We report on the control and modification of optical transitions in 40× GaN/AlN heterostructure superlattices embedded in GaN nanowires by an externally applied bias. The complex band profile of these multi-nanodisc heterostructures gives rise to a manifold of optical transitions, whose emission characteristic is strongly influenced by polarization-induced internal electric fields. We demonstrate that the superposition of an external axial electric field along a single contacted nanowire leads to specific modifications of each photoluminescence emission, which allows to investigate and identify their origin and to control their characteristic properties in terms of transition energy, intensity and decay time. Using this approach, direct transitions within one nanodisc, indirect transitions between adjacent nanodiscs, transitions at the top/bottom edge of the heterostructure, and the GaN near-band-edge emission can be distinguished. While the transition energy of the direct transition can be shifted by external bias over a range of 450 meV and changed in intensity by a factor of 15, the indirect transition exhibits an inverse bias dependence and is only observable and spectrally separated when external bias is applied. In addition, by tuning the band profile close to flat band conditions, the direction and magnitude of the internal electric field can be estimated, which is of high interest for the polar group III-nitrides. The direct control of emission properties over a wide range bears possible application in tunable optoelectronic devices. For more fundamental studies, single-nanowire heterostructures provide a well-defined and isolated system to investigate and control interaction processes in coupled quantum structures.
The covalent functionalization of GaN and AlN surfaces with organosilanes is demonstrated. Both octadecyltrimethoxysilane and aminopropyltriethoxysilane form self-assembled monolayers on hydroxylated GaN and AlN surfaces, confirmed by x-ray photoelectron spectroscopy and atomic force microscopy. The monolayer thickness on GaN was determined to 2.5±0.2nm by x-ray reflectivity. Temperature-programmed desorption measurements reveal a desorption enthalpy of 240kJ∕mol. The realization of micropatterned self-assembled monolayers and the hybridization of deoxyribonucleic acid molecules on biofunctionalized GaN surfaces are shown.
Ga N ∕ Al N quantum dots were investigated as optical transducers for field effect chemical sensors. The structures were synthesized by molecular-beam epitaxy and covered by a semitransparent catalytic Pt top contact. Due to the thin (3nm) AlN barriers, the variation of the quantum dot photoluminescence with an external electric field along the [0001] axis is dominated by the tunneling current rather than by the quantum confined Stark effect. An increasing field results in a blueshift of the luminescence and a decreasing intensity. This effect is used to measure the optical response of quantum dot superlattices upon exposure to molecular hydrogen.
In this work we analyse the performance of Pt- and Ni-based Schottky metallizations on AlxGa1−xN (x = 0, 0.31). An intermediate thin Ti layer is shown to enhance the thermal stability of Pt/Au, and leads to an increase of the Schottky barrier height. Pt/Ti/Au contacts on GaN provide a barrier height of 1.18 ± 0.07 eV, increasing up to 2.0 ± 0.1 eV on Al0.31Ga0.69N. Further improvement of Schottky contacts is achieved by surface passivation with plasma-enhanced chemical vapour deposited SiO2 or SixNy, which reduces the leakage current by two orders of magnitude and structural modifications in the metal due to thermal ageing.
The photoluminescence intensity of group III nitrides, nanowires, and heterostructures (NWHs) strongly depends on the environmental H(2) and O(2) concentration. We used this opto-chemical transducer principle for the realization of a gas detector. To make this technology prospectively available to commercial gas-monitoring applications, a large-scale laboratory setup was miniaturized. To this end the gas-sensitive NWHs were integrated with electro-optical components for optical addressing and read out within a compact and robust sensor system. This paper covers the entire realization process of the device from its conceptual draft and optical design to its fabrication and assembly. The applied approaches are verified with intermediate results of profilometric characterizations and optical performance measurements of subsystems. Finally the gas-sensing capabilities of the integrated detector are experimentally proven and optimized.
This paper reports about the first piezoresistive pressure sensor for high operating temperatures using single crystalline, n-type /spl beta/-SiC piezoresistors on Silicon On Insulator (SOI) substrates. The new Silicon Carbide On Insulator (SiCOI) layer structure prevents a leakage current flow through the substrate at high temperatures up to 723 K. The sensor was tested in the temperature range between room temperature and 573 K. The sensitivity of the device at room temperature is approximately 20.2 /spl mu/V/VkPa. This corresponds to a longitudinal gauge factor of -32 in the [100]-direction. The Temperature Coefficient of Sensitivity (TCS) is -0.16 %K/sup -1/ at 573 K.
