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Structure, Defects, Mechanical andOptical Properties of HexagonalSemiconductor Gase1lxsx Single

Crystals at O<X<0.4V.V. Atuchin1, Yu.M. Andreev2, S.A. Bereznaya3, T.A.

Gavrilova1, G.V. Lanskii2, T.D. Malinovskaya3, A.N. Moro-zov3, Z.V. Korotchenko3, L.D. Pokrovsky1, S.Yu. Sarkisov3

'Institute ofSemiconductor Physics, SB RAS, 630090, Lavrent 'evAve., 13, Novosibirsk, Russia, atuchin@,thermo. isp. nsc. ru

Institute ofMonitoring ofClimatic and Ecological Systems SB RAS,634021, Academicheskii 1013, Tomsk, Russia, yuandreev@,imces.ru3Siberian Physico-Technical Institute, 634034, Lytkin Str, 28, Tomsk,

Russia, malin@,elefot.tsu.ru

Abstract - Structure, defects, mechanical and optical propertiesof GaSe1_Sx, O<x<0.4, matters and single crystals are presented.

Index Terms- Semiconductor, GaSe1 xSx, hardness, optical dam-age.

The layer structure GaSe crystal, which belongs to 6 m2 crystalsymmetry and is known since 1972 [1], has attractive potential use innuclear radiation detection, electro-optical devices and high effi-ciency IR and THz range nonlinear frequency converters. Coherentsources in the mid-IR and Far-IR are of prime importance for mo-lecular spectroscopy, eye-save medical instruments, radar and remotesensing of atmospheric constituents, so as for numerous militaryapplications such as target tracking, obstacle avoidance, and infraredcountermeasures. Among binary and ternary semiconductor crystalsGaSe has certainly been most studied both theoretically and experi-mentally and shown a number of top physical properties. Unfortu-nately, the crystals posses' also easy cleavage on a 100 plane, toosoft (Mohs hardness is O) and have fairly low melting temperature.Nowadays growth technologies are characterized by low, <20-30%,

1-4244-0346-4/07/$20.00 © 2007 IEEE

180 Siberian Conference on Control and Communications SIBCON-2007

yield of relatively high quality single-crystal boules and a few procents of high quality boules. These circumstances resulted in ex-tremely sprayed data on principle physical properties of GaSe andtendency to improve it by doping with the same chemical groupelements or growth of solid solutions. From our point of view one ofthe best dopant for GaSe is sulfur and best solutions are GaSelxSxsolutions. Grown both doped GaSe:S and solid solutions GaSe1xSx,mixing ratio x<0.03, single crystals are characterized by improvedoptical quality in relation to pure GaSe, so as nonlinear optical andmechanical properties. Noticeable shift of short-wavelength trans-parency end to shorter wavelength range, that lower nonlinear ab-sorption for near IR radiation and linear absorption coefficient inTHz range, is also fixed [2-5]. Further improvement is waited forwith S content increase. On the other hand physical properties anddefects of doped GaSe crystals and GaSe1lxSx solid solutions, specialat x>0.03, are not studded in detail yet.

In this study structure, defects, optical and mechanical propertiesof pure, S doped solid solution GaSe1xSx, x=0.045, 0.067, 0.133,0.216, 0.29, and 0.4, single crystals are studded. The material syn-theses and crystal growth technology are presented elsewhere [2]. Toreveal the system of defects formation of which is possible in GaSe,as a first step we study the real defect structure of GaSe1lxSx ingotsyielded by chemical synthesis. The samples cut from the ingots werenear 1 cm2 in area and -500 ptm in thickness. Thin flakes suitable fortransmittance electron microscopy (TEM) analysis were produced byaccurate exfoliation of the samples after removing top surface con-taminated layer (Fig.la,b). In many cases one could see moire pic-ture for GaSe0.98S0.02 (Fig.lb), possibly coursed by relative smallrotation and shift of atomic layers. Selected area electron diffraction(SAED) patterns in most cases show crystal spot system with hex-agonal symmetry related to (001) GaSe (Fig.2). The presence ofsmall crystals grains with random variations -6° around [001] hasbeen detected. In Fig.2a superposition of two patters from two adja-cent GaSe grains with relative rotation 7.50 is displayed. RHEEDpatterns show identity of it cell parameters with g- and f-polytypicGaSe crystals, confirm presence of two adjacent grains and presenceofweak splitting of the reflexes within the angle -2°. So, the angle

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a bFig. 1. Surface microstructure x 190000 of GaSeo.933So.067 material (a), and

SEM picture of high optical quality GaSeo.6SO4 (b)

a bFig.2. SAED pattern of grains (a), and for (001) GaSeo.6SO4 single crystal (b)

domains seems formation of small be appeared due to sulfur doping.The inclusions of -0.2 nm polycrystalline GaSe and SeO2 have beenfound by scanning over the flake area (Fig.3a). As it seems this oxidephase is generated by chemical reaction of excessive Se with residualoxygen in the ampoule.

a bFig.3. RHEED pattern of SeO2 polycrystal (a), stripe-type (b)

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Improved technology let us to grow high quality initial matterand single solid solution crystals. They were studded in the sameway as synthesized matter. The GaSe1l-Sx, x<0.4, crystals shown ussaved crystal structure at good structural uniformity (Fig. 1 c) andhigh optical quality. Such defects as dark stripes and Ga drops(Fig.3b) where also found. From Fig.4 it goes that short-wavelengthtransmission edge is still show shifting versus mixing ratio x in-crease. It has resulted in lower two-photon absorption for near IRand THz emission. Besides, according to our data, optical transpar-ency is often improving and it means that the yield of high qualitycrystals is enlarged. But growth technology needs in further

70 .....I''''I6 improvement to exclude de-60 7 fects (Fig.3b). Nonlinear prop-

- 84" 850 erties have to become lowerg40 because S atomic weight is 4.9

30 i times less to Se, but frequency20 conversion efficiency depends10 on a set of physical properties0 0.6 1 2 and has to be specially esti-

0.60 0.62 10 20x, mated.Fig.4. Transparency spectra of pure The doping with sulfurand modified GaSe crystals with x renders an appreciable influ-equal to: 1 - 3; 2 - 2; 3 - 0.1; 4 -0 ence on the mechanical proper-(high quality); 5 - 0.01; 6 -0 (high ties of gallium selenide despitequality); 7 - 3; 8 - 7; 9 -0 (low of the fact that atoms of sulfurquality); 10 - 10; 11 - 5 wt%. mainly introduce to the anion

sublattice, isomorphically replacing selenium. Cleaving the crystalsdoped with sulfur and mixed along the cleavage planes is more diffi-cult than the pure ones. It is possible to explain stronger binding be-tween the layers by partial stacking of layers to the f-polytype in whichanions are located closer to cations (directly above them), that results inappearance of some ionic component in bonds along with weak Van derWaal forces. Also, presumably modified GaSe crystals contain moreanion vacancies resulting in appearance of not involved in formation ofcovalent bonds electrons of atoms of gallium, which can form bondswith anions of adjacent layer. Microhardness data measured for mod-ernized GaSe crystals are given in Table I.

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Table I. Microhardness (H) of GaSel,S, crystals versus chemical composi-tion

Crystal charge Dopant content 'rystal composition, at% I, kg/mm2composition vt % At% Ga SeGaSe - - 51.6 48.4 147+3.0GaSeo.995S.0005 0.1 0.232 - - 118+33.7GaSe0.9S0.j 3 6.664 - - 177+29GaSeo.8SO.2 5 10.8 53.5 36.7 209+166.7GaSeO.7SO.3 7 14.8 50.41 36.35 642+144.6GaSeo.6SO.4 10 20.2 50.22 30.93 420+131.9

In general, it is possible to reckon for modified crystals that themicrohardness grows with increasing sulfur concentration in galliumselenide. This effect is very promising for practical fabrication ofnonlinear optical elements with modified GaSe crystals (doped andsolid solutions).

Damage threshold under the dye (0.73 and 0.78 ptm), Er3+:YAG(2.94 ptm) and CO2 (9.2-9.7 ptm) laser pump appears identical with10% measurement accuracy for solid solutions GaSe1l-S, at0.133<x<0.4. For 10 ns dye laser pulses it is 145 MW/cm2 and 85±5MW/cm2 for 40 ns Er3+:YAG laser pulses. Possible reason is in weak(<50/O) thermal conductivity changing in the direction of layers.Manifold increasing of thermal conductivity in the direction or-thogonal to layers do not influence on the heat diffusion to the pe-riphery of high optical quality crystals because low temperaturegradient in the direction ofpump beam propagation.

CONCLUSIONSModernized technology of syntheses of modified GaSe materials

and growth of modified single crystals are developed with use pre-liminary results of investigation of crystal structure and defects.GaSexS1-x crystals show saved crystal structure of pure GaSe at bet-ter structural uniformity and higher optical quality at least at x<0.4,and also better yield of high quality crystals with shift of short-wavelength transparency end well correlated with mixing ratio in-crease. Mechanical properties, namely hardness, are increasing andanisotropy is decreasing versus x increase, that makes modified

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GaSe crystals attractive for mechanical treatment and use in appliedsystems. Detailed study of usefulness of modified GaSe crystals forfrequency conversion has to be carried out.

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