ManuscriptWe report the magnetic and transport properties of Ga1-xMnxP synthesized via ion implantation followed by pulsed laser melting over a range of x, namely 0.018 to 0.042. Like Ga1-xMnxAs, Ga1-xMnxP displays a monotonic increase of the ferromagnetic Curie  temperature with x associated with the hole-mediated ferromagnetic phase while thermal annealing above 300 ºC leads to a quenching of ferromagnetism that is accompanied by a reduction of the substitutional fraction of Mn. However, contrary to observations in Ga1-  xMnxAs, Ga1-xMnxP is non-metallic over the entire composition range. At the lower temperatures over which the films are ferromagnetic, hole transport occurs via hopping conduction in a Mn-derived band; at higher temperatures it arises from holes in the valence band which are thermally excited across an energy gap that shrinks with x

Farshchi, R.

Scarpulla, Michael

English

The University of Utah: J. Willard Marriott Digital Library

UU IR Author Manuscript UU IR Author ManuscriptU n iv e r s i ty  o f  U ta h  I n s t i t u t i o n a l  R e p o s i to r yA uthor M anuscriptCompositional tuning of ferrom agnetism  in Gai_xM nxPR. Farshchi1’2, M. A. Scarpulla1’2, P. R  Stone1’2, K. M. Yu2,1. D. Sharp1'2, J.W. Beeman2, H. H. Silvestri1’2, L.A. Reichertz2, E.E. Haller1’2, and O.D. Dubon1,2Dept, o f Materials Science and Engineering, Univ. o f California, Berkeley, CA 94720 “Lawrence Berkeley National Laboratory, Berkeley, CA 94720A bstractWe report the magnetic and transport properties o f Gai_xMnxP synthesized via ion implantation followed by pulsed laser melting over a range o f x, namely 0.018 to 0.042. Like Gai.xMnxAs, Gai-xMnxP displays a monotonic increase o f the ferromagnetic Curie temperature with x associated with the hole-mediated ferromagnetic phase while thermal annealing above 300 °C leads to a quenching of ferromagnetism that is accompanied by a reduction o f the substitutional fraction o f Mn. However, contrary to observations in Gaj. xMnxAs, Gai.xMnxP is non-metallic over the entire composition range. At the lower temperatures over which the films are ferromagnetic, hole transport occurs via hopping conduction in a Mn-derived band; at higher temperatures it arises from holes in the valence band which are thermally excited across an energy gap that shrinks with x.UU IR Author Manuscript UU IR Author ManuscriptU n iv e r s i ty  o f  U ta h  I n s t i t u t i o n a l  R e p o s i to r yA uthor ManuscriptIntroductionThe ability to tune both electronic (charge) and magnetic (spin) functionalities in the same material is essential to the field o f spin-based electronics, or spintronics [1 ]. Promising candidates for this purpose are III-Mn-V ferromagnetic semiconductors, where a few atomic percent Mn atoms occupy the group III (cation) sub-lattice [2]. The substitutional Mn atoms act as acceptors, and in the case o f the archetypal ferromagnetic semiconductor Gai_xMnxAs, the holes they provide mediate the inter-Mn exchange. Despite extensive efforts to understand the nature o f inter-Mn exchange, limited progress has been made in elucidating the interplay between hole transport and ferromagnetism in Ga-Mn-pnictide systems beyond Gai_xMnxAs. Here we report the dependence of magnetism and hole transport on composition in Gai_xMnxP formed by ion implantation and pulsed-laser melting (II-PLM).Ion implantation o f a few atomic percent Mn into the III-V semiconductor lattice followed by pulsed-laser melting is a proven processing route for creating ferromagnetic semiconductors [3-6]. Rapid solidification o f the molten implanted layer upon pulsed- laser irradiation traps the Mn atoms on substitutional lattice positions. The II-PLM process has been used to synthesize Gai_xMnxAs films with magnetic and transport properties similar to films grown by low-temperature molecular beam epitaxy (LT-MBE) [3, 4, 6 ]. More recently II-PLM has been used to synthesize the carrier-mediated ferromagnetic phase in Gai_xMnxP [5, 6 ]. An important difference between these two systems is the larger hole binding energy in Gai_xMnxP, namely 0.4 eV [7], compared to 0.11 eV in Gai_xMnxAs [8]. One therefore expects holes to be more strongly localized in Gai_xMnxP, which may affect the ferromagnetic properties o f this material.2UU IR Author Manuscript UU IR Author ManuscriptU n iv e r s i ty  o f  U ta h  I n s t i t u t i o n a l  R e p o s i to r yA uthor ManuscriptSample PreparationGaP wafers (001) were implanted with Mn+ ions over a dose range between1 S 9  1 ^ 94x10 cm' and 2x10 cm' at an implantation energy of 50 keV. The wafers were then cleaved into approximately 5mm x 5mm square samples and irradiated with a single 0.4 J/cm“ pulse from a KrF excimer laser (X =  248nm, FWHM = 18ns), homogenized to a spatial uniformity o f ±5% by a crossed-cylindrical lens homogenizer. Upon laser irradiation, the amorphous implanted layer melts and regrows via liquid-phase epitaxy, hence restoring the crystallinity o f the ion damaged layer. Finally, an extended HC1 etch was carried out to remove a highly twinned surface layer leaving a single crystalline, single phase Gai.xMnxP film [6 ].Rutherford backscattering spectrometry (RBS) in combination with particle- induced X-ray emission (PIXE) analysis in the <110> and <111> axial channeling directions show that 70 to 85% of the Mn atoms occupy substitutional sites (Mnoa) and that the remaining Mn atoms do not occupy interstitial sites but instead are incommensurate with the lattice (i.e., at random locations). This level o f substitutionality is not unlike Gai_xMnxAs films o f higher Mn concentration grown by LT-MBE, which contain on the order o f 20% non-substitutional Mn [9].The concentration profile o f Mn in the resulting Gai_xMnxP layer was measured by secondary ion mass spectrometry (SIMS). After processing, the Gai_xMnxP films were characterized by a thickness in the range of 60-100 nm and a Mn concentration profile that reaches a maximum approximately 30 nm below the surface. We define x as the peak Mnoa concentration. A maximum value o f Mnoa is given by the product o f overall substitutional Mn fraction (from RBS/PIXE analysis) and maximum Mn concentration3UU IR Author Manuscript UU IR Author ManuscriptU n iv e r s i ty  o f  U ta h  I n s t i t u t i o n a l  R e p o s i to r yA uthor Manuscript(from SIMS). We thus arrive at Mnoa mole fractions (x) ranging from 0.018 to 0.042 for the samples studied.Results and DiscussionThe dependence o f ferromagnetic Curie temperature (Tc) on x as measured with a superconducting quantum interference device (SQUID) magnetometer is presented in Figure 1. Magnetization versus temperature was measured along a <110> in-plane direction in a field o f 50 Oe (after saturation to 50 kOe), and data are shown for four compositions in Figure la. The Curie temperatures for these and other Gai-xMnxP films made by II-PLM are plotted versus x in Figure lb. Contrary to previous reports [10], Tc increases monotonically with x. The linear relationship between Tc and x observed for Gai_xMnxP is not unlike that reported for Gai_xMnxAs and described by mean-field models for hole-mediated exchange [11, 12]. This similarity between Gai_xMnxP and Gai_xMnxAs is consistent with the fact that the Mn L3.2 X-ray magnetic circular dichroism in the two materials is nearly identical [13]. However, it is somewhat surprising given the more strongly localized nature o f Mn in Gai_xMnxP as discussed below. Linear extrapolation o f Tc to room temperature indicates that a composition x o f approximately 0.18 may be required for room temperature ferromagnetism in this system.Four-point sheet resistance measurements using pressed-on indium contacts in the van der Pauw geometry as a function o f temperature are shown in Figure 2. The samples are insulating over the entire composition range. For samples with larger x (and higher Tc), there exists a distinct change in thermally activated transport between the low- temperature and the high-temperature resistivities. We attribute this to two distinct4UU IR Author Manuscript UU IR Author ManuscriptU n iv e r s i ty  o f  U ta h  I n s t i t u t i o n a l  R e p o s i to r yA uthor Manuscripttransport mechanisms [5]: i) one characterized by a smaller activation energy at lower temperatures corresponding to hole hopping in a Mn-derived band and ii) a larger activation energy mechanism associated with thermal excitation o f holes from the Mn band to the valence band at higher temperatures. By comparison of the high temperature activation energies (slopes) as a function of composition, we find that the excitation energy for holes into the valence band decreases with increasing x. The charge hopping regime is not as apparent for the samples with lower x due to reduced inter-Mn wavefunction overlap leading to a strong increase in the activation barrier for hopping.In general the charge hopping temperature regime corresponds to that o f the ferromagnetic state o f Gai_xMnxP. Thus, holes mediating ferromagnetism in Gai-xMnxP are strongly localized in contrast to the picture ascribed to Gai_xMnxAs in which delocalized or weakly localized valence-band holes mediate ferromagnetism [1 1 ].The presence of an excitation gap for localized holes in a Mn-derived band into the valence band is manifested in the infrared (IR) photoconductivity spectra o f these films. In these measurements a constant bias is applied across the sample, and the electrical current through it is measured as a function of photon energy. As a result a spectrum of the photoconductive response o f the sample is obtained. Figure 3 shows a photoconductivity spectrum measured at 5 K for Gai_xMnxP with x ~ 0.032. The spectrum is characterized by a threshold in photoconductive response near 70 meV. Below this edge there is no signal as has been confirmed by the measurement o f the far IR photoconductivity spectrum using a Fourier transform spectrometer with greater sensitivity in this photon energy range (inset o f Figure 3). Similar experiments were carried out previously for x ~ 0.042 and yielded a photoconductivity response threshold5UU IR Author Manuscript UU IR Author ManuscriptU n iv e r s i ty  o f  U ta h  I n s t i t u t i o n a l  R e p o s i to r yA uthor Manuscriptat a considerably lower energy of 26 meV [5]. The compositional dependence o f both the threshold and the activation barrier for hole transport paints a picture o f holes localized in a Mn-derived band that is separated from the valence band by an energy gap that shrinks with increasing x.Similar to Gai_xMnxAs, carrier-mediated ferromagnetism in Gai_xMnxP is not stable against thermal annealing above 300 °C [9, 14]. Figure 4a shows magnetization versus temperature for samples annealed at 300 °C - 400 °C for 15 minutes in flowing N 2 gas. These measurements show a significant decrease o f Tc upon thermal annealing, which as reflected by PIXE measurements is accompanied by a relocation o f a fraction of Mn atoms from substitutional to incommensurate (random) sites.Figure 4b presents Tc versus x. The tuning of x is achieved by variation o f the implantation dose (as presented in Figure 1) and by thermal annealing o f samples having an initial (prior to annealing) Mnoa mole fraction o f x ~ 0.034. The stronger dependence o f Tc on x for the series o f annealed samples suggests that relocation o f Mnoa to incommensurate sites leads to the partial compensation o f the remaining Mnoa or to the formation of defect complexes that render a fraction o f Mnoa ferromagnetically inactive. We note that a similar effect has been observed in Gai_xMnxAs films grown by LT-MBE[9]. Finally, the thermal instability o f Mnoa in Gai_xMnxP at temperatures above 300 °C demonstrates that any high-temperature ferromagnetism in Mn-containing GaP that has been subjected to extended exposure to such temperatures is unlikely to originate from the dilute Gai_xMnxP (alloy) phase.S u m m a r y6UU IR Author Manuscript UU IR Author ManuscriptU n iv e r s i ty  o f  U ta h  I n s t i t u t i o n a l  R e p o s i to r yA uthor ManuscriptWe have determined the compositional dependence o f Tc in Gai-xMnxP. The films are non-metallic for x ranging from 0.018 to 0.042. and hopping conduction is observed at temperatures over which the films are ferromagnetic. At higher temperatures the sheet resistance is governed by holes that are thermally excited into the valence band across an energy gap that shrinks with increasing x. Finally, Mnoa in Gai_xMnxP is not stable to thermal annealing above 300 °C, which causes a strong decrease o f Tc accompanied by a relocation of a fraction o f Mnca to incommensurate sites.AcknowledgementsThis work is supported by the Director, Office o f Science, Office o f Basic Energy Sciences, Division of Materials Sciences and Engineering, o f the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. P.R.S. acknowledges support from an NDSEG Fellowship.UU IR Author Manuscript UU IR Author ManuscriptU n iv e r s i ty  o f  U ta h  I n s t i t u t i o n a l  R e p o s i to r yA uthor ManuscriptReferences[1] G.A. Prinz, Science 282 (1998) 1660.[2] H. Ohno, Science 281 (1998) 951.[3] M. A. Scarpulla, O. D. Dubon, K. M. Yu, O. Monteiro, M. R. Pillai, M. J. Aziz, and M.C. Ridgway, Appl. Phys. Lett. 82 (2003) 1251.[4] M.A. Scarpulla, U. Daud, K.M. Yu, O. Monteiro, Z. Liliental-Weber, D. Zakharov, W. Walukiewicz, and O. D. Dubon. Physica B 340-342 (2003) 908.[5] M.A. Scarpulla, B.L. Cardozo, R. Farshchi, W.M. Hlaing Oo, M.D. McCluskey, K.M. Yu, and O.D. Dubon, Phys. Rev. Lett. 95 (2005) 207204.[6 ] O.D. Dubon, M.A. Scarpulla, R. Farshchi and K.M. Yu, Physica B 376-377 (2006) 630.[7] B. Clerjaud, J. Phys. C: Solid State Phys. 18 (1985) 3615.[8] M. Linnarsson, E. Janzen, B. Monemar, M. Kleverman and A. Thilderkvist, Phys. Rev. B 55 (1997) 6938.[9] K. M. Yu, W. Walukiewicz, T. Wojtowicz, I. Kuryliszyn, X. Liu, Y. Sasaki, and J. K. Furdyna, Phys. Rev. B 65 (2002) 201303.[1 0 ] N. Theodoropoulou, A.F. Hebard, M.E. Overberg, C.R. Abernathy, S.J. Pearton, S.N.G. Chu, and R.G. Wilson, Phys. Rev. Lett. 89 (2002) 107203.[11] T. Dietl, H. Ohno, F. Matsukra, J. Cibert, and D. Ferrand, Science 287 (2000) 1019.[12] T. Jungwirth, K. Y. Wang, J. Masek, K. W. Edmonds, J. Konig, J. Sinova, M. Polini, N. A. Goncharuk, A. H. MacDonald, M. Sawicki, A. W. Rushforth, R. P.U n i v e r s i t y  o f  U t a h  I n s t i t u t i o n a l  R e p o s i t o r yA u th o r  M a n u s c r ip tCampion, L. X. Zhao, C. T. Foxon, and B. L. Gallagher, Phys. Rev. B 72 (2005)165204.c \ [13] P.R. Stone, M.A. Scarpulla, R. Farshchi, I.D. Sharp, E.E. Haller, O.D. Dubon,Ci—i&J.W. Beeman, K.M. Yu, J. Denlinger, H. Ohldag, and E. Arenholz, Appl. Phys.>cc~h Lett. 89 (2006) 012504.troHg [14] S. J. Potashnik, K. C. Ku, S. H. Chun, J. J. Berry, N. Samarth, and P. Schiffer,PPC Appl. Phys. Lett. 79 (2001) 1495.nH>—*■•T3r+CcI—l>cr~ttr0H£PMjh-> cnl-fcrt-9UU IR Author Manuscript UU IR Author MaU n i v e r s i t y  o f  U t a h  I n s t i t u t i o n a l  R e p o s i t o r yA u th o r  M a n u s c r ip tFigure Captions:Figure 1. a) Magnetization versus temperature for x ~ 0.018 (circles), 0.029 (squares), 0.034 (diamonds), and 0.038 (triangles). Hysteresis loops for x ~ 0.034 measured at 5K (filled squares) and 3OK (open squares) are also shown (inset), b) Variation of Tc with x. Measurements for additional samples are included.Figure 2. Dependence of sheet resistance on temperature for x ranging from 0.018 to 0.042. Distinct regimes of thermally activated transport are observed for samples with higher values of x (and larger Tc).Figure 3. Main panel: IR photoconductivity spectrum from a film with x ~ 0.032. The photoconductivity threshold near 70meV is confirmed by the photoconductivity spectrum measured using a far IR Fourier transform spectrometer (inset). Features in the spectrum associated with the instrument response have not been removed.Figure 4. a) Temperature dependence of bulk magnetization for II-PLM samples with x ~ 0.034 after (post-PLM) thermal annealing for 15 minutes at various temperatures. The sample annealed at 400 °C shows near complete loss of ferromagnetism, b) Dependence of Tc on x. The Mnca mole fraction is tuned by varying the implantation dose (squares) and the annealing temperature (triangles).UU IR Author Manuscript UU IR Author ManuscriptU n i v e r s i t y  o f  U t a h  I n s t i t u t i o n a l  R e p o s i t o r yA u th o r  M a n u s c r ip t3 . 50 . 5oCOCDC2 . 5■\  .  tCD32 * .  *  I.  ♦  »  51 . 5•  ■  ♦  *cCDE"  ^  ▲o21•  \  \  ■Field (Oe)0 1 0F i g u r e  1 aUU IR Author Manuscript UU IR Author ManuscriptU n i v e r s i t y  o f  U t a h  I n s t i t u t i o n a l  R e p o s i t o r yA u th o r  M a n u s c r ip toM n  m o l e  f r a c t i o n  xG aF i g u r e  1 bU n i v e r s i t y  o f  U t a h  I n s t i t u t i o n a l  R e p o s i t o r yA u th o r  M a n u s c r ip tc  cI—I &  >t roHgP13Ccr>nn .T3cci—i &  >t roH2ccr>n3 .T31 0O "COE. cO> >; >0 )CO01—00_ cCO1 0 '1 0 '1 0 'rT •  x-0.018■ x-0.029♦ x-0.034a x~0.038▼ x-0.042■_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_l_0 0 . 0 2 0 . 0 4  1 / T  ( K ' 1 )F i g u r e  20 . 0 6 0 . 0 8UU IR Author Manuscript UU IR Author ManuscriptU n i v e r s i t y  o f  U t a h  I n s t i t u t i o n a l  R e p o s i t o r yA u th o r  M a n u s c r ip t0  1 0 0  2 0 0  3 0 0  4 0 0  5 0 0  6 0 0  7 0 0  8 0 0  e n e r g y  ( m e V )F i g u r e  3UU IR Author Manuscript UU IR Author ManuscriptU n i v e r s i t y  o f  U t a h  I n s t i t u t i o n a l  R e p o s i t o r yA u th o r  M a n u s c r ip t•  II-PLM only ■ 3 2 5 °C♦ 375 °C a 4 0 0 °CT ( K )F i g u r e  4 aUU IR Author Manuscript UU IR Author ManuscriptU n i v e r s i t y  o f  U t a h  I n s t i t u t i o n a l  R e p o s i t o r yA u th o r  M a n u s c r ip tM n  m o l e  f r a c t i o n ,  xG aF i g u r e  4 b

Compositional tuning of ferromagnetism in Ga1-xMnxP

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Compositional tuning of ferromagnetism in Ga1-xMnxP

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