AlGaAs Layers Research Articles

GaAs-based red micro-light-emitting diodes with an oxide perimeter region for improved external quantum efficiency

Red micro-light-emitting diodes (μ-LEDs) with AlGaInP/GaInP multiple quantum wells are fabricated with an oxide perimeter region to control the current injection path. When the values of the external quantum efficiency (EQE) of the 30 μm-size μ-LED with the oxide perimeter region are compared with those of the device without the oxide perimeter region, an improvement as high as 40% is observed at current densities <80 A/cm2. From the finite-difference time-domain simulation of the light-extraction efficiency (LEE), which shows that the LEE of the device with the oxide perimeter region is ∼12% smaller than that of the device without the oxide perimeter region, it can be seen that the increased EQE is attributed to the improvement of the internal quantum efficiency (IQE). Since the oxide perimeter region limits the current paths to the sidewalls of the μ-LED chips, the nonradiative recombination via sidewall defects is considered suppressed, resulting in the improvement of the IQE. The oxidation of the AlGaAs layer utilized in this work is easy to implement and accurately controllable, suitable for mass-production of high-efficiency red μ-LEDs for display applications.

High-efficiency edge-emitting laser diodes with oxidation confinement stripe structure

High-efficiency, low-cost, high-power laser diodes (LDs) are in the spotlight due to the growing market demand. This study proposes a new edge-emitting LD structure that utilizes the facile oxidization properties of the high aluminum (Al) component AlGaAs material and simplifies LD fabrication to only one-step lithography. The key innovation is incorporating a ∼200 nm thick high-Al AlGaAs layer in the vertical epitaxial structure. This layer serves two purposes: 1) The low refractive index improves vertical mode confinement, enabling a thinner overall epitaxial structure. 2) Oxidation of this layer forms a dielectric isolator and a current aperture. Experimental LDs exhibited a peak efficiency of 77.8 %, peak power of 33.6 W, vertical divergence angle of 19.4°, and horizontal divergence angle of 8.8°. Compared to conventional edge-emitting LD, this LD demonstrates a 39 % lower thermal resistance, better optoelectronic performance, a simpler process, and a lower production cost.

Self-consistent Analysis for Optimization of AlGaAs/GaAs Based Heterostructure

The heterostructures are suitable for developing high-performance electronic and optoelectronic devices. In this work, a significant interest in the design and analysis of compound semiconductor Aluminium Gallium Arsenide (AlGaAs) and Gallium Arsenide (GaAs) heterostructures has been realized. These structures are fabricated with alternating layers of GaAs (a direct bandgap material) and AlGaAs (an indirect bandgap material) and have been used to design a range of high-performance devices, including lasers, solar cells, and field-effect transistors. A 30 nm AlGaAs consisting of a middle layer between two GaAs layers with a GaAs substrate has been reported. This work has been carried out at 300 K utilizing a quantum transport and self-consistent method for the proposed AlGaAs/GaAs one-dimensional heterostructure with a gate length of 2 nm and a voltage varying from 0 to 0.1 V. The measured values of doping density (nd) and electron density (ne) of AlGaAs/GaAs one-dimensional heterostructure are 8.96 × 1011 cm−3 and 2 × 1026 cm−3, respectively. The system response to geometric changes in several parameters has been realized. Hence, confined restricted states were computed using wave functions and energies. The GaAs layer on top of quantum well heterostructure interfaces has been used to modulate the wave functions (eigenstates) resulting in pseudo-one dimensional or small-dimension eigenstates. In this work, a comprehensive analysis of 1D AlGaAs/GaAs heterostructure through benchmarking with several homo-structures (various thicknesses) has been performed.

Stochastic simulation of electron transport in a strong electrical field in low-dimensional heterostructures

Abstract In this paper we develop a stochastic simulation algorithm for electron transport in a DA-pHEMT heterostructure. Mathematical formulation of the problem of electron gas transport in the heterostructure in the form of a coupled system of Poisson, Schrödinger and kinetic Boltzmann equations is given. A Monte Carlo model of electron transport in DA-pHEMT heterostructures which accounts for multivalley parabolic band structure, as well as relevant formulas for calculating electron scattering rates and scattering phase functions on polar optical, intervalley phonons and on impurities are developed. The results of a computational experiment involving the solution of the system of Poisson–Schrödinger–Boltzmann equations for the AlGaAs/GaAs/InGaAs/GaAs/AlGaAs heterostructure are presented. The distribution of electrons by energy subband in the main and satellite valleys and the field dependences of the electron drift velocity in each valley are calculated. It was discovered that there is no spatial transfer of electrons into wide-gap AlGaAs layers due to high barriers created by modulated-doped impurities. A comparative analysis of the electron drift velocities in the studied DA-pHEMT heterostructures and in the unstrained layer of the InGaAs is given.

Trapping Layers Prevent Dopant Segregation and Enable Remote Doping of Templated Self-Assembled InGaAs Nanowires.

Selective area epitaxy is a promising approach to define nanowire networks for topological quantum computing. However, it is challenging to concurrently engineer nanowire morphology, for carrier confinement, and precision doping, to tune carrier density. We report a strategy to promote Si dopant incorporation and suppress dopant diffusion in remote doped InGaAs nanowires templated by GaAs nanomembrane networks. Growth of a dilute AlGaAs layer following doping of the GaAs nanomembrane induces incorporation of Si that otherwise segregates to the growth surface, enabling precise control of the spacing between the Si donors and the undoped InGaAs channel; a simple model captures the influence of Al on the Si incorporation rate. Finite element modeling confirms that a high electron density is produced in the channel.

Estimation of interaction parameters in the Al-Ga-As-Sn-Bi system

The development of GaAs based high power side-input photovoltaic converters requires thick (50–100 μm) transparent gradient refraction layers that can be grown by liquid phase epitaxy. Such thick layers can also be used in LED structures. To solve the problem of AlxGa1-xAs conductivity reduction at the x∼40% point a five-component, Al-Ga-As-Sn-Bi system is proposed. The interaction parameters in the liquid phase (αij) in the Al-Ga-As-Sn-Bi system are determined within the framework of a quasi-regular solutions model. For an AlxGa1-xAs solid solution growing from a Ga-melt containing 10 at.% of Bi (as a neutral solvent) and 15 at.% of Sn (as an n-type dopant), liquidus and solidus isotherms for 900 °C are modeled based on the calculated αij. Satisfactory agreement between calculated and experimental data has been obtained. Hall data show that AlGaAs layers grown from Bi-containing melts have n-type conductivity. Doping by tin during growth from mixed Ga–Bi melts makes it possible to increase the electron concentration in the AlGaAs layer.

Al0.35Ga0.65As/InGaP heterojunction solar cell based on temperature-graded growth

The p-Al0.35Ga0.65As/n-InGaP heterojunction solar cells are promising competitors compared with conventional InGaP or AlGaAs solar cells as the heterogeneous combination can overcome the demerits of each material. However, InGaP has an optimized growth temperature much lower than that of AlGaAs in metal-organic vapor phase epitaxy (MOVPE) growth. Therefore, a challenge arises from oxygen contamination at the hetero-interface during temperature adjustment for obtaining MOVPE-grown high-quality p-AlGaAs/n-InGaP heterojunction. Here we report that a temperature-graded layer (TGL) of AlGaAs can solve the issue of growth temperature mismatch, and significantly improve the voltage performance of AlGaAs/InGaP solar cells. Absolute electroluminescence measurement and atomic force microscopy confirmed a smooth interface with less non-radiative recombination after TGL was applied. The temperature-graded growth is verified able to improve the AlGaAs/InGaP interface quality and provides a scalable method for AlGaAs-based heterogeneous growth.

Application of TiO2/Ag/TiO2 as an Ohmic Contact to an AlGaAs Layer in a GaAs Solar Cell

This paper investigates the possibility of using a nanolaminate TiO2/Ag/TiO2 structure as a transparent conductive coating on GaAs solar cells. A novel result is that this structure forms an Ohmic contact to Al-rich AlGaAs, which is used as a “window” layer in GaAs-based solar cells. The TiO2/Ag/TiO2 structure is deposited by RF magnetron sputtering at room temperature. This nanolaminate coating has good optical and electrical properties: a high transmittance of 94% at 550 nm, a sheet resistance of 7 Ω/sq, and a figure of merit (FOM) of 105 × 10−3 Ω−1. These properties are the result of the presence of a discontinuous layer of Ag between two thin layers of TiO2. The morphology of a discontinuous layer of Ag nanogranules is confirmed by the observation of a cross-section of a sample with high-resolution transmission electron microscopy (HRTEM) and EDX analyses. Current–voltage diode characteristics of GaAs solar cells measured under standard test illumination at 1000 W/m2 are analyzed. The formation of an Ohmic contact is explained by the Fermi-level pinning effect caused by nanosized Ag particles in the nanolaminate TiO2/Ag/TiO2 structure. The obtained results demonstrate a new application of oxide−metal−oxide (OMO) coatings as Ohmic contacts to III-V compound semiconductors.

Epitaxial lift-off method for GaAs solar cells with high Al content AlGaAs window layer

The epitaxial GaAs substrate lift-off (ELO) technique is widely used to produce thin film III–V semiconductor optoelectronic devices. However, the hydrofluoric (HF) acid used in the process forbids the incorporation of active AlGaAs device layers with high Al content due to its selective etching. In this work, a new ELO method is presented, which allows for the protection of AlGaAs layers against the HF acid attack. The method is used here to prepare and analyse thin film GaAs solar cells containing Al0.85Ga0.15As window layer. We employ a sacrificial GaAs buffer and device perimeter pre-processing that covers the exposed edges with back contact metals and electroplated Cu. A comparison of the epitaxially lifted GaAs solar cells with identical cells prepared by substrate etching demonstrates identical photovoltaic figures of merit and confirms the viability of the AlGaAs protection approach. In addition to applications in thin film photovoltaics, this ELO method can be applied to solid-state lasers and single-photon emitters employing AlGaAs-based Bragg reflectors, among other III–V semiconductor optoelectronic devices.

Simulation and Analysis of the Optical Characteristics of Cylindrical Micropillars with InAs/GaAs Quantum Dots

The optical characteristics of vertical cylindrical micropillars with AlGaAs distributed Bragg reflectors and InAs/GaAs quantum dots, which are designed for the fabrication of single-photon sources, have been studied. The effect of parameters such as the inclination angle of sidewalls, partial oxidation of AlGaAs layers, and deviation of quantum dots from the central axis of a micropillar on the Purcell factor and the radiation extraction efficiency has been numerically simulated by the finite-difference time-domain method. The allowable ranges of the listed parameters have been determined for cylindrical vertical 920-nm micropillars. The comparison of the calculations performed with the refined refractive indices of the used materials at cryogenic temperatures with the measured characteristics of the fabricated micropillar structures has confirmed the adequacy of the used models.

(Invited) Achieving High-Power Single-Mode Operation in Vertical-Cavity Surface-Emitting Lasers Via Scalable, Higher-Order Mode Suppression Techniques

Achieving high-power single-mode operation in Vertical-Cavity Surface-Emitting Lasers (VCSELs) has received renewed interest because of applications that span facial recognition and 3D imaging in mobile telephones, Light Detection and Ranging (LiDAR) for autonomous vehicles, and Pulse Amplitude Modulation (PAM-4) for high-speed (>100 Gbps) data transmission within data centers. A key challenge with oxide-confined VCSELs is that the strong refractive index contrast between the oxidized ~AlAs aperture and surrounding AlGaAs layers combined with the oxide aperture size create a photonic cavity that fundamentally supports multiple transverse modes. When the size of the aperture is reduced to create a single-mode cavity, the output power is limited by the electrically pumped gain region within the aperture. There is thus a fundamental trade-off between size and power which manifests itself in a limited capability to simultaneously achieve high power and single mode operation. Various methods have been employed by groups working on VCSELs to overcome this limitation. Neglecting external cavity methods which are fundamentally not scalable, the most promising results have been achieved by groups using etched surface relief where a pattern is etched in the top layer or layers to change the standing wave pattern in the cavity and reduce modal gain by lowering the magnitude of the electric field in regions overlapping with the quantum wells, which provide laser gain. A downside of this approach is that material is removed, impacting the uniformity of current injection across the area of the device. A key question of interest is therefore how to achieve mode suppression in large area devices in a way that does not impact current injection. In this presentation, novel methods for mode control that rely on spatial variation of modal gain across the area of the VCSEL aperture will be discussed, and results on high-power single-mode operations of VCSELs will be presented. Two distinct mode control methods will be reviewed. The first method of mode control uses deposited optical coatings on the top surface of the VCSEL to spatially modify the electric field standing wave in the quantum well heterostructure region. Through patterning the coating in either a pillar or anulus, specific modes can be suppressed by selectively changing the gain available for those modes. These filters have been made using both SiO2/TiO2 stacks and alternatively a single layer of silicon. The capability to pattern these layers in a wafer-scale process makes this method attractive for volume manufacturing. The second method of mode control utilizes impurity-induced layer disordering via patterned diffusion of Zn to selectively intermix Distributed Bragg Reflector (DBR) layers, thereby creating a top laser mirror whose reflectivity varies spatially across the aperture. Designing the diffused region to maximize overlap with high-order modes while minimizing overlap with the fundamental mode provides mode suppression and enhanced single-mode power. An added benefit of this approach is a reduction in resistance through the device top DBR layers brought about by the higher p-doping level in the Zn-diffused regions, minimizing Ohmic losses and heat generation. Through the use of strain control in the diffusion mask, diffusion front shape is also shown to be controllable in a way that can be used to minimize the impact of the diffused region on fundamental mode reflectivity. Tensile and compressive strain of diffusion masks are shown to have differing effects on vertical (into the device) versus lateral (across the area of the device under the mask) diffusion, allowing more precise spatial control of reflectivity. Using these methods, single-mode output powers in excess of 10 mW have been demonstrated, with side-mode suppression ratios in excess of 30 dB. These results will be discussed, as well as further work to enhance VCSEL performance. Figure 1

All-optical light control with a AlGaAs-based metal-PhC cavity via multiple optical Tamm states

All-optical light control is the key technology for realizing optical computing, ultrahigh-speed information processing, and optical neural network. In this work, all-optical light control is investigated with a AlGaAs-based metal-photonic crystal (PhC) cavity, based on photorefractive effect and optical Tamm state. The proposed structure can generate multiple optical Tamm states in the stop-band of the PhC. That makes the optical properties of the proposed structure highly dependent on the refractive index of the AlGaAs layer. Through turning on and off the gate laser, the refractive index of the AlGaAs layer slightly changes. This tiny variation will strongly influence the intensity of the signal light. Simulated results show the normalized intensity of the reflected signal light can be tuned from ~0.0 to ~0.5 under suitable conditions. Meanwhile, this all-optical light control can be adjusted by changing the incident angle or the position of the signal light.

Modeling free-carrier absorption in ultrathin III-V solar cells with light management.

Ultrathin III-V solar cells with proper light management have become more attractive than their optically thick counterparts as they are less expensive and lightweight, can maintain photon absorption, and have high radiation tolerance for space-related applications. Comprehensive optical modeling efforts have provided pathways to improve device efficiency in ultrathin GaAs solar cells with light trapping structures. Usually, the absorption mechanism known as free-carrier absorption (FCA) is ignored in these models due to the ultrathin layers and the direct bandgap of GaAs. This manuscript reports the significance of considering FCA as a parasitic loss caused by the optical enhancement in highly doped non-active layers between the ultrathin solar cell and backside light trapping structures. We model FCA based on Drude theory in a p-type AlGaAs layer behind ultrathin GaAs solar cells with a planar mirror and cylindrical gratings. Our results show that, depending on the AlGaAs thickness and doping concentration, free carriers will absorb transmitted photons and reduce the backside reflectance, degrading the current and voltage output from ideal conditions. One example shows that for a 300 nm-thick GaAs solar cell, the Ag mirror's peak reflectance decreases nearly 12% when the AlGaAs back layer is 800 nm-thick at a doping concentration of 4x1019 cm-3. Notably, the cylindrical grating designs with 38.5%, 46.5%, and 64.9% AlGaAs coverage resulted in an absolute efficiency reduction of 0.6%, 1.8%, and 2.9% at a doping concentration of 4x1019 cm-3, respectively. This novel study demonstrates that FCA in non-active layers must be properly addressed in the device design to progress the efficiency of ultrathin III-V solar cells with light trapping structures.

A Refined Prediction Parameter for Molecular Alignability in Stretched Polymers and a New Light-Harvesting Material for AlGaAs Photovoltaics.

Light-harvesting concentrators have a high potential to make highly efficient but precious energy converters, such as multijunction photovoltaics, more affordable for everyday applications. They collect sunlight, including diffusively scattered light, on large areas and redirect it to much smaller areas of the highly efficiency solar cells. Among the best current concepts are pools of randomly oriented light-collecting donor molecules that transfer all excitons to few aligned acceptors reemitting the light in the direction of the photovoltaics. So far, this system has only been realized for the 350–550 nm wavelength range, suitable for AlGaInP photovoltaics. This was achieved by using acceptor molecules that aligned during mechanical stretching of polymers together with donors, that stay random in that very same material and procedure. However, until recently, very little was known about the factors that are responsible for the alignability of molecules in stretched polymers and therefore it was difficult to find suitable donors and acceptors, as well as for other spectral ranges. Recently, a structural parameter was introduced with a high predictivity for the alignability of molecules that contain rigid band-like structures or linear aromatic π-systems. However, for light concentrators in more red spectral ranges, molecular systems often contain larger and extended, planar-like π-systems for which the previously reported parameter is not directly applicable. Here, we present a refined prediction parameter also suitable for larger plane-like structures. The new parameter depends on the number of in-plane atoms divided by out-of-plane atoms as determined by computational geometry optimization and additionally the planar aspect ratio for molecules that contain only in-plane atoms. With the help of this parameter, we found a new system that can efficiently collect and redirect light for the second 500–700 nm AlGaAs layer of current world-record multijunction photovoltaics. Similarly, as the previously reported system for the blue-green layer, it has also overall absorption and re-directioning quantum efficiencies close to 80–100%. Both layers, together, already cover about 75% of the energy in the solar spectrum.

Theoretical modeling and analyzing structural characteristics of AlGaAs/GaAs negative electron affinity array cathode with optically and electrically injected variable bandgap

In order to obtain high emission current efficiency of the AlGaAs/GaAs NEA array cathode, this array cathode has two ways to form electron emission, i.e. optical injection and electrical injection. The two-dimensional continuity equation of electronic transport in the variable bandgap emission array is solved numerically by using the finite volume method thereby obtaining the emission current and emission current efficiency. Simulation obtains the optimal parameter range for each layer of the AlGaAs/GaAs NEA array cathode under both optical injection and electrical injection. The results show that the optimal angle of incident light for the array cathode under light injection is 10°–30° for selecting an array micro-nano column with a duty cycle of 2/3. Under the condition of light injection, the P-type variable bandgap AlGaAs layer array micro-nano column height ranges from 0.3–0.6 μm. Under the condition of electrical injection, the height of the micro-nano column of the P-type variable bandgap AlGaAs layer array is 0.1–0.3 μm. The optimal thickness range of N-type variable bandgap AlGaAs layer, N-type AlGaAs layer, and P-type AlGaAs layer under light injection are 0.5–2.5 μm, 0.5–1.0 μm and 0.2–0.5 μm, respectively. The optimal thickness range of N-type variable bandgap AlGaAs layer, N-type AlGaAs layer, and P-type AlGaAs layer under electrical injection conditions are 2–3 μm, 0.8–1.2 μm and 0.1–0.3 μm, respectively. The optimal doping concentration of P-type AlGaAs layer and N-type AlGaAs layer under light injection are range from 5×10<sup>18</sup> to 1×10<sup>19</sup> cm<sup>–3</sup> and from 1×10<sup>18</sup> to 5×10<sup>18</sup> cm<sup>–3</sup>, respectively. The optimal doping concentration of the P-type AlGaAs layer and the N-type AlGaAs layer under electrical injection range from 1×10<sup>18</sup> to 5×10<sup>18</sup> cm<sup>–3</sup> and from 5×10<sup>17</sup> to 1×10<sup>18</sup> cm<sup>–3</sup>, respectively. The maximum efficiency of the emission current under the light injection is 35.04%, and the maximum emission current per unit length is 10.3 nA/μm. The maximum efficiency of the emission current under electrical injection is 31.23%, and the maximum emission current per unit length is 105.5 μA/μm. Electric injection cathode does not need expensive and complex drive laser system, and the light injection control mode is simple, so light injection or electrical injection control mode can be chosen as needed. The research of array cathode, which integrates the advantages of many advanced technologies, is of great significance for enriching the cathode array cathode emission theory and expanding its application field.

Selective oxidation of AlGaAs aperture layers of a Vertical-cavity surface-emitting laser with a generation wavelength of 850 nm

A study of the technology of selective oxidation of the buried AlGaAs layer used as an aperture layer in the structure of a Vertical-cavity surface-emitting laser has been carried out. Oxidation process was made as thermal oxidating in a humidified nitrogen atmosphere. The conditions of the oxidation process are described, images of the oxidation results and the dependence of the growth rate of the oxidized layer on the process temperature are presented. A technology for the formation of an oxide current aperture has been developed for vertical cavity surface emitting lasers with a generation wavelength of 850 nm, which makes it possible to accurately control the size and shape of the resulting aperture.

Signature of linear-in- k Dresselhaus splitting in the spin relaxation of X -valley electrons in indirect band gap AlGaAs

GaAs/AlGaAs quantum well (QW) system is utilized to investigate the electron spin relaxation in the satellite X-valley of indirect band gap Al0.63Ga0.37As epitaxial layers through polarization resolved photo-luminescence excitation spectroscopy. Solving the rate equations, steady state electronic distribution in various valleys of AlxGa1-xAs is estimated against continues photo carrier generation and an expression for the degree of circular polarization (DCP) of photoluminescence coming from the adjacent quantum well (QW) is derived. Amalgamating the experimental results with analytical expressions, the X-valley electron spin relaxation time ({\tau}_S^X) is determined to be 2.7 +/- 0.1 ps at 10 K. To crosscheck its validity, theoretical calculations are performed based on Density Functional Theory within the framework of the projector augmented wave method, which support the experimental result quite well. Further, temperature dependence of {\tau}_S^X is studied over 10-80 K range, which is explained by considering the intra-valley scattering of carriers in the X-valley of indirect band gap AlGaAs barrier layer. It is learnt that the strain induced modification of band structure lifts the degeneracy in X-valley, which dominates the electron spin relaxation beyond 50 K. Furthermore, the DCP spectra of hot electrons in indirect band gap AlGaAs layers is found to be significantly different compared to that of direct bandgap AlGaAs. It is understood as a consequence of linear k dependent Dresselhaus spin splitting and faster energy relaxation procedure in the X-valley. Findings of this work could provide a new horizon for the realization of next generation spin-photonic devices which are less sensitive to Joule heating.

Mid-wave infrared transmittance filters in suspended GaAs subwavelength gratings

The high-Q resonances observed in subwavelength gratings support many applications, including optical phase arrays, refractive index sensing, enhancement of nonlinear effects, and spectral filtering. Integrating these functions with active devices opens new possibilities for microscale optical systems. GaAs high-contrast gratings (HCGs) simplify the integration of resonant structures with active III–V semiconductor devices. These structures are particularly useful in the mid-wave infrared (5–7 μm), overcoming several limitations of the material systems presented in previous works. By selectively removing an underlying AlGaAs layer, a high index contrast is introduced between suspended GaAs gratings and air without using an exotic low-index substrate. Furthermore, epitaxial growth enables precise device thickness. In this work, we demonstrate GaAs/air HCG transmittance filters for operation in the mid-wave infrared. A numerical study investigates the dependence of filter performance on substrate proximity. We fabricate experimental filters using an HBr-based plasma etch and a selective wet etch. Optical characterization demonstrates quality factors >50 and transmittance peak efficiency >50%. The observed filter performance is comparable to other mid-wave infrared subwavelength grating filters. This work concludes with discussion of experimental nonidealities, emphasizing the considerable influence of grating size and optical measurement conditions. The suspended GaAs HCG filters show potential for direct integration with active devices such as photodetectors and lasers.

Luminescence from Droplet-Etched GaAs Quantum Dots at and Close to Room Temperature.

Epitaxially grown quantum dots (QDs) are established as quantum emitters for quantum information technology, but their operation under ambient conditions remains a challenge. Therefore, we study photoluminescence (PL) emission at and close to room temperature from self-assembled strain-free GaAs quantum dots (QDs) in refilled AlGaAs nanoholes on (001)GaAs substrate. Two major obstacles for room temperature operation are observed. The first is a strong radiative background from the GaAs substrate and the second a significant loss of intensity by more than four orders of magnitude between liquid helium and room temperature. We discuss results obtained on three different sample designs and two excitation wavelengths. The PL measurements are performed at room temperature and at T = 200 K, which is obtained using an inexpensive thermoelectric cooler. An optimized sample with an AlGaAs barrier layer thicker than the penetration depth of the exciting green laser light (532 nm) demonstrates clear QD peaks already at room temperature. Samples with thin AlGaAs layers show room temperature emission from the QDs when a blue laser (405 nm) with a reduced optical penetration depth is used for excitation. A model and a fit to the experimental behavior identify dissociation of excitons in the barrier below T = 100 K and thermal escape of excitons from QDs above T = 160 K as the central processes causing PL-intensity loss.

Temperature Dependence of Carrier Extraction Processes in GaSb/AlGaAs Quantum Nanostructure Intermediate-Band Solar Cells.

From the viewpoint of band engineering, the use of GaSb quantum nanostructures is expected to lead to highly efficient intermediate-band solar cells (IBSCs). In IBSCs, current generation via two-step optical excitations through the intermediate band is the key to the operating principle. This mechanism requires the formation of a strong quantum confinement structure. Therefore, we focused on the material system with GaSb quantum nanostructures embedded in AlGaAs layers. However, studies involving crystal growth of GaSb quantum nanostructures on AlGaAs layers have rarely been reported. In our work, we fabricated GaSb quantum dots (QDs) and quantum rings (QRs) on AlGaAs layers via molecular-beam epitaxy. Using the Stranski–Krastanov growth mode, we demonstrated that lens-shaped GaSb QDs can be fabricated on AlGaAs layers. In addition, atomic force microscopy measurements revealed that GaSb QDs could be changed to QRs under irradiation with an As molecular beam even when they were deposited onto AlGaAs layers. We also investigated the suitability of GaSb/AlGaAs QDSCs and QRSCs for use in IBSCs by evaluating the temperature characteristics of their external quantum efficiency. For the GaSb/AlGaAs material system, the QDSC was found to have slightly better two-step optical excitation temperature characteristics than the QRSC.