Geometric Electrode Effects on the Induced Photocurrent in Polycrystalline Diamond Based X- Ray Dosimeters

At a Glance

Metadata	Details
Publication Date	2015-01-01
Journal	VNU Journal of Science: Natural Sciences and Technology (Vietnam National University)
Authors	K.S. Al Mugren
Analysis	Full AI Review Included

Geometric Electrode Effects in PCD Dosimeters: 6CCVD Technical Analysis

This documentation provides an expert analysis of the findings regarding the geometric electrode effects on induced photocurrents in polycrystalline CVD (PCD) diamond X-ray dosimeters. This research is critical for optimizing diamond sensor design, particularly concerning active area linearity and transient response.

Executive Summary

Application Focus: Development and characterization of tissue-equivalent polycrystalline CVD diamond dosimeters for high-fidelity X-ray detection applications.
Material and Structure: Devices utilized thermal grade MPCVD PCD material with a sandwich structure featuring thermally evaporated Aluminum (Al) contacts forming circular pads of varying diameters (2 mm to 7 mm).
Key Performance Metric (Linearity): Photocurrent response showed significant deviation from linearity with respect to dose rate, quantified by the Fowler relation exponent (A = 0.73 to 0.9). Higher bias voltages improved linearity (A closer to 1).
Geometric Finding: A non-linear relationship (under-response) was observed between the detector active area (pad size) and sensitivity, strongly suggesting performance is limited by edge effects rather than bulk properties under broad beam irradiation.
Transient Response: The 5%-95% rise time was primarily governed by the applied bias voltage and the material quality (defect concentration), showing substantial increase with higher electric fields, indicative of shallow trapping/detrapping effects.
Engineering Takeaway: Achieving optimal dosimeter performance requires precision control over material quality (minimizing defects/traps) and meticulous control over electrode geometry and deposition for mitigating edge effects.

Technical Specifications

Parameter	Value	Unit	Context
Material Type	Polycrystalline CVD Diamond	N/A	Thermal Grade
Sample 1 Dimensions	20 x 20	mm²	CVD plate area
Sample 2 Dimensions	10 x 10	mm²	CVD plate area
Sample 1 Thickness	0.3 (300)	mm (µm)	Standard Thickness
Sample 2 Thickness	0.5 (500)	mm (µm)	Standard Thickness
Electrode Pads (Sample 1)	4x (7, 6, 6, 5)	mm dia	Circular Al electrodes
Electrode Pads (Sample 2)	3x (5, 4, 2)	mm dia	Circular Al electrodes
Metalization Material	Aluminum (Al)	N/A	Thermally evaporated
Metalization Thickness	100	nm	Standard film thickness
Maximum Applied Bias (S1)	120	V	Used for dose rate tests
Maximum Applied Bias (S2)	200	V	Used for dose rate tests
Dark Current (S1, 120V)	~200	pA	Low-noise operation
Linearity Exponent (A, S1)	0.73 - 0.81	N/A	Non-linear response (ideal A = 1)
Linearity Exponent (A, S2)	~0.9	N/A	Improved linearity
E-Field (S1 120V/0.3mm)	4000	V/cm	Highest tested field strength
Rise Time (5%-95%) Range	1000 - 2000+	ms	Dependent on bias and material quality

Key Methodologies

The experimental approach focused on fabricating sandwich-type detectors and measuring their electrical response under controlled X-ray irradiation conditions.

Material Preparation:
- Polycrystalline CVD diamond plates (Thermal Grade) were sourced from Element Six LTD (UK).
- Samples were chemically cleaned using 60 ml of hydrochloric acid (HCl) and 20 ml of nitric acid (HNO₃).
Metalization and Contact Fabrication:
- Both top and back surfaces of the diamond samples were metalized via thermal evaporation with 100 nm thick Aluminum (Al).
- Circular electrodes of precise diameters (2 mm to 7 mm) were defined on the surface.
Mounting and Contacting:
- Devices were mounted onto Printed Circuit Boards (PCBs) (2.2 x 1.6 cm²).
- Gold paste was used on the center of the deposited circles.
- Fine (25 µm thickness) gold wires were attached using Conductive Silver Epoxy under a microscope, secured for 3 hours, and connected to copper pads on the PCB via silver paint.
Measurement Setup:
- X-ray source: Oxford Instruments tube with a Molybdenum target anode, operated at 50 kVp.
- Dose rate was varied by adjusting the tube current (range: 50 µA to 1000 µA).
- Current-time plots were acquired using a Keithley 487 picoammeter/voltage source.
- Measurements included both un-collimated (broad beam) and collimated (using a 12 cm brass tube with a 2 mm hole) X-ray beams for analysis of spatial/edge effects.
Data Analysis:
- Custom LabVIEW VI was used for semi-automated analysis of mean peak current, standard deviation, and 5%-95% rise time over the final 15 seconds of a 30-second exposure.
- Data was fit to the Fowler relation ($I = I_{d} + R D_{r}^{A}$) to quantify linearity (A exponent).

6CCVD Solutions & Capabilities

This research confirms the potential of MPCVD diamond for advanced dosimetric applications but highlights significant material purity and fabrication challenges, particularly concerning electrode design and transient response control. 6CCVD is uniquely positioned to meet and exceed the material and customization requirements identified in this study.

Applicable Materials & Purity Optimization

The paper used “Thermal Grade” PCD. To achieve superior performance, especially better linearity (A closer to 1) and reduced time-dependent effects (shorter rise times), researchers require Detector Grade diamond with ultra-low impurity levels.

Recommended 6CCVD Material	Key Advantage over Thermal Grade	Replication/Extension Rationale
Detector Grade Polycrystalline CVD (PCD)	Minimized trap/defect density; enhanced charge collection efficiency (CCE).	Directly addresses the material quality limitations driving non-linearity (A < 1) and long rise times observed in the research.
High Purity Single Crystal CVD (SCD)	Near-perfect trap distribution; superior consistency and intrinsic linearity.	Ideal for high-precision dosimetry, providing the basis for “near-perfect values achievable” mentioned in the literature review.
Boron-Doped Diamond (BDD)	Potential for customized ohmic contacts or p-type layers, stabilizing dark current response.	Relevant for exploring alternative contact schemes to improve stability and sensitivity consistency across different pad areas.

Customization Potential & Electrode Engineering

The study emphasized that geometric factors (pad area, edge effects) and specific metalization (Al contacts) are crucial. 6CCVD offers the full range of fabrication services needed to replicate, test, and optimize these structures at scale.

Custom Dimensions and Thicknesses: The paper utilized 0.3 mm and 0.5 mm thick PCD. 6CCVD routinely provides PCD wafers up to 125 mm diameter and custom thicknesses ranging from 0.1 µm up to 500 µm, and substrates up to 10 mm.
Precision Geometry and Etching: The required circular electrode patterns (2 mm to 7 mm diameter) can be achieved via 6CCVD’s custom laser cutting services or advanced photolithographic processes, ensuring precise pad area definition crucial for minimizing the reported edge effects.
Advanced Metalization Capability: While the paper used Al, 6CCVD offers a broad suite of high-reliability metal contacts optimized for high-performance radiation detection:
- Metals Available: Au, Pt, Pd, Ti, W, Cu.
- Benefit: Custom Ti/Pt/Au stacks can provide superior ohmic contacts and thermal stability compared to single-layer Al, potentially reducing the reported dark current inconsistencies and noise issues.

Engineering Support & Application Acceleration

The non-linear response to bias and the complex transient behavior indicate that material selection, crystal quality (grain boundaries in PCD), and contact engineering are tightly coupled performance factors.

Expert Consultation: 6CCVD’s in-house PhD engineering team specializes in diamond material physics and can assist with material selection to optimize performance parameters such as maximizing linearity (moving A → 1) and minimizing transient response (reducing rise time) for X-ray dosimetry projects.
Testing and Validation: We support customized metallization recipes and offer validation services to ensure the optimal combination of material grade (SCD vs. PCD), thickness, and contact scheme is selected for specific radiation detector design applications.

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

View Original Abstract

The aim of the study is to develop tissue equivalent CVD diamond based dosimeters with a carbon based electrodes. We have initially studied the dependence of induced X-ray photocurrent amplitude and time response as a function of electrode area under broad beam X-ray irradiation. For this purpose, we have fabricated circular electrode sandwich tested structures based on thermal grade polycrystalline CVD diamond with different 3 diameters. The X-ray photocurrent increases with increasing dose rate sub linearly. Although the photocurrent increases with increasing electrode diameter, the increase in current is less than expected from the increase in active area. To further investigate, a small detector based upon 4 separate circular pad was tested for sensitivity with respect to detector area, applied bias voltage and incident x-ray doserate. Additionally the rise time of these detectors was evaluated with respect to the same criteria from these tests there was a non-linear relationship of under response between detector area and sensitivity under broad beam irradiation, the cause of which has been attributed to edge effects, supported by the results from collimated beam measurements. The results from the rise time tests appear to indicate that the time dependent components of the detector response are affected primarily by the applied bias upon the detector and the quality of the material, with additional doserate related effects scaling with bias. The higher quality material was found to have a far greater dependence on applied bias than the lower quality sample, which varied little. Keywords: CVD Diamond, Dosimetric Characteristic, Electrical Contact, Radiation Detector, Thermal Grade polycrystalline diamond.

Tech Support

Original Source

DOI: None