J.-T. Zettler

In situ monitoring of GaN metal-organic vapor phase epitaxy by spectroscopic ellipsometry

Epitaxy of high-quality GaN on sapphire requires a rather sophisticated substrate preparation prior to the GaN epilayer growth, namely nitridation of the substrate’s surface, growth of a GaN nucleation layer at a relative low temperature, and reduction of the defect density of this layer by a subsequent annealing step. For studying both, the detailed mechanisms of this complex procedure and its growth parameter dependencies, we attached an in situ spectroscopic ellipsometer to a nitride metal-organic vapor phase epitaxy reactor. First, the high-temperature dielectric function of GaN was measured using samples from different suppliers. Based on these data, the effect of growth parameter variations on the crystal quality of GaN epilayers could be monitored in situ. In particular, we determined the threshold temperature and the duration of the substrate nitridation under ammonia as well as the thermal threshold and duration of the nucleation layer transformation. Additionally, based on the in situ measuremen...

Real-time monitoring of MOVPE device growth by reflectance anisotropy spectroscopy and related optical techniques

Abstract Reflectance anisotropy spectroscopy (RAS/RDS) so far has been mostly used for basic growth studies in both molecular beam epitaxy (MBE) and metal-organic vapor-phase epitaxy (MOVPE). Due to its sensitivity to the uppermost atomic monolayers, RAS became a very versatile tool for investigating surface stoichiometry, surface reconstruction and surface morphology especially under gas-phase conditions. Meanwhile, however, the performance and adaptability of RAS to standard MOVPE systems has been enhanced significantly and RAS sensors now can also be used for MOVPE device growth monitoring and control. Therefore, after a brief introduction to the basic surface physics and surface chemistry causing the optical signatures, this paper concentrates on device related applications. Examples will be given concerning the optical response to both n-type and p-type GaAs doping levels and the real-time measurement of ternary compound composition for reaching lattice matched growth. The optical surface response during the growth of a complete GaAs/InGaP heterojunction bipolar transistor is visualized. The result indicates on a monolayer level either consistency or deviation from the intended growth process.

On the temperature dependence of the InP(001) bulk and surface dielectric function

The bulk dielectric function, the surface reflectance anisotropy, and the surface dielectric anisotropy of InP(001) were determined from room temperature up to 875 K. Measurements were performed in-situ on as-grown samples in both a metal-organic vapour phase epitaxy and a chemical beam epitaxy (CBE) system using a rotating analyser type ellipsometer (SE) and a reflectance anisotropy spectrometer (RAS). The temperature dependence of the bulk critical points was deduced by performing a direct line shape analysis of the SE spectra. A high-temperature bulk dielectric function database for optical in-situ studies was established by applying a combined cubic spline/harmonic oscillator interpolation scheme to the measured data. Three InP(001) surface reconstructions were found by reflection high-energy electron diffraction (RHEED) measurements in the CBE system: (2 x 1) and c(4 x 4) under phosphorus-rich conditions and a (2 x 4) reconstruction under reduced phosphorus supply. Characteristic RAS spectra are assigned to these reconstructions. The temperature shift of the main features of the surface dielectric anisotropy spectra are compared to those of the bulk critical points.

Characterization of chemically prepared Si-surfaces by uv-vis and IR spectroscopic ellipsometry and surface photovoltage

Abstract Roughness and oxide coverage of silicon (Si) surfaces after various chemical treatments were investigated using ex situ spectroscopic ellipsometry (SE) both in the ultraviolet-visible (uv-vis) and in the infrared (IR) spectroscopic region. For control and optimization of electronic interface properties the large-signal field-modulated surface photovoltage technique was used for contactless measurement of the surface band-bending and the energetic distribution of interface states Dit(E). Applying a special hydrogen (H)-termination procedure, very smooth Si(111) surfaces without any native oxide coverage were prepared, characterized by a U-shaped intrinsic surface state distribution and very low surface state density Dit.min

Growth oscillations with monolayer periodicity monitored by ellipsometry during metalorganic vapor phase epitaxy of GaAs(001)

In this letter we report on the observation of growth oscillations with monolayer periodicity by ellipsometry. An oscillation amplitude of δ〈e1〉=0.05 was measured using an optimized spectroscopic in situ ellipsometer whose wavelength was tuned to the 2.65 eV resonance energy of the arsenic dimers covering the GaAs (001) growth surface. The monolayer periodicity was verified by parallel monitoring of the growth with reflectance anisotropy spectroscopy (RAS).

In situ monitoring of InAs-on-GaAs quantum dot formation in MOVPE by reflectance-anisotropy-spectroscopy and ellipsometry

Abstract The formation of InAs quantum dots (QDs) on GaAs(001) during metalorganic vapour phase epitaxy (MOVPE) was studied for the first time in situ by reflectance-anisotropy-spectroscopy (RAS/RDS) and spectroscopic ellipsometry (SE). To analyze in detail the optical response of this highly lattice mismatched system, first RAS and SE spectra were taken for well defined InAs coverages (every 0.5 monolayers). During the initial wetting layer growth the surface dimer configuration changes from a c (4 × 4) GaAs-like to a (2 × 4) InAs-like reconstruction. After the transition to 3-dimensional growth characteristic features in the RAS spectra indicate an anisotropic shape of the growing QDs. The transition from two dimensional to three dimensional growth was monitored in real time. Substrate temperatures and growth rates have been systematically changed and the postgrowth evolution of the self assembling islands was studied. By taking RAS and ellipsometry transients at characteristic photon energies and by comparing our experimental results to calculations based on an anisotropic effective medium model, we gained insight into the evolution of sample properties such as QD density and (averaged) QD size.

Real-time calibration of wafer temperature, growth rate and composition by optical in-situ techniques during AlxGa1−xAs growth in MOVPE

In this paper we report a novel technique, based on optical measurements, to measure the true temperature of the wafer surface during epitaxial growth. It will be shown that this temperature can deviate considerably from the susceptor temperature measured with thermocouples or pyrometers. For this purpose we employed combined in-situ reflectance anisotropy spectroscopy and spectroscopic reflectance measurements in a number of different metal-organic vapor phase epitaxy (MOVPE) reactors. Measurements have been performed on rotating and non-rotating samples during growth of GaAs, AlAs and AlxGa1� xAs. We demonstrate that in a single growth run the reading of a conventional thermocouple can be calibrated to the true wafer temperature, the growth rate can be determined and process calibration for the AlGaAs composition can be established. r 2002 Elsevier Science B.V. All rights reserved.

H-terminated silicon: spectroscopic ellipsometry measurements correlated to the surface electronic properties

Abstract A correlation was established between the morphological structure of the Si surface caused by chemical preparation and its electronic interface properties by combined monitoring of (i) the surface roughness and hydrogen and oxide coverage by ex situ (UV-VIS-IR) and in situ (UV-VIS) spectroscopic ellipsometry (SE), and (ii) the density D it,min and energetic distribution D it ( E ) of interface states by surface photovoltage (SPV) measurements. With these systematic investigations a wet-chemical H-termination procedure was optimized and very smooth Si(111) surfaces without any native oxide coverage were prepared, characterized by an intrinsic surface state distribution and a very low surface state density D it,min 10 cm −2 eV −1 . On these surfaces the resonant absorption due to the Si–H bonds was directly observed by infrared ellipsometry.

Surface processes responsible for reflectance-anisotropy oscillations during molecular beam epitaxy growth of GaAs(001)

Abstract Reflectance anisotropy spectroscopy (RAS) oscillations during molecular beam epitaxy (MBE) growth of singular GaAs(001) were studied at substrate temperatures between 500°C and 610°C. RAS spectra were measured and related to surface reconstructions determined by reflection high-energy electron diffraction (RHEED). The RAS signal sensitively monitors surface changes when growth is initiated. With opening of the Ga shutter an immediate change in the RAS response indicates the modified As adsorption/desorption equilibrium due to the presence of Ga atoms on the surface. During growth of the first GaAs monolayers RAS oscillations mirror the oscillation of the monolayer step density in island growth mode. From all experimental evidence it must be concluded that the step density modulates the As dimer coverage because As desorbs predominantly in regions close to the island boundaries.

In situ characterization of GaAs growth in nitrogen atmosphere during MOVPE: a comparison to hydrogen atmosphere

The influence of the carrier gas atmosphere is investigated in situ for GaAs in the temperature range between 670 and 990 K by means of reflectance anisotropy spectroscopy (RAS). During growth (dynamic conditions), diffusion lengths of the growth determining Ga-species are nearly identical for both carrier gases. The investigation of arsenic desorption revealed that comparable processes and activation energies are determined for both ambients below about 950 K. However, studies at a constant AsH 3 partial pressure show that the carrier gas has a big influence on arsenic surface coverage. This observation explains why the group V sources are exploited to a higher degree in a nitrogen ambient [H. Hardtdegen, Electrochem. Soc. Proc. 96 (2) (1996) 49: H. Hardtdegen, P. Giannoules. III-Vs Rev. 8 (3) (1995) 34].