Varatharajan Rengarajan

SUBLIMATION GROWTH OF SIC SINGLE CRYSTALS

The technological potential of silicon carbide (SiC) single crystals for highpower, high-temperature, and high-frequency electronic devices has been recognized for several decades; however, such applications have been greatly hindered by problems related to bulk crystal growth. SiC bulk crystal growth technology has recently achieved drastic improvement and enabled the growth of large high-quality single crystals. This chapter overviews the recent achievements in SiC bulk crystal growth aimed at producing high-quality largediameter crystals, highlighting the improvement of the crystal diameter enlargement process and the reduction of crystallographic defects in SiC crystals.

ZnO semiconductors for lighting

Intentionally doped n-type bulk ZnO has been grown by patented melt technique at Cermet and was used as a substrate for homo-epitaxial growth of p-type ZnO films. The n-type ZnO has a carrier concentration on the order of 1018cm-3 with a mobility of 113cm2/Vs, which is good for optical devices. Secondary ion mass spectroscopy (SIMS) profile shows a very uniform distribution of n-type dopant in the ZnO. Excellent transmission from the sharp absorption edge through the visible portion of the spectrum indicates that as grown n-type ZnO is perfect for any optical device applications. P-type ZnO thin films were successfully grown by MOCVD technique on n-type ZnO substrate to form ZnO based p-n junction structure. Cadmium and magnesium doped ZnO films were also grown by MOCVD and resulted in tunable bad gap energy of ZnO based alloy. Ohmic contact layer on n-type ZnO was formed by using Ti/Au and on p-type ZnO was formed by using Ni/Au. The current-voltage (I-V) characteristics of the ZnO based p-n junction exhibited rectification when reverse biased with a breakdown voltage of 10 V and turn-on voltage of 3.3 V. Post anneal of p-type ZnO films showed big improvement on the I-V characteristics. Electroluminescence (EL) spectra obtained from devices driven to 40mA are dominated by a peak at 384nm.

Growth of Large Diameter 6H SI and 4H n+ SiC Single Crystals

SiC single crystals are grown at II-VI by the seeded sublimation technique. The process has been scaled up and optimized to support commercial production of high-quality 100 mm diameter, Semi-Insulating (SI) 6H substrates and 100 mm 4H n+ substrates. The growth process incorporates special elements aimed at achieving uniform sublimation of the source, steady growth rate, uniform doping and reduced presence of background impurities. Semi-insulating 6H substrates are produced using precise vanadium compensation. Vanadium doping is optimized to yield SI material with very high resistivity and low capacitance. Crystal quality of the substrates is evaluated using a wide variety of techniques. Specific defects, their interaction and evolution during growth are described with emphasis on micropipes and dislocations. The current quality of the 6H SI and 4H n+ crystals grown at II-VI is summarized.

Propagation and Density Reduction of Threading Dislocations in SiC Crystals during Sublimation Growth

SiC single crystal wafers grown by sublimation exhibit relatively high dislocation densities. While it is generally known that the overall dislocation density tends to decrease throughout crystal growth, there has been a limited quantitative analysis of such trend. In this study, we measured the density of threading dislocations in the wafers sliced from several SiC boules. Although the dislocation density in the wafers sliced from different boules could differ by orders of magnitude, a consistent empirical relationship was found between the dislocation density (ρ) and the axial wafer position within the crystal ( w ): ρ is proportional to w (−0.5) . Monte Carlo simulations were performed based on two assumptions: (i) during growth the threading dislocations move randomly in the lateral directions, and (ii) two dislocations of opposite sign annihilate when they come within a critical distance between them. Good agreement was achieved between the model and experimental results. The critical distance determined from the simulations was in the range between a few hundred Ă and a micron.

Transition Metal-Doped ZnO: A Comparison of Optical, Magnetic, and Structural Behavior of Bulk and Thin Films

Transition metal-doped ZnO bulk crystals and thin films have been investigated to determine the effects of transition metal incorporation on optical, magnetic, and structural properties of ZnO. A modified melt growth technique was used to grow bulk Zn 1-x Mn x O, Zn 1-x Co x O, and Zn 1-x Fe x O. Optical transmission measurements show an apparent shift in absorption edge with increasing transition metal incorporation. Raman spectroscopy also shows increasing lattice disorder with increasing transition metal concentration. ZnO thin films doped with Ni, Co, and Gd were grown by metalorganic chemical vapor deposition (MOCVD). While the Co-doped thin films showed antiferromagnetic behavior, magnetic hysteresis was observed in the Ni-doped and Gd-doped thin films. Structural quality was verified with X-ray diffraction (XRD), and optical properties were investigated using room temperature photoluminescence (PL) and optical transmission measurements. Properties of ZnO:TM bulk crystals and thin films are compared and used to discuss possible origins of ferromagnetism in these materials.

Room Temperature Ferromagnetic Properties of Epitaxially Grown Zn 1-x Co x O Thin Films

We have conducted a systematic magnetic characterization of a series of Zn 1-x Co x O samples with different cobalt composition. The Zn 1-x Co x O thin films were epitaxially grown by metal organic chemical vapor deposition (MOCVD) on quartz and ZnO substrates. The Co composition was adjusted by controlling the bubbler temperature and carrier gas flow. The magnetization measurements were performed using a Quantum Design MPMS system, which utilizes a superconducting quantum interference device (SQUID) magnetometer. Magnetic hysteresis curves were observed at 5K which persisted up to 300K, possibly characteristic of ferromagnetic behavior. Temperature dependent magnetization was recorded under both zero-field cooled (ZFC) and field cooled (FC) conditions. Changes of magnetization were observed under ZFC and FC conditions in some samples from 5K up to 300K. Composition-dependent changes in magnetization were also observed among samples with different cobalt doping, indicative of ferromagnetism related directly to cobalt incorporation. Magnetic field dependent magnetization at various temperatures with field up to 5 Tesla suggests the Zn 1-x Co x O layers were not paramagnetic.