13.1 The UV spectral range The ultraviolet-visible boundary is at about 390 nm, where the 1978 CIE eye sensitivity curve has a value of 0.1% of its maximum value. This chapter concentrates on materials issues of III-V nitrides, on devices emitting in the ultraviolet (UV, X < 390 nm), and on devices emitting in the violet near the UV-visible boundary (390-410 nm). Although the latter devices emit in the visible spectrum, they are frequently classified as UV devices. For UV devices, we will differentiate between devices having a GaInN active region (X > 360 nm) and devices having an AlGaN active region (X < 360 nm).

There are two classifications of the UV spectrum, both of which are shown in Fig. 13.1. The UV-A (315-390 nm), UV-B (280-315 nm), and UV-C (< 280 nm) classification is based on a convention established during the Second International Congress on Light in 1932 (International, 1932). UV-A radiation from the sun penetrates the earth’s atmosphere (including clouds) and, due to the high energy of UV photons, creates damage to the skin, particularly to the deeper layers of the skin. UV-A radiation also causes cataracts (the clouding of the eye’s lens), which can lead to total blindness. Both UV-B (partially absorbed by the earth’s ozone layer) and UV-C radiation (mostly absorbed by ozone layer) create serious damage to skin and eyes. Because no natural light with X < 280 nm exists at the earth’s surface (i. e. in the UV-C range), this wavelength range is also referred to as the solar-blind range.

An alternative classification of UV radiation has evolved over time. This alternative classification has the following categories: Extreme UV (10-100 nm); vacuum UV (100­200 nm); deep UV (200-320 nm); and near UV (320-390 nm).