Enhancement of Curie temperature in Ga1−xMnxAs/Ga1−yAlyAs ferromagnetic heterostructures by Be modulation doping

Abstract

The effect of modulation doping by Be on the ferromagnetic properties of Ga 1-x Mn x As is investigated in Ga 1-x Mn x As/Ga 1-y Al y As heterojunctions and quantum wells. Introducing Be acceptors into the Ga 1-y Al y As barriers leads to an increase of the Curie temperature T C of Ga 1-x Mn x As, from 70 K in undoped structures to over 100 K with the modulation doping. This increase is qualitatively consistent with a multi-band mean field theory simulation of carriermediated ferromagnetism. An important feature is that the increase of T C occurs only in those structures where the modulation doping is introduced after the deposition of the magnetic layer, but not when the Be-doped layer is grown first. This behavior is expected from the strong sensitivity of Mn interstitial formation to the value of the Fermi energy during growth.

Direct evidence of the Fermi-energy-dependent formation of Mn interstitials in modulation-doped Ga1−yAlyAs/Ga1−xMnxAs/Ga1−yAlyAs heterostructures

“…In contrast, significantly lower T C 's were found in inverted MDHs (IMDHs), in which the modulation doped barrier was grown first, followed by the quantum well. It was suggested [13] that in I-MDH holes transferred from the barrier lower the Fermi energy level during the growth of the Ga 1-x Mn x As, causing an increase in the concentration of the compensating Mn I , and thus lowering the T C .…”

“…In all cases the width of the QW was 5.6 nm, the value of x in the well was 0.062, and the total thickness of each barrier was 13.5 nm. Details of the growth conditions were reported elsewhere [13]. Both the Ga 1-y Al y As and Ga 1-x Mn x As layers in the MDHs were grown at the same low temperature of 210ºC.…”

“…Briefly, the Mn concentration in the Ga 1-x Mn x As quantum well is kept constant in this approach, while the hole density in the well is controlled through p-type doping of the Ga 1-y Al y As barrier [13,14]. It was shown that introducing Be acceptors into the Ga 1-y Al y As barriers in all low-temperature-grown Ga 1-y Al y As/Ga 1-x Mn x As/Ga 1-y Al y As QW structures leads to an increase of T C from 70 K in undoped heterostructure to over 100 K in modulation doped heterostructure (MDH) [13]. Also it has been observed that T C strongly depends on the MDH growth sequence.…”

Definitive evidence of interlayer coupling betweenGa1−xMnxAslayers separated by a nonmagnetic spacer

“…23,28 Since the FM exchange in Ga 1-x Mn x As is hole mediated, 6,29 the FM transition temperature of a Ga 1-x Mn x As is increased when it is adjacent to an Al y Be z Ga 1-yz As layer. For our samples, this means that the top and bottom Ga 1-x Mn x As layers will exhibit very different M(T) curves -unless the two layers are coupled across the spacer.…”

“…14,16,17,18 In this way, we determined the thickness, Mn concentration x 20,21 , and M for the individual layers in each of the samples studied. 22 Three 1 cm x 2 cm rectangular samples were prepared by molecular beam epitaxy on GaAs substrates 23 with the following layer structure (starting at the substrate interface): a 16 ±2 nm bottom layer of FM x = 0.05 ±0.015 Ga 1-x Mn x As, a non-magnetic GaAs spacer, an 8 ±2 nm top layer of FM x = 0.05 ±0.015 Ga 1-x Mn x As, and a 25 ±1.5 nm Al 0.25 Be 0.31 Ga 0.44 As cap. 24 We observe that nominally 25 the three samples are structurally identical except for the spacer thickness, which is 12 ±1 , 6 ±1 , and 3 ±1 nm, respectively.…”