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

Summary

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.

Abstract

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 2×1 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.

Introduction

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 m....

Protocol

1. Preparing Sr and Ti Precursors for ALD Experiments

1. Load the clean, dry saturators and new precursors into the antechamber of a glove box. Follow the glove box'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 Equipment.
2. Store the strontium precursor (strontium bis(triisopropylcyclopentadienyl) [Sr(ⁱPr₃Cp)₂]) and t....

Results

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 "smiley face" 2×1 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 also demonstra.......

Discussion

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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
MBE	DCA	M600
Cryopump for MBE	Brooks Automation, Inc.	On-Board 8
Residual Gas Analyzer for MBE	Extorr, Inc.	XT200M
ALD Reaction Chamber	Huntington Mechanical Laboratories		Custom manufactured, hot-wall, stainless steel, rectangular (~20 cm long, 460 cm3)
ALD Saturator	Swagelok/Larson Electronic Glass	See comments	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. Larson Electronic Glass catalog number: SP-075-T.
Manual Valves for Saturators	Swagelok	SS-DLVCR4-P and 6LVV-DPFR4-P	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.
ALD Valves	Swagelok	6LVV-ALD3TC333P-CV
ALD System Tubing	Swagelok		316L tubing of various sizes. This group uses inner diameter of 1/4"
ALD power supply	AMETEK Programmable Power, Inc.	Sorensen DCS80-13E
ALD Temperature Controller	Schneider Electric	Eurotherm 818P4
ALD Valve Controller	National Instruments	LabView	Program developed within the group
XPS	VG Scienta
RHEED	Staib Instruments	CB801420	18 keV at ~3° incident angle
RHEED Analysis System	k-Space Associates	kSA 400
Digital UV Ozone System	Novascan	PSD-UV 6
Ozone Elimination System	Novascan	PSD-UV OES-1000D
Titanium tetraisopropoxide (TTIP)	Sigma-Aldrich	87560	Flammable in liquid and vapor phase
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Strontium bis(triisopropylcyclopentadienyl)	Air Liquide	HyperSr	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
Ge (001) wafer	MTI Corporation	GESBA100D05C1	4", single-side polished Sb-doped wafer with ρ ≈ 0.04 Ω-cm
Argon (UHP)	Praxair
Deionized Water			18.2 MΩ-cm