References of "Sofer, Zdenĕk"
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See detailDirect electro-optical pumping for hybrid CdSe nanocrystal/III-nitride based nano-light-emitting diodes
Mikulics, Martin; Arango, Y.C.; Winden, Andreas et al

in Applied Physics Letters (2016), 108

We propose a device concept for a hybrid nanocrystal/III-nitride based nano-LED. Our approach is based on the direct electro-optical pumping of nanocrystals (secondary excitation) by electrically driven ... [more ▼]

We propose a device concept for a hybrid nanocrystal/III-nitride based nano-LED. Our approach is based on the direct electro-optical pumping of nanocrystals (secondary excitation) by electrically driven InGaN/GaN nano-LEDs as the primary excitation source. To this end, a universal hybrid optoelectronic platform was developed for a large range of optically active nano- and mesoscopic structures. The advantage of the approach is that the emission of the nanocrystals can be electrically induced without the need of contacting them. The proof of principal was demonstrated for the electro-optical pumping of CdSe nanocrystals. The nano-LEDs with a diameter of 100 nm exhibit a very low current of 8 nA at 5V bias which is several orders of magnitude smaller than for those conventionally used. The leakage currents in the device layout were typically in the range of 8 pA to 20 pA/cm2 at 5V bias. The photon-photon down conversion efficiency was determined to be 27%. Microphotoluminescence and microelectroluminescence characterization demonstrate the potential for future optoelectronics and highly secure “green” information technology applications. [less ▲]

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See detailReduction of skin effect losses in double-level-T-gate structure
Mikulics, Martin; Hardtdegen, Hilde; Arango, Y. C. et al

in Applied Physics Letters (2014), 105

We developed a T-gate technology based on selective wet etching yielding 200 nm wide T-gate structures used for fabrication of High Electron Mobility Transistors (HEMT). Major advantages of our process ... [more ▼]

We developed a T-gate technology based on selective wet etching yielding 200 nm wide T-gate structures used for fabrication of High Electron Mobility Transistors (HEMT). Major advantages of our process are the use of only standard photolithographic process and the ability to generate T-gate stacks. A HEMT fabricated on AlGaN/GaN/sapphire with gate length Lg=200 nm and double-stacked T-gates exhibits 60 GHz cutoff frequency showing ten-fold improvement compared to 6 GHz for the same device with 2 um gate length. HEMTs with a double-level-T-gate (DLTG) structure exhibit up to 35% improvement of fmax value compared to a single T-gate device. This indicates a significant reduction of skin effect losses in DLTG structure compared to its standard T-gate counterpart. These results agree with the theoretical predictions. [less ▲]

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See detailInGaN nano-LEDs for energy saving optoelectronics
Marso, Michel UL; Mikulics, Martin; Winden, Andreas et al

in Conference Proceedings The 10th International Conference on Advanced Semiconductor devices and Microsystems (2014, October)

Vertically integrated III-nitride nano-LEDs designed for operation in the telecommunication-wavelength range were fabricated and tested in the (p- GaN/InGaN/n-GaN/sapphire) material system. We found that ... [more ▼]

Vertically integrated III-nitride nano-LEDs designed for operation in the telecommunication-wavelength range were fabricated and tested in the (p- GaN/InGaN/n-GaN/sapphire) material system. We found that the band edge luminescence energy of the nano-LEDs could be engineered by their size and by the strain interaction with the masked SiO2/GaN substrates; it depends linearly on the structure size. The results of reliability measurements prove that our technological process is perfectly suited for long-term operation of the LEDs without any indication of degradation effects. The presented technology shows strong potential for future low energy consumption optoelectronics. [less ▲]

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See detailNovel Double-Level-T-Gate Technology
Fox, Alfred; Mikulics, Martin; Hardtdegen, Hilde et al

in Conference Proceedings The 10th International Conference on Advanced Semiconductor devices and Microsystems (2014, October)

We developed a novel double-level-T-gate technology based on wet etching of a metal gate interlayer. With the help of this technological process we prepared T-gate feet with widths as small as 200 nm. The ... [more ▼]

We developed a novel double-level-T-gate technology based on wet etching of a metal gate interlayer. With the help of this technological process we prepared T-gate feet with widths as small as 200 nm. The major advantage of our process is its use of only standard optical lithography. It allows the fabrication of 100 nanometer size T-gates for transistors. High electron mobility transistors (HEMTs) were fabricated on an AlGaN/GaN/sapphire material structure with an original gate length Lg of 2 μm. Their cutoff frequency of 6 GHz was improved to 60 GHz by etching the gate to a 200 nm length double T-gate contact. [less ▲]

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See detailImpact of thermal annealing on nonequilibrium carrier dynamics in single-crystal, freestanding GaAs mesostructures
Mikulics, Martin; Hardtdegen, Hilde; Adam, Roman et al

in Semiconductor Science & Technology (2014), 29

We report on the impact of thermal annealing to carrier transport and transient, subpicosecond photoresponse of freestanding GaAs mesostructures. Our measurements included micro-photoluminescence and dark ... [more ▼]

We report on the impact of thermal annealing to carrier transport and transient, subpicosecond photoresponse of freestanding GaAs mesostructures. Our measurements included micro-photoluminescence and dark current and responsivity studies as well as optical femtosecond characterization. The fabricated GaAs mesostructures consisted of both mesowires and platelets that were integrated into coplanar striplines to form a photoconductive switch. We demonstrate that an optimized annealing process of our mesostructures, performed at 600 ◦C for 20 min, led to restoring bulklike properties of our freestanding devices. They exhibited dark currents below 600 pA at 10 V bias, responsivity of 0.2 A W−1 at 30 V, and mobility as high as 7300 cm2 V s−1. The annealed freestanding GaAs photodetectors were characterized by subpicosecond carrier relaxation dynamics with negligible trapping and a cutoff frequency of 1.3 THz. The latter characteristics make them excellent candidates for THz-bandwidth optoelectronics. [less ▲]

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See detailNon-uniform distribution of induced strain in gate-recessed AlGaN/GaN structure evaluated by micro-PL measurements
Mikulics, Martin; Hardtdegen, Hilde; Gregušová, Dagmar et al

in Semiconductor Science & Technology (2012), 27(10), 105008-105008

Micro-photoluminescence (μ-PL) studies were performed on AlGaN/GaN heterostructure field effect transistors (HFETs) with different gate-recessing depths. It was found that μ-PL is the method of choice for ... [more ▼]

Micro-photoluminescence (μ-PL) studies were performed on AlGaN/GaN heterostructure field effect transistors (HFETs) with different gate-recessing depths. It was found that μ-PL is the method of choice for detecting dry etching damage and simultaneously recording strain and stress in the HFET GaN layer. Lateral sub-μm resolved mapping shows that the strain in the GaN layer after recessing is partially relaxed and non-uniform. [less ▲]

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See detailResidual strain in recessed AlGaN/GaN heterostructure field-effect transistors evaluated by micro photoluminescence measurements
Mikulics, Martin; Hardtdegen, Hilde; Winden, Andreas et al

in Physica Status Solidi C. Current Topics in Solid State Physics (2012), 9(3-4), 911-914

The residual strain in AlGaN/GaN heterostructure field-effect transistors (HFETs) with recessed gates and their non-recessed counterparts were investigated by micro photoluminescence measurements (μ-PL ... [more ▼]

The residual strain in AlGaN/GaN heterostructure field-effect transistors (HFETs) with recessed gates and their non-recessed counterparts were investigated by micro photoluminescence measurements (μ-PL). It is found that strain relaxation accounts for the observed sheet carrier density reduction after gate recessing. The usefulness of the method for device processing optimization is demonstrated. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. [less ▲]

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See detailFemtosecond and highly sensitive GaAs metal–semiconductor–metal photodetectors grown on aluminum mirrors/pseudo-substrates,
Mikulics, Martin; Adam, Roman; Sofer, Zdenek et al

in Semiconductor Science & Technology (2010), 25(7), 75001

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See detailGrowth and properties of GaN and AlN layers on silver substrates
Mikulics, Martin; Kočan, Martin; Rizzi, Angela et al

in Applied Physics Letters (2005), 87

We report on the preparation and properties of GaN and AlN layers grown by molecular-beam epitaxy on silver metal substrates. X-ray diffraction rocking curves show polycrystalline character of GaN with ... [more ▼]

We report on the preparation and properties of GaN and AlN layers grown by molecular-beam epitaxy on silver metal substrates. X-ray diffraction rocking curves show polycrystalline character of GaN with high preferential GaN 11-22 orientation. An intermetallic phase of Ga3Ag is found at the GaN/Ag interface. On the other hand, AlN layers exhibit a monocrystalline structure with a growth direction of 0002 . Schottky diodes prepared on GaN layers show good rectifying behavior and relatively low leakage current 10−3 A/cm2 . These results indicate that the III-nitride growth on metallic substrates might be used for low-cost and large-area electronic and photonic devices. [less ▲]

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