For many years, scanning capacitance microscopy (SCM) has been an important method to observe and to analyse the carrier distributions and pn junctions in semiconductor materials and devices.SCM provides differential capacitance (dC/dV) images corresponding to carrier polarity and concentrations, which are important information forsemiconductordevices. For instance, SCM can identify p-type and n-type regions in semiconductors and map pn junctions. With SCM, one can profile the source and the drain regions as well as the channel length of a metal-oxide-semiconductor (MOS) device. It is well known that a typical SCM setup is based on an atomic force microscope (AFM) equipped with a conductive probe to synchronously provide AFM images and the corresponding carrier concentration profiles. Unfortunately, prior studies reported that the AFMlaser system may lead to photoperturbation problems in SCM measurements, including distorted junction profiles and false dC/dV signals. For a pn junction, the AFM laser could induce electron-hole pairs in the depletion region during SCM measurements. The photo-generated electron-hole pairs are separated by a built-in electric field, perturbing the dC/dV signals nearby the junction region. For this reason, a dark-mode SCM system has been developed to clearly observe pn junctions without photoperturbation problems. However, it is interesting to know if there is any physical information hid in the photoperturbed junction images observed by typical SCM. For a pn junction under continuous-wave (CW) laser illumination, the depletion region reaches a steady state but thermal equilibrium. Since dC/dV signals are very sensitive to the carrier polarity and concentration, it is expected that the dC/dV image may respond to the carrier distribution of a depletion region in steady state. In this work, we performed dark-mode SCM measurements combining with external laser illumination for investigating the subtle variation in pn junctions under various illumination intensities. Two series of samples were prepared by patterned n-type silicon wafers and boron ion implantation. The window width and the spacer of the pattern was 0.8 um and 2 um, respectively. The first series of the samples were treated by microwave annealing (MA) for 300 s after boron implantation. This series of samples was denoted as MA300. Another series of samples were treated by the same MA process followed by furnace annealing (FA) at 550 ^oC for 6 hours. This series of samples was denoted as MA+FA. The MA power is about 1800 W. In general, the effective temperature of MA for boron activation is less than 500 ^oC. A Bruker D3100 scanning probe microscope was used to obtain the cross-sectional dC/dV images of pn junctions. For all of the samples, the boron depth profiles were obtained by secondary ion mass spectrometer (SIMS). For all of the samples, there was no significant difference between the SIMS boron profiles since MA is a low-thermal-budget process. For the samples MA300, illumination-induced contrast reversal was clearly observed from the dC/dV images of the lateral pn junctions. It is found that the contrast reversal behavior depends on the intensity of the illumination. Due to the recombination of photo-generated electron-hole pairs and space charges, the p-side and the n-side of the junction region exhibits electron-rich and hole-rich, respectively. As a result, the dC/dV images of the depletion regions with photo-illumination show a contrast reversal behavior. In addition, the influence of the electron drift from p-side to n-side on the dC/dV signals was also observed. For the vertical junctions, SCM cannot observe obvious contrast reversal since the effective space charge density in vertical direction is lower than that in lateral direction. In addition, for the samples MA+FA the dC/dV images exhibit a similar behavior before and after laser illumination. Section analysis of the dC/dV images revealed that FA induces boron deactivation, reducing the electric field in the lateral junction and leading to a shift of the electrical junction. Since the electric field in the lateral junction is reduced by FA, the dC/dV signal intensity induced by the electron drift from p-side to n-side also decreased. In summary, we have employed dark-mode SCM with external laser illumination to observe the image contrast reversal of pn junctions formed by boron implantation and MA treatment. For an abrupt pn junction, one can observe clear contrast reversal from the dC/dV image of the depletion region. Our experimental results indicate that the contrast reversal behavior in the dC/dV image of a depletion region can be an indicator of the space charge density.

1. POSTER: Image contrast reversal of depletion regions in scanning capacitance microscopy, Zip archive data, at least v2.0 to extract, 0MB

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