AlGaN/GaN High Electron Mobility Transistors (HEMTs) are capable of achieving high breakdown voltage, low operating resistance and large switching speed due to the excellent performance shown by III-N structures. The paper presents selected details of technological experimental work on high voltage (HV) AlGaN/GaN-on-Si HEMTs fabricated with multifinger structures and gate widths of up to 40×1 mm. The electrical isolation of individual devices was elaborated using Al+ implantation. The ions were implanted up to a depth of 200 nm in order to produce an effective damage and isolation up to the non-conducting AlGaN buffer layer. The influence of the ion energy (in the range 208-385 kV) and the ion dose (in the range 8.5x1012-1.4x1013cm-2) on the effectiveness of the fabricated isolation was found. The properties of the fabricated ohmic contacts (using Ti/Al/Mo/Au and Ti/Al/TiN/Cu metallization schemes) with emphasis put on the technology of recess etching were studied. The impact of various pretreatment, applied before deposition of the gate metallization, on electrical parameters of multifinger devices was analysed. The tested pretreatment methods included oxide removal in HCl-based solution, and O2 or BCl3 plasma treatment, with the lowest gate leakage current obtained for the latter. The results of fabrication of the HV HEMTs with single field-plate structures with various dielectrics (Si3N4 or Al2O3) are discussed. The characterization results within the paper cover electrical (I-V characteristics), structural (TEM, XRD), topographical (AFM) and elemental (EDS mapping) analyses.
This work was supported by The National Centre for Research and Development under Agreement nr TECHMATSTRATEG1/346922/4/NCBR/2017 for project "Technologies of semiconductor materials for high power and high frequency electronics"
Novel optical sensors the most often require thin films or surface structures with strictly controlled properties, playing a critical role in them by initiating or modifying their sensorial responses. Selected results of research on atomic layer deposited (ALD) metallic oxides will be shown, regarding their applicability for thin functional coatings in lossy mode resonance (LMR) and long period grating (LPG) optical fiber sensors. Basically amorphous films of tantalum oxide (TaxOy), zirconium oxide (ZrxOy) and hafnium oxide (HfxOy) below 200 nm were deposited at relatively low temperature (LT) of 100°C. The optical, structural, topographical, tribological, hydrophilic and chemical stability properties of the films and their technological controllability were analysed. The TaxOy was selected and successfully applied as an oxide coating in LPG sensor. As chemically robust in alkali environment (pH over 9) it allowed to gain a potential for fabrication of regenerable/reusable biosensor. Additionally, ALD technique was tested as a tool for tailoring sensorial properties of LMR sensors. The double-layer coatings composed of two different materials were experimentally tested for the first time; the coatings were composed of plasma-enhanced chemical vapour deposited (PECVD) silicon nitride (SixNy) followed by much thinner ALD TaxOy. That approach yielded operating devices, ensuring fast overlay fabrication and easy tuning of the resonant wavelength at the same time. The LT ALD TaxOy films turned out to be slightly overstoichiometric (y/x approx. 2.75). Therefore, the issue of TaxOy chemical composition was studied by secondary ion mass spectroscopy, Rutherford backscattering spectrometry and x-ray photoelectron spectrometry.
Fabrication of approx. 3 THz Al0.15Ga0.85As/GaAs QCLs grown by Molecular Beam Epitaxy equipped with Ta/Cu or Ti/Cu waveguide claddings will be presented.
Our previous studies showed that copper layers as the waveguide claddings are most promising in THz QCLs technology. The theoretical predictions showed that lasers with Ti/Cu or Ta/Cu claddings (where Ti and Ta play the role of diffusion barriers and improve adhesion) show the smallest waveguide losses when compared with other metals. The main important issue of the presentation will be the wafer bonding of the QCL active region and GaAs receptor wafer. We will compare the results of ex-situ and in-situ bonding technology. The structures were tested by optical microscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDXS). Our studies show that it was necessary to apply at least 5 nm-thick diffusion-barrier layers, as well as to keep all of the process temperatures below 400C in order to ensure the barrier tightness. The next important issue was control of composition of metallic claddings, in order to provide the control of the refractive index profiles of the claddings.
The ridge structure lasers were fabricated with ridge width in the range 100 – 140 µm, formed by dry plasma etching in BCl3/Cl2/Ar mixture in ICP RIE system.
The lasers operated with threshold current densities of approx. 1.2 kA/cm2 at 77 K and the Tmax = 130 K, when fed by 100-300 ns current pulses supplied with 0.3-1 kHz repetition frequencies.
*This research is supported by The National Centre for Research and Development (bilateral cooperation, project no. 1/POLTUR-1/2016) and TUBITAK (Scientific and Technical Research Council of Turkey) project number 215E113.
Conditions of fabrication of first-order distributed-feedback surface gratings designed for single-mode Al0.45Ga0.55As/GaAs quantum cascades lasers with the emission wavelength of about 10 μm are presented. The 1-μm-deep rectangular-shaped gratings with the period of about 1.55 μm and duty cycle in the range of 65% to 71% made by the standard photolithography are demonstrated. The wavenumber difference of about 7 cm−1 at 77 K is observed for the radiation emitted by lasers fabricated from the same epitaxial structure with ridge widths in the range of 15 to 25 μm. Moreover, the emission wavelength of the lasers could be tuned with temperature at a rate of 1 nm/K in the temperature range of 77 to 120 K. The full width at half maximum of the emitted spectra is ∼0.4 cm−1.
Conditions of fabrication of first order distributed-feedback surface gratings designed for single-mode Al0.45Ga0.55As/GaAs quantum cascades lasers with the emission wavelength of about 10μm are presented. The 1 μm-deep rectangular-shaped gratings with the period of about 1.55 μm and duty cycle in the range of 65-71% made by the standard photolithography are demonstrated. The wavenumber difference of about 7 cm-1 at 77 K is observed for the radiation emitted by lasers fabricated from the same epitaxial structure with ridge widths in the range of 15-25 μm. Moreover, the emission wavelength of the lasers could be tuned with temperature at a rate of 1 nm/K in the temperature range of 77-120 K. The full width at half maximum of the emitted spectra is ~ 0.4 cm-1.
We report research results with regard to AlGaAs/GaAs structure processing for THz quantum-cascade lasers (QCLs). We focus on the processes of Ti/Au cladding fabrication for metal–metal waveguides and wafer bonding with indium solder. Particular emphasis is placed on optimization of technological parameters for the said processes that result in working devices. A wide range of technological parameters was studied using test structures and the analysis of their electrical, optical, chemical, and mechanical properties performed by electron microscopic techniques, energy dispersive x-ray spectrometry, secondary ion mass spectroscopy, atomic force microscopy, Fourier-transform infrared spectroscopy, and circular transmission line method. On that basis, a set of technological parameters was selected for the fabrication of devices lasing at a maximum temperature of 130 K from AlGaAs/GaAs structures grown by means of molecular beam epitaxy. Their resulting threshold-current densities were on a level of 1.5 kA/cm2. Furthermore, initial stage research regarding fabrication of Cu-based claddings is reported as these are theoretically more promising than the Au-based ones with regard to low-loss waveguide fabrication for THz QCLs.
We report our research on processing of AlGaAs/GaAs structures for THz quantum-cascade lasers (QCLs). We focus on
the processes of fabrication of Ti/Au claddings for metal-metal waveguides and the wafer bonding with indium solder.
We place special emphasis on the optimum technological conditions of these processes, leading to working devices. The
wide range of technological conditions was studied, by use of test structures and analyses of their electrical, optical,
chemical and mechanical properties, performed by electron microscopic techniques, energy dispersive X-ray
spectrometry, secondary ion mass spectroscopy, atomic force microscopy, fourier-transform infra-red spectroscopy and
circular transmission line method. On the basis of research a set of technological conditions was selected, and devices
lasing at the maximum temperature 130K were fabricated from AlGaAs/GaAs structures grown by molecular beam
epitaxy (MBE) technique. Their threshold-current densities were about 1.5kA/cm2. Additionally we report our initial
stage research on fabrication of Cu-based claddings, that theoretically are more promising than the Au-based ones for
fabrication of low-lossy waveguides for THz QCLs.
Progress in quality of ultrathin superconducting niobium nitride films for fabrication technology of single photon
detectors is here presented. The films deposited on Al2O3 single crystals reveal excellent both superconducting and
structure properties but the films deposited on Si single crystals have really worse parameters. High epitaxial quality of
NbN and NbTiN films grown on the Al2O3 substrates is proved by HRXRD and HRTEM studies. The results of the
studies on both NbN and NbTiN films reveal one cubic NbN phase with NaCl-type structure, and the planes of NbN are
correlated with Al2O3 crystal orientation. The critical temperatures of NbN and NbTiN films with thickness of few nm
grown on the Al2O3 and Si substrates are in range 4K ÷ 7K, but post-grown annealing of the films at 1000°C in Ar
increases temperature about 10K. Moreover, the NbTiN film deposited on sapphire at optimized conditions and annealed
discloses the best superconducting properties - critical temperature of 14 K as well as extremely high critical current
density of 12·106 A/cm2. This is the best results measured on so thin superconducting films and not reported up to now.
The improvement in superconductor parameters is explained here due to reduced strain and defects by high temperature
annealing of the film. Structural analysis on the annealed NbTiN films by XRD measurement confirms that FWHM of
the 111 Bragg reflection is extremely narrow, about value of 10 arcsec characterising the best single crystals.
For the growth of many high-quality photonic devices, especially surface emission lasers with a vertical resonance
cavity (Vertical External Cavity Surface Emitting Laser, VECSEL) very important is to know the actual concentration of
the carriers in each layer laser structure resulting from the presence of unintentional impurities. Studies of doping profile
of this type multilayer structure can be carried out only by comparing the measured capacitance - voltage characteristics
with the calculated theoretically. The paper presents results of research VECSEL structures with different numbers of
quantum wells 4, 8, 12, 16, produced by molecular beam epitaxy. Measurements of the capacitance - voltage
characteristics has been performed using mercury probe in the system for automatic measurement of C-V, I-V, G–V. The
results of C-V measurements and numerical simulations have confirmed the possibility to control the level of
unintentional impurities in the different layers of the laser VECSEL structure. The lowest concentrations of unintentional
impurities were obtained for structures with highest power output.
LPE growth of Ga1-xAlxAsySb1-y on (100) GaSb substrates has been investigated for wide range of aluminum content in the melt, xAl1=0.01 - 0.06, various growth temperatures, and various amount of supersaturation. Epilayers were characterized by means of XRD, TEM, EPXMA, and SIMS. It has been found that LPE growth at Tapproximately equals 5300C produces good quality Ga1-xAlxAsySb1-y layers with Al content in the solid up to x equals0.24 and latice mismatch (delta) a/a not exceeding 5*10-4. As for the growth of higher aluminum content alloys at higher temperatures Tequals590 - 6000C, good results have been obtained unless the Al content in the melt does not exceed xAl1equals0.02 giving perfectly matched Ga1- xAlxAsySb1-y epilayers with Al content in the solid by up to x equals0.3. By introducing an interlayer, either of the lattice matched Ga0.91In0.09As0.08Sb0.92 or Ga0.70Al0.30As0.03Sb0.97, LPE growth from the melt with Al content up to xAl1equals0.06 becomes possible and enables fabrication of Ga1-xAlxAsySb1-y layers with Al content in the solid as high as xequals0.62. Ga1-xAlxAsySb1-y layers obtained from the melt with xAl1equals0.04 were characterized by lattice mismatch (Delta) a/aequals(8-9)-10-4, an increase of (Delta) a/a to 2.2*10-3 was observed for epilayers obtained from the melt with xAl1equals0.06.
The paper reports on the design and fabrication of LPE-grown (formula available in paper) heterojunction photodetectors operating in the 2-2.4 micrometers wavelength region. Experiments on LPE growth of high-x- content quaternaries as well as optimization of device processing has been carried out. LPE growth at Tapproximately equals 530DEGC enabled obtaining lattice matched heterostructures with 19% indium in the active layer In (formula available in paper) and photodetectors with (lambda) co=2.25micrometers . By increasing the temperature of epitaxial growth to 590DEGC In (formula available in paper)heterostructures (with 23%indium content suitable for photodetectors with (lambda) co=2.35 micrometers have been obtained. Mesa-type photodiodes were fabricated by RIE in Ccl (formula available in paper) plasma and passivated electrochemically in (formula available in paper). These devices are characterized by differential resistance up to (formula available in paper) and the detectivity in the range (formula available in paper), in dependence on the photodiode active area cutoff wavelength.