C. Jagadish

Large energy shifts in GaAs‐AlGaAs quantum wells by proton irradiation‐induced intermixing

Proton irradiation and subsequent rapid‐thermal annealing are used to create intermixing in GaAs‐Al0.54Ga0.46As quantum wells of various thicknesses. Very large energy shifts (up to 200 meV) with no apparent saturation have been observed even up to a dose of about 4×1016 cm−2. This effect is explained in terms of the dilute irradiation damage and the evolution of discrete (point) defects during annealing. In comparison to heavy ion irradiation effects, high point defect fraction in the case of light ions leads to efficient intermixing with large energy shifts. Although much of the proton energy loss occurs across the quantum wells, the generated defect density is dilute, and hence good recovery in photoluminescence intensities is achieved after rapid thermal annealing.

Ion damage buildup and amorphization processes in AlxGa1-xAs

The nature of keV ion damage buildup and amorphization in AlxGa1−xAs at liquid‐nitrogen temperature is investigated for various Al compositions using Rutherford backscattering channeling, transmission electron microscopy, and in situ time‐resolved‐reflectivity techniques. Two distinct damage buildup processes are observed in AlxGa1−xAs depending on Al content. At low Al content, the behavior is similar to GaAs whereby collisional disorder is ‘‘frozen in’’ and amorphization proceeds with increasing dose via the overlap of damage cascades and small amorphous zones created by individual ion tracks. However, some dynamic annealing occurs during implantation in AlGaAs and this effect is accentuated with increasing Al content. For high Al content, crystallinity is retained at moderate ion damage with disorder building up in the form of stacking faults, planar, and other extended defects. In the latter case, amorphization is nucleation limited and proceeds abruptly when the level of crystalline disorder exceeds ...

Picosecond carrier lifetime in GaAs implanted with high doses of As ions: An alternative material to low‐temperature GaAs for optoelectronic applications

Nonstoichiometric GaAs obtained by implantation with 2 MeV arsenic ions at 1015 cm−2 dose is studied. As‐implanted samples show a <200 fs lifetime of photocarriers and low resistivity due to hopping, with mobility less than 1 cm2/V s. Annealing of the samples at 600 °C leads to substantial recovery of postimplant damage, as seen from Rutherford backscattering channeling spectra and mobility increase to about 2000 cm2/V s, but photocarrier lifetime is still about 1 ps. These parameters are similar to those of low‐temperature GaAs annealed at 600 °C, and make arsenic implanted GaAs an interesting material for optoelectronic applications.

Point defects in MeV ion-implanted silicon studied by deep level transient spectroscopy

Abstract This paper reviews recent work using deep level transient spectroscopy (DLTS) for studying point defects in crystalline silicon implanted with H, B, C, O, Si, Ge and Sn ions. Doses between 107 and 1010 cm−2 and energies from 0.4 to 8 MeV were used. Different intrinsic and impurity-related defects like divacancy and vacancy-oxygen centers are identified and their formation has been studied as a function of dose, dose rate, sample depth, implantation temperature and ion mass. Recombination between vacancies and Si self-interstitials is found to play a major role and only a few percent of the generated vacancies form stable defects. Furthermore, in direct contrast to that for damage accumulation at doses above ∼ 1012 cm−2, the production of vacancy-type defects increases with increasing implantation temperature and decreases with increasing dose rate. These effects are qualitatively simulated using a simple model for the defect generation kinetics and attributed to enhanced vacancy annihilation by overlapping Si self-interstitials from adjacent ion tracks.

Wavelength shifting in GaAs quantum well lasers by proton irradiation

Proton irradiation followed by rapid thermal annealing was used to selectively induce layer intermixing and thus shift the emission wavelengths of GaAs–AlGaAs graded-index separate-confinement-heterostructure quantum well lasers. Up to 40 nm shifts were observed in 4 μm ridge waveguide devices irradiated to a dose of 1.5×1016 cm−2. Although the wavelength shifts were accompanied by some degradation in the lasing threshold current and differential quantum efficiency, they were still quite acceptable at moderate wavelength shifts. This technique provides a simple and promising postgrowth process of integrating lasers of different wavelengths for wavelength-division-multiplexing applications.

In/sub 0.5/Ga/sub 0.5/As/GaAs quantum dot infrared photodetectors grown by metal-organic chemical vapor deposition

We report the growth by low-pressure metal-organic chemical vapor deposition, fabrication, and characterization of ten-layer In/sub 0.5/Ga/sub 0.5/As/GaAs quantum dot infrared photodetectors. Normal incidence photoresponse of the detector was obtained at 5.9 /spl mu/m. The 77-K peak responsivity was 5.6 mA/W with the detectivity D/sup */ of 1.2/spl times/10/sup 9/ cm/spl middot/Hz/sup 1/2//W at the bias of 0.4 V.

Electrical isolation of GaN by MeV ion irradiation

This work was partly supported by Conselho Nacional de Pesquisas (CNPq, Brazil) under Contract No. 200541/ 99-4.

Novel impurity-free interdiffusion in GaAs/AlGaAs quantum wells by anodization and rapid thermal annealing

A novel impurity-free interdiffusion technique utilizing pulsed anodization and subsequent rapid thermal annealing at temperatures near 900 °C was reported. Enhanced interdiffusion was observed in the presence of an anodized GaAs capping layer in GaAs/AlGaAs quantum well structures. Transmission electron microscopy studies show evidence of interdiffusion. Photoluminescence spectra from interdiffused samples show large blue shift and no significant linewidth broadening. Possible mechanism of interdiffusion was discussed.

Proton-irradiation-induced intermixing of InGaAs quantum dots

Proton irradiation was used to create interdiffusion in In0.5Ga0.5As quantum dots (QDs), grown by low-pressure metalorganic chemical vapor deposition. After 25-keV proton irradiation, the QD samples were annealed at two temperatures (700 or 750 °C) for 30 s. It was found that much lower annealing temperatures were needed to recover the photoluminescence signals than in the quantum-well case. Large blueshifts (120 meV) and narrowing of the photoluminescence spectra were seen. Various doses (5×1013–1×1015 cm−2) and implant temperatures (20–200 °C) were used to study the interdiffusion processes in these samples. In QD samples, much lower doses were required to achieve similar energy shifts than reported in quantum-well samples.

Effect of irradiation temperature and ion flux on electrical isolation of GaN

We study the evolution of sheet resistance of n-type GaN epilayers irradiated with MeV 1H and 12C ions. Results show that both implantation temperature (varied from 77 up to 423 K) and ion beam flux affect the process of electrical isolation in the case of irradiation with 12C ions. This behavior is consistent with significant dynamic annealing occurring in GaN during MeV light-ion bombardment, which suggests a scenario where the centers responsible for electrical isolation are defect clusters or anti-site-related defects. Dynamic annealing causes simple ion-beam-generated Frenkel pairs to annihilate (or cluster) during irradiation at liquid nitrogen temperature and above. These beam-flux and irradiation-temperature effects are not observed during bombardment with lighter 1H ions, which produce very dilute collision cascades. A qualitative model is proposed to explain temperature and flux effects in GaN in the MeV light-ion bombardment regime used for electrical isolation.