Monolayer Depth Resolution Research Articles

Single Atomic Defect Conductivity for Selective Dilute Impurity Imaging in 2D Semiconductors.

Precisely controlled impurity doping is of fundamental significance in modern semiconductor technologies. Desired physical properties are often achieved at impurity concentrations well below parts per million level. For emergent two-dimensional semiconductors, development of reliable doping strategies is hindered by the inherent difficulty in identifying and quantifying impurities in such a dilute limit where the absolute number of atoms to be detected is insufficient for common analytical techniques. Here we report rapid high-contrast imaging of dilute single atomic impurities by using conductive atomic force microscopy. We show that the local conductivity is enhanced by more than 100-fold by a single impurity atom due to resonance-assisted tunneling. Unlike the closely related scanning tunneling microscopy, the local conductivity sensitively depends on the impurity energy level, allowing minority defects to be selectively imaged. We further demonstrate subsurface impurity detection with single monolayer depth resolution in multilayer materials.

In situ Auger electron spectroscopy of complex oxide surfaces grown by pulsed laser deposition

The authors report in situ Auger electron spectroscopy (AES) of the surfaces of complex oxide thin films grown by pulsed laser deposition (PLD). The authors demonstrate the utility of the technique in studying elemental composition by collecting characteristic Auger spectra of elements from samples such as complex oxide thin films and single crystals. In the case of thin films, AES studies can be performed with single unit cell precision by monitoring thickness during deposition with reflection high energy electron diffraction. The authors address some of the challenges in achieving in situ and real-time AES studies on complex oxide thin films grown by PLD. Sustained layer-by-layer PLD growth of a CaTiO3/LaMnO3 superlattice allows depth-resolved elemental composition analysis during the growth process. The evolution of the Auger spectra of elements from individual layers was used to perform compositional analysis with monolayer-depth resolution.

Initial stages of growth of iron on silicon for spin injection through Schottky barrier

AbstractThe characterization of ferromagnet‐semiconductor interfaces with monolayer (ML) depth resolution is an important issue in the development of spin injection devices. In this article, highly resolved depth distributions of Fe have been measured during the initial stages of growth of Fe on Si (100) at room temperature by in situ high‐resolution Rutherford backscattering spectrometry. Extensive in‐diffusion of Fe has been observed even for the coverage of 0.0325 ML of Fe. At this coverage the Si crystal structure is apparently still conserved. Every second Si layer is depleted of Fe, thus giving rise to compositional oscillations of Fe. At higher coverages strong interdiffusion occurs resulting in the formation of silicides at the interface. Even at 9.2 ML of Fe coverage, no pure Fe layers were observed.

Zero‐energy SIMS depth profiling: the role of surface roughness development with XeF2‐based etching

AbstractTo obtain monolayer depth resolution in the zero‐energy SIMS concept, an almost atomically flat surface during the electron beam‐induced etching (EBIE) is crucial. However, the surface roughness observed with Si EBIE and XeF2 shows the same characteristics as observed during spontaneous etching of Si with XeF2. The subsurface fluorine concentration in a Si EBIE process (even in a mass transport limited regime) can influence the unactivated formation of the SiFx reaction layer as well, and therefore, contribute to observed roughness development. SiO2 EBIE leads to extreme pit formation due to preferential etching of Si at the SiO2/Si interface. In general, preferential etching can deteriorate or enhance the depth resolution as shown on a SiGe epilayer structure. Finally, the capabilities of zero‐energy SIMS are shown on an ultra‐shallow SiGe structure demonstrating a narrow surface transient and a very good depth resolution in the surface region. Copyright © 2010 John Wiley & Sons, Ltd.

Depth-profiling the composition of bimetallic nanoparticles using medium energy ion scattering

The near monolayer depth resolution of medium energy ion scattering is utilized to develop a probe of the depth dependent composition of bimetallic nanoparticles supported on planar oxide supports. The approach fits spectra of scattered ion intensity versus ion energy at well-defined scattering angles taking into account the asymmetric line shape in such spectra and also the depth dependent loss processes encountered by incident ions as they pass through the bimetallic particles.

High-resolution RBS: a powerful tool for atomic level characterization

A compact high-resolution RBS (HRBS) system consisting of a simple magnetic spectrometer and a small accelerator is developed. The developed HRBS system has several attractive features, e.g. capability of depth profiling with monolayer depth resolution, small footprint, reasonably short measuring time. Several examples of application are presented, which include depth profiling of ultrashallow As implants in silicon, nitrogen depth profiling in ultrathin SiO x N y films and strain profiling in HfO 2/Si(0 0 1). These examples show that HRBS is a powerful tool for atomic level characterization.

Hyperfine interaction studies with monolayer depth resolution using ultra-low energy radioactive ion beams

A variety of nuclear techniques rely on the incorporation of radioactive atoms to investigate the microscopic structural, electronic and magnetic properties of a material. In the past, ion implantation has been utilized to introduce these radioactive probes, resulting in a depth distribution of typically several hundreds of Å, and damaging the sample. Both implantation-related deficiencies are incompatible with the ever shrinking sizes relevant in nanostructures. This problem can be circumvented by using ultra-low energy ion beams – of the order of 5 eV, i.e. below the displacement energy of the substrate atoms. Consequently, the radioactive probes are “deposited” on top of the sample, without generating damage to the substrate. Since the implantation chamber is in vacuo connected with the molecular beam epitaxy deposition chamber, the probe layer can be introduced at any stage during the sample growth (from surface to interface) – with monolayer depth resolution. As an example, we discuss the ultra-low energy ion deposition of 111 In in Cr, followed by analysis with perturbed angular correlation spectroscopy. The aim of the study is to explore the magnetic ordering of Cr thin films.

Surfacestructure and dipole moments of RbI(001) determined byhigh-resolution medium-energy ion scattering

The surface structure of RbI(001) was determined directly by meansof medium-energy ion scattering (MEIS) with a monolayer depthresolution. The rumpled surface structure determined here wascompared with semi-classical shell-model calculations. On the basisof a simple model regarding the lattice site ion as a point chargeaccompanied by a dipole moment, we evaluated the dipole moments ofthe top- and second-layer I- and Rb+ ionsself-consistently from the relaxed lattice positions determined bymeans of MEIS and from the polarizabilities estimated systematically fromexperimental refraction data. The balance between a short-rangeforce and a long-range coulombic one determined the equilibriumpositions of the top- and second-layer ions; these are consistentwith those derived by means of MEIS within the experimentaluncertainties of 0.02 Å. This indicates that thepolarizablities derived from the refraction data and the interatomic(short-range) potentials of the Born-Mayer type are applicable.

On the limitations introduced by energy spread in elastic recoil detection analysis

Improvements in experimental techniques have led to monolayer depth resolution in heavy ion elastic recoil detection analysis (HI-ERDA). Evaluation of the spectra, however, is not trivial. The spectra, using even the best experimental set-up, are subject to finite energy resolution of both extrinsic and intrinsic origin. A proper account for energy spread is necessary to extract the correct depth profile from the measured spectra. With calculation of the correct energy (or depth) resolution of a given method, one can decide in advance whether or not the method will resolve details of interest in the depth profile. To achieve the best depth resolution, it is also possible to find optimum parameters for the experiments. The limitations introduced by the energy spread effects are discussed. An example for simulation is shown for high energy resolution HI-ERDA measurements. Satisfactory agreement between the simulated and the measured HI-ERDA spectra taken by 60 MeV 127 I 23+ ions on highly oriented pyrolythic graphite (HOPG) sample is found, in spite of the non-equilibrium charge state of the recoils and the difference in the stopping powers caused by the given charge state of the incident ion and the recoil, which are not taken into account. To achieve more precise data evaluation these effects should be included in simulation codes, or all the subspectra corresponding to different recoils charge states should be measured and summed.

Limitations to depth resolution in ion scattering experiments

The energy spectra of scattered ions in Rutherford backscattering and elastic recoil scattering experiments are commonly described using stopping power and straggling data. These, in turn, are based on statistical approximations and thus neglect the characteristics of individual ion–atom collisions. This simplified analysis is not justified when considering experiments with high-energy resolution. We have performed theoretical and experimental studies to understand the shape of high-energy resolution ion scattering spectra. The impact parameter dependent energy loss in single ion–atom collisions has been calculated using the semi-classical approximation (SCA). We show that limitations for extracting monolayer depth resolution become obvious from this non-statistical approach. Experiments were performed using a medium energy ion scattering (MEIS) system. For the case of 100 keV protons scattered on sulfur atoms, good agreement between the theoretical description and experimental data was found. The influence of thermal vibrations of the sample atoms on the backscattering spectra is discussed.

Dipole moments of the top and second layer ions of RbI(0 0 1) surface determined by high-resolution medium energy ion scattering

The heights of the top layer ions of RbI(0 0 1) surfaces from the second layer I − and Rb + ions, respectively were determined directly by medium energy ion scattering (MEIS) with a monolayer depth resolution. The balance between a short-range force and a long-range Coulombic one gives the local electric fields at the relaxed lattice positions of the first and second layer ions. As the short-range interaction, we employed the pair potential which reproduces well the elastic constants of the bulk RbI crystal. Multiplication of the electric field by the polarizability (bulk) deduces the dipole moments of the top and second layer I − and Rb + ions. The present MEIS results generally agree with the theoretical predictions based on the shell model.

Rumpled relaxation of TiC(001) and TaC(001) determined by high-resolution medium-energy ion scattering spectroscopy

The rumpled relaxations of TiC(001) and TaC(001) with strong covalent bonds were determined precisely by medium energy ion scattering using a toroidal electrostatic analyzer with an excellent energy resolution of $\ensuremath{\Delta}E/E=9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}.$ It allows a monolayer depth resolution and thus a simple triangulation method using the ion shadowing effect achieved the accuracy of 0.01 \AA{} in depth for the displacement of atoms near a surface region. The surface relaxation (+: outwards) and rumpling [+: displacement of C toward vacuum side larger than that of Ti(Ta)] were estimated to be +0.1\ifmmode\pm\else\textpm\fi{}0.3% and +3.5\ifmmode\pm\else\textpm\fi{}0.5% for TiC(001) and +1.4\ifmmode\pm\else\textpm\fi{}0.3% and +4.9\ifmmode\pm\else\textpm\fi{}0.5% for TaC(001). The present result is consistent with the recently reported calculation using the full-potential linear muffin-tin orbital method but inconsistent with the predictions using the tight-binding model and the full-potential linearized augmented plane-wave method.

Anomalous Surface Peaks Observed in the Backscattering Spectra from Amorphous Si andSiO2Films for Medium Energy He Ions

Anomalous surface peaks were first observed in the backscattering spectra for 60--110 keV ${\mathrm{He}}^{+}$ incidence on amorphous Si(40 \AA{}) and ${\mathrm{SiO}}_{2}(10--70\AA{})$ films grown on Si(001) substrates. Our toroidal electrostatic analyzer with a monolayer depth resolution made it possible to detect the surface peaks, which can be explained well from nonequilibrium charge exchange processes near the surface regions. The peak height and width provide the quantitative information about the local electron capture and loss cross sections at the surface together with the charge exchange region from the top atomic plane toward the vacuum side. The electron capture and loss cross sections derived are compared with those for gaseous targets and the charge exchange region near the surface is discussed in the context of the surface stopping powers.

Ion beam analysis with monolayer depth resolution

The paper is concerned with the analysis of surfaces and near-surface layers with monolayer depth resolution by means of high resolution Rutherford backscattering (HRBS) and elastic recoil detection (HERDA) of ions with an energy of a few MeV, in combination with an electrostatic spectrometer. With this instrument, which has recently been set up at the 6 MV Pelletron accelerator of the Max-Planck-Institut für Metallforschung in Stuttgart, depth resolutions of 0.1 nm are obtained in HRBS and 0.3 nm in HERDA experiments. This paper gives a short outline of the design and performance of the spectrometer followed by various examples of applications. These comprise examples showing the analyzing power of the instrument, the analysis of an X-ray mirror by HRBS, the study of the initial oxidation of surfaces of aluminum single crystals by HERDA and recent results concerning charge exchange in ion backscattering.

Depth-profiles in compositionally-graded amorphous silicon alloy thin films analyzed by real time spectroscopic ellipsometry

Real-time spectroscopic ellipsometry (RTSE) has been applied to characterize composition depth-profiles in compositionally-graded amorphous silicon–carbon alloy (a-Si 1− x C x :H) thin films, prepared using continuous variations in the flow ratio z( t)=[CH 4]/{[SiH 4]+[CH 4]} during r.f. plasma-enhanced chemical vapor deposition (PECVD). In order to calculate the dielectric functions of a-Si 1− x C x :H alloys for any value of x, a new parameterization of the measured dielectric functions was established using the recently-derived Tauc–Lorentz (T-L) model. This model is found to provide improved fitting of the measured dielectric functions of a-Si 1− x C x :H over the analyzed energy region of 2.2<E<4.2 eV, in comparison to previous parameterization schemes. Applying the T-L parameterization, the depth-profiles in the C-content were then analyzed using a virtual interface approximation. For several a-Si 1− x C x :H graded layers having triangular variations in the C-content (0< x<0.24) over 25–130 Å thick layers, we found very good agreement between the analyzed depth-profiles and those predicted from z( t) based on individually-deposited samples. In the depth-profile of the thinnest C-graded layer (∼25 Å), monolayer depth resolution was evident with an average compositional uncertainty of±0.009.

Ion Beam Analysis with Monolayer Depth Resolution

Abstract The paper is concerned with the analysis of surfaces and near-surface layers with monolayer depth resolution by means of high resolution Rutherford backscattering (HRBS) and elastic recoil detection (HERDA) of ions with an energy of a few MeV, in combination with an electrostatic spectrometer. With this instrument, which has recently been set up at the 6 MV Pelletron accelerator of the Max-Planck-Institut fur Metallforschung in Stuttgart, depth resolutions of 0.1 nm are obtained in HRBS and 0.3 nm in HERDA experiments. This paper gives a short outline of the design and performance of the spectrometer followed by various examples of applications. These comprise examples showing the analyzing power of the instrument, the analysis of an X-ray mirror by HRBS, the study of the initial oxidation of surfaces of aluminum single crystals by HERDA and recent results concerning charge exchange in ion backscattering.

Analysis of completed commercial semiconductors using EPMA

The development of specialized analytical instruments such as the SIMS, XPS, ISS etc., all with truly incredible abilities in certain areas, has given rise to the notion that electron probe microanalysis (EPMA) is an old fashioned and rather inadequate technique, and one that is of little or no use in such high technology fields as the semiconductor industry. Whilst it is true that the microprobe does not possess parts-per-billion sensitivity (ppb) or monolayer depth resolution it is also true that many times these extremes of performance are not essential and that a few tens of parts-per-million (ppm) and a few tens of nanometers depth resolution is all that is required. In fact, the microprobe may well be the second choice method for a wide range of analytical problems and even the method of choice for a few.The literature is replete with remarks that suggest the writer is confusing an SEM-EDXS combination with an instrument such as the Cameca SX-50. Even where this confusion does not exist, the literature discusses microprobe detection limits that are seldom stated to be as low as 100 ppm, whereas there are numerous element combinations for which 10-20 ppm is routinely attainable.

Oscillatory evolution of an ion bombarded InSb(100) surface

Low energy ion-scattering and ion-recoil (LEIS and LEIR) spectroscopies have been used to monitor the structure and composition of an InSb(100) surface during continuous irradiation with argon ions (4.8 keV) directed at grazing incidence. Oscillations in both the scattered and recoil ion intensities have been observed during all bombardment sequences (for doses 2 × 10 16 ions cm 2 ). The periods of these oscillations show some dependence on the experimental geometry, and are close to those predicted for sputter-removal of one or two surface layers. Interestingly, the surface shows no sign of becoming amorphous even following excess ion bombardment. Details of the variations in surface composition are assigned, not only to layer-by-layer depth profiling, but also by rapid diffusion of Sb to the surface from the Sb depleted lattice. These data indicate that LEIS and LEIR can be used to monitor composition profiles with monolayer depth resolution such that different states of the same surface can be achieved simply and controllably by terminating ion bombardment following a pre-determined irradiation dose.

A direct method to produce and measure compositional grading in AlxGa1−xAs alloys

We present a method to calibrate the profile of Al mole fraction versus depth, deposited in growth of graded AlxGa1−xAs alloys in a molecular-beam epitaxy machine. A computer is used to either ramp the Al oven temperature (analog alloy), or pulse the Al beam (digital alloy), with a fractional monolayer depth resolution that permits averaged alloy profiles corresponding to a range of different design profiles to be obtained. The profiles are measured in calibration runs by using a fast picoammeter to track the ion-collector current of the beam flux monitor ion gauge (facing the ovens), and integrating the ion current with time. Parabolic quantum wells are grown by either technique and the corresponding measured profiles are compared to each other and to the design profile. The ability of the digital-alloy technique to obtain almost arbitrarily varying graded-alloy profiles is illustrated.

Direct imaging and theoretical modelling of the atomistic morphological and chemical structure of semiconductor heterointerfaces

Cathodoluminescence images (CLI's) of GaAs/AlGaAs and GaAs/InGaAs interfaces are presented. A novel experimental approach “CL-wavelength imaging (CLWI)” is demonstrated for the first time. A complete CL spectrum is recorded at every pixel within 100 ms by scanning of the exciting e-beam across the interface area and detecting the spectrally resolved light with a reticon. This enables us to measure directly the lateral fluctuations of recombination energy of GaAs quantum well (QW) excitons and thus the variation of GaAs QW thickness Lz with monolayer depth resolution. Eventually three-dimensional images of the atomic morphology of GaAs/AlGaAs heterointerfaces with submicron resolution are obtained. In addition to lattice matched quantum well systems, strained layer GaAs/InGaAs/GaAs quantum wells are investigated. Direct images of dislocations by CLI are presented. Using time-resolved CLI the carrier lifetime around these strain induced dislocations is mapped and observed to decrease drastically at the dislocation lines.