We show that N-doped ZnO films grown on sapphire can exhibit significant (∼1018 cm−3) room-temperature p-type behavior when sufficient nitrogen (N) is incorporated and the material is annealed appropriately. Substitutional N on the oxygen (O) sublattice is a deep acceptor; however, shallow acceptor complexes involve N, H, and zinc vacancies (VZn). Combining secondary ion mass spectrometry, Raman-scattering, photoluminescence, and Hall-effect data, we establish the evolution of N from its initial incorporation on a Zn site to a final shallow acceptor complex VZn_NO_H+ with an ionization energy of ca. 130 meV. This complex is responsible for the observed p-type behavior.

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Kasama, Takeshi; Thuvander, M.; Siusys, A.; Gontard, L.C.; Kovács, A.; Yazdi, Sadegh; Duchamp, M.; Dunin-Borkowski, R.E.; Gustafsson, A.; Sadowski, J. Doping mechanisms of Mn in GaAs nanowires (NWs) that have been grown self-catalytically at 600 C by molecular beam epitaxy (MBE) are investigated using advanced electron microscopy techniques and atom probe tomography. Mn is found to be incorporated primarily in the form of non-magnetic tetragonal Ga 0.82 Mn 0.18 nanocrystals in Ga catalyst droplets at the ends of the NWs, while trace amounts of Mn (22 6 4 at. ppm) are also distributed randomly in the NW bodies without forming clusters or precipitates. The nanocrystals are likely to form after switching off the reaction in the MBE chamber, since they are partially embedded in neck regions of the NWs. The Ga 0.82 Mn 0.18 nanocrystals and the low Mn concentration in the NW bodies are insufficient to induce a ferromagnetic phase transition, suggesting that it is difficult to have high Mn contents in GaAs even in 1-D NW growth via the vapor-liquid-solid process. V C 2015 AIP Publishing LLC.

Semiconductor nanowires have been identified as a viable technology for next-generation infrared (IR) photodetectors with improved detectivity and detection across a range of energies as well as for novel single-photon detection in quantum networking. The GaAsSb materials system is especially promising in the 1.3-1.55 μm spectral range. In this work we present bandgap tuning up to 1.3 μm in GaAs/GaAsSb core-shell nanowires, by varying the Sb content using Ga-assisted molecular beam epitaxy. An increase in Sb content leads to strain accumulation in shell manifesting in rough surface morphology, multifaceted growths, curved nanowires, and deterioration in the microstructural and optical quality of the nanowires. The presence of multiple PL peaks for Sb compositions 12 at.% and degradation in the nanowire quality as attested by broadening of Raman and x-ray diffraction peaks reveal compositional instability in the nanowires. Transmission electron microscope (TEM) images show the presence of stacking faults and twins. Based on photoluminescence (PL) peak energies and their excitation power dependence behavior, an energy-band diagram for GaAs/GaAsSb core-shell nanowires is proposed. Optical transitions are dominated by type II transitions at lower Sb compositions and a combination of type I and type II transitions for compositions 12 at.%. Type I optical transitions as low as 0.93 eV (1.3 μm) from the GaAsSb for Sb composition of 26 at.% have been observed. The PL spectrum of a single nanowire is replicated in the ensemble nanowires, demonstrating good compositional homogeneity of the latter. A double-shell configuration for passivation of deleterious surface states leads to significant enhancement in the PL intensity resulting in the observation of room temperature emission, which provides significant potential for further improvement with important implications for nanostructured optoelectronic devices operating in the near-infrared regime.

Self-catalyzed growth of axial GaAs1−xSbx nanowire (NW) arrays with bandgap tuning corresponding to the telecommunication wavelength of 1.3 µm poses a challenge, as the growth mechanism for axial configuration is primarily thermodynamically driven by the vapor-liquid-solid growth process. A systematic study carried out on the effects of group V/III beam equivalent (BEP) ratios and substrate temperature (Tsub) on the chemical composition in NWs and NW density revealed the efficacy of a two-step growth temperature sequence (initiating the growth at relatively higher Tsub = 620 °C and then continuing the growth at lower Tsub) as a promising approach for obtaining high-density NWs at higher Sb compositions. The dependence of the Sb composition in the NWs on the growth parameters investigated has been explained by an analytical relationship between the effective vapor composition and NW composition using relevant kinetic parameters. A two-step growth approach along with a gradual variation in Ga-BEP for offsetting the consumption of the droplets has been explored to realize long NWs with homogeneous Sb composition up to 34 at.% and photoluminescence emission reaching 1.3 µm at room temperature.