PhysicaB325(2003)300–307 Shallow donors in a triple graded quantum well under electric and magnetic field H. Sari a, *, E. Kasapoglu a , I. Sokmen b a Department of Physics, Cumhuriyet University, 58140 Sivas, Turkey b Department of Physics, Dokuz Eyl . ul University, ’ Izmir, Turkey Received 8 July 2002 Abstract The binding energy of the donor impurity in triple graded GaAs–(Ga,Al)As quantum wells is calculated by using a variational approach. The results have been obtained in the presence of a uniform magnetic and electric field applied alongthegrowthdirectionasafunctionoftheimpurityposition.Weshowthatbychangingtheintensityoftheexternal magnetic and electric field, a large spread in the range of the donor binding energy may be obtained. In triple graded quantum wells (TGQWs), the donor binding energy has much stronger electric field dependence than in the square quantum well structures. Our calculations have revealed the dependence of the impurity binding not only on the magnitude but also the field direction, and impurity location in the TGQWs. r 2002 Elsevier Science B.V. All rights reserved. PACS: 71.55 Keywords: Triple quantum wells; Hydrogenic impurities; Electronic confinement; Graded well 1. Introduction In recent years, a considerable amount of work has been devoted to the study of artificial low- dimensional systems, such as semiconductor het- erostructuresduetotheirinterestingbasicphysical properties and the possible technological applica- tions as in photodetector and optoelectronical devices. Among the various systems under current investigation, the quantum wells (QWs) have attained considerable theoretical experimental attention. The QWs are commonly taken to be symmetric, but the asymmetric QWs give new tunable properties which are very important for deviceapplications[1–6].Withinthelastfewyears, there has been great interest in the electronic propertiesintheasymmetricquantumwell(AQW) structures because they are the ideal systems for the study of terahertz electromagnetic radiation from semiconductor heterostructures [7–14]. This interest is mainly due to the possible applications of quantum devices high-speed electronics and to the generation and detection of terahertz band- width signals. In order to get sufficient tuning range, different QW structures such as stepped QW and asymmetric coupled double QW have *Corresponding author. E-mail address: sari@cumhiriyet.edu.tr (H. Sari). 0921-4526/02/$-see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0921-4526(02)01543-0