Applied Surface Science 54 (1992) 1-7 North-Holland

applied surface science

Photodecomposition of precursors for metal organic vapour phase epitaxy

W o l f g a n g R i c h t e r , P a u l K u r p a s a n d M a r g r e t W a s c h b ü s c h

Berlin Unirersity q( T««hnology, Hardenbergstrasse 36, D-IO00 Berlin 12, Germany

Received 28 May 1991; accepted for publication 2 September 1991

The excimer laser induced photodecomposition of the arsine precursor in metal organic vapour phase epitaxy (MOVPE) was monitored in situ with temporal resolution by coherent anti-Stokes Raman scattering (CARS). Simultaneously, the local gas temperature was derived from the CARS spectra. The results show that the 193 nm ArF line leads to a nearly 100% decomposition efficiency at low total pressures hut to a considerably reduced efficiency at higher total pressures. The temporal development of the efficiency occurs in two steps. From the CARS-derived local gas temperature it is concluded that the first step essentially results from photodissociation, while in the second one thermal decomposition is dominant.

1. Introduction

The s t a n d a r d p rocesses for growing epi taxia l s emiconduc to r layers a re in genera l dr iven by the rmal energy. T h e r m a l energy a lone is u t i l ized to d e c o m p o s e the p r ecu r so r molecu les (in C V D ) and to cont ro l the growth kinet ics on the surface (in C V D as weil as MBE) . The t e m p e r a t u r e s n e e d e d to ob ta in sa t is factory growth results , are however of ten qui te high (600 to 1000 ° C) so that o the r but the des i r ed p rocesses such as so l id-s ta te diffusion, in ter face recons t ruc t ion or in ter face re- ac t ions are s t rongly s t imula ted , too. Fo r this rea- son research is concen t r a t i ng on new or modi f i ed growth techniques which al low lower growth tem- p e r a t u r e s to be used.

In m e t a l o rgan i c v a p o u r p h a s e ep i t axy ( M O V P E ) with its main app l i ca t ion in the growth of I I I - V semiconduc to r s (A1, Ga, I n / A s , P) the s t anda rd p recur so r s a re alkyls for the G r o u p II I e l emen t s and hydr ides for the G r o u p V e l emen t s [1]. F o r G a A s , for example , the s t a n d a r d total growth reac t ion reads

G a ( C H » ) 3 + A s H 3 -* G a A s + 3 CH 4. (1)

In M O V P E the high growth t e m p e r a t u r e s (600-

700 o C) are r equ i r ed in o r d e r to ob ta in a substan- t ial decompos i t ion of the V-hydr ides . A poss ible r e p l a c e m e n t of the hydr ides by me ta lo rgan ic G r o u p V precursors which d e c o m p o s e at Iower t e m p e r a t u r e s [2] has not been fully successful up to now. Even though such a r e p l a c e m e n t is s t rongly p r o m o t e d today by safety aspects, the qual i ty of the layers grown has not been com- p le te ly sat isfactory. The main reason for this is very p robab ly the e n h a n c e d carbon incorpora t ion into the layers which de t e r i o r a t e s the e lectr ical p rope r t i e s in most cases. The carbon concent ra - t ion on the o the r hand may be r e duc e d to near ly unde t e c t a b l e levels when a tomic hydrogen is re- leased by a d s o r b e d V-subhydr ides at or nea r the surface. A n o t h e r poss ible reason may also be given by the la rger pur i ty which can be achieved at p resen t for the hydr ides as c o m p a r e d to the me ta lo rgan ic compounds . Fo r this reason modif i - ca t ions of the s t anda rd M O V P E process as given in eq. (1), are still the subject of research. Al te r - nat ive forms of energy s t imula t ing the growth process can be suppl ied by e lect r ica l energy ( p l a s m a - M O V P E ) o r by p h o t o n s ( p h o t o - M O V P E ) .

(/169-4332/92/$05.00 © 1992 EIsevier Science Publishers B.V. All rights reserved

2 W. Richtet" /Photode«omposition ofpre«ursors for MOVPE

It has heen shown in recent years in many publications that it is possible to grow epitaxial semiconductor layers at considerably lower tem- peratures if photons are used to stimulate the growth kinetics. The growth of GaAs layers ho- moepitaxially on GaAs has been the example for such studies on photoinduced or photoassisted growth [3,4]

It was also verified that growth could only be achieved if the photon energy was within the spectral region where the precursor molecules exhibit the strong absorption in the gas phase as shown in fig. 1 [5]. It was found that with 193 nm radiation (ArF) depending on the substrate tem- perature growth is either enhanced or eren fully photon controlled (fig. 2a). With the 351 nm (XeCI) laser line in contrast no growth enhance- ment was observed at all (fig. 2b). With 248 nm radiation which falls within an absorption band of trimethylgallium, hut is not absorbed by A s H » gallium deposition was observed [3]. Thus gas- phase decomposition of AsH 3 seems to be an important first step in photoinduced growth of GaAs from TMG and AsH,» On the other hand, the growth was found to be laterally confined to the illuminated area of the substrate seleetive

l ~1/\ 20 \ ArF KrCI KrF XeCI N 2 XeF

_ > ' ' , 1 I I I u \

9 Is F~ I:: [ \ ~ - - TEG

~:D ' / X ~ - TNAs

l f / I \ 1 _J TMG

01 "-- .~a~~-- .~- i - ~ _ L , J , " ~ L _ 180 2 2 0 2 6 0 3 0 0 3 4 0

X[nm) Fig. 1. Light absorption efficiency (molar extinction) of vari- ous MOVPE precursor materials in the UV spectral region. Marked are the wavelengths of the most common exeimer

laser lines and of the nitrogen laser line.

growth), which indicates that the relevant gas- phase decomposition processes occur very close to the surface [6].

In these growth studies no further attempt was made to reveal the dissociation mechanism and to arrive at optimized parameters for photoinduced epitaxial growth.

The alm of the present report is, therefore, to investigate the AsH~ photodissociation process in more detail. Time-resolved coherent anti-Stokes Raman scattering measurements of the AsH~ partial pressure and the gas temperature were performed as a function of total pressure and the kind of carrier gas. The results reveal different behaviour for the two carrier gases (N 2, H 2) used and especially an strong influence of the total pressure.

2. Experimental

The experiments were performed in a horizon- tal MOVPE reactor at 300 K. The arrangement is outlined schematically in fig. 3. For photodissoci- ation an excimer laser was utilized which illumi- nated the gas flow perpendicular to the flow direction and normal to the substrate surface. Concentration and temperatures were measured by coherent anti-Stokes Raman scattering (CARS) [7]. In this four-wave mixing process the rotational and vibrational transitions of the molecules give rise to sharp spectral lines, whose energy positions are characteristic for the excita- tion levels involved. While the concentration is obtained from the overall intensity of the CARS spectral lines, the temperature ean be derived from the relative intensities of the CARS lines, since they reflect the occupation of the various energy levels. Thus, CARS offers the advantage of determining simultaneously both, concentra- tion and temperature, for preeursors as weil as for the carrier gas [8].

The local resolution was a few mm in beam direction and much less in flow direction. Time resolution is determined, as can be seen from fig. 3b, by the pulse width and the time stability of the lasers involved (Nd:YAG, dye laser and ex- eimer laser). The pulse width was in the fange

W. Richter / Photodecomposition of precursors for MOVPE 3

r ( ~ m / h )

10

0.1

0.01

800 700 600 500 ~k ,u . . . . . . . \/ uv: ~ = 193om ~\ / e°org,-- ,3 m,Jc~2

\ P A s H 3 = 340 Pa

PTMG = 28Pa

i ~ , 1.18 , I 1.0 1 2 1. 1.6 2.0 2.2

400 T(K)

(a) I I

1000 l / T ( )

K

r ( F m / h )

12

10

8

6

4

2

0

TD =

9 =

Ptot

PAsH3 =

I ; I I I 6 1 0 0 2 40

i i I I i I I I i

• da rk

o X = 193 nm o /

[] X = 351 nm

energy = 15 m J / c m 2 ~ o

786 K

10 c m / s

1.0 x 104 Pa

400 Pc (b) I I I

8O PTMG (Pa)

Fig. 2. (a) UV irradiation influence to the temperature dependence of the GaAs growth rate in MOVPE. A significant stimulation is achieved below 650 K. (b) T M G pressure dependence of the GaAs growth rate for different irradiation conditions. At 351 nm no

stimulation occurs, since the absorption is negligible (see fig. 1).

from 10 to 20 ns. Together with the time jitter of the excimer laser this resulted in a time resolu- tion of approximately 20 ns. The laser pulse repe- tition rate was 10 Hz. The time delay between the excimer laser pulse and the CARS laser pulses could be varied from - 1 0 0 ns up to 50 ms and larger if neccessary. A cross section view perpen- dicular to the gas flow direction of the reactor used is given in fig. 4. Special care had to be taken in the design of the reactor windows in order to avoid deposition on them.

The concentration and therefore the degree of decomposition was determined by evaluating the integral over the AsH 3 CARS spectra and nor- malizing it with respect to the reactor input par- tial pressure. Temperatures for the carrier gas as well as for AsH 3 were obtained from the rota- tional structure in the CARS spectra. For hydro- gen the pure rotational spectrum can be used directly to determine the temperature, while for nitrogen and AsH 3 the vibrational-rotational structure of the vibrations at 2330 and 2115 cm-1,

4 I4/. Richter / Photodecomposition qfprecursors [br MOVPE

0, ) £xcimer[Qser

UV:193nm 0 - 120 m J/cm 2

~ - « \ i ! / /

CARS faser

b)

e x e i m e r l a s e r

CARS l a s e r

[ de lay At I

A At. = -100 ns ... 50 m s

Fig. 3. (a) Schematic arrangement for arsine photodecomposition by an excimer laser and optical analysis by CARS. (b) Temporal correlation between the excimer photodecomposition pulse and the CARS analysis pulse.

respect ively, had to be analyzed. This imposes some diff icul t ies in the case of A s H 3 because as a consequence of the two d i f ferent and small ma-

excimer laser q

liner tube H2-flushing

BNd : YAG ~susceptor L(532 nm) f ~substrate

clye laser IR heaters

Fig. 4. Cross section through the MOVPE reactor in the substrate region, showing the optical viewports with the ex- cimer laser beam for arsine decomposition (vertical) and the

CARS laser beam t)r analysis (horizontal).

ments of iner t ia there is cons ide rab le over lap be tween d i f ferent t ransi t ions . The re fo re , for AsH~ the t e m p e r a t u r e was d e t e r m i n e d with the help of a ca ta logue of spec t ra es tab l i shed before - hand (fig. 5). Since the ars ine v i b r a t i o n a l - r o t a - t ional s t ruc ture and the a p p r o p r i a t e ro ta t iona l bands of hydrogen are more than 1000 cm 1 apar t , dyes had to be changed in o r d e r to mea- sure both spectra . For this reason only a few m e a s u r e m e n t s were p e r f o r m e d on thc ca r r i e r gas t e m p e r a t u r e for hydrogen as a car r ie r gas.

In no case non - the rma l d is t r ibut ions were ob- served and all the da ta could be desc r ibed with the he lp of the Bol tzmann d i s t r ibu t ion funct ion within the range of p ressures (0.5 to 5 × 10 4 Pa) and t ime delays (20 ns to 20 ms) used.

3. Results

The resul ts of the ars ine decompos i t ion for hydrogen and n i t rogen as car r ie r gases and for

144 Richter /Photodecomposition of precursors for MOVPE

two different total pressures (but always the same partial pressure) are shown as a function of time delay between excimer laser pulse and CARS laser pulse in fig. 6. All time-resolved spectra start approximately at 20 ns given by the time resolution. They all show negligible decomposi- tion for times larger than 20 ms. This is the time determined by the average gas flow velocity in the reactor. The forced flow results in an average movement of the excimer illuminated gas volume of approximately 50 /xm per ms and, as a conse- quence, after 20 ms the photo-modified gas vol- urne has moved out of the CARS beams. All spectra exhibit also a first plateau in the decom- position at intermediate time delays close to 1/xs. This can be seen more pronounced in hydrogen than in nitrogen. Its appearanee suggests that the decomposition is operative on two different time scales.

Remarkable changes are observed with in- creasing total pressure for both carrier gases. The maximum level of decomposition decreases with increasing pressure. This is shown more explicitly

t

AsH~

BOG K

AsH 3

I

!00 K [ / .

AsH,o

700 K

AsH3 I

400 K , I LI

i ' i

AsH 3

BO0 K )

AsH;

BO0 K

I I [ I 2100 2105 2110 2115 2100 2105 2110 2115

Raman shift. / cm -I

Fig. 5. Extract f rom the exper imenta l ly de te rm~ned catalog of A s H 3 C A R S spec t ra for t e m p e r a t u r e de te rmina t ion . Note the fine s t ruc ture and the i r regular i ty of the spectral line intensi-

ties, specially at 800 K.

in fig. 7, where the decomposition is plotted versus total pressure at different time delays. A strong decrease is observed when the total pres- sure is increased beyond 104 Pa.

The temperatures measured during the AsH 3 decomposition (fig. 8) correspond to the decom- position data in fig. 6. They also exhibit quite a significant dependence on the total pressure. At higher pressures the tempera ture of AsH 3 as well as of N 2 turns out to be much lower. In addition, the evolution with time is less prominent as com- pared to low partial pressures. In the latter case the tempera ture shows a pronounced maximum around 1 p~s, which is the time where approxi- mately also the decomposition displays a plateau (fig. 6).

4. Discussion

The most remarkable result is the quite strong decrease of the decomposition of AsH 3 with in- creasing total pressure but constant partial pres- sure. This could be caused by increased colli- sional de-excitation of the 193 nm excited AsH 3 state with increasing density. The photoexcitation or decomposition of Group V hydrides ( N H » P H » AsH 3) in molecular beams has been the subject of many studies [9,10]. It was shown that 193 nm excitation promotes AsH 3 from the ground state (X) to its lowest excited singlet stare AsH3(A). From there it can either fragment into AsH 2 and H or be excited further (fig. 9) [9]. The strong decrease in decomposition must then be interpreted as being due to the de-excitation of the AsH3(A) state.

The second interesting fact is the appearance of the plateaus in the decomposition for time delays below 1 /xs. While the first increase on the decomposition follows essentially the laser pulses convoluted with the time resolution, the second increase after approximately 1 p,s has to be of different origin. The time scales involved but also the increase observed in tempera ture suggest that the second increase is due to thermal decomposi- tion. The temperatures of AsH 3 and N?, how- ever, show different time evolution especially at low pressure (fig. 8). The AsH 3 temperature is cut oft sharply at 500 K. This very probably

bZ Richter / Photodecomposition of precursors,for MOVPE

o~

0 - r - - I

° t - - t ~ q

0

0 0 CD

100

50

0

100

50

' " 1 ' ' " 1 ' ' ' I l i ' " l ' ' " 1 " ' " [ ' ' " 1 ' '

AsH3 in Nz

\

\ /

Ptot = 10 4 Pa

ù , , I , , , I . . . . I , , , , 1 , , , , I . . . . I . . . . I ,

n s 1 ~ts 1 m s

' '1 ' " 1 ' ' " 1 ' ' " [ ' ' " 1 ' ' " 1 . . . . I ' '

AsHa in N2

Ptot = 2 1 0 4 Pa

ns i ,Lzs I m s

100

50

I01

' '" l ' '"1 ' '"I . . . . [ . . . . T ' "l ' "I ' "

AsH 3 in H2 o / / ~ ~ \ ~

, \ I

, \ F \ \ + Ptot = 104 Pa \

, , , I , , , , I , , , , 1 , , ù 1 , , J , I , , , , I . . . . J l

ns 1 /zs I ms

• '"I ' '"I .... I ' '"I ' '"[ ' ''q ' '"I '"

AsHa in Hz

Ptot = 2 ' 1 0 ~ Pa

. ns 1 /zs 1 ms

delay Fig. 6. T e m p o r a l d e v e l o p m e n t o f the a r s i n e decomposition efficiency for d i f f e r e n t c a r r i e r g a s e s a n d tota l p r e s s u r e v a l u e s . T h e

efficiency is reduced by i n c r e a s i n g tota l p r e s s u r e . A two-step structure is observed, which is most pronounced in H 2.

reflects the well known thermal decomposit ion behaviour of AsH 3 [8] which indicates the onset of decomposit ion of arsine at this temperature. The fact that at larger total pressures the temper- ature increase is much less, can be explained by

the larger heat capacity at higher pressures. The same amount of energy (or even less if radiative de-excitation is involved) is distributed among a larger number of molecules. It is also observed that the temperatures of both nitrogen and arsine

I00

1 75

ô 5o

Fn O

c~ 25 E O CD

o AL -- 11 # s /

I I I I i }

1 2 3 4 5 6

t o t a l p r e s s u r e / 10 4 Pa - - »

Fig. 7. T o t a l p r e s s u r e dependence of the decomposition efficiency at d i f f e r e n t s t a g e s o f the t e m p o r a l d e v e l o p m e n t .

14d Richter / Photodecomposition of precursors for MOVPE 7

900

T g00

700

600 E

~" 500

400

300

900

T S00

700

6OO K E

500

400

300

I I I I I I I

AsH 3 in N z ~ Ptot = 104 Pa

Z' / ~ AsH3 x ~

[ I I I I 1 I

ns 1 ,~s 1 ms I I I 1 [ I I

AsH 3 in N e Ptot : 2 ' 10 4 Pa

ns

AsH~ ~ o

N2

I I I I 1 I 1

1 ],s 1 ms

Delay »

Fig. 8. Temporal development of the local temperature of precursor and carrier gas for different total pressure values.

are nearly the same at this higher pressure. This demonst ra tes that at these time scales and pres- sures thermal equilibrium is reached.

10 AsH~ AsHy* + (3-y)H

~ 6 [IÄsH3 lA) ,~D,,AsH2 (A) +H

I AsH 2 (X} + H

2 1~d l l ~ d e excifo.fion

0 1 AsH 3 (X)

Fig. 9. Energy diagram of the possible electronic excitations of arsine due to ArF laser irradiation. The excited state can

either result in a decomposition or in a de-excitation.

5. Summary and conclus ions

It is shown that the photodecompos i t ion effi- ciency of AsH 3 depends strongly on photon en- ergy as weu as on total pressure. For total pres- sures above 10 4 Pa t ime-resolved measurements show a decrease from nearly 100% to approxi- mately 20%. This experimental fact is in terpreted as due to de-excitation of the intermediate AsH3(A) state before dissociation takes place. It is fur ther shown that substantial t empera ture in- creases (up to 800 K) appear especially at low pressures, which also contr ibute to the decompo- sition process. In conclusion, we believe that the choice of growth parameters besides photon pa- rameters in p h o t o - M O V P E can have significant influence on the decomposi t ion as well as on the growth. Careful optimizat ion of all parameters is neccessary.

Acknowledgement

This work was suppor ted Volkswagenwerk (AZ 1/64061).

by the Stiftung

References

[1] G.B. Stringfellow, Organometallic Vapor Phase Epitaxy: Theory and Practice (Academic Press, Boston, MA, 1989).

[2] A. Brauers, J. Cryst. Growth 107 (1991) 281. [3] P. Balk, M. Fischer, D. Grundmann, R. Lückerath, H.

Lüth and W. Richter, J. Vac. Sci. Techno[. B 5 (1987) 1453.

[4] V.M. Donnelly, D. Brasen, A. Appelbaum and M. Geva, J. Vac. Sci. Technol. A 4 (1986) 716.

[5] M. Fischer, R. Lückerath, P. Balk and W. Richter, Chemtronics 3 (1988) 156.

[6] D. Grundmann, J. Wisser, R. Lückerath, W. Richter, H. Lüth and P. Balk, lnst. Phys. Conf. Ser. No. 91 (1988) ch. 8, p. 797.

[7] J.W. Nibler and G.V. Knighten in: Topics in Current Physics, Ed. A. Weber (Springer, Berlin, 1979) ch. 7.

[8] R. Lückerath, P. Balk, D. Grundmann, A. Hertling and M. Fischer, Chemtronics 2 (1987) 199.

[9] B. Koppitz, Z. Xu and C. Wittig, Appl. Phys. Lett. 52 (1988) 860.

[10] R.D. Kenner, F. Rohrer, R.K. Browarzik, A. Kaes and F. Stuhl, Chem. Phys. 118 (1987) 141.