Low-temperature growth of GaN by atomic nitrogen based on a dielectric barrier discharge

*Corresponding author. Tel.: #82-2-3290-3260; fax: #82-2-928-3584.

E-mail address: [email protected] (D. Byun)

Journal of Crystal Growth 210 (2000) 478}486

Low-temperature growth of GaN by atomic nitrogen basedon a dielectric barrier discharge

Joosung Kim!, Dongjin Byun!,*, Jin-sang Kim", Dong-Wha Kum"

!Department of Materials Science and Engineering, Korea University, 1, 5-ka, Anam-dong, Sungbuk-ku, Seoul 136-701, South Korea"Division of Metals, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea

Received 24 August 1999; accepted 30 October 1999Communicated by G.B. Stringfellow

Abstract

GaN "lms were deposited on sapphire(0 0 0 1) and Si(1 0 0) substrates by MOCVD using an atomic nitrogen sourcebased on a dielectric barrier discharge (DBD) method. Molecule nitrogen and trimethylgallium (TMG) were separatelydelivered to the substrates. Wurtzite GaN "lms, with no trace of cubic GaN, were successfully grown on a-Al

2O

3substrates even at relatively low temperatures ((8003C). Sapphire substrate RMS roughness was 5.01 and 4.93 As beforeand after the exposure to DBD N-source, respectively. This shows negligible irradiation damage of accelerated N`

2ion as

well as the e!ect of smoothening the substrate surfaces with DBD N-source. The PL results exhibited small luminescenceat the spectral region of blue and UV but a luminescence around the yellow region (2.5 eV) was detected. This is caused byoxygen impurity from AES analysis. ( 2000 Elsevier Science B.V. All rights reserved.

1. Introduction

GaN and related compounds have attractedstrong interest owing to their great potential for thedevelopment of optoelectronic devices in the blueand UV spectral region as well as for high-powerand high-temperature electronics [1,2]. MOCVDhas already proven its capability to grow high-quality epitaxial "lms and hetero-structures notonly of conventional III}V compound semiconduc-tors but also group III nitrides. Since the pioneer-ing work by Amano et al. [3] on MOCVD of GaN

using ammonia and trimethylgallium (TMG), am-monia has been commonly used as a nitrogensource for III}V nitride growth including AlN, InNand GaN.

Even though the use of NH3

is well adopted,several workers have made e!orts to supply high#ux of active nitrogen atom to the substrate surfaceunder low temperature. This is because of the factthat metalorganic source such as a Ga-source iseasily decomposed at relatively low temperaturesbut commonly used ammonia gas as an N-sourcerequires high temperature to provide su$cient ni-trogen atom from ammonia due to its high bindingenergy. Therefore, conventional MOCVD tech-niques for the growth of GaN were employed athigh temperatures of &10003C. But this highgrowth temperature has a negative side. Thermal

0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 7 5 9 - 9

Fig. 1. Schematic view of dielectric barrier discharge equipment for atomic nitrogen source was shown.

expansion di!erence between substrates and grown"lm makes a break or crack on grown "lm [4].Besides, this high growth temperature gives a limitto substrate materials together with other crystallo-graphic reasons for the growth of III}V nitrides(e.g. sapphire or SiC). Also for indium incorpora-tion to the nitride materials, low temperature isrequired [5].

For the supply of high #ux of N-source at lowtemperature, some researchers tried to use alterna-tive nitrogen precursors like hydrazine (N

2H

2) or

hydrazoic acid (N3H). Even though this approach

has its own bene"ts, documentation is rather lim-ited because on-site synthesis is needed and hand-ling of the precursors requires precaution. On theother hand, several workers employed variousmethods such as microwave [6] or RF remoteplasma [7,8]. However, these methods needed lowoperation pressure such as MBE for the purpose ofproducing stable plasma and e!ective reaching ofactive nitrogen source up to the surface of sub-strate. Therefore, the development of new plasmasource is required to supply and deliver largeamount of active nitrogen with stable discharge inrelatively high gas pressure. Young et al. studiedthe formation of atomic nitrogen from pureN

2molecule in microwave discharge [9], and the

recombination of nitrogen atoms and the nitrogenafter glow have been well identi"ed [10,11].

In this study, the dielectric barrier discharge(DBD) method was used as a plasma source, whichhas stable discharge in relatively high gas pressure.DBD is a type of discharge, which has at least onedielectric material as a barrier between electrodes.DBD is a type of controlled microarc that can beoperated at high gas pressures and excimer formedunder ultraviolet light generation produces nitro-gen atoms. DBD is simple and we could get highnitrogen atom #ux even at high pressure.

2. Experiments

Fig. 1 shows the schematic design of homemadeDBD equipment for the generation and supply ofactive nitrogen source. Quartz tube which hasa relatively high dielectric constant (e

3"3.9) has

been used as a dielectric barrier material in thedischarge zone. A discharge electrode, tungsten (W)or molybdenum (Mo) was used as power electrodeat the center of quartz tube and aluminum radiatoris for ground at the outer quartz.

To generate micro-discharge between powerelectrode (W or Mo) and inner wall of quartz tube,alternate current (AC) was supplied between W-rodand Al-electrode. For the purpose of e!ective cool-ing originating in micro-discharge, aluminum hasa round shape with a large number of "ns. Cooling

479J. Kim et al. / Journal of Crystal Growth 210 (2000) 478}486

Table 1Growth conditions of GaN by DBD-MOCVD are summarized

1st step GaN bu!er growth Temperature (3C) 500}560Time (min) 0.5}5Flow rate (sccm) TMG 0.09

N2

(Carrier gas) 600}1000N

2(DBD) 400}600

DBD Frequency (kHz) 200}300Power 100}300 W

2nd step GaN "lm growth Temperature (3C) 700}900Time (min) 30}60Flow rate (sccm) TMG 0.09}0.18

N2

(Carrier gas) 600}1000N

2(DBD) 400}800

DBD Frequency (kHz) 200}300Power 100}300 W

water and cooling fan were also set up at the regionof aluminum radiator. Tungsten (W) electrode waspressed into stainless tube, which was extended toquartz tube using ultra-Torr joint to insulate an-other unit. AC power was supplied at the stainlesstube. The end quartz tube was connected to nitro-gen inlet.

When high-purity nitrogen gas #owed in thedischarge gap through four holes of stainless tube,micro-penning discharge between electrodes disso-ciates N

2molecule and produced active nitrogen

atoms, which were used as an N-source. From thecharacteristic afterglow of yellowish orange light,the formation of atomic nitrogen could be con-"rmed. AC electric power and frequency for con-tinuous discharge reaction were determined by thegap between dielectric barrier and electrode, RFpower and gas pressure.

The system is equipped with an impedancematching with serial inductor to "t the frequencyrange of 200}300 kHz. Impedance matching wasachieved by tuning the frequency at constantcapacitance (C) and reactance (¸). With this match-ing network, we could get an RMS voltage betweenpower and ground electrodes of about 3 kV.

Active nitrogen species from discharge zone weresupplied at the front part of growth reactor. Thehorizontal-type MOCVD reactor consists of aquartz tube and an IR ramp-heating element.Trimethylgallium (TMG) was supplied into a reac-

tor as a Ga-precursor with N2

as a transfer andcarrier gas. Thermocouple buried in the graphitesusceptor was another unit separately insulatedand the positive DC bias was applied to ther-mocouple to exclude any e!ect of N`

2ion.

The di!erence between the new DBD-MOCVDprocess and the conventional MOCVD process isthat NH

3as an N-source has been superseded as

active nitrogen source from the DBD method andtwo-step MOCVD process without high-temper-ature nitridation. At the "rst stage of growthGaN-bu!er layer was grown for a short time at500}6003C, and at second stage thick GaN layerwas grown at the "lm growth temperature. Growthconditions and process variables for each stage arearranged in Table 1.

a-Al2O

3(0 0 0 1) and Si(1 0 0) were used as sub-

strates. Before etching, both substrates weredegreased, cleaned and rinsed in trichroloethy-lene (TCE), acetone, methanol and deionized (DI)water for 10 min, respectively. After degreasing,H

2SO

4: H

3PO

4"3 : 1 solution was used for a-

Al2O

3etching and HF (10%) solution for both

substrates. Cleaned substrates were placed ona graphite susceptor in a horizontal-type reactor.Prior to growing, substrates were thermally cleanedin N

2ambient over 1003C for growth lasting 10 min.

DBD active nitrogen source has an advantage oflowering the growth temperature due to the re-placement of nitrogen source. So in this study we

J. Kim et al. / Journal of Crystal Growth 210 (2000) 478}486480

Fig. 2. The calculated #ow rate of N-atom and dissociation e$ciency of N2

molecules was shown for 1 l/min N2

supply and twodi!erent pressures: (j) 100 Torr and (v) 200 Torr. The calculation was based on previous study [13].

choose the way to con"rm the possibility of low-temperature growth of GaN.

The surface morphology of the samples wascharacterized using atomic force microscope(AFM) and scanning electron microscopy (SEM).The crystalline quality was characterized by X-raydi!raction (XRD). Auger electron spectroscopy(AES) was carried out to analyze the impurity distri-bution in the "lm. The optical properties of the "lmswere characterized by photoluminescence (PL). De-fects in grown "lms were analyzed using cross-sec-tional transmission electron microscopy (TEM).

3. Results

The Lewis}Rayleigh afterglow (orange yellow)has proven the existence of atomic nitrogen up toan extended length from delivery ends, which is theresult of nitrogen recombination into molecules:

N#N"N2#hm (Afterglow).

The amount of active N-atoms can be estimated bymeasuring the afterglow intensity. Commonly,mass spectroscopy, electron spin resonance (ECR)absorption spectroscopy and NO titration could be

used to quantify the amount of N-atoms. But in thisstudy accurate titration for the amount of N-atomsfrom DBD source was not performed. The #ux anddissociation e$ciency of active N-atom were esti-mated from a previous report on NO titration ofN-atom with DBD [12].

Fig. 2 shows the calculated #ow rate of N-atomand dissociation e$ciency of N

2molecule based on

the previous study [12]. This result showed the RFpower dependence of #ux and dissociation e$cien-cy for 1 l/min N

2gas. Dissociation e$ciency

([NH]/[N2]) of N

2molecule was increased linearly

with increasing power. Flow rate of N-atom is1017}1018/s in the region of applied voltage of10}150 W. Nearly close to our GaN growth condi-tion, at "xed 150 W powers, the dissociation e$-ciency is 0.21% for 100 Torr and 0.16% for200 Torr. This e$ciency corresponds to 15.5 and6 lmol/min, respectively. With increased reactorpressure, the dissociation e$ciency was decreased.From these results we can obtain the #ow of activenitrogen up to [NH]+100 lmol/min with controlvariable for DBD dissociation system.

To activate GaN growth by an alternative nitro-gen source, many methods such as PECVD wereused. But using plasma has some problem such as


Fig. 3. The substrate damage was con"rmed by measuringN`

2ion current with and without applied DC bias to susceptor.

N`2

ion currents were measured using various powers as a func-tion of DC bias applied at: (a) 5 Torr and (b) 12 Torr.

Fig. 4. AFM surface images of Si(1 0 0): (a) before and (b) afterthe exposure to DBD N-source for 20 min are presented. RMSroughness of Si before and after the exposure was 14.1 and13.7 As , respectively.

ion damage of the "lm and substrate surface. Toinvestigate the damage of nitrogen ion possiblyformed during DBD dissociation of N

2, the ion

current was measured at the substrate. DC bias wasapplied at the graphite susceptor and N`

2ion cur-

rent at reactor pressures of 5 and 12 Torr usingdi!erent applied power with varying voltages wasmeasured s seen in Fig. 3. For the reactor pressureof 5 Torr, no ion current was measured but nega-tive current with no DC bias under 200 W wasdetected as seen in Fig. 3a. With increased appliedpower, the negative current was increased. N`

2ion

current was also measured without DC bias and isshown in Fig. 3. Negative current was measureddue to the electrons produced by DBD source.Positive current was measured when the appliednegative DC bias was increased. At the DBD power125, 150 and 200 W the critical negative DC biases

is !60, !40 and !19 V, respectively. Above thiscritical negative DC bias, the ion current increasedlinearly with increasing negative DC bias. Whenover 250 W power was used, positive ion currentwas detected without DC bias. Positive ion cur-rents are 136, 245 lA at 250, 300 W, respectively. At12 Torr of reactor pressure, positive ion (N`

2) cur-

rent was not measured for 250 and 300 W. Thisstems from molecule}ion and ion}ion collision, andlifetime of N`

2is decreased. This implies that both

active nitrogen atoms and N`2

ions formed fromDBD discharge area to substrate.

The substrate damage was con"rmed by measur-ing the N`

2ion current. Under the same condition

of "lm growth (pressure, temperature, #ow) DBDN-sources were supplied at the surface of the sub-strate and compared with the bare substrate withan atomic force microscope (AFM). Fig. 4 shows


Fig. 5. XRD spectra of GaN with various main growth temper-atures: (a) 5503C; (b) 7003C; (c) 7503C; (d) 8003C; (e) 8503C and (f)9003C without bu!er layer are presented. GaN growth wascarried out with 600 sccm N

2supply and 280 W (240 kHs)

power.

Fig. 6. XRD spectra of GaN on sapphire with bu!er layergrown at 5403C for various bu!er growth times: (a) 30 s; (b)1 min and (c) 2 min are presented. GaN main growth at 7003Cwith 600 sccm N

2supply and 300 W (230}240 kHs) power is

shown.

the AFM images of Si(1 0 0) substrate surface withand without DBD nitrogen exposure. Si(1 0 0) sub-strate was exposed to the DBD N-source for20 min. RMS roughnesses were 14.1 and 13.7 As(average values are 11.2 and 10.7 As ) before andafter the exposure to DBD N-source, respectively.For sapphire substrate RMS roughnesses were 5.01and 4.93 As before and after the exposure, respec-tively. This implies that there was negligible ir-radiation damage of accelerated N`

2ion and

showed the e!ect of smoothening the substratesurfaces.

In contrast to NH3, the DBD source provides

a higher amount of active nitrogen atom even atlow temperatures. That is, in the conventionalMOCVD with the use of NH

3, the formation of

nitrogen atom from ammonia molecules is verylittle at bu!er growth temperature (&5003C) com-pared with GaN growth temperature (&10003C).But using DBD as a nitrogen source, the supply ofnitrogen atom was not in#uenced by temperature.For the growth of GaN, the condition of growthtemperature and DBD N-source were optimized"rst. Then the e!ect of low-temperature GaN bu!erlayer on sapphire substrate was investigated.

X-ray di!raction patterns of GaN grown on sap-phire at various growth temperatures without buf-fer layer are shown in Fig. 5. No trace of cubic GaN

was observed, and the peaks of wurtzite GaNwhich was dominated by the (0 0 0 2) re#ectionwhen the "lms were grown above 7003C. But under6003C, a weak re#ection of (1 0 11 0), (1 1 21 0), wasobserved as seen in Fig. 5. Above 8003C, the(1 1 21 0) peak was observed but under 7503C therewas no trace of (1 1 21 0) peak. This implies thatGaN grown at 7003C exhibited good crystallinityand good surface morphology.

The bu!er growth temperature was set at500}5603C with various bu!er growth times at7003C main growth temperature, and the growthtemperature was "xed at 5403C. Fig. 6 shows theXRD pattern of GaN with bu!er layer grown at5403C for various growth times. Bu!er growthchosen from the XRD analysis was the growth for1 min at 5403C. Fig. 7 shows the SEM image ofGaN grown at 7003C with bu!er layer.

The cross-sectional TEM bright-"eld micro-graph of GaN grown at 7003C for 60 min withbu!er layer grown at 5403C for 1 min is presentedin Fig. 8. The TEM image revealed the columnargrowth morphology of hexagonal GaN near theinterface between GaN and the substrate. Aftercolumnar growth of GaN "lms, the lateral growthwas enhanced. Hexagonal GaN domains are notfully connected with each other so as to form a con-tinuous layer, but near the surface 2-D (lateral)


Fig. 7. Surface morphology of GaN grown at 7003C with thebu!er layer grown at 5403C for 1 min.

Fig. 8. The cross-sectional TEM bright "eld micrograph ofGaN grown at 7003C with bu!er layer grown at 5403C for 1 min.

Fig. 9. PL spectrum of DBD-MOCVD-grown GaN shown inFig. 7.

growth was predominant. Adjacent columns jointogether at the thickness of about 0.25 lm anddislocations were dense in this region. This impliesthat there was little misorientation between col-umns.

The optical property of DBD-MOCVD-grownGaN on sapphire at 7003C was characterized byPL as seen in Fig. 9. The PL results exhibited smallluminescence at the spectral region of blue and UV

but a luminescence at the yellow region was detec-ted. Yellow region ((2.5 eV) intensity was pre-dominant.

Auger electron spectroscopy (AES) was carriedout for the impurity distribution in the "lm as seenin Fig. 10. Carbon (C) and oxygen (O) distributionat the surface was due to surface contamination.However, inside the "lm, oxygen (O) was distrib-uted uniformly throughout the thickness. In theDBD-MOCVD system oxygen can be intermixeddue to the oxidation of power electrode material(molybdenum). MoO may contribute to the oxygencontamination in the "lms. The oxygen contamina-tion was reduced with the use of tungsten (W) rodas power electrode, but could not be removed com-pletely.

DBD active nitrogen source has an advantage; itis possible to lower the growth temperature result-ing in the broader choice of substrates such as Siand GaAs. It is con"rmed by the growth of GaN onthe Si(0 0 1) substrate. Fig. 11 shows the XRDspectra of GaN on Si for various growth temper-atures. XRD analysis always exhibited the peaks ofwurtzite GaN. GaN grew along the c-axis above7003C, but below 6003C GaN grew as polycrystal-line. Lattice mismatch between Si(1 0 0) and GaN islarge about &17%, but GaN(0 0 0 1)/Si(0 0 1) waspossible with DBD-MOCVD.

Fig. 12 shows the surface image of the GaN"lm on Si(1 0 0). Compared to GaN on sapphire


Fig. 10. Impurity distribution in DBD-MOCVD-grown GaN characterized by AES.

Fig. 11. XRD spectra of GaN/Si(1 0 0) grown at various temper-atures: (a) 6003C, (b) 7003C, (c) 8003C and (d) 9003C withoutbu!er layer are presented. GaN growths were carried out with500 sccm N

2supply and 250 W (260 kHz) power.

Fig. 12. SEM micrograph of GaN/Si(1 0 0).

substrate, GaN on Si substrate exhibited roughsurface. It is suggested that the rough surface is theconsequence of large lattice mismatch.

The growth rate of the "lm was 1.8}9 lm/h bySEM and TEM analyses. But more studies areneeded to get high-quality GaN. With optimizedconditions it would be possible to grow a betterquality GaN with DBD-MOCVD.

4. Summary

GaN "lms were grown on both sapphire and Sisubstrate by DBD-MOCVD. Noble nitrogensource was developed based on the dielectric bar-rier discharge (DBD) method, and employed in thenew type of growth system. Wurtzite GaN "lms,with no trace of cubic GaN, were successfullygrown on a-Al

2O

3substrates even at 7003C. RMS


roughnesses of sapphire substrate were 5.01 and4.93 As before and after the exposure to the DBDN-source, respectively. This indicates negligible ir-radiation damage of accelerated N`

2ion as well as

the e!ect of smoothening the substrate surfaces.Yellow luminescence occurred at around 2.5 eV butsmall luminescence occurred at the blue region.Our PL study suggests that gallium vacancy ordivacancies were responsible for the yellowluminescence. This is caused by oxygen impurityfrom AES analysis.

A new MOCVD process to grow III-nitride "lmsemploying new nitrogen source based on dielectricbarrier discharge was developed.

Acknowledgements

This work was supported by The Korea Scienceand Engineering Foundation (Grant No. 98-0300-08-01-03) and KIST-2000 Research Program(Grant No. 2V00253).

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Low-temperature growth of GaN by atomic nitrogen based on a dielectric barrier discharge

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