Thin heterostructured

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. 

Epitaxy. There is often a sharp orientation relationship between a singlecrystal substrate and a thin-film deposit, depending on the crystal structures and lattice parameters of the two substances. When such a relationship exists, the deposit is said to be in epitaxy with the substrate. The simplest relationship is parallel orientation, and this is common in semiconductor heterostructures, but more complex relationships are often encountered. 

The photoelectrochemical behavior of ZnSe-coated CdSe thin Aims (both deposited by vacuum evaporation on Ti) in polysulflde solution has been described by Russak and Reichman [112] and was reported to be similar to MIS-type devices. Specifically, Auger depth profiling showed the ZnSe component of the (ZnSe)CdSe heterostructures to convert to ZnO after heat treatment in air, thus forming a (ZnO)CdSe structure, while the ZnO surface layer was further converted to a ZnS layer by cycling the electrode in polysulfide electrolyte. This electrochemically generated ZnS layer provided an enhanced open-circuit potential compared to CdSe alone. Efficiencies as high as 5.4% under simulated AM2 conditions were recorded for these electrodes. 

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). 

An example "double heterostructure" OLED shown in Figure 7c uses an ITO coated glass substrate, upon which a hole transporting layer, typically composed of a tertiary amine (eg, IV,IV-biphenyl-A IV7-bis(3-methylphenyl)l-l biphenyl-4,4 diamine, abbreviated TPD), a thin film of an emissive material such as aluminum-8-hydroxyquinoline(Alq3) and an electron-transporting layer (often an oxidiazole derivative) are sequentially deposited in vacuum (Fig. 

There are different criterion of how to classify solid-solid interfaces. One is the sharpness of the boundary. It could be abrupt on an atomic scale as, for example, in III-IV semiconductor heterostructures prepared by molecular beam epitaxy. In contrast, interdiffusion can create broad transitions. Surface reactions can lead to the formation of a thin layer of a new compound. The interfacial structure and composition will therefore depend on temperature, diffusion coefficient, miscibility, and reactivity of the components. Another criterion is the crystallinity of the interface. The interface may be crystalline-crystalline, crystalline-amorphous, or completely amorphous. Even when both solids are crystalline, the interface may be disturbed and exhibit a high density of defects. 

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. 

I. Takeuchi, T. Venkatesan, Wide band gap ZnO and MgZnO heterostructures for future optoelectronic devices. In Thin Films and Heterostructures for Oxide Electronics, ed. by S.B. Ogale (Springer, Berlin Heidelberg New York 2005) pp 301-330... 

In general, AIN crystal samples of suitable size and of high quality have not been available for measurements of IR spectra. Only a limited number of experimental results have been published [12], In very small samples, Collins et al [13] measured the IR absorption and reflectivity spectra, and obtained TO = 666.7 cm 1, LO = 916.3 cm 1, e(oo) = 4.84, and s(0) = 9.14. Carlone et al [14] obtained Ei(LO) and Ei(TO) modes near 800 and 610 cm 1, respectively. MacMillan et al [15] reported IR reflectance of AIN thin fihns in the reststrahl region, and discussed their results using Lorenz oscillators. However, these data are not conclusive. Recently, Wetzel et al reported IR reflection in AlGaN heterostructures [16],... 

FIGURE 2 Cross-sectional TEM micrograph of a GaN thin film grown on a high temperature (1100°C) AIN buffer laycr/on-axis 6H-SiC (0001) substrate heterostructure. 

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