Modified Liquid Phase Epitaxy Research Articles

Gallium Phosphide Solar Cell Structures with Improved Quantum Efficiencies

Gallium phosphide (GaP) solar cell structures with improved quantum efficiencies were realized using a modified liquid phase epitaxy (LPE) technique and diodes formed using semi-transparent Schottky contacts. The improvement is due to the addition of a small amount of aluminum to the gallium and phosphorus containing LPE melt. The Al reduces the background concentration of oxygen in the melt, which is known to produce deep trap states in GaP. Additionally, it was found that by depositing an aluminum (Al)-rich AlGaP layer on top of the active GaP and then selectively etching it away, the surface morphology of the active layer was significantly improved. Thus, the modified LPE technique eliminates the major problem of meniscus lines associated with the standard LPE method.

Narrow bandgap mid-infrared photodetectors based on InAsSbP quantum dots

The results of investigations of mid-infrared photodetectors based on InAsSbP quantum dot (QD) grown on InAs(100) substrate by modified liquid phase epitaxy are presented. The atomic force microscope measurements have shown that the surface density of grown QDs is (4–8) × 109 cm−2. Also, the morphology and crystalline quality of grown QDs are investigated by a scanning tunneling microscope. Photodetectors based on n-InAs(100) substrate with InAsSbP QDs on its surface were fabricated in the form of a photoconductor cell. The photoresponse spectrum extended up to 4 μm was observed. The optical properties of fabricated structures were investigated under He–Ne laser irradiation with wavelength of 1.15 μm. It was found that the relative surface conductance increases by 16% at power density of 0.15 W/cm2. Capacitance hysteresis with maximal remnant capacitance of 2.17 nF at 103 Hz was observed as well.

Improved High-Energy Response of AlGaAs/GaAs Solar Cells Using a Low-Cost Technology

We report on an AlGaAs/GaAs solar cell with a significantly increased high-energy response that was produced via a modified liquid phase epitaxy (LPE) technique. This technique uses a one-step process in which the solid–liquid equilibrium Al-Ga-As:Zn melt in contact with an n-type vendor GaAs substrate simultaneously getters impurities in the substrate that shorten minority carrier lifetimes, diffuses Zn into the substrate to create a p–n junction, and forms a thin p-AlGaAs window layer that enables more high-energy light to be efficiently absorbed. Unlike conventional LPE, this process is performed isothermally. In our “double Al” method, the ratio of Al in the melt (“Al melt ratio”) that was used in our process was two times more than what was previously reported in the record 1977 International Business Machines (IBM) solar cell. Photoluminescence (PL) results showed our double Al sample yielded a response to 405 nm light (“blue light”), which was more than twice as intense as the response from our replicated IBM cell. The original 1977 cell had a low-intensity spectral response to photon wavelengths under 443 nm (Woodall and Hovel in Sol Energy Mater Sol Cells 29:176, 1990). Secondary ion mass spectrometry results confirmed the increased blue light response was due to a large reduction in AlGaAs window layer thickness. These results proved increasing the Al melt ratio broadens the spectrum of light that can be transmitted through the window layer into the active GaAs region for absorption, increasing the overall solar cell efficiency. Our enhanced double Al method can pave the way for large-scale manufacturing of low-cost, high-efficiency solar cells.

An investigation of exciton behavior in type-II self-assembled GaSb/GaAs quantum dots

We report the investigation of exciton dynamics in type-II self-assembled GaSb/GaAs quantum dots. The GaSb/GaAs quantum dots (QDs) were grown using a modified liquid phase epitaxy technique. Statistical size distributions of the uncapped QDs were investigated experimentally by field-emission scanning electron microscopy (SEM) and atomic force microscopy (AFM), and theoretically by an eight-band k · p calculation, which demonstrated a dissolution effect. Furthermore, the low-temperature luminescence spectra of type-II GaSb/GaAs QDs with a thick capping layer exhibit well-resolved emission bands and LO-phonon-assisted transitions in the GaSb wetting layer. However, the luminescence lines quench at temperatures above 250 K, which is attributed to the weak quantum confinement of electrons participating in indirect exciton recombination. It was demonstrated that the room temperature stability of the excitons in type-II GaSb/GaAs QDs could be achieved by growing thin a capping layer, which provides strong quantum confinement in the conduction band and enhances the electron–hole Coulomb interaction, stabilizing the excitons.

Chiral Porous Metacrystals: Employing Liquid-Phase Epitaxy to Assemble Enantiopure Metal-Organic Nanoclusters into Molecular Framework Pores.

We describe the fabrication of hybrid yet well-ordered porous nanoparticle (NP) arrays with full three-dimensional periodicity by embedding nanometer-sized metal-organic clusters (MOCs) into metal-organic frameworks (MOFs). Although conventional NP@MOF encapsulation procedures failed for these fairly large (1.66 nm diameter) NPs, we achieved maximum loading efficiency (one NP per pore) by using a modified liquid phase epitaxy (LPE) layer-by-layer approach to grow and load the MOF. The preformed NPs, homochiral Ti4(OH)4(R/S-BINOL)6 clusters (Ti-MOC, BINOL = 1,1'-bi-2-naphthol), formed a regular lattice inside the pores of an achiral HKUST-1 (or Cu3(BTC)2, BTC = 1,3,5-benzenetricarboxylate) MOF thin film. Exposure to the different enantiomers of methyl lactate revealed that the NP@MOF metacrystal is quite efficient regarding enantiomer recognition and separation. The approach presented here is also suited for other MOF types and expected to provide a substantial stimulus for the fabrication of metacrystals, crystalline solids made from nanoparticles instead of atoms.

Investigation of InAsSbP quantum dot mid-infrared sensors

Abstract. This work presents the results of investigations of a low bias mid-infrared(IR) photoconductive cell (PCC) with InAsSbP quantum dots (QDs). The self-assembled nanostructures were grown on an InAs(100) substrate by modified liquid phase epitaxy. The coarsening of the QDs due to Ostwald ripening was discussed. Hysteresis with a remnant capacitance of 0.483 pF and contra-directional oscillations on the PCC's capacitance–voltage characteristic at 78 K were observed. Additionally, peaks at 3.48, 3.68 and 3.89 μm on the room temperature photoresponse spectrum of a quantum dot photoconductive cell were detected. Room temperature photo-sensing properties were investigated under an irradiation of 3.39 μm as well. At a power density of 0.07 W cm−2, the surface resistance of quantum dot PCC was reduced by up to 7 %. A current responsivity of 0.2 mA W−1 was measured at an applied voltage of 8 mV.

InAsSbP quantum dot mid-IR photodetectors operating at room temperature

A modified liquid phase epitaxy was applied to grow InAsSbP quantum dots on InAs (100) substrate. Low bias and room temperature photoconductive cells with InAsSbP nanostructures were developed. In comparison with InAs based photoconductive cell, a red shift and additional peaks at 3.4μm, 3.68μm and 3.89μm on photoresponse spectrum of quantum dot photoconductive cell were observed. Additionally, the spectrum dependence on bias voltage at room temperature is presented. Under irradiation of 3.39μm, the surface resistance was reduced up to 7% at power density of 0.07W/cm2. At the bias of 8mV current responsivity of 0.2mA/W was measured.

Rib waveguides based on Zn-substituted LiNbO 3 films grown by liquid phase epitaxy

The fabrication of epitaxially grown Zn-substituted LiNbO 3 (Zn:LiNbO 3) waveguide films and rib waveguides is reported and detailed investigations about microstructure, morphology and optical waveguide properties are provided. Zn:LiNbO 3 films were grown on congruent X-cut LiNbO 3 substrates by a modified liquid phase epitaxy in solid–liquid coexisting solutions. The homogeneously Zn-substituted films exhibit high crystalline perfection and extremely flat surfaces with averaged surface roughness of rms = 0.2–0.3 nm. At the film/substrate interface a Zn-containing transient layer has been observed, which allows the growth of elastically strained Zn:LiNbO 3 film lattices. X-ray diffraction reciprocal-space measurements prove the pseudomorphic film growth. The refractive index difference between substrate and film depends on the zinc substitution content, which increase with rising growth temperatures. For films with 5.3 mol% Zn (Δ n o ≈ +5 × 10 −3) only ordinary ray propagation was observed, while for films with 7.5 mol% Zn (Δ n o ≈ +8 × 10 −3, Δ n e ≈ +5 × 10 −3) both modes, TM and TE propagate. Stress-induced refractive index changes are in the order of Δ n ≈ 10 −4. In rib waveguide microstructures singlemode propagation with nearly symmetrical field distribution has been observed. To demonstrate the potential of the proton exchange-assisted dry-etching technique interferometer microstructures were fabricated.

High quality of InAsSb epilayer with cutoff wavelength longer than 10 μm grown on GaAs by the modified LPE technique

The InAsSb epilayers with a cutoff wavelength of 11.5 μm were successfully grown on highly lattice-mismatched semi-insulating (1 0 0) GaAs substrate by the modified liquid phase epitaxy (LPE) technique. Fourier transform infrared (FTIR) transmission spectrum revealed a strong band gap narrowing for this alloy. The electrical properties were investigated by the Van der Pauw measurements at 300 and 77 K. InAsSb epilayers showed high Hall mobilities being 11,800 cm 2/V s at room temperature (RT). After an annealing treament for 10 h, the electron mobility at 77 K were improved from 1730 cm 2/V s (prior to annealing) to 13,470 cm 2/V s. Wet etching was used to display the surface etch pits prior to and after annealing treatment, showing that the mobility improvement was due to the reduction of the etch pits density.

Wavelength and temperature characteristics of BiYbIG film/YIG crystal composite structure for magneto-optical applications

We report the wavelength and temperature characteristics of novel Bi-substituted rare-earth iron garnet films grown on a YIG substrate by a modified liquid phase epitaxy (LPE) technique. The Faraday-rotation spectrum was measured by the magneto-optically modulated dual-frequency technique with the wavelength varied from 800 nm to 1700 nm. The resultant Bi0.37Yb2.63Fe5O12 (BiYbIG) LPE film/YIG crystal structure showed an increased Faraday-rotation coefficient due to Bi3+-ion doping on the dodecahedral sites of the iron garnet without increasing absorption loss; therefore, a good magneto-optical figure of merit, defined by the ratio of Faraday rotation and optical absorption loss, has been achieved (21.5 deg/dB and 30.2 deg/dB at 1300-nm and 1550-nm wavelengths, respectively, at room temperature). In addition, since the Yb3+ and Y3+ ions provide opposite contributions to the wavelength and temperature characteristics of the Faraday rotation, the resultant BiYbIG LPE film/YIG crystal structure showed a flat Faraday-rotation curve versus wavelength and temperature. The Faraday-rotation wavelength coefficient was reduced to 0.06 %/nm at 1550-nm wavelength. The Faraday-rotation temperature derivative was reduced to 0.006 deg/°C at 1300-nm wavelength and 0.007 deg/°C at 1550-nm wavelength, respectively.

Thick M-type barium hexaferrite films grown on garnet substrates

The deposition and characterization of thick M-type barium hexaferrite (BaFe12O19) films produced by a modified liquid phase epitaxy deposition technique are reported. The films are deposited on (111) oriented single crystal garnet (Gd3Ga5O12) substrates. The thickness of the films ranged between 45 and 80μm with growth rates of up to 40μm∕h. This growth rate is about five times greater than films grown on substrates of (111) magnesium oxide. Although the films are relatively thick, the ferrimagnetic resonance linewidth is remarkably narrow (∼50Oe) for external fields applied along the c axis.

Liquid phase epitaxy growth of bismuth-substituted yttrium iron garnet thin films for magneto-optical applications

The novel Bi-substituted rare-earth iron garnet films were grown by the modified liquid phase epitaxy (LPE) technique for use as a 45° Faraday rotator in optical isolators. First, single crystals of Y 3Fe 5O 12 (YIG), with a lattice constant of 1.2378 nm, were grown by means of the Czochralski method. Using the seed crystal of YIG instead of the conventional non-magnetic garnet of Gd 3Ga 5O 12 as a substrate, a film of BiYbIG was grown by means of the LPE method from Bi 2O 3–B 2O 3 fluxes. The structural, magnetic and magneto-optical properties of BiYbIG LPE film/YIG crystal composite have been investigated using directional X-ray diffraction, electron probe microanalysis, vibrating sample magnetometer and near-infrared transmission spectrometry. The saturation magnetization 4πM s has been estimated to be approximately 1200 G. The Faraday rotation spectrum was measured by the method of rotating analyzer ellipsometry with the wavelength varying from 800 to 1700 nm. The resultant Bi 0.37Yb 2.63Fe 5O 12 LPE film/YIG crystal composite showed an increased Faraday rotation coefficient due to doping Bi 3+ ions on the dodecahedral sites of the magnetic garnet without increasing absorption loss, therefore a good magneto-optic figure of merit, defined by the ratio of Faraday rotation and optical absorption loss, has been achieved of 21.5 deg/dB and 30.2 deg/dB at 1300 nm and 1550 nm wavelengths, respectively, at room temperature. Since Yb 3+ ions and Y 3+ ions provide the opposite contributes to the wideband and temperature characteristic of Faraday rotation, the values of Faraday rotation wavelength and temperature coefficients were reduced to 0.06%/nm and 0.007 deg/°C at 1550 nm wavelength, respectively.

Growth and Electrical Properties of New Semiconductor Compound ZnCdHgTe

Growth technology and electric properties of the new semiconductor compound ZnCdHgTe obtained by modified liquid phase epitaxy technique on (111) CdTe single crystal substrates are described. Principal attention is focused on current-voltage characteristics of the heterostructures ZnxCdyHg1-x-yTe/CdTe. The mechanism of carriers transfer, in particular, multi-mode regime governed by the charge states localized at the interface, is studied. Results of numerical experiment performed for the first time for examined heterostructures are in good qualitative and quantitative agreement with the data of electric measurements.

High-efficiency multicrystalline silicon solar cells by liquid phase-epitaxy

Thin-film silicon cells produced on crystalline silicon substrates have the potential to achieve high cell efficiencies at low cost. We have used a modified liquid-phase epitaxy growth process to produce very smooth, high-quality silicon films on multicrystalline silicon substrates. Photoconductivity decay measurements indicate that the minority carrier lifetimes in these layers are at least 10 μs, sufficient to achieve cell efficiencies in excess of 16%. This efficiency potential is confirmed in small area cells, which have displayed efficiencies up to 15.4%. Further improvements up to 17% efficiency are possible in the short term, even without the introduction of any light-trapping schemes into the device structure.

Process and device characterization of high voltage gallium arsenide P- i- N layers grown by an improved liquid phase epitaxy method

GaAs P- i- N layers with an i-region net doping of less than 10 12 cm −3 were grown on P + and N + substrates by a modified liquid phase epitaxy (LPE) method. Doping profiles and structural data obtained by varius characterization techniques are presented and discussed. A P +- P- i- N- N + diode with a 25 μm-wide i-region exhibits a breakdown voltage of ∼ 1000 V, a t rr of ∼ 50 ns, and reverse current densities (at V R = 800 V) of − 3 × 10 −6 A/cm 2 at 25°C and ∼ 10 −2 A/cm 2 at 260° C.

First SiC dynistor

A four-layer dynistor has been made from SiC for the first time. Switching voltage is in the range from 30–50 V and switching time is about 10−8 s. The structure has been fabricated with a modified liquid-phase epitaxy (LPE).