F. J. Grunthaner

Optically controlled absorption modulator based on state filling of InxGa1−xAs/GaAs quantum wells

We report the first demonstration of an optically controlled absorption modulator based on state filling in a periodically doped InxGa1−xAs/GaAs multiple quantum well structure. Differential absorption of approximately 104 cm−1 is observed in the quantum wells of our test structure at saturation pump powers. Photoluminescence and time‐resolved modulation measurements confirm the predicted behavior of carrier recombination and give a measure of enhanced carrier lifetime of approximately 1 ms. These initial results show the potential for developing these structures into optically addressed spatial light modulators.

Cathodoluminescence and transmission electron microscopy study of dark line defects in thick In0.2Ga0.8As/GaAs multiple quantum wells

The spatial distribution of the long‐wavelength luminescence in thick In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) has been investigated using cathodoluminescence (CL) imaging and spectroscopy. The CL spectra show defect‐induced broad bands between 1000≲λ≲1600 nm. These bands exhibit spatial variations which correlate with the dark line defects (DLDs) observed in the λ=950 nm exciton luminescence imaging. Transmission electron microscopy showed that [110]‐oriented misfit dislocations occur primarily at the substrate‐to‐MQW and GaAs capping layer‐to‐MQW interfaces. The large spatial variation of the luminescence intensities indicates that the DLDs observed in CL images are caused by the presence of nonradiative recombination centers occurring in the MQW region located between the interface misfit dislocations. This study provides new information describing the origin and nature of DLDs and differs from previous models which have regarded the electronic nature of dislocation cores as the primary mechanis...

Near‐infrared cathodoluminescence imaging of defect distributions in In0.2Ga0.8As/GaAs multiple quantum wells grown on prepatterned GaAs

The defect distribution in a highly strained In0.2Ga0.8As/GaAs multiple‐quantum‐well (MQW) structure grown on a patterned GaAs substrate is examined with cathodoluminescence imaging and spectroscopy in the near infrared. By spatially correlating the luminescence arising from the MQW exciton recombination (λ≊950 nm) with the longer wavelength (1000≲λ≲1200 nm) luminescence arising from the defect‐induced recombination, we demonstrate that it is possible to determine the regions of highest film quality in both the mesa and valley regions. The present approach enables a judicious determination of the optimal regions to be used for active pixels in InGaAs/GaAs spatial light modulators.

ULTRAHIGH VACUUM ARCJET NITROGEN SOURCE FOR SELECTED ENERGY EPITAXY OF GROUP III NITRIDES BY MOLECULAR BEAM EPITAXY

The key technical challenge in the molecular beam epitaxial (MBE) growth of group III nitrides is the lack of a suitable source of incorporatable nitrogen. In contrast with the growth of the other III–V compound semiconductors by MBE, direct reaction of N2 with excess group III metal is not feasible, because of the high bond strength of dinitrogen. An incorporatable MBE nitrogen source must excite N2 forming a beam of atomic nitrogen, active nitrogen (N2*), or nitrogen ions. rf and electron cyclotron resonance sources use electron impact excitation to obtain atomic nitrogen and in the process generate a wide variety of excited ions and neutrals. Experiments have shown that ionic species in the beam degrade the morphology of the epitaxial layer and generate electrically active defects. Recent theoretical studies have predicted that ground state atomic nitrogen will successfully incorporate into the growing GaN surface, while atomic nitrogen in either of the excited doublet states will lead to etching. In t...

Observation of a correlation between twin orientation and substrate step direction in thin GaAs films grown on intentionally misoriented Si (100)

A transmission electron microscope study of GaAs grown on Si tilted 4° off (100) has for the first time revealed that of the two twin variants propagating to the GaAs film surface, a substrate tilt about a 〈011〉‐type direction favors one over the other.

Ambipolar diffusion anisotropy induced by defects in nipi‐doped In0.2Ga0.8As/GaAs multiple quantum wells

The influence of strain‐induced defects on the ambipolar diffusive transport of excess electrons and holes in the δ‐doped InGaAs/GaAs multiple quantum well system has been examined with a new technique called electron‐beam‐induced absorption modulation (EBIA). The excess carrier lifetime and diffusion coefficient are obtained by a one‐dimensional diffusion experiment that utilizes EBIA. An anisotropy in the ambipolar diffusion along both high‐symmetry 〈110〉 directions is found, and this is seen to correlate with the distribution of dark line defects observed in cathodoluminescence.

Electron beam-induced absorption modulation imaging of strained In0.2Ga0.8As/GaAs multiple quantum wells

We have examined the effects of electron‐hole plasma generation on excitonic absorption phenomena in nipi‐doped In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) using a novel technique called electron beam‐induced absorption modulation imaging. The electron‐hole plasma is generated by a high‐energy electron beam in a scanning electron microscope and is used as a probe to study the MQW absorption modulation. The influence of structural defects on the diffusive transport of carriers is imaged with a μm‐scale resolution.

Optical and structural characterization of InAs/GaAs quantum wells

Three InAs/GaAs single quantum wells of 2, 3, and 4 monolayer thickness were characterized using optical and structural techniques. The results of high-resolution transmission electron (HRTEM) microscopy and optical studies which combine absorption, photoluminescence (PL), photoreflectance and cathodoluminescence are presented. Using the polarization modulated absorptance technique we observed two absorption features in our samples at 77 K. On the basis of their polarization properties and comparison with an envelope function calculation, these structures are assigned to transitions between the confined heavy-hole and confined and unconfined electron levels. Photoreflectance spectra of the 3- monolayer sample in 77-300 K range show only the fundamental quantum well transition. The temperature dependence of this transition is approximately linear with a slope of 2.2·10 −4 eV/K which is significantly lower than in both constituent materials. Comparison to the absorption data reveals that the PL spectra are affected by the carrier diffusion and therefore do not provide direct measure of the exciton density of states. Therefore, photoluminescence results alone do not provide unequivocal information about the fundamental transition energy or the interface quality in quantum wells. The HRTEM images indicate that while the interfaces of the 2-monolayer sample are smooth and the well thickness is uniform, the 4-monolayer sample has uneven interfaces and contains domains of 2, 3, and 4 monolayers. In agreement with these observations, absorption features broaden with the increased well width. Scanning cathodoluminescence images of the 2- monolayer sample present no evidence of dislocations, which is consistent with the HRTEM observations.

AES and EELFS Studies of Initial Stages of Growth of GaAs/InAs/GaAs Heterostructures.

We report on a systematic study of the composition and structure of GaAs/InAs/GaAs quantum wells using Auger Electron Spectroscopy (AES), Extended Energy Loss Fine Structure (EELFS), and Reflection High Energy Electron Diffraction (RHEED) techniques. Double heterostructures with InAs thickness ranging from 2 to 10 monolayers, capped by 2 to 10 monolayers of GaAs, were grown by MBE using a variety of techniques, including those employing sequential, interrupted, and delayed shutter timing sequences. AES peak ratios are compared with model calculations to monitor compositional development of the multilayers. The AES results are correlated with RHEED measurements to determine MBE growth parameters for optimal control of the stoichiometry and surface morphology. EELFS was used to monitor strain in the buried InAs layers. The AES results show departure from smooth laminar growth of layers of stoichiometric InAs on GaAs at temperatures below 420 C and above 470 C. AES results on the quantum well structures suggest floating InAs layers on top of the GaAs and/or facet formation in the GaAs layers. The EELFS results, when compared to bulk InAs, indicate the presence of strain in the buried InAs quantum well.

REDUCED OXIDATION STATES AND RADIATION-INDUCED TRAP GENERATION AT Si/SiO2 INTERFACE*

Thin thermal SiO2 films (< 80 A) grown on Si (100) substrates are irradiated with electrons from 0 to 20 eV during in situ XPS measurements. Both oxide/vacuum surface states and Si (+3) species at the Si/SiO2 interface are generated and allowed to relax during the course of the measurements. The results are correlated with the presence of a strained layer of SiO2 (~ 20 A) at the interface that we had previously reported.