Chung-Lin Wu ( 吳忠霖 ) Department of Physics. Colloquium (2008/12/1) Department of Physics, National Dong Hwa University

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1 Electronic Structures of III-Nitride Heterojunctions Chung-Lin Wu ( 吳忠霖 ) Department of Physics National Cheng-Kung University it Colloquium (2008/12/1) Department of Physics, National Dong Hwa University

2 Outline: Heteroepitaxial Growth of InN on Silicon Commensurate Lattice Match Band Alignment of InN/AlN Interface Synchrotron Radiation Photoelectron Spectroscopy (SR-PES) Polarization-Induced Band Alignments at Opposite Polarity (Anion- and Cation-Polar) InN/GaN Interfaces Opposite Dipole at Interfaces Three-dimensional Visualization of III-Nitride Semiconductor Band Lineups Cross-sectional sectional Scanning Photoelectron Microscopy/ Spectroscopy (XSPEM/XSPES) Conclusions

3 Heteroepitaxial Growth of InN on Silicon - Molecular Beam Epitaxy (MBE) Growth - Commensurate Lattice Match

layers/sec) atomic abrupt interface Real-time monitoring RHEED Mass

4 Molecular Beam Epitaxy (MBE) Characteristics Well controlled UHV evaporation Low growth rate and low growth temperature good thickness control (atomic layers/sec) atomic abrupt interface Real-time monitoring RHEED Mass spectrometer t Molecular beam generation zone Elements mixing zone Substrate crystallization zone

5 Interface of Heteroepitaxy Perfect Case of Heteroepitaxy: Lattice Match, Strain Free Van der Waals Epitaxy: Incommensurate Commensurate with Strain: Coherently Strain (with Small Mismatch) Commensurate Interface with Coincident Lattice Match Incommensurate (in this case with 3:4) Heteroepitaxy Misfit Dislocation with Large Difference in Lattice Constants

7 High-Resolution TEM Imaging of InN/AlNHeterojunction Atomically abrupt interfaces are clearly visible in TEM images and SAED patterns Wurtzite stacking of AlN and InN epilayers Perfectly commensurate InN/AlN heterointerface

8 8:9 Commensurate Lattice Match at the Heterointerface of InN and AlN 8:9 commensurate lattice supercell p formed by InN (000-1) and AlN (000-1) planes (plan view) and comparison to the actual TEM image (cross-section view)

9 Summary: A new route to reach perfect heteroepitaxyin III-nitrides-on- Si system with magically matched lattice constants: 2:1 - β-si 3N 4 4( (0001)/Si(111) ( ) 5:4 - AlN(0001)/Si(111) 8:9 - InN(0001)/AlN(0001) C.-L. Wu, J.-C. Wang, M.-H. Chan, T. T. Chen, and S. Gwo, "Heteroepitaxy of GaN on Si(111) realized by coincident-interface AlN/β-Si 3 N 4 (0001) double buffer layers, " Appl. Phys. Lett. 83, 4530 (2003). S. Gwo, C.-L. Wu, C.-H. Shen, W.-H. Chang, T. M. Hsu, J.-S. Wang, and J.-T. Hsu, "Heteroepitaxial growth of wurtziteinn films on Si(111) exhibiting strong near-infrared photoluminescence at room temperature, " Appl. Phys. Lett. 84, 3765 (2004). C.-L. Wu, C.-H. Shen, H.-W. Lin, H.-M. Lee, and S. Gwo, Direct evidence of 8:9 commensurate heterojunction formed between InN and AlN on c plane, Appl. Phys. Lett. 87, (2005) C -L Wu C -H Shen H -Y Chen S -J Tsai H -W Lin H -M Lee S Gwo T -F Chuang and H -S C.-L. Wu, C.-H. Shen, H.-Y. Chen, S.-J. Tsai, H.-W. Lin, H.-M. Lee, S. Gwo, T.-F. Chuang, and H.-S. Chang, and T.M. Hsu, The effects of AlN buffer on the properties of InN epitaxial films grown on Si(111) by plasma-assisted molecular-beam epitaxy, J. Crystal Growth 288, 247 (2006)

10 Band Alignment of InN/AlN Interface - Probing by Synchrotron Radiation Photoelectron Spectroscopy (SR-PES) - Huge Difference in Various Measured Band Offset Values

11 Photoelectric effect Einstein, Nobel Prize 1921 Photoemission as an analytical tool Kai Siegbahn, Nobel Prize 1981

12 Photoelectron Spectroscopy (PES) Photoelectron Spectroscopy (PES) is the most widely used surface analysis technique because of its relative simplicity in use and data interpretation. XPS ESCA UPS PES X ray Photoelectron Spectroscopy py Electron Spectroscopy for Chemical Analysis Ultraviolet Photoelectron Spectroscopy Photoelectron Spectroscopy

Energy Spectrum selectable energy beam Sharp leading edge of valence band maximum high accuracy of VBM position

13 SR- PES Characterization Synchrotron Radiation Excitation (An intense radiation with broad energy band) High Brightness 10 5 of times higher than conventional X-ray tube Monochromatic Light high spectral resolution Wide Energy Spectrum selectable energy beam Sharp leading edge of valence band maximum high accuracy of VBM position determination Intense core level peak excellent signal-to-noise characteristics Valence Band Maximum (VBM) Core Level (CL)

14 Band Alignment Measurement- PES Characterization Probing Semiconductor Heterojunction Interfaces (Band Offsets): Electrical Transport Measurement space-average technique affected by contact performance, leakage current etc Photoelectron Spectroscopy Measurement spatial localized technique (short mean free path of photo-excited electron) relatively unaffected by contact performance (grounded to avoid charging) Band Offset of A/B Heterojunction Synchrotron Radiation Photoelectron Conduction Band Maximum (CBM) Valence Band Maximum (VBM) A/ B ΔE V E A CL E A VBM E B CL E B VBM Core Level (CL) A/ B ΔE CL Substrate (B) Overlayer (A)

15 Valence-Band Discontinuity of InN/AlN Interface Photoelectron Spectroscopy (PES) is used to measure the valence-band discontinuity at the InN/AlN heterojunction. The value of ΔE V can be evaluated from the energy difference between In and dal core-levels l in the heterojunction ti sample and the core-level positions relative to the respective valence-band maxima (E VBM )of InN and AlN epilayers. Synchrotron Radiation ~2 nm nm = +

16 PES Spectra of InN/AlN AlN, InN, and AlN Photon Source Synchrotron radiation (hv=110 ev) Lattice strain relaxation at the InN/AlN heterojunction without strain-induced piezoelectric field effect Determination of E VBM locations VBM linear extrapolation of leading edge AlN (E ΔE G = 6.2 ev) CL CL = ev InN (E G ~ 0.65 ev) ΔE C E InNN In 4d E InNN VBM E AlN Al 2p E AlN VBM InN Epilayer ΔE V =3.10 ± 0.02 ± ev 0.04eV AlN Epilayer ± 0.04 ev Type-I Heterojunction

Simulation of InN/AlNHeterojunction Photoelectron Spectrum C.-L. Wu, C.-H. Shen, and S.

88, 032105 (2006) InN/AlNheterojunction photoelectron spectrum includes signals from both InN and AlN parts.

Relative contribution of InN and AlN layers is determined by the core-level peak intensities of the InN/AlNheterojunction

17 Simulation of InN/AlNHeterojunction Photoelectron Spectrum C.-L. Wu, C.-H. Shen, and S. Gwo, Valence band offset of wurzite InN/AlN heterojunction determined by photoelectron spectroscopy, Appl. Phys. Lett. 88, (2006) InN/AlNheterojunction photoelectron spectrum includes signals from both InN and AlN parts. Simulation is based on the summation of InN (A) and AlN (B) epilayer spectra with the measured ed VBO value (3.10 ev). Relative contribution of InN and AlN layers is determined by the core-level peak intensities of the InN/AlNheterojunction photoelectron spectrum. Excellent agreement is observed between the line shapes of experimental and simulated spectra. Accuracy of measured VBO value Strain-free nature in the InN/AlN interface

18 Valence Band Offset (VBO) Values of III-Nitride Heterojunctions InN/AlN Heterojunction Group ΔE V Method Growth Ref. Martin et al ± 0.20 XPS ECR-MBE C.-L. Wu et al ± 0.04 SR-PES PAMBE King et al ± 0.17 XPS PAMBE Appl. Phys. Lett. 68, 2541 (1996) Appl. Phys. Lett. 88, (2006) Appl. Phys. Lett. 90, (2007) GaN/AlN Heterojunction Group ΔE V Method Growth Ref. Sitar et al. 1.4 Optical - Thin Solid Films 200, 311 (1991) Martin et al. 0.8 ± 0.3 XPS ECR-MBE Appl. Phys. Lett. 65, 610 (1994) Bermudez et al ± XPS UPS MOCVD J. Appl. Phys. 79, 110 (1996) Martin et al ± 0.24 XPS ECR-MBE Appl. Phys. Lett. 68, 2541 (1996) Waldrop et al ± 0.07 XPS GSMBE Appl. Phys. Lett. 68, 2879 (1996) King et al. 0.8 ± 0.2 XPS GSMBE J. Appl. Phys. 84, 2086 (1998)

19 Polarization-Induced Band Alignments at Opposite Polarity InN/GaN Interfaces - Polarization Discontinuity across the Heterojunction - Special Designed InN/GaN Heterojunctions Unstrained Tensile Compressive

Polarizations in III-Nitrides: Spontaneous

Spontaneous polarization is a consequence of

Al, Ga, In: cation atoms N: anion atoms Wurtzite

polarization coefficient is large Presence of

crystal - + - + F. Bernardiniet al, Phys. Rev.

20 Polarizations in III-Nitrides: Spontaneous Polarization (polarization in unstrained lattice) Spontaneous polarization is a consequence of wurtzite symmetry. Al, Ga, In: cation atoms N: anion atoms Wurtzite lacks mirror symmetry in c-axis Spontaneous polarization coefficient is large Presence of polarization dipole and E- field in an unstrained crystal F. Bernardiniet al, Phys. Rev. B 56, R10024 (1997) III-Nitride AlN GaN InN c 0 /a Ideal wurtzite: c 8 = = a 3 Nonideality P SP

21 Polarizations in III-Nitrides: Piezoelectric Polarization (polarization caused by strain) Piezoelectric polarization is a consequence of strain existing in the crystalline structure. Large lattice match between AlN, GaN, and InN (12% for InN/AlN; 10% for InN/GaN; 2% for GaN/AlN) Piezoelectric polarization coefficient is large Presence of polarization dipole and E- field in a strained crystal Unstrained Tensile Compressive F. Bernardiniet al, Phys. Rev. B 56, R10024 (1997); S. Adachi et al, J. Appl. Phys 58, R1 (1985) a- a C P pz e e C 0 13 = a0 C33 a : lattice constant of strained layer a 0 : lattice constant t of unstrained layer e ij : piezoelectric constant C ij : elastic constant

22 Polarization in Heterojunction (case in InN/GaN Heterojunction) Polarization (Spontaneous and Piezoelectric) as the source of built-in electric field Fixed polarization charges is induced at the heterojunction interface Interface charge equal to negative divergence of polarization Spontaneous and piezoelectric polarizations are comparable Fixed charges Free electrons ΔP ρ = P ρ = lim Δ x 0 Δx Specially designed (In/Ga- ρ Δ x = σand = N-polar) lim ΔP nearly strain-free Δx 0 InN/GaN Heterojunctions to observe spontaneous polarization Top Bottom effects. σ = ( P P ) InN GaN

23 InN, GaN, InN/GaN Heterojunctions with Different Polarities Plasma-assisted Molecular-Beam Epitaxy (PA-MBE) Growth Sample Growth Conditions (Temperature, Flux, etc) Polarity Controlled by the Growth Substrate/Template Polarity Determined by Chemical Etching and RHEED Pattern (Surface Reconstruction) InN Epilayer In-polar (In-Polarity) GaN Epilayer GaN Template GaN Epilayer GaN Template GaN Epilayer GaN Template Sapphire Sapphire Sapphire InN Epilayer GaN Epilayer InN/GaN Heterojunction InN Epilayer N-polar (N-Polarity) GaN Epilayer AlN Buffer Layer GaN Epilayer AlN Buffer Layer GaN Epilayer AlN Buffer Layer Si Substrate Si Substrate Si Substrate

24 PES Spectra of InN and GaN Epilayers with Different Polarities No apparent energy differences between core levels to VBMs of InN and GaN with different polarities Two components for In 4d core level peak of InN Epilayer (HCl Chemical Etching + in situ Thermal Annealing) - B (Bulk State): Dominant Bulk Component - S (Surface-related State): Chemical Shift of Surface Atom Bond InN (E in 4d -E VBM ) GaN (E Ga 3d -E VBM ) In/Ga- Polar 16.71± eV ± eV N-Polar ±0.02 ev ±0.02 ev

25 Crystalline Properties of InN/GaN Heterojunction Pseudomorphic/fully relaxed transition occurs within the 1 monolayer growth of In- and N-polar InN on GaN epilayer (Interface Control). From the synchrotron-radiation XRD spectra of the InN/GaN heterojunction samples, the compressive strain in thin InN epilayers can be obtained to be 04% 0.4% (c=5.728 Å) and 0.2% (c=5.718 Å) of In- and N-polarities, respectively. (c=5.706 Å for thick InN epilayer) In-plane lattice mismatch of InN@GaN: (a GaN -aa InN )/a InN >-10%

26 PES Spectra of InN/GaN Heterojunctions Thickness of InN Overlayer is around 2 nm B Δ E = 2.23eV CL S ΔE E = ev CL B S E V of InN/GaN heterojunctions as a function of crystal polarity Non-Commutativity

27 PES Spectra of InN/GaN Heterojunctions Thickness of InN Overlayer is around 2 nm B Δ E = 2.23eV CL S ΔE E = ev CL B S

The divergence of spontaneous polarization induce a interfacial charge of density σ, and form a

The voltage drop across of this capacitor with distance L d ( Debye length of InN).

Ga polar CL N polar InN ( σ ) ( % σ ) = E + d E E + d E Ga int In Ga int In K K K K εinn ε0 ε In /

28 The divergence of spontaneous polarization induce a interfacial charge of density σ, and form a interfacial microscopic i capacitor. The voltage drop across of this capacitor with distance L d ( Debye length of InN). A increase of decrease of Ga photoelectron energy across the capacitor will be InN 0 kt ev = σ ε ε int d, where d LD 1.25nm 2 ε ε = = en InN 0 Sum of polarization-induced interface charge density ( ΔE ) ( ΔE ) CL In / Ga polar CL N polar InN ( σ ) ( % σ ) = E + d E E + d E Ga int In Ga int In K K K K εinn ε0 ε In / Ga polar InNε 0 N polar σ int + % σ int = d = 0.54 ev ε InNε 0 ε InNε 0 σi nt + % σint = 0.54 = C/m = cm d Hi h 2DEG d it b hi d ith t d i High 2DEG density can be achieved without doping Polarization Doping High Breakdown Down Voltage

The Spontaneous polarization difference between InN InN GaN and GaN: ΔPP = P P SP SP SP σ + % σ = [( P P ) P ] + [( P P% ) P ] P P

4% strain in the c-axis) InN 2 PZ =0.0047 C/m (0.2% strain in the c-axis) 2 Δ P SP = 0.

633 III-Nitride AlN GaN InN c 0 /a 0 1.6010 b 1.6259 b 1.6116 b P sp (C/m 2 ) -0.120 a -0.081 b -0.090 c -0.074 a -0.029 b -0.

29 The Spontaneous polarization difference between InN InN GaN and GaN: ΔPP = P P SP SP SP σ + % σ = [( P P ) P ] + [( P P% ) P ] P P % int int InN InN GaN InN InN GaN SP PZ SP SP PZ SP InN InN PSP PPZ P% PZ 2 = 2( Δ ) ( + ) = C/m InN 2 PZ = C/m (0.4% strain in the c-axis) InN 2 PZ = C/m (0.2% strain in the c-axis) 2 Δ P SP = C/m Spontaneous polarization vs Crystal parameters the c/a ratio must differ from the ideal ration of (8/3)=1.633 III-Nitride AlN GaN InN c 0 /a b b b P sp (C/m 2 ) a b c a b c a b c ref: (a) F. Bechstedt et al, Phys. Rev. B 62, 8003 (2000) (b) F. Bernardini et al, Phys. Rev. B 56, R10024 (1997) (c) F. Bernardini et al, Phys. Rev. B 63, (2001)

30 Summary: Have the 1 st direct evidence of spontaneous polarization effect in III-nitride heterojunctions (large core level shift and VBO modification). Polarity plays a significant role in band alignment of InN/GaN heterojunction. A new degree of freedom in the design of III-nitride heterojunction devices!!! How to play the polarity? Growth, Pattern-assistance, Intermixing Interface Atoms Chung-Lin Wu, Hong-Mao Lee, Cheng-Tai Kuo, Chia-Hung Hsu, and Shangjr Gwo, Polarization-Induced Valence-Band Alignments at Anion- and Cation-polar InN/GaN Interfaces, Appl. Phys. Lett. 91, (2007)

31 Three-dimensional Visualization of III-Nitride Electronic Structures Band Lineup of InN/GaN heterojunction - Nonpolar cross-section Presence or absence of surface band bending at InN surfaces - Polar vs. Nonpolar surface Metal contact to nonpolar InN surface

32 High-Quality III-Nitride Heterostructure Plasma-assisted assisted molecular-beam epitaxy (PA-MBE) growth InN (2.4 μm)/gan (1.8 μm)/aln (1 μm)/si (111) substrate Strain-free nitride heterojunctions (via an interface-controlled growth technique) Layer a (Å) c (Å) InN GaN AlN c-plane

33 Conventional PES Measurement on Heterojunction c-plane in situ Cr0ss-sectional Measurement Easily cleaving sample Silicon substrate Large detection area Thick layer Spatial resolution Small beam size Cross-Sectional SPEM/S UHV Cleaved a-plane Surface - No spatial information - Limited detection depth (small photoelectron escape depth) - Surface coupling - Polarization coupling

(via an interface-controlled growth technique) Cleaving surface Non-polar

34 Cleaving the High Quality InN/GaN Heterojunction Plasma-assisted molecular-beam epitaxy (PA- MBE) growth Strain-free nitride heterojunctions (via an interface-controlled growth technique) Cleaving surface Non-polar surface No Spontaneous polarization effects Only can be achieved on III-nitride/Si system

35 Scanning Photoelectron Microscope (SPEM) NSRRC (Hsinchu Hsinchu, Taiwan) S b Submicron Resolution

Cross-Sectional SPEM Scanning PhotoElectron Microscopy

- Focused soft x-ray (zone plate + order sorting

mode - μ-pes mode Vacuum Cleaved III-Nitride

films grown on Sisubstrates - Clean and oxide-free

36 Cross-Sectional SPEM Scanning PhotoElectron Microscopy (SPEM) - Synchrotron radiation (NSRRC, Hsinchu Taiwan) - Focused soft x-ray (zone plate + order sorting aperture) Two operation o modes: - Chemical mapping mode - μ-pes mode Vacuum Cleaved III-Nitride Heterojunction Samples - Cleavability of III-nitride films grown on Sisubstrates - Clean and oxide-free cross-sectional surface - Nonpolar a-plane μ-pes μ at heterojunction interfaces

37 Chemical Mapping/Valence Band Offset (VBO) Measurements Cross-sectional sectional SEM Image ev μ-pes Spectra - Core-levels and VBM - Direct VBO Measurement Cross-sectional sectional Chemical Mapping By SPEM ΔE CL =1.98 ev CL N-polarity: 1.69 ev In-polarity: 2.20 ev ev

38 PES Measurement of As-Grown a-plane InN Synchrotron Radiation a-axis Epitaxial a-plane InN Photoelectron hν = 380 ev a-plane InN Epilayer r-plane Sapphire E V E C 0.65 ev

39 Comparison with Surface Band Bending of InN Direct evidence of Flat surface bands on the UHV-cleaved a-plane InN surface

Comparison with Surface Band Bending on c-plane InN Significant

represented areas of Au-free and Au/InN boundary - Energy

40 Comparison with Surface Band Bending on c-plane InN Significant downward band bending in the c-axis - PES measurements on two represented areas of Au-free and Au/InN boundary - Energy separation between the VBM and the Fermi edge on the polar c-plane 1.54 ev - Energy Separation between the VBM and the Fermi edge on non-polar a-plane (cross-sectional measurement) 0.50 ev E In 4d =1829eV 18.29

Have the 1 st direct measurement of heterojunction VBO in the cross-sectional view. No electron accumulation on UHV cleaved InN a-plane. Lighting the application of InN devices!

41 Have the 1 st direct measurement of heterojunction VBO in the cross-sectional view. No electron accumulation on UHV cleaved InN a-plane. Lighting the application of InN devices!!! p-type doping InN Chung-Lin Wu ( 吳忠霖 ), Hong-Mao Lee ( 李弘貿 ), Cheng-Tai Kuo ( 郭承泰 ), Chia-Hao Chen ( 陳家浩 ), and Shangjr Gwo ( 果尚志 ), Cross-sectional scanning photoelectron microscopy and spectroscopy of wurtzite InN/GaN heterojunction: Measurement of intrinsic band lineup, Appl. Phys. Lett. 92, (2008) Chung-Lin Wu ( 吳忠霖 ), Hong-Mao Lee ( 李弘貿 ), Cheng-Tai Kuo ( 郭承泰 ), Chia-Hao Chen ( 陳家浩 ), and Shangjr Gwo ( 果尚志 ), Absence of Fermi-Level Pinning at Cleaved Nonpolar InN Surfaces, Phys. Rev. Lett. 101, (2008)

42 Conclusions: Reach the perfect heteroepitaxy and nano-epitaxy in III- Nitride-on-Si system Commensurate Lattice Match A comprehensive understanding of InN based heterojunctions o Heterojunction Band offsets (InN/AlN) Polarization Effects (InN/GaN) 3D Virtualization of Electronic Structure (InN/GaN/AlN)

43 Acknowledgements : National Tsing-Hua University, Dept. of Physics Prof. S. Gwo ( 果尚志教授 ) MBE Growth C.-H. Shen ( 沈昌宏 ) H.-W. Lin ( 林弘偉 ) H.-M. Lee ( 李弘貿 ) PES Measurement C.-T. Kuo ( 郭承泰 ) National Synchrotron Radiation Research Center Dr. T.-W. Pi ( 皮敦文博士 ) Dr. C.-H. Chen ( 陳家浩博士 ) Dr. C.-H. Hsu ( 徐嘉鴻博士 )

Thermal Stress and Strain in a GaN Epitaxial Layer Grown on a Sapphire Substrate by the MOCVD Method

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