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InAs/InAsSb strain balanced superlattices for optical detectors: Material properties and energy band simulations
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Image of FIG. 1.
FIG. 1.

(Color online) HR-XRD spectra of the 004 reflection. (a) Measured data of SQW samples 1-5. The scans are offset for clarity. (b) Comparison between measured data of MQW sample 6 and a simulation.

Image of FIG. 2.
FIG. 2.

(Color online) The 4 K PL spectra of InAsSb layers grown on InAs and labeled according to the Sb content in the QWs. (a) A series of samples consisting of one SQW with a width of 20 nm embedded in InAs. (b) Six period SL structures with InAs and InAsSb layer thicknesses of 20 nm per 40 nm period.

Image of FIG. 3.
FIG. 3.

(Color online) 4 K PL peak energies of InAsSb layers grown on InAs are plotted as a function of Sb content in the QWs. The symbol × (★) denotes the SQW (MQW) samples. The InAsSb unstrained bandgap is displayed as a solid line. The dashed-dotted line shows the InAsSb bandgap strained on the InAs substrate. The dotted (dashed) line is the result of the type IIa transition energy simulation for the case of the SQW (MQW) samples.

Image of FIG. 4.
FIG. 4.

The 4 K PL peak energies are plotted as a function of Sb content for the InAsSb QWs (•). Also included are the calculated bandgaps of unstrained InAsSb (solid line) and InAsSb strained on GaSb (dashed-dotted line), as well as the predicted type IIb transition energies (dashed line). Good agreement with the measured data was achieved by assuming that 30% of the InAsSb bandgap bowing occurs in the conduction band, and 70% in the valence band.

Image of FIG. 5.
FIG. 5.

(Color online) Measured PL energy (×) as a function of InAs0.925Sb0.075 QW width on InAs substrates compared with simulation. The solid (dashed) line is computed with a type IIb (IIa) alignment in which the heavy holes (electrons) are confined in the well. The fact that there is very little change in confinement for a QW width < 10 nm strongly suggests the type IIb alignment. The inset shows a sketch of the two different band alignment cases.

Image of FIG. 6.
FIG. 6.

(Color online) HR-XRD (004) measurement of sample 9 compared to two simulations: (top) assuming both InAsSb QWs (16 nm, 5 nm) have a Sb content of 0.075, and (bottom) different compositions with the Sb content of the 5 nm QW changed to 0.055. Unfortunately, XRD measurement is not very sensitive to a small composition change in a single quantum well of this width; however, the bottom simulation does appear to be closer to the measured data. The spectra are offset for clarity.

Image of FIG. 7.
FIG. 7.

(Color online) Band structure simulation of an InAs QW in InAsSb strained on GaSb with periodic boundary conditions at 77 K. The Sb composition in the InAsSb layers is 0.21. This structure is strain balanced on a GaSb substrate. The dotted lines are the 1st quantized energy levels of the electron confined in the InAs layer and the heavy hole in InAsSb. Ψ2 is plotted so that the zero of probability coincides with the corresponding quantized energy level.

Image of FIG. 8.
FIG. 8.

(Color online) Probability distribution for electrons and heavy holes in InAsSb/InAs SL structures on GaSb substrates at 77 K for a Sb content in InAsSb of 0.21 and periods of 40 nm (a), 20 nm (b), and 10 nm (c). Periodic boundary conditions were chosen so as to simulate the SL. The probability distribution functions are normalized to one over one period. With decreasing period, the wavefunction overlap strongly increases. While electrons (holes) are confined in the InAs (InAsSb), only the electron wavefunction penetrates significantly into the barrier.

Image of FIG. 9.
FIG. 9.

Simulation results for InAsSb/InAs strain balanced SL structures on GaSb at 77 K. (a) Expected transition energy as a function of Sb composition in InAsSb for different SL periods. (b) Calculated squared optical matrix elements for one period as a function of Sb composition in InAsSb for different SL periods. me denotes the mass of an electron.

Image of FIG. 10.
FIG. 10.

Calculated transition energy (a) and optical matrix element for one period (b) of InAs/(In)GaSb SL structures at 77 K as a function of period. InAs and (In)GaSb layers are of the same thickness.

Image of FIG. 11.
FIG. 11.

Optical response of a strain balanced InAs0.79Sb0.21/InAs SL detector with a period of 40 nm. The circular detector aperture has a diameter of 200 μm. The vertical dashed lines are the positions of the calculated lowest four energy transitions. The significant sub-bandgap response is due to the type II transitions. Due to the low matrix element (see Fig. 9), resulting from the long period, the type II response is very weak, as predicted. For comparison, in gray, we present the response of an InAs0.91Sb0.09 n-p+ homojunction detector lattice matched to GaSb (LM) with a 1 μm thick absorber layer. The diameter of the circular aperture of this detector is 300 μm. Both measurements were done under zero bias condition at 77 K. The simulated transitions, shown as dashed vertical lines, include strain and quantization energies in all cases.


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Table I.

Summary of the PL samples grown on InAs substrates. Composition and thickness values were measured via HR-XRD.

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Table II.

Parameters used for simulations. Parameters are taken from the work of Vurgaftman (Ref. 27) unless otherwise stated. The bandgap (Eg ) as a function of temperature (T) is given in the Varshni form: Eg (T) = Eg αT2 /(T + β). γ describes the linear expansion coefficient of the lattice parameter. Note that parameters such as band gap and carrier masses are regarding the Γ-point.

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Table III.

Bowing parameters of ternary alloys used for simulations. Only the non-zero bowing coefficients (cbow ) defined as Y (ABC) = xYAB  + (1 − x)YAC − x(1 − x)cbow are displayed. Parameters are taken from the work of Vurgaftman (Ref. 27) unless otherwise stated.

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Table IV.

Comparison of InAsSb/InAs with InAs/(In)GaSb SL structures for the case of a detector cutoff at 10 μm and 5 μm at an operating temperature of 77 K.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: InAs/InAsSb strain balanced superlattices for optical detectors: Material properties and energy band simulations