LP-MOCVD growth of CuAlSe2 epitaxial layers

Journal of Crystal Growth 126 (1993) 635-642 North-Holland

LP-MOCVD growth of

CuA1Se 2

~o. . . . . . .

CRYSTAL GROWTH

epitaxial layers

S. C h i c h i b u ~, A. Iwai a, S. M a t s u m o t o a a n d H. Higuchi b a Department of Electrical Engineering, Faculty of Science and Technology, Keio University, 3-14-1, Hiyoshi, Yokohama 223, Japan t, Bentec Corporation, Hid Shinjuku Daini Building, 1.26-9, Shinjuku, Tokyo i60, Japan Received 20 July 1992; manuscript received in final form 4 September 1992

Epitaxiai growth of CuAISe 2 has been performed on GaAs and GaP substrates by LP-MOCVD technique using cyclopentadienylcoppertriethylphosphine, trimethylaluminum, and dimethylselenium as respective Cu, AI, and Se precursors. Growth orientation of CuAISe 2 depends on the substrate orientation, i.e., the epitaxial layers are oriented toward the appropriate d~rection which has the smallest lattice mismatch. The carbon incorporation from trimethylahminium has been reduced by increasing the growth temperature and by increasing the dimethylselenium molar flow rate. The carbon incorporation has also been reduced about one order of magnitude by using the new AI precursor, ethyldimethylaminealane. The epitaxial layers have shown red, broad photoluminescence at 77K.

1. I n t r o d u c t i o n

The widegap members of C u - I I I - V I 2 ternary chalcopyrite compound semiconductors such as C u A I S 2 , C u A l S e 2 , and CuGaS 2 have been consk!cr~d ~e be promising materials for light-emitting diodes and semiconductor lasers operating in a short-wavelength region. In additien, doableheterostrueture (DH) lasers of a Cu(AlxGal_ x) (SySet_y) 2 pentanary active layer with a CuA1 (S vSe ~_,.)2 cladding structure lattice-matched to a GAP000) substrate have been expected for blue-light emitting lasers by using epitaxial growth techniques. Among the above compounds, CuAISe2 has an energy gap of 2.67 eV [1] at room temperature, which corresponds a blue-light region, and shows a p-type conduction property [2]. Hence, strucvrope,,l,.s tual, optical, and e,,.ct.,cal ~o "; " "¢'~ of r,,at. .... CuA1Se z have been studied by many workers [3-8]. However, excitonic a n d / o r band-edge blue-light emission has not yet been reported though CuAlSe z has a direct band gap [9]. New approaches in growing high-purity CuAISe z and other chalcopyrite epitaxial layers have recently been performed by using epitaxial

growth techniques applicable for Al-containing materials such as iodine vapor transport [10], m e t a l o r g a n i c chemical v a p o r deposition (MOCVD) [11-14], and molecular-beam epitaxy (MBE) [15-17]. A single crystalline CuAISe 2 has been grown by MBE and a photoluminescence spectrum was shown [16]. By using a MBE method, new C u - A I - S e systems of Cu3AISe 5 [15], CusAISe 4 [17], and C u 3 A l 3 S e 4 [17] were also reported to be present in addition to the stoichiometric CuAISe 2. Their results [11-17] have suggested that non-equilibrium growth techniques such as MOCVD and MBE are suitable for the growth of Al-containing multinary chalcopyrite compounds and thier alloys. In fact, these techniques have been successful for the growth of many I I I - V and II-VI compounds and alloys. However, epitaxial growth and properies of ~,, At ~ have not been sL~,,.,,,.u .,,~,:~Aby ,,,,.,,.. ~¢ar,vr~ .... In this work, we have performed low-pressure (LP) MOCVD growth of CuAISe 2 epitaxial layers on GaAs and GaP substrates using cyclopentadienylcoppertriethylphosphine (CpCuTEP) [18], trimethylaluminum (TMAI), and dimethylselenium (DMSe) as respective Cu, AI, and Se precursors. In order to reduce the carbon incorpora-

0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

S. Chichibu et al. / LP-MOCVD growth of CuAISe z epitaxial layers

636

tion from TMAI, a new organo-aluminum precursor of ethyldimethylaminealane (EDMAAI) was also used. Growth orientations, carbon incorporation, and photoluminescence were studied.

2. Experimental procedure The growth system used in this study is shown schematically in fig. 1. This LP-MOCVD apparatus (Bentec SCV-2000QR) was previously used for the growth of high-purity GaAs (n < 10 t4 cm -s and /.tn> 100,000 cm 2 V -t s -t at 77 K) and AIGaAs using trimethylgallium (TMGa) and tertiarybutylarsine (tBAs) [19]. The C p C u T E P transfer line was heated at 75°C to prevent its condensation on the inner wall of a stainless steel pipe [11]. A SiC-coated graphite susceptor in the hot-water (75°C) circulated vertical reactor was heated by RF induction. The source precursors were CpCuTEP, TMAI, and DMSe. EDMAAI was also used for comparison. The reactor pressure was 5.3 × 104 Pa. In order to improve the thickness uniformity, lowpressure growth was employed. Pd-purified H 2 was used as a carrier gas. The total H 2 flow rate

into the reactor was 2.5 l / m i n . T h e growth temperature was varied from 500 to 660°C. The typical growth rate was about 0.05 / x m / h . H 2 flow rate into C p C u T E P bubbler was set at 1 l / m i n and the bubbler temperature was 66°C. The molar flow rate of CpCuTEP could not be estimated due to the lack of vapor pressure data. TMAI flow rate was varied from 0.1 to 1 p . m o l / m i n in order to control the solid composition of Cu and AI to be unity. DMSe flow rate was varied from 50 to 3 0 0 / ~ m o l / m i n . For the M O C V D growth of I I I - V compounds, input molar ratio of group-V and group-III sources ( V / I I I ratio) is an important growth parameter. To represent the gasphase input molar ratio of anion and cation in MOCVD growth of CuAISe 2, we use the V I / I I I ratio instead of the V I / ( I + l i d ratio because the CpCuTEP molar flow rate could not be estimated, as stated above. The substrates used were liquid-encapsulated Czochralski (LEC) undoped semi-insulating GaAs with orientations of (100), (110), ( l l l ) A , and ( l l l ) B . LEC S-doped GAP(100) substrate was also used. The surface morphology was observed with a Nomarski interference microscepe and scanning

Purified

CpCuTEP

[~

-- o u t

Hot Water In TMA I or EDMAA I

0 O 0 0 0

DI~Se Fig. 1. Schematic diagram of L P - M O C V D apparatus.

0 0 0 0

EXHAUST

s. Chichibu et al. / LP-MOCVD growth of CuAISe 2 epitaxial layers

637

electron microscope (SEM: JEOL JSM-5200). A lattice image was observed with transmission electron microscope (TEM: J E O L JEM2000FXII). Crystal orientation was evaluated by X-ray diffraction measurements with the 0-20 method using the Cu K a line. The solid composition of Cu, AI, and Se was estimated by a twoelectron-probe X-ray mieroanalyzer. One was an wavelength-dispersive electron-probe microanalyzer (EPMA: Shimadz EPMA-8705) and the other was an energy-dispersive X°ray microanalyzer (EDX: JEOL JEM-2000FXII). Residual impurities were evaluated by secondary-ion mass spectrometry (SIMS: ATOMIKA SIMS-6500). Photoluminescence (PL) spectra were obtained at 77 K. The 365 nm line of a super high-pressure Hg lamp (USH-250D) extracted by an interference filter was used as an excitor.

3. Results and discussion Figs. 2a-2d show the typical surface morphology of CuAISe 2 epitaxial layers grown on (a) GaAs(100), (b) GAP(100), (c) GaAs (110), and (d) GaAs ( l l l ) B substrates observed by Nomarski interference microscope. The growth temperature was 600°C and the epitaxial CuAISe 2 thickness was about 0.4 /x~fi. An almost fiat surface with a fcw hillocks in the (110) dircction is observed on the GaAs(100) surface. In addition, a few cross-hatches (crack-lines) in the (100) and (010) directions are observed on the CuAISez/ GaAs(100) surface, presumably due to the lattice mismatch and the different thermal expansion coefficient between GaAs and a-axis of CuAISe 2The morphology of the epitaxial layers grown on GAP(100) and GaAs(ll0) substrates are not so good as shown in figs. 2b and 2c. The epitaxial layer grown on GaAs(lll)B tends to lift off, as is

Fig. 2. Surface morphologies of CuAISe2 epitaxial layers grown on (a) GaAs(100),(b) GAP(100),(c) GaAs(110), and (d) GaAs(l I 1)B substrates observed by Nomarski interference microscope. Epitaxial films were grown at 600°C and [DMSe]/[TMA1]= 500. Marker represents20 tim.

S. Chichibu et al. / LP-MOCVD growth of CuAISe 2 epitaxial layers

638

shown in fig. 2d. The solid compositions of these layers are estimated by EDX and EPMA to be about C u : A I : S e = 1:1:2. Figs. 3 a - 3 d show the X-ray diffraction patterns of CuAISe 2 epitaxial layers grown on (a) GaAs(100), (b) GAP(100), (c) GaAs(ll0), and (d) G a A s ( l l l ) B substrates. The diffraction peaks correspond with (001), (100), (110) and (102), (112), and (112) orientations of bulk CuAISe 2 in figs. 3a-3d, respectively, as inscribed in the figures. The diffraction pattern of the CuAISe2/ GaAs(111)A structure was almost the same as the pattern of the C u A I S e 2 / G a A s ( l l l ) B structure. The calculated lattice constants are a = 0.561 nm and c = 1.090 nm, on the assumption that these layers have a ehalcopyrite tetragonal structure. The epitaxial layer grown on GaAs(100) substrate

CuAISez / GoAs (100)

was oriented toward (001) direction for CumlSe2, i.e., c-axis oriented. However, the epitaxial layer on GAP(100) was oriented toward (100) direction, i.e., a-axis oriented. In addition, the layer on GaAs(ll0) was oriented toward (110) and (102) directions, with the former being dominant. The epitaxial layer on G a A s ( l l l ) A and ( l l l ) B oriented toward (112) direction of chalcopyrite structure. The lattice spacings along (100) (a-axis) and (001) (c-axis) directions for CuAISe 2 are not the same, but c equals 1.94a due to its uniaxial distortion of the chalcopyrite structure [9]. Then, the above-mentioned substrate-dependent growth with appropriate directions is considered to be related to a difference in lattice mismatch between the epitaxiai layer and the substrates. The

CuK~

o

3

CuAtSez/GAP(100)

6 I--

,,it

8 z uJ i-. z

Z u.J i-z I--4

8

;0

°

>--

>-


< n~ a

2O

f

?!

b 30

40

50

60

70

80

20

30

(deg)

20

3.

o

t3

,¥

40

50 60 20 (deg)

CuK=

70

80

CuK~

CuNSe2/GaAs (111) B CuA[Sez

/ CmAs(110)

I-U3

Z

uJ FZ b.4

rr

oc

c 20

/

Z

LLI I-Z

3'0

4'0 2g

5'0

10

(deg)

7b

8o

\J d

20

5.o

3'o 28

6'o

7b

so

(deg)

Fig. 3. X-ray diffraction patterns o f CuAISe 2 epitaxial layers grown on (a) GaAs(100), (b) GAP(100), (c) GaAs(ll0), and (d) GaAs( I I ! )B substrates. Epitaxial films were grown at 600°C and [DMSe]/[TMA1] = 500. Marker represents 20 tzm.

639

s. Chichibu et al. / LP-MOCVD growth of CttAISe 2 epitaxial layers

values of lattice mismatch are 0.6% for CuAISe2(001) / GaAs(100), 0.6% along (100) and 3.4% along (001 ) for CuAISe 2(100)/GaAs(100), 0.01% along (001) and 2.9% along (100) for CuAISe 2 (100)/GAP(100), 2.9% for CuAISe2(001) / GAP(100), 0.5% along (110) and 3.4% along (001) for CuAISe2(110)/GaAs(110), 1.9% along (102) and 0.64% along (100) for CuAISe2(102)/ GaAs(110), and 0.5% along (110) and 1.9% along (102) for CuAISe2(112)/GaAs(111). Namely, the growth direction would be determined by the lattice mismatch of CuAISe2/substrate resulting in (001)-oriented single epitaxial growth for for CuAI CuAISe2/GaAs(100), (10u/-ortented -"\ . Se2/GAP(100), and (112)-oriented for CuAISe2/ G a A s ( l l l ) A and ( l l l ) B . However, (110)- and (102)-oriented polycrystalline growth occurs for CuAISe2/GaAs(110). These experimental findings that the epitaxial growth direction depends strongly on the lattice mismatch between substrate (zincblende structure) and epitaxial layer (chalcopyrite structure) coincide with the results obtained for the CVD-grown ZnSiAs 2 [20], liquid phase epitaxy (LPE)-grown ZnSnP 2 [21], MOCVD-grown CuGa(SxSet_x) 2 [13], and CVD-grown CuGaS 2 [22].

Fig. 4 shows a cross-sectional TEM micrograph of the CuAISe2(001)/GaAs(001) interface observed from the (110) direction of the substrate. As shown in fig. 4, the lattice image dearly appears both in the substrate and the epitaxial layer. In the epitaxial layer, some lattice defects can be found. However, no amorphous phase is found. This is true across much wider areas on different regions than in fig. 4. Thus, it is concluded that this grown film is the epitaxial single crystalline CuAISe 2. In order to investigate the growth mode for the MOCVD growth of CuAISe:, the growth rate of CuAISe 2 is plotted in fig. 5 as a function of the reciprocal of the growth temperature. The AI precursor was TMAI and the V I / I I I ratio was set at 500. The growth rate increases rapidly with increasing the growth temperature from 570 to about 600°C. On the other hand, the growth rate begins to decrease in the high temperature region above 660°C. The rate-limiting step of the CuAISe 2 growth below 600°C seems to be a decomposition a n d / o r a surface reaction of some precursor. CpCuTEP and DMSe are considered to be sufficiently decomposed for the epitaxial growth because the decomposition temperature

A

0 0 V

CuA[Se2 epi GaAs (100) sub.

!

!

lOnm Fig. 4. Cross-sectionalTEM micrographof CuAISe2/OaAs(100) interface.The growthtemperaturewas 600°Cand [DMSe]/ [TMAll= 500.

S, Chichibu et aL / LP.MOCVD growth of CuAISe2 epitaxial layers

640

lOs

TEMPERATURE (%) 600 700 i

i

I I

TMAI 600"C ~I/~.:500

500

i I

E 0.1 LU F-

<

Se Al Cu

10s

CpCuTEP TMAI DMSe ~/~=500

"~10 ~

"1" 1"--

z

0,01

8

ID

I

I

,

i

I

,

102

I

1.1 1,2 1.3 1000/Tg (K "l) Fig, 5. Dependence of the growth rate on the growth temperature. The group-Ill precursorwas TMAI and Vl/lll = 500.

!

1.0

lo

lbo

360 400

260

DEPTH (nm) Fig, 6, SIMS profile of C'uAISe: grown on GaAs(10O)sub-

strate. The growth conditions were T~= 6IR)°Cand [DMSe]/ [TMAI]= 500, o f CpCuTEP has been reported to be 122-124°C

[11], and the growth of ZnSe using dimethylzinc and DMSe has been confirmed at a growth temperatare of as low as 500°C (growth rate: 3 - 4 p r o / h i [23]. Thus, considering the AIGaAs growth with TMAI, TMGa, and AsH 3, the undecomposed precursor for the CuAISe z growth may be TMAI due to a strong A I - C bond strength. Hence the rate-limiting step seems to be a surface reaction of TMAI. In the MOCVD growth for the reaction-limited regime, carbon incorporation from incompletely decomposed reactants is likely to occur. To investigate the carbon incorporation, SIMS profiles of CuAISez grown at 600°C are shown in fig. 6. The input molar ratio of DMSe and TMAI ( V I / I I I ratio) was 500. The vertical axis is a count rate because the absolute concentration cannot be determined due to the absence of reference samples. In the epitaxial layer, carbon incorporation is clearly observed. The count rate of carbon in the epitaxial layer is shown in fig. 7 as a function of reciprocal of growth temperature. The carbon concentration decreases with increasing growth temperature up to 620°C. Both the increase of carbon concentration (in fig. 7) and the decrease of growth rate (in fig. 5) with decreasing growth temperature seem to correlate strongly with the insufficient decomposition of TMAI. One of the growth methods possible to de-

crease the carbon incorporation is to increase the anion-to-cation molar flow ratio (VI/III ratio), as is the case with the MOCVD growth of GaAs [24], in which the carbon incorporation from TMGa is greatly reduced by increasing the AsH 3to-TMGa ratio I V / I l l ratio). Fig. 8 shows the SIMS count rate of carbon as a function of V I / I I I ratio grown at 600°C. The count rate of carbon decreases about three orders of magnitude with increasing the V I / I I I ratio from 100 to 1000,

?00 '

TEMPERATURE ('C) 600

I

I

l

SIMS

,~- 10s LU

~Z om

/o

prim.ion: Cs÷

/

?

10a

0 Z

50(

I

103

n" <

/V

19

0/

CpCu'fEP TMAi DMSe "~Il'tl'f =.500

1.1 1.2 1000/Tg (K ~)

1.3

Fig. 7. Count rate of C as a function of growth tcraperature. Vcrtica~ axis shows the count rate of C in SIMS measurements.

S. Chichibu et al. / LP-MOCt/'D growth of Ctc4lSe 2 epitaxial layers

106

,~ 10

SIMS prim.ion: Cs*

& m 10;

600"C 'VI/BI=500 - - -

CpCuTEP TMAI DMSe Tg = 600"(:

i

EDMAAI - - -

641

101 1C 10

w p-

ot

Z o

I

........ , ........ , ........ , ........ 10z 103 10~ 10S

~ [ / 1 RATIO Fig. 8. Count rate of C as a function of input VI/lll molar flow ratio. The group-ill precursorwas TMAI and the growth temperaturewas 600~C.

This suggests that the existence of excess DMSe and its decomposed reactants would help in breaking the strong A I - C bond of TMAI and its decomposed reactants such as dimethylaluminum and monomethylaluminum. However, the detailed mechanisms are not clear at the present stage. Recently, in the MOCVD and metalorganic molecular-beam epitaxy (MOMBE) growth of I I I - V compound semiconductors, uew organoaiuminum precursors with low calbon incorporation have attracted much attenlion. Among man,: new organoaluminum materials, we now select ethyldimethylaminealane ((C2HsXCH 3)2NAIH 3: EDMAA1) as an alternative source precursor for the MOCVD growth of CuAIS%. EDMAAI is one of the adduct compounds of trialkylamine and alane, which is liquid at room temperature and has no AI-C direct bond. Then, carbon incorporation from the AI precursor is expected to be reduced because of the stability of ethyldimethylamine and the instability of alane. Fig. 9 shows the SIMS profile of CuAISe2 grown at 600°C and V I / I I I = 500 using CpCuTEP, EDMAAI, and DMSe. The carbon incorporation decreases about one order of magnitude by using EDMAAI compared with TMA1 (fig. 6) for the same growth conditions except for the AI precursor. Considering the lower decomposition temperatures of CpCuTEP (122-124°C) [11] and EDMAAI (below 100°C), the cause of the residual

\ ' - ~ . . . = - ~ - - - . ~ .....

,x,

1

|

0

i

i

100 200 300 DEPTH (rim)

400

Fig. 9 SIMS profile of CuAISe, grown on GaA.~tltD)substrate. The group-Ill precursor was EDMAAI and VI/IIi = 500. carbon contaminant seems to be an incomplete decomposition of DMSe. However, the advantage of using EDMAAI has been shown clearly in reducing the carbon incorporation. Fig. l0 shows the 77 K PL spectrum of CuAIS% grown on GaAs(100) substrate at 600°C using CpCuTEP, TMAI, and DMSe with a V l / l l l ratio of 500. The spectrum exhibited no band-edge emission, but only deep emission ranging from .80 5' to 820 nm in the same way as the bulk material [4,8] and MBE-grown material [16]. Although the origin of the deep emi~ion is not

"-t. ¢}

MOCVD

CpCuTEP

CuAlSe2/C~As (100)

TMA[ DMSe Tg= 600"C

77K ex. Hg 365nm

).U3 Z LU Z _J Q_

~17]][=500

J

do

660

WAVELENGTH

900 (nm}

Fig. 10. 77 K photoluminescence spectrum of CuAISe 2 grown on GaAs (100) substrate. The growth conditions were [DMSe]/ITMAI] = 580, Tg -- 600,°C, and Pg = 400 Torr. The Hg 365 nm line was used as an excitor.

642

S. Chichibu et al. / LP-MOCVD growth of CuAISe 2 epitaxial layers

clear, the crystalline imperfection seems to be the cause of the absence of band-edge emission.

4. Conclusion Epitaxial layers of CuAISe 2 have been grown on GaAs and GaP substrates by LP-MOCVD technique using CpCuTEP, TMAI, and DMSe. The growth orientation of CuA1Se 2 depended on the substrate orientation. The epitaxaal growth orientations of CttAISe 2 were (001), (100), and (112) for GaAs(100), GAP(100), and GaAs(lll)A and (111)B substrates, respectively. The orientations were (110) and (102) for GaAs(ll0) substrate. The carbon incorporation from TMAI has been reduced by increasing the growth temperature for the reaction-limited regime of TMAI, and increasing VI/III ratio at 600°C. Excess feeding of DMSe (and its decomposed reactants) seems to help the TMAi decomposition. In addition, the carbon incorporation has been reduced by about one order of magnitude by using EDMAAI instead of TMAI. Band-edge luminescence has not been observed at 77 K.

Acknowledgments The authors would like to thank Benkan Corporation for the support of many super-clean parts in producing the MOCVD apparatus. They also want to acknowledge S. Hachiya of Trichemical Laboratory, K. Hirahara of Shin-Etsu Chemical Co., Ltd., and H. Sakurada of Tomoe Shokai Co., Ltd., for the help in purchasing metalorganic precursors. The authors are grateful to Y. Takaoka, T. Mitani, and M. Sakurazawa for EPMA, TEM and EDX, and SIMS measurements, respectively. They would also like to thank F. Ishihara and H. Uji for their contribution of fabricating the specimen for TEM observations. Drs. A. Kamata, H. Yoshida, and M. Kushibe of

Toshiba Corporation are acknowledged for helpful discussions. This work has been partly supported by a Grant-in-Aid for JSPS fellows.

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