The determination of interface state energy distribution of the H-terminated Zn/p-type Si Schottky diodes with high series resistance by the admittance spectroscopy

Abstract

Analysis of Zn/p–Si Schottky diodes (SDs) with high resistivity has been given by admittance spectroscopy. The importance of the series resistance in the determination of energy distribution of interface states and especially their relaxation time in the SDs with high resistivity has been considered. The effect of the series resistance on capacitance–conductance/frequency characteristics has been given by comparing experimental data with theoretical data. The interface state density N_ss from the admittance spectroscopy ranges from 1.0×10¹² cm⁻² eV⁻¹ in 0.720-E_v eV to 2.03×10¹² cm⁻² eV⁻¹ in 0.420-E_v eV. Furthermore, the relaxation time ranges from 4.20×10⁻⁵ s in (0.420-E_v) eV to 3.20×10⁻⁴ s in (0.720-E_v) eV. It has been seen that the interface state density has a very small distribution range (1.0–2.03×10¹² cm⁻² eV⁻¹) that is ascribed to the predominant termination with hydrogen of the silicon surface after HF treatment.

Introduction

Metal–semiconductor (MS) contact is one of the most widely used rectifying contacts in electronics industry [1], [2], [3]. Due to the technological importance of the Schottky barrier diodes (SBDs) which are of the most simple of the MS contact devices, a full understanding of the nature of their electrical characteristics is of greater interest. Many SBDs are not intimate MS contacts but have, instead, metal-interfacial layer-semiconductor (MIS) structure unless specially fabricated. Therefore, the nonideal behavior observed in electrical characteristics of SBDs has been generally attributed to the effect of interface layer properties [2], [3], [4], [5], [6], [7]. On the other hand, the series resistance R_s (between the depletion region and ohmic contact) of the neutral region of the semiconductor bulk also plays an important role in capacitance–voltage (C–V), current–voltage (I–V), capacitance–frequency (C–f) and conductance–frequency (G–f) characteristics of SBDs, and it causes that the interface state density N_ss and their relaxation time obtained from admittance spectroscopy become different from those that would be expected [8], [9], [10], [11], [12]. Furthermore, some investigations [8], [9], [10] have reported an anomalous peak in forward C–V characteristics. The origin of such peaks has been ascribed to the interface states [7] and to the series resistance effect [8], [9], [10], [11]. It has been seen that peaks in the forward C–V characteristics appear because of the series resistance.

In general, the C–f and G–f plots in the idealized case are frequency-independent [10], [11], [12], [13], [14], [15], [16], [17]. However, this idealized case is often disturbed due to the presence of an interfacial layer between MS, interface states at the interfacial layer and semiconductor interface [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], imperfect ohmic back-contacts (and therefore minority carrier injection effect) [17], [18], and series resistance of the neutral region of the SBDs [8], [9], [10], [11], [12]. Owing to the above effects or non-idealities, the C–f plots differ significantly from that expected for an ideal Schottky contact.

In the present study, the characteristic parameters of the Zn/p-Si SBDs terminated with hydrogen by HF dips were calculated by using the forward bias I–V, C–f and G–f measurements, that is, by the admittance spectroscopy. The admittance spectroscopy gives the important information about the density or energy distribution of the interface states and their time constants. This method accounts for the interfacial layer capacitance. The experimental C–f and G–f data were fitted to theoretical equations with the consistent interface state and their time constant values by taking into account the series resistance and interfacial layer capacitance of the devices.

For the preparation of the H-terminated Zn/p-Si SBDs, the previously polished p-type Si wafer was cleaned by using the traditional RCA clean with the final dip in diluted HF for 30 s. The RCA cleaning procedure: 10 min boil in NH₄OH+H₂O₂+6H₂O followed by a 10 min boil in HCl+H₂O₂+6H₂O. The RCA clean with HF dip shows a predominant coverage of the surface with hydride groups. Grundner and Jacob [19] have showed that the RCA cleaning with “HF” dip results in a hydrophobic surface. In the same way, Gräf et al. [20] have reported that, immediately after the HF treatment, high resolution electron energy loss spectroscopy (HREELS) shows a predominant coverage of the surface with hydride groups. Pietsch et al. [21] have found that after HF etching the surface is predominantly terminated by various hydrides. This hydrogen termination is well established and the fluorine termination can be removed efficiently by a short final rinse with water and is replaced by OH groups. The predominant termination with hydrogen is responsible for a variety of remarkable properties of the silicon surface after HF treatment. Chemically, H-terminated surface behaves completely hydrophobic and is passivated against reoxidation that allows a short handling time before the regrowth of oxide. Electronically, it is remarkably inactive with a largely reduced density of surface states in Si energy band gap due to the covalent satisfaction of all surface bonds. The surface after HF treatment is unreconstructed with an undisturbed bulk-like arrangement of surface atoms [21]. Heating to about 850°C for 2 or 5 min leads to desorption of hydrogen [21], [22]. Furthermore, Kampen and Mönch [22] have also indicated that such treatments remove the native oxide layer and result in H-terminated clean Si surfaces; that is, Si atoms at the cleaned Si wafer surface are terminated by hydrogen. Again, as mentioned in Ref. [23], silicon surfaces may be easily terminated with hydrogen by HF dips. As known, the intrinsic surface states present at the semiconductor–vacuum interface before contact with the metal are an important factor in Schottky barrier formation or Fermi level pinning at the interface. Briefly, hydrogen is well known to saturate the dangling band of the Si surface [19], [20], [21], [22], [23], [24], [25].