Depth-Resolved and Excitation Power Dependent Cathodo-Luminescence of MBE Grown Cubic GaN Epilayers
- PDF / 1,108,425 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 6 Downloads / 204 Views
661
Mat. Res. Soc. Symp. Proc. Vol. 482 © 1998 Materials Research Society
0.4
0.3
g = 3.2"1021 cm-3 s'l
X
/
.
\
0.2 CU
0.1 g
3 1 1.11026cm- s-
0. 0 2.2
2.6
3.0
3.4
photon energy (eV)
Fig. 1: Room temperature CL-spectra at high excitation (lower curve) and low excitation (upper curve) conditions (Vb= 1 2 kV, Ib= 15 nA)
of 200 cm 2/gs as measured by Hall effect measurements [5]. The thickness of the layers investigated is about 1.6 gim. Cathodoluminescence is measured in a Zeiss DSM 950 scanning electron microscope (SEM). The emitted light is collected by an elliptical mirror which focuses the CL-light into an UV-transparent glass fibre. A Spex 0.5 m monochromator disperses the signal spectrally. A 1P28 photomultiplier operating in the photon counting mode is used for detection. The electron energy and beam current is varied between 4 to 30 keV and 0.1 to 30 nA, respectively. By varying the diameter of the electron beam (e-beam) the excitation density can be changed by more than 7 orders of magnitude. RESULTS AND DISCUSSION At room temperature and under high e-beam excitation conditions (acceleration voltage Vb= 12 kV, e-beam current lb= 15 nA) the CL spectrum of the cubic GaN sample is dominated by the near band emission at 3.2 eV with a full width at half maximum (FWHM) of 55 meV (lower curve in Fig. 1). To our knowledge, this is the narrowest room temperature linewidth reported so far for cubic GaN. A detailed lineshape analysis of this spectrum reveals that the dominating peak consists of contributions from an excitonic and a band-accceptor (e, A0) transition [6]. Similar observations have been made by Yang et al.[7], however the FWHM of the room temperature CL of their samples was at least twice as large as in our case. Using the free exciton energy of 26 meV [8] we calculate a band gap energy of 3.230 eV for c-GaN at room temperature. This value is in excellent agreement with already published values measured with photoreflection [9]. No deep level emission can be seen under this excitation conditions.
662
1022
1021
. . Ec
108 6
gedeep
X
%"
1019
Ev
Ide ep
"Tj 1017 1016 1015 1020
1021
1022
1023
1024
1025
1026
1027
1
electron-hole pair generation rate (cm-3 s- )
Fig.2: CL-intensity versus electron-hole pair generation rate for the excitonic transition Ix and the deep 2.4 eV band Ideep. The symbols are the experimental data and the curves are the calculated intensity. The inset shows schematically the used recombinationmodel.
By reducing the excitation intensity of the e-beam a broad deep band at 2.4 eV is observed in the CL-spectrum which is depicted as the upper curve in Fig.1. For better comparison both spectra have been normalized to the near band edge emission. The corresponding electron-hole pair generation rates g shown in Fig.1 are calculated by an expression given in Ref. 10 IbEb(1 -7)
qV(2.Eg +M) where Ibis the e-beam current, Eb the electron energy, Eg the semiconductor gap energy, V the e-h pair generation volume, q the electron charge, y = 0.5 is the fractio
Data Loading...
