# III-Nitride Short Period Superlattices for Deep UV Light Emitters
## 5 Deep UV Light Emitters Based on AlN/AlGa(In)N SPSLs  
The ability to change the bandgap in SPSL by several hundred meV, by changing the thickness of the well by 1 ML, opened up a very simple path to the implementation of the DHS LEDs. Since SPSL with particular barrier/well thicknesses has its effective bandgap, we can consider them to form a heterostructure in the sense that an SPSL-based active layer with a smaller effective bandgap is sandwiched between n- and p-type SPSL- based cladding layers with a larger bandgap. A schematic cross-section of a typical DHS for a sub-300 nm LED is shown in Figure 9. The structure consists of six main parts: (1) thin, \(<\)100 nm, AlN buffer; (2) thick, \(\sim\)200 nm, Al\({}_{0.7-0.8}\)Ga\({}_{0.3-0.2}\)N buffer for reduction of dislocation density in a device structure (this layer can be replaced by AlN/GaN SPSL--\"dislocation absorber\"); (3) n-type AlN(5 ML)/Al\({}_{0.92-0.97}\)Ga\({}_{0.08-0.03}\)(In)N(2 ML) SPSL cladding layer of \(\sim\)400 nm; (4) undoped AlN(5 ML)/Al\({}_{0.92-0.97}\)Ga\({}_{0.08-0.03}\)(In)N(3 ML) SPSL active region (5 pairs); (5) p-type AlN(5 ML)/Al\({}_{0.92-0.97}\)Ga\({}_{0.08-0.03}\)(In)N(2 ML) SPSL cladding layer of \(\sim\)200 nm; (6) thin, \(\sim\)2-5 nm, p-type contact layer with the composition of SPSL's well material or pure GaN:Mg. As can be seen from the TEM cross-section of this device, shown in the same Figure 9, the barrier/well interfaces are quite sharp, and the thickness of the well in the active region is greater than in the adjacent n- and p-type barriers. The current-voltage (I-V) characteristic of a typical 260 nm DHS mesa-LED is shown in Figure 10a. The LED turns on at -6.0 V, and has a relatively high differential series resistance \(R_{ser}\) of 115 \(\pm\) 5 \(\Omega\) under forward bias from 8 V to 12 V. The \(R_{ser}\) of mesa diodes is the sum of the contact (\(R_{cont}\)), spreading (\(R_{sp}\)), and vertical (\(R_{ver}\)) resistances. The contact resistance of a 160 \(\upmu\)m diameter diode, obtained from specific contact resistance, is \(R_{cont}\)\(\sim\)90 \(\Omega\). The estimated spreading resistance of this LED is \(R_{sp}\)\(\sim\)20 \(\Omega\), taking into account the resistivity of n-type AlGaN buffer layer (not shown in the inset of Figure 10a). The resistance of the etched part of the mesa, corresponding to transport across the SPSL layers, is \(R_{ver}\approx\rho_{1}\times(h/A)\) where \(\rho_{1}\) is the perpendicular (along the growth direction) resistivity of the SPSL, \(h\) is the height of the mesa (\(\sim\)300 nm, with the thickness of p-SL \(\sim\)200 nm), and \(A\) is the contact area. Finally, we obtain \(R_{ver}\)\(\sim\)5 \(\Omega\), resulting in \(\rho_{1}\)\(\sim\)50 \(\Omega\) cm for a p-type SPSL. Comparing this to the in-plane conductivity (\(\rho_{2}\)) of the p-type SPSL obtained from Hall measurements, \(\rho_{2}\)\(\sim\)4 \(\Omega\) cm, we obtain the conductivity anisotropy \(\rho_{1}/\rho_{2}\)\(\sim\)13. These simple considerations indicate relatively low \(\rho_{1}\), considering the high AlN fraction in these SPSLs, and underscore the importance of reducing the contact resistance to p-type materials. However, it is also important to lower \(R_{sp}\) by optimization of the buffer layer thickness and its resistivity. Similar I-V characteristics were reported by different teams for LEDs emitting near the same deep UV range of the wavelengths [42; 43; 44; 45; 91; 92; 93; 94; 95; 96; 97; 98; 99; 100; 101; 102; 103; 104; 105; 106; 107; 108; 109; 110; 111; 112; 113; 114; 115; 116; 117; 118; 119; 120; 121; 122; 123; 124; 125; 126; 127; 128]. The typical electroluminescence spectra of DHS LEDs based on AlN/AlGa(In)N SPSLs are shown in Figure 10b. All these spectra were obtained on LEDs with a similar geometry, and with the same currents. A significant decrease in EQE is evident for devices emitting in ever deeper UV.