MOVPE-grown high CW power InGaAs/InGaAsP/InGaP diode lasers

Al-free InGaAs/InGaAsP/InGaP laser structures grown by MOVPE have yielded new records in performance. CW front-facet-emitted powers of 5.8 W are achieved from optimized double-quantum-well lasers with 100 μm wide stripes and 1 mm long cavity lengths. Devices with a 0.5 mm cavity length exhibit total power conversion efficiencies as high as 59%. A novel facet passivation technique, consisting of laser-assisted deposition of ZnSe, is shown to increase the COD level by 50%. Single-mode, CW output powers of 400 mW are obtained from triple-core ARROW lasers fabricated by a two-step MOVPE growth process.     

InGaAsP/InP Heterostructure Doped by Cobalt

The preparation of a new type of InGaAsP/InP heterostructures for light emitting diodes is presented. The active region of the heterostructure contains thin layers of nondoped InGaAsP (λ=1.3μ) alternating with barrier layers of InGaAsP (λ=1.1μ) doped by cobalt. Properties of prepared heterostructures were tested on edge emitting diodes with the stripe geometry. Curves of the minority carrier lifetime and the optical power versus current are compared with those obtained on classical bulk InGaAsP/InP double heterostructure. It is shown that the new structure is very eligible for construction of high‐ speed noncoherent 1.3μ diodes operating at low pumping current.     

AlGaAs/InGaP interfaces in structures prepared by MOVPE

Al_0.3Ga_0.7As/In_1−xGa_xP structures were prepared by low-pressure MOVPE. Lattice matched and strained ones with top In_1−xGa_xP layers as well as reverse ones with top Al_0,3Ga_0,7As layers were examined. The structures were studied by photoluminescence, X-ray and atomic force microscope (AFM) methods. An additional photoluminescence peak from the Al_0.3Ga_0.7As/In_1−xGa_xP interface was observed in our samples and it was attributed to a type-II band offset. A conduction band offset of 0.121 eV was measured in the Al_0.3Ga_0.7As/In_0.485Ga_0.515P lattice-matched structure and a linear dependence of the conduction band offset on In_1−xGa_xP composition, with a zero offset in the Al_0.3Ga_0.7As/In_0.315Ga_0.685P structure, was determined. The valence band discontinuity had a nearly constant value of 0.152 eV.     

Precise structure control of GaAs/InGaP hetero-interfaces using metal organic vapor phase epitaxy and its abruptness analyzed by STEM

Fabrication of abrupt InGaP on GaAs (InGaP/GaAs) and GaAs on InGaP (GaAs/InGaP) hetero-interfaces has been difficult using metal organic vapor phase epitaxy (MOVPE) due to the exchange of P and As during the fabrication steps. Indium (In) surface segregation during InGaP growth also degrades the abruptness. Here, the MOVPE gas-switching sequence to fabricate atomically abrupt hetero-interfaces was optimized and the effects of this optimization on the hetero-interface abruptness were quantitatively evaluated using the Z-contrast method with scanning transmission electron microscopy (STEM). Results revealed that (a) in the fabrication of InGaP/GaAs hetero-interface, the GaAs top layer should be stabilized using As-source gas supply, and the excess As layer on GaAs should be terminated using an additional supply of Ga species, and (b) in the fabrication of GaAs/InGaP interface, the InGaP layer should be grown using the flow modulation method to suppress In surface segregation. In conclusion, the abruptness of hetero-interfaces of InGaP/GaAs and GaAs/InGaP was improved by using these optimized gas-switching sequences.     

In-situ RHEED study on the effect of light irradiation on Ge/Si heteroepitaxial growth by GeH4 source MBE

Effects of light irradiation on growth processes in the initial stage of growth of Ge films on (100)Si substrates by gas source molecular beam epitaxy using GeH₄ have been investigated by in-situ RHEED observations. It has been found that the growth rate is enhanced by an Ar⁺ ion laser and a D₂ lamp irradiation, and, moreover, that the growth process sequence of the Ge films irradiated by the Ar⁺ laser and D₂ lamp is the same as that without irradiation. The irradiation effect on the photolysis of GeH₄ gas has been examined by mass spectroscopy. The increase of the growth rate is due to the photolysis of GeH₄ and the excitation of substrate surfaces by vacuum-ultraviolet light in the D₂ lamp irradiation, and is due to only the excitation of substrate surfaces in the Ar⁺ laser irradiation.     

RHEED studies of the growth of Si(001) by gas source MBE from disilane

The growth of Si(001) from a gas source molecular beam epitaxy system (Si-GSMBE) using disilane (Si₂H₆) was investigated using reflection high-energy electron diffraction (RHEED). The surface reconstructions occurring between 100 and 775°C were studied as a function of both substrate temperature and surface coverage. We report the first observation of (2x2) and c(4x4) reconstructions during growth at substrate temperatures near 645°C using Si₂H₆. All growth was found to be initiated by the formation of three-dimensional (3D) islands which coalesced at substrate temperatures above 600°C. The surface reconstruction was found to change from a disordered to an ordered (2x1)+(1x2) structure at 775°C via intermediate (2x2) and c(4x4) phases. Thereafter, growth was found to proceed in a 2D layer-by-layer fashion, as evidenced by the observation of RHEED intensity oscillations. This technique has been used, for the first time, to calibrate growth rates during Si-GSMBE. The intensity oscillations were measured as a function of both substrate temperature and incident beam flux. Strong and damped oscillations were observed between 610 and 680°C, in the two-dimensional growth regime. At higher temperatures, growth by step propagation dominated while at lower temperatures, growth became increasingly three-dimensional and consequently oscillations were weak or absent. Similarly, there was a minimum flux limit ( <0.16 SCCM), below which no oscillations were recorded.     

AlGaN/GaN HEMTs regrown by MBE on epi-ready semi-insulating GaN-on-sapphire with inhibited interface contamination

In this work, we show that, by carefully designing the subsurface Fe doping profile in SI-GaN templates grown by MOVPE and by optimizing the MBE regrowth conditions, a highly resistive GaN buffer can be achieved on these epi-ready GaN-on-sapphire templates without any addition of acceptors during the regrowth. As a result, high-quality high electron mobility transistors can be fabricated. Furthermore, we report on the excellent properties of two-dimensional electron gas and device performances with electron mobility greater than 2000 cm²/V s at room temperature and off-state buffer leakage currents as low as 5 μA/mm at 100 V.     

GaSb/GaAs异质结热光伏电池材料的MBE生长

利用分子束外延方法(MBE)在GaAs (001)衬底上外延生长了GaSb薄膜,并对GaSb薄膜进行了高温退火研究,利用高分辨透射电子显微镜(HRTEM)、原子力显微镜(AFM)、Hall效应(Hall Effect)和低温光荧光谱(LTPL)等手段对薄膜的晶体质量、电学性质和光学性质进行了研究.发现直接生长的GaSb膜表面平整,空穴迁移率较高.研究发现30 s、650℃的快速热退火可消除位错等缺陷,显著提高GaSb薄膜的光学质量.     

固态源分子束外延法生长InGaP/GaAs异质结构的热力学分析

本文建立了采用分子束外延法制备InGaP/GaAs异质结构的热力学模型,其中考虑了两个重要的因素,由晶格失配引起的内在应力和InP的脱附.所得到的模型与现有实验结果匹配较好.该模型的实验结果表明在InGaP的生长过程中,生长温度,In/Ga束流比及合金组分之间的相互关系,同时也与实验数据相吻合.该模型对于其他气相沉积生长方式也具有一定的适用性.     

InGaAsP/InP multiple quantum wells grown by gas-source molecular beam epitaxy

We have grown In₁-_xGa_xAs_yP₁-_y/InP multiple quantum well structures with 1.3 μm excitonic absorption at room temperature by gas-source molecular beam epitaxy. In-situ composition determination in GaAs₁-_xP_x and InAs_xP₁-_x was carried out by measuring group-V-induced intensity oscillations of reflection high-energy electron diffraction. Based on the in-situ composition calibration for these ternary end members, Ga and As compositions in the quaternary compound, In₁-_xGa_xAs_yP₁-_y, were controlled successfully. Measurements by X-ray rocking curve, low-temperature photoluminescence and absorption spectroscopy indicate that high-quality In₁-_xGa_xAs_yP₁-_y/InP multiple quantum well samples were obtained.     

The transition layers in AlGaAs/GaAs and InGaAsP/GaAs heterostructures grown by LPE

AlGaAs/GaAs and InGaAsP/GaAs heterostructues were grown by liquid phase epitaxy (T_o = 800–770 °C), the transition layers (TLs) were measured with Auger profiling. The Auger profiling of the InGaAsP/GaAs heterostructures after a long heating (1–6 hours, 770 °C) has been made too. The analysis of the experimental data led to the conclusion that the fundamental values of the TL in above mentioned systems are about 10 and 1 to 2 nm, correspondingly. This difference between the minimum widths of the TL in AlGaAs/GaAs and InGaAsP/GaAs system is connected with the different speed of solid state diffusional processes which occur at the initial stpeps of liquid – solid contact.     

Improvements in a-plane GaN crystal quality by AlN/AlGaN superlattices layers

Non-polar (1 1 2¯ 0) a-plane GaN films have been grown by low-pressure metal-organic vapor deposition on r-plane (1 1¯ 0 2) sapphire substrate. We report on an approach of using AlN/AlGaN superlattices (SLs) for crystal quality improvement of a-plane GaN on r-plane sapphire. Using X-ray diffraction and atomic force microscopy measurements, we show that the insertion of AlN/AlGaN SLs improves crystal quality, reduces surface roughness effectively and eliminates triangular pits on the surface completely.     

ZnO growth on Si with low-temperature ZnO buffer layers by ECR-assisted MBE

High-quality ZnO films were grown on Si(1 0 0) substrates with low-temperature (LT) ZnO buffer layers by an electron cyclotron resonance (ECR)-assisted molecular-beam epitaxy (MBE). In order to investigate the optimized buffer layer temperature, ZnO buffer layers of about 1.1 μm were grown at different growth temperatures of 350, 450 and 550 °C, followed by identical high-temperature (HT) ZnO films with the thickness of 0.7 μm at 550 °C. A ZnO buffer layer with a growth temperature of 450 °C (450 °C-buffer sample) was found to greatly enhance the crystalline quality of the top ZnO film compared to others. The root mean square (RMS) roughness (3.3 nm) of its surface is the smallest, compared to the 350 °C-buffer sample (6.7 nm), the 550 °C-buffer sample (7.4 nm), and the sample without a buffer layer (6.8 nm). X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurements were carried out on these samples at room temperature (RT) in order to characterize the crystalline quality of ZnO films. The preferential c-axis orientations of (0 0 2) ZnO were observed in the XRD spectra. The full-width at half-maximum (FWHM) value of the 450 °C-buffer sample was the narrowest as 0.209°, which indicated that the ZnO film with a buffer layer grown at this temperature was better for the subsequent ZnO growth at elevated temperature of 550 °C. Consistent with these results, the 450 °C-buffer sample exhibits the highest intensity and the smallest FWHM (130 meV) of the ultraviolet (UV) emission at 375 nm in the PL spectrum. The ZnO characteristic peak at 438.6 cm⁻¹ was found in Raman scattering spectra for all films with buffers, which is corresponding to the E₂ mode.     

Alloy phase separation in InAs/InAlAs/InP nanostructure superlattices studied by finite element calculation

The alloy phase separation effect in InAlAs spacer layer of InAs/InAlAs nanostructure superlattices was studied by two-dimensional finite element calculation. The calculation results showed that InAs islands with wide top would prefer to induce “V”-like In-rich InAlAs arms above InAs islands in InAlAs spacer layer, while InAs islands with narrow top would promote the formation of “I”-like In-rich InAlAs arms above InAs islands in InAlAs spacer layer which corresponded well with the experimental results reported in Ref. [9].     

Low dark current In0.53Ga0.47As/InP SAM avalanche photodiodes grown by gas-source MBE

In_0.53Ga_0.47As/InP avalanche photodiodes (APDs) with separate absorption and multiplication (SAM) regions have been grown by gas-source molecular beam epitaxy (GSMBE). These mesa-structure diodes, with a mesa diameter of 150 μm, exhibit a very low dark current of 17 nA, and primary unmultiplied dark current of less than 1.0 nA at 90% of the breakdown voltage. A gain as high as 100 is observed near the breakdown. Good uniformities of punch-through voltage and breakdown voltage were obtained on a 16 mm×16 mm wafer. Also, a premature breakdown was observed in the In_0.53Ga_0.47As/InP SAM-APDs with a V-shaped defect density of 25,000 cm^-2.     

Microampere laser threshold at 80°C with InGaAs/GaAs/InGaP buried heterostructre strained quantum well lasers

An extremely low CW threshold current of 670 μA and a high slope efficiency of 0.14 W/A at a high junction temperature of 80°C were obtained with a 200 μm long Al-free InGaAs/GaAs/InGaP buried heterostructure (BH) quantum well laser grown by three-step metal organic vapor phase epitaxy (MOVPE). The maximum energy conversion efficiency of a 500 μm long laser was as high as 50% at a output power level of 1 mW. Regrowth conditions of InGaP layers were found to be crucial for planarizing the grown surface to realize the high performances.     

Precise thickness measurement within a few monolayers by X-ray diffraction from InGaAs/GaAs strained-layer superlattices

We propose a technique to measure the thickness of a GaAs layer with a precision of a few monolayers (MLs) by high-resolution X-ray diffraction (HRXRD) from InGaAs/GaAs strained-layer superlattices (SLSs) on GaAs substrates. Using this technique to monitor the GaAs growth rate, we have successfully controlled the Ga beam flux within ±1% in molecular beam epitaxy (MBE) growth for continuous 40 runs during four days by increasing the Ga cell temperature to compensate the decrease of the Ga beam flux caused by the consumption of the Ga source. Precise thickness measurements are also demonstrated in the growth on InP substrates by using InAlGaAs/InGaAs SLSs and InAlGaAs/InAlAs SLSs.     

InGaAs/InAlAs in-plane superlattices grown on slightly misoriented (110) InP substrates by molecular beam epitaxy

InGaAs/InAlAs in-plane superlattices (IPSLs) composed of InAs/GaAs and InAs/AlAs monolayer superlattices were grown using molecular beam epitaxy. The substrates were misoriented (110) InP tilting 3° toward the [00 ] direction. We grew half monolayers of AlAs and GaAs and single monolayers of InAs alternately, keeping regular arrays of single monolayer steps. The structures were evaluated by transmission electron microscopy (TEM). In a transmission electron diffraction pattern from the ( 10) cross-section, we observed two types of superstructure spot pairs double-positioned in the [001] direction, indicating the formation of the intended IPSL structures. In a cross-sectional TEM dark-field image, we observed the InGaAs/InAlAs superlattice structures formed almost in the [001] direction. The mean period of the superlattices was approximately 4 nm, which was comparable to the terrace width expected from the substrate tilt angle. However, IPSL structures were not completely formed, i.e., the lateral interfaces meandered along the growth direction, and partial disorderings were often observed. The photoluminescence spectrum from the IPSL had a peak corresponding to the InGaAs (2 nm thick)/InAlAs (2 nm thick) superlattice in addition to a peak corresponding to the In_0.5Al_0.25Ga_0.25As alloy.     

Novel carbon materials obtained by reactions of C60 fullerene with phosphorus at high temperature

C₆₀ fullerene reacted with phosphorus at high temperatures to graphitic materials with strongly differing properties. Several spectroscopic investigations as Raman, ESR, XPS and ³¹P-NMR as well as TEM and ESEM have shown that the structural order of the mostly micro-crystalline materials increased with increasing reaction temperatures and decreasing phosphorus content. At suitable preparation conditions carbon nanotubes are formed, which are disordered in most cases, but partly exhibit high structural order (single-wall nanotubes), too. The prepared materials are very hard, e.g. harder than silica, intensely coloured and electrically conductive. Their hardness decreased with rising phosphorus content, and their conductivity increased with decreasing phosphorus content.     

GaN/GaAs(1 0 0) superlattices grown by metalorganic vapor phase epitaxy using dimethylhydrazine precursor

Superlattices of cubic gallium nitride (GaN) and gallium arsenide (GaAs) were grown on GaAs(1 0 0) substrates using metalorganic vapor phase epitaxy (MOVPE) with dimethylhydrazine (DMHy) as nitrogen source. Structures grown at low temperatures with varying layer thicknesses were characterized using high resolution X-ray diffraction and atomic force microscopy. Several growth modes of GaAs on GaN were observed: step-edge, layer-by-layer 2D, and 3D island growth. A two-temperature growth process was found to yield good crystal quality and atomically flat surfaces. The results suggest that MOVPE-grown thin GaN layers may be applicable to novel GaAs heterostructure devices.