Photonic crystal surface emitting lasers with naturally-formed periodic

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Photonic crystal surface emitting lasers with naturally-formed periodic ITO structures Han-Lun Chiu, Kuo-Bin Hong, Kuan-Chih Huang, and Tien-Chang Lu ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01530 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 2, 2019

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Photonic crystal surface emitting lasers with naturallyformed periodic ITO structures Han-Lun Chiu, Kuo-Bin Hong, Kuan-Chih Huang, Tien-Chang Lu* Department of Photonics, College of Electrical and Computer Engineering, National Chiao Tung University, 1001 Ta-Hsueh Rd., Hsinchu 30010, Taiwan *Correspondence and requests for materials should be addressed to: [email protected] (T.C.L.)

We have successfully demonstrated GaAs-based photonic crystal surface emitting lasers with low threshold current density of 0.45 kA/cm2 by increasing the thickness of indium-tin-oxide (ITO) top cladding layer appropriately. The thicker ITO top cladding layer contributes to lower scattering loss at the surface and more uniform carrier injection. Furthermore, periodic patterns are formed naturally on the surface of ITO layer during the deposition process, resulting in the deflection of the output beam with an angle of 4.5 degrees from the vertical direction and maintaining a small divergence angle. The turn-on voltage, series resistance and slope efficiency of the laser pumped at room temperature are 2.09 V, 5.10 Ω and 0.24 W/A, respectively. Lasing wavelengths of different laser devices can be varied from 913.3 nm to 954.4 nm with several lattice constants designed from 265 to 280 nm. Based on the simple fabrication process, great energy efficiency, small divergence output and possible beam steering capability by adjusting the periodicity of top cladding layer, such kind of lasers have great potential to be applied in the field of 3D optical sensing, such as vehicle light detection ad ranging, facial identification, environmental sensing, and so on. KEYWORDS: photonic crystal, electrically-driven device, surface emitting laser, ITO, grating effect, beam deflection 1 ACS Paragon Plus Environment

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Semiconductor lasers have been used widely in our daily lives in fields of optical communication,(1) optical storage,(2) displays,(3) and even 3D sensing.(4) Many kinds of semiconductor lasers such as edgeemitting lasers,(5,6) vertical-cavity surface emitting lasers (VCSELs),(7,8) distributed feedback (DFB) lasers,(9,10) and photonic crystal surface emitting lasers (PCSELs) have been fabricated and investigated around the world for decades.(11-14) Among all, with advantages of great mode-controlling ability, large surface emitting area, extremely small output divergence, two-dimensional array capability and high output power, the photonic crystal surface emitting laser (PCSEL) is not only considered as a powerful light source but also a great candidate of optical sensor.(15-19) By designing the photonic crystal (PC) structure properly, characteristics such as emission polarization, lasing wavelength and beam emitting angle could be controlled precisely.(20-24) In our previous research, we have demonstrated a novel way to make PCSELs incorporated indium-tin-oxide (ITO) as the top cladding layer, with advantages of low refractive index, low light absorption and great conductivity that can greatly ease the fabrication process and achieve excellent optical properties of devices.(25-27) The optical confinement in the active and photonic crystal regions have been increased due to the low refractive index of ITO, leading to a good threshold performance of laser devices. Moreover, because of the fast, simple deposition process and low deposition temperature, it is convenient and cost-effective to fabricate PCSELs by applying ITO as the top cladding layer.

In this report, the thickness of ITO cladding layer is increased to 400 nm in order to reduce the threshold current in comparison to the PCSELs with a thinner ITO layer of 200 nm.(27) The aim is that the thicker ITO layer could reduce the vertical optical field penetrating through the ITO and reach the surface to reduce the scattering loss at the ITO surface and at the same time avoid carriers accumulated at the corner of the injecting area, which may lead to non-uniform carrier injection and result in poor threshold performance.(28,29) Furthermore, a periodic structure is formed naturally in the ITO layer during the ITO deposition process. Consequently, output beam is split into two with an approximately 4.5 2 ACS Paragon Plus Environment

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degree deflecting angle from the vertical direction. This result is originated from the inhomogeneous ITO layer which directly causes periodic variation of the refractive index, resulting in the mode-shifting situation from the original Г point and bringing out the deflection of the output beam. This exciting phenomenon reveals that PCSELs with the beam-steering function could be designed and fabricated easily without changing the photonic crystal structure and applying the regrowth process. What’s more, it is possible to manipulate the beam deflecting angle by setting the grating period in the ITO surface properly.(30,31) Such a PCSEL with the low threshold current density and beam-steering capability may be applied to be an optical sensor used in mobile phones or vehicle LIDARs in the future.

Materials and Methods Sample Preparation and Design of Photonic Crystal The complete PCSEL structure is illustrated in Figure 1a. Epitaxial layers were grown by metalorganic chemical vapor deposition (MOCVD) method, including an n-GaAs substrate, a 100 nm-thick graded index (GRIN) n-AlxGa1-xAs (x = 0-0.4) layer, a 980 nm-thick n-Al0.4Ga0.6As cladding layer, an nGaAs separate confinement heterostructure (SCH) layer, an active region containing 3 pairs of InGaAs/GaAs multi quantum wells (MQWs), a p-GaAs SCH layer, a 100 nm-thick p-Al0.4Ga0.6As cladding layer, a 100 nm-thick GRIN p-AlxGa1-xAs (x = 0-0.4) layer, and a 100-nm-thick p+ GaAs contact layer. A square lattice of 125 × 125 μm2 photonic crystal area with circular air holes were designed and optimized in advance with lattice constant a = 275 nm to acquire λ = 941 nm for the lasing wavelength, and the filling factor (FF) was set to be 17.6 % approximately. The refractive index of each layer and the normalized field distribution in the vertical direction of PCSEL structure calculated by the finite element method are shown in Figure 1b. The MQW, photonic crystal and ITO regions are labeled and separated by the black dashed lines. The red curve indicates the normalized field distribution in the structure, while the blue curve represents the refractive index variation between different layers. Due to the low refractive 3 ACS Paragon Plus Environment

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index of ITO, the optical field is pushed towards the PC and the MQW regions, leading to better confinement both for PC and active regions.

Fabrication of Photonic Crystal and Formation of ITO grating The photonic crystal structures were firstly defined by an E-beam lithography system and then etched through p+ GaAs contact layer, p-GRIN layer and p-AlGaAs cladding layer for 300 nm in total by an inductively coupled plasma reactive ion etching (ICP-RIE) system. Next, a 145 × 145 μm2 square mesa was fabricated by wet-etching method, and the current confinement structure was then constructed by passivated silicon nitride (SiNx) around the PC area for the carrier injection through the PC region. Afterwards, the ITO cladding layer and the electrodes were deposited by the electron gun (E-gun) evaporator to finish the whole fabrication process. During the ITO deposition process, periodic height difference of 30-40 nm was naturally generated, which forms the grating structure of the ITO layer. The top-view image of the device taken by optical microscope and the cross-sectional SEM image of photonic crystals are shown in Figure 1c,d, respectively. The lattice constant, etching depth and ITO thickness were measured as 275 nm, 300 nm and 400 nm, which were all consistent with our design.

Results and Discussion Electrical Characteristics of PCSELs The power-current-voltage (L-I-V) curve of the PCSEL with lattice constant a = 275 nm is shown in Figure 2a. The pumping conditions were set as follows: pulse width = 1 μs, duty cycle = 0.1 %, temperature = 300 K. As it shows, the threshold current Ith is measured as 95 mA, corresponding to the threshold current density Jth of 0.45 kA/cm2. This value is much smaller than PCSELs with thinner ITO cladding layers (200 nm) fabricated in our previous works (Jth = 0.84kA/cm2).(27) The reason is that PCSELs with a thicker ITO cladding layer could avoid carriers accumulated at the corner of the injecting area, which may lead to inefficient carrier injection and result in higher threshold current. At the same time, as Figure 4 ACS Paragon Plus Environment

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1b indicates, the thicker ITO can prevent the optical field from penetrating to ITO/air interface to reduce the scattering loss. Figure 2c shows the near field patterns under 120 mA current injection, showing no carriers accumulated at the corner. The turn-on voltage, series resistance and slope efficiency are measured as 2.09 V, 5.10 Ω and 0.24 W/A, respectively. Figure 2b shows the polarization profile of the output emission measured by inserting a polarizer plate between the PCSEL sample and the detector at the normal direction. The degree of polarization (DOP = (Imax - Imin)/(Imax + Imin), where Imax and Imin represent the maximum and the minimum detected signal, respectively) is measured as 6.1%, showing that there is no specific polarized direction of the output beam.

Optical Characteristics of PCSELs We fabricated PCSELs with different lattice constants of 265 nm, 270 nm, 275 nm and 280 nm for comparison and the corresponding lasing wavelengths were measured as 913.3 nm, 927.0 nm, 941.1 nm and 954.4 nm, respectively. The top-view SEM images of the photonic crystal structure are shown in Figure 3a. The photoluminescence (PL) spectrum of the wafer and the normalized lasing spectra of PCSELs measured under 300 mA current injection with varying photonic crystal lattice constants are shown in Figure 3b. The peak wavelength of the PL spectrum is 934.8 nm. Figure 3c shows the measured threshold current densities of 0.86 kA/cm2, 0.64 kA/cm2, 0.45 kA/cm2 and 0.75 kA/cm2 of PCSELs with lattice constants of 265 nm, 270 nm, 275 nm and 280 nm, respectively. As we can see, lower threshold current density values are obtained from PCSELs with lasing wavelength of 927.0 nm and 941.1 nm, whereas the other two PCSELs show higher threshold values due to the fact that the lasing wavelength of these PCSELs are far away from the PL peak wavelength of 934.8 nm.

Figure 4a shows the angle-resolved electroluminescence (AREL) result of the PCSEL. A detector was fixed on a rotator which was placed 7 cm above the PCSEL, and then the AREL measurement was operated through varying the angle of the rotator precisely in order to detect the emission signals from 5 ACS Paragon Plus Environment

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different angles. The pumping conditions were set as follows: pulse width = 1 μs, duty cycle = 0.1 %, temperature = 300 K, injecting current = 200mA. The measured result reveals that there are two separate beam spots existing in the x direction with about 4.5 degree deflecting angle. This phenomenon is originated from the shift of the Г point on the photonic band diagram, which would be discussed in detail in the followings.

Figure 4c shows the ITO surface topography measured by the alpha step system. As it appears, height difference of 30-40 nm with about 8 to 10 μm periods is observed at the surface of the ITO layer, leading to the beam deflecting phenomenon. The periodically arranged pattern in ITO at the x direction as shown in schematic picture of Figure 1a would be originated from the internal stress evolution of ITO layers grown on the PC patterns when ITO layers are growing thicker and could be manipulated by deposition condition, such as chamber pressure, temperature, E-gun source orientation, and so on. The exact forming mechanism requires further study. However, these patterns in ITO layer could result in the difference in carrier injection and refractive index difference along the x-direction. Figure 4d reveals striped-liked nearfield patterns on the emitting surface when the injecting current is increased to 200 mA, which exactly matches to the periodic pattern of the ITO layer.

Photonic Crystal Band Structure Figures 5a, b show the measured PC band diagrams below and above the threshold condition. Solid lines represent the calculated bands near the Г point with the original photonic crystal structure, showing a good match between measured and calculated PC bands. Many repeated measured photonic bands shown in Figure 5a below the threshold condition result from Fabry-Perot interference between top and bottom interfaces of the sample. When introducing a periodic ITO layer along the x direction ( to X(0 ° )), additional photonic bands arise to couple with the original PC bands as shown as the cyan dotted lines in Figure 5b. These new bands create a shifted band edge condition away from the  point by the effect of 6 ACS Paragon Plus Environment

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hybridization of two PC structures stacking vertically. As shown in Figure 5b, the measured lasing mode is located at the intersection of a photonic crystal band and another cyan dotted band, indicating that the lasing mode is shifted from the original Г point calculated without ITO structure, leading to the deflection of the output beam.

To support the argument that the beam deflection feature of PCSEL is resulting from the periodiclike ITO structure, we first constructed a two-dimensional PCSEL model combined with an ITO grating with 31 sets of unit cells in the x-direction using the finite element simulation. Here the lattice constant of PC unit cell, the period and the duty cycle of ITO grating were set to 270 nm, 8.37 m and 50%, respectively. The refractive indices in the unit cell were defined through the effective index method. (32) Based on the calculation, a new flat band edge near the 0.032 (π/a) was created as shown in Figure 5b which matches to the measured band edge above the lasing threshold. To further observe the far field divergence effect, we have performed the three-dimensional model to calculate the specific eigenmodes of periodic PC slab combined with an ITO grating having a period varied from 6.48 to 10.8 m and the depth of grating was 40 nm. The periods of ITO grating would be equal to 24 to 40 pairs of PC unit cells. Figure 4b shows the deflection angle decreases with increasing the period of ITO and the calculated deflecting angles match to the measurement result shown in Fig. 4(a). Nevertheless, the measurement results show a rather broadening and asymmetric divergence angles that could be due to the variation in the grating periods that formed naturally during the ITO deposition process.

As can be expected, the periods of ITO pattern can be further adjusted artificially so that the deflecting angle of output emission can be steered accordingly. A similar result was reported previous by exploiting in-plane hybridizing PC patterns with two different lattice constants.(21,33) The deflection angle of output emission was determined by the lowest common multiple period between these two lattice constants. Here, we demonstrated another relatively easy way to realize the same functionality with 7 ACS Paragon Plus Environment

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vertically stacked hybridizing PC patterns and show these interesting phenomena to make such PCSEL a candidate to be developed into 3D optical sensors equipped with the dynamic modulation ability of beam angle controlling which is a crucial and necessary function for optical sensors.

Conclusion GaAs-based PCSELs with low threshold current density of 0.45 kA/cm2 are designed and fabricated. The turn-on voltage, series resistance and slope efficiency are measured as 2.09 V, 5.10 Ω and 0.24 W/A, respectively. By applying different lattice constants of photonic crystals, the lasing wavelength can be controlled from 913.3 nm to 954.4 nm. The unpolarized output beam which is split into two spots with approximately 4.5 degree deflecting angle owing to the naturally-formed ITO periodic structure to realize vertically stacked hybridizing two PC structures. Such kind of low threshold current PCSEL equipped with the beam angle controlling ability has great potential to be applied into the fields of 3D sensing as a key light source element in the near future.

Acknowledgement This work has been supported in part by the Ministry of Science and Technology in TAIWAN under Contract Number MOST 106-2221-E-009-112-MY3 and MOST 107-2119-M-009-016. The authors would like to acknowledgment Nano Facility Center (NFC), Center for Nano Science and Technology (CNST) in NCTU, and also the Industrial Technology Research Institute (ITRI) for their technical support.

Contribution H.L.C., K.B.H., K. C. H., and T. C. L. initiated the study. K.B.H. performed the numerical calculation and simulation. H.L.C. and K. C. H. performed the optical experiments. H.L.C., K.B.H., and T. C. L. wrote the manuscript. All authors analyzed the calculated and experimental data. All authors discussed the results and commented on the manuscript.

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Corresponding Authors Tien-Chang Lu (email: [email protected]).

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Figure. 1. (a) The three-dimensional schematic diagram of a PCSEL with two output emission beams. Photonic crystals covered by the ITO cladding layer are etched through the p-GaAs, GRIN, and p-AlGaAs cladding layers for 300 nm. The ITO layer exhibits a naturally formed periodic structure. The output emission shows two deflected beams with narrow divergence due to the effect of ITO periodic patterns. (b) The refractive index of each layer and the normalized optical field distribution in the PCSEL structure at the vertical direction. The MQW, photonic crystal and ITO regions are labeled. (c) The top-view image of the PCSEL taken by an optical microscope. The region of silicon-nitride (SiNx), photonic crystals (PC), mesa and metal are labeled. (d) The cross-sectional scanning electron microscope image of cleaved photonic crystal structures covered by the ITO layer. The thickness of ITO is 400 nm, etched hole depth is 300 nm, and the lattice constant is 275 nm.


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Figure. 2. (a) The power-current-voltage (L-I-V) characteristics of the PCSEL with a lattice constant a = 275 nm. The insets show the lasing spectrum with 941.1 nm lasing peak position and current-dependent linewidth. The linewidths of spectra drop sharply from 15.6 nm to 1.2 nm clearly indicating the transition from spontaneous radiation to the lasing action. (b) The polarization characteristic of the PCSEL, showing no specific polarized direction. (c) The near-field pattern measured under 120 mA current injection, indicating a rather uniform current injection into the emission aperture with a thick ITO layer.


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Figure. 3. (a) The top-view images of photonic crystals with different lattice constants of 264.4, 269.8, 275.0, and 280.8 nm. (b) Lasing spectra of PCSELs with different lattice constants indicated by different colors. Lasing wavelengths are labeled above peaks correspondingly. The linewidths for PCSELs with four lattice constants are approximately 1.4, 1.5, 1.2, and 1.3 nm, approaching to the spectral limit of approximately 1 nm in our system. The black curve is the photoluminescence (PL) spectrum of the wafer without the photonic crystal structure. (c) Threshold current density values of PCSELs with different lattice constants.


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Figure. 4. (a) The angle-resolved electroluminescence (AREL) results of the PCSEL. The red and blue curves illustrate the measured intensity by a detector rotated along x and y directions, respectively. The divergence angle of device ranges from 2 to 5 degrees. A relatively large divergence may result from the non-uniform etching condition of the PC holes and texturing of ITO layers. And these factors might also contribute to no specific polarization direction observed in Figure 2b. (b) Simulated far-field emission angles for PCSELs with ITO periods varies from 6.48 μm to 10.8 μm. N is the ratio between ITO period and lattice of PC of 270 nm. Calculated deflection angles for different N are presented in the inset. (c) The ITO surface topography measured by the alpha step system. Rapid changing periods is due to the PC patterns. Large periods with 8 to 10 m can be observed. (d) The striped-like near-field pattern measured under 200 mA current injection. The period of striped pattern matches to the large period observed in (c).


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ACS Photonics

Figure. 5. The measured photonic band diagrams (a) below and (b) above the threshold condition. The left and right sections of both figures indicate the measurement along the x and y direction, respectively. Solid lines represent the calculated bands near the Г point with the original photonic crystal structure, and cyan dotted lines indicate the calculated photonic crystal bands with ITO structure considered.


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Photonic crystal surface emitting lasers with naturally-formed periodic ITO structures Han-Lun Chiu, Kuo-Bin Hong, Kuan-Chih Huang, Tien-Chang Lu

This figure briefly illustrates that an ITO cladding layer with a periodic surface deposited on the photonic crystal surface emitting laser can be treated as a built-in diffraction grating structure to easily demonstrate the fascinating feature of laser beam deflection through accompanying simple fabrication process.


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Photonic crystal surface emitting lasers with naturally-formed periodic

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