Ingan/Gan Distributed Feedback Laser Diodes with Surface Gratings and Sidewall Gratings

micromachines

Article InGaN/GaN Distributed Feedback Laser Diodes with Surface Gratings and Sidewall Gratings

Zejia Deng 1,2, Junze Li 1,2,*, Mingle Liao 1,2, Wuze Xie 1,2 and Siyuan Luo 1,2

1 Microsystems and Terahertz Research Center, China Academy of Engineering Physics, Chengdu 610200, China; [email protected] (Z.D.); [email protected] (M.L.); [email protected] (W.X.); [email protected] (S.L.) 2 Institute of Electronic Engineering, China Academy of Engineering Physics, Mianyang 621999, China * Correspondence: [email protected]

Received: 15 September 2019; Accepted: 12 October 2019; Published: 14 October 2019

Abstract: A variety of potential applications such as visible light communications require laser sources with a narrow linewidth and a single wavelength emission in the blue light region. The gallium nitride (GaN)-based distributed feedback laser diode (DFB-LD) is a promising light source that meets these requirements. Here, we present GaN DFB-LDs that share growth and fabrication processes and have surface gratings and sidewall gratings on the same epitaxial substrate, which makes LDs with diﬀerent structures comparable. By electrical pulse pumping, single-peak emissions at 398.5 and 399.95 nm with a full width at half maximum (FWHM) of 0.32 and 0.23 nm were achieved, respectively. The surface and sidewall gratings were fabricated alongside the p-contact metal stripe by electrical beam lithography and inductively coupled plasma etching. DFB LDs with 2.5 µm ridge width exhibit a smaller FWHM than those with 5 and 10 µm ridge widths, indicating that the narrow ridge width is favorable for the narrowing of the line width of the DFB LD. The slope eﬃciency of the DFB LD with sidewall gratings is higher than that of surface grating DFB LDs with the same ridge width and period of gratings. Our experiment may provide a reliable and simple approach for optimizing gratings and GaN DFB-LDs.

Keywords: GaN laser diode; distributed feedback (DFB); surface gratings; sidewall gratings

1. Introduction Gallium nitride (GaN) laser diodes (LDs) are potentially used in displays, medical application, visible light communications (VLC), etc [1–4]. Some applications require a single peak and narrow-linewidth laser source. Distributed feedback lasers diodes (DFB-LDs) with these advantages have attracted extensive concern in both academia and the industry. In optical atomic clocks, an extremely narrow-linewidth blue laser is required to aim at atomic cooling transition [5]. In medical diagnostics, the DFB LD is a promising light source for ﬂuorescence spectroscopy, where the emission wavelength can be precisely targeted [6]. GaN-based DFB LDs have been achieved by buried gratings [7–9], sidewall gratings, and surface gratings [10–13]. In the beginning, the ﬁrst order DFB LD was achieved by establishing the buried gratings [14]. Subsequently, the third and 39th DFB LDs were fabricated by etching sidewall gratings [15–17]. Additionally, the single longitudinal mode emissions of the 10th DFB LDs with surface gratings were realized under optical pumping and electrical impulse driving [18,19]. In contrast to burying gratings, DFB LDs with sidewall and surface gratings do not require high-cost and hard crystal regrowth processing, preventing the device from being damaged in the regrowth process [20,21]. Surface gratings and sidewall gratings are preferred for DFB LDs due to simpler fabrication processes. Though there have been varieties of DFB LDs fabricated using surface gratings or sidewall gratings,

Micromachines 2019, 10, 699; doi:10.3390/mi10100699 www.mdpi.com/journal/micromachines MicromachinesMicromachines2019 2019, 10, 10,, 699 x 2 of 210 of 10

are achieved on different epitaxial wafers; there are no reports on GaN DFB-LDs that share growth to the best of our knowledge, all of them, regardless of grating type, are achieved on different epitaxial and fabrication processes and have surface gratings and sidewall gratings on the same epitaxial wafers;substrate. there Therefore are no reports, it is inevitable on GaN DFB-LDsthat there that are distinct share growth differen andces fabricationin the optical processes and electrical and have surfaceproperties gratings of D andFB LD sidewalls with gratings different onkinds the sameof grating epitaxial structures substrate.. It isTherefore, difficult to it systematically is inevitable that therecompare are distinct and analyze differences the performance in the optical of DFB and LD electricals with propertiessurface and of sidewall DFB LDs gratings with di, becausefferent kindsthey of gratingare fabricated structures. on diverse It is diffi epitaxialcult to systematically wafers. Hence, compare in order andto further analyze un thederstand performance the performance of DFB LDs withdifferences surface of and DFB sidewall LDs with gratings, different because structures they, it are is necessary fabricated to on fabricate diverse DFB epitaxial LDs with wafers. different Hence, intypes order ofto gratings further on understand the same theepitaxial performance substrate, di whichfferences can of provide DFB LDs some with experimental different structures, evidencesit is necessaryfor the optimal to fabricate design DFB of DFB LDs LD withs. different types of gratings on the same epitaxial substrate, which can provideIn this somework, experimental the finite-difference evidences-time for-domain the optimal (FDTD) design tool was of DFB used LDs. to analyze the influence of theIn this width work, of the the ridge finite-di andff erence-time-domain the types of gratings (FDTD) on the tool properties was used of DFB to analyze LDs. Based the influence on the of thedesigned width of structure the ridge of and LDs, the DFB types-LDs of with gratings surface on the and properties sidewall gratings of DFB LDs.of different Based onperiods the designed and structureridge widths of LDs, were DFB-LDs fabricated with on surface the same and epitaxial sidewall wafer. gratings Fabry of– diPérotfferent (F– periodsP) LDs andwith ridge the same widths wereridge fabricated widths were on the also same fabricated epitaxial on wafer. the same Fabry–P epi-waferérot (F–P) for comparison LDs with the. The same well ridge-proportioned widths were alsograting fabricateds were ondefine thed same alongside epi-wafer the p- forcontact comparison. metal stri Thepe by well-proportioned electrical beam lithography gratings were(EBL)defined and alongsideinductively the coupled p-contact plasma metal (ICP) stripe etching. by electrical The morphology beam lithography characteristics (EBL) and of inductivelyall the LDs coupledwere observed by a scanning electron microscope (SEM) with an FEI Nova NanoSEM 450. The electrical plasma (ICP) etching. The morphology characteristics of all the LDs were observed by a scanning and optical characteristics of the DFB LDs, including the linewidth, slope efficiency, and threshold electron microscope (SEM) with an FEI Nova NanoSEM 450. The electrical and optical characteristics current, were measured and analyzed. of the DFB LDs, including the linewidth, slope efficiency, and threshold current, were measured and2. Simulation analyzed. and Fabrication 2. SimulationAccording and to Fabricationthe coupled mode theory, the coupling coefficient can be achieved approximately by the following equation [22]: According to the coupled mode theory, the coupling coefficient can be achieved approximately by  the following equation [22]: 2n2 - n 1  sin π m   = ( ) (π ) (1) 2 n2 n1 sin m γ κ = 1 − m (1) λ1 m where m is the grating order, λ1 is the wavelength, n1 is the effective modal index in the broad area where m is the grating order, λ1 is the wavelength, n1 is the effective modal index in the broad area including the sections of the ridge and grating, n2 is the effective modal index in the section of the including the sections of the ridge and grating, n2 is the effective modal index in the section of the ridge, and γ is the duty-cycle of gratings. ridge, and γ is the duty-cycle of gratings. The effective modal index (neff) of the waveguide in a GaN LD can be calculated using the The effective modal index (n ) of the waveguide in a GaN LD can be calculated using the finite-difference-time-domain (FDTD)eff , where the epitaxial structure and device structure of the finite-difference-time-domain (FDTD), where the epitaxial structure and device structure of the simulation can be found in the parameters displayed in the following sections. Generally, there is a simulation can be found in the parameters displayed in the following sections. Generally, there is decreasing trend in modal index with ridge width at a certain etch depth. Typical optical field adistributions decreasing trend are given in modal in Figure index 1, with with ridge ridge widths width of at 2.5 a and certain 5 µm. etch It is depth. obvious Typical that a reduction optical field distributionsof the waveguide are given width in Figure restriction1, with the ridge optical widths field of 2.5more and, resu 5 µm.lting It isin obviousa decreasing that a reductionin mode of thenumber waveguide. width restriction the optical field more, resulting in a decreasing in mode number.

FigureFigure 1.1 OOpticalptical field ﬁeld profiles proﬁles for for ridge ridge widths widths of of(a) ( a2.5) 2.5 µmµ mand and (b)( 5b )µm 5 µ. m.

TheThe refractive refractive modal modal index index increases increases with with ridge ridge width widt andh gradually and gradually approaches approach the maximumes the valuemaximum when value the ridge when width the ridge is relatively width is relatively large, which large is, which shown is in shown Figure in2 a.Figure It is 2a. worthy It is worthy pointing outpointing that coupling out that lengthcouplingkL lengthhas a greatĸL has inﬂuence a great influence on the characteristics on the characteristics of the DFB of the LDs. DFB Figure LDs. 2b shows the coupling length kL as a function of the ridge width when the etch depth was 500 nm and MicromachinesMicromachines2019 2019, ,10 10,, 699 x 3 of3 of10 10

Figure 2b shows the coupling length ĸL as a function of the ridge width when the etch depth was the500 cavity nm and length the cavi wasty 600 lengthµm. wa Fors 600 the µm. sidewall For the grating, sidewall the grating, broad sectionthe broad including section including the ridge the and gratingridge and had grating a width ha twiced a width the size twice of thethe sectionsize of the of the section ridge, of whereas the ridge, for whereas the surface for t grating,he surface the broadgrating, section the broad possessed section a fixed possesse widthd ofa fixed 80 µm. width Obviously, of 80 µm.kL showed Obviously a decreasing, ĸL show trended a decreasing as the ridge widthtrend increased.as the ridge It width must beincrease pointedd. It out must that be this pointed calculation out that was th basedis calculation on the fundamental was based on mode; the infundamental the high order, mode mode; in thekL hadhigh aorder relatively, mode large ĸL ha value,d a relatively leading tolarge a moderate value, leading reflectivity. to a moderate Since the surfacereflectivity. gratings Since had the a surface greater gratings width (and had thus a greater refractive width index) (and inthus the refractive broad area index) for the in samethe broad ridge widtharea for (compared the same withridge the width sidewall (compared gratings), with they the hadsidewall a bigger gratings refractive), they index had a di biggerfference refractive∆n = (n 2 indexn ). As difference demonstrated △n = in(n2 Equation − n1). As (1), demonstrated the coupling in length EquationkL was (1), proportional the coupling to refractive length ĸL indexwas − 1 diproportfferenceional∆n whento refractive the order index and difference duty ratio △ ofn thewhen gratings the order were and defined. duty ratio As such, of the the gratingskL of the we DFBre LDdefined. with surface As such gratings, the ĸL was of the slightly DFB higherLD with than surface that of gratings the one withwas slightly sidewall higher gratings than when that the ofridge the widthsone with of thesidewall two kinds gratings of structures when the ridge were identical.widths of the two kinds of structures were identical.

FigureFigure 2.2 (a) Refractive index index as as a a function function of of the the ridge ridge width width.. ( (bb)) C Couplingoupling length length ĸLkL asas a afunction function of of thethe ridge ridge widthwidth ofof thethe sidewallsidewall andand surfacesurface gratings.gratings. EtchingEtching depth: 500 nm.

AccordingAccording to to the the Bragg Bragg condition, conditionΛ, =Λλ =m /λ2nm/eﬀ2nandeff and the characterizationthe characterization of the of emission the emissi at aroundon at 403around nm of 40 the3 nm F–P of LDs the fabricatedF–P LDs fabricated on the epitaxial on the wafer,epitaxial the wafer, periods the of periods the 10th-order of the 10th and-order 20th-order and Bragg20th-order grating Bragg were grating calculated were and calculated found to and be 824found and to 1648 be 824 nm, and respectively. 1648 nm, Anotherrespectively. parameter Another for theparameter gratings fo isr thethe dutygratings cycle is γthe= (dutyΛ o)cycle/Λ, whereγ = (Λo −denotes o)/Λ, where the widtho denotes of the the etched width trench.of the etched A duty − cycletrench. of aroundA duty cycle 0.8 was of around adjusted 0.8 for wa thes adjusted gratings. for the gratings. TheThe designed designed complete complete structures structure ofs theof the DFB DFB LDs LD withs with surface surface and sidewall and sidewall gratings grating are depicteds are indepicted Figure3 in. It Figure is worth 3. It noting is worth that noting the surface that th gratingse surface hadgratings a larger had grating a larger width grating than width the than sidewall the gratingsidewall and grating were muchand we widerre much than wider the width than ofthe the width ridge of waveguide, the ridge waveguide, while the width while of the the width sidewall of gratingthe sidewall was closergrating to wa thats closer of the toridge that of waveguide. the ridge waveguide. The LDs wereThe LDs fabricated were fabricated from the from AlInGaN the epi-structureAlInGaN epi composed-structure compo of n-typesed andof n- p-typetype and epitaxial p-type layersepitaxial and layer InGaNs and/GaN InGaN multiple/GaN multiple quantum wellsquantum (MQWs) wells designed(MQWs) designed for the emission for the emission of 403 nm, of 40 including3 nm, including a lower a cladding lower cladding layer of layer 850 nmof Al850.0750 nmGa Al0.9250.075N:Si,Ga0.925 a 60-nm-thickN:Si, a 60-nm In-0.03thickGa 0.97In0.N:Si03Ga0.97 lowerN:Si waveguidelower waveguide layer, a multiplelayer, a multiple quantum quantum well with threewell 6.5-with or three 2-nm-thick 6.5- or 2 undoped-nm-thick GaN undoped quantum GaN barriersquantum and barriers two 2.7 and nm two In 0.12.7Ga nm0.9 NIn0.1 wells,Ga0.9N an wells upper, waveguidean upper waveguide layer of doped layer 60 of nm doped InGaN:Mg, 60 nm a InGaN20 nm Al:Mg0.13, Ga 0.87 20 N:Mgnm Al electron0.13G0.87N blocking:Mg elect layer,ron blocking a 450 nm Allayer,0.05Ga a 0.95450N:Mg nm Al upper0.05Ga cladding0.95N:Mg upper layer, andcladding a 10 nm layer GaN:Mg, and a subcontact10 nm GaN layer.:Mg subcontact The F–P LDs layer. and The DFB LDsF–P wereLDs fabricatedand DFB LDs on thewere same fabricated epi-wafer on by the the same same epi manufacturing-wafer by the processsame manufacturing except for the process building ofexcept the surface for the and building sidewall of gratings the surface of DFB and LDs. sidewall The 2.5-, gra5- andting 10-s ofµm-wide DFB LDs Ni/.Au The p-contact 2.5-, 5- stripes and with10-µm a thickness-wide Ni/Au of 10 p/-30contact nm were stripes formed with ona thickness the wafer of by 10/30 electron nm were beam formed evaporation on the and wafer a rapid by electron beam evaporation and a rapid thermal process. The 150-nm-thick SiO2 layer was deposited thermal process. The 150-nm-thick SiO2 layer was deposited on the whole wafer by plasma-enhanced chemicalon the whole vapor wafer deposition by plasma (PECVD)-enhanced as the c hardhemical mask vapor of etching deposition of gratings (PECVD) and as ridge the waveguideshard mask of for DFBetching LDs. of The grating 250-nm-thicks and ridge positive waveguides polymethyl for methacrylateDFB LDs. Th (PMMA)e 250-nm resist-thick was positive used topolymethyl deﬁne the gratingsmethacrylate with periods(PMMA of) 824resist and was 1648 used nm, to the define patterns the grating of the gratingss with period were constructeds of 824 and on 1648 both nm sides, the of thepattern p-contacts of the stripes grating ands w perpendicularere constructed to on them both with sides this of method the p-contact of electron stripes beam and lithographyperpendicular (EBL), to

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them with this method of electron beam lithography (EBL), the model of the EBL was Raith i-line theplus model with of a the write EBL resolution was Raith of i-line 7 nm plus. The with pattern a writes resolution of grating ofs 7and nm. ridge The patterns waveguides of gratings were andtransferred ridge waveguides to the epi-structure were transferred of the wafer to the through epi-structure reactive of ion the etching wafer through based on reactive CHF3/SF ion6 gas etching and basedthe dry on etching CHF3/SF o6f gasinductively and the dry coupled etching plasma of inductively (ICP) using coupled Cl2/BCl plasma3 gas (ICP) in succession using Cl2./ BClThe3 gas300 innm succession.-thick SiO2 The insulating 300 nm-thick layer SiOwas2 insulating formed on layer the was wafer formed by on PECVD the wafer, and by the PECVD, 2-, 4.5 and- and the 2-,8.5 4.5--µm and-wide 8.5- openingµm-wides on openings the ridge on w theere ridge given were shape given by dry shape etching by dry the etching SiO2 la theyer SiO using2 layer CHF using3/SF6 CHFmixed3/SF gas6 mixed before gas the before 50/250 the nm 50 /Ti/Au250 nm metal Ti/Au pad metal was pad deposited. was deposited. The back The side back of side the of wafer the wafer was wasthinned thinned and andpolished polished to a to thickness a thickness of of120 120 µmµ,m, and and then then an an n- n-contactcontact Ti/Pt/Au Ti/Pt/Au metal metal layer layer with aa thicknessthickness ofof 5050/50/250/50/250 nmnm waswas depositeddeposited onon it.it. The processed samplesample was cut into barsbars withwith aa cavitycavity lengthlength ofof 600600 µµmm,, andand bothboth frontfront andand backback facetsfacets werewere uncoated.uncoated.

Figure 3. The structures of processed distributed feedback laser diodes (DFB-LDs) with surface gratings andFigure sidewall 3 The gratings, structures as well of processed as the diagrammatic distributed drawing feedback of laser the cross diodes proﬁle (DFB of- theLDs) gallium with surfacenitride (GaN)-basedgratings and epitaxial sidewall wafer. gratings, as well as the diagrammatic drawing of the cross profile of the 3. Resultsgallium and nitride Discussions (GaN)-based epitaxial wafer.

3.1.3. Results The DFB and LDs Discussions with the Surface Gratings

3.1. TheTable DFB1 compares LDs with the the structureSurface Gratings parameters of di ﬀerent manufactured LDs. Devices 1 and 4 are F–P LDs with ridge widths of 2.5 and 10 µm, respectively, that are compared with the fabricated DFB-LDs with surfaceTable 1 gratings.compares Devices the structure 2 and 3pa haverameters the same of different ridge width manufactured and width LDs. and dutyDevice ratios 1 ofand surface 4 are gratings,F–P LDs butwith they ridge have widths diﬀerent of 2.5 grating and periods10 µm, respectively, of 824 and 1648 that nm. are The compared same parameters with the alsofabricated apply toDFB Devices-LDs with 5 and surface 6 except gratings for the. Device ridges width. 2 and 3 Allhave of the them same were ridge fabricated width and from width the sameand duty epitaxial ratio waferof surface and gratings shared the, but same they processing. have different grating periods of 824 and 1648 nm. The same parameters also apply to Devices 5 and 6 except for the ridge width. All of them were fabricated from the same epitaxialTable wafer 1. The and structure shared parameters the same ofprocessing. the fabricated distributed feedback laser diodes (DFB-LDs) of surface gratings and Fabry–Pérot (F–P) LDs. Table 1 The structure parameters of the fabricated distributed feedback laser diodes (DFB-LDs) of surface gratingSamples and Fabry–Pérot Device (F–P) LD 1s. Device 2 Device 3 Device 4 Device 5 Device 6 Ridge width 2.5 µm 2.5 µm 2.5 µm 10 µm 10 µm 10 µm Sample Device 1 Device 2 Device 3 Device 4 Device 5 Device 6 Width of grating of each side - 40 µm 40 µm - 40 µm 40 µm PeriodRidge of gratings width 2.5 z µm 8242.5 µm nm 16482.5 µm nm 10 µm - 10 824 µm nm 10 1648 µm nm Width of grating of each side - 40 µm 40 µm - 40 µm 40 µm Duty ratio of grating - 80% 80% - 80% 80% Period of gratings - 824 nm 1648 nm - 824 nm 1648 nm Duty ratio of grating - 80% 80% - 80% 80% The structures of the ridge and gratings of the DFB LDs with surface gratings were well formed, as shownThe structure in Figures4 .of The the high ridge resolution and grating SEMs graphof the in DFB Figure LDs4a showswith surface a lateral grating views of we there DFB well LDformed with, as surface shown gratings, in Figure thus 4. The displaying high resolution the general SEM structural graph in featuresFigure 4 ofa show the device.s a lateral The view actual of the DFB LD with surface gratings, thus displaying the general structural features of the device. The

Micromachines 2019, 10, 699 5 of 10 Micromachines 2019, 10, x 5 of 10 parametersactual parameter of thesridge of the and ridge the gratingand the ofgrating the processed of the processed DFB LD are DFB shown LD are in Figure shown4b, in which Figure were 4b, whichclose to we there designedclose to the values designed shown values in Table shown1. Figure in T4ablec presents 1. Figure the 4 sectionalc presents feature the section of theal surface feature ofgratings the surface with a grating periods of with 1648 a nm. period It can of be 1648 concluded nm. It thatcan the be gratingsconcluded have that the the desired grating qualitys have and the a desireddepth of quality 481 nm and close a depth to the headof 481 of nm the close upper to waveguidethe head of layer, the upper which waveguide means that layer they, basicallywhich means meet thatour designthey basically requirements. meet our design requirements.

Figure 4.4 ScanningScanning electron electron microscope microscope (SEM) (SEM) images images of of ( (aa)) an an oblique oblique view view of the fabricated DFB laser diodes diodes with with surface surface gratings, gratings, (b ()b a) top a top vision vision of the of the20th 20th order order surface surface gratings gratings alongside alongs a metalide a metalstripe, stripe and (,c )and a sectional (c) a sectional structure structure of the surfaceof the surface gratings gratings perpendicular perpendicular to the facetto the of facet LDs. of LDs.

The spectral m measurementseasurements we werere conducted by a fiber fiber spectrum analyzer (BIM (BIM6002,6002, Brolight Brolight,, Hangzhou, China)China) withwith aa resolution resolution of of 0.16 0.16 nm nm under under a pulseda pulsed operation operation of aof 500 a 500 nspulse ns pulse length length and 1kHzand 1kHz repetition repetition frequency. frequency Figure. 5Figure shows 5 the shows emission the spectrum emission chart spectrum of the chart DFB LDs of the (Devices DFB 1–6)LDs with(Device surfaces 1–6 gratings) with surface and F–P grating LDs. Alls and of them F–P wereLDs. operated All of them at the we impulsivere operated condition at the ofimpulsive 1.2 times conditionthe threshold of 1.2 current times bythe the thresho fiber spectrometer.ld current by the Since fiber the spectrometer. emission peaks Since of those the emission LDs had dipeaksfferent of thoseintensities LDs underhad different certain testintensities currents, under the dicertainfferences test in currents, the position the difference of the bottoms in ofthe the position spectrums of the of bottomLDs were of causedthe spectrum by thes normalization of LDs were processcaused of by dividing the normalization by the different process highest of dividing intensities. by Thethe differentemission peakshighest of intensities Devices 1–6. The were emission located peaks at 402.13, of Device 398.86,s 398.50,1–6 we 402.86,re located 400.50 at and402.13, 400.86 398. nm,86, 398.50respectively., 402.86 Obviously,, 400.50 and the 40 DFB0.86 LDs nm, had respectively. a shorter lasing Obviously, wavelength the DFB than theLDs F–P had LDs a short wither the lasing same wavelengthridge width because than the of F the–P interactionLDs with the between same gratings ridge width and multiple because quantum of the interaction wells. The between offset of gratingsthe 2~3 nmand emission multiple wavelength quantum wells between. The theoffset DFB of andthe the2~3 F–Pnm laseremission also confirmswavelength the between modulation the DFBeffect and of the the grating. F–P laser The also full confirmwidth ats halfthe maximamodulation (FWHM) effect corresponding of the grating.to The DFB full LDs width for Devices at half maxima2, 3, 5, and (FWHM) 6 were corresponding 0.37, 0.32, 0.52, to andDFB 0.50LDs nm,for Device respectively.s 2, 3, 5, Weand can 6 we concludere 0.37, 0.32 that, 0.52 the, DFB and LDs0.50 nmexhibited, respectively a narrower. We emissioncan conclude width that and the a singleDFB LD peaks exhibit emissioned a becausenarrower of emission the modulation width ofand the a singlegratings, peak while emission the F–P because LDs for of Devices the modulation 1 and 4 had of distinctthe gratings multimode, while characteristics. the F–P LDs for Additionally, Devices 1 andthe FWHM4 had distinct of the emissionmultimode of thecharacteristics DFB LD with. Additionally the same ridge, the seemsFWHM to of have the rarelyemission been of related the DFB to LDthe periodwith the of same the gratings, ridge seem whiles to the have DFB rarely LD with been the related 2.5 µ mto ridgethe period width of had the a lowergratings FWHM, while than the DFBthat withLD with the 10 theµm 2.5 ridge µm width.ridge width This can had be a partly lower explained FWHM than by the that fact wit that,h the as depicted10 µm ridge in Figure width1,. Thisthe simultaneous can be partly oscillation explained ofby lateral the fact modes that, sharingas depicted the samein Figure longitudinal 1, the simultaneous mode number oscillation eventually of lateralresults inmodes a wider sharing spectral the width. same Inlongitudinal addition, according mode number to the equation eventuallyL = resultλm/2sn in —where a wider n spectral2.5 eff eff ≈ width.and L = In600 addition,µm—the according expectedfree to the spectral equation range L (FSR) = λm of/2 theneff— longitudinalwhere neff modes≈ 2.5 and ofthe L = F–P 600 laser µm could—the expectedbe approximately free spectral 0.05 nm range at around (FSR) 400 of nm. the Therefore, longitudinal the DFB modes LDs ofpossibly the F showed–P laser multi-mode could be approximatelyoperation, but this 0.05 phenomenonnm at around was 400 not nm been. Therefore, confirmed the becauseDFB LD ofs possibly the lack showof theed measurement multi-mode operationcondition,of but the this high phenomenon resolution spectrum was not andbeen the confirmed lateral far because field. of the lack of the measurement condition of the high resolution spectrum and the lateral far field.

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Figure 5 5. EmissionEmission spectrograms spectrograms of of fabricated fabricated laser laser diodes diodes (Device (Devicess 1 1–6)–6) driven driven by by electrical electrical impulses impulses with a 500 ns pulse w widthidth and a 1 kHz repetition rate. 3.2. The DFB LDs with the Sidewall Gratings 3.2. The DFB LDs with the Sidewall Gratings In addition, the DFB LDs with sidewall gratings were fabricated on the same epitaxial wafer, and In addition, the DFB LDs with sidewall gratings were fabricated on the same epitaxial wafer, they shared the same processing steps with the surface grating DFB LDs. The structure parameters of and they shared the same processing steps with the surface grating DFB LDs. The structure the fabricated LDs are shown in Table2. Devices 7 and 8 had the same ridge width of 2.5 µm, gratings parameters of the fabricated LDs are shown in Table 2. Devices 7 and 8 had the same ridge width of width of 1.25 µm, and 80% duty ratio of gratings, but they had diﬀerent grating periods of 824 and 2.5 µm, gratings width of 1.25 µm, and 80% duty ratio of gratings, but they had different grating 1648 nm, respectively. The same parameters for DFB LDs with sidewall gratings except for the widths periods of 824 and 1648 nm, respectively. The same parameters for DFB LDs with sidewall gratings of the ridge and gratings were applied to Devices 10 and 11. Device 9 was the F–P LDs with a ridge except for the widths of the ridge and gratings were applied to Devices 10 and 11. Device 9 was the width of 5 µm, which was in contrast with the DFB LDs for Devices 10 and 11. F–P LDs with a ridge width of 5 µm, which was in contrast with the DFB LDs for Devices 10 and 11.

Table 2. Structure parameters of the fabricated DFB LDs of sidewall gratings and F–P LDs. Table 2 Structure parameters of the fabricated DFB LDs of sidewall gratings and F–P LDs. Sample Device 7 Device 8 Device 9 Device 10 Device 11 Sample Device 7 Device 8 Device 9 Device 10 Device 11 RidgeRidge width width 2.5 2.5 µmµm 2.5 2.5 µmµ m5 µm 5 µm5 µm 5 µm5 µm 5 µm Width of gratings of each side 1.25 µm 1.25 µm - 2.5 µm 2.5 µm Width of gratings of each side 1.25 µm 1.25 µm - 2.5 µm 2.5 µm Period of gratings 824 nm 1648 nm - 824 nm 1648 nm Period of gratings 824 nm 1648 nm - 824 nm 1648 nm DutyDuty ratio ratio of of gratings gratings 8 80%0% 80 80%% - -80% 80%80% 80%

FFigureigure 66 shows shows that that the the DFB DFB LD LD with with 20th 20th order order sidewall sidewall gratings grating wass was well well fabricated. fabricated Figure. Figure6a 6isa theis the top top view view of the of DFB the LD DFB with LD sidewallwith sidewall gratings, grating ands it, depictsand it depicts the structure the structure of ridge, of sidewall ridge, sidewallgratings, grating doubles etching, double grooves, etching andgroove thes, p-contact and the p metal-contact on themetal ridge. on the Figure ridge6b,c. Figure shows 6b,c the actualshows theparameters actual parameters of the ridge of and the gratings ridge andof the grating processeds of device, the processed which are device, basically which in conformityare basically to thein conformvalues inity Table to 2 the. Sidewall values gratings in Table with2. Sidewall a period grating of 1648s nm, with a dutya period ratio of approximately1648 nm, a duty 80%, ratio and of a approximatelywidth of 1.25 µ m 80 of%, each and side a width alongside of 1.25 the ridgeµm of waveguide each side were alongside observed. the Sinceridge thewaveguide corresponding were observedetchings processes. Since the were corresponding under the same etchings experimental processes condition, were under the the surface same and experimental sidewall gratings condition, had thea similar surface etching and sidewall depth and gratings sectional had morphology, a similar etc ashing shown depth in and Figure sectional4. The morphology notches of the, as gratings shown inhad Figure a tilt angle4. The as notches a result of of the the gratings ICP etch ha process,d a tilt so angle the average as a result duty of ratio the ofICP the etch fabricated process gratings, so the averagewas greater duty than ratio 80% of [the23, 24fabricated]. Moreover, grating thes simulation was greater results than showed 80% [23,24 that]. the Moreove influencer, the of thesimulation relative resultslarge grating showed angle that of the 70 ◦influencecould be ignoredof the relative [25], off largeering grating a high enoughangle of reflectivity 70° could for be theignored DFBLDs. [25], offering a high enough reflectivity for the DFB LDs.

MicromachinesMicromachines 20192019,, 1010,, x 699 7 7of of 10 10 Micromachines 2019, 10, x 7 of 10

Figure 6 SEM images of a top view of the fabricated DFB laser diodes with 20th order sidewall gratingsFigureFigure 6.6. (SEMa) An images images overall of ofview a a top top of view the view ofDFB the of LD thefabricated with fabricated double DFB DFlaser etchingB diodeslaser grooves diodes with 20th; ( withb) ordera low20th sidewallpower order and sidewall gratings. (c) a highgratings(a) An power overall. (a) perspectiveAn view overall of the viewof DFBthe of ridge LD the with DFBwaveguide double LD with etchingand double the grooves; sidewall etching ( bgratings grooves) a low power;along (b) a it.low and power (c) a high and power (c) a highperspective power perspective of the ridge of waveguide the ridge andwaveguide the sidewall and the gratings sidewall along gratings it. along it. The emission spectrum of the DFB LDs with sidewall gratings and the F–P LDs for Devices 7–11 TThecanhe be emission seen in spectrum spectrum Figure 7, of ofand thethe the DFB DFB LDs LDs LD we withs rewith operated sidewall sidewall at gratings a grating current ands andaround the the F–P 20%F LDs–P LDabove fors Devicesfor threshold Device 7–11s current7can–11be can seen. T behe inseenemission Figure in Figure7 ,peaks and 7, the ofand LDsthe the Device were LDs operated swe 7–re11 operated we atre a current399. at59 a , current around399.95, around 20%403.76 above, 401.20% threshold4 aboveand 401. threshold current.77 nm, respectivcurrentThe emission. Tely.he emissionT peakshe FWHM of thepeakss Devicesof ofthe the DFB 7–11 Device LD weres fors 7 399.59, –Device11 we 399.95,res 7 399., 8, 1059 403.76,,, and399. 401.49511, we403 andre.76 0., 401.7725 401., 0.423 nm,and, 0.42 respectively.401., and77 0.nm,48 nm,respectivThe FWHMsrespectively,ely. T ofhe the FWHMwhich DFB LDsswe ofre forthe slightly Devices DFB LD higher 7,s 8,for 10, Device than and the 11s 7 were resolution, 8, 10 0.25,, and 0.23, of11 thewe 0.42, re fiber and0.25 spectrometer. 0.48, 0.23 nm,, 0.42 respectively,, and In 0. the48 contextnm,which respectively, were of comparing slightly which higher the wemultimode thanre slightly the resolution morphology higher than of the of the F ﬁber– P resolution LD spectrometer.s with ofthe the ridge In fiber the of context spectrometer.5 µm during of comparing lasing,In the tcontextthehe emission multimode of comparing spectrum morphology the of themultimode of DFB F–P LD LDs smorphology showed with the a ridge single of F of– peakP 5 LDµm sbecause duringwith the lasing,of ridge the existence the of emission5 µm ofduring the spectrum gratings. lasing, of Similartthehe emission DFB to LDs the spectrum showed previous aof single results, the DFB peak the LD becauseDFBs showed LD ofs of thea single sidewall existence peak grating ofbecause thes gratings.ha ofd thea lower existence Similar lasing toof thethe wavelength previousgratings. thanSimilarresults, the tothe F – theP DFB LD previouss LDs with of the sidewall results, same the gratingsridge DFB width. LD hads aInof lower addition, sidewall lasing gratingthe wavelength DFBs haLDds awith thanlower a the 2.5 lasing F–P µmLDs ridge wavelength with width the hathansamed athe ridge lower F–P width. FWHMLDs with In addition, than the thsameose the ridge with DFB the LDswidth. 5 with µm In aaddition, ridge 2.5 µ m width, ridge the DFBthus width LDindicating hads with a lower a that2.5 FWHM µma narrow ridge than width ridge those widthhawithd a the loweris 5favoµm FWHMrable ridge for width, than the narrowing thusthose indicating with of the the that5 µmline a narrowwidth ridge width,of ridge the widthDFBthus LDindicating is favorable. According that for to thea narrowthe narrowing spectrum ridge of characteristicwidththe linewidth is favos,rable of we the couldfor DFB the speculate LD. narrowing According that of t he tothe theeffect line spectrumwidth of grating of characteristics, the period DFB sLD on. theAccording we line could width speculate to theof LD spectrum thats is not the verycharacteristiceﬀect evident of grating. s, we periods could onspeculate the line that width the of effect LDs isof not grating very evident.periods on the line width of LDs is not very evident.

Figure 7. Emission spectrograms of processed laser diodes for Devices 7–11 driven by electrical Figureimpulses 7 Emission with a 500 spectrograms ns pulse width of and processed a 1 kHz laser repetition diodes rate. for Devices 7–11 driven by electrical impulsesFigure 7 withEmission a 500 spectrogramsns pulse width of and processed a 1 kHz repetition laser diodes rate. for Devices 7–11 driven by electrical 3.3. The Comparison of Properties of the DFB LDs with Surface and Sidewall Gratings impulses with a 500 ns pulse width and a 1 kHz repetition rate. 3.3. TheMeanwhile, Comparison based of Properties on the good of the linewidth DFB LDscharacteristics with Surface and of Sidewall DFB LDs Gratings with a ridge width of 2.5 µm, 3.3.the The characteristics Comparison ofof DFBProperties LDs withof the sidewall DFB LDs and with surface Surface gratings and Sidewall and theGratings F–P LD were compared in Meanwhile, based on the good linewidth characteristics of DFB LDs with a ridge width of 2.5 the case that their ridge widths were all 2.5 µm. The sidewall gratings need lower costs for writing µm, Meanwhilethe characteristics, based on of theDFB good LD slinewidth with sidewall characteristics and surface of DFB grating LDs swith and a theridge F –widthP LD ofwe 2.5re a smaller area of patterns than surface gratings which use EBL. The light output power and voltage comparedµm, the characteristics in the case that of the DFBir ridge LDs widthwith ss wereidewall all 2.5 and µm. surface The sidewall gratings grating and thes need F–P lower LD wecostres versus current (L–I–V) characteristics of the LDs were tested under the pulsed driving condition with a forcompared writing ina smallthe caseer area that of the patternir ridges than width surfaces were gratings all 2.5 whichµm. The us esidewall EBL. The grating light outputs need lowerpower cost ands pulse width of 1 µs and a pulse repetition frequency of 10 kHz at room temperature to avoid generating voltagefor writing versus a small currenter area (L of– Ipattern–V) characteristicss than surface of gratings the LD whichs we reus etested EBL. Theunder light the output pulsed power driving and excess heat. Figure8 shows L–I–V characteristics of the DFB LDs and the F–P LDs with the same conditionvoltage versus with a current pulse width (L–I– ofV) 1 characteristicsµs and a pulse ofrepetition the LD sfrequency were tested of 10 under kHz at the room pulsed temperature driving condition with a pulse width of 1 µs and a pulse repetition frequency of 10 kHz at room temperature

Micromachines 2019,, 10,, x 699 8 8of of 10 10 to avoid generating excess heat. Figure 8 shows L–I–V characteristics of the DFB LDs and the F–P LDridges with width the of same 2.5 µ ridgem. The width values of of2.5 the µm. threshold The value currentss of the of threshold Devices 1,current 2, 3, 7,s andof Device 8 weres 1, between 2, 3, 7, and450 and8 we 560re between mA, and 450 their and slope 560 mA, efficiencies and their were slope approximately efficiencies we 0.24,re approximately 0.12, 0.17, 0.13, 0.2 and4, 0.190.12, W 0./17A,, 0.13,respectively. and 0.19 The W/A deviations, respectively of the. The threshold deviations current of the and threshold slopes ecurrentfficiency and of slopes the LDs efficiency with the of same the LDsstructures with the were same less structures than 8% we andre 13%, less than respectively. 8% and 13 However,%, respectively. all of the However, F–P and all DFB of the LDs F– testedP and DFBshowed LDs comparatively tested showed high comparatively threshold currents high threshold and lower current slope esffi andciencies, lower which slope was efficienc mainlyies deemed, which wasto owe mainly to the deemed performance to owe ofto the the epitaxial performance wafer. of Morethe epitaxial intuitive wafer and. detailedMore intuitive results areand shown detailed in resultsTable3. are shown in Table 3.

Figure 8 8. LightLight output output power power–current–voltage–current–voltage characteristics characteristics of of Device Devicess 1, 1, 2, 2, 3, 3, 7, 7, and and 8 8 operated in pulsed mode with a 1 µs pulse width and a 10 kHz repetition rate.

Table 3. Electrical properties of the fabricated DFB LDs and F–P LDs. Table 3 Electrical properties of the fabricated DFB LDs and F–P LDs. Sample Device 1 Device 2 Device 3 Device 7 Device 8 Sample Device 1 Device 2 Device 3 Device 7 Device 8 TypeType of ofgratings gratings - -Surface Surface Surface Surface SidewallSidewall SidewallSidewall OrderOrder of ofgratings gratings - -10th 10th 20th 20th 10th10th 20th20th Threshold current (518 12) mA (539 4) mA (550 10) mA (458 7) mA (485 19) mA Threshold current (518 ± ±12) mA (539 ±± 4) mA (550 ± 10) mA (458 ±± 7) mA (485± ± 19) mA The slopes (235 15) (116 3) (172 11) (129 7) (189 6) The slopes (235 ± ±15) (116 ±± 3) (172 ±± 11) (129 ±± 7) (189± ± 6) eﬃciency mW/A mW/A mW/A mW/A mW/A efficiency mW/A mW/A mW/A mW/A mW/A FWHM 1.96 nm 0.37 nm 0.32 nm 0.25 nm 0.23 nm FWHM 1.96 nm 0.37 nm 0.32 nm 0.25 nm 0.23 nm

FFromrom the data inin FigureFigure8 8and and Table Table3, it3, can it can be seenbe seen that that DFB DFB LDs LD withs with the samethe same ridge ridge width width and anda lower a lower grating grating period period exhibited exhibit lowered slope lower effi slopeciency efficiency because ofbecause the higher of thevalue higher of kL comparedvalue of ĸL to comparethose withd to high those order with gratings high ord [26].er Ingrating addition,s [26 the]. In slope addition, efficiency the ofslope DFB efficiency LDs with of sidewall DFB LD gratingss with sidewallwas slightly grating highers wa thans slightly that of high the DFBer than LDs that with of surface the DFB gratings LDs with but smallersurface thangrating F–Ps LDsbut becausesmaller thanof the F scatter–P LDs losses because and of di theffraction scatter losses losses from and the diffraction gratings. losses Besides, from the the FWHM grating ofs DFB. Beside LDss, with the FWHMsidewall of gratings DFB LDs was with slightly sidewall lower gratings than that wa ofs DFBslightly LDs lower with surfacethan that gratings, of DFB andLDs this with may surface have gratings,been due and to the this fact may that have the be sidewallen due gratingto the fact DFB th LDat the had sidewall a narrower grating effective DFB LD width had in a thenarrower whole effectivestructure width compared in the to whole the surface structure grating compared of the DFB to the LD surfa withce the grating same ridgeof the width. DFB LD Given with its the smaller same ridgegrating width area,. theGiven sidewall its smaller grating grating DFBLD area, had the fewer sidewall transverse grating modes, DFB thus LD ha resultingd fewer in transverse a smaller modesFWHM, thus compared resulting to thein a surface smaller grating FWHM DFB compared LD. Additionally, to the surface the linewidthgrating DFB of theLD.DFB Additionally, LDs with thea single linewidth peak of emission the DFB showed LDs with a drastic a single reduction peak emission compared show toed that a drastic of the reduction F–P LDs with compared multiple to thatlasing of peaks.the F–P LDs with multiple lasing peaks.

4. Conclusions Conclusions InIn conclusion, the the GaN GaN-based-based DFB LD LDss that integrat integrateded with the surface and sidewall grating gratingss wewerere investigated by the FDTD method.method. The surface and sidewall grating gratingss alongside the p p-contact-contact metal stripe on the ridge waveguide were fabricated by by EBL and ICP etching on the same epitaxial wafer. The DFB LD with the 20th order, 80% duty-cycle surface gratings showed a single wavelength

Micromachines 2019, 10, 699 9 of 10 wafer. The DFB LD with the 20th order, 80% duty-cycle surface gratings showed a single wavelength emission at 398.86 nm with an FWHM of 0.32 nm under the electrical pulsed driving, and the DFB LD with the 20th order, 80% duty-cycle sidewall gratings obtained a peak emission at 399.95 nm with an FWHM of 0.23 nm. Additionally, both of the DFB LDs showed a narrower linewidth compared to that of the F–P LDs. Moreover, the FWHM of the DFB LDs with the ridge width of 2.5 µm was obviously lower than that of the DFB LDs using the same type of gratings with ridge widths of 5 or 10 µm, which indicates that the narrow ridge width was favorable for the narrowing of the linewidth. Furthermore, the sidewall grating DFB LDs possessed a slightly higher slope eﬃciency than that of the surface grating DFB LDs with the same ridge width and period of gratings. Given that, the DFB LD with sidewall grating required a lower fabrication cost and achieved better device performance compared to the surface grating DFB LDs, which makes it a better choice for these applications. In addition, in order to further improve the performance of the DFB LDs, the optimization of the structure of the gratings and the conduction of the cavity surface coating process are required to reduce threshold current and improve slope eﬃciency. Additionally, the side-mode suppression ratio and the high resolution spectral measurement are also required in future work.

Author Contributions: Conceptualization—Z.D. and J.L.; data curation—Z.D. and M.L.; methodology—Z.D. and M.L.; project administration—J.L.; supervision—J.L.; validation—W.X. and S.L.; writing original draft—Z.D. Funding: This work was funded by Science Challenge Project (No. TZ2016003-2), National Natural Science Foundation of China (No. 61804140), and National Key R&D Program of China (No. 2017YFB0403103). Acknowledgments: The authors would like to thank the Nanofabrication facility in Suzhou Institute of Nano-tech and Nano-bionics (CAS) for equipment access. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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