Top-emitting 940-nm thin-film VCSELs transferred onto aluminum heatsinks

Thin-film vertical cavity surface emitting lasers (VCSELs) mounted onto heatsinks open up the way toward low-power consumption and high-power operation, enabling them to be widely used for energy saving high-speed optical data communication and three-dimensional sensor applications. There are two conventional VCSEL polarity structures: p-on-n and n-on-p polarity. The former is more preferably used owing to the reduced series resistance of n-type bottom distributed Bragg reflection (DBR) as well as the lower defect densities of n-type GaAs substrates. In this study, the p-on-n structures of thin-film VCSELs, including an etch stop layer and a highly n-doped GaAs ohmic layer, were epitaxially grown in upright order by using low-pressure metalorganic chemical vapor deposition (LP-MOCVD). The p-on-n structures of thin-film VCSELs were transferred onto an aluminum heatsink via a double-transfer technique, allowing the top-emitting thin-film VCSELs to keep the p-on-n polarity with the removal of the GaAs substrate. The threshold current (Ith) and voltage (Vth) of the fabricated top-emitting thin-film VCSELs were 1 mA and 2.8 V, respectively. The optical power was 7.7 mW at a rollover point of 16.1 mA.

www.nature.com/scientificreports/ tight control over thickness, composition and doping concentration for a large number of DBR periods is an indispensable prerequisite for achieving the necessary high reflectivity.
In the present paper, the p-on-n structures of top-emitting thin-film VCSELs, including an etch stop layer and a highly n-doped GaAs ohmic layer, are epitaxially grown in upright order using metalorganic chemical vapor deposition (MOCVD). A transfer approach to release and transfer the thin-film VCSELs from the growth substrate onto an aluminum heatsink is described in detail. The thermal conductivity of Aluminum (2.47 W/ cm K), 4.57 times higher compared to that of a GaAs substrate (0.54 W/cm K), enables the high-power VCSELs operation due to the higher rollover current with the lowered thermal resistance. The top-emitting 940-nm thin-film VCSELs are successfully fabricated onto aluminum heatsinks via a transfer approach that utilizes a double-transfer process with a polyimide carrier, an aluminum heatsink, and adhesive materials such as wax, silver-filled epoxy. This double-transfer technique allows the top-emitting thin-film VCSELs to keep p-on-n polarity after transferring the upright-order VCSELs onto the aluminum heatsink. Figure 1 shows schematic top-emitting 940-nm thin-film VCSELs transferred onto an aluminum heatsink. The fabricated thin-film VCSELs are investigated using optical spectrum and light-current-voltage (L-I-V) curves.

Results
Double-transfer technique for top-emitting thin-film VCSELs mounted onto an aluminum substrate. The p-on-n structures of top-emitting 940-nm thin-film VCSELs are epitaxially grown on n-GaAs substrates in upright order using MOCVD, as shown in Fig. 2. Compared to the conventional p-on-n structures of bulk VCSELs, the p-on-n structures of thin-film VCSELs included additional layers, such as the etch stop layer and n-GaAs contact layer, while the p-GaAs contact layer, MQWs, two oppositely doped n-, and p-DBRs Figure 1. Schematic p-on-n structures of top-emitting 940-nm thin-film VCSELs transferred onto an aluminum heatsink. The top-emitting 940-nm thin-film VCSELs with p-on-n polarity are successfully fabricated onto an aluminum heatsink by using a double-transfer technique. The thin-film VCSELs with p-on-n polarity, placing p-DBRs above the active region, enable high-performance thin-film VCSELs owing to the reduced series resistance of n-DBRs as well as the lower defect densities of n-type substrates. www.nature.com/scientificreports/ remained unchanged (see "Methods"). The etch stop layer was inserted to keep the epilayers of thin-film VCSELs intact during the removal of a GaAs substrate in NH 4 OH-based etchant. The heavily n-doped GaAs ohmic layer can enhance the electrical characteristics of thin-film VCSELs as a way of alleviating the electrical loss stemming from the considerably thick and moderately doped GaAs substrates 17 . Figure 3 shows the fabrication procedures of thin-film VCSELs onto an aluminum heatsink via a doubletransfer technique. The double-transfer technique that transferred the thin-film VCSELs onto foreign substrates twice was carried out as follows: Fig. 3a shows that the p-on-n structures of top-emitting 940-nm thin-film VCSELs, including the etch stop layer and n-GaAs ohmic layer, are grown via MOCVD in upright order. Figure 3b shows the finished front processes of thin-film VCSELs, including mesa etching, p-metal deposition, and selective oxidation of Al x Ga 1-x As before separating thin-film VCSELs from the growth substrate. A Mesa etching of n-and p-DBRs and MQWs was carried out in order to isolate the VCSELs. A grid pattern of p-ohmic contact with a combination of Ti/Pt/Au was deposited on the highly p-doped GaAs layer. The 10-μm oxide-confined aperture of each VCSEL was defined with selective oxidation of Al x Ga 1-x As with high Al contents. Figure 3c shows the thin-film VCSELs mounted on a polyimide carrier by using adhesive wax. The adhesive wax was uniformly applied on the polyimide carrier at low temperature, which was bonded to the thin-film VCSELs with controlled pressure. The highly n-doped GaAs ohmic layer is exposed after removing the GaAs substrate and etch stop layer, as shown in Fig. 3d. The etch stop layer was selectively etched after the GaAs substrate removal, which prevented the NH 4 OH-based etchant from penetrating into the n-GaAs ohmic layer of the thin-film VCSELs during removal of the GaAs substrate. Figure 3e shows the wax-bonded thin-film VCSELs mounted onto an aluminum heatsink. After a combination of AuGe/Ni/Au was deposited on the n-GaAs ohmic layer, the wax-bonded thinfilm VCSELs were integrated with silver-filled epoxy onto the aluminum heatsink. The wax-bonded thin-film VCSELs were completely bonded onto the aluminum heatsink during the curing of the silver-filled epoxy at 150 ℃ for one and a half hours. Figure 3f shows the top-emitting 940-nm thin-film VCSELs transferred onto the aluminum heatsink using the double-transfer technique. The polyimide carrier was separated by selectively removing the wax, thereby the top-emitting 940-nm thin-film VCSELs mounted onto the aluminum substrate were successfully fabricated.
Operation of top-emitting 940-nm thin-film VCSELs with p-on-n polarity. Figure 4a shows a topview scanning electron microscopy (SEM) image of the fabricated 940-nm top-emitting thin-film VCSELs onto the aluminum heatsink. The surface of the thin-film VCSELs mounted onto the aluminum heatsink was considerably smooth as shown in Supplementary Fig. S1. Figure 4b shows the cross-sectional SEM image, obtained using focused ion beam (FIB) milling, of the fabricated thin-film VCSELs where the n-and p-DBR layers as well as the MQWs are clearly visible. No significant dislocations of the transferred thin-film VCSELs onto the alu- www.nature.com/scientificreports/ minum heatsink were observed in the VCSEL structure, demonstrating the feasible double-transfer technique for transferring the thin-film VCSELs onto foreign substrates. Figure 4c shows an optical microscopy image of the fabricated 940-nm top-emitting thin-film VCSELs onto the aluminum heatsink using a probe station. The full width at half maximum (FWHM) of lasing spectra at 8 mA was approximately 6.98 nm. The invisible laser emission of the 940-nm thin-film VCSELs at injection current of 8 mA was observed to be a purple beam using a conventional microscope 5 . As can be seen from Fig. 4d, the peak wavelength of the emitting light from the thin-film VCSELs was 941 nm which corresponded to the sharp reflectance dip near 940 nm shown in Supplementary Fig. S2. Figure 5 shows the L-I-V characteristics of the thin-film VCSELs onto the aluminum heatsink under continuous wave (CW) operation. The dot-dashed line indicates the threshold current of 1 mA. The peak power of the thin-film VCSELs was 7.7 mW at input current of 16.1 mA. A few deformations of contact layers, as shown in the SEM image of Fig. 5, might contribute to an increase in series resistance, resulting in a rather high threshold voltage of approximately 2.8 V and steeper upward tilt to the slope of the I-V curve of thin-film VCSELs.

Discussion
The top-emitting 940-nm thin-film VCSELs transferred onto the aluminum heatsink were fabricated using the double-transfer technique, thereby the p-on-n polarity of the thin-film VCSELs remained unchanged. This approach does not require inverted growth which could have adverse effects on epi layers of the thin-film VCSELs. The care of the thin-film VCSELs was carefully taken during the double-transfer of the thin-film VCSELs onto foreign substrates. There was no discernible dislocation in the structures of the fabricated thin-film VCSELs, which successfully demonstrated the double-transfer technique for realization of high-power thin-film VCSELs mounted onto heatsinks. The adhesive wax was used to transfer the thin-film VCSELs onto the polyimide carrier with the removal of the GaAs substrate and etch stop layer. The etch stop layer was resistant to the NH 4 OH-based etchant that was used in order to etch the GaAs substrate, preventing the etchant from penetrating into epilayers of thin-film VCSELs. The wax-bonded thin-film VCSELs were integrated onto the aluminum heatsink using the silver-filled epoxy. The high thermal conductivity of silver-filled epoxy promoted efficient heat transfer to the aluminum heatsink, potentially contributing to extending the rollover point. The top-emitting 940-nm thin-film VCSELs were fabricated after separating the polyimide carrier from the wax-bonded thin-film VCSELs.
The threshold current and voltage of the top-emitting 940-nm thin-film VCSELs were 1 mA and 2.8 V, respectively. The series resistance of thin-film VCSEL was approximately 160 Ω, which could be degraded due to the irregular surface of the n-ohmic contact layer caused by the contraction stress of silver-filled epoxy during www.nature.com/scientificreports/ epoxy curing. This might contribute to the increase in the series resistance, resulting in the steeper curve of the I-V curve. A thermal resistance (R th ) of the fabricated thin-film VCSELs can be extracted from the temperature rise with dissipated electrical input power as followed by an equation 8,11,22 .
where T is the temperature rise obtained from device active region, P diss is the change of dissipated electrical power, and λ is the output wavelength shift. Figure 6a shows the wavelength shift as a function of dissipated electrical input power obtained from the fabricated thin-film VCSEL under continuous wave (CW) operation. The λ/ P diss value of the fabricated thinfilm VCSEL was 0.0776 nm/mW. Figure 6b shows the output spectrum of the fabricated thin-film VCSEL at input current of 8 mA under the controlled temperature from 10 to 85 °C. The output spectrum peak positon of the thin-film VCSEL showed the gradual red-shift from 939.3 to 944.6 nm with temperature rise. The output spectrum intensity of thin-film VCSEL was significantly degraded above 70 °C. The thermal wavelength shift as a function of the varied temperatures from 10 to 85 °C is shown in Fig. 6c. The ·λ/ ·T value was 0.0713 nm/K. Therefore, the extracted R th value of the fabricated thin-film VCSEL was 1088 K/W. The experimentally measured thermal resistance of the thin-film VCSELs mounted on the aluminum heatsinks was comparable to that of other thin-film VCSELs 8,23 .
Further performance enhancement of thin-film VCSELs could be realized in combination with direct transfer to ideal heatsinks by using metal electroplating instead of adhesive materials. The Au and Cu electroplating, in accordance with high thermal conductivities of 4.01 and 3.17 W/cm K 14 , respectively, can be used as efficient electrical and thermal passages, leading to further improved high-power thin-film VCSELs. Furthermore, this approach may reduce the thermal resistance induced by the irregular surface at bonding interface. Therefore, the fabricated top-emitting 940-nm thin-film VCSELs could exhibit superior thermal performance by using metal electroplating, leading to low-power consuming and high-power thin-film VCSELs.  www.nature.com/scientificreports/ In summary, the p-on-n structures of thin-film VCSELs were epitaxially grown via MOCVD in upright order. The double-transfer technique allowed top-emitting 940-nm thin-film VCSELs to keep the p-on-n polarity after transferring the thin-film VCSELs onto the aluminum heatsink. The threshold current and voltage of the fabricated thin-film VCSELs were estimated to be 1 mA and 2.8 V, respectively. The threshold voltage could be degraded due to the deformation of the n-GaAs ohmic layer. The peak power of the thin-film VCSELs was 7.7 mW at input current of 16.1 mA. We believe that the low-power consumption and high-power thin-film VCSELs fabricated using this double-transfer technique will enable the widespread use of thin-film VCSELs in three-dimensional depth sensing systems and modern lidar in automotive driving systems.

Methods
Epitaxial growth. The p-on-n structures of 940-nm thin-film VCSELs were grown via MOCVD in upright order as shown in Fig. 2. The used precursors were trimethylgallium (TMGa), trimethylaluminum (TMAl), trimethylindium (TMIn), arsine (AsH 3 ), and phosphine (PH 3 ). Silane (SiH 4 ) was used as a dopant source for n-GaAs, whereas carbon (CBr 4 ) was utilized for p-doping. The etch stop layer was inserted between the n-GaAs ohmic contact layer and n-GaAs buffer layer on n-GaAs (100) substrates. The three InGaAs/AlGaAs quantum wells were sandwiched between the 18-pair p-DBRs and 38-pair n-DBRs. The In and Al compostions of InGaAs/ AlGaAs MQWs are 10.6 and 35%, respectively. The spectral reflectance spectrum exhibited a sharp reflectance dip near 940 nm caused by cavity resonance. (Supplementary Fig. S2). The Al x Ga 1-x As with high Al fractions was included above the MQWs to define the 10-μm oxide-confined aperture size of thin-film VCSELs using selective oxidation of Al x Ga 1-x As.
Device characterization. The L-I-V characteristics of top-emitting 940-nm thin-film VCSELs with continuous wave (CW) operation were measured via a Keithley 2602B at room temperature. The Keithley current source was used to bias the thin-film VCSELs with electrical probing. A silicon photodiode (Hamamatsu, 2201 photodiode) was adopted in order to collect the 940-nm emission above the thin-film VCSELs. The emission spectrum of 940-nm thin-film VCSELs was investigated by using an Ocean optics Maya 2000 PRO Spectrometer. www.nature.com/scientificreports/