Corso FS02: Materiali e Dispositivi per optoelettronica, spintronica e nanofotonica Modulo 1: Crescita epitassiale di materiali semiconduttori (Giorgio Biasiol)

1
Corso FS02 Materiali e Dispositivi per
optoelettronica, spintronica e nanofotonicaModul
o 1 Crescita epitassiale di materiali
semiconduttori (Giorgio Biasiol)

• Program
• General concepts in epitaxy
• Epitaxial techniques Molecular Beam Epitaxy
  (MBE)
• Epitaxial techniques Metal Organic Chemical
  Vapor Deposition (MOCVD)
• Low-dimensional semiconductor nanostructures

2
PART I GENERAL CONCEPTS IN EPITAXY

• Applications of compound semiconductors
• Introduction to epitaxial techniques
• Basic concepts in epitaxy
• Crystallography of zinc-blende lattices

3
Applications of compound semiconductors

• H. S. Bennett, "Technology Roadmaps for Compound
  Semiconductors, http//nvl.nist.gov/pub/nistpubs/
  jres/105/3/j53ben.pdf
• International Technology Roadmap for Compound
  Semiconductors (ITRCS) Bulletin Board,
  http//www.eeel.nist.gov/812/itrcs.html

4
Inorganic semiconductor materials

• Inorganic semiconductors can be roughly divided
  into two categories
• Elemental semiconductors, belonging to the group
  IV of the periodic table (Si, Ge)
• Compound semiconductors, synthetic materials not
  existing in nature, composed of elements from
  groups III (Al, Ga, In) and V (N, P, As, Sb), or
  from groups II and VI.

5
Crystal form of semiconductors

• Most semiconductors crystallize in a form
  identical to diamond.
• In compound semiconductors, group-III and group-V
  atoms alternate within the unit cell (zincblende).

Unit cell of GaAs. The side is about 0.56nm
6
From silicon to compound semiconductors

• Traditional devices (electronics, computing)
  silicon-based.
• New advanced devices based on synthetic compound
  semiconductors.

7
New possibilities provided by compound
semiconductors

• These materials allow to overcome some limits
  intrinsic to Si technology
• In many cases, differing from Si, they have
  direct band gaps ? light emitters, ? both
  electronic and optoelectronics applications
• They generally have a larger electron mobility ?
  devices are faster and less power-consuming
• They allow a great flexibility in the fabrication
  of materials with the desired features, and in
  the combination of two or more materials in a
  device (heterostructures).

8
Semiconductor Heteroepitaxy Road-map

• Richness and variety of III-Vs ?
  high-performance "band-gap engineered"
  heterostructures and devices with optical and
  electronic properties difficult to achieve in
  other materials.

9
Trends of compound semiconductors vs. Si

• Communications products to replace computers as
  key driver of volume manufacturing.
• Present and future volume products include
• cell phones and video phones
• Bluetooth appliances
• Optoelectronics (lasers, diodes, sensors)
• automotive electronics that add functionality of
  home and office to cars and trucks.

10
Some applications of compound semiconductors

• Optoelectronic devices (LED, LASER) for the
  production and sensing of light, and for
  telecommunications
• High-speed transistors (HEMT), used, e.g., in
  mobile telephony and satellite systems.

11
Compound semiconductors market share
12
Material choices for device applications
Optoelectronics

• Application examples optical communications,
  displays, sensors) ? wavelength ranges within
  which materials emit and absorb light efficiently

GaN-related 0.3-0.6 µm
GaP-related 0.5-0.7 µm
GaAs-related 0.8-1.0 µm
InP-related 1.3-1.7 µm
InSb-related 2-10 µm
13
Material choices for device applications
Electronics

• Applications examples wireless communications
  based on high-frequency RF or microwave carriers,
  radars, and magnetic-field sensors)
• ?
• trade-offs between performance and material
  robustness during device manufacture and
  operation. In practice, GaAs-related materials
  are the most common, but InP-related materials
  and InSb-related materials also have important
  applications.

14
Electron confinement

• Many of these devices involve structures based on
  electron confinement. This effect limits
  electronic motion to two, one or zero dimensions.
• Such structures are composed of layers of a
  material where electrons are confined,sandwiched
  in layers acting asan energy barrier. They
  arecalled quantum wells, wiresor dots,
  depending ondimensionality.

Section of a LASER structure based on a GaAs QW
embedded in AlGaAs barriers.
15
Typical sizes to observe quantum confinement

• Conduction electrons in semiconductors have
  wavelengths of the orderof 10nm.
• ? To observe quantumconfinement effects,
  quantumwells (wires, dots) must havesizes
  around 10nm.
• Next-generation devices (e.g.,quantum cascade
  lasers)may include layerslt1nm thick.

Energy profile of a GaAs/AlGaAs quantum well and
electronic wave functions of two confined levels.
16
Bulk vs. Quantum-confined devices
Si MOSFET single bulk material, doping by
diffusion or implantation typical sizes mm
InGaAs/AlGaAs p-HEMT abrupt heterostructures,
planar (d-) doping
VCSEL (left upper and lower DBRs and active
region right blow-up of active region) 100s of
layers of different materials withsub-ML
precision
typical sizes lt 10 nm
17
Why Epitaxy?

• Sizes lt 10nm
• ? structure and composition control with accuracy
  better than the single atomic monolayer (0.3nm)
• Semiconductor growth techniques that allow this
  control are called epitaxial techniques.
• Growth takes place on planar, single-crystal
  substrates, atomic layer by atomic layer.

18
Introduction to Epitaxial Techniques
19
Crystallization and film growth

• Amorphous no ordered structures
• Polycrystalline randomly oriented grains,
  oriented grains, highly oriented grains
• Single crystal bulk growth, epitaxy e p i
  (upon) t a x i ? (ordering)

20
Growth Processes

• Bulk techniques (massive semiconductors, wafers)
  Si, compounds semiconductors.
• Epitaxy (higher cost of the growth process)
  high control of interfaces ? thin films, quantum
  confined systems.
• Epitaxy film growth phenomenon where a relation
  between the structure of the film and the
  substrate exists ? single crystalline layer grown
  on a single crystal surface. Film and substrate
  of the same material homoepitaxy. Film and
  substrate are of different materials
  heteroepitaxy

21
Growth techniques for bulk semiconductors
1Crystal pulling (Czochralski method)
The CZ technique consists of dipping an oriented
seed into the molten charge. Solid-liquid
equilibrium is established and the seed is pulled
out to obtain a large crystal. The melt will
freeze following the crystallographic orientation
of the seed. The monocrystalline seed is
suspended to a pulling rod and rotated during the
growth. The pulling rod is then lifted and the
melt crystallizes at the interface of the seed by
forming a new crystal portion. Dislocation-free
conditions. GaAs crystals have been grown since
20 years
22
Growth techniques for bulk semiconductors
2Horizontal/Vertical Bridgman and Vertical
Gradient Freeze
The method consists of a boat which is translated
across a temperature gradient in order to allow
the molten charge contained in the boat to
solidify starting from an oriented seed. The
solidification can be achieved by moving either
the boat or the furnace. An excess of Group V
(As, P) is necessary to control the melt
composition. Horizontal method used for
polycrystals (D-shaped wafers). Vertical method
more popular wafer uniformity, minimized thermal
gradients (reduced dislocation density).
23
Epitaxial techniques

• LPE near-equilibrium technique fast,
  inexpensive, poor thickness/interface control (OK
  only for bulk growth)
• MBE, MOCVD slower, monolayer control on
  thickness and composition ? heterostructures,
  quantum confined systems, band-gap engineering

Interest for both studies of fundamental
physics/materials science and for commercial
applications of advanced devices
24
Comparison of MBE and MOCVD
Feature MBE MOCVD
Source materials Elemental Gas-liquid compounds
Evaporation Thermal, e-beam Vapor pressure, Carrier gas
Flux control Cell temperature Mass flow controllers
Switching Mechanical shutters Valves
Environment UHV H2-N2 (10-1000mbar)
Molecular transport Ballistic (mol. beams) Diffusive
Surface reactions Physi-chemisorbtion Chemical reactions
25
Advantages-disadvantages of MBE and MOCVD
Feature MBE MOCVD
Thickness/composition control -
Process simplicity (ballistic transport, physisorption) - (hydrodynamics, chemical reactions)
Abrupt junctions lt1ML (Shutters) 3ML (Valves)
In-situ characterization (RHEED) Uncommon (RAS)
Purity (UHV) - (C incorporation)
Health, safety (solid sources) - (H2, Highly toxic gases)
Growth rates (GaAs) 1mm/h Up to 4mm/h
Wafer capacity 7X6, 4X8 10X8, 5X10
Environment UHV (sub)atmospheric pressure
Graded composition layers - (thermal evaporation) (mass flow control)
Defect density - (oval defects)
Downtime -
26
Hybrid techniques

• Gas source MBE, Metal Organic Molecular Beam
  Epitaxy (MOMBE), Chemical Beam Epitaxy (CBE).
• Principle using group V or/and group III gas
  sources in a UHV MBE environment.
• Developed in order to combine advantages (but
  also disadvantages!) of MBE and MOCVD.

27
Device applications and epitaxy
Products range from a commercial epiwafer
supplier Source http//www.iqep.com
MATERIAL SYSTEMS DEVICES APPLICATIONS
  GaAs  AlGaAs  InGaP  InGaAs  InSb    MESFETs   HEMTs   PHEMTs   HBTs   Lasers    Mobile Telephony   Global Positioning Systems (GPS)   Satellite Systems   Direct Broadcast Satellite (DBS)   Paging   Wireless LAN / Wireless Cable   Automotive Radar
  InP  InGaAs  InAlAs  InGaAlAs  InGaAsP  InGaAsN    DH, QW, DFB Lasers   LEDs   VCSELs   Detectors   HBTs    Optical Fiber Communications   Sensors   Infra-Red Cameras   Wireless Communications
  GaAs  AlGaAs  InGaAs  Pseudomorphic  InGaAlAs  InGaAsP    DH, QW,    Pseudomorphic Lasers   VCSELs   HEMTs   FETs   Solar Cells   Detectors    Fiber Amplifiers, Gigabit Ethernet   Medical Systems   Solid State Laser Pumps   CD, Minidisc   GPS   Automotive   Satellite Systems
  InGaP  InAlP  GaN  InGaN  InGaAlN    Visible Lasers   UHB LEDs   Visible VCSELs   HBTs   DJ Solar Cells    Displays   DVD / CD   Illumination   Pointers / Bar Code   Wireless Communications   Satellite Systems   Medical Applications
High-speed electronics
Optoelectronics
28
Basic concepts in epitaxy

• J. B. Hudson, Surface Science An
  Introduction, Butterworth-Heinemann, Boston,
  1992
• I. V. Markov, Crystal Growth for Beginners,
  World Scientific, Singapore, 1995
• A. Pimpinelli and J. Villain, Physics of Crystal
  Growth, Cambridge University Press, 1998
• T. F. Gilbert, Methods of Thin Film Deposition,
  http//www.engr.sjsu.edu/cme/cmecourses/MatE270/

29
Supersaturation

• Growth rate is thermodynamically limited by
  chemical potential difference between fluid phase
  and fluid-surface equilibrium
• Dm m meq supersaturation driving force
  for film growth
• Dm must be positive for growth to take place ( ?
  energy gain by adding atoms to the solid phase)
• Real growth rates are limited by other factors
  (mass transport, reaction kinetics)

30
Molecular flux

• Molecular flux of molecules hitting a
  cross-sectional area in a time unit J
  molecules/(m2sec)
• Maxwell-Boltzmann distribution ?

31
Deposition rate

• The flux of molecules of the surface leads to
  deposition, with the rate of film growth
  depending on J
• Example Silane (SiH4) in VPE
• P 0.001 Torr (1 Torr 133 Pa)
• M Si 28 g/ mol and H 1 g/ mol
• rfilm 2.33 g/mol r 50nm/sec
• T 400C 673K
• NA 6.02X1023

32
Mean free path

• d molecular diameter 0.5nm,
• R 8.31 J/(mol K)
• T RT (300K)
• 1 Torr 133 Pa
• 10-5 Torr ? l 3m (e.g. As4 in MBE)
• P
• gt10 Torr ? l lt 30mm (MOCVD)

33
Flow regimes

• The magnitude of l is very important in
  deposition. This determines how the gas
  molecules interact with each other and the
  deposition surface. It ultimately influences
  film deposition properties.
• The flow of the gas is characterized by
  theKnudsen number Kn l / L, where L is a
  characteristic dimension of the chamber (given).
• Kn gt 1 the process is in high vacuum (molecular
  flow regime).
• Kn lt 0.01 the process is in fluid flow regime.
• In between there is a transition region where
  neither property is necessarily valid.

34
Steps for Deposition to Occur

• Every film regardless of deposition technique
  (PVD, CVD, sputtering, thermally grown) follows
  the same basic steps to incorporate molecules
  into the film.
• Absorption/desorption of gas molecule into the
  film Physisorption Chemisorption
• Surface diffusion
• Nucleation of a critical seed for film growth
• Development of film morphology over time

All processes must overcome characteristic
activation energies Ei, with rates ri ?
exp(Ei/kT), depending on atomic details of the
process ? Arrhenius-type exponential laws
35
Physi- and Chemisorption

• Physisorption precursor state, often considered
  as having no chemical interaction involved (van
  der Waals).Ea 100meV/atom
• Chemisorption dissociation of precursor
  molecule, strong chemical bond formed between the
  adsorbate atom or molecule and the substrate.Ea
  a few eV/atom (gt substrate sublimation energy)

Chemisorption reaction rate R k ns0 Q k
reaction rate constant naexp(-Ea/kT), na
characteristic atomic vibration frequency, ns0
ML surface concentration, Q fractional surface
coverage
36
Surface Diffusion

• Overall surface energy can be minimized if the
  atom has enough energy time to diffuse to a low
  energy add site (i.e., step or kink).
• The reaction rate (in molecules/cm2s) for surface
  diffusion is given as

withns surface concentration of reactant,nd
characteristic diffusion frequency 1014s-1Ed
migration barrier energy In unit time the adatom
makes nd attempts to pass the barrier, with a
probability of exp (-Ed/kT) of surmounting it on
each try. Ed ltlt Ea ? surface diffusion is far
more likely than desorption.
37
Diffusion coefficient, diffusion length
Diffusion coefficient (mean square displacement
of the random walker per unit time)
with a lattice constant Adatom lifetime before
desorption
Diffusion length (characteristic length within
which the adatom can move)
Measurable quantity!
38
Nucleation

• Homogeneous nucleation takes place in the gas
  phase (only in MOCVD), parasitic reactions
• Heterogeneous nucleation takes place on the film
  surface

39
Competing processes in nucleation

• Gain in bulk free energy DGv with respect to
  individual atoms
• Loss of surface free energy with respect to
  individual atoms
• ?
• For a stable film, a critical size nuclei is
  needed.
• With embryos smaller than that, the surface
  energy is to large and the overall reaction is
  thermodynamically unfavorable (the overall DG is
  positive).
• With larger nuclei, the free energy from
  converting a volume of atoms to solid overcomes
  the added surface energy (the overall DG is
  negative).

40
Energetics of homogeneous nucleations
g
vapor
r
nucleus
Bulk contributionDGv -Dm / vf Dm mv-mf
kT ln(p/p0) supersaturation, vf V / NA
molecular volume Surface contributiong
surface energy per unit area Total energy change
on cluster formationDG (4p/3) r3 DGv 4p r2
g lt0 gt0
41
Critical nucleus
unstable equilibrium!

• Critical radius for stable nucleation (only for
  positive supersaturation)
• d(DG)/dr 0
• ?

(a few atoms) Thomson-Gibbs equation
universal results (liquid and crystal phases)
42
Heterogeneous nucleation
vapor
nucleus
substrate

• Youngs equation gsv gfs gvf cos q, q
  wetting angle
• gsv ? gfs gvf (highly reactive substrate
  surfaces) ? cos q ? 1 q 0 or undefined ?
  wetting
• gsv lt gfs gvf (poorly reactive substrate
  surfaces) ? cos q lt 1 0ltqlt? ? no wetting
• gsv gfs gvf (metastable situation)
• Typical case 3 lattice-mismatched, strained
  heteroepitaxy
• Strain energy (needed to adjust to substrate
  lattice) depends on gfs and increases linearly
  with film thickness
• ? If at 0 thickness gsv ? gfs gvf , at some
  critical thicknessgsv lt gfs gvf will be
  realized ? 2D wetting layer 3D islands

43
Energetics of heterogeneous nucleations
gvf
vapor
nucleus
q
substrate
r
q
Volume of nucleus Surface area of nucleus
44
Energetics of heterogeneous nucleations
gvf
vapor
nucleus
q
substrate
r
q
same as hom. nucl., no dependence on q

• q 0 ? DG 0 3D droplets thermodynamically
  unfavored ? wetting of continuous 2D film
• q p ? DG DGhom no influence of substrate

45
Growth modes
gsv ? gfs gvf
gsv ? gfs gvf
gsv lt gfs gvf
FM growth The interatomic interactions between
substrate and film materials are stronger and
more attractive than those between the different
atomic species within the film material. VW
growth opposite situation.SK growth occurs for
interaction strengths somewhere in the middle.
46
Examples Frank-van der Merwe growth
Layer-by-layer growth (Frank - van der Merwe) is
the most used epitaxial process in semiconductor
device production. It is most often realized for
lattice matched combinations of semiconductor
materials with high interfacial bond energies
(i.e., AlxGa1-xAs/GaAs).
TEM micrograph of the active region of a
lattice-matched AlInAs/GaInAs QCL grown by
MBECho et al., J. Cryst. Growth 227-228, 1 (2001)
47
Examples Stranski-Krastanov growth
Stranski-Krastanov - grown islands can be
overgrown by the same barrier material as the
substrate, to form buried quantum dots,
completely surrounded by a larger band gap
barrier material. These dots are optically active
due to their damage-free interfaces and are very
well suited for studies of quantum phenomena.
They are very promising systems for laser
production, once high enough uniformity is
achieved
AFM image of uncapped InAs/GaAs quantum dots
formed just afted the critical thickness on a
wetting layer showing monolayer-high 2D islands.
The sample is MBE-grown at TASC.
48
Crystallography of zinc-blende lattices
0.56 nm
49
Technologically important surfaces

• Real surfaces surface relaxation,
  reconstruction, faceting
• Examples
• (100) reconstruction linked to As/Ga ratio on
  surface, depends on supersaturation
• n11 needed to satisfy electron counting
  criterion (electrons from dangling bonds must be
  on states below EF), charge neutrality. E.g.,
  311A breaks into -233 facets

• n11
• Alternating k X (100) h X 111 with k/h
  (n-1)/2

(01-1) cross section
50
Equilibrium shape of crystalsJ. Y. Tsao,
Material Fundamentals of Molecular Beam Epitaxy
(Academic Press, Boston, 1993)
Wulff theorem equilibrium crystal shape
minimizes total surface free energy g
(anisotropic) specific surface free energy n
local surface orientation Construction given
g(n) ? set of planes ? ng(n) from origin, passing
through g(n). Equilibrium shape inner envelope
of these planes.

• Low-energy planes are favored and more extended
  (g closer to origin)
• g(n) has cusps for lowest-energy orientations
  (generally high-simmetry, low-Miller index
  planes) ? flat facets
• As T increases g(n) gets less cusped ?
  disappearence of facets as TgtTr (roughening T for
  each facet), until spheric shape for isotropic
  g(n)

51
Equilibrium shape of GaAsN. Moll et al., Phys.
Rev. B 54, 8844 (1996)

• 100, 011, 111A and 111B considered
  (lowest-energy from experience)
• Calculation of absolute surface energies as a
  function of chemical potentials and related
  surface reconstructions
• As-rich environments (usual for MBE, MOCVD) all
  four orientation coexist in equilibrium, with
  small (10) differences in surface energy
• Applicable to InAs and other III-Vs with similar
  surface reconstructions