Study of Raman Scattering on InP/InGaAs/InP HEMTs

Abstract

AbstractRaman scattering studies have been recently used to relate the strain in the semiconductor layer structure with the line shape of allowed modes. It can yield important information about the nature of the solid on a scale of the order of a few lattice constants. It can also provide an evaluation on the carrier concentration in the channel layer of high electron mobility transistors (HEMTs). In this investigation, Raman scattering was used to study the effect of varying the In mole fraction (x) from 0.53 to 0.81 in the InxGa1−xAs channel layer of InGaAs/InP heterostructures. The effect of varying the doping concentration in the donor layer from 6×1017/cm3 to 2.5×1018/cm3, and the effect of varying the In0.75Ga0.25As channel thickness from 140 Å to 260 Å are also reported.A two-mode Raman characteristic for all InxGa1−xAs/InP HEMTs is clearly seen, with the two LO modes (InAs-like LO and GaAs-like LO) located at 229cm−1 and 268.6cm−1, respectively. At a Raman frequency of 347cm−1, a small peak is observed due to InP LO mode. As the In composition increases from 0.53 to 0.81, the InAs-like LO mode peak intensity increases while that of GaAs-like LO mode decreases. The peak intensity ratio of InAs-like LO mode and GaAs-like LO mode increases from 0.78 to 1.10. By increasing the doping concentration in the donor layer (ND), there is also an increase in the carrier concentration in the InGaAs channel assuming that the donors are fully ionised. The coupled mode between the InGaAs longitudinal optical phonons and electrons in the InGaAs channel shifts continuously to a low wave number with the increasing ND in the InP donor layer. The increase in the lnGaAs channel thickness from 140Å to 260Å causes the InAs-like LO mode peak to shift to a lower wave number from 235.5 to 228.5cm−1. There is no change in the GaAs-like LO peak position located at 268.4cm−1.