Optical Investigation of the Localization Effect in the Quantum Well Structures

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1 Department of Physics Shahrood University of Technology Optical Investigation of the Localization Effect in the Quantum Well Structures Hamid Haratizadeh IPM, SCHOOL OF PHYSICS, JULY

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3 Quantum Well Structures AlGaN GaN Substrate 3

4 Recombination Processes Intristic:>>> no impurity 1-Band to Band Recombination 2-Free Exciton (FE) Recombination Extristic:>>>> with impurity shallow level deep level Intra Band Inter Band 4

5 III-Nitride Nitrides s Semiconductors (a) Cubic zincblende structure (b) Hexagonal wurtzite structure Wurtzite is thermodynamically stable for bulk AlN, GaN and InN. E g (GaN)=3.43 ev (Bo Monemar 1974) E g (AlN)=6.20 ev (P. B. Perry et al. 1978) E g (InN)= ev (J. Wu et al. 2002) 5

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7 Polarization Fields Piezoelectric polarization appears in the presence of strain Lattice mismatch strain Ppz = 2εxx( e31 e33c13 / C33) Thermal strain Along the [0001] direction (Cmn is elastic tensor and εxx is in-plane strain) Spontaneous polarization exists in polar semiconductors with a wurtzite or lower symmetry crystal structure Fixed direction along the [0001] c-axis in the wurtzite lattice In the Al x In y Ga 1-x-y N alloy systems (Vegard-like rule): P SP ( x, y) = xp + yp + (1 x AlN SP InN SP y) P GaN SP The energy bands are tilted and carriers are localized at the opposite interfaces The reduction E of the recombination energy (QCSE) 7

8 Effect of the Polarization Fields on Carrier Localization The in-plane extension of the hole wave function is smaller than the average Al-Al distance in barriers. Moreover, both electron and hole envelope functions are pushed apart from each other by the electric field. M.Gallart et. al. Physica. Status Solidi A 180, 127 (2000) 8

9 Confirmation of the Localization Effect by the Transient PL Above the Mott density, a decrease of the decay time should be expected, but an almost similar decay time was measured for all samples. In the heavily Si-doped samples: free electrons-to-localized holes and an almost constant decay time over the PL spectrum is evidence for the dominant role played by the localized holes. Time Decay (ps) 700 Undoped (a) PL Intensity (arb. units) Decay Time (ps) 700 Si: cm -3 (b) PL Intensity (arb. units) Photon Energy (ev) Photon Energy (ev) 9

10 MQW samples with Different Well Width T=2 K 45 Å 30 Å 15 Å Residual doping ~ cm Å splitting 10 mev FWHM ~ 15 mev 30 Å FWHM ~ 25 mev 45 Å splitting 25 mev FWHM ~ 20 mev PL Intensity (log scale) LO (45 Å) LO (30 Å) 2nd LO (15 Å) Photon Energy (ev) Well width fluctuation by 2 ML. The main broadening mechanism is the well width fluctuation. The hole localization at lower interface of the QW determines the radiative process at low temperature in terms of spectral shape, line width. 10

11 PL Mapping PL intensity (arb. units) A cross-section TEM image of the undoped 4.5 nm wide well sample. Energy-filtered TEM (EFTEM) to produce chemical distribution profiles, using inelastically scattered electrons with energy-losses characteristic of the specific Ga and Al elemental species. The EELS study was used to investigate the variation in the Al concentration between the wells with a near nanometer resolution. AlGaN-MQW 30 Å, 2 K (b) PL Intensity (arb. units) AlGaN-MQW 15 Å, 2 K (a) PL Intensity (arb. units) GaN/AlGaN-MQW 45 Å, 2 K (c) Photon energy (ev) Photon energy (ev) Photon energy (ev) 11

12 Ternary III-V-N Structures Diluted N-containing III-V ternary or quaternary semiconductor alloys GaNAs InAsN InPN GaPN Quaternary Ultradilute Nitrogen Concentration: 0.01 < x < 0.1 % Intermediate Nitrogen Concentration: 0.1 < x < 2 % High Nitrogen Concentration: 2 < x < 5 % InGaNP InGaNAs 12

13 Applications IR Diode Laser for Optical Fibers Communications Vertical Cavity Surface Emitting Laser (VCSEL) Multi-Junction Solar Cells 13

14 Adding N to InGaAs Creating of fluctuations localized Center Large Band-Gap Bowing Increasing of Effective Mass Reduction of Band-Gap Large Band Offset in CB InGaAs InGaNAs 14

15 Band Anticrossing model Splitting of CB into two Subband ( E +, E - ) Reduction of Fundamental Band-gap I. A. Buyanova et al,. Nitride Semiconductor. Research. (2001) & C. Skierbiszewski et al, Appl. Phys. Lett. 76, 2409 (2000) 15

16 Effect of N on the Band Gap, FWHM and PL Intensity of In x Ga 1-x N y As 1-y SQW Normalized PL Intensity(arb.units) In x Ga 1-x N y As 1-y T=150 K ev ev ev P=25 mw (2) x=0.30 y= (1) x=0.354 (3) y= x=0.37 y=0.006 Normalized PL Intensity(arb.units) In x Ga 1-x N y As 1-y T=2 K P=25 mw (3) X=0.37 y= ev ev ev (2) X=0.30 y= (1) X=0.354 y= Energy(ev) Energy(ev) samples FWHM (mev) (T=2K) mev decrease of FWHM (mev) (T=150K) band-gap per %1 N 16

17 The Role of N on the Exciton Localization E( g) αt 2 = E(0) β + T α = β = 347 K 4 ev / K Normalized Intensity (arb.units) In 0.36 GaN As SQW Exc. power=25mw T=10 K T=25 K T=33 K T=40 K T=45 K T=50 K T=55 T=60 K T=70 K T=80 K T=90 K T=110 K T=130 K T=150 K T=220 K T=300 K Energy(ev) 17

18 The Role of N on the Exciton Localization In GaN As/GaAs In 0.3 SQW GaN As/GaAs MQW In 0.36 GaN As/GaAs SQW PL Intensity(arb.units) In GaN As/GaAs SQW Different Excitation Intensity T=2 K ev PL Intensity(arb.units) P=40 mw ev p=10 mw ev P=4 mw 1.00 ev p=27 mw p=5 mw 0.99 ev In 0.36 GaN As SQW Different Excitation Intensity T=2 K In GaN As MQW Different Excitation Intensity T=2 K PL Intensity(arb.units) P=5 mw P=0.5 mw p=27 mw P=10 mw ev ev 1.001eV ev Energy (ev) Energy(eV) Energy (ev) 18

19 InGaNAs Suitable for optoelectronic application Optical devices and lasers with long wavelength (1300 nm & 1550 nm) Quantum structures Why Annealing? Adding N to InGaAs (less than few percent) Increase optical efficiency Advantage: Red shift the energy emission to close to the interest wavelength Disadvantage: Increase nonradiative centers (Decrease the Optical Efficiency)! Low temperature growth and non-equilibrium condition Thermal Annealing Increase defect and nonradiative centers Improvement the optical efficiency 19

20 Annealing Processes Series 1: In x Ga 1-x N y As 1-y /GaAs Single Quantum Well (SQW) Substrate: undoped (001) oriented GaAs Buffer: 300 nm undoped GaAs Cap Layer: 100 nm undoped GaAs Well Thickness: 7 nm In Content (x) = 35.4% N Content (y) = 0.36% 100nm GaAs Cap Layer 7nm InGaNAs Growth Technique: HVPE Growth Temperature: 495 c Annealing Temperature: 900 c Annealing Time: 0, 5, 15 and 30 seconds 300nm GaAs Buffer Layer Substrate GaAs (oo1) GaAs GaAs 20

21 Annealing Processes 1A, 1B, 1C and 1D Normalized PL Intensity (arb. un.) InGaNAs/GaAs SQW 1A, 1B, 1C, 1D 2 k, 27 mw RTA (30 Second) RTA (15 Second) 1A 1B 1D 1C RTA (5 Second) As Grown Photon Energy (ev) FWHM (mev) Annealing Time (Second) Peak Position (ev) Annealing Time (Second) 21

22 Annealing Processes Series 2: In x Ga 1-x N y As 1-y /GaAs Single Quantum Well (SQW) Substrate: undoped (001)Oriented GaAs Buffer: 300 nm undoped GaAs Cap Layer: 100 nm undoped GaAs Well Thickness: 7.2 nm In Content (x) = 37% N Content (y) = 0.59% 100nm GaAs Cap Layer 7.2nm InGaNAs Growth Technique: HVPE Growth Temperature: 495 c Annealing Temperature: 900 c Annealing Time: 0, 5, 15 and 30 seconds 300nm GaAs Buffer Layer Substrate GaAs (oo1) 22

23 Annealing Time vs. N content 1A, 1B, 1C, 1D and FWHM (mev) 2A, 2B, 2C, 2D 40 In Ga N As SQW (square) In 0.37 Ga 0.63 N As SQW (circle) T=2K, 25mW PL Intensity(arb. unit) In 0.37 Ga 0.63 N As A, 2B, 2C, 2D Solid Symbol In Ga N As A, 1B, 1C, 1D Open Symbol 30 sec RTA 15 sec. RTA 5 sec. RTA Annealing Time (Second) as grown Photon Energy (ev) 23

24 Series 3: In x Ga 1-x N y As 1-y /GaAs Multi Quantum Well (MQW) Substrate: undoped (001)Oriented GaAs Buffer: GaAs Cap Layer: 50 nm undoped GaAs Well Thickness: 5.7 nm Barrier Thickness: 19.3 nm In Content (x) = 30% N Content (y) = 1.5% Growth Technique: MOVPE (520 c) Thermal Annealing Processes: Well Barrier 50nm GaAs Cap Layer 5.7nm InGaNAs 19.3nm GaAs 5.7nm InGaNAs 19.3nm GaAs 5.7nm InGaNAs 19.3nm GaAs 5.7nm InGaNAs 19.3nm GaAs 5.7nm InGaNAs GaAs Substrate GaAs (oo1) Rapid Thermal Annealing (RTA) at N 2 atmosphere and 650 c for 15 seconds. Reactor Annealing (R) at H 2 + AsH 3 and 650 c for 30 minuets. 24

25 1A, 1C,and 3A, 3B 1A : as-grown 1C : RTA at 900 c for 15 s InGaNAs/GaAs SQW As grown 15s RTA PL Intensity(arb. unit) 1A 1C FWHM (mev) A : as-grown 80 PL Intensity (a.u.) Photon Energy(ev) 60 3B : RTA at 650 c and N 40 2 atmosphere for 15 s 20 as-grown Annealing Temperature ('c) 0.01 as-grown Annealing Temperature ('c) 3B x = 34%, y = 1% Growth by MOVPE at 510 c Annealing Time : 10 min 25

26 Multi Thermal Annealing Processes 3A, 3B, 3C and 3D In 0.3 GaN As/GaAs MQW T=2K, Power Exc.=40 mw RTA+R+RTA PL Intensity (arb. units) As grown As grown RTA RTA+R RTA+R+RTA * 100 RTA RTA+R Photon Energy (ev) InGaNAs/GaAs MQW T=2K, Power Exc.=4 mw RTA + R + RTA PL Intensity (arb. units) As grown As grown RTA RTA+R RTA+R+RTA 100 RTA RTA + R Photon Energy (ev) 26

27 Multiple Thermal Annealing Processes 3A PL Intensity (arb-unit) As grown (3A) T=2K 40 mw 2 mw PL Intensity (arb-unit) RTA (3B) T=2K 40 mw 4 mw 3B Photon Energy (ev) Photon Energy (ev) 3C PL Intensity (arb-unit) RTA+R (3C) T=2K 40 mw 4 mw PL Intensity (arb-unit) RTA+R+RTA (3D) T=2K 40 mw 4 mw 3D Photon Energy (ev) Photon Energy (ev) 27

28 Structural Changes Ga-As : 2.45 Å In-N : 2.14 Å Ga-N : 1.95 Å As grown Annealed 28

29 Conclusions: Adding Nitrogen to InGaAs Reduction of band-gap (Long Wavelength (IR region)). Increasing Localized Center due to Potential fluctuations Asymmetry of PL spectra S-shape behavior of peak position of PL spectra Decrease Optical Efficiency by Increase in non-radiative Centers. Thermal annealing. Improvement the Optical Efficiency by Suitable Thermal annealing Changes in N Surrounding from Ga4 -N to Ga3In-N Causes Reduction of Strain and Potential Fluctuation. Increases Optical Efficiency Thermal annealing temperature optimum is around c To avoid escaping of nitrogen from compound during annealing, the samples should be annealed in presence of N 2 gas. 29

30 Photoluminescence (PL) Setup 30

31 Excitation Laser Microscope Objective InAs QDs ~5 nm ~10 µm ~1.5 µm ~35 nm GaAs Detection Energy (ev) PL Intensity (arb. units) ss GaAs InAs GaAs p s pp ss s p Wetting Layer QD x50 E pp - E ss = 30 mev GaAs x50 pp (T = 4 K) 31

32 PL spectra GaN ev Normalized PL Intensity (arb. units) ev ev ev Photon Energy (ev) Undoped Barrier doped Dual doped Well doped Normalized PL Intensity (arb. units) Si: Barrier doped T=2 K ev ev ev ev Undoped Low-doped ( cm -3 ) Medium-doped ( cm -3 ) High-doped ( cm -3 ) Photon Energy (ev) 32

33 PhotoLuminescence Setup Time-Resolved PL Setup 4 µs 200 fs 33

34 (a) Undoped PL Intensity (arb. units) 50 ps 100 ps 150 ps 200 ps 300 ps 400 ps 500 ps 600 ps Photon Energy (ev) Intensity Intensity Wavelength Time 34