GaP Films Research Articles

Low temperature plasma enhanced deposition of GaP films on Si substrate

Amorphous and microcrystalline GaP films were deposited on Si substrates by time modulated plasma enhanced deposition from trimethylgallium and phosphine using constant hydrogen plasma at a temperature of 250–380 °C. Amorphous GaP films obtained at constant low radio-frequency (RF) power (20 W) mode exhibit the broad feature at 350–360 cm−1 and a shoulder at 370–390 cm−1 in Raman spectra. Amorphous films have smooth surface with root-mean-square (RMS) roughness decreasing from 0.9 to 0.2 nm with increasing deposition temperature from 250 to 380 °C. Small amounts of 3–5 nm GaP nanocrystal inclusions in the amorphous matrix are formed at 380 °C. An increase of RF power to 100 W during Ga and P deposition steps leads to the formation of a GaP crystalline phase as confirmed by transmission electron microscopy. Two peaks in the Raman spectra at 365 and 402 cm−1, which correspond to GaP TO-LO duplet, were observed in this case. However, the microcrystalline GaP layers have rough surface with RMS roughness of 6 nm. A critical role of the hydrogen plasma for GaP growth mechanism is demonstrated. Indeed an effect of the phosphorous accumulation in the deposition chamber during phosphine plasma assisted decomposition step is shown. The hydrogen plasma treatment after P deposition step allows one to avoid an influence of excess phosphorous to the growth process providing a way to obtain ALD like growth of GaP with good thickness uniformity.

Detection of African Trypanosomes Using Asymmetric Double-Split Ring Based THz Sensors

Human African Trypanosomiasis is a highly dangerous tropical disease for which no fast, reliable, and sensitive screening tests are available to date. In this paper, the foundation of a novel electromagnetic sensor concept for aptamer-based direct detection of African trypanosome parasite layers is examined. Special attention is given to the sensitivity of asymmetric double-split ring resonators regarding different resonance modes in dependence on ring gap and target film dimensions. A specific sensor array design is then prototyped and tested using photoresist and actual trypanosome parasite coatings. Its response to different types and mixtures of cells in buffer solution is examined in detail to determine detectability of the parasite in varying dielectric contrast.

Radio frequency sputter deposition of Cu2ZnSnS4 thin films with a temperature-controlled reflector wall: Effects of H2 addition to the sputtering gas

Cu2ZnSnS4 (CZTS) thin films were deposited from a quaternary ceramic compound target using radio-frequency magnetron sputtering in Ar and Ar–10% H2 to investigate the effects of H2 addition to Ar on the film structure and optical and electrical properties. The sputter-deposition system was equipped with a cylindrical reflector wall between the target and substrate to enhance the probability of reimpingement of Zn and S atoms re-evaporated from the growing thin-film surface to the film surface. The thin films deposited in both Ar and Ar–10% H2 were identified as kesterite Cu2ZnSnS4 with 〈112〉 preferred orientation by X-ray diffraction measurements and Raman spectroscopy. The crystallite size evaluated from the half width at full-maximum of the 〈112〉 diffraction peak increased from 10 to 12 to 24nm with the H2 addition to Ar at substrate temperatures<200°C. The lattice spacing of (112) also increased by 2–3nm with the H2 addition to Ar. Cross-sectional scanning electron microscopy images of the thin films revealed that the thin-film structure changed from a fine to coarse columnar structure with the H2 addition to Ar. X-ray photoelectron spectroscopy analysis indicated that the O content decreased, whereas the Cu content increased with the H2 addition. The band gap range of the CZTS thin films deposited in Ar–10% H2 was 1.4–1.5eV, which was narrower than that of the films deposited using Ar. The Hall mobility and carrier density were not greatly affected by the H2 addition to Ar. Although other factors than O content, such as cation ratio, cation/anion ratio, and film crystallinity affect band gap and other film properties, from the fact that the H2 addition affected the lattice spacing and the band gap clearly, it was concluded that suppression of the incorporation of oxygen into CZTS thin films by the H2 addition to Ar was the foremost factor for the decrease in the lattice spacing and the narrowing of the band gap.

N-GaP/p-Si heterojunction solar cells fabricated by PE-ALD

Significant progress in photovoltaic conversion of solar energy can be achieved by new technological approaches that will improve the efficiency of solar cells and make them appropriate for mass production. A new technological approach for the growth of III-V compounds on Si substrates using low temperature plasma-enhanced atomic layer deposition (PE-ALD) is explored in the paper. This technique, which consists of alternatively changing the phosphorus and gallium atom source flows providing the growth of one monolayer by cycle, was developed for the growth of GaP films on Si substrates in a standard PECVD setup at 380 °C using PH3 and TMG (Trimethylgallium) as sources of III and V atoms. First (n)GaP/(p)c-Si anisotype heterojunction solar cell structures fabricated by PE-ALD exhibit open circuit voltage values similar to that obtained for (n)a-Si:H/(p)c-Si heterojunctions fabricated using the same (p)c-Si substrates. However (n)GaP/(p)c-Si solar cells demonstrates a potential to extend a high quantum efficiency in the short wavelength region due to lower absorption losses in the GaP emitter layer.

Fabrication of Single Crystal Gallium Phosphide Thin Films on Glass

Due to its high refractive index and low absorption coefficient, gallium phosphide is an ideal material for photonic structures targeted at the visible wavelengths. However, these properties are only realized with high quality epitaxial growth, which limits substrate choice and thus possible photonic applications. In this work, we report the fabrication of single crystal gallium phosphide thin films on transparent glass substrates via transfer bonding. GaP thin films on Si (001) and (112) grown by MOCVD are bonded to glass, and then the growth substrate is removed with a XeF2 vapor etch. The resulting GaP films have surface roughnesses below 1 nm RMS and exhibit room temperature band edge photoluminescence. Magnesium doping yielded p-type films with a carrier density of 1.6 × 1017 cm−3 that exhibited mobilities as high as 16 cm2V−1s−1. Due to their unique optical properties, these films hold much promise for use in advanced optical devices.

Pulsed laser deposition of Al x Ga1–x As and GaP thin films onto Si substrates for photoelectric converters

Pulsed laser deposition is used to produce AlGaAs and GaP thin films (with a thickness of less than 1 μm) on Si substrates. Methods for reducing the number of structural defects in the films are analyzed, and the effect of strains upon AlGaAs/Si and GaP/Si heterostructures is established by Raman spectroscopy. The application of Al0.3Ga0.7As and GaP films as wide-gap windows of silicon photoelectric converters is examined. The spectral characteristics of photocells based on Al0.3Ga0.7As/Si and GaP/Si heterostructures are studied. The heterostructures can be used as the first p–n junction of a Si-based multijunction photoelectric converter.

The structural state of epitaxial GaP films of different polarities grown on misoriented Si(001) substrates

The structure of GaP films grown by molecular-beam epitaxy on vicinal Si(1113) substrates has been studied by X-ray diffraction. It is established that the crystalline lattice of a pseudomorphic film rotates about the axis toward increasing deviation from the singular orientation, while the subsequent relaxation leads to rotation in the opposite direction. This is valid for the films of both (001) and (001) polarities. Differences between the surface morphologies of relaxed and pseudomorphic GaP films are revealed.

GaP-interlayer formation on epitaxial GaAs(100) surfaces in MOVPE ambient

The challenge to embed a single monolayer of phosphorus during epitaxial gallium arsenide (GaAs) growth triggers numerous questions regarding practical preparation, effective analysis, and fundamental consideration of the resulting interlayers. Beyond better understanding of III-V heterointerface formation processes, precise interlayer incorporation may enable enhanced interface design, effective diffusion barriers, and advanced band structure engineering. We employ metalorganic vapor phase epitaxy (MOVPE) in various growth modes (continuous, with interruptions, pulsed, surface exchange) targeting the most abrupt incorporation of thinnest GaP films in the GaAs(100) matrix. The intensities of higher order interference fringes in high resolution X-ray diffraction (HR-XRD) serve as a measure of the effective GaPxAs1−x film thickness and P concentration, which is compared to compositional analysis based on scanning transmission electron microscopy (STEM). In situ reflection anisotropy spectroscopy (RAS) provided us with insights to the GaAs(100) surface configurations relevant during the P interlayer preparation.

Thin film GaP for solar cell application

A new approach to the silicon based heterostructures technology consisting of the growth of III-V compounds (GaP) on a silicon substrate by low-temperature plasma enhanced atomic layer deposition (PE-ALD) is proposed. The basic idea of the method is to use a time modulation of the growth process, i.e. time separated stages of atoms or precursors transport to the growing surface, migration over the surface, and crystal lattice relaxation for each monolayer. The GaP layers were grown on Si substrates by PE-ALD at 350°C with phosphine (PH3) and trimethylgallium (TMG) as sources of III and V atoms. Scanning and transmission electron microscopy demonstrate that the grown GaP films have homogeneous amorphous structure, smooth surface and a sharp GaP/Si interface. The GaP/Si heterostructures obtained by PE-ALD compare favourably to that conventionally grown by molecular beam epitaxy (MBE). Indeed, spectroscopic ellipsometry measurements indicate similar interband optical absorption while photoluminescence measurements indicate higher charge carrier effective lifetime. The better passivation properties of GaP layers grown by PE-ALD demonstrate a potential of this technology for new silicon based photovoltaic heterostructure

Optical properties of oxygen doped diamond-like carbon thin films

In this work, oxygen doped diamond-like carbon thin films were deposited on silicon substrates by a RF-PECVD method using mixture of methane (CH4), argon (Ar) and oxygen (O2) precursor gases. The refractive index (n), extinction coefficient (k) and thickness of the thin films were measured by spectroscopic ellipsometry method (SE). The absorption coefficient (α) and optical band gap were also derived. The structure of the films was investigated by Raman spectroscopy. Fourier Transform Infra-Red (FTIR) spectroscopy was used to measure the transmittance in the midwave infrared (mid IR) region and to identify existing bonds in the structure of the films. The results showed that these films were ideal to use them as antireflective coatings for silicon optics in mid IR. Oxygen doping did not significantly change optical properties; and did not have undesirable effect on the antireflection property. Among the films, the oxygen doped diamond-like carbon film that was deposited with 5.6% volume oxygen had highest refractive index, lowest optical gap and minimum film deposition rate.

Coherent phonon spectroscopy characterization of electronic bands at buried semiconductor heterointerfaces

We demonstrate an all-optical approach to probe electronic band structure at buried interfaces involving polar semiconductors. Femtosecond optical pulses excite coherent phonons in epitaxial GaP films grown on Si(001) substrate. We find that the coherent phonon amplitude critically depends on the film growth conditions, specifically in the presence of antiphase domains, which are independently characterized by transmission electron microscopy. We determine the Fermi levels at the buried interface of GaP/Si from the coherent phonon amplitudes and demonstrate that the internal electric fields are created in the nominally undoped GaP films as well as the Si substrates, possibly due to the carrier trapping at the antiphase boundaries and/or at the interface.

The effect of substrate temperature, Cu/Sn ratio and post-annealing on the phase-change and properties of Cu2SnS3 film deposited by ultrasonic spray pyrolysis

In this work, Cu2SnS3 (CTS) films were prepared by a simple ultrasonic spray pyrolysis method on soda-lime glass substrates with different Cu/Sn precursor ratio and substrate temperature. The structural, component, morphological, optical and electrical properties of the films were investigated. XRD and Raman results confirmed that the Cu2SnS3 films were polymorphism with tetragonal phase and monoclinic phase. Sn-rich precursor and higher substrate temperature may change the dominant phase from tetragonal CTS phase to monoclinic CTS phase. Optical band gap of the films decreased from 1.87 to 1.03 eV with increasing Cu/Sn precursor ratio under 350 °C. All the films showed p-type conductivity with a minimum resistivity of 3.5 × 10−3 Ω cm. Additionally, the CTS films were annealed at 500 °C in sulfur atmosphere to improve the crystallinity and sulfur content. After annealing, a narrow band gap and pure CTS film was obtained, which could be a promising absorber layer material for high efficiency solar cell.

Self-assembled nc-Si-QD/a-SiC thin films from planar ICP-CVD plasma without H2-dilution: a combination of wide optical gap, high conductivity and preferred 〈220〉 crystallographic orientation, uniquely appropriate for nc-Si solar cells

Spontaneous miniaturization and rapid synthesis of self-assembled nc-Si-QDs of 〈220〉 orientation in high crystalline nc-Si-QD/a-SiC thin films of high conductivity and wide optical gap is obtained in ICP-CVD, from (SiH4 + CH4)-plasma, without H2-dilution.

Evolution of silicon bulk lifetime during III–V‐on‐Si multijunction solar cell epitaxial growth

AbstractThe evolution of Si bulk minority carrier lifetime during the heteroepitaxial growth of III–V on Si multijunction solar cell structures via metal‐organic chemical vapor deposition (MOCVD) has been analyzed. In particular, the impact on Si lifetime resulting from the four distinct phases within the overall MOCVD‐based III–V/Si growth process were studied: (1) the Si homoepitaxial emitter/cap layer; (2) GaP heteroepitaxial nucleation; (3) bulk GaP film growth; and (4) thick GaAsyP1‐y compositionally graded metamorphic buffer growth. During Phase 1 (Si homoepitaxy), an approximately two order of magnitude reduction in the Si minority carrier lifetime was observed, from about 450 to ≤1 µs. However, following the GaP nucleation (Phase 2) and thicker film (Phase 3) growths, the lifetime was found to increase by about an order of magnitude. The thick GaAsyP1‐y graded buffer was then found to provide further recovery back to around the initial starting value. The most likely general mechanism behind the observed lifetime evolution is as follows: lifetime degradation during Si homoepitaxy because of the formation of thermally induced defects within the Si bulk, with subsequent lifetime recovery due to passivation by fast‐diffusing atomic hydrogen coming from precursor pyrolysis, especially the group‐V hydrides (PH3, AsH3), during the III–V growth. These results indicate that the MOCVD growth methodology used to create these target III–V/Si solar cell structures has a substantial and dynamic impact on the minority carrier lifetime within the Si substrate. Copyright © 2015 John Wiley &amp; Sons, Ltd.

The Study of the Electrothermal Heating Pipe Membrane Resistance Test Equipment

In order to detect the electrothermal heating tube internal electrothermal film resistance and the relationship between the two electrode gap, this thesis used design using a testing equipment: The two ends of the measurement period from two electrodes, and the probe spacing can be adjusted ; The two measuring points into hair heat pipe internal contact with the resistive film, using the bleeder circuit, the indirect measuring resistance membrane resistance and the relationship between the two measuring points spacing; then analyze the measurement data, the relationship between the two electrode gap and electrothermal film resistance. The use of such equipment, the measured data, we can analyze the electrothermal film heating pipe internal heating membrane resistance and two measurement of the end head distance is linear change. The linear relationship between the use of electrothermal film heating tube internal electrothermal membrane resistance, can estimate the range of internal electric heating film heating pipe resistance from two the distance between the electrodes, get the change range of heating tube power.

Growth of antiphase-domain-free GaP on Si substrates by metalorganic chemical vapor deposition using an in situ AsH3 surface preparation

Antiphase-domain (APD) free GaP films were grown on Si(100) substrates prepared by annealing under dilute AsH3 in situ in an MOCVD reactor. LEED and AES surface analysis of Si(100) surfaces prepared by this treatment show that AsH3 etching quickly removes O and C contaminants at a relatively low temperature (690–740 °C), and creates a single-domain “A-type” As/Si surface reconstruction. The resulting GaP epilayers grown at the same temperature are APD-free, and could thereby serve as templates for direct growth of III-V semiconductors on Si. This single chamber process has a low thermal budget, and can enable heteroepitaxial integration of III-Vs and Si at an industrial scale.

Tailoring Si(100) substrate surfaces for GaP growth by Ga deposition: A low-energy electron microscopy study

For GaP-on-Si(100) heteroepitaxy, currently considered as a model system for monolithic integration of III–V semiconductors on Si(100), the surface steps of Si(100) have a major impact on the quality of the GaP film. Monoatomic steps cause antiphase domains in GaP with detrimental electronic properties. A viable route is to grow the III–V epilayer on single-domain Si(100) with biatomic steps, but preferably not at the expense of reduced terrace widths introduced by miscut substrates. We have performed in situ investigations of the influence of Ga deposition on the kinetics of surface steps and terraces of Si(100) at substrate temperatures above 600 °C by low-energy electron microscopy. Starting from nearly equally distributed TA and TB terraces of a two-domain Si(100) surface, submonolayer deposition of Ga results in a transformation into a surface dominated by TA terraces and biatomic DA steps. This transformation is reversible, and Si(100) with monoatomic steps is recovered upon termination of the Ga flux. Under conditions of higher coverages (but still below 0.25 monolayer), we observe restructuring into a surface with TB dominance, similar to the findings of Hara et al. [J. Appl. Phys. 98, 083515 (2005)]. The occurrence and mutual transformations of surface structures with different terrace and step structures in a narrow range of temperatures and Ga deposition rates is discussed.

Effect of Sn doping on properties of transparent ZnO thin films prepared by thermal evaporation technique

Sn doping is used as an effective method to increase conductivity and photosensitivity of highly transparent nanostructured ZnO thin films, prepared by multisource vacuum evaporation followed by air annealing. The microstructural characterizations reveal formation of polycrystalline ZnO and ZnO:Sn films, with grain size ∼16–20nm and preferred orientation along (002) plane. Increased electrical conductivity by a factor ∼102 on doping, coupled with the enhancement of transmittance (80–90% in visible range) and photoconductivity lends these wide band gap films (∼3.21eV–3.24eV) application in photovoltaics. Fast response to ethanol (5–7s) indicates suitability of these films in gas sensors.

Controlling the polarity of metalorganic vapor phase epitaxy-grown GaP on Si(111) for subsequent III-V nanowire growth

Nanowire growth on heteroepitaxial GaP/Si(111) by metalorganic vapor phase epitaxy requires the [-1-1-1] face, i.e., GaP(111) material with B-type polarity. Low-energy electron diffraction (LEED) allows us to identify the polarity of GaP grown on Si(111), since (2×2) and (1×1) surface reconstructions are associated with GaP(111)A and GaP(111)B, respectively. In dependence on the pre-growth treatment of the Si(111) substrates, we were able to control the polarity of the GaP buffers. GaP films grown on the H-terminated Si(111) surface exhibited A-type polarity, while GaP grown on Si surfaces terminated with arsenic exhibited a (1×1) LEED pattern, indicating B-type polarity. We obtained vertical GaAs nanowire growth on heteroepitaxial GaP with (1×1) surface reconstruction only, in agreement with growth experiments on homoepitaxially grown GaP(111).

Atomic scale analysis of the GaP/Si(100) heterointerface byin situreflection anisotropy spectroscopy andab initiodensity functional theory

A microscopic understanding of the formation of polar-on-nonpolar interfaces is a prerequisite for well-defined heteroepitaxial preparation of III-V compounds on (100) silicon for next-generation high-performance devices. Energetically and kinetically driven Si(100) step formations result in majority domains of monohydride-terminated Si dimers oriented either parallel or perpendicular to the step edges. Here, the intentional variation of the Si(100) surface reconstruction controls the sublattice orientation of the heteroepitaxial GaP film, as observed by in situ reflection anisotropy spectroscopy (RAS) in chemical vapor ambient and confirmed by benchmarking to surface science analytics in ultrahigh vacuum. Ab initio density functional calculations of both abrupt and compensated interfaces are carried out. For P-rich chemical potentials at abrupt interfaces, Si-P bonds are energetically favored over Si-Ga bonds, in agreement with in situ RAS experiments. The energetically most favorable interface is compensated with an intermixed interfacial layer. In situ RAS reveals that the GaP sublattice orientation depends on the P chemical potential during nucleation, which agrees with a kinetically limited formation of abrupt interfaces.