Status on long-wavelength InP waveguide heterojunction phototransistors Samuel Dupont, Vincent Magnin, Manuel Fendler, Filippe Jorge, Sophie Maricot, Jean-Pierre Vilcot, Joseph Harari, Didier Decoster Institut d'Electronique de Microélectronique et de Nanotechnologie, UMR CNRS 8520 Université des Sciences et Technologies de Lille Villeneuve d’Ascq France

Introduction Generalities about HPT Why lateral illumination ? Scholar study of a side illuminated HPT Optical modeling Alignment tolerance Optical structure optimization 3T waveguide HPT making of Structure Characterization HPTs state of art What performances could we expect? Outline GHz µm²

Offers gain compared to PIN-diode or UTC Good noise performances (/ APD) Compatible with HBT fabrication process Several configurations: –Top / substrate / side illumination –2T / 3T Specific applications: injection locked oscillators, clock recovery setups… Heterostructure phototransistor

Heterojunction bipolar PhotoTransistor principle InGaAs Collector InGaAs Base InP Emitter holes from base current photocreated holes

HPT: electrical modelling Electric field Carrier densities Conditions: -3-T HPT, I b = 50 µA - V ce = 1.5 V - darkness (—) - 3 mW optical input (  ) Conditions: - 3-T HPT, I b = 10 µA - V ce = 1.5 V - 0, 1, 5, 10 mW optical input P opt NPN HPT behavior under optical illumination P opt

Why lateral illumination? More flexibility for the design : Optimisation of device in terms of electronic behaviour + optimisation of device to improve optoelectronic efficiency. To decorrelate light absorption and carrier transport directions So, in a first approach, to allow the separate optimisation of optoelectronic (efficiency,…) and electronic (bandwidth,…) properties h e - ; h +

Introduction Scholar study Optical modeling Optical structure optimization 3T HPT making of Structure Characterization HPT state of art Side illumination requires optical guiding properties of the device Need a specific design to optimize the coupling efficiency  BPM simulations of the structure

InPEmitter 0.3 µm InGaAsBase 0.1 µm InGaAsCollector 0.28 µm InPCollector 0.2 µm InGaAsSub-collector 0.2 µm Inspired of: H. Kamitsuna, Y. Matsuoka, N. Shigekawa, “Ultrahigh-speed InP/InGaAsP DHPTs for OEMMICs”, IEEE Trans. Microwave Theory Tech., vol. 49, no. 10, (2001), pp Lensed fibre Optical study Absorbing layers Example of a phototransistor: top illuminated HPT structure Device size: 6x4 µm² Spot size:  m … but study in the case of lateral illumination !

InP substrate air Internal responsivity Side illumination: 0.52 A/W TE 0.64 A/W TM (Top illumination: 0.37 A/W) Device size: 6x4 µm² N InP (emitter 0.3 µm) P+ InGaAs (base 0.1 µm) InGaAs (collector 0.28 µm) N InP (collector 0.2 µm) N+ InGaAs (sub-collector 0.2 µm) 2D BPM modeling of side illuminated HPT Simulation of light propagating inside the device = 1.55 µm spot  : 2.4 µm air substrate

Carriers generation rate Most of the light is absorbed along the 1 st 5 µm

Tolerance to the fibre position emitter base collector sub-collector airsubstrate 0.62 A/W = 1.55 µm spot  : 2.4 µm air substrate Optimal injection is centered on the base layer Misalignment tolerance +/- 0.5 µm (10% of the maximum)

Considering a typical HPT structure: Changing from top illumination to side illumination can result in: 0.52 A/W TE 0.62 A/W TM 0.37 A/W top +/- 0.5 µm alignment tolerance Optical guiding properties of the device are not optimized Introduction Scholar study Optical modeling Optical structure optimization 3T HPT making of Structure Characterization HPT state of art

Introduction Scholar study Optical modeling Optical structure optimizations 3T HPT making of Structure Characterizations HPT state of art What do we want ? A more efficient light collection How to get it ? Get a better light confinement  Add a confinement layer

InPEmitter 0.3 µm InGaAsBase 0.1 µm InGaAsCollector 0.28 µm InPCollector 0.2 µm InGaAsSub-collector 0.2 µm Spot size Device size: 6x4 µm² InGaAsPconfinement w Insertion of an InGaAsP Optical confinement layer Optimization parameter: Its thickness w Modified structure To get better guiding properties:

N InP (emitter 0.3 µm) P+ InGaAs (base 0.1 µm) InGaAs (collector 0.28 µm) Q µm N InP (collector 0.2 µm) N+ InGaAs (sub-collector 0.2 µm) 2D-BPM modeling Modified structure:

Better absorption Lower losses Without With Comparison: with and without confinement layer: 2D-BPM modeling More efficient light collection  increased response Find the optimal confinement layer width

Device optimisation Optimal confinement layer width: w = 0.5 µm Increase / saturation / decrease of R with w W R = 1.55 µm

0.74 A/W Optimal injection is centered on the absorbing region Misalignment tolerance +/ µm (10% of the maximum) emitter base collector confinement sub-collector airsubstrate Tolerance to the fibre position w = 0.5 µm = 1.55 µm spot  : 2.4 µm air substrate

Side illumination: up to µm Internal responsivity increase: 16% more compared to non optimized structure up to 2x better than vertical illumination up to 6 dB more microwave power Internal responsivity (A/W) Comparison: responsivities

Introduction Scholar study Optical modeling Alignment tolerance Optical structure optimization 3T HPT making of Structure Characterization HPT state of art Side illumination is more efficient Optimized structure gives about twice the responsivity (with the same absorbing layers)  6 dB more microwave power

Introduction Scholar study Optical modeling Alignment tolerance Optical structure optimization 3T HPT making of Structure Characterization HPT state of art Light collection: Can we find more efficient structures ?  Thicker absorbing layer  Thicker confinement layer (not without consequences on bandwidth!)

Absorption and confinement layers widths optimization: -Two polarizations -Two  1.55 µm ; 1.3 µm - Two fibres  cleaved ; lensed  Trade off between several optimal values 0.5 µm < w < 0.8 µm = 1.55 µm = 1.3 µm Device definition w InGaAs = 0.49 µm

E B B InGaAs (p++) InGaAsP (n+) Substrate InP (I.) InP (n+) InGaAs (n-) C InGaAsCap layer 0.1 µm InGaAsBase 0.1 µm InGaAsCollector 0.4 µm InPSubstrate InGaAsPCladding 0.7 µm InPEmitter 0.1 µm Device defined to get an optimum light collection: 90% internal efficiency (8 µm long device ; lensed fibre) Developed structure

Phototransistor-guide (HPT) - Triple mesa structure - Polymide bridge - Self-aligned base process Emitter contact Base contact Collecteur contact Simple heterostructure But: - device performance relies on the final cleaving process (couple of microns difference in cleaving decrease either the bandwidth (too long) or the efficiency (too short). - no possible integration with double heterostructure HBT

E E C B C B Cleaving axis Fabricated device Device fabricated at IEMN Optical micrographySEM micrography Device size after cleaving 4x8 µm² DC currant gain : around 200

Introduction Scholar study Optical modeling Alignment tolerance Optical structure optimization 3T HPT making of Structure Characterization HPT state of art S parameters extraction –f t, f max –equivalent model Opto-microwave parameters –optical f c

Microwave properties

Noise comparison HPT versus PIN + HBT Frequency (GHz) Equivalent Input Noise (pA.(Hz)-½) p-i-n/HBT HPT - Equivalent input noise : Noise comparison HPT versus PIN + HBT  Advantage can be taken from HPT use

Device size: 4x8 µm² Optical gain cut-off frequency > 45 GHz Opto-microwave properties

Introduction Scholar study Optical modeling Alignment tolerance Optical structure optimization 3T HPT making of Structure Characterization HPT state of art 3T side illuminated HPT: –Trade off optimization (1.3 µm, 1.55 µm) –8x4 µm² after cleaving –w InGaAs = 0.5 µm –w InGaAsP = 0.7 µm –DC gain: 200 –f t = 60 GHz –f copt = 45 GHz

Introduction Scholar study Optical modeling Alignment tolerance Optical structure optimization 3T HPT making of Structure Characterization HPT state of art Electrical DHBT can go up several hundreds of GHz BUT: –Optical HPT needs a sufficient absorbing layer –Side illumination requires a confinement layer However 100 GHz operation has been reported

Cut-off frequency state of art Dashed line consistent with emitter-base junction capacity limitation top side 2T side 3T

Conclusion HPT type:TopSideNote: Fabrication: cleaving A.R. coating: + Gain: == Fc: ~ Side: e - transit time < Alignment: + Side: Waveguide coupling? Responsivity: + S/N: +

Conclusion Optimal light collection requires side illuminated structures Side illuminated structures can be optimised Up to 2x responsivity, 4x microwave power (6dB) Best S/N results should be obtained with side illuminated structures BUT: increased technological difficulties