P-type doping of beryllium chalcogenides

In this contribution, we present first results on the p-type doping of beryllium-containing II–VI compounds such as BeTe, (BeMg)Te, (BeZn)Se and (BeMgZn)Se grown by molecular beam epitaxy (MBE) using a nitrogen plasma source. These materials are a very promising alternative to ZnSSe and MgZnSSe ternaries and quaternaries, respectively. The p-type dopability of (BeMg)Te:N as determined by means of van-der-Pauw measurements and far-infrared reflectivity (FIR) spectroscopy almost exceeds that of ZnTe, and free hole concentrations NA - ND of more than 1 × 1020 cm−3 have been reached. Heavily p-doped ternary (BeMg)Te:N layers are obtained easily which are lattice matched to GaAs and have a high structural quality. The sheet and contact resistance of such layers have been below 5 × 10−3 Ωcm and 7 × 10−3 Ωcm2, respectively. Therefore, this material is the ideal replacement for ZnTe:N, which is used nowadays as top contact layer in ZnSe-based blue-green laser devices. (BeZn)Se and (BeMgZn)Se ternaries and quaternaries seem to behave similarly to their sulfur-containing counterparts; an increase of the band gap leads to a decrease of the free hole concentration. Several (BeMgZn)Se/(BeZn)Se single quantum well (SQW) and multiple quantum well (MQW) light-emitting devices (LEDs) have been fabricated which all show a bright, deep blue emission at low turn-on voltages. One reason for the low operating voltage is that the valence band of BeTe is aligned rather well with that of GaAs, and together with the high p-type dopability of BeTe, a smooth transition from the p-GaAs substrate to p-ZnSe via a BeTe/ZnSe pseudograding is possible.