Optical Spectroscopy IV
Semiconductor Quantum Wells

 Objective:

We will study a semiconductor sample containing alternating layers of GaAs and AlGaAs, which yield finite square well potentials along the axis perpendicular to the layers. The layers are thin enough to exhibit quantum confinement, so the ground state energy depends on the thickness of the well.  We will measure the emission energy of these quantum wells, and compare our results with theoretical predictions using both infinite and finite square well analysis.  We also seek to understand the shape of the high energy tail on our emission peaks in the context of thermal energy and the Boltzmann distribution.

Background:

We will investigate the structure shown (not to scale!) below:

GaAs layers are sandwiched between AlGaAs layers, which have a higher conduction band energy.  Hence, when electrons are excited by the laser, they get trapped in the thin GaAs layers, and quantum confinement shifts their emission energies as shown to the right of the structure.  The GaAs emission energy without quantum confinement is 1.512eV at 77K.  A peak near this energy should be evident in your measured spectrum.  You should also observe 3 emission peaks above this energy which correspond to the 3 quantum wells in the structure.  The ground state energy of electrons in each quantum well is the difference between the measured peak energy and the energy without quantum confinement (~1.512eV).  These measured energies should be compared with the predictions of infinite and finite square well theories.  Please note that electrons in GaAs behave as if they were lighter than free electrons so you will need to adjust the mass as follows: m* = 0.067me (m* is the effective mass in GaAs, and me is the free electron mass).  For the finite square well predictions, you will need the AlGaAs energy (1.933eV at 77K) to determine the barrier height.

Procedure:

Emission from the quantum wells is weak at room temperature, so we will eventually cool the sample with liquid nitrogen to 77K.  When you are ready to start the lab, your instructor will take you through this process.  Please note that it takes approximately 1 hour for the system to cool to 77K.  In the meantime, draw a block diagram of the experimental setup (here's an image of the cryostat) and review the instructions below.

CAUTION: THE Nd:YAG LASER EMITS INTENSE RADIATION AND EXPOSURE TO THE EYES AND SKIN MUST BE AVOIDED. PAY PARTICULAR ATTENTION TO REFLECTIONS OF THE LASER BEAM, AS THESE ARE SOMETIMES DIFFICULT TO PREDICT.

Analysis: