DETECTION OF SURFACE CONTAMINANTS ON GaAs BY MEANS OF SURFACE ENHANCED RAMAN SCATTERING

DETECTION OF SURFACE CONTAMINANTS ON GaAs BY MEANS OF SURFACE ENHANCED RAMAN SCATTERING Consiglio Nazionale delle Ricerche (CNR), Institute for Nanostructured Materials (ISMN), Area della Ricerca di Roma,P.O. Box 010- 00016 Monterotondo Scalo, Roma (Italy), E-mail:quaglian@mlib.cnr.it. The purpose of this study was to show that by using SERS (Surface Enhanced Raman Scattering) spectroscopy it is possible to detect and characterize species adsorbed onto semiconductor surfaces, even if they are present at low concentration. An arsenic passivation layer has been deposited on GaAs layer directly after growth in the molecular beam epitaxy (MBE) reactor. Usually such passivation is applicable for further surface characterizations. As matter of fact this layer serve to protect the surfaces against contamination in air after removing the samples from the MBE growth reactor. Part of this As-cap layer has been removed by thermal desorption in an UHV chamber and island silver films with different thickness have been deposited. Silver films with were deposited by thermal evaporation under ultra-high vacuum. During the evaporation the deposition rate was controlled by a quarz thickness monitor and different sections of the sample allowing a comparison between silver coated and not coated areas100 Å thickness. metal island film is composed of nanometer size metal particles, the island structures. This film could be considered as a random array of individual particles. The Raman measurements were performed at room temperature on the samples in air for ex situ Raman spectra collection. (The deposition of arsenic onto a low temperature substrate yields an AMORPHOUS overlayer Careful examination of the spectra in the low frequency region shows (evidences) vibrational mode structure which could be assigned to amorphous arsenic The Raman spectra show a broad structure at about 225 wavenumbers that is characteristic for AMORPHOUS ARSENIC(alfa As). (amorphous STATE of arsenic is SEMICONDUCTING) Arsenic DETECTION SENSITIVITY of Raman technique (=30Å) After Ag deposition new peak was found at xx which coul been ascribed to Raman mode of amorphous arsenic. Without Ag-layer only the longitudinal optical (LO) phonon modes of GaAs can be detected, WHILE with the Ag film amorphous As modes can be seen IN ADDITION. (With growing thickness of Ag the band of ???? rapidly decreases in intensity) The optimum AMOUNT of enhancement is related to the thickness of Au film. The measurements show that too much Ag has a detrimental effect on the SERS signal. Therefore, the optimun thickness of Ag film is 100 Å. ON THE OTHER HAND The presence of Ag layer decreases the intensity of the longitudinal optical (LO) phonon modes of GaAs. This band slowly decreases in intensity with increasing thickness of Ag film. In the presence of silver film a reduced incident laser radiation reachs the surface of GaAs layer because the light penetrates through the silver layer before to reach the GaAs. It is not easy to give a ROUGH ESTIMATION of the cap thickness from the intensity of the GaAs Lo phonon mode (if the thickness of the As cap is in the order of the penetration depth of the light or less) Silver island films with thickness of =100 Å were deposited onto epitaxial GaAs, by simple evaporation. The deposition rate was very low because it was necessary to get a rough metal surface to obtain SERS enhancement. It is well known that a slowly formed film tends to grow more in height, forming island structures. Figure 1 shows a schematic sketch of the used samples. Raman measurements were performed on several places inside and outside the discontinuous Ag film deposited on our samples. We observed some differences between the coated and uncoated surfaces for glass and Si but not for GaAs, InP, InAs and silver. In Figure 3 we compare the Raman spectra from the glass sample, in the presence of the Ag film the spectrum drastically changes. (1) an decreased intensity of the band at 294 cm-1; (3) the appearance of the band at 1400 cm-1 probably due to the presence of amorphous arsenic on the GaAs surface. This band is absent on the Ag uncoated section of the sample and it was not observed from the other semiconductors coated with the Ag film. For this reason we can exclude that its origin is due to impurities in the Ag film, as in that case we should have observed this structure in all systems with an Ag overlayer. In our opinion, this band has an enhanced Raman cross-section due to the presence of the silver film. This work shows that species present on surface materials overlayed with a discontinuous silver film have a surface enhanced Raman signal. In particular, these results on semiconductor materials prove that surface species due to contamination can be detected by means of SERS effect depositing an Ag island film on the semiconductor surface. these new bands which appear in SERS spectra may correspond to species which are also present on the (material) surface, although at a very low concentration. These results demonstrate the sensitivity of SERS and suggest that Raman scattering by contaminants can be easily observed by SERS. Therefore, it is evident that with both methods the observed Raman signal was surface enhanced, in fact normal Raman scattering of adsorbates on semiconductors is too weak to be clearly observed. This extension of SERS spectroscopy to semiconductor materials is important for developing SERS as a new surface sensitive probe in material science. We demonstrate that by using Ag overlayer method surface species are clearly observed by SERS.