Far infrared (FIR) radiation of high-power pulsed laser incident normal to the surface of GaAs/metal tunnel structures with a self-consistent Schottky barrier gives rise to a change in the tunnel conductance. It has been shown that the observed photoresistive effects are caused by ponderomotive forces of the radiation field on the free electron plasma in the junctions. The change of tunnel conductance rises linearly with increasing intensity at low power levels and proceeds into a strongly superlinear dependence at high intensities. It is shown that this superlinearity is a result of an enhancement of the local radiation field in the near zone of diffraction by inhomogeneities at the metal-semiconductor interface and depends strongly on the roughness of the metal electrode. Experimental results are compared to a nonlinear extension of the theory of electron redistribution due to the radiation pressure

Aggarwal, I. D.

Almeida, J.

Coluzza, C.

Courjon, D.

Cricenti, A.

Generosi, R.

Gilligan, J. M.

Margaritondo, G.

Perfetti, P.

Spajer, M.

Tolk, N. H.

English

Christophe, Humbert

Buck, M.

Calderone, A.

Vigneron, J.-P.

Meunier, V.

Champagne, B.

Zheng, Wan-Quan

Tadjeddine, A.

Thiry, Paul A.

Peremans, A.

Archive Ouverte en Sciences de l'Information et de la Communication

In Situ Monitoring of the Self-Assembly of p-Nitroanilino Terminated Thiol on Gold: a Study by IR-vis Sum-Frequency Generation Spectroscopy

We study the electronic structure and the optical properties of germanium clusters up to about 10(3) atoms, by using a semi-empirical tight-binding approach. Germanium nanocrystals are related to quantum dots growing, by a self-organization process, in sapphire matrices. The optical properties of such dots are experimentally found to be strongly influenced by quantum confinement effects.. We investigate these effects from the theoretical point of view, calculating the optical spectra in the single-particle approximation. Inclusion of the electron-hole interaction is found to yield visible effects

M. Palummo

R. Del Sole

G. Onida

AIR Universita degli studi di Milano

Optical properties of germanium nanocrystals

Shul'man, A.

Ganichev, Sergey

Kotel'nikov, I.

Dizhur, E.

Prettl, Wilhelm

Ormont, A.

Fedorov, Y.

Zepezauer, E.

University of Regensburg Publication Server

Near-zone field effect of FIR laser radiation on tunnel current through the Schottky barrier under plasma reflection condition

The study of semiconductor interfaces and of solid interfaces in general requires novel instrument capable to investigate the lateral fluctuations of properties on a microscopic scale. We present the first result of a major effort in that framework, whose main objective is the exploitation of the unique characteristics of free electron laser (FEL) infrared sources. The background was provided by our previous development of FEL-based techniques to measure interface barriers with high accuracy and reliability. Quite recently, we were able to implement similar investigations with high lateral resolution. The key elements were the use of a small-tip optics fiber and its coupling with a scanning module; in this way, we achieved and verified the condition of near-field microscopy including a lateral resolution much below the wavelength value. Our discussion includes a presentation of the first scanning near-field optical microscopy (SNOM) images obtained with an FEL and data on small and microscopic-scale fluctuations of semiconductor interface barriers. The likely development of this exciting new field will also be discussed, in particular considering the new proposals for powerful FELs in the low-wavelength spectral range

Infoscience - École polytechnique fédérale de Lausanne

Interface applications of scanning near-field optical microscopy with a free electron laser

We report on a vibrational study of the self-assembly of para-nitroanilino-dodecane-thiol (p-NAT, NO2-C6H4-NH-(CH2)12-SH) molecules on polycrystalline gold surfaces, by means of sum-frequency generation (SFG). In the investigated spectral range (1150 to 1400 cm-1), the evolution of the vibrational fingerprint indicates that important reorientations of the molecules occur during film growth, until a stable conformation is reached slightly before saturation coverage. The experimental data are interpreted with the help of ab initio calculations from which the geometrical orientation of the self-assembled molecules can be deduced.</p

Humbert, Ch

Vigneron, J. P.

Zheng, Wan Quan

Thiry, P. A.

Repository of the University of Namur

In situ monitoring of the self-assembly of p-nitroanilino terminated thiol on gold:A study by IR-vis sum-frequency generation spectroscopy

PALUMMO, MAURIZIA

ONIDA, GIOVANNI

DEL SOLE, RODOLFO

We report reflectance anisotropy spectroscopy (RAS) results for the controlled dosing of atomic hydrogen onto a clean 3 degrees offcut Si(001)-(1 x 2) sample, cut so as to expose a single domain under ultra-high vacuum (UHV) conditions. Upon dosing approximately 0.8 monolayers of atomic hydrogen onto this surface at room temperature, followed by annealing, large changes in the RAS lineshape are observed. These are attributed to terrace disorder, step roughening and to the formation of the monohydride Si(001)-(1 x 2)-H phase, respectively, with increased annealing temperature. However, a (1 x 2) LEED pattern is observed throughout H adsorption and after annealing. We compare the RAS spectrum for the monohydride phase with microscopic calculations of the optical response and interesting agreement is found in the region of 3.2 eV

J. Power

W. Richter

Monohydride formation on vicinal Si(001) investigated by reflectance anisotropy spectroscopy

phys. stat. sol. (a) 175, 289 (1999)Subject classification: 78.66.Fd; 72.20.Ht; 72.30.+q; 72.40.+w; 73.40.Gk; S7.12Near-Zone Field Effect of FIR Laser Radiationon Tunnel Current through the Schottky Barrierunder Plasma Reflection ConditionA. Ya. Shulman1 (a), S. D. Ganichev (b), I. N. Kotelnikov (a), E. M. Dizhur (c),W. Prettl (b), A. B. Ormont (a), Yu. V. Fedorov (a), and E. Zepezauer (b)(a) Institute of Radioengineering and Electronics, Russian Academy of Sciences,103907 Moscow, Russia(b) Institut fuÈ r Experimentelle und Angewandte Physik, UniversitaÈt Regensburg,D-93040 Regensburg, Germany(c) Institute for High Pressure Physics, Russian Academy of Sciences, 142092 Troitsk,Russia(Received May 8, 1999)Far infrared (FIR) radiation of high-power pulsed laser incident normal to the surface of GaAs/metal tunnel structures with a self-consistent Schottky barrier gives rise to a change in the tunnelconductance. It has been shown that the observed photoresistive effects are caused by ponderomo-tive forces of the radiation field on the free electron plasma in the junctions. The change of tunnelconductance rises linearly with increasing intensity at low power levels and proceeds into astrongly superlinear dependence at high intensities. It is shown that this superlinearity is a result ofan enhancement of the local radiation field in the near zone of diffraction by inhomogeneities atthe metal±semiconductor interface and depends strongly on the roughness of the metal electrode.Experimental results are compared to a nonlinear extension of the theory of electron redistribu-tion due to the radiation pressure.1. IntroductionThe conductance of tunnel Schottky junctions formed by highly doped n-GaAs and asemitransparent metal electrode on its surface is changed by normally incident electro-magnetic radiation with frequency in the far infrared below the plasma edge of theelectron gas in GaAs. The electromagnetic wave being totally reflected by the plasmatransfers momentum to the electrons of the semiconductor. This leads to a spatial redis-tribution of the electrons due to the radiation pressure and yields a photoresistive ef-fect resulting from the corresponding change in the shape of the self-consistent Schott-ky-barrier [1 to 3]. The change of tunnel conductance in response to pulsed far-infrared(FIR) radiation rises linearly with increasing intensity at low power levels and proceedsinto a strongly superlinear dependence at high intensities. This superlinearity is a resultof an enhancement of the local radiation field in the near-zone owing to the diffractionby inhomogeneities at the metal/semiconductor interface. Because of the high dopinglevel the width of the Schottky barrier is of the order of 20 to 30 nm which is much less1 e-mail: ash@cplire.ru; Fax: (7-095) 203-8414; Tel.: (7-095) 203-4987A. Ya. Shulman et al.: Near-Zone Field Effect of FIR Laser Radiation on Tunnel Current 289than the extent of near-zone field variations of the order of mm. Therefore, the fieldstrength in the Schottky barrier region is to be of the same order as at the inhomogene-ities themselves and the onset of the nonlinear response depends essentially on thegrade of the surface inhomogeneity of the metal electrodes.Here we present experimental results for n-GaAs/Al tunnel junctions and comparethem to a nonlinear extension of the theory of electron redistribution due to radiationpressure. Both theory and experiment yield an effective enhancement of the radiationintensity close to the semiconductor/metal-gate interface compared to the incidentplane wave up to about 105.2. Theory2.1 Reconstruction of the Schottky barrierThe equation of motion of a degenerate electron plasma in the semiconductor underthe action of the electrostatic barrier field Est and an electromagnetic wave fieldE1;B1  can be written asmnx dvdt ÿrpx ÿ nx eEx ÿ nx ecv B1 ÿmnx vt:Here Ex  Estx  E1x; t is the electric field, B1 is the magnetic field, px is thepressure of the electron plasma with density nx and vx; t is the electron drift veloc-ity. The incident radiation is directed along the x-axis normally to the semiconductorsurface.The time-averaged Lorentz force acting on the electrons may be represented as pon-deromotive force originated by the divergence of the Maxwell stress tensorFi  dTikdxk ; where Txx 18pjE2st ÿj2jE21j ÿ12jB21j and j is the dielectric constant of the GaAs lattice. The equilibrium condition of theplasma elemental volume is dv=dt  0, that isÿ ddxpÿ eNEstx  ddx Txx  0 :The integration of this relation gives us the equation for the electrostatic field of thebarrier in the depletion layer with account for the radiationj8pE2st  NFx ÿ25m j16pwpw 2jE21j :Here F is the electrostatic barrier potential, m is the Fermi energy of the semiconductorelectron plasma, N is the ionized donor concentration in the semiconductor, wp is theplasma frequency, w is the frequency of the incident radiation.2.2 Change in the tunnel currentThe tunnel current of the irradiated junction may be presented in a simplified formI*V*; T*; J / 10de f e; T* ÿ f e eV*; T exp ÿG*e;V*; jE1j2 : 1290 A. Ya. Shulman et al.Here f e;T is the Fermi distribution function of the electrons with temperature T, e isthe electron energy, V is the bias voltage without irradiation, V*  V ÿ DUL,DUL  U*L ÿUL is the change in the voltage drop on the load resistance (see Fig. 1).The ª*º denotes the values of the correponding quantities during irradiation. The elec-tron temperature T* takes into account possible electron heating in the radiation field.The quasi-classical expression for the barrier transparency now depends on the electro-magnetic wave amplitude and may be written asG*e;V; jE1j2  2mhwpFbedFFÿ eFÿ 25Ke umvuuut : 2Here FbV is the band-bending height at the semiconductor±metal interface, u is thehigh-frequency potential related to the intensity J of the incident radiation,u  e2 jE1j24mw2 JNc: 3The coefficient Ke describes the electric field enhancement at the semiconductor sur-face. The change in the tunnel current due to incident radiation isDIV*; T*; J  I*V*; T*; J ÿ IV; T; J  0 : 4A quantitative description of the near-zone field effect needs to introduce an effectiveintensity Je  KeJ and to take into account that only a small fraction h of the junctionarea is affected by the enhanced near-zone field. Thus, we obtain for the total currentresponseDIt  h DIV*; T*; Je  1ÿ h DIV*; T; J  0 : 5The important feature of Eq. (5) should be stressed. The two terms on the right-handside have opposite signs since the barrier transparency and the electron temperatureare increased in the small region of high near-zone field while the current through theremaining area is decreased owing to a drop in the bias during the laser pulse ifRL 6 0. As a result, at high intensities the observed response is formed by a delicateNear-Zone Field Effect of FIR Laser Radiation on Tunnel Current 291Fig. 1. Schematic representation of the re-construction of the Schottky barrier due tothe incident FIR radiation. The dotted linerepresents the reconstructed barrier. Insets:Experimental set-up using a standardphotoconductivity measurement circuit(left). Reflectivity of the free electron gasin n-GaAs showing the plasma reflectionedge (right)balance between these two terms giving rise to a rather high sensitivity of the numer-ical analysis of the experimental data to inconsistence of the theoretical model with realphysics of the phenomenon.And finally, the equation for the response DUL  RL DI in the measurement circuit,RL DI V*; T*; Jÿ  V*ÿ V  0 : 6Another important variable in Eq. (6) is the electron temperature T* which is calcu-lated in the electron temperature approximation with taking into account the electronmomentum loss due to ionized impurities and the energy loss due to nonequilibriumLO and intervalley G$ L phonons. Taking the approach [5, 6] to the electron±pho-non heating of the dependence of the electron temperature T* on the intensity J wasderived in the following formT*  hw0=ln 1 12uehw0tetN0w0;T0@ 1A ; 7where w0 is the phonon frequency, N0 is the equilibrium Bose distribution, ue  uJe(see Eq. (3)), te and t are the energy and momentum relaxation time of the electrongas, respectively. The expression Eq. (7) is derived in high frequency approximationwt > 1. The ratio of the relaxation times te=t  a was chosen as one of the fitting para-meters being independent of J in addition to the Ke and h characterizing the electrody-namic part of the problem.Since the phonon heating is also present in the system the increase of the electrontemperature in L-valley is to be taken into consideration. This was done by introducingone more parameter b by means of the relation DT*L  b DT*G.3. Experimental ResultsTunnel semiconductor/metal Schottky junctions have been prepared by evaporation un-der various conditions of aluminum on MBE grown n-doped GaAs (2) with2 1018 cmÿ3 donor concentration. Special efforts were made to obtain semitransparentmetal electrodes with different grades of inhomogeneity. Scanning electron micrographsof a smooth metal film (type A) and a rough film (type B) are shown in Fig. 2. An292 A. Ya. Shulman et al.Fig. 2. Scanning electron micrographs of the metal films of Schottky-barrier junctions for variousfilm morphologies. The smooth film is left (type A) and the rough film is right (type B)additional important condition for thework is that the prepared metal±semi-conductor structures are in fact tunnel-ing junctions. This has been proved bymeasurements of tunneling characteris-tics by means of tunneling spectroscopytechnique.The experimental set-up of the photo-conductivity measurements is sketchedin Fig. 1. Pulses of a high-power FIRmolecular laser [7] have been appliedwith 40 ns duration and up to 2 MW/cm2peak intensity. The experimental resultspresented here were obtained at 90 mmNear-Zone Field Effect of FIR Laser Radiation on Tunnel Current 293Fig. 3. Intensity dependence of the responsefor two kinds of tunnel junctions. Data fortype A and B samples correspond to low andhigh near-zone enhancement of the radiationfield, respectively. The dashed line shows thelinear function of the intensity. The full linerepresents the theoretical curve calculatedaccording to Eq. (5) at T*  T and V*  VFig. 4. Upper part: response of rough elec-trode tunnel junction (type B) with highnear-field enhancement at positive bias.Squares are measurements, full lines repre-sent theoretical curves calculated accordingto Eq. (5) at T*  T for different values ofthe enhancement coefficient Ke. In the calcu-lated response at low intensities and small Keoscillations occur which are due to the inter-play between increase of barrier transparancyand drop of bias during the laser pulse asdiscussed after Eq. (5). Lower part: Mea-sured and calculated conductance of one ofthe tunnel n-GaAs/Al junctions. Fitting thecalculation to experimental data has beenused to determine junction parameters likethe height of the Schottky barrier and the io-nized impurity concentration. These para-meters are essential for calculating the near-field enhancement shown in the upper part20 physica (a) 175/1wavelength using NH3 as laser active medium. The intensity incident on the sampleshas been controlled by calibrated teflon attenuators. The measurements were carriedout at room temperature and at liquid nitrogen temperature.Fig. 3. shows the different degrees of nonlinearity of the photoresistive response oftype A and type B samples obtained at room temperature. The onset of superlinearityoccurs for the sample with smooth electrodes (type A) at about 500 kW/cm2 whereasthe response of the other sample type with rough electrodes (type B) is superlinear inthe whole intensity range of this measurement. The deviation of linear response inagreement to the measurement requires a field enhancement parameter Ke  103 andan active area fraction h  2:5 10ÿ2 resulting in the full line in this figure.The experimental data for the rough electrode junction (type B) are replotted inFig. 4, lower part, and compared to fittings using various intensity enhancement factorsKe and active area fractions h. An agreement to the extremely large nonlinearity of themeasured results is obtained with an effective enhancement of the local intensity ashigh as 6 104. An incident 90 mm radiation intensity of the order of 1 MW/cm2, corre-sponding to 18 kV/cm electric field amplitude, may lead at enhancement coefficient Keequal to 105 to a locally enhanced field as high as 1:6 MV/cm on a spatial scale of theorder of 1 mm. Under plasma reflection condition, the respective amplitude of the lat-eral electric field inside the Schottky barrier is about 75 kV/cm.We would like to note that this is the first observation of a giant radiation field sur-face enhancement in the far-infrared region whose values are comparable to those ob-served in the visible region for giant surface Raman scattering or surface second harmo-nic generation. As a result, theoretical models explaining the surface enhancement byplasma resonance effects in the metal film should be questioned because the character-istic frequencies of the plasma effects of well-conducting metals lie in the UV regionand cannot be responsible for an effect in the FIR region.294 A. Ya. Shulman et al.Fig. 5. Images of Al electrodes on GaAs surface for two subtypes of type B samples (thick metalfilm) with lower (left part) and higher (right part) grade of film inhomogeneity. The images areobtained by means of a He±Ne laser scanning microscope. The diameter of each electrode is about750 mmThe strong superlinearity of the response leads to a large radiation-induced change ofthe sample voltage which is comparable to the bias voltage. As the current±voltagecharacteristics is also strongly nonlinear, the evaluation of the photoconductivity dataneeds a very accurate knowledge of the current±voltage relation in the dark and atirradiation. A numerical method to calculate such a characteristic has been developedyielding good agreement to the measurements displayed in Fig. 4, upper part. Thesecalculations were used for the exact determination of the junction parameters like bar-rier height and doping level since the calculated response turned out to be very sensi-tive on these parameters.In order to explain the enhancement of the local electromagnetic field and the non-linear response of the junctions, a new approach to the diffraction theory for a conduct-ing screen with aperture has been suggested [8]. This procedure allows to carry out aqualitative or a semiquantitative evaluation of the near-zone field without cumbersomenumerical solutions of integral equations. The simple calculations predict the field en-hancement in a small aperture of order of l=d, where l is the radiation wavelength andd is the skin depth of metal film. For l  10ÿ2 cm and d  5 10ÿ6 cm this gives a fieldenhancement of about 103 and a corresponding effective intensity enhancement ofabout 106 experimentally observed here. Moreover, this approach allows to suggest anexplanation of the observed dependence of the enhancement on the metal film thick-ness. It turned out that the responses of junctions formed by thick (but semitranspar-ent!) metal films (type B) with two subtypes with lower (Fig. 5 left part) and higher(Fig. 5, right part) grade of film inhomogeneity have similar strong nonlinearity (com-pare Figs. 4 and 6). The thickness of the metal electrode appears quite essential for theNear-Zone Field Effect of FIR Laser Radiation on Tunnel Current 295Fig. 6. Left part: photoresistive response at low bias and photo-e.m.f. as a function of intensity. are the measured data,  denote the data points selected for the least-square search of optimalparameters Ke; h; a; b,  represent the calculated response obtained by numerical solution of thecircuit equation Eq. (6) for the bias voltage V* with using the optimal parameters. The dashed lineshows the smooth of the initial data set. Right part: temperature of G-valley electrons as a functionof the incident radiation intensity determined from photoresistive response at low bias and photoe.m.f. The solid line is calculated from the theoretical formula Eq. (7) and circles () again repre-sent the solution of the circuit equation Eq. (6) for the hot electron temperature with optimal val-ues Ke; h;b20*enhancement of near field. The enhancement increases with increasing film thickness aslong as the film thickness is less than the skin depth of the radiation field.In all experimental situations described until now electron heating is not important.In samples with small field enhancement, like type A, electron heating has not beenobserved [3]. In junctions of type B with very large field enhancement the effect ofelectron heating on the response is negligible at high bias voltages. However, at lowbias voltages or without bias voltage electron gas heating in the surface layer of thesemiconductor gets important in rough electrode samples (type B) with very high fieldenhancement. The electron heating is causing an e.m.f. and affects also the photocon-ductive signal at low and medium biases. The e.m.f. as a function of intensity is pre-sented in Fig. 6 together with results of numerical calculations of tunneling of heatedelectrons across the barrier which is modified by the ponderomotive force of the near-zone field. As a result, it was found that heating of electrons not only in the G-valleybut also in the L-valley must be taken into account as well as the generation of non-equilibrium LO phonons [5]. From this analysis the magnitude of the effective enhance-ment coefficient of the radiation intensity was confirmed and the temperatures of hotelectrons have been determined (Fig. 6), being as high as 2000 K at the maximumincident intensity of the order of 1 MW/cm2.Acknowledgements Financial support by Russian Foundation for Basic Researchesand Deutsche Forschungsgemeinschaft is gratefully acknowledged.References[1] S. D. Ganichev, K. Gloukh, I. N. Kotelnikov, N. A. Mordovets, A. Ya. Shulman, and I. D.Yaroshetskii; Soviet Phys. ± J. Exper. Theor. Phys. 75, 495 (1992).[2] A. Ya. Shulman, Proc. Internat. Semicond. Dev. Res. Symp., 1995, Vol. 1 (p. 229).[3] S. D. Ganichev, A. Ya. Shulman, I. N. Kotelnikov, N. A. Mordovets, and W. Prettl, Internat.J. Infrared and Millimeter Waves 17, 1353 (1996).[4] A. Ya. Shulman, I. N. Kotelnikov, S. D. Ganichev, E. M. Dizhur, A. B. Ormont, E. Zepezauer,and W. Prettl, Proc. 24th Internat. Conf. Physics of Semiconductors, Jerusalem 1998.[5] I. N. Kotelnikov, A. Ya. Shulman, S. D. Ganichev, N. A. Varvanin, B. Mayerhofer, andW. Prettl, Solid State Commun. 97, 827 (1996)[6] A. Ya. Shulman, phys. stat. sol. (b) 204, 136 (1997).[7] S. D. Ganichev, I. N. Yassievich, and W. Prettl, Phys. Solid State 39, 1703 (1997).[8] A. Ya. Shulman, Proc. Workshop Surface and Interface Optics '99, Sainte Maxime (France),May 4 to 8, 1999; phys. stat. sol. (a) 175, 279 (1999).296 A. Ya. Shulman et al.: Near-Zone Field Effect of FIR Laser Radiation on Tunnel Current

We observed chemical contrast with subwavelength resolution on a III-V heterostructure by a scanning near-field optical microscope (SNOM) working in the external reflection mode and coupled with a Free Electron Laser (FEL). SNOM reflectivity images revealed features that were not present in the corresponding shear-force (topology) images and are due to localized lateral changes in the optical properties of the sample. The data indicate an optical spatial resolution well below the diffraction limit of lambda/2 with most optical images having better lateral resolution than topographic images

Herold, G.

Chiaradia, P.

Chemical contrast observed at a III-V heterostructure by scanning near-field optical microscopy

http://epub.uni-regensburg.de/2216/1/gani7.pdf

Near-zone field effect of FIR laser radiation on tunnel current through the Schottky barrier under plasma reflection condition

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Infoscience - École polytechnique fédérale de Lausanne

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AIR Universita degli studi di Milano

University of Regensburg Publication Server

University of Regensburg Publication Server

Infoscience - École polytechnique fédérale de Lausanne