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Improving the Al2O3 / GaAs junction

Using a simple barium (Ba) evaporation technique in a molecular oxygen background, it is now possible to grow crystalline BaO films on GaAs (100) substrates even at room temperature. Such a BaO buffer layer has been found to decrease the density of interface defects at the Al2O3/GaAs(100) junction
A Finnish-Russian collaboration is claiming that it has grown the first crystalline BaO films on a III-V substrate.

Crystalline oxide films are potential parts in the construction of high-quality insulator/III-V junctions which are needed for various components such as transistors, detectors, and solar cells.

The density of disorder-induced defects is expected to decrease at crystalline interfaces compared to the junctions with amorphous insulator such as Al2O3/GaAs. However, it is not straightforward to manufacture crystalline insulator/III-V junctions because III-V surfaces are easily oxidised, leading to the formation of an amorphous oxidised III-V interface layer.

The oxidation of III-V surfaces is an energetically favoured process, and it is very difficult (or impossible) in practice to avoid the oxygen contact of III-V surfaces during growth of the junctions. From the viewpoint of large-scale manufacturing, it is beneficial that the insulator/III-V junctions can be synthesised at low temperatures and that the junctions tolerate high-temperature post annealing without harmful structural changes.

What's more, the band gap of an insulator should be large enough to provide a high confinement barrier for electric carriers. BaO with a band gap of 4.3 eV and a dielectric constant of 33 is a potential insulator for many III-V's.

Now researchers at the University of Turku and Tampere University of Technology in Finland and the Ioffe Institute in Russia have shown that crystalline BaO films can be grown at the temperatures between room temperature (RT) and 400 °C. They evaporated Ba from a solid source in the background of O2 gas injected via a leak valve to a vacuum chamber.
The O2 partial pressure was 1 - 10 × 10-7 mbar in these experiments. A Ba ayer was deposited first on GaAs(100) producing a Ba-induced c (8×2) reconstruction for the starting template, of which low-energy-electron-diffraction (LEED) pattern is shown in Figure 1(a) below.

The Al2O3/GaAs junctions for ex-situ photoluminescence characterisation were grown using a home-made atomic-layer-deposition (ALD) tool, connected directly to the vacuum chamber, using trimethylaluminum and water pulses. Al2O3 films were grown on the top of crystalline BaO/GaAs on the clean GaAs(100) substrate before transferring the samples via air for photoluminescence measurements.

The diffraction spots in Figure 1(b) show a crystalline structure for a 10-monolayer thick BaO film on GaAs(100). Large scale scanning-tunnelling-microscopy image (1300 nm × 1300 nm) from the same BaO/GaAs junction reveals a two-dimensional film showing the epitaxial character of the junction.

If the ratio of the O2 pressure and Ba flux was too high or low, no diffraction spots were observed, and the film contained different phases of BaOx, according to X-ray photoemission measurements.

The zoomed image in the inset of Figure 1(c) shows that the top-most surface of the BaO film has a row structure.

The projections of intensity edges of a differentiated scanning-tunnelling-spectroscopy curve (Figure 1(d)), the band gap of the BaO film was estimated  to be 4.2 ± 0.1 eV.

The comparison of photoluminescence (PL) spectra from the Al2O3 -capped samples in

Figure 2(a) shows that the peak intensity of the GaAs emission is higher for the sample which contains the crystalline BaO buffer than for the sample without the buffer, indicating lower interface defect density for the Al2O3/BaO/GaAs sample.

This is associated to the crystalline nature of the BaO/GaAs interface. X-ray photoemission spectra from both Al2O3/GaAs and Al2O3/BaO/GaAs are similar (except for the Ba peaks), and the fitting analysis of the Ga 3d spectra (inset of Figure 2(a)) for the both samples shows the presence GaAs bulk component and one extra component shifted by +0.6 eV.

Figure 2(b) shows the PL intensity as a function of the post heating of Al2O3/BaO/GaAs. Heating between 350 and  550 °C does not change the intensity appreciably, but heating at 700 °C reduces the PL intensity, indicating an increase in the interface defect density. The PL intensity from the Al2O3/GaAs junction with the post heating behaves in the same way as for Al2O3/BaO/GaAs.

Further details of this work have been published in the paper, "Growth and properties of crystalline barium oxide on the GaAs(100) substrate," by M. Yasir, et al., Applied Physics Letters 103, 191601 (2013). http://dx.doi.org/10.1063/1.4828794

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