A quantum cascade laser (QCL) having a bias-neutral design and a semiconductor with multiple layers of AlxIn1-xAs/InyGa1-yAs. The first active region barrier has a thickness of less than fourteen angstroms, and the second active region barrier has a thickness of less than eleven angstroms. The lower active region wavefunction overlaps with each of the injector level wavefunctions. Also, the laser transition is vertical at a bias close to roll-over. The injector level 3′ is above a lower laser level 3, the injector level 2′ is below the lower laser level 3, and the active region level 2 is confined to the active region. The lower laser level 3 is separated from the active region level 2 by the energy of the LO phonon. The remaining active region states and the remaining injector states are either above the lower laser level 3 or significantly below the active region level 2.

TECHNICAL FIELD

This invention relates to quantum cascade lasers.

BACKGROUND ART

A quantum cascade laser (QCL) is a multilayer semiconductor laser, based only on one type of carrier (usually electrons). A schematic diagram of a QCL is shown inFIG. 1. It consists of multiple layers of AlxIn1-xAs/InyGa1-yAs having different compositions x and y, typically grown using molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD) techniques. Electric current in these devices is injected along the x-axis, perpendicular to the grown layers. An insulator confines the current under the contact stripe, preventing it from spreading in the y-direction. When carriers reach the gain section, they emit photons through intersubband radiative transitions (see below). Waveguide and cladding layers confine emitted light around the gain region and direct it along the z-axis. A laser of this type is described in U.S. Pat. No. 5,457,709.

The gain section of a QCL usually consists of 20 to 60 identical gain stages. A gain stage consists of approximately 20 very thin InxGa1-xAs and AlyIn1-yAs layers (1-5 nm) with alternating bandgap material compositions (quantum wells and barriers). A schematic of the conduction band diagram of one gain stage under applied electric field is shown inFIG. 2. In an ideal case each carrier emits one photon in each gain stage.

As claimed in U.S. Pat. No. 5,457,709, the layers within the stage are usually divided into two regions: the active region and the energy relaxation region (injector). The active region is designed for light emission through carrier radiative intersubband transitions (transition from level3to level2inFIG. 2), while the energy relaxation region (injector) is used for energy relaxation of carriers before injection into the next stage.

Carrier population inversion between the upper and lower laser levels (levels3and2inFIG. 2), required for lasing, can be achieved when the upper laser level lifetime, τ3, is longer than the lower laser level lifetime, τ2. As claimed in U.S. Pat. No. 5,457,709, this condition is met when the energy spacing between levels2and1, E21, is designed to be substantially equal to the energy of the longitudinal optical (LO) phonon (˜35 meV in case of InP-based QCLs). In this case τ2and lower laser level population are substantially reduced. This scheme is often called the single-phonon design.

QCL performance can be substantially improved employing a so-called two-phonon resonance design (see U.S. Pat. No. 6,751,244) instead of the single-phonon resonance design described above. A schematic conduction band diagram for this design is shown inFIG. 3. The active region in this case is composed of at least four quantum well/barrier pairs instead of at least three for the single-phonon design. The lasing transition occurs between energy levels4and3. Significantly, energy spacings E32and E21are both substantially equal to LO phonon energy, leading to short τ3and τ2. Since the energy spacing between the lower laser level and the lowest active region level E31(˜70 meV) is increased by a factor of two compared to E21in case of the single-phonon resonance design (˜35 meV), the two-phonon resonance design has an advantage of reduced carrier population on the lower laser level3due to reduced carrier thermal backfilling for this state from the lowest active region state1.

The last active region barrier (the so-called extraction barrier), inFIG. 3that illustrates the two-phonon design, is usually thicker than the other active region barriers (except the first active region barrier, called the injection barrier). This helps to confine the lower laser level within the active region, increasing its overlap with the upper laser level and, as a consequence, increasing the laser transition matrix element. However, a thicker extraction barrier leads to longer electron extraction time from the active region to the injector, as discussed in Reference 1 (citation provided below). Longer extraction time increases global transit time of an electron across an active region stage, leading, in turn, to lower maximum current density for the same doping level. See Reference 2 (citation provided below) and references therein for details.

In the Bound-to-Continuum design, described in U.S. Pat. No. 6,922,427, the laser transition occurs between the upper laser level and delocalized lower laser levels, as shown inFIG. 4. Since the lower laser levels are delocalized, there is no electron extraction bottleneck that slows electron transport in the two-phonon case. However, since the laser transition is diagonal, the corresponding matrix element is lower and linewidth is larger. Both lower laser transition matrix element and larger linewidth reduce the active region differential gain, lowering laser performance.

The Single Phonon Resonance-Continuum Depopulation design, presented in Reference 3 (citation provided below), combines vertical laser transition, characteristic to the two-phonon design, and delocalized carrier extraction, characteristic to the bound-to-continuum design. However, this combination is realized only at bias close to roll-over, when the lowest injector state in the previous stage and the upper laser level are close to resonance. Indeed, according to the band diagram presented in Reference 3, calculated approximately at roll-over bias, there is an injector level located just below the lower laser level. Therefore, at slightly lower bias, these levels align with each other due to the Stark effect. Since there is a strong active region/injector coupling, these levels become delocalized at lower bias. As follows from typical QCL voltage vs. current (IV) characteristics, current starts flowing through a superlattice at bias significantly below its roll-over value. Therefore, delocalization of the lower laser level leads to an increase in threshold current density, reducing laser dynamic range, maximum optical power and wall plug efficiency.

The goal of the present invention is to achieve vertical transition and fast carrier extraction in a broad bias range, i.e. to introduce a bias independent design method. Stability of QCL parameters at lower bias has been consistently overlooked in QCL design so far.

DISCLOSURE OF INVENTION

This document discloses a quantum cascade laser having a bias-neutral design. In one aspect, the invention is a quantum cascade laser having a semiconductor with only one type of carrier. The semiconductor has multiple layers of AlxIn1-xAs/InyGa1-yAs, where x is a number between (but not including) 0 and 1, and y is a number between (but not including) 0 and 1. An electric current is injected along an x-axis perpendicular to the multiple layers. And an insulator confines the electric current under a contact stripe and prevents the electric current from spreading in a y-axis parallel to the multiple layers.

The quantum cascade laser also has a first active region barrier with a thickness of less than fourteen angstroms. It has a second active region barrier with a thickness of less than eleven angstroms.

In addition, the quantum cascade laser has a bias independent design with strong active region/injector coupling. The bias independent design is characterized by several features noted here, including an active region and multiple injector states. The active region includes lower active region states each having a lower active region state wavefunction. And each of the multiple injector states has an injector level wavefunction. At least one of the lower active region wavefunctions overlaps with at least one of the injector level wavefunctions.

Also, the bias independent design has a laser transition that is vertical at a bias close to roll-over. Additionally, the injector level3′ is above a lower laser level3, the injector level2′ is below the lower laser level3, and the active region level2is confined to the active region. The lower laser level3is separated from the active region level2by the energy of the longitudinal optical (LO) phonon. The remaining active region states (that is, other than active region level2) and the remaining injector states (that is, other than injector level2′ and injector level3′) are located either above the lower laser level3or significantly below the active region level2.

Consequently, the lower laser level3is well-confined in the active region in a broad bias range and the laser transition from laser level4to the lower laser level3is vertical.

In another aspect of the invention, the multiple layers are of Al0.78In0.22As/In0.72Ga0.28As. The semiconductor has a strain of −2.0% in the Al0.78In0.22As layers and a strain of 1.3% in the In0.72Ga0.28As layers. In addition, the thickness of the first active region barrier is eleven angstroms (11 A), and the thickness of the second active region barrier is eight angstroms (8 A).

In another aspect of the invention, the multiple layers are of Al0.85In0.15As/In0.75Ga0.25As. The semiconductor has a strain of −2.5% in the Al0.85In0.15As layers and a strain of 1.5% in the In0.75Ga0.25As layers. Additionally, the thickness of the first active region barrier is eleven angstroms (11 A), and the thickness of the second active region barrier is six angstroms (6 A).

BEST MODE FOR CARRYING OUT THE INVENTION

The detailed description set forth below in connection with the appended drawings is intended as a description of presently-preferred embodiments of the invention and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the functions and the sequence of steps for constructing and operating the invention in connection with the illustrated embodiments. However, it is to be understood that the same or equivalent functions and sequences may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention.

A design generally becomes unstable when there is a strong active region/injector coupling and when there are injector (or active region) states close, in energy space, to the lower laser level. This leads to lower laser level delocalization due to its anticrossing with these states. Conduction band diagrams for an unstable design, presented inFIGS. 5A and 5B, illustrate this problem. The laser transition is designed to be vertical at bias close to resonance between the lowest injector state in the previous stage and the upper laser level4(91 kV/cm), as shown inFIG. 5A. At lower bias of 80 kV/cm, the level2′ approximately aligns with the lower laser level. As a consequence, the lower laser level wavefunction is delocalized at this bias, spanning from the active region into the injector. Therefore, the laser transition matrix element reduces from 1.39 nm down to 1.05 nm when bias changes from 91 kV/cm to 80 kV/cm, increasing laser threshold.

Design stability can be improved by reducing active region/injector coupling using a thicker extraction barrier. However, as discussed above, a thicker extraction barrier leads to a longer electron extraction time from the active region to the injector and therefore to a lower maximum current density for the same doping level.

Here we disclose a method to achieve design stability while simultaneously increasing active region/injector coupling. In particular, we found that instead of increasing extraction barrier thickness, stability can be achieved by designing all the injector states to be higher or significantly lower than the lower laser level. As a consequence, despite increased active region/injector coupling, the lower laser level is well confined in the active region in a broad bias range since all the injector states are significantly away from resonance with this level.

FIGS. 6A and 6Bshow an embodiment of this design method. Similar to the structure presented inFIGS. 5A and 5B, the laser transition is designed to be vertical at bias close to roll-over (FIG. 6A). Also, there is a significant overlap between the active region and injector states indicating a strong active region/injector coupling. In general, the coupling is strong when the thickness of the extraction barrier is substantially equal to the thickness of the other injector barriers. To improve design stability, the injector state3′ was designed to be above the lower laser level3. When bias reduces, the level3′ moves higher relative to the lower laser level3due to the Stark effect. As a consequence, anticrossing is not possible between these states in the whole bias range up to roll-over. The injector level2′ is designed to be deep inside the injector at roll-over, significantly below the lower laser level3. Therefore, even though the energy spacing between these levels reduces when bias reduces, the level2′ is below the lower laser level3in a broad bias range. Note, that the active region level2is also well confined in the active region. This is in contrast with the approach used in Reference 3 (citation provided below), where this level was essentially replaced by a miniband.

FIG. 6Bshows that at 70 kV/cm, significantly below roll-over, the lower laser level is still well confined in the active region. This is a direct consequence of the fact that in this broad bias range the lower laser level does not come into resonance with any of the injector states. As a consequence, the laser transition matrix element is the same for both biases. On the other hand, since the lower active region and injector states are close to each other and there is a strong active region/injector coupling, the lower active region wavefunctions and injector levels wavefunctions overlap with each other. This large overlap leads to a small carrier extraction time from the active region to injector compared to the two-phonon case or any other designs where a thick extraction barrier is used for confinement of the active region levels.

The structure presented inFIGS. 6A and 6Bserves as an example of how bias independent structure with a strong active region/injector coupling can be designed. There are other alternative approaches. However, the fundamental idea behind them is that the lower laser level should not come into resonance with any injector levels, or other active region levels, in a broad bias range. We have found that design stability can be achieved when close to roll-over condition if the lower laser level3and the active region level2, located just below level3, are separated by approximately the energy of the LO phonon, while the rest of the active region and injector states are located either above the low laser level3or significantly below the level2. This ensures that the lower laser level is well confined in the active region in a broad bias range and that the laser transition is vertical from level4to level3. This is the case for design inFIGS. 6A and 6B. In addition, carrier extraction time from the active region to injector is reduced since the active region and injector states located below the active region level2overlap with each other (these levels are close to each other in energy space, i.e. close to anticrossing, and the extraction barrier is thin, leading to large active region injector coupling). Simultaneous realization of vertical laser transition and fast carrier extraction from the active region to injector in a broad bias range leads to lower laser threshold current density and increased dynamic range. This translates to higher optical power and wall plug efficiency for lasers based on bias independent design principles.

Design stability can be further improved by reducing the thickness of the first and second active region barriers (FIG. 7) relative to the extraction barrier and the injector barriers, while keeping the active region and injector quantum wells thicknesses approximately the same. In this case, average lower laser level wavefunction tends to shift towards the first large active region quantum well that is located between the first and the second active region barriers. This leads to more vertical laser transition and better design stability.

However, for the same material composition, i.e. for the same potential barrier height, employment of thin first and second active region barriers reduces laser transition energy. For short wavelength QCLs (i.e. the wavelength is less than 5 μm), thin means the first and second active region barriers are less than eleven Angstroms. The reduction in transition energy can be compensated using higher strain material with larger potential barrier height that in turn increases laser transition energy. In addition to improved design stability, employment of high barriers with thin first and second active region barriers also suppresses carrier leakage from the upper laser level. Thermal carrier leakage from the upper laser level4to continuum states located above the barriers (designated as ‘C’ inFIG. 7) reduces laser population inversion at a given pumping current density and therefore increases laser threshold current density. Another carrier leakage path is due to carrier excitation to level5, located in the active region right above level4, and subsequent scattering to states other than level4. Employment of higher potential barrier increases energy spacings EC4and E54and therefore suppresses thermal carrier leakage to these states. (EC4and E54are defined inFIG. 7.)

It is generally considered that an increase in barrier height also increases linewidth γ43of the laser transition. As described in Reference 4 (citation provided below), the proportionality of the linewidth γ43is given by:
γ43˜EC2·Δ2·Λ2·Σ(Ψ42(zk)−Ψ32(zk))2(Formula 1)

where ECis conduction band offset (barrier height), Δ is interface roughness length, Λ is roughness correlation length, Ψ is electron wavefunction, zkis interface location, and summation is done over all interface k where either the upper or lower laser level wavefunction is non-zero. Laser threshold current density is inversely proportional to linewidth. Therefore, increase in linewidth for a composition with higher barriers can offset laser performance improvements due to suppressed carrier leakage from the upper laser level and better design stability.

Formula 1 shows that interfaces where one of the squared laser levels wavefunctions reaches maximum and the other one minimum have the largest contributions to linewidth broadening. First, this means that to minimize linewidth the laser transition should be vertical in space, i.e. the upper and lower laser levels wavefunctions should be both tightly bound in the active region.FIG. 8shows a conduction band diagram of an exemplary QCL structure with a vertical laser transition. This structure is based on the Non Resonant Extraction (NRE) design approach claimed in U.S. Patent Application Publication 2009/0213890. At the interfaces of the first and the second active region barriers the upper laser level squared wavefunction inFIG. 8reaches maximum and the lower laser level squared wavefunction is at minimum. Therefore, the four interfaces (two for each barrier) are expected to contribute most to linewidth broadening.

Formula 1 was derived assuming that thicknesses of the layers at hand are larger than length of interface roughness Δ. Typical thicknesses for the first and the second active region barriers are in the range of two to five monolayers (six to fifteen Angstroms). Depending on epi-growth quality, a barrier thickness of two to five monolayers is comparable to, or even less than, total roughness at both interfaces of a barrier 2Δ. In other words, the whole barrier thickness d may be irregularly graded as shown inFIG. 9. In this case, Formula 1 cannot be directly applied to calculate laser transition linewidth since it was derived assuming d>2Δ. In particular, Formula 1 cannot predict dependence of laser transition linewidth on thicknesses of the first and the second barriers. However, one may expect that since the whole thickness of the barrier may be graded, linewidth could be reduced employing thinner barriers since this reduces overlap between the laser levels wavefunctions and the graded barriers.

To demonstrate that narrow electroluminescence (EL) can be achieved for high strain QCL structures with the thin first and second active region barriers, we experimentally characterized the following three structures with laser emission at ˜4.6 μm. For a base level performance we used the NRE structure shown inFIG. 8that is based on Al0.64In0.36As/In0.67Ga0.33As composition with strain of −1.1% in the AlInAs layers and strain of 1.0% in the InGaAs layers. Structures #2and #3are based on Al0.78In0.22As/In0.72Ga0.28As (strain of −2.0% and 1.3%) and Al0.85In0.15As/In0.75Ga0.25As (strain of −2.5% and 1.5%) compositions, respectively.

Each of the three structures is bias independent, designed using non resonant extraction principles and grown and processed under the same conditions. Band offsets, first and second active region barriers thicknesses, and measured EL FWHM (full width at half maximum) for the three structures are summarized inFIG. 11.

Laser transition increase due to higher band offset in Structures #2and #3was compensated by transition energy reduction due to narrower first and second active region barriers. As a consequence, the total nominal thickness of the first and the second active region barriers progressively reduces from 25 Angstroms for Structure #1to 19 Angstroms for Structure #2to 17 Angstroms for Structure #3.

The data inFIG. 11show that EL FWHM reduces from 26 meV for Structure #1down to 20 meV for Structure #3, despite ˜50% increase in potential barrier height for Structure #3. This is consistent with the assumption that lower laser wavefunctions overlap with the barriers leads to narrower linewidth.FIG. 11data also demonstrate that this effect can be stronger than linewidth broadening due to larger potential barriers. Therefore, in addition to a better design stability and suppressed carrier leakage, high strain QCL structures with ultra thin first and second active region barriers also offer advantage of narrow laser transition linewidth.

Improvement in laser transition linewidth directly translates to improvement in laser characteristics.FIG. 10compares light vs. current characteristics for QCLs with the same dimensions processed from Structure #2(the upper line) and Structure #1(the lower line). Threshold current density for Structure #2is approximately 20% lower than that for Structure #1. These data are in accordance with the fact that structure #2has ˜20% narrower linewidth and that threshold current density is inversely proportional to linewidth.

In conclusion, we have presented QCL design methods that improve laser design stability with bias change, reduce laser transition electroluminescence, and improve laser temperature characteristics. QCLs based on a high strain composition with narrow first/second active region barriers and designed using bias independent design principles have superior performance compared to QCLs engineered using traditional design approaches.

While the present invention has been described with regards to particular embodiments, it is recognized that additional variations of the present invention may be devised without departing from the inventive concept.

REFERENCES

INDUSTRIAL APPLICABILITY

This invention may be industrially applied to the development, manufacture, and use of quantum cascade lasers.