Defect redistribution in postirradiation rapid-thermal-annealed InN

We have applied positron annihilation to study point defects in 2 MeV exp 4 He exp + -irradiated and subsequently rapid-thermal-annealed (RTA) InN grown by molecular-beam epitaxy. The irradiation fluences ranged from 5×10 exp 14 to 2×10 exp 16 cm exp −2. The irradiation primarily produces donor defects but the subjects of this work are the acceptor-type defects produced in lower concentrations: VIn, in addition to negative-ion-type defects. The heat treatment results in a redistribution of the irradiation-induced point defects. The In vacancies near the film-substrate interface appear restructured after the RTA process, possibly influenced by growth defects near the interface, while deeper in the InN layer, the defects produced in the irradiation are partially removed in the annealing. This could be responsible for the improved transport properties of the annealed films.Peer reviewe

The Bright Side and the Dark Side of Hybrid Organic Inorganic Perovskites

The previously developed bistable amphoteric native defect (BAND) model is used for a comprehensive explanation of the unique photophysical properties and for understanding the remarkable performance of perovskites as photovoltaic materials. It is shown that the amphoteric defects in donor (acceptor) configuration capture a fraction of photoexcited electrons (holes) dividing them into two groups: higher energy bright and lower energy dark electrons (holes). The spatial separation of the dark electrons and the dark holes and the k-space separation of the bright and the dark charge carriers reduce electron hole recombination rates, emulating the properties of an ideal photovoltaic material with a balanced, spatially separated transport of electrons and holes. The BAND model also offers a straightforward explanation for the exceptional insensitivity of the photovoltaic performance of polycrystalline perovskite films to structural and optical inhomogeneities. The blue-shifted radiative recombination of bright electrons and holes results in a large anti-Stokes effect that provides a quantitative explanation for the spectral dependence of the laser cooling effect measured in perovskite platelets

Mechanisms of Schottky Barrier Control on n-Type Germanium Using Ge3N4 Interlayers

The influence of a few monolayers of crystalline and amorphous Ge3N4 on the Schottky barrier height of n-type Ge has been investigated. Low temperature capacitance–voltage measurements are used to accurately determine the barrier height. Both amorphous and epitaxial Ge3N4 effectively eliminate pinning of the Fermi level at the metal/Ge interface. Metal/Ge3N4/n-Ge contacts therefore show a linear dependence of the Schottky barrier height with metal work function. Our results indicate that the Fermi level unpinning is achieved mainly due to the passivation of interface states related to defects at the metal/Ge interface. Aluminum on amorphous and epitaxial Ge3N4 delivers barrier heights of 0.09 ± 0.05 and 0.0 ± 0.1 eV, respectively, resulting in Ohmic behavior. The formation of epitaxial Ge3N4 requires temperatures above 600 °C, whereas amorphous layers can be formed at much lower temperatures. Amorphous Ge3N4 can therefore be used to form Ohmic contacts at a low thermal budget.status: publishe

Strain relaxation of CdTe films growing on lattice-mismatched substrates

We have deposited CdTe films by laser-assisted epitaxy approach and investigated the influence of substrate and film thickness on the film properties. Grown on Si(001), GaAs(001), and quartz substrates; the CdTe films exhibit preferential orientation along the cubic CdTe(111) direction. When the films are thin (<500 nm), a blueshift of the band gap and splitting of valence bands were observed. These results are attributed to the existence of residual strains induced by mismatch of the film lattice constant with that of the substrate, and by their difference in thermal expansion coefficients. The bulk band-gap energy of 1.5 eV was achieved on the surface of thick CdTe films grown on Si(001) substrate, indicating that strain was almost completely relaxed in this case. Our results demonstrate that by a proper selection of substrate and film thickness it is possible to grow film semiconductors with band gap approaching those of bulk crystals

Strain relaxation of CdTe films growing on lattice-mismatched substrates

We have deposited CdTe films by laser-assisted epitaxy approach and investigated the influence of substrate and film thickness on the film properties. Grown on Si(001), GaAs(001), and quartz substrates; the CdTe films exhibit preferential orientation along the cubic CdTe(111) direction. When the films are thin (<500 nm), a blueshift of the band gap and splitting of valence bands were observed. These results are attributed to the existence of residual strains induced by mismatch of the film lattice constant with that of the substrate, and by their difference in thermal expansion coefficients. The bulk band-gap energy of 1.5 eV was achieved on the surface of thick CdTe films grown on Si(001) substrate, indicating that strain was almost completely relaxed in this case. Our results demonstrate that by a proper selection of substrate and film thickness it is possible to grow film semiconductors with band gap approaching those of bulk crystals

Arsenic Antisite-Related Defects in Low-Temperature MBE Grown GaAs

GaAs layers grown by the molecular beam epitaxy (MBE) method at low temperatures (200-degrees-C) and also MBE samples grown at 300-degrees-C highly doped with Be or Si show high concentrations of As antisite-related defects in the optical absorption and in the magnetic circular dichroism of the absorption (MCDA). With optical detection of EPR it is shown that these antisite-related defects have properties similar to those of the EL2 defects except for the EL2 bleaching characteristics and the so called zero phonon line. Their spin-lattice relaxation time is strongly reduced compared with that of EL2+. These differences may be related to their high concentrations, which are of the order of approximately 10(19)-10(20) cm-3. In MBE samples grown at higher temperatures (325-degrees-C, 400-degrees-C) a new As antisite-related defect was detected with a reduced As-75 hyperfine splitting as compared with that of EL2+. This new As antisite-related defect has properties very similar to those of another As antisite-related defect previously detected in horizontal Bridgman n-type GaAs. A comparison of four different As antisite-related defects with similar reduced As-75 hyperfine splittings is presented

Transition metal elements as donor dopants in CdO

CdO has been shown to achieve a high electron concentration N (>1021cm-3) and at the same time a high mobility μ (>100cm2/Vs) when doped with conventional shallow dopants (In or Ga), and consequently making it a transparent conducting oxide with very low resistivity ρ<10-4ωcm. In this work, the properties of CdO thin films doped with a series of transition metal elements (CdO:TM) with partially filled 3d and 4d shells, including Sc, Ti, V, Cr, Fe, Y, Mo, and W, were investigated. We find that doping with these TM elements can effectively increase the N in CdO to a maximum N (Nmax) of ∼(7-12)×1020cm-3 with a dopant concentration xmax of 4-7 %. However, unlike CdO:In, the μ of CdO:TM films drops rapidly from >100 to <10cm2/Vs as the dopant concentration x increases, so that they can only achieve a minimum ρ of ∼(1-2)×10-4ωcm, ∼ a factor of 2-3 higher than that in CdO:In. As a result, free-carrier absorption and plasma reflection effects limit their optical transparency to <1200 nm. For most 3d TM dopants, a qualitatively higher d-donor level Ed,donor gives rise to higher EF,max or a higher Nmax. Although at low x, the optical band gap Eopt of CdO:TM follows the calculated values due to free-carrier effects, as x increases, Eopt values are significantly higher than the calculated values. This is believed to be an effect of the anticrossing interaction of the localized d-levels and the extended CdO conduction-band (CB) states, giving rise to a lower occupied E- and an upper unoccupied E+ subband. The restructured CBs have much flatter dispersion, which also results in a much higher effective mass me∗, hence it can also explain the much lower μ of CdO:TM films with high N