High-temperature operation of broadband bidirectional terahertz quantum-cascade lasers

Terahertz quantum cascade lasers (QCLs) with a broadband gain medium could play an important role for sensing and spectroscopy since then distributed-feedback schemes could be utilized to produce laser arrays on a single semiconductor chip with wide spectral coverage. QCLs can be designed to emit at two different frequencies when biased with opposing electrical polarities. Here, terahertz QCLs with bidirectional operation are developed to achieve broadband lasing from the same semiconductor chip. A three-well design scheme with shallow-well GaAs/Al0.10Ga0.90As superlattices is developed to achieve high-temperature operation for bidirectional QCLs. It is shown that shallow-well heterostructures lead to optimal quantum-transport in the superlattice for bidirectional operation compared to the prevalent GaAs/Al0.15Ga0.85As material system. Broadband lasing in the frequency range of 3.1–3.7 THz is demonstrated for one QCL design, which achieves maximum operating temperatures of 147 K and 128 K respectively in opposing polarities. Dual-color lasing with large frequency separation is demonstrated for a second QCL, that emits at ~3.7 THz and operates up to 121 K in one polarity, and at ~2.7 THz up to 105 K in the opposing polarity. These are the highest operating temperatures achieved for broadband terahertz QCLs at the respective emission frequencies, and could lead to commercial development of broadband terahertz laser arrays.

given bias field to avoid instabilities due to electric-field domain formation. Dual-color lasing was also realized in ref. 11 due to two separate intersubband radiative transitions within the same quantum-wells of the superlattice. However, it was argued that such a lasing characteristic is detrimental to the QCL operation, and it is better to suppress multi-photon transitions from the same wells to maintain stable electrical operation in the QCL 12 . A terahertz QCL with large bandwidth was also realized by using strong tunnel-coupling for injection transport in a two-well design 13 . However, such a strategy is less likely to work for resonant-phonon design schemes, which suffer from large parasitic currents if thin injector barriers (large tunnel-coupling) are used.
One of the unique features of QCLs is that they could be designed for lasing independently at two different frequencies for positive and negative electrical bias respectively 14 . This is possible because of the unipolar carrier transport in intersubband lasers. The gain bandwidth for lasing in resonant-phonon terahertz QCLs is typically in the range of 0.2-0.5 THz. When properly designed, bidirectional QCLs could potentially increase the gain bandwidth to ~1 THz without compromising the performance characteristics significantly, since entire gain is available for a given operating polarity. There have been very few reports of bidirectional terahertz QCLs. The first bidirectional terahertz QCLs were demonstrated with a four-well GaAs/Al 0.15 Ga 0.85 As design scheme with one-well injector and resonant-phonon depopulation 15,16 . In this work, we show that improved performance could be realized for bidirectional QCLs when they are implemented with the more robust three-well design scheme; however, in the following, it is argued that shallow-wells (with 10%-Al barriers) need be employed to achieve the design flexibility that is needed for achieving gain at separate frequencies in opposing polarities. For such designs, the maximum operating temperature is better or at-par with that of any other previously developed broadband terahertz QCLs. Shallow-wells also lead to lower threshold current-densities compared to the threshold current-densities in three-well terahertz QCLs that are predominantly based on 15%-Al barriers 17 , which is an additional benefit in the presented designs.

Results
Design parameters for three-well bandstructure. To understand the importance of utilizing shallow-wells in a three-well bidirectional QCL design as proposed here, it is instructive to know the typical design parameters for three-well terahertz QCLs with best temperature performances, which are listed in Table 2. QCLs with variable-height barriers 18,19 (that are still primarily based on GaAs/Al 0.15 Ga 0.85 As heterostructures) are not included in the table since their design parameters are similar to that in ref. 3. Arguably, the two most important design parameters for such resonant-phonon designs are the tunneling parameters Δ inj and Δ col , which refer to the energy-splitting (anticrossing) between the tunneling subbands when they are resonantly aligned for injection and extraction respectively. As is evident from the compilation in the table, optimum transport is achieved for Δ inj ~ 2-2.5 meV and Δ col ~ 4-5 meV. The degree of diagonality of the radiative transition is represented by the oscillator-strength f osc ; however, f osc primarily impacts the operating current-densities 20 while not affecting the maximum operating temperature T max 3 sensitively. In general, the coupling strength needs to be reduced by utilizing thicker tunnel barriers for more "vertical" (large f osc ) designs lest the low-bias parasitic current-density becomes too large to allow the superlattice to reach the required bias alignment for lasing. Figure 1 shows a hypothetical design for a three-well bidirectional terahertz QCL structure in GaAs/Al 0.15 Ga 0.85 As. It is designed for optical gain centered around frequencies of 3.1 THz (E 43 ~ 12.9 meV) and 3.7 THz (E 43 ~ 15.4 meV) in positive and negative polarity operation respectively. The wide-well serves as the injector-well in both polarities; however, the role of the two narrower wells is interchanged in opposing polarities, which bring the required asymmetry to the design. Similarly, the roles of injection and extraction tunnel barriers is interchanged with the operating polarity. The purpose of the design in Fig. 1 is to show that when tunneling parameters Δ inj and Δ col are optimized for one  Table 1. Multi-color/broadband terahertz QCLs listed in chronological order of development (for QCLs operating without the assistance of an applied magnetic-field). Design types such as BTC, RP and SA are acronyms for active-region designs based on "bound-to-continuum", "resonant-phonon" and "scatteringassisted" schemes respectively. Key operation parameters are listed, where T max is the maximum operating temperature in pulsed operation, J th is the threshold-current density at the specified temperature, and ν is the center-frequency of the lasing spectra from a typical ridge-cavity QCL.

Shallow-wells for bidirectional resonant-tunneling.
Scientific RepoRts | 6:32978 | DOI: 10.1038/srep32978 polarity (in this case, for positive-bias as in Fig. 1a), the corresponding values in opposing polarity veer away from that optimally desired as per Table 2. This is because, in general, for designs with relatively deep wells including that with GaAs/Al 0.15 Ga 0.85 As heterostructures, the injector barrier (the 5.2 nm thick barrier) needs to be kept thicker in comparison to the extraction barrier (the 4.5 nm thick barrier) to keep ∆ ∆  inj c ol , which could only be realized for one polarity. While we did not grow and test the structure presented in Fig. 1, it will most likely perform well only in positive-polarity. In reverse-polarity the performance is likely to degrade significantly in comparison owing to a large parasitic current (due to large Δ inj ~ 2.4 meV) and poor extraction efficiency (due to small Δ col ~ 2.7 meV).
Broadband bidirectional QCL (Design A). The undesired asymmetry for tunneling parameters in opposite polarities for designs based on GaAs/Al 0.15 Ga 0.85 As could be avoided by utilization of shallow-well heterostructures. With deep-wells, the extraction barrier has to be thinner than the injection barrier for aforementioned reasons. However,  Table 2. Key design and operation parameters of previously reported terahertz QCLs based on three-well resonant-phonon design scheme (all listed QCLs are in the GaAs/Al 0.15 Ga 0.85 As material-system). QCLs emitting in the range of ~3-4 THz with T max > 150 K are listed in chronological order of publication. Δ inj is the energy splitting (anticrossing) at injection resonance, Δ col is the corresponding value for extraction resonance, and f osc represents the normalized radiative oscillator-strength at design bias. for shallow-wells, the extraction barrier needs to be made thicker than usual, because now the extraction subbands are energetically located closer to the continuum in the injector well, which requires use of a thicker-than-usual tunnel barrier to maintain the desired tunnel coupling Δ col . Consequently, for a shallow-well QCL design, the thickness of the injection and extraction barriers could be kept similar while still maintaining desired Δ inj and Δ col . Figure 2 shows the re-designed three-well bidirectional terahertz QCL structure with a GaAs/Al 0.10 Ga 0.90 As superlattice. The barrier height is now lowered to ~90 meV from ~135 meV for GaAs/Al 0.15 Ga 0.85 As. As can be seen from the layer thicknesses, the injection and extraction barriers are now of the same thickness (6.5 nm), and consequently, the tunneling parameters Δ inj ≡ Δ 1′4 and Δ col ≡ Δ 32 could be kept similar for both polarities. The design in Fig. 2 (labeled BIDR3W160, or "Design A") was realized with smaller tunnel couplings compared to that in Table 2, to keep the parasitic leakage current small. The parasitic leakage in general increases for such shallow-well designs due to carrier-scattering into the energy continuum over the barriers 17 . Design A was the first trial for a bidirectional three-well terahertz QCL, hence, the radiative frequencies were made only slightly different in opposite polarities, centered around 3.0 THz and 3.4 THz for positive and negative polarity operation respectively, which allows a design with almost similar design parameters for either polarity including f osc . Figure 3 shows light-current (L-I) and current-voltage (I-V) curves for a representative ridge-cavity QCL for Design A in both polarities. The L-I were recorded with a room-temperature pyroelectric detector (model number: Gentec THz 2I-BL-BNC with THz-WC-13), and the absolute power was calibrated using a thermopile power meter (model number: Scientech AC2500 with AC25H) as is reported without any corrections to the detected signal. A winston-cone was used in front of the laser's facet for power collection. This QCL operated up to T max ~ 147 K with J th ~ 400 A/ cm 2 low-temperature threshold current-density in positive polarity, and up to T max ~ 128 K with J th ~ 420 A/cm 2 in negative polarity. Representative spectra for both polarities are also shown as insets; the typical frequency coverage was from 3.1-3.5 THz in positive polarity, and from 3.3-3.7 THz in negative-polarity. The highlights of the experimental results are the wide combined spectral coverage from 3.1-3.7 THz from a single QCL, as was originally intended from such a design. Secondly, the use of shallow-wells leads to low threshold current-densities, compared to typical values for such three-well terahertz QCLs (as in Table 2). The reduced operating current-density is due to weaker interface-roughness scattering in shallow-well superlattices that utilize thicker tunnel barriers 17 , and is a useful practical feature to keep low electric power dissipation for such QCLs in operation. These characteristics are realized without a significant degradation in maximum operating temperatures, which are better than that from all other reports of previously reported broadband terahertz QCLs (as in Table 1).

Dual-color bidirectional QCL (Design B).
To show the flexibility of the three-well resonant-phonon design scheme for bidirectional operation at any desired frequencies, a second design was implemented in which the radiative frequency for opposite polarities was significantly separated by ~1 THz. The design (labeled BIDR196B, or "Design B") is shown in Fig. 4. As with Design A, the injector and the extraction barriers are of the same thickness (6.22 nm) due to the choice of shallow-well heterostructures, which leads to similar tunneling parameters Δ inj and Δ col for either polarity. The increased asymmetry in the radiative frequency is achieved by increasing the asymmetry in the thickness of of the two active-region wells. If the tunneling parameters Δ inj and Δ col have to be kept same, the asymmetry is then reflected primarily in the radiative oscillator strength f osc , which now becomes different for opposing polarities. However, as mentioned previously, f osc affects the temperature performance less sensitively and hence, different f osc values do not limit the available design flexibility for bidirectional operation at desired frequencies.
The experimental results from a representative QCL (wafer VB0788) for Design B are shown in Fig. 5. The QCL emitted from 3.4-3.8 THz and operated up to a maximum temperature of 121 K with low-temperature J th ~ 360 A/cm 2 in positive-polarity operation, and emitted from 2.6-2.8 THz and operated up to 105 K with low-temperature J th ~ 700 A/cm 2 in negative-polarity. The higher operating current-density and lower T max for negative-polarity operation is due to large parasitic leakage current in the superlattice that is a characteristic of lower radiative-frequency in this polarity. In general, high parasitic current-densities are a characteristic of three-well resonant-phonon QCLs, especially when designed for operation below 3 THz.

Discussion
In conclusion, we have demonstrated high-temperature operation of broadband/dual-color terahertz QCLs based on the bidirectional three-well resonant-phonon superlattices. Designs based on three-well periods are robust with respect to growth fluctuations and are easier to optimize due to fewer variable parameters in comparison to bandstructures with greater number of quantum-wells. Design flexibility is demonstrated by developing a QCL with wide spectral coverage across a bandwidth of 0.7 THz centered around 3.5 THz for one QCL, whereas dual-color operation is demonstrated for a second bidirectional QCL with ~1 THz frequency separation when the QCL is operated in opposing polarities. The maximum operating temperature is better than that of previously reported broadband and multi-color terahertz QCLs, which were all developed with more number of quantum-wells in the repeated QCL bandstructure. It is argued that the key enabling characteristic of achieving robust bidirectional operation in the three-well structures is the use of shallow-well heterostructures based on GaAs/Al 0.10 Ga 0.90 As, which allows optimization of resonant-tunneling electron-transport in either polarity. Bidirectional terahertz QCLs may be a better alternative to achieve broadband gain at higher temperatures from the same active medium when compared to QCL designs with heterogeneous cascade structures for development of broadband terahertz laser arrays on monolithic semiconductor chips, and for applications in multi-color terahertz spectroscopy and sensing.

Methods
The bidirectional QCL structure in Design A (BIDR3W160) was grown by molecular beam epitaxy (wafer VB0682) with 160 cascaded periods, leading to an overall thickness of 8 μm. The bidirectional QCL structure in Design B (BIDR196B) was grown by molecular beam epitaxy (wafer VB0788) with 196 cascaded periods, leading to an overall thickness of 10 μm. For operation in both polarities, 10 nm thick GaAs contact layers were grown with 5 × 10 19 cm −3 doping followed by 50 nm thick layers with 5 × 10 18 cm −3 doping on either side of the active superlattice for both wafers. A 300 nm thick Al 0.55 Ga 0.45 As etch-stop layer was grown as a layer preceding the entire stack. Cu-Cu based metallic waveguides were fabricated using standard terahertz QCL fabrication techniques. A sequence of Ta/Cu/Au were deposited as both top (20/200/100 nm) and bottom (20/200/100 nm) metallic layers. Fabry-Pérot ridge cavities were processed by wet-etching using H 2 SO 4 :H 2 O 2 :H 2 O etchant in 1:8:80 concentration. To operate in both polarities, the ridges are processed with highly-doped GaAs contact layers left intact beneath the metal cladding layers of the cavities. Fabricated devices were cleaved and indium soldered on a copper mount, wire-bonded and mounted on the cold-stage of a Stirling cryocooler for characterization.