Luminescence Lifetime Monitor

Wafercheck

Features

• Excitation with ultrashort laser pulses from diode laser (down to 50 ps FWHM)
• Excitation waveIength 375 to 1550 nm;
• Wavelength selection with easy to use filter wheel
• Data acquisition electronics on PC-card using TCSPC
• Integrated laser power meter
• Detector options: MCP-PMT, PMT, SPAD

Applications

• Monitoring of time-resolved photoluminescence from Semiconductors
• On-line process analysis and quality control
• Time response characterisation of opto-electronic devices
• Specific designed for GaAs and GaN wafers

Time-resolved Photoluminescence (TRPL) article awarded by "Laser Focus World".
A Brief Description

TRPL is a very powerful tool for the characterisation and investigation of semiconductor materials. It is non-destructive and involves just light as a probe. It can be used on raw material as well as on devices and structures. These features qualify the method for basic research, testing and quality control. The WaferCheck system is a complete and easy to use setup for such measurements. At extraordinarily low cost it permits measurements that previously could only be performed by the best equipped laboratories.

In the semiconductor industry the application of TRPL is focused mainly on the measurement and identification of electron-hole recombination rates. The length of time a photoexcited carrier can remain in the conduction (or valence) band is an important parameter directly related to material quality and device performance. The fluorescence lifetime decay is an indicator to characterise semiconductors e.g. for use in photovoltaic devices (solar cells) or photodetectors. With all its potential the method has been mainly used as a research tool with little incorporation into the fabrication/quality control side until now. The main reason for this is that TCSPC systems which are suitable for many of the materials of interest, systems with sub nanosecond resolution and high energies per pulse, were found to be prohibitively expensive in the past.

The application of TRPL in quality controll of III-V semiconductors has been demonstrated for many different purposes [1-3]. Its routine use in regular production can drasticly improve yield and quality for many types of semiconductor as the first installations of the WaferCheck system show.

TRPL is also of great interest for the latest nitride semiconductors used in optoelectronics. With PicoQuant's ultraviolett and blue picosecond diode lasers the method becomes a lot more accessible for use on these materials. An excellent example for the use of TRPL for blue InGaN LEDs is reported by Pophristic et. al. [4] :

"Recently, there has been world-wide interest in the use of nitride semiconductors (e.g., GaN, InN, and AlN) for optoelectronic devices such as lasers and light-emitting diodes (LEDs). The large changes in physical properties such as band gap, crystal structure, phonon energy, and electronegativity difference between GaN and GaAs, demonstrate that nitride semiconductors are fundamentally distinct from traditional III–V semiconductors. In spite of the impressive progress made in recent years in the development of LEDs and lasers, significant work needs to be done in terms of the optimization of device performance. In order to achieve this goal, the physics underlying the operation of these devices must be better understood. Furthermore, new diagnostic techniques for the characterization of materials and devices will greatly aid in the long-term commercialization of this technology. It has been recognized that under typical growth conditions there is a positive enthalpy for indium mixing in GaN. Electron microscopy and cathodoluminescence of InGaN have demonstrated the existence of nanometer- and micron-scale regions of high indium concentration. Regions of high indium concentration have a lower band gap than bulk InxGa1-xN; therefore, either excitons or carriers are expected to spatially localize in these low-energy regions on very fast time scales. It has been hypothesized that the nanoscale regions of high indium concentration are critical to LED operation.Recently, time-resolved photoluminescence (TRPL) has been used to examine nitride semiconductor multiple quantum wells (MQWs) InGaN films at room temperature. Time-resolved electroluminescence measurements have been reported on Nichia LEDs, consisting of a single InGaN layer doped with Zn. In this letter, we present results of time-resolved PL measurements on LED wafers based on InGaN/GaN MQWs. We have found dramatic differences in the time-resolved kin etics between bright and dim devices."

References

[1] "Time Resolved Photoluminescence Studies on Transferred Thin Film InP Epilayers", Augustine, G., Keyes, B., N.M. Jokerst, Rohatgi, A.; Ahrenkiel, R., Proceeding of the IEEE InP Related Material Conference, France, April, 1993

[2] "Effect of Implantation Dose on Photoluminescence Decay Times in Intermixed GaAs/AlGaAs Quantum Wells" P.G. Piva, S. Charbonneau, I.V. Mitchell, R.D. Goldberg , Appl. Phys. Letts. 68(16):2252-2254, 1996

[3] "Carrier Dynamics of Anti-Stokes Photoluminescence in Staggered-Band Lineup AlxGa1-xAs/GaInP2 Heterostructures", Yong-Hoon Cho, J.J. Song, D.S. Kim, H. Lim, and B.D. Choe, Proceeding of SPIE '98, San Jose, USA

[4] "Time-resolved photoluminescence measurements of InGaN light emitting diodes", M.Pophristic and F.H. Long, Rutgers University; C. Tran, I.T. Ferguson and R.F. Karlicek, Jr., EMCORE Coperation. Published in Applied Physics Letters, Vol. 73, Number 24, 14 December 1998