S. Noor Mohammad

Growth of GaN nanowires by direct reaction of Ga with NH3

Semiconducting, single crystal wurtzite GaN nanowires have been grown by direct reaction of metal Ga with NH3 in a tube furnace. This paper discusses the growth mechanism. Nanowires grow only between 8251C and 925 o C. Their diameters vary between 20 and 150 nm and depend directly on temperature and NH3 flow rate. Wires as long as 500mm have been fabricated; once wires have formed, their length increases directly with time in the reactor. There are three different stages in the process, each of which has its own mechanism. First, a nearly amorphous GaN matrix forms, followed by growth of hillocks of thin GaN platelets. Finally, nanowires emerge from the edges of the platelets in characteristic directions. This analysis can be used as a guide for controlling GaN wire diameters and lengths. Strategies for growth of thinner and thicker nanowires are suggested. Thicker cylindrical structures denoted as rods grow from the face of the platelets. Description of their growth mechanism requires further study. r 2001 Elsevier Science B.V. All rights reserved.

Electrical, thermal, and microstructural characteristics of Ti/Al/Ti/Au multilayer Ohmic contacts to n-type GaN

The electrical, thermal, and microstructural characteristics of Ti/Al/Ti/Au (30 nm/100 nm/30 nm/30 nm) multilayer Ohmic contacts to n-GaN (doping level 5×1017u200acm−3) were studied. The lowest contact resistivity derived from the annealed contact was ρS=3.0×10−6Ωu200acm2. The contacts were robust and showed high-thermal stability. X-ray diffraction and Auger electron spectroscopy studies were made to investigate the microstructure of the annealed contacts. The key to the success of the contact was the Ti layers placed on both sides of the Al layer. Upon annealing, there occurred both in-diffusion and out-diffusion of the Ti layer in intimate contact with the GaN film. The in-diffusion of this led to the formation of TiN, while the out-diffusion of this led to the formation of Ti–Al alloys. The second Ti layer also in-diffused and out-diffused during annealing. However, due to the presence of Au, the out-diffusion was marginalized, and the in-diffusion was higher than the out-diffusion. The in-diffusion led to th...

Low-resistance Ti/Al/Ti/Au multilayer ohmic contact to n-GaN

A metallization scheme has been developed for obtaining low ohmic contacts to n-GaN with a low contact resistance. The metal contact is a Ti/Al/Ti/Au composite with layers that are respectively 30, 100, 30, and 30 nm thick. Contacts with a specific contact resistivity ρs, as low as 6.0×10−7u200aΩu200acm2 for a doping level of 1.40×1020u200acm−3 were obtained after annealing the sample for 30 s at 750u200a°C in a rapid thermal annealer. The Ti placed on top of the traditional Ti/Al contact appears to have the advantage of tying up the excess Al; therefore it does not form a mottled contact. Some of the additional Ti–Al intermetallic alloys that are formed also have beneficial effects. The Ti–Au layer forms a robust upper portion of the composite, which enables the contacts to have high-temperature applications.

Electrical characteristics of thin film cubic boron nitride

Abstract Cubic boron nitride (c-BN), if properly developed, can be a very promising material for electronic applications, such as ultraviolet (UV) detectors and UV light emitting diodes operable at wavelengths in the deep UV regime. Its energy bandgap is favorable also for field-effect transistors for high-power microwave applications. Yet, electrical characteristics of c-BN have hardly been reviewed during the past years putting together its important features. The present article attempts to describe the latest developments in electrical properties of c-BN taking into account metal–semiconductor contacts, interface states, lattice relaxation and impurity states, NEA, and other related topics. In conclusion, current status and the future prospects for c-BN films have been discussed.

Temperature and doping-dependent resistivity of Ti/Au/Pd/Au multilayer ohmic contact to n-GaN

The temperature (both measurement temperature and annealing temperature) and doping dependence of specific contact resistivity of Ti/Au/Pd/Au multilayer ohmic contact have been studied. The metallization for the contact involves the deposition of the composite metal layer Ti/Au/Pd/Au (200 A/600 A/400 A/500 A) on n-GaN. When the contacts were annealed at 800u200a°C for 30 s in air it was observed that the contact resistivity decreases with increasing doping concentration of the n-GaN, but increases with increasing measurement temperature of the contact. Within the framework of the transmission line measurement method, the specific contact resistivity for doping ND=6×1017u2009cm−3 was ρS=1.0×10−4u2009Ωu200acm2, and for doping ND=1020u2009cm−3 was ρS=2.38×10−9u2009Ωu200acm2. The resistivity ρS=1.0×10−4u2009Ωu200acm2 for ND=6×1017u2009cm−3 decreased to ρS≈8×10−7u2009Ωu200acm2 after a second annealing. Thus annealing, some times more than once, appears to play an important role in shaping the best value of the resistivity. The physical cause underlying the ...

InAs nanowires and whiskers grown by reaction of indium with GaAs

Free-standing InAs nanowires and whiskers were grown employing reaction of indium (In) liquid and vapor with GaAs substrate. The arsenic (As) atoms resulting from this reaction were transported by a flow of N2 or NH3 to the growth location where they reacted with In to produce InAs nanowires and whiskers. Scanning electron microscopy, energy dispersive x-ray spectroscopy, and transmission electron microscopy of the products indicate that the diameter of the nanowires and whiskers ranges from 15 nm to 2 μm depending on the growth temperature, the composition is InAs, and the structure is zinc-blende crystal with [110] or [100] growth direction. The As source and growth mechanism were discussed. The method for synthesis involved no any template, catalyst, toxic As source, nor even lattice matched substrate.

Systematic study of effects of growth conditions on the (nano-, meso-, micro)size and (one-, two-, three-dimensional) shape of GaN single crystals grown by a direct reaction of Ga with ammonia

A series of experiments have been conducted to systematically study the effects of growth conditions (NH3 flow rate, growth temperature, chamber pressure, and growth location) on the size (nano, meso, or micro) and the shape (one, two, or three dimensional) of GaN single crystal products grown by a direct reaction of Ga with NH3. A growth map with a wider range of experimental parameters was developed; it has three distinct zones. The size and shape of the products in every zone were found to depend on both temperature and NH3 flow rate with other growth conditions fixed. An effective surface diffusion length consisting of the Ga atomic surface diffusion length and the GaN molecular surface diffusion length, and the anisotropy of the Ga surface diffusion length and the GaN growth rate in different growth directions were introduced into the growth model, in such a way that it allowed successful explanation of all observed results. The optimal growth parameters could thus be determined, which conclusively d...

Electrical characteristics of AlxGa1−xN Schottky diodes prepared by a two-step surface treatment

Near-ideal Schottky barrier contacts to n-type Al0.22Ga0.78N have been developed by a two-step surface treatment technique. Plasma etching of the AlxGa1−xN surface prior to Schottky metal deposition, combined with sequential chemical treatment of the etched surface, holds promise for developing high quality low-leakage Schottky contacts for low noise applications and for recessed gate high electron mobility transistors. In this work, the effect of postetch chemical treatment of the n-type Al0.22Ga0.78N surface on the performance of the Ni∕Au based Schottky contact has been investigated. Three different types of chemical treatment: viz, reactive ion etching, reactive ion etching plus dipping in hot aqua regia, and reactive ion etching plus dipping in hot KOH, are studied. Detailed current-voltage studies of three different surface treated diodes and a comparison with as-deposited diodes reveal significant improvement in the diode characteristics. The latter surface treatment yields Ni∕Au Schottky diodes wi...

High-transparency Ni/Au bilayer contacts to n-type GaN

A unique metallization scheme has been developed for obtaining both Schottky and low-resistance Ohmic contacts to n-GaN. It has been demonstrated that the same metallization can be used to make both Schottky and Ohmic contacts to n-GaN using a Ni/Au bilayer composite with Ni in contact to GaN. Using this metallization, contacts with a specific contact resistivity, ρs, as low as 6.9×10−6 Ωu200acm2 for a doping level of 5.0×1017 cm−3 was obtained after annealing the sample for 10 s at 800u200a°C in a rapid thermal annealer. The presence of only (111)Au and (111)Ni peaks in the x-ray diffraction (XRD) pattern of as-deposited samples indicates that both metals participate to form epitaxial or highly textured layers on the basal GaN plane. When the contact layer is annealed, Au and Ni react with GaN creating interfacial phases. Both XRD and transmission electron microscopy confirm that Ni3Ga and Ni2Ga3 intermetallic phases together with Au and Ni based face-centered-cubic solid solutions, are formed during annealing. ...

Temperature, electric field, and doping dependent mobilities of electrons and holes in semiconductors

Abstract A new formula for electron and hole mobilities in semiconductors is presented. Although empirical, it is accurate and widely applicable to a number of semiconductors, such as Si, Ge, GaAs, InP, etc. The formula is simple, and yet predicts temperature and field dependence of electron and hole mobilities very well. To our knowledge, the present model is more general than any other model (both empirical and theoretical) available in the literature. Because of very simplistic nature, it promises to be highly useful for analytically modeling the current-voltage characteristics of transistors.