Abrosimov, N.V.

Astrov, Yu.A.

Choporova, Yu.Yu.

Hübers, H-W.

Klopf, J.M.

Knyazev, B.A.

Kovalevsky, K.A.

Pavlov, S.G.

Redlich, B.

Shastin, V.N.

Tsyplenkov, V.V.

Zhukavin, R.Kh.

English

Repositorium für Naturwissenschaften und Technik

Relaxation of Coulomb States in Semiconductors Probed by FEL Radiation R.Kh. Zhukavin1, K.A. Kovalevsky1, V.V. Tsyplenkov1, S.G. Pavlov2, H-W. Hübers2,3, Yu.Yu. Choporova4,5, B.A. Knyazev4,5, J.M. Klopf6, B. Redlich7, N.V. Abrosimov8, Yu.A. Astrov9, V.N. Shastin1 1Institute for Physics of Microstructures, Nizhny Novgorod, Russia, zhur@ipmras.ru 2Institute of Optical Sensor Systems, German Aerospace Center (DLR), Berlin, Germany 3Department of Physics, Humboldt-Universität zu Berlin, Berlin, Germany 4Budker Institute of Nuclear Physics, Novosibirsk, Russia 5National Research Novosibirsk State University, Novosibirsk, Russia 6Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany 7FELIX Laboratory, Radboud University, Institute of Molecules and Materials, Nijmegen, The Netherlands 8Leibniz-Institut für Kristallzüchtung (IKZ), Berlin, Germany 9Ioffe Institute, St. Petersburg, Russia  The typical semiconductors such as silicon and germanium with embedded donors and acceptors have been intensively studied since 1950th [1]. The main reason was the importance of doped semiconductors in the production of various electronic devices based on the properties of p-n junction or Schottky-junction. The new interest to Coulomb centers in semiconduc-tors has arisen from several new directions including single-atom electronics, quantum logic and lasing based on impurity centers. For use of Coulomb cen-ters in the frame of any of those fields one should know such properties of bound electron as energy spectrum and relaxation rates. The experimental me-thods being used for the investigations compose a very broad set of different techniques involving trans-port and optics and their combinations under various conditions. Last two decades the appearance of new user facilities based on THz free electron lasers (FELs) emitting high power short THz pulses pro-vides the unique opportunity for studying of impurity centers in semiconductors under resonant excitation of chosen energy levels using single color pump-probe technique [2-4]. In particular, the lifetimes of different donors and acceptors in silicon have been measured to localize the time scale of the existing relaxation rates. More recently similar experiments have been performed to explore impurities in germa-nium. Here the authors present a review of pump-probe experiments devoted to lifetime measurements of excited donor states in silicon and germanium un-der the uniaxial stress. As well known the relaxation of shallow donor electrons in semiconductors under low temperatures is governed by the emission of acoustic or optical pho-nons. The elemental semiconductors such as silicon and germanium have multi-valley structure of the conduction band that, together with appropriate ener-gy gaps between states of the Coulomb centers, allow intervalley electron scattering processes. In silicon those are accompanied by the emission of f-type or g-type optical or acoustic phonons. The former corres-ponds to e.g. [100]-[010] transition while the latter to [100] – [-100] transition in the reciprocal space. The uniaxial stress applied along [100] crystal axis re-moves the degeneracy resulting in splitting of valleys and corresponding energy levels. Thus uniaxial stress can modify relaxation rates if changes the energy gaps. Fig. 1 demonstrates the energy diagram of phosphorus donor in silicon under the stress as an example. The excited states move parallel to their “parent” valleys while the singlet ground state 1s(A1), which contains equal contributions from 6 valley un-der zero stress, moves nonlinearly losing the contribu-tion of upper valleys.   Fig. 1. Energy diagram for phosphorus donor in silicon as a function of uniaxial stress along [110] (left) or [100] (right) crystallographic orientation. Only lower excited states and ground states are shown as well as position of conduction band valleys 2Δ and 4Δ. © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(http://creativecommons.org/licenses/by/4.0/). EPJ Web of Conferences 195, 07008 (2018) https://doi.org/10.1051/epjconf/201819507008TERA-2018 The silicon and germanium crystals doped by dif-ferent donors were taken for sample preparation. The silicon samples were cut along 100 axis while germa-nium samples were cut along 111 axis. Those direc-tions were chosen to provide the maximum valley splitting for given stress value. Typical donor concen-trations were in the range of ND=1-5×1015 cm−3. The samples were shaped into parallelepipeds with dimen-sions of ~7×5×2 mm3. Each sample was placed into the special module designed to apply uniaxial stress. The stress module containing one sample and individ-ual stress value was mounted in an optical liquid he-lium flow cryostat and cooled to about 5 K. The achieved stress value in each module was estimated from the splitting of the donor levels as measured by absorption spectroscopy. Relaxation times of particu-lar electronic states were derived from the time-resolved pump-probe technique. This involved the frequency-selective excitation of the investigated sample with an optical pulse, where the excitation photon energy corresponded to the energy of the tran-sition between the probed electronic states. To meas-ure the characteristic decay time τ, the pulse duration of optical excitation must be much shorter than τ and the time delay between pump and probe pulses must be appropriate to resolve τ. The overall data set was obtained using pump-probe experimental setups at several free electron laser facilities: FELIX (The Netherlands), FELBE (Germany) and NovoFEL (Russia). Details of the pump-probe setups are given elsewhere. Spectral width of the FEL emission on half width on half maximum (FWHM) was better than 1 % of a FEL wavelength. The typical micropulse duration was about 10 ps for FELIX and FELBE and 100 ps for NovoFEL.  Fig. 2 demonstrates the pump-probe signals ob-tained for Si:P stressed along [100] when pumped into the lower stress component of 2p0 level for two values of uniaxial stress – 80 and 650 bar. It is clear seen from the picture that higher stress value results in longer lifetime indicating suppression of the f-type phonon emission. Similar results have been obtained for other donor in silicon such as bismuth and arsenic. It has been obtained that typical lifetimes for donors in germanium are much longer (0.5-3 ns) than for acceptors in germanium (150-250 ps) as well as do-nors (100-200 ps) and acceptors (30-100 ps) in sili-con. The stress applied along [111] axis for Ge:Sb results in increase of lifetime for 2p± level from 1.8 ns to about 3.2 ns that can be explained by switching off the intervalley relaxation.  Fig. 2. Pump-probe signal for Si:P under two values of un-iaxial stress (80 bar and 650 bar) along [100]. This work was partly supported by the joint German-Russian project of the Russian Foundation for Basic Research (RFBF Project No. 18-42-520064, 18-502-12077-DFG) and of the Deutsche Forschungsgemein-schaft (DFG No. 389056032). References 1. Ramdas, A. K., Rodriguez, S.  Spectroscopy of the solid-state analogues of the hydrogen atom: donors and acceptors in semiconductors, Rep. Prog. Phys. 1981, V. 4, P. 1291-1387. 2. Hübers, H.-W., Pavlov, S. G., Lynch, S. A., Green-land, Th., Litvinenko, K. L., Murdin, B., Redlich, B., van der Meer, A. F. G., Riemann, H., Abrosimov, N. V., Becker, P., Pohl, H.-J., Zhukavin, R. Kh., Shastin, V. N. Isotope effect on the lifetime of the 2p0 state in phosphorus-doped silicon // Phys. Rev. B. 2013. V. 88, P. 035201(1-5). 3. Pavlov, S. G., Deßmann, N., Pohl, A., Shuman, V. B., Portsel, L. М., Lodygin, А. N., Astrov, Yu. A., Winnerl, S., Schneider, H., Stavrias N., van der Meer, A. F. G., Tsyplen-kov V. V., Kovalevsky, K. A., Zhukavin, R. Kh., Shastin, V. N., Abrosimov, N. V., Hübers, H.-W. Dynamics of nonequi-librium electrons on neutral center states of interstitial mag-nesium donors in silicon // Physical Review B. 2016. V. 94, P. 075208(1-8).  4. Choporova, Yu. Yu., Gerasimov, V. V., Knyazev, B. A., Sergeev, S. M., Shevchenko, O. A., Zhukavin, R. Kh.,  Abrosimov, N. V., Kovalevsky, K. A., Ovchar, V. K., Hübers, H-W., Kulipanov, G. N., Shastin, V. N., Schneider, H., Vinokurov, N. A., Kulipanov, G. N. First terahertz-range experiments on pump–probe setup at Novosibirsk free elec-tron laser // 2016.  Physics Procedia. V. 84, P. 152-156.     2EPJ Web of Conferences 195, 07008 (2018) https://doi.org/10.1051/epjconf/201819507008TERA-2018

Relaxation of Coulomb States in semiconductors probed by FEL radiation

https://oa.tib.eu/renate/bitstream/123456789/11669/1/epjconf_tera2018_07008.pdf

Relaxation of Coulomb States in semiconductors probed by FEL radiation

Abstract

Similar works

Full text

Available Versions

Repositorium für Naturwissenschaften und Technik