Properties of InMnP (0 0 1) grown by MOVPE

ARTICLE IN PRESS Journal of Crystal Growth 310 (2008) 4046–4049

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Properties of InMnP (0 0 1) grown by MOVPE M. Pristovsek , A. Philippou 1, B. Ra¨hmer, W. Richter 2 ¨t Berlin, Institut fu ¨ r Festko ¨rperphysik, Hardenbergstraße 36, D-10623 Berlin, Germany Technische Universita

a r t i c l e in f o

a b s t r a c t

Article history: Received 20 November 2006 Received in revised form 11 June 2008 Accepted 17 June 2008 Communicated by R. Bhat Available online 27 June 2008

We investigated growth and incorporation of Mn into InP (0 0 1) by metal-organic vapour phase epitaxy (MOVPE). Depending on the Mn/In ratio and temperature we found four different incorporation regimes. Flat mirror like layers with high Mn incorporation were either produced around 510  C or around 600  C. At higher temperatures or higher Mn-fluxes, the surfaces roughened. We achieved a maximum Mn incorporation around 0.6%, estimated by X-ray diffraction. The corresponding hole concentration was 1:7  1017 cm3 . The hole activation energy for the Mn acceptor in variable temperature Hall measurements was 220 meV, comparable to the onset of a broad photoluminescence. Due to this high activation energy the layers showed no spin polarization. & 2008 Elsevier B.V. All rights reserved.

PACS: 75.50.Pp 61.72.Vv 81.15.Gh Keywords: A1. Doping A3. Metal-organic vapour phase epitaxy B2. Magnetic materials B2. Semiconducting III–V materials

1. Introduction Manganese and other transition metal alloys with III–V semiconductors have attracted much interest in the last few years due to the new ferromagnetic compound semiconductor Ga1x Mnx As [1]. The maximum concentration of manganese in the case of most materials is relatively low, typically far below 10%. For GaAs it is 7–8% and results in Curie temperatures up to 170 K. Such high incorporations are usually achieved in MBE (molecular beam epitaxy) using low-temperature growth (less than 250  C). Attempts to grow GaMnAs in MOVPE (metal-organic vapour phase epitaxy) resulted in the formation of Mn-rich clusters which were embedded defect free in the surrounding heavily Mn-doped GaAs matrix [2,3]. These clusters are ferromagnetic at room temperature, but do not induce spin polarization comparable to homogenous GaMnAs [3]. Other materials like InMnP received little attention, despite the very first report on diluted magnetic III–V semiconductors (DMS) about photo-induced magnetization in InP:Mn [4]. In the paper

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E-mail address: [email protected] (M. Pristovsek). Present address: Qimonda Dresden GmbH & Co. OHG, Ko¨nigsbru¨cker Str. 180, 01099 Dresden, Germany. 2 Present address: Dipartimento di Fisica Universita` di Roma II: Tor Vergata, Via della Ricerca Scientifica 1, I-00133 Roma, Italy. 1

0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.06.069

the Mn concentration was given as 8  1018 cm3 without the actual hole concentration. Under illumination a magnetization with T C ¼ 12 K was reported [4]. A newer work on In0.9Mn0.1P nano-clusters (3 nm diameter) reported T C ¼ 25 K [5]. However, the authors assumed that size effects strongly influenced the magnetic coupling. Shon et al. [6] claimed to have produced InMnP:Zn diluted magnetic semiconductor (DMS) by Mn indiffusion into an InP:Zn p-doped epilayer. Using EDX a maximum Mn concentration of 3% in the near surface region was found. However, there is strong evidence that the observed magnetization originates from MnP and InMnP clusters. First, there were two unidentified peaks in the X-ray diffraction (XRD) at 2Y  37 and  50 . Second, the magnetization curves show two regimes, one where the magnetization drops rapidly until 50 K and a long tail with T C above 300 K. This indicates the formation of MnP inclusions/clusters, because MnP has a T C of 292 K and a second magnetization regime below 50 K [7]. Furthermore, by using ion implantation into undoped InP, no p-type conductivity could be achieved [6,8], which is assumed to be a prerequisite for coupling between spins to achieve a DMS. The electronic and optical properties of the Mn acceptor in InP have been studied mostly by low-temperature photoluminescence (PL) [9–11]. Mn was also intentionally incorporated as p-dopant, but showed low activation with a maximum hole concentration in the middle 1015 cm3 range in MOVPE [11], while in liquid phase

ARTICLE IN PRESS M. Pristovsek et al. / Journal of Crystal Growth 310 (2008) 4046–4049

epitaxy (LPE) a maximum hole concentrations of 3  1017 cm3 was reported [12]. A newer study of manganese and other transition metals in InP using bulk growth crystals reported a maximum hole concentration below 3  1016 cm3 [13]. The low hole concentration was explained by the relatively high activation energy for the manganese acceptor of 220 meV, found by temperature dependent Hall, absorption and PL measurements. It was also noted that the absorption band became much broader upon high manganese incorporation [13]. A systematic ab inito theory study of V, Cr, and Mn in several III–V-semiconductors, including InP, suggested InP:Cr as the material with the highest exchange interaction [14]. But this study did not consider the problem of carrier activation to mediate the exchange interaction. Thus, the questions of whether Mn is a good choice for an InP based DMS is still open. Therefore, in this work we present our investigation on InMnP and InMnP:Zn grown by MOVPE. Using MOVPE instead of implantation or indiffusion allows us very good control and reproducibility. Furthermore, since we use in situ characterization for growth control and stabilize our samples by PH3, we can exclude surface effects or annealing damage.

2. Experimental procedure The samples were grown in a horizontal double wall quartz MOVPE reactor under in situ control using reflectance anisotropy spectroscopy (RAS). The carrier gas was either hydrogen or nitrogen at 100 mbar and 3 l/min. Precursors’ partial pressures were PH3 (100 or 200 Pa) and TMIn (typical 0.5 Pa). The manganese source was bis(cyclopentadienly)manganese (bcpMn) with partial pressures between 1 and 40 mPa. bcpMn is a viscous liquid with a vapour pressure of 5 Pa at 20  C [15]. The expected decomposition temperature is around 450  C similar to the related compound tricarbonyl–cyclopentadienyl–manganese (tcpMn) [16]. For additional p-doping dimethyl–Zinc:triethyl–amine adduct was used, with typical partial pressures from 0.2 to 1 Pa. The growth was controlled in situ using RAS, mainly to see the effect of doping and the onset of roughening. The Mn concentration in the layers was determined from XRD. Surface topography was measured using ex situ atomic force microscopy (AFM). The electrical characterization was done with variable temperature/variable field van der Pauw Hall measurements on typical 2 mm thick layers on InP:Fe substrates. PL was measured in a helium cooled cryostat using an InGaAs-photodiode and excited via an Arþ or HeNe-laser.

3. Results and discussion 3.1. Growth of InMnP In MBE, high concentrations of Mn in GaMnAs are incorporated by low-temperature growth, below 250  C. Such low growth temperatures are possible, since the materials are supplied in elemental form as beams of atoms or dimers/tetramers. On the other hand, in MOVPE, the precursors require a certain minimum temperature for decomposition to release their elements. In the case of precursors for InP the temperatures for 50% decomposition are about 375  C for TMIn and 500  C for PH3, i.e. 250  C above the MBE growth temperatures. Unfortunately, the bcpMn decomposition temperature is unknown, although it is expected around 450–500  C, similar to the closely related compound tcpMn [16]. The foremost task was to develop a set of reasonable growth conditions for InMnP growth. Fig. 1 summarizes the conditions for several growth regimes. At too low temperatures, the PH3

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rough

>650° C ~600° C

smooth InMnP cluster cluster & rough

~500° C

smooth InP:Mn

<500° C

whisker Mn flux

Fig. 1. Different growth regimes during MOVPE of InP:Mn.

decomposition is insufficient while In and Mn are still decomposed. As a result whiskers form. For temperatures above 500  C the layer becomes smooth even for higher bcpMn fluxes with some occasional rhomboedric clusters (which indicates rhomboedric MnP, Fig. 2 left). Exceeding a certain bcpMn flux or a certain temperature, the surface gets completely covered by rhomboedric clusters (Fig. 2 middle). With increasing temperature the maximum bcpMn flux before the formation of clusters and roughening decreases (Fig. 3). This finding can be explained, assuming at 500  C bcpMn is not yet fully decomposed. Then at higher temperatures the same bcpMn flux would result in a higher surface Mn concentration. Around 600  C a fourth regime is present: even with relatively high bcpMn fluxes the surface is smooth with some pits and some rectangular step structures (Fig. 2 right). The latter may be MnInP clusters, although we could not measure their composition. To estimate the Mn concentration in the InMnP layers, we grew several superlattices at 500 and 600  C. The typical XRD of a sample grown at 500  C is shown in Fig. 4. The rocking curve is very similar to that of a bulk layer of InMnP with the same thickness. We never observed clear satellite peaks, indicating a strong Mn segregation even at 500  C. The kinematic calculation for InMnP (like shown in Fig. 4) was done using the known rhomboedric MnP elastic constants and assuming an extrapolated lattice constant of 0.61 nm for the hypothetical cubic MnP; similar to the case of MnAs for GaMnAs [1]. The resulting mean Mn concentration in the layer for the simulation for the sample in Fig. 4 was around 0.2%. The maximum concentration measured for bulk layers by XRD did not exceed 0.6%. To measure correct p-doping concentration usual an n-type background has to be overcome. This background—mostly originating from an interfacial layer of dopants—can be as high as 1  1016 cm1 , depending on the surface preparation of the InP:Fe substrates, layer thickness, temperature, and precursors. We achieved best results, with nearly semi-insulating buffer layers, when etching the InP:Fe substrate for at least 10 min in an H2 SO4 : H2 O2 : H2 O (3:1:1) etching solution and growing the buffer layer at 650  C or below. Fig. 5 summarizes the obtained type of doping. To achieve an acceptor concentration exceeding 1  1016 cm1 the In/Mn ratio must exceed 1% bcpMn in the gas phase, nearly independent of the temperature. The maximum acceptor concentration achieved at room temperature was 1:7  1017 cm1 without ex situ annealing, comparable to the highest values reported for InP:Mn LPE [12] and more than two orders of magnitudes above a previous report on MOVPE-grown InP:Mn [11]. Annealing did not influence the hole concentrations. The mobilities were about 130–140 cm2 =V s for the highest concentrations. Assuming for the thick layers grown for Hall effect measurements a mole fraction of 0.6% Mn, then the activation is less than 5  106 at room temperature, which would imply an activation

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Fig. 2. Surfaces after growth in several regimes. Left: at 500  C before roughening with a cluster spot. Middle: 520  C in the rough cluster regime. Right: at 600  C.

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Fig. 3. Transition from smooth to rough as a function of temperature, In/Mn ratio, and PH3 partial pressure in the intermediate temperature region.

energy of some 100 meV. We measured the activation energy by temperature variable Hall measurements, also using varying fields to check for spin polarization effects. The inset in Fig. 6 shows the carrier concentration for a typical sample. The activation energy for the Mn acceptor was always around 220 meV, independent of In/Mn ratio with hydrogen or nitrogen carrier gas. No spinpolarization was found. PL of Mn-doped samples showed a broad feature, which can be separated in at least three main peaks (Fig. 6). The centre energies for these peaks were 1.169, 1.139, and 1.092 eV. The latter two are 38.5 meV separated phonon replica of the first peak, as previously reported [9]. The energy of the first peak corresponds reasonably to the activation energy of the Mn acceptor of 222 meV estimated from temperature dependent Hall measurement of the same sample. The value of 222 meV was also reported for a recent study using Hall, IR-adsorption and PL [13]. Using (optical) deep level transient spectroscopy an activation of 230 meV was reported [11]. A corresponding PL peak was reported at 1.182 eV [9] resp. 1.14 eV [10] both ð230  10Þ meV below the band gap. All smooth samples without clusters were paramagnetic, contrary to Ref. [6], despite the relatively high Mn content. Using variable field Hall effect measurements, no anomalous Hall effect was observed, thus no spin polarization was present. Without co-doping this is most likely due to the fact that all Mn acceptors are frozen out below 100 K, which is above the expected Curie temperature.

31.65 ω−2Θ (°)

31.70

31.75

Fig. 4. o22Y scan of a superlattice sample, grown with PH3 ¼ 200 Pa, TMIn ¼ 0:2 Pa, bcpMn ¼ 10 mPa at 500  C. Two calculation were made, one for a homogenous layer and one for a superlattice. Only the calculation for the homogenous layer agrees to the measurement, indicating a strong Mn segregation.

temperature (°C) 575

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1.2 1.3 reciprocal temperature (1000/K)

650nm In0.9981Mn0.0019P

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1.4

Fig. 5. Type of doping (either n-type background/semi-insulating or p-type Mn) in InP:Mn on InP:Fe substrates as a function of temperature and In/Mn ratio.

To achieve a higher hole concentration down to 4 K we co-doped with Zn. Unfortunately, to avoid roughening only medium range Zn-doping was possible without strongly reducing the Mn-amount. The resulting carrier concentrations were still in

ARTICLE IN PRESS M. Pristovsek et al. / Journal of Crystal Growth 310 (2008) 4046–4049

photon energy (eV) 1.5

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not exceed 1:7  1017 cm3 , indicating a relatively high activation energy. Using PL and temperature dependent Hall measurements, we estimated an activation energy of 220–230 meV, comparable to values in literature. Without clusters, the samples were paramagnetic and showed no spin polarization. As a conclusion, InP:Mn does not seem to be a promising candidate as a diluted magnetic semiconductor, due to the high activation energy of the Mn acceptor.

Acknowledgements Part of this work was supported by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 296).

InP:Mn 10-4

800

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References

900

1000

1100 1200 wave length (nm)

1300

1400

Fig. 6. Photoluminescence of undoped InP and InP:Mn with about 0.5% Mn. The inset shows the temperature dependent Hall data of the same sample.

the mid to upper 1017 cm3 range, and probably not high enough to achieve ferromagnetic coupling and spin polarization. The mobilities with Zn co-doping were 50–100 cm2 =V s, comparable to Zn-doping only. No anomalous Hall effect and thus no spinpolarization was observed with or without Zn co-doping.

4. Conclusion We have grown and characterized InP:Mn and InP:(Mn,Zn). Manganese showed strong segregation even at 500  C. The maximum manganese incorporation before the onset of cluster formation was about 0.6%. The resulting hole concentration did

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