Photo luminescence
The simplest example of light emission from a semiconductor occurs for direct excitation and recombination of an EHP, as depicted as photoluminescence.
Photo luminescence:
The Photoluminescence is shown in figure:
- If the recombination occurs directly rather than via a defect level, band gap light is given off in the process. For steady state excitation, the recombination of EHPs occurs at the same rate as the generation, and one photon is emitted for each photon absorbed.
- Direct recombination is a fast process; the mean lifetime of the EHP is usually on the order of 10-8 s or less.
- Thus the emission of photons stops within approximately 10-8 s after the excitation is turned off.
- Such fast luminescent processes are often referred to as fluorescence.
- In some materials, however, emission continues for periods up to seconds or minutes after the excitation is removed.
- These slow processes are called phosphorescence, and the materials are called phosphors.
- An example of a slow process is shown in Fig. given below.
- This material contains a defect level (perhaps due to an impurity) in the band gap which has a strong tendency to temporarily capture {trap) electrons from the conduction band.
- The events depicted in the figure are as follows;
(a) An incoming photon with hv1 > Eg is absorbed, creating an EHP;
(b) the excited electron gives up energy to the lattice by scattering until it nears the bottom of the conduction band;
(c) the electron is trapped by the impurity level Er, and remains trapped until it can be thermally reexcited to the conduction band
(d); (e) finally direct recombination occurs as the electron falls to an empty state in the valence band, giving off a photon {hv2) of approximately the
band gap energy.
- The delay time between excitation and recombination can be relatively long if the probability of thermal reexcitation from the trap (d) is small. Even longer delay times result if the electron is retrapped several times before recombination.
- If the trapping probability is greater than the probability of recombination, an electron may make several trips between the trap and the conduction band before recombination finally occurs.
- In such material the emission of phosphorescent light persists for a relatively long time after the excitation is removed.
Fluorescent lamp:
One of the most common examples of photoluminescence is the fluorescent lamp. Typically such a lamp is composed of a glass tube filled with gas
(e.g., a mixture of argon and mercury), with a fluorescent coating on the inside of the tube. When an electric discharge is induced between electrodes in
the tube, the excited atoms of the gas emit photons, largely in the visible and ultraviolet regions of the spectrum. This light is absorbed by the luminescent
coating, and the visible photons are emitted. The efficiency of such a lamp is considerably better than that of an incandescent bulb, and the wavelength
mixture in light given off can be adjusted by proper selection of the fluorescent material.