|United States Patent
,   et al.
October 10, 1995
Unipolar semiconductor laser
This application discloses, to the best of our knowledge, the first
unipolar laser. An exemplary embodiment of the laser was implemented in
the GaInAs/AlInAs system and emits radiation of about 4.2 .mu.m
wavelength. Embodiments in other material systems are possible, and the
lasers can be readily designed to emit at a predetermined wavelength in a
wide spectral region. We have designated the laser the "quantum cascade"
(QC) laser. The QC laser comprises a multilayer semiconductor structure
that comprises a multiplicity of essentially identical undoper "active"
regions, a given active region being separated from an adjoining one by a
doped "energy relaxation" region. In a currently preferred embodiment each
active region comprises three coupled quantum wells designed to facilitate
attainment of population inversion. In the currently preferred embodiment
the energy relaxation regions are digitally graded gap regions. However,
other energy relaxation regions are possible. The unipolar plasma in a
unipolar laser can be manipulated by means of an electric "control" field,
facilitating, for instance, beam steering or external control of the modal
gain of the laser. Means for accomplishing this are discussed.
Capasso; Federico (Westfield, NJ);
Cho; Alfred Y. (Summit, NJ);
Faist; Jerome (Scotch Plains, NJ);
Hutchinson; Albert L. (Piscataway, NJ);
Luryi; Serge (Bridgewater, NJ);
Sirtori; Carlo (Summit, NJ);
Sivco; Deborah L. (Warren, NJ)
AT&T IPM Corp. (Coral Gables, FL)
April 4, 1994|
|Current U.S. Class:
||372/45; 372/43 |
|Field of Search:
References Cited [Referenced By]
U.S. Patent Documents
|5311009||May., 1994||Capasso et al.||250/214.
"Possibility of the Amplification of Electromagnetic Waves in a
Semiconductor with a Superlattice", by R. F. Kazarinov et al., Soviet
Physics-Semiconductors, vol. 5, No. 4, Oct., 1971, pp. 707-709.
"Evaluation of the Feasibility of a Far-infrared Laser Based on
Intersubband Trransitions in GaAs Quantum Wells", by S. I. Borenstain,
Applied Physics Letters, vol. 55 (7), 14 Aug. 1989, pp. 654-656.
"Feasibility of Far-infrared Lasers Using multiple Semiconductor Quantum
Wells," by Q. Hu, Applied Physics Letters, vol. 59(23), 2 Dec. 1991, pp.
"Possibility of Infrared Laser in a Resonant Tunneling Structure," by A.
Kastalsky et al., Applied Physics Letters, vol. 59 (21), 18 Nov. 1991, pp.
"Carrier Transport and Intersubband Population Inversion in Coupled Quantum
Wells", W. M. Yee et al., Applied Physics Letters, vol. 63 (8), 23 Aug.
1993, pp. 1089-1091.
"Periodic Negative Conductance by Sequential Resonant Tunneling Through an
Expanding High-field Superlattice Domain", by K. K. Choi et al., Physical
Review B, vol. 35, No. 8, 15 Mar. 1987-I, pp. 4172-4175.
"Band Nonparabolicity Effects in Semiconductor Quantum Wells," by D. F.
Nelson et al., Physical Review B, vol. 35, No. 14, 15 May 1987, pp.
"Quantum-well Intersub-band Electroluminescent Diode at .lambda.=5.mu.m,"
by J. Faist et al., Electronics Letters, vol. 29, No. 25, 9 Dec. 1993, pp.
"Phonon Limited Intersubband Lifetimes and Linewidths in a Two-dimensional
Electron Gas," by J. Faist, Applied Physics Letters, vol. 64 (7), 14 Feb.
1984, pp. 872-874.
"Pseudo-quaternary GaInAsP Semiconductors: A new Ga.sub.0.47 In.sub.0.53
As/InP Graded Gap Superlattice and its Applications to Avalanche
Photodiodes", by F. Capasso, Applied Physics Letters, vol. 45 (11), 1 Dec.
1984, pp. 1193-1195.
"Staircase Solid-State Photomultipliers and Avalanche Photodiodes with
Enhanced Ionization Rates Ratio," by F. Capasso, IEEE Transactions on
Electron Devices, vol. ED-30, No. 4, Apr. 1983, pp. 381-390.
"Mid-infrared Field-tunable Intersubband Electroluminescence at Room
Temperature by Photon-assisted Tunneling in Coupled-quantum Wells," by J.
Faist, Applied Physics Letters, vol. 64(9), 28 Feb. 1994, pp. 1144-1146.
Primary Examiner: Davie; James W.
Attorney, Agent or Firm: Pacher; Eugen E.
1. An article comprising a unipolar semiconductor laser, said laser
a) a multilayer semiconductor structure that comprises doped semiconductor
material of only a first conductivity type; and
b) means for applying a voltage across said multilayer semiconductor
characterized in that
c) said multilayer structure comprises a multiplicity of essentially
identical active regions, a given of said active regions being separated
from an adjoining active region by an energy relaxation region;
d) said active region comprises two or more coupled quantum wells,
associated with said coupled quantum wells being at least second and third
energy states for charge carriers of the first conductivity type, with
said third energy state being higher than said second energy state,
associated with said energy states being, respectively, second and third
wavefunctions, said active region selected to provide reduced spatial
overlap between said third and second wavefunctions;
e) said energy relaxation region is selected to provide for substantial
energy relaxation of charge carriers of the first given conductivity type
in the energy relaxation region when a normal operating voltage is
applied, at least some of said charge carriers being introduced into the
energy relaxation region from said active region; and
f) at least some of the charge carriers of the first conductivity type
undergo a radiative transition from the third to the second energy state.
2. Article according to claim 1, wherein said active region comprises three
or more coupled quantum wells, associated with said coupled quantum wells
being at least first, second and third energy states for said charge
carriers, with the first energy state being lower than the second energy
state when said normal operating voltage is applied, associated with the
first energy state is a first wavefunction; at least some of said charge
carriers undergo a transition from the second to the first energy state
and are introduced into the energy relaxation region from the first energy
3. Article according to claim 2, wherein the active region is substantially
undoped, and the energy relaxation region comprises doped semiconductor
4. Article according to claim 3, wherein the active region is not
intentionally doped, and the energy relaxation region is doped n-type.
5. Article according to claim 2, wherein the third, second and first energy
states are associated with third, second and first quantum wells
respectively, with an energy barrier between said first and second quantum
wells, and another energy barrier between said second and third quantum
6. Article according to claim 1, wherein said energy relaxation region is a
digitally graded gap region comprising a multiplicity of semiconductor
7. Article according to claim 6, wherein said digitally graded gap region
comprises alternating layers of a first and a second semiconductor
8. Article according to claim 1, wherein said energy relaxation region
comprises a doped continuously graded or step-wise graded gap region.
9. Article according to claim 1, wherein said energy relaxation region
comprises a doped well of width sufficient to provide energy relaxation of
the carriers substantially to the bottom of an energy band in the well.
10. Article according to claim 1, wherein said energy relaxation region
comprises a doped superlattice region that comprises a multiplicity of
11. Article according to claim 6, wherein said multiplicity of
semiconductor layers of the digitally graded gap region are selected to
provide a Bragg reflector for charge carriers of the first conductivity
type having an energy that corresponds to the third energy state.
12. Article according to claim 1, wherein the first conductivity type is
n-type conductivity, and the charge carriers of the first conductivity
type are electrons.
13. Article according to claim 1, wherein said multilayer semiconductor
structure is selected to provide a waveguide for photons of energy
corresponding to said radiative transition from the third to the second
14. Article according to claim 13, wherein said waveguide comprises a core
that comprises said active region, and further comprises a doped
semiconductor region having a refractive index that is higher than the
refractive index of said active region.
15. Article according to claim 1, wherein the active region comprises
i) AlGaAs and GaInAs;
ii) InP and GaInAs; or
iii) AlInAs and GaInAs.
16. An article according to claim 1, wherein said first energy state is
lower than said second energy state by an amount greater than or equal to
h.nu..sub.op when the normal-operating voltage is applied, where h is
Planck's constant and .nu..sub.op is an optical phonon frequency.
17. An article comprising a unipolar semiconductor laser comprising an
active region, associated with the laser is a longitudinal axis, the laser
a) a multilayer structure that comprises doped semiconductor material of
only a first conducting type, associated with said multilayer structure
being a plane parallel to the layers of the multilayer structure; and
b) means for applying a voltage across said multilayer semiconductor
structure such that a current can be caused to flow in a direction
substantially normal to said plane parallel to the layers of the
CHARACTERIZED IN THAT
c) the article further comprises means for applying an electric control
field to the laser such that at least a component of said control field
lies in the plane parallel to the layers of the multilayer structure.
18. An article according to claim 17, wherein the means of c) comprise at
least one pair of substantially parallel planar conductors disposed
substantially perpendicularly to the plane parallel to the layers of the
19. An article according to claim 17, wherein the means of c) comprise a
transducer for generating an acoustic wave in the laser such that a
spatially varying control field results.
20. An article according to claim 17, wherein the means of c) comprise
means for applying stress to the laser.
FIELD OF THE INVENTION
This application pertains to the field of injection semiconductor lasers.
BACKGROUND OF THE INVENTION
R. F. Kazarinov et al. (Soviet Physics-Semiconductors, Vol. 5(4), p. 707
(1971)) predicted the possibility of amplification of electromagnetic
waves in a semiconductor superlattice structure. Since the publication of
this seminal paper, the feasibility of a unipolar quantum well
semiconductor laser has been considered by many workers in the field. See,
for instance, S. J. Borenstain et al., Applied Physics Letters, Vol.
55(7), p. 654 (1989); Q. Hu et al., Applied Physics Letters, Vol. 59(23),
p. 2923 (1991); A. Kastalsky et al., Applied Physics Letters, Vol. 59(21),
p. 2636 (1991); and W. M. Yee et al., Applied Physics Letters, Vol. 63(8),
p. 1089 (1993). However, to the best of our knowledge, the prior art does
not disclose observation of lasing in any of the proposed unipolar
Those skilled in the art are aware of the advantages potentially offered by
some types of unipolar injection laser. Among these are a frequency
response that is not limited by electron/hole recombination, a narrow
emission line because the line-width enhancement factor is (theoretically)
zero, and a weaker temperature dependence of the lasing threshold than in
conventional (i.e., bipolar) semiconductor lasers. Furthermore,
appropriately designed unipolar semiconductor lasers can have an emission
wavelength in the spectral region from the mid-infrared (mid-IR) to the
submillimeter region, exemplarily in the approximate range 3-100 .mu.m,
that is entirely determined by quantum confinement. The emission
wavelength can be tailored using the same heterostructure material over
the above mentioned wide spectral region, a portion of the spectrum not
easily accessible with diode semiconductor lasers. Furthermore, unipolar
lasers can use relatively wide bandgap, technologically mature materials
(e.g., utilize GaAs- or InP-based heterostructures), without reliance on
temperature sensitive and difficult to process small bandgap
semiconductors such as PbSnTe. Such unipolar lasers could, for instance,
be advantageously used for pollution monitoring, industrial process
control and automotive applications.
Prior art proposals for unipolar quantum well semiconductor lasers
typically involve use of resonant tunneling structures. For instance, W.
M. Yee et al., (op. cit.) analyzed two coupled quantum well structures,
each of which contains an emission quantum well sandwiched between energy
filter wells, the coupled quantum well assembly sandwiched between n-doped
injector/collector regions. The energy filter wells, respectively, have
only one quasibound state (E.sub.1 and E.sub.3, respectively), and the
emission quantum well has more than one quasibound state, with
intersubband transitions taking place between two of the states
(E.sub.2.sup.(2) and E.sub.2.sup.(1)). When, by means of an applied
electric field, E.sub.1 becomes substantially aligned with
E.sub.2.sup.(2), electrons from the injector can resonantly populate
E.sub.2.sup.(2). If, at the same applied field, E.sub.3 is substantially
aligned with E.sub.2.sup.(1) then the latter can be resonantly depleted
into the collector. If the time constant for the former process is longer
than the time constant for the latter then a population inversion in the
emission quantum well could, at least in principle, be achieved.
As those skilled in the art will appreciate, the optical output power
obtainable from a structure of the type analyzed by Yee et at., (i.e., a
structure that comprises a single set of coupled quantum wells) is
typically too small to be of practical interest. In principle this
shortcoming can be remedied by provision of a structure that comprises a
multiplicity of said sets. However, it is known that, for fundamental
reasons, lasing typically cannot be achieved in such a structure. For
instance, application of a voltage across such a multi-quantum well
structure that comprises doped regions will typically result in a
non-uniform field in the device, with attendant negative resistance and
instability. See, for instance, K. K. Choi et al., Physical Review B, Vol.
35 (8), p. 4172 (1987).
In view of the considerable potential commercial and scientific value of a
unipolar semiconductor laser, especially one that can be designed to emit
in the mid-IR spectral region, such a laser would be of substantial
interest. This application discloses such a laser, to be referred to as
the quantum cascade (QC) laser.
SUMMARY OF THE INVENTION
In a broad aspect this application discloses a unipolar semiconductor
injection laser. More specifically, the instant invention is embodied in
an article that comprises a unipolar semiconductor (typically III-V
semiconductor) laser that comprises a multilayer semiconductor structure
containing doped semiconductor material of only a first conductivity type
(typically n-type), and means for applying a voltage (including a normal
operating voltage ) across said multilayer semiconductor structure.
Significantly, the multilayer structure comprises a multiplicity (e.g., ten
or more) of essentially identical (i.e., exhibiting at most only
substantially unavoidable minor differences) "active" regions, a given one
of said active regions being separated from an adjoining active region by
an "energy relaxation" region. Each of said active regions comprises two
or more coupled quantum wells. Associated with said at least two coupled
quantum wells are at least two (but preferably three or more) energy
states, said at least two (or three) energy states to be referred to,
respectively as third and second (or third, second and first) energy
states. The third energy state is at a higher energy than the second
energy state, which in turn is (under appropriate bias) at a higher energy
than the first energy state. The third, second and first energy states
will also be referred to as the n=3, 2 and 1 states, respectively.
Associated with said first, second and third energy states are,
respectively, first, second and third wavefunctions. A wavefunction is
"associated" with an energy state in a well if the centroid of the modulus
square of the wavefunction is located in the well.
An important aspect of preferred embodiments of the QC laser is selection
of the active region such that there is provided reduced spatial overlap
between said third and second wavefunctions in a given active region. By
"reduced spatial overlap" we mean herein overlap that is less than the
overlap between the wavefunctions of states E.sub.2.sup.(2) and
E.sub.2.sup.(1) of QW2 of any of the two structures of FIG. 1 of W. M. Yee
et al., Applied Physics Letters, Vol. 63(8), p. 1089. Techniques for
computing the overlap between wavefunctions in quantum well structures are
known to those skilled in the art and do not require exposition. See, for
instance, G. Bastard, "Wave Mechanics Applied to Heterostuctures", Les
Editions de Physique, Paris 1990, incorporated herein by reference. See
also D. F. Nelson et al., Physical Review B, Vol. 35 (14), p. 7770 (1987).
At least some (desirably substantially all) of the charge carriers undergo
a radiative transition from the third to the second energy state.
The energy relaxation regions are selected to be thick enough to provide
substantial energy relaxation and randomization of motion of charge
carriers of the given conductivity type in a given graded energy
relaxation region when a normal operating voltage is applied. Differently
stated, and with reference to FIG. 1, the thickness of a given energy
relaxation layer 12 is selected to facilitate relaxation of charge
carriers from the energy of the n=1 energy state 119 to the energy of the
n=3 energy state 122 of the next active region, such that the carriers can
tunnel into the n=3 state 122 of the next active layer. Exemplarily, the
thickness is 1-2 .lambda..sub.E, where .lambda..sub.E is the energy mean
free path of the carriers in the energy relaxation region. At least some
of the charge carriers are introduced into the given energy relaxation
region by tunneling from said first energy state through a barrier layer.
Typically, the active regions are at most lightly doped (exemplarily less
than about 2-3.times.10.sup.16 cm.sup.-3) and preferably undoped (not
intentionally doped), and the graded gap regions are doped (desirably not
greatly exceeding about 10.sup.17 cm.sup.-3, to minimize free carrier
absorption) to exhibit conductivity of the given type, typically n-type.
The energy relaxation region can be continuously (analog) or stepwise
graded, but in currently preferred embodiments is digitally graded.
A further significant aspect of the layer structure is the provision of
optical confinement of the lasing mode. This is accomplished by
appropriate selection of layer compositions to result in a waveguide core
of higher effective refractive index than that of the cladding layers.
As will be described in more detail below, the reduced overlap between the
third and second energy states in preferred embodiments materially
contributes to attainment of a relatively long lifetime (.tau..sub.32) of
the charge carriers in the third energy state. (The carders are provided
to the third energy state by tunneling from a first adjoining energy
relaxation region). The lasing-relevant transition is the photon-emitting
transition from the third to the second energy state. Carriers typically
are removed from the second energy state by tunneling or phonon scattering
into the first energy state. Associated with the carriers in the second
energy state is a lifetime .tau..sub.21, which must be less than
.tau..sub.32, to achieve population inversion between energy levels 3 and
2. Lifetime .tau..sub.32 can be made relatively large by appropriate
choice of the wavefunction overlap, and .tau..sub.21 can be made
relatively small by, e.g., provision of an appropriately thin (e.g., less
than 10 or 5 nm) barrier between the quantum wells associated with said
first and second energy states. Preferably the active region is formed
such that, with a normal operating voltage applied, the energy difference
.DELTA.E.sub.21 between second and first energy states is about
h.nu..sub.op or larger, where h is Planck's constant and .nu..sub.op is
the relevant optical phonon frequency. The preferred layer structure is
also designed to provide, under a normal operating voltage, for tunneling
of the carriers from the well associated with the first energy state into
a second adjoining graded gap region.
The provision of energy relaxation regions is a significant aspect of the
described embodiment of the invention, since it inter alia makes possible
lasing in the multi-period structure before the appearance of negative
resistance, thereby overcoming a significant shortcoming of previously
proposed unipolar laser structures. The provision of reduced overlap of
the third and second wavefunctions makes possible attainment of larger
values of .tau..sub.32 than are typically attainable in prior art
structures, and is another significant aspect of preferred embodiments.
A still further significant aspect is the substantial absence of dopant
from the active regions of the QC laser, facilitating narrowing of the
luminescence spectrum, thereby increasing the peak gain. The absence of
doping is to be compared with the structure disclosed by J. Faist et al.,
(Electronics Letters, Vol. 29 (25), p. 2230 (Dec. 1993)) wherein, in
addition to the graded gap region, a portion of the active region is
doped. The structure exhibited a broad luminescence peak and was not
capable of lasing, due to the absence of waveguiding layers and the doping
of a portion of the active region.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically depicts a portion of the conduction band diagram of an
exemplary laser according to the invention;
FIG. 2 is a schematic representation of the dispersion of the n=1, 2 and 3
energy states, respectively;
FIG. 3 shows exemplary data on the emission spectrum of a laser according
to the invention, for various drive currents;
FIG. 4 provides data on optical power vs. drive current for another
exemplary laser according to the spectrum;
FIG. 5 shows a portion of the spectrum of FIG. 3 in high resolution;
FIG. 6 shows data on threshold current vs. laser temperature for a still
further laser according to the invention, indicating a value of 112K for
the laser parameter T.sub.o ; and
FIG. 7 shows refractive index profile and mode intensity of an exemplary QC
FIGS. 8 and 9 schematically illustrate an exemplary superlattice energy
relaxation region; and
FIG. 10 schematically depicts an exemplary laser according to the invention
DETAILED DESCRIPTION OF SOME PREFERRED EMBODIMENTS
Among the difficulties that have to be overcome before a practical unipolar
laser can be achieved is the following: In III-V semiconductor materials
of current interest, the excited state (e.g., E.sub.2.sup.(2) of FIG. 1 of
Yee et al.) non-radiative lifetime (.tau..sub.NR) typically is quite small
(exemplarily about 1 ps), if the intersubband separation (e.g.,
E.sub.2.sup.(2) -E.sub.2.sup.(1) of FIG. 1 of Yee et at.) corresponds to a
wavelength less than about 40 .mu.m. This is typically due to the
possibility in these materials of non-radiative decay due to optical
phonon emission, for E.sub.2.sup.(2) -E.sub.2.sup.(1)
.gtoreq.h.nu..sub.op, the optical phonon energy (typically about 30 meV).
For E.sub.2.sup.(2) -E.sub.2.sup.(1) <h.nu..sub.op, .tau..sub.NR is much
larger, typically a few hundred picoseconds, being limited primarily by
acoustic phonon scattering. J. Faist et al., Applied Physics Letters, Vol.
64(7), p. 872, (Feb. 1994).
Those skilled in the art will appreciate that a unipolar laser requires
design features that make it possible to increase the excited state
lifetime to a value substantially greater than is associated with prior
art structures, such that a significant population inversion can be
attained for transition energies.gtoreq.h.nu..sub.op. Exemplarily this is
achieved by means of spatial separation between the ground state and the
first excited state, with the relevant quantum wells being asymmetric and
FIG. 1 schematically depicts the conduction band diagram of a portion of
the multilayer semiconductor structure of an exemplary unipolar laser
according to the invention under positive bias conditions corresponding to
an electric field of 10.sup.5 V/cm. The quantum wells exemplarily are
Ga.sub.0.47 In.sub.0.53 As (to be referred to as "GaInAs"), and the
barriers are Al.sub.0.48 In.sub.0.52 As"). The depicted portion comprises
one period 10 of a (multiperiod) multilayer semiconductor structure, with
the two adjoining periods also indicated. Each period comprises an active
region 11 and an energy relaxation region 12, the former being
substantially undoped (typically not intentionally doped), and the latter
being doped. The energy relaxation region exemplarily is a digitally
graded gap region. However, other energy relaxation regions are possible.
Electrons are tunnel injected into active region 11 through barrier 110.
The active region includes coupled asymmetric quantum wells 111 and 113,
separated by barrier 112. Associated with wells 111 and 113 are third
energy state 117 and second energy state 118, respectively. Wavy line 120
indicates the photon-assisted tunneling transition responsible for
luminescence (and lasing) in the exemplary structure. Electrons in the
second energy state tunnel through barrier 114 into quantum well 115,
occupying therein first energy state 119. Furthermore, electrons tunnel
through barrier 116 from well 115 into energy relaxation region 12. One of
the adjoining active regions shown in FIG. 1 depicts the modulus square of
the wavefunction associated with the first, second and third energy
As can be seen from FIG. 1, the wavefunction of the n=3 state can extend
into the neighboring quantum well (e.g., 113) and beyond, possibly
resulting in some "leakage" of electrons out of the n=3 state. This occurs
because the QC laser typically operates under a strong applied electric
field. The main effect of the extension of the wavefunction into the
continuum typically is a decrease of the injection efficiency, as
electrons are escaping into the continuum instead of making a radiative
transition. We believe that such escape can be reduced or prevented by
appropriate choice of layer thicknesses, such that the layers form a Bragg
reflector for the n=3 carriers (exemplarily electrons).
More specifically, it is possible to design, at least if the energy
relaxation region is a digitally graded gap region, the energy relaxation
region so as to behave as a Bragg mirror for the electrons at the energy
of the n=3 level. Provision of an appropriately designed Bragg mirror can
result in reflection of electrons back into the n=3 level, and thus
prevent their escape into the continuum. Exemplarily, such Bragg
reflection can be achieved if
l.sub.w,j k.sub.w +l.sub.b,j k.sub.b =.pi., (1)
where l.sub.w,j and l.sub.b,j are the thicknesses, respectively, of the
j-th well and barrier of the digitally graded region, and k.sub.w and
k.sub.b the wavevector of the electron in the n=3 energy state.
For electrons having relatively low energy, the digitally graded region
behaves as a medium with an effective conduction band edge difference
.DELTA.E.sub.eff, measured with respect to the well (exemplarily GaInAs)
conduction band edge, where
.DELTA.E.sub.eff,j =.DELTA.E.sub.c [l.sub.b,j /(l.sub.b,j +l.sub.w,j)]. (2)
In equation 2, .DELTA.E.sub.c is the conduction band discontinuity between
the well and barrier materials. Selecting .DELTA.E.sub.eff,j of the j-th
well/barrier pair of the digitally graded region such that the conduction
band edge of the region is substantially flat under the electric field
designed for lasing (which is close to the value required for the "flat
band" condition in the graded region), and using equations (1) and (2),
one can ensure enhanced electron confinement in the n=3 state while
allowing simultaneous transport through the digitally graded region of the
electrons that did relax inside the active region.
Whereas a Bragg reflector is an optional feature of QC lasers that have the
n=3 and n=2 states associated with different quantum wells, it is
considered to be a necessary feature for lasers that have the n=3 and n=2
states associated with one and the same quantum well.
Exemplarily, the active region 11 contains, going from higher to lower
layer in the layer sequence, the following sequence of undoped layers: 4.5
nm barrier 110, 0.8 nm well 111, 3.5 nm barrier 112, 3.5 nm well 113, 3.0
nm barrier 114, 2.8 nm well 115, and 3.0 nm barrier 116.
For clarity's sake, FIG. 1 does not show the conduction band edge of
digitally graded gap region 12, only the effective band edge 121
associated with the region. The graded gap region exemplarily consist of
an AlInAs/GaInAs superlattice with constant period that is shorter than
the electron thermal de Broglie wavelength (exemplarily of order 30 nm) in
the material, and varying duty cycle to obtain a graded gap
pseudoquaternary alloy. See, for instance, F. Capasso et al., Applied
Physics Letters, Vol. 45(11), p. 1193 (1984). Exemplarily the graded gap
region consists of the following n-doped (e.g., 1.5.times.10.sup.17
cm.sup.-3 Si) layer sequence, arranged to give an effective gap that
varies from lower to higher values in going from left to right in FIG. 1:
1.8 nm well/1.2 nm barrier; 1.6/1.4 nm; 1.3/1.7 nm; 1.1/1.9 nm; 0.9/2.1
nm; 0.7/2.3 nm; 0.6 nm well.
As disclosed above, FIG. 1 pertains to the case of a QC laser with
digitally graded gap energy relaxation region, and with an applied bias
electric field of about 10.sup.5 V/cm, sufficient to provide an electric
field that approximately equals the opposing quasi-electric field
associated with the conduction band grading, thus resulting in a
substantially staircase-shaped conduction band edge. Those skilled in the
art will appreciate that, in the absence of an applied bias, the band
diagram of the structure has an overall sawtooth shape. See, for instance,
Capasso et al., IEEE Transactions on Electron Devices, Vol. ED 30(4), p.
We have carried out calculations which show that, at the depicted
substantially "flat band" condition, graded gap energy relaxation regions
12 are quasi-neutral. Electrons relax in the energy relaxation regions and
are injected by tunneling into the n=3 excited state (the third energy
state). The tunneling rate through trapezoidal barrier 110 is extremely
high, exemplarily about 5 ps.sup.-1, ensuring efficient filling of the n=3
level. The electronic states of the Al.sub.0.48 In.sub.0.52 As/Ga.sub.0.47
In.sub.0.53 As coupled well structure of FIG. 1 were calculated in the
envelope function approximation for various electric fields. The material
parameters used are: .DELTA.E.sub.c (conduction band
discontinuity)=0.52eV, m.sub.e.sup.* (GaInAs)=0.043 m.sub.o, m.sub.e.sup.*
(AlInAs)=0.078 m.sub.o, where m.sub.o is the free electron mass. For the
nonparabolicity coefficient we used .gamma.=1.13.times.10.sup.-18 m.sup.2.
Nonparabolicities were taken into account using the method of D. F. Nelson
et at. (op. cit.).
FIG. 2 is a schematic representation of the dispersion of the n=1, 2, 3
energy states parallel to the layers of the multilayer structure, with
k.sub..parallel. being the corresponding wavevector component. The bottom
of the subbands correspond to the first, second and third energy states.
The subbands are nearly parallel, due to the small non-parabolicities of
k.sub..parallel. not too far (typically .ltorsim.10 meV) from the bottom
(k.sub..parallel. =0) and for not too large transition energies. As a
result, electrons making radiative transitions to a lower subband (e.g.,
from n=3 to n=2) will all emit photons of essentially the same energy. The
joint density of states of these transitions is therefore substantially
delta function-like (in the absence of broadening). If a population
inversion is created between the n=3 and n=2 states then the gain spectrum
of the transition will be correspondingly narrow (collision limited),
nearly symmetric, and typically much less sensitive to thermal broadening
of the electron distribution than that associated with interband
transitions in semiconductor diode lasers. Numeral 20 refers to the
optical phonon-assisted transition from the n=3 level to the n=2 level.
This process, in structures according to the invention, is between states
of reduced wavefunction overlap and is accompanied by a large momentum
transfer. Consequently, the relevant relaxation time (.tau..sub.32) can be
relatively long, estimated about 4.3 ps at a bias of 10.sup.5 V/cm in the
above described embodiment. This can ensure population inversion between
the n=3 and n=2 shares, since the n=2 state empties into the n=1 state
with a relaxation time estimated to be about 0.6 ps. This efficient
relaxation is provided by quantum well 115. Strong inelastic relaxation
via optical phonons with nearly zero momentum transfer occurs between
strongly overlapped and closely spaced n=2 and n=1 subbands, as shown in
FIG. 2. Finally, the tunneling escape time out of the n=1 state into the
adjoining energy relaxation region typically is extremely short,
exemplarily less than about 0.5 ps. This further facilitates population
The above described structure also facilitates injection into the excited
state E.sub.3 by reducing the tunneling escape probability into the
continuum. The estimated escape time .tau..sub.esc is 6 ps, which leads to
an injection efficiency .eta..sub.in =.tau..sub.esc /(.tau..sub.esc
+.tau..sub.32).congruent.0.6. The radiative efficiency of the n=3 to n=2
laser transition is estimated to be .tau..sub.32 /.tau..sub.R
.apprxeq.3.times.10.sup.-4 at a field .about.10.sup.5 V/cm, where
.tau..sub.R is the spontaneous emission lifetime, estimated to be about 13
ns. Calculations show that the product of .vertline.z.sub.32
.vertline..sup.2 .tau..sub.32 (where z.sub.32 =1.5 nm is the n=3 to n=2
transition matrix element) is weakly dependent on the electric field. The
low or substantially zero doping level in the active region strongly
reduces the linewidth of the n=3 to n=2 electroluminescence, as compared
to more highly doped coupled wells, thus enhancing the peak material gain
for the same radiative efficiency. Electric field tunable
electroluminescence, up to room temperature, has recently been observed by
us in similar AlInAs/GaInAs coupled-quantum-well heterostructures.
We have incorporated a multiplicity of the above described active
region/energy relaxation region units into a waveguide structure. The
resulting unipolar device lased at a wavelength of about 4.2 .mu.m. To the
best of our knowledge, this is the first observation of laser action in a
unipolar quantum well semiconductor structure.
Table 1 shows the layer sequence of an exemplary laser according to the
invention. The 33.2 nm digitally graded region corresponds to
below-defined "digital grating II", followed by below-defined "digital
grating I", with the latter being above the former in the sequence of
Table 1. The active region/digitally graded energy relaxation region
sequence was repeated 25 times. Each unit of the sequence consisted of an
undoped active region (3.0 nm barrier/2.8 nm well; 3.5 nm/3.5 nm; 3.5 nm
0.8 nm; 4.5 nm barrier) and one n-type (1.5.times.10.sup.17 cm.sup.-3)
"digital grating II". The 3.0 nm barrier corresponds to layer 110 of FIG.
1 and is, under a normal operating voltage, the most "upstream" layer of a
given active region 11. The 14.6 nm digitally graded region corresponds to
"digital grating I", and the 18.6 nm digitally graded region corresponds
to "digital grating II". The compositions of the core and cladding regions
were selected such that, at the lasing wavelength, the effective
refractive index of the core region is larger than the effective indices
of both the lower and upper cladding regions. The refractive index profile
70 of the layer structure is shown in FIG. 7, together with the calculated
intensity profile 71 of the lasing mode. The confinement factor of the
mode was calculated to be 0.496. A significant aspect of the waveguide
structure that corresponds to the refractive index profile of FIG. 7 is
the presence of doped (10.sup.17 cm.sup.-3) 300 nm GaInAs layers that
correspond to retractive index regions 72 and 73, which sandwich the
active region that corresponds to refractive index region 74 and which
significantly enhance mode confinement by providing large refractive index
steps. Those skilled in the art will appreciate that such enhancement
layers are novel, being unsuitable for use in conventional diode lasers
since they would introduce unacceptable loss due to interband absorption.
The entire multilayer structure was grown epitaxially, by MBE, on a
conventional n.sup.+ -doped InP wafer.
.uparw. GaInAs 2.0 .times. 10.sup.20
Contact Sn doped
layer GaInAs 1.0 .times. 10.sup.18
AlGaInAs 1.0 .times. 10.sup.18
.uparw. AlInAs 5.0 .times. 10.sup.17
AlInAs 1.5 .times. 10.sup.17
.uparw. AlGaInAs 1.5 .times. 10.sup.17
core Active region
GaInAs 1.0 .times. 10.sup.17
AlGaInAs 1.5 .times. 10.sup.17
AlGaInAs 1.5 .times. 10.sup.17
Digitally graded x25
undoped 21.1 .dwnarw.
GaInAs 1.0 .times. 10.sup.17
AlGaInAs 1.5 .times. 10.sup.17
.uparw. AlInAs 1.5 .times. 10.sup.17
Doped n.sup.+ InP
Digital Grating I
AlInAs 1.2 nm
GaInAs 6.5 nm
AlInAs 1.2 nm
GaInAs 4.5 nm
AlInAs 1.2 mn
Digital Grating II
GaInAs 1.8 nm
AlInAs 1.2 nm
GaInAs 1.6 nm
AlInAs 1.4 nm
GaInAs 1.3 nm
AlInAs 1.7 nm
GaInAs 1.1 nm
AlInAs 1.9 nm
GaInAs 0.9 nm
AlInAs 2.1 nm
GaInAs 0.7 nm
AlInAs 2.3 nm
GaInAs 0.6 nm
The thus produced multilayer wafer was conventionally lithographically
processed into mesa etched ridge waveguides of width 12 .mu.m. The length
of the waveguides (varying from 0.5 to 2.8 mm) was defined by cleaving.
This also was conventional. The cleaved facets provided the reflection
means that define the laser cavity. Facet reflectivity was about 0.27.
Conventional ohmic contacts were provided to the top contact layer and to
the substrate. An exemplary device, of length 500 .mu.m, was soldered to a
ceramic holder, mounted in a conventional flow dewar and tested by
injection of 20 ns current pulses (10.sup.-3 duty cycle) and measurement
of the emission spectrum by conventional means (Nicolet Fourier transform
IR spectrometer). FIG. 10 schematically depicts the above described
exemplary laser according to the invention. Numerals 101-104 refer to the
substrate, 500 nm AlInAs layer, 33.2 nm digitally graded layer, and 300 nm
GaInAs layer, respectively. The digitally graded layer 103 consists of
digital grating I above digital grating II. Numerals 105 and 106 refer to
the active region and the 18.6 nm digitally graded layer (digital grating
II), respectively. Layers 105 and 106 are repeated several (e.g., 25)
times. Numerals 107-110 refer to the 14.6 nm digitally graded (digital
grating I) layer, the second 300 nm GaInAs layer, the further 21.1 nm
active region, and the further 18.6 nm digitally graded (digital grating
II) layer, respectively. Numerals 111-115 refer to the 1000 nm cladding
layer, the 1500 nm cladding layer, the 30 nm graded contact layer, the 670
nm GaInAs contact layer, and the 20 nm highly doped (2.times.10.sup.20
cm.sup.-3) GaInAS contact layer, respectively. Numerals 116 and 117 refer
to conventional metal contact layers. Those skilled in the art will
appreciate that FIG. 10 is schematical, with some conventional features
(e.g., re-growth) not shown, and that layer thicknesses and other
dimensions are not drawn to scale or in proportion.
FIG. 3 shows exemplary results at 10K. Above a device current of about 850
mA, corresponding to a threshold current density of about 15 kA/cm.sup.2,
the signal amplitude increases abruptly by orders of magnitude,
accompanied by dramatic line-narrowing. This is direct manifestation of
laser action. In other QC lasers we have observed lasing at liquid
nitrogen temperature, with powers approaching 20 mW for a 1.2 mm long
cavity under pulsed operation. Lasing temperatures as high as 125K have
been attained. Design and packaging optimization is expected to result in
QC lasers capable of CW operation at even higher temperatures.
FIG. 4 shows optical power vs. device current at 10K for a laser
substantially as described, but of length 720 .mu.m. The threshold current
density is 11 kA/cm.sup.2, corresponding to 8.7 V across the device, with
peak optical power from a single facet of about 8.5 mW. This power was
limited by the collection efficiency (40%) of the apparatus and by the
divergence of the beam (.+-.40.degree.) normal to the layers. FIG. 5 shows
a portion of the spectrum at higher resolution. Well defined, nearly
equally spaced (.DELTA..nu.=2.175 cm.sup.-1) longitudinal modes were
observed. The linewidth of the dominant mode was about 0.3 cm.sup.-1,
presently limited by heating effects and mode hopping during the pulse.
Theory predicts, for single longitudinal mode continuous wave (cw)
operation, a Schawlow-Townes linewidth with negligible linewidth
enhancement factor, compared to conventional semiconductor lasers.
The laser wavelength essentially does not shift in the current range of
FIG. 4, indicating that the electron density in the n=3 state is locked at
the threshold value. Using measured results and estimated values of
.tau..sub.32 and .tau..sub.esc (.tau..sub.esc is the escape time from the
n=3 state into the continuum), we estimate a population inversion n.sub.s
=1.7.times.10.sup.11 cm.sup.-2, comparable to the electron density in the
graded gap region.
In other experiments, we obtained from a 1.2 mm long laser an output power
of 30 mW at 10K and 4 mW at 125K, and from a 2.8 mm long laser a power of
130 mW at 10K.
FIG. 6 shows data on the temperature dependence of the threshold current of
a 1.2 mm QC laser. The data shows that the conventional laser parameter
T.sub.o had the value 112K, indicative of deskably low temperature
dependence of the threshold. The observed lasing mode was polarized normal
to the layers of the multilayer structure, due to the selection rule for
Digital grading of the energy relaxation regions is not required, and
conventional (continuous or discontinuous) "analog" grading is
contemplated. Indeed, the energy relaxation region need not be a graded
gap region, and all other means for attaining carder energy relaxation in
the region between adjacent active regions are contemplated. Exemplary of
such other means are a doped, relatively thick, uniform quantum well, and
a superlattice region.
The former exemplarily is doped to a level of order 10.sup.17 /cm.sup.3,
and has a thickness (e.g., 1-2 .lambda..sub.E) selected to result in
energy relaxation of the carriers to the bottom of the band in the region.
Deskably, the composition of the quantum well (e.g., Al.sub.x Ga.sub.1-x
As) is selected such that, under the bias required for lasing, the
conduction band edge in the well is essentially lined up with the n=3
energy state of the adjacent (downstream) active region.
FIGS. 8 and 9 illustrate the design of an exemplary superlattice energy
relaxation region, with the former showing the relevant aspects of the
band structure under zero bias and the latter showing the aspects under
lasing bias. Active region 11 is essentially the same as the active region
of FIG. 1, and numeral 81 designates the superlattice energy relaxation
region, with numerals 82-85 designating energy states. The quantum wells
of the superlattice are selected such that, upon application of the lasing
bias, energy states 82-85 become substantially aligned, forming miniband
90, with the miniband desirably positioned such that carriers from first
energy state 119 can readily enter the miniband, and carriers can readily
enter third energy state 117 of the adjacent (downstream) active region.
Still furthermore, the unipolar laser need not be n-doped but can, at least
in principle, be p-doped, with the carriers preferably being the light
holes to facilitate tunneling in appropriately strained layer
semiconductors such as In.sub.x Ga.sub.1-x As.
QC lasers can be implemented by conventional techniques (exemplary
comprising MBE) in a variety of semiconductor systems. Exemplarily of such
systems are AlGaAs/InAs, InP/GaInAs, and AlInAs/GaInAs, where the first
member of a pair is the barrier material, and the second member is the
Quantum well structures similar to those described above (but differing
therefrom inter alia with respect to doping of the active region, and
lacking a waveguide structure) were recently described in the literature.
J. Falst et al., Electronics Letters, Vol. 29 (25), p. 2230 (Dec. 1993),
and J. Faist et al., Applied Physics Letters, Vol. 64, p. 1144, (1994),
both incorporated herein by reference. These papers disclose observation
of luminescence, but do not report observation of lasing. Indeed, the
structures disclosed in these papers were not designed, and were
unsuitable, for laser action.
Unipolar lasers have the unique property that carriers in the active region
form a unipolar plasma. We have discovered that the spatial position of
such a plasma can be controlled by means of an appropriate electric field,
to be designated the "control" field. The ability to control the spatial
position of the plasma in the laser structure makes possible, for
instance, controlled steering of the laser output beam, and external
control of the modal gain of the laser, i.e., control of the overlap of
the optical wave cross section with the spatial profile of the gain.
In general, the electric control field will be applied such that at least a
component of the control field lies in the plane of the layer structure
(that is to say, in a plane normal to the direction of current flow in the
layer structure). For instance, the control field can be applied in the
direction perpendicular to the axis of the laser cavity by one or more
sets of parallel planar conductors, possibly but not necessarily,
integrated with the laser structure. Exemplarily, the planar conductors
are metallic layers deposited over the sidewall dielectric.
Another contemplated approach to the application of the control field
utilizes the piezo-electric property of the laser material, especially if
the QC laser is implemented in III-V heterostructure materials. For
instance, generation of an acoustic wave (either as a traveling or a
standing wave) can result in a spatially varying electric field in the
material, and thus can result in an acoustoelectric interaction between
the acoustic wave and the carriers. For instance, an acoustic wave,
propagating in the direction parallel to the axis of the laser cavity, can
establish a periodic structure in the density of carriers (which, in turn,
leads to periodic spatial structure in the optical gain and the refractive
index) which can, for instance, serve as a "grating" for distributed
feedback. This effect, for instance, makes it possible to vary the
frequency of the lasing mode by varying the frequency of the acoustic
Those skilled in the art will appreciate that the piezo-electrically
generated control field need not be due to an acoustic wave but can be
generated by other means, e.g., by application of static or time-varying
* * * * *