Spatial correlation effects in the charged impurity distribution on the electronic properties of δ‐doped structures

Abstract

Dedicated by the coauthors to Professor Jozef T. Devreese on the occasion of his 65th birthday PACS 71.55. Eq, 73.50.Dn, 73.63.Hs In this paper we will show that the δ-doped GaAs/AlGaAs/GaAs quantum barrier is an ideal system to study deep centers in narrow doping layers with high doping density. By varying the Al content in the barrier, the distance between the Fermi-level and the deep level can be tuned and therefore the number of populated states. By applying hydrostatic pressure this number can be further increased. Our measurements show that in these δ-doped barrier structures electrons are trapped under hydrostatic pressure and that the quantum mobility is enhanced. This mobility enhancement can be explained by MC calculations which include the effect of the spatial correlation of the charge distribution when the deep state is the DX center and has a negative charge state. The fact that we do not observe a PPC effect or a significant change in the mobility after illumination at high pressure suggest that the Fermi-level is pinned by a non-metastable deep state like the A 1 state or that the DX center is modified.1 Introduction The idea of positioning individual atoms on a surface is very appealing and has many futuristic applications other than a 5-atom font size [1]. Using STM atoms can be moved around and placed on the surface in any configuration. Thus we can in principle create an artificial lattice. Especially in the semiconductor field this is an attractive concept because ordering of the doping atoms is expected to increase the electrical activity [2], reduce dopant fluctuations [3] and increase the carrier mobility [4]. However for practical applications too many atoms are involved thus the ordering of impurity atoms should be self-assembling. This can be accomplished by using a modified substrate surface, which has sites to which doping atoms preferentially go. At high enough growth temperatures the impurity atoms are allowed to diffuse to these sites. This technique has been applied to reconstructed surfaces [2,5] and misaligned substrates [6]. Most of these studies were concerned with structural properties and some with the consequences of such ordering on the electronic properties [7]. These growth studies are however difficult and require a lot of samples. There is fortunately a different approach to study spatial correlation effects by obtaining ordering in the charge distribution of randomly ordered impurity atoms. This ordering cannot be as large as a full spatial ordering introduced during growth, but the degree of ordering can be changed within one structure. Such ordering in the charge distribution can be obtained using the DX state associated with the Si donor in GaAs. By applying hydrostatic pressure electrons are trapped in these states. In current models this donor state is either neutral [8] or negatively [9] charged. Thus when DX centers are populated we have a so-called multi-valence system with either positively charged and