Abstract

LED is revolutionizing humans lighting environments.  There have been timelines established to replace incandescent or fluorescent lamps worldwide.  However, the current white light LED is based on GaN semiconductor that is epitaxially deposited on either sapphire or SiC wafers.  Due to the non-equilibrium growth of GaN lattice on mismatched Al2O3 substrate, or defects ridden SiC, there are ample dislocations (e.g. 109/cm2) that may reduce the internal quantum efficiency of photon emission.  Worse still, the dislocations may propagate irreversibly upon heating.  This is why LED will dim with time.  In order to avoid LED chips from overheating, the heat blocking epoxy insulator on the circuit board is replaced by diamond like carbon (DLC).  The result is a dramatic decrease of the chip temperature.

Diamond can be the substrate itself for growing epitaxial GaN.  Better still, single crystal AlN can be deposited directly on diamond surface.  Boron doped diamond has the highest mobility of holes, and silicon doped AlN can boost electron mobility.  The AlN on diamond is capable to emit ultraviolet (UV) light with high intensity.  Such UV light can excite phosphors for the emission of different colors, including white light with balanced RGB distribution.

There are many possibilities of making super LED with diamond.  Unfortunately, diamond wafers are not available commercially.  However, synthetic diamond crystals can be made cheaply by a noval seeding technology (DiaCan™).  These diamond crystals are embedded in a ceramic matrix to form diamond islands wafer (DIW).  DIW is the enabling substrate for making super LED in the near future.

Light Revolution

LED is replacing conventional lamps as the mainstream lighting device.  The LED business is booming, in particular, for the so called high brightness LED.

Fig. 1: LED shipments have been escalating with high brightness LED increasing in proportions.

The white light LED technologies are covered by patents of few large companies (e.g. yellow YAG phosphors triggered by blue emission of GaN by Nichia).  It is difficult to enter this business without paying a lot of royalties.  In fact, most LED manufacturers (e.g. Taiwanese) are making little money compared to those with strong IP portfolios.

Fig. 2: The tight licensing network of high brightness LED.

Diamond Technologies

LED is based on semiconductors, in particular, nitride semiconductors with wurtzitic structure such as GaN.  GaN is epitaxially deposited by MOCVD (Metal Organic Chemical Vapor Deposition) on substrate of either sapphire (Al2O3) or SiC.  Sapphire has a large lattice mismatch (16%) with GaN, consequently, the hetero epitaxy is ridden with defects (e.g. dislocation density in the order of 109/cm2).  On the other hand, SiC substrate is grown by vapor deposition (e.g. by CREE).  Due to the non-equilibrium process of condensation, SiC itself contains high density of defects.  Due to the presence of densely distributed dislocations in the GaN lattice currently employed, photons emitted by the combination of electrons and holes can rapidly annihilate at the defect site with the release of heat.  Additional problems associated with high brightness LED is the thermal degradation due to the propagation of defects.  In fact, with high current (e.g. 1A) application, high brightness LED can easily burn out.

The above limitations can be removed by using diamond materials.  Diamond is the king of semiconductors with many superlative properties.  For example, diamond’s thermal conductivity is 60 times of Al2O3; and 4 times of SiC.  Additionally, boron doped diamond is the most efficient p-type semiconductor with the highest hole mobility (1600 cm2/Vs), even under high carrier (1018/cm3) concentration.  Consequently, diamond can become the super LED with built in heat spreader.

Table 1: Semiconductor Properties


Although diamond is the crown of semiconductors, diamond wafers for making LED is not available.  This problem is solved by developing diamond islands wafers (DIW).  DIW uses the cheap industrial diamond crystals synthesized under ultrahigh pressure as “bricks”.  These bricks are cemented in a ceramic matrix (e.g. alumina) to form a rigid wafer that is highly thermally stable.  DIW can be used to grow nitride LED with MOCVD with conventional processing.

Fig. 3: The concept of diamond islands wafer and the photos of prototype.

Synthetic diamond crystals can be most efficiently synthesized by using the DiaCan™ technology developed by the first author.  This technology can allow diamond crystals to grow on planted seeds with patterned distribution under ultrahigh pressure (e.g. in a cubic press).  Due to the controlled growth environment, similar shaped (e.g. cubicles) diamond crystals can be made with high yield.  The production cost of making such diamond crystals is low.  For examples, 30/40 mesh (600 to 425 microns) of diamond crystals can be produced with a cost of about 2000 per USD.


Fig. 4: The DiaCan™ technology with the formation of diamond crystals on seeded micron diamond particles (left diagram).  By lowering the growth temperature (e.g. 1250℃) under ultrahigh pressure (e.g. 5.2 GPa), diamond cubicles may be formed (right diagram).

Another technology is to deposit diamond like carbon (DLC) on printed circuit board (PCB) for mounting LED chips  For example, DLC may replace epoxy as the insulating layer on the aluminium board.  In this case, electroplated copper is plated on DLC as circuits.  DLC has a thermal conductivity (500 w/mK) that is three orders of magnitude higher than epoxy (0.5 w/mK).  Moreover, DLC is a black body that can radiate infrared (IR) heat toward surrounding (e.g. to air molecules).  Consequently, DLC coating can dissipate heat much more effectively than conventional PCB for LED.

Fig. 5: DLC can coat PCB with different surface smoothness.

With the above technologies available for thermal management, super LED can be fabricated with diamond as substrate or as P type semiconductor.

Fig. 6: Super LED architecture with an UV LED sandwiched between diamond films and mounted on DLC coated PCB.

DLC PCB

Conventional GaN based LED chips were tested with DLC coated PCB.  It was found that temperatures differences among chips on the same board could be homogenized to less than 1 ℃.

Fig. 7: Chip temperatures were highly uniform with DLC coated PCB.  Under the same conditions, uncoated LED chip may vary by more than 3℃.