Low-threshold T-shaped
Quantum Wire Lasers byArm-arm Current Injection
ISSP, Univ. of Tokyo, and CREST, JST,
Bell Lab, Lucent TechnologiesA
S.M.Liu, M. Yoshita, M. OKano, T. Ihara, H.
Itoh, H. Akiyama, L. PfeifferA, K.WestA, andK.BaldwinA

24aXL-10

1

Contentz Introductionz Characteristics of our low threshold lasers,

includingarm-arm current injection scheme;
emission spectra and images;
I-L and I-V curves.
z Summary

24aXL-102

Introduction
zWhy low threshold?

LOW THRESHOLD lasers are attractive for opticalcommunication systems because of the tight packing density,
low power consumption and high modulation bandwidth.
z How to get low-threshold?

Ideal QUANTUM WIRE lasers are predicted to show ultralow
threshold current of several µA due to small volume of active
region and large density of states at the subband edges.
24aXL-103

Theoretical threshold current for 3,2,and 1D system(A. Yariv APL 53, 1033 (1988))

In which, the transparency carrier density

Assuming infinite potential well depth;
Involving only the first quantized states.

Quantum well lasers: 100 µA; Quantum wire lasers: 2-3 µA.
24aXL-104

Previous experimental results I (V-groove)
V-groove quantum wire lasers grown on patterned substrates

V-groove QWRs
9 nm X 80-100 nm
Ith=50 mA

1989: Kapon et al, Phys. Rev. Lett. 63, 430

24aXL-105

Previous experimental results II (V-groove)
Lowest threshold current of V-groove quantum wire lasers

3-period V-groove QWRs with Si3N4 mask
10 nm X 35 nm
Ith=0.19 mA
1994: Tiwari et al, Appl. Phys. Lett. 64, 3536

24aXL-106Previous experimental results III (T-shaped)
T-shaped quantum wires provide smaller size and higher array density

15-period 7 nm X 7nm T-wires
Ith=0.4-0.7 mA @ 4K
1994: Wegscheider et al, Appl. Phys. Lett. 65, 2510

24aXL-107

Previous experimental results IV (DFB)

InGaAsP/InP QWR DFB laser
9 nm x 24 nm
EB lithography, dry and wet etching,
MOCVD growth
cw 1.5 µm-wavelength lasing
Ith= 2.7 mA @ r.t.
2005: Yagi et al, Appl. Phys. Lett. 87, 223120

24aXL-108

Current injection schemes of T-wire lasers
arm wellstem wellarm-stem injection

arm-arm injection

24aXL-12 Okano et al.
24aXL-109

MBE growth of arm-arm injection laser
Cavity length: 500 µm
HR/HR coating by Au(300nm/50nm)

arm wellp-contactside cladding

C-doped MQWs
upper claddingstem MQWs
lower cladding

n-contact

Si-doped MQWs

Si-doped buffer layers

n+ GaAs substrate

24aXL-10

Current confinementin the single arm well10

EL and lasing spectra @ 30K• No EL or lasing from stem well indicating
current confinement in the arm well;• Single mode lasing above threshold;• Similar spectra from 30-70K.

24aXL-1011

EL images @ 30K

GaAs n+ substrate

0.01mA

Cladding layer

Si-doped layers
Cladding layer
Stem MQWs
Cladding layer
C-doped MQWs

2µm
42

196

Tight optical confinement
above Ith

24aXL-10

12

Power out & voltage vs current
20-period T-wire laser
6nm X14nm per wire

c.w.
HR/HR
L=500 µm

I-L
V-I

24aXL-1013

Temperature dependence

Threshold current: 0.27-0.76 mA
Differential quantum efficiency: 12%-8.2%
24aXL-1014

Summaryz Arm-arm injection scheme makes current

confinement in the single arm well;z A very low threshold current of 0.27 mA has beenachieved @ 30K from a 500 µm device under cw
operation;
Lasing mechanism will be presented by
Dr. Yoshita 24aXL-11

24aXL-10

15