Cathodoluminescence of Rare Earth Ions in Semiconductors and - PowerPoint PPT Presentation

Cathodoluminescence of Rare Earth Ions in Semiconductors and Insulators Leon Maurer International Materials Institute for New Functionality in Glass and Lehigh University Department of Physics REU – p. 1/1

Terms in the Title Q: What is Cathodoluminescence(CL)? – p. 2/1

Terms in the Title Q: What is Cathodoluminescence(CL)? A: Light emission caused by electron bombardment. – p. 2/1

Terms in the Title Q: What is Cathodoluminescence(CL)? A: Light emission caused by electron bombardment. Q: What are the rare earth ions (REIs) in question? – p. 2/1

Terms in the Title Q: What is Cathodoluminescence(CL)? A: Light emission caused by electron bombardment. Q: What are the rare earth ions (REIs) in question? A: Rare earth atoms have been doped into our sample and assume a +3 ionization state. – p. 2/1

Terms in the Title Q: What is Cathodoluminescence(CL)? A: Light emission caused by electron bombardment. Q: What are the rare earth ions (REIs) in question? A: Rare earth atoms have been doped into our sample and assume a +3 ionization state. For the curious: Q: Are rare earth elements rare? A: Not particularly – they are all significantly more abundant than gold. Q: Are rare earth elements “earth”? A: No, “earth” is an archaic word for oxide. – p. 2/1

Applications Electrically pumped lasers, light sources, and light amplifiers REIs are already used in optically pumped lasers REIs have consistent emissions in the visible spectrum independent of host material create a display from different REIs in same host Wide Bandgap Semiconductors Transparent to visible light Efficient REI emission – Ions can be electrically excited Can use semiconductor tricks – can make PN junctions to inject electrons – p. 3/1

Experimental Setup – p. 4/1

Interaction with the beam – p. 5/1

Interaction with the beam Collisions create electron hole pairs Electron Energy = 20 KeV 620 nm Photon Energy ≈ 2 eV – p. 5/1

What do we see? Intensity vs. Wavelength 25000 line 1 20000 Intensity (arb units) 15000 10000 5000 0 200 300 400 500 600 700 800 Wavelength (nm) – p. 6/1

Questions Is the process efficient? What is the limiting factor? How do the REIs become excited? Directly by beam electrons hitting the REIs? Electron hole pairs transferred from the base material? Is there an intermediate trap? – p. 7/1

The Model 2 Energy Level System N total Ions, N e excited, and N g in the ground state p pump rate, k decay rate, τ = 1 k decay time constant d dt N e = pN e − kN g = pN e + kN e − kN d dt N e = − kN e – p. 8/1

The Model 2 Energy Level System N total Ions, N e excited, and N g in the ground state p pump rate, k decay rate, τ = 1 k decay time constant d dt N e = pN e − kN g = pN e + kN e − kN d dt N e = − kN e Solutions: N e = pN 1 − e − ( p + k ) t � � p + k N e = pN p + k e − kt – p. 8/1

The Model and Spot Mode In spot mode, the beam just dwells on one spot. If we wait a moment: 1 − e − ( p + k ) t � pN = pN � N e ≈ lim p + k p + k t →∞ – p. 9/1

The Model and Spot Mode In spot mode, the beam just dwells on one spot. If we wait a moment: 1 − e − ( p + k ) t � pN = pN � N e ≈ lim p + k p + k t →∞ However, this only really tells us about the ratio k p . N e ≈ pN N p + k = 1+ k p – p. 9/1

Spot Mode and Saturation Current vs. Intensity 900 experimental data intensity fit 800 700 600 Intensity (arb units) 500 400 300 200 100 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 Current (uA) Output saturates – efficiency is not the limiting factor Intensity is less than for photoluminescence (PL) – fewer REIs are being excited That rules out direct excitation and direct host transfer – p. 10/1

The Model and Line Mode 1 1 line 1 line 1 0.9 0.9 0.8 0.8 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0 0.1 0 200 400 600 800 1000 0 200 400 600 800 1000 The beam only shines on a spot part of the time. Finding the steady state is done numerically. The frequency of the scan is varied. The time average is fit to the data, determining k . – p. 11/1

Does The Model Work? Frequency vs. Intensity 8000 experimental data intensity fit 7000 6000 Intensity (arb units) 5000 4000 3000 2000 1000 0 0 500 1000 1500 2000 2500 3000 3500 4000 4500 Frequency (hz) k = 1 . 46 ∗ 10 6 p = 1 . 60 ∗ 10 7 τ = 6 . 82 ∗ 10 − 5 seconds is similar to the relaxation time of Eu This does not show evidence for a trap – p. 12/1

REIs in Insulator – Er in Glass Intensity vs. Wavelength 2400 line 1 2200 2000 Intensity (arb units) 1800 1600 1400 1200 1000 800 300 350 400 450 500 550 600 650 Wavelength (nm) Shows a drop compared to Er in semiconductors. Indicates that direct excitation is not the mechanism at work. We’re still investigating this material. – p. 13/1

Conclusion and Summary Difference in intensity between CL and PL, and dependence on host material, suggests direct excitation is not at work. However, the measured time constant is similar to that of Eu, meaning that if there is a trap, it is faster. Still, something besides the number of REIs and the efficiency must be the limiting factor – traps are likely to be it. More measurements are needed – taking data is slow with the current setup. – p. 14/1