# III-Nitride Short Period Superlattices for Deep UV Light Emitters
## 4 AlN/AlGa(In)N SPSL Doping and Ohmic Contacts
Fitting lines 1, 2, and 3 were calculated using BESST simulator, assuming AlN barriers in the SPSL to be relaxed. It is clearly seen that the hole concentration varies very little with the temperature in SPSLs, and is highly dependent on the temperature (almost 1.5 orders of magnitude) in a uniformly Mg-doped Al0.05 Ga0.95N layer. Similar experimental results were reported by many different teams [105; 106; 107; 108; 109; 110; 111; 112; 113]. The valence band edge profile in AlN(\(\sim\)0.90 nm)/Al0.03 Ga0.97N(\(\sim\)0.52 nm) SPSL, computed by the BESST simulator [76] at room temperature, are shown in Figure 7. The position of the Fermi level in this SPSL corresponds to the zero-energy horizontal line shown in Figure 7a. The acceptor levels in Al0.03 Ga0.97N-well and AlN-barrier layers are indicated by the dashed lines. It is clearly seen that the acceptor levels in the quantum wells are above the Fermi level. Thus, these acceptors should be not activated. In the AlN barriers, the Fermi level crosses the acceptor energy levels, yielding a partial activation of acceptors in these layers. The degree of activation depends  
Figure 6: Hole concentration in Al\({}_{x}\)Ga\({}_{1-x}\)N alloys and AlN/AlGa(In)N SPSLs. (**a**) The dependence on average Al concentration in epitaxial layers (black symbols) and in SPSLs (red symbols and green box); (**b**) The temperature dependence of the hole concentration in Al0.05 Ga0.95N epitaxial layer (\(\sim\)0.5 μm thick, filled circles) and in AlN/Al\({}_{x}\)Ga\({}_{1-x}\)N SPSLs (open symbols). Lines 1, 2, and 3 are the results of the applied BESST simulator [76]. Figure 7: Valence band layout in AlN/Al0.03 Ga0.97N SPSL, computed for room temperature (**a**) and band diagram and heavy (HH), light (LH), and split-off (SH) hole concentrations in this sample (**b**). The acceptor level positions were estimated by neglecting the effect of the hole confinement in the quantum wells. on temperature, creating hole tunneling from barriers to wells, which results in the exponential dependences of hole concentration, as shown in Figure 6.  
Figure 7b shows the distributions of the heavy, light, and split-off holes in the same SPSL obtained by self-consistent solution of the Poisson and Schrodinger equations [76], accounting for the complex valence band structure of III-nitrides [116]. It is clearly seen that the total hole density in such an SPSL mostly depends on the heavy hole concentration. The contribution of the split-off holes can be neglected, due to the fact that this subband in the SPSL is far below the heavy and light subbands. Note, the holes in the SPSL are accumulated in the wells, partially penetrating into the AlN barriers. One could conclude that the efficiency of Mg activation is a very weak function of temperature in wide bandgap AlN/Al\({}_{x}\)Ga\({}_{1-x}\)(In)N SPSL (E\({}_{g}\) from \(\sim\)4.2 to \(\sim\)5.3 eV) [107; 108]. Thus, these SPSLs are very attractive for fabrication of transparent low resistive ohmic contacts for deep UV light emitters. Various contact metallization schemes have been applied for Mg-doped p-GaN, including Au, Ni, Ti, Pd, Pt, Au/Ni, Au/Pt, Au/Cr, Au/Pd, Au/Mg/Au, Au/Pt/Pd, Au/Cr/Ni, Au/Pt/Ni, Au/Ni/Pt, Au-Zn/Ni, Si/Ni/Mg/Ni, etc. [117]. Oxidized Au/Ni is the most common ohmic contact to p-GaN, due to the relatively low specific contact resistance (\(\rho_{c}\)) and simplicity of fabrication [118]. It was shown that formation of crystalline NiO and Ni\({}_{3}\)Ga\({}_{4}\) play an essential role in the reduction of the specific contact resistance of p-GaN [118; 119; 120; 121]. While the formation of NiO yields a reduction of the specific contact resistance, the formation of Ni\({}_{3}\)Ga\({}_{4}\) results in the eventual increase in the contact resistance [119; 120; 121]. The lowest specific contact resistance \(\rho_{c}\)\(\sim\)9.2 \(\times\) 10\({}^{-6}\) and 2 \(\times\) 10\({}^{-6}\)\(\Omega\) cm\({}^{2}\) at 150 \({}^{\circ}\)C were achieved for p-type Mg-doped GaN and Al\({}_{x}\)Ga\({}_{1-x}\)N layers [121; 122].