NANO LETTERS
Spin-Polarized Light-Emitting Diodes with Mn-Doped InAs Quantum Dot Nanomagnets as a Spin Aligner
2005 Vol. 5, No. 2 209-212
Subhananda Chakrabarti,† Michael A. Holub,† Pallab Bhattacharya,*,† Tetsuya D. Mishima,‡ Michael B. Santos,‡ Matthew B. Johnson,‡ and Douglas A. Blom§ Solid State Electronics Laboratory, Department of Electrical Engineering and Computer Science, UniVersity of Michigan, Ann Arbor, Michigan 48109-2122, Department of Physics and Astronomy, and Center for Semiconductor Physics in Nanostructures, UniVersity of Oklahoma, Norman, Oklahoma 73019, and Metal and Ceramics DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Received August 26, 2004; Revised Manuscript Received December 9, 2004
ABSTRACT We have fabricated and characterized surface-emitting, spin-polarized light-emitting diodes with a Mn-doped InAs dilute magnetic quantum dot spin-injector and contact region grown by low-temperature molecular beam epitaxy, and an In0.4Ga0.6As quantum dot active region. Energy-dispersive X-ray and electron energy loss spectroscopies performed on individual dots indicate that the Mn atoms incorporate within the dots themselves. Circularly polarized light is observed up to 160 K with a maximum degree of circular polarization of 5.8% measured at 28 K, indicating high-temperature spin injection and device operation.
In recent years, considerable interest has been directed toward electrical spin injection in semiconductors, which is an important step toward the realization of spin-electronic (or spintronic) devices1,2 wherein electron charge and spin are manipulated simultaneously. Several spin injection systems combining ferromagnetic and nonmagnetic semiconductors have been demonstrated using either III-V or II-VI dilute magnetic semiconductors (DMS).3,4 III-Mn-V DMS, such as (Ga,Mn)As5,6 and (In,Mn)As,7,8 have been shown to exhibit well-ordered ferromagnetism at low temperatures and for dilute Mn concentrations and may be used as spin aligners for injection of spins into nonmagnetic semiconductors.9 The Curie temperatures (TC) in these alloys are still low (TC < 160 K)10 compared to most ferromagnetic metals, which results in correspondingly low temperatures of operation. However, we have recently shown that Curie temperatures above 300 K may be obtained in Mn-doped InAs dilute magnetic quantum dots (DMQDs),11 making InAs:Mn DMQDs a promising candidate for high-temperature operation of spin-based devices. Knowledge of the magnetic moment distribution within the QD heterostructure and epitaxial control thereof is a * Corresponding author. E-mail address:
[email protected]. † University of Michigan. ‡ University of Oklahoma. § Oak Ridge National Laboratory. 10.1021/nl048613n CCC: $30.25 Published on Web 12/29/2004
© 2005 American Chemical Society
critical step toward realizing QDs with an embedded giant Zeeman splitting for use in spintronic devices. In this letter, we describe the low-temperature molecular beam epitaxy (LT-MBE) of InAs:Mn self-organized quantum dots exhibiting room-temperature ferromagnetism and demonstrate their application in a spintronic device. Specifically, we report high-temperature, electrical spin injection arising from InAs: Mn QD nanomagnets embedded in the p-type ohmic contact layer of a surface-emitting light-emitting diode. Our demonstration of a working spin-polarized light-emitting diode (spin-LED) provides evidence that the fringe field of the dots suffices to align hole spins preferentially along the dot axis of magnetization, thereby altering the degree of circular polarization of the device luminescence according to optical selection rules.12 The InAs:Mn QD samples considered in this study were grown by solid-source LT-MBE on semi-insulating, (100)oriented GaAs substrates in a Varian GEN-II chamber under As-stabilized conditions. Reflection high-energy electron diffraction (RHEED) was used to monitor the surface reconstruction during growth. It is important to note that at growth temperatures typically used for the self-assembly of InAs QDs (Tg ) 480-520 °C), Mn surface and phase segregation are very dominant.13 This necessitates the growth of InAs:Mn QDs at much lower temperatures. Thus, all InAs: Mn QD layers were grown at temperatures in the range of
250-280 °C under an As4 beam equivalent pressure of ∼8 × 10-6 Torr onto a 500 nm GaAs buffer layer exhibiting (2 × 4) surface reconstruction at Tg ) 610 °C. The GaAs and InAs growth rates used were 0.72 µm/h and 0.07 µm/h, respectively. The growth temperature was measured by a thermocouple radiatively coupled to an In-bonded, molybdenum substrate holder. Following surface oxide removal and GaAs buffer layer growth, the substrate temperature was quickly ramped down to a temperature in the range of 250-280 °C with GaAs being grown continuously throughout. Once the substrate stabilized at the desired growth temperature, a 50 nm LTGaAs buffer layer was deposited followed by 1-10 InAs: Mn QD layers separated by 40-60 nm GaAs barriers. Details of the low-temperature, slow InAs growth rate, InAs:Mn QD formation are as follows. A streaky RHEED pattern was observed during the initial growth stage, indicating a twodimensional growth mode. After deposition of a 2.0 ML InAs wetting layer, the Mn effusion cell shutter was opened for the growth of 2.4 ML of InAs:Mn QDs, upon which the RHEED pattern became dim and then spotty. A 45 s growth interruption was allowed for dot formation before capping with GaAs. All InAs:Mn multilayer structures were grown with Mn compositions in the range of 7-25 at. % as determined by secondary ion mass spectroscopic (SIMS) analysis on similarly grown InGaMnAs quantum wells. Finally, the InAs:Mn QD multilayer structure was capped with 30 nm of GaAs, cooled to 100 °C, and promptly removed to minimize in-situ annealing effects from the substrate heater and high-temperature effusion cell radiation. The microstructure and chemical composition of the InAs: Mn QD samples were examined by cross-sectional transmission electron microscopy (XTEM) and X-ray energy dispersive spectroscopy (EDS). For XTEM imaging, a JEOL JEM-2000FX TEM at the University of Oklahoma was used, and we find that the QDs are well-formed with heights and base widths ranging from 10 to 16 nm and 13-25 nm, respectively. EDS analysis was performed by a Hitachi HF2000 cold field-emission TEM at the Oak Ridge National Laboratory. Cross-sectional specimens were prepared by mechanical thinning and dimpling, followed by Ar+ ion milling. We measured EDS spectra from a QD and its immediately surrounding LT-GaAs layer with a ∼1 nmdiameter electron beam. As shown in Figure 1, the spectrum taken from the QD layer clearly exhibits both Mn and In peaks, whereas the spectrum taken from the surrounding LT-GaAs layer yields only random background peaks. Though this result may indicate that Mn and In reside predominantly within the QDs, it is important to note that the QD EDS spectrum truly corresponds to the column of InAs:Mn QDs and LT-GaAs defined by the interaction volume of the electron beam with the InAs:Mn QD specimen. As such, it is not known unambiguously from the EDS spectra alone whether the Mn atoms have incorporated either substitutionally for group-III cations or as MnAs nanoclusters - within the dots only, formed a shell around the dots, or distributed between the InAs QDs and the GaAs matrix within the same plane. 210
Figure 1. Energy dispersive spectroscopy spectra taken from an InAs:Mn quantum dot layer and GaAs cap showing that the quantum dots contain both Mn and In.
Figure 2. Electron energy loss spectroscopy Mn L-edge crosssectional profile of a single InAs:Mn quantum dot having height and base width of 10 and 14 nm, respectively. Inset shows schematically the approximate scan position and direction of the electron beam probe.
For further insight into the distribution of Mn atoms within and around the QDS, we sought to examine cross-sectional InAs:Mn QDs specimens via electron energy loss spectroscopy (EELS) using Mn L2,3 edge profiling at the base of individual dots. In transition metals with unoccupied 3d states, such as Mn, the L ionization edges exhibit very sharp, intense peaks (so-called white lines) at the near-edge region and may be easily distinguished. Specifically, the L3 line arises from the transition of 2p3/2 electrons to 3d3/23d5/2 while the L2 line results from transitions from 2p1/2 to 3d3/2 states, and their intensities are consequently proportional to the number of Mn atoms within the excitation volume of the electron beam. EELS line scanning, with a lateral resolution of 2 nm, can thus provide a qualitative description of the Mn distribution within and around a single InAs:Mn QD. Figure 2 shows the results of an EELS L-edge line scan on a single, nearly-pyramidal InAs:Mn QD 10 nm in height and 14 nm in width; the inset depicts schematically the approximate location and direction of the EELS line scan. We find that the intensity of the Mn L3,2 lines rise dramatically when the electron beam probe is scanned over an InAs:Mn Nano Lett., Vol. 5, No. 2, 2005
Figure 3. Temperature-dependent, field-cooled magnetization obtained by superconducting quantum interference device magnetometry of a 10-layer InAs:Mn quantum dot sample with a 0.2 T in-plane, applied field.
QD and extinguish when the beam is focused outside of the dot. The intensity of the L3,2 lines peak at the approximate dot center, wherefrom the intensity drops symmetrically on either side up to the dot boundary. Such a distribution of Mn atoms correlates with the fractional dot volume within the excitation path. This observation provides fairly convincing evidence that the Mn incorporates within the InAs:Mn QDs rather than forming a metallic shell of Mn around the dot or in the GaAs matrix for which uniform distributions would have been observed by EELS. Several dots were scanned in an identical fashion, yielding similar results. We measured the magnetic properties of the InAs:Mn QD samples using a Quantum Design MPMS-5 superconducting quantum interference device (SQUID) magnetometer. Fielddependent magnetic moment measurements of our sample reveal an apparent hysteresis loop, which indicates ferromagnetic ordering of the InAs:Mn QD multilayer samples.11,14 Hysteresis and remanence are observed with a magnetic field applied either parallel or perpendicular to the sample surface. Since the magnetic moment for the perpendicular magnetic field is smaller than that for the parallel field, the sample has an in-plane easy axis.11,14 We performed field-cooled, temperature-dependent magnetization measurements on our InAs:Mn QD sample in a 0.2 T magnetic field applied along the [110] direction. The results are shown in Figure 3. The contribution of the InAs:Mn multilayer structure is isolated from the total SQUID response after subtraction of the diamagnetic contribution from the GaAs substrate and sample holder. We estimate a Curie temperature of 345 K for the InAs:Mn QD layers through linear extrapolation of the hightemperature, inverse susceptibility. The observance of high TC and remanence in an InAs:Mn multilayer QD sample suggests that such structures might be suitable for the hightemperature operation of a surface-emitting, spin-polarized light emitter without reliance on large, external magnetic fields. The spin-LEDs are grown by MBE on n+-GaAs(100) substrates and consist of a 0.7 µm thick n+-GaAs layer, followed by four strain-coupled In0.4Ga0.6As active region Nano Lett., Vol. 5, No. 2, 2005
Figure 4. Schematic heterostructure of spin-LED with InAs:Mn/ GaAs:Be spin aligner.
QD layers and an i-GaAs spacer layer of thickness d, and finally a InAs:Mn QD/GaAs:Be spin-aligner layer. A schematic heterostructure of the spin-LED is shown in Figure 4. To ensure high luminescence efficiency for the In0.4Ga0.6As QD active region, the nonmagnetic layers of the spin-LED are grown in an MBE chamber optimized for the growth of optoelectronic materials using substrate temperatures of 610 °C and 500 °C for the GaAs and In0.4Ga0.6As QD layers, respectively. A judicious choice for the thickness of the GaAs layer separating the active region QDs from the spin-aligner layer is made based on recent photoluminescence experiments performed on a similar structure.15 Following growth of the spacer layer, an amorphous arsenic cap is deposited at room temperature before transferring through air into a second MBE chamber optimized for the growth of IIIMn-V diluted magnetic semiconductors. Mesa-shaped diodes having 200-600 µm diameters and ring ohmic contacts were fabricated using standard optical lithography, wetchemical etching, and metallization techniques. The electroluminescence of the spin-LED was investigated in a closed-cycle helium cryostat with an external, air-gap electromagnet generating a magnetic field parallel to the spinLED emission direction, i.e., using the Faraday geometry. Initially, the spin-LED was cooled from room temperature to 28 K in the presence of a 5 kOe magnetic field applied along the surface normal, establishing a net perpendicularly magnetization in the InAs:Mn QD layers. Polarizationresolved electroluminescence was then recorded upon heating in zero magnetic field using an achromatic quarter-wave plate and Glan-Thompson calcite linear polarizer to analyze the degree of circular polarization of the spin-LED emission, given by Pcirc ) [I(σ+) - I(σ-)]/[I(σ+) + I(σ-)] where I(σ() is the intensity of light with ( helicity integrated across the spectral width of the quantum dot electroluminescence. The spin-LED was DC forward-biased with 35 mA. A temperature-independent background polarization, likely 211
Mn nanomagnet spin-LED emission is observed for temperatures
