Conclusions
In this study, we have explored and developed the process for fabrication of long wavelength vertical cavity surface emitting lasers (LW-VCSELs). We have achieved the following benchmarks with the assistance of modern epitaxial technology provided by metal organic chemical vapor deposition (MOCVD) system.
(1) Optimization of gain mediums:
We have obtained impact factors for InGaAlAs based multiple quantum wells (MQWs). The amount of compressive strain strongly influences the threshold current density. However, the net strain amount has to be calculated and balanced when applying more quantum well numbers to increase the gain. In addition, the impurity concentration strongly influences the performance of edge emitting lasers (EELs) due to the extent of the internal loss. The overall optimization of these factors makes us obtaining low threshold current density of 1.45 kA/cm2 in EELs with 300 µm cavity length and 30%/85% facet coatings.
(2) High reflectivity InP/InGaAlAs-based distributed Bragg reflectors (DBRs):
We have grown InP/InGaAlAs and InAlAs/InGaAlAs DBRs with excellent electrical and optical properties using MOCVD and the growth interruption technique.
The DBRs show low resistance with an estimated resistance per DBR pair of 1.2×10-5 Ωcm2 and 2.2×10-5 Ωcm2 for the InP/InGaAlAs and InAlAs/InGaAlAs DBRs, respectively. The maximum reflectivity of both DBRs exceeds 99% with a stopband width of 110 nm for InP/InGaAlAs DBR and 100 nm InAlAs/InGaAlAs DBR.
Although the InP/InGaAlAs DBRs have better optical and electrical properties, the InAlAs/InGaAlAs DBRs has much lower growth complexity. Both DBR structures should be applicable for fabrication of long wavelength VCSELs in 1.5~1.6 µm range.
(3) High reflectivity InP/Airgap DBRs:
We have designed, fabricated, and demonstrated a rigid InP/airgap structure with high reflectivity at 1.54 µm using InGaAs as the sacrificial layer. The 3-pair InP/airgap DBR structure with 5 λ /4 thick InP layer was fabricated from the MOCVD grown InP/InGaAs structure using H2SO4 solution as etching agent. The InP/airgap DBR has a peak reflectivity at 1.54 µm with a stopband width of about 200 nm. The InP/airgap DBR structure was rigid and stable and should be applicable for 1.5 µm VCSELs.
(4) Demonstration of fusion techniques:
We have setup a wafer fusion systemand established the wafer fusion techniques and process conditions. Smooth fusion interfaces have been observed with cross-section SEM images. The crystal quality of InGaAlAs MQW do not degrade.
The reflectance spectra, stopband and maximum reflectivity of GaAs/AlAs DBR before and after fusion process do not change. Reliable fusion mechanical intensity, smooth fusion interface, and good optical and electrical characteristics of fusion interfaces provide good basement for further fabrication of LW-VCSELs.
(5) Successful demonstration of optically pumped InP-based LW-VCSELs:
We also demonstrated the optically pumped InP-based VCSELs with the 35 pairs InP/InGaAlAs DBRs and 10 pairs SiO2/TiO2 top dielectric mirrors and a 2λ thick cavity composed periodic strain compensated MQWs to fully utilize the gain in every quantum well. The optically pumped VCSELs operated at room temperature with the
threshold pumping power of 30 mW. The wavelength of the output beam is 1562 nm.
The minimum linewidth above threshold is 1 nm limited by the resolution of the spectrometer. The equivalent threshold current density is calculated to be 2 kA/cm2 when taking into account the absorption of the pumping light in the cladding layers and the reflection at the surface.
(6) Successful demonstration of optically pumped LW-VCSELs made by wafer fusion:
We demonstrated the optically pumped VCSEL structure with the fused bottom 30 pairs GaAs/AlAs DBR, InGaAlAs MQW and the fused top 25 pairs GaAs/AlAs DBR. The optically pumped double fused VCSELs operated at room temperature with the threshold pumping power density of 5 kW/cm2 and emitted laser at 1527nm.
The equivalent threshold current density is calculated to be 4 kA/cm2. We attributed this two-fold value compared with the threshold current density obtained in InP-based LW-VCSEL to the strong absorption of pumping light.
(7) Demonstration of regrowth and buried tunnel junction devices:
We have demonstrated the prototype long wavelength light emitter with buried tunnel junction by regrowth technique in MOCVD reactor. Smooth regrown interface and flat wafer surface have been obtained. The preliminary emission shows the good current blocking scheme outside the buried tunnel junction area. The built-in index-guiding characteristics provided by the buried tunnel junction should be applicable in single mode LW-VCSELs.
Future Works
Although many benchmarks have been done, our long-term goal to fabricate electrically driven continuous wave (CW) LW-VCSELs with single mode operation has yet to be fulfilled. All previous steps will provide precious information and
experience in the future. To achieve this goal, the following actions require to be done:
(1) Improvement of dielectric mirror quality:
In hydride type of VCSEL, the quality of the dielectric mirror is important.
When the device is electrically driven, the access heat and high surface carrier density between the epitaxial layer and dielectric mirror easily damage the surface and coating materials. The combinations of a-Si/Al2O3 will be tested since they have larger refractive index difference and better thermal conductance in comparisons to the combinations of SiO2/TiO2.
(2) Optimization of p-type contacts:
The common feature in our electrically driven LW-VCSELs discussed in chapter 7 is the relatively high operation voltage. One main contribution of the high operation voltage is the poor ohmic condition of p-type contacts. The insertion of a small bandgap material, e.g. InGaAs, between the p-type metal contact and InP layer should be helpful to reduce the operation voltage.
(3) Development of C doping in In-contained layers grown by MOCVD:
To further increase the tunneling efficiency of the tunnel junction, the abrupt junction interface is essential for tunneling current with low reverse bias voltage.
P-type dopant with low diffusion characteristics, such as C, is more appropriate for heavily doping layer in tunnel junction instead of the highly diffusive p-type dopant, Zn. Although the C doping is relatively difficult in In-contained materials with percentage of In more than 50% , the special epitaxial technique has to be established first in the future.
(4) Optimization of serial resistance in p-type DBRs:
In our demonstration of electrically driven double-fused LW-VCSEL, the
relatively high operation voltage is the main problem. Optimization of the p-type doping in GaAs/AlGaAs DBRs and fusion conditions will be the initial steps in the future.
(5) Design and fabrication of air-gap type LW-VCSELs for optical pumping:
LW-VCSELs with extremely high reflectivity air-gap DBRs is potentially applicable in wavelength tunable devices. However, the heat dissipation and current aperture formation are quite difficult. The regrowth technique and buried tunnel junctions can be further applied in the process of air-gap type VCSELs.
(6) Analysis of transverse mode characteristics in LW-VCSELs:
Single mode operation of VCSEL is very important in fiber communications. We need to study the transverse mode characteristics and understand more about the scheme of mode patterns in VCSELs before we design and fabricate single-mode LW-VCSELs.