Physica 116B (1983) 527-536 Paper presented at ICDS-12 North-Holland Publishing Company Amsterdam, August 31 - September 3, 1982

DEFECT STRUCTURE TRANSITIONS IN SILICON INDUCED BY Q.SWITCHED LASER ANNEALING

A . G . C U L L I S

Royal Signals and Radar Establishment, Malvern, Worcs. WR14 3PS, England

During the last few years, a very large number of investigations have been directed towards elucidating the varied and often complex phenomena associated with Q-switched laser annealing of materials. Most work has been carried out upon elemental Si and signif icant advances in crystal growth research have occurred. The control led, rapid quenching that has been achieved for the f i r s t time has enabled new defect structure transit ions and novel impurity behaviour to be observed. This paper surveys many of the new phenomena that have been revealed.

I. INTRODUCTION

The application of Q-switched laser radiation to the annealing of semiconductor layers is import- ant for a number of reasons. Clearly, i t is possible to induce in the layers structural and electr ical modifications which have potent ia l ly useful electronic device applications. Moreover, the way in which these modifications are accom- plished involves hitherto unachievably high heating and cooling rates. This processing has revealed novel crystal growth and high speed reso l id i f i ca t ion phenomena that have marked out a new area of fundamental research. I t is this area of rapid advance that w i l l be the focus of the present paper, with part icular reference to the behaviour of semiconductor Si. The vigorous ac t iv i ty in this area is par t icu lar ly c lear ly documented in a number of conference proceedings (see, for example, Gibbons et al [ l ]and Appleton and Celler [2]).

The annealing process typ ica l ly employs short (~l-lOOnsec) pulses of radiation in or near the v is ib le region of the spectrum. Q-switched ruby and Nd-YAG lasers are commonly used and these are sometimes mode-locked to reduce the pulse length s t i l l further. Material to be annealed can be processed with a single large area radia- t ion spot although i t is often necessary to render the laser beam spat ia l ly uniform and to eliminate local energy density f luctuations [3]. Al ternat ively, a small (~O-lO0~m)radiation spot size can be used and a large sample area is covered by overlapping irradiated regions by means of a scanning process [41. However, th is la t te r method may give r ise to small-scale annealing nonuniformities.

2. FUNDAMENTALS OF ANNEALING PROCESS

Many experimental investigations have given in- formation about the mechanism by which Q-switch- ed laser annealing occurs. Much early work [4-7] indicated that transient surface melting played a central role, although an al ternat ive anneal- ing mechanism was proposed [8], th is being based

upon the postulated presence of a dense, long- l i fet ime plasma of photoexcited carr iers. How- ever, the la t te r was not supported by time- dependent optical r e f l ec t i v i t y measurements at various wavelengths [9land the transient melting model has been confirmed by other subsequent studies. Indeed, as further described in section 4, a range of important dopant segregation phenomena strongly indicate that melting occurs [I0,II] . Most recently, independent time-resolved measurements of several d i f ferent Si physical properties have provided f ina l confirmation: namely, the determination of the veloci t ies of atoms leaving the irradiated surface [12], the measurement of the heat-induced variat ion in matrix la t t i ce parameter [13], the estimation of the instantaneous matrix electron temperature [14] and the observation of the optical trans- missivity of the irradiated material [15,16]. Time-resolved Raman scattering measurements [17] have also been shown [18] to be in accord with the transient melting model. All these studies are consistent with the rapid thermalization of energy deposited by the laser pulse (see, also, the discussion by Bloembergen et al [ i 9 ] ) . As shown by recent computer modelling work [20], i f the energy is transformed into l a t t i ce phonons in a small fract ion of the pulse duration time, transient melting is the inevitable dominant annealing phenomenon. Nevertheless, i t w i l l be most important to quantify the effects of very short-term diffusion [21] and containment [22] of excited carriers.

The behaviour of the laser-induced molten surface layer has been modelled by computer heat-flow calculations. A typical result [23] for a Q- switched laser pulse of a few tens of nanoseconds duration shows that, with a beam energy density above the surface melting threshold, after melt formation commences the l iqu id layer thickness increases rapidly while substantial i r rad iat ion continues. The veloci ty of the l iqu id-so l id interface often l ies in the range 5-10m/s and the melt quickly reaches i ts maximum depth which may be as great as ~l~m depending upon beam energy density. Resol id i f icat ion next takes

528 A.G. Cullis / Q-switched laser annealing

place with a l i qu i d - so l i d in ter face ve loc i ty t yp i ca l l y in the range l-5m/s, as has been ver i - f ied by d i rect measurements [24] of t rans ient conduct iv i ty behaviour. The l a t t e r ve loc i ty is a pa r t i cu la r l y important parameter since i t controls the behaviour observed during reso l id - i f i c a t i o n , which in turn determines the charact- e r i s t i cs of the f i na l sol id phase. The reso l id - i f i c a t i o n in ter face ve loc i ty can be varied over a wide range by changing the background temper- ature of the substrate material [25]or by a l t e r - ing the laser pulse length I26,271. This ve loc i ty control gives the important a b i l i t y to test the appl icat ion of crystal growth models to the many, diverse observations that have been made.

3. LATTICE DISORDER REMOVAL AND MODIFICATION

Q-switched laser annealing research has focused most in tens ive ly on the rec rys ta l l i za t i on of ion implantat ion damage in Si [4-6,28,291. The extent of penetrat ion of the laser-induced melt is pa r t i cu la r l y important [30] and the nature of the defect t rans i t ions observed at moderate energy densi t ies of i r r ad ia t i on depends upon the character of the i n i t i a l ion-induced l a t t i c e disorder. I f ion implantat ion has led to the formation of an amorphous surface layer , anneal- ing with t y p i c a l l y 30ns ruby laser pulses can y ie ld residual defects ranging from polycrystals to d is locat ion networks, though a pulse with a high enough energy can give single crystal f ree of extended defects [311. The various structure t rans i t ions are shown pa r t i cu la r l y c lea r l y by cross-sectional transmission electron microscope (TEM) work [32], as i l l u s t r a ted in Fig. l for an As + ion-implanted layer. A low energy density rad ia t ion pulse of just 0.2J/cm 2 p a r t i a l l y melts the amorphous layer, the outer region of which then r e s o l i d i f i e s as po lyc rys ta l l i ne material since i t is not an contact with the underlying crystal matrix (Fig. Ib) . With increasing energy density of i r r ad ia t i on , the amorphous Si is melted to the crystal in ter face. However, re- s o l i d i f i c a t i o n continues to give material of poor crystal qua l i t y up to ~O.7J/cm 2 and there is good evidence that the t rans ient melt at the posi t ion of deepest penetration is s i gn i f i can t l y undercooled - see section 5. Nevertheless, at O.85J/cm2 (Fig. Id) the melt temperature has risen s u f f i c i e n t l y to give ep i tax ia l recrys- t a l l i z a t i o n upon the underlying crysta l . However, the reformed single crystal layer s t i l l contains many defects, p r i nc ipa l l y inc l ined microtwin lamellae. For an energy density of ~IJ/cm 2 the melt ac tua l ly just penetrates the crystal matrix although an i r regu la r buried band of point defect clusters s t i l l remains in the annealed c rys ta l l i ne layer. Occasional V-shaped dis loca- t ion pairs or ig inate in the defect band. U l t i - mately, when the energy density of the i r r ad ia - t ing pulse is increased to 1.2J/cm 2, the trans- ient melt removes a l l the dense surface damage produced by implantat ion so that r e s o l i d i f i c a - t ion y ie lds high qua l i t y ep i tax ia l r e c r y s t a l l i z - ation and a Si layer f ree of a l l extended crysta l lographic defects (Fig. I f ) .

b

f ' 0.2pro =

Figure 1 : TEM images of cross-sections of (001) Si implanted with 4xlOl515OkeV-As+/cm2 a) as-implanted, b) O.20J/cm2, c) O.35J/cm 2, d) O.85J/cm2, e) l.OOJ/cm 2 and f ) 1.20J/cm 2, (After Cul l is et al [32].)

I f i n i t i a l ion implantat ion gives only a buried defect c luster damage band, the polycrystal t rans i t ions described in the above sequence do not occur. In th is case, the most prominent defects to be produced occur when the melt f ront , at deepest penetrat ion, intersects the damage band [33] . This is i l l u s t r a t e d in Fig. 2 for high-dose B + ion implanted Si. The sharp in te r - face in Fig. 2b represents the level to which the melt f ront penetrated, the point defect clusters i n i t i a l l y present above th is posi t ion having been removed. Occasional d is locat ion pairs nucleated in the i n te r fac ia l region. Once again, of course, a l l extended defects could be el iminated from the annealed layer by much deeper melting with a s u f f i c i e n t l y high energy density laser pulse.

The behaviour of point defects in laser annealed layers has been the subject of a number of stud- ies. Photoluminescence work [34] has demonstra- ted by P bound-exciton luminescence observations that the laser annealing of high-dose P+ ion implanted Si can place the dopant atoms on sub- s t i t u t i ona l l a t t i c e si tes in the r eso l i d i f i ed Si with high e f f i c iency . However, point defect

A. G. Cullis / Q-switched laser annealing 529

Figure 2 : TEM images of cross-sections of ( I l l ) Si implanted with lO 16 IOOkeV-B+/cm2: a) as-implanted, b) 1.5J/cm 2. (After McMahon et al [33].)

damage deep in the crystal produced by channelled ions during implantation can also give rise to additional important effects. Indeed, these defects are subjected to a heat pulse during the laser annealing sequence and can transform by solid-phase processes into photoluminescent W- centres [35] with emission at l.Ol9eV. Similar defect transformations have been observed in studies using deep level transient spectroscopy [36]. In addition, low concentrations of point defects can also be produced by laser annealing in unimplanted Si crystal [37].

Pulses of Q-switched laser radiation can also be used to recrystall ize Si layers deposited on various substrates. Amorphous layers deposited on single crystal Si substrates can exhibit excellent epitaxial recrystall ization under suitable irradiation conditions [38,39]. However, i f free-volume is present in the i n i t i a l layers void information can occur upon annealing. I f an as-deposited layer is actually single crystal but contains epitaxy-related defects, these can be substantially modified by transient laser melting. This is i l lustrated in Fig. 3 for epitaxial Si on sapphire where the i n i t i a l layer contains arrays of inclined microtwin lamellae. By laser-melting down to the hetero-interface twin growth can be par t ia l ly suppressed during recrystall ization of the Si although, as shown in Fig. 3b, many dislocations are introduced into the annealed single crystal material [40].

Studies have also been carried out on the effects of laser annealing upon diffusion-induced defects in Si. Small dislocation loops intro- duced by P-diffusion can be removed by laser

Figure 3 : TEM images of cross-sections of Si layers on sapphire: a) as-deposited (with ion-impLanted surface), b) annealed at l.IJ/cm c (ruby, 30ns). (After Cullis et al [40].)

melting and, indeed, the depth of melting i t se l f has been measured by observations of this phenomenon [41]. When extended dislocations are present in the i n i t i a l material they can change their shape in the annealed, resol idi f ied region and dislocation spl i t t ing reactions of a complex nature can occur [42] .

4. TRAPPING AND SEGREGATION OF DOPANTS

When the conventional dopants such as B, P and As are implanted into Si crystal they can be in- corporated almost exclusively onto substitutional lat t ice sites by laser annealing [4,43]. In fact, even i f dopants exhibit a particularly low equilibrium solubi l i ty in solid Si, the speed of the resol idi f icat ion interface during annealing is great enough to physically trap nonequilib- rium high concentrations of the impurities on latt ice sites. This process increases the effective segregation coefficient (k') of a dopant and has been observed in a number of cases, including those of In, Sb, Bi, Pt and C in Si [25,27,43-47]. The dopant solubi l i ty enhancements that can be achieved are sometimes extremely large: in the case of Pt in Si the maximum solubi l i ty is almost three orders of magnitude greater than the equilibrium level [25] . The impurity trapping phenomenon also exhibits a dependence upon the crystallographic orientation of the irradiated Si surface [47 ] . This is demonstrated for Bi in Si in Fig. 4 where i t is clear that, for recrystall ization inter- face velocities of up to about 4m/s, trapping is more pronounced for ( I l l ) Si than for (OOl) Si. This is l ike ly to be at least in part a con-

530 A. G. Cullis / Q-switched laser annealing

O.:q

0 2

0 1

0101

0001

//,III~H.. , ,'

l I

i I

i I I

k o , 7 x i 0 " 4

I 1 i J I 2 3 4

v ( m / s e c )

Figure 4 : In ter face segregation coe f f i c i en t fo r Bi in Si as a funct ion of ve loc i t y and c rys ta l lograph ic o r i en ta t i on . (Af ter Baeri et al [47 ] . )

0-

sequence of the greater undercooling expected to be associated wi th a ( I I I ) Si in te r face moving at any given ve loc i t y (see also section 5). The supersaturated so l id solut ions formed by solute t rapping are, of course, metastable. However, in the case of C in Si , layers supersaturated by about two and a ha l f orders of magnitude requi re temperatures of ~IO00°C fo r rapid impur i ty pre- c i p i t a t i o n to take place [46].

I f an except iona l ly large concentrat ion of a low s o l u b i l i t y dopant is present in a laser annealed layer not a l l (and in some cases very l i t t l e ) of the impur i ty is s u b s t i t u t i o n a l l y trapped in the so l id . One consequence often observed is that

a large amount of the impur i ty segregates out to the sample surface [10,44,48]. This phenomenon is analogous to that which occurs during the process of zone re f i n i ng . However, the l a te ra l d i s t r i b u t i o n of the segregated mater ia l is also often not uniform. Indeed, the p rec i p i t a t e phase can occur as microscopic channels which form the boundaries of minute c r y s t a l l i n e Si ce l l s . This e f fec t has been observed fo r a number of im- planted dopants in Si [44] and has been shown to be due to the occurrence of cons t i t u t i ona l super- cool ing in the r e s o l i d i f y i n g t rans ient melt [11,25]. Calculat ions [49,501 employing morpho- log ica l s t a b i l i t y theory [511 have shown that the experimental observations can be accounted fo r in a quan t i t a t i ve manner. A typ ica l segrega- t ion ce l l s t ructure given by laser annealed In + ion implanted Si is shown in Fig. 5. Channels of me ta l l i c In form the ce l l wal ls and i t is c lear that the ce l l dimensions depend sens i t i ve- ly upon the ve loc i t y of the r e c r y s t a l l i z a t i o n in ter face. The Figure demonstrates that as the ve loc i t y of the in ter face is changed from 4 to 2m/s (using substrate tempRrature ~ o n t r o l ) the ce l l s ize changes from ~450Ato 850A [49] .

I t is important to note tha t not a l l low solu- b i l i t y dopants give segregation ce l l s t ructures of the type jus t described. I f the impur i ty has only a low s o l u b i l i t y in even molten Si prec ip- i t a t i o n can occur in a more uniform manner before r e s o l i d i f i c a t i o n is complete. This is the case fo r excess implanted C in Si where some segrega- t ion towards the surface is evident [46]. How- ever, implanted Ar in Si is so extremely inso lu- ble that p r e c i p i t a t i o n in the form of gas bubbles can take place at an ear ly stage in the melt ing sequence so that the f i na l bubble depth d i s t r i - bution is s im i la r to the i n i t i a l Ar implant d i s t r i b u t i o n [ I I I .

A method other than ion implantat ion that has been used to introduce large quant i t i es of im- pu r i t i es in to Si is the d i r ec t laser a l loy ing of

Figure ~ : TEM images of (001) Si implanted wi th 3xlO 15 170keY In+/cm 2 and annealed at 1.5J/cm c (ruby, 30ns). Note dependence of ce l l size on substrate background temperature: a) 77°K, b) 300°K and c) 410°K. (Af ter Cu l l i s et al [491.)

A.G. Cullis / Q-switched laser annealing 531

Figure 6 : TEM image and d i f f r ac t i on pattern from Pt ~ilm on Si a f ter Nd-YAG annealing at 18 MW/cm c. (After Poate et al [52].)

deposited metal f i lms. The resul t [52] of laser melting a deposited Pt layer on Si is shown in Fig. 6 and, once again, segregation ce l ls are produced due to melt supercooling. Simi lar ce l l formation has been observed [53] a f ter the laser a l loy ing of Co and Mo f i lms on Si and other, larger ce l ls ascribed to melt convection ef fects in non-uniformly i r rad ia ted areas were also i den t i f i ed .

5. ULTRA-RAPID RESOLIDIFICATION PHENOMENA

Transient annealing with Q-switched laser radia- tion has revealed a range of new high-speed, liquid-phase crystal growth phenomena. The preceding sections were primarily concerned with work involving resol idi f icat ion interface veloc- i t ies in the range l-5m/s. However, even higher velocities can be achieved by use, especially, of short laser pulse lengths and short wave- length radiation. Important results in this regime were obtained by groups at Harvard [54] and IBM [55 ]. These workers were f i r s t to demonstrate that at exceptionally high quench rates achieved, for example, using mode-locked laser pulses i n i t i a l l y crystall ine Si is render- ed amorphous. In this case, the resol idi f icat ion interface velocity is so high that atomic re- ordering cannot occur.

The condit ions required for the formation of amorphous material have been quant i f ied in studies [56 ] which employed large-area uniform 2.5ns laser pulses of u l t r a - v i o l e t rad ia t ion . For (001) Si, Figs, 7a and b demonstrate that a very th in (~IOOR) amorphous surface layer is produced just ~bove the threshold for melting (about O.15J/cm c) and that the thickness of the amorphous material i n i t i a l l y increases as the

Figure 7 : TEM images of cross-sections of Si layers irradiated with 2.5ns 347nm pulses: (OOl) Si - a) O.20J/cm2, b) O.27J/cm 2 and c) O.40J/cm2; ( I l l ) Si - d) O.20J/cm 2, e) O.5J/cm 2, f) O.55J/cm 2 and g) O.9J/cm 2. (After Cullis et al [561.)

radiation energy density is raised. However, above ~O.3J/cm amorphization abruptly ceases to occur and, instead, high quality single crystal free of extended defects is formed (Fig. 7c). This behaviour is to be contrasted with that exhibited by ( I l l ) Si. For material of the latter orientation surface amorphization begins at low energy densities as before (Fig. 7d). However, amorphous Si continues to form U~D to ~O.55J/cm2, with thicknesses of up to ~800)~ due to the deeper melting (Fig. 7f). Furthermore, although for irradiation above this energy density crystall ine material is produced, the annealed layer is heavily defective and contains many inclined microtwin lamellae (Fig. 7g). The microstructure resulting from this defect trans- i t ion [56] is also shown in the plan-view TEM image of Fig. 8a. With further increase in radiation energy density (almost up to the damage threshold) or increase in the pulse length the defect density is reduced: compare the few stacking fault defects remaining in ( I l l ) Si after irradiation at 1.2J/cm 2 with a 16ns pulse (Fig. 8b).

I t is interesting to note that the amorphous Si produced by rapid quenching is structurally similar to that produced by conventional means

532 A. G. Cullis / Q-switched laser annealing

Figure 8 : TEM images of ( I I I ) Si irradiated with 347nm pulses: a) 2.5ns, O.9J/cm2; b) 16ns, 1.2J/cmZ.

(for example, by ion implantation) and i t is essentially impurity-free [56,571. However, under more extreme conditions of irradiation surface oxidation can occur [581.

The various defect t r ans i t i ons described above are summarized in Fig. 9, which also shows the re la t ionsh ip of the rad ia t ion energy densi ty to the computed ve loc i t y of the 1412% isotherm (normal melting temperature) [56 ] . I t is imme- diately clear that the energy density thresholds which were identif ied previously can be taken to correspond to thresholds in the resol idi f icat ion velocity. Thus, higher velocities are needed for the formation of an amorphous solid on (OOl) Si (~18m/s) than on ( I I I ) Si (~15m/s). While c r y s t a l l i n e mater ia l is formed below these thresholds, profuse defect nucleat ion occurs s p e c i f i c a l l y on ( I I I ) Si fo r r e s o l i d i f i c a t i o n in ter face ve loc i t i es down to about 6m/s.

The curve given in Fig. 9 represents an approx- imation since the velocity computation did not take into account undercooling which is l ike ly to occur in the transient melt. The formation of an amorphous solid would then be possible when the undercooling at the resol idi f icat ion interface fa l ls below the melting point of that phase. This concept is consistent with the

p- k- o

14

m MELT ONSET 2 . 5 n s e c 347nm si AMORPHOUS

(o01)

~ : (O01~)RYSTAL

l _(111) 1 ~ ~ c

AM'()RPHOUS

(111) ~ ~

TWINNED & FAULTED CRYSTAL ( t o -- 6 m / s e c )

I I i I 0"2 0.4 0'6 0.8

ENERGY DENSITY (J/cm2)

Figure 9 : Computed peak ve loc i t y of 1412°C isotherm as a funct ion of pulse energy density showing surface melt ing threshold and or ientat ion-dependent amorphization and defect formation regimes. (Af ter Cu l l i s et al [56 ] . )

r e l a t i v e ease of formation of amorphous Si on ( I I I ) surfaces since, due to the d i f f i c u l t y of plane nucleat ion on th i s crysta l o r i en ta t i on , i t s associated undercooling during r e s o l i d i f i c a - tion would be larger than for (001) Si crystal. Furthermore, i t has been theoretically predicted [59,601 that amorphous Si should undergo a f i r s t order melt ing/solidif ication phase transition a few hundred °C below the crystal melting temper- ature. Some experimental measurements support this proposition [61] but the issue remains rather contentious [62].

Further informat ion about the melt ing and so l i d - i f i c a t i o n behaviour of amorphous Si can be obtained when very short laser pulses are used to rap id ly anneal p re -ex is t ing amorphous layers (formed, fo r example, by ion implantat ion) [63, 64]. Indeed, explosive r e c r y s t a l l i z a t i o n events have been observed under su i tab le conditions[641. However, i n t r i gu ing sub-threshold behaviour also occurs. Figure lOa shows an In + ion implanted layer which i n i t i a l l y had a uniform s t ruc ture . Extremely short- term melt ing wi th a low energy densi ty, 2.5ns, ruby laser pulse appears to give a very undercooled t rans ient l i qu i d which re- s o l i d i f i e s d i r e c t l y back in to an amorphous state whi le the implanted In segregates in to a narrow band ac tua l ly w i th in the amorphous layer [641 . Further conf i rmat ion that a normal melt is form- ed during th is very fas t annealing sequence is given by the observation of a ce l l s t ructure in the segregation band (Fig. IOb), which almost

A.G. Cullis / Q-switched laser annealing 533

Figure I0 : TEM images of (001) Si implanted with 10 16 170keV-ln +/cm 2 and annealed at O.40J/cm2 (ruby, 2.5ns, 77°K substrate): a) cross-section showing In segregation band in amorphous layer, b) plan-view showing segregation cell structure. (After Cull is et al [64].)

cer ta in ly is due to the occurrence of const i tu t - ional supercooling in the reso l id i fy ing l i qu id . This is thought to be the f i r s t observation of impurity segregation during the direct sol id- i f i ca t ion of an amorphous phase and the accom- panying di f fus ion behaviour indicates that amorphous Si is not, for example, simply a glassy state [64].

Experiments of the type just described give considerable insight into the fundamental nature of amorphous Si. When considered together with the wide-ranging basic research outl ined ear l ier in th is a r t i c le , i t is evident that studies of laser annealing phenomena are leading to most important advances in the areas of high speed so l id i f i ca t ion and crystal growth.

6. ACKNOWLEDGEMENTS

The author is indebted to colleagues at RSRE, Bell Laboratories (Murray H i l l , USA) and the University of Catania ( I ta l y ) for the i r col lab- oration in those items of work mentioned above which were carried out in the author's group at RSRE.

Copyright (~) Control ler HMSO, London, 1982

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[7] Auston, D.H., Surko, C.M.,Venkatesan,T.N.C., Slusher, R.E. and Golovchenko, J.A., Time resolved r e f l e c t i v i t y of ion-implanted s i l icon during laser annealing, Appl. Phys. Lett. 33 (1978) 437-440.

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534 A. G. Cullis / Q-switched laser annealing

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