Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
1997-01-17
2001-04-03
Mintel, William (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S021000, C257S022000, C257S184000, C257S185000, C438S057000, C438S073000, C438S074000, C438S077000
Reexamination Certificate
active
06211529
ABSTRACT:
ORIGIN OF THE INVENTION
The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected to retain title.
FIELD OF THE INVENTION
This invention relates to quantum-well devices for detecting infrared (“IR”) electromagnetic radiation.
BACKGROUND AND SUMMARY
Objects emit infrared radiation according to their temperature. An object at room temperature (i.e., 300° K.), for example, emits infrared radiation that has a peak at around 8.5 &mgr;m. Even in complete darkness, i.e., in the absence of visible optical wavelengths, the infrared radiation emitted from the object can be detected. That detected radiation can be processed with an infrared-radiation detector to generate an image.
Infrared radiation detectors operating in the range of 8-15 &mgr;m have been used in night vision, navigation, flight control, weather monitoring, security, surveillance, and chemical detection. The earth's atmosphere is transparent to 8-12 &mgr;m radiation, and infrared-radiation detectors operating in this range are thus used in telescopes, communication systems, and in defense. IR scanner data has also been used to map sulfur dioxide fumes from quiescent volcanos.
The early IR detectors were intrinsic detectors. An intrinsic photodetector takes advantage of optical radiation's capability of exciting a photocarrier, e.g., an electron. Such a photo-excited electron or “photoelectron” is promoted across the band gap from the valence band to the conduction band and collected. The collection of these photoelectrons produces a flow of electrons, which is detected as a current.
An intrinsic photodetector requires that an incoming photon from the radiation to be sensed is sufficiently energetic to promote an electron from the valence band to the conduction band. Hence, the energy of the photon =h&ngr; needs to be higher than the band gap E
g
of the photosensitive material.
Quantum well detectors are more sensitive. Quantum well photodetectors can be used to form quantum well infrared photodetectors (“QWIPs”)that are sensitive to 6-25 &mgr;m infrared radiation. A quantum well is formed by packaging a relatively thin layer of a first semiconductor (typically GaAs) between adjacent layers of a second semiconductor (typically Al
x
Ga
1−x
As). These semiconductor materials have a gap of inherent energies, “a band gap”, between them. The materials are used to form an energy “well” in the semiconductor. That well can capture photons generated by the incoming radiation. The electrons are promoted by the photon from a ground state within the well to an excited state.
Spectral response of the detectors has been adjusted by controlling the band gap. However, detection of long wavelength radiation, such as infrared radiation, requires a small band gap; e.g., around 62 meV. These low band gap materials are characterized by weak bonding and low melting points.
The art responded by forming multi-quantum well structures (MQW) made of large band gap semiconductors. Positions of the energy levels in an MQW structure are primarily determined by the well height and width. For example, the energy level separation and the depth of the quantum well are increased as the thickness of the GaAs layer is decreased. The well's height also depends on the band gap of the Al
x
Ga
1−x
As layer and the relative proportions of Al and Ga (“x”) in the Al
x
Ga
1−x
As. The intersubband energy, i.e., the energy between the ground state E
1
and the first excited state, defines many of the essential characteristics of the quantum well.
Quantum well infrared photodetectors operate based on photoexcitation of an electron between ground and a first excited state in the quantum well. The basic operation of a single well is shown in FIG.
1
.
The band gap
110
of the Al
x
Ga
1−x
As
112
is different from the band gap
120
between the GaAs layers
122
. This difference forms the well which captures the electrons. These photoelectrons can escape from the well and are collected as photocurrent.
The band gap of Al
x
Ga
1−x
As can be changed by varying x. This hence changes the height of the well and allows changing the energy required to capture an electron, the “intersubband transition energy.”
An intrinsic infrared photodetector, as described above, increases the energy of an electron using one (or many) photons, and detects the resultant photoelectrons. The photon needs to be sufficiently energetic to increase the energy of the electron sufficiently to promote the electron from the valence band
130
to the conduction band
132
. This has been called interband operation, signifying the electron's promotion from one band to another band.
The intersubband system shown in
FIG. 1
promotes the electrons between subbands—here from one subband
101
to another subband
106
. Intersubband transitions operate between confined energy states, i.e., quantum wells associated with either the conduction band
132
or valence band
130
in the quantum well. The promotion is effective at holes
100
in the quantum well.
Different kinds of intersubband transitions exist. A bound-to-bound transition is formed when both the ground state
104
, and the excited state
106
of the excited electrons are bound within a quantum well
100
.
A multi-quantum well system is schematically shown in FIG.
2
. Like the
FIG. 1
system, the quantum wells generate photocurrent following intersubband absorption between two bound energy levels. A bound-to-bound intersubband absorption requires the infrared wavelengths to excite an electron from the ground state
220
a
to a bound excited state
222
within the well. The electron then tunnels through the edge of the well via quantum tunneling shown as
230
, to an unbound and continuous level above the well level, “the continuum level”
210
. The bias on the well excites a flow of electrons through the continuum. This flow of electrons is detected as photocurrent.
The sensitivity of the detector is a function of efficiency of the photocurrent detection, i.e., the amount of detected photocurrent sensitivity is degraded by noise in the detector. Since infrared radiation has less energy than higher frequency electromagnetic radiation such as visible electromagnetic radiation, the system generates relatively less photocurrent. This has provided a unique challenge to enhancing detector efficiency.
Dark current is a source of noise in QWIPs. Dark current is, as the name implies, current that flows in the dark, i.e., even when radiation to be detected is not reaching the QWIP. The dark current in a QWIP originates from three main mechanisms, quantum mechanical tunneling, thermally assisted tunneling and thermionic emissions.
Quantum mechanical tunneling from well to well through the barriers (shown as
224
), also called sequential tunneling, occurs independent of temperature. This occurs to a very small extent, and dominates the dark current at very low temperatures.
Thermally-assisted tunneling
226
is based on thermally excited quantum tunneling through the tip of the barrier into the continuum
210
. At medium temperatures, e.g., around 45° K. for an 8-9 &mgr;m detector, thermally-assisted tunneling governs the dark current.
At the more usual high temperatures, greater than 45° K., classical thermionic emissions
228
dominate the dark current. A thermionic emission occurs when the electrons are promoted by thermionic processes, i.e. without an incoming photon.
It is highly desirable to reduce the dark current to make a more sensitive detector, i.e., a detector with higher signal to noise ratio. However, it is also desirable that the detector produce as much photocurrent as possible.
The bound-to-bound system requires a photoexcitation energy, E
P
240
in order to excite it from one state to another. This energy E
P
is less than the energy for thermionic emission E
P
242
. Since the bound level E
P
is within the quantum well, thermionic emission is only
Gunapala Sarath
Lin True-Lon
Liu John K.
Park Jin S.
Sundaram Mani
California Institute of Technology
Fish & Richardson P.C.
Mintel William
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