Yeeloy Lam

Tunneling injection lasers: a new class of lasers with reduced hot carrier effects

In conventional quantum-well lasers, carriers are injected into the quantum wells with quite high energies. We have investigated quantum-well lasers in which electrons are injected into the quantum-well ground state through tunneling. The tunneling injection lasers are shown to have negligible gain compression, superior high-temperature performance, lower Auger recombination and wavelength chirp, and better modulation characteristics when compared to conventional lasers. The underlying physical principles behind the superior performance are also explored, and calculations and measurements of relaxation times in quantum wells have been made. Experimental results are presented for lasers made with a variety of material systems, InGaAs-GaAs-AlGaAs, InGaAs-GaAs-InGaAsP-InGaP, and InGaAs-InGaAsP-InP, for different applications. Both single quantum-well and multiple quantum-well tunneling injection lasers are demonstrated.

Photoluminescence and electro‐optic properties of small (25–35 nm diameter) quantum boxes

The luminescence and electro‐optic properties of buried 25–35 nm quantum boxes have been measured. The quantum boxes were defined by a combination of molecular beam epitaxial growth and regrowth, electron beam lithography, and dry etching. The photoluminescence from 35 nm boxes shows a blue shift of ∼15 meV compared to the bulk luminescence and an enhancement, taking into account the fill factor. An enhanced effective linear electro‐optic coefficient, rl, is observed for the quantum boxes.

Comparison of steady-state and transient characteristics of lattice-matched and strained InGaAs-AlGaAs (on GaAs) and InGaAs-AlInAs (on InP) quantum-well lasers

Numerical techniques are developed to study the output spectra and to solve the multimode coupled rate equations including transverse electric (TE) and transverse magnetic (TM) propagations for In/sub x/Ga/sub 1-x/As-Al/sub 0.3/Ga/sub 0.7/As and In/sub 0.53+x/Ga/sub 0.47-x/As-Al/sub 0.48/In/sub 0.52/ quantum-well lasers. Optical properties are calculated from a 4*4 k*p band structure, and strain effects are included with the deformation potential theory. It is found that an introduction of 1.4% compressive strain to the quantum well results in roughly 3-4 times improvement in the intrinsic static characteristics in terms of lower threshold current, greater mode suppression and lower nonlasing photon population in the laser cavity. The authors identify the effect of strain on the large signal temporal response. They also include calculated CHSH Auger rates in their model. >

Carrier capture and relaxation in narrow quantum wells

In separate confined heterostructure (SCH) lasers, injected electrons and holes thermalize into a quantum well after diffusion through the outer cladding layers. The carriers move towards equilibrium by emitting optical phonons. In narrow quantum wells, as compared to the 1-2 ps required in bulk semiconductors, this phonon emission process can be considerably slowed down due to the 2-D density of states and the nature of the electron-optical phonon interaction. This process has been studied theoretically using a Monte Carlo program which allows us to see the carrier distribution as a function of time. Typical times for carrier relaxation are 10-15 ps for a 50 /spl Aring/ GaAs well with Al/sub 0.30/Ga/sub 0.70/As barriers and /spl sim/5 pS for a 200 /spl Aring/ well. These calculations have been complemented by time-resolved photoluminescence measurements on SCH structures where the relaxation time from a 3D distribution into In/sub 0.20/Ga/sub 0.80/As/GaAs wells is measured at T=200 K. Carrier relaxation times of 50, 41, 22, and 17 ps are obtained for wells of sizes 30, 40, 50, and 100 /spl Aring/, respectively. The results show clearly that the use of narrow quantum wells in low threshold lasers will pose a serious limitation to the efficiency and small-signal modulation bandwidth of these devices. >

Auger recombination rates in compressively strained In/sub x/Ga/sub 1-x/As/InGaAsP/InP (0.53<or=x<or=0.73) multiquantum well lasers

The authors have experimentally determined Auger recombination rates in compressively strained In/sub x/Ga/sub 1-x/As/InGaAsP/InP MQW lasers for the first time. The Auger recombination rates were derived from the measured turn-on delay times during large-signal modulation of single-mode lasers. The Auger coefficient increases from 5+or-1*10/sup -30/ to 13+or-1*10/sup -30/ cm/sup 6/ s/sup -1/ as the indium composition in the quantum well active region, x, increases from 0.53 to 0.73.<<ETX>>

Monte Carlo studies on the well‐width dependence of carrier capture time in graded‐index separate confinement heterostructure quantum well laser structures

The total carrier capture time and the quantum well width are both important parameters affecting the graded‐index separate confinement heterostructure (GRINSCH) quantum well laser modulation speed limit. However, discrepancies exist in the literature on the well‐width dependence of the carrier capture times. To study this phenomenon, we have developed a Monte Carlo technique to simulate carrier relaxation in GRINSCH quantum well structures. Our results show that the carrier capture time increases with the density of carrier injection. Furthermore, depending on the concentration of injected carriers, the capture time will either decrease, remain the same, or increase with increases in the well width. At lasing conditions, the times are more or less independent of the well width up to 100 A. We compare our calculations to published experiments and find good agreements.

Studies of carrier relaxation in low dimensional structures

Abstract We have made theoretical and experimental studies of the carrier relaxation process in quantum wells and quantum wires. Monte Carlo studies of the relaxation process in wells and wires indicate that the relaxation times can be very large in narrow structures. Femtosecond differential transmission spectra measurements on similar structures grown by molecular beam epitaxy give relaxation times in good agreement with the theoretically calculated values.

Monte Carlo analysis of the carrier relaxation processes in linear- and parabolic-GRINSCH quantum well laser structures

The carrier relaxation process is widely acknowledged to have a strong bearing on the modulation limit of quantum well lasers. As a first and crucial step toward achieving a better understanding of this phenomenon, we have developed a numerical technique to study such processes in graded-index separate confinement heterostructure quantum well laser structures having an arbitrary grading profile. We base our approach on ensemble Monte Carlo simulation of the carrier transport in the 3-D graded-index region and in the 2-D quantum well. We also introduce a technique to handle the carrier capture and re-emission processes within the Monte Carlo method. The results obtained from our calculations for a number of structures with quantum well sizes 50-100 /spl Aring/ indicate that the overall carrier capture time is about 5-7.5 ps under low injection condition for the linearly graded structures, and significantly longer for the parabolically graded structures. On the other hand, the carrier capture efficiency is found to be higher for the parabolic graded-index structures. We also compare our calculations to published experiments and find good agreement. >

Effects of strain on the high speed modulation of GaAs- and InP-based quantum-well lasers

A small-signal numerical analysis of pseudomorphic GaAs- and InP-based Fabry-Perot quantum-well lasers using calculated optical gain spectra with strain effects included is reported. Examination of the effect of lifetime broadening shows that the resonance frequency increases at a rate of approximately 250-MHz/meV reduction in the lifetime broadening for a GaAs-based strained layer laser. The modulation speed is limited by either device heating or facet damage. If the limitation is imposed by the optical power then the modulation speed increases as the laser cavity becomes shorter and the number of quantum wells increases. If the limitation is imposed by the injection current density, however, then the modulation speed decreases for the laser with shorter cavity length. The highest modulation speed is given by an optimum well number. A resonance frequency of approximately 16 GHz is predicted for a pseudomorphic GaAs-based laser with 30% excess In and average output power of approximately 5 mW. >

Monte Carlo simulation of gain compression effects in GRINSCH quantum well laser structures

Gain compression is widely acknowledged to be a serious limitation to the ultimate modulation bandwidth of a semiconductor laser. We have developed a numerical technique to study the gain compression effects in graded-index separate confinement heterostructure (GRINSCH) quantum well laser structures, This technique is based on the combination of the Monte Carlo simulation of the carrier dynamics in the device while under intense stimulated photon emission, and the calculation of the optical gain using a 4/spl times/4 k/spl middot/p Hamiltonian. From the simulated results, we calculated a gain compression coefficient /spl epsiv/=1.1/spl times/10/sup -17/ cm/sup 3/ for a linearly graded quantum well laser structure having a 50 /spl Aring/ In/sub 0.2/Ga/sub 0.8/As well. We find good agreement between our results and published experiments. We have also demonstrated that our calculation method is capable of simulating the gain dynamics in the laser structure, such as those studied with femtosecond pump-probe experimental techniques. >