Quantum-well heterostructure lasers

The various features peculiar to the operation of quantum-well semiconductor lasers are described and illustrated with data on single- and multiple-quantum-well AlxGa1-xAs-GaAs heterostructures grown by metalorganic chemical vapor deposition (MO-CVD). Photo-pumped and p-n diode lasers (injection lasers) are described that are capable of continuous room temperature (CW 300 K) operation. The basic problems of carrier collection, thermalization, and quantum-well band filling are considered and have made clear the limits on single quantum-well laser operation and how these can be overcome with multiple quantum-well active regions. The idea that the steplike density-of-states of a quantum-well heterostructure can improve the operation of a semiconductor laser is shown to be valid. Also, it is shown that phonon participation in the operation of a quantum-well laser, which was not anticipated, is a major (even dominant) effect, with perhaps the phonon emission itself in the compact active region being stimulated. Besides the obvious freedom that quantum-well layers offer in how the active region of a semiconductor laser can be designed, quantum-well lasers are shown to exhibit a lesser sensitivity of the threshold current density on temperature, which is explained in terms of the step-like density-of-states and the disturbed electron and phonon distributions in the quantum-well active regions. Values as high assim437degC have been obtained for T0in the usual expressionJ_{th}(T) = J_{th}(0) exp (T/T_{0}). Since photopumped multiple-quantum-well MO-CVD AlxGa1-xAs-GaAs heterostructures have operated as CW 300 K lasers with only 5-10 mW of photoexcitation (uncorrected for focusing and window losses,lambda sim 5145Å), it is suggested that quantum-well laser diodes can be made that will operate at ∼1 mA or even less excitation.     

Continuous operation of visible-spectrum in1-xGaxP1-zAszlaser diodes (6280 Å, 77 K)

Stripe-geometry In1-xGaxP1-zAsz(x approx 0.84-0.86, z approx 0.38-0.42) double heterojunction laser diodes, grown by liquid phase epitaxy (LPE) on vapor phase epitaxy (VPE) GaAs1-yPysubstrates, are described that operate (CW, 77 K) in the visible atlambda approx 6280-6360Å with differential quantum efficiencyeta_{ext} sim 28percent and power output in the range 1-7 mW.     

Continuous room-temperature photopumped laser operation of visible-spectrum LPE In1-xGaxP1-zAsz(λ ~ 6700 Å)

A modified version of the standard step cooled LPE growth process is described for the growth of visible-spectrum In1-xGaxP1-zAszdouble-heterostructure (DH) lasers on GaAs1-yPy(y sim 0.30). The quality of the quaternary epitaxial layers is improved by growing a "thick" uniform active region (∼1000 Å) by means of a multiple-layer stack of "thin" (∼100 Å) layers. Each of the "thin" layers grown (cyclically) from an equilibrium melt in the short growth period of <100 ms does not make a transition to the diffusion-limited conditions employed in standard LPE growth processes. This method of liquid-phase epitaxy has been used to grow In1-xGaxP1-zAsz-In1-x'Gax'- P1-z'Asz'(x sim 0.87, z sim 0.44; x' sim 0.66, z' lsim 0.01) DH's that operate as photopumped CW 300 K lasers atlambda sim 6700Å.     

Fabrication of short GaAs wet-etched mirror lasers and theircomplex spectral behavior

A versatile fabrication technique for GaAs-AlGaAs wet-etched mirror lasers is presented. This technique works independently of the Al concentration in the cladding layers up to a value of 70%, and it requires four photolithography steps. Ridge waveguide lasers have been successfully processed using a double heterostructure (DHS) as well as graded index separate confinement heterostructures (GRINSCH) having different quantum-well (QW) active layers. This technique is used to fabricate short-cavity lasers in GRINSCH structures having GaAs multiple-quantum-well (MQW) or bulk active layers. Laser operation was obtained in a 29-μm-long device using a 5-QW structure. Short lasers with QW active layers show a complex spectral behavior. These lasers operate at higher current densities (~20 kA/cm²) and emit light at more than one wavelength. This implies that higher order transitions are involved which is not the case when using a bulk GaAs active layer. Besides the two peaks corresponding to the n=1 and n=2 transitions, we found an intermediate peak which corresponds presumably to the forbidden transition E1-HH2     

Superlinear dependence of gain on current density in GaAs injection lasers

Measurements of threshold-current density and incremental efficiency on homostructure and heterostructure GaAs lasers show an increasingly superlinear dependence of gaingon current densityJwith increasing temperature. The indexnin the relationg propto J^{n}satisfies the relationn = [1 + (kT/E_{t})^{2}]^{1/2}where Etis a measure of the band-tail depth.     

Solid-phase epitaxial Pd/Ge ohmic contacts to In1-xGaxAsyP1-y/InP

A Pd/Ge metallization to InGaAsP/InP semiconductors, formed with solid-phase epitaxy (SPE) technique, has been investigated in this study. With this method, ohmic contacts with low specific contact resistance (rho_{c} approx 2.3 times 10^{-6}Ω.cm²) have been achieved on p-type In0.53Ga0.47As(p approx 1.8 times 10^{19}/cm³). The same contact scheme also gives low specific contact resistance (rho_{c} approx 6 times 10^{-7}Ω.cm²) on n-type In0.53Ga0.47As (n approx 1.0 times 10^{19}/cm³). Excellent surface morphology is observed in all the samples, and the contacts do not deteriorate for at least 4 h at temperatures between 300 and 500°C.     

On the dark currents in germanium Schottky-barrier photodetectors

It is shown that for experimentally observed values of Schottky-barrier height of metal-n-type germanium photodetector structures, the dominant component of dark current can be due to the injection of minority carriers, rather than to the usual majority-carrier component. For barrier heights approaching 0.6 eV, for doping concentrationsN_{d} lsim 10^{15}cm^-3, for short minority-carrier lifetimetau_{p} lsim 1µs, for narrow base widthW_{b} lsim 10µm, or combinations of these conditions, the minority-carrier injection ratio approaches unity and these devices behave in the same way as p⁺-n junctions, with identical dark currents. The low-temperature fabrication requirements and processing simplicity of germanium Schottky barriers makes these devices more attractive under these conditions than germanium p-n junction photodetectors.     

Quantum-well Inp-Inl−xGaxPl−zAsz heterostructure lasers grown by liquid phase epitaxy (LPE)

By means of computer-controlled liquid-phase epitaxy (LPE), undoped single-and multiple-quantum-well InP-In_l-xGa_xP_l-zAs_z (x∼0.13, z∼0.29) heterostructures of uniform well (Lz∼150 ?) and coupling barrier size (200–300? ) have been grown and examined in photolumi-nescence. Undoped samples are used to eliminate problems with impurities and to make possible study of the fundamental properties of quaternary quantum-well heterostructures. For a single quaternary quantum-well heterostructure of small enough well size (Lz∼150?), which still collects holes but not electrons (electron scattering length >L_Z), hot-electron (p²/2m_n∼ΔE_c) recombination from E_c(InP) to bound holes in the valence-band well (i.e., to E_v(InGaPAs)), leads to stimulated emission in a band ∼ 80 meV below E_g(InP) and thus to an estimate of Δ E_v∼ 80 meV (ΔE_c∼2×ΔE_v) for the InP-InGaPAs valence-band discontinuity. On rectangular samples of multiple quantum-well LPE InP-In_1-xGa_xP_1-zAs_z heterostructures laser operation has been identified on LO phonon sidebands located below the lowest confined-particle transitions. A 5-barrier, 6-well quaternary quantum-well heterostructure exhibits phonon-assisted laser operation shifted in energy below confined-particle transitions by an InGaAs-like phonon (ˉhω_L0∼31 meV) and for an 11-barrier, 12-well heterostructure shifted by an InP-like phonon (ˉhω_L0∼43 meV). The 11+12 quaternary quantum-well heterostructure is shown to be capable of operating as a room temperature continuous (CW, 300K) laser (ˉhω<E_g ) .     

Low-resistance ohmic contacts to AlGaAs/GaAs and In0.52Al0.48As/In0.53Ga0.47As modulation-doped structures obtained by halogen lamp annealing

The successful application of short-term halogen lamp annealing to form ohmic contacts to AlGaAs/GaAs and In0.52Al0.48As/ In0.53Ga0.47As modulation-doped structures is demonstrated. Use of Ti in the electron-beam evaporated metallization scheme and a two-step annealing cycle give contacts with reproducibly good electrical and morphological characteristics. Minimum values of specific contact resistancerho_{c} = 4.0 times 10^{-7}and6.0 times 10^{-7}Ω.cm²for AlGaAs/GaAs and In0.52Al0.48As/In0.53Ga0.47As, respectively, are measured. Corresponding values of the transfer resistance Rcare 0.12 ± 0.02 and 0.18 ± 0.05 Ω.mm. These values are the lowest achieved with lamp annealing and are comparable to the best obtained with transient furnace annealing.     

Narrow-band injection locking of a spectrally dense molecular laser system--HgBr (B → X)

Narrow-band injection locking of a discharge driven HgBr (Brightarrow X) laser withDeltaomega(FWHM) < 0.02 cm^-1(Deltalambda < 0.005Å) is reported. For injection fluxes as low as 3.3 W/cm²(Deltaomega= 0.015cm^-1), approximately 80 percent of the laser output Was in this band. These results and previous work are analyzed in terms of a Simple Fokker-Planck diffusion model of the HgBrB^{2}Sigmarotational manifold.     

Effect of band shapes on carrier distribution at high temperature

This paper indicates both experimentally and graphically how the distribution of carriers changes above a critical temperature Tc, which is related to the density of states ρ in the band tails. Thus,kT_{c} = E_{0}, whererho = rho_{0} exp (DeltaE/E_{0}). The emission spectrum shifts with changing excitation (band filling) at lower temperatures (T < T_{c}) and stays at a constant value independent of excitation at higher temperatures (T > T_{c}). Experimental results of photoluminescence studies with GaAs crystals show that the quantity E0, obtained from the dependence of the peak energy upon the degree of excitation is the same as the quantitykT'_{c}, obtained from the dependence of the peak energy upon temperature. This is in agreements with the analysis. For lasing diodes, the threshold will increase steeply with increasing temperature above Tc, if the excited carriers are located mostly in the exponential states of the band tail.E_{0} = (10 sim 20)meV and Tc= (120 sim 250degK) are typical for crystals of heavily compensated GaAs.     

High-transconductance p-channel modulation-doped AlGaAs/GaAs heterostructure FET's

p-channel modulation-doped AlGaAs-GaAs heterostructure FET's (p-HFET's) employing two-dimensional hole gas (2DHG) were fabricated under various geometrical device parameter conditions. The p-HFET characteristics were measured at 300 and 77 K for the following three device-parameter ranges: the gate length Lg(1-320 µm), the gate-source distance Lgs(0.5-5 µm), and the layer thickness dt(35-58 nm) of AlGaAs beneath the gate. Based on the obtained results, a high-performance enhancement-mode p-HFET was fabricated with the following parameters:L_{g} = 1µm,L_{gs} = 0.5µm, andd_{t} = 35nm. The achieved extrinsic transconductance gmwas 75 mS . mm^-1at 77 K. This experimental result indicates that a gmgreater than 200 mS . mm^-1at 77 K Can be obtained in 1-µm gate p-HFET devices.     

Temperature dependence of the lasing characteristics of the 1.3 µm InGaAsP-InP and GaAs-Al0.36Ga0.64As DH lasers

We report here our experimental observations on the temperature dependence of threshold current, carrier lifetime at threshold, external differential quantum efficiency, and gain of both the 1.3 μm InGaAsP-InP and GaAs-AlGaAs double heterostructure (DH) lasers. We find that the gain decreases much faster with increasing temperature for a 1.3 μm InGaAsP DH laser than for a GaAs DH laser. Measurements of the spontaneous emission observed through the substrate shows that the emission is sublinear with injection current at high temperatures for the 1.3 μm InGaAsP DH laser. Such sublinearity is not observed for GaAs DH lasers in the entire temperature range 115-350 K. The experimental results are discussed with reference to the various mechanisms that have been proposed to explain the observed temperature dependence of threshold of InGaAsP DH lasers. We find that inclusion of a calculated nonradiative Auger recombination rate can explain the observed temperature dependence of threshold current, carder lifetime at threshold, gain, and also the sublinearity of the spontaneous emission with injection current of the 1.3 μm InGaAsP-InP DH laser. Measurement of the nonradiative component of the carrier lifetime (τA) as a function of injected carrier density (n) shows thattau_{A}^{-1} sim n^{2.1}which is characteristic of an Auger process.     

Dispersion of the group velocity refractive index in GaAs double heterostructure lasers

The dispersion of the refractive index corresponding to the group velocityn*_{1}has been measured as a function of wavelength. It is obtained from the longitudinal mode spacing of GaAs buried heterostructure lasers at threshold. The dependence ofn*_{1}on wavelength contains an approximately constant term due to the refractive index n1and a strongly dispersive component due to-lambda (partialn_{1}/ partiallambda). For a given spectral bandwidth, the dispersion ofn*_{1}causes a temporal broadening of a pulse as it passes through the medium. This dispersive effect is shown to contribute to the width of 0.65 ps long pulses obtained recently from mode locked semiconductor lasers. By reducing the length of the laser, the dispersive effect is reduced and it is suggested that pulses as short as 10^-13s should be obtainable from such mode locked semiconductor lasers.     

Bandgap narrowing in moderately to heavily doped silicon

A model of bandgap reduction in silicon through the stored electrostatic energy of majority-minority carrier pairs is developed and compared with experimental results in the doping range from 3 × 10¹⁷to 1.5 × 10²⁰/cm³at room temperature. An analytic expression for the bandgap reduction in nondegenerate material is obtaineddeltaepsilon _{g} = 3q^{2}/(16pi epsilon) cdot (q^{2}n/epsilonkT)^1/2having a square-root dependence on the majority carrier concentration. At room temperature this becomesdeltaepsilon _{g} = 22.5 (n/10^{18})^{1/2}meV. In degenerate material, the bandgap reduction is independent of temperature, following the relationshipdelta epsilon _{g} = 162 (n/10^{20})^{1/6}meV. The experimental data at room temperature are in excellent agreement with this theory. Plots of bandgap narrowing as a function of doping level are presented for a number of temperatures.     

Narrow spectral width surface emitting LED for long wavelength multiplexing applications

We discuss a novel surface emitting InGaAsP LED with a relatively narrow spectrum obtained by the integral growth of a 2.6 μm thick semiconductor absorbing, or filtering, layer on a conventional double heterostructure device. The full width at half maximum for the filtered LED with peakpower atlambda_{0} = 1.29 mum isDeltalambda = 850Å (63 meV),compared with an unfiltered LED half width ofDeltalambda = 1300Å (97 meV). Half width measurements were made at a drive current density of 10 kA/cm². The optimization of the filtered LED is considered, and it is found that a substantial reduction of the power in the short wavelength spectral tail can be obtained without significantly decreasing the total LED output power. For these reasons, the filtered LED has potential applications for low cost wavelength division multiplexing systems where a low overlap of spectral output between adjacent wavelength channels is required. In addition, the narrow spectrum may prove useful in moderate bit rate (> 100 Mbits/s) multimode transmission systems where the output from broad spectrum, conventional surface emitting LED's leads to unacceptably high levels of signal dispersion.     

Graded barrier single quantum well lasers--Theory and experiment

Optical gain characteristics of quantum well heterostructure lasers are investigated in a broad range from a weakly quantized system to a strongly quantized one utilizing the band-to-band model withkselection rule, the detailed band structure of the quantum well heterostructures, and degenerate statistics. The results are compared to the experimentally observed threshold current densities in graded barrier single quantum well lasers. It is found that the threshold current densities of these quantum well lasers are well explained by the theoretical results in the range of well width from 75 to 300 Å. The lowest threshold current density observed is 245 A/cm²in a heavily doped 100 Å single quantum well laser with a 150μm stripe width and 550μm length.     

Temperature Characteristics of Gain Profiles in 1.3-$ muhbox{m} p$-Doped and Undoped InAs/GaAs Quantum-Dot Lasers

The modal gain and differential gain of 1.3-$muhbox{m}break p$ -doped and undoped InAs/GaAs quantum-dot (QD) lasers have been investigated as a function of injection current under different operation temperatures. The results show that $p$ -doping improves the modal and differential gains in QD lasers at high temperatures. Exponential decrease in the differential gain profiles were observed in both types of lasers from 20 $^{circ}hbox{C}$ to 80 $^{circ}hbox{C}$. Theoretical calculations based on the rate equation model for the undoped QD laser gain at different temperatures are presented.      

Operation of strained-layer (In,Ga)As quantum well lasers preparedon (112)B GaAs substrate

The operation of (In,Ga)As quantum well lasers prepared on [112]-oriented GaAs substrates is reported. Threshold current densities as low as 187 A/cm² for a 1.57 mm-long device have been obtained under pulsed-mode operation. The demonstration of laser action in a heterostructure grown on the [112]-oriented substrate is important for the design of blue-green light emitters by second harmonic generation     

Optimization and characterization of index-guided visible AlGaAs/GaAs graded barrier quantum well laser diodes

Index-guided, single and multiple stripe, visible laser diodes (lambda = 6950-7150Å) have been fabricated and characterized. These structures utilize a graded barrier quantum well laser structure having high aluminum composition (x = 0.60-0.85) confining layers to obtain low threshold current. The use of thin AlAs quantum well barrier layers allows short wavelengths to be obtained from the quantum size effect in binary GaAs wells without the need for alloy AlxGa1-xAs wells. Index-guiding is accomplished by use of either a complementary self-aligned structure or a shallow mesa laser structure allowing stabilized single-mode laser operation.