Atomic Hydrogen-assisted Molecular Beam Epitaxy Research Articles

Theoretical analysis of GaAs/AlGaAs quantum dots in quantum wire array for intermediate band solar cell

A GaAs quantum dot (QD) array embedded in a AlGaAs host material was fabricated using a strain-free approach, through combination of neutral beam etching and atomic hydrogen-assisted molecular beam epitaxy regrowth. In this work, we performed theoretical simulations on a GaAs/AlGaAs quantum well, GaAs QD and QD array based intermediated band solar cell (IBSC) using a combined multiband k·p and drift-diffusion transportation method. The electronic structure, IB band dispersion, and optical transitions, including absorption and spontaneous emission among the valence band, intermediate band, and conduction band, were calculated. Based on these results, maximum conversion efficiency of GaAs/AlGaAs QD array based IBSC devices were calculated by a drift-diffusion model adapted to IBSC under the radiative recombination limit.

Optical properties of multi-stacked InGaAs/GaNAs quantum dot solar cell fabricated on GaAs (311)B substrate

Quantum dot solar cells (QDSCs) comprised of 10 stacked pairs of strain-compensated InGaAs/GaNAs QD structure have been fabricated by atomic hydrogen-assisted molecular beam epitaxy. A homogeneous and high-density QD array structure with improved in-plane ordering and total density of ∼1012 cm−2 has been achieved on GaAs (311)B grown at 460 °C after stacking. The external quantum efficiency (EQE) of InGaAs/GaNAs QDSC increases in the longer wavelength range due to additive contribution from QD layers inserted in the intrinsic region. The short-circuit current density measured for QDSC is 17.2 mA/cm2 compared to 14.8 mA/cm2 of GaAs reference cell. Further, an increase in EQE due to photocurrent production by 2-step photon absorption has been observed at room temperature though it is still small at around 0.1%.

Photoluminescence from GaAs nanodisks fabricated by using combination of neutral beam etching and atomic hydrogen-assisted molecular beam epitaxy regrowth

We have fabricated GaAs nanodisk (ND) structures by using a combination of neutral beam etching process and atomic hydrogen-assisted molecular beam epitaxy regrowth. We have observed clear photoluminescence (PL) emissions from GaAs NDs. The peak energy showed a blueshift due to the quantum confinement in three spatial dimensions, and it agreed with the theoretically estimated transition energy. The PL results also showed that the cap-layer disks act as radiative recombination centers. We have confirmed that the PL emission originates from the GaAs NDs, and our approach is effective for the fabrication of high quality ND structures.

Transmission electron microscopy study of GaInNAs(Sb) thin films grown by atomic hydrogen-assisted molecular beam epitaxy

The quaternary GaInNAs is a promising material system for use in next generation multijunction photovoltaic devices. We have investigated the effect of introducing antimony on the growth by using transmission electron microscopy and energy dispersive x-ray (EDX) spectroscopy. Two-dimensional growth was observed in GaInNAs films with striation features associated with compositional fluctuation and nanometer scale elemental segregation on the growth front. On the contrary, GaInNAsSb films exhibit uniform contrast throughout. EDX profile indicates uniform compositional distribution, as antimony atoms suppress the surface mobilites of adatoms resulting in a lower probability to generate the favored bonds, such as Ga-N and In-As.

Reduction of carrier concentrations of [formula omitted]-[formula omitted] films by atomic hydrogen-assisted molecular beam epitaxy

We have grown intentionally undoped β-FeSi2 thin films on Si(111) substrates by atomic hydrogen-assisted molecular beam epitaxy. The conductivity of β-FeSi2 films changed from p to n-type, and the carrier concentration decreased drastically from the order of 1019 to that of 1016cm−3. These results show that the atomic hydrogen played an important role to decrease the number of Si vacancies acting as acceptors.

Growth of multi-stacked InAs/GaNAs quantum dots grown with As2 source in atomic hydrogen-assisted molecular beam epitaxy

Abstract We have investigated the effect of using As 2 source on the self-assembly process of 5 layer stacked InAs quantum dots (QDs) on GaAs(0 0 1) grown by atomic hydrogen-assisted molecular beam epitaxy. The deposition thickness of each QD layer was varied from 1.32 to 2.32 monolayers (MLs), and InAs QDs were embedded by a GaNAs strain-compensating layer (SCL). By using As 2 , high-density QDs with higher aspect ratio were formed during the stage of QD formation. A mono-modal size distribution with size fluctuation in diameter of 9.2% was obtained for As 2 sample, which was better than that of 13.6% obtained for As 4 sample. On the other, As 4 sample showed a bimodal size distribution in the initial stage of QD formation. The size distribution gradually improved with increasing InAs deposition and eventually saturated to become larger QDs with a mono-modal size distribution for As 4 sample. The saturated QDs exhibited almost the same PL properties emitting at around 1100 nm with a narrow linewidth of 40 meV at 77 K for both As 2 and As 4 samples.

The effect of spacer layer thickness on vertical alignment of InGaAs/GaNAs quantum dots grown on GaAs(3 1 1)B substrate

We fabricated multiple stacked self-organized InGaAs quantum dots (QDs) on GaAs (3 1 1)B substrate by atomic hydrogen-assisted molecular beam epitaxy (H-MBE) to realize an ordered three-dimensional QD array. High quality stacked QDs with good size uniformity were achieved by using strain-compensation growth technique, in which each In 0.35Ga 0.65As QD layer was embedded by GaNAs strain-compensation layer (SCL). In order to investigate the effect of spacer layer thickness on vertical alignment of InGaAs/GaNAs QDs, the thickness of GaNAs SCL was varied from 40 to 20 nm. We observed that QDs were vertically aligned in [3 1 1] direction when viewed along [0 1 −1], while the alignment was inclined when viewed along [−2 3 3] for all samples with different SCL thickness. This is due to their asymmetric shape along [−2 3 3] with two different dominant facets thereby the local strain field around QD extends further outward from the lower-angle facet. Furthermore, the inclination angle of vertical alignment QDs became monotonously smaller from 22° to 2° with decreasing SCL thickness from 40 to 20 nm. These results suggest that the strain field extends asymmetrically resulting in vertically tilted alignment of QDs for samples with thick SCLs, while the propagated local strain field is strong enough to generate the nucleation site of QD formation just above lower QD in the sample with thinner SCLs.

Structural properties of multi-stacked self-organized InGaAs quantum dots grown on GaAs (3 1 1)B substrate

We have investigated the structural properties of multi-stacked layers of self-organized In 0.4Ga 0.6As quantum dots (QDs) embedded with GaN 0.007As 0.993 strain compensation layers (SCLs) grown on GaAs (3 1 1)B substrate by atomic hydrogen-assisted molecular beam epitaxy. Symmetrical lens-shaped QDs are observed along [ 0 1 1 ¯ ] , while their shape is asymmetric along [ 2 ¯ 3 3 ] with two different dominant facets. Further, QDs are vertically aligned in the growth direction when viewed along [ 0 1 1 ¯ ] , while the alignment is inclined at an angle of 22° with respect to the growth direction when viewed along [ 2 ¯ 3 3 ] . The inclination angle is in good agreement with the result of resonant diffuse X-ray scattering sheets in reciprocal space mapping around GaAs (3 1 1) lattice point. We believe that the local strain field around QD extends further outward from the lower-angle facet, thereby the vertical alignment is tilted along the direction of stronger strain field.

Improvement of GaInNAsSb films fabricated by atomic hydrogen-assisted molecular beam epitaxy

The effect of incorporation of antimony in GaInNAs films grown by atomic hydrogen-assisted molecular beam epitaxy (MBE) has been investigated. We show that the rate of incorporation of N and In forming GaInNAs do not depend on the Sb beam flux. However, the incorporation of Sb is strongly dependent on the Sb/As2 flux ratio. Introducing a small amount of Sb (<∼1%) significantly improves the photoluminescence (PL) emission efficiency of GaInNAs, but Sb concentration of >1% rapidly degrades the PL intensity, though a large redshift can still be achieved. Therefore, there is an optimum amount of Sb for the growth of low-strained GaInNAs films to improve the overall optical quality.

Fabrication of homojunction GaInNAs solar cells by atomic hydrogen-assisted molecular beam epitaxy

The effect of atomic hydrogen irradiation during molecular beam epitaxy (MBE) on the electrical properties of GaInNAs thin films and the characteristics of homojunction GaInNAs solar cells are presented. We found that an optimum amount of atomic H flux irradiated during MBE growth of GaInNAs material is effective for suppressing or passivating the defects that act to mainly trap the electrons. Furthermore, an enhanced layer-by-layer growth improves the uniformity of GaInNAs layers thereby reducing the segregation and clustering of N atoms. A p–i–n Ga 0.984In 0.016N x As 1− x solar cell fabricated by atomic H-assisted MBE shows a short-circuit current density of 22.2 mA/cm 2.

Strain-compensated InAs/GaNAs quantum dots for use in high-efficiency solar cells

We have investigated GaAs-based p-i-n quantum dot solar cells (QDSCs) with 10 up to 20 stacked layers of self-assembled InAs quantum dots (QDs) grown by atomic hydrogen-assisted molecular beam epitaxy. The net average lattice strain was minimized by using the strain-compensation technique, in which GaNAs dilute nitrides were used as spacer layers. The filtered short-circuit current density beyond GaAs bandedge was 2.47 mA/cm2 for strain-compensated QDSC with 20 stacks of InAs QD layers, which was four times higher than that for strained QDSC with identical cell structure.

Growth of GaNAs films with As 2 source in atomic hydrogen-assisted molecular beam epitaxy

We have investigated the effect of using As 2 source instead of a more commonly used As 4 source on the crystalline quality of GaN x As 1− x thin films grown by atomic hydrogen-assisted molecular beam epitaxy. Improved structural and optical properties of GaNAs thin films were obtained by using As 2 source. The nitrogen atoms were incorporated into GaAs at a more stable rate under As 2 flux than As 4 flux, and a two-dimensional nucleation growth mode was promoted for growth of GaN x As 1− x with As 2 source. As a consequence, the surface roughness measured for a 500 nm-thick Ga 0.008N 0.992As sample grown with As 2 flux was 1–2 monolayers, which was three times more smoother than that for As 4 sample. The photoluminescense measurements showed an improved potential fluctuation of 78.1 meV and twice the intensity at room temperature for As 2 sample.

Carrier mobility characteristics in GaInNAs dilute nitride films grown by atomic hydrogen-assisted molecular beam epitaxy

We have investigated the electrical properties of GaInNAs dilute nitride films grown by atomic hydrogen-assisted molecular beam epitaxy. We found that although the hole mobilities in Be-doped p-GaInNAs films exhibit a temperature dependence nearly identical to that for the homoepitaxial p-GaAs films, the electron mobilities in Si-doped n-GaInNAs films are strongly affected by the introduction of nitrogen into Ga(In)As. Further, the degree of scattering by the ionized impurity-like centers generated by N atoms decreased with increasing Si doping, while neutral impurity-like scattering became more dominant with increasing Si doping. These results suggest that the decrease of electron mobility and carrier concentration in Si-doped n-GaInNAs films is strongly correlated with the presence of N and Si atoms.

Effect of growth temperature on the properties of Ga(In)NAs thin films by atomic hydrogen-assisted RF-MBE

The effect of growth temperature on the crystal quality of Ga 1− y In y N x As 1− x thin films by atomic hydrogen-assisted molecular beam epitaxy (H-MBE), and the characteristics of GaAs/Ga 1− y In y N x As 1− x heterojunction solar cells were investigated. A high growth temperature of ∼520 °C resulted in both an improved crystal quality and solar cell performance. The PL peak intensity measured for GaN 0.003As 0.997 film was three times higher than that for the sample grown at a more conventional growth temperature of 480 °C. Furthermore, the highest electron mobility of 250 cm 2/V s was obtained for the sample grown at 520 °C. An unoptimized p-GaAs/i–n-GaInNAs heterojunction solar cell grown by H-MBE showed a maximum quantum efficiency of 50% and good diode factor of 1.4, respectively.

MBE growth of InAs self-assembled quantum dots embedded in GaNAs strain-compensating layers

We fabricated self-assembled InAs quantum dots (QDs) embedded with a combined set of a GaNAs strain-compensating layer (SCL) and an InGaAs strain-reducing layer (SRL) by using atomic hydrogen-assisted molecular beam epitaxy (H-MBE) with a RF plasma nitrogen source. By inserting an InGaAs SRL between the InAs QD layer and GaNAs SCL, we were able to achieve a significant improvement of optical properties. A 1.3 μ m -range photoluminescence (PL) emission was clearly obtained at 300 K from QDs embedded in a 5 nm-thick In 0.15Ga 0.85As SRL followed by a 20 nm-thick GaN 0.008As 0.992 SCL. Further, PL emission from the first excited state of QDs was also observed under a high excitation intensity of ∼ 20.0 W / cm 2 .

CORRUGATED SURFACES FORMED ON GaAs (331)A SUBSTRATES: THE TEMPLATE FOR LATERALLY ORDERED InGaAs NANOWIRES

Morphology evolution of high-index (331) A surfaces during molecular beam epitaxy (MBE) growth have been investigated in order to uncover their unique physic properties and fabricate spatially ordered low dimensional nanostructures. Atomic Force Microscope (AFM) measurements have shown that the step height and terrace width of GaAs layers increase monotonically with increasing substrate temperature in conventional MBE. However, this situation is reversed in atomic hydrogen-assisted MBE, indicating that step bunching is partly suppressed. We attribute this to the reduced surface migration length of Ga adatoms with atomic hydrogen. By using the step arrays formed on GaAs (331) A surfaces as the templates, we fabricated laterally ordered InGaAs self-aligned nanowires.

Multiple stacking of self-assembled InAs quantum dots embedded by GaNAs strain compensating layers

We have investigated a growth technique to realize high-quality multiple stacking of self-assembled InAs quantum dots (QDs) on GaAs (001) substrates, in which GaNxAs1−x dilute nitride material was used as a strain compensation layer (SCL). The growth was achieved by atomic hydrogen-assisted rf molecular beam epitaxy, and the effect of strain compensation was systematically investigated by using high-resolution x-ray diffraction measurements. By controlling the net average lattice strain to a minimum by covering each QD layer with a 40-nm-thick GaN0.005As0.995 SCL, we obtained a superior QD structure with no degradation in size homogeneity. Further, no dislocations were generated even after 30 layers of stacking, and the area density of QDs amounted to as high as 3×1012cm−2. The photoluminescence peak linewidth was improved by about 22% for QDs embedded in GaNAs SCLs as the accumulation of lattice strain with increasing growth of QD layers was avoided, which would otherwise commonly lead to degradation of size homogeneity and generation of dislocations.

Optimization of Ga(In)NAs thin film growth by atomic hydrogen-assisted molecular beam epitaxy

ABSTRACTThe effect of growth temperature on the crystal quality and optical properties of Ga(In)NAs films was investigated over a range of 340 ∼ 520 °C. We found that Ga(In)NAs films fabricated at lower growth temperatures generally result in an improved crystal quality. An XRD linewidth of as low as 45 arcsec was obtained for a 1 µm-thick Ga0.94In0.06N0.01As0.99 thin film grown at 380 °C. This is ∼1/2 of that grown at the conventionally-adopted growth temperature of 480 °C. After annealing, an improved optical property represented by a higher PL intensity compared to the conventional growth method (annealed, Tgrowth = 480 °C) was also obtained in the 1 µm-thick Ga0.94In0.06N0.01As0.99 thin film grown at low temperature of 380 °C.

Growth of self-assembled GaInNAs quantum dots by atomic-H assisted RF molecular beam epitaxy

Self-organized Ga x In 1− x N y As 1− y quantum dots (QDs) were fabricated on GaAs (001) substrates by atomic hydrogen-assisted molecular beam epitaxy with a radio frequency nitrogen plasma source (RF-MBE). We have investigated the effect of N 2 flow rate on growth dynamics and optical properties of GaInNAs QDs. With increasing N 2 flow rate, both the QD size and size fluctuation became significantly larger possibly due to phase separation and non-uniformity of N atom concentration, thereby causing linewidth broadening of PL emission of band-to-band transition. We found that N 2 flow rate during the growth of GaInNAs QDs on GaAs (001) in RF-MBE strongly affects the growth kinetics and optical properties. Further, appropriate post-growth annealing was found to be effective in improving PL emission efficiency of both thin GaInNAs films and QDs. Finally, we obtained high-quality 1.3-μm wavelength PL emission with a narrow linewidth of 98.4 meV at room temperature from optimally annealed GaInNAs QDs.

Growth of high-quality GaAs/Si films for use in solar cell applications

The characterization of thin film GaAs/Si solar cell structures grown by low-temperature atomic hydrogen-assisted molecular beam epitaxy (H-MBE) is presented. With assistance of atomic hydrogen irradiation in MBE growth, an enhanced two-dimensional growth and sharp photoluminescence peaks with linewidths of less than 10 meV at 77 K were obtained. We also found that GaAs/Si layers grown at low temperatures were strained, and gradually relaxed with increasing layer thickness. Further, by introducing an optimally designed Bragg reflector (BR) superlattice structure, remarkable improvements of spectral response were observed in GaAs/Si solar cells near the bandedge region of 750–850 nm, which is often degraded in common GaAs/Si solar cells. Suppression of threading dislocation densities due to lattice mismatch and thermal expansion coefficient difference was achieved by the combined approach of BR superlattice insertion and low-temperature growth scheme by H-MBE.