Short-period Superlattice Barriers Research Articles

Dependences of the Transport Scattering Time and Quantum Lifetime on the Two-Dimensional Electron Gas Density in Modulation-Doped Single GaAs Quantum Wells with AlAs/GaAs Short-Period Superlattice Barriers

The dependences of the transport scattering time {\tau}t, quantum lifetime {\tau}q, and their ratio {\tau}t/{\tau}q on the density ne of the electron gas in modulation-doped single GaAs quantum wells with AlAs/GaAs short-period super-lattice barriers are investigated. The experimental dependences are explained in terms of electron scattering by remote ionized donors with an effective two-dimensional concentration n*R and background impurities with a three-dimensional concentration nB. An expression for n*R(ne) is obtained including the contribution of X-valley electrons localized in AlAs layers to the suppression of scattering by the random potential of remote donors. It is shown that the experimentally observed abrupt increase in {\tau}t and {\tau}q with an increase in ne above a certain critical value nec is related to a decrease in n*R. It is established that the drop in {\tau}t/{\tau}q observed for electron densities ne > nec occurs because scattering by the random potential of background impurities in this two-dimensional system with a decrease in n*R limits an increase in {\tau}t more considerably than an increase in {\tau}q.

Photoluminescence study of self-assembled GaAs quantum wires on (631)A-oriented GaAs substrates

The authors report a low-temperature photoluminescence (PL) study of multiple GaAs layers grown between AlAs(0.6 nm)/GaAs(0.6 nm) short-period superlattice barriers (SLBs) simultaneously grown on both GaAs(631)A and (100) substrates. Five GaAs-layers of different nominal thicknesses (LW, ranging from 12 to 2.4 nm) were grown by molecular beam epitaxy. By using (631)A-oriented substrates a self-organized and highly ordered corrugation is obtained in the growth of the GaAs layers, and at the end of the SLB growth, flat surfaces are found. Whereas, for the (100)-oriented sample, flat interfaces are confirmed after the growth of GaAs and SLB layers. By reducing LW below ∼3.6 nm in the (631) sample, strong quantum wire (QWR)-like confinement is achieved as deduced from polarized PL spectroscopy where polarization degrees as large as 0.43 are obtained. The PL emission energy of the (631)-QWRs is redshifted, as compared with the transitions of the (100)-oriented quantum wells, when LW is reduced. The authors explain this energy shift by the widening of the effective thickness of the confinement regions in the GaAs layers.

Infrared optical absorbance of intersubband transitions in GaN/AlGaN multiple quantum well structures

Intersubband transitions in Si-doped molecular beam epitaxy grown GaN/AlGaN multiple quantum wells on c-plane sapphire were investigated using the Fourier-transform infrared optical absorption technique. Several GaN quantum well samples were grown with either AlGaN bulk or GaN/AlGaN short period superlattice barriers. The measurements were made in a waveguide configuration utilizing a facet polished at 45° to the c plane. The integrated area of the intersubband transitions in several waveguides cut from different location of the wafer was measured, from which we estimated the two-dimensional electron gas density (σ). The measured values of σ are about two orders of magnitude larger than the Si doping level of ∼8×1017 cm−3, which is consistent with the polarization effects, particularly considering the large number of GaN/AlGaN interfaces. The internal quantum efficiency of the intersubband transitions was estimated to be on the order of 40% for samples with superlattice barriers.

Improved uniformity of self-organized In 0.53Ga 0.47As/In 0.52Al 0.48As quantum wires grown on (7 7 5)B-oriented InP substrate by molecular beam epitaxy

Uniformity of self-organized In 0.53Ga 0.47As/In 0.52Al 0.48As quantum wire (QWR) structures grown on (7 7 5)B-oriented InP substrate by molecular beam epitaxy was improved by changing substrate temperature. Both surfaces of In 0.53Ga 0.47As and In 0.52Al 0.48As layers grown at 580°C on the (7 7 5)B InP substrates were corrugated with different periods and amplitudes, which leads to rather poor uniformity of the (7 7 5)B In 0.53Ga 0.47As/In 0.52Al 0.48As QWRs. Surface of an In 0.52Al 0.48As barrier layer grown at 540°C was found to become almost flat. An improved In 0.53Ga 0.47As/In 0.52Al 0.48As QWR structure was fabricated by growing an In 0.53Ga 0.47As QWR layer at 580°C and In 0.52Al 0.48As barrier layers at 540°C. Their full-width at half-maximum (FWHM) values of photoluminescence (PL) observed in the QWRs at 12 K became as small as 19 meV, which is about 40% smaller than that (31 meV) of the previous In 0.53Ga 0.47As/In 0.52Al 0.48As QWRs grown at 580°C. In addition, more improved uniformity (17 meV at 12 K) was realized by using (In 0.53Ga 0.47As) 2(In 0.52Al 0.48As) 2 short period superlattice barriers grown at 540°C. These reduced PL-FWHM values are smaller than those of any InGaAs QWRs emitting 1.1– 1.35 μm PL reported so far.

Sub-picosecond intersub-band electron scattering times in GaN/AlGaN superlattices grown by molecular beam epitaxy

GaN quantum wells were studied, grown by plasma-assisted molecular beam epitaxy with a sub-band spacing of ∼740 meV (λ=1.67 µm). The GaN quantum wells are clad on both sides with short-period superlattice barriers. Using the time-resolved pump-and-probe technique, with 1.55 µm pump and 1.70 µm probe wavelength, an intersub-band electron scattering time of 370 fs was measured.

Growth and Characterization of GaN/AlGaN Superlattices for Near-Infrared Intersubband Transitions

We report the results of the growth by molecular beam epitaxy of GaN/Al x Ga 1-x N superlattice (SL) structures with single or double quantum wells and thick Al x Ga 1-x N or short-period SL barriers. The dependence of intersubband absorption on the well width for single well SL structures was systematically investigated. For GaN quantum wells interleaved with SL barriers, the upper state electron confinement was maintained by Bragg reflection from the minigaps formed within the barriers. In addition, evidence of electron transfer from doped SL barriers to undoped GaN quantum wells was obtained. Abrupt interfaces and coherent periodicities of the SL structures were confirmed by transmission electron microscopy and X-ray diffraction measurements. Intersubband absorption at peak wavelengths between 1.4 and 4.2 μm has been observed.

Growth and Characterization of GaN/AlGaN Superlattices for Near-Infrared Intersubband Transitions

We report the results of the growth by molecular beam epitaxy of GaN/AlxGa1—xN superlattice (SL) structures with single or double quantum wells and thick AlxGa1—xN or short-period SL barriers. The dependence of intersubband absorption on the well width for single well SL structures was systematically investigated. For GaN quantum wells interleaved with SL barriers, the upper state electron confinement was maintained by Bragg reflection from the minigaps formed within the barriers. In addition, evidence of electron transfer from doped SL barriers to undoped GaN quantum wells was obtained. Abrupt interfaces and coherent periodicities of the SL structures were confirmed by transmission electron microscopy and X-ray diffraction measurements. Intersubband absorption at peak wavelengths between 1.4 and 4.2 μm has been observed.

Influence of growth conditions for GaAs quantum wells and composition fluctuations in Al(Ga)As barriers on exciton and electron localization

Quantum wells (QWs) with short-period superlattice barriers are grown by molecular-beam epitaxy in two-dimensional island nucleation or step-flow growth mode. A pronounced roughening of step edges as imaged by atomic force microscopy for samples grown either at high As 4-partial pressure or on misoriented substrates with As-terminated steps is observed. Micro-photoluminescence spectra of the GaAs wells and their barriers reveal a rich fine structure. The narrow lines are attributed to exciton localization by potential fluctuations at the interface. Since the fine structure is observed in ternary barriers too, it is concluded that compositional fluctuations at QW interfaces are crucial. The interface roughness correlation lengths are measured by diffuse X-ray scattering. Another localization effect is connected with the appearance of heavy-mass X-electrons in the short-period superlattice barriers on both sides of the QW. Thus, their high screening capability results in unexpected high mobilities at high carrier densities. Transport correlation lengths are compared with those of the interface roughness.

Growth-mode-induced surface morphology and its relation to optical properties of GaAs single quantum wells

Quantum wells with short period superlattice barriers are grown by molecular beam epitaxy in growth modes which range from two-dimensional island nucleation to step flow. These different growth modes result in different surface morphologies as imaged by atomic force microscopy. Micro-photoluminescence of the quantum wells and of their barriers reveal spectra with a rich fine structure. These narrow lines in the micro-photoluminescence spectra are attributed to exciton localization by potential fluctuations at the interface. The formation of a pronounced roughening of step edges for samples grown either at high As 4 -partial pressure or on misoriented substrates with As-terminated steps is observed, which is accompanied by the appearance of a pronounced fine structure in photoluminescence and a broadening of its distribution.

InAs channel heterostructure-field effect transistors with InAs/AlSb short-period superlattice barriers

We report on the implementation of InAs-channel heterostructure-field-effect transistors (HFETs) fabricated with InAs/AlSb short-period superlattice barriers. The InAs/AlSb superlattice barrier structure is advantageous for InAs/AlSb HFETs because of its improved chemical stability against oxidation when compared to pure AlSb, and its compatibility with silicon as an n-type dopant during growth by molecular beam epitaxy. The structures examined here consist of a 200-Å-wide InAs quantum well inserted between 25/25 Å InAs/AlSb superlattice barriers that provide a 0.5 eV conduction band discontinuity between the quantum well and the superlattice barrier. Fabricated HFET devices display complete channel modulation, confirming the field-effect operation at room temperature. In addition, we demonstrate the modulation doping of an InAs quantum well clad by silicon-doped InAs/AlSb superlattice barriers.

Realization of high performance doped-channel MISFETs in highly strained AlGaAs/InGaAs/AlGaAs based quantum wells

Doped channel metal-insulator-semiconductor field effect transistors (DC-MISFETs) in the highly strained Al x Ga 1- x As/ In 0.26Ga 0.74As/Al y Ga 1- y As (0< x, y≤1.0) and ((AlAs) m (GaAs) n )/In 0.26Ga 0.74As/( (AlAs) p (GaAs) q ) single quantum wells grown via MBE on GaAs (100) substrates have been examined with a view towards improved performance via control of growth using RHEED behavior of static and dynamic surfaces. Breakdown voltages over 20 V and over 40 V have been realized in structures containing AlGaAs alloy and equivalent Al content short period superlattice (SPSL) barriers, respectively. Low gate leakage currents and output conductance as low as 150 μS/mm have been realized. Increasing the In content to 30% and using heavy Si doping in the well, DC-MISFETs containing SPSL barriers have yielded drain currents of 1.2 A/mm and breakdown voltages of ∼10 V.

Effect of barrier configuration on excitonic recombination in Ga 0.47In 0.53As/Al 0.48In 0.52As multi quantum well structures

Photoluminescence and photoluminescence excitation spectroscopy on Ga 0.47In 0.53As multi quantum wells confined either by homogeneous ternary Al 0.48In 0.52As barriers or by Ga 0.47In 0.53As/Al 0.48In 0.52As short-period superlattice (SPS) barriers show that the confinement by SPS barriers improves the edge luminescence significantly. The spectral width of the free-exciton absorption and the low-temperature emission peak as well as the Stokes shift between emission and excitation spectra are reduced as compared to samples clad by homogeneous ternary Al 0.48In 0.52As barriers. The dominant low-temperature emission line in the SPS-clad Ga 0.47In 0.53As multi quantum wells is assigned to intrinsic excitonic recombination. The small Stokes shift of the excitonic luminescence line can be explained by intrinsic relaxation processes caused by monolayer fluctuation of the well width and by statistical fluctuation of the composition of the ternary Ga 0.47In 0.53As well material.

Effect of barrier configuration on excitonic recombination in Ga .47In .53As/Al .48In .52As multi quantum well structures

Photoluminescence and photoluminescence excitation spectroscopy on Ga .47In .53As multi quantum wells confined either by homogenous ternary Al .48In .52As barriers or by Ga .47In .53As/Al .48In .52As short-period superlattice (SPS) barriers show that the confinement by SPS barriers improves the edge luminescence significantly. The spectral width of the free-exciton absorption and the low-temperature emission peak as well as the Stokes-shift between emission and excitation spectra are reduced as compared to samples clad by homogenous ternary Al .48In .52As barriers. Based on temperature-dependent emission and excitation measurements, the dominant low-temperature emission line in the SPS-clad Ga .47In .53As multi quantum wells is assigned to intrinsic excitonic recombination.