Bandedge

Figure 21. The energy band diagram (only the conduction band is shown) calculated for the silicon/electrolyte interface with a potential drop of 5 V and different radii of curvature. Ec is the conduction bandedge in the bulk and Ecs is the conduction bandedge at the surface. AE AEj, AE1/2, and AE1/5 are the possible tunneling energy ranges for different radii of curvature. The distribution of occupied states at the interface, Dred, is also schematically indicated. After Zhang.24...

Fig. 2.10. Photobleaching relaxation kinetic curves at the bandedge of absorption spectrum of a CdS colloid obtained in the excess of Cd2+ ions with the particles of various sizes. [CdS] = 10 M, [DCH] = 210 3 M, [TG] = 5-10 3 M. Illumination at X < 360 nm (UFS-1), Cell length 1 is 10 cm, T = 20°C.

Another example for the same kind of effect is n-RuS2 studied extensively by Kuhne and Tributsch (1986). For this material (bandgap 1.3 eV) the valence band is located in the dark at about 0 eV (SCE). The bands are shifted downwards by as much as 2 eV upon illumination. Interestingly, the photocurrent resulting in 02 evolution occured around +1.1 V(SCE), the valence band being located, however, at +2.3 eV, i.e. considerably below the E0(H20/02)-value. The bandedge movement is seen best in capacity measurements which give Hatband potentials as shown in Fig. 7. 

Bandedge and optical functions of AIN. 2 Raman and IR studies of AIN. 3 Photoluminescence/cathodoluminescence of clean samples of AIN... 

The large span of applications of GaN based semiconductors has focused attention on the properties of these compounds. The aim of this Datareview is to quote recent results on the bandedge of GaN. Also, since GaN will be the basis of blue-UV laser diodes, for instance as the guiding layer, the determination of its optical constants will be examined. 

A more detailed k.p description of the valence band dispersion, in particular beyond the quasi cubic model, can be found in [1,4,5,7,8,11,12], The valence bandedge separations are then given by ... 

From the previous data, one can conclude that the bandedge structure of a-GaN, including the effect of heteroepitaxial strain, is well understood. The remaining difficulty is in finding the correct crystal field parameter A , for strain free GaN. Clearly, this point will be solved soon, with inproving sample quality. Similarly, the optical constants of a-GaN, measured by a variety of techniques, are in overall... 

On increasing the temperature, B and even C free excitons are thermally populated and their contribution can be observed in the bandedge luminescence of undoped GaN (see FIGURE 1). At helium temperature weak luminescence peaks located 19 - 20 meV higher in energy than the A line [6,13,19,21] are frequently observed. Their attribution to n - 2 states of the A exciton can be used to evaluate the A Rydberg. Doing so leads to values of 25 2 meV. 

Photocurrent spectra of a p-type GaN epilayer have also been measured [7], From the onset of photoconductivity spectra, it was suggested that metastable centres at 1.1, 1.40, and 2.04 eV above the valence bandedge were responsible for the PPC in Mg-doped GaN, and that Ga vacancies may be responsible for PPC in n-type GaN [7], The spectral dependence of the optical cross section of the PPC related impurities in n-type epilayers has also been measured and the results are shown in FIGURE 4, from which an optical ionisation energy of about 2.7 eV was obtained [8], The correlation between the... 

A phenomenon which is related to the PPC effects is the optical metastability in GaN. Optical metastability has been observed in GaN epilayers and InGaN/GaN multiple quantum wells [24-26]. Optical metastability in bulk GaN was manifested by a photoinduced decrease in the output intensity of the bandedge transition at 365 nm followed by an increase in the output intensity of a new emission band at 378 nm [25], The recovery time associated with the observed optical metastability is very long (weeks). The cause for such an effect was attributed to the presence of traps in bulk GaN [25], In... 

For wide bandgap semiconductors, such as III-V nitrides, the thermal techniques (DLTS, DDLTS, ICTS) can only detect deep levels which are energetically located within 1 eV of either bandedge for practical measurement conditions. To access deep levels near midgap optical excitation of carriers (ODLTS, OICTS) is necessary. 

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