Voltage control of ferromagnetism at room temperature in GaCrN (p-i-n) device structures

Transcription

1 Voltage control of ferromagnetism at room temperature in GaCrN (p-i-n) device structures N. A. El-Masry 1, J. M. Zavada 2, J. G. Reynolds 1, C. L. Reynolds Jr. 1, Z. Liu 1 and S. M. Bedair 2 1 Department of Materials Science & Engineering 2 Department of Electrical &Computer Engineering North Carolina State University, Raleigh, NC 27695, USA Abstract We have demonstrated a room temperature dilute Magnetic semiconductor (DMS) based on GaCrN epitaxial layers grown by metalorganic chemical vapor deposition (MOCVD). The saturation magnetization M s was increased when the GaCrN film is incorporated in a (p-gan/i-gacrn/n- GaN) device structure, due the proximity of mediated holes present in the p-gan layer. Zero field cooling (ZFC) and field cooling (FC) were measured to ascertain the absence of superparamagnetic behavior in the films. A (p-gan/i-gacrn/n-gan) device structure with room temperature ferromagnetic (FM) properties that can be controlled by an external applied voltage has been fabricated. In this work we show that the applied voltage controls the ferromagnetic properties, by biasing the (p-i-n) structure. With forward bias, ferromagnetism in GaCrN layer was increased nearly 20 fold of the original value. Such enhancement is due to carrier injection of holes into the Cr deep level present in the i-gacrn layer. A memory effect for the FM behavior in the (p-i-n) GaCrN device structure that persisted for 42 hours after the voltage bias was turned off. These measurements also support that the observed ferromagnetism in the GaCrN film is not due to superparamagnetic clusters but instead is a hole-mediated phenomena. 1

2 Dilute magnetic semiconductors (DMS) have been reported for Mn doped III-V compounds at low operating temperatures. The low magnetic ordering temperature limits the room temperature potential applications [1, 2]. DMS based on transition metals (TM) doped GaN, such as Mn and Cr, have been widely investigated for potential room temperature (RT) spintronic applications [3-8]. Several theoretical models predicted that ferromagnetic behavior in DMS such as GaMnN and GaCrN is hole mediated ferromagnetism via an exchange interaction with localized transition metals (TM) spins in the DMS films [9]. Holes in this material system can be generated through several approaches. The most direct way to generate holes in DMS is by doping these films; however, such an approach was not successful due to the deep impurity level band of the transition metals in GaN and doping results in high resistivity material [10]. However, under favorable growth conditions, it was shown that holes needed for carrier mediation can be available if the Fermi level is located within the impurity bands formed by Mn or Cr in GaN films [10]. A partially filled impurity band can create the holes required for the FM exchange interaction. Based on the above concept, we were able to modulate the FM behavior of GaMnN by adding dopants to manipulate the position of the Fermi level [10]. It was also demonstrated that holes can be available from an adjacent p-gan layer to the DMS film [11-13]. The enhancement or depletion of these holes in the p-gan film result in an increase or decrease in the FM properties of the DMS film [11-13]. We also have demonstrated electric field control of FM in an (i-gamnn/p-gan/n-gan) (i-p-n) structure at room temperature [14]. Reverse bias of the (i-gamnn/p-gan/n-gan) (i-p-n) structure resulted in hole depletion in the p-gan film and loss of the FM properties. In this approach holes are not physically injected into the GaMnN film, but their concentrations and proximity are the reason for this control of the FM properties. The electric field control in this approach reduces rather than enhances the FM properties from its original value at the zero bias condition [14]. In the current work, we show an approach to enhance the hole concertation and correspondingly the FM properties of DMS films by carrier injection across a (p-i-n) junction. The (p-i-n) structure in the current study is (p-gan/i-gacrn/n-gan), where the DMS layer is located within the junction depletion region. Holes can be injected from the p-gan across the valence band discontinuity barrier into both the i-dms layer and the n-gan layer. Some of the injected holes will be trapped by the deep level formed by the Cr ions that is ~2 ev above the valence band [5]. Forward bias of the junction reduces the barrier for carrier injection across the junction and thus 2

3 the concentration of holes trapped in the Cr deep level will be enhanced. With reverse bias, both carrier injection and the proximity effect of holes in the p-type GaN will not impact the FM properties of the GaCrN film. Thus, the FM properties of GaCrN DMS film can be enhanced or minimized by biasing this (p-i-n) structure. Also, due to the deep level nature of the Cr band [5], the injected holes trapped in this Cr band can have a carrier lifetime that is different from the conventional carrier lifetime. This can result in the potential of hole retention in the Cr deep level, and thus, the FM properties of GaCrN can be sustained for a relatively long time after forward biased is switched off. This proposed structure can be the basis of an approach for a non -volatile memory cell that can impact the field of spin electronics. The GaCrN/n-GaN device was grown by MOCVD on (n-gan/i-gan/sapphire) templates. The Si-doped n-gan layer had an electron concentration of ~3 x cm -3. The thickness of the GaCrN layer was ~ 0.2 m, and the Cr concentration was ~10 20 Cr atoms/cm 3. The GaCrN film was capped with ~20nm of undoped GaN to prevent oxidation of the GaCrN film. The as-grown structure, i.e., without a top p-type GaN film, allowed measurement of the FM properties without any contribution of holes from a p-gan layer. Subsequently, prior to regrowth of the top p-gan layer, part of the undoped 20nm GaN cap layer was etched off by high temperature heating under ammonia [15]. A Mg-doped p-gan layer with a thickness of 0.42 m was then grown with an activated hole-concentration 3 x cm -3 as measured by the van der Pauw Hall technique. We further performed magneto-transport measurements as a function applied field at room temperature to provide detailed information regarding the magnetic ordering in the GaCrN (p-i-n) device. Anomalous Hall Effect (AHE) measurements were performed using the standard fourpoint probe van der Pauw configuration. While the Hall resistance for the p-type GaN film was linear, which indicated that it follows the Ordinary Hall effect (OHE), that for the GaCrN (p-i-n) structure was non-linear indicating Anomalous Hall behavior. Similar behavior was observed in our investigation of GaMnN (i-p-n) structures. 14 In both of these FM structures, the magnetic field B splits the GaN valance band that leads to an exchange interaction between holes and localized TM-ions 3d spins, thus resulting in the observed Anomalous Hall Effect (AHE). The growth of the p-gan layer produced the (p-i-n) structure (p-gan/i-gacrn/n-gan/sapphire). A schematic of the fabricated GaCrN (p-i-n) device structure is shown in Fig. 1(a). The size of each GaCrN (p-i-n) device was 3x3 mm 2. After etching, Ohmic contacts were deposited on the p-gan 3

4 and n-gan layers, and the current-voltage curves (I-V) of the devices were measured using a Keithley electrometer. A Quantum Design superconducting quantum interference device (SQUID) was used to investigate the temperature dependence of magnetism in the GaCrN-based structure. Zero field cooling (ZFC) and field cooling (FC) curves were measured to ascertain the presence of any superparamagnetic effects in the GaCrN films. The magnetic properties of the GaCrN structure, with/without an adjacent p-gan layer and with/without biasing the p-i-n junction, were also examined using a MicroMag model 2900 alternating gradient magnetometer (AGM). Figure 1 (a) & (b) The (I-V) characteristic of a representative device presented in Fig. 1(b) shows reasonable forward and reverse bias performance, albeit there is a relatively high series resistance due to non-optimized metal contacts. Prior to epitaxial growth of the p-gan layer, the crystalline quality of the n-gan/i-gacrn structure was analyzed by X-ray diffraction (XRD) using a Rigaku diffractometer. The XRD pattern revealed intense (0002) GaN and (0006) sapphire reflections with no extraneous peak at 37.5 o, which would have been indicative of the presence of a CrN secondary phase. The lack of extra peaks in the XRD pattern that might be associated with the GaCrN layer confirms that the entire structure is single-phase Wurtzite GaN. Thus, the XRD data show that the FM behavior reported in this paper cannot be attributed to the presence of clusters that are known to exhibit 4

5 superparamagnetic behavior. The presence or lack of superparamagnetic clusters will be further addressed in this paper by performing zero field cooling and other measurements related to carrier mediation. Figure 2 Figure 2 shows the FM hysteresis curves at 300K as measured by the AGM of the n-gan/i- GaCrN heterostructure measured prior to p-gan regrowth compared to that of the n-gan/i- GaCrN/p-GaN heterostructure. The saturation magnetization has increased fourfold to 4 emu/cm 3 by introducing the top p-gan layer. These results are very similar to those observed previously in our work of GaMnN structures, where a gain of magnetization was observed in p- GaN/GaMnN heterostructures [11]. In both cases, GaMnN and GaCrN epitaxial films, the saturation magnetization can be increased due to carrier mediation (hole) at room temperature (RT) from an adjacent p-gan layer. The Mn-ion concentration in those GaMnN films was ~10 20 cm 3, which is approximately the same as the Cr-concentration in the GaCrN films here. In the previous study [11], several GaMnN (i-p-n) structures were grown with different p-gan layer thicknesses. The magnitude of saturation magnetization M s in the GaMnN films was shown to depend on the hole-concentration and thickness of the non-depleted p-gan film [11-13]. As was reported earlier [11] the hole wave function in the p-gan interacts with TM-ions even when the 5

6 p-gan and GaMnN DMS layers are separated by an AlGaN barrier at the interface as thick as 25nm [11]. Thus, the presence of a GaN cap layer with thickness that is less that 20nm, at the interface between p-gan and GaCrN, as shown in Figure 1(a) does not impede the hole/cr-ion interaction. It can also be concluded that the FM behavior of GaCrN observed in Fig 2 is not due to Cr clusters in the semiconductor film. Figure 3 (a), (b), and (c) A superconducting quantum interference device (SQUID) was used to investigate the temperature dependence of magnetism in the GaCrN structure. Zero field cooling (ZFC) and field cooling (FC) curves were measured to ascertain the presence of any superparamagnetic effects in the GaCrN film. Following a room temperature hysteresis loop measurement in the SQUID, the applied magnetic field was reduced to zero and the sample cooled from 300K to 4K. Then, under a small applied field (B) ~100 Oe, the sample was heated to 300K. The measured ZFC and FC curves are shown in Fig 3(a). If the magnetization in the GaCrN film were caused by superparamagnetic clusters, the ZFC curve would show a peak at a temperature known as the blocking temperature of the clusters which then drops off as the temperature is cooled to zero. This blocking temperature is dependent on particle size and composition [16]. Also, there is a clear separation in the ZFC and FC curves up to 400K, and in the absence of the 100 Oe applied field, the measured moment is diminished. The ZFC and FC curves support our earlier 6

7 conclusion that the measured ferromagnetism in the GaCrN-based structure is not due to superparamagnetic cluster behavior but can be attributed to a hole-mediated mechanism [16]. Figure 3(b) shows the hysteresis curves of the device structure measured in the SQUID at 1.8K and 300K. Figure 3(c) shows that this structure is ferromagnetic at 1.8K with a remnant magnetization M s of ~21 emu/cm 3 and a coercively H c of 326 Oe. Figure 4 A GaCrN device structure was then fabricated to investigate whether the observed enhanced FM behavior can be controlled by an external applied voltage to the (p-i-n) junction. In Fig. 4 the saturation magnetization, as measured by AGM at room temperature, is shown as a function of applied bias voltage. Under forward bias (V F ) from 0 to 1.2V, the saturation magnetization M s dramatically increases from ~ 1 to 4.8 emu/cm 3 when the applied voltage reaches 0.9 V. We attribute this improvement to injection of holes from the p-gan layer across the valence band discontinuity barriers into both the i-gacrn DMS and n-gan layers of the junction to augment those holes trapped by the Cr-ion deep level formed in the GaCrN film. The injected or trapped holes in the GaCrN deep level band will thus increase the number of interacting holes with the Cr-ions with a resultant increase in FM properties. With increased forward bias, more holes are injected into the GaCrN film and M s increases. With further increase in V F, M s did not change appreciably. This may be due to the buildup of positive charge, due to hole-injection, trapped in the i-layer, which in essence reduces the forward biased voltage across the junction. The 7

8 threshold voltage (V t ) where there is a sudden increase in the magnetic moment shown in Fig. 4 is less than 1V which is low relative to a GaN p-n junction. This can be explained based on the fact that the contact potential in this structure is expected to be less than that of GaN due to Fermi level pinning in the GaCrN layer at the Cr-ion level ~2eV above the valence band. Under reverse bias (V R ), we did not observe any significant change in the M s. By reverse biasing the junction, there is essentially no hole injection, and secondly the Fermi level is pinned within the Cr band. Therefore, the FM properties will be independent of voltage under reverse biased conditions. It should be mentioned that our explanations for the above data shown in Fig. 4 are based on simple arguments applied to a complicated structure. Modeling based on the nature and the role of the Cr-ion band in this (p-i-n) structure is beyond the scope of this work. However, these data show clear unambiguous evidence that ferromagnetism in the GaCrN-based structure is hole-mediated and does not originate from Cr-ion clusters. It should be mentioned that the current approach of voltage control of FM properties is enhanced with forward bias. This current approach differs from our earlier result in GaMnN [14], where the DMS film is placed on the top of a GaN p-n junction and the electric field control in this approach only reduces the FM properties from its original value at zero bias condition. The FM properties in the GaCrN (p-i-n) device described here were found to have a magnetic memory effect. The saturation magnetization enhancement that resulted under forward bias conditions remained even after decreasing V F from 1.2 to 0V. The RT M s remained constant at ~ 2.5 emu/cm 3 for several hours as indicated in Fig. 5. We have also observed that this persistent increase in magnetization slowly decays after switching off the forward voltage bias, and M s returns to its original value of ~ 1 emu-cm -3 after 42 hours as shown in Fig 5. A possible explanation for the persistent ferromagnetism is that trapping of holes on the Cr impurity band occurs in the GaCrN layer by forward biasing the junction and induces the magnetic memory effect. The trapped holes then need to be thermally activated to be released to the valance band with a lifetime that can be very long due to the large activation energy on the order of 2eV. These results were repeatable and confirmed that hole-mediated FM behavior exists in a GaCrN thin film layer that is embedded in a (p-i-n) structure. We reported a similar memory effect with photoluminescence (PL) from GaN films having a high density of defects. The PL intensity was 8

9 maintained for days after the laser source was turned off. This observation was explained by hole-trapping, either at the interfaces or defects within the GaN film [17]. Figure 5 In summary, we have fabricated a (p-i-n) [p-gan/i-gacrn/n-gan] device structure with room temperature FM properties that can be controlled by an external applied voltage. The demonstrated properties establish GaCrN as an important dilute magnetic semiconductor material. Applied voltage control of the FM properties of the (p-i-n) structure has been demonstrated. With forward biasing the (p-i-n) structure, the saturation magnetization in the GaCrN layer was increased approximately four times of its original value providing evidence that FM in the GaCrN film is hole-mediated. We also observed a memory effect for the FM behavior in the (p-i-n) GaCrN structure where FM persisted for 42 hours after the voltage bias was turned off. These measurements also indicate that the FM in GaCrN films is not due to second phases (or superparamagnetic clusters). These results may be useful in nonvolatile magnetic memory devices with low refresh rate. Voltage control of FM can have a great technological impact on the field of spin electronics due to the possible integration of a FM semiconductor with non magnetic semiconductors. The voltage control of FM, coupled with the 9

10 observed memory effect, can impact the field of quantum information technologies that are based on the manipulation of spin states in semiconductors. Acknowledgment: This work is supported by the National Science Foundation, Grant No. ECCS The authors would like to acknowledge Dr. Nicoleta Kaluza, and Dr. Yong Suk Cho of Institute of Bio- und Nanosystems (IBN-1), Jülich Aachen Research Alliance (JARA) for supplying the magnetic GaCrN wafer. References 1. H. Ohno, D. Chiba, F. Matsukura, T. Omiya, E. Abe, T. Dietl, Y. Ohno & K. Ohtani, Nature (London) 408, 944 (2000). 2. H. Ohno, J. Magn. Magn. Mater. 200, 110 (1999). 3. M.L. Reed, N.A. El-Masry, H.H. Stadelmaier, M.K. Ritums, M.J. Reed, CA Parker, J.C. Robert and S.M. Bedair, Applied Physics Letters 79 (21), 3473 (2001). 4. M.L. Reed, M.K. Ritums, H.H. Stadelmaier, M.J. Reed, C.A. Parker, S.M. Bedair, and N.A. El- Masry, Materials Letters 51 (6), 500 (2001). 5. A.Y. Polyakov, N.B. Smirnov, A.V. Govorkov, N.V. Pashkova, A.A. Shiensky, S.J.Pearson, M.e. Overberg, C.R. Abernathy, J.M. Zavada, R.G. Wilson. J. Applied Physics 93, 5388(2003). 6. H.X. Lui, S.y. Wu, R.K. Singh, L. Gu., N.R. Dilley, L. Montes, M.B. Simmonds. Appl. Phys. Lett. 85, 4076 (2004). 7. M. Hashimoto,Y.K.Zhou, M. Kanamura, H. Katayama-Yoshida and H.Asahi. J. Crystal Growth, 251, 327(2003). 8. Y.S. Chu, N. Kaluza, V. Guzenko, T. Schapers, H. Hardtdegen, H.P. Bochem, U. Breuer, M.R. Ghadmi, M. Fecioru-Morariu, B. Beschoten and H. Luth, Phys. Stat. Sol. (a) 204,72 (2007). 10

11 9. T. Dietl, H. Ohno, and F. Matsukura, IEEE Trans. Electron Devices 54, 945 (2007). 10. M.J. Reed, F.E. Arkun, E.A. Berkman, NA Elmasry, J. Zavada, M.O. Luen, M.L. Reed and S.M..Bedair, Applied Physics Letters 86, (2005). 11. F. E. Arkun, M. J. Reed, E. A. Berkman, N. A. El-Masry, J. M. Zavada, M. L. Reed and S. M. Bedair, Appl. Phys. Lett. 85, 3809(2004). 12. N.A. El-Masry, J.M. Zavada, N. Nepal, and S.M. Bedair, Rare Earth and Transition Metal Doping of Semiconductor Materials: Synthesis, Magnetic properties and room temperature spintronics, Woodhead Publishing Series in Electronic and optical materials (Editors: Dierlof, V., Ferguson, I.T; Zavada, J.M.); Ch. 12 p. 429 (2016). 13. A. M. Mahros, M. O. Luen, A. Emara, S. M. Bedair, E. A. Berkman, N. A. El-Masry, and J. M. Zavada, Appl. Phys. Lett. 90, (2007). 14. N. Nepal, M. Oliver Luen, J. M. Zavada, S. M. Bedair, P. Frajtag and N.A. El-Masry. Appl. Phys. Lett. 94, (2009). 15. P. Frajtag, AM Hosalli, GK Bradshaw, N. Nepal, N.A. El-Masry, S.M. Bedair, Applied Physics Letters 98 (14), (2011). 16. M.F. Hansen and S. Morup, J. Mag. and Magnetic Material 203, 214 (1999) 17. V. A. Joshkin, J. C. Roberts, F. G. McIntosh, S. M. Bedair, E. L. Piner and M. K. Behbehani, Appl. Phys. Lett. 71, 234 (1997). 11

12 Figure Captions: Figure 1. a) Schematic of the fabricated p-gan/i-gacrn/n-gan, (p-i-n) device structure. b) I-V curve of a representative GaCrN (p-i-n) device. Figure 2. AGM room temperature measurements for comparison of magnetization characteristics for a GaCrN sample without the p-gan layer to that for the GaCrN sample, as part of the p-i-n structure, with a p-gan layer, indicating a 4x increase in M s. Figure 3. (a) Temperature dependence of FM of the GaCrN structure under FC to 1.8K (red line) and heating up at ZFC (blue line). (b) Magnetization curves measured by SQUID at 1.8K and 300K. (c) The M s and H c at 1.8K. Figure 4. Variation of magnetic moment at room temperature in the GaCrN (p-i-n) device with applied voltage. With reverse bias the magnetic moment is nearly constant. Under forward bias the magnetic moment increases from ~1 emu/cm 3 to a saturation level at ~ 4.8 emu/cm 3. Decreasing the applied voltage back to 0 V, M s remained at the saturation level for a long time period. Figure 5. Saturation magnetization decay at room temperature as a function of time after the applied forward bias was turned off. 12

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