Shouleh Nikzad

Delta-doped electron-multiplied CCD with absolute quantum efficiency over 50% in the near to far ultraviolet range for single photon counting applications

We have used molecular beam epitaxy (MBE) based delta-doping technology to demonstrate nearly 100% internal quantum efficiency (QE) on silicon electron-multiplied charge-coupled devices (EMCCDs) for single photon counting detection applications. We used atomic layer deposition (ALD) for antireflection (AR) coatings and achieved atomic-scale control over the interfaces and thin film materials parameters. By combining the precision control of MBE and ALD, we have demonstrated more than 50% external QE in the far and near ultraviolet in megapixel arrays. We have demonstrated that other important device performance parameters such as dark current are unchanged after these processes. In this paper, we briefly review ultraviolet detection, report on these results, and briefly discuss the techniques and processes employed.

Delta-doped CCDs: high QE with long-term stability at UV and visible wavelengths

Delta-doped CCDs, developed at JPLs Microdevices Laboratory, have achieved stable 100% internal quantum efficiency in the visible and near UV regions of the spectrum. In this approach, an epitaxial silicon layer is grown on a fully-processed commercial CCD using molecular beam epitaxy. During the silicon growth on the CCD, 30% of a monolayer of boron atoms are deposited on the surface, followed by a 15

Direct detection and imaging of low-energy electrons with delta-doped charge-coupled devices

angstrom silicon layer for surface passivation. The boron is nominally incorporated within a single atomic layer at the back surface of the device, resulting in the effective elimination of the backside potential well. The measured quantum efficiency is in good agreement with the theoretical limit imposed by reflection from the Si surface. Enhancement of the total quantum efficiency in the blue visible and near UV has been demonstrated by depositing antireflection coatings on the delta-doped CCD. Recent results on antireflection coatings and quantum efficiency measurements are discussed.

Ultraviolet antireflection coatings for use in silicon detector design

We report the use of delta-doped charge-coupled devices (CCDs) for direct detection of electrons in the 50–1500 eV energy range. We show that modification of the CCD back surface by molecular beam epitaxy can greatly improve sensitivity to low-energy electrons by introducing an atomically abrupt dopant profile to eliminate the dead layer. Using delta-doped CCDs, we have extended the energy threshold for detection of electrons by over an order of magnitude. We have also measured high gain in response to low-energy electrons using delta-doped CCDs. The effect of multiple electron hole pair production on the observed signals is discussed. Electrons have been directly imaged with a delta-doped CCD in the 250–750 eV range.

Metal–dielectric filters for solar–blind silicon ultraviolet detectors

We report on the development of coatings for a charged-coupled device (CCD) detector optimized for use in a fixed dispersion UV spectrograph. Because of the rapidly changing index of refraction of Si, single layer broadband antireflection (AR) coatings are not suitable to increase quantum efficiency at all wavelengths of interest. Instead, we describe a creative solution that provides excellent performance over UV wavelengths. We describe progress in the development of a coated CCD detector with theoretical quantum efficiencies (QEs) of greater than 60% at wavelengths from 120 to 300 nm. This high efficiency may be reached by coating a backside-illuminated, thinned, delta-doped CCD with a series of thin film AR coatings. The materials tested include MgF(2) (optimized for highest performance from 120-150 nm), SiO(2) (150-180 nm), Al(2)O(3) (180-240 nm), MgO (200-250 nm), and HfO(2) (240-300 nm). A variety of deposition techniques were tested and a selection of coatings that minimized reflectance on a Si test wafer were applied to functional devices. We also discuss future uses and improvements, including graded and multilayer coatings.

Atomic layer deposition of magnesium fluoride via bis(ethylcyclopentadienyl)magnesium and anhydrous hydrogen fluoride

We report on the fabrication of metal-dielectric thin film stacks deposited directly onto silicon substrates for use as ultraviolet bandpass filters. Integration of these filters onto silicon improves the admittance matching of the structure when compared to similar designs fabricated on transparent substrates, leading to higher peak transmission or improved out-of-band rejection if used with a Si-based sensor platform. Test structures fabricated with metallic Al and atomic layer deposited Al2O3 were characterized with spectroscopic ellipsometry and agree well with optical models. These models predict transmission as high as 90% the spectral range of 200-300 nm for simple three-layer coatings.

Enhanced quantum efficiency of high-purity silicon imaging detectors by ultralow temperature surface modification using Sb doping

A new process has been developed to deposit magnesium fluoride (MgF2) thin films via atomic layer deposition (ALD) for use as optical coatings in the ultraviolet. MgF2 was deposited in a showerhead style ALD reactor using bis(ethylcyclopentadienyl)magnesium and anhydrous hydrogen fluoride (HF) as precursors at substrate temperatures from 100 to 250 °C. The use of HF was observed to result in improved morphology and reduced impurity content compared to other reported MgF2 ALD approaches that use metal fluoride precursors as the fluorine-containing chemistry. Characterization of these films has been performed using spectroscopic ellipsometry, atomic force microscopy, and x-ray photoelectron spectroscopy for material deposited on silicon substrates. Films at all substrate temperatures were transparent at wavelengths down to 190 nm and the low deposition temperature combined with low surface roughness makes these coatings good candidates for a variety of optical applications in the far ultraviolet.

Single Photon Counting UV Solar-Blind Detectors Using Silicon and III-Nitride Materials

A low temperature process for Sb doping of silicon has been developed as a backsurface treatment for high-purity n-type imaging detectors. Molecular beam epitaxy (MBE) is used to achieve very high dopant incorporation in a thin, surface-confined layer. The growth temperature is kept below 450°C for compatibility with Al-metallized devices. Imaging with MBE-modified 1k×1k charge coupled devices (CCDs) operated in full depletion has been demonstrated. Dark current is comparable to the state-of-the-art process, which requires a high temperature step. Quantum efficiency is improved, especially in the UV, for thin doped layers placed closer to the backsurface. Near 100% internal quantum efficiency has been demonstrated in the ultraviolet for a CCD with a 1.5nm silicon cap layer.

Performance and prospects of far ultraviolet aluminum mirrors protected by atomic layer deposition

Ultraviolet (UV) studies in astronomy, cosmology, planetary studies, biological and medical applications often require precision detection of faint objects and in many cases require photon-counting detection. We present an overview of two approaches for achieving photon counting in the UV. The first approach involves UV enhancement of photon-counting silicon detectors, including electron multiplying charge-coupled devices and avalanche photodiodes. The approach used here employs molecular beam epitaxy for delta doping and superlattice doping for surface passivation and high UV quantum efficiency. Additional UV enhancements include antireflection (AR) and solar-blind UV bandpass coatings prepared by atomic layer deposition. Quantum efficiency (QE) measurements show QE > 50% in the 100–300 nm range for detectors with simple AR coatings, and QE ≅ 80% at ~206 nm has been shown when more complex AR coatings are used. The second approach is based on avalanche photodiodes in III-nitride materials with high QE and intrinsic solar blindness.

Delta-doped back-illuminated CMOS imaging arrays: progress and prospects

Abstract. Metallic aluminum mirrors remain the best choice for high reflectance applications at ultraviolet wavelengths (90 to 320 nm) and maintain good performance through optical and infrared wavelengths. Transparent protective coatings are required to prevent the formation of an oxide layer, which severely degrades reflectance at wavelengths below 250 nm. We report on the development of atomic layer deposition (ALD) processes for thin protective films of aluminum fluoride that are viable for application at substrate temperatures <200°C. Reflectance measurements of aluminum films evaporated in ultrahigh vacuum conditions, and protected mirrors encapsulated with ALD AlF3 are used to evaluate the far ultraviolet (90 to 190 nm) and near ultraviolet (190 to 320 nm) performance of both the ALD material and the underlying metal. Optical modeling is used to predict the performance of optimized structures for future astronomical mirror applications.