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Claims  |
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What is claimed is:
1. A light emitting device comprising:
a first cladding region of semiconductor material having conduction and
valence allowed energy bands defined by corresponding band edges spaced
apart by an energy band-gap;
a second cladding region of semiconductor material having conduction and
valence allowed energy bands defined by corresponding band edges spaced
apart by an energy band-gap;
a first quantum well region of semiconductor material having conduction and
valence allowed energy bands defined by corresponding band edges spaced
apart by an energy band-gap, said first quantum well region which is
adjacent to said first cladding region having at least one energy state
residing in said conduction band of said first quantum well region; and
a second quantum well region of semiconductor material having conduction
and valence allowed energy bands defined by corresponding band edges
spaced apart by an energy band-gap, said second quantum well region
sandwiched between said first quantum well region and second cladding
region, said first and second quantum well regions forming a type-II
heterojunction, said second quantum well region having at least one energy
state which is lower in energy than said energy state in said conduction
band of said first quantum well region and is within said valence band of
said second quantum well region, under a bias voltage each of said energy
states residing within said allowed energy bands of said adjacent cladding
region and simultaneously residing within at least one of the band-gaps of
said adjacent quantum well region and the other cladding region.
2. The light emitting device of claim 1 further including means for
applying a bias to said device sufficient to cause emission of
electromagnetic energy between said energy state in the conduction band of
said first quantum well region and said energy state in the valence band
region of said second quantum well region.
3. The light emitting device of claim 1 wherein said first quantum well
region is InAs and said second quantum well region is GaSb.
4. The light emitting device of claim 1 further including a first tunnel
barrier disposed between said first cladding region and said first quantum
well region and a second tunnel barrier between said second cladding
region and said second quantum well region.
5. The light emitting device of claim 4 wherein said first and second
tunnel barriers are each comprised of a layer of semiconductor material
having a band-gap wider than the band-gap of said adjacent quantum well
region to provide carrier confinement and to act as an insulator, said
first and second barriers being sufficiently thin to permit carrier
tunneling.
6. The light emitting device of claim 4 wherein said first cladding region,
said first tunnel barrier and said first quantum well region form a type-I
tunnel junction and wherein said second quantum well region, said second
tunnel barrier and said second cladding region form a type-II tunnel
junction.
7. The light emitting device of claim 6 wherein said first and second
cladding regions are each comprised of an analog graded quaternary
AlInAsSb alloy layer or digitally graded InAs/AlSb superlattice, and
wherein said first tunnel barrier, said first quantum well region, said
second quantum well region and said second tunnel region are
AlSb/InAs/GaSb/AlSb, respectively, under said bias voltage said energy
state in the conduction band of said InAs residing within the conduction
band of said first cladding region and simultaneously residing within the
band-gap of said GaSb and said energy state in the valence band of said
GaSb residing within the conduction bands of said InAs and said second
cladding region and simultaneously within the band-gap of said first
cladding region.
8. The light emitting device of claim 4 wherein said first cladding region,
said first tunnel barrier and said first quantum well region form a
type-II tunnel junction and wherein said second quantum well region, said
second tunnel barrier and said second cladding region form a type-I tunnel
junction.
9. The light emitting device of claim 8 wherein said first and second
cladding regions are each comprised of an analog graded ternary AlGaSb
alloy layer or digitally graded GaSb/AlSb superlattice, and wherein said
first tunnel barrier, said first quantum well region, said second quantum
well region and said second tunnel region are AlSb/InAs/GaSb/AlSb,
respectively, under said bias voltage said energy state in the conduction
band of said InAs residing within the valence bands of said first cladding
region and said GaSb and simultaneously residing within the band-gap of
said second cladding region and said energy state in the valence band of
said GaSb residing within the band-gap of said InAs and simultaneously
within the valence band of said second cladding region.
10. The light emitting device of claim 4 wherein said first cladding
region, said first tunnel barrier and said first quantum well region form
a type-I tunnel junction and wherein said second quantum well region, said
second tunnel barrier and said second cladding region form a type-I tunnel
junction.
11. The light emitting device of claim 10 wherein said first cladding
region, said first tunnel barrier, said first quantum well region, said
second quantum well region, said second tunnel region and said second
cladding region are InAs/AlSb/InAs/GaSb/AlSb/GaSb, respectively.
12. The light emitting device of claim 1 further including a tunnel barrier
disposed between said first and said second quantum well regions.
13. The light emitting device of claim 12 wherein said tunnel barrier is
comprised of a layer of semiconductor material having a thickness selected
to manipulate the spatial interband coupling between said first and second
quantum well regions, said barrier being sufficiently thin to permit
carrier tunneling.
14. The light emitting device of claim 1 further comprising a plurality of
the device of claim 1 stacked in series to form a superlattice structure
so as to produce cascade photon emissions.
15. The light emitting device of claim 4 further comprising a plurality of
the device of claim 4 stacked in series to form a superlattice structure
so as to produce cascade photon emissions.
16. The light emitting device of claim 6 further comprising a plurality of
the device of claim 6 stacked in series to form a superlattice structure
so as to produce cascade photon emissions.
17. The light emitting device of claim 7 further comprising a plurality of
the device of claim 7 stacked in series to form a supeflattice structure
so as to produce cascade photon emissions.
18. The light emitting device of claim 8 further comprising a plurality of
the device of claim 8 stacked in series to form a superlattice structure
so as to produce cascade photon emissions.
19. The light emitting device of claim 9 further comprising a plurality of
the device of claim 9 stacked in series to form a superlattice structure
so as to produce cascade photon emissions.
20. The light emitting device of claim 10 further comprising a plurality of
the device of claim 10 stacked in series to form a superlattice structure
so as to produce cascade photon emissions.
21. The light emitting device of claim 12 further comprising a plurality of
the device of claim 12 stacked in series to form a superlattice structure
so as to produce cascade photon emissions. |
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Claims |
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Description |
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Foreign priority benefits under Title 35, U.S. Code .sctn.119 are claimed
for Canada Application No. 2,150,499, filed May 30, 1995.
FIELD OF THE INVENTION
This invention relates to semiconductor light emitting devices and more
particularly, to quantum well semiconductor light emitting devices having
improved radiative efficiency, wavelength tuning and carrier injection
efficiency.
BACKGROUND OF THE INVENTION
The emission wavelength of the semiconductor light emitting devices such as
lasers developed to date is mainly in the near infrared (IR). This is
limited by the band-gap of the semiconductor material in the active region
where the stimulated recombination of electrons and holes across the
band-gap gives the emission of electromagnetic radiation. Longer
wavelength semiconductor light sources, particularly in naturally
occurring atmospheric IR transmission bands (3-5 .mu.m and 8-12 .mu.m),
are demanded by many military and civilian applications such as free-space
communications, medical diagnostics, atmospheric pollution monitoring, and
IR radar for aircraft and automobiles. There have been many attempts
devoted to developing long wavelength IR sources by employing intersubband
transitions in artificial quantum well (QW) semiconductor heterostructures
since an original proposal by Kazarinov et al. in Soviet Phys. Semicond.
Vol. 5 (4), 1971. The wavelength of such an IR source due to intersubband
transition is determined, not by the band-gap, but by the smaller energy
separation of conduction subbands arising from quantum confinement in QW
heterostructures made from relatively wide band-gap semiconductor
materials. Therefore, the emission wavelength can be tailored over a wide
spectral range from mid-IR to sub-millimeter by merely changing QW layer
thickness. One recent development in such an intersubband light emitting
device is reported by Faist et al. in Science, Vol. 264, pp. 553-556, Apr.
22, 1994 and in Electronics Letters, Vol. 30 (11), 1994, who demonstrate a
so called quantum cascade laser which consists of 500 layers, some as thin
as 0.8 nm.
A difficulty associated with prior art intersubband light emitting devices
is that a high carrier injection efficiency is hard to achieve without
reducing population inversion which is essential to lasing action. Such a
difficulty would also limit the practical range of wavelength tuning. An
issue of more concern for intersubband light emitting devices is that the
electron non-radiative relaxation between subbands with energy separation
higher than the optical phonon energy (.about.30 meV) is very fast due to
optical phonon scattering, and its typical relaxation time (.about.1 ps)
is at least three order of magnitude smaller than the radiative time (>1
ns), resulting in a very low radiative efficiency (<10.sup.-3). There is a
need for a QW semiconductor IR source having a improved radiative
efficiency by suppressing non-radiative relaxation loss, and a high
carrier injection efficiency without difficulties and restrictions in
practical implementation.
SUMMARY OF THE INVENTION
The invented semiconductor light emitting devices comprise two spatially
coupled active QW regions residing in conduction and valence bands
respectively, where the valence band-edge in one QW is higher in energy
than the conduction band-edge of the other QW. This unique band-edge
alignment between the two QWs provides a novel way to establishing
efficient population inversion with a high carrier injection efficiency,
and to easily manipulating transition rates and tuning emission wavelength
in a broad range. The two coupled active QW regions are sandwiched between
cladding layers that serve as the emitter/collector and are separated from
the quantum well regions by thin tunnel barriers. Each QW region contains
at least one energy state formed by the quantum size effect. The energy
state in the conduction band QW is higher in energy than the energy state
in the spatially adjacent valence band QW.
Under an appropriate bias, each of the above mentioned two energy states
residing within the allowed energy bands of the adjacent cladding layer
and simultaneously residing within at least one of the band-gaps of the
adjacent quantum well region and the other cladding region, electrons are
injected from the emitter to the energy state in the conduction band QW.
These electrons cannot travel directly to the collector at that high
energy level because the energy level resides in at least one of the
band-gaps of the adjacent quantum well region and the collector layer. The
electrons relax to the lower energy state in the valence band of the
adjacent QW thereby causing light to be emitted at a wavelength inversely
proportional to the difference in energy of the two states. Since the
lower energy level lies in the allowed energy band of the collector layer,
the electrons then travel through to the collector layer. The electrons at
the lower energy level are prevented from traveling back to the emitter
region because the lower energy level resides in at least one of the
band-gaps of the adjacent quantum well region and the emitter layer.
The light emitting devices of the present invention utilize
spatially-diagonal transitions between an energy state in the conduction
band of one QW and an energy state in the valence band of the adjacent QW,
which differ from either the conventional intraband resonant schemes as
reported by Faist et al. in Science, Vol. 264, Apr. 22, 1994 or an
interband resonant tunneling approach based on intrawell intersubband
transitions as shown by Yang et al. in Applied Physics Letters, Vol. 59
(2), 1994 and by Esaki et al. in U.S. Pat. No. 5,079,601, leading to an
improved radiative efficiency since the non-radiative relaxation via
optical phonon scattering will be suppressed in the interband transitions.
The present invention provides a system in which sufficient population
inversion and efficient carrier injection can be more easily realized, and
which has a greater tolerance in practical implementation, facilitating
customization to a wide wavelength range. In addition, the resonant
tunneling injection scheme in the present invention allows the same type
of carrier emitter/collector (n- or p-type) to be located in the two ends
of a device, making cascade photon emission possible by periodically
stacking many repeated QW regions separated by a gap-graded cladding
region which serves as the collector for one pair of active QWs and the
emitter for the next pair.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention, as exemplified by a preferred embodiment, is described with
reference to the drawings in which:
FIG. 1 is a schematic illustration of the energy band profile of an
elementary light emitting device of the present invention in a forward
biased state; and
FIG. 2 is a cross-sectional view of an edge of a light emitting device of
the present invention; and
FIG. 3 is a schematic illustration of the energy band profile of a specific
embodiment of the light emitting device with n-type emitters/collectors in
a forward biased state according to the present invention; and
FIG. 4 is a schematic illustration of the energy band profile of a specific
embodiment of the light emitting device with p-type emitters/collectors in
a forward biased state according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring to the drawings, FIG. 1 is a schematic illustration of the energy
band profile of an elementary light emitting device in a forward biased
state useful in explaining the invention. The device of FIG. 1 comprises a
first QW active region 12 and a second QW active region 14 sandwiched
between a first cladding layer 16 and a second cladding layer 18. The
conduction band-edge of the first QW active region 12 is lower than the
valence band-edge of the second QW active region 14. Cladding layer 16 is
separated from the first QW active region 12 by tunnel barrier 20 and
cladding layer 18 is separated from the second QW active region 14 by
tunnel barrier 22. The first QW region 12 is separated from the second QW
region 14 by tunnel barrier 24 which can be zero thickness. The band edges
of barrier layers 20, 22 and 24 are not shown and may vary for particular
material systems, but it should be understood that their band-gaps must be
wide enough to provide the necessary carrier confinement. Barrier layers
20, 22 and 24 can vary in thickness, but should be thin enough so that
carriers can tunnel through the layers.
As shown in FIG. 1, at least one ground energy state E.sub.e is formed in
the conduction band of the first QW region 12 and one ground energy state
E.sub.h which is lower than E.sub.e in energy is formed in the valence
band of the second QW region 14. The energy state E.sub.e resides in the
band-gap of the second QW region 14 and simultaneously in the conduction
band of the cladding layer 16. The energy state E.sub.h resides in the
band-gap of the cladding layer 16 and simultaneously in both conduction
bands of the cladding layer 18 and the first QW region 12. This allows a
finite penetration of the wave-function for state E.sub.h into the
conduction band of the first QW region 12, which can enhance the spatial
overlap between states E.sub.e and E.sub.h and is beneficial to the
optical transition between the two states. In FIG. 1, both cladding layers
16 and 18 are made of n-doped semiconductor materials, which serve as the
electron emitter and collector respectively. The first cladding layer 16,
the tunnel barrier 20 and the first QW layer 12 form a type-I tunnel
junction. The second QW layer 14, the tunnel barrier 22 and the second
cladding layer 18 form a type-II tunnel junction. A type-I tunnel junction
is such that the conduction band-edge of the emitter is higher in energy
than the conduction band-edge of the collector. In addition, a type-I
tunnel junction is formed with the conduction band-edge of the collector
higher in energy than the valence band-edge of the emitter. In contrast,
in type-II tunnel junctions, the valence band-edge of the emitter is
higher in energy than the conduction band-edge of the collector or
alternatively the conduction band edge of the emitter is lower in energy
than the valence band-edge of the collector.
Under an appropriate forward bias, i.e. the cladding layer 18 biased
positively with respect to the cladding layer 16, electrons will tunnel
through from the conduction band of layer 16 through layer 20 to the
energy state E.sub.e as depicted by arrow 26. The electrons in the state
E.sub.e can hardly tunnel directly through to the layer 18 because state
E.sub.e resides in the band-gap of layer 14 which is thick enough to
efficiently block the direct tunneling of the electrons at state E.sub.e.
The electrons relax to state E.sub.h as depicted by arrow 27. The
relaxation results in the emission of electromagnetic radiation depicted
by arrow 28 of energy .eta..omega.. The wavelength of the emitted light
will be inversely proportional to the energy difference E.sub.e -E.sub.h,
which can be easily tuned in a broad spectral range (e.g. from mid- to
far-IR). The recombined electrons in state E.sub.h then tunnel through
layer 22 to the cladding layer 18 as depicted by arrow 29. This is the
only path for the electrons in state E.sub.h leaving state E.sub.h since
E.sub.h resides in the band-gap of layer 16 preventing electrons from
tunneling back to 16. Since states E.sub.e and E.sub.h are localized
mainly in layer 12 and layer 14 respectively, the relaxation between the
two states can be easily manipulated to be much slower than the tunneling
process for carriers from layer 14 to layer 18, facilitating the
establishment of sufficient population inversion between the two states.
Furthermore, because the electrons relax from a conduction band state to a
valence band state, in contrast to the transition between two conduction
states in the quantum cascade laser as described by Faist et al. in
Science, Vol. 264, Apr. 22, 1994, and the intrawell intersubband
transitions as shown by Yang et al. in Appl. Phys. Lett. Vol. 59 (2),
1994, the non-radiative relaxation via the phonon scattering will be
greatly suppressed and a higher radiative efficiency can be achieved in
the device of the present invention. Thus, both efficient optical
transitions and near 100% carrier injection efficiency can be easily
achieved with a light emitting device formed with the band-edge alignments
and energy state arrangements in accordance with the present invention.
A further embodiment includes integrating many elementary device structures
into a superlattice structure as shown in FIG. 2 by periodically stacking
repeated QW regions separated by a gap-graded cladding region 17 which
serves as the collector for one pair of QWs and the emitter for the next
pair. Heavily doped electrode contact layers 35 and 36 are provided at the
top and bottom of the superlattice structure as shown in FIG. 2.
Additional cladding layers 37 and 38 are inserted adjacent to the layers
35 and 36 respectively as shown in FIG. 2. The refractive indexes of the
cladding layers 37 and 38 are lower than the average refractive index of
the QW regions and the regions 17 so that light is confined in the region
between the cladding layers 37 and 38, as is conventional in such
light-emitting devices. Such a specific preferred light emitting device
structure with n-type emitter/collectors in a forward biased state is
illustrated in a nearly lattice-matched InAs/AlSb/GaSb type-II QW system
as shown in FIG. 3. The cladding region 17 can be constructed by either
analog graded quaternary AlInAsSb alloy material or digitally graded
binary InAs/AlSb alloy superlattice. The conduction band-edge of layer 17
which is n-doped is nearly flat in an appropriate forward bias. The QW
layer 14, the adjacent runnel barrier 22 and the adjacent layer 17 form a
type-II tunnel junction. Layer 17, the adjacent tunnel barrier 20 and the
adjacent QW layer 12 form a type-I tunnel junction. Each layer in the
quantum well regions can vary in thickness, providing great design
flexibility. In each period, since the valence band edge of GaSb is higher
in energy than the conduction band edge of InAs by about 0.15 eV, strong
spatial coupling between the conduction band in the InAs layer 12 and the
valence band in the GaSb layer 14 exists in the coupling window region
defined between the conduction band edge of InAs and the valence band edge
of GaSb. Under a forward bias, the whole band-edge diagram of the device
looks like an energy staircase in which an electron tunnels from the layer
17 through the barrier 20 to state E.sub.e in QW layer 12, and relaxes to
the lower energy state E.sub.h in the neighboring QW layer 14 causing the
emission of a photon with an energy E.sub.e -E.sub.h, and then tunnels
from state E.sub.h through the barrier 22 to the next layer 17 and on
towards another period of stairs, leading to cascade photon emission,
which will further improve the device performance.
The emission wavelength, determined by the energy separation between the
two states, can be entirely tailored by adjusting the InAs and GaSb well
layer thicknesses, over a wide spectral range from the mid-IR to the
submillimeter wave region (.about.100 .mu.m). The short wavelength cutoff
(.about.2 .mu.m) is limited by the conduction band-offset between InAs and
GaSb. However, a shorter wavelength in the near IR spectrum including the
important 1.3 .mu.m and 1.55 .mu.m for optical fiber communication can be
realized by using a further wider band-gap material AlGaSb instead of
GaSb. In addition, the transition rate and spectrum can be manipulated by
varying the AlSb barrier thickness between InAs and GaSb well layers to
change the spatial overlap of wave-functions for the two states, providing
a mean to the optimization of device performance.
The preferred embodiment of the invention has been described above, in
which the cladding layers are made of n-doped semiconductor materials,
i.e. both the emitter and collector are n-type. However, several
variations of the elementary device structure can be made possible by
using either two p-type cladding layers or one n-type cladding layer and
one p-type cladding layer with an appropriate rearrangement of energy
states E.sub.e and E.sub.h in respect to the band-edge alignment of the
whole structure without departing from the spirit of the invention. In one
embodiment, a similar structure to that of FIG. 3 is provided but having
p-doped analog graded AlGaSb as the cladding layer 17 instead of n-doped
AlInAsSb, in which the valence band-edge of the AlGaSb cladding layer 17
is nearly flat under an appropriate forward bias. In this embodiment as
shown in FIG. 4, the InAs layer 12, the adjacent tunnel barrier 20 and the
adjacent AlGaSb cladding layer 17 form a type-II tunnel junction, and
AlGaSb cladding layer 17, the adjacent tunnel barrier 22 and the adjacent
GaSb layer 14 form a type-I tunnel junction. Upon applying a forward bias
to this device, holes tunnel from the valence band of AlGaSb layer 17
through the adjacent AlSb barrier layer 22 to the energy state E.sub.h
residing in the band-gap of the adjacent InAs QW layer 12 and
simultaneously in the valence band of the adjacent AlGaSb cladding layer
17, the holes then relax to the energy state E.sub.e residing in both
valence bands of the adjacent AlGaSb cladding layer and the adjacent GaSb
QW layer 14 and simultaneously in the band-gap of the second adjacent
AlGaSb cladding layer, causing the emission of photons.
In accordance with the present invention, the light emitting devices
utilize spatially-diagonal transitions between an energy state in the
conduction band of one QW and an energy state in the valence band of the
adjacent QW rather than intersubband transitions within a conduction band
(or valence band). The utilization of the energy states in this manner,
together with the unique band-edge alignment of the layers and the
resonant tunneling injection, allows for a great improvement in both
radiative efficiency and carrier injection efficiency, and facilitates
achieving a sufficient population inversion. Additionally, the emission
wavelength can be more easily customized by selecting material system and
thicknesses with much tolerance to create the desired energy difference
between the two states.
The QW light-emitting devices can be manufactured by any known processes
for creating semiconductor devices, such as the growth of epitaxial layers
on a substrate. One of the most widely used and versatile techniques for
growing epitaxial layers is molecular beam epitaxy (MBE). For example, by
using MBE a light-emitting device comprising a superlattice structure as
shown in FIG. 4 is grown on a GaSb substrate in a sequence from the bottom
to the top as shown in FIG. 2. Layers 35, 36, 37 and 38 are chosen to be
p-type doped. The thickness of layers 12, 14, 20, 22 and 24 in FIG. 4 are:
10 nm, 1.9 nm, 2.5 nm, 2 nm and 0, respectively. This construction and
combination of thicknesses produces emitted light with a wavelength of 10
micrometers.
Although the invention has been specifically described and illustrated with
respect to the preferred embodiments which can represent applications of
the principles of the invention, it should be understood by those skilled
in the art that all variations and modifications in form and details may
be made therein without departing from the spirit and scope of the
invention which should be limited only by the scope of the claims.
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