Micro-Raman Study of Charge Carrier Distribution and Cathodoluminescence Microanalysis of Porous gap Membranes
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(U
S~Bulk
•t•
GaP
Porous GaP
'U
360
380
400
420
Raman shift (cm' 1)
FIG. 1. SEM Micrograph of a porous membrane fabricated on substrate with (111) - orientation,
FIG. 2. Raman spectra from bulk (Il )oriented substrate and porous GaP membrane. the initial surface. Fig. 1 shows a SEM micrograph of the top surface of an anodized (I 11)Aoriented sample. One can see that the average pore and skeleton thickness is about 50 nm. The micro-Raman measurements on as-grown and porous GaP were performed at room temperature in a near backscatterin.1 geometry using a triple Jobin-Yvon spectrometer with a spectral resolution less than 0.5 cm' . The samples were excited by the 530.9 nm line of a Kr÷ ion laser. In order to avoid local heating, 2 the power of the laser beam was set up at 1 mW in a spot of I ýtm on the sample surface. The CL experiments were performed in a SEM equipped with Oxford Instruments MonoCL2 cathodoluminescence imaging and spectral analysis system, and cryogenic specimen stages. The CL was excited with a continuous electron beam at normal incidence, and measured using a retractable parabolic mirror collector. Spectra were collected over the wavelength range 250-900 nm using Hamamatsu R943-02 high sensitivity photomultiplier with a 1200 line/mm grating, blazed at 500 rum. They were collected over a range of beam energies (Eb = 15-30 keV) and beam currents (Ib = 0.25-100 nA) for specimen temperatures between -80-295 K. The spectral data were collected from -7000 Ptm 2 regions to reduce electron beam induced effects. The spectra were converted from wavelength to energy space, and corrected for monochromator dispersion and total instrument response. RESULTS Micro-Raman Scattering The Raman spectrum of as-grown (111)-oriented substrates displays the typical behavior of highly n-type doped GaP (Fig. 2). According to the selection rules for a (111)-oriented surface, both the TO- and LO-phonon modes are allowed in GaP. The TO-phonon band appears at a frequency of 365 cm'. The inherent to undoped material pure LO-phonon is no longer observed due to the interaction of lattice vibrations with the free-carrier plasma. This kind of interaction gives rise to the observation of LO-phonon-plasmon-coupled modes (LOPC), usually denoted by L+ and L. 8 Unlike the case of GaAs, the L. mode cannot be observed in GaP due to strong damping 9 caused by the low mobility of the free carriers. The L+ mode shows an upward frequency shift in comparison with that of the pure LO-phonon and its line-width is considerably broadened. The peak position co and line-width Ao) equal 406.4 cm 1 and 12.8 cmrespectively (see Table 1).
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TABLE 1. Parameters of Raman modes related to as-grown crystals and porous GaP membranes (for pure LO-phonon in undoped GaP: w = 402.3 cm" and AG) = 1 cm-) As-grown GaP 0), cm-I AG), cm-" LO L+, LOPC Surface related mode
-
-
406.4
12.8
-
-
Porous GaP W,cm-I A0, cm402.0 3.6 404.5 6.2 399.2 16.5
Measurements of RS in free-standing porous membranes (Fig. 2) show quite a different behavior. Wherea
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