Unpinned behavior of the electronic properties of a p-GaAs(Cs,O) surface at room temperature

surface science ELSEVIER

Surface Science 331-333 (1995) 1250-1255

Unpinned behavior of the electronic properties of a p-GaAs(Cs,O) surface at room temperature V.L. Alperovich a,b,*, A.G. Paulish a, A.S. Terekhov a,b a Institute of Semiconductor Physics, 630090 Novosibirsk, Russian Federation b Novosibirsk State University, 630090 Novosibirsk, Russian Federation

Received 6 August 1994; accepted for publication 2 December 1994

Abstract

The evolution of the surface band bending, photovoltage, and recombination rate is experimentally studied in situ by means of photoreflectance and photoluminescence techniques under deposition of cesium and oxygen on the surface of epitaxial p- and n-type GaAs layers at room temperature. The evolution of the band bending is explained in terms of Fermi level pinning by initial and adatom-induced donor-like and acceptor-like surface states. For the surface of p-type GaAs multiple reversible variations of the electronic properties are observed under alternate Cs and 02 deposition. This unpinned behavior proves that adatom-induced surface states dominate over defect-induced states. Hysteresis in the dependences of photovoltage and photoluminescence intensity on the band bending shows that variations of the concentrations or cross sections of surface capture and recombination centers take place at the first cycles of Cs and 02 deposition. Keywords: Gallium arsenide; Metal-semiconductor interfaces; Photoluminescence; Schottky barrier; Surface electronic phenomena; Surface

photovoltage

1. Introduction

The nature of electronic states on the surface of GaAs and other compound semiconductors is still a controversial problem, being important both for fundamental surface science and device applications. The unified defect model (UDM) proposed by Spicer et al. [1,2] attributed the surface states to universal intrinsic defects of a semiconductor arising at an early stage of adsorption of alien atoms. According to UDM, these intrinsic defects are responsible for the Fermi level pinning on the GaAs surface in a

* Corresponding author.

midgap position. This pinning left small opportunities for the control of the electronic properties of the surface by further treatments. Later it was experimentally proved that the creation of intrinsic surface defects can be avoided by deposition of a protective arsenic overlayer on MBE-grown (100)GaAs layers [3], appropriate sulfur treatment [4,5], deposition of a xenon interlayer [6], or by lowering the temperature of deposition down to approximately 100 K [7,8]. Recently we observed unpinned behavior of the Fermi level under successive deposition of cesium and oxygen on p-type GaAs epitaxial layers at room temperature [9,10], with the clean surfaces of the layers being prepared by an oxide-free technique [11]. Thus, the results reported in Refs. [3-10] prove

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V.L. Alperovich et al. / Surface Science 331-333 (1995) 1250-1255

that the domination of adatom-induced [12-14] rather than defect-induced surface states is an inherent property of carefully prepared interfaces of GaAs with adsorbate layers. However, the role of adatominduced surface states is not studied well enough even for the interfaces that are important for applications. The aim of this paper is to study in detail the influence of initial and adatom-induced surface states on various electronic properties of (100)GaAs(Cs,O) surfaces, namely, on the surface band bending, trapping, and recombination.

2. Background of photoreflectance and photoluminescence techniques Optical modulation techniques proved to be very useful for in situ investigations of semiconductor surfaces during epitaxial crystal growth and interface formation [15-23]. In particular, photoreflectance (PR), which is a contactless form of electroreflectance, is an effective tool for the determination of the built-in electric field F s near the surface [15-19]. The magnitude of F~ can be determined from the positions of the extrema of Franz-Keldysh oscillations observed in photoreflectance spectra [15-18]. In the regime of small photomodulation of the surface field this procedure yields the maximal nearsurface value of the electric field of a depleted Schottky layer [18]. The magnitude of band bending q~s, in its turn, is calculated from F s and the doping concentration, according to the Schottky model for a depleted space charge region. Together with the band bending, the photoreflectance technique allows one to study the evolution of the surface photovoltage (SPV). It was shown by Kanata et al. [19] that the PR amplitude ( A R / R ) o is proportional to the magnitude of the modulated SPV arising under modulated illumination. The magnitude of SPV can be easily calculated for a semiconductor-metal junction, using the condition of compensation of the photocurrent by the restoring thermionic and tunneling currents [24,25]. This approach implies that the metal is an ideal reservoir both for electrons and holes. However, this assumption usually is not obeyed for a clean surface or a surface with submonolayer amount of adsorbates. In the latter cases SPV originates from: (i) the screening of the surface

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field by free photoexcited electrons and holes, and (ii) additional nonequilibrium charge captured by the surface states [26]. Therefore, SPV is determined by capture and recombination rates of electrons and holes on the surface. These processes, in turn, depend on the concentration, energy spectrum, and capture cross-sections of the respective surface states. A large number of unknown parameters hinders the quantitative theoretical analysis of the surface photovoltage. In this paper only qualitative considerations on the evolution of SPV under Cs and 0 2 deposition are given. The photoluminescence (PL) technique enables one to study both radiative and nonradiative recombination on the surface [20,21]. Nonradiative surface recombination leads to a decrease of the intensity of near-bandgap PL that originates from radiative recombination in the bulk. Thus, the variations of the intensity of bulk PL reflect the variations of the effective surface recombination velocity [20]. For high values of the surface recombination velocity S > > D / L , the photoluminescence intensity IrE is inversely proportional to S. Here L and D are the bulk electron diffusion length and diffusion coefficient, respectively. The calculation of S requires detailed knowledge of the microscopic characteristics of the surface recombination centers. In a simple case of recombination via isolated surface level, the Schockley-Reed approach yields that S reaches a maximum value when the Fermi level is close to the middle of the bandgap. Similar result was obtained for a continuous energy distribution of recombination centers [20].

3. Experimental The epitaxial n- and p-type GaAs layers used in the present study were grown by liquid phase epitaxy on semi-insulating GaAs substrates. The concentration of dopants (Si for n-doping and Ge or Zn for p-doping) was determined from Hall and voltagecapacitance measurements and was in the range of Nn(p) = ( 1 - 1 0 ) × 1 0 I7 c m - 3 . The thickness of the layers was in the range of 6-10 /zm. The oxide-free low-temperature cleaning (LTC) of the surface included treatment in a solution of HC1 in isopropyl alcohol, transfer to a high vacuum

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EL. Alperouich et al. / Surface Science 331-333 (1995) 1250-1255

chamber without exposure to air, and heat cleaning. The details of the technique are described in Refs. [9-11]. Using this technique, it is possible to obtain an atomically clean surface by heat cleaning at relatively low temperatures T = 400 ° C [11]. Cesium and oxygen were deposited from channel sources by thermal decomposition of cesium chromate and barium peroxide. Photoreflectance spectra and the integral intensity of near-bandgap photoluminescence were measured in situ, in the U H V chamber. A laser beam with a wavelength of 632.8 nm and a power density of 0.05-1.5 m W / c m 2 was used for the excitation of photoluminescence and photomodulation of reflectivity. A halogen lamp and grating monochromator were used for measuring PR spectra. It should be noted that the surface photovoltage averaged over the period of modulation causes a systematic reduction of the measured values of band bending [25,26]. To minimize the error related to SPV, we measured the PR spectra at the lowest possible level of pump light intensity. Additional measurements at various light intensities and at elevated temperatures allowed us to estimate the error in gos to be below 0.1 eV.

4. Results and discussion

Earlier the low-temperature surface preparation technique enabled us to observe domination of adatom-induced donor-like surface states over initial surface donors on a clean surface of p-type GaAs [9,10]. To elucidate the possible role of initial surface acceptors, which may be present on GaAs treated by the LTC technique, in this work we studied also the evolution of the electronic properties on the surface of n-type GaAs(Cs,O). Photoreflectance spectra were measured on a clean surface of p- and n-type GaAs and after successive deposition of cesium and oxygen contained Franz-Keldysh oscillations, which enabled us to determine the surface electric field and then calculate the magnitude of band bending [9,10]. In the top of Fig. 1 the evolution of band bending is shown for three cycles of C s - O deposition on p- and n-type GaAs. Cesium deposition on a clean surface of p-type GaAs led to a fast increase of band bending from q~s = 0.3 eV to q~s = 0.6 eV at coverages 0 < 0.1 ML, then to satura-

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0 10 2200 10240 102 D e p o s i t i o n o f Cs (ML) a n d 0 z (L) Fig. 1. The evolution of band bending (top, circles), photoreflectance amplitude ( A R / R ) o (middle, squares), and photoluminescenee intensity IpL (bottom, triangles) for the first three cycles of alternate Cs and 0 2 deposition on the surface of p-type (open symbols) and n-type (closed symbols) GaAs. tion and a subsequent decrease of q~s by 0.05 eV at 0.2 < 0 < 1.0 ML. This " o v e r s h o o t " behavior was characteristic of the first Cs deposition. Larger values (up to 0.15 eV) of the overshoot were observed on MBE-grown structures [10]. Exposure to oxygen led to a decrease of q~s down to 0.25 eV. Under alternate depositions of Cs and 0 2, we observed reversible switching of band bending between large " c e s i u m " and small " o x y g e n " values. On p-type GaAs the switching was observed for a broad range of the heat cleaning temperatures T = 4 0 0 - 6 0 0 ° C. On n-type GaAs small but reproducible variations of q~s around 0.6 eV were observed only at the first C s - O cycle (see closed circles in Fig. 1). The Fermi level on the n-type GaAs surface was essentially pinned under further alternate deposition of Cs and 0 2• The reversible switching of q~s proves that the position of the Fermi level on p-type GaAs(Cs,O) surface is controlled by adatom-induced rather than defect-induced surface states. The initial band bending q~s = 0.3 eV on a clean p-type GaAs surface is

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EL. Alperovich et al. / Surface Science 331-333 (1995) 1250-1255

determined by some initial donor-like surface states. This value is smaller than those obtained on clean surfaces of MBE-grown p-type (100)GaAs decapped from a protective As overlayer [27,28]. Probably, this is due to the different energy position of the initial surface donors on the clean surfaces prepared by the LTC technique. At small Cs coverages 0 < 0.1 M L the increase of q~s with increasing 0 is caused by the creation of isolated Cs-induced donor-like surface states [8-13], which lie higher in energy as compared to the initial surface donors. The electrons from these states are donated to the bulk of p-type GaAs. Consequently, the surface is charged positively until the Fermi level lines up with the energy of Cs-induced surface donors. At larger doses of Cs the Fermi level remains pinned by the surface donors. The decrease of q~s with increasing 0 is explained by the lowering of the energy of Cs-induced surface donors due to influence of surface dipoles and overlapping of electron wave functions of adsorbed Cs atoms [13]. Exposure to oxygen leads to the discharge of the surface due to oxidation of cesium. It is seen from Fig, 1 that as the number of C s - O deposition cycles increased, the band bending after oxygen exposure decreased down to % = 0.1 eV. One can conclude that some kind o f passivation of initial surface donors rather than creation of new defect-induced surface states takes place under successive C s - O deposition. The pinned behavior of band bending on n-type GaAs layers shows that a large number of initial acceptor-like surface states, which lie approximately 0.65 eV below the bottom of the conduction band, is present on a clean surface prepared by LTC. However, a small decrease of q~ by about 50 meV at Cs coverages 0 < 0.2 was observed, probably, due to partial compensation of the initial negatively charged acceptors by positively charged Cs-induced donors. This compensation proves that at small 0 the energy of Cs-induced donors is higher than that of initial acceptors. With increasing 0, the energy of Cs-induced donors decreases [13] and eventually goes below the acceptor levels. Consequently, the donors become neutral and q~ returns to its initial value, as is seen from Fig. 1. The pinning of the Fermi level on n-type GaAs shows that new surface acceptors, which lie below the initial acceptor levels, are not created under alternate deposition of Cs and 0 2, at

least at concentrations sufficient to move the Fermi level, We believe that the initial acceptor states are present on the surface of both n- and p-type GaAs prepared by LTC. On p-type GaAs the surface acceptors can limit the increase of q~s at the deposition of cesium, because the movement of the Fermi level above acceptor levels involves the charging of a large number of acceptor states. This argument is in accord with the experiment: the maximal value of q~s = 0.7 eV observed at room-temperature on p-type GaAs(Cs) [10] is most likely determined by the position of surface acceptors rather than by the position of Cs-induced surface states [7,8,13]. The presence of the initial surface acceptors, which pin the Fermi level on n-type GaAs surface and restrict the

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Band Bending (eV) Fig. 2. Photoreflectance amplitude ( A R / R ) o (top) and photolumincscence intensity IpL (bottom) as functions of band bending in the first three cycles of Cs and 02 deposition on the surface of p-type GaAs. Open triangles correspond to a clean surface, open circles correspond to the surface after Cs deposition, and closed circles correspond to the surface after 02 deposition. Each further cycle is shifted to the right along the horizontal axis for clarity. This shift is denoted by dashed horizontal arrows, which connect the last point of a previous Cs-O cycle with actually the same point showing the beginning of the next cycle. The solid arrows show the direction of the evolution of ( A R / R ) o and 1pu under cesiation and oxidation.

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V.L. Alperovich et aL/ Surface Science 331-333 (1995) 1250-1255

Fermi level movement on p-type GaAs surface, is the reason of differences between the results of our room-temperature experiments on chemically prepared surfaces and similar experiments performed on cleaved ( l l 0 ) G a A s surfaces at low temperatures [7,8]. It is seen from the middle and bottom of Fig. 1 that the PR amplitude and PL intensity vary reversibly together with the band bending under successive deposition of Cs and 0 2 on p-type GaAs. This behavior reflects reversible variations of the surface photovoltage [19,24-26] and effective recombination velocity [20]. One simple reason for these variations is the dependence of SPV and S on q~s, which should reveal itself even if the concentration and cross-sections of surface capture and recombination centers are fixed. This hypothesis qualitatively explains the experimental results: as seen from Fig. 1, ( A R / R ) o increases, and IpL decreases with increasing q~s, as they should according to theoretical considerations [19,20,24-26]. To study the question more thoroughly, in Fig. 2 we plotted the dependences of ( A R / R ) o and IpL on ~s for the first three cycles of C s - O deposition on p-type GaAs. The plot was made using the data shown in Fig. 1. For the first C s - O cycle a pronounced hysteresis is seen in both dependences. One can see that at a certain stage of Cs deposition, ( A R / R ) o decreased by almost one order of magnitude while the band bending remained practically constant. A vertical section of the plot IpL(q~s) shows an increase of photoluminescence at a constant value of q~. The observation of the hysteresis proves that the variation of the SPV and S are due not only to the variations of q~s, but also to some irreversible changes of the concentrations and, probably, cross-sections of the surface capture and recombination centers. One can also conclude that the calculation of SPV in a model of metal-semiconductor junction, which implies one-to-one dependence of SPV on q~s [24,25], is not sufficient to describe the situation on a surface with localized electronic states. It is seen from Fig. 2 that at the second C s - O cycle the hysteresis became less pronounced; almost one-to-one dependences were observed at the third cycle. This means that only reversible changes of the surface electronic properties take place when the surface is already covered with a layer of cesium oxide.

5. Conclusions In conclusion, the evolution of band bending, photovoltage, and recombination velocity under successive deposition of Cs and 0 2 on the surface of pand n-type GaAs epitaxial layers is studied in situ by photoreflectance and photoluminescence techniques. The behavior of band bending on p-type GaAs at increasing Cs coverage is in qualitative accord with the theoretical model [13] that describes the evolution of adsorbate-induced electronic surface states from an isolated adatom-induced surface state at low coverages to Schottky barrier formation at larger (monolayer or more) coverages. On n-type GaAs the Fermi level is pinned by a large number of initial acceptor-like surface states. These acceptor states limit the maximal band bending under deposition of cesium on the surface of p-type GaAs. For p-type GaAs the hysteresis in the dependences of the surface photovoltage and recombination velocity on the band bending was observed. This proves that complicated irreversible changes of concentration and properties of surface capture and recombination centers take place at the first cycles of C s - O deposition.

Acknowledgements The authors are grateful to N.A. Yakusheva and N.S. Rudaya for supplying epitaxial GaAs structures. This work was partly supported by the Russian Foundation for Fundamental Research under Grant No. 93-02-15177 and by the Scientific Program "Russian Universities" under grant 3H-354.

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