Luminescence Study for Band Discontinuity in Free-Standing CdZnS/ZnS Strained Layer Multi-Quantum Wells
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III. RESULTS AND DISCUSSION A. Cd0.3Zn0.7SIZnS strained layer MQW Figure 1 shows PL spectra in the Cdo.3Zno.7S/ZnS MQWs with the CdZnS well width of 2.1 nm and 7.6 nm. The ZnS barrier width is 8.1 nm in both samples. PL measurements at 1.4 K show intense, sharp excitonic emission in the blue-ultraviolet spectral region and no emissions due to deep levels. Figure 2 shows PL peak energy as a function of CdZnS well width. With decreasing CdZnS well width, the PL peaks shift to higher energy, as expected from a quantum well model. The quantum transition energies were calculated by a finite-potential well model[10] including the effect of exciton binding energy and elastic strain. It is assumed that the emission peak in CdZnS/ZnS MQW is attributed to the heavy-hole-free-exciton transition. Therefore, adding the binding energy of a free-exciton, which we assume to be 36 meV (equal to the free-exciton energy in bulk ZnS[6]), to the emission peak photon energies, we obtain the transition energy between n=1 electron quantized level and n=1 heavy hole quantized level.
exc. 325 nm
MJ 2.2I
2.6
2.4
0
a-
~1.4
3.4
3.6
'C
CD CD
K
3.6
~Bandgap of CdZnS
'C 3.4
0 P- 3.4
2.8 3.0 3.2 ENERGY (eV)
Fig.l. PL spectra from the as-grown Cd0.3Zn0.7S/ZnS MQWs with CdZnS well widths of 2.1 nm and 7.6 nm.
4 3.8
uJ 3.5 Z
2.1 nm
0
3.8 -3.7 0 M 3.6 z
7.6 nm
1.4 K
SI
3.2
I
I
I
3
N 2.8
S3.3
'0
0 2.6
3--
3.2
2.4
0
2
10 6 8 4 CdZnSe WELL WIDTH (nm)
12
Fig.2. PL peak energy as a function of CdZnS well width. The solid lines show the calculated results. The calculated curves are for transitions between n=I electron and n=1 heavy hole, between n=2 electron and n=2 heavy hole, and between n=3 electron and n=3 heavy hole. Points show the experimental data.
0
0.2
0.8 0.6 0.4 Cd COMPOSITION X
1.0
Fig.3. Bandgap and PL peak energy of cubic CdxZn i-xS. Points show the experimental data.
496
There are no reports of bandgap data for cubic CdxZnl-xS over the entire composition range because bulk CdS crystallizes exclusively in hexagonal structure. Therefore, we used the cubic CdxZn 1-xS bandgap, as shown in Fig. 3, which was estimated from the ZnS bandgap of 3.840 eV at 1.4 K and the dependence of the FA (free-to-acceptor) peak energy on the alloy composition of cubic CdxZnl-xS.[4] The CdxZnl-xS bandgap Eg is then given by Eg=3.84-2.0781x+0.82662x 2.
(1)
In a free-standing CdZnS/ZnS MQW, the equilibrium in-plane lattice constant axy of the MQW is given by[9]
axy=(aZnSGZnSLZnS+aCdZnSGCdZnSLCdZnS)/(GZnSLZnS+GCdZnSLCdZnS), (2)
(3)
Gi=2(Ci 1I+2C1 2 )(Xl-CiI 2 /C1i 1),
where i denotes the material (ZnS or CdZnS), aZnS and aCdZnS are the lattice constants of the bulk ZnS (i.e., 5.4093 A) and CdZnS, LZnS and LCdZnS are the ZnS barrier and CdZnS well width, and Ci 11and Ci1 2 are the elastic constants of material i. The lattice constant of the CdxZnl-xS alloy is determined by assuming a linear variation of the lattice constant with x (Vegard's law). A lattice constant of cubic CdS is 5.832 A. From Eqs. (2,3), the eq
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