Optically Detected Magnetic Resonance of Group IV and Group VI Donors in Al 0.6 Ga 0.4 As/GaAs Heterostructures
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OPTICALLY DETECTED MAGNETIC RESONANCE Alo. 6 Gao. 4 As/GaAs HETEROSTRUCTURES
OF
GROUP
IV AND GROUP VI DONORS IN
E. GLASER*, T.A. KENNEDY*, B. MOLNAR*, AND M. MIZUTA** *Naval Research Laboratory, Washington, D.C., 20375 U.S.A. **Fundamental Research Laboratory, NEC Corporation, 34 Miyukigaoka, 305, Japan
Tsukuba
ABSTRACT The influence of chemically different donor species on the nature of shallow donor states in Alo. 6 Gao. 4 As/GaAs heterostructures has been investigated by optically detected magnetic resonance (ODMR). Previous theoretical work by Morgan predicts a triplet state for group IV donors and a singlet state for group VI donors. ODMR experiments were performed on as-grown and implanted Si-, Se-, and S-doped epitaxial layers of Alo. 6 Gao. 4 As grown on (001) GaAs substrates. The effective-mass states are modified by the heteroepitaxial strain in these layers. The Si donors are characterized as quasiindependent valley states. The Se and S donors have valley-orbit splitting energies (i.e. chemical shifts) of 19-20 meV . The results indicate that Si, Se, and S donors are on the lattice sites in the metastable state of DX.
I.
INTRODUCTION
The doping of AlGaAs crystals with donor atoms from either group IV or group VI results in the formation of both shallow and deep levels [1]. In current models these levels are associated with the same defect, the DX center [2,3]. However, the microscopic structure of the DX center is a subject of considerable debate (4,5]. Magnetic resonance experiments performed on n-doped Al-Ga..--As/GaAs heterostructures with x20.35 have been most successful in elucidating the nature of the shallow, metastable state associated with the X-point conduction band minima. The symmetry of the shallow state was revealed recently by optically detected magnetic resonance (ODMR) experiments (6] on Si-doped epitaxial layers of AlAs and Al-Ga_.=As (xkO.35). The symmetry of the state was independently verified by electron paramagnetic resonance (EPR) experiments on similar samples [7]. Deep level transient spectroscopy (DLTS) experiments have been performed to study the influence of different chemical species on the nature of the deep state [8,9]. The thermal activation energies of the deep state in AlGaX__As (0.30•x•0.43) doped with group IV impurities (Si, Sn, Ge) were found to decrease with increasing mass number of the impurity. However, there is a constant thermal activation energy (9] for samples doped with a variety of group VI donor atoms (S, Se, Te). The shallow and deep aspects of the DX problem must meet in the central cell. Chemical shifts have been observed in the ionization energies for various shallow donor and acceptor impurities in GaP [10]. An anomalous deepening of the ground state donor level was found for Ge in GaP [11,12]. Thus, we are motivated to study the chemical effects on the nature of the shallow donor states in AlGaAs/GaAs heterostructures. II.
EXPERIMENTAL ASPECTS
The ODMR experiments described in this work were performed on three epitaxial layers (1-1.5 ±m)
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