Abstract
The Fermi level position in (Ga,Mn)N has been determined from the period-analysis of GaN-related Franz-Keldysh oscillation obtained by contactless electroreflectance in a series of carefully prepared by molecular beam epitaxy GaN/Ga1−xMnxN/GaN(template) bilayers of various Mn concentration x. It is shown that the Fermi level in (Ga,Mn)N is strongly pinned in the middle of the band gap and the thickness of the depletion layer is negligibly small. For x > 0.1% the Fermi level is located about 1.25–1.55 eV above the valence band, that is very close to, but visibly below the Mn-related Mn2+/Mn3+ impurity band. The accumulated data allows us to estimate the Mn-related band offsets at the (Ga,Mn)N/GaN interface. It is found that most of the band gap change in (Ga,Mn)N takes place in the valence band on the absolute scale and amounts to −0.028 ± 0.008 eV/% Mn. The strong Fermi level pinning in the middle of the band gap, no carrier conductivity within the Mn-related impurity band, and a good homogeneity enable a novel functionality of (Ga,Mn)N as a semi-insulating buffer layers for applications in GaN-based heterostuctures.
Highlights
Dilute ferromagnetic semiconductors are in the focus of research interest since they combine functionalities of semiconductors and magnetic materials providing a prolific playground for both basic research and technology viable applications[1,2]
It provides a way to determine the built-in electric field in semiconductor structures via an analysis of Franz-Kieldysh oscillation (FKO) which appears in the Contactless electroreflectance (CER) spectrum for samples with medium or strong electric fields[24,25,26,27,28]
The method is based on the determination of the strength of the built-in electric field in an insulating cap layer overgrown on the studied material by means of an analysis of the period of Franz-Keldysh oscillation observed in contactless electroreflectance spectroscopy
Summary
Dilute ferromagnetic semiconductors are in the focus of research interest since they combine functionalities of semiconductors and magnetic materials providing a prolific playground for both basic research and technology viable applications[1,2]. Our approach enables to independently test earlier reports of the position of the Fermi level in (Ga,Mn)N and, allows to determine the band offset between GaN and (Ga,Mn)N. Both are important parameters for an integration of the material in high power nitride devices. The optical studies allowed us to establish a strong mid-gap Fermi level pinning and a lack of a depletion layer in (Ga,Mn)N Those results, in combination with already established insulating character[18,19] and a sizable dielectric strength of 5 MV/cm[20], strongly favor (Ga,Mn)N as an insulating buffer material for applications in (high power) nitride devices. The accumulated results allow us to estimate the Mn-related band offsets in (Ga,Mn)N which we find to take place predominantly in the valence band and amounting to −28 ± 8 meV per %Mn
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