1.5 {\mu}m Epitaxially Regrown Photonic Crystal Surface Emitting Laser Diode

We present an InP-based epitaxially regrown photonic crystal surface emitting laser diode, lasing in quasi- CW conditions at 1523nm.

In this paper we demonstrate an epitaxially re-grown InP PCSEL operating at 1523 nm at room temperature. The realization of epitaxially regrown PCSELs at this wavelength, as opposed to ones via wafer fusion [29], is a critical step in providing a route to engineer high-power sources with high beam quality. x II. DEVICE DESIGN AND SIMULATION Figure 1 shows a schematic of our InP based epitaxially regrown PCSEL structure, where the structure consists of, from bottom to top, 3.3 µm of n-doped InP, five 6 nm AlGaInAs quantum wells (separated by 8 nm AlGaInAs barrier layers), a p-doped 243 nm PC layer (consisting of GaInAsP and InP with air containing voids), 1.8 µm of p-doped InP, and capped with a p+ InGaAs cap layer. The PC is a circular atom in a square lattice. Assuming an average refractive index for the PC layer of 3.24, the structure is simulated using the Ritz-Iteration method [30]. The mode shows an overlap with the QWs of 4.7% and overlap with the photonic crystal of 16.4 %. Figure 2 (a) shows a conventional cross-sectional transmission electron microscope (TEM) image of our regrown photonic crystal grating layer (details later). The bright, central region corresponds to a crystallographic void formed within the etched feature; these are encapsulated laterally by GaInAsP and InP above. Such voids are observed within each of the grating holes. Due to the lattice-matched nature of the structure there is minimal contrast between the GaInAsP layer and adjacent InP layers. Figure 2 b) shows a schematic of the TEM image shown in Fig 2 a). The simulation of band structure is calculated by 2D plane-wave (PW) expansion method [31]. The dotted lines, denoted !""_$ and !""_& show the cross section through the material stack used to calculate the refractive index of the atom, and field materials in the PC, hereafter referred to as region a and b, respectively [32]. Figure 3 shows the simulated photonic band structure for our PC calculated utilizing the structural information from Fig 2, where an r/a of 0.17 (where r/a is the ratio between radius and period of the photonic crystal) is determined, as is the position and height of the void. The k-vector is plotted to 0.2 (2π/λ), which was determined to be the collection angle of the optics used to measure the device (described later). As expected from the symmetry of the PC, four modes are obtained. A splitting of the bands at the Γ-point results in the high density of states from which lasing may occur. The insets in Fig 3 show band splitting of modes A and B. Shown adjacently are the in-plane electric field highlighting that bands C and D are leaky and bands A and B are non-leaky and therefor the likely lasing modes.
On these base epitaxy structures, 200 nm of SiO2 was deposited by plasma enhanced chemical vapour deposition. A square lattice, circular unit cell photonic crystal, with a period of 470 nm , was defined by electron-beam lithography in PMMA and etched into the SiO2 by reactive ion etching with CHF3/Ar chemistry. This acts as a hard mask for the etching of the underlying semiconductor. Through this hard mask, the semiconductor is etched to a depth of approximately 170 nmjust above the p-cladding layer atop the AlGaInAs active region with a CHF4/ H2 -based inductively coupled plasma etch. SEM images of test structures indicate that the etched InGaAsP PC layer is not modified by the regrowth process. See schematic in Fig 2 b).
The SiO2 hard mask is then removed and epitaxial regrowth is undertaken. Immediately prior to regrowth, the wafer is uv/ozone cleaned (UVCOS uv/ozone cleaner) followed by 1 minute in 10:1 buffered HF. Regrowth was performed in an AIXTRON 2400 G2 Planetary MOVPE reactor at 100 mbar, utilising trimethylindium (TMIn) and trimethylgallium  Following re-growth, a 200 µm x 200 µm square mesa is patterned with photolithography and etched in a solution of sulphuric acid and hydrogen peroxide. This etches the regrown material, to a depth of 100 nm. A 200 nm -thick SiO2 passivation layer is then deposited across the quarter wafer, with contact windows etched into this with a CHF3 -and Arcontaining reactive ion etch. Following the opening of these contact windows, Ti/Pt/Au is deposited on the top surface of the quarter wafer. A 200 µm square contact with a "lollipop" shaped aperture of 100 nm diameter in the centre is created via a conventional metal lift-off process, the contact shape is illustrated in Fig 1 a). 57% of the device emission area is covered by this contact. A Ni/Au/Ge/Ni/Au n-type contact is then deposited on the bottom side of the quarter wafer, via electron-beam evaporation. This is then annealed at 400 °C for 1 minute. Thick Ti/Au bond-pads are added to the top surface of the quarter wafer, by an electron-beam evaporation and liftoff process.

IV. RESULTS
Devices were measured at 15 °C under quasi-cw conditions using a 10% duty cycle and 10μs pulse width. Surface emission was collected using a NA =0.34 lens, and was focused into a multi-mode fibre-optic cable. Electro-luminescence spectra were measured using an optical spectrum analyser with a resolution of 0.1 nm. Figure 4 a) shows the current-power characteristics of a typical device showing a threshold current of 640 mA (J=1.6 kA/ '( ). An average slope efficiency of ~0.002 W/A is obtained, which is low due to the circular symmetry of the PC [33,34] and the masking of the PCSEL emission by the contacts. Figure 4 b) shows the sub-threshold electroluminescence at 450 mA. Two main peaks are observed at 1512 nm and 1527 nm, attributed to the modification of the spontaneous emission spectrum by the PC . Figures 4 c) and 4 d) show the electroluminescence spectra of the same device at 700 and 900 mA, respectively. A clear lasing peak is observed at 1523 nm in both cases. Figure 5a) shows the EL spectrum of the PCSEL in Fig 4 c) plotted over a narrower wavelength range. Figure 5 b) shows the simulated optical density of states (ODOS) for our photonic crystal. The 2 peaks are attributed to the 2 band edges of the photonic crystal. This ODOS is calculated by integrating the simulated band structure over the range of k-vectors that are expected to be collected through our measurement system (NA = 0.34). The observed features are in good agreement with the peak position of the long wavelength peak showing excellent agreement with the experimental spectra shown in Fig 5 a), which are the non-leaky modes shown in Fig 3.

V. SUMMARY
We have reported the realisation of an epitaxially regrown PCSEL at 1.5 μm, operating quasi-CW at room temperature opening a new route to surface emitting lasers in InP materials.