Memory device

Abstract: A memory device consists of an array of resonant tunnel diodes in the form of pillars which are formed by selective etching. Each pillar includes first and second barriers (B1, B2) disposed between terminal regions (T1, T2) and a conductive region (CR1) between the barriers. The diameter of the pillars is typically of the order of 20-50 nm and is sufficiently small that the device exhibits first and second relatively high and low stable resistive states in the absence of an applied voltage between the terminal regions. The device can be used as a non-volatile memory at room temperature.

Description: This invention relates to a memory device and a method of fabrication thereof. It is known that resonant tunnelling diodes (RTDs) exhibit a negative differential resistance characteristic which can be used as a memory. A recent review of RTDs given in F. Capasso, S. Sen and F. Beltram in High-Speed Semiconductor Devices edited by S.M. Sze, John Wiley & Sons, New York, 1990. Reference is also directed to EP-A-0297778 and EP-A-0320110. A typical RTD consists of a substrate with two overlying charge barriers spaced by a conductive region. Because the layers that make up the barriers and the well are very thin, fabricated on the nanometre scale, the device exhibits quantum conductivity effects and can exhibit different stable states each with a different resistance level. Switching between the states can be achieved by applying a predetermined voltage across the device. However, hitherto, such devices have required the continuous application of a bias voltage to maintain the different resistance states. In accordance with the invention, there is provided a memory device comprising first and second terminal regions, first and second barrier means disposed between the terminal regions and a conductive region between the first and second barrier means, wherein said barrier means and the conductive region are disposed within a pillar and their dimensional extent is configured to be sufficiently small that the device exhibits first and second relatively high and relatively low stable resistive states in the absence of a voltage applied between the terminal regions. In accordance with the invention, the memory device can be operated without the requirement for a continuous applied bias voltage. The values of the resistive states is substantially greater than in the previously reported RTDs. The ratio of the value of the first and second states may lie within a range of 5:1 to 500:1. Typically, the ratio exceeds 250:1 at room temperature, which significantly exceeds a typical comparable value for the prior art of 10:1. The invention also extends to a method of fabricating the memory device, which includes selectively etching a substrate formed with overlying layers that define the first and second barrier means and the conductive region so as to form pillars at least some of which are of a diameter for exhibiting the relatively high and low conductivity states in the absence of a voltage applied between the terminal regions. The pillars typically have diameters within a range of 20 to 50 nm. The first and second barrier means may comprise any suitable double barrier structure and one possible example comprises AlAs/GaAs heterojunctions. The conductive region between the barrier means may comprise a i-GaAs layer. The etching process may be carried out by evaporating a metallic material such as gold onto the substrate to form a granular layer of discrete islands, and then plasma etching material between the islands so as to form the pillars. A conductive overlayer that bridges the pillars may be provided in order to make contact collectively with the first terminal regions. Connection to the second terminal regions may be achieved through the substrate by means of contact, e.g. AuGeNi that is annealed to the substrate. In order that the invention may be more fully understood an embodiment thereof will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a sectional view through a substrate for use in forming a memory device according to the invention; Figure 2 is an electron beam micrograph of a pillar structure for use in a device according to the invention, at a magnification of 60,000; Figure 3 is a schematic sectional view of a memory device in accordance with the invention, that utilises pillars formed as shown in Figure 2; Figure 4 illustrates current-voltage characteristic across the terminals of the device shown in Figure 3; Figure 5 is a plot of the peak current and valley current associated with the relative low and high resistance states of the device shown in Figure 2, the current being plotted on a logarithmic scale for 50 trials, for two different devices; Figure 6a is a graph of current versus bias voltage for a memory device according to the invention which is initially in a first relatively low resistance state and is then switched to a relatively high resistance state by increasing the bias voltage V; Figure 6b corresponds to Figure 6a and illustrates that the device stays in the relatively low resistance state if the bias remains positive; Figure 6c illustrates that when the device is pulsed at -5V for one second, the device returns to its original condition; and Figure 7 is a graph of the current through the device as a function of time as the device switches between the low and high resistance states. The memory device according to the invention is shown in Figure 3 and makes use of a pillar array structure which is shown in the electron micrograph of Figure 2. The pillars of the array are of a non-uniform diameter and each contains a RTD. The method of forming the pillar array will now be described with reference to Figure 1. A n + GaAs substrate 1 is provided by conventional techniques e.g. MBE or MOCVD, with a n GaAs layer 2, which itself is overlaid by i-GaAs layer 3 and an associated i-AlAs layer 4. The layers 3, 4 form a AlAs/GaAs heterojunction together constitute a first barrier region. An i-AlAs layer 5 and associated i-GaAs 6 form a second similar barrier region overlying the first. A conductive region of i-GaAs material 7 is disposed between the first and second barrier regions 3, 4; 5, 6. The second barrier region is overlaid by a n-GaAs layer 8 which itself is overlaid by a n + GaAs layer 9. In order to define the pillars, a thin metallic film is deposited onto the surface 10 of the semiconductor by an evaporation technique. In this example, gold is used, and for film thicknesses less than around 10 nm, the gold forms a granular layer composed of discrete islands as described in B. Abeles, Appl. Sold. Sci. 6, 1 (1976). The resulting gold islands 11 shown schematically in Figure 1 partially mask the underlying material so that it can be selectively etched to form the pillars. In this example, the gold mask was approximately 5 nm thick and the sample was etched for three minutes in 10 mTorr SiCl 4 /AR at a ratio of 1:2 with a power density of 4 mW/cm 2 . This selectively removed the material between the dots and produced an array of semiconductor pillars as shown in Figure 2. Due to the random dimensions of the dots 11, the pillars have a range of diameters and typically are in the range of 20 to 50 nm. The pillars thus formed typically have a height of 350 nm. Some of the pillars are partially connected to form larger structures. Typically, the pillar array extends over a sample surface of 5 x 5 mm, although the sample size is not limited to this value by the fabrication procedure. Referring now to Figure 3, this shows an example of the memory device according to the invention. Four pillars P1 -4 are shown, each of different diameter. Considering the pillar P1 by way of example, it includes first and second barrier regions B1, B2 formed from portions of the layers 3, 4 and 5, 6 respectively, the barrier regions being separated by a conductive region CR1 formed from a portion of layer 7. A first terminal region T1 comprising portions of layers 8, 9 overlies the barrier region B2, and a second terminal region formed of portions of layers 1, 2 underlies the first barrier region B1. A top contact C1 to the pillar array is constructed by evaporating a 30 x 30 micron gold pad so to form a conductive region which bridges the pillars P to provide a contact collectively to their first terminal regions. Collective contact is made to the second terminal regions by means of a back contact C2 which is annealed onto the substrate 1. In this embodiment, the back contact is formed of AuGeNi and is annealed at 400°C for 10 seconds. In use, a voltage V is applied between the contacts C1 and C2 from a variable source (not shown). The current through the device as a function of the bias voltage V, is shown in Figure 4, and exhibits hysteresis. Starting at zero bias voltage V, the device is initially in a high resistance state. As the bias voltage V is moved negatively, the current jumps sharply at -2.4 V and the device takes up a low resistance state. The device stays in the low resistance state until the bias crosses zero and increases to 1.4 V. Here the device switches back to the high resistance state and the ratio of the current peak for the high resistance state to the current valley for the low resistance state, is typically in excess of 250:l and with a typical maximum value of 500:1. By way of comparison, RTDs constructed according to the prior art (i.e. without pillar formation) showed a room temperature peak to valley ratio of 3:1 with no hysteretic switching behaviour. By room temperature, we mean a temperature of 300°K ± 10°K. In order to test the stability and reproducibility of the results shown in Figure 4, a series of hysteresis measurements were made on two different devices and the results are shown in Figure 5. The current in the low resistance state (labelled peak current) is compared with the current in the high resistance state (labelled valley current) measured at a bias of 1 V following a particular switching event. The results from the first device under test as shown as solid triangles for the peak current and solid circles for the valley current. The device exhibited a large variability in both the peak and valley currents. However, the peak to valley ratio of the device never dropped below 5:1 and sometimes was as high at 400:1. After 50 trials, no degradation in device performance was observed. The second device having its peak and valley currents shown as open triangles and circles, was tested for comparison over 13 trials. The peak and valley currents lie within the same set of values as the first device and no degradation was observed. The device can thus be used as a zero-bias two-terminal memory device which exhibits a non-volatile characteristic at room temperature. The low resistance state can represent one stored bit of information, which can be erased by switching the device into the high resistance state. The process is demonstrated in Figure 6. Starting at zero bias in the low resistance state, as shown in Figure 6a, the device switches to the high resistance state at 1.7 V. This corresponds to erasing the previously stored information. Thereafter, the bias voltage V was switched off for 10 minutes and then the state of the device was probed by applying a positive voltage, and measuring a resistance. The results are shown in Figure 6b where the current remains low as the bias voltage V is increased. Information can be restored by applying a large enough negative bias -V, which switches the device to the low resistance state. Figure 6c shows the same trace taken after pulsing the device with -5V for one second. The original characteristics (Figure 6a) are reproduced, thus indicating the storage of an information bit. Further measurements show that the high resistance state remains stable at zero-bias for an unlimited time, while the low resistance is stable for at least one hour before eventually switching to the high resistance state. The theory of operation of the device is not fully understood and the following discussion is presented by way of non-limiting explanation. It is believed that trapped charge in the well between the barrier regions B1, B2 has a strong influence on the energy of the resonant level for the well formed between the barriers. In the low resistance state, the resonant level is accessible. However, as the bottom of a conduction band moves through the resonance energy, as the bias voltage V is increased, the trapped charge escapes. This switches the device to the high resistance state. If the switching behaviour is due to a resonant process, it is to be expected that the switching time should be similar to that observed in a typical resonant tunnelling device. In Figure 7, it can be seen that the current as a function of time, as the device switches from the low to the high resistance state, falls in less than 10 -7 seconds, and is then followed by a slower exponential decay. This suggests an initial resonant switch followed by a slower decharging of the trapped states. Thus, the device described herein can be used as a non-volatile memory at room temperature and can be switched between its stable states by appropriately pulsing the bias voltage V. Alternative switching means, for switching the device between the states may be possible, for example, additional electrode structures or other processes.

Claims: A memory device comprising: first and second terminal regions (T1, T2), first and second barrier means (B1, B2) disposed between the terminal regions, and a conductive region (CR1) between the first and second barrier means, characterised in that said barrier means and conductive region are disposed within a pillar and their dimensional extent is configured to be sufficiently small that the device exhibits first and second relatively high and relatively low stable resistive states in the absence of a voltage applied between the terminal regions. A memory device according to claim 1 including switching means (V) for switching the device between said first and second states. A memory device according to claim 2 wherein the switching means comprises means for applying a switching voltage to the terminal regions (T1, T2). A memory device according to any preceding claim which exhibits said relatively high and low resistive states at room temperature. A memory device according to claim 4 wherein at room temperature, the ratio between the values of the first and second states exceeds 250:1. A memory device according to claim 4 wherein at room temperature, the ratio between the values of the first and second states lies between 5:1 and 500:1 A device according to any preceding claim including a substrate formed with overlying layers (1...9) which provide the first and second barrier means and the conductive region, and which have been formed into a plurality of pillars (P1- P4) at least some of which are of a diameter for exhibiting said relatively high and low conductivity states in the absence of a voltage applied between the terminal regions. A device according to claim 7 wherein a plurality of the pillars have diameters within a range of 20 to 50 nm. A device according to claim 7 or 8 wherein the first and second barrier means comprise AlAs/GaAs heterojunctions. A device according to claim 9 wherein the conductive region between the barrier means comprises a i-GaAs layer. A device according to claim 7, 8 or 9 wherein the terminal regions include respective layers of n-GaAs material. A device according to any one of claims 7 to 11 including a conductive overlayer (C1) bridging the first terminal regions of the individual pillars. A device according to claim 12 wherein the overlayer comprises a vapour deposited metallic layer. A method of fabricating a memory device according to any one of claims 7 to 13 including selectively etching a substrate (1) formed with overlying layers (1-9) that define the first and second barrier means and the conductive region, so as to form said pillars. A method according to claim 14 including evaporating a metallic material onto the substrate such as to form a granular layer of discrete islands, and plasma etching material between the islands to form the pillars. A method according to claim 15 including forming by vapour deposition a conductive overlayer (C1) that bridges the pillars to provide a contact collectively to the first terminal regions thereof. A method according to claim 14, 15 or 16 including attaching a contact (2) to the substrate (1) so as to provide a collective electrical connection to the second terminal regions of the pillars. A method according to claim 17 wherein the contact is of AuGeNi and is annealed to the substrate. A method according to any one of claims 14 to 18 wherein the substrate is of GaAs material and prior to the etching is formed with first and second overlying GaAs/AlAs heterojunctions spaced by a i-GaAs layer.

Inventors: ALPHENAAR BRUCE, DURRANI ZAHID ALI KHAN, ALPHENAAR, BRUCE, DURRANI, ZAHID ALI KHAN
Applicants: HITACHI EUROP LTD, HITACHI EUROPE LIMITED
Application year: 1995
Main classification: H01L29
IPC codes: H01L29, H01L27, G11C11