Toshikazu Suda

Band-gap energy and electron effective mass of polycrystalline Zn3N2

Zn3N2 polycrystalline films with n+-type conductivity have been grown by metalorganic chemical vapor deposition and rf-molecular beam epitaxy with carrier concentration in the range between 1019 and ∼1020cm−3. Oxygen contamination without an intentional doping was found to be a cause of high electron concentration, leading to a larger band-gap energy due to Burstein-Moss shift. The significant blue shift of the optical band gap Eopt with increasing carrier concentration ne obeys the relation Eopt=1.06+1.30×10−14ne2∕3. This evaluation enables the conclusion that the actual band-gap energy of Zn3N2 is 1.06eV. Electron effective mass m* for Zn3N2 has been deduced from Fourier transform infrared reflectivity measurements to be (0.29±0.05)mo.

Epitaxial growth of zinc phosphide

We report the first epitaxial growth of Zn3P2. The Zn3P2 epitaxy was obtained on (100) GaAs and ZnSe/GaAs by metalorganic chemical vapor deposition at growth temperatures of 350–400 °C and V/II ratios above 0.8 using dimethylzinc and PH3. The stoichiometry was assessed by x‐ray photoelectron spectroscopy at Zn/P=1.5. A Hall mobility of 310 cm2/Vs at 300 K was obtained with a hole concentration of 3.5×1016 cm−3 for Zn3P2 grown at 380 °C. The absorption edge deduced is 1.51 eV at 300 K. Photoluminescence data suggest four acceptor levels (45, 65, 86, and 104 meV at 12.8 K).

Electrical Properties of Woodceramics

Woodceramics have recently attracted much attention as ecomaterials at low cost. Electrical properties of the woodceramics (WCM hereafter) have been characterized in the range 10–70% relative humidity and for temperatures from −20 to 100°C. The WCM bulk has been prepared by burning MDF board impregnated with phenolic resin at 650 and 750°C. Electrical resistance decreased linearly with increasing temperature, indicating the negative temperature coefficient like semiconductor. Relative humidity dependence of the resistance also indicated excellent linear characteristics between 10 and 70% RH measured here. Activation energies of 0.21, 0.15, 0.05, and 0.01 eV have been revealed from van der Pauw method. The excellent linearity for humidity and temperature is prominent advantages of WCM which may be useful as a new humidity and also temperature sensor.

N‐type zinc phosphide grown by molecular beam epitaxy

We report on the results of Hall effect and photoluminescence (PL) in n‐type Zn3P2 grown by molecular beam epitaxy. The Zn3P2 thin films indicated n‐type conductivity instead of the usual p‐type conductivity due to a strong self‐compensation effect with Hall mobility and carrier concentration of 3–7×103 cm2/Vs and 3–9×1010 cm−3 at room temperature, respectively. Donor levels of 0.01 and 0.73 eV from the conduction band were identified by resistivity and Hall effect measurements. The PL spectra show donor‐acceptor pair emission near 1.41 eV at 20 K ascribed to an acceptor level of 0.26 eV from the valence band.

Zn3P2 epitaxial growth by MOCVD

Abstract Epitaxial layers of zinc phosphide (Zn 3 P 2 ) have been grown, for the first time, on (100) GaAs substrate by metalorganic chemical vapor deposition (MOCVD) using dimethylzinc (DMZ) and 5% phosphine (PH 3 ) diluted with H 2 gas as reactants. The PH 3 gas was cracked at 800 ° C to lower the growth temperature for the purpose of suppressing the occurence of microcracks in the layers. The growth temperature and PH 3 /DMZ molar ratio were varied from 280 to 480 ° C and from 0.2 to 3.2, respectively. Reflection high energy electron diffraction (RHEED) shows a distinct and clear spot pattern indicating high crystalline quality and three-dimensional epitaxial growth at growth temperatures between 350 and 400 ° C, and V/II ratio above 0.8. The resistivity decrease as the growth temperature increases because of an increase in acceptor concentration. The absorption edge evaluated from the transmissivity and reflectivity data is 1.51 eV at 300 K.

Zn3P2 thin films grown on glass substrates by MOCVD

Zinc phosphide (Zn 3 P 2 ) polycrystalline thin films have been grown by low-pressure metalorganic chemical vapor deposition on glass substrates. Electrical properties have been evaluated for the films grown at growth temperatures from 200 to 400°C and with the P/Zn molar ratio fixed at 5.7. The growth of Zn 3 P 2 is limited by surface reaction at growth temperatures below around 240°C and by mass transfer from the gas phase to the surface at 260-380°C. The preferential orientation along the c-axis became dominant at 260-290°C. The rate for decomposition of Zn 3 P 2 and the gas-phase reaction had to be considered above around 390°C, The electrical conductivity of the films was found to increase with raising growth temperature. From the temperature dependence of electrical conductivity, four levels were obtained with activation energies of 0.06 eV, 0.16 eV, 0.21 eV and 0.66 eV for the Zn 3 P 2 films. The concentration of levels increased as the growth temperature was raised.

Zn3P2 photovoltaic film growth for Zn3P2/ZnSe solar cell

Abstract Zinc phosphide (Zn 3 P 2 ) thin films have been grown by photo-metalorganic chemical vapor deposition (photo-MOCVD) and also MOCVD onto n-type ZnSe single crystal aiming at p-Zn 3 P 2 /n-ZnSe solar cells. A good diode characteristic was observed in p-Zn 3 P 2 /n-ZnSe heterojunctions fabricated with annealing in hydrogen at 400°C. An open-circuit voltage ( V oc ) of 0.4–0.5 V, a fill factor of 0.6, and a short-circuit current of 2–9 μA/cm 2 were obtained under simulated air mass 1.5 illumination without anti-reflective coating. One of the reason with this small short-circuit current is the high resistivity and ohmic contact problem of ZnSe. A spectral response of V oc indicates a good band-pass behavior of the heterojunction between 1.5 and 2.7 eV, which are bandgaps of Zn 3 P 2 and ZnSe, respectively.

Zinc phosphide epitaxial growth by photo-MOCVD

Abstract Zinc phosphide (Zn3P2) has been grown by photo-metalorganic chemical vapor deposition (photo-MOCVD) using dimethylzinc and phosohine. A low-pressure mercury lamp was used to photodissociate the reactant gases. For the Zn3P2 films grown at growth temperatures between 320 to 410°C and the P/Zn molar ratio of 5.7, the observation of reflective high-energy electron diffraction indicates the distinct and clear spot pattern confirming the epitaxial growth of Zn3P2 films on the GaAs substrate. The resistivity of those films decreases monotonically as the growth temperature increases irrespective of the growth method, while the resistivity of Zn3P2 films grown by photo-MOCVD is lower than that grown by MOCVD. The ultraviolet radiation resulted in lowering the optimum growth temperature and improving the Zn3P2 film crystallinity. Heterojunctions of p-Zn3P2/n-ZnSe fabricated as a demonstration show good diode characteristics.