October_AMP_Digital

Andy Phillips

Lake Shore

Cryotronics Inc.

Westerville, Ohio

ADVANCED MATERIALS & PROCESSES •

OCTOBER 2014

ith the continuing demand for

higher computing performance, sig-

nificant research is being aimed at

characterizing novel materials for semiconduc-

tor use. Characterization of carriers as well as

unwanted impurities in materials will continue

to be an important step in the development of

next-generation semiconductor devices. Vari-

ous measurement techniques for Hall conduc-

tivity, carrier concentration, and mobility

measurements, as well as Raman and x-ray

spectroscopy, help to understand these materi-

als. However, many commercially available

technologies offer limited utility because they

do not account for material responses as a func-

tion of temperature, or their magnetic fields are

fixed, so it is impossible to differentiate mobil-

ities and carriers.

Many measurement platforms also do not

allow for noninvasive characterization of wafer-

scale materials or they require labor-intensive

bonding and packaging, making them imprac-

tical in current semiconductor materials test-

ing environments. New approaches to

nondestructive measurement for early stage,

temperature dependent materials characteriza-

tion under high magnetic field, as well as device

level, variable measurement testing will be ex-

plored, particularly as it relates to Hall analysis.

Temperature and early-stage

materials characterization

Analyzing at low temperatures is a common

method for isolating specific material phenom-

ena. Characterizing at variable temperatures

can also yield important insights into underly-

ing conductivity mechanisms. In particular, the

cryogenic environment reduces the inherent

noise of electronic materials, lessening its im-

pact on measurements. Certain carrier trans-

port properties are easier to detect at low

temperatures as well.

In some semiconductor materials, free car-

riers can be “frozen out” at cryogenic tempera-

tures while the intrinsic carrier concentration

or activation energy can be determined from

the temperature dependency of the carrier den-

sity. Knowing the material’s mobility and tem-

perature dependence can also help identify

concentrations of impurities and gauge poten-

tial saturation transconductance.

Continuous wave terahertz

For more than 20 years, researchers have

used terahertz frequency spectroscopy for ma-

terials characterization. The energy of terahertz

waves is low enough to couple to the free car-

rier motion in semiconductors. As a noncon-

tact, quasi-optical technique, terahertz

spectroscopy is ideal for characterizing the con-

ductivity of bulk semiconductors, ultrathin epi-

layers, and buried thin films in pre-device stage

heterostructures.

Terahertz spectroscopy at cryogenic tem-

peratures can expose properties not apparent

at room temperature and allows carrier con-

centration and semiconductor mobility to be

tuned. However, most commercially available

THz systems lack the necessary cryogenic and

magnetic environments required for targeted

semiconductor materials research, and if they

do have them, THz energy is usually generated

outside the testing environment. With these

optical cryostat-based systems, THz beams

must pass through windows—reducing signal

power and causing spectral distortion—and

their optics are difficult to align, which can lead

to repeatability issues.

However, all of this is changing with im-

provements in how THz energy is generated

and applied to materials under test. Newer

con-

Nondestructive Variable Temperature

Materials Characterization

for Semiconductor Research

To meet the

rigorous

demands of

next-generation

computer

technology, new

approaches to

nondestructive

measurement

for early stage,

temperature

dependent

materials

characterization

are needed.

Continuous wave-THz spectroscopy system emitter and detector devices

for semiconductor materials characterization.