A study was performed to evaluate the performance of a recently developed type of Fe-based spin-light-emitting diodes (spin-LEDs) that incorporate wetting layers (WLs). [The term "wetting layer" has two slightly different meanings as explained below.] Light beams emitted by the WL Febased spin-LEDs were found to exhibit the same high degree of circular polarization as do those of previously developed Fe-based spin-LEDs, but differ in one very important aspect: they are an order of magnitude brighter than those emitted by their previously developed counterparts. As a consequence, the WL Fe-based spin-LEDs function reliably at room temperature, whereas their previously developed counterparts do not.

This Energy-Band Diagram, not to scale, is representative of a WL Fe-based spin-LED.
In a typical previously developed spin-LED, electron-hole recombination takes place in either a GaAs quantum well (QW) or an InAs quantum dot (QD). The growth of an InAs QD proceeds as follows: InAs is deposited on GaAs and initially forms a two-dimensional layer that is highly strained because of the large mismatch between the GaAs and InAs crystal lattices. This layer, which is what has previously been meant by "wetting layer," has a critical thickness of 1.7 monolayers. The InAs QDs are grown on the WL using an indium flash procedure. Optical-pumping experiments have led to the finding that the WL results in strong (30 percent) circular polarization that remains constant across the temperature range of 5 to 100 K. This degree of polarization corresponds to a ratio τRS ≈ 1, where τR is the radiative relaxation time and τS is the spin relaxation time. This finding prompted the development and study of the present WL Fe-based spin-LEDs, which differ from prior Fe-based InAs-QD spin LEDs in that they incorporate WLs but not InAs QDs.

The layers now denoted as WLs in the present WL Fe-based spin-LEDs differ from the WLs of the prior Fe-based InAs-QD spin LEDs. The layers now denoted as WLs could be characterized more accurately as quantum wells. They have thicknesses between 3 and 4 nm and compositions of InxGa1-xAs, where 0.25 ≤ × ≤ 0.35.

The figure presents the energy-band diagram of a specific WL Fe-based spin-LED, considered in the study reported here, that incorporates three WLs constituting thinner, deeper quantum wells at the middle of a thicker (40-nm-thick), shallower GaAs quantum well. The Al0.1Ga0.9As barrier to the left of the WLs is n-doped, while the A0.3Ga0.7As barrier to the right of the WLs is p-doped. The leftmost 15 nm of the Al0.1Ga0.9As is doped heavily (to a number density 1019 cm-3) to form a Schottky barrier with a 10-nm-thick Fe contact.

The principle of operation is as follows: A magnetic field oriented through the thickness (along a horizontal line in the figure) is applied by an external magnet to saturate the out-of-plane magnetization of the Fe contact. Under this condition, spin-polarized electrons (predominantly in the spin-down state) from the Fe contact tunnel through the reverse-biased Fe/Al0.1Ga0.9As Schottky barrier, while unpolarized holes are injected from a p-doped GaAs buffer layer. Electrons and holes are captured by the WL QWs. The recombination of electrons and holes in the WL QWs results in emission of photons with a degree of circular polarization corresponding to the degree of polarization of the electrons.

In experiments, intensities of electroluminescence from WL Fe-based spin-LEDs was found to be typically an order of magnitude greater than the corresponding intensities of prior spin-LEDs in which recombination takes place in quantum wells that do not contain narrower, deeper WL quantum sub-wells. Moreover, even though only a small fraction of prior spin-LEDs were found to emit at room temperature, all WL spin-LEDs were found to emit at room temperature.

This work was done by Athos Petrou of the Research Foundation of the State University of New York for the Office of Naval Research.


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Evaluation of Performance of WL Fe-Based Spin-LEDs

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This article first appeared in the February, 2009 issue of Defense Tech Briefs Magazine.

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