Shakil Pittal

Characterization of pulsed laser deposited zinc oxide

Abstract The pulsed laser deposition of zinc oxide films (ZnO) has been studied as a function of laser wavelength, and substrate temperature. Optical emission spectroscopy of the laser produced plume was used to characterize the deposition process. The deposited films were characterized by X-ray diffractometry, Auger electron spectroscopy, and scanning electron microscopy. Highly textured (002) ZnO films deposited at substrate temperatures of 300 °C with laser wavelengths of 532 nm and 248 nm. However, the energy fluence of 248 nm radiation controls the degree of texturing, allowing highly textured films to be deposited at room temperature.

Real-time monitoring and control during MOVPE growth of CdTe using multiwavelength ellipsometry

Abstract New multi-wavelength in-situ ellipsometer hardware and data analysis software are described. The hardware can simultaneously acquire accurate ellipsometric data at 12 wavelengths in less than 1 s, is simple and compact and is well suited for in-situ monitoring. The data analysis software implements a “virtual interface” approach to determine in real time the characteristics (growth rate and composition) of the near-surface region of the film. These new tools were used to study the metal-organic vapor phase epitaxy (MOVPE) growth of CdTe on GaAs. From a post-deposition analysis of the in-situ data, dielectric constants of CdTe at growth temperature were obtained. Non-uniformity in the CdTe film thickness, and film nucleation during the initial stages of growth, were also observed in the post-deposition analysis. The determined CdTe dielectric constants were utilized in subsequent depositions to determine the growth rate of a CdTe film in real time. Feedback control of the CdTe growth rate was effected by connecting an analog control voltage line from the data acquisition/analysis computer to the Cd mass flow controller.

Real-time control of HgCdTe growth by organometallic vapor phase epitaxy using spectroscopic ellipsometry

The use of spectroscopic ellipsometry for monitoring the vapor phase epitaxial growth of mercury cadmium telluride (Hg1−xCdxTe) in real-time is demonstrated. The ellipsometer is used to perform system identification of the chemical vapor deposition reactor used for the growth of CdTe and to measure the response of the reactor to different growth conditions. The dynamic behavior of the reactor is also studied by evaluating the gas transport delay. The optical constants of Hg1−xCdxTe are determined at the growth temperature for different compositions.In-situ real-time composition control is performed during the growth of Hg1−xCdxTe. The required target compositions are attained by the ellipsometer and appropriate corrections are also made by the controller when a noise input in the form of a temperature variation is introduced.

Application of spectroscopic ellipsometry for real-time control of CdTe and HgCdTe growth in an OMCVD system

A multi-wavelengthinsitu spectroscopic ellipsometer system is described. The hardware can acquire accurate ellipsometric data at 44 wavelengths in less than one second, is simple and compact, and is well suited forin-situ monitoring of chemical vapor deposition. The software used for data analysis is capable of determining the growth rate and composition of the growing layer in real time. These tools were used to study the organometallic chemical vapor deposition of CdTe, HgTe, and HgCdTe on GaAs. We could obtain the dielectric constants of these materials at the growth temperature and also the growth rate and composition of the layers in real time. Feedback control of CdTe growth was performed by connecting an analog control voltage line from the data acquisition/ analysis computer to the dimethylcadmium mass flow controller. Using dielectric constants of HgCdTe for two different compositions at the growth temperature, composition control of HgCdTe was attempted in a similar manner.

Ellipsometry study of non-uniform lateral growth of ZnO thin films

Abstract Ellipsometry is normally used to characterize layered structures which have a uniform properties laterally across the area of the probe beam. Ex situ ellipsometry was used to study the lateral variation in film thickness introduced purposely in ZnO films grown by magnetron sputtering.

Spectroscopic ellipsometric monitoring of electron cyclotron resonance plasma etching of GaAs and AlGaAs

In situ real time spectroscopic ellipsometry measurements were made during electron cyclotron resonance plasma etching of radio frequency biased GaAs and AlGaAs samples. Gas mixtures used were CH4/H2/Ar, pure H2, and pure Ar. Ellipsometry provided information about damage to the surface region and AlGaAs epilayer thickness. For the methane mixture GaAs etch, damage appeared in the form of redshifted and broadened E1 and E1+Δ1 critical point features in a surface layer several tens of nm thick. The damage layer began forming within a few seconds after the start of etching, and stabilized within 1 min. Hydrogen etching caused a thicker damage layer with greater redshifting and broadening, while argon caused relatively little damage. Possible mechanisms for the redshifting are discussed. During etching of an AlGaAs/GaAs heterostructure with the methane mixture, the same redshifting and broadening effects were seen in the AlGaAs critical point structure. The AlGaAs thickness was determined from the real time ...

Real-time monitoring and control during MBE growth of GaAs/AlGaAs Bragg reflectors using multi-wavelength ellipsometry

Abstract A new multi-wavelength in situ ellipsometer capable of acquiring accurate ellipse-metric data at 44 wavelengths from 415 to 750 nm in less than 1 s has been directly mounted on a molecular beam epitaxy (MBE) growth system. In contrast to single-wavelength ellipsometers, enough measured data is available to allow calculation of layer thickness, composition, temperature and exact angle of incidence. In situ monitoring and real-time analysis was used to control the growth process of GaAs/AlGaAs Bragg reflectors with a center wavelength of 1000 nm. The layer thickness is controlled very accurately even though ellipsometric data was acquired only every 3 s. The accuracy of shutter timing can be controlled very precisely, even allowing for slow ellipsometric acquisition rates and substrate wobble due to MBE substrate rotation. The control algorithm for the two reflectors did not attempt to control the Al composition of an individual AlGaAs layer, but the measured composition was used to adjust the Al cell temperature for the next AlGaAs layer. For purposes of comparison, the FastDyn fitting routine was used with another reflector to simultaneously control the thickness and surface composition of the AlGaAs layers. An overview of the hardware and software integration on the MBE system will be given. The in situ measurements during the growth control were later compared with ex situ measurements made with the spectroscopic ellipsometer system variable angle spectroscopic ellipsometry (VASE).

Hardware and software for in-situ process monitor and control using multiple wavelength ellipsometry

Abstract We have built and tested a low-cost ellipsometer which acquires data in the spectral range 400–800 nm at multiple (up to 44) wavelengths simultaneously in less than 1 s. A personal computer is used to acquire data and analyze the data in real time for studies of time dependent phenomena. In addition, this permits feedback control which we have demonstrated on epitaxial growth of CdTe on GaAs by metal organic chemical vapor deposition.

Real-time monitoring and control during MBE growth of GaAs/AlGaAs Bragg reflectors using multiwave ellipsometry

New multi-wavelength in-situ ellipsometer acquiring accurate ellipsometric data at 44 wavelength form 415 to 750 nm in less than 1s is directly mounted on a MBE growth system. Compared to single wavelength ellipsometers enough measured data are available to have access to layer thickness, composition, temperature and exact angle of incidence. In- situ monitoring and real time analysis was used to control the process of GaAs/AlGaAs Bragg reflectors with center wavelength of 1000 nm. The layer thickness is controlled very accurately even though ellipsometric data was acquired only every 3 seconds. The accuracy of shutter timing can be made very precisely even for slow ellipsometric acquisition rates and substrate wobble due to MBE substrate rotation. The control algorithm for two reflectors did not attempt to control the Al composition of an individual AlGaAs layer, but the measured composition was used to adjust the Al cell temperature for the next AlGaAs layer. In comparison for another reflector, the FastDyn fitting routine were used to simultaneously control the thickness and surface composition of the AlGaAs layers. An overview about the hardware and software integration on the MBE system will be given. The in-situ measurements during the growth control were later compared with ex-situ measurements made with spectroscopic ellipsometer system VASE.