STM system overview

Chapter 2: Instrumentation and Methodology

2.1 Scanning tunneling microscopy (STM) system

2.1.3 STM system overview

Figure 2.6: Schematic illustration of the spectroscopy scan. (a) Measurement is taken on each pixel point of the grid over the area we would like to scan. Whenever the tip moves to the new pixel point, the tip height is initialized to the set tunneling junction resistance and hold its position before sweeping the voltage. (b) Two modes that we can use to obtain DOS-vs.-V spectrum. One is to measure I-vs.-V and then calculate dI/dV-vs.-V from it. The other is to use the lock-in technique to derive signal LIX, which is proportional to (dI/dV) so that dI/dV-vs.-V can be measured directly.

Although, in theory, the lock-in technique is a better way to obtain the conductance map data, this method could take up to several days to complete a scan so that the average location of the tip relative to the sample may have drifted significantly. Additionally, for low- temperature measurements, time is more limited because of a finite evaporation rate of liquid helium. The presence of AC noises from electrical cables could also introduce excess noises to the lock-in response and make the measured response unstable. Therefore, for most experiments, we only measured I-vs.-V for STS. On the other hand, our controller provides a built-in lock-in function module that can be used for direct (dI/dV)-vs.-V measurements so that we can choose the lock-in option when needed.

current we measure can be as small as a few pico-amperes (pA) and hence require a current amplifier to amplify the signal to be detectable. Any environmental electromagnetic radiation can readily introduce substantial noise currents. Therefore, it is essential to optimize the shielding of electromagnetic noises so that the feedback can respond to the real signal.

The microscope used in this thesis was previously designed and built by Ching-Tzu Chen and Nils Asplund[33]. It was subsequently modified by Andrew Beyer [34] and Marcus Teague [35]. Additional modifications to the STM head were made by the author and will be described in detail later. Here, we introduce the overall STM setup first.

The main body of the STM probe is a stainless steel stick with several partitions to block thermal radiations and with several copper cold fingers to enhance the thermal conduction to the STM can (Figure 2.7 (a)). The STM can is a metal tube that provides protection and vacuum seal to the STM probe. The STM can be separated into the top part and bottom part.

The top part is made of stainless steel, while the bottom part is made of copper.

For cryogenic experiments, good thermal isolation between the top and bottom parts of the STM probe and the STM can is essential. Stainless steel has low thermal conduction at low temperatures and is a solid material with sufficient mechanical strength. Therefore, both the top part of the STM probe and STM can are made of stainless steel to prevent heat load from top to bottom.

The bottom part of the STM probe is the STM head. More details about the STM head will be given in the next subsection. The STM can is separable so we can load the sample directly inside the glove box in an argon environment. This design is crucial when handling air- sensitive samples. We insert the STM probe into the STM can and then move it to the glovebox. The glove box is filled with argon gas so that it is safe to expose the air-sensitive sample in argon after it has been annealed or etched and then loaded into the STM head.

After the sample is loaded, an indium wire is used as a gasket between the top and bottom parts of the STM can. Finally, both parts are tightened together with a set of screws.

Figure 2.7: STM probe and its Dewar. (a) The STM probe consists of a probe body and a probe head. The STM head includes a tube scanner, piezo drives, and sample stage. The entire STM probe is protected by the STM can, which can be separated to the top part and the bottom part.

The bottom part is made of copper for better thermal conduction in liquid helium. The charcoal box is placed at the bottom of the bottom can as a cryogenic pump to enhance the vacuum more.

The top part is made of stainless steel to reduce the thermal load from room temperature. The angle valve is used to seal the STM probe so that we can vacuum it. (b) The STM probe is placed inside the STM Dewar. The Dewar is installed on an air table to reduce mechanical vibration. Dewar jacket needs to be vacuumed to enhance thermal isolations. The Dewar can be filled with different cryogens like liquid nitrogen (LN2) or liquid helium (LHe) to lower the temperature of the Dewar and STM probe. A 7-T superconducting magnet is installed at the bottom of the STM Dewar. It can only be used when it is fully immersed into liquid helium and cooled below the superconducting transition temperature. Several wires connect the STM head to the feedthrough at the top of the probe. The bias current signal is sent through the pre- amplifier first before it enters the STM controller. The temperature sensor and heater are controlled by a temperature controller.

The STM probe with the outer can is capped by an angle valve so we can evacuate the argon gas inside the STM can by first using a roughing pump and then a turbopump. Even though a high vacuum is not necessary for an air-stable sample like graphite or gold, keeping the STM probe in a high vacuum environment is still a good measure to prevent potential contamination and keep the STM system under a premier condition. Additionally, the voltage required for activating the piezo drives can be as high as 400 V, which could cause discharge and damage the piezo drive in the absence of a high vacuum. Finally, for measurements at cryogenic temperatures, it is essential to evacuate residual air from the STM system to prevent the liquidation of air at liquid nitrogen temperature (77K) and ice formation of air at liquid helium temperature, which could cause damage to the STM head. Moreover, an ultrahigh vacuum is particularly crucial for cryogenic measurements because even a very thin layer of liquefied air or ice on the sample surface could complicate or even nullify STM studies of the sample.

With a turbopump, the pressure at room temperature can reach ~ 10⁻⁶ 𝑡𝑡𝑡𝑡𝐶𝐶𝐶𝐶 if the STM probe was baked before the insertion of a sample. A small copper box full of charcoal is placed at the bottom of the STM can, which can be activated at low temperatures as a cryogenic pump.

The final pressure reaches as low as ~ 10⁻¹⁰ 𝑡𝑡𝑡𝑡𝐶𝐶𝐶𝐶 at liquid helium temperature. After pumped down to a vacuum level of ~ 10⁻⁶ 𝑡𝑡𝑡𝑡𝐶𝐶𝐶𝐶, the STM probe is inserted into a steel vacuum jacket mounted in an Oxford cryogenic Dewar. The Oxford Dewar is set on a three- inch-thick aluminum plate mounted on four air-damped pneumatic legs to reduce mechanical vibrations.

We can transfer cryogen such as liquid nitrogen (LN2) or liquid helium (LHe) into the STM dewar to cool down the STM probe. The dewar jacket needs to be vacuumed before transferring cryogen; otherwise, the air inside the dewar jacket will be frozen and cause damages. As we warm up the Dewar, evaporated cryogen will build up pressure inside the vacuum-sealed dewar jacket quickly. A pressure release valve is used to release the excess pressure and to prevent the potential risk of explosion.

There is a superconducting magnet at the bottom of the STM dewar. The magnet consists of a solenoid of superconducting wires with a superconducting transition temperature significantly higher than liquid helium temperature. Hence, after liquid helium transfer and immersing the entire solenoid in liquid helium, the solenoid becomes superconducting, which can hold a very high current in the persistent current mode without dissipating any energy, as long as the current stays below the critical current of the material. The superconducting magnet is an exceedingly power-efficient way to generate a strong magnetic field for a long time. Our superconducting magnet can hold currents up to 90 A and a maximum field of 7 T with controller shown in figure 2.8(d).

To ensure sufficient cooling power to the sample, we use copper as the material for the bottom part of the STM can. The material of the STM head consists of molybdenum, Macor, sapphire, and aluminum, all non-magnetic to avoid unwanted interferences with the applied magnetic field.

Conducting wires for various purposes connect the STM head to the top of the STM probe.

The bias voltage, bias current, piezo drives are controlled by the STM controller (Figure 2.8 (b)). We use the Nanonis controller as our STM controller, which includes HVS4 (high voltage supply), HVA4 (fine approach control), PMD4 (coarse approach control), SC4 (8 output and 8 input ports), and RC4 (the brain of all controllers and connect to the computer).

The output signals include the bias voltage and back gate of the STM, and HVA 4 for fine approach control. The input signal is the bias current from the pre-amplifier. The controller connects to the white power or power conditioner in our lab to minimize electrical noises from other facilities in the building. Bias current ground also connects to the white power ground, while bias voltage ground shares the same ground with the controller.

A current pre-amplifier, Femto DLPCA-200 Variable Gain Low Noise Current Amplifier (figure 2.8 (a)), is used to amplify the current before inputting into the controller. This amplifier has a Low Noise (LN) mode and High Speed (HS) mode. The Low Noise mode can amplify current with a gain from 10³ to 10⁹ and the High Speed mode can amplify current with a gain from 10⁵ to 10¹¹. Generally, a smaller gain has a higher level of

background noise, which can be even higher than our bias current setpoint. Therefore, we choose as high gain as possible most of the time.

Figure 2.8: Instruments used for the STM system. (a) DLCP-200 Variable Gain Low Noise Current Amplifier (picture adapted from manual). (b) Nanonis STM controller sets. The instruments from top to bottom are HVA4, PMD4, HVS4, SC4, and RC4. (c) LakeShore Model 340 Temperature controller. (d) Superconducting magnet controller and current reversing switch.

The conducting wires from the temperature sensor and heater at STM head go to a LakeShore Model 340 Temperature Controller (figure 2.8 (c)). The temperature sensor is CX-1050-SD- 4L Cernox Resistor, with a working range from 4K to 325K. The Cernox resistive temperature sensor is chosen for its low-temperature compatibility and negligible sensitivity to the magnetic field.

Our STM lab is at the subbasement of the building. Sound absorbing sponges are attached to all interior walls of the lab. Outside the lab, we cover the wall with sound-reflecting curtains.

Additionally, the air table, together with the STM dewar in the lab, is enclosed by a wooden box with sound-absorbing sponges on both the interior and exterior surfaces. Outside the wooden box, we put four pieces of aluminum walls with sound-absorbing sponges attached to both sides of each wall to surround the wooden box. Here the metallic walls are used to shield electromagnetic radiation. Several heavy bricks are placed on the air table to enhance the total mass of the system, thereby reducing the vibrational amplitude induced by mechanical noise. Lastly, the most significant noise is the 60 Hz noise from the power lines.

Therefore, we turn the light off during all STM experiments. Additionally, low noise BNC cables are used for all electrical connections. Despite all the efforts, the ultimate noise level is often determined by other activities (such as the operation of a nearby elevator, running of large vacuum pumps in other research groups, etc.) outside the STM lab, which are unfortunately beyond our control. This situation can be mitigated by concentrating more measurements during the evenings and weekends when external activities are much reduced.