Fabrication and study of few-layer PtSe2 films
Few-layer PtSe2 films are promising candidates for applications in high-speed electronics, spintronics and photodetectors. We reported the fabrication of epitaxially aligned PtSe2 films using one-zone selenization of pre-sputtered platinum layers on the c-plane sapphire substrate. The use of elevated temperature led to enhanced crystallinity and better electrical properties.1 Moreover, we have identified the Se : Pt ratio as a parameter controlling the charge carrier mobility in the films. The mobility increases more than twice as the ratio changes in a narrow interval around a value of 2.2 Combining a semiconductor with a superconductor on a single platform is expected to provide devices with improved performance. we report on the fabrication of a few-layer PtSe2 film on top of an NbN substrate. We found the selenization parameters preserving the chemical and structural integrity of both the PtSe2 and NbN films. The PtSe2 film alignment can be tuned by varying the nitrogen flow rate through the reaction chamber.3
Fig. 1 Se : Pt ratio vs. charge carrier mobility and carrier concentration of PtSe2 films prepared by selenization of 1 nm thick Pt layer at 550 °C during 30 min with different N2 flow rate
Fig. 2. (a) XRD pattern of NbN substrate layer and PtSe2 films prepared by selenization of 3 nm thick platinum layer at 550 °C on the NbN substrate using different nitrogen flow rates and annealing times. (b) Schematic sketch showing the arrangement of a PtSe2 films on an NbN / sapphire substrate.
1, Sojková, M., Dobročka, E., Hutár, P., Tašková, V., Pribusová Slušná, L., Stoklas, R., Píš, I., Bondino, F., Munnik, F., and Hulman, M.: High carrier mobility epitaxially aligned PtSe2 films grown by one-zone selenization, Applied Surface Sci 538 (2021) 147936.
2. Hrdá, J., Tašková, V., Vojteková, T., Pribusová Slušná, L., Dobročka, E., Píš, I., Bondino, F., Hulman, M., and Sojková, M.: Tuning the charge carrier mobility in few-layer PtSe2 films by Se: Pt ratio, RSC Adv. 11 (2021) 27292. (APVV 17-0352, 17-0560, 19-0365, VEGA 2/0059/21, CEMEA ITMS 313021T081)
3. Sojková, M., Hrdá, J., Volkov, S., Vegso, K., Shaji, A., Vojteková, T., Pribusová Slušná, L., Gál, N., Dobročka, E., Šiffalovič, P., Roch, T., Gregor, M., and Hulman, M.: Growth of PtSe2 few-layer films on NbN superconducting substrate, Applied Phys. Lett. 119 (2021) 013101.
Schottky barrier height inhomogeneity in 4H-SiC surface barrier detectors
We investigate the Schottky barrier height of Au/Ni/4H-SiC structures designated for detection of ionizing radiation. The current-voltage and capacitance-voltage measurements were utilized for analyzing of three diode parameters – barriere height, ideality factor and series resistance. We found out that the barrier height is not possible to analyse with only one barriere height of the whole area of a diode. Following we used the analyzis of Gauss barrier height distribution. The experimental current-voltage curves we simulataed with two diffirent influences of seriers resistance on structures using two algorithms. In the first case we assumed that the Schottky diode is created from a lot of parallel non-interacting smaller diodes and each has a different series resistance. In the second case we also assumed a lot of parallel Schottky diodes but with one common series resitance. In both approaches we searched three parameters – mean value of the barrier Gauss distribution, mean square deviation of the distribution and series resistance of diode.
Results shows that current-voltage measurements are good fitted with the first approach while the second has much higher variance with real measurements.
Osvald, J., Hrubčín, L., and Zaťko, B.: Schottky barrier height inhomogeneity in 4H-SiC surface barrier detectors, Applied Surface Sci 533 (2020) 147389.
Tailored growth of MoS2 few-layer films
Few-layer MoS2 films are promising candidates for applications in many areas, such as photovoltaics, photocatalysis, nanotribology, lithium batteries, hydrogenation desulfurization catalysis and dry lubricants, mainly due to their characteristic electronic, optical and catalytic properties. In general, two orientation options of the MoS2 layers are possible – horizontal (with the c-axis perpendicular to the plane of the substrate) and vertical (where the c-axis is parallel to the substrate), having different physicochemical properties. In the preparation of these materials by the sulfurization of molybdenum layers, the initial thickness of the molybdenum proved to be a critical parameter affecting the final orientation of the MoS2 layers. Unlike standard CVD chambers, where the reaction takes place in a two-zone furnace, we use a single zone furnace where we have a substrate and sulfur together at a high temperature in the center of the furnace. The aim of the work was to study the influence of other annealing parameters on the orientation of layers. The heating rate has been shown to be a critical parameter for the growth mechanism where rapid sulfurization leads to the growth of vertical MoS2 layers and slow sulfur evaporation leads to horizontal growth even for thicker initial molybdenum layers.
GIWAXS reciprocal space maps of MoS2 films on the c-plane sapphire substrate prepared from 3 nm thick Mo layers at 800 °C during 30 min with the heating rate of (a) 25 °C / min, (b) 5 °C / min and (c) 0.5 °C / min. The peaks at ~ 1 Å-1 originate from the (002) diffraction planes.
In addition, the single-zone sulfurization method used allowed the growth of MoS2 on the surface of the CVD microcrystalline diamond layers. This experimental design results in a sulfur-rich environment during the process, and diffusion of sulfur into molybdenum at temperatures below that required for formation of molybdenum carbide prevents the formation of the latter at the Mo-diamond interface. This finding may open a way for growing MoS2 layers on substrates which are otherwise susceptible to a chemical reaction with molybdenum. We have also shown that horizontal and vertical growth of the MoS2 layers is possible, depending on the thickness of the Mo layer as in the case of unstructured substrates. The combination of unique diamond properties and ultra-thin MoS2 layers with tunable crystallographic orientation can offer material properties relevant to a wide range of applications.
SEM images of MoS2 layers grown from (a) 1 nm, (b) 3 nm and (c,d) 6 nm thick Mo films deposited on the microcrystalline CVD diamond substrate. In (d), standing MoS2 flakes are seen on the edge of a diamond crystallite.
- Sojková, M., Végso, K., Mrkývkova, N., Hagara, J., Hutár, P., Rosová, A., Čaplovičová, M., Ludacka, U., Skákalová, V., Majková, E., Šiffalovič, P., and Hulman, M.: Tuning the orientation of few-layer MoS2 films using one-zone sulfurization, RSC Adv. 9 (2019) 29645-29651. IF 3.049, Q SJR 1
- Sojková, M., Šiffalovič, P., Babchenko, O., Vanko, G., Dobročka, E., Hagara, J., Mrkývková, N., Majková, E., Ižák, T., Kromka, A., and Hulman, M.: Carbide-free one-zone sulfurization method grows thin MoS2 layers on polycrystalline CVD diamond, Sci Rep. 9 (2019) 2001. IF 4.011, Q SJR 1
Study of semiconductor detectors of ionizing radiation based on 4H-SiC epitaxial layer
Semiconductor detectors of ionizing radiation are important area of research activity because of their utilization in many areas of human activities like monitoring radiation in nuclear energetics, medicine, cosmic research and so on. Because detectors operate at harsh environment by definition, there is much effort to search for base semiconductor material with high radiation and temperature resistance. Silicon carbide is very promissing material. Our research is also concentrated on radiation resistance characteristics of prepared detector samples and their comparision with standartly used Silicon detectors. Picture below shows comparison of Si and SiC detectors and their energy resolution worsering after 5 MeV electrons irradition. You can see that Si detector lost its energy resolution of testing X-ray peaks generated by 241Am radioisotope after dose of 1 kGy but SiC is still relatively good after dose of 30 kGy.
Zaťko, B., Hrubčín, L., Šagátová, A., Osvald, J., Boháček, P., Zápražný, Z., Sedlačková, K., Sekáčová, M., Dubecký, F., Skuratov, V.A., Korytár, D., and Nečas, V.: Schottky barrier detectors based on high quality 4H-SIC semiconductor: electrical and detection properties, Applied Surface Sci 461 (2018) 276-280.
Hrubčín, L., Gurov, J.B., Zaťko, B., Mitrofanov, S.V. Rozov, S.V., Sedlačková, K., Sandukovskij, V.G. Semin, V.A., Nečas, V., and Skuratov, V.A.: Characteristics of Si and SiC detectors at registration of Xe ions, J. Instrument. 13 (2018) P11005.
Thermo-mechanical analysis of uncooled La0.67Sr0.33MnO3 microbolometer made on circular SOI membrane
We have introduced design, technology as well as measurement of physical parameters of antenna-coupled microbolometer, used for broadband detection of terahertz electromagnetic spectrum. This microbolometer features an application of La0.67Sr0.33MnO3 layer (shortened LSMO) grown on multilayered material stack ensuring lattice matching of the sensing LSMO layer to silicon. By virtue of bulk micromachining of silicon-on-insulator substrates, the sensing structure is built on thin suspended membrane to provide weak thermal link, increasing thermal response, thus sensitivity. Additionally, it helps suppressing excitation of in-plane guided surface modes that will otherwise deteriorate the antenna radiation diagram and affect the spectral responsivity of the sensor. Finally, the operation of sensor is demonstrated using a molecular laser setup at 762 GHz (and also 1.4 THz) emission line. The parameters of the fabricated microbolometers are analyzed in terms of response and time constant. Optimal working temperature of the detectors is about 65 °C.
Figure 1. Pictorial view of antenna-coupled microbolometer, featuring a bolometer disk placed in the center of circular drum membrane defined on SOI substrate. This disk is electrically connected to log-periodic antenna and converts the incident electromagnetic energy to Joule heat. The temperature of the bolometric disk is measured electrically through its resistance, accessible on the extended pads in the corners of the chip.
Figure 2. Photograph of the antenna-coupled microbolometer chip used in the tests.
Figure 3. x-y scan of 762-GHz laser beam over antenna-effective area; color-coded is the normalized lock-in amplifier signal magnitude.
Rýger, I., Lobotka, P., Steiger, A., Chromik, Š., Lalinský, T., Raida, Z., Pítra, K., Zehetner, J., Španková, M., Gaži, Š., Sojková, M., and Vanko, G.: Uncooled antenna-coupled microbolometer for detection of terahertz radiation, J. Infrared, Millimet., Terahertz Waves 42 (2021) 462–478.
Š.Chromik, M.Španková, M.Talacko, E.Dobročka,T.Lalinský: Some peculiarities at preparation of Bi4Ti3O12 films for bolometric applications, Applied Surf. Sci. 461 (2018) 39. https://doi.org/10.1016/j.apsusc.2018.06.059
Lalinský, T., Dzuba, J., Vanko, G., Kutiš, V., Paulech, J., Gálik, G., Držík, M., Chromik, Š., and Lobotka, P.:Thermo-mechanical analysis of uncooled La0.67Sr0.33MnO3 microbolometer made on circular SOI membrane. Sensors Actuators A 265 (2017) 321–328.
Kutiš, V., Paulech, J., Gálik, G., Lalinský, T., Vanko, G., Hrabovský, J., Jakubec, J., : Thermal analysis of microbolometer In: Proc. 23th Inter. Conf. on Applied Phys. of Cond. Matter (APCOM 2017). Eds. J. Vajda and I. Jamnický. Bratislava: FEI STU 2017. ISBN 978-80-227-4699-1. P. 267-271.
Detectors of neutrons based on 4H-SiC high quality epitaxial layer
We have prepared and optimized 4H-SiC detector structures with Schottky contact for detection of fast and thermal neutrons. Typicaly used silicon detectors have short lifetime due to high radiation damage coefficient of neutrons. New semiconductor materials with higher radiation resitance are demanded where SiC is one of the most importante candidate, which is able to operate also at high temperatures (more then several hundred of degrees Celsius). The expected utilization of SiC detectors is also close to nuclear or fusion reactors for monitoring of neutron flux. The conversion layer type is able to change sensitivity of detectors for various type of neutrons (like thermal or fast). Our goal was the optimization of electrical and detection properties using simulation tools and following realization of detectors and its experimental verification.
The spectral response of 4H-SiC detector of thermal neutrons with different thickness of conversion layer (left picture). The response of 15 MeV fast neutrons with and without HDPE conversion layer (right picture).
Zaťko, B., Šagátová, A., Sedlačková, K., Nečas, V., Dubecký, F., Solar, M., Granja, C., : Detection of fast neutrons from D-T nuclear reaction using 4H-SiC radiation detector. Inter. J. Modern Phys.: Conf. Ser. 44 (2016) 1660235.
Sedlačková, K., Šagátová, A., Zaťko, B., Nečas, V., Solar, M., Granja, C., : MCNPX simulations of the silicon carbide semiconductor detector response to fast neutrons from D-T nuclear reaction. Inter. J. Modern Phys.: Conf. Ser. 44 (2016) 1660226.
Zaťko, B., Šagátová, A., Sedlačková, K., Boháček, P., Sekáčová, M., Kohout, Z., Granja, C., Nečas, V., :Radiation detector based on 4H-SiC used for thermal neutron detection. J. Instrument. 11 (2016) C11022.
Zaťko, B., Hrubčín, L., Šagátová, A., Boháček, P., Dubecký, F., Sedláčková, K., Sekáčová, M., Arbet, J., Nečas, V., and Skuratov, V.: Particle detectors based on 4H-SiC epitaxial layer and their properties In: ASDAM 2016. Eds. Š. Haščík et al. IEEE 2016. ISBN 978-1-5090-3081-1. P. 141-144.