Haluk Sankur

Broadband gradient-index antireflection coating for ZnSe

A gradient-index antireflection coating spanning the 0.6–12.6-μm wavelength range for a ZnSe substrate was designed, fabricated, and tested. The index profile used was a fifth-order polynominal that smoothly matches the substrate. The coating was fabricated using a discrete layer approximation for the gradient-index design using ZnSe and CaF2 deposited by laser-assisted evaporation. Total thickness of the coating was 3 μm. Transmission measurements show good agreement with calculated performance. Reflectivity per surface is <3% (compared with 17% for uncoated ZnSe) in the wavelength band encompassing 4.5 octaves from 0.6 to 12.6 μm.

High‐quality optical and epitaxial Ge films formed by laser evaporation

High‐quality thin films of Ge were deposited by pulsed laser evaporation of molten Ge. Films deposited on 300 °C substrates showed very smooth morphologies and single‐crystal grain structures. Energetic ions in the vapor stream, generated by the laser‐induced plasma, were observed to affect nucleation and bulk‐film growth. Films deposited on ambient temperature substrates by laser evaporation were denser, harder, and exhibited higher values of refractive index and lower levels of intrinsic stress than the films deposited by thermal evaporation.

Deposition of optical thin films by pulsed laser assisted evaporation.

Thin films of several refractory metal oxides and Ge were deposited by pulsed laser evaporation using a TEA CO(2) laser. Films deposited on ambient temperature substrates had a polycrystalline microstructure. Ge films deposited on 300 degrees C substrates were single crystalline. The refractive indices of these films were higher than indices of films deposited by conventional evaporation techniques and were bulk values for HfO(2) and ZrO(2). The crystalline microstructure and high packing density of the films were attributed to the effect of energetic ions in the laser-induced plasma.

Microstructure and composition of composite SiO2/TiO2 thin films

The microstructure and composition of the SiO2/TiO2 analog (codeposited) and digital (thin layer pairs) gradient‐index films were examined by transmission electron microscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry, x‐ray photoelectron spectroscopy, scatterometry, and in situ ellipsometry. Both analog and digital structures were amorphous as‐grown. The SiO2 was incompletely oxidized at some of the interfaces in the layered structures and in the codeposited films, indicated competition from the TiO2 for available oxygen during growth. Digital structures with thin (65 A) layers remained well defined after annealing at 900 °C, but their order was completely destroyed by 1100 °C. Structures with thick (500 A) layers remained intact up to 1100 °C, with the TiO2 crystallizing throughout the layer width. Both anatase and rutile TiO2 crystallites were present in the layered and codeposited films after a high‐temperature anneal (T≳650 °C), while only the anatase phase was observed for ...

Intrinsic stress and structural properties of mixed composition thin films

Intrinsic stress and microstructure of mixed composition films were investigated for several binary systems of IR optical materials. These properties were measured for the entire range of compositions and for mixing obtained by codeposition and by layering of alternate pure components. The variation of stress with composition was observed to be significantly different, depending on the method of mixing. Microstructural analysis revealed a corresponding difference in the grain structure of the films. Low compressive stress films were obtained by coevaporation of high tensile stress materials. These can be used to produce thick and mechanically stable gradient-index optical coatings.

Fabrication of refractive microlens arrays

Fabrication issues of microlens arrays, made by first forming photoresist microlenses, by patterning and reflowing photoresist islands under temperature, and then transferring this into the substrate by a dry etch process, were studied. Photoresist microlenses were reliably fabricated within a range of aspect ratios. The desired sag of the microlenses in the substrate was controllably achieved by adjusting the etch selectivity. Etching behavior of fused silica in mixtures of fluoroform with oxygen or sulfur hexafluoride was studied in detail. High quality microlens arrays were fabricated in fused silica, silicon and germanium, and selected lenses were characterized.

Modeling of in situ monitored laser reflectance during MOCVD growth of HgCdTe

An effective way to in situ monitor the metalorganic chemical vapor deposition (MOCVD) of HgCdTe/CdTe/ZnTe on GaAs or GaAs/Si substrates is presented. Specular He-Ne laser reflectance was used to in situ monitor the growth rates, layer thickness, and morphology for each layer in the grown multilayer structure. In situ monitoring has enabled precise measurements of ZnTe nucleation and CdTe buffer layer thicknesses. Monitoring the constancy of reflectance during the thicker CdTe buffer growth where absorption in the CdTe reduces reflectance to just the surface component has led to optimum buffer growth ensuring good quality of subsequently grown HgCdTe. During the interdiffused multilayer process (IMP) HgCdTe growth, because multiple interfaces are present within the absorption length, a periodic reflectance signal is maintained throughout this growth cycle. A theoretical model was developed to extract IMP layer thicknesses from in situ recorded experimental data. For structures that required the growth of a larger band gap HgCdTe cap layer on top of a smaller band gap active layer, in situ monitored reflectance data allowed determination of alloy composition in the cap layer as well. Continuous monitoring of IMP parameters established the stability of growth conditions, translating into depth uniformity of the grown material, and allowed diagnosis of growth rate instabilities in terms of changes in the HgTe and CdTe parts of the IMP cycle. A unique advantage of in situ laser monitoring is the opportunity to perform “interactive” crystal growth, a development that is a key to real time MOCVD HgCdTe feedback growth control.

Thin-film deposition by laser-assisted evaporation

Laser-assisted evaporation of materials has several unique features and is being used increasingly as a thin-film deposition process. Characteristics of this technique and design parameters of a laser-assisted evaporation system are discussed. Types of source material, scanning and focusing of the laser beam, optical elements to deliver laser power to the sources, and emplacement of these elements to deposit thick films are explained.

In situ optical monitoring of OMVPE deposition of AlGaAs by laser reflectance

The feasibility of laser reflectometry for in situ monitoring and control of OMVPE AlGaAs was demonstrated. The optical constants, refractive index and extinction coefficient, of the AlGaAs alloys at the growth temperature were obtained for 633 nm wavelength. These data were used in closed loop control of thickness and composition during deposition of heterostructure layers. Laser reflectometry was also instrumental in the observation of unintentional composition gradients at the substrate-epilayer interfaces in atmospheric pressure vertical flow reactors.

Complete In Situ Laser Monitoring of HgCdTe/CdTe/ZnTe Growth onto GaAs Substrates

Abstract Specular HeNe laser reflectance has been used as a continuous in situ monitor of growth rate, thickness and composition during the growth of multilayer films of Hg 1- x Cd x Te(MCT)/CdTe/ZnTe onto GaAs (100)10° → (110) substrates. Accurate in situ monitoring has enabled thickness measurement of the thin ZnTe nucleation layer which has been shown to critically influence the X-ray rocking curve widths in the CdTe buffer layer. Monitoring of the alternate layers of HgTe and CdTe in the interdiffused multilayer process (IMP) has identified growth rate instabilities and subsequent improvement in uniformity. This, the first complete in situ monitoring of mercury cadmium telluride (MCT) metalorganic chemical vapor deposition (MOCVD) growth, has resulted in new insights into the growth process.