# III-Nitride Short Period Superlattices for Deep UV Light Emitters
## 1 Introduction  
The invention of semiconductor double heterostructure laser [1; 2] and the concept of a semiconductor superlattice (SL) [3] can be considered as the foundation of modern semiconductor p-n junction-based light emitters, lasers, and light-emitting diodes. Double heterostructure (DHS) is a semiconductor \"sandwich\" where a layer of narrow band gap semiconductor (referred to as an active layer or a well, depending on its thickness) is placed between n-type and p-type layers (cladding layers) of the wide bandgap semiconductors. Under forward bias, the electrons and holes are injected from the cladding layers into an active layer, and well confined there. Confinement of these injected carriers leads to very effective radiative recombination in direct bandgap semiconductors. The SL periodic layered structure of different crystalline semiconductors, allows for engineering of a bandgap of active (referred as a well) and cladding (referred as a barrier) layers when their thickness is of about a few nanometers. Note, these structures must be grown into two-dimensional (2D) growth mode, in order to get a flat/abrupt interface between all layers. The SLs based on the direct bandgap semiconductors in different combinations with p-n DHS are used for design and fabrication of light emitters operating in very wide range of wavelengths, ensuring the needs of optical communication, medicine, security, lighting, and agriculture [4; 5; 6; 7; 8; 9; 10; 11]. We will discuss only p-n junction-based structures in this review. III-Nitrides, AlN (E\({}_{\text{g}}\) = 6.2 eV, [12; 13]), GaN (E\({}_{\text{g}}\) = 3.4 eV, [13]), InN (E\({}_{\text{g}}\) = 0.7 eV, [14; 15; 16]), and their AlGaInN, AlGaN, AlInN, InGaN alloys, are direct bandgap semiconductors which would significantly influence the development of new optoelectronic and light-emitting devices throughout the 21 st century [17; 18; 19]. Most III-nitride (III-N) light-emitting and laser diodes contain different SLs with a period exceeding 4 nm [20; 21; 22; 23]. These SLs, in addition to the bandgap engineering, allow for reducing the dislocation density propagating into an active layer of III-N light-emitting diodes (LEDs) grown on the heavily lattice-mismatched substrates, like silicon and sapphire [24; 25; 26]. Strain engineering in an active layer of LEDs allows for modification of an internal quantum efficiency of radiative carrierrecombination [27; 28]. The design and fabrication of such light emitters, based on \"long period\" SLs, are well described in many books and reviews [29; 30; 31; 32; 33; 34; 35; 36]. III-N LEDs based on very short period superlattices (SPSLs, sometimes referenced as digital alloys, DA), periodic structures of GaN/AlN, AlGaN/AlN, AlGaInN/AlN, InGaN/GaN, InN/GaN, and InAlN/GaN having a few monolayer thick wells and barriers, and a period not exceeding 2 nm, are very attractive for the design and fabrication of a new generation of light emitters. The main important difference between SPSLs and SL with a long period, is that carriers tunneling through barriers between quantum wells (QWs) in SPSLs already affect energy levels, and even lead to the formation of minibands (at least in the conduction band). Bandgap behavior of the InGaN/GaN, InN/GaN, and InAlN/GaN SPSLs, and their applications in visible and infrared light emitters, are well summarized in recent publications [37; 38; 39; 40]. The GaN/AlN, AlGaN/AlN, and AlGa(In)N/AlN (C\({}_{\text{In}}\) < 0.02 mol fraction) SPSLs are very attractive for deep ultraviolet light emitters [41; 42; 43; 44; 45; 46; 47; 48]. One of the attractive features of AlGa(In)N/AlN SPSLs relates to the formation of very sharp heterointerfaces over the entire range of compositions, which makes it possible to obtain well/barrier thicknesses comparable to the interatomic distance, and to make tunneling the main carrier transport mechanism.