M. Beaudoin

Optical absorption edge of semi-insulating GaAs and InP at high temperatures

The temperature dependences of the optical absorption edges of semi-insulating GaAs and InP have been measured from room temperature to 905 °C and 748 °C, respectively, with accuracies of ±1 °C at room temperature and ±5 °C at 900 °C. The temperature dependence of the optical absorption edge is adequately reproduced by an Einstein model although the Varshni model gives an improved fit to the band gap. Finally, the widths of the absorption edges are correlated with ionicity.

Film thickness and composition monitoring during growth by molecular beam epitaxy using alpha particle energy loss

The α-particle energy loss method (AEL) has been implemented in situ to monitor film thickness during growth by molecular beam epitaxy. For InP and GaAs substrates recoil implanted with α-particle emitters, we have been able to measure thickness and composition of deposited GaAs, AlGaAs and InGaAs in real time. The AEL method yields in situ real time results comparable in accuracy to those obtained by ex situ scanning electron microscope and high-resolution x-ray diffraction measurements.The α-particle energy loss method (AEL) has been implemented in situ to monitor film thickness during growth by molecular beam epitaxy. For InP and GaAs substrates recoil implanted with α-particle emitters, we have been able to measure thickness and composition of deposited GaAs, AlGaAs and InGaAs in real time. The AEL method yields in situ real time results comparable in accuracy to those obtained by ex situ scanning electron microscope and high-resolution x-ray diffraction measurements.

The applicability of implanted -sources to thickness and stoichiometry measurements of thin films

A method for determining both the thickness and the average stoichiometry of thin films is presented. The method is based on implanting radioactive -sources in the substrate prior to layer growth and measuring the energy loss of the -particles as they traverse the layer. Information about the stoichiometry is obtained through the comparison of the energy loss of -particles of different initial energies. Experimental examples for the utilization of this method are presented, in which Sb was grown on Si substrates, GaAs, InAs and AlAs on GaAs and YBCO on YSZ. The experimental precision which can be expected using the method is discussed, together with specific scenarios in which it could be advantageously applied.

Extension of the Rosencwaig-Gersho-Fernelius photothermal deflection spectroscopy model to account for multiple reflections in epitaxial samples

The Fernelius extension to the Rosencwaig–Gersho theory for photothermal deflection spectroscopy (PDS) is modified to take into account multiple reflections and light trapping within a thin film-on-substrate system where both the thin film and the substrate can be absorbing. The extended model is used to simulate the PDS signal from bulk GaAs and GaNAs epilayers grown on GaAs substrates. While the PDS magnitude signal shows a strong dependence on the light trapping effects, the phase signal remains almost insensitive to this effect. However, the PDS relative phase is very sensitive to the sample thickness.

In situ thickness measurements in molecular beam epitaxy using alpha particle energy loss

Abstract The α-particle energy loss method has been implemented in situ to monitor film thickness during growth by molecular beam epitaxy. For InP and GaAs substrates dosed with 500–1500 Bq of α-particle emitters, we have been able to measure thickness in situ of deposited GaAs and InP, to an accuracy of 6 nm in 180 s of counting time. The corresponding growth rate accuracy for growth rates on the order of 0.3 nm/s was ±0.01 nm/s. The accuracy and counting time improvements expected with the use of a stronger marking source are also discussed.

Multilevel infrared coupling of excitons in quantum-well semiconductors

We study the effects of multilevel mixing of the 1s and 2s states of the E1-HH1 and E2-HH1 excitons in the emission spectra of an undoped quantum well. This is done by investigating the E1-HH1 exciton emission spectra in the presence of an intense CO/sub 2/ laser near resonance with the transition between El and E2. Our results show that, depending on the frequency of the CO/sub 2/ laser, these spectra are quenched peculiarly. We explain these phenomena based on the frequency dependence of the mixing configurations of the exciton states and infrared enhancement of the nonradiative decay rates of E1-HH1 excitons. We also study the effect of the nonparabolicity of the hole subband (HH1) in the infrared mixing of the E1-HH1 and E2-HH1 excitons.

In situ real time monitoring of thickness and composition in MBE using alpha particle energy loss

The α-particle energy loss method (AEL) has been implemented in situ to monitor film thickness and composition during growth of GaAs, InP and LaF 3 based materials by molecular beam epitaxy (MBE). In the AEL method, a 228 Th source is used to recoil implant a 5 mm diameter region of the surface of the wafers with the α-emitter daughter isotope 224 Ra prior to growth. The implanted nuclei decay with a half life of 3.7 days through a sequence of daughters which emit alpha particles at different energies. Deposition on the surface causes the emission lines to be shifted to lower energies due to energy loss in the film. For substrates marked with a low activity (∼ 30 kBq; similar to activity of smoke detectors) we are able to measure film thickness with ± 6 nm uncertainty and growth rate with ± 0.01 nm/s uncertainty in real time. By measuring the relative growth rates of the different materials, AEL also allows us to infer the composition of a ternary laver film as well as the sticking coefficients rates directly at different growth temperatures.

Diffusion studies of Ra and Pb in GaAs by the alpha-particle energy loss method

The temperature dependence of the diffusion of lead in GaAs is determined by measuring the modification to the energy spectrum of emitted alpha particles from the decay chain of implanted 212Pb atoms. Diffusion rates are measured for temperatures up to 900 °C. Higher rates are observed for the diffusion in silicon-doped GaAs than in semi-insulating GaAs. An upper limit for the diffusion of radium in GaAs is similarly obtained from the decay of the 224Ra isotope. Implications for the use of implanted alpha sources for thickness monitoring during epitaxial film growth by the alpha-particle energy loss method are discussed.