42^nd 2000 Electronic Materials Conference
Symposium: Epitaxy for Devices

Influence of misfit dislocations on the mobility in pseudomorphic high electron mobility transistors based on In_xAl_1-xAs / In_0.75Ga_0.25As / InP structures.

R. Hsing, M. Naidenkova, and M.S. Goorsky
Department of Materials Science and Engineering UCLA Los Angeles, CA 90095-1595
R. Sandhu, M. Wojtowicz, T.P. Chin, T.R. Block, and D.C. Streit
TRW Electronic Technology Division Space and Electronics Group Redondo Beach, CA 90278

The reduction of the misfit dislocation density at the interfaces between the channel and the barrier layers improves the electronic properties in InP-based pHEMT structures. Misfit dislocations that form primarily at the bottom interface between the channel and the buffer layer cause interfacial roughness at the top interface between the channel and the donor supply layer. This roughness is found to be a major cause of the mobility reduction. We demonstrate this influence of misfit dislocations on the surface morphology and transport properties of In_xAl_1-xAs / In_0.75Ga_0.25As / InP pHEMT structures with lattice matched (X_In = 0.52) and tensile strained (X_In = 0.48) buffers with channel thickness in the range of 15-40nm.
Both 60° mixed dislocations and 90° edge dislocations form at the interface between the strained In_0.75Ga_0.25As channel and the lattice matched InAlAs buffer or barrier layers. However, misfit dislocations were not present for the same channel structure grown on the tensile strained buffer layer, demonstrating that strain compensation leads to beneficial properties in these pHEMT structures. The surface morphology was also much smoother (4-6 Å r.m.s.) than that for the lattice matched case (10-20 Å), and the room temperature mobility measurements show much higher values for the structures grown on the tensile strained buffer layer. With increasing dislocation density in the lattice-matched barrier structures, the room temperature mobility drops from 12,000 cm²/V×s (channel thickness = 150 Å) to 4,500 cm²/V×s (channel thickness = 400 Å).
For a given sample that possesses misfit dislocations, the differences in carrier scattering along different crystallographic directions corresponds to asymmetric distribution of the different misfit dislocations as determined by magneto-transport Hall bar measurements. In addition, the electrons that occupied the lowest quantum level showed a decreasing mobility with increasing temperature, while the electrons that occupied the second lowest quantum level showed a slight increase in mobility with temperature. Scattering of the lowest energy electrons is a direct result of the fluctuating potential caused by the imperfect In_xAl_1-xAs / In_0.75Ga_0.25As interface (channel/spacer) as modeling shows that these carriers are confined near the top In_0.75Ga_0.25As / InAlAs barrier interface. The electrons at the second quantum level are distributed to both interfaces and show signs of strong scattering by the dislocations, as confirmed by modeling. The use of strain compensating buffer layers can eliminate the formation of misfit dislocations and roughness at the channel interfaces and thus lead to improved device performance.