Observation of low-T GaAs growth regimes by real-time ellipsometry

The molecular beam epitaxial growth of low temperature (LT) GaAs films has been studied by real-time ellipsometry. A modification in a GaAs (001) surface by cooling under a specific As₂ flux caused a change in the ellipsometry data. The thermocouple reading of this change was used as a signature to indicate the reproducible substrate temperature for the growth of LT-GaAs layers. The origin of this surface modification was studied by reflection high energy electron diffraction. The growth regimes of LT-GaAs layers were studied by real-time ellipsometry. The dielectric properties of the epitaxial layer and the critical thickness for epitaxial growth were extracted for various growth conditions. The microstructure beyond the critical point was found to be composed of amorphous as well as crystalline forms of GaAs.     

Diode structures from amorphous low-temperature GaAs

The effect of annealing on the electrical properties of a GaAs diode structure, which incorporated a nominally undoped low-temperature (LT) layer on top of conventionally grown p-type GaAs, is examined. Unannealed GaAs grown by molecular beam epitaxy at substrate temperatures below 250°C is amorphous and highly resistive. Annealing at high temperatures converts the undoped LT-GaAs from amorphous to single crystal material. The annealed material is n-type. The current-voltage characteristics of the LT on p-type GaAs structures showed greater asymmetry, with lower reverse leakage currents, as the anneal temperature was increased above 400°C. This reflects the improved crystal quality of the LT layer.     

利用双低温缓冲层和插入应变超晶格技术制备高质量InP-on-GaAs复合衬底的MOCVD生长

提出一种结合双低温缓冲层和应变超晶格优势的高质量InP-on-GaAs复合衬底制备技术. 研究发现LT-InP/LT-GaAs的双低温缓冲层比单一低温InP缓冲层的聚集应变的效果更为显著. 并且,双低温缓冲层中的低温GaAs层存在一个最优生长厚度. 当低温InP生长厚度一定,低温GaAs层的生长厚度达到优化生长厚度时,LT-InP/LT-GaAs双低温缓冲层能达到调节应变的最佳状态. 最后,通过插入InGaP/InP应变超晶格,并且优化其在外延层中的插入位置,得到了高质量的InP-on-GaAs的复合衬底,2μm厚的InP外延层XRD-ω/2θ扫描的半高宽小于200" .     

Application of low temperature GaAs to GaAs/Si

Low Temperature grown GaAs (LT-GaAs) was incorporated as a buffer layer for GaAs on Si (GaAs/Si) and striking advantages of this structure were confirmed. The LT-GaAs layer showed high resistivity of 1.7 × 10⁷ ω-cm even on a highly defective GaAs/Si. GaAs/Si with the LT-GaAs buffer layers had smoother surfaces and showed much higher photoluminescence intensities than those without LT-GaAs. Schottky diodes fabricated on GaAs/Si with LT-GaAs showed a drastically reduced leakage current and an improved ideality factor. These results indicate that the LT-GaAs buffer layer is promising for future integrated circuits which utilize GaAs/Si substrates.     

Electrical properties of low-temperature GaAs grown by molecular beam epitaxy and migration enhanced epitaxy

Measurements on low-temperature GaAs epitaxial layers (LT-GaAs) grown by molecular beam epitaxy and migration enhanced epitaxy showed that the excess arsenic incorporated during growth played a crucial role in determining their electrical properties. The electrical transport in LT-GaAs grown by a standard molecular beam epitaxy proceeded mainly via a hopping process, which showed a higher activation energy and onset temperature than those usually observed in lightly doped semiconductors. Using migration enhanced epitaxy to grow LT-GaAs, we were able to substantially reduce the density of As-rich defects and to achieve a good Hall mobility in Be-doped LT-GaAs. The study presented here indicates that, with controlled excess arsenic incorporation during growth, LT-GaAs can vary in a range of conduction properties and thus can be engineered for different device applications.     

Defects in molecular beam epitaxial GaAs grown at low temperatures

We have utilized a variable energy positron beam and infrared transmission spectroscopy to study defects in GaAs epilayers grown at low temperatures (LT-GaAs) by molecular beam epitaxy. We have measured the Doppler broadening of the positron-electron annihilation gamma ray spectra as a function of positron implantation energy. From these measurements, we have obtained results for the depth profiles of Ga monovacancies in unannealed LT-GaAs and Ga monovacancies and arsenic cluster related defects in annealed LT-GaAs. We have also studied the effects of the Si impurities in annealed LT-GaAs. The infrared transmission measurements on unannealed LT-GaAs furnish a broad defect band, related to As antisites, centered at 0.370 eV below the conduction band.     

利用双低温缓冲层和插入应变超晶格技术制备高质量InP-on-GaAs复合衬底的MOCVD生长

提出一种结合双低温缓冲层和应变超晶格优势的高质量InP-on-GaAs复合衬底制备技术．研究发现LT-InP/LT-GaAs的双低温缓冲层比单一低温InP缓冲层的聚集应变的效果更为显著．并且,双低温缓冲层中的低温GaAs层存在一个最优生长厚度．当低温InP生长厚度一定,低温GaAs层的生长厚度达到优化生长厚度时,LT-InP/LT-GaAs双低温缓冲层能达到调节应变的最佳状态．最后,通过插入InGaP/lnP应变超晶格,并且优化其在外延层中的插入位置．得到了高质量的InP-on-GaAs的复合衬底,2μm厚的lnP外延层XRD-ω/2θ扫描的半高宽小于200．     

Effect of dopants on arsenic precipitation in GaAs deposited at low temperatures

High resolution x-ray diffraction using synchrotron radiation was used to characterize GaAs grown by MBE at low temperatures (LT-GaAs) LT-GaAs grown at 225°C is nonstoichiometric and exhibits a 0.15% lattice expansion along the growth direction. Annealing LT-GaAs results in arsenic clusters with a well-defined orientation relationship with the GaAs matrix and a relaxation of the LT-GaAs lattice. The arsenic precipitation corresponds to a classical case of diffsion controlled nucleation and growth followed by coarsening. While the rates of growth and coarsening in the n-doped and the p-doped samples are observed to be identical, the effects of the superlattice seem to accelerate the precipitation kinetics in the p-n superlattice sample. The enhanced coarsening in the p-n superlattice sample is consistent with a previously propsed model involving interaction between charged precipitate and arsenic defects.     

In-situ monitoring and control for MOCVD growth of AlGaAs and InGaAs

We have used spectroscopic ellipsometry to perform real-time monitoring during metalorganic chemical vapor deposition growth of AlGaAs (on GaAs) and InGaAs (on GaAs and InP). Optical constants for these materials were obtained up to growth temperatures of 600 to 700°C. This information permits real-time extraction of composition and layer thickness from the raw ellipsometric data at sample rates on the order of 0.5 Hz. We describe closed-loop control of composition and total layer thickness on AlGaAs-based structures, including Bragg reflectors. In-situ data obtained on double-heterostructure quantum-well laser structures demonstrate that spectroscopic ellipsometry is an extremely powerful monitoring and quality-control tool, giving important real-time information on complex structures that would be difficult and time-consuming to obtain after growth.     

Interaction of low-temperature surfactant-grown InAs superlattice layers with arsenic precipitates

Double crystal X-ray diffraction (DCXRD), transmission electron microscopy (TEM) and photoluminescence (PL) measurements were used to study the effect of post-growth annealing temperature on the structural properties of novel low growth (250 °C) and normal growth (450 °C) temperature InAs surfactant-mediated grown materials. Under conditions of “arsenic-free” growth, high-quality InAs-GaAs superlattices are obtained at low temperatures (LT) even at thicknesses as high as 3 monolayers (MLs) for InAs. The interaction of the built-in strain fields and the point defects in LT-GaAs both before and after annealing has been studied in detail. DCXRD studies show that the thickness of the LT-grown InAs layers decreased by up to 0.7 ML as the annealing temperature increased to 550 °C. There is evidence from the DCXRD and TEM that the LT InAs-GaAs superlattice structure starts to distort at annealing temperatures above 450 °C. In comparison, the sample grown at normal temperature, 450 °C, still retained the periodicity of the superlattice layers up to an annealing temperature of 650 °C without any change in the thickness of either the InAs (wetting layer) or GaAs.     

High-power narrow-band terahertz generation using large-aperturephotoconductors

Large-aperture biased photoconductive emitters which can generate high-power narrow-band terahertz (THz) radiation are developed. These emitters avoid saturation at high fluence excitation and achieve enhanced peak power spectral density by employing a thick layer of short-lifetime low-temperature-grown GaAs (LT-GaAs) photoconductor and multiple-pulse excitation. THz waveforms are calculated from the saturation theory of large aperture photoconductors, and a comparison is made between theory and measurement. A direct comparison of the multiple-pulse saturation properties of THz emission from semi-insulating GaAs and LT GaAs emitters reveals a strong dependence on the carrier lifetime. In particular, the data demonstrate that saturation is avoided only when the interpulse spacing is longer than the carrier lifetime     

Capacitance study of electron traps in low-temperature-grown GaAs

Electron traps in GaAs grown by MBE at temperatures of 200–300°C (LT-GaAs) were studied. Capacitance deep level transient spectroscopy (DLTS) was used to study the Schottky barrier on n-GaAs, whose space-charge region contained a built-in LT-GaAs layer ∼0.1 μm thick. The size of arsenic clusters formed in LT-GaAs on annealing at 580°C depended on the growth temperature. Two new types of electron traps were found in LT-GaAs layers grown at 200°C and containing As clusters 6–8 nm in diameter. The activation energy of thermal electron emission from these traps was 0.47 and 0.59 eV, and their concentration was ∼10¹⁷ cm⁻³, which is comparable with the concentration of As clusters determined by transmission electron microscopy. In LT-GaAs samples that were grown at 300°C and contained no arsenic clusters, the activation energy of traps was 0.61 eV. The interrelation between these electron levels and the system of As clusters and point defects in LT-GaAs is discussed. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 4, 2004, pp. 401–406. Original Russian Text Copyright ? 2004 by Brunkov, Gutkin, Moiseenko, Musikhin, Chaldyshev, Cherkashin, Konnikov, Preobrazhenskii, Putyato, Semyagin.     

Transient current spectroscopy and frequency dispersion studies of low temperature GaAs and Al0.3Ga0.7As metal-insulator-semiconductor diodes

Low temperature (LT)-grown GaAs and Al_0.3Ga_0.7As metal-insulator-n⁺-GaAs (MIN) diodes have been fabricated and their electrical properties analyzed. Studies were carried out to evaluate the interfacial quality of the LT layer and the underlying n⁺-GaAs layer using transient current spectroscopy (TCS) and capacitance-frequency (C-f) characterization. TCS studies on LT-GaAs revealed a high concentration of a continuum of states anda dominant electron trap with an activation energy of 0.52eV. In LT-Al_0.3Ga_0.7As, a shallow trap at 0.36eV and two deep level traps at 0.85eV and 1.12eV were observed. Frequency dispersion was observed to be less for LT-GaAs samples with an AlAs barrier layer than without an AlAs barrier layer. However, LT-Al_0.3Ga_0.7As MIN diodes displayed a smaller frequency dispersion than LT-GaAs MIN diodes. Upon further investigation into MISFET devices, it was found that LT-Al_0.3Ga_0.7As MISFET devices had better transconductance frequency dispersion characteristics than LT-GaAs MISFET devices did.     

Thermal stability of MISFET with low-temp molecular-beamepitaxy-grown GaAs and Al0.3Ga0.7As gate ins

GaAs and Al_0.3Ga_0.7As epilayers grown at LT by MBE were used as insulators in the fabrication of MISFET devices. Parametric changes were used to evaluate the thermal stability of MISFET, to identify failure mechanisms and validate the reliability of these devices. The LT-Al_0.3Ga_0.7As MISFET showed superior thermal stability. The degradation in the performance of MISFET with 1000 Å thick LT-GaAs gate insulator was worse than those of the MESFET. On the other hand, MISFET with 250 Å thick LT-GaAs gate insulators exhibited stable characteristics with thermal stressing, LF (low frequency) noise studies on the TLM structures of MISFET layers exhibited 1/f noise in the LT-Al_0.3Ga_0.7As samples and 250 Å LT-GaAs samples; whereas the 1000 Å thick LT-GaAs samples exhibited 1/f^3/2 noise, which was attributed to: (i) the thermal noise generated at the interface of the insulator, and (ii) the active layer due to the outdiffused metallic arsenic. Reverse gate-drain current degradation experiments were carried out at 120°C, 160°C, 200°C, and 240°C. Transconductance frequency dispersion studies were carried out before and after thermal stress on these MISFET. The transconductance of MISFET with 1000 Å LT-GaAs gate insulators was degraded by 40% at 100 kHz after thermal stress. The rest of the samples exhibited stable characteristics. These results indicate that composition changes had occurred at the interface in thicker LT-GaAs MISFET structures. Thinner LT-layers are ideal for achieving higher transconductance and better thermal stability without sacrificing the power capability of MISFET     

Real-Time monitoring of III-V molecular beam epitaxial growth using spectroscopic ellipsometry

We report the real-time monitoring of monolayer thickness changes in AlAs and GaAs layer growth on rotating GaAs substrates using spectroscopic ellipsometry (SE). A phase-modulated spectroscopic ellipsometer was integrated with a III-V MBE system by triggering spectral acquisition synchronously with substrate rotation. Absolute thickness accuracy was verified using ex situ SE measurement. Reasonable agreement was also obtained between in situ growth rate measurements by SE and reflection high energy electron diffraction. The precision and speed of this method appears suitable for real-time control of quantum devices, such as resonant-tunneling diodes.     

Ultrafast all-optical switching with an asymmetric Fabry-Perotdevice using low-temperature-grown GaAs: material and device issues

For future telecommunications systems to take full advantage of the optical fiber bandwidth, it will be necessary to have components responding at picosecond speeds. The only way currently known to achieve these speeds is using all-optical switching. By using low-temperature-grown GaAs (LT-GaAs) in a compact asymmetric Fabry-Perot device, we have achieved ultrafast all-optical switching with large bandwidth, high contrast ratio, low insertion loss, and low switching energy. In this paper, we discuss the dependence of the switch performance on the mirror bandwidth and reflectivity, and the LT-GaAs layer thickness and growth conditions. We develop guidelines for the optimization of the device design to maximize the bandwidth and contrast ratio     

Mobility of modulation doped AlGaAs/low-temperature MBE-grown GaAs heterostructures

A study of the mobility of a novel modulation doped heterostructure in which the channel region is made of low-temperature molecular beam epitaxially grown GaAs (LT-GaAs) and all other layers are grown at normal temperatures is presented for the first time. The resistivity of the as-grown samples(in- situ annealed) is very high, as is that of single layers of bulk LT-GaAs. However, in the presence of light, the resistivity of the LT-GaAs modulation-doped field effect transistor (MODFET) is significantly lower, facilitating reliable Hall measurements. We speculate that the observed decrease in resistivity of the LT-GaAs MODFET is due to the formation of a two-dimensional electron gas (2DEG) at the heterointerface under illumination. A number of samples grown under different growth conditions were investigated. Mobilities for these samples were found to be in the range of 250 to 750 cm²Vs at 300K and ∼3000 to 5500 cm²Vs at 77K. A first-order computer simulation was implemented to calculate the mobility of the 2DEG using the relaxation-time approximation to solve the Boltzmann equation, taking into account different scattering mechanisms. Scattering by the arsenic clusters and by ionized impurities in the LT-GaAs MODFET channel are found to be the two dominant mechanisms limiting the mobility of the LT-GaAs MODFET samples. Experimental values are in good agreement with theoretical results.     

Photoreflectance Spectroscopy Study of LT-GaAs Layers Grown on Si and GaAs Substrates

The mechanical strains and densities of surface charge states in GaAs layers grown by low-temperature (LT) molecular-beam epitaxy on Si(100) and GaAs(100) substrates are investigated by photoreflectance spectroscopy. Lines corresponding to the fundamental transition (E _g ) and the transition between the conduction band and spin-orbit-split valence subband (E _g + Δ _SO ) in GaAs are observed in the photoreflectance spectra of Si/LT-GaAs structures at 1.37 and 1.82 eV, respectively. They are shifted to lower and higher energies, respectively, relative to the corresponding lines in GaAs/LT-GaAs structures. Comparing the spectra of the Si/LT-GaAs and GaAs/LT-GaAs structures, it is possible to estimate mechanical strains in LT-GaAs layers grown on Si (by analyzing the spectral-line shifts) and the density of charge-carrier states at the GaAs/Si heterointerface (by analyzing the period of Franz–Keldysh oscillations).     

原子层沉积超薄氮化铝薄膜的椭圆偏振光谱法表征

采用原子层沉积(ALD)工艺在硅衬底上生长了35 nm以下不同厚度的超薄氮化铝(AlN)晶态薄膜。利用椭圆偏振光谱法在波长275~900 nm内测量并拟合薄膜的厚度及折射率和消光系数等光学参数。利用原子力显微镜(AFM)表征AlN晶粒尺寸随生长循环次数的变化,计算得到薄膜表面粗糙度并用于辅助椭偏模型拟合。针对ALD工艺特点建立合适的椭偏模型,可获得AlN超薄膜的生长速率为0.0535 nm/cycle,AlN超薄膜的折射率随着生长循环次数的增加而增大,并逐渐趋于稳定,薄膜厚度为6.88 nm时,其折射率为1.6535,薄膜厚度为33.01 nm时,其折射率为1.8731。该模型为超薄介质薄膜提供了稳定、可靠的椭圆偏振光谱法表征。     

Electrical characteristics of low temperature-Al0.3Ga0.7As

In this work, we present electrical characterizations of n⁺ GaAs/low temperature (LT)-Al_0.3Ga_0.7As/n⁺ GaAs resistor structures in which the LT layers are grown at nominal substrate temperatures of 250 and 300°C. The resistivity and V_tfl parameters of these LT-Al_0.3Ga_0.7As layers are compared with those of LT-GaAs and Al_0.3Ga_0.7As grown at a normal growth temperature of 720°C. Low-temperature Al_0.3Ga_0.7As layers exhibit resistivities as high as 10¹² ohm-cm, nearly four orders of magnitude higher than that of LT-GaAs, and V_tfl values as high as 45 V, over twice that of LT-GaAs. We also find that the LT-Al_0.3Ga_0.7As materials grown at 250 and 300°C appear to show opposite and contradictory trends with respect to resistivity and V_tfl. We propose that this result can be explained by residual hopping conduction in the 250°C material. Temperature dependent conductivity measurements confirm the presence of a hopping mechanism in LT-Al_0.3Ga_0.7As grown at 250°C and yield activation energies of 0.77 and 0.95 eV for LT-GaAs and LT-Al_0.3Ga_0.7As, respectively.