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.

Photoluminescence and Raman scattering measurement were carried out to study the effect of varying the dopant concentration (ND) in the InP donor layer of In0.53Ga0.47As/InP high-electron mobility transistor (HEMT) structures. Assuming that the donors are fully ionized, the carrier concentration in the InGaAs channel layer was found to increase when the doping concentration in the donor layer was increased. From PL, a red shift is observed for the E1–HH1 peak with increasing doping concentration. Both E1–HH1 and E2–HH1 transitions can only be observed for doping concentration greater than 1.5×1018cm−3. Raman characteristic was clearly seen with the two LO modes (InAs-like LO and GaAs-like LO) located at 226 cm−1 and 268 cm−1, respectively. The coupled mode between the InGaAs longitudinal optical phonons and the electrons in the InGaAs channel was observed to shift 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 allows for non-destructive characterization of HEMT structures.

The effect of growth interruption on the top interface of the InGaAs layer in InP/In0.53Ga0.47As/InP heterostructures grown by molecular beam epitaxy was studied by Hall measurements, photoluminescence (PL) spectroscopy and high-resolution X-ray diffraction (HR-XRD). PL results from multiple quantum well structures (three: stacked InGaAs quantum wells of 10, 20 and 40 Angstrom width lattice matched to 300 Angstrom InP barriers) indicated an optimum interruption time of similar to 70 s for obtaining the minimum PL line width. Lattice matched high-electron mobility transistor structures were grown using different growth interruption times similar to MQW structures. A maximum electron mobility of 3500 cm(2)/Vs and 15 900 cm(2)/V s at 300 K and 77 K, respectively, was measured. This time approximately corresponds to the minimum PL line width obtained for the quantum well structure. The variation of the electron mobility with the growth interruption time was attributed to the formation of a thin InAsP or InGaAsP layer on the surface of the InGaAs at the InGaAs/InP heterointerface similar to MOVPE-grown samples. PL and HR-XRD results indicate that a similar phenomenon may be operative in MBE-grown layers also, and an optimum interruption time may provide abrupt interfaces with better electrical and optical properties. (C) 2000 Elsevier Science B.V. All rights reserved.

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 In x Ga 1-x As channel layer of InGaAs/InP heterostructures. The effect of varying the doping concentration in the donor layer from 6×10 17 /cm 3 to 2.5×10 18 /cm 3 , and the effect of varying the In 0.75 Ga 0.25 As channel thickness from 140 Å to 260 Å are also reported. A two-mode Raman characteristic for all In x Ga 1-x As/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 (N D ), 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 .

The effect of varying In mole fraction from x=0.53–0.81 in InxGa1−x As channel layer of InGaAs/InP high electron mobility transistor (HEMT) structure was studied by Hall measurements, photoluminescence spectroscopy and Raman scattering measurements. When x=0.75 a maximum electron mobility of 6130 and 35 600 cm2/V s at 300 and 77 K, respectively was measured. The sheet carrier concentration in the channel did not change with x, and was nearly the same with an average value of 1.5×1012/cm2. A narrow photoluminescence linewidth of 15 meV was measured indicating the high quality of the epilayers. A red shift in the position of the PL peak energy from 0.86 to 0.76 eV was observed with an increase in x. A two-mode Raman characteristic was clearly seen with the two LO modes (InAs-like LO and GaAs-like LO) located at 226 and 268 cm−1, respectively. When the In mole fraction was increased in the channel, the InAs-like LO mode peak intensity increased while that of GaAs-like LO mode decreased. Correspondingly, the peak intensity ratio of InAs-like LO mode and GaAs-like LO mode increased from 0.78 to 1.10.

The effect of varying In mole fraction from x=0.53–0.81 in InxGa1−x As channel layer of InGaAs/InP high electron mobility transistor (HEMT) structure was studied by Hall measurements, photoluminescence spectroscopy and Raman scattering measurements. When x=0.75 a maximum electron mobility of 6130 and 35 600cm2/Vs at 300 and 77K, respectively was measured. The sheet carrier concentration in the channel did not change with x, and was nearly the same with an average value of 1.5×1012/cm2. A narrow photoluminescence linewidth of 15meV was measured indicating the high quality of the epilayers. A red shift in the position of the PL peak energy from 0.86 to 0.76eV was observed with an increase in x. A two-mode Raman characteristic was clearly seen with the two LO modes (InAs-like LO and GaAs-like LO) located at 226 and 268cm−1, respectively. When the In mole fraction was increased in the channel, the InAs-like LO mode peak intensity increased while that of GaAs-like LO mode decreased. Correspondingly, the peak intensity ratio of InAs-like LO mode and GaAs-like LO mode increased from 0.78 to 1.10.