Tunneling Ionization of AutolocalizedDX−Centers in Terahertz Fields

Abstract

Tunneling ionization of DX centers in Al Gal, Sb has been observed in terahertz radiation fields.Tunneling times have been measured for autolocalized and on-site deep impurities. It is shown that in one case the tunneling time is smaller and in the other larger than the reciprocal temperature multiplied by a universal constant due to the different tunneling trajectories. This allows one to distinguish in a direct way between the two types of configuration potentials of impurities. PACS numbers: 71.55.i, 72.20.Ht, 72.40.+w Defect engineering of material properties is one of the key issues of present semiconductor technology. Two remarkable examples of high current interest are the FL2 defect in GaAs [1] and the DX center in III-V ternary alloys like A1GaAs, A1GaSb, etc. [2 -4]. The EL2 centeris a growth defect that is responsible for the semiinsulating properties of GaAs, whereas DX centers are dopant-induced defects. These defects have been studied extensively in the past, stimulated by the phenomenon of metastability that both defects have in common. Here we will deal with the DX center. Shallow n dopants in ternary III-V alloys coexist in two charged states as ionized effective-mass-like donors and as deep negatively charged DX centers. By light illumination the DX center can be made electrically and optically inactive.The large difference between optical e, and thermal activation energies and the presence of persistent photoconductivity observed for these centers has been explained by a model of large lattice relaxation leading to autolocalization [5,6]. Figure 1 presents two adiabatic potential diagrams with a shift in the configuration coordinate of the equilibrium position, which correspond to electron-phonon coupling with [ Fig. 1(a)] and without [ Fig. 1(b)] autolocalization. The configuration of Fig. 1(a) is usually assumed to apply to DX centers, giving a big difference between e, and eT [2 -6]. The configuration of Fig. 1(b) corresponds to onsite impurities. In this case the difference between e, and eT is usually small but can also be very large as shown by Henry and Lang [7] for "state 2" oxygen in GaP. The large difference has been introduced by taking two different vibrational frequencies for the occupied state and the unoccupied state, respectively. The details of the adiabatic potential configuration are of great importance for the nonradiative capture of free carriers. Extremely small capture cross section may result from autolocalization but also from lack of intersection of the bound state and the conduction band as observed in GaP [7].Here we demonstrate that tunnel ionization in terahertz fields [8 -10] allows in a simple way a clear-cut distinction between the two types of potential configurations shown in Fig. 1. The tunneling time, caused by the rearrangement of the lattice during detachment of the electron, is systematically different for both situations. To be more precise, the tunneling time is in one case smaller, in the other bigger, than the reciprocal temperature multiplied by a un...

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