Effect of p-doping on the temperature dependence of differentialgain in FP and DFB 1.3-μm InGaAsP-InP multiple-quantum-well lasers

The temperature dependence of differential gain dG/dn for 1.3-μm InGaAsP-InP FP and DFB lasers with two profiles of p-doping was obtained from RIN measurements within the temperature range of 25°C-65°C. Experiments showed that the change of the active region doping level from 3·10¹⁷ cm^-3 to 3·10¹⁸ cm^-3 leads to a 50% increase of the differential gain for FP lasers at 25°C. Heavily doped devices also exhibit more rapid reduction of the differential gain with increasing temperature. The effect of active region doping on the energy separation between the electron Fermi level and electronic states coupled into the laser mode explains the observations. The temperature dependence of differential gain for DFB devices strongly depends on the detuning of the lasing wavelength from the gain peak     

Carrier transport related analysis of high-power AlGaN/GaN HEMTstructures

Shubnikov-de Haas (SdH) oscillation and Hall measurement results were compared with HEMT DC and RF characteristics for two different MOCVD grown AlGaN-GaN HEMT structures on semiinsulating 4H-SiC substrates. A HEMT with a 40-nm, highly doped AlGaN cap layer exhibited an electron mobility of 1500 cm²/V/s and a sheet concentration of 9×10¹² cm at 300 K (7900 cm²/V/s and 8×10¹² cm^-2 at 80 K), but showed a high threshold voltage and high DC output conductance. A 27-nm AlGaN cap with a thinner, lightly doped donor layer yielded similar Hall values, but lower threshold voltage and output conductance and demonstrated a high CW power density of 6.9 W/mm at 10 GHz. The 2DEG of this improved structure had a sheet concentration of n_SdH=7.8×10¹² cm^-2 and a high quantum scattering lifetime of τ_q=1.5×10^-13 s at 4.2 K compared to n_SdH=8.24×10¹² cm^-2 and τ_q=1.72×10^-13 s for the thick AlGaN cap layer structure, Despite the excellent characteristics of the films, the SdH oscillations still indicate a slight parallel conduction and a weak localization of electrons. These results indicate that good channel quality and high sheet carrier density are not the only HEMT attributes required for good transistor performance     

Room temperature stimulated emission from a CdTe/CdMgTesuperlattice laser structure in the red spectral range

We have investigated the optical response of a red emitting CdTe/CdMgTe laser structure after optical excitation. For the first time, stimulated emission has been observed in this material system at room temperature, demonstrating the potential for the development of CdTe/CdMgTe laser structures in the visible spectral range. An analysis of our experimental data yields a threshold carrier density of about 3.5·10¹² cm^-2 and a net gain coefficient of about 95 cm^-1 at T=300 K     

Monte Carlo study of electron relaxation in quantum-wire laserstructures

Very-low threshold currents are expected to be achieved in quantum-wire lasers owing to the singularity in the density of states occurring at the bandedge. On the other hand, the high-speed modulation of quantum-wire lasers may be limited by carrier relaxation processes that are greatly affected by the reduction in the momentum space. In this paper, we calculate the electron relaxation times for GaAs/AlGaAs wires of various cross sections assuming that electrons are injected in a thermal distribution at the edge of the potential well formed by the barrier. The relaxation times are extracted from the time evolution of the carrier distribution as the electrons come to thermal equilibrium with the lattice. The Monte Carlo method is used to simulate the details of the relaxation process with the inclusion of electron-bulklike phonon, electron-electron and electron-hole interactions. We find that the electron relaxation times range from 120 ps for the 100×100 Å wire to 30 ps for the 200×200 Å wire for a carrier density of 10¹⁸ cm^-3. When the electron-hole interaction is included into the calculations, the equilibration time for the 100×100 Å wire is reduced to ≈50 ps. Screening effects are incorporated using the Thomas-Fermi formalism. At a carrier concentration of 10¹⁶ cm^-1, the equilibration times for the corresponding wire sizes are 20 and 5 ps. Thus, the relaxation time calculated within the limits of our model decreases with an increased wire cross section. This trend indicates the presence of a trade-off between speed and efficiency in quantum-wire lasers considering that the threshold current is decreased by reducing the wire cross section     

Strong increase of the derivative of the carrier-induced indexchange of semiconductor lasers at low injected carrier density

The carrier-induced index change of a semiconductor laser was measured for injected carrier density ranging from 3 × 10¹⁶ cm^-3 to 2 × 10¹⁸ cm^-3. A strong nonlinear behavior between index change and carrier density is observed. The derivative of the index change versus carrier density at low carrier density can be 15 times larger than the derivative of the index change at high carrier density     

High performance poly-Si TFTs fabricated using pulsed laserannealing and remote plasma CVD with low temperature processing

Key technologies for fabricating polycrystalline silicon thin film transistors (poly-Si TFTs) at a low temperature are discussed. Hydrogenated amorphous silicon films were crystallized by irradiation of a 30 ns-pulsed XeCl excimer laser. Crystalline grains were smaller than 100 nm. The density of localized trap states in poly-Si films was reduced to 4×10¹⁶ cm^-3 by plasma hydrogenation only for 30 seconds. Remote plasma chemical vapor deposition (CVD) using mesh electrodes realized a good interface of SiO ₂/Si with the interface trap density of 2.0×10¹⁰ cm^-2 eV^-1 at 270°C. Poly-Si TFTs were fabricated at 270°C using laser crystallization, plasma hydrogenation and remote plasma CVD. The carrier mobility was 640 cm²/Vs for n-channel TFTs and 400 cm²/Vs for p-channel TFTs. The threshold voltage was 0.8 V for n-channel TFTs and -1.5 V for p-channel TFTs. The leakage current of n-channel poly-Si TFTs was reduced from 2×10^-10 A/μm to 3×10^-13 A/μm at the gate voltage of -5 V using an offset gate electrode with an offset length of 1 μm     

Measurement of differential carrier lifetime in vertical-cavitysurface-emitting lasers

Measurements of differential carrier lifetimes on gain-guided proton-implanted vertical-cavity surface-emitting lasers with device size as a parameter are reported. The lifetimes were obtained from laser impedance measurements and from small-signal modulation optical response at subthreshold currents. A simple small-signal equivalent circuit was used to correct the optical data and to extract the carrier lifetimes from the impedance data. Carrier lifetimes ranged from 4.2 ns at 0.04 mA, to about 0.6 ns at a bias close to threshold. The measured carrier lifetimes were used to calculate the corresponding threshold carrier density (n_th~6×10¹⁸ cm^-3) and recombination parameters     

Noise performance of InGaAs-InP quantum-well infraredphotodetectors

Dark current noise measurements were carried out between 10 and 10 ⁴ Hz at T=80 K on two InGaAs-InP quantum-well infrared photodetectors (QWIPs) designed for 8-μm infrared (IR) detection. Using the measured noise data, we have calculated the thermal generation rate, bias-dependant gain, electron trapping probability, and electron diffusion length. The calculated thermal generation rate (~7×10 ²² cm^-3·s^-1) is similar to AlGaAs-GaAs QWIPs with similar peak wavelengths, but the gain is 50× larger, indicating improved transport and carrier lifetime are obtained in the binary InP barriers     

1.3-μm n-type modulation-doped AlGaInAs/AlGaInAsstrain-compensated multiple-quantum-well laser diodes

In this paper, we report the fabrication and characterization of 1.3-μm AlGaInAs/AlGaInAs laser diodes (LDs) with an n-type modulation-doped strain-compensated multiple-quantum-well (MD-SC-MQW) active region and a linearly graded index separate confinement heterostructure. The barrier in the MD-SC-MQW active region contains the 28 Å Si-doped modulation-doped region and two 29 Å surrounding undoped regions that serve to prevent the overflow of Si doping atoms into the wells. We investigate the threshold current density, infinite current density, differential quantum efficiency, internal quantum efficiency, internal optical loss, threshold gain (for the cavity length of 300 μm), and transparency current density as a function of doping concentration in the n-type AlGaInAs barrier for the 1.3-μm MD-SC-MQW LDs. The theoretical and experimental results show that the optimum doping concentration of doped barriers is 5×10 ¹⁸ cm^-3. With this optimum condition, the 3.5-μm ridge-striped LDs without facet coating will exhibit a lower threshold current and a higher differential quantum efficiency of 18 mA and 52.3% under the CW operation as compared to those of 22 mA and 43% for the undoped active region, respectively. In addition, a high characteristic temperature of 70 K, a low slope efficiency drop of -1.3 dB between 20 and 70°C, and a wavelength swing of 0.4 nm/°C for the LDs operated at 60 mA and 8 mW can be obtained in the LDs with doped barriers     

Low-threshold room-temperature operation of injectorless quantum-cascade lasers: influence of doping density

The influence of the doping density in the active sections of InP-based injectorless quantum cascade lasers, emitting at 6.8 mum, is investigated. The doping sheet density is varied in the range 2.5-8.6times10¹⁰ cm^-2. Lasing is observed in the whole range, with a threshold current density as low as 1.2 kA/cm² at 300 K for the smallest doping sheet density of 2.5times10 ¹⁰ cm^-2. Further improvement has been made by additionally increasing the number of periods in the active region from 40 to 60. With the same doping level of 2.5times10¹⁰ cm^-2 record low threshold current densities of 0.73 kA/cm² at 300 K were achieved     

Experimental studies of proton-implanted GaAs-AlGaAsmultiple-quantum-well modulators for low-photocurrent applications

We describe the first attempts to control photocurrent, and thus power dissipation, in surface-normal multiple-quantum-well (MQW) modulators. We have made detailed experimental studies of proton-implanted p-i-n GaAs-Al_xGa_1-xAs MQW modulators having barrier layers of x=0.3, 0.45, and 1.0. Structures were implanted to levels of 1×10¹² cm^-2, 1×10¹³ cm^-2, and 1×10¹⁴ cm ^-2. Photocurrent progressively decreased with increasing implant-dose and barrier mole fraction (x). Exciton linewidths showed a strong voltage and implant dose dependence, demonstrating a tradeoff between photocurrent and modulation performance. We obtained our best results with x=1.0 barriers. For example, 1×10¹³ cm^-2-implanted asymmetric Fabry-Perot modulators were realized in which the optical performance was similar to that of unimplanted devices. The photocurrent responsivity was, however, only 0.007 A/W at 12.5 V bias. We report measurements of carrier lifetime in these materials that show the reduction in photocurrent arises from a reduction in lifetime due to implant-induced damage. In addition, the reduced lifetime decreases the optically-excited quantum-well carrier population, leading to an increase in cw saturation intensity. Specifically, 1×10¹³ cm^-2-implanted devices with x=1.0 have a saturation intensity of roughly 45 kW/cm², while unimplanted devices have 3.5 kW/cm². Asymmetric self electro-optic effect devices (A-SEED's) are demonstrated, and power dissipation issues associated with the use of low-photocurrent modulators in integrated systems are discussed     

Experimental study of gain and output coupling characteristics of aCW chemical oxygen-iodine laser

Gain and output coupling characteristics of the CW chemical oxygen-iodine laser (COIL) are determined experimentally by means of varying the output coupling method. Under the conditions that the Cl₂ flow rate is 11.8 mmol/s, the I₂ molar flow rate is from 20 to 50 μmol/s, and the duct pressure is 200 Pa, the following were obtained from the experimental data: maximum values of output power of 58 W, and optimal output coupling factor of 1.50%, a resonator efficiency of 4.8%, an unsaturated small-signal gain of 1.55×10^-3 cm^-1, a threshold small-signal gain of 1.31×10^-3 cm^-1, a saturation intensity of 1150 W/cm², intraresonator losses of 9%, and an atomic iodine concentration of 2.85×10¹⁴ cm^-3. A comparison of these results to the published data of other COIL systems is presented     

Properties of a tunneling injection quantum-well laser: Recipe for`cold' device with a large modulation bandwidth

A quantum-well laser in which electrons are directly injected into the lasing quantum well by resonant tunneling is proposed and demonstrated. The preliminary GaAs-based devices, grown by molecular beam epitaxy have an 80-Å In_0.1Ga_0.9As active single quantum well and AlAs tunneling barriers. I_th is 15 mA in a single-mode ridge device and the differential gain is ~2×10^-16 cm^-2. The principle of operation promises a `cold' laser at high injection levels, and therefore Auger recombination and chirp are expected to be suppressed. In addition, tunneling of carriers into the active well as the potential to achieve large modulation bandwidths     

Characteristics of laser diodes with a partially intermixedGaAs-AlGaAs quantum well

Laser diodes (LD's) with a partially intermixed quantum-well (QW) active layer are fabricated by Zn out-diffusion from a p-cladding layer to the QW region. The dependencies of the degree of intermixing, measured by the photoluminescence (PL) shift, on Zn concentration of the p-cladding layer (P_clad) and the Al content of the guiding layer (X_g) in a separate-confinement-heterostructure (SCH) are investigated. P_clad changes in the range from 1×10 ¹⁸ cm^-3 to 4×10¹⁸ cm^-3 and X_g changes in the range from 0.21-0.37. When P_clad is 2×10¹⁸ cm^-3 and X_g is 0.37, large bandgap energy shift of 96.1 meV is observed. The lasing wavelengths of the LD's, with the partially intermixed QW, are blue-shifted linearly with increasing P_clad and X_g. For the bandgap energy shift of 66.8 meV by PL, the threshold current density is increased by 33% from that of the nonintermixed LD. Reliability of LD's with the partially intermixed QW is investigated for the first time. In spite of a large degree of intermixing the reliability of the LD with the partially intermixed QW of 66.8 meV energy shift by PL is the same as the nonintermixed one, which is confirmed by the aging test of 2500 hours at 45°C with the output power of 1 W under CW operation     

Emission cross sections and spectroscopy of Ho3+ laserchannels in KGd(WO4)2 single crystal

The spectroscopic properties of Ho³⁺ laser channels in KGd(WO₄)₂ crystals have been investigated using optical absorption, photoluminescence, and lifetime measurements. The radiative lifetimes of Ho³⁺ have been calculated through a Judd-Ofelt (JO) formalism using 300-K optical absorption results. The JO parameters obtained were Ω₂=15.35×10^-20 cm², Ω ₄=3.79×10^-20 cm², Ω₆ =1.69×10^-20 cm². The 7-300-K lifetimes obtained in diluted (8·10¹⁸ cm^-3) KGW:0.1% Ho samples are: τ(⁵F₃)≈0.9 μs, τ( ⁵S₂)=19-3.6 μs, and τ(⁵F₅ )≈1.1 μs. For Ho concentrations below 1.5×10²⁰ cm^-3, multiphonon emission is the main source of non radiative losses, and the temperature independent multiphonon probability in KGW is found to follow the energy gap law τ_ph ^-1(0)=βexp(-αΔE), where β=1.4×10^-7 s^-1, and α=1.4×10³ cm. Above this holmium concentration, energy transfer between Ho impurities also contributes to the losses. The spectral distributions of the Ho³⁺ emission cross section σ_EM for several laser channels are calculated in σ- and π-polarized configurations. The peak a σ_EM values achieved for transitions to the ⁵I₈ level are ≈2×10^-20 cm² in the σ-polarized configuration, and three main lasing peaks at 2.02, 2.05, and 2.07 μm are envisaged inside the ⁵I₇→⁵I₈ channel     

Optical Gain of V—groove Zn1—xCdxSe／ZnSe Quantum Wires

The subband structures, distributions of electron and hole wave functions, state density, optical gain spectra, and transparency carrier density of the V-groove Zn 1-x Cd x Se/ZnSe quantum wires are investigated theoretically using four band effective-mass Hamiltonian, which takes into account the effects of the valence band anisotropy and the band mixing. The biaxial strain effect for quantum wires is included in the calculation. The compressive strain in the Zn 1-x Cd x Se wire region increases the energy separation between the uppermost subbands. The optical gain with xy -polarized light is enhanced, while optical gain with z -polarized light is strongly decreased. The xy -polarized optical gain spectrum has a peak at around 2.541 eV, with the transparency carrier density of 0.75×10 18 cm -3 . The calculated results also show that the strain tends to increase the quantum confinement and enhance the anisotropy of the optical transitions.     

Interferometric measurement of the linewidth enhancement factor ofa 1.55-μm strained multiquantum-well InGaAs/InGaAsP amplifier

The linewidth enhancement factor of an InGaAs/InGaAsP strained multiquantum well optical amplifier was measured interferometrically. It varied from 3 to 18 over the wavelength range from 1500 to 1600 nm with injection currents varying from one to four times the lasing threshold of the uncoated device. A rate equation model gave differential gain and refractive index change per carrier, respectively, in the range 0.3 to 2.5×10^-15 cm² and -5 to -8×10^-20 cm³     

Gain spectra of quantum wires with inhomogeneous broadening

The effects of fabricational variations on the gain spectra of quantum wires are calculated within the limits of first-order perturbation theory. Gain spectra and density of states for 50-Å-radius and 150-Å-radius cylindrical quantum wires are calculated and plotted for several different fabrication tolerances. The wave functions for a finite, cylindrical potential are calculated and a quasicritical radius, below which the carriers are weakly confined by the potential, is established. This sets a lower limit on quantum wire size. Upper limits on the size of quantum wells, quantum wires, and quantum boxes are also discussed. The threshold current and differential gain of quantum-wire lasers and quantum wire array lasers are calculated. These calculations indicate a possible reduction in threshold current of one to two orders of magnitude as compared to the best quantum-well lasers     

Optically pumped intersubband lasers based on quantum wells

A far infrared (FIR) laser based on intersubband transitions in quantum wells is proposed where a pumping laser is used to create population inversion in the structure. The goal is to develop a structure which operates essentially as a 4-level laser, to minimize bottlenecking of the lower laser state. Multiple quantum wells can be used in the active laser of these structures to enhance the laser gain and the minimum required reflectivity in the cavity structure. The possibility of using both conduction and valence band quantum-well structures are investigated. Our study shows that, due to high intersubband scattering rates in the valence band structure, the creation of population inversion is more difficult and requires a high pumping power density while in the conduction band structure, population inversion can be achieved by a moderate pumping power density. The maximum population inversion in the conduction band structure is estimated to be 2.1×10¹¹ cm², which requires a pumping power density 2 kW cm^-2 for a single quantum well. The threshold power as well as the minimum required reflectivity of the cavity structure for the conduction band scheme are estimated for different well numbers     

Tensile-strained GaInAsP-InP quantum-well lasers emitting at 1.3μm

Tensile-strained GaInAsP-InP quantum-well (QW) lasers emitting at 1.3 μm are investigated. Low-pressure metalorganic chemical vapor deposition (LP-MOCVD) is used for crystal growth. High-resolution X-ray diffraction shows good agreement with theoretical simulation, photoluminescence spectra have good energy separation between light-hole and heavy-hole bands due to biaxial tension. The lowest threshold current density for infinite cavity length J_th/N_w ^∞ of 100 A/cm² is obtained for the device with -1.15% strain and N_w=3. The amount of strain which gives the lowest J_th/N_w^∞ experimentally clarified is around -1.2%. Threshold current of a buried-heterostructure (BH) laser is reduced to be as low as 1.0 mA. Enhanced differential gain of 7.1×10^-16 cm² is also confirmed by measurements of relative intensity noise. Much improved threshold characteristic with the feasibility of submilliamp threshold current can be achievable by optimizing the BH structure. The tensile-strained QW laser emitting at 1.3 μm with very low power consumption is attractive for the light source of fiber in the loop system and optical interconnection applications