Cryogenic Near-Infrared Photoluminescence Measurement System
MNIRPL-532-LT
Cryogenic Near-Infrared Photoluminescence Measurement System MNIRPL-532-LT
 
Photoluminescence (PL) is a technique for non-destructively evaluating the electronic states, defects, and impurity levels in a material. It works by irradiating the material with light to excite electrons, then spectrally resolving and detecting the light (luminescence) emitted as those excited electrons return to the ground state. By analyzing the energy position, intensity, and width of the emission spectrum, the band structure, lattice perfection, trace impurity distribution, and other properties of the material can be determined.

This system uses a 532 nm laser as the excitation light source and is designed mainly to measure emission in the near-infrared region. It is well suited to characterizing devices that emit in the near-infrared region and photodetectors that operate in the near-infrared wavelength range.

The system is equipped with a cryostat that cools the sample down to 4 K, creating a cryogenic environment for low-temperature photoluminescence measurements. This enables detailed analysis of fine emission structures and exciton emission that are difficult to observe at room temperature.

Measurements can provide information such as:
  • Calculation of band gap energy (for small- to medium-band-gap materials)
  • Detection of emission from bound and free excitons
  • Estimation of donor and acceptor impurity concentrations
  • Non-destructive detection of lattice and interface defects
  • Evaluation of epitaxial growth processes
Because evaluation is non-contact and non-destructive, it is an effective characterization tool across a wide range of stages, from materials development to mass-production inspection and reliability evaluation.

■Specifications
Excitation wavelength 532 nm
Measurement wavelength range 1000 nm to 3000 nm
Temperature range 4–350 K
Sample holder Sample size: up to 10 mm square
Excitation wavelength cleanup Prism dispersion method
Spectrometer Optical configuration: Czerny-Turner mount
Focal length: 250 mm
Aperture ratio: F/4.3
Detector InAs detector
Signal processing Lock-in detection

■PL Spectrum of a Ge Substrate (Room Temperature)
PL spectrum of a Ge substrate (room temperature)

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

■Basic Principle of Photoluminescence (PL)

Photoluminescence (PL) is an optical technique in which a material is irradiated with high-energy light (excitation light), exciting electrons in the valence band to higher energy levels such as the conduction band, and the light (photons) emitted as they return to the ground level is detected and analyzed. This phenomenon is widely used to evaluate electronic states and diagnose material quality, mainly in semiconductor materials and phosphors.

As shown in the figure, electrons at the ground level (in most cases, the top of the valence band) absorb the energy of the externally applied excitation light and are excited to a higher energy level (typically the conduction band). This creates electron-hole pairs.

Diagram of the basic principle

However, excited electrons do not necessarily recombine immediately and undergo radiative transitions. In many cases, they pass through relaxation mechanisms such as those shown below before recombining.

  • Non-radiative relaxation: A process in which excited electrons lose energy as heat through interaction with lattice vibrations (phonons). No photons are emitted in this process.
  • Trapping by defect or impurity levels: Electrons may be temporarily captured at intermediate levels formed by defects or impurities in the crystal lattice, and when they return from there to the ground level, light with energy lower than the band gap 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 band gap, but is influenced by the various relaxation pathways within the material.

PL is a highly sensitive evaluation technique that can be applied to samples without contact or damage. In addition to analyzing band structures and energy levels, it can comprehensively provide information on crystallinity, defect density, impurity distribution, and more. It is regarded as an especially important technique for materials development and quality control of semiconductor devices, and for the evaluation of quantum dots and low-dimensional structures.

■What Is Internal Quantum Efficiency (IQE)?

Internal quantum efficiency (IQE) is a metric that indicates the fraction of carriers (electrons and holes) generated in a semiconductor that recombine radiatively (are emitted as light).

Definition:
IQE = number of radiatively recombined carriers / total number of recombined carriers

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

■Evaluating Relative Internal Quantum Efficiency (IQE) by Low-Temperature PL Measurement

In this method, the PL intensity at low temperature (typically around 10 K) is regarded as the maximum emission from radiative recombination, and the relative internal quantum efficiency (IQE) is calculated by comparing it with the PL intensity at room temperature (300 K).

Assumptions:
・At low temperature, non-radiative recombination is suppressed and radiative recombination is dominant.
・Therefore, the PL intensity at low temperature can be regarded as the ideal (maximum) emission intensity.
・At room temperature, non-radiative recombination increases and the PL intensity decreases.

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 by this method is a relative value, not an absolute value, but it is very effective for evaluating the influence of temperature-dependent non-radiative recombination.

■Steps for Calculating Internal Quantum Efficiency (IQE) by Low-Temperature PL Measurement
  1. Temperature-dependent measurement of PL spectra
    Cool the sample using a cryostat and measure photoluminescence (PL) spectra at multiple temperatures from 10 K (or below) up to room temperature (300 K).
  2. Calculation of integrated PL intensity
    Integrate the PL spectrum at each temperature over wavelength to obtain the total PL intensity (integrated intensity).
    I(T) = ∫ PL(λ, T) dλ
  3. Calculation of relative IQE
    Calculate the 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 influence of temperature-dependent non-radiative recombination. Note that the IQE obtained by this method is not an absolute value but a relative evaluation.
  4. Evaluation of non-radiative recombination (optional)
    To extract the activation energy of non-radiative recombination from the temperature dependence of the PL intensity, perform an analysis based on the Arrhenius equation.
    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 a metric that indicates the ratio of the number of photons emitted externally from a semiconductor device (e.g., an LED or solar cell) to the number of carriers (electrons) injected or generated.

Definition:
EQE = number of photons emitted externally / number of injected electrons

EQE represents the actual emission efficiency of the device, and is measured by absolute PL measurement using an integrating sphere or by measurement under electrical drive.

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

External quantum efficiency (EQE) can be calculated by measuring the light emitted from a device (spectral radiant flux) with an integrating sphere and comparing it with the number of injected electrons. This section describes a method using the radiant flux in the "2π direction (forward hemisphere)" measured by the integrating sphere.

  1. Measurement of spectral radiant flux with an integrating sphere
    Use an integrating sphere to measure the spectral radiant flux Φλ [W/nm] of the light emitted by the device. Calculate the number of emitted photons from the measured radiant flux at each wavelength.

    Formula for the number of photons:
    Nphoton = ∫ (Φλ × λ) / (h × c) dλ
    where
    λ: wavelength [m]
    h: Planck constant
    c: speed of light
  2. Correction to all directions (4π conversion)
    The integrating sphere measures only the light in the forward hemisphere (2π solid angle). Assuming isotropic (Lambertian) emission, the number of photons emitted into all space can be approximated as twice the measured value.

    Ntotal ≈ 2 × Nphoton
  3. Calculation of the number of injected electrons
    Determine the number of electrons per unit time from the current I [A] flowing through the device.

    Nelectron = I / q
    where q = 1.602 × 10-19 C (elementary charge)
  4. Calculation of 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

    In this way, the actual external emission efficiency of the device can be evaluated through integrating sphere measurement.

■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,
which indicates the fraction of photons generated in the emitting layer that actually escape from the device.

The extraction efficiency is affected by total internal reflection due to refractive index differences, device structure (reflective layers, patterning), light absorption, and other factors.

IQE is the emission efficiency inside the material, while EQE is the efficiency of the light that "actually exits," including the extraction efficiency. The maximum IQE is 100%, and EQE is normally lower than that.