Double heterostructure

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... 

DH = double heterostructure, homo = homostructure, and SH = single heterostructure. 

VG Kozlov, V Bulovic, PE Burrows, and SR Forrest, Laser action in organic semiconductor waveguide and double-heterostructure devices, Nature, 389 362-364, 1997. 

Double helixes, self-recognition in the self- assembly of, 16 803 Double-heterojunction (DH) structures, for LEDs, 22 173, 174, 175 Double heterostructure (DH), 14 844 Double heterostructure laser diodes, 14 700 Double hetero structure OLEDs, 22 216 Double-immunodiffusion technique, 9 753-754... 

Excited-state relaxation, in photochemical technology, 19 109-111 Excitons. See also Frenkel exciton in double heterostructure OLEDs, 22 217... 

P. Zory and L. D. Comerfold, Grating-coupled double-heterostructure AlGaAs diode lasers, IEEE J. Quant. Electron QEll, 451-457 (1975). 

Penmans P, Eorrest SR (2001) Very-high-efficiency double-heterostructure copper phthalo-cyanine/Cgo photovoltaic cells. Appl Phys Lett 79 126... 

Several heterostructure geometries have been developed since the 1970s to optimize laser performance. Initial homojunction lasers were advanced by the use of heterostructures, specifically the double-heterostructure device where two materials are used. The ability 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 more thin layers to allow for improved electron and hole confinement as well as optical field confinement. 

A series of LEDs with different active-layer thicknesses was grown by low-pressure MOCVD, with the structure given in the schematic in Fig. 6. The LEDs used were double-heterostructure, edge-emitting devices wherein p-type and n-type semiconductors sandwich an undoped, low-band-gap energy semiconductor (active layer). The p-n junction double-heterostructure is more efficient at trapping electrons and holes within the active layer for recombination, enhancing EL efficiency. 

Figure 6 Schematic diagram of the double-heterostructure p-n junction diode used for chemical sensing experiments. Electrical contact is made to the top and bottom surfaces with metal films. (Adapted from Ref. 3.)...

Fig. 7. Schematic of light emitting diodes (a) single-layer device (b) single heterostructure (c) double heterostructure.

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. 

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.

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

Electrically detected magnetic resonance (EDMR) is conceptually similar to ODMR, i.e. the magnetic resonance is observed through spin-dependent electrical rather than optical properties of a sample. Virtually all of the EDMR in GaN-based materials reported to date has bear performed on LEDs and so the device type will serve as a basis for the organisation of this section. Three basic device types have been studied m-i-n-n+ diodes, double heterostructures (DHs) and single quantum wells (SQWs). Some details on these structures can be found elsewhere in this volume [35] and in the original work. 

An example of this is the achievement of Alo.32Gao.68N/GaN SQWs with 4 nm well thickness by RMBE, with PL (77 K) at 343 nm and a linewidth of 11 nm [10]. Additionally, Alo.1Gao.9N/GaN double-heterostructure LEDs with electroluminescence at 371 nm and a narrow linewidth of 8 nm [70] were grown by this method (FIGURE 6). Recently, the first GaN-based LED on Si was reported in [71], employing RMBE to grow a GaN/AlGaN double-heterostructure onto an n-type Si substrate. 

FIGURE 6 Electroluminescence of InGaN/GaN and AlGaN/GaN double heterostructure LEDs grown by PMBE [70] and RMBE [27], respectively. 

FIGURE 1 Comparison of optical absorption and gain spectra of a GalnN/GaN double heterostructure [12], The development of the spectra from absorption to gain with increasing pump power (from top to bottom) is indicative of free-carrier lasing with no influence of localised states. 

After we reported in 1992 p-n junction type GaN LEDs of which the external quantum efficiency was 1.5% [5], we continued to increase this efficiency in homo p-i-n GaN LEDs, single heterostructure (SH) AlGaN/GaN LEDs and asymmetric double heterostructure (ADH) AlGaN/GalnN/GaN LEDs. Over the past years, brightness has been increased from 0.2 cd to 2.5 cd for the above mentioned p-n junction types of LED. The external quantum efficiency was raised to 3.9% at 20 mA as a result of the ADH AlGaN/GalnN/GaN LEDs [7,8],... 

FIGURE I Optical gain spectra for a GalnN/GaN double heterostructure at room temperature for various pump power densities [14],... 

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