Carrier-confining layers

Y. Ohmori, M. Uchida, K. Muro, and K. Yoshino, Effects of alkyl chain lengths and carrier confinement layer on characteristics of poly(3-alkylthiophene) electroluminescent diodes, Solid State Commun., 80 605-608, 1991. 

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... 

This confinement yields a higher carrier density of elections and holes in the active layer and fast ladiative lecombination. Thus LEDs used in switching apphcations tend to possess thin DH active layers. The increased carrier density also may result in more efficient recombination because many nonradiative processes tend to saturate. The increased carrier confinement and injection efficiency faciUtated by heterojunctions yields increasing internal quantum efficiencies for SH and DH active layers. Similar to a SH, the DH also faciUtates the employment of a window layer to minimise absorption. In a stmcture grown on an absorbing substrate, the lower transparent window layer may be made thick (>100 /tm), and the absorbing substrate subsequendy removed to yield a transparent substrate device. 

The two-dimensional carrier confinement in the wells formed by the conduction and valence band discontinuities changes many basic semiconductor parameters. The parameter important in the laser is the density of states in the conduction and valence bands. The density of states is gready reduced in quantum well lasers (11,12). This makes it easier to achieve population inversion and thus results in a corresponding reduction in the threshold carrier density. Indeed, quantum well lasers are characterized by threshold current densities as low as 100-150 A/cm, dramatically lower than for conventional lasers. In the quantum well lasers, carriers are confined to the wells which occupy only a small fraction of the active layer volume. The internal loss owing to absorption induced by the high carrier density is very low, as Httie as 2 cm . The output efficiency of such lasers shows almost no dependence on the cavity length, a feature usehil in the preparation of high power lasers. 

Fig. 3. The lattice-matched double heterostmcture, where the waves shown in the conduction band and the valence band are wave functions, L (Ar), representing probabiUty density distributions of carriers confined by the barriers. The chemical bonds, shown as short horizontal stripes at the AlAs—GaAs interfaces, match up almost perfectly. 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.

It was also reported that microcrystallites of layered semiconductor Pblj were prepared in colloidal form. The spectrum of such a colloidal solution consisted of three absorption bands in the UV which were considerably bu hifted from the absorption threshold of macrocrystalline Pblj. These results were explained by carrier confinement in three differently sized crystallites, each a single layer ( 7 A) thick. However, complexes of Pbl2 with iodide have similar absorption bands, and it seems at the present time that additional experiments have to be carried out to ascertain the colloidal nature of the absorbing species. Size quantization was also reported for colloids of red Hgl2 in acetonitrile... 

Controlled introduction of impurities forms the basis of much of semiconductor technology indeed p-type (acceptor-doped) and n-type (donor-doped) layers and the junctions between them control carrier confinement, carrier flow and ultimately the device characteristics. Achieving both n-type and p-type conductivity has traditionally proved to be a challenge in wide-bandgap semiconductors. 

Quantum Confinement Model. To account for the formation of micropores of less than a few nanometers formed on p-Si, Lehmann and Gosele in the early 1990s postulated that instead of the depletion layer, which is involved in macropores, quantum carrier confinement is responsible for the formation of the micropores on p-Si. The confinement occurs due to an increase in band-gap energy and energy barrier caused by the quantum size porous structure, which prevents the carriers from entering the wall regions of the PS as illustrated in Fig. 8.61. Due to the quantum confinement the pore walls are depleted of carriers and thus do not dissolve during the anodization. 

Single-layer and heterojunction organic LEDs have been fabricated. Single-layer organic LEDs have a low efficiency, due in part to the low probability for exciton formation in the thin film. Heterojunction organic LEDs are much more efficient, since the carrier confinement provided by the heterointerface increases the probability of exciton formation. 

With the advent of sophisticated techniques such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MCXTVD), synthesis of heterostructure such as multiple quantum wells or superlattices with precise interface layer down to one monolayer have been routinely possible. This not only allows modulation of electronic properties such as carrier confinement and concentration profile, but also optical confinement and wave guiding properties with appropriate choice of refractive indices of the materials. Such precise controls over the growth and material properties have opened the field of band gap engineering . 

To attain electroluminescence from the PbBr-based layered perovskite, we employ the heterostmcture device where emissive layer of the PbBr-based layered perovskite are combined with organic carrier-transporting layers. In 1990, Adachi et. al. demonstrated that carriers and excitons to be confined within emissive layer (EML) in organic double heterostructure (DH) device consisting of electron-transporting layer (ETL), EML and hole-... 

The carrier confinement enlarges effective electric field applied to the OXD7/AlLi interface and increase electron-injection. As a result, it is supposed that driving voltage is lowered and EL efficiency is enhanced in the DHD. In other words, employment of CuPc as hole-injection and hole-transporting layer provides good device performance of the DHD. 

AlN layer contained a high density (10 cm ) of threading dislocations (TDs). As mentioned by the above authors, it is necessary to reduce the density of TD and use a carrier confinement structure such as a double heterostructure or a QW structure to improve the internal quantum efficiency. 

Figure Bl.22.4. Differential IR absorption spectra from a metal-oxide silicon field-effect transistor (MOSFET) as a fiinction of gate voltage (or inversion layer density, n, which is the parameter reported in the figure). Clear peaks are seen in these spectra for the 0-1, 0-2 and 0-3 inter-electric-field subband transitions that develop for charge carriers when confined to a narrow (<100 A) region near the oxide-semiconductor interface. The inset shows a schematic representation of the attenuated total reflection (ATR) arrangement used in these experiments. These data provide an example of the use of ATR IR spectroscopy for the probing of electronic states in semiconductor surfaces [44]-...

Fig. 2. Schematic diagram of active layer stmctures employed in LEDs under forward bias showing the conduction band (CB) and valence band (VB). The simplest devices employ (a) a homostmcture active layer wherein the bandgap is constant throughout the device. More advanced stmctures consist of (b) single and (c) double heterostmctures. Heterostmctures faciUtate the confinement and injection of carriers in the active region where the carriers may...

Fig. 4. Schematic cross section and the band diagram of a double heterostmcture showing the band-edge discontinuities, AE and AE used to confine carriers to the smaller band gap active layer, (a) Without and (b) with forward bias. See text.

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). 

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