Semiconductor Characterization
Figure 5: SCM differential capacitance (dC/dV) versus active dopant concentration on n-type and p-type staircase samples.
end has a sharp tip used to scan a sample surface that is coated with a thin oxide layer. An AC bias is applied to the sam- ple with respect to the probe tip, thereby forming a metal-oxide-semiconductor (MOS) capacitor structure. During each cycle of the AC bias, the majority carriers in the semiconductor sample undergo localized depletion and accumulation directly below the SCM tip, thereby gen- erating a capacitance-voltage (CV) curve as illustrated in Figure 4b. Te magni- tude of the slope of the CV curve in the depletion mode and its polarity are deter- mined by the concentration and type of majority carriers in the vicinity of the tip contact, respectively. Te SCM tech- nique measures differential capacitance with respect to the bias voltage (dC/dV) as the tip is raster scanned across the sample surface. Te phase of the signal represents the type of carriers, and its magnitude represents the concentration of carriers. Figure 5 shows the relation- ship between active carrier concentra- tion with dC/dV. Here, a positive dC/dV indicates p-type carriers, and negative indicates n-type carriers. Te SCM tip is scanned across the sample surface and allows mapping spatially resolved images of carrier types and concentration with high spatial resolution (∼5 nm). Spatial resolution depends on tip dimension and geometry. Although SCM is ideal for p/n junc-
Figure 6: SSRM setup showing the required high vacuum, common back contact, MOS device cross sec- tion to be imaged, diamond probe tip, DC bias and logarithmic current amplifier, and spreading resistance relation to active dopant density.
tion delineation, the technique’s sen- sitivity at high carrier concentration (>1020
cm−3 ) is lower, and quantitative
analysis is challenging. Te SCM response within the depletion of a p/n junction depends on imaging parameters including AC and DC sample bias, as well as relative dopant concentration between the p and n sides. A typical p/n junction is charac- terized by a double inversion layer within the depletion zone. Tis feature has been reported [17] and is attributed to the space charge density described in an ideal p/n junction [18]. Scanning spreading resistance
Figure 7: (a) Schematics of an optical modulator; (b) phase map by electron holography; (c) differential capacitance (dC/dV) map by SCM.
2021 May •
www.microscopy-today.com
microscopy (SSRM). Much like SCM, SSRM is an AFM-based technique that is used for mapping active dopant distribu- tion on semiconductor surfaces. It uses a conductive probe tip that is scanned over a sample surface while a DC bias is applied to the sample with respect to the tip [19]. Te experimental setup for SSRM, shown in Figure 6, requires an inert or
39
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92
