Origin of C60 surface reconstruction resolved by atomic force microscopy

Surface adsorption of C 60 affects its chemical and electronic properties. Numerous studies have reported observation of bright and dark fullerenes on metal surfaces that suggest extensive surface reconstruction; however, the underpinning mechanism of the reconstruction remains under debate. Here we report tip-functionalized noncontact atomic force microscope measurements which unambiguously reveal that C 60 fullerenes adsorb with three well-defined adsorption heights on the Cu(111) surface, consistent with theoretical reports of top-layer hollow sites, single-atom vacancies, and surface nanopits. Using single-molecule resolution Δ f ( z ) measurements we identify well-defined adsorption heights specific to each site, confirming the presence of a complex vacancy model for C 60 monolayers on metal surfaces

Cross-sectional Nanoscale Resolution Mapping of Potential and Current Distribution in 3D Structure of Vertical Cavity Surface Emitting Laser iii-v Nanostructures

Vertical cavity surface emitting lasers (VCSELs) hold a major promise in the telecommunication and data interfacing due to their efficient manufacturing pathways and prospects of seamless integration with microelectronics components. VCSEL structures include multilayer Distributed Bragg Reflection (DBR) surrounding an active cavity that typically have multiple quantum wells (QWs) and in some devices quantum dots (QDs) layers [2]. The properties, morphology and quality of multiple buried layers and interfaces are crucial for the development of novel devices, improving device performance and optimization of production processes. Unfortunately, accessing these layers to explore these generally three-dimensional (3D) structures is often a laborious (e.g. via cross-sectional transmission or scanning electron microscopies, EMs) task. Significantly, the sample preparation can also change properties of the material and the device studied (e.g. Ga ion implantation during FIB milling) and usually allows to see only a very limited part of the wafer. Furthermore, the EM does not allow to access local physical properties of the device – such as local electric potential, current density and heat generation, all being extremely crucial to the device performance. Here we report for the first time the direct observation of local electric potential and conductance in the bulk of VCSEL stack by using combination of the Ar-ion beam exit cross-section polishing (BEXP) that creates an oblique section with sub-nm surface roughness through the VCSEL structure [2] combined with the material sensitive scanning probe microscopy (SPM). We used three different SPM measurement modes – nanomechanical local elastic moduli mapping via Ultrasonic Force Microscopy (UFM), surface potential mapping via Kelvin Probe Force Microscopy (KPFM) [3] and mapping of injected current (local conductivity) via Scanning Spreading Resistance Microscopy (SSRM). These allowed to observe the resulting geometry of the device, including active cavity MQW, and to obtain profiles of differential doping of the DBR stack, profile of electric potential in the active cavity, and spatial variation of current injection in the individual QW in MQW area. In conclusion, this approach opens unique novel possibility to directly explore the physical phenomena of operation of VCSELs and other iii-v devices, helping to advance the manufacturing of these these devices, as well as opening insight into the fundamental electronic and atomistic phenomena in these complex nanostructured materials [4]

3D mapping of nanoscale physical properties of VCSEL devices

There is clear lack of methods that allows studies of the nanoscale structure of the VCSEL devices1 that are mainly focused on the roughness of the DBR, or using FIB cross-sectioning and TEM analysis of failed devices to observe the mechanism of the degradation. Here we present a recently developed advanced approach that combines Ar-ion nano-cross-sectioning with material sensitive SPM2 to reveal the internal structure of the VCSEL across the whole stack of top and bottom DBR including active area. We report for the first time the direct observation of local mechanical properties, electric potential and conductance through the 3D VCSEL stack. In order to achieve this, we use beam exit cross-section polishing that creates an oblique section with sub-nm surface roughness through the whole VCSEL structure that is fully suitable for the subsequent cross-sectional SPM (xSPM) studies. We used three different SPM measurement modes – nanomechanical local elastic moduli mapping via Ultrasonic Force Microscopy (UFM) 3, surface potential mapping via Kelvin Probe Force Microscopy (KPFM) and mapping of injected current (local conductivity) via Scanning Spreading Resistance Microscopy (SSRM). xSPM allowed to observe the resulting geometry of the whole device, including active cavity multiple quantum wells (MQW), to obtain profiles of differential doping of the DBR stack, profile of electric potential in the active cavity, and spatial variation of current injection in the individual QW in MQW area. Moreover, by applying forward bias to the VCSEL to initiate laser emission, we were able to observe distribution of the potential in the working regime, paving the way to understanding the 3D current flow in the complete device. Finally, we use finite element modelling (FEM) that confirm the experimental results that of the measurements of the local doping profiles and charge distribution in the active area of the VCSEL around the oxide current confinement aperture. While we show that the new xSPM methodology allowed advanced in-situ studies of VCSELs, it establishes a highly efficient characterisation platform for much broader area of compound semiconductor materials and devices. REFERENCES. 1. D. T. Mathes, R. Hull, K. Choquette, K. Geib, A. Allerman, J. Guenter, B. Hawkins and B. Hawthorne, in Vertical-Cavity Surface-Emitting Lasers Vii, edited by C. Lei and S. P. Kilcoyne (2003), Vol. 4994, pp. 67-82. 2. A. J. Robson, I. Grishin, R. J. Young, A. M. Sanchez, O. V. Kolosov and M. Hayne, Acs Applied Materials & Interfaces 5 (8), 3241-3245 (2013). 3. J. L. Bosse, P. D. Tovee, B. D. Huey and O. V. Kolosov, Journal of Applied Physics 115 (14), 144304 (2014)

Imaging 3D nanostructure of III-V on Si via cross-section SPM: quantum wells and nanowires - defects, polarity, local charges

Merging unique performance of compound semiconductor (CS) III-V materials in optoelectronics, high frequency and power devices with mature Si manufacturing is a holy grail of modern semiconductor technology. The difference between lattice constants, processing, and chemistry are just a few major challenges to be resolved. With practically non-existing methods for studying nanoscale physical properties of these buried structures, we developed a new concept for fast and efficient 3D nanoscale resolution quantitative mapping of physical properties of CS materials and devices. We combine novel nano-sectioning using variable energy Ar ion beam targeted at the edge of the sample to create a perfectly flat oblique near-atomic flat section through all layers of interest, and the material sensitive scanning probe microscopy (SPM), to reveal 3D morphology, composition, strain and crystalline quality via local physical properties – mechanical and piezoelectric moduli, nanoscale heat conductance, workfunction and electrical conductivity. We can observe the propagation of antiphase domains (APD) from the GaAs-Si interface through the 3D structure, reporting for the first time APD effect on electronic properties of multiple quantum wells that are electrically short the structure evident on charge distribution nanomaps. In GaN nanowires, we directly observe NW/Si substrate interface, and unexpectedly find the in-NWs domains of the opposite polarity via piezoelectric moduli maps. The novel paradigm will make a disruptive change on how 3D structure and physical properties of CS and microelectronics materials and devices are currently studied

STUDYING THE MOBILITY AT THE FREE INTERFACE THROUGH SURFACE PATTERN DECAY: EXPERIMENTS ON POLYMERIC FILMS

We studied the time evolution of the dynamics of a (3 kg/mol molecular mass) polystyrene film patterned with a sinusoidal waveform with various wavelengths, spanning a range from μm to hundreds of nm. The analysis of the pattern decay by white light interferometry and atomic-force microscopy techniques, allowed to determine that the viscous flow diffusion controls the surface flattening in a wide temperature range: from the glass transition temperature of the polymer to tens of degrees above it. Moreover, we have simulated the polystyrene dynamics in the glassy state showing that the surface diffusion would overcome the viscous flow in a region some degrees below the glass transition temperature, determining a dynamic decoupling. We have also developed a new procedure through the application of multiple techniques and with the use of different polymers to realize a quasi-sinusoidal molds (PFPE) with an aspect ratio lower than 0.1 and wavelengths spanning from 450 to 4500 nm, to use a master for the patterning of the polystyrene films