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.     

MOCVD外延碲镉汞薄膜的生长工艺选择

金属有机化合物汽相沉积（ＭＯＣＶＤ）技术是制备用于红外焦平面阵列（ＩＲＦＦＰＡ）的高质量碲镉汞（ＨｇＣｄＴｅ）薄膜材料的重要手段。文中讨论了汞源温度、生长温度、衬底材料及互扩散多层工艺（ＩＭＰ）等因素在ＭＯＣＶＤ外延生长碲镉汞薄膜过程中的作用机理,并选择适合的生长条件获得了质量优良的碲镉汞薄膜。     

A model of the interdiffused multilayer process

The interdiffused multilayer process (IMP) is a novel approach to growing Hg_1−xCd_xTe. In this process, alternating thin films of HgTe and CdTe are grown and allowed to interdiffuse resulting in a bulk material of constant composition. A model of the IMP must include the effects of both the deposition of new material and the interdiffusion of the material. It must also be able handle the flush phases of the IMP where the growth rate decays to zero. Existing approaches to modeling epitaxial growth of Hg_1−xCd_xTe treat growth and interdiffusion as separate, sequential steps resulting in numerical stability problems, pseudodiffusion effects, or flush phase modeling problems. The model presented here, however, is based on an incremental balance where growth and diffusion occur simultaneously, resulting in a model exhibiting none of the difficulties mentioned above. The IMP growth model is integrated with a model for calculating reflectance from a laser directed at near normal incidence angle. The predicted reflectance is compared to experimental measurements and showed a good preliminary fit when the model employed default parameters. The agreement is greatly improved after parameter fitting.     

GaN生长速率的研究

采用在位监控方法研究了MOCVD系统中GaN材料的外延生长速率与NH3流量、TMGa流量、Ⅴ/Ⅲ比等生长参数的关系．GaN生长速率随NH3流量的提高先增加后减小,而随TMGa流量的增加线性的增加．在不同NH3流量的情况下,GaN生长速率随TMGa流量增加的速率不同．GaN的生长速率与Ⅴ/Ⅲ比没有直接的关系,而与NH3,TMGa等条件有关．实验结果表明,MOCVD系统中存在着较强的预反应．预反应的程度与TMGa的流量成正比．     

Continuous in situ growth rate extraction using pyrometric interferometry and laser reflectance measurement during molecular beam epitaxy

We describe a new, universal, and easy-to-use method for real-time thickness and growth rate extraction based on in situ optical monitoring during molecular beam epitaxy of GaAs/AlAs layered structures. The method is based on a novel least-square phase extraction algorithm and a model-reference comparison scheme. The new method demonstrates a significantly reduced growth rate estimation error (<2%) on both pyrometric interferometry (PI) and laser reflectance (LR) monitored data over conventional methods. Based on preliminary studies, the new method also shows excellent dynamic tracking of growth rate with a thickness resolution of only 30 nm when the growth rate is intentionally changed.     

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.     

Modeling of Copper SIMS Profiles in Thin HgCdTe

Anomalous secondary-ion mass spectroscopy (SIMS) profiles of copper in thin pieces of HgCdTe are explained using the model used for diode formation by ion milling and ion implantation. In this model, the SIMS ion beam injects mercury interstitials into the HgCdTe as it etches the HgCdTe. The interstitials fill metal vacancies and kick copper off the metal lattice sites. The copper interstitials then diffuse either to the surface being etched, where it is removed and detected by the SIMS instrument, or deeper into the HgCdTe, where it annihilates vacancies. Good agreement between model predictions and experimental SIMS profiles are obtained.     

Orientation dependence of HgCdTe epitaxial layers grown by MOCVD on Si substrates

Orientation dependence of HgCdTe epilayers grown by MOCVD on Si substrates was studied. Substrate orientation is considered to be one of the most sensitive factors to enable hetero-epitaxial growth on silicon substrates, especially in the case of a low temperature growth process. The present work was carried out with characterized features of a low temperature process for HgCdTe growth on Si and using a thin CdTe buffer layer. The (100), (100) misoriented toward [110], (311), (211), (111), and (331) oriented Si substrates were used in the present work. The best results were obtained on (211)Si substrates with an x-ray full width at half maximum of 153 arc sec for a 5 (im thickness HgCdTe layer and 69 arc sec for a 10 um thickness layer. It was found that the effective lattice mismatch of CdTe/Si heterosystem was reduced to 0.6% (for the 611 lattice spacing of CdTe and 333 spacing of Si) in the case of (133)CdTe/(211)Si.     

Low pressure MOCVD growth of buried heterostructure laser wafers using high quality semi-insulating InP

Semi-insulating Fe doped InP has been grown by low pressure MOCVD at 100 mbar and 630° C. Complete activation of Fe below the solubility limit of 5 × 10₁₆ cm^-3 has been achieved by reducing the PH₃ concentration during crystal growth to the lowest value required to maintain good surface morphology of the layer. Diffusion of the Fe dopant and dopant spikes at the interface between the substrate and grown layer can be minimized by ensuring that the total Fe concentration in the layer does not exceed the diffusion threshold of 2 × 10¹⁷ cm⁻³. Growth of Fe doped InP around a double heterostructure mesa formed by reactive ion etching produces a structure without either growth of InP on the mesa or notches at the mesa sidewalls, even with minimal overhang of the dielectric mask. Examination of regrown heterostructures shows no evidence of interdiffusion of Fe and Zn, indicating that Fe diffusion has been successfully prevented. Completed lasers have threshold current densities of 2.5 kA/cm² at 20° C and initial aging results which indicate that these devices have good lasing characteristics and potentially high reliability.     

Modeling of Recombination in HgCdTe

Minority carrier recombination lifetime calculations for narrow-gap semiconductors are of direct practical interest in establishing whether a material’s recombination is extrinsically or intrinsically limited, and therefore in guiding research and development programs regarding material quality improvements. We describe efforts to obtain accurate electronic band structures of HgCdTe alloy-based materials with infrared energy gaps and employ them to evaluate Auger recombination lifetimes. We use a 14-band k · p formalism to compute and optimize electronic band structures, and use the obtained electronic energies and matrix elements directly in the numerical evaluation of Auger and radiative lifetimes.     

Improved arsenic doping in metalorganic chemical vapor deposition of HgCdTe andin situ growth of high performance long wavelength infrared photodiodes

Controlled and effective p-type doping is a key ingredient forin situ growth of high performance HgCdTe photodiode detectors. In this paper, we present a detailed study of p-type doping with two arsenic precursors in metalorganic chemical vapor deposition (MOCVD) of HgCdTe. Doping results from a new precursortris-dimethylaminoarsenic (DMAAs), are reported and compared to those obtained from tertiarybutylarsine (TBAs). Excellent doping control has been achieved using both precursors in the concentration range of 3 × 1015-5 × 10¹⁷ cm⁻³ which is sufficient for a wide variety of devices. Arsenic incorporation efficiency for the same growth temperature and partial pressure is found to be higher with DMAAs than with TBAs. For doping levels up to 1 × 10₁₇ cm⁻³, the alloy composition is not significantly affected by DMAAs. However, at higher doping levels, an increase in the x-value is observed, possibly as a result of surface adduct formation of DMAAs dissociative products with dimethylcadmium. The activation of the arsenic as acceptors is found to be in the 152–50% range for films grown with DMAAs following a stoichiometric anneal. However, a site transfer anneal increases the acceptor activation to near 100%. Detailed temperature dependent Hall measurements and modeling calculations show that two shallow acceptor levels are involved with ionization energies of 11.9 and 3.2 meV. Overall, the data indicate that DMAAs results in more classically behaved acceptor doping. This is most likely because DMAAs has a more favorable surface dissociation chemistry than TBAs. Long wavelength infrared photodiode arrays were fabricated on P-on-n heterojunctions, grownin situ with iodine doping from ethyl iodide and arsenic from DMAAs on near lattice matched CdZnTe (100) substrates. At 77K, for photodiodes with 10.1 and 11.1 (im cutoff wavelengths, the average (for 100 elements 60 × 60 μm² in size) zero-bias resistance-area product, R₀A are 434 and 130 ohm-cm², respectively. Quantum efficiencies are ≥50% at 77K. These are the highest R₀A data reported for MOCVDin situ grown photodiodes and are comparable to state-of-the-art LPE grown photodiodes processed and tested under identical conditions.     

Low threshold injection laser in HgCdTe

Narrow-stripe transverse injection lasers in HgCdTe have been successfully fabricated and operated. The double heterostructure of the laser was grown by molecular beam epitaxy on a {111} CdZnTe substrate. The n type base was extrinsically indium doped but other p type layers were only doped by deviation from stoichiometry. Laser emission at 3.4 μm and 3.56 μm was obtained at 78K under pulsed current conditions. Threshold current density as low as 90 A/cm² at 40K was achieved. Over 65K, the threshold current variation with temperature showed a sharp increase due to a drift-current leakage in the structure.     

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.     

MBE growth and characterization of in situ arsenic doped HgCdTe

We report the results of in situ arsenic doping by molecular beam epitaxy using an elemental arsenic source. Single Hg_1−xCd_xTe layers of x ∼0.3 were grown at a lower growth temperature of 175°C to increase the arsenic incorporation into the layers. Layers grown at 175°C have shown typical etch pit densities of 2E6 with achievable densities as low as 7E4cm⁻². Void defect densities can routinely be achieved at levels below 1000 cm⁻². Double crystal x-ray diffraction rocking curves exhibit typical full width at half-maximum values of 23 arcsec indicating high structural quality. Arsenic incorporation into the HgCdTe layers was confirmed using secondary ion mass spectrometry. Isothermal annealing of HgCdTe:As layers at temperatures of either 436 or 300°C results in activation of the arsenic at concentrations ranging from 2E16 to 2E18 cm⁻³. Theoretical fits to variable temperature Hall measurements indicate that layers are not compensated, with near 100% activation after isothermal anneals at 436 or 300°C. Arsenic activation energies and 77K minority carrier lifetime measurements are consistent with published literature values. SIMS analyses of annealed arsenic doping profiles confirm a low arsenic diffusion coefficient.     

MOCVD of bandgap-engineered HgCdTe p-n-N-P dual-band infrared detector arrays

We report the implementation of recent advances in metalorganic chemical vapor deposition (MOCVD) for in situ growth of four-layer HgCdTe mid wave/ long wave (MW/LW) simultaneous dual-band 64 × 64 infrared detector arrays. This independently accessed, simultaneous, double-heterojunction p-n-N-P dualband detector has two back-to-back stacked photodiodes grown on CdZnTe (100) substrates. The LW photodiode is a p-on-n heterojunction grown on top of an MW N-on-P heterojunction photodiode. Secondary ion mass spectrometry depth profiles of these 28 μ m thick p-n-N-P dual-band films show four well-defined regions of alloy composition and doping, and agree well with the device design. 64 × 64 arrays of dual-band detectors were fabricated from these films using electron cyclotron resonance dry etching and CdTe passivation, and hybridized to a dual-band readout chip. Two bump inter-connects in each unit cell provide independent electrical access to the back-to-back MW and LW photodiodes, and allow the MW and LW photocurrents to be separate and independent. The dualband infrared focal plane arrays (IRFPAs) spectral response data at 78K are well-behaved and are fully consistent with that observed in individual singleband LW p-on-n and MW N-on-P heterojunction devices of the same design. The hybridized 64 × 64 duai-band FPAs have MW and LW average in-band quantum efficiencies of 79 and 67%, and median D* values of 4.8 × 10¹¹ and 7.1 × 10¹⁰ cm-√Hz/W, in the respective spectral bands at 78K. The data demonstrate that MOCVD has progressed significantly toward being a practical and viable vapor phase in situ growth technology for advanced bandgap-engineered HgCdTe detector arrays.     

Modeling of arsenic activation in HgCdTe

We present a theoretical examination of the behavior of arsenic atoms in Hg_1−xCd_xTe for x=0.3, focusing on the thermodynamic conditions that most closely mimic molecular beam epitaxial growth and subsequent annealing steps. We show that, for molecular beam epitaxial growth where tellurium-saturated conditions apply, arsenic incorporates onto the cation sublattice and becomes inactive. A significant fraction of these arsenic atoms are bound to mercury vacancies. We also propose a model of the activation, which involves transfer of the arsenic from the cation to the anion sublattice. The model suggests that activation anneals must be done at high enough temperatures to surmount an activation barrier, and that the phase field from tellurium- to mercury-rich conditions should be traversed slowly enough so that the cation vacancies are not filled before the site transfer can be completed.     

MOVPE growth of (Al,Ga)InP-based laser structures monitored by real-time reflectance anisotropy spectroscopy

Using a specially-designed spectrometer enabling combined reflectance anisotropy spectroscopy (RAS) and reflectance measurements on rotating substrates in a commercial MOVPE reactor, we report the first full-spectroscopic RAS-monitoring of (Al,Ga)InP-based 650 nm laser growth. First, a spectral database was built up from systematic studies of AlGaInP RAS signatures for different Al compositions, doping levels and growth temperatures. These data are subsequently used for the interpretation of characteristic RAS fingerprints taken throughout the entire laser growth process. From the analysis of characteristic changes in the RAS spectra even small deviations from the optimum process (doping levels, composition, etc.) which would effect the performance of the final device can be detected. (This paper previously appeared in Vol. 29, No. 1, 2000, with Fig. 5 inadvertantly printed in black and white. It is reprinted here with Fig. 5 in color.)     

MOVPE growth of (Al, Ga)InP-based laser structures monitored by real-time reflectance anisotropy spectroscopy

Using a specially-designed spectrometer enabling combined reflectance anisotropy spectroscopy (RAS) and reflectance measurements on rotating substrates in a commercial MOVPE reactor, we report the first full-spectroscopic RAS-monitoring of (Al,Ga)InP-based 650 nm laser growth. First, a spectral database was built up from systematic studies of AlGaInP RAS signatures for different Al compositions, doping levels and growth temperatures. These data are subsequently used for the interpretation of characteristic RAS fingerprints taken throughout the entire laser growth process. From the analysis of characteristic changes in the RAS spectra even small deviations from the optimum process (doping levels, composition, etc.) which would effect the performance of the final device can be detected.     

Arsenic incorporation during MBE growth of HgCdTe

We discuss the equilibrium model of the amphoteric behavior of arsenic in HgCdTe and its applicability to material grown by molecular beam epitaxy. Suggestions are made on how to achieve active incorporation by manipulating the surface orientation, or by using precursors that provide steric hindrance.     

用微波反射法测HgCdTe中少数载流子寿命

介绍了用微波反射法测量ＨｇＣｄＴｅ中的少数载流子寿命,分析了其测量原理,并与接触式的光电导衰减法进行了比较。