Hybrid white light sources based on layer-by-layer assembly of nanocrystals on near-UV emitting diodes

We present the design, growth, fabrication, integration and characterization of alternative hybrid white light sources based on the controlled layer-by-layer assembly of nanocrystals on UV-emitting nitride diodes for adjustable white light parameters. We hybridize CdSe/ZnS core–shell nanocrystals of different sizes (1.9–3.2–5.2 nm) on InGaN/GaN LEDs as a near-UV excitation source at 383 nm for efficient pumping. The first device includes layer-by-layer assembly of dichromatic cyanand red-emitting nanocrystals (λPL = 504–615 nm) leading to the tristimulus coordinates (x = 0.37, y = 0.46); the second device uses the trichromatic combination of layer-by-layer hybridized cyan-, yellowand red-emitting nanocrystals (λPL = 504–580–615 nm), yielding (x = 0.38, y = 0.48). Such layer-by-layer hybridization offers the advantages of precisely controlling individual nanocrystal film thicknesses and order in addition to concentrations. By utilizing such multiple combinations of nanocrystals in the assembly, the light parameters are well controlled and adjusted. Leveraging rapidly advancing UV technology into efficient lighting with nanocrystal based color conversion, it is critical to develop and demonstrate hybrid light sources on UV pumping platforms. (Some figures in this article are in colour only in the electronic version) White light emitting diodes (WLEDs) are promising devices for their potential use in many lighting applications, including in the display and automotive industries [1]. Up to the present, several approaches to generating white light have been exploited including multi-layer monolithic fabrication, multi-chip combinations and color conversion using phosphor molecules commonly pumped by blue-emitting nitride LEDs [2]. Among these approaches, the colorconversion technique using phosphors has been the most successful one and has been commercialized [2–4]. For phosphors, however, it is difficult to control the granule size and to mix and deposit uniform films, which disadvantageously results in undesired visible color variations [5]. Also, the phosphors’ photoemission properties are not easy to adjust and, consequently, the color parameters of its resulting white light are not as easy to tune as desired [2]. As an alternative light source, color-conversion WLEDs that rely on nanocrystal (NC) emitters instead of phosphors have recently been demonstrated to overcome these disadvantages [6–9]. In our previous research work, such hybrid light sources using multiple nanocrystals pumped by blue nitride LEDs have also been shown to allow their color parameters to be readily tuned as an additional advantage [6, 7]. Unlike using phosphors, employing such a combination of semiconductor nanocrystals makes color tuning possible because their peak emission wavelength can conveniently be controlled with 0957-4484/07/405702+04$30.00 1 © 2007 IOP Publishing Ltd Printed in the UK Nanotechnology 18 (2007) 405702 S Nizamoglu and H V Demir crystal size, enabling sensitive color tuning as the crystals can be synthetically prepared, each with a narrow size distribution for a comparatively narrow photoluminescence spectrum. In such hybrid NC-WLEDS, though, nanocrystal efficiency and hybridization significantly affect the device performance. In previous research work, mostly blue LED platforms are used as the excitation sources [6–9]. However, ultraviolet (UV) emitting LEDs provide a better platform for nanocrystal-based white light generation for a number of reasons. First, at UV wavelengths, the absorption of nanocrystals is higher than in blue and, as a result, with UV pumping, thinner nanocrystal films are needed to achieve white light generation compared to blue pumping. Second, with a UV pump source, the white light emission results solely from the photoemission of the nanocrystals. This implies that the generated white light does not directly depend on the LED platform but only on the combination of NC emitters, facilitating easier color tuning. Third, in the near future UV LEDs are expected to reach significantly higher optical power levels as they are increasingly aggressively pushed forward by other industrial, wide-scale, high-power applications (e.g. photolithography, inkjet printing, resin curing, etc.). In this regard, the famous Japanese LED maker Nichia (where blue LEDs were invented [3]) has announced the production of UV LEDs with output optical powers up to 5 W in the short term [10]. To leverage rapidly advancing UV technology into efficient lighting with nanocrystal-based color conversion, it is critical to develop and demonstrate hybrid light sources on UV pumping platforms. Very recently Ali et al showed white light generation on a commercial UV LED coated with blended CdSeS nanocrystals [11]. This work is particularly impressive for the synthesis of high-quality CdSeS NCs and white photoemission from the NC mixture under UV pumping. In this work we independently introduced and demonstrated an alternative hybrid light source that relies on the layer-by-layer assembly of multiple nanocrystals carefully hybridized on our near-UV emitting nitride diodes in a controlled manner for adjustable white light parameters for the first time. Such layer-by-layer hybridization offers advantages of controlling precisely the individual NC film thicknesses and NC film order in addition to setting the concentrations and overall film thickness. Here we report the design, growth, fabrication, integration and characterization of these hybrid NC–LED sources. In this work, we use CdSe/ZnS core–shell nanocrystals (emitting at λPL = 504, 580 and 615 nm) integrated on InGaN/GaN n-UV LEDs that we developed as a UV excitation source at λLED = 383 nm for our nanocrystals. Here, rather than blending NCs into a single film, we incorporate these nanocrystals of different sizes (1.9, 3.2 and 5.2 nm) layer by layer in adjacent thin films of a few hundred nanometers in thickness. Our first hybrid device includes the layer-bylayer assembly of cyanand red-emitting NCs (λPL = 504 and 615 nm). For this dichromatic NC combination, the tristimulus coordinates (x, y) are (0.37, 0.46) with correlated color temperature Tc = 4529 K and color rendering index Ra = 43.1. Our second device uses the layer-by-layer hybridization of a trichromatic combination of cyan-, yellowand red-emitting NCs (λPL = 500, 540 and 620 nm), with (x, y) = (0.38, 0.48), Tc = 4474 K and Ra = 67.6, respectively. The use of such nanocrystal combinations Figure 1. Photoluminescence (PL) and absorption spectra of our CdSe/ZnS core–shell nanocrystals in thin films at room temperature. enables the color properties of the resulting emission such as tristimulus coordinates, correlated color temperature and color rendering index to be controlled and adjusted. For white light generation, we use CdSe/ZnS core–shell nanocrystals of crystal sizes 1.9, 3.2 and 5.2 nm (with a size distribution of ±5%) from Evident. The emission colors of these nanocrystals are cyan, yellow and red, tuned using the quantum size effect across the visible spectral range with their corresponding photoluminescence (PL) peaks at 504 nm, 580 nm and 615 nm, respectively. These nanocrystals have high PL quantum yields ranging between 30 and 50%. Such core–shell NCs are shown to yield even higher efficiencies up to 66% [12]. We used similar types of nanocrystals in our previous work for white light sources integrated on blue nitride LEDs [6, 7] and nanocrystal-based UV scintillators integrated on Si detectors [13]. In this work, we prepare highconcentration NC solutions to vortex-mix into the host polymer of poly(methyl-methacrylate) (PMMA). We evaporate NC films in micro-droplets of 25 μl by drop-casting at 70–100 ◦C for optimal film formation and complete the polymerization process for each layer in the assembly. Figure 1 shows the photoluminescence and absorption spectra of our CdSe/ZnS core–shell nanocrystals in thin films of the host PMMA. We use InGaN/GaN-based LEDs with a peak wavelength of 383 nm in the near-ultraviolet as the pump source for the entire hybrid devices presented in this paper. Figure 2 shows the design of our LEDs. For epitaxial growth, we use a GaN dedicated metal–organic chemical vapor deposition (MOCVD) system. First, we begin with a 14 nm thick GaN nucleation layer. To increase the crystal quality of the subsequent epitaxial layers, we then grow a 200 nm thick GaN buffer layer. For the n-type contact, we grow a 690 nm thick Si-doped epitaxial layer. For the active layers, we continue the epi-growth with five 2–3 nm thick InGaN wells and GaN barriers at a growth temperature of 720 ◦C. We finish our growth with the p-type layers that consist of a 50 nm thick Mg-doped AlGaN layer and a 120 nm thick Mg-doped GaN layer as the contact cap. Finally, we activate the Mg dopants at 750 ◦C for 15 min. We used similar growth steps for the development of our GaNbased quantum electroabsorption modulators [14]. Our device fabrication follows standard semiconductor processing procedures as in our previous work [14–16]. These include photolithography, thermal evaporation (metallization),