The effects of KrF pulsed laser and thermal annealing on the crystallinity and surface morphology of radiofrequency magnetron sputtered ZnS:Mn thin films deposited on Si

Abstract

Thin films of ZnS:Mn (800 nm) have been deposited by rf magnetron sputtering onto 100 mm diam n-type single-crystal (100) Si wafers. Specifically for use as active layers in thin film electroluminescent devices, the films need a postdeposition annealing treatment to enhance their luminescent properties. Inherent to the later process step are structural modifications of the phosphor layer which form the basis of this study. Both pulsed laser and thermal postannealing techniques have been investigated. Reported are the induced crystalline and surface morphology modifications via x-ray diffraction and atomic force microscopy analysis. As-grown and thermally treated films were cubic in nature and no significant grain growth or reorientation occurred while heating up to 700 °C. Pulsed (∼ 20 ns duration) KrF laser treated samples were annealed at power densities from 10.76 to 24.27 MW/cm2 under 10.34 bar of argon pressure. Beam quality and diagnostics were emphasized during laser irradiation with particular attention brought to energy and pulse duration measurements. It has been demonstrated that at the power densities used, a gradual phase transition from cubic to hexagonal is occurring while the average crystallite size remains constant. Surface analysis highlights concomitance between the phase transition and the smoothening of the irradiated surface. A one dimensional thermal model of the pulsed laser annealing process shows that a surface temperature for crystalline ZnS equating to the transition temperature should be reached at 17 MW/cm2, significantly below the numerically evaluated melting threshold of 30.5 MW/cm2. Combining experimental and theoretical results, it is concluded that the phase transition occurs in the solid state.

Lattice misfit versus performance of thin film electroluminescent structures

“…This conclusion was found to be in agreement with the work from Cattel and Cullis 16 who investigated similar films and substrates. According to our previous work, 15 the observed diffraction peaks in Fig. 1͑a͒ are attributed to the ͑111͒ and ͑311͒ reflection planes.…”

“…1͑a͒ are reported from our previous work, 15 and have been corrected for a quanta acquisition time of 20 s. In Ref. 15, we observed that the ZnS:Mn films exhibit predominantly the zincblende structure with a preferred ͗111͘ orientation. In Ref.…”

“…The applied power of the x-ray tube was 35 kVϫ25 mA and the angular step was 0.005°w ithin the studied 25°-62°range. The quanta acquisition time was 5 s for NTU228, 15 and 20 s for NTU398 and NTU259.…”

“…In Ref. 15, we also investigated the thermal annealing effects on the crystallinity of ZnS:Mn and concluded that thermal treatments up to 700°C for 1 h do not induce grain growth or recrystallization of ZnS:Mn when grown onto Si ͑100͒ substrates. This conclusion was found to be in agreement with the work from Cattel and Cullis 16 who investigated similar films and substrates.…”

“…Thermally annealed ZnS:Mn thin films directly deposited on Si did not recrystallize at temperatures up to 700°C, 15,16 whereas those grown onto Y 2 O 3 recrystallize at 600°C ͑Fig. 2͒.…”