Stacked multiple quantum well superlattice infrared detector

Details Stacked multiple quantum well superlattice infrared detector Stacked multiple quantum well superlattice infrared detector

1993-11-15

2003-03-18

Crane, Sara (Department: 2811)

Active solid-state devices (e.g., transistors, solid-state diode

Thin active physical layer which is

Heterojunction

C257S184000, C257S440000

Reexamination Certificate

active

06534783

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the detection of electromagnetic radiation with a multiple quantum well (MQW) superlattice structure, and more particularly to the sensing of long wavelength infrared radiation (LWIR) with an MQW superlattice in which photoexcited charge carriers are transported through the superlattice under the influence of an applied bias voltage.
2. Description of the Related Art
MQW superlattice LWIR detectors made of heterojunction materials, such as GaAs/GaxAll xAs, provide good design flexibility for spectral response. The detection of LWIR with an MQW sensor has been reported in several publications, such as Levine et al., “Bound-to-Extended State Absorption GaAs Superlattice Transport Infrared Detectors”,
J. Applied Physics Letters,
Vol. 64, No. 3, Aug. 1, 1988, pages 1591-1593; Levine et al., “Broadband 8-12 &mgr;m High-Sensitivity GaAs Quantum Well Infrared Photodetector”,
Applied Physics Letters,
Vol. 54, No. 26, Jun. 26, 1989, pages 2704-2706; Hasnain et al., “GaAs/AlGaAs Multiquantum Well Infrared Detector Arrays Using Etched Gratings”,
Applied Physics Letters,
Vol. 54, No. 25, Jun. 19, 1989, pages 2515-2517; Levine et al., “High-Detectivity D*=1.0×1.0
10
cm {square root over (Hz)}/W GaAs/AlGaAs Multiquantum Well &lgr;=8.3 &mgr;m Infrared Detector”,
Applied Physics Letters,
Vol. 53, No. 4, Jul. 25, 1988, pages 296-298.
The principal of operation for an MQW superlattice IR detector is illustrated in FIG.
1
. The basic device consists of a periodic heterostructure of GaAs quantum wells
2
and AlGaAs barrier layers
4
. The GaAs quantum well layers are doped with an n-type dopant, such as silicon, to provide electrons in the ground states of the wells for intersubband detection. The superlattice is sandwiched between a pair of heavily n-doped GaAs contact layers
6
and
8
, with contact layer
6
functioning as an electron emitter and contact layer
8
as an electron collector during sensor operation. Ohmic contacts
10
and
12
on the opposed contact layers provide access to apply a bias voltage across the superlattice.
The thickness of each quantum well layer
2
is sufficiently small, generally about 20-60 Angstroms and most preferably about 40 Angstroms, that quantum effects are significant. The thickness of each barrier layer
4
is generally about 40-300 Angstroms, and most preferably about 140 Angstroms. The superlattice period is thus preferably about 180 Angstroms. It is generally preferred that the superlattice have about 20-30 periods. GaAs quantum well layers
2
are heavily doped n-type with a donor impurity such as Ge, S, Si, Sn, Te or Se. A particularly preferred dopant is Si at a concentration of about 1×10
18
-5×10
18
cm
3
, and most preferably about 2×10
18
cm
−3
. Lattice match and thermal coefficient considerations, impurity concentrations and fabrication techniques are known in the art.
Although a GaAs/AlGaAs superlattice is preferred, other materials may also be used. For example, it may be desirable to use materials such as InGaAs/InAlAs on InP, SiGe on Si, or HgCdTe. In general, superlattices fabricated from III-V, IV-IV and II-VI semiconductor materials are suitable. The MQW superlattice detectors are particularly suited for the detection of LWIR, but the sensors in general are applicable to the detection of radiation and other wavelength regimes, and no limitation to LWIR for the present invention is intended.
The potential energy barrier height of the barrier layers
4
is about 160 mev above the potential energy barrier height of the quantum wells
2
for GaAs/AlGaAs. For LWIR with peak detection of about 12 microns, the energy gap between the bound state and the excited state for electrons in the quantum wells is about 100 mev, with the first electron excited state in the quantum wells lying above the conduction band edge of the barrier layers.
Incident infrared photons excite electrons from the quantized baseband of the wells to extended excited states in a continuous conduction subband, which has an energy level greater than the conduction band floor for the barrier layers. The excited electrons are then accelerated towards the collector contact
8
by an electric field created by an externally applied bias voltage source V
b
. Under normal sensor operating conditions, the bias voltage causes the mean-free path of electrons in the subband
8
to be sufficiently large for the electrons to travel under the applied field through the superlattice, producing a photocurrent that is measured as an indication of the magnitude of incident radiation. An ammeter
14
can be inserted in the circuit between contact layers
10
and
12
for this purpose.
The sensitivity of an MQW superlattice infrared detector can be severely limited by high levels of dark current. This current consists primarily of electrons which tunnel through the intervening barrier layers
4
between the ground states of adjacent quantum wells
2
. The tunneling current can be reduced by increasing the widths of the barrier layers
4
. However, any such increase in the barrier layer width reduces the device's radiation hardness, which is inversely related to its thickness.
An improvement upon the detector as described thus far is disclosed in pending U.S. patent application Ser. No. 07/457,613, filed Dec. 27, 1989 by Sato et al., “Dark Current-Free Multiquantum Well Superlattice Infrared Detector”, and assigned to Hughes Aircraft Company, the assignee of the present invention. Under this approach the barrier layers
4
are kept thin, but a thicker (generally about 800-3,000 Angstroms) tunneling current blocking layer is provided at the end of the superlattice in the path of the tunneling electrons. The blocking layer
16
, which is preferably formed from the same material as the barrier layers
4
, eliminates most of the tunneling current component of the photodetector's dark current. This in turn allows the individual barrier layers
4
to be made thinner, thus enhancing the detector's quantum efficiency and increasing its radiation hardness.
The prior MQW superlattice IR detector illustrated in
FIG. 1
requires both a relatively large number of quantum well/barrier layer periods, generally 20-30, and a relatively high bias voltage of about 3 volts to obtain sufficiently high quantum efficiency. The large number of superlattice periods can cause detector failure if the detector is operated in a harsh radiation environment. Furthermore, since the number of electrons that are absorbed in the superlattice before reaching the collector contact increases with the initial distance of the electrons from the collector contact, the portions of the superlattice that are distant from the collector contact reduce the overall quantum efficiency. The relatively high bias voltage requirement can lead to a high noise-to-signal ratio and large power consumption.
SUMMARY OF THE INVENTION
The present invention seeks to provide an MQW superlattice radiation detector that is operable with a substantially reduced bias voltage compared to prior detectors, and has a higher quantum efficiency, lower noise level and lower power consumption.
These goals are achieved with a radiation detector in which a plurality of MQW superlattice detectors are structurally formed in a unitary stack, but are electrically connected in parallel. Electrical contact layers are provided between each adjacent pair of superlattices and at opposite ends of the stack, with the adjacent superlattices sharing common contact layers between them. The number of quantum well/barrier layer periods per superlattice can be reduced to about 20-30 divided by the number of superlattices in the stack. This reduces the required bias voltage across the parallel-connected superlattices by a corresponding amount, with an accompanying increase in quantum efficiency and a reduction in noise and power consumption.
The different superlattices can either be similar in structure and composition, or the thickness

Affiliated with

Also associated with

Rating

Stacked multiple quantum well superlattice infrared detector does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Stacked multiple quantum well superlattice infrared detector, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Stacked multiple quantum well superlattice infrared detector will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3084474