RU2744440C1 - Method of writing and reading information for permanent memory elements of neuromorphic systems - Google Patents

Abstract

FIELD: memory structures.

SUBSTANCE: invention relates to the field of memory structures using a biological model, in particular to a method for recording and reading information for permanent memory elements of neuromorphic systems. The technical result is achieved due to the method, which includes changing the conductivity of a single crystal of black phosphorus, as well as recording and reading, which are performed due to the controlled drift of vacancies near the metal-black phosphorus contact when a voltage is applied to the contact from less than -2 V to -10 V for recording and +2 V to more than -2 V for reading.

EFFECT: technical result consists in providing information recording for a long-term resistive memory system with a single recording and reading channel without the need to use external electric fields of the transistor gate to record information.

1 cl, 1 dwg

Description

Recently, resistive memory structures have attracted considerable interest for their use in electronics and systems for the implementation of neuromorphic computations. In this case, the operating parameter is the conductivity of the material, which can be controlled by external parameters. To date, several types of materials have been developed that have the ability to change conductivity up to several orders of magnitude under the action of voltage / current, radiation, etc. In practical implementation, the method of recording information should be selected depending on the required type of memory cell.

A known method of recording information based on the effect of an electric field on the conductivity of a single crystal of black phosphorus [Black phosphorus nonvolatile transistor memory, Dain Lee, Yongsuk Choi, Euyheon Hwang, Moon Sung Kang, Seungwoo Lee and Jeong Ho Cho, Nanoscale, 8, 9107, (2016) ] - prototype. With this method, thin layers of a monocrystal of black phosphorus serve as a conductive channel of a field-effect transistor. The conductivity in the channel is controlled by the rechargeable region of the gate dielectric, which is based on gold nanoparticles. Recharging is carried out when a gate voltage is applied, due to the tunneling of charge carriers between a layer of gold nanoparticles (gate dielectric) and a monocrystalline film of black phosphorus (conducting channel of the transistor). The electric field arising after recharging makes it possible to maintain the number of carriers in the channel when the voltage at the gate is turned off and the use of such a recording method to implement a permanent non-volatile resistive memory.

The disadvantage of this recording method - the prototype, is the need to use external electric fields, and, therefore, a separate control electrode (gate) with a complex type of rechargeable gate dielectric for recording information, in addition to the usual metal contacts used to read the state of the cell. At the same time, in a number of problems of neuromorphic computations, it is required to create simple elements of long-term memory, where information is written and erased through the same channel as reading it.

The objective of the present invention is to develop a method for recording information for a long-term resistive memory system with a single recording and reading channel without the need to use external electric fields of the transistor gate for recording information.

The problem is solved by changing the conductivity of a single crystal of black phosphorus, while recording and reading are performed due to the controlled drift of vacancies near the metal-black phosphorus contact when a voltage is applied to the contact from less than -2 V to -10 V for writing and from +2 V to -2 V for reading.

Black phosphorus is the most stable allotropic modification of phosphorus, it has a layered crystal structure with an orthorhombic crystal lattice. The conductivity of black phosphorus crystals is determined by the presence of vacancies in the crystal structure, which leads to the formation of charged centers of the acceptor type. Heating the crystal by the flowing electric current leads to the appearance of vacancy drift in the current field, and the drift is significant even far up to the melting temperature of the bulk material.

For a separate metal-black phosphorus contact, when a bias voltage is applied, a highly asymmetric character of differential conductance is observed depending on the sign of the bias voltage due to the presence of a built-in electric field in the contact. The drift of vacancies near the metal-black phosphorus contact changes the built-in electric field in the contact area, and accordingly, changes the conductivity of the metal-black phosphorus contact.

With such a structure of the contact conductivity, when low (in the range from +2 V to more than -2 V in the model structure) voltages of both signs are applied, the sample demonstrates a stable, well reproducible behavior of differential conductivity in different cycles, which makes it possible to use voltages in the range from +2 V to greater than -2 V to read device status. The application of large (from less than -2 V to -10 V in the model structure) negative voltages leads to a significant increase in the current through the sample, its local heating and stimulates the drift of vacancies from the black phosphorus-metal contact, reducing the conductivity of the contact.

This change in conductivity is irreversible and retains a certain state when the field is switched off: the reverse process of vacancy drift is impossible, since, due to the sharply asymmetric current-voltage characteristic of the contact, heating effects are unattainable with the reverse (positive) polarity of the electric field. Thus, in the proposed recording method, a controlled decrease in differential conductivity is realized when a voltage pulse of a certain polarity is applied, and multiple readings of the state of the device without changing it.

The use of such a method for recording information will significantly simplify the method for recording information due to the absence of the need for a control electric field of the gate, and, therefore, due to the use of a single recording and reading channel, which is important for the implementation of neuromorphic systems. The proposed recording method also makes it possible to control AC voltage pulses of sufficient amplitude, since the state of the sample will change only at one half of the voltage oscillation period.

FIG. 1 shows a plot of the dependence of the differential conductance dl / dV for the model structure on the electric voltage V applied to the metal-black phosphorus contact in one voltage cycle. The direction of voltage change for the curves on the graph is shown by arrows. A change in the conductivity of the contact is seen when a negative bias voltage is applied.

The suggested recording method is as follows. The application of low (in the range from +2 V to -2 V in the model structure) voltages of both signs (or an alternating voltage of a similar amplitude) to the metal-black phosphorus contact makes it possible to repeatedly read the state of the device without changing it. The application of large (in the range from less than -2 V to -10 V in the model structure) negative voltages (or an alternating voltage of the same amplitude) controllably increases the resistance of the metal-black phosphorus contact, this state remains stable when the voltage is disconnected and can be read many times when voltages are applied in the range from +2 V to -2 V.

Based on the current-voltage characteristic of FIG. 1, the following examples of using the write and read method can be given:

Example 1

A voltage of +2 V is applied. The conductivity of the contact is stable, does not depend on the duration of the voltage application, information is read (conductivity value).

Example 2

The applied voltage is 0 V. The conductivity of the contact is stable, does not depend on the duration of the voltage application, information is read (conductivity value).

Example 3

The applied voltage is more than -2 V. The conductivity of the contact is stable, does not depend on the duration of the voltage application, information is read (conductivity value).

Example 4.

The applied voltage is less than -2 V. The conductivity of the contact changes markedly over time, information is recorded.

Example 5.

A voltage of -6 V is applied. The conductivity of the contact changes markedly over time, and information is recorded.

Example 6.

The applied voltage is -10 V. The conductivity of the contact changes markedly over time, information is recorded.

Example 7.

Applied voltage -11 V. Heating of the contact by the flowing current leads to melting of a single crystal of black phosphorus, the dependence of conductivity on time changes uncontrollably, reading and writing information is impossible.

Claims (1)

A method of recording and reading information for permanent memory elements for neuromorphic systems, including changing the conductivity of a single crystal of black phosphorus, characterized in that recording and reading are performed due to the controlled drift of vacancies near the metal-black phosphorus contact when a voltage is applied to the contact from less than -2 V to -10 V for writing and +2 V to more than -2 V for reading.

Images