Layer perfection in ultrathin InAs quantum wells in GaAs(001)

Abstract

X-ray standing wave (XSW), x-ray diffraction, and photoluminescence (PL) measurements were used to assess the layer perfection and positions of 1 and 1/2 monolayer (ML) InAs quantum wells buried in GaAs(001). Local structure in the 1-ML films was evaluated using x-ray absorption fine structure (XAFS) measurements. Growth temperature effects were studied in a series of samples produced by metal organic vapor phase epitaxy (MOVPE) between 400 and $600\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ The XSW coherent position of the In atoms decreases with increasing temperature in the 1-ML samples, and the optimal growth temperature is near $550\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ as evidenced by the coherent position of $1.15\ifmmode\pm\else\textpm\fi{}0.02$ and the relatively high coherent fraction of $0.72\ifmmode\pm\else\textpm\fi{}0.08.$ With decreasing growth temperature the XSW measurements may suggest segregation of In atoms, which results in an incorporation of the In into multiple layers. The segregation appears to be reduced at the higher temperatures due to the favorable kinetic conditions created in the MOVPE environment. Low-temperature PL measurements indicate that the sharpest and most intense In-excitonic emission is obtained from the 1-ML sample grown at $530\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ For the $\frac{1}{2}$-ML samples, growth temperatures of 400 and $600\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ produce similar standing wave results, although the PL reveals the higher temperature sample to be of far superior quality, due to excessive carbon incorporation at $400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ In-As bond-length distortions found in the XAFS measurements agree with a macroscopic elastic description of the pseudomorphic epitaxy.