Phase Transitions in Amorphous Germanium under Non-Hydrostatic Compression

At a Glance

Metadata	Details
Publication Date	2022-06-24
Journal	Crystals
Authors	Jianing Xu, Lingkong Zhang, Hailun Wang, Gao Yan, Tingcha Wei
Institutions	Nanjing University of Aeronautics and Astronautics, Shanghai Institute of Ceramics
Citations	2
Analysis	Full AI Review Included

Executive Summary

This research investigates the structural phase transitions of amorphous Germanium (a-Ge) under extreme hydrostatic and non-hydrostatic compression, revealing that shear stress fundamentally alters the material’s transformation pathway.

Shear Stress Control: Non-hydrostatic compression (inducing high shear stress) suppresses the typical pressure-induced crystallization pathway observed under hydrostatic conditions.
Pathway Divergence (Hydrostatic): Under hydrostatic pressure, a-Ge transforms into the crystalline beta-Sn structure (above ~11 GPa), which subsequently yields the metastable, quenchable ST12 phase upon decompression.
Pathway Divergence (Non-Hydrostatic): Under non-hydrostatic pressure (high shear), a-Ge undergoes a reversible amorphous-to-amorphous transition: Low-Density Amorphous (LDA) to High-Density Amorphous (HDA) phase, occurring at a higher critical pressure (~14.1 GPa).
Suppression of Crystallization: The presence of giant shear stress hinders the atomic ordering required for the formation of the crystalline beta-Sn phase, stabilizing the HDA amorphous structure up to 30 GPa.
Reversible Transition: The LDA-HDA transition under shear is reversible, contrasting sharply with the irreversible crystallization pathway seen under hydrostatic conditions.
Engineering Implication: These findings demonstrate that shear stress acts as a critical, controllable thermodynamic variable for tuning the structural stability and phase selection of Ge, opening routes for synthesizing novel metastable phases for advanced electronics.

Technical Specifications

Parameter	Value	Unit	Context
Hydrostatic Transition Pressure (a-Ge to beta-Sn)	~11	GPa	Compression using Ethanol PTM.
Non-Hydrostatic Transition Pressure (LDA to HDA)	~14.1	GPa	Compression without PTM (shear stress induced).
Maximum Non-Hydrostatic Pressure	~33	GPa	Achieved during compression cycle.
DAC Diamond Culet Size	300	µm	Used for high-pressure generation.
Gasket Hole Diameter	~100	µm	Sample chamber size.
Pressure Gradient (Non-Hydrostatic)	Up to ~10	GPa	Pressure difference from center to edge of sample chamber (at 30 GPa external pressure).
ALS Synchrotron XRD Wavelength	0.4959	A	Non-hydrostatic experiment setup.
APS Synchrotron XRD Wavelength	0.3100	A	Hydrostatic experiment setup.
Raman Laser Wavelength	532	nm	Used for high-pressure Raman measurements.
Raman TO Mode Frequency (LDA a-Ge)	~280	cm^-1	Transverse Optic phonon mode observed at low pressure.
Raman Spectral Resolution	Better than 1	cm^-1	Achieved using 2400 lines·mm^-1 grating.

Key Methodologies

The phase evolution of amorphous Germanium (a-Ge) was investigated using Diamond Anvil Cells (DACs) combined with synchrotron radiation techniques under two distinct pressure environments.

Sample Preparation: Amorphous Ge samples were fabricated using a magnetron sputtering system.
Pressure Generation: High pressure was generated using DACs with 300 µm culets. Samples were loaded into laser-drilled stainless-steel gasket holes (~100 µm diameter).
Hydrostatic Compression: Ethanol was used as the Pressure Transmitting Medium (PTM) to maintain isotropic pressure. Maximum pressure reached was ~18 GPa.
Non-Hydrostatic Compression: No PTM was used within the DAC chamber to maximize shear stress/strain and generate a non-hydrostatic environment. Maximum pressure reached was ~33 GPa.
Pressure Calibration: Pressure was monitored using the fluorescence R1-R2 line shift of a micro-sized ruby ball standard.
Structural Characterization (XRD):
- In situ high-pressure Angle-Dispersive XRD (AD-XRD) was performed at the Advanced Light Source (ALS) and Advanced Photon Source (APS).
- X-ray beam sizes were ~30 x 30 µm² (ALS) and ~4 x 4 µm² (APS).
Vibrational Characterization (Raman):
- High-pressure Raman spectrum measurements were conducted using a 532 nm laser.
- Raman shift and Full Width at Half Maximum (FWHM) were tracked to monitor the LDA-HDA transition and the degree of structural disorder.

Commercial Applications

The ability to control the phase transition pathways of Germanium using shear stress has significant implications for advanced electronic and materials engineering fields.

Advanced Semiconductor Devices:
- Germanium is a foundational semiconductor. Controlling its amorphous and metastable crystalline phases (ST12, HDA) is crucial for next-generation transistors and integrated circuits, especially those operating under mechanical strain.
Phase-Change Memory (PCM):
- The quenchable ST12 phase (formed under hydrostatic decompression) and the stable HDA phase (formed under shear) represent high-density, potentially metallic states. These metastable structures are highly relevant for developing fast, high-density non-volatile memory materials.
Strain Engineering and Nanomechanics:
- The study confirms that shear stress is an effective tool for inducing specific displacive phase transformations. This principle can be applied in nanomechanics to engineer materials with enhanced properties (e.g., ultra-hard materials like h-diamond, as referenced in the conclusion) by utilizing non-hydrostatic DAC techniques.
High-Pressure Materials Synthesis:
- Non-hydrostatic compression provides a pathway to synthesize novel metastable polymorphs that are inaccessible through standard hydrostatic or thermal routes, offering new materials for superconductivity and high-performance ceramics.
Electronic Materials Reliability:
- Understanding how a-Ge behaves under non-hydrostatic stress is essential for predicting the reliability and performance of Ge-based microelectronic components subjected to mechanical loads or internal stresses during manufacturing.

View Original Abstract

As the pioneer semiconductor in transistor, germanium (Ge) has been widely applied in information technology for over half a century. Although many phase transitions in Ge have been reported, the complicated phenomena of the phase structures in amorphous Ge under extreme conditions are still not fully investigated. Here, we report the different routes of phase transition in amorphous Ge under different compression conditions utilizing diamond anvil cell (DAC) combined with synchrotron-based X-ray diffraction (XRD) and Raman spectroscopy techniques. Upon non-hydrostatic compression of amorphous Ge, we observed that shear stress facilitates a reversible pressure-induced phase transformation, in contrast to the pressure-quenchable structure under a hydrostatic compression. These findings afford better understanding of the structural behaviors of Ge under extreme conditions, which contributes to more potential applications in the semiconductor field.

Tech Support

Original Source

DOI: https://doi.org/10.3390/cryst12070898

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