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.     

Dynamic crystallization of II—VI compounds with a gaseous control of vapour concentrations

The method for dynamic crystallization of II—VI compounds with a gaseous control of vapour concentrations in the growth zone is developed. The predeterminated change of the transport gas flows which carry the vapours of the initial elements into the reaction zone together with the change of additional inert gas flows conducted directly to the crystallizer entries gives the posibility to displace the growth zone during the process of synthesis. It results in the considerable increase in crystal number and size and leads to best quality of grown crystals.     

Debye temperature and melting point of II‐VI and III‐V semiconductors

In this paper, simple relations are proposed for the calculation of Debye temperature θ_D and melting point T_mof II‐VI and III‐V zincblende semiconductors. Six relations are proposed to calculate the value of θ_D. Out of these six relations, two are based on plasmon energy data and the others on molecular weight, melting point, ionicity and energy gap. Three simple relations are proposed to calculate the value of T_m. They are based on plasmon energy, molecular weight and ionicity of the semiconductors. The average percentage deviation of all nine equations was calculated. In all cases, except one, it was estimated between 3.34 to 17.42 % for θ_D and between 2.37 to 10.45 % for T_m. However, in earlier correlations, it was reported between 10.59 to 33.38% for θ_D and 6.96 to 14.95% for T_m. The lower percentage deviation shows a significant improvement over the empirical relations proposed by earlier workers. The calculated values of θ_D and T_m from all equations are in good agreement with the available experimental values and the values reported by different workers. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Pyrolysis characteristics of iodine precursors for gas source n-type doping of II–VI compounds

The selection criteria and pyrolysis characteristics of iodine precursors were investigated to determine the best compounds for the n-type doping of CdTe during gas source molecular beam epitaxy. Ethyliodide and allyliodide were found to have the most suitable properties and to produce iodine dimers for pyrolysis temperatures above 650 and 600°C, respectively. The ethyliodide doping of CdTe was studied and produced highly conductive CdTe layers with room temperature electron concentrations as high as 3 × 10¹⁸ cmsu-3 with a mobility of ˜ 460 cm²/V·s.     

Compositional effects accompanying near equilibrium vapour growth of solid solution crystals of the types IV‐VI and II‐VI

Near equilibrium evaporation‐condensation in a sealed ampoule leads to almost full compositional reproduction of a solid solution if it consists of components having comparable vapour pressures; this can be qualitatively interpreted by domination of entropy increase. Nevertheless, even vestigial separation requires closer characteristics, since it may prove crucial – particularly for properties of semiconducting solid solutions. Maximum component separation allowed by a small temperature difference is described here in terms of thermodynamics and kinetics of solid‐vapour and vapour‐solid phase transitions. Theoretical models of the determining effects having different character are shortly described, and their applicability areas are determined. Experimental data collected for crystal growth of numerous semiconducting solid solutions of the II‐VI and IV‐VI type support the conclusion drawn from the models that the near equilibrium crystal growth from the vapour in a closed system ensures the highest degree of compositional uniformity. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

高效大功率LED用蓝宝石图形衬底的制备

首先在蓝宝石衬底上旋涂PMMA/copolymer双层胶,然后再进行电子束光刻处理,随后溅射铝膜,并结合剥离技术,从而制备图形化金属铝膜;最后再采用两步热处理法,使图形铝膜发生固相外延反应.在此过程中,侧重于电子束光刻和高温热处理的工艺优化研究.SEM测试结果表明,图形化铝膜在450℃低温氧化热处理24h后,其图形形貌没有发生明显改变;在温度低于1200℃的高温热处理1h后,图形仍然存在.HRXRD分析表明,图形化铝膜在450℃低温热处理24h后再在1000℃高温热处理1h,可在蓝宝石衬底上形成氧化铝外延薄膜;并且相比于基片,氧化铝外延薄膜的结晶质量还有所提高.本方法成功实现了高效大功率用LED蓝宝石图形衬底的制备.     

Growth and Characterization of Direct-connecting AlGaAs/GaAs TJS Light Emitting Device on SI GaAs Substrate by LPE

High output power (above 3 mW/facet) AlGaAs/GaAs Transverse-Junction Stripe light emitting diodes have been grown on Semi-Insulating (100) GaAs substrates by Liquid Phase Epitaxy. these light emitting diodes utilize a “Direct-connecting” transverse-junction stripe structure, which can confine the transverse-current and reduce the series resistance. By thinning the thickness of the “effective active-layer” of this structure, a room-temperature pulsed lasing operation is also achieved with a threshold current as low as 35 mA and a peak wavelength around 904 nm. This “Direct-connecting” transverse-Junction Stripe light emitting device with a Metal-Semiconductor Field Effect Transistor on an electrical isolated semi-insulating substrate in the future.     

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.     

MOVPE Growth and Characterisation of Zn‐Doped (GaIn)P Layers with Respect to Surface Structure and Ordering

(GaIn)P grown on (001)GaAs substrates by metal‐organic vapour phase epitaxy (MOVPE) is highly ordered material. In this work the effect of Zn doping on the epitaxial crystal growth, the ordering behaviour and the surface morphology is investigated. Zn‐doped (GaIn)P layers, grown with phosphine (PH₃), tertiarybutylphosphine (TBP) and ditertiarybutylphosphine (DitBuPH) as phosphorous precursors, reveal a strong drop of the binary growth rate of InP r_InP with increasing Zn/III ratio, whereas r_GaP remains nearly constant. The Zn doping efficiency and the ordering behaviour are observed to be dependent on the misorientation of the substrates. Finally, the surface morphology as a function of the different parameters was analysed by atomic force microscopy (AFM), and no considerable change of the growth mechanism was found for Zn‐doped layers in comparison to undoped layers.     

Bridgman Growth and Characterization of LuAlO3—Ce3+ Scintillator Crystals

Chemical and structural effects in LuAlO₃-Ce³⁺ single crystals grown by the Bridgman method were studied using spectral emission, x-ray, etching and optical techniques. The Ce concentration distribution was found to exhibit the normal freeze behavior with partition coefficient of 0,17. The expansion of the unit cell volume due to incorporation of Ce was measured. Dislocation etch pits were revealed on (010), (100) and (001) crystal faces. Thermal expansion behavior of LuAlO₃ was studied along the major crystallographic axes in between the room temperature and 1000 °C. The scintillation performance of 5 × 5 × 10 mm³ samples was measured using a 662 keV gamma source with a shaping time of 1.2 μs. The light yield was increasing from 40% BGO to 70% BGO with increasing of the Ce³⁺ content from 0.13 at.% to 0.9 at.%.     

AgGaSe2 on {100} and {110} GaAs—Special Features of Epitaxial Growth

AgGaSe₂ thin epitaxial layers onto {100} and {110} oriented GaAs substrates were prepared by flash evaporation technique and investigated by the RHEED method (reflection high energy electron diffraction). Special epitaxial relationships were found and the results will be discussed.     

Comment on “Debye temperature and melting point of II‐VI and III‐V semiconductors” [Cryst. Res. Technol. 45, No. 9, 920–924 (2010)]

In the present work, we have readjusted some empirical parameters obtained by Kumar et al. in their work which contains some numerical errors.     

Replacement of hydrides by TBAs and TBP for the growth of various III–V materials in production scale MOVPE reactors

Besides the standard group V precursors AsH₃ and PH₃, so-called alternative precursors like TBAs and TBP (tertiary-butyl-arsine and tertiary-butyl-phosphine) are more and more important in today's MOVPE processes. A lot of publications have demonstrated that these precursors can be successfully used for the growth of different III–V materials. In this study we want to demonstrate that TBAs and TBP can be used as the group V precursor in a complete family of production scale reactors. It is shown that these precursors can be used for the growth of InP-based as well as for GaAs-based materials. The reactors that have been employed are medium scale reactors (AIX 200/4; 1 × 2 inch, 3 or 4 inch or 3 × 2 inch capability) and large scale Planetary Reactors®, in particular the AIX 2400 system (15 × 2 inch or 5 × 4 inch). Materials that have been grown are (Al)GaInP on GaAs and GaInAsP on InP. The lower cracking energy of these precursors compared to PH₃ and AsH₃ allows one to use lower growth temperatures and lower V/III ratios, particularly in combination with the high cracking efficiencies of the used reactors. For the growth of GaInAsP on InP, the consumption of TBP and TBAs is up to 8 times lower than using PH₃ and AsH₃. GaInP on GaAs could be grown with a V/III ratio as low as 25 in a Planetary Reactor®. Good crystalline quality is demonstrated by DCXD (e.g. for GaInP: FWHM = 35 arcsec, substrate 32 arcsec). PL intensity and growth rate are not affected by using the alternative precursors. The compositional uniformity is similar to layers grown with arsine and phosphine (e.g. 1.5 nm uniformity for GaInAsP (λ = 1.5 μm) on 2 inch; approximately 1 nm uniformity for GaInP) [1,2]. The purity of the grown layers depends mainly on the quality of the TBP and TBAs. Using high purity TBP, InP revealed background carrier concentration in the mid 10¹⁴ cm⁻³ regime. Our investigation shows that TBP and TBAs can replace phosphine and arsine in state of the art MOVPE reactors. Both for single and multi-wafer production MOVPE reactors these compounds can be used successfully for the growth of the entire material spectrum in the Al---Ga---In---As---P system.     

The Growth and Characterization of Hydrogen (Deuterium) — Bonded Ferroelectric Crystals of Schultenites PbHAsO4, PbDAsO4 PbHPO4 and PbDPO4

Single crystals of lead hydrogenarsenate, lead hydrogenphosphate and deuterated analogues have been grown from water solution, further, by means of various chemical reactions in water solutions and polyacrylamide hydrogel. The growth conditions, morphology of crystals, their imperfections and some physical properties have been studied.     

Growth and characterization of pure and lithium doped L‐alanine single crystals for NLO devices

Single crystals of pure and lithium substituted L‐alanine are successfully grown by slow evaporation method at constant temperature of 32°C. The effect of lithium dopant on crystal properties has been studied. Powder and single crystal XRD analysis confirms the structure and change in lattice parameter values for the doped crystals. The crystals were characterized by solubility studies, density, melting point measurements, FTIR and UV‐Vis‐NIR techniques. Thermal and mechanical stability of crystals were tested by TGA/DTA and micro hardness analysis. NLO activity of the crystals is found to be increased in the presence of lithium ions. The dielectric constant and dielectric loss of the crystals were studied as a function of frequency. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Using a Phase‐field‐like Approach for the Calculation of Melt Flow and Interface Shape during Czochralski Growth

A phase‐field‐like approach is introduced into the commercial general‐purpose program FIDAP^TM to calculate the melt‐crystal interface for a quasi‐stationary approach of the Czochralski growth. Temperature and flow field are solved using the segregated solver of the FIDAP^TM software.     

Numerical Simulation of Temperature and Flow Field in the Melt for the Vapour‐pressure‐ controlled Czochralski Growth of GaAs

The influence of the melt flow on the temperature field and interface during the vapour‐pressure‐controlled growth of GaAs was studied numerically with the commercial general‐purpose program FIDAP^TM. The thermal boundary conditions for the domain of seed, crystal, boron oxide and crucible were taken from a global calculation for an equipment used at the IKZ to grow 6″ crystals. Due to the large melt volume the buoyancy forces become rather strong and have to be counteracted by reasonable rotation rates. Preliminary results have been obtained for iso‐ and counter‐rotation showing that the flow field exhibits structures on small scales. High rotation rates are needed to counteract the buoyancy flow efficiently and to achieve a smooth flat interface. Even if the the flow structure is not resolved in detail, the interface shape can be deduced form the calculations.     

On the Design of Double‐Ellipsoid Mirror Furnace and its Thermal Characteristics for Floating‐Zone Growth of SrxBa1‐xTiO3 Single Crystals

We have designed a double ellipsoid mirror furnace for floating‐zone crystal growth using lamps with rectangular filaments. Its thermal characteristics were studied using an alumina tube for several system configurations. A simple comparison with a commercial furnace that used cylinder lamps for the heating profile was also conducted. By adjusting lamp orientation and positions, one could modify heating profiles easily. In general, the thermal characteristics of the furnace were consistent with the model's prediction [J. Crystal Growth 173 (1997) 561]. The effects of growth chamber and heat pipe were further illustrated. Furthermore, a suitable system configuration leading to better heating uniformity and lower thermal gradients near the growth interface was found for the floating‐zone growth of Sr_xBa_1‐xTiO₃ single crystals.