Mbe Growth And Characterization Of Er/O And Er/F Doped Si Light Emitting Structures
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Er flux together with oxygen or fluorine atoms was supplied by sublimation of Er 20 3 or ErF3, using high temperature effusion cells developed and fabricated by our group. For Er/O doped samples, the growth was nominally carried out at 420'C, at which a twodimensional growth mode (a 2x I reconstruction was observed by reflection high-energy electron diffraction (RHEED)) could be maintained with a moderate Er/O flux corresponding to an equivalent bulk Er concentration of lxl019 cm' 3. Above this concentration, the RHEED pattern degraded, implying a more severe surface roughness. The RHEED pattern eventually developed to be bulk like when the Er bulk doping concentration exceeded 2x 1020 cm'3 . Secondary ion mass spectrometry (SIMS) measurements showed that the Er profiles are sharp and well defined for growth at 420'C with different Er/O impinging fluxes. The Er incorporation levels can be 5x10' 9 cm'3 without precipitates, as confirmed by studies of cross-section transmission electron microscopy (XTEM), which is more than one order of magnitude higher than the previously
reported value of -lxlO' 8 cm' 3 .5 For a sample grown at > 500'C, the doping profile is significantly broadened due to Er segregation. The effects of surface segregation and accumulating surface roughening were stronger for growth of ErF3 doped structures at the same temperature (420'C), which in turn results in a smeared Er profile. To overcome this problem, multiple layer structures have been prepared using a temperature modulation method, i.e., growth at 350'C with ErF3 flux and at a higher temperature (600°C) without ErF3 flux. In this way, 2000A thick layers with average concentrations of NE, >1019 cm'3 have been produced, that appeared to be without defects in studies with XTEM. 6 Furthermore, SIMS measurements using a Cs" ion beam showed that the F incorporation level can be about three times higher than that of Er in Si in agreement with a chemical ratio of ErF 3. In contrast, a gradual decrease of the 0 OFri .... incorporation level in time with the O/Er ratio S:/O 3 Er changing from 1.5 to 0.7 in Er/O doped Si layers 171 EL at 38 K
due to
Er2 0
3
dissociation during sublimation at
high temperatures (>=2000 'C). The photoluminescence (PL) measurements were performed using the 514.5-nm line
of an Ar+ laser for excitation. The PL was then
dispersed by a double grating monochromator and detected by a North-Coast liquid nitrogen cooled Ge detector. The electroluminescence (EL) measurements were carried out using Er doped layers with several groups of Al contact bars with different electrode areas (l0xl00-200xl00 gtm 2V ) and separation distances (150-200 gtm). During the measurements, 200 Hz 50% duty cycle square pulses without a dc bias were applied on a pair of face-to-face connected Al Schottky junctions.7 The Er ions can be then excited by a hot electron impact excitation process at an area
ad acent to the electrode under reverse bias. The 134
co
a-,
(b)
C CD -
Er+
PL at 2 K
co U1) .c
E
Si-TO (a) -A -,,,__...__...__..__.
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