Phonons in surface photovoltage of GaAs

Volume 47A, number 3

PHYSICS LETTERS

25 Match 1974

P H O N O N S IN S U R F A C E P H O T O V O L T A G E O F G a A s A. MORAWSKI and J. LAGOWSKI Institute of Physics, Pohsh Academy of Sciences, Al. Lotnik6w 32/46, 02-668 Warsaw,Poland Received 21 January 1974 Oscillatory surface photovoltage is reported in GaAs at 4.2°K, characterized by two series of minima. Dominating series is attributed to the capture of photoexcited electrons by surface states with emission of phonons. Second, weak series coincides with oscillations in photoconductivity. The surface photovoltage is defined as the change of surface potential occuring under illumination. The analysis of surface photovoltage as a function of the energy of the incident light has proven to be very useful for determining the parameters of the surface states [1]. So far such analysis has been limited to the region o f subbandgap illumination. The present paper presents high resolution spectra of surface photovoltage near and above the intrinsic absorption edge. GaAs, n-type (111) wafers with a bulk resistivity of about 0.4 [2 cm were employed in this study. The surface photovoltage generated by chopped light (19 Hz) was detected capacitively on semitransparent reference electrode. The contribution from Dember effect to the photovoltlage under study was negligible. Details of the measuring system have been reported elsewhere [2]. The measurements were carried out at liquid helium temperature. The wafers were etched in 1HF + 3HNO 3 + 2H20. Typical results obtained with (111) surface of GaAs at 4.2°K are shown in fig. 1. It is seen that for X < 0.82/am the surface photovottage exhibits well pronounced oscillations in considerable energy interval. It should be noted that no oscillations were found for lower incident photon energy. The photovoltage oscillations can be resolved into two series shifted in energy scale: the large oscillations dominating the spectrum (minima denoted as series n) and weaker series l which only slightly affects the spectrum. As shown in fig. 2 the minima occur at the energies E(/) ~ 1.520 +0.041 l(eV) ;

(1) E(n) ~ 1.483 + 0.043n (eV)

for I and n series respectively. It is of importance to

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!~ • /

/

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1.7

I

I

7

6

,, "

-

h,~ (eV) 1.6

1.5

'1

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4

3

2

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,

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j=

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l

,

+

1~

E =1 522eV

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,

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,

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.7

i .6

I .75

I 80 WAVELENGTH

(~m)

Fig. 1. Typical surafce photovoltage spectrum of (111) surface of n-type GaAs at liquid helium temperature. I

I

Ga A s

I

I

I

I

f

4.2 ° K

I

19

~'~ 1.8

z W

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.L' s e r i e s s zxE=4t.3meV s , , ¢ ' ~ , l e . n " s e r i e s ,,,,e"~ 4 " z~E =43meV

17

16

Eg. ~

Eo=1.483 eV

I 1

I 2

I 3

I 4

1.5 I 5

I 6

I 7

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14

ORDER NUMBER OF MINIMA

Fig. 2. The energies of surface photovoltage minima as a function of the order numbers assigned to the minima. note that the l series coincides with previously reported oscillation in photoconductivity of GaAs [3,4] There are two basic mechanisms responsible for the 219

Volume 47A, number 3

PHYSICS LETTERS

surface photovoltage. 1) the standard photovoltage resulting from photoinjection of excess carriers [e.g. 5], 2) photovoltage due to the change in the net electric charge in surface states under illumination. In the case 1) the photovoltage is accompamed by photoconductance, while in the case 2) no detectable photoconductance is required. Let us assume that analogously to other osctllatory photoeffects the present oscillations are related to the emission of phonons by electrons photoexcited from the heavy-hole band. The period of oscillations z3~ can be related to phonon energy ~w [e.g. 6] AxE = h6o(1

+me/mhh )

(2)

where m e and mhh are the effective masses of electron and heavy hole, respectively. In GaAs me/mhh = 0.12 -+ 0.01 [4] and for l series/~w = 36.5 -+ 0.5 meV i.e. equal to longitudinal optic phonon energy h w t at q ~ 0 [7]. For n series/~o = 38.3 -+ 0.3 meV i.e. shghtly above the allowed energy of bulk phonons. The l series which coincides with oscillations in photoconductivity can be attributed to the mechanism 1). The series n, dominating on photovoltage, was not seen in photoconductivity and it could be explained by mechanism 2). Thus the process responsible for large oscillations would involve the capture of photoexcited electrons by shallow surface states with emission of phonons of the energy 38.3 meV. Minima in photovoltage correspond to the maxima in number of electrons trapped by surface states i.e. to the case when electron energy differs from the trapping

220

25 March 1974

surface level by a multiple of phonon energy. The energy level of the surface states revolved is E c - E t ~ 35 meV. Speculating about small but significant difference between/iw = 38.3 meV and the allowed energy of bulk phonons it can:be noticed that in the capturing of electrons by surface states the surface phonons may be involved. If such locahzed surface vibrations were associated with Impurity atom absorbed at the surface, the fact that rico was shghtly higher than ~/w L would not be surprising. The authors would like to thank Professor L. Sosnowskl and dr. J. Chroboczek for helpful discusslons. They are also indebted to Mr. A. ~wiatek for assistance in experiments.

References [1] H.C. Gatos and J. Lagowskl, J. Vac. Sol. Technol 10 (1973) 130. [2] J. ]2agowskl, C.L. Balestra and H.C. Gatos, Surface Sel 29 (1972) 213. [3] R.E. Nahory, Phys. Rev. 178 (1969) 1293. [4] R.W. Shaw, Phys. Rev. B3 (1971) 3283. [5] D.R. Frankl, in Electrical properties of semiconductor surfaces (Pergamon, Oxford 1967). [6] H.Y. Fan, Proc Int. Conf. Semiconductor Physics, Moscow, 1968, p. 135. [7] A. Mooradian and G.B. Wright, Solid State Commun. 431 (1966).