Photoluminescence and lasing in a high-quality T-shaped quantum wires

M. Yoshita, Y. Hayamizu, Y. Takahashi, H. Itoh, and H. Akiyama

Institute for Solid State Physics, Univ. of Tokyo and CREST, JST

L. N. Pfeiffer and K. W. WestBell Laboratories, Lucent Technologies

FOPS, at Stanley Hotel, Estes Park, CO, USA (2004.8)

1. Characterization of T-wires, single-T-wire lasers

AFM, PL, PLE, PL scan, Lasing

2. Exciton, biexciton, and plasma in our best-quality single T-wire

PL, Absorption/Gain spectra measured by Cassidy’s method

3. Exciton Mott transition picture does not work. New picture is needed!!!

Cleaved-edge overgrowth with MBE

In situCleave

(001) MBE Growth (110) MBE Growth

[110]

[001]

GaAssubstrate

600oC 490oC

by L. N. Pfeiffer et al., APL 56, 1679 (1990).

490oC GrowthHigh Quality

T-wire

Interface control by growth-interruption annealing

(by M. Yoshita et al.

JJAP 2001)

Atomically flat

interfaces

600oC Anneal

armwell6nm

stemwell14nm

(Akiyama et al. APL 2003)

PL and PLEspectra

1D free exciton

small Stokes shift

1D continuum states

armwell

stemwell

T-wire

E-field

// to wire

_ to wire// to arm wellI

E-field

PLE

Absorption= 80-90 /cm (=5x10-4), or T= 1-2% @ L=0.5mm cavityfor single T-wire

Cavity length 　 500 m

Probability of Photon

Probability of Electron

Single quantum wire laser

=5x10-4

Scanning micro-PL spectra

ContinuousPL peak over 20 m

PL width < 1.3 meV

scan

T=5K

T-wire T-wirestem well stem well

500m gold-coated cavity

Threshold 5mW

(Hayamizu et al, APL 2002)

Lasing in a single quantum wire

Excitation power dependence of PL

Single quantum wire T=4K

M. Yoshita, et al.

Free exciton

Biexciton+Exciton

Electron-hole plasma

Den

sit

y

Single quantum wire T=4K

n1D = 2.6 x 104 cm-1 (rs = 30 aB)

n1D = 1.7 x 105 cm-1 (rs = 4.6 aB)

n1D = 1.2 x 106 cm-1

(rs = 0.65 aB)

n1D ~ 102 cm-1

aB ~13nm

EB =3meV

•No peak shift

•Gradual & symmetric broadening

Single quantum wire T=4K

Biexciton

Plasma

PL from 1D-continuum band edge

▼ plasma band edge (low energy edge of plasma PL) starts at biexciton energy and shows red shift.

exciton band edge, (onset of continuum states)exciton ground and excited states show no shift.

M. Yoshita, et al., submitted to PRL, but

Lasing & many-body effects in quantum wires

E. Kapon et al. (PRL’89) Lasing in excited-states of V-wiresW. Wegscheider et al. Lasing in the ground-state of T-wires, no energy shift, (PRL’93) excitonic lasingR. Ambigapathy et al. PL without BGR, strong excitonic effect in V-wires (PRL’97) L. Sirigu et al. (PRB’00) Lasing due to localized excitons in V-wiresJ. Rubio et al. (SSC’01) Lasing observed with e–h plasma emission in T-wiresA. Crottini et al. (SSC’02) PL from exciton molecules (bi-excitons) in V-wiresT. Guillet et al. (PRB’03) PL, Mott transition form excitons to a plasma in V-wiresH. Akiyama et al. Lasing due to e–h plasma, no exciton lasing in T-wires

(PRB’03)

F. Rossi and E. Molinari (PRL’96)F. Tassone, C. Piermarocchi, et al. (PRL’99,SSC’99)S. Das Sarma and D. W. Wang (PRL’00,PRB’01)

Theories

“1D exciton Mott transition”

eg. D. W. Wang and S. Das Sarma, PRB 64, 195313 (2001).

・ reduction of exciton binding energy

・ red shift of the band edge (band-gap renormalization (BGR))

Physical picture of 1D exciton–plasma transition

Increase of e–h pair density causes

the exciton Mott transition Our PL results show

band edge

exciton level

no energy shift of the exciton band edge

plasma low-energy edges appear at the bi-exciton energy positions, and show BGR

no connection, but coexistence of two band edges

no level-crossing between the band edges and the exciton level

θee

eE

ll

l

I22

2

sinR4)R1(

R)1(A)(

B. W. Hakki and T. L. Paoli JAP. 46 1299 (1974)

11

R1

ln1pp

l

min

sum/FSR

I

Ip

R ：Reflectivity

c

Eln

： Absorption coeff.

D. T. Cassidy JAP. 56 3096 (1984)

Absorption/gain measurement based on Cassidy’s analysis of Fabry-Perot-laser emission below threshold

Free Spectral Range

Point

Absorption Spectrum by Cassidy method

Excitation Light ： cw TiS laser at 1.631eV

WaveguideEmission

Polarizationparallel toArm well

Spectrometer with spectral resolution of

0.15 meV

Cassidy’s Method

Single wire laser, uncoated cavity mirrors

Excitation Light ： cw TiS laser at 1.631eV

WaveguideEmission

Polarizationparallel toArm well

Stripe shape

Spectrometer with spectral resolution of

0.15 meV

Spontaneousemission

Measurement of absorption/gain spectrum

Cassidy’s Method

8.3mW

Absorption/gain spectrum (High excitation power)

Electron-Hole Plasma

EFEEBE

Gain

Absorption

Hayamizu et al. unpublished

8.3mW

1. Exciton peak and continuum onset decay without shift.

2. Gap between exciton and continuum is gradually filled.

3. Exciton changes to Fermi edge

Electron-Hole Plasma

ExcitonHayamizu et al. unpublished

Conclusions

Exciton-Mott-transition picture does not work. New picture is needed.

1. As e-h density is increased, exciton peak and exciton band edge (the onset of continuum states above excitons) decay with NO shift.

2. Exciton band edge does NOT connect to plasma band edge (the low-energy edge of plasma). They even co-exist. Therefore, these edges NEVER cross exciton peak.

3. The exciton-plasma evolution is NOT like an abrupt metal-insulator transition, but a gradual crossover.

4. Lasing is caused by plasma gain, but the gain spectral shape is NOT proportional to 1D density of states, probably due to Coulomb interactions.

5. Exciton gradually changes to Fermi edge in plasma.6. Biexciton PL gradually changes to plasma PL without shift.

•The gain peaks appear

2meV below biexciton energy.

•Gain peaks have symmetric

shape and no similarity

to 1D Density of States.

Gain peaks of 20-wires laser

•The gain peaks are broadened

with slight red shifts.

(001) and (110) surfaces of GaAs

(001) (110)

[001]

[110]

[110]

[001]

(By Yoshita et al. APL 2002)

Theory1D exciton and continuum states

Intensity vs. excitation powerSingle quantum wire T=4K

Plasmars = 0.65 aB

Quadratic increase

BiexcitonSingle quantum wire T=4K

T. Guillet et al. (PRB’03) Mott transition form an exciton gas to a dense plasma in very-high-quality V-wire

eg. D. W. Wang and S. Das Sarma, PRB 64, 195313 (2001).

・ red shift of the band edge (band-gap renormalization (BGR))

・ reduction of exciton binding energy

Physical picture of 1D exciton–plasma transition

Increase of e–h pair density causes

the exciton Mott transition

band edge

exciton level

meV 3.37EB

(Zn,Cd)Se/ZnSe samples

Thickness 5 nm

Quantum well