Saturated gain in GaN epilayers studied by variable stripe length technique Rui Li Journal Club, Electrical Engineering Boston University J. Mickevičiusa and G. Tamulaitis Institute of Materials Science and Applied Research, Vilnius University, Saulėtekio 9-III, LT Vilnius, Lithuania M. S. Shur Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, New York and Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, New York Q. Fareed, J. P. Zhang, and R. Gaska Sensor Electronic Technology, Inc., 1195 Atlas Road, Columbia, South Carolina JOURNAL OF APPLIED PHYSICS 99,

Outline Introduction to GaN VSL technique Results

Introduction to GaN GaN Direct Band gap Wurtzite Crystal Structure High Optical Gain, 25000cm -1 predicted High joint density of states Application Blue/UV LEDs – Display Blue LD – Blu-ray Disc Playstation3

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Variable Stripe Length Technique One Dimensional Amplifier Amplified Spontaneous Emission

Time-resolved VSL technique L. Dal Negro et al., Appl. Phys. Lett., 82, 4636 (2003)

Gain Saturation in VSL Analysis of gain saturation behavior in GaN based quantum well lasers Vehse et. al. Aachen phys. stat. sol (c) 0, (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 10 8 and 4×10 9 cm -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 g peak = 6500cm -1

ASE dependence on I ex The dependence faster than I ex 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= λ 2 /2nL=2.3nm g thr =(2L) -1 ln(R 1 R 2 ) -1 =2200cm -1 R1,R2 are the reflection coefficients of the interfaces 2.1nm L AirGaN sapphire

Light-induced Transient Grating τ G : Characteristic grating decay time τ R : Carrier life time D a : Diffusion coefficient Λ: Grating Spacing

Comparison between the 2 samples Dislocation densities Carrier life times Gain S ns 7300cm -1 S2 4 × ps 3600cm -1 Ratio~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.

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