Multilayers doping

Doping multilayers are formed by alternating n-type and p-type doping. This results in a smooth modulation of the bands, as illustrated in Fig. 9.31, because electrons transfer from the n-type to the p-type layer. The maximum amplitude of the modulation is equal to the shift of the Fermi energy between the n-type and the p-type material. There is incomplete transfer when the layer width is less than the depletion width, so that the modulation amplitude is smaller. For this 

The photoconductivity response of a-Si H nipi structures has an extremely long recombination lifetime. A brief exposure to illumination causes an increase in the conductivity which persists almost indefinitely at room temperature (Kakalios and Fritzsche 1984). An example of this persistent photoconductivity is shown in Fig. 9.32. The decay time exceeds 10 s at room temperature and decreases as the temperature is raised, with an activation energy of about 0.5 eV. 

E efect equilibration provides a plausible explanation of the persistent photoconductivity (Kakalios and Street 1987). According to this model, defect generation is enhanced by the long lifetime of holes in the p-type region of the nipi structure. Holes in the p-type layers 

The charge-induced defect creation mechanism is too slow to be significant at low temperature and the electronic recombination effects reestablish themselves. Low temperature measurements (0-100 K) have been performed using an IR probe beam to modulate the excess carrier density that is in the band tail states (Hundhausen, Ley and Carius 

The carrier lifetime is longer in the nipi structures than for bulk a-Si H, but the excess carrier lifetimes decrease below a few minutes when the temperature is raised above 50 K. It is concluded that the tunneling recombination mechanism is present at low temperatures, but is obscured by the defect creation mechanism at elevated temperatures. 

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Gu, Z., and Xu, S. (2010) Large scale synthesis of N-doped multilayered graphene sheets by simple... 

Toujou, E, Yoshikawa, S., Homma, Y., Takano, A., Takenaka, H.,Tomita, M., Li, Z., Hasegawa,T., Sasakawa, K., Schuhmacher, M., Merkulov, A., Kim, H.K., Moon, D.W, Hong, T, Won, X (2004) Evaluation of BN-delta-doped multilayer reference materials for shallow depth profiling in SIMS round-robin test. Applied Surface Science, 231-232,649-652. 

Figure 4 shows the basic constmetion of the devices used in different appHcations, involving the deposition of multilayers of i -SiH of intrinsic (/), doped ), and closely aUied films, such as amorphous siHcon nitride, SiN, and transparent conducting oxide (TCO). As in crystalline... 

Although a great number of compound semiconductor devices make use of epitaxy to form the cote vertical stmcture of the device, ion implantation (qv) is a powerful tool in creating both horizontal and vertical modifications to a device. Ion implantation can be used to dope a semiconductor either fi- or / -type by using appropriate species. Implantation can also be used to render a region semi-insulating or to initiate multilayer intermixing. 

Yonemura, H., Yanagita, M., Horiguchi, M., Nagamatsu, S. and Yamada, S. (2008) Characterization of mono- and multilayered films with Mn +-doped ZnS nanopartides and luminescence properties of the monolayered films prepared hy applying magnetic fields. Thin Solid Films, 516, 2432—2437. 

A typical multilayer thin film OLED is made up of several active layers sandwiched between a cathode (often Mg/Ag) and an indium-doped tin oxide (ITO) glass anode. The cathode is covered by the electron transport layer which may be A1Q3. An emitting layer, doped with a fluorescent dye (which can be A1Q3 itself or some other coordination compound), is added, followed by the hole transport layer which is typically a-napthylphenylbiphenyl amine. An additional layer, copper phthalocyanine is often inserted between the hole transport layer and the ITO electrode to facilitate hole injection. 

In the vapor-deposited OLED community, a number of approaches have been employed to produce white light emission. White OLEDs have been demonstrated based on multilayer structures, e.g., stacked backlights [153,168], multidoping of single-layer structures [145], phosphorescent monomer-excimer emission layers [169] and on doping of phosphorescent materials into separate bands within the emission zone, called a tri-junction [170]. The trijunction device has produced the highest white OLED efficiency of 16% external quantum efficiency demonstrated thus far [171]. 

An optical microcavity produced by the latter process has been applied to tune the emission from erbium-doped PS [Zh6], Erbium compounds like Er203 are known to exhibit a narrow emission band at 1.54 pm, which is useful for optical telecommunications. Several methods have been used to incorporate erbium in PS. A simple and economical way is cathodic electrochemical doping. External quantum efficiencies of up to 0.01% have been shown from erbium-doped PS films under electrical excitation [Lo2]. The emission band, however, is much broader than observed for Er203. This drawback can be circumvented by the use of an optical cavity formed by PS multilayers. In this case the band is narrowed and the intensity is increased because emission is only allowed into optical cavity modes [Lo3]. 

Multilayer Langmuir-Blodgett films doped with a cyanine dye have been deposited on the surface of a quartz multimode optical fiber and the fluorescence properties investigated. 49 The fluorescence intensity of the films was found to be a periodic function of the number of layers due to the waveguide properties of the films. 

Figure 6.25. Valence band photoemission spectra of 1 ML Ceo on a Ag(lOO) surface as a function of potassium doping. Also shown are the spectra of the clean Ag(lOO) surface and of a Ceo multilayer (bottom). All binding energies are referred to the L f of polycrystalline silver. Reprinted from Surface Science, Vols. 454-456, C. Cepek, M. Sancrotti, T. Greber and J. Osterwalder, Electronic structure of K doped Ceo monolayers on Ag(OOl), 467 71, Copyright (2000), with permission from Elsevier.

Wang P, Metzger RM, Bandow S, Maruyama Y (1993) Superconductivity in Langmuir-Blodgett multilayers of fullerene (Cgo) doped with potassium. J Phys Chem 97 2926—2927... 

Excitation Eunctions of O2 and 02-Doped Ar Eilms. Resonances can be best identified by the structures they produce in excitation functions of a particular energy-loss process (i.e., the incident-electron energy dependence of the loss). Fig. 7 is reproduced from a recent study [118] of the electron-induced vibrational and electronic excitation of multilayer films of O2 condensed on the Pt(lll) surface and shows the incident electron energy dependence of major losses at the indicated film thickness and scattering angles. Also shown in this figure is the scattered electron intensity of the inelastic background... 

Several examples have been reported recently of solution-processed multilayer electroluminescence devices incorporating semiconductor nanocrystals as the active recombination centers (16-18, 164). Recently, attention has also turned to hybrid electroluminescent devices involving transition metal-doped nanocrystals (104, 165-167). Although many challenges remain, including more specific exploitation of the dopants in many cases, the devices demonstrated to date represent a new direction in application of doped semiconductor nanocrystals made possible by the compatibility of these luminescent nanocrystals with solution processing methodologies. 

In one recent study, Mn2+-doped CdS nanocrystals grown with a ZnS passivating shell were used as the recombination centers in direct current (dc) electroluminescent devices (104). The Mn2+ CdS/ZnS nanocrystals were prepared by the inverse micelle procedure (102) (see Section II.C) and these colloids were incorporated into a multilayer device structure by spin-coat... 

Full-color CL in a range from UV to visible has been successfully realized by combining three lanthanide ions in a multilayer sample of stacked GaN QD planes (Hori et al., 2005). The sample consists of 3 planes of Eu-doped, 4 planes of Tm-doped and 10 planes of Tb-doped GaN QDs (repeated 5 times). Intense white light was also observed at RT upon exciting the... 

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