Influence of Internal Electric Fields on the Ground Level Emission of GaN/AlGaN Multi-Quantum Wells
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GaN substrate, so that the GaN QWs are unstrained. Conversely, samples grown on the 100 nm thick Al0.15Ga0.85N buffer layer (which is completely relaxed) result in strained QWs. Each sample consisted of 10 GaN quantum wells. In each set, four samples of wellwidth Lw=2,3,4, and 5 nm (measured by double crystal X-ray diffraction) were grown and analyzed. The barrier width and composition were kept constant in all samples (Al0.15Ga0.85N barriers of thickness LB=10 nm). Set C was grown by MOCVD according to the structure already described for set B but with quantum well widths of respectively, 1, 2, 3, 6, and 9 nm. The optical measurements were performed either under cw excitation (325 nm line of a He-Cd laser) or under a pulsed excitation (4th harmonic of a Nd-YAG laser). The samples were kept in a variable temperature closed cycle cryostat. The spectral resolution was always better than 0.2 meV. DISCUSSION The systematic analysis of all samples belonging to sets A and B results in the wellwidth dependence of the n=1 ground level emission displayed in Fig. 1 (symbols). In this figure the experimental data are compared to the theoretical curves obtained by the analytical models discussed in the following.
3,65 3,60
Energy (eV)
3,55 Bulk GaN
3,50 3,45 3,40 3,35
QWs on GaN buffer QWs on AlGAN buffer 12
-2
Theory: QW on GaN (n = 10 cm )
3,30 3,25 1,5
12
-2
Theory: QW on AlGaN (n = 10 cm )
2,0
2,5
3,0
3,5
4,0
4,5
5,0
5,5
6,0
Well width (nm)
Figure 1. Well-width dependence of the ground level emission energy of GaN/AlGaN QWs grown on GaN, at 10 K. The curves are calculated by means of Eq. 4 The horizontal line indicates the unstrained bulk energy gap of GaN.
The main features of the experimental data shown in figure 1 can be summarized as follow:
1) for Lw≥3 nm the emission energy falls below the bulk energy-gap; 2) the observed well-width dependence differs considerably from the usual square well model (Lw-2 dependence); 3) for a given well-width, the ground level emission energy is different in the two sets of samples, i.e. it depends dramatically on the AlGaN buffer thickness;
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4) despite the difference in the absolute energy value, the well-width dependence of the ground level emission is similar in the two sets of samples, i.e. it does not depend on the thickness of the AlGaN buffer; 5) The emission energy blue-shifts with increasing the photo-generated charge density (data not shown). The detailed analysis of the results obtained on sets A and B has been already presented in [9]. Figure 2 shows the emission energy of samples belonging to set C. As can be easily noticed, the linear dependence of energy on the quantum well width holds only for the narrower wells (first 3 points) whereas the wider wells exhibit a considerable deviation from such trend. The blue-shift displayed by the emission energy of the samples with larger well widths indicates that the role of the well dimensions must be considered, in this case, with greater attention.
3.70
Energy (eV)
3.65 3.60 3.55 3.50 3
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