High quality GaN grown by MOVPE

Several nitrogen precursors have been used for the growth of GaN in MOVPE, but so far the best results were obtained using NH₃, even though NH₃ does not produce a significant amount of active species at the growing interface. To produce active species from N₂ or NH₃, a remote plasma-enhanced chemical vapour deposition (RPECVD) process has been implemented. In addition, nitrogen metalorganic precursors, triethylamine and t-butylamine, were also used. To accurately control the critical parameters of the MOVPE of GaN, we have implemented a laser reflectometry equipment, which allows a real-time in situ monitoring of the different steps of the growth, i.e. nitridation of the substrate, nucleation, heat treatment, and deposition. Using an appropriate buffer layer, GaN grown on sapphire using NH₃ as nitrogen precursor, shows sharp low temperature photoluminescence lines (4 meV at 9 K), whereas other nitrogen precursors did not lead to comparable electronic quality.     

Incorporation of phosphorus during gas phase epitaxy

The PH₃ desorption rate can be reduced and the decomposition rate increased, thereby increasing the P incorporation efficiency by replacing TMIn with InCl created by the pyrolysis of DEIn. InCl generated by cracking DEInCl and uncracked PH₃ could be used for CBE growth of InP provided that H on the PH₃ can be used to remove Cl from the InCl. Evidence is presented that this is possible. Evidence is also provided that P is more readily incorporated during OMVPE growth using TBP than PH₃ because PH₂ is a primary pyrolysis product, and it is less likely to desorb and more likely to decompose than PH₃.     

Two-level growth of potassium niobate KNbO3 single crystals: a new growth method for ABO3-type materials

Based on our recent study on the directional growth of KNbO₃ single crystals, a new growth scheme called the two-level growth (TLG) has been developed. By using the TLG method, high quality transparent KNbO₃ single crystals can be prepared with high reproducibility. The technique includes two growth stages. In level-one growth, a dimensionally defined (shaped) KNbO₃ seed with high crystal perfection is fabricated. The target crystal with desired dimensions and an architectural structure can be obtained in the second level of the growth. A process flow chart for the TLG method has been presented and six representative types of the growths have been introduced. Generally, crystals grown by TLG have much less crystal imperfections than those obtained from the conventional way. Technological merits of the TLG scheme have been studied, indicating that this growth scheme can also be employed to other oxide perovskite ABO₃ single crystal growths.     

Enhanced two-dimensional growth of MOVPE InN films on sapphire (0 0 0 1) substrates

MOVPE growth of InN on sapphire substrates is compared using two different designs of horizontal reactor. The major difference between the two designs is a variation in the reactant-gas flow-spacing between the substrate and the ceiling of the quartz chamber: 33 mm for the Type A reactor and 14 mm for Type B. Compared with the Type A reactor, the Type B reactor brings about InN films with a larger grain size. This is especially true when InN is grown at 600°C using the Type B reactor, in which case the two-dimensional (2D) growth of InN is found to be extremely enhanced. An investigation of the NH₃/TMIn molar ratio dependence of the surface morphology of grown InN films using the two reactors suggests that the enhanced 2D growth is attributed to the decrease in the effective NH₃/TMIn ratio in the growth atmosphere. Even using the Type A reactor, a film with enhanced 2D growth can be obtained when the NH₃/TMIn ratio is considerably low (1.8×10⁴). The enhanced 2D growth results in a smaller XRC-FWHM (full-width at half maximum of the X-ray rocking curve) (1500 arcsec), than that for a 3D-grown film (5000 arcsec).     

Growth of SiGe bulk crystals with uniform composition by utilizing feedback control system of the crystal–melt interface position for precise control of the growth temperature

We developed an automatic feedback control system of the crystal–melt interface position to keep the temperature at the interface constant during growth, and demonstrate its successful application to grow Ge-rich SiGe bulk crystals with uniform composition. In this system, the position of the crystal–melt interface was automatically detected by analyzing the images captured using in situ monitoring system based on charge-coupled-devices camera, and the pulling rate of the crucible was corrected at every 1 min. The system was found to be effective to keep the crystal–melt interface position during growth even when the variation of the growth rate is quite large. Especially, the interface position was kept for 470 h during growth of Ge-rich SiGe bulk crystal when we started with a long growth melt of 80 mm. As a result, a 23 mm-long Si_0.22Ge_0.78 bulk crystal with uniform composition was obtained thanks to the constancy of the growth temperature during growth through the control of the interface position. Our technique opens a possibility to put multicomponent bulk crystal in a practical use.     

ZrO2对Ho∶(Y0.7Sc0.3)2O3激光陶瓷致密化和光学性能的影响

2 μm 波段处于人眼安全波长,在医疗、加工、红外探测与对抗,以及大气环境监测等军、民两用领域有着重要潜在和实际应用。Ho³⁺掺杂倍半氧化物陶瓷具有宽的吸收和发射光谱、高热导率以及低声子能量等优点,是一类重要的 2 μm 波段激光材料。通过材料固溶原理,可以实现光谱更加宽化,这使其有可能成为一类性能优异的中红外固体激光材料。本文以商业Y₂O₃、Sc₂O₃以及Ho₂O₃粉体为原料,添加少量ZrO₂(原子比为0~1.0%)作为烧结助剂,采用真空预烧,结合热等静压烧结的工艺,成功制备出高透明的0.5%Ho∶(Y_0.7Sc_0.3)₂O₃陶瓷。研究了ZrO₂掺杂浓度(0~1.0％)对Ho∶(Y_0.7Sc_0.3)₂O₃激光陶瓷致密化过程和光学性能的影响。通过添加ZrO₂有效抑制了高温下Ho∶(Y_0.7Sc_0.3)₂O₃陶瓷晶粒的生长,掺杂1.0％ZrO₂的Ho∶(Y_0.7Sc_0.3)₂O₃陶瓷经1 690 ℃下真空预烧结4 h和1 600 ℃/190 MPa热等静压烧结3 h后,其透过率在1 100 nm处达到79.1％(厚度为4.4 mm),接近理论透过率。     

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.     

A comparative study of GaAs- and InP-based HBT growth by means of LP-MOVPE using conventional and non gaseous sources

Low-pressure metalorganic vapor phase epitaxy (LP-MOVPE) growth of carbon doped (InGa)P/GaAs and InP/(InGa)As heterojunction bipolar transistors (HBT) is presented using a non-gaseous source (ngs-) process. Liquid precursors TBAs/TBP for the group-V and DitBuSi/CBr₄ for the group-IV dopant sources are compared to the conventional hydrides AsH₃/PH₃ and dopant sources Si₂H₆/CCl₄ while using TMIn/TEGa in both cases. The thermal decomposition of the non gaseous sources fits much better to the need of low temperature growth for the application of carbon doped HBT. The doping behavior using DitBuSi/CBr₄ is studied by van der Pauw Hall measurements and will be compared to the results using Si₂H₆/CCl₄. Detailed high resolution X-ray diffraction (HRXRD) analysis based on 004 and 002 reflection measurements supported by simulations using BEDE RADS simulator enable a non-destructive layer stack characterization. InGaP/GaAs HBT structures designed for rf-applications are grown at a constant growth temperature of T_gr=600°C and at a constant V/III-ratio of 10 for all GaAs layers. P-type carbon concentrations up to P = 5·10¹⁹cm⁻³ and n-type doping concentrations up to N = 7·10¹⁸cm⁻³ are achieved. The non self-aligned devices (A_E = 3·10 μm²)_show excellent performance, like a dc-current gain of B_max = 80, a turn on voltage of V_offset = 110 mV (Breakdown Voltage V_CEBr,0 > 10 V), and radio frequency properties of f_T/f_max = 65 GHz/59 GHz.

In the non-gaseous source configuration the strong reduction in the differences of V/III-ratios and temperatures during HBT structure growth enable easier LP-MOVPE process control. This is also found for the growth InP/InGaAs HBT where a high dc-current gain and high transit frequency of f_T= 120 GHz are achieved.     

Use of V/III ratio to produce heterostructures in ordered GaInP

Compositional ordering in Ga_0.52 In_0.48 P results in a significant reduction in bandgap energy. The degree of order is known to be a strong function of the input partial pressure of the P precursor during growth due to the effect of this parameter on the surface reconstruction. This raises the possibility of producing heterostructures by simply changing the flow rate of the P precursor during growth. The change in bandgap energy at order/disorder (O/D) heterostructures formed by decreasing the P partial pressure during the OMVPE growth cycle is graded over several thousands of Å when PH₃ is used as the P precursor. Examination of the transmission electron microscope image and the photoluminescence (PL) spectrum reveals that the ordered structure in the lower layer persists far into the upper layer. Similarly, disorder/order (D/O) structures produced in this way yield PL spectra indicative of graded composition at the heterostructure when the P precursor is PH₃. The abruptness is not affected by interruptions in the growth cycle for as long as one hour. Similar heterostructures produced using tertiarybutylphosphine (TBP) as the P precursor are distinctly different. Both D/O and O/D heterostructures can be produced by abruptly increasing the TBP flow rate during the growth cycle. PL studies show two distinct peaks closely corresponding to those observed for single layers grown using these conditions. Surface photoabsorption spectroscopy was used to monitor the transition in surface reconstruction. The change was found to be limited by the dynamics of the mass flow controller. The only difference detected which might explain the difference in behavior between PH₃ and TBP is that atomic force microscopy scans show the layers grown using TBP are covered by monolayer and bilayer (6 Å in height) steps. Growth under similar conditions using PH₃ produces bunched steps, much larger than 6 Å in height.     

Influence of phosphine flow rate on Si growth rate in gas source molecular beam epitaxy

As reported by other authors, we have also observed that the Si growth rate decreases with increasing phosphine (PH₃) flow rate in gas source Si molecular beam epitaxy using phosphorous (P) as a n-type dopant. Why small quantity PH₃ can affect Si growth rate? Up to now, the quantitative characterization of PH₃ flow influence on Si growth rate is little known. In this letter, the PH₃ influence will be analyzed in detail and a model considering strong P surface segregation and its absorption of hydrogen will be proposed to characterize the effect.     

Growth of cadmium mercury thiocyanate dimethylsulphoxide single crystal for laser frequency doubling

This paper reports the growth of an organometallic nonlinear optical (NLO) complex crystal, Cadmium mercury thiocyanate dimethyl-sulphoxide (CdHg(SCN)₄(H₆CP₂OS)₂, CMTD), which was grown from aqueous solution by a temperature lowering method. Its powder SHG intensity is higher than that of CMTC. The crystal size of CMTD can reach 25×23×15mm³. The solubility curve, and solubility variation with pH value are also reported. The growth habits of crystals of CMTD grown under different conditions are discussed and the optimized condition for the growth CMTD using a temperature-lowering method from aqueous solution are also discussed.     

Ambient-condition growth of superconducting YBa2Cu4O8 single crystals using KOH flux

YBa₂Cu₄O₈ is a stoichiometric oxide superconductor of T_c80 K. Unlike YBa₂Cu₃O_7−δ, this compound is free from oxygen vacancy or twin formation and does not have any microscopic disorder in the crystal. Doping with Ca raises its T_c to 90 K. The compound is a promising superconductor for technological application. Up to now, single crystals have not been grown without using specialized apparatus with extremely high oxygen pressure up to 3000 bar and at over 1100 °C due to the limited range of reaction kinetics of the compound. This fact has delayed the progress in the study of its physical properties and potential applications. We present here a simple growth method using KOH as flux that acts effectively for obtaining high-quality single crystals in air/oxygen at the temperature as low as 550 °C. As-grown crystals can readily be separated from the flux and exhibit a perfect orthorhombic morphology with sizes up to 0.7×0.4×0.2 mm³. Our results are reproducible and suggest that the crystals can be grown using a conventional flux method under ambient condition.     

MOVPE growth of GaInP/GaAs hetero-bipolar-transistors using CBr4 as carbon dopant source

Carbon doping of GaAs with carbon tetrabromide (CBr₄) in low pressure MOVPE has been investigated and applied to the fabrication of GaInP/GaAs HBTs. Especially the hydrogen incorporation and the associated acceptor passivation has been studied. The hydrogen found in single GaAs:C layers is predominantly incorporated during cooling the sample under AsH₃ after growth, n-Type capping layers can block this H indiffusion and GaAs:C base layers in HBTs show much lower H concentrations than GaAs:C single layers without a cap. A further reduction of acceptor passivation is possible by optimization of the growth procedure. First HBTs processed from layers with a base that was doped using CBr₄ show promising DC and HF performance (β = 45, for 2 × 20 μm² devices).     

Structure of a copper sodium phosphate glass by neutron scattering with isotopic substitution

The structure of a sodium copper phosphate glass containing approximately equimolar concentrations of CuO, Na₂O and P₂O₅ (0.3CuO·0.35Na₂O·0.35P₂O₅) was studied. From the total structure factor, the oxygen environment of P and Cu can be established. The first- and second-neighbour environment of Cu can be measured more accurately by taking first differences between the normalised scattering intensities for specimens containing ⁶³Cu and ⁶⁵Cu. The first-neighbour shell is found to be narrow with a mean radius of 0.2 nm - almost identical to the value found in similar crystals and with a coordination number of 4.0 (as in the crystal). Further oxygens are observed at about 0.25 nm, suggesting that the shell exhibits Jahn-Teller distortions, again as in the crystal, where two atoms are found at the longer distance. The Cu---Cu distribution has been measured directly using second difference techniques. The Cu---Cu signal is extremely weak - due to the low Cu concentration (about 7%) but a peak near 0.6 nm is observed. This corresponds to the mean distance calculated for a random dispersion of Cu atoms in the glass but the peak is so narrow that the Cu environment is clearly not random. In particular, it is possible that the Cu is ‘solvated’ by a shell of Na---O polyhedra.     

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.     

Stable Al-Cu-Co decagonal quasicrystals

Stable Al₆₅Cu₂₀Co₁₅ and Al₆₅Cu₁₅Co₂₀ decagonal quasicrystals, sometimes using a few percent of Si to replace Al, can be grown directly from the melt as single grain decaprisms of up to 1 cm in length. Some salient features of their solidification processes, growth morphologies, surface structures, high resolution electron microscopic images, two dimensional quasicrystalline structure, relationship to one-dimensional quasicrystals, crystalline approximants, and dislocations are presented.     

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%.     

Surface crystallization of rare-earth aluminosilicate glasses

Surface crystallization in a rare-earth aluminosilicate glass (Nd₂O₃–Al₂O₃–SiO₂–TiO₂) was studied using an isothermal method and the crystal growth rate of the glasses was evaluated as a function of the composition. For measuring the surface crystal growth rate, two different methods: measurement of the crystal layer in the longitudinal and lateral growth direction. It was found that crystallization proceeded by surface crystallization only and TiO₂ did not act as a nucleating agent. The growth rate was strongly dependent on the viscosity of glass and agreed with prediction from the Preston model using the known viscosity and melting temperature. As the Si/Nd and Si/Al ratios decreased, the crystal growth rate increased. TiO₂ and Nd₂O₃ played the role of network modifier, which decreased the viscosity of the glass, facilitating crystallization of the rare-earth aluminosilicate glass.     

Hydrogen and nitrogen ambient effects on epitaxial growth of GaN by hydride vapour phase epitaxy

This study presents the influence of the composition of the carrier gas on the growth of GaN by HVPE. Since no hydrogen is introduced in the vapour phase, the deposition is expected to be controlled by Cl desorption in the form of GaCl₃, as has been proposed for GaAs. However, our published model predicts much lower growth rates than those observed. We can account for both the observed parasitic deposition and GaN growth rate if we assume that GaCl₃ is not at its equilibrium pressure in the deposition zone and where nucleation takes place on the walls as well as on the substrate. This yields a high rate of parasitic nucleation even though the nominal supersaturation is vanishing small. Very little growth takes place on the substrate where the equilibrium pressure of GaCl₃ is reached. We describe similar experiments performed with a H₂/N₂ mixture as the carrier gas. In this case, we expect GaN deposition to be controlled by desorption of Cl as HCl, which is known as the H₂ mechanism. It is speculated that the results show the existence of a new growth mechanism.     

CdZnTe伽马射线探测器的能谱特性分析

CdZnTe(CZT)探测器目前在国内外天体物理研究中占有重要的地位。本文采用蒙特卡洛软件GEANT4模拟了CZT伽马射线探测器对伽马射线的能谱响应,研究了电子和空穴的输运特性、外加偏压、探测器厚度等因素对平面型探测器的能谱特性的影响规律。结果表明,在电子收集效率较高时,能量分辨率明显受迁移率寿命积比值((μτ)_e/(μτ)_h)的影响,比值越小,能量分辨率越好。增加工作电压可以提高载流子的收集效率和探测器的能量分辨率。在电子收集效率较高时,增大厚度可以弱化空穴信号贡献,提高能量分辨率。漂移程与晶体厚度之比(μτ)_eE/d可以用来估算平面型CZT探测器对低能射线的收集效率,并计算出其对应关系。