Technical Insight
Superlattices improve electrical properties (Research Review)
The low doping efficiency in III-nitrides is an inherent property of a deep acceptor in wide bandgap materials. For the p-type dopant Mg in AlGaN, the activation energy increases with Al content, which leads to a decrease in the doping efficiency compared to bulk GaN. J Sheuet al. (Elec. Dev. Lett. 2001 22(4) 160) describe the incorporation of Mg-doped Al0.15Ga0.85As strained-layer superlattices (SLS) to improve p-dopant activation. Hall measurements showed a higher conductivity as a result of improved Mg activation efficiency, which was associated with strain-induced piezoelectric fields in the AlGaAs SLS. The authors (from Taiwan s National Central University and Epistar) applied the same AlGaN SLS design to a blue InGaN/GaN LED, and were able to re-duce the operating voltage from the 3.8 V typical of conventional devices to 3.0 V (at 20 mA). Grown on a sapphire substrate, the LED consisted of a nucleation layer and thick Si-doped GaN layer, followed by 5 In0.3Ga0.7N/GaN QWs (3 nm wells, 7 nm barriers). A layer of Mg-doped GaN was followed by the AlGaN/GaN 10-pair SLS (in the ratio 16 nm : 8 nm) and a NiAu transparent contact. Considerable improvements in the electrical properties were measured: the hole carrier concentration was 3 1018/cm3, or 30 times that of a bulk AlGaN layer. In addition, a very low contact resistance of 4 106 O.cm2 and series resistance of 18 O were obtained, compared to 40 O in an AlGaN device without the SLS.
