Multiquantum well

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. 

Several heterostructure geometries have been developed since the 1970s to optimize laser performance. Initial homojunction lasers were advanced by the use of heterostmctures, specifically the double-heterostmcture device where two materials are used. The abiUty of the materials growth technology to precisely control layer thickness and uniformity has resulted in the development of multiquantum well lasers in which the active layer of the laser consists of one or mote thin layers to allow for improved electron and hole confinement as well as optical field confinement. 

Volume 24 Applications of Multiquantum Wells, Selective Doping,... 

Although the rocking curve from a superlattice or multiquantum well stmcture may be quite complex, there are a number of common features which can be used for the analysis. Assuming that we have a substrate of material A (e.g. GaAs) and a superlattice or MQW with a stack of AB layers, where B is the alloy (e.g. Gai- As), as illustrated in Figure 6.9, the rocking curve will show the following features ... 

On semiconductors light emission is induced by injection of electrons into the conduction band and subsequent band-to-band radiative recombination with holes (Fig. 38a). The process is reminiscent of electroluminescence or cathodolumines-cence and works with p-type substrates only (at n-type specimens no hole is available at the surface). Tunnel biases of 1.5-2 V are necessary in the case of GaAs, for instance. Figure 38b is a photon map of a GaAlAs/GaAs multiquantum well obtained by Alvarado et al. [140], The white stripes are regions where photons are emitted and correspond to the GaAs layers. The lateral resolution is about 1 nm and is limited by the diffusion distance of minority carriers. In Sec. 5.1 we have seen an example of the application of this technique in the case of porous silicon layers. 

Fig. 38. Photon emission at a semiconductor, (a) Principle of the process at a p-type semiconductor, (b) Photon map taken on a GaAlAs/GaAs multiquantum well. Electrons are injected into GaAlAs and recombine in p-GaAs, from which light is emitted (after [140]).

CL(oo,oo n) is realized by superlattice structures such as Si and Ge multiquantum wells, in which n values correspond to the silicon layer thickness. 

New kinds of dilute-nitride type-II InAsN/GaSb laser diodes on InAs substrate with "W" or "M" design are theoretically investigated. For these laser diodes, designed to qjerate at 3.3 im at room temperature, the total threshold current densities are calculated. Under the hypothesis of a total loss coefficient a = 50 cm", these multiquantum well laser structures present a calculated threshold current density J lower than 1.1 kA/cm. ... 

Dingle R. (1987), Semiconductors and Semimetals Applications of Multiquantum Wells, Selective Doping, and Superlattices, Vol. 24, Academic Press, New York. 

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. 

Agrawal. N, C. M. Weinert, H.4. Ehrke, G. G. Mekonnen, D. Franke, C. Bornholdt, and R. Langenhorst (1995) "Fast 2x2 Mach-Zehnder optical space switches using InGaAsP-InP multiquantum- well structures" IEEE Photonics technology letters, Vol. 7, No. 6, page no. 644-645, June 1995. 

D. Delprat, A. Ramdane, A. Ougazzaden, H. Naka-jima, M. Carre Integrated multiquantum well distributed feedback laser-electroabsorption modulator with a negative chirp for zero bias voltage. Electron. Lett. 33, 53-55 (1997)... 

Click, M., Reinhart, F. K., Weimann, G., and Schlapp, W., Quadratic electro-optic light modulation in a GaAs/AlGaAs multiquantum well heterostructure near the ex-citonic gap, Appl. Phys. Lett, 48y 989 (1986). 

Chen, T.R., Chen, RC., Ungar, J., and Bar-Chaim, N. 1995. High-power operation of multiquantum well DFB lasers at 1.3 fim. Elect. Lett. 31 1344—1345. 

Lester, L.F., Schaff, W.J., Song, X., and Eastman, L.F. 1991. Optical and RF characteristics of short-cavity-length multiquantum-well strained-layer lasers. IEEE Phot Tech. Lett. 3 1049-1051. 

Morton, RA. et al. 1992. 25 GHz bandwidth 1.55 jo, GalnAsP p-doped strained multiquantum-well lasers. Elec. Lett. 28 2156-2157. 

Partin (1983a,b, 1984, 1985, 1987, 1988) and Partin and Thrush (1984) have prepared Pbi- tEU (Tei Se (x = 0.015, y = 0.020) films by MBE. This alloy is either covered with a PbTe film (20 nm thick) to form a single-quantum-well laser, or deposited onto PbTe (100) or (111) substrates for diode lasers. It is assumed that there is a good lattice match to PbTe. It is shown that the laser emission energy as a function of composition increases as the relative Eu content increases. The photoluminence properties at 4.2 K, in PbTe/PbEuTeSe multiquantum wells have been reported by Goltros et al. (1985). 

Sun, H. D.> Makino, T., Segawa, Y., Kawasaki, M., Ohtomo, A., Tamura, K., Koinuma, H. (2002). Enhancement of exciton binding energies in ZnO/ZnMgO multiquantum wells. Journal of Applied Physics, 91,1993-1997. 

Figure 3.11 Photoluminescence spectra of GaN/AICaN multiquantum wells of different thicknesses grown on nonpolar o-plane and polar c-plane CaN bulk substrates.

At this stage, it is too early to compare the optical properties of polar and nonpolar lasers. However, our results on homoepitaxial multiquantum wells and optically pumped lasers prove that it is possible to grow high-quality nitride structures along with nonpolar directions. It goes without saying that further research is necessary. 

H Sugawara, K Itaya, G Hatakoshi. Emission properties of InGaAlP visible light-emitting diodes employing a multiquantum-well active layer. Jpn J Appl Phys 33 5784, 1994. 

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