ZnSe Buffer Layer Research Articles

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 & 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.

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.

Band alignment in ZnCdTe/ZnTe and ZnCdSe/ZnSe SQWstructures grown on GaAs(100) by MBE

Molecular-beam-epitaxy-grown ZnTe/CdZnTe/ZnTe andZnSe/ZnCdSe/ZnSe strained single-quantum-well structures withnon-doping layers were investigated by cathodoluminescence (CL)and deep-level transient spectroscopy (DLTS). The activationenergies for deep levels in ZnTe and ZnSe buffer layers grownon GaAs were determined by the DLTS spectra. Moreover, anadditional DLTS peak that depends on the quantum well (QW) parametersand correlates with the QW emission line position in the CLspectra was observed. This peak is interpreted as an emissionof electrons from a ground level in the QW. Obtained DLTS andCL results were used for the estimation of the conduction bandoffset parameter QC.

MBE growth and RHEED characterization of MnSe/ZnSe superlattices on GaAs (1 0 0) substrates

[(MnSe) x /(ZnSe) y ] 60 superlattices ( x=4–16 Å, y∼12 Å) are grown on GaAs(1 0 0) substrates by molecular beam epitaxy (MBE) and are characterized by in situ reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), photoluminescence (PL) and photoreflectance (PR). MnSe in the superlattice structure is confirmed to be zincblende (ZB) by RHEED analysis. Lateral lattice spacings a 0 1 ̄ 1 of the individual MnSe and ZnSe layers in the superlattice are measured from the interval of the RHEED streaks. The lateral lattice spacing of the superlattice with MnSe layers thinner than 4 Å is fixed to that of the ZnSe buffer layer throughout the 60-period superlattice growth, that is, the superlattices grow coherently to the ZnSe buffer layer. Superlattices with thicker MnSe layers show increases in the lateral lattice spacing as the growth proceeds, and the RHEED pattern becomes spotty. Coherently grown samples show PL and PR signals due to excitonic transitions in the MnSe/ZnSe type-I superlattice. A small Stokes shift of 2 meV indicates the high quality of the superlattices. The energy gap of the ZB MnSe is estimated to be 3.4 eV at 13.5 K. Mn-related luminescence bands are observed at 2.1 and 1.7 eV, and the intensity of the 1.7 eV band is very small in coherently grown samples.

Photoluminescence characterization of MBE grown Zn 1− xBe xSe

We report photoluminescence (PL) studies of high-quality Zn 1− x Be x Se films grown by molecular beam epitaxy (MBE) on GaAs substrates by use of a novel growth method of Be–Zn co-irradiation before the growth of a thin ZnSe buffer layer. Samples show double-crystal X-ray linewidth as narrow as 23 arcsec. Low-temperature (13 K) PL of undoped samples showed free exciton emission, which defined the bandgap and showed that this system gives bandgap bowing. In addition, there was dominant deep bound excitonic recombination. We also suggest that deep PL can be caused by low structural quality of films, rather than by specific impurity states. Nitrogen-doped samples (with net acceptor concentrations up to 2×10 17 cm −3) show strong impurity related photoluminescence.

High crystalline quality ZnBeSe grown by molecular beam epitaxy with Be–Zn co-irradiation

High crystalline quality ZnBeSe epilayers with different compositions were grown on GaAs substrates by molecular beam epitaxy using Be–Zn co-irradiation of the III–V surface and a ZnSe buffer layer. A (1×2) reflection high-energy electron diffraction pattern was formed after the Be–Zn co-irradiation indicating the formation of Be and Zn dimers on the GaAs surface. A two-dimensional growth mode was observed throughout the growth of the ZnSe buffer layer and ZnBeSe epilayer. Narrow X-ray linewidth as low as 23 arcsec with the etch pit density of mid 10 4 cm −2 were obtained. The linewidth of the dominant excitonic emission is about 2.5 meV at 13 K for the near-lattice-matched ZnBeSe layer. For a nitrogen-doped sample, capacitance–voltage measurements showed a net acceptor concentration of 2.0×10 17 cm −3. In addition, the use of a BeTe buffer layer and of a Zn-irradiation with a ZnSe buffer layer were also investigated.

Cu(In,Ga)Se2 solar cells with a ZnSe buffer layer: interface characterization by quantum efficiency measurements

We investigate Cu(In,Ga)Se 2 -based solar cells with a new ZnSe buffer layer deposited by metal-organic vapour deposition and compare their electronic properties to reference cells using a standard CdS buffer layer. The best solar cell with a ZnSe buffer layer achieves an efficiency of 11.6%. We further investigate a large series of solar cells with varied thickness of both types of buffer layers by means of quantum efficiency measurements in equilibrium and under light and voltage bias. The characterization of the devices concentrates on the analysis of the collection of photogenerated holes from the buffer layer. We introduce a new method to determine the recombination probability of holes at the buffer/absorber interface. We find a similar interface recombination probability of about 40% for both devices, those with a ZnSe buffer layer and those with a CdS buffer layer. An anomalous enhancement of the quantum efficiency measured under current bias is ascribed to a barrier modulation effect which is caused by light absorbed in the buffer layer.

Luminescence properties of CdS quantum dots on ZnSe

The luminescence properties of self-assembled CdS quantum dots (QDs) were studied. CdS QD structures formed on ZnSe buffer layers without capping layers showed intense photoluminescence (PL). The PL peak position could be controlled by changing the amount of CdS deposited. Introduction of the capping layer enhanced the PL intensity. The buffer layer material selected also affected the luminescence properties of the CdS QDs, and brighter PL with the higher transition energy was observed by replacing the ZnSe buffer layer with ZnSSe. A light emitting diode structure based on CdS QDs sandwiched in the pn junction of ZnSe was fabricated and bright blue or green luminescence from current injection was observed at 77 K as well as at room temperature.

Structural and optical studies of ZnCdSe/ZnSe/ZnMgSSe separate confinement heterostructures with different buffer layers grown by molecular beam epitaxy

A great improvement of optical quality was observed from a ZnCdSe/ZnSe/ZnMgSSe single quantum well separate-confinement heterostructure grown on GaAs substrate with ZnSe/ZnCdSe strained-layer superlattices (SLS) buffer layers by molecular beam epitaxy. Transmission electron microscopy images showed that a pitted surface was generally formed after the desorption of GaAs substrate in a II–VI chamber. However, this pitted surface was smoothed out by the growth of SLS buffer layer. Near room temperature photoluminescence indicated that the carrier collection efficiency of quantum wells in the sample with SLS buffer layer is better than the sample only with a ZnSe buffer layer.

Molecular beam epitaxy of CdS/ZnSe heterostructures

We investigated the growth of CdS/ZnSe type II quantum-well structures, produced by MBE for the first time. Epitaxy was carried out with compound sources on GaAs(0 0 1) substrates. The best results were achieved on ZnSe buffer layers and at rather low substrate temperatures ( T=170°C). Photoluminescence measurements showed intense luminescence bands in the energy range from 2.0 to 2.65 eV depending on the quantum-well thickness. The PL bands are quite narrow (16–43 meV). X-ray diffraction profiles of superlattices revealed satellite peaks up to the fourth order. The critical thickness for the coherent growth of CdS on ZnSe was determined by detecting the quenching of the PL intensity as a function of the quantum-well thickness; it is about 4.5 nm.

MOVPE growth of ZnS on (1 0 0)GaAs using dimethylzinc and ditertiarybutyl sulphide precursors

Epitaxial ZnS films have been grown on (1 0 0)-oriented GaAs substrates by atmospheric pressure metal-organic vapour phase epitaxy (MOVPE), using dimethylzinc (DMZn) and ditertiarybutyl sulphide (DTBS) precursors for obtaining high quality layers. A detailed study of the growth kinetics has enabled us to determine the influence of the deposition temperature T g and the precursor partial pressures on the growth rate, surface morphology and crystalline quality of the layers. For the precursor partial pressures used here, high growth rates reaching 3.5 μm/h have been achieved. X-ray measurements result in a diffraction linewidth as low as 200 arcsec for films obtained with a ZnSe buffer layer. These data are a clear indication of the good crystalline quality of the films, when it is considered that a value of 180 arcsec is measured for the GaAs substrate. The double-crystal X-ray measurements give additional support to this conclusion, since the values of the FWHM are 74 and 25 arcsec for the above film and the substrate, respectively. Under optimised growth conditions, the photoluminescence spectra are dominated by excitonic emissions, with a free exciton linewidth of 4.7 meV. The experimental data clearly show the good quality of the epitaxial layers obtained with the DMZn/DTBS combination, used here for the first time to the best of our knowledge.

High-resolution x-ray diffraction investigations of highly mismatchedII-VI quantum wells

High-resolution x-ray diffraction (HRXRD) was used to systematically investigate CdSe and ZnTe quantum wells one to three monolayers thick sandwiched between a ZnSe buffer and cap layer grown at different substrate temperatures. For comparison high-resolution transmission electron microscopy (HRTEM) measurements were performed which were evaluated by digital analysis of lattice images. The x-ray diffraction profiles show typically two main layer peaks. Their intensity ratio depends critically on the quantum well thickness and varies only weakly with the thickness of the ZnSe layers. The total Cd or Te content determined from comparisons of experimental and simulated (004) -2 scans is well confirmed by the results from digital analysis of HRTEM lattice images. For quantum well thicknesses larger than 1.5 (ZnTe) or 2.0 (CdSe) monolayers, no simulation parameters could be found to achieve good agreement between theoretical and measured diffraction profiles. This transition is more clearly visible in diffraction profiles of asymmetrical reflections. By HRTEM measurements, this could be correlated to the occurrence of stacking faults at these thicknesses. The formation of quantum islands detected by HRTEM was not reflected in the HRXRD -2 scans.

Study of thin ZnSe buffer layers for growth of GaAs on CaF2/Si(111) heterostructures

Effects of a ZnSe buffer layer for heteroepitaxial growth of GaAs layers on CaF2 layers were investigated. It was found that wettability of ZnSe was better than that of GaAs on the CaF2(111) layer through the contact angle measurement of the ZnSe and GaAs islands by an atomic force microscope (AFM). It was found by secondary ion mass spectrometry (SIMS) measurement that a ZnSe buffer layer suppressed diffusion of Ca and F into the GaAs layer which was found in the case of direct growth and produced an abrupt interface between the CaF2 layer and GaAs layer. Analysis of the initial stages of GaAs growth revealed that nucleation density was increased and crystallinity was better when the ZnSe buffer layer was grown on the CaF2 surface. It is considered to contribute to better crystallinity of GaAs layers in the subsequent growth.

The annealing effects on ZnCdSe/ZnSe quantum wells and ZnSe/GaAs interfaces

This study investigated the thermally induced interdiffusion in ZnCdSe/ZnSe quantum wells and the outdiffusion of Ga atoms from the GaAs substrate by photoluminescence (PL), secondary ion mass spectrometry (SIMS), and contactless electroreflectance (CER) spectroscopy. The quantum well structures grown by molecular beam epitaxy were thermally annealed at temperatures between 250 and 700 °C. According to our results, the 15 K PL main peak disappeared when the annealing temperature reached 550 °C and above for samples grown with a 0.1 μm ZnSe buffer (sample A). In contrast, the PL main peak disappeared when the annealing temperature reached 600 °C and above for samples grown with 0.85 μm ZnSe buffer (sample C). In addition, for sample A, two extra PL peaks around 2.0 and 2.3 eV were observed when the annealing temperature reached 500 °C and above; those peaks were observed only when the annealing temperature reached 700 °C for sample C. SIMS results indicated the interdiffusion of Cd in the ZnCdSe/ZnSe quantum well regions in both samples. However, only sample A revealed a strong outdiffusion of Ga atoms from the substrate into the epilayer side, but a weaker one in sample C. The main peaks in PL spectra and the intersubband transitions in CER spectra disappeared owing to the Cd interdiffusion in the wells and the defect-related transitions introduced by the Ga outdiffusion onto the epilayer side. Based on the SIMS, PL, and CER spectra results, a thicker ZnSe buffer layer can increase the thermal stability of ZnCdSe/ZnSe quantum wells grown on GaAs substrates because of its effectiveness in mitigating the outdiffusion of Ga atoms into buffer layers and the interdiffusion of quantum well regions.

Role of V-shaped stacking faults in Au/n-type ZnMgSSe:Cl Schottky diodes

The microstructural, optical, and electronic characteristics of both Cl-doped ZnMgSSe epilayers and Schottky diodes grown by the molecular beam epitaxy method on n+-type GaAs (100) substrates with different thicknesses of Cl-doped ZnSe buffer layers were investigated via three measurements: transmission electron microscopy, photoluminescence, and deep level transient spectroscopy. The V-shaped stacking faults, originating at or near the ZnSe:Cl (buffer layer)/GaAs interface and extending into the ZnMgSSe:Cl epilayer, were discovered to have the evident inclination to exist in samples with thicker ZnSe:Cl buffer layers and higher growth temperatures. Evidence also revealed that the stacking faults not only produced poor optical quality of ZnMgSSe:Cl thin film, but also created more interface state densities at the Au/n-type ZnMgSSe:Cl Schottky junction. Such results are the main factors to damage the I–V characteristics of Au/n-type ZnMgSSe:Cl Schottky diode, including the less forward conduction current d...