Semiconducting particles undergo blue shift with decrease of size, why?
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Answer:
Explanation:15.4 ANNEALING EFFECT
Annealing of GaInNAs is known to produce a significant blue shift of its band gap. The origin of this band gap shift has been widely discussed. In this section, we re-examine this phenomenon, and we compare the behavior of GaNAs, GaInNAs and GaNAsSb. For this purpose, we have grown a set of samples consisting of 0.1-μm thick layers of each alloy. To determine their compositions, the samples were analyzed by SIMS to measure the N concentrations, and XRD to measure the lattice parameters. The N levels measured by SIMS were calibrated from the determination of GaNAs composition by XRD, assuming Vegard law. The nitrogen compositions were found to be very close to 2% for the three samples. The In and Sb concentrations of the quaternary layers were found to be 5 and 8.1%, respectively, as deduced from the lattice parameters. The GaNAs layer was tensely strained, the GaInNAs was almost lattice-matched to GaAs, and the GaNAsSb was slightly compressively strained.
PL and photoreflectance (PR) spectra of the III-V-N layers are shown in Figure 15.7, for as-grown and annealed (750°C, 10 min) samples. Details of the PR optical characterization can be found in the present book, as presented in the chapter by Misiewicz and Kudrawiec. The three alloys show a significant blue shift of their emission (PL) and their absorption (PR), after annealing. At RT, the tensely strained GaN0.02As0.98 layer has light hole band gap of 1.103 eV, which is shifted to 1.115 eV after annealing. In the almost lattice-matched Ga0.95In0.05N0.02As0.98 layer, heavy and light hole transitions are not resolved, and the band gap shifts from 1.077 to 1.104 eV. The compressively strained GaN0.02As0.90Sb0.08 layer has a heavy hole band gap of 0.941 eV, shifted to 0.985 eV by thermal annealing. Table 15.1 summarizes these results. The band gap blue shift is thus observed in GaNAs, GaInNAs and GaNAsSb alloys, but with different values, 12, 27, and 44 meV, respectively. The quaternary alloys are more sensitive to the anneal than the ternary. It is worth noting that the largest shift is observed in GaNAsSb.
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Figure 15.7. Photoluminescence and photoreflectance (solid line: experimental, dashed line: calculated) of (a) GaNAs, (b) GaInNAs, (c) GaNAsSb samples, before and after annealing.
Table 15.1. Characteristics of three layers with similar N concentrations: GaNAs, GaInNAs, and GaNAsSb
Alloy GaNAs GaInNAs GaNAsSb
N composition (%) 2.05 1.9 2.0
In composition (%) 0" 5.0 0
Sb composition (%) 0 0 8.1
Egap before annealing (eV) 1.103 (e-lh) 1.077 0.941 (e-hh)
Egap after annealing (eV) 1.115 1.104 0.985
Anneal-induced band gap shift (meV) + 12 +27 +44
EGaAs-Egap before annealing (meV) +322 +348 +484
EGaAs-Egap after annealing (meV) +310 +321 +440
Biaxial strain contribution to (EGaAs-Egap) (meV) +49 +6 -15
In or Sb contribution to (EGaAs-Egap) (meV) 0 +80 +153
N contribution to (EGaAs-Egap) before annealing (meV) normalized to N = 1% +133 +138 +173
N contribution to (EGaAs - Egap) after annealing (meV) normalized to N = 1% +127 +124 +151
Experimental band gaps are given for as-grown and annealed samples. The contributions to the band gap are evaluated for the different alloys.