Applied Surtacu Scienct: 011/61 Iig92) 224-227 ~ ~ Norlh-Holland

s u r f a c e S c i e n c e

As desorption from GaAs and InAs surfaces studied by improved high-energy electron reflectivity measurements

Hiroshi Yamaguch i and Yoshiji Hor~koshi N ~ f Basit' Resear~'h Laborawries, Mttsa~hhlo-shL Tokyo 180, Japall

Reeei~red ~} November Iggl; accepled for publicathm 3 December 1991

The electron beam specular reflectivby of GaA~ and InAt Ug)l ) sllrtaces at several temperatures was quamitallvely e~aluated by measuring the cutl~nl through tin electrode in I'ron I of the tluorescence screen. At the reconstrucUtm transition, the refleclivhy as a [unclion of substrate temperalur¢ changes discontinuously with a hysteresis cycle fur InAs and changes continuously without a bystelesis for GaAs. This differgnce indit:ales Ihal the order of surface struclure Iransitkn~ differs between these B~u surfilces, The reflectivil~ changes during As desllrption show lhut the desorption pr(ices~ ig sensilive to slit[ace slruclure transition and that the desorption at the soucture Iransilion differs between differem orders of surhlce struclurc Iransitions

L l n t ~ u c t | o n

"[here arc many kinds of reconstructions ob- served on the polar surfaces of l l l - V compound semiconductors [l,2]. Various reconstructions are well known to be due to different surface stoi- chlometries 1"1-6]. The atomic surface structures for some of these reconstructions have been made clear by recent developments in several surface characterization methods, especially in scanning tunneling microscopy [6,7]. The elementary sur- face processes that determine surface reconstruc- tions, however, have not been well studied. Sur- face structure transition is one of the most ioter- esting phase transitions in two-dimensional sy.x- terns and it provides imporlant information on the chemical kinetics related to the phase transi- tions. We used an improved reflec:ion high-en- ergy electron diffraction ( R H E E D ) technique to investigate the influenee of surfaee structure transition on As desorption. This analytical tech- nique allows accurate in-situ determination of electron beam refleetivity by directly measuring the reflected electron beam current [8,9]. This paper shows that As desorption strongly depends on the surface recu~str ,~ion and that the de- sorption at the structure transition greatly differs

for different orders of surface structure transi- tions.

2. Experiments and discussion

In this improved R H E E D method, the re- flected electron beam intensity is obtained by measuring the current through a small electrode placed in front of the fluorescence screen. This technique improves the linearity and signal-to- noise ratio, allowing a more quantitative R H E E D observation. The details of the experimental setup are explained elsewhere [91. The R H E E D accel- eration voltage ~,as 2(I kV. Unless otherwise men- tioned, the incidence azimuth was [110] and the incidence angle was 0.9 ° . An infrared pyrometer was used to measure substrate temperature.

Electron beam speeular refloctivity of GaAs and InAs (001) surfaces was measured under an As pressure af ~ 2 . 5 × 1O * Tort (fig. I). We first increased and then decreased the substrate temperature. Because the rate of temperature change was ~ I°C/mln, thermal equilibrium was established for each observation. For both lnAs and OaAs, a i~,ofo|d pattern corresponding ro an

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0,015

0.010

H. Yamaguchi. }~ Horikoshi / As dt'sorpUo~/ from GaAs arid i:tlA~ surfaces

IncreJ~ng T InAs (001) ~ - e~ree=l.o T } 1St run

, In©r~aln 0 T t 2nd fun d~reulng T I

2-fo~ pattern

0.005 G 2"I * t !

P~I : 2AJ¢I0, TOt. , . ~ *

[110] uzlmuth 4.told pattern 0 0"00410 450 490 530

0.04 ! G~Afl (001) P~"=2'3xl&°T°rr

Inc~e~lng X 0 ,03 ~ d ~ l n g T

0,02 a-fold - -~ 3-|oTd panern pmttern

It 10] ~zlm.th . . ~ . . ~ p~ttDrn

o,01

o . . . . . . . . . . i 0'0540 570 60o 030 650

Substrete Temperature (oc) Fig. I. Specular reflectlvity versus substrate temperature.

225

As-stable (2 x 4) surface was observed at low substrate temperatures, and a fourfold pattern corresponding to a metal-stable ( 4 × x ) ( x = 2, 6, 8) surface was observed at high sabstrate temperatures. The refleetivity change at the re- construction transition clearly differs between InAs and GaAs surfaces.

With lnAs. the electron reflectivity changed discontinuously as a function of the temperature, with a 10°C-wide hysteresis cycle. This result was obtained reproducibly: the rcflcctivity changes during two different runs coincide. This suggests that the surface structure transition from As-sta- ble (2 × 4} to In-stable (4 × 2) is a first-order transition and that surfaces with a coverage inter- mediate between these two surfaces are not sta- ble. Moison et al. [10] reported that the recon- struction transition for lnAs (001) has a hysteresis cycle. Our results dear ly show that, due to a first-order surface structure transition, a physical parameter - the electron reflectivity or, probably, the As coverage itself depends on substrate tem- perature discontinuously. With GaAs, on the other hand, the reflectivity changed gradually as a function of the temperature, passing through an intermediate (3 x 1) surface without hysteresis. The transition is higher-order for GaAs (001), and a mixed 1"3x t) surface can exist for any degree of As coverage.

The influence of this structure transition on the As desorption process was then examined in detail when the As supply to the GaAs (001) surface was terminated (fig. 2). When the initial As pressure, 5.7 × 10 -~ Torr, was reduced to 1.5 × I0-7 Tor r by closing the shutter in fl'ont of the As effusion cell, the refleetivity R initially decreased due to _s desorption from the surface and then reached a stable value R o. Fig. 2b plots the logarithm of R - R o and shows the observed R H E E D patterns. It is clearly seen that the re- flectivity decreases exponentially and that the time constant changes when the surface recon- struction changes from (2 × 4) to (3 × 1). Similar results (not shown) were obtained at other sub- strate temperatures. The time required for the transition of surface reconstruction changed when the temperature was changed, but at every sub- strate temperature the reconstruction transition occurred e~actly when the time constant changed. The temperature dependence of the time con- stants gives activation energies of 2.65 eV for (2 x 4) surfaces and 3,4 eV li)r (3 X 1) surfaces. The activation energy thus differs between differ- ent surface reconstructions, and the results indi- cate that desorption is sensitive to the surface structure.

The time constants measured under several diffraction conditions for each reconstruction (ta-

220 H. Vamagtlchi. Z Honkr~hi / As de~rpzion ~rmn Gtvls mul It~4s ~ur#l£~es

0.0f°'°2[ ----. (a) =

I~ o : O.Ol O3

"t • = 6f~5°C I

0,0 4 ,0 8 .0 TIm~ (Sl

'°+! %-

10,41 T ~ = 605eC ..'

0.O 2.0 4.:$

Tirr.~ Is)

Fig. 2. Change ;n the RHEf iD specul~ beam r~:fl¢cfivity when the A~ supply to th~ surface is Icrminaled. T'.;. ~ t,tcu- sured r~I'le¢liVity (//I is plotted in a linear scale in (a) and th*: difft:renc¢ R - Ro, v, here Re is the saturated re.flec0vily, ;s

plotted in a log mate in (b}.

ble 11 have nearly the same values except when the incidence azimuth is the exact [110] direction and the incidence angle is 1.35 °, which nearly corresponds to an out-of-phase condition for (2 ×

Table I Time e(,~stant of eleel,on rcfleclivity dcca; during A~ desurp- tion from GaAs sur';accs

]ncidenc': azimuth Incidence Timcc+mstanl(sl

angle During (2x41 During (3 x 1) (deg)

Exazt [1 lO~ u.a5 125 0.51 1,14 2.03 n.67 L35

I 3" from I 11 O] 0.85 1.23 0.43 LI4 1~=14 Jk49 135 2.03 0.26

~-5° from [llOJ 0.85 1.35 0.47 1.14 1~6 0.49 1~5 120 0.44

InAo (001)

~"- As ghutter ctaeed

g v 440DC +2 $

4 5 o C

[tfOl ~lmuth

T!me Fig, k Chance in RItEED specular beam r¢flectivity of an In {I)WO surface when the As supply was terminated at various s£~ostrmc tumpuratures. (Arrows sho,x. ~vhen the RHEED

pattern changed from twt)fold Io fnurfold,)

4) reconstruction i l l ] . Under this condition, the rcflcctivity did not show exponential time depen- denc, ' , - probably because of muhiple scattering or resonances [12]. Even under the out-of-phase eondhious, the time constants were almost the same when the azimuth was slightiy off [110]. The time constant with which reflectivity decreases is not necessarily with which As desorbs because reflectivity may not be a simple linear function of As coverage. Although the refleetix'ity also de- pends on the surface reconstruction and the sur- face disorder, the time constant tar As dcsorption itself probably changes due to the reconstruction transition. This is because the change in the time constant of refleetivity decrease is too large to be explained by a nonlinear dependence of retlectiv- ity on As coverage and because the same values were obtained under almost all diffraction condi- tions.

Fig. 3 shows the change in specular beam reflectivity and the transition point of the surface structure from (2 × 4) to (4 x 2) when the As supply to the lnAs (001) surface was terminated at several substrate temperatures, in contrast to the GaAs surface, this surface clearly shows a reflectivity decrease whose slope has a minimum

H. Yaraaguchi, Z Horikoshi / As desorplion from GaAs and lnAs surfaces

value in the neighborhood of the transition and increases after tile transition. Especially, at tem- peratures lower than 430°C, the refleetivity reached a stable value without a transition to an In-stable surface. Similar phenomena (not shown) were observed at different azimuths and inci- dence angles. The As desorption rate probably reaches a minimum in the neighborhood of the transition. These results suggest that, for an lnAs (001) surface, the existence of a metastable state due to the first-order transition slows the rate of As desorpfion in the neighborhood of the transi- tion [13].

The refiectivity change during a twofold recon- struction can be fitted by an exponential function, and the Arrhenius plot of the decay constant gives an activation energy of 1.65 eV. This value is close to the value obtained from the tempera- ture dependence of the I I [ / V ratio with which the transition was observed during the growth in ref. [14]. The reflectivity change during a fourfold pattern was bet ter fitted with an exponential curve when a [010] azimuth was used than when a [110] azimuth was used. The activation energy of an As desorption process during a fourfold pattern, 4.2 eV, was evaluated from the reflectivity change at this [010] azimuth. The activation energy differs for different reconstructions, as it does for the GaAs surface.

3. Conclusions

We explored the As desnrptlon from GaAs and lnAs (00t) surfaces by using an improved R H E E D analysis that allows accurate in-situ de- termination of electron beam reflectivity by meas- uring the reflected electron beam current. With an InAs surface, the electron reflcefivity under As pressure changes discontinuously as a function of the temperature with a hysteresis cycle, indi-

227

caring first-order surface structure transition. The metastable state due to the first-order transition suppresses the As desorpdon near the structure transition. This phenomenon was not observed with a GaAs surface because the order oi" sarfaee structure transition is higher. The activation en- ergy of As desorption from both GaAs and IBAs is sensitive to the surface reconstruction.

Aekn0wledgement

We thank Dr. Tatsuya Kimura for his continu- ous encouragement of this work.

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