Cryogenic Infrared Photoluminescence Measurement System
MIRPL-532-LT
Cryogenic Infrared Photoluminescence Measurement System MIRPL-532-LT
 
Photoluminescence (PL) is a nondestructive technique for evaluating electronic states, defects, and impurity levels in materials. Light is used to excite electrons, and the luminescence emitted as the excited electrons return to the ground state is spectrally analyzed and detected. Analysis of emission peak energies, intensities, and spectral widths provides information about the material's band structure, lattice perfection, and trace impurity distribution.

This system is equipped with a 532 nm laser as its excitation source and is primarily designed to measure emission in the infrared region. It is suitable for characterizing infrared light-emitting devices and photodetectors operating in the near-infrared wavelength range.

The system incorporates a cryostat that cools the sample to 4 K, enabling infrared photoluminescence measurements under cryogenic conditions. This allows detailed analysis of fine emission features and excitonic emission that are difficult to observe at room temperature.

Measurements can provide the following information:
  • Determination of bandgap energy (for materials with medium or small bandgaps)
  • Detection of bound-exciton and free-exciton emission
  • Estimation of donor and acceptor impurity concentrations
  • Nondestructive detection of lattice and interface defects
  • Evaluation of epitaxial growth processes
Its noncontact, nondestructive measurement capability makes it an effective characterization tool across a wide range of stages, from materials development to production inspection and reliability evaluation.

■Specifications
Excitation Wavelength 532 nm
Measurement Wavelength Range 600 nm–10,000 nm
Temperature Range 4–300 K
Excitation Wavelength Cleanup Prism-based spectral filtering
Spectrometer FT-IR spectrometer

*Specifications and appearance are subject to change without notice for product improvement.

■Basic Principles of Photoluminescence (PL)

Photoluminescence (PL) is an optical technique that detects and analyzes the light (photons) emitted when a material is illuminated with high-energy excitation light. Electrons in the valence band are excited to higher energy levels, such as the conduction band, and subsequently return to the ground state, emitting light in the process. This phenomenon is widely used to evaluate electronic states and assess material quality, particularly in semiconductors and phosphors.

As illustrated in the diagram, electrons at the ground-state energy level, often the top of the valence band, absorb energy from incident excitation light and are excited to a higher energy level, typically the conduction band. This process creates electron–hole pairs.

Diagram illustrating the basic principles of photoluminescence

However, excited electrons do not necessarily recombine immediately through a radiative transition. In many cases, they undergo relaxation processes such as those described below before recombination occurs.

  • Non-radiative Relaxation: Excited electrons lose energy as heat through interactions with lattice vibrations (phonons). No photons are emitted during this process.
  • Trapping by Defect or Impurity Levels: Electrons may be temporarily trapped at intermediate energy levels formed by defects or impurities in the crystal lattice. When they subsequently return to the ground state, light with an energy lower than the bandgap may be emitted.

Because of these processes, the emission spectrum observed in PL measurements does not necessarily directly reflect the energy difference between the incident photons and the bandgap. It is also influenced by the various relaxation pathways within the material.

PL is a highly sensitive, noncontact, nondestructive characterization technique. In addition to analyzing band structures and energy levels, it can provide comprehensive information about crystallinity, defect density, and impurity distribution. It is particularly important in materials development and quality control for semiconductor devices, as well as in the characterization of quantum dots and low-dimensional structures.

■What Is Internal Quantum Efficiency (IQE)?

Internal quantum efficiency (IQE) indicates the fraction of carriers (electrons and holes) generated in a semiconductor that recombine radiatively, emitting light.

Definition:
IQE = Number of carriers that recombine radiatively / Total number of recombined carriers

IQE is an important parameter for evaluating the effects of non-radiative recombination in materials and structures.

■Evaluating Relative Internal Quantum Efficiency (IQE) Using Low-Temperature PL Measurements

In this method, the PL intensity at low temperature, typically around 10 K, is treated as the maximum emission due to radiative recombination. Relative internal quantum efficiency (IQE) is calculated by comparing this intensity with the PL intensity at room temperature (300 K).

Assumptions:
• Non-radiative recombination is suppressed at low temperature, and radiative recombination dominates.
• The low-temperature PL intensity can therefore be regarded as the ideal maximum emission intensity.
• At room temperature, increased non-radiative recombination reduces the PL intensity.

Approximate formula for relative IQE:
IQE300K = I300K / I10K

where:
I300K: Integrated PL intensity at room temperature
I10K: Integrated PL intensity at low temperature (maximum emission)

The IQE obtained using this method is a relative value rather than an absolute value, but it is highly useful for evaluating the effects of temperature on non-radiative recombination.

■Steps for Calculating Internal Quantum Efficiency (IQE) Using Low-Temperature PL Measurements
  1. Measure Temperature-Dependent PL Spectra
    Cool the sample using a cryostat and measure photoluminescence (PL) spectra at multiple temperatures from 10 K or below to room temperature (300 K).
  2. Calculate the Integrated PL Intensity
    Integrate the PL spectrum over wavelength at each temperature to obtain the total PL intensity (integrated intensity).
    I(T) = ∫ PL(λ, T) dλ
  3. Calculate Relative IQE
    Calculate relative IQE from the ratio of the integrated PL intensity at room temperature (300 K), I300K, to the intensity at low temperature (10 K), I10K.

    Formula:
    IQE300K = I300K / I10K

    where:
    I300K: Integrated PL intensity at room temperature
    I10K: Integrated PL intensity at low temperature (maximum emission)

    This ratio serves as an indicator of the effects of temperature on non-radiative recombination. The IQE obtained using this method is a relative assessment rather than an absolute value.
  4. Evaluate Non-radiative Recombination (Optional)
    Perform an Arrhenius analysis of the temperature dependence of PL intensity to extract the activation energy of non-radiative recombination.
    I(T) = I0 / (1 + C × exp(-Ea / kT))
    where:
    Ea: Activation energy of the non-radiative process
    k: Boltzmann constant

■What Is External Quantum Efficiency (EQE)?

External quantum efficiency (EQE) is the ratio of the number of photons emitted outside a semiconductor device (such as an LED or solar cell) to the number of injected or generated carriers (electrons).

Definition:
EQE = Number of photons emitted outside the device / Number of injected electrons

EQE represents the actual light-emission efficiency of a device. It is measured using methods such as absolute PL measurements with an integrating sphere or measurements under electrical excitation.

■Measuring External Quantum Efficiency (EQE) with an Integrating Sphere (2π Measurement)

External quantum efficiency (EQE) can be calculated by measuring the light emitted by a device (spectral radiant flux) using an integrating sphere and comparing it with the number of injected electrons. The following method uses the radiant flux measured over the forward hemisphere (2π solid angle).

  1. Measure Spectral Radiant Flux with an Integrating Sphere
    Use an integrating sphere to measure the spectral radiant flux Φλ [W/nm] emitted by the device. Calculate the number of emitted photons from the measured radiant flux at each wavelength.

    Photon calculation formula:
    Nphoton = ∫ (Φλ × λ) / (h × c) dλ
    where:
    λ: Wavelength [m]
    h: Planck constant
    c: Speed of light
  2. Correct for Emission in All Directions (4π Conversion)
    The integrating sphere measures only the light emitted into the forward hemisphere (2π solid angle). Assuming isotropic emission (Lambertian radiation), the total number of photons emitted into all directions can be approximated as twice the measured value.

    Ntotal ≈ 2 × Nphoton
  3. Calculate the Number of Injected Electrons
    Calculate the number of electrons per unit time from the current I [A] supplied to the device.

    Nelectron = I / q
    where q = 1.602 × 10-19 C (magnitude of the electron charge)
  4. Calculate External Quantum Efficiency (EQE)
    Calculate EQE from the ratio of the number of photons emitted in all directions to the number of injected electrons.

    EQE = (2 × Nphoton) / Nelectron
    or:
    EQE = (2 × Nphoton × q) / I

    This allows the actual external light-emission efficiency of the device to be evaluated using integrating sphere measurements.

■Relationship Between External Quantum Efficiency (EQE) and Internal Quantum Efficiency (IQE)

EQE and IQE are related by the following equation:

EQE = IQE × ηout

where:
ηout is the light extraction efficiency,
representing the fraction of photons generated within the light-emitting layer that actually escape from the device.

Light extraction efficiency is affected by total internal reflection caused by refractive index differences, device structures such as reflective layers and surface patterning, and optical absorption.

IQE describes the light-emission efficiency inside the material, while EQE describes the efficiency of light actually emitted outside the device. The maximum IQE is 100%, whereas EQE is typically lower.