ZnSe Buffer Research Articles

Structural study of ZnSe films grown on substrate with InxGa1-xAs and Al1-xGaxAs buffer layers: strain, relaxation and lattice parameter

ZnSe layers of various thickness were grown on (001) GaAs substrates, using InxGa1-xAs or Al1-xGaxAs as buffer layers by molecular beam epitaxy and were studied by high-resolution x-ray diffraction. The principal structural characteristics of ZnSe layer and buffer layer were determined using several reflections, such as (004) and two pairs of coupled asymmetric reflections, namely (224), (-2-24) and (115) (-1-15). In order to evaluate their validity, the experimental data obtained from these reflections were handled by means of two known expressions found in the literature. We have found the relaxation process of ZnSe layers is well described by a geometrical model including the thermal strain and small strain due to work hardening. The relaxation process is faster for ZnSe grown on ternary buffer layers despite the fact that, some buffer layers are pseudomorphically grown to the substrate; therefore we conclude that not only the lattice mismatches have effect on the relaxation process but also the surface state of the buffer layer has an influence in this process.

Formation of self-assembled ZnTe quantum dots on ZnSe buffer layer grown on GaAs substrate by molecular beam epitaxy

Self-assembled ZnTe quantum dot structures were grown by molecular beam epitaxy on GaAs substrates with a 200 nm ZnSe buffer layer. Surface morphology was studied by atomic force microscopy. A three-dimensional Volmer–Weber growth mode was identified. Two types of dots were observed. Strong photoluminescence observed at 1.9–2.2 eV was attributed to emission from the large type II ZnTe quantum dots. Emission from the smaller ZnTe quantum dots was observed at an energy of around 2.26 eV. The density of the larger and smaller dots was approximately 10 8/cm 2 and 10 9/cm 2, respectively.

Improvement in crystallinity of ZnSe by inserting a low-temperature buffer layer between the ZnSe epilayer and the GaAs substrate

This article demonstrates the effect of the thin low-temperature-grown ZnSe buffer layer (LT-ZnSe) in improving crystallinity of ZnSe-based films grown on GaAs substrate. Especially, the density of stacking fault defects, which is normally observed in ZnSe-based films grown on GaAs substrate, can be well reduced by inserting a thin ZnSe buffer layer grown at sufficiently low temperature between ZnSe/GaAs heterostructure. A ZnSe film with stacking fault densities as low as ∼2×10 6/cm 2 was obtained by growing on the LT-ZnSe/GaAs buffer layers, in contrast, ∼8×10 8/cm 2 was obtained by directly growing on GaAs substrate. The Frank-type stacking faults was dominant for films grown with LT-ZnSe, meanwhile, Shockley-type stacking faults was dominant for films grown without LT-ZnSe buffer. At the initial stages of ZnSe growth, three-dimensional growth mode is considerably suppressed with LT-ZnSe buffer. The improvement of crystallinity in the ZnSe film with LT-ZnSe and GaAs buffer is also evidenced by low-temperature photoluminescence spectroscopy and high-resolution X-ray diffraction, which show very large intensity ratio of near-band-edge emission to deep level emission and narrow X-ray diffraction peak width of (0 0 4) rocking curve with 92 arcsec, respectively.

ZnSe buffer prepared by iodine-enhanced chemical vapour deposition for Cu(In,Ga)(Se,S)-based solar cells

Abstract Cu(In,Ga)(Se,S) 2 -based solar cells with an alternative ZnSe buffer prepared by an iodine-enhanced low-pressure chemical vapour deposition (CVD) from a powder source were investigated. The application of iodine allowed acceptable transport rates even at substrate temperatures below 300°C, which is essential to avoid damage to the predeposited absorber. So far, the most efficient cells provided with a CVD ZnSe buffer yielded efficiencies of more than 80% of reference cells with CdS buffer from a chemical bath. The losses are due to a reduced open circuit voltage and fill factor whereas the short circuit current density was improved as a consequence of less absorption losses in the buffer. The photocurrent was observed to be voltage dependent, which affects open circuit voltage and fill factor. In addition, measurements of the voltage-dependent quantum efficiency as well as the junction capacitance revealed a reduced doping density and an enlarged space charge region of the Cu(In,Ga)(Se,S) 2 absorber in solar cells with a ZnSe buffer prepared by CVD.

Magnetic CdSe-based quantum dots grown on Mn-passivated ZnSe

In this letter we describe the properties of self-assembled CdSe quantum dots (QDs) grown on Mn-passivated ZnSe buffers. We show that the Mn deposited on the ZnSe surface during the passivation process acts as a nucleating seed for self-assembled QD formation. For moderate amounts of Mn deposition, the dots grown in this way show a significant improvement in size uniformity compared to CdSe dots grown on ZnSe without Mn passivation. Using photoluminescence, we also show that the dots exhibit large Zeeman splitting, indicating that this growth method is suitable for fabricating magnetic QDs that exhibit strong spin polarization effects.

ZnSTeSe metal-semiconductor-metal photodetectors

High-quality quaternary ZnSTeSe epitaxial layers were successfully grown by molecular beam epitaxy (MBE). It was found that a ZnS/sub 0.18/Se/sub 0.82/ layer was automatically formed in between the ZnSe buffer layer, and the ZnS/sub 0.17/Te/sub 0.08/Se/sub 0.75/ epitaxial layer, when we increased the ZnS cell temperature. ZnSTeSe metal-semiconductor-metal (MSM) photodetectors were also fabricated for the first time. It was found that we could achieve a photocurrent to dark current contrast higher than five orders of magnitude by applying a 10-V reverse bias. We also found that the maximum photoresponsivity is about 0.4 A/W under a 10-V reverse bias. Such a value suggests that the ZnSTeSe MSM photodetector is potentially useful in the blue-UV spectral region.

Investigations of atomic diffusion at CIGSSe/ZnSe interfaces with heavy ion elastic recoil detection analysis (HI-ERDA)

The performance of thin film chalcopyrite solar cells based on a p-Cu(In,Ga)(S,Se) 2/n-ZnO pn-junction can be improved significantly by inserting a buffer at the interface. Besides the widely used CdS, ZnSe is a promising alternative. Such a layer can be most successfully grown by halogen supported chemical vapour deposition (H-CVD) at temperatures of approximately 300°C. With buffers grown at higher temperatures, the solar cells show lower efficiencies, possibly caused by thermally activated atomic interdiffusion. Heavy ion elastic recoil detection analysis (HI-ERDA) has been used to study depth-dependent atomic concentration profiles. These measurements revealed diffusion of indium from the Cu(In,Ga)(S,Se) 2 absorber material into the ZnSe buffer. However, a quantitative determination of the diffusion parameters is difficult, since surface and interface roughness of the polycrystalline material system affect the measured energy spectra of the sample constituents. Therefore, the diffusion coefficient has been calculated from separate measurements of evaporated indium layers on ZnSe single crystals, annealed in argon atmosphere. Since the results were still largely influenced by the sample's surface roughness, additionally, atomic force microscopy (AFM) measurements have been performed to determine the surface morphology. With these additional information, it was possible to extract the diffusion coefficient of indium in ZnSe. So for the first time, it is possible to analyse qualitatively and quantitatively atomic diffusion processes by ERDA for the material system and the temperatures used for solar cell fabrication.

Optical investigation of CdSe/ZnSe quantum nanostructures

We have used resonant and non-resonant Raman scattering as well as photoluminescence and cathodoluminescence experiments to study the structure and composition properties of CdxZn1−xSe formed by migration enhanced epitaxy of CdSe layers on ZnSe buffers. The spectral change of the photoluminescence maximum correlates with the increase of the Cd content, depending on the nominal CdSe thickness in the 1.5–3.0 ML range. The inhomogeneous broadening of the photoluminescence band is caused by the composition difference between the two-dimensional mixed CdxZn1−xSe layer and the inserted islands with larger Cd concentration. This is confirmed by phonon frequency changes in resonant Raman scattering for samples with different nominal CdSe thicknesses as well as in Stokes and anti-Stokes frequency changes observed in the 1.5 ML sample. Attention is paid to the role of defects on Raman scattering and photoluminescence for the 3.15 ML sample.

HRTEM observation of CdSe/ZnSe SQWs grown on vicinal GaAs substrate

Microscopic structural properties of CdSe/ZnSe single quantum wells grown on vicinal GaAs substrate are investigated by cross-sectional high-resolution transmission electron microscope. The interface of ZnSe buffer and GaAs substrate, the surface of top ZnSe, and CdSe well are observed. At the ZnSe/GaAs interface, the lattice structure is strongly smeared, indicating that structures other than ZnSe and GaAs are formed. Stacking faults are generated at this region. A distinct step and terrace structure is observed at the surface of top ZnSe. The averaged terrace width coincides with the nominal value of 5° vicinal surface. Well layer is tilted with regard to the (2 0 0) lattice plane by 5°, and no misfit dislocation is found. Strain mapping of the obtained lattice image is performed.

Growth and Characterisation of Mg-Based Low Dimensional Systems for Blue-Green Optoelectronics

In this paper a brief overview of the state-of-the-art of the growth and characterisation of Mg-based II-VI heterostructures is reported. Recent studies on MgSe binary compound, ZnMgSe ternary alloy and ZnSe/ZnMgSe multiple quantum wells (MQWs) grown by metalorganic vapour phase epitaxy MOVPE are presented. The low pressure MOVPE growth of MgSe, ZnMgSe and ZnSe/ZnMgSe MQWs on (100)GaAs by using ditertiarybuthylselenide, dimethylzinc:triethylammine (Me 2 Zn:Et 3 N), and bismethylcyclopentadienylmagnesium [(MeCp) 2 Mg] as Se, Zn and Mg precursors, respectively is reported. MgSe layers turned out to be mosaics made of (111)- and (100)-oriented MgSe crystals in their rocksalt phase; no signatures of the zincblend (ZB) phase was observed. ZnMgSe ternary epilayers were however grown in the ZB phase throughout the compositional range studied (0.10<x c <0.46). The ZnMgSe solid-vapour distribution curve deviates from linearity due to the different Mg and Zn alkyl vapour diffusion coefficients. A detailed structural analysis of MOVPE-grown ZnSe/ZnMgSe MQWs on (100)GaAs is also reported for the first time. Several ZnSe/Zn 0.83 Mg 0.17 Se MQWs having 16 monolayers (ML)(4.4 nm) thick wells, an alloy barrier thickness ranging between 32 ML (10.5 nm) and 16 ML (4.5 nm) and a number of periods varying between 6 and 12 were deposited on GaAs after a 4.2 nm thin pseudomorphic ZnSe buffer layers. The MQW modulation of as-grown samples was shown by secondary ion mass spectrometry (SIMS) elemental depth profiling measurements. The crystalline properties of ZnSe/Zn 0.83 Mg 0.17 Se MQWs were investigated by high resolution X-ray diffraction (HRXRD), whose spectra were simulated by means of a XRD model within the kinematical approximation, allowing us to determine the sample built-in strains and alloy composition. Whilst the latter value confirmed what expected on the basis of the MOVPE solid-vapour distribution curve, the determination of residual elastic strain in the MQWs structures allowed a systematic investigation as function of MQWs parameters. In the case of 6 periods ZnSe/Zn 0 83 Mg 0.17 Se MQWs having 32 ML thick barriers (total thickness 105 nm) diffraction patterns recorded in asymmetrical configuration indicate almost no relaxation of the structure, within the HRXRD sensitivity. HRXRD of ZnSe/ Zn 0.83 Mg 0.17 Se MQWs structures grown under the same conditions (well and barrier width of 16 ML and Mg content equal to 0.17) but with a different number of periods (6, 9, 12) and so with different total thickness (68, 98, 127 nm) show that zeroth and first order satellite peaks broaden with increasing the number of periods even for nominally pseudomorphic MQWs as a result of strain fluctuations and/or defect nucleation in the MQW structure. Between 9 and 12 periods MQWs structures begin to plastically relax. 10K PL spectra of ZnSe/ZnMgSe MQWs with the same well width show a sharp peak that red-shifts with the Mg content in the barriers. This can be ascribed to an increased tensile strain in the ZnSe well dominating over the confinement effect. Fixing the Mg amount in the barrier, the PL peak red-shifts with increasing the well width.

Spectral Ellipsometry of Multilayer ZnS/ZnSe Heterostructures

By the method of spectral ellipsometry with binary modulation of the polarization state the dispersions of the refractive index n, absorption coefficient k, and layer thickness in ZnS/ZnSe multilayer structures grown by the chemical gas‐phase deposition method from heteroorganic compounds on GaGaAs‐substrates with ZnSe buffer layers have been determined. The efficiency of local ellipsometric measurements (with a light beam size less than 150 × 500 μm) permitting mapping of the parameters of structures with A2B6 layers up to a few microns thick has been demonstrated. The optical properties of oxide layers formed on the zinc selenide surface have been investigated. Multilayer structures (ZnSe/ZnS)n/ZnSe/GaAs with a pronounced exciton absorption and specific features in the reflection spectra coinciding in energy with exciton transitions, as well as Bragg mirrors with a reflection coefficient up to 99% in the blue region of the spectrum, have been studied.

MBE growth of MnTe/ZnTe superlattices on GaAs (1 0 0) vicinal substrates

MnTe/ZnTe and MnSe/ZnSe superlattices are grown by molecular beam epitaxy (MBE) on GaAs(1 0 0) substrates 5°vicinal toward [ 0 1 ̄ 1 ] direction. Crystal structures, lateral lattice spacings, and atomic step arrays on the growing surface are investigated by in situ reflection high energy electron diffraction (RHEED). The superlattices are grown on ZnTe or ZnSe buffer layers which are lattice relaxed with respect to the GaAs substrate and have uniform array of atomic steps on the surface. MnTe and MnSe layers grow coherently, respectively, on the ZnTe and ZnSe buffer layers and show RHEED patterns indicating a uniform array of atomic steps when their thicknesses are smaller than 1–2 ML. High-quality (MnTe/ZnTe) 60 and (MnSe/ZnSe) 60 vicinal superlattices can be prepared for the MnTe or MnSe layers thinner than 1–2 ML.

Cd-free buffer layers for CIGS solar cells prepared by a dry process

ZnSe buffer layers for Cu(ln,Ga)Se 2/buffer/ZnO solar cells have been prepared by metal organic chemical vapor deposition (MOCVD). Using photoassisted MOCVD, deposition temperatures down to 265°C are possible. It is shown that deposition temperatures well below 300°C are essential as well as deposition times not much longer than 3 min. Higher temperatures and longer deposition times lead to absorber degradation. With optimized buffer deposition efficiencies of 11% have been obtained on CIGS absorbers from the Siemens pilot production line.

Raman scattering studies of the ZnSe/GaAs interface

AbstractRaman spectroscopy was used to study the band bending at the interface of ZnSe/GaAs hetero‐structures. A series of samples, which contained a ZnSe buffer layer, 0–35 nm thick, grown at a lower temperature than the much thicker ZnSe epilayer, by metal–organic chemical vapor phase deposition, were investigated. Compared with that of the GaAs substrate, an enhancement of the intensity of the LOGaAs phonon was found in samples grown without and with a thick (≥28 nm) buffer layer, but not in a sample grown with a 4 nm thick buffer layer. The enhancement is attributed to the electric field induced Raman scattering, resulted from a strong band bending on the GaAs side of the hetero‐structure. The results suggest that the direction of the interfacial electric field on the GaAs side will reverse with increasing buffer layer thickness. Between this reversal, a near flat band condition can be achieved, as was found in a sample grown with a 4 nm buffer layer. This suggestion is consistent with the concomitant improvement of the structure of the epilayer and of the interfacial quality of the hetero‐junction, which unpins the Fermi level and affects the band bending. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Improvement in the quality of ZnSe epilayers grown on (0 0 1) GaAs by the low temperature growth of a thin ZnSe buffer layer

The surface morphology, optical and structural properties of ZnSe epilayers grown on (0 0 1) GaAs at 450°C using metalorganic chemical vapor phase deposition are found to be greatly influenced by the initial growth of a thin ZnSe buffer layer at the low temperature of 360°C. Moreover, there is an optimum thickness for the buffer layer on which the quality of the epilayer grown is significantly better, as indicated by four independent quality gauges. In comparison to epilayers grown without a buffer layer, epilayers grown on a buffer layer of optimum thickness around 28 nm show: (1) about 3 orders of magnitude reduction in the density of growth hillocks, (2) narrow line width and about 3 orders of magnitude increase in the intensity of excitonic photoluminescence, (3) much narrower X- ray rocking curve peaks, and (4) mono-layer high surface steps with atomically smooth terraces in between.

Structural properties of MOVPE-grown ZnMgSe epilayers and ZnSe/ZnMgSe MQWs on (100) GaAs

Abstract The structural characterisation of MOVPE-grown ZnMgSe and ZnSe/ZnMgSe multiple quantum wells (MQWs) is reported. Zn 0.83 Mg 0.17 Se epilayers and 6 periods ZnSe/Zn 0.83 Mg 0.17 Se MQWs having 4.5 nm thick ZnSe wells and 10.5 nm thick alloy barriers were deposited on (1 0 0) GaAs after a thin pseudomorphic ZnSe buffer. No macroscopic misorientations between ZnMgSe epilayers and GaAs lattice has been observed, but a broadening of the alloy (4 0 0) peak indicates the occurrence of extended defects. Reciprocal space map (RSM) measurements of a Zn 0.83 Mg 0.17 Se epilayer show a slight asymmetry of the alloy peak along the growth direction, ascribed to an inhomogeneous relaxation. Diffraction patterns recorded in asymmetrical configuration indicate almost no relaxation of the MQWs structure, within the technique sensitivity. RSMs in the vicinity of the (4 0 0) lattice point show a characteristic diffuse scattering around the MQW satellite peaks, indicating that the MQWs structure is beyond the onset of plastic relaxation.

Electrical characterization of defects in Cu(In,Ga)Se 2 solar cells containing a ZnSe or a CdS buffer layer

Cu(In,Ga)Se 2 based solar cells with a ZnSe buffer layer deposited by metal-organic vapor deposition are compared to reference cells with a usual CdS buffer. Current–voltage characteristics were measured at different intensities of illumination (up to 100 mW/cm 2) in the temperature range between 80 and 300 K. We observe a current roll-over for cells with ZnSe buffer at much higher temperatures than for cells with CdS buffer, which seems to originate from defects either in the buffer volume, or at the buffer/absorber interface. Also wavelength-dependent I( V) measurements were performed before and after annealing at 380 K. A distortion of the I( V) curve measured under red illumination occurs for most of the cells with CdS buffer, but cannot be observed in cells with ZnSe buffer. This effect can be attributed to deep levels in the CdS volume.

Thickness dependent surface morphologies and luminescent properties of ZnSe epilayers grown on (0 0 1) GaAs by metalorganic chemical vapor phase deposition

ZnSe epilayers were grown on (0 0 1) GaAs substrate by using metalorganic chemical vapor phase deposition. The growth was carried out by first growing a 30 nm thick ZnSe buffer layer at a lower temperature of 360°C, followed by the main growth at 450°C. Surface morphologies and photoluminescence (PL) of the ZnSe layers with various thicknesses were investigated. We found that the ZnSe buffer layer contains (1) high density of islands about 80 nm in size and (2) low density of holes about 100–1500 nm in diameter. These two features develop in different ways during the growth at higher temperature. The places containing small islands become rougher first and then smoother, with an increase in layer thickness. After the thickness exceeds 210 nm, the surface becomes atomically flat. The holes, however, favor the three-dimensional growth mode and develop into growth hillocks. Energy dispersive X-ray spectroscopy showed that the hole is mainly made up of Ga–Se compounds, consistent with the earlier reports that Ga 2Se 3 compounds are the source for the growth of hillocks. PL study showed that the donors, acceptors and the deep level emissions associated defects are mainly concentrated at the GaAs/ZnSe interface.

Molecular beam epitaxy of MnAs/ZnSe hybrid ferromagnetic/semiconductor heterostructures

We report the use of molecular beam epitaxy to create hybrid ferromagnetic/semiconductor heterostructures composed of MnAs and ZnSe, with a Curie temperature of 325 K. The presence of a ZnSe buffer layer exclusively stabilizes the type-B orientation of MnAs, in which the (1̄101) MnAs and (001) ZnSe planes are parallel. Single-phase type-B αMnAs/ZnSe heterostructures yield magnetic properties comparable to those reported in the literature for MnAs/GaAs heterostructures. Variations in growth conditions also permit the stabilization of a strained, nonferromagnetic phase that can coexist with the ferromagnetic phase even at room temperature.

Low pressure MOVPE growth and structural properties of ZnMgSe epilayers on (1 0 0)GaAs

The low-pressure MOVPE growth of ZnMgSe on (1 0 0)GaAs is reported. ZnMgSe alloys were deposited after a thin pseudomorphic ZnSe buffer layer by using dimethylzinc : triethylammine (Me 2Zn : Et 3N), ditertiarylbutilselenide ( tBu 2Se) and bis(methylcyclopentadienyl)magnesium [(MeCp) 2Mg]. Zn 1− x Mg x Se (0.10< x<0.46) epilayers were grown at 330°C, 304 mbar and a high VI/II ratio in the vapour. Under these conditions the growth is limited by the mass transport of the II-group alkyls and good control of the composition was achieved. Rutherford backscattering spectrometry measurements allowed to determine the epilayer stoichiometry and deposited dose. The ZnMgSe solid–vapour distribution curve deviates from linearity due to the different Mg and Zn alkyl vapour diffusion coefficients, whose ratio turns out to be D (MeCp)2Mg/ D Me2Zn=0.410. The epilayer crystalline properties were studied by double-crystal X-ray diffraction (DC-XRD) and high-resolution reciprocal space mapping (RSM) measurements. Rocking curve FWHM values of 540–900 nm thick Zn 1− x Mg x Se were in the 5–12 mrad range, indicating the occurrence of extended defects in the epilayers. RSM measurements in the vicinity of the (400)-peak of a Zn 0.844Mg 0.156Se/ZnSe/(1 0 0)GaAs sample showed a slight asymmetry of the ternary alloy peak along the growth direction, ascribed to an inhomogeneous relaxation of the epilayer.