Properties of thick semi-insulating GaN - CERN fileRD50, Florence, 2004.10.14-16 J.Vaitkus...

RD50, Florence, 2004.10.14-16 J.Vaitkus

Properties of thick semi-insulating GaNJ.Vaitkus 1) , E. Gaubas1) , V.Kazukauskas1), A.Blue 2),

W.Cunningham2), K.M.Smith2) and P.Gibart 3)

1) Institute of Materials Science and Applied Research, Vilnius University, Lithuania2) Department of Physics and Astronomy, Glasgow University, Glasgow, UK3) LUMILOG,Ltd., France


Maybe ever RD50 in Vilnius?


Motivation & short history♪ Two years ago - here in Florence a first demonstration of GaN as amaterial for ionising radiation detection.

It was proposed that GaN has to be more radiation hard than neighboringmaterials, as SiC, Si, GaAs due to different chemical bonding: a larger ioniccomponent causes bigger density of material.

≈0.42.42.2 - 40.510Breakdown voltage, [MV/cm]

109.7εR

5.36.23.23.22.32.33.5Density [g/cm3]

10Ga – 20; N -102513-2043Displacement [eV]

518136eV/µm for MIPs

4.38.9-8.44-4.83.613e-h pair creation [eV]

1.2x1072.0x1072.0x1071.0x107Saturated electron drift velocity (cm/s)

≤400305050-1150.01- 0.0054501200µh [cm2/Vs]

≤85001000370800-10001-1014501800µe [cm2/Vs]

1.43.393.033.31.71.125.5Eg [eV]

31/3331/714/614/614146Z

GaAsGaN6H-SiC4H- SiCa-Si(H)SiDiamondProperty


Motivation & short history♪ Two years ago was only a surprise that 2.5 micron thicksamples were tested.

A half year ago was shown that semi-insulating GaN is quiteradiation hard: the detectors were “alive” after high fluences:

Properties of GaN detectors before and after irradiation. Sample irradiation /fluence CCE,% /@

bias, V I10V, nA cm-2 /character τfast, µs

Non-irradiated 95 / 30 0.06 /barrier 0.1-0.5

X-rays (10 KeV) / 600 MRad

100 / 26 5.50 /barrier 0.08

Neutrons / 5 1014 cm-2,

(reactor, 100 KeV) 1015 cm-2, 1016 cm-2.

77 / 28 10 / 30

5 / 16

0.35 /resist. 0.65 /barrier 0.23 /resist.

0.0150.02

<0.01

Protons, 24 GeV/ 1016 cm-2. 13.6 / 30 0.40 /barrier 0.034

A low bias was related to breakdowns at the defects


The promising results related to radiation hardness“allowed”

1. to “force” Lumilog Ltd., to grow thicker samples (12 microns);

2. to ask Tokushima University & Nitride Ltd., to repeat the growth ofsemi-insulating GaN epi-layers to have a possibility for better statistics and moredetail investigation.

Both companies supplied epi-GaN layers on two inches wafers:one - 12 micron thick and three – 2.5.micron thick

3. to receive a reaction of South Carolina University & Sensors, Ltd.,and to discuss a possibility to grow free standing thick high resistivity GaN forcharacterisation.


Next steps:

♪ The key issue is to perform in a short term thetests that for other materials has many years history.

Now it is known: GaN properties depends on materialquality, mostly induced by substrate induced highdislocation density.

Also, the knowledge of different defects: vacancy type point defects,shallow and deep donor-acceptor pairs, and their transforms under heavyirradiations by different particles is necessary.


♪ The presenting some data characterizing the SI-GaN materialproperties before and after irradiation

and

♪ to characterize the difference of thin and thick GaN epi-layers.

This talk:


Samples

◙ non-doped n-type GaN epilayers of 12 µm thickness grownby MOCVD on sapphire substrates using n-GaN buffer

◙ non-doped n-type GaN epilayers of 2.5 µm thickness grown byMOCVD employing ammonia and trimethylgallium as

precursors, and different trimethylgallium (TMG) flow rates and growth temperatures

☼ 10-keV X-ray irradiation with the dose of 600 Mrad☼ 100�keV neutrons with the fluence

of 5 × 1014 cm-2 , 1015 cm-2 and 1016 cm-2

☼ 24 GeV/c protons with fluence of 1016 cm-2

☼ C.C.E. testing - 5.48 MeV Am241 α-particles

Irradiations


Photoluminescence spectra at ~10 ns pulse excitation

2.5 µm MOCVD 2TGM 2.5 µm MOCVD 4TGM 12 µm MOCVD BL

2,0 2,5 3,0 3,510-2

10-1

100

101

102

epi-GaN 2.5 µm 2TMG 0.12 mW 0.6 mW 1.5 mW 3 mW 5 mW 7.5 mW 10 mW

PL

Inte

nsity

(arb

. uni

ts)

Photon Energy (eV) 2,0 2,5 3,0 3,510-2

10-1

100

101

102

epi-GaN 2.5 µm 4TMG 0.12 mW 0.6 mW 1.5 mW 3 mW 5 mW 7.5 mW 10 mW

...\InGaN040304.opj

PL

Inte

nsity

(arb

. uni

ts)

Photon Energy (eV)2,0 2,2 2,4 2,6 2,8 3,0 3,2 3,4 3,6

10-1

100

101

102

epi-GaN 12 µm BL TMG 9 mW 2.9 mW 0.95 mW

PL In

tens

ity (a

rb. u

nits

)

Photon Energy (eV)

Synchronous enhancement of the intensity of UV band, with well expressed edge luminescencestructure, is observed in 12 µm thick layer relatively to those of 2.5 µm thickness.

The native defects concentration estimated in 2.5 µm thick layer from the luminescence spectra dynamics withthe excitation density: “yellow” trap (point defects VGa) : NY <1015 cm-3, “blue” levels (dislocation-related): NB≥1018 cm-3.


Comparison photoluminescence spectra in GaN, grown @ different TMGa flow

2,0 2,5 3,0 3,5

10-1

100

101

epi-GaN

UVB

BB

YB

Pex= 0.12 mW 4 TMG 2 TMGPL

inte

nsity

(a.

u.)

Photon energy (eV)

The native defects concentration increases with TMG flow rate in MOCVDgrown intrinsic n-type GaN layers

Enhancement of the intensity of the PL bands attributed to defects, vacancy complexes (YB) and dislocations (BB)due to an increase in native defect concentration with trimethylgallium (TMG) flow.


Photoluminescence spectra variations in different GaN materials

Comparison of PL spectra under cw excitation of the same intensity in the intrinsic n-type MOCVD (1-3) and HVPE (4) grownGaN layers of 2.5 µm thickness (1, 2), by using TMG flow rates 2 (1) and 4 (2), relatively to the baseline regime, and of 12 µmthickness (3) compared with the Mg doped p-type GaN (5).

2,0 2,5 3,0 3,510-2

10-1

100

101

511

43

21

MOCVD n-typeepi GaN 2.5 µm1 (1030) 2 TMG2 (1047) 4 TMGepi GaN 12 µm3 baseline HVPE4 free-standing MOCVD p-type

5 Mg 6x1019 cm-3

PL

inte

nsity

(a.u

.)

Photon energy (eV)


Photoluminescence spectra variations with excitation density before and after irradiation

PL spectra under cw excitation of the same intensity in as-grown 2.5 µm GaN epitaxial layer (1) and afterirradiation by x-rays (2) and neutrons with fluence 5x1014 cm-2 (3), respectively.

2,0 2,5 3,0 3,510-2

10-1

100

101

102

103

11

2

3

UVB

BBYB

PL

inte

nsity

(arb

. uni

ts)

Photon energy (eV)2,0 2,5 3,0 3,5

0

20

40

6011

2 blue x 1.8

3 violet x 3.6

UVB

BBYB

PL

inte

nsity

(nor

mal

ised

ti e

xc.p

eak)

Photon energy (eV)

D-A

Formation of non-radiative recombination centres!


Photoconductivity (CPC) and microwave absorption (MWA)

0 100 200 300

101

102 MWA decays excited by 10 ns pulse of intensity

Iex =1 a.u. 0.65 0.28

UM

WA (a

.u.)

t (ns)0 2000 4000 6000 8000

10-1

100

2

1

1 CPC excited with 355 nm light pulse of 30 ps2 MWA excited with 355 nm light pulse of 30 ps

t (µs) N

orm

alize

d ph

otor

espo

nse

UCP

C, M

WA

/Um

ax

ab

Variation of the initial MWA transients as a function of excitation density (a) in MOCVDas-grown GaN 2.5 µm thick epi-layer, and asymptotic CPC and MWA decays (b). CPCand MWA transients exhibit the same excess carrier decay rate in the asymptotic of aphotoresponse.


Photoconductivity (CPC) and microwave absorption (MWA) in GaN epi-layer (2.5µm).

TRPL decay (1) collated with the initial stages of CPC (2,4,5) and MWA (3,6) decays in as-grown (1-3) andirradiated by neutrons (4) and protons (6) of fluence 1016 cm-2 and x-rays (5)The trapping caused long-tail decay amplitude decreases with radiation induced defects density (curves 4 and 6) in veryheavily irradiated material due to diminished carrier diffusion (increases scattering by defects inside crystallite) towardscrystallite boundary.

For large fluence of proton irradiation (curve 6) the asymptotic decay amplitude crucially falls down, while for the samefluence of neutron irradiation in the range of 1016 cm-2 the asymptotic decay component is non-resolvable (curve 4).

1 10 100

101

102

103

104

2

64

3

5

1

UM

WA,

CPC

, IPL

1/2 (

arb.

units

)

t (ns)0 50 100 150 200

101

102

103

104

105

2

6

1 TRPL 266 nm light 20 ps pulse non-irradiated2 CPC 355 nm light 30 ps pulse non-irradiated3 MWA 355 nm light 10 ns pulse non-irradiated4 CPC 355 nm light 30 ps pulse 1016 cm-2 neutron irradiated5 CPC 355 nm light 30 ps pulse x-ray irradiated6 MWA (UMWA*10) 10 ns pulse 1016 cm-2 proton irradiated

4

3

5

1

UM

WA,

CPC

, IPL

1/2 (

arb.

units

)

t (ns)


Trapping and disorder

0 5 10 15100

101

102

(a)

as-grown x-rays 600 Mrad irradiated neutrons irradiated 5x1014 n/cm-2

I CPC

(a.

u.)

t (ms)

0 2 4 6 8 1010-1

100

101

102

tα=0.3 (msα)

I CP

C (

a.u.

)

tα=0.7 (msα)

0 1 2 3 4 5

(b)

Variation of the asymptotic photoconductivity decay due to irradiation measured under UV photoexcitation on a semi-log scale (a) andwithin a stretched-exponent approximation (b). Irradiation by neutrons is seen to change the time-stretching factor from 0.3 to 0.7.

This implies that the character of carrier motion changes from percolation on an infinite cluster of dislocation net to that on a fragmentedcluster.

Disorder facilitates capture ofcarriers into relatively shallowlevels, characterized byincrease in capture time withoccupation of trapping centersand a carrier-density dependentcapture cross-section, thusreducing the occupation of therecombination centers.

Infinite dislocations netin the as-grown layer

Fragmented fractonsof crystallites afterirradiations


Thermally stimulated current and polarisation

0.003 0.006 0.009

10-14

10-13

10-12

10-11

10-10

10-9 1016 cm-2 neutrons, GaN

TSC

, A

0.74 & 0.04 eV0.70 & 0.06 0.73 & 0.080.72 & 0.080.71 & 0.18

1/T, 1/K

I-V - 0.22 (eV)IV - 0.22 & 0.11VII - 0.18 & 0.120.1680.1850.220.280.320.320.360.45

0,004 0,006 0,008

10-13

10-12

10-11

0.56 eV0.015

0.2

0.21

0.16

0.18

0.270.33

TSC

,A

1/T (1/K)

0.49

GaN 1e15 neutronsExcitation - white light, Bias 1V

0.10

0,003 0,004 0,005 0,006

10-12

10-11

10-10

Bias 1 VMultiple heating, excitation at 150 K

GaN 1e15 neutrons

TSC

, A

1/T, 1/K

0.18 eV

0.004 0.006 0.008

10-14

10-13

10-12

10-11

GaN, neutrons 1015 cm-2

TSC, bias 1 V dark Linear fit

(+) (-)

(+)

134 meV

I, A

1/T, 1/K

TS Current - blue, TS Depolarisation - black

TS Current - multiple annealing

TS Current at different bias


MODEL

TEM image


Detection of 5.48 MeV Am241 α-particles

0 20 40 60 8010-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

12µm GaN Forward Reverse

I (A

)

Bias (V)

Run 1 Run 2 Run 3

0 50 100 150 200 250

0

100

200

300

400α spectra for 12µm thick GaN

Cou

nts

(au)

Channel Number

0V 10V 20V 40V 50V 60V

0 2 4 6 8 10

0,1

1

10

eV/A

rmst

rong

Range (µm)

Alpha particle in GaN

C.C.E. = ~ 50 %


Conclusions# The correlated investigations of the photoluminescence (PL) spectroscopy,contact photoconductivity (CPC) and microwave absorption (MWA) transientshave been performed to clarify a role of grown-in and radiation-induced defects inthe MOCVD- and HVPE-grown epitaxial GaN layers.The TS current and polarisation results show the changes of inhomogeneitystructure caused by irradiation.

# Irradiation by x-rays of 600 Mrad and neutrons of 5×1014 cm-2 fluence induces anincrease of the non-radiative trap density, which yields PL quenching in all theobserved bands: ultraviolet (UV), blue (B), and yellow (Y).The trapping caused long-tail decay amplitude decreases with radiation induceddefects density for very large fluences of neutron and proton irradiation in the rangeof 1016 cm-2.

# Radiation defects modify the grown-in structure (characterized by by time-stretching factor β =0.7) of an infinite cluster of dislocations net by formation oflocalized fractons characterized by time-stretching factor of 0.3.


Report from crystal growers:----- Original Message -----To: [email protected]: Monday, October 11, 2004 12:43 PMSubject: Re: High resistivity or semiinsulating GaN•

Dear Juozas,• We will be able to grow thick layers of NID GaN on sapphire (~50 µm), the final

layer will however exhibits a bow.Pierre

P.S. Free standing 50 µm thick will be too brittle, the thickness should be at least200 µm.We can re-growth a conducting layer on the SI GaN to make a back contact.

The cost of SI-GaN wafer will be much less than SiC.

Thank You for attention !

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Properties of thick semi-insulating GaN - CERN fileRD50, Florence, 2004.10.14-16 J.Vaitkus...

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