Properties of InP(110) surfaces, InP(110)−Ag interfaces and InP(110)−Ag schottky diodes contribution of the temperature

Surface Science 189/190 (1987) 315-321 North-Holland, Amsterdam

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PROPERTIES OF InP(ll0) SURFACES, InP(ll0)-Ag INTERFACES AND I n P ( l l 0 ) - A g S C H O T r K Y D I O D E S Contribution of the temperature M. D U M A S , M. B E N K A C E M , J . M . P A L A U a n d L. L A S S A B A T I ~ R E Laboratoire d'Etudes des Surfaces, Interfaces et Composants, UA CNRS No. 04 0787, Universitd des Sciences et Techniques du Languedoc, Place Eugene Bataillon, 34060 Montpellier-Cedex, France

Received 31 March 1987; accepted for publication 15 April 1987

We present the results obtained on cleaved InP(ll0) surfaces by Auger electron spectroscopy (AES) and Kelvin measurements. We first study the electronic properties of the surface just after cleavage at room temperature and liquid nitrogen temperature. Then, we increase the temperature up to 670 K and follow the modification of the free surface stoichiometry by AES. We repeat the same experiments after the deposition of three Ag monolayers. From these experiments, we deduce information on the relative concentration of In-Ag-P in the surface layer. On the free surface the stoichiometry remains practically unchanged in the range 110-473 K. Above these temperatures, the stoichiometry is modified particularly above 480 K: the P concentration decreases and the In concentration increases. P desorption is yet important. Different results were obtained when the sample was Ag covered. At T = 450 K the P concentration increases and the concentration of Ag decreases. There is an out-diffusion of P and in-diffusion of Ag which noticeably modify the layer stoichiometry. In the second part of the paper, we report results on Ag-InP(ll0) Schottky diodes obtained at room temperature and after annealing of the diodes. We compare the evolutions of the properties of the InP(ll0)-3 ML Ag interface with the evolution of the electrical properties of the diodes. Afterwards, the results are analysed in terms of interdiffusion and change in the stoichiometry.

1. Introduction C o m m o n l y , a n n e a l i n g is u s e d in o r d e r to o p t i m i z e the electrical characteristics of S c h o t t k y diodes, T h e r e a s o n s of this i m p r o v e m e n t are n o t really known. C h e m i c a l , s t o i c h i o m e t r i c effects, or species m i g r a t i o n c a n b e i n v o k e d [1,2]. I n this p a p e r we d e s c r i b e s o m e e x p e r i m e n t s d e v o t e d to this p r o b l e m . T h e effect of t e m p e r a t u r e is o b s e r v e d first o n a free surface, s e c o n d o n a surface c o v e r e d w i t h a few m o n o l a y e r s of m e t a l , third o n the diodes.

2. Experimental T h e d o p i n g of the [ n P s a m p l e s is n = 5 x 1015 c m -3 n o n - i n t e n t i o n a l l y d o p e d , p = 2 X 1015 c m -3 Z n d o p e d . 4 x 8 m m (110) surfaces are o b t a i n e d by 0 0 3 9 - 6 0 2 8 / 8 7 / $ 0 3 . 5 0 9 E l s e v i e r S c i e n c e P u b l i s h e r s B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g D i v i s i o n )

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M. Dumas et al. / Properties of lnP(llO) surfaces

cleavage under UHV (ultra high vacuum) base pressure 10 -l~ Torr. The experimental set up is similar to that described elsewhere [3]. Surface characterization and silver evaporation are performed under UHV conditions. Annealing of the diodes and diode characterization are performed in an other arrangement under primary vacuum. The contact potential difference is measured with a small Kelvin probe allowing topographical studies [4]. Auger electron spectroscopy (AES) is performed with a standard single pass cylindrical mirror analyser (OPC/03 RIBER) with a resolution A E / E of about 0.5% and a 3 keV coaxial electron gun. The thickness of the thin deposited layers is obtained by using the Smith and Southworth [5] method for the 120 and 1860 eV peak of phosphorus. Escape depths are calculated by the Seah and Dench relation [6].

3. Results and discussion

3.1. Temperature effect on surface properties Surfaces properties have been studied by AES and by our topographic Kelvin method [7]. In the temperature range 100-300 K, the (110) cleaved surface exhibits the previously described electrical behaviour [7], i.e. band bending on n and p types due to cleavage defects leading to a pinning of the Fermi level in the upper half part of the gap when the density of the cleavage surface states is high enough [8,9]. Bands are flat on very well cleaved areas showing that even on cleaving at 100 K the intrinsic surface states do not return to the gap. On the other hand, increasing the temperature above 400 K gradually produces surface states. Results for AES signal versus temperature are shown in fig. 1. Heating the InP(ll0) cleaved surface leads first of all, to the increase of the P percentage and subsequently to an increase of the In percentage above 500 K. Thus, thermal decomposition of InP, resulting in P evaporation and In cluster formation [1,2] probably occurs after a previous migration of P from the bulk to the surface. Such a process is very interesting, because it may produce deep phosphorus vacancies, which we will later link to the diode characteristics.

3.2. Thermal diffusion through Ag Three monolayers of silver have been deposited under UHV on the cleaved surface and they have been annealed by steps of 50 K up to 600 K. Silver was chosen because it is generally considered to be unreactive [10-12]. Moreover we never observed out-diffusion on In through a thick layer of silver contrarily to what happened with aluminium [7]. The AES results of fig. 2 show that out-diffusion a n d / o r intermixing occurs at 500 K. Clearly, the increase of In

317

M. Dumas et al. / Properties of lnP(l lO) surfaces

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and especially of P is correlated to the temperature for which InP begins to decompose (fig. 1).

3.3. Effect of annealing on InP / Ag Schottky diodes The diodes are achieved by deposition under U H V of 1 m m diameter silver dots on the cleaved face. They are characterized by the classical forward In I(V) and reverse 1/Cz(V) methods. Afterwards all the diodes are heated by steps of 50 K up to 550 K. Each annealing stage lasts 20 min. Between each stage the diodes are cooled to room temperature in order to be characterized. For a better understanding of the annealing effects, a lot of new diodes was made on a cleaved surface previously heated to 550 K and studied in the same way. Results are shown in fig. 3.

3.3.1. n-type Because the Schottky barrier is small, only I(V) results can be obtained. The experimental values of the barrier q~B are always in the 420-480 meV

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range for both types of surfaces, close to the value previously reported [13]. On the other hand, the effect of preliminary heating the surface at 550 K is particularly visible on the ideality factor n. While it improves continuously in the case of the unheated surface, it is damaged by annealing in the case of the heated surface. These results support the hypothesis that the barrier height is determined by interface states with steady energy levels but whose density and geometrical repartition is very sensitive to the initial physico-chemical state of the surface.

3.3.2. p-type The results are different for p-type diodes. For the two surfaces, the ideality factor is optimized by an annealing temperature in the 450-500 K range. For the two surfaces, ~B slowly increases in the first stages of the annealing and then decreases for higher temperatures at the same time that n is damaged.

M. Dumas et al. / Properties of InP(l lO) surfaces

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Nevertheless, the variations of ~8 are moderate. So, the previous hypothesis of a barrier due to the pinning of the Fermi level by interface states, whose energy level is almost constant remains valid. Concerning their spacial distribution, it may be envisaged that the effect of the annealing temperature is stronger for acceptors (mainly active on n-type) than for donors (mainly active on p-type) either because they are submitted to opposite electric field, or because each of these kinds of states is associated with different processes of physico-chemical interaction differently activated by temperature, such as creation and migration of In or P vacancies. For example, the P vacancies, of higher density in the case of the annealed surface, could be more active on n-type diodes. It is remarkable that the annealing effect mainly influences on the ideality factor which may be an indicator of a modification of the shape of the barrier. Another indication for this modification is given by the comparison between the experimental I(V) and C(V) barrier. In fig. 3, it is evident that the shape of the barrier is very different from the classical parabolic one in the case of the previously annealed surface. Whereas a good correlation is exhibited between I(V) and C(V) barrier of the diodes made on the unheated surface and annealed at 350-400 K, the C(V) barrier is dramatically too small for the diodes made on the heated surface. We observe that such a situation occurs when the former diodes are annealed at 550 K which is the temperature used for annealing the latter surface. 4. Conclusion The results presented in this paper indicate that alteration of the electrical properties of InP surfaces occurs in the temperature range usually used for annealing the Schottky diodes. Out-diffusion of In and P through three monolayers of Ag is also observed in this temperature range. Then, in the interface intermixing process we can envisage: (i) that only In or P atoms diffuse into Ag; (ii) that also Ag atoms diffuse into InP. Our experimental results show that intermixing leads to deep modifications in the space charge zone. But such a modification is also obtained without silver deposition. Thus we can suppose that the effect of the temperature on the diode characteristics is essentially due to the creation of defects in InP over several atomic layers. If interdiffusion of Ag occurs, we think that it is limited to a few atomic layers. Indeed, a deep inclusion of Ag atoms in the InP pattern would produce electrical effects which we have not observed. To summarize, we think that the barrier height is determined by the energy level of states, associated with InP defects, whose density distributions inside the space charge zone is responsible for the particularity of the electrical characteristics.

M. Dumas et al. / Properties of lnP(l lO) surfaces

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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

Sng Chu, C. Jodlank and W.D. Johnston, Solid State Sci. Technol. (1983) 2393. J. Massies and F. Lemaire-Dezaly, J. Appl. Phys. 55 (1984) 3136. J.M. Palau, E. Testemale, A. Ismail. L. Lassabat6re, J. Vacuum Sci. Technol. 21 (1982) 6. J.M. Palau, E. Testemale and L. Lassabat6re, J. Vacuum Sci. Technol. 19 (1981) 192. J.F. Smith and H.N. Southworth, Surface Sci. 122 (1982) L619. M.P. Seah and W.A. Dench, Surface Interface Anal. 1 (1979) 2. A. Ismail, A. Ben Brahim, M. Dumas and L. Lassabat6re, Surface Sci. 178 (1986) 158. A. Ismail, A. Ben Brahim, J.M. Palau and L. Lassabat6re, Surface Sci. 164 (1985) 43. A. Ismail, J.M. Palau and L. Lassabat6re, J. Phys. (Paris) 45 (1984) 1717. A. McKinley, G.J. Hugues and R.H. Williams, J. Phys. C10 (1977) 4545. A. McKinley, A.W. Parker and R.H. Williams, J. Phys. C13 (1980) 6723. T. Kendelewicz, W.G. Petro, I. Lindau and W.E. Spicer, J. Vacuum Sci. Technol. B2 (1984) 453. [13] L. Lassabat~re, A. Ismail, J.M. Palau and A. Ben Brahim, Surface Sci. 168 (1986) 336.