Tuning of the alloy composition of Zn/sub 1-x/Cd/sub x/Se quantum wells by submonolayer pulsed beam epitaxy

Zn/sub 1-x/Cd/sub x/Se ternary alloys are the basis of quantum wells (QWs) employed in the elaboration of blue-green LEDs and lasers. In the case of molecular beam epitaxy (MBE) the ternary alloy is produced by the simultaneous exposure of the substrate to the vapor beams of the Cd, Zn and Se effusion cells. One of the disadvantages of this method is that it is necessary to stop the growth, modify the cell temperatures and wait for several minutes (sometimes more than one hour), if one wants to grow another well in the same sample with a different alloy composition. In this work we present a new approach to grow Zn/sub 1-x/Cd/sub x/Se QWs with different composition without changing the effusion cells temperature: submonolayer pulsed beam epitaxy (SPBE). To grow the alloy the substrate is exposed to Cd-Zn-Se cycles, only one cell is open while the others remain closed. The relevant characteristic of this method is that the exposure time of the substrate to the Cd flux is limited in such a way that the surface is only partially covered by Cd. Under our typical growth conditions (Ts: 250 - 300/spl deg/C) a [001]-Cd or Zn terminated surface presents a c(2/spl times/2) surface reconstruction with a maximum coverage of Q/sub Cd/, /sub Zn//spl sim/0.5 (half monoatomic layer), therefore, we deposit Q/sub Cd/<0.5. Afterwards, Zn is deposited on this surface, we have identified two simultaneous processes during this step: (i) the filling of the Cd vacancies left by the interrupted flux (0.5 - Q/sub Cd/), and, (ii) the chemical interaction between Zn and Cd that leads to substitution of Cd atoms by Zn atoms. Next, selenium is deposited and the growth continues up to a given number of cycles determined by the desired QW thickness. In this way we are able to control the alloy composition and QW thickness by the SPBE growth.