This research was supported by the National Science Foundation (Awards CMMI-1437050 and DMR-1207342), the Office of Naval Research (Grant N00014-10-10489), and the Air Force Office of Scientific Research (Grant FA9550-14-1-0090).

1. Preparing Sr and Ti Precursors for ALD Experiments
<ol>
	<li>Load the clean, dry saturators and new precursors into the antechamber of a glove box. Follow the glove box&#39;s loading procedure to ensure proper purging of air and moisture. Transfer the materials into the main chamber. 
	Note: This group uses in-house built saturators (see Figure 3) with components purchased commercially. Details of the saturator assembly can be found in the List of Specific Reagents and Equipmen...

<ol>
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The authors have no competing financial interests to disclose.

The cleanliness of the Ge substrate is the key to success when growing the epitaxial perovskite using ALD. The amount of time a Ge substrate spends between degreasing and deoxidization, and the amount of time between deoxidization and STO deposition, should be kept at a minimum. Samples are still subject to contaminant exposure even under the UHV environment. Prolonged exposure may lead to redeposition of adventitious carbon or Ge reoxidation, resulting in poor film growth. This group has employed a widely-used degreasin...

Perovskite materials are becoming increasingly attractive due to their highly symmetric cubic or pseudocubic structure and myriad of properties. These materials, with general formula ABO3, consist of A atoms coordinated with 12 oxygen atoms and B atoms coordinated with six oxygen atoms. Owing to their simple structure, yet wide range of potential elements, perovskite materials provide ideal candidates for heterostructure devices. Epitaxial oxide heterostructures boast ferromagnetic,1-3 anti/ferroelectric,4 multiferroic,5-8 superconductive,7-12 and magnetoresistive functionalities.13,14 Many of these desirable electronic properties are interfacial and thus dependent on clean, abrupt transitions between materials. The nearly identical structure and lattice constants shared between members of the perovskite family allow for excellent lattice matching and, therefore, high quality interfaces. Readily lattice-matched to each other as well as some semiconductors, perovskite oxides are now being turned to in next generation metal-oxide-semiconductor electronics.
Monolithic integration of crystalline oxides with silicon, first demonstrated with perovskite strontium titanate, SrTiO3 (STO), by McKee and colleagues,15 was a monumental step towards the realization of electronic devices with perovskite-semiconductor incorporation. Molecular beam epitaxy (MBE) is the primary technique for epitaxial growth of oxides on silicon because of the layer-by-layer growth as well as the tunable oxygen partial pressure necessary to control amorphous, interfacial SiO2 formation.16-19 Typical MBE growth of STO on Si (001) is achieved by Sr-assisted deoxidation of SiO2. Under the ultra-high vacuum (UHV) conditions, SrO is volatile and subject to thermal evaporation. Since SrO is thermodynamically preferred over strontium metal and SiO2, deposition of Sr scavenges oxygen from the SiO2 layer and the resulting SrO evaporates from the surface. During this process the silicon surface experiences a 2&#215;1 reconstruction at the surface that forms rows of dimerized silicon atoms. Conveniently, &#189; monolayer (ML) coverage of Sr atoms on the reconstructed surface fills in the gaps created by these dimer rows.20 The &#189; ML coverage provides a protective layer that, with careful control of oxygen pressure, can prevent or control interfacial SiO2 formation during subsequent oxide growth.21-23 In the case of STO (and perovskites with similar lattice match), the resulting lattice is rotated 45&#176; in-plane such that (001)STO&#8214;(001)Si and (100)STO&#8214;(110)Si, allowing registry between the Si (3.84 &#197; Si-Si distance) and STO (a = 3.905 &#197;) with only slight compressive strain on the STO. This registry is necessary for high quality interfaces and the desired properties they possess.
Silicon became industrially significant due to the high quality of its interfacial oxide, but SiO2 use is being phased out for materials capable of equivalent performance at smaller feature sizes. SiO2 experiences high leakage currents when ultra-thin and this diminishes device performance. The demand for smaller feature sizes could be met by perovskite oxide films with high dielectric constants, k, that provide performance equivalent to SiO2 and are physically thicker than SiO2 by the factor k/3.9. Furthermore, alternative semiconductors, like germanium, offer potential for faster device operation due to higher electron and hole mobilities than silicon.24,25 Germanium also has an interfacial oxide, GeO2, but in contrast to SiO2, it is unstable and subject to thermal deoxidation. Thus, 2&#215;1 reconstruction is achievable by simple thermal annealing under UHV, and a protective Sr layer is unnecessary to prevent interfacial oxide growth during perovskite deposition.26
Despite the apparent ease of growth offered by MBE, atomic layer deposition (ALD) provides a more scalable and cost effective method than MBE for the commercial production of oxide materials.27,28 ALD employs doses of gaseous precursors to the substrate that are self-limiting in their reaction with the substrate surface. Therefore, in an ideal ALD process, up to one atomic layer is deposited for any given precursor dosing cycle and continued dosing of the same precursor will not deposit additional material onto the surface. Reactive functionality is restored with a co-reactant, often an oxidative or reductive precursor (e.g., water or ammonia). Previous work has demonstrated the ALD growth of various perovskite films, such as anatase TiO2, SrTiO3, BaTiO3, and LaAlO3, on Si (001) that had been buffered with four-unit-cell thick STO grown via MBE. 29-34 In purely MBE growth of crystalline oxides, &#189; monolayer coverage of Sr on clean Si (001) is enough to provide a barrier against SiO2 formation under the pressures native to the technique (~10-7 Torr). However, under typical ALD operating pressures of ~1 Torr, previous work has shown that four unit cells of STO is required to avoid oxidizing the Si surface.29
The procedure detailed here utilizes the instability of GeO2 and achieves monolithic integration of STO on germanium via ALD without the need of an MBE-grown buffer layer.26 Furthermore, the Ge-Ge interatomic distance (3.992 &#197;) on its (100) surface allows for an analogous epitaxial registry with STO that is observed with Si (001). Though the procedure presented here is specific to STO on Ge, slight modifications may allow for the monolithic integration of a variety of perovskite films on germanium. Indeed, direct ALD growth of crystalline SrHfO3 and BaTiO3 films have been reported on Ge.35,36 Additional possibilities include the potential gate oxide, SrZrxTi1-xO3.37 Finally, building on previous studies of ALD perovskite growth on a four-unit cell STO film on Si (001)29-34 suggests that any film that could be grown on the STO/Si platform could be grown on an ALD-grown STO buffer film on Ge, such as LaAlO3 and LaCoO3.32,38 The multitude of properties available to oxide heterostructures and remarkable similarity between perovskite oxides suggest this procedure could be utilized to study previously difficult or impossible growth combinations with such an industrially viable technique.
Figure 1 depicts the schematic of the vacuum system, which encompasses ALD, MBE, and analytical chambers connected by a 12-foot transfer line. The samples can be transferred in vacuo between each chamber. The baseline pressure of the transfer line is kept at approximately 1.0&#215;10-9 Torr by three ion pumps. The commercial angle-resolved ultraviolet and X-ray photoelectron spectroscopy (XPS) system is maintained with an ion pump such that the pressure in the analytical chamber is kept at approximately 1.0&#215;10-9 Torr.
The ALD reactor is a rectangular custom-built stainless steel chamber with a volume of 460 cm3 and length of 20 cm. A schematic of the ALD reactor is shown in Figure 2. The reactor is a hot wall, continuous cross-flow type reactor. Samples placed in the reactor have a clearance of 1.7 cm between the top surface of the substrate and the chamber ceiling and 1.9 cm between the bottom of the substrate and the chamber floor. A heating tape, powered by a dedicated power supply, is wrapped around the chamber from the inlet to approximately 2 cm beyond the exhaust port and provides temperature control of the reactor walls. A temperature controller adjusts the power input to the heating tape according to a temperature measurement taken by a thermal couple located between the heating tape and exterior reactor wall. The reactor is then completely wrapped with three additional heating tapes of constant power provided by a variac, and a final layer of fiberglass wool with aluminum foil covering provides insulation to promote uniform heating. The power output of the variac is adjusted such that the idling temperature (when the dedicated power supply is turned off) of the reactor is approximately 175 &#176;C. The reactor is passively cooled via ambient air. The substrate temperature is calculated using the linear-fit equation (1), where Ts (&#176;C) is the temperature of substrate and Tc (&#176;C) is the temperature of the reactor wall, obtained by directly measuring a substrate fitted with a thermocouple. A temperature profile exists along the flow direction of the chamber due to the cold gate valve that connects the reactor to the transfer line; the temperature profile perpendicular to the flow direction is negligible. The temperature profile causes a richer Sr deposition at the leading edge of the sample, but the composition variation along sample is small (less than a 5% difference between the leading and trailing edges of the sample) according to XPS.31 The exhaust of the reactor is connected to a turbomolecular pump and a mechanical pump. During the ALD process, the reactor is pumped by the mechanical pump to maintain the pressure at around 1 Torr. Otherwise, the reactor pressure is maintained below 2.0&#215;10-6 Torr by the turbomolecular pump.
(1) Ts=0.977Tc + 3.4
The MBE chamber is maintained at a baseline pressure of approximately 2.0&#215;10-9 Torr or below by a cryogenic pump. The partial pressure of various species in the MBE chamber is monitored by a residual gas analyzer. The background pressure of H2 is around 1.0&#215;10-9 Torr, while those of O2, CO, N2, CO2, and H2O, are less than 1.0&#215;10-10 Torr. In addition, the MBE chamber is also equipped with six effusion cells, a four-pocket electron beam evaporator, an atomic nitrogen plasma source and an atomic oxygen plasma source with high-precision piezoelectric leak valve, and a reflection high energy electron diffraction (RHEED) system for real-time in situ growth and crystallization observations. The sample manipulator allows the substrate be heated up to 1000 &#176;C using an oxygen-resistant silicon carbide heater.

<table border="0" cellpadding="0" cellspacing="0" width="963">
	<tbody>
		<tr height="20">
			<td height="20" style="height:20px;width:285px;">MBE</td>
			<td style="width:235px;">DCA</td>
			<td style="width:143px;">M600</td>
			<td style="width:301px;"></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Cryopump for MBE</td>
			<td>Brooks Automation, Inc.</td>
			<td>On-Board 8</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Residual Gas Analyzer for MBE</td>
			<td>Extorr, Inc.</td>
			<td>XT200M</td>
			<td></td>
		</tr>
		<tr height="23">
			<td height="23" style="height:23px;">ALD Reaction Chamber</td>
			<td>Huntington Mechanical Laboratories</td>
			<td>N/A</td>
			<td>Custom manufactured, hot-wall, stainless steel, rectangular (~20 cm long, 460 cm3)</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">ALD Saturator</td>
			<td>Swagelok/Larson Electronic Glass</td>
			<td>See comments</td>
			<td>Custom-built from parts supplied by Swagelok and Larson Electronic Glass. The saturator is made out of 316 stainless steel and Pyrex. All parts are connected via butt welding. Swagelok catalog numbers:SS-4-VCR-7-8VCRF, SS-4-VCR-1, SS-8-VCR-1-03816, SS-8-VCR-3-8MTW, 316L-12TB7-6-8, SS-8-VCR-9, SS-4-VCR-3-4MTW, SS-T2-S-028-20&#160; Larson Electronic Glass catalog number: SP-075-T</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Manual Valves for Saturators</td>
			<td>Swagelok</td>
			<td>SS-DLVCR4-P and 6LVV-DPFR4-P.</td>
			<td>Both diaphragm-sealed valves are used interchangably by this group. The specific connectors (VCR male/female/etc.) to use will depend on the actual system design.</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">ALD Valves</td>
			<td>Swagelok</td>
			<td>6LVV-ALD3TC333P-CV</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">ALD System Tubing</td>
			<td>Swagelok</td>
			<td></td>
			<td>316L tubing of various sizes. This group uses inner diameter of 1/4&#34;</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">ALD power supply</td>
			<td>AMETEK Programmable Power, Inc.</td>
			<td>Sorensen DCS80-13E</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">ALD Temperature Controller</td>
			<td>Schneider Electric</td>
			<td>Eurotherm 818P4</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">ALD Valve Controller&#160;</td>
			<td>National Instruments</td>
			<td>LabView</td>
			<td>Program developed within the group</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">XPS</td>
			<td>VG Scienta</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">RHEED</td>
			<td>Staib Instruments</td>
			<td>CB801420</td>
			<td>18 keV at ~3&#176; incident angle</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">RHEED Analysis System</td>
			<td>k-Space Associates</td>
			<td>kSA 400</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Digital UV Ozone System</td>
			<td>Novascan</td>
			<td>PSD-UV 6</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Ozone Elimination System</td>
			<td>Novascan</td>
			<td>PSD-UV OES-1000D</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Strontium bis(triisopropylcyclopentadienyl)</td>
			<td>Air Liquide</td>
			<td>HyperSr</td>
			<td>Mildly reactive to air and water. Further information supplied by Air Liquide can be found at https://www.airliquide.de/inc/dokument.php/standard/1148/airliquide-hypersr-datasheet.pdf</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Titanium tetraisopropoxide (TTIP)</td>
			<td>Sigma-Aldrich</td>
			<td>87560</td>
			<td>Flammable in liquid and vapor phase</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Ge (001) wafer</td>
			<td>MTI Corporation</td>
			<td>GESBA100D05C1</td>
			<td>4&#34;, single-side polished Sb-doped wafer with &#961; &#8776; 0.04 &#937;-cm</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Argon (UHP)</td>
			<td>Praxair</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Deionized Water</td>
			<td>N/A</td>
			<td>N/A</td>
			<td>18.2 M&#937;-cm</td>
		</tr>
	</tbody>
</table>

epitaxial growth of perovskite strontium titanate on germanium via atomic layer deposition

Atomic layer deposition (ALD) is a commercially utilized deposition method for electronic materials. ALD growth of thin films offers thickness control and conformality by taking advantage of self-limiting reactions between vapor-phase precursors and the growing film. Perovskite oxides present potential for next-generation electronic materials, but to-date have mostly been deposited by physical methods. This work outlines a method for depositing SrTiO3 (STO) on germanium using ALD. Germanium has higher carrier mobilities than silicon and therefore offers an alternative semiconductor material with faster device operation. This method takes advantage of the instability of germanium's native oxide by using thermal deoxidation to clean and reconstruct the Ge (001) surface to the 2&#215;1 structure. 2-nm thick, amorphous STO is then deposited by ALD. The STO film is annealed under ultra-high vacuum and crystallizes on the reconstructed Ge surface. Reflection high-energy electron diffraction (RHEED) is used during this annealing step to monitor the STO crystallization. The thin, crystalline layer of STO acts as a template for subsequent growth of STO that is crystalline as-grown, as confirmed by RHEED. In situ X-ray photoelectron spectroscopy is used to verify film stoichiometry before and after the annealing step, as well as after subsequent STO growth. This procedure provides framework for additional perovskite oxides to be deposited on semiconductors via chemical methods in addition to the integration of more sophisticated heterostructures already achievable by physical methods.

Figures 5 and 6 show typical X-ray photoelectron spectra and RHEED images from a cleaned and deoxidized Ge substrate. A successfully-deoxidized Ge substrate is characterized by its &#34;smiley face&#34; 2&#215;1 reconstructed RHEED pattern.26,39 In addition, Kikuchi lines are also observed in the RHEED images, which indicate the cleanliness and long range order of the sample.40 The sharpness and intensity of the diffraction pattern a...

Watch this Scientific Journal Video about Epitaxial Growth of Perovskite Strontium Titanate on Germanium via Atomic Layer Deposition at JoVE.com

Epitaxial Growth of Perovskite Strontium Titanate on Germanium via Atomic Layer Deposition

This work details the procedures for the growth and characterization of crystalline SrTiO3 directly on germanium substrates by atomic layer deposition. The procedure illustrates the ability of an all-chemical growth method to integrate oxides monolithically onto semiconductors for metal-oxide semiconductor devices.

Atomic layer deposition (ALD) is a commercially utilized deposition method for electronic materials. ALD growth of thin films offers thickness control and ...

The overall goal of this procedure is to grow high dielectric constant crystalline perovskite oxides in a scalable process to function as gadoxides directly on geranium, a future microelectronics platform. This method can answer key questions in the micro electronics field which is can crystalline oxides which bind across the interface produce films with extremely low trap densities and interface states? The main advantage of this technique is it's easy to implement in a manufacturing setting and should be readily extendable to three dimensional device architectures.Generally individuals new to this method will struggle because of lack of minorities with handling the precursors and that having timing the sequence to produce steps for their particular hardware in the reactor. We first had the idea for this method when collaborators grouped perovskites directly on geranium using molecular beam epitaxy and we thought to adapt their process to atomic layer deposition. Visual demonstration of this method is critical because the sample cleaning, transfer step and timing need to be modified for other systems while ensuring a clean and reconstructed growth surface.For this protocol use a heavily doped intact geranium if electrical measurements of the film are needed otherwise all doping levels and doping types are acceptable. Place the geranium substrate polished side up into a small beaker. Add about 1 cm of acetone and place the beaker in a bath sonicator, sonicate it for ten minutes.Decant the majority of the acetone but do not pour out or flip the geranium substrate. Then rinse the walls of the beaker with isopropyl alcohol until about 1 cm full. Then pour off most of it.Refill the beaker with the same volume of isopropanol and repeat the sonication. Repeat the rinse procedure, this time using water. Be careful to not disturb the substrate.Then remove the substrate with tweezers and dry under an inert gas such as nitrogen. Now clean the substrate in a UV ozone cleaner for 30 minutes. When 8 minutes remain start venting the load lock so the substrate can be immediately loaded into the vacuum system when the cleaning is completed.Place the substrate polished side down into a 20 by 20 mm holder. Ensure the substrate is flush with the bottom of the holder. After the vacuum system has completely vented open the load lock.Place the sample holder into an open carrier cart position by aligning the tabs of the holder with the channels of the open cart. Then close the load lock and turn on the load lock turbo molecular pump. When the pressure in the load lock has dropped sufficiently low open the load lock gate valve and move the cart through the transfer line.Next transfer the germanium substrate into the MBE chamber. Adjust the sample to an appropriate height relative to the MBE sample heating stage. Then ramp the substrate temperature to 550 degrees Celsius at 20 degrees Celsius per minute.Once at 500 degrees Celsius ramp the temperature up to 700 degrees Celsius at 10 degrees Celsius per minute. After holding the sample at 700 degrees Celsius for an hour cool the sample to 200 degrees Celsius, dropping the temperature by 30 degrees Celsius per minute. Next analyze the substrate by reflection high energy electron diffraction to confirm the two by one reconstructed surface.In preparation set the ALD reactor temperature to 225 degrees Celsius and let it warm up. Heat the bis-triisopropyl cyclopentadianyl strontium to 130 degrees Celsius in a saturator wrapped with heating tape. At the same time heat the titanium tetraisopropoxide to 35 to 40 degrees Celsius in the heating tape wrapped saturator.Regulate the room temperature water vapor flow into the ALD system via the needle valve attached to the saturator. Keep the water pressure around 0.1 torr. Also maintain constant precursor temperatures throughout the deposition process.As soon as the ALD reactor temperature is stable quickly and carefully transfer the sample in situ to the ALD reactor. The time between obtaining a two by one reconstructed surface and loading the sample into the ALD is critical. Minimize this time to avoid any background contamination of the surface.Crystalline films will not grow on a contaminated surface. Isolate the ALD reactor from the transfer line and switch the exhaust port of the ALD reactor from the turbo molecular pump to the mechanical pump. Turn on the flow controller to use an inert gas such as argon and maintain an operating pressure of about 1 torr during the entire growth process.After keeping the system stable for 15 minutes program the ALD. First set the unit cycle ratio of strontium to titanium to be 2:1. Then set the strontium and titanium unit cycles to a two second dose for the strontium or titanium precursor each followed by a 15 second argon purge, a one second dose of water and another 15 second argon purge.Next set the number of cycles for the required deposition thickness. The pressure increases during dosing in our system. Dosing times and pressures needed to reach saturation are likely ALD systems specific and will need to be determined for different systems.As soon as the ALD deposition is completed carefully transfer the sample to the annealing chamber. There heat the sample to 650 degrees Celsius at 20 degrees Celsius per minute under UHV conditions. Once at 650 degrees Celsius hold the temperature for five minutes and then cool the sample back to 200 degrees Celsius at the same rate.Ultimately use RHEED to evaluate the annealing result and if needed additional layers can be deposited onto the substrate by repeating the procedure. RHEED imaging of a cleaned and deoxidized germanium substrate showed that a successful deoxidation could be characterized by its smiley faced two by one reconstructed pattern. Kikuchi lines were observed in the RHEED images indicating the substrate had good cleanliness and long range order.The sharpness and intensity of the RHEED pattern after the deposition and annealing of STO film demonstrated good epitaxial growth. The germanium 3D X-ray photo electron spectrum was free of oxidized germanium peaks with a zero valent germanium peak observed at 30 electron volts. Pre deposition, 36 cycle and 155 cycle depositions are shown respectively in red, brown and black.The X-ray diffraction pattern of epitaxial STO on germanium had the expected characteristic peaks at 22.8 degrees, 46.5 degrees and 66 degrees. The epitaxial nature of the film was directly confirmed by cross sectional high resolution transmission electron microscopy. This showed high quality epitaxial registry between the STO and germanium and abrupt transitions between layers.After watching this video you should have a good understanding of how to grow mono crystalline perovskites directly on germanium using atomic layer deposition. This approach is not restricted to strontium titinate. While attempting this procedure it is important to ensure that the germanium surface has the two by one reconstructions.It's also free of carbon and oxygen contamination that can occur in the transfer lines if the sample is held there for days. Following this procedure are the measures like electrical measurements with MOS capacitor structures can be performed to investigate that interface trap density, film dilation constant and each carrier. After its development this technique paved the way for researchers in the micro electronics field to explore germanium structures that require extremely low equivalent oxide thicknesses.

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Chemistry

12.2K vistas.  The University of Texas at Austin. The overall goal of this procedure is to grow high dielectric constant crystalline perovskite oxides in a scalable process to function as gadoxides directly on geranium, a future microelectronics platform. This method can answer key questions in the micro electronics field which is can crystalline oxides which bind across the interface produce films with extremely low trap densities and interface states? The main advantage of this technique is it's easy to implement in a manufacturing setting...