Consumer applications of VCSEL arrays demand larger sizes and improved reliability. Ge-substrates are drop-in replacements for GaAs, whilst offering additional benefits. Thinner Ge-substrates are readily available due to photovoltaic supply chains, offering a cheaper cost per wafer. We report on device performance of identical 940-nm VCSELs, grown on 675µm, 500µm and 225µm thick Ge-substrates. Threshold current densities vary by less than 10μA/µm2 and 36μA/µm2 at the wafer centre between 675µm and, 500µm and 225µm respectively. A 3nm wavelength shift with decreasing substrate thickness is also observed. Results show a potential route to larger manufacturing volumes with lower cost per wafer.
1-port and 2-port multi-mode interference reflectors (MMIR) are excellent components for Photonic Integrated Circuits, being highly reflective and easy to fabricate. We demonstrate InAs-Quantum-Dot MMIR lasers, where the high reflectivity is particularly advantageous, with lower threshold current than Fabry-Perot ridge lasers with the same cavity length e.g. 6mA compared to 46-mA. The threshold current density of the 1-mm MMIR laser is equivalent to the Fabry-Perot laser with a 3-mm cavity length. MMIRs have a higher optical slope efficiency, indicating mirror reflectivity above 85%.
InAs QD lasers emitting in the 1.3-μm-region have suitable device properties important for integrated applications and growth on silicon. Sensing applications have encouraged further development of these wavelengths for high-volume-manufacturing. Assessment of epitaxial wafers is demonstrated here by fabrication of oxide isolated broad-area edge-emitting-lasers and on-wafer characterization of 150-mm p-doped InAs QD wafers grown via MBE. We report on spatial variations through Power-Current-Voltage-Wavelength measurements with Jth of EELs calculated using current spreading structures. A 9 nm decrease in center-to-edge emission wavelengths is observed for 2mm devices, with a threshold current density variation of approximately 0.63 kA/cm2 for a particular epitaxial design.
Emerging consumer applications of VCSEL arrays demand larger sizes and improved reliability. Significant wafer bow seen on a 150-mm GaAs-substrate wafer can impact fabrication, characterisation, and yields. It has been reported that Ge-substrates are drop-in replacements for GaAs, but also have additional benefits. We report on the spatial performance of identical 940 nm VCSELs, grown on both types of 200-mm substrate. Threshold current densities vary by 0.1μA/cm2 at the wafer centre, and a 0.78% and 0.59% decrease in centre-to-edge emission wavelengths for Ge and GaAs respectively. Results show a potential route to larger manufacturing volumes with lower costs per wafer.
We report on a study using VCSEL Quick Fabrication (VQF) devices for the rapid assessment of epitaxial structures designed for emission at 894nm grown on 100mm substrates. A comparison of measured VQF device results to the epitaxial design specification allows for the extraction of key variances across the wafer and the identification of their potential causes. We also demonstrate the applicability of this technique for the assessment of uniformity and reproducibility of 150mm VCSEL wafers for emission at 940nm, identifying the potential sources for observed variations in device performance that impact in specification device yield.
We demonstrate high-gain InAs QDs targeting C-band and L-band using a five QD-layer structure grown via MOCVD, with a photoluminescence broadening of ~55 meV. Lasers were fabricated with cavity lengths from 2000-µm down to 333-µm, with cleaved un-coated facets. Threshold current density increases monotonically with temperature over a range of 300 K to 380 K for all cavity lengths with a factor of 3.0 and 3.4 increase for the longest and shortest cavities, respectively. Measurements of lasing spectrum reveal that even the shortest cavity maintains a stable increase in wavelength up to 390 K with no observable transition to the excited state.
We introduce direct n-doping of quantum dots together with modulation p-doping as a technique to reduce both the threshold current density and the temperature dependence of threshold current density in 1.3um emitting quantum dot lasers. Threshold current density in 1mm long QD lasers with cleaved and uncoated facets is effectively halved at both 27°C and at 97°C when using co-doping as compared to the undoped case. Results indicate that modulation p-doping can improve the threshold current temperature dependence and direct n-doping reduces the magnitude of threshold current density and that the benefits of each is maintained when used together.
We report on the epi-design and characterisation of VCSELs for atomic sensors, including miniaturised clocks and magnetometers. To understand how epi-design impacts device performance and separate this from effects of growth and fabrication, we employ techniques to study the interplay between optically-active gain medium and the cavity-resonance. We experimentally determine the net modal-gain spectrum of VCSEL material using a single-pass stripe-length method covering the range of pumping and hence gain requirements of VCSELs. This is compared to photovoltage spectroscopic measurements, which are used to determine the quantum well transition energies and cavity resonance, aiding further optimisation of device design.
The strain-induced wafer bow for VCSEL epitaxial structures grown on GaAs substrates is measured and compared to that of Ge substrates. We find that the ~ 160 μm height difference between the centre and edge of a GaAs wafer results in a significant temperature gradient and hence has a large effect on oxidation rate in the high-Al layer in the top DBR of the epi-structure. We measure a resultant centre-to-edge variation in oxidation length of ~ 3 μm for a GaAs wafer. We assess the contributions of wafer bow and epi-layer non-uniformity, as well as temperature variation in the furnace, and find that the effect of the bow dominates.
We employ a Very Quick Fabrication (VQF) method to rapidly produce oxide confined VCSELs across a 150 mm GaAs substrate wafer to assess the impact on device performance. By measuring threshold current density between 20 and 70 ℃, we find ~ 25 ℃ variation in the temperature corresponding to the alignment of the spectral peak of gain with the cavity resonance wavelength. However, we still find that the threshold current density at zero detuning, is lower for edge devices, which we attribute to material variation.
We disentangle the different contributions to device performance to isolate the effect of material variation. We compare this remaining spatial non-uniformity to that of VCSELs grown on Ge substrates.
A simplified fabrication process for VCSELs which employs oxidation-vias for definition of the laser aperture and bond pad is applied to a full 150mm wafer as a technique for material characterisation. This Quick Fabrication process produces representative VCSELs, with performance comparable to standard process VCSELs, with threshold currents for 8μm oxide-aperture devices measured between 0.8 and 1.3mA for both device types. The redshift of the lasing wavelength and threshold currents are used for rapid assessment of the VCSEL wafers.
Development of a quick fabrication (QF) method for commercial wafer characterisation based on rapid feedback of VCSEL performance. We report on the design of the fabrication process including the systematic removal of time-consuming steps of planarization, oxidation and substrate lapping, and the associated impact on device performance and yield. We show comparable performance of the oxide-confined QF etched trench VCSELs and full process devices and we show that unoxidised devices behave as large aperture oxidised devices. Further, we demonstrate similar performance of substrate-lapped and -unlapped VCSELs between 1.0-1.2 Ith with a difference in current tuning typically 0.064nm/mA.
High-volume low-cost production of vertical cavity surface emitting lasers (VCSELs) will allow their exploitation in new commodity markets. We report the successful scaling up from research level fabrication to produce oxide confined VCSELs across a whole 150mm wafer. On-wafer light-current-voltage (L-I-V) and spectral measurements are analyzed to determine the cross-wafer variations in threshold current, threshold current densities and emission wavelength, which is compared with reflectivity measurements taken immediately after growth. We examine the dependence of VCSEL performance on fabrication parameters over a range of device dimensions to assess whether variation arises from non-uniformity of the epitaxial material or wafer processing.
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