What is PL Spectroscopy?
Photoluminescence (PL) spectroscopy is a non-destructive optical characterization technique that probes the electronic structure of materials by measuring the wavelength and intensity of light emitted following laser excitation at a specified wavelength. The resulting emission spectrum provides insights into the material properties. Configured with the geometry of a confocal microscope, PL microscopy enables high-resolution mapping of these properties to assess material uniformity.
Why use PL Spectroscopy?
Photoluminescence (PL) analysis provides a rapid, non-destructive way to evaluate material quality and uniformity. Analyzing PL spectra can reveal information about optical emission characteristics, defects, impurities, and recombination behavior that can impact device performance. Confocal PL microscopy enables high-resolution mapping of the uniformity of these properties along a sample, across a wafer, and through the depth below transparent surfaces.
- Multiple laser options – Available excitation wavelengths across near-UV, visible, and near-IR
- Sample mapping – across 300mm wafers or small, high-resolution areas
- Broad material compatibility – semiconductors, 2D materials, perovskites, and quantum materials.
Identify process-induced changes
Detect variations caused by fabrication, annealing, or packaging processes.
Map wafer non-uniformity
Map spatial variations across thin films, devices, and wafers.
Enable targeted root-cause analysis
Localize material and process variations that can be further investigated with complementary techniques.
Working Principle
Photoluminescence (PL) spectroscopy measures the light emitted from a material after it is excited with a laser. Different excitation wavelengths can be used to optimize the measurement for specific material systems and optical transitions. The absorbed energy promotes electrons to higher-energy states, and as these electrons relax and recombine, they emit photons with characteristic wavelengths. In a confocal microscope geometry, the emitted light is collected with high spatial resolution, enabling localized analysis of material quality, optical properties, defects, strain, and recombination behavior.
Equipment Used for PL
Our photoluminescence and Raman spectroscopy capabilities span multiple instrument platforms, allowing us to tailor laser excitation wavelength, confocal optics, and measurement conditions to each sample and application. We also integrate these spectroscopy studies with complementary analytical techniques to deliver a more complete understanding of a sample’s composition, structure, defects, and material properties.
- Confocal Raman and photoluminescence (PL) microscope designed for advanced materials and semiconductor characterization
- Excitation sources:
- 355 nm.
- 532 nm.
- 300 mm wafer capability: Motorized vacuum wafer stage supports automated mapping and characterization of semiconductor wafers up to 300 mm in diameter.
- TrueSurface™ Topography tracking: Maintains optimal focus across rough, curved, patterned, and non-planar samples by automatically following surface topography.
- Vibration-isolated platform: enables high stability measurements.
- Configurable spectral resolution: Multiple spectrometer grating options can support both wide spectral range acquisitions and high-resolution characterization.
- Multiple Excitation Lasers:
- 455 nm.
- 532 nm.
- 785 nm.
- Laser Power with precision controls: 0.1 mW power increments.
- Spatial Resolution: Better than 0.5 micron.
- Confocal Depth Resolution: Better than 2 micron.
- Maximum image area: 101.6 mm x 76.2 mm.
Key Differentiators
- Excitation in the UV, VIS, and NIR for various sample types.
- Micron scale information can be obtained.
- Topography-aware measurements – TrueSurface™ tracking maintains focus across patterned and non-planar semiconductor structures.
- Integrated analytical workflows – PL results can guide targeted SEM, FIB, TEM, SIMS, XPS, and other follow-on analyses.
Strengths
- Non-destructive characterization.
- Can measure samples through glass and other transparent containers.
- Confocal microscopy provides depth resolution for non-destructive depth profiling into semi-transparent surfaces.
Limitations
- Laser confocal microscope configuration probes a small spot/volume, around 1um, which requires multiple spots to adequately sample heterogeneous materials. Fast mapping of many particles or locations can be beneficial.
- For materials with strong absorption at the laser wavelength, this technique may only probe the surface.
- PL spectroscopy can reveal the presence and distribution of defects, but complimentary techniques will be needed to determine their exact composition or structure.
Sample Information
Depending on the project, outputs can include raw emission spectra, tables of emission band peak position and intensity, and 1D or 2D scans.
Investigating the uniformity of a GaAs wafer by mapping the shift in the luminescence peak position.
Examining non-uniformity in a Diamond sample, comparing the map of the intensity of the Raman peak (left) with the map of the intensity of a specific diamond luminescence band (right).
What we accept:
Sample must be stable under laser irradiation; reduced power can be used to mitigate. Strongly absorbing samples can be sensitive.
Use Cases
Semiconductors
Defect mapping, wafer uniformity assessment, epitaxial layer thickness, doping concentrations, process-induced damage evaluation, strain analysis, and failure analysis.
Power Electronics
Defect characterization in SiC and GaN devices, material quality assessment, process optimization, thermal characterization under electrical bias, failure analysis and reliability investigations.
Solar Cells
Defect characterization, material quality assessment, emission uniformity mapping, process optimization, and failure analysis of silicon, III-V, perovskite, and thin-film photovoltaic materials.
2D Materials
Material quality assessment, defect characterization, emission uniformity mapping, and process optimization for atomically thin semiconductor materials and heterostructures.
Photonics & Optoelectronics
Optical emission characterization and development of LEDs, lasers, waveguides, and other photonic materials.
Complementary Techniques
- Raman Spectroscopy: performed on the same instrument platform, Raman spectroscopy complements PL by providing information on material identification, crystal phase, stress/strain, crystallinity, and molecular bonding.
- FTIR & AFM-IR: provide complementary molecular identification and chemical bonding information, particularly for polymers, organics, and other infrared-active materials.
- SEM-EDS: correlates PL features with surface morphology while providing spatially resolved elemental composition.
- XRD: identifies crystal structure, phase composition, crystallinity, and residual stress to complement optical emission measurements.
- XPS: Characterizes surface chemistry, oxidation state, and chemical bonding to help explain changes in photoluminescence behavior.
- TEM: reveals crystal defects, dislocations, interfaces, and atomic-scale structure responsible for observed photoluminescence characteristics.
Quantifies heat flow for material optimization. Explore
Characterizes thermal and mechanical properties of soft materials. Explore
Rapid, non-destructive molecular fingerprinting across materials. Explore
Identifies and quantifies small organic molecules in mixtures. Explore
Measures material dimension changes with temp, time, or force. Explore
Why Choose Covalent?
Covalent combines advanced photoluminescence instrumentation with a broad portfolio of materials characterization and failure analysis services. Our confocal PL/Raman spectroscopy platforms support multiple excitation wavelengths and configurable spectral acquisition, enabling measurements to be tailored to your specific application.
Additionally, Covalent helps customers understand the underlying cause of observed behavior. PL results can be integrated with our in-house spectroscopy, profilometry, SEM, XPS, FIB, TEM, and other analytical techniques to accelerate root-cause investigations, process optimization, and materials development efforts.
Frequently Asked Questions
Identifying the right test can be complex, but it doesn’t have to be complicated.
Here are some questions we are frequently asked.
What information does PL provide?
PL analysis can reveal optical emission characteristics, material quality, defect-related luminescence, strain effects, Emission spectra can be reported along with mapping of emission spectra features (e.g. defect luminescence intensity, peak shift due to strain, etc.) to assess spatial variations.
What excitation wavelengths are available?
Across our confocal PL/Raman spectroscopy platforms, laser excitation wavelengths of 355, 405, 532, 638, and 785 nm are available to support a range of semiconductor and advanced materials applications.
Is PL analysis non-destructive?
Yes. PL is generally considered a non-destructive optical characterization technique that preserves the sample for additional testing and analysis. For strongly absorbing or laser-sensitive materials, excitation power can be reduced to minimize localized heating or damage.
What spatial resolution can be achieved?
Depending on the objective and measurement conditions, photoluminescence measurements can achieve spatial resolution of a few microns.
What size regions can be mapped?
Automated measurements can characterize variability at selected locations across individual devices, thin films, coupons, packaged components, and semiconductor wafers up to 300 mm.