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Passivation and optical coating of high-power diode-laser facets at Ferdinand-Braun-Institut P. Ressel, U. Spengler, A. Mogilatenko, A. Knigge, and G. Tränkle Ferdinand-Braun-Institut Leibniz-Institut für Höchstfrequenztechnik Symposium “Optical Coatings for Laser Applications”, Buchs, CH, 11.04.2019

Outline

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Introduction Ferdinand-Braun-Institut für Höchstfrequenztechnik (FBH) Diode lasers: specifics and applications

Laser diode facet passivation and optical coating Facet passivation and optical coating @ FBH Passivation quality assurance

Summary and outlook

Symposium “Optical Coatings for Laser Applications”, Buchs, CH, 11.04.2019

Introduction – Ferdinand-Braun-Institut für Höchstfrequenztechnik (FBH)

3 Symposium “Optical Coatings for Laser Applications”, Buchs, CH, 11.04.2019

Institute within Forschungsverbund Berlin e.V., Member of Leibniz Association Shareholders: State of Berlin / Federal Republic of Germany Founded in: 1992

Staff: 305 (incl. 150 scientists &

PhD candidates) from 24 nationalities

Budget / Turnover (2018): 37.9 M€ (incl. 23.0 M€ project revenues)

Partner of / Joint Labs: Research Fab Microelectronics Germany (FMD) Technische Universität Berlin Humboldt-Universität zu Berlin Goethe-Universität Frankfurt a. M. BTU Cottbus-Senftenberg

Introduction: FBH - Semiconductors for New Applications & Markets

4 Symposium “Optical Coatings for Laser Applications”, Buchs, CH, 11.04.2019

Microwave & optoelectronic components – key devices for: Health & nutrition, climate & energy, mobility, security, communications, …

International center – full value chain & covering all competencies: MMICs & power electronic devices High-power diode lasers (NIR to UV spectral range) & UV light-emitting diodes

Successful in knowledge and technology transfer : University cooperation (joint labs) Strategic partner of the industry Spin-offs

Technology of Laser Diodes (LDs) at FBH

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Chip Design

Chip Technology

III-V epitaxy (MOVPE) on 2 – 4“ wafers

Clean-room wafer processing

LD facet passivation and optical coating

Mounting and Assembly

Laser modules and systems

Prototypes Mounted laser bars

Laser module for space applications

Pulse laser source

Facet passivation equipment

Processed LD wafers LD characteristics simulation


Introduction: Diode lasers

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Diode lasers grow with the total laser market keeping a rough 40 % market share

Diode lasers instrumental in all these sectors


Introduction: Diode lasers

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Pros: Compact

Electrical pumping of semiconductor chips allows unprecedented small form-factors for laser systems

Highly efficient Wall-plug efficiencies >60 % (in special cases >70 %)

are routinely achieved

Cons: Laser beams highly divergent

Beam-shaping optics required

Lifetime issues, e.g., in the past Semiconductor bulk and facet degradation


single LD bar 1 cm long, 1 - 6 mm wide, 0.1 mm thick

bar stack mounted on heat-sink

S.G. Strohmaier et al., Proc. of SPIE Vol. 10086 (2017), 100860C

LD passivation and optical coating – general motivation

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Issue Solution „as-cleaved“-LDs emit

light from both (front and rear) facets,

Ras-cleaved 30 %

Optical coating of both facets Anti-reflective (AR)

coating of front facet

High-reflective (HR) coating of rear facet

Immediate oxidation of LD facets after cleaving the facets in air

Native oxides limit maximum output power and device lifetime

Facet passivation prior to optical coating:

Removal/avoidance of native oxides

plus sealing of cleaned facets

Native oxides comprise of Ga2O3, As2O3, Al2O3…

thermodynamically unstable:

• 2 GaAs + As2O3 Ga2O3 + 4 As • As reacts with O2 or H2O to As2O3

diff = P/I increases by coating



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Necessity of LD facet passivation prior to optical coating:

Native-oxide/semiconductor interface with high concentration of nonradiative centers Nnr: 1014 cm-2eV-1

Interfacial reactions during lasing lead to even higher Nnr (thermo-/photodynamic instability) due to defect creation and growth

Nonradiative recombination leads to T increase in the LD facet region; if a threshold value is exceeded, then a positive feedback loop starts (thermal runaway), leading to local facet melting and lasing breakdown – COMD (catastrophic optical mirror damage)

SE micrograph of a 10-µm-wide COMD region on LD front facet

optical micrograph of a 3-µm-wide COMD region on LD front facet

COMD


Lifetest @ 1 W of 5 LDs with insufficient passivation – all fail with COMD


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Native oxides on LD facets have to be removed/avoided prior to optical coating!

Compact manual system for bar cleavage and coating in ultra-high-vacuum

Source: SVT Associates

Avoidance? By cleaving the laser bars in UHV and in-situ sealing the freshly cleaved facets low throughput highest quality

Removal? Low-energetic (<80 eV) ion or plasma etching of LD facets and subsequent sealing/optical coating high throughput standard to high quality

Automated system for Optical Coating using Ion Beam Deposition

Source: Veeco Instr.


LD passivation and optical coating at FBH

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LD bar cleaving in air

Reactive (atomic hydrogen) etching of native oxides without applying ions or plasmas: lowest possible energy impact

Epitactic ZnSe growth for sealing: low interfacial state density on arsenides, phosphides

Proprietary process based on facet cleaning with atomic hydrogen and ZnSe sealing DE 10221952 B4; EP 1514335 B1; US 7338821 B2



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Comparison with state-of-the-art – patents below are representative for a certain class of passivation technology

Mitsubishi US 6,744,074 2004

Corning US 6,618,409 2003

FBH EP 1,514,335 2006

IBM Rüschlikon US 5,063,173 1992

Native oxide removal/avoidance

Ar plasma or ion etch (25..100 eV)

H2 plasma etch (20 eV)

Atomic hydrogen etch (1 eV)

E2-process Bar cleaving in UHV

sealing Si coating (partially oxidized)

-Si:H coating ZnSe epitaxy Si ( = 980 nm) ZrO2 ( = 800 nm)

advantages

• Standard equipment

• Industry baseline

• Standard equipment

• Standard to high quality

• No defect generation during native-oxide-removal

• High quality

• No native oxides at all

• Highest quality

drawbacks • Generation of crystal defects

• Still some generation of crystal defects

• Al-containing oxides not eliminated

• MBE+IBS equipment

• Low throughput



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Experimental MBE system, 2000 – 2013 • Manual single chamber system

Automated MBE system, 2010 – • Separate process chambers for atomic

hydrogen cleaning and ZnSe growth • Higher throughput Ressel et al., “Novel Passivation Process for the Mirror Facets of Al-Free

Active-Region High-Power Semiconductor Diode Lasers”, IEEE Photon. Techn. Lett. 17 (2005) 962.



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Requirements

Deposition of dense and transparent dielectric layers for AR and HR coatings Sufficiently dense to stop indiffusion of gases or H2O (thus avoiding reoxidization of LD facets)

Transparent for LD laser wavelengths (400 – 1100 nm)

Variable conditions for damage-free deposition of layers adjacent to sealing or semiconductor layers

Layer thickness errors <1 %

Technique of choice: Ion beam sputtering (IBS)

Source: scia Systems



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Employment of IBS tool from Oxford Instruments

Deposition of dielectrics as:

Al2O3, TiO2, Ta2O5, SiO2

• AR coatings made from Al2O3

Reflectivities 0.05 .. 30 % depending on LD device type

• HR coatings from Al2O3/TiO2 and SiO2/Ta2O5 stacks Typically 95..98 %

In-situ native-oxide etch possible prior to

deposition for standard quality facet coatings



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Achievements in device lifetime at high optical power

• Single-mode RW-LDs (980 nm) with 5-µm-wide aperture withstand aging @1 W for more than 25 kh

• Facet load during aging @ 1 W is approx. 20 MW/cm2!


technique cw facet load

lifetime notes

passivated/coated using FBH method

20 MWcm-2 > 25 kh Conditions of FBH´s recent life-test

40 MWcm-2 > 5 kh Conditions of FBH´s actual step-stress-test

low-energy ion etch of native oxides

20 MWcm-2 2..5 kh industry baseline

• The FBH facet passivation method allows for more than twice the optical power density compared to the industry baseline

• Inferior to E2-technique only, but with much higher throughput

LD passivation – quality assurance

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Several analytical techniques: RHEED (reflection high-energy electron

diffraction) on H-cleaned GaAs-wafers

RHEED pattern of a H-cleaned GaAs(001) surface in [100] direction

Streaky RHEED patterns on GaAs(001) test pieces are a necessary but not sufficient condition for having achieved adequate cleaning of the LD bar facets

XRD (X-ray diffraction) on H-cleaned and

ZnSe-coated GaAs-wafers

XRD from a H-cleaned GaAs(001) surface coated with approx. 60 nm ZnSe

Similarly, high-quality diffractograms are a necessary but not sufficient condition for having achieved adequate cleaning of the LD bar facets


LD passivation – quality assurance

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Several analytical techniques: XTEM (Cross-sectional transmission electron microscopy) on focused-ion-beam-prepared slices from LD facets

ZnSe

QWs* AlGaAs waveguide

Oxide layer

QWs* ZnSe

Oxide layer QWs* AlGaAs

waveguide

Device lifetime corresponds to quantum well (QW*) cleanliness – only oxide-free ZnSe/QW interfaces guarantee high device lifetimes!

Continuous oxide layer between ZnSe and QWs – early device failure

ZnSe/QW interface oxide-free – high device lifetime


LD passivation and optical coating – quality assurance

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COMD-tests and Life-tests – use of RW-LDs 980 nm with a single lateral mode


RW-LD 980 nm should „rollover“ - failure with COMD will disqualify the lot

Check of COMD-limit prior to lifetests

RW-LD sketch: emission aperture width 5 µm

LD passivation and optical coating – quality assurance

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Introduction of step-stress-tests to shorten time to result (TTR)

Actually, a step-stress-test of 5 RW-LDs 980 nm with the LDs surviving at least 2 kh @ 2 W is required for (re)qualifying the passivation/coating process at FBH


Till recently, a 10-kh (later 5 kh)-lifetest of 5 RW-LDs 980 nm @ 1 W, 25 °C, was required for (re)qualifying the passivation/coating process at FBH

TTR decreased from 7 months (life-test @ 1 W) to 1-3 months (step-stress-test)

Summary and outlook

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FBH with proprietary process for passivation and optical coating of LD facets damage-free atomic hydrogen cleaning of air-cleaved facets and in-situ ZnSe coating patent application in 2003; german, european, and US patent granted several licenses to LD companies

Process quality strongly enhanced in comparison to industry baseline, i.e. in-situ ion etch of native oxide; inferior to low-throughput UHV-facet-cleavage/coating only cw facet loads of 40 MWcm-2 for >5 kh (RW-LD @980 nm)!

Permanent quality control based on life-tests of passivated/coated LDs and analytic tools as RHEED, XRD, and XTEM as well

Actual work on improvement of bar stack temperature and atomic hydrogen flux control during passivation


Thank you. P. Ressel, U. Spengler, A. Mogilatenko, A. Knigge, and G. Tränkle Ferdinand-Braun-Institut Leibniz-Institut für Höchstfrequenztechnik Symposium “Optical Coatings for Laser Applications”, Buchs, CH, 11.04.2019

passivation and optical coating of high ... - rhysearch.ch · symposium “optical coatings for...

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