Modulation-doping in 3–5 μm GaAs/AlAs/AlGaAs double barrier quantum well infrared photodetectors: an alternative to achieve high photovoltaic performance and high temperature detection

In this work we propose new detector designs, which allow achieving mid-infrared photovoltaic (PV) detection at temperatures as high as 180 K. The devices, which are grown by molecular beam epitaxy, are modulation-doped (MD) double barrier quantum well infrared photodetectors (QWIPs) based on AlGaAs/AlAs/GaAs. As the photocurrent spectra and I–V characteristics (in the dark and under infrared illumination) show that the dopant location is a relevant design parameter regarding the performance of PV QWIPs, we begin our work with a comparison of the performance of a set of MD samples (where we have varied the dopant location in the AlGaAs barriers) with respect to a well-doped sample of nominally the same structure. We find that the responsivity and detectivity of the MD devices seem to be higher than those of the well-doped detector, specially when the dopant is located in the substrate-sided barrier. Then, in order to improve the dark current-limited performance, we designed a new set of substrated-sided MD detectors that exhibit an extremely low dark current, even at high temperatures, otherwise no drop in the zero bias peak responsivity. Therefore, the association of the notable PV signal detection in the 3–5 μm range of these MD detectors together with the dark current reduction of the new structures has allowed us to achieve a 140 K zero bias peak responsivity of 0.015 A/W and a 180 K zero bias peak responsivity of 0.01 A/W at 4.4 μm.     

Structural properties of 10 μm thick InN grown on sapphire (0001)

The structural properties of a 10 μm thick In-face InN film, grown on Al₂O₃ (0001) by radio-frequency plasma-assisted molecular beam epitaxy, were investigated by transmission electron microscopy and high resolution x-ray diffraction. Electron microscopy revealed the presence of threading dislocations of edge, screw and mixed type, and the absence of planar defects. The dislocation density near the InN/sapphire interface was 1.55×10¹⁰ cm⁻², 4.82×10⁸ cm⁻² and 1.69×10⁹ cm⁻² for the edge, screw and mixed dislocation types, respectively. Towards the free surface of InN, the density of edge and mixed type dislocations decreased to 4.35×10⁹ cm⁻² and 1.20×10⁹ cm⁻², respectively, while the density of screw dislocations remained constant. Using x-ray diffraction, dislocations with screw component were found to be 1.2×10⁹ cm⁻², in good agreement with the electron microscopy results. Comparing electron microscopy results with x-ray diffraction ones, it is suggested that pure edge dislocations are neither completely randomly distributed nor completely piled up in grain boundaries within the InN film.     

STEM-study of 1.3 μm InAs/InGaAs quantum dot structures

We investigated InAs-Dots-in-a-well structures emitting near 1.3 μm by bright field and Z-contrast mode in a scanning transmission electron microscope. The chemically sensitive Z-contrast mode is found to give direct information on the actual position of the InAs-Dots inside the embedding well, while the bright field mode monitors the strain fields. Comparing a series of structures, we found that the most symmetric design is realized by an nominally asymmetric growth. These symmetric structures exhibit the best performance with respect to photoluminescence spectra and laser threshold current density.     

Annealing effect on InAs islands on GaAs(0 0 1) substrates studied by scanning tunneling microscopy

Post-annealing effects on InAs islands grown on GaAs(0 0 1) surfaces have been investigated by scanning tunneling microscopy (STM) connected to molecular beam epitaxy (MBE). It is found that for islands grown by 1.6 ML InAs deposition at 450 °C, post-annealing at 450 °C in an As₄ atmosphere causes dissolving of the InAs islands. In contrast, for larger islands obtained by 2.0 ML InAs deposition at 450 °C, the post-annealing leads to coarsening of the islands. The result can be explained in terms of a critical nucleus in heterogeneous nucleation.     

Structural characterization of Ga-doped Mg0.1Zn0.9O layers grown on ZnO/α-Al2O3 templates by P-MBE

In this study, structural properties of epitaxial Ga-doped Mg_0.1Zn_0.9O layers grown on ZnO/α-Al₂O₃ templates by plasma-assisted molecular beam epitaxy have been investigated by high-resolution transmission electron microscopy (HRTEM), and high resolution X-ray diffraction (HRXRD). From analysis of the diffraction pattern, the monocrystallinity of the Mg_0.1Zn_0.9O layer with hexagonal structure is confirmed. The orientation relationship between Mg_0.1Zn_0.9O and the template is determined as (0 0 0 1)_Mg0.1Zn0.9O(0 0 0 1)_ZnO(0 0 0 1)_Al2O3 and [ [ ]_ZnO[ . The density of dislocations near the top surface layers measured by plan-view TEM is about 3.610¹⁰ cm⁻², one order of magnitude higher than the value obtained for ZnO layers on α-Al₂O₃ with a MgO buffer. Cross-sectional observation revealed that the majority of threading dislocations are in the [0 0 0 1] line direction, i.e. they lie along the surface normal and consist of edge, screw, and mixed dislocations. Cross- sectional TEM and X-ray rocking curve experiments reveal that most of dislocations are edge dislocations. The interface of Mg_0.1Zn_0.9O and ZnO layers and the effect of excess Ga-doping in these layers have been also studied.     

GaAsN/AlAs/AlGaAs double barrier quantum wells grown by molecular beam epitaxy as an alternative to infrared absorption below 4 μm

In this work, we report on the design, growth and characterization of GaAsN/AlAs/AlGaAs double barrier quantum well infrared detectors to achieve intraband absorption below 4 μm. Due to the high effective mass of N-dilute alloys, it is common for these N-containing double barrier quantum well structures to have more than one bound state within the quantum well, enabling the possibility of achieving multispectral absorption from these confined levels to the quasi-bound. Based on a transfer matrix calculation we will study the influence of the potential parameters, in particular the well width and the introduction of a GaAs spacer layer in between the N-well and the AlAs barriers. We will compare the case in which there are two confined levels with the case in which only one level is bound, like in the conventional AlGaAs/AlAs/GaAs structures. On the basis of the simulation, we have grown and characterized some N-containing double barrier detectors. Moreover, an optimization of the post-growth annealing treatments of the GaAsN quantum well structures has also been performed. Finally, room temperature absorption measurements of both as-grown and annealed samples are presented and analyzed.     

1.3 μm range effectively cylindrical In0.53Ga0.47As/In0.52Al0.48As quantum wires grown on (2 2 1)A InP substrates by molecular beam epitaxy

Vertically coupled quantum wires (QWRs) have been made by alternately stacking nominally 3.6 nm thick In_0.53Ga_0.47As self-organized QWR layers and 1 nm thick In_0.52Al_0.48As barrier layers on (2 2 1)A-oriented InP substrates by molecular beam epitaxy. The surface of In_0.53Ga_0.47As QWR layers was corrugated at an amplitude of 1.1 nm and period of 27 nm, and lateral confinement potential is induced by their thickness modulation. The wavelength of photoluminescence (PL) from the stacked QWRs at 15 K becomes longer from 1220 to 1327 nm with increasing total number of stacked QWR layers, N_SL, from 1 to 9, while PL full-width at half-maximum is reduced from 22 to 8.6 meV. The PL intensity with the polarization parallel to the wire direction, I, is 1.30 times larger than that with the normal polarization, I, when N_SL=1. The PL intensity ratio, I/I, reaches as large as 4 when N_SL=9, indicating successful control of relative strength between vertical confinement and lateral confinement of carriers. The value of I/I obtained for the stacked QWRs with N_SL=9 is the same value as cylindrical QWRs have. The results indicate that effectively cylindrical QWRs with the best uniformity and 1.3 μm range emission were realized by stacking of self-organized QWR layers.