Phys. Status Solidi A 207, No. 1, 19–23 (2010) / DOI 10.1002/pssa.200982638 p s sa

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applications and materials science

New MBE growth method for high

quality InN and related alloys using in situ monitoringtechnology

Tomohiro Yamaguchi*,1 and Yasushi Nanishi2

1Research Organisation of Science & Engineering, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan2Department of Photonics, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan

Received 12 July 2009, accepted 3 October 2009

Published online 9 December 2009

PACS 61.05.jh, 68.55.ag, 81.05.Ea, 81.15.Hi

*Corresponding author: e-mail to-yama@fc.ritsumei.ac.jp, Phone: þ81 77 561 2884, Fax: þ81 77 561 3994

A new radio-frequency plasma-assisted molecular beam

epitaxy (RF-MBE) method, named droplet elimination by

radical beam irradiation (DERI), is utilized for the growth of

InN. This method is composed of two series of growth

processes: (1) InN growth under an In-rich condition and

(2) consecutive nitrogen radical beam irradiation. This method

is simple, enabling the reproducible growth of a high-quality

thick InN film with a flat surface. This method is also effective

for monitoring a nitrogen radical beam that is responsible for

InN growth. This new growth method is also applied to InGaN

growth. The results indicate a preferable Ga capturing process

on an InGaN growing surface, particularly when the samples

are grown under a metal-rich condition.

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1 Introduction Almost 7 years have passed since theband gap of InN was reported to be narrower than 1.9 eV[1–4]. Now, it is widely accepted that the band gap of InN isaround 0.65 eV. Because of this new finding, InN and relatedalloys are currently attracting considerable attention forapplication to high-frequency electronic devices, infrared-light-emitting devices, and high-efficiency tandem solar celldevices. The crystal qualities of InN and related alloys,however, hinder the application of these materials to actualdevices, as these materials are the most difficult to grow dueto their low dissociation temperature and the extremely highequilibrium vapor pressure of nitrogen [5].

Molecular beam epitaxy (MBE) has widely been usedfor the growth of InN and related alloys [1–3, 6–8], as MBEwith a radio-frequency (RF) plasma source has essentialadvantages over metal-organic chemical vapor deposition(MOCVD). In this growth method, neutral and ionizedexcited state nitrogen atoms and molecules can be generatedseparately by plasma sources, which enable us to growInN even at temperatures lower than the dissociationtemperature.

In the MBE growth of nitrides, it is well known that thegrowth under aN-rich condition results in a poor-quality filmwith a rough surface due to the lack ofmigration. The growth

under a metal-rich condition leads to a high-quality film, butmetal droplets are formed on the surface. In the case of GaN,the desorption rate of Ga fromGametal is higher than that ofN from GaN [8]. This indicates that the Ga droplets formedon the surface can easily be removed by thermal treatmentprocess such as annealing. In the case of InN under an In-richcondition, on the other hand, it is difficult to remove Indroplets by thermal treatment, as InN decomposes beforeIn droplets are evaporated [8].

Thus, the optimum window for the growth of a high-quality InN film with a flat surface is very narrow. Inaddition, a practicalmethod of precisely controlling theV/IIIratio during growth remains difficult. One of the mainreasons for this difficulty is the lack of an in situmonitoringtechnology for a nitrogen radical beam, which includesneutral and ionized excited state nitrogen atoms andmolecules.

In this study, droplet elimination by radical beamirradiation (DERI) [9] is utilized as a new InN RF-MBEgrowthmethod. This methodmakes reproducible and simplehigh-quality InN growth possible. Thismethod also providesa new and effective nitrogen radical beam monitoringmethod. The application of this method to InGaN growth isalso investigated.

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20 T. Yamaguchi and Y. Nanishi: New MBE growth method for high quality InN and related alloysp

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Figure 1 Results for InN films grown without nitrogen radicalbeamirradiation: (a)SurfaceSEMimageand(b)XRD2u�vprofile(InN(0002), In(101), GaN(0002), and Al2O3(0006)).

Figure 2 Results for InN films grown with nitrogen radical beamirradiation: (a) Surface SEM image and (b) XRD 2u�v profile(InN(0002), In(101), GaN(0002), and Al2O3(0006)).

Figure 3 (online color at: www.pss-a.com) In situ RHEEDobservation results: (a) RHEED intensity variation during InNgrowth under In-rich condition and consecutive nitrogen radicalbeam irradiation; (b)RHEEDpattern (e//h11�20i) after InNgrowthunder In-rich condition; and (c) RHEED pattern (e//h11�20i) afterconsecutive nitrogen radical beam irradiation.

2 Experimental InN and InGaN are grown in aconventional MBE system (EpiQuest RC2100NR) equippedwith conventional effusion cells for In and Ga. An activenitrogen radical beam is generated by a commercializednitrogen plasma source (SVT Associates 6.03). Thestructures of InN and InGaN can be monitored in situ byreflection high-energy electron diffraction (RHEED)analysis using kSA400 (k-Space Associates). In this study,MOCVD-grown GaN templates with a thickness of1.7–2.0mm on Al2O3(0001) are used as substrates.

3 New MBE growth method DERI DERI iscomposed of two series of growth processes. The firstprocess is InN growth under an In-rich condition for formingIn droplets. The second process is consecutive nitrogenradical beam irradiation for eliminating In droplets.

Figure 1a shows a typical scanning electron microscopy(SEM) image of the InN sample grown under an In-rich con-dition. Indium droplets are observed on the sample surface.Figure 1b shows the XRD 2u�v profile of this correspond-ing film. The metal In(101) peak is clearly identified at�33.08, in addition to peaks of InN(0002), GaN(0002), andAl2O3(0006) at 31.3, 34.5, and 41.78, respectively.

After consecutive nitrogen radical beam irradiation, theIn droplets formed after the In-rich growth process are totallyeliminated, as can be observed in the SEM image in Fig. 2a.The XRD 2u�v profile shown in Fig. 2b also confirms theelimination ofmetal In from the surface after nitrogen radicalbeam irradiation. Instead of the lack of the metal In peak, anincrease in InNpeak intensity is observed.An increase in InNthickness is also confirmed by cross-sectional SEM

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observation after nitrogen radical beam irradiation. Theseresults can be explained by the process in which the metal Indroplets formed by an In-rich growth process are totallyeliminated and transformed into InN epitaxially on anunderlayer by nitrogen radical beam irradiation.

In-droplet formation during In-rich growth and elimin-ation by nitrogen radical beam irradiation can be monitoredin situ by the RHEED intensity variation of an InNdiffraction pattern during MBE growth, as shown inFig. 3a. RHEED intensity decreases markedly after the startof InN growth under an In-rich condition. This probablycorresponds to a surface stoichiometry change from aN-stabilized surface to an In-stabilized surface. TheIn-stabilized surface would be formed by a two-monolayer-thick In wetting layer, as is observed elsewhere[10, 11]. As the growth continues under an In-rich condition,RHEED intensity decreases further gradually, but the InNdiffraction pattern is still observable, as shown in Fig. 3b.This corresponds to the accumulation of In, that is, anincrease in In droplet size. During consecutive nitrogenradical beam irradiation after stopping In supply, RHEEDintensity gradually increases as In droplet size decreases, andeventually, it returns to its original value when the surfacereturns to being N-stabilized. The RHEED pattern after thenitrogen radical beam irradiation is shown in Fig. 3c.

These results indicate that we can monitor In-dropletformation and elimination processes simply by in situRHEED intensity observation. Thus, stoichiometry controlthroughout the growth process can be automatically

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Figure 5 Relationship between time of InN growth under In-richcondition (T) and time required to recover originalRHEEDintensityunder nitrogen radical beam irradiation (t).

achieved by successive nitrogen radical beam irradiationwith the in situ monitoring of RHEED intensity. When Indroplets become sufficiently large, we could even grow a60-nm-thick filmwith a flat surface only by a single nitrogenradical beam irradiation process. By repeating the DERImethod, we can essentially grow very thick InN filmswithout encountering troublesome problems on In-dropletformation.

4 Quality of InN films grown by DERI Thesurface morphology of the InN films grown by repeatingthe DERI method is always better than or at least similar tothat of the InN films grown by a conventional methodat a very precisely controlled V/III ratio. A flat and(H3�H3)R308 reconstructed surface [12, 13] can alwaysbe observed after nitrogen radical beam irradiation.

The crystallographic quality of the films grown byrepeating the DERI method is similar to that of the InN filmsgrown by the conventional method. The full-width at half-maximum (FWHM) of the X-ray rocking curves (XRCs) ofthe InN films with a thickness of approximately 500 nm are9.0� 1.0 and 27.0� 2.0 arcmin for (0002) and (10–12)reflections, respectively. Here, the FWHM of the XRCs ofthe GaN templates used are 6.5� 0.5 and 11.0� 1.0 arcminfor (0002) and (10–12) reflections, respectively.

The electrical properties, investigated by Hall-effectmeasurements, between these two different films grown byrepeating the DERI method and the conventional method areshown in Fig. 4. The electrical properties of the films grownby repeating the DERI method show lower residual carrierconcentrations and higher Hall mobilities than those of thefilms grown by the conventional method. The carrierconcentration and Hall mobility of the InN films of app-roximately 500 nm thickness grown by repeating the DERImethod are 1.7� 0.2� 1018/cm3 and 1300� 100 cm2/V s,respectively. By increasing the film thickness, the electricalproperties improve with the crystal quality [9]. When InNlayers are grown under an In-rich condition, the In vacancyconcentration should be automatically controlled to reducedlevels. Thismight offer another advantage and potential overthe conventional method.

Thus, we can obtain higher-quality InN films, in terms ofsurface morphology and electrical properties, reproduciblyby using the simple DERI process.

Figure 4 (online color at:www.pss-a.com)Electrical properties ofInN films with thicknesses of approximately 500 nm and 3.5mmgrown by repeating DERI method and conventional method.

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5 Radical beam monitoring using DERI TheDERI method also provides a new and effective radicalbeam monitoring method. Again, we focus on a RHEEDintensity variation cycle during InN growth under an In-richcondition and consecutive nitrogen radical beam irradiation,as shown in Fig. 3a. Here, we define the time of InN growthunder an In-rich condition as T and the time required torecover the original RHEED intensity under nitrogen radicalbeam irradiation as t. Thus, T is proportional to the quantityof excess In supplied to the growing surface. From the ratioof T to t, we can estimate radical beam intensity under thecorresponding RF power and nitrogen gas flow rate. As canbe observed in Fig. 5, linear and proportional relationshipsbetween T and t are experimentally obtained. This indicatesthat t under a constant growth time T is also related tonitrogen radical beam intensity. t under a constant growthtimeT (120 s) is plotted as a function ofRFpowerwithN2 gasflow rate as a parameter, as shown in Fig. 6. From this curve,we can estimate radical beam intensity, which is responsiblefor InN growth, as a function of RF power and N2 gas flowrate.

Combining these results with those of optical absorptionor emission spectroscopy from a nitrogen RF plasma, radicalelements from neutral, ionized, atomic, or molecular excitedspecies responsible for InN growth should be more preciselyidentified. Thus, this proposed growth method provides anew and effective radical beam monitoring tool, which hasnot been successfully developed thus far.

Figure 6 (online color at: www.pss-a.com) t under constantgrowth time T (120 s) as function of RF powerwith N2 gas flow rate.

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Figure 7 XRD 2u�v profile of InGaN grown with various RFpowers between 100 and 1000W under fixed In and Ga molecularbeams, and N2 gas flow rate.

Figure 8 Estimated In compositions of InGaN.

Figure 9 XRD2u�v profiles of InGaN samples with andwithoutnitrogen radical beam irradiation after InGaNgrowth under ametal-rich condition.

6 Growth of InGaN alloy The application of theDERI method to InGaN alloy growth is investigated.Constant In and Ga molecular beams with In and Ga ratiosof 35 and 65% are supplied with various RF powers under afixed N2 gas flow rate of 2 sccm. The growth condition variesfrom N-rich to metal-rich as the RF power changes from1000 to 100W. After the growth of InGaN, consecutivenitrogen radical beam irradiation is carried out.

XRD 2u�v profiles and the estimated alloy compo-sitions are shown in Figs. 7 and 8, respectively. The alloycomposition in the grown layer remains almost constant at anIn-to-Ga supply ratio when the RF power reduces from 1000to 500W. The supplied metal would be incorporated into theInGaN grown layer. The In molar fraction in the InGaNgrown layer decreases with a decrease in RF power below500W. The InN(0002) peak appears at �31.38 and itsintensity increases with a decrease in RF power. Eventually,only the peaks of GaN and InN are detected and no alloys aregrown under an RF power of less than 200W.

When consecutive nitrogen radical beam irradiation isnot carried out after the growth of InGaN, the metal In(101)peak is observed at�33.08, instead of the InN(0002) peak, ascan be observed in Fig. 9. These results suggest that Ga ispreferentially captured over In, particularly when thesamples are grown under a metal-rich condition and In ispreferentially swept out from the growing interface. Indiumdroplets should form on an InGaN surface, as in the case of

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InN growth by DERI. Thus, InN growth occurs duringconsecutive nitrogen radical beam irradiation.

A similar selective process between In and Ga isobserved in InGaN grown on GaN substrates with mis-orientation angle [14, 15]. In these cases, Ga is morepreferentially captured at growth steps with a highermisorientation angle than In and In is forced to be sweptaway from the growth steps. The selective process betweenIn and Ga is found to be enhanced by increasing III/V ratioand growth step density.

These experimental results may imply that Ga-richInGaN preferably grows in surrounding dislocations(particularly screw-type threading dislocations), whichwould be one of the growth step sites, and a wider bandgap barrier forms in a surrounding threading dislocation line.This barrier should suppress the adverse effect of dislocationon the optical properties of InGaN.

7 Conclusion A new RF-MBEmethod, named DERI,is utilized for the growth of InN. This method is composed oftwo series of growth processes, namely, In-rich growth andconsecutive nitrogen radical beam irradiation. DERI is asimple method of obtaining high-quality InN films repro-ducibly. As the films are grown under ametal-rich condition,it is considered that In-vacancy-related point defects can beautomatically controlled and minimized. Thus, the DERImethod may lead to breakthroughs in the application of InNand related alloys to electronic and optical devices. TheDERI method also provides a new and effective method ofmonitoring a nitrogen radical beam, which is responsible forInN growth. It is also applied to the growth of InGaN alloys.The preferential incorporation of Ga over In on an InGaNgrowing surface is clearly observed when the samples aregrown under a metal-rich conditions.

Acknowledgements The authors would like to thankDr. N.Maeda of NTT Photonics Laboratories and Prof. T. Araki and Prof.M. Takeuchi of Ritsumeikan University for a fruitful discussion.This work was supported by the MEXT through Grant-in-Aids forScientific Research in Priority Areas ‘‘Optoelectronics Frontier byNitride Semiconductor’’ #18069012 and Scientific Research (A)#21246004. One of the authors (T.Y.) was also supported by Grant-in-Aid for Young Scientist (B) #21760237 from the MEXT.

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