AlGaN/GaN异质结载流子面密度测量的比较与分析

Comparison of measuring methods of sheet carrier density in AlGaN/GaN heterostructures

对MOCVD技术在蓝宝石衬底上生长的不同Al组分AlGaN/GaN异质结进行了范德堡法Hall测量和电容-电压(C-V)测量,发现Hall测量载流子面密度值大于C-V测量值,并且随着AlGaN层Al组分的增加,两种测量值都在增加,同时它们的差值也在增加.认为产生这一结果的原因有两方面.一方面,Ni/Au肖特基金属淀积在AlGaN/GaN异质结上,改变了AlGaN势垒层的表面状态,使得一部分二维电子气(2DEG)电子被抽取到空的施主表面态中,从而减小了AlGaN/GaN异质结界面势阱中的2DEG浓度.随着势垒层Al组分的增加,AlGaN层产生了更多的表面态,从而使得更多的电子被抽取到了空的表面态中.另一方面,由于C-V测量本身精确度受到串联电阻的影响,使得测量电容小于实际电容,从而低估了载流子浓度.

Hall measurement with Van der Pauw method and the Capacitance-Voltage（C-V）characteristics method are performed on AlGaN/GaN heterostructures with different Al contents grown on sapphire substrates by metalorganic chemical vapor deposition.It is found that the value of sheet carrier density obtained from Hall measurement is larger than that deduced from C-V carrier density profile,and both values,as well as the difference between them increase with increasing Al content.This result is ascribed to two reasons.On the one hand,Ni/Au Schottky contact deposited on AlGaN/GaN heterostructure changes the surface states of the AlGaN barrier layer.Some electrons in the two-dimensional electron gas（2DEG）are extracted to the void surface donor states,and consequently the 2DEG sheet carrier concentration is reduced.And with the Al content increasing,the more the surface states of the AlGaN layer,the more the electrons are extracted to the void surface donor states.On the other hand,the precision of C-V measurement is influenced by the series resistance,which causes underestimation of the magnitude of the depletion-layer capacitance and hence the carrier concentration.