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The effect of varying the dopant concentration (ND) in the InP donor layer of In0.53Ga0.47As/InP high-electron mobility transistor (HEMT) structure was studied by Raman scattering measurements. The carrier concentration in the InGaAs channel was found to increase when the doping concentration in the donor layer was increased assuming that the donors are fully ionized. The coupled mode between the InGaAs longitudinal optical phonons and the electrons in the InGaAs channel shifts continuously to a lower wave number with the increase in the value of ND in the InP donor layer. The correlation between the observed Raman shift with the carrier concentration in the channel layer can be used to characterize the HEMT structures nondestructively.

Raman 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 InxGal.xAs channel layer of InGaAs/InP heterostructures. The effect of varying the doping concentration in the donor layer from 6x101 7 /cm 3 to 2.5xl 01 8 /cm 3 , and the effect of varying the In 0 . 75 Ga 0 . 25 As channel thickness from 140 A to 260 A are also reported.A two-mode Raman characteristic for all InxGalxAs/InP HEMTs is clearly seen, with the two LO modes (InAs-like LO and GaAs-like LO) located at 229cm1 and 268.6cml, respectively. At a Raman frequency of 347cml, 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 140A to 260A causes the InAs-like LO mode peak to shift to a lower wave number from 235.5 to 228.5cml. There is no change in the GaAs-like LO peak position located at 268.4cm-.