4H-SiC defects

Nicolò Piluso

Outline 2 -  Introduction

Overview 4H-SiC (growth, properties, application) -  Defects

Bulk Epitaxial layer

-  Characterization methods

Photoluminescence Raman Spectroscopy DLTS µ-PCD KOH molten Optical microscopy, TEM, SEM Scattered Light

4H-SiC lattice (Wurtzite unit cell) Siatom

Catom

Introduction

at 300K Si GaAs 4H-SiC GaN

Eg(eV) 1.12 1.4 3.2 3.4

Ec (MV/cm) 0.25 0.3 2.2 3

µn (cm2/Vs) 1350 8500 1000 1000

εr 11.9 13 10 9.5

Vsat (cm/s) 1x107 1x107 2x107 3x107

λ (W/cmK) 1.5 0.5 3-5 1.3

Semiconductor properties

3

Some devices are pushing Si boundary SiC and GaN offer promise for improvement

Slide from Prof. S. E. Saddow

Applications of Power Devices

Introduction 4

Growth Methods: PVT - Bulk (thick, high doping) CVD - Epitaxy (thin film, mid-low doping)

PVT

CVD

cutting

Ingot - Bulk

Epitaxy

5

From Ch3, Kimoto-Cooper Book

Micropipe defects are indeed located at the center of a large spiral on the surface of the SiC boule, and

the diameters of the pinholes range from 0.5 µm to several micrometers

Micropipe

TSD TED-BPD

Bulk Defects

6

> C/Si Ratio > Polarity (Si or C face) > Hexagonality of the polytype - 4H-SiC (0.5 Hex), more stable in C-rich ambient - 6H-SiC (0.33 Hex)

Polytype mixing during boule growth ! switching, coalescence, nucleation !

Occurrence of spiral growth around TSD that is generated to remedy to

mismatch of polytype

7

BPD generation during boule growth

A doping variation affects the BPD generation due to the lattice

parameters variation.

Most TEDs are formed by conversion from BPDs

along the growth direction during growth.

A stress reduction could reduce the BPD generation

8

From Ch5,p144 Kimoto-Cooper Book

TSD generation and removal TSD

BPD TED

BPD

Vendor Value EPD = TSD+TED+BPD

9 Defects reduction (Bulk)

10

From Ch5,p161 Kimoto-Cooper Book

Epi Defects

From Ch4,p97 Kimoto-Cooper Book

11

CVD process Si-rich ! Carrot

C-rich ! Triangle

Epi Defects

Extended epi defect Current Density (cm-2) Down Fall 0.1

Triangular defects / Comets 0.2

Carrots / Pit 0.5-1

Stacking Faults 1

Dislocations Vs Epi defects 12

0

10

20

30

40

50

60

70

80

90

100

0 5000 10000 15000 20000 25000 30000

Carrots

0 5000 10000 15000 20000 25000 30000

0 20 40 60 80

100 120 140 160 180 200

3500 3700 3900 4100 4300 4500 4700 4900 5100

Stacking Faults

0 4000 8000 12000 16000 20000 24000 28000 32000

PVT growth direction

ingo

t

seed

EPD

EPD

High density

Low density

The spread observed is mainly due to uncorrect EPD value declared. EPD decrease along the bulk growth. The EPD value is, typically, detected by etching a wafer within or on the edge of the ingot. This method produce a reference value for all the ingot, but, truly speaking, it is only an approximate value for all the wafers.

EPD=TSD+TED+BPD

Epi defect: Step bunching and roughness 13

-Step Flow -Epitaxial process parameters -No relevant effects on device performances - Crucial growth parameters: Temperature, growth rate, Si/C

AFM

Rq < 1nm

Stacking Faults 14

4H- SiC

SF

Band gap diagramme

4H- SiC

15 Impacts of epi defects on devices Current

Density (cm-2) Defect SBD MOSFET, JFET Pin, BJT, Thristor,

IGBT

~ 0 Micropipe VB reduction (by 50-80%)

500 TSD (without pit) NO NO NO, but can causes local reduction of carrier lifetime

3000

TED (without pit) NO NO NO, but can causes local reduction of carrier

lifetime

1000

BPD (including interface

dislocation, half loop array)

NO, but can cause degradation of MPS Diode

NO, but can cause degradation of body

diode

Bipolar degradation (increase of on-

resistence and leakage current)

0.01 - 1 In-grown SF VB reduction (by 20-50%) V

B reduction (by 20-50%)

VB

reduction (by 20-50%)

0.1 - 1 Carrot, triangular defect V

B reduction (by 30-70%)

V

B reduction (by 30-70%)

VB

reduction (by 30-70%)

0.1 - 1 Down fall VB reduction (by 50-90%)

V

B reduction (by 50-90%)

VB

reduction (by 50-90%)

From Kimoto-Cooper book, Fundamentals of Silicon Carbide Technology, 2014

Spatially resolved micro-photoluminescence and micro-Raman setup

• Confocal apparatus • Grating monochromator

• XY motorized stage

• CCD detector

PL or Raman map

325 nm UV laser 633 nm Vis laser

PL or Raman signals

x

y

XY motorized stage

Sample

Objective

Evaluations of: -  Crystallographic defect (PL and Raman) -  Doping (PL and Raman) -  Stress (Raman) -  Polytype inclusion (PL and Raman)

17

Stacking Faults Doping

-40000 -20000 0 20000 40000

775,6

776,0

776,4

776,8

777,2

777,6

Stress free value

TO R

aman

Shi

ft (c

m-1

)

Distance (µm)

Vendor B Vendor C BridgestoneVendor A

Stress

-30 000

-20 000

-10 000

0

10 000

20 000

30 000

Y (µ

m)

x103-20 0 20X (µm)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

4000 µm

0

200

400

600

800

1 000

1 200

1 400

1 600

1 800

2 000

2 200

2 400

2 600

Inte

nsity

(cnt

)

750 760 770 780 790 800 810 820Raman Shift (cm-1)

798

.0

767

.6

789

.5

0

50

100

150

200

250

300

350

400

450

500

550

600

650

Inte

nsity

(cnt

)

740 750 760 770 780 790 800 810Raman Shift (cm-1)

776

.8

0

200

400

600

800

1 000

1 200

1 400

1 600

1 800

2 000

2 200

2 400

2 600

Inte

nsity

(cnt

)

730 740 750 760 770 780 790 800 810 820Raman Shift (cm-1)

797

.6

770

.4

777

.3

786

.4

6H

4H 4H+6H

(+15R ?)

Polytype inclusion analysis (HeNe)

Transverse optical mode (TO)

19

Im

Vc

Sii

Ci

VSi

"  Intrinsic (interstitial, vacancy, antisite) Sii,Ci, Vsi, Vc, Csi, etc

"  Extrinsic (impurity) "  Complex

carbon

silicon

[000

1]

Point Defects

Perfect lattice Point defects

As

Energy levels of major deep levels observed in as grown n-type and p-type 4H-SiC epitaxial layers

EH6/7 is a carbon vacancy

Also, the origin of the Z1⁄2 center was identified as the acceptor

levels of a carbon vacancy

After Annealing at 1600°C, Z1/2 and EH6/7 remain important levels

Z1/2 = Ec-0.63eV EH6/7 ~ Ec/2 eV

µ-PCD and DLTS methods 20

Reducing of the on-resistance in bipolar devices

21

Implantation process produces a damage on the lattice crystal. By performing an annealing at high temperature (T > 1600 °C) the crystal

is partially recovered. However, an agglomeration of defects is still present and

observed by PL characterization.

An optimization of Ion dose and Ann. temperature is on going

Defects from implantation

0,00E+00

5,00E-01

1,00E+00

1,50E+00

2,00E+00

2,50E+00

3,00E+00

350 400 450 500 550 600 650

Inte

nsity

Wavelength (nm)

Defect @ 470nm (2.6eV) Gap

Not implanted

Implanted + HT Decreasing the source dose we observe a large decrease of the

point defects peak observed by PL

1.5 2.0 2.5 3.0

0.000

0.001

0.002

0.003

0.004

0.005

0.006

Inten

sity (

a.u.)

Energy (eV)

body (1650°C) body (1700°C) body (1750°C)

Increasing the annealing temperature the peak at 1.6 eV (EH6/7) decreases and the peak at 2.6 eV (Z1/2) increases

22 Candela tool (scattered light)

23

100 µm

Triangles

Carrots

Stacking Faults Macrostep Particle

Particle

EpiDefects

EPI 4H-SiC defects (Candela CS920 Images)

Conclusions 24

-T. Kimoto, Japanese Journal of Applied Physics 54, 040103 (2015)

-T. Kimoto, J.A. Cooper, FUNDAMENTALS OF SILICON CARBIDE TECHNOLOGY, 2014 John Wiley & Sons Singapore Pte. Ltd.

-G. Feng, et al. Physica B 404 (2009) 4745–4748

-M.B.J. Wijesundara and R. Azevedo, Silicon Carbide Microsystems for Harsh Environments

-F. La Via, Silicon Carbide Epitaxy, Research Signpost 2012

-Owner studies

References

" Continuous quality improvement

" Wide choice in characterization techniques

" Different quality from different vendors

" Non destructive VS destructive characterization methods (dislocation density)

Non destructive (PL) Destructive (KOH etch)