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A new mid-infrared (MIR) Vertical Cavity Surface Emitting Laser (VCSEL) structure is proposed. We have integrated to the VCSEL structure both an oxide aperture for lateral confinement, and a sub-wavelength high-contrast-grating top mirror. Upon the GaSb-based half-VCSEL, we have grown a metamorphic AlGaAs heterostructure to enable thermal oxidation and grating mirror fabrication steps. A methodology based on optimization and anti-optimization methods has been used to design the optical grating, with improved parameter tolerances regarding processing errors. Finally, we show the complete fabrication of an electrically-pumped MIR monolithic VCSEL structure implementing both oxide confinement and a subwavelength grating top mirror.
© 2013 Optical Society of America
1. Introduction
Vertical-cavity surface-emitting lasers (VCSEL) have already demonstrated their excellent ability as compact sources for molecular spectroscopic measurements. For these applications, the mid-infrared (MIR) wavelengths (λ > 2 µm) represents a major field of investigations since they allow the detection of molecules in various domains including environment (polluting gas detection,...), medicine (illness diagnostics, ...), food market or industrial process control (gas emission during process, security, ...). A general issue for the long-wavelength VCSELs is the inherent thick epitaxial structure (about t = 14µm for λ = 2.3 µm) leading to detrimental electro-thermo-optical properties. Although, for emission at λ > 2 µm, the antimonide alloys are the better system to reach high performance lasers, this system still suffer from the lower development of the material processing technology compared to the conventional GaAs and InP options. Nevertheless, excellent results have been recently demonstrated near 2.3 µm and 2.6 µm with monolithic, or hybrid dielectric-semiconductor Sb-based structures [1,2]. Moreover, even if InP-based technology allows high performing electrically-pumped (EP) VCSELs up to 2.3 µm, emission beyond 2.6 µm can only be realized with GaSb-based active layers. In this paper, we propose to integrate both a lateral oxide electrical / optical confinement, and a sub-wavelength grating high contrast mirror (HCG), within a Sb-based VCSEL structure emitting in the MIR spectral range. In order to define a simple and proven way for electro-optical confinement, we propose to use a metamorphic approach with (Al)GaAs layer to be oxidized [3,4]. This approach combines the advantages of a well-controlled oxidation of an AlGaAs layer with the high-efficiency of a Sb-based active region for emission in the MIR. In addition, in order to overcome the problems caused by the laser structure thickness and the inherent electro-optical-thermal issues, the use of HCG mirror allows to control directly through its design the lasing wavelength, to stabilize the beam polarization while maintaining transverse single-mode operation. In this work, since its optical properties depend on multiple geometrical parameters, the HCG design is refined by an original method of optimization based on particle swarm optimization algorithm [5] combined with an anti-optimization one [6]. Moreover, the development of this mirror implies technological targets as the etching control of semiconductor alloys on a nanometric scale, but also the wet oxidation of these alloys. We finally present the results obtained on electrically-pumped characterizations of MIR-VCSELs structures, for which we included an oxide aperture for lateral confinement and a HCG as the top high-reflectivity output mirror, both based on AlxOy/GaAs heterostructures.
2. Optimization Method for the design of MIR HCG-VCSELs
2.1 High contrast grating mirror structure
A HCG is composed of a diffraction grating with a high index contrast between the slab and surrounding materials. By tailoring the period of the grating to sub-wavelength dimensions, one suppresses all the diffraction orders, except the 0th order. In such a regime, it has been shown that [7, 8] very high reflectivity on a wide spectral band can be achieved when the two or more propagating Bloch modes sustained by the grating waveguide interfere destructively at the output interface, thus cancelling each other in the transmission through the grating, and reflecting back all the incident power.
Therefore, a properly-designed HCG mirror is an extremely efficient mirror with almost 100% reflectivity over a large bandwidth which can reach Δλ/λ0 = 30% [9,10]. This extraordinary optical property makes HCG very attractive to replace Bragg reflectors in VCSELs by using only one layer, instead of about twenty or more quarter wavelength layers required in the case of MIR VCSELs [11]. Furthermore, besides a 10 fold reduction in thickness, HCG mirrors are polarization sensitive due to their 1D symmetry and can be designed to fix the polarization of VCSEL structures [9,12,13].
Another asset of the HCG structures is that one can take advantage of the highly angular dependent reflectivity to ensure that laser emission can only be obtained on the fundamental transverse mode [14].
The HCG structure presented in Fig. 1 is made of a GaAs grating (n = 3.3) with a period Λ, a groove width Le, a slab width Lf and a thickness Tg. The grating layer is combined to an AlxOy low index layer (n = 1.66) of thickness TA obtained by wet oxidation of an Al0.98GaAs layer [15]. The GaAs grating layer is not completely etched resulting in a GaAs sublayer of thickness TL to prevent delamination during the oxidation process and enhance mirror performances.
Fig. 1 Design of the HCG mirror structure. (a) Scheme of the GaAs/AlxOy high contrast grating structure. The light propagates upwards from the GaAs substrate at normal incidence. (b) Reflectivity spectra for transverse magnetic (TM) and transverse electric (TE) modes of the optimum design exhibiting a 369 nm large and 99.5% high reflectivity stopband for RTM. A good polarization selectivity is obtained with RTE<80% for the whole stopband.
2.2 Design method
The reflectivity of the grating structure is numerically computed for transverse magnetic (TM) and transverse electric (TE) modes by rigorous coupled-wave analysis (RCWA) [16]. The light excitation is at normal incidence to the grating surface and propagates from the substrate as it is the case in the studied VCSEL configuration. To ensure laser emission with improved polarization stability, the HCG is designed to provide a reflectivity RTM>99.5% while keeping RTE below 90% for the largest possible bandwidth (cf. Figure 1(b)). These requirements are quantitatively defined by a merit factor MF which mainly represents the normalized bandwidth Δλ/λ0 centered at λ0 multiplied by a Gaussian weighted average of the reflection coefficient RTM [17].
The parameters of the design are then automatically adjusted by a particle swarm optimization algorithm (PSOA) [5] which maximizes MF, and thus provides the best HCG mirror for the VCSEL application.
From a technological point a view, it is important to have not only a design that exhibits good performances, but also large fabrication tolerances. HCGs have been shown to exhibit good tolerance values of more than 20 nm on the different lengths (Le, Lf, Tg) [18] and have allowed laser operation with parameter deviations as large as 10% on the grating period for instance [19]. However, since the optimization process can result in a critical point with less than 1 nm of tolerance [20], a robust design algorithm based on anti-optimization [6] has been combined with the PSOA to provide a HCG mirror which exhibits both a high MF and large fabrication tolerances.
2.3 Results
The robust optimization algorithm has been used to design a mirror centered at λ0 = 2.3 µm by adjusting the set of parameters {Tg, FF, TA, Λ, TL} with FF = Lf/Λ being the Fill Factor of the grating. For the anti-optimization algorithm, we have fixed a lower bound on each parameter tolerance range, by considering the different capabilities of the involved fabrication steps. A minimum tolerance of the AlxOy thickness ΔTA = ± 50 nm has been chosen since the wet oxidation process renders the control of the AlxOy thickness difficult due to a thickness change of the Al0.98GaAs layer during its oxidation. Even though a 50 nm tolerance requirement is large, it has been shown that HCGs exhibit good tolerances on the thickness of the low index sublayer [21, 15] and is not critical to the algorithm convergence.
The etching of the grating slab is another challenging step of the fabrication process and minimum tolerance values of ΔTg = ± 20 nm and ΔFF = ± 0.02 have been chosen to limit the impact of etching imperfections on the mirror performances. The tolerance requirements of the grating period ΔΛ and the GaAs thickness ΔTL have been set to 3 nm and 1 nm respectively, since the e-beam nanolithography enables such accuracy.
The robust optimization algorithm returns a design with a large high-reflectivity bandwidth of 369 nm for RTM > 99.5% (Fig. 2) centered at λ0 = 2.309 µm. The final structure whose dimensions are given in Table 1 fulfills all the tolerance requirements with ΔTA = ± 100 nm, ΔTg = ± 31 nm and ΔFF = ± 0.064 (Table 1). The tolerances on the equivalent parameters (Λ, FF, TA) are quite similar to those presented in ref [19]. The variation ranges given in Table 1 are computed for each parameter independently while keeping the other dimensions at their optimum values. However the fabrication process can result in errors made on several parameters simultaneously. The anti-optimization algorithm can take into account this aspect by varying several dimensions of the structure during the evaluation of the robustness. A statistical study performed on 30 000 tests on the optimum design which parameters are simultaneously and randomly varied within the tolerance requirements shows that 99.9% of the mirrors fulfill the reflectivity requirements with RTM > 99.5% and RTE < 90% at λ0 = 2.3 µm.
Table 1. Optimum parameters of the HCG obtained by the robust optimization algorithm for a mirror centered at λ0 = 2.3 µm. Tolerance values which ensure a RTM > 99.5% at λ0 are presented for each design length.
3. Metamorphic growth of VCSEL structure on GaSb
The VCSEL structure is grown by solid-source molecular beam epitaxy (MBE) in a RIBER C21E machine equipped with two valved As and Sb cracker cells providing As2 and Sb2. Growth runs were realized on epi-ready Te-doped (100)-oriented GaSb substrates whose temperature was monitored with an optical pyrometer and calibrated using the (1x3) (2x5) reconstruction change. The surface oxide of the substrate was thermally desorbed around 550°C under Sb2 overflow in order to prevent the Sb desorption from GaSb. The bottom distributed N-doped Bragg mirror (DBR) consists of 24 pairs of Te-doped (1018 cm−3) lattice-matched AlAsSb/GaSb, and is grown at 510 °C. The 3λ/4 cavity is grown at 470°C, including Al0.35Ga0.65As0.03Sb0.97 cladding layers surrounding five compressively-strained 10 nm-thick Ga0.65In0.35As0.08Sb0.92 quantum wells (QWs) separated by 15 nm of AlGaAsSb barriers. Above the cavity, at the first node of electromagnetic field standing wave, a thin GaSb p++ (15 nm at 1019cm−3)/ InAs n++(15 nm at 1019cm−3) tunnel junction is inserted. The ambipolar role of Si (p-type for Sb-based materials and n-type for As-based materials) is used to dope the TJ in order to limit atoms interdiffusion at the interface.
Above the latter GaSb lattice-matched structure, a metamorphic growth of N-type AlGaAs-based heterostructure is realized, in which two high Al content layers are inserted to achieve by wet thermal oxidation, respectively an oxide confinement aperture, and a fully oxidized low index layer below the HCG. This heterostruture, directly grown on the GaSb/InAs tunnel junction, is composed of GaAs (304 nm)/Al0.98GaAs (50 nm)/GaAs (841 nm)/Al0.98GaAs (355 nm)/GaAs (713 nm). The lower thin AlGaAs layer is placed at a field node and is devoted to the oxide aperture, while the upper one will serve as the low index layer for insuring a high reflectivity of the HCG. Additionally, thanks to this metamorphic growth of GaAs-based materials, we can benefit from stable etch process for the grating fabrication.
A N-type GaAs layer is then grown directly on the highly doped GaSb/InAs tunnel junction, simply by switching the group III elements. Due to the large lattice mismatch between the InAs and the GaAs (~7%), the strain is relaxed after 1 ML only. After a temporary spotty RHEED pattern, a clear streaky (2x4) pattern, denoting a smooth As-stabilized GaAs surface, is recovered after only 20 ML growth. The position of this metamorphic interface close to the node of the field will ensure minimal optical losses from the generated defects. Indeed, the growth of antimonide material on GaAs [22, 23] or Si [24] substrate and oppositely the growth of GaAs on GaSb have been recently studied [25]. This couple of materials presents the advantage to form defect-free growth capability by confining the propagation of the Lomer misfit dislocations in the plane orthogonal to the growth direction [26]. To study the crystalline quality of the material, high resolution X-ray diffraction (HRXRD) has been carried out using a high resolution Philips X-Pert goniometer. We have used the CuKα1 wavelength from a line focus. The Fig. 2 presents the HRXRD pattern of the VCSEL metamorphic heterostructure on a GaSb (001) substrate, around the (004) reflection. This spectrum evidences a very high crystalline quality and sharp interfaces for the Sb-based materials. In addition, the angular position of the Ga(Al)As-diffraction peaks shows a complete relaxation of these layers.
The optical reflectivity measured with a Nicolet Fourier transform IR (FTIR) spectrometer shows a stop-band centered at 2.27 µm with a cavity resonance dip at 2.21 µm. This slight detuning can be interpreted by a lower growth rate than estimated during the metamorphic growth of AlGaAs layers. The electroluminescence measurement of this heterostructure was performed just after the growth, on a sample with wide metallic contact pads (500 µm x 500µm) under pulsed operation (1 µs / 21 kHz) at a driving current of 500 mA. The electroluminescence spectrum exhibits a resonantly enhanced emission at 2.21 µm, but the electroluminescence measurement of edge emitted light shows that the QWs gain is centered around 2.32 µm. The reflectivity and electroluminescence spectra measured from the top of the structure are given in Fig. 3.
Fig. 3 Reflectivity (experimental and simulation) of the metamorphic half VCSEL after growth and electroluminescence signal of the epitaxial structure.
4. Fabrication of MIR VCSEL structures with HCG mirrors and oxide aperturing
The fabrication process of the HCG-VCSEL is similar to the one of a standard VCSEL. The overall process flow is as follows: the top grating is first realized by nanolithography and inductively coupled plasma reactive ion etching (ICP-RIE), then top ring and substrate backside metallizations are applied, later mesas are formed by ICP-RIE, and afterwards the selective lateral thermal oxidation of both Al0.98GaAs layers simultaneously, is carried out. Subsequently, SiO2 surface passivation and contact pad deposition are realized. We present now with more insight each of the main fabrication steps for the fabrication of the HCG-VCSEL.
The fabrication of HCG mirror is carried out using the following steps: hard SiOx mask deposition by plasma-enhanced chemical vapor deposition (PECVD), electron-beam lithography of spin-coated PMMA, ICP-RIE etching of the grating. The etching profile of the grating is critical since it strongly modifies its optical properties. Ideally this profile would be rectangular since the latter geometry presents the highest fabrication tolerance. In any case, the most critical parameter is the grating depth, which fixes the interference conditions between the Bloch modes, and then enables the broadband high reflectivity. In our case, the optimization method described in section 2 gives us a range between 682 and 773 nm for the etch depth, to achieve a sufficient reflectivity with maximum tolerance ranges of the HCG parameters.
To obtain nearly vertical sidewalls, we have developed ICP-RIE etching recipes based on CHF3 for the hard mask etch, and Cl2/N2/Ar for the GaAs layer etch. The Fig. 4 shows the obtained etch profile. The selected GaAs etch was relatively slow, in order to control the groove depth. You may note that the sidewalls have a low roughness thereby minimizing scattering losses.
Fig. 4 Cross section image by Scanning Electron Microscopy (SEM) of the fabricated HCG-VCSEL after the ICP-RIE processing.
Hence, a first mesa is etched around the grating down to the N-type GaAs layer below the topmost thick Al0.98GaAs layer, to enable the lateral oxidation of this latter, and the formation of the low-index underneath the HCG. Then a top metal ring contact (TiPtAu) is deposited around the mesa on the N-doped GaAs layer etched surface (Fig. 5). A second larger mesa coaxial with the first one is etched enabling the lateral oxidation of the second Al0.98GaAs layer serving as electrical and optical confinement aperture (see Fig. 6(a)).
Fig. 6 Images of the HCG-VCSEL during the technological processing. (a) Focused Ion Beam (FIB) image of HCG-VCSEL showing the interface Al0.98GaAs /AlOx of the confinement layer. (b) In situ near-infrared microscope image of the both Al0.98GaAs laterally oxidized layers within the VCSEL structure.
This successive realization of these two aligned mesas, will allow to realize the oxidation of the both Al-rich layers in a single run. The goal is to seal completely the oxidation of the upper AlGaAs layer below the HCG, to have a bulk low index layer below the overall surface of the HCG. Since this AlGaAs layer is thicker and that the oxidation proceed from a smaller diameter of mesa, this sealing will occur quite rapidly.
At the same time the second deeper oxide layer forming the confinement aperture should be carefully controlled in order to obtain the aimed final aperture diameter. This may be achieved thanks to the relative low thickness of the layer and the initial large diameter of the mesa from which this second oxidation will proceed.
Then, a single oxidation run for both Al0.98GaAs layers is done. The oxidation of the layer below the grating completes first and the process is stopped once the oxide apertures (see Fig. 6(b)) are observed to be slightly smaller than the top grating size (see Fig. 6(a)) . This lateral wet thermal oxidation was performed at 400°C and lasted 102 minutes, and have been monitored in situ by a dedicated infrared imaging setup (see Fig. 6(b)) [27]. At this point, it is interesting to emphasize that the oxidation kinetics and the oxide structural quality of these metamorphic Al0.98GaAs layers is identical to the one obtained with conventional lattice-matched layers on GaAs. Finally, the HCG-VCSEL fabrication is completed by the PECVD deposition of a SiOx passivation layer, followed by the deposition of the upper contact pad.
5. Characterizations of the HCG-VCSEL
Optical characterizations under electrical pumping were performed at room temperature and in pulsed mode on the fabricated VCSELs, with gratings size ranging from 19 to 35 µm and oxide apertures ranging from 12 to 28 µm respectively. The voltage-current characteristics show a turn-on voltage of 0.9V to 3V for an applied current around 5mA, with increasing voltage drops from large to smaller apertures. This trend corresponds to the increase of the series resistance due to current crowding for small aperture sizes. In our designed VCSEL, the goal was not to use small confinement apertures but rather defining an injected carrier profile matching the HCG size. The use of metamorphic AlGaAs layers to realize this lateral confinement has proven its efficiency in several previous works [4, 28, 29]. We have then chosen to define oxide aperture smaller than the HCG mirror to ensure a good overlap between the electromagnetic field profile and the mirror area.
Despite the good electrical characteristics and the good electroluminescence measured just after the epitaxial growth, we couldn’t observe laser operation with our devices. On Fig. 7, we present the measure spectrum under pulsed electrical pumping, showing a narrow peak at 2.235 µm due to the resonant cavity effect of the structure. The fact that laser action is not reached is attributed to a lack of reflectivity around 2.25 µm of the HCG mirror, together with a large and unfavorable detuning between the cavity mode and the QW gain centered 2.32 µm. The assumption of low reflective HCG has been confirmed by a cross-section observation with a focus ion beam (FIB) (Fig. 6(a)), revealing substantial deviations of the geometrical parameters of the grating (grating depth, fill factor) from the design. An estimation of the HCG reflectivity with the measured parameters gives a maximum reflectivity around 97.8% at the resonance wavelength, indeed not sufficient to support laser operation.
Fig. 7 Electroluminescence spectrum of the MIR HCG-VCSEL in pulsed regime taken at room temperature.
6. Conclusion
In conclusion, we have presented a thorough study on an original GaSb-based VCSEL structure for operation in the MIR range. This MIR VCSEL structure incorporates a lateral oxide confinement and the replacement of the top Bragg mirror by HCG grating mirror. Both elements are formed using the AlGaAs material system metamorphically grown on the GaSb platform, and are expected to significantly improve the performances of the MIR VCSELs and the ability to shape through the design of the grating the output beam properties. We have demonstrated in this work, an original way for designing the VCSEL structure by optimizing the HCG properties while allowing a wide tolerance on geometrical parameters, enabling acceptable margins for the errors during the fabrication process. We have presented the first complete fabrication of such MIR VCSEL structure, including the combination by epitaxial growth of antimonide half-VCSEL structure and metamorphic AlGaAs heterostructures, and the technological process very similar to the standard process flow for near-infrared oxide-apertured VCSELs. Finally, we show the first demonstration of electrical pumping and light emission at room temperature on this innovative VCSEL structure.
Acknowledgments
This study was supported by the French National Research Agency (ANR), by the program Blanc under the project Marsupilami, Grant NT09-505624. Also, the authors acknowledge the support of RENATECH (the French Network of Major Technology Centres) within the technological platform of LAAS-CNRS.
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