N. C. Giles

Role of copper in the green luminescence from ZnO crystals

Electron paramagnetic resonance (EPR), photoluminescence, and infrared optical absorption have been used to investigate a ZnO crystal before and after a thermal anneal for 1 h in air at 900 °C. The sample was an undoped high quality crystal grown by the chemical vapor transport method. In addition to shallow donor impurities, the crystal contained trace amounts of copper ions. Prior to the thermal anneal, these ions were all in the Cu+ (3d10) state and the observed luminescence at 5 K, produced by 364 nm light, consisted of a broad structureless band peaking at 500 nm. After the high-temperature anneal, the Cu2+ (3d9) EPR spectrum was observed and the luminescence had changed significantly. The emission then peaked near 510 nm and showed structure identical to that reported by Dingle [Phys. Rev. Lett. 23, 579 (1969)]. Our data reaffirm that the structured green emission in ZnO is associated with Cu2+ ions. We suggest that the unstructured green emission (observed before the high-temperature anneal) is don...

Temperature dependence of the free-exciton transition energy in zinc oxide by photoluminescence excitation spectroscopy

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies are used to track the temperature dependence of the A exciton energy (EXA) in undoped bulk ZnO crystals grown by the seeded-chemical-vapor-transport method. For T>150 K, the edge emission becomes broad as the A exciton recombination and its longitudinal-optical (LO) phonon replica become superimposed. We use PLE to determine the temperature dependence of EXA by monitoring the broad green emission commonly observed in as-grown ZnO crystals, and thus have established the energy difference between the EXA and PL emission peak energies. The PL emission at 3.26 eV at room temperature is shown to be offset by about 50 meV to lower energy than the actual EXA transition. The temperature dependence of the energy difference between the EXA and PL peaks is compared with predictions based on the lineshape function for the EXA– LO recombination. At 300 K, the PL is predominantly composed of EXA– LO recombination. Further, the temperature depe...

Production of Nitrogen Acceptors in ZnO by Thermal Annealing

Nitrogen acceptors are formed when undoped single crystals of zinc oxide (ZnO) grown by the chemical-vapor transport method are annealed in air or nitrogen atmosphere at temperatures between 600 and 900 °C. After an anneal, an induced near-edge absorption band causes the crystals to appear yellow. Also, the concentration of neutral shallow donors, as monitored by electron paramagnetic resonance (EPR), is significantly reduced. A photoinduced EPR signal due to neutral nitrogen acceptors is observed when the annealed crystals are exposed to laser light (e.g., 364, 442, 458, or 514 nm) at low temperature. The nitrogens are initially in the nonparamagnetic singly ionized state (N−) in an annealed crystal, because of the large number of shallow donors, and the light converts a portion of them to the paramagnetic neutral acceptor state (N0).

Remote hydrogen plasma doping of single crystal ZnO

We demonstrate that remote plasma hydrogenation can increase electron concentrations in ZnO single crystals by more than an order of magnitude. We investigated the effects of this treatment on Hall concentration and mobility as well as on the bound exciton emission peak I4 for a variety of ZnO single crystals–bulk air annealed, Li doped, and epitaxially grown on sapphire. Hydrogen increases I4 intensity in conducting samples annealed at 500 and 600 °C and partially restores emission in the I4 range for Li-diffused ZnO. Hydrogenation increases carrier concentration significantly for the semi-insulating Li doped and epitaxial thin film samples. These results indicate a strong link between the incorporation of hydrogen, increased donor-bound exciton PL emission, and increased n-type conductivity.

Production of native donors in ZnO by annealing at high temperature in Zn vapor

Zinc oxide crystals grown by the seeded chemical vapor transport method have been annealed in zinc vapor at 1100 °C for 30 min. These thermochemical reduction treatments produce a deep red coloration in the crystals and increase their n-type electrical conductivity. Electron paramagnetic resonance (EPR), optical absorption, and Hall measurements were used to monitor changes in the crystals. After an anneal, an intense optical absorption band is present that extends from the band edge out to approximately 550 nm, and the EPR signal near g=1.96 (due to shallow donors and/or conduction-band electrons), the free-carrier absorption, and the Hall electron concentration are all larger. Hydrogen was not present during these anneals, thus leaving oxygen vacancies and zinc interstitials as candidates for the added donors. Neutral oxygen vacancies are produced at high temperature by the additive-coloration mechanism, and are responsible for the broad near-edge absorption band. The observed increase in the number of ...

Growth of high mobility n‐type CdTe by photoassisted molecular beam epitaxy

We report details of the successful controlled substitutional doping of CdTe films with indium. These n‐type films were prepared using a new technique, photoassisted molecular beam epitaxy, in which the substrate is illuminated during the deposition process. In the present work, an argon ion laser was used as an illumination source. The incident light was found to produce immediate and significant changes in the electrical properties of the films. In particular, highly activated n‐type CdTe:In layers resulted.

Growth of (100)CdTe films of high structural perfection on (100)GaAs substrates by molecular beam epitaxy

Growth of epitaxial (100) CdTe films on (100) GaAs substrates by molecular beam epitaxy is discussed. X‐ray diffraction, UV reflectance, photoluminescence, and transmission electron microscopy techniques were employed to characterize the film specimens. The high structural perfection of the layers was evidenced by line dislocation densities of ≤104/cm2 at the free surface of films ≂6.6 μm thick and by measurable excitonic photoluminescence (∼1.504 eV) at room temperature. The CdTe epilayers were smooth and mirrorlike in appearance.

Microstructure, dangling bonds and impurities in activated carbons

Wide-angle X-ray diffraction (WAXD), magnetometry, electron spin resonance (ESR) and Raman spectroscopy are used to investigate three activated PICA carbons (viz. GX203, P1400, Med 50) with surface areas of 1000, 1150 and 2000 m2/g, respectively. Although WAXD showed the absence of any minerals in these carbons, temperature and magnetic field variations of the magnetization shows significant magnetic impurities with concentrations in the order GX203>Med 50>P1400. Analysis of WAXD and Raman spectra shows these carbons to consist of graphitic crystallites with La≃30 A and Lc≃10 A. All three carbons give an ESR line due to dangling bonds with g=2.0028(3) but the linewidth ΔH and the spin concentration Ns are strongly affected by air/oxygen. The linewidths are reduced from ΔH∼4 Oe for the as-received samples to ΔH∼1.2 Oe for the evacuated samples. The Ns values are enhanced upon evacuation, with the enhancement factors of about 40, 130 and 670 for GX203, P1400 and Med 50 respectively, correlating well with the increase in the relative surface areas of these carbons. Magnetization studies using a SQUID magnetometer show that below about 30 K, all three carbons yield Curie-law variations with Ns∼1019/gram, in good agreement with the Ns values obtained from the room temperature ESR studies on evacuated samples.

Further characterization of oxygen vacancies and zinc vacancies in electron-irradiated ZnO

Electron paramagnetic resonance (EPR) has been used to monitor oxygen vacancies and zinc vacancies in a ZnO crystal irradiated near room temperature with 1.5MeV electrons. Out-of-phase detection at 30K greatly enhances the EPR signals from these vacancies. Following the electron irradiation, but before illumination, Fe3+ ions and nonaxial singly ionized zinc vacancies are observed. Illumination with 325nm laser light at low temperature eliminates the Fe3+ signal while producing spectra from singly ionized oxygen vacancies, neutral zinc vacancies, and axial singly ionized zinc vacancies. This light also produces EPR spectra from zinc vacancies having a OH− ion at an adjacent oxygen site. The low-temperature response of the irradiated crystal to illumination wavelengths between 350 and 750nm is described. Wavelengths shorter than 600nm convert Fe3+ ions to Fe2+ ions and convert neutral oxygen vacancies to singly ionized oxygen vacancies. Neutral zinc vacancies are formed by wavelengths shorter than 500nm as...

p‐type CdTe epilayers grown by photoassisted molecular beam epitaxy

We report the first successful substitutional doping of CdTe films with antimony. These p‐type epilayers were prepared using a new technique, photoassisted molecular beam epitaxy, in which the substrate is illuminated during the film deposition process. Illumination was found to produce immediate and favorable changes in the electrical properties of the CdTe:Sb films such that highly activated p‐type layers resulted. CdTe:Sb films grown under similar conditions, but in the absence of illumination, were found to be insulating.