Gas source molecular beam epitaxy growth of InAlP band offset reduction layers on p-type ZnSe

One of the major problems in the application of wide-band gap II–VI compound semiconductors to laser diodes is the difficulty of achieving low resistance ohmic contact to p-type ZnSe due to the large valence band offset between metal electrode and ZnSe. To solve this problem, the use of an intermediate p-type InAlP layer to p-type ZnSe as a valence band offset reduction layer is studied by gas source MBE (molecular beam epitaxy). It is found that the surface morphology of the In_0.5Al_0.5P layers grown on (001) ZnSe becomes better as growth temperature is decreased. In the initial stage of InAlP growth on ZnSe, the facets are formed and a rough surface morphology is produced at a high growth temperature of 500°C. Lower temperature growth at about 350°C and the use of the MEE (migration enhanced epitaxy) growth method produces a better surface morphology. Be doping of InAlP layers at low growth temperatures is also studied. It is found that hole concentrations as high as 2 × 10¹⁸ cm⁻³ are easily obtained for p-type InAlP layers grown at 350°C, although a higher Be cell temperature is required than that for a 500°C grown p-type InAlP. These results suggest that the Be-doped InAlP layer can be used as an intermediate layer to form the low resistance ohmic contact to p-type ZnSe.