bluuebunny/crossref_metadata_2025 huggingface dataset:
120 °C fabrication technology for a-Si:H thin film transistors on flexible polyimide substrates
Authors: Sazonov A. , Nathan A.
Published: July 2002

In this article, we report a fabrication process for hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFT) at 120 °C on flexible Kapton® substrates for large-area imaging applications. The samples are based on the bottom-gate inverted staggered TFT structure. Initially, both sides of the substrate are coated by amorphous silicon nitride (a-SiNx:H), followed by 120 nm of aluminum (Al) film for the gate. After gate patterning, a trilayer is deposited at 120 °C by plasma-enhanced chemical vapor deposition comprising of 250 nm a-SiNx:H gate dielectric, 50 nm a-Si:H, and 250 nm top (passivation) a-SiNx:H. After opening the contact windows, we deposit 35 nm of n+ a-Si:H at 120 °C. Next, a 1 μm Al top contact layer is deposited. The a-Si:H films are deposited from a gas mixture of silane (SiH4) and hydrogen. For the n+ a-Si:H layer, a hydrogen-diluted (1% PH3+99% SiH4) mixture is used. The a-SiNx:H films are deposited from a helium-diluted mixture of silane, ammonia and nitrogen. Dry etching is used except for the metal layers, where wet etching is used. The TFTs show an off-current less than 10−12 A, and an on-current of more than 10−6 A, thus giving an on/off current ratio greater than 106. The effective device mobility, μeff, is about 0.4 cm2/V s.
Progress on optimization of p-type GaAs/AlGaAs quantum well infrared photodetectors
Authors: Shen A. , Liu H. C. , Buchanan M. , Gao M. , Szmulowicz F. , Brown G. J. , Ehret J.
Published: July 2002

We report the optimization of barrier thickness and well doping density for GaAs/AlGaAs p-type quantum well infrared photodetectors covering the 3–5 μm wavelength region. We investigated a series of samples with barrier widths varying from 10 to 50 nm and found that the optimum barrier thickness is about 20 nm. For devices operating at about 100 K, the optimum two-dimensional doping density is found to be in the range 1–2×1012 cm−2, which maximizes the background limited infrared performance temperature and dark current limited detectivity.
Design and preparation of a 33-layer optical reflection filter of TiO2–SiO2 system
Authors: Wang Xinrong , Masumoto Hiroshi , Someno Yoshihiro , Chen Lidong , Hirai Toshio
Published: July 2002

Multilayer optical reflection filters with several kinds of graded refractive index profiles were designed. The effects of the number of layers of multilayer films and grading functions of refractive index profiles on the optical filter characteristics were examined by using simulation. A 33-layer optical reflection filter with a linear refractive index profile was found to be optimal in terms of both optical performance and manufacture. The designed 33-layer TiO2–SiO2 reflection filter was fabricated by helicon plasma sputtering. The optical performance of the prepared multilayer film agreed well with that of the designed filter. The measured transmittance spectrum exhibited a sharp cutoff stop band around a central wavelength of 1340 nm and a wide pass region with high transmittance of about 90%. The reflectance of the stop band was greater than 99.0% in the wavelength region from 1208 to 1518 nm.
Schottky diode measurements of dry etch damage in n- and p-type GaN
Authors: Cao X. A. , Zhang A. P. , Dang G. T. , Ren F. , Pearton S. J. , Shul R. J. , Zhang L.
Published: July 2002

n- and p-type GaN was exposed to inductively coupled plasma of N2, H2, Ar, or Cl2/Ar, as a function of source power (0–1000 W) and rf chuck power (20–250 W). For n-GaN, there was a strong reduction in diode reverse breakdown voltage and an increase in forward and reverse currents, while for p-GaN the reverse breakdown voltage increased. These results are consistent with creation of point defects with shallow donor nature that increase the conductivity of initially n-type GaN or decrease the conductivity of p-type GaN. Annealing at 750 °C under N2 produced significant recovery of the electrical properties, while wet etch removal of 500–600 Å of the surface produced a full recovery. For completed n-type mesa diodes exposed to Ar or Cl2/Ar discharges, the low bias forward currents increased by several orders of magnitude. The exposed surfaces became N2 deficient in all cases.
Examination of MBE GaAs/Al0.3Ga0.7As superlattices by Auger electron spectroscopy
Authors: Erickson L. Peter , Phillips Bradway F.
Published: July 2002

MBE GaAs/Al0.3Ga0.7As superlattices have been examined by Auger electron spectroscopy (AES) using Ar+ sputter depth profiling and line scans across the crater wall left after depth profiling. The superlattice was grown by MBE with ten periods of alternating Al0.3Ga0.7As and GaAs layers of 200 and 50 Å thickness, respectively, at a substrate temperature of 700 °C. AES sputter depth profiling clearly indicated the periodic nature of the structure through all ten periods, although the Al signal did not appear to drop to zero in the GaAs layers, probably due to sputter effects. The use of the standard elemental Auger sensitivity factor for Al (0.070 at 5 kV) yielded the correct atomic concentration of Al in the Al0.3Ga0.7As layers (15% for x=0.3), however, this was not true for either Ga or As. By using an AES line scan up the sputter crater wall, the superlattice layers were spread out by a factor of ∼2400. The periodicity of the structure was more strongly evident than that seen by AES depth profiling with the Al signal dropping to zero in the scanned crater wall regions corresponding to the 50 Å GaAs layers. The expanded superlattice was also examined by absorbed current and secondary electron imaging and by AES mapping.
Interference effect in multivalley quantum well structures
Authors: Chang Yia-Chung , Ting D. Z.-Y.
Published: July 2002

The energy spectra of a multivalley quantum well structure are investigated by both the tight-binding and the effective-mass methods. It is shown that the energy spectrum as a function of the quantum well width exhibits an interference pattern due to the mixing of Bloch states of the well material with wave vectors near the two equivalent conduction band minima.
Poly(chloromethylstyrene): A high performance x-ray resist
Authors: Choong Hsia S. , Kahn Frederic J.
Published: July 2002

X-ray lithographic performance of poly(chloromethylstyrene) (PCMS), a high performance negative e-beam resist, was investigated. Superior resolution, i.e., resist profile and linewidth control, was obtained by x-ray exposure due to the essentially square x-ray exposure profile and the significantly reduced amount of backscattered electrons from the substrate. Sensitivity and resolution were evaluated as a function of polymer molecular parameters. Dry etching characteristics were evaluated under polysilicon, SiO2 and Al etching conditions. Excellent resolution, sensitivity and dry process compatibility were obtained and attributed to the high absorption of chlorine to Pd Lα radiation, the high chemical reactivity of the benzyl chloride moiety and the high plasma stability of the aromatic styrene moiety. Sensitivity of 300 000 molecular weight PCMS is 25–30 mJ/cm2 (D0.5g) to Pd Lα radiation; resolution is 1 μm in 1 μm final thickness resist with vertical walls at D0.9g (70 mJ/cm2), and submicron in thinner layers. Dry etch resistance is comparable to that of AZ-type photoresist. Postexposure deep UV hardening minimizes thermally induced flow and profile distortion even at temperatures greater than 300 °C. Lithographic performance of PCMS compares very favorably with other leading x-ray resists including the mixture of poly(2,3-dichloropropyl acrylate) and poly(glycidyl methacrylate-co-ethyl acrylate) (DCOPA), poly(allyl methacrylate-co-hydroxyethyl methacrylate) (EK88) and chloromethylated polystyrene (CMS).
Persistent photoconductivity in AlGaAs–GaAs heterostructures
Authors: Nathan M. I. , Heiblum M. , Klem J. , Morkoç H.
Published: July 2002

Persistent photoconductivity (PPC) is studied in modulation doped AlGaAs–GaAs heterostructures in the conductivity parallel to the heterojunctions. The results indicate that the macroscopic fields of the heterojunction and traps primarily in the GaAs rather than AlGaAs are responsible for the phenomenon. PPC is also reported in n+ GaAs–undoped AlGaAs–n+ GaAs heterostructures for current perpendicular to the heterojunctions.
The effect of aluminum composition on silicon donor behavior in AlxGa1−xAs
Authors: Salmon L. G. , D’Haenens I. J.
Published: July 2002

The behavior of Si in AlxGa1−xAs grown by molecular beam epitaxy (MBE) has been studied. Temperature-dependent Hall measurements were made to investigate the doping behavior of Si in MBE grown AlxGa1−xAs across the composition range 0.2<x<0.8. Aluminum composition was determined using photoluminescence. Doped AlxGa1−xAs layers were separated from underlying semi-insulating GaAs substrates by an unintentionally doped AlxGa1−xAs spacer layer and a 0.2 μm GaAs buffer layer. Low-temperature mobility of the structures proved that no two-dimensional electron gas was formed at the GaAs/AlxGa1−xAs interface. Therefore, the data obtained are characteristic of bulk AlxGa1−xAs. Si donor activation energy increased from ∼6 meV for GaAs to ∼200 meV for x=0.33 and decreased for x>0.6. Net electron concentration and mobility were also strongly dependent on aluminum composition.
Si-doped GaAs/AlGaAs TJS laser by MBE
Authors: Mitsunaga K. , Fujiwara K. , Nunoshita M. , Nakayama T.
Published: July 2002

The effect of high temperature annealing on the properties of silicon-doped GaAs/AlGaAs double heterostructure (DH) grown by molecular beam expitaxy (MBE) and its application to the fabrication of transverse junction stripe (TJS) lasers are reported. In spite of the amphoteric nature of Si, it was found that the high temperature annealing gave little influence on the electrical and optical quality of the n-type DH wafer. The TJS laser using Si-doped GaAs/AlGaAs wafer has been oscillated cw at room temperature and exhibited low threshold current of 30 mA and high quantum efficiency of 60%.
Structural dependent electrical characteristics of submicron gallium arsenide devices
Authors: Grubin H. L. , Kreskovsky J. P.
Published: July 2002

Numerical studies of the transient and dc electrical behavior of submicron N+N−N+ gallium arsenide structures are discussed. It is shown that the transient results are dominated, during the first fraction of a picosecond, by displacement current contributions. Velocity overshoot is less important. Under dc conditions and high bias levels, submicron effects may be masked by transport within the N+ regions.
Influence of slight deviations from TaSi2 stoichiometry on the high-temperature stability of tantalum silicide/silicon contacts
Authors: Oppolzer H. , Neppl F. , Hieber K. , Huber V.
Published: July 2002

The influence of deviations from TaSi2 stoichiometry in cosputtered, amorphous Ta–Si films on interface reactions in contacts to silicon at 900 °C was investigated by TEM of thin cross sections through the contact windows. For Si rich films, the excess Si precipitates epitaxially in the contacts, whereas pits are formed for Ta rich films. Since the lateral Si transport involved ranges over several tens of microns, even a very small deviation from TaSi2 stoichiometry leads to strong degradation of the contacts. The interfacial oxide has no influence on the contact reactions. Since slight deviations from disilicide stoichiometry cannot be avoided with cosputtering, the realization of a low-resistive, high-temperature-stable TaSi2 interconnection system with good contacts to Si would additionally require a high-temperature-resistant barrier layer. Experiments with a crystalline TaSi2 pad and a TaN0.8 barrier layer show insufficient stability at 900 °C, but suggest that most of the large lateral Si transport happens during the initial stage of crystallization.
Microstructural investigations of refractory metal silicide films on silicon
Authors: Magee T. J. , Woolhouse G. R. , Kawayoshi H. A. , Niemeyer I. C. , Rodrigues B. , Ormond R. D. , Bhandia A. S.
Published: July 2002

In a closely correlated study, we have investigated phase transformations and grain growth phenomena together with measurements of electrical resistivity in the silicides of tungsten, molybdenum, and tantalum. In the case of WSi2, we find the tetragonal structure at all temperatures of anneal between 700 and 1100 °C. Furthermore, the activation energy, 0.65 eV, derived from grain growth measurements is close to the value, 0.66 eV, derived from the increase in electrical conductivity. In the case of MoSi2, we find a mixture of hexagonal and tetragonal phases after annealing at 700 °C and pure tetragonal phase at 800 °C and above. Once again the activation energies derived from grain growth and electrical conductivity measurements are very similar. TaSi2 gives anomalous results compared with the other two silicides. Electrical conductivity shows an asymptotic behavior with increasing anneal temperature. This is probably related to the fact that tetragonal TaO2 was found in the films.
Influence of the interfacial oxide on titanium silicide formation by rapid thermal annealing
Authors: Pramanik D. , Saxena A. N. , Wu Owen K. , Peterson G. G. , Tanielian M.
Published: July 2002

The interaction of titanium films with single crystal silicon during rapid thermal annealing (RTA) has been studied by Auger analysis and SEM. The diffusion of silicon in titanium to form a silicide has been investigated as a function of the thickness of the interfacial silicon dioxide between the film and the substrate. For a clean interface the diffusion is initiated at lower temperatures, approximately 600 °C. Ion beam mixing of the interface caused by the implantation of heavy ions, such a arsenic, through the titanium film helps to render the interfacial oxide ineffective and thereby facilitates Si diffusion into the film. The presence of the interfacial oxide has been shown to affect the smoothness of the final silicide layer and the silicide–silicon interface. Silicide films produced from ion-mixed films have been found to have smoother surfaces and interfaces than nonion-mixed samples. Application of ion-mixed films to devices has been studied.
Aluminum Schottky barrier formation on arsenic capped and heat cleaned MBE GaAs(100)
Authors: Eglash Stephen J. , Williams M. D. , Mahowald P. H. , Newman Nathan , Lindau Ingolf , Spicer W. E.
Published: July 2002

The fabrication and characterization of an aluminum Schottky barrier on an arsenic capped and heat cleaned GaAs(100) surface is described. The GaAs surface was prepared by molecular beam epitaxy (MBE), and covered with an arsenic cap for protection during transfer through air. The arsenic cap was sublimed by heating, and its removal was monitored by photoemission spectroscopy utilizing synchrotron radiation. After the formation of an aluminum Schottky barrier on this surface, the interfacial position of the Fermi level was measured by photoemission spectroscopy, and the barrier height was measured by I–V and C–V techniques.
Some etch properties of doped and undoped silicon oxide films formed by atmospheric pressure and plasma-activated chemical vapor deposition
Authors: Gualandris F. , Pignatel G. U. , Rojas S. , Scannell J.
Published: July 2002

Silicon oxide films formed using two different chemical vapor deposition (CVD) techniques have been characterized by selected etching properties, both in dry (plasma) etching and in wet (chemical) etching. Dry etching was accomplished using CHF3+O2 in a reactive ion etching (RIE) system. The films investigated are undoped silicon oxide, phosphorus-doped silicon oxide at 4 mol % P2O5, and phosphorus-doped silicon oxide at 9.5 mol % P2O5. The effects of densification, i.e., thermal treatments carried out at 900 °C in an inert atmosphere (N2) and at 920 °C in steam, are discussed in detail. A decrease in etch rate is observed and considered to be an indication of the structural changes that lead to less porous and more stable films.
Homo- and heteroepitaxial growth of high quality ZnSe by molecular beam epitaxy
Authors: Park R. M. , Mar H. A. , Salansky N. M.
Published: July 2002

ZnSe layers (1–2 μm thick) have been grown by molecular beam epitaxy on both (111) and (100) oriented ZnSe substrates. The layers were grown on atomically clean, single-crystalline ZnSe surfaces prepared in situ by an Argon-ion sputtering/annealing process. Homoepitaxial ZnSe layers grown on (111) ZnSe exhibited microtwinning as indicated by reflection high energy electron diffraction (RHEED) observations together with zero detectable excitonic photoluminescence emission. In contrast, layers grown on (100) ZnSe had smooth surfaces, indicated by a streaky RHEED pattern, and exhibited strong 4.2 K excitonic emission at 2.7980 eV (I0α). This can be contrasted with the Ga-bound excitonic emission at 2.7972 eV (IGa20) observed from layers grown on (100) GaAs substrates.
Factors affecting the growth of an integrated Ga1−xInxAs/InP PIN–FET by molecular beam epitaxy
Authors: Scott E. G. , Wake D. , Livingstone A. W. , Andrews D. A. , Davies G. J.
Published: July 2002

A combination of electrical and optical properties makes GaInAs lattice matched to InP a suitable material for the fabrication of an integrated PIN-FET receiver for use in telecommunications applications at 1.55 μm. As steps towards integration, the individual devices in such a receiver have been fabricated using molecular beam epitaxy (MBE) material. With the use of careful substrate cleaning it has been demonstrated that significant improvements in the epitaxial quality of GaInAs layers can be produced. JFETs have been fabricated from this material and using a novel self-aligning source/drain contact method, source contact to channel resistance has been minimized by the formation of submicron source to gate spacings. These devices had a dc transconductance of 60 mS/mm for a 3 μm gate length. Similarly PIN diodes have been fabricated. The high field region of the device contains low doped GaInAs, Nd–Na approximately 5×1015 cm−3 with corresponding μ300>9000 cm2 V−1 s−1. For monolithic integration, the two components require an intervening isolation region. The choice of isolation material as well as the optimization of growth conditions for the integrated structure together with preliminary device results are discussed.
Effect of dopant implantation on the properties of TaSi2/poly-Si composites
Authors: Vaidya S. , Retajczyk T. F. , Knoell R. V.
Published: July 2002

Refractory metal silicide/poly-Si composites are replacing poly-Si in VLSI circuits in order to reduce runner resistance without sacrificing poly-Si MOS compatibility. In a typical integrated circuit (IC) fabrication sequence, the gate serves as a mask for the formation of self-aligned sources and drains. It is, therefore, exposed to various heavy implants of B, P, or As. These implants subsequently redistribute during further processing. This paper investigates the effect of dopant implantation and redistribution on the mechanical, structural and electrical properties of silicided poly-Si. Implantation increases the sheet resistance of 2500 Å of sintered TaSi2 by a factor of 2–3. This coincides with a decrease in the silicide stress, proportional to the implant range in TaSi2, and to a lesser degree, the implant dose. These stress changes, however, recover by 500 °C, while the TaSi2 resistivity returns to its nominal value (2–2.5Ω/⧠) following a 900 °C heat cycle. Doses as high as 1E16As/cm2 do not render TaSi2 amorphous. If the underlying LPCVD poly-Si is initially undoped, the nature of the impurity appears to influence subsequent poly-Si recrystallization. With P/As, the poly-Si grains grow during a high temperature anneal. However, with B, the grains remain needle-like. This recrystallized poly-Si grain structure is nevertheless significantly different from the large, equiaxed grains developed when the poly-Si is doped in a PBr3 gas flow, prior to TaSi2 deposition. N-type impurities readily redistribute from the silicide into poly-Si by 950 °C. Thus, a single n+ source/drain implant can effectively dope the gate, buried gate-substrate contacts and junctions in NMOS circuits. The corresponding gate metal work function φm is comparable to the reported values for n+ poly-Si. B, on the other hand, is retained in the silicide. This results in a somewhat lower φm value than expected for p+ poly-Si.
Theoretical studies of the intrinsic quality of GaAs/AlGaAs interfaces grown by MBE: Role of kinetic processes
Authors: Singh Jasprit , Bajaj K. K.
Published: July 2002

We present a model for the growth of III–V compound semiconductors by molecular beam epitaxy which is based on an atomistic Monte Carlo simulation and a statistical fluctuation theory. This model allows one to understand the microscopic nature of the growth front and the interface as well as to identify the critical kinetic parameters responsible for their quality. We find that under normally employed growth conditions the cation surface migration rate is the most important parameter in controlling the surface and interface quality.
Material effects on the cracking efficiency of molecular beam epitaxy arsenic cracking furnaces
Authors: Lee Reuy-Lin , Schaffer William J. , Chai Young G. , Liu David , Harris James S.
Published: July 2002

We have constructed a solid arsenic source cracking furnace and have compared the cracking patterns with four different cracking baffle materials: graphite, tantalum, PBN, and molybdenum. The results show that a definite baffle material effect is observed. For graphite, molybdenum, and PBN, greater decomposition is favored at high temperatures, with the PBN baffles being the least effective. For tantalum baffles, an optimal temperature window is obtained and the dimer content drops at temperatures above 850 °C.
Low pressure chemical vapor deposition boro-hydro-nitride films and their use in x-ray masks
Authors: Dana S. S. , Maldonado J. R.
Published: July 2002

This paper describes the characterization of amorphous B–H–N films deposited by low pressure chemical vapor deposition, and their use in the fabrication of mask substrates for a step and repeat x-ray lithography system utilizing a storage ring source. The chemical composition of the films was analyzed by ion beam techniques. Because the experimental results indicate that the chemical composition differs from stoichiometric boron nitride, it is proposed to call them ‘‘boro-hydro-nitride’’ (B–H–N) films. The stress and optical properties of the films were measured for various values of the deposition parameters. It is shown that these properties can be correlated to the hydrogen content of the films. Membranes ∼2 μm thick with a central area 6 cm in diameter are presently fabricated for x-ray mask substrates.
Growth of silicon homoepitaxial thin films by ultrahigh vacuum ion beam sputter deposition
Authors: Schwebel C. , Meyer F. , Gautherin G. , Pellet C.
Published: July 2002

Silicon homoepitaxial films have been grown by ion beam sputter deposition using an ultrahigh vacuum (UHV) apparatus with in situ diagnostic equipment. The deposition conditions are characterized and the beginning of single crystal growth occurs at 250 °C. Films of high crystalline and morphological quality are obtained at deposition temperatures above 700 °C, where good doping element transfer efficiency from the target to the film is observed. Room temperature bulk mobility is found for film thicknesses as low as 0.5 μm and deposition temperatures near 700 °C.
Deposition properties of silicon films formed from silane in a vertical-flow reactor
Authors: Foster Derrick W. , Learn Arthur J. , Kamins T. I.
Published: July 2002

The growth of silicon films by low pressure chemical vapor deposition in an unique vertical-flow reactor and in a conventional tube reactor is studied, with emphasis on the vertical-flow reactor. For a hydrogen-carried silane process in the vertical-flow reactor, the growth rate depends on both the partial pressure and the flow rate of silane. A growth rate expression incorporating both of these parameters is derived which accounts for a nearly linear dependence on either parameter for small values and saturation at large values. No dependence on hydrogen partial pressure is observed. An Arrhenius-type dependence of growth rate on temperature is observed for both type of reactors, with an activation energy of 1.5 eV. Thickness uniformity of the films deposited in the vertical-flow reactor is compared to that found in the conventional reactor. For high temperatures (above 650 °C) depletion effects degrade the thickness uniformity of the deposited films. Such effects are more pronounced in the conventional reactor than in the vertical-flow reactor. These effects can be attributed to the shorter gas flow path in the vertical-flow reactor. The vertical-flow reactor can be operated without a temperature gradient along the deposition chamber so that uniform film properties can be obtained over the entire load.
Current transport mechanisms in GaAs/AlAs tunnel structures grown by metal–organic chemical vapor deposition
Authors: Bonnefoi A. R. , Chow D. H. , McGill T. C. , Burnham R. D. , Ponce F. A.
Published: July 2002

Elastic and inelastic tunneling processes are investigated in GaAs–AlAs–GaAs double heterojunctions grown in the [100] direction by metal–organic chemical vapor deposition (MOCVD). The AlAs quantum barriers in the heterostructures studied are doped p-type with Mg. Theoretical calculations of tunneling currents are performed and compared with experimental I–V data. It is found that for structures with thin AlAs barriers, the dominant current transport mechanism at low temperatures is tunneling through the AlAs band gap at both the Γ and X points. This is consistent with inelastic processes observable in first (dI/dV) and second (d2I/dV2) derivative spectra obtained with modulation techniques. A simple model, developed for calculating impurity-assisted tunneling currents, shows that the role of barrier impurities becomes more important as the barrier is grown thicker. Implications of some of these results for resonant tunneling heterostructures consisting of two AlAs quantum barriers separated by a GaAs quantum well are discussed. Experimental second derivative spectra showing reproducible features are also presented for these double barrier structures.
Near surface contamination of silicon during reactive ion beam etching with chlorine
Authors: Charvat Peter K. , Krueger E. Eric , Ruoff Arthur L.
Published: July 2002

The concentration and distribution of chlorine incorporated into silicon substrates during reactive ion beam etching is investigated. Rutherford backscattering spectrometry and Auger sputter profiling are employed to analyze the single crystal silicon samples. Data are presented for samples etched at various energies, current densities, and angles of incidence. The effect of temperature, dopant concentration, and crystalline orientation are also discussed. The amount of chlorine retained by the substrate decreases sharply with increasing energy and temperature. The quantity of chlorine retained increases linearly with current density, and is insensitive to doping level and orientation. Chlorine incorporation increases with angle of incidence as A/cos (bθ), where θ is measured with respect to the surface normal. Samples etched at low energies show surface roughening and thick oxide growth. The data are in large part accounted for by a combination of direct and recoil implantation. However it is argued that radiation enhanced diffusion is responsible for results obtained at high current densities and large angles to the beam. Implications for reactive ion etching are discussed.
Electrical behavior of GaAs–AlAs heterostructures
Authors: Woodward T. K. , McGill T. C. , Burnham R. D.
Published: July 2002

We report an experimental study of the electrical behavior of GaAs–AlAs–GaAs heterostructures grown by metal–organic chemical vapor deposition. The structures consisted of a layer of AlAs several thousand angstroms thick sandwiched between layers of GaAs which were a few microns thick. The top layer of GaAs was doped degenerately n-type with Se, while the bottom layer was nondegenerately doped. Capacitance–voltage (C–V) and curent–voltage (I–V) curves were obtained as a function of temperature, illumination, and rate of data acquisition. Deep-level transient spectroscopy (DLTS) measurements were also made. The C–V showed hysteresis near zero bias with the capacitance being larger when the voltage was swept from reverse to forward bias in the dark. The C–V displayed a light sensitive peak near zero bias. With illumination, the capacitance was greater, and no hysteresis was observed. We explain these phenomena as being due to deep levels near the AlAs–GaAs interface; DLTS has confirmed this. I–V curves taken in darkness also showed hysteresis. We take this as further evidence of deep levels. Additionally, capacitance failed to level off in reverse bias, indicating a lack of inversion in the samples.
Oxidation of hydrogen doped tantalum films on silicon
Authors: Ohfuji Shin-ichi , Hashimoto Chisato
Published: July 2002

Hydrogen doping of Ta films before thermal oxidation affects the dc leakage current in oxidized Ta films. Metal-insulator-semiconductor (MIS) capacitors of Al/Ta2O5 (40 nm thick)/Si-substrate structures have been prepared for the electrical measurements. The Ta films were reactively sputtered on p-type Si substrates in a mixture of Ar and H2 gases suitable for hydrogen doping. The hydrogen doping was found to be effective in reducing the leakage current in the Ta2O5 films to be less than 10−3 times that of undoped films, especially after low temperature oxidation at 400 °C. Secondary ion mass spectrometry (SIMS) analysis showed that the incorporation in the Ta2O5 films of Si from the substrate was decreased by the presence of hydrogen during sputtering. A possible explanation for the reduction of leakage current caused by hydrogen doping is that the prevention of Si entry into Ta2O5 films reduces the density of oxygen vacancies induced by unsaturated SiOx(x<2). This is thought to suppress the Poole–Frenkel-type conduction in the Ta2O5 films.
Nitrogen, oxygen, and argon incorporation during reactive sputter deposition of titanium nitride
Authors: Williams D. S. , Baiocchi F. A. , Beairsto R. C. , Brown J. M. , Knoell R. V. , Murarka S. P.
Published: July 2002

For the reactive sputter deposition of titanium nitride, stress and resistivity of the films has been measured as a function of the processing variables target power, substrate bias, pressure, and N2/Ar ratio. These studies were limited to the conditions that produce titanium nitride of stoichiometry near 1. Through Rutherford backscattering spectroscopy, the changes in stress and the conductivity of the films as a function of the processing variables were interpreted in terms of nitrogen, argon, and oxygen concentration in the films. The increase in resistivity of the films correlates with increased oxygen incorporation and the increase in compressive stress of the films correlates with increased argon incorporation. The amount of oxygen in the films appears to produce a unique value of resistivity but the argon concentration that produces a given compressive stress is a function of the processing parameters that control argon incorporation.
An x-ray photoelectron spectroscopy and low-energy electron diffraction controlled surface preparation of Si(100) prior to epitaxial growth of GaAs
Authors: Contour J. P. , Massies J. , Arnaud d’Avitaya F.
Published: July 2002

Low-energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS) analyses have been performed after the different steps of a Si(100) low temperature cleaning procedure prior to GaAs heteroepitaxial growth. The residual oxygen and carbon contamination observed after deoxidation by HF–ethanol under dry nitrogen is not completely eliminated after heating to 750 °C under ultrahigh vacuum, in spite of the observation of a clear 2×1 LEED pattern. The complete removal of oxygen and carbon is only achieved after deposition of a thin GaAs prelayer and its evaporation at 750 °C.
Evaluation of interface defects and the effect of iodine impurity in low-resistivity metal-organic chemical vapor deposition-grown ZnS films on GaAs
Authors: Kawakami Yoichi , Taguchi Tsunemasa , Hiraki Akio
Published: July 2002

Characteristics of the structural defects and impurities, which appear at the interface and within a few μm layers of an epitaxial ZnS grown by low-pressure metal-organic chemical vapor deposition (MOCVD) on a GaAs substrate, have extensively been elucidated by means of a 2 MeV transmission-electron microscope, Rutherford backscattering spectroscopy (RBS)/channeling, secondary ion mass spectrometry (SIMS), and 4.2 K photoluminescence measurements. It is found that the unintentionally doped, relatively thick-ZnS films grown onto the (100) substrates, exhibit a defect density of about 4×1015 cm−3. These films are of a high-epitaxial quality, although the ZnS/GaAs interface has a high density of interfacial disorder induced by lattice mismatch. The variation of the lattice constant in the epitaxial ZnS as a function of thickness can be qualitatively interpreted by considering the influence of a tensile strain in ZnS/GaAs structure. Bound-excitonic lines and two types of the edge-emission bands were observed at 4.2 K; the origin of the acceptor impurities is discussed in relation to SIMS results. We have, for the first time, found that the I donor impurity can be successfully incorporated in the ZnS epitaxial layer during MOCVD growth using HI or I2 gas. Consequently, a highly conductive ZnS:I layer having a resistivity of about 1 Ω cm and an electron concentration of about 1017 cm−3 is obtainable and exhibits a strong-blue emission band which is ascribed to the self-activated center in the vicinity of 460 nm at room temperature.
Surface structure and interface formation of Si on GaAs(100)
Authors: Bachrach R. Z. , Bringans R. D. , Olmstead M. A. , Uhrberg R. I. G.
Published: July 2002

The interfacial development of Si on GaAs(100) has been studied using core level photoemission. The studies included arsenic rich and gallium rich starting surfaces and a variety of annealing procedures. Si deposited at various coverages below 10 monolayers (ML) onto a room temperature substrate yields a disordered surface. For coverages below 2 ML, subsequent annealing leads to a predominantly single domain 1×2 surface. Deposition onto a heated substrate in the range of 450–600 °C yields the 1×2 reconstruction directly. Growth of a GaAs–Si alloy yielded a 1×2 surface with diffraction spot broadening indicating considerable strain.
Mechanism for annealing-induced changes in the electrical characteristics of Al/GaAs and Al/InP Schottky contacts
Authors: Newman N. , Spicer W. E. , Weber E. R.
Published: July 2002

We report a systematic study of the electrical properties of Al diodes formed on GaAs (110) and InP (110) surfaces. Intimate Schottky barrier diodes were fabricated by in situ metal deposition in ultrahigh vacuum on atomically clean cleaved n-type and p-type surfaces. Current–voltage (I–V) measurements as a function of temperature were performed to determine the Schottky barrier heights using thermionic emission theory. For the unannealed Al diodes, our results indicate that the interface Fermi level of n-type and p-type samples pins at essentially the same position within the band gap of GaAs and InP. Annealing-induced changes in the electrical characteristics of the Al diodes can be completely accounted for by a shift in the Fermi level pinning position toward the valence band maximum of the GaAs and InP. We show that this is the only possible alternative because the annealing-induced increase in the barrier height of the diodes formed on n-type material is equal in magnitude to the decrease in the barrier height of the diodes formed on p-type material. Previous reports have attributed the increase in the apparent barrier height of n-type samples, which occurs upon annealing, to the formation of a larger band-gap semiconductor at the interface (i.e., AlxGa1−xAs and AlxIn1−xP) or to changes in the current transport process due to an increased rate of trapping carriers at deep levels near the interface. These mechanisms can be ruled out because they are inconsistent with the new data for diodes formed on p-type material. Because of the unique properties of Al and its interaction with the semiconductor substrate, we show that the results of this study provide a particularly critical test of different models of the Schottky barrier.
Study of CoSi2/Si strained layers grown by molecular beam epitaxy
Authors: Kao Y. C. , Wang K. L. , de Fresart E. , Hull R. , Bai G. , Jamieson D. , Nicolet M-A.
Published: July 2002

In this paper, we report a study of the strain and its relaxation of epitaxial CoSi2 grown on Si by molecular beam epitaxy (MBE). The strain is measured by a x-ray rocking curve technique and misfit dislocations are determined by transmission electron microscopy (TEM) as the strain relaxes. Results show that the critical thickness of pseudomorphic growth is about 30 nm for growth temperature of 550 °C although there is still a residual strain remained for the thicker films. No apparent complete relaxation of the strain is obtained. Thermal annealing of CoSi2 films is performed and the results of the strain relaxation are discussed.
Chemically assisted ion beam etching of polycrystalline and (100)tungsten
Authors: Garner Charles
Published: July 2002

A chemically assisted ion beam etching (CAIBE) technique is described which employs an ion beam from an electron bombardment ion source and a directed flux of ClF3 neutrals. This technique enables the etching of tungsten foils and films in excess of 40 μ thick with good anisotropy and pattern definition over areas 30 mm2, and with a high degree of selectivity. (100)tungsten foils etched with this process exhibit preferred orientation etching, while polycrystalline tungsten films exhibit high etch rates. This technique can be used to pattern the dispenser cathode surfaces serving as electron emitters in traveling wave tubes to a controlled porosity.
Ionized cluster beam deposition
Authors: Knauer W. , Poeschel R. L.
Published: July 2002

An experimental cluster beam facility of the type used by Takagi was developed and tested. Since the cluster generation rate (of silver clusters in graphite crucibles) was small and not consistently observable, a basic study of the generation process was undertaken. Theoretical evaluation has revealed that clusters are formed on crucible internal surfaces and not in the vapor stream which passes out of the crucible through a nozzle, as had been assumed earlier. Experiments have confirmed this heterogeneous cluster growth model, showing that clusters occur consistently only when a temperature distribution is maintained within the crucible which favors cluster formation on surfaces near the crucible nozzle. A remaining problem is the low rate of cluster generation. This rate needs to be raised if metal clusters are to be used in practical film deposition apparatus. An alternative process which involves gaseous rather than metallic clusters is also under study. It is well known that isentropic expansion of gases, pressurized to a few atmospheres, results in homogeneous nucleation and can produce appreciable quantities of clusters with thousands of atoms. In order to utilize such gaseous clusters for film deposition applications, an assist mode is used in which films are grown by simultaneous vapor deposition and bombardment with energetic noble gas clusters. So far, a suitable cluster beam source has been developed, and deposition of gold films on GaAs substrates has been initiated, whereby ionized argon cluster beams provide the energy.
Microstructure fabrication and transport through quantum dots
Authors: Randall J. N. , Reed M. A. , Moore T. M. , Matyi R. J. , Lee J. W.
Published: July 2002

We report the microfabrication techniques used to produce devices which study electronic transport through quantum dots. Molecular-beam epitaxy, electron-beam lithography, and reactive ion etching have been utilized in this effort. The minimum physical lateral size of the dots reported here is 0.1×0.2 μm. Transport shows some degradation in I–V characteristics with respect to much larger resonant tunneling diodes. Current densities observed suggest the electrical size of diodes is smaller than the physical size. A surface depletion region of 500 Å may account for this effect. Telegraph noise is observed as a result of single electron trapping in the structure. No clear evidence of lateral quantization has been observed.
Morphology of thermally etched GaAs substrate and molecular-beam epitaxial layers grown on its substrate
Authors: Saito Junji , Nanbu Kazuo , Kondo Kazuo
Published: July 2002

The surfaces of a thermally etched GaAs substrate and a GaAs epitaxial layer grown on its substrate by molecular-beam epitaxy have been observed for the first time using reflection high-energy electron diffraction (RHEED) and a Nomarski contrast microscope. The surface morphology of the thermally etched GaAs substrate degraded, taking on an orange-peel appearance, as the etching depth was increased. When a GaAs epitaxial layer with a thickness of 150 Å was grown on a thermally etched substrate, the RHEED pattern became streaked, indicating that the surface was smooth on an atomic scale. However, using a Nomarski contrast microscope, it was observed that the surface morphology of the epitaxial layer remained rough with wave longer than the optical wavelength, in the same way as the substrate. Moreover, an epitaxial layer grown on a conventionally cleaned GaAs substrate was thermally etched, and the surface was observed using RHEED, a Nomarski contrast microscope, and electron microscopy of platinum–carbon replica, in order to reveal the morphology. The morphology of the thermally etched epitaxial layer was smooth and quite different from that of the thermally etched substrate. As far as the origin of the surface degradation in the thermally etched substrate is concerned, it is believed that residual carbon contamination on the substrate surface acts as a mask, preventing the sublimation of GaAs.
Radiation damage in boron nitride x-ray lithography masks
Authors: King Paul L. , Pan Lawrence , Pianetta Piero , Shimkunas Alex , Mauger Philip , Seligson Daniel
Published: July 2002

The optical and mechanical properties of boron nitride vapor deposited at 400 °C are shown to degrade when exposed to synchrotron radiation. The extent of the damage and the rate at which the damage occurs are similar to that first reported by Johnson et al. Transmission through membranes of boron nitride was measured in situ during exposure to x rays. Membranes darkened considerably with the transmission through a typical membrane falling from 50% to 20% after absorbing ∼250 kJ/cm3 of x rays. Changes in local film stress were measured with a simple cantilever technique. Films originally in tension (∼5E8 dyne/cm2) were found to become compressive after absorbing 300 kJ/cm3 of x rays. Both forms of damage responded well to annealing. Fourier transform infrared (FTIR) and near-edge x-ray absorption measurements were made to discern the structural differences between degraded and unexposed films. No significant structural changes were observed. Boron nitride films deposited at higher temperatures (600 °C) proved to be much less susceptible to radiation damage. In a related development, films produced through the pyrolysis of borazine appeared to be completely immune to radiation damage as were silicon and silicon nitride membranes. FTIR spectra indicate that less hydrogen is present in the 600 °C chemical-vapor deposition (CVD) and the pyrolytic boron nitride films than in the 400 °C CVD films. As proposed by Johnson et al., hydrogen is implicated as an intermediary in the boron nitride damage mechanism.
Radiation stability and damage mechanisms in x-ray membranes
Authors: Vladimirsky Y.
Published: July 2002

The radiation stability of x-ray membranes is of major importance for future development of x-ray lithography and x-ray microscopy. The primary concern in this respect is loss of dimensional stability due to the radiation damage. The dose required to introduce sensible distortion of the pattern on a membrane is far below that necessary for the radiolysis of the membrane material. The ability of a material to resist generation and accumulation of the radiation defects is related to its band structure. Mechanisms of radiation defect generation, protective mechanisms, and possible ways to control the radiation resistance are discussed.
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u investigation of TiN formation on top of TiSi2
Authors: Willemsen M. F. C. , Kuiper A. E. T. , Reader A. H. , Hokke R. , Barbour J. C.
Published: July 2002

TiN formation on TiSi2 has been studied using in situ Rutherford backscattering spectroscopy and Auger electron spectroscopy. Two systems were investigated: (i) the reaction of Ti with nitrogen and (ii) the nitridation of TiSi2. When a Ti film on Si is annealed in N2 or NH3, two reactions occur simultaneously: TiN forms at the surface and TiSi2 at the interface. Oxygen, dissolved in the original Ti film, is expelled from the growing silicide and is piled up at the TiN/TiSi2 interface. Around 600 °C, the complete conversion of the upper nitrogen-containing layer into TiN is retarded by this oxygen and a TiNxO1−x layer remains detectable between the TiN and TiSi2 layers. At 750 °C, the TiN layer is formed very rapidly and further growth is blocked by the TiSi2 layer that has developed underneath. Nitridation of TiSi2 requires a temperature of at least 800 °C. Starting at the exposed surface, the silicide layer is gradually converted into TiN. Some of the Si released in this reaction segregates to the surface and forms islands. Most of the Si, however, segregates to the Si substrate where epitaxial regrowth occurs. Nitridation of TiSi2, therefore, results in rough layer structures, in contrast to nitridation of Ti on Si. The differences in layer morphology for these two processes are clearly evidenced by transmission electron microscopy cross sections.
High-throughput nanolithography using an oxygen-plasma resistant two-layer resist system
Authors: Lin P. S. D. , Gozdz A. S.
Published: July 2002

LaB6-based electron beam nanolithography systems suffer from low beam current, resulting in low throughput for nanometer-sized patterns. This drawback is further aggravated by the low sensitivity of the standard high-resolution resist [poly(methylmethacrylate) (PMMA)] and by the reduction of intraproximity effect. We have investigated the lithography aspects (contrast, resolution, exposure latitude, throughput, and statistics) of two-layer resist systems consisting of a sensitive positive electron beam resist poly(3-butenyltrimethylsilane sulfone) (PBTMSS) as the top layer, and a variety of materials such as diamondlike carbon film or polyimide as the bottom layer. In these systems, exposed and developed PBTMSS is first oxidized to SiO2, which masks etching of the bottom layer. The pattern is in turn transferred to the substrate either by wet or dry etching using the bottom layer as a mask. Grating patterns with 0.2- and 0.1-μm pitch can be routinely fabricated on semiconductors, and 0.05-μm pitch patterns have been demonstrated. To achieve such high resolution, a low-strength developer is used, but a throughput about 30 to 50 times higher than that of PMMA can still be obtained. Gratings now can be patterned in a few hours over square-centimeter areas. Such a task would require days for PMMA. Implications of our results on lithography system clock rate and resist resolution–sensitivity tradeoff are also discussed.