Effective Interface State Density Research Articles

Characteristic comparison of AlGaN/GaN enhancement-mode HEMTs with CHF3 and CF4 surface treatment

In this study, enhancement-mode (E-mode) AlGaN/GaN HEMTs that underwent CHF3 and CF4 plasma treatment beneath the gate metal were fabricated. These treatments were applied because, although previous studies have formed AlF3 compound layers after fluorine-based plasma treatment to suppress the polarization-induced charge density, the surface negative charges still influenced the device gate leakage current and trap density. In the device in this study, unlike in previous CF4 plasma-treated GaN E-mode devices, the hydrogen atoms of the CHF3 plasma were introduced to compensate for vacancies by donating an electron to a vacancy acceptor level, thereby reducing the number of vacancy induced traps. Based on the measured subthreshold slope (SS) and the effective interface state density (Dit) results, the SS value of a CHF3-treated HEMT was 80 mV/decade and the Dit was 1.23 × 1012 cm−2. Moreover, the CHF3-treated HEMT exhibited a current gain cut-off frequency, a maximum oscillation frequency, and an output power of 6.7, 26, and 14.8 dBm (302 mW/mm), respectively. The 1/f noise measurement results of the CHF3-treated HEMT indicated that the flicker noise-induced generation-recombination noise and gate leakage-induced generation-recombination noise were also improved. Therefore, the CHF3-treated HEMT has great potential for use in low-distortion power amplifiers and logic control circuits.

The effective density of induced interface states of ionized cluster beam deposited Ag/n-Si Schottky junctions

The series of Ag/n-Si(1 1 1) Schottky junctions, prepared by the ionized cluster beam deposition method for an Ag ion acceleration voltage of U a=0 V, 300 V, 600 V and 1 kV, exhibits strong excess capacitances, the peaks of which gradually shift from reverse to forward bias voltages. These C– U measurements, the first such reported, are interpreted on the basis of the existence of external bias-dependent induced interfacial net charge density. It has been found that either reverse or forward excess capacitance of pertinent samples of the series is directly related to the induced interfacial charges above. From the above data an effective density of interface states is extracted which is characterized by the bias-dependent sharp spikes distributed along the semiconductor energy gap.

Manifestations of the deneutralization of mobile charges in SiO2 in the spectroscopy of the silicon-oxide interface

The total number of mobile ions in the oxide film in a Si-based MOS structure is determined by the conventional methods of recording capacitance-voltage and dynamic current-voltage characteristics. The fraction of ions in the neutral state at the Si-SiO2 interface is determined. Spectroscopy of the interface reveals a peak of the effective density of interface states. It is shown that the number of states in this peak corresponds to the number of neutralized particles. The mechanism for neutralization of the mobile charge of ions is discussed.

4H-SiC MOSFETs utilizing the H2 surface cleaning technique

The H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> cleaning technique was examined as the precleaning of the gate oxidation for 4H-SiC MOSFETs. The device had a channel width and length of 150 and 100 μm, fabricated on the p-type epitaxial layer of 3×10/sup 16/ cm/sup -3/. The gate oxidation was performed after the conventional RCA cleaning, and H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> annealing at 1000/spl deg/C. The obtained channel mobility depends on the pre-cleaning process strongly, and was achieved 20 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /N s in the H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> annealed sample. The effective interface-state density was also measured by the MOS capacitors fabricated on the same chips, resulting 1.8×10/sup 12/ cm/sup -2/ from the photo-induced C-V method.

Silicon-Nitride for Amorphous Silicon Thin-Film Transistors

ABSTRACTNitrogen-rich amorphous silicon-nitride films a-Si1−xNx:H were prepared by glow-discharge decomposition of gas mixtures of ammonia and silane. With increasing nitrogen content the spin density, Ns, decreases from 4×1018cm−3 (x = 0.55) to 3×1017cm−3 (x = 0.67). These films were used as a gate dielectric in amorphous silicon TFTs. The TFTs are characterized by measurements of the transfer characteristics and by a transient current spectroscopy (TCS). Nitrogen-rich dielectrics with Ns&lt;1018cm−3 have little influence on the transfer characteristic, however, they tend to have a lower sensitivity to bias stress. Using a device-quality nitride (x = 0.64) the properties of the TFTs were varied in two ways: 1) doping of the a-Si:H film with phosphine or diborane and 2) exposure of the nitride film to an oxygen plasma. The variation of the areal density of defect states, Nd, with Ec-EF suggests that the effective density of interface states, Ni, and the characteristic of undoped TFTs are determined by interface defects of the a-Si:H film. The plasma treatment introduces oxygen into a thin superficial layer of the nitride. By varying the exposure time te it is possible to change the properties of the TFTs continuously from nitride like to oxide like.

Amorphous Silicon Thin-Film Transistors Modified by Doping and Plasma Treatment of the Nitride

ABSTRACTThin—film transistors (TFTs) were prepared by the glow—discharge deposition of amorphous silicon nitride (a—SiNx:H) and amorphous silicon (a—Si:H). The properties of these TFTs were varied in two ways: a) doping of the amorphous silicon film with phosphine or diborane and b) exposure of the a—SiNx:H film to an oxygen plasma prior to the deposition of the a—Si:H layer. The TFTs are characterized by measurements of the transfer characteristic, ISD(VG) and of the effective density of interface states, Ni(E), using a transient current spectroscopy (TCS). The dependence of Ni(E) on the Fermi—level position in the a—Si:H film suggests that for EC—EF &gt; 0.6eV this quantity is mainly determined by interface related defect states whereas for Ec,—EF &lt;0.6eV it is determined by doping—induced defect states. The exposure to the oxygen plasma results in a reduction of Ni in both the upper and lower half of the gap and in an improvement of the characteristic, in particular in p—channel TFTs. These changes are discussed in terms of the chemical-equilibrium or defect—pool concept.

Amorphous Silicon Thin—Film Transistors with Non—Uniform Distribution of Gap States

ABSTRACTA theoretical study of the properties of a-Si:H thin-film transistors (TFTs) has been performed using uniform and non-uniform distributions of gap states in the a-Si:H layer. The potential profile in the semiconductor is calculated in a straightforward way by solving Poisson's equation numerically. From the band bending the source-drain current ISD(VG) of the TFT is predicted. In addition we calculate the transient discharge current I(t) generated by switching the gate voltage from positive to negative values. In experimental studies such transients are analyzed to obtain an effective density of interface states, Ni(E). We find that N;(E) depends little on the spatial variation of gap states, N(E,x), but sensitively on the energy distribution of N(E,x). It is shown that experimental N;(E) curves and ISD(VG) characteristics from the same sample can be fitted by the same N(E,x) which however cannot be unequivocally determined.

TCS study of n- and p-channel amorphous silicon thin-film transistors

We apply a transient current spectroscopy (TCS) to n-channel and, for the first time, also to p-channel TFTs made with a-Si:H and a-SiN x :H. The data are analysed in terms of an effective density of interface states, N i (E). We find that N i (E) has a maximum near midgap and a minimum in each half of the band gap, the lower one being less pronounced than the upper one. The TFT properties are modified by exposing the nitride to an oxygen plasma prior to the deposition of the a-Si:H film. Short exposures (t e 2 as a gate insulator. We also study the effect of bias-temperature stress (BTS) on the properties of our TFTs. Positive (negative) BTS causes a metastable increase of N i (E) predominantly in the lower (upper) half of the band gap. Our results suggest that the metastable defects but not the stable ones are caused by chemical equilibration within a defect pool.

DLTS study of defect creation in amorphous-silicon thin-film transistors

The metastable changes in a-Si:H/a-SiN x:H TFTs caused by prolonged application of high gate voltages and light irradiation have been studied for the first time by transient current spectroscopy, a DLTS technique. We find that the effective density of interface states, N i, which most likely is determined by a spatially non-uniform distribution of states in the a-Si:H film, increases during voltage stress by up to a factor of 2. The energy distribution of ΔN i suggests that the defects created are Si-dangling bonds with an areal density of approximately 10 12 cm −2. We find that ΔN i depends appreciably stronger on the stress temperature than the density of light-induced defects, indicating that voltage-induced defect creation is energetically less favorable than the light-induced effect.

Effect of nitridation on the density of interface states in W–Ti/n-GaAs Schottky diodes

The effective density of interface states that exist at the metal–semiconductor Schottky barrier formed between W–Ti and silicon implanted (n‐type) GaAs has been characterized using forward‐biased capacitance–voltage (C–V) measurements. The Schottky diodes were fabricated by sputtering from a tungsten titanium target under different nitrogen gas ambients of 0%, 6%, and 12%. The nitrogen incorporation and interface stability were verified by secondary ion mass spectroscopy (SIMS) and Rutherford backscattering (RBS). The results of this study clearly show that the lowest effective density of interface states was obtained for the highest nitrogen ambient of 12% at a deposition rate of 10 nm/min.

Detection of Electronic Defects in Strip-Heater Crystallized Silicon Thin Films

ABSTRACTElectronic defects in strip-heater crystallized silicon thin films have been investigated with capacitance-voltage (C-V),deep-level spectroscopic, and scanning-electron microscopic techniques. For electrical characterization the crystallized silicon films were used to fabricate inverted metal-oxide-silicon capacitors in which degenerately doped bulk silicon substrates provided the gate electrode. High-frequency C-V characteristics yield effective fixed-charge densities in the oxide of ≤ 2×1011 cm−2. Trap-emission spectra, recorded with deep-level transient spectroscopy on both p-type and n-type capacitors, indicate a continuous distribution of deep levels throughout the silicon bandgap. The Si-SiO2 interface is considered to be the principal source of this deep-level continuum, since the films are essentially single crystal with a low density of subgrain boundaries; the effective interface-state density is ≤ 2.5×1010 eV−1 cm−2. A discrete energy level, detectable above the background continuum,appears in the upper half of the silicon bandgap; it may identify a point defect in the bulk of the silicon film with a spatially uniform density of approximately 1×1013 cm−3. On lateral p-n junction diodes, electron-beam-induced-current images reveal enhanced diffusion of arsenic along structural defects intersecting the junction.