Semiconductor

24 Dec 2013

Solar Cells

Optical Property - Photoconductivity   Topography (left image) and photocurrent (right images) with laser source on (top right image) and laser source off (bottom right image) show the effect of light in current generation; note the increased amount of photo-induced current in the upper image.   Park AFM Productivity in Action Jeongjin Lee, Inhee Choi, Surin Hong, SuSeung Lee, Young In Yang, Younghun Kim, Jongheop Yi; Construction of pcAFM module to measure photoconductance with a nanoscale spatial resolution;  Ultramicroscopy 108 (2008) 1090– 1093  Equipment: Park Systems XE-150 Abstract: A photoconductive atomic force microscopy (pcAFM) module was designed and the performance was tested. This module consisted of three units: the conductive mirror-plate, the steering mirror and the laser source. The module with a laser irradiation unit was equipped to a conventional conducting probe atomic force microscopy (C-AFM) instrument to measure photoconductance in a nanoscale resolution. As a proof-of-concept experiment, the photo-conductance of aggregated fullerene on indium tin oxide (ITO) substrate was measured with this module. The electrical signals (currents) of aggregated fullerene under the conditions of laser on/off at about -10V sample bias voltage were -100 to -160 nA and 0 to -20 nA, respectively. Results indicated that the pcAFM with this module allowed one to observe photoinduced changes of electrical properties in nanodevices with nanoscale spatial resolution.   Fig 3. (a)3-D topography of the contact point for the I/V curve measurement and (b) the measured I/V curve. The resistance was decreased by a factor of 5~100. The arrow in (a) indicateds the point at the slope of the sub-circle line, ca, 10 nm height from the boom. At this point, the electrical signals (currents) of aggregated fullerene under the conditions of laser on/off at about -10V sample bias voltage were -100 to -160nA and 0 to -20nA, respectively. Sample:   -  Solar Cell Image Conditions:   -  Contact  Mode AFM  -  Tr-PCM  -  Force ( ?? nN)  -  Scan Speed (?? Hz) System Requirement:    -  Closed-loop AFM System  -  Conductive AFM The Benefits Park AFM series offers many modes that are well suited for phtoonic devices and materials. In particular, Time-resolved Photocurrent Mapping (Tr-PCM) offers the ability to obtain the conductivity data with the AFM laser turned off (at alternative scan lines) so  time resolved photo-induced current can be measured: a) with topography b) with nanoscale spatial resolution c) without interference from the AFM laser source.

23 Dec 2013

SrTiO3 Surfaces by Using Park Systems AFM

SrTiO3 Surfaces by Using Park Systems Atomic Force Microscope John G. Connell and S. S. Ambrose Seo Department of Physics and Astronomy University of Kentucky, Lexington, KY 40506 General considerations Atomically flat SrTiO3 surfaces are necessary for epitaxial thin film and heterostructure growth.  The common method of substrate surface preparation requires the use of BHF (buffered HF) [1].  This new method does not use any acid-etchant in its process.   Park AFM System Overview   Surface topography of the substrates is necessary in order to characterize the substrates for future heterostructure growth.  The Park AFM’s non-contact tapping mode is perfect for imaging the step-terrace formations of SrTiO3 as well as other formations on the substrate surface.  The non-contact tapping mode allows us nanometer resolution without disturbing the surface of the substrate.   Substrate surface evolution via acid-ecthantless method Figure 1.a and f show the topography and line profile of the SrTiO3 substrate after annealing at 1000C for 1 hr. in air, respectively.   Figure 1  Notice the ~2 nm hills on the surface of figure 1 a.  Figure 1 b and g show the sample after the acid-less etchant technique.  Figure 1.c and h shows the sample after annealing at 1000C for 1 hr. in air.  Figure 1 d and i shows the sample after a second etchant treatment.  Figure 1.e and j shows the sample after annealing the substrate at 1000C.   Notice from Figure1.a through c how the ~2nm hills are reduced.  In d and e the resolution of the Park AFM allows us to see the small double steps and pits, which are clear in the average line profile.   Conclusion The Park AFM’s non-contact tapping mode easily gives us publishable images time after time.  We plan to continue to use it as an integral part of our research as well as one of our primary methods of atomically flat substrate characterization.   References   [1] Masashi Kawasaki, Kazuhiro Takahashi, Tatsuro Maeda, Ryuta Tsuchiya, Makoto Shinohara, Osamu Ishiyama, Takuzo Yonezawa, Mamoru Yoshimoto, and Hideomi Koinuma  Science 2 (1994) 1540-1542.

23 Dec 2013

Nanoscale Surface Photovoltage

Spectroscopy Investigation of Nanostructures Utilizing the Park AFM Instruments Case Study: Zinc Oxide Nanostructures BY T.A. Merz, D.R. Doutt, and L.J. Brillson Department of Physics, Ohio State University General Considerations Zinc Oxide (ZnO) is increasingly gaining attention in the research community because of its excellent bulk properties -- high exciton binding energy, high sensitivity to adsorbate charge transfer, and high carrier mobility -- but also, more recently, because of its remarkable ability to grow nanostructures.  There has been much research done in the development of growth procedures and increasing the understanding of the growth mechanisms for induced nanostructures and their use in devices[i],[ii]. However, there has been very little work done on the examination of electronic properties on the nanoscale and how these properties feed into the nucleation and growth of said nanostructures or how these electronic properties affect the ability to make reliable Schottky contacts to ZnO[iii]. Furthermore, the exploration of nanostructures has yet to venture into the realm of spontaneous structure growth. The ability to understand and thus potentially control how electronic, morphological, and chemical disparities all play a synergistic role in the behavior of ZnO devices should be a high priority of all those currently investigating ZnO as an electronic material and especially those that are looking to use it to make nanoscale devices.  The investigation of this trinity of defects -- electronic, morphological, and chemical -- on the nanoscale is by no means an easy task and might explain the relative lack of work on the topic.  There are a number of techniques that are capable of looking at  one or two of these properties on a localized scale, most notably SEM with depth resolved cathodoluminescence or AFM/KPFM, but being able to examine all three simultaneously at a reasonable price has evaded many well meaning attempts. Figure 1 shows one sample of these spontaneous nanostructures formed on a Zn-polar ZnO surface as imaged by a Park AFM. The alignment of these nanostructures along crystallographic directions is shown in Fig. 1 as well as a close up of a string of nano-mounds highlighting the trenches surrounding the base of each mound, shown as a break out on the right side. This map emphasizes the complexity and compound nature of the spontaneous nanostructure growth that is spontaneously possible. It is through the scanning probe revolution that we are able to offer a cluster of three techniques that allow one to probe the electronic, morphological, and chemical landscape of any semiconducting sample in ambient conditions.  The use of AFM and KPFM allow for the simultaneous mapping of the morphological and electronic features of a sample while the inclusion of nanoscale surface photovoltage spectroscopy (SPS) allows for the probing of chemical defects at isolated positions, which can be selected from either AFM or KPFM maps, or over large regions. Figure 1. Spontaneous nanostructure growth on the Zn-polar surface of ZnO.   Park AFM System and Nanoscale Surface Photovoltage Spectroscopy Overview  At the core of the nanoscale SPS is the Park AFM/KPFM with the Enhanced EFM module.  The Park's unique open head design allows for an optical fiber to be brought into close proximity to the sample's surface.  This geometry permits us to illuminate the sample surface with monochromatic light while the AFM/KPFM mode is enabled and scanning.  With increasing photon energy above characteristic threshold values, the light pumps carriers into and out of surface states within the semiconductor bandgap, changing the band bending and the Fermi level at the surface, which can be simultaneously monitored with the KPFM module.  Changes that are seen in the KPFM signal may then be interpreted as movement of the Fermi level because of the population/depopulation of such surface or defect states through optical excitation.  The energy of photons at which the KPFM signal changes offers the relative energy of the transition and therefore the characteristic energy of the defect level and the sign of the change in KPFM signal pins down whether the characteristic energy is relative to the valence band or the conduction band. This allows for the exact determination of defect states within the bandgap on a localized scale: a marked improvement over traditional luminescence techniques where only energy differences are possible to determine. Figure 2 shows the schematic setup of a nanoscale SPS setup. The metalized cantilever, PSPS, and laser diode are all contained within the Park AFM/KPFM system. Figure 2. Schematic of the nanoscale SPS monochromatized light source positioned near a Park Systems KPFM cantilever. The tip of the fiber optic cable is 2 cm from the metalized tip and illuminates a 5 mm diameter spot.   Investigation of ZnO Nanostructures Figure 3 shows two simultaneous maps (a) and (b) acquired with a Park AFM and Enhanced EFM mode respectively.  The two maps are from the Zn-polar surface of an air exposed ZnO single crystal.  Three separate regions, of two distinct types, are visible in the maps. The three regions are labeled with markers (1), (2), and (3).  Regions (1) and (3) show a high concentration of nano-mounds as evident in the AFM map; the regions also show a markedly lower EFM signal than region (2) which is relatively smooth topographically. Figures 3(c), (d), and (e) show corresponding SPS spectra from the localized points (1), (2), and (3). For the regions with nanostrcuture growth there is an obvious extra feature in the SPS spectra beside the bandgap response at 3.34 eV, that is absent in the smooth region's SPS spectra. The feature in Fig. 3 (c) and (e) are from a surface defect at 1.7-1.8 eV below the conduction band, which is connected by other experimental evidence to the VZn[iv]. The use of all three of these techniques together allows a complete picture of the topographical, electronic, and chemical structure on the nanoscale. Figure 4 (a) shows an AFM map highlighting the area near one isolated nano-mound from another Zn-polar. Four SPS spectra were taken from four separate places on the AFM map, each of the four places farther from the tip of the nano-mound than the last. Figure 4 (b), (c), (d), and (e) show the SPS spectra from these four points which correspond to positions at the peak of the nano-mound (b), at the edge of the nano-mound (c), 350 nm from the edge of the nano-mound, and 750 nm from the edge of the nano-mound. At each of these locations there is an obvious change in the defect level at 2.1 eV, demonstrating a resolution for chemical analysis of better than 100 nm. A more complete analysis and demonstration of the nanoscale SPS is written up elsewhere[v]. Figure 3. (a) AFM map of ZnO(000 ī) surface with two adjoining surface morphologies: a 1.27 nm rms smooth surface region (2) bounded by surfaces with clustered nano-mounds (1 and 3). (b) KPFM map from the same area. Regions (1) and (3) show lower CPD (larger Φ) than (2). SPS spectra (c) and (e), respectively show clear onsets at 1.7–1.8 eV that are absent in spectrum (2).   Figure 4. (a) Topographical map of a single nano-mound on a ZnO(0001) face. Spectra show: (b) no evidence of a 2.1 eV feature, (c) a positive CPD change at 2.1 eV, (d) a larger CPD slope change at 2.1 eV at the nano-mound edge, and (e) no evidence of gap states atop nano-mound.   Conclusion The ability to simultaneously image topographical, chemical, and electronic local environments on the nanometer scale is not only a highly powerful technique for understanding the basic physics of materials, but it is also is very useful in examining device structures and their failure mechanisms[vi]. Through the ability to examine and interpret material behavior in a complete way, non-destructive way it may become possible to then control material behavior with these insights. The combination of AFM/KPFM/SPS offers the perfect complement of techniques to measure these three different, but related, local environments. The Park AFM/KPFM has proven to be superior to other designs by having an open design where additions and complementary equipment may be included by the end user to flexibly change their system to do the unique tasks that they wish to accomplish which in turn further both scanning probe technology and basic science. Note This work was supported by the National Science Foundation under Award No. DMR-0803276 (Verne Hess and Charles Ying) under a Research Experience for Undergraduates. References [i] S.J. Pearton, D.P. Norton, D.P., K. Ip, Y.W. Heo, T. Steiner, Prog.Mater. Sci. 50 (2005) 293. [ii] L. Schmidt-Mende, J.L. MacManus-Driscoll, Mater. Today 10 (2007) 40. [iii] L.J. Brillson, H.L. Mosbacker, M.J. Hetzer, Y. Strzhemechny, G.H. Jessen, D.C. Look. G. Cantwell, J. Zhang, and J.J. Song, Appl. Phys. Lett. 90 (2007) 102116. [iv] Y. Dong, F. Tuomisto, B.G. Svensson, A.Y. Kuznetsov and L.J. Brillson, Phys. Rev. B. 81 (2010) 081201. [v] T.A. Merz, D.R. Doutt, T. Bolton, Y. Dong, and L.J. Brillson, Surf. Sci. Lett. (doi:10.1016/j.susc.2010.12.021). [vi] C. Lin, T.A. Merz, D.R. Doutt, M.J. Hetzer, J. Joh, J.A. del Alamo, U.K. Mishra, L.J. Brillson, Appl. Phys. Lett. 95 (2009) 033510.

23 Dec 2013

Zinc Oxide Surfaces

Nanoscale AFM and KPFM Mapping of Localized Charge and Recombination Centers on Chemically Active ZnO Surfaces Case Study: Zinc Oxide Surfaces   General Considerations In recent years, ZnO has gained popularity among researchers due to its excellent ability to grow nanostructures with a variety of unique geometries. Advances in ZnO growth and processing have enabled many opto- and microelectronic applications with nanoscale dimensions such as laser and blue/UV light emitters, yet the impact of surface morphology and near surface defects on optical emission has remained largely unexplored. Techniques such as atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) coupled with depth resolved cathodoluminescence spectroscopy (DRCLS) are capable of exploring these features on a nanoscale. Surface morphological features produce near surface defects that affect surface potentials and recombination velocity. This highlights the importance of overall surface quality and surface preparation. Consequently, AFM roughness values coupled with KPFM potential maps may act as a predictive tools for assessing surface optoelectronic quality for nanoscale optical devices. We have used nanoscale DRCLS, AFM, and KPFM to measure the spatial distribution of defects within the outer 10-100nm of the surface of ZnO single crystal wafers grown by melt, hydrothermal, and vapor-phase transport methods. Park AFM System Overview At the core of our nanoscale measurements is the Park AFM/KPFM with the Enhanced EFM module. The high lateral and z-axis resolution allows for roughness measurements below unit cell dimensions. This coupled with KPFM potential maps and DRCLS emission allows for accurate determination of optical efficiency. Furthermore, the True Non-Contact mode successfully maintains the tip-sample distance of only a few nanometers allowing for measurements that do not disrupt the surface or its features. This capability is vital for obtaining accurate information about near or subsurface defects and feature that they relate to. Investigation of ZnO Surfaces Figure 1 shows DRCLS spectra from two ZnO single crystal sample grown by hydrothermal methods from different sources with relatively high 1(a) and low 1(b) deep level optical emission at that can be de-convolved into contributions from both ~2.1 and 2.5 eV. Previous work has identified the deep level emission at 2.1 and 2.5 eV to be related to zinc vacancies (VZn) and oxygen vacancies (VO), respectively[i]. We may understand the relative densities of these defects by taking the ratio of NBE to maximum defect  emission for a given probe energy (EB). For 1(a) (hydrothermal-16) we see that I(NBE)/I(2.1-2.5 eV) at EB = 2 keV results in a ratio R( 2 keV) = 203. The deep level emission changes with excitation energy, first decreasing for EB = 5 keV, then increasing such that R(10 keV) = 84. Figure 1(b) (hydrothermal-1) on the other hand, shows near-surface emission with R(2 keV) = 933. All higher EB spectra show a relative increase in defect emission such that R(5 - 20 keV) ~ 155, i.e. ~6x lower NBE emission efficiency. Figure 1. DRCLS spectra of (a) high and (b) low near-surface defect ZnO. The latter shows order-of-magnitude changes in deep level defect emission with depth.[ii]   Figure 2 shows AFM and KPFM maps for both hydrothermal-16 and -1 imaged simultaneously with a Park AFM in Enhanced EFM mode. AFM topography for Hydrothermal-16 (a) reveals rms roughness values of ~2.022 and average roughness values of ~1.505. On the other hand, topography for Hydrotherm-1 2(b) reveals rms roughness value of ~0.644 nm and average roughness values of ~ 0.287 nm. Hydrothermal-16 shows potential changes 2(c) consistent with surface morphology 2(a) due to defects at and below the surface. Circular surface pits (red) with depths ranging from ~2 - 20 nm exhibit potential decreases of greater than 120 mV with increasing radial distance. Conversely, Hydrothermal-1 shows laterally non-uniform potential variations 2(d) due to densities of electrically active defects below the surface. These potential variations of ~ 50 mV do not correlate with any morphological features and are consistent with a smoother surface. This is also consistent with DRCLS spectra that indicate a strong increase in defect densities just below the free surface. The clear reduction of native point defects and surface asperities indicated by both AFM and DRCLS is due to chemomechanical polishing and is a powerful example of the importance of surface preparation. Figure 2. AFM morphology maps (a) and (b) corresponding to hydrothermal-16 and hydrothermal-1, respectively, in Figure 1(a) and 1(b). KPFM potential maps (c) and (d) acquired simultaneously with AFM maps (a) and (b), respectively.ii   We can identify how surface morphology impacts the efficiency of NBE optical emission by introducing native point defects and competing pathways by plotting the ratio of NBE/defect emission as a function of rms and average surface roughness. Figure 3 shows R(2 keV) versus average (a) and rms (b) roughness values for a wide collection of hydrothermal, melt, and vapor phase transport grown ZnO surfaces. We find that rough surfaces exhibit R values ~200 while smoother surface have R > 13,000. Furthermore, both (a) and (b) plots reveal a well-defined threshold dependence. We find that R increases rapidly when surface roughness decreases below a ZnO unit cell of c = 0.52 nm. This suggests that lattice imperfections at the unit cell dimension, rather than extended morphological defects, dominate recombination and NBE intensity. Figure 3. The ratio of NBE to deep level intensities I(3.3 eV)/I(2.1-2.5 eV) for EB = 2 keV for hydrothermal, melt, and vapor phase transport grown ZnO as a function of AFM (a) average roughness and (b) rms roughness displays a well defined threshold dependence that increases rapidly for average roughness <0.3 nm and rms roughness <0.5 nm.ii   The increase in the ratio of NBE to deep level emission and the corresponding relative emission efficiency of NBE radiation is due to lower densities of defects (ND). The sharp increase for roughness’ below 0.5 nm corresponds to a strong ND decrease when roughness is due only to monolayer step edges. Using a modified formalism derived for spherical pores we may estimate the surface charge density within surface asperities as well the overall carrier concentration near the surface[iii].  This formalism yields a hemispherical surface pore charge density of 4.2+0.8 x 1011 cm-2 and a carrier concentration of 8.4+0.5 x 1015 cm-3 accounts for the observed 100 meV potential decreases away from the pore perimeter over distances of ~ 125 nm. These potential variations can thus provide a measure of fixed surface charge density within the pore and a bulk free carrier density at the surface.ii   Conclusion In summary, AFM reveals large variations in surface roughness, asperities, and extended surface features related to growth method and subsequent surface treatments. KPFM surface and subsurface potential variations acquired simultaneously reveal systematic correlations with defect distributions measured by DRCLS. These results stress the importance of surface roughness and surface preparation to minimize surface recombination by free carriers due to defects. The Park AFM scanning probe microscopes non-contact scanning capabilities has proven itself an important tool for understanding material properties and electronic structure without disrupting the surface. The ability to probe the outer few nanometers of any surface and identify the distribution of electrically active defects using KPFM is a powerful tool that can be used to predict surface and optoelectronic quality for nanoscale optical device.   Note  This work was supported by the National Science Foundation under Award No. DMR-0513968 (Verne Hess). References [i] Y. Dong, F. Tuomisto, B.G. Svensson, A.Y. Kuznetsov and L.J. Brillson, Phys. Rev. B. 81 (2010) 081201. [ii] D. Doutt, H. L. Mosbacker, G. Cantwell, J. Zhang, J. J. Song, and L. J. Brillson, Appl. Phys. Lett. 94, 042111 (2009). [iii] D.C. Look, Z.Q. Fang, S. Soloviev, T.S. Sudarshan, and J.J. Boeckl, Phys. Rev. B. 69 (2004) 195205 By D.R. Doutt  and L. J. Brillson

23 Dec 2013

AlGaN/GaN HEMT Reliability

Park AFM instruments create new ways to understand GaN HEMT reliability issues Case Study: AlGaN/GaN HEMT reliabilityBy Chung-Han Lin, D. R. Doutt, and L. J. Brillson General considerations AlGaN/GaN high electron mobility transistors (HEMTs) are promising devices for future high power and high speed application due to their high breakdown voltage, high current density, and high temperature capability1.  However, high voltage and current generate large piezoelectric stress2 and high temperature3, challenging our understanding of AlGaN/GaN HEMT device reliability and device failure.  High temperature generated inside the operating device combines with other effects such as field-induced mechanical stress, occurs in different localized areas and makes device failure difficult to understand and predict. Park AFM System Overview At the core of our nanoscale measurements is the Park AFM system with the Enhanced EFM module. The high lateral resolution allows for investigating highly localized failure regions with small device dimensions. Coupled with KPFM potential maps and depth-resolved cathodoluminescence spectroscopy (DRCLS), this allows for accurate investigation of defect generation or even failure mechanisms of AlGaN/GaN HEMTs. Furthermore, the True Non-Contact mode successfully maintains the tip-sample distance of only a few nanometers allowing for measurements that do not disrupt the surface or its features. This capability is vital for obtaining accurate electronic and morphological information near subsurface defects. Surface potential evolution of AlGaN/GaN HEMT under stress  We have already shown that the Park AFM system has excellent spatial resolution for investigating ZnO nanostructures.4 Here we use the Park atomic force microscopy (AFM) with Kelvin probe force microscopy (KPFM) capability and DRCLS to investigate the reliability issues of start-of-the-art AlGaN/GaN HEMT under off-state (high voltage, low current) stress. With this nanoscale technique, we are able to measure both where and how electric-field-induced stress degrades AlGaN/GaN HEMTs, and the electrically-active defects produced at local degradation areas, thereby allowing us to predict where device failure will occur.  Figure 1.  DC output characteristics and IG-OFF as a function of Off-state stress VDS = 10 – 30 V and VGS = -6 V. Inset shows DC-IV characteristics before and after stress.[5]   Figure 1 shows DC-IV and IG-OFF before and after OFF-state stress.  Gate leakage current (IG-OFF) increases by 2.6x with VDG, rising sharply above a 28 V critical voltage after Off-state stress.  The decrease of the drain current (IDS) and the increase of IG-OFF indicate the deterioration of device performance due to off-state stress.  With increasing Off-state stress, KPFM reveals micron-scale patches of lower potential/Fermi level (EF). Figures 2 display the KPFM and AFM results, of gate (G), drain (D), and source (S) at two representative area of a AlGaN/GaN HEMT without surface passivation.  The region between G & D and G & S is defined as extrinsic drain and source, respectively.  The KPFM images show a striking evolution of surface potential with increasing stress voltage VDS as low surface potential patches begin to grow.  Initially, the lowest potential area extends along and on both sides of the gate.  Between VDG = 24 and 32 V, the lowest potential area shifts to the gate edge on the drain side.  As VDG increases further, this lowest potential area (marked as black-dashed circle) expands and extends from the gate edge across the extrinsic drain region. The surface potential at this extended area decreases from ~ 0.4 V to -1.6 V.  AFM shows no surface morphology changes during these potential changes from the rectangular gate edge to the drain.  Figure 2. KPFM results which shows the evolution of surface potential under Off-state stress at two representative areas of AlGaN/GaN HEMTs.  AFM images show the topography results of these two representative areas at VDG = 36 V. The dashed circles show regions where potentials change faster.[5]   Increasing Off-state VDS further results in a broken gate, a crater forming at  15 µm away from the beginning of the extrinsic drain region, and thus device failure. Images obtained by scanning electron microscope (SEM) system before and after off-state stress were shown in figure 3.   Correlating the potential and morphology results versus electrical stress, one can link device failure with lower surface potential area.  As stress voltage increases, device properties gradually degrade toward failure - faster for VDG above a ~28 V critical voltage.  Although no AFM or SEM morphology changes can be observed before device fail, surface potential changes dramatically with stress.  Furthermore, failure occurs close to the region of lowest potential. Failure is defined here as a crater in the extrinsic drain region.  Likewise, the lower the surface potential, the more probable failure occurs there.  Therefore, surface potential appears to be a strong indicator for device failure: (i) low potential (e.g., decreasing by > 1.5 eV) patches indicate likely failure points, (ii) a patch with the lowest potential is likely the initial point of failure, (iii) the lower the potential, the closer to device failure. Figure 3. The SEM image indicates where failure occurs with increasing Off-state stress. (a) whole device, and (b) enlarge view of black rectangle in (a) after device failure occurs. [5]       DRCLS result at one of the lowest surface potential areas (point 9 in figure 4) in the extrinsic drain region after Off-state stress not only show the 3.45 eV near band edge emission (NBE), but also a 2.2 eV yellow band (YB) and 2.8 – 3.0 eV blue band (BB) are evident.  Figure 4 also shows that the surface potential correlated with the YB/NBE ratio very well, i.e. the lower the surface potential, the higher the YB/NBE ratio.  The formation of the low surface potential patches was caused by the generation of electrically-active defects due to field-induced stress VDS.  These defects will move the EF position and thus affect the surface potential which can be detected by KPFM technique.  A more complete analysis and detail explanation can be found elsewhere5. Figure 4. Surface potential and average YB/NBE intensity ratio increases with Off-state stress.  YB/NBE increases most at lower potential regions 6, 8, 9, and 10. Higher potential patches display slower changes. 1 and 2 correspond to extrinsic drain and drain-side gate edge of an unstressed reference device.[5]   Conclusion The ability to simultaneously image topographical and electronic properties on the nanometer scale is not only a highly powerful technique for understanding the basic physics of materials, but it is also is very useful in examining device structures and their failure mechanisms. Here, we demonstrate the failure-related surface potential evolution with increasing off-state stress on a nanometer scale.   The low potential patches appear and then expand at extrinsic drain region indicate where electrically-active defects grow, and where device failure occurs.  The lowest surface potential region is likely the device failure occurs due to highest defect density.  With superior sensitivity and spatial resolution, the Park AFM/KPFM can be used to investigate device reliability issues without time consuming lifetime tests.    Note This work is supported by the Office of Naval Research Grant No. N00014-08-1-0655 (Dr. Paul Maki and Harry Dietrich) Reference 1Y. F. Wu, D. Kapolnek, J. P. Ibbetson, P. Parikh, B. P. Keller, and U. K. Mishra, IEEE Trans. Electron Dev., 48, 586 (2001) 2J. A. del Alamo and J. Joh, Microelectron Reliab., 49, 1200 (2009) 3C.-H. Lin, T. A. Merz, D. R. Doutt, M. J. Hetzer, J. Joh, J. A. del Alamo, U. K. Mishra, and L. J. Brillson, Appl. Phys. Lett., 95, 033510 (2009) 4T. A. Merz, D.R. Doutt, T. Bolton, Y. Dong, and L.J. Brillson, Surface Science (Letters), 605, L20 (2011) 5C.-H. Lin, D. R. Doutt, U. K. Mishra, T. A. Merz, and L. J. Brillson, Appl. Phys. Lett., 97, 223502-1 (2010)

23 Dec 2013

Quantum Dots/Photonic Devices

Optical Properties – SERS/NSOM/Topography       Gold nanoparticles are imaged with high resolution.  Gold nanoparticles are known to promote surface-enhanced Raman spectroscopy (SERS).  In Ref. 1 below, the authors use this property to demonstrate an ultrasensitive analytical method by exploiting a Raman probe covalently bound to gold nanoparticles; the biological marker to be detected was adsorbed on functionalized gold nanoparticles.  The technique resulted in the ability to detect marker concentrations down to the picomolar (pM) level.   Relevant Publications using the XE-series AFM and SERS/NSOM 1.  Anna Rita Bizzarri, Salvatore Cannistraro; Surface-enhanced Raman spectroscopy combined with atomic force microscopy for ultrasensitive detection of thrombin; Analytical Biochemistry 393 (2009) 149–154 Equipment: Park Systems XE-100   2.  Nanfang Yu, Ertugrul Cubukcu, Laurent Diehl, Mikhail A. Belkin, Kenneth B. Crozier, Federico Capasso, David Bour, Scott Corzine, and Gloria Höfler; Plasmonic quantum cascade laser antenna; APPLIED PHYSICS LETTERS 91, 173113 (2007) Equipment: Park Systems XE-120   3.  Sangwook Oh, Chankyeong Hyon, Sanghoon Sull, Sungwoo Hwangb, Yongju Park; Detection and volume estimation of semiconductor quantum dots from atomic force microscope images; Rev. Sci. Instrum., Vol. 74, No. 11, (2003) 4687 - 4695 Equipment: Park Systems XE-100   Fig 4. (color online) Mid-ir apertureless-NSOM imaging of plasmonic laser antennas. (a) Top and middle panels: simultaneous AFM topography and NSOM image for a resonant antenna with L=12 μm fabricated on the facet of a λ=7.0 μm QCL. Bottom panel: corresponding line scan along the antenna axis. (b) Top and middle panels: simulaneous AFM topography and NSOM image for a resonant antenna with L=0.76 μm fabricated on the facet of a λ=5.3 μm QCL. Bottom panel: corresponding line scan along the antenna axis. (c) Top and middle panels: simultaneous AFM topography and NSOM image of an antenna with L=0.79 μm fabricated on the facet of a λ=5.3 μm QCL but with orientation normal to the laser field. Bottom panel: corresponding line scan along the antenna axis. The polarization of the incident electric field is indicated in each NSOM image.  

23 Dec 2013

Graphene (Step Height)

Metrology - (True-NC-AFM, Contact AFM and STM) Graphene is a sheet of carbon atoms bound together with double electron bonds (called a sp2 bond) only one-atom-thick. The atoms in graphene are densely packed in a honeycomb style crystal lattice. Graphene is extracted from graphite. Graphite itself consists of many graphene sheets stacked together. It has been shown that step height measurements taken on graphene sheets show an inconsistency in the reported value.  This inconsistency is seen both among different AFMs, as well as, when using different modes on the same AFM. With the Park AFM, inconsistency is removed.  The images on the left are of Graphene on SiC imaged by True-Non Contact AFM, Contact AFM, and STM using the Park Systems AFM. The graphene step height measurements show consistent results among the three modes.         Sample: Graphene on SiC Courtesy of : Georgia Institute ofTechnology. Image Conditions: - True Non-Contact - Contact AFM - STM - Scan size (5x2.5 um) - Scan rate (1Hz) - Z servo gain (1) - Pixel (512 x 512) System Requirement: - True Non-Contact - Contact Mode - STM - Decoupled XY closed-loop scanner - Flexure guided Z-scanner with fast response(10kHz)  

23 Dec 2013

Graphene Membrane/Graphite

Mechanical Properties - Nanoindentation Graphite is a layered compound composed of carbon atoms.  It is important industrially for its electrical and lubricating properties.  Graphite is made up of layers of graphene, which is thought to be on of the strongest materials to be tested with a breaking strength 200 times that of steel.   Relevant Publication using the Park AFM and Nanoindentation Title: Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene Authors: C. Lee, X.Wei, J.W. Kysar, J. Hone Journal: SCIENCE  321(18) JULY 2008 p. 385 System: Park Systems XE-100 We measured the elastic properties and intrinsic breaking strength of free-standing monolayer graphene membranes by nanoindentation in an atomic force microscope. The force-displacement behavior is interpreted within a framework of nonlinear elastic stress-strain response, and yields second- and third-order elastic stiffnesses of 340 newtons per meter (N m-1) and -690 N m-1, respectively. The breaking strength is 42 N m-1 and represents the intrinsic strength of a defect-free sheet. These quantities correspond to a Young’s modulus of E = 1.0 terapascals, third-order elastic stiffness of D = –2.0 terapascals, and intrinsic strength of sint = 130 gigapascals for bulk graphite. These experiments establish graphene as the strongest material ever measured, and show that atomically perfect nanoscale materials can be mechanically tested to deformations well beyond the linear regime.  

23 Dec 2013

Surface Topography Considerations of Patterned Sapphire Substrates for Blue/Green Light Emitting Diode

General Considerations Nitride-based semiconductor materials (i.e., GaN, InGaN, AlN) have attracted considerable attention in the past years for their applications to electronic and optoelectronic devices, including blue/green light emitting diodes (LEDs), laser diodes (LDs), and solid state transistors.1 Researchers have long known of the properties of the group-III nitrides, including their wide bandgaps and robust nature, but had not been able to find a suitable substrate on which to grow thin films of these compounds. To be practical, the substrate needed a crystalline structure that matched GaN (utilized as standard n/p-type contact) and it had to be available in large sizes at a reasonably low cost. Recent advancements in the epitaxial thin film growth techniques and substrate material selections partially addressed this problem. However, the lattice parameters of sapphire, the substrate of choice for most LED manufacturers, are not an exact match to GaN (16% lattice mismatch), which results in a high density of crystalline defects (i.e., dislocations, point defects) 2 SiC is better suited as a substrate material, but is still very costly and also generates stacking mismatch boundaries in the device structure, which significantly shorten the device lifetime. Large area Si substrates (4”, 6” and beyond) are rapidly becoming the substrate of choice for electronic device manufacturers, however substrate mismatch effects are still present and a high degree of surface cracking (due to the built-in strain) occurs when the critical film thickness is surpassed. In addition, Si substrates are less resistant from a mechanical standpoint when compared to both sapphire and SiC. To further enhance the brightness of blue/green LEDs grown by metal organic chemical vapor deposition (MOCVD), most of the established manufacturers in the Asian region (Japan, Korea, Taiwan, and China) have adopted surface patterning techniques for both (a) the sapphire to epitaxial film interface (sapphire substrate patterning achieved by substrate preparation techniques) and (b) epitaxial film to fabricated contact interface (p‑side roughening achieved by MOCVD epitaxial growth methods). These methods, if properly designed and implemented, allow for a more efficient coefficient of light extraction from the LED device to be achieved.3 For both patterning cases, proper information concerning the surface morphology is vital to achieving target requirements. Most of the traditional microscopy techniques (i.e., optical microscopy, scanning electron microscopy, etc.) have been found to have major limitations in terms of applicability to monitoring the above mentioned surface roughening techniques. Included are spatial resolution limitations, sample preparation techniques, reliability and reproducibility issues, amount of time spent to collect data and so on. Atomic force microscopy (AFM) offers a simple, efficient, and non‑destructive alternative to investigating the various types of patterns achievable for both sapphire substrate and top LED surface cases. Traditional AFM tools have been designed with the core of the scanning mechanism relying on an XYZ piezoelectric tube scanner. As such, crosstalk and nonlinearity were inherently built in this design. The Park AFM scan system, which has been developed by Park Systems decoupled the XY and Z-scanners and was introduced as the next generation AFM design. As a result, the True Non-Contact mode was enabled.   Park AFM System Overview For all products of the Park AFM, the Z-scanners controlling the vertical movement of the probe tip, which is fundamental to acquiring the surface morphology information, is completely decoupled from the XY-scanner, which moves the sample in the XY directions. This fundamental architectural design change provides the user with significant operational advantages enabling the True Non-Contact mode. Physical separation of the XY and Z-scanners allows for complete removal of the background curvature and effectively eliminates the crosstalk and nonlinearity issues that are intrinsic to conventional piezoelectric tube based AFM systems. This uniquely designed Park AFM scan system not only increases the data collection speed by at least 10× compared to conventional piezoelectric tube scanners, but also minimizes the error due to the inherent nonlinearity of the scanner itself. Figure 1. Cross-section view of Park AFM’s scanning system. Perfectly decoupled XY-Z scanner enables independent movement of Z scanner, which makes True Non-Contact scanning with optimal feedback speed. The Park AFM scan system is a core feature that gives the competitive edge to the Park AFMs. Park Systems’ innovative scanner design separates the Z-scanners from the XY-scanner, enabling exceptional Z-servo performance, orthogonality, and scan accuracy. (See Figure 1.) The Z-scanners, which controls the vertical movement of the AFM tip and is fundamental to acquiring the surface morphology information, is completely decoupled from the XY-scanner which moves a sample in X and Y horizontal directions. From the structure, Park AFMs remove background curvature from a fundamental standpoint, and effectively eliminates the crosstalk and nonlinearity problems that are intrinsic to conventional piezoelectric tube based AFM systems. The Z-scanners is designed to have a higher resonant frequency than conventional piezoelectric tube scanners. For this reason, a stacked piezoelectric actuator is used for the Z-scanners with a high push-pull force when appropriately pre-loaded. Since the Z‑servo response of the Park AFM scan system is very accurate, the probe can precisely follow the steep curvature of a sample without crashing or sticking to the surface. In the Park AFM scan system, the XY-scanner is a Body Guided Flexure scanner, which is used to scan a sample in the X and Y directions only. The flexure hinge structure of the XY-scanner guarantees highly orthogonal 2D movement with minimum out-of-plane motion. The 2D flexure stage of the Park AFM scan system has only 1 nm ~2 nm of out-of-plane motion for the scan range of 50 μm, compared to the inherent 80 nm by the piezoelectric tube scanner of conventional AFMs over the same scan range. (See “Technology behind the Park AFM” for detail.) The symmetrical flexure scanner design also makes possible to place much larger samples on the sample stage than could normally be accommodated by a piezoelectric tube scanner. Furthermore, the symmetry enables to keep the scanner balanced even when a sample is loaded, so that the dynamics of the XY-scanner is not distorted by the sample holder and/or the loaded sample. Since the flexure scanner only moves in the XY direction, it can be scanned at much higher rates (10 Hz ~50 Hz) than would be possible with a standard AFM. The Park AFM not only achieves a structural design innovation that yields a trend setting AFM performance, but it also brings state-of-the-art improvements to the electronics. The Park AFM Control Electronics incorporates advanced digital circuitry with precision software and hardware components that empower high speed and high capacity data processing, which are designed to enable the scanner, the core unit of the AFM, to provide efficient, accurate and fast control, and to facilitate the acquisition of stable images even beyond a scan speed of 10 Hz. Besides the high speed measurement ability, Park AFM’s electronics controls the movement of the AFM system precisely by the closed-loop scan system, which is indispensable to map each additional property to the very point of enhanced topographic details. Even though an AFM system can acquire data with multiple modes, unless the system displays the exact position of measurement, it needs software correction (or calibration) to map the data on the exact position. Correction by software remapping usually works well when the imaging area is comparatively small, but closed-loop scan is applicable on any imaging area without distortion.   Patterned Sapphire Substrates (PSS) Investigation Figure 2. Surface morphology at the 20 μm scale of two different patterned sapphire substrate samples (a) smooth dome surfaces and clean areas between domes and (b) both dome top surfaces and areas between domes show damage and/or residue. Images were acquired with an XE-150 AFM instrument in True Non-Contact mode at 0.2 Hz with 256 pixel resolution.   Shown in Figures 2 are AFM surface topography images (True Non-Contact mode, amplitude channel) at the 20 μm scale of two different types of patterned sapphire substrates (PSS) specimens acquired utilizing an XE-150 manufactured and commercialized by Park Systems. Circular features (i.e., domes) are noticeable with diameters ~ 5 μm and about 1.7 μm in height independent on the scan axis direction. Although the patterns were manufactured by the same method for both cases, the material in Figure 2 (a) displays smooth, featureless dome surfaces with clean areas between these domes, while the sample in Figure 2b shows a certain amount of surface damage and/or residue for both top dome and areas in between. Figure 3 represents corresponding AFM surface topography views (Tapping Mode, amplitude channel) at the 20 μm scale of the same PSS samples (as shown in Figure 2) acquired utilizing a piezoelectric tube scanner instrument manufactured and distributed by a different vendor. Several image discrepancies are easily noticeable between Figures 3 and 2. As such, in Figures 3, • The features appear to be cylindrical (to some extent elongated along the slow axis scan). • Features heights are ~ 1.5 μm along the fast scan axis (X) and in the 1.1 ~ 1.4 μm range along the slow scan axis (Y). • No significant differences are noted in areas between the features of these two samples. Figure 3. Surface morphology at the 20 μm scale of the same two patterned sapphire substrate samples shown in Figure 2. The images show (a) asymmetrical features (somewhat elongated along the slow axis scan) with clean areas between the features and (b) asymmetrical shape features displaying damage and/or residue in the areas in between. These images were acquired with a conventional piezoelectric tube scanner instrument in tapping mode at 1.0 Hz and 512 × 256 pixel resolution.   The only relevant imaging information in Figures 3 appears to be the fact that the sample in Figure 3 (b) (if compared to Figure 3 (a)) has some amount of surface damage and/or residue between the observed features, somewhat similar to the information contained in Figures 2, but incomplete. The images in Figure 2 also indicate damage and/or residue for the top dome areas. Direct optical microscopy investigation of the PSS samples discussed above was performed in order to evaluate the exact nature (i.e., shape and size) of the features imaged by AFM techniques. It was found (not shown) that the sapphire pattern features were round, about 4 μm ~5 μm in diameter and approximately 1.6 μm tall for both samples discussed above. Based on these findings, the data displayed by Figure 2, which was acquired utilizing the XE-150 was validated. The question of imaging discrepancies between the two types of AFM instrument still remains. To further investigate, several images of the sample shown in Figure 2 (a) were taken utilizing the XE-150 under various scan conditions. Figure 4 shows three-dimensional views of (a) True Non-Contact mode at 0.2 Hz, (b) True Non-Contact mode at 1.0 Hz, and (c) intermittent contact mode at 1.0 Hz. No major (< 20%) scan parameters adjustments were employed during the image acquisition. As easily observable, all three images look similar with no significant feature changes (size, height) generated by the employment of different scan modes and minor (< 20%) scan parameters adjustments. Even when the XE-150 is operated utilizing the exact main scan conditions as for the piezoelectric tube instrument (i.e.,intermittent contact mode, 1.0 Hz), the image details as shown in Figure 4 (c) are very different from the piezoelectric scanner results as shown in Figure 3 (a). Perhaps additional scan parameters conditions need to be examined for the piezoelectric tube scanner tool in order to further improve its sample surface tracking and data acquisition/rendering capabilities. It is not obvious from a simple user’s perspective.   Conclusion Patterned sapphire substrates have been proven to significantly enhance the light extraction coefficients of blue/green LEDs, and their use has been experiencing a steady growth across all LED manufacturers. Information concerning the exact pattern shapes is vital to achieving proper light extraction efficiency. The atomic force microscopy technique provides a simple, easy to use, non-destructive method to analyzing sapphire substrates prior to epitaxial film growth. In addition to sapphire surface quality from a quality control standpoint, atomic force microscopes can provide exact measurements/imaging of various patterns employed by LED manufactures to better extract light from the devices. The Park AFM instruments manufactured and distributed by Park Systems have been proven superior to traditional piezoelectric tube based counterparts for these applications. The newest architectural designs of the Park AFM tools, comprising of decoupled XY and Z-scanners and achievement of True Non-Contact mode, enable these instruments to record the correct surface morphology information with a wide scan parameter range and flexibility.   Note Patterned sapphire substrate samples courtesy of undisclosed LED manufacturer.   References 1. S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, T. Yamada, and T. Mukai, Jpn. J. Appl. Phys. Part 2 34, L1332 (1995). 2. J. Nause, III-V Review 12 , 28 (1999). 3. S.J. Chang, Y.K. Su, Y.C. Lin, R. W. Chuang, C.S. Chang, J.K. Sheu, T.C. Wen, S.C. Shei, C.K. Kuo, and D.H. Fang, Phys. Stat. Solidi (c) 7, 2253 (2003).

23 Dec 2013

Atomic Force Microscopy Investigation of 1D Structures Utilizing the Park AFM Instruments

General Considerations Zinc Oxide (ZnO) – a wide bandgap (3.4 eV) II-VI compound semiconductor with a stable wurtzite structure (a=0.325 nm, c=0.521 nm) – offers tremendous potential for electronic, optoelectronic, and magnetoelectronic device applications. To date, it has attracted intensive research efforts for its unique properties and versatile applications in transparent electronics, ultraviolet light emitters, piezoelectric devices, chemical sensors and spin electronics. Based on its remarkable physical properties and the motivation of device miniaturization, large efforts have been focused on the synthesis, characterization, and device applications of ZnO nanomaterials. As such, an assortment of ZnO nanostructures, such as nanowires, nanotubes, nanorings, and nano-tetrapods have been successfully grown by a variety of methods, including chemical vapor deposition, thermal evaporation, electrodeposition, etc. These structures have been subjected to electrical transport, UV emission, gas sensing, and ferromagnetic doping studies, and considerable progress has been achieved. The application prospects of ZnO nanostructures largely rely on the ability to control their location, alignment and packing density. Vertically aligned nanowires/nanorods have promising applications for electron field emitter, vertical transistor, and light emitting diodes (LEDs), thus have attracted enormous attention. Although the vertical alignment of ZnO nanostructures can be assisted by an electric field, for most cases, the alignment is realized by lattice matching between ZnO and the substrate utilized. Several types of epitaxial substrates have been utilized, including sapphire, GaN, ZnO film coated substrates, SiC, and Si. Although sapphire has been widely used as the epitaxy substrate for vertical growth of ZnO nanowires, it can be seen that GaN could be an even better candidate since it has a similar crystal structure and lattice constants to ZnO. Nanowires grown on GaN epilayers showed better vertical alignment than those produced on sapphire. The additional advantage of employing GaN as a substrate material instead of sapphire (and/or ZnO-based) rests in the fact that GaN has much better electrical properties and it is much easier to be doped with dopants to achieve p-type material. Figure 1. Hexagonally grown ZnO nanorod.   Direct measurements of the mechanical, piezoelectrical, optical, and magnetic properties of individual nanostructues are rather challenging since the traditional measurement methods used for bulk material do not apply. Typically, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been employed to observe and measure physical dimensions and orientation of nanorods/nanowires. However, these conventional methods exhibit major limitations. Among these are spatial resolutions, sample preparation techniques, amount of time spent to collect data and so on. In addition, besides information on size and orientation no other physical properties characteristics may be investigated by both the TEM and SEM methods. On the other hand, atomic force microscopy (AFM) offers a simple, efficient, and non-destructive alternative to not only investigating mechanical properties (size, orientation, elastic properties, etc.) but also electric and magnetic characteristics of ZnO nano-structures. Traditional AFM tools have been designed with the core of the scanning mechanism relying on an XYZ piezoelectric tube scanner. As such, cross-talk and non-linearity were inherently built in this design. The Park AFM scan system, which has been developed by Park Systems decoupled the XY and Z-scanners and was introduced as the next generation AFM design. As a result, the True Non-Contact mode was enabled.   Park AFM System Overview Figure 2. Based on Park AFM Scanning Technology, Park AFMs provide various SPM modes with outstanding stability and applicability.   The Park AFM scan system is a core feature that gives the competitive edge to the Park AFMs. Park Systems’ innovative scanner design separates the Z-scanner from the XY-scanner, enabling exceptional Z-servo performance, orthogonality, and scan accuracy. The Z-scanner, which controls the vertical movement of the AFM tip and is fundamental to acquiring the surface morphology information, is completely decoupled from the XY-scanner which moves a sample in X and Y horizontal directions. From the structure, Park AFMs remove background curvature from a fundamental standpoint, and effectively eliminates the cross-talk and non-linearity problems that are intrinsic to conventional piezoelectric tube based AFM systems. The Z-scanner is designed to have a higher resonant frequency than conventional piezoelectric tube scanners. For this reason, a stacked piezoelectric actuator is used for the Z-scanner with a high push-pull force when appropriately pre-loaded. Since the Z‑servo response of the Park AFM scan system is very accurate, the probe can precisely follow the steep curvature of a sample without crashing or sticking to the surface. In the Park AFM scan system, the XY-scanner is a Body Guided Flexure scanner, which is used to scan a sample in the X and Y directions only. The flexure hinge structure of the XY-scanner guarantees highly orthogonal 2D movement with minimum out-of-plane motion. The 2D flexure stage of the Park AFM scan system has only 1-2 nm of out-of-plane motion for the scan range of 50 μm, compared to the inherent 80 nm by the piezoelectric tube scanner of conventional AFMs over the same scan range. (See “Technology behind the Park AFM” for detail.) The symmetrical flexure scanner design also makes possible to place much larger samples on the sample stage than could normally be accommodated by a piezoelectric tube type scanner. Furthermore, the symmetry enables to keep the scanner balanced even when a sample is loaded, so that the dynamics of the XY-scanner is not distorted by the sample holder and/or the loaded sample. Since the flexure scanner only moves in the XY-direction, it can be scanned at much higher rates (10 Hz ~ 50 Hz) than would be possible with a standard AFM. The Park AFM not only achieves a structural design innovation that yields a trend setting AFM performance, but it also brings state-of-the-art improvements to the electronics. The Park AFM Control Electronics incorporates advanced digital circuitry with precision software and hardware components that empower high speed and high capacity data processing, which are designed to enable the scanner, the core unit of the AFM, to provide efficient, accurate and fast control, and to facilitate the acquisition of stable images even beyond a scan speed of 10 Hz. Besides the high speed measurement ability, Park AFM’s electronics controls the movement of the AFM system precisely by the closed-loop scan system, which is indispensable to map each additional property to the very point of enhanced topographic details. Even though an AFM system can acquire data with multiple modes, unless the system displays the exact position of measurement, it needs software correction (or calibration) to map the data on the exact position. Correction by software remapping usually works well when the imaging area is comparatively small, but closed-loop scan is applicable on any imaging area without distortion.   General Investigation of ZnO Nanorod Samples Shown in Figures 2 are True Non-Contact AFM surface topography images at the 5 μm x 5 μm scale of ZnO nanorod samples grown on GaN-like substrates. For the GaN growth, sapphire was the substrate of choice. These ZnO nanorod specimens were grown by placing the GaN template in a solution of zinc nitrate and hexamethyl-tetramine held at a temperature of 60 °C. Both vertically and horizontally oriented nanorods are observed. Figure 3 (a) displays a three dimensional (3D) view of the scan area, while Figure 3 (b) shows both the top image view and line profile analysis results. Based on the data of Figure 3 (b), the typical height for the vertically oriented specimens is in the 0.3 nm to 0.6 nm range, while for the horizontal case, the length is in the 1.1 μm to 2.0 μm range with diameters up to 1.2 μm. Figure 3. Surface morphology at the 5 μm × 5 μm scale of ZnO nanorod samples grown on GaN templates (a) 3D view and (b) top view with line analysis information. The image was acquired using high aspect ratio tip probes at 0.15 Hz with 256 x 256 pixel resolution.   Figures 3 are scanning electron microscopy (SEM) images (courtesy of Bell Labs, Lucent Technologies) of the same specimen as shown in Figures 2. The image magnifications are 15,000× for Figure 4 (a) and 30,000× for Figure 4 (b), respectively. For better imaging, a 45 degrees sample tilt angle was employed. Similar to the AFM imaging case, both horizontally and vertically oriented ZnO nanorods are observed with dimensions in the 0.30 nm to 0.50 nm height (vertical orientation), 1-2 μm length (horizontal orientation), and up to 1 μm diameter (horizontal orientation). Considering the fact that SEM imaging is performed at a 45 degree sample tilt angle, the physical nanorod dimensions based on the SEM evaluation are a very close match to the exact sizes directly observed by AFM investigation and shown in Figures 2. Figure 4. Scanning Electron Microscopy (SEM) images of the ZnO nanorod sample shown in Figures 3 (courtesy of Bell Labs, Lucent Technologies). Image magnification is 15,000x for figure (a) and 30,000x for figure (b), respectively. A 45 degrees sample tilt was employed.   Conclusion One-dimensional (1D) semiconductor nanostructures, such as rods, wires, belts, and tubes have attracted much attention in recent years due to their unique properties and possibility to utilize them as building blocks for electronic, photonic, and life-science applications. ZnO is a direct-bandgap semiconductor with a large exciton binding energy, exhibiting near ultraviolet emission and piezoelectricity, bio-safe and biocompatible. Intensive research has been focused on fabricating one-dimensional ZnO nanostructures and in correlating their morphologies with their size-related optical, electrical, and magnetic properties. Among the 1D nanostructures, ZnO nanorods/nanowires have been widely studied because of their easy formation and device applications. AFM offers a direct, non-destructive, and easy to operate characterization method, which not only allows for exact mechanical properties measurements of ZnO nanostructures, but also provides means to enhancing the physical properties investigation by utilizing advanced scanning probe modes. The Park AFM instruments manufactured and distributed by Park Systems are equipped with state-of-the-art capabilities both in terms of standard (i.e., mechanical) and advanced (i.e., electrical, magnetic) options. The newest architectural design of the Park AFM tools comprising of decoupling of XY and Z-scanners and achievement of True Non-Contact scanning, enables these instruments with a wide range of scan parameters and scope of applications therein.   Note Park Systems would like to thank Dr. Hock Ng of Bell Labs, Lucent Technologies for providing the ZnO nanorod samples and his technical contribution to preparing this document.   References 1. Z. Fan and J.G. Lu, article in press. 2. S.Y. Lee, E.S. Shim, H.S. Kang, S.S. Pang, and J.S. Kang, Thin Solid Films 437, 31 (2005). 3. S.J. Pearton, W.H. Heo, M. Ivill, D.P. Norton, and T. Steiner, Semicond. Sci. Technol. 19, R59 (2004). 4. S. Fujita, S.W. Kim, M. Ueda, and S. Fujita, J. Cryst. Growth 272, 138 (2004). 5. H. Zhou, and Z. Li, Mater. Chem. Phys. 89, 326 (2005). 6. D.W. Zeng, C.S. Xie, M. Dong, R. Jiang, X. Chen, A.H. Wang, J.B. Wang, and J. Shi, and A. Krost, Supperlatice Microst. 36, 95 (2004). 7. Z. Fan and J.G. Lu, article in press. 8. H.J. Fan, F. Fleischer, W. Lee, K. Nielsch, R. Scholz, M. Zacharias, U. Gosele, A. Dadgar, Appl. Phys. Lett. A79, 1865 (2004).