R. C. Tu

Contactless electroreflectance study of strained Zn0.79Cd0.21Se/ZnSe double quantum wells

We have studied various excitonic transitions of strained Zn0.79Cd0.21Se/ZnSe double quantum wells, grown by molecular beam epitaxy on (100) GaAs substrates, using contactless electroreflectance (CER) at 15 and 300 K. A number of intersub-band transitions in the CER spectra from the sample have been observed. An analysis of the CER spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state and the nth heavy (light)-hole band state. The conduction-band offset Qc is used as an adjustable parameter to study the band offset in the strained Zn0.79Cd0.21Se/ZnSe system. The value of Qc is determined to be 0.67±0.03.

Near-band-edge optical properties of molecular beam epitaxy grown ZnSe epilayers on GaAs by modulation spectroscopy

A study of near-band-edge optical properties of ZnSe epilayers grown on GaAs substrates using various modulation techniques is presented. We compare the contactless electroreflectance (CER) and piezoreflectance spectra to ascertain that our ZnSe epilayers of 1.2 μm in thickness grown on GaAs substrates are under a biaxial tensile strain. The defect-related transitions near the ZnSe/GaAs interface are also compared by identifying the photoreflectance and other spectra. In addition, in order to observe the temperature-dependent energy splitting and strains, we present a detailed investigation of the heavy-hole and light-hole related transition energies as a function of temperature in the 15–200 K range by identifying the excitonic signatures in the CER spectra. We have also calculated the energy splitting between heavy-hole and light-hole valence bands by utilizing the temperature-dependent elastic constants for ZnSe and the thermal expansion coefficients for ZnSe and GaAs. Both the experimental result and ...

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 quant...

Photoluminescence properties of Zn1-xMgxSe on misoriented GaAs substrates by molecular beam epitaxy

We have investigated the photoluminescence (PL) properties of Zn1−xMgxSe epilayers grown on GaAs substrates with different misorientation angles by molecular beam epitaxy. According to the data measured by PL and by electron-probe microanalysis, the Mg incorporation in the Zn1−xMgxSe epilayer decreases with increasing misorientation angle. In addition, the PL spectra showed that the full width at half maxima of the band-edge excitonic emission and the intensity of the defect-related donor-acceptor emission in Zn1−xMgxSe epilayers decreased appreciably when a substrate with a misorientation angle of 15° was used.

Effects of thermal annealing on photoluminescence and structural properties of (ZnSe)2(CdSe)n short-period-superlattices multiple quantum wells

This work investigates how thermal annealing affects the optical and structural properties of (ZnSe)2(CdSe)n short-period-superlattices multiple quantum wells (SPS MQW) grown by molecular beam epitaxy using photoluminescence (PL) and high-resolution transmission electron microscopy (HRTEM) techniques. A characteristic that differentiates annealed SPS MQW from annealed conventional quantum wells is that much greater blueshifts can be observed in annealed SPS MQW as the annealing temperature rises above 400 °C. We attribute the larger blueshifts to thermally induced interdiffusion of Zn and Cd atoms between alternate ZnSe and CdSe layers. Furthermore, the PL emission in annealed (ZnSe)2(CdSe)n SPS MQW quenches at higher temperatures and yields a larger value for the activation energy than in as-grown SPS MQW. HRTEM images of samples annealed at 450 °C for 30 min clearly indicate that SPS structures remain in the quantum well regions of as-grown SPS MQW, but intermix and disappear in the well regions of anne...

THE STRUCTURAL AND OPTICAL PROPERTIES OF HIGH QUALITY ZNTE GROWN ON GAAS USING ZNSE/ZNTE STRAINED SUPERLATTICES BUFFER LAYER

This work studied the structural and optical properties of ZnTe epilayers grown on GaAs substrates with ZnSe/ZnTe strained superlattices buffer layers. Material properties were characterized using cross-sectional transmission electron microscopy, photoluminescence (PL), contactless electroreflectance (CER), and piezoreflectance (PzR). PL spectra clearly distinguished the strong free exciton peaks, weak donor-acceptor pair, Y lines, and oxygen-bound exciton peaks, indicating the high quality of the films. In addition, the CER and PzR spectra were compared to ascertain that ZnTe epilayers of 1.5 μm in thickness grown on GaAs substrates are under a biaxial tensile strain. An attempt was also made to identify the origins of the near-band-edge transitions of ZnTe epilayer in the CER and PzR spectra by comparing these spectra with PL spectra and the second harmonic frequency CER. By doing so, the interference below the band gap of ZnTe could be effectively eliminated. Moreover, the energy splitting between heav...

Structural and Optical Studies of ZnCdSe/ZnSe/ZnMgSSe Separate Confinement Heterostructures with Different Buffer Layers

Detailed structural and optical studies of ZnCdSe/ZnSe/ZnMgSSe separate confinement heterostructures (SCH) grown on ZnSe, ZnSe/ZnSSe strained-layer superlattices (SLS), and GaAs buffer layers at the II–VI/GaAs interface have been carried out by employing transmission electron microscopy, variable temperature photoluminescence (PL), and contactless electroreflectance (CER) measurements. A significant improvement on the defect reduction and the optical quality has been observed by using either the ZnSe/ZnSSe SLS or GaAs as the buffer layers when compared to that of the sample using only ZnSe as the buffer layer. However, the sample grown with the SLS buffer layers reveals a room temperature PL intensity higher than that of the sample grown with a GaAs buffer layer, which may still suffer from the great ionic differences between the II–V and III–V atoms. Using 15 K CER spectra, we have also studied various excitonic transitions originating from strained Zn0.80Cd0.20Se/ZnSe single quantum well in SCH with different buffer layers. An analysis of the CER spectra has led to the identification of various excitonic transitions, mnH (L), between the mth conduction band state and the nth heavy (light)-hole band state. An excellent agreement between experiments and theoretical calculations based on the envelope function approximation model has been achieved.

Temperature dependence of the band-edge exciton of a epilayer on GaAs

We have measured the temperature dependence of the spectral features near the direct bandgap of a film, grown by molecular beam epitaxy on a (001) GaAs substrate, in the temperature range between 15 and 300 K using contactless electroreflectance. The parameters that describe the temperature variations of the energy and broadening function of the band-edge exciton have been evaluated.

Optical properties of ZnSe1−xSx epilayers grown on misoriented GaAs substrates

The strain induced heavy hole and light hole exciton splitting of ZnSe1−xSx (x<0.1) epilayers grown on misoriented GaAs (001) substrates has been studied by reflectance spectroscopy. The heavy hole and light hole exciton energies are determined by the composition of the layers. It was concluded that for both thin (largely unrelaxed strain) and thick epilayers (with largely relaxed strain) misorientation of the substrate results in increase of x, i.e., in increasing incorporation of sulphur. However, the additional strain expected due to the increasing incorporation of sulphur with misorientation was not observed due to partial strain relief of the epilayers grown on misoriented substrates. The optical quality of the epilayers is tilt angle dependent and is best for about 10° tilt from the (001) direction.

Contactless electroreflectance and piezoreflectance studies of temperature-dependent strain in ZnTe/GaAs heterostructures with ZnSe/ZnTe superlattice buffer layers

The temperature-dependent optical properties of ZnTe epilayers grown on GaAs substrates by molecular beam epitaxy with and without ZnSe/ZnTe strained-layer superlattice (SLS) buffer layers have been studied using contactless electroreflectance (CER) and piezoreflectance (PzR). Our ZnTe epilayers of 1.5 μm in thickness grown on GaAs substrates are under a biaxial tensile strain according to the results shown in CER and PzR spectra. Furthermore, the strain induced energy splitting between heavy- and light-hole valence bands in the ZnTe epilayer can be reduced by using the ZnSe/ZnTe SLS buffer layers. We have also justified the temperature-dependent energy splitting between heavy- and light-hole valence bands for ZnTe through theoretical calculations. Discrepancy between experiments and calculations indicates that the residual mismatch-induced strain as well as the thermally induced strain during cooling must be taken into account at the same time.