Quantum-well

Multiquantum weUs (QWs) are multilayered structures consisting of sequences of interchanging thin layers (3-10 nm) of two different semiconductors, each 

In recent years improvements in the technological processing has lowered the density of surface electron states in the best MOS structures from 10 to 10 cm eV . The methods commonly used to investigate surface electron states are the field effect, the frequency dependence of the surface conductivity and C-V, deep level transient spectroscopy (DLTS), and electron paramagnetic resonance (EPR) methods [27, 95-99], 

Harrick [100] proposed to use internal reflection spectroscopy to investigate the surface electron states in the dielectric-semiconductor systems. In contrast to the indirect methods of the field-effect and surface conductivity measurements, measurement of the optical absorption allows one to determine the spectral distribution of the surface states directly. However, assuming that the surface state 

The properties of surface states at the SiOa-Si interface could also be inferred by analysis of IR absorption spectra of silicon oxide [94, 108-110]. Althongh the IR absorption of the Si—O—Si stretching mode does not allow for direct determination of the density of surface states at the interface or the traps in the oxide, it does supplement techniques such as the C-V method, RBS analysis, and AES. 

The correlation between the midgap interfacial state density Du and the thickness strain in thermally grown Si02 films has been studied [110]. The At decreases with the oxidation temperature and with the oxide film thickness. The frequency of the Si—O stretching vibrations increases with increasing oxidation temperature and film thickness. The peak position v is expressed as a function of the average Si—O—Si bond angle as 

The dimension at which quantum confinement becomes important, Ax, is derived from the Heisenberg uncertainty principle (see Section 1.2.1). It is found that 

The energy of an electron in a quantum well can be calculated using the approach outiined in Section 2.3.6. If it is assumed that the electron is free, and trapped by an infinite boundary potential, the same equations for a free electron in a metal apply. Thus, the energy, E, of a free electron in a rectangular parallelepiped with edges a, b and c is given by Equation (2.15)  

Exactly the same equations apply to holes, where the effective mass replaces m. The energy levels that arise from trapped holes are called hole subbands. 

The optical properties of quanmm wells are described in Section 14.11.1. 

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With a standard image intensifier, characterized by a gain of more than x10,000, the quantum well effect is clearly avoided. Nevertheless this very high gain reduces the image dynamics unless strong attenuation is introduced at the tube output (iris or neutral filter). Also a standard intensifier is bulky, affected by pincushion distortion and magnetic fields which can be a serious limitation in some applications. 

Maranowski K D, Gossard A C, Unterrainer K and Gornik E 1996 Far-infrared emission from parabolioally graded quantum wells Appl. Rhys. Lett. 69 3522-4... 

Figure C2.16.ll. Changes in the tlireshold eurrent density of diode lasers resulting from new stRieture eoneepts. A homojunetion diode laser was first demonstrated in 1962. SH and DH stand for single and double heterostaieture, respeetively. The best laser perfonuanee is now obtained in quantum well (QW) lasers.

A logical consequence of this trend is a quantum w ell laser in which tire active region is reduced furtlier, to less tlian 10 nm. The 2D carrier confinement in tire wells (fonned by tire CB and VB discontinuities) changes many basic semiconductor parameters, in particular tire density of states in tire CB and VB, which is greatly reduced in quantum well lasers. This makes it easier to achieve population inversion and results in a significant reduction in tire tlireshold carrier density. Indeed, quantum well lasers are characterized by tlireshold current densities lower tlian 100 A cm . ... 

Mews A et al 1994 Preparation, oharaoterization and photophysios of the quantum dot quantum well system CdS/HgS/CdS J. Phys. Chem. 98 934... 

GaAs, GaAlAs, and GaP based laser diodes are manufactured using the LPE, MOCVD, and molecular beam epitaxy (MBE) technologies (51). The short wavelength devices are used for compact disc (CD) players, whereas the long wavelength devices, mostly processed by MBE, are used in the communication field and in quantum well stmctures. 

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). 

The uncertainty principle, according to which either the position of a confined microscopic particle or its momentum, but not both, can be precisely measured, requires an increase in the carrier energy. In quantum wells having abmpt barriers (square wells) the carrier energy increases in inverse proportion to its effective mass (the mass of a carrier in a semiconductor is not the same as that of the free carrier) and the square of the well width. The confined carriers are allowed only a few discrete energy levels (confined states), each described by a quantum number, as is illustrated in Eigure 5. Stimulated emission is allowed to occur only as transitions between the confined electron and hole states described by the same quantum number. 

Fig. 5. Energy levels of electrons and heavy holes confined to a 6-nm wide quantum well, Iuq 53GaQ 4yAs, with InP valence band, AE and conduction band, AE barriers. In this material system approximately 60% of the band gap discontinuity Hes in the valence band. Teasing occurs between the confined...

An even wider range of wavelength, toward the infrared, can be covered with quantum well lasers. In the Al Ga As system, compressively strained wells of Ga In As are used. This ternary system is indicated in Figure 6 by the line joining GaAs and In As. In most cases the A1 fraction is quite small, X < 0.2. Such wells are under compressive strain and their thickness must be carefully controlled in order not to exceed the critical layer thickness. Lasers prepared in this way are characterized by unusually low threshold current density, as low as ca 50 A/cm (l )-... 

Quantum well lasers ia this system typically use ternary Iuq 53GaQ 47AS wells and biaary InP barriers. AH quaternary lasers, ie, lasers ia which both the wells and barriers are formed by quaternary compounds, are also being developed. These stmctures can be lattice matched or strained. 

AlGaAs quantum well infrared photodetector (QWIP) focal planes have achieved sufficient sensitivity out to 10-p.m wavelength to result in scene temperature sensitivity of ca 0.2°C when the focal plane is cooled to 77 K. Spectral sensitivity is shown in Eigure 9c and array information is given in Table 1. The supedattice, a newer tool for achieving controlled activation energy, should present many alternative infrared detection techniques. 

Eig. 13. Absorption between confined energy levels in a quantum well infrared photodetector (QWIP). The energy difference between the... 

P. S. Zory, A., Quantum Well Easers, Academic Press, Inc., San Diego, Calif., 1993. 

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.

In photoluminescence one measures physical and chemical properties of materials by using photons to induce excited electronic states in the material system and analyzing the optical emission as these states relax. Typically, light is directed onto the sample for excitation, and the emitted luminescence is collected by a lens and passed through an optical spectrometer onto a photodetector. The spectral distribution and time dependence of the emission are related to electronic transition probabilities within the sample, and can be used to provide qualitative and, sometimes, quantitative information about chemical composition, structure (bonding, disorder, interfaces, quantum wells), impurities, kinetic processes, and energy transfer. 

Figures Monochromatic CL image (recorded at 1.631 eV) of quantum well boxes, which appear as bright spots. ...

Quantum well interface roughness Carrier or doping density Electron temperature Rotational relaxation times Viscosity Relative quantity Molecular weight Polymer conformation Radiative efficiency Surface damage Excited state lifetime Impurity or defect concentration... 

Figure 3 Composite plot of 2 K excitonic spectra from 11 GaAs/Alo,3Gao 7AS quantum wells with different thicknesses. The well width of each is given next to its emission peak.

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