Study of metal-semiconductor interface states using deep level transient spectroscopy

This paper describes the principle of the determination of interface-state parameters by deep level transient spectroscopy (DLTS) and presents a new, simple and exact method to discriminate the DLTS signal due to the emission from interface states from that from bulk traps. The n-type Au-GaAs and Cr-GaAs interfaces have been investigated by the technique. The results obtained in the investigation have revealed the dependences of the energy position, density and capture cross section for the interface states on the metal deposited onto the semiconductor surface, which is consistent with the theoretical prediction by Yndurain and the experimental results obtained by other authors.     

A deep level transient spectroscopy study on the interface states across grain boundaries in multicrystalline silicon

Degrading the recombination activities of grain boundaries (GBs) is essential to improve the efficiency of multi‐crystalline silicon (mc‐Si) based solar cells. We apply the deep level transient spectroscopy technique to detect interface states at Σ3 and Σ9 GBs in mc‐Si. The density of interface states close to midgap is found comparable for both as‐grown GBs. Gettering or hydrogenation leads to shallower states with a smaller capture cross section and lower density. Recombination activity reduction for Σ3 GBs is stronger than for Σ9 GBs especially after hydrogenation. Both the analysis approach and experimental results could be applied for a specific GB engineering of mc‐Si based solar cells. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Energy band alignment at ferroelectric/electrode interface determined by photoelectron spectroscopy

The most important interface-related quantities determined by band alignment are the barrier heights for charge transport, given by the Fermi level position at the interface. Taking Pb(Zr,Ti)O3(PZT) as a typical ferroelectric material and applying X-ray photoelectron spectroscopy(XPS), we briefly review the interface formation and barrier heights at the interfaces between PZT and electrodes made of various metals or conductive oxides. Polarization dependence of the Schottky barrier height at a ferroelectric/electrode interface is also directly observed using XPS.     

A study of interface characteristics in HfAlO/p-Si by deep level transient spectroscopy

Deep level transient spectroscopy (DLTS) and high-frequency capacitance-voltage (HF-CV) measurement are used for the investigation of HfAlO/p-Si interface. The so-called “slow” interface states detected by HF-CV are obtained to be 2.68 × 10¹¹ cm⁻². Combined conventional DLTS with insufficient-filling DLTS (IF-DLTS), the true energy level position of interfacial traps is found to be 0.33 eV above the valance band maximum of silicon, and the density of such “fast” interfacial traps is 1.91 × 10¹² cm⁻² eV⁻¹. The variation of energy level position of such traps with different annealing temperatures indicates the origin of these traps may be the oxide-related traps very close to the HfAlO/Si interface. The interfacial traps’ passivation and depassivation effect of postannealing in forming gas are shown by comparing samples annealed at different temperatures.     

Photoelectron spectroscopy of Ar/Cu(100) interface states

Buried interface states in Ar/Cu(100) were studied by means of one- and two-photon photoemission experiments. With increasing Ar overlayer thickness, a transition from broad electron scattering resonances in the Ar conduction band into a hydrogen-like series of quasi-bound states at the Ar/Cu interface was observed. The thickness dependence of energies and lifetimes is compared to theoretical resonance positions and linewidths derived from a parameterized one-dimensional potential. PACS 73.20.-r; 73.40.Ns; 79.60.-i; 78.47.+p     

High-sensitivity transient spectroscopy using tunable diode lasers

Experimental techniques have been developed to monitor transient infrared absorptions using lead-salt tunable diode lasers. The techniques are easily implemented, yield sensitivities which are limited by detector noise at 10^–5 level of absorbance, and have a response time on the order of one microsecond. The transient absorption detection techniques are high frequency versions of the sweep integration technique pioneered by Jennings [Appl. Opt.19, 2695 (1980)]. TDL modulation rates of 100 kHz and 500 kHz allow for absorption sampling rates of 200 kHz and 1 MHz, respectively. In order to reproducibly achieve near-detector-noise-limited sensitivities for 100 kHz TDL modulation rates, an automated analog subtraction circuit has been developed which removes the effects of minor TDL power variations. At the 500 kHz modulation rate, digital filtering techniques are used to remove the effects of this power variation.     

Deep-level spectroscopy by analysis of isothermal capacitance transients

Summary Deep-level spectrometry of semiconductor has been carried out by measuring isothermal capacitance transients. From the recorded transients, DLTS signals have been reconstructed by software to give Arrhenius plots. Transients have been acquired in two time ranges to increase the emissivity interval on which to observe the trap decay. The method has been tested on Mo/GaAs Schottky barriers and a comparison has been carried out with results obtained on the same diodes using a commercial DLTS system. The authors of this paper have agreed to not receive the proofs for correction.     

Improvement in the determination of accuracy of enthalpy of ionization from the isothermal deep level transient spectroscopy

In this paper we propose to use a combination of a modified isothermal Deep Level Transient Spectroscopy (IDLTS) method with optical excitation and a filtering method of multi-exponential decays for IDLTS signal represented by a nonexponential transient capacitance. Excellent agreement with published results is achieved using this method for the investigation of a hole trap response (HL4) interfering with other traps in Schottky barriers that were fabricated on an epitaxial GaAs layer grown by VPE on bulk N-GaAs.The authors express sincere thanks to I. Thurzo and F. Dubecký of the Slovak Academy of Sciences who provided samples for these experiments and for useful advice.     

Influence of surface states on deep level transient spectroscopy in AlGaN/GaN heterostructure

Deep level transient spectroscopy(DLTS) as a method to investigate deep traps in AlGaN/GaN heterostructure or high electron mobility transistors(HEMTs) has been widely utilized.The DLTS measurements under different bias conditions are carried out in this paper.Two hole-like traps with active energies of E_v + 0.47 eV,and E_v + 0.10 eV are observed,which are related to surface states.The electron traps with active energies of E_c-0.56 eV are located in the channel,those with E_c-0.33 eV and E_c-0.88 eV are located in the AlGaN layer.The presence of surface states has a strong influence on the detection of electron traps,especially when the electron traps are low in density.The DLTS signal peak height of the electron trap is reduced and even disappears due to the presence of plentiful surface state.     

Absolute atomic densities determined by auger electron spectroscopy

The secondary electron distributionN(E) obtained with a spherical grid retarding field analyser is stored in a multichannel analyser. The experimental intensities of Auger lines are accurately determined by numerically substracting the background in theN(E) distribution and taking the area under the resulting peaks. Broadening of the lines due to several experimental factors, the multiple structure of the lines and the characteristics energy losses are taken into account. The absolute atomic densities on the surface are deducted from the Auger line intensities by a simple theoretical model. A comparison is made with atomic densities on the surface which are known either from the crystal structure (cleaved muscovite) or from Rutherford backscattering experiments (thin layer of Ag) or simply from the specific weight in the case of bulk materials (C, Cu, Ag). The maximum deviation is smaller than a factor of 2. Generally, the values differ by about 30%, which shows that AES, performed in this way, can give reliable quantitative results for densities ranging from a fraction of a monolayer to the bulk material.     

Method of observation of low density interface states by means of X-ray photoelectron spectroscopy under bias and passivation by cyanide ions

X-ray photoelectron spectroscopy (XPS) measurements under bias can observe low density interface states for metal-oxide-semiconductor (MOS) diodes with low densities. This method can give energy distribution of interface states for ultrathin insulating layers for which electrical measurements cannot be performed due to a high density leakage current. During the XPS measurements, a bias voltage is applied to the rear semiconductor surface with respect to the ∼3 nm-thick front platinum layer connected to the ground, and the bias voltage changes the occupation of interface states. Charges accumulated in the interface states shift semiconductor core levels at the interface, and thus the analysis of the bias-induced shifts of the semiconductor core levels measured as a function of the bias voltage gives energy distribution of interface states. In the case of Si-based MOS diodes, the energy distribution and density of interface states strongly depend on the atomic density of silicon dioxide (SiO₂) layers and the interfacial roughness, respectively. All the observed interface state spectra possess peaked-structures, indicating that they are due to defect states. An interface state peak near the Si midgap is attributable to isolated Si dangling bonds at the interface, while those above and below the midgap to Si dangling bonds interacting weakly with Si or oxygen atoms in the SiO₂ layers. A method of the elimination of interface states and defect states in Si using cyanide solutions has been developed. The cyanide method simply involves the immersion of Si in KCN solutions. Due to the high Si-CN bond energy of ∼4.5 eV, the bonds are not ruptured at 800 °C and upon irradiation. The cyanide treatment results in the improvement of the electrical characteristics of MOS diodes and solar cells.     

Femtosecond high-sensitivity, chirp-free transient absorption spectroscopy using kilohertz lasers

A simple method is demonstrated for high-sensitivity, chirp-free measurements of femtosecond (fs) transient absorption over the entire bandwidth of a white-light continuum probe. This technique uses phase-sensitive detection, with spectral scanning and simultaneous adjustment of the time delay between pump and probe pulses; it permits a direct measurement of spectra undistorted by chirp at all time scales, limited only by the resolution of the fs source. The method is applied to study the ultrafast relaxation dynamics of pi-conjugated oligomers and semiconductor nanocrystals.     

Visible-super-resolution infrared microscopy using saturated transient fluorescence detected infrared spectroscopy

A scanning visible-super-resolution microscope based on the saturation behaviour of transient fluorescence detected infrared (TFD-IR) spectroscopy is proposed. A Gaussian IR beam, a Gaussian visible beam and a Laguerre-Gaussian (LG) visible beam are used to obtain two separate two-color excitation fluorescence (2CF) images of the sample. The final image is obtained as the difference between the two recorded images. If the peak intensity of the LG beam is high enough to induce saturation in the fluorescence signal, the image can, in principle, have unlimited spatial resolution. A ∼3-fold improvement in transverse resolution over the visible diffraction limit (and far exceeding the IR diffraction limit) is easily achievable in present experimental setups.     

Nondestructive investigation of interface states in high‐k oxide films on Ge substrate using X‐ray absorption spectroscopy

The unoccupied electronic structures of 5 nm thick high permittivity (k) oxides (HfO₂, ZrO₂, and Al₂O₃) and SiO₂ films on Ge substrates were examined using O K‐edge X‐ray absorption spectroscopy. Comparative studies with those on Si substrates showed contrasts in the conduction bands, which should be due to the formation of interface states. In the Al₂O₃ and SiO₂ films, GeO₂ layers are formed at the interface and they suppress in part the formation of detrimental germanate phases. In contrast, in the HfO₂ and ZrO₂ films, no signature of the Ge‐oxide phase is observed but some germanate phases are expected to prevail, suggesting a degradation of the gate oxide characteristics. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Time-dependent current response of semiconductor multi-barriers

The transient current response of n-doped GaAs—AlGaAs multiple barrier systems is studied within the time-dependent Schrödinger equation. Open boundary conditions are designed to minimize spurious reflections at the simulation boundaries. A local time-dependent Hartree-exchange-correlation-potential is used to self-consistently account for the inter-carrier Coulomb interaction. Application to the resonant-tunneling double barrier reveals current oscillations which are due to charging and discharging of the well. Current oscillations are also obtained in the transient for multi-barrier systems     

Band offsets of epitaxial LaAlO3/TiO2 interface determined by X-ray photoelectron spectroscopy

Room-temperature ferromagnetism in doped anatase TiO₂ has previously been observed, ferromagnetic semiconductor heterostructures based on anatase TiO₂ can thus provide a new opportunity to study spin-dependent transport phenomena at room temperature. An accurate determination of barrier heights or band offsets at the TiO₂-based heterojunctions is of great importance for the spintronics application with semiconductors. X-ray photoelectron spectroscopy with high-energy resolution was used to determine the band offsets of epitaxial LaAlO₃/TiO₂ heterojunction on SrTiO₃(001) substrate fabricated by pulsed laser deposition. Results showed an upward band bending of 0.48(0.03) eV when the film thickness of the overlayer LaAlO₃ above 4 unit cells. The valence band offset obtained is about 0.35(0.16) eV. Assuming bulk band gaps for the LaAlO₃ and TiO₂ epitaxial films, the associated conduction band offset is about 2.95(0.16) eV. These results show that LaAlO₃ can be an ideal tunneling barrier for TiO₂-based heterojunctions.     

Tracking coherent low frequency vibrational information of Rh101 in ground and excited electronic states by broadband transient grating spectroscopy

Time-and frequency-resolved broadband transient grating(BB-TG) spectroscopy is used to distinguish between ground-and excite-electronic state vibrational coherence at different wavelengths. Qualitative theoretical analysis using double-sided Feynman diagrams indicates that a superposition of ground and excited state vibrational coherence are contained in the ground state absorption(GSA) and stimulated emission(SE) overlap band, while only the excited state is contained in the excited state absorption(ESA) band. The TG experiment, in which a white light continuum(WLC) is adopted as a probe, is conducted with rhodamine101(Rh101~+) as the target molecule. Fourier analysis of TG dynamics in a positive delay time range at specific wavelengths enables us to distinguish the low-frequency vibrational modes of Rh101 in ground-and excite-electronic states.