Long Wavelength Surface Emitting Lasers: Introduction
- Fig. 1: Mounted VCSEL
Vertical Cavity Surface Emitting Laser diodes (VCSELs) are semiconductor devices with light emission perpendicular to the chip surface. They are highly attractive for applications in optoelectronics, since they offer several advantages compared to conventional edge-emitting (in-plane) laser diodes, such as:
- low electric power consumption,
- capability of on-wafer testing,
- simplified fiber coupling and packaging,
- longitudinal single-mode emission spectrum, and
- suitability for 2D-array integration.
- Fig. 2: Processed wafer
In the past years, several projects have been concerned with the development and optimization of GaAs-based VCSELs in the near infrared (<1.3µm) at the Walter Schottky Institute. Different concepts for current confinement have been realized, such as blocking layers and selective oxidation.
Also, VCSELs with quantum dots as active material have been demonstrated in continuous-wave mode operation at room temperature.
- Fig.3: Bandgap energy and wavelength vs. lattice constant
Particular interest with respect to fiberoptical communication systems exists for those VCSELs that are emitting at wavelengths around 1.31µm or 1.55µm because of minimum dispersion or absorption, respectively, in silica fiber. Eligible material system of compound semiconductors for these wavelength are
- (GaIn)(NAs) on GaAs substrate for 1.31µm, and
- (InGaAl)As, (InGa)(AsP), and (AlGa)(AsSb) on InP for both 1.31µm and 1.55µm.
However, the realization of VCSELs with reasonable characteristics suffers from several technological challenges related to the required materials, especially in the 1.55µm wavelength case:
- Low refractive index-contrast of Distributed Bragg Reflector (DBR) mirrors
- Poor thermal conductivity of ternary- (quaternary-) compound semiconductor DBRs
- Poor light amplification performance at elevated temperatures
- Selective oxidation (of InAlAs) expected to cause material damage
Long Wavelength Surface Emitting Lasers: Device Structure
- Fig.4: Fabrication process of the buried tunnel junction
To overcome these obstacles, the device concept of VCSELs with a Buried Tunnel Junction (BTJ) in conjunction with a dielectric DBR has been developed at the WSI. The fabrication as well as the functional principle of BTJs are shown in fig. 4. A heavily p+/n+ doped layer-pair of low-bandgap material is prepared by Mo-lecular Beam Epitaxy (MBE). It provides an ohmic-type tunnel interface that translates a current from high-resistive p-doped to low-resistive n-doped material, which substantially reduces heat generation in the device. Then, a circular or elliptic mesa is formed in a Reactive Ion Etching (RIE) process and regrown ('buried') in a second epitaxy run with n-doped InP. If a voltage is applied as indicated, the n/p+ junction aside the rest of the tunnel junction has reverse bias and therefore blocks the electrical current, which instead goes through the BTJ. This mechanism confines the current to the active region of the device, which is essential for proper VCSEL operation. Since the tunnel interface sheet resistance is as low as 3x10-6 Ωcm2, which is competitive to advanced p-side metal contacts, the voltage drop is negligible.
- Fig.5: Complete VCSEL structure
Due to the integration of the BTJ, the VCSELs are slightly thicker at the center (ΔL / Leff >> 1%) wherefore the boundary conditions of the optical field yield an efficient wave-guiding that is of comparable magnitude to that ofsteam-oxidized GaAs-based VCSELs. The optical and the electrical guidance by the BTJ are conveniently self-aligned. In the complete BTJ-VCSEL structure (fig. 5) the BTJ is found upside down, and the InP substrate is totally removed. The active region where the light is generated is placed above the BTJ. For a λ=1.55µm device, it comprises five 8nm thick compressively strained InGaAlAs quantum wells. The front side mirror atop the active region is an epitaxial DBR with about 35 layer-pairs of InGaAlAs/InAlAs yielding a calculated power reflectivity of 99.4%. The back side mirror consists of a dielectric layer stack as a DBR in conjunction with a gold termination that boosts the reflectivity to approximately 99.75%. Because of the higher refractive index contrast of the dielectric materials (e.g. amorphous CaF2/Si for λ=1.55µm),the dielectric DBR is significantly thinner compared to an equivalent InP-based semiconductor DBR and has a smaller thermal resistance, even though the resistivity of the dielectric materials can be higher. This is important for the heat sinking in the Laser. For example, a device with a BTJ diameter DBTJ of 10µm enabling a dielectric back reflector could have a thermal resistance of 3000 K/W Instead of 8000 K/W with an equivalent InP-based semiconductor DBR.
- Fig.6: Cross-section of a VCSEL structure
Current is injected through the integrated heat sink of electroplated gold (+) and a gold pad on the top (-), as indicated by the animation in fig. 5. It can be seen that except for a thin layer of ~100nm (pink and red in fig. 5) the whole current path is n-doped. This is especially important for the current injection laterally around the dielectric material (dark green in fig. 5) where a p-doping would cause too much heat.
Long Wavelength Surface Emitting Lasers: Results
VCSELs at 1.55µm wavelength
- Fig.7: Optical output power and driving voltage vs current
Characteristics of a VCSEL with circular BTJ of 20µm diameter are shown in fig. 7. It features an output power of 7mW, which is the highest reported output power for electrically pumped long-wavelength VCSELs. At the threshold current of 6mA, the voltage drop is just about 0.9V, and due to a low series resistance of 11 Ohms, the operation voltage at 7mW is still below 1.5V. VCSELs with small elliptical BTJs of 2µm x 3µm exhibit a low threshold current of 0.43mA, a slope efficiency of 30% and an output power of 0.7mW in continuous-wave mode (cw, i.e. not pulsed) operation at 20°C. These devices also have a threshold voltage of about 0.9V.
- Fig.8: Temperature characteristics of BTJ-VCSELs at 1.55µm wavelength
Exemplary temperature-dependent characteristics are given in fig. 8 (left) for a device with an elliptical BTJ, 5µm x 6µm in diameter. Lasing activity in cw-mode operation can be sustained up to 110°C, and the maxi-mum output power at 80°C still amounts to 0.5mW. This outperforms any other reported VCSELs for 1.55µm wavelength [1,2]. The curve to the right hand side describes the dependency of the threshold current on the external temperature. A minimum is observed at 40°C, which can be explained by a stronger overlap of the spectral distribution of the active material's optical gain with the eigenfrequency of the laser resonator.
VCSELs between 1.3µm and 2.0µm wavelength
The concept of long-wavelength VCSELs in the (InGaAl)As/InP material system employing a BTJ has proved suitability for a wavelength range from 1.3µm to 2.0µm. Fig. 9 and 10 show output power versus current in cw-mode operation at room temperature for VCSELs at 1.8µm and 2.0µm wavelength, which is the longest wavelength for electrically pumped VCSELs in cw-mode operation at room temperature that is known to the author. Moreover, excellent performance is achieved with 1.83µm-VCSELs, such as output powers exceeding 1mW with large devices and threshold currents as low as 190µA with small devices. Lasing activity in cw-mode operation has been demonstrated to sustain up to 90°C. These properties can be attributed to a relatively high index step in the InGaAs/InAlAs epitaxial front DBR, strongly strained InGaAs quantum wells in the active region, and a high-reflective Al2O3/Si back DBR with practically no optical absorption.
- Fig.9: PI-characteristics for VCSELs with emission at 1.8µm and …
- Fig.10: … 2µm wavelength for several diameters
BTJ-VCSELs have also been made for a wavelength of 1.3µm, with only minor modifications to devices at 1.55µm, such as 6 quantum wells, more layer pairs in the front side DBR, and materials with higher bandgap in general. Typical PI- and VI-characteristic for these devices are illustrated in fig. 11, the spectrum in fig. 12 shows a side mode suppression ratio (SSR) of as much as 50dB.
- Fig.11: Typical PI- and VI-characteristics for VCSELs at 1.3µm
- Fig.12: Spectrum with high side mode suppression ratio
Long Wavelength Surface Emitting Lasers: Applications
- Fig.13: Measured beam profiles of VCSELs at 1.55µm with BTJ diameters of 5, 10, and 13µm
In most areas of application for long-wavelength VCSELs - fiberoptical com-munications and gas monitoring - single-mode emission is required not only longitudinally, what is accomplished with any VCSEL, but also transversally (see fig. 12). This means that in a cross-section of the laser beam the intensity should be distributed like in the upper half of fig. 13. In addition, the optical field's polarization must be pure and stable. This is supported by elliptically shaped BTJs that are easy to manufacture since they are defined by standard photomask lithography.
Trace Gas Sensing
- Fig.14: Setup for trace gas sensing with single-mode lasers
A highly encouraging application for VCSELs is the monitoring of trace gases such as CH4, HCN, H2O, HCl, N2O, CO, and CO2, which have typical absorption lines in the infrared. To determine the types of gases and their con-centration, the electrical current through a single-mode VCSEL is modulated, the inner temperature of the VCSEL follows, and so does the emission wavelength. Tuning ranges of more than 5nm are achievable. The laser beam travels through a probe chamber with the gas mixture to analyze and the transmitted light intensity is measured by a detector (fig. 14). Then, the relative intensity loss com-pared to the evacuated chamber is computed. Since this simple yet efficient and fast-response method requires less than 1mW of laser power, VCSELs are the light sources of choice due to low power consumption, low price and eye-safety reasons.
- Fig.15: Measurement of water vapor absorption lines with 1.8µm-VCSEL
Typical modulation frequencies for this technique are in the kHz regime and above, which renders time-resolved concentration measurements in chemical processes possible. On the other hand, this 'chirp' effect is too slow to interfere in multi-channel fiber-optical communication systems with modulation frequencies in the GHz regime. Fig. 15 shows experimental data obtained with a 1.8µm BTJ-VCSEL, several absorption lines of water vapor were observed.
- Fig.16: Modulation characteristics of a VCSEL at 1.55µm for optical communications
The InGaAlAs/InP material system is known to be well suited for direct modulation of laser diodes for high-speed communication systems at 1.55µm. Therefore the BTJ-VCSELs, which utilize this material in the active region, show excellent intrinsic modulation characteristics (fig. 16). In conjunction with the low series resistances that are typical for the BTJ concept, trans-mission at 10Gbit/s is feasible. Fig. 17 shows eye diagrams of a back-to-back measurement at 5 and 10Gbit/s, respectively.
- Fig.17: Eye diagrams of a back-to-back fiberoptical transmission system with 1.55µm-VCSELs at 5 (left) and 10 GBit/s (right).