Present status and future prospects of Bi-containing semiconductors M. Yoshimoto and K. Oe Dept. Electronics, Kyoto Institute Technology Japan
Acknowledgement RBS: Prof. K. Takahiro (Kyoto Inst. Tech.), Dr. A. Chayahara (AIST) Photoreflectance: Prof. T. Kita (Kobe Univ.) EXAFS: Prof. M. Tabuchi, Prof. Y. Takeda (Nagoya Univ.) Raman spectroscopy: Prof. Harima, Dr. P. Verma (Kyoto Inst. Tech.) Post. Doc. Dr. W. Huang, Dr. G. Feng Graduate students: Mr. S. Murata, Mr. Y. Tanaka, Ms. Y. Tominaga, Mr. K. Yamada, Mr. T. Fuyuki
Background 0utline MOVPE growth of GaAsBi and InAsBi RBS Raman EXAFS: substitutional incorporation of Bi Photoluminescence, Photoreflectance: temperature-insensitive E PL, E g MBE Growth of GaAsBi GaAsBi growth: surfactant-like effect of Bi atom GaNAsBi and InGaAsBi: expansion of luminescence wavelength GaAs/GaAsBi multi-quantum wells: smooth interface w/o segregation Device-quality epilayers Laser emission from GaAsBi by photo-pumping Issue of GaAsBi growth Summary
Earliest days of epitaxy of Bi-containing semiconductors MBE growth of InSbBi to obtain III-V alloys with the narrowest possible band gap. 1 K. Oe, S. Ando, K.Sugiyama: Jpn. J. Appl. Phys. 20 (1981) L303. 2 A.J.Noreika, W.J. Takei, H. Francombe and C.E.C Wood: J.Appl.Phys. 53 (1982) 4932 Key to growth K. Oe, et.al:jjap low-temperature growth no growth of InSbBi Sb/In>1 mirror-like surface Sb/In 1 InSbBi rough surface (Sb inclusion) Sb/In<1 InSb sub.
Why low ΔEg/ΔT? Wavelength-Division Multiplexing (WDM) GaInAsP laser diode (LD): temperature dependence of bandgap and refractive index fluctuation of lasing wavelength LD equipped with Peltier device drawback: cost, energy consumption Laser diode Δλ/ΔT: 0.1 nm/k Dense WDM (example) λ Division: 0.4 nm/channel Materials with low ΔEg/ΔT LD with an emission of temperature-insensitive wavelength: elimination of Peltier device Proposal of GaInAsBi as a active-layer materials of LD K. Oe and H. Asai, Proc. Electronic Materials Symp. 95, Izu, Japan p.191 Semiconductor : GaAs Semimetal: GaBi Alloy : GaAsBi
X-ray diffraction pattern MOVPE growth of GaAsBi Low-pressure MOVPE Sources: TIPGa, TMBi, TBAs, Substrate: GaAs(100) Growth temperature: 365 o C Growth rate: 1μm/h K.Oe, JJAP 41 (2002) 2801 Successful epitaxial growth Lattice constant increases with GaBi molar fraction. GaBi molar fraction: Rutherford backscattering spectroscopy Thermally stable after anneal in As pressure at 560 o C for 30min
Determination of GaBi molar fraction x for GaAs 1-x Bi x Rutherford backscattering spectroscopy(rbs) GaBi Molar Fraction(%) 4.0 2.0 Fitting Function x=6.93 Δ2θ 0.0 0 0.2 0.4 0.6 Angular Spacing in X-ray diffraction Δ2θ(deg) K. Takahiro, et.al, J. Electron. Matter. 32 (2003)34. M.Yoshimoto, et.al, JJAP 42 (2003) L1235 determination of GaBi molar fraction by X-ray diffraction
Angular scan in Rutherford backscattering spectroscopy Angular yield profile for [100] and [110] channel Yield(Bi)=Yield(Ga+As) Bi atoms are located exactly on substitutional sites. Note Interstitial site in a zinc-blend lattice [100]: shadowed, [110]: visible K. Takahiro, et.al, J. Electron. Matter. 32 (2003)34.
Raman spectroscopy and EXAFS of GaAsBi and InAsBi GaBi-like and InBi-like modes substitutional incorporation of Bi atoms P. Verma, et.al JAP 89(2001) 1657 The majority of Bi atoms substituted the As site of InAsBi H.Ofuchi, et.al JJAP 38 (1999)Suppl. 38-1, 545
Temperature dependence of PL for GaAsBi K.Oe, JJAP 41 (2002) 2801
Photoreflectance spectra of GaAsBi GaBi molar fraction (%) GaAs 0.995 Bi 0.005 Franz-Keldysh oscillation due to built-in electric field Decrease in bandgap of GaAs 1-x Bi x with increasing GaBi molar fraction J.Yoshida, et al., JJAP 42 (2003) 371
Temperature dependence of bandgap of GaAsBi 1.5 x = 0 GaBi molar fraction ΔE g /ΔT 150-300K (mev) Bandgap Energy (ev) 1.4 1.3 x = 0.005 x = 0.013 x = 0.026 0 (GaAs) -0.42 0.005-0.24 0.013-0.23 0.026-0.15 Temperature-insensitive bandgap 1.2 0 50 100 150 200 250 300 Temperature (K) J.Yoshida, et al., JJAP 42 (2003) 371
Drawbacks of MOVPE growth of GaAsBi MOVPE: insufficient decomposition of metalorganic at low T sub difficulty in incorporation of In with existence of Ga at low T sub segregation of Bi on surface MBE: low-temperature growth without decomposition process no Bi segregation: desorption from surface
X-ray diffraction Bi-flux dependence MBE Growth of GaAsBi INTENSITY (arb. units) 4 2 0 GaAs(004) K α 1 GaAsBi(004) K α 1 K α 1 K α 1 K α 1 K α 2 K α 2 65.5 66 66.5 2 θ (deg) Bi flux (x10 8 Torr) 0 2 4 6 8 Thickness: 0.5 μm Substrate temperature: 380 o C Ga flux: 3 10-7 Torr As flux: 8 10-6 Torr Lattice constant increases with Bi flux, followed by saturation. RBS: Substitutional incorporation of Bi M.Yoshimoto, et.al, JJAP 42 (2003) L1235
GaBi molar fraction vs. Bi flux and substrate temperature (a) (b) GaBi MOLAR FRACTION x (%) 4.0 2.0 T sub =380 o C Bi flux 2.8x10 8 Torr 0.0 0 5 10 Bi FLUX (x10 8 Torr) 360 380 400 420 SUBSTRATE TEMPERATURE ( o C) M.Yoshimoto, et.al, JJAP 42 (2003) L1235
Effect of As flux on MBE growth of GaAsBi Thickness: 0.5 μm Substrate temperature: 380 o C Ga flux: 3 10-7 Torr Bi flux: 2 10-8 Torr As/Ga>1 As/Ga 1 As/Ga<1 Bi is incorporated with a limited As flux. M. Yoshimoto, et.al, Mater. Res. Soc. Symp. Proc. 891 (2006) 0891-EE11-06
Photoluminescence from GaAsBi R.T. PL spectra of GaAsBi grown at different T PL peak energy vs. GaBi molar fraction Intensity (arb. unit) 350 o C 360 o C 380 o C PL peak emission (ev) 1.4 1.3 1.2 1.1 0.9 1.0 1.1 1.2 1.3 Photon energy (ev) 1.0 0 1 2 3 4 5 6 GaBi molar fraction (%) Luminescent GaAsBi can be obtained by low-temperature MBE growth (<400 o C), probably due to a surfactant-like effect of Bi atoms. W.Huang, et.al, JAP 98(2005) 053505
Expansion of luminescence wavelength to longer wavelength - GaNAsBi Plasma-assisted MBE GaAs buffer layer (thickness: 100nm, T sub 500 o C) GaNAsBi substrate temperature: 350~400 o C source: Ga (10-7 Torr), As (10-6 Torr), Bi (10-8 Torr) plasma activated nitrogen(13.56mhz) Key to Bi incorporation: narrow process window for As flux low-temperature growth (<400 ) [110] M. Yoshimoto, et.al, JJAP, 43 (2004) L845 GaNAsBi RHEED - [110]
GaNAsBi: composition determination GaBi molar fraction by RBS GaN molar fraction by SIMS 10 6 As SIMS YIELD (arb. units) 10 4 10 2 GanAsBi GaAs sub. Ga Ga+N 10 0 0 200 400 600 DEPTH (nm) Substitutional incorporation of Bi atoms were also confirmed by channeling RBS. M. Yoshimoto, et.al, MRS Symp. Proc. 891 (2006) 0891-EE11-06 Bi
X-ray diffraction of GaNAsBi Lattice matched to GaAs W.Huang, et.al, JAP 98(2005) 053505 Constant supplies of Ga, As, Bi
PL emission from lattice-matched GaNAsBi Ga(N 0.34 Bi 0.66 ) z As 1-z W.Huang, et.al, JAP 98(2005) 053505 M. Yoshimoto, et.al, 16th IPRM, Kagoshima, Japan, 2004, IEEE #04CH37589, p.501. Ga(N 0.34 Bi 0.66 ) z As 1-z : Lattice matched to GaAs PL emission in the optical fiber communication waveband
Photoluminescence of GaNAsBi Low temp. coefficient Temperature coefficient of the PL peak energy 0.14 mev/k (150-300k) = 1/3 InGaAsP W.Huang, et.al, JAP 98(2005) 053505
Growth of GaAsBi Multi-quantum wells Intensity (arb. units) GaAsBi [110] [110] [110] [110] [110] [110] GaAs GaAsBi Layer-by-layer growth Y. Tominaga, et.al, APL 93 (2008)131915 0 60 120 180 Time (sec.)
(400) X-ray diffraction pattern Intensity (arb. units) GaAs 0.948 Bi 0.052 layer / GaAs layer = 7nm / 14nm, 14periods Simulation fitting Experimental data -3rd -5th -6th -4th -7th -2nd MQW 0th -1st GaAs +1st +3rd +2nd 61.0 62.0 63.0 64.0 65.0 66.0 67.0 68.0 2θ (degree) Intensity (arb. units) -1st Smooth interface Laue function: N =N-2 1 2 3 4 5 67 12 11 10 9 8 0th 65.2 65.4 65.6 65.8 66. 2θ (degree)
Cross-sectional TEM image GaAs 0.952 Bi 0.048 /GaAs MQWs (Growth temperature:350 C) Y. Tominaga, et.al, APL 93 (2008)131915
Quantum size effect Photoluminescence (PL) spectra of GaAs 1-x Bi x /GaAs MQW Normalized intensity (arb. units) 1.0 0.8 0.6 0.4 0.2 Excitation wavelength : 488nm (Ar + laser) GaAs 0.948 Bi 0.052 / GaAs = 3~12nm / 14nm, 10~15 periods 7nm 5nm 12nm 3nm 0.0 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 Photon energy (ev) RT PL peak energy (ev) 1.16 1.14 1.12 1.10 1.08 1.06 GaAs 0.949 Bi 0.051 thin film RT 2 4 6 8 10 12 Thickness of GaAsBi layer (nm) Y. Tominaga et al.:pss(c) 5, 2719 (2008)
PL at a wavelength of 1.3µm Normalized PL intensity (arb. units) 1.2 1.0 0.8 0.6 0.4 0.2 0.0 PL spectra of GaAs 1-x Bi x / GaAs MQWs GaAs 1-x Bi x / GaAs = 7nm / 14nm, 5~10 periods 3.1% 4.8% 6.8% 8.1% 9.3% 10.9% RT 1.0 1.1 1.2 1.3 1.4 1.5 Wavelength (µm) PL peak wavelength (µm) 1.35 1.30 1.25 1.20 1.15 1.10 1.05 1.00 0.0 2.0 4.0 6.0 8.0 10.0 12.0 Bi content (%) Y. Tominaga, et.al, APL 93 (2008)131915
Thermal stability Annealing:10 minutes under N 2 flow GaAs 0.984 Bi 0.016 /GaAs MQW XRD intensity (arb. units) -3rd -2nd MQW 0th GaAs -1st 1st Annealing temperature 500 o C 600 o C 700 o C 800 o C 900 o C 64.5 65.0 65.5 66.0 66.5 67.0 2θ (degree) Y. Tominaga, et.al, APL 93 (2008)131915
Laser emission from GaAs 1-x Bi x by photo-pumping GaAs 0.975 Bi 0.025 Intensity (arb. units) 6x10 4 5x10 4 4x10 4 3x10 4 2x10 4 1x10 4 0 RT GaAsBi GaAs 3.3mJ/cm 2 2.5mJ/cm 2 2.4mJ/cm 2 900 950 1000 1050 1100 Wavelength (nm) Integrated intensity (arb. units) RT 1.0 1.5 2.0 2.5 3.0 3.5 Pumping density (mj/cm 2 ) Y. Tominaga, et.al, Appl. Phys. Express 3 (2010) 062201
Laser emission from GaAs 1-x Bi x by photo-pumping GaAs 0.975 Bi 0.025 Intensity (arb. (arb. units) units) 6x10 6x10 4 4 5x10 5x10 4 4 4x10 4 4x10 4 3x10 4 3x10 4 2x10 4 2x10 4 1x10 4 1x10 4 0 0 RT 240K GaAsBi GaAs 3.3mJ/cm 2 0.91mJ/cm 2.5mJ/cm 2 2 0.83mJ/cm 2.4mJ/cm 2 2 0.72mJ/cm 2 900 950 1000 1050 1100 900 950 1000 1050 1100 Wavelength (nm) Wavelength (nm) Integrated intensity (arb. units) Integrated intensity (arb. units) 240K RT 0.3 0.4 1.0 0.5 1.5 0.6 2.0 0.7 2.5 0.8 3.0 0.9 3.5 1.0 Pumping density (mj/cm Pumping density (mj/cm 2 ) Y. Tominaga, et.al, Appl. Phys. Express 3 (2010) 062201
Temperature dependence of lasing wavelength GaAs 0.975 Bi 0.025 1.35 10 1 GaBi molar fraction ΔE PL /ΔT 150-300K (mev/k) 0 (GaAs) -0.42 0.025(Laser) -0.18 0.025(PL) -0.15 Low-temperature coefficient of lasing wavelength Emission energy (ev) 1.30 1.25 1.20 Y. Tominaga, et.al, Appl. Phys. Express 3 (2010) 062201 T 0 =83K 100 150 200 250 300 Temperature (K) 10 0 10-1 Threshold pumping density (mj/cm 2 ) 31
Issue of GaAsBi growth Key to InSbBi growth (1981) low-temperature growth no growth of InSbBi Sb/In>1 mirror-like surface Sb/In 1 InSbBi rough surface (Sb inclusion) Sb/In<1 InSb sub. narrow window of Sb/In ratio Key to GaAsBi growth (at present) low-temperature growth narrow window of As/Ga ratio As/Ga>1 As/Ga 1 As/Ga<1
Issue of GaAsBi growth The essence of growth conditions for GaAsBi-based alloys is exactly the same as that of InSbBi growth in the early 80s!! Conventional essence low temperature growth (<400 o C) As (or Sb) flux adjustment in a limited range on the brink of As (or Sb) shortage on the growing surface growth condition B (flux etc.) single crystalline luminescent process window for GaAsBi growth laser emission growth condition A (temperature etc.) very narrow process window for laser emission An innovative growth technique is expected for further improvement in GaAsBi-based alloys.
Summary MOVPE growth of GaAsBi and InAsBi Bi atoms occupy substitutional sites (RBS, Raman, EXAFS). A single-peak PL (10 300K). Temperature dependence of E g of GaAs 0.974 Bi 0.026 is 1/3 of the value of GaAs. MBE growth of GaAsBi Key to growth: (1)Control of As flux within narrow limits, (2)low-temperature growth. A surfactant-like effect: Luminescent GaAsBi grown at low temperature (<400 o C). Expansion of luminescence wavelength to longer wavelength GaNAsBi/GaAs: fairly luminescent (1.4 μm emission). InGaAsBi/InP: weak luminescence (extremely low temperature growth <300 o C). MQW structure: abrupt interface w/o segregation, thermally stable (<800 o C), luminescent (1.3 μm@10.9%bi), quantum-size effect. Device-quality epilayer Laser-emission can be obtained, however, very narrow process window. The essence of growth conditions for GaAsBi-based alloys is exactly the same as that of InSbBi growth in the early 80s!! An innovative growth technique is expected for further improvement in GaAsBibased alloys.