Photoluminescence

Post date: Feb 8, 2014 3:58:03 PM

PL is a good characterization tool due to high internal efficiency (a measure of optically generated electron-hole pairs recombining radiatively thereby emitting light).  In addition, PL is a contactless and non-destructive tool.

PL can identify impurities that produce radiative recombination processes.

I use PL on my GaAs-on-GaSb heterostructures to investigate its optical quality. In particular, low temperature PL is employed in order to minimize thermally activated non-radiative recombination processes. 

The following are the steps taken during a low temperature PL experiment in our lab:

1. My samples are mounted on the copper plate cold finger of an APD closed-cycle Helium cryostat and cooled to 10 K or 13 K.

2. A Uniphase Model 2010 Ar+ laser (488 nm) is used to optically excite the samples.  The laser is mechanically chopped at 200 Hz.

3. Luminescence was collected via suitable optics and dispersed using a Spex500M spectrometer. 

4. The spectrum is detected through a cooled Hamamatsu GaAs photomultiplier tube (PMT) and then fed to a computer for storage. 

5. A 340 Lakeshore temperature controller stabilizes the temperature inside the He-cryostat.

6. For samples with high PL intensity, such as HEMT 261, the sensitivity is set to 500 mV.

7. For my GaAs-on-GaSb, the sensitivity is set to 10 uV. 

Next, the things I have to know and understand: 

1. The resolution used and how it is set. 

Further notes on PL and the set-up:

1. The energy of the laser is greater than the band gap energy of GaAs, thereby generating electron-hole pairs.

2. The electron-hole pairs recombine radiatively and photons are emitted.

3. The Coulombic interaction between electron and hole can lead to an excited state, known as free exciton (FE) or (F, X), in which the electron and hole are bound to each other. 

4. The energy of a FE is less than the band gap energy.

5. Bound exciton (BE) refers to a positively charged excitonic ion, formed when a free hole becomes bound to a neutral donor (DO, X), or when a free electron to a neutral acceptor (AO, X).

6. At 10 K and 13 K, the GaAs-related peak in the PL spectra is seen at 8180 angstroms (1.516 eV).

7. The peak at 8300 angstroms (1.493) is a carbon-related transition.

8. The penetration depth of the 488 nm laser is about 0.08 micrometer.

This is a 11 K PL spectra of a GaAs showing sharp peaks at 8180 angstroms and 8300 angstroms.