Materials exhibiting reversible resistance switching are attractive for many of today's semiconductor devices, including non-volatile random-access memory devices. However, previous efforts in the art to reversibly vary electrical performance have exhibited numerous drawbacks. For example, some capacitance-switching semiconductor devices, such as doped Schottky-junction diodes, require relatively large amounts of electrical power (voltage) to switch to, and maintain, a particular capacitance state. Still further, such a device completely loses its capacitance state when the power is withdrawn. Current leakage, and associated heat-build up, are also especially problematic with these switchable semiconductor devices. Thus, high power consumption, current leakage and poor retention characteristics make these devices unsuitable for many practical applications.
Other efforts in the art have taught that several different resistance-switching technologies can be triggered by voltage. This phenomenon has sometimes been called an EPIR (Electrical Pulse Induced Resistance) switching effect. EPIR semiconductor devices are disclosed in U.S. Pat. No. 3,886,577 (Buckley). In the Buckley devices, a sufficiently high voltage (50 V) is applied to a semiconductor thin film in which an approximately 10 micron portion, or filament, of the film may be set to a low resistivity state. Filament size is highly dependant on the amount of current flowing through the device. The device may then be reset to a high resistance state by the action of a second high current pulse (150 mA). However, the set voltage is strongly affected by the number of switching cycles performed. Thus, these devices generally exhibit high power consumption and poor cycle fatigue performance.
Recent efforts in the art have investigated ferroelectric and magnetoresistive materials for non-volatile memory applications. These materials, however, suffer from cycle fatigue and retention problems. Moreover, many magnetoresistive oxide devices require magnetic switching fields and have low operating temperatures.
Some thin film materials in the perovskite family, especially in colossal magnetoresistive (CMR) thin films, have exhibited reversible resistance changes upon application of an electrical stimuli in a magnetic field. It has been recently found that some transition metal oxides in the perovskite family exhibit resistance-switching under a voltage trigger in the absence of a magnetic field. Indeed, the recent observation of the electrical pulse induced resistance (EPIR) change effect in perovskite oxide thin films at room temperature and in the absence of a magnetic field has drawn much attention. See, e.g., “Electric-Pulse Induced Reversible Resistance Change Effect in Magnetoresistive Films,” S. Q. Liu, N. J. Wu, A. Ignatiev, Applied Physics Letters, Vol. 76, No. 23 (2000). In these previous efforts, a Pr1-xCaxMnO3 (PCMO) oxide film placed between two electrodes served as an EPIR device. The resistance states of such simple structured semiconductor devices were switchable by the application of a voltage trigger. The trigger could directly increase or decrease the resistance of the thin film sample depending on voltage polarity. Such voltage triggering phenomenon can be useful in a variety of device applications, including non-volatile memory devices such as resistance random access memory (RRAM) devices.
These early devices, however, required relatively high voltage triggers and the EPIR effect was found to be cycle dependant. The EPIR effect, measured as the ratio between the resistance states, was found to decrease as the number of triggering events increased. Thus, the high power requirements and lack of resistance state stability plagued these early EPIR compositions and devices. Although the basic mechanism responsible for the EPIR effect is still under investigation, there exists a need in the art to develop improved resistance-switching semiconductor devices for potential application in different technology areas.
Thus, there is a need in the art for resistance-switching semiconductor devices having low power consumption. Still further, there is a need in the art for such semiconductor devices having low voltage leakage and high retention of the respective low and high resistance states. There is also a need in the art for resistance-switching semiconductor devices having improved cycle fatigue performance.
One basic phenomenon of interest concerns a two-terminal device, which has two stable states—one with a lower resistance RL, the other with a higher RH—that can be switched back and forth by applying a voltage pulse or a current pulse. These two states can be used for digital memory; for example, the higher resistance state may be called the “On-state”, and the lower resistance state may be called the “Off-state”. The voltage used to switch the off-state to the on-state may be called the “Set-voltage”, and the one to switch the on-state to the off-state the “Reset-voltage.”
In nearly all the cases reported in the past with various oxide thin film configurations, the as-fabricated device has a very high resistance value (even higher than the RH later used for the on-state.) Previous devices do not show any resistance switching phenomenon until a “forming” process is first performed. The forming process places the device under a high voltage (larger in magnitude than the set- and reset-voltage) for a certain duration, sometimes in a high vacuum environment. In doing so, it may cause ion (or metal) migration from electrode into the insulator oxide or formation of ionic vacancies in the oxide, creating localized states that facilitate electron (or hole) movement. Apparently the mechanism of forming is still under debate. The nature and density of the paths of localized conduction (called “filament” in the literature) created by the forming process, however, are difficult to control. Moreover, there is a scalability problem in such device in that the resistance of the formed device does not show a clear dependence on the area in the normal way (namely, the resistance being inversely proportional to the device area). For filamentary conduction, it appears that the resistance switching property will be lost when the device (cell) area is smaller than the characteristic area of either the localized conducting path or the average area between such paths. The value of such a limiting size is currently unknown.
Several switching mechanisms have been proposed over the years for the oxide thin film devices. These include electron trapping and release by injected Au (from electrode) inside SiO2, formation and rupture of a conducting filament in NiO, TiO2, Cr—SrZrO3 (SZO) and CeO2, Schottky barrier modification through interface-trapped charges in Pr0.7Ca0.3MO3 (PCMO) with Ti top electrode, and field-induced electrochemical migration of ions (oxygen vacancy) between PCMO and Ag top electrode interface. The latter two may be regarded as an interface phenomenon, which does not depend on the film thickness as long as the thickness of the interface layer is very small. More commonly, the filamentary mechanism for switching has received much support; its origin is thought to be due to forming. For example, conduction is thought to be switched on by metallic Ni formation in NiO, by local point defects around multi-valent Ti ions in TiO2, by free carrier release due to Cr3+ to Cr4+ underneath the anode, and by valence-shifted CeOx domains inside the insulating CeO2 matrix. Just as forming, this filamentary mechanism for switching is not reliable for long-term operation and may not be reproducible from device to device. It is also likely that the interface phenomenon mentioned above may involve filaments formed locally across the interface. (The as-fabricated device is in the high-resistance state.)
According to the literature, although there is usually a characteristic set- (reset-) voltage in a given device, there is not a clear connection between this voltage and the material system. In particular, in some systems set/reset switching is possible without changing the polarity of the voltage, as in Nb2O5, ZrO2, TiO2, NiO and MgO, which we call unipolar, whereas in other systems switching requires alternate positive and negative biases, as in Cr (V)-SZO, PCMO and CeO2, which we call bipolar. It is not clear why some of these oxides are unipolar and others are bipolar. It is also not clear why the switching voltage is low in some systems while high in others. Whether a resistance memory is unipolar or bipolar will impact the circuit design. For practical applications, a relatively low set-reset-voltage close to 1 V is desired, since this is the typical operating voltage in an integrated circuit.
Although the current invention is not limited to perovskite, the inventors have found many compositions of perovskite provide good resistance switching films for non-volatile memory devices. Perovskite material systems have traditionally been used for capacitors, dielectrics, piezoelectrics, pyroelectric, and other related applications. In such applications, it is advantageous to have very low conductivity, i.e., very high resistivity, to minimize dielectric loss and/or leakage of stored charge. Perovskite compositions for such applications generally avoid conducting dopants. In those circumstances where dopants are used, a relatively small amount (typically less than about 2 atomic percent) is used to compensate for valence mismatch in the insulator matrix, for example a small amount of Nb5+ can be used to compensate for the small amount of Ti3+ in BaTiO3. Perovskite material systems may also be used for conductors, electrodes, and other related applications. In these applications, however, very high conductivity, i.e., very low resistivity, is desired to minimize ohmic loss and power consumption. Therefore, the preferred compositions generally include a very high amount of conducting components (typically greater than about 70 atomic percent). Prior efforts in the art have thus taught either very highly-doped or minimally-doped perovskite materials systems for various technological applications. Here, the inventors have surprisingly found a significant and reproducible resistance-switching phenomenon upon mid-range doping of some perovskite material systems.
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