Size dependent micro-photoluminescence spectra

The PL spectra of the self-organized disk-like GaN dots with different coverage at 10 K were shown in Fig. 4-6. The 10 K PL spectra of GaN and Al0.11Ga0.89N epilayers were shown as reference in Fig. 4-6 (a) and (b), respectively. The peak emission situated at 3.48 eV is ascribed to the near band edge emission (NBE) of GaN [43]. In Fig. 4-6 (b), the PL peak located at 3.651 eV corresponds to the NBE emission of the AlxGa1-xN epilayer, whose Al composition of the 11% was confirmed by x-ray diffraction analysis of the epitaxial layers [43]. The PL peak energy located at 3.594 eV, which is 57 meV lower than NBE, was not a phonon replica because the longitudinal optical phonon energy is 94 meV for Al0.11Ga0.89N. It was suggested to be a defect-related emission from the Al0.11Ga0.89N epilayer in Fig.4-6 (b) or Al0.11Ga0.89N buffer and capping layer in Fig. 4-6 (c) to (e). In Fig.4-6, the PL intensity is normalized according to the intensity of NBE from the Al0.11Ga0.89N buffer layer or capping layer.

Figs. 4-6(c) to (e) show the PL spectra of the GaN QDs grown on Al0.11Ga0.89N buffer layers with different GaN coverage. When GaN coverage is less than 8.2MLs, the dot density is too rare for the dot emission to be detected. In current study, the optical spectroscopy of GaN dots is confined to the samples of 9.1, 10.9, and 13.6MLs. All the samples for PL studies were capped with 30 nm Al0.11Ga0.89N layer whose Al content was the same as the buffer layer (11%), as shown in Fig. 3-1 (b). The PL labeled as IQD

is attributed to the luminescence emitted from the GaN dots. Note that the peak positions of INBE from the Al0.11Ga0.89N buffer layer and/or capping layer are roughly the same for all three GaN dots, indicating the aluminum composition fluctuation of AlGaN buffer layers and capping layers is negligible in these samples. Furthermore, no luminescence related to the wetting layer was observed in PL at 10 K or RT. The absence of the luminescence related to the wetting layer is most likely due to the fact that the carriers photogenerated in the wetting layer transfer to the adjacent GaN dots

and then recombine there [44].

The peak energy position and full width at half maximum (FWHM) of PL spectra at 10 K are summarized in Fig. 4-7. The PL peak position of the disk-like GaN dots grown under 9.1, 10.9, and 13.6MLs of GaN coverage situate at 3.546, 3.510, 3.487 eV at 10 K, respectively. The PL peak energy of GaN epilayer is 3.480 eV at 10 K for reference. We found that all of the GaN QD samples exhibited large blue-shifts in the PL spectra compared with GaN epilayer (from 36 to 62 meV at RT), even though the shape of QDs are disk-like. Ramvall et al. [45] calculated the energy shift of the electronic level to fit the experimental data of GaN quantum dots and showed that energy shift as large as 125.9 meV is possible for a quantum dot of 10 nm in diameter and 3.5 nm in height. To analyze the line width dependence of dot-related emission peak (IQD) on the size distribution, PL peaks were fitted by Gaussian curve. All the peak energy and FWHM of PL peaks were derived from the fitting value. The red-shift in the energy with the increasing GaN coverage is attributed to the decrease in the quantum confinement energy as the dot size is increased. In order to further explain the blue-shift with reducing dot size in the PL spectra quantitatively, we consider an electron or a hole in a rectangular box for simplicity. Therefore, the confinement energy of the ground state is expressed as: effective mass of electron or hole in x, y, and z directions, respectively. For a disk-like dot, the in-plane sizes d and dx y are much larger than the height dz = d. Therefore,

Where, the electron effective mass mez equals 0.2 m , the hole effective mass m0 hz equals 1.0 m , m0 0 is the electron rest mass[33]. In Fig. 4-8, the solid curve is plotted by using equation (2). The triangular points represent the PL data. The horizontal bars are obtained from AFM measurements to represent the size fluctuation. We show that the experimental results fit in with this simplified model quantitatively.

In addition to an increasing confinement shift with decreasing physical size of the dots, an increased inhomogeneous broadening can be also observed. From Fig. 4-7, the FWHM of the PL peak from GaN dots range from 56 to 88 meV at 10 K. It was 11.6 meV for GaN epilayer. Compared with bulk GaN, the FWHM of dot is relatively larger which may originate from the dot size fluctuation. In order to illustrate that the larger FWHM of QD PL spectra is mainly induced by size fluctuation, we will calculate the size fluctuation induced FWHM. The method is demonstrated in Fig. 4-9, where the error bar of x-axis denotes the height inhomogeneity. Fig 4-10 shows the PL FWHM at 10 K and RT as well as the size fluctuation induced FWHM. From comparison, we find that the PL FWHM at RT is relatively larger than FWHM at 10 K and the size fluctuation induced FWHM, indicating the carrier-phonon scattering increases with increasing temperature. In addition, the FWHM at 10 K is close to the size fluctuation induced FWHM. Therefore, we can confirm that the PL FWHM of IQD is derived from size inhomogeneity and the increasing of the FWHM with the decreasing size is attributed to the increasing of the size fluctuation.

3.4 3.5 3.6 3.7 I

I

I

10K

Normalized micro-PL Intensity (a.u.)

Photon energy (eV)

(e) 9.1 MLs

(d) 10.9 MLs

(c) 13.6 MLs

(b) AlGaN

(a) GaN

I

I

I

57meV

Fig. 4-6 Normalized μ-PL spectra at 10 K of (a) GaN epilayer, (b) Al0.11Ga0.89N epilayer, and the GaN samples grown on Al0.11Ga0.89N buffer layers with different GaN coverage of (c) 9.1 MLs, (d) 10.9 MLs, and (e) 13.6 MLs.

9 10 11 12 13 14 3.48

3.50 3.52 3.54 3.56 3.58

peak energy

GaN coverage (MLs )

Peak ener gy (eV)

Fig. 4-7 The peak energy and FWHM of μ-PL spectra of the GaN samples grown on Al0.11Ga0.89N buffer layers with different GaN coverage at 10 K.

bulk GaN

10 K

20 40 60 80 100 120 FWHM

FWH M ( m eV )

5 10 1 5 0

20 40 60 80 100

Peak Shift (meV)

Dot height (nm)

Micro-PL RT Calculation GaN E _g ~3.42 eV (RT)

Fig. 4-8 Comparison with the PL experimental data and calculation derived from the disk-like rectangular box model.

Fig. 4-9 Demonstration of size distribution FWHM.

5 10 15

0 20 40 60 80

100 Micro-PL RT

Calculation

Peak Shift (meV)

Size distribution FWHM

Dot height (nm)

6 7 8 9 0

40 80 120 160 200 240

RT GaN bulk

RT-PL FWHM

Size distribution FWHM

10 K-PL FWHM

10K GaN bulk

Fig. 4-10 Comparison with FWHM of PL spectra and size distribution.

FWHM (me V )

Dot height (nm)

4.3 Temperature dependent micro-PL spectra