GaN Device from an

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Injection of Spin-Polarized Electrons into a GaN/AlGaN Device From an Electrochemical Cell - Evidence for an Extremely Long Spin Lifetime Anup Kumar, Eyal Capua, Claudio Fontanesi, Raanan Carmieli, and Ron Naaman ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01347 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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Injection of Spin-Polarized Electrons into a GaN/AlGaN Device From an Electrochemical Cell Evidence for an Extremely Long Spin Lifetime Anup Kumara, Eyal Capuaa, Claudio Fontanesi,b Raanan Carmieli,c and Ron Naaman*a a

Dept. of Chemical and Biological Physics, Weizmann Institute of Science, Rehovot 76100, Israel.

b

Dept. of Engineering ‘Enzo Ferrari’, University of Modena and Reggio Emilia, Via Vivarelli 10, 41125 Modena, Italy c

Dept. of Chemical Research Support, Weizmann Institute, Rehovot 76100, Israel

Email: [email protected]

Abstract

Spin-polarized electrons are injected from an electrochemical cell through a chiral selfassembled organic monolayer into a GaN/AlGaN device in which a shallow two-dimensional electron gas (2DEG) layer is formed. The injection is monitored by a microwave signal that indicates a coherent spin lifetime that exceeds 10 milliseconds at room temperature. The signal was found to be magnetic field independent; however, it depends on the current of the injected electrons, on the length of the chiral molecules, and on the existence of 2DEG.

Key words: Chirality, Interface, Two dimensional electron gas, microwave, electrochemistry

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The field of spintronics is typically associated with information technology, as a possible option for replacing the commonly used microelectronics. The combination of semiconductors and microwave radiation is attractive for spintronics-related applications and they were also considered for quantum computing technology, because of the ability to manipulate and probe spins, either of electrons or nuclei.1,2 The combination of electrons’ spin and microwave radiation was also proposed and demonstrated as a means of sensing bio-processes.3,4,5,6,7 However, any attempt to apply the spin/microwave technology faces the problem of a relatively short spin lifetime and low sensitivity, especially when operation at room temperature is considered. Here we present a method for detecting electron spin polarization by microwave at room temperature, when the spins are injected from an electrochemical cell through a monolayer of chiral molecules (see Fig. 1) into a GaN/AlGaN device in which a shallow two-dimensional electron gas (2DEG) layer is formed. We found a surprisingly long electron spin lifetime in the GaAs/AlGaAs, that exceeds 10 milliseconds. The concept presented also allows probing the electrons’ spin in electrochemical complex systems, which occurs in a biologically relevant environment at physiological relevant temperatures, by a miniature detector with remote sensing with very high sensitivity. The sensitivity scales with the spin lifetime. Hence while the spin lifetime in semiconductors is typically in the sub-microsecond range in previous reports, but in the present device it is on the millisecond sale. The device is based on the chiral-induced spin selectivity (CISS) effect.8,9 Namely, the spin preference in electron conduction is through chiral molecules. Which spin is preferred depends on the molecule, its handedness, and the direction of the electron transfer. Various methods were applied in the past for detecting spin-polarized current. In most of them ferromagnetic electrodes served as the spin analyzer. Recently a similar device as used here was applied for detecting spin-polarized electron transfer through a chiral monolayer via Hall measurements.10 Electron paramagnetic resonance studies were combined with electrochemistry in the past and offered important insights into the oxidation-reduction process.11 The microwavebased method presented here provides the ability to detect spins, without the use of any magnetic materials or magnetic fields and without applying optical methods. Results We performed time-resolved microwave absorption/emission studies. The experimental setup is presented schematically in Figure 1. A two-electrode electrochemical cell was inserted within an 2 ACS Paragon Plus Environment

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EPR spectrometer microwave cavity; the charge is injected into the GaN/AlGaN device, which serves as the working electrode (Figure 1a). The electrochemical cell is of original design and is machine carved from a full polymethyl methacrylate (PMMA) rod, and it is placed in the sample holder microwave cavity (Fig. 1). Due to the size of the sample holder the resonator Q-factor was very low and therefore the MW absorption range very broad around 0.6 GHz. Charge injection in the GaN semiconductor is obtained by applying a suitable external potential to oxidize/reduce benzoquinone; the latter has a 1mM concentration in an aqueous PBS-buffered solution at pH=7. Figure 1b presents a typical cyclic voltagram (CV)

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benzoquinone (BQ) in PBS solution, when the potential refers to the potential between the working and counter electrodes. The GaN working electrode was coated with a covalently bound self-assembled monolayer (SAM) of chiral SHCH2CH2CO-{Ala-Aib}n-COOH (n=5,9 indicates AL5 or AL9, respectively) polyalanine, and as a reference an achiral 3-mercapto propanoic acid (MPA) monolayer was used (See Figure 5). Time-resolved microwave measurements at 9.49 GHz were carried out to monitor in operando spin-polarized injection into the GaN semiconductor.

CV of BQ in PBS (pH ~ 7)

Figure 1 (a) Details concerning the microwave time-resolved experimental set-up. An electrochemical cell containing GaN/AlGaN working electrode (WE) and counter electrode (CE) is inserted into microwave (MW) cavity. (b) The cyclic voltagram (CV) of benzoquinone (BQ) in PBS solution.


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The microwave signal was monitored when a pulse of +1 V followed by a pulse of -1V was applied for 1 sec, between the GaN and Pt electrodes. A kinetic trace appeared in the timeresolved microwave experiment, as seen in Figure 2a. Figure 2b shows a schematic view of the whole triggering and synchronization scheme. The charge injected into the GaN semiconductor is driven by the electrochemical process and the spin detection is performed by the microwave. The microwave signal vs time transients were also collected as a function of the magnetic field. The microwave signal was found to be independent of the magnetic field, applied perpendicular to the working electrode, up to a field of 1.2 Tesla. The microwave time-resolved measurements show that the charge injected into the semiconductor causes an oscillation to appear in the signal, which is found at about 20 ms after switching the potential from 0 to +1 V. The oscillation corresponds to a coherent sequence of absorption-emission and the oscillations decay on a time scale of about 15 msec. To understand the mechanism underlying the occurrence of the oscillation in the microwave time-resolved signal, experiments were performed varying i) the nature of the charge injection/extraction process (oxidation/reduction regime), ii) the nature of the organic SAM, length, and achiral/chiral, and iii) the substrate. t (ms)

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Figure 2: a) Time-resolved microwave signal recorded during application of a constant +1 V potential, 1 second duration pulse. GaN functionalized with AL5, 1mM benzoquinone in aqueous PBS buffer, pH = 7 solution. b) Microwave (blue frame) and potential pulse (black frame, showing the first 250 ms of the total 1 s pulse) time window, showing the triggering and timing of the EPR microwave signal sampling (200 ms time window). Note that the EPR microwave signal sampling time window has been suitably triggered in time (in different experiments) to properly sample the full, 1 second potential pulse. The blue frame shows a survey of a full 200 ms microwave signal vs. the time transient (the relevant zoom in the 0 to 45 ms time range is reported in Fig.1a). The microwave signal vs time transients are also collected


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as a function of the magnetic field. Thus, a single EPR signal vs the time transient is a section of 3D EPR signal intensity as a function of both the magnetic field and time.

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Figure 3 a) Microwave signal measured under anodic (red line) and cathodic (blue curve) regimes. b) Normalized microwave signal, chiral SAM: AL5 (red line), AL9 (blue line). c) The signal in the case of an achiral MPA monolayer (red line) and a bare GaN electrode (black line). In the inset,, a long time behavior of the signal when no chiral molecules are adsorbed. Figure 3a presents the signal observed in the oxidation process when i) a potential of +1V is applied between the working and counter electrode (blue curve) and ii) in the reduction process, when a pulse of -1V is applied (red curve). The phase of the microwave signal is reversed between the two cases, indicating that in one case the first peak in the signal relates to absorption of the radiation, whereas in the other to induced emission. The results are consistent with injecting spin-polarized electrons into the solid-state device in the oxidation process, and injecting electrons with the opposite spin in the reduction process. When an n-doped GaN device was used as the working electrode, no signal could be detected. We concluded that the signal observed required use of a 2DEG layer. Figure 3b provides insight into the effect of the length of the chiral oligopeptide. It presents the microwave signals when the device is coated either with AL5 or AL9 polypeptide. The signals are about the same in the two cases; however, note that the electrochemical current is larger for the shorter oligomer; it is 71 and 5 mA for the AL5 and AL9 oligomers, respectively. (The current values are reported in Fig. 3b) Hence, this means that the spin polarization in the 5 ACS Paragon Plus Environment

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case of the longer oligopeptide is about an order of magnitude higher than for the shorter oligopeptide. Remarkably, Figure 3c shows that in the case of an achiral SAM, there is no microwave signal that can be detected. In the inset of Figure 3c the survey of the time-dependent microwave signal is shown, again with no evidence of any absorption both for the device coated with the achiral SAM (MPA) and for the bare GaN device, despite the high electrochemical current of 85 and 98 mA for the MPA-coated and bare devices (See also Fig. 7), respectively (current values are reported in the legend of Fig. 3c). Even in this latter case, there is no evidence of any microwave signal. In all of our measurements we observed that the electrochemical current scales with the length of the SAM, which is a well-documented effect.12 Figure 4a shows the amplitude of the microwave signal as a function of the pulse potential. As the potential increases, the maximum current peak increases linearly (see Fig. 2b), so does the amplitude of the microwave signal. This linear behavior is presented in Fig. 4b both for the positive and negative peaks. Because the amplitude is a linear function of the current, one could expect that the signal observed when the device is coated with AL5 will be about ten times larger than that obtained when the device is coated with AL9. However, as shown clearly in Fig. 3b, this is not the case, indicating the much more efficient role that AL9 plays as a spin filter. These results are consistent with previously reported results on spin polarization by oligopeptides.10,13

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Figure 4. a) Time-resolved microwave intensity as a function of the applied potential. A functionalized AlGaN/GaN/AL5 chiral interface in 1mM benzoquinone PBS, pH = 7, buffer solution. b) The amplitude of the microwave signal as a function of the applied potential. We found that the intensity of the microwave signal is proportional to the power of microwave in resonator cavity (Figure 7). The signal depends linearly on the power till intensity of mW is obtained, following this intensity the signal saturates. Notably, after denaturation/desorption of AL-5 from the surface by application of high voltage in the electrochemical cell, i.e., 3V, the microwave signal is gradually reduced after a few CV scans. This observation confirms the contribution of the chiral monolayer to the signal, hence the importance of the CISS effect (Figure 8). In former studies10 we already demonstrated that with the present device the Hall signal reverse its sign upon switching the chirality of the molecules for the same molecule on the charge polarization. Discussion The results presented here are surprising for several reasons: i)

There is a strong microwave absorption/emission that does not depend on the external magnetic field (Fig. 9).

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The signal appears only when the electrochemical current is at its maximum.

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The signal has a coherence time of milliseconds at room temperature.

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The signal depends on the existence of 2DEG. Different electron spin relaxation lifetimes at room temperature were reported for GaN,

ranging from 35 to 300 nsec.14,15 It was also observed that electron spins can be injected ballistically into GaN16 and spin injection into a high-mobility two-dimensional electron gas confined at an (Al,Ga)As/GaAs interface with spin decay lengths on the order of 2μm was reported.17 In previous studies, we have found efficient spin injection from chiral molecules selfassembled on the surface into the bulk of a GaN/AlGaN device.18 We propose here that our observations result from electrons originating from the electrochemically driven benzoquinone reduction/oxidation process, which are spin polarized by the chiral monolayers and injected efficiently into the device. When the concentration of the spins is high enough, at the peak of the electrochemical current, a strong exchange interaction due to 2DEG and via the RKKY mechanism19,20,21 induces a magnetic state that survives as long


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as the spin concentration is high. This model explains all of our observations: the spin density at the peak of the electrochemical current can be estimated at about 1016 electrons (spins)/cm3. This value is based on an electrochemical current of about 1 µA, a spin lifetime of 10±5 msec (as measured by the time profile of the microwave signal), a surface area of 1 cm2, and a distance between the surface to the 2DEG of 30 nm. When the magnetic state is formed, the spin state is stabilized by the exchange interaction and therefore, splitting in the spin states occurs and the microwave absorption/emission can be observed. Indeed, splitting in spin states was observed before in the type of device we used.22 This is why an external magnetic field has no effect on the microwave signal we observed. The long lifetime of the signal depends on the period in which the spin concentration is high, i.e., the spin population pumped by the current driven by the externally applied potential and filtered by the chiral interface. When the spin concentration is lower than the required threshold for forming the ferromagnetic state, the lifetime becomes very short and the concentration is too low to be detected by microwave absorption. Conclusions In the present work we were able to detect an electrochemical process by microwave absorption. The special properties of the 2DEG GaN/AlGaN device increase the sensitivity of the detection. This study is another confirmation of the CISS effect and it demonstrates the ability to inject efficiently electron spins from chiral molecules into semiconductors at room temperature. This efficiency may be explained by the lack of a Schottky barrier, which hinders an efficient process when the electrons are injected from ferromagnets.23,24 Experimental Materials and Methods. Solvents were purchased from Merck, Baker, or Bio-Lab (A.R /HPLC grade). Mercapto-propanoic acid (MPA) and benzoquinone (BQ) were purchased from Aldrich and the series of oligopeptides i.e., SHCH2CH2CO-{Ala-Aib}x-COOH (where x = 5, 9: in the text referred to as AL5 and AL9, respectively) were purchased from Genemed Synthesis, Inc. All chemicals were used without further purification. The AlGaN/GaN HEMT Epi wafers with epitaxial layers of i-GaN (1800 nm), i-AlGaN (20nm), and i-GaN (2nm) on sapphire-substrate were purchased from NTT AT. Additionally, AlGaAs/GaAs HEMT with epitaxial layers of iGaAs (11nm), i-AlGaAs (10nm), i-n+AlGaAs (12nm) i-AlGaAs (3nm), and i-GaAs (200nm) was purchased from IQE.


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Device Fabrication. The Hall devices were fabricated by standard photolithography in a class 1000 clean room. Ohmic contacts were obtained by annealing a metallic multilayer i.e., Ti (20 nm)/Al (100nm)/Ni (40 nm)/Au (40 nm) at 900°C. The dimensions of the working electrode surface are 3 mm (width) × 4 mm (length) for molecular adsorption. The metal contacts were passivated using a high-quality UV-glue. Similarly, Au and n-type Si substrates with identical dimensions were used for a series of cross-check experiments (Figure 7). A multi-metallic stack of Ni (5 nm), Au (60 nm), Ge (30 nm), Ni (20 nm), and Au (100 nm) was deposited on GaAs for achieving ohmic conduction after annealing at 380°C. Formation of monolayers over GaN devices.

Four different molecular systems, mercapto-

propanoic acid (MPA), SHCH2CH2CO-{Ala-Aib}5-COOH (AL-5), and SHCH2CH2CO-{AlaAib}9-COOH (AL-9) were adsorbed on GaN following the procedure described below. GaN devices were boiled with hot acetone and ethanol for 15 min, followed by 10 seconds of sonication. The cleaned substrates were etched for 30 sec in 6 M HCl, rinsed in water, and then dried under a N2 stream. Then the samples were treated with UV/ozone oxidation (Ultra violet Ozone Cleaning System, UVOCS) for 30 min, and then placed immediately in the incubation solution (1 mM oligopeptide in toluene). The solution vials were kept under N2 and then placed in a desiccator for 72 h in the dark. After adsorption, the samples were rinsed with toluene and dried with an N2 stream for measurement. Formation of monolayers over GaAs devices. The devices were cleaned in boiling acetone and isopropanol for 10 minutes each, dried by N2, and treated with UV/ozone oxidation (UVOCS) for 20 min. This procedure was immediately followed by a 5-second immersion in a 2% HF solution, then 30 seconds in 25 % ammonia solution to remove the native oxide on the exposed GaAs surface. Devices were washed with water, dried with N2, and put into vials containing SAM solution (1 mM in toluene). The vials were filled with N2, sealed, and put into dark desiccators for 72 hours. At the end, the devices were cleaned with toluene and ethanol. Fourier Transform Infrared (FT-IR) spectroscopy: Monolayer formation was analyzed by Fourier transform infrared spectroscopy (FTIR) in grazing-angle attenuated total reflectance mode (GATR-FTIR), by using a ThermoScientific FTIR instrument (Nicolet 6700) equipped with a VariGATR accessory (Harrick Scientific) with a single reflection Ge crystal. Spectra were collected by accumulating a minimum of 500 scans per sample with a clean GaN surface as a reference, and mounting the GaN sample at a Brewster angle of incidence of 67.4°. In addition, 9 ACS Paragon Plus Environment

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all spectra were collected while purging the VariGATR attachment and FTIR instrument with N2 along the infrared beam path to minimize the absorption due to both atmospheric moisture and CO2. Spectra were analyzed and processed using OMNIC software. The IR spectrum of the mercapto-propanoic acid (MPA) monolayer (Fig. 5) exhibits two welldefined peaks at 2847 and 2917 cm-1 due to the symmetric and asymmetric CH2 stretching vibrations, respectively. The IR spectra of the chiral oligopeptides SHCH2CH2CO-{Ala-Aib}xCOOH exhibit characteristic stretching frequency peaks at ~ 1660 cm-1, corresponding to the amide I band and at 1536 cm-1; the latter corresponds to the amide II band. 1.0

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measurements. The two-electrode assembly i.e., the Pt-wire counter electrode and the device functionalized with chiral/achiral molecules as the working electrode were used for measurements. This set-up was placed inside the EPR resonator cavity. The electrochemical measurements were performed using a Keithley 2636A source unit. A constant potential (V = 1 V) was applied on the device for a specific time and the microwave signal was recorded using a Bruker EPR spectrometer. Figure 6 presents the microwave signal observed with bare surfaces of GaN (blue), Au (green), and Si (red).

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When the GaN device is base, microwave signal from the benzoquinone (BQ) could be observed only when its concentration was above 100 mM (Figure 10).


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Detailed about the sample preparation, characterization of the samples, and results from controlled experiments (PDF). Acknowledgement We thank Dr. Karen Michaeli for many helpful discussions. We acknowledge the support from the John Templeton Foundation, the Israel Science Foundation, and the European Research Council under the European Union's Seventh Framework Program (FP7/2007-2013) / ERC grant agreement n° [338720]. C.F. acknowledges financial support for this research by University of Modena and Reggio Emilia (Department of Engineering ‘Enzo Ferrari’), through “Spin Dependent Electrochemistry”, FAR2016.

References (1) Dutt, M. V. G.; Childress, L.; Jiang, L.; Togan, E.; Maze, J.; Jelezko, F.; Zibrov, A. S.; Hemmer, P. R.;. Lukin, M. D. Quantum Register Based on Individual Electronic and Nuclear Spin Qubits In Diamond, Science, 2007, 316, 1312-1316. (2) d’Amico, I.; Biolatti, E.; Rossi F.; De Rinaldis, S.; Rinaldis, R.; Cingolani, R. GaN Quantum Dot Based Quantum Information/Computation Processing, Superlattices and Microstructures, 2002, 31, 4,117-125. (3) McGuinness, L. P.; Hall, L. T.; Stacey, A.; Simpson, D. A.; Hill, C. D.; Cole, J. H.; Ganesan, K.; Gibson, B. C.; Prawer, S.; Mulvaney, P.; Jelezko, F.; Wrachtrup, J.; Scholten, R. E.; Hollenberg, L. C. L.

Ambient Nanoscale Sensing With Single Spins Using Quantum

Decoherence, New J. Phys. 2013, 15, 073042. (4) Grotz, B.; Beck, J.; Neumann, P.; Naydenov, B.; Reuter, R.; Reinhard, F.; Jelezko, F.; Wrachtrup, J.; Schweinfurth, D.; Sarkar, B.; Hemmer, P. Sensing External Spins With NitrogenVacancy Diamond, New J. Phys. 2011, 13, 055004. (5) Staudacher, T.; Shi, F.; Pezzagna, S.; Meijer, J.; Du, J.; Meriles, C. A.; Reinhard, F.; Wrachtrup J. Nuclear Magnetic Resonance Spectroscopy on A (5-Nanometer)3 Sample Volume, Science 2013, 339, 561-563.


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Chiral molecules on GaN

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MW source Benzoquinone in PBS


GaN Device from an

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