Nominal In0.18Ga0.82N (1 nm)/GaN (10 nm) MQWs are grown using trimethylindium (TMIn), triethylgallium (TEGa) and NH3 as described in [18] and coated by a p-GaN layer doped in the 1017-cm−3 range using TMGa, NH3 and bis(cyclopentadienyl)magnesium (Cp2Mg). Electroluminescence (EL) measurements shown in Figure 4 were carried out on a probe station under continuous-wave (CW) operation and ambient conditions on single standing LED wires. As shown in the inset, the current is injected into the wires from a 2-μm radius metallic tip on the external sidewall p-doped layer and collected through the n-core wire, the AlN/SiN x interface and the 275-μm-thick Si substrate
(phosphorus-doped with a 10−2 Ω cm resistivity). EL spectra for different CW currents ranging from 2 to 60 μA have been obtained for high voltage bias between 40 and 20 V. This high turn-on voltage (V on) can be attributed to the electrical injection Entinostat that involves two barriers coming from the wire/Si and wire/tip interfaces in addition to the resistive
behaviour of the Si substrate. The AlN layer has a bandgap of approximately 6.2 eV and a conduction band offset with respect to Si (GaN) Selleck GSK1904529A estimated to be approximately 2.3 (2.1) eV [19, 20]. These barriers do not explain however the very high V on of the device. For a comparison, the electron injection through a thick AlGaN/AlN epilayer has been reported to be only about 4 V [21]. Therefore, the high turn-on voltage can be mainly attributed selleck compound to the contact between the metallic tip and the p-doped part of the structure. This assumption has been confirmed by the connection
of an assembly of wires by indium titanium oxide exhibiting V on ~ 10 V [13]. The EL spectra exhibit a violet emission centred at 420 nm and no defect band (the usual yellow band being close to 550 nm). These results demonstrate the possibility to make a wire-based LED device on silicon by MOVPE. A weaker low-energy contribution is also measured at 460 nm. The origin of these two contributions has been assigned MycoClean Mycoplasma Removal Kit by cathodoluminescence mapping [5] to the presence of both radial (420 nm) and axial (460 nm) MQWs inside the wires (note that these luminescence peaks are also measured for wires that are not coated by the Mg-doped GaN shell). The 40-nm shift of the wavelength could be attributed to the variations of the In composition, well thickness and/or to the influence of the electric field [18] corresponding to the c- or m-plane MQW growth orientations. The influence of the internal electric field on the luminescence wavelength is negligible due to the small thickness of the wells (estimated to be 1 nm by TEM observations). This point is also confirmed by the lack of any significant peak shifts with increasing current density.