A low-noise microwave HEMT using MOCVD

Low-noise high-electron-mobility Transistors (HEMT's) with AlGaAs/GaAs heterostructures have been successfully fabricated using normal pressure metal-organic chemical vapor deposition (MOCVD). Hall mobilities of the two-dimensional electron gas at the interface are 8030 and 14 8000 cm²/V . s at 300 and 77 K, respectively, with an undoped Al0.3Ga0.7As spacer layer of 100 Å. The HEMT's with 0.65-µm-long and 200-µm-wide gates have exhibited a noise figure of 1.13 dB with 10.8 dB of associated gain at 12 GHz, and a dc transconductance of 280 mS/mm. These values are comparable to other reported HEMT devices using molecular-beam epitaxy (MBE).     

Grown-film silicon transistors on sapphire

A method was developed for depositing silicon films by the pyrolytic decomposition of SiH4on single crystal sapphire. Electron diffraction and Laue reflection examinations of the films shows single-crystal patterns. The silicon film has a Hall mobility of 135 cm²/volt-second at a hole density of 10¹⁷/cm³. Insulated-gate field-effect transistors with a transconductance of 1000 µmho at 5 ma were made with dimensions of 10 µ source-to-drain spacing and an active distance of 120 µ. This value compares favorably with similar units made on bulk silicon and is very encouraging for the possibilities of thin-film silicon devices.     

Thermal annealing of proton-irradiated silicon solar cells

Solar cells made from 1.5- and 10-Ωċcm p-type silicon, with silver-titanium evaporated electrodes, were irradiated by 4.6-MeV protons at room temperature to fluences ranging from 1 × 10¹⁰to 1 × 10¹²protons/cm². The photovoltaic current-voltage characteristics, the photovoltaic spectral response, and the minority carrier diffusion length were studied as the solar cells were annealed isochronally to temperatures up to 600°C. The proton radiation damage annealed in two stages, the first occurring between 50° and 150°C, and the second between 350° and 450°C. The removal of proton damage in this manner differs markedly from the annealing reported for 1-MeV electron damage, where practically no recovery of the photovoltaic properties is observed below 350°C. At any selected annealing temperature, the 10-Ω ċ cm cells were observed to recover to a slightly greater degree than the 1.5-Ω ċ cm type.     

Laser-recrystallized silicon thin-film transistors on expansion-matched 800°C glass

Laser-recrystallized silicon thin-film transistors (TFT's) have been fabricated, for the first time, on a novel, potentially low-cost glass substrate, The 0.5-µm-thick silicon films were deposited along with appropiate dielectric layers on Corning Code 1729 glass substrates and recrystallized using an argon ion laser. The n-channel enhancement-mode transistors were made using conventional IC device fabrication procedures modified to have a maximum processing temperature of 800°C. Transistor's made in the recrystallized silicon show field-effect electron mobilities as high as 270 cm²/V.s, approximately 15 times that of comparable devices made in as-deposited polycrystalline-silicon films. The recrystallized silicon devices also exhibit lower threshold voltages and lower leakage currents than do comparable polycrystalline-silicon devices.     

Ballistic transport and properties of submicrometer Silicon MOSFET's from 300 to 4.2 K

The characteristics of submicrometer silicon MOSFET's have been measured from 300 to 4.2 K, and the mobility versus temperature and carrier velocity versus longitudinal field as a function of temperature have been plotted. Effective mobilities in 500-µm-square devices as high as 25 000 cm²/V . s at 4.2 K have been observed. Mobilities of this magnitude represent mean free path lengths that could lead to ballistic transport in submicrometer devices. Effective mobilities in 0.2-µm devices were only 800 cm²/V . s at 4.2 K due to high-field effects. The mobility versus effective channel length for 0.2-, 0.7-, and 1.7-µm devices operating at drain voltages of 0.1 V has been plotted, and it has been observed that the mobility is greatly reduced in short-channel devices. The mobility versus longitudinal field was studied, resulting in the observation that ballistic transport is inhibited by the high fields in devices operating at 0.1 V. Similar high-field effects should limit the effects of ballistic transport in high-mobility semiconductors such as submicrometer GaAs FET's Operating at nominal supply voltages.     

Effects of subgrain boundaries on carrier transport in zone-melting-recrystallized Si films on SiO2-coated Si substrates

The transport properties of zone-melting-recrystallized Si films on SiO2-coated Si substrates have been studied by the fabrication and characterization of thin-film resistors and n-channel MOSFET's. Subgrain boundaries, which are the predominant crystal defects in the films, have a relatively low trapping state density (7-8 × 10¹¹cm^-2) and low resistance. N-channel MOSFET's fabricated in the films exhibit high surface electron mobilities (∼ 640 cm²/V-s) for electron transport either parallel or perpendicular to the subgrain boundaries.     

MOSFET Characteristics in low-temperature plasma-enhanced chemical vapor deposited epitaxial silicon

The fabrication of high-quality MOSFET's using low-temperature (750-800°C) Plasma-Enhanced Chemical Vapor Deposited (PECVD) epitaxial silicon is reported here for the first time. The fabricated devices include PMOS transistors with hole channel mobilities of 213 cm²/V.s (versus 218 cm²/V.s in bulk silicon controls) and NMOS transistors with electron channel mobilities of 520 cm²/V.s (versus 560 cm²/V.s in bulk silicon controls), and with an on-current to off-current ratio of 10⁷. These results indicate that epitaxial silicon films deposited by the PECVD technique are of high quality, even though the epitaxial deposition temperature was only 750-800°C.     

Ceramic ferroelectric field effect studies

The ferroelectric field effect has been observed in a semiconducting thin film of n-type tin oxide deposited on a ferroelectric lead zirconate-titanate ceramic substrate. The semiconductor was deposited by electron gun evaporation onto a thermally depolarized (randomly oriented) substrate which permitted the carrier concentration of the film to be enhanced or depleted depending on the direction of polarization of the substrate. Typical average resistivity values of 200-Å films are approximately 0.1 Ω.cm for the depoled state, 0.01 Ω.cm for the enhanced state, and 100-1000 Ω.cm for the depleted state. "On"-"off" ratios as high as1.7 times 10^{5}have been observed in a single device. The transition from enhancement to depletion is quasi-continuous due to the small size and random orientation of the individual crystallites in the ceramic. Conductance measurements during this transition have yielded field effect mobilities in the range 7-10 cm²/V.s; and maximum average carrier densities in the range 0.5-1.0 × 10²⁰carriers/cm3. The tin oxide-ceramic devices described here suffer from the long-term drift that is characteristic of many field effect devices. The resistivity of a device stored in the depleted state will decrease from 3 to 4 orders of magnitude in times between 10⁴-10⁵minutes.     

p-Channel MODFET's using GaAlAs/GaAs two-dimensional hole gas

p-channel MODFET's were fabricated in the GaAlAs/GaAs system and the properties of the hole gas system tested to ascertain its suitability for complementary logic. The Hall mobilities on a Ga.5Al.5As/GaAs modulation-doped hole gas structure were measured to be 3650 cm²V^-1s^-1and 54000 cm²V^-1s^-1with sheet carrier concentration of 1 × 10¹²cm^-2and 7.76 × 10¹¹cm^-2at 77 and 4.2 K, respectively. The measured transconductances of 1.5-µm gate-length MODFET's on this structure were measured to lie in the range of 28-35 mS.mm^-1at 77 K. The field mobility measured on long gate-length MODFET's was approximately 3200 cm²V^-1s^-1at 77 K. Using test structures for measuring current voltage characteristic in the hole-gas system, low field drift mobility was measured to be 3000 cm²V^-1s^-1and velocities of 3 5 × 10⁶cm.s^-1were measured at electric fields of 3-4 kV.cm^-1at 77 K. The Schottky barriers showed low leakage and a barrier height of 0.7 to 0.8 eV. Calculations indicate that transconductances of approximately 100 mS.mm^-1should be achievable in this system for similar gate lengths.     

High-performance 8—14- µm Pb1-xSnxTe photodiodes

Photovoltaic detectors of Pb1-xSnxTe, sensitive in the 8- 14-µm spectral region, with near-theoretical performance characteristics, have been fabricated by a surface inversion technique. These detectors have exhibited quantum efficiencies of up to 45 percent limited by surface reflection), zero-bias resistance-area products of 21 Ωċcm²and background-limited detectivities of 1.5 × 10¹¹cm ċ Hz^1/2/W. A theoretical model consistent with experimental results is presented. Such high-performance devices are useful in guidance, reconnaissance, surveillance, ranging, and communication systems.     

p-Channel MOSFET's in LPCVD PolySilicon

p-channel MOSFET's have been fabricated in LPCVD polysilicon. A 5000-Å n⁺poly acts as the gate electrode on which a 500-Å thermal oxide is grown to act as the gate insulator. Then a 1500-Å LPCVD polysilicon layer is deposited at 620°C and is subsequently boron doped to form the conductive channel. Devices with channel length as small as 2 µm show well-behaved transistor characteristics. The drive current and leakage current are as suitable for usage as load element in memory applications. At large gate voltages the accumulation hole mobility is 9 cm²/V.s. The drain-to-source breakdown voltage exceeds -20 V.     

High mobility two-dimensional electron gas inInP/Ga0.47In0.53As heterojunctions grown bylow-pressure organometallic vapour phase epitaxy

The two-dimensional electron gas in InP/GaInAs heterojunctions, grown by LP-OMVPE, showed Hall mobilities as high as 164000 and 103000 cm²/Vs at 4 K and 80 K, respectively. A maximum Hall mobility of 172000 cm²/Vs was measured at 20 K. Shubnikov-de Haas oscillations measured at 200 mK in magnetic fields up to 20 T indicated the total absence of parallel conduction. By illuminating the sample it was possible to populate the second electrical sub-band at a total carrier density n_s=4.4×10¹¹ cm^-2     

p-type thin-film transistors with vacuum-deposited crystallized copper-doped germanium films

Copper-doped 10-nm-thick vacuum-deposited Ge films between vitreous aluminosilicate insulator films can be crystallized at 400°C, with hole mobilities of 80 cm²/V.s. They yield stable p-type TFT's with 10⁵on/off ratio which are process-compatible with n-type CdSe TFT's and thus usable for complementary on-board shift registers in active matrix displays.     

Utilization of NiSi2as an interconnect material for VLSI

The applicability of NiSi2as an interconnect material was investigated using narrow (5 µm- × 2600-µm) lines. 2500-Å-thick silicide lines were thermally oxidized to form a passivation layer of SiO2for the next metallization level. Isolation of more than 50 V for 2200-Å SiO2is achieved. The interconnect resistivity following the oxidation is 1.2-1.4 Ω. The maximum current capability of the lines was found to be > 5 × 10⁶A/cm²and their stability under prolonged high current densities was demonstrated. We propose a scheme to increase the local metallization-level density using NiSi2as an interconnect.     

Low-temperature polysilicon thin-film transistors fabricated fromlaser-processed sputtered-silicon films

In an effort to develop a simple low-temperature high-performance polysilicon thin-film transistor (TFT) technology, we report a fabrication process featuring laser-crystallized sputtered-silicon films. This top Al-gate coplanar TFT process subjects the substrate to a maximum temperature of 300°C, and produces devices with mobilities up to 450 cm²/Vs, on/off current ratios greater than 10⁷ , without using a post-hydrogenation step. We believe these results represent the highest performance TFT's to date fabricated from sputtered silicon films     

An investigation of i-AlGaAs/n-GaAs doped-channel MIS-like FET's (DMT's)—Properties and performance potentialities

Doped-channel MIS-like FET's (DMT's) based upon an i-AlGaAs/n-GaAs structure have been investigated in detail for the purpose of clarifying their properties and performance potentialities. The DMT is unique in having two operation modes, a depletion-layer modulation mode and an electron accumulation mode, both of which are experimentally demonstrated through capacitance-voltage characteristics. Analytical and experimental results shows that the maximum drain current IDSmaxis more than 2.5 times that for a conventional n-AlGaAs/GaAs 2DEGFET. gmmaxand IDsmaxvalues obtained for 0.5- µm gate DMT's are very high, 310 mS/mm (410 mS/mm) and 650 mA/mm (800 mA,/mm) at 300 K (77 K), respectively, fmaxis 48 GHz. fTis as large as 45 GHz, which is the best data ever reported in 0.5-µm gate FET's. Moreover, the estimated electron saturation velocity is outstandingly large, 1.5 × 10⁷cm/s (2 × 10⁷cm/s) at 300 K (77 K), even for a thin GaAs channel layer with a 3 × 10¹⁸cm^-3doping level, while Hall electron mobility is not reasonably so high, being typically 1850 cm²/V . s (1650 cm²/V . S). Preliminary power performances are also studied at 28.5 GHz. An 18-dBm (225-mW/mm) saturation output power, 6.4-dB linear gain, and 15-percent power added efficiency are achieved. A further performance improvement may be easily accomplished by gate length reduction, structure optimization, and so on. Consequently, it has been proved that DMT's have great feasibility for high-speed and high-frequency high-power device applications.     

The effect of channel boron implants on electron mobility inNMOSFET's

The Hall mobilities and Hall concentrations of channel electrons in boron-implanted NMOSFETs were measured at 77 and 300 K. At both temperatures, the mobilities were found to decrease with increasing implantation dose (10¹¹-10¹² cm^-2) only for electron concentrations <2×10¹² cm^-2, the effect being more pronounced at 77 K. It is suggested that the mobility degradation is mainly due to impurity scattering     

Sputtered molybdenum-silicon contact

The results of the study of sputtered molybdenum-silicon contact is reported. The contact resistance has been found to be in the range of 3-6 µΩ ċ cm²when the composite structure is annealed at 1050°C for 30 min in nitrogen ambient. Application of passivating layer of boron-doped glass over the molybdenum and subsequent annealing at the said temperature reduced this to 2-3 µΩ ċ cm².     

Source—Drain contact resistance in CMOS with self-aligned TiSi2

The contact resistance between TiSi2and n⁺-p⁺source-drain in CMOS is studied for a variety of junction profiles and silicide thicknesses. It is shown that the measured contact resistance is consistent with the transmission-line model for electrically long contacts. The contact contribution to the total device series resistance can be significant if excessive silicon is consumed during silicide formation. Contact resistivities of 3 × 10^-7and 1 × 10^-6Ω . cm²can be obtained for 0.15-0.20-µm-deep arsenic and boron junctions, respectively, if the interface doping concentration is kept at 1 × 10²⁰/cm³. Furthermore, low-temperature measurements show that the contact resistivity is nearly constant from 300 to 77 K, as would be expected from a tunneling-dominated current transport at the TiSi2-n⁺and TiSi2-P⁺interfaces.     

Electron microprobe analysis of impurity heterogeneities in thermally grown silicon oxide

The electron probe X-ray microanalyzer is a powerful tool for studying impurity distribution and motion in thin films. This analytical instrument is capable of detecting metallic impurities present in areas as small as 1 × 10^-6mm²and in concentrations of greater than 1 × 10¹⁹atoms/cm³. The analysis requires no sample preparation and is essentially a nondestructive test. This instrument was used to examine unoxidized and oxidized silicon surfaces and a finished microcircuit. With the electron microprobe, aluminum-bearing regions approximately one microns in diameter were detected on the bare surface of mechanically polished silicon slices. These aluminum-rich regions are believed to be alumina abrasive used in polishing. If these regions are not removed by chemical etching they will generate oxide defects during oxidation. These defects were found to contain Al (1 × 10²¹atoms/cm³and Na (1 × 10²⁰atoms/cm³). Other oxide defects, i.e., pinholes, generated during oxidation varied in size from 0.5 to 5.0 microns and were found to contain Na (1×10²¹atoms/cm³) and K (5×10²¹atoms/cm³). Mg and Ca (1 × 10²⁰atoms/cm³) were occasionally observed in these defects. After oxidation, all these impurities could be removed with a hot hydrochloric acid and deionized water rinse; surprisingly, this treatment reduced the silicon surface charge in the MOS structure (X_{0} cong 1500Å) by approximately 1.4 × 10¹¹charges/cm². The surface charge could be further reduced by heating the oxidized wafer at 900°C in a silicon nitride tube.