D. Drews

Near-field Raman spectroscopy of semiconductor heterostructures and CVD-diamond layers

Abstract Raman scattering from semiconductor surfaces as well as from bulk and CVD diamond samples has been detected with high spatial resolution using a near-field optical microscope operating in the detection mode. First- and secondorder near-resonant Raman scattering involving the 250 cm −1 longitudinal optical (LO) phonon of ZnSe in an epitaxial ZnSe layer on GaAs has been studied in the vicinity of the heterointerface. CVD diamond samples were investigated using the near field as well as a conventional optical microscope. A positional dependence of the scattering from diamond and graphitic carbon has been found in the near-field experiments.

STUDY OF HYDROGEN AND METHANE MODIFICATION OF CVD DIAMOND BY XAS AT THE CARBON K-EDGE

Abstract The surface of a textured, (110) oriented, diamond film produced by chemical vapor deposition (CVD) was investigated using surface-sensitive X-ray absorption spectroscopy at the carbon 1s absorption edge. For surface modification, the film was exposed to different in-situ hot filament treatments involving atomic hydrogen and methane radicals. In the pre-edge absorption spectrum, we observe different resonances characteristic for transitions in non-diamond carbon species. They can be attributed to sp2-coordinated carbon atoms in amorphous or graphitic carbon phases and to C-H∗ resonances due to hydrocarbon surface components. The changes in the pre-edge structure during the hot-filament treatments are discussed in terms of hydrogen etching and growth of solid carbon.

The phototransformation of C60 thin films on GaAs(100) studied by in situ Raman spectroscopy

Raman spectra in the region of the pentagonal pinch mode Ag(2) of C60 were taken in situ during the deposition of C60 on the GaAs(100) surface at different temperatures. For very low coverages, only the feature corresponding to the pentagonal pinch mode of pristine C60 is visible. The onset of polymerization under laser irradiation occurred at thicknesses of about 15 nm which is attributed to a suppressive effect on the polymerization process due to the interaction of C60 with the substrate surface. The line shape for the feature due to photopolymerized C60 was different at each temperature indicating distinct polymeric states at different temperatures. These different states are discussed in comparison to recent theoretical calculations. Additionally, the photopolymerization due to irradiation after growth was investigated in situ.

Enhanced Raman signals in thin solid films formed from fullerene, copperphthalocyanine, or perylene derivates and incorporated metal clusters

Abstract Surface enhanced Raman spectroscopy (SERS) experiments have been carried out in thin organic films with incorporated metal clusters. Copperphthalocyanine, N , N ′-dimethyl-3,4,9,10-perylenedicarboximid (MPP) and fullerene have been used as matrix materials, while the clusters were formed from silver, gold, or copper. In these systems, maximum Raman cross-section enhancement values of the molecular matrix have been found to be around 70. The effects have been attributed to resonant excitation of local plasmon modes in the metal clusters. The Raman spectra are discussed with respect to spectral shape and Raman intensity enhancement in terms of a semiclassical microscopic SERS approach. In particular, the different effects of homogeneous and inhomogeneous plasmon line broadening on the SERS efficiency are extensively discussed.

Characterization of MOVPE grown InPSb/InAs heterostructures

Abstract InPSb was grown by low pressure metal-organic vapour phase epitaxy (LP-MOVPE) on InAs at temperatures between 520°C and 570°C. The samples were analysed by Raman and photoluminescence (PL) spectroscopy and high resolution X-ray diffraction (XRD). XRD as well as PL spectroscopy demonstrate the high quality of the deposited material. Sb contents of x = 0.31–0.34 in InP 1− x Sb x were investigated and lattice matching on InAs was achieved as controlled by XRD. Raman spectra of micron thick InPSb layers display InSb-like and InP-like phonon features and an additionally weak structure related to liberated Sb. p-type doping of the InPSb with diethylzinc or H 2 S causes a change of the lattice constant as indicated by XRD. The Raman spectra of the doped samples reveal changes in the lineshape of especially the InP-like phonon structure. Moreover, these changes differ for n- and p-type doping, which is interpreted in terms of free carrier contributions to the spectra via plasmon-phonon coupling. The predicted miscibility gap and its consequences for future opto-electronic applications are discussed with respect to the growth conditions and the doping.

Raman monitoring of molecular beam epitaxial growth of GaN on GaAs (100) and Si (111)

Epitaxial layers of GaN were grown by molecular beam epitaxy on GaAs (100) and Si (111) substrates. Nitrogen was provided from a rf-plasma source while elemental Ga was evaporated from a Knudsen cell. The growth process was performed in an ultrahigh vacuum chamber that is optically aligned with a multichannel Raman spectrometer. This setup allows Raman spectra to be taken online, i.e., during the growth process. Utilizing resonant Raman scattering conditions, spectra with a sufficient signal-to-noise ratio were taken even at the growth temperature of 600 °C. For both substrates the evolution of compressive strain at the interface was monitored from the frequency shift of the substrate phonons in the initial phase of the growth process.

The preparation of Sb contacts to molecular beam epitaxial ZnSe on GaAs(100) monitored by Raman spectroscopy

Epitaxial layers of ZnSe on GaAs(100) grown by molecular beam epitaxy (MBE) were capped with an amorphous selenium layer in order to protect the ZnSe surface against contamination during exposure to air. After transport the capping layer was removed again under ultrahigh vacuum conditions in a thermal desorption process, which was monitored by Raman spectroscopy. The spectra taken continuously while annealing the sample reveal the crystallization of the amorphous layer and then the complete desorption of Se at about 150°C. Thereafter, Sb was evaporated onto such clean ZnSe surface at room temperature (RT). The formation of the Sb contact was again monitored by Raman spectroscopy. Hereby the explosive phase transition from an initially amorphous to a crystalline Sb overlayer was observed online for the first time. In addition, the Sb desorption process was monitored for annealing up to 280°C. After this process new features occur in the Raman spectra, which are discussed in terms of scattering induced by interface roughening and cluster formation at the interface.

Raman monitoring of selenium decapping and subsequent antimony deposition on MBE-grown ZnSe(100)

Epitaxial layers of ZnSe were grown on GaAs(100) substrates using molecular beam epitaxy at a substrate temperature of 300°C. Immediately after growth the samples were capped with thick layers of amorphous Se as a protection against contamination of the ZnSe surface during the exposure to air, e.g. during transport. Such samples were then investigated using Raman monitoring. This new technique is realised by the combination of a specially designed ultrahigh vacuum (UHV) chamber with a triple Raman spectrometer equipped with multichannel detection. As a result, Raman spectra can be taken on-line and in situ from the sample surface during desorption or deposition processes. While Raman spectra from the capped samples taken at room temperature show only a broad structure centred around 255 cm -1 , which is characteristic for amorphous Se, the evolution of Raman spectra during the annealing of the samples indicates the formation of crystalline Se for temperatures above 100°C, as can be judged from the appearance of the well known phonon features of trigonal Se. Further annealing results in the complete desorption of the Se cap. Thereafter Sb was deposited on such clean ZnSe(100) surfaces. The formation of the Sb overlayer was investigated for the first time using Raman monitoring. In this case the on-line technique allows the so called explosive phase transition in the overlayer, i.e. from the amorphous to the crystalline state of Sb, to be observed without interrupting the deposition process.

The influence of a selenium interlayer on the In/GaAs(100) interface formation

Abstract The modification of clean GaAs(100) surfaces by in situ deposition of molecular selenium was investigated by synchrotron photoelectron emission spectroscopy. The Se deposition onto GaAs(100) at elevated temperature (330°C) leads to the formation of a Ga 2 Se 3 -like layer on the surface exhibiting a (2 × 1) LEED pattern. In addition, the selenium modification induces a reduction in band bending of 0.2 eV on n-doped GaAs while the Fermi level position on p-type GaAs is also shifted by 0.1 eV towards the conduction band. The subsequent In deposition results in a strong reaction of the In with the topmost Se and causes a further shift of the Fermi level of approximately 0.3 eV towards the conduction band for both types of doping. For large In coverages the saturation value for the Fermi level is located 1.25 eV on n-doped and 1.00 eV on p-doped Se modified GaAs(100) above the valence band, which is higher than the value observed for the unmodified In/n-GaAs(100) interfaces (0.75 eV).

Raman monitoring of ternary compound formation: ZnSxSe1 − x on GaAs(100)

Abstract Expitaxial layers of ZnS x Se 1 − x were grown on GaAs(100) at room temperature by molecular beam epitaxy (MBE) using compound sources for the evaporation of both ZnSe and ZnS. The formation of the ternary compound was monitored on-line by taking Raman spectra from the sample surface, i.e. continuously during the growth process. The spectra display ZnS x Se 1 − x -related scattering processes up to third order under resonant excitation using the 2.808 eV (441.6 nm) emission line of a HeCd laser. From the frequency positions of the ZnSe- and ZnS-like longitudinal optical (LO) phonon modes the sulfur content x was determined. The Raman scattering intensity displays a characteristic modulation upon deposition time due to Fabry-Perot interference of the incident as well as the scattered light. From the period of this modulation, the growth rate was calculated. Furthermore, photoluminescence (PL) spectra were taken in situ using the 3.81 eV (325 nm) emission line of the HeCd laser. They display the near band edge emission of the ZnS x Se 1 − x layer in the blue spectral region.