Low-temperature scanning system for near- and far-field optical

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1 Journal of Microscopy, Vol. 209, Pt 3 March 2003, pp Received 10 August 2002; accepted 25 October 2002 Low-temperature scanning system for near- and far-field optical Blackwell Publishing Ltd. investigations D. V. KAZANTSEV, C. DAL SAVIO, K. PIERZ, B. GÜTTLER & H.-U. DANZEBRINK Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, Braunschweig, Germany Key words. Cold finger cryostat, low-temperature microscopy, NSOM, photoluminescence spectroscopy, quantum dots, scanning near-field optical microscopy, SNOM. Received 10 August 2002; accepted 25 October 2002 Summary A combined system for far- and near-field optical spectroscopy consisting of a compact scanning near-field optical microscope and a dedicated spectrometer was realized. The set-up allows the optical investigation of samples at temperatures from 10 to 300 K. The sample positioning range is as large as mm 3 and the spatial resolution is in the range of 1.5 µm in the far-field optical microscopy mode at low temperatures. In the scanning near-field optical microscope mode the resolution is defined by the microfabricated cantilever probe, which is placed in the focus of a double-mirror objective. The tip-to-sample distance in the scanning near-field optical microscope is controlled by a beam deflection system in dynamic scanning force microscopy mode. After a description of the apparatus, scanning force topography images of selfassembled InAs quantum dots on a GaAs substrate with a density of less than one dot per square micrometre are shown, followed by the first spectroscopic investigations of such a sample. The presented results demonstrate the potential of the system. Introduction Decreasing the dimensions is of great importance for advances in many fields, such as microelectronics, micromechanics and also biotechnology. Nanoanalytical, optical measurement techniques for the characterization of such structures are urgently required. There is a strong demand for spatially resolved, analytical information right down to the size of quantum structures and molecules. The transition into the submicrometre range is, however, a turning point in the application of optical techniques because the spatial resolution Correspondence: H.-U. Danzebrink. Tel ; fax: ; Hans-Ulrich.Danzebrink@ptb.de of conventional optical methods is limited by diffraction. Fundamental microanalytical tools, such as conventional photoluminescence- and Raman-spectroscopy, are therefore not available for nanoanalytics. A possible solution to this problem is the combined application of scanning near-field optical microscopy (SNOM) (Paesler & Moyer, 1996; Ohtsu, 1998) and spectroscopy. The spatial resolution of a SNOM is mainly determined by the geometry of its near-field probe if the distance between surface and probe is significantly smaller than the optical wavelength. A spatial resolution of < 100 nm can be achieved in this way. Apart from high spatial resolution, low temperature is required for investigations of quantum dots (QDs), as these show largely increased photoluminescence (PL) emission lines only at low temperatures. Low-temperature optical set-up We succeeded in building a low-temperature optical scanning system, based on a commercially available cryostat with a helium-cooled cold finger (CryoVac). A principal sketch of the system in the cryostat is presented in Fig. 1. All elements are mounted inside the vacuum chamber of the cryostat. The sample is glued directly onto the cold finger. Its temperature is therefore defined by the helium flux, which is electronically controlled to obtain the desired temperature. The general advantage of the design based on a cold-finger cryostat (Behme et al., 1997; von Freymann et al., 2000) instead of a bath cryostat is that the piezoscanner is always at room temperature so that no scan range reduction caused by low temperatures occurs (Kazantsev, 1998). Furthermore, no special requirements of the components are to be taken into account apart from their vacuum compatibility. The whole system consists of two units: the optical system, which is fixed to a closed-loop piezoscanner (x, y, z: 100 µm; Piezosysteme Jena); and the cold finger and the sample, which are mounted on a stepper motor stage (X, Y, Z: 5 mm; CryoVac) The Royal Microscopical Society

2 200 D. V. KAZANTSEV ET AL. Fig. 1. Principal sketch of the low-temperature optical scanning system showing the optical system (OS) which is fixed to the piezoscanner (PS) as well as the cold finger (CF) and the sample (S) which are mounted on a stepper motor stage (SMS). The coarse positioning of the sample, like adjusting the optics to an area of interest or bringing the sample into focus, as well as the coarse approach in the case of SNOM mode are done by the stepper motor stage. The fine scanning of the optical system with respect to the sample is then performed by the closed-loop piezoscanner. Because the precision of the stepper motor stage is better than 1 µm it is also possible to carry out a sample scan by using the coarse stage only. This is interesting for those experiments in which a spatial optical resolution of 1 3 µm is already high enough. A conventional microscope objective is then used in a confocal arrangement as a laser scanning microscope with the coarse stage used for the surface raster mapping. In this case, the objective provides a significantly higher optical throughput and light collection efficiency than the SNOM mode. Light is guided to and from the microscope system with optical fibres via vacuum-tight throughputs. Thus, the only media in the optical path which might bring in extra Raman lines is the glass of the objective. This is advantageous in comparison with those micro-raman systems where the light is sent through the cryostat windows, as the additional Raman bands from the windows often conceal the weak signal from the sample surface. The bandpass filter to suppress the Raman lines of the input fibre can be inserted between the fibre collimator and the microscope objective. There is also an option to use polarization filters and retarders in order to investigate the polarization properties of the scattered light or PL signal. After

3 LOW TEMPERATURE SCANNING SYSTEM 201 Table 1. Parameters of the low-temperature scanning system. Temperature of the sample 6 K 300 K Precision to control the sample temperature 0.05 K XYZ positioning range 5 mm XY positioning stability better than 1 µm per 4 h Fine XYZ scanning range µm 3 Thermal drift of the system while cooling from µm in the sample plane room to liquid helium temperature Z positioning (for focusing) 4 mm Excitation efficiency 9% to deliver light from the laser to the sample with the monomode fibre; 31% in the case of the multimode fibre Light collection efficiency 25% in the case of collection from the sample with the multimode fibre; monomode fibres are not used for light collection Fig. 2. Principal sketch of the SNOM head (SH) consisting of a doublemirror objective system. The tip sample distance is monitored by beam deflection. The beam from the laser diode (LD) is focused onto the cantilever probe (CP) and reflected towards the quadrant photodiode (QP). interaction with the sample surface, the collected optical signal is analysed by a Fourier Transform spectrometer (Bruker Matrix). The parameters of the system developed are listed in Table 1. SNOM head microscope objective Besides using standard microscope objectives in the optical system, it is also possible to implement a specially designed SNOM head. This head replaces a standard objective in the turret of a conventional microscope. The threads and working distances of the head therefore correspond to those of standard objectives. A room temperature version of the head has already been described by Dal Savio et al. (2002). A principal sketch of the SNOM head is presented in Fig. 2. A microfabricated cantilever probe with an apertured tip is located at the focus of a double mirror objective. The objective allows focusing of the excitation light beam into the tip and collection of the scattered light from the tip (Dziomba et al., 1999, 2001). The optics consists entirely of mirrors. Thus avoiding refractive optical components, the SNOM head can be used in a wide spectral range from the UV to the IR. The separation distance between tip and surface is maintained by monitoring the damping of the tip vibrating normal to the surface. To measure the tip vibration, the beam of an auxiliary laser is focused onto the cantilever and the reflected beam is detected by a four-quadrant photodiode. Simultaneously, SNOM as well as SFM, images are recorded to obtain optical and topographical information. An optical resolution better than 170 nm (at λ = 1064 nm) has been achieved with this head in the room temperature configuration. Because the spatial resolution of the SNOM head is mainly defined by the quality and geometry of the near-field probes this value is not a limitation of the SNOM itself. For higher resolutions, probes with smaller apertures will soon be implemented. Quantum dot samples Sample fabrication The samples with self-assembled InAs QDs were grown by molecular beam epitaxy on semi-insulating GaAs(100) wafers. After growing a buffer layer of 500 nm of GaAs at a substrate temperature (T S ) of 600 C, the temperature was lowered over a period of 1 min and stabilized for 1 min at T S = 540 C. Without substrate rotation the InAs was deposited at a slow growth rate of 0046 monolayers per s in a cycled mode (5 s growth and 3 s growth interruption).

4 202 D. V. KAZANTSEV ET AL. Fig. 3. Topography image of the sample surface (InAs QDs on GaAs substrate). The atomic terraces of the GaAs substrate can be seen. Because of the nonconcentric position of the In-evaporation cell with respect to the substrate in the molecular beam epitaxy chamber, an almost linear gradient of InAs coverage across the diameter of the wafer is produced. With regard to the nominal InAs coverage in the centre of the wafer the InAs gradient is ±15% resulting in a variation of the density of InAs QDs from 0 to 80 µm 2 across the 2 wafer. After deposition of nominal 1.7 ml of InAs in the centre of the wafer, the QDs were capped by 50 nm of GaAs. This cap layer forms a potential barrier for the carriers captured in the QDs and also protects the QDs from degradation caused by oxidation. SFM measurements with the SNOM head objective The topography image of the sample (Fig. 3) was recorded at room temperature with the SNOM head described above in SFM mode and showing InAs QDs on a GaAs substrate. To make the QDs visible for SFM scanning, the cap layer deposition, which is necessary for the fabrication of QD samples used for optical investigations, was left out in the growth procedure. Figure 3 demonstrates that the vertical stability of the set-up with the SNOM head inserted is high enough over a short time scale to image even atomic terraces on the surface. The typical height of the imaged QDs of 6 10 nm is directly seen in the figure, whereas their lateral diameter could be estimated by deconvolution with the tip shape to nm. Micro-PL investigations Micro-PL investigations were carried out to characterize the QD emission (objective used: Leica, Apoplan 50). The samples consist of a layer of InAs QDs grown on GaAs covered with a 50 nm GaAs cap layer (as described above). To estimate the optimal working temperature, the dependence of the PL scattering efficiency with respect to the temperature was recorded using a Nd-YAG laser (λ = 1064 nm) for excitation. The result of this measurement was that no further significant increase of the PL yield occurs at temperatures below 100K. For the following studies, the sample was always cooled to 30K. Photoluminescence spectra were acquired at different positions on the sample. A blue shift of the PL peak energy was found when this position moved towards the region with a low QD density. This effect is explained by the decrease in the QD dimensions for lower densities. In order to study the emission of single or few QDs, measurements were carried out in a sample region where the QD density was estimated to be less than one QD per square micrometre. For this density and the resulting QD dimensions the emission energies were too high for Nd-Yag and we therefore used the He-Ne laser instead. At this energy, the dots are no longer resonantly excited; instead, excitation occurs via the conduction band of the GaAs. The generated charge carriers may diffuse before they relax and recombine in the QDs. In Fig. 4 photoluminescence spectra measured at 30 K using different excitation power densities (100, 10, 1 W cm 2 ) are shown. For the lowest excitation density (Fig. 4a) a single dominant peak appears with an energy of ev. This emission is due to recombination involving electron/hole pairs in the ground states of the InAs QDs on GaAs (Steer et al., 1996; Buckle et al., 1999). The width of this peak is in the order of 8 mev (with a spectrometer resolution of 0.25 mev). Although the lateral resolution of the optical system (> 1.5 µm) should allow for the excitation of a single QD, the measured linewidth is larger than expected for single dots (Fafard et al., 1994; Gammon et al., 1996; Dekel et al., 1998). This broadening is attributed to diffusion of carriers in the surrounding GaAs. Nevertheless, the sharpness of the peak (8 mev) indicates that only a few QDs are excited. PL experiments of InAs QDs on GaAs using conventional optics and medium density samples result in widths of 35 mev (e.g. Steer et al., 1996). As the intensity is increased (Fig. 4b) peaks at higher energies (1.22, mev) are clearly visible. These signals are attributed to recombination from excited states of the QDs and the differences between the energies correspond to results published elsewhere (Steer et al., 1996; Buckle et al., 1999). When the excitation intensity is increased further (Fig. 4c) the ground state is saturated and the excited states begin to dominate the spectrum (Heitz et al., 2001). To show the lateral distribution of the PL signals, a series of photoluminescence spectra is presented in Fig. 5 collected along a 8 µm line (1 spectrum per micrometre) on the same sample as in Fig. 4. The marked features show PL emission from QDs of different dimensions.

5 LOW TEMPERATURE SCANNING SYSTEM 203 Fig. 4. Photoluminescence spectra for different excitation power densities: (a) 1 W cm 2, (b) 10 W cm 2, (c) 100 W cm 2 all at λ = nm. Fig. 5. Photoluminescence spectra collected from different positions on the sample (excitation: λ = nm). The marked sharp peaks correspond to PL emission from the QDs. Conclusions An optical measuring system was set up which allows investigation of sample surfaces using either conventional far-field or scanning near-field optical methods. In order to characterize the luminescence behaviour of only few QDs, samples with a low QD density were fabricated (InAs QDs on GaAs). Densities of < 1 dot per µm 2 have been produced, as demonstrated by applying the scanning force mode of the SNOM head which provided topography images of single QDs and even atomic terraces during surface scanning. Low-temperature (T = 30K) micro-pl measurements carried out in the conventional optical microscopy mode were performed for optical characterization of the dots. A comparison

6 204 D. V. KAZANTSEV ET AL. of spectra from areas of different QD densities revealed a blue shift of the PL emission when low-density areas of the sample were measured. The width of the PL line in this area is only 8 mev, indicating that only a few QDs were excited. After an increase in the excitation power density, recombination, not only from the ground state, but also from excited states of the QDs, was detected. Also, to show the lateral distribution of the PL signals, spectra were recorded on different positions of the QD sample. More investigations are required to further characterize the system, e.g. of other samples and/or in near-field mode. The results achieved to date indicate that the realized system is a promising tool for micro- and most probably also nanoanalytic investigations. Acknowledgements The authors would like to thank Th. Dziomba (PTB), Dr P. Dawson (Manchester University) and Dr F.-J. Ahlers (PTB) for valuable discussions. We also thank our partners Surface Imaging Systems GmbH (Dr H.-A. Fuß), Bruker Optik GmbH (Dr R. Schlipper) and NanoWorld Services GmbH (Th. Sulzbach) for the good co-operation in the BMBF-project Near-field optical microscopy and -spectroscopy. Thanks are due to Dr G. von Freymann and Prof M. Wegener (Karlsruhe University) for very valuable advices during the initial state of the setup of our low-temperature system. This work is financially supported by the Federal German Ministry of Education and Research (BMBF project 13 N7643/6). References Behme, G., Richter, A., Sueptitz, M. & Lienau, Ch (1997) Vacuum nearfield scanning optical microscope for variable cryogenic temperatures. Rev. Sci. Instrum. 68, Buckle, P.D., Dawson, P., Hall, S.A. et al. (1999) Photoluminescence decay time measurements from self-organized InAs/GaAs quantum dots. J. Appl. Phys. 86, Dal Savio, C., Wolff, H., Dziomba, T., Fuss, H.-A. & Danzebrink, H.-U. (2002) A compact sensor-head for simultaneous scanning force and near-field optical microscopy. Precision Engineering, 26, Dekel, E., Gershoni, D., Ehrenfreund, E., Spektor, D., Garcia, J.M. & Petroff, M. (1998) Multiexciton Spectroscopy of a Single Self-Assembled Quantum Dot. Phys. Rev. Lett. 80, Dziomba, T., Danzebrink, H.U., Lehrer, Ch, Frey, L., Sulzbach, Th & Ohlsson, O. (2001) High-resolution constant-heigh imaging with apertured silicon cantilever probes. J. Microsc. 202 (1), Dziomba, T., Sulzbach, Th, Ohlsson, O., Lehrer, Ch, Frey, L. & Danzebrink, H.U. (1999) Ion beam-treated silicon probes operated in transmission and cross-polarized reflection mode near-infrared scanning near-field optical microscopy (NIR-SNOM). Surf. Interface Anal. 27, Fafard, S., Leon, R., Leonard, D., Merz, J.L. & Petroff, P.M. (1994) Visible photoluminescence from N-dot ensembles and the linewidth of ultrasmall Al y In 1 y As/Al x Ga 1 x As quantum dots. Phys. Rev. B, 50, von Freymann, G., Luerssen, D., Rabenstein, C. et al. (2000) Near-field photoluminescence imaging of single defects in a ZnSe quantum-well structure at low temperatures. Appl. Phys. Lett. 76 (2), Gammon, D., Snow, E.S., Shanabrook, B.V., Katzer, D.S. & Park, D. (1996) Homogeneous Linewidths in the Optical Spectrum of a Single Gallium Arsenide Quantum Dot. Science, 273, Heitz, R., Born, H., Guffarth, F., Stier, O., Schliwa, A., Hoffmann, A. & Bimberg, D. (2001) Existence of a phonon bottleneck for excitons in quantum dots. Phys. Rev. B, 64 (241305), 1 4. Kazantsev, D.V. (1998) A simple scanning head for scanning near-field optical microscope. Ultramicroscopy, 71 (1 4), Ohtsu, M. (1998) Near-Field Nano/Atom Optics and Technology. Springer, Tokyo. Paesler, M.A. & Moyer, P.J. (1996) Near-Field Optics: Theory, Instrumentation, and Applications. Wiley-Interscience, New York. Steer, M.J., Mowbray, D.J., Tribe, W.R. et al. (1996) Electronic energy levels and energy relaxation mechanisms in self-organized InAs/GaAs quantum dots. Phys. Rev. B, 54 (24),