# III-Nitride Short Period Superlattices for Deep UV Light Emitters
## 4 AlN/AlGa(In)N SPSL Doping and Ohmic Contacts  
The efficiency of deep UV LEDs is very sensitive to the doping level of p- and n-type emitters. There are no significant issues with n-type doping of III-Ns compounds and their alloys, since Si, a common n-type dopant, behaves as a shallow donor, even in wide bandgap AlGaN alloys [87]. Although the activation energy of Si significantly increases in AlN [88; 89; 90], the resistivity of Si-doped AlN/Al\({}_{\mathrm{x}}\)Ga\({}_{1-\mathrm{x}}\)(In)N (0.05 \(<\) x \(<\) 0.1) SPSLs is very low, 0.015-0.040 \(\Omega\)-cm, and the electron concentration exceeds 10\({}^{19}\) cm\({}^{-3}\)[91; 42]. Such n-type SPSLs can also be used as contact layers. Using a Ti/Al/Ti/Au stack annealed at 700 \({}^{\circ}\)C, specific contact resistance of the order of 10\({}^{-5}\)\(\Omega\)\(\cdot\)cm\({}^{2}\) was obtained for AlN/AlGa(In)N SPSL with \(\sim\)5.1 eV bandgap [92]. Unfortunately, for III-Ns compounds and their alloys, there is only one convenient p-type dopant, Mg, the experimentally determined activation energy of which varies from \(\sim\)120 to \(\sim\)220 meV in GaN [93; 94], and reaches more than 500 meV in AlN [94; 95]. A detailed analysis of the Mg activation energy in p-AlGaN epitaxial layers over the entire composition range was recently published [96]. It should be noted that if Mg-doped AlGa(In)N is grown using MOCVD, then Mg activation is required. This can be done by both rapid thermal annealing at elevated temperatures [97; 98] and holding the sample under an electron beam irradiation [99; 100; 101]. It was also shown that Mg-O co-doping reduces acceptor activation energy in GaN [102; 103], as in AlN [104]. However, this method of doping is not widely used since oxygen can react with aluminum and gallium, forming undesirable oxides of these metals, especially when used in molecular beam epitaxy. The hole density at room temperature in Al\({}_{\mathrm{x}}\)Ga\({}_{1-\mathrm{x}}\)N:Mg alloys with Mg concentration at \(\sim\)10\({}^{19}\)-10\({}^{20}\) cm\({}^{-3}\) is shown in Figure 6a. Note that all Mg concentrations were obtained using secondary ion mass spectrometry (SIMS) [25; 103]. A significant decrease in the hole concentration with an increase in the Al content in AlGaN is consistent with an increase in the activation energy of Mg in the wide bandgap layers. For AlN/Al\({}_{\mathrm{x}}\)Ga\({}_{1-\mathrm{x}}\)(In)N (0.03 \(<\) x \(<\) 0.08) SPSLs with 2-3 ML thick wells, and periods of the order of 6-8 MLs, the average AlN concentration in SPSLs can be changed in the range of \(\mathrm{y}_{\mathrm{ave}}\) = 0.5-0.8. The average Mg concentration in these SPSLs is usually at the level of 10\({}^{19}\) cm\({}^{-3}\). The concentration of holes can be at the level of 10\({}^{18}\) cm\({}^{-3}\), even in SPSLs with high average AlN content, as is seen in Figure 6a. Such structures were obtained using both MBE and MOCVD methods [25; 41; 42; 45; 91; 105; 106; 107; 108; 109; 110]. Figure 6b shows the results of temperature-dependent Hall characterization of three SPSLs and one AlGaN layer. The Mg concentration was \(\sim\)10\({}^{19}\) cm\({}^{-3}\) in SPSLs, and \(\sim\)3 \(\times\) 10\({}^{19}\) cm\({}^{-3}\) in AlGaN. Figure 5: Experimental optical reflectance and cathodoluminescence (CL)-obtained effective bandgaps of AlN/Al\({}_{0.08}\)Ga\({}_{0.98}\)N SPSLs vs its period, grown with two nominal well thicknesses (2 and 3 monolayers (MLs), blue and green symbols, respectively) [45]. Theoretical simulations based on two approaches [46; 76] are shown by continuous dashed and solid curves, respectively. Note that the composition of the well was in the range 0.03 \(<\) x \(<\) 0.08 which is very similar to the Al concentration in the AlGaN thick layer. Average AlN concentrations of these SPSLs were yave = 0.65, 0.55, and 0.45. The average SPSL compositions, yave, were obtained using X-ray diffraction [61]. The hole density, here, corresponds to the hole concentration averaged over the full SPSL thickness.