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A near-field scanning microwave microscope for characterization of inhomogeneous photovoltaics
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Image of FIG. 1.
FIG. 1.

(a) Layout of NSMM with green arrows detailing microwave power flow from the VNA, through the phase shifter and bias-T, and down to the probe tip. The bias-T separates the DC current from the microwave signal, allowing the former to pass on through to a preamplifier for detection. During scanning, the tip is held stationary, while the sample is controlled with an X, Y, Z piezo stage. The sample is scanned in the illuminated or dark state with the tip-sample distance controlled by use of tuning-fork feedback. (b) RLC model of tip/sample interaction composed of the parallel coupling capacitance (Ccoupling) and resistance (Rcoupling) in series with the bulk sample capacitance (Csample) and resistance (Rsample). Rcoupling accounts for the thin water film typically found on samples in ambient conditions. At the operating frequencies, Rcoupling ≫ 1/ωC and can be neglected. The capacitance between the sample and the outer conductor (Cout) can be ignored as it is very large compared to Csample. The inductance is not important for the samples being measured in this frequency regime.

Image of FIG. 2.
FIG. 2.

SEM image of mechanically cut, Pt-Ir tip with radius roughly 100 nm.

Image of FIG. 3.
FIG. 3.

(a) Schematic of probe tip construction with cut Pt-Ir tip inserted into a coaxial cable. 3(b) Model of tuning fork holder with set screw that provides variable torque between the tuning fork and the probe tip.

Image of FIG. 4.
FIG. 4.

Tuning fork probe amplitude as a function of frequency, showing typical responses of the tuning fork in contact (solid) and out of contact (dashed) with the NSMM tip. Contact results in roughly halving the measured Q and decreasing the resonant frequency by 0.42 kHz.

Image of FIG. 5.
FIG. 5.

Microwave reflection magnitude (S11) versus frequency with the tuning fork in contact (solid) and out of contact (dashed) with the NSMM tip. Removing the tuning fork resulted in an increase in both the reflection minimum of 29 dBm and shift in position by 0.35 MHz.

Image of FIG. 6.
FIG. 6.

(a). Sample topography showing 5 μm wide trench with side walls of roughly 350 nm in height. 6(b). Sample S11 amplitude at 2.26 GHz with evident distinction between the grounded gold region (left), exposed quartz trench (center), and biased gold region (right).

Image of FIG. 7.
FIG. 7.

Single horizontal topography scan across the test sample showing the 5 μm quartz trench with sample tilt of 1 μm for every 50 μm laterally.

Image of FIG. 8.
FIG. 8.

Typical reflection frequency response with minimum tuned to −78 dBm and a normalized phase shifter setting of 0° is shown by a dashed line (sharpest dip). Small variations in the phase shifter highlight the sensitivity of |S11| to the microwave phase-tuning circuit.

Image of FIG. 9.
FIG. 9.

Measured current density versus internal voltage for CIGS. The measurements reveal a 75% fill factor and 17.125% efficiency (ratio of the maximum power point to the incident light power density). Both the dark state current density (solid) and the illuminated state (dashed) are shown.

Image of FIG. 10.
FIG. 10.

Processed CIGS data with fixed tip-sample height across a 30 × 30 μm2 scan area. 10(a) Topography in the dark state showing grains several micrometers across with height variation on the order of 1.5 μm. 10(b) DC current data in the dark state with low absolute values corresponding to concentrations of raised grains. 10(c) S11 data in the dark state with sharp contrast likely corresponding to variation in depletion due to trapped charge localized at or near grain boundaries. 10(d) Topography in the illuminated state. Prominent grains can be matched with those of the dark state indicating that illumination does not substantially affect tuning-fork feedback response. 10(e) DC current data in the illuminated state with less contrast at grain boundaries due light-generated charge reducing depleted regions. For both the dark and illuminated current images, quenching of the data occurred as current values periodically saturated the preamplifier. 10(f) S11 data in the illuminated state with decreased contrast at the grain boundaries; once again caused by light-generated charge reducing depleted regions and contrast in local capacitance. The square area enclosed by the solid lines corresponds to the area magnified in Fig. 11.

Image of FIG. 11.
FIG. 11.

Zoomed in S11 images from solid, highlighted region in FIG 10. 11(a) S11 data in the dark state showing two grains also found in the topography. 11(b) S11 data in the illuminated state with one grain having disappeared. Because this feature is still present in the illuminated topography scan, its absence is attributed to a reduction in local depleted regions at the grain and a subsequent loss in capacitance contrast.


Generic image for table
Table I.

Spot size and intensity values for various wavelengths of the Thorlabs multi-channel fiber-coupled laser source.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: A near-field scanning microwave microscope for characterization of inhomogeneous photovoltaics