Direct proof of two-electron occupation of Ge-DXcenters in GaAs codoped with Ge and Te

Abstract

In this paper we present the first direct and unambiguous evidence that the total number of electrons which can be trapped on GQ-DX centers is exactly twice as large as the number of electrons which can be bound on Gc-A \ donor states. It should be emphasized that our reasoning does not require any technological information, about either doping or the compensation of a sample.PACS numbers: 71.55. Eq, 61.72.Vv It is well known that many substitutional donors in semiconducting compounds, beside normal hydrogenlike donor states, can also form highly localized electronic states whose energy relative to the T conduction band minimum is highly pressure dependent. For some of these states there exist potential barriers for both electron emission and electron capture processes, leading to nonequilibrium occupation of these states by electrons at low temperatures-i.e., so-called metastable effects. These are called DX states. There are also other localized states with no apparent metastable effects present-see, e.g., the results of anitcrossing between a shallow donor state and a highly localized one, both originating from the same impurity, observed for w-InSb [l] or GaAs:Ge [2]. For simplicity, following some authors, such highly localized states will throughout this paper be called "A\ states," just to distinguish them from DAMike ones-no matter whether the name is justified or not. There are some rare but very interesting cases for which both types of highly localized states (originating from the same impurity or defect) coexist and can both be populated by electrons in the same experiment. Such a population can be induced, e.g., by hydrostatic pressure which shifts the appropriate electronic levels down with respect to the conduction band minimum and the Fermi level. However, at low temperatures (i.e., well below a certain temperature T c ) due to potential barriers for emission and capture of electrons the population of DX-likc states is nonequilibrium (frozen) and the pressure shift of the corresponding energy level does not influence its occupation. At T<^T C its population can be changed only in a persistent way by means of illumination. While the pressure shift of the corresponding energy level can change the population of ZXY-like states only at T> T c , for A\ states there is no such restriction. Accordingly, in such materials as nominally undoped /2-InSb [3], GaSb:S [4], or GaAs:Ge [5], in which both ZXY-like and A \ states are present, hydrostatic presure measurements of the Hall coefficient reveal that at low temperatures the pressureinduced capture of free electrons is governed by a level whose energy position as well as pressure shift differ from those observed for temperatures high enough to populate DX states.One of the most exciting problems related to DX centers was the charge state of the defect occupied with electrons. Since the first suggestions of Khachaturyan, Weber, and Kamiriska [6] and Chadi and Chang [7] that DX is a negatively charged donor (i.e., occupied with two electrons) subjected t...

Metastable One- and Two-Electron donor States in GaAs and CdF2

“…At the ambient pressure, the strongly localized D0 and D -states, are resonant with the conduction band. The energy levels of these states calculated for zero pressure are located at 75 meV and 115 meV., respectively, in agreement with experimental data [2]. The calculated pressure coefficients (-9 meV/kbar) for both the states are nearly equal to each other and also agree with the experimental values [2].…”

“…For GaAs, the results received for the energy positions and pressure coefflcients of the D0 and D-donor states agree with the experimental results [2] for the DX center. This provides a strong suggestion that in GaAs under hydrostatic pressure the DX center can be identified with the substitutional D -state.…”

“…These are: the weakly localized (hydrogen-like) states (d0) of shallow energy levels, strongly localized one-electron states (D0), and strongly localized two-electron states (D -). These three types of donor states were observed in GaAs under hydrostatic pressure [1,2] and in Al x Ga1-x Αs [3]. The one-electron donor states of different localization, i.e., d0 and D0, were observed in CdF2 [4].…”

Brief Review of the Effects of Pressure on Wolframite-Type Oxides

“…Regarding the slope change observed at 16.9 GPa, it is a consequence of a direct to indirect band-gap transition caused by the modification of the electronic band structure under compression [50]. Such band crossing has been also observed in other wolframites, for instance InTaO4 [15].…”

Photoionization of Ge¯-DX State in GaAs

“…One of the features characteristic for Ge-impurity in GaAs is coexistence of Ge--DX state lying about 110 meV above the bottom of the conduction band, with another strongly localized state the socalled Α 1 or G0 state lying about 30 meV below the former [1]. Some theoretical calculations show that for Ge --DX states the broken-bond model usnally accepted for DX-centres in HI-V compounds is appropriate [2].…”