The influence of the film-substrate interface on the defect densityand other properties of sputter-deposited amorphous hydrogenated silicon

The properties of sputter-deposited amorphous hydrogenated silicon have been found to vary considerably as a function of the film thickness for d<1 μm. This behavior can be interpreted as follows. The defect density decreases exponentially from 2×10¹⁷ cm^-3 at the substrate interface to values below 10¹⁶ cm^-3 in the bulk. A corresponding change in the Urbach energy E₀ indicates that structural inhomogeneities are the reason for the change of the density of states. As a consequence, the ημτ product drops by four orders of magnitude from 1 to 0.01 μm. With electron spin resonance measurements, additional defects that are directly located at the interface are detected. These additional defects might be caused by the creation of a-Si dangling bond-like defects on the surface of the SiO ₂ substrate due to the sputter process     

Bulk and surface properties of amorphous hydrogenated fluorinatedsilicon grown from SiF4 and H2

A detailed study of the growth of amorphous hydrogenated fluorinated silicon (a-Si:H, F) from a DC glow discharge in SiF₄ and H₂ is discussed. The electrical properties of the films can be varied over a very wide range. The bulk properties of the best films that were measured included an Urbach energy E_u =43 meV, a deep-level defect density N_s=1.5×10¹⁵ cm^-3, and a hole drift mobility of 8×10^-3 cm² V^-1 s^-1, which reflects a characteristic valence band energy of 36 meV. It was found that E_u, N _s, and the density of surface states N_ss are related to each other. Under the deposition condition of the films with the best bulk properties, N_ss reaches its highest value of 1×10¹⁴ cm^-2. It is suggested that in growth from SiF₄/H₂, the density of dangling bonds at the growing surface is very sensitive to the deposition conditions     

Analysis of Hg1-xCdxSe alloys inelectroreflectance

An electrolyte electroreflectance study on Hg_1-xCd_xSe alloy films is first reported. The critical point energies E₀ and E₁ are obtained for different CdSe composition samples and material homogeneities are also discussed     

Error performance of a digitally processed binary ESK frequencyhopping receiver in the presence of large Doppler

Analysis is made of the effects of Doppler on the error rate performance of a low data rate binary FSK frequency hopping receiver, employing a discrete Fourier transform (DFT) technique for baseband detection. Bit detection decision is made by locating the maximum of the DFT outputs which, in the frequency domain, are assumed to be separated by 1/T where T is the bit period. Both the worst case and average error performances are obtained and presented as a function of E_b/N₀ for various values of M where E_b/N₀ is the signal bit energy-to-noise density ratio and M is the degree of freedom associated with the Doppler uncertainty window. The E _b/N₀ degradation as a function of M is also presented     

Asymptotic performance of M-ary orthogonal modulation ingeneralized fading channels

The asymptotic (M→∞) probability of symbol error P_e,_m for M-ary orthogonal modulation in a Nakagami-m fading channel is given by the incomplete gamma function P(m, mx) where x=In 2/(E_b/N₀) and E_b is the average energy per bit. For large signal-to-noise ratio this leads to a channel where the probability of symbol error varies as the inverse mth power of E_b/N₀. These channels exist for all m⩾1/2. The special case of m=1 corresponds to Rayleigh fading, an inverse linear channel     

Asymptotic performance of M-ary orthogonal signals inworst case partial-band interference and Rayleigh fading

It is shown that for worst-case partial-band jamming, the error probability performance (for fixed E_b/N_I) becomes worse with increasing M for (M>16). The asymptotic probability-of-error is not zero for any E_b/N _I(>ln 2), but decreases inverse linearly with respect to it. In the fading case, the error-probability performance (for fixed E_b/N₀) improves with M for noncoherent detection, but worsens with M for coherent detection. For large E_b/N₀ the performance of the Rayleigh fading channel asymptotically approaches the same limit as the worst case partial-band jammed channel. However, for values of M at least up to 4096, the partial-band jammed channel does better. While it is unlikely that an M-ary orthogonal signal set with M>1024 will be used in a practical situation, these results suggest an important theoretical problem; namely, what signal set achieves reliable communication     

Characterization of a-Si:H and a-SiGe:H p-i-n and Schottkyjunctions by admittance circuit modeling

The midgap density of states (MGDOS) and junction properties of a-Si:H and a-SiGe:H alloys are investigated by admittance measurements on p-i-n solar cells. An expression for the capacitance is derived from the frequency, temperature, and bias dependence of the deep state response. The measured admittance is analyzed with an equivalent circuit model. The density of states g₀, attempt-to-escape frequency, and the Fermi energy are adjusted to fit the measured admittance. Close agreement is found with the measured capacitance and conductance over a wide range of temperature and frequency. The single junction model is shown to apply equally well to p-i-n and Schottky diodes, justifying the neglect of the n-i junction and thin doped layers in the p-i-n device analysis. Fitted values of g₀ are confirmed by values of g₀ obtained from the limiting capacitance at high temperature     

Performance limits of coded multilevel DPSK in cellular mobileradio

Performance limits of coded multilevel differential PSK (MDPSK) in multipath Rayleigh fading channels are described. The simple Gaussian metric is assumed for reasons for practicality even though it is not the maximum likelihood. The channel cutoff rate, R₀ of MDPSK is analyzed based on the metric. Account is taken of additive white Gaussian noise (AWGN), cochannel interference, and multipath channel delay spread. For the analysis of the spectrum efficiency of a cellular mobile radio system employing coded MDPSK, its service area is defined as the area in which, with a bit rate of R information bit/symbol (R⩽R₀), reliable communications are possible. Three optimal information bit rates are determined from the channel cutoff rate to minimize the required average signal energy per information bit-to-noise power spectral density ratio (E_b/N₀) to maximize the tolerable r.m.s. delay spread τ_r.m.s. and to maximize the spectrum efficiency     

Quantization loss in convolutional decoding

The loss in quantizing coded symbols in the additive white Gaussian noise (AWGN) channel with binary phase-shift keying (BPSK) or quadrature phase-shift keying (QPSK) modulation is discussed. A quantization scheme and branch metric calculation method are presented. For the uniformly quantized AWGN channel, cutoff rate is used to determine the step size and the smallest number of quantization bits needed for a given bit-signal-to-noise ratio (E_b/N₀) loss. A nine-level quantizer is presented, along with 3-b branch metrics for a rate-1/2 code, which causes an E_b/N₀ loss of only 0.14 dB. These results also apply to soft-decision decoding of block codes. A tight upper bound is derived for the range of path metrics in a Viterbi decoder. The calculations are verified by simulations of several convolutional codes, including the memory-14, rate-1/4 or -1/6 codes used by the big Viterbi decoders at JPL     

Amorphous silicon thin-film transistor with fluorinated siliconoxide ion stopper

We propose fluorinated silicon oxide (SiOF) as the ion-stopper of bottom-gate amorphous silicon thin film transistors (a-Si:H TFTs). The low dielectric constant SiOF on both the back-channel of the TFT and the crossover regions of gate/data lines can contribute to reducing the RC delay of the gate pulse signal in active-matrix liquid-crystal displays. Besides, the a-Si:H TFT with a SiOF stopper shows an improved performance compared to the widely-employed silicon nitride (SiN_x ) stopper TFT, because the fluorine incorporation reduces the interface state density between a-Si:H and SiOF     

The cutoff rate for on-off keying

It is well known that on-off keying (OOK), i.e. the transmission of a pulse for a data one (mark) and transmission of no signal for a data zero (space), exhibits a 3 dB inferiority to antipodal signalling, in terms of the signal-to-noise ratio required to achieve a specified bit error probability. When the channel is characterized by the cutoff rate R₀, this 3 dB inferiority persists, in the sense that the required E_b/N₀ to operate at a given value of R₀ is 3 dB greater for the OOK channel than for the antipodal channel. However, it is shown that, if the definition of cutoff rate is generalized in an appropriate way to reflect the fact that signal energy is expended only in the transmission of marks, part of this penalty is recovered. In fact, it is shown that, in the limit of low R₀, there is no penalty in signal-to-noise ratio. Even at rates of realistic interest, the penalty is significantly less than the expected 3 dB     

Silicon oxynitride and silicon oxynitride-silicon interface: aphotoemission study

A study of silicon-oxynitride materials and their interfaces with amorphous silicon using synchrotron radiation photoemission is discussed. The effect of the hydrogen atoms as an intralayer at the silicon-oxynitride-a-Si interface is also analyzed. The data relative to the bulk silicon oxynitride show that the valence band edge gradually shifts toward a higher binding energy as the [O]/[N] ratio (A) is increased. For the silicon-oxynitride-a-Si heterojunctions, the valence band discontinuities ΔE_v ranged from 1.1±0.15 eV in the case of A=0.08 up to 4.6±0.15 eV in the case of A=2.02. The effect of the hydrogen intralayer was to reduce the ΔE_v by 0.5±0.15 eV. The observed intralayer-induced effect is attributed to modification of the interface dipoles. A simple model calculation, based on electronegativity arguments, gives a reduction of 0.4 eV for A=0 and 0.46 for A corresponding to pure silicon dioxide (SiO₂)     

High field effect mobility deuterated amorphous silicon thin-filmtransistors based on the substitution of hydrogen with deuterium

The characteristics of amorphous silicon hydrogen and deuterium thin-film transistors (a-Si:H/a-Si:D TFT) were studied. The deuterated and hydrogenated amorphous silicon channels were prepared by first annealing the as-deposited a-Si:H layer at 550°C in N₂ environment to expel all the hydrogen atoms out of the films, then the D ₂ or H₂ plasma were applied to treat the amorphous silicon layers. The field effect mobility of the conventional hydrogen TFT is usually smaller than 1 cm²/V-s. It was found that substitution of hydrogen with deuterium improved the field effect mobility of the TFT. The maximum field effect mobility of a-Si:D TFT obtained from the saturation region was 1.77 cm²/V-s     

An a-Si:H/a-Si,Ge:H bulk barrier phototransistor with a-SiC:Hbarrier enhancement layer for high-gain IR optical detector

The design and fabrication of a high-gain amorphous silicon/amorphous silicon germanium (a-Si:H/a-Si,Ge:H) bulk barrier phototransistor for infrared light detection applications are reported. The a-Si,Ge:H material featured a lower energy gap and is suitable for the absorption of longer wave light, but it also leads to a low breakdown voltage and high dark current. An additional a-SiC:H thin-film layer was used at the collector/base interface in the conventional amorphous bulk barrier phototransistor to enhance the function of the bulk barrier and obtain high optical gain     

Optimal probability-of-error thresholds and performance for twoversions of the sign detector

Expressions are obtained for specifying the optimal error probability (minimum P_e) thresholds λ₀₁ and λ₀₂ for the traditional and modified sign detectors, respectively. These thresholds are shown to depend on the parameters p, P₁, and M where: M is the number of observations z_i used in the test statistic; P₁=P(H₁ ) is the prior probability for hypothesis H₁ that signal s₁ is present and 1-P₁ =P(H₀) corresponds to the hypothesis H₀ that signal s₀ is present; and p=Pr{z_i⩾0|H₁} with s₀=0 for the traditional sign detector and p=Pr{z_i⩾λ|H₁ }=Pr{z_i<λ|H₀} with λ =(s₀+s₁)/2 for the modified sign detector. The expressions for λ₀₁ and λ₀₂, are given explicitly, and shown to be independent of P₁ for sufficiently large M. Optimal P_e versus M performance curves, corresponding to both versions of the sign detector, are obtained for a representative range of values for p and P₁     

Dependence of thin-oxide films quality on surface microroughness

The effects of silicon surface microroughness on electrical properties of thin-oxide films, such as breakdown electric field intensity (E_BD) and time-dependent dielectric breakdown (Q_BD), have been studied, where the microroughnesses of silicon and silicon dioxide surfaces are evaluated by the scanning tunneling microscope (STM) and the atomic force microscope (AFM), respectively. An increase of surface microroughness has been confirmed to severely degrade the E_BD and Q_BD characteristics of thin-oxide films with thicknesses of 8-10 nm and to simultaneously decrease channel electron mobility. An increase of surface microroughness has been demonstrated to originate mainly from wet chemical cleaning processing based on the RCA cleaning concept, particularly the ammonium-hydrogen-peroxide cleaning step. In order to keep the surface microroughness at an initial level, the content ratio of NH₄OH/H₂O₂/H₂ O solution has been set at 0.05:1:5 and the room-temperature DI water rinsing has been introduced right after the NH₄OH/H₂O₂/H₂O cleaning step in conventional RCA cleaning procedure     

An approximation for the phase offset sensitivity of Viterbidetected continuous phase modulation signals

An approximation is presented for the loss in performance when the Viterbi algorithm receiver for CPM (continuous phase modulation) signals is subject to a phase offset. Pairs of symmetric error events are used to derive a closed-form result that approaches the asymptotic expression, based solely on minimum distance, as E_s/ N₀ approaches infinity. The improved accuracy of the approximation is demonstrated by comparison with a Monte Carlo simulation     

Capacitance allocation and its role in the performance ofdoubling-circuit pulsed gas lasers: its application to the N2laser

An investigation of the optical output for all possible combinations between the capacitances, C₁ and C ₂, in a doubling circuit N₂ laser is presented. It is shown that a maximum optical output appears when C ₁=C₂ for constant total capacitance. The maximum value increases when the total capacitance increases and the system approaches saturation for capacitance values higher than 20 nF each. This behavior of the optical energy is due to a similar behavior of the current, which becomes maximum when the best coupling of the two loops of the system is achieved through capacitance equality, and the oscillatory behavior of the system is minimized. When this equality is disturbed, either with weak (C₁>C₂ ) or strong (C₂<C₁) coupling of the system, the current and optical outputs decrease. In both cases, the undesirable oscillatory behavior of the system increases. The electric parameters R₁,L₁ of the spark-gap loop and R₂,L₂ of the laser channel loop are calculated     

An InGaAs-InP position-sensing photodetector

The design and fabrication of a position-sensing photodetector that is sensitive to a wavelength of 1.55 μm are reported. This device is based on the photo-lateral effect and uses an In_0.53Ga_0.47As absorbing layer that is lattice matched to an InP substrate. The output signal (I₁- I₂)/(I₁+I₂) varies linearly with respect to the position of the incident beam. The responsivity of 0.75 A/W is uniform over the length of the photodetector. The device is intended for video rate applications and has a -3-dB cutoff frequency of 7 MHz     

Absolute and relative nonlinear optical coefficients of KDP, KD*P,BaB2O4, LiIO3, MgO:LiNbO3,and KTP measured by phase-matched second-harmonic generation

Both absolute and relative nonlinear optical coefficients of six nonlinear materials measured by second-harmonic generation are discussed. A single-mode, injection-seeded, Q-switched Nd:YAG laser with spatially filtered output was used to generate the 1.064-μm fundamental radiation. The following results were obtained: d₃₆(KD*P)=0.38 pm/V, d₃₆(KD*P)=0.37 pm/V, |d₂₂(BaB ₂O₄)|=2.2 pm/V, d₃₁(LiIO₃ )=-4.1 pm/V, d₃₁(5%MgO:MgO LiNbO₃)=-4.7 pm/V, and d_eff(KTP)=3.2 pm/V. The accuracy of these measurements is estimated to be better than 10%