Magnetic Properties of P-Type GaMnP Grown by Molecular-Beam-Epitaxy

 

Mark E. Overberg, Brent P. Gila, Cammy R. Abernathy, and Stephen J. Pearton

 

Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611

 

Nikoleta A. Theodoropoulou, Kevin T. McCarthy, Stephen B. Arnason, and Arthur F. Hebard

 

Department of Physics, University of Florida, Gainesville, Florida 32611

 

 

ABSTRACT

 

            Growth by molecular-beam epitaxy of the dilute magnetic alloy GaMnP:C is reported.  The GaMnP:C contains 9.4 % Mn as determined by Auger electron spectroscopy, and is single phase as determined by x-ray diffraction, reflection high-energy electron diffraction, and transmission electron microscopy.  Both magnetization and magnetotransport data are reported.  The results show the anomalous Hall effect, negative magnetoresistance, and magnetic hysteresis at 10 K, indicating that Mn is incorporating into the GaP:C and forming the ferromagnetic semiconductor GaMnP:C.  Temperature-dependent magnetization and anomalous Hall data show that magnetic behavior persists to at least 200 K, which is the highest value reported to date for a III-V based dilute magnetic semiconductor.

            Currently, a large research effort is centered upon methods to exploit the property of electron spin in device structures.(1-6)  Recently it has been hypothesized that “spintronic” devices that utilize the quantum properties of the electron spin wavefunction will allow significant advances in the development of electro-optic switches, ultra-sensitive magnetic field sensors, and, particularly, quantum-based logic and memory for high speed computation.(1-6)  However, it has been found that directly mating electronic materials (semiconductors) with spin materials (ferromagnetic metals) leads to interfacial problems such as the formation of a magnetically dead interfacial layer(7-9) due to the dissimilar nature of the materials’ crystal structure, bonding, physical and chemical properties.  Consequently, only low spin injection efficiencies have been reported.(10)  Another solution is the use of dilute magnetic semiconductors (DMS), which consist of semiconductor hosts heavily doped with substitutional magnetic ions, for spin injectors.

            Several theories have been presented on the nature of DMS-related ferromagnetism.(11,12)  In one theory based on the bound magnetic polaron (BMP) model, Curie temperatures (TC) have been calculated for 5% Mn in various III-V and II-VI semiconductors.  In this calculation, a concentration of free holes equal to 3.5x1020/cm3 has been assumed.(12)  To date, the highest reported experimental TC value for a DMS III-Mn-V material is 110 K for GaMnAs.(13)  The p-type III-V DMS material GaMnP is predicted to have a TC of roughly 100 K.(12)  Although the TC is well below room temperature, the close lattice matching between GaP and Si may allow GaMnP layers to be directly integrated as spin injection layers with currently established Si CMOS technology.  A DMS based upon GaP is also interesting as GaP may be highly doped either n-type (using a Si dopant such as SiBr4) or p-type (using a C dopant such as CBr4).  This allows the high carrier concentrations required by current DMS theories.  In this paper, we demonstrate the growth of thin film ferromagnetic GaMnP:C.

            GaMnP:C films were grown on In-mounted (100) epi-ready GaP substrates by Gas Source Molecular Beam Epitaxy in an INTEVAC Gas Source Gen II.  A surface oxide desorption step was performed by heating the GaP substrate under a P overpressure for 5 minutes at a temperature of 600 °C.  After the oxide desorption, a 150 Å GaP:C layer was grown at a substrate temperature of 600 °C.  This layer was grown to buffer the GaMnP:C layer from the substrate/epilayer interface.  Following the GaP:C growth, GaMnP:C was grown for 31 minutes, also at a substrate temperature of 600 °C, to a total thickness of approximately 3000 Å.  The C flux was provided by the pyrolysis of CBr4 at the substrate surface.  The CBr4 was introduced into the growth chamber from a bubbler using a He carrier gas at a flow of 1 sccm and a pressure of 10 torr.  The CBr4 bubbler was held at a temperature of 3.3 °C.  Shuttered effusion ovens charged with 7 N (99.99999 % pure) Ga and 4 N (99.99 % pure) Mn provided the group III and the magnetic dopant fluxes.  Reactive phosphorous was provided by thermally cracking phosphine (PH3) gas into both P and P2 species.  The cracker was held at a temperature of 1050 °C during the film growth.  During the oxide desorption step, a PH3 flow rate of 5 sccm was used, while the flow rate during the GaP:C and GaMnP:C growth was 2 sccm.

X-ray diffraction (XRD) measurements were performed in a Philips APD 3720 powder diffractometer.  Transmission electron microscopy (TEM) measurements were performed in a JEOL 200CX.  Compositional information was provided by Auger Electron Spectroscopy (AES) in a Perkin-Elmer PHI 6600 system.  Magnetization and magnetotransport measurements were performed respectively in a Quantum Design superconducting quantum interference device (SQUID) Magnetic Properties Measurement System and in a Quantum Design Physical Properties Measurement System equipped with a Linear Research LR700 AC impedance bridge.  Hall measurements were also taken at room temperature in a custom built system using a 0.8 T electromagnet.  Crystal quality was measured in-situ using a Staib reflection high-energy electron diffraction (RHEED) gun set to 15 kV.

For the growth of the GaMnP:C layer, the Mn effusion oven was set to a temperature of 724°C, resulting in a total Mn concentration of 9.4 at% as determined by AES depth profiling.  The Mn cell temperature was chosen after a study of Mn flux versus Mn incorporation to be discussed elsewhere.(14)  The growth rate of the GaP:C layer was determined to be 72 Å/min, while the growth rate of the GaMnP:C layer was determined to be 93 Å/min.  RHEED measurements of the GaMnP:C surface clearly showed a spotty pattern.  This 3D growth indicates the acceptable quality of the GaMnP:C layer despite the large fraction of Mn within the material.  Subsequent x-ray diffraction of the GaMnP:C revealed only the presence of peaks corresponding to cubic GaP; no second phase peaks were found.  All the peaks seen in the x-ray scan have previously be seen in GaP and GaP:C epilayers grown in the same Varian Gen II.  A cross-sectional TEM photograph of the GaMnP:C is given in Figure 1 (top).  No spots indicating second phase precipitates or superparamagnetic Mn clusters are visible, to a resolution of 50 Å.  The lack of precipitates is also evident when investigating the selected area diffraction (SAD) pattern of the GaMnP:C layer (Figure 1, bottom).  One extra set of spots (in addition to those from GaP) was found in the SAD photograph of the GaMnP:C layer.  However, this set of spots was found to correspond to a shift of the GaP pattern, indicating that there exists a degree of tilt to the GaMnP:C grains.  The TEM photograph also shows a rough GaMnP:C surface, which produces the 3D pattern seen by RHEED.  This result shows the need for continued optimization of the GaMnP:C growth conditions.  Homoepitaxial GaP is normally grown at 600 °C.  Therefore, low-temperature epitaxy of GaP is not required to obtain the necessary concentration of Mn within the material while avoiding the formation of second phases, as was the case with GaMnAs.(13)

                Magnetization versus magnetic field (M-H) and versus temperature (M-T), measured by SQUID magnetometry, are shown in Figure 2.  For the M vs. H measurement, H was parallel to the sample plane and the measurement temperature was 10 K.  From the figure, a nonlinearity in M is observed with a clear amount of hysteresis.  The coercive field is ~195 Oe and the saturation magnetization corresponds to 0.005 Bohr magnetons/Mn atom.  M vs. T was determined by taking the difference of field-cooled and zero-field cooled measurements at 500 Oe.  These data indicates that the observed ferromagnetism persists to approximately 250 K.  While MnP has a ferromagnetic phase with TC as high as 291 K(15), there is no evidence for its presence in our samples, provided the Mn content is kept below 10 % at a growth temperature of 600 °C.  Magnetotransport properties of the GaMnP:C material were investigated in the temperature range between 10 K and 350 K for magnetic field sweeps between –7 T and +7 T.  The Hall (transverse) and sheet (longitudinal) resistances were measured after applying In ohmic contacts to the GaMnP:C.  The magnetotransport data are given in Figures 3 and 4.  The concentration of holes at 300 K was found to be 3.0 X 1018/cm3.  The sheet resistance showed clear negative magnetoresistance up to 200 K, with the value of DR/R at 20 K equal to 0.35 %.  For the Hall resistance measurements, hysteresis is observed in the data at 50 K, 100 K, and 200 K.  At 200 K, there is a time-dependent drift in the measurement of comparable value, that is believed to be due to either contact resistance or a surface potential due to a non-ohmic component to the contacts.  The hysteresis at 50 K and 100 K is consistent with the existence of an anomalous component to the Hall resistance.  The Hall resistance data given in Figure 3 show the shift in the data as the field is ramped to +7 T and then reversed, indicating the existence of hysteresis consistent with the SQUID magnetometry data.

In summary, we have found that MBE growth of GaP:C with a high concentration of Mn resulted in the formation of the ferromagnetic phase GaMnP:C.  Negative magnetoresistance was observed up to 200 K, while the anomalous Hall effect was observed at 50 K, 100 K, and 200 K.  The lack of any detected second phases by x-ray diffraction analysis and cross sectional TEM of the GaMnP:C material in combination with the observed 3D RHEED pattern of the final epitaxial surface indicates that the ferromagnetic MnP phase is not forming within the film.  Therefore, we infer that the observed ferromagnetism within the material is due to the formation of GaMnP:C.  This result is significant on several levels.  First, ferromagnetism within epitaxial GaMnP:C is reported.  Second, the experimental results show that the ferromagnetism persists to at least 200 K, far greater than predicted by current theories.  This raises the possibility of further increases in the TC within the GaMnP:C material by refined growth procedures to: improve the film morphology, optimize the degree of Mn incorporation, and to optimize the concentration of free holes.  By incorporating GaMnP:C layers with current Si-based structures, room temperature spintronic devices may be possible.

 

ACKNOWLEDGEMENTS

 

      The authors would like to thank E. Lambers of the MAIC at the University of Florida for his assistance with the Auger Electron Spectroscopy analysis.  Support for this work was provided by the U.S. Army Research Office under grant no. ARO- DAAG55-98-1-0216 and by the National Science Foundation under grant nos. DMR-9705224 and DMR-0101438.

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FIGURE CAPTIONS

 

Figure 1:  Cross sectional TEM photograph of the GaMnP:C after growth (top).  The length scale is 4000 Å.  Selected area diffraction pattern (bottom) from the same sample.

Figure 2:  Magnetization of GaMnP:C with Mn = 9.4 % as determined by Auger depth profiling, versus (a) field at 10 K and versus (b) temperature at 500 Oe.  Magnetic field applied parallel to the sample plane.

Figure 3:  Hall resistance data of GaMnP:C with Mn = 9.4 % at 50 K.  The magnetic field was applied perpendicular to the sample plane.  The contact configuration is indicated.

Figure 4:  Magnetotransport sheet resistance data of GaMnP:C with Mn = 9.4 %.  Magnetic field applied perpendicular to the sample plane.