Er-Implanted Porous Silicon: a Novel Material for Si-Based Infrared LEDs
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Ion implantation in the MeV range has been used to dope erbium into single-crystal silicon.1 • In this work, we proposed and demonstrated that porous Si is an excellent medium for obtaining a practical level of optically-active erbium by implanting with a commercial implanter below 200 keV, in contrast to using an MeV implanter which is typically a low current machine and more expensive. Most importantly, visible light from porous Si may be used to pump optically active erbium. Since Si is transparent to 1.54 pm (which is the result of 413 12 -4 4115/2 transition of Er-+) no loss could occur because of self-absorption. Recent measurements have suggested' 2 that the solubility of Er in Si is 1.3 x 10' 8/cm 3 at 900'C. Precipitates take the form of platelets of erbium suicide 100 to 300A in diameter and I•A thick. The photoluminescence (PL) saturates at 5 x 10' 7/cm3 , below the apparent solubility 3 3 16 limit.' 2 In this meeting it was reported that the actual solubility is in range of 10 /cm .' 4 Researchers have applied a variety of methods to increase the solubility1 and optical efficiency of erbium in silicon. Based on the published literature'-1 8 one can infer that optical inefficiency is predominantly caused by erbium's limited solubility in silicon (which is influenced by the recrystallization process) and by a dependence on the presence of oxygen. Why Porous Si is an Excellent Host for Er a) Porous Si has a higher bandgap (1.8 to 2.0 eV) than bulk Si (1.1 eV); thus, as shown 5 in Figure 1 is stronger IR emission and weaker temperature dependence are expected.' 94378
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(a) GO.,3, In,62As.Po. 12(EG-0.807 AV) e
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C(c)
0
I
+
10_
_d
(g)CdS (E,= 2.42 eV) (h)a-Si:H(EG= 1.8eV)
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Figure 1
7(e)
(b)Si (EG= 1.12 eV) InP (E = 1.27 eV) = 1.43eV) (1hGaAs(EG = 1.67 eV) As (EG A.1 Gao,. (I ZnTe (E,=, 2.26 eV)
100 200 300 TEMPERATURE, K
Er0 + emission intensity as a function of temperaturefor various semiconductor 15 hosts.
b) As mentioned above a number of researchers"-"8 have shown that erbium with oxygen coordination results in an enhancement of light efficiency. Erbium in silicon nanostructures (or even meso- or macrostructures) can easily acquire oxygen because the surface area of the porous Si is enormous and the average distance between the erbium and surface oxygen is on the order of nanometers.
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c) In contrast to Er in bulk Si, Er in porous Si cannot precipitate because porous Si consists of free-standing, quantum-confined structures with distinct physical boundaries. d) Recovery of damage resulting from Er implantation into porous Si at 190 keV should occur at much lower annealing temperatures than in bulk Si at MeV. Defects such as vacancies or interstitials created by Er implantation need only travel a relatively small distance to reach the free surface (sink) as compared to MeV implantation into bulk Si'2 where defects must travel rather long distances (several hundred/thousand Angstroms) to reach the sink. Annealing at 900'C or higher is used12 to recrystallize bulk Si; howev
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