MOCVD of SrS and SrS:Ce Thin Films for Electroluminescent Flat Panel Displays
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Conditions: Films were deposited in an CVD reactor constructed in-house (to be described elsewhere). Depositions were carried out at 20 torr pressure with an H 2S/Ar ratio of 0.2 v/v. Total flow through reactor was 1200 SCCM resulting in a 4.0 min. reactor residency time. Reactor feed lines and chamber walls were heated to 230-240 °C. Substrate temperatures ranged from 250-550 'C with a 30 min. deposition time. Post-deposition cool down was carried out under an 0.2 v/v H2 S/Ar atmosphere with the samples not being exposed to air until their temperature was below 60 'C. Samples were stored in a vacuum desiccater post-deposition. Analysis: Film thicknesses were measured on 2020 Dektac profiler. X-ray diffraction (XRD) analysis was conducted on a Rigaku "Rotoflex" 200B fitted with a position sensitive detector. FWHM measurements were conducted on a Philips APD 2320 i/20 with a 0.015 deg. step. Auger electron spectroscopy was conducted using a linear detector. Depth profiling was carried out using a 1.7 KeV Ar+ sputter gun. Rutherford backscattering spectroscopy (RBS) was performed using 6.0 MeV 4 He2 + with a Au/Si standard. Elastic recoil spectroscopy (ERD) was conducted using 2.0 MeV 4 He 2 + with a Kapton (C22H 10 N2 0 5 ) standard. Analysis was carried out using the program RUMP. For EL measurements, deposited films were overcoated with a barium tantalum oxide dielectric and aluminum test dots. EL luminance and emission spectra were taken at 60 Hz AC. RESULTS SrS: Strontium sulfide was deposited from Sr(thd) 2 and H2 S on EL panel structures and .. * _ silicon reference samples. The films were I . 10000 I... .... colorless and hig hly transparent with thickness 8000 averaging 5000 A for a 30 min. deposition time. showed good adhesion to the substrates "(scotchtape test) with no indication of S 6000 . delamination even with very thick films (2.5 um). Films were readily etched with aqueous solutions 4000 with liberation of H2 S, a common characteristic of Group 2 sulfides. The growth rate of the material appears to be fairly insensitive to substrate v- 2000 temperature (Fig. 1) with only a gradual reduction 0 _of rate at lower temperatures. This would imply ... .. - I that the growth rate is precursor limited and not in 200 300 400 500 600 a kinetic regime. Substrate Temperature(C) XRD of the cubic SrS deposited on Si or glass at 500 'C shows the films to have a Figure 1: SrS Growth vs. Temperature predominant texturing with about 25% (30 min. deposition time). . A small amount of material is also present. In contrast, deposition onto the ZnS surface of the EL panel substrates yields films with nearly equal amounts of and with a significant amount of . This would seem to indicate that the growth surface, in this case ZnS, has some influence the orientation of the growing film. Growth texturing is also affected by the growth temperature (Fig 2). Depositions at low temperature seem to produce primarily oriented material with some and . As the temperature is increased, the and reflections increase in intensity while t
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