T. O. Sedgwick

Low-temperature selective epitaxial growth of silicon at atmospheric pressure

Epitaxial Si has been grown selectively on oxide‐patterned substrates from 850 down to 600u2009°C for the first time in the Si‐Cl‐H system at atmospheric pressure. Si deposition was achieved by hydrogen reduction of dichlorosilane in an ultraclean system using a load lock. Epitaxy was achieved at low temperatures only when the hydrogen was purified to remove traces of H2O and O2 implying that an oxygen‐free environment is the most important factor controlling epitaxy at low temperatures. Cross‐sectional transmission electron micrographs reveal perfect crystallinity in the epitaxial layer and a totally clean and featureless interface between epitaxy and substrate.

Selective SiGe and heavily As doped Si deposited at low temperature by atmospheric pressure chemical vapor deposition

Selective epitaxial Si with added As for n‐type doping or Ge for band gap modulation can be deposited in an ultraclean atmospheric pressure chemical vapor deposition system down to temperatures as low as 550–750u2009°C. Depositions are carried out in a hydrogen ambience using dichlorosilane, HCl, and arsine or germane. The additives, arsine and germane enhance the Si deposition rate at low temperatures so that practical deposition rates can be achieved. HCl, which is used to control deposition selectivity with respect to oxide or nitride, decreases the deposition rate so that final growth rates are in the range 1–5 nm/min. Rutherford backscattering spectrometry was used to measure As concentrations as high as 1×1021/cm3 and Ge concentrations as high as 50%. A selective growth process for SiGe has been used to grow p‐type metal–oxide–semiconductor field effect transistors and resonant tunneling diodes which display excellent device characteristics.

Selective growth of Si/SiGe resonant tunneling diodes by atmospheric pressure chemical vapor deposition

Atmospheric pressure chemical vapor deposition is used to grow p‐type Si/Si1−xGex double‐barrier resonant tunneling structures on unstrained substrates, with a Si0.75Ge0.25 well clad by Si barriers. The current‐voltage I(V) characteristics at T=77 and 4.2 K exhibit current peaks and negative differential resistance regions corresponding to resonant tunneling through well‐resolved heavy‐ and light‐hole subbands in the well. Device quality is comparable to Si/SiGe resonant tunneling structures grown by molecular beam epitaxy. The in situ substrate cleaning and selective growth capabilities of atmospheric pressure chemical vapor deposition are used for the first successful selective growth of resonant tunneling structures through an oxide mask. The resulting diodes exhibit good resonant tunneling characteristics. The selective growth process is promising for the fabrication of small vertical heterostructure devices.

Epitaxial garnet films by organometallic chemical vapor deposition

Epitaxial films of Y3Fe5O12, Eu3Fe5O12, (Eu, Y)3Fe5O12, and Er3Fe5O12 l-2µm thick have been chemically vapor deposited on <111> GGG and <100> SmGG garnet substrates from 1000°C to 1200°C in an oxygen atmosphere from metal organic source compounds. These source compounds which are used here for the first time in chemical vapor deposition are tris 2, 2, 6, 6, tetramethyl 3, 5 heptanedionate complexes, (thd or M(thd)3) of the metals used. In the reactor, the individual compounds are volatilized in a helium carrier in separate source containers. The vapors are then combined, and premixed without reaction at about 300°C with a large excess of 02 and passed with high velocity, ∿500 cm/sec, onto an r.f. heated substrate. The growth rate under these conditions is 0.4 - 0.8µm/hr. X-ray double diffraction, glancing angle X-ray and microprobe analyses were employed to characterize the crystallinity and stoichiometry, respectively, of the resulting garnet films. They were single crystal and exhibited a lattice constant dependent upon the rare earth to Fe ratio.The Eu containing films were not pseudomorphic probably due to the large lattice mismatch between substrate and film in most cases. The erbium iron garnet films were apparently close to pseudomorphic as determined by measured film and substrate lattice constants.

Strain relaxation in silicon‐germanium microstructures observed by resonant tunneling spectroscopy

We have measured the resonant tunneling current–voltage I(V) characteristics of strained p‐Si/Si1−xGex double‐barrier microstructures ranging from 1.0 to 0.1 μm in lateral extent. The bias spacing between resonant current peaks in the I(V) reflects the energy separation of the Si1−xGex quantum well subbands, which is partially determined by the strain. As the lateral size of the structures decreases, we observe consistent shifts in the I(V) peak spacing corresponding to strain energy relaxation of ∼30% in smaller structures. An additional I(V) fine structure is observed in the 0.1 μm device, consistent with lateral quantization due to nonuniform strain.

Very narrow SiGe/Si quantum wells deposited by low‐temperature atmospheric pressure chemical vapor deposition

The optical, structural, and electrical properties of very narrow SiGe quantum wells grown by ‘‘ultra‐clean’’ atmospheric pressure chemical vapor deposition (APCVD) are investigated. X‐ray reflectivity data reveal abrupt interfaces with a root‐mean‐square roughness of not more than 0.2 nm. For the first time narrow (4.3 meV) excitonic photoluminescence (PL) spectra were obtained from APCVD grown samples containing SiGe wells with 12.5% to 32.5% Ge. For the narrowest wells PL doublets are observed which are attributed to atomic steps at the SiGe/Si interfaces. The PL and x‐ray diffractometry data show that process deposition control for well and barrier width is within the monolayer range. Resonant tunneling diodes fabricated with 2.5‐mm‐wide Si0.75Ge0.25 wells show world record peak to valley ratios of 4.2. Magneto‐transport measurements performed at high magnetic fields of two‐dimensional hole gases exhibit pronounced Hall plateaus and well‐defined Shubnikov de Hass oscillations, indicating high material...

INHOMOGENEOUS STRAIN IN INDIVIDUAL QUANTUM DOTS PROBED BY TRANSPORT MEASUREMENTS

Resonant tunneling measurements are used to probe the inhomogeneous strain in individual SiGe quantum dots. Current–voltage characteristics of strained Si/SiGe resonant tunneling diodes of diameter D⩽0.25u2009μm exhibit additional fine quasi-periodic structure in the resonant peaks. The fine structure is consistent with lateral quantization in the SiGe quantum well due to in-plane confining potentials arising from inhomogeneous strain, which we calculate by finite element techniques for various D. Quenching of the fine structure by a magnetic field is consistent with the effective length scale of the strain-induced potential.

Fabrication of three‐terminal resonant tunneling devices in silicon‐based material

Laterally gated three‐terminal resonant tunneling devices have been fabricated from Si/Si1−xGex double‐barrier structures grown by atmospheric pressure chemical vapor deposition. The gate is insulated from the submicrometer vertical channel by a low‐temperature oxide and the entire fabrication scheme is compatible with current silicon technology. At T=77 K the resonant peak current can be modulated by 25% by applying a moderate gate voltage; at T=4.2 K, current modulation reaches 50%. We present calculations demonstrating that devices fabricated from optimized Si/Si1−xGex structures will pinch off fully at moderate gate voltages and operate at liquid nitrogen temperatures.

The influence of silicon heat treatments on the minority carrier generation and the dielectric breakdown in MOS structures

The minority carrier lifetime of Si and the dielectric breakdown of SiO2 on Si has been investigated as a function of various high temperature treatments preceding the formation of the SiO2 layer.Annealing wafers in H2 or certain H2 -containing ambients prior to oxidation lea to a dramatic decrease in the number of breakdown defects found in capacitors. The higher the temperature the more effective is the defect removal. Using this process the defect density could be reproducibly controlled at ≤10 defects/cm , and in some cases wafers with no defects were found. The defects appear to be related to some airborn contamination and can be increased by exposure to air and to certain aqueous cleaning steps.By “soaking≓ the Si wafers in an equilibrium gas mixture containing SiH4 as well as HCl, it was possible to prevent etching of the Si but yet expose the wafer to approximately 4% HC1 for longer times and at higher temperatures, 12 75‡C, than is possible with the well known HCl-oxidation process. It was found that this treatment will remove Au, Fe, and Cu from intentionally contaminated wafers but at rates much slower than would be expected from bulk-diffusion, rate-limited transport. Soaking at 1275‡C led to minority carrier lifetimes comparable but not significantly better than for HCl-oxidized wafers.

SiGe/Si quantum wells with abrupt interfaces grown by atmospheric pressure chemical vapor deposition

Abstract Atmospheric pressure chemical vapor deposition has been used to grow SiGe/Si quantum well structures on (001) oriented Si substrates. SiCl 2 H 2 and GeH 4 were used as reactive gases in a H 2 atmosphere. The hydrogen ambient is shown to greatly facilitate the deposition of quantum wells with abrupt interfaces in the temperature range of 550–750 °C. The interface roughness is determined to be less than two monolayers, as shown by X-ray reflectivity, X-ray diffractometry data and the characteristics of resonant tunnel diodes showing a peak to valley ratio of 4.2. Photoluminescence spectra with resolved lines of no-phonon and phonon assisted recombination processes are observed.