Microstructure of spark-processed blue luminescent CdTe, GaSb, and InSb

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Thin Solid Films 281-282 (1996) 552-555

Microstructure of spark-processed blue luminescent CdTe, GaSb, and InSb A Gudino-Martinez u, C. Falcony '', C. Vazquez-Lopez b,*, H. Navarro u, M.A. Vidal u, 1. Araujo-Osorio c, lG. Cabanas-Moreno C h

• Univ. Aut. deSan Luis Potost, /leO, Ave. Karacorum 1470. Lomas 4 Sec, SLP, Mexico Depto. de Flsica delC1NVESTAV delH'N, Apdo. Postal 14-740. Mexico 07000, D.F.• Mexico "lnstituto Politecnico Nacional, E. S.J.Q.J. B., AP. 75-373, Mexico 07300, D.F., Mexico

Abstract Low frequency spark-discharges were applied to single crystalline wafers of CdTe, GaSb, and InSb. The samples were characterized by photoluminescence spectroscopy at room temperature using anexcitation wavelength of325 nm. The morphology was determined by optical, scanning electron and atomic force microscopy. Inspite ofthe morphological differences occurring at micrometric scales, the spectral regions in which luminescence is observed are almost the same. The origin of the blue luminescence could be an opening of the bandgap due to a quantum confinement effect. Inorder to estimate the size of the confinement regions, theeffective mass approximation model was used. The obtained values of the size of these confinement regions (d) were: for Cd'Fe, d:= 30 A. for GaSb. d =50 Aand for Insb, d = 25 A. Keywords: Spark-processed materials; Porous semiconductors; Luminescence

1. Introduction The spark-processing method is a new and useful alternative to prepare porous materials [1]. It has some potentially desirable features compared to the electrochemical etching method that has been extensively used in silicon [2,3J, such as: (a) It is a non-wet approach that could beused toprepare optoelectronic devices; (b) the Iuminesccnt area affected can be chosen at will, (c) the emission can be tuned in some materials bychanging the parameters of preparation (wafertip separation. substrate temperature. ambient gas, pressure. spark frequency, current andvoltage, etc.) [4], (d) the same equipment and procedures can beused for the preparation of any type of semiconductor materials. The list of materials whose luminescence properties are substantially modified when they areexposed tohigh voltage discharges includes the following semiconductors and semimetals: Si, Ge, GaAs, Sb, Bi, Sn, As, and Te [5 J" It seems that the luminescence properties of these spark-processed materials aregeneral features produced by the generation of nanocrystalline regions [6]. Themorphology of the craters produced byelectric discharges depends onthe melting point, the thermal diffusivity and the thermal conductivity of the '"Corresponding author. Telephone and fax: (525) 147-1096. E-mail: [email protected]. 0040-6090/96/$15.00 © 1996 Elsevier Science SA AU rights reserved PIIS0040-6090( 96) 08720·2

substrate, asisthe case ofcrater formation inmetals bypulsed arcs [7J. In this work experimental PL spectra of spark-processed Cd'Ie, GaSb and InSb are reported. By using the effective mass approximation model the size ofthe confinement region possibly responsible for the luminescent properties is estimated.

2. Experimental A unipolar spark generator with a repetition rateof 20 Hz and voltages of50 000 volts was used for thespark-processing of the samples studied. The sample was glued with silver paste to a copper plate biased as the cathode. in order to dissipate the heating produced by the sparks. A tungsten wire whose tipwas separated I mm from the sample was used as anode. The tipwas prepared byelectrochemical etching inan aqueous solution of KOH I M. All spark treatments were performed in air, at room temperature and for 1 h. Once the samples were prepared we did not notice any appreciable heating of the copper plate, The PLspectra were determined at room temperature using the325 nm line ofa He-Cd laser. Inorder to avoid scattered Raleigh light from tile sample, the

Gudifio·Mart(/Iez etal./Thi/lSolid Films 281-282 (1996) 552-555

553

3. Results and discussion

input of the spectrometer was covered with a blocking UV filter. In order to determine the influence ofthe tungsten tip, spark-processed samples were prepared using asa tip an edge of a piece of the same material asthe sample. No differences in the luminescence spectra were observed using a tungsten tip or the corresponding sample tip, in spite of lhe presence of Wrevealed by microanalysis of the former samples.

At optical microscopic scales some differences are observed among the samples: (a) The crater ofCdTe has two delimited regions: a highly dispersive region in which there are dark holes surrounded by clear granular regions, that are composed of smaller particles, and a dark corona region also

d) CdTe

16

e) GaSb

f)lnSb

1000

:.:

Fig. t. Scanning electron micrographs (SEM) obtained attheedge of spark-produced craters and AFM images obtained onglobular regions. Fig. I(a), 70 X54 ILm region of SP-CdTe; Fig. t (b), 160X 120 ILm region of SP·GaSb; Fig. l(c), 160X 120 JLm region of SP-InSb. Fig. l(d), 1.6 X1.6XO.2 ILm 3·0 AFM image ofaspark-processed region in CdTe; Fig. 1(e), 1Xl JLmx200 A3-D AFM image 'L>fa spark-processed region in GaSb; Fig. I (f), O.7XO.6ILm x200 A3-DAFM image of a spark-processed region iu lnsb,

Gudilio-Martfnez etal./ThinSolid Films 281-282 (1996) 552-555

554

composed ofgranularregions; (b) the crater ofGaSb. ispizzalike shaped, whose corona is composed by clear microcrystallites, and the central region is formed by dark holes surrounded by reddish-yellowish granular regions; (c) the crater of InSb is formed by dark regions in the centre, surrounded by a clear corona. These materials have thestronger luminescent regions situated at the corona. In Fig. 1,SEM micrographs are shown foreach materi~1. InFig. 1(a) the image of a spark-produced hole of 20 IJ.m IS depicted, surrounded bygrains with sizes inthe range of 1000 Ato 31J.m. In Fig. 1(b) andFig. 1(c) cauliflower-like structures with holes of 20 JLm diameter corresponding to GaSh andInSb, respectively areshown. Pores inGaSb aresmoother than inInSb. Thecauliflowerstructure ofInSb isalso rougher. Theoptical and SEM images reveal a fractal like trend in the morphology of the samples. One can speculate that this behaviour could continuedown tonanometric scales. Inorder to determine the mesostructure of the spark-processed samples atomic force microscopy (AFM) i~ages were obtain~d. Fig. I (d) shows a representative AFM Image corresponding to a spark produced crater region in CdTe. In the central portion of this image a 4000 Aporous region is obse.rved. In this image nanometric particles arenot observed. InFig. 1( e) anAFM image of the porous region ofGaSb is shown. Pores of2000 Aindiameter andsome smaller features are observed. InFig. 1(f) a 0.7 X 0.61J.m micrograph ispresented. Features even smaller than 1000 Aare revealed. From Fig. I (d), 1(e) and 1(f), no granular regions of nanometric sizes can be determined with AFM, possibly due to the large roughness of the crater and thedimensions (If, .". AFM tip [8], as was thecase of AFM images obtained in porous silicon [9]. The PL luminescent spectra corresponding to the corona region of thecraters of CdTe, GaSb, and JnSb are shown in Fig. 2. The curve corresponding to CdTe consists.of a main peak centred at 3700 A., anda shoulder at 4700 A. The PL spectrum of GaSb is composed of a broad peak centred at 4000 A and a stronger peak centred at 5000 A. The PL spectrum of InSb has a main peak centred at 4900 Aand a smaller one centred at 3900 A. Following the arguments of

SP-Cdre

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UJ

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Z UJ

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o

ff3

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Z

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Table 1 Materials parameters forthecalculation of theconfinement region average diameters using Eq. (I) and the PL peaks of Fig. 2. E(d)h (E(d),) is the heavy hole (lighthole) energy exciton transition in eV; cpsis the relative dielectric constant

CdTe GaSb JnSb

m*e

m~

n:lh '"

e;

eps

E(d)h

E(d).

d(A)

0.09 0.041 0.014

0.8 0.28 0.4

0.12 0.05 0.016

1.49 0.7 0.17

10.4 15.7 17.0

2.62 2.30 2.53

3.21 3.30 3.18

30 50 25

M.H. Ludwig et al. [5], if the luminescence is produced by an opening of the bandgap due to a quantum confinement effect, an estimation of the average of the particle diameter can be obtained using the effective mass approximation [ 10,11 J. In this model theground state foranexcitonic transition can bedescribed as a function of theparticle diameter, dby: E(d) =Eg +(h2/2£f)( 11m: + 11m:) - 3.572e 2/fd

(1)

where Eg is the optical gap energy of the bulk, m: and m~ are the effective masses oftheelectrons and theholes, respectively, and f is therelative dielectric constant of the mate~iaJ.. Eq. ( 1) is written inc.g.s. units. Thesecond term of the right hand side represents theconfinement energy of electrons and holes while the third term is the coulombic interaction. In Table 1thematerials parameters [12] andtheposition ofthe PLpeaks E(d) obtained from Fig. 2 are jndicat~d. I~ the ~ast column theresults of the calculation for thegram size using Eq. (1) are. also indicated: for CdTe, d = 30 A; for GaSb, d == 50 Aand for InSb, d = 25 A. For these calculations, in each PL spectrum the high energy peak is considered to be associated with theionization energy ofthelight hole exciton, while the ION energy peak is considered tooriginate from the heavy hole exciton.

4. Conclusions Inconclusion, wehaveobserved visible blue luminescence in spark-processed CdTe, GaSb, andInSb. The m~rphology ofthe processed materials isdominated byfractal-hkeporous regions and aggregated spherical regions. B~ AFM we did not observe grains smaller than 1000 A, possibly due to the inherent limitations of the technique for studying samples with high surface roughness. It may be useful to consider mechanisms of confinement such as strain, or chemical or electrical potentials that might confine the carriers in a small region of thegrains.

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4000

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Acknowledgements

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WAVELENGTH(A) Fig. 2. Room temperature photoluminescence from the spark-processed samples of CdIe, GaSb. andJnSb.

This work was partially supported byConsejo Nacional de Ciencia y Tecnologfa (CONACYT), Mexico. Two of the authors (lA.O. and J.G.C.M.) acknowledge COFAA-IPN

GlIdi,io-M(//'t(lIez et lIf.ITllill SolidFilm» .?81-282 (1996) 552-555

fellowships. The technical assistance of R. Fragoso and B. Zendejas is alsoacknowledged.

References [I] R.E. Hummel and 5.-5. Chang, Appl. Phys. Lett.. 61 (1992) 1\.1 12] L.T. Canham. Appl. Pllys. Lett.. 57 ( 1990) 1046. [3] V. Lehmann and U.Goesele, Appl. Phys. Lett., 58 ( 1991) 856. [4] D. Ruter, T. Kunze and W. Buuhofer, IIppl. Pllys. u«. 64 (1994) 3006. [5] M.H. Ludwig, R.E. Hummel and M. Stora, 71,ill Solid Fi/m.v. 255 (1994) 103.

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[6J M.II. Ludwig, RE. Hummel and S.-S. Chung. 1. Vile. Sci. Technol. B. /2 (1994) 3023. [7] M.J. Vasile andG.W. Kammlott, 1. Ap#l'hy.I·" ./6 (1975) 618. [8] A Park Scientific Instruments Atomic Force Microscope model Autoprobe CP ill contact mode, was used for AFM images. The tip was of the Ultralevers (TM) class.conical, with a rmJius of curvature of 100Aand a height of 0.8 p.m. [9] F. Ruiz, C. Vazquez-Lope>, J. Gonzalez Hernandez, D.D, Allred. G. Romero, R. Pefia and G. Torres, J. Vac. Sci. Technol. II. 12 ( 1994) 2565. [ 10] L.E. Brus,J. Chem. Pllys., 80 (1984)4403. [II] L.E. Brus, J. Pl1y.v. Chem. Solids. 90 (1986) 2555. [ 12J Landolt-Bornsteln Numerical Data and Functiaual Relaliollsllips ill Science and Teclilll1fll~Y, ed. O. Madelung, Springer-Verlag, Berlin. 1982. New Series111/23a, Cap.2.1.28 and2.1.16,