H. Z. Wu

Far-infrared reflection spectra of PbSrSe thin films grown by molecular beam epitaxy

We report the far-infrared (FIR) reflection measurements of Pb1-xSrxSe thin films with different Sr concentrations grown by molecular beam epitaxy on BaF2 substrates. Sr composition and lattice parameter have been assessed by x-ray diffraction. By fitting the FIR reflection spectra and by comparison with the results of binary PbSe and SrSe thin films, both PbSe-like and SrSe-like optical phonon reflection bands in ternary PbSrSe thin films have been identified. Also, the carrier concentration and mobility of PbSrSe thin films have been extracted from the theoretical calculation of the reflection spectra, and are found to be in agreement with reported results from Hall measurements.

Low-threshold optically pumped λ=4.4 μm vertical-cavity surface-emitting laser with a PbSe quantum-well active region

We report pulsed emission from an optically pumped lead-salt vertical-cavity surface-emitting laser with a PbSe/PbSrSe quantum-well active region. The lasing wavelength of λ=4.44 μm is nearly constant over the temperature range 200–280 K, and the threshold is only 10.5 kW/cm2 at 260 K, where the gain peak and cavity mode are in resonance. Over 330 mW of peak power is emitted at 260 K, into a circular beam whose divergence angle increases with pump intensity.

Above-room-temperature optically pumped 4.12 μm midinfrared vertical-cavity surface-emitting lasers

Optically pumped lead salt vertical-cavity surface-emitting lasers (VCSELs) with a nine period PbSe/PbSrSe quantum well active region operating above room temperature in pulsed mode are reported. The gain peak and cavity mode of the VCSEL structure are in resonance at 300 K. A power output of 40 mW is obtained at room temperature and it does not show saturation. The room-temperature threshold pump density is 200 kW/cm2. The lasing wavelength of λ=4.12 μm remains nearly constant over a temperature range of 280–310 K.

Band gaps, effective masses and refractive indices of PbSrSe thin films: Key properties for mid-infrared optoelectronic device applications

Key properties of PbSrSe thin films grown by molecular beam epitaxy have been studied for mid-infrared optoelectronic device applications. Detailed knowledge of the material parameters for the device design is required. The material parameters considered are: temperature-dependent band gaps, composition (or band gap)-dependent effective masses, and energy-dependent refractive indices. The study has been carried out by a combination of temperature-dependent photoluminescence and absorption measurements with the theoretical models on PbSrSe thin films of Sr compositions of as high as 0.276. The derived empirical equations for band gaps, effective masses, and refractive indices have been employed successfully in PbSe/Pb0.934Sr0.066Se multiple quantum well mid-infrared laser systems, for studying the band offsets and subband behavior. We have shown that the derived material parameters clearly promise of being applied to other PbSrSe thin films and PbSe/PbSrSe heterostructure systems for their optoelectronic a...

Continuous wave optically pumped lead-salt mid-infrared quantum-well vertical-cavity surface-emitting lasers

Continuous wave optically pumped mid-infrared lead-salt quantum-well vertical-cavity surface-emitting lasers with an eleven-period PbSe/PbSrSe quantum-well active region are demonstrated. The cw emissions are observed up to 230 K at the emission wavelengths around 4.03 μm. The lowest threshold pump density of 2.6 kW/cm2 appears at 190 K, 65 °C lower than that of the pulsed measurements. This indicates the temperature difference between the active region and the heat sink of the episide-up mounted on-wafer testing under cw pumping.

Unambiguous observation of subband transitions from longitudinal valley and oblique valleys in IV–VI multiple quantum wells

PbSe/PbSrSe multiple-quantum-well (MQW) structures were grown on BaF2(111) substrates by molecular-beam epitaxy and characterized by Fourier transform infrared transmission spectroscopy. To reduce unwanted Fabry–Perot interference fringes, the top surface of the MQW samples was coated with an anti-interference film, enabling clear observation of subband transitions without superposed interference fringes. Transition energies involving longitudinal and oblique valleys were unambiguously resolved and are in good agreement with calculations made using the envelope wave function approximation.

Molecular beam epitaxy growth of PbSe on BaF2-coated Si(111) and observation of the PbSe growth interface

Epitaxial growth of PbSe/BaF2/CaF2 heterostructures was carried out by molecular beam epitaxy (MBE) on Si(111) wafers. Successful transfer of 3-μm-thick PbSe epilayers was accomplished by bonding the MBE-grown samples face down to polished copper plates followed by the removal of the silicon substrate by dissolving the BaF2 buffer layer in water. High-resolution x-ray diffraction measurements demonstrated that the PbSe epilayer maintained high-crystalline quality after transfer. In addition, optical Nomarski characterization of the exposed growth interface showed sets of parallel straight step lines consistent with glide of dislocations in the primary {100}〈110〉 glide system. Such features are evidence of the large thermal expansion mismatch strain that occurred in these layers.

Study of band structure in PbSe/PbSrSe quantum wells for midinfrared laser applications

The electronic states in PbSe/Pb0.934Sr0.066Se multiple-quantum-well structures (MQWs) grown by molecular-beam epitaxy have been investigated both theoretically and experimentally for the midinfrared laser applications. With the aid of combined temperature-dependent photoluminescence and absorption measurements on a Pb0.934Sr0.066Se thin film for the effective masses and temperature-dependent band gaps, we find that the PbSe/PbSrSe MQWs have type-I band alignment and the conduction band offset ratio is Qc=0.82±0.03. The calculation, taking into account the strain, carrier confinements, and the multivalley band structure, can well explain both the observed luminescence peak energies and the temperature coefficient of the luminescence peaks as a function of well thickness.

Transfer of PbSe/PbEuSe epilayers grown by MBE on BaF2-coated Si(111)

Abstract Epitaxial growth of PbSe/PbEuSe/BaF 2 /CaF 2 heterostructures was carried out by a molecular beam epitaxy on Si(111) wafers. Successful transfer of 3-μm thick PbSe/PbEuSe epilayers was accomplished by bonding the MBE-grown samples face down to polished copper plates followed by the removal of the silicon substrate by dissolving the BaF 2 buffer layer in water. Nomarski microscopy and surface profile characterization showed that the transferred PbSe/PbEuSe layer had mirror-like morphology. High-resolution X-ray diffraction measurements demonstrated that the PbSe/PbEuSe epilayers maintained high crystalline quality after transfer, and surface analysis by X-ray photoelectron spectroscopy indicated complete removal of BaF 2 buffer layer. The above results indicate that bonded PbSe/PbEuSe could be used as a new substrate material for regrowth of high quality PbSe-related heterostructure devices.

Molecular beam epitaxial growth of IV–VI multiple quantum well structures on Si(111) and BaF2(111) and optical studies of epilayer heating

IV–VI semiconductor multiple quantum well (MQW) structures (PbSe/PbSrSe) were grown on Si(111) and BaF2(111) substrates by molecular beam epitaxy. Structural and optical properties of the MQW structures have been studied using reflection high-energy electron diffraction, high resolution x-ray diffraction (HRXRD), Fourier-transform infrared (FTIR) transmission, and midinfrared photoluminescence (PL). Numerous satellite diffraction peaks and narrow linewidths of the HRXRD rocking curves indicate the high crystalline quality of the structures grown on both Si and BaF2. Longitudinal and oblique valley subband transitions without superposed interference fringes were observed in FTIR differential transmission spectra. Continuous wave midinfrared photoluminescence was also observed from both sets of samples. Comparison of FTIR and PL spectra allows determination of localized epilayer heating in the MQW samples due to optical pumping. For a typical laser pumping power of 9.1 W/cm2 the heating in a MQW layer on Si...