The feasibility of the active layers and DBRs discussed in previous two chapters is examined by the performance of optically pumped LW-VCSELs. This chapter reports the structures and the characteristics of LW-VCSELs for optical pumping, including the InP-lattice-matched and wafer-fused structures.
6-1 Structures and Characteristics of Optically Pumped LW-VCSELs Based on InP/InGaAlAs DBRs
The Structure of LW-VCSEL
To further validate the viability of the DBR structures for long wavelength VCSELs, we have grown a laser structure based on the InP/InGaAlAs DBRs using the growth interruption technique. Figure 6-1 shows the laser structure. The epitaxial layers were grown on an n-type InP substrate. The first step was to grow the 5/4λ thick InGaAlAs and InP calibration layers followed by the 35 pairs InGaAlAs/InP DBRs. The interruption time was 0.2 minute. The laser reflectometry monitored the epitaxial growth. The 5/4λ thick InGaAlAs and InP calibration layers were first grown to check the growth rate and growth conditions before the entire DBRs were grown. The 2λ thick periodic gain cavity was grown in the second step. At the same time, another InP dummy wafer was loaded in MOCVD for photoluminescence (PL) measurement. The laser structure has InGaAlAs cladding layers of 2λ thick with a band edge emission peak at 1.1 μ m. Three sets of strain compensated multi-quantum wells (SCMQWs) were placed at the anti-nodes of the electric standing wave field within the 2λ thick cavity to increase the enhancement factor of
MQW active regions. Each set of SCMQWs consisted of five InGaAlAs (strain = 1.37%, thickness = 5.5 nm) quantum wells and four InGaAlAs (strain = -0.6%, thickness = 9.3 nm) barriers and the PL wavelength was tuned at 1.51 μm, which is 40 nm blue-shifted from the target emission wavelength to insure the proper operation of the VCSELs at room temperature [1]. Two InP space layers of half λ thickness grown on the top and the bottom of the cavity were served to protect the InGaAlAs layer from being oxidized during the post processing. Finally, the wafer was coated with 10 pairs SiO2/TiO2 top dielectric mirrors to form a complete VCSEL structure.
The Results of Optical Pumping
Figure 6-2 shows the reflectivity and PL measurements during every process step. All of the reflectivity results were normalized to the reflectivity of Au film.
Figure 6-2(a) shows the reflectivity curve of 35 pairs InGaAlAs/InP DBRs. The center wavelength of the stop-band is located at 1555 nm. Figure 6-2(b) shows the reflectivity of the half cavity VCSEL. The PL spectrum of the active regions grown on the InP dummy wafer in the same run is shown in Figure 6-2(c). The PL peak is 1510 nm and the full width half maximum (FWHM) is 54 nm. The peak of shorter wavelength was the signal belonged to the previous grown layers on the InP dummy wafer. Figure 6-2(d) shows the reflectivity of the complete VCSEL structure and the Fabry-Perot dip is located at 1558 nm. Figure 6-2(e) demonstrates the PL spectrum of the complete VCSEL structure. The peak wavelength is coincided with the Fabry-Perot dip and the FWHM is measured to be 3.3 nm. The equivalent quality factor, Q, is estimated about 470 in the vertical direction.
The complete VCSEL structure was placed in an optical pumping system as shown in Figure 6-3. The pumping source was a continuous wave operated Ti:sapphire laser. The wavelength of the Ti:sapphire laser was tuned at 990 nm
although the pumping wavelength at slightly beyond 1.1 μm would be more desirable to avoid absorption of the InP (λg = 0.9 μm) space layers and InGaAlAs (λg = 1.1 μm) cladding layers. The diameter of the pumping beam entered from the
top dielectric DBRs was estimated to be 30 μm. Figure 6-4 shows the pumping result of the VCSEL. The threshold pumping power is 30 mW at room temperature.
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 red shift of the peak wavelength from the Fabry-Perot dip was attributed to the local heating caused by the strong absorption. 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. Compared this number with the best threshold current density obtained in chapter 4, which is about 1.45 kA/cm2 shown in Figure 4-11, the relatively large threshold current density might be attributed to two reasons. One is the non-optimized quantum well structure in terms of the amount of net strain in periodic gain structure. The other is the relatively large cavity loss such as the absorption of the pumping power by the 2λ thick cladding layers in the laser structure. All in all, the quality of the DBRs and the active region have been basically qualified in this demonstration for the first step in LW-VCSEL process.
6-2 Structures and Characteristics of Optically Pumped LW-VCSELs with Wafer-fused GaAs/AlAs DBRs
High reflectivity DBR and high gain active region are important for LW-VCSEL.
In this section, we first find the influence of the distance between fusion interface and active region on active region photoluminescence intensity. Second, we compare the
PL signal of the InGaAlAs MQW before and after fusion process. The reflectivity of DBR before and after fusion process is also compared. Finally, we demonstrate VCSEL structures with wafer-fused DBRs for optical pumping.
Influence of Distance between Fusion Interface and Active Region
A stack of 290 nm InP / MQW / 290 nm InP / 180 nm InGaAs epitaxial layers as shown in Figure 6-5 was grown by MOCVD on InP substrate. The MQW structures consisting of a 70 nm In0.52Al0.48As inner cladding layer followed by six In0.39Ga0.46Al0.15As barriers with thickness of 7 nm, and six In0.73Ga0.2Al0.07As wells with thickness of 6nm, then another inner cladding layer In0.52Al0.48As with thickness of 70 nm. The two InP layers served as spacers. InGaAs was the etching stop layer.
Another wafer was grown with 25 pairs of GaAs/AlAs on a GaAs substrate.
Before the fusion process, the samples were cleaved into four pieces with dimension of 10*12 mm2. Three of these four samples were subsequently wet etched to change the thickness of the top InP layers. The top InP layers of these four samples are 290 nm, 190 nm, 140 nm and 90 nm. These four samples were fused with GaAs wafers with dimension of 10*12 mm2 at 600°C for one hour in hydrogen atmosphere.
After fusion, the InP substrates and InGaAs stop layers were selective wet chemical etched with HCl : H2O = 3 : 1 and H2SO4 : H2O2 : H2O = 1 : 1 : 10. The process flowchart was shown in Figure 6-5.
Dependence of the PL peak intensity on the distance from the fusion interface is shown as Figure 6-6. The PL peak intensity decreases as the distance from the interface decreases. It has been shown by Ram et al [2] that all dislocations occurring during fusion are localized to within 200 nm from the fused junction. The migrating dislocation might degrade the crystal quality and decrease the PL peak intensity. The dislocations that migrate to MQW increase as the distance from the interface decrease
along with the decrease of the PL peak intensity. Therefore, we should take the distance between fusion interface and active region into consideration when we design the VCSEL structure.
Comparisons of Spectra before and after Wafer Fusion Process
The MQW wafer with the same structure in the last subsection was grown.
Another wafer is the DBR consisting of a stack of 25 pairs 115 nm GaAs / 132 nm AlAs layers with grown on GaAs substrate by MOCVD. Before the fusion process, the MQW and DBR wafers were cleaved into 10*12 mm2 pieces. The PL of one MQW sample was measured using a 532 nm laser with 4.4 mW. The reflectivity of one DBR sample was also measured. After the fusion process with the MQW and DBR samples at 600°C in hydrogen atmosphere, the InP substrate and InGaAs etch stop layer were removed by wet etching. The process flowchat is shown in Figure 6-7.
Figure 6-8 shows the PL spectra of the sample before and after the wafer fusion.
Due to the high reflectivity of the DBR, the PL peak intensity of the sample after fusion was 4.4 times higher than that before fusion process. The peak wavelength of MQW retained at 1521 nm after fusion process. The FWHM of the PL curve for the sample after fusion is 29 nm which is narrower than that (63 nm) of sample before fusion. An optical resonant cavity was formed after the fusion, which modified the peak wavelength and shrink the PL FWHM.
The DBR reflectivity spectra before and after fusion process are shown in Figure 6-9. The stop-band width of the DBR is the same before and after the fusion process.
The maximum reflectivity of DBR is higher than 95%. As mentioned before, stop-band width and maximum reflectivity of DBR are very important for VCSEL device. It was confirmed that the fusion process does not affect these two key characteristics of DBR.
Optical Pumping of VCSEL Structure with Double-fused DBRs
The fusion process flow chart of double-fused VCSEL structure is shown in Figure 6-10. The bottom mirror is a 30-period GaAs/AlAs DBR. The active region is a periodic gain structure consisting of three sets of five InGaAlAs quantum wells and barriers. The three sets of MQW are located at where the antinodes of standing wave pattern locate in the resonant cavity. The top mirror is a 25-period GaAs/AlAs DBR.
One distinct feature is that the cavity length of the wafer-fused VCSEL structure has to be m*λ+0.5*λ (m = 0, 1, 2…) to avoid the anti-node of the electrical field locating at the fusion interface to prevent strong absorption. In addition, the top layer of the DBRs has to be GaAs to avoid the oxidation of AlAs layers during the fusion process. The designed VCSEL structure is shown in Figure 6-11.
Before fusion process, the MQW and DBR wafers were cleaved into 10*12 mm2 pieces. After pre-fusion process as mentioned in the previous section, the MQW and bottom DBR wafers were fused at 600°C in hydrogen at one atmosphere. After first fusion process, the InP substrate and InGaAs etch stop layer were removed by wet etching. In the second fusion process, the fused sample was fused with another 25-period GaAs/AlAs top DBR with a 30 nm InGaP stop layer. Then the GaAs substrate and the InGaP stop layer were removed by wet etching sequently.
The final double-fused VCSEL was placed in the optical pumping system. The setup of optical pumping system is as the same as mentioned in the last section. The wavelength of the pumping laser was tuned at 900nm with 0.2 pico second pulses and a repetition rate of 76 MHz. Figure 6-12 shows the pumping result of the double-fused LW-VCSEL. The threshold pumping power density is about 5 kW/cm2 at room temperature. The inset shows the VCSEL emission spectrum at the pumping power density of 8 kW/cm2. Peak emission wavelength is 1527 nm and the FWHM is 1.8 nm. The equivalent threshold current density is calculated to be 4 kA/cm2. We
believe that this two-fold value compared with the previous threshold current density comes from the strong absorption of pumping light. The relatively thick top DBR absorbs some percentage of the incident light. The fusion interfaces, especially the top one, play an important recombination center for the pumping light due to the induced defects and QW-like band discontinuity at the fusion interfaces.
References
[1] J. Piprek, Y. A. Akulova, D. I. Babic, L. A. Coldren and J. E. Bowers, Appl. Phys.
Lett., v72, no.15, p1814, 1998.
[2] R. J. Ram, J. J. Dudley, J. E. Bowers, L. Yang, K. Carey, S. J. Rosner, and K.
Nauka, J. Appl. Phys., v78, no.6, p4227, 1995.