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Thick, crack-free blue light-emitting diodes on Si111 usinglow-temperature AlN interlayers and in situ SixNy masking

A. Dadgar,a) M. Poschenrieder, J. Blasing, K. Fehse, A. Diez, and A. KrostFakultat fur Naturwissenschaften, Institut fur Experimentelle Physik, Otto-von Guericke Universitat

Magdeburg, Universitatsplatz 2, 39016 Magdeburg, Germany

Received 19 November 2001; accepted for publication 21 March 2002

Thick, entirely crack-free GaN-based light-emitting diode structures on 2 in. Si 111 substrates weregrown by metalorganic chemical-vapor deposition. The 2.8-m-thick diode structure was grownusing a low-temperature AlN:Si seed layer and two low-temperature AlN:Si interlayers for stressreduction. In currentvoltage measurements, low turn-on voltages and a series resistance of 55 were observed for a vertically contacted diode. By in situ insertion of a SixNy mask, theluminescence intensity is significantly enhanced. A light output power of 152 W at a current of 20mA and a wavelength of 455 nm is achieved. 2002 American Institute of Physics. DOI: 10.1063/1.1479455

Due to the lack of large GaN substrates commerciallyavailable, GaN-based light emitters are usually grown onsapphire or SiC substrates by metalorganic chemical-vapordeposition MOCVD . Growth on these substrates is eitherexpensive SiC or requires elaborate etching and contacting.Especially for low-power illumination or signaling, a reduc-tion in device cost is of great interest. A significant reductionin device cost can be achieved by simpler processing, e.g.,using vertical contacts and by using inexpensive substrates.Vertical contacts can be easily realized on conducting SiCand Si substrates. Therefore, SiC has already been used as analternative to sapphire but the substrate cost is orders of mag-nitude higher than for Si. Furthermore, using Si substratesmakes it possible to integrate light emitters with Si electron-ics. In most reports on light-emitting-diode LED structures

grown by MOCVD or molecular-beam epitaxy on Si, thedevices usually suffer from cracking at layer thicknesses ofmore than 1 m MOCVD , low output power, and highseries resistance, as well as a high turn-on voltage in somecases.1 5 A method to obtain bright, crack-free LEDs on Sihas recently been proposed by patterning the substrate.6 Thismethod does give good results, but thickness fluctuations atthe edge of the diodes occur. Another way to eliminate crack-ing of GaN on Si is by using low-temperature LT AlNinterlayers,7 first introduced by Amano and co-workers forthe growth of thick crack-free AlGaN on GaN.8 Althoughthis method improves the overall layer quality, it does intro-duce new dislocations at the upper AlN/GaN interface that

can degrade device performance.9,10 Here, we present amethod to grow thick 2.8 m , crack-free GaN-basedLED structures on planar Si.

MOCVD growth was performed with an AIXTRONAIX-200/4 RF-S reactor. An n-type Si 111 substrate wasetched prior to growth using H2SO4 :H2O2 :H2O 3:1:1 andHF 5% .11 This procedure results in an oxide-free,hydrogen-terminated Si surface and no high-temperature an-nealing step needs to be applied. The samples investigatedwere grown with an approximately 20-nm-thick AlN:Si seed

layer with a few monolayers Al predeposition around 720 Cusing TMAl and ammonia sources and hydrogen as the car-rier gas. Subsequently, a 500 nm Si-doped GaN buffer,followed by 1015 nm LT-AlN:Si, GaN:Si, and a secondLTAlN:Si layer were grown. Upon this, about 200 nm ofGaN were grown followed by an in situ SixNy mask for somesamples. With this mask, it is likely to reduce the dislocationdensity as reported by Lahreche and co-workers.12 The majorproblem with masking in our case is the coalescence thick-ness, which is usually in the range of a few microns and eventhicker for Si-doped GaN. To achieve coalescence well be-low the critical thickness of GaN on Si 1 m , the maskthickness was chosen as thick as possible for a coalescencethickness of only a few hundred nanometers. Fast coales-cence was assisted by growth at high ammonia flow. From

other samples we conclude that the GaN island distanceabove the mask is in the range of 15 m. After this ap-proximately 800-nm-thick GaN:Si coalescence layer, fiveInGaN/GaN:Si quantum wells were grown at 760C fol-lowed by an AlGaN:Mg blocking layer and 100200 nmGaN:Mg. P-type activation was performed in the growthchamber under N2 gas at 800C after growth. The 2 in.wafer obtained by this method is completely crack free. Al-loyed Ni/Au top contacts, 360 m in diameter, and Al/Aubackside contacts on the n-type Si substrate were used forelectrical characterization. The samples were characterizedwithout any further structuring directly on the wafer. Forpower measurements, a LED on a 1 mm2 chip was

mounted and ultrasonically bonded into an epoxy LED hous-ing.

The wafer curvature was determined by x-ray measure-ments as described in Ref. 7. We obtain a radius of 7.8 m,which is large compared to other samples on Si.7 A largeradius does indicate a low total stress of the layers and isimportant to avoid breaking of the substrate when processingdevices.

From the wafer curvature we determine the total stressof the layers to be 0.30 GPa. X-ray diffraction measurementsof the symmetric 0002 and asymmetric (2024) reflectionsgive values of 3.1935 and 5.1811 for the a- and c-latticea Electronic mail: armin.dadgar@physik.uni-magdeburg.de

APPLIED PHYSICS LETTERS VOLUME 80, NUMBER 20 20 MAY 2002

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constants, respectively. From these values the stress can bedetermined to be 0.58 GPa. In this case, the error is relativelylarge since the accuracy in determining a and c is reduced bybroadening of the x-ray peaks. This broadening is likely tobe due to differently strained GaN regions. This can beclearly seen for the top InGaN/GaN layers Fig. 1 . In the(2024) reciprocal space maps all superlattice SL peaks(SL1, SL0, SL1, SL2 are shifted in qx with respect tothe main GaN reflection. The shift is not due to partial relax-ation since in this case all SL peaks would be shifted diago-nally towards the origin of the reciprocal space. Thus, thedisplacement of the SL peaks originates from a differenta-lattice constant for this upper part of the structure. This

difference in lattice constants originates in a decoupling andpartial relaxation of the different GaN layers by the LT-AlNlayers. Additionally, a reduction in dislocation density by theLT-AlN layers in combination with the SixNy in situ maskfrom 1010 to 109 cm2 is observed. Details on strainrelaxation and dislocation reduction will be published else-where.

The rocking curve of the 0002 reflection gives a fullwidth at half maximum of 637 arcsec, which is among thebest values published for GaN on Si. Taking into account thatthe GaN buffer and top layers are likely grown with differenta and, consequently, c lattice constants, the broadening israther small. In highly resolved x-ray diffractometry 2scans Fig. 2 we observe high quality of the InGaN/GaNmultiquantum wells. The InGaN quantum-well thickness isdetermined by x-ray reflectometry measurements to be 1.9nm. With this result the In concentration can be determinedfrom highly resolved x-ray diffractometry 2 scans to be15%.

Currentvoltage measurements Fig. 3 show similar be-havior as the devices on sapphire13 with a turn-on voltage of2.52.8 V and series resistances around 55 for the bestdiodes. The low turn-on voltages show that the potential dropat the AlN:Si/n-Si-substrate interface is rather negligible, incontrast to other reports using high-temperature AlN seed

layers,5

and not a principal problem of vertically contactedlight-emitting diodes on Si. We assume most of the series

resistance to be due to the p-type contact and the Si-doped

LT-AlN layers.In the power measurements, an output power of 152 Wat 20 mA and an emission wavelength of 455 nm Fig. 4 isdetermined. This is already sufficient for simple signalingapplications. At currents above 35 mA we observe a degra-dation in device performance, which is due to the relativelyhigh series resistance in combination with the poor thermalconductance of the LED housing. At 40 mA an output powerof 274 W is achieved.

The external light output power and efficiency 1 ofthe device is still low compared to commercially availablehigh-brightness diodes but significantly higher when com-pared to samples grown on silicon and comparable tosamples grown on sapphire by other groups.5,1417 It is likelythat the light intensity can be enhanced significantly by asuited structuring of the sample because, compared tosamples on sapphire and SiC, most light is absorbed by thesubstrate and the InGaN/GaN layers since no side facetswere etched or any device separated. Even for diodesmounted into epoxy a significant amount approximately80% of all light emitted by the active layers in the upper

FIG. 1. Reciprocal space map around the GaN (2024) reflection. As can beclearly seen, the superlattice SL peaks are slightly shifted in qx with re-spect to the main GaN reflection, indicating a different a-lattice constant vertical dashed line but no relaxation partial relaxation: diagonal dashedline .

FIG. 2. X-ray 2 scan of a diode sample showing the measurement upper curve and simulation lower curve . Besides the superlattice peaks,pendellosungs fringes from the total SL and the cap layer can be seen,indicating the high quality of the InGaN/GaN interfaces.

FIG. 3. I V characteristics of a vertically contacted light-emitting diode

grown on Si substrate. The step in the I V characteristic below 2 V is dueto leakage currents as reported for devices on sapphire.

3671Appl. Phys. Lett., Vol. 80, No. 20, 20 May 2002 Dadgar et al.

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part of the structure is totally reflected at the GaN/air or

GaN/epoxy interface.A drastic improvement in device performance by the in-sertion of the in situ SixNy mask is observed in electrolumi-nescence for samples with and without this layer. For thesample with the SixNy in situ masking the electrolumines-cence intensity increases by about a factor of 5. For thegrowth of thick GaN layers on silicon LT-AlN interlayers areneeded. But despite a strong reduction in the threading dis-location density by LT-AlN interlayers for GaN layers grownon sapphire as reported by Amano and co-workers,9 we didobserve that new dislocations are introduced at the LT-AlN/GaN interface at least for layers grown on Si. This is espe-cially problematic since the maximum layer thickness above

a LT-AlN layer is around 1 m, thus the amount of defectscannot be reduced by growing thick layers. Introducing aSixNy mask the dislocation density can be reduced,

12 whichis important for good device performance.

In conclusion, 2.8-m-thick, crack-free LED struc-tures were grown by MOCVD on 2 in. Si 111 substrates

using LT-AlN interlayers. Output power of 152 W at 20 mAand 455 nm is already sufficient for low-power signaling andlighting applications. A further reduction in series resistanceis likely possible by optimization of the LT-AlN layers.

Part of this work is financially supported by the Deut-sche Forschungsgemeinschaft in the framework of ContractNo. KR1239/10-1. The authors gratefully acknowledge the

cooperation of D. Bimberg, Technical University, Berlin.

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FIG. 4. Power vs current of a LED 360 m diam simply mounted on anapproximately 1 mm2 die in an epoxy LED dome. At 35 mA Ohmic heatingstarts to significantly reduce device performance. The peak wavelength isaround 455 nm.

3672 Appl. Phys. Lett., Vol. 80, No. 20, 20 May 2002 Dadgar et al.

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