Impurity pairs and relaxation of excitation in silicon doped with group III and V impurities

Abstract

PACS 71.55.Cn, 72.40.+w In Si doped with group III and V impurities at concentrations of 10 16 -10 18 cm −3 infrared absorption bands of impurity pairs (IPs) appear and there is a slow relaxation of polarization hopping photoconductivity (PC) in a microwave (MCW) electric field. IPs create a broad spectrum of local states with ionization and excitation energies both lower and higher than those of single-impurity atoms. It is shown that ionization of IPs in compensated Si and a slow relaxation of re-charge after extrinsic excitation give a principal contribution to PC, due to hopping transitions of carriers between ionized and neutral impurity atoms in IPs induced by a MCW electric field.It has been established [1] that in Si doped with various group III and V impurities at concentrations N > 10 16 cm −3 at low temperatures (T < 20 K) in an 8 mm microwave (MCW) electric field a slow (10 −5 s) component of extrinsic photoresponse U(MCW) appears. It was concluded that the phenomenon is due to an increase of hopping polarization conductivity σ [2] caused by accumulation of captured charge carriers in long-lived excited 1S states with large orbitals, split from the ground states of donors and acceptors by valley -orbit or spin -orbit interactions. However, some subsequent experiments have contradicted the existence of long-lived excited states. Therefore we suggest a new concept of the polarization hopping photoconductivity (PC) in a MCW electric field.We suppose that extrinsic photoexcitation of doped and compensated Si leads to ionization of impurity pairs (IPs) [3,4], i.e. a group of impurities, localized within some distance r of the order of a few Bohr radii a. Hops of non-equilibrium charge carriers between ionized and neutral impurities in IPs, induced by a MCW electric field, should give a principal contribution to polarization hopping PC.To clarify the situation, consider a system of two electrons and two protons, localized at various distances r/a -one-electron terms of the H 2 molecule (Fig. 1) [5]. The solid curves in Fig. 1 correspond to the ground H(1S) + H(1S), the first excited H(1S) + H(1P) and the ionized H + + H(1S) states. The differences between the dashed and solid lines correspond to decreases of ionization and excitation electron energies E as compared to that for r/a = ∞. It is seen from Fig. 1 that significant differences correspond to r/a = 2 -5. Figure 1 also shows the dependence of the contribution of MCW polarization conductivity dσ(MCW)/d(r/a) on r/a, calculated for an 8 mm MCW field for random distribution of atoms [2]. It is clear that the most important hopping transitions again correspond to r/a = 2 -5. Pairs of group III and V impurities are rather like the H 2 molecule, so it could be expected that the impurity atoms in IPs, localized within r/a ~ 2 -5, give the most important contribution to hopping PC in a MCW field.