Michał Leszczyński

Elimination of AlGaN epilayer cracking by spatially patterned AlN mask

The inherent problem in III-nitride technology is the cracking of AlGaN layers that results from lattice mismatch between AlGaN and GaN. In case of thin substrates (30–90μm), such as, bulk GaN grown by the high-pressure/high-temperature method, the bowing of AlGaN∕GaN strained structures becomes an additional problem. To eliminate cracking and bowing, AlGaN layers were grown on GaN substrates with an AlN mask patterned to form 3–15μm wide windows. In the 3μm window, the AlGaN layer was not cracked, although its thickness and Al composition exceeded critical values for growth on nonpatterned substrates. Dislocation density in the windows was of 5×106∕cm2.

Lateral Control of Indium Content and Wavelength of III–Nitride Diode Lasers by Means of GaN Substrate Patterning

A patterned GaN/sapphire template with separate regions angled between 0.4 and 2° to the wurtzite c-plane was used to grow a 50 nm In0.1Ga0.9N layer. The photoluminescence wavelength varied between 403 and 389 nm according to the increased regions angle. The indium content measured using X-rays was reduced in regions with a higher miscut angle. Patterned freestanding GaN with separate regions angled by 0.35 and 0.85° to the c-plane was used to fabricate ridge-waveguide diode lasers. Each laser stripe was placed inside one of the angled regions. Lasing wavelengths of 405.8±0.2 and 401.0±1 nm were obtained for devices grown in those regions.

Hybrid reciprocal lattice: application to layer stress determination in GaAlN/GaN(0001) systems with patterned substrates

Epitaxy of semiconductors is a process of tremendous importance in applied science and optoelectronic industry. Controlling of defects introduced during epitaxial growth is a key point in manufacturing devices of high efficiency and durability. In this work, we demonstrate how useful hybrid reflections are on the study of epitaxial structures with anisotropic strain gradients due to patterned substrates. High accuracy to detect and distinguish elastic and plastic relaxations are one of the greatest advantages of measuring this type of reflection, as well as the fact that it can be exploited in symmetrical reflection geometry on a commercial high-resolution diffractometer.

Synthesis and Structure of Alkylzinc 3,5-Diphenylpyrazolates: Dramatic Influence of Steric and Solvent Effects.

The reaction of R2Zn (R = Et, (t)Bu) with 3,5-diphenylpyrazole results in the formation of three structurally diverse alkylzinc pyrazolates: a novel dinuclear tetrahydrofuran solvate, an unprecedented trimeric structure, and a spiro trinuclear aggregate. Structural analysis of the resulting complexes provides a new look at the aggregation and stabilization of alkylzinc species.

Electrically driven terahertz radiation of 2DEG plasmons in AlGaN/GaN structures at 110 K temperature

We experimentally observed a terahertz (THz) radiation of electrically driven 2D electron gas (2DEG) plasmons in AlGaN/AlN/GaN structures at T = 110 K. The grating with a period of 1.0 μm and a filling factor of 0.35 was used to couple electromagnetic radiation out from the plasmonic sample excited in a pulsed regime. The peak power radiated from ungated 2DEG plasmons at a frequency of 5.0 THz under an electric field of 450 V/cm was up to 940 nW. The intensity of the radiation was sufficient to measure spectra with a conventional far-infrared Fourier transform spectrometer. The analysis of the data revealed that the 2DEG plasmon radiation was superimposed with the black-body radiation of the sample and electroluminescence of the impurities. The strategy to reach higher powers of THz emission for practical applications is discussed.

Nitride based laser diodes on substrates with patterned AlN mask

We report on electrical and optical properties of InGaN violet laser diodes, grown on high-pressure grown, almost dislocation free GaN substrates. In these structures, mechanical strain has been reduced by a partially relaxed AlN mask with stripe shaped windows. The dislocation density of the mask was 1010∕cm2. In the window areas, where we located the laser stripes, the dislocation density was below 3×106∕cm2. We fabricated three types of laser diodes with aluminum composition in the cladding of 8%, 13%, and 16%. All devices remained crack-free. The laser threshold current decreased with increasing Al composition in the cladding.

Properties of violet laser diodes grown on bulk GaN substrates

High pressure grown GaN bulk crystals, because of their low defect density, are atractive for the use as substrates for blue-violet laser diode fabrication. These laser diodes are characterized by a low density of dislocations (8×104-1×105 cm-2) and thus they possibly have the best crystalline quality ever reported for this type of nitride devices. Previously, we demonstrated that these lasers are able to emit a very high optical power under pulse operation. In the present paper we will demonstrate the details of their room temperature CW operation, giving good prognostics for the further development of these devices. Preliminary estimation of the internal losses indicated a very low internal absorption in the range of 5 cm-1. The characterization of the aged devices did not reveal any dark lines or facet degradation. A correlation between the device lifetime and p-type layers growth methods will be suggested here.

GaN substrates with variable vicinal angles for laser diode applications

GaN c-plane substrates were patterned to obtain 30-70 μm wide differently angled regions. The patterning technique was based on multilevel pattern photolithography and ion etching and was similar to the one used for the diffraction optics elements fabrication. The region angles were between 0.2 and 3.4 degrees with respect to the c-plane. It is shown that photoluminescence and cathodoluminescence wavelengths of InGaN/GaN quantum wells grown by metalorganic vapor phase epitaxy depend on each regions angle. Laser diodes grown on freestanding patterned GaN are also demonstrated. The lasing wavelength of chips grown in differently angled substrate regions are different. We attribute those differences to indium content differences in each of the angled regions.

Rational Design of Noncovalent Diamondoid Microporous Materials for Low-Energy Separation of C6-Hydrocarbons

Selective separation of gases/vapors with similar physicochemical properties involves energetically costly distillation processes. Alternative separation processes based on shape-selective molecular sieving, taking place on porous frameworks (or membranes), are less energy demanding but require an optimal balance between selectivity and diffusion kinetics (permeability). Herein, we report a rational strategy to select an optimal soft noncovalent microporous material (NPM) suitable for the low-energy separation of C6-hydrocarbons with kinetic diameters in the range of 4.3-6.3 Å. This strategy is based on a Cambridge Structural Database search of diamondoid NPMs with a low packing factor, leading to the selection of an oxidotetrazinc cluster based diamondoid NPM network named DiaMM-1 containing tetrahedral voids of 336 Å3 (tetrahedron insphere diameter of 5.8 Å accessible through 8.2 Å triangular windows) suitable for this separation. Based on this result the fluorinated analogue DiaMM-2 was designed and synthesized. DiaMM-1 and DiaMM-2 exhibit permanent porosity and high thermal stability. The optimal combination of molecular crystal softness, pore size, and decoration of pore surface of DiaMM-1,-2 leads to high adsorbate diffusivity and low adsorption energy, allowing fast separation of hexane isomers and benzene/cyclohexane mixtures at low temperature.

Straightforward Synthesis of Single-Crystalline and Redox-Active Cr(II)-carboxylate MOFs

We report on a facile and environmentally friendly synthetic approach for single-crystalline chromium(II) carboxylate metal-organic frameworks (i.e., Cr3(BTC)2·3H2O (1) and Cr(hfipbb)·H2O (2) at room temperature in water. Both MOFs can be easily dehydrated, affording single-crystalline materials with open Cr(II) sites. In addition, the redox activity and porosity of the resulting Cr(II) MOFs were examined.