Gallium Manganese Arsenide - Growth

Growth

Like other DMSs, (Ga,Mn)As is formed by doping a standard semiconductor with magnetic elements. This is done using the growth technique molecular beam epitaxy (MBE), whereby crystal structures can be grown with atom layer precision. In (Ga,Mn)As the manganese substitute into gallium sites in the GaAs crystal and provide a magnetic moment. Because manganese has a low solubility in GaAs, incorporating a sufficiently high concentration for ferromagnetism to be achieved proves challenging. In standard MBE growth, to ensure that a good structural quality is obtained, the temperature the substrate is heated to, known as the growth temperature, is normally high, typically ~600°C. However, if a large flux of manganese is used in these conditions, instead of being incorporated, segregation occurs where the manganese accumulate on the surface and form complexes with elemental arsenic atoms. This problem was overcome using the technique of low-temperature MBE. It was found, first in (In,Mn)As and then later used for (Ga,Mn)As, that by utilising non-equilibrium crystal growth techniques larger dopant concentrations could be successfully incorporated. At lower temperatures, around 250°C, there is insufficient thermal energy for surface segregation to occur but still sufficient for a good quality single crystal alloy to form.

In addition to the substitutional incorporation of manganese, low-temperature MBE also causes the inclusion of other impurities. The two other common impurities are interstitial manganese and arsenic antisites. The former is where the manganese atom sits between the other atoms in the zinc-blende lattice structure and the latter is where an arsenic atom occupies a gallium site. Both impurities act as double donors, removing the holes provided by the substitutional manganese, and as such they are known as compensating defects. The interstitial manganese also bond antiferromagnetically to substitutional manganese, removing the magnetic moment. Both these defects are detrimental to the ferromagnetic properties of the (Ga,Mn)As, and so are undesired.

The temperature below which the transition from paramagnetism to ferromagnetism occurs is known as the Curie temperature, T_C. Theoretical predictions based on the Zener model suggest that the Curie temperature scales with the quantity of manganese, so T_C above 300 K is possible if manganese doping levels as high as 10% can be achieved. After its discovery by Ohno et al., the highest reported Curie temperatures in (Ga,Mn)As rose from 60 K to 110 K. However, despite the predictions of room-temperature ferromagnetism, no improvements in T_C were made for several years.

As a result of this lack of progress, predictions started to be made that 110 K was in fact a fundamental limit for (Ga,Mn)As. The self-compensating nature of the defects would limit the possible hole concentrations, preventing further gains in T_C. The major breakthrough came from improvements in post-growth annealing. By using annealing temperatures comparable to the growth temperature it was possible to pass the 110 K barrier. These improvements have been attributed to the removal of the highly mobile interstitial manganese.

Currently, the highest reported values of T_C in (Ga,Mn)As are around 173 K, still well below the much sought room-temperature. As a result, measurements on this material must be done at cryogenic temperatures, currently precluding any application outside of the laboratory. Naturally, considerable effort is being spent in the search for an alternative DMS that does not share this limitation. In addition to this, as MBE techniques and equipment are refined and improved it is hoped that greater control over growth conditions will allow further incremental advances in the Curie temperature of (Ga,Mn)As.