Ga Interdiffusion Research Articles

Se interlayer in CIGS absorption layer for solar cell devices

A CIGS absorber layer with high gallium contents in the space-charge region can reduce the carrier recombination and improve the open circuit voltage Voc. Therefore, controlling Ga grading on top of CIGS thin film solar cells is the main objective of this experiment. To reduce Selenium (Se) vacancy, it is important that the diffusion of Ga elements into Se vacancy between Mo back contact and CIGS absorption layer would be controlled. In order to reduce Se vacancy and confirm Ga inter-diffusion, two CIGS solar cells were fabricated by converting CIG precursor with and without Se interlayer. The copper-indium metallic precursors were fabricated corresponding to the sequence CuIn/In/Mo/STS on stainless steel (STS) substrates by sequential direct current magnetron sputtering while Se layer was evaporated by rapid thermal annealing (RTA) system to obtain a Se/CuIn/In/Mo/STS stack. CuGa precursor layer was also fabricated on the Se/CuIn/In/Mo/STS stack. Finally, both CuGa/Se/CuIn/In/Mo/STS and CuGa/CuIn/In/Mo/STS stacks were selenized at 500°C for 1h. It was clearly observed from the secondary ion mass spectroscopy (SIMS) and X-ray diffraction (XRD) that there was a change between the fabricated CIGS absorption layers and the amount of Ga elements. Furthermore, the Ga elements gradually decreased from the top to the bottom layer of the CIGS absorption layer. We also discussed the effect of Se interlayer in the CIGS absorption layer and its influence on the solar cell’s performance.

Fabrication of InxGa1−xN/GaN QDs with InAlGaN capping layer by coaxial growth on non-(semi-) polar n-GaN NWs using metal organic chemical vapor deposition for blue emission

Merits of InAlGaN capping layer over self-assembled InxGa1−xN/GaN quantum dots coaxially grown on n-GaN nanowires using MOCVD.

Influence of Varying Cu Content on Growth and Performance of Ga-Graded Cu(In,Ga)Se2 Solar Cells

Cu(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin-film solar cells with Ga-graded absorber layers and a [Cu]/([In] + [Ga]) ratio varying between 0.5 and 1.0 were prepared by coevaporation and investigated. Except for the sample with a final [Cu]/([In] + [Ga]) ratio of 1.0, the samples were Cu-poor at all times during the evaporation. The variation in copper was found to influence the material properties in several ways: 1) Changing the Cu content had a strong impact on In and Ga interdiffusion, resulting in decreased Ga gradients in samples with large Cu deficiency; 2) the Cu-poor Cu(In,Ga) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> phase was detected in absorbers with [Cu]/([In] + [Ga]) ratios of 0.65 and below; and 3) the grain size changed significantly with the Cu variation. We observe a trend of reduced solar cell efficiencies for [Cu]/([In] + [Ga]) ratios of 0.65 and below, with an efficiency of 13.4% for the sample with a [Cu]/([In] + [Ga]) ratio of only 0.5, i.e., far from stoichiometry. We tentatively attribute the efficiency loss to a high concentration of point defects caused by the Cu deficiency.

Enhanced performance of ultra-thin Cu(In,Ga)Se2 solar cells deposited at low process temperature

To investigate the process temperature on the growth of ultra-thin (≤500nm) Cu(In,Ga)Se2 (CIGSe) absorbers and the corresponding performance of solar cells, the process temperature was set to 610°C and 440°C, respectively. It was found that the low process temperature (440°C) could reduce the inter-diffusion of Ga–In and thus result in a higher back [Ga]/([Ga]+[In]) ([Ga]/[III]) grading than at the temperature of 610°C. The higher back [Ga]/[III] grading at 440°C was evidenced to both electrically and optically contribute to the efficiency enhancement of the solar cells in contrast to the lower back [Ga]/[III] grading at 610°C. It was also implied that the high back [Ga]/[III] grading was beneficial to the collection of carriers generated from the back-reflected light.

Influence of substrate and its temperature on the optical constants of CuIn1−xGaxSe2 thin films

We investigate the influence of substrate and its temperature on the optical constants of CuIn1−xGaxSe2 (CIGSe) thin films using the transfer-matrix method. The optical constants of a CIGSe layer on top of a transparent conducting oxide (TCO) layer were calculated considering the realistic optical constants of the TCO layer after CIGSe deposition. It was found that TCO substrates could influence the optical constants of CIGSe layers and that the ITO (Sn doped In2O3) substrate had a greater impact than IMO (Mo doped In2O3) for the CIGSe (x = 0.4) film when compared to a reference on bare glass substrate. Additionally, the varied substrate temperatures did not impact the optical constants of CGSe (x = 1). For CIGSe (x = 0.4), the refractive index n stayed relatively independent although at low temperature the grain size was reduced and the Ga/(Ga+In) profile was altered compared to that at high temperature (610 °C). In contrast, the extinction coefficient k at low temperature showed higher absorption at longer wavelengths because of a lower minimum bandgap (Eg,min) originating from reduced inter-diffusion of Ga–Se at a low substrate temperature.

Fabricating Ni–Mn–Ga microtubes by diffusion of Mn and Ga into Ni tubes

Tubes of the ferromagnetic shape-memory alloy Ni–Mn–Ga of composition near the Ni2MnGa Heusler phase can be used, alone or combined in structures, in magnetic actuators or magnetic refrigerators. However, fabrication of Ni–Mn–Ga tubes with sub-millimeter diameter by classical cold or hot drawing methods is hampered by the brittleness of the alloy. Here, we demonstrate a new process, where Ni–Mn–Ga tubes are fabricated by interdiffusion of Mn and Ga into drawn, ductile Ni tubes with 500 and 760 μm inner and outer diameters. After interdiffusion and homogenization of Mn and Ga at 1000 °C for 24–36 h, Ni–Mn–Ga tubes with ∼300 and ∼900 μm inner and outer diameters were obtained with homogeneous radial composition distribution, independently of the diffusion sequences (i.e., Mn and Ga diffused sequentially or simultaneously). Longitudinal composition was uniform over lengths of ∼1 mm, but variable over longer length due to incomplete process control. For two of the three diffusion sequences, a sizeable (20–80 μm) region exhibiting Kirkendall pores formed at the outer surface of the tubes. Magnetization values as high as ∼60 emu/g were measured, which is comparable to the magnetization of the Ni2MnGa Heusler phase. X-ray diffraction on the tube with the highest magnetization confirmed the room-temperature structure as cubic austenite.

디지털 합금 InGaAlAs 다중 양자 우물의 열처리 온도에 따른 발광 특성

The effect of rapid thermal annealing (RTA) on the optical properties of digital-alloy InGaAlAs multiple quantum well (MQW) structures have been investigated by using photoluminescence (PL) and time-resolved PL measurements as a function of RTA temperature. The MQW samples were annealed from to for 30 s in a nitrogen atmosphere. The MQW sample annealed at exhibited the strongest PL intensity and the narrowest FWHM (Full width at half maximum), indicating the reduced nonradiative recombination centers and the improved interfaces between the wells and barriers. The MQW samples annealed at and showed the decreased PL intensities and blueshifted PL peaks compared to -annealed sample. The blueshift of PL peak with increasing RTA temperatures are ascribed to the increase of aluminum due to intermixing of gallium (Ga) and aluminum (Al) in the interfaces of InGaAs/InAlAs short-period superlattices. The decrease of PL intensity after annealing at and are attributed to the interface roughening and lateral composition modulation caused by the interdiffusion of Ga and Al and indium segregation, respectively. With increasing RTA temperature the PL decay becomes slower, indicating the decrease of nonradiative defect centers. The optical properties of digital-alloy InGaAlAs MQW structures can be improved significantly with optimum RTA conditions.

Molecular beam epitaxy growth of InSb1−xBix thin films

Molecular beam epitaxy growth for InSb1−xBix thin films on (100) GaAs substrates is reported. Successful Bi incorporation for 2% is achieved, and up to 70% of the incorporated Bi atoms are at substitutional sites. The effects of growth parameters on Bi incorporation and surface morphology are studied. Strong In and Ga inter-diffusion induced by Bi incorporation is observed and discussed.

Ion beam-mixing effects in nearly lattice-matched AlInN/GaN heterostructures by swift heavy ion irradiation

Ion beam-induced intermixing is of great interest and has been observed in various systems such as metal–semiconductor interfaces. However, similar effects in III-nitrides have not been studied in any detail, yet. In the present study, swift heavy ion-induced intermixing of nearly lattice-matched Al(1−x) In x N/GaN heterostructures have been investigated using Rutherford backscattering spectrometry and high-resolution X-ray diffraction techniques. Inter-diffusion of Ga and In elements across the Al(1−x) In x N/GaN interface and the consequent formation of a new mixed quaternary alloy (AlGaInN) layer have been observed. The influence of electronic energy loss of SHI on intermixing effects has been studied.

A study of CuGaSe 2 thin film growth from multilayered precursors

In the growing process of CuGaSe 2 thin films for solar cells, a bilayered GaSe/CuSe structure has been widely used as a precursor and characteristic of the bilayered precursors which have been studied in many research groups. It is well known that interdiffusion of Cu and Ga by concentration difference is a prerequisite for growing CuGaSe 2 films from the GaSe/CuSe precursor. In the interdiffusion process, it is considered that the thinner each precursor layer is, the smaller activation energy and time needed to form the single phase CuGaSe 2. In this study, the comparison of CuGaSe 2 thin films from the multilayered precursor and the bilayered precursor was focused on in order to confirm the multilayered precursor to be a promising candidate for obtaining the single phase CuGaSe 2 thin films. These results show that the CuGaSe 2 pathway is dependent on the precursor structure. A bilayer precursor stack reveals that a phase transition from CuSe to CGS single phase accompanied by the growth of CuSe(006) plane and Cu 2-xSe (111). However, GaSe/CuSe precursors do not show any intermediate phase transformations for the 4 and 6 sequences.

Effect of interdiffusion on band structure and intersubband absorption coefficient of GaAs/GaAlAs double quantum well

The influence of Ga and Al atoms interdiffusion on the band structure and absorption coefficient of coupled double quantum wells are investigated on the basis of a potential composed of four modified Wood–Saxon potentials. Wave functions of the real potential are expressed as a superposition of the stationary state wave functions of the “basal” potential. It is shown that interdiffusion leads to the smearing of the rectangular profile of the potential and to the disappearance of the barrier between the wells, as a result of which, the degradation of doublet character of the electron energetic spectrum is observed. It is also shown that interdiffusion leads to the significant change in oscillator strength and in the enhancement of the thermal stability of the absorption spectrum of the considered heterostructure.

Optoelectronic evaluation of the nanostructuring approach to chalcopyrite-based intermediate band materials

Nanostructured chalcopyrite compounds have recently been proposed as absorber materials for advanced photovoltaic devices. We have used photoreflectance (PR) to evaluate the impact of interdiffusion phenomena and the presence of native defects on the optoelectronic properties of such materials. Two model material systems have been analyzed: (i) thin layers of CuGaSe 2 ( E g =1.7 eV) and CuInSe 2 (1.0 eV) in a wide/low/wide bandgap stack that have been grown onto GaAs(0 0 1) substrates by metalorganic chemical vapor deposition (MOCVD); and (ii) thin In 2S 3 samples ( E g =2.0 eV) containing small amounts of Cu that have been grown by co-evaporation (PVD) intending to form Cu x In y S z ( E g ∼1.5 eV) nanoclusters into the In 2S 3 matrix. The results have been analyzed according to the third-derivative functional form (TDFF). The valence band structure of selenide reference samples could be resolved and uneven interdiffusion of Ga and In in the layer stack could be inferred from the shift of PR-signatures. Hints of electronic confinement associated to the transitions at the low-gap region have been found in the selenide layer stack. Regarding the sulphide system, In 2S 3 is characterized by the presence of native deep states, as revealed by PR. The defect structure of the compound undergoes changes when incorporating Cu and no conclusive result about the presence of ternary clusters of a distinct phase could be drawn. Interdiffusion phenomena and the presence of native defects in chalcopyrites and related compounds will determine their potential use in advanced photovoltaic devices based on nanostructures.

Direct Evidences of Enhanced Ga Interdiffusion in InAs Vertically Aligned Free-Standing Nanowires

Direct evidences of enhanced Ga interdiffusion in InAs free-standing nanowires grown at moderate temperatures by molecular beam epitaxy on GaAs (111)B are presented in this work. Scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction measurements in coplanar and grazing incidence geometries show that nominally grown InAs NWs are actually made of an In0.86Ga0.14As alloy. Unlike typical vapor-liquid-solid growth, these nanowires are formed by diffusion-induced growth combined with strong interdiffusion from substrate material. Based on the experimental results, a simple nanowire growth model accounting for the Ga interdiffusion is also presented. This growth model could be generally applicable to the molecular beam heteroepitaxy of III-V nanowires.

Effect of Al and Ga interdiffusion on the electronic states in GaAs/Ga1−xAlxAs semiconductor superlattice

The effect of interdiffusion of Al and Ga atoms on the confining potential and band structure of a three-dimensional superlattice, composed of initially spherical GaAs/Ga1−xAlxAs quantum dots, is investigated in the framework of the modified Wood-Saxon potential model. It is shown that the interdiffusion leads to the disappearance of the quantum dots’ spherical symmetry and to the broadening of the superlattice energy minibands.

Effect of Growth Parameters on Composition Distribution in Superlattice-in-Well Structure by Submonolayer Deposition Technique

The superlattice-in-well structures were grown using a cycled submonolayer AlGaAs/GaAs deposition technique. The optical quality of Al–Ga interdiffusion in AlGaAs/GaAs superlattice was investigated by measuring the photoluminescence of samples grown at temperature from 610◦C to 630◦C. Results show that Al composition can be modulated under some growth temperature or period. Effect of the growth interrupt in the growth process of superlattice on film optical quality is also discussed. Especially, the role played by the period of superlattice in the process of obtaining high quality film material with low composition is investigated in detail.

Quantitative compositional profiles of enhanced intermixing in GaAs/AlGaAs quantum well heterostructures annealed with and without a SiO2 cap layer

Cross-sectional scanning transmission electron microscopy together with energy-dispersive x-ray spectroscopy (EDX) was used to quantitatively analyze quantum well intermixing (QWI) of a GaAs quantum well (QW) structure with AlGaAs barriers for samples capped with SiO2 or uncapped. Energy-dispersive x-ray analysis shows inter-diffusion of Ga and As into the SiO2 layer and Si into top GaAs layer. The enhanced QWI due to rapid thermal annealing (RTA) with the cap layer produces a square-like broadening of the QWs, while the QW broadening remains mostly Gaussian like for uncapped samples. A photoluminescence blueshift is seen for both capped and uncapped samples after RTA. Void-like defects have been observed to form and coalesce at higher anneal temperatures in the top GaAs layer at the interface with the SiO2. This is consistent with the Kirkendall process occurring at the interface producing the voids and also resulting in the production of interstitial atoms which diffuse employing the ‘kick-out’ mechanism to produce the flat-bottomed QW broadening.

Rapid thermal annealing effects on self-assembled quantum dot and quantum ring structures

Low temperature photoluminescence spectroscopy is used to analyze the effects of the postgrowth thermal annealing on the electronic structure ad carrier dynamics of GaAs∕AlGaAs quantum dot and quantum ring structures grown by droplet epitaxy. All the samples show a large increase in the photoluminescence efficiencies after the thermal treatment due to sizeable reduction in the material defectivity. Modifications of the photoluminescence band, which depend on thermal annealing temperature, are found and quantitatively interpreted by means of a simple model based on the Al–Ga interdiffusion.

Thermal stability of C-doped GaAs/AlAs DBR structures

Thermal stability of heavily carbon-doped and undoped DBRs has been investigated by reflectivity measurements and Raman spectroscopy. These analytical techniques are used to study the effect of heavy C-doping on Al–Ga interdiffusion during subsequent high-temperature anneals. Reflectivity spectra around the DBR stop-band wavelength clearly show that the growth-rate is reduced due to etching associated with the CBr4 precursor used, but they also indicate that no Al–Ga interdiffusion that could significantly degrade the DBR performance takes place for any samples during annealing. The results are supported by Raman spectra, which indicate the positions of the LO and LOPC modes do not change when the DBRs are annealed, whether the DBRs are doped or not. Simulations of Al–Ga interdiffusion at GaAs/AlAs DBR interfaces indicates that intermixing up to ∼15nm on either side of each interface will not affect the reflectivity of the DBR stack significantly. The observed small changes in the stop-band central wavelength and peak reflectivity due to annealing is most likely a consequence of increased surface roughness resulted from annealing.

Thermal-induced intermixing effects on the optical properties of long wavelength low density InAs/GaAs quantum dots

The effect of post-growth rapid thermal annealing on the photoluminescence properties of long wavelength low density InAs/GaAs (001) quantum dots (QDs) with well defined electronic shells has been investigated. For an annealing temperature of 650 °C for 30 s, the emission wavelength and the intersublevel spacing energies remain unchanged while the integrated PL intensity increases. For higher annealing temperature, blue shift of the emission energy together with a decrease in the intersublevel spacing energies are shown to occur due to the thermal activated In–Ga interdiffusion. While, this behaviour is commonly explained as a consequence of the enrichment in Ga of the QDs, the appearance of an additional exited state for annealing temperatures higher than 650 °C suggests a variation of the intermixed QDs's volume/diameter ratio toward QDs's enlargement.

Optical loss and interface morphology in AlGaAs∕GaAs distributed Bragg reflectors

It is shown that n-type doping of AlGaAs∕GaAs distributed Bragg reflectors (DBRs) grown by metal-organic vapor-phase epitaxy has a profound negative impact on the performance of vertical-cavity surface-emitting lasers (VCSELs) based on such mirrors. Using an intracavity contact scheme, 1.3-μm-range InGaAs VCSELs with and without doping in the bottom DBR are directly compared. Doped mirrors lead to lower slope efficiency, lower output power, and higher threshold current. From x-ray diffraction, high-accuracy reflectance measurements, and atomic force microscopy studies, it is suggested that this performance degradation is due to the doping-enhanced Al–Ga interdiffusion, leading to interface roughening and increased scattering loss.