At present the interest to Coulomb impurity centers in semiconductors, particularly in silicon and germanium, is revived due to their natural zero-dimensional origin . The specific properties of such centers and advancement in modern technology allow one to create, a qubit with optically controlled coherent states [1], or a source of the THz coherent radiation which utilizes the conventional laser scheme or stimulated Raman scattering [2]. Such applications require accurate knowledge of optical excitation and relaxation processes within an impurity center.

In weakly and moderately doped semiconductors, the lifetime of excited states for a shallow impurity center is controlled by phonon-assisted relaxation. Recently [3], the relaxation times for arsenic donor states in bulk germanium have been calculated; these values are encouraging and suggest that the population inversion and THz lasing can be realized under optical pumping.

The present work is devoted to studying the low-temperature relaxation of the excited states of As donors in Ge crystal using a pump-probe technique. We show that the lifetime of lower odd parity 2p states are close to one ns. At the same time, experimental study of the inverse relaxation rate for the first excited state 1s(T₂) yields value not longer than 160 ps. The data obtained are compared with the results of theoretical calculations [3] and confirm the possibility to reach THz amplification on the 2p – 1s(T₂) transitions of optically excited As donors in Ge.

1. K.J. Morse, R. J. S. Abraham, A.D. Abreu et al., Sci. Adv. 3, e1700930, (2017).
2. S. G. Pavlov, R. Kh. Zhukavin, V. N. Shastin et al., Phys. Stat. Sol. (b) 250, 9 (2013).
3. V.V. Tsyplenkov, V.N. Shastin, Semiconductors, 52, 1573 (2018).

Zhukavin R. Kh., Kovalevskii K.A., Choporova Yu. Yu. et al. (Collaboration)

JETP Letters 110, issue 10 (2019)