Luminescent Semiconductors

Abstract

The investigation of the optical properties of inorganic solids is currently a very topical theme, not least because of the dramatic changes being made in the use of illuminants. In fact, the energy saved by replacing incandescent bulbs and even energy-saving lamps with the highly efficient light-emitting diodes (LEDs) is considerable, [1,2] since approximately 20 % of the electric energy is used for illumination purposes. In this context two different technologies are employed to produce white light: [3] either three semiconductor diodes in the colors blue/green/red are used (multi-chip LED), or a blue diode is coated with a yellow phosphor (usually Ce-YAG; Figure 1) or with a green and red phosphor (phosphor-converted LEDs). In each case the primary radiation is brought about by semiconductor luminescence. Semiconductors that emit efficiently in the long wavelength range have been known for some time, yet surprisingly the great breakthrough in their use in lighting technology did not come until the semiconductor Ga/InN, which emits in the near UV/blue/green range, was developed in the 1990s.[4] For other applications too, such as display backlighting, general and medical sensor technology, or photovoltaics, luminescent inorganic semiconductors are the future materials of choice owing to their usually hightemperature and long-term stability, environmental tolerance, and their non-toxicity.From a chemical materials viewpoint the number of semiconductor materials with a large band gap used in the optical area is, however, rather limited.[5] Normally these are typically binary II/VI or III/V semiconductors or their mixed crystals. Thus ZnO, ZnS, and CdS have been investigated thoroughly at a relatively early stage, whereas today III/V semiconductors are more likely to be used. GaN, which emits in the near-UV (NUV) range at a wavelength of 364 nm, is particularly suitable for applications with short wavelength emissions. Moreover, there is an adequately efficient emission since unlike GaP, GaN is a direct band-gap emitter, that is, the transition takes place with conservation of momentum. With indirect emitters, the electrons in the conduction band have a different momentum than the holes in the valence band, the transitions are therefore forbidden, and such materials are unsuitable for applications as illuminants. In addition, GaN is relatively insensitive towards defects so that a high efficiency can also be achieved with a high defect concentration. Substitution of some of the Ga ions by In leads to a decrease of the band gap so that depending on the In content blue and green light-emitting diodes, up to an emission wavelength of l max = 540 nm, can be obtained (Figure 1).Modern red and yellow LEDs also contain III/V semiconductor compounds, for example, (AlGaIn)P, in which the wavelength can be varied by variation of the cation ratio. At 650 nm an internal efficiency of almost 100 % is achieved, which however, falls considerably at shorter wavelengths. Moreover, to date it is not possible to obtain efficient hig...

Nanosegregation and Neighbor‐Cation Control of Photoluminescence in Carbidonitridosilicate Phosphors

“…[1][2][3][4][5] Phosphors for white LEDs should have good thermal stability and conversion efficiency…”

“…[3,[6][7][8] The yellow-emitting phosphor (Y,Gd) 3 (Al,Ga) 5 O 12 :Ce 3+ is a well-known commercial example, but the lack of red emission results in cold white light and a low color-rendering index (CRI). [1,7,9,10] Nitride-based materials are more covalent than oxides, which improves thermal stability, and their greater crystal field splitting increases the red emission leading to warmer white light.…”

Comparative analysis on the photoluminescence properties of Cs2BF6:Mn4+ (B = Ge, Si, Zr, Ti) red phosphors for WLEDs

“…The Mn 4+ ‐activated fluoride phosphors have been attracted more and more attentions recently . Mn 4+ is a transition metal ion with 3d 3 external electronic structure.…”

“…10 The Mn 4+ -activated fluoride phosphors have been attracted more and more attentions recently. 7,[11][12][13][14][15][16][17] Mn 4+ is a transition metal ion with 3d 3 external electronic structure. The broad excitation and narrow emission band of Mn 4+ as the consequence of its unique electronic configuration makes it proper for red phosphors.…”

Nanosegregation and Neighbor‐Cation Control of Photoluminescence in Carbidonitridosilicate Phosphors

“…[1][2][3][4][5] Phosphors for white LEDs should have good thermal stability and conversion efficiency and an excitation wavelength range in the UV to blue region (370-460 nm). [1][2][3][4][5] Phosphors for white LEDs should have good thermal stability and conversion efficiency and an excitation wavelength range in the UV to blue region (370-460 nm).…”