Ferromagnetism and near-infrared luminescence in RE-GaN via diffusion

NC STATE UNIVERSITY

M. O. Luen1, N. Nepal1, P. Frajtag2, J. M. Zavada1, E. Brown3, U. Hommerich3 ,

S. M. Bedair1, and N. A. El-Masry2 1Electrical and Computer Engineering, North Carolina State University, Raleigh, NC 27695 USA2Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695 USA3Department of Physics, Hampton University, Hampton, VA 23668 USA

Ferromagnetic RE-GaN films via MBE

Ferromagnetism vs. Annealing & Doping

Effects of GaN Growth Parameters Emission Intensity vs. Si-doping

Ferromagnetism vs. Si-doping Emission Intensity vs. Annealing Time

Ferromagnetism vs. Annealing Time

Ms “saturates” beyond 9 hours of annealing

Magnetic Saturation vs. Annealing Time for GaN:Nd (N039-08)

0.E+00

2.E-06

4.E-06

6.E-06

2 4 6 8 10 12

Anneal Time (hrs)

Ms

(e

mu

)

* We have shown that n-type GaN:Si grown by MOCVD and ex-situ doped with Nd and Er via diffusion exhibits room temperature ferromagnetism and near-infrared and infrared luminescence as well. All Nd-doped samples showed emission in the near-infrared (~1060nm and ~1350nm) while the Er-doped samples’ emission was weak at ~1546 nm.

* Substrate conditions play a role in magnetic behavior as both Si and RE atoms compete for gallium sites, and increased Si-doping leads to decreased saturation magnetization. Luminescent intensity varies slightly over the Si doping range, and emission intensity also varies with annealing time.

* To the best of our knowledge, this is the first demonstration of above room temperature ferromagnetism and emission in GaNdN via diffusion.

Low-dispersion wavelength

Low-losswavelength

Nd

Er

Nd3+Er3+

* Nd emits at ~1.3μm : the low-dispersion regime for optical fiber communications. Low dispersion = faster transmission.

* Er emits at ~1.5μm : the low-loss regime for optical fiber communications. Low loss = fewer amplifiers.

* Similar outer e- config. mean similar chemical behavior:Nd: atomic no.=60; e- config.=[Xe]4f4,6s2; 3µβ per trivalent ionic state Er: atomic no.=68; e- config.=[Xe]4f12,6s2; 3µβ per trivalent ionic state

1

Effect of Anneal Time on Ms for GaN:Nd(Similar experimental conditions (800C, 3sccm silane

flow, no H2))

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 2 4 6 8 10 12 14 16

Anneal Time (hr)

Ms (

mic

ro-e

mu

)

Er Nd3sccm silane flow favorsFM in Nd-diffused GaN:Si

N089-08: GaN: n-type (Si: 1.5sccm) Nd(blue square), Er(red dot), n=~1.4e18Effect of Anneal Time on Ms

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 2 4 6 8 10 12 14 16

Anneal Time (hrs)

Ms

(m

icro

-em

u) Nd Er

1.5sccm silane flow is favored For FM in Er-diffused GaN:Si

* FM coupling saturates at Longer anneal times.

* Ms levels-off for +9hr Anneal times.

* 16nm diffusion depth @ 15hr anneal.

* 50% reduction in silane: 3X improvement in Ms(Er) while Ms(Nd) appears unaffected.

* 32nm diffusion depth @ 15hr anneal.

Rare earth doped semiconductor based devices that utilize both the ferromagnetic properties (through electron spin) and atomic-like emission of RE-GaN to store and transmit information can have reduced size and more efficient transmission across optical fiber. Most recent studies on dilute magnetic semiconductor (DMS) materials have been focused on GaMnAs and InMnAs

Motivation

Summary

Er in n-GaN (med. Si-doping) for varying annealing times.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

3 5 7 9 11 13 15

Annealing Time (hrs)

Ms

(mic

ro-e

mu)

1000 1100 1200 1300 1400 1500

0.0

0.2

0.4

0.6

0.8

note: all samples diffused with Nd under similar conditions.

Excitation: 578 nm

~1350nm

~1064nm

Nd:YAG (MOPO)0.3m Spectrometer1 m gratingInGaAs detectorFilter: RG1000Scan: 950-1550 nm @ 100nm/minSlit: 3mm

Nd diffused on GaN Samples

Em

issio

n In

ten

sit

y (

a.u

.)

Wavelength (nm)

Sample Name/No. Substrate Chrac. MOL# 48, N210-07 med. silane MOL# 49, N211-07 high. silane MOL# 70, N098-08 low. silane MOL# 73, N105-08 no silane

(undoped)

Nd-doped GaN:Si (2hr anneal)

* Increased Si-doping has little effect on emission intensity, but decreases saturation magnetization of GaN:Nd.

* Nd does not have to be substitutional for emission.

* ~1um emission has application in future optical fiber telecommunication windows.

Emission Intensity vs. Si-doping

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0 1 2 3 4 5 6

Silane Flow (sccm)

Em

issi

on

Inte

nsi

ty (

a.u

)

1450 1500 1550 16000.0000

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

~1546 nm

Note: Same Si-doping conditions

Er doped GaN Samples

2nd order

Excitation: 810 nm

Ti:Sapphire LaserPump power: ~0.95 watt (output beam)laser power ~350 mW on the sample0.3m Spectrometer2 m gratingInGaAs detectorFilter: RG1000Scan: 1450-1590 nm @ 20nm/minSlit: 3mm

Em

issi

on

Inte

nsi

ty (

a.u

.)

Wavelength (nm)

MOL # Anneal Time MOL#59 9 hrs MOL#60 12 hrs MOL#61 4 hrs MOL#62 3 hrs MOL#68 15 hrs

Er-doped GaN:Si (Si: 3sccm)

* Increased diffusion time increases emission to a point and then intensity decreases.

* Emission intensity very weak so fluctuations are insignificant.

* ~ +9hr anneal is best for Er-IR emission.

Emission Intensity vs. Anneal Time

0.E+00

1.E-04

2.E-04

3.E-04

4.E-04

5.E-04

6.E-04

3 5 7 9 11 13 15

Anneal Time (hrs)

Em

issi

on In

tens

ity

(a.u

.)

Ref: S. Dhar, PHYSICAL REVIEW B 72, 245203 (2005)

MBE–grown GaGdN shows average colossal magnetic moment (4000μβ) compared to atomic magnetic moment of 8μβ. Gd concentration: 7x1015/cm3 to 2x1019/cm3.

Ref: M. Hashimoto, Jpn. J. Appl. Phys. Vol. 42 (2003) Pt. 2, No. 10A

MBE-grown GaEuN with 2 at.% Eu with 0.1μβ per Eu ion at 300K and 200mT applied magnetic field.

Conventional approach to ferromagnetic RE-GaN

Nd Er

This work is supported by the U. S. Army Research Office Grant No. W911NF-07-1-0603.

Rare Earth Properties

SIMS Profile for RE-Diffused GaN

RE-GaN films via Diffusion

Computed Values of the Curie Temperature (using the Zener model description) for various p-type semiconductors. Ref: T. Dietl, et. al., Science, Vol. 287, 2000, p. 1019.

* Tc (GaMnAs) = 140 K and Tc (InMnAs) = 90 K * Tc (GaMnN) > 300 K Tc (GaN:RE) > 300 K* According to Zener model, predicted Tc for GaMnN is above room temperature

Ref. National Laboratory for Advanced Tecnology and

nano Science, Nottingham Univ.

Curie Temperature of GaMnAs

ZnTeZnSe

ZnOInAs

InPGaSb

GaAsGaP

GaNAlAs

AlPGe

Si

10 100 1000Curie Temperature (K)

NC State University reported RT-FM in GaMnN (2000), press release.

Sources Ga: TMGa,N: NH3Si: SiH4

RE-GaN layer:Tdif = 800 C Pbase = ~ 2x10-9 Torr Laser: KrF @ 248nmRE film: 10nm < t < 40nm

Sapphire (Al2O3)

GaN (1µm)

RE-GaN

* The magnetic film shows a strong saturation magnetization of 5.7 µemu.

* Blue curve is representative of pre-diffusion saturation magnetization of samples.

RE-doped GaN achieved via diffusion has benefits of ease of “growth” and low cost. Technique demonstrates both optical and magnetic response in samples.

Layer structure

-7.E-06

-5.E-06

-3.E-06

-1.E-06

1.E-06

3.E-06

5.E-06

7.E-06

-1.E+04 -8.E+03 -6.E+03 -4.E+03 -2.E+03 0.E+00 2.E+03 4.E+03 6.E+03 8.E+03 1.E+04

Applied Field (Oe)

Sa

tura

tio

n

Ma

gn

eti

za

tio

n (

em

u)

n-GaN Template

RE diffused n-GaN

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

0 16

32

48

64

80

96

11

2

12

8

14

4

15

9

17

5

19

1

Depth from Surface (nm)

Co

un

ts/s

ec

Nd in GaN:Si (1.5 sccm silane)

Nd in GaN:Si (3.0 sccm silane)

Si for Nd in GaN:Si (1.5sccm silane)

Si for Nd in GaN:Si (3.0 sccm Silane)

* Similar outer e- configuration means similar chemical behavior, therefore Er diffusion profile approximates Nd diffusion profile.* Ting et al. and Chen et al. calculated diffusion constant and coefficient for GaErN via diffusion at 800°C for 7.5 and 21 hours to be D0 = 1.8x10-12cm2/s, D(800°C) = 2x10-17cm2/s.* We estimate a smaller DNd(800°C) ~ 1x10-17cm2/s due to larger atomic size of Nd.

Ref: Y-S Ting et al. Optical Materials, 24, 515- 518 (2003) Ref: C-C Chen et al. Solid-State Electronics, 47, 529-531 (2003)

1x1018/cm3

2x1017/cm3

* Vacancy and/or Defect-mediated Diffusion.* Increased silane means more Si competing with RE for Gallium Vacancies.* H diminishes Ms via: H-complex passivation of VGa or e- donation which compensates VGa.

D(Nd in GaN) ~ 1x10-17cm2/s @ 800°C D(Si in GaN) ~ 6.5x10-11cm2/s @ ~900°C

Si occupies VGa and stays

Effect of H2 and Silane Flow on Ms of GaN:Nd(Similar experimental conditions (800C, 2hr anneal))

0

0.5

1

1.5

2

2.5

3

0 1 2 3 4 5 6 7

Silane Flow (sccm)

Ms

(mic

ro-e

mu

) w/o H2

w/ H2

Poly. (w/oH2)

Ref: R. Jakiela et al. Phys. Stat. Sol. (c), vol. 3, issue 6, 1416-1419 (2006)

Ref: A. F. Wright et al., J. Appl. Phys.,vol. 90, 1164 (2001)

Silane Flow vs. Saturation Magnetization (GaN:Nd, 800C, 2hr anneal)

0

0.5

1

1.5

2

2.5

3

0 1 2 3 4 5 6 7Silane Flow (sccm)

Ms

(mic

ro-e

mu)

Ms decreases with increased Si-dopingSaturation Magnetization vs. Anneal

Time for Different Si-doping

1

2

3

4

5

8 10 12 14 16Annealing Time (hrs)

Sa

tura

tio

n M

ag

neti

za

tio

n

(mic

ro

-em

u) 3sccm

1.5sccm

Saturation Magnetization vs. Si-doping for Er-GaN

-5.E-06

-4.E-06

-3.E-06

-2.E-06

-1.E-06

0.E+00

1.E-06

2.E-06

3.E-06

4.E-06

5.E-06

-5.E+03 -3.E+03 -1.E+03 1.E+03 3.E+03 5.E+03

Applied Field (Oe)

Sa

tura

tio

n M

ag

ne

tiza

tio

n (

em

u)

3sccm

1.5sccm

Saturation Magnetization vs. Si-doping in GaN:Nd

-4.E-06

-3.E-06

-2.E-06

-1.E-06

0.E+00

1.E-06

2.E-06

3.E-06

4.E-06

-5.E+03 -3.E+03 -1.E+03 1.E+03 3.E+03 5.E+03

Applied Field (Oe)

Sat

ura

tio

n M

agn

etiz

atio

n

(em

u)

Low Si-doped

Med. Si-doped

High Si-doped

Nd

Er

We are unsure of the cause of Ms saturation for annealing beyond 9 hours. One explanation: localized high Er concentration (due to shallow diffusion) causing anti-ferromagnetic/ ferromagnetic competition between additional, nearby RE ions. This competition proposed to occur between substitutional and interstitial RE ions

T. Dietl, Physica E, 35 (2006) 293-299