Development of nanoscale local-bandstructure analyzing system
Osamu Takeuchi, Shoji Yoshida, Hidemi Shigekawa (University of Tsukuba, CREST)

We will develop a measurement system of local bandstructure at semiconductor surfaces with ananometer resolution using scanning tunneling microscopy (STM). Our two goals are establishing aconcrete analyzing procedure and releasing a completed measurement system for it. Knowledge oflocal amount of surface bandbending is not only important for developing future nanodevices but for
general STM measurements on any semiconductor substrate because it affects effective bias voltage.
Recently, we have developed a new technique to measure nanoscale variation of surface
photovoltage (SPV) simultaneously with STM topography. Note that, SPV is photo-induced change
in surface bandbending, which appears as modulation in effective bias voltage in STM measurement.
Our method provides local SPV for a range of bias voltage without varying probe-sample distance inaddition to current-voltage characteristics of STM at the measurement point. For instance, left figure
shows how SPV is varied by bias-induced change in local bandbending. Right figure shows how SPV
is modulated by nanoscale silver islands with one monolayer thickness (outlined by green lines).
Although, our method provides extensive information about local bandstructure at semiconductor
surfaces, it still has several drawbacks that commonly appears in local bandstructure measurementswith scanning probe microscopy: SPV, Kelvin probe (KP) and barrier height (BH) measurement.
These measurements are, in general, strongly affected by chemical and physical properties of probe
apex, which makes interpretation of measured data difficult. In addition, the measurements requires
special equipments and skills to use them. Consequently, they have not yet been in general usage.
In this project, we improve theoretical and technical analyzing procedures to extract applicatively
meaningful parameters from combination of SPV, KP and BH measurements by STM. For this purpose, we first develop a measurement system with which a same sample can be easily measured byvariety of analyzing techniques. This is required to examine the effect of probe apex properties andcheck the theoretical models. The system will be developed using a digital signal processor (DSP)
board, which undertake all the tasks of servoing z and/or bias, modulating z and/or bias, scanning xy
and/or bias, phase-locked loop (PLL) detection of external modulation, phase sensitive detection ofmodulated signals and data processing for analysis, so that we can easily switch between variety ofmeasurement techniques without purchasing, modifying nor (dis)connecting/connecting electronics.
Replacement of the large part of analog circuits in the conventional STM controllers with a concurrent DSP processor of high performance and noble software logic will make the system costless.
Thus, we plan the system to be released from a domestic STM company as the standard STM
controllering system so that all future STM users have our new technique without extra cost.

Light ON

IntrinsicSPVLight OFFSPV underbias voltage

5nm
Bias-dependent SPV for clean Si(111)-7x7 at 300K.

Spatially modulated SPV for Ag/Si(001)-2x3 at 80K.