First germanium laser on silicon
Band Gap: It is the amount of energy required to free an outer shell electron from its orbit about the nucleus to become a mobile charge carrier, able to move freely within the solid material. The band gap generally refers to the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band which is found in insulators and semiconductors. The valence electrons are bound to individual atoms, as opposed to conduction electrons (found in conductors and semiconductors), which can move freely within the atomic lattice of the material.
Bandgap in semiconductor
In order for an electron to jump from a valence band to a conduction band, it requires a specific minimum amount of energy for the transition. The required energy differs with different materials. Electrons can gain enough energy to jump to the conduction band by absorbing either a phonon (heat) or a photon (light). A material with a small but nonzero band gap (arbitrarily defined as < 3 eV, although some definitions place the upper limit at 4 eV) is referred to as a semiconductor. A material with a large band gap is called an insulator. In conductors, the valence and conduction bands may overlap, so they may not have a band gap.
Direct band gap or an indirect band gap: In semiconductor physics, the band gap of a semiconductor is always one of two types, a direct band gap or an indirect band gap. In direct band gap semiconductor such as gallium arsenide an electron near the bottom of the conduction band annihilates a hole near the top of the valence band, releasing the excess energy as a photon. In case of semiconductors with indirect band gap this kind of electron hole recombination is almost impossible because the conservation of crystal momentum would be violated. For electron hole recombination to occur in an indirect band gap material, the process must also involve the absorption or emission of a phonon, where the phonon momentum equals the difference between the electron and hole momentum. The involvement of the phonon makes the process of electron hole recombination much less likely to occur in a given span of time. This is why light-emitting and laser diodes are almost always made of direct band gap materials, and not indirect band gap semiconductors like germanium and silicon.
Limitations of the existing electronic silicon chips: To increase a chips’ computational capacity higher-bandwidth connections are needed to send data to memory. But conventional electrical connections will soon become impractical, because they’ll require too much power to transport data at ever higher rates.
Emerging technology – Integrate optical and electronic components on silicon chips:
Transmitting data with lasers could be much more power-efficient, but this requires a cheap way to integrate optical and electronic components on silicon chips. For example monolithic lasers on Si are ideal for high volume and large-scale electronic-photonic integration. Chip assembly is a painstaking process in which layers of different materials are deposited on a wafer of silicon, and patterns are etched into them. Inserting a new material into this process is difficult: it has to be able to chemically bond to the layers above and below it, and depositing it must be possible at the temperatures and in the chemical environments suitable to the other materials. Monolithically integrated lasers on Si have long been one of the biggest challenges for Si-based electronic-photonic integration. Present day lasers made of materials such as gallium arsenide are difficult to integrate on to a Si chip.
First germanium laser
Now researchers from MIT have demonstrated the first laser built from germanium (on an Si wafer) that operates at room temperature and can produce wavelengths of light useful for optical communication. They choose Ge because of its pseudo-direct gap properties and compatibility with Si complementary metal oxide semiconductor (CMOS) technology. (Almost all major chip manufacturers have already begun integrating germanium into the manufacturing process, since the addition of germanium increases the speed of silicon chips.) The researchers have demonstrated at room temperature photoluminescence, electroluminescence and optical gain from the direct gap transition of band-engineered Ge-on-Si using tensile strain and n-type doping. They report the first experimental observation of lasing from the direct gap transition of Ge-on-Si at room temperature using an edge emitting waveguide device.
Geranium as such is a indirect band gap semiconductor. In order to make Geranium behave as a direct band gap semiconductor the researchers ‘doped’ Ge with phosphorous, which has five outer electrons. Germanium has only four outer electrons, “so each phosphorous gives us an extra electron,” says one of the researchers, Lionel C. Kimerling. The extra electron fills up the lower-energy state in the conduction band, causing excited electrons to, effectively, spill over into the higher-energy, photon-emitting state.
According to the researchers, phosphorous doping works best at 10 to the power of 20 atoms per cubic centimeter of germanium. So far, the group has developed a technique that can add 10 to the power of 19 phosphorous atoms to each cubic centimeter of germanium, “and we already begin to see lasing,” Kimerling says.
In their next step, in order to increase the likelihood of the excited electrons to spill over into the photon-emitting state the researchers “strained” the germanium i.e. pried its atoms slightly farther apart than they would be naturally — by growing it directly on top of a layer of silicon. Both the silicon and the germanium were deposited at high temperatures. But silicon doesn’t contract as much as germanium when it cools. The atoms of the cooling germanium tried to maintain their alignment with the silicon atoms, so they ended up farther apart than they would ordinarily be. Changing the angle and length of the bonds between germanium atoms also changed the energies required to kick their electrons into the conduction band.
According to Kimerling, “The ability to grow germanium on silicon is a discovery of this group and the ability to control the strain of those germanium films on silicon is a discovery of this group.”
The research team included, Jifeng Liu, Xiaochen Sun, Rodolfo Camacho-Aguilera, Lionel C. Kimerling, and Jurgen Michel.
Source: http://web.mit.edu/newsoffice/2010/first-germanium-laser.html
February 5, 2010