Heterostructures, confined

A more effective carrier confinement is offered by a double heterostructure in which a thin layer of a low-gap material is sandwiched between larger-gap layers. The physical junction between two materials of different gaps is called a heterointerface. A schematic representation of the band diagram of such a stmcture is shown in figure C2.l6.l0. The electrons, injected under forward bias across the p-n junction into the lower-bandgap material, encounter a potential barrier AE at the p-p junction which inliibits their motion away from the junction. The holes see a potential barrier of... 

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. 

Band gap engineetring confined hetetrostruciutres. When the thickness of a crystalline film is comparable with the de Broglie wavelength, the conduction and valence bands will break into subbands and as the thickness increases, the Fermi energy of the electrons oscillates. This leads to the so-called quantum size effects, which had been precociously predicted in Russia by Lifshitz and Kosevich (1953). A piece of semiconductor which is very small in one, two or three dimensions - a confined structure - is called a quantum well, quantum wire or quantum dot, respectively, and much fundamental physics research has been devoted to these in the last two decades. However, the world of MSE only became involved when several quantum wells were combined into what is now termed a heterostructure. 

A new chapter in the uses of semiconductors arrived with a theoretical paper by two physicists working at IBM s research laboratory in New York State, L. Esaki (a Japanese immigrant who has since returned to Japan) and R. Tsu (Esaki and Tsu 1970). They predicted that in a fine multilayer structure of two distinct semiconductors (or of a semiconductor and an insulator) tunnelling between quantum wells becomes important and a superlattice with minibands and mini (energy) gaps is formed. Three years later, Esaki and Tsu proved their concept experimentally. Another name used for such a superlattice is confined heterostructure . This concept was to prove so fruitful in the emerging field of optoelectronics (the merging of optics with electronics) that a Nobel Prize followed in due course. The central application of these superlattices eventually turned out to be a tunable laser. 

Stripe—geometry gain—guided AlGaAs-GaAs quantum well heterostructure lasers have been fabricated from masked hydrogenation to produce the resistive regions necessary to current confinement. 

F. Capasso, Graded-Gap and Superlattice Devices by Band-gap Engineering W. T. Tsang, Quantum Confinement Heterostructure Semiconductor Lasers... 

Semiconductor-based lasers have been further developed from the simple model depicted in Figure 2.14. The predictions by Kroemer and Alferov in the early 1960s stated that the concentrations of electrons, holes, and photons would become much higher if they were confined to a thin semiconductor layer between two other layers (Kroemer, 1963). Since then, sophisticated configurations for semiconductor heterostructures lasers have been made possible due to the development of fabrication techniques (Wilson and Hawkes, 1998 Kasap, 2001). 

Fig. 7. In an effort to improve tlie performance of the basic semiconductor laser, researchers developed tlie buried-heterostructure laser. In this configuration, the p-n junction is reduced to a tube that runs the length of the semiconductor crystal. This tube is surrounded by layers of semiconductor whose wide band gap raises the electrical barrier confining charge earner s within the tube. The wide-band-gap material also confines the. photons produced at the junction. The laser beam spreads because of diffraction occurring where the beam emerges from tlie face of tire device...

Fig. 3. The lattice-matched double heterostructure, where the waves shown in the conduction band and the valence band are wave functions, T(x), representing probability density distributions of carriers confined by the barriers. The chemical bonds, shown as short horizontal stripes at the AlAs—GaAs interfaces, match up almost perfecdy. The wave functions, sandwiched in by the 2.2 eV potential barrier of AlAs, never see the defective bonds of an external surface. When the GaAs layer is made so narrow that a single wave barely fits into the allotted space, the potential well is called a quantum well. Because of the match in the atomic spacings between GaAs and AlAs, 99.999% of the interfacial chemical bonds are saturated.

Haus, J. W. Zhou, H. S. Honma, I. Komiyama, H. Quantum confinement in semiconductor heterostructure nanometer-size particles, Phys. Rev. 1993,47, 1359. 

Mg Quantum confinement PL of MgZnO/ZnO hetero- and double-heterostructures grown by PLD [53]... 

The results are arranged according to the aim of doping or alloying, namely (1) n-type conductivity, (2) bandgap-tuning and quantum confinement, (3) potential p-type conductivity, and (4) ferromagnetic behavior. In addition, (5) ZnO/BaTiC>3 heterostructures as combination of ZnO with fixed and BaTiC>3 with electrically switchable polarization are included... 

Fig. 7.30. Photoluminescence spectra (2K) of PLD MgZnO-ZnO-MgZnO quantum well heterostructures on sapphire with nominal thickness of the ZnO quantum well of 25, 12, 6, and 3nm [53]. The blueshift of the excitonic peak combined with the intensity enhancement is a clear indication of optical confinement in the ZnO layer...

First successful ZnO device demonstrations as for example stable homo-and heteroepitaxial pn-junctions and LED structures, thin film scintillators, and quantum well structures with optical confinement, and oxide-based Bragg reflectors, and high-quality Schottky contacts are based on PLD grown thin films. Several techniques as for example the PLD in UHV conditions (laser MBE), and gradient and combinatorial PLD, and high-pressure PLD for nano-heterostructures show the innovative potential of the advanced growth technique PLD. 

In modem heterostructure lasers, the actual active layer is usually very thin, much thinner than the optical mode to be supported by its optical gain. Therefore, it is convenient to introduce the so-called optical confinement factor T, which relates the material optical gain gmlt to the net optical gain geff seen by the lasing mode [6] ... 

Then, why do Nichia s GaN lasers work at all The answer lies with two other serious non-uniformities present in the laser (1) an accidental non-uniformity within the quantum well due to spinodal decomposition [7], and (2) the intentional use of quantum wells and heterostructures to define sheets of high population inversion and photon confinement. The former effect creates a granulated, random array of InN-rich quantum boxes within the intended quantum well volume, which confine the population inversion within these nanoscopic regions of high optical quality. While the random... 

Both n- and p-type doping is possible in SiC and it has an excellent power amplification device performance. As a result it is a direct competitor of the nitrides in this application. Nitrides form direct band-gap heterostructures allowing the placing of carriers at the interface and carrier confinement. In addition, nitrides can form good ohmic contacts, which are imperative for power devices. 

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