Metal contacts in terahertz quantum cascade lasers (Vol. 42, No. 6)

Schottky contacts should be avoided in electrically-pumped semiconductor laser devices because they cause an extra voltage drop at metal-semiconductor interfaces - wasting power, overheating device active region and degrading performance. Metal stacks of Ni/Ge/Au and Ti/Pt/Au are commonly employed to form ohmic contacts with n-type and p-type III-V semiconductors, respectively. The optical loss of these ohmic metal contacts is negligible in the visible-light/near-infrared range (~hundreds of THz) due to their much lower Plasmon frequencies (1-10 THz). The optical properties of these metals are therefore not a concern in conventional semiconductor diode lasers.

This drastically changes if the lasing frequency approaches the terahertz range (10¹² Hz), i.e., THz quantum cascade lasers (QCLs) that are based on GaAs/AlGaAs multiple quantum-well structures. The high tangent loss of the commonly-used metals in this frequency range could become a substantial part of the total waveguide loss of the lasers, however metal stacks (such as Ti/Au and Ta/Cu/Au) that exhibit low optical loss in the terahertz frequency range from non-ohmic contacts with III-V semiconductors. Researchers often face a dilemma when picking the metals - ohmic or non-ohmic contacts?

We experimentally investigated the electrical and optical behaviours of THz QCLs with four different Au- and Cu-based metal contacts. The QCL device with non-alloyed Ta/Cu/Au exhibits the lowest threshold current density and the highest lasing temperature in pulsed mode. The better performance is attributed to the lower optical loss of the device waveguide in spite of the formation of a Schottky contact. The findings clarify an important issue that will help researchers design and fabricate THz QCLs operating at higher temperatures and eventually at room temperature.