**7. GaAs optical properties**

There are two main categories in which the optoelectronic devices can be categorized. First is the set of those devices in which electrical current get converted into electromagnetic radiation i.e. light. Second is the set of those devices in which light is converted into electric current. An example of the first category of devices is the LEDs. Optoelectronic devices as well as light-emitting devices have GaAs in them. Macroscopic evidence is available for the interaction of matter with light and it is made of four components, namely, incident, reflected, transmitted, and scattered component [52, 54, 55].

Absorption of a photon in a semiconductor can happen in a number of ways. This is commonly known as interbond absorption. This happens when in the conduction band an electron is excited up after having absorbed a photon in the valence band. The momentum is unchanged in case of direct gap semiconductor while there is an increase in electron's energy. The momentum is though shifted in case of an indirect gap material. This is made possible by a decrease in bandgap energy with the increase in temperature and a photon being either absorbed or emitted. The two factors responsible for this are lattice vibration (phonons) and thermal expansion. The increase of lattice constant is due to the thermal expansion. This further leads to

#### **Figure 9.**

*The heterojunction has a higher quantum efficiency since the carriers are localized GaAs [63].*

the change in the periodic potential as seen by the electron. The band structure is altered due to these changes [56–59].

Temperature and bandgap shift are related as per the following formula

$$E\_{\xi} = E\_{\xi} - \frac{\alpha T^2}{T + \beta} \tag{12}$$

where

Eg - band gap energy

T - temperature

Eg (0) - bandgap at 0 K having units same as that of energy α has units the same as that of temperature/energy.

Prominently used in optoelectronic as well as microelectronic devices, Gallium arsenide, GaAs is considered a good semiconducting material, having high electron mobility. In the semiconductor material, through the interaction of electron and photons, sunlight is converted into electricity directly by photovoltaic cells [60–62].

Electronic excitation of luminescence is the reason, it is also called as optical radiation. State energy is emitted in the form of EM radiation when the excited electrons move back to the ground. Depending upon the electronic excitation created originally, there are four different types of luminescence, namely,

Photoluminescence � incident light ! electronic excitation Radioluminescence � ionizing radiation <sup>β</sup> � rays ! electronic excitation Cathodoluminescence � electron beam ! electronic excitation Electroluminescence � electrical field ! electronic excitation

The functioning of LEDs is by electroluminescence. Using a functional bias, electric current which includes holes and electrons are forwarded to the device. Light is emitted by the recombination of these holes and electrons. In order to increase efficiency, heterostructure LEDs are used. The holes and the electrons, collectively called the carriers are confined in a small spatial region in order to achieve this. Due to the localization of carriers in GaAs, there is higher quantum efficiency at the heterojunction. Hence, it is only in the i-GaAs region, that the recombination takes place. **Figure 9** draws a clear picture of the emitted wavelength in case of both LEDs as shown in **Figure 9** [63–66].

## **8. GaAs elastic properties**

The physical knowledge of the materials namely the phase transitions, interatomic forces and the mechanical features apart from many other features are better understood by examining the elastic properties of the material.
