It is known that some defects undergo a structural transformation to a metastable configuration when capturing a carrier. However, their roles in affecting device performance are not widely recognized. Here we show that metastable oxygen defects can cause recoverable degradation of InAs/AlSb HEMTs (high-electron mobility transistors).

Recently, we performed electrical stress tests on InAs/AlSb HEMTs and observed a recoverable degradation in some of the devices. The degradation is manifested as negative shifts of the transconductance peak and threshold voltage, which nearly completely recover after two days at room temperature. The recoverable nature of the degradation suggests that metastable defects are involved. The threshold shift indicates an increase of donor concentration or a decrease of acceptor concentration in the device. No degradation of mobility in the channel is observed, indicating that the responsible defects are in the AlSb barrier instead of the InAs channel.

 

We propose that the recoverable degradation is caused by a pre-existing defect in the top AlSb barrier being converted into more positive metastable configurations upon capturing the injected holes that are generated by impact-ionization in the channel. To identify the responsible defects, we performed a thorough survey of the defects in AlSb such as intrinsic defects, dopants, and contaminants, using first-principles calculations. We found that among all the candidates, oxygen impurities, both substitutional and interstitial, can account for the observed degradation. More specifically, both oxygen defects undergo large structural changes upon capturing two holes and the resulting states are metastable.

 

It is notable that the metastability of the oxygen defects in these devices does not originate from an energy barrier, but instead from a totally different mechanism. The Fermi level in the AlSb layer is pinned in the lower part of its band gap by the adjacent InAs layer, which ensures low electron concentration in AlSb. This leads to slow electron capture by the metastable defects. Furthermore, upon capturing two holes, the emptied electronic level associated with an oxygen defect is shifted far above the Fermi level due to the large structural relaxation. This energy-level shift ensures slow hole emission from the defect and slow electron tunneling from the adjacent layer. This mechanism results in lifetimes of metastable defects that are consistent with the experiments, and is further confirmed by additional annealing experiments that are done under bias.

 

The work was supported by ONR MURI under Grant No. N-00014-08-10665 and by the McMinn Endowment at Vanderbilt University.