TABLE OF CONTENTS Elemental silicon has been widely used for realization of photodetectors and photovoltaics. Both Si and Ge have found wide applications in APDs. Silicon based optoelectronic materials are crystalline Si, SiGe, SiGeC and other IV-IV alloys, poly- and amorphous Si, SiGe and porous Si. A basic limitation of Si and Ge arises from the fact that they have an indirect bandgap and therefore quantum efficiency is low. Some of the important optical properties of Si and Ge are presented in table 10.1.
| Property | Si | Ge |
|---|---|---|
| Transparent regions ( ?m) | 1.1 6.5 | 1.8 15 |
| Dielectric constant | 11.9 | 16 |
| Refractive index (optical) | 3.455 | 4.001 |
| Optical-phonon energy (eV) | 0.063 | 0.037 |
| Phonon mean free path ( ) | 76 (electron) | 105 |
| 55 (hole) |
Optoelectronic devices can be divided into three groups: (i) devices that convert electrical energy into optical radiation (LED and laser), (ii) devices that detect optical signals (photodetectors), and (iii) devices that convert optical radiation into electrical energy (photovoltaic cell). An optoelectronic system contains in addition optical waveguiding structures and modulation methods, electrical waveguides where necessary and electronic systems.
Energy states between which transitions may take place are those in the conduction and valence bands and the impurity states within the bandgap. Basic transitions in a semiconductor may be classified as: (i) interband (between conduction and valence band), (ii) conduction or valence band to donor or acceptor and donor to acceptor, and (iii) intraband involving states in the conduction or valence...
