Quantum dot based pressure switch

A semiconductor heterostructure based pressure switch comprising: first and second small bandgap material regions separated by a larger bandgap material region; a third small bandgap material region within the region of larger bandgap material, the third material region and larger bandgap material region defining at least one quantum dot; and, first and second electrodes electrically coupled to the first and second small bandgap material regions, respectively, wherein the electrodes are sufficiently proximate to said quantum dot to facilitate electron tunneling there between when a pressure is applied to the bandgap material defining the quantum dot.

FIELD OF THE INVENTION

The present invention relates generally to pressure sensors, and more particularly to pressure activated switches.

BACKGROUND OF THE INVENTION

Quantum dots are a well known phenomena and have been employed to fabricate various optoelectronic devices including semiconductor lasers, optical amplifiers, light emitting diodes, digital circuits and the like. Generally, quantum dots are formed in III-V elements. (See, e.g. text entitled “Self Assembled InGaAs/GaAs Quantum Dots” by Robert Willardson et al. and published by Academic Press (1999) ISBN-0-12-75169-0.) The text includes many examples of how quantum dots embedded in semiconductor substrates are also commercially available. For example, Evident Technologies of Troy, N.Y. sells semiconductor materials such as InP having quantum dots (see, e.g. www.evidentech.com). Devices employing quantum dots are capable of high speed operations as compared to conventional semiconductor devices.

It is known to be desirable to sense pressing, or pressure, exerted on certain structures. It is further known to be desirable to provide switches that have sharp response curves to such pressing or pressures as they traverse a desired transition pressure, such that there is a highly focused transition between switching states at the transition pressure. The present invention contemplates a quantum dot pressure device exhibiting high speed operation and selective switching in response to a sensed pressure.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a semiconductor heterostructure based pressure switch comprising: first and second small bandgap material regions separated by a larger bandgap material region; a third small bandgap material region within the region of larger bandgap material, the third material region and larger bandgap material region defining at least one quantum dot; and, first and second electrodes electrically coupled to the first and second small bandgap material regions, respectively, wherein the electrodes are sufficiently proximate to said quantum dot to facilitate electron tunneling there between when a pressure is applied to the bandgap material defining the quantum dot.

According to another aspect of the invention, a pressure switch comprises a semiconductor substrate having a thinned portion indicative of an active area which will deflect upon application of a force thereto. The substrate comprises carriers, and a quantum dot is formed in the substrate and within the active area, whereby when the force is applied to the active area, the quantum dot and the substrate exhibit a transition to enable a current to flow through the substrate proportional to the magnitude of the applied force.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Epitaxial techniques may be used to produce semiconductor based structures having abrupt changes in semiconductor materials, such that the change from one material to another occurs on the order of one molecular monolayer (on the order of a few Angstroms). Where the two or more disparate materials have different bandgaps, such semiconductor structures are conventionally known as heterostructures. The bandgap is the energy difference between the top of the valence band and the bottom of the conduction band in semiconductors. Due to the energy difference across a bandgap, an energy barrier for carriers (e.g. electrons or holes) is typically created in the smaller-bandgap material. For purposes of explanation, the present invention will be discussed as it relates to the carriers of interest being electron carriers, however, it should be understood that an alternative implementation may be carried out using hole carriers.

If the energy barrier is significantly larger than the thermal energy (kT) of the carriers where (k is Boltzmann's constant and T is the absolute temperature), the carriers are effectively confined to the smaller-bandgap material. If the region of confinement is sufficiently small (i.e., on the order of the wavelength of an electron or about 20 nanometers (nm) or less), quantum effects play a significant role. In this case, not only are the electrons confined to the smaller bandgap material region, but also to specific energy levels rather than the entire energy continuum that is available to electrons in large structures, due to energy quantization.

Where the sufficiently small confinement is only in one dimension, the structure is commonly known as a quantum well. That is, in a quantum well particles are confined in one dimension, forcing them to occupy a planar region. Quantum confinement takes place when the quantum well thickness becomes comparable to the de Broglie wavelength of the carriers, leading to energy levels called “energy subbands”, wherein, the carriers only have discrete energy values.

Where sufficiently small three-dimensional (3-D) confinement is provided, the structure is commonly known as a quantum dot. Thus, in a quantum dot, electrons, holes, or electron-hole pairs are confined in three dimensions. The confinement region may be on the order of a few nanometers up to a few hundred nanometers, which leads to quantized energy levels and to the quantization of charge in units of the elementary electric charge. The structure and theory of operation of quantum dots is well known.

In a quantum structure, varying the confinement dimension as well as the barrier height controls the number and values of the supported discrete energy levels. In addition to confinement area size and energy barrier height, strain can be used to vary the discrete energy level values. The quantitative effect of the strain on the energy levels can be predicted using deformation potential theory. The details of the calculation depend on the nature of the particular semiconductor and whether electrons or holes are being implemented. In the case of electrons in direct-bandgap semiconductors, such as GaAs or InAs, the change in energy level ΔE produced by strain is given by:
ΔE=Ξ(000)(εxx+εyy+εzz)  (1)
where Ξ(000)is the dilation deformation potential for the direct bandgap conduction band valley, which depends on the particular material being strained, and εxx, εyy, and εzzare the components of the strain tensor in the x, y, and z directions, respectively. Similar equations have been derived for indirect bandgap semiconductors, such as silicon (Si) and germanium (Ge), as well as for strain effects in the valence (hole) band.

According to an aspect of the present invention, a highly sensitive pressure switch that uses the change of the energy band levels with applied strain may be provided. A cross-sectional schematic of such a device10is shown inFIG. 1. Device10generally includes a substrate20. In an exemplary configuration, substrate20includes a thinned region that defines a diaphragm25. Substrate20supports an emitter region30and collector region40. Electrode35is electrically coupled to emitter30, and electrode45is electrically coupled to collector region40. Substrate20also includes a portion27positioned between emitter and collector regions30,40. Portion27is generally vertically aligned with diaphragm25. Portion27has at least one quantum dot50formed therein. An operating potential can be applied to electrodes35and45to bias the device.

Quantum dot50is typically made of a small bandgap material, such as Ge or SiGe (the bandgap of Ge is around 0.67 eV, the bandgap of SiGe varying according to the respective compositions of the Si and Ge, but typically around 0.91 eV for a SiGe composition of (50% Ge, 50% Si)), and is surrounded by a larger bandgap material such as Si (the bandgap of silicon is around 1.14 eV). Of course, other material systems providing for a suitable bandgap differences may be used, such as AlAs/GaAs, GaAs/InAs, InGaAs/GaAs, InP and other heterostructure materials.

Substrate20is typically made of the larger bandgap material, such as Si. In the embodiment illustrated inFIG. 1, diaphragm25is etched in the backside of substrate20so that an applied force or pressure (P) applied to the top surface of the device is transferred to dot50as a strain. The thickness of diaphragm25can be varied to provide for a range of strains depending on the pressure range of interest. The diaphragm25deflects upon application of a force or pressure, P, thereto.

Emitter and collector regions30,40are typically made of the smaller bandgap material, such as Ge. Electrodes35,45are placed sufficiently close to quantum dot50so that quantum tunneling of electrons from the electrodes35,45is facilitated. As indicated, the construction of such devices as shown inFIG. 1can be implemented by many different techniques. In an exemplary configuration, emitter and collector regions30,40have a doping density of about 1018or 1019carriers/cm3or higher, a substrate thickness and diaphragm thickness of about 5 mils and about 0.25-2 mils, respectively, with the thickness varying according to the pressure to be measured (e.g. thinner diaphragm for smaller pressures); the quantum dot being about 10 nanometers (nm) in size, with the spacing between the quantum dot and corresponding edge of each respective emitter/collector region on the order of 1-10 nm, and typically about 5 nm.

FIGS. 2 and 3illustrate an operational principle of device10according to an aspect of the present invention. If the strain induced by pressure P (FIG. 1) is such that the energy level100within the dot50is aligned with the Fermi Level110in the electrodes (FIG. 2), electron tunneling is supported between electrodes35,45and dot50. If no voltage is applied between electrodes35,45, the rate of tunneling in both directions is predicted to be equal such that no net current flows. However, if a small bias voltage is applied, so the collector40voltage is slightly higher than the emitter30voltage, the tunneling rate from emitter30through the dot50to collector40exceeds the tunneling rate from collector40through dot50to emitter30, such that a measurable net current will flow.

On the other hand, if the amount of strain is such that energy level100in quantum dot50is not aligned with the Fermi Level110in the electrodes (FIG. 3), tunneling is not supported between electrodes35,45and dot50—as electrons at the Fermi energy level110of electrodes35,45are not supported by dot50. In this case, a corresponding small bias voltage between the emitter and collector does not cause current to flow.

Thus, according to an aspect of the present invention, measuring the current between collector40and emitter30(using electrodes35,45, for example) provides an indication of the amount of strain applied to dot50, and hence the amount of pressure (P,FIG. 1) applied to diaphragm25(FIG. 1).

Referring now also toFIG. 4, there is shown an idealized current output as a function of applied strain of a device10. Where the pressure P (FIG. 1) induced strain is such that energy level100is aligned with the Fermi energy level110of the collector40and emitter30(region200), the net current is high. At other strain values (regions210,220) the current is near zero. Thus, the measured current output between electrodes35,45after biasing device10, provides a good indication of applied pressure P (FIG. 1), such that device10may be used as a pressure switch.

In principle, device10(FIG. 1) can be designed so that when zero pressure is applied to diaphragm25(FIG. 1), the energy levels of quantum dot50(FIG. 1) are not aligned with the Fermi levels in the emitter and collector (FIG.3)—such that no current flows under zero pressure. For measurable current to flow (i.e., to turn “on”the switch), pressure is applied to sufficiently move the energy levels in quantum dot50(FIG. 1) into conformance with the Fermi levels of the emitter and collector regions30,40(FIGS. 1,2). Alternatively, device10(FIG. 1) can in principle be designed so that the energy levels are aligned at zero applied pressure (FIG. 3), and pressure must be applied to turn the switch “off” (FIG. 2).

By way of further, non-limiting explanation only, in the case of three dimensional confinement, the smaller the dot the fewer discrete energy levels that are provided, and the more widely spaced these discrete energy levels are. In order for discrete energy states to play a role in the pressure sensing, quantum dot50(FIG. 1) should be sufficiently small so that the energy spacing between the levels is significantly larger than the thermal energy—as greater changes in temperature are required to cause an energy level transition than if quantum dot50(FIG. 1) were larger. This provides for more energy levels that are more closely spaced to one another. It should also be noted that the changes in effective mass affect the tunneling probability even if quantum dot50(FIG. 1) is not sufficiently small for the effect to be measurable.

Thus, according to an aspect of the present invention, a quantum dot sensor can also operate based upon another physical mechanism, not directly relating to the particular discrete energy values in the quantum dot. More particularly, the probability of tunneling through an energy barrier can be approximated by
P=exp(−k√{square root over (m)}t√{square root over (W)})  (2)
where P is the tunneling probability, k is a constant (equal to 0.34), m is the relative effective mass of the carrier, t is the thickness of the barrier through which the carrier must tunnel, and W is the height of the energy barrier. From equation (2) it can be seen that any changes in the effective mass of the carriers affects the tunneling probability, and hence the output current of the device. When strain is applied to a semiconductor, the effective mass of the carriers in that semiconductor changes. In piezoresistive sensors, the effective mass affects the carrier mobility and hence changes the resistance of the device. According to an aspect of the present invention, the effective mass change due to applied strain alters the tunneling probability of carriers into and out of quantum dot50. This in turn changes the output current between collector40and emitter30and may be used to provide an indication of the experienced strain, and hence applied pressure. Thus, a pressure switch comprises a semiconductor substrate having a thinned portion indicative of an active area which will deflect upon application of a force thereto. The substrate comprises carriers, such as electrons (or holes) and a quantum dot is formed in the substrate and within the active area. When a force is applied to the active area, the quantum dot and the substrate exhibit a transition to enable a current to flow through the substrate proportional to the magnitude of the applied force.

It should also be noted that while the principle of operation of such a pressure sensor may be similar to a single electron transistor, a significant difference lies in the realization that energy levels are moved using an electric field applied via a gate electrode in a single electron transistor, while strain is applied to move the energy levels in the herein-disclosed quantum dot pressure switch.

Those of ordinary skill in the art may recognize that many modifications and variations of the present invention may be implemented without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention.