Search

The Ultraviolet Transient Astronomical Satellite (ULTRASAT) is a scientific UV space telescope that will operate in geostationary orbit. The mission, targeted to launch in 2024, is led by the Weizmann Institute of Science (WIS) in Israel and the Israel Space Agency (ISA). Deutsches Elektronen Synchrotron (DESY) in Germany is tasked with the development of the UV-sensitive camera at the heart of the telescope. The camera's total sensitive area of ~90mm x 90mm is built up by four back-side illuminated CMOS sensors, which image a field of view of ~200 deg2. Each sensor has 22.4 megapixels. The Schmidt design of the telescope locates the detector inside the optical path, limiting the overall size of the assembly. As a result, the readout electronics is located in a remote unit outside the telescope. The short focal length of the telescope requires an accurate positioning of the sensors within +-50 mu along the optical axis, with a flatness of +-10 mu. While the telescope will be at around 295K during operations, the sensors are required to be cooled to 200K for dark current reduction. At the same time, the ability to heat the sensors to 343K is required for decontamination. In this paper, we present the preliminary design of the UV sensitive ULTRASAT camera., Comment: "UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XXII", Proc. SPIE 11821, Optics and Photonics, 11821-21 (August, 2021). Contact authors Rolf Buehler (rolf.buehler@desy.de) and Merlin Barschke (merlin.barschke@desy.de)

An electron-spin resonance technique has been used to examine the influence exerted by the degree of compensation on spin effects in a set of nonmagnetic Ge:As semiconductor samples in the vicinity of the insulator–metal phase transition at temperatures 2 K ≤ T ≤ 80 K. It was found that the behavior of the paramagnetic susceptibility, specific to the Pauli paramagnetism and manifested in the spin density resonance, is drawn deep into the insulator state and is observed up to rather large degrees of compensation K ≈ 0.7 at temperature 20 K ≥ T ≥ 5–10 K. Outside this temperature range, both the quantities rapidly grow with increasing compensation and their behavior approaches the Curie law characteristic of the strong insulator state. The reason for this behavior is that the degeneracy is lifted at high temperatures and, at low temperatures, electron states are localized at the Fermi level as a result of the Coulomb blockade by compensating acceptors of the most closely lying donor states and the resulting formation of a narrow Coulomb band in the metallized impurity band. At temperature T ≥ 5–10 K, the gap is blurred by thermal excitations and has no effect on the manifestation of the Pauli paramagnetism. The behavior of the line width of the electron-spin resonance is determined by the scattering on quasi-stationary electric dipoles created by the Coulomb blockade at low temperatures and by the scattering on phonons at high temperatures. The behavior of the g-factor is qualitatively correlated with the specific features of the paramagnetic susceptibility and spin density: with decreasing temperature it weakly grows in the range of the Pauli paramagnetism and then rapidly increases in the range in which the Coulomb blockade is manifested and a transition occurs to the Curie law. By an analogy with ferromagnetic materials in which such a fast increase in the g-factor is observed, the possibility of a compensation caused ferromagnetic coupling of spins localized on donors. The microscopic model of such a coupling is discussed.

Features of magnetic-field-dependent microwave absorption in HgSe samples doped with Co and Ni impurities in different concentrations are investigated. The electron-spin resonance spectra of weakly coupled Co atoms and peculiarities of the magnetic-absorption variation in a magnetic field passing through the zero value are established. The main parameters of the electron-spin resonance spectra and temperature and angular dependences of microwave absorption in weak fields are determined.

The electron-spin-resonance spectra of transition metal ions Fe in the HgSe matrix are analyzed. The spectra have an appearance typical of the Fe3+ ion, i.e., they consist of five fine-structure lines with a splitting that corresponds to a weak crystal field. The spectra are observed at temperatures of up to 50 K. The spectral lines are deformed and have a Dyson shape. An analysis of the temperature dependences of the line amplitudes shows that a transition from paramagnetic ordering to ferromagnetic ordering occurs with decreasing temperature, with the Curie temperature T ≈ 7 K.

The Ultraviolet Transient Astronomical Satellite is a scientific space mission carrying an astronomical telescope. The mission is led by the Weizmann Institute of Science in Israel and the Israel Space Agency, while the camera in the focal plane is designed and built by Deutsches Elektronen Synchrotron in Germany. Two key science goals of the mission are the detection of counterparts to gravitational wave sources and supernovae. The launch to geostationary orbit is planned for 2024. The telescope with a field-of-view of $\approx200$deg$^2$, is optimized to work in the near-ultraviolet band between $220$ and $280$nm. The focal plane array is composed of four $22.4$-megapixel, backside-illuminated CMOS sensors with a total active area of 90x90mm$^2$. Prior to sensor production, smaller test sensors have been tested to support critical design decisions for the final flight sensor. These test sensors share the design of epitaxial layer and anti-reflective coatings (ARC) with the flight sensors. Here, we present a characterization of these test sensors. Dark current and read noise are characterized as a function of the device temperature. A temperature-independent noise level is attributed to on-die infrared emission and the read-out electronics` self-heating. We utilize a high-precision photometric calibration setup to obtain the test sensors` quantum efficiency (QE) relative to PTB/NIST-calibrated transfer standards ($220$-$1100$nm), the quantum yield for $\lambda < 300$nm, the non-linearity of the system, and the conversion gain. The uncertainties are discussed in the context of the newest results on the setup`s performance parameters. From three ARC options, Tstd, T1 and T2, the latter optimizes out-of-band rejection and peaks in the mid of the ULTRASAT operational waveband (max. QE $\approx80\%$ at $245\mathrm{nm}$). We recommend ARC option T2 for the final ULTRASAT UV sensor.

Organic photodiodes (OPDs) for its interesting optoelectronic properties has the potential to be utilized with complementary metal-oxide-semiconductor (CMOS) circuit for imaging, automotive, and security based applications. To achieve such a hybrid device as an image sensor, it is imperative that the quality of the OPD remains high on the CMOS substrate and that it has a well-connected optoelectronic interface with the underneath readout integrated circuit (ROIC) for efficient photogeneration and signal readout. Here, we demonstrate seamless integration of a thermally deposited visible light sensitive small molecule OPD on a standard commercial CMOS substrate using optimized doped PCBM buffer layer. Under a standard power supply voltage of 3 V, this hybrid device shows an excellent photolinearity in the entire bias regime, a high pixel sensitivity of 2 V/Lux.sec, a dynamic range (DR) of 71 dB, and a low dark leakage current density of 1 nA/cm2. Moreover, the integrated OPD has a minimum bandwidth of 400 kHz. The photoresponse nonuniformity being only 1.7%, achieved under research lab conditions, strengthens the notion that this fully-CMOS compatible technology has the potential to be applied in high-performance large-scale imaging array.

Samples of a gapless HgSe semiconductor with different iron impurity concentrations are investigated. HgSe:Fe samples are examined by the electron-spin-resonance technique. Multiple resonance lines caused by unpaired spins of different origins are analyzed. The properties of electrons localized at shallow impurities are described using a hydrogen-like model. The effect of an internal field on the resonance lines is established. It is found that the conduction of HgSe is not only nonparabolic, but also nonspherical.

Magnetoresistance oscillations are considered in the case of microwave-radiation absorption in HgSe samples with a different Fe-impurity concentration. From the simultaneous analysis of the field and temperature dependences of the Shubnikov–de Haas oscillations, the effective mass, the Dingle temperature, and the quantum-limit field corresponding to the Fermi level are obtained. A method for analyzing the spectra with several oscillation frequencies, i.e., the beating effect, is proposed. The results are compared with data obtained by Hall measurements.