Scanning-probe Single-electron Capacitance Spectroscopy

Summary

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.

Abstract

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 material^1,2,3. 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 dopants^1,2. 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^½ at 0.3 K ⁵.

Introduction

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 l....

Protocol

1. Protocol

1. Initial setup of microscope and electronics.
   1. 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 in Figure 2.
   2. In addition to the bias voltage and tunneling current coaxial wires, provide at least two other coaxial wires ....

Results

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.......

Discussion

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.......

Materials

List of materials used in this article

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