High-Speed Non-Volatile Optical Memory
Published on Jan 23, 2020
We have proposed, fabricated, and studied a new design of a high-speed optical non-volatile memory. The recoding mechanism of the proposed memory utilizes a magnetization reversal of a nanomagnet by a spin-polarized photocurrent. It was shown experimentally that the operational speed of this memory may be extremely fast above 1 TBit/s. The challenges to realize both a high-speed recording and a high-speed reading are discussed. The memory is compact, integratable, and compatible with present semiconductor technology. If realized, it will advance data processing and computing technology towards a faster operation speed.
The first important application of the high-speed memory is chip-to-chip interconnection [2,3]. Its purpose is to transfer data from one silicon chip to another in the shortest possible time. In a silicon chip electrical memory, like Dynamic random-access memory (DRAM), Static random-access memory (SRAM), Phase-change memory (PRAM), Flash, Resistive random-access memory (ReRAM)  and Magnetoresistive random-access memory (MRAM)  , is dense, but electrical memory has a relatively moderate operational speed. The moderate speed for the data transfer inside the chip is acceptable and it does not limit a high speed of the total data transfer inside a chip. This is because, inside the chip, there is a large number of connection lines and the data transfer happens in parallel. Even though the transfer speed of each line is moderate, the overall transfer speed is fast. The case of a chip-to-chip interconnection is different, because the number of connection lines between chips is limited. It is important to transfer data through each line at a substantially faster speed. Optical interconnections between chips are used [2,3] in order to achieve a high-speed data transfer. A high-speed optical buffer memory is required to efficiently utilize the high-speed capabilities of the optical data links.
The purpose the buffer memory is to match the in-parallel data transfer inside a chip with the in-serial data transfer between chips. The buffer memory does not need to be dense, but it must be very fast. Another important application of the proposed high-speed optical memory is a buffer memory for an optical network router. It is worth noting that even though high-speed optical fiber links are widely installed worldwide, only a small fraction of their high-speed capacity is used. At present, the speed of a network is mostly limited by the speed of the network routers. The network router switches data streams between different nodes in a network. The function of the network router is to receive the data package from one channel, to store it, and send it to the second channel.
Since the availability of the second channel is basically unknown, non-volatile memory has to be used in the router. Routers made of electrical components are installed in modern optical networks. Routers, which can process data at speeds up to 10 GBit/s, are already commercially available. Now significant efforts are applied to fabricate routers that can process data at speeds of 40 GBit/s. It is a challenging task, because the required operational speed is significantly above the speed limit of most of the electrical components. A further possible increase of the operational speed of the router, which is made of electrical components is, therefore, questionable. The use of a high-speed non-volatile optical memory might be the only option to increase the operational speed of an optical router and subsequently to increase the overall speed of the Internet.
Memory Design, Fabrication, and Recording Principle
The memory consists of micro-sized memory cells integrated on a semiconductor wafer. A bit of data is stored by each cell. Each cell consists of three parts: an optical waveguide, a semiconductor-made photo detector, and a nanomagnet made of a ferromagnetic metal. In the case of the memory with MTJ, instead of a single layer of ferromagnetic metal two layers are used, which are separated by a tunnel barrier. Figure 2 shows a memory cell of a prototype fabricated on GaAs substrate. The AlGaAs waveguide is transparent for light of = 800 nm. It is used to deliver the optical pulses to the memory cell. On top of the AlGaAs waveguide the p-i-n GaAs photo-detector is fabricated. The photo-detector absorbs the light. On the top of GaAs photo detector there is a Fe nanomagnet. The size of the nanomagnet is sufficiently small so that it is a single-domain state and it has two stable magnetization directions along its easy axis. The data is stored as a magnetized direction in the nanomagnet. Additionally, the nanomagnet functions as a top contact for the photo-detector.
Memory cell of a prototype fabricated on GaAs substrate. The transparent AlGaAs waveguide is used to deliver optical pulses to the memory cell. The p‐i‐n GaAs photodetector absorbs the optical pulses. It converts spin information in an optical pulse into the spin polarization of photoexcited electrons. The data is stored as a magnetized direction in the nanomagnet, which is fabricated on top of the detector. The nanomagnet is also used as a top electrode for the detector.
Figure 3 shows integration of two memory cells and explains principle of high speed recording. There are two waveguide inputs. One input is for data pulses and one input is for the clock pulse. The clock pulse is used to select for recording a single pulse from a sequence of the data pulses. Polarization of data pulses and the clock pulse are linear and mutually orthogonal. Optical paths were split so each memory cell is illuminated by the data pulses and the clock pulse. The lengths of waveguides are adjusted so that the phase difference between the clock and data pulses is lambda/4 at each memory cell. At the first memory cell the clock pulse came at the same time with first data pulse.
Therefore, these two pulses are combined into one circularly polarized pulse. Since only the first pulse is circularly polarized, only this pulse excites spin-polarized electrons, changes magnetization, and is memorized. All other data pulses are linearly polarized, they do not excite spin-polarized electrons and they have no effect on the magnetization. For the second memory cell, the clock pulse is slightly delayed relative to the data pulses and it comes together with the second data pulse. Only the second pulse is circularly polarized and can be memorized by the second memory cell. Therefore, each data pulse can be memorized by individual memory cell. The closer the pulses can be placed relative to each other, the more data can be transformed through one line and the faster recording speed of the memory can be achieved. The minimum interval between pulses, at which a pulse can be recorded without any influence of nearest pulse, determines the recording speed of the memory.
High-Speed Injected of Photo-Excited Electrons into a Nanomagnet
The recoding mechanism of the proposed memory consists of two steps (See Figure 1). As was shown above, we have successfully verified an ultra-high demultiplexing for the proposed recording method. The second step of the recording is magnetization reversal of a nanomagnet by spin-polarized photo-excited electrons. This step can be done at a moderate speed. The spin lifetime in the semiconductor is short. For GaAs it does not exceed 100 picoseconds at room temperature. In the design of the proposed memory a short optical pulse excites spin-polarized electrons in the semiconductor photo-detector. Under the applied voltage the photo-excited electrons are injected into the nanomagnet. In the case when the injection time is longer than the spin lifetime, the spin information is lost inside the semiconductor before it is injected into the nanomagnet. Additionally, for the same energy of the optical pulse, the amplitude of the injected current lowers for the longer injection time.
Since the magnetization reversal has a threshold with respect to the amplitude of the injected current, the long injection time significantly limits the possibility of the magnetization reversal by the light. The injection time shorter than 100 picoseconds is essential to achieve magnetization reversal by the light with respect to both the electron spin lifetime and the amplitude of the injected current. The injection time should be as short as possible. At least it should be shorter than the spin lifetime in the semiconductor. In order to verify and to optimize the injection of photo-electrons into a nanomagnet, we have fabricated a simplified memory cell comparing to that of Figure 2. We have removed the waveguide and we have made a p-contact directly to the p-region of the photo-detector.
Except for this simplification, the structure remains the same as in Figure 2. A femtosecond pulse laser was used as the light source with pulse width of 140 fs and the pulse repetition rate of 80 MHz. The laser beam was focused into a spot of 4-m diameter exactly on top of nano contact (Figure 5b). Since the size of the nanomagnet and its connecting electrode are much smaller that the laser spot, almost all of the light is absorbed by the photo-detector. The time response of the p-i-n photo detectors was measured by a sampling oscilloscope on a 50-Ohm load resistor connected between the nanomagnet and the metallic contact to p-GaAs layer of the detector. We have found that the injection time strongly depends on the contact resistance between the nanomagnet and the detector. The lowest contact resistance is required to achieve the acceptably-short injection time.
In order to reduce the contact resistance we have used a delta-doped InGaAs contacting layer. The same fabrication method was used as was described in the previous chapter. By varying the doping concentration of the InGaAs layer, three samples with the contact resistance of 7000, 250, and 30 Ohm/m2 were fabricated. All contacts were Ohmic. Figure 5b shows the time evolution of photo-excited current. For the sample with the contact with the contact resistance of 30 Ohm/μm2, the injection time was 80 ps, which is shorter than the spin lifetime in GaAs. From Figure 5b it could be concluded the lowest metal/semiconductor contact resistance is a critical parameter for the design of this memory. It should be lower than 100 Ohm/µm2
Memory with a MTJ Electrode
nanomagnet) was discussed. There is another possible design of the memory, where a magnetic tunnel junction (MTJ) is used instead of the nanomagnet. There are merits and demerits of such design. Major merit of the memory with a MTJ electrode is its ability for both electrical and optical reading and writing. Due to this property, the reading/recording from/into the memory can be done either by an electrical current, which makes possible a data exchange between the memory and other electrical components inside the chip, or by light, which provides data exchange with other chips.
Therefore, this memory design is quite suitable for chip‐to‐chip connection as a high‐speed buffer memory. The demerit of this design is that the recording can be done not only by a circularly‐polarized optical pulse, but also by a linear‐polarized pulse. It makes it more difficult to use the high‐speed demultiplexing scheme of Figure 3. In contrast to the memory with a nanomagnet, in the case of the memory with a MTJ electrode there are additional limitations on pulse intensities and the number of pulses in a pulse package that could be recorded. The magnetization direction of a ferromagnetic metal may be reversed by a spin current. The relaxation of spin current in the ferromagnetic metal induces a spin torque , which may reverse the magnetization of the metal.
This effect is used as a writing method in magnetic random access memories (MRAM) . In the memory design with a MTJ electrode, the spin current may be generated not only in the semiconductor detector, but also at the tunnel junction. Only circularly-polarized light photo-excites a spin-polarized current in the detector. In contrast, a pulse of any polarization may induce a spin-current and the spin torque at the tunnel barrier. The tunnel barrier itself functions as a spin polarizer. The ability of the magnetization reversal by both spin-polarized and spin-unpolarized photo-currents has some advantages and disadvantages for the memory with a MTJ electrode, which are discussed below. Figure 8 shows an example of the memory with a MTJ electrode.
It consists of a GaAs p-i-n diode with a MTJ electrode and a side electrode in close vicinity of the MTJ electrode. The MTJ is made of two ferromagnetic metals separated by an MgO tunnel barrier. The “free” layer of the MTJ is in contact with the GaAs photodiode. The top layer of the MTJ is the “pin” layer. The photo-induced electrons may reverse magnetization of the “free” layer, but the magnetization of “pin” layers should remain the same. The magnetization direction of “free” and “pin” layers may be either in-plane, as shown in Figure 8, or perpendicular to the plane. The resistivity of the MTJ depends on mutual orientation of magnetization of “free” and “pin” layers. The “free” layer has two opposite stable magnetization directions.
The data is stored as a magnetized direction in the “free” layer. The side contact is used for the electrical recording and reading. There is a delta-doped nn-InGaAs layer (which is not shown in Figure 8) between the “free” layer of MTJ and n-GaAs. As was explained before, the nn-InGaAs layer is used to achieve the lowest contact resistance between the MTJ and n-GaAs layer. Additional difference of the design of Figure 8 from the design of Figure 2 is that the p-i-n detector is embedded into the AlGaAs waveguide. The AlGaAs was wet-etched before the growth of the detector.
In the memory with a MTJ electrode recording and reading can be done by an electrical current. The recording and reading methods are similar to those that are currently used in the MRAM . For the electrical reading, a voltage is applied between the MTJ and the side contact. The resistivity of the MTJ is different for two opposite magnetization directions of the “free” layer with respect to the magnetization direction of the “pin” layer. The stored data is sensed by the current flowing through the MTJ. For the electrical recording, a larger voltage is applied between the MTJ and the side contact. The charge current, which flows through the tunnel barrier, induces the spin current. The relaxation of the spin current causes a spin‐transfer torque . The spin‐transfer torque is a consequence of the transfer of spin angular momentum from a spin current.
If the current is sufficient, the magnetization of the “free” layer is reversed by the spin‐transfer torque and data is memorized. By changing polarity of the applied voltage, the magnetization of the “free” layer can be turned either in parallel or opposite to the magnetization of the “pin” layer. The method of the optical reading for this memory is based on the control of the optical gain of a p-i-n junction by a MTJ electrode. The p-i-n GaAs junction absorbs the light, when there is no current injection into the p-i-n junction. When under a direct bias the current is injected, the absorption can be compensated and light can be amplified, because of the stimulated emission. A bias voltage is applied between the back electrode and the top of the MTJ. Since the different resistance of the MTJ, the current injected into the p-i-n GaAs is different for the opposite magnetization directions of the “pin” layer.
The bias voltage should be optimized so that for the parallel magnetization directions of the MTJ electrodes, the injection current is sufficient to produce an optical gain in the p-i-n junction and, for the opposite magnetization directions, the injection current should not be sufficient to create any gain. Therefore, the direct-biased p-i-n junction with a MTJ works as an optical switch. When the magnetization of the “free” layer is parallel to the magnetization of the “pin” layer, a light pulse can pass through the junction. When the magnetization of the “free” layer is reversed, the light pulse is blocked
The high-speed reading can be done using the magneto-optical (MO) effect of the magnetization-dependent loss. We have demonstrated experimentally the read-out of nanomagnets of sizes larger than 1 m with a high SNR and a high on/off ratio. However, in the case of nanomagnet diameter of 1 m or smaller, the on/off ratio and the SNR sharply decrease. This makes difficult to use this MO read-out method for nanomagnets of a smaller size. The problem can be resolved using a spin-injection from a nanomagnet into a gain region of an optical amplifier. The spin-polarized electrons, which are injected from a nanomagnet into the amplifier, spread over a larger volume in the amplifier. This makes the effective volume of a MO material larger and the read-out easier. The read-out efficiency may be improved even more in a memory with a MTJ electrode. The resistance of the MTJ significantly changes for two opposite directions of the “free” layer due to the TMR effect. In the case when the MTJ is used as an electrode of an optical amplifier, the reversal of the magnetization may switch on/off the optical amplifier. This reading method is very effective. It provides a large SNR and a high on/off ratio for reading.
Paper Submitted by Vadym Zayets, Spintronics Research Center, National Institute of Advanced Industrial Science and Technology
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