Two-dimensional (2D) materials represent an ultrathin material class with unique properties. For example, graphene as the first 2D material reported combines high electrical conductivity and high transparency making it ideally suited as transparent contact layer in GaN-based LEDs. Graphene has been grown by plasma-enhanced CVD and integrated into GaN-based light emitting devices for the visible and for the UV spectral range by a transfer-free approach. Pronounced lateral current spreading, and a reduced turn-on voltage indicate the suitability of our concept.
We report on the characterization of V-defects in GaN-based heterostructures via scanning force microscopy techniques.
The diameter and density of the V-defects are found to strongly depend on growth thickness and temperature of the top
layer, respectively, while no correlation between the V-defect formation and the type of doping could be identified.
Kelvin probe force microscopy measurements revealed for both, n- and p-doped GaN top layers, a decrease of the Kelvin
voltage within the V-defects, which indicates an enhanced work function of the facets of the V-defects with respect to
the planar surface. Surprisingly, an increase of the current flow within the V-defects is found by conductive atomic force
microscopy in case of the n-doped top layer, while current flow into the V-defect is suppressed for the p-doped top layer.
For a consistent explanation of these results we suggest a model, which is based on an enhanced electron affinity of the
{10-11}-surfaces within the V-defects as compared to the planar (0001)-surface.
We demonstrate the potential of Kelvin Probe Force Microscopy (KPFM) for analyzing degradation effects in GaN-based
laser diodes (LDs). Thereby, the surface potential at the mirror facet was measured locally for both, unbiased LDs
and LDs exposed to a well-defined current. In the unbiased case, our KPFM measurements demonstrate the impact of
aging on the mirror facet, which we attribute to a photon enhanced facet oxidation. In case of an externally applied
voltage, the local variation of the Kelvin voltage across the heterostructure layer sequence is analyzed. A clear
correlation between macroscopic I-V-characteristics and the microscopic data obtained with the KPFM is found.
High-brightness light-emitting diodes (LED) based on AlGaInP combines the possibility to achieve high efficiency with the flexibility of tuning the emission wavelength over a large range of the visible spectrum. For optimizing the device characteristics an accurate determination of the electronic properties, like e. g. the voltage drop across the semiconductor layer sequence, is desirable. We demonstrate the potential of Kelvin Force Microscopy for quantitative investigations of the voltage drop across the heterostructure layers of an operating AlGaInP LED. The surface potential was measured for external biases between -2.0 V and +1.86 V. By subtracting the zero bias result the voltage drop could be extracted quantitatively. In the low voltage regime, most of the voltage drops in the active layer. Above +1.5 V an additional voltage drop occurs on the p-side of the device, i. e. outside the active layer sequence, which reduces the efficiency of the LED. By comparing experimental data with simulations we will discuss possible mechanisms of these findings.
KEYWORDS: Silicon, Waveguides, Diodes, Atomic force microscopy, Semiconducting wafers, Oxides, Microscopy, Near field scanning optical microscopy, Aluminum, Scanning electron microscopy
The spatial and time resolved characterization of electronic devices by scanning probe microscopy demands the fabrication of proximal probes with well defined properties. To fulfill these requirements micromachining is the most appropriate technique, as it allows probe fabrication in a batch process with highest reproducibility. In this paper we describe the development of electrical and thermal near-field probes which can be employed for high frequency scanning force microscopy (HFSFM) and scanning thermal microscopy (SThM) respectively. Both probes have been completely fabricated in a micromachining batch process based on an almost identical technological design. For electrical imaging by HFSFM a coplanar wave guide probe was developed. The probes wave guide properties have been characterized by network analysis. A novel thermal probe consisting of a Schottky diode at the tip of a silicon cantilever was developed for SThM. Preliminary results on electrical and thermal characterization will be presented.
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