Post-growth Thermal Annealing Research Articles

Spectral broadening due to post-growth annealing of a long-wave InGaAs/GaAs quantum dot infrared photodetector with a quaternary barrier layer

We present the effect of post-growth rapid thermal annealing on multilayer, uncoupled In0.50Ga0.50As/GaAs quantum dot infrared photodetectors (QDIPs) with a combination of quaternary In0.21Al0.21Ga0.58As and GaAs capping. Long wave photoresponse (~0.11eV or 10.2μm) with narrow spectral width (14%) is obtained from as-grown QDIPs. The spectral width increases to 45% for QDIP annealed at 800°C. It is likely that a large inter-diffusion effect because of annealing changes the composition of the quantum dots and capping layer, thus increasing the QDIP spectral width. Dark current as well as noise current is decreased for 650°C annealed QDIP compared to as-grown detector. Passivation of as-grown defect by post growth annealing predicted the improvement of dark current. Such improvement and broadening of characteristics may render such QDIPs suitable for use in broadband infrared imaging applications.

Tuning in spectral response due to rapid thermal annealing on dot-in-a-well infrared photodetectors

Here we investigated the effect of post-growth rapid thermal annealing (RTA) on dot-in-a-well infrared photodetectors. In a photoluminescence (PL) study, we initially observed a small red shift in the ground-state PL peak upon annealing to 650°C but then saw the usual blue shift as the annealing temperature increased. We also observed increases in the dark current as the annealing temperature increased up to 700°C but a sudden decrease in the dark current at 750°C. Activation energy is calculated using temperature-dependent PL and dark current measurements. The photoresponse peak was observed at 6.41μm for the as-grown device. As we increased the annealing temperature to 800°C, the peak response shifted to 9.52μm. Thus, we achieved wavelength tunability of peak photoresponse using the post-growth annealing technique.

Influence of post-growth rapid thermal annealing on the transport and lasing characteristics of terahertz quantum-cascade lasers

We investigate the effect of post-growth rapid thermal annealing (RTA) on the transport and lasing characteristics of terahertz quantum-cascade lasers (THz QCLs) operating in a frequency range between 4.88 and 4.94 THz. The emission frequencies are blue shifted by about 80 GHz after RTA, which is attributed to a shift of the gain maximum to higher frequencies due to composition grading at the interfaces between the quantum wells and barriers of the annealed wafer pieces. The optical output power of the annealed THz QCLs is reduced, which is explained by a broadening of the levels due to the annealing process.

Rapid thermal annealing and photoluminescence of type-II GaSb single monolayer quantum dot stacks

The effects of rapid thermal annealing on the photoluminescence emission obtained from ten-layer stacks of GaSb/GaAs type-II single monolayer quantum dots and Stranski–Krastanow grown quantum rings have been studied and interpreted. Post-growth rapid thermal annealing was performed with proximity capping at temperatures from 550 °C to 800 °C, resulting in an increase in photoluminescence emission intensity and a blue shift in peak energy in both types of stacks, together with changes in the activation energy for thermal quenching. This behaviour originates from Sb–As intermixing and changes in morphology of the nanostructures formed using the two different growth mechanisms.

N-type ZnO and Al-doped ZnO transparent conductive films prepared by an aqueous solution deposition technique

We exploited an aqueous solution deposition technique to fabricate N-type ZnO and Al-doped ZnO (AZO) films with environmentally benign process at a very low temperature. ZnO, Zn, and Al were used in this study, instead of Zn(CH3COO)2, Al(NO3)3, and AlCl3 employed in previous investigations. Furthermore, the processing temperature was about 80°C which was lower than those of other fabrication techniques. The experimental results demonstrated that the resistivity of pristine ZnO and AZO films without any post-growth thermal annealing can be as low as ~2.0×10−2Ωcm and ~8.4×10−3Ωcm, respectively, and the transmittances were approximately 90% and 85%, respectively.

Analysis of thermal cycle-induced dislocation reduction in HgCdTe/CdTe/Si(211) by scanning transmission electron microscopy

High threading dislocation densities limit the operability of infrared focal plane arrays based on large lattice-mismatched heterostructures such as HgCdTe/CdTe/Si. Recently it has been shown that post-growth thermal cycle annealing can routinely reduce the surface etch pit density from >5×106cm−2 to as low as 9×105cm−2. To fully exploit the procedure, a deeper understanding of the inherent dislocation dynamics is needed. In this work, we employ scanning transmission electron microscopy to analyze cross-sectional samples of HgCdTe/CdTe/Si prepared using site-specific focused ion beam milling. A key factor in this work is the use of defect decorated samples, which has allowed for a correlation of surface etch pits to dislocation segments observed in cross-section images. We have observed that the previously reported oval-shaped etch pits are likely associated with Shockley partial type dislocations, and that triangular etch pits are associated with perfect dislocations. This suggests the likelihood that interaction between mobile Shockley partial and other dislocation types are responsible in part for the observed reduction in top surface etch pit density. These studies provide a deeper understanding of dislocation reduction processes which are critical for the realization of high performance infrared detectors based on low-cost, lattice-mismatched substrates.

Post-Growth Annealing Effects on the Photoluminescence of ZnO Nanoparticle-Based Discs

The luminescence of ZnO exhibits a band-edge UV emission peak and a broad emission in the blue-green-red regions associated with the deep level defects as well as the oxygen vacancies and zinc interstitials in the discs. The mechanism of the defect is attributed to the electronic transitions from near conduction band-edge to the deep level acceptors and transitions from the deep donor levels to the valence band. In this study, post-growth annealing was conducted to investigate the mechanism involved in the visible luminescence of ZnO nanoparticle discs prepared. Post-growth thermal annealing can be applied as a new technique in controlling the optical properties of ZnO discs.

Improved Optoelectronic Properties of Rapid Thermally Annealed Dilute Nitride GaInNAs Photodetectors

We investigate the optical and electrical characteristics of GaInNAs/GaAs long-wavelength photodiodes grown under varying conditions by molecular beam epitaxy and subjected to postgrowth rapid thermal annealing (RTA) at a series of temperatures. It is found that the device performance of the nonoptimally grown GaInNAs p-i-n structures, with nominal compositions of 10% In and 3.8% N, can be improved significantly by the RTA treatment to match that of optimally grown structures. The optimally annealed devices exhibit overall improvement in optical and electrical characteristics, including increased photoluminescence brightness, reduced density of deep-level traps, reduced series resistance resulting from the GaAs/GaInNAs heterointerface, lower dark current, and significantly lower background doping density, all of which can be attributed to the reduced structural disorder in the GaInNAs alloy.© 2012 TMS.

A detailed investigation on the impact of post-growth annealing on the materials and device characteristics of 35-layer In0.50Ga0.50As/GaAs quantum dot infrared photodetector with quaternary In0.21Al0.21Ga0.58As capping

The effect of post-growth rapid thermal annealing on 35-layer In0.50Ga0.50As/GaAs quantum dot infrared photodetector (QDIP) with quaternary In0.21Al0.21Ga0.58As capping has been investigated. Transmission electron microscopy showed some as-grown defects were removed by post growth annealing treatment. An increase in the compressive strain in the heterostructure due to annealing was identified from X-ray diffraction curve. A two-color photoresponse in the long-wave region (8.5 and 10.2μm) was observed in both as-grown device and those annealed at 650°C temperature. A three-fold enhancement in peak responsivity was observed in the QDIPs annealed at 650°C (1.19A/W) compared to that in the as-grown (0.34A/W). Detectivity also increased by two fold from as-grown to 650°C annealed device. The changes are attributed to the removal of as-grown defects and dislocations during epitaxial growth. These removals changed the confinement potential profile, which resulted in an improvement in the detectivity and responsivity of the annealed sample.

Strain-free ring-shaped nanostructures by droplet epitaxy for photovoltaic application

Droplet epitaxy is a flexible nanomaterial growth technique and is a potential method to fabricate advanced electronic and optoelectronic devices. Here, we report strain-free GaAs/Al0.33Ga0.67As quantum ring solar cells fabricated by droplet epitaxy technique. Photoluminescence is used to study the electronic structure of the lattice-matched GaAs/Al0.33Ga0.67As quantum ring solar cells. Post-growth thermal annealing is used to improve the optical quality of the solar cell as well as device efficiency. A power conversion efficiency of 1.8% is demonstrated from a prototype quantum ring solar cell. This work opens new opportunities for quantum dot solar cells with strain-free nanostructures.

Theoretical optimization of multi-layer InAs/GaAs quantum dots subject to post-growth thermal annealing for tailoring the photoluminescence emission beyond 1.3 μm

In this paper, we present theoretical analysis and computation for tuning the ground state (GS) photoluminescence (PL) emission of InAs/GaAs quantum dots (QDs) at telecommunication window of 1.3–1.55 μm by optimizing its height and base dimensions through quantum mechanical concepts. For this purpose, numerical modelling is carried out to calculate the quantized energy states of finite dimensional QDs so as to obtain the GS PL emission at or beyond 1.3 μm. Here, we also explored strain field altering the QD size distribution in multilayer heterostructure along with the changes in the PL spectra, simulation on post growth thermal annealing process which blueshifts the operating wavelength away from the vicinity of 1.3 μm and improvement of optical properties by varying the thickness of GaAs spacing. The results are discussed in detail which will serve as an important information tool for device scientist fabricating high quality semiconductor quantum structures with reduced defects at telecommunication wavelengths.

Impact of post-growth thermal annealing and environmental exposure on the unintentional doping of CVD graphene films

The effect of vacuum annealing followed by exposure to oxygen and water vapor on the unintentional doping of CVD-grown graphene was investigated. CVD graphene samples were cycled between room temperature and 500 °C in vacuum while in situ Raman measurements were recorded. During the heating and cooling cycle, a hysteresis in the Raman response due to the desorption of p-dopants was observed. Upon exposure to O2 gas or air, a blue shift in the Raman response with respect to the as grown film was observed which was due to increased adsorption of p-dopants on the sample. Experiments showed that a combination of water vapor and oxygen is more effective in p-doping the samples than just oxygen and that the doping effects are reversible in both cases. Electrical measurements performed on back-gated field effect graphene devices indicate that shifts in the Dirac point correlate well to the shifts in the Raman peak positions as well as changes found in XPS and Kelvin Probe measurements, verifying the changes in doping of the graphene.

Hg based II–VI compounds on non‐standard substrates

AbstractThe Army Research Laboratory (ARL) is pursuing scalable infrared (IR) material technologies to meet the growing need for large format arrays. A two pronged approach is being undertaken at ARL using II–VI semiconductor materials. The first encompasses the well developed IR material system HgCdTe but grown on large area, extremely lattice mismatched Si(112) substrates. To overcome the high dislocation density associated with the large mismatch within the as‐grown material, ARL has successfully pursued a postgrowth thermal cycle annealing (TCA) process to reduce the dislocation density by an order of magnitude. Current results show HgCdTe/Si material can consistently be achieved with dislocation density values on the order of 1 × 106 cm−2. The second approach involves developing HgCdSe material using lattice matched, large area, and commercially available III–V substrates, specifically GaSb. Preliminary results indicate that quality HgCdSe growth by molecular beam epitaxy (MBE) can be achieved and that the bandgap of the material can be controlled through the Cd/Se ratio of fluxes used during growth.

Ag–TiO2–Ag core–shell–satellite nanowires: Facile synthesis and enhanced photocatalytic activities

Ag–TiO2–Ag core–shell–satellite nanowires were fabricated by hydrothermal growth and post-growth thermal annealing. After annealed at 600°C for 8h, core–shell Ag–TiO2 nanowires evolved into core–shell–satellite nanowires, with crystalline Ag core, shell layer of crystalline TiO2 and isolated Ag particles scattering on the shell‐like satellites. The morphologies, microstructures, absorption properties, and photocatalytic activities of the resultant nanostructures were investigated. Ag–TiO2–Ag core–shell–satellite nanowires showed remarkable enhancement of photocatalytic activity, in comparison with the pristine Ag–TiO2 nanowires, and the crystalline TiO2 component in core–shell–satellite nanowires. The enhancement could be attributed to both the enhancement of photon absorptions induced by core–shell–satellite morphology and the improvement of TiO2 crystallinity induced by annealing.

Paramagnetic shift in thermally annealed CdxZn1−xSe quantum dots

The photoluminescence of annealed CdxZn1−xSe quantum dots (QDs) under the influence of an external magnetic field has been studied in this paper. Post-growth annealing was performed for different annealing times. Above a critical annealing time, the QD luminescence shows a pronounced red-shift of the Zeeman split magnetic subcomponents. This observation is in contrast to the blue-shift caused by the diamagnetic behavior that is usually observed in non-magnetic QDs. We attribute our finding to the paramagnetism caused by the mixing of heavy and light hole states. Hence, post-growth thermal annealing treatment might be employed to render undoped epitaxial QDs intrinsically magnetic in a controlled manner. Two theoretical models were developed: a few-particle model to account for excitonic complex effects and a multiband calculation that describes the valence band hybridization. Contrasting the two models allowed us to unambiguously elucidate the nature of such an effect.

Molecular beam epitaxial growth and characterization of nitrogen δ-doped AlGaAs/GaAs quantum wells

The authors carry out δ-doping at the middle of AlGaAs/GaAs quantum wells employing molecular beam epitaxy and varying the nitrogen coverage up to 0.5 monolayers. Transmission electron micrography and x ray diffraction indicate the introduction of a nitrogen δ-doped layer with precisely controlled position and nitrogen coverage. Photoluminescence spectra obtained for the samples show clear redshift of spectral peak positions depending on the amount of nitrogen, suggesting the band structure is modified by the δ-doping. The growth can be carried out at a substrate temperature of 560 °C. The growth temperature, which is high compared with that of standard dilute nitride compounds, could suppress the formation of growth-induced defects, resulting in the weak effect of post-growth thermal annealing on the characteristics of room-temperature photoluminescence.

The influence of post-growth annealing on the optical properties of InAs quantum dot chains grown on pre-patterned GaAs(100)

We report on the effect of post-growth thermal annealing of [011]-, []-, and [010]-oriented quantum dot chains grown by molecular beam epitaxy on GaAs(100) substrates patterned by UV-nanoimprint lithography. We show that the quantum dot chains experience a blueshift of the photoluminescence energy, spectral narrowing, and a reduction of the intersubband energy separation during annealing. The photoluminescence blueshift is more rapid for the quantum dot chains than for self-assembled quantum dots that were used as a reference. Furthermore, we studied polarization resolved photoluminescence and observed that annealing reduces the intrinsic optical anisotropy of the quantum dot chains and the self-assembled quantum dots.

How do InAs quantum dots relax when the InAs growth thickness exceeds the dislocation-induced critical thickness?

A simple critical thickness for generating lattice misfits is insufficient to describe the onset strain relaxation in InAs quantum dots (QDs). A predominant dot family is shown to relieve its strain by In/Ga interdiffusion, rather than by lattice misfits, at the onset of strain relaxation. This argument is based on photoluminescence spectra, which show the emergence of a fine blueshifted transition at the onset of strain relaxation, along with a low-energy transition from a dot family degraded by lattice misfits. From the analysis of the temperature-dependent blueshift and energy separation between the ground and excited-state transitions, the blueshift is attributed to In/Ga interdiffusion. Transmission electron microscopy suggests a relaxation-induced indium migration from the interdiffused dot family to the dislocated dot family. Post-growth thermal annealing can further relieve strain by inducing more In/Ga interdiffusion in the interdiffused dot family and more dislocations in the dislocated dot family. This study explains the co-existence of strong carrier confinement in the QDs and enormous misfit-related traps in the capacitance-voltage spectra, and an elongated QD electron-emission time.

Interface properties of (In,Ga)As/GaAs quantum wells grown by solid-phase epitaxy

(In,Ga)As/GaAs quantum wells (QWs) are successfully fabricated via a thermally induced structural transition from deposited amorphous material to epitaxial films, also known as solid-phase epitaxy (SPE). Although exact processes occurring during the epilayer formation are unknown, it is shown that the method allows the fabrication of high quality QWs with abrupt and symmetric composition profiles. As discussed here, the analysis of the chemical interface (composition profile) of the SPE-grown QWs, and its comparison with the element profiles of similar heterostructures grown by conventional molecular beam epitaxy (C-MBE) provides further insight into the SPE processes. In particular, we find that regardless of the fabrication method (SPE vs C-MBE), the smooth variation of the element concentration with the position across the interface is remarkably well described by a sigmoidal function. Such functional dependence is determined by fundamental processes occurring during the growth; thus suggesting that the basic mechanisms of interface formation are similar in SPE and C-MBE. Finally, the effect of self- and post-growth thermal annealing on SPE QWs is also discussed.

A systematic study of the nitridation of SnO 2 nanowires grown by the vapor liquid solid mechanism

SnO 2 nanowires (NWs) with diameters of 50 nm and lengths ≥10 μm have been grown at 800 °C on 1.0 nm Au/Si(001) via the vapor liquid solid mechanism and the low pressure chemical vapor deposition. These exhibited clear peaks in the X-ray diffraction corresponding to the tetragonal rutile crystal structure of SnO 2 and a broad-symmetric photoluminescence (PL) spectrum, centered around 560 nm due to structural-related defect states, energetically located in the upper half-band-gap of SnO 2. We find that post-growth thermal annealing of the SnO 2 NWs over a broad range of temperatures, i.e. 400–1000 °C and high flow of O 2 does not change their crystal structure or optical properties. In contrast the nitridation of SnO 2 NWs using NH 3 leads to their elimination above 500 °C. Lower temperatures did not favor the nitridation even using extended nitridation times, hydrogen, lower ramp rates or a two-step-temperature process which are effective in the case of In 2O 3 and Ga 2O 3. However the nitridation of SnO 2 NWs was promoted by HCl, supplied in-situ via the sublimation of NH 4Cl, which reacts with Sn and SnO 2 leading to the formation of the intermediate SnCl 4, which reacts in turn with NH 3 giving tin nitride at temperatures between 400 and 500 °C . We discuss the effect of the nitridation and thermal annealing on the PL spectra.