Variable magnetic field Hall effect, photoluminescence, and capacitance-voltage ͑CV͒ analysis have been used to study InN layers grown by plasma assisted molecular beam epitaxy. All three techniques reveal evidence of a buried p-type layer beneath a surface electron accumulation layer in heavily Mg-doped samples. Early indications suggest the Mg acceptor level in InN may lie near 110 meV above the valence band maximum. The development of p-type doping techniques offers great promise for future InN based devices.

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We present the structural and photoluminescence properties of 30 keV gadolinium implanted and subsequently annealed zinc oxide (ZnO) single crystals. Rutherford backscattering and channeling results reveal a low surface region defect density which was reduced further upon annealing. For low implantation fluence, around 85% of the Gd atoms are estimated to be in sites aligned with the ZnO lattice, while for higher fluences the Gd is largely disordered and likely forms precipitates. The Raman spectra of the implanted samples show defect-induced modes, which match the one-phonon density of states for the most heavily implanted samples. Annealing eliminates these features implying the removal of Gd-associated lattice disorder. Low temperature photoluminescence spectra revealed a red-shift in the defect emission, from green to orange/yellow, indicating the suppression of a deep level, which is thought to be due to oxygen vacancies. It is suggested that the orange/yellow emission is unmasked when the green emission is quenched by the presence of the implanted Gd atoms.

The optical properties of bulk ZnO ion implanted with nitrogen ions, at an energy of 23 keV have been studied as a function of implantation fluence and electron beam (EB) annealing conditions. Nuclear reaction analysis and Raman results have revealed the implanted N concentration and its structural changes with respect to various nitrogen ion fluences. The optical properties of nitrogen implanted bulk ZnO were investigated by low temperature photoluminescence measurements. An enhanced peak at 3.235 eV has been attributed to donor-accepter pair (DAP) emission involving the implanted N acceptor in ZnO. The emission near 3.3085 eV is attributed to a free electron to acceptor transition. We also report a broad band emission feature at ∼3.09 eV in the nitrogen implanted with 1–2×1015 ions cm−2 and EB annealed at 800–900 °C. This is assigned to a thermally activated nitrogen acceptor transition as it is unique only to nitrogen implanted samples. An ionization energy of 377 meV indicates that this line may correspond to a significantly less shallow acceptor level. In addition an increase in the intensity and dominance of this DAP line in nitrogen implanted samples over the other acceptor transitions was observed with increasing annealing time and temperatures. It is shown that EB annealing offers a method of enhanced nitrogen activation when compared to a more conventional furnace approach.

Stimulated Stokes emission has been observed from silicon crystals doped by antimony donors when optically excited by radiation from a tunable infrared free electron laser. The photon energy of the emission is equal to the pump photon energy reduced by the energy of the intervalley transverse acoustic (TA) g phonon in silicon ( 2:92 THz). The emission frequency covers the range of 4.6 -5.8 THz. The laser process occurs due to a resonant coupling of the 1s E and 1s A 1 donor states (separation 2:97 THz) via the g-TA phonon, which conserves momentum and energy within a single impurity center. DOI: 10.1103/PhysRevLett.96.037404 PACS numbers: 78.47.+p, 41.60.Cr, 42.65.Es, 71.55.Cn In the past few years, significant progress has been made towards silicon based lasers. Silicon is an indirect band gap semiconductor. Therefore, it is difficult to realize an efficient process for light amplification. This fact compels the search for nontraditional approaches for light generation. In the near infrared region, numerous approaches (see Ref.[1] for a review) to overcome this difficulty, such as silicon nanocrystals [2,3], Si=SiO 2 [4] and Si=SiGe [5] superlattices, porous silicon [6], erbium-doped silicon [7], and silicon light-emitting diodes [8], have been attempted. Recently, the first silicon lasers operating at 1:540 and 1:675 m based on stimulated Raman scattering have been reported [9,10]. The Raman effect in silicon occurs via scattering of photons by optical phonons of the crystal. The strongest Stokes emission is due to the threefold degenerate short-wavelength optical modes at the center of the Brillouin zone of silicon [11].The first silicon laser was realized by infrared optical excitation of group-V donor centers embedded in a silicon host lattice [12,13]. For this type of laser, the interaction between phonons and electrons is essential. Except zonecentered optical phonons, intervalley acoustic and optical phonons have been found to play a decisive role in energy and momentum relaxation for nonequilibrium charge carriers in silicon [14,15]. Strong resonant intervalley phononimpurity interaction has been observed in absorption spectra of Bi donors in silicon [16]. When the phonon energy does not exactly coincide with the energy between two impurity states, the contributions from the electronic impurity state and the phonon-related part form a ''mixed'' state, as in silicon doped by Ga [17]. These interactions play an important role in the formation of population inversion between excited states, eventually leading to lasing on particular intracenter transitions in Si:Bi [18] as well as in Si:As [19]. A different situation occurs in silicon doped by antimony (Si:Sb). The energy of the transverse acoustic g-TA intervalley phonon and the energy between the 1s E and 1s A 1 states are almost equal ( 12 and 12.27 meV, respectively). This enhances the nonradiative electronic relaxation between these states. Terahertz intracenter laser emission in Si:Sb has been observed when the crystal was excited by radiation fr...

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We have investigated the importance of intervalley (Γ–Χ) electron transfer between Γ-point quantum well states and X-point barrier states in GaAs-based quantum cascade lasers with indirect band gap AlAs barriers. A series of samples has been studied in which the energy separation between the coupled injector/upper laser levels and the lowest confined X state in the injection barrier is varied. We demonstrate that for lasing to occur, electron injection into the upper laser level must proceed via Γ states confined below the lowest X state in the injection barrier. The limit this places on the minimum operating wavelength (λ≈8 μm) for the present laser design is overcome by utilizing a double injection barrier to achieve lasing at λ=7.2 μm.

We have directly determined with pump/probe spectroscopy the light hole (LH1) excited state lifetime for the lowest heavy hole to light hole intrawell subband transition (HH1-LH1) for three prototype samples of Si/SiGe strain-symmetrized multi-quantum well structures, designed to have the final LH1 state increasingly unconfined. The transition energy is below the optical phonon energy. We find that a decay time of 20 ps for sample 1 with a well width of 5.0 nm lengthens to 40 ps for sample 3 with a well width of 3.0 nm, in good agreement with the design. In addition, we have measured the lifetime for holes excited out of the well, from which we determine the lifetime for diagonal transitions (back into the well) to be of approx. several hundred picoseconds.