Phonons and Holes in Magnesium Doped GaN
- PDF / 329,167 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 68 Downloads / 220 Views
P doping of gallium nitride by incorporation of magnesium in the layers was controlled recently. Only Popovici et al. [1] have published the results of a Raman study on p type GaN. In the present communication, we report on the interaction of the free hole gas with the axial A1(LO) or planar EI(LO) phonon modes, evidenced by Raman scattering: the observed coupled phononplasmon mode is found very different from the corresponding one evidenced in silicon doped (n type) GaN. We compare the experimental data with the lineshape calculated within a dielectric model, using the results of electrical measurements. These results are also compared with infrared reflectivity spectra.
EXPERIMENT The characteristics of the p type samples used for this study are given in Table I. The hole concentrations and mobilities were obtained from Hall measurements. Two GaN layers were grown at 830°C by molecular beam epitaxy (MBE) using ammonia as nitrogen precursor [2]. Two other layers were grown at 1080 0 C by metalorganic vapor phase epitaxy (MOVPE) in nitrogen and ammonia flows. The orientation of the GaN c-axis is normal to the surface of the sapphire substrate. TABLE I Sample characteristics sample
(a) (b) (c) (d)
growth
MBE MBE MOVPE MOVPE
hole concentration Np (cm"3)
hole mobility IH (cm 2V s-
undoped 3. 101 7. 10"7 3. 1018 333
Mat. Res. Soc. Symp. Proc. Vol. 512 © 1998 Materials Research Society
8 5 2
)
Micro-Raman spectra were recorded at room temperature, with a Dilor spectrometer, in backscattering geometry along or perpendicular to the c-axis, using the 488 nm line of an Ar÷ laser as excitation source. A 100x microscope objective was employed to focus on the (0001) surface or on the edge of the GaN crystal, with a spatial resolution of 1 gtm ; the scattered light was detected with a charge coupled device. Low temperature Raman measurements and annealing in situ were also performed for the present study. Room temperature infrared (IR) measurements were done with a Fourier transform spectrometer Bruker IFS 113V at nearly normal incidence of polarized infrared radiation. IR reflectivity spectra were detected in a spectral range between 90 and 10000 cm-1 with a resolution ranged from 1 cm-l in the far-IR region to 5 cm' in the near-IR region. The reflected radiation intensity from the gold mirror was used as reference. RAMAN EXPERIMENTAL RESULTS Figure 1 shows typical room temperature micro-Raman spectra, in the Y (x'x')z and ' (zz)x' backscattering configurations (where x' is an in-plane axis normal to the crystal edge). In the former we observe the non polar E 2 mode and the high frequency component L÷ of the Al(LO) phonon-plasmon coupled mode, labelled CPPM(A1 ). T
It is only weakly shifted towards high (C
frequencies from the uncoupled LO phonon frequency ; its linewidth y is nearly the same as for undoped GaN. E(LO) phonon-plasmon mode, labelled
(b)
a)
-EI 2
'
-I
(x'x')z _(d)
ELO)The
CPPM(E 1 ), may be evidenced in the E'(zz)x' configuration. The activation of thiso forbidden » mode may be achieved thro
Data Loading...
