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High p-type conductivity in Phosphorus-doped ZnO films.
|Arjun Mandal , Subhananda Chakrabarti|
Indian Institute of Technology Bombay (IITBOMBAY), Powai, Mumbai 400076, India
ZnO is a crucial member in the arena of exciton-based UV emitters because of its wide band gap (3.37eV) and large exciton binding energy (60meV) at room temperature. Fabrication of p-type ZnO is a big challenge as undoped ZnO is unintentionally n-type due to the presence of native point defects (oxygen vacancies and zinc interstitials). In this work, ZnO thin films were deposited on semi insulating <100>InP by Pulsed Laser Deposition (PLD) technique at 400ºC in an oxygen ambient of 75mTorr followed by Rapid Thermal Annealing (RTA) of the samples at 500ºC, 550ºC, 600ºC and 650ºC. Atomic force microscopy (AFM) images revealed the growth of ZnO with roughness varying between 9.26nm to 22.28nm (root-mean-square) within a scanned area of 1µm2. It was observed from the XRD results of grown ZnO <002> that the full width at half maxima (FWHM) of the annealed samples were narrower (~0.1428º) compared to as grown one (~0.2652º) due to improved crystallinity. Absorption coefficients calculated from the Reflectance data of UV/VIS spectroscopy decreased for the annealed samples (from 2.56um-1 to 1.02 um-1 at 360 nm). Temperature dependent (80K to 310K) Van Der Pauw Hall measurements confirmed stable p-type conductivity for the ZnO samples (resistivity~ .024 Ω-cm) annealed at 550ºC, 600ºC and 650ºC. The highest hole concentrations and Hall mobilities were measured to be 2.52x1018cm-3and 397cm2/V-sec at 80K while their values at 310K were 5.41x1020cm-3 and 19.8cm2/V-sec respectively. This is the best combination of hole concentration and Hall mobility reported so far for any phosphorus doped p-type ZnO films. At 80K, the hole diffusivity calculated was 2.73 cm2/s for the phosphorus impurities. From Arrhenius plots, Acceptor activation energy of 25.8±0.7 meV was calculated for P acceptors . Our future works are paving towards the fabrication and characterization of UV LEDs. We acknowledge Central SPM Facility of IIT Bombay for AFM study and DST for financial assistance.
Presentation: Oral at E-MRS Fall Meeting 2009, Symposium C, by Subhananda Chakrabarti
See On-line Journal of E-MRS Fall Meeting 2009
Submitted: 2009-06-25 12:13 Revised: 2009-08-10 17:01