R. Grey

Strained and strain-balanced quantum well devices for high-efficiency tandem solar cells

Abstract The state of GaAs/InGaAs quantum well solar cell research is reviewed. The effect of strain upon the GaAs/InGaAs cells is discussed and the limits to a strained GaAs/InGaAs cell established. The strain-balance approach is suggested as a means of overcoming the limits inherent to the strained approach and the principle is demonstrated in two differing device configurations. The strain-balance devices show enhanced efficiencies over their strained counterparts and in one case, comparable efficiency to a good GaAs control cell. The application of these cells to tandem structures is discussed, indicating the potential for a substantial efficiency enhancement.

Investigation of impact ionization in thin GaAs diodes

The electron and hole multiplication coefficients, M/sub e/ and M/sub h/, respectively, have been measured in thin GaAs homojunction PIN and NIP diodes and from conventional ionization analysis the effective electron and hole ionization coefficients, /spl alpha/ and /spl beta/, respectively, have been determined. The nominal intrinsic region thickness w of these structures ranges from 1.0 /spl mu/m down to 25 nm. In the thicker structures, bulk-like behavior is observed; however, in the thinner structures, significant differences are found. As the i-regions become thinner and the electric fields increase, the M/sub e//M/sub h/ ratio is seen to approach unity. The experimental results are modeled and interpreted using a semianalytical solution of the Boltzmann equation. In thin (w/spl les/0.1 /spl mu/m) devices the dead space effect reduces effective ionization coefficients below their bulk values at low values of carrier multiplication. However, overshoot effects compensate for this at extremely high fields (/spl ges/1/spl times/10/sup 3/ kV/cm).

Tailoring of internal fields in InGaAs/GaAs multiwell structures grown on (111)B GaAs

We present a study of internal field distributions in strained InGaAs/GaAs multiple quantum wells in p‐i‐n structures grown on (111)B‐oriented GaAs. Room temperature photocurrent spectroscopy shows clear blueshifting of the e1‐hh1 transition as the well fields are reduced by external bias. The relative length of total well to total barrier material is shown to be an important factor in determining the well and barrier fields. We demonstrate a photocurrent contrast ratio of 4.5:1 for only 3 V applied bias across a 25 quantum well In0.13Ga0.87As p‐i‐n diode and discuss the implication of our results to the design of high performance electro‐optic modulators and self electro‐optic effect devices in this material system.

Avalanche multiplication in Al/sub x/Ga/sub 1-x/As (x=0 to 0.60)

Electron and hole multiplication characteristics, M/sub e/ and M/sub h/, have been measured in Al/sub x/Ga/sub 1-x/As (x=0-0.60) homojunction p/sup +/-i-n/sup +/ diodes with i-region thicknesses, w, from 1 /spl mu/m to 0.025 /spl mu/m and analyzed using a Monte Carlo model (MC). The effect of the composition on both the macroscopic multiplication characteristics and microscopic behavior is therefore shown for the first time. Increasing the alloy fraction causes the multiplication curves to be shifted to higher voltages such that the multiplication curves at any given thickness are practically parallel for different x. The M/sub e//M/sub h/ ratio also decreases as x increases, varying from /spl sim/2 to /spl sim/1 as x increases from 0 to 0.60 in a w=1 /spl mu/m p/sup +/-i-n/sup +/. The Monte-Carlo model is also used to extract ionization coefficients and dead-space distances from the measured results which cover electric field ranges from /spl sim/250 kV/cm-1200 kV/cm in each composition. These parameters can be used to calculate the nonlocal multiplication process by solving recurrence equations. Limitations to the applicability of field-dependent ionization coefficients are shown to arise however when the electric-field profile becomes highly nonuniform.

Plastic relaxation of metamorphic single layer and multilayer InGaAs/GaAs structures

The plastic relaxation of multilayer structures of strained InGaAs grown above critical thickness on GaAs is reported and compared with the relaxation of single layers and with theory. We show that a composite structure, taken as a whole, follows the same relaxation law as observed in single layers. However, departures of the strains of some component layers from theory show that misfit dislocations are easily pinned at an interface. Implications for the design of relaxed buffer layer growth are discussed.

PARTIAL SCREENING OF INTERNAL ELECTRIC FIELDS IN STRAINED PIEZOELECTRIC QUANTUM WELL LASERS : IMPLICATIONS FOR OPTOELECTRONIC INTEGRATION

The spectral electroluminescence characteristics of broad‐area (Al)GaAs/In0.23Ga0.77As/(Al)GaAs single quantum well separate confinement heterostructure lasers grown on (111)B GaAs have been studied under forward biased current injection. A room‐temperature threshold current density of 750 A/cm2 is measured for a 1000 μm laser. The subthreshold electroluminescence spectrum blue shifts with increasing current up to the point of lasing threshold. Our measurements reveal that lasing is achieved while there is a strong residual or ‘‘unscreened’’ electric field across the quantum well. Based on these observations we outline how piezoelectric quantum wells can be used to monolithically integrate a quantum well laser with a blue‐shifting electroabsorption modulator.

EFFECT OF STRAIN RELAXATION ON FORWARD BIAS DARK CURRENTS IN GAAS/INGAAS MULTIQUANTUM WELL P-I-N DIODES

The effect of the dislocation line density produced by the relaxation of strain in GaAs/InxGa1−xAs multiquantum wells where x=0.155–0.23 has been studied. There is a strong correlation between the dark line density, observed by cathodoluminescence, before processing of the wafers into photodiode devices, and the subsequent low forward bias (<1.5 V) dark current densities of the devices. A comparison is made of the correlation between the reverse bias current density and dark line density and it is found that, in this range of strain, the forward bias current density varies more. Two growth methods, molecular beam epitaxy and metal organic vapor phase epitaxy, have been used to produce the wafers and no difference between the growth methods has been found in dark line or current density variations with strain.

Controlling the performance of GaAs–AlGaAs quantum-cascade lasers via barrier height modifications

A series of GaAs/AlGaAs quantum-cascade lasers has been studied in which the confinement of the upper lasing level is systematically varied. This is achieved by modifying the aluminum composition, and hence the height, of a single barrier in each active region. Increasing the height of the barrier increases the upper laser level lifetime, while decreasing the optical transition matrix element. We find an optimum barrier composition (Al0.4Ga0.6As), with the sample containing this barrier exhibiting a significantly improved low-temperature threshold current density (3.8 kA/cm2) and peak power output (∼800 mW) relative to previously reported GaAs-based quantum-cascade lasers. The temperature performance of all the samples is very similar, indicating that thermal activation of electrons from the upper laser level is not the dominant factor limiting high-temperature operation.

CHARACTERIZATION OF GAAS/INGAAS QUANTUM WELLS USING PHOTOCURRENT SPECTROSCOPY

We report on characterization studies of high quality metal‐organic vapor phase epitaxy and molecular beam epitaxy grown GaAs/InGaAs quantum wells, set within p‐i‐n diodes, to determine the well widths, indium mole fractions, and conduction band offset. We present photocurrent spectra containing a larger number of transitions than revealed in photoluminescence or photoluminescence excitation experiments. The energies of these transitions have been modeled using a theoretical characterization tool known as ‘‘contouring,’’ which is used in this strained system for the first time. This has enabled determination of the conduction band offset in GaAs/InGaAs quantum wells, to a value between 0.62 and 0.64, for a range of indium fractions between 0.155 and 0.23. As a final, additional check on our results, we compare the field dependence of the e1‐hh1 exciton transition energy with our theoretical calculations and find good agreement.

Relaxation of strain within multilayer InGaAs/GaAs pseudomorphic structures

Strained‐layer superlattice structures (SLSs) have been grown in InGaAs/GaAs with various GaAs barrier layer thicknesses. Photoluminescence measurements indicate that, in structures with thin barriers, the strained layers interact leading to the relaxation of strain, even though each individual well does not exceed the critical thickness for a single quantum well. These results suggest that a fuller understanding of the mechanisms by which strain relaxes in SLSs is important in order that the constraints on the design of devices using such structures can be known.