Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Field effect transistor
Reexamination Certificate
1999-09-15
2004-04-13
Wille, Douglas A. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Field effect transistor
C257S014000
Reexamination Certificate
active
06720589
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the field of so-called low dimensional semiconductor devices. More specifically, the present invention relates to semiconductor devices which use so-called quantum dots to either store charge or detect incident radiation. The devices can be used for a range of applications, but are particularly intended for use as optically activated devices such as optical detectors or memory structures.
BACKGROUND OF INVENTION
The electrons and holes in an ideal bulk semiconductor have a continuous spectrum of energy states. Confinement of the carriers in one or more dimensions modifies this energy spectrum by a quantisation of the k-vector along the confinement direction(s). In a quantum dot, the motion of the carriers is restricted in all three spatial dimensions. Consequently, the energy spectrum of the dots consists of a series of discrete levels. As the size of the quantum dot reduces, the energy spacing between these discrete levels increases. The maximum number of electrons which can occupy each electron level is two, corresponding to the up and down spin states. Similarly, each hole level has an occupancy of two. Optical transitions occur between the discrete electron and discrete hole levels.
There is a need for an optical detector which is capable of detecting a single photon. Recently, this need has been heightened by the advent of quantum cryptography of optical signals. In essence, quantum cryptography relies upon the transmission of data bits as single particles in this case photons, which are indivisible. One way in which the data can be encoded is via the polarisation of the electric field vector of the photons. The key component of such a system is a detector which can respond to individual photons. It has been proposed that quantum cryptography can be used to transmit the key for the encryption of data. Examples of information which might be encrypted in this way are internet data or data from automatic teller machines.
Single photon detection is also useful as a low level light detection means for spectroscopy, medical imaging or astronomy. An optimum signal to noise ratio is achieved when a single photon is detected, as the noise is then limited by the source and it is completely independent of noise arising due to the amplifier or detector itself.
A single photon detector could also be used for time-of-flight ranging experiments where the distance of an object from a fixed point is measured by calculating the time over which a single photon takes to return to a detector. This technique can also be used to scan the surface of an object, even a distant object, to form a spatial image of its surface depth, thickness etc.
Single photon detectors are available in the form of photo multiplier tubes (PMT) and single photon avalanche photo diodes (SPAD). PMTs have the disadvantage of having low quantum efficiency, being expensive, bulky, mechanically fragile, requiring high biasing voltages and cooling. They can also be damaged and can require a long settling time after exposure to high light levels or stay magnetic fields. On the other hand, SPADs have the disadvantage of having a relatively low gain and high dark count rates, especially when operated at higher repetition rates. They are also expensive and require high bias voltages and external cooling.
Applications for memory devices are well known. Memory devices which use quantum dots are also know, for example, Imamura et al. Jpn. J. Appl. Phys. Vol. 34 (1995) L1445-L1447. Here, the device has a plurality of InAs quantum dot of different sizes.
Upon illumination, an electron-hole pair is excited in the quantum dot. Due to the biasing of the structure, the electron is swept vertically down through the structure into an ohmic metal contact. The hole is trapped in the quantum dot Due to the movement of electrons, a photocurrent flows. Only a finite number of holes can be stored in a dot. If this finite number is reached, no further holes can be stored in the dots and hence no photocurrent due to dissociated electrons can flow. Thus, stored charge can by detected by a lack of photocurrent.
The carrier trapping properties of quantum dots is also illustrated in Yusa et al Appl. Phys. Lett. 70 (1997) 345. Here, a plurality of quantum dots are used to show trapping effects which occur when a two dimensional electron gas (2DEG) is illuminated.
SUMMARY OF INVENTION
The device of the present invention can be primarily configured as an optical detector which is capable of detecting single photons or an optical memory. Other device configurations are also possible.
In a first aspect, the present invention provides a semiconductor device comprising first and second active layers separated by a first barrier layer, means for applying electric field normal to the first and second active layers and detecting means for detecting a change in a characteristic of the first active layer, wherein the first active layer is a quantum well layer capable of supporting a two dimensional carrier gas and the second active layer comprises a plurality of quantum dots.
Initially, a second aspect of the present invention will be discussed where the device is configured as an optical detector which is sensitive to detect a single photon. The detector of the second aspect comprises first and second active layers separated by a first barrier layer, and detecting means for detecting a change in a characteristic of the first active layer, wherein the first active layer is a quantum well layer capable of supporting a two dimensional carrier gas and the second active layer comprises at least one quantum dot, the device further comprises means for separating a photo-excited electron-hole pair.
Preferably the means for separating an electron-hole pair will be provided by a means for applying an electric field normal to the active layers. However, the device may be fabricated such that the internal field of the device allows separation of photo-excited electron-hole pairs.
The device is capable of detecting a single photon. This is because optical illumination of the device leads to a change in the charge occupancy of the quantum dots and this, in turn induces a change in a transport or optical characteristic of the first active layer. The first active layer will have excess carriers provided by a doped barrier layer or a gate etc.
Absorption of a single photon by the device results in a change in the occupancy of a quantum dot by one carrier and this in turn induces a change in a transport or optical characteristic of the first active layer. A single photon incident on the device will photo-excite one electron-hole pair within the device. One of these photo-excited carriers is trapped by a quantum dot and induces a change in a characteristic of the first active layer. For simplicity, it will be assumed that the photo-excited hole is trapped within the quantum dot. However, it will be appreciated by those skilled in the art that the electron can be the photo-excited carrier which is trapped within the dot
The detector of the second aspect is configured to detect the presence of a single photon either by the size of the device, the total number of dots in the second active layer, the layer structure of the device or in the actual detection mechanism of the device.
Preferably, to reliably detect single photons irradiating the device, the number of active dots in the device is less than 100,000, more preferably less than 10,000. To avoid confusion, the term active dot is intended to mean a dot in the active area of the device which is capable of trapping carriers during normal operation of the device. Dots which cannot trap charge or which are outside the active area are not active dots. Also, the term active area is intended to mean the area of the device which is subjected to the separating means and which contributes to a change in the characteristic of the first active layer. In some cases, the field will be applied by a gate and a mesa will be etched to define the device boundaries, the active area here will be the
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Wille Douglas A.
LandOfFree
Semiconductor device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Semiconductor device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Semiconductor device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3202987