Techniques of GaN crystal Growth

   Techniques of GaN crystal
                    Growth
Courtesy of Fu Xiaoling. Used with permission.

Outline

Major properties of GaN

Significance of growing bulk GaN crystal

Methods we have achieved

Challenges we still have

GaN devices impact on the future

Properties of GaN

1.high melting point (2791K) at a

correspondingly high equilibrium

pressure of nitrogen(45Kbar)

2. wide, direct bandgap of 3.39ev

3.high thermal conductivity

4.high electron saturation velocity

Why GaN ?

Great potential applications in highpower

electronics capable of operation

at elevated temperatures and high

frequencies.

Difficulties we have met

Difficult to get native substrates in high

quality and large quantities

Even the slight lattice mismatch induce

misfit dislocations at the interface,which

could develop cracks in crystals that

degrade the performance of devices

Substrates we have used

Have tried sapphire(Al2O3), SiC, MgO,

MgAl2O4, LiAlO2,LIGaO2, ZnO, MoS2 et al.

Most common use: sapphire

Better quality in many cases

Available up to inches in diameter

Inexpensive

Main Methods we have got

Vapor phase epitaxy(HVPE)

Molecular beam epitaxy (MBE)

Metalorganic chemical vapor

deposition(MOCVD)

Crytal growth of GaN (HVPE)

Long history and widely used because of high growth

rate

Hydride vapour phase epitaxy(HVPE) attract renewed

interest to produce thick, strain-relieved buffer layers.

Comments on VPE

Highly suitable for GaN films due to high growth rate

and bandgap engineering in near-UV spectral range

is possible

However, the very high growth temperature of VPE

processes, between 1000 and 1200 degrees make Si

and O impurity concentration high, which makes

electron concentration high

Growth rate:100um/h have been achieved.

Crystal growth of GaN(MBE)

1. Nitrogen gas cannot be

directly used for GaN growth.

Nitrogen (N2) must be

dissociated prior to reaching the

surface of the substrate in order

to incorporate in GaN.

2. Low temperature

Comments on MBE

Advantages:

High purity growth

Hydrogen free environment

Possibility to use plasma or Laser assisted growth

Disadvantages:

Need ultra-high vacuum

Low growth rate

Very expensive

Crystal growth of GaN(MOCVD)

Basic MOCVD reaction describing GaN deposition process:

Ga(CH3)3+NH3??GaN

For high optical quality material it's necessary to grow GaN at

temperatures as high as 1080 degrees.

The deposition of a low temperature "buffer"layer of AlN on

sapphire substrates was a key discovery in improving surface

morphology and crystalline quality of GaN.

Comment on MOCVD

Advantages:

High growth rate

Large-area growth capability

Very high quality film

Intermediate cost

Disadvantages:

High temperature

Tendency to preact

Various new technologies

Modified VPE process: Sublimation sandwich

method(SSM):

good structural quality for films grown at very high rates

Lateral epitaxial overgrowth(LEO) by MOCVD:

Reduce defect density tremendously, but only in the windows, not on

the mask

enable the controlled deposition of low-dimensional microstructures

such as quantum wires and dots, don't cause any damage or

contamination at sample surface.

Challenges

Need maturer technology:

Larger size

Defect free (Has been reduced from 108–1010 cm-2 to

107 cm-2 not low enough)

Low cost

Doping issues.

Impact on the future

Enable the fabrication of LEDS, lasers,

detectors, transistors and so on

Save lots of energy

Harmless

Major References

Fabrication and performance of GaN electronic devices, Materials

Science and Engineering: R: Reports, Volume 30, Issues 3-6, 1

December 2000, Pages 55-212

S. J. Pearton, F. Ren, A. P. Zhang and K. P. Lee

Progress and prospects of group-III nitride semiconductors, Progress in

Quantum Electronics, Volume 20, Issues 5-6, 1996, Pages 361-525

S. N. Mohammad and H. Morkoç

GaN heteroepitaxial growth techniques, Journal of Microwaves and

Optoelectronics, Vol.2, No.3,July 2001, P22-29

Nasser N.M, Ye Zhi zhen, Li Jiawei and Xu Ya bou

Growth and applications of Group III-nitrides, Journal of applied

Physics, 31(1998) 2653-2710

O Ambacher.

Nitride semiconductors––impact on the future world, Journal of

Crystal Growth, Volumes 237-239, Part 2, April 2002, Pages 905-911

Isamu Akasaki.

Bárbara Scarlett Betancourt Morales

CAF

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