ZnTe Superlattices Research Articles

Light up-conversion mechanism of ZnSe–ZnTe superlattices

The specially designed ZnSe–ZnTe superlattice (SL), type-II SL, has the light up-conversion effect that SL emits a photoluminescence (PL) peak under the irradiation of the excitation light with the lower energy than the PL peak. The excitation intensity dependence of the energy of up-converted PL peak obeys the characteristic of type-II SL. The PL peak intensity has the super-linear dependence on the excitation intensity. The five kinds of samples in this work showed these phenomena. The model to explain these situations has been proposed.

Excitation intensity dependence of up-converted emission in ZnSe–ZnTe superlattice grown by MBE

The photoluminescence (PL) spectra of the type II ZnSe–ZnTe superlattices with dual-subband structures have two emission peaks. The PL peak at the higher energy side appears by the excitation even with the light, with the middle energy between the PL peaks. The emission energy of the up-converted light shows dependence similar to the case of direct excitation (high-energy excitation) on the excitation intensity. The emission energy increases with increase in the excitation intensity. The up-converted and the direct excited emissions are due to the same state in the superlattice.

Observation and origin of the photoluminescence spectrum showing multi-band structure from strained layer superlattice of ZnS–ZnTe

In photoluminescence (PL) spectra of a ZnS–ZnTe superlattice grown on GaAs substrate a multi-band structure with equal energy spacing of about 90 meV has been observed. This multi-band structure is shown to be explainable with a one-dimensional harmonic oscillator model where electrons and holes are confined separately in approximately parabolic potential wells formed by space charges at the p–n heterojunctions. The origin of the PL spectrum is attributed to electron–hole recombinations from electrons being in Boltzmann distribution among levels n=0, 1, 2,… in n-ZnS to the holes at n=0 level in p-ZnTe. However, an appreciably higher electron temperature than the lattice temperature is necessary for explaining the observed spectral shape, although the temperature during decay shows a tendency of cooling down. Thermal quenching of the luminescence as a whole is thought to be due to hole release from the level in ZnTe.

High-efficiency p–i–n detectors for the visible spectral range based on ZnSTe–ZnTe superlattices

We demonstrate that ZnSTe–ZnTe superlattices, which are grown lattice matched on GaAs substrates by molecular-beam epitaxy, are well suited as a material for light detection in the visible spectral range. Due to their type-II band alignment, these superlattices have a small band gap compared to ZnSe and allow light detection for photon energies between 2.1 and 3.2 eV. In combination with ZnMgSSe, the response curve of the diodes shows a strong dependence on the applied reverse bias, so that they can be used as two-color devices. The obtained external quantum efficiencies are as high as 70%, and dark currents as low as 10−12 A/mm2 were obtained.

Up-conversion effect of ZnSe–ZnTe superlattices with modulated periodicity

A light up-conversion effect has been observed in the ZnSe–ZnTe superlattices with a modulated periodicity. A photoluminescence (PL) peak at 2.1 eV is observed in the samples even under excitation by an He–Ne laser (633 nm, 1.96 eV). The samples, which we grow on InP by molecular beam epitaxy, consist of a combination of two kinds of superlattices. The PL spectrum, measured by using an He–Cd (325 nm, 3.81 eV) or Ar + (488 nm, 2.54 eV) laser as the excitation source, indicates two peaks due to the respective superlattices at 1.9 and 2.1 eV. The intensity of the up-converted peak has a super-linear dependence on the excitation intensity. The mechanism of the effect has been explained by a dual-subband model.

Time-Resolved Photoluminescence Measurements on ZnSe-ZnTe Superlattices

Time-resolved photoluminescence of ZnSe–ZnTe superlattices with different Te concentrations has been measured in the picosecond time region. The samples show typical S1 (blue band) and S2 (green band) emissions in the time-integrated spectra. In the time-resolved spectra, both samples exhibit an asymmetrically broadened shallow component at the high-energy side of the luminescence band, indicating the presence of shallow localized excitons due to long-range potential fluctuations.

Picosecond photoluminescence properties of CdTeZnTe superlattices prepared by hot-wall epitaxy

CdTeZnTe strained-layer superlattices (SLSs) are grown on GaAs substrates by hot-wall epitaxy (HWE). SLSs are studied by X-ray diffraction, photoluminescence (PL) and transmission electron microscopy (TEM). Cross sectional TEM image of SLSs shows that the critical thickness of the CdTe well layer is about 9–12 Å. Picosecond time-resolved PL spectra reveal that the exciton luminescence bands in SLS are originating from both free and bound exciton recombination. Dynamical processes of photo-excited carrier relaxation and annihilation in CdTeZnTe are discussed.

Emission mechanism of blue and green bands in ultrathin ZnSeZnTe superlattices

The electron-phonon interaction strength (Huang-Rhys parameter S) of ultrathin ZnSeZnTe superlattices has been estimated. The values of S are 3.5 and 7.4 for S1 blue and S2 green bands caused by the Te isoelectronic traps. Using these values, we have constructed configuration coordinate diagrams to explain absorption and emission mechanisms in the ZnSeZnTe superlattices.

Ultrathin pseudomorphic layers of ZnTe in CdTe/(Cd,Zn)Te superlattices: A direct optical probe of the mixed-type band configuration.

A range of studies, such as transmission electron microscopy, x-ray diffraction, and optical spectroscopy, applied to a series of CdTe/ZnTe superlattices with ultrathin ZnTe layers, confirmed the ability to grow, in CdTe, ultrathin pseudomorphic layers of ZnTe with limited Zn segregation and of high crystalline and optical quality, at least for layer thicknesses up to a few monolayers. Subsequently, we employed the spatially selective incorporation of monomolecular isoelectronic planes of ZnTe as a perturbative probe of the exciton states in a mixed-type CdTe/(Cd,Zn)Te superlattice. The effect of the ZnTe monolayer as a function of position on the heavy- and light-hole transitions of this system was determined by optical spectroscopy. Our results confirmed experimentally the type-I nature of the heavy-hole excitons and demonstrated the possibility to directly tune the ``indirectness'' of the light-hole exciton, depending on the position of the planar perturbation. Theoretical calculations were found in qualitative agreement with all these results.

Planar isoelectronic perturbation as a probe of the mixed type band configuration in CdTe/(Cd,Zn)Te superlattices

We employed the spatially selective incorporation of monoatomic isoelectronic planes of ZnTe as a perturbative probe of the exciton states in a mixed type CdTe/(Cd,Zn)Te superlattice. In order to identify the effect of the ZnTe monolayer on the heavy and light hole transitions, a range of optical spectroscopy techniques were applied to our samples. Our results confirm experimentally the type I nature of the heavy hole excitons and demonstrate the possibility to directly tune the “indirectness” of the light hole exciton, depending on the position of the planar perturbation. Theoretical calculations are in qualitative agreement with all these results.

Optically pumped [formula omitted] lasers on [formula omitted] substrates

Optically pumped lasing has been observed up to 100 K in a Cd 0.2Zn 0.8Te ZnTe superlattice grown on a GaAs Si substrate. At 100 K, rapid degradation of the lasing signal is seen. This result is inferior to that previously reported for a similar structure grown on a GaAs bulk substrate, for which room-temperature lasing was seen with no degradation. X-ray rocking curve and morphology measurements suggest that the II–VI epilayer has limited ability to heal the extra defects that are present when a GaAs Si , rather than a bulk GaAs, substrate is used.

Calculations of electronic states and optical matrix elements of freestanding (100) (CdTe) n(ZnTe) n superlattices

We present a study of the electronic and optical properties of CdTeZnTe superlattices using an empirical pseudopotential complex bandstructure approach. The mixing between the light and heavy-hole states is examined as a function of sublayer width and valence-band offset, and is found to be strongly dependent upon the width, but little affected by change in offset. However, the resulting wavefunctions are sensitive to the offset, and this will have an effect upon the properties of these superlattices.

ZnTe and CdTe: ZnTe superlattices grown by MOVPE

Epitaxial layers of ZnTe and CdTe: ZnTe superlattices have been grown by MOVPE on {100} and {⦶111}B GaAs substrates at temperatures down to 300°C using di-isopropyl telluride. In contrast to growth at 410°C using di-ethyl telluride, these layers exhibit excellent morphology. Using RHEED, however, it has been possible to show that the morphology remains better on the {⦶111}B. ZnTe has also been grown on {100} GaSb.

Tem Studies of CD:ZN:TE-Based II-VI Superlattices and Epitaxial Layers

ABSTRACTA TEM study is presented charting the development of a MOVPE growth process for the deposition of CdTe//ZnTe superlattices. In addition, MBE grown (Cd,Zn)Te//CdTe superlattices deposited onto GaAs and InSb substrates are compared.

Optical Investigation of the Band Offset of CdxZn1−xTe /ZnTe and ZnSexTe1−x/ZnTe Superlattices

ABSTRACTWe have analyzed photoluminescence spectra from CdxZnl−xTe /ZnTe and ZnSexTel−x/ZnTe strained layer superlattices grown by MBE, and obtained the band offsets by fitting to theory. We find that the valence band offset of the CdTe/ZnTe system is quite small (-50± 160 meV). In CdxZnl−xTe /ZnTe superlattices, the electrons and heavy holes are confined in the CdxZn1−xTe layers (type I), while the light holes are confined in the ZnTe layers (type II). On the other hand, the photoluminescence data from the ZnSexTe1−x /ZnTe superlattices suggest that the band alignment is type II, with a large valence band offset (−907 ± 120 meV). We also investigated the band bowing in the ZnSexTel−x alloys by optical spectroscopy, and found that there is only a small component of bowing in the valence band, while most of the bowing occurs in the conduction band. Based on our results for band alignments, we evaluate the prospects for minority carrier injection in wide bandgap heterostructures based on ZnSe, ZnTe, and CdTe.

Properties of HgTe:ZnTe Strained Layer Superlattices Grown by MOVPE

ABSTRACTHgTe:ZnTe superlattices have been grown by thermal MOVPE at temperatures down to 325°C. At this temperature, interdiffusion is sufficiently low to make superlattice periods as low as 45A practicable. The results of theoretical calculations of the electronic structure of these materials are also reported. These show that the electronic structure may be significantly different to that found for the HgTe:CdTe system due to the large biaxial compression present in the HgTe well layers.

Atomic Layer Epitaxy of Wide Bandgap II-VI Compound Semiconductor Superlattices

ABSTRACTZnSe, ZnTe and ZnSe-ZnTe strained-layer superlattices (SLS's) have been successfully grown by atomic layer epitaxy (ALE) using molecular beam epitaxy (MBE-ALE). The ideal ALE growth, i.e., one monolayer per cycle of opening and closing the shutters of the constituent elements, was obtained for ZnSe in the substrate temperature range of 250-350° C. However, for ZnTe, precise control of the Te beam intensity is needed to obtain the ALE growth. Optical properties of the (ZnSe)l-(ZnTe) 1 SLS were evaluated by photoluminesence. ZnSe films were also grown by ALE using metalorganic molecular beam epitaxy (MOMBE-ALE). Diethylzinc (DEZn), diethylsulfur (DES) and diethylselenium (DESe) were used as source gases for Zn, S and Se, respectively. The ALE growth of ZnSe was achieved at substrate temperature between 250 and 300° C which is about 150° C lower than that for the conventional MOMBE.

Optical absorption study of MOCVD grown CdTeZnTe superlattices

The absorption spectra of MOCVD grown CdTeZnTe superlattices were studied at different temperatures in the range of 300 to 15 K. Superlattices with periods varying between 30 and 80 Å and CdTe well thicknesses between 12 and 46 Å were measured. The lowest absorption band detected is assigned to a transition to the first confined state. A second distinct absorption line assigned to a transition to the second confined energy level can be seen for samples with a well width exceeding 25 Å. The experimental results are found to be reasonably well fitted to the energies calculated using the Kronig-Penney model, disregarding strain.

Band offset of the ZnSe–ZnTe superlattices: A fit to photoluminescence data by k⋅p theory

The ZnSe–ZnTe superlattices have attracted considerable attention as possible blue/green light emitters. Although these superlattices have been successfully fabricated and show intense photoluminescence, there are many basic issues about this system which still remain unresolved. The most important of them is the value of the valence band offset between ZnSe and ZnTe. We have studied the band structure of ZnSe–ZnTe superlattices. Our calculations are based on second order k⋅p theory and include the effects of strain and spin-orbit splitting on the superlattice band structure. We have investigated the dependence of the superlattice band gap on the valence band offset. Based on the assumption that the photoluminescence from the superlattice corresponds to a bound exciton at a Te1 isoelectronic center in ZnSe, we have fit the experimental photoluminescence data with k⋅p theory to obtain the best value of the valence band offset. The value we find is 0.97±0.10 eV. Alternatively, assuming that the photoluminescence was due to band-to-band transitions we obtain a valence band offset of 1.20±0.13 eV. We have also calculated the superlattice band gap as a function of the constituent material layer thicknesses for the first valence band offset quoted. We expect these results to be important in gaining an understanding of the value of the valence band offset, and the nature of the photoluminescence from this system.

Growth and characterisation of ZnTe and ZnTeCdTe superlattices on GaAs substrates

Epitaxial layers of ZnTe and CdTeZnTe superlattices have been grown by atmospheric pressure MOVPE on {;100}; GaAs substrates and characterised by RHEED, SEM and TEM. Although they are of high crystal quality, the ZnTe layers exhibit a characteristic ridged morphology due to anisotropic facetting. These layers are found by TEM to contain a high density of misfit dislocations extending throughout a 2 μm thick layer. In contrast, CdTeZnTe superlattices grown under the same conditions are shown to have a much improved surface morphology but with a characteristic assymetric rectangular network of slip traces and cracks. These are attributed to the thickness of the CdTe layers and the high growth temperature causing the strain in the system to be largely relieved.