Unstrained GaAs Research Articles

Strain effects in InGaAs/HaAs superlattices

Estimates are made of the effect of strain on the conduction and valence bands of In x Ga 1- x As wells confined by unstrained GaAs barriers. Strain leads to a large splitting between the heavy and light holes which enhances in-plane mobilities, while the heavy hole band itself remains fairly stationary.

High-efficiency carrier collection and stimulated emission in thin (50 Å) pseudomorphic InxGa1−xAs quantum wells

Efficient collection of photoexcited electrons and holes by thin, strained quantum wells is demonstrated in an In0.16Ga0.84As-GaAs pseudomorphic quantum well heterostructure laser. The undoped laser structure, which was grown by molecular beam epitaxy, consists of five 50 Å pseudomorphic In0.16Ga0.84As quantum wells separated by thick (700 Å), unstrained GaAs confining layers. Despite the fact that the quantum wells are undoped, decoupled, and of dimensions which are known to be too small to provide efficient carrier collection in unstrained AlxGa1−xAs-GaAs structures, 77 K photopumped laser operation is achieved on quantum well transitions (λ∼870 nm) at a threshold excitation intensity of 9.3×103 W/cm2. At photoexcitation intensities even as high as five times the threshold value, spontaneous emission from the quantum well is more than 700 times as intense as the confining layer luminescence. Based upon these photoluminescence results and some simple physical arguments, we suggest that carrier collection is enhanced in pseudomorphic quantum wells.

Optical Properties of Pseudomorphic InxGa1−xAs Quantum Wells

ABSTRACTThe luminescence properties of MBE-grown pseudomorphic InxGa1−xAs—GaAs quantum-well structures are examined as a function of photoexcitation intensity and temperature. The structures examined consist of single In0.28Ga0.72As or (isolated) multiple In0.16Ga0.84As pseudomorphic wells sandwiched between thick, unstrained GaAs confining layers. Low-temperature photoluminescence spectra for these samples, which range in quantum-well thickness from 17 Å to 50 Å, consist of a single feature attributable to transitions associated with n = 1 electron and j = 3/2, Mj = 3/2 > - hole states. Spectral widths of these peaks are very narrow (7–11 meV), even for a heavily spike-doped sample (Si, ND ∼ 1018 spike-doped at well center). Emission intensities for the quantum-well structures are studied as a function of excitation intensity over the range 3 × 102 ≤ Pex ≤ 6 x 1O4 W/cm2, and one of the samples (x = 0.16, 50 Å undoped wells) prepared as a laser structure is shown to support stimulated emission at an excitation intensity < 104 W/cm2. The excellent luminescence properties of these structures are shown to degrade rapidly with increasing temperature, with radiative efficiencies dropping more than two orders of magnitude over the temperature range 20K – 180K. One possible explanation for this behavior is proposed.

Optical absorption and photoluminescence studies of thin GaAs layers in GaAs–AlxGa1−xAs double heterostructures

A three-layered AlxGa1−xAs–GaAs–Alx Ga1−xAs structure has been used to measure the optical absorption and photoluminescence in thin GaAs layers prepared by liquid-phase epitaxy. The results presented here are for lightly doped n-type GaAs with free-carrier concentrations near 1016 cm−3; however, the technique can be used for arbitrarily doped material. The absorption coefficient α was measured between 1.4 and 2.2 eV at 2 and 298 K. The absorption strength at the band gap was found to be (1.15×104)±1000 cm−1 at 2 K and (0.99×104)±1000 cm−1 at 298 K. At 1.96 eV, the energy of the He–Ne laser commonly used for photoexcitation of GaAs, α at 298 K was measured to be 4.4×104 cm−1. A value of 3.8 meV for the room-temperature exciton binding energy was inferred from the temperature dependence of the interband absorption strength. This value together with previous reflectivity data for high-purity GaAs gives the energy gap of pure unstrained GaAs at 298 K as 1.424±0.001 eV. The effects of strain due to lattice mismatch in the three-layered structures were observed in the absorption edge at 2 K. The calculated photoluminescence spectrum obtained through the principle of detailed balance from the absorption data agrees well with the measured photoluminescence at 298 K. A comparison of the photoluminescence from the excited surface and back surface permits assignment of an upper limit of 5×104 cm/sec for the room-temperature GaAs–AlxGa1−xAs interface recombination velocity for x ≈ 0.5. This comparison of the front and back photoluminescence also shows that the minority-carrier diffusion length at 298 K for a sample with an electron concentration of 2×1016 cm−3 is at least 2.5 μ. The data presented here can be used to calculate the intrinsic carrier concentration ni, the thermal radiative generation rate G, and the radiative constant B (=Rr/np). The values at 298 K (corrected where necessary to a band-gap energy Eg =1.424 eV) are as follows: ni =1.8×106 cm−3, G =4500 cm−3/sec, and B=1.4×10−9 cm3/sec.