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Time Resolved Photoluminescence, TRPL

The time resolved photoluminescence laboratory, TRPL, is located in the cleanroom area of the Physics House. It is well equipped with lasers, cryostats and several different optical detection systems.

The material characterisation are mainly focussed on to study different semiconductor crystals, either grown at the department or obtained from research collaborations. In a photoluminescence experiment lasers are used to excite carriers from the valence band over the bandgap to the conduction band. These excited carriers will recombine either directly or via defects in the material, and emitting a photon with a specific energy and wavelength.

The emitted emission from the sample, the photoluminescence, are collected and investigated with grating spectrometers to separate the different wavelenghts and photon energies.

Since many of the energy levels related to defects in a crystal are very small, the sample is usually cooled, sometimes to temperatures < 2 Kelvin to avoid temperature effects. This is done in He cryostates where liquid Helium or Nitrogen are used.

Specific for time resolved photoluminescence is that we are using pulsed laser to also observe the decay of the different emissions which gives further information about the recombination processes. To study the time behaviour of the emission we are using different detection systems with different time resolution and sensitivity.

The pulsed lasers also gives an increasing possibility to use non-linear crystals to increase the laser photon enrgy by doubling, tripling and quafrupling. Due to this our laboratory is specially well suited for time resolved spectroscopy of wide bandgap materials like SiC and Nitrides.

Laser systems

The main laser system is a Coherent modelocked Ti:sapphire laser, MIRA , which can operate in both pico- and femto- second mode. It is pumped by a solid state 18W Coherent Verdi laser. The MIRA produces pico- or femto.-second pulses in a tunable range from 700nm to 1000nm, with a repetition rate of 76 MHz. The average power is ~1 W.

The laser system also has an regenerative amplifier, Coherent RegA, which is used to amplify the pulses from the MIRA, and reducing the repetition frequency, typically down to 250 kHz. The average output power is comparable to the output from the MIRA, which means that the enrgy per pulse is about 300 times higher from the RegA as compared to the MIRA. The reduction in repetition rate is also an advantage since it makes it possible to study slower recombination process. A frequency of 250 kHz corresponds to a time between each pulse of 4 usec, as compared to 12 ns for the pulses from the MIRA.

The RegA is also pumped by the Verdi laser. It has inside a Ti:sapphire cavity which is made pulsing by an Q-switch. Pulses from the MIRA is injected into the cavity by an cavity dumper and are amplified during the interaction with the Q-switched pulses. When the amplification is reaching a maximum at the peak of the original Q-switch puls, which usually requires 25-30 cycles, it is ejected out from the cavity by the Cavity Dumper. The output pulses are compressed by multiple pass reflection through a grating. The RegA is optimized to work at 800 nm, and has very limited tuning possibilities. 

The pulses from both the MIRA and the RegA can be frequency doubles, and tripled by a XXX tripler. Using the MIRA output this gives tunable pulses in the range of 350-500 nm and 240-330nm, with average powers of 200 mW and 50 mW, respectively. Using the RegA output we can have laser ligth at 400nm and 267 nm, with average powers 60 mw and 20 mW, respectively.

We have also an extension of the tripler, to obtain quadrupling. This has however not yet been implemented, but are of interest to obtain laser pulses for widebandgap materials such as AlN and its Alloys.

As a complement to the large frame laser system we have a smaller diode pumped YLF laser from ADLAS , with emisson at 1060 nm. This has built in doubler and triple crystal and produces laser pulses at 515 nm and 340 nm, which are separated by selective mirrors from the fundamental frequency. The pulse width from the diode pumped laser are ~30-50 ns, and with a tuneable repetition rate from 500 Hz - 50 kHz. This makes it a good complement to the other laser, and are specially suitable for studying recombinations in indirect bandgap materials such as SiC. It is also useful for slower recombination processes in direct bandgap materials like nitrides and ZnO.

Detection systems

The main detection system is a synchroscan STREAK camera from Hamamatsu. It iss well suited for measurements togehter with the MIRA laser, and has a time resolution down to 2 picosecond. In our case we are using a 50 cm  single grating spectrometer in front of the streak input, which reduces the timeresolution to 15-20 ps. The spectrometer can be by-passed to improve the time resolution but with the sacrifice of spectral resolution. The maximum useful time range for the synchroscan streak camera is  2 ns.

In addition to the synchroscan mode we also have a fast sweep unit for the streak camera. This is very useful to use together with the laser output from the RegA where we can increase the useful time range up to 160 us. The fast seep unit can also be used together with the slower diod pumped laser systems.  

As a complement to the streak camera we also have different photon counting systems. This gives higher sensitivity but can anly be used with spectrometers and gives detection at a single wavelength when used with a spectrometer. Typically we use a 450mm Jobin-Yvon spectrometer for this measuremeents. The oldest detection system is a standard single photon counting technique based on time-to-amplitude conversion, TAC and detection of the pulses with multichannel detection system. We also have a more modern pico-second time analyzer which have multiple psingle photon detection possibilities.   

In addition we also have a Photon counter averaging which are useful for slower decays and repetition rate.  

During time we have also developped time resolved measurements to a useful carrier lifetime mapping technique used for SiC. In this case we use the diode pumped laser together with metal packed PMT, and an averaging digital oscilloscope to measure the photoluminscence decay at room temperature, which are related to the minority carrier lifetime in the material. Together with this setup we also have a extensive number of electrical equipment such as High voltage SMU, high current power supplies, that are used for electroluminscence and bipolar degredation studies of SiC material and devices.

Cryostat Systems

Most of our measurements are performed at low temperature and for this we have a liquid He cooled bath cryostat. This have under good conditions a 48 hours holding time, and the temperature can be varied from 1.6 to 300 K. In addition we also have a high temperature cryostate which can be used with liquid N2 in the temperature range from 77 Kto 500 K. This also has the advanatge of a large optical aperture and can be used for full 2" wafers and together with the mapping system 

Responsible for this page: Fredrik Karlsson
Last updated: 04/26/10