Progress in Crystal Growth and Characterization of Materials
ReviewMetamorphic InAs(Sb)/InGaAs/InAlAs nanoheterostructures grown on GaAs for efficient mid-IR emitters
Introduction
The mid-infrared (mid-IR) spectral region contains intense fundamental vibrational molecular transitions as well as two atmospheric transmission windows of 3–5 μm and 8–13 μm, which makes it crucial for spectroscopy, gas sensing, noninvasive medical tests, security and industry [1]. Over the last couple of decades such a variety of applications has driven significant development of the mid-IR semiconductor emitters. In particular, many efforts have been focused on breaking the “3 μm barrier” at room temperature (RT) in order to reach the important wavelength range around 3.3 μm [2]. One of the main approaches to achieve this goal is to employ interband lasers with a type-I active region. First attempts to achieve continuous wave (cw) operation beyond 3 μm have been performed on GaInAsSb/AlGaAsSb quantum wells (QWs) [3]; however, the threshold current density of such lasers was three times higher as compared to those emitting at 2.24 μm. Shterengas et al. suggested that degradation of laser characteristics is caused by thermal excitation of holes from the type-I QW into the waveguide rather than non-radiative recombination and Auger effects [4]. This statement was confirmed later by absorption measurements [5] as well as time-resolved photoluminescence (PL) [6]. The problem of thermal escape was partially solved in 2005 when Grau et al. proposed the quinternary AlGaInAsSb material as a barrier for GaInAsSb QW [7]. Such barrier allows one to control the valence band offset independently of the lattice parameter and the bandgap. Thus, the thermal escape of holes has been reduced in the type-I QW heterostructures. Nevertheless, cw laser operation was demonstrated only after several years by the reason of growth complexity of the quinternary compounds [8], [9], [10]. Currently, type-I QW heterostructures with a number of cascades are generally used in order to achieve higher values of an output power. Recently the research group from Stony Brook University fabricated cascade type-I GaInAsSb/AlGaInAsSb QW laser diodes (LDs) with carrier recycling between QW gain stages. Such LDs showed RT emission in cw mode with the output power of 980, 500 and 360 mW near 3, 3.18 and 3.25 μm, respectively [11]. Although rather high output power have been demonstrated near 3.2 μm, laser performance still degrades with increasing the emission wavelength, which indicates that this material system has probably reached its limitations.
Such shortcomings as a poor electrical confinement due to small valence band offsets and losses due to the thermal escape of the carriers, which are typical for type-I systems, could be significantly reduced in type-II QW heterostructures [12], [13], [14]. It should be noted that the reduced overlap of electron and hole wave functions associated with spatially indirect optical transitions is often considered as a weak point of type-II systems as it leads to higher threshold current density. However, a direct comparison of the mid-IR absorption spectra of type-I and type-II QW heterostructures has revealed no significant difference in saturation gain between them [15]. Therefore, lasers based on type-II superlattices (SLs) or multiple QWs should display improvements in the performance at longer wavelengths. Since 1996, when the type-II QW lasers first demonstrated an emission beyond 3 μm with an output power up to 270 mW [16], the design of mid-IR type-II “W” lasers was greatly optimized. In 2011 Vurgaftman et al. presented the description of the internal carrier generation and transport processes in interband cascade lasers (ICLs) and suggested novel design which provides complete rebalancing of electron and hole densities in the active region [17]. Type-II cascade “W” InAs/GaInSb ICLs with such design demonstrated RT emission at 3.5 μm with the high output power up to 592 mW [18], [19].
Another approach to achieve emission beyond 3 μm employed quantum cascade lasers (QCLs). In particular, the InAs/AlSb is rather widely considered as the most attractive system for the development of short-wavelength QCLs due to the high conduction band offset of 2.1 eV and the large Г-L distance of 0.73 eV in InAs [2]. However, the first 3.1 μm emission was achieved only in 2006 when the InAs spacers in the laser waveguide were replaced by short-period InAs/AlSb superlattices with a sufficiently wide bandgap [20]. Such an approach allowed one to fabricate the QCLs emitting even below 3 μm [21] that was a challenge for many years. One of the great assets of QCLs is their outstanding temperature stability. Optical phonon scattering, being the dominant non-radiative recombination channel in QCLs, is much less temperature sensitive as compared to the thermal escape or Auger recombination processes. This provides a possibility of operation of QCLs at rather high temperatures. In 2012 Laffaille et al. fabricated InAs/AlSb QCLs operating at 3.22 μm up to 423 K in a pulse mode [22]. One should note that GaAs/AlGaAs is another material system successfully used to fabricate QCLs. However, such QCLs cannot operate below 8 μm due to the small conduction band offset between GaAs and GaAlAs [23].
Pseudomorhpic growth of the 3–5 μm mid-IR heterostructures requires “6.1 Å-family” substrates, such as GaSb and InAs, which are more expensive and less developed as compared to GaAs and InP ones. Many techniques have been proposed to overcome the lattice mismatch problem such as wafer bonding [24], patterned substrate epitaxy [25], compliant substrates [26], [27], and metamorphic technology [28]. All of them except the latter one require special preparation of the wafers before the epitaxial growth that increases the fabrication complexity. In the metamorphic approach, a graded buffer layer is grown on a commercial substrate to relax most of the strains and accumulate the majority of threading dislocations (TDs) [29]. Such technology allows one to achieve material with the desired lattice constant, namely, virtual substrate (VS) and fabricate low-defect-density heterostructures with acceptable surface and structural quality. The residual strain accumulated at the top region of InAsP, InAlAs or InGaAs metamorphic buffer layer (MBL) is generally compensated by an In inverse step (ΔIn), which represents the difference between the maximum composition of MBL and that of VS. Usually, MBLs with steplike [30], [31] or linear-graded [32], [33], [34] composition profiles were used in metamorphic structures due to the easier growth process and the possibility to use a Dunstan's relaxation theory [35], [36]. The Dunstan's model allows one to quantify the strained region at the top of MBL and, therefore to calculate the optimum ΔIn value, which is crucial for metamorphic growth as it greatly affects the residual stresses in VS and the active layers as well as their critical thicknesses. The lack of such complete relaxation theory for compositionally nonlinear-graded MBLs and scarce experimental studies restrict their utilization in the structures. However, several models predict a wider top region free of TDs for nonlinear-graded buffers as compared to steplike or linear-graded ones [36], [37]. In particular, Choi et al. reported the effectiveness of convex-graded buffers InxAl1-xAs (x = 0.15–0.50) for metamorphic In(Ga,Al)As layers grown on GaAs (001) [38]. The structures with convex-graded MBL demonstrated better optical and structural properties as compared to those with linear- and concave-graded MBLs. This is in good agreement with the results of comparative study of metamorphic In(As,Sb)/InGa(Al)As heterostructures fabricated on GaAs substrates via different types of graded InxAl1-xAs (x = 0.05–0.83) MBLs [39]. It was shown using the cross-sectional transmission electron microscopy (TEM) that the convex-graded MBL, characterized by the steep at the bottom and smooth at the top increase in the In content, results in more efficient stress relaxation at the bottom part of the MBL, ensuring thicker TD-free region and half an order decrease in the TD density as compared to the linear-graded one.
One more way to reduce the TD density in the active region of the structures is the insertion of SLs into MBL. Galiev et al. reported on the influence of the InGaAs/InAlAs strain-compensated SLs and the additional inverse steps incorporated into MBL on the PL properties [40]. SLs were used to create local fields of elastic strain that do not introduce additional strain in the buffer as they have the opposite sign and compensate each other. Such fields promote the lateral bending of TDs. It was also shown that the use of additional inverse steps inserted into different parts of the MBL does not reduce the TD density in the structures and rather leads to degradation of transport and optical properties.
While many efforts have been focused on optimization of MBL design, the processes of strain relaxation have not been completely understood yet, thus, molecular beam epitaxy (MBE) growth of the MBLs is still attributed to a category of art. Despite all the difficulties it was shown that metamorphic approach could be successfully used for fabrication of short- and mid-wavelength IR emitters. In particular, ultra-low threshold current density (≤70 A/cm2) as well as high temperature stability of metamorphic 1.5 μm quantum dot lasers grown on GaAs substrates have been demonstrated [41], [42], [43], [44]. However, only several research groups reported on fabrication of metamorphic mid-IR lasers emitting beyond 3 μm. Such lasers generally employ type-I GaIn(As)Sb/AlGaIn(As)Sb QW heterostructures grown on GaSb [45], [46] and, therefore, undergo all the problems inherent to the type-I 6.1 Å system.
In order to achieve emission beyond 3 μm we proposed recently the novel approach, which implies inserting one or several ultrathin, of the order of 1 monolayer (ML) in thickness, InSb layers into an InAs/InGaAs QW [39]. This leads to formation of a type-II InSb/InAs QW inside the type-I InAs/InGaAs QW [47]. Similar InSb/InAs nanostructures were previously grown pseudomorphically on InAs substrates and exhibited bright RT PL [48], [49] and electroluminescence [50], [51] in the 3–6 μm range. Four years later, Lu et al. fabricated such InSb/InAs structures on GaAs substrates using thick InAs buffer layers, which demonstrated low-temperature luminescence near 3.5 μm under optical and injection pumping. It was also noted that the absence of RT emission was caused by the high density of TDs inherent for such uniform buffers [52]. Recently, we reported on metamorphic InSb/InAs/InGaAs/InAlAs separate confinement heterostructures grown by MBE on GaAs, which demonstrated bright 3.2–3.5 µm RT PL [53]. The convex-graded InAlAs buffer was chosen as the MBL in such heterostructures as it provides lower TDs density in comparison with linear-graded one [39]. One should note that InSb/InAs/InGaAs/InAlAs QW heterostructures are almost out of shortcomings, which are inherent for type-I and type-II systems for a number of reasons. First, they employ only binary and ternary compounds with one group-V element, which makes the composition and strain control easier as compared to the structures with quaternary and quinternary compounds. Second, the use of type-I InAs/InGaAs QWs with “W” type-II InSb/InAs active region provides good both electron and hole confinement which allows one to reduce the probability of thermal escape of the carriers. Third, the type-II InSb insertion does not lead to weak overlap of the electron and hole wave functions in the active region due to its small thickness (∼1 ML). And finally, it employs a GaAs substrate platform widely used in photonics and electronics. This paper gives an overview of our recent detailed studies of structural and optical properties of such metamorphic heterostructures prospective for GaAs-based mid-IR emitters.
Section snippets
Optimization of MBL growth
To implement the epitaxial growth of InSb/InAs/InGaAs/InAlAs heterostructures on highly mismatched GaAs substrates, MBL with different distribution profiles of the In content over the depth can be used, including steplike [54], [55], [56], linear [33], [47], or nonlinear [37], [38]. Despite the steplike profile is simple in implementation when using MBE setups and makes it possible to achieve rather high electron mobility in the In0.75Ga0.25As/In0.75Al0.25As QWs at low temperatures [55], the
Strain relaxation in convex-graded InxAl1-xAs MBL (x = 0.05–0.79)
This chapter presents the results of detailed studies of structural properties of In0.75Ga0.25As/InAlAs QW heterostructures with convex-graded InxAl1-xAs MBLs and discuss main mechanisms of elastic strain relaxation revealed by using a combination of X-ray reciprocal space mapping (RSM) with the data of spatially-resolved selected area electron diffraction (SAED) implemented in a transmission electron microscope. RSM is commonly used for characterization of MBLs and provides information on the
Effect of the inverse step value at the InAlAs MBL/VS interface
The lack of complete relaxation theory for non-linear graded MBLs leads to impossibility of the accurate calculation of the inverse step (ΔIn), which affects greatly the stress distribution in the metamorphic heterostructures. Therefore, the optimum design of the structures, including the optimum ΔIn value, could be approached only by experimental studies. Recently we have reported on strain optimization in such metamorphic InSb/InAs/InGaAs/InAlAs QW heterostructures by variation of ΔIn [53].
Conclusions
The paper reviews the results of comprehensive study of the MBE growth peculiarities as well as structural and optical properties of the metamorphic In(Ga,Al)As heterostructures with a combined type-II/type-I InSb/InAs/InGaAs QW grown on GaAs (001) substrates via the convex-graded InxAl1-xAs MBL, which demonstrate bright RT PL in the 3–3.6 μm spectral range.
First, we optimized the MBE growth conditions of the InxAl1-xAs (xmax ≤ 0.77–0.89) convex-graded MBLs as well as elastic stress profiles
Acknowledgments
The authors are thankful to A.A. Sitnikova for TEM measurements, Dr. D.A. Kirilenko for SAED measurements, Dr. M.V. Baidakova and Dr. M.A. Yagovkina for XRD and RSM measurements. The work was supported in part by the Russian Foundation for Basic Research (Project #18-02-00950). Structural characterization of the samples was performed using equipment owned by the Joint Research Center “Material science and characterization in advanced technology”.
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