METHOD FOR OBTAINING GaAs FLAT SURFACES

Abstract: The present invention refers to a method for obtaining GaAs atomic flat surfaces, by microlithography and chemical etching on GaAs substrates of mesas of predetermined dimensions and orientation, followed by the growth on these mesas of epitaxial layers made from arsenic saturated gallium solution in usual liquid phase epitaxy equipments.

Description: MEIHUl f Oh' ϋBlfilh'lNL barts f i I .UlhF fΕH The present" invention refers to a method for obtaining iJ A H t surf ares use as substrates an epitaxial growth proresEpε. BAL GRUUND DF TH INVENT ION 31 is. I nowti that the crystals grown in quasi eqm 13 br ιum conditions are naturally bounded by the atomic -Hat sur-races whirfi are rhar rter 3 serl by low Mi l ler indt?:, vaJ upε , i « e- crystal 11 m * flat faces oh F type. The substrates for τ.n ra f-pitaMfil growth are rut ed a ter l t pJant * surf c s with tr>r oπentat-πn lose to the crystal locjraphi - lanes f , 10 * * ,< or • .1 i 1 > , From t chno ogi r al point of vi rw the preparation o-l substrate. • involves ori ntation, cutting and pol ishing operations and v. he orientati n precision of the substrate surfaces aqainst that nt the crystal log rap hi r planes depends on the 3.c Λirs * r.y at these procedures, being usual 1 super i or 1 y L 1 mi ted to <. 1... . ti '„ ! or soffiε' of the growth methods, as f or the vapoi phas'- π epi td„y, an intentional mi son en tar. ion of '..» . ,. 5 " against tnt- crystal loπraphic planes is preferer hor other epιta:;ιai growtπ methods, 3:, +01 liquid phase epitaxy, an as qood as possible * orientation is pref erod, in order to avoid the macrostep formation that could negatively influence the properties of some 1 * :<■•? , especi l I v those wherein thr epitaxi l grown active las rs have smal l thi :l nesses, 1 than •' .> .1 urn. 11"! the paper ''L ;tremc-J y -f l t layer surfaces m liquid phase ita y πf GaAs and Al,,Ba^„ ,As" pub ished n Journal of Crystal 1 C-n αw+h r "? . 1980, p. " .43, the authors , U. hor loci- , M. i elεh ann L " .,. Bauser have oroposed a method to obtain tne atomi M t crystal line -tacp? b-' liquid phase epitaxial growth ori structurcα substrates with mesa areas separated ty narrow ana snai ow grooves. The mesa structure dimensions sr€. comprised between 300*300 urn * and iOOO-HOOO pir". The qrowth proceeds bv the coolinq o-f the growth solution on a 1ϋ „ « .2 * * 0 C interval, starring i-rom 62 ... >"£<• " ' L, and thicl nesses o-f 5, ..10 uπi ate obtained. The αepth of the grooves between the mesa structures are of about 1 urn. The εur-faces obtained n tru s wa> are later than the best surfaces obtained bv polishing. The authors suggest tnat, using the mentioned method and rhang ng in the cooling interval a growth solution w th another, epitaxial structures wi th a high interface perfect on can be obtained. The mentioned method presents di advantages since the groove shallow depth does not allow the growth of tnict* structures and large area flat surfaces, especially in the rase of large initial substrate misorientatioπ , of about 0.5 , due to the groove filling during the growth. The increasing of the groove αeptri presents the inconvenience that the grooves can not be εuficientjy cleaned BLther b\ the removing of the growth solution when the growth stops or by the replacement with the ne,; * i qrowth solution when the growth continues from this ne;:t solution, especially xri the case of Panish's method -tor liquid phase Another d sadvantage consists in the fact that the preparation of the atomic flat surfaces needed for the further epιta:ci£*l structure deposition s done at relative) 1 - nigh temperatures, where from the termod namical point of view tnere atre still possible fluctuations in the atomic arrangement ana the critical dimension, which determines tπe distance between the succesi ve atomic steps of lateral growth mechanism starting from dislocations, is relatively small, of about it' rμm. UHHAH,' OF I UL I NVbN I LU I he of 1 he pre s ent UK'SΠII OΠ I < ** to obtain I arge at ran i r fl t * surfaces, of superior ηusl i tv . The problem -olverl bv the present invention consists in the opti ati n of both mes ■ prepara ion arm pit xi l growth on these mesas,. The method according to this in ention v o s the above mentioned di sadvantage pro- dinq the formation, in usual condit on,, on G m substrates, rutte wi th a better man u.1 1 ' precis] ori against the ι » rr -,'si d I i oriraphi r plans ** ;, oi mes '* ** * with 0. mm" di ensions, on erπc-d witn the I sit ger eαqr- ε i orni the t l l direction and separated by πrriovr * - of 1 '„ „ „ ** μ depth and '.H'li ~ι width, -ioi lowed b-' the growth on these mesas of epita ial l yers, o-f a maximum th rlness f ■' μm, t h<= growth proceeding from a arseni saturated gal l ium solution ot „b„ „ „ ! mm thiclness, cooled with a rate of v. b„ „ „ 1 „ fi 'b mi n from "* i-ι' '.. - " ," (• !' ! " 'ϊ unti l the gal l i m so I i d i -', i rat i on t mpe ture, fol lowed bv the removing of the surplus gal l ium solution,, using usual merhaniral means, atter tho eni r xi a i grr.wth prores * * **** ha ** ceased and the substrate treated in thi way has been e trac ed f r orti t he ep i tax i a I equ pemen t „ l h DE-.B KIPTJUN OF " I HI-: DRkWllMGfe in the ft-Uowings an example ot the in.entior arcompl i shement is given, ret erenro being marie to the Fib, 1 » » „ -i wh i rh represent , ME„ 1 r perspecti ve v ew of a mesa; Fib,, * , a longitudinal section through mesa, w tr ~ grown C I-, c* cross-sert J on of a mesa w th grown epi ta,.ι - ~ iav*er Fjϋ. * •* ••-, a detail in a cros =~-sect i on ot a mesa. SPECIFIC DESCRIPTION The process o-f obtaining large surfaces with -flat atomic crystalline races starts f-rom substrates the surface of which is as close as possible to the crystalline face to be obtained. Between the two mentioned planes there is a mi απentation angle θ 0 . As it is I* nown , an angle & 0 equal to 0.1 is in present, for semiconductor material suostrates, a catalog characteristics for many producers. On such wafers, using microlithographical and chemical etching procedures, mesas of 0.5-*5 mm" " dimensions, oriented with the Larger edge along the C1103 directions, separ ted bv grooves of 2u.. ,4' urn depth and 200 urn width. FIG. i presents the perspective v ew of a mesa which has a length L and a width 1. The mesa is bounded by an upper surface 1, lateral surf*aces 2, 3, 4 and 5 and edges o, 7, 8 and 9. The mesa is oriented with the edges parallel to the CH I and III103 directions to allow a facile chip formation, especiall in laser diode case. The rectangular shape of the upper face 1, so that the edqεs - • > and 8, parallel to the Lil'.' direction 3.r longer than the edαes " ana 9, parallel to the CllOJ direction, has been chosen αue to the negative influence on the growth process of the edges pat allel tc the CIIOJ direction, influence which will be explained ιr. connection with FIG. 2 and 7. On the upper surfaces 1 ot the mesas atomic flat crystalline faces are obtained by epitaxial growth. In FIB. 2, a longitudinal section through a mesa, attei the growth of an epitaxi l layer 10, ι~ presented. The epitaxial l yer j > " ■ has a maximum height h*-.* ] and a minimum height n m - The epitaxial layer 10 is upper! y bounded by a crystalline face I i „ Between the initial surface of the mesa 1 and the- flat crystalline face there is a misoπentation angle θ. The relation between U and B Q ~ tf = tJ 0 „ The geometrical el e entr noticed in F-1D, 2 are conned ed by the relation -,li | v | -h m ; L=tg H-fc-J. f-or an angle 8 equal to *- > . l and a L value equal to 5 mm a 10 urn value results tor the ιh*> | --h (Tl ■ difference. The change n the orien at on of thr- upper esa sur-f re 1 during the growth process is a r DEIIJ I * of dif erent growth velocities in the direction perpendicular to εt plane di fferent -i ram the crystal lographi c plane U00) and ir, the direction perpendicular to the crystal Lographi c plane <10* " ') , the lattf-r being the smallest one. " ialinc, into account the values of these velocities l I" is found that the minimum thicl-ness of thε- grown l yer h m =5...1 urn is su-f . j ciε-nt to obtairi a f lat l yer on a mesa of a length L= mm, so that the maximum thicl-ness o-f the grown layer hεt to be > urn. The maximum value o-f the grown layer thid-ness determines also the maximum depth of the grooves between mesas, lafing into account the fact that in the epitaxial growth exist*. a gener l tendence for groove filling and tor surface smoothing dυκ j to a greater growth velocity insino the groove, a groove* 1 depth a ** least equal to h-v* is needed, a double value bei nq preferable. During the etching and -further growth processes, lateral surfaces 2...b are obtained. These surfaces can be crystal lograpic faces o-f the (311) type or fa(j. ii) or close to them. T-f the mesa i oriented in the L110J direction, then the latera 1 faces . and 4 are of the A type and the other and 5 ~.re ot the fa type. The H type*- faces, " '■ and f-, β.re represented n Fib. '. in fig. " "■„ whirlι represents a cross-section ot the mesa, lateral faces of the H type, V and 4, arε' presented. In hese o figures one ran observe that the growth velocities into the directions perpendicular to the faces A and fa, having the symbols v^ and v*^, are different from each other and different from the growth velocity v ci003» the growth velocity in the perpendicular to the C1003 plane direction, these three velocities being in the relation vg<v* * * ι t ) ** |<vp 1 . • z:r am these resons results the fact that the mesa extends laterally more in the case presented in FIG. 3 than in the case presented n FIG. 2. New growing seeds from one face to the other can be generatd at edges. In the FIG. 2 case crystalli ation seeds can be generated from the faces 3 and 5 towards the face 11 and in the FIG. 3 case crystalli ation seeds can be generated from the face 11 towards the faces 2 and 4. To obtain a surface 11 as flat as possible, as ew new crystalli ation seeds as possible sire desirable, therefore the length of the edges 7 and 9 should be less than the length of the edges 6 and 8, and the mesas should be oriented with the longer edge in the C110II direction,as in FIG. 1 A ratio β / c jQ Q3=1.7 was found. Due to this ratio value, the width of the mesa grows during the growth process. A detail of the cross-section is presented in FIG. 4. The following notations have been made:: Δl/2 - the increment, of the mesa width, AB - •• the line joining the edges before and after the growth, h - the thickness of the grown layer, <~^ι - the angle made by the normal CA to the plane C1003 with the line AB, - 2 ~ the angle made by the normal BD to the A type plane with the line AB. The following relations hold; From these relations results: «< j =53.9 and BC= Δl 2=1.4*h. The v-.idth of the grooves should be at least the double of this value. For a thickness of the grown layer of 20 urn, the width of the groove should be at least 56 urn. Taking into account the the preponderent groove filling, an initial mesa separation of about 100...200 um is prefered. So, in the process of the flat surface formation by the epιta * :ιaL growth according to the invention, relatively deep grooves are used, which can not n cleaned ot the growth solution in situ, therefore, in the method according to the invention, the growth continues until the gallium sol ι rii -f i cati on temperature, and the gal lium is swept off by mechanical means outside the growth equi pement , using a very tine brush. In this way, the substrate smoothing and the atomic fiat surface formation is separated from the manufacturing of tne epi ta,:ι a! strurture needed for devices, an operation that ir performed by a latter epi axial qrowth process. The decrease of the ma mum growth temperature presents thr advantage of obtaining the l st atomic layers at l w temperatures, in conditions of extremely low growth vplππt p* r and supersatur ti ns. in these conditions the critical dimension d of the growth in the lateral propagation mecham sm around the crystalli ation seeds increases I , . „ . lo times and the ratio between the height h,*, of an atomic step and this critical dimension d is very small . The h ( -,/d ratio final I v thr qualit y of the grown surface and as smal l as possible values i ox thi ratio are prefered. Moreover, at low final temperatures, of nppro;:ι matel y " _.' ■ •> I * . which art- three times lower than the usual temperatures, of approximately 9')') , the atomic arrangement fluctuations are very much reduced, all wing the atomic flat surface formation. " In obtain epitaxial l yers of 2 * <> urn thici ness different values tor the initial temperature of * the cooling cycle, or tne cooling ratr* and or the gallium solution thiciness can be used. For applying the method according to the? invention usual liquid phase epi axy eηuipements are used. Rel tively low values of the solution thickness, of 0.5...1 mm, are used, these values beinq compatible with cooling rates of >.». . „ .1. b L/miπ, which avoid both the homogeneous nucieation phenomenon involving a maτeπal loss and a too l ng duration ot the whole process, lo ensure a thicl-ness of 'u urn, the initi l temperaturt- of the coolinn cycle should be 750... " * * B u i_ ■■ so that the initial solution. As saturated, is comprising enough material to at? deposited, fh s nventions presents the following advantages: - superior quality flat surfaces are obtained; — c t sΛt-f rteπt rhiclness of the grown layer is obtained to cover with aromi fiat layers large area ot the substrate; - the i -' nal τern or ure J s verv εmal 1 , that smooths verv ucn these surfaces.

Claims: WHMI lb Cl AiMLl' IS; H method tor obtaining 1-ιaπs atomic flaτ surfaces, b. mi crol J l-hogi-aphy and chemical etching on fϊa a sub-ilraies nt rπesa= of prerlel ei* mi nerl di ension and or i ori tat J on , fol lowed by t ho growth on these mesas ot epitaxial J vers made from a senic •sa urareπ gal I i urn solution in usual liqtuπ phase 1 epi ta; * *- equi pe ents,, characteri ed D*' the tact thai in orαer to obtain large atomic l t sui* faces of superior quali y provides τ he formation, in usual conditi n, on barts substrates, rutted with c betτ-er than 0. I precisi n acj i n*=vr the {['>'" rr st l I orjraphi r [i I ant"- * , of mesas with • "* . f- •" • hi rum" d ensi oπs . oriented with the larger edqo along the L110J direction and separated by grooves of ' ' - * ...4i.' urn depth and L * ' " >- ■ urn wi τh, followed by the growth on these mesas of epita i l layers, ot a ma:;ι mum thicl-ness of '.:• ι urn the growth proceeding from a arsenic saturated q-fllium sαluti on of 0.5...1 mm thici-ness, cooled with a rate of 0.5...1.5 C/min ______ from ?0.. r 78 1"1 C until the gallium sol ld f i carion temperature, followed b the removing of the surplus gallium solution, using usual mechanical means, when th epitasn.- growth process has ceased and the substrate treated in thi way has αeen extracted from the opi tc ;ι a! equipement. (5t) Bib] lαqraphy : LA - - 7 s 8469 h . " Japanese l-'atent 35699/78 Roma*nian Patent 7T.'453 ϋerman Patent 27l ϋ4

Inventors: LEPSA MIHAIL ION, PETRESCU-PRAHOVA IULIAN BASARA, LEPSA, MIHAIL, ION, PETRESCU-PRAHOVA, IULIAN, BASARAB
Applicants: INST DE FIZICA ATOMICA, INSTITUTUL DE FIZICA ATOMICA
Application year: 1991
Main classification: C30B19
IPC codes: C30B19, H01L21