Fundamental studies of zincblende nitride structures for optoelectronic applications

Lead Research Organisation: University of Cambridge
Department Name: Materials Science & Metallurgy

Abstract

Over the last fifteen years there has been a revolution in terms of the availability and efficiency of GaN based light emitting diodes (LEDs) that emit principally blue light but also green light. These LEDs have found widespread application in displays, local monochrome lighting and most importantly so-called Solid State Lighting (SSL), in other words LED light bulbs. In an LED bulb, the white light is produced by mixing the blue light from an LED with yellow light from a phosphor. Some of the LED light is transformed into yellow light by the phosphor in a process called "down-conversion". The biggest advantage of LED bulbs is that they are much more efficient than incandescent or compact fluorescent light bulbs. Lighting uses about 20% of global electricity production, and has been identified as the single most wasteful component of domestic electricity use. SSL based on GaN LEDs, has the potential to reduce the consumption of energy used by lighting by a factor of 4. Down-converting blue light intrinsically wastes energy, and if the phosphor could be replaced by LEDs emitting other colours (green and red) with similar performance as the blue LED, the efficiency of SSL could be improved by 15 - 20%. At the moment, according to the US Department of Energy (Bardsley N et al. 2015 "US Department of Energy 2015 Solid-State Lighting R&D Plan"), the Power Conversion Efficiencies (PCE) of blue and green LEDS when operated to give sufficient light to illuminate a room are 60% and 22% respectively. The target PCE figures for the year 2020 from this report are blue 80% and green 35%. To achieve the stated improvements in PCE means that the IQE of blue and green emitters has to be increased significantly. Yet despite decades of research and development the best IQE values for both blue and green emitters have plateaued with little promise of any further significant improvements being achieved using the conventional technology.
In this program we propose to develop a new form of technology that could lead to LEDs with significant increases in IQE. At the moment the GaN based crystals that make up the active light emitting regions of LEDs are such that the atoms are arranged in a hexagonal pattern. This has the consequence that when an electron is injected into the crystal to generate light, the light emission process is slow, leaving plenty of time for the electron to lose energy by other processes that do not lead to light emission. These "non radiative" processes limit the IQE of LEDs. In this work we propose to produce cubic GaN crystals in such a form that the time for electrons to generate light is greatly reduced, thus offering up the possibility of LEDs with increased efficiency.
There are several challenges to be overcome in achieving this goal. Firstly the fabrication of cubic GaN without too many mistakes in the crystal's structure is very difficult. The main problem to be overcome is a natural tendency for the crystals to have faults in the way layers of atoms stack. We intend to study in depth the crystal growth process enabling us to eliminate this problem and produce crystals with many fewer faults. Secondly we must be able to control the ability of the GaN to conduct electricity so that we can successfully fabricate LEDS. We will investigate the processes whereby the conductivity may be limited with the aim of producing high conductivity material. Thirdly we will determine the details of the light emission process to determine whether the promise of higher efficiency is fulfilled.
 
Description It has been discovered that the emission from cubic quantum wells (QWs) is strongly polarized at room temperature. This means that LEDs based on such QWs can be used in devices such as displays making them significantly more efficient. Currently, the need for a separate polariser in LCD displays means that upto 50% of the light is thrown away.
Exploitation Route We are currently in discussion with a company called Kubos Semiconductors Ltd to develop this technology in to commercial devices
Sectors Digital/Communication/Information Technologies (including Software),Electronics

 
Description The findings have formed the basis of a patent application
First Year Of Impact 2019
Sector Digital/Communication/Information Technologies (including Software),Electronics
Impact Types Economic

 
Title Research data supporting "Effect of growth temperature and V/III-ratio on the surface morphology of MOVPE-grown cubic zincblende GaN" 
Description Figure 1: XRD intensity profile through the 1-103wz and 113zb reflections for the samples grown on a 4° miscut substrate at 875°C and a V/III-ratio of 76 (a), and 1200 (b). Figure 4: Feature sizes in (a) [110] and (b) [1-10] directions extracted from 2D-FFT of AFM height data of zb-GaN epilayers grown at different growth temperatures and a constant V/III-ratio of 76. (c) Variation of the aspect ratio of surface features with growth temperature. (d) Variation of root mean square surface roughness with growth temperature. Figure 5: Zb-GaN content determined by XRD as a function of the GaN epilayer growth temperature. Figure 8: Feature sizes in (a) [110] and (b) [1-10] directions extracted from 2D-FFT of AFM height data of the zb-GaN epilayers grown at different V/III-ratios and a constant growth temperature of 875 °C. (c) Variation of aspect ratio of surface features with V/III ratio. For (a) to (c), there are no data points for the sample grown at a V/III-ratio of 15, as it was not possible to extract feature sizes using the same 2D-FFT method as for other sample in the series. (d) Variation of root mean square surface roughness with V/III-ratio. The labels i, ii and iii indicate the proposed growth regimes. Figure 11: Relative intensities of the zb-GaN XRD peaks for samples in the V/III-ratio series at a constant growth temperature of 875 °C. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
 
Title Research data supporting "Effect of growth temperature and V/III-ratio on the surface morphology of MOVPE-grown cubic zincblende GaN" 
Description Figure 1: XRD intensity profile through the 1-103wz and 113zb reflections for the samples grown on a 4° miscut substrate at 875°C and a V/III-ratio of 76 (a), and 1200 (b). Figure 4: Feature sizes in (a) [110] and (b) [1-10] directions extracted from 2D-FFT of AFM height data of zb-GaN epilayers grown at different growth temperatures and a constant V/III-ratio of 76. (c) Variation of the aspect ratio of surface features with growth temperature. (d) Variation of root mean square surface roughness with growth temperature. Figure 5: Zb-GaN content determined by XRD as a function of the GaN epilayer growth temperature. Figure 8: Feature sizes in (a) [110] and (b) [1-10] directions extracted from 2D-FFT of AFM height data of the zb-GaN epilayers grown at different V/III-ratios and a constant growth temperature of 875 °C. (c) Variation of aspect ratio of surface features with V/III ratio. For (a) to (c), there are no data points for the sample grown at a V/III-ratio of 15, as it was not possible to extract feature sizes using the same 2D-FFT method as for other sample in the series. (d) Variation of root mean square surface roughness with V/III-ratio. The labels i, ii and iii indicate the proposed growth regimes. Figure 11: Relative intensities of the zb-GaN XRD peaks for samples in the V/III-ratio series at a constant growth temperature of 875 °C. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
 
Title Quantum Wires in Cubic group III Nitrides 
Description GaN-related structures in the cubic/zincblende phase are known as a promising alternative to the more widely-known wurtzite/hexagonal GaN semiconductors, and may be used to achieve improved efficiencies for long-wavelength (including Green amber and red) LEDs. Semiconductors, such as those comprising GaN, are known to give rise to photo-luminescent and electro-luminescent properties, where such semiconductors may be used in LED or photo-diode devices. Group-Ill nitride semiconductors generally offer a wide range of optoelectronic applications including LEDs, and laser diodes emitting in the blue, green, and red spectral region. However, conventional LED sources still require separate polarisation filters when used in display technology, which inherently reduces the transmission of light, and thus reduces efficiency of the system. Thus, it would be advantageous to obtain a polarised light source for use in LD or LCD displays, or other such devices requiring a polarised light source. 
IP Reference PC928981GB 
Protection Patent application published
Year Protection Granted 2019
Licensed No
Impact None so far