Notes: We have carried out a comprehensive study of the Raman spectra of a-Si:H films produced by the glow discharge (GD) and radio frequency sputtering (RFS) deposition techniques. The results show that the short-range disorder (bond-angle deviation), as measured by the width of the TO band (ΓTO), is larger in RFS than in GD a-Si:H films. The intermediate-range disorder (dihedral angle deviation), as measured by the ratio of the intensity of the TA band to that of the TO band (ITA/ITO), is generally larger in RFS than in GD a-Si:H films. However, while the ITA/ITO values of RFS films remain relatively close to those of GD films when the interior is probed, the near surface of RFS films shows much larger values evidencing the existence of a significant disorder gradient along the growth axis. Together, these results indicate that the network order and homogeneity of RFS amorphous silicon is lower than those of GD for substrate temperatures that produce the hydrogenated material. These structural differences are interpreted in terms of the differences between the two film growth processes and are believed to be the reason for the poorer transport properties of RFS a-Si:H films. © 1995 American Institute of Physics.

Notes: The photoluminescence (PL) at λ≤850 nm of boron-doped porous Si films anodically etched, passively etched, annealed, and reactive ion etched (RIE) under systematically varied conditions is described and discussed. As previously observed, the PL yield η of films etched from 7 to 20 Ω cm wafers in HF/H2O/C2H5OH solutions rapidly degraded during 40 mW illumination in air at 488 nm. In addition, it was totally quenched by O2 annealing or RIE but not by H2 RIE. However, the yield of films etched in HF/H2O only decreased by less than 10% following similar illumination for 7 h, and O2 annealing or RIE reduced it by 0%–50% only. The instability under illumination, the effects of oxygen annealing and RIE, Auger line shape measurements, and surface charge buildup during Auger analysis are consistent with a significantly higher density of weak Si—Si bonds in films etched in ethanol-containing solutions. These bonds are apparently broken by photoinduced oxidation, oxygen annealing, or O2 RIE, but the oxygen does not efficiently passivate the newly generated dangling bonds. While the peak emission wavelength λmax of the films etched from these wafers shifted with annealing and etching conditions, it invariably peaked in the ∼7600–8600 A(ring) range; η reached ∼1.7%. H2 RIE had no effect on η, but λmax was blueshifted. This behavior is attributed to similar breaking of weak Si—Si bonds followed by efficient passivation of the newly generated dangling bonds.The blueshift results from the higher H/Si ratio; the invariant yield is believed to be limited by the density of dangling bonds at the SiOxHy/Si interface which is apparently unaffected by the treatments. It is therefore believed that the PL is at least partially due to SiHx or SiOxHy complexes on the crystallite and column surfaces, which are similar to those found at the surface of some H-rich a-Si:H. Size effects, however, cannot be ruled out. In contrast to films etched from 7 to 20 Ω cm wafers, λmax of those prepared from 1 to 2 Ω cm substrates was consistently ∼7000 A(ring) after a brief exposure to air. While η exhibited a complex dependence on the O2 RIE power, λmax was unchanged by either O2 or H2 plasmas. The emission from heavily doped ∼0.02 Ω cm films was usually undetectable. However, H2 RIE resulted in observable PL. © 1995 American Institute of Physics.

Notes: A study of plasma-film interactions present during the deposition process of a-Si:H prepared by radio frequency (rf) sputtering in 10 mTorr of either He, Ar, or Xe and 0.5 mTorr H2, at rf power levels of 0.27–3.3 W/cm2 (50–600 W), on nominally unheated substrates, is presented. Measurement techniques included scanning electron microscopy (SEM), infrared (IR) and optical absorption, and electron-spin resonance (ESR). The SEM images indicated that the films prepared in He/H2 at low power are morphologically porous and/or columnar; the evidence suggests that as the rf power is increased, the islands coalesce to form larger ones and the images appear glassy and cracked. IRabsorption spectra indicated that the porous films undergo post-depositional oxidation. A comparison between the observed deposition rates and the rates expected from previously published sputtering yields indicated that resputtering of the growing film is a dominant feature in films deposited in Ar/H2 and Xe/H2 but a weaker one in those grown in He/H2. The dependence of the total H concentration, as determined from the 640 cm−1 IR wagging mode absorption, on the deposition rate, is in reasonable agreement with the kinetic model of Moustakas et al.; the comparison with that model indicates that the product of the sticking coefficient and capture cross section for hydrogen is much lower when depositing in He/H2 than in Ar/H2 or Xe/H2. Theanalysis of the concentration of hydrogen in dihydride and trihydride configurations, as determined from the 850–890 cm−1 IR bending mode absorption, indicated that this quantity is determined by the number of sites available for these configurations and a random statistical process. The number of these available sites is generally much higher in He/H2 sputtered films than in Ar/H2 or Xe/H2 ones, and in the former sputtering medium also sharply increases with decreasing rf power. These observations are in excellent agreement with the picture of dihydride and trihydride configurations residing at internal surfaces, the area of which increases as the rf power is decreased as the film resputtering process weakens and the morphology becomes increasingly porous and/or columnar. Analysis of the ESR measurements indicates that the density of unpaired bonds sharply increases as the dihydride and trihydride, and apparently the internal surface area, also increase. It is also concluded that the density of weak bonds, essentially constructed by the pairing of dangling bonds on the internal surfaces of the film and invisible by ESR, is much largerin the films prepared with He than with Ar or Xe. The optical absorption measurements in the Tauc region indicate that the He/H2 films are considerably more disordered than the Ar/H2 ones; the subgap Urbach slopes of the former are also much larger (as high as 490 meV) than the former and indicate that band tail states originate, to a considerable extent, from weak SiSi bonds created as mentioned above. Finally, it should be noted that the films deposited in Ar/H2 at high power approach device quality, in spite of the relatively low effective substrate temperature, which is around 100–150 °C. The analysis may thus indicate that the source of the special care and attention to the various deposition conditions required in the synthesis of high photovoltaic quality rf-sputtered films may be due to the double role of plasma-film processes, which are both responsible for destroying incipient porous and/or columnar morphology yet introduce other defects into the growing film.

Notes: Hydrogenated amorphous germanium carbide (a-Ge1−xCx@B:H ) films were prepared by rf sputtering, at various rf power levels and mixtures of Ar, H2, and propane. As in the case of a-Si:H rf sputtered under similar conditions, the concentration of Ge–H bond, as determined by the IR absorption spectra, and the Tauc determined optical gap, generally increase as the rf power is decreased. The optical gap of the a-Ge1−xCx@B:H films range from 0.85–2.3 eV, and the electron-spin-resonance defect spin densities from 6.5×1017 to 3×1018 spins/cm3. Auger spectroscopy was used to determine the C/Ge ratio and indicated that in most of the samples, this ratio was (approximately-less-than)0.15. Isochronal annealing up to 400 °C indicated that (i) Ge-C segregation effects already initiated at 100 °C are greatly enhanced above 300 °C, (ii) at 300 °C C–H bonds are formed at the expense of Ge–H ones, and (iii) all of the hydrogen bonded to Ge and most of that bonded to carbon evolve out of the sample at or slightly below 400 °C.

Notes: We have measured the femtosecond dynamics of the nonlinear optical response χ(3) in polydiethynylsilane (C4H2SiBu2)x, a novel class of π-conjugated polymers incorporating Si, using degenerate four-wave mixing and photoinduced absorption techniques. In resonance conditions at 620 nm we found χ(3)=3×10−9 esu, which decays in a record time of 135 fs, followed by a slower decay component of 750 fs, without a long tail even at high laser intensities. This material is an excellent candidate for nonlinear optical devices in the sub-THz frequency range.

Notes: It is shown that the bulk hydrogen concentration in radio-frequency (rf) sputtered films of hydrogenated amorphous silicon (a-Si:H) can be determined from reflected electron energy-loss spectroscopy signals obtained even during ion beam etching of the film. As a result, depth profiling, with a 50-A(ring) depth resolution of the hydrogen concentration, and with 1 at. % accuracy of the a-Si@B:Hx/a-Si@B:Hy multilayer, is achieved. Using this technique to depth profile annealed multilayer samples of rf sputter-deposited a-Si:H, it was found that despite the loss of hydrogen at elevated temperatures the original interface widths are unchanged even after a 500 °C, 1 h anneal. This result contrasts sharply with previous measurements of hydrogen motion in glow-discharge a-Si:H and indicates a significant difference between the two types of a-Si:H with respect to hydrogen motion at elevated temperatures.

Notes: Arrays of ultraviolet–violet (indium tin oxide)/[copper phthalocyanine (CuPc)]/[4,4′-bis(9-carbazolyl)biphenyl (CBP)]/[2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4oxadiazole (Bu-PBD)]/CsF/Al organic light-emitting devices, fabricated combinatorially using a sliding shutter technique, are described. Comparison of the OLED electroluminescence and CBP photoluminescence spectra indicates that the emission originates from the bulk of that layer. In arrays of devices in which the thickness of the CuPc and Bu–PBD were varied, but that of CBP was fixed at 50 nm, the optimal radiance R was obtained at CuPc and Bu–PBD thicknesses of 15 and 18 nm, respectively. At 10 mA/cm2, R was 0.38 mW/cm2, i.e., the external quantum efficiency was 1.25%; R increased to ∼1.2 mW/cm2 at 100 mA/cm2. © 2001 American Institute of Physics.

Notes: Bright multilayer organic light-emitting devices (OLEDs) containing both perylene-doped [4,4′-bis(9-carbazolyl) biphenyl (CBP)] and [4-(dicyano-methylene)-2-methyl-6-(p-dimethyl aminostyryl)-4H-pyran (DCM1)-doped tris-(8-hydroxy quinoline) Al (Alq3)] are described. The electroluminescence spectra consist of blue and green bands at 453, 487, and 524 nm due to perylene and a red band at 600 nm due to DCM1, although these two emitting layers are separated by a layer of [2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (Bu-PBD)]. As the thickness of the hole-injecting [N,N′-diphenyl-N,N′-bis(1-naphthyl-phenyl)-1, 1′-biphenyl-4,4′-diamine (NPB)] and perylene:CBP layers increases and that of the DCM1:Alq3 layer decreases the perylene emission intensifies strongly relative to the DCM1 emission. For 350 Å thick NPB, 350 Å thick perylene:CBP, 100 Å Bu-PBD, and 150 Å DCM1:Alq3, the brightness reaches 3750 Cd/m2 at 20 V, the efficiency is 2.4 Cd/A at 19 V, and the (x,y) CIE coordinates are well within the white region. However, as the bias is increased the intensity of the perylene emission increases relative to the DCM1 emission. Both the layer thickness and bias dependence are believed to result from changes in the recombination zone and in the field- and cathode-mirror-induced quenching of DCM1 singlet excitons. © 2002 American Institute of Physics.

Notes: The behavior of bright, efficient, low-driving-voltage blue organic light-emitting diodes based on amino-oxadiazole-fluorenes (AODFs) with Al2O3/Al cathodes is described. It is shown that the thin Al2O3 buffer layer sharply enhances current injection, increases the device efficiency, and reduces the driving voltage; the performance of devices with the optimal oxide buffer layer thickness approaches those with Mg0.9Ag0.1 cathodes. The effects of the Al2O3 buffer layer are believed to result from the removal of interface gap states induced by defects and chemical bonds between the AODF and Al, which trap carriers and quench singlet excitons nonradiatively. © 1997 American Institute of Physics.

Notes: Light emitting devices were fabricated from 2,5-dialkoxy derivatives of poly(p-di ethynylene phenylene-p-di phenylene vinylene) (PDEPDPV) sandwiched between indium tin oxide (ITO) and Al. The current–voltage (I–V) curve, electroluminescence (EL) intensity-voltage (IEL–V) curve, and the EL spectra were identical in forward and reverse bias. The I–V curves were also symmetric under illumination, with I≈0 and V=0, suggesting a negligibly small internal electric field. At high bias voltage, carrier injection was found to be dominated by tunneling at the interfaces. At low bias voltage, tunneling among localized states at the Fermi level prevailed. The behavior is discussed in relation to Fermi-level pinning at defect states in the interfaces with the ITO and Al. © 1995 American Institute of Physics.