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Quantification of Hydrogen Concentrations in Surface and Interface Layers and Bulk Materials through Depth Profiling with Nuclear Reaction Analysis
Chapters
Summary March 29th, 2016
We illustrate the application of 1H(15N,αγ)12C resonant nuclear reaction analysis (NRA) to quantitatively evaluate the density of hydrogen atoms on the surface, in the volume, and at an interfacial layer of solid materials. The near-surface hydrogen depth profiling of a Pd(110) single crystal and of SiO2/Si(100) stacks is described.
Transcript
The overall goal of nuclear reaction analysis with the resonant 15 nH nuclear reaction is to measure the density of absorbed hydrogen atoms on solid surfaces, and to determine the concentration versus depth distribution of absorbed hydrogen in the volume of materials. Clarifying the hydrogen content on surfaces, in the near surface region, and at shallow interfaces of solids is a key question in many fields of fundamental material science and engineering. The main advantage of nuclear reaction analysis is that it reveals the concentration and depth location of hydrogen quantitatively, nondestructively, and with nanometer depth resolution.
Hydrogen profiling with NRA supports research of hydrogen storage and purification materials. Fuel cell and hydrogenation catalysts, hydrogen retention and embrittlement, device fabrication and hydrogen-related reliability concerns in semi-conducted technology. In this video, we demonstrate a unique combination of NRA with surface science instrumentation for the quantification of a hydrogen-layered densities on atomically-controlled target surfaces and at shallow interfaces.
These nuclear reaction analysis experiments take place at the University of Tokyo's MALT accelerator facility. Measurements of surface hydrogen are done on beam line 1E in this ultra-high vacuum chamber. This chamber has been loaded with a palladium 110 single crystal sample and is held at less than 10 nanopascals at room temperature.
This top view schematic of the sample chamber gives an overview of the equipment layout. The nitrogen ion beam line including a deflector and a Faraday cup comes in at the left. In addition, there is an ion gun for sputtering as well as an input for hydrogen.
The chamber is equipped to perform low energy electron diffraction and Auger electron spectroscopy for INC2 preparation of targets. The final two instruments are quadrupole mass spectrometer shown at the bottom of the schematic and a scintillation detection system at the right. The sample is held by a sample holder on an XYZ stage near the center of the chamber and can be viewed through a viewport.
This picture provides an example of the sample holder with a sample that is currently in the chamber. Tantalum wires support a single crystal specimen. The holder also facilitates electrical and thermal measurements.
Begin with cleaning the target surface in the chamber through sputtering and annealing. Operate the XYZ stage to position the sample in the center of the chamber. In addition, rotate the sample to properly align it.
The sample should face the gas doser between the ion gun and the viewport. To fine adjust the angle, turn on the ion gun power supply. Adjust emission control to 20 milliamps.
Observe the sample through the viewport while fine adjusting its angle. The goal is to have the mirror image of the ion gun filament visible on the sample surface. Once fine adjustment is complete, change the ion gun power supply setting for the beam energy to 800 electron volts.
Next, at the bottom of the chamber, close the NEG pump gate valve. Use a variable leak valve to introduce 9 millipascal argon gas into the chamber. For sputter ion current reading, consult the digital tester connected between the sample and ground.
Confirm that the current is around two microamps for the 10-minute duration of the sputter. Stop the sputtering by closing the variable leak valve and turning off the ion gun power supply. Continue preparations by bringing liquid nitrogen to the manipulator and adding about 100 milliliters to the cryostat.
Stay at the manipulator to make electrical connections for annealing. Connect the filament heater lids to the heater power supply. Also, connect the thermocouple feedthrough to a digital tester to monitor the temperature.
Ground the filament by connecting it to the chamber body. Finally, connect the sample contact to the bias voltage power supply. At this point, set up the high voltage power supply.
Apply a sample bias of one kilovolt. Proceed to anneal the sample at 1, 000 Kelvin and oxidize it at 750 Kelvin. After annealing and oxidation, prepare to do low energy electron diffraction on the sample.
Observe the diffraction pattern and look for a clear structure with bright spots and low background noise as in this example. Be prepared to repeat the sputtering, annealing, oxidation, and reduction steps as necessary. The next step is to align the nitrogen ion beam to the single crystal target for nuclear reaction analysis.
Center the sample in the XY plane and adjust the Z position to the height of the mass spectrometer front aperture. Rotate the sample back to face the beam line. Next, lower the sample holder to put the beam profile monitor into position for nuclear reaction analysis.
Set a camera on the window flange to transmit beam profile images to the control room. Return to the manipulator and remove all electrical contacts to the sample. After this, attach the sample current line.
Now, prepare for introducing the ion beam. Set the electrostatic deflector voltage on the beam line to 8, 500 volts. Enter the control room to continue.
There, switch the current integrator from standby mode to operate. This schematic represents a portion of the accelerator beam line before the ion beam is stirred to the different experiment beam lines. The experiment beam lines are also represented.
There are four components that are important for this protocol. BM03 is a 90-degree sector magnet. It selects the ion beam energy during depth profiling.
FC04 is a Faraday cup that can be inserted into the beam to read the ion beam current and prevent the beam from reaching the sample. MQ04 is a magnetic quadrupole lens used to focus the beam onto the sample. And BM04 is a bending magnet that directs the beam into the beam lines.
Take steps to adjust the ion beam energy and direct the beam onto the target in the vacuum chamber. Set the MQ04 magnetic quadrupole lens, XCC and the YCC lens parameters to approximately focus the beam. Open the gate valves between the accelerator and the beam line and then open the Faraday cup, FC04.
Use the monitor to observe the ion beam profile on the quart's plate in the target chamber. With this feedback, fine-tune the bending magnet, BM04 and the magnetic quadrupole lens settings. The goal is to obtain a well-focused ion beam in the center of the profile monitor plate.
Close the Faraday cup and record the parameters before returning to the beam line. Back on beam line 1E, use the filament heater to flash heat the palladium sample to 600 Kelvin then set the filament heater to about 3.6 amps to maintain the sample temperature at 145 Kelvin. Isolate the chamber from the accelerator and NEG pump before exposing the sample to 2, 000 langmuirs of hydrogen at 145 Kelvin.
Turn off the filament heater and when the temperature reaches 80 Kelvin, adjust the hydrogen background pressure to be one micropascal. Back in the control room, arrange for the nitrogen ion beam to have the desired starting energy. For this experiment, ensure that Faraday cup 04 registers a beam current of 10 to 20 nanoamps.
Next, enter the parameters for the energy scan and the acquisition time into the control software before starting automated data acquisition. These are typical parameter values for BM03 to control the energy scan. Values are given for selecting the initial energy, the final energy, and the change in energy with each step.
The acquisition time is 50 seconds. Measurements of bulk and interface hydrogen are done on beam line 2C. This manipulator has been removed from the beam line and is ready with a new sample in the sample holder.
For these measurements, the sample is a thin film of silicon dioxide on a silicon 100 surface. With the sample aligned parallel to the transfer tube access, tighten the clamp screws to secure it in place. Retract the sample into the transfer tube and secure it with a locking screw.
Move the manipulator to its position on the beam line and reinstall it on the gate valve. When the system is ready, lower the sample into position for the NRA measurement. As in this schematic, align the sample surface normal to the incident beam.
Use a beam line camera and monitor for this purpose. At the manipulator head, connect the sample current line to the control room current integrator. Move to the control room to continue.
Coursely align the beam by setting the bending magnet and the magnetic quadrupole lens parameters. Observe the beam profile on the profile monitor while further optimizing the parameters for beam transmission and keeping the profile on target. Next, set the BM03 parameter to determine the starting beam energy for the scan.
At the computer, enter the desired parameters for the energy scan and start the automated scan of energy to acquire a depth profile. This near surface depth profile is from single crystal palladium that has had its 110 surface exposed to hydrogen gas. The experiment was performed in beam line 1E with a hydrogen background pressure of 1.3 micropascals.
The bottom horizontal axis gives the nitrogen ion beam energy. The top axis provides a measure of the depth based on the stopping power of palladium. The open symbols correspond to an experiment with palladium that was pre-exposed in 100 seconds to 2, 000 langmuirs of hydrogen gas at 145 Kelvin.
The data were taken at 90 Kelvin. The profile can be decomposed into a peak at depth zero in black and a plateau in blue. The area of the peak provides information on hydrogen surface density.
In this case, the coverage is 1 1/2 hydrogens per surface palladium atom. The plateau reveals that hydrogen has been absorbed to a depth of at least 20 nanometers. The gray and black symbols are for experiments with no pre-dosage of hydrogen.
These data were taken at 170 Kelvin. These plots represent data from a series of experiments with silicon dioxide on silicon performed in beam line 2C. As before, the ion energy is shown on the bottom axis.
The depth along the top. The positions of the interface between the materials is indicated by a vertical dashed line. All profiles show two peaks indicating a non-uniform distribution of hydrogen including hydrogen just a few nanometers in front of the silicon oxide silicon interface.
The experiments in this video demonstrate that the NRA technique can distinguish surface-absorbed and volume or interface-absorbed hydrogen in solid targets. NRA furthermore quantifies the hydrogen content at their respective depth locations without destroying the sample material. Please be aware that especially the temperature and pressure during hydrogen exposure, very critically influence the depth distribution of palladium-absorbed hydrogen.
If these experimental parameters are changed, hydrogen profile is different from those shown in this video will likely result. After watching this video, you should have a good impression of how nuclear reaction analysis measurement is performed at MALT facility to quantitatively determine hydrogen densities on surfaces and in the interior of solid materials through nanoscale depth profile.
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