Tip-induced band bending and its effect on local barrier height measurement studied by light modulated scanning tunneling spectroscopy

Local barrier height (LBH) of Si(001) surface was studied by light-modulated scanning tunneling spectroscopy (LM-STS), which enables the observation of the tip-sample-dependent LBH with or without photoillumination simultaneously. The bias voltage and tip-sample distance dependence of LBH were comprehensively understood by the tip-induced band bending (TIBB), which influences the scanning tunneling microscopy and spectroscopy (STM/STS) in measurement of the local electronic structures of semiconductors. A marked decrease in surface photovoltage caused by photocarrier tunneling at shorter tip-sample distance was also shown. On the basis of these results, a method to measure LBH free of TIBB is discussed.


Introduction
As the scale of target materials decreases, scanning tunneling microscopy and spectroscopy (STM/STS) has been playing an important role as a powerful method of analyzing physical and chemical phenomena because of its high potential to probe the local electronic structures of materials.Actually, since its development, a considerable number of studies have been devoted to the utilization of STM/STS for analyzing the characteristics of electronic structures such as local band structures [1] and local barrier height (LBH) [2-4].
However, as has been pointed out, STM/STS parameters, such as bias voltage, tunneling current and tip-sample distance, influence the sample characteristics [5,9].
One of the critical problems is the tip-induced band bending (TIBB).As is discussed in this paper, it is difficult to accomplish spectroscopic analysis on semiconductor surfaces by conventional methods because the bias voltage between the tip and the sample induces band bending in the sample surface underneath the STM tip [6].
Figure 1 shows the characteristics of TIBB depending on tip-sample distance.The measurement conformation among the STM tip, tunnel gap and semiconductor surface forms a metal-insulator-semiconductor (MIS) structure.When the tip-sample distance is sufficiently large, since the bias voltage is mostly applied to the tunnel gap between the sample and the tip, the band bending in the sample surface is small as shown in Fig.  and extensively applied to the analysis of semiconductor devices [6][7][8].However, since the tip-sample distance cannot be maintained constant when the bias voltage is changed [5], these conventional methods cannot be used for the detailed analysis of TIBB.
In this study, we analyzed TIBB on a Si(001) surface by measuring LBH using light-modulated-scanning tunneling spectroscopy (LM-STS) [9].Since this method enables the observation of the tip-sample-dependent LBH with or without photoillumination simultaneously, we can precisely analyze the tip-sample dependence of TIBB.

Experimental
Measurements were performed under ultrahigh vacuum (T=300 K) for a Si(100) 2x1 clean sample surface (p-type, 10 Ωcm).A laser diode (635nm) or a He-Cd laser (442nm), mechanically chopped at 100Hz, was used for photoillumination to perform LM-STS.Figure 3 shows an example of LM-STS for SPV measurement.After setting the tip position under the conditions of V=-3.0V and I=1nA, an I-V curve was obtained under With a similar process to that employed for SPV analysis, two I-Z curves under photoilluminated (red) and dark (blue) conditions are obtained simultaneously from the LM-STS spectrum as shown in Fig. 4. Thus, each value of LBH is calculated as Here, Φ tip , Φ sample , V and h represent the work functions of the STM tip (W) and the Si sample, the applied bias voltage and the Planck constant, respectively.Now, let us examine TIBB by LM-STS.The tip-sample distance dependence of LBH was measured by the method explained in section 2 using several tungsten tips.despite the limited distance change of 0.1nm for one tip..

Results and Discussions
Note that the LBH in Fig. 5 is not a real one, but an apparent LBH, which is deduced from the derivative of I-Z curves (eq.( 1)).If there is no TIBB, LBH should be constant because the slope of I-Z curve should be constant.In contrast, the decrease in LBH in Fig. 5, with the decrease in the tip-sample distance, indicates the increase in TIBB that lowers the intensity of tunnel current resulting in a deviation from the exponential function in eq. ( 1) [10].
Let us see the results obtained under dark condition first.In Fig. 5   Since SPV is realized by the photoexcited carriers, the observed change in SPV may be caused by the efficient tunneling of the photoexcited carriers at a shorter tip-sample distance that results in a suppression of the SPV effect [9].As assumed by the results of LBH, change in SPV is larger at higher bias voltage.This may be due to that tunneling of the photoexcited carriers is more effective for the higher bias voltage because of the decrease in LBH.
To examine the effect of photoillumination on LBH, we measured the tip-sample distance dependence of LBH by changing photoillumination intensity.A setpoint was determined as -2V and 1nA. Figure 7 shows the results, where the tunnel gap is photoilluminated continuously without chopping.In such a condition, since thermal expansion effect can be reduced, we can increase the intensity of photoillumination compared to the case of chopped light.When there is no band bending, LBH can be observed correctly regardless of photoillumination.Even in the case where band bending exists, LBH can be observed almost constant if TIBB is weak and little tip-sample distance dependent.In such a case, LBH obtained under photoillumination can be lower than that in the dark, because of the effect of SPV, which is slightly shown for the large distance region in shown.On the basis of these results, a method to measure LBH free of TIBB was discussed.Horizontal axis represents the distance of tip retraction from the set point.
Figs.1(b) and 1(a).TIBB increases with the total applied bias voltage.Although this effect is well recognized and suggested by some experimental results[6], its characteristic properties have not yet been well examined experimentally because these issues are directly related to the STM/STS measurement mechanism itself.

Figure 5
Figure 5 shows the tip-sample distance of LBH obtained for two different tips ((a), (c) and (e) for one tip, and (b), (d) and (f) for the other tip) under dark (blue) and photoilluminated (red) conditions, respectively.To examine the bias dependence of LBH, we obtained I-Z curves at various bias voltages, three of which are shown here.With this method, since LBH with or without photoilumination can be obtained simultaneously, the tip-sample dependence of TIBB can be analyzed as the effect of SPV taken into consideration together.After setting the STM tip with the conditions of sample bias voltage (-2V) and tunneling current (1nA), we wait for a while with a weak feedback under a chopped light to settle the tip position just to measure 1nA (center of the two I-V curves in Fig.4).Then, feedback is turned off completely and the bias voltage is changed to that used in the experiment.With this process, we can keep the same tip-distance of the set point throughout the measurements for different bias voltages.Thus, LM-STS spectra were obtained as the tip-sample distance was increased from the set point.

Fig. 5
Fig.5 photoilluminated condition, with the decrease in the tip-sample distance, indicates a decrease in the effect of photoillumination depending on the tip-sample distance.

Figure 6
Figure 6 shows the result.Tip-sample distance was varied by setting the bias voltage of the servo condition with a constant tunneling current set point of 1nA.The green spectrum was obtained at the position of 0 nm in Figs.5(a), 5(c) and 5(e).Blue and green spectra were obtained at two different positions of -0.35 nm (closer) and 0.24nm (far) from the set point.The central part of each spectrum is missing due to the difficulty in calculating the shift of the two I-V curves for that region as explained in section 2.

Fig. 6
Fig.6 photoexcited carriers at a shorter tip-sample distance that results in a suppression of the SPV effect.Actually, with the increase in photoillumination, LBH increases to close to the flat-band condition as shown in Fig.2(b).This is due to the reduction of TIBB caused by the increased photo carriers introduced by the increase in photoillumination.Therefore, although it is not perfect, the undesirable effects on LBH measurement can be suppressed by photoillumination of higher intensity.Since tunneling of the photoilluminated carriers at a shorter tip-sample distance suppresses the SPV effect, we should measure LBH at a large distance.A dull tip, which forms a large tip-sample distance as the set point of the tip position determined by the tunneling current and bias voltage, is suitable to measure a better LBH.

Fig. 5
Fig. 5 Tip-sample distance and bias-voltage dependences of LBH obtained by two different tips ((a), (c) and (e) for one tip and (b), (d) and (f) for another) under dark (blue) and photoilluminated (red)) conditions, respectively.Horizontal axis represents the distance of tip retraction from the set point.

Fig. 6
Fig. 6 Tip-sample distance dependence of SPV.The green spectrum was obtained at the position of 0 nm in Figs.5(a), 5(c) and 5(d).Blue and green spectra were obtained at two different positions of -0.35 nm (closer) and 0.24nm (far) from the set point.Horizontal axis represents the distance of tip retraction from the set point.

Fig. 7
Fig. 7 Tip-sample distance dependence of LBH under various intensities of photoillumination.Horizontal axis represents the distance of tip retraction from the set point.