Schottky Barrier Diodes Based on Freestanding Polycrystalline Diamond Membranes

At a Glance

Metadata	Details
Publication Date	2025-10-23
Journal	Advanced Electronic Materials
Authors	Dmitry Shinyavskiy, Chenyu Wang, L. J. Suter, Matthias Muehle, Jung‐Hun Seo
Institutions	University at Buffalo, State University of New York, Fraunhofer USA
Analysis	Full AI Review Included

Executive Summary

This research introduces a novel vertical Schottky barrier diode architecture utilizing freestanding Polycrystalline Diamond Membranes (PCDm), overcoming limitations associated with conventional thin-film PCD devices grown on rigid substrates.

Core Innovation: The freestanding PCDm format enables dual-side processing, allowing for asymmetric contact engineering: a high-quality growth surface for the Schottky contact and the sp²-rich nucleation surface for the ohmic contact.
Vertical Architecture Advantage: This geometry promotes direct carrier transport, significantly reducing lateral resistance caused by grain boundaries, a major issue in planar PCD diodes.
High Performance: The devices demonstrate excellent rectifying behavior with an average On/Off ratio of approximately 10³ (at +5 V), surpassing most previously reported PCD Schottky diodes.
Breakdown Field: A high breakdown field of 0.25 MV cm^-1 was achieved, demonstrating suitability for high-voltage applications.
Material Properties: The top (Schottky) surface exhibits superior crystalline quality (narrower Raman peak, lower sp² content) compared to the bottom (ohmic) surface, which has higher sp² content and structural disorder, facilitating ohmic contact formation.
Thermal Stability: The Schottky junction maintains rectifying behavior across the entire tested temperature range (20 °C to 150 °C), confirming thermal stability.

Technical Specifications

Parameter	Value	Unit	Context
PCDm Thickness	3.5	µm	Final thickness after RIE-ICP cleanup
On/Off Ratio (Average)	≈10³	N/A	Measured at +5 V bias
Breakdown Field	0.25	MV cm^-1	Calculated based on 3.5 µm thickness
Breakdown Voltage (RT)	-86	V	Measured at room temperature (RT)
Breakdown Voltage (150 °C)	-67	V	Measured at 150 °C
Schottky Barrier Height (Φ_B)	1.06	eV	Extracted from Richardson plot
Built-in Potential (V_bi)	0.98	V	Extracted from C-V plot (RT, 100 kHz)
Ideality Factor (n)	1.47	N/A	Measured at RT (close to ideal value of 1)
Sheet Resistance (R_s)	37	Ω/square	Measured on bottom surface (TLM)
Contact Resistance (R_c)	27	Ω	Measured on bottom surface (TLM)
Top Surface RMS Roughness	≈98	nm	Pronounced microstructures
Bottom Surface RMS Roughness	≈5.5	nm	Significantly smoother and more uniform
Top Surface Grain Size	≈1200	nm	Highly crystalline, columnar structure
Bottom Surface Grain Size	≈500	nm	Smaller grain size, consistent with nucleation
Top Surface sp²/sp³ Ratio	0.009	N/A	Low non-diamond carbon content
Bottom Surface sp²/sp³ Ratio	0.012	N/A	Higher non-diamond carbon content
Top Surface PL Slope	1.75	a.u. cm^-1	Lower hydrogen incorporation
Bottom Surface PL Slope	3.23	a.u. cm^-1	Greater hydrogen incorporation

Key Methodologies

The vertical PCDm Schottky diode fabrication relies on a sequence of growth, release, transfer, and dual-side metallization steps:

PCD Film Growth (Figure 1a(i)): Boron-doped PCD film was synthesized on (100) Si/SiO₂ wafers using Microwave Plasma-Enhanced Chemical Vapor Deposition (MPECVD).
Mask Deposition (Figure 1a(ii)): A bi-layer metal etching mask (Cr/Ni, 40/320 nm) was deposited via photolithography and electron-beam evaporation, defining micro-sized etching holes.
PCD Etching: Inductively Coupled Plasma-Reactive Ion Etching (ICP-RIE) using a gas mixture of O₂ and CF₄ (4:1 ratio) was used to etch the PCD layer, replicating the hole pattern.
Membrane Release (Figure 1a(iii)): The underlying SiO₂ sacrificial layer was removed by immersing the sample in 49% Hydrofluoric Acid (HF), releasing the PCD layer as a freestanding membrane (PCDm).
Flip-Transfer (Figure 1a(iv)): The released PCDm (top surface facing upward) was flip-transferred onto a foreign Si substrate using a micro-transfer printing technique, with SU-8 acting as a temporary adhesive, exposing the bottom surface.
Bottom Surface Cleanup: A brief 1 min RIE-ICP etching was performed on the exposed bottom surface to remove any residual Si.
Ohmic Contact (Anode) (Figure 1a(v)): A Ti/Au (10/150 nm) ohmic contact was deposited onto the bottom (nucleation) surface using electron-beam evaporation.
Membrane Detachment: The PCDm was carefully detached by dissolving the SU-8 adhesive layer in acetone.
Schottky Contact (Cathode) (Figure 1a(vi, vii)): A 200 nm-thick Molybdenum (Mo) Schottky contact was deposited onto the top (as-grown) surface using a shadow mask and a sputtering system.

Commercial Applications

The unique properties of PCDm, including wide bandgap, high breakdown field, chemical inertness, and superior thermal conductivity, make this technology suitable for demanding electronic environments:

Power Electronics: High-efficiency power switches and diodes, leveraging the high breakdown field (0.25 MV cm^-1) and excellent rectifying behavior.
Extreme Environment Electronics: Devices requiring stable electrical performance and chemical inertness in harsh conditions (e.g., high temperature, radiation).
Bioelectronics: Applications where biocompatibility and chemical inertness are critical.
Optoelectronics: Solar-blind photodetectors, utilizing diamond’s wide bandgap.
Hybrid Device Architectures: The membrane format enables easy heterogeneous integration and vertical stacking with existing semiconductor devices and circuits (e.g., Si, GaN, SiC) for 3D-integrated electronics.
Flexible Electronics: Enhanced mechanical compliance due to the membrane configuration supports emerging flexible device platforms.

View Original Abstract

Abstract Polycrystalline diamond (PCD) thin films have been widely used as a coating material to enhance surface properties or protect against wear and tear. However, their implementation as an electronic material has been hindered by inconsistent semiconducting properties arising from their polycrystalline nature and associated processing challenges. In this study, the first demonstration of vertical Schottky barrier diodes fabricated using freestanding PCD membranes (PCDm) is presented, which addresses these limitations by enabling dual‐side access to the PCDm. The Schottky contact is formed on the high‐quality growth surface with larger grains and high sp 3 carbon content, while the ohmic contact is placed on the smoother, sp 2 ‐rich bottom side. This configuration enables distinct contact optimization on each surface, eliminating the trade‐offs encountered in conventional planar devices based on thin‐film PCD. The devices exhibit an excellent rectifying behavior with an on/off ratio of ≈10 3 and a breakdown field of 0.25 MV cm −1 —among the highest reported for PCD‐based Schottky barrier diodes. The result paves the way for the development of high‐performance electronic devices based on freestanding and transferable PCDm, positioning it as a cost‐effective and scalable wide‐bandgap semiconductor for next‐generation electronics.

Tech Support

Original Source

DOI: https://doi.org/10.1002/aelm.202500409