Laser heterojunction

The first semiconductor lasers, fabricated from gallium arsenide material, were formed from a simple junction (called a homojunction because the composition of the material was the same on each side of the junction) between the type and n-ty e materials. Those devices required high electrical current density, which produced damage ia the region of the junction so that the lasers were short-Hved. To reduce this problem, a heterojunction stmcture was developed. This junction is formed by growing a number of layers of different composition epitaxially. This is shown ia Figure 12. There are a number of layers of material having different composition is this ternary alloy system, which may be denoted Al Ga his notation, x is a composition... 

Because there are two changes ia material composition near the active region, this represents a double heterojunction. Also shown ia Figure 12 is a stripe geometry that confines the current ia the direction parallel to the length of the junction. This further reduces the power threshold and makes the diffraction-limited spreading of the beam more symmetric. The stripe is often defined by implantation of protons, which reduces the electrical conductivity ia the implanted regions. Many different stmctures for semiconductor diode lasers have been developed. 

Eig. 10. Schematic of various LED and laser diode stmctures where S signifies material of a lower energy band gap (a) homojunction, (b) double-heterojunction (DH), and (c) multiquantum well (MQW) stmctures. 

Semiconductor laser diodes are widely used in CD players, DVDs, printers, telecommunication or laser pointers. In the structure, they are similar to LEDs but they have a resonant cavity where laser amplification takes place. A Fabry-Perot cavity is established by polishing the end facets of the junction diode (so that they act as mirrors) and also by roughening the side edges to prevent leakage of light from the sides of the device. This structure is known as a homojunction laser and is a very basic one. Contemporary laser diodes are manufactured as double heterojunction structures. 

N. Holonyak, Jr. and M.H. Lee, Photopumped III-V Semiconductor Lasers Henry Kressel and Jerome K. Butler, Heterojunction Laser Diodes... 

Double helixes, self-recognition in the self- assembly of, 16 803 Double-heterojunction (DH) structures, for LEDs, 22 173, 174, 175 Double heterostructure (DH), 14 844 Double heterostructure laser diodes, 14 700 Double hetero structure OLEDs, 22 216 Double-immunodiffusion technique, 9 753-754... 

Figure 5. Qualitative electronic state (miniband) band diagrams envisaged for 8Ag,(2-p)X,pY-SOD as a new material for a chemistry approach to a resonance tunneling quantum dot transistor and a heterojunction multiple quantum dot laser array.

Henry Kressel and Jerome K. Butler, Heterojunction Laser Diodes... 

Figure 17 shows an energetic scheme for the electron transfer processes taking place at the dye-sensitized heterojunction of such a device. Electron injection from the sensitizer s excited state into the conduction band of Ti02 is followed by regeneration of the dye by hole injection into the hole transport material (HTM). Conduction-band electrons in the metal oxide, as well as holes in the organic medium, are then transported by electronic conduction to the anode and the cathode, respectively. Pulsed picosecond laser photolysis has shown that the hole injection from the oxidized dye sensitizer [Ru (dcbpy)2(NCS)2]" into the spiro-MeOTAD... 

General Motors Research Laboratories (GMR). Molecular beam epitaxial (MBE) growth of Pbi xEuxSeyTei y lattice-matched to PbTe substrates has been used to fabricate double heterojunction diode lasers, including quantum well, large optical cavity (LOG) devices, the first such TDL structures reported for a lead-salt compound. These Mesa stripe... 

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