This paper describes the fabrication of nitrogen-polar AlxGa1-xN/AlN (x = 0, 0.1) quantum dot sup... more This paper describes the fabrication of nitrogen-polar AlxGa1-xN/AlN (x = 0, 0.1) quantum dot superlattices integrated along GaN nanowires for application in electron-pumped UV sources. The nanowires are grown using plasma-assisted molecular-beam epitaxy on n-type Si(111) wafers using a low-temperature AlN nucleation layer. Growth conditions are tuned to obtain a high density of non-coalesced nanowires. To improve the uniformity of the height along the substrate, the growth begins with a base long nanowire (~900 nm), with a diameter of 30-50 nm. The AlxGa1-xN/AlN active region is 400 nm long (88 periods of quantum dots), long enough to collect the electron-hole pairs generated by an electron beam with an acceleration voltage £ 5 kV. The spectral response is tuned in the 340 to 258 nm range by varying the dot/barrier
Nanowire photodetectors are investigated because of their compatibility with flexible electronics... more Nanowire photodetectors are investigated because of their compatibility with flexible electronics, or for the implementation of on-chip optical interconnects. Such devices are characterized by ultrahigh photocurrent gain, but their photoresponse scales sublinearly with the optical power. Here, we present a study of single-nanowire photodetectors displaying a linear response to ultraviolet illumination. Their structure consists of a GaN nanowire incorporating an AlN/GaN/AlN heterostructure, which generates an internal electric field. The activity of the heterostructure is confirmed by the rectifying behavior of the current-voltage characteristics in the dark, as well as by the asymmetry of the photoresponse in magnitude and linearity. Under reverse bias (negative bias on the GaN cap segment), the detectors behave linearly with the impinging optical power when the nanowire diameter is below a certain threshold (≈ 80 nm), which corresponds to the total depletion of the nanowire stem du...
Intersubband optoelectronic devices rely on transitions between quantum-confined electron levels ... more Intersubband optoelectronic devices rely on transitions between quantum-confined electron levels in semiconductor heterostructures, which enables infrared (IR) photodetection in the 1-30 μm wavelength window with picosecond response times. Incorporating nanowires as active media could enable an independent control over the electrical cross-section of the device and the optical absorption cross-section. Furthermore, the three-dimensional carrier confinement in nanowire heterostructures opens new possibilities to tune the carrier relaxation time. However, the generation of structural defects and the surface sensitivity of GaAs nanowires have so far hindered the fabrication of nanowire intersubband devices. Here, we report the first demonstration of intersubband photodetection in a nanowire, using GaN nanowires containing a GaN/AlN superlattice absorbing at 1.55 μm. The combination of spectral photocurrent measurements with 8-band k·p calculations of the electronic structure supports t...
This paper assesses the effects of Ge-doping on the structural and optical (band-to-band and inte... more This paper assesses the effects of Ge-doping on the structural and optical (band-to-band and intersubband) properties of GaN/AlGaN multi-quantum wells designed to display intersubband absorption in the short-wave, mid-and far-infrared ranges (SWIR, MIR, and FIR, respectively). The standard c-plane crystallographic orientation is considered for wells absorbing in the SWIR and MIR spectral regions, whereas the FIR structures are grown along the nonpolar m-axis. In all cases, we compare the characteristics of Ge-doped and Sidoped samples with the same design and various doping levels. The use of Ge appears to improve the mosaicity of the highly lattice-mismatched GaN/AlN heterostructures. However, when reducing the lattice mismatch, the mosaicity is rather determined by the substrate and does not show any dependence on the dopant nature or concentration. From the optical point of view, by increasing the dopant density, we observe a blueshift of the photoluminescence in polar samples due to the screening of the internal electric field by free carriers. In the intersubband absorption, on the other hand, there is a systematic improvement of the linewidth when using Ge as a dopant for high doping levels, whatever the spectral region 2 under consideration (i.e. different quantum well size, barrier composition and crystallographic orientation).
In this paper, we study band-to-band and intersubband characteristics of GaN/AlN heterostructures... more In this paper, we study band-to-band and intersubband characteristics of GaN/AlN heterostructures (planar and nanowires) structurally designed to absorb in the short-wavelength infrared region, particularly at 1.55 µm. We compare the effect of doping the GaN sections with Si and Ge, and we discuss the variation of free-carrier screening with the doping density and well/nanodisk size. We observe that nanowire heterostructures consistently present longer photoluminescence decay times than their planar counterparts, which supports the existence of an in-plane piezoelectric field associated to the sheer component of the strain tensor, leading to lateral electron-hole separation. We report intersubband absorption covering 1.45 µm to 1.75 µm using Ge-doped quantum wells, with comparable performance to well-studied Si-doped planar heterostructures. We also report comparable intersubband absorption in Si- and Ge-doped nanowire heterostructures indicating that the choice of dopant is not an ...
The impact of the barrier thickness on the performance of In 0.17 Ga 0.83 N multiple-quantum-well... more The impact of the barrier thickness on the performance of In 0.17 Ga 0.83 N multiple-quantum-well (MQW) solar cells is studied. When the barrier thickness is reduced from 9.0 to 3.7 nm, the effect of the internal polarization fields on the MQW band structure results in a blueshift of the cell photoresponse. At the same time, the overlap of the fundamental electron and hole wave-functions in the quantum wells increases and the carrier extraction by field-assisted tunneling is enhanced, impacting the external quantum efficiency and fill-factor of the cells. The experimental results show that the performance of the thinner-barrier cells studied in this work is superior, or at least comparable to the performance of their thickerbarrier counterparts, in spite of the smaller total thickness of their absorbing region. This is due to their higher external quantum efficiency (37% at 370 nm) and improved fill-factor (62%), which result in a conversion efficiency of η = 0.82%.
This paper describes the fabrication of nitrogen-polar AlxGa1-xN/AlN (x = 0, 0.1) quantum dot sup... more This paper describes the fabrication of nitrogen-polar AlxGa1-xN/AlN (x = 0, 0.1) quantum dot superlattices integrated along GaN nanowires for application in electron-pumped UV sources. The nanowires are grown using plasma-assisted molecular-beam epitaxy on n-type Si(111) wafers using a low-temperature AlN nucleation layer. Growth conditions are tuned to obtain a high density of non-coalesced nanowires. To improve the uniformity of the height along the substrate, the growth begins with a base long nanowire (~900 nm), with a diameter of 30-50 nm. The AlxGa1-xN/AlN active region is 400 nm long (88 periods of quantum dots), long enough to collect the electron-hole pairs generated by an electron beam with an acceleration voltage £ 5 kV. The spectral response is tuned in the 340 to 258 nm range by varying the dot/barrier
Nanowire photodetectors are investigated because of their compatibility with flexible electronics... more Nanowire photodetectors are investigated because of their compatibility with flexible electronics, or for the implementation of on-chip optical interconnects. Such devices are characterized by ultrahigh photocurrent gain, but their photoresponse scales sublinearly with the optical power. Here, we present a study of single-nanowire photodetectors displaying a linear response to ultraviolet illumination. Their structure consists of a GaN nanowire incorporating an AlN/GaN/AlN heterostructure, which generates an internal electric field. The activity of the heterostructure is confirmed by the rectifying behavior of the current-voltage characteristics in the dark, as well as by the asymmetry of the photoresponse in magnitude and linearity. Under reverse bias (negative bias on the GaN cap segment), the detectors behave linearly with the impinging optical power when the nanowire diameter is below a certain threshold (≈ 80 nm), which corresponds to the total depletion of the nanowire stem du...
Intersubband optoelectronic devices rely on transitions between quantum-confined electron levels ... more Intersubband optoelectronic devices rely on transitions between quantum-confined electron levels in semiconductor heterostructures, which enables infrared (IR) photodetection in the 1-30 μm wavelength window with picosecond response times. Incorporating nanowires as active media could enable an independent control over the electrical cross-section of the device and the optical absorption cross-section. Furthermore, the three-dimensional carrier confinement in nanowire heterostructures opens new possibilities to tune the carrier relaxation time. However, the generation of structural defects and the surface sensitivity of GaAs nanowires have so far hindered the fabrication of nanowire intersubband devices. Here, we report the first demonstration of intersubband photodetection in a nanowire, using GaN nanowires containing a GaN/AlN superlattice absorbing at 1.55 μm. The combination of spectral photocurrent measurements with 8-band k·p calculations of the electronic structure supports t...
This paper assesses the effects of Ge-doping on the structural and optical (band-to-band and inte... more This paper assesses the effects of Ge-doping on the structural and optical (band-to-band and intersubband) properties of GaN/AlGaN multi-quantum wells designed to display intersubband absorption in the short-wave, mid-and far-infrared ranges (SWIR, MIR, and FIR, respectively). The standard c-plane crystallographic orientation is considered for wells absorbing in the SWIR and MIR spectral regions, whereas the FIR structures are grown along the nonpolar m-axis. In all cases, we compare the characteristics of Ge-doped and Sidoped samples with the same design and various doping levels. The use of Ge appears to improve the mosaicity of the highly lattice-mismatched GaN/AlN heterostructures. However, when reducing the lattice mismatch, the mosaicity is rather determined by the substrate and does not show any dependence on the dopant nature or concentration. From the optical point of view, by increasing the dopant density, we observe a blueshift of the photoluminescence in polar samples due to the screening of the internal electric field by free carriers. In the intersubband absorption, on the other hand, there is a systematic improvement of the linewidth when using Ge as a dopant for high doping levels, whatever the spectral region 2 under consideration (i.e. different quantum well size, barrier composition and crystallographic orientation).
In this paper, we study band-to-band and intersubband characteristics of GaN/AlN heterostructures... more In this paper, we study band-to-band and intersubband characteristics of GaN/AlN heterostructures (planar and nanowires) structurally designed to absorb in the short-wavelength infrared region, particularly at 1.55 µm. We compare the effect of doping the GaN sections with Si and Ge, and we discuss the variation of free-carrier screening with the doping density and well/nanodisk size. We observe that nanowire heterostructures consistently present longer photoluminescence decay times than their planar counterparts, which supports the existence of an in-plane piezoelectric field associated to the sheer component of the strain tensor, leading to lateral electron-hole separation. We report intersubband absorption covering 1.45 µm to 1.75 µm using Ge-doped quantum wells, with comparable performance to well-studied Si-doped planar heterostructures. We also report comparable intersubband absorption in Si- and Ge-doped nanowire heterostructures indicating that the choice of dopant is not an ...
The impact of the barrier thickness on the performance of In 0.17 Ga 0.83 N multiple-quantum-well... more The impact of the barrier thickness on the performance of In 0.17 Ga 0.83 N multiple-quantum-well (MQW) solar cells is studied. When the barrier thickness is reduced from 9.0 to 3.7 nm, the effect of the internal polarization fields on the MQW band structure results in a blueshift of the cell photoresponse. At the same time, the overlap of the fundamental electron and hole wave-functions in the quantum wells increases and the carrier extraction by field-assisted tunneling is enhanced, impacting the external quantum efficiency and fill-factor of the cells. The experimental results show that the performance of the thinner-barrier cells studied in this work is superior, or at least comparable to the performance of their thickerbarrier counterparts, in spite of the smaller total thickness of their absorbing region. This is due to their higher external quantum efficiency (37% at 370 nm) and improved fill-factor (62%), which result in a conversion efficiency of η = 0.82%.
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Papers by Akhil Ajay