Micro-Raman study of columnar GaAs nanostructures

Transcription

1 phys. stat. sol. (a) 202, No. 8, (2005) / DOI /pssa Micro-Raman study of columnar GaAs nanostructures Pavel Prunici *, 1, Gert Irmer 1, Jochen Monecke 1, Lilian Sirbu 2, and Ion Tiginyanu 2 1 Institute of Theoretical Physics, TU Bergakademie Freiberg, Bernhard-von-Cotta-Strasse 4, Freiberg, Germany 2 Institute of Applied Physics, Technical University of Moldova, Boulevard Stefan cel Mare 168, 2004 Chisinau, Moldova Received 8 March 2004, revised 6 October 2004, accepted 27 January 2005 Published online 8 June 2005 PACS Fs, Ea, Yz Micro-Raman scattering spectra of bulk and porous GaAs and of individual nanocolumns were studied. The GaAs nanocolumns have radii of nm and lengths between 5 and 25 µm. The experimental data are compared with the results of calculations predicting coupled LO-phonon plasmon and coupled Fröhlich-plasmon modes using a dielectric function derived on the basis of an appropriate twodimensional effective medium theory. 1 Introduction There are many Raman scattering investigations of the charge carrier concentration and mobility in bulk GaAs samples over broad concentration and temperature ranges (see e.g. [1 3]). Relatively few papers are devoted to the study of charge carriers in porous GaAs, which attracted considerable attention (see e.g. [4 8]) due to its potential applications in different fields of electronics and photonics [9]. Here we present the results of Raman scattering analysis of bulk, porous and of nano-columnar GaAs. The charge carrier concentration along individual cylindrical columns as well as its radial distribution inside the cylinders could be obtained by interpreting the experimental spectra on the basis of an effective dielectric function theory [10 14]. New bands related to Fröhlich vibrational modes appear in the Raman scattering and infrared reflectivity spectra due to porosity. The properties of the Fröhlich modes strongly depend on symmetry type and degree of porosity as well as on the properties of the material filling the pores [13, 14]. 2 Experimental results and discussion Porous and columnar structures of GaAs were fabricated using electrochemical etching of (100)-oriented n-gaas substrates in an aqueous solution of HCl. An example of the porous medium consisting mainly of well oriented nanocolumns made by scanning electronic microscopy is presented in Fig. 1. The Raman scattering spectra of the samples were excited with a Nd:YAG laser and with a Ti:sapphire laser pumped by an Ar + laser. The excitation wavelengths were nm and 950 nm, respectively. The incident laser beam was focused to a diameter of ~1 µm. In order to avoid sample heating the power of the exciting beam was limited to a range between 2 and 14 mw. For the low temperature measurements (77 K), the samples were mounted to the cold finger of a cryostat. The scattered light was * Corresponding author: Pavel.Prunici@physik.tu-freiberg.de; Phone: , Fax:

2 phys. stat. sol. (a) 202, No. 8 (2005) / Fig. 1 An example of a porous medium consisting mainly of nanocolumns. analysed by a Jobin Yvon T64000 triple monochromator and multichannel detection was done by a nitrogen cooled CCD. In order to improve the spectral resolution, we made our measurements mainly using the Ti:sapphire laser with the wavelength 950 nm. In addition to this, GaAs samples are more transparent for this wavelength and the laser beam penetrates deeper into the sample. In this case the theoretical interpretation of the spectra is easier than in the case of an opaque sample. Typical experimental data are presented in Fig. 2. One can see that, in addition to the coupled LOphonon plasmon modes L + and L of the bulk material (Fig. 2b), coupled Fröhlich-plasmon modes appear in the porous structures (Fig. 2a). The Raman modes in the columnar structures were calculated with a two-dimensional effective medium theory (MGI2d, [13, 14]), which takes the plasmons into account. Coupled Fröhlich-plasmon modes appear which are frequency shifted in comparison with the LO-phonon plasmon modes of the bulk material. The frequency shift depends on the scattering geometry (see Fig. 3). For the Raman spectra shown in Fig. 2 the angle θ is near 90, were θ denotes the angle between the wave vector k of an excitation and the cylinder direction (Fig. 4b). We also observed changes in charge carrier concentration and mobility along the individual cylinders of the porous media at room temperature and at 77 K. One of the measured cylinders can be seen in Fig. 4a. Measuring Raman scattering at different points along the individual cylinders, a dependency was observed, which is illustrated in Fig. 5. We see that the position of the LO-phonon plasmon mode L + Fig. 2 a) Typical experimental Raman spectra of porous GaAs (solid line), their fit (dashed line) by using Lorentzians (dotted lines), b) experimental Raman spectra of bulk GaAs.

3 1564 P. Prunici et al.: Micro-Raman study of columnar GaAs nanostructures Fig. 3 Calculated peak positions in dependence of the angle between GaAs cylinder axes and wave vector k of the excitation. The numbers at the curves correspond to the bands of Fig. 2. a) b) Fig. 4 a) Example of a measured cylinder and b) theoretical configuration of the probe and notations. Fig. 5 Frequency positions of the plasmon mode in different points of an individual cylinder.

4 phys. stat. sol. (a) 202, No. 8 (2005) / Fig. 6 Poisson and Schottky equation modelling for GaAs cylinders with radius of 75 nm and 50 nm. Fig. 7 Carrier distribution in GaAs cylinders with different radii. changes from 412 cm 1 (at the top of the cylinder) up to cm 1 (at the root of the cylinder). There is an inhomogeneous charge distribution along the cylinder. Applying a two-dimensional effective medium theory (MGI2d, [13, 14]), which takes plasmons into account, the charge carrier concentrations at these positions were calculated to vary between cm 3 and cm 3. The L + -frequency in the porous region was ~422 cm 1 corresponding to ~ cm 3. In the homogeneous GaAs substrate we obtained 450 cm 1 corresponding to cm 3. The decrease of the charge carrier concentration in the porous layer and in the nanocolumns compared to the bulk value can be understood calculating the charge carrier distribution in cylinders of different radii by solving the Poisson equation. In Figs. 6 and 7 the results are shown using the following input parameters: n = cm 3, µ = 2000 cm 1, φ 0 = 0.7 ev (pinning of the Fermi energy level at the surface), T = 77 K. We observe a decrease of the carrier concentration in the middle of the cylinders with decreasing cylinder radii and a smooth transition of the charge carrier concentration from the middle of a cylinder of given radius to the depletion layer near the surface. In this way the experimentally obtained smaller L + -values compared to those of the substrate can be explained. We note that the simple Schottky model is not able to explain the experimental results. It results in a depletion layer of thickness: 2 f0 e0es l D ª n near a surface and in an unchanged carrier concentration for r below the depletion layer, Fig Conclusions Micro-Raman scattering spectroscopy proves to be a powerful tool for the purpose of characterizing charge carriers both in porous layers and individual nanocolumns of III V compounds. The information concerning free carrier concentration, mobility and depletion layers, obtained from the analysis of the Raman spectra, depends upon the morphology of the porous structures. We succeeded in study phonon spectra in individual nanocolumns of GaAs. The observed Raman modes are discussed on the basis of an appropriate effective medium theory.

5 1566 P. Prunici et al.: Micro-Raman study of columnar GaAs nanostructures Acknowledgement This work was done in the frame of the research group: Transportvorgänge in porösen Systemen der Verfahrens- und Geotechnik supported by the Deutsche Forschungsgemeinschaft (DFG). References [1] A. Mooradian and G. B. Wright, Phys. Rev. Lett. 16(22), (1966). [2] R. Fukasawa and S. Perkowitz, Phys. Rev. B 50(19), (1994). [3] F. Vallee, F. Ganikhanov, and F. Bogani, Phys. Rev. B 56(20), (1997). [4] T. Takizawa, S. Arai, and M. Nakahara, J. Appl. Phys. 33, L643 (1994). [5] I. M. Tiginyanu, G. Irmer, J. Monecke, and H. L. Hartnagel, Phys. Rev. B 55, 6739 (1997). [6] A. Anedda, A. Serpi, V. A. Karavanski, I. M. Tiginyanu, and V. M. Ichizli, Appl. Phys. Lett. 67, 3316 (1995). [7] P. Schmuki, J. Fraser, C. M. Vitus, M. J. Graham, and H. S. Isaacs, J. Elecrochem. Soc. 143, 3316 (1996). [8] D. J. Lockwood, P. Schmuki, H. J. Labbe, and J. Fraser, Physica E 4, 102 (1999). [9] J. Bell, Opto Laser Eur. 60, 31 (1999). [10] G. Irmer, M. Wenzel, and J. Monecke, Phys. Rev. B 56, 9524 (1997). [11] R. Ruppin and R. Englman, Rep. Prog. Phys. 33, 144 (1970). [12] J. Monecke, phys. stat. sol. (b) 155, 437 (1989). [13] A. Sarua, J. Monecke, G. Irmer, I. M. Tiginyanu, G. Gärtner, and H. L. Hartnagel, J. Phys. C 13, 6687 (2001). [14] I. M. Tiginyanu, A. Sarua, G. Irmer, J. Monecke, S. M. Hubbard, D. Pavlidis, and V. Valiaev, Phys. Rev. B 64, (2001).