Nitride semiconductors and their applications Part II Nitride

Nitride semiconductors and their applications Part II: Nitride semiconductors

Nitride papers published

Nitride-based semiconductors • A III-V semiconductor in which N is one of the elements. Examples include Al. N, Ga. N, In. N, and alloys such as Alx. Ga 1 -x. N. • These materials have high melting points (strong bond with N) and a wide span of bandgaps (from 0. 7 e. V for In. N to 3. 4 e. V for Ga. N to 6. 2 e. V for Al. N).

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

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.

Short history 1971 Ga. N LED demonstrated (Pankove) 1986 “High” quality Ga. N grown (Akasaki) 1988 P-type Ga. N grown (Akasaki); Nakamura starts work on Ga. N 1990 Two-flow MOCVD system developed (Nakamura) 1991 High quality p-type Ga. N grown; first pn-junction Ga. N LED created 1992 Zn. Se-Cd. Zn. Se blue laser developed (3 M) 1993 Commercial blue Ga. N LEDs introduced (Nichia) 1996 Room temperature nitride LDs developed (Nichia) 1999 Commercial nitride LDs introduced (Nichia) 2004 SONY markets blue laser DVD writers (23 GB/layer)

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

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

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

Buffer layer Dislocation High quality Ga. N Low quality Ga. N (0. 2 mm) Al. N or Ga. N buffer layer (0. 05 mm) Substrate

p-type Ga. N • Problem: No one could dope Ga. N 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!

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

Current research on nitrides • • Fundamental physics Improving crystal quality (still very poor) Ultraviolet lasers Lattice matching with quaternary alloys (Al. Ga. In. N) • Nitride heterostructures and accompanying applications

Nitride heterostructures

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).

Research questions • Even undoped, carrier densities in Al. Ga. N/Ga. N heterostructures is 10 to 100 times larger than those in similar (Al. Ga. As/Ga. As) systems. What is the source of these carriers? • Carrier mobilities in Al. Ga. N/Ga. N heterostructures are 10 to 100 times lower than in the Al. Ga. As/Ga. As system. What are the principle mechanisms limiting the mobility?

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

Pseudomorphic growth Al. Ga. N lattice matched non-lattice matched Al. Ga. N/Ga. N interface • Ga. N and Al. N have ~2. 5% lattice mismatch • Grown on polar c-axis • Spontaneous and induced piezoelectric fields are present

Band structure • When Al. Ga. N barrier is thick enough to pull defect level above bottom of Ga. N 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 Ga. N well

Electron transfer

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

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 = e. E/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.

Limiting factors Scattering mechanisms • Coulomb fields • Phonons • Alloy disorder

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.

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

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).

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

2 -DEG mobilities

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 – Al. N spacer

Al. N spacer

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 cm 2/V s without a Al. N spacer. • Using a Al. N spacer seems a promising way to improve the conductivity of nitride heterostructures.

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

Subband structure

Comparison with experiment

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

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