GaP Buffer Layer Research Articles

High-quality single InGaAs/GaAs quantum dot growth on a silicon substrate for quantum photonic applications

Developing non-classical light sources for use in quantum information technology is a primary goal of quantum nanophotonics. Significant progress has been made in this area using quantum dots grown on III/V semiconductor substrates. However, it is crucial to develop quantum light sources based on silicon wafers to facilitate large-scale integration of electronic circuits and quantum photonic structures. We present a method for the direct heteroepitaxial growth of high-quality InGaAs quantum dots on silicon, which enables the fabrication of scalable and cost-effective quantum photonics devices that are compatible with silicon technology. To achieve high-quality GaAs heterostructures, we apply an intermediate GaP buffer and defect-reducing layers on a silicon substrate. The epitaxially grown quantum dots exhibit optical and quantum-optical properties similar to reference ones based on conventional GaAs substrates. The distributed Bragg reflector used as a backside mirror enables us to achieve bright emission with up to (18 ± 1)% photon extraction efficiency. Additionally, the quantum dots exhibit strong multi-photon suppression with g(2)(τ) = (3.7 ± 0.2) × 10−2 and high photon indistinguishability V = (66 ± 19)% under non-resonant excitation. These results indicate the high potential of our heteroepitaxy approach in the field of silicon-compatible quantum nanophotonics. Our approach can pave the way for future chips that combine electronic and quantum photonic functionality.

Effects of CdTe selenization on the electrical properties of the absorber for the fabrication of CdSexTe1-x/CdTe based solar cells

CdTe thin film solar cells are the most successful thin film photovoltaic devices in terms of production yield. In the last few years, by changing the original solar cell structure, the efficiency was increased from around 17 % up to 22 %. In particular two main innovations have been introduced: a CdSexTe1-x layer, that narrows the band gap of the absorber near the junction, and a large band gap buffer layer (i.e. MgxZn1-xO). These allow to improve the absorption in the long and in the short wavelength region, respectively. Generally, CdSexTe1-x is formed either by the deposition of a CdSe layer at the junction subsequently mixed with CdTe or by directly depositing a CdSexTe1-x compound, we have instead introduced a different method where CdSexTe1-x is formed by treating a thin CdTe layer at high temperature in selenium atmosphere. The finished device has been fabricated by substituting CdS with SnO2 and efficiencies exceeding 14 %, with current densities higher than 26 mA/cm2 , are obtained by a low substrate temperature process. In this work we analyze and compare our older standard FTO/SnO2/CdS/CdTe with the novel FTO/SnO2/CdSexTe1-x/CdTe devices by capacitance voltage, drive level capacitance profiling, and admittance spectroscopy. The results highlight the effect of the selenization process in terms of carrier concentration, carrier profile, and formation of deep and shallow defects. The overall performance of the completed devices is also presented and compared.

Direct Heteroepitaxy and Selective Area Growth of GaP and GaAs on Si by Hydride Vapor Phase Epitaxy

Direct heteroepitaxy and selective area growth (SAG) of GaP and GaAs on Si(100) and Si(111) are implemented by low‐pressure hydride vapor phase epitaxy (LP‐HVPE), which are facilitated by buffer layers grown at 410–490 °C with reactive gas mixing directly above Si substrates. High‐density islands observed on GaP buffer layers on Si result in rough morphology and defect formation in the subsequent GaP layers grown at 715 °C. The impact of growth temperature of GaAs buffer layers on the crystal quality of GaAs/Si is studied. A decreased nucleation temperature significantly improves the morphology and crystalline quality of the overall GaAs growth on Si. It is observed that Si(111) substrates are favorable for both GaP and GaAs growths in comparison with Si(100). In SAGs of GaP/Si and GaAs/Si, the high selectivity innate to HVPE is maintained in the used unconventional growth regime. The spatially resolved photoluminescence mapping reveals the material quality of GaAs/Si is enhanced by defect filtering by SAG. The outcomes of this work will pave the way of III–V/Si integration realized by cost‐effective HVPE for photonic device applications.

Heteroepitaxy of GaAsP and GaP on GaAs and Si by low pressure hydride vapor phase epitaxy

Direct heteroepitaxy of GaAsP and GaP on GaAs and on Si by low-pressure hydride vapor phase epitaxy (HVPE) is investigated as prior studies for photovoltaics and non-linear optics applications. When growing GaAsP on GaAs, it is found that the ambient gas during substrate pre-heating influences the ternary composition as well as the crystalline quality of the subsequent growth. GaAs0.72P0.28 with bandgap energy of 1.76 eV has been achieved, which would be suitable for a top cell in Si tandem solar cell structures. Growth of GaP was investigated on planar GaAs as a prior study for realizing orientation patterned (OP) GaP on OP-GaAs. Threading dislocations caused by the 3.6% lattice mismatch between GaP and GaAs are suppressed by adjusting the GaCl flow, achieving a low full width at half maximum of 146 arcsec for the X-ray diffraction omega scan. Direct heteroepitaxy of GaAsP on Si aiming for achieving a GaAsP/Si dual junction solar cell is demonstrated. The inherent problem of initiating nucleation during the direct heteroepitaxy of III-V on Si by HVPE is overcome by utilizing the vapor mixing approach to grow a low-temperature GaP buffer layer on Si, followed by a GaAsP layer grown by conventional HVPE.

Controllable antiphase domain density in dilute nitride GaPN/GaP heterostructures on silicon

Formation and propagation of the antiphase domains in dilute nitride GaPN/GaP epitaxial heterostructures grown on Si (001) by plasma assisted molecular beam epitaxy (PA-MBE) on silicon is studied. Role of the layer composition, substrate orientation and growth conditions are discussed. Composition of the dilute nitride film was studied by X-ray diffraction (XRD) while the effect of the antiphase disorder in GaP buffer layer on GaPN epilayer structural properties was studied by transmission electron (TEM) and scanning electron microscopy (SEM). Controllable transition between antiphase disordered and monodomain film depending on the concentration of incorporated nitrogen is demonstrated – transition to the monodomain film occurs in dilute nitride GaPN layers starting low with 0.4% of incorporated nitrogen. Control of the antiphase disorder allows to tune mean film polarity and second order nonlinear optical response of III-phosphide heterostructures.

GaNP-based photovoltaic device integrated on Si substrate

Gallium phosphide is an important material in terms of III-V semiconductors integration on Si. In this work we study photovoltaic properties of GaP:Be/GaNP (Eg ~ 2.0eV)/GaP:Si p-i-n heterostructure grown on Si (100) with GaP buffer by plasma-assisted molecular beam epitaxy. Correlation between the structural and optoelectronic properties of the fabricated device was studied by scanning and transmission electron microscopy (TEM), electron beam induced current (EBIC) and deep-level transient spectroscopy (DLTS) techniques. The I–V characteristic of the fabricated mesa diode demonstrates short circuit current of 2.2 mA/cm2 and open circuit voltage of 0.8 V. TEM studies of the epilayer structural properties demonstrate high density of antiphase domains in the n-doped GaP buffer layer; however, they found to be annihilated in dilute nitride layer, where mainly threading dislocations were formed. EBIC investigation shows that crystalline imperfections of the heterostructure leads to poor carriers transport affecting the energy conversion efficiency. We performed numerical modeling concerning presence of the structural defects and discuss their influence on the diode performance. The carried out study is the initial step on the way to development of the growth technique of the GaP-based dilute nitride direct bandgap materials on Si for photonic and photovoltaic applications.

Growth of GaP1 − x − yAsyNx on Si substrates by chemical beam epitaxy

Chemical beam epitaxy has been used to grow layers of GaP1 − xNx, GaP1 − yAsy, and nearly lattice-matched layers GaP1 − x − yAsyNx on Si substrates. To address the issue of antiphase domain generation associated with the growth of polar semiconductors on Si, misoriented Si(001) substrates have been used combined with a carefully designed GaP buffer layer growth. The reflection high-energy electron diffraction pattern exhibits a (2 × 4) surface reconstruction after GaP buffer layer and GaP(As,N) graded layer growth, indicating the good surface quality and planarity of the grown layers. Sample composition was obtained by simultaneous acquisition of Rutherford backscattering spectrometry and nuclear reaction analysis, indicating a linear dependence of N and As mole fractions on the flux of their respective precursor. GaP1 − x − yAsyNx layers grown on Si substrates have a lattice mismatch not larger than ±0.005 for N contents in the range 0.02 < x < 0.05. High-resolution X-ray diffraction reciprocal space maps demonstrate a good crystalline quality. Intense photoluminescence spectra have been measured in all GaP1 − xNx and GaP1 − x − yAsyNx layers, as it is expected for direct bandgap materials. Two wide overlapped emission peaks are observed in all the spectra, most likely related to near bandgap recombination. The position of the higher energy peak for GaP1 − xNx and GaP1 − x − yAsyNx layers has been compared to bandgap energy calculations using the band anticrossing model, showing good agreement.

Heteroepitaxy of GaP Nucleation Layers on Si by Molecular Beam Epitaxy

The growth of single crystalline GaP nucleation and buffer layers on Si substrates by molecular-beam epitaxy is demonstrated. The use of two different regimes of migration-enhanced epitaxy was studied. It was found that p–n junctions can be created due to the interdiffusion during the growth of GaP layer on p-type Si substrate.

Study of the structural and optical properties of GaP(N) layers synthesized by molecular-beam epitaxy on Si(100) 4° substrates

The structural and optical properties of GaP and GaPN layers synthesized by molecular-beam epitaxy on Si(100) substrates misoriented by 4° are studied. The possibility of producing GaP buffer layers that exhibit a high degree of heterointerface planarity and an outcropping dislocation density of no higher than ~2 × 108 cm–2 is shown. Emission from the Si/GaP/GaPN structure in the spectral range of 630–640 nm at room temperature is observed. Annealing during growth of the Si/GaP/GaPN structure makes it possible to enhance the room-temperature photoluminescence intensity by a factor of 2.6, with no shift of the maximum of the emission line.

Development of a new thin film composition for SnS solar cell

The work is devoted to the development of a new thin film composition for SnS solar cell based on semiconductor heterojunction n-Zn(O,S)/p-SnS by the combination of two low-cost liquid methods, namely electrodeposition and Successive Ionic Layer Adsorption and Reaction (SILAR). Structure and optical properties of the obtained layers have been investigated. The p-SnS absorber is fabricated by combining the two methods. The creation of the main SnS film was carried out by sulfurization of the electrodeposited Sn thin film. The elimination of the shunt leakage was made by SILAR deposition of nanocrystalline SnS into pores of the main SnS. All SnS films are single-phase and consisted of tin monosulfide with orthorhombic structure of Herzenbergite. Wide band gap and transparent buffer layer n-Zn(O,S) is created by air annealing of ZnS thin film prepared by SILAR. Series and shunt resistances as well as a height of the rectifying barrier of this n-Zn(O,S)/p-SnS heterostructure were investigated due dark current–voltage characteristics. The diode and electronic parameters of this new thin film composition is superior ones for the obtained by using tin sulfide absorber produced only by SILAR.

MBE growth of GaP on a Si substrate

It is shown that single-crystal GaP buffer layers can be formed on a Si substrate by molecular-beam epitaxy, with the “migration-enhanced epitaxy” procedure applied in the stage in which the nucleating layer is formed. When a GaP layer is produced on a p-type silicon substrate, a p-n junction is created in a natural way between the p-Si substrate and the surface n-Si layer produced by the diffusion of phosphorus into the substrate during the course of the epitaxial growth of GaP. This p-n junction can be used as the first junction of a silicon-based multijunction photovoltaic converter.

Structural properties of AlGaP films on GaP grown by gas-source molecular-beam epitaxy

The growth of Al0.85Ga0.15P on GaP using gas-source molecular-beam epitaxy is investigated using in situ high-energy electron diffraction, high-resolution x-ray diffraction, atomic-force microscopy, and scanning electron microscopy. Growth temperature and phosphorus flux were varied. The 1.0-μm AlGaP films were grow on a GaP buffer layer and capped with GaP. The investigation indicates that a growth temperature of 490°C and a cracked PH3 flux of 2.7sccm resulted in the best AlGaP quality, while maintaining very good GaP quality.

EFFECTS OF THE FORMATION METHOD OF EARLY GAP BUFFER MONOLAYERS ON THE STRAIN STATE OF GAAS FILMS ON VICINAL SI(001) SUBSTRATES

A significant dependence of the strain state of GaAs film lattice grown by molecular−beam epitaxy (MBE) on the nucleation method of early GaP buffer layers (50 nm) on the vicinal substrate Si(001) 4° around the axis was discovered. GaP growth started layer−by−layer with a gallium or a phosphorus sublayer. If GaP nucleated with a gallium sublayer, the GaAs film has a significant lattice rotation around the axis. If the buffer starts forming with a phosphorus layer the GaAs film evidently rotates around the axis. The film relaxation degree ex- ceeds 100%, and the film is in a laterally strained state. Analysis was carried out using the triclinic distortion model. A reciprocal space scattering map was obtained using X−ray diffraction in a three−axis low resolution setup. The map clearly shows that the GaAs film lattice is rotated.

Deposition by liquid epitaxy and study of the properties of nano-heteroepitaxial structures with quantum dots for high efficient solar cells

The paper presents experimental results showing the possibility of liquid-phase epitaxy with a pulse-cooled substrate nano-heteroepitaxial structures with Ge quantum dots and GaP matrix for production of single-junction solar cells working under sun light concentration. The preparation of the surface of Si wafers suitable for growing of these structures is described. The optimal technological conditions for growing of the of GaP buffer layer on the Si wafer from supersaturated solutions of melts Sn are applied. Structures with Ge quantum dots are grown at different conditions and study of their structural and optical properties is performed. The surface morphology of the grown materials is studied by AFM microscopy. The photoluminescence spectra of nano-heteroepitaxial structures with Ge quantum dots are presented and analyzed.

InAs Quantum Dot Growth on a Thin GaNP Buffer Layer on GaP by Metalorganic Chemical Vapor Deposition

InAs quantum dots (QDs) on a 2-nm-thick GaNP buffer layer on GaP were investigated using low-pressure metalorganic chemical vapor deposition. The InAs QDs density was significantly increased from 2.6×109 to 2.0×1010 cm-2 by increasing the nitrogen composition from 0 to 4.5% in GaNP. The formation characteristics of InAs QDs were also investigated at various InAs supply amounts from 0.7 to 2.5 monolayers (ML). The generation of QDs on the GaNP and GaP buffer layers started at the InAs supplies of 1.0 and 1.1 ML, respectively. Since the lattice mismatches were large, these supply amounts were definitely smaller than that on GaAs.

Study of GaP single crystal layers grown on GaN by MOCVD

The performance of GaN based devices could possibly be improved by utilizing the good p-type properties of GaP layer and it provides the possibility of the integration of InAlGaN and AlGaInP materials to produce new devices, if high quality GaP compounds can be grown on III-nitride compounds. In this paper, the growth of GaP layers on GaN by metalorganic chemical vapor deposition (MOCVD) has been investigated. The results show that the GaP low temperature buffer layer can provide a high density of nucleation sites for high temperature GaP growth. Using a 40 nm thick GaP buffer layer, a single crystal GaP layer, whose full-width at half-maximum of the (1 1 1) plane measured by double crystal X-ray diffraction is 580″, can be grown on GaN. The V/III ratio plays an important role in the GaP layer growth and an appropriate V/III ratio can improve the quality of GaP layer. The GaP:Mg layer with hole carrier concentration of 4.2 × 10 18 cm −3 has been obtained.

GaP layers grown on GaN with and without buffer layers

The growth of GaP layer on GaN with and without buffer layers by metal-organic chemical vapour deposition (MOCVD) has been studied. Results indicate that the GaP low temperature buffer layer can provide a high density of nucleation sites for high temperature (HT) GaP growth. These sites can promote the two-dimensional (2D) growth of HT GaP and reduce the surface roughness. A GaP single crystal layer grown at 680 °C is obtained using a 40-nm thick GaP buffer layer. The full-width at half-maximum (FWHM) of the (111) plane of GaP layer, measured by DCXRD, is 560 arcsec. The GaP layer grown on GaN without low temperature GaP buffer layer shows a rougher surface. However, the FWHM of the (111) plane is 408 arcsec, which is the indication of better crystal quality for the GaP layer grown on GaN without a low temperature buffer layer. Because it provides less nucleation sites grown at high growth temperature, the three-dimensional (3D) growth is prolonged. The crystalline quality of GaP is lightly improved when the surface of GaN substrate is pretreated by PH3, while it turned to be polycrystalline when the substrate is pretreated by TEGa.

Comment on “Experimental evidence for N-induced strong coupling of host conduction band states inGaNxP1−x: Insight into the dominant mechanism for giant band-gap bowing”

A recently observed pinned peak in photoluminescence excitation spectroscopy studies (PLE) of $\mathrm{Ga}{\mathrm{N}}_{x}{\mathrm{P}}_{1\ensuremath{-}x}$ epilayers, that remained stationary with nitrogen concentration, was attributed to a transition from the valence band edge to either the ${t}_{2}({\mathrm{X}}_{3})$ or ${t}_{2}(L)$ conduction bands by Buyanova et al. [Phys. Rev. B 69, 201303(R) (2004)]. Using absorption and PLE studies on carefully prepared samples, we show that this pinned peak is merely an artifact that arises from the GaP buffer layer and is not associated with the $\mathrm{Ga}{\mathrm{N}}_{x}{\mathrm{P}}_{1\ensuremath{-}x}$ epilayers.

Material properties of graded composition In xGa 1−xP buffer layers grown on GaP by organometallic vapor phase epitaxy

Graded In x Ga 1− x P buffer layers were grown on GaP substrate in order to prepare optically transparent substrate for optoelectronic structures. In our contribution we present results from the optimization of the growth process. We found different optimal growth conditions for GaP buffer layer growth and subsequent deposition of In x Ga 1− x P layers with graded value of x In. Structures with final composition x In = 0.33 and 0.27 were prepared using grading rates of 10% x In /μm and 5% x In /μm. The buffer structures were step graded with the compositional step between two adjacent layers 1% x In and 3% x In and the step width of 100, 200 and 300 nm.The subject of our investigation was the influence of structure design and growth conditions on buffer properties. Layer composition was determined using X-ray diffraction. Photoluminescence measurements served to find the composition at which the band gap changes from indirect to direct one. Surface morphology and surface roughness was characterized by atomic force microscopy and scanning electron microscopy. Transmission electron microscopy in cross-sectional view configuration was used for revealing the buffer microstructure. We found out that the V/III ratio has to be optimized during the buffer growth in order to avoid the degradation in the top part of the buffer structure.

Performance analysis ofa‐Si:H p–i–n solar cells with and without a buffer layer at the p/i interface

Light soaking experiments have been conducted on a-Si:H p-i-n solar cells with a silicon carbide buffer layer at the p/i interface. The rate of light induced degradation in the performance of these solar cells is higher in the initial stages of light soaking and assumes the same levels as the cells without a buffer layer with prolonged light soaking. Computer modelling has revealed that a graded band gap buffer layer at the p/i interface containing a slightly acceptor doped defective layer next to the p-layer improves the initial performance of a-Si:H p-i-n solar cells. The modelling also reveals that the effect of the buffer layer on solar cell performance depends critically on the configuration and composition of the buffer layer. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)