1. Nitride semiconductors and their applications
Part II: Nitride semiconductors

2. Nitride papers published

3. Nitride-based semiconductors
A III-V semiconductor in which N is one of the elements. Examples include AlN, GaN, InN, and alloys such as AlxGa1-xN.
These materials have high melting points (strong bond with N) and a wide span of bandgaps (from 0.7 eV for InN to 3.4 eV for GaN to 6.2 eV for AlN).

4. Applications High-temperature, high-power electronics
Ultraviolet (UV) radiation detectors
feedback systems in furnaces and engines
solar-blind missile early warning systems
astronomical applications
LEDs and LDs

5. Blue LEDs and LDs Full color day-visible displays
Energy efficient lighting
Traffic lights, replace every 6 months, LEDs, every 5-10 years (60,000 hours)
Consumer lighting applications.
Higher storage density on optical media (>50 GB on a single DVD)
Faster on-time
Fluorescent photosensitizers accumulate preferentially in cancerous cells.

8. Short history 1971 GaN LED demonstrated (Pankove)
1986 “High” quality GaN grown (Akasaki)
P-type GaN grown (Akasaki); Nakamura starts work on GaN
Two-flow MOCVD system developed (Nakamura)
High quality p-type GaN grown; first pn-junction GaN LED created
ZnSe-CdZnSe blue laser developed (3M)
Commercial blue GaN LEDs introduced (Nichia)
Room temperature nitride LDs developed (Nichia)
1999 Commercial nitride LDs introduced (Nichia)
SONY markets blue laser DVD writers (23 GB/layer)

9. Nitride problems 1. Inability to grow good quality crystals
2. Inability to grow p-type crystals

10. Crystal quality
Problem: No lattice matched substrates, high growth temperature results in convection currents
Sapphire is closest but is 15% off.
SiC is too expensive
MOCVD growth too fast for good control (few mm/min)
Solution: Buffer layers, new growth system
First grow GaN or AlN buffer layer
Two-flow MOCVD system
Still many many dislocations in material (1010 cm-2) but dislocations don’t matter?

11. Two-flow MOCVD
Nakamura, Harada, and Seno, J. Appl. Phys 58, 2021 (1991)

12. Buffer layer Dislocation High quality GaN Low quality GaN (0.2 mm)
AlN or GaN buffer layer
(0.05 mm)
Substrate

13. p-type GaN Problem: No one could dope GaN p-type Solution:
At first, LEEBI (Low Energy Electron Beam Irradiation)
Later, annealing at 700°C in non H-containing gas
Hydrogen was passivating the acceptors!

14. Factors leading to success
Small bureaucracy (N. Ogawa and S. Nakamura)
A 3.3 M$ USD gamble (1.5% of annual sales)
Large companies tend to be conservative, both in funds and in research outlook

15. Current research on nitrides
Fundamental physics
Improving crystal quality (still very poor)
Ultraviolet lasers
Lattice matching with quaternary alloys (AlGaInN)
Nitride heterostructures and accompanying applications

16. Nitride heterostructures

17. Heterostructure usefulness
Charge carriers are spatially separated from the (now) ionized impurity atoms, leading to higher carrier mobilities.
Electrons form a 2-Dimensional Electron Gas (2-DEG).

18. Research questions
Even undoped, carrier densities in AlGaN/GaN heterostructures is 10 to 100 times larger than those in similar (AlGaAs/GaAs) systems. What is the source of these carriers?
Carrier mobilities in AlGaN/GaN heterostructures are 10 to 100 times lower than in the AlGaAs/GaAs system. What are the principle mechanisms limiting the mobility?

19. Origin of carriers Current theory: surface donor defects on AlGaN
Concentration of surface defects and transfer of electrons to GaN well is enhanced by strain-induced electric field.

20. Pseudomorphic growth AlGaN GaN lattice matched non-lattice matched
AlGaN/GaN interface
GaN and AlN have ~2.5% lattice mismatch
Grown on polar c-axis
Spontaneous and induced piezoelectric fields are present

21. Band structure
When AlGaN barrier is thick enough to pull defect level above bottom of GaN well, electrons begin to transfer to well.
Formation energy of donor defects is reduced because electrons can drop to lower energy level by transferring to GaN well

22. Electron transfer

23. Comparison with experiment
Smorchkova et al., J. Appl. Phys 86, 4520 (1999)

24. Transport properties Semiconductor with no applied field:
Electrons move randomly with an average velocity of zero
Mean time between electron-electron collisions is t.
With an applied field:
Electrons have an acceleration of a = eE/m*
Average velocity of electrons is at, parallel to field.
The mobility is a measure of how easily charge carriers respond to an applied electric field.

25. Limiting factors Scattering mechanisms Coulomb fields Phonons
Alloy disorder

26. Coulomb scattering
Electrons are affected by the long-range Coulomb fields of randomly distributed ionized donor atoms.
Thicker barriers move ionized surface donors further away from carriers.
Large 2-DEG densities screen the effect of these Coulomb fields.

27. Phonon scattering Phonons are lattice vibrations in a crystal.
Acoustic phonons
Both types of atoms move “in-phase”
Low energy vibrations
Optical phonons
Atoms of different types move “out-of-phase”
High energy vibration

28. Phonon scattering
Phonons scatter carriers by creating small fluctuating dipoles between atoms (piezoelectric mode).
Phonons scatter carriers by disturbing the periodicity of the crystal lattice (deformation potential mode).

29. Alloy disorder Electron wavefunction penetrates into AlGaN barrier.
Al and Ga atoms are distributed randomly in AlGaN
Randomly varying potential scatters electrons.

30. 2-DEG mobilities

31. Improving the mobility
Strategies:
Reduce 2-DEG density
Smaller Al alloy fractions
Thinner barriers
(Highest mobility heterostructures have Al fractions of ~10% and barrier thicknesses of ~130 Å)
Reduce alloy disorder scattering
AlN spacer

32. AlN spacer

33. Conclusions
Nitride-based semiconductors are a promising field for a wide variety of new technological applications.
2-DEG mobilities are limited by two factors:
Coulomb scattering (N < 2 x 1012 cm-2)
Alloy disorder scattering (N > 4 x 1012 cm-2)
We predict maximum low temp mobilities of 105 cm2/V s without a AlN spacer.
Using a AlN spacer seems a promising way to improve the conductivity of nitride heterostructures.

34. Subband structure
Confining potential results in quantized energy levels.
Trial wavefunction

35. Subband structure

36. Comparison with experiment

37. Comparison with experiment
Smorchkova et al., J. Appl. Phys 86, 4520 (1999)

38. Comparison with experiment
Smorchkova et al., J. Appl. Phys 86, 4520 (1999)