Low dislocations density GaN/sapphire for optoelectronic devicesLow dislocations density GaN/sapphire for optoelectronic devices

B. Beaumont, J-P. Faurie, E. Frayssinet, E. Aujol and P. Gibart, Lumilog, 06220-Vallauris-FRANCE

It is nowadays well established that threading dislocations (TDs) are degrading the performances and the operating lifetime of optoelectronics GaN based devices (LDs and UV-LEDs). GaN/sapphire layers have been grown by Metal Organic Vapour Phase Epitaxy (MOVPE). A silicon nitride layer is deposited using a SiH4/NH3 mixture prior to the growth of the low temperature GaN buffer layer. Such a process induces a 3D nucleation at the early beginning of the growth, resulting in a kind of intrinsic random mask ELO process. This produces a significant decrease of the TDs density compared to the best GaN/sapphire templates. GaN layers with TD density as low as 7×107cm-2 were obtained (as measured by atomic force microscopy (AFM), cathodoluminescence and transmission electron microscopy (TEM)). The two-step epitaxial lateral overgrowth technology (2S-ELO) allows decreasing the TDs around 107cm-2. These templates are suitable for fabricating LDs. Regrowth by HVPE on these ELO GaN/sapphire further decreases the TDs density below 5×106cm-2.

GaN/template:The Si/N treatment of the sapphire substrate creates an intrinsic random mask, thus resulting in a micro-ELO process

0 50 100 150 200 250

0

5

10

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25 Standard GaN/Sapphire

U.L.D GaN/Sapphire

Ref

lect

ivity

sig

nal

Time (mn)

Comparison between reflectivity spectra recorded during the growth of GaN/sapphire standard epilayer and Ultra Low dislocation (ULD) GaN/sapphire. Arrows indicate where the growth starts

High Resolution image of the interface in a ULD GaN/sapphire sample.

Cross-sectional bright field image of the ULD GaN/sapphire sample showing the interface region.

GaN/ELO: Two steps ELOTemplate 2S-ELO 1S-ELOTemplate 2S-ELO 1S-ELO

Epitaxial growth of 2µm GaN

deposition of SiN

Selective Epitaxy (SAE)

Lateral Overgrowth

Selective Epitaxy (SAE) and Lateral

Overgrowth

Cross section along the [10-10] zone axis of a 2S-ELO film at the end of the second step

CL map of a GaN layer grown with the two-step process. Each dark spot corresponds to a merging TD at the surface

Free standing GaN (HVPE)

Characteristics

Sample FWHM of near band gap PL at 10K

TRPL (A exciton)

TDs density measured by AFM and CL

Standard <3meV 80 ns 5×108cm-2

ULD <2meV 220 ns 7×107cm-2

2S-ELO <1meV 375ns 5×106cm-2

Between stripes

Free-standing 1×106cm-2

Full wafer

Devices: UV detectors and LEDs

250 300 350 400 450

10-5

10-4

10-3

10-2

10-1

100

ELOG GaN

GaN / SapphireNo

rma

lize

d R

esp

on

sivi

ty

Wavelength, nm

250 300 350 400 450

1x10-5

1x10-4

1x10-3

1x10-2

1x10-1

1x100

No

rmal

ized

Res

po

nsi

vit

y

Wavelength (nm)

250 300 350 400 450

10-5

10-4

10-3

10-2

10-1

100

ELOG GaN

GaN / SapphireNo

rma

lize

d R

esp

on

sivi

ty

Wavelength, nm

250 300 350 400 450

10-5

10-4

10-3

10-2

10-1

100

ELOG GaN

GaN / SapphireNo

rma

lize

d R

esp

on

sivi

ty

Wavelength, nm

250 300 350 400 450

1x10-5

1x10-4

1x10-3

1x10-2

1x10-1

1x100

No

rmal

ized

Res

po

nsi

vit

y

Wavelength (nm)

250 300 350 400 450

10-5

10-4

10-3

10-2

10-1

100

ELOG GaN

GaN / SapphireNo

rmal

ized

Re

spo

nsi

vit

y

Wavelength, nm

ELOG GaN

Micro-ELO GaN

GaN/Sapphire

CL map of a free standing GaN grown by HVPE and separated from the sapphire substrate, TDs density 106cm-2Images from MFA, Budapest

Data from U. Politechnica, Madrid

From Yasan et al, APL, 81, 2151(2002)