Dephasing by optical phonons in GaN defect single-photon emitters

At a Glance

Metadata	Details
Publication Date	2023-05-29
Journal	Scientific Reports
Authors	Yifei Geng, Jialun Luo, Len van Deurzen, Huili Grace Xing, C. Jena
Institutions	Cornell University
Citations	10
Analysis	Full AI Review Included

Technical Documentation & Analysis: GaN Single-Photon Emitters and the Role of MPCVD Diamond

This document analyzes the research paper “Dephasing by optical phonons in GaN defect single-photon emitters” and outlines how 6CCVD’s expertise in MPCVD diamond materials can address the challenges identified, positioning diamond as a critical platform for next-generation quantum applications.

Executive Summary

The research investigates the temperature-dependent dephasing mechanisms in Gallium Nitride (GaN) Single-Photon Emitters (SPEs), yielding key insights relevant to solid-state quantum platforms:

Novel Dephasing Mechanism: Unlike SPEs in diamond (NV centers) and SiC, where dephasing is typically dominated by low-energy acoustic phonons (T⁵ or T⁷ power laws), GaN SPE dephasing is governed by the absorption/emission of low-energy optical phonons (E₂(low) mode).
Critical Phonon Energy: The extracted optical phonon energy responsible for dephasing is approximately 19 meV, matching the zone center energy of the GaN E₂(low) Raman-active mode.
Linewidth Broadening: The Zero Phonon Line (ZPL) linewidth (FWHM) increases monotonically with temperature, evolving from a Gaussian lineshape (spectral diffusion dominated, <50 K) to a Lorentzian lineshape (phonon dominated, >125 K).
Indistinguishability Challenge: High-temperature FWHM values (up to 7.12 meV at 270 K) result in broad ZPLs, posing a significant challenge for generating indistinguishable photons required for quantum communication and computation.
Diamond as Alternative: The findings highlight the need for alternative, highly stable host materials. 6CCVD’s high-purity Single Crystal Diamond (SCD) offers a platform with distinct, well-characterized phonon coupling mechanisms, potentially enabling narrower, more stable ZPLs for integrated quantum devices.

Technical Specifications

The following hard data points were extracted from the experimental results concerning GaN SPE performance and material properties:

Parameter	Value	Unit	Context
Temperature Range Studied	10 - 270	K	Range for ZPL emission spectra measurement.
ZPL Wavelength Range	600 - 700	nm	Observed range for GaN SPEs.
Emitter E3 Center Energy (10 K)	1916	meV	Corresponds to 650.1 nm.
Emitter E4 Center Energy (10 K)	1820.2	meV	Corresponds to 684.5 nm.
Low-T FWHM (E3, 10 K)	0.88	meV	Gaussian component (spectral diffusion limited).
High-T FWHM (E3, 270 K)	7.12	meV	Lorentzian component (phonon limited).
Low-T FWHM (E4, 10 K)	0.72	meV	Gaussian component (spectral diffusion limited).
High-T FWHM (E4, 270 K)	6.82	meV	Lorentzian component (phonon limited).
Extracted Optical Phonon Energy (ħω_op)	19 ± 0.5	meV	Matches GaN E₂(low) zone center energy (~18 meV).
Saturation Pump Power (P_sat)	650	µW	Measured without Solid Immersion Lens (SIL).
Photon Collection Enhancement	4 - 5	Factor	Achieved using 2.5 µm radius hemispherical SIL.
Second Order Correlation (g⁽²⁾(0))	0.17 (E3), 0.19 (E4)	N/A	Confirms single-photon emission (< 0.5).
GaN Layer Thickness	~4	µm	Semi-insulating GaN epitaxial layer.
Substrate Thickness	430	µm	Sapphire substrate.

Key Methodologies

The experimental approach combined advanced material growth, nanofabrication, and cryogenic optical spectroscopy to characterize the GaN SPEs:

Material Growth: Semi-insulating GaN epitaxial layers (~4 µm thick, Ga-polar) were grown via Hydride Vapor Phase Epitaxy (HVPE) on 430 µm thick sapphire substrates.
Solid Immersion Lens (SIL) Fabrication: Hemispherical SILs (radius 2.5 µm) were fabricated on top of individual SPEs using Focused Ion Beam (FIB) milling to enhance photon collection efficiency.
Surface Charge Mitigation: A 30 nm Al layer was sputtered prior to FIB milling to prevent ion beam deflection due to surface charge accumulation, followed by a wet etch to remove the Al.
Optical Excitation: A custom confocal scanning microscope was used, employing a 532 nm pump laser for excitation.
Photoluminescence (PL) Collection: The collected PL was split 50:50 into a spectrometer and a Hanbury-Brown and Twiss setup (using PMA hybrid 40 detectors and a MultiHarp150 correlator) for g⁽²⁾(τ) measurements.
Cryogenic Spectroscopy: Samples were mounted in a cryostat for temperature-dependent measurements (10 K to 270 K). A 0.7 NA objective with a correction collar was used for cryogenic PL collection.
Data Analysis: ZPL spectra were fitted using a Voigt function (convolution of Gaussian and Lorentzian components) to extract the temperature-dependent FWHM (f_V).

6CCVD Solutions & Capabilities

The research highlights the critical need for high-quality materials and precise engineering to manage phonon interactions and achieve narrow ZPLs for quantum applications. 6CCVD specializes in providing the foundational diamond materials necessary to advance this field, offering superior alternatives to GaN for specific SPE requirements.

Applicable Materials for Quantum Emitters

The challenges faced by GaN SPEs (broad ZPL due to optical phonon coupling) underscore the advantages of diamond, where SPEs like the Nitrogen-Vacancy (NV^-) or Silicon-Vacancy (SiV^-) centers exhibit different, often more controllable, dephasing mechanisms.

Application Requirement	6CCVD Material Recommendation	Technical Rationale
High Coherence SPE Host	Optical Grade Single Crystal Diamond (SCD)	Essential for minimizing spectral diffusion (Gaussian component) and achieving long coherence times (T₂). Our SCD features Ra < 1nm polishing, crucial for high-fidelity optical interfaces.
Integrated Quantum Devices	High Purity Polycrystalline Diamond (PCD)	Available in large formats (up to 125mm diameter) for scaling integrated photonic circuits and control electronics, offering superior thermal management compared to GaN/Sapphire.
Electrically Addressable SPEs	Boron-Doped Diamond (BDD)	Customizable doping levels allow for the creation of p-n junctions or conductive layers necessary for electrical injection or Stark shift control of SPEs.

Customization Potential for Advanced Integration

The paper utilized focused ion beam (FIB) milling to create 2.5 µm radius Solid Immersion Lenses (SILs) on the GaN surface. Replicating or extending this level of integration requires highly precise, customizable substrates.

Custom Dimensions and Thickness: 6CCVD supplies SCD and PCD plates/wafers with custom dimensions. We offer precise thickness control for SCD (0.1 µm to 500 µm) and PCD (0.1 µm to 500 µm), enabling the creation of thin membranes or optimized layers for subsequent nanofabrication (e.g., etching, FIB milling for SILs).
High-Precision Polishing: Our industry-leading polishing (Ra < 1nm for SCD, Ra < 5nm for inch-size PCD) ensures an atomically smooth surface, which is vital for minimizing scattering losses and achieving high-quality optical interfaces required for SIL fabrication and high-NA objectives.
Integrated Metalization: 6CCVD offers internal metalization services (Au, Pt, Pd, Ti, W, Cu). This capability is crucial for integrating control electronics directly onto the diamond substrate, supporting the development of integrated quantum platforms that require electrical addressing or heating elements.

Engineering Support

6CCVD’s in-house PhD team specializes in the material science of wide bandgap semiconductors for quantum applications. We can assist researchers and engineers in selecting the optimal diamond material (SCD, PCD, or BDD) and crystal orientation to minimize phonon coupling and spectral diffusion for similar Single-Photon Emitter (SPE) projects.

We offer global shipping (DDU default, DDP available) to ensure rapid delivery of custom-engineered diamond solutions worldwide.

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

View Original Abstract

Abstract Single-photon defect emitters (SPEs), especially those with magnetically and optically addressable spin states, in technologically mature wide bandgap semiconductors are attractive for realizing integrated platforms for quantum applications. Broadening of the zero phonon line (ZPL) caused by dephasing in solid state SPEs limits the indistinguishability of the emitted photons. Dephasing also limits the use of defect states in quantum information processing, sensing, and metrology. In most defect emitters, such as those in SiC and diamond, interaction with low-energy acoustic phonons determines the temperature dependence of the dephasing rate and the resulting broadening of the ZPL with the temperature obeys a power law. GaN hosts bright and stable single-photon emitters in the 600-700 nm wavelength range with strong ZPLs even at room temperature. In this work, we study the temperature dependence of the ZPL spectra of GaN SPEs integrated with solid immersion lenses with the goal of understanding the relevant dephasing mechanisms. At temperatures below ~ 50 K, the ZPL lineshape is found to be Gaussian and the ZPL linewidth is temperature independent and dominated by spectral diffusion. Above ~ 50 K, the linewidth increases monotonically with the temperature and the lineshape evolves into a Lorentzian. Quite remarkably, the temperature dependence of the linewidth does not follow a power law. We propose a model in which dephasing caused by absorption/emission of optical phonons in an elastic Raman process determines the temperature dependence of the lineshape and the linewidth. Our model explains the temperature dependence of the ZPL linewidth and lineshape in the entire 10-270 K temperature range explored in this work. The ~ 19 meV optical phonon energy extracted by fitting the model to the data matches remarkably well the ~ 18 meV zone center energy of the lowest optical phonon band ( $$E_{2}(low)$$ <mml:math xmlns:mml=“http://www.w3.org/1998/Math/MathML”><mml:mrow><mml:msub><mml:mi>E</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>l</mml:mi><mml:mi>o</mml:mi><mml:mi>w</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math> ) in GaN. Our work sheds light on the mechanisms responsible for linewidth broadening in GaN SPEs. Since a low energy optical phonon band ( $$E_{2}(low)$$ <mml:math xmlns:mml=“http://www.w3.org/1998/Math/MathML”><mml:mrow><mml:msub><mml:mi>E</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>l</mml:mi><mml:mi>o</mml:mi><mml:mi>w</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math> ) is a feature of most group III-V nitrides with a wurtzite crystal structure, including hBN and AlN, we expect our proposed mechanism to play an important role in defect emitters in these materials as well.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41598-023-35003-z