Lasers quantum well

A logical consequence of this trend is a quantum w ell laser in which tire active region is reduced furtlier, to less tlian 10 nm. The 2D carrier confinement in tire wells (fonned by tire CB and VB discontinuities) changes many basic semiconductor parameters, in particular tire density of states in tire CB and VB, which is greatly reduced in quantum well lasers. This makes it easier to achieve population inversion and results in a significant reduction in tire tlireshold carrier density. Indeed, quantum well lasers are characterized by tlireshold current densities lower tlian 100 A cm . ... 

Epitaxial crystal growth methods such as molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) have advanced to the point that active regions of essentially arbitrary thicknesses can be prepared (see Thin films, film deposition techniques). Most semiconductors used for lasers are cubic crystals where the lattice constant, the dimension of the cube, is equal to two atomic plane distances. When the thickness of this layer is reduced to dimensions on the order of 0.01 )J.m, between 20 and 30 atomic plane distances, quantum mechanics is needed for an accurate description of the confined carrier energies (11). Such layers are called quantum wells and the lasers containing such layers in their active regions are known as quantum well lasers (12). 

An even wider range of wavelength, toward the infrared, can be covered with quantum well lasers. In the Al Ga As system, compressively strained wells of Ga In As are used. This ternary system is indicated in Figure 6 by the line joining GaAs and In As. In most cases the A1 fraction is quite small, X < 0.2. Such wells are under compressive strain and their thickness must be carefully controlled in order not to exceed the critical layer thickness. Lasers prepared in this way are characterized by unusually low threshold current density, as low as ca 50 A/cm (l )-... 

Quantum well lasers ia this system typically use ternary Iuq 53GaQ 47AS wells and biaary InP barriers. AH quaternary lasers, ie, lasers ia which both the wells and barriers are formed by quaternary compounds, are also being developed. These stmctures can be lattice matched or strained. 

Quantum well lasers, 22 180 Quantum wells, 14 844 Quantum yield, 8 256 19 75. See also Photocatalytic quantum yield (quantum efficiency)... 

Arakawa, Y., Yariv, A., 1986, Quantum-well lasers gain, spectra, dynamics, IEEE J. Quantum Electron. 22(9) 1887-1899. 

Production of single crystals with carefully controlled varying composition is vital to make semiconductor devices. The production of crystals for quantum well lasers illustrates how carefully such syntheses can be controlled. 

The electronic and magnetic properties of nanolayers are important in devices formed from electronic materials that are more conventional. We have already discussed quantum well lasers (see Chapter 8) and giant magnetoresistance (GMR) devices used for hard disk read heads (see Chapter 9). Quantum well lasers may be an important component of light-based computers. Other possibilities include magnets with unusual properties (Section 11.2). 

New physics such as the fractional quantum Hall effect has emerged from non-magnetic semiconductor heterostructures. These systems have also been a test bench for a number of new device concepts, among which are quantum well lasers and high electron mobility transistors. Ferromagnetic 111-Vs can add a new dimension to the III-V heterostructure systems because they can introduce magnetic cooperative phenomena that were not present in the conventional III-V materials. 

Of course, there is a considerable amount of inhomogeneous broadening due to compositional fluctuations, as in any other temary/binary quantum well laser material. However, this broadening is detrimental rather than beneficial for the laser, since it reduces the peak optical gain. 

P.S. Zory, Jr., ed.. Quantum Well Lasers, Academic Press, San Diego, 1993. 

Nanocomposites in the form of superlattice structures have been fabricated with metallic, " semiconductor,and ceramic materials " " for semiconductor-based devices. " The material is abruptly modulated with respect to composition and/or structure. Semiconductor superlattice devices are usually multiple quantum structures, in which nanometer-scale layers of a lower band gap material such as GaAs are sandwiched between layers of a larger band gap material such as GaAlAs. " Quantum effects such as enhanced carrier mobility (two-dimensional electron gas) and bound states in the optical absorption spectrum, and nonlinear optical effects, such as intensity-dependent refractive indices, have been observed in nanomodulated semiconductor multiple quantum wells. " Examples of devices based on these structures include fast optical switches, high electron mobility transistors, and quantum well lasers. " Room-temperature electrochemical... 

Zory P S Jr (ed) 1993 Quantum Well Lasers (Boston Academic)... 

InGaN/GaN multiple quantum well laser heterostructures... 

Double-heterostructure lasers are fabricated in form of various stripegeometry laser diodes. Improved production technology allows even the fabrication of single- and multiple-quantum-well lasers [218, 219]. 

Figure 2. Spontaneous and stimulated emission of a quantum-well laser with 80-90 S wells obtained by optical pumping. (Adapted from Ref. 39.)...

Another method of increasing the modulation bandwidth is to decrease the photon lifetime. This is most easily accomplished by decreasing the laser diode cavity length. This, however, increases the laser threshold current level, and as a result, lasers with extremely low threshold current levels are required for this method. Utilizing a 40-/rm-long AlGaAs multiple quantum well laser, a modulation bandwidth of 50 GHz has been achieved. 

Quantum well semiconductor lasers with both single and multiple active layers have been fabricated. Quantum well lasers with one active are called single-quantum-well (SQW) lasers and lasers with multiple quantum well active regions are called multiquantum-well (MQW) lasers. The layers separating the active layers in a multiquantum well structure are called barrier layers. Typical examples of the energy band diagram of both SQW and MQW are schematically represented in Fig. 18. 

A key feature associated with quantum well laser structures as compared to their bulk counterparts is that the laser threshold current is greatly reduced by the effect of the modified density of states. If the density of states of the bulk and quantum well structures is given as above, one can express the optical gain coefiftcient as... 

Owing to the additional confinement of carriers to within the quantum well structure, the resultant threshold current density becomes considerably less than for bulk double heterostructure devices. Additionally, quantum well lasers generally have a narrower gain spectrum (for similar bias currents to DH structures), a smaller lasing linewidth of the lasing modes, a reduced temperature dependence, and the potential for achieving higher modulation frequencies. 

The active-layer thickness of an SQW laser is typically less than 10 nm, which is to be compared to 0.1 /xm for a DH laser. The threshold current for a typical SQW laser is typically less than 1 mA, while DH structures have threshold currents of several tens of milliamperes. The spectral width of the lasing emission from an SQW laser is usually less than 10 MHz as compared to 100 MHz for a typical DH structure. The output power from single quantum well laser is on the order of 100 mW, although... 

Recent experimentation with semiconductor lasers has revealed many new features of the spectral behavior. New theories have been formulated to explain this behavior and are directing attention to the new quantum-well lasers for improved spectral behavior. 

Fig. 2. The calculated value of the a parameter of a quantum well laser as a function of the well width.

Conventional SC lasers have been demonstrated with upper modulation frequency f of -llGHz. The predicted improvement in the value of fj. in the quantum well lasers should give rise to values of fp -20-30 GHz. 

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