Buried Tunnel Junction Research Articles

Impact of Device Topology on the Performance of High-Speed 1550 nm Wafer-Fused VCSELs

A detailed experimental analysis of the impact of device topology on the performance of 1550 nm VCSELs with an active region based on thin InGaAs/InAlGaAs quantum wells and a composite InAlGaAs buried tunnel junction is presented. The high-speed performance of the lasers with L-type device topology (with the largest double-mesa sizes) is mainly limited by electrical parasitics showing noticeable damping of the relaxation oscillations. For the S-type device topology (with the smallest double-mesa sizes), the decrease in the parasitic capacitance of the reverse-biased p+n-junction region outside the buried tunnel junction region allowed to raise the parasitic cutoff frequency up to 13–14 GHz. The key mechanism limiting the high-speed performance of such devices is thus the damping of the relaxation oscillations. VCSELs with S-type device topology demonstrate more than 13 GHz modulation bandwidth and up to 37 Gbps nonreturn-to-zero data transmission under back-to-back conditions at 20 °C.

Long-Cavity M-Plane GaN-Based Vertical-Cavity Surface-Emitting Lasers with a Topside Monolithic Curved Mirror

We report long-cavity (60.5 λ) GaN-based vertical-cavity surface-emitting lasers with a topside monolithic GaN concave mirror, a buried tunnel junction current aperture, and a bottomside nanoporous GaN distributed Bragg reflector. Under pulsed operation, a VCSEL with a 9 µm aperture had a threshold current density of 6.6 kA/cm2, a differential efficiency of 0.7%, and a maximum output power of 290 µW for a lasing mode at 411 nm and a divergence angle of 8.4°. Under CW operation, the threshold current density increased to 7.3 kA/cm2, the differential efficiency decreased to 0.4%, and a peak output power of 130 µW was reached at a current density of 23 kA/cm2.

Efficient Tunnel Junction Lithographic Aperture 940 nm VCSEL

We report on oxide free 940 nm VCSELs based on a buried tunnel junction acting as a lithographic aperture and achieving power conversion efficiency above 40%. We present the fabrication flow implementing a 2-step epitaxial growth as well as a detailed electro-optical characterization of the fabricated devices. The tunnel junction based VCSELs benefit from a uniform current distribution thanks to a low resistive n-doped top DBR. The mesa-free lithographically defined aperture enables the arrangement of compact arrays constituted of small single mode emitters. Such array configuration allows to scale up the generated optical power while maintaining a diffraction-limited Gaussian beam and limiting the impact of thermal rollover.

Single-Mode High-Speed 1550 nm Wafer Fused VCSELs for Narrow WDM Systems

High power single-mode wafer fused 1550 nm VCSELs with an active region based on InGaAs quantum wells are fabricated. An InP-based optical cavity and two AlGaAs/GaAs distributed Bragg reflector heterostructures were grown by molecular-beam epita oy. The current and optical confinements are provided by a lateral-structured buried tunnel junction with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim ~6~\mu \text{m}$ </tex-math></inline-formula> diameter and etching depth of ~ 20 nm. The VCSELs demonstrate ~ 3.4 mW single-mode continuous-wave output optical power and a threshold current about 2 mA at 20°C. The output optical power exceeds 1 mW at 70°C. A −3dB modulation bandwidth > 13 GHz is obtained at 20°C. Non-return-to-zero data transmission under back-to-back condition of ~ 37 Gbps is shown.

20-Gbps 1300-nm range wafer-fused vertical-cavity surface-emitting lasers with InGaAs/InAlGaAs superlattice-based active region

1300-nm vertical-cavity surface-emitting lasers (VCSELs) were fabricated by wafer fusion (WF) technique and studied. The active region based on InGaAs/InAlGaAs superlattice was grown by molecular-beam epitaxy (MBE). Current and optical confinement was provided by composite n ++ -InGaAs / p ++ -InGaAs / p ++ -InAlGaAs buried tunnel junction (BTJ) realized by selective etching and overgrowth by n-InP. AlGaAs/GaAs distributed Bragg reflectors grown by MBE were applied on both sides of the cavity by WF and substrate removal techniques. The devices with BTJ diameter of 5 μm demonstrated a stable single-mode lasing with threshold current <1.3 mA and output optical power >6 mW and operation in a wide temperature range. The measured −3 dB bandwidth was more than 8 GHz at 20°C and about 5.5 GHz at 85°C, the eye diagrams were open with a bit rate up to 20 Gbps using nonreturn-to-zero (NRZ) modulation standard at 20°C. Using 5-tap feedforward equalizer, the NRZ transmission at 25 Gbps was demonstrated up to 5 km single-mode fiber at 20°C. The developed VCSELs represent a platform for further significant performance improvement.

High Power Single Mode 1300-nm Superlattice Based VCSEL: Impact of the Buried Tunnel Junction Diameter on Performance

High power single mode wafer-fused 1300-nm VCSELs with a gain region based on InGaAs/InAlGaAs short period superlattice are fabricated. An InP-based optical cavity and two AlGaAs/GaAs distributed Bragg reflector heterostructures were grown by molecular beam epita&#x00F5;y. The current and optical confinement is provided by a lateral-structured buried tunnel junction with etching depth of <inline-formula> <tex-math notation="LaTeX">$\sim 25$ </tex-math></inline-formula> nm. It is shown that optimal diameter of the buried tunnel junction for high-power single mode emission is <inline-formula> <tex-math notation="LaTeX">$\sim 5$ </tex-math></inline-formula>-<inline-formula> <tex-math notation="LaTeX">$6~\mu \text{m}$ </tex-math></inline-formula>. The VCSEL demonstrates more than 6 mW single mode continuous-wave power and a threshold current less than 1.5 mA at 20 &#x00B0;C. The output optical power exceeds 1 mW at 85 &#x00B0;C. A -3dB modulation bandwidth up to 8 GHz and 6 GHz is obtained at 20 &#x00B0;C and 85 &#x00B0;C, respectively. The gain coefficient of <inline-formula> <tex-math notation="LaTeX">$\sim 650$ </tex-math></inline-formula> cm^&#x2212;1 and the transparency current density of <inline-formula> <tex-math notation="LaTeX">$\sim 630$ </tex-math></inline-formula> A/cm² are estimated at zero gain-to-cavity detuning (\sim 60 &#x00B0;C). The ultimate low internal optical losses about 0.08 &#x0025; per round-trip (distributed losses &#x007E;3.2 cm^&#x2212;1) at 20 &#x00B0;C and 0.13 &#x0025; per round-trip (distributed losses &#x007E;5.5 cm^&#x2212;1) at 100 &#x00B0;C were obtained.

Inverted N-polar blue and blue-green light emitting diodes with high power grown by metalorganic chemical vapor deposition

The N-polar orientation of GaN may be an option for the development of longer wavelength visible optoelectronics because of its higher indium uptake. N-polar InGaN LEDs with an inverted p-side down configuration and buried tunnel junctions were grown by metalorganic chemical vapor deposition. Fabricated devices in the wavelength range of 450–509 nm showed record high light output power for N-polar LEDs, up to 0.21 mW on-wafer at 20 A/cm2 with an emission wavelength of 470 nm. These results represent an improvement in performance of more than one order of magnitude over previously reported N-polar LEDs, demonstrating potential for the N-polar orientation in the nitride optoelectronics space.

Impact of transverse optical confinment on performance of 1.55 μm vertical-cavity surface-emitting lasers with a buried tunnel junction

The impact of transverse optical confinement on the static and spectral characteristics of 1.55 μm vertical-cavity surface-emitting lasers (WF-VCSEL) with a buried tunnel junction (BTJ) n++-InGaAs/p++-InGaAs/p++-InAlGaAs, implemented using molecular-beam epitaxy and wafer fusion. It was found that for VCSELs with a tunnel junction (TJ) etching depth of 15 nm, the single-mode lasing occurs up to 8 μm BTJ mesa size due to a relatively weak lateral optical confinement, while the effect of a saturable absorber (SA) appears when the BTJ mesa size is less than 7 μm. Enhancing lateral optical confinement by increasing the BTJ etching depth up to 20 nm leads to suppression of the SA effect at the BTJ mesa size of 5-6 μm, but simultaneously limits the maximum single-mode optical power. According to obtained results an increase in the spectral mismatch between the maximum of the gain spectrum of the active region and the resonance wavelength of the WF-VCSEL up to ~ 35-50 nm will make it possible to suppress the undesirable SA effect in a wide range of the BTJ mesa sizes maintaining the single-mode lasing. Keywords: vertical-cavity surface-emitting laser, wafer fusion, molecular beam epitaxy, single-mode operation, saturable absorber.

Высокоскоростные одномодовые вертикально-излучающие лазеры спектрального диапазона 1550 нм

The results of complex studies of static and dynamic performance of 1550 nm-range VCSELs, which were created by direct bonding (wafer fusion technique) InAlGaAs/InP optical cavity wafers with AlGaAs/GaAs distributed reflector wafers grown by molecular beam epitaxy, are presented. The VCSELs with a buried tunnel junction diameter less than 7 μm demonstrated a single-mode lasing with a side-mode suppression ratio more than 40 dB; however, at diameters less than 5 μm, a sharp increase in the threshold current is observed. It is associated to the appearance of a saturable absorber due to penetration of optical mode into the non-pumped regions of the active region. The maximum single-mode output optical power and the–3 dB modulation bandwidth reached 4.5 mW and 8 GHz, respectively, at 20°C. The maximum data rate at 20ºC under non-return-to-zero on-off keying modulation was 23 Gb/s for a short-reach link based on single-mode fiber SMF-28. As the length of the optical link increased up to 2000 m, the maximum data rate dropped to 18 Gbit/s. The main factors affecting the high-speed operation and data transmission range are defined and discussed, and the further ways to overcome themit are proposed.

1550 nm range high-speed single-mode vertical-cavity surface-emitting lasers

The results of complex studies of static and dynamic performance of 1550 nm-range VCSELs, which were created by direct bonding (wafer fusion technique) InAlGaAs/InP optical cavity wafers with AlGaAs/GaAs distributed Bragg reflector wafers grown by molecular beam epitaxy, are presented. The VCSELs with a buried tunnel junction diameter less than 7 μm demonstrated a single-mode lasing with a side-mode suppression ratio more than 40 dB; however, at diameters less than 5 μm, a sharp increase in the threshold current is observed. It is associated to the appearance of a saturable absorber due to penetration of optical mode into the non-pumped regions of the active region. The maximum single-mode output optical power and the -3 dB modulation bandwidth reached 4.5 mW and 8 GHz, respectively, at 20oC. The maximum data rate at 20oC under non-return-to-zero on-off keying modulation was 23 Gb/s for a short-reach link based on single-mode fiber SMF-28. As the length of the optical link increased up to 2000 m, the maximum data rate dropped to 18 Gbit/s. The main factors affecting the high-speed operation and data transmission range are defined and discussed, and the further ways to overcome themit are proposed. Keywords: VCSEL, wafer fusion, molecular beam epitaxy, single-mode operation, high-speed performance.

Characterization of lasing regimes of 1.3 µm vertical-cavity surface-emitting lasers based on a short-period InGaAs/InGaAlAs superlattice

Lasing regimes of 1.3 µm vertical-cavity surface-emitting lasers (VCSELs) based on a short-period In0.57Ga0.43As/In0.53Ga0.27Al0.2As superlattice and two Al0.9Ga0.1As/GaAs distributed Bragg reflectors on a GaAs substrate were characterized in this study. VCSEL crystals were mounted on crystal holders with a radio frequency SMA connector and an optical fiber with ferrule connector (FC) for investigation. Current–voltage and output-power–current characteristics, optical and radio frequency spectra, and polarization stability of laser emission were experimentally investigated in the pump current range of 0–14 mA. The threshold current of the studied VCSELs was established to be 1.7 mA. The maximum output optical power at the output of the optical fiber of the VCSEL based on a short-period In0.57Ga0.43As/In0.53Ga0.27Al0.2As superlattice and the current-induced emission wavelength shift were demonstrated to be 1.34 mW and 0.5 nm/mA, respectively. The small-signal modulation frequency was in the GHz range. The investigated VCSELs can be used in the development of radio photonic devices for telecommunications. Further improvement of the static and dynamic properties of VCSELs is possible by optimizing the design of the short-period superlattice and buried tunnel junction.

High conductivity n-Al0.6Ga0.4N by ammonia-assisted molecular beam epitaxy for buried tunnel junctions in UV emitters

Highly doped n-Al0.6Ga0.4N can be used to form tunnel junctions (TJs) on deep ultraviolet (UVC) LEDs and markedly increase the light extraction efficiency (LEE) compared to the use of p-GaN/p-AlGaN. High quality Al0.6Ga0.4N was grown by NH3-assisted molecular beam epitaxy (NH3 MBE) on top of AlN on SiC substrate. The films were crack free under scanning electron microscope (SEM) for the thickness investigated (up to 1 µm). X-ray diffraction reciprocal space map scan was used to determine the Al composition and the result is in close agreement with atom probe tomography (APT) measurements. By varying the growth parameters including growth rate, and Si cell temperature, n-Al0.6Ga0.4N with an electron density of 4×1019 /cm3 and a resistivity of 3 mΩ·cm was achieved. SIMS measurement shows that a high Si doping level up to 2×1020 /cm3 can be realized using a Si cell temperature of 1450 °C and a growth rate of 210 nm/hr. Using a vanadium-based annealed contact, ohmic contact with a specific resistance of 10−6 Ω·cm2 was achieved as determined by circular transmission line measurement (CTLM). Finally, the n-type AlGaN regrowth was done on MOCVD grown UVC LEDs to form UVC TJ LED. The sample was processed into thin film flip chip (TFFC) configuration. The emission wavelength is around 278 nm and the excess voltage of processed UV LED is around 4.1 V.

Wafer‐fused 1300 nm VCSELs with an active region based on superlattice

The 1300 nm range vertical-cavity surface-emitting lasers with the active region based on InGaAs/InGaAlAs superlattice are fabricated using molecular-beam epitaxy and the double wafer-fusion technique. Lasers with the buried tunnel junction diameter of 5 μm have shown single-mode CW operation with the output optical power of ∼6 mW at 20°C. Opened eye diagrams are observed up to 10 Gbps.

Влияние латерального оптического ограничения на характеристики вертикально-излучающих лазеров cпектрального диапазона 1.55 &amp;mu;m с заращенным туннельным переходом

The impact of transverse optical confinement on the static and spectral characteristics of 1.55 µm vertical-cavity surface-emitting lasers (WF-VCSEL) with a buried tunnel junction (BTJ) n++-InGaAs/р++-InAs/р++-InAlGaAs, implemented using molecular-beam epitaxy and wafer fusion. For lasers with a tunnel junction (TJ) etching depth of ~ 15 nm, it was found that the single-mode lasing occurs up to 8 μm BTJ mesa size due to a relatively weak lateral optical confinement, while the effect of a saturable absorber (SA) appears when the BTJ mesa size is less than 7 µm. Enhancing lateral optical confinement by increasing the BTJ etching depth up to ~ 20 nm leads to suppression of the SA effect at the BTJ mesa size of 5–6 µm, but at the same time limits the maximum single-mode optical power. According to the results of the analysis, an increase in the spectral mismatch between the maximum of the gain spectrum of the active region and the resonance wavelength of the WF-VCSEL up to ~35-50 nm will make it possible to suppress the undesirable SA effect in a wide range of the BTJ mesa sizes maintaining the single-mode lasing.

The Effect of a Saturable Absorber in Long-Wavelength Vertical-Cavity Surface-Emitting Lasers Fabricated by Wafer Fusion Technology

An investigation has been performed of 1.55-μm vertical-cavity surface-emitting lasers based on heterostructures with a buried tunnel junction (BTJ) with a height difference of 15 nm. The devices are obtained by wafer fusion of heterostructures grown by molecular beam epitaxy and provide single-mode lasing at a BTJ diameter of up to 8 μm. A decrease in the BTJ size leads to a sharp increase in the threshold current, the output optical power, and the resonance frequency at the lasing threshold. Stable single-mode lasing takes place due to the smoothed boundary of the buried surface relief, which induces a gradual change in the profile of the effective refractive index in the lateral direction with the effective current confinement retained. This makes it possible to reduce significantly the transverse optical confinement factor for the higher-order modes even at a large BTJ size. However, at a small BTJ size, it leads to the formation of a saturable absorber in unpumped parts of the active region.

Effect of saturable absorber in 1.5 μm wafer-fused vertical cavity surface-emitting lasers

The static characteristics of 1.5 μm wafer-fused VCSEL with the thin n++-InGaAs/p++-InGaAs/p++-InAlGaAs tunnel junction (TJ) were investigated. The devices with the 8 μm diameter of the buried tunnel junction (BTJ) demonstrate effective single-mode lasing. A rapid increase in the threshold current, accompanied by an abrupt change in the output optical power is observed for devices with BTJ-diameter less than 7 μm. The observed behavior can be explained by increasing in the overlap of the optical mode with the unpumped (passive) part of the active region, which leads to the additional optical absorption. On the one hand, since the high-order transverse modes have the higher optical losses due to larger overlap with the unpumped part of the active region, then lasing via fundamental mode is more preferable even at large size of BTJ region. On the other hand, the reduction of effective index step between the BTJ region and the regions outside BTJ (caused by using small TJ etching depth) and the lateral smoothing of the difference in the height of the surface relief after TJ regrowth results in the significant drop of the transverse optical confinement factor and rise of absorption. Moreover, the absorption effect can be also strongly enhanced by increasing the heatsink temperature due to decreasing in the gain-to-cavity detuning.

Room-temperature continuous-wave operations of GaN-based vertical-cavity surface-emitting lasers with buried GaInN tunnel junctions

We report GaN-based vertical-cavity surface-emitting lasers (VCSELs) with buried GaInN tunnel junctions (TJs). Under room-temperature continuous-wave operation, the VCSEL with an 8 μm aperture exhibited a low operating voltage of 5.3 V at 10 kA cm−2 and a differential resistance of 110 Ω. In addition, the VCSEL with a 10 μm aperture showed a threshold current of 14.4 mA and a peak output power of 2.0 mW. An influence of an absorption loss at the GaInN TJ on laser performance was estimated using an analysis of a one-dimensional optical intensity profile based on a cross-sectional STEM image.

1.55-μm-Range Vertical-Cavity Surface-Emitting Lasers, Manufactured by Wafer Fusion of Heterostructures Grown by Solid-Source Molecular-Beam Epitaxy

The results are presented of a study and optimization of the conditions under which heterointerfaces of the GaAs–InGaAsP type are formed via the direct intermolecular wafer bonding (fusion) of a heterostructure of an active region on an InP substrate and distributed Bragg reflectors on GaAs substrates upon the fabrication of hybrid heterostructures for long-wavelength vertical-cavity surface-emitting lasers (VCSELs). The heterostructures are grown by solid-source molecular-beam epitaxy. It is shown that, in the case of the incomplete removal of oxide films during preparation of the wafers before fusion and/or in the presence of adsorbed water on the wafer surfaces, the fused interface contains a large number of amorphous inclusions, which are most likely oxides of Group-III elements. Optimization of the conditions in which a buried tunnel junction is formed on the surface of the heterostructure on an InP wafer made it possible to reduce the surface roughness to 1 nm and to ensure that the thickness of the GaAs–InGaAsP fused interface does not exceed 5 nm, with no dislocations or other extended defects found at the fused heterointerfaces. The 1.55-μm VCSELs fabricated from the hybrid heterostructures created using the developed technology demonstrate efficient lasing under continuous-wave pumping in a wide temperature range, which is indicative of the high optical quality of the fused heterointerfaces in the VCSEL structure.

The fabrication technology of VCSELs emitting in the 1.55 μm waveband

The paper presents the results on fabrication technique of a 1,55 μm wafer-fused vertical-cavity surface-emitting lasers (VCSELs) based on the InAlGaAsP/InP optical cavity and AlGaAs/GaAs distributed Bragg reflectors (DBRs) grown by solid-source molecular beam epitaxy. The optical cavity InAlGaAsP/InP containes an active region based on multiple InGaAs quantum wells, n++/p++-In (Al)GaAs buried tunnel junction (BTJ), InGaAsP intracavity contact layers and n-InP spreading layers. The top and bottom AlGaAs/GaAs DBRs were grown on GaAs substrate, while the optical cavity InAlGaAsP/InP was grown on InP substrate. The main fabrication proceseses of 1,55 μm VCSELs such as: BTJ fabrication, dry etching of first mesa on top DBR, wet etching of second VCSEL mesa, forming of the ohmic contacts and passivation of the VCSEL structure were described in details.

Theoretical Analysis of Tunnel-Injected Sub-300 nm AlGaN Laser Diodes

Electrically-pumped AlGaN-based edge-emitting laser diodes with a buried tunnel junction (TJ) for sub-300 nm emission are designed in this paper. Hole injection is one of the major concerns for the design of ultraviolet (UV) lasers based on this material system. The use of a low-resistive TJ as an intracavity contact within the devices will offer an opportunity to replace highly resistive p-type AlGaN-based cladding and contact layers by their n-doped counterparts. This advanced polarization-engineered interband TJs will lead to improved hole injection and a significantly reduced threshold voltage. The thermal properties of the tunnel-injected devices are thoroughly studied theoretically. For the demonstration of continuous-wave operating lasers, possible improvements in terms of better thermal management of the device are also discussed. Our improved design allows CW-operating AlGaN lasers with a threshold current density of <; 8 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and maximum optical output power of > 220 mW, yielding a wall-plug efficiency of > 2.8%.