Mocvd Erbium Sources *
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MOCVD ERBIUM SOURCES" ANTON C. GREENWALD', WILLIAM S. REES, Jr.'*AND UWE W. LAY** Spire Corporation, One Patriots Park, Bedford, MA 01730-2396 Florida State University, Dittmer Laboratory of Chemistry, Tallahassee, FL 32306 ABSTRACT The overall objective of this research is to develop source materials for doping AIGaAs. We compared Er(CsH5 )3 to Er{N[Si(CH 3) 3] 2) 3 for purity, decomposition kinetics and doping of germanium films deposited from Ge(CH 3)4 in a hydrogen atmosphere. Cyclopentadienyl erbium left large amounts of carbon both in pure metal films, and in the germanium film, at low pressure and temperatures to 850'C. Bis-(tri-methylsilyl) erbium amide decomposed cleanly without carbon, nitrogen or silicon in the deposited film. INTRODUCTION The ability to dope Ill-V compound semiconductors with rare-earth elements could make it possible to produce ionic luminescence whose wavelength is temperature independent. Stable erbium-doped GaAs and AlGaAs diode lasers offer particularly great promise because their 1538 nm wavelength matches that needed for efficient pumping of the Er-doped fibers used in optical amplifiers. Rare earth doping of III-V semiconductors by melt growth processes has generally led 23 to broad photoluminescence (PL) spectra. Ion implantation doping has produced narrow PL spectra, but results were sensitive to annealing parameters. Because light emitting diodes and lasers are now typically fabricated by metalorganic chemical vapor deposition (MOCVD), and
because recent successful results have been reported with rare earth doping of I[-V semiconductors by MOCVD,"5 this approach to achieving narrow-line, temperature-independent, solidstate light sources has reduced risk. MOCVD is possible if and only if a volatile erbium source compatible with MOCVD of III-V compounds exists. Finding an improved erbium source that contained no oxygen was the objective of this research program. Possible structures for volatile erbium compounds are
shown in Figure 1. The betadiketonates (Figure Ia) contain oxygen and were not considered. Er(CH 5 ) 3 and Er(N[Si(CH 3) 312 }3 (Figures lb and lc) were tested. SYNTHESIS
The cyclopentadienyl erbium material was purchased from a commercial vendor and had tested impurity levels below 1 ppma. The amide was investigated after good results were reported for MOCVD of ZnSe using similar precursors.9 Er(N[Si(CH 3)3] 2}3 was synthesized from LiN[Si(CH 3)3] 2 and ErCI3, forming lithium chloride and the erbium-amide.'0 The principal impurity in the resulting product is LiN[Si(CH 3)3] 2, which was separated out by successive sublimations at 5 x 10-2 torr and 80°C. The final residue was sublimed at 140'C, giving a pink powder. The purity of this product is indicated by the NMR data shown in Figure 2. The single very sharp peak implies a total contaminant concentration less than 0.1 ppma. Decomposition kinetics of Er{N[Si(CH 3)3]2) 3 are shown in the thermal gravimetric analysis (TGA) in Figure 3. The decomposition point of this material is 257°C. It sublimes rapidly at 175°C.
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