The semiconductor heterostructures with shallow impurities in quantum wells (QW) are fruitfully applied lately for development of photodetectors and sources of the Terahertz spectral range. Their attractiveness is caused by the fact that an impurity state energy in heterostructures may vary with changing QW parameters (such as composition of components, QW width, spatial position of an impurity layer) in contrast to bulk doped semiconductors in which this energy is unchangeable. Such advantage may be used for variation of the THz laser emission spectrum and THz photodetector spectral sensitivity.

In the present work we have studied the dependence of the ground state energy, E₀, of the boron impurity in QW (stressed layers of Si_0.88Ge_0.12 in the Si/Si_0.88Ge_0.12 heterostructures) on the spatial position in the well and its width. The energy was determined from the temperature dependence of the Hall coefficient in the range of impurity deionization and from the long wavelength edge of the impurity photoconduction.

It has been found that the magnitude of E₀ in QW is less as compared to the bulk Si_0.88Ge_0.12 and it decreases with increasing a QW width and shifting the impurity d-layer from the QW center to its edge. For example, the magnitude of E₀ decreases by ~ 50 % with variation of the QW width from 150 to 200 A.  For the QW width of 200 A and a shift of impurity from the QW center to its edge it varies by 1.5 to 2 times. It has been shown that variation of E₀ in QW as compared to the bulk material is caused by the change of the effective mass of free carriers, which connected basically with splitting of the light and heavy hole subbands due to internal stress and the effect of size quantization on the energy spectrum, rather than by spatial restriction of wave functions of  the carriers localized on impurities by the barrier potential, which leads only to a stronger localization near an impurity center and, consequently, to an increase of E₀.

From measurements of conduction via the impurity band in the range from 30 down to 2 K under electric fields heating up charge carriers we have determined the magnitude of carrier localization radius r (the Bohr radius of an impurity state). It has been found that its value increases with shifting of impurity position to the QW edge. It has been shown that relationship between r and an impurity state energy in QW does not obey the formula r~ (E₀)^-1/2 following from the Bohr model for the shallow impurities in bulk semiconductors.

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