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Abstract
We report III-nitride vertical-cavity surface-emitting lasers (VCSELs) with buried tunnel junction (BTJ) contacts. To form the BTJs, GaN TJ contacts were etched away outside the aperture followed by n-GaN regrowth for current spreading. Under pulsed operation, a BTJ VCSEL with a 14 µm diameter aperture showed a lasing wavelength of 430 nm, a threshold current of ∼20 mA (12 kA/cm2), and a maximum output power of 2.8 mW. Under CW operation, an 8 µm aperture VCSEL showed a differential efficiency of 11% and a peak output power of ∼0.72 mW.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Vertical-cavity surface-emitting lasers (VCSELs) have been studied because of many advantages, such as low threshold current, small device size, high-speed modulation, and circular beam profile with small divergence angle [1–3]. GaN-based VCSELs have great potential for applications that require wavelengths from ultraviolet to green, including optical communications, sensors, projectors, and displays [3–5]. However, the device performance is limited by many challenges, such as fabricating highly reflective mirrors, achieving vertical and lateral carrier confinement, and obtaining efficient lateral current spreading [6]. Several groups have demonstrated various structures to improve the maximum output power through increasing lateral optical confinement and reducing internal loss [7–14]. For example, to reduce optical loss caused by transparent conductive oxide (TCO), a GaN tunnel junction (TJ) contact and an n-GaN current spreading layer has been employed [12,15,16]. Thus far, GaN-based TJ VCSELs have only been demonstrated with an ion implanted current aperture design, which can lead to additional optical loss when the mode overlaps with the ion implanted area [9]. In the GaAs-based material system, oxide apertures have led to a leap in VCSEL performance but are difficult to apply to GaN-based VCSELs [3,17].
Buried tunnel junctions (BTJs) are a promising alternative for defining current apertures and have been demonstrated in InP- and GaAs-based VCSELs [1,3] and in GaN-based light emitting diodes (LEDs) [18–20]. A BTJ can be achieved by selectively etching the highly doped n++- and p++-type layers outside the aperture and then covering the remaining p-type layer with a lightly doped n-type material in the next epitaxial growth. Therefore, a blocking pn-junction layer can be formed outside the BTJ, while within the BTJ area the p++n++-junction has a low resistance due to tunnel effects. Applying an equal bias to both the BTJ and the blocking layer results in an effective lateral current confinement to the BTJ region [3]. Moreover, the n-type layer regrowth can be performed so that the thickness difference due to the etching of the TJ outer region is maintained up to the upper epitaxial layer. This difference can enhance lateral optical confinement when the resonance wavelength inside the BTJ region is longer than the outside [3,21,22]. In addition, BTJ structures do not sacrifice thermal conductivity unlike other aperture designs that use thermally resistive materials such as dielectric, oxide, and air-gap apertures [3,11,23]. Despite those benefits, it has not previously been demonstrated for GaN-based VCSELs because of difficulties to form effective TJ contacts.
In this letter, we report the first GaN-based VCSELs with BTJ current apertures. BTJs were formed by etching the TJ contacts outside of apertures followed by n-GaN regrowth for current spreading. A 14 µm BTJ VCSEL showed a maximum output power of 2.8 mW at a lasing wavelength of 430 nm under pulsed operation, and an 8 µm BTJ VCSEL showed a peak output power of 0.72 mW with a differential efficiency of 11% under continuous-wave (CW) operation. High current confinement was maintained to over 8 volts.
2. Experimental methods
Figure 1 shows the epitaxial structure of a fabricated flip-chip nonpolar VCSEL with a BTJ contact. The nonpolar device would be expected to have good electron-hole wave function overlap even with a thick quantum well (QW) without quantum-confined Stark effect [16,24–26] and to give smooth surface morphology after substrate removal using photoelectrochemical (PEC) etching [27]. The main epitaxial structure with an active 28 nm single QW ending with p++-GaN was grown by metal-organic chemical vapor deposition (MOCVD) on a bulk nonpolar m-plane GaN substrate with an intentional -1˚ miscut in the c-direction, as described in Table 1. This structure was designed to place a null of a cavity mode at the highly doped TJ contacts to reduce internal loss, which is similar as the previously reported paper [15]. The doping concentrations and thicknesses in Table 1 were calculated based on secondary ion mass spectrometry (SIMS) and X-ray diffraction (XRD) measurement on test structures, but it might differ due to variation of MOCVD growth and Mg clustering effect [28]. After the first growth, the surface of p++-GaN was treated with buffered HF for 5 min to reduce a Mg-rich film. This cleaning was followed by 10 nm n++-GaN regrowth to form TJ contacts [15,29]. After the second growth, the surface roughness was measured by atomic force microscopy (AFM), which showed a root mean square (RMS) value of 2 nm and ridge-like surface features along the a-direction as shown in Fig. 2(a). Then, ∼40 nm GaN, including p++- and n++-GaN layers, outside of the apertures was etched to form a current blocking layer by reactive ion etching (RIE) with Cl2 gas. Next, Al ion implantation (dose: 1015 cm−3, energy: 10 keV) was performed to protect the active layer during PEC undercut etching [30,31], to prevent sidewall leakage, and to reduce current leakage passing through the lightly doped pn-junction at high voltages [32]. As shown in Fig. 1, the implantation was kept 8 µm away from the BTJ apertures to avoid optical loss caused by mode overlap with the implanted GaN layer. Next, n-type GaN layers starting with lightly doped n-GaN were regrown by MOCVD for current spreading while forming a current blocking layer outside TJ contacts. A mesa etch was then performed by RIE followed by deposition of a 17 period Ta2O5/SiO2 distributed Bragg reflector (DBR). After Ti/Au metal contact deposition, flip-chip bonding was carried out on a sapphire carrier coated with Ti/Ni/Au/In. The GaN substrate was then removed by PEC etching of a sacrificial MQW (3 × InGaN/GaN). Then, Ti/Au was deposited after RIE etching with SiCl4 gas for 5 sec to improve n-contact [33]. Finally, a 12 period Ta2O5/SiO2 DBR with a Ta2O5 spacer was deposited next to n-GaN. As shown in Fig. 2(b), it was observed that the thickness difference from aperture etching remained after the n-GaN regrowth and propagated through the bottom DBR.
Fig. 1. Schematic of a flip-chip buried tunnel junction (BTJ) VCSEL with dual dielectric distributed Bragg reflectors (DBRs).
Fig. 2. (a) A surface roughness measured after n++-GaN regrowth and (b) a cross-section image of a BTJ VCSEL taken using a focused ion beam (FIB). The RMS roughness was ∼2 nm.
Table 1. Epitaxial structure grown on m-plane GaN substrate for BTJ VCSELs. (EBL: electron blocking layer, UID: unintentionally doped, SQW: single quantum well, MQW: multiple quantum well)
3. Results and discussion
The light-current-voltage (LIV) characteristics of a 14 µm BTJ VCSEL were analyzed under pulsed operation with a pulse width of 500 ns and a duty cycle of 0.5% at room temperature, as demonstrated in Fig. 3. The threshold current density (Jth) and voltage (Vth) were 12 kA/cm2 and 9.8 V, respectively. The device was measured up to a current of 100 mA which gave an output power (Pmax) of 2.8 mW at 66 kA/cm2. The differential efficiency (ηd) was 2.8% at a current density of 12 kA/cm2. Pmax and ηd were higher than previously reported GaN-based TJ VCSELs with Al ion implanted apertures [11,15,16]. We believe that the major improvement was achieved by reducing the optical loss caused by Al ion implantation and increasing lateral optical confinement. Nevertheless, the threshold current density was somewhat similar to VCSELs with ion implanted apertures [15,16]. This may be because the lasing wavelength of 430 nm was misaligned by ∼20 nm with the peak wavelength of spontaneous emission at ∼410 nm. We believe that other longitudinal modes shorter than the lasing wavelength did not appear due to high optical losses caused by absorption from the GaN [34] and scattering [16].
Fig. 3. LIV characteristics of a 14 µm diameter aperture VCSEL measured under pulsed operation up to a current of 100 mA. An inset shows the spontaneous emission (dashed line) and lasing (solid line) spectra measured before and after the VCSEL processing, respectively.
Figure 4 shows near-field images of lasing VCSELs with various aperture sizes taken by a microscope camera. Fundamental transverse mode operation was not observed in any of devices. The relative refractive index difference (Δneff/neff) is calculated to be ∼4% (Δneff/neff ≈ Δλ/λ), where Δλ is the difference in resonance wavelength outside and inside the BTJ region [3,7,21,22]. Thus, this high index difference is likely to cause a strong index guiding which can support high order modes. However, the lasing modes generally did not follow typical linearly-polarized (LP) modes of a circular aperture design but were probably dictated by rough surface morphology as shown in Fig. 2(a) [7,11]. A smooth surface and well calculated etch depth to control the refractive index step will be needed for fundamental mode lasing.
Fig. 4. Near field images of various BTJ VCSELs taken under pulsed operation. All images were aligned in the same crystallographic direction.
Next, an 8 µm BTJ VCSEL was measured under CW operation as shown in Fig. 5. The device started to lase at a Jth of 10 kA/cm2 and a Vth of 7.2 V which rolled over at a current density of 25 kA/cm2 at a voltage of 8.2 V. The device lased up to a Pmax of 0.72 mW with a ηd of 11%, which were higher than those previously demonstrated for TJ VCSELs using ion implanted apertures (a Pmax of 0.14 mW with a ηd of 1.1%) [35]. This improved performance is likely due to lower optical loss outside the aperture than the ion implant aperture design. However, this device with TJ contacts completely grown by MOCVD showed a turn on voltage (Von) of ∼4.2 V with a differential resistivity (ρd) of 6.67×10−5 Ω·cm2 after lasing, which is higher than VCSELs employing MOCVD/molecular-beam epitaxy (MBE) hybrid TJ growth (a Von of ∠ 4 V and a ρd of ∼1×10−5 Ω·cm2) [16,35]. The resistance of the TJ contacts might be higher due to the p++-GaN deactivation during the n++-GaN MOCVD regrowth [15]. This high operating voltage leads to increased device temperature and limits the maximum output power. The TJ contacts can be improved using lateral activation for the p++-GaN layer when the sidewall is exposed to air after the aperture etching step [36]. However, a method preventing the p++-GaN layer from being deactivated during the next regrowth step must be considered. In addition, a thicker n-GaN layer especially on the p-side may improve current spreading and thermal dissipation, but it may cause multiple longitudinal modes by reducing the mode spacing [35].
To analyze the breakdown voltage of the pn-junction current blocking layer, test structures without TJ contacts and DBRs were processed along with the BTJ VCSELs as illustrated in the inset in Fig. 6. The p++/n++-GaN layer in the test structure was etched off during the aperture etch step. As shown in Fig. 6, the current started flowing through the current blocking layer at ∼8 V under CW operation which was slightly lower than the voltage when the power rolled over in Fig. 5. If a leakage current outside the aperture begins to flow, it would decrease injection efficiency which would degrade the ηd and accelerate heating by extra input current. Given that other 8 µm devices also rolled over at a voltage of 8.2∼8.9 V, the device performance might be limited by the leakage through the current blocking layers. Although the measured current leakage (e.g. 0.002 kA/cm2 at 8.5 V) in the test structure seems insignificant for device performance, it might differ in the BTJ VCSELs because of operational and structural differences, such as the variation of the current blocking layers, different operating temperatures, and the presence of the DBRs. Further studies are necessary to investigate the effect of leakage current outside BTJ area especially for different temperatures. To improve reliability and the maximum output power of devices, it will be desirable to increase the breakdown voltage of the pn-junction. The early breakdown of the current blocking layer may be due to dry etch damage and residues before n-GaN regrowth, which can be improved by reducing the severity of the dry etching condition and using surface treatments before regrowth. Additionally, the breakdown voltage can be increased by using lower doped and wider bandgap materials for pn-junction.
Fig. 6. IV characteristics of a test structure without TJ contacts and DBRs under CW operation. The test structure is illustrated as an inset in the IV curve. The diameter of the current blocking pn-junction was 26 µm and the outer area to the edge of the mesa was implanted with Al ions.
4. Conclusion
In summary, we have demonstrated GaN-based nonpolar VCSELs with BTJ current apertures lasing at a wavelength of 430 nm. A 14 µm BTJ VCSEL lased up to a Pmax of 2.8 mW under pulsed operation which was higher than reported TJ VCSELs with ion implant apertures. We suggest that higher lateral confinement and lower optical loss compared to the ion implanted apertures lead to dramatic improvement in output power. The high order modes in near field images also suggests that a strong index-guiding occurred by a thickness difference between inside and outside the BTJ area. Finally, an 8 µm BTJ VCSEL showed a Pmax of 0.73 mW at a voltage of 8.2 V, which might be limited by current leakage through the current blocking layer and a thermal effect.
Funding
Solid State Lighting and Energy Electronics Center, University of California Santa Barbara.
Acknowledgments
A portion of this work was carried out in the UCSB nanofabrication facility, the UCSB Materials Research Laboratory (MRL), which is supported by the NSF MRSEC program (NSF DMR 1720256), and the California Nano System Institute’s (CNSIs) Center for Scientific Computing at UCSB.
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