Deep levels in AlGaN induce not only serious degradation in AlGaN/GaN HEMTs such as drain current instability, noise enhancement and lack of long-term reliability, but also significant reduction of light emitting efficiency in optical devices using AlGaN materials. In spite of its importance, a very few results have been reported for the deep levels in AlGaN, in particular for near-midgap levels.
The purpose of this study is to investigate near-midgap deep levels in AlGaN by analyzing capacitance transient in AlGaN Schottky and MIS diodes.
The Ni/AlGaN Schottky and Ni/AlON/AlGaN MIS diodes were fabricated on a 0.8µm thick AlxGa1-xN(0<x<0.26) layer grown on a sapphire substrate by metal organic vapor phase epitaxy. Typical values of electron concentration and mobility at room temperature were 2 x 1017 cm-3 and 100 cm2/Vs, respectively.
First we characterized capacitance transients in AlGaN Schottky diodes, using a filling bias of +0.5V and an emission bias of -5V at room temperature. The gradual increase in capacitance was observed at -5V, indicating the electron emission from the deep levels in AlGaN. This capacitance transient continued for a long time. For example, the saturation of measured capacitance could not be seen even for a measurement time of 4x104sec. This indicated that the transient included the electron emission response from deep levels with activation energies of 1eV or deeper.
In order to characterize the deep levels in detail, capacitance deep level transient spectroscopy (DLTS) measurement was carried out, using a filling bias of +0.5V and an emission bias of -2V. To focus on extremely deep levels around 1eV or deeper, rate window was set to the order of several seconds, which is much longer than several milliseconds typically used in DLTS. The DLTS signal, measured with a rate window of 3.9sec, showed a large peak around 350K. By analyzing DLTS spectra, the activation energy and capture cross section were estimated to be 0.9eV and 3x10-15cm2, respectively. By considering partial ionization of deep levels in the depletion layer, the trap density was calculated to be 1x1016cm-3. At the higher temperature side to this peak, in addition, further increase was observed in the DLTS signal. Thus, it is likely that deeper levels exist near midgap.
|