The electrical and microstructural properties of Au/Ti and Au/Pd/Ti contacts to p+-GaAs (C-doped, 5×1018cm−3) were investigated. Current-voltage measurements as a function of temperature showed that the Ohmicity of the Au/Ti contact improved upon annealing. However, the annealed binary contact featured Au spiking into the GaAs making it unsuitable for HBT applications. The addition of a Pd diffusion barrier layer between the Au and Ti metallisation layers prevented spiking, but resulted in a decrease in the Ohmicity of the contact.

For all the contact systems it was found that thermionic-field emission dominates the current transport mechanism across the metal-semiconductor interface between the temperature range 198K and 348K. The Au/Pd/Ti contact structure shows HBT potential, however higher epilayer doping levels will be required to produce satisfactory specific contact resistivities.

Near-theoretical-minimum values of specific contact resistivity, ρc (in the mid-to-low E-8 Ω-cm2 range) have been achieved for Ni-based contacts to moderately doped (2E18 cm−3) n-type InP. In each case these values are an order of magnitude lower than those previously achieved. These ultra-low resistivities are shown to result when the metallurgical interaction rate between the contact metal and the semiconductor is sufficiently reduced. Several methods of reducing the metal-InP reaction rate and thus achieving lowered resistivity values are demonstrated. We show, for instance, that the introduction of a thin (100Å) Au layer at the metal-InP interface retards metal-semiconductor intermixing during sintering and results in a ten-fold reduction in pc. Another method consists of ensuring the perfection of the near-surface InP lattice prior to and during contact deposition process. Use of this technique has enabled us to fabricate, for the first time, Ni-only contacts with pc values in the low E-8 Ω-cm2 range. We present an explanation for these observations that is based upon the magnitude of the In-to-P atomic ratio at the metal-InP interface.

The development of shallow ohmic contacts for the thin p-InGaAs layer (p≤1*1019 cm−5) of InP-based heterojunction bipolar transistors is a severe challenge to contact technology. While standard metallizations reveal several drawbacks, Pd/Ge-based systems emerged as promising candidates. In this work, Zn or Cd was implanted into the inner Pd contact layer for lowering contact resistivity. We present a study of the electrical and metallurgical properties of Pd/Ge contacts to p-InGaAs, and the influence of Zn and Cd implanted into the metallization. The resistivity of implanted and annealed (450-575 °C) contacts is reduced up to one order of magnitude compared to the unimplanted contact. Backside SIMS measurements show that annealing leads to a very limited interdiffusion at the interface. Cd and Zn diffuse into the InGaAs layer to a depth of approx. 30 and 40 nm, respectively. Due to these features, this implanted Pd/Ge contact scheme is a promising candidate for shallow contacts to p-InGaAs.

Ohmic contacts to p-type InP with an In0.47Ga0.53As buffer layer and an interposed superlattice of 50 Å In0.47Ga0.53As/ 50 Å InP have been investigated. Initial studies of contacts to In0.47Ga0.53As/ InP without the superlattice structure have shown that Pd/Zn/Pd/Au metallization produced a lower specific contact resistance (pc = 1.1 × 10−4 Ω cm2) than Pd/Ge/Au, and over a wider range of anneal temperature than Au/Zn/Au. The incorporation of the superlattice in the p-In0.47Ga0.53As/ InP structure resulted in Pd/Zn/Pd/Au contacts with pc of 3.2 × 10−5 Ω cm2 as-deposited and 7.5 × 10−6 Ω.cm2 after a 500 °C anneal. The presence of Pd/Zn in the metallization was shown as important in reducing pc. Significant intermixing of the metal layers and In0.47Ga0.53As occured at ≥ 350 °C, as revealed by Rutherford backscattering spectrometry.

A method to realize high quality SiGe/Si heterostructures where surface segregation seriously deteriorates the interface integrity is discussed. After clarifying the mechanism of surface segregation, a new technique, called segre-gant-assisted growth (SAG), where atoms having a strong segregation tendency are introduced at heterointerfaces is proposed and its advantages are demonstrated. Intersubband transition of electrons in the conduction band can be clearly observed even in narrow quantum wells (QWs), and the well width dependence reflecting the square shape potential is obtained in the absorption peak energy. Gas source MBE (GSMBE), which is considered to be quasi-SAG with hydrogen generated at the growth front acting as a segregant, is shown to provide high quality SiGe/Si heterostructures with abrupt interfaces. Highly efficient band edge luminescence is observed in the QWs grown by the SAG method, especially by GSMBE, and the quantum confinement effect is confirmed. Electroluminescent diodes providing band edge luminescence are fabricated by this method, suggesting a high potential for SiGe/Si heterostrucutres in device applications.

Heteroepitaxial growth of 3C-SiC on Si in gas source molecular beam epitaxy ( GSMBE ) was carried out by a combination of carbonization of a Si surface and subsequent crystal growth on it using hydrocarbon radicals and Si2H6. The carbonization process and the initial stage of the subsequent growth during the intermittent supply of Si2H6 have been studied by a reflection high-energy electron diffraction (RHEED) observation. A Si surface was chemically converted to 3C-SiC at 750°C, and homoepitaxial growth on the carbonized layer could be obtained at 1000°C. Si atoms generated by thermal decomposition on a surface would react with hydrocarbon radicals, forming SiC through a layer by layer growth mode.

The crystalline quality of ZnS films grown on arsenic covered Si(100) surfaces is shown to be improved as compared to films grown on bare silicon surfaces by MBE (molecular beam epitaxy). Employing RGA (residual gas analyzer) and RHEED (reflection high energy electron diffraction) techniques, we found that a strong initial adsorption of sulfur on bare silicon surfaces led to the formation of disordered silicon-sulfide surfaces. This disordered surface initiated three-dimensional growth of ZnS and resulted in poor crystalline quality. An arsenic overlayer was found to be effective in preventing the interaction of sulfur with the silicon surface and thereby maintained surface ordering. X-ray rocking curve analysis indicated higher crystallinity in ZnS films grown on arsenic covered surfaces.

Epitaxial (100) CdTe layers have been grown by organometallic vapor phase epitaxy (OMVPE) on GaAs and Si substrates. A thin layer of CdTe was first grown by atomic layer epitaxy (ALE) on GaAs substrates followed by thicker CdTe layer by conventional organometallic vapor phase epitaxy (OMVPE). This process resulted in high quality (100) CdTe on GaAs substrates. On Si substrates, direct growth of CdTe resulted in only polycrystalline layers. Hence, a thin Ge buffer layer grown at low temperature followed by an interfacial layer of ZnTe was used to get high quality (100) CdTe on Si. The process developed here eliminates the high temperature (>850°C) deoxidation step generally required when Si substrates are used. The CdTe layers were characterized by X-ray diffraction and optical microscopy. X-ray rocking curve with full width at half maximum (FWHM) of about 260 arcsec has been obtained for a 4 um thick CdTe layer. The results presented demonstrate novel techniques to control the hetero-interfaces in order to grow high quality CdTe on GaAs and Si substrates.

Chemical adsorption of gases onto semiconductor surfaces results in a change in the density of electronic surface states. The corresponding change in the number of surface charges gives rise to a measurable change in the bulk charge distribution within the semiconductor. This phenomenon forms the basis of a novel solid-state chemical sensor device using the intensity of photoluminescence (PL) as a measurement technique. We have observed changes in the PL intensity from MOVPE-grown GaAs/Al0 3Ga0 7As heterostructures due to the chemisorption of SO2 onto “photowashed” GaAs surfaces. The photowashing technique is believed to reduce the density of preexisting surface states by removing As from the GaAs surface oxide. The PL signal-to-noise ratio and PL sensitivity characteristics of these chemical sensors may be dramatically improved by varying semiconductor structure. We have observed a range of PL enhancements upon exposure to 0.6% SO2 in flowing N2 from heterostructure layers with various thicknesses and doping densities. These results are explained by comparison with a FEM numerical model derived to correlate the relative PL intensity from various heterostructures to the surface charge density.

The deposition of thin films on III-V compound semiconductors is encountered with various interfacial problems. We present a new approach using a modified plasma technique in a phosphorous ambient at substrate temperatures ranging from 250 to 350 °C which covers two import aspects: Firstly, a good insulator quality due to the plasma process. Secondly, an appropriate surface pre-treatment and defect passivation by a phosphine-like cleaning process using a solid phosphorous source. The analysis of the film adhesion yields improved adhesive properties due to the high activation energy of a plasma process for surface reactions, and by preventing a phosphorus-related defects. The phosphorous concentration in the SiO2 films has to be adapted in order to prevent electrical degradation of the SiO2-InP interface. The influence of phosphorous deficiency and excess phosphorous will be discussed.

The chemical state of (NH4)2S treated (100)GaAs surfaces exposed to ambient conditions for several days was correlated with barrier height measurements. Surface chemistry was characterized by Auger electron spectroscopy and barrier heights were calculated from C-V measurements obtained with a Hg probe. Results show that surfaces are chemically and electrically unstable for several hours following the sulfide treatment. The chemical and electrical states continually changed during ambient exposure up to 300 hours. Although strictly speaking, the surfaces were not passivated, the presence of sulfur did inhibit the formation of Ga and As oxides and the incorporation of carbon. In addition, stable, low barrier heights were observed after ambient exposure for several hours. Barrier heights from C-V measurements using deposited Au and Al contacts were compared to the barrier heights obtained with a Hg probe.

Process chemicals in use today are quite pure, depending on the grade used but can easily be contaminated with metal ions through improper handling or storage techniques. Such metal impurities, if deposited on wafer surfaces during processing can increase reverse-bias junction leakage, degrade oxide breakdown strength and increase metal oxide semiconductor capacitor leakage which in turn can adversely affect the function of ultra large scale integrated (ULSI) circuits. Because of these device effects and since metal contamination can come from several sources it is important to know the deposition level and mechanism on Si wafers from a given process solution. HF based process solution have been investigated due to their historical use in patterning, etching, and their increased use in advanced wafer cleaning processes.

Multi-contaminant experiments have been designed to study the probability of, and mechanism for deposition. It is shown that in general single element deposition is predicted by electrochemical considerations based on redox reactions, however, potential complex synergistic interactions can cause deviations from the simple theory. Specifically, Sn is shown to inhibit the deposition of Ag, and Mo does not deposit in proportion to the amount of Mo in solution, but does deposit on wafers in the presence of Cu and/or Ag. Detailed analysis of Cu and Ag from BOE and Cu from HF shows the deposition: is statistically uniform over bare Si wafers, is independent of substrate doping and rinse time, does not occur on SiO2 is linear with time (up to 25 min) and solution concentration ( up to 500 ppb), shows an Arrhenius behavior with temperature (15 - 55°C) and increases surface micro-roughness. Interpretation of these results with an Evans diagram indicates the deposition is mass transport limited, allowing a first order quantitative theoretical interpretation of the deposition process.

The adsorption of Al, Ga and Si on the Si(001) surface is studied by the ab initio molecular dynamics (Car-Parrinello) method based on the norm-conserving pseudopotential. In the stable structures obtained for half mono-layer coverage( ө = 1/2), these ad-atoms form dimers, but the dimer configurations are different. Al and Ga atoms form parallel dimers whose dimerization direction is parallel to that of substrate Si-dimers, while adsorbed Si atoms form (dense) orthogonal dimers. The electronic origin of the difference in the stable configurations among Al, Ga and Si ad-atoms is analyzed by calculating the local density of states (LDOS) of each atom.

This study explores the effects of two chemical vapor cleaning chemistries on silicon surfaces. The silicon surfaces are not significantly roughened by exposure to either process. Trace amounts of fluorine are found on the surfaces exposed to 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (HFAC). A thin silicon nitride film forms on the silicon surface as a result of exposure to the HMDS process and is attributed to the H2/N2 plasma treatment used in the first step of the process.

We studied surface cleaning using photoexcited fluorine gas diluted with hydrogen (UV/F2/H2). We found that UV/F2/H2 cleaning selectively removes native Si oxides from thermal oxides without etching the bulk Si. After UV/F2/H2 cleaning, hydrogen atoms terminate almost all the dangling bonds on the Si surface, and fluorine atoms terminate the few remaining bonds. UV/F2/H2 cleaning also flattens the Si surface. We applied UV/F2/H2 cleaning to Si epitaxy and obtained single-crystal Si films with preannealing and growth temperatures as low as 600°C, 150°C lower than for conventional methods. UV/F2/H2 cleaning is a good dry precleaning method for various processes that include Si epitaxy.

In-situ native-oxide removal is critical for epitaxial single-crystal silicon deposition, for polysilicon emitters and contacts and for ultrathin gate dielectric films in integrated circuit (IC) fabrication. We have developed an in-situ, thermally-driven, anhydrous hydrogen fluoride (AHF)-based native-oxide removal technique in which the wafer is treated by AHF at low temperatures (300-400°C) and a short (10 sec) 950°C ‘spike’ in AHF-H2 immediately prior to Si deposition. This process removes native oxides formed by standard wet cleans such as HC1:H202 and NH4OH:H202, as well as native oxides formed by the clean-room ambient. Further, the technique is an effective pre-clean for both polysilicon and epitaxial silicon deposition. This flexibility, combined with other salient features such as simplicity and a low thermal budget, make the process eminently suited for IC fabrication.

There have been several studies of second harmonic generation (SHG) from chemically-modified vicinal Si(111) wafers. The SH fields contain one-fold (ψ) and three-fold (3ψ) symmetry contributions, which originate respectively from the terrace and surface step atoms. The phase of these contributions are different for native oxides, and are a function of the frequency of the incident radiation, ω. To identify the origin of these different phases for the terrace and step SH fields, we use a classical anharmonic oscillator model based on two assumptions: (a) a significant fraction of Si atoms at the steps have dangling bonds when oxides are formed below ∼850°C, and (b) these step atoms are linked to atoms at the bottom of the steps by O-Si-O groups following annealing at >900°C.

STM observation of the wet-chemical treated Si(001) surface was carried out. On the surface treated by 1%-HF solution (pH=2), the STM images were always rough and hardly exhibited the atomic feature. The STM images on the surface treated by HCl:HF=19:1 solution (pH<1) after 1%-HF dipping were also rough in a macroscopic scale but they occasionally shown atomlike corrugation in a magnified scale. The atomlike corrugation tends to align toward [110] directions which may reflect the ordered structure on Si(001) surface. This result indicates that in the case of etching by the HCl:HF=19:1 solution, the microscopic roughness on Si(001) surface tends to decrease, which might be explained by the steric hindrance effect during extremely slow etching.

Direct wafer bonding is a viable technique for fabricating high-voltage devices. An understanding of the microstructure and electrical behavior of the bonded interface is critical for device fabrication. In this paper, we investigated the microstructure of the silicon-to-silicon bonded interface using cross-sectional transmission electron microscopy and the corresponding electrical behavior using spreading resistance probing. Results indicate that oxide precipitates were present at the bonded interface when Czochralski silicon wafer were used in the process. Oxide precipitates were noticeably absent from the bonded interface when float zone wafers were bonded to each other. We find that oxide precipitates at the interface arise not due to the residual oxide at the surface prior to wafer bonding but due to gettering of oxygen from the Czochralski wafer. Spreading resistance measurements show occurrence of a high resistivity region at the bonding interface whether or not oxide precipitates are present.

We describe an approach for rapid evaluation of thin film interfacial reactions using a combination of temperature-ramped in situ measurements of sheet resistance, calorimetry and stress. Electrical, mechanical and thermal measurements at elevated temperatures provide detailed reaction information which is unavailable in room temperature measurements. Kinetic data is particularly useful in making comparisons with mechanistic models. The following examples are discussed:

1. Effects of interfacial oxygen on Ta as a diffusion barrier between Cu and Si,

2. Effects of interfacial oxygen on the Cu/Mg reaction to form CuMg2 and Cu2Mg,

3. Effects of the density of internal interfaces (grain boundaries) on Al2Cu dissolution and precipitation in Al-Cu alloys.