Dramatic Changes in Thermoelectric Power of Germanium under Pressure - Printing n–p Junctions by Applied Stress

At a Glance

Metadata	Details
Publication Date	2017-03-14
Journal	Scientific Reports
Authors	Igor V. Korobeinikov, Н. В. Морозова, Vladimir V. Shchennikov, Sergey V. Ovsyannikov
Institutions	M.N. Mikheev Institute of Metal Physics, University of Bayreuth
Citations	19
Analysis	Full AI Review Included

Dramatic Changes in Thermoelectric Power of Germanium under Pressure: Printing n-p Junctions by Applied Stress

Executive Summary

This documentation analyzes the research paper, “Dramatic Changes in Thermoelectric Power of Germanium under Pressure,” focusing on its implications for stress-controlled semiconductor technology and linking the required experimental capabilities to 6CCVD’s ultra-hard MPCVD diamond materials and engineering services.

Core Achievement: Demonstration that moderate applied stress can dramatically and reversibly tune the electronic transport properties of conventional germanium, effectively transforming it into a ‘smart’ material.
Novel Application: Suggestion of new innovative applications, specifically the stress-controlled ‘printing’ of micro- and nanoscale n-p diodes and n-p-n junctions on germanium surfaces.
Conduction Inversion: Applied pressure above a few GPa shifts electrical conduction from intrinsic (or n-type) to strongly p-type, evidenced by high positive Seebeck coefficients (+150 to +200 µV/K).
Mechanism: The p-type dominance is attributed to a pressure-driven splitting of the “heavy” and “light” holes bands, leading to enhanced hole mobility and carrier transfer.
Phase Stability: Irreversible pressure cycles lead to the formation of the metastable simple tetragonal st12 polymorph (Ge-III), characterized by persistent n-type semiconducting conductivity (S ~ -150 to -250 µV/K) upon pressure release.
Reversibility Threshold: The n-p inversion is fully reversible only if the applied pressure remains below approximately 2 GPa.
6CCVD Relevance: The utilization of extreme pressure and the proposed ‘printing’ technology necessitate ultra-hard, dimensionally stable materials and custom metalization solutions, aligning perfectly with 6CCVD’s SCD and PCD capabilities.

Technical Specifications

Parameter	Value	Unit	Context
Maximum Applied Pressure (Investigation)	20	GPa	Pressure range covering multiple phase transitions.
Phase Transition (Ge-I $\rightarrow$ Ge-II, Semiconductor $\rightarrow$ Metal)	~10	GPa	Transition to the metallic $\beta$-Sn phase.
Critical Pressure for Reversible n-p Inversion	< 2	GPa	Pressure below which the conduction shift is reversible upon release.
Temperature of Measurement	Room	°C	All thermoelectric measurements performed at RT.
Initial Carrier Concentration (Bulk Ge Ingots)	~10¹⁴	cm^-3	Samples used were intrinsic or compensated semiconductors.
Peak Seebeck Coefficient (Pressure-Induced p-type Ge-I)	+150 to +200	µV/K	Observed after irreversible compression above 4 GPa.
Seebeck Coefficient (Metallic Ge-II, $\beta$-Sn Phase)	+12 to +17	µV/K	Weakly varied positive values in the high-pressure metal phase.
Seebeck Coefficient (Metastable Ge-III, n-type)	-150 to -250	µV/K	Observed upon decompression from high pressures (>10 GPa).
Direct Band Gap (Ge-I)	0.8	eV	Established from near-infrared absorption spectra.
Germanium Anvil Size (Typical)	200 x 200 x 30	µm³	Thin disc samples used in flat anvil cells.

Key Methodologies

The experiment successfully characterized the Seebeck effect in germanium under extreme hydrostatic and quasi-hydrostatic conditions using specialized high-pressure systems.

High-Pressure System: Automated mini-press utilizing modified Bridgman-type high-pressure cells.
Anvil Configuration:
- Flat Anvils: Used for measurements up to 20 GPa, consisting of sintered diamond culets (~600 µm diameter).
- Concave Anvils: Used for lower pressure measurements (up to 7 GPa) to provide more uniform, quasi-hydrostatic pressure distribution across the bulk samples.
Pressure Transmitting Medium: Limestone (soft CaCO₃-based material) functioned as both the pressure-transmitting medium and the sample gasket.
Sample Preparation: Germanium samples were cut into thin discs (~30 µm thickness) for flat anvil measurements. Original samples for optical studies were double-polished to 15-20 µm thickness.
Thermoelectric Measurement: The Seebeck coefficient (S) was determined at room temperature by measuring the thermoelectric voltage (U) generated across a small temperature difference ($\Delta T$) using the relationship $S = -U/\Delta T$.
Temperature Gradient Generation: A temperature difference (a few Kelvin) across the sample thickness was induced by electrical current heaters attached to the upper anvils.
Structural Verification: Recovered metastable polymorphs (Ge-III, st12 phase) were verified using standard structural techniques, including X-Ray Diffraction and Raman Spectroscopy (Raman peak at ~300 cm^-1 for Ge-I; distinct peaks for Ge-III).

6CCVD Solutions & Capabilities

This research into stress-induced thermoelectric shifts and the resulting ‘printing’ of semiconductor junctions highlights a critical need for materials capable of operating under and inducing extreme, controllable stress—a domain where MPCVD diamond excels. 6CCVD is positioned to provide the specialized materials necessary to advance this research from foundational discovery to applied device engineering.

Applicable Materials

To replicate or extend research on stress-controlled semiconductor modification (Ge, SiGe, or Diamond-based sensors), 6CCVD recommends the following materials:

Ultra-Hard Single Crystal Diamond (SCD): Required for fabricating the robust, high-purity micro-anvils or nanoindenter tips proposed for the ‘printing’ technology (Figure 7). SCD offers superior yield strength and wear resistance compared to sintered diamonds used in the current study, enabling higher precision and repeatable stress application.
- Specification Focus: SCD Substrates up to 500 µm thickness, polished to Ra < 1 nm for ideal interface quality.
Polycrystalline Diamond (PCD): Suitable for large-area tooling and specialized pressure cells requiring extreme mechanical stability and excellent thermal management.
- Specification Focus: Inch-size PCD Wafers (up to 125mm) with surface polish Ra < 5 nm.
Boron-Doped Diamond (BDD): BDD is an intrinsically robust, p-type wide-bandgap semiconductor. It serves as an exceptional reference or substrate material for studying hole conduction phenomena under pressure, relevant to the p-type inversion observed in Ge.
- Specification Focus: Heavy Boron-Doped PCD or SCD films for high-conductivity electrical contact layers.

Customization Potential

The experimental setup requires highly specific geometries and precise electrical contacts—core competencies of 6CCVD.

Custom Dimensions and Etching: 6CCVD provides in-house laser cutting and shaping services necessary to create the small, precise anvil tips or micro-device structures (e.g., hard tips for ‘printing’) as discussed in the paper.
Advanced Metalization: Accurate thermoelectric measurements require stable, low-resistance electrical contacts under high pressure. 6CCVD offers internal metalization capability using high-performance metals such as Ti/Pt/Au or W/Cu layers, ensuring robust electrical integration with the semiconductor samples.
Thermal Management: Diamond’s superior thermal conductivity enables precise control over the $\Delta T$ required for accurate Seebeck measurements, addressing a critical component of the methodology.

Engineering Support

This research opens doors to next-generation stress-controlled n-p switches and memory devices.

6CCVD’s in-house PhD engineering team specializes in MPCVD diamond applications in extreme environments (high pressure, high temperature, thermoelectric devices). We provide consultation and design support for projects focused on stress-controlled semiconductor electronics and advanced thermoelectric materials like Ge or SiGe alloys, aiding researchers in material selection, orientation specification, and optimizing integration methodologies.

For custom specifications or material consultation related to high-pressure studies or stress-controlled n-p junction projects, visit 6ccvd.com or contact our engineering team directly.

Tech Support

Original Source

DOI: https://doi.org/10.1038/srep44220