Stability of Zn doping profile in modulation-doped multiple quantum well structure

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Abstract
Stability of the Zn doping profile in InGaAs/InGaAsP modulation-doped multiple quantum well (MD-MQW) structures grown by low-pressure metalorganic vapor-phase epitaxy (MOVPE) has been investigated by secondary ion mass spectrometry and transmission electron microscope with wedge-shaped samples. Although excellent stability of Zn profile is confirmed in an as-grown sample with modulation doping (d=3 nm CZn=1×1018 cm−3), the modulation-doping structure diminishes after the second epitaxial regrowth of a p-InP layer (CZn=1×1018 cm−3) carried out by either liquid phase epitaxy or MOVPE, which is applicable for buried heterostructure lasers. However, the modulation-doping profile is successfully preserved even after regrowth of the p-InP layer for 90 min in a sample constructed of an undoped InP clad layer instead of a p-InP clad layer superposed on the MD-MQW layers. A Zn diffusion coefficient in the modulation-doped region is extremely small, having a value of less than 7×10−18 cm2/s. The maximum Zn concentration in the modulation-doped region in barrier layers was found to be 2×1018 cm−3 for obtaining a stable modulation-doping structure. It is proposed that the suppression of both interstitial Zn atoms and subsequently produced interstitial group III atoms, which are generated in the p-InP clad layer via a kick-out mechanism and diffuse into the MD-MQW region, is important for preserving modulation-doping structure.