A. Molina-Sánchez*, J. Segura-Ruiz, N. Garro, A. Garcia-Cristobal, A. Cantarero, F. Iikawa,

C. Denker, J. Malindretos, and A. Rizzi

Niza, 9 de Mayo de 2011

III-N NanowiresInN: Properties and accumulation layer

Electronaccumulation at the surface(films)

Reassigned bangap around 0.67 eV

Transport experiments point theexistence of the electron

accumulation layer

rrgDD 2

2

3

2

conductance g vs the radius r

6.1rg measured

Nano Letters B 9, 1567 (2009)

III-N NanowiresInN NWs: Photoluminescence experiments

Courtesy of J. Segura-Ruiz & C. Denker

0.6 0.7 0.8 0.9 1.0

PL,

PLE

yie

ld (a

rb. u

nits

)

Energy (eV)

InN NWs samples grown under differentconditions

Photoluminescence

Blue‐shift of the emission energy (differentelectron concentration?)

Broadening increases with the emissionenergy.

Photoluminescence excitation

Blue‐shift and slope are different for eachsample. Higuer slopes for samples with higherabsorption edges.No exciton related peaks .

III-N NanowiresSelf-consistent modeling

Schrödinger eq.

Wave functions :   Poisson eq.

n(r)= f (EF)||2

Fermi level and electron density

V(r)Potential energy

Until convergence

Donor surface statesrelease electronsinside the NW

n(r)

InN nanowire Nss+

Uniform distribution of donors in the volume

+ND+ Electrons occupy the

energetic levelsdetermined by thepotential energy

EFermi

n(r)

V(r) Nss+

radius

energy

Calculation of conduction and valence band states

Optical absorption (comparisonwith experiments)

0 10 20 30 40

-1.0

-0.5

0.0

1017

1018

1019

V -

E c (eV

)

r (nm)

EF-Ec

n (c

m-3

)

(2,0)

(1,0)

III-N NanowiresPotential profile and electron distribution

Electron density.  Highconcentration near to the NW surface

The potential energy fallsat the NW surface. Itdetermines the electronicstates

The potential energyconfines the electronwave functions near tothe NW surface

ND = 31017 cm-3

Nss = 11013 cm-2

++

III-N NanowiresOptical absorption of InN NWs. Influence of size

Blue‐shift of the absorption edge fordecreasing NW size

0.6 0.7 0.8 0.9 1.0 1.1

15 30 45

0.7

0.8

0.9

-bulk

(a

rb. u

nits

)

Energy (eV)

Ene

rgy

(meV

)R (nm)

50

40

20

10

ND = 31017 cm-3

Nss = 11013 cm-2

++

Reduction of the absorption intensitywith respect to the non‐degeneratebulk spectrum

Confinements effects are onlyAppreciable for radius < 15 nm

III-N NanowiresOptical absorption of InN NWs. Influence of doping

0.65 0.70 0.75 0.80 0.85

(a

rb. u

nits

)

Energy (eV)

ND = 1017 cm-3

Nss = 1012 cm-2

Nss = 1013 cm-2

+

++

ND = 51017 cm-3

Nss = 1012 cm-2

Nss = 1013 cm-2

+

++

Different effects of ND and Nss on the absorption profile+ +

The increase of ND blue‐shifts theabsorption edge

+

The increase of Nss reduces theabsorption intensity

+

0

EFEc V(r)

Ev V(r)

III-N NanowiresComparison with PL and PLE experiments

0.6 0.7 0.8 0.9 1.0

PL,

PLE

yie

ld (a

rb. u

nits

)

Energy (eV)

G532

G041

G136

G044

(i) Excitation and  relaxation(ii) Emission

Eemission

Pem

Prel

EexcitationPabs

The PLE spectra exhibit a blue‐shift Agreement of theoretical absorption with PLE spectra for excitation energy < 0.9 eVThe PL broadening increases with the increasing of the absorption edge

PLE experiment

InN NWs grown under different conditions

III-N NanowiresComparison with PL and PLE experiments

The determined doping concentrations are in the range of the reported experimental values

Sample ND (1017 cm‐3) Nss (1013 cm‐2)

G532 0.8 2.5

G041 2.0 2.4

G136 5.0 1.7

G044 7.5 1.4

Doping level is associated to the growth conditions

Broadening increases with the Fermi energy

EFEc V(r)

Ev V(r)

Small Fermi energy

EF

Ec V(r)

Ev V(r)

High Fermi energy

Eemission Eemission

III-N NanowiresConclusions

• Effective‐mass approximation is an alternative to solveself‐consistent problems. For instance, the determination of the electron distribution in InN NWs.

• Optical absorption of InN NWs with electron accumulationlayer exhibits a blue‐shift that depends on:

• NW size.• Doping concentration.

• Agreement of the theoretical absorption spectra with thePLE spectra (deduction of the doping level).

THANK YOU FOR YOUR ATTENTION

III-N NanowiresQuestions

01 ii ?

noyesEND

Initial Potential EnergyV(r)

eEV c

1

2

3

4

Schrödinger Eq.

electronic structure

charge density& 

Fermi energy 

Poisson Eq.electrostatic potential i(r)

Check convergence