New aluminium precursors for MOMBE (CBE): a comparative study

Recently different new Al precursors have been developed to improve the electrical and optical quality of AlGaAs layers grown by MOMBE (CBE), since AlGaAs layers still suffer from the high incorporation of oxygen and carbon. Three approaches are introduced and results obtained from Al_xGa_1−xAs layers (0 < x ≤ 1) are discussed. APAH, a double ring structure molecule, was found to yield AlGaAs layers with high contents of carbon and nitrogen. The use of an Alane-adduct decreases impurity concentrations and improves optical properties. However, TIBAl is superior and provides highest PL response together with carrier concentrations below p = 10¹⁶ cm^-3. Even though the concept of coordinative saturation is promising, results achieved by TIBAl showed that trialkyls could also be well suited for AlGaAs, assuming that they are properly synthesized.     

Transmission electron microscopy observation of GaAs/Al0.3Ga0.7As T-shaped quantum well structure fabricated by glancing angle molecular beam epitaxy on GaAs(100) reverse-mesa etched substrates

GaAs/Al_0.3Ga_0.7As multi-layer structures were grown on GaAs (100) reverse-mesa etched substrates by glancing angle molecular beam epitaxy (GA-MBE). A(111)B facet was formed as a side-facet. Surface migration of Ga and Al atoms from the (100) flat region to the (111)B side-facet region has been investigated to fabricate T-shaped GaAs/AlGaAs quantum wells (QWs) under the condition that Ga and Al atoms impinge only an the (100) flat region and do not impinge on the (111)B side-facet. Observation of T-shaped GaAs/AlGaAs quantum wires (QWRs) by cross-sectional transmission electron microscopy (TEM) revealed that there is no migration of Al atoms from the (100) to the (111)B facet region at a substrate temperature (T_s) as high as 630°C, under a V/III ratio of 28 (in pressure ratio). On the other hand, very thin GaAs epitaxial layers grown on the (111)B side-facet region owing to the Ga migration were observed for substrate temperatures of 600 and 630°C. It was found that the mass flow of Ga atoms from the (100) region to the (111)B side-facet region increases, with the thermal activation energy of 2.0 eV, as the substrate temperature increases from 570 to 630°C. The GA-MBE growth on a reverse-mesa etched GaAs substrate at a low temperature 570°C or lower is desirable to fabricate a nm-scale GaAs/AlGaAs QWR structure with nm-scale precision.     

A difference between initial growth stages of the AlGaAs/GaAs and GaAs/AlGaAs heterostructures produced by contact replacement of solutions

The method of liquid epitaxial growth of GaAs/AlGaAs/GaAs heterostructures when the change of solutions occurs due to pushing off one melt by another is discussed. It has been shown theoretically and experimentally that the initial stages of film growth after the change of the binary Ga As melt on the ternary Al Ga As melt differ from that when the Ga As solution pushes off the Al Ga As liquid. The difference is caused by the inequality of the diffusion coefficients of As and Al in a multicomponent Al Ga As liquid (D_Al > D_As). As a result, the growth of an AlGaAs film begins immediately in the case when the Al Ga As solution pushes off the Ga As liquid but in the opposite case the dissolution of an underlying AlGaAs solid is unavoidable and depends little on degree of a saturation of the Ga As washing solution. These peculiarities must be taken into account in discussions of abruptness and other properties of LPE-grown AlGaAs/GaAs and GaAs/AlGaAs heterojunctions.     

Growth of heavily C-doped GaAs/AlGaAs MQW structures by MOVPE for 2–3 μm normal incidence photodetectors

The growth and intersubband optical properties of high quality heavily doped p-type GaAs/AlGaAs multiple quantum well (MQW) structures are reported. The MQWs were fabricated by the atmospheric pressure metalorganic vapor phase epitaxy process using liquid CCl₄ to dope the wells with C acceptors (N_a ≈ 2 × 10¹⁹ cm⁻³). A constant growth temperature was maintained for the entire structure while different V/III ratios were used for the well and barrier regions. By this process it is possible to achieve both high C doping densities in the wells and to simultaneously obtain good quality AlGaAs barriers. Fourier transform infrared spectroscopy measurements on heavily doped 10-period MQW structures reveal a new absorption peak at 2 μm with an effective normal incidence absorption coefficient of 4000 cm⁻¹. Photocurrent measurements on mesa-shaped diodes show a corresponding peak at 2.1 μm. The photodiodes exhibit a symmetrical current-voltage characteristic and a low dark current, which are indicative of a high quality MQW structure and a well-controlled C doping profile. The 2 μm absorption represents the shortest wavelength ever reported for any GaAs/AlGaAs or InGaAs/AlGaAs MQW structure and should be very useful for implementing multicolor infrared photodetectors.     

In-situ photoluminescence and capacitance-voltage characterization of InAlAs/InGaAs regrown heterointerfaces by molecular beam epitaxy

This paper characterizes the electronic properties of InAlAs/InGaAs molecular beam epitaxy (MBE) regrown interfaces by combined use of in-situ photoluminescence surface state spectroscopy (PLS³) and capacitance-voltage (C-V) techniques. It is shown that the interface state density at the continuously grown InAlAs/InGaAs interface is low and comparable with that of the uninterrupted AlGaAs/GaAs interface, and that the effect of the growth interruption was surprisingly small, whereas the same interruption resulted in almost 10² times reduction of the PL efficiency for the AlGaAs/GaAs system. It is also shown that a high density of surface states exists at the MBE InGaAs surface. Interruption after the growth of the bottom InAlAs layer in the InAlAs/InGaAs/InAlAs system also leads to appreciable generation of interface states, but it is much smaller as compared with the case of the AlGaAs/GaAs/AlGaAs system.     

Se-doped AlGaAs grown on GaAs(111)A by molecular beam epitaxy

The electrical properties of Se-doped Al_0.3Ga_0.7As layers grown by molecular beam epitaxy (MBE) on GaAs(111)A substrates have been investigated by Hall-effect and deep level transient spectroscopy (DLTS) measurements. In Se-doped GaAs layers, the carrier concentration depends on the misorientation angle of the substrates; it decreases drastically on the exact (111)A surface due to the re-evaporation of Se atoms. By contrast, in Se-doped AlGaAs layers, the decrease is not observed even on exact oriented (111)A. This is caused by the suppression of the re-evaporation of Se atoms, by Se---Al bonds formed during the Se-doped AlGaAs growth. An AlGaAs/GaAs high electron mobility transistor (HEMT) structure has been grown. The Hall mobility of the sample on a (111)A 5° off substrate is 5.9×10⁴ cm²/V·s at 77 K. This result shows that using Se as the n-type dopant is effective in fabricating devices on GaAs(111)A.     

Low temperature growth of AlGaAs by MOMBE (CBE) using trimethylamine alane

In order to gain further insight into the surface chemistry of AlGaAs growth by metalorganic molecular beam epitaxy, we have investigated the deposition behavior and material quality of AlGaAs grown at temperatures from 350 to 500°C using trimethylamine alane (TMAA), triethylgallium (TEG) and arsine (AsH₃). Though the Al incorporation rate decreases with decreasing temperature, Ga-alkyl pyrolysis, and hence Ga incorporation rate, declines more rapidly. Thus the Al content increases from X_AlAs = 0.25 at 500°C to X_AlAs = 0.57 at 350°C. Below 450°C, the Ga incorporation rate appears to be determined by the desorption of diethylgallium species, rather than interaction with adsorbed AlH₃. Similarly, carbon incorporation is enhanced by 2 orders of magnitude over this temperature range due to the increasingly inefficient pyrolysis of the Ga-C bond in TEG. Additionally, active hydrogen from the TMAA1, which normally is thought to getter the surface alkyls, is possibly less kinetically active at lower growth temperatures. Contrary to what has been observed in other growth methods, low growth temperatures produced a slight decrease in oxygen concentration. This effect is likely due to reduced interaction between Ga alkoxides (inherent in the TEG) and the atomic hydrogen blocked Al species on the growth surface. This reduction in oxygen content and increase in carbon concentration causes the room temperature PL intensity to actually increase as the temperature is reduced from 500 to 450°C. Surprisingly, the crystalline perfection as measured by ion channeling analysis is quite good, χ_min≤5%, even at growth temperatures as low as 400°C. At 350°C, the AlGaAs layers exhibit severe disorder. This disorder is indicative of insufficient Group III surface mobility, resulting in lattice site defects. The disorder also supports our conclusions of kinetically limited surface mobility of all active surface components.     

Heterointerface study of perturbed LPE growth of AlGaAs/InGaP single heterostructure

This paper presents the perturbed growth of Al_0.7Ga_0.3As/In_0.5Ga_0.5P single heterostructure on a GaAs substrate by liquid-phase epitaxy. The AlGaAs-InGaP heterointerface was characterized by scanning electron microscopy, photoluminescence, Auger electron spectroscopy, and transmission electron microscopy. Evidence is provided showing that a small amount of droplets, after the slider operation of the In_0.5Ga_0.5P epitaxial growth, mixed with the Ga-rich AlGaAs melt, is sufficient to attack the In_0.5Ga_0.5P underlying layer. Even with complete melt removal, there is still a partial dissolution at the “flat” Al_0.7Ga_0.3As-In_0.5Ga_0.5P heterojunction. The Auger depth profiles reveal the composition-depth transition width at this interface to be 560Å from the 90%-10% of Al (or Ga, As, and In) Auger profile; however, the P atoms penetrate deeply into the Al_0.7Ga_0.3As layer due to the partial dissolution of In_0.5Ga_0.5P layer. By high-resolution electron-micrograph analysis, some dislocations are observed at the heterojunction leading to nonradiative recombination and to poor optical device performance, even though the heterointerface observed by scanning electron microscopy is very flat.     

Fabrication of GaAs quantum wires by metalorganic molecular beam epitaxy and their optical properties

We have grown GaAs quantum wires having nominal cross-sectional dimensions of 20×20 nm² buried in AlGaAs layers, by lateral metalorganic molecular beam epitaxy on the terraced sidewalls of mesa-grooved ( ) substrates. In the photoluminescence spectrum of this sample at 77 K, a dominant emission has been observed at a peak wavelength of 780 nm which corresponds to a blue shift of 80 meV from the GaAs bulk transition. Emission spectroscopy from different positions and imaging by cathodoluminescence have demonstrated that this emission was generated from the sidewalls, indicating that it originates from the quantum wire.     

Growth of carbon-doped base GaAs/AlGaAs HBT by gas-source MBE using TEG, TEA, TMG, AsH3, and Si2H6

High-performance carbon-doped-base GaAs/AlGaAs heterobipolar transistors (HBTs) were grown by gas-source MBE using only gaseous sources including dopant sources. The AlGaAs emitter layer was doped with Si from uncracked SI₂H₆ (n = 9 × 10¹⁷ cm^-3), and the base layer (92.5 nm) was doped with carbon from TMG (p = 4 × 10¹⁹ cm^-3). From SIMS analysis it was confirmed that a well-defined emitter-base junction with sharp carbon profile was obtained. The base-current ideality factor from the Gummel plot was 1.47, and the emitter-base junction ideality factor was 1.12. A high DC current gain of 53 was obtained at a current density of 4 × 10⁴ A/cm². The device characteristics of our carbon-doped HBTs were found to be stable under current stress.     

Influence of decomposition parameters on agglomeration process and total soda content in precipitated Al(OH)3

Grain size composition of precipitated Al(OH)₃ is dependent on the mechanism of decomposition process of the caustic solution which determines crystal growth process, agglomeration process and secondary nucleation. Because the literature data shows that the growth rate is very low, the agglomeration process plays an important role in increasing the initial particle size. On the other hand, the agglomeration process enables the inclusion of impurities by the grain boundary of sticking Al(OH)₃ particles, above all, the inclusion of soda Na₂O.

In this paper we investigate the influence of caustic soda concentration Na₂O_(c), that is the supersaturation of the solution, seed charge and seed grain size on the agglomeration and secondary nucleation processes and total soda content in precipitated Al(OH)₃. The results have shown that the factor which causes the increase of the agglomeration process also causes the increase of total soda content in precipitated Al(OH)₃.     

CBE growth of high-quality AlGaAs/GaAs heterostructures for HEMT applications

AlGaAs/GaAs heterostructures were grown by chemical beam epitaxy using triethylgallium, triisobutylaluminium and pure arsine in flow control mode with hydrogen as carrier gas. For substrate temperatures of 580°C and V/III ratios of 10, high quality AlGaAs layers are obtained; heterostructures show abrupt and smooth interfaces. Modulation doping with silicon evaporated from a conventional effusion cell gives two-dimensional electron gases with carrier densities up to 1×10¹² cm^-2. Mobilities of 70000 cm²/V·s are obtained at 77 K for carrier densities of 4×10¹¹ cm^-2. The lateral homogeneity of the heterostructures in layer thickness, composition and doping level is excellent. Perfect morphology with defect densities of about 100 cm^-2 is observed. High electron mobility transistors (gate length 0.3 nm) fabricated from quantum well structures show a transconductance of about 380 mS/mm.     

Molecular beam epitaxy growth and properties of GaAs/(AlGa)As p-type heterostructures on (100), (011), (111)B, (211)B, (311)B, and (311)A oriented GaAs

We report on a series of Be-doped GaAs/AlGaAs two-dimensional hole gas (2DHG) structures grown on (110), (111)B, (211)B and (311)B oriented substrates and compare their properties with high-mobility samples grown on (311)A using Si doping. The samples were prepared and grown under the same conditions so as to render them comparable. They are found to have mobilities which are strongly anisotropic within the plane. The highest mobility is found on the (110) surface with 100,000 cm² V⁻¹ s⁻¹, while the (211) surface gave the lowest values 10,000 cm² V⁻¹ s⁻¹. However, the later samples are found to have quantum Hall effect critical currents of >70 μA: an exceptionally high value for a hole gas which makes them suitable for metrology. All the samples show strong low-field positive magnetoresistance with resistance increases of up to 30% at magnetic fields of only 0.1 T. The presence of this feature on all the different planes shows that it does not depend upon the details of the band structure. It is identified with the lifting of the degeneracy of the spin sub-bands by the asymmetrical potential giving rise to a classical two-band magnetoreresistance.     

Strained AlGaInP quantum wire lasers

We report on the fabrication of strained separate-confinement heterostructure AlGaInP visible laser diodes with multiple quantum wire active regions formed in situ during gas source molecular beam epitaxy. Lateral composition modulation with a period on the order of 100 å perpendicular to the growth direction occurs spontaneously during the growth of the (GaP)₂/(InP)₂ short-period superlattice active region by the strain-induced lateral-layer ordering (SILO) process. Individual laser diodes fabricated from the same wafer but emitting parallel and normal to the wire axis exhibit highly anisotropic properties, including large difference in threshold current, emission wavelength, and opposite polarizations. These effects are explained in terms of the strained quantum wire potential. The additional degree of quantum confinement in quantum wire structure reduces the threshold current density by a factor of >2.7 when compared with an otherwise identical quantum well laser.     

以溶液法PEDOT∶PSS作为空穴传输层的免光刻背接触硅太阳电池

对PEDOT∶PSS(聚(3,4亚乙二氧基噻吩)-聚(苯乙烯磺酸))薄膜与Mg、Al和Ag三种金属接触后的I-V特性曲线进行了测试分析,发现Mg和Al与PEDOT∶PSS薄膜接触后呈现高电阻特性,可以起到绝缘隔离层的作用。在此基础上,以PEDOT∶PSS作为空穴传输层,以LiF作为电子传输层,以PEDOT∶PSS与Mg/Al的接触作为隔离层,不采用光刻工艺,设计制备了只需一次掩膜工艺的背接触太阳电池。通过在PEDOT∶PSS上采用热丝氧化升华技术制备MoO_x层,通过优化LiF薄膜的厚度,在抛光硅片上初步实现了开路电压最高为592 mV和效率最高为10.13%的背接触太阳电池。采用金属辅助腐蚀制备硅纳米线陷光结构改善前表面陷光效果,得到了开路电压为587 mV,短路电流密度为35.57 mA/cm²,填充因子为69.97%,效率为14.61%的背接触太阳电池。     

β-Ga2O3晶体金刚石线锯切割的表面质量研究

本文探究了往复式金刚石线锯的工艺参数对β-Ga₂O₃单晶沿(001)晶面切片时表面质量的影响,从压痕断裂力学理论角度探究了金刚石线锯切割β-Ga₂O₃单晶过程中磨粒行为和材料去除机理。实验从各向异性角度分析了切割方向对(001)面β-Ga₂O₃单晶切割片表面质量的影响,并采用SEM和SJ-210粗糙度测试仪探究了工艺参数对金刚石线锯切割后的晶片表面质量的影响。实验结果表明,增大锯丝速度或减小材料进给速度都能降低亚表面损伤层深度及表面粗糙度,有效改善晶片表面质量。     

Strong enhancement of the optical and electrical properties, and spontaneous formation of an ordered superlattice in(111)B AlGaAs

Strong enhancement in the luminescence intensity is observed in Al_0.22Ga_0.78As epitaxial layers grown on misoriented (111)B GaAs as compared to those simultaneously grown on (100) GaAs. For a 1° misorientation the luminescence intensity is almost 1° to 1000 times that of the (100) layers, depending on the growth temperature. Room temperature electron mobility for 3° misoriented (111)B Al_0.18Ga_0.82As is 19% higher than that for side-by-side grown (100). The strong luminescence associated with a large red shift of 90 meV and the 19% mobility enhancement are related to the long range composition ordering in (111)B AlGaAs, which is observed by cross-sectional transmission electron microscopy in a 280 å Al_0.4GaAs quantum well heterostructure with Al_0.7GaAs barriers grown on (111)B GaAs substrates.     

Influence of initial growth parameters on the structural and optical properties of GaAs on (001) Si

The structural and optical properties of GaAs on (001) Si substrates were investigated by transmission electron microscopy (TEM) and low-temperature photoluminescence (PL). It was found that the success of the two-step growth technique is controlled by the quality (morphology and defect density) of the low-temperature grown AlGaAs nucleation layer. GaAs epilayers grown on low V/III ratio AlGaAs nucleation layers exhibit improved surface morphologies and structural properties. These results were confirmed by optical measurements where it was shown that the best PL response was obtained from GaAs epilayers in which the initial AlGaAs nucleation layers were deposited at a low V/III ratio.     

Atomic structure of Al55(Cr1−xMnx)15Si30 metallic glasses observed by neutron diffraction

An atomic structure of Al₅₅(Cr_1−xMn_x)₁₅Si₃₀ (x = 0, 0.49,1) metallic glasses was studied by neutron diffraction. An advantage of the neutron diffraction technique was fully exploited by utilizing the negative scattering length of Mn to form a neutron zero scattering ‘alloy’ for the component Cr_0.51Mn_0.49 in this quaternary Al---(Cr, Mn)---Si alloy. This allows the atomic distribution of the resulting quasibinary Al---Si metallic glass to be derived directly. Moreover, the (Al, Si)---TM (TM = Mn, Cr) and TM---TM pair correlations were also extracted by taking appropriate linear combinations of the atomic structures for the Al₅₅(Cr_1−xMn_x)₁₅Si₃₀ (x = 0, 0.49, 1) metallic glasses. A sharp first peak in the (Al,Si) ---TM pair correlations thus obtained led to the conclusion that a strong attractive interaction exists between (Al, Si) and TM atoms and, hence, that the presence of the TM atoms is responsible for the formation of an amorphous phase.     

Optical anisotropy of(11N) and vicinal(001) quantum wells

We investigated the in-plane optical anisotropy of (11N) quantum wells both experimentally and theoretically. A high-quality GaAs/AlGaAs quantum-well structure was grown on a (113)A substrate by molecular beam epitaxy. Polarized photoluminescence measurements at 77 K revealed an in-plane anisotropy of heavy-hole and light-hole excitonic transitions in this sample. The experimental results were compared with the polarization anisotropy of the optical matrix elements which were calculated within the Luttinger-Kohn scheme with a 4×4 Hamiltonian. It is shown that the observed anisotropy in the (113) quantum well, as well as the recently-reported in-plane optical anisotropy in vicinal (001) quantum wells, can be well explained as being due to the polarization dependence of the optical matrix elements.