Electrical conductivity and minority carrier diffusion in thermally oxidized PbTe thin films

ARTICLE IN PRESS Physica B 405 (2010) 1058–1061

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Electrical conductivity and minority carrier diffusion in thermally oxidized PbTe thin films E. Shufer a, Z. Dashevsky a,, V. Kasiyan a, E. Flitsiyan b, L. Chernyak b, K. Gartsman c a

Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel Physics Department, University of Central Florida, Orlando, Florida 32816-2385, USA c Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel b

a r t i c l e in f o

a b s t r a c t

Article history: Received 25 August 2009 Received in revised form 3 October 2009 Accepted 4 November 2009

Thermally oxidized 0.1 and 1 mm thick n-type PbTe:In films were studied in this work. Two main processes induced during the thermal treatment in oxygen atmosphere were identified. These are the formation of an oxide phase on the surface and generation of acceptor states of oxygen along grain boundaries inside a film. The latter process causes inversion of the type of electrical conductivity in PbTe from n to p. Electron beam-induced current (EBIC) measurements of minority electron diffusion length in oxidized 0.1 mm thick PbTe:In film showed diffusion length increase with increasing temperature similar to the wide band gap semiconductors. & 2009 Elsevier B.V. All rights reserved.

Keywords: Lead telluride film Thermal oxidation Transport properties EBIC effect

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . Experimental details . . Results and discussion. Conclusions . . . . . . . . . Acknowledgments . . . . References . . . . . . . . . .

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1. Introduction Lead chalcogenides (LC) is a group of semiconductors which possesses a narrow direct energy band. This makes them attractive for the production of thin film IR-detectors for various applications [1,2]. Vacuum evaporated PbSe films have been reported to demonstrate photosensitivity after thermal treatment at oxygen containing ambience, which also led to the appearance of oxides such as—PbO, PbO2, PbSeO3 [3,4]. The oxygen in the oxide phases is ‘‘tightly bound’’ (T-oxygen). Photosensitive films of lead chalcogenides also contain weakly bound excess oxygen (W-oxygen) which is oxygen atoms that flowed through preferred diffusion paths at grain surfaced during elevated temperature

 Corresponding author. Tel.: + 972 86472573; fax: + 972 8 6472946.

E-mail address: [email protected] (Z. Dashevsky). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.11.004

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thermal treatment. These excessive oxygen atoms, due to their acceptor nature in LC, function as electron acceptor sights, and cause the appearance of potential barriers at the grain boundaries (GBs). Neustroev and Osipov offered a model that explains qualitatively and quantitatively the electrical and photo-electrical behavior of polycrystalline oxidized LC films [1,5,6]. According to the model, the oxygen-generated acceptor states at the GBs of n-type grains capture electrons from the interior of grains creating p-type inversion layers (ILs) at the grain boundaries. When voltage bias is generated on such a film, the only mobile charge carriers are holes, which are the minority charge carriers in the originally n-type film. This creates an integral p-type behavior of a film grown from an n-type source. This effect is known as the conductivity type inversion (CTI). In this paper, we study electrical conductivity and minority carrier diffusion in thermally oxidized PbTe films with the main focus on the oxide phase of this semiconductor.

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2. Experimental details The n-type PbTe doped with In (PbTe:In) thin films used in this research were prepared by physical vapor deposition (PVD) (using the electron gun) on 100 mm thick pseudo-crystalline polyimide substrate. The main advantages of the polyimide substrate are related to a small value of Young’s modulus, which prevents creation of micro-cracks due to sample cooling, and transparency in the mid-IR range. Unique properties of PbTe:In are in the effect of Fermi level pinning induced by doping [7]. This effect results in a stable free carrier concentration independent of doping level fluctuations in the sample. For the In-doped lead telluride, the Fermi level is pinned at 70 meV above the conduction edge, giving rise to the electron concentration of 5  1018 cm  3 at T= 4.2 K. Thermal oxidation of PbTe films was carried out in a reactor which is consisted of a resistance-heated furnace and a cylindrical stainless-steel tube containing the samples. The oxidation temperature was in the range from 250 to 400 1C. The experimental setup was computer-controlled with temperature accuracy within 71 1C. The chemical and phase composition, structure and surface morphology of the starting and annealed PbTe films were investigated using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). The type of conductivity was evaluated by the local thermal probe method (Seebeck coefficient measurements) [8]. Nickel contacts, prepared by thermal evaporation technique, were used for electrical measurements. The current–voltage characteristics measurements on these contacts confirmed their Ohmic nature. Hall effect measurements were carried out at the magnetic field of 0.8 T over 4.2–300 K temperature range in Janis Research cold-finger cryostat SH-4-1 using a closed cycle refrigerator RDK 101D. The photoresponse was studied at the same temperature range under excitation with a GaAs light emitting diode (LED). Variable temperature electron beam-induced current (EBIC) experiments were performed at 5 kV accelerating voltage, in-situ in Phillips XL30 scanning electron microscope fitted with a hot stage and an external temperature controller [9]. A relatively low electron beam accelerating voltage insured excitation within PbTe films, but not in the underlying substrate [9 and references therein].

3. Results and discussion XRD analysis of PbTe:In film deposited on polyimide substrate revealed a texture with the axis in [1 0 0] direction perpendicular to the substrate. AFM images of PbTe films display a granular structure with the grain size ranging from 200 to 300 nm and the root mean square surface roughness from 10 to 15 nm. Annealing of PbTe films leads to the formation of oxide phases with composition and distribution over the film thickness dependent on the annealing temperature and duration (Fig. 1). The oxidation product that was formed on the PbTe surface is close to that of PbTeO3 [10]. The oxidized layer thickness is up to 100 nm and it practically does not increase when the annealing duration exceeds 1 h. It is well known that during the early stages of oxide growth, a surface reaction is the rate limiting oxidation factor and the oxide thickness varies linearly with time. As the oxide layer becomes thicker, an oxidant must diffuse through it to react at the oxidePbTe interface. The reaction becomes then diffusion limited. The oxide thickness after duration of oxidation, tO , is given by [11] sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2DCo x ð1Þ tO ; C1

Fig. 1. XPS component analysis of PbTe:In films versus distance from the film surface for various oxidation durations, tO, at 400 1C. 1, 2—oxygen, 3, 4—lead, 5, 6—tellurium. 1, 3, 5—sO = 1 h; 2, 4, 6—sO = 3 h.

where D is the diffusion coefficient of the oxidizing species; Co is the equilibrium concentration of the species (oxygen) at the oxidation temperature; C1 is the number of molecules of the oxidizing species in a unit volume of the oxide (PbTeO3). The oxide thickness was estimated based on the distance (from the surface into the material) at which the oxygen concentration falls below 3 at %. Accounting for the experimental dependence of the oxide layer thickness on oxidation temperature and duration, the diffusion coefficient of oxygen in oxidized PbTe was estimated as D=5.5  10  6 exp( 0.45/kT). Here k is the Boltzmann constant, and T is the absolute temperature. The measured initial (prior to oxidation) room temperature value, S =  190 mV/K, for Seebeck coefficient of 1 mm thick PbTe:In film indicates n-type conductivity. Hall effect measurements showed electron concentration on the level of n =5  1018 cm  3 at 4.2 K. The same type of electrical conductivity was observed for 0.1 mm thick PbTe:In film prior to oxidation (S=  260 mV/K). Higher S value for a thinner film can be explained by ambient oxygen (acceptor in PbTe) absorption on PbTe surface (the impact of this additional absorption is greater for a thinner film). The inversion of the electrical conductivity type as a function of oxidation duration was demonstrated by positive Seebeck coefficients at 300 K for two groups of PbTe films with the thickness of 0.1 and 1 mm (Fig. 2). According to the model suggested in [1,3,4], oxygen diffuses along the grain boundaries during the thermal treatment and generates acceptor states on the surface of n-type grain, which are able to capture electrons from the grain interior. This gives rise to p-type inversion layers at the grain boundaries and, as a result, the current flow in the film is exclusively due to the drift of holes in the inversion channels along the grain boundaries. Schematic view of oxidized PbTe film is shown in inset of Fig. 2. Because our goal was to study fully oxidized PbTe films, and since the XPS data in Fig. 1 indicate the presence of oxygen down to 100 nm from the surface, the majority and minority carrier transport measurements, explained below, were solely focused on the 0.1 mm thick films. In EBIC testing, employed for the minority carrier diffusion length measurements, we used a built-in electric field present at the grain boundaries for charge collection (see Ref. [9] for details).

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Fig. 2. Seebeck coefficient for PbTe:In films as a function of oxidation duration at 400 1C. 1—1 lm film thickness. 2–0.1 lm film thickness. Inset: Schematic view of the PbTe film after oxidation.

Fig. 3. Temperature dependence of conductivity of PbTe:In films taken in darkness (solid symbols) and under illumination (open symbols). 1—as grown film; 2—after oxidation during 0.25 h; 3—after oxidation during 0.5 h.

After tO Z0.25 h, the Hall coefficient for these films is positive and practically constant at all temperatures (4.2–300 K) and the hole concentration is on the level of pE2  1018 cm  3. The electrical conductivity, s in darkness is shown in Fig. 3 as a function of temperature. For the PbTe film prior to oxidation, s decreases with increasing temperature due to electron scattering on lattice vibrations. This temperature dependence, however, is weaker than that for bulk PbTe crystals due to additional carrier scattering on grain boundaries [1,12]. For oxygen treated PbTe films the electrical conductivity is thermally activated above 150 K and increases sharply with temperature in agreement with

s  expðEa =kTÞ;

ð2Þ

upon oxidation of PbTe films activation energy Ea, estimated from (2) is about 3875 meV for sample 2 (Fig. 3) and then decreases with increasing oxidation time, Ea = 2475 meV for sample 3. The

Fig. 4. Experimental dependence of minority carrier diffusion length on temperature in 0.1 mm thick PbTe film after 0.5 h oxidation. Inset: Ahrenius plot of L vs. T.

first increase of activation energy corresponds to the formation of a non-homogeneous structure of oxide phase along the grain boundaries inside a PbTe film. The increase of oxidation time leads to homogenization and formation of a wide-gap semiconductor phase corresponds to the decrease of activation energy. The electrical conductivity stabilizes and even shows a slight increase below 150 K due to hopping conductivity and tunneling [1]. Under illumination (Fig. 3) for n-type films the photoresponse appears only at temperatures lower than 25 K like in PbTe(In) single crystals due to transition electrons from indium impurity level to the conduction band [13]. For p-type films after oxidation a noticeable photoreponse observed at temperature beginning from room temperature due to the heterojunction structure [1,4]. EBIC measurements revealed that the diffusion length, L, of minority electrons in the p-type 0.1 mm thick oxidized PbTe:In film increases with increasing temperature, T. Similar trends were previously observed in GaN [14] and ZnO [15], and this provides an additional evidence for transition from the narrow band PbTe semiconductor into the wide band gap PbTeO3 semiconductor. The diffusion length increase was modelled (cf. Fig. 4) with the following expression [14]:   EA;T ; L ¼ L0 exp  2kT

ð3Þ

where L0 is a scaling factor, EA,T is the thermal activation energy. Inset of Fig. 4 shows the Ahrenius plot for L vs. T in 0.1 mm thick PbTe:In film after 0.5 h oxidation. The fit using Eq. (3) yields the activation energy of 365 710 meV. The latter parameter represents carrier delocalization energy, since it determines the increase of the diffusion length due to reduction of recombination efficiency [15]. The temperature-induced increase of L may be attributable to the growing lifetime of non-equilibrium minority electrons in the conduction band due to a smaller recombination capture cross section for these carriers at elevated temperatures [15]. The value of activation energy, which is comparable to that in wide band gap p-ZnO semiconductor [16], is another indication for a transition from the PbTe phase to the oxide phase.

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4. Conclusions Oxidation of polycrystalline PbTe:In films leads to nonhomogenous formation of the oxide (PbTeO3) phase on the surface and generation of oxygen acceptor states on grain boundaries. Temperature dependent EBIC measurements in oxidized thin layer allowed to estimate the minority carrier diffusion length which is 1.7 70.1 mm at T=273 K and increases with increasing temperature that is typical for wide band semiconductor.

Acknowledgments This research was partially supported by the Israeli Ministry of Defense and NATO (SfP 981939; CLG 983122). References [1] Z. Dashevsky, High photosensitive films of lead chalcogenides, in: A.A. Balandin, K.L. Wang (Eds.), Handbook of Semiconductor Nanostructures and Devices, American Scientific Publishers, 2006 (Chapter 11).

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