3.46 PHOTONIC MATERIALS AND DEVICES Lecture 15: III-V Processing

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1 3.46 PHOTONIC MATERIALS AND DEVICES 15: III-V Processing Double Hetero structure laser (band structure engineering) AlGaAs GaAs AlGaAs e - E n hν P h + X n x I d < 1 μm 1. Large refractive index active region 2. Low E active region g η d is increased faster inversion for same injection current light concentrated for stimulated emission light (guided) confinement carrier (electron and hole) confinement 100 of J th Confinement Γ DH SQW SCH Γ=1 Γ Δ nd 2 Γ Δ nd DH: Double Heterostructure SQW: Single Quantum Well SCH: Separate Confinement Heterostructure 3.46 Photonic Materials and Devices 15: III-V Processing Prof. Lionel C. Kimerling Page 1 of 11

2 DH g g p n as d th g( ν) f g ( ν) ρ bulk ( ν) n th g p : peak gain SQW g g p higher T stability g( ν) f g ( ν) ρ QW ( ν) n th ρ(e) Density of States of QW ) QW 2 ρν) ( = states / cm for de = kt 2m r = d E 1 E 2 E 3 E λ 2 m r g P threshhold 2 hd g τ r multilevel gain Threshhold current density R th = threshold recombination rate J th = er th Cnp R cm 3 s 1 ( cm 3 ) 2 = cm s J th = R th 6.4 ka cm -2 μm l w d 3 1 J th decreases with d 3.46 Photonic Materials and Devices 15: III-V Processing Prof. Lionel C. Kimerling Page 2 of 11

3 DH: d 0.2 μm J th = 1.2 ka/cm 2 I th ma d < 300 Å SQW: Jth < 180 A/cm 2 SQW 1. E levels quantized QW transitions 2. ρν P ν ( )(2D) more efficient, g = const ( ) 3. g saturates 2 4. QW states/cm 2 DH states/cm in d = 1000Å 5. Confinement optimized by separation SCH Strained Layers Strain (compressive) raises the LH sub band reduces carriers to invert J th η d C V d<300å with band filling, transition g p are useless (a) 3.46 Photonic Materials and Devices 15: III-V Processing Prof. Lionel C. Kimerling Page 3 of 11

4 III-V Compound Semiconductor Processing 1. Substrate Preparation GaAs, InP 2. Epitaxial layer growth LPE, MBE, MOCVD, CVD 3. Etch Dry (RIE), wet 4. Contacts Au, silicides, metals 1. Process Constraints A. CSBH laser provides (CSBH: Channeled-Substrate Buried Heterostructure) lateral optical and electrical confinement. i. grow InP:Fe SI layer ii. iii. etch channel grow InP/InGaAsP/InP DH in channel B. APD detector (SAM) i. grow InGaAs/InP het. ii. SiNx dielectric deposition iii. etch contact window iv. diffuse p+ contact/junction v. implant p- guard ring Both devices employ deposited dielectrics for AR coatings (APD) and facet reflectors (laser). 2. Issues A. Groups V volatility i. incongruent vaporization of P from T > 360 C ii. as from T > 600 C Solution: group V overpressure or stable dielectric cap layer. iii. RIE creates group III rich suffice Solution: lower T, lower E, high Z (Z: atomic number) 3.46 Photonic Materials and Devices 15: III-V Processing Prof. Lionel C. Kimerling Page 4 of 11

5 B. Preferential etch of V groove Solution: surface prep. C. Metallization reactions Solution: barriers or stable phases D. Degradation of η i Solution: defect control, life testing 3. Epitaxial Growth A. Dislocation density B. Stoichiometry Concept: Substrate: Single crystal film bonded to a single crystal substrate with a common interface and the lattice of the film having a definite orientation w.r.t. the substrate lattice. semi infinite thickness Surface: atomically flat (ledges) (bond reconstruction) Film: homogeneous, 2D (x, y >> t) (phase separation?) Interface: sharp (interdiffusion) Tangential forces: sinusoidal in a 0 Growth Modes E fs = film/substrate bond strength E ff = film/film bond strength W = E fs = relative strength of bonds to E ff substrate a a η=lattice misfit = s f a f a 0 Si(100) 2 1 or GaAs(100) rows of AS V-termination flat surface 3.46 Photonic Materials and Devices 15: III-V Processing Prof. Lionel C. Kimerling Page 5 of 11

6 Epitaxy equilibrium: low deposition rate high T (surface diffusion) ΔG (system energy) minimize N f (film atoms) Coherency (dislocations) variables: a, E ff, h 2. minimize energy E = 2 Bh Film thickness strain elastic constant Coherent: η= (strained) Incoherent: η= +δ (relaxed) Frank-Vander Merwe 1D harmonic chain δ=strain relief by dislocations separation of parallel misfit dislocations: b S = δ 1 η (relaxed) = + b cos λ S projection of b on plane of interface Critical h c minimize E vs. E dislocation Matthews Blakelee b h h = ln c +1 c 8 πη (1+ υ) b h c b 4η 10 4 b 1 Å h = 100 h c of η=10 (Å) c Å η% 3.46 Photonic Materials and Devices 15: III-V Processing Prof. Lionel C. Kimerling Page 6 of 11

7 Morphology (wetting) μ = f G N ML: monolayers Nucleation barrier to clustering γ 3 c/ v ΔG* = 8π 3 ρ 2 Δ η F( ) 2 0 density of unstrained film Deposition w = 1 μ 0 1 monolayer coverage G μ 1 2 f-f clustering G w = 2 μ 1 2 layer growth G Stronski-Kranstanov: G after one monolayer Volmer-Weber: G initially (no wetting) F( ) Δ η 2 η N* = 3 16πγ c/v [Δ η)] F( ρ 0 I N Γexp ( ΔG*/RT) = s Morphology + Coherency are determined by nucleation barriers ΔG* for dislocation formation clustering Metastability is common 3.46 Photonic Materials and Devices 15: III-V Processing Prof. Lionel C. Kimerling Page 7 of 11

8 4. Contacts stable selective low R c low T deposition adhesion Eutectics Au(Be) P Au(Ge) n small process window o RTA unreliable R < 10 c for lasers 5 Ω cm 2 surface defects pin E F contact resistance (Schottky Barrier) for n-gaas p-inp heavily doped epilayer under contact Silicides Stable o undefined interface R c Metals reactive with compounds defects, dissociation phase stability 3.46 Photonic Materials and Devices 15: III-V Processing Prof. Lionel C. Kimerling Page 8 of 11

9 AB dominant 500 C Ga MA x dominant MB y AB MB y dominant TiGa 4 Ti 500 C MB y TiGa 3 TiAs As No phase dominant 300 C PtAs 2 PtGa 3 PtAs 2 + PtGa GaAs 3.46 Photonic Materials and Devices 15: III-V Processing Prof. Lionel C. Kimerling Page 9 of 11

10 NiP (conductor) In 3.46 Photonic Materials and Devices 15: III-V Processing Prof. Lionel C. Kimerling Page 10 of 11

11 Adhesion: local structural relaxation ion beam mixing chemical bonding (Cu / Al 2 O 3 with excess 2 ) Interdiffusion Polycrystal: grain boundary diffusion 3E a D bulk = E a D disloc 4 D bulk (T MP ) D gb 1 TMP 2 E a Dgb 2 D= D bulk + f gb Diffusion Barrier (Ti/Pt)Au o high T MP o chemically stable Intermetallic Compound Coherent Interface refractory TM: Cr, Ni, Ta, Ti, Hf Dielectric Deposition SiO 2, SiO x N y, SiN x sputter PECVD e-beam facets, isolation, diffusion masks Etch Wet etch (Br:CH 3 OH, HCl) o layer stop H 2 SO 4 : H 2 O 2 : H 2 O o v-groove Dry etch (CF 2 Cl 2 ), (HBr, HI) o Anisotropy o Photoelectrochemical etch anisotropy 3.46 Photonic Materials and Devices 15: III-V Processing Prof. Lionel C. Kimerling Page 11 of 11