We hereby offer a comprehensive analysis of various factors that could potentially enable terahertz quantum cascade lasers (THz QCLs) to achieve room temperature performance. We thoroughly examine and integrate the latest findings from recent studies in the field. Our work goes beyond a mere analysis; it represents a nuanced and comprehensive exploration of the intricate factors influencing the performance of THz QCLs. Through a comprehensive and holistic approach, we propose novel insights that significantly contribute to advancing strategies for improving the temperature performance of THz QCLs. This all-encompassing perspective allows us not only to present a synthesis of existing knowledge but also to offer a fresh and nuanced strategy to improve the temperature performance of THz QCLs. We draw new conclusions from prior works, demonstrating that the key to enhancing THz QCL temperature performance involves not only optimizing interface quality but also strategically managing doping density, its spatial distribution, and profile. This is based on our results from different structures, such as two experimentally demonstrated devices: the spit-well resonant-phonon and the two-well injector direct-phonon schemes for THz QCLs, which allow efficient isolation of the laser levels from excited and continuum states. In these schemes, the doping profile has a setback that lessens the overlap of the doped region with the active laser states. Our work stands as a valuable resource for researchers seeking to gain a deeper understanding of the evolving landscape of THz technology. Furthermore, we present a novel strategy for future endeavors, providing an enhanced framework for continued exploration in this dynamic field. This strategy should pave the way to potentially reach higher temperatures than the latest records reached for Tmax of THz QCLs.

Terahertz quantum cascade lasers (THz QCLs) are advanced semiconductor devices that emit terahertz frequency electromagnetic radiation through a quantum mechanical process known as intersubband transitions. These lasers operate in the terahertz frequency range, which lies between the microwave and infrared regions of the electromagnetic spectrum.1,2 This range of frequencies is often referred to as the “THz gap” due to the lack of widespread available effective sources. THz QCLs hold significant potential for a wide scope of applications due to their ability to generate terahertz radiation. This technology finds application in fields such as spectroscopy,3–5 imaging,5 sensing,6 astronomy,7 and communication.8 For instance, THz QCLs have shown promise in nondestructive testing,9 in security screening,10 in pharmaceutical analysis,11 and even in biomedical imaging,12,13 offering the capability to identify hidden objects and materials with remarkable precision. In recent years, researchers have been actively exploring innovative methods to enhance the temperature performance, efficiency, and portability of THz QCLs to unlock their full potential across various domains.

Since 2012, when a Tmax of ∼200 K was achieved,14 progress in the temperature performance of THz QCLs remained relatively stagnant until 2019 when a GaAs/Al0.25Ga0.75As THz QCL achieved a Tmax of ∼210 K.15 It was not until 2021 that a breakthrough was achieved, pushing the Tmax to ∼250 K,16 and subsequently ∼261 K,17 facilitating the launch of the first portable THz QCL. However, this portable device still required thermoelectric cooling (TEC), and the elevated Tmax was attainable only in pulsed operation. Worth noting is that other research groups have not reported similar Tmax values, highlighting the considerable challenge at hand.

Hence, despite their promising potential demonstrated since their first introduction in 2002,18 THz QCLs have faced limitations in terms of portability. This limitation stems from the necessity for either bulky cooling equipment, which compromises the ability to be portable and compact, or portable devices that rely solely on TEC but are unable to generate sufficient output powers in the milliwatt range. As a result, these devices remain inappropriate for numerous real-world applications. Consequently, the central objective in this field is to achieve operation at room temperature while also attaining substantial lasing output power.

To further advance the temperature capabilities of THz QCLs, a deeper comprehension of the underlying physics and the obstacles surmounted to reach the latest Tmax records16,17 is imperative. By integrating findings from the latest studies, we could open avenues for potentially uncovering fresh perspectives and gaining novel insights and offer a new strategy for future research. Here is a short summary of the progress, sequential advancements, and challenges overcome in attaining the current Tmax achievements. A central factor that was limiting the temperature performance of standard vertical-transition THz QCLs was the thermally activated longitudinal optical (LO) phonon scattering from the upper laser level (ULL) to the lower laser level (LLL).19 Overcoming this restraint necessitated the development of highly diagonal structures that significantly mitigate thermally activated LO phonon scattering.20–22 However, in these highly diagonal structures, another challenge emerged in the form of thermally activated leakage into the continuum,22 particularly when using barriers with only 15% Al content.21 A leakage of charged carriers into excited bound states23,24 persisted even with barriers containing 30% Al. Combining high barriers with thin wells successfully elevated the excited and continuum states’ energies, effectively suppressing these leakage pathways.24–26 Careful engineering led to the manifestation of clear negative differential resistance (NDR) behavior in current-voltage (I-V) curves even at room temperature,24–27 indicating the mitigation of thermally activated leakage paths. This approach ensured a clean n-level system, where transport occurs only within the active laser states, “n” representing the number of active subbands.24–26 It follows that the primary strategy has been to devise THz QCLs that support clean n-level systems, particularly at higher temperatures. This approach was instrumental in achieving the recorded highest Tmax of ∼25016 and ∼261 K,17 and it is the approach adopted in our research endeavors as well.

The designs that accomplished the remarkable Tmax of ∼25016 and ∼261 K17 employ a two-well (TW) configuration, supporting a clean three-level system. This design closely resembles prior successful variations (referred to as Design HB2 in Ref. 25) that also exhibit a clean three-level structure. Other designs that effectively suppressed thermally activated leakage pathways include a resonant-phonon (RP) design introduced in 201624 and a split-well direct-phonon (SWDP) configuration proposed in 2019.26 Nonetheless, the reasons behind disparate Tmax values among designs with similar attributes remain unclear, prompting ongoing investigations.

Our work goes beyond a mere analysis; it represents a nuanced and comprehensive exploration of the intricate factors influencing the performance of THz QCLs. By proposing a strategic approach for attaining room temperature performance, we strive to not only consolidate but to forge new pathways in the optimization of THz QCLs. Unlike prior studies, which delved into isolated facets of THz QCL optimization, our contribution lies in the synthesis of these individual findings, presenting a holistic perspective for the first time. This strategic approach brings together diverse structural considerations in our analysis, paving the way for a more profound understanding and optimization of THz QCLs. We believe this novel insight holds the potential to make a significant and lasting contribution to the advancement of this field.

Based on our research, the fundamental explanation for the lack of consistency in the performances of similar structures, such as the designs in Refs. 16 and 17 and Design HB2 in Ref. 25, relies on the interfaces’ quality.27 We conducted a systematic study of these structures using simulations based on nonequilibrium Green’s functions (NEGFs).27–33 Various scattering mechanisms, including elastic and inelastic scattering processes, are taken into account in the simulations. These mechanisms encompass ionized impurity scattering (IIS), interface roughness (IFR) scattering, and optical and acoustic phonons, as well as alloy disorders, as documented in prior studies.31,32,34,35 To calculate the gain in a self-consistent manner, we employed a comprehensive quantum-kinetic approach, following linear response theory.36 We drew the conclusion that the performance should be similar for all three designs;16,17,25 however, the experimental temperature performance is very different, and we attribute it mostly to the interface’s quality.27,37 Although the quality of the interfaces is high in Design HB2 in Ref. 25 and in previous works, it seems that the thickness fluctuations of the interfaces in Refs. 16 and 17 are of just about an atomic layer, as opposed to about one monolayer in Ref. 25, and this minimal improvement in the interfaces’ flatness has a huge impact on the temperature performance. We would like to point out that interfaces with thickness fluctuations of just one monolayer are still considered of high quality. Assuming the interfaces’ can be grown in the highest flatness, we should acknowledge the other factors that are still limiting the temperature performance of THz QCLs.

Previous investigations have pointed to the effect of doping on THz QCLs as one of the main factors still limiting their temperature performance.38–40 The impact that the doping density and its spatial position have on THz QCLs has been studied by more than one group, bringing up many controversial results. In 2016, a study that involved experimental results from a three-well RP design41 led to the conclusion that high diagonality as a strategy cannot succeed on its own and must be paired with increased doping. Tmax was found to improve significantly when the doping was increased from 3 × 1010 to 6 × 1010 cm−2. The values reported were around 135 and 177 K, respectively. Unfortunately, the improvement did not extend to the samples where the doping was multiplied four times, six times, and eight times. This was attributed to carrier induced band bending. For split-well direct-phonon designs, the results showed the contrary: Tmax decreased as the doping increased from 3 × 1010 to 6 × 1010 cm−2.38,40 Nevertheless, as doping and temperature increased, a considerable rise in gain broadening and a decrease in the dephasing time were observed. These effects were attributed to IIS. Recently, we have proposed another RP design, called the split-well resonant-phonon (SWRP) design, and unlike the previous RP schemes, the temperature performance of this design did not improve with increasing doping from 3 × 1010 to 6 × 1010 cm−2.42 However, the temperature performance appeared to be better than the one observed for SWDP designs,38,40 due to a decreased overlap between the doped region and the active laser states. The detrimental effects of the increased doping density were still attributed to significant additional line broadening due to the increased doping concentration.

It has also been noted in the past that the number of electrons in the ULL could be a potential limitation to the THz QCLs’ temperature performance.15 This is assumed to be due to the fact that optical losses primarily occur from parasitic reabsorption by free electrons engaged in transport.15 This limitation could be overcome by the engineering of the doping concentration, position, and doping region width.15 

In 2022, two different studies were published with contradicting results. On the one hand, one of the studies states that higher doping concentration gives more effective carriers through the designed structures, augments the flowing current and populations in the expected subbands, and, thus, increases the optical gain and current dynamic range.43 On the other hand, another group reached the conclusion that the highest optical gain is obtained under undoped conditions and proposed a doping method wherein doped and undoped modules appear, in turn, in a periodical structure.44 

Seemingly, the effects of doping concentration vary from design to design, and its spatial location is also crucial.44 High doping conditions, although leading to higher currents and populations, can be detrimental to the laser performance when significant band bending happens. As a result, some parameters such as oscillator strength, optical gain, and population inversion are also influenced.

In Figs. 1(a) and 1(b), we show the band diagram of two previously presented direct-phonon structures. As stated before, the record Tmax was also achieved in a direct-phonon design.16,17 Both these structures display a clear NDR signature up to room temperature.25,26 The doped regions are marked in both designs. Figure 1(a) is the band structure for a TW design.25 The design is a simple scheme with direct-phonon depopulation.25 In TW designs, the doped region is normally at the center of the well, but here, we marked the whole wide well as it is effectively doped due to dopants’ diffusion. The second design is a SWDP45 [Fig. 1(b)]. The SWDP design also has a direct-phonon depopulation scheme, with the advantage of a thin intrawell barrier that enables the manipulation of the energy separation between the LLL and the injection level. In this scheme, the width of the thin intrawell barrier can be tuned to make the energy separation the exact LO phonon energy and, thus, achieve the fastest LLL depopulation.46 As can be seen, there is a large overlap between the doped region and the active laser states in both designs. We assume the large overlap between the doped region and active laser states is a significative detrimental factor. Previous studies have also proved that the electroluminescence from two otherwise identical samples with different doping profiles shows a higher and narrower peak for the sample with a setback on the doping profile.47 

FIG. 1.

(a) Band diagram of two sequential periods termed module i (left, marked by dashed-dotted box) and module i + 1 (right) of the TW THz QCLs with Al0.3Ga0.7As barriers, corresponding to energy levels of the device. The doped region is marked in the structure. Widths of the layers are written in units of monolayers. (b) Band diagram of two sequential periods termed module i (left, marked by dashed-dotted box) and module i + 1 (right) of the SWDP THz QCLs with Al0.3Ga0.7As barriers, corresponding to energy levels of the device. The doped region is marked in the structure (by the sides of the thin intrawell barrier). Widths of the layers are written in units of monolayers.

FIG. 1.

(a) Band diagram of two sequential periods termed module i (left, marked by dashed-dotted box) and module i + 1 (right) of the TW THz QCLs with Al0.3Ga0.7As barriers, corresponding to energy levels of the device. The doped region is marked in the structure. Widths of the layers are written in units of monolayers. (b) Band diagram of two sequential periods termed module i (left, marked by dashed-dotted box) and module i + 1 (right) of the SWDP THz QCLs with Al0.3Ga0.7As barriers, corresponding to energy levels of the device. The doped region is marked in the structure (by the sides of the thin intrawell barrier). Widths of the layers are written in units of monolayers.

Close modal

In order to better understand the mechanisms still limiting the temperature performance, we conducted a theoretical study on the TW direct-phonon design utilizing simulations based on NEGF simulations.29–33 This scheme was chosen as a representative for direct-phonon designs. We calculated the contribution of the different scattering mechanisms to the optical gain by eliminating one of the mechanisms from the calculation each time. In Fig. 2, the optical gain as a function of photon energy is plotted for these different scenarios. We can see from this plot that, despite having a lesser effect than other mechanisms, IFR scattering greatly impacts optical gain. As mentioned before, based on our research, the main contribution to IFR scattering relies on the interfaces’ quality.28 We assume that also in our designs, optimizing the quality of the interfaces will lead to better temperature performance. It can also be observed that the contribution of the IIS is much more significant. We hypothesize that, by reducing the overlap between the doped region and the active laser states, the line and gain broadenings will be reduced due to reduced IIS, ultimately contributing to an improved temperature performance. The anticipated enhancement from reducing this scattering mechanism is expected to be substantially greater than the improvement expected from mitigating IFR. Additionally, we can see from Fig. 2 that the most significant contribution to the reduction in the optical gain is the electron-electron (e-e) scattering, but this mechanism cannot be manipulated.

FIG. 2.

Optical gain as a function of frequency for the TW design (representative for direct-phonon designs) displaying the contribution of the different scattering mechanisms to the optical gain.

FIG. 2.

Optical gain as a function of frequency for the TW design (representative for direct-phonon designs) displaying the contribution of the different scattering mechanisms to the optical gain.

Close modal

Based on these results, we suggested two other novel designs with a clean n-level system, with an enhanced doping profile in comparison with the TW and SWDP designs. The first one is a two-well injector direct-phonon (TWI-DP) structure [Fig. 3(a)],48 which combines both two-well injector and direct-phonon scattering schemes. The TWI-DP design keeps the direct-phonon scheme for the depopulation of the LLL, while overcoming its main disadvantages, such as the large overlap between the doped and active laser regions. Tmax observed in this device was ∼170 K, and this was achieved with a relatively high doping density. Additionally, new encouraging results were obtained recently using this same scheme after some initial optimization, which led to a Tmax of ∼235 K.17 It was noted that in this structure, the dephasing time of the resonant tunneling is not a dominant mechanism at high temperatures, as opposed to other structures previously studied.48 This underscores the effectiveness of our strategic approach in genuinely enhancing the temperature performance of THz QCLs.

FIG. 3.

(a) Band diagram of two sequential periods termed module i (left, marked by dashed-dotted box) and module i + 1 (right) of the TWI-DP THz QCLs with Al0.3Ga0.7As barriers, corresponding to energy levels of the device. The doped region is marked in the structure. Widths of the layers are written in units of monolayers. (b) Band diagram of two sequential periods termed module i (left, marked by dashed-dotted box) and module i + 1 (right) of the SWRP THz QCLs with Al0.3Ga0.7As barriers, corresponding to energy levels of the device. The doped region is marked in the structure (by the sides of the thin intrawell barrier). Widths of the layers are written in units of monolayers.

FIG. 3.

(a) Band diagram of two sequential periods termed module i (left, marked by dashed-dotted box) and module i + 1 (right) of the TWI-DP THz QCLs with Al0.3Ga0.7As barriers, corresponding to energy levels of the device. The doped region is marked in the structure. Widths of the layers are written in units of monolayers. (b) Band diagram of two sequential periods termed module i (left, marked by dashed-dotted box) and module i + 1 (right) of the SWRP THz QCLs with Al0.3Ga0.7As barriers, corresponding to energy levels of the device. The doped region is marked in the structure (by the sides of the thin intrawell barrier). Widths of the layers are written in units of monolayers.

Close modal

The second design is a highly diagonal SWRP scheme [Fig. 3(b)],42,49,50 based on the same design principles as the SWDP design previously described in Refs. 26, 32, 38, and 45. However, in the SWDP device, there is a high overlap between the doped region and the active laser states26,45 and in the new SWRP design, this overlap is reduced, and effects related to gain broadening should be lower.42,49,50 This design reached an improved Tmax of ∼131 K compared to the Tmax of ∼120 K previously achieved with the reference SWDP device (with the same barriers’ composition and similar lasing frequency).45 

Considering all the above, we can see that some initial engineering of the doping profile shows encouraging results. However, the interfaces’ quality in these designs is still not ideal. We expect results beyond the current state-of-the-art once the quality of the interfaces is improved. In offering this perspective, our work introduces a new strategic approach that focuses on optimizing both the doping profile and interface quality, paving the way for unprecedented advancements in the temperature performance of THz QCLs.

Moreover, when developing the two novel designs, the key consideration was the possible improvement due to the reduced overlap between doped and active laser areas. Given this, we believe that additional tuning and modification of the injection coupling, oscillator strength, and doping density and profile could result in improved temperature performance. Understanding the influence of the doping density, profile, and spatial position is critical, and the appropriate optimization could lead to possible improvements in the devices’ temperature operation.

In conclusion, we presented a comprehensive analysis of various factors that could potentially enable THz QCLs to achieve room temperature performance. We believe that after the right optimization of the interfaces’ quality and flatness, the next step toward the improvement of THz QCLs should be the optimization of the doping density, its spatial location, and its profile. This is based on our results from different structures, such as the SWRP and the TWI-DP schemes for THz QCLs, which allow efficient isolation of the laser levels from excited and continuum states. Additionally, the doping profile has a setback in these schemes that lessens the overlap of the doped region with the active laser states. Our research contributes significantly to the advancement of THz QCLs by providing a thorough and nuanced analysis of the multifaceted factors influencing their performance. Beyond a mere examination of individual aspects, we present a strategic approach that aims to propel THz QCLs toward achieving room temperature performance. In synthesizing and integrating the results from prior studies through a holistic lens, our work not only consolidates existing knowledge but also yields new insights. This marks a departure from the conventional exploration of isolated aspects of THz QCL optimization, as our contribution lies in drawing novel conclusions from this synthesized body of knowledge. As we pave the way for a more comprehensive understanding and optimization of THz QCLs, we anticipate that our strategic approach will set the stage for groundbreaking advancements in this field. This strategy should pave the way to potentially reach higher temperatures than the latest records reached for the Tmax of THz QCLs.

The authors would like to acknowledge the Israel Science Foundation (ISF) (Grant No. 1755/23) for its grant. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by the National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under Contract No. DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.

The authors declare no conflicts to disclose.

All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

Nathalie Lander Gower: Formal analysis (equal); Investigation (equal); Writing – original draft (equal); Writing – review & editing (equal). Shiran Levy: Formal analysis (supporting); Investigation (supporting). Silvia Piperno: Formal analysis (supporting); Investigation (supporting). Sadhvikas J. Addamane: Resources (supporting). John L. Reno: Resources (supporting). Asaf Albo: Funding acquisition (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – review & editing (equal).

The data that support the findings of the study are available from the corresponding author upon reasonable request.

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