Cross-sectional Scanning Capacitance Microscopy Research Articles

Dislocations at coalescence boundaries in heteroepitaxial GaN/sapphire studied after the epitaxial layer has completely coalesced

We have performed cross-sectional scanning capacitance microscopy (SCM), cathodoluminescence (CL) microscopy in the scanning electron microscope (SEM) and transmission electron microscopy (TEM) all on the same few-micron region of a GaN/sapphire sample. To achieve this, it was necessary to develop a process flow which allowed the same features viewed in a cleaved cross-section to be traced from one microscope to the next and to adapt the focused ion beam preparation of the TEM lamella to allow preparation of a site-specific sample on a pre-cleaved cross-section. Growth of our GaN/sapphire samples involved coalescence of three-dimensional islands to form a continuous film. Highly doped marker layers were included in the sample so that coalescence boundaries formed late in the film growth process could be identified in SCM and CL. Using TEM, we then identified one or more dislocations associated with each of several such late-coalescing boundaries. In contrast, previous studies have addressed coalescence boundaries formed earlier in the growth process and have shown that early-stage island coalescence does not lead to dislocation formation.

Measurement of 3-dimensional dopant distribution in InGaAs microdiscs grown selectively on Si (111)

The control of the dopant profile in 3-dimentional InGaAs microdiscs on Si (111) is essential for their device applications. However, such profiles can never be controlled by simply changing the supply of dopant precursors during the growth of microdiscs. This is because a variety of crystal planes, such as (111), {−110}, and irregular planes near the corners, surround a hexagonal pillar of InGaAs and the incorporation efficiency of dopant elements depends significantly on the kind of planes involved. We here observed the distributions of sulfur and zinc in p-i-n InGaAs microdiscs by both cross-sectional scanning capacitance microscopy (SCM) and secondary-ion mass spectrometry using focused ion beam (NanoSIMS). Even though the InGaAs shell was grown on the microdiscs using dimethylzinc (DMZn), no p-type region was found on the top of the microdiscs and the p-type region existed on the sidewall of the discs alone. This result suggested that the zinc incorporation efficiency on InGaAs (111) plane is much lower than that on {−110} planes. Complete encapsulation of the microdiscs with p-type region was possible by the post-diffusion of zinc during exposure to a mixture of tertiarybutylarsine (TBAs) and DMZn after the growth of InGaAs microdiscs.

Characterization of process-induced defects in SiC MOSFETs by cross-sectional cathodoluminescence

Cross-sectional cathodoluminescence (CL) and scanning capacitance microscopy (SCM) measurements were carried out for silicon carbide (SiC) metal–oxide–semiconductor field-effect transistors (MOSFETs) to investigate process-induced defects. The DI defect-related line at 426 nm and a broad luminescence at approximately around 430–470 nm, which were produced by ion implantation, were observed in addition to the near-band-edge emission. CL images showed that the densities of nonradiative recombination and DI centers were high near the source region. Moreover, DI centers existed even in the n-drift region located 10 µm from the surface. These results indicate that many types of defects diffuse and interact with each other during annealing even in the area where dopant atoms are not implanted. The annealing process not only activates dopant atoms but also induces the diffusion of unstable native defects and transforms their structure into more thermally stable defects such as DI centers.

Nanoscale calibration of n-type ZnO staircase structures by scanning capacitance microscopy

Cross-sectional scanning capacitance microscopy (SCM) was performed on n-type ZnO multi-layer structures homoepitaxially grown by molecular beam epitaxy method. Highly contrasted SCM signals were obtained between the ZnO layers with different Ga densities. Through comparison with dopant depth profiles from secondary ion mass spectroscopy measurement, it is demonstrated that SCM is able to distinguish carrier concentrations at all levels of the samples (from 2 × 1017 cm−3 to 3 × 1020 cm−3). The good agreement of the results from the two techniques indicates that SCM can be a useful tool for two dimensional carrier profiling at nanoscale for ZnO nanostructure development. As an example, residual carrier concentration inside the non-intentionally doped buffer layer was estimated to be around 2 × 1016 cm−3 through calibration analysis.

Probing at Nanoscale Underneath the Gate Oxides in 4H-SiC MOS-Based Devices Annealed in N<sub>2</sub>O and POCl<sub>3</sub>

In this paper a comparative study of the impact of N2O and POCl3 annealing on the SiO2/SiC system is presented, combining nanoscale electrical characterization of SiC surface doping by scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM) to the conventional capacitance-voltage (C-V) and current-voltage (I-V) measurements on MOS-based devices. A significant reduction of the interface states density (from 1.8×1012 to 5.7×1011 cm-2eV-1) and, correspondingly, an increase in the carrier mobility (from 19 to 108 cm2V-1s-1) was found moving from N2O to POCl3 annealing. Furthermore, SSRM measurements on bare p+-type SiC regions selectively exposed to N2O and POCl3 at high temperature provided the direct demonstration of the incorporation of N or P-related donors in the SiC surface, leading to a partial compensation of substrate acceptors during N2O treatment and to an overcompensation during POCl3 annealing. Finally, cross-sectional SCM profiles performed on epitaxial n-doped 4H-SiC with 45 nm SiO2 (subjected to post deposition annealing in the two ambients) allowed to quantify the active donors concentrations associated to P or N incorporation under the gate oxide, showing almost a factor of ten higher doping (4.5×1018cm-3 vs 5×1017cm-3) in the case of P related donors.

A look underneath the SiO2/4H-SiC interface after N2O thermal treatments

The electrical compensation effect of the nitrogen incorporation at the SiO2/4H-SiC (p-type) interface after thermal treatments in ambient N2O is investigated employing both scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM). SSRM measurements on p-type 4H-SiC areas selectively exposed to N2O at 1150 °C showed an increased resistance compared to the unexposed ones; this indicates the incorporation of electrically active nitrogen-related donors, which compensate the p-type doping in the SiC surface region. Cross-sectional SCM measurements on SiO2/4H-SiC metal/oxide/semiconductor (MOS) devices highlighted different active carrier concentration profiles in the first 10 nm underneath the insulator–substrate interface depending on the SiO2/4H-SiC roughness.The electrically active incorporated nitrogen produces both a compensation of the acceptors in the substrate and a reduction of the interface state density (Dit). This result can be correlated with the 4H-SiC surface configuration. In particular, lower Dit values were obtained for a SiO2/SiC interface on faceted SiC than on planar SiC. These effects were explained in terms of the different surface configuration in faceted SiC that enables the simultaneous exposition at the interface of atomic planes with different orientations.

A Nanoscale Look in the Channel of 4H-SiC Lateral MOSFETs

In this work the field effect mobility measured on lateral n-channel MOSFETs in 4H-SiC with Al implanted body was correlated with the interface trap density measured on MOS capacitors. The test devices were fabricated on samples subjected to different post implantation annealing conditions (i.e. with or without a protective carbon capping layer) and to an identical post-oxidation annealing in N2O. Despite the improved interfacial morphology, a reduction of the peak mobility (from 40 to 24 cm2V-1s-1) was observed using the carbon capping layer. An increase in the density of interface traps was consistently found. Nanoscale measurements of the active dopant concentration in the SiC channel region by cross-sectional scanning capacitance microscopy showed an higher compensation of p-type SiC for the sample processed without the capping layer, which indicates a more efficient incorporation of nitrogen at the SiO2/SiC interface.

Nonequilibrium carrier distribution in semiconductor photodetectors: Surface leakage channel under illumination

The nonequilibrium carrier distribution in an InGaAs/InP avalanche photodiode under light illumination is obtained by cross-sectional scanning capacitance microscopy combined with numerical simulation. The sheet density of negative surface charge is determined to be 1.85×1010 cm−2 on the native-oxidized InGaAs (110) face. This surface charge is found responsible for the accumulation of minority holes, which leads to an inversion layer at the sidewall surface of device in the absorption region under illumination exceeding 0.1 mW/cm2. The inversion depth increases up to 200 nm along with the enhancement of excitation intensity. This work suggests that a surface leakage channel may form in semiconductor photodetectors through detection light excitation.

Scanning capacitance microscopy investigation on InGaAs/InP avalanche photodiode structures: Light-induced polarity reversal

Cross-sectional scanning capacitance microscopy is applied to study the carrier distribution as well as its variation under irradiation in an InGaAs/InP avalanche photodiode. The photocarriers excited by the stray light of atomic force microscope laser beam lead to a dramatic deviation of the dC/dV profile in the unintended-doped absorption layer, and even cause the reversal of signal polarity. The existence of surface potential and its impact on the spreading of photocarriers near the cleaved face are demonstrated as the main origins of the light-induced dC/dV reversal. The effect provides experimental information on the distribution property of photoelectric process in devices.

Doping characterization of InAs∕GaAs quantum dot heterostructure by cross-sectional scanning capacitance microscopy

In order to better understand dopant incorporation in quantum dot infrared photodetectors, the application of cross-sectional scanning capacitance microscopy (SCM) has been used to investigate carrier occupation/distribution in a multilayer InAs∕GaAs quantum dot (QD) heterostructure for different doping techniques. The doping schemes in the QD structure include direct doping (in InAs QD layers) and remote doping (in GaAs barrier layers), each with different doping concentrations. The SCM image suggests that large band bending occurs due to highly doped, remote-doping layers, thereby causing electron redistribution in direct-doping layers. The experimental result is supported by a band structure calculation using the Schrödinger–Poisson method by NEXTNANO3.

Characterization of Al-doped ZnTe layer using scanning capacitance microscopy and Kelvin probe force microscopy

Al-doped ZnTe layer in ZnTe LED fabricated by Al thermal diffusion has been characterized using cross-sectional scanning capacitance microscopy (SCM) and Kelvin probe force microscopy (KFM), and the results were verified by comparing with the result of the electron beam induced current (EBIC) measurements. Both SCM and KFM images showed contrast changes between Al diffusion layer and p-type substrate, indicating the conductivity type conversion in ZnTe. The distance from the surface of ZnTe to the contrast change position was around 1.8 μm, in consistent with the result of EBIC measurement. The surface potential of the Al diffusion layer was found to be almost constant, implying that the diffusion layer is relatively homogenous in depth direction. Thus, SCM and KFM are capable of providing the electrical information on ZnTe LED.

Improving the Accuracy of Modified Shift-and-Ratio Channel Length Extraction Method Using Scanning Capacitance Microscopy

A modified shift-and-ratio (MS&R) method of extracting the effective channel length (Leff) has been tested on metal oxide semiconductor (MOS) transistors fabricated by state-of-the-art CMOS technology. The value of Leff generated by this method is more reasonable than the original shift-and-ratio method with much less computation time involved. We show the correctness of the MS&R method by comparing the Leff extracted with the channel length obtained by cross-sectional scanning capacitance microscopy (SCM) measurement. In addition, we show that a higher off current corresponds to a smaller Leff measured by our method.

Scanning capacitance microscopy investigations of InGaAs/InP quantum wells

In this work, cross-sectional scanning capacitance microscopy (SCM) is used to investigate InGaAs/InP (latticed matched) quantum wells grown by metal-organic vapor phase epitaxy. Using n-doped InP as barriers with different doping levels, different InGaAs wells structures (5, 10 and 20 nm) were investigated. The capability of SCM to detect electrons in the quantum wells is demonstrated, showing in addition, a systematic and consistent trend for the different well widths and barrier doping levels. The SCM results are qualitatively consistent with electron distribution obtained for 1D Poisson/Schrödinger simulation. Finally, resolution issues in SCM are discussed in terms of tip averaging effects.

Electrical characterization of InGaAs/InP quantum wells by scanning capacitance microscopy

In this work, cross-sectional scanning capacitance microscopy (SCM) is used to investigate InGaAs/InP quantum wells. It is demonstrated that SCM is indeed capable of detecting the electrons in the quantum wells and that the SCM signal shows a systematic trend for the different wells. Clear dips in the dC/dV signal are observed at the InGaAs quantum wells and imply carrier densities higher than the surrounding barriers. It is also shown that the depletion regions in the barriers adjacent to the wells can be resolved. The results show that geometric tip-averaging effects significantly influence the imaging of electrons in quantum wells and limit the lateral resolution.

Ultrathin oxides for the SCM analysis of sub-micron doping profiles

Attenuated total reflection (ATR) spectroscopy and spectroscopic ellipsometry (SE) have been used to characterize oxides used for the scanning capacitance microscopy (SCM) technique. SCM has been used to study boron and phosphorous doped Si test structures epitaxially grown on (100) Si substrates. SCM samples have one-dimensional (1D) doping profiles with sub-micron features, with staircase-like steps in the unipolar sample and a smoother profile in the bipolar sample, as obtained by secondary ion mass spectrometry (SIMS) profiling. Cross-sectional SCM results obtained on samples oxidized by the standard low-temperature UV-ozone method are presented, discussed and compared to results obtained on cleaved samples oxidized by a simple exposure to air. The results show that the native oxide covering a (110) cleaved section may yield SCM images of sufficient quality, with no contrast reversal on a wide range of doping levels, as well as observed on sections prepared with the UV-ozone technique. However, the long-term stability of the SCM signal on native oxides is poor, and UV-ozone oxidation can be used to recover a valid SCM signal. Realistic ultrathin oxide thickness data obtained by SE on (110) substrates are presented together with ATR results, which confirm the superior quality of UV-ozone oxides with respect to other kinds of oxides.

Product development and yield enhancement through failure analysis of integrated circuits with scanning capacitance microscopy

The application of scanning capacitance microscopy (SCM) to failure analysis of integrated circuits is described. Both top-down (parallel to the wafer surface) and cross-sectional SCM measurements were performed on product lots containing yield-limiting failures identified with electrical testing. The SCM images of functioning devices were compared with those of failed devices to note any differences in free carrier distributions. Using this approach, the root causes of failures in two different product lots—one bulk Si technology and one silicon-on-insulator technology—were identified. These two examples are described in detail, as well as the subsequent corrective actions taken to improve yield.

Application of SCM for the microcharacterization of semiconductor devices

The article presents an overview of the current status of scanning capacitance microscopy (SCM) with respect to applications in the failure analysis of modern semiconductor devices. Examples for top-down and cross-sectional SCM imaging and the importance of an optimized sample preparation for improved SCM results will be discussed. An outlook on SCM requirements for future-technology generations highlights the usefulness of high aspect ratio tips for SCM measurements.

Method for the study of semiconductor device operation using scanning capacitance microscopy

A new method for studying semiconductor device operation with cross-sectional scanning capacitance microscopy (SCM) is described. The technique uses a modified, commercial SCM system to image device cross sections while dc voltages are incrementally applied to bias test structures between nonconducting and conducting states. The novel test structure design and packaging simplify sample preparation while enabling the application of voltages to discrete regions of the semiconductor device. By recording sequential SCM images as a function of bias voltage, the process of switching a device “off” and “on” can be visualized to study carrier movement in the active region of the device. Two sample SCM sequences of metal–oxide–semiconductor field-effect transistors, one fabricated on a bulk silicon substrate and the other on a silicon-on-insulator substrate, are presented to show typical carrier movement in these devices.

Direct measurement and characterization of n+ superhalo implants in a 120 nm gate-length Si metal–oxide–semiconductor field-effect transistor using cross-sectional scanning capacitance microscopy

We have directly measured nanoscale electronic features associated with a 120 nm physical gate length p-channel silicon metal–oxide–semiconductor field-effect transistor device structure including n+ superhalo implants using cross-sectional scanning capacitance microscopy (SCM). A dc bias-dependent voltage series of SCM images representing nine bias conditions from 2 to −2 V in 0.5 V steps was obtained. The SCM contrast observed varies with the ac and dc bias applied to the sample and allows delineation of the device features, including the p+ source and drain contacts, p+ source and drain extensions, p+ polycrystalline silicon gate, electrical p–n junction, n-well, and n+ superhalo implants. It is demonstrated that the superhalo implant features are imaged only under specific SCM bias conditions. Detailed analysis of the resulting SCM contrast indicates an apparent channel length of 73±11 nm, and reveals clear asymmetry in the individual lobes of the n+ superhalo implant features.

Characterization of GaAs/AlGaAs laser mesas regrown with semi-insulating GaInP by scanning capacitance microscopy

In this work, cross-sectional scanning capacitance microscopy (SCM) is used to investigate GaAs/AlGaAs buried heterostructure lasers. Laser mesas are buried with GaInP:Fe selectively regrown by hydride vapor phase epitaxy. It is shown that a complete 2D map of the electrical properties of device structure including, delineation of regrown interfaces and electrical nature of the regrown GaInP layer can be obtained. The behavior of the SCM signal with ac bias is used to verify the semi-insulating nature of the regrown layer at different locations of the sample. The measured SCM signal for the regrown GaInP:Fe layer is uniformly zero, indicating very low free carrier densities and confirming semi-insulating properties. This observation strongly suggests, in addition, uniform Fe incorporation in the regrown layers, close to and far away from the mesa. Finally, a nanoscale feature in the SCM contrast appearing as a bright (dark) spot in dC/dV mode (feedback bias mode) is observed at the mesa sidewall close to the interface between the regrown GaInP:Fe and the p-cladding layer. The origin of this contrast is discussed in terms of local band-bending effects and supported by 2D Poisson simulations of the device structure.