The research discussed here was supported by the Michigan State University Institute for Quantum Sciences and the National Science Foundation DMR-0305461, DMR-0906939, and DMR-0605801. K.W. acknowledges support from a U.S. Department of Education GAANN Interdisciplinary Bioelectronics Training Program fellowship.

1. Protocol
<ol>
	<li>Initial setup of microscope and electronics.
	<ol>
		<li>Begin with a cryogenic-capable scanning probe microscope with associated control electronics. The microscopes used for the research described here use inertial translation to "walk" the sample towards and away from the tip along ramps13 (made from a conducting material such as copper, brass, or stainless steel to enable them to transmit bias voltage to the sample) as part of a Besocke design STM14, schematically shown i...

<ol>
	<li>Gasseller, M., DeNinno, M., Loo, R., Harrison, J. F., Caymax, M., Rogge, S., Tessmer, S. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single-Electron+Capacitance+Spectroscopy+of+Individual+Dopants+in+Silicon.">Single-Electron Capacitance Spectroscopy of Individual Dopants in Silicon.</a> Nano Lett. 11, 5208-5212 (2011).</li><li>Kuljanishvili, I., Kayis, C., Harrison, J. F., Piermarocchi, C., Kaplan, T. A., Tessmer, S. H., Pfeiffer, L. N., West, K. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Scanning-probe+spectroscopy+of+semiconductor+donor+molecules.">Scanning-probe spectroscopy of semiconductor donor molecules.</a> Nat. Phys. 4, 227-233 (2008).</li><li>Tessmer, S. H., Kuljanishvili, I., Kayis, C., Harrison, J. F., Piermarocchi, C., Kaplan, T. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanometer-scale+capacitance+spectroscopy+of+semiconductor+donor+molecules.">Nanometer-scale capacitance spectroscopy of semiconductor donor molecules.</a> Physica B. 403, 3774-3780 (2008).</li><li>Yoo, M. J., Fulton, T. A., Hess, H. F., Willett, R. L., Dunkleberger, L. N., Chichester, R. J., Pfeiffer, L. N., West, K. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Scanning+Single-Electron+Transistor+Microscopy:+Imaging+Individual+Charges.">Scanning Single-Electron Transistor Microscopy: Imaging Individual Charges.</a> Science. 276, 579-582 (1997).</li><li>Urazhdin, S., Tessmer, S. H., Ashoori, R. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+simple+low-dissipation+amplifier+for+cryogenic+scanning+tunneling+microscopy.">A simple low-dissipation amplifier for cryogenic scanning tunneling microscopy.</a> Rev. Sci. Instrum. 73 (2), 310-312 (2002).</li><li>Williams, C. C., Hough, W. P., Rishton, S. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Scanning+capacitance+microscopy+on+a+25+nm+scale.">Scanning capacitance microscopy on a 25 nm scale.</a> Appl. Phys. Lett. 55 (2), 203-205 (1989).</li><li>Tessmer, S. H., Glicofridis, P. I., Ashoori, R. C., Levitov, L. S., Melloch, M. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Subsurface+charge+accumulation+imaging+of+a+quantum+Hall+liquid.">Subsurface charge accumulation imaging of a quantum Hall liquid.</a> Nature. 392, 51-54 (1998).</li><li>Tessmer, S. H., Kuljanishvili, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Modeling+single-+and+multiple-electron+resonances+for+electric-field-sensitive+scanning+probes.">Modeling single- and multiple-electron resonances for electric-field-sensitive scanning probes.</a> Nanotechnology. 19, 445503-445510 (2008).</li><li>Kuljanishvili, I., Chakraborty, S., Maasilta, I. J., Tessmer, S. H., Melloch, M. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Modeling+electric-field-sensitive+scanning+probe+measurements+for+a+tip+of+arbitrary+shape.">Modeling electric-field-sensitive scanning probe measurements for a tip of arbitrary shape.</a> Ultramicroscopy. 102, 7-12 (2004).</li><li>Martin, J., Akerman, N., Ulbricht, G., Lohmann, T., Smet, J. H., von Klitzing, K., Yacoby, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Observation+of+electron-hole+puddles+in+graphene+using+a+scanning+single-electron+transistor.">Observation of electron-hole puddles in graphene using a scanning single-electron transistor.</a> Nat. Phys. 4, 144-148 (2008).</li><li>Ashoori, R. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrons+in+artificial+atoms.">Electrons in artificial atoms.</a> Nature. 379, 413-419 (1996).</li><li>Ashoori, R. C., Stormer, H. L., Weiner, J. S., Pfeiffer, L. N., Pearton, S. J., Baldwin, K. W., West, K. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single-electron+capacitance+spectroscopy+of+a+few+electron+box.">Single-electron capacitance spectroscopy of a few electron box.</a> Physica B. 189, 117-124 (1993).</li><li>Frohn, J., Wolf, J. F., Besocke, K., Teske, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Coarse+tip+distance+adjustment+and+positioner+for+a+scanning+tunneling+microscope.">Coarse tip distance adjustment and positioner for a scanning tunneling microscope.</a> Rev. Sci. Instrum. 60 (6), 1200-1201 (1989).</li><li>Besocke, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+easily+operable+scanning+tunneling+microscope.">An easily operable scanning tunneling microscope.</a> Surf. Sci. 181, 145-153 (1987).</li><li>Urazhdin, S., Maasilta, I. J., Chakraborty, S., Moraru, I., Tessmer, S. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-scan-range+cryogenic+scanning+probe+microscope.">High-scan-range cryogenic scanning probe microscope.</a> Rev. Sci. Instrum. 71 (11), 4170-4173 (2000).</li><li>Ashoori, R. C., Stormer, H. L., Weiner, J. S., Pfeiffer, L. N., Pearton, S. J., Baldwin, K. W., West, K. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single-electron+capacitance+spectroscopy+of+discrete+quantum+levels.">Single-electron capacitance spectroscopy of discrete quantum levels.</a> Phys. Rev. Lett. 68 (20), 3088-3091 (1992).</li></ol>

A detailed explanation of the theoretical basis for this experimental method is given in References 8 and 9 and discussed with respect to the scenario of subsurface dopants in Reference 2; the overview presented here will therefore be brief and conceptual. The tip is treated as one plate of a capacitor, and the conducting layer underlying the sample comprises the other plate. If the DC voltage is applied such that electrons are pulled toward the tip, and if there is a dopant atom situated between the underlying conductin...

Subsurface Charge Accumulation (SCA) imaging is a low-temperature method capable of resolving single-electron charging events. When applied to the study of dopant atoms in semiconductors, the method can detect individual electrons entering donor or acceptor atoms, permitting characterization of the quantum structure of these minute systems. At its heart, SCA imaging is a local capacitance measurement6 well-suited for cryogenic operation. Because capacitance is based on electric field, it is a long-range effect that can resolve charging beneath insulating surfaces6. Cryogenic operation permits investigation of single-electron motion and quantum level spacing that would be unresolvable at room temperature1,2. The technique can be applied to any system in which electron motion below an insulating surface is important, including the charging dynamics in two-dimensional electron systems at buried interfaces7; for brevity, the focus here will be on studies of semiconductor dopants.
At the most schematic level, this technique treats the scanned tip as one plate of a parallel-plate capacitor, although realistic analysis requires a more detailed description to account for the curvature of the tip8,9. The other plate in this model is a nanoscale region of the underlying conducting layer, as shown in Figure 1. Essentially, as a charge enters a dopant in response to a periodic excitation voltage, it gets closer to the tip; this movement induces more image charge on the tip, which is detected with the sensor circuit5. Similarly, as the charge exits the dopant, the image charge on the tip is decreased. Hence the periodic charging signal in response to the excitation voltage is the detected signal - essentially it is capacitance; thus this measurement is often referred to as determining the C-V characteristics of the system.
During the capacitance measurement, the only net tunneling is between the underlying conductive layer and the dopant layer - charge never tunnels directly onto the tip. The lack of direct tunneling to or from the tip during the measurement is an important difference between this technique and the more familiar scanning tunneling microscopy, although much of the hardware for this system is essentially identical to that of a scanning tunneling microscope. It is also important to note that SCA imaging is not directly sensitive to static charges. For investigations of static charge distributions, scanning Kelvin probe microscopy or electrostatic force microscopy is appropriate. Additional cryogenic methods for examining local electronic behavior exist which also have good electronic and spatial resolution; for example, scanning single-electron transistor microscopy is another scanning probe method capable of detecting minute charging effects4,10. SCA imaging was originally developed at MIT by Tessmer, Glicofridis, Ashoori, and co-workers7; moreover, the method described here can be considered as a scanning probe version of the Single-Electron Capacitance Spectroscopy method developed by Ashoori and co-workers11. A key element of the measurement is an exquisitely sensitive charge-detection circuit5,12 using high electron mobility transistors (HEMT); it can achieve a noise level as low as 0.01 electrons/Hz&frac12; at 0.3 K, the base temperature of the cryostat in Reference 5. Such a high sensitivity allows observation of single-electron charging in subsurface systems. This method is suited for the study of electron or hole dynamics of individual or small groups of dopants in semiconductors, with typical dopant areal densities on the order of 1015 m-2 in a plane geometry2. An example of a typical sample configuration for this type of experiment is shown in Figure 1. The dopant layer is typically positioned a few tens of nanometers beneath the surface; it is important to know the precise distances between the underlying conducting layer and the dopant layer and between the dopant layer and the sample surface. In contrast to tunneling, capacitance does not fall off exponentially but instead essentially decreases in inverse proportion to the distance. Hence, the dopant depth could in principle be even deeper than tens of nanometers beneath the surface, as long as some reasonable fraction of electric field lands on the tip. For all of the aforementioned cryogenic local probes of electronic behavior, including the technique described here, spatial resolution is limited by the geometric size of the tip and by the distance between the subsurface feature of interest and the scanning probe tip.

<table border="1">
		<tbody>
		 <tr>
			<td></td>
			<td></td>
			<td></td>
			<td>Equipment</td>
		 </tr>
		 <tr>
			<td>Besocke-design STM</td>
			<td>Custom</td>
			<td></td>
			<td>References 14 and 15</td>
		 </tr>
		 <tr>
			<td>Control electronics for STM</td>
			<td>RHK Technology</td>
			<td>SPM 1000 Revision 7</td>
			<td></td>
		 </tr>
		 <tr>
			<td>Lock-in amplifier</td>
			<td>Stanford Research Systems</td>
			<td>SR830</td>
			<td></td>
		 </tr>
		 <tr>
			<td>Curve tracer</td>
			<td>Tektronix</td>
			<td>Type 576</td>
			<td></td>
		 </tr>
		 <tr>
			<td>Oscilloscope</td>
			<td>Tektronix</td>
			<td>TDS360</td>
			<td></td>
		 </tr>
		 <tr>
			<td>Multimeter</td>
			<td>Tektronix</td>
			<td>DMM912</td>
			<td></td>
		 </tr>
		 <tr>
			<td>Wire bonder</td>
			<td>WEST&middot;BOND</td>
			<td>7476D</td>
			<td>with K~1200D temperature controller</td>
		 </tr>
		 <tr>
			<td>Soldering iron</td>
			<td>MPJA</td>
			<td>301-A</td>
			<td></td>
		 </tr>
		 <tr>
			<td>Cryostat</td>
			<td>Oxford Instruments</td>
			<td>Heliox</td>
			<td></td>
		 </tr>
		 <tr>
			<td></td>
			<td></td>
			<td></td>
			<td>Material</td>
		 </tr>
		 <tr>
			<td>Pt/Ir wire, 80:20</td>
			<td>nanoScience Instruments</td>
			<td>201100</td>
			<td></td>
		 </tr>
		 <tr>
			<td>GaAs wafer</td>
			<td>axt</td>
			<td>S-I</td>
			<td>For the mounting chip</td>
		 </tr>
		 <tr>
			<td>99.99% Au wire, 2 mil diameter</td>
			<td>SPM</td>
			<td></td>
			<td>For the mounting chip</td>
		 </tr>
		 <tr>
			<td>99.99% Au wire, 1 mil diameter</td>
			<td>K&amp;S</td>
			<td></td>
			<td>For wire bonding</td>
		 </tr>
		 <tr>
			<td>Indium shot</td>
			<td>Alfa Aesar</td>
			<td>11026</td>
			<td></td>
		 </tr>
		 <tr>
			<td>Silver epoxy</td>
			<td>Epo-Tek</td>
			<td>EJ2189-LV</td>
			<td>Any low-temperature-compatible conductive epoxy is acceptable</td>
		 </tr>
		 <tr>
			<td>HEMT</td>
			<td>Fujitsu</td>
			<td>Low Noise HEMT</td>
			<td></td>
		 </tr>
		</tbody>
 </table>

scanning-probe single-electron capacitance spectroscopy

The integration of low-temperature scanning-probe techniques and single-electron capacitance spectroscopy represents a powerful tool to study the electronic quantum structure of small systems - including individual atomic dopants in semiconductors. Here we present a capacitance-based method, known as Subsurface Charge Accumulation (SCA) imaging, which is capable of resolving single-electron charging while achieving sufficient spatial resolution to image individual atomic dopants. The use of a capacitance technique enables observation of subsurface features, such as dopants buried many nanometers beneath the surface of a semiconductor material1,2,3. In principle, this technique can be applied to any system to resolve electron motion below an insulating surface.
As in other electric-field-sensitive scanned-probe techniques4, the lateral spatial resolution of the measurement depends in part on the radius of curvature of the probe tip. Using tips with a small radius of curvature can enable spatial resolution of a few tens of nanometers. This fine spatial resolution allows investigations of small numbers (down to one) of subsurface dopants1,2. The charge resolution depends greatly on the sensitivity of the charge detection circuitry; using high electron mobility transistors (HEMT) in such circuits at cryogenic temperatures enables a sensitivity of approximately 0.01 electrons/Hz&frac12; at 0.3 K 5.

The chief indicator of a successful measurement is reproducibility, much as in other scanning probe methods. Repeated measurements are very important for this reason. For point capacitance spectroscopy, taking many measurements in succession at the same location helps to increase the signal-to-noise ratio and identify spurious signals.
Once a feature of interest has been identified within the charge accumulation image and capacitance spectroscopy has been performed, interpretation of the C-V d...

Watch this Scientific Journal Video about Scanning-probe Single-electron Capacitance Spectroscopy at JoVE.com

Scanning-probe Single-electron Capacitance Spectroscopy

Scanning-probe single-electron capacitance spectroscopy facilitates the study of single-electron motion in localized subsurface regions. A sensitive charge-detection circuit is incorporated into a cryogenic scanning probe microscope to investigate small systems of dopant atoms beneath the surface of semiconductor samples.

The integration of low-temperature scanning-probe techniques and single-electron capacitance spectroscopy represents a powerful tool to study the ...

scanning-probe-single-electron-capacitance-spectroscopy

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