Chikara Onodera

A new 1.47 eV defect-luminescence band in MOCVD-grown CdTe on (100) GaAs

Abstract We have studied the temperature dependence of emission intensity and peak energy, photoluminescence excitation and time-resolved spectra (TRS) of the LO-phonon replicated 1.47 eV band in CdTe films grown on (100) GaAs substrates by a low-pressure metalorganic chemical vapour deposition (MOCVD) method. The TRS studies indicate that there is no observable energy shift toward lower photon energy after time delay. The excitation spectrum represents an exciton resonant peak at 1.598 eV, suggesting that the contribution of free excitons to the 1.47 eV band is predominant. This evidence is also revealed by the thermal quenching of the emission intensity which shows that a thermal liberation of excitons from the localized center takes place. We therefore propose that the observed 1.47 eV band looks like the Y line series, attributable to recombination of excitons at an extended defect, which has been already observed in ZnSe.

Band Offsets in CdZnS/ZnS Strained-Layer Quantum Well and Its Application to UV Laser Diode

Conduction- and valence-band offsets have been estimated as a function of well layer thickness in CdxZn1-xS/ZnS strained-layer superlattices with various compositions using the model-solid theory. Relatively large values of approximately 180-260 meV in the conduction-band offset can be obtained for x=0.2-0.3. The valence-band offset depends strongly on the well layer thickness, while the conduction-band offset is nearly independent of the well layer thickness up to 200 A. This strained-layer structure was applied to fabricate ultraviolet (UV) laser diodes with multiple quantum wells. Stimulated emission can be observed either under optical pumping at RT or under pulsed injection at 30 K in the spectral range of 357-390 nm. A spectral narrowing in the emission spectrum with increasing current in the UV injection diode was clearly observed in the vicinity of 375 nm at 30 K.

Effects of Hg Annealing on Photoluminescence Spectra of CdTe Crystals and MOCVD-Grown CdTe/GaAs Films

Annealing effects on CdTe crystals and CdTe/GaAs films in Hg have been investigated through the spectral changes of PL (photoluminescence) at 4.2 K. Before annealing, the PL spectrum of CdTe crystals or films is dominated by the excitonic-emission lines, the edge-emission band and two broad bands. After annealing, a marked decrease in intensity of the broad and edge-emission bands in the bulk crystals is observed, while an increase in intensity of the broad bands in the MOCVD-grown CdTe is significant. We will point out from the PL investigations that the Hg annealing plays an important role in the improvement of both the surface and bulk properties.

Effect of H2 heat-treatment of excitonic emission in MOCVD-grown CdTe films on (100) GaAs substrates

Abstract CdTe epitaxial growth can be accomplished on near-oriented (100) GaAs substrates at 350 °C by low-pressure metalorganic chemical-vapour deposition (MOCVD) using dimethylcadmium and dimethyltelluride, the latter of which was thermally pre-cracked. Nominally undoped film is originally under compressive strain for a thickness of about 1.2 μm as expected from the valence-band splitting in the exciton reflectance spectra. Intense excitonic-emission lines associated with the free-exciton, principal neutral-donor and neutral-acceptor bound-exciton (BE) lines at 1.596, 1.5931, 1.5904 and 1.5985 eV appear at 4.2 K, respectively. The linewidths of the donor and acceptor BE lines are less than 0.8 meV. The effect of post-growth annealing at temperatures between 150 and 550 °C in H 2 leads to a marked reduction in intensity of the 1.5931 and 1.5904 eV BE lines, whilst no observable energy shifts and further splitting of the excitonic lines appear.

Excitonic Properties in CdxZn1-xS/ZnS Quantum Wells

We have performed a theoretical study of the exciton properties in the CdxZn1-xS/ZnS quantum wells (QWs) with taking into account the strain effect due to a ZnS buffer layer. The variation of the exciton binding energy that is caused by the strain effect due to the ZnS buffer layer is very small. The variation of the effective band gap energy is larger than that of the exciton binding energy under the strain due to the ZnS buffer layer. The effect of the electric field on the exciton binding energy is calculated using a variational approach. The exciton binding energy decreases with increasing the electric field, and decreases significantly at the wide well widths due to the spatial separation between the electron and the hole. The diamagnetic shift of the exciton is calculated under the magnetic field, and is less than 0.11 meV.

Effect in Changes of Conduction and Valence Band Offsets on Exciton Binding Energy in CdxZn1-xSe/ZnSySe1-y Single Quantum Wells

We calculate the induced-strain effects in CdxZn1-xSe/ZnSySe1-y single quantum wells (SQWs). The conduction (Vc) and valence (Vh) band offsets in this system depend greatly on the alloy contents x and y. The variation in exciton binding energy due to the variations in Vc and Vh with varying y is very small.

Photoluminescence and Reflectance Spectra of CdTe films on GaAs(100) Substrates

Photoluminescence (PL) and reflectance spectra were measured on CdTe films grown on GaAs(100) substrates by metal organic chemical vapor deposition (MOCVD). By comparing the PL spectra with the reflectance spectra in a certain exciton energy range, we identified heavy- and light-hole exciton transitions in the CdTe films on the GaAs(100) substrates. The energy separation between the heavy- and light-hole exciton transitions was estimated to be 3.1 meV. This energy separation was caused by a compressive strain due to a difference in thermal expansion coefficient between the grown CdTe films and GaAs substrates. Bound exciton emissions and the 1.42 eV band in the PL spectra were also studied on the basis of their temperature dependence.

Proposal for ZnS/MgxBeyZn1-x-yS Quantum Wells

We propose the use of MgxBeyZn1-x-yS barriers combined with ZnS wells because MgxBeyZn1-x-yS barriers can be lattice-matched to ZnS wells by adjusting magnesium (Mg) (x) and beryllium (Be) ( y) contents. With increasing x and y, conduction and valence band offsets in ZnS/MgxBeyZn1-x-yS single quantum wells (SQWs) increase and thus the effective band gap energy can be controlled by adjusting x and y. In addition, the exciton binding energy increases. We indicate that the use of MgxBeyZn1-x-yS barriers can reduce the induced strain in the ZnS wells and is useful for designing ultraviolet applications using ZnS wells.

Strain Effects on Exciton Transition in CdTe Films on GaAs(100) Substrates

We performed photoluminescence (PL) and reflectance characterization of CdTe films on GaAs substrates. Separate emission peaks for heavy- and light-hole free excitons due to induced compressive strain were observed. The strain in a CdTe film is estimated to be e=-5.28×10-4. By analyzing the PL and reflectance spectra, both heavy- and light-hole free exciton binding energies are estimated to be 9.9 meV. By analyzing the energy separation between the heavy- and light-hole free exciton reflectance dips, the magnitude of the residual compressive strain is determined to be homogeneously distributed within a CdTe film. From the CdTe film thickness dependence of heavy- and light-hole free exciton linewidths, the CdTe film quality is observed to become poor with decreasing CdTe film thickness.

Photoluminescence Analysis of Deep Acceptor in CdTe Films on GaAs(100) Substrates

In this study, we used photoluminescence (PL) measurement for analysis of deep acceptor states in unintentionally doped CdTe films on GaAs substrates. We analyzed PL and time-resolved spectra (TRS) in the vicinity of the 1.42 eV band. By analyzing the peak shift of the 1.42 eV band as a function of time after pulsed excitation, the bound-to-bound reaction constant for the unintentionally doped CdTe film on a GaAs substrate is estimated to be W0=4.0×107 s-1. The impurity concentration in an unintentionally doped CdTe film is estimated to be N=2.1×1016 cm-3. By analyzing the TRS of the 1.42 eV band as a function of time after pulsed excitation, the acceptor binding energy is estimated to be 105 meV. The acceptor binding energy is reasonably comparable to that in the case of a VII-donor-doped CdTe. The acceptor bound exciton, which can probably be attributed to a recombination of excitons bound to a complex acceptor consisting of a Cd vacancy and two Cl donors, is observed. From these features, it appears that the origin of the deep acceptor in unintentionally doped CdTe films on GaAs substrates is a complex acceptor consisting of a Cd vacancy and a Cl donor.