Deep donor levels (D X centers) in III-V semiconductors

Abstract

D X centers, deep levels associated with donors in III-V semiconductors, have been extensively studied, not only because of their peculiar and interesting properties, but also because an understanding of the physics of these deep levels is necessary in order to determine the usefulness of III-V semiconductors for heterojunction device structures. Much progress has been made in our understanding of the electrical and optical characteristics of DX centers as well as their effects on the behavior of various device structures through systematic studies in alloys of various composition and with applied hydrostatic pressure. It is now generally believed that the DX level is a state of the isolated substitutional donor atom. The variation of the transport properties and capture and emission kinetics of the DX level with the conduction-band structure is now well understood. It has been found that the properties of the deep level when it is resonant with the conduction band, and is thus a metastable state, are similar to its characteristics when it is the stable state of the donor. And it has been consistently found that there is a large energy difference between the optical and thermal ionization energies, implying that this deep state is strongly coupled to the crystal lattice. The shifts in the emission kinetics due to the variation in the local environment of the donor atom suggest that the lattice relaxation involves the motion of an atom (the donor or a neighboring atom) from the group-III lattice site toward the interstitial site. Total energy calculations show that such a configuration is stable provided that the donor traps two electrons, i.e., has negative U. Verification of the charge state of the occupied DX level is needed as well as direct evidence for its microscopic structure.

Experimental study of negative photoconductivity inn-PbTe(Ga) epitaxial films

“…DX centers in III-V and II-VI semiconductors may be considered a classic example of mixed valence impurity behavior, providing the formation of impurity states with negative correlation energy U. 11 The present state of the theory offers no possibility to predict correctly whether an impurity would reveal a mixed valence behavior in the lead telluride or its solid solutions. It was established experimentally that in addition to the group-III elements In, Tl, and Ga, the transition metal Cr ͑Refs.…”

Anion vacancies as a source of persistent photoconductivity in II-VI and chalcopyrite semiconductors

“…This is the case for the "DX center" [14], where a donor atom such as Te in AlGaAs [15] or Cl in CdZnTe [16] exhibits the conducting "DLS-above-PHS" β-type behavior in its normal substitutional configuration, but after a large lattice relaxation involving bond breaking [17], it exhibits the insulating "DLS-below-CBM" α-type behavior. The hallmark of the DX centers is the phenomenon of persistent photoconductivity (PPC), arising from the light induced configuration change from the non-conducting ground state to the metastable conductive state.…”

Neutron scattering studies of nanomagnetism and artificially structured materials

“…Interestingly, the photoinduced magnetism coincides with the metalinsulator transition in these systems. While photoinduced metal-insulator transitions in semiconductors are well known [147], the creation of collective states, such as superconductivity and magnetism, is a recent development, with important implications.…”