Saturated gain in GaN epilayers studied by variable stripe length technique

Rui LiJournal Club, 3.05.07

Electrical Engineering

Boston University

J. Mickevičiusa and G. TamulaitisInstitute of Materials Science and Applied Research, Vilnius University, Saulėtekio 9-III,

LT-10222 Vilnius, Lithuania

M. S. ShurDepartment of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, New York

12180 and Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, New York 12180

Q. Fareed, J. P. Zhang, and R. GaskaSensor Electronic Technology, Inc., 1195 Atlas Road, Columbia, South Carolina 29209

JOURNAL OF APPLIED PHYSICS 99, 103513 2006

Outline

• Introduction to GaN

•VSL technique• Results

Introduction to GaN

• GaNDirect Band gapWurtzite Crystal Structure

• High Optical Gain, 25000cm-1 predictedHigh joint density of states

• ApplicationBlue/UV LEDs – DisplayBlue LD – Blu-ray Disc

Playstation3

Blu-ray Disc

Variable Stripe Length Technique

• One Dimensional Amplifier• Amplified Spontaneous Emission

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 110

-3

10-2

10-1

100

101

102

103

104

L (cm)

I (a

.u.)

g=10

g=-10g=0.001

Time-resolved VSL technique

L. Dal Negro et al., Appl. Phys. Lett., 82, 4636 (2003)

Gain Saturation in VSL

d ,, , ( )

d spon

I Lg L I L J f I

L

( ) 1f I I

Analysis of gain saturation behavior in GaN based quantum well lasersVehse et. al. Aachen phys. stat. sol (c) 0 , 43-47 (2002)

Information from the population inversion level is needed!

ε is related to the depletion of the excited state population due to the ASE.

Pump Diffraction Effect

• The pump along the stripe is not homogeneous due to Fresnel diffraction caused by the edge of the slit.

• ASE intensity from short stripe length is unreliable.

• Place the slit closer

The slit is placed 3cm from the sample

Applicability conditions and experimental analysis of the variable stripe length method for gain measurements

L.Dal Negro et al.

Optics Communications 299 (2004)

Sample Preparation

• Low pressure metal-orgainc chemical vapor deposition (MOCVD) 11μm on AlN buffer layers, on saphire by migration ehanced MOCVD(MEMOCVD).

• Dislocation densities 108 and 4×109cm-2 for samples S1 and S2.

Experiment

Pump: Fourth harmonic (266nm) of the Nd:YaG laser (pulse duration 4ns), focused into a 30μm wide stripe.

For L: 2~10μm

S1: g=7300cm-1

S2: g=3600cm-1

ASE spectrum

• Emission peak red shifts• Gap between Quasi-Fermi

Energies is lowered for long stripe length. The short-wavelength side emission band is saturated more rapidly.

Gain Spectrum

1: L= 3μm 2L= 6μm

2: L= 5μm 2L= 10μm

gpeak = 6500cm-1

ASE dependence on Iex

2em exI I3

exem II

• The dependence faster than Iex

2 is usually considered as an indication of stimulated emission.

• The undetected emission propagating perpendicularly depletes the population

1mm

10μm

VCSEL Configuration

• Fabry-Parot Resonance ∆λ= λ2/2nL=2.3nm

• gthr=(2L)-1ln(R1R2)-1=2200cm-1

R1,R2 are the reflection coefficients of the interfaces

2.1nm

L

AirGaN

sapphire

Light-induced Transient Grating

τG: Characteristic grating decay time

τR: Carrier life time

Da: Diffusion coefficient

Λ: Grating Spacing

Comparison between the 2 samples

Dislocation densities

Carrier life times

Gain

S1 108 2ns 7300cm-1

S2 4×109 960ps 3600cm-1

Ratio ~2 ~2

• Dislocations increases the density of nonradiative recombination centers

• The carrier life times are comparable with the pump duration (4ns), so the population inversion level depends on the carrier life time.

Conclusion

• Gain values as high as ~7500cm-1 is observed. • Gain saturation limits the applicability of the VSL

technique in high gain materials.

Make the layer thinner, focus the pump to a narrower stripe.

• VSL technique is useful to compare different GaN samples.

Thank you!