Quantum dot cascade laser
© Zhuo et al.; licensee Springer. 2014
Received: 12 February 2014
Accepted: 12 March 2014
Published: 25 March 2014
We demonstrated an unambiguous quantum dot cascade laser based on InGaAs/GaAs/InAs/InAlAs heterostructure by making use of self-assembled quantum dots in the Stranski-Krastanow growth mode and two-step strain compensation active region design. The prototype generates stimulated emission at λ ~ 6.15 μm and a broad electroluminescence band with full width at half maximum over 3 μm. The characteristic temperature for the threshold current density within the temperature range of 82 to 162 K is up to 400 K. Moreover, our materials show the strong perpendicular mid-infrared response at about 1,900 cm-1. These results are very promising for extending the present laser concept to terahertz quantum cascade laser, which would lead to room temperature operation.
42.55.Px; 78.55.Cr; 78.67.Hc
KeywordsQuantum dot Quantum cascade laser MBE Mid-infrared
Quantum cascade lasers are semiconductor laser sources based on intersubband transitions in multiple quantum well systems . Their unique operation principle and good performance have established themselves as the leading tunable coherent semiconductor source in the infrared and terahertz ranges of the electromagnetic spectrum [2–10]. Although quantum cascade lasers have experienced rapid development, several drawbacks still exist. First of all, the intersubbands transition nature leads to relatively narrow gain spectrum and, consequently, narrow spectrum tunability . Moreover, due to intersubband selection rules, the emitting light is polarized in the growth direction, which makes surface emission impossible. Another drawback is that due to numerous in-plane scattering paths that the electrons undergo and decrease the upper lasing state lifetime, the threshold current is increased and the wall plug efficiency is decreased [12–17]. An appealing and ambitious route to tackle these difficulties is to explore quantum dot cascade laser (QDCL) [17, 18], by substituting the quantum wells (QWs) in the active region with self-assembled quantum dots (QDs).
The development of QDCL using self-assembled QDs as substitute for QWs in the active region faces two challenges: (1) the QDs' size and controllability, implying the effective of three-dimensional (3D) quantum confinements, i.e., the prerequisite of realizing the ‘phonon bottleneck’ effect and (2) the adjustable energy levels, which satisfy critical requirements of injection and extraction efficiency. Here, our design targets precisely these challenges: first, two-step strain compensation mechanics using InGaAs/GaAs/InAs/InAlAs material system can realize controllable InAs QDs on tensile-strained InAlAs layers; second, the population inversion is achieved between lower levels of coupled InAs QDs and upper hybrid QW-dominated lasing states.
Figure 1 depicts the growth mechanics of coupled InAs QDs in the QDCL wafer. In order to restrain the appearance of unavoidable InAs quantum dashes on In0.53Ga0.47As, In0.52Al0.48As, and In0.53Al0.24Ga0.23As layers lattice-matched to InP substrate, the InAs QDs are grown on tensile-strained In0.44Al0.56As and caped by GaAs to increase the lattice mismatch between InAs and embedding materials system. This GaAs/InAs(QDs)/In0.44Al0.56As triple layer is a QDs-embedded composite layer which is partially strain-compensated, but still tensile-strained as a whole. This approach points out that the distillation of the first step of the two-step strain compensation mechanics brings on two advantages: the feasible route for forming self-assembled InAs QDs and the flexibility in quantum engineering. The second step of two-step strain compensation mechanics is using In0.6Ga0.4As layers to compensate the QDs-embedded composite layers in active region and using In0.6Ga0.4As/In0.44Al0.56As layers in the injection/collection regions, aiming at strain compensation in one period of QDCL.
The QDCL structure was grown by molecular beam epitaxy (MBE) combined with metal-organic chemical vapor deposition (MOCVD). The epitaxial layer sequence starting from the n-doped InP substrate was as follows: 1.3 μm InP cladding layer (Si, 2.2 × 1016 cm-3), 0.3-μm-thick n-In0.53Ga0.47As layer (Si, 4 × 1016 cm-3), 30 QDCL stages, 0.3-μm-thick n-In0.53Ga0.47As layer (Si, 4 × 1016 cm-3), 2.5 μm upper cladding (Si, 2.6 × 1016 cm-3), and 0.6 μm cap cladding (Si, 1 × 1019 cm-3). The active core of QDCL is based on a bound-to-continuum design. The layer sequence, with four material compositions, starting from the injection barrier is as follows (in angstroms, and InAs in monolayer (ML)): 44.1/13.7/14.7/28.7/9.6/4.71ML(InAs)/15.8/25.3/8.4/4.15ML(InAs)//16.8/22.4/7.5/3.68ML with In0.44Al0.56As in bold, In0.6Ga0.4As in regular, GaAs in bold and italic, and InAs QD layer in italic style, and underlined layers correspond to the doped layers (Si, 1.5 × 1017 cm-3). Only InP was grown by MOCVD. For InAs QDs, the nominal growth rate was 0.41 ML/s, and the substrate temperature was kept at 510°C during MBE growth. After the QD layer was deposited, 30 to 60 s of ripening time was given under As4 protection.
The wafer was processed into double-channel ridge waveguides using conventional photolithography and wet chemical etching. The detail of fabrication is identical to . The average core width is 16 μm, and the waveguides were cleaved into 3-mm-long bars. The laser spectral measurements were carried out using two Fourier transform infrared (FTIR) spectrometers (Bruker Equinox 55 Bruker Corporation, Billerica, MA, USA; and Nicolet 8700, Thermo Fisher Scientific, Hudson, NH, USA). The emitted optical power from laser was measured with a calibrated thermopile detector placed directly in front of the cryostat with a corrected collection efficiency of 15%.
In order to demonstrate the role of QDs in the active region further, we also performed the subband photocurrent measurements. The wafer was processed into circular mesa with a diameter of about 340 μm using conventional photolithography and wet chemical etching. The etch depth was down to the substrate. The Ti/Au ohmic contact was deposited onto the top surface of the circular mesa as the top contact with a diameter of 170 μm and onto the upper surface of the substrate as the bottom contact. For the PC measurements, the incident light, namely, the infrared (IR) beam from the FTIR spectrometer, was perpendicular to the mesa upper surface; and for our structure on the mesa upper surface, the area exposed to the light occupies about 75% of the total area.
Results and discussion
We present here a novel design to form upper hybrid QW/QD lasing states and lower pure QD lasing states to realize the ‘phonon bottleneck’ effect. A general scheme of the electron energy versus in-plane wave vector relations is shown in Figure 3b. Although the states still have free particle-like dispersion skeleton in the direction parallel to the layers, the lateral quantum confinement breaks the subbands into quasi-continuous or discrete states. The upper hybrid subband (consists of hybrid-quantized states of QWs and QDs) is quasi-continuous, but the lower QD subband consists of widely separated in-plane energy states due to the lateral confinement of QDs. An electron in the upper quasi-continuous subband which relaxes to lower quantized states is difficult to obtain due to lack of appropriate final states. As a consequence, the relaxation time for the single-phonon process is increased. This implies that the nonradiative LO-phonon-assisted electron relaxation time in a QD is enhanced by a factor that depends on the lateral size of the QD. Figure 3c depicts the relevant energy levels and the electron injection/extraction sketch.
Figure 4c shows the light power (L) versus current (I) characteristics of laser for different heat sink temperatures. A peak optical power of more than 140 mW at 82 K was measured, with a threshold current density of about 4 kAcm-2. The large threshold current density may stem from a number of factors, including the broad gain spectrum, the energy misalignment between injector and bound state 9, electron leakage to higher spurious states, over-discrete and inhomogeneous lower energy states due to size inhomogeneity of QDs, possible parasitical bound state between states 9 and 8, extraction efficiency of electron from low miniband not optimized, and thermal backfilling. Figure 4d shows the temperature dependence of the threshold current for another typical laser. A T0 value of 400 K is obtained within the temperature range of 82 to 162 K. This relative high T0 is also the inherent characteristic of QDs-based lasers [29–31].
In conclusion, we believe that the reported structure does show quantum dot characteristics from the AFM, TEM, EDS, EL, T0, and PC measurements and to some extent, limited phonon bottleneck effects. Moreover, by improved design of the QDs-based active region of our device, in particular, aiming at the controllability on QDs size and smart two-step strain compensation, we also believe that the overall performance of QDCLs will be a great leap forward. What is more, our QDCL design concept can be transplanted to terahertz quantum cascade laser design, paving a new way for room temperature operation.
atomic force microscopy
energy dispersion spectra
Fourier transform infrared
molecular beam epitaxy
metal-organic chemical vapor deposition
quantum dot cascade laser
transmission electron microscopy
This work was supported by the National Research Projects of China (Grant Nos. 2013CB632800, 60525406, 60736031, and 2011YQ13001802-04).
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