Sublimation Source Research Articles

Controlled Growth of 15R-SiC Single Crystals by the Modified Lely Method

Bulk single crystals of the 15R-polytype of SiC have been grown by the sublimation physical-vapor-transport technique. 15R-SiC crystal growth was stabilized on a 15R-SiC seed using near-thermal-equilibrium growth conditions and a stoichiometric SiC sublimation source material. The polarity of the seed surface is shown to be an important factor in preparation of the 15R-polytype. The dual seed method (simultaneous growth on the Si- and C-face) was employed to demonstrate that 15R-polytype growth occurred only on the Si-face, while 6H- and 4H-polytype growth occurred on the C-face. The incorporation of nitrogen donors and boron acceptors was also shown to be a function of seed polarity, with nitrogen incorporation on the C-face two times that on the Si-face, and boron incorporation on the C-face one-half that on the Si-face.

Controlled Growth of 15R-SiC Single Crystals by the Modified Lely Method

Bulk single crystals of the 15R-polytype of SiC have been grown by the sublimation physical-vapor-transport technique. 15R-SiC crystal growth was stabilized on a 15R-SiC seed using near-thermal-equilibrium growth conditions and a stoichiometric SiC sublimation source material. The polarity of the seed surface is shown to be an important factor in preparation of the 15R-polytype. The dual seed method (simultaneous growth on the Si- and C-face) was employed to demonstrate that 15R-polytype growth occurred only on the Si-face, while 6H- and 4H-polytype growth occurred on the C-face. The incorporation of nitrogen donors and boron acceptors was also shown to be a function of seed polarity, with nitrogen incorporation on the C-face two times that on the Si-face, and boron incorporation on the C-face one-half that on the Si-face.

Low temperature solid-source molecular-beam epitaxy growth of Al-free quantum well laser diodes using a GaP-decomposition source

Low threshold aluminum-free, InGaP/GaAs/InGaAs quantum well laser diodes have been grown at low temperature on GaAs using a GaP sublimation source for phosphorous and an atomic hydrogen cracker source for substrate surface preparation. The growth cycle was conducted entirely below 470 °C so that it is compatible with growth on GaAs very large scale integrated circuits, as in the epitaxy-on-electronics monolithic optoelectronic integration processes. The room temperature pulsed threshold current density of broad-area stripe contact laser diodes was consistently on the order of 0.38 kA/cm2. InGaP grown with the GaP cell is found by secondary ion mass spectroscopy to contain significant amounts of oxygen, particularly in Be- and Si-doped layers.

Electrical and optical properties of phosphorus doped Ge 1− yC y

In situ n-type doping was investigated for Ge 1−yC y/Si heteroepitaxial layers (y∼0.001) for a potential optoelectronic material compatible with Si. Using a solid GaP sublimation source for phosphorus doping, epitaxial Ge 1− y C y films were in situ doped on Si(100) substrates during solid source molecular beam epitaxy and we compare their electrical and optical properties with those of epitaxial Ge on Si. Infrared absorption revealed red shifts in the absorption of visible light with increasing P doping for both Ge 1− y C y and Ge. The index of refraction decreases for Ge 1− y C y layers compared with Ge. Free carrier absorption increased with increasing phosphorus concentrations, following a wavelength dependence of ~ λ 3.1 in the region of 10–20 μm for heavily doped material. Addition of C did not affect the incorporation of P donors in the grown layers or the electrical activation of the donors. An increase in the electron mobility for heteroepitaxial Ge 1− y C y layers compared with Ge was observed for the doping levels studied.

Growth of 6H and 4H silicon carbide single crystals by the modified Lely process utilizing a dual-seed crystal method

The influence of polarity on the SiC crystal growth has been demonstrated using a dual-seed technique to grow on both the C- and Si-face seed simultaneously. For the investigated range of growth conditions, 4H-SiC crystals were grown on the C-face of 6H-SiC seed crystals with on-axis orientation, when the growth rate exceeded 1.2 g/h. We were never able to grow 4H-SiC on the Si- face of 6H- or 4H-SiC seed crystals under these growth conditions. The incorporation of nitrogen is shown to be 2–3 times higher in crystals grown on the C-face than on the Si-face, and is independent of both polytype and 8° off-axis orientation. The addition of more than 15% silicon powder to the SiC sublimation source resulted in polycrystalline growth.

Residual effects of chromium gettering on the outgassing behavior of a stainless steel vacuum vessel

Long-lasting beneficial effects on plasmas after chromium gettering have been observed in ISX and TFTR. These benefits include a reduction in radiated power, lower Z eff, and the capability to operate at higher densities. This paper reports laboratory experiments performed to further investigate the underlying mechanism for this enhanced plasma performance. A stainless steel vacuum vessel and a chromium sublimation source were assembled to simulate the arrangement in TFTR. A controlled air leak was introduced, and the residual gases were monitored by a quadrupole mass spectrometer before, during, and after the deposition of a chromium film that covered 50% of the vacuum wall. The pumping speed of the film and the amount of gas sorbed as functions of time have been measured. Rate-of-rise measurements were made before deposition and after saturation of the film with oxygen. The results indicate that the residual effects of the chromium film on the vacuum are due to reduced outgassing of the stainless steel surface.

A practical sublimation source for large-scale chromium gettering in fusion devices

This paper describes the fabrication and testing of a large-scale chromium sublimation source that resembles the VARIAN Ti-ball TM in its design. The device consists of a hollow chromium sphere with a diameter of approximately 3 cm and an incandescent filament for radiation heating from inside the ball. We also discuss the gettering technique utilizing this source. The experimental arrangement consists of an ultrahigh vacuum (UHV) system instrumented for total and partial pressure measurements, a film thickness monitor, thermocouples, an optical pyrometer, and appropriate instrumentation to measure the heating power. The results show the temperature and corresponding sublimation rate of the Cr-ball as functions of input power. In addition, an example of the total pumping speed of a gettered surface is shown.

A carbon reaction studied by crossed molecular beamsa)

The chemiluminescent reaction C2+NO→CN(B 2Σ+) +CO+2.89 eV has been studied in a crossed molecular beam configuration. The carbon beam was obtained from a graphite sublimation source operating at 2550° K. The beam (principally C, C2, C3) was pulsed by a rotating slotted disk before it crossed the NO beam. Electronically excited molecules formed in reaction, CN(B 2Σ+), were detected as they rapidly decayed to the ground state. Photon time-of-arrival measurements identified the reactant as C2. The vibrational energy disposal within CN(B 2Σ+) was derived from wavelength resolved measurements. Reaction energetics and the signal intensity indicated the companion product, CO(X1Σ). This study indicates that energy deposition into vibrational degrees of CN(B 2Σ+) disagrees mildly with ’’prior expectation’’ (Levine, Bernstein, and co-workers). The reaction probably proceeds on a potential energy surface that correlates to C2(X′ 3Πu)+NO(X 2Π).