Name: all-electronic nanosecond-resolved scanning tunneling microscopy: facilitating the investigation of single dopant charge dynamics
Uploaded: 2018-01-19T14:00:00
Description: Watch this Scientific Journal Video about All-electronic Nanosecond-resolved Scanning Tunneling Microscopy: Facilitating the Investigation of Single Dopant Charge Dynamics at JoVE.com

We would like to thank Martin Cloutier and Mark Salomons for their technical expertise. We also thank NRC, NSERC, and AITF for financial support.

1. Initial Setup of Microscope and Experiments
<ol>
	<li>Begin with an ultrahigh vacuum cryogenic-capable STM and associated control software. Cool the STM to cryogenic temperatures. 
	NOTE: Throughout this report, ultrahigh vacuum refers to systems that achieve &#60;10 x 10-10 Torr. The STM should be cooled to cryogenic temperatures; this is especially important when investigating the charge dynamics of dopants, which are thermally activated at modest temperatures. Other chambers may be at room tempera...

     

<ol>
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A., Guo, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantum+Transport+in+Gated+Dangling-Bond+Atomic+Wires.">Quantum Transport in Gated Dangling-Bond Atomic Wires.</a> Nano Lett. 17, 322-327 (2017).</li><li>Kolmer, M., Zuzak, R., Dridi, G., Godlewski, S., Joachim, C., Szymonski, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Realization+of+a+quantum+Hamiltonian+Boolean+logic+gate+on+the+Si(001):H+surface.">Realization of a quantum Hamiltonian Boolean logic gate on the Si(001):H surface.</a> Nanoscale. 7, 12325-12330 (2015).</li><li>Schofield, S. 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The variant of TR-STS in which the pump pulse is not applied is comparable to conventional STS, except that the system is being sampled at a high frequency rather than continuously. If the durations of the probe pulses are appropriate (&#62;&#915;LH), the TR-STS signal acquired without the pump pulse can be multiplied by a constant proportional to the experiment&#39;s duty cycle to coincide exactly with a conventional STS measurement. This is only possible because the measurements are made without the...

Scanning tunneling microscopy (STM) has become the premier tool in nanoscience for its ability to resolve atomic-scale topography and electronic structure. One limitation of conventional STM, however, is that its temporal resolution is restricted to the millisecond timescale because of the limited bandwidth of the current preamplifier1. It has long been a goal to extend STM&#39;s temporal resolution to the scales on which atomic processes commonly occur. The pioneering work in time-resolved scanning tunneling microscopy (TR-STM) by Freeman et. al.1 utilized photoconductive switches and microstrip transmission lines patterned on the sample to transmit picosecond voltage pulses to the tunnel junction. This junction-mixing technique has been used to achieve simultaneous resolutions of 1 nm and 20 ps2, but it has never been widely adopted due to the requirement of using specialized sample structures. Fortunately, the fundamental insight gained from these works can be generalized to many time-resolved techniques; even though the bandwidth of the STM&#39;s circuitry is limited to several kilohertz, the non-linear I(V) response in STM allows faster dynamics to be probed by measuring the average tunnel current obtained over many pump-probe cycles. In the intervening years, many approaches have been explored, the most popular of which are briefly reviewed below.
Shaken-pulse-pair-excited (SPPX) STM takes advantage of the advancements in ultrafast pulsed laser technologies to achieve sub-picosecond resolution by directly illuminating the tunnel junction and exciting carriers in the sample3. Incident laser light creates free carriers that transiently enhance conduction, and modulation of the delay between the pump and probe (td) allows dI/dtd to be measured with a lock-in amplifier. Because the delay between the pump and probe is modulated rather than the laser&#39;s intensity, as in many other optical approaches, SPPX-STM avoids photo illumination-induced thermal expansion of the tip3. More recent extensions of this approach have extended the timescales over which SPPX-STM can be used to investigate dynamics by utilizing pulse-picking techniques to increase the range of pump-probe delay times4. Importantly, this recent development also provides the ability to measure I(td) curves directly rather than via numerical integration. Recent applications of SPPX-STM have included the study of carrier recombination in single-(Mn, Fe)/GaAs(110) structures5 and donor dynamics in GaAs6. Applications of SPPX-STM face some restrictions. The signal SPPX-STM measures depends on free carriers excited by the optical pulses and is best suited to semiconductors. Additionally, although the tunneling current is localized to the tip, because a large area is excited by the optical pulses, the signal is a convolution of the local properties and material transport. Finally, the bias at the junction is fixed at the measurement timescale so that the dynamics under study must be photoinduced.
A more recent optical technique, terahertz STM (THz-STM), couples free-space THz pulses focused on the junction to the STM tip. Unlike in SPPX-STM, the coupled pulses behave as fast voltage pulses allowing for the investigation of electronically driven excitations with sub-picosecond resolution7. Interestingly, the rectified current generated from the THz pulses results in extreme peak current densities not accessible by conventional STM8,9. The technique has been used recently to study hot electrons in Si(111)-(7x7)9 and image the vibration of a single pentacene molecule10. THz-pulses naturally couple to the tip, however, the necessity to integrate a THz source to an STM experiment is likely to be challenging to many experimenters. This motivates the development of other widely applicable and easily implementable techniques.
In 2010, Loth et al.11 developed an all-electronic technique where nanosecond voltage pulses applied on top of a DC offset electronically pump and probe the system11. The introduction of this technique offered a critical demonstration of unambiguous and practical applications of time-resolved STM to measure previously unobserved physics. Although it is not as fast as junction mixing STM, which preceded it, applying microwave pulses to the STM tip permits arbitrary samples to be investigated. This technique does not require any complicated optical methodologies or optical access to the STM junction. This makes it the easiest technique to adapt to low temperature STMs. The first demonstration of these techniques was applied to the study of spin-dynamics where a spin-polarized STM was used to measure the relaxation dynamics of spin-states excited by the pump pulses11. Until recently, its application remained limited to magnetic adatom systems12,13,14 but has since been extended to the study of carrier capture rate from a discrete mid-gap state15 and charge dynamics of single arsenic dopants in silicon15,16. The latter study is the focus of this work.
Studies on the properties of single dopants in semiconductors have recently attracted significant attention because complementary metal oxide semiconductor (CMOS) devices are now entering the regime where single dopants can affect device properties17. In addition, several studies have demonstrated that single dopants can serve as the fundamental component of future devices, for example as qubits for quantum computation18 and quantum memory19, and as single atom transistors20,15. Future devices may also incorporate other atomic-scale defects, such as the silicon dangling bond (DB) which can be patterned with atomic precision with STM lithography21. To this end, DBs have been proposed as charge qubits22, quantum dots for quantum cellular automata architectures23,24, and atomic wires25,26 and have been patterned to create quantum Hamiltonian logic gates27 and artificial molecules28,29. Moving forward, devices may incorporate both single dopants and DBs. This is an attractive strategy because DBs are surface defects that can easily be characterized with STM and used as a handle to characterize single dopant devices. As an example of this strategy, DBs are used in this work as charge sensors to infer the charging dynamics of near-surface dopants. These dynamics are captured with the use of an all-electronic approach to TR-STM that is adapted from the techniques developed by Loth et al.11
Measurements are performed on selected DBs on a hydrogen terminated Si(100)-(2x1) surface. A dopant depletion region extending approximately 60 nm below the surface, created via thermal treatment of the crystal30, decouples the DB and the few remaining near-surface dopants from the bulk bands. STM studies of DBs have found that their conductance is dependent on global sample parameters, such as the concentration of dopants and the temperature, but individual DBs also show strong variations depending on their local environment16. During an STM measurement over a single DB, the current flow is governed by the rate at which electrons can tunnel from the bulk to the DB (&#915;bulk) and from the DB to the tip (&#915;tip) (Figure 1). However, because the conduction of the DB is sensitive to its local environment, the charge state of nearby dopants influences &#915;bulk (Figure 1B), which can be inferred by monitoring the DB&#39;s conductance. As a result, the conductance of a DB can be used to sense the charge states of nearby dopants, and can be used to determine the rates at which the dopants are supplied electrons from the bulk (&#915;LH) and lose them to the STM tip (&#915;HL). To resolve these dynamics, TR-STS is performed around the threshold voltages (Vthr) at which the tip induces ionization of near-surface dopants. The role of the pump and probe pulses is the same in the three time-resolved experimental techniques presented here. The pump transiently brings the bias level from below to above Vthr, which induces dopant ionization. This increases the conductance of the DB, which is sampled by the probe pulse which follows at a lower bias.
The techniques described in this paper will benefit those wishing to characterize dynamics occurring on the millisecond to nanosecond timescale with STM. While these techniques are not limited to studying charge dynamics, it&#39;s crucial that the dynamics are manifested via transient changes in the conductance of states that can be probed by STM (i.e., states on or near the surface). If the conductance of the transient states does not differ significantly from the equilibrium state, such that the difference between the transient and equilibrium currents multiplied by the probe pulse duty cycle is smaller than the systems noise floor (typically 1 pA), the signal will be lost in the noise and will not be detectable by this technique. Because the experimental modifications of commercially available STM systems required to perform the techniques described in this paper are modest, it&#39;s anticipated these techniques will be widely accessible to the community.

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all-electronic nanosecond-resolved scanning tunneling microscopy: facilitating the investigation of single dopant charge dynamics

The miniaturization of semiconductor devices to scales where small numbers of dopants can control device properties requires the development of new techniques capable of characterizing their dynamics. Investigating single dopants requires sub-nanometer spatial resolution, which motivates the use of scanning tunneling microscopy (STM). However, conventional STM is limited to millisecond temporal resolution. Several methods have been developed to overcome this shortcoming, including all-electronic time-resolved STM, which is used in this study to examine dopant dynamics in silicon with nanosecond resolution. The methods presented here are widely accessible and allow for local measurement of a wide variety of dynamics at the atomic scale. A novel time-resolved scanning tunneling spectroscopy technique is presented and used to efficiently search for dynamics.

The results presented in this section of the text have been previously published15,16. Figure 3 illustrates the behavior of an example selected DB with conventional STM. A conventional I(V) measurement (Figure 3A) clearly depicts a sharp change in the conductance of the DB at Vthr = -2.0 V. This behavior is also observed in STM images taken at -2.1 V (...

Watch this Scientific Journal Video about All-electronic Nanosecond-resolved Scanning Tunneling Microscopy: Facilitating the Investigation of Single Dopant Charge Dynamics at JoVE.com

All-electronic Nanosecond-resolved Scanning Tunneling Microscopy: Facilitating the Investigation of Single Dopant Charge Dynamics

We demonstrate an all-electronic method to observe nanosecond-resolved charge dynamics of dopant atoms in silicon with a scanning tunneling microscope.

The miniaturization of semiconductor devices to scales where small numbers of dopants can control device properties requires the development of new ...

The overall goal of electronic time-resolved scanning tunneling microscopy is to study dynamic processes occurring on the atomic scale. In these experiments, the charged dynamics of dopants in silicon are studied. These methods can be used to investigate interesting phenomena in the field of nanotechnology and physics such as the rate at which dopants in semiconductors are supplied with electrons or how tunneling currents can be used to drive the magnetic resonance of individual atoms.The main advantage of these techniques is that fast dynamics can be studied without the need to integrate optics into the tunneling junction. These techniques are not limited to low temperatures or ultra-high vacuum scanning tunneling microscopes. To begin, cool the scanning tunneling microscope capable of ultra-high vacuum to cryogenic temperatures.Ensure that the microscope's tip is equipped with high frequency wiring capable of about 500 megahertz. Then, connect an arbitrary function generator with at least two channels to the tip. Prepare the cycles of voltage pulse pairs used for pump-probe experiments by configuring the arbitrary function generator so that the pump and probe voltage pulses are generated independently and summed before being fed into the tip.Next, apply the DC bias voltage used for imaging in conventional spectroscopy to the sample. Then, connect two radio frequency switches to the arbitrary function generator's output channels. Configure the switches so that the tip will be grounded during scanning, tunneling, imaging, and conventional spectroscopy.Collect the tunneling current for all measurements through a preamplifier connected to the sample. Using a silicon carbide scriber, scratch the back of a silicon wafer. Then, use a pair of glass microscope slides to gently snap the sample off the wafer.Make sure to use ceramic tweezers to handle silicon crystals. Affix the sample to a scanning, tunneling microscope holder. Once secured in the holder, introduce it to the ultra-high vacuum chamber.Then, degas the sample by resistively heating it to 600 degrees Celsius and holding it at that temperature for at least six hours in the ultra-high vacuum. Once the sample has been cooled, use the same chamber to degas a tungsten filament by resistively heating the filament to 1, 800 degrees Celsius and waiting for the system to recover to base pressure. Then, remove the oxide from the sample surface by flashing the sample to 900 degrees Celsius and holding it at that temperature for 10 seconds before cooling it back to room temperature.Progressively flash the sample to higher temperatures while attempting to reach a final flash of 1, 250 degrees Celsius. Let the sample cool to room temperature after each flash. To ensure the sample remains clean, abort flashes where the pressure rises above nine times 10 to the negative 10 Torr.Then, leak hydrogen gas into the chamber at a pressure of one times 10 to the negative six Torr and heat the tungsten filament to 1, 800 degrees Celsius. This will crack the gas into atomic hydrogen. Hold the sample in these conditions for two minutes before flashing the sample back to 1, 250 degrees Celsius.After five seconds at 1, 250 degrees Celsius, cool it back down to 330 degrees Celsius. Hold the temperature at 330 degrees Celsius for one minute and then simultaneously close the hydrogen leak valve and turn off the tungsten filament. Let the sample cool to room temperature.Finally, approach the tip to sample by engaging the current feedback controller with a sample bias of negative 1.8 volts, the current setpoint of 50 picoamperes, and using the control software's auto approach function. Take area scans of the surface on the order of 50 by 50 nanometers to verify the surface's quality. There should be large terraces separated by single atom steps in a defect rate of less than 1%The dimer rows of the silicon one zero zero two by one reconstruction should be clearly resolved.Dangling bonds appear as bright protrusions in field state images. Look for an area on the sample surface free from large surface defects by taking large area scans that are 50 nanometers by 50 nanometers. To characterize the electronic ringing, position the tip over a hydrogen atom on the surface.Because each surface silicon atom is terminated with a hydrogen atom, this is equivalent to positioning the tip over the dimer rows on the surface. Turn off the current feedback controller and then set the DC voltage to minus 1.0 volts, then pump voltage to minus 0.5 volts, the probe voltage to minus 0.5 volts, the width of the pump and probe pulses to 200 nanoseconds, and the rise and fall time of the pulses to 2.5 nanoseconds. Then, send a series of trains of pump and probe pulses where the relative delay of the pump and probe is swept from minus 900 nanoseconds to 900 nanoseconds.Ringing is manifested through smaller amplitude oscillations in the tunneling current on both sides of the origin. By increasing the rise times on the pulses from 2.5 nanoseconds to 25 nanoseconds, a strong suppression of the ringing is observed. Eliminate ringing by increasing the rise and fall times of the pulses to obtain the most accurate spectroscopic results.However, be aware that increasing the width of the pulse will decrease the resolution. Position the tip over a silicon dangling bond which appear as bright protrusions at negative tip sample biases. Turn off the current feedback controller and send a train composed of only the probe pulse used as a reference signal with a repetition rate of 25 kilohertz.Over a series of pulse trains, sweep the bias of the probe pulse over a range of 500 millivolts from the DC bias of negative 1.8 volts. Each pulse should have a different bias. This step is equivalent to standard IV spectroscopy but with reduced duty cycle.Ensure that the width of the pulses are long enough to get a signal to noise ratio of at least 10. Send a train composed of pump pulses set at a fixed bias so that the DC voltage added to the pump voltage is greater than the threshold voltage with a repetition rate of 25 kilohertz. Next, send a train composed of the pump pulses with the probe pulses followed by a delay of 10 nanoseconds.Compare the probe only in the pump and probe signals by plotting them in the same graph. Any hysteresis in little signals is an indication of dynamics that can be probed with time-resolved STM techniques. To measure relaxation dynamics, position the tip of the microscope over a silicon dangling bond and turn off the current feedback controller.Then, send a train composed of pump pulses set at a fixed bias such that the DC voltage added to the pump voltage is greater than the threshold voltage with a repetition rate of 25 kilohertz. Next, send another train of pump and probe pulses. Ensure that the probe pulses have an amplitude smaller than the pumps and comparable to the range at which hysteresis occurs.Finally, sweep the delay between the pump and probe pulse up to several tens of microseconds. For determination of excitation dynamics, again send a train composed of pump pulses so that DC voltage added to the pump voltage is greater than the threshold voltage with a repetition rate of 25 kilohertz. Sweep the duration of the pump pulse from several nanoseconds to several hundred nanoseconds.Then, send a train of pump and probe pulses. The probe pulses should have an amplitude smaller than the pumps and comparable to the range at which hysteresis occurs. Using conventional scanning tunneling microscopy, silicon dangling bonds occur as bright protrusions in field state images negative sample bias.In some cases, they display a sharp change in the conductance at minus two volts. For this specific dangling bond, images taken at minus 2.1 volts, minus two volts, and minus 1.8 volts also show this. In these time-resolved measurements, a pump-pulse transiently brings the system above the voltage threshold and immediately after a probe-pulse, interrogates the conductance of the transient state.To maximize the visibility of the hysteresis, the duration of the probe-pulse should be shorter than the relaxation rate of the high conducted state. When measuring dopant relaxation dynamics, the pulse sequences vary in delay between the pump and probe. A single exponential decay is extracted which settles to a fixed offset with time.For excitation times, the pulse sequence pump voltage width is changed. By sweeping the width of the pump, the average rate at which the dopant is ionized is mapped. It is important that you characterize any electronic ringing in your microscope setup.Ringing can result in signal artifacts and must be eliminated. Also, be aware that eliminating ringing by extending pulse length results in a loss of time resolution and will need to be optimized. While we have demonstrated time-resolved scanning tunneling microscopy to study dopants in silicon, these techniques can be applied to many other physical systems.The main advantage of these techniques is that no optical systems need to be integrated with your microscope. Don't forget that working with cryogens and high voltages can be dangerous. Always ensure a safe working environment and follow proper safety protocol when setting up and using your microscope.

Research

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Engineering

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