Technology and electronic properties of PHEMT AlGaAs/In$_{y(z)}$Ga$_{1-y(z)}$As/GaAs compositionally graded quantum wells

Graded In$_y$Ga$_{1-y}$As quantum well epitaxial technology is developed for engineering the band potential profile. The crystal structure of the samples is clarified by high-resolution X-ray diffraction. The influence of quantum-well bending on the crystal and electron transport properties is studied on one- and two-side $\delta$-doped Al$_{0.23}$Ga$_{0.77}$As/In$_y$Ga$_{1-y}$As/Al$_{0.23}$Ga$_{0.77}$As PHEMT heterostructures. The highest InAs content gradient reached is 1.2%/nm for the mean InAs content $y$ = 0.2. Optimization of the InAs content grading leads to an increase in the electron mobility and concentration. This effect is related to the straightening and deepening of the quantum-well potential profile. In addition, the electron wavefunction shifts toward the quantum-well center, thus reducing electron scattering.