n-Si structures

ELSEVIER

Thin SolidFilms276 (1996) 183-186

Electronic properties of thin Au/nanoporous-Si/n-Si structures Th.'Dittrich a, K. Kliefoth a, I. Sieber a, j. Rappich a, S. Rauscher a, V.Yu. Timoshenko b a Hahn-Meitner.lnstitut, Rudower Chaussee 5, D-12489 Berlin, Germany b Moscow State University, Department of Physics, 119899 Moscow, Russia

Abstract Ultrathin nanoporous Si layers (UPSL) were prepared on n-Si(100) by anodization in aqueous NH4F solution starting from an electrochemically hydrogenated surface. The thickness of the UPSL was controlled with field emission scanning electron microscopy. The interface state density (/)it) of UPSL was measured with a field-dependent pulsed surface photovoltage technique. The value of/)it normalized to the surface area of UPSL is about 1.3 x 10~l eV-~ cm -2. Au/UPSL/n-Si structures were characterized with temperature-dependent currentvoltage measurements. The room-temperaturebarrier height and the ideality factorat the Au/UPSL interface were 0.75 eV and 1.8, respectively. The temperature dependence of the reverse current of A u l U P S L I n - S i structures showed two regions with activation energies at 120 meV and about 60 meV for temperatures below and above 200 K, respectively. Strong near-infrared electroluminescence was observed for Au/UPSL/ n-Si structures. The results are discussed on the basis of the role of Si nanostructure surface conditioning with regard to the porous Si electronic properties. Keywords: Nanostructures;Silicon;Electrochemistry;Surfaceand interfacestates

1. Introduction Barrier heights (tpa) at porous Si (por-Si) interfaces and recombination centres influence the conditions for carrier injection and radiative or non-radiative recombinatiG~'~. They are important for the electronic properties of possible photonic devices such as light-emitting diodes (LEDs). Por-Sibased LEDs have been fabricated, for example, on ITO/ por-Si/p-Si [1] or Au/por-Si/n-Si [2] structures. These structures showed rectifying behaviour and electroluminescence (EL) under forward bias. The large ideality factors of the current-voltage characteristics (n > 3) were interpreted in Ref. [ 1] with an additional voltage drop in the por-Si layer due to charging of interface states. The respective estimated value of the interface state density (Dit) was 9.6 × 10 ~3eVc m - 2 [ 1 ]. The EL efficiency could be increased strongly by filling pores with In or AI in a certain spatial region of the por-Si layer [2]. The por-Si/p-Si contact is rectifying [3,4]. Measurements with a large signal photovoltage (SPV) showed that the potentials at the por-Si/p-Si and por-Si/n-Si interfaces are about +0.47 and -0.05 V [5]. It. should be remarked that the bands at the hydrogenated c-Si surfaces are nearly flat for n-Si and inverted for p-Si [6,7 ]. We think that hydrogenated Si nanostructure surfaces determined the potential at the por-Si/c-Si interface. The Au/mesopor-Si interface forms a rectifying contact (tt~ = 0.74 eV, n = 3.2) 0040-6090/96/$15.00 © 1996ElsevierScienceS.A.All rightsreserved SSD! 0040 -6090 ( 95 ) 08087-2

[8]. The electrical transport in por-Si is electric-field enhanced and governed by a Poole-Frenkel mechanism [9]. The ambipolar diffusion length of mesopor-Si ranges from 30 to 90 nm [10]. Very thin por-Si layers with efficient passivated Si cluster surfaces would be important for further improvement of por-Si device properties. The preparation of ultrathin por-Si layers (UPSL) on n-Si was recently shown for potential-controlled anodization in aqueous NH4F solutions [ 11 ]. This work aims at characterizing electronic properties of UPSL prepared in aqueous NH4F solutions. The density of interface states, the characteristics of the Au/UPSL contact and radiative recombination were investigated with SPV, current voltage I(U) and EL measurements, respectively.

2. Experimental Wafers of n-Si(100) (4.5 fl cm resistivity) were precleaned in HNO3:H202:H20 (1:1:6) for 1 min and treated in 1 M NH4F (pH 4.0) for I min. AI back-contacts were sputtered on the wafers after precleaning. The electrochemical preparations were carried out in a teflon cell sealed with a viton ring of 1 inch diameter to the wafer serving as a working electrode. APt ring and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively.

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T.h. Dittrich et al. /Thin Solid Fihns 276 (1996) 183-186

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The pH of the 0.2 M NH,:F electrolyte was adjusted to 3.2 with H2SO4 and controlled with a calibrated pH meter (Knick). All solutions were made from analytical graded chemicals. The por-Si preparation started with electrochemical hydrogenation carried out in the same manner as described recently for n-Si(111) surfaces [ 12] with control of the dark current transient [ 13]. Por-Si was formed under illumination (halogen lamp, 10 mW cm- 2) at low potentials. Fig. 1 shows the dependence of the photocurrent on the applied voltage of n-Si(100) in 0.2 M NH4F electrolyte (pH 3.2). Por-Si forms in the potential region below the first photocurrent maximum [ 14 ]. The por-Si layer thickness was adjusted with the value of the flowed charge [ 11 ]. The asanodized UPSL was free ofoxidic species (Fourier transform infrared investigations [ 15] ). Au dots with 1 mm diameter and semitransparent I mm × 5 mm contacts were evaporated on as-prepared UPSL for I(U) and EL measurements. Field emission scanning electron microscopy (HITACHI $4100) was used for the control of the thickness of the UPSL. The SPV measurements [ 16-18] were carried out after electrochemical hydrogenation of n-Si and after UPSL preparation in the same experimental configuration as described recently [6]. The SPV pulses were excited with a laser diode (wavelength, 902 nm; pulse length, 100 ns; and power, 150 W) and detected with a TEKTRONICS transient recorder via an SnO2 electrode divided by a mica spacer from the wafer surface. I(U) measurements were carried out with a KEITHLEY voltage source 230 and electrometer 617 in the temperature range from 100 to 325 K. EL was investigated with Si and InGaAs photodetectors with high-impedance pre-amplifiers (EMM) and with a SPM-2 monochromator (SiO2 prisma).

3, Results and discussion Fig. 2 shows the distribution of/)it for the electrochemically hydrogenated n-Si(100) surface and for the n-Si(100) surface covered with a 20 nm thick UPSL. The distributions of D=t have a minimum about E-E~ = 0.25 eV (E~ is the Fermi-level position of intrinsic Si). The values of D~=in the

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minimum are about 1 x 10 '~ and 2 x 10 t2 eV- i cm-2 for the hydrogenated and for UPSL/n-Si(100). It can be expected that all surface states of the por-Si clusters are coupled to the Si bulk for the UPSL. The surface area of por-Si is very large and ranges up to 600 m 2 cm-3 for nanoporous Si [ 19] and up to 230 m 2 cm -3 for mesoporous Si [20]. According to these values the value of D. of the UPSL can be estimated and amounts about 1.3× l0 ~' eV -~ cm -2. It can be concluded that the electronic passivation of the Si cluster surfaces is comparable with that of electrochemically hydrogenated n-Si surfaces. The origin of the electronic states at the hydrogenated Si surface and at the Si nanostructure surfaces of the UPSL is not very clear up to now. Dangling bonds (DB) can be excluded as a possible reason for the investigated surface states since the DB concentration per unit area of an asanodized por-Si film [ 21 ] is nearly 1.5 orders of magnitude lower than the concentration of rechargeable centres measured in this work. Fig. 3 shows I(U) curves of Au/n-Si(100) and Au/ UPSL/n-Si(100) at room temperature. The thickness of the UPSL was 60 nm. Both structures are rectifying. The I(U) curve ofAu/n-Si(100) is determined by thermionic emission and the values of tpa and n can be obtained by using the well-

T.h. Dittrich et al. /Thin Solid Films 276 (1996) 183-186

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known equations in Ref. [ 22]. The values of q~ and n are 0.75 eV and 1.2 for Au/n-Si(100). Thermionic emission and surface recombination processes contribute dominantly to the I(U) curve [22] of Au/UPSL/n-Si(100). The thermionic emission dominates in the voltage region from 0.4 to 0.6 V. The values of ff'n and n are 0.75 eV and 1.8 for Au/UPSL/ n-Si(100). These values characterize even the Au/UPSL contact since the bands at the UPSL/n-Si(100) junction were flat (proven by photovoltage). Please note the unexpectedly low value ofn was probably caused by the excellent hydrogen passivation of the as-prepared UPSL as shown above. The temperature dependence of the I(U) curves was rather complicated and quite different for Au/n-Si(100) and Au/ UPSL/n-Si(100). A detailed analysis of the temperature dependence is still in progress. The situation is less complicated for the temperature dependence of the reverse currents being a superposition of carrier generation and field enhanced carrier transport through the por-Si layer [9]. The Arrhenius plots of the reverse currents at U = - 1 V given in Fig. 4 are characterized by an ensemble of activation energies. The activation energy (EA) of the Au/n-Si(100) decreased with decreasing temperature in contrast to Au/UPSL/n-Si(100) for which EA increases with decreasing temperature. Activation energies decreasing with decreasing temperatures are known from generation/recombination processes in amorphous Si and can be related to a temperature-dependent shift of the equilibrium energy level between recombination and generation from tail states [ 23 ]. The temperature dependence of the reverse current of Au/UPSL/n-Si(100) contains mainly two activation energies at 120 meV and about 60 meV for temperatures below or above 200 K, respectively. The reason for the increase of EA of the reverse current with decreasing temperature should be connected with the temperature-dependent influence of additional barriers between neighboured or undulated Si nanostructures on the transport in UPSL. Strong and stable EL was observed on Au/UPSL/nSi(100)/A! structures. Fig. 5 shows the I(U) curve and the potential dependence of the EL intensity from - 20 V to + 20 V. The I(U) curve is resistor like in forward bias while thermal effects decreased slightly the resistivity for high volt-

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age. EL set on at about 2 V. The investigated device emitted light in the near-infrared region (see the inset of Fig. 4) with two broad peaks at about 1 200 and 1 500 nm. The corresponding photon energies are known as the starting point from the so-called Vis and IR photoluminescence bands [ 24 ]. We think that intrinsic disorder-induced surface states at Si nanostructures [25] are responsible for the infrared EL of UPSL. It should be remarked that UPSL oxidized in air also exhibit red EL [26]. Finally, we have shown that ultrathin por-Si layers with very low Dit c a n be prepared in aqueous NH4F solutions. Possible application potentials of such por-Si layers were demonstrated in the cases of Schottky barriers and LED structures. Acknowledgements

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