Annealing Of GaAs Research Articles

Impact of Pocket Geometry on Quantum Dot Lasers Grown on Silicon Wafers

Epitaxially grown quantum dot (QD) lasers in narrow pockets on patterned silicon photonics wafers present a key step toward full monolithic integration of on‐chip light sources. However, InAs QD lasers grown in deep and narrow pockets demonstrate limited performance and reliability compared to planar‐grown counterparts. Herein, InAs QD lasers are grown in patterned SiO2 pockets atop planar thermal cyclic annealed GaAs on (001) Si substrate with reduced threading dislocation density, enabling detailed study of how pocket geometry impacts device performance. Fabry–Pérot lasers with cleaved facets exhibit strong variation in performance based on the dimensions of the pocket, wherein thermal and optical metrics improve with increasing pocket width. Devices lase up to a maximum stage temperature of 115 °C with an extrapolated lifetime of 2.2 years at 80 °C for material grown in 50 μm by 3900 μm pockets. This study addresses, the ongoing challenge of optimizing pocket‐grown devices to planar equivalent performance.

Transient THz Emission and Effective Mass Determination in Highly Resistive GaAs Crystals Excited by Femtosecond Optical Pulses

We present comprehensive studies on the emission of broadband, free-space THz transients from several highly resistive GaAs samples excited by femtosecond optical pulses. Our test samples are characterized by different degrees of disorder, ranging from nitrogen-implanted to semi-insulating and annealed semi-insulating GaAs crystals. In our samples, we clearly observed transient THz emissions due to the optical rectification effect, as well as due to the presence of the surface depletion electrical field. Next, we arranged our experimental setup in such way that we could observe directly how the amplitude of surface-emitted THz optical pulses is affected by an applied, in-plane magnetic field. We ascribe this effect to the Lorentz force that additionally accelerates optically excited carriers. The magnetic-field factor η is a linear function of the applied magnetic field and is the largest for an annealed GaAs sample, while it is the lowest for an N-implanted GaAs annealed at the lowest (300 °C) temperature. The latter is directly related to the longest and shortest trapping times, respectively, measured using a femtosecond optical pump-probe spectroscopy technique. The linear dependence of the factor η on the trapping time enabled us to establish that, for all samples, regardless of their crystalline structure, the electron effective mass was equal to 0.059 of the electron mass m0, i.e., it was only about 6% smaller than the generally accepted 0.063m0 value for GaAs with a perfect crystalline structure.

High-resolution analogue of time-domain phonon spectroscopy in the transmission electron microscope.

Femtosecond photoexcitation of semiconducting materials leads to the generation of coherent acoustic phonons (CAPs), the behaviours of which are linked to intrinsic and engineered electronic, optical and structural properties. While often studied with pump-probe spectroscopic techniques, the influence of nanoscale structure and morphology on CAP dynamics can be challenging to resolve with these all-optical methods. Here, we used ultrafast electron microscopy (UEM) to resolve variations in CAP dynamics caused by differences in the degree of crystallinity in as-prepared and annealed GaAs lamellae. Following in situ femtosecond photoexcitation, we directly imaged the generation and propagation dynamics of hypersonic CAPs in a mostly amorphous and, following an in situ photothermal anneal, a mostly crystalline lamella. Subtle differences in both the initial hypersonic velocities and the asymptotic relaxation behaviours were resolved via construction of space-time contour plots from phonon wavefronts. Comparison to bulk sound velocities in crystalline and amorphous GaAs reveals the influence of the mixed amorphous-crystalline morphology on CAP dispersion behaviours. Further, an increase in the asymptotic velocity following annealing establishes the sensitivity of quantitative UEM imaging to both structural and compositional variations through differences in bonding and elasticity. Implications of extending the methods and results reported here to elucidating correlated electronic, optical and structural behaviours in semiconducting materials are discussed. This article is part of a discussion meeting issue 'Dynamic in situ microscopy relating structure and function'.

Monolithic growth of GaAs/Al0.3Ga0.7As multiple quantum well structure on Ge substrate with low defects: theoretical and experimental correlation of the structural and optical properties

High quality III–V multiple quantum well (MQW) heterostructure on germanium (Ge) substrate is grown by molecular beam epitaxy. The propagation of defects and dislocations from the Ge/GaAs interface towards the active layer is suppressed via the adapted novel growth strategy. The cross-sectional transmission electron microscopy images showed the active layer of the MQW structure with reduced anti-phase domains and dislocations due to the introduction of migration enhanced epitaxy (MEE) and three-step annealed GaAs buffer layer. The optical properties are compared with another sample having similar heterostructure on GaAs substrate. The variation in full width half maxima with the well thickness has been analyzed via the correlation between photoluminescence (PL) result and calculated penetration depth of electron wave functions into the barrier material. The effect of substrate on the hetero-interface of MQW structure is also investigated. Rapid thermal annealing (RTA) is carried out on the sample with Ge substrate in order to explore the change in optical properties of the MQW structure. PL study indicates insignificant effect of low temperature RTA treatment on the thinner QWs. However, improvement in the optical property such as increment in activation energy and three order enhancement in the PL intensity was observed for the thicker QWs, owing to the amelioration in their structural property.

Below-bandgap photoluminescence from GaAs substrates induced by pre-MBE-growth treatments

We report the observation of below-GaAs-bandgap photoluminescence (PL) emission from semi-insulating GaAs substrates subjected to thermal annealing during the standard pre-MBE-growth processes. The below-GaAs-bandgap luminescence from defects were investigated using a combination of PL techniques including below-gap-excitation (BGE) and backside illuminated (BI) PL. Using BGE and BI PL, defects deep within the substrates were probed, and their spatial positions along the sample were analyzed. A PL peak at 1000 nm was observed after pre-bake annealing at 300°C, and further peaks at 905, 940 and 1150 nm were found after oxide desorption annealing at 600°C. These are attributed to the Ga-vacancy related defect, Ga-vacancy-complex defect, As-vacancy defect, and InGaAs states, respectively. This is the first report of the formation of such optically-active defects after annealing of GaAs at moderate temperature ranges (≤600°C), providing guidelines to distinguish desired electronic states for device applications from those that arise from defects which often confuse, and also degrade the device performances.

(Invited) Potential and Activities of III-V/Si Tandem Solar Cells

III-V compound/Si tandem solar cells are expected to have great potential of space and terrestrial application because of high efficiency, light weight and low cost potential. However, problems to be solved are high density of dislocations and large residual strain in III-V compound on Si in the case of hetero epitaxy of III-V compound materials on Si substrates. There are other approaches such as epitaxial lift-off and direct bonding technologies in order to prevent such problems. This paper presents efficiency potential of III-V/Si tandem solar cells with efficiencies of more than 35% under 1-sun AM1.5G and effects of dislocation upon solar cell properties of III-V-on-Si single-junction solar cells and III-V/Si tandem solar cells. Because III-V/Si system has large misfit and difference in thermal expansion coefficient, generation of misfit and thermal stress induced dislocations affect on solar cell properties. Effects of dislocations upon minority-carrier lifetime are derived from considering one dimensional transport of minority carriers to dislocations. In order to realize high efficiency solar cells with similar efficiency by homo-epitaxially grown solar cells, low density dislocation of less than 3x105 cm-2 is necessary. The paper also reviews approaches on reduction in dislocation density in III-V compound films on Si and improvements in efficiencies of III-V compound single-junction solar cells on Si substrates and III-V/Si tandem solar cells. Especially, effective dislocation density reduction in III-V compound films on Si substrates by thermal cycle annealing (TCA) and insertion of strained-layer super-lattices (SLS) in our study are demonstrated. As a result of combination of TCA process and SLS insertion, high quality GaAs films with a dislocation density of 1-2x106 cm-2 have been realized. Reduction in dislocation density of GaAs-on-Si layers due to TCA and SLS insertion have been explained by coalescence of dislocations under high temperature and high stress condition and dislocation movement towards edge and interface by mismatch stress, respectively. In addition, effectiveness of thicker-layer growth, selective area growth, low-temperature growth for hetero epitaxy of III-V compound films on Si substrates are also reviewed. We have demonstrated 20% efficiency (under 1-sun of AM1.5G) with hetero-epitaxially grown GaAs single-junction solar cells on Si. By employing high quality GaAs films on Si, heteroface GaAs thin films solar cells with an Al0.8Gao.2As window layer were fabricated on Si substrates. The thickness of Si substrates was 200mm. The n-GaAs base layer (carrier concentration of 5x1017 cm-3) with a thickness of 1.3-1.5mm was grown on an Al0.6Ga0.4As/GaAs super lattice (SL) layer (20nm/100nm x 5pairs), In0.1Ga0.9 As/GaAs SLS (10nm/10nm x 5 pairs), and thermal cycle annealed GaAs buffer layer (TCA:9000C x 5 times, 2mm thick) which were grown on Si substrates. The thickness and carrier concentration of p-GaAs emitter layer were 0.5 mm and 2x1018 cm-3, respectively. The total epitaxial layer thickness was about 3.5-5 mm. A Si3N4 layer was used as an anti-reflection film. Ti/Ag was used as electrode metal. The cell area was 1 cm2. The total-area conversion efficiency of 18.3% at AM0 and 20.0% at AM1.5G were the highest ever reported for GaAs-on-Si single-junction solar cells. Such high-efficiency GaAs-on-Si cells have been attained by combining TCA, SLS and SL buffer layers. As a result of such a combination, high efficiency GaAs-on-Si solar cells have been realized using high quality GaAs films with a dislocation density of 3.7x106 cm-2. The first space flight of forty-eight 2 cm x 2 cm GaAs-on-Si solar cells with an average AM0 total-area efficiency of 16.9% have been carried out using the Engineering Test Satellite No. 6 (ETS-VI). The GaAs-on-Si solar cells have been demonstrated to be more radiation-resistant on space than GaAs-on-GaAs solar cells and 50, 100 and 200-mm-thick Si space solar cells. These results show that the III-V/Si-based multi-junction solar cells have great potential for space applications. Ohio State University has also demonstrated 17.1% (AM0) efficiency with GaAs single-junction solar cells by using Ge buffer layer. Nagoya Inst. Tech. has achieved 22.1% with GaAs/Si tandem solar cell by hetero epitaxy. Most recently, Fraunhofer ISE has demonstrated 27.9 % efficiency (under 48.3-suns of AM1.5D) InGaP/GaAs/Si 3-junction solar cells by using epitaxial lift-off and direct bonding. Future prospects are also discussed. Further improvement in efficiency for GaAs-on-Si single-junction solar cells and III-V/Si multi-junction solar cells can be obtained by reducing dislocation densities to less than 3x105 cm-2.

Lithium-diffused and annealed GaAs: An admittance spectroscopy study

We study lithium-diffused and annealed GaAs by admittance spectroscopy in the frequency range $10--{10}^{6}\mathrm{Hz}$ and the temperature range 30--300 K. Li diffusion turns the GaAs semi-insulating but subsequent out-diffusion of Li increases the conductivity and makes the samples p type. It is demonstrated that the conduction in semi-insulating Li-diffused GaAs is due to thermally activated carriers in the valence-band percolating around insulating metallic precipitates. At high frequencies the ac conductivity is proportional to ${\ensuremath{\omega}}^{x},$ with x being close to unity value, independent of temperature. We suggest that the percolation may be due to metallic precipitates formed during in-diffusion of lithium and following cooling. After subsequent annealing the ac conductivity becomes proportional to ${\ensuremath{\omega}}^{s}$ at high frequencies with the value of s decreasing with increasing temperature. The temperature dependency of s suggests a correlated barrier hopping mechanism in a band of defects. We attribute these defects to gallium vacancies ${V}_{\mathrm{Ga}}$ and gallium antisites ${\mathrm{Ga}}_{\mathrm{As}}.$

Frequency-dependent conductivity in lithium-diffused and annealed GaAs

We report on measurements of the frequency-dependent conductivity in lithium-diffused (Li-diffused) and annealed GaAs in the frequency range 10–106Hz and the temperature range 30–300K. Li diffusion into GaAs reduces the free carrier concentration and can make the material semi-insulating. Using admittance spectroscopy we show that the conduction in semi-insulating Li-diffused GaAs is due to hopping between localized centers. These centers are suggested to be associated with the gallium vacancy VGa and the gallium antisite GaAs. Furthermore, percolation is apparent in as-diffused samples, indicating that the material contains metallic precipitates. We suggest that the percolation is due to Li precipitation or formation of GaLi clusters during in-diffusion of lithium. At high frequency the AC conductivity is proportional to ωs. The value of s decreases with increasing temperature which suggests a correlated barrier hopping mechanism.

Impurity band in lithium-diffused and annealed GaAs: Conductivity and Hall effect measurements

Originally n-type Si-doped GaAs turns p-type after Li indiffusion and consequent annealing at $200--600\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ Temperature-dependent conductivity and Hall effect measurements carried out in the temperature range 30--300 K reveal conduction via impurity bands made up of shallow acceptors in addition to the valence-band conduction. Li diffusion into GaAs reduces the free-carrier concentration which leads to electrical resistivity as high as ${10}^{7}\ensuremath{\Omega}\mathrm{cm}.$ Annealing highly resistive samples at temperatures above $200\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ significantly decreases the room-temperature resistivity to 0.1--1 $\ensuremath{\Omega}\mathrm{cm},$ depending on the annealing temperature. For samples annealed at $300\ensuremath{-}400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ we observe a metallic-type conduction which contributes to the conduction even at room temperature. In samples annealed at $500--600\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ the temperature dependence of the resistivity indicates that impurity conduction sets in at temperatures below 100 K. We discuss this conduction change in relation to enhanced gallium vacancy ${(V}_{\mathrm{Ga}})$ and gallium antisite $({\mathrm{Ga}}_{\mathrm{As}})$ concentration in Li indiffused and annealed samples.

Role of As precipitates on ultrafast electron trapping in low-temperature-grown GaAs and AlGaAs alloys

The role and interplay of basic structure parameters of arsenic precipitates on the ultrafast trapping of conduction band electrons have been studied in a series of low-temperature-grown GaAs and AlGaAs epilayers grown at various temperatures ranging from 170 up to 325 °C and annealed at 600 °C. Cross-sectional electron-transmission microscopy was used to determine the density and size of the precipitated arsenic clusters as a function of the growth temperature. The dependence of the electron trapping times (τ) on the spacing (R) and radius (α) of the arsenic precipitates has been systematically investigated by time-resolved pump–probe transient transmission spectroscopy. The electron trapping times follow a τ∝R3/α law which holds for low-temperature-grown GaAs and is applicable for low-temperature-grown AlGaAs as well. This trend generalizes the electron trapping mechanism for low-temperature-grown and annealed GaAs and related semiconductor alloys.

Electron and trap dynamics in As-ion-implanted and annealed GaAs

The ultrafast dynamics of As-ion-implanted and annealed GaAs is investigated using transmission pump–probe measurements. Carrier recombination time was found to increase from 4 to 40 ps with increasing annealing temperature. At lower annealing temperatures, the transmitted optical signal is dominated by induced absorption and at higher annealing temperatures this effect is replaced by induced transparency.

Characterization of carbon-doped GaAs grown by metalorganic vapor-phase epitaxy

For carbon-doped p-GaAs layers grown by metalorganic vapor-phase epitaxy (MOVPE), the two parameters, k and α, respectively, used for indicating strain state and activity ratio of the C-doped epilayers, are important for demonstrating the quality of GaAs : C layers. By comparing the data from Hall effect and double-crystal X-ray diffraction (DCXRD) measurements made on as-grown and annealed GaAs : C layers grown by atmospheric pressure MOVPE (AP-MOVPE) using trimethylgallium (TMGa), arsine (AsH 3) and carbon tetrachloride (CCl 4), the lattice mismatch and resultant strain state, and hydrogen passivation of GaAs : C layers on GaAs substrates were investigated as well as their changes after annealing. Furthermore, a relation was established between the lattice mismatch (Δ a ⊥/ a GaAs) and hole concentration p (cm −3), Δ a ⊥/ a GaAs=−8×10 −24 kp/ α. With this equation the relation between k and α can be determined from the relation between the measured (Δ a ⊥/ a GaAs) and p. If one of the two parameters, k and α, is known, the other could be just then determined. This method for determining the values of both k and α, not involving the traditional use of the secondary ion mass spectrometry (SIMS), is not only relatively simple and cheap, but also in fairly good agreement with the experimental results.

Reduced thermal conductivity in low-temperature-grown GaAs

Thermal conductivity of low-temperature-grown GaAs(LT GaAs) was measured at room temperature using a self-heated photolithographically patterned platinum wire on the surface of the sample. Finite element calculations were performed to extract the thermal conductivity from the nonlinear I–V characteristic of the wires. For LT GaAs grown at a substrate temperature of 240 °C, the thermal conductivity was found to be only 23% of the value for stoichiometric GaAs. Rapid thermal annealing of the sample at 650 °C for 30 s increased the thermal conductivity to 46% of the GaAs value. Strong phonon scattering by point defects could account for reduced thermal conductivity in the as-grown material. The reduced thermal conductivity in the annealed material, however, is not consistent with our current understanding of the defects in annealed LT GaAs.

Identification of individual and aligned microdefects in bulk vertical Bridgman- and liquid encapsulated Czochralski-grown GaAs

A systematic study of microdefects (precipitates) in undoped and Si-doped vertical Bridgman (VB)-grown and updoped liquid encapsulated Czochralski (LEC)-grown GaAs crystals was performed using infra-red (IR) and differential interference contrast (DIC) microscopy, diluted sirtl-like etchants used with light (DSL) etching, transmission electron microscopy (TEM) and X-ray topography. Apart from hexagonal As precipitates, which were reported in many publications, inclusions of crystalline As 2O 3 and As 2O 5 have been identified for the first time by selected-area diffraction (SAD) and from lattice spacing of Moiré patterns in undoped GaAs. The crystalline precipitates are often associated with voids. In Si-doped annealed GaAs, however, only voids have been found. The average size of precipitates in LEC material is about 100 nm, while in VB they can reach 600 nm. Most of the precipitates decorate dislocations (DPs), but some were also found in the dislocation-free matrix as single inclusions or linear arrays of precipitates (LAPs). The latter defect pattern has not yet been reported. We shall present a tentative formation mechanism of LAPs. From the TEM analysis it follows that the nature of these precipitates is the same as that of the DPs. It has been established that microdefects are surrounded by large (compared to the size of precipitates themselves) zones which have recombinative properties for photo-generated carriers, leading to the formation of hillocks during photo-etching before the defects are dissolved to form S-pits. The S-pits formed during DSL etching and jet etching in H 2SO 4:H 2O 2:H 2O solution used to prepare TEM specimens are elongated and run along the [1 1 ̄ 0] direction. From TEM it follows that the large microdefects in VB material have the form of a regular tetrahedron with the side walls bounded by {111} planes and the edges along the 〈110〉 directions.

Spectroscopic Identification of theSiO2Complex in Oxygen-Implanted GaAs:Si

Oxygen-implanted and annealed GaAs:Si epitaxial layers have been investigated by low-temperature Fourier transform infrared absorption spectroscopy. Several new local vibrational modes are observed in the range from 614 to $641{\mathrm{cm}}^{\ensuremath{-}1}$. Isotope shifts and splittings allow the unambiguous chemical identification of the defect involved as ${\mathrm{SiO}}_{2}$. A microscopic structure model is developed which provides, based upon a simple fit, the correct vibrational frequencies. Evidence is presented that deep levels associated with this complex are causing the high sheet resistance of the layers.

High-carrier-density electron dynamics in low-temperature-grown GaAs

Pump-probe differential transmission measurements examine high-carrier-density phenomena in as-grown and annealed GaAs samples grown at temperatures from 210 to 270 °C. We observe trap saturation and Auger recombination, and accurately model the measurements on annealed samples with a simple two level rate equation, allowing us to extract the trapped-electron lifetimes.

Optical properties of α-irradiated and annealed Si-doped GaAs

The influence of irradiation by α particles and subsequent isochronal annealing on n-GaAs doped with silicon, was investigated. Photoluminescence (PL) studies revealed the formation of an induced radiation center at 1.486 eV. In addition to the PL investigation we also employed capacitance–voltage (C–V) measurements and deep level transient spectroscopy (DLTS). The C–V measurements indicate a slight reduction in n-type carriers in treated samples and the DLTS spectra showed a number of commonly observed deep levels in irradiated samples (not annealed), which disappear after annealing. Based on the experimental evidence we propose that the 1.486 eV band is due to a donor-acceptor-pair transition. Alternative mechanisms are also discussed.

Pulsed Electron Beam Annealing of GaAs after High Dose Implantation of Hydrogen

Pulsed Electron Beam Annealing of GaAs after High Dose Implantation of Hydrogen

Comparative study of microdefects in dislocation-free, heavily Si doped VB GaAs by DSL etching, NIR phase contrast microscopy, TEM and X-ray diffuse scattering

Microdefects in as-grown and annealed VB GaAs (Si > 5 x 10 18 cm −3) have been studied by four methods. The nature and density of microdefects has been established by TEM: perfect or faulted interstitial type dislocation loops lying in the 110 and 111 planes have been found. Direct comparison between the results of TEM study and other methods has been performed. Photo-etching reveals microdefects in the form of microroughness. Large loops (micrometer size) in as-grown and annealed material could be clearly discerned by this method. NIR phase contrast microscopy is effective in analysing the loops which are enlarged during high temperature annealing. Diffuse X-ray scattering is suitable for qualitative assessment of the density and type of microdefects. From etching experiments a speculative argument has been obtained on the involvement of Si in the formation of interstitial type faulted loops.

On Correlations between Extended Defects Formation and Electron Concentration Changes Caused by Annealing of GaAs:Te

Extended type defects in heavily doped GaAs:Te (n 1 x 10 19 cm-3 ) after annealing at 700°C and 1150°C were studied by transmission electron microscopy, by high resolution technique and energy-dispersive X-ray analysis. Assuming, according to the literature that these defects are enriched in impurity atoms it is suggested that this solely is not sufficient to explain changes of electrical properties during annealing. Estimated amount of atoms involved in faulted dislocation loops seems to be too small, energy-dispersive X-ray microanalysis of precipitates did not show large differences in composition with the matrix. PACS numbers: 61.72.Nn, 61.72.Vv, 72.80.Ey