Shallow donor and deep DX-like center in InAlN layers nearly lattice-matched to GaN
Nonintentionally doped 200-nm-thick In0.16Al0.84N/n(+)-GaN samples were grown by metal-organic vapor phase epitaxy and used for the electrical characterization of InAlN. In the temperature range 180-400 K, the forward current of Schottky diodes is dominated by a tunneling mechanism below 1.2 V. Capacitance and conductance-temperature characteristics were measured at 1 MHz in the 90-400 K range and at various voltages. The conductance vs temperature reveals two peaks D-1 and D-2, which are attributed to bulk states in InAlN. Their characterization by admittance spectroscopy gives thermal activation energies of approximate to 68 meV and 290 meV, and thermal capture cross section of 9.7 x 10(-17) cm(2) and approximate to 6.2 x 10(-15) cm(2), respectively. The same levels are also revealed by extracting the temperature dependence of the carrier density in the neutral region of InAlN from I-V-T characteristics on the Schottky diode. A partial carrier freeze out is demonstrated and discussed in the framework of an existing theory for DX centers. The use of this approach is supported by the evidence of persistent photoconductivity effects, which strongly indicate the presence of DX centers in our material. It results that each donor in InAlN would exist in two distinct lattice configurations, a substitutional one (D-1, hydrogenic state) and a lattice-distorted one (D-2, DX state). From secondary ion mass spectrometry data, theoretical grounds, and previous experimental evidence in the AlxGa1-xN system, oxygen is the most probable candidate for such an unintentional dopant.