Liquid Phase Epitaxial Layers Research Articles

Layer-by-Layer Grafting Dye on Chiral MOF Thin Films for Circularly Polarized Luminescence

Metal-organic frameworks with chiral feature (chirMOFs) are attracting great attention on circularly polarized luminescence (CPL). However, developing new efficient strategy to achieve or improve CPL properties is an urgent task. Herein, new chiral MOF thin films prepared by liquid-phase epitaxial layer by layer (lbl) growth method (SUR-chirMOF) are composed of D- or L-camphorate (D/Lcam) and aminopyrazine (Pr-NH2) by using liquid phase epitaxial layer by layer (lbl) method. The resulted Zn2(D/Lcam)2Pr-NH2 SURchirMOF shows strong chirality and luminescence but weak CPL emission at 390 nm. After lbl modifying a dye molecule FluoresceinIsothiocyanate (FITC), the chiroptical Zn2(D/Lcam)2Pr-NH-FITC SURchirMOFs with ~7 times CPL signal improvement and ~3 times glum value amplification are obtained. This work provides a new strategy to develop chiral MOF thin films for CPL improvement using lbl grafting approach.

Liquid-phase epitaxial growth of multiple MOF thin films with high lattice mismatch

We have prepared a series of multiple (bi-varietal and tri-varietal) pillar-layered MOF thin films [Cu2L2dabco]n (L = bdc, bpc and tpdc) with high lattice mismatch (up to 77.8%) using a liquid-phase epitaxial layer by layer method.

CdTe thin films as protective surface passivation to HgCdTe layers for the IR and THz detectors

The properties of thin CdTe cap layers grown using the hot-wall epitaxy (HWE) method under low-temperature technological conditions (T ≤ 373 K) to Hg1-xCdxTe liquid phase epitaxial layers (LPE) have been investigated. It has been shown that the CdTe films deposited using this low-temperature method, can be used as the passivation coatings for the infrared (IR) detectors based on the HgCdTe epitaxial layers grown on the CdZnTe monocrystalline substrates, not affecting the HgCdTe properties. It has been found that CdTe films with the thickness d ≥ 360 nm are continuous and possess effective protective properties in application to Hg1-xCdxTe/CdZnTe IR detectors. The manufactured photodiode detectors demonstrate relatively good parameters: the detectivity D* = 2.7 × ·1010 cmW−1Hz1/2 (T = 80 K) for the long-wave range (8–10.5 μm) IR photodiodes (x ≈ 0.22) and noise-equivalent temperature difference NRTD ≈ 20 mK for mid-wavelength (3–5 μm) photodiodes.

Ohmic metal/Hg1-xCdxTe (x ≈ 0.3) contacts

Some technological features of p-type Hg 1-x Cd x Te (x ≈ 0.3) liquid phase epitaxy layers grown on Cd 1-y Zn y Te (y ≈ 0.04) substrates are briefly discussed. Energy dispersive analysis of mercury-cadmium-telluride (MCT) layers and metal contacts (Au(In)/Cr(Mo,Ti)) to MCT layers together with their current-voltage characteristics at T ≈ 80 and 300 K are considered. It is shown that Cr(Mo,Ti)/MCT contacts have ohmic characteristics or close to them at T ≈ 80 and 300 K for both n- and p-type MCT layers. This gives the opportunity to form ohmic contacts to n- and p-type pads in one technological cycle. The contact resistance Rc is much smaller as compared to the HgCdTe p-n junction resistance R0[ RcA (< 10–2 Ω cm2) ≪ R0A (> 103 Ω cm2), where R0 is the zero bias diode resistance at T = 80 K, and A is the MCT (x ≈ 0.3) junction area]. So, such contacts are appropriate for the fabrication of photovoltaic HgCdTe detectors.

Liquid phase epitaxial layer by layer dipping assembly of metal-organic framework thin films and their physical property

Metal-organic framework (MOF) is a new kind of inorganic-organic hybrid porous ordered crystal material, which is connected by metal nodes and organic ligands through coordination bond. Because of its large specific surface area, high stability, diverse structure and adjustable function, MOF has received wide attention. The improvements in preparation and functionalization of MOF thin films expand their application fields. In this paper, the method for assembly of surface coordinated metal-organic framework thin films (SURMOF) by liquid phase expitaxial layer-by-layer dipping method is introduced, and the physical properties of some SURMOFs in optics, electricity and other aspects are summarized, and the application prospect of SURMOF is prospected as well.

Properties of heavily impurity-doped PbSnTe liquid-phase epitaxial layers grown by the temperature difference method under controlled Te vapor pressure

We propose the use of heavily impurity-doped Pb1-xSnxTe/PbTe epitaxial layers grown via the temperature difference method under controlled vapor pressure (TDM-CVP) liquid-phase epitaxy (LPE) for the preparation of IV–VI compounds for mid- to far-infrared optical device applications. A flat surface morphology and the distribution of a constant Sn concentration for 0.05≤x≤0.33 were observed in the epitaxial layers using electron-probe microanalysis. The segregation coefficient of Sn in Pb1-xSnxTe grown via TDM-CVP LPE (Tg=640°C) was xSSn／xLSn=0.28. The appearance of the Fermi level pinning and persistent photoconductivity effects in In-doped PbSnTe were also proposed; we estimated that the activation energies of these processes were 2.8 and 39.7meV, respectively, based on the In-doped Pb1-xSnxTe carrier profile as a function of ambient temperature. In Hall mobility measurements, Sn was assumed to be a main scattering center in the Pb1-xSnxTe epitaxial crystals. The impurity effect was also observed in Pb1-xSnxTe epitaxial growth, similar to the effects observed for Tl-doped PbTe bulk crystals. We concluded that the heavily doped Pb1-xSnxTe crystals grown via TDM-CVP LPE can be used to fabricate high-performance mid- to far-infrared optical devices.

Radiation detector based on 4H-SiC used for thermal neutron detection

In this work we have focused on detection of thermal neutrons generated by 239Pu–Be isotopic neutron source. A high quality liquid phase epitaxial layer of 4H-SiC was used as a detection region. The thickness of the layer was 70 μ m and the diameter of circular Au/Ni Schottky contact was 4.5 mm. Around the Schottky contact two guard rings were created. The detector structure was first examined as a detector of protons and alpha particles for energy calibration. Monoenergetic protons of energies from 300 keV up to 1.9 MeV were used for detector energy calibration and a good linearity was observed. The energy resolution of 35 keV was obtained for 1.9 MeV protons. The 6LiF conversion layer was applied on the detector Schottky contact. In the experiment we used different thicknesses of conversion layers from 5 μ m up to 35 μ m. Measured detected spectra show two parts corresponding to alpha particles detection in lower energy channels and 3H in higher energy channels. We have also performed simulations of thermal neutron detection using MCNPX (Monte Carlo N-particle eXtended) code. The detection efficiency and the detector response to thermal neutrons was calculated with respect to the 6LiF layer thickness. The detection efficiency calculation is found to be in good agreement with the experiment.

HgCdTe PHOTODIODES FOR INFRARED MID-WAVELENGTH REGION

HgCdTe photodiodes of relatively large area (Ø = 0.5 mm) for mid-wave infraredregion (MWIR) were fabricated on the base of liquid phase epitaxial layers. Characteristics of these photodiodes were estimated to be applicable as those working in background limited performance (BLIP) mode. It was shown that in spectral range of atmospheric transparency λ = 3 to 5 µm obtained dynamical resistance of HgCdTe photodiodes allow to realize their detectivity D* that are restricted by fluctuations of background radiation with specific power of photon fluxes Weff ≈ 3.3·10-4 - 4.4·10-5 W/cm2

Polarity inversion and growth mechanism of AlN layer grown on nitrided sapphire substrate using Ga–Al liquid‐phase epitaxy

Previously, we developed Ga–Al liquid‐phase epitaxy (LPE) to grow AlN layers on a surface nitrided sapphire substrate using Ga–Al solutions. We found that polarity inversion occurs at the interface between the nitrided sapphire layer and the LPE layer. As the partial pressure of oxygen influences LPE growth, our current study investigated this growth by controlling oxygen in supplying nitrogen gas. Combined with the previous study, we discuss these results in light of the polarity inversion model and growth mechanism for the Ga–Al LPE technique. Oxygen atoms are believed to dissolve in the solution and get trapped on the surface of nitrided sapphire. The trapped oxygen atoms form a symmetrical octahedral structure, which resets the polarity of the nitrided sapphire. Consequently, chemically stable aluminum‐polar AlN can be grown in the solution.

Mid-infrared photoconductive properties of heavily Bi-doped PbTe p–n homojunction diode grown by liquid-phase epitaxy

We fabricated a heavily Bi-doped (xBi≈2×1019cm−3) PbTe p–n homojunction diode that detects mid-infrared wavelengths by the temperature difference method (TDM) under controlled vapor pressure (CVP) liquid phase epitaxy (LPE). The photocurrent density produced by the heavily Bi-doped diode sample is approximately 20 times and 3 times greater than that produced by an undoped and heavily In-doped sample, respectively. By varying the ambient temperature from 15K to 225K, the detectable wavelength is tunable from 6.18μm to 4.20μm. The peak shift of the detectable wavelength is shorter in the heavily Bi-doped sample than in the undoped sample, consistent with our previously proposed model, in which Bi–Bi nearest donor–acceptor pairs are formed in the heavily Bi-doped PbTe liquid phase epitaxial layer. Current–voltage (I–V) measurements of the heavily Bi-doped diode sample under infrared exposure at 77K indicated a likely leak in the dark current, arising from the deeper levels. From the dark I–V measurements, the activation energy of the deep level was estimated as 0.067eV, close to the energy of the deep Tl-doped PbTe acceptor layer. We conclude that the deep level originates from the Tl-doped p-type epitaxial layer.

Issues in HgCdTe Research and Expected Progress in Infrared Detector Fabrication

The purpose of this paper is to review the trends in HgCdTe research, illustrating the discussed ideas with the latest results obtained at DEFIR (CEA-LETI and Sofradir joint laboratory). The beginning of this paper is devoted to an extended introduction to today’s issues concerning HgCdTe photodiode performance enhancement. In fact, very high-quality material is mandatory for ultrahigh performance at low temperature as well as for high noise operability at high operating temperature (HOT). Therefore, a strong effort has been carried out during the last few years for lattice-matched CdZnTe substrate and HgCdTe active layer growth improvement, leading to very large substrates and ultraflat liquid-phase epitaxy layers. The same analysis holds for diode process quality and passivation. Therefore, the photodiode process has been completely revisited in order to optimize HOT operability. Also necessary for the next generation of HOT devices, some significant progress has been made in small-pixel-pitch interconnection on silicon read-out circuits. Indeed, the first 10-μm focal-plane arrays (FPAs) have been successfully fabricated this year in the mid-wave infrared (IR) band. The latest avalanche photodiode (APD) realizations are also presented with 15-μm-pitch FPAs for passive imaging and 30-μm-pitch ultrafast arrays running at 1.5 kHz full frame rate. Linear-mode photon counting using APDs is also briefly discussed. Finally, the paper concludes on more complex structures for the third generation of IR detectors, discussing the latest achievements in dual-band FPA fabrication in various spectral bands.

Growth of Single-Phase 2H-SiC Layers by Vapor–Liquid–Solid Process

We attempted the vapor–liquid–solid (VLS) growth of SiC film in Si-Li solution using gaseous CH4 as a carbon source at 900 oC. A 100-m-thick liquid-phase epitaxy (LPE) layer was obtained on a 4H-SiC (0001) substrate under CH4 pressure of 0.9 MPa. X-ray diffraction (XRD) and a high-resolution transmission electron microscope (HR-TEM) measurement showed that the LPE layer was single-phase 2H-SiC. We concluded that VLS growth in Si-Li solution using gaseous CH4 as a carbon source is useful for growing single-phase 2H-SiC.

Comparison of the current characteristics of photodiodes formed on CdHgTe films grown by molecular-beam and liquid-phase epitaxy for the 8-12-μm spectral range

This paper presents a comparison of the dark currents and differential resistance of photodiodes, obtained using p-type cadmium-mercury telluride (CdHgTe) layers doped with boron ions, grown by molecular-beam epitaxy (MBE) and liquid-phase epitaxy (LPE). The volt-ampere responses for diodes on CdHgTe layers with composition x=0.215, grown by MBE and LPE (x≈0.222) are characterized by various saturation currents (reverse bias −0.25V) of 1–2 and 5–10nA, even though the band gap is greater for the latter. In this case, the maximum differential resistance is 4×109 and 5×107Ω for diodes based on MBE and LPE layers, respectively. The experimental data are compared with the calculated values. Numerical modelling showed that, for MBE structures with low biases, the dark current is limited by the diffusion current and by the Shockley-Read-Hall current outside the n-p junction, whereas, in LPE structures, there is a substantial contribution of currents through traps in the depletion region.

Liquid-phase epitaxy of 2H–SiC film on a (0001) 4H–SiC substrate in Li–Si melt

We first applied a liquid-phase epitaxy (LPE) method to the growth of 2H–SiC to obtain an oriented 2H–SiC layer. The LPE growth was performed on a (0001) 4H–SiC substrate in a Li–Si–C melt at 850 °C. A LPE layer grew on the carbon face of the (0001) 4H–SiC substrate with the area of about 1 × 1 mm 2. The thickness and morphology of the LPE layer were confirmed by scanning electron microscope (SEM), and the polytype of the LPE layer was determined using a high-resolution transmission electron microscope (HR-TEM) and a Raman spectroscopy. SEM measurements revealed that the thickness of the LPE layer was 50 μm and that this layer developed with a hexagonal shape. HR-TEM observations and Raman spectral analysis showed that 2H–SiC layer grows on 4H–SiC substrate with the relationship of (0001) 2H–SiC (0001) 4H–SiC. In this study, we first succeeded to grow <0001>-axis-oriented 2H–SiC films and concluded that the LPE technique is applicable to the growth of oriented 2H–SiC films in the Li–Si melt.

On the influence of solution density on the formation of macroscopic defects in the liquid phase epitaxy of GaN

This paper reports on a new simple method for avoiding particle-induced macroscopic defects using the liquid phase epitaxy (LPE) of GaN as an example. In a series of growth experiments by LPE of GaN it is demonstrated that the number of particle-induced macrodefects in the epitaxial layers correlates strongly and reproducibly with the density of the solution. In solutions with a density higher than that of the deleterious particles, the particles float on the solution and hence are hindered to get into contact with the seed, which is placed at the bottom of the crucible. Consequently, so-called depressions — a typical particle-induced defect in GaN–LPE layers — are avoided. The principle of avoiding the formation of macroscopic defects originating from particles by adapting the density of the solution (density criterion) should be generally applicable to solution growth processes, regardless of the material system.

Liquid Phase Epitaxial Growth of Si Layers on Si Thin Substrates from Si Pure Melts under Near-Equilibrium Conditions

The growth of Si epitaxial layers on Si substrates from Si pure melts was attempted under near-equilibrium conditions by dipping-type liquid phase epitaxial (LPE) growth, in order to eliminate the doping effect from metal solutions on the purity of the Si epitaxial layers. Si epitaxial layers can be grown on Si substrates from a Si pure melt only when the temperature of the Si growth melt is kept 1–2°C above the melting point of Si (1414°C) and the growth melt is cooled just after the substrate is dipped in the growth melt. The Si substrate, Si epitaxial layers, and Si polycrystals can be clearly distinguished in the electron back-scattering diffraction pattern (EBSP) image of their cross sections. The growth rate of Si LPE layers during cooling clearly increases as the amount of overheating of the growth melt decreases from 2 to 1°C and the cooling rate increases from 0.4 to 1.0 mm/min.

Understanding of crystal growth mechanisms through experimental studies of semiconductor epitaxy

Elementary crystal growth mechanisms in solution and vapor growths are discussed taking semiconductor as a model material. Solution growth mechanism is argued at first taking liquid phase epitaxy (LPE) as a model system. On the surface of the LPE layer, macrosteps are often generated as a result of step bunching. By studying the behavior of the macrosteps using photoluminescence image and chemical etching of the cleaved cross section, it was found that the shape of the macrostep is governed by bulk diffusion and growth kinetics. This has been confirmed by space experiments. It is concluded that the growth on the macrostep terrace is conducted by steps generated at screw dislocations. A technique, which is called microchannel epitaxy, was developed to realize macrostep-free and dislocation-free layers. Vapor growth mechanism is discussed next taking molecular beam epitaxy of III–V compound as a model system. Here, the mechanism of inter-surface diffusion is investigated. The surface diffusion of group-III elements between facets was studied by changing group-V pressure. It was found that the direction of the inter-surface diffusion is reversed twice as the group-V pressure is increased. This has been attributed to the different group-V pressure dependence of group-III ad-atom lifetime on different facet. It was concluded that in MBE the growth on the facet is conducted by birth and spread of two-dimensional nuclei and by mass transports from next facet and from effusion cell.

Nonstoichiometric deep levels in Mg-doped GaP epitaxial layers

We have investigated the relation between deep levels in Mg-doped p-type GaP liquid phase epitaxy (LPE) layers and stoichiometry of the surface of the substrates by PHCAP measurement. Concentration of a deep donor level at E C−1.9–2.1 eV is higher in an n-type undoped GaP substrate annealed with applying phosphorus vapor pressure of 20 kPa than in sample annealed beneath a carbon cover. Next, Mg-doped LPE layers are grown on substrates that have been pre-annealed under phosphorus vapor pressure just before the growth. The densities of deep levels at E V+0.85 and E V+1.5 eV in long-time (2 h) pre-annealing sample are greatly decreased, but a deep level at E C−1.9–2.1 eV shows opposite tendency. The latter is thought to be identical to a deep level detected in the substrate, probably phosphorus interstitial atoms.

Properties of Bi-doped PbTe layers grown by liquid phase epitaxy under controlled Te vapor pressure

Effects of Bi doping in PbTe liquid-phase epitaxial layers grown by the temperature difference method under controlled vapor pressure (TDM-CVP) are investigated. For Bi concentrations in the solution, xBi, lower than 0.2 at.%, an excess deep-donor level (activation energy Ed≈0.03–0.04 eV) appears, and Hall mobility is low. In contrast, for xBi>0.2 at.%, Hall mobility becomes very high, while carrier concentration is in the range of 1017 cm−3. Inductive coupled plasma (ICP) emission analysis shows that, for xBi=1 at.%, Bi concentration in the epitaxial layer is as high as NBi=2.3–2.7 × 1019 cm−3. These results indicate that Bi behaves not only as a donor but also as an acceptor, and the nearest neighbor or very near donor-acceptor (D-A) pairs are formed, so that strong self-compensation of Bi takes place. Carrier concentration for highly Bi-doped layers shows a minimum at a Te vapor pressure of 2.2 × 10−5 torr for growth temperature 470°C, which is coincident with that of the undoped PbTe.

Properties of Bi-doped PbTe epitaxial layers and pn junction diodes grown by TDM-CVP

Effects of Bi-doping in PbTe liquid-phase epitaxial layers grown by the TDM-CVP have been investigated. For Bi concentration in the solution, x Bi, lower than 0.2 at.%, Hall mobility is low. In contrast, for x Bi>0.2 at.%, Hall mobility is high, while carrier concentration is in the range 10 17 cm −3. However, ICP emission analysis shows that, for x Bi=1.0 at.%, Bi concentration in epitaxial layer is N Bi=2.3–2.7×10 19 cm −3. These results indicate that Bi behaves not only as a donor but also as an acceptor; the nearest neighbor or very near DA pairs are formed. Carrier concentration for Bi-doped layers takes a minimum value at a Te vapor pressure of 2.2×10 −5 Torr for growth temperature 470°C, which is coincident with that of the undoped PbTe. And broad contact pn junctions with highly Bi-doped layers easily cause laser emission compared to undoped junctions. The result suggests that the nearest lattice site Bi–Bi DA pairs behave as strong radiative centers in PbTe.