Deep-Level Impurities: A Possible Guide to Prediction of Band-Edge Discontinuities in Semiconductor Heterojunctions

Abstract

Since transition-metal impurity levels are found to be aligned with respect to each other within a group of isovalent semiconductor compounds, as if a common reference level existed for them, we propose to use this fictitious level for the band alignment in semiconductor heterojunctions. This rule leads to a valence-band discontinuity in Gai_ x Al x As/GaAs heterojunction of A£ v =(0.34 ±0.05)xAis g (AlAs/GaAs) in agreement with the most recent measurements. Predictions of several III-V on III-V and II-VI on II-VI heterojunction band-edge discontinuities are also given.PACS numbers: 73.40.Lq, 71.25.Tn, 71.55.Fr The band-edge discontinuity in semiconductor heterojunctions (HJ) is among the most important parameters characterizing such a structure. Both the experimental results and the predictions by different theoretical models scatter dramatically in spite of the great technological importance of these structures and the necessity of knowing a reasonably accurate value of the discontinuity for the purpose of device engineering. Recent reviews of the subject strongly reflect this situation. l~3 There is a clear indication that the band discontinuities are not only the result of the bulk properties of both constituents of the HJ, but may also depend on the way that the HJ is prepared. Among the most obvious of these variable parameters is the crystallographic orientation of the substrate, the possible reconstruction of the interface, a more or less graded transition from one constituent to the other, the formation of a dipole layer, and other technological factors as discussed by Bauer and Sang. 2 Therefore, two extreme approaches to the problem can be found.(i) The existing universal approaches 4 " 7 are only of limited value in predicting band-edge discontinuities.The key argument is that they are based only on the bulk properties of the constituents. None of them nor the other more elaborate theoretical computations 8 takes into account the complete situation present in reality at the interface (see, e.g., the review by Bauer and Sang 2 ).(ii) In certain well-selected and well-prepared HJ's the discontinuity can be related to bulk properties alone. These restrictions would include, for instance, the formation of an abrupt junction, the absence of extra dipole layers and, therefore, electrically neutral surfaces [e.g., (110) in tetrahedrally coordinated compounds] , and to some extent the choice of the constituents of the HJ from only one ' 'class" of semiconductors, e.g., III-V or II-VI compounds. In this case the band-edge discontinuities are expected to fulfill the properties of "linearity and transitivity" as discussed by Kroemer. 1 To separate the technology-dependent factors from the factors governed only by the bulk properties, it is necessary either to determine experimentally or to predict in a reasonably accurate way the latter contribution.From the experimental side, the achievements of molecular-beam-epitaxy or metal-organic chemicalvapor-deposition growth techniques allow for preparation of a...

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