John Batey

Low‐temperature deposition of high‐quality silicon dioxide by plasma‐enhanced chemical vapor deposition

Thin films of high‐quality silicon dioxide have been deposited at low temperatures by plasma‐enhanced chemical vapor deposition. A deposition rate much lower than that used in conventional plasma‐enhanced processes is found to be crucial in obtaining material with reproducible, good properties. Controlled, slow deposition is achieved by using very low flow rates of reactive gases, together with a much higher flow of inert carrier gas to ensure uniformity. Films deposited at usual high deposition rates (∼500 A/min) exhibit irreproducible and poor electrical properties and are porous. Those deposited slowly (∼60 A/min) have very reproducible properties, are relatively dense and exhibit very good electrical integrity. Oxides deposited using a substrate temperature of 350 °C compare favorably with those deposited at 700°C using atmospheric‐pressure chemical vapor deposition and can be deposited routinely over a wide range of oxide thickness. Deposition at 275 °C results in similar properties but with increase...

Energy band alignment in GaAs:(Al,Ga)As heterostructures: The dependence on alloy composition

The band alignment in GaAs:(Al,Ga)As heterostructures has been investigated over the full range of alloy composition. The valence‐band discontinuity ΔEv is determined by measuring the activation energy for thermionic emission of holes from p‐GaAs over an undoped, square (Al,Ga)As barrier. The use of p‐type structures to measure ΔEv circumvents a number of complications involved in the measurement of ΔEc. The parameters required for analysis are determined by different measurements on the same structures and the analysis is performed so that the activation energy, extrapolated to zero bias, yields ΔEv directly. It is found that ΔEv is a linear function of the aluminum mole fraction xAl: ΔEv ≂0.55xAl (eV) (0≤xAl≤1). The validity of these data is supported by measurements of ΔEc in the direct band‐gap regime, where complementary values of ΔEv and ΔEc add up to the expected band‐gap difference. This relationship provides a simple description of the full band alignment in this heterosystem and should prove val...

Low hydrogen content stoichiometric silicon nitride films deposited by plasma‐enhanced chemical vapor deposition

We have deposited silicon nitride films by plasma‐enhanced chemical vapor deposition (PECVD) at 250 °C with properties similar to films prepared at 700 °C by low‐pressure chemical vapor deposition (LPCVD). Films are prepared using silane and nitrogen source gases with helium dilution. The film properties, including N/Si ratio, hydrogen content and electrical quality are most sensitive to changes in the silane flow rate during deposition. For films deposited under optimized conditions at a substrate temperature of 250 °C, current versus voltage measurements in metal‐insulator‐semiconductor structures show the onset of carrier injection at 3–4 MV/cm, slightly lower than LPCVD films. When bias‐stressed to 2 MV/cm, capacitance versus voltage measurements show some hysteretic behavior and evidence for positive fixed charge, similar to LPCVD films. For the optimized films: N/Si=1.33±.02; refractive index (λ=6328 A)=1.980±0.01; dielectric constant (1 MHz) ∼7.5; density=2.7±0.1; and the etch rate in 10% buffered ...

Unpinned GaAs MOS capacitors and transistors

Metal-oxide-semiconductor (MOS) capacitors and field-effect transistors (MOSFETs) in the GaAs semiconductor system using an unpinned interface are described. The structures utilize plasma-enhanced chemical-vapor deposition (PECVD) for the silicon-dioxide insulator on GaAs that has been terminated with a few monolayers of silicon during growth by molecular beam epitaxy. Interface densities in the structures have been reduced to approximately 10/sup 12/ cm/sup -2/.eV/sup -1/. High-frequency characteristics indicate strong inversion of both p-type and n-type GaAs. The excellent insulating quality of the oxide has allowed demonstration of quasi-static characteristics. MOSFETs operating in depletion mode with a transconductance of 60 mS/mm at 8.0- mu m gate lengths have been fabricated.<<ETX>>

Energy band‐gap discontinuities in GaAs:(Al,Ga)As heterojunctions

The measurement of the energy band discontinuities in GaAs:(Al,Ga)As heterostructures is described. These values are deduced from the activation energy governing current transport in the direction perpendicular to the heterojunction interface. Using complementary structures (fabricated with an Al mole fraction of ≂0.38), we study both electron and hole transport to independently measure the conduction and valence‐band discontinuities respectively. The results obtained are self‐consistent and indicate that the total band‐gap difference distributes approximately in the ratio 60:40 between the conduction band and valence band. Measurement of the conduction‐band discontinuity for an Al mole fraction of ≂0.24 yields a similar ratio. Preliminary measurements of the valence‐band discontinuity in structures fabricated using (Al,Ga)As with an indirect band gap (Al mole fraction of ≂0.60) are also described.

Electrical characteristics of very thin SiO 2 deposited at low substrate temperatures

Very thin (≲ 100-Å) films of SiO2have been deposited by a modified plasma-enhanced chemical-vapor deposition (PECVD) process at very low substrate temperatures (≲ 350°C). Low flow rates of reactive gases and a high flow of inert carrier gas were used to lower the deposition rate, ensuring improved dielectric properties and good control over film thickness. Measurements made on MOS capacitors of current-voltage characteristics, electrical breakdown, interface trap density, and mobile ion drift indicate that these very thin PECVD films are approaching thermally grown SiO2in quality and may be suitable as gate dielectrics in device applications.

Plasma-enhanced CVD of high quality insulating films

Abstract A modified plasma-enhanced chemical vapor deposition process (He-PECVD) is described, and used to deposit high quality silicon-based insulators at low substrate temperature (≤ 350°C). This process utilizes very high levels of helium dilution of the reactive gases to eliminate many of the problems normally associated with plasma deposition. SiO 2 films deposited by He-PECVD approach the standards of high quality thermally grown oxide and very thin films (≤ 100 A) with excellent electrical integrity can be deposited over large areas routinely. MOSFETs incorporating deposited gate oxides highlight the fundamental difference between deposition and growth by revealing a strong correlation between the ultimate device performance and pre-deposition silicon surface treatment. By minimizing the extent of process-induced surface damage, good quality deposited oxide FETs can be fabricated without the need for additional high temperature annealing. The He-PECVD process can also be applied to the deposition of Si 3 N 4 films, also with marked improvements in film quality.

High-quality deposited gate oxide MOSFET's and the importance of surface preparation

The fabrication and electrical characteristics of MOSFETs incorporating thin gate oxides deposited by a modified plasma-enhanced chemical-vapor-deposition (PECVD) process are reported. The gate oxide deposition and all subsequent steps were carried out at or below 400 degrees C. These results represent the first demonstration of near-thermal-gate oxide quality. MOSFETs fabricated using a low-temperature PECVD gate oxide process without requiring a high-temperature anneal. The ultimate performance of the deposited oxide devices is shown to be critically dependent on the degree of process induced microroughness of the starting silicon surface. Low-temperature effective mobility measurements are used to compare inversion-layer scattering mechanisms in these devices.<<ETX>>

Spectroscopic and electrical studies of GaAs metal–oxide semiconductor structures

Recent studies of plasma‐enhanced chemical vapor deposition of SiO2 upon Si epitaxial layers grown in situ upon epitaxial GaAs layers, with the deposited interfacial Si of order 10 A, strongly suggest that greatly reduced state densities at SiO2/GaAs interfaces have been attained with this approach. We report spectroscopic ellipsometry results for molecular‐beam epitaxy (MBE) Si/GaAs samples both before and after SiO2 deposition, as well as before and after annealing. The ellipsometric results indicate that the SiO2/GaAs interface is <9 A thick. The amount of unreacted silicon ranged from 3±3 to 0.4±0.2 A equivalent thickness for the best fit cases. Ellipsometric results for samples without the deposited SiO2 suggest of order 0.4‐A unoxidized silicon at the interface. Results of x‐ray photoelectron spectroscopy (XPS) studies of the samples (without deposited SiO2) suggest that all silicon is being oxidized prior to SiO2 deposition for at least some of the ‘‘unpinned’’ samples. Raman studies find no Si–Si ...

Thin-film transistors incorporating a thin, high-quality PECVD SiO/sub 2/ gate dielectric

Thin-film transistors (TFTs) have been made that incorporate a thin ( approximately 380 AA), high-quality plasma-enhanced chemical vapor deposition (PECVD) SiO/sub 2/ film as the gate dielectric in a staggered-inverted structure. Threshold voltages and mobilities have been found to be in the range of 1.6-2.4 V and 0.20-0.25 cm/sup 2/ V/sup -1/ s/sup -1/, respectively, where the exact values are dependent on the measurement technique used. Very low gate leakage currents (<10/sup -11/ A) were recorded when measured using a ramped I-V technique, even for electric fields as high as 5*10/sup 6/ V/cm.<<ETX>>