Nanosurfaces


Research Mentor: Robert Nemanich

Participants: Aaron Jarvis and Harrison Stratton


Overview:

Located in the Psychology North building of the Tempe ASU campus, the Surface Science Lab aims to study the electronic properties of varying materials that are being investigated for use in the development of new types of transistor devices. In this lab multiple different analysis techniques are used to describe the structure and function of metallic oxides that are deposited on the surface of substrate wafers. There are currently many different projects being investigated in this lab involving silicon oxide, sapphire, lithium niobate, and diamond substrates acting as the surface for the deposition of metal oxide thin films. An ultra high vacuum chamber, which consists of a 47 foot transfer line with ten different deposition and analysis chambers attached to the line to allow for the exposure of the sample to different temperatures and atmospheres without compromising the very low vacuum pressures. Two different photo emission processes, X-ray and Ultra violet, are used to characterize the surfaces of the thin films with the x-ray being used to determine which elements are present on the surface and the UV used for determination of the work function and fermi level of the sample. The goal of the research team for the summer is to deposit a hafnium and vanadium oxide sandwich on the surface of a silicon oxide wafer which will hopefully display a capacitance effect that can be advantageous when coupled with a transitor.



1.0 Materials


1.1 Silicon:
Silicon is one of the most common materials on Earth and is most usually found in the form of silicon oxide, which is commonly known as beach sand to many individuals. Silicon itself is a metal although it rarely occurs in this state on its own in Nature and as its oxide makes up about a quarter of the Earth's crust. Silicon oxide is very useful because it is a semiconductor which means that the gap between the conduction band and the valence electron band is small and can be altered using other atoms to change the conductive properties of the material. This method of introducing foreign species of atom into the silicon matrix is called doping and is the principle mechanism used in developing semiconductor transistors.

1.2 Vanadium
This is a metallic element that is not found in metallic form in nature but instead is stored in many different crystal materials, one of the most common being magnetite. Oxides of Vanadium are used to catalyze the production of sulfuric acid as well as introduced into steel to increase the strength of the steel alloy. In our case, vanadium is being used as a dielectric material for the purpose of storing charge as a capacitive material. This is not commonly found in transistors but is one of the most common materials used in our lab.

1.3 Hafnium
In its oxidized form, Hafnium is an inert inorganic compound with a band gap of approximately 6 eV which is close to the band gap of silicon oxide. This material is currently being used as a dielectric material for capacitors in DRAM applications and is currently being investigated for use as a gate dielectric material in microprocessors. This is a colorless, odorless solid that is often referred to as Hafnia.

2.0 Background


2.1 Band Theory of Solids
This is the main theory we are using to investigate the properties of the materials we are testing in our lab. It suggests that the electron energy levels in solids are separated in quantized bands that have a distinct structure based on the orientation of the atoms in the crystal lattice. The number of electrons in each atom as well as the individual position of electrons within their energy levels in the atoms also determine the shape of the band in the crystal material. We are manipulating the positioning of these atoms to bend the shape of the bands to create special features in the material which will eventually determine how electrons will flow in the material. This has been done for many years already to create integrated circuits in silicon but we are investigating new materials that will hopefully improve performance and lower energy consumption of devices in the future.

There are two major bands in solids that are of utmost importance when considering the flow of electrons, which are the conduction band and the valence band. The valence band represents electrons that are more tightly bound to the nucleus and do not participate in the conduction of electrons. The conduction band, as its name implies, sits above the valence band and electrons in this band are less tightly bound to the nucleus and are thus more free to move from atom to atom. In between these two bands the Fermi energy is found, which represents the chemical potential of the material and is essentially the surface of where electrons can be found. The Fermi energy will always be in between the conduction and valence bands, but the positioning of the energy level in reference to the bands determines the properties of electronic conduction of the material. When the conduction band and valence band overlap the Fermi energy the material is deemed a conductor as electrons can easily pass to the conduction band where they can then flow to another atom. An insulator has a band diagram that has a large gap between the valence and conduction bands which prevents electrons from jumping this potential barrier and being conducted to other atoms. A semiconductor will have a small band gap with the Fermi energy usually located close to the conduction band so when external electric fields are applied the two can be made to overlap creating a temporary conducting region.

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3.0 Equipment:


3.1 Oxygen Plasma

The oxygen plasma is generated using a heating coil coupled with an RF transmitter to excite and ionize the gas which is then accelerated through a glass tube using an electric potential difference. The plasma is funneled through the top of the vacuum chamber down onto the sample surface where it serves as a cleaning agent. When the plasma is on it emits a faint blue light that is best observed without the lights overhead and creates a dull glow within the sample chamber. Hydrocarbons can collect on the sample surface before it is loaded into the vacuum chamber as well as when in the transfer line and the oxygen plasma is used to heat those molecules off the surface liberating them as carbon dioxide. The sample is only cleaned for a duration of thirty seconds to avoid any etching effects that could result from overheating in the plasma. After the sample has been cleaned the chamber must be equalized back the pressure of the transfer line which is held constant at about 2.0 x 10-9 torr. This pumping process takes about 10 minutes using a high velocity turbo pump that is attached to the vacuum chamber exterior.
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Oxygen Plasma Operating Under Low Light Conditions

3.2 X-Ray Photo-emission Spectrometer

This device is used for the determination of the presence of different atoms on the surface of the sample wafer. High energy electrons are accelerated through a potential difference incident on the surface of a metal source which serves to emit the x-ray photons. Once the x-rays are emitted by the source they are directed at the surface of the material at an angle of around 30 degrees from the plane of the sample. The x-rays are emitted at a range of energies so when they contact the surface of the sample they transmit their energy corresponding to the potential difference with which the electrons were initially accelerated. This energy that is transmitted to the sample causes electrons to be ejected from the sample material if they are of the appropriate wavelength. These electrons, once ejected from the sample, accumulate in a collector which then is able to detect the kinetic energy of the emitted electrons. This kinetic energy represents the electron orbital from which the electron was ejected giving us an accurate picture of which electron states are present in the highest density. By analyzing the presence of specific electron states we are able to determine which elements are present on the sample surface and can detect the presence of any contaminants. If contaminants are present the sample can be cleaned again using the oxygen plasma and if there are no contaminants present then the electron energy levels associated with the materials deposited on the surface should be the only spectra present in the analysis.
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XPS Chamber

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XPS Spectrum Silicon 2p Orbital

3.3 UV Photo-emission Spectrometer

The photons from this device are much lower energy than those of the x-ray spectrometer and are used to determine the fermi level and the work function of the materials deposited on the surface. These two electron properties, fermi level and work function, will determine how the material can be used in a transistor. First a helium gas is introduced into the vacuum chamber from an outside source and is cooled using liquid nitrogen to condense any contaminants that may be present in the gas. Then the gas is excited using a high voltage source in order to excite the electrons in the helium gas corresponding to the frequency of ultraviolet light. Once the electrons that are excited decay back down to the ground state, they emit a photon of ultra violet light which is then directed at the surface of the material to be studied. The incident photons cause the electrons in the material to become excited and eventually are emitted from the surface then these electrons are collected and analyzed to identify the fermi level and work function of the material.

3.4 Molecular Beam Epitaxy

This chamber is used for the deposition of metal films onto the surface of the wafer substrate and can be very precisely controlled to allow for the creation of films as small as one angstrom. At the bottom of the chamber metal sources are arranged around the diameter of the chamber approximately thirty centimeters away from the sample stage. An electron gun, which accelerates electrons at high voltage is used to directly heat the surface of the metal source to a point where metal atoms begin to sublimate off the surface of the material. An oxygen atmosphere is introduced into the chamber to form the metal oxide before the compound reaches the sample surface. The sample is positioned facing the ground so as the metal atoms drift up in the chamber they slowly become affixed to the face of the sample. A small crystal resonator is located at the bottom of the chamber near the source and is used to determine the thickness of the material being deposited. As the resonant frequency of the crystal changes an analyzed converts this frequency to a measurement of thickness that can be correlated to what is being deposited on the sample. There are multiple different source metals so alternating layers of metal oxide can be created on the sample without having to move to another chamber. Once the process is completed the chamber must once again be brought back down to vacuum using the turbo pump which can take up to half an hour.

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MBE Chamber

3.5 Ultra High Vacuum Chamber

The vacuum chamber is the most integral component of the entire system as it is the location where all testing, transfer, and deposition takes place. The system consists of a 47 foot long transfer line that is essential for moving the samples from one chamber to another along a mobile cart transfer system. The cart is on rails within the transfer line and has three slots where sample stages can be inserted then transferred along the line. Before the sample can be mounted it first must be introduced through the load locker which undergoes a long pressurization process which takes around thirty minutes to complete. Once the locker is pressurized the sample is lowered manually into the locker and attached to the loading arm which is sealed within a long stainless steel tube. The loading arm is rotated and either extended or retracted by the manipulation of an external permanent magnet that is affixed to the exterior of the tube. This is to allow for the researcher to manipulate the loading arm while maintaining the vacuum pressure within the system. Once the sample has been affixed to the load arm, the load locker is then closed and re vacuumed to a pressure of approximately 10^-9 torr. After this has finished the valve to the transfer line is opened and the sample can be affixed to the sample cart using the loading arm. This is a very delicate process and can prove to be quite difficult requiring one to be very gentle when manipulating the sample. The cart is moved along the transfer line using a wheel that is located at one end of the line pulling a wire that is tied to the base of the cart. The cart must be moved very slowly to ensure the safe passage of the sample through the line. If it moved too fast the sample can bounce out of the cart and fall to the bottom of the transfer line where it cannot be easily retrieved.
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The Transfer Line


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Transfer Rod


Currently Vanadium Oxide is being deposited in a layer that is approximately 2 nm thick between alternating layers of Hafnium Oxide. The Vanadium layer will be annealed at 400 C for one of our tests and left without the heat treatment in another. The annealing process is reported to form islands within the Vanadium Oxide that will hopefully provide different quantum effects in regards to the bending of the electron energy levels being observed. X-Ray photoemission spectrometry will be used to ascertain the positioning of the correct elements on the surface of the silicon substrate and following that the sample will be analyzed with Ultra Violet Photoemission Spectrometry to calculate the band gap. We are expecting results for the band gap to be approximately around the range of 4 eV in order to allow for significant conducting and insulating qualities.

4.0 Tutorial


4.1 Preparing the sample

In the video below is a quick introduction to how the silicone sample is attached to the holder which will be placed into the Ultra High Vacuum.




5.0 The Experiment:

We are characterizing the electrical properties of thin films deposited on silicon dioxide substrates for use in semiconductors devices. Our experiment will consist of a comparison of three different samples one of them being a control and the other two will be identical except for the inclusion of an annealing phase in one of the samples. The variance in the samples is described below:

Sample 1: Control
This wafer was cleaned using the oxygen plasma then characterized using XPS and UPS to make sure the sample was completely clean and the band structure was what is expected of silicon. The sample was then transferred to the MBE where a 5 nm layer of hafnium oxide was deposited on top of the SiO2 layer then moved back to the XPS and UPS. In final analysis the hafnium appeared thick enough to not allow any silicon to show through and demonstrated semi conducting properties as was expected.

Sample 2: No Heat Treatment

The sample was cleaned and analyzed just like the control to make sure it is clean and ready for deposition. The sample was then moved to the MBE chamber where a 2 nm thick layer of hafnium oxide was deposited on top of the wafer. The MBE source was then switched to vanadium and a 1 nm thick layer of vanadium oxide is grown on top. Finally, a 2nm thick layer of hafnium completes the sandwich and then the wafer is removed from the MBE. This sample was taken to the XPS and UPS for characterization where it is expected to display properties such as a low band gap and a negative capacitance.

Sample 3: Heat Treatment

This sample is exactly the same as the second except that it is annealed at 600 degrees inside the Oxygen Plasma chamber to form islands in the vanadium oxide layer. It is annealed before the final layer of hafnium is deposited and it is expected that these islands of vanadium will alter the capacitive properties of the material stack.