Co-adsorption of cesium and oxygen on GaAs(001) surfaces studied by metastable de-excitation spectroscopy

Surface Science 402–404 (1998) 683–686

Co-adsorption of cesium and oxygen on GaAs(001) surfaces studied by metastable de-excitation spectroscopy Kenji Yamada a,*, Junko Asanari b, Masamichi Naitoh b, Satoshi Nishigaki b a Ishikawa National College of Technology, Tsubata, Ishikawa 929-03, Japan b Kyushu Institute of Technology, Tobata, Kitakyushu 804, Japan Received 30 July 1997; accepted for publication 19 September 1997

Abstract Metastable de-excitation spectroscopy (MDS ) has been employed for monitoring the electronic structure variation in Cs- and oxygen-adsorption processes at a Ga rich p-GaAs(001)-(4×2) surface. MDS spectra at low Cs coverages showed a peak at 2.6 eV below E due to filling of Ga dangling bonds by charges from Cs 6s. At moderate coverages a Cs 6s-induced peak (6s˜) appeared F just below E with an abrupt increase at around h*~0.5, whereas contribution from substrate valence states remained on the MDS F spectrum. This suggests that the Cs-induced electronic states are not fully delocalized. Upon admission of oxygen onto the surface, O 2p-induced states (2p˜) appeared with multiple peak structures, implying direct bonding of oxygen with substrate atoms even at the initial oxygenation stage. Variations in the Cs 6s˜ and O 2p˜ emissions with oxygen supply showed enhanced oxygen uptake induced by the charge transfer from the Cs 6s˜ states. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Cesium; Gallium arsenide; Metastable de-excitation spectroscopy; Oxygen

1. Introduction The co-adsorption system of alkali-metals (AM ) and oxygen at III–V semiconductor surfaces has attracted much attention of surface scientists not only from a technological point of view in developing negative-electron-affinity cathodes or catalysts for surface oxidation [1–4], but also as a prototype of study on the electron transfer process leading to dissociative chemisorption. Many workers have proposed initial oxygen adsorption modes by using mainly photoemission spectroscopy, for example, existence of a molecular-oxygen precursor state [1,3], or oxygen bridge bonding between alkali and substrate atoms [4]. * Corresponding author. E-mail: [email protected] 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 97 ) 0 09 4 6 -1

Although those phenomena seem to be explained by roles played by adsorbed AM atoms, most authors have not established any direct relations with variation of adsorbate-induced charge states. Metastable de-excitation spectroscopy (MDS) probes occupied local density of states (LDOS) at average positions of the approaching He* atoms [5]. Using this method oxygen adsorption on AM-saturated Si and Ge surfaces [6 ], was investigated, and it was found that the process of electron transfer from AM atoms to oxygen depends on how the AM-induced s states develop on the topmost layer. In this paper, the results of an MDS study on the variation of local electronic structure during Cs and oxygen co-adsorption on the GaAs(001) surface are report. It will be shown that variation

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in the Cs 6s-induced states (6s˜) proceeds via three steps and oxygen adsorbs dissociatively by taking electrons from the Cs 6s˜ states at the initial adsorption stage.

2. Experimental procedures The experimental apparatus has been described previously [7]. A metastable-atom source in a pulsed hot-cathode discharge mode was used, producing a beam of excited helium atoms (He*) mainly in the 23S state (excitation energy 19.8 eV ). It also produces photons (hn=21.2 eV ), which reach the target earlier than He*. The time-offlight technique makes it possible to obtain both UPS and MDS spectra from exactly the same surface [7]. The energy of emitted electrons E is measured from the vacuum level of initial clean GaAs(001) surfaces. The position of the Fermi energy in MDS is not strictly determined, and is marked on the following spectra at the maximum energy of electrons ejected from Cs 6s-induced states. Low-energy thresholds of UPS spectra are used to monitor the work function variation. The p-type GaAs(001) specimen were cleaned by repeated Ar+ ion sputtering followed by annealing. This surface is known to have a Ga top-layer with the (4×2) arrangement of double Ga dimers [8]. Cs was deposited by a carefully degassed alkali dispenser. Pressure variations due to the supplied oxygen gas was recorded and afterwards integrated to estimate the amount of supplied oxygen at each dosage.

3. Results and discussion 3.1. Cs adsorption From the initial clean GaAs(001)-(4×2) surface a spectrum shown in Fig. 1a was obtained. It exhibits a broad structure having a width of about 12 eV, which is similar in shape to a result of the GaAs(110) surface by a He+ probe [9]. This is, therefore, interpreted as originating from resonance ionization of incident He* atoms followed by Auger neutralization (AN ) [5].

Fig. 1. A series of MDS spectra in the course of Cs adsorption on a GaAs(001) surface, showing growth of a Cs 6s-induced peak. (a) Clean GaAs(001) surface. (bn) (n=1–10) Cs-deposited surfaces with n=deposition cycle, in each of which the same amount of Cs was deposited.

Upon adsorption of Cs atoms, the spectrum quickly extended to an energy region higher than the AN maximum energy ( Fig. 1b1). This means Auger de-excitation (AD) starts to occur at alkaliadsorbed sites. After repeating Cs deposition cycle both a huge peak P and a double-peak P 1 2 appeared, which can be assigned to Cs 6s˜ states and Cs 5p orbitals, respectively [10]. In addition to these structures, MDS spectra showed weak multiple peaks in the range 4~12 eV. The AN spectrum in Fig. 1a reflects a convolution of upward and downward transition densities. Selfdeconvolution or first derivative can be used to extract LDOS. Since the multiple-peak structure in the range 4~12 eV ( Fig. 1b1–b3) agrees well with those in a first derivative of Fig. 1a [11], this structure is considered as electron emission by the

K. Yamada et al. / Surface Science 402–404 (1998) 683–686

interaction of incident He* 2s with the substrate GaAs valence states. In Fig. 2a the intensities of peak P are plotted 1 as a function of the Cs deposition cycle. The effective coverage h* on the horizontal axis from a criterion of saturation in the MDS 6s˜ peak intensity was estimated. The curve of 6s˜ intensity variation consists of three stages. In the first stage I (h*≤0.25), the peak intensity is nearly zero. By taking into account the fact that the work function variation reached −2.3 eV as shown in Fig. 2B, it is possible to estimate that Cs atoms adsorb ionically on the Ga-rich surface. An occurrence of heavy charge transfer from the Cs 6s˜ states to Ga dangling bonds is expressed in the emergence of a new peak marked by open triangle at 2.6 eV below E in curves b1–b3. In the second stage II (h*~ F 0.5), the P intensity rapidly increased whereas the 1 Dw decreased moderately. The peak P is due to 1 the AD process or the autodetachment process of He−*. This point is discussed in Ref. [11]. In the third stage III (0.6≤h*≤1.0), the Dw value

Fig. 2. (a) Variation in the intensity of the Cs 6s-induced states as a function of the deposition cycle. (b) Variation in the work function with increasing the Cs coverage. The solid lines are only a guide for the eyes.

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remains constant whereas the P intensity grad1 ually increased up to the saturation. This means Cs atoms adsorb neutrally in this stage. 3.2. Oxygen adsorption on a Cs/GaAs(001) surface Fig. 3a shows an MDS spectrum for a Cs/GaAs(001) surface prepared in the same condition as Fig. 1b10. The surface was then exposed to oxygen at room temperature (Fig. 3b–d), which was followed by annealing at 680 K ( Fig. 3e). By oxygen adsorption the Cs 6s˜ peak P drastically 1 decreased in intensity and new peaks appeared, as shown in the Fig. 3b. The former observation means that electrons flow out from the Cs 6s˜ state. This is supported also by an increase in the Cs 5p P peak height on the Fig. 3b, implying that the 2

Fig. 3. MDS spectra in the course of oxygen adsorption on a Cs-saturated GaAs(001) surface, showing a marked decrease in the Cs 6s˜ intensity and growth of O 2p˜-induced multiple peaks. (a) Cs-saturated GaAs(001), (b–d) oxygen-adsorbed surfaces and (e) after annealing the surface of (d) at 680 K.

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Cs 5p orbitals become exposed to He* atoms when oxygen atoms are adsorbed, stripping Cs atoms of their s˜ electrons. It is also noted that the energetic position of the peak P was shifted upward by 1 about 0.8 eV. This would reflects, via the Madelung potential, mainly an increase in the amount of ionic oxygen adsorbates around Cs atoms. A multiple-peak structure P appeared at the 3 beginning of oxygen adsorption (Fig. 3b), displacing the structure at ~8 eV due to substrate valence state. A peak at 5.6 eV in Fig. 3b corresponds to the Ga–O–Ga binding energy in a UPS result [12]. This means direct bonding of oxygen with substrate atoms even at the initial oxygenation stage, which shows clear contrast to the oxygen bonding to Cs in the Cs/Si(001) interface [10]. The other peaks at 9.1 and 11.9 eV in the P structure may 3 include various electronic states such as bonding, non-bonding or antibonding orbitals formed between oxygen and substrate atoms. When the O/Cs/GaAs surface ( Fig. 3d ) was heated at 680 K, the multiple peak P was con3 verted to a multiple peak P∞ . The annealing gives 3 to the surface enough thermal energy for desorption of surface atoms except Ga oxide [12]. It is thought that a new peak at 3.7 eV corresponds to the oxidized GaAs. This process of thermal oxidation promotion is similar to that at O/Cs/Si surfaces [13]. The substrate dependence of alkali and oxygen adsorption was demonstrated. Recently, Balasubramanian et al. [14] reported a rearrangement of Cs and As surface layers during oxidation of Cs/(As-rich) GaAs(001) surfaces. In order to clarify the relation between the AM-induced charge transfer and the surface structure, MDS experiments are required also for As-rich GaAs(001) surfaces.

4. Conclusion The LDOS at both Cs-adsorbed and Cs plus oxygen-co-adsorbed GaAs(001)-(4×2) (Ga-rich) surfaces has been extracted by MDS. It was shown

that Cs atoms initially adsorb ionically giving their 6s electrons to dangling bonds of adjacent Ga dimers and then neutrally at other sites including dimer vacancies. This observation presents a new point in the development of alkali-induced electronic states as compared with that at the Si(001) surface. The emergence of multiple bonding structures for adsorbed oxygen is related to such a complicated system of alkali/III–V semiconductor interface.

Acknowledgements This work was partly supported by a Grant-inAid for Scientific Research from the Ministry of Education, Science and Culture of Japan, and by the Foundation for C&C Promotion.

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