Mn-doped III-V Semiconductors Research Articles

Carrier-induced ferromagnetism in the insulating Mn-doped III-V semiconductor InP

Although InP and GaAs have very similar band-structure their magnetic properties appear to drastically differ. Critical temperatures in (In,Mn)P are much smaller than that of (Ga,Mn)As and scale linearly with Mn concentration. This is in contrast to the square root behaviour found in (Ga,Mn)As. Moreover the magnetization curve exhibits an unconventional shape in (In,Mn)P contrasting with the conventional one of well annealed (Ga,Mn)As. By combining several theoretical approaches, the nature of ferromagnetism in Mn doped InP is investigated. It appears that the magnetic properties are essentially controlled by the position of the Mn acceptor level. Our calculations are in excellent agreement with recent measurements for both critical temperatures and magnetizations. The results are only consistent with a Fermi level lying in an impurity band, ruling out the possibility to understand the physical properties of Mn doped InP within the valence band scenario. The quantitative success found here reveals a predictive tool of choice that should open interesting pathways to address magnetic properties in other compounds

Ferromagnetism and impurity band in a magnetic semiconductor: InMnP

We have synthesized ferromagnetic InMnP, a member of the III-Mn-V ferromagnetic semiconductor family, by Mn ion implantation and pulsed laser annealing. Clear ferromagnetic hysteresis loops and a perpendicular magnetic anisotropy are observed up to a Curie temperature of 42 K. Large values of negative magnetoresistance and magnetic circular dichroism as well as an anomalous Hall effect are further evidence of a ferromagnetic order in InMnP. An effort is made to understand the transport mechanism in InMnP using the theoretical models. We find that the valence band of InP does not merge with the impurity band of the heavily doped ferromagnetic InMnP. Our results suggest that impurity band conduction is a characteristic of Mn-doped III-V semiconductors which have deep Mn-acceptor levels.

Bridging semiconductor and magnetism

Carrier-induced ferromagnetism and its manipulation in Mn-doped III-V semiconductors, such as (In,Mn)As and (Ga,Mn)As, offer a wide variety of phenomena that originate from the interplay between magnetism and semiconducting properties, forming a bridge between semiconductor and magnetism. A review is given on the electrical manipulation of magnetism, its understanding, and potential applications both from the physics point of view and from the technological point of view. The electric-field study on magnetism is now being extended to magnetic metals, leading to an energy efficient way of magnetization reversal important for future semiconductor integrated circuit technology, yet another route to bridge semiconductor and magnetism in a fruitful way.

Chemical trends of magnetic interaction in Mn-doped III-V semiconductors

The trends of magnetic coupling strength in Mn-doped III-V semiconductors are explained using a physically transparent band-coupling model, based on first-principles calculations. According to this model, the stability of the ferromagnetism in Mn-doped III-V semiconductors should increase with both the strength of p-d coupling and an effective coupling range parameter, α. However, these two quantities counteract to each other, i.e., increased p-d coupling strength means a decreased α value. Therefore, this competition will lead to the non-monotonic variation of ferromagnetic interaction in Mn-doped common-cation III-V semiconductors as the anion becomes heavier. Our results suggest that Mn-doped GaAs and AlAs are optimal materials for high TC spintronics, in good agreement with experimental observations.

ChemInform Abstract: Electric‐Field Controlled Ferromagnetism in MnGe Magnetic Quantum Dots

AbstractReview: 54 refs.

Electric-field controlled ferromagnetism in MnGe magnetic quantum dots

Electric-field control of ferromagnetism in magnetic semiconductors at room temperature has been actively pursued as one of the important approaches to realize practical spintronics and non-volatile logic devices. While Mn-doped III-V semiconductors were considered as potential candidates for achieving this controllability, the search for an ideal material with high Curie temperature (Tc>300 K) and controllable ferromagnetism at room temperature has continued for nearly a decade. Among various dilute magnetic semiconductors (DMSs), materials derived from group IV elements such as Si and Ge are the ideal candidates for such materials due to their excellent compatibility with the conventional complementary metal-oxide-semiconductor (CMOS) technology. Here, we review recent reports on the development of high-Curie temperature Mn0.05Ge0.95 quantum dots (QDs) and successfully demonstrate electric-field control of ferromagnetism in the Mn0.05Ge0.95 quantum dots up to 300 K. Upon the application of gate-bias to a metal-oxide-semiconductor (MOS) capacitor, the ferromagnetism of the channel layer (i.e. the Mn0.05Ge0.95 quantum dots) was modulated as a function of the hole concentration. Finally, a theoretical model based upon the formation of magnetic polarons has been proposed to explain the observed field controlled ferromagnetism.

Room-Temperature Electric-Field Controlled Ferromagnetism in Mn0.05Ge0.95 Quantum Dots

Room-temperature control of ferromagnetism by electric fields in magnetic semiconductors has been actively pursued as one of important approaches to realize practical spintronic and nonvolatile logic devices. While Mn-doped III-V semiconductors were considered as potential candidates for achieving this controllability, the search for an ideal material with high Curie temperature (T(c) > 300 K) and controllable ferromagnetism at room temperature has continued for nearly a decade. Recently, Mn(0.05)Ge(0.95) quantum dots (QDs) were demonstrated to have a T(c) above 300 K. However, the field control of ferromagnetism based on hole-mediated effect remained at low temperatures and thus prohibited spintronic devices operable at ambient environment. Here, we report a successful demonstration of electric-field control of ferromagnetism in the Mn(0.05)Ge(0.95) quantum dots up to 300 K. We show that, by using quantum structure, high-quality material can be obtained and effective hole mediation due to quantum confinement effect can be achieved. Upon the application of gate bias to a metal-oxide-semiconductor (MOS) capacitor, the ferromagnetism of the channel layer, that is, the Mn(0.05)Ge(0.95) quantum dots, was manipulated through the change of hole concentration. Our results are fundamentally and technologically important toward the realization of room-temperature spin field-effect transistors and nonvolatile spin logic devices.

Ferromagnetism of low-dimensional Mn-doped III-V semiconductor structures in the vicinity of the insulator-metal transition

The structural and transport properties of GaAs/Mn/GaAs/InxGa1−xAs/GaAs quantum wells (x≈0.2) with Mn δ-layer (4–10 at. %), separated from the well by a GaAs spacer, have been studied. The hole mobility in the investigated structures has exceeded the values known for magnetic III-V heterostructures by two orders of magnitude. For structures with the conductivity of the metal type, we have succeeded to observe at low temperatures Shubnikov–de Haas oscillations just confirming the two dimensionality (2D) of the hole energy spectrum. Exactly those 2D holes promote the ferromagnetic ordering of the Mn layer. That has been proven by (i) observing maxima (at 25–40 K) in temperature dependencies of the resistance, which positions agree with calculated values of Curie temperatures (for structures with the indirect interaction of Mn atoms via 2D holes), and (ii) revealing the negative spin-dependent magnetoresistance (NMR) as well as the anomalous Hall effect (AHE), which values are also in good agreement with calculations relating to ferromagnetic 2D III-V systems. As for the structures with the insulator type of the conductivity, their NMR and AHE features evidence the phase separation—the sample fragmentation with the formation of mesoscopic ferromagnetic areas separated by paramagnetic strata of the high tunnel conductivity.

Electronic structure of Mn-doped III-V semiconductor quantum dots

The electronic structure of GaAs and InAs quantum dots (QDs) containing a single substitutional Mn impurity is investigated in the envelope-function formalism. The Mn impurity in these compounds is known to be a shallow acceptor in the configuration ${d}^{5}+h$ and characterized by a strong antiferromagnetic $sp\text{\ensuremath{-}}d$ exchange interaction between the hole $(h)$ and the Mn ion. Our model for the hole states is based on the Luttinger Hamiltonian and the Coulomb potential with a central-cell correction that accounts for the observed binding energy and the effective $g$ factor in the bulk. The binding energy as well as the exchange contribution is found to increase with decreasing QD size. However, in contrast with the case of spherical nanocrystals (NCs), the binding energy in lens-shaped self-assembled QDs in the low-confinement limit is lower than that in the bulk because of their highly anisotropic shape. With an on-center impurity, NCs retain the bulk ${T}_{d}$ symmetry and the ground state is a $j=3/2$-like ${\ensuremath{\Gamma}}_{8}({T}_{d})$ level. In self-assembled QDs it splits into two doublets: ${\ensuremath{\Gamma}}_{6}$ $(|{j}_{z}|=1/2)$ and ${\ensuremath{\Gamma}}_{7}$ $(|{j}_{z}|=3/2)$ of ${D}_{2d}$, which mix in the presence of in-plane asymmetry, both belonging to ${\ensuremath{\Gamma}}_{5}$ of the reduced symmetry ${C}_{2v}$. The order and the splitting between the doublets depend on the degree of confinement and the strain-induced separation between the light- and heavy-hole valence bands. In lattice-matched GaAs/(Ga,Al)As QDs the ground-state doublet is $|{j}_{z}|=3/2$-like in the low-confinement limit. As the lateral size decreases there is a rapid crossover to a $|{j}_{z}|=1/2$-like ground state in QDs of typical sizes. On the other hand, in strained InAs/GaAs QDs the ground state is always $|{j}_{z}|=3/2$-like and the splitting relatively large. The $sp\text{\ensuremath{-}}d$ coupling with the Mn spin $S=5/2$ finally leads to a splitting of the ground-state doublet into six doubly-degenerate levels. The components are close to one another as the effective exchange parameters are an order of magnitude smaller than in the bulk. Our results thoroughly contradict the previously adopted picture based on treating the confinement potential as a small perturbation to the bulk impurity levels. We also consider the lowest two-hole states: the ground state in InAs/GaAs QDs is a singlet almost uncoupled to the Mn spin. We deduce the zero-field fine structure of the excitonic transitions and compare the results with the recently reported photoluminescence spectra.

Transport in the metallic regime of Mn-doped III-V semiconductors

The standard model of Mn doping in GaAs is subjected to a coherent potential approximation (CPA) treatment. Transport coefficients are evaluated within the linear response Kubo formalism. Both normal and anomalous contributions to the Hall effect are examined. We use a simple model density of states to describe the undoped valence band. The CPA band structure evolves into a spin split band caused by the $p\text{\ensuremath{-}}d$ exchange scattering with Mn dopants. This gives rise to a strong magnetoresistance, which decreases sharply with temperature. The temperature $(T)$ dependence of the resistance is due to spin-disorder scattering (increasing with $T$), CPA band-structure renormalization, and charged impurity scattering (decreasing with $T$). The calculated transport coefficients are discussed in relation to the experiment, with a view of assessing the overall trends and deciding whether the model describes the right physics. This does indeed appear to be the case, bearing in mind that the hopping limit needs to be treated separately, as it cannot be described within the band CPA.

Large magnetic moments and anomalous exchange coupling in As-doped Mn clusters

We report electronic and magnetic properties of pure and arsenic-doped manganese clusters from density functional theory using spin-polarized generalized gradient approximation for the exchange-correlation energy. We see for both the cases, noncollinear treatment of spins are indeed necessary. The arsenic atom stabilizes manganese clusters, which is further accompanied by the ferromagnetic $\mathrm{Mn}\text{\ensuremath{-}}\mathrm{Mn}$ coupling for ${\mathrm{Mn}}_{2}\mathrm{As}$ and ${\mathrm{Mn}}_{4}\mathrm{As}$ clusters, resulting in large cluster magnetic moments, 9 and $17\phantom{\rule{0.3em}{0ex}}{\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$, respectively. We show in doped clusters, exchange coupling are anomalous and behave quite differently from the Ruderman-Kittel-Kasuya-Yosida-type predictions. Finally, it is suggested that, if grown in the low temperature MBE, the giant magnetic moments due to ferromagnetic Mn ordering and their large enough exchange coupling parameter in ${\mathrm{Mn}}_{2}\mathrm{As}$ and ${\mathrm{Mn}}_{4}\mathrm{As}$ could play an important role on the ferromagnetism and on the observed large variation in the Curie temperature of the Mn-doped III-V semiconductors.

Magnetic phase structure of Mn-doped III-V semiconductor quantum wells

We present a self-consistent calculation of magnetic phase structure in modulation-doped dilute magnetic III-V semiconductor quantum wells. The spin-split subband structure is calculated at finite temperature and magnetic-field effects on the carrier-induced magnetism are examined in a weak-field regime. The exchange correlation of free carriers is shown to enhance the ferromagnetic tendency. And the temperature ${T}_{\mathrm{th}},$ below which the system spontaneously becomes fully spin polarized, increases as the doping concentration ${N}_{a}$ in nonmagnetic barriers increases. Hysteresis loop of the magnetic quantum well predicts that the remnant magnetization is enhanced as one increases the modulation-doped acceptor impurity concentration.