Best sample for our SPV measurements
Dimensions:
The sample dimensions should be between 5x5 and 10x10 mm2, because our SPV probe electrode is 5x5mm2 and the space on the sample holder is limited. The sample could be a rectangle or could have irregular shape. The important point concerning the size is that the sample should not be too small. If it is unavoidable, we can use samples as small as 2x2mm2, but this could lead to lower and noisier signal. The substrate thickness should be standard ~300-500 um.
Doping:
In general an undoped (nano)structure grown on a highly doped substrate seems to be the most favorable combination for SPV measurements. This situation would be further favorable if the doping of the substrate and the residual doping of the nanostructure are of the same type, because in this case the direction of the band bending will be the same all over the system (substrate and structure) and therefore no subtraction of SPV signals from different parts of the sample will occur.
Density:
In the case of quantum dots (QD) or quantum wires (QWR) it is preferable that their surface density is high in order to have larger optical absorption and therefore larger SPV signal. In this sense multi-layered QD or QWR samples are good solutions. Detection of good SPV signal from single quantum wells (QWs) is possible (sometimes this depends on the presence of a substrate related background and its magnitude).
Potential wells:
The SPV signal from QWs, QWRs or QD would be lager if their barriers are lower because of the following reasons. The carriers generated in the potential well of the QW, QWR or QD have to escape in the barrier region, where they are separated by the built-in electric field thus giving rise to SPV effect. The carrier escape from the well could be via thermal emission or via field assisted tunneling. At room temperature the thermal emission predominates and therefore the barrier height is an important parameter for obtaining a good SPV signal.
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We are aware that it could be not possible to satisfy all the requirements, but this description could serve as a guide if one has to choose among several samples.