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Abstract
We achieve the continuous-wave (CW) lasing of electrically-injected, first-of-their-kind vertical-cavity surface-emitting lasers (VCSELs) that use a subwavelength monolithic high-refractive-index-contrast grating (MHCG) mirror. The MHCG, unlike the well-known high-refractive-index-contrast grating (HCG) is neither a membrane suspended in the air nor a structure that requires a cladding layer. The MHCG is patterned in a semiconductor material atop the VCSEL cavity creating an all-semiconductor laser. Static measurements show CW operation of the VCSELs from room temperature up to 75 °C. The VCSEL with a 13.5 μm current oxide aperture diameter operates with quasi-single mode emission from threshold to rollover. Our results open a way to produce all-semiconductor surface emitting lasers emitting at wavelengths from the ultraviolet and the visible (GaN-based) to the infrared (InP- and GaSb-based) with a reduced vertical thickness and thus we believe the manufacturing costs potentially will be reduced by approximately up to about 90% in comparison to the typical DBR VCSELs. Our VCSELs have immediate and emerging applications in optical communication, illumination, sensing, and as light sources in photonic integrated circuits.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Since the very first experimental demonstration of a vertical-cavity surface-emitting laser (VCSEL) that uses a high refractive-index-contrast grating (HCG) as a coupling mirror [1,2], numerous studies of HCG VCSELs have demonstrated the possibility of single mode operation, high-speed modulation, and tunability over a broad spectral range [3]. Since HCG mirrors with a broad power reflectance near 1.0 can be designed for all VCSEL emission wavelengths of interest and can be fabricated in all modern material systems used in optoelectronics, the HCG mirror is a versatile solution for VCSELs emitting at wavelengths where distributed Bragg reflectors (DBRs) with sufficient index contrast are not available. To date room temperature electrically-injected VCSELs with HCG mirrors have been demonstrated with peak emission wavelengths of 850 nm [1,4], 1060 nm [5], 1320 nm [6], and 1550 nm [7,8]. Electrically injected HCG VCSELs emitting at four distinct wavelengths around 985 nm were reported in [9], while laser emission under optical pumping around 400 nm and 1550 nm was shown in [10,11] and in [12–14], respectively.
The physics behind the very high optical power reflectance of the standard HCG [15] requires that the grating must be made of high refractive index material that is surrounded by low refractive index material. The same applies to two interesting variations of the HCG - the zero-contrast grating (ZCG) [16] and the hybrid grating reflector (HG) [17] where a high refractive index slab or a cap layer lies directly adjacent to the grating in order to exploit additional resonances between the grating and the slab modes. This requirement is the key reason why only a few such designs have been realized for use in VCSELs, of which an HCG membrane suspended in air [1–9] is the original, quintessential design for use in micro-electro-mechanical tunable VCSELs. If such an HCG is used as a coupling mirror for an electrically driven VCSEL the low refractive index material – an air gap or a dielectric - must be incorporated in the VCSEL optical cavity. This HCG VCSEL design has serious practical problems with current injection, heat dissipation, and it may negatively impact the mechanical stability of the device.
The subwavelength monolithic high index contrast grating (MHCG), a special case of an HCG, was first proposed and demonstrated by Goeman et al. in 1998 [18] and then later demonstrated by Lee et al. in 2009 [19]. The experimental power reflectance (R) of these MHCGs exceeded 0.85 (85%) at 1550 nm [18] and 0.90 (90%) at 450 nm [19], respectively. In our recent work [20] we demonstrated 0.93 (93%) power reflectance at ~1030 nm with an MHCG constructed of GaAs. In all the cases the gratings were characterized by a large polarization selectivity. The high R of an MHCG is independent of the refractive index contrast between the grating and the material which lies beneath the grating (called the cladding) [21]. A special case of such a situation is when the grating is made of the same material as the cladding. In other words, an MHCG may be monolithically integrated with its substrate and may thus be composed of a single material layer such as gallium arsenide, indium phosphide, gallium nitride, silicon, or another material, with a shallow grating pattern [22]. A VCSEL design that incorporates one or two MHCG mirrors [23,24] inherits the advantages of the HCG VCSEL including a decrease in the amount of material that must be grown by epitaxy and opens a way to manufacture VCSELs emitting at wavelengths that are currently not possible with semiconductor DBRs, when the DBRs have a low contrast between the alternating low index and the high index DBR material layers. The MHCG thus enables visible light and ultraviolet (UV) VCSELs based on GaN, and infrared VCSELs based on InP, GaAs, and GaSb [22,25]. The MHCG VCSEL also improves upon several issues found in HCG VCSELs. We expect MHCG VCSELs compared to HCG VCSELs will have improved: 1) heat dissipation; 2) current injection efficiency; and 3) mechanical stability for the price of no dynamic tunability and a small high power reflection bandwidth. In addition to a significantly simplified epitaxial design [23] we believe MHCG VCSELs will be well suited for monolithic integration with silicon. In this paper we report the first electrically-injected VCSELs emitting at ~980 nm that employ an MHCG. We use a composite DBR and MHCG top coupling mirror and demonstrate continuous wave (CW) lasing up to 75 °C.
2. Design and fabrication
2.1 VCSEL design
A schematic view and a top down view of our MHCG DBR VCSEL are shown in Fig. 1 and Fig. 2 respectively. This novel VCSEL is a top-emitting, gallium arsenide-based single cavity VCSEL with two mesas and two oxide current confinement apertures. Electrical connections are realized by a top metal ring contact and a bottom half- to three-quarter metal-ring contact which are connected to high frequency ground-signal-ground pads deposited on planarizing B-staged bisbenzocyclobutene (BCB). The epitaxial structure is based on a state-of-the-art communication 980 nm VCSEL design [26]. On the surface of an ~450 μm n-doped gallium arsenide substrate an ~1.58 μm thick n-doped GaAs buffer layer is grown. Following the buffer layer we grow 37 pairs of n-doped GaAs/Al0.9Ga0.1As DBRs with simple linear compositionally-graded interfaces. The multi-quantum-well active region consisting of 5 compressively-strained InGaAs quantum wells (QWs) and 6 GaAsP barrier layers is embedded in the center of a half-lambda optically-thick cavity. The interfaces between the active zone and the top and bottom DBRs are realized by AlGaAs graded layers of various compositions. The top coupling mirror is composed of a 5.5 period DBR of p-doped GaAs/Al0.9Ga0.1As again with simple linear compositionally-graded interfaces capped with an ~425 nm p-doped GaAs phase-matching layer. The last 30 nm of the phase-matching layer is highly doped (~1020 cm−3) to form a low resistivity electrical contact with a top-contact Au/Zn/Au metal ring. The top-most part of the epitaxial structure is composed of a 341 nm-thick undoped GaAs layer for the MHCG mirror patterning separated from the rest of the structure with a 20 nm-thick GaInP etch-stop layer that is lattice-matched to GaAs. Two Al-rich 20 nm-thick layers for oxide aperture formation are in the first nodes of the optical field intensity standing wave closest to the active region, one each at the interfaces of the top and bottom DBRs and the half-lambda-thick optical cavity. The calculated optical field intensity profile and real refractive index distribution in the top part of the VCSEL structure is presented in the Fig. 3(a).
Fig. 2 SEM images of the MHCG DBR VCSEL (Device A, see section ‘Results’): a) an entire device; b) mesa region; and c) cross-section of the top part of the cavity (FIB cut, with added thin layer of Pt to aid the FIB process); d) the surface MHCG region. Optical microscope images of the device connected with a GSG prober: e) illuminated by an outside light source; f) below the threshold (with a forward bias I = 3 mA); and g) above the threshold (with I = 12 mA).
Fig. 3 a) simulated optical field intensity profile (blue) and real refractive index at 980 nm both versus distance z in the top part of the MHCG DBR VCSEL structure; and b) simulated power reflectance (R) of the top MHCG DBR outcoupling mirror and 5.5 top DBR periods (inset) as seen from the optical cavity at normal incidence for TM polarized light and ranges of fundamental mode wavelengths for devices A (blue), B (green) and C (red) from threshold to rollover at 25 °C. In the simulations the real measured geometry of the MHCG mirrors were used (see Fig. 2(d)). The Inset in b) is the simulated R of our 5.5 period DBR into air (thus without the MHCG and phase matching layer) as seen from the optical cavity at normal incidence.
2.2 MHCG design
The MHCG geometry is: 1) a period L = 0.500 μm; and 2) a stripe height H = 0.241 μm. We fabricate the MHCGs with a small variation of fill factor in the range from F = 0.37 to F = 0.45. The fill factor F is defined as the ratio of the stripe width to the grating period. Our experimental grating parameters result in a nominal power reflectance of the entire outcoupling mirror of 0.9999 for TM-polarized light (the only non-zero component of the electrical field vector is perpendicular to the stripes) at 980 nm. The fill factors F of the three MHCG VCSELs presented in this paper range from 0.38 to 0.40 and the actual processed shapes of the cross-sections are not perfectly rectangular which suggests a reasonably high manufacturing tolerance of the MHCG mirror as predicted in our earlier theoretical work [18,20].
2.3 Fabrication methods
The processing of our VCSELs involved planar processing procedures developed and used at the TU Berlin for standard DBR VCSEL processing. Additionally, the processing included the electron beam lithography (EBL) patterning of the MHCG gratings. In the first step the wafer sample was covered with AR.P6200.09 EBL resist and the MHCG fields, 100 μm x 100 μm in size, were patterned using a Raith ELPHY Plus EBL tool. After development the patterns were etched using Cl/BCl3/Ar in an inductively coupled plasma reactive-ion etching (ICP-RIE) reactor. Once the MHCGs were made, standard planar techniques for processing VCSELs were used. Annular patterns in photoresist to define the areas where we will next place top p-metal ring contacts were lain down on top of MHCG fields using standard contact UV-lithography. Then the exposed parts of the MHCGs together with the undoped InGaP etch-stop layer were removed using a mixture of wet- and dry-etching techniques. Au/Zn/Au top metal contact rings were then deposited using thermal evaporation. In the following step, top mesas were patterned and dry-etched using a Cl/BCl3 plasma in an ICP-RIE reactor. The etching was stopped after passing through the active region at the fifth bottom DBR pair to open access to both the top and bottom Al-rich layers, which after selective thermal wet-oxidation for 130 minutes at 420 °C and 50 mbar formed the top and the bottom oxide apertures. A second dry-etching step was then performed in order to form the second mesa and reach the (n+)GaAs buffer layer that lies just under the bottom n-doped DBR. The sample was then planarized using a photo-sensitive BCB spin-on film, which was selectively removed to expose the top mesa together with the top p-metal contact rings and create openings for the bottom n-metal contact deposition. The sample was covered with an ~300 nm thick PECVD-deposited SiN layer to increase the mechanical stability of the BCB and to improve the adhesion of metal to the BCB surface. The SiN was then patterned and etched by RIE to once again expose the top mesas and bottom contact openings. In the final step, Ni/AuGe/Au GSG pads were deposited using thermal evaporation in a standard photoresist lift-off process. This uncommon choice of metals for the GSG pads comes from an idea to use the ground part of the GSG pad as the bottom n-metal contact. Thanks to this approach the processing was simplified by removing one lithography step (we thus skipped our standard bottom n-metal contact lithography and deposition step since the GSG pad metal served also as n-contact metal) without any noticeable deterioration of the device performance.
The mask set used in the processing is designed in such a way that several identical unit cells (in the form of a matrix) of devices are made. Every unit cell is divided into 16 rows and 15 columns which results in 240 VCSELs in each unit cell. The mesa diameters are varied within every column from 18 μm (Row 0) to 31 μm (Row F) with a 0.5 μm step between Rows 0 to 4 and with a 1.0 μm step between Rows 4 to F. Nine different MHCG mirror geometries were patterned on the sample in a one-full-column-of-devices-per-design manner. All the MHCG types have - by design - identical period L = 0.500 μm and thickness H = 0.298 μm, while we varied the fill factor from 0.47 to 0.55 in 0.01 steps. Due to the inaccuracy of the MHCG processing, all the resulting MHCG patterns have a fill factor and thickness smaller than the design value by about 0.1 and 60 nm, respectively.
3. Results
3.1 Static measurements
Devices A and B have a top mesa diameter of 31 μm while Device C has a top mesa diameter of 28 μm. We measure the oxide aperture diameters (with use of FIB cuts) to be 16.5 μm for VCSEL A and B and 13.5 μm for VCSEL C. As seen in Fig. 4(a) all the devices have typical diode I-V curves. One can also see well pronounced threshold and rollover currents in the L-I-V curves taken at 25 °C. The threshold currents are equal to 6.18 mA, 6.75 mA, and 5.48 mA, which correspond to threshold current densities equal to ~2.89 kA/cm2, ~3.16 kA/cm2 and ~3.83 kA/cm2 for Devices A, B and C, respectively. The maximal optical output power at 25 °C is equal to 0.39 mW, 0.30 mW, and 0.20 mW for Devices A, B and C, respectively. In Fig. 4(b) we present the threshold current and optical output power at rollover as a function of the heat-sink temperature for Device A. The data is extracted from the L-I-V curves measured at temperatures from 15 °C to 75 °C with a 10 °C step. The VCSEL exhibits a maximal output power of 0.45 mW at 15 °C and a minimal threshold current of 6.18 mA at 25 °C. Our reference VCSEL structures (with 15.5 top DBR periods – but otherwise identical to our MHCG VCSELs) have threshold currents below 1 mA and several milliwatts of output power for the same oxide aperture diameters. This is a clear indication that optical losses are much higher in our MHCG mirrors and the power reflectance properties of our MHCG mirrors are likely non ideal. The main reasons for high losses are the non-ideal shape of the MHCGs as seen in cross-section SEM images of the gratings and roughness of the sidewalls of the grating stripes.
Fig. 4 a) light output power-voltage-current characteristics for devices A, B, and C taken at 25 °C; and b) threshold current (blue) and optical output power at rollover (red) vs. heat-sink temperature for device A.
3.2 Spectral measurements
Emission spectra a little below the VCSEL threshold (at a forward bias of 5 mA) and above the threshold taken at 25 °C for Devices A and C are presented in Fig. 4(b) and 5(a), respectively. Device A as well as Device B whose spectra is not shown emit in a double lateral mode over the full range of forward bias currents from threshold to rollover. Device C in contrast emits in a quasi-single mode over the full range of forward bias currents from threshold to rollover with a maximal side mode suppression ratio (SMSR) equal to 36 dB at 11 mA. Emission spectra for a device identical to Devices A and B but without the MHCG mirror as shown in Fig. 5(c) indicates that lasing is not possible for VCSELs with the 5.5 period top DBR but without the MHCG. Reference VCSELs with a 15.5 period top AlGaAs DBR and without the MHCG show strong multimode operation for oxide aperture diameters of ~5 μm and larger [24]. Hence, the single and double-mode operation of the MHCG DBR VCSELs with such large oxide aperture diameters (13.5 and 16.5 μm) must result directly from the influence of the MHCG portion of the top coupling mirror. This phenomenon cannot be simply explained by a small angular tolerance for the high power reflectance of our MHCG mirrors as is shown in [27] and will be discussed in detail in a future publication after a deeper study is performed.
Fig. 5 Emission spectra: a) of the double mode VCSEL A; b) of the single mode VCSEL C; and c) of a device of nominally the same size as VCSELs A and B but without the MHCG for various forward bias currents taken at a 25 °C ambient temperature. For both Devices A and C the emission spectrum taken at 5 mA is below the laser threshold.
3.3 Thermal resistance
The fundamental mode peak emission wavelengths for Device A are extracted from the emission spectra taken at 25 °C using bias currents from threshold to 20 mA in 1 mA steps. The peak emission wavelengths are a linear function of the dissipated power (Pdiss) defined as the electrical power (I∙V) minus the optical output power taken at the same forward bias current. The slope of this function (dλ/dPdiss) for Device A is 0.0492 nm/mW. Additionally, if the peak emission wavelengths taken at a fixed forward bias current are plotted versus the heat-sink temperature, this function is also linear with the slope (dλ/dT) of 0.0786 nm/K. Finally, having these two numbers, the thermal resistance (Rth = dT/dPdiss) of Device A is calculated and is found to be equal to 0.626 K/mW, which is close to the lowest values (~0.4 to 0.6 K/mW) of Rth ever reported for VCSELs [28,29].
4. Conclusions
We demonstrate continuous-wave lasing of VCSELs emitting at 980 nm with a composite DBR MHCG top coupling mirror. This is the first demonstration of lasing from an electrically-injected VCSEL that includes an MHCG. All our reported devices reach lasing in the CW regime at 15 °C up to at a maximal ambient temperature of 75 °C. We obtained a maximal optical output power of 0.45 mW at 15 °C. The optical output power of all the measured devices may likely be improved by perfecting the geometric precision of the top grating mirror to reduce optical losses. The VCSEL with an electrical aperture diameter of ~13.5 μm operates with quasi-single mode emission at 25 °C for all driving currents from the threshold to 20 mA with a maximal SMSR of 36 dB at 11 mA. The MHCG enables the fabrication of VCSELs in material systems which have been difficult or even impossible to use up until now. In the case of the mature gallium-arsenide-based technology, the use of two MHCGs to replace the two DBRs would enable a radical 90% or greater reduction in a given VCSEL’s vertical thickness. In this first demonstration with one MHCG in concert with a 5.5 period DBR we reduced the vertical VCSEL epitaxial thickness by nearly 30%.
Funding
German Research Foundation via the Collaborative Research Center (Sonderforschungsbereich) 787; Project OPUS 2014/15/B/ST7/05258, Polish National Science Centre; Polish National Science Centre ETIUDA scholarship 2015/16/T/ST7/00514.
Acknowledgments
M. Gębski would like to thank: 1) M. Wasiak from the Lodz University of Technology (LUT) for support and many discussions on the idea of the MHCG VCSEL; 2) P. Moser from the Technical University Berlin (TUB), now at Stanford University, for processing discussions; 3) N. Srocka, P. Schnauber, and T. Hauser from TUB for general cleanroom support; 4) M. Marciniak from LUT for her help with the grating device processing development; 5) M. Dems from LUT for providing a framework for numerical simulations; and 6) M. Narodovich from TUB for performing FIB cuts of the devices. Finally, he would like to express his sincerest special thanks to R. Schmidt and S. Bock from the TUB for their daily expert technical and processing support.
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