S. S. Pei

Electronic transport in chemical vapor deposited graphene synthesized on Cu: Quantum Hall effect and weak localization

We report on electronic properties of graphene synthesized by chemical vapor deposition (CVD) on copper then transferred to SiO2/Si. Wafer-scale (up to 4 in.) graphene films have been synthesized, consisting dominantly of monolayer graphene as indicated by spectroscopic Raman mapping. Low temperature transport measurements are performed on microdevices fabricated from such CVD graphene, displaying ambipolar field effect (with on/off ratio ∼5 and carrier mobilities up to ∼3000 cm2/V s) and “half-integer” quantum Hall effect, a hall-mark of intrinsic electronic properties of monolayer graphene. We also observe weak localization and extract information about phase coherence and scattering of carriers.

High power mid-infrared interband cascade lasers based on type-II quantum wells

We report a high power mid-infrared interband cascade laser operating at temperatures up to 170 K. The threshold current densities of this laser are considerably lower than the previously reported values in cascade lasers. The structure was grown by molecular beam epitaxy on a GaSb substrate and comprises 23 periods of active regions separated by digitally graded multilayer injection regions. A peak optical output power of ∼0.5 W/facet and a slope of 211 mW/A per facet, corresponding to a differential external quantum efficiency of 131%, are observed at 80 K and at a wavelength of ∼3.9 μm.

Near-room-temperature mid-infrared interband cascade laser

A 25-stage interband cascade laser with a W active region and a third hole quantum well for the suppression of leakage current has exhibited lasing in pulsed mode up to 286 K. A peak output power of 160 mW/facet and a slope efficiency of 197 mW/A per facet (1.1 photons per injected electron) were measured at 196 K. Above 200 K, the characteristic temperature was higher (T0=53 K) and the threshold current densities lower than for a previously reported W interband cascade laser without the third hole quantum well.

Auger coefficients in type-II InAs/Ga1−xInxSb quantum wells

Two different approaches, a photoconductive response technique and a correlation of lasing thresholds with theoretical threshold carrier concentrations have been used to determine Auger lifetimes in InAs/GaInSb quantum wells. For energy gaps corresponding to 3.1–4.8 μm, the room-temperature Auger coefficients for seven different samples are found to be nearly an order-of-magnitude lower than typical type-I results for the same wavelength. The data imply that at this temperature, the Auger rate is relatively insensitive to details of the band structure.

Grating-tuned external-cavity quantum-cascade semiconductor lasers

Grating-coupled external-cavity quantum-cascade lasers were studied for temperatures from 80 to 230 K. At 80 K, a tuning range of ∼65–88 nm are obtained for 4.5 and 5.1 μm laser amplifiers, respectively. The tuning ranges for both narrowed substantially with increasing temperature, to ∼23 nm at 203 K. The threshold varied slowly versus wavelength, while the efficiency appeared to be close to optimum toward wavelengths shorter than the free running wavelength.

Thermal Transport in Graphene Nanostructures: Experiments and Simulations

a Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907 b School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907 c Department of Mechanical Engineering, Iowa State University, Ames, IA 50011 d School of Physics, Georgia Institute of Technology, Atlanta, GA 30332 e Department of Electrical and Computer Engineering, University of Houston, Houston, Texas 77204 f Department of Physics, Purdue University, West Lafayette, IN 47907 g School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907

Large-scale graphitic thin films synthesized on Ni and transferred to insulators: Structural and electronic properties

We present a comprehensive study of the structural and electronic properties of ultrathin films containing graphene layers synthesized by chemical vapor deposition based surface segregation on polycrystalline Ni foils then transferred onto insulating SiO2/Si substrates. Films of size up to several mm’s have been synthesized. Structural characterizations by atomic force microscopy, scanning tunneling microscopy, cross-sectional transmission electron microscopy (XTEM), and Raman spectroscopy confirm that such large-scale graphitic thin films (GTF) contain both thick graphite regions and thin regions of few-layer graphene. The films also contain many wrinkles, with sharply-bent tips and dislocations revealed by XTEM, yielding insights on the growth and buckling processes of the GTF. Measurements on mm-scale back-gated transistor devices fabricated from the transferred GTF show ambipolar field effect with resistance modulation ∼50% and carrier mobilities reaching ∼2000 cm2/V s. We also demonstrate quantum tra...

Mid-infrared interband cascade lasers with quantum efficiencies >200%

An external differential quantum efficiency exceeding 200% has been observed from 4 μm InAs/InGaSb/AlSb interband cascade lasers under 1 μs pulses and 0.1% duty cycle at 80 K. By increasing the pulse lengths and the repetition rates, average powers >16 mW have been measured with 5 μs pulses at 10% duty cycle, the internal quantum efficiency and the internal loss are determined to be 220% and 14 cm−1, respectively.

ROLE OF INTERNAL LOSS IN LIMITING TYPE-II MID-IR LASER PERFORMANCE

We report an experimental and theoretical investigation of internal losses in optically pumped type-II lasers with InAs/GaSb/Ga1−xInxSb/GaSb superlattice active regions. Whereas the losses are found to be moderate at 100 K (11–14 cm−1), they increase rapidly with increasing temperature (to 50–120 cm−1 at 200 K). Comparison with a detailed numerical simulation shows that the internal losses play a much more important role than Auger recombination or carrier/lattice heating in limiting the laser performance at high temperatures. Calculations of the temperature-dependent intervalence absorption cross sections show that losses of the magnitude observed experimentally can easily occur if one does not take special care to avoid resonances in all regions of the Brillouin zone. Practical design guidelines are presented. The superlattice lasers yield maximum peak output powers of up to 6.5 W per facet at 100 K and 3.5 W per facet at 180 K, threshold incident pump intensities as low as 340 W/cm2 at 100 K, and Shock...

Novel type‐II quantum cascade lasers

A new class of quantum cascade lasers based on type‐II quantum wells is analyzed. In these novel mid‐ and long‐wavelength IR lasers, not only can a population inversion be easily created with a nearly 100% current injection efficiency, but also the nonradiative loss from the optical phonon scattering can be greatly suppressed. A general description of how the lasing threshold current depends on the injection, radiative, and inversion efficiencies is formulated to illustrate the expected improvements over the recently reported quantum cascade laser. Also, the features that distinguish quantum cascade lasers from traditional bipolar lasers are discussed in the context of the carrier transport in equivalent circuit models to illustrate the advantages of quantum cascade lasers for high power mid‐ and long‐wavelength IR source applications.