The effect of AlN buffer layer on properties of AlxIn1 − xN films on glass substrates

Al_xIn_1 − xN (AlInN) films with x = 0.36 and 0.55 were grown on glass substrate by pulsed direct-current reactive sputtering. X-ray photoelectron spectroscopy depth profiles revealed that oxygen diffused from glass substrate to AlInN films at temperatures ≧ 300 °C. After applying AlN buffer layer, the crystallinity of AlInN films was markedly improved without oxygen contamination observed. The AlN-buffered AlInN films are c-axis-oriented with low full-width-at-half-maximum of 2.9°-3.5°, fine-grained, and low electron concentration, which are comparable with AlInN films grown by other high-temperature processes. AlN buffer layer is proved to be good seeding and diffusion-barrier layers for AlInN films deposited on glass substrates.     

Molecular beam epitaxy of GaSb on GaAs substrates with AlSb/GaSb compound buffer layers

GaSb films with AlSb/GaSb compound buffer layers were grown by molecular beam epitaxy on GaAs (001) substrates. The crystal quality and optical properties were studied by high resolution transition electron microscopy and low temperature photoluminescence spectra (PL), respectively. It was found that the AlSb/GaSb compound buffer layers can restrict the dislocations into GaSb epilayers. The intensity of PL spectra of GaSb layer becomes large with the increasing the periods of AlSb/GaSb superlattices, indicating that the optical quality of GaSb films is improved.     

Effect of GaAs substrate misorientation on InxGa1 − xAs crystalline quality and photovoltaic performance

This paper presents the quality of In_xGa_1 − xAs (0 < x < 0.2) layers grown on GaAs substrate with different miscut angle (2° and 15°) by metal organic chemical vapor deposition. The crystalline quality of In_xGa_1 − xAs layers was found to strongly depend on indium content and substrate misorientation. The In_0.16Ga_0.84As solar cells with PN structure were grown on a 2°- and 15°-off GaAs substrates. It was found that the photovoltaic performance of In_0.16Ga_0.84As solar cell grown on 2°-off GaAs substrate was better than that of In_0.16Ga_0.84As grown on a 15°-off GaAs substrate, because the In_xGa_1 − xAs films grown on 15°-off GaAs substrate shows a highly strain relaxation in active layer of solar cell, which causes the high dislocation density at the initial active layer/In_xGa_1 − xAs graded layer interface.     

Growth and electrical characterization of Zn-doped InAs and InAs1 − xSbx

The electrical properties of Zn doped InAs and InAsSb layers grown on semi-insulating GaAs by metal organic vapour phase epitaxy, using dimethyl zinc as the p-type dopant source, have been studied. The influence of dopant flow rate, V/III ratio and substrate orientation on the electrical properties of these InAs and InAs_1 − xSb_x layers have been studied at a few appropriate growth temperatures. A promising group V source, tertiary butyl arsenic was used as an alternative to arsenic hydride in the case of InAs growth. The electrical properties of the InAs and InAs_1 − xSb_x epitaxial layers were mainly studied by the Hall effect. However, surface accumulation in these materials results in deceptive Hall results being extracted. A two layer model (assuming the layer to consist of two parallel conducting paths viz. surface and bulk) has therefore been used to extract sensible transport properties. In addition, conventional Hall measurements ignores the high electron to hole mobility ratio in InAs and InAsSb leading to erroneous transport properties.     

Selective doping of conformal GaAs layers grown by hydride vapour phase epitaxy on Si substrates studied by spatially resolved optical techniques

High quality GaAs layers on silicon substrates were grown by conformal growth. In this technique, the GaAs conformal growth is initiated on the sidewalls of GaAs seed stripes (typically 10 μm wide with a pitch of 200 μm) grown conventionally on a Si substrate, and developed inside a cavity formed by the silicon substrate and an overhanging dielectric cap layer. The conformal layers were grown by hydride vapour phase epitaxy (HVPE) and doped with Si or Zn. The properties of these layers were studied by micro-Raman, cathodoluminescence (CL) and micro-photoluminescence (PL). The free carrier concentration was obtained with a micrometric spatial resolution. Both n-type (Si) and p-type (Zn) doping was achieved, showing the feasibility of selective doping and p–n homojunctions.     

Effect of a ZnSe buffer layer on the surface, structural, and optical properties of the ZnTe/ZnSe/GaAs heterostructures

Lattice-mismatched ZnTe epilayers on GaAs (1 0 0) substrates with and without ZnSe buffer layers were grown by using molecular beam epitaxy. AFM, XRD, and TEM measurements were performed to investigate the surface and structural properties of the ZnTe thin films. Photoluminescence, Raman scattering, and TEM measurements showed that the crystallinity of a ZnTe epilayer grown on a GaAs substrate was remarkably improved by using a ZnSe buffer layer. Photoreflectance measurements showed that the strain of the ZnTe layer with the ZnSe buffer layer was smaller than that without the ZnSe buffer layer. These results indicate that ZnTe epitaxial films grown on GaAs substrates with ZnSe buffer layers hold promise for potential applications in optoelectronic devices operating in the blue-green spectral region.     

Improved growth of GaN layers on ultra thin silicon nitride/Si (1 1 1) by RF-MBE

High-quality GaN epilayers were grown on Si (1 1 1) substrates by molecular beam epitaxy using a new growth process sequence which involved a substrate nitridation at low temperatures, annealing at high temperatures, followed by nitridation at high temperatures, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. The material quality of the GaN films was also investigated as a function of nitridation time and temperature. Crystallinity and surface roughness of GaN was found to improve when the Si substrate was treated under the new growth process sequence. Micro-Raman and photoluminescence (PL) measurement results indicate that the GaN film grown by the new process sequence has less tensile stress and optically good. The surface and interface structures of an ultra thin silicon nitride film grown on the Si surface are investigated by core-level photoelectron spectroscopy and it clearly indicates that the quality of silicon nitride notably affects the properties of GaN growth.     

GaAs metal‐semiconductor‐metal photodetectors (MSM‐PD'S) with AlGaAs cap and buffer layers

Abstract

GaAs metal‐semiconductor‐metal photodetectors with AlGaAs cap and buffer layers have been fabricated and studied. It is shown that the trap‐induced effects which result from the GaAs surface trap states can be avoided by adding an AlGaAs cap layer. In addition, we used an AlGaAs buffer layer to reduce the interfacial charge effects between the GaAs substrate and the GaAs absorption layer. The dark currents were less than 1 nA and the low frequency internal gain was dramatically improved. The results also show that a complete depletion can occur even at biases below 0.5 V.     

Evolution of the optical transitions in AlxGa1 − xAs/GaAs quantum well structures grown on GaAs buffers with different surface treatments by molecular beam epitaxy

Al_0.3Ga_0.7As/GaAs Quantum Well structures were grown by molecular beam epitaxy (MBE) on a 500 nm thick GaAs buffer layer subjected to the following surface processes: a) in-situ Cl₂ etching at 70 °C and 200 °C, b) air-exposure for 30 min. The characteristics of these samples were compared to those of a continuously grown sample with no processing (control sample). We obtained the quantum wells energy transitions using photoreflectance spectroscopy as a function of the temperature (8-300 K), in the range of 1.2 to 2.1 eV. The sample etched at 200 °C shows a larger intensity of the quantum well peaks in comparison to the others samples. We studied the temperature dependence of the excitonic energies in the quantum wells (QWs) as well as in GaAs using three different models; the first one proposed by Varshni [4], the second one by Viña et al. [5], and the third one by Pässler and Oelgart [6]. The Pässler model presents the best fitting to the experimental data.     

Low temperature formation of Si1 − x − yGexSny-on-insulator structures by using solid-phase mixing of Ge1 − zSnz/Si-on-insulator substrates

We proposed the low temperature formation technique of strain-relaxed Si_{1 − x − y}Ge_xSn_y-on-insulator (SGTOI) structures. We found that the solid-phase reaction and the formation of single and uniform Si_{1 − x − y}Ge_xSn_y layer on an insulator after annealing SiO₂/Ge_1 − zSn_z/SOI structures even at a temperature as low as 400 °C. We characterized the crystalline structure of SGTOI, and investigated the effects of annealing, Sn incorporation, and a SiO₂ cap layer on the solid-phase reaction between Ge_1 − zSn_z and SOI layers. The solid-phase reaction is enhanced with a higher Sn content and a thicker SiO₂ cap layer, and then Si_{1 − x − y}Ge_xSn_y layers are more rapidly formed. The SGTOI layer exhibits very low mosaicity and have good crystallinity.     

Explanation of Unusual Photoluminescence Behavior from InAs Quantum Dots with InAlAs Capping

The effect of different kinds of cap layers on optical property of InAs quantum dots (QDs) on GaAs (100) substrate was studied. Temperature dependent photoluminescence (PL) indicates that the PL integrated intensity from the ground state of InAs QDs capped with an intermediate InAIAs layer drops very little as compared to QDs capped with a thin InGaAs or GaAs cap layer from 15 K up to room temperature. PL integrated intensity ratio of the first excited to ground states for InAs QDs capped with an intermediate InAIAs layer is unexpectedly decreased with increasing temperature, which are attributed to phonon bottleneck effect. A virtual barrier is proposed to describe this physics process and shows good agreement with experimental results when fitting the curve with the value of the virtual barrier 30 meV.     

Low-temperature liquid-phase epitaxy growth from Ga–As–Bi solution

Measurements of Ga–As–Bi liquidus composition at 600 °C for Ga–Bi–GaAs range are reported. High quality GaAs layers and Al_0.28Ga_0.72As/GaAs single quantum well (SQW) structures with different well thicknesses are grown from a mixed Ga + Bi solvent by low-temperature liquid-phase epitaxy method. They are characterized by the Hall effect and photoluminescence (PL) measurements. Significant changes in the layer electrical properties are observed with varying the Bi content in the solution. The layers grown from a mixed solution with 20–30% Bi content are n-type with carrier concentration of 10¹⁴ cm^− 3 and mobility of 40 000 cm²/Vs at 77 K, while p-type high resistivity layers are obtained from a solution with more than 70% Bi content. PL spectra of the SQW structures grown from Ga + 25% Bi solution at 2 K show that the roughness at the interface is less than two monolayers.     

90°coupling in (Fe/Cr/Fe)AFM/Cr/Fe system epitaxially grown on GaAs(0 0 1)

Single-crystalline (Fe/Cr/Fe)_AFM/Cr/Fe structures were epitaxially grown on atomically flat GaAs(0 0 1). Choosing the same thickness of the antiferromagnetically (AFM) coupled Fe layers in the bottom (Fe/Cr/Fe)_AFM structure, their net magnetization is balanced to zero, in particular up to a spin-flop transition when the field is applied along the [1 1 0] direction. For the Cr thicknesses at which the top Fe layer is weakly magnetically coupled to the bottom (Fe/Cr/Fe)_AFM structure, at low fields, the magneto-optical Kerr effect and/or SQUID signal from the sample corresponds to the top Fe layer only. An influence of the Cr spin structure on the magnetization reversal in the Fe layer is reported. In particular, a strong increase of coercivity (by a factor of 12) is found at low temperatures. A 90° coupling is detected which affects the minor loops measured along the [−1 1 0] and [1 0 0] directions.     

探究GSMBE制备GaAsBi薄膜中生长条件对Bi浓度的影响

为了探究GaAsBi薄膜生长中生长条件与Bi浓度的关系,我们利用气态源分子束外延(GSMBE)技术,通过改变每个GaAsBi单层的生长温度、AsH_3压和Bi源温度,在半绝缘GaAs(100)衬底上生长GaAsBi的多层结构。通过二次离子质谱(SIMS)及透射电镜X光谱(EDX)测试得出:GaAsBi中Bi的浓度随着生长温度的升高而降低,且生长速率越慢表面偏析和再蒸发严重,可导致Bi浓度下降趋势更明显;Bi浓度随着AsH_3压的升高而减小,在As_2和Ga束流比在0.5~0.8之间几乎成线性变化,远不如固态源MBE敏感;此外,Bi源温度升高,Bi掺入的浓度也会增大,但是当生长温度大于420℃时,Bi就很难凝入。     

Optimizing the spacer layer thickness of vertically stacked InAs/GaAs quantum dots

Vertically stacked multilayers of self-organized InAs/GaAs quantum dots (QDs) structures with different GaAs intermediate layer thicknesses varying between 2.8 and 17 nm are grown by solid source molecular beam epitaxy (SSMBE) and investigated by photoluminescence spectroscopy (PL). For 17 nm thick GaAs spacer, the PL spectra show two well separated features attributed to the formation of two QDs family with a bimodal size distribution indicating no correlation between the dots in different layers. In the meanwhile, the structures having thinner spacer thickness demonstrate single PL peaks showing an enhancement of high energy side asymmetrical broadening when increasing the excitation power. The corresponding emission energies exhibit a red shift when the spacer layer thickness decreases and correlated with the enhancement of the vertical electronic coupling as well as the rise of the QD's size in the upper layers induced by the build up of the strain field along the columns. The spacer thickness of 8.5 nm is found to yield the best optical properties.     

The role of the Zn buffer layers in the structural and photoluminescence properties of ZnO films on Zn buffer layers deposited by RF magnetron sputtering

Highly c-axis oriented ZnO thin films were grown on Si (100) substrates with Zn buffer layers. Effects of the Zn buffer layer thickness on the structural and optical qualities of ZnO thin films were investigated for the ZnO films with the buffer layers 90, 110, and 130 nm thick using X-ray diffraction (XRD), photoluminescence (PL) and atomic force microscopy (AFM) analysis techniques. It was confirmed that the quality of a ZnO thin film deposited by RF magnetron sputtering was substantially improved by using a Zn buffer layer. The highest ZnO film quality was obtained with a Zn buffer layer 110 nm thick. The surface roughness of the ZnO thin film increases as the Zn buffer layer thickness increases.     

Study of GaAs spacer layers in InAs/GaAs vertically aligned quantum dot structures

We investigated vertically aligned InAs/GaAs QD structures, grown by atomic layer molecular beam epitaxy, with a number N of layers and with spacer thicknesses d. QD alignment and structure quality were checked by transmission electron microscopy. The dependencies of carrier capture, decay dynamics and existence of quenching channels on the design parameters N and d were studied by time resolved photoluminescence (PL), PL excitation (PLE) and PL temperature-dependent measurements. Our results show that the carrier capture and the radiative efficiency of the QDs are negatively affected by increasing the number of QD layers and by reducing the spacer thicknesses; this effect is likely to be related to the increase of defect concentrations in GaAs spacers, due to relaxation of an increasingly large strain.     

Improvement of InN layers deposited on Si(111) by RF sputtering using a low-growth-rate InN buffer layer

We investigate the influence of a low-growth-rate InN buffer layer on structural and optical properties of wurtzite nanocrystalline InN films deposited on Si(111) substrates by reactive radio-frequency sputtering. The deposition conditions of the InN buffer layer were optimized in terms of morphological and structural quality, leading to films with surface root-mean-square roughness of ~ 1 nm under low-growth-rate conditions (60 nm/h). The use of the developed InN buffer layer improves the crystalline quality of the subsequent InN thick films deposited at high growth rate (180 nm/h), as confirmed by the narrowing of X-ray diffraction peaks and the increase of the average grain size of the layers. This improvement of the structural quality is further confirmed by Raman scattering spectroscopy measurements. Room temperature PL emission peaking at ~ 1.58 eV is observed for InN samples grown with the developed buffer layer. The crystal and optical quality obtained for InN films grown on Si(111) using the low-growth-rate InN buffer layer become comparable to high-quality InN films deposited directly on GaN templates by RF sputtering.     

Effects of substrate miscut on threading dislocation distribution in metamorphic GaInAs/AlInAs buffers

Metamorphic GaInAs/AlInAs buffers with a total mismatch of 2 % with respect to GaAs have been grown by metal organic chemical vapor deposition on GaAs substrates with miscuts of 2°, 7° and 15° toward (111)A. The buffer grown on 2° miscut substrate exhibits a dislocation pile-up formation which increases the threading dislocation density greatly. However, using 7° and 15° off substrates eliminates the dislocation pile-up and decreases the dislocation density by an order of magnitude compared with that on the 2° substrate miscut, and the best crystalline quality of buffer is obtained on the 15° substrate miscut. Experimental results show that the buffers grown on the 7° and 15° miscut substrate exhibit shallower trenches on the surface and a weaker strain field in the InGaAs cap layers, and these help to suppress the occurrence of dislocation pile-up, leading to a lower threading dislocation density.     

Fabrication of a depletion mode GaAs MOSFET using Al2O3 as a gate insulator through the selective wet oxidation of AlAs

A 1 μm thick undoped GaAs buffer layer, a 1500 Å thick n-type GaAs layer, an undoped 500 Å thick AlAs layer and a 50 Å thick GaAs cap layer were consecutively grown by molecular beam epitaxy (MBE) on a [100] oriented semi-insulating GaAs substrate. The AlAs layer was oxidized in a N₂ bubbled H₂O vapor ambient at 400°C for 3 h and fully converted to Al₂O₃ for use as a gate insulator. The I–V characteristics, having a maximum drain current of 10.6 mA, a current cut-off voltage of −4.5 V and a maximum transconductance value of 11.25 mS/mm, indicate that the selective wet thermal oxidation of AlAs/GaAs was successful in producing a depletion mode GaAs MOSFET.