improvement in the crystalline quality of homoepitaxial diamond films by oxygen plasma etching of...

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Journal of Crystal Growth 285 (2005) 130–136

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Improvement in the crystalline quality of homoepitaxialdiamond films by oxygen plasma etching of mirror-polished

diamond substrates

Michinori Yamamoto, Tokuyuki Teraji�, Toshimichi Ito

Division of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University,

2-1 Yamada-oka, Suita, Osaka 565-0871, Japan

Received 20 July 2005; accepted 17 August 2005

Communicated by K.W. Benz

Abstract

We have investigated the effect of crystalline defects existing in mirror-polished single-crystalline diamond substrates

on the electronic quality of diamond films homoepitaxially grown on them. High-pressure/high-temperature-

synthesized diamond substrates were exposed to quasi-electron-cyclotron-resonance (quasi-ECR) oxygen plasma to

etch the substrate surface layers E1mm in thickness, where a substantial amount of polishing-process-induced

crystalline defects were supposed to remain. Defects other than these were also expected to be etched out during the

oxygen quasi-ECR process. Homoepitaxial diamond films were subsequently grown on them using a high-power

microwave-plasma chemical-vapor-deposition (MPCVD) method. The crystalline quality of the grown films was

characterized electronically using cathodoluminescence measurements. The results clearly verify that the proposed

etching process for mirror-polished diamond substrates can effectively suppress defect formation in the MPCVD

diamond films homoepitaxially grown on them.

r 2005 Elsevier B.V. All rights reserved.

Keywords: A1. Defect; A1. Etching; A2. Epitaxial; A3. Chemical vapor deposition; B1. Diamond; B1. Oxygen plasma

e front matter r 2005 Elsevier B.V. All rights reserve

ysgro.2005.08.019

ng author. Tel.:+816 6879 7703;

7704.

esses: [email protected] (T. Teraji),

ka-u.ac.jp (T. Ito).

1. Introduction

Since diamond has a large bandgap, high carriermobilities and negative electron affinity of itshydrogen-terminated surface, it is expected to be apotential material for such high-performanceelectronic devices as highly sensitive ultraviolet(UV) light detectors [1], UV light emitters [2] and

d.

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M. Yamamoto et al. / Journal of Crystal Growth 285 (2005) 130–136 131

highly efficient electron emitters [3,4]. One of themost important research subjects for fabricatingsuch diamond devices is to develop a well-reproducible fabrication process that allows areasonably high growth rate of a sufficientlyhigh-quality diamond.

We have been developing a high-rate growthprocess suitable for homoepitaxial diamond filmsusing a high-power microwave-plasma chemicalvapor deposition (MPCVD) method and havedemonstrated that high-quality homoepitaxialdiamond films can be grown at deposition rateshigher than 2 mm/h [5–11]. Here, ‘‘high quality’’means that the concerned specimen yields intensefree-exciton (FE) recombination emissions incathodoluminescence (CL) spectra taken even atroom temperature (RT) and whose RT carriermobility is over 1000 cm2/Vs.

Such homoepitaxial films often show two CLpeaks in the visible light region as well, whichcould be related to gap states originally existing inthe substrate or those created unintentionally inthe homoepitaxial films during the homoepitaxialgrowth process employed [5,6,8]. One of them, anemission band peaked at 520 nm, is characteristicof RT CL spectra taken from high-pressure/high-temperature-synthesized (HPHT) type-Ib sub-strates although the emission band can be decom-posed into an H3 center peaked at 503 nm and itsreplica at low temperatures such as liquid-nitrogentemperature [12]. We refer to this emission band as‘‘NV emission’’ because it originates from acomplex state of nitrogen and vacancies. Thisemission is, however, hardly observed in homo-epitaxial films grown on transparent substrates,such as type-IIa diamond, under identical growthconditions [10,11]. The FE and NV emissions areusually uniformly observed in their CL imagesexcept for a number of point-like regions. Thus, itis recognized that the observed NV emissionoccurred in the type-Ib substrate due to substan-tially long diffusion lengths of electron-beam-excited carriers in the high-quality homoepitaxialdiamond overlayer [6,11].

The other broad emission band that peaked at420 nm is called the band A (BA) emission, whichoriginates from dislocations in the diamond [12].In contrast to the relatively homogeneous FE and

NV emission features, the BA emission imagesshow localized features which could reflect thelateral distribution of the dislocations in thediamond film. This emission band is still underinvestigation since the defect formation mechan-ism concerned has not yet been well understood.For example, we have reported in our previouspaper that the emissions occur mainly near theinterface between the homoepitxial overlayer andthe HPHT substrate [6]. This indicates that suchdislocations may be formed in the overlayerregions adjacent to the defective regions of theHPHT substrate. Thus, an efficient process forremoving such surface and subsurface defects ofHPHT substrates must be developed to fabricatehigh-quality homoepitaxial diamond films repro-ducibly well which may be suitable for electronicdevice applications.

In this study, a definitely effective pretreatmentprocess for HPHT diamond substrates has beensuccessfully developed. We have found that asubstrate etching process using oxygen plasmabefore the homoepitaxial growth can have asubstantial effect on improvements in the surfacemorphology and crystalline quality of MPCVDhomoepitaxial diamond films.

2. Experimental procedure

Mechanically polished HPHT type-Ib (1 0 0) ortype-IIa (1 0 0) single crystals of size3.0� 3.0� 0.5mm3 were used as substrates. Aftera series of chemical cleaning processes, includinghigh-temperature annealing in a hydrogen atmo-sphere, were performed [7], homoepitaxial dia-mond films were successively deposited on thesesubstrates using high-power MPCVD under opti-mized conditions as follows [8–10]: a microwavepower of 3800W, a total gas pressure of 120Torr,a total gas flow of 200 sccm, a methane concentra-tion of 4.0% and substrate temperatures of 1000—1050 1C. The 5-h growth duration resulted in filmthicknesses ranging from 15 to 18 mm. Before thehomoepitaxial growth process, the surface andsubsurface regions of some HPHT substrates wereetched using oxygen ‘‘quasi’’-electron-cyclotron-resonance (hereafter quasi-ECR) plasma created

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0.0 0.2 0.4 0.6 0.8 1.00

1

2

3 600 W

400 W

Etc

hed

dept

h [µ

m]

Etching time [h]

Fig. 1. Etched depth of diamond after oxygen quasi-ECR

plasma etching process as a function of etching time under two

different etching conditions: the microwave power and total gas

pressure were, respectively, (i) 600W and 0.01Torr (triangles),

and (ii) 400W and 0.1Torr (squares). A positive voltage of 30V

was applied to the samples during the etching process.

M. Yamamoto et al. / Journal of Crystal Growth 285 (2005) 130–136132

in a magnetic field. (We use the term quasi-ECR.This is because the present process is not an idealECR plasma process in the strict sense of the wordsince the pressure employed is too high forelectrons to complete a cyclotron orbital withoutscattering. A more suitable term for the presentplasma process may be a ‘‘magneto-active micro-wave plasma process’’ [13].) The microwavepower, total gas pressure and oxygen gas flowused were 600W, 0.01Torr and 100 sccm, respec-tively. A positive voltage of 30V was applied tothe sample during the etching process and theholder current at the voltage was 0.1A. In order toverify the microwave power dependence on thediamond etching rate, a different etching conditionwith a microwave power of 400W and a total gaspressure of 0.1 Torr, otherwise the same, was alsoexamined. The surface morphology of homoepi-taxial films thus grown was characterized using aNomarski-type optical microscope (OM) or scan-ning electron microscope (SEM). CL measure-ments were carried out at RT to characterize thecrystalline quality of the diamond films deposited.An acceleration voltage of 15 kV and probingcurrent of 5� 10�7A were selected for the CLmeasurements while SEM images were taken at anacceleration voltage of 15 kV and a probingcurrent of 1� 10�11A.

3. Results and discussion

Fig. 1 shows the etched depth of the HPHTdiamond substrates measured as a function of theetching time. At a lower microwave power of400W, the etched depth saturated at about0.25 mm within an etching time shorter thanE10min. In the case of low oxygen plasmadensity, where the oxidization progressed mainlyat the reaction front, a charging-up phenomenonresultantly occurred on the HPHT diamond sur-face due to irradiation of plasma-created high-density electrons that were accelerated by thepositively biased voltage. In the case of a highermicrowave power of 600W, the etched depth wasalmost proportional to the etching time. Theetching rate under this condition was estimatedat E3 mm/h. It was found from the results

obtained under the two different conditions thatmicrowave power is very crucial for controlling thediamond-etching rate and that the process condi-tion including the power of 600W is moreappropriate for etching a thicker subsurface regionof an HPHT diamond substrate than several mm indepth. Thus, in this study, the above conditionwith a 600-W microwave power was employed asan appropriate oxygen quasi-ECR plasma etchingprocess.

Fig. 2 shows typical OM images taken from (a)an as-received HPHT substrate with a mirror–pol-ished surface, (b) a homoepitaxial diamond filmdeposited on the as-received substrate whosesurface was chemically cleaned with our standardprocedure before diamond deposition [7] (withoutthe quasi-ECR etching pretreatment, Sample #1),(c) an HPHT substrate after a 30-min oxygenquasi-ECR plasma etching process with a magni-fied image in the inset, and (d) a homoepitaxialdiamond film grown on such an HPHT substrateetched similarly using oxygen quasi-ECR plasma(Sample #2). The etching period of 30mincorresponded to the diamond-etched depth of1.5 mm. Here, each edge line of these OM imagesis parallel to each edge of a rectangular-shapedsamples, which corresponds roughly to the /100Scrystalline direction. As shown in Fig. 2(a),

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Fig. 2. Optical microscope images of (a) an as-received HPHT (1 0 0) substrate whose surface was mirror-polished, (b) a diamond film

deposited directly on such an as-received substrate, (c) a HPHT (1 0 0) substrate after oxygen plasma etching with an inset of a

magnified image, and (d) a diamond film deposited on an oxygen-plasma-etched HPHT (1 0 0) substrate.


numerous line-like features appeared on themirror-polished surface of an HPHT diamond,which had been generated during the mechanicalpolishing process employed. The polishing-pro-cess-induced features disappeared gradually withprogressing high-power MPCVD process, asdemonstrated in the OM image (Fig. 2(b)). Theregions featured by the polishing-process-induceddefects have a higher step density than the flatsurface regions that could be formed after anapparent lateral growth from step edges on the(1 0 0) diamond surface [14]. We found that suchan apparent lateral growth occurs at methaneconcentrations substantially higher than thoseemployed in a conventional growth process, whichis one of the advantages of the present high-powerMPCVD [5–9]. By contrast, in the case of oxygen-plasma-etched HPHT substrates, such polishing-process-induced features were almost completelyremoved while circular shaped etch pits withdiameters of 5–10 mm appeared in the OM image,as demonstrated in Fig. 2(c). These pits, having adensity of (1477)� 104 cm–2, are supposed to

reflect the crystalline defects generated in thesubstrate during the HPHT growth process. Inthe present oxygen plasma etching process, apreferential etching occurred around a number ofpoints where such defects existed significantly.These pits disappeared morphologically after thehigh-power MPCVD homoepitxial process. As aresult, flat surface regions thus appeared. Apossible mechanism for flattening the diamondoverlayer surface is considered to originate fromrelatively higher growth rates at the points wherethe diamond is locally etched than those in the flatregions so that the free energy of the systemconcerned can be lowered by reduction in thesurface area of the system.

Figs. 3(a)–(d) show typical RT CL images takenfrom two homoepitaxial surfaces of Samples ]1and ]2. The measured wavelengths of 235 and420 nm correspond to FE recombination emissionsand Band-A emissions, respectively. Edge lines ofCL images are also parallel to edges of rectan-gular-shaped samples, similar to the case ofOM images. The lateral distribution of the FE

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Fig. 3. Typical CL images taken at room temperature from two homoepitaxial specimens fabricated without (Sample ]1, (a) and (b))

and with oxygen quasi-ECR plasma etching process for the HPHT substrates (Sample ]2, (c) and (d)). The measured wavelengths

correspond to free-exciton recombination emissions that peaked at 235 nm ((a) and (c)) and dislocation-related band-A emissions

peaked at 420 nm ((b) and (d)).


emissions was in a relationship complementary tothat of the BA emissions, as we have previouslyreported [6,11]. The BA emission distribution isdominantly point-like or line-like for Sample ]1whereas no line-like feature was observed inSample ]2. The line-like feature suggests that thedefects could have originated from polishing-process-induced marks on the substrate, whichare evidenced in Fig. 2(a). Their disappearancefrom the CL images in the case of 1.5-mm-thickoxygen plasma etching indicates that the crystal-line defects could have existed in the substratesurface and subsurface regions of the HPHTsubstrate employed. The measured density ofpoint-like BA emissions was (472)� 104 cm�2,being roughly comparable with the density of theetch pits appearing after the oxygen plasmaetching. The pit density observed remainedroughly unchanged between the etched diamonddepths of 3 and 20 mm. These results can beunderstood as follows: (1) such point-like disloca-tions are formed at or near the defect sites on the

HPHT substrate surface. (2) Such defect sites inHPHT diamond are formed during the HPHTgrowth process. (3) Thus, line-like defectiveregions generated in the homoepitaxial overlayerextend along the growth direction from thesubstrate surface. (4) The observed BA emissionimages demonstrate point-like but broad features,suggesting that the carrier recombination canoccur even after substantially long diffusions of apart of the electron-beam-excited carriers in ahigh-quality homoepitaxial overlayer. This featureis consistent with our previous reports mentioningthat point-like dislocation defects exist near theinterface between the homoepitaxial diamond filmand the HPHT substrate [6]. Direct evidences arerequired to understand accurately the distributionof defects in the homoepitaxial films.

Curves (a) and (b) in Fig. 4 show respectively,RT CL spectra of Samples #1 and #2 which weretaken at typical positions in their dominant surfaceregions. Intense FE and NV emissions wereobserved in both films while the corresponding

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200 300 400 500 600 700 800

(b)

(a)

CL

Inte

nsity

[arb

. uni

ts]

Wavelength [nm]

(c)

Fig. 4. Typical CL spectra taken in the wavelength region from

200 to 800 nm at room temperature from homoepitaxial

diamond films grown using different substrate treatments: (a)

Sample ]1 (type-Ib substrate, without oxygen quasi-ECR

plasma etching), (b) Sample ]2 (type-Ib substrate, with oxygen

quasi-ECR plasma etching process), and (c) Sample ]3 (type-

IIa substrate, with oxygen quasi-ECR plasma etching process).


BA emission intensities were negligibly small. Inorder to verify whether the substrate emissioneffect was superimposed on the measured CLspectrum, CL spectra were measured from adiamond film grown homoepitaxially on a type-IIa crystal substrate after a similar oxygen quasi-ECR etching process (Sample ]3). In contrast tothe cases of Samples ]1 and ]2, the NV emissionintensity observed was negligibly small in the caseof Sample ]3 (curve (c) in Fig. 4), indicating thatthe NV emissions measured for Samples ]1 and ]2could have originated from their respective HPHTsubstrates. Furthermore, the FE emission intensityincreased substantially in the case of the type-IIasubstrate, suggesting that the density of non-radiative centers determining the CL intensitymay depend at least on the impurity density andcrystalline quality of the substrate surface. Thismeans that the present high-power MPCVDprocess combined with the oxygen plasma etchingprocess is still not enough for a sufficient reductionof non-radiative defects in the homoepitaxialdiamond.

Besides further optimization of the MPCVDprocess, we propose utilization of an appropriatebuffer layer in order that the information on thesubstrate defects such as dislocation and other

non-radiative defects may effectively disappear inthe buffer layer and hardly be extended to thehomoepitaxial film. Such an advanced processusing an appropriate buffer layer combined withsufficient etching of the substrate surface isexpected to further improve the crystalline qualityof the homoepitaxial diamond overlyer.

4. Conclusions

We have applied an oxygen plasma etchingprocess before homoepitaxial diamond growth toeffectively remove crystalline defects existing in thesubstrate surface and subsurface regions. Polish-ing-process-induced features almost completelydisappeared while a number of etch pits appearedon the homoepitaxial surface, indicating a sub-stantial influence of intrinsic defects existing in theHPHT substrate employed. The oxygen quasi-ECR plasma etching of the HPHT substrates iseffective in removing such substrate surface andsubsurface defects, especially the polishing-pro-cess-induced features. Furthermore, the density ofthe point-like defects also decreased slightly byemploying the proposed etching process beforehomoepitaxial growth. In order to further improvethe crystalline quality of homoepitaxial diamondfilms, a process composed of the present quasi-ECR plasma etching and insertion of an appro-priate buffer layer is proposed with a precisecontrol of the MPCVD parameters for thehomoepitaxial diamond growth.

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improvement in the crystalline quality of homoepitaxial diamond films by oxygen plasma etching of...

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