R.H. Hopkins

Semi‐insulating 6H–SiC grown by physical vapor transport

Semi‐insulating 6H–SiC crystals have been achieved by using controlled doping with deep‐level vanadium impurities. High resistivity undoped and semi‐insulating vanadium‐doped single‐crystals with diameters up to 50 mm were grown by physical vapor transport using an induction‐heated, cold‐wall system in which high purity graphite materials constituted the hot zone of the furnace. Undoped crystals were p‐type due to the presence of residual acceptor impurities, mainly boron, and exhibited resistivities ranging up to 3000 Ω cm. The semi‐insulating behavior of the vanadium‐doped crystals is attributed to compensation of residual acceptors by the deep‐level vanadium V4+(3d1) donor located near the middle of the band gap.

Growth of large SiC single crystals

We have grown 6H-polytype SiC single crystal boules up to 60 mm in diameter by the physical vapor transport process at 2300 o C. [0001] oriented substrate wafers prepared from these undoped crystals exhibit resistivities of up to 10 5 Ω cm and etch pit defect densities of 10 4 -10 5 cm -2 . Epitaxially-grown microwave MISFIT structures exhibit 5 GHz cutoff frequency; the highest reported to date

Laser Properties of Nd +3 and Ho +3 Doped Crystals with the Apatite Structure

A great variety of compounds occur in nature or have been synthesized in the laboratory that crystallize with the apatite structure. We have investigated a number of the apatites and found them to be excellent laser hosts for neodymium and holmium. The apatites described in this paper were grown using the Czochralski method, have low optical losses in the pump and emission spectral regions for neodymium and holmium, and the hosts have been developed to readily accept large concentrations of doping ions. This paper describes the crystal growth, physical properties, spectroscopy, and laser performance of this family of new laser materials.

Large diameter 6H-SiC for microwave device applications

6H-polytype SiC single crystals with diameters up to 50 mm and lengths up to 75 mm have been grown in the c-and a-axis directions by physical vapor transport (PVT) at growth rates of 0.25 to 1 mm h -1 . Undoped crystals grown from purified source material reveal residual impurity concentrations in the 10 16 cm -3 range and resistivities up to 1000 Ω-cm. N + crystals with resistivities < 0.02 Ω-cm have been produced by controlled nitrogen doping. PVT-grown SiC crystals are characterized by dislocation densities of 10 4 to 10 5 cm -2 and can also exhibit micropipe defects in the 10 2 to 10 3 cm -2 range

SiC boule growth by sublimation vapor transport

Abstract Silicon carbide is an attractive candidate for high power and high temperature electronics due to its inherent high thermal conductivity, large saturated drift velocity, high breakdown strength and large bandgap. A review of the material properties which influence semiconductor device characteristics is presented, and recent advances in crystal growth technology leading to the preparation of 25 mm and larger wafers for “silicon-like” device fabrication processes are reviewed. A sublimation vapor transport system is described and preliminary results on growth of 6H-SiC boules are presented.

Physical Vapor Transport Growth and Properties of SiC Monocrystals of 4H Polytype

The physical vapor transport technique can be employed to fabricate large diameter silicon carbide crystals (up to 50 mm diameter) exhibiting uniform 4H-polytype over the full crystal volume. Crystal growth rate is controlled to first order by temperature conditions and ambient pressure. 4H-polytype uniformity is controlled by polarity of the seed crystal and the growth temperature. 4H-SiC crystals exhibit crystalline defects mainly in the form of dislocations with densities in the 10 4 cm -2 range and micropipe defects, the latter having densities as low as 10 cm -2 in best crystals. Electrical conductivity in 4H-SiC bulk crystals ranges from 10 15 Ω cm) at room temperature.

4H-SiC MESFET's with 42 GHz f/sub max/

We report for the first time the development of state-of-the-art SiC MESFETs on high-resistivity 4H-SiC substrates. 0.5 /spl mu/m gate MESFETs in this material show a new record high f/sub max/ of 42 GHz and RF gain of 5.1 dB at 20 GHz. These devices also show simultaneously high drain current, and gate-drain breakdown voltage of 500 mA/mm, and 100 V, respectively showing their potential for RF power applications.

DEEP LEVEL TRANSIENT SPECTROSCOPIC AND HALL EFFECT INVESTIGATION OF THE POSITION OF THE VANADIUM ACCEPTOR LEVEL IN 4H AND 6H SIC

Hall effect, deep level transient spectroscopy (DLTS) and optical absorption measurements were employed in concert to determine the position of the vanadium acceptor level in vanadium and nitrogen doped 6H and 4H SiC. Hall effect results indicate that the acceptor position in 4H SiC is at 0.80 eV beneath the conduction band edge, and 0.66 eV for the 6H polytype. The DLTS signature of the defect in the 4H polytype showed an ionization energy of 0.80 eV and a capture cross section of 1.8×10−16 cm−2. The optical absorption measurements proved that the levels investigated are related to isolated vanadium, and therefore the vanadium acceptor level. Based on the DLTS measurements and secondary ion mass spectroscopy data, the maximum solubility of vanadium in SiC was determined to be 3.0×1017 cm−3. At these incorporation limits and with the depth of the level, the vanadium acceptor level could be used in the creation of semi‐insulating silicon carbide.

A critical look at the performance advantages and limitations of 4H-SiC power UMOSFET structures

A realistic performance projection of 4H-SiC UMOSFET structures based on electric field in the gate insulator consistent with long-term reliability of insulator is provided for the breakdown voltage in the range of 600 to 1500 V. The use of P/sup +/ polysilicon gate leads to higher breakdown voltage as the Fowler Nordheim injection from the gate electrode is reduced. It is concluded that the insulator reliability is the limiting factor and therefore the high temperature operation of these devices may not be practical.

Silicon ribbon growth by the dendritic web process

Abstract Silicon dendritic web is a unique mode of ribbon growth in which crystallographic and surface tension forces, rather than shaping dies, are used to control crystal form. The single crystal webs, typically 2–4 cm wide, have been made into solar cells which exhibit AM1 conversion efficiencies as high as 15.5%. During crystallization, silicon webs effectively segregate metal impurities to the melt ( k eff ≈ 10 −5 ) so that the use of cheaper, less pure silicon as feedstock for crystal growth appears feasible. Recent studies described here indicate that higher growth output rates can be achieved by control of the thermal profiles in the web itself and in the melt from which the crystal grows. The improvements stem from an enhancement in the dissipation of latent heat and a reduction in stress within the crystals. To sustain high output rates for prolonged periods will require melt replenishment during growth.