Focused Ion Beam Technology for Optoelectronic Devices

J.P. Reithmaier, L. Bach, A. Forchel

Technische Physik, University of Würzburg, Germany

Abstract. High-resolution proximity free lithography was developed using InP as anorganic resist for ion beamexposure. InP is very sensitive on ion beam irradiation and show a highly nonlinear dose dependence with a contrastfunction comparable to organic electron beam resists. In combination with implantation induced quantum wellintermixing this new lithographic technique based on focused ion beams is used to realize high performance nanopatterned optoelectronic devices like complex coupled distributed feedback (DFB) and distributed Bragg reflector(DBR) lasers.

INTRODUCTION

High-resolution lithographic techniques gainedmore and more importance for the realization ofadvanced microelectronic and optoelectronic devicesdue to reduced structure dimensions and the demandon size control in the nanometer range. The control ofstructure dimensions below 0.1 µm by opticallithography is quite sophisticated. Improved resolutionis possible by high-resolution e-beam writing systems.Both techniques are based on exposure of organicresists and need additional mask and structure transfersteps to form semiconductor nanostructures. Thispattern transfer process causes additional deviationsand has to be corrected. Important factors are depth offocus for optical lithography that limits the exposure ofmulti-level surfaces, and the proximity effect for e-beam exposure caused by backscattered electrons.

Also with focused ion beams (FIB) high-resolutionexposure is possible in the range well below 100 nm.Due to the large penetration depth and the lowprobability to create secondary electrons, proximityeffects can be neglected. In contrast to the otherlithographic techniques, FIB can also applied formaskless patterning. Different effects can be used, likeimplantation induced quantum well intermixing1 ordirect sputtering of material.2 Unfortunately, sputteringneeds relatively high ion doses and causes significantcrystal damage, which may significantly degradedevice performances.

Ion beam enhanced wet chemical etching, whichwas demonstrated first in the AlGaAs/GaAs materialsystem,3-5 needs several orders of magnitude lowerdoses and strongly reduces the damage problem byselectively removing the highly damaged area. Thistechnology could be successfully transferred to InPwhich allow the implementation of FIB based nanopatterning techniques to the fabrication of telecom-munication related optoelectronic devices.6-9

In this paper the newly developed nano fabricationtechnique based on FIB exposure of InP as inorganicresist material is reviewed. Examples will bediscussed, where this technique was applied for thefabrication of optical feedback gratings to realizesingle mode emitting lasers and monolithicallyintegrated devices.

MASKLESS PATTERNING

For implantation, an FIB system from Eiko Corp. isused with a beam diameter (FWHM) of about 30 nm.10

For maskless patterning, 100 keV Ga+ ions weredirectly implanted into InP and the damaged regionswere removed by a 10% HF acid in an ultrasonic bathat 80 °C. In Fig. 1, the dose dependence of the etchdepth is plotted which show a resist like strongly non-linear characteristic. Below an ion dose of 1.5×1013

cm-3 etching is completely stopped while above 2×1013

cm-3 the etch depth saturates to about the ion

penetration depth (not taking into account channelingeffects).

FIGURE 1. Dose dependence of etch depth in InP after 100keV Ga implantation and 10 min etching in 10% HF.

Because of the high contrast function the etchingstops at a fixed ion dose level. This can be seen in asecondary electron microscopy (SEM) image of thecross section of such a line in Fig. 2. The etch front ishighlighted by a dashed line which coincides quitenicely with the dose profile calculated by Monte Carlosimulations. The feature size at the surface is 25 nmand broadens to about 40-50 nm due to scatteringeffects. Because the dose is quite low, only 125 ionsare involved within an area of 25×25 nm2 andstatistical deviations are already significant. Due to thehigh sensitivity of InP on ion beam implantation, thewriting speed can be as fast as e-beam exposure oforganic resists.

FIGURE 2. SEM cross-section of wet chemically etchedsingle implanted line. The dashed line highlights the edge ofthe groove.

In addition to the creation of a damage networknear the surface, which is removed by the abovediscussed wet chemical etching process, an essentialpart of the ions penetrate much deeper into thesemiconductor due to channeling in crystaldirection.11,12 In (100) InP the penetration depth for100 keV Ga ions is in the range of about 300 nm. If

quantum wells are within this depth, the channeledions can induce thermal intermixing effects in spatialalignment to the etched patterns. In Fig. 3, theimplantation induced thermal intermixing effect isillustrated which creates a local band gap shift.13

FIGURE 3. Schematic illustration of implantation inducedthermal intermixing effect in semiconductor quantum films.Starting from an untreated quantum film (left) the transitionenergy is shifted to higher energies due to a narrowing of thewell caused by material interdiffusion.

GRATING FABRICATION

Both effects can be used separately orsimultaneously to fabricate gratings for opticalfeedback in single mode emitting devices likedistributed feedback (DFB) or distributed Braggreflector (DBR) lasers. If only the material contrast isused, an index coupled grating is built while by usingthe intermixing effect, a gain coupled grating isformed.14 Best results in terms of wavelength controland device performance are achieved by combiningboth effects in so-called complex coupled gratings.9

FIGURE 4. Schematic illustration of the definition oflaterally complex coupled gratings by FIB technology. Theoptical feedback is based on the overlap with the evanescentpart of the propagating optical mode.

In Fig. 4, the grating fabrication technique isillustrated. First, a conventional ridge waveguide(RWG) laser is processed by optical lithography andwet chemical etching. The etch depth is controlled by aGaInAsP etch stop layer. Second, the grating geometryis defined by direct Ga implantation without any maskprocess. The highly damaged parts lateral to the ridge

Ga+

are removed by HF. The ridge itself is still protectedby the previous etch mask. Afterwards, the sample isannealed by a rapid thermal annealing step (typicalparameters: 700 °C, 60 s) to intermix the underlyingquantum wells and to form a self-aligned bandgapshifted absorption grating to the index surface grating.

In Fig. 5, an SEM image of an FIB defined gratingis shown after the wet chemical etch process. Due tothe proximity free exposure, the grating is well definednear to the ridge. Shadowing of the slightly under-etched ridge causes a well controlled lateraldisplacement of the grating of about 100 nm. Therectangular unpatterned area in Fig. 5 was formed bybeam shadowing of the top under-etched contact layer,which is broken away after the process.

FIGURE 5. Top-view of a lateral grating after the wetchemical etch process. The unpatterned area at the end of theridge is caused by beam shadowing of an under-etched partof the top contact layer (not visible any more).

The grating depth is controlled by a secondGaInAsP etch stop layer, the groove width by theexposed ion dose. The cross section of a grating line isnearly rectangular with a width and depth of about 80nm. The grating period is about 240 nm. The bandgapshift caused by the intermixing is in the order of 40meV.

DEVICE EXAMPLES

The definition of 1st order gratings by conventionaltechniques for DFB or DBR lasers is still sophisticatedbecause an additional overgrowth process is neededand the wavelength control is difficult. With this newapproach the device processing can be stronglysimplified and the fabrication yield improved. In thefollowing two examples will be briefly discussed.

In Fig. 6, the schematic structure of a DBR laserwith an active gain section and a passive DBR sectionis shown. Except for the grating definition by FIB, allprocess steps are identical to a conventional RWG

process technology. The contact layers from gain andDBR sections are separated and only the gain sectionis pumped.

FIGURE 6. Schematic structure of a DBR laser with gainand passive grating sections.

Despite the simple fabrication technology, thedevice performance is very high.9 In Fig. 7, the lightoutput characteristic of such a device with a totaldevice length of 600 µm is shown. The light outputwas detected from the cleaved backside of the device.The device has a very low threshold current of 8 mAand very high differential efficiency of 0.374 W/A.The inset shows the emission spectrum with a highside mode suppression ratio (SMSR) of 48 dB. Thewavelength is precisely defined by the grating periodand the fabrication yield is nearly 100%. Lifetimemeasurements show no degradation up to 10000 h.7

FIGURE 7. Light output characteristic (solid line) andSMSR values (open dots) for a DBR laser with 300 µmgrating and 300 µm gain sections. The inset shows theemission spectrum at a drive current of 70 mA.

For wavelength division multiplexing (WDM)systems, light sources with different emission wave-lengths are necessary. By using the newly developedgrating fabrication technique by FIB, the integration ofDFB lasers with different grating periods is stronglysimplified and the total fabrication yield is quite high.

gratingsection

gain section

In Fig. 8, a realized device design is shown whichintegrates four DFB lasers emitting at differentwavelengths.8 The light of all 4 lasers is coupled into asingle output ridge waveguide which simplifies fibercoupling.

FIGURE 8. Schematic illustration of the monolithicalintegration of four DFB lasers with a passive waveguidenetwork.

By simultaneously operating all four devices, fourdifferent wavelengths controlled by the grating periodscan be detected from the output port as plotted in Fig.9. Due to negative detuning of the emission wave-lengths, the bandgap absorption in the unpumpedwaveguide sections could be minimized and a totaloutput power for each wavelength of about 5 mWcould be obtained.

FIGURE 9. Emission spectrum from the single waveguideoutput by simultaneous operation of all 4 DFB lasers at atotal drive current of 300 mA. Four equidistant emissionlines can be seen related to the grating periods of theintegrated DFB lasers.

ACKNOWLEDGMENTS

The supply of epitaxial laser structures by AlcatelCorp. Res. Center, Opto+, and the financial support bythe European Community (LTR project NANOLASE,IST project BigBand) and the State of Bavaria isgratefully acknowledged.

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