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Materials Science

Time-Resolved Photoluminescence (TRPL)

Charge carrier dynamics in semiconductors

Charge carrier dynamics in semiconductors are determined by the architecture and function of the respective device and directly reflect the nature and quality of wafer materials. This makes precise and efficient measurement techniques of the free charge carrier lifetime essential for characterizing these systems. For particular classes of semiconductors, the characteristic charge carrier lifetime is highly dependent on the nature and dimensions of the materials and interfaces involved. Furthermore, surface effects, passivation and the energy transfer efficiency of sensitizers as well as the presence of dopants, impurities and defect sites can introduce significant variations in this parameter. Since the photoluminescence of semiconductors is a direct monitor of the charge carrier dynamics, the general methodology of time-resolved photo-luminescence (TRPL) via time-correlated single photon counting (TCSPC) and the periphery technology are highly suited for the analysis of the phenomena that determine fast charge carrier dynamics in a semiconductor. As a result, the mechanism that determines the charge carrier dynamics within a particular system can be characterized directly down to the sub-nanosecond time scale.

Generalized setup of a fluorescence lifetime imaging microscopeTime-Resolved Photoluminescence (TRPL) via Time-Correlated Single Photon Counting (TCSPC) is particularly suited for fast charge carrier dynamics in the nanosecond regime associated with common III-V and II-VI direct semiconductors such as GaAs and CdTe as well as for  the somewhat slower dynamics of the more complex I-III-VI2 CIGS, organic and dye sensitized systems that span the nano- to microsecond domain. In TCSPC, one measures the time between sample excitation by a pulsed laser and the arrival of the emitted photon at the detector. TCSPC requires a defined “start”, provided by the electronics steering the laser pulse or a photodiode, and a defined “stop” signal, realized by detection with single-photon sensitive detectors. The measurement of this time delay is repeated many times to account for the statistical nature of the fluorophores emission. The delay times are sorted into a histogram that plots the occurrence of emission over time after the excitation pulse.

General layout of a fluorescence spectrometerTRPL measurements can be performed with different set-ups - the two most general configurations are either spectrometers or confocal microscopes.

Consequently the essential components of a set-up for measuring fluorescence lifetimes are:

  • pulsed laser source (diode laser, LED or multi-photon excitation)
  • single photon sensitive detector
  • means of separate fluorescence signal from excitation light (monochromator or optical filters)
  • TCSPC unit to measure the time between excitation and fluorescence emission

PicoQuant offers several systems that can be used for TRPL:

FluoTime 300FluoTime 300

High Performance Fluorescence Lifetime Spectrometer

The FluoTime 300 "EasyTau" is a fully automated, high performance photoluminescence lifetime spectrometer with steady-state and phosphorescence option. The system features an ultimate sensitivity with a record breaking 24.000:1 Water Raman SNR. The FluoTime 300 contains the complete optics and electronics for recording TRPL decays by means of Time-Correlated Single Photon Counting (TCSPC) or Multichannel Scaling (MCS). The system is designed to be used with picosecond pulsed diode lasers, LEDs or Xenon lamps. Multiple detector options enable a large range of system configurations. The FluoTime 300 can be used to study photoluminescence between a few picoseconds and several seconds


FluoTime 200FluoTime 200

Modular Fluorescence Lifetime Spectrometer

The FluoTime 200 spectrometer is a high performance photoluminescence lifetime system featuring modular construction, single photon timing sensitivity and research flexibility. It contains the complete optics and electronics for recording TRPL decays by means of Time-Correlated Single Photon Counting (TCSPC). The system can be used with femtosecond or picosecond lasers. With the FluoTime 200, decay times down to a few picoseconds can be resolved.


WaferCheck 150WaferCheck 150

Semiconductor Wafer Analyzer

The WaferCheck system is a complete and easy to use set-up for Time-Resolved Photoluminescence (TRPL) measurements. It can measure the luminescence lifetime from semiconductor materials in a range from picoseconds to microseconds. The luminescence lifetime directly depends on the electron-hole recombination rates which reflect material purity, doping levels, stress etc. The system is specially designed for monitoring GaAs or GaN wafers before processing or in on-line process control.


MicroTime 100 - upright time-resolved confocal microscopeMicroTime 100

Upright time-resolved confocal microscope

The MicroTime 100 is an idea tool for the study of time-resolved photoluminescence of solid samples such as wafers, semiconductors or solar cells. It  can also be used for mapping purposes or to measure intensity dependent TRPL. The system is based on a conventional upright microscope body that permits easy access to a wide range of sample shapes and sizes. The MicroTime 100 can be supplied with either manual scanning or with a 2D piezo scanner with either µm or cm resolution..


The following core components are needed to build a system capable of TRPL measurements (either spectrometer or microscope based), which are (partly) available from PicoQuant:

Generalized setup of a fluorescence lifetime imaging microscope

General layout of a fluorescence spectrometer

Time-Resolved Photoluminescence (TRPL)  can be used for several application such as:

  • Identify or separate materials by their photoluminescent lifetime
  • Analyze injection efficiencies in solar cells
  • Analyze efficiencies of LEDs, OLEDs
  • Characterize quantum dot
  • Monitor an/or control the quality of wafers
  • ...

Determining electron-hole diffusion lengths in
perovskite solar cells

Determining electron-hole diffusion lengths inperovskite solar cellsA critical parameter in understanding the photophysics of semiconductor solar cells is the diffusion length of the photo-excited electrons and holes in the material. Time-resolved photoluminescence quenching experiments are a valuable tool for determining diffusion lengths. The example shows data obtained from mixed halide and triiodide organometal perovskite layers in presence of either an electron (blue) or hole (red) quenching layer, or a PMMA coating (black). The decay curves were recorded at 780 nm, corresponding to the peak emission of both materials. The measured decay dynamics can be fitted to a diffusion model, allowing to derive diffusion lengths. Here, the diffusion length of the electrons and holes in the mixed halide perovskite was 1 μm while the triiodide material featured a much shorter length of 100 nm, correlating well with performance of these materials as solar cells.

Set-up:

Reference: S. D. Stranks et al., Science, 342 (2013), p.341


TRPL of a GaAsP quantum well system

Transient TRPL spectrum of a quantum well Transient TRPL spectrum of a quantum well illuminated at 595 nm and measured with a fluorescence lifetime spectrometer showing (a) the layer structure of the quantum well and (b) the time-resolved emission spectrum (TRES) of the wafer. The first peak at 650 nm stems from the Al0.4Ga0.6As-barrier, the peak around 735 nm from the GaAsP quantum well and the peak around 860 nm from the n-GaAs layer and the GaAs substrate. Each spectral channel can be described with a three-component exponential model. The average lifetime and the longest component of the fits are displayed. The measurement exemplifies the correlation of characteristic charge carrier dynamics in material specific spectral channels of the multi-component system.

Set-up:


Excitation spectra of the 650 nm peak of the Al0.4Ga0.6As-barrier (blue), of the quantum well layer( light green) and of the n-GaAs-layer and GaAs substrate (dark green). The spectrum of the quantum well layer shows a prominent drop in intensity around 650 nm indicating the interaction with the barrier layer. The n-GaAs-layer and GaAs substrate on the other hand show an increase in intensity around 650 nm, which correlates with the absorption edge in the barrier at wavelengths longer than the barrier band gap. The rectangles illustrate the band gaps of the corresponding layers.

Set-up:


Excitation intensity dependent time resolved photoluminescence of the GaAsP quantum well. A 635 nm laser was focused onto a 100 µm spot (FWHM) and the emission was separated using a 665 nm longpass filter. The inset shows a saturation effect in the average lifetime for increasing intensity in the excitation. The average lifetime approaches a fixed value, as expected for high injection conditions in Shockley-Read-Hall determined photoluminescence.

Set-up:


Latest 10 publications related to TRPL

The following list is an extract of 10 recent publications from our bibliography that either bear reference or are releated to this application and our products in some way. Do you miss your publication? If yes, we will be happy to include it in our bibliography. Please send an e-mail to info@picoquant.com containing the appropriate citation. Thank you very much in advance for your kind co-operation.