Pseudovertical Schottky Diodes on Heteroepitaxially Grown Diamond

At a Glance

Metadata	Details
Publication Date	2022-11-13
Journal	Crystals
Authors	Jürgen Weippert, Philipp Reinke, Fouad Benkhelifa, Heiko Czap, Christian Giese
Institutions	Fraunhofer Institute for Applied Solid State Physics
Citations	7
Analysis	Full AI Review Included

Technical Documentation & Analysis: Pseudovertical Schottky Diodes on Heteroepitaxially Grown Diamond

Executive Summary

This documentation analyzes the fabrication and performance of pseudovertical Schottky diodes utilizing heteroepitaxially grown diamond (HET-Dia) layers. The findings highlight the potential of diamond for power electronics while underscoring the critical need for high-quality, low-defect substrates, a core offering of 6CCVD.

Device Structure: Pseudovertical Schottky diodes were fabricated using p^- Boron-Doped Diamond (BDD) drift layers (10¹⁵-10¹⁶ cm^-3) grown on highly doped p⁺ BDD layers (10¹⁹-10²⁰ cm^-3).
Performance Metrics: The best diodes achieved a high breakdown field of 1.5 MV/cm and an excellent ideality factor (n) of 1.06, demonstrating strong rectification capabilities.
Barrier Characteristics: The Schottky barrier height (Φ_SB) showed a systematic correlation with the ideality factor, reaching an ideal value of 1.43 eV for n = 1.
Conduction Mechanisms: Forward current was dominated by thermionic emission (TEM) at higher voltages, preceded by a low-voltage shunt conductance region.
Material Limitation: Reverse leakage current and yield were severely limited by crystal defects inherent to heteroepitaxial growth, specifically penetration twins and boundaries, which act as high-conductivity channels.
6CCVD Value Proposition: The reported limitations (leakage, low BFOM of 810 W/cm²) can be overcome by transitioning to 6CCVD’s high-quality, low-defect Homoepitaxial Single Crystal Diamond (SCD) substrates.

Technical Specifications

The following hard data points were extracted from the device characterization and material analysis:

Parameter	Value	Unit	Context
Maximum Breakdown Field (E_BD)	1.5	MV/cm	Achieved by best diode (Sample I)
Breakdown Voltage (V_BD)	105	V	Assigned value at 1 A/cm² current density
Baliga Figure of Merit (BFOM)	810	W/cm²	Calculated for best analyzed diode
Ideality Factor (n)	1.06	-	Best rectifying diode fit
Ideal Schottky Barrier (Φ₁)	1.43 ± 0.01	eV	Extrapolated value for n = 1
p⁺ Layer Doping (N_B)	5 x 10¹⁹ to 1 x 10²⁰	cm^-3	Range across samples I, II, III (SIMS)
p^- Layer Doping (N_A-N_D)	5 x 10¹⁴ to 1.4 x 10¹⁶	cm^-3	Range across samples I, II, III (CV)
p^- Layer Thickness (Drift Layer)	~0.7	µm	For diode achieving 1.5 MV/cm
Ohmic Contact Annealing Temp.	1120	K	Ti/Pt/Au contacts
Schottky Contact Diameter Range	100, 150, 200, 300	µm	Fabricated diode sizes
Reverse Conduction Mechanism	Space-Charge-Limited	-	Observed at higher reverse voltages

Key Methodologies

The pseudovertical diode structure required complex multi-step MPCVD growth, polishing, and fabrication techniques:

Substrate Preparation: Ir/YSZ/Si(001) buffer layers were sputtered onto 2” Si wafers (N⁺⁺/As).
- YSZ (45 nm) grown at 1050 K (2:1 Ar/O₂).
- Ir (120 nm) grown at 975 K (Pure Ar).
Diamond Nucleation (BEN): Bias-Enhanced Nucleation performed at 1050 K, 5% CH₄/H₂ ratio, 700 W MW power, and 400 V DC bias.
Bulk Growth (Quasi-Intrinsic): Growth into 300 µm thick single-crystalline diamond in an ellipsoid reactor at 1100 K, 12 kW MW power, 1.7% CH₄/H₂, 1.5% O₂, and 20 ppm N₂ (N₂ acts as growth mediator in [001] direction).
Substrate Separation & Polishing: Wafers were laser-cut (9 x 9 mm²), separated from Si using KOH, and polished (laser + mechanical) to achieve a smooth surface.
p⁺ Layer Deposition: Short CVD step at 1100 K, 9 kW power, 1.5% B/C doping ratio (resulting in 10¹⁹-10²⁰ cm^-3).
Intermediate Polishing: Mechanical polishing was performed after p⁺ layer growth.
p^- Layer Deposition: Growth at 1100 K, 8 kW power (residual doping resulted in 10¹⁵-10¹⁶ cm^-3).
Tilted Polishing: A final tilted mechanical polish was used to expose both the p⁺ and p^- layers for contact access.
Surface Termination: Nitrating acid treatment was applied to remove hydrogen termination and create a well-defined oxygen-terminated surface.
Metalization: Ohmic contacts (Ti/Pt/Au) were sputtered and annealed at 1120 K. Schottky contacts (Ti/Pt/Au) were deposited via E-beam evaporation and lift-off (unannealed).

6CCVD Solutions & Capabilities

This research validates the potential of BDD diamond for high-performance power devices. However, the use of heteroepitaxial growth introduced critical defects (penetration twins, stacking faults) that severely limited yield and performance (BFOM of 810 W/cm²).

6CCVD specializes in high-quality homoepitaxial MPCVD diamond, offering direct solutions to overcome the material limitations encountered in this study.

Applicable Materials: Eliminating Defect-Induced Leakage

The primary challenge identified was leakage current stemming from crystal defects inherent to the HET-Dia/Ir/YSZ/Si system. 6CCVD offers superior homoepitaxial materials:

High-Purity Single Crystal Diamond (SCD): To replicate and significantly improve upon this research, 6CCVD recommends using High-Purity SCD substrates for the quasi-intrinsic layer, followed by homoepitaxial growth of the doped layers. This eliminates the Ir/YSZ buffer and the associated high density of threading dislocations and penetration twins (the source of the leakage shown in Figure 5A).
Boron-Doped Diamond (BDD): We provide precise, customized doping for the active layers:
- Heavy Boron Doped SCD: For the p⁺ layer (target doping 10¹⁹-10²⁰ cm^-3) to ensure low-resistance ohmic contact.
- Light Boron Doped SCD: For the p^- drift layer (target doping 10¹⁵-10¹⁶ cm^-3) to maximize breakdown voltage and minimize leakage.
Thickness Control: 6CCVD guarantees precise thickness control for both SCD and PCD layers from 0.1 µm up to 500 µm, allowing engineers to optimize the drift layer thickness for higher absolute breakdown voltages (V_BD > 105 V).

Customization Potential: Scaling and Advanced Processing

The fabrication process relied on complex, non-scalable steps like tilted mechanical polishing and small 9 x 9 mm² pieces. 6CCVD offers scalable solutions:

Requirement in Paper	6CCVD Capability	Benefit to Researcher/Engineer
Small 9 x 9 mm² pieces	PCD Wafers up to 125mm	Enables true wafer-scale production and high-volume manufacturing.
Tilted Mechanical Polishing	Precision Laser Cutting & Deep Etching	Provides a scalable, high-resolution method for contacting buried p⁺ layers, replacing the complex and yield-limiting tilted polish.
Ti/Pt/Au Metalization	In-House Metalization Services	We offer the required Ti/Pt/Au stack, plus other standard power electronics metals (Au, Pt, Pd, Ti, W, Cu), applied under cleanroom conditions.
Surface Quality (Polishing)	Ultra-Smooth Polishing	SCD polishing to Ra < 1 nm and inch-size PCD polishing to Ra < 5 nm, ensuring optimal interface quality for Schottky contacts and minimizing surface scattering losses.

Engineering Support

The paper concluded that future efforts must focus on reducing impurities and defects to improve material properties and achieve a decent breakdown field for the majority of diodes.

6CCVD’s in-house PhD team specializes in optimizing MPCVD growth recipes to minimize the formation of (111) stacking faults and threading dislocations, which are known to harbor sp² carbon moieties and promote vertical leakage. We offer consultation and custom material development for similar High-Voltage Diamond Power Electronics projects, ensuring high yield and superior device performance far exceeding the BFOM reported in this heteroepitaxial study.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Substrates comprising heteroepitaxially grown single-crystalline diamond epilayers were used to fabricate pseudovertical Schottky diodes. These consisted of Ti/Pt/Au contacts on p− Boron-doped diamond (BDD) layers (1015-1016 cm−3) with varying thicknesses countered by ohmic contacts on underlying p+ layers (1019-1020 cm−3) on the quasi-intrinsic diamond starting substrate. Whereas the forward current exhibited a low-voltage shunt conductance and, for higher voltages, thermionic emission behavior with systematic dependence on the p− film thickness, the reverse leakage current appeared to be space-charge-limited depending on the existence of local channels and thus local defects, and depending less on the thickness. For the Schottky barriers ϕSB, a systematic correlation to the ideality factors n was observed, with an “ideal” n = 1 Schottky barrier of ϕSB = 1.43 eV. For the best diodes, the breakdown field reached 1.5 MV/cm.

Tech Support

Original Source

DOI: https://doi.org/10.3390/cryst12111626

References

2020 - Diamond power devices: State of the art, modelling, figures of merit and future perspective [Crossref]
2020 - Diamond semiconductor performances in power electronics applications [Crossref]
2016 - Single crystal diamond wafers for high power electronics [Crossref]
2019 - 3 GHz RF measurements of AlGaN/GaN transistors transferred from silicon substrates onto single crystalline diamond [Crossref]
2015 - Power diamond vertical Schottky barrier diode with 10 A forward current [Crossref]
2019 - Performance Improved Vertical Diamond Schottky Barrier Diode with Fluorination-Termination Structure [Crossref]
2020 - Characterization of Schottky Barrier Diodes on Heteroepitaxial Diamond on 3C-SiC/Si Substrates [Crossref]
2020 - Epitaxial diamond on Ir / SrTiO 3 / Si(001): From sequential material characterizations to fabrication of lateral Schottky diodes [Crossref]
2020 - Influence of Different Surface Morphologies on the Performance of High-Voltage, Low-Resistance Diamond Schottky Diodes [Crossref]