Spin transport and carrier-mediated magnetism in magnetic semiconductor heterostructures

198

Spin transport and carrier-mediated magnetism in magnetic semiconductor heterostructures Nitin Samarth

The study of spin-dependent phenomena involving free carriers in magnetic semiconductor heterostructures has seen rapid advances over the past year, allowing fresh insights into spin transport, localization and carrier-mediated magnetism. These experiments have been enabled by the realization of extrinsically and intrinsically doped magnetic semiconductor heterostructures derived from the II-VI and Ill-V semiconductors. Addresses The Department of Physics, 104

Davey

Laboratory, The

State University, University Park, PA 16802, e-mail: [email protected] Current Opinion 3:198-202

in Solid State

& Materials

Pennsylvania

USA;

Science

1998,

Electronic identifier: 1359-0286-003-00198 0 Current Chemistry ISSN Abbreviations 2DEG two-dimensional IQHE MR MS RKKY

heterostructures has revealed a remarkably rich arena in which the interplay between low-dimensional magnetism and quantum confined electronics can be systematically manipulated to examine fundamental problems in electronic spin scattering, spin coherence and collective magnetism [4]. Due to difficulties in electrical doping, much of the earlier work focused on the effects of optically-created electron-hole pairs (excitons). However, recent advances in materials growth that include both the successful doping of II-VI MS heterostructures [5**,6’*,7,8”] and the invention of new MS heterostructures derived from the III-V semiconductors [9”,10”,11’,12’] have enabled the study of free carrier effects. These developments raise fresh questions concerning spin-related effects in heterostructures and also comprise important first steps towards the successful realization of spin-dependent electronic devices.

1359-0286

electron gas

integer quantum Hall effect magnetoresistance magnetic semiconductor Ruderman-Kittel-Kasuya-Yosida

Introduction Semiconductor heterostructures have elicited enormous scientific and technological interest for the past two decades, resulting in fundamentally important discoveries such as integer and fractional quantum Hall effects, as well as useful applications in contemporary optoelectronics such as quantum well lasers [l]. The study of magnetic heterostructures formed using metallic multilayers has followed a parallel but separate track, again with substantial interest in both the fundamental and technological aspects of phenomena such as giant magnetoresistance [Z]. In an attempt to bridge these somewhat disparate fields, there have been steadily growing efforts aimed at combining magnetic phenomena with semiconductor heterostructures, with the broad technological aim of controlling spin-spin interactions between quantum confined electronic states and localized magnetic moments, thereby developing a basis for ‘spin electronics.’ One way of achieving this goal is to directly incorporate magnetic atoms into the crystalline lattice of a semiconductor heterostructure. This strategy has been exploited for many years by fashioning magnetic semiconductor heterostructures, most commonly derived from wide bandgap II-VI magnetic semiconductors (MSs) such as (Zn,Mn)Se and (Cd,Mn)Te [3]. Magneto-optical spectroscopy of such

Spin transport and localization 2DEGs

in magnetic

The two-dimensional electron gas (ZDEG) formed in modulation-doped semiconductor heterostructures has become a well-established model 2D system for fundamental studies of electron-electron interactions both in the presence and absence of disorder. By extending this concept to include spin-spin interactions between a ZDEG and local moments, Smorchkova and Samarth [S”] have realized a magnetic ZDEG in which spindependent effects are maximized whilst simultaneously maintaining the sample mobility in a regime which enables clean studies of quantum transport [6”,7]. The generic structure employed is described by the conduction band profile in Figure 1: in such structures, carriers are transferred from an n-doped ZnSe layer to an MS quantum well, created by periodically inserting fractional monolayers of MnSe into (Zn,Cd)Se. The best mobility obtained so far for the nonmagnetic ‘host’ material (ZnSe/(Zn,Cd)Se) is - 104 cm*/Vs [ 13”]; the introduction of magnetic atoms into such a lattice degrades this mobility and hence it is important to design optimal architectures that simultaneously maximize spin effects and the ZDEG mobility. It is noted that the first attempts to exploit the spin-degree of freedom in magnetic ZDEGs were frustrated either by contributions of opposite sign from spin-orbit effects [ 141 or by low mobility samples [ 151. In magnetic ZDEGs, the ferromagnetic s-d exchange interaction between conduction electrons and local moments results in an enhanced electronic spin splitting that is given by:

Spin transport and carrier-mediated

magnetism in magnetic semiconductor heterostructures

Samarth

199

Figure 2

Figure 1

Fractional monolayers MnSe

(a) of

-0

4

a

12

-

B (Tesla) (b)

1

I

I

I

I

I

I

I

,130

-10 nm wide (Zn,Cd)Se quantum well Current Opinion in Solid State & Materials Science

Illustration

of the conduction

band energy

profile

in a magnetic

dopants reside in the barrier region (n-ZnSe) separated by an intrinsic region (i-ZnSe).

which

7-\ %x 110

2DEG

structure created by modulation-doping a (Zn,Cd)Se quantum well region that contains fractional monolayers of MnSe. The n-type

-

-r.L

is spatially

0

\ 1

I\

I

I

4

I

I

8

12

I

L10 16

B (Tesla) AE = gpBB + f (v)(N,

a)

where g is the intrinsic electronic g-factor, B is the magnetic field, pB is the Bohr magneton, f (w) is the wavefunction overlap between the confined state and local moments, a is the s-d exchange integral, N, is the number of unit cells per volume and is the sample magnetization. The latter is empirically described by a modified Brillouin function B,,,($tBB/knT,rf) for S = 5/z, in which the temperature, T is empirically resealed to an effective temperature (T,, = T + T,) where T, accounts for short-ranged antiferromagnetic spin-spin correlations between the Mn ions. As the intrinsic g-factor for electrons is small (e.g. g -1.1 in (Zn,Cd)Se [13”]), the spin splitting is dominated by the second term in Equation 1. This may be viewed in terms of a field- and temperature-dependent ‘effective’ g-factor that can reach values as large as -100. The classic signature of a ZDEG with a modest degree of disorder is the integer quantum Hall effect (IQHE) in which the application of a magnetic field perpendicular to the ZDEG plane results in a vanishing longitudinal sheet resistance (p,,) and a quantized Hall resistance (p,, = v(h/vez)) when an integer number (v) of Landau levels are filled. In magnetic ZDEGs, detailed studies of the IQHE and of the quantum oscillations in pXXreveal that-as a result of the exchange-enhanced spin splitting - the

Current Opinion in Solid State & Materials Science

Longitudinal

and transverse

sheet resistance

(p,, and pxY, respectively)

in a magnetic PDEG sample at (a) T = 400mK and (b) T = 4.2K. The filling factors (v) indicate that the Landau levels are spin resolved from the onset of quantum

oscillations

at -2T

(i.e. from n = 8 onwards).

The

anomalous shapes of the IQHE plateaus are probably the result of sample disorder. The measurements shown here were made using standard Hall bar techniques.

Landau levels involved in quantum transport are completely spin resolved even at relatively high temperatures (Figure 2). Furthermore, a direct measurement of this spin splitting using magneto-optical spectroscopy [ 16’1 and the subsequent construction of a Landau level diagram clearly show that there is a substantial spin polarization of the ZDEG in modest magnetic fields at low temperatures. Figure 3 shows that in the quantum regime - defined by OCR > 1 where FIJI is the cyclotron frequency and T is the quantum lifetime-all the IQHE states of interest are purely separated by a cyclotron gap; the character of the IQHE states in such 2DEGs is therefore fundamentally different from that of traditional 2DEGs where pairs of IQHE states such as v = 1,2 or v = 3,4 may be spinresolved but are separated by a spin-gap. Ironically, the introduction of large spin effects in magnetic 2DEGs renders the energy level structure of these 2DEGs closer to

200

Optical and magnetic materials

that envisaged in theories of the quantum ignore the presence of spin [ 171.

Hall effect that

Figure 3

In addition to providing a model system for studying spinpolarized quantum transport, magnetic ‘2DEGs also constitute a new testing ground for the interplay between electron-electron interactions, spin polarization and disorder. For instance, Figure 2 shows a striking background magnetoresistance (MR) that is positive at low fields and negative at high fields; these characteristics are present even when the magnetic field is parallel to the plane of the ZDEG, indicating that there are important contributions that stem from the spin-splitting of electronic states and/or the magnetization of the sample. Although there is no detailed model for negative MR at present, the behavior is qualitatively consistent with the suppression of spin-disorder scattering as the paramagnetic landscape is smoothed by a magnetic field. The positive MR in weakly localized samples (k,l, > 1, where k, is the Fermi wave vector and 1, is the elastic scattering length) has been attributed to the effects of the spin splitting on the disorder-modified electron-electron interactions, extending to 2D a perturbative field theory [ 181 that had earlier explained similar behavior in 3D MS alloys [19]. A deeper examination of the MR in gated magnetic 2DEGs as a function of carrier density and temperature, however, shows that this interpretation is at best incomplete [ZO’,Zl”], particularly because the perturbative constraint imposed by the theory (k&_ >> 1) excludes significant regimes of experimental interest.

Free carrier induced ferromagnetism dimensional magnetic hole gases

in two-

In the magnetic 2DEGs described above, the magnetization of the paramagnetic lattice is largely unaffected by the presence of a ZDEG. This is consistent with the wellestablished view that the Mn-Mn exchange in II-VI magnetic semiconductors is dominated by short-ranged antiferromagnetic superexchange; contributions from carrier-mediated mechanisms such as the BloembergenRowland and RKKY (Ruderman-Kittel-Kasuya-Yosida) interactions are unimportant [3]. A recent mean-field calculation [Z”] has re-examined the carrier-mediated exchange in heavily doped II-VI MSs in the presence of delocalized or weakly localized carriers. The principal prediction is that a dominant ferromagnetic RKKY Mn-Mn interaction can be produced by manipulating factors such as the carrier concentration, the dimensionality of the sample, quantum confinement and perhaps - disorder. In a simplified picture that ignores the influence of disorder and electron-electron interactions, the Curie-Weiss temperature, 0, associated with the carrier-mediated RKKY exchange is given by (apart from some constants): 0 - p(E,)

I2 f (ur),

(2)

0

4

12

16

Current Opinion in Solid State 8 Materials Science Landau level diagram for the magnetic PDEG sample whose transport data are shown in Figure 2. Solid (dashed) lines correspond to spin down (spin up) states. The dark solid line shows the variation of the Fermi energy with magnetic field. Parameters used in this calculation are: E, = 7 meV at B = 0, me* = 0.14 m. and T = 360mK. The spin splitting parameters used are To = 2.1 K and a saturation conduction band spin splitting of 12.9 meV; these were obtained by fitting magneto-optical data.

where p(E,) is the density-of-states of the carriers at the Fermi energy, E,, I is the s-d (p-d) exchange integral for electrons (holes), and f (w) represents the wavefunction overlap of the carriers with the magnetic moments in the lattice. It is noted that-in contrast to metals-the period of the oscillatory RKKY interaction, here, is much longer than the nearest-neighbor magnetic-ion distance. Consequently, the interaction is purely ferromagnetic and a ferromagnetic phase will result if the RKKY exchange overwhelms the antiferromagnetic superexchange (i.e. 0 > T,). As 0 is enhanced by increasing the density-of-states and the carrier-ion exchange, an attractive system for observing a ferromagnetic transition is a magnetic ZD hole gas (ZDHG) in a II-VI MS quantum well where both the effective mass and the p-d exchange are much larger than for electrons. To verify this prediction, Haury et a/. [8”] carried out a magneto-optical study of modulation p-doped (Cd,Mn)Te quantum wells containing a ZDHG with a sheet concentration in the range (1.6-3.2) x l@* holes cm-z, as deduced from the Moss-Burstein shift between photoluminescence (PL) excitation and photoluminescence spectra. The key observation is that the PL spectra reveal a distinct spin splitting at zero magnetic field below a critical temperature T, = 1.8K, indicating the onset of a ferromagnetic phase;

Spin transport and carrier-mediated

magnetism in magnetic semiconductor heterostructures Samarth

such behavior is not observed in an undoped control sample. From the temperatureand field-dependence of the PL spectra, the Curie-Weiss temperature 0 = T, - To is determined; an extension of Equation 2 to include the effects of disorder and electron-electron interactions allows one to relate 0 to a Fermi liquid parameter with a ‘reasonable’ value, providing credible support for essential predictions of the mean-field theory developed in [Z?“].

Carrier-induced ferromagnetism I I I-V heterostructures

in magnetic

Although holes enable carrier-mediated ferromagnetism in II-VI MS heterostructures, present limitations on doping and the intrinsic value of the carrier-ion exchange make it difficult to achieve carrier-induced ferromagnetism at high temperatures. An exciting alternative has begun to emerge from the fabrication of new families of MS heterostructures that incorporate Mn into a III-V semiconductor resulting in the formation of the alloys (In,Mn)As [23,24] and (Ga,Mn)As [lO”]. The divalent Mn ion substituted on group III lattice sites acts as an acceptor, resulting in a large concentration of holes (-1020 holes/cm-3) that can mediate an RKKY interaction. Furthermore, the exchange-coupling between holes and Mn ions is relatively strong (e.g. N& -3 x 104K in (Ga,Mn)As: about 3 times larger than in (Cd,Mn)Te) such that carrier-induced ferromagnetism can be observed at temperatures as high as 110K [ZS”]. Several III-V-based MS heterostructures have been examined recently: (In,Mn)As/GaSb [lo”], (Ga,Mn)As/GaAs [ 11’1, (Ga,Mn)As/AlAs [1’2’], (In,Mn)As/(Ga,Al)Sb [26] and (Ga,Mn)As/(Ga,In)As [27’]. The focus here is on two key studies that reinforce the picture of hole-mediated spininteractions. The conjecture that the ferromagnetic exchange in (Ga,Mn)As is a hole-mediated RKKY interaction is supported by two experimental observations [25”]: first, magnetization studies show that (Ga,Mn)As layers separated by a nonmagnetic (Al,Ga)As spacer layer become magnetically decoupled when the barrier in the valence band is increased; second, the coupling between (Ga,Mn)As layers systematically decreases with the thickness of a nonmagnetic GaAs spacer layer. Both observations are qualitatively indicative of a hole-mediated coupling. Quantitative support also emerges from a mean-field model for the carrierion RKKY exchange which predicts a Curie temperature that is reasonably consistent with observed values (F Matsukara, H Ohno, A Shen, Y Sugawara, unpublished data). The picture of hole-mediated ferromagnetism in III-V MS is also demonstrated by photo-magnetization studies of (In,Mn)As/GaSb heterostructures with a staggered type-II band alignment [10”]. The composition of the (In,Mn)As layer-and hence the hole concentration-is chosen so that the layer is paramagnetic but close to that required for a ferromagnetic phase transition. When the sample is illuminated with broadband white light at low temperatures

201

(T = SK), a persistent ferromagnetic phase is induced, as evidenced by magnetization studies [lo”]. The experiment essentially demonstrates that holes excited in the GaSb layer migrate to the (In,Mn)As layer where a ferromagnetic phase transition occurs when a critical hole density is exceeded.

Conclusions In summary, the past year has witnessed phenomenal advances in the experimental study of free carrier effects in MS heterostructures, primarily as a result of the invention of new materials. Studies of these spin-engineered systems also raise further fundamental and technological possibilities that hopefully will be addressed in the near future. A few of these are worth mentioning. A magnetically-induced insulator-quantum Hall liquid transition is observed in strongly localized magnetic ZDEG samples (IP Smorchkova, N Samarth, unpublished data). The complete spin polarization of the ZDEG possibly provides a new universality class for fundamental studies of this transition. The mean field model developed in [ZZ”] makes definite predictions about 1D and OD magnetic 2DEGs (i.e. quantum wires and dots). If this model is indeed correct, the singular nature of the density-of-states in such low-dimensional structures should yield interesting physical effects for example, the nonintuitive idea that in a 1D electron gas, the ferromagnetism is enhanced at reduced carrier densities. The rapid development of advanced III-V MS architectures will no doubt continue towards the realization of proof-of-concept devices; for instance, preliminary experiments [27’] already provide a basis for designing spindependent resonant tunneling devices.

References and recommended

reading

Papers of particular interest, published within the annual period of review, have been highlighted as: l l

of special interest * of outstanding interest

1.

Weisbuch C, Vinter B: Quantum Semiconductor Strucfures: Fundamenfals and Applications. Boston: Academic Press; 199 1.

2.

Heinrich B, Bland JAC: Ultrathin Magnetic Springer-Verlag; 1994.

3.

Diet1 T: Diluted magnetic semiconductors. In Handbook on Semiconductors, Vol 3b. Edited by Moss TS. Amsterdam: North. Holland; 1994:i 251.

4.

Awschalom DD, Samarth N: Spin spectroscopy and coherence in magnetic quantum structures. In Dynamical properties of unconventional magnetic systems. Edited by Skjeltorp AT. Amsterdam: Kluwer Academic; 1998, in press. [NATO ASI Series]

Structures.

Berlin:

5. ..

Smorchkova IP, Samarth N: Fabrication of n-doped magnetic semiconductor heterostructures. Appl Phys Leff 1996,69:1640-l 642. This paper establishes a successful scheme for coupling a PDEG to large local concentrations of magnetic moments without significantly sacrificing mobility. 6. ..

Smorchkova IP, Samarth N, Kikkawa JM, Awschalom DD: Spin transport and localization in a magnetic two-dimensional electron gas. Phys Rev Leff 1997,78:3571-3574.

202

Optical and magnetic materials

This is a detailed studv of auantum and diffusive transoort in maanetic 2DEG.s. It essentially demonstrates the creation of a highly spin-pol&ed PDEG at large Landau level filling factors and also addresses the anomalous background MR in these systems using quantum correction theories. 7.

IP, KikkawaJM, Samatth N, Awschalom DD: Quantum transport and magneto-optics in a magnetic two-dimensional electron gas. In Proceedings of the 47~ Conference on Magnetism

Haury A, Wasiela A, Arnoult A, Cibert J, Tatarenko S, Diet1 T, Merle d’Aubigne Y: Observation of a ferromagnetic transition induced bv a two-dimensional hole oas in modulation-dooed CdMnTe quantum wells. Phys Rev L& 1997, 79:513-514. ’ This paper describes the first realization of a ferromagnetic Mn-Mn coupling in a II-VI MS, and is an elegant demonstration of “spin-engineered” design based on the predictions in 1231.

*

Kivelson S, Lee DH, Zhang SC: Global phase diagram quantum Hall effect Phys Rev B 1992,46:2223-2238.

18.

Lee PA, Ramakrishnan TV: Magnetoresistance of weekly disordered electrons. Phys Rev B 1981, 26:4009-4012.

19.

Sawicki M , Diet1 T, Kossut J, lgalson J, Wojtowicz T, Plesiewicz W: Influence of s-d exchange interaction on the conductivity of CdMnSe:ln in the weakly localized regime. Phys Rev Lett 1986, 56:508-511.

Materials. J Appl Phys 1997, 81:4858-4860.

8. ..

l

17.

Smorchkova

and Magnetic

9.

Modulated Semiconductor Structures. Physica B 1998, in press. Steady-state and time-resolved magneto-optical spectroscopy is employed to study the spin splitting and coherent spin dynamics in magnetic 2DEGs.

Ohno H. Shen A. Matsukura

F Oiwa A. Endo A. Katsumoto

S. Ive Y:

(Ga,MnjAs: a new diluted magnetic semiconductor based’on GaAs. Appl Phys Lett 1996, 69:363-365.

This is the first report of the growth of a new MS derived from the technologically important material GaAs. It describes details of the epitaxial growth conditions for successful fabrication of this material. 10. ..

Koshihara S, Oiwa A, Hirasawa M, Katsumoto S, lye Y, Urano C, Takagi H, Munekata H: Ferromagnetic order induced by photogenerated carriers in magnetic Ill-V semiconductor heterostructures of (In,Mn)As/GaSb. Phys Rev Lett 1997, 78:4617-4620. This paper demonstrates the role of holes in mediating a ferromagnetic exchange coupling in (In,Mn)As through the observation of a persistent ferromagnetism created by optically excited holes. It also raises the interesting prospect of optical control of spin properties in hybrid ferromagnetlsemiconductor heterostructures. 11. .

Shen A, Ohno H, Matsukura F, Sugawara Y, Ohno Y, Akiba N, Kuroiwa T: (Ga,Mn)As/GaAs diluted magnetic semiconductor superlattice structures prepared by molecular beam epitaxy. Jpn J Appl Phys 1997, 36:L73-L75. This paper describes the epitaxial growth of (Ga,Mn)As/GaAs superlattices, as well as structural and magneto-transport studies of these samples. Hayashi T, Tanaka M, Seto K, Nishinaga T, Ando K: Ill-V based magnetic(GaMnAs)/nonmagnetic(AIAs) semiconductor superlattices. Appl Phys Lett 1997, 71 :1825-l 827. This paper provides a demonstration of the growth of (Ga,Mn)As)/AIAs MS superlattices by low-temperature molecular beam epitaxy, showing a suppression of ferromagnetism at lower dimensionality.

20. .

13.

Kikkawa JM, Smorchkova IP, Samarth N, Awschalom DD: Room temperature spin memory in two-dimensional electron gases. Science 1997, 27711284-l 287.

Although not the subject of this review, this paper bears on the potential development of spin-dependent devices. The essential result is that a nonmagnetic 2DEG can sustain optically injected angular momentum for surprisingly long times even at room temperature. 14.

Grabecki G, Diet1 T, Sobkowicz P, Kossut J, Zawadski W: Quantum transport studies of grain boundaries in p-Hg,,Mn,Te. Appl Phys Leti 1984,45:1214-l 216.

15.

Scholl S, Schafer H, Waag A, Hommel D, Von Schierstedt K, Kuhn-Heinrich B, Landwehr G: Shubnikov de Haas oscillations in modulation doped CdTe/CdhlnTe quantum well structures. Appl Phys Lett 1993,62:301 O-301 2.

16. .

Kikkawa JM, Smorchkova IP, Samarth N, Awschalom DD: Optical studies of spin precession in magnetic two-dimensional electron gases. In Proceedings of the 8th international Conference on

IP, Kikkawa JM, Samarth N, Awschalom

DD:

Spin-dependent transport in a magnetic two-dimensional electron gas. In Proceedings of the 8th international Conference

on Modulated Semiconductor Structures. Physica B 1998, in press. The studies of spin-transport in [S”] are extended to gated magnetic 2DEGs, so that the anomalous MR can be examined as a function of sheet concentration. The paper raises questions about the rigorous application of quantum correction theories to the MR in these systems. 21. ..

Smorchkova IP, Flack FS, Samarth N, Kikkawa JM, Crooker SA, Awschalom DD: Spin transport and optically-probed coherence in maanetic semicondudor heterostructures. In Proceedinos of the 12tE International Conference on the Electronic Propertiegof Two. Dimensional Systems. Physica E 1998, in press. This is a comprehensive review-of recent investigations into the transport as well as steady-state and time-resolved magneto-optical phenomena in undoped magnetic quantum structures as well as magnetic 2DEGs. 22. l

.

Diet1 T. Haurv A. Merle d’Aubiane Y: Free carrier-induced ferromagnetism in structur& of diluted magnetic semiconductors. Phys Rev B 1997,55:R3347-3350.

This paper describes a theoretical calculation of the carrier-mediated RKKY inter&ion in MS systems of varying dimensionality. Most importantly, it points out the conditions required to achieve ferromagnetic exchange in these systems. 23.

Munekata H, Ohno H, von Molnar S, Segmiiller A, Chang LL, Esaki L: Diluted magnetic Ill-V semiconductors. Phys Rev Lett 1994, 63:1849-1852.

24.

Ohno H, Munekata H, Penney T, von Molnar S, Chang LL: Magnetotransport properties of p-type (In,Mn)As diluted magnetic Ill-V semiconductors. Phys Rev Lett 1992,68:2664-2667.

12. .

..

Smorchkova

in the

25. ..

Ohno H, Matsukura F, Shen A, Sugawara Y, Akiba N, Kuroiwa T: Ferromagnetic (Ga,Mn)As and its heterostructures. In Proceedings of the 8th International Conference on Modulated Semiconductor Heterostructures. Physica B 1998, in press. This paper contains a comprehensive review of recent investigations into the materials science and physics of (Ga,Mn)As. 26.

27. .

Shen A, Matsukara F, Sugawars Y, Kuroiwa T, Ohno H, Oiwa A, Endo A, Katsumoto S, lye Y: Epitaxy and properties of InMnAs/AlGaSb diluted magnetic Ill-V semiconductor heterostructures. Appl Surf Sci 1997,113/114:183-188.

Shen A, Ohno H, Matsukara F, Liu H C, Akiba N, Sugawara Y, Kuroiwa T, Ohno Y: Superlattice and multilayer structures based on ferromagnetic semiconductor (Ga,Mn)As. In Proceedings of the 12th international Conference on the Electronic Properties of TwoDimensional Svstems. Phvsica E 1998. in mess. Apart from descricng the preparation of ‘new superlattices based on (Ga,Mn)As, this paper also demonstrates initial results of I-V spectroscopy of resonant tunneling diodes containing (Ga,Mn)As.