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Thermal analysis of GaN-based photonic membranes for optoelectronics

This paper demonstrates a non-invasive, all-optical method for characterizing the in-plane thermal conductivity of GaN-based photonic membranes by combining two-laser Raman thermometry with photoluminescence measurements to accurately account for all laser-induced power balances.

Original authors: Wilken Seemann, Mahmoud Elhajhasan, Julian Themann, Katharina Dudde, Guillaume Würsch, Jana Lierath, Joachim Ciers, Åsa Haglund, Nakib H. Protik, Giuseppe Romano, Raphaël Butté, Jean-François Carlin

Published 2026-02-11

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CC BY 4.0

Original authors: Wilken Seemann, Mahmoud Elhajhasan, Julian Themann, Katharina Dudde, Guillaume Würsch, Jana Lierath, Joachim Ciers, Åsa Haglund, Nakib H. Protik, Giuseppe Romano, Raphaël Butté, Jean-François Carlin, Nicolas Grandjean, Gordon Callsen

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Tiny Heat Map: Solving the Mystery of the "Glow-in-the-Dark" Membrane

Imagine you are a tiny detective trying to measure how heat moves through a single sheet of tissue paper. But there’s a catch: this tissue paper is actually a high-tech, microscopic semiconductor membrane (made of a material called GaN) used to build the lasers in your electronics.

Even worse, this "tissue paper" is a tiny light bulb. Every time you try to shine a light on it to measure its temperature, it starts glowing brightly, blinding your sensors and making it impossible to tell if the heat you're seeing is from your measurement tool or the membrane itself.

This paper describes how a team of international scientists built a "special pair of sunglasses" and a "double-laser trick" to solve this problem.

1. The Problem: The Blinding Light Bulb

In the world of microchips, we use "membranes"—ultra-thin sheets of material—to make things like tiny lasers (VCSELs). To make these devices better, engineers need to know exactly how well they move heat away. If heat gets trapped, the device melts or fails.

Usually, scientists use a single laser to heat a spot and then "listen" to the Raman signal (a type of light vibration) to see how hot it got. But these membranes are photonic, meaning they are designed to emit light.

The Analogy: Imagine trying to measure the temperature of a glowing ember by shining a flashlight on it. The light from the ember is so bright that it washes out your flashlight, making your thermometer useless. This is what happens to the scientists; the membrane's own light "blinds" the measurement.

2. The Solution: The Two-Laser Dance (2LRT)

To fix this, the researchers used a technique called Two-Laser Raman Thermometry (2LRT).

Instead of using one laser to do two jobs, they split the work between two specialized lasers:

The Heater (The Stove): One laser acts like a tiny, controlled stove, heating up a specific spot on the membrane.
The Probe (The Thermometer): A second laser stays separate. Its only job is to "peek" at the vibrations of the atoms to read the temperature.

By keeping the "stove" and the "thermometer" separate, they can move the thermometer around the heated spot to see how the heat spreads out.

The Analogy: It’s like trying to study how heat moves through a piece of bread. Instead of using a single candle (which is both the heat and the light), you use a heating element to warm the bread and a separate, tiny infrared sensor to map out the heat patterns without adding any extra light of its own.

3. The "Power Balance" Accounting

The scientists also had to become expert accountants. When they shine a laser on the membrane, the energy doesn't just turn into heat. Some of it bounces off (reflection), some goes through (transmission), some scatters like a disco ball (scattering), and some is turned into light by the membrane (re-emission).

If you don't account for all that "missing" energy, your math will be wrong. The team used another technique (Photoluminescence) to calculate exactly how much light the membrane was "stealing" and turning into brightness, allowing them to calculate the true amount of heat being generated.

4. What They Discovered: The "Rough Road" Effect

The team tested different membranes to see how their surfaces affected heat flow. They found that:

Smooth surfaces allow heat to zip through easily.
Rough surfaces (or adding extra layers) act like speed bumps or roadblocks for heat.

They even found that if the etching process (how the membrane is made) creates tiny "channels" or cracks in a certain direction, the heat travels faster in one direction than the other—like water flowing faster down a groove in a wooden plank than across the grain.

Why does this matter?

By mastering this "all-optical" way to see heat, scientists can now design much more powerful, efficient, and reliable tiny lasers and electronics. They have essentially created a high-definition thermal camera for the microscopic world, allowing us to see how heat "flows" through the building blocks of future technology.

Technical Summary: Thermal Analysis of GaN-based Photonic Membranes for Optoelectronics

1. Problem Statement

The development of high-performance optoelectronic devices, such as Vertical-Cavity Surface-Emitting Lasers (VCSELs), increasingly relies on semiconductor membranes (SMs). These membranes are often nanostructured (e.g., containing quantum wells) to serve as light sources. However, characterizing the thermal properties of these membranes presents two major challenges:

Non-invasiveness: Standard thermal measurement techniques often interfere with the delicate nanostructures or require physical contact that is impossible for free-standing membranes.
Optical Interference: Photonic membranes are designed to be bright light emitters. In conventional One-Laser Raman Thermometry (1LRT), a single laser acts as both the heat source and the temperature probe. Because the laser-induced heating is only monitored at the exact heat spot, it is difficult to determine the actual volume of heating or the absorbed power, especially when the sample's own luminescence (photoluminescence) interferes with the Raman signal.

2. Methodology

The researchers developed an all-optical, non-invasive approach using Two-Laser Raman Thermometry (2LRT) combined with Time-Resolved Photoluminescence (TRPL).

Two-Laser Raman Thermometry (2LRT): The setup uses two independent lasers: a continuous-wave UV heating laser ( $\lambda_{heat} = 325\text{ nm}$ ) and a visible probe laser ( $\lambda_{probe} = 532\text{ nm}$ ). This allows the heating spot to be fixed while the probe laser scans the surrounding area, enabling the mapping of temperature gradients across the membrane.
Power Balance Disentanglement: To accurately determine the heat power ( $P_{heat}$ ), the team accounted for the entire laser power balance, including reflected ( $P_{ref}$ ), transmitted ( $P_{trans}$ ), scattered ( $P_{scat}$ ), and re-emitted ( $P_{rad}$ ) light.
TRPL Integration: Because the membranes are highly emissive, the re-emitted light ( $P_{rad}$ ) could lead to errors in thermal calculations. The team used TRPL to measure the internal quantum efficiency and decay dynamics of the InGaN quantum wells (QWs), allowing them to quantify the re-emitted power and ensure the accuracy of the absorbed power calculation.
Numerical Modeling: The experimental temperature maps were analyzed using a 3D heat transport equation solver (COMSOL) and compared against ab initio simulations of the linearized phonon Boltzmann transport equation (BTE) to extract the in-plane thermal conductivity ( $\kappa_{in-plane}$ ).

3. Key Contributions

Advanced Characterization Protocol: Demonstrated a robust method to decouple heating from probing, specifically designed for highly emissive, nanostructured photonic membranes.
Thermal Imaging Capability: Provided high-spatial-resolution temperature maps that allow for the direct visualization of thermal anisotropy and the impact of structural variations.
Comprehensive Power Accounting: Established a methodology to correct for the "optical noise" (re-emitted light) inherent in photonic devices, which is critical for accurate thermometry at cryogenic temperatures.

4. Results

Thermal Conductivity Measurement: For the best-performing membrane (Sample 2), the measured $\kappa_{in-plane}$ was $165^{+16}_{-14}\text{ W m}^{-1}\text{K}^{-1}$ . This showed excellent agreement with the ab initio simulation result of $177\text{ W m}^{-1}\text{K}^{-1}$ .
Impact of Surface Roughness: The study found that backside roughness significantly affects thermal transport. Sample 2 (smooth backside) exhibited the highest conductivity. In contrast, Sample 3, which utilized AlN/AlInN etch-stop layers to achieve an ultra-smooth surface, showed a reduction in $\kappa_{in-plane}$ by more than half. This is attributed to increased phonon scattering at the additional interfaces and acoustic mismatch.
Thermal Anisotropy: The 2LRT maps revealed thermal anisotropy in the membranes, likely caused by radial etch channels formed during the electrochemical etching process.
Validation of the Method: The team confirmed that for temperatures above $100\text{ K}$ , the re-emitted light ( $P_{rad}$ ) is a negligible fraction of the total power, but its inclusion is vital for accurate measurements at cryogenic temperatures.

5. Significance

This work provides a vital toolset for the semiconductor industry, particularly for the design of UV and blue VCSELs. By enabling the precise measurement of thermal conductivity in nanostructured membranes, researchers can better optimize heat dissipation in high-power optoelectronic devices. Furthermore, the methodology paves the way for studying advanced phonon transport regimes (such as non-Fourier heat conduction) at cryogenic temperatures, which is essential for the next generation of quantum communication and computing technologies.