Piotr Caban

Tunable GaN/AlGaN ultraviolet detectors with built-in electric field

We present the optical and the electrical properties of GaN/AlGaN structures that can be used as tunable ultraviolet photodetectors. The photosensitivity spectrum of the structures can be tuned in the range 3.5–3.85 eV by external voltage. The photosensitivity of the sample in the 3.5 eV spectral range increases about 500 times when the reverse bias changes from −2 to −6 V. The effect can be explained by changes in the electric field in the GaN/AlGaN structure. The field is generated by the internal electric polarization and the external bias. We present also a numerical modeling of the electric field, the potential profiles, and the current flow in such structures. Our modeling shows that at low bias a two-dimensional (2D)-electron gas at the AlGaN/GaN interface screens the electric field generated by spontaneous polarization. The lack of the field stops transport of photoexcited holes. The holes that are accumulated on the interface causes fast electron recombination and reduces photocurrent. The extern...

Step-edge-induced resistance anisotropy in quasi-free-standing bilayer chemical vapor deposition graphene on SiC

which results in macro steps � 10 nm in height. In this report, we study the qualitative and quantitative effects of SiC steps edges on the resistance of epitaxial graphene grown by chemical vapor deposition. We experimentally determine the value of step edge resistivity in hydrogen-intercalated QFS-bilayer graphene to be � 190Xlm for step height hS ¼ 10 nm and provide proof that it cannot originate from mechanical deformation of graphene but is likely to arise from lowered carrier concentration in the step area. Our results are confronted with the previously reported values of the step edge resistivity in monolayer graphene over SiC atomic steps. In our analysis, we focus on large-scale, statistical properties to foster the scalable technology of industrial graphene for electronics and sensor applications. V C 2014 AIP Publishing LLC .[ http://dx.doi.org/10.1063/1.4896581]

Growth Rate and Thickness Uniformity of Epitaxial Graphene

The paper provides a deeper understanding of key-parameters of epitaxial graphene growth techniques on SiC. At 16000C, the graphene layer is continuous and covers a large area of the substrate. Significant differences in the growth rate could be observed for different reactor pressures and the polarity of SiC substrates as well as for the substrate miscut and surface quality. In addition, graphene thickness uniformity and mechanism of ridges creation was examined.

Integrated AlGaN quadruple-band ultraviolet photodetectors

Monolithically integrated quadruple back-illuminated ultraviolet metal‐semiconductor‐metal photodetectors with four different spectral responsivity bands were demonstrated on each of two different AlxGa1-xN heterostructures. The average of the full-width at half-maximum (FWHM) of the quantum efficiency peaks was 18.15 nm for sample A, which incorporated five 1000 nm thick epitaxial layers. In comparison, the average FWHM for sample B was 9.98 nm, which incorporated nine 500 nm thick epitaxial layers. (Some figures may appear in colour only in the online journal)

AlGaN quadruple-band photodetectors

Quadruple back-illuminated ultraviolet metal-semiconductor-metal photodetectors with four different spectral responsivity bands were demonstrated. The average of the full-width at half-maximum (FWHM) of the quantum efficiency peaks was 9.98 nm.

The Influence of Growth Temperature on Oxygen Concentration in GaN Buffer Layer

The influence of growth temperature on oxygen incorporation into GaN epitaxial layers was studied. GaN layers deposited at low temperatures were characterized by much higher oxygen concentration than those deposited at high temperature typically used for epitaxial growth. GaN buffer layers (HT GaN) about 1 μm thick were deposited on GaN nucleation layers (NL) with various thicknesses. The influence of NL thickness on crystalline quality and oxygen concentration of HT GaN layers were studied using RBS and SIMS. With increasing thickness of NL the crystalline quality of GaN buffer layers deteriorates and the oxygen concentration increases. It was observed that oxygen atoms incorporated at low temperature in NL diffuse into GaN buffer layer during high temperature growth as a consequence GaN NL is the source for unintentional oxygen doping.

Structural Characterization of GaN Epitaxial Layers Grown on 4H-SiC Substrates with Different Off-Cut

The influence of surface preparation of 4H-SiC substrates on structural properties of GaN grown by low pressure metalorganic vapour phase epitaxy was studied. Substrate etching has an impact on the crystallographic structure of epilayers and improves its crystal quality. The GaN layers were characterized by RBS/channelling and HRXRD measurements. It was observed that on-axis 4H-SiC is most suitable for GaN epitaxy and that substrate etching improves the crystal quality of epilayer.