Characterization of GaN using thermally stimulated current and photocurrent spectroscopies and its application to UV detectors

Deep levels in insulating GaN grown by metalorganic vapor phase epitaxy (MOVPE) have been studied using thermally stimulated current (TSC) and photocurrent (PC) spectroscopies. Five main levels (0.11, 0.24, 0.36, 0.53 and 0.62 eV) were observed by TSC measurements in the as-grown undoped GaN. PC measurements showed three deep levels located within bandgap at 1.32, 1.70 and 2.36 eV, respectively. We found that three of the levels, located at 0.24, 0.36 and 0.53 eV, were eliminated by annealing at 1000°C under N₂ for six hours, whereas the 0.62 eV level density increased after annealing. In addition, both the responsivity and on/off times of GaN metal-semiconductor-metal (M---S---M) detectors degrade with increasing concentration of the 0.62 eV trap. We have also found that this trap can be effectively reduced by increasing the ammonia flow rate during the MOVPE growth. Accordingly, a high responsivity ( 3200 A/W) UV detector with an improved response time, from 8 to 0.4 ms, was fabricated on GaN grown under the optimized conditions.     

Surface analysis of different oriented GaAs substrates annealed under bismuth flow

Several orientations of GaAs substrates, including (1 0 0), (4 1 1), (1 1 1) and (5 1 1) have been annealed in a metalorganic vapour phase epitaxy (MOVPE) horizontal reactor at different annealing temperatures and under different trimethyl-bismuth (TMBi) flux. Surface morphology of the annealed GaAs substrates was investigated by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results show islands formation on all the studied samples. The density and size of Bi islands vary greatly with annealing temperature and TMBi flow. For different substrate orientations, the activation energies were deduced from Arrhenius plot of island density. Except for (5 1 1) oriented GaAs, all the studied orientations show the same activation energy of 1.8 eV. For low annealing temperature 420 °C, and under different Bi flux, each oriented substrate shows a specific behaviour. For higher temperatures 700 °C and above Bi islands are totally removed and the substrates are smooth. Surface change of (1 0 0) oriented GaAs substrate was in situ monitored by laser reflectometry.     

Progress in Modeling of III-Nitride MOVPE

This review provides an introduction to III-Nitrides MOVPE process modeling and its application to the design and optimization of MOVPE processes. Fundamentals of the MOVPE process with emphasis on transport phenomena are covered. Numerical techniques to obtain solutions for the underlying governing equations are discussed, as well as approaches to describe multi-component diffusion for typical regimes during MOVPE. Properties of common industrial MOVPE reactor types like close spaced showerhead reactors, rotating disk reactors and Planetary Reactors are compared in terms of underlying working principles and generic process parameter dependencies.The main part of the paper is devoted to reviewing gas phase and surface reaction mechanisms during MOVPE. The process design in particular for MOVPE of III-Nitrides is determined by complex gas phase reaction kinetics. Advances in the modeling and predicting of these processes have contributed to understanding and controlling these phenomena in industrial scale MOVPE reactors. Detailed kinetics and simplified surface kinetic approaches describing the incorporation of constituents into multinary solid alloys are compared and a few application cases are presented. Differences in thermodynamic and kinetic properties of multi-layered structures of different compositions such as InGaN, AlGaN can cause enrichment of the adsorbed layer by certain group III atoms (indium in case of InGaN and gallium in case of AlGaN) that translate into specific features of composition profiles along the growth direction.An intrinsic feature of III-nitride materials is epitaxial strain that shows up in different forms during growth and affects both deposition kinetics and material quality. In case of InGaN MOVPE there is a strong interplay between indium content and strain that has direct influence on distribution of material composition in the epitaxial layers and multi-layered structures. Epitaxial strain can relax via different routes such as nucleation and evolution of the extended defects (dislocations), layer cracking and roughening of the surface morphology. Simulation approaches that address coupling of growth kinetics with strain and defect dynamics are discussed and exemplified.     

New dopant precursors for n-type and p-type GaN

Tetraethylsilane (TeESi) and bis(ethylcyclopentadienyl)Mg (ECp₂Mg) were employed as Si and Mg dopant precursors for MOVPE growth of n-type and p-type GaN films, respectively. In Si doping, the electron concentration was observed to increase with the increase of the TeESi flow rate. The temperature dependence of the Hall mobility showed good agreement with n-type GaN films grown using different dopant precursors (SiH₄, GeH₄, Si₂H₆). The donor activation energy was estimated to be 27 meV, which is almost the same as the literature values. In Mg doping, we also found that the Mg concentration increases as the ECp₂Mg flow rate increases. All of Mg-doped samples in this study showed p-type conduction after annealing. The acceptor activation energy was estimated to be 170 meV, which was close to the reported values.     

In situ surface passivation of III–V semiconductors in MOVPE by amorphous As and P layers

A new process for chemical passivation of III–V semiconductor surfaces in metalorganic vapour phase epitaxy (MOVPE) is developed. A passivation layer is deposited directly after growth in the reactor. It consists of amorphous arsenic or a double-layer package of amorphous phosphorus and arsenic, which are grown by photo-decomposition of the group-V hydrides. These layers (caps) serve to protect the surfaces against contamination in air after removing the samples from the MOVPE growth reactor. Such passivation is applicable e.g. for a two-step epitaxy or for further surface characterizations.     

Correlation of ordering formation and surface structure in (GaIn)P using modulated MOVPE

Modulated metalorganic vapour phase epitaxial growth (MOVPE) is used to clarify the role of the surface conditions on the ordering behaviour in ternary (GaIn)P layers. The alternating deposition of GaP and InP layers with individual thicknesses of up to one monolayer is successfully used for the growth of (GaIn)P bulk layers lattice matched to (100) GaAs substrates with various off-orientations. The layer quality and the degree of ordering are investigated using high-resolution X-ray diffraction (XRD), transmission electron microscopy (TEM), and photoluminescence spectroscopy (PL), respectively. The application of modulated growth conditions for the deposition of (GaIn)P bulk layers has a strong influence on the degree of ordering achieved in the intermediate growth temperature regime where the highest degree of ordering occurs under continuous MOVPE. Beside a new boundary structure observed in layers grown under modulated flux conditions, the successful growth of highly ordered (GaIn)P layers grown using the modulated MOVPE technique support the model that up to 2 monolayers of the (GaIn)P growth surface are involved in the ordering formation process.     

Thick GaN layers on sapphire with various buffer layers

Thick GaN layers deposited in HVPE system on composite substrates made on sapphire substrates in Metalorganic Vapour Phase Epitaxy (MOVPE) system have been investigated. The following substrates were used: (00.1) sapphire substrates with AlN, AlN/GaN and GaN thin layers. The crystallographic structure and the quality of the epitaxial thick GaN layers were determined. Comparison of the three types of thick layers was performed. Significant differences were observed. It was found that thick GaN deposited on the simplest MOVPE‐GaN/sapphire composite substrate has comparable structure's properties as the other, more complicated. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

MOVPE Growth of Wide Band-Gap II—VI Compounds for Near-UV and Deep-Blue Light Emitting Devices

We report on the growth by metalorganic vapour phase epitaxy of high structural and optical quality ZnS, ZnSe and ZnS/ZnSe multiple quantum well (MQW) based heterostructures for applications to laser diodes operating in the 400 nm spectral region. High purity ^tBuSH, ^tBu₂Se and the adduct Me₂Zn:Et₃N were used as precursors of S, Se and Zn, respectively. The effect of the different MOVPE growth parameters on the growth rates and structural properties of the epilayers is reported, showing that the crystallinity of both ZnS and ZnSe is limited by the kinetics of the incorporation of Zn, S and Se species at the growing surface. Very good structural and optical quality ZnS and ZnSe epilayers are obtained under optimized growth conditions, for which also dominant (excitonic) band-edge emissions are reported. The excellent ZnS and ZnSe obtained by our MOVPE growth matches the stringent requirements needed to achieve high quality ZnS/ZnSe MQWs. Their structural properties under optimized MOVPE conditions are shown to be limited mostly by the formation of microtwins, a result of the intrinsic high lattice mismatch involved into the ZnS/ZnSe heterostructure. Despite the large amount of defects found, the optical quality of the MQWs turned out to be high, which made possible the full characterization of their electronic and lasing properties. In particular, photopumped lasing emission up to 50 K in the 3.0 eV energy region are reported for the present MQWs heterostructures under power excitation density above 100 kW/cm².     

石墨衬底上铝诱导法制备多晶硅薄膜

利用磁控溅射技术在石墨衬底上制备了石墨/a-Si/Al和石墨/Al/a-Si叠层结构,采用常规退火(CTA)和快速热退火(RTA)对样品进行退火,系统研究了不同退火条件对多晶硅薄膜制备的影响.利用X射线衍射(XRD),拉曼光谱(Raman)对制备的多晶硅薄膜进行表征,并利用谢乐公式计算了晶粒尺寸,结果表明制备的多晶硅薄膜具有高度(111)择优取向,结晶质量良好,利于后续外延制作多晶硅厚膜电池.基于实验结果,建立了铝诱导晶化模型,很好的解释了实验现象.     

MOVPE of GaN using a specially designed two-flow horizontal reactor

GaN epilayers have been grown on (0001) sapphire substrates with a specially designed two-flow horizontal metalorganic vapor phase epitaxy (MOVPE) reactor. Epilayers with flat and smooth surfaces were obtained at the growth temperature of 950°C with relatively low source supply rates. This indicates a relatively high growth efficiency of the reactor. Characterization by photoluminescence, X-ray diffraction and Hall measurements reveal that the epilayers are of reasonably high quality.     

Metalorganic chemical vapor epitaxy and doping of ZnMgSSe heterostructures for blue emitting devices

Blue-green semiconductor laser diodes operating at room temperature are still the domain of wide bandgap II–VI compound semiconductors. CW operation at room temperature and hours of lifetime were reported. However, the conductivity control, defect generation and the ohmic contacts still need improvement. Therefore we focused our work on the MOVPE growth and the optimization of ZnMgSSe/ZnSSe/ZnSe heterostructures as well as on nitrogen doping of ZnSe. To verify the layer quality characterization was carried out by X-ray diffraction, electron probe microanalysis, electrical measurements and photoluminescence. ZnMgSSe/ZnSSe/ZnSe and ZnSSe/ZnSe quantum wells and superlattices were grown to investigate structural as well as interface properties. Electron beam and optical pumping was used to clarify the laser mechanism and to clarify the suitability of a MOVPE process to grow laser quality material. The electrical compensation of ZnSe doped with nitrogen is still controversially discussed whereas high n-type doping with chlorine was reproducible achieved. ZnSe: N doped at different growth conditions (II/VI ratio, growth temperature, nitrogen supply) using N₂ excited in a plasma source or by the use of nitrogen containing precursors was investigated to study the compensation mechanisms.     

In situ processing of III–V semiconductors: Mile stones and future prospects

In situ processing combined with metalorganic vapor phase epitaxy (MOVPE), molecular beam epitaxy, or chemical beam epitaxy appears to be an attractive method for fabricating sophisticated optoelectronic devices such as buried heterostructure lasers, vertical cavity surface emitting lasers, and photonic integrated circuits. Successful reduction of residual contaminants at the regrowth interface and improvement in the optical and electrical quality of the regrown layer has been achieved by using in situ processing techniques. Device fabrication is alrady taking advantage of this kind of technology. Nevertheless, interface quality between an in situ etched layer and a regrown layer has not yet reached the status of continuously grown interfaces. In this paper, progress of in situ processing is reviewed mainly focusing on our recent studies on in situ HCl gas etching in MOVPE. The approach of two-step HCI gas etching has proven superior to obtain clean regrowth interfaces, leading to the conclusion that the in situ processing can be widely used for advanced optoelectronic device fabrication.     

Behaviour of vicinal InP surfaces grown by MOVPE: exploitation of AFM images

InP substrates and epilayers grown by MOVPE have been studied by AFM. For different misorientation angles, we observed the surface of the substrate after annealing under phosphine (PH₃) and of the epilayers under different growth conditions such as growth temperature T_g and trimethylindium (TMI) partial pressure. After annealing the terrace width corresponds to the nominal value of misorientation angle measured by X-ray diffraction. We observed different topographies and roughnesses for the grown layers corresponding to different growth modes. We propose, taking into account the roughness of the surface, a calculation of the step height and terrace width. For 2D nucleation (θ ≤ 0.2° and T_g = 500°C) and step flow mode, the roughness is low while it is increased by step bunching (θ ≥ 0.5° and T_g ≥ 580°C). Moreover we have examined the surface morphology for different misorientation angles and determined the influence of growth conditions (growth temperature, indium partial pressure) on the growth mechanism. At T_g = 580°C the increase of the TMI partial pressure in the reactor enhances the step bunching and leads to larger terraces.     

Alternative precursors for MOVPE growth of InN and GaN at low temperature

We report on the use of dimethylhydrazine (DMHy) and tertiarybuthylhydrazine (TBHy), as alternative nitrogen precursor for GaN low-temperature growth, as well as to improve the InN growth rate. Lowering the GaN growth temperature, would allow growing InN/GaN heterostructures by MOVPE, without damaging the InN layers. Increasing the low InN MOVPE growth rate is of major importance to grow reasonably thick InN layers. In this respect, triethylindium (TEIn) was also used as an alternative to trimethylindium (TMIn).     

半绝缘磷化铟中与非化学配比有关的深能级缺陷

在不同的化学配比条件下制备了半绝缘磷化铟材料,其中包括配比和富铟熔体中的铁掺杂以及磷气氛和磷化铁气氛下高温退火非掺杂晶片.在这些半绝缘磷化铟材料中检测到了与非化学配比有关的深能级缺陷.通过对大量的原生掺铁和非掺退火半绝缘磷化铟材料中的缺陷的研究,发现原生深能级缺陷与材料的电学参数质量密切相关.迁移率低、热稳定性差的掺铁半绝缘磷化铟材料中有大量的能级位于0.1～0.4eV之间的缺陷.高温退火非掺磷化铟抑制了这些缺陷的产生,获得了迁移率高、均匀性好的高质量半绝缘材料.根据这些结果,我们提出了一种通过控制化学配比制备高质量半绝缘磷化铟材料的方法.     

Growth monitoring by reflectance anisotropy spectroscopy in MOVPE reactors for device fabrication

Reflectance anisotropy spectroscopy (RAS) has proved its capability to study surface processes during metalorganic vapour phase epitaxy (MOVPE) growth of a variety of III–V compounds. However, these investigations up to now have been mostly restricted to specialized research reactors. Therefore, we studied the feasibility of in-situ monitoring by RAS during growth on two production-type MOVPE reactors: horizontal 2 inch single wafer reactor AIX 200 and Planetary Reactor™ AIX 2000 for 5 × 3 inch. The slight modifications of the reactors necessary to gain normal incidence optical access to the sample do not alter the properties of the grown materials. While in the horizontal reactor the strain-free optical window allows one to obtain well-resolved RAS spectra the signals in the multiwafer reactor are affected by the anisotropy of the ceiling plate. Even in this case RAS spectra can be extracted. First measurements on rotating samples in the horizontal reactor demonstrate the possibility to obtain RAS spectra by multitransient spectroscopy. As an application monitoring of the growth of p-type layers for the base of GaInP/GaAs hetero-bipolar-transistors (HBTs) is discussed. The linear electro-optic effect (LEO) gives information on doping type and doping level. Time-resolved transients at specific energies are used to study the impact of different switching schemes on the properties of the base-emitter interface.     

Growth of InN films and nanostructures by MOVPE

Since a few years, a lot of research efforts have been devoted to InN, the least known of the semiconducting group-III nitrides. Most of the samples available today have been grown using the molecular beam epitaxy technique, and fewer using the metal organic vapor phase epitaxy (MOVPE) method. Whatever the method, the growth of InN is extremely challenging, in particular due to the fact that no lattice matched substrate is available.     

MOVPE of II–VI materials

The development of II–VI MOVPE is reviewed, contrasting the narrow bandgap materials with the wide bandgap. Common issues are the need to grow the layers at lower temperatures than their III–V cousins in order to avoid point defects. This means that II–VI MOVPE occurs in a surface kinetic regime for precursor decomposition and has stimulated a lot of research on alternative precursors. The narrow bandgap II–VI growers have settled on dimethyl cadmium (DMCd) combined with diisopropyl telluride (DIPTe) and a liquid Hg source but wide bandgap growers are split between pyrolytic and photo-assisted growth. Recent progress in p-type doping has enabled the demonstration of some new devices, including two colour infrared detectors and the first MOVPE grown green emitting laser structure. The common theme appears to be hydrogen passivation of the Group V dopant and some novel precursor solutions to this problem are discussed.     

Large scale manufacturing of compound semiconductors by MOVPE

As more compound semiconductor devices reach large volume manufacturing levels, a trend toward the use of the MOVPE technique is clear. In this paper we examine the criteria needed for MOVPE equipment suitable for large scale manufacturing. We find that although uniformity and device performance are necessary, reproducibility is also critical, along with high throughput and low operating costs. These points are illustrated by actual examples including MMIC power amplifiers, HB-LEDs, and solar cells. A realistic COO model provides a tool for evaluating MOVPE systems of different capacities. In situ control of key parameters during growth is now feasible, and will become an important method for increasing reproducibility and throughput. Lastly we look at the prospects for automation, for decreasing labor costs as well as wafer handling. This is likely to first have an impact on systems for the growth of electronic device structures on large (100 and 150 mm) wafers.     

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).