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/cm<sup>2</sup>. 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.
We report on the fabrication and performance of amorphous oxide thin-film transistors with In-Ga-Zn-O deposited by
RF magnetron reactive sputtering for semiconductor channel layer. The influence of the electrical transport properties of
the channel on the electrical parameters of thin-film transistors has been determined. By optimizing process parameters
depletion-mode n-channel devices with maximum field-effect mobility (μ<sub>FE</sub>) 10.1 cm<sup>2</sup>/Vs, threshold voltage V<sub>th</sub>=-4.85V
and on to off current ratio (I<sub>on</sub>/I<sub>off</sub>)=2.1x10<sup>2</sup> have been demonstrated.
Thin film chemical compounds can be obtained as a result of sputtering of dielectric target (High-Frequency sputtering)
or sputtering of conductive target in reactive atmosphere (Direct Current or pulsed sputtering). Achievement of high
efficiency of thin films deposition is possible when conductive target is sputtered, as its surface is not covered
(or is covered only partially) with dielectric compound. Aim of this work was obtaining aluminum oxide thin films with
highest efficiency, using reactive pulsed magnetron sputtering of aluminum target in Ar+O<sub>2</sub> atmosphere.
In-Ga-Zn-O thin films fabricated by reactive RF magnetron sputtering have been investigated for their compositional,
structural, morphological, electrical and optical properties. All resulting films present the amorphous microstructure, and
root mean square roughness below 0.6 nm. The variation of the oxygen content in the deposition atmosphere from 0% to
0.9% results in the formation of a-IGZO thin films consisting of 15-29% indium, 16-28% gallium, 10-13% zinc and
30-60% oxygen, which significantly differs from the InGaZnO<sub>4</sub> target composition. IGZO thin films present
the transmittance in range of 75% to 90% for VIS-NIR wavelengths. Mechanism of free electrons generation via oxygen
vacancies formation is proposed to determine the relation between oxygen content in the deposition atmosphere and
the transport properties of the IGZO of the thin films.
Diffractive optical elements are designed and demonstrated as elemental units in photonic gas sensors. Diffraction gratings are written on specially designed photosensitive polymers using photolithographic techniques, as well as on multilayer metal/metal oxide thin film structures. Photonic sensors are implemented using grating structures as the elemental units for the detection of the external agent. These gratings are designed from such materials that show response to the external agent and the sensitivity is increased through the design of the grating. The principle of operation is based on the grating's diffraction efficiency variations due to index of refraction alterations and/or geometrical changes of the grating structure (e.g., groove depth, groove spacing) to external factors. The advantageous characteristics of the presented integrated sensor are the fully reversible behavior at ambient operating conditions, without the need for additional heating or light exposure. Applications of these sensitive photonic sensors so far include water vapor, hydrocarbons, and alcohol detection. The optical designs are based on diffraction efficiency measurements, and incorporate a monochromatic optical source and simple optoelectronic detection components. The photonic sensor integration is based on bulk optics approach.
Materials involved in gas sensing applications have been deposited by various methods in thin film form. Variation of the optical properties of the thin films were investigated under butane and ozone exposure using the m-lines technique. Effective index variations down to 10<sup>-4</sup> were observed. Concentrations of 100 ppm of butane diluted in the air or in nitrogen were detected.
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 530<SUP>DEG</SUP>C enabled obtaining lattice matched heterostructures with 19% indium in the active layer In (formula available in paper) and photodetectors with (lambda) <SUB>co</SUB>=2.25micrometers . By increasing the temperature of epitaxial growth to 590<SUP>DEGC In (formula available in paper)heterostructures (with 23%indium content</SUP> suitable for photodetectors with (lambda) <SUB>co</SUB>=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.
LPE growth of Ga<SUB>1-x</SUB>Al<SUB>x</SUB>As<SUB>y</SUB>Sb<SUB>1-y</SUB> on (100) GaSb substrates has been investigated for wide range of aluminum content in the melt, x<SUB>Al</SUB><SUP>1</SUP>=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 530<SUP>0</SUP>C produces good quality Ga<SUB>1-x</SUB>Al<SUB>x</SUB>As<SUB>y</SUB>Sb<SUB>1-y</SUB> layers with Al content in the solid up to x equals0.24 and latice mismatch (delta) a/a not exceeding 5*10<SUP>-4</SUP>. As for the growth of higher aluminum content alloys at higher temperatures Tequals590 - 600<SUP>0</SUP>C, good results have been obtained unless the Al content in the melt does not exceed x<SUB>Al</SUB><SUP>1</SUP>equals0.02 giving perfectly matched Ga<SUB>1- x</SUB>Al<SUB>x</SUB>As<SUB>y</SUB>Sb<SUB>1-y</SUB> epilayers with Al content in the solid by up to x equals0.3. By introducing an interlayer, either of the lattice matched Ga<SUB>0.91</SUB>In<SUB>0.09</SUB>As<SUB>0.08</SUB>Sb<SUB>0.92</SUB> or Ga<SUB>0.70</SUB>Al<SUB>0.30</SUB>As<SUB>0.03</SUB>Sb<SUB>0.97</SUB>, LPE growth from the melt with Al content up to x<SUB>Al</SUB><SUP>1</SUP>equals0.06 becomes possible and enables fabrication of Ga<SUB>1-x</SUB>Al<SUB>x</SUB>As<SUB>y</SUB>Sb<SUB>1-y</SUB> layers with Al content in the solid as high as xequals0.62. Ga<SUB>1-x</SUB>Al<SUB>x</SUB>As<SUB>y</SUB>Sb<SUB>1-y</SUB> layers obtained from the melt with x<SUB>Al</SUB><SUP>1</SUP>equals0.04 were characterized by lattice mismatch (Delta) a/aequals(8-9)-10<SUP>-4</SUP>, an increase of (Delta) a/a to 2.2*10<SUP>-3</SUP> was observed for epilayers obtained from the melt with x<SUB>Al</SUB><SUP>1</SUP>equals0.06.
The effects of various chemical treatments on (100) GaSb surface with the aim to develop procedures of polishing of GaSb substrates, surface preparation prior to LPE growth, metal and dielectric deposition, fabrication of patterns have been examined. We show that chemomechanical polishing in Br<SUB>2</SUB> - ethylene glycol followed by anodic oxidation and oxide removal enables to fabricate damage free GaSb surface with the roughness of about 1.5 nm. Surface treatment in 30 HCL-1HNO<SUB>3</SUB> followed by 5%HCL etch gives the best results for surface cleaning prior to metal deposition. The optimum pre-epitaxial treatment includes the use of 1M Na<SUB>2</SUB>S solution and H<SUB>2</SUB> anneal. For features patterning 60HCL-1H<SUB>2</SUB>O<SUB>2</SUB>-1H<SUB>2</SUB>O enables etching at rate of approximately 4 micrometers /min, however, to achieve highly anisotropic etching of small size features the use of Ccl<SUB>4</SUB>/H<SUB>2</SUB> plasma is the most suitable.