power of 4.7 mW at 100 mA DC current injection. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Mg-doped GaN epilayers are analyzed by Raman and low-energy electron-excited nanoluminescence (LEEN) spectroscopies before and after oxygen ambient annealing at temperatures from 450 to 550°C. Annealing as low as 450°C shows the appearance of a local vibrational mode of the Mg Ga acceptor. Correspondingly, LEEN emission at 2.8 eV increases and that at 3.27 eV decreases after annealing in oxygen ambient. On the other hand, electron beam treatment decreases 2.8 eV emission and increases 3.27 eV emission. These luminescence properties are explained by a donor-acceptor pairs transition model due to hydrogen-related deep and shallow donors and Mg Ga acceptor.

The electrical properties of an n-type GaN wafer and epitaxial layer were evaluated using terahertz time-domain spectroscopic ellipsometry (THz-TDSE). The electrical properties, such as carrier density and mobility, were determined by fitting the reflective spectra of the sample using the Drude model. The measured properties of the GaN by THz-TDSE are consistent with those obtained by conventional Hall methods. For the GaN epitaxial layer on sapphire substrates, the thickness was obtained by THz-TDSE and compared with other measurements. THz-TDSE has shown to be a suitable technique for evaluating the electrical properties of n-type GaN materials non-destructively and without contacts.

We demonstrated how annealing of the sputtered AlN buffer layer (sp-AlN) on r-plane sapphire could be used to produce a high-crystalline-quality a-plane GaN (a-GaN). The sp-AlN with large grains was confirmed by annealing at 1600 °C in N2 ambient, consequently its crystalline orientation and quality were significantly improved. Moreover, it was found that a-GaN grown on annealed sp-AlN showed better crystalline quality, including a reduction of basal stacking fault density, than a-GaN grown on untreated sp-AlN (as sputtered). The implication is that the a-GaN growth method using annealed sp-AlN is an effective way to obtain high crystalline quality.

We executed a feasibility study of a novel laser structure composed of GaN nanowire (NW) and GaInN/GaN multi-quantum-shell (MQS) active layers. We predicted that the novel structure would increase of the optical confinement factor Γ because of its three-dimensional active layers. We analyzed the optical field distribution of the waveguide in the structure by the effective refractive index method, and the optical confinement factor was calculated. When the active layers had five periods of 4 nm-thick well layers and 9 nm-thick barrier layers, the confinement factor was estimated to be 6.5%. This is more than three times higher than that of a conventional planar multi-quantum-well (MQW) structure. The threshold current density estimated from the gain curve was shown to be 44% lower than that of a conventional MQW laser.

We report on the electrical properties of AlInN/GaInN heterostructures fabricated with InN molar fractions of 0 to 0.6 in the GaInN layer. High-density two-dimensional electron gases are formed near the interfaces of AlInN/AlN/GaInN at InN molar fractions of 0.3 and 0.6. The Al0.82In0.18N/AlN/Ga0.4In0.6N/GaN heterostructure field-effect transistors exhibited static characteristics. The maximum drain-source current reached a value of 0.26 A/mm (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the electrical properties of AlGaN/GaInN heterostructures fabricated with various InN molar fractions from 0 to 0.60 in GaInN on a GaN template. The sheet carrier density of the AlGaN/GaInN heterostructure monotonically increased with increasing InN molar fraction in GaInN, reaching 5.0 x 10 13 cm ―2 at an InN molar fraction of 0.60. The Al 0.30 Ga 0.70 N/Ga 0.40 In 0.60 N heterostructure exhibited static field-effect transistor (FET) characteristics.

We optimized the initial GaN growths of nonpolar m- and a-plane GaN grown on liquid phase epitaxy (LPE-) GaN substrates. Such nonpolar LPE-GaN substrates were produced by cutting slices from c-plane LPE-GaN bulk single crystals grown by the Na-flux method. We investigated the qualities of epitaxially grown GaN layers on the LPE-GaN substrates, with different polishing methods, which are chemical mechanical polishing and plasma dry etch polishing. We found that the crystalline quality of epitaxial GaN on m-plane GaN is very sensitive to the surface polishing even the minute unevenness in GaN substrate. Our experiments also indicated that a low initial growth rate was necessary to obtain high-crystalline-quality epitaxial m-plane GaN. In contrast, high-crystalline-quality a-plane GaN was obtained even with a high initial growth rate, indicating that the crystalline quality of a-plane GaN is not sensitive to surface roughness. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We evaluated AlGaN-based 355 nm UV laser diodes prepared under two different Mg activation conditions. The annealing processes for Mg activation in p-layers were carried out under N2 or O2 ambient to investigate the effect of the ambient gas on the laser characteristics. The threshold current densities and operating voltages of the UV laser diodes were improved by annealing under O2 compared with those under N2. We then estimated the injection efficiencies of the laser diode structures by considering the internal quantum efficiencies of optical excitation and electrical excitation. The internal quantum efficiency of the electrically excited spontaneous emission from the laser structure annealed under O2 reached 50% at a carrier density of 7.0×1018 cm-3, while the structure annealed under N2 required a 1.8-fold higher carrier density of 1.2×1019 cm-3 to reach the same internal quantum efficiency. In addition, the internal quantum efficiency estimated from optical excitation reached 50% even at a carrier density of 3.0×1018 cm-3. This implies that the injection efficiencies in the UV laser diode structures annealed under N2 and O2 were 25% and 45%, respectively. Mg activation by O2 annealing is effective for increasing the injection efficiency. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated the transparent electrodes using indium tin oxide (ITO) for UV light-emitting diodes (LEDs). The carrier concentration of ITO was increased from 1.1×1018 to 1.5×1021cm-3 after annealing under N2 ambient at 600 °C. The fundamental absorption edge was also shifted to a shorter wavelength owing to the increase in carrier concentration, through the so-called the Burstein-Moss effect. The annealed ITO forms an ohmic contact with p-GaN with a specific contact resistance of 1.2×10-3Ωcm2. The light output power of a 350 nm LED with the annealed ITO contact is 1.2 times higher than that with a conventional Ni/Au contact. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the fabrication of GaInN-based solar cells using GaInN/GaInN superlattices as active layers and also as underlying layers beneath the active layers. We obtained pit-free surfaces, even with a high InN molar fraction, using the superlattices. As a result, the maximum external and internal quantum efficiencies reached 60%, and 88%, respectively. The open-circuit voltage of the soalr cells was 1.77 V, the short-circuit current density was 3.08 mA/cm2, and the fill factor was 70.3%. A conversion efficiency of 2.46% was achieved at room temperature under simulared 1.5 sun × AM1.5G illumination using a solar simulator. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)