MBE. Application to key materials. - T.Vijaykumar. Outline. Introduction Technical data Effusion cells Growth mechanism RHEED Application Quantum dots HEMT High Tc Superconductors GMR Summary. Invented in late 1960’s at Bell Laboratories by J. R. Arthur and
Application to key materials
A. Y. Cho.
Molecular beam epitaxy (MBE) is performed with different types of semiconducting materials like:
i) Group IV elemental semiconductors like Si, Ge, and C
ii) III-V-semiconductors: arsenides (GaAs, AlAs, InAs), antimonides
like GaSb and phosphides like InP
iii) II-VI- semiconductors: ZnSe, CdS, and HgTe
Electrons move through GaAs five times faster than through silicon.
Epitaxy: Growth of film with a crystallographic relationship with the substrate
Types: Homoepitaxy & Heteroepitaxy.
For good epitaxy:
deposition rate -
The Gas-Source MBE (GS-MBE)
group-V materials are hydrides such as arsine (AsH3) or phosphine (PH3)
Metalorganic MBE (MO-MBE)
group-III materials are metalorganic compounds.
e.g., TEGa and TMIn
Solid-Source MBE (SS-MBE)
group-III and -V molecular beams.
Cylindrical crucible offers good charge material capacity, but uniformity decreases as charge material is depleted. It offers excellent long-term flux stability, but permits large shutter flux transients.
Conical crucible offers reduced charge material capacity, excellent uniformity, and poor long-term flux stability, and permits large shutter flux transients.
SUMO crucible offers excellent charge material capacity, excellent uniformity, excellent long-term flux stability, and minimal shutter-related flux transients.
- do not decompose or outgas impurities even when heated to 1400ºC.
- combines the evaporation and cracking of elements like P,S, As, Se, Te etc.
At very high temperature of substrate, there are many different possible surface diffusion mechanisms:
epitaxial growth is ensured by-
RHEED is sensitive for surface structures and reconstructions.
a) GaAs(100) - 1x1
b) GaAs(100) - 2x1
electron beam is incident in the (110) with 8.6 keV
Different stages of layer-by-layer growth by nucleation of 2D islands and the corresponding intensity of the diffracted RHEED beam.
(b) Transmission through high and narrow crystal;
(c) Transmissionthrough short and wide crystal;
(d) Diffraction from nearly flat asperities.
Substrate temperatures - 580ºC-650ºC.
Requires an As overpressure to prevent the surface from becoming Ga rich.
GaAs, there is a large window for which there is both unity sticking and sufficient mobility.
Ternary or quaternary compounds - the window becomes smaller
- differences in the relative bond strengths of the different group III adatoms.
RHEED can be used to determine the minimum amount of As required to maintain the proper stoichiometry by measuring the incorporation ratio.
Values of the incorporation ratio for GaAs is 1.3 to 1.8.
(100) is the predominant substrate orientation for MBE growth of compound semiconductors.
Growth of InGaAlAs on InP:
Incorporating In into AlxGa1-xAs will decrease the bandgap.
In.52Al.48As - 1.49 eV 0.74 eV - In.53Ga.47As
- small enough bandgap to detect light at the important wavelengths of 1.3 mm and 1.55 mm.
structures based on highly lattice mismatched materials.
mean free path and the de Broglie wavelength of free carriers exceed the critical sizes of structures.
carriers experience a three-dimensional quantum confinement.
InAs grows layer-by-layer till critical coverage - wetting layer (WL).
After θc=1.6 ML (w~0.5 nm), Stranski-Krastanow 3D growth occurs.
relaxation of the elastic energy which builds up as the thickness of mismatched epilayers increases.
arsenides (InGaAs/AlGaAs ,InAs/InGaAs, InAlGaAs /AlGaAs )
phosphides (InAs/InP, InP/InGaP ),
antimonides and nitrides (GaN/AlN),
Ge/Si and SiGe/Si
Bennett, Magno, and Shanabrook Appl. Phys. Lett. 68 (4), 22 (1996)
E - emission energy
Eg - quantumdot bandgap energy
Ee - electron confinement energy
Eh - hole confinement energy
Nt: Exciton binding energy is neglected.
C. K. Chia et al., J. Crystal. Growth, 288, 57-60 (2006)
increases in the Tc in thin films of copper oxide
compressive epitaxial strain.
Under compressive epitaxial strain a much larger increase in Tc is possible. Requires the choice of a suitable system and substrate combination
the lattice deformations associated with the strain fundamentally modify the
energy scales, leading to the formation and condensation of the superconducting pairs.
the Cu and O atoms of the CuO2 planes are expanded.
"214" thin film on a SrLaAlO4 substrate
in-plane compression and an out-of-plane expansion.
- redox reaction at interface is a serious problem fabricating tunnel junctions using high-Tc Cuprates,
J. P. Locquet et al., Nature 394, 453 (1998)
The graphite-like array of boron (shown in black)
Josephson tunnel junctions (MgB2/AlOx/MgB2)
fabricated on sapphire -C substrates below 300° C.
K. Ueda, S. Saito, K. Semba, T. Makimoto and M. Naito, APL 86 , 172502 (2005)
Dr. Takashi Mimura - inventor of HEMT (
- FET with a junction between two materials with different band gaps as the channel instead of an n-doped region.
Features of GaN HEMT
high frequency power transistors for RF transmission applications covering the 1-50 GHz band.
inherently higher transconductance,
good thermal management and
higher cutoff frequencies.
prime candidate for high power microwave applications.
M. Aslf Khan, J. N. Kuznia, et al, Appl. Phys. Lett., 65(9):1121
extremely low noise device in terrestrial and space telecommunications systems, radio telescopes in the area of astronomy, DBS receivers and a car navigation receivers.
In 1985, HEMT was announced as a unique microwave semiconductor device with the lowest noise characteristics in the world.
- The antenna is less than 30 centimeters in diameter,
- prepared in 1986, using MBE
- magnetoresistance is much larger than that of the intrinsic magnetoresistance of the Fe layers themselves
Oscillatory coupling - a very general property of almost all transition-metal magnetic multilayered systems
the nonferromagnetic layer comprises one of the 3d, 4d, or 5d transition metals or one of the noble metals
The oscillation period was found to be just a few atomic layers, typically about 10 Å, but varying up to ~20 Å.
- ferromagnetic cobalt layers separated by thin copper layers, was found to exhibit very large GMR effects even at room temperature
GMR sandwiches can achieve sensitivities to such fields of perhaps as much as five hundred times greater than conventional materials.
For copper thickness (typically 1.9 nm) - anti-parallel magnetic coupling between successive cobalt layers in zero applied field.
the magnitude of the interlayer exchange coupling decreases much more rapidly with increasing Cu thickness.
"spin valve" effect:
GMR read heads allows the reading of extremely small magnetic bits at an areal density of 2.69 gigabits per square inch.
One minute please…………..
IBM J . RES. DEVELOP. VOL 42 NO. 1 JANUARY 1998