Surface photoabsorption monitoring of the growth of GaAs and InGaAs at 650°C by MOCVD

By monitoring the cyclic behavior of surface photoabsorption (SPA) reflectance changes during the growth of GaAs at 650°C and with sufficient H₂ purging time between the supply of trimethylgallium and AsH₃, we have been able to achieve controlled growth of GaAs down to a monolayer. Our results show, as confirmed by photoluminescence (PL) measurements, the possibility of growing highly accurate quantum well heterostructures by metalorganic chemical vapor deposition at conventional growth temperatures. We also present our PL measurements on the InGaAs single quantum wells grown at this temperature by monitoring the SPA signal.     

The growth and characterization of AlGaAs double heterostructures for the evaluation of reactor and source quality

AlGaAs double heterostructures are grown by low-pressure metalorganic chemical vapor deposition to evaluate the level of oxygen contamination in different trimethylaluminum sources. Effects of arsine purifiers, misoriented substrates, atmospheric exposure of the growth chamber, and possible phosphorus contamination are also studied. Extensive characterization is performed on these films by a variety of methods, including high-resolution x-ray diffraction, photoluminescence (PL), time-resolved photoluminescence, and secondary-ion mass spectrometry. The PL intensities for structures grown with the low-alkoxide grade are reproducibly much greater than those grown with the regular-grade TMA1. The use of AsH₃ purification improves the PL intensity.     

In-situ reflectance monitoring during MOCVD of AlGaN

We report in-situ optical reflectance monitoring during the metalorganic chemical vapor deposition (MOCVD) growth of (Al)GaN. In addition to the well-known thin film interference effect which enables a real-time determination of growth rate, we show that several insights about the MOCVD growth process can be gained by using this simple yet powerful technique. Illustrations from a variety of applications for in-situ reflectance monitoring, specifically the study of growth evolution, the control of alloy fractions, and the use of growth rate to gauge surface kinetics and gas injection will be reported.     

Strain from modified interface compositions in InGaAs/InP superlattices

Thin strained regions have been inserted at the interfaces of lattice-matched InGaAs/lnP superlattices to assess growth conditions for tailoring of localized compositional changes and for studying As-P intermixing behavior during heterojunction growth. Also, precise growth rates of binary composition layers were determined from specially designed superlattices using strained layers of common anion compounds inserted periodically into InP and GaAs. Growth rates of fractional monolayers are found to be identical to thick layer growth rates. When thin InAs, GaAs, GaP, ALAs, or AIP layers were inserted at the InGaAs/lnP heterojunctions, the measured strain at either one or both interfaces was equal to the strain predicted from the growth rate x time product. Excess strain seen in some cases is due to a change in As-P intermixing and this component can be separated from the predicted strain. Insertion of Ga-compounds at the InP-grown-on-InGaAs interface causes interface roughening which degrades the superlattice. For all other compositions the thin, highly strained regions are not detrimental to the crystalline quality of the periodic structure.     

Ion beam mixing characteristics of MOCVD grown InGaAs/GaAs superlattices

The ion beam mixing behavior of InGaAs/GaAs strained layer superlattice structures grown by metalorganic chemical vapor deposition was studied using secondary ion mass spectroscopy and Rutherford backscattering channeling. The fluence dependence of intermixing by MeV Kr⁺ irradiation has been investigated. Significant intermixing occurs for fluences much lower than for similar intermixing in other superlattice systems (i.e. ALAs/GaAs). The intermixing exhibits no temperature dependence for fluences of 2 x 10¹⁵ to 5 x 10¹⁵ cm⁻² which sharply contrasts with the behavior of the AlAs/GaAs superlattice system which shows a strong temperature dependence, including a miscibility gap, in the temperature range 523 to 973K. Samples irradiated at 573K retain a high degree of crystallinity when compared to lower temperature irradiations indicating that the InGaAs/GaAs superlattice can be disordered and still retain crystallinity.     

MOCVD生长1．06μm InGaAs／GaAs量子阱LDs

用低压MOCVD生长应变InGaAs/GaAs量子阱,采用中断生长、应变缓冲层(SBL)、改变生长速度和调节Ⅴ/Ⅲ等方法改善InGaAs/GaAs量子阱的光致发光(PL)质量。PL结果表明,10s生长中断结合适当的SBL生长的量子阱PL谱较好。该量子阱应用于1.06μm激光器的制备,未镀膜的宽条激光器(100μm×1000μm)有低阈值电流密度(110A/cm2)和高的斜率效率(0.256W/A,per.facet)。     

Passivation of carbon acceptors during growth of carbon-doped GaAs,InGaAs, and HBTs by MOCVD

Carbon dopedp-type GaAs and In_0.53Ga_0.47As epitaxial layers have been grown by low-pressure metalorganic chemical vapor deposition using CC1₄ as the carbon source. Low-temperature post-growth annealing resulted in a significant increase in the hole concentration for both GaAs and In_0.53Ga_0.47As, especially at high doping levels. The most heavily doped GaAs sample had a hole concentration of 3.6 × 10²⁰ cm⁻³ after a 5 minute anneal at ≈400° C in N₂, while the hole concentration in In_0.53Ga_0.47As reached 1.6 × 10¹⁹ cm⁻³ after annealing. This annealing behavior is attributed to hydrogen passivation of carbon acceptors. Post-growth cool-down in an AsH₃/H₂ ambient was found to be the most important factor affecting the degree of passivation for single, uncapped GaAs layers. No evidence of passivation is observed in the base region of InGaP/GaAs HBTs grown at ≈625° C. The effect ofn-type cap layers and cool-down sequence on passivation of C-doped InGaAs grown at ≈525° C shows that hydrogen can come from AsH₃, PH₃, or H₂, and can be incorporated during growth and during the post-growth cool-down. In the case of InP/InGaAs HBTs, significant passivation was found to occur in the C-doped base region.     

A comparison of TMGa and TEGa for low-temperature metalorganic chemical vapor deposition growth of CCI4-doped inGaAs

Factors which influence the alloy composition and doping level of CCl₄-doped In_0.53Ga_04.7As grown at low temperatures (450°C < T_g < 560°C) by low-pressure metalorganic chemical vapor deposition (MOCVD) have been investigated. The composition is highly dependent on substrate temperature due to the preferential etching of In from the surface during growth and the temperature-dependent growth efficiency associated with the Ga source. The lower pyrolysis temperature of TEGa relative to TMGa allows the growth of CCl₄-doped InGaAs at lower growth temperatures than can be achieved using TMGa, and results in improved uniformity. High p-type doping (p ∼ 7 × 10¹⁹ cm^-8) has been achieved in C-doped InGaAs grown at T = 450°C. Secondary ion mass spectrometry analysis of a Cdoping spike in InGaAs before and after annealing at ∼670°C suggests that the diffusivity of C is significantly lower than for Zn in InGaAs. The hole mobilities and electron diffusion lengths in p⁺-InGaAs doped with C are also found to be comparable to those for Be and Zn-doped InGaAs, although it is also found that layers which are highly passivated by hydrogen suffer a degradation in hole mobility. InP/InGaAs heterojunction bipolar transistors (HBTs) with a C-doped base exhibit high-frequency performance (f_t = 62 GHz, f_max=42 GHz) comparable to the best reported results for MOCVD-grown InP-based HBTs. These results demonstrate that in spite of the drawbacks related to compositional nonuniformity and hydrogen passivation in CCl₄-doped InGaAs grown by MOCVD, the use of C as a stable p-type dopant and as an alternative to Be and Zn in InP/ InGaAs HBTs appears promising.     

Effect of Se-doping on deep impurities in AlxGa1−xAs grown by metalorganic chemical vapor deposition

We have studied the effect of Se-doping on deep impurities in Al_xGa_1−xAs (x = 0.2∼0.3) grown by metalorganic chemical vapor deposition (MOCVD). Deep impurities in various Se-doped Al_xGa_1−xAs layers grown on GaAs substrates were measured by deep level transient spectroscopy and secondary ion mass spectroscopy. We have found that the commonly observed oxygen contamination-related deep levels at E_c-0.53 and 0.70 eV and germanium-related level at E_c-0.30 eV in MOCVD grown Al_xGa_1−xAs can be effectively eliminated by Se-doping. In addition, a deep hole level located at E_y + 0.65 eV was found for the first time in Se-doped Al_xGa_1-xAs when Se ≥2 × 10¹⁷ cm⁻³ or x ≥ 0.25. The concentration of this hole trap increases with increasing Se doping level and Al composition. Under optimized Se-doping conditions, an extremely low deep level density (N_t less than 5 × 10¹² cm⁻³, detection limit) Al_0.22Ga_0.78As layer was achieved. A p-type Al_0.2Ga_0.8As layer with a low deep level density was also obtained by a (Zn, Se) codoping technique.     

Wavelength tuning in strained layer InGaAs-GaAs-AlGaAs quantum well lasers by selective-area MOCVD

Selective-area growth and regrowth using conventional atmospheric pressure metalorganic chemical vapor deposition is investigated for wavelength tuning in strained layer In_xGa_1-xAsGaAs-Al_y Ga_1-yAs quantum well lasers. Growth inhibition from a silicon dioxide mask is the mechanism used for the selective-area growth rate enhancement. By varying the width of the oxide stripe opening, differences in the growth rate yield different quantum well thicknesses, and hence different lasing wavelengths for devices on the same wafer. Both two-and three-step growth processes are utilized for selective-area epitaxy of strained layer In_xGa_1-xAs-GaAs quantum well active regions, with lasers successfully fabricated from the three-step growth. Scanning electron microscopy and transmission electron microscopy indicate that the absence of an oxide mask during Al_yGa_1-yAs growth is essential for successful device operation. A wide wavelength tuning range of over 630Å is achieved for lasers grown on the same substrate.     

Growth,characterization, and modeling of ternary InGaAs-GaAs quantum wells by selective-area metalorganic chemical vapor deposition

A computational diffusion model is used to predict thickness and composition profiles of ternary In_xGa_1-xAs quantum wells grown by selective-area, atmospheric pressure metalorganic chemical vapor deposition (MOCVD), and its accuracy is investigated. The model utilizes diffusion equations and boundary conditions derived from basic MOCVD theory, with reaction parameters derived from experimental results, to predict the concentration of each column III constituent throughout the concentration boundary layer. Solutions to these equations are found using the two-dimensional, finite element method. The growth thickness profiles of GaAs, InP, and In_xGa_1-xAs deposited by selective-area MOCVD are observed by conventional profilometry, and compositions are measured indirectly by laser emission wavelengths. The data presented show that the model accurately predicts growth thickness and composition profiles of ternary III-V materials grown by selective-area MOCVD.     

MOCVD生长高反射率AlN/GaN分布布拉格反射镜

利用金属有机物化学气相沉积(MOCVD)方法在蓝宝石c面衬底上制备出高反射率AlN/GaN分布布拉格反射镜(DBR).利用分光光度计测量,在418 nm附近最大反射率达到99%.样品表面显微照片显示,有圆弧形缺陷和少量裂纹出现;在缺陷和裂纹以外的区域,DBR具有较为平坦的表面,其粗糙度在10μm×10μm面积上为3.3 m左右.样品的截面扫描电镜(SEM)照片显示,DBR具有良好的周期性.对反射率和表面分析的结果表明,该样品达到了制备GaN基垂直腔面发射激光器(VCSEL)的要求.     

Effect of growth pressure on indium incorporation during the growth of InGaN by MOCVD

The effect of the growth pressure on the In incorporation in InGaN thin films, grown by metalorganic chemical vapor deposition (MOCVD) have been investigated. The InGaN thin films were grown by varying the growth pressures, while maintaining all other growth parameters constant. Photoluminescence and high resolution x-ray diffraction (XRD) measurements showed that the In incorporation in the InGaN thin film was drastically increased with decreasing growth pressures. XRD analysis also revealed that the In concentration in the films was increased by 7.5% as the growth pressure was decreased from 250 torr to 150 torr. This can be attributed to the enhanced mass transportation of precursor gases through the boundary-layer on the substrate in the MOCVD system.     

In situ spectral reflectance monitoring of III-V epitaxy

Near normal incidence spectral reflectance was used to monitor the growth of ALAs, GaAs, and AlGaAs films by metalorganic chemical vapor deposition in real time. The simultaneous acquisition of reflectance data over a wide spectral bandwidth allows compositional discrimination between layers and greater thickness sensitivity than single wavelength measurements. The potential of this technique for application to device structures was demonstrated by moni-toring the fabrication of AlAs/AlGaAs visible distributed Bragg reflectors.     

The impact of MOCVD growth ambient on carrier transport,defects, and performance of CdTe/CdS heterojunction solar cells

CdTe solar cells were fabricated by depositing CdTe films on CdS/SnO₂/glass substrates in various metalorganic chemical vapor deposition growth ambient with varying Te/Cd mole ratio in the range of 0.02 to 15. The short-circuit current density (J_sc) showed a minimum at a Te/Cd ratio of 0.1 and increased on both sides of this minimum. The open-circuit voltage (V_oc) was found to be the highest for the Te-rich growth ambient (Te/Cd∼6)and was appreciably lower (600 mV as opposed to 720 mV) for the stoichiometric and the Cd-rich growth conditions. This pattern resulted in highest cell efficiency (12%) on Te-rich CdTe films. Auger electron spectroscopy revealed a high degree of atomic interdiffusion at the CdS/CdTe interface when the CdTe films were grown in the Te-rich conditions. It was found that the current transport in the cells grown in the Cd-rich ambient was controlled by the tunneling/interface recombination mechanism, but the depletion region recombination became dominant in the Te-rich cells. These observations suggest that the enhanced interdiffusion reduces interface states due to stress reduction or to the gradual transition from CdS to CdTe. The hypothesis of reduced defect density in the CdTe cells grown in the Te-rich conditions is further supported by the high effective lifetime, measured by time-resolved photoluminescence, and the reduced sensitivity of quantum efficiency to forward/light bias.     

Improved quality of AlxGa1−x as grown on se-Doped AlxGa1−x as substrate-Layers by metalorganic chemical vapor deposition

Al_xGa_1−xAs epilayers were grown directly on different Al_xGa_1−xAs substrate-layers by metalorganic chemical vapor deposition (MOCVD). The quality of Al_xGa_1−-xAs layers was significantly improved when Se-doped Al_xGa_1−xAs substrate-layers were used. Al_0.13Ga_0.87As epilayers with excellent morphology, optical, and crystal quality were grown on Se-doped Al_0.26Ga_0.74As. The full width at half maximum of the bound exciton peak as low as 4.51 meV was measured by low-temperature (14.9K) photoluminescence. The improvement is attributed to a Se passivation effect at the surface of Se-doped Al_xGa_1−xAs substrate-layers. Results suggest that Se will reduce and delay the formation of native oxides.     

Insights into MOCVD process control as revealed by laser interferometry

In-situ laser interferometry has been used to study the nucleation of CdTe on basal plane sapphire in MOCVD growth. The nucleation delay and nucleation thickness were seen to be influential in the determination of overall buffer layer quality. This nucleation process was seen to be highly complex and the nucleation delay and nucleation layer thickness were seen to vary for constant conditions of nucleation II:VI ratio and temperature. A possible link between nucleation delay and nucleation temperature has been commented on. The results for CdTe on sapphire suggest that for consistent metalorganic chemical vapor deposition (MOCVD) growth conditions from run to run during the nucleation stage, the nucleation delay and thickness may still vary as a function of subtle differences in the substrate growth surface. This argument is further substantiated by our observations using laser interferometry to study the nucleation of CdTe on Si (211) in MOCVD, where different surface preparatory treatments were employed. Laser interferometry was found to be sensitive to differences in surface treatment of the Si substrates prior to ZnTe/CdTe growth. The benefits of the use of interferometry to study and to control the nature of complex epitaxial systems is discussed.     

Growth of InGaN HBTs by MOCVD

The design and growth of GaN/InGaN heterojunction bipolar transistors (HBTs) by metalorganic chemical vapor deposition (MOCVD) are studied. Atomic-force microscopy (AFM) images of p⁺InGaN base layers (∼100 nm) deposited under various growth conditions indicate that the optimal growth temperature is limited to the range between 810 and 830°C due to a trade-off between surface roughness and indium incorporation. At these temperatures, the growth pressure must be kept above 300 Torr in order to keep surface pit density under control. An InGaN graded-composition emitter is adopted in order to reduce the number of V-shaped defects, which appear at the interface between GaN emitter and InGaN base and render an abrupt emitter-base heterojunction nearly impossible. However, the device performance is severely limited by the high p-type base contact resistance due to surface etching damage, which resulted from the emitter mesa etch.     

The role of the low temperature buffer layer and layer thickness in the optimization of OMVPE growth of GaN on sapphire

In agreement with previous work,¹² a thin, low temperature GaN buffer layer, that is used to initiate OMVPE growth of GaN growth on sapphire, is shown to play a critical role in determining the surface morphology of the main GaN epilayer. X-ray analysis shows that the mosaicity of the main GaN epilayer continues to improve even after several μm of epitaxy. This continuing improvement in crystal perfection correlates with an improvement in Hall mobility for thicker samples. So far, we have obtained a maximum mobility of 600 cm²/V-s in a 6 μm GaN epilayer. Atomic force microscopy (AFM) analysis of the buffer layer and x-ray analysis of the main epilayer lead us to conclude that the both of these effects reflect the degree of coherence in the main GaN epitaxial layer. These results are consistent with the growth model presented by Hiramatsu et al., however, our AFM data indicates that for GaN buffer layers partial coherence can be achieved during the low temperature growth stage.     

Optical properties of InN films grown by MOCVD

By means of optical absorption, photolumines-cence (PL), Raman scattering and ellipsometry, optical properties of indium nitride (InN) films grown by metal organic chemical vapor deposition (MOCVD) are investigated. Through absorption and PL measurements, it is proven that the band gap of high quality InN is 0.68 eV, which agrees with the recently reported value, 0.7 eV. By analysis of the Raman scattering spectrum, the comparatively low background concentration of electron results in a smaller band gap value. The transition energy of wurtzite InN at critical point is determined by ellipsometric spectra. In addition, the complex refractive index of InN at energy ranging from 0.65 to 4.0 eV is obtained for the first time.