Scanning Kelvin Probe Microscopy

NREL uses scanning kelvin probe microscopy (SKPM) to measure two-dimensional distributions of contact potential difference (CPD) between the tip and the sample with resolution in the nanometer range.

The CPD can be converted to the work function of the sample if the measurement is performed under thermoequilibrium state; and it is the electrical potential when the sample is illuminated or a bias is applied to the sample.

How It Works

In SKPM, the atomic force microscopy (AFM) system operates in non-contact mode, and a conductive tip is oscillated at the first resonant frequency of the cantilever over the sample surface as it is scanned laterally. The topographic data is taken by controlling the atomic force between the tip and sample. In addition to the atomic force, a long-range electrostatic force also exists between the tip and sample, which is determined by the CPD between them. The electrostatic force is detected by applying an ac voltage to the tip and using a lock-in amplifier. The ac voltage frequency is set at either the second resonant frequency of the cantilever or a low frequency (~20 kHz) that is far off the first resonant frequency, in order to avoid the interaction between the topographic and electrical signals. The electrostatic force is zero when the CPD is completely compensated by a dc voltage applied to the tip. In this case, the CPD is equal to the applied dc voltage.

In our laboratory, we have SKPM capability both in air and in UHV. SKPM measurement on solar cell cross-sections help to identify and assess p-n junctions, while SKPM performed on the surface of semiconductors allows the investigation of local electrical potential and band bending distribution.

Examples of Scanning Kelvin Probe Microscopy Capabilities

Left: Scanning kelvin probe microscopy is widely used to measure surface work functions and electrostatic potentials on nanoscale circuits, devices, and materials; this schematic shows the measurement capabilities of the technique when a device sample is in the dark. Right: This schematic shows the measurement capabilities of the scanning kelvin probe microscopy technique when a device sample is illuminated.

Schematics of band diagrams of the tip and sample in SKPM measurement. Kelvin probe measures the work function difference between the tip and the sample when in thermoequilibrium state (a), and measures the electrical potential when the sample is illuminated (b), which is the sum of the difference in workfunction and internal/external-applied voltages.

Left: Scanning kelvin probe microscopy image showing vertical bands of gray, white, and black that represent measured electrical potential in a cross section of a gallium indium phosphide/gallium arsenide tandem device sample. Center: Atomic force microscopy image showing topography of the cross section of the gallium indium phosphide/gallium arsenide tandem device sample represented at left but in more detail; image appears as vertical bands of gray with light and dark diagonally striped areas at left, then a narrow black band, and then a lighter gray band. Right: Graph showing the averaged potential profile at top, tunneling, and bottom junctions of the gallium indium phosphide/gallium arsenide tandem device sample shown in images at left.

(a) SKPM potential and (b) corresponding AFM topographic images from the cross-section of a GaInP2/GaAs tandem-junction solar cell. (c) Averaged potential profile along the vertical direction in (a), showing the potential at the top, tunneling, and bottom junctions.

Left: Graph showing distribution of the electric field in a hydrogen-doped amorphous silicon solar cell sample containing buffer layers. Right: Graph showing distribution of the electric field in a hydrogen-doped amorphous silicon solar cell sample that does not contain buffer layers.

Distributions of the electric field in a-Si:H n-i-p solar cells with (a) and without (b) buffer layers between the n and i and i and p layers. The electric field is deduced by taking the first derivative of the SKPM potential measurement. It is clear from the data that the incorporation of buffer layers improved the electric field distribution.

Left: Scanning kelvin probe microscopy image showing high-contrast areas of light and dark that look like platelets and represent measured electrical potential in a sample copper indium gallium selenide thin-film device. Center: Atomic force microscopy image showing more distinct and jagged light and gray areas at high resolution that represent the measured electrical potential in the sample copper indium gallium selenide thin-film device shown in the image at left. Right: Graph showing jagged line profile of the sample copper indium gallium selenide thin-film device shown at far left in scanning kelvin probe microscopy image.

(a) Electrical potential measured by SKPM and (b) corresponding AFM topographic image from Cu(In,Ga)Se2 thin films. The SKPM data shows a higher electrical potential or an upward band bending at grain boundaries. (c) Line profile along the arrows in (a).

Left: Scanning kelvin probe microscopy image showing electrical potential, as indicated in scattered round dark areas among lighter areas, in the p-layer of a hydrogen-doped amorphous silicon device sample. Right: Atomic force microscopy topographic image of a hydrogen-doped nanocrystalline amorphous silicon device; image contains bright white rounded shapes scattered among smaller, rounded gray shapes.
Left: Graph of the measured potential values of the p-layer of the hydrogen-doped amorphous silicon device sample shown above. Right: Graph showing deduced local open-circuit voltage values for the p-layer of the nanocrystalline hydrogen-doped amorphous silicon device shown above.

(a) SKPM potential and (b) AFM topographic images taken on the p-layer of a-Si:H and nanocrystalline (nc-Si:H) mixed-phase n-i-p device, showing that the nc-Si:H phase aggregate to clusters in size of ~500 nm. From the measured potential values shown in Fig. (c), local Voc distribution on the mixed-phase device was deduced as shown in Fig. (d).

Contact

Chun Sheng Jiang

Acting Group Manager-Analytical Microscopy and Imaging

Chun.Sheng.Jiang@nrel.gov
303-384-6687

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