HVPE-GaN grown on MOCVD-GaN/sapphire templateand ammonothermal GaN seeds: Comparison of structural, optical,and electrical properties

Tomasz Sochacki a,b,n, Zachary Bryan c, Mikolaj Amilusik a,b, Milena Bobea c,Michal Fijalkowski a, Isaac Bryan c, Boleslaw Lucznik a,b, Ramon Collazo c, Jan L. Weyher a,Robert Kucharski d, Izabella Grzegory a, Michal Bockowski a,b, Zlatko Sitar c

a Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Polandb TopGaN Ltd., Sokolowska 29/37, 01-142 Warsaw, Polandc Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7919, USAd Ammono S.A., Prusa 2, 00-493 Warsaw, Poland

a r t i c l e i n f o

Article history:Received 2 January 2014Received in revised form28 January 2014Accepted 14 February 2014Communicated by T. PaskovaAvailable online 21 February 2014

Keywords:A1. CharacterizationA3. Hydride vapor phase epitaxyB1. GaNB1. NitridesB2. Semiconducting III–V materials

a b s t r a c t

Crystallization of GaN by the HVPE technique on an MOCVD-GaN/sapphire template with photo-lithographically patterned Ti mask and ammonothermally-grown GaN crystals (Am-GaN) was studiedand compared. Structural and optical properties of the obtained free-standing HVPE-GaN revealed thatAm-GaN seeds produced material of much higher quality in terms of etch pit density (EPD), X-ray rockingcurves, and excitonic emission than the sapphire-based templates. The crystallization run on the Am-GaN seed, annealed before the growth in H2þNH3 atmosphere, resulted in the HVPE-GaN of an averageEPD of 5�104 cm�2, (002) rocking curve width of 22″, and photo-luminescence as narrow as 130 meV.However, small change in quality was observed when the Am-GaN seed was annealed in N2þNH3

atmosphere prior to growth. In turn, the Hall effect measurements showed that the HVPE-GaN grown onMOCVD-GaN/sapphire template possessed lower free carrier concentration and higher mobility than theHVPE-GaN grown on the Am-GaN seeds. These results were associated to the higher growth rate of theexamined HVPE-GaN sample.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Due to a relatively high growth rate (4100 μm/h) anda possibility to crystallize high-quality and high-purity material, theHVPE technology is the technology of choice for the production of GaNsubstrates [1–4]. It is based on GaN deposition on foreign substrates(typically sapphire or GaAs), which enables the growth of large-diameter GaN crystals. The significant differences between the latticeconstants and thermal expansion coefficients of the foreign substrateand the nitride film cause significant lattice bow and high threadingdislocation density; e.g., when GaN is grown by the HVPE on sapphire,the bowing radii of the (00.1) crystallographic planes can be below10m and the threading dislocation density is well above 106 cm�2.

GaN crystals of higher structural quality can be grown from thesolution in supercritical ammonia [5]. The ammonothermal GaNcrystals (Am-GaN) are crystallographically extremely flat, withbowing radii of the crystallographic planes up to 100 m and

dislocation density of the order of 104 cm�2. Recently, it has beenshown that the Am-GaN crystals can be successfully used as seedsfor the HVPE growth and that it was possible to combine the HVPEand ammonothermal methods and crystallize structurally perfectHVPE-GaN [6]. Crack-free and up to 1.1 mm thick HVPE-GaN layerswere obtained. The free-standing HVPE-GaN crystal sliced fromthe seed showed high structural as well as optical quality [6].

In this paper, we compare structural, optical, and electricalproperties of the HVPE-GaN grown on an MOCVD-GaN/sapphiretemplate with a photo-lithographically patterned Ti mask and ontwo Am-GaN seeds with different surface treatments and discussthe results in terms of the growth mode, morphology, crystal-lographic quality, and optical and electrical properties.

2. Experimental procedures

A lab-fabricated, horizontal, quartz HVPE reactor with a rotat-ing quartz susceptor designed for 2 in. substrates was used asa growth apparatus [7,8]. Three different substrates/surface treat-ments were used to grow thick GaN crystals and compared: an

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Journal of Crystal Growth

http://dx.doi.org/10.1016/j.jcrysgro.2014.02.0200022-0248 & 2014 Elsevier B.V. All rights reserved.

n Corresponding author.E-mail address: tsochacki@unipress.waw.pl (T. Sochacki).

Journal of Crystal Growth 394 (2014) 55–60

MOCVD-GaN/sapphire template with a photo-lithographicallypatterned 20 nm thick Ti mask with 3 mm openings and 9 mmpitch, and two differently annealed (before the crystallizationruns) ammonothermally grown (00.1) substrates. The latter sub-strates were chemo-mechanically polished and offcut �0.31 in them direction. These two substrates were of high structural qualitywith FWHM of the X-ray rocking curve for (002) reflection ofabout 50″ and bowing radii of the (00.1) crystallographic planesof about 20 m. Their free carrier concentration was below5�1018 cm�3.

Due to the use of two different seeds, the MOCVD-GaN/sapphire template and the Am-GaN, two different growth condi-tions (Process A and Processes B and C, see Table 1 where detailson all pertinent growth parameters are given) were applied thatyielded the best results for each particular seed. The HVPE crystal-lization process was always carried out between 1030 and 1060 1Cwhile the GaCl synthesis took place at 870 1C in all growthexperiments. The growth time was 8 h. It should be noted thatfor the case of the Am-GaN seeds two different annealingprocesses at 1045 1C, in absence of metal precursor, were per-formed before the growth in two types of atmosphere: H2þNH3

and N2þNH3. In both cases, the annealing time was 20 min. Inthe case of the growth on the MOCVD-GaN/sapphire template,a nitridation step was applied (for details see [8]) before theannealing process. The nitridation step was not performed for theAm-GaN seeds.

Following the growth processes, the original substrate/seedwas removed and 1 cm2 samples were diced from the grownHVPE-GaN crystals; they were studied by means of optical micro-scopy with differential interference contrast (DIC) and high-resolution X-ray diffraction (HRXRD) using a Philips X'Pert Materi-als Research Diffractometer with a CuKα X-ray source(λ¼1.54056 Å). On and off-axes ω-rocking curves (XRCs) wererecorded in a double axis configuration using a four-bounce Ge(220) monochromator and an open slit on the detector side. Defect

selective etching (DSE) in molten KOH–NaOH eutectic [9] anddefect selective photo-etching using potassium peroxydisulphate,K2S2O8 (KSO-D) under UV illumination [10] were performed inorder to reveal dislocations threading to the growth surface. Thevan der Pauw Hall method was applied to examine the free carrierconcentration in the HVPE-grown GaN. Low-temperature (LT,�3 K) photo-luminescence (PL) spectra were acquired using acontinuous-wave He–Cd laser (λ�325 nm) along with a closed-cycle helium cryostat, Princeton Instruments Acton SP2750 0.75 mhigh resolution monochromator with 3600 grooves/mm grating,and a PIXIS: 2KBUV cooled charge-coupled device camera.It should be noted that for the HRXRD, DSE, Hall and LT PLmeasurements the samples were mechanically polished on both(00.1) and (00.�1) surfaces and chemo-mechanically polished ontheir (00.1) surfaces.

3. Results and discussion

3.1. Growth on MOCVD GaN/sapphire template

Fig. 1a shows a free-standing (FS) HVPE-GaN crystal separatedfrom the sapphire-based template during the cool-down step(Process A in Table 1). The average growth rate, determined fromthe change of mass before and after the experiment, was 130 μm/h.The crystal was not cracked but several pits were found on the as-grown crystal surface. Fig. 1b presents a typical, hillock-richmorphology of the as-grown HVPE-GaN surface. The DSE revealedtwo distinct types of pits: large hexagonal pits, and smaller pits thatwere either hexagonal or round (see Fig. 1c). The large hexagonalpits were correlated with the growth hillocks. The total etchpit density (EPD) was 5�106 cm�2 while the density of thelarge hexagonal etch pits was one order of magnitude lower,5�105 cm�2. It should be noted that the dislocation density inthe MOCVD-GaN/sapphire template was of the order of 109 cm�2.

Striations

Rough piramidal

growth

Fig. 1. (a) Free standing HVPE-GaN grown on sapphire (as-grown crystal); (b) DIC image of the as-grown surface morphology; (c) surface after DSE process revealingthreading dislocations; (d) cross-section after KSO-D mode process.

T. Sochacki et al. / Journal of Crystal Growth 394 (2014) 55–6056

Thus, the dislocation density in the newly grown HVPE materialdecreased three orders of magnitude. The mechanism for thisphenomenon was explained and described by Valcheva et al. [11].Fig. 1d shows a cross-section of the FS HVPE-GaN after the KSO-Dprocess. Two different regions were revealed in the material: thebottom part (substrate side) showing rough, pyramidal features,and the upper, relatively featureless part, showing some striations,presumably arising from the nonuniform impurity incorporation. Asa general trend, the KSO-D process yielded etch rates inverselyproportional to the free electron concentration [10].

3.2. Growth on Ammono-GaN seeds

Different starting growth conditions (annealing at two differentatmospheres, see Table 1) did not change the surface morphologyof the HVPE-GaN grown on the Am-GaN seeds. Fig. 2a shows theHVPE-GaN crystal grown on the Am-GaN seed annealed in theH2þNH3 atmosphere and Fig. 2b shows its surface morphology.Only 1–3 hexagonal hillocks were found on the entire 1 in. crystalsurface. Two different growth rates were, however, measured atthe same crystallization conditions (see processes A and B inTable 1). For the Am-GaN seed annealed in the H2þNH3 atmo-sphere the growth rate was 110 μm/h. Annealing in the N2þNH3

atmosphere led to a lower growth rate, of about 90 μm/h.A representative image of the HVPE-GaN surface after the DSEprocess is shown in Fig. 2c. The average EPD was of the order of5�104 cm�2, which was similar to the average dislocation densityin the Am-GaN seed. Three types of pits could be distinguishedaccording to the size: large, medium, and small (see Fig. 2d). Thedensity of the large etch pits was around 103 cm�2. According toWeyher et al. [12] the biggest etch pits in GaN should be correlatedto the screw dislocations, the smallest ones to the edge disloca-tions, while the medium pits should represent mixed dislocations.

Fig. 3 shows cross-sectional images of Am-GaN/HVPE-GaNcrystals cleaved along the m-plane after the KSO-D processes:Fig. 3a for H2þNH3 (Process B in Table 1) and Fig. 3b for N2þNH3

(Process C in Table 1). For the crystal annealed in H2þNH3, theetching revealed rough features at the interface that were a fewmicrometers in size, while the annealing in N2þNH3 produceda relatively uniform and featureless interface. As it was alreadyshown [7] that the annealing in H2þNH3 atmosphere led to

decomposition of the Am-GaN seed surface. At these annealingconditions, gallium nitride loses its thermodynamic stability and,therefore, the (0001) Am-GaN surface is etched. The HVPE-GaNaway from the interface was homogeneous, indicating relativelyuniform electrical properties; however, some striations werefound also away from the interface, as seen in Fig. 3.

3.3. Structural, optical and electrical characterizations

The XRCs of the symmetric (002) and skew-symmetric (302)reflections for HVPE-GaN grown on patterned MOCVD-GaN/sap-phire template (Process A in Table 1) and Am-GaN (Process B inTable 1) are shown in Fig. 4. The full-width at half maximum(FWHM) values of the (002), (102), and (302) reflections corre-sponding to each process are summarized in Table 2.

HVPE-GaN deposited on the GaN/sapphire template exhibitedrelatively poor structural quality as revealed by significantly largerXRC FWHM values. These results were expected due to thegeneration of threading dislocations and lattice bow at the poorlylattice- and thermally-matched GaN/sapphire interface. However,epilayers grown on Am-GaN exhibited very intense, narrow Braggpeaks, indicative of excellent crystallinity and low dislocationdensity. While the Am-GaN seeds yielded HVPE material of veryhigh quality in general, small differences in quality were observedas a function of different annealing conditions prior to growth.Annealing under H2þNH3 atmosphere resulted in higher qualityfilms, as exhibited by very narrow rocking curves. Interestingly,photo-etching of these samples revealed a pyramidal structure atthe homoepitaxial interface, thus indicating a variation in carrierconcentration. It is known that the growth on different crystal-lographic facets incorporates vastly different amounts of oxygen atotherwise identical growth conditions [13]; two orders of magni-tude different oxygen concentrations were observed for Ga- andN-polar GaN when grown simultaneously on the same wafer[14,15]. From these observations, it was concluded that H2þNH3

annealing roughened the initially smooth (00.1) Am-GaN surfaceand exposed the pyramidal facets. As such, GaN growth started asthe pyramidal growth and proceeded as basal plane, Ga-polargrowth once coalescence was achieved (after �5 mm). The initialgrowth on pyramidal facets produced GaN with higher oxygenconcentration, which resulted in the lower photo-etching ratesand contrast revealed in Fig. 3a. This process is similar to themaskless lateral epitaxial overgrowth process [16–18] and offersa pathway to further improve the quality of native GaN via HVPE.

LT PL spectra acquired from the (0001) surface of the FS HVPE-GaN obtained by the three different processes are shown in Fig. 5aand b. Several clearly distinguishable donor-bound exciton (DBX)peaks dominated the spectra of all three samples. A detailedspectrum of the Am-GaN/HVPE-GaN annealed in H2þNH3 (Pro-cess B in Table 1) is shown in Fig. 5a. Free excitons (FXs) related toholes from the A (XA) and B (XB) valence bands are located at3.4785 eV and 3.4840 eV (not shown), respectively. The XB wasmuch weaker than the XA and was only visible under a higherexcitation power than what was used in Fig. 5a. The dominantdonor-related peaks located at 3.4714 eV and 3.4724 eV areassigned as A excitons bound to neutral O (O0XA) and Si (Si0XA),respectively. Several other distinct excitons bound to neutral orionized impurities located at 3.4661 eV, 3.4705 eV, 3.4720 eV,3.4729 eV, 3.4736 eV, 3.4754 eV, and 3.4762 eV and attributed tothe A and B valence bands are labeled as A0XA, DþXA, D

0?XA, IOX2a,

ISiX2a, O0XB, and Si0XB, respectively, according to the assignmentsmade by Freitas et al. [19]. All samples possessed the same weakand broad yellow-green luminescence, therefore only the NBEluminescence is shown for comparison in Fig. 5b. The FWHM ofthe O0XA was the lowest at 130 μeV for the Am-GaN/HVPE-GaNannealed under H2þNH3 and the highest at 410 μeV for HVPE-GaN

Table 1Surface treatment and growth parameters.

Process A B CSubstrate Patterned

GaN/sapphireAm-GaN Am-GaN

Substrate size (in.) 2 1 1Nitridation 1060 1C – –

H2, 1600 sccmNH3, 200 sccm60 min

Annealing 1060 1C 1045 1C 1045 1C10%H2þN2

4000 sccmH2, 4000 sccm N2, 4000 sccm

NH3, 120 sccm NH3, 200 sccm NH3, 200 sccm20 min 20 min 20 min

Growth temperature (1C) 1060–1030 1045 1045Ga precursor TGa¼870 1C TGa¼870 1C TGa¼870 1C

HCl, 30 sccm HCl, 24 sccm HCl, 24 sccmN2, 500 sccm H2, 500 sccm H2, 500 sccm

N precursor NH3, 600 sccm NH3, 480 sccm NH3, 480 sccm10%H2þN2,4000 sccm

H2, 4000 sccm H2, 4000 sccm

Total pressure (mbar) 800 800 80Growth time (h) 8 8 8

T. Sochacki et al. / Journal of Crystal Growth 394 (2014) 55–60 57

Edge dislocation

Screw dislocation

Mixed dislocation

Fig. 2. (a) HVPE-GaN grown on Am-GaN seed annealed in H2þNH3; (b) DIC image of the as-grown surface; (c) HVPE-GaN surface after the DSE process revealing threadingdislocations; (d) three types of etch pits correlated with threading dislocations.

HVPE -GaN

NaG-mANaG-mA

HVPE -GaN

Interface

Interface

Striations Striations

Fig. 3. Cross-section after PEC of (a) GaN grown on Am-GaN seed annealed in atmosphere of H2þNH3 (Process B in Table 1); (b) GaN grown on Am-GaN seed annealed inatmosphere of N2þNH3 (Process C in Table 1).

Fig. 4. XRCs of (a) symmetric (002) and (b) skew-symmetric (302) reflections for FS HVPE-GaN grown on patterned MOCVD-GaN/sapphire template (blue curve) and Am-GaN (red curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

T. Sochacki et al. / Journal of Crystal Growth 394 (2014) 55–6058

grown on MOCVD-GaN/sapphire template. Similar to the XRDresults, PL revealed that the Am-GaN seeds produced a bettermaterial than the sapphire-based templates. While Processes Band C both produced high-quality material, Process B (annealing inH2þNH3 atmosphere) produced noticeably sharper PL spectra,indicative of a well-ordered lattice. The DBX to FX intensity ratiowas similar for the HVPE-GaN grown by Processes B and C;however, it was much lower for the HVPE-GaN grown onMOCVD-GaN/sapphire template. This indicated that ProcessesB and C produced samples with similar impurity incorporationwhile the sample produced in Process A had a lower impurityconcentration.

The Hall effect measurements were conducted using the vander Pauw method within ohmic contacts. As shown in Table 3these unintentionally doped HVPE-GaN crystals exhibited n-typeconductivity (with both O and Si as donors, accordingly to the PLspectra) and relatively high mobilities. The overall resistivity in allsamples ranged from 0.1 Ω cm to 0.3 Ω cm. The HVPE-GaN grownon the MOCVD-GaN/sapphire template possessed the lowest freecarrier concentration and the highest mobility, suggesting that thismaterial incorporated the least amount of unintentional impuri-ties, which was also consistent with the LT PL spectra. Presumably,this was the consequence of higher supersaturation at the pre-sence of N2 in the carrier gas used in the growth of this sample(see Table 1), which resulted in a higher growth rate (130 μm/h) [20].As was determined before [21], higher growth rate in the polar,nonpolar and semi-polar directions was observed when thenitrogen carrier gas was used instead of hydrogen gas. Therefore,the growth on patterned GaN/sapphire was faster and the higherpurity material was obtained. Interestingly, almost two ordersof magnitude higher dislocation density in this crystal had notangible effect on mobility. Since the growth rate on the Am-GaNseeds was lower due to different growth conditions, whichresulted in an increase in unintentional impurity concentration,the mobility decreased due to carrier scattering regardless ofhigher crystalline quality. Small differences in the mobility andthe free carrier concentration for the samples grown in theprocesses B and C were caused by the difference in the growthrate (110 μm/h and 90 μm/h for the Am-GaN seeds annealed priorto growth in H2þNH3 and N2þNH3, respectively).

4. Conclusions

Three processes for the production of FS HVPE-GaN crystalswere studied and compared. Generally, the Am-GaN seeds resultedin much higher crystalline quality HVPE material than the sap-phire-based, patterned MOCVD-GaN templates, as evidenced bythe narrow XRD RCs and narrow low temperature excitonicemission: (002) RCs as narrow as 22″ and exciton peaks as narrowas 130 meV were obtained from crystals grown on Am-GaN seedsannealed in H2þNH3 before the crystallization process. A masklesslateral overgrowth process in which growth occurred on thepyramidal planes initially and later turned into the basal planegrowth was proposed to explain the higher quality of this material.A difference in the growth rates was, in turn, proposed to explainvariation in free carrier concentration and mobility in obtainedHVPE-GaN crystals. The highest purity material was grown at thehighest growth rate. An impact of threading dislocation density onthe mobility of free carriers was not observed.

Acknowledgments

This research was supported by the National Centre forResearch and Development: PBS 1/B5/7/2012 and by the EuropeanUnion within the European Regional Development Fund, throughgrants: Innovative Economy (POIG.01.04.00-14-153/11) and Inno-vative Economy (POIG.01.01.02-00-008/08).

Table 2HRXRD data for differently processed HVPE-GaN.

HVPE-GaN from XRC – FWHM (arcsec)

(002) (102) (302)

MOCVD-GaN/sapphire template Process A 1168 1203 279Am-GaN – H2þNH3 Process B 22 17 35Am-GaN – N2þNH3 Process C 183 121 44

3.468 3.472 3.476 3.480

XASioXB

OoXBISiX2a

IOX2a

D+XA

Do?XA

AoXA

SioXA

OoXA

Log

Inte

nsity

(a. u

.)

Energy (eV)

Fig. 5. Near band edge PL spectra of FS HVPE-GaN taken at �3 K; (a) detailedspectra of Am-GaN/HVPE-GaN annealed under H2þNH3; (b) comparison plot ofAm-GaN/HVPE-GaN annealed under N2þNH3 and H2þNH3 and HVPE-GaN grownon MOCVD-GaN/sapphire template.

Table 3Carrier concentrations and mobility of unintentionally doped HVPE-GaN.

HVPE-GaN from n (1/cm3) μ (cm2/Vs)

MOCVD-GaN/sapphire template Process A 2�1016 1107Am-GaN – H2þNH3 Process B 4�1016 882Am-GaN – N2þNH3 Process C 8�1016 743

T. Sochacki et al. / Journal of Crystal Growth 394 (2014) 55–60 59

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