Dilute nitrides are only a subgroup of a much broader class of materials, the highly mismatched alloys (HMAs) whose electronic structure is determined by the anticrossing interaction. Group II-VI dilute oxide (II-O-VI) semiconductors with the anions partially replaced by highly electronegative isoelectronic O atoms are a direct analog of the III-V diluted nitrides. It has recently been demonstrated that group II-O-VI alloys in which highly electronegative O partially replaces the group VI element show behaviors that are similar to those of III-N-V alloys. For example, a dramatic O-induced reduction of the band gap has been reported in Cd1-yMnyOxTe1-x and ZnOxSe1-x. The electronic band structure of the dilute oxides is determined by an anticrossing interaction between localized states of O and the extended states of the semiconductor matrix.
In most instances, e.g. N in GaAs or O in CdTe, the localized states are located within the conduction band and consequently a relatively wide lower subband is formed. This is manifested as a reduction of the energy band gap. A narrow band can be formed only if the localized states occur well below the conduction band edge. Such a case is realized in ZnTe and Zn1-yMnyTe alloys where the O level is located roughly 0.2 eV below the conduction band edge. The BAC model predicts that the anticrossing interaction of the O states with the extended conduction band states in the Zn1-yMnyTe will lead to the formation of a narrow band of intermediate states. With multiple band gaps that fall within the solar energy spectrum, Zn1-yMnyOxTe1-x is extremely well suited for the proposed high efficiency multi-band single-junction solar cells.
The figure on the left shows the PR spectrum from Zn0.88Mn0.12Te samples implanted with 3.3 mole % followed by PLM with laser energy fluence of 0.15 J/cm2. Two optical transitions at ~1.8 and 2.6 eV that are distinctly different from the fundamental band gap transition of the matrix Zn0.88Mn0.12Te (EM=2.32 eV) are observed in Fig. 2. These transitions can be attributed to transitions from the valence band to the two conduction subbands, E+ (~2.6 eV) and E- (~1.8 eV) formed as a result of the hybridization of the localized O states and the extended conduction band states of ZnMnTe. The strong photomodulated transition signals indicate the extended nature of these electronic states and the substantial oscillator strength for the transitions. Using the BAC model, the substitutional O content of the Zn0.88Mn0.12OxTe1-x alloys is estimated to be x»0.01.
Dilute III-V nitrides and II-VI oxides are HMAs in which small amount of the V or VI hosts are substituted by highly electronegative N and O, respectively. Conversely, when a larger, more metallic element substitutes the anions, such as in GaN1-xAsx, ZnS1-xTex and ZnSe1-xTex, it is predicted that the localized impurity states will be located near the host valence band edge and will induce a similar anticrossing interaction that will cause the valence band to restructure. This type of behavior is also expected in dilute GaSbxAs1-x and GaBixAs1-x alloys. The electronic structure of these HMAs can be well described the valence band anticrossing (VBAC ) model. The bandgap reduction is primarily the result of an upward shift of the GaAs-related heavy and light hole bands induced by an anticrossing interaction with those of the impurity atoms. This interaction also modifies the position of the spin-orbit split-off band, and as a result the spin-orbit splitting energy rises with x.