InGaAs-AlAsSb quantum cascade lasers

Transcription

1 InGaAs-AlAsSb quantum cascade lasers D.G.Revin, L.R.Wilson, E.A.Zibik, R.P.Green, J.W.Cockburn Department of Physics and Astronomy, University of Sheffield, UK M.J.Steer, R.J.Airey EPSRC National Centre for III-V Technologies, University of Sheffield, UK

2 Materials systems for QCLs Substrate Well Barrier Year Minimal wavelength, µm InP As In 0.53 Ga 0.47 As As Al 0.48 In 0.52 As 1994 ~ 3.6, strain- compensated GaAs GaAs AlGaAs 1998 ~ 7 InAs InAs AlSb 2003 ~ 4.6 InP In 0.53 Ga 0.47 As AlAs As 0.56 Sb < 3 First demonstration of intersubband emission in the l~3-5µm range First realization of l~4.3µm QC laser on InGaAs/AlAsSb/InP

3 In 0.53 Ga 0.47 As As AlAs AlAs 0.56 Sb system miniband Active region Bridging region Needs in shorter wavelength use materials with higher DE c Eg, ev Advantages: Lattice matched to InP (no strain). Very high conduction band offset (~1.6 ev). State-of the art waveguide with InP or InAlAs claddings, advanced processing Observed intersubband absorption up to l<2µm (Georgiev et al, J.Vac.Sci.Tech. B 19, 1747 (2001). Problems: Difficult epitaxial growth - AlAs 0.56 Sb 0.44 is in miscibility gap (uncertainties in material quality and heterojunction roughness), Shortage in data of parameters for AlAs 0.56 Sb 0.44.

4 Growth of In Ga 0.47 As As AlAs AlAs 0.56 Sb 0.44 MBE n+ InP (100), growth temperature 480 o C Growth rates: 1 ML/s for In 0.53 Ga 0.47 As and In ML/s for AlAs 0.56 Sb Al 0.48 As X-ray diffraction - mismatch a/a < Growth interruption up to 10 s maintaining continuous As flux at every interface to minimize fluctuation in composition across the heterointerfaces. (Gopal et al, APL, 80, 4696 (2002))

5 In 0.53 Ga 0.47 As As AlAs AlAs 0.56 Sb 0.44 LEDs #1938, 95kV/cm #1940, 80kV/cm Energy (ev) Distance (nm) Distance (nm) Conduction band profiles for samples #1938 (3 QW in the active region) and #1940 (4 QWs in the active region). The moduli squared of wave functions in the active region are shown. Calculated wavelengths: l=4.1 µm (#1938) l=4.8 µm (#1940)

6 TEM images: #1940 sample, overall and one period Highly doped In 0.52 Al 0.48 As AlAs 0.56 Sb lighter layers In 0.53 Ga 0.47 As 20 periods of the active and bridging regions In 0.53 Ga 0.47 As/ Bridging region AlAs 0.56 Sb 0.44 Active region In 0.53 Ga 0.47 As Low doped 6Å AlAs 0.56 Sb 0.44 InP substrate

7 Intersubband emission from LED samples Intensity (a.u.) Wavelength (µm) #1940 #1938 T=10 K Norm. Intensity (a.u.) Polarization angle ( o ) Energy (mev) Electroluminescence spectra at 10K Cleaved edge of 400µm mesa Voltage (V) V 14 #1940 T=10 K TM polarization Current density (A cm -2 ) Optical power (nw) Samples were driven by 2µs 50kHz pulses Inset - the integrated optical power as a function of polarization angle. #1938:? em. =4.1µm, (designed for 4.1µm) #1940:? em. =5.3µm, (designed for 4.8µm) Voltage versus current and optical power versus current characteristics for sample #1940. Revin et al, APL, 84, 1447 (2004).

8 In 0.53 Ga 0.47 As As AlAs AlAs 0.56 Sb QCL structures Plasmon enhanced waveguide High-doped In As In 0.52 Al 0.48 As Low-doped In 0.52 Al 0.48 As In 0.53 Ga 0.47 As spacer Core region In 0.53 Ga 0.47 As spacer low-doped InP substrate TM mode profile Measured waveguide losses are ~ 5 cm -1 Technique details in Revin et al, JAP, 95, 7584 (2004)

9 Intersubband emission from InGaAs - AlAsSb QC devices Intensity, a.u Wavelength, µm #2001 #1998 #2005 Intensity, a.u T=20 K Polarization angle, TM mode Energy, mev Normalized low temperature spontaneous emission (I ~ 2kA/cm 2 ) #1998:? em. =4.1µm, (designed for 4.1µm) #2001:? em. =5.1µm, (designed for 4.8µm) #2005:? em. =3.15µm, (designed for 2.9µm) Emission was still observed at temperatures up to 240K.

10 Emission from InGaAs AlAsSb QCL structures Intensity,a.u. intersubband emission interband emission (mainly TE polarised) Wavenumber, cm -1 Current density ka/cm 2 Spontaneous emission for #2001 sample, driven by 2µs pulse, 10kHz. T=20K Interband emission at high current (voltage) is originated from impact ionization in cladding layers Voltage, V T=77K #2001 # Current density, ka/cm 2 Typical I-V characteristics taken at 50ns 5kHz Higher doping was needed

11 Improved design for InGaAs - AlAsSb QCL Energy (ev) ACTIVE REGION #2060, 93kV/cm injection barrier ACTIVE REGION Distance (nm) 3 exit barrier 2 1 Higher doping in the bridging region cm -3 Optimized injection and exit barriers thickness 4 samples designed for l~4.2µm LASING I th from ~6 to 20kA/cm 2

12 l~4.36mm laser emission from InGaAs - AlAsSb QCL Intensity (a.u.) Wavelength (µm) (4) T=240K Energy (mev) (3) T=20K (2) (1) Voltage (V) K 300K 20K 100K 120K 140K 160K 180K 200K 220K 5, 240K Current density (ka/cm 2 ) Peak Output Power (mw) Normalized spontaneous emission and laser spectra for sample #2060 T=20, 240K. Current density, ka/cm 2 : 1 3.7, 2 5.8, 3 6.2, Threshold current (T=20K): 5.9 ka/cm 2 V-I and L-I characteristics for sample #2060 for various heat sink temperatures. Driven pulses 100ns, 10kHz. T 0 ~ 150K

13 Narrowing of emission line for l~3.15µm QCL Intensity, a.u. 1.0 I=1A, 39.33meV I=2A, 36.73meV 0.8 I=3A, 33.31meV I=4.2A, 29.5meV I=5A, 31.17meV Energy, mev Light Intensity, a.u Current density, ka/cm 2 FWHM, mev Spontaneous emission spectra for sample #2083 for different driven current. T=20K Linear L - I dependence for sample #2083. Peak width is decreased from 39meV to 29meV. Existence of some gain?

14 The shortest l~3µm intersubband emission 1.0 Wavelength, µm Intensity, a.u T=20K I=1.2kA/cm 2 1µs, 50 khz Energy, ev Spontaneous emission from In 0.53 Ga 0.47 As AlAs 0.56 Sb 0.44 QCL with InAs monolayers in the middle of QWs in the active region Wilson et al, APL 78, 413 (2001)

15 Conclusions First demonstration of intersubband emission from InGaAs/AlAsSb QC structures in the l~3-5µm range First realization of l~4.3µm QC laser on InGaAs/AlAsSb/InP The observation of the shortest l~3µm intersubband emission for quantum cascade devices InGaAs/AlAsSb system - excellent potential candidate for short wavelength QCLs with high temperature performance Acknowledgments to EPSRC UK and EU FP6 Funds d.revin@sheffield.ac.uk