The influence of transition metals (TMs) on the hydriding/dedydriding critical point of NaAlH4 has been studied using the chemical potential method. The theoretical results predict that, if the reaction system reaches equilibrium, most of the TM additives significantly increase its hydriding/dedydriding critical point in the sequence Pd>Co>Zr>Ni>Nb>Hf>Ti>Mn>Fe>V>Cu>Cr and therefore have destabilizing effects on NaAlH4 in the same sequence. Experimentally, the isothermal dehydriding kinetics of NaAlH4 is studied only with Fe, Ti, and FeTi additives. The experimental results show that the destabilizing effect of Fe and Ti additives with coarse particles is very low while more effective destabilizing ability is achieved with fine FeTi particles. These experimental results suggest that the predicted destabilizing effect of TM additive is hindered kinetically due to the coarse particles of additives, and therefore experimental verification of the destabilizing effect of TMs should be performed with nanosized particles.

Zr mono-doped and (Zr,N) co-doped ZnO are investigated by the first-principles calculations. It is found that Zr prefers to substitute Zn site under most growth conditions. The passive (N-Zr-N) complexes create a fully occupied impurity band above the valence-band maximum (VBM) of ZnO, which helps p-type conductivity by reducing the ionization energy, consistent with a new approach to overcome the doping asymmetry [Y.F.Yan, J.B.Li, S.H.Wei, and M.M. Al-Jassim, Phys. Rev. Lett. 98 (2007) 135506]. In comparison with (Ga,N) co-doping, (Zr,N) is found to be probably better dopants to push p-type conductivity in ZnO through the new approach with easier formation of the passive impurity band.

The influence of defect charge state on the magnetism of Cu doped ZnO as well as the Cu defects clustering have been investigated by the first-principles calculations. We demonstrate that p-type ZnO:Cu could have ferromagnetic (FM) property, but n-type ZnO:Cu would not have local magnetic moment. Furthermore, the neutral substitutive Cu defects are found to be favorable in clustering, which maintains the FM ordering.

We distill from first-principles spin-polarized total-energy calculations some practical rules for predicting the magnetic state (ferromagnetic/antiferromagnetic/paramagnetic) of substitutional transition-metal impurity with different charge state in various host crystal groups Ⅳ, Ⅲ-Ⅴ, Ⅱ-Ⅵ, Ⅰ-Ⅲ-Ⅵ2, and Ⅱ-Ⅳ-Ⅴ2 semiconductors. The basic mechanism is the stabilization of a ferromagnetic bond between two transition metals if the interacting orbitals are partially-occupied. These rules are then subjected to quantitative tests, which substantiate the mechanism of ferromagnetism in these systems.We discuss cases where current electronic structure calculations agree with these rules, and identify a few cases where conflicts exist. The effect of doping on transition-metal magnetic properties is also covered by these rules by considering the oxidation state changes due to doping. In addition, we systematically apply these rules to ideal substitutional impurities, contrasting our predictions with experiment. Discrepancies may be used to assess the role of various nonidealities such as presence of additional dopants, precipitates, clusters, or interstitial sites.

The efficiency of CuInSe2 based solar cell devices could improve significantly if CuGaSe2, a wider band gap chalcopyrite semiconductor, could be added to the CuInSe2 absorber layer. This is, however, limited by the difficulty of doping n-type CuGaSe2 and, hence, in its alloys with CuInSe2. Indeed, wider-gap members of semiconductor series are often more difficult to dope than lower-gap members of the same series. We find that in chalcopyrites, there are three critical values of the Fermi energy EF that control n-type doping: (i) EnF,pin is the value of EF where the energy to form Cu vacancies is zero. At this point, the spontaneously formed vacancies (=acceptors) kill all electrons. (ii) EnF,comp is the value of EF where the energy to form a Cu vacancy equals the energy to form an n-type dopant, e.g., CdCu. (iii) EnF,site is the value of EF where the formation of Cd-on-In is equal to the formation of Cd-on-Cu. For good n-type doping, EnF,pin, EnF,comp, and EnF,site need to be as high as possible in the gap. We find that these quantities are higher in the gap in CuInSe2 than in CuGaSe2, so the latter is difficult to dope n-type. In this work, we calculate all three critical Fermi energies and study theoretically the best growth condition for n-type CuInSe2 and CuGaSe2 with possible cation and anion doping. We find that the intrinsic defects such as VCu and InCu or GaCu play significant roles in doping in both chalcopyrites. For group-II cation (Cd, Zn, or Mg) doping, the best n-type growth condition is In/ Ga-rich, and maximal Se-poor, which is also the optimal condition for stabilizing the intrinsic InCu/GaCu donors. Bulk CuInSe2 can be doped at equilibrium n-type, but bulk CuGaSe2 cannot be due to the low formation energy of intrinsic Cu-vacancy. For halogen anion doping, the best n-type materials growth is still under In/ Ga-rich, and maximal Se-poor conditions. These conditions are not best for halogen substitutional defects, but are optimal for intrinsic InCu/GaCu donors. Again, CuGaSe2 cannot be doped n-type by halogen doping, while CuInSe2 can.

The need for spin-injectors having the same zinc-blende-type crystal structure as conventional semiconductor substrates has created significant interests in theoretical predictions of possible metastable “half-metallic” zinc-blende ferromagnets, which are normally more stable in other structure-types, e.g., NiAs. Such predictions were based in the past on differences △bulk in the total energies of the respective bulk crystal (forms szinc blende and NiAs). We show here that the appropriate criterion is comparing difference △epi(as) in epitaxial total energies. This reveals that even if △bulk is small, still for MnAs, CrSb, CrAs, CrTe, △epi＞0 for all substrate lattice constant as, so the zinc-blende phase is not stabilized. For CrS we find △epi(as)＜0, but the system is antiferromagnetic, thus not half-metallic. Finally, zinc-blende CrSe is predicted to be epitaxially stable for as＞6.2 Å and is half metallic.

The wider-gap members of a semiconductor series such as diamond→Si→Ge or AlN→GaN→InN often cannot be doped n-type at equilibrium. We study theoretically if this is the case in the chalcopyrite family CuGaSe2→CuInSe2, finding that: (i) Bulk CuInSe2 (CIS, Eg=1.04 eV) can be doped at equilibrium n-type either by Cd or Cl, but bulk CuGaSe2 (CGS, Eg=1.68 eV) cannot; (ii) result (i) is primarily because the Cu-vacancy pins the Fermi level in CGS farther below the conduction band minimum than it does in CIS, as explained by the “doping limit rule; (iii) Cd doping is better than Cl doping, in that CdCu yields in CIS a higher net donor concentration than ClSe; and (iv) in general, the system shows massive compensation of acceptors (CdⅢ,VCu) and donors (ClSe,CdCu, InCud).

We report density-functional calculations of the ferromagnetic (FM) stabilization energy δ=EFM-EAFM for differently oriented Mn pairs in Ⅲ-V's (GaN, GaP, GaAs) and chalcopyrite (CuGaS2, CuGaSe2, CuGaTe2) semiconductors. Ferromagnetism is found to be the universal ground state (δ＜0) in all cases. The order of FM stability in Ⅲ–V’s is GaN＞GaP＞GaAs, whereas in chalcopyrites it is CuGaS2＞CuGaSe2＞CuGaTe2. Considering both groups, the order is GaN →GaP→GaAs→CuGaS2→CuGaSe2→GaSb≈CuGaTe2. The stronger FM stabilization in Ⅲ-V's is attributed to the stronger covalent coupling between the Mn 3d and the anion p orbitals. In contrast to expectations based on Ruderman-Kittel-(Kasuya)-Yosida, (i)all Mn-Mn pair separations show FM, with no FM to antiferromagnetic oscillations and, (ii) FM is orientationally dependent, with <110> Mn-Mn pairs being the most FM.

The electronic and magnetic properties of Mn doping at either cation sites in the class of Ⅰ-Ⅲ-Ⅵ2 chalcopyrites are studied by first-principles calculation. It is found that Mn doping at the Ⅲ site provides holes and stabilizes the ferromagnetic interaction between neutral Mn defects; the neutral MnoCu also stabilizes the ferromagnetism, although it provides electrons to the conduction band, instead of holes. The ferromagnetic stability is generally weaker when the cation or the anion becomes heavier in these chalcopyrites, i.e., along the sequences CuAlS2→CuGaS2→CuInS2 and CuGaS2→CuGaSe2→CuGaTe2. Interestingly, CuAlO2 in the chalcopyrite structure is predicted to have lower FM energy than CuAlS2 despite its lighter anion and shorter bonds. In general, Ⅲ site substitution gives stabler ferromagnetism than Cu substitution. Thus, the preferred growth conditions are Cu-rich and Ⅲ-poor, which maximize MnⅢ replacement. In n-type samples, when MnⅢ is negatively charged, the antiferromagnetic coupling is preferred. In p-type samples, the ground state of positively charged Mn+Cu is also antiferromagnetism. The main feature of the calculated electronic properties of Mn defect at either Cu or Ⅲ site is explained using a simple picture of dangling bond hybride and crystal-field resonance.

The quest for combining semiconducting with ferromagnetic properties has recently led to the exploration of Mn substitutions not only in binary (GaAs, CdTe), but also in ternary semiconductors such as chalcopyrites ABⅢX2Ⅵ. Here, however, Mn would substitute any of the two metal sites A or B. The site preference of Mn doping in CuMⅢX2Ⅵ chalcopyrite is crucial because it releases different type of carriers: electrons for substitution on the Cu sites, and holes for substitution on the MⅢ sites. Using first-principles calculation we show that Mn prefers the MⅢ site under Cu-rich and Ⅲ-poor conditions, and the Cu site under Ⅲ-rich condition. We establish the chemical potential domains for pure CuAlS2, CuGaS2, CuInS2, CuGaSe2, and CuGaTe2 stability. We show that the solubility of Mn on the MⅢ (Cu) site increases (decreases) as the Fermi level moves toward the conduction-band minimum (n-type conditions). It is further found that domains of chemical stability of all these chalcopyrites may be largely reduced by Mn incorporation.