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Abstract
We demonstrate a high power InP-based quantum cascade laser (QCL) (λ ∼ 9 µm) with high characteristic temperature grown by metalorganic chemical vapor deposition (MOCVD) in this article. A 4-mm-long cavity length, 10.5-µm-wide ridge QCL with high-reflection (HR) coating demonstrates a maximum pulsed peak power of 1.55 W and continuous-wave (CW) output power of 1.02W at 293 K. The pulsed threshold current density of the device is as low as 1.52 kA/cm2. The active region adopted a dual-upper-state (DAU) and multiple-lower-state (MS) design and it shows a wide electroluminescence (EL) spectrum with 466 cm−1 wide full-width at half maximum (FWHM). In addition, the device performance is insensitive to the temperature change since the threshold-current characteristic temperature coefficient, T0, is as high as 228 K, and slope-efficiency characteristic temperature coefficient, T1, is as high as 680 K, over the heatsink-temperature range of 293 K to 353 K.
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1. Introduction
Long-wavelength infrared (LWIR, λ = 8–12 µm) quantum cascade lasers (QCLs) [1–3] are widely applied in chemical and biological sensing [4–6], infrared spectroscopy [5,7], infrared countermeasures [8], and free-space communications [9], because they correspond to atmospheric window and molecular fingerprint absorption bands. Although the room temperature continuous-wave (CW) operation of LWIR QCLs was early achieved in 2002 [10–12], the development of high power LWIR lasers has been dropped behind because of the technological challenges, like the short lifetime of the upper laser level, the increased nonradiative leakage of the injection carrier, low voltage efficiency, and high waveguide losses [13–16]. Recently, LWIR-QCLs are widely concerned again due to their great commercial value and many related distinguish researches. For years molecular beam epitaxy (MBE) grown LWIR QCL has held the dominant position in the performance of the lasers [2,17,18], because it can epitaxy sharp interfaces and control the layer thickness precisely. However, it is very difficult for MBE to satisfy industrial demands for the reasons of the low production capacity and high cost. Therefore, metalorganic chemical vapor deposition (MOCVD) becomes a competitive choice for III-V semiconductor manufacturing in industrial environments due to its large capacity, high efficiency and low cost. In addition, the quality of MOCVD grown hetero-interfaces is comparable to that of the MBE grown samples for many QCL practical applications [19,20]. Many groups have carried out researches on MOCVD grown LWIR QCL and achieved significant successes [1,3,21–25]. However, few groups can achieve a light source which maintains high power and high characteristic temperatures simultaneously to work in extreme environments. For example, although QCL emitting at 8.7 µm with dual-upper-state (DAU) design was reported to obtain fairly high characteristic temperatures by Fujita [26,27], the power of the device was less than 400 mW in CW mode at room temperature. In addition, although M. Troccoli et al. [24,28] and F. Xie et al. [1] used MOCVD to grow QCLs with relatively high CW power at 8.9 µm and 10 µm, respectively, the performance of their lasers degraded at high temperatures. Moreover, Botez et al. achieved both high pulse and CW power with high characteristic temperatures for QCLs at 8µm [3,25,29], but the multi-component active area design in their work can be inconvenient for industrial production.
Therefore, we draw on the idea of using DAU and multiple-lower-states (MS) design to achieve high population inversion, and adapted the diagonal transition design to achieve high power and high characteristic temperatures. A 4-mm-long cavity length, 10.5-µm-wide ridge QCL grown by MOCVD with high-reflection (HR) coating demonstrates a maximum pulsed peak power of 1.55 W and CW output power of 1.02W at 293 K. Performances of the device have great temperature stability. It shows a threshold-current characteristic temperature coefficient, T0, as high as 228 K, and slope-efficiency characteristic temperature coefficient, T1, as high as 680 K, over the 293K-353 K heatsink-temperature range. In addition, devices with the proposed active region design exhibit a 466 cm−1 (57.8 meV) wide electroluminescence (EL) spectrum.
2. Material design and epitaxy
The active region of the QCL was specially designed to achieve high output power, broad gain, and high characteristic temperatures simultaneously in this work. The structure of the active region includes 50 stages of strain-compensated In0.593Ga0.407As/In0.362Al0.638As quantum wells and barriers. The layer sequence of one period, starting from the injector barrier is as follows (thickness in nanometers): 2.8/3.0/1.25/6.27/1.0/5.35/0.88/4.85/1.09/4.16/1.22/3.66/1.42/3.15/1.68/3.3, In0.362Al0.638As barrier layers were in bold, In0.593Ga0.407As quantum well layers were in roman, and doped layers (Si, 1.5*1017 cm-3) were underlined. In addition, the active region was sandwiched by two 3.4-µm-thick InP cladding layers (Si, 2.2*1016 cm-3) and a 0.6-µm-thick cap layer (Si, 5*1018 cm-3) was deposited on the top of cladding layer.
A calculated energy diagram for QCL based on a In0.593Ga0.407As/In0.362Al0.638As is shown in Fig. 1. The structure of the active region adopt the high-CW-power design of the 6.8 µm-emitting QCL reported by Fujita et al. [30], and extended the DAU and MS design to QCL with longer wavelength at 9 µm. The main points of the active region design include the following aspects.
Fig. 1. Band diagram of QCL based on In0.593Ga0.407As/In0.362Al0.638As under an applied electric field of 48 kV/cm.
The transitions to multiple lower states from two upper states (state 4 and state 3) allow many transition channels, thus achieving broad-gain spectra [30]. By the calculation and optimization, the double upper energy level (level 4 and level 3) and the lower energy level 1 and level 2 were designed diagonally to make the electrons of the upper energy level highly localized to achieve high electrons number inversion. The lifetimes of upper states E4 and E3 in this design were 1.27 ps and 2.02 ps, respectively. Furthermore, the wave function of the upper states overlapped strongly with the injector miniband (the width is ∼78 meV). This leaded to a strong coupling of the injector ground state inj and the laser upper state, $\hslash\mathrm{\Omega}\sim6.5~\mathrm{meV}$. The designed injection efficiency was about 65%. In addition, the diagonal transition reduced the overlap between the wave functions of state E4 and E5, thus leading to a reduction in the carrier leakage from state 4 to state 5. It can significantly increase T0 and T1 of the LWIR QCLs because the carrier leakage mainly happens from the upper-laser-states to the energy states above (state 5) followed by relaxation to low-energy active-region levels for tall barrier devices like this [31].
A strain-compensated structure was adopted to increase the energy separation between the state 5 and state 4 to 66 meV. Because E54 of this structure is slightly higher than the QCL with conventional structure and comparable with the QCL with DAU design reported by Fujita et al [26,30]. The carrier leakage of the upper laser state is further suppressed. Carrier leakage is part of the total injection efficiency ηinj,tot which is the product of the tunneling-injection efficiency and the pumping efficiency ηp = 1-Jleak/Jth (Refs. 3 and 31). That is, Jth$\propto$1/(ηinj,tot, τup), so carrier- leakage suppression lowers the Jth value because of a higher total injection efficiency. Therefore, the suppression of carrier leakage is an important reason for the high characteristic temperature in the QCL structure.
The injector sheet-doping density, ns, was kept as low as 0.68*1011 cm-2 to suppress the thermal backfilling of the lower lasing level [27,31,32]. Because the temperature dependence of the Jth is partly decided by the thermal backfilling, such a low ns can increase T0 significantly.
The structures were fully grown using a low-pressure (100 mbar) MOCVD equipped with a close-coupled showerhead growth chamber [19]. Group III precursors were trimethylindium (TMIn, In (CH3)3), trimethyl-gallium (TMGa, Ga (CH3)3), and trimethylaluminum (TMAl, Al (CH3)3), and group V precursors were arsine (AsH3) and phosphine (PH3). The n-type dopant precursor was diluted monosilane (SiH4) balanced in hydrogen (0.02% in H2). The QCL wafer was grown on a n-doped (Si, 2*1017 cm-3) InP substrate. For the designed structure, the growth of the active region was performed at the thermocouple reading temperatures of ∼680 °C with V/III ratio of about 145. The growth of the waveguide and cladding layers was performed at the thermocouple reading temperatures of ∼690 °C. Growth rates of the active region (In0.593Ga0.407As/In0.362Al0.638As) were ∼0.2 nm/s, and the growth rates of waveguide and cladding layers (n-type doped InP) were ∼0.6 nm/s.
For material characterizations, the optical microscopy, atomic-force microscopy (AFM), High resolution X-ray diffraction (HRXRD), were used to characterize the morphology, structure and composition of the epitaxial layers.
The structural investigations were performed by HRXRD measurements. The XRD ω-2θ scan of this wafer and its corresponding simulation are displayed in Fig. 2(a). The HRXRD rocking curve reveals well-resolved superlattice peaks up to 15th order, which is an indication of high material quality and consistent periodicity. The calculated periodicity of the QCL stages based on the spacing of the satellite peaks is 45.08 nm, which agrees well with the nominal value of 44.69 nm. The closeness of simulated and experimental data also indicate an excellent control over layer thickness, material composition, and interface switching across the entire 50 periods layer structure. Figure 2(b) shows the enlarged view of the satellite peaks with their full width at half maximum (FWHM) labelled aside. The FWHM of the satellite peaks are from 13 to 17 arcsec, which indicate consistent periodicity through the entire structure. In addition, since the carrier leakage is mostly interface roughness (IFR) triggered [33], the high T1 which results from low carrier leakage implies small IFR of the structure. AFM is an important method to obtain surface morphology. Figures 4(c) and (d) show that step edges are smooth with kinks occurring roughly at 200 nm intervals. As can be seen, the observation of these surface steps indicates that the growth process of the epitaxial layers undergoes a step-flow mode.
Fig. 2. (a) XRD ω/2θ scan measured near InP (004) diffraction condition (blue, top) and the simulated curve (red, bottom) calculated based on the QCL epitaxial structure. (b) Partially enlarged view of satellite diffraction peaks. The full-width at half-maximum (FWHM) of satellite diffraction peaks are labelled in arcsec. AFM images (c) (5 × 5 µm2 scan) and (d) (1 × 1 µm2 scan) of the strain compensation sample grown on InP substrate.
3. Fabrication and characterization of the devices
The epitaxial wafer was fabricated into a FP cavity device which is similar to those previously reported [34]. Firstly, double channels were etched and filled with semi-insulating InP:Fe for better heat dissipation around the ridge. Then a 450nm-thick SiO2 layer was deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) for insulation, and a Ti/Au layer was deposited by electron beam evaporation for electrical contact. An additional 4µm-thick gold layer was electroplated to further improve heat dissipation. And a Ge/Au/Ni/Au metal contact layer was deposited on the substrate which was thinned to 140 µm. The waveguides were then cleaved into 4-mm-long bars. The back facet was high-reflectance (HR) coated with ZrO2/Ti/Au/Ti/Al2O3 and the front facet was uncoated for the measurement of edging emitting power. The double channel ridge structure with an average ridge width of 10.5 µm was fabricated by photolithography and wet chemical etching. Due to the anisotropy of two-dimensional materials, the device with the above ridge structure had higher transverse resistivity and less leakage and was beneficial to reduce electrical loss. To further improve the heat dissipation and achieve higher output power, the device was mounted epi-side down on the diamond heatsink with indium, which was subsequently soldered on copper heat sinks. The output power of the device was measured by a calibrated thermopile detector with a collection efficiency of near 100%. The spectral characterization was performed by a Fourier transform infrared spectrometer (FTIR) equipped with a deuterated triglycine sulphate (MCT) detector with a resolution of 0.125 cm-1 in rapid scan mode. The EL spectrum was measured in pulsed mode (2 µs, 50 kHz) at various voltages and a temperature of 300 K using a FTIR spectrometer in step-scan mode, together with a cryogenically cooled MCT detector and a lock-in amplifier.
4. Experimental results and analysis
The Light-current-voltage (L-I-V) curve for the device is shown in Fig. 3. The current was kept lower than the roll-over to prevent the device from burning out. Within the tested current range, Fig. 3(a) shows that the CW working voltage range was from 10.1 V to 11.88 V. The maximum CW optical output power reached to 1.02W and the threshold current density (Jth) was 1.88 kA/cm2 at 293 K. At the same temperature, the maximum slope efficiency was about 0.95 W/A and the maximum wall-plug-efficiency (WPE) was 5% at the current of 1.53 A. To characterize the temperature performance of the device, the CW output power of the laser was tested from 293 K to 353 K. The CW output power maintained at 250 mW and the WPE was 1.5% at 353 K. In addition, the 8.1% maximum WPE, 1.51 W maximum peak power and a lower Jth of 1.52 kA/cm2 were obtained in pulsed mode for the QCL at 293 K in Fig. 3(b). Since the highest front-facet WPE of the 8-11 um MOCVD-grown QCL which hold potential to CW work is 10.6% for λ∼ 8 µm QCL [35], the 8.1% front-facet WPE in this research is relatively high for MOCVD-grown λ∼ 9 µm QCL. When the temperature increased to 353 K, the maximum peak power and the pulsed WPE decreased to 0.86 W and 4.5%, respectively.
Fig. 3. (a) The L-I-V and WPE experimental results with different temperatures in CW operation. The right colored solid line represents the power curve, and the right colored dashed line represents the WPE curve. (b) The L-I-V and WPE experimental results with different temperatures in pulse operation (480 ns, 20 kHz). The right colored solid line represents the power curve, and the right colored dashed line represents the WPE curve. (c) Threshold current density and slope efficiency around threshold as functions of heat-sink temperature in pulsed operation (480 ns, 20kHz). (d) Emission spectrum at current slightly above threshold at 293 K. The inset shows the two-dimensional intensity distribution of the laser beam.
Based on the measured L-I-V curves, the threshold current density and slope efficiency of the device with different temperatures were extracted in Fig. 3(c). The pulsed characteristic temperatures of the threshold-current density (T0) and slope efficiency (T1) can be calculated using the usual exponential fits as follows:
where T is the heat sink temperature, $\textrm{J}_{th}$ is the threshold current density, $\eta$ is the slope efficiency. The characteristic temperature T0 and T1 are determined to be 228 K and 680 K, respectively. The value of T1 of the device is more than 150 K higher than the MOCVD grown 8 µm QCL reported in Ref. [3] and Ref. [35] while T0 of the device is comparable to the devices in both papers. The high T1 results from the efficient carrier-leakage suppression by the diagonal transition structure and large E54 of the devices. The high T0 also indicates that the thermal backfilling of lower-laser-states is effectively reduced.Figure 3(d) shows the device lasing spectrum at room temperature. Emission spectrum centering at 1103cm–1 (≈9.0 µm) was obtained by FTIR with 0.125 cm-1 resolution in rapid scan mode. The inset shows the two-dimensional intensity distribution of the laser beam at a distance of 580 mm from the lens. It can be clearly seen that the spot in the far field is a standard fundamental transverse mode.
In order to measure the EL spectra of the device, the back facet of the strip cavity was destroyed to avoid possible feedback in the cavity [36]. EL spectra measured in pulsed operation (50 kHz, 2 µs) with various voltage at 300 K are shown in Fig. 4. It shows a side peak and a main peak simultaneously. The side peak may come from the unequal oscillator strength of the transition between the two upper-laser-states to the lower-laser-states [30]. By fitting the raw data with multiple Lorentzian peaks, the FWHM of the main peak was extracted to be the FWHM of the EL spectra and it changed from 466 cm-1 to 376 cm-1 as the voltage raised from 8.2 V to 11 V. The broad EL FWHM in this article is mainly contributed from the DAU and MS design of the active region [27,32]. the EL FWHM of the QCL in this article is 200cm-1 (25 meV) higher than the bound-to-bound QCL [27] and comparable to the previous 8.7 µm DAU device [26]. The large FWHM of the EL spectrum increases nonresonant intersubband losses and reduces peak gain, thus leading to a higher laser threshold current density [15,37]. The inset of Fig. 4 shows the subthreshold amplified spontaneous emission spectrums at different current levels. It shows a clear spectrum narrowing with increasing current and exhibits a wide FWHM of over 275 cm-1 at 0.72 Ith. Based on the subthreshold amplified spontaneous emission spectrum, we believe the proposed QCL is highly suitable for broadband tuning applications.
Fig. 4. Intersubband EL spectra of the QCL with destroyed back facet for various voltages at 300 K. The inset is the subthreshold amplified spontaneous emission spectrum of the 4 mm long, 10.5 um wide, HR coated, buried heterostructure QCL in pulsed mode (80 kHz, 3 µs) at 300 K.
5. Conclusion
In conclusion, a high power, broad gain, temperature stable LWIR QCL grown by MOCVD is demonstrated. The active region adopted a DAU and MS design to achieve high characteristic temperatures and high power. A 4-mm-long cavity length, 10.5-µm-wide ridge QCL with the proposed active region demonstrated a maximum pulsed peak power of 1.55 W and CW output power of 1.02W at 293 K. The maximum FWHM of the EL spectrum is as wide as 466 cm−1 for the device. In addition, T0 and T1 of the laser are as high as 228 K and 680 K, respectively, which indicates that the performance of the device is insensitive to temperature change.
Funding
National Key Research and Development Program of China (2020YFB0408401); National Natural Science Foundation of China (61991430); Youth Innovation Promotion Association of the Chinese Academy of Sciences (2022112); Key projects of the Chinese Academy of Sciences (YJKYYQ20190002, QYZDJ-SSW-JSC027, XDB43000000, ZDKYYQ20200006).
Acknowledgments
The authors would like to thank Ping Liang and Ying Hu for their help in device processing.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data underlying the results presented herein are not publicly available currently but can be obtained from the authors upon reasonable request.
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