In-situ Al0.24Ga0.24In0.52As surface cleaning procedure using hydrogen radicals for molecular beam epitaxy regrowth

In-situ hydrogen radical treatment in a processing chamber attached to a molecular beam epitaxy (MBE) system is successfully applied for the native oxide removal from AlGaInAs surfaces exposed to air prior to MBE regrowth. The influence of cleaning conditions is studied on regrown GaInAs layers and AlGaInAs/GaInAs single quantum well (SQW) structures. At optimum cleaning conditions no deterioration of the quality of the treated material appears. In addition, reduction of the interface carbon concentration is achievable.     

Highly reproducible and defect-free MOVPE overgrowth of InGaAsP-based DFB gratings

The paper reports on a study aiming to develop a highly reproducible process for the MOVPE overgrowth of first-order gratings made by reactive ion etching in InGaAsP. MOVPE parameters were elaborated, which guarantee both nearly perfect preservation of the gratings and an almost defect-free surface of the regrown InP layer. Whereas the former goal calls for a low growth temperature, the latter was found to be achievable only above a critical growth temperature depending on the growth rate adjusted. As a good compromise, a regrowth starting temperature of 550°C and an extremely high V/III ratio in the initial stage of regrowth have been chosen. Furthermore, a well-adjusted concentration of AsH₃ has been added during the heat-up cycle.     

Low-threshold strained multi-quantum well lasers fabricated by selective metalorganic vapor phase epitaxy without a semiconductor etching process

Strained InGaAsP multi-quantum well (MQW) buried hetero- (BH) laser diodes (LDs) on a p-InP substrate were fabricated by selective metalorganic vapor phase epitaxy (MOVPE). In the laser fabrication process, both the strained MQW active layer and current blocking structure were directly formed by selective MOVPE without a semiconductor etching process. This novel laser fabrication process produces extremely uniform device characteristics that are essential to the deployment of optical subscriber systems. Furthermore, important device design parameters (e.g. the active stripe shape or the leakage current path configuration) are precisely controlled by only the epitaxial growth steps. This highly controllable laser fabrication method results in a very low-threshold current with excellent uniformity (I_th = 1.78 ± 0.19 mA) for 20 consecutive LDs (L = 200 μm with 70%–90% coatings).     

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.     

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.     

Metalorganic vapor phase epitaxy of III–V-on-silicon: Experiment and theory

The integration of III–V semiconductors with Si has been pursued for more than 25 years since it is strongly desired in various high-efficiency applications ranging from microelectronics to energy conversion. In the last decade, there have been tremendous advances in Si preparation in hydrogen-based metalorganic vapor phase epitaxy (MOVPE) environment, III–V nucleation and subsequent heteroepitaxial layer growth. Simultaneously, MOVPE itself took off in its triumphal course in solid state lighting production demonstrating its power as industrially relevant growth technique. Here, we review the recent progress in MOVPE growth of III–V-on-silicon heterostructures, preparation of the involved interfaces and fabrication of devices structures. We focus on a broad range of in situ, in system and ex situ characterization techniques. We highlight important contributions of density functional theory and kinetic growth simulations to a deeper understanding of growth phenomena and support of the experimental analysis. Besides new device concepts for planar heterostructures and the specific challenges of (001) interfaces, we also cover nano-dimensioned III–V structures, which are preferentially prepared on (111) surfaces and which emerged as veritable candidates for future optoelectronic devices.     

In situ mesa etching and immediate regrowth in a HVPE reactor for buried heterostructure device fabrication

Mesa etching in a hydride vapour-phase epitaxy (HVPE) reactor has been studied. Etched depth, underetching and shape of the mesas have been analysed as a function of partial pressures of active gases (HCl, PH₃ and InCl), stripe orientation and etching temperature. The experimental results show that the depth and undercut can be etched independently. We propose qualitative mechanisms for etching each of the emerging crystallographic planes ((0 0 1), (1 1 0) and {1 1 1}). In situ mesa etching with immediate regrowth was applied to the fabrication of buried heterostructure Fabry–Perot lasers. No surface contamination due to exposure to ambient and low process time are advantages of this technique.     

Growing of bulk crystals and structuring waveguides of fluoride materials for laser applications

We report on recent progress in the synthesis, the crystal growth and the epitaxial growth of fluoride and other laser materials. Results on the fabrication of single crystalline waveguides for dielectric down - and upconversion lasers pumped by semiconductor diode lasers are summarized. Epitaxial growth (molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), pulsed laser deposition (PLD)) and surface modifying techniques (high energy ion implantation, ion diffusion) have been applied in several laboratories. Progress in techniques fabricating optical waveguides from glassy media is addressed as well. Particular emphasis is given on the structuring (wet etching, chemical polishing, ion beam etching) of fluoride crystals for the purpose of obtaining 2-D and 1-D optical waveguides. Results on the structuring of LiYF₄ by wet and ion beam etching are reported. With respect to laser action, the generation of short wavelength light by upconversion (UC) processes, stimulated Raman scattering (SRS) and second harmonic generation (SHG) is discussed. Reports on the first crystalline waveguide lasers of fluoride crystals LiYF₄ and LaF₃, both doped with neodymium, are presented.     

Defect reduction in GaN epilayers and HFET structures grown on (0 0 0 1)sapphire by ammonia MBE

The quality of GaN epilayers grown by molecular beam epitaxy on substrates such as sapphire and silicon carbide has improved considerably over the past few years and in fact now produces AlGaN/GaN HEMT devices with characteristics among the best reported for any growth technique. However, only recently has the bulk defect density of MBE grown GaN achieved levels comparable to that obtained by MOVPE and with a comparable level of electrical performance. In this paper, we report the ammonia-MBE growth of GaN epilayers and HFET structures on (0 0 0 1)sapphire. The effect of growth temperature on the defect density of single GaN layers and the effect of an insulating carbon doped layer on the defect density of an overgrown channel layer in the HFET structures is reported. The quality of the epilayers has been studied using Hall effect and the defect density using TEM, SEM and wet etching. The growth of an insulating carbon-doped buffer layer followed by an undoped GaN channel layer results in a defect density in the channel layer of 2×10⁸ cm⁻². Mobilities close to 490 cm²/Vs at a carrier density of 8×10¹⁶ cm⁻³ for a 0.4 μm thick channel layer has been observed. Growth temperature is one of the most critical parameters for achieving this low defect density both in the bulk layers and the FET structures. Photo-chemical wet etching has been used to reveal the defect structure in these layers.     

Epitaxial growth on optical gratings for distributed feedback GaAs injection lasers

Good quality epitaxial overgrowth of Al_0.3Ga_0.7As on corrugated GaAs and Al_0.12Ga_0.88As surfaces has been achieved by molecular-beam epitaxy. The electrical properties of the interface appear to be equivalent to those prepared by LPE without growth interruption. The corrugations were third order Bragg gratings of ～0.37 μm period and ～0.20 μm depth and were formed by ion milling. Separate confinement heterostructure injection lasers with these periodic corrugations in the optical cavity have demonstrated lasing behavior characteristic of distributed feedback. They have operated with room-temperature threshold current densities as low as 2.2 kA/cm². These results suggest that the MBE overgrowth is a useful technique for the fabrication of integrated optoelectronic structures which include active devices.     

Effective reduction of bowing in free-standing GaN by N-face regrowth with hydride vapor-phase epitaxy

Free-standing GaN films prepared with hydride vapor-phase epitaxy (HVPE) technique usually show bowing resulting from the high densities of defects near the N-polar face after separation from the original substrates. To solve the problem, a simple technique has been developed. A GaN layer was regrown on the N-polar face of the free-standing GaN by HVPE. High-resolution X-ray diffraction (HRXRD) measurements were performed to compare the bowings among GaN films before laser lift-off (LLO), after LLO, and after regrowth. The apparent reductions of XRD full-width at half-maximum (FWHM), along with the increase of XRD peak intensity, after regrowth clearly demonstrate the effectiveness of this method to eliminate bowings of the free-standing GaN films.     

Selectively embedded growth by chemical beam epitaxy for the fabrication of InGaAs/InP double-heterostructure lasers

In order to fabricate InGaAs/InP double-heterostructure (DH) lasers, a novel selectively embedded one-step growth by chemical beam epitaxy (CBE) was adopted. Before the selective CBE growth, 6–8 μm wide channels on an n-InP substrate were undercut by wet chemical etching through a 170 nm thick SiO₂ film mask. A 6 μm wide stripe-geometry DH laser structure with an active layer of 0.14 μm thickness was grown selectively with good planarity into the channels and operated by a pulse.     

Characterization of p-type ZnSe grown by MOVPE; excitonic emission lifetime measurements

Time-resolved photoluminescence (TRPL) measurements are made on p-type nitrogen-doped ZnSe grown by photoassisted metalorganic vapor phase epitaxy (MOVPE) together with post-growth thermal annealing, in order to investigate optical quality of the layers. It is suggested that the annealing degrades the layer quality and the MOVPE samples have more non-radiative recombination centers compared with MBE samples. A key issue for high quality p-ZnSe by MOVPE seems to be optimization of annealing conditions.     

自支撑氮化硅膜结构制备工艺优化

自支撑氮化硅膜结构一般是基于微纳加工技术来制备的。为了提高膜结构的品质,本文分别对自支撑氮化硅膜结构制备中的干法刻蚀参数和各向异性湿法腐蚀参数进行了研究和优化,其中干法刻蚀参数主要包括反应气体配比和刻蚀时间,各向异性湿法腐蚀参数主要包括腐蚀剂浓度和腐蚀温度。在不同的参数组合下进行实验,使用光学显微镜观察并比较不同样品的表面形貌,得到了较理想的参数组合。在干法刻蚀的反应气体中加入少量O₂可改善刻蚀效果,反应气体配比V(SF₆)∶V(CHF₃)∶V(O₂)=6∶37∶3,刻蚀时间2 min。湿法腐蚀中腐蚀剂在质量分数25%处达到最大的硅腐蚀速率,同时氮化硅表面形貌也较理想。     

Computational studies of the transient behavior of horizontal MOVPE reactors

Dynamic models of the mass transfer in horizontal reactors used for metalorganic vapor phase epitaxy (MOVPE) of compound semiconductors have been developed and used to study the duration of transients on the substrate during the growth of heterostructures. Dispersion of reactants during gas switching can be detrimental to the abruptness of the interfaces. A solution to this problem is the operation of MOVPE reactors at very high flow rates and low pressures, but this leads to low conversions of the expensive precursors. Our simulations indicate that critical Péclet numbers can be identified, beyond which no significant reduction in the duration of transients on the substrate occurs when increasing the gas velocity. The substrate always reached a new steady state much faster than the entire reactor. Symmetry of filling and purging with respect to transients was also observed. Performance diagrams connecting optimal operating conditions with reactor geometry have been constructed. Time-dependent models of the MOVPE process can help identify optimal operating conditions and reactor shapes leading to abrupt interfaces with maximum precursor utilization.     

Growth reactions and mechanisms in chemical beam epitaxy (CBE)

The rapidly increasing interest in chemical beam epitaxy (CBE) and related ultra-high vacuum (UHV)-based epitaxial growth techniques has arisen primarily as a result of their expected technological advantages compared to either the conventional molecular beam epitaxy (MBE) or metalorganic vapour phase epitaxy (MOVPE) processes. The CBE-related techniques do, however, provide the additional important advantage, compared to MOVPE, that the UHV environmental allows in-vacuo analytical techniques to be used to provide in-situ characterization regarding the composition and crystallography of the growing layers, and also vital information regarding the growth mechanisms involved. The present paper reviews the current understanding relating to III–V CBE reaction mechanisms, and highlights specific topics which require further investigation.     

Highly reflective AlGaAsSb/InP Bragg reflector at 1.55 μm grown by MOVPE

The metalorganic vapor-phase epitaxy growth of a highly reflective 24-pair AlGaAsSb/InP-distributed Bragg reflector (DBR) is reported for the first time. The influence of the growth parameters such as the V/III input ratio, the growth temperature and the pressure, the total H₂ flow, the gas velocity and the switching sequence of the source gases at the interfaces has been deeply investigated and optimized to achieve stable growth conditions. The DBR achieves a reflectivity as high as 99.5% around 1.55 μm, a uniform stable composition, and an excellent crystal quality over the 2 inch wafer, with a surface free of crosshatch and a defect density below 1/cm². For the optical characterizations, measurements of linear and nonlinear reflectivity, transmission, pump-probe and photoluminescence were done. The interfaces and bulk layers of InP/AlGaAsSb/InP heterostructures were analyzed by transmission electron microscopy. High resolution X-ray diffraction measurements were used to determine the composition shift in the growth plane of the DBR. The measurements show the high quality of the growth and demonstrate that thick AlGaAsSb/InP heterostructures can be grown by metalorganic vapor-phase epitaxy (MOVPE), and in particular DBRs above 1.31 μm.     

Effects of pressure and temperature on RD detected growth oscillations

We report on the vacuum chemical epitaxy (VCE) growth of GaAs from triethylgallium and arsine at varying partial pressures of arsine and hydrogen. In situ, monolayer growth oscillations were, for the first time, detected in a hydrogen environment using reflectance difference (RD). These results offer the possibility to link surface mechanisms occuring during chemical beam epitaxy (CBE) with those taking place in metalorganic vapour phase epitaxy (MOVPE) and may lead to the observation of growth oscillations also during MOVPE. Finally, the behaviour of the RD signal as a function of substrate temperature is studied over a wider temperature interval than has previously been reported, giving further information about surface processes.     

Material optimisation for AlGaN/GaN HFET applications

An optimisation of some growth parameters for the epitaxy of AlGaN–GaN based heterostructure field effect transistors (HFET) at low pressure in a new 3 _* 2″ MOVPE reactor is presented. Some possible processes for the growth of semi-insulating buffers have been identified and are described. TEM analysis shows that the insulating character is not due to a high density of dislocations, whereas SIMS analysis shows that classical impurity (Si, O and C) concentrations are in the same range as in conductive undoped layers. Further studies are needed to identify the traps responsible for the compensation of the GaN layers. The properties of the two-dimensional electron gas (2DEG) located at the AlGaN–GaN interface can be tuned by modifying the characteristics of the AlGaN layer and of the insulating buffer. The best mobility (1500 cm² V⁻¹ s⁻¹ for n6×10¹² cm⁻²) is obtained when using a thick buffer layer, whereas the sheet carrier density is found to increase with the Al content in the undoped supply layer and reaches 1.1×10¹³ cm⁻² for a composition of 24%.     

Growth of freestanding GaN using pillar-epitaxial lateral overgrowth from GaN nanocolumns

Dislocation-free and strain-free GaN nanopillars, grown on Si by molecular beam epitaxy, were used as nanoseeds for a new form of epitaxial lateral overgrowth (ELO) by metalorganic vapour phase epitaxy (MOVPE) until full coalescence. Such overgrown GaN films are almost relaxed and were used as templates for producing thick GaN layers by halide vapour phase epitaxy (HVPE). The final GaN film is easily separated from the starting Si substrate. This is henceforth a new technology to produce freestanding GaN. The GaN crystal quality was assessed by transmission electron microscopy (TEM), photo- and cathodoluminescence (PL, CL). It was seen that the pillar-ELO is produced from a limited number of nanopillars. Some dislocations and basal stacking faults are formed during the coalescence. However, those that propagate parallel to the substrate do not replicate in the top layer and it is expected that the thickened material present a reduced defect density.