GB2306769A

GB2306769A - Radiation detector or mixer

Info

Publication number: GB2306769A
Application number: GB9521155A
Authority: GB
Inventors: Donold Dominic Arnone; Charles Smith
Original assignee: Toshiba Cambridge Research Centre Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 1995-10-16
Filing date: 1995-10-16
Publication date: 1997-05-07
Anticipated expiration: 2015-10-16
Also published as: GB2306769B; GB2306771B; GB2306772A; GB9521998D0; GB9521155D0; GB9521995D0; GB2306771A; GB2306772B

Abstract

A radiation detector 1 comprises an active layer within an appropriate structure (eg a HEMT structure) in which a two-dimensional electron or hole gas is induced. Gate means (21, 23, 25, 27, 37, 39) confines a pool of carriers within a confinement region (15). Antenna means (3, 5) couples electromagentic radiation to the confinement region. Conduction across the potential barrier confining the "quantum dot" of carriers enables conduction. The device may have a response up to 100 GHz or more.

Description

RADIATION DETECTOR The present invention relates to a semiconductor detector for electromagnetic radiation.

Many different semiconductor structures are known which are capable of detecting electromagnetic radiation. However, the detector or mixer according to the present invention is particularly adapted to detection of electromagnetic radiation in the submillimetre, far-infrared or mid-infrared wavelength ranges.

This particular detector or mixer utilises the phenomenon of single-electron charging as it occurs in mesoscopic, low-dimensional structures. The main concept used in the design of this detector is embodied in a device comprised of one or more electron tunnelling barriers which show nonlinearities in their current vs. voltage (I vs. V) characteristic.

The invention derives from a proposal that far-infrared electromagnetic radiation might be detected by irradiating a tunnelling barrier or barriers and monitoring the change in current passing through or over the barrier (see D.V. Averin and K.K. Likharev in H. Geabet (Ed.), Single Charge Tunnelling, NATO ASI Series, Ch-9, pp 323, 324).

Sub-millimetre frequencies herein refer to photon frequencies in the approximate range 100GHz to 240GHz or in equivalent energy units to 0.2meV to 1meV. far-infrared frequencies refer to the approximate range 1meV to 30meV, and mid-infrared frequencies to the range 30meV to 250meV.

The photoresponse of a mesoscopic system to irradiation at a frequency hco is extracted by measuring the change in some quantity when the system is irradiated.

Typically this quantity is the current through Qr voltage across the system. One modulates the irradiation of the system at low frequency ( e, and measures the change in current or voltage at frequency oe using standard phase sensitive techniques. Xe is typically much lower than the microwave or infrared frequency hco. Single electron tunnelling devices become photon counters when operated at frequencies where the photon energy hole is large compared to the voltage scale of the dc nonlinearity. For hco/e comparable or smaller than this voltage scale, the photoresponse of the device will be dominated by dc rectification of the high frequency signal.Thus, a mesoscopic system may be used as a photodetector in photon counter mode where hco/e is large compared to the voltage scale of nonlinearity. For hco/e below this level, the sytem may be used as a dc rectifier.

One system which possesses nonlinearities of large magnitude over voltage scales tiwle lying in the microwave and infrared region is the single electron charging device. This is a known form of so-called quantum-effect semiconductor device. In this structure, electron motion is confined in all three dimensions. Such confinement is often realised by placing depleting Schottky gates on the surface of a semiconductor heterostructure which itself contains a two dimensional electron gas. In certain instances, single electron charging manifests itself in one-dimensional and two-dimensional systems.

It is well known that quantum-effect devices can be made by arranging layers of semiconductor layers of different band gaps, such that it is possible to induce a quantum well adjacent an interface. Carriers can be confined in the well layer so that the current flowing in the well layer, between source and drain regions can be modulated by means of a control potential applied to any overlying gate electrode.

The carriers, usually of high mobility, may exist in two dimensions to behave as a "two-dimensional electron gas" (2DEG) or they may be influenced by means of applied electrode potentials to exist substantially in only one dimension, i.e. as a "onedimensional electron gas" (1DEG) otherwise sometimes referred to as a "quantum wire".

Of course, the majority carriers can be electrons or holes so equivalent devices utilising a two-dimensional hole gas (2DHG) or one-dimensional hole gas (1DHG) can also be realised. However, for simplicity, the generic terms 2DEG or 1DEG or simply "carrier gas" will be used herein and should be understood as encompassing both possibilities, unless specifically indicated to the contrary.

As an extrapolation of such confinement, it is possible to arrange barrier potentials in three dimensions to confine a puddle of 100 or so electrons. This puddle is commonly referred to as a "quantum dot" or quantum box". In this structure, electron motion is confined in all three dimensions.

This kind of quantum dot confinement has previously been realised by placing four or more depleting Schottky gates in a turnstile arrangement on the surface of a semiconductor heterostructure which itself contains a 2DEG. The gates squeeze the electron gas such that the remaining two degrees of freedom are impaired. This additional squeezing or confinement produces tunnelling barriers around the dot through which electrons must pass if they are to enter or exit the dot. Such transport through the dot is then affected by applying an external current or voltage to the dot.

This external bias raises the energy of electrons, allowing them to pass over and/or tunnel through the tunnelling barriers.

Figures 1 and 1A of the accompanying drawings show a heterostructure 111 in which a substrate 113 has formed thereon, a GaAs layer 115, above which is formed an AlGaAs layer 117 A plurality of gates 119,121,123,125,127,129 and 131 are situated above the AlGaAs layer. These are shown in more detail in Figure 1A.

Electrons in a 2DEG induced in the active layer 115 are squeezed by a gate voltage (negative) applied to the gate electrodes. A negative voltage removes electrons and can electrostatically shape the electron gas to a desirable pattern, in the present case, that of a quantum dot.

Referring to Figure 1A, the black squares and rectangles are front gates which deplete the electron gas underneath, as mentioned above. Barriers are formed between all adjacent gates. The arrows show two possible conduction paths through the device.

A quantum dot is formed at the centre of the gates.

Single electron charging, commonly referred to as Coulomb Blockade, manifests itself in the current-voltage characteristics of quantum dots when the confinement length is sufficiently small ( < -300nm) and the number of electrons is sufficiently small (several hundred or less). The single electron charging energy, commonly referred to as the Coulomb energy, is the energy penalty incurred by having to add a whole electron to the dot when electrostatics require only a fractional amount of charge to produce neutrality between the dot and the surrounding reservoir of electrons. This charging energy is -e2/2C , where C is the capacitance of the dot.

A consequence of this charging energy is that if one measures conductance or resistance through the dot as a function of voltage or current applied across the dot, one should see "oscillations" in the conductance/resistance with a period of e/C.

Alternatively, one can measure conductance/resistance as function of the confinement width and see similar oscillations. Oscillations arise in the latter case because the capacitance changes with confinement width, and therefore so does the charging energy e2/2C necessary for an electron to enter/exit the dot. For dots created by means of depleting Schottky gates, the confinement width is varied via the voltage Vfg applied to these gates. Thus one expects to see oscillations in conductance/resistance as a function of gate bias Vfg.

Figure 2 of the accompanying drawings is a schematic representation of conductance through a quantum dot as a function of source-drain bias. The conductance oscillations are separated by a Coulomb energy e2/2C.

Figure 3 shows the measured conductance through a quantum dot as a function of front gate voltage. The solid line shows the effect of a negative swept voltage and the broken line, a positive swept voltage. The peaks are due to passage of an additional electron into and out of a dot.

A new form of electromagnetic irradiation detector has now been devised which permits a considerably simplified construction in comparison with the aforementioned "turnstile" arrangement and potentially, allows for better confinement of the quantum box and/or better coupling of the radiation to the quantum box region. Thus, in accordance with the present invention, there is provided a detector for electromagnetic radiation, the detector comprising an active layer for induction of a two dimensional carrier gas therein, gate means for confining a pool of carriers from the carrier gas within a confinement region of the active layer and antenna means for coupling electromagnetic radiation to said confinement region.

According to a described embodiment hereinbelow, the confinement region of the active layer is located within a recessed region of the device. This permits the gate means to be formed as a plurality of "side gates". However, it is also preferred for the gate means to comprise at least one secondary gate for restricting the width of the carrier gas.

With the recessed form of construction, the antenna means can extend into the recessed region. Such antenna means may comprise a pair of substantially triangular members for impedance matching (in the manner of microwave microstrip). The apex of the triangular members can extend into the recessed region. These triangular features can be, e.g. in the form of equilateral or isosceles triangles.

Other forms of matched antenna are possible, for example a log-periodic structure (i.e.

in the manner of a low-frequency Yagi antenna).

In the described preferred embodiments, the active layer is formed on an angled facet of an etched layer structure. The gate means comprises a front gate overlying the active layer and at least one back gate constituted by a layer or layers or the etched layer structure.

In any event, it is convenient to form the active layer as part of a high electron mobility transistor structure. In the described embodiment, source and drain regions are arranged so that the active layer provides electrical conduction therebetween.

In order to maximise the photo-induced current or voltage, a practical device may in some cases be realised by providing a device comprising a plurality of such detectors, i.e. a minimum of two such detectors but more preferably, an array of up to one hundred or more such detectors. Any device comprising a plurality of such detectors is preferably provided on a single semiconductor substrate.

The present invention will now be explained in more detail by reference to the following description of a preferred embodiment and with reference to the accompanying drawings in which Figure 1 shows a cross section through a known device utilising coulomb blockade; Figure 1A shows a plan view of the device of Figure 1, showing the turnstile arrangement of gate electrodes; Figure 2 shows a schematic representation of conductance through a quantum dot in the presence of Coulomb blockade as a function of source-drain bias across the dot; Figure 3 shows a plot of measured conductance through a quantum dot in the presence of Coulomb blockade as a function of front gate voltage; Figure 4 shows a plan view of a first embodiment of a radiation detector device according to the present invention; Figure 5 shows a cross-section through the device shown in Figure 4;; Figure 6 shows a cross-section through a radiation detector of a second embodiment of the present invention; and Figure 7 shows a cross-section through a radiation detector of a third embodiment of the present invention.

Referring now to Figures 4 and 5, there is shown a radiation detector 1 according to the present invention. As seen in plan view (Fig. 4), a first antenna member 3 and a second antenna member 5 each are in the shape of an equilateral triangle. One apex 7 of the first antenna member 3 and one apex 9 of the second antenna member 5 point towards one another. Thus, the edge 11 of first antenna member 3, remote from its apex 7 and the edge 13 of the second antenna member 5, remote from its apex 9, are parallel to each other and each is disposed outwardly. The aforesaid apexes 7, 9 are separated by a gap 15.

A source region 17 and a drain region 19 are situated either side of the gap 15 along an imaginary line at right angles to an imaginary line joining the two apexes 7,9 of the antenna members. First, second, third and fourth side gates 21, 23, 25, 27 are buried in the device structure and are each provided with a respective electrical connection 29,31,33,35.

The first and second side gates 21, 23 are both disposed one side of the imaginary line joining the source and drain regions 17, 19 but respectively either side of the imaginary line joining the apexes 7, 9. Similarly the third and fourth side gates 25, 27 are both disposed at the other side of the imaginary line joining the source and drain regions 7, 9, opposite to the first and second side gates 21, 23. However, the third and fourth side gates 25, 27 are also disposed on opposite sides of their source and drain regions 7, 9. Thus, the side gates 21, 23, 25, 27 are in positions (as seen in plan view) roughly corresponding to the corners of an imaginary square bounding the gap 15 between the apexes 7, 9.

Also, within the gap 15 and the aforementioned imaginary square but either side of the aforementioned imaginary source-drain line, are first and second secondary gates 37, 39 or "plungers", also buried in the structure. Each secondary gate 37, 39 is provided with a respective electrical contact 41, 43.

The first and second antenna members 3, 5, are each provided with a respective electrical connection 45, 47, extending from the respective outwardly-facing sides 11, 13 thereof. In use, these may be left open circuit or connected to ground.

Figure 5 shows how, along one axis (the source-drain axis), a recess 49 is etched in a layer structure 51 grown on a GaAs substrate 53. The antenna members 3, 5 are formed on the structure 51 and extend into the recess 49 so that their facing apexes 7, 9 are lowermost. Between the apexes is an uncovered region 55 at the bottom of the recess 49.

The layer 51 is comprised of the following layers formed in turn, upon the substrate 53, prior to selective etching of the recess 49, namely an n+ GaAs back gate layer 57 of 200nm thickness, an AlAA'GaAs superlattice 59, an undoped GaAs layer 60 of 100nm thickness, a p GaAs side gate layer 61 of 100nm thickness, and a conventional HEMT structure 63 (which of course includes an active layer). The triangular antenna members 3, 5 are deposited on top of the structure, after selective etching to form the recess 49.

The HEMT structure 63 extends into the side gate layer 61 to divide it into the first and second secondary side gates 37, 39. The primary side gates 21, 23, 25, 27 are not visible in this cross-section.

In use, a bias voltage between the source 17 and drain 19 includes a two-dimensional electron gas in the active layer of the HEMT 63. The back gate and secondary gates 37, 39 are used to deplete-out all except a narrow conduction region in the centre region 55. Then, bias voltages applied to the primary side gates 21, 23, 25, 27 confine carriers in a quantum dot. Current flowing between source and drain is interrupted until radiation is received via the antenna members 3,5 to provide the additional energy required to allow conduction across the potential barriers which confine the quantum dot.

Figure 6 shows a second embodiment of a detector 71 according to the invention. It comprises a layered structure 73 which has been selectively etched to produce an angled side facet 75 intersecting edges of layers in the structure 73.

A high electron mobility transistor (HEMT) heterostructure 77 is formed by regrowth over the side facet 75. Over the heterostructure 77 is formed an oxide insulating layer 79 and a front gate electrode 81.

The layer structure 73 comprises a lower p-Si layer 83 (50nm), a middle p-Si layer 85 (l00nm) and an upper p-Si layer 87 (50nm) which are respectively separated by lower and upper SiO2 insulating layers 89, 91.

Over a region 93 of the HEMT structure 77 opposite the middle p-Si layer 85 is formed a surface-mounted antenna pattern 95 which can have the "bow tie" configuration used in the first embodiment or could consist of concentric circles in log periodic fashion.

In any event, the middle of the antenna 95 corresponds to a "quantum dot" confinement region produced in an active layer of the HEMT structure 77. A 2DEG is induced by application of a potential bias between an n-type source region 97 in contact with a lower end of the HEMT structure 77, and an n-type drain region 99 in contact with an upper end 101 of the HEMT structure. The source region 97 and drain region 99 have respective ohmic contacts 103, 105.

In use, the confinement potential barriers are induced by applying bias potentials to the front gate 81 and back gates constituted by the p-Si layers 83, 85, 87. Again, radiation received by the antenna 95 enables conduction across the "quantum dot" in the confinement region.

As shown in Figure 7, a third embodiment of a detector 151 according to the present invention is somewhat analogous to that of the first embodiment.

A recess 153 is etched in a layer structure 155 consisting of a 2000A (100) n+GaAs back gate layer 157 formed on a 500 prn SI substrate 159. The recess divides the n+GaAs layer into respective side gates 161, 163 and to reveal a surface 165 which acts as a regrowth interface.

On the regrowth interface surface 165 are grown as a regrowth structure 167, in order, a 300A GaAs layer 169, a isooA AlAs'GaAs superlattice 171, a 300A active GaAs layer 173, and a 650A top structure 175, consisting of, in order, and AlGaAs spacer layer 177, an n+AlGaAslayer 179 and a GaAs cap layer 181.

A double-triangular antenna (not shown) is formed over the cap layer 181, as in the first embodiment, extending apex-wise into the recess 153. In use, a 2DEG 183 within the active GaAs layer 173 adjacent the AlGaAs spacer layer is restricted by the applied potentials so that a quantum dot 185 is confined in the 2DEG plane, of the bottom of the recess 153.

In the light of this description, modifications of the described embodiment, as well as other embodiments, all within the scope of the present invention as defined by the appended claims, will now become apparent to persons skilled in this art

Claims

CLAIMS 1. A detector for electromagnetic radiation, the detector comprising an active layer for induction of a two dimensional carrier gas therein, gate means for confining a pool of carriers from the carrier gas within a confinement region of the active layer and antenna means for coupling electromagnetic radiation to said confinement region.
2. A detector according to claim 1, wherein the confinement region of the active layer is located within a recessed region of the device.
3. A detector according to claim 1 or claim 2, wherein said gate means comprises a plurality of side gates.
4. A detector according to claim 3, wherein the gate means further comprises at least one secondary gate for restricting the width of the carrier gas.
5. A detector according to any of claims 24, wherein said antenna means extends into the recessed region.
6. A detector according to any of claims 2-5, wherein said antenna means comprises a pair of substantially triangular members.
7. A detector according to claim 6, wherein an apex of each substantially triangular member extends into the recessed region.
8. A detector according to claim 6 or claim 7, wherein said substantially triangular members are substantially equilateral.
9. A detector according to any of claims 2-5, wherein said antenna means is configured as a log-periodic antenna.
10. A detector according to claim 1, wherein the active layer is formed on an angled facet of an etched layer structure.
11. A detector according to claim 10, wherein the gate means comprises a front gate overlying the active layer and at least one back gate constituted by a layer or layers of said etched layer structure.
12. A detector according to any preceding claim, wherein the active layer is part of a high electron mobility transistor structure.
13. A detector according to any preceding claim, further comprising source and drain regions arranged so that the active layer provides electrical conduction therebetween.
14. A device for detecting electromagnetic radiation, the device comprising a plurality of detectors according to any preceding claim.

14. A detector for electromagnetic radiation, the detector being substantially as hereinbefore described with reference to any of Figures 46 of the accompanying drawings.

15. A device for detecting electromagnetic radiation, the device comprising a plurality of detectors according to any preceding claim.

Amendments to the claims have been filed as follows CLAIMS 1. A detector for electromagnetic radiation, the detector comprising an active layer for induction of a two dimensional carrier gas therein, gate means for confining a pool of carriers from the carrier gas within a confinement region of the active layer and antenna means for coupling electromagnetic radiation to said confinement region, wherein said gate means comprises a plurality of embedded side gates.

2. A detector according to claim 1, wherein the confinement region of the active layer is located within a recessed region of the device.

3 A detector according to any of claims 1 and 2, wherein the gate means further comprises at least one secondary gate for restricting the width of the carrier gas.

4. A detector according to any of claims 2-3, wherein said antenna means extends into the recessed region.

5. A detector according to any of claims 2-4, wherein said antenna means comprises a pair of substantially triangular members.

6. A detector according to claim 5, wherein an apex of each substantially triangular member extends into the recessed region.

7. A detector according to claim 5 or claim 6, wherein said substantially triangular members are substantially equilateral.

8 A detector according to any of claims 2-4, wherein said antenna means is configured as a log-periodic antenna.

9. A detector according to claim 1, wherein the active layer is formed on an angled facet of an etched layer structure.

10. A detector according to claim 9, wherein the gate means comprises a front gate overlying the active layer and at least one back gate constituted by a layer or layers of said etched layer structure.

11. A detector according to any preceding claim, wherein the active layer is part of a high electron mobility transistor structure.

12. A detector according to any preceding claim, further comprising source and drain regions arranged so that the active layer provides electrical conduction therebetween.

13. A detector for electromagnetic radiation, the detector being substantially as hereinbefore described with reference to any of Figures 4-6 of the accompanying drawings.