GB2314674A

GB2314674A - Optically operable semiconductor device

Info

Publication number: GB2314674A
Application number: GB9613494A
Authority: GB
Inventors: Jeremy Henley Burroughes; Riichi Katoh
Original assignee: Toshiba Cambridge Research Centre Ltd; Toshiba Corp
Current assignee: Toshiba Europe Ltd; Toshiba Corp
Priority date: 1996-06-27
Filing date: 1996-06-27
Publication date: 1998-01-07
Anticipated expiration: 2016-06-27
Also published as: JPH1022516A; GB2314674B; GB9613494D0

Abstract

An optically operable semiconductor device comprises means for photo-inducing free carriers in a predetermined region 9 and input means 7, 11 for causing the number of carriers in a first part of the predetermined region 9 to be different from the number of carriers in a second part of the predetermined region. Output means 15, 19 is provided for producing an electrical output independent upon the relative difference in numbers of carriers between the first and second parts of the predetermined region 9. The region 9 comprises a pair of quantum bars 3, 5 and the device acts as an optically activated switch.

Description

OPTICALLY OPERABLE SEMICONDUCTOR DEVICE The present invention relates to a semiconductor device in which carriers are confined within predetermined regions. There have been many proposals for devices in which carriers are trapped in a discrete puddle, e.g. at about 100 electrons. Such a puddle is commonly referred to as a "quantum dot" or "quantum box", although the carriers may exhibit classical or quantum behaviour, depending on confinement conditions. There are several ways in which quantum dots may be isolated. Typically, a discrete region of a twodimensional electron gas (2DEG) may be isolated by depleting-out surrounding areas using an appropriate gate arrangement.

The device of the present invention is an optically operable device in which free carriers are induced in one or more predetermined regions in response to incident electromagnetic radiation and the distribution of those carriers is influenced by the relative potentials applied to inputs of the device. Thus, a first aspect of the present invention now provides an optically operable semiconductor device comprising means for inducing free carriers within a predetermined region in response to incident electromagnetic radiation, means for causing the number of carriers in a first part of the predetermined region to be different from the number of carriers in a second part of the predetermined region, and output means for producing an electrical output in dependence upon the relative difference in numbers of carriers between the first and second parts of the predetermined region.

As described in more detail hereinbelow, the device may comprise first and second predetermined regions, each respectively for isolating a group of photo-induced carriers. The input means can be arranged to determine the relative difference in numbers of carriers between the first and second parts of one of these predetermined regions and that in itself, causes a relative difference in the numbers of carriers between first and second part of the other predetermined region which is adjacent. The output means is then responsive to the relative difference in numbers of carriers between the first and second parts of this other predetermined region.

Conveniently, such pairs of predetermined regions isolating respective groups of carriers are elongate and are preferably arranged parallel to one another. In a described embodiment hereinbelow, each of these regions is a one-dimensional quantum wire of finite length. In any event, these pairs of regions of trapped carriers can be considered to be "quantum bars".

The or each means for inducing the free carriers within a predetermined region may comprise a respective isolated region of a 2DEG. A number of different ways of fabricating a device having predetermined electron gas confinement regions are known in the art. For example, heterostructures of silicon, GaAs or the like, doped to have respective layers of opposing conductivity types may be employed. Alternatively, layers of different semiconductors having respective different forbidden bandgaps, preferably those with direct band gaps may be used, for example, GaAs/AlGaAs, InP/AlInAs, GaN/AIGaN. In the device according to the present invention, the predetermined region or regions must in any event be configured such that they may be exposed to electromagnetic radiation capable of inducing free carriers within those regions.

For the avoidance of doubt, the device according to the present invention may be operable by electromagnetic radiation of an appropriate wavelength but in principle, is applicable to devices operable within the infrared, visible or ultraviolet wavelength ranges. Any reference to "light", "photooperation" or "optical" herein, is to be interpreted thus.

Preferably, the device also comprises respective screening means so that only the respective predetermined regions of the wafer are exppsed to the radiation and the remaining areas are shielded therefrom.

The invention will now be explained in more detail by way of the following non-limiting description of a preferred embodiment and with reference to the accompanying drawings in which: Fig. 1 shows a schematic of a basic device structure according to the present invention; Fig. 2 shows the device structure of Fig. 1 in operation when exposed to electromagnetic radiation; Fig. 3 shows the device structure of Figs. 1 and 2 when the input terminals thereof are short-circuited; and Fig. 4 shows operation of a device such as shown in Figs. 1-3, when operated as a photo-activated switch.

As shown in Figure 1 of the accompanying drawings, the basic device structure 1 comprises a pair of parallel but mutually separated quantum bars 3, 5. A first input terminal 7 is positioned adjacent a first end 9 of the second quantum bar 5 and a second input terminal 11 is positioned adjacent the other end 13 of the second quantum bar 5. A first output terminal 15 is positioned adjacent a first end 17 of the first quantum bar 3 and a second output terminal 19 is positioned adjacent the other end 21 of the first quantum bar 3.

As shown in Figure 1, the structure is not illuminated with an optical beam and so no charges are induced in the quantum bars 3, 5, nor in the output terminals 15, 19.

Figure 2 shows the situation in which the device is illuminated with optical radiation ho. The optical radiation induces free carriers in the quantum bars, which are then influenced by the potential applied across the input terminals 7, 11.

A potential difference applied across the input terminals 7, 9 such that the first terminal 7 is positive and the second input terminal 11 is negative.

Each then induces a corresponding opposite charge at the adjacent relevant end of the second quantum bar 5. Thus, the first end 9 of the first quantum bar 5 becomes relatively negative and the second end 13 of the second quantum bar 5 becomes relatively positive. This in turn induces opposite charges in the first quantum well 3. Thus, a positive charge is induced at the first end 17 of the first quantum well 3 and a relatively negative charge is issued by the second end 21 of the first quantum bar 3.

These charges are then transferred with opposite polarity to the output terminals 15, 19. The first output terminal 15 becomes relatively negative in response to the relatively positive charge at the first end 17 of the first quantum bar 3. Similarly, the second output terminal 19 becomes relatively positive in response to the negative charge at the second end 21 of the first quantum bar 3.

Figure 3 shows the situation when the input terminals are short-circuited. Zero net charge is retained in the input or output terminals, or in the quantum bars. The free carriers in the quantum bars become depleted.

Figure 4 shows how the device shown in Figures 1-3 operates as a photo-activated switch.

A logic pulse train Vdc is represented as the input. The output signal (broken line) is only transferred across the device upon application of an optical pulse. However, the output signal is switched off when the clearance operation is performed by shorting the input terminals as shown in Figure 3.

Under illumination, the substrate may conduct as well as the quantum bars 3, 5. Therefore, it is highly desirable to provide shielding so that only the quantum bars are illuminated and all other areas of the device are shielded from the radiation.

Also, upon illumination, the response time of the device will be dictated by the time taken for electrons to be excited into the conduction band. This can be improved by: i. Including a thin line of degenerate levels in each quantum bar 3, 5 by (say) ion implantation. These then provide a source from which the electrons can be excited.

ii. If, for example, the quantum bars are formed in a GaAs layer, growing that layer at a lower temperature so that As is precipitated-out. Such precipitates then act as efficient recombination centres at low bias.

In the light of this disclosure, 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. An optically operable semiconductor device comprising means for inducing free carriers within a predetermined region in response to incident electromagnetic radiation, input means for causing the number of carriers in a first part of the predetermined region to be different from the number of carriers in a second part of the predetermined region, and output means for producing an electrical output in dependence upon the relative difference in numbers of carriers between the first and second parts of the predetermined region.

2. A device according to claim 1, wherein the predetermined region in which carriers are isolated is a first predetermined region, further means being provided for isolating carriers within a second predetermined region, whereby the input means functions to determine the relative difference in numbers of carriers between the first and second parts of the first predetermined region which determines the relative difference in numbers of carriers between first and second parts of the second predetermined region so -that the output means produces the electrical output in dependence upon the relative difference in numbers of carriers between the first and second parts of the second predetermined region.

3. A device according to claim 2, wherein each of the first and second predetermined regions is elongate.

4. A device according to claim 3, wherein each of the first and second predetermined regions is a one-dimensional wire of finite length.