GaN High Power Electronics
In Proceedings of the Electrochemical Society (September 2011), pp. 2209-2209
Wide band gap (WBG) semiconductor devices can condition electrical power more efficiently than silicon devices primarily because they have much larger breakdown fields. This enables the WBG to be more heavily doped to achieve a given breakdown voltage so that the devices have a lower on-resistance, less switching loss, and they can switch more rapidly. WBGs have the additional advantage of being able to operate efficiently at higher temperatures, which reduces the cooling requirements and frees up valuable space. Even though GaN has a larger critical field and higher electron mobility than SiC, SiC has been the WBG of choice because, until very recently, higher quality SiC substrates were available. Recently, 2” high quality GaN crystals have been grown in Poland whose crystal quality meets or exceeds that of SiC. This, the fact that GaN based transistors can have a significantly lower on-resistance, and they can be used as a stepping stone to future AlGaN devices with even larger breakdown fields, makes GaN an attractive material to investigate for high power applications now. They can have a smaller on-resistance because the channel through which the electrons flow is located at the interface between epitaxially grown films, as opposed to an interface between an amorphous dielectric and a semiconductor film. In addition, the electrons are in a 2-dimensional electron gas (2DEG) in which the electrons have a higher mobility than they do in the bulk, as opposed to a lower mobility. Less expensive, but poorer quality hydride vapor phase epitaxy (HVPE) substrates, which are still preferable to hetero-substrates are also available. We have used them, as well as sapphire substrates, to fabricate Schottky diodes focused on applications that require a breakdown voltage > 600 V to determine if GaN devices are appropriate for this regime, as well as better understand how defects degrade device properties. We determined that GaN diodes can operate in this regime, but that the problem of C incorporation during the growth of device structures by the metal organic chemical vapor deposition (MOCVD) technique will have to be dealt with if these devices are to reach their true potential. Carbon has not been a serious problem for GaAs or InP, as it appears that growth conditions can be adjusted so that the C incorporation from the methyl groups in trimethyl-gallium or -indium (TMG or TMI) is in the low to mid 1014 cm-3. However, due to the apparent difficulty of thermally removing the H atoms from NH3, the N precursor, the growth temperature is typically ~1050oC, as opposed to ~ 600 – 700 oC, so the CH3 group is more likely to decompose before it reacts. Increasing the V/III ratio and increasing the growth pressure reduces the C incorporation, but it is still significant. C incorporation has not been seen as an important problem for RF high electron mobility transistors (HEMTs) or light emitting/detecting devices because they are doped more heavily (>1017 cm-3) than the background carrier concentration of the C (1 – 5 x1016 cm-3). C can substitute for N and act as a deep acceptor, substitute for Ga and act as a shallow donor, or incorporate as an interstitial and create empty states near the center of the energy gap. More work has to be done to determine how the C is incorporated. We show that one possible way to circumvent this problem is to fabricate diodes on the low doped HVPE substrates, but then the substrate resistance becomes a problem. This points to a need to develop a process for growing more heavily doped, high quality HVPE substrates with a low doped, high quality film grown on top of it.
Identifying threading dislocations in GaN films and substrates by electron channelling
Blackwell Publishing Ltd, Journal of Microscopy, Vol. 244, No. 3. (1 August 2011), pp. 311-319, doi:10.1111/j.1365-2818.2011.03538.x
Electron channelling contrast imaging of threading dislocations in GaN (0002) substrates and epitaxial films has been demonstrated using a conventional polepiece-mounted backscatter detector in a commercial scanning electron microscope. The influence of accelerating voltage and diffraction vector on contrast features denoting specific threading dislocation types has been studied. As confirmed by coordinated transmission electron microscopy analysis, electron channelling contrast imaging contrast features for edge-type threading dislocations are spatially smaller than mixed-type threading dislocations in GaN. This ability to delineate GaN edge threading dislocations from mixed type was also confirmed by defect-selective etch processing using molten MgO/KOH. This study validates electron channelling contrast imaging as a nondestructive and widely accessible method for spatially mapping and identifying dislocations in GaN with wider applicability for other single-crystal materials.
Dual temperature process for reduction in regrowth interfacial charge in AlGaN/GaN HEMTs grown on GaN substrates
physica status solidi (c), Vol. 8, No. 7-8. (19 May 2011), pp. 2053-2055, doi:10.1002/pssc.201001059
The effects of growth temperature and Mg compensation doping on the structural and electrical properties of AlGaN/AlN/GaN high electron mobility transistor (HEMTs) structures grown on low threading dislocation density bulk GaN substrates by metalorganic chemical vapor deposition were investigated. The background electron concentration in the regrown GaN was found to decrease from 1.5×1018 cm-3 to 2×1016 cm-3 as the growth temperature was reduced from 1100 °C to 950 °C. A dual temperature process was then employed for growth of the GaN base layer and AlGaN/AlN/GaN top heterostructure in the HEMT along with Mg doping of the GaN base to compensate residual Si donors at the regrown interface. Using this approach, AlGaN/AlN/GaN HEMTs were produced that had a sheet carrier density as high as 1.1×1013 cm-3 with a room temperature mobility of 1600 cm2/Vs. The incorporation of a thin AlN layer at the regrown interface was found to reduce the sheet carrier density of the overall structure.
On the reduction of efficiency loss in polar c -plane and non-polar m -plane InGaN light emitting diodes
physica status solidi (c), Vol. 8, No. 5. (14 March 2011), pp. 1560-1563, doi:10.1002/pssc.201000893
We have undertaken a series of experiments in InGaN light emitting diode (LED) structures both on polar (c-plane) and non-polar (m-plane) GaN substrates with and without magnesium doped AlGaN electron blocking layers (EBLs) on the p-side of the p-n junction to shed the much needed light on the carrier injection and transport. The LEDs grown on c-plane bulk GaN substrates without EBL show 40% peak electroluminescence (EL) intensity, while the EL peak intensity of the LEDs grown on m-plane bulk GaN substrates without EBL is 30% of that with EBL. However, optical measurements for internal quantum efficiency (IQE) reveal that the IQE values of LEDs with and without EBL are comparable for both cases, which are in the range of 80-85% for m-plane variety and 50% for c-plane variety. Furthermore, with varying Al composition (15%, 8%, 0%) in the EBL, the EL intensity for both m-pane and c-plane LEDs decrease progressively as the Al composition decreases. When highly conductive and transparent Ga doped ZnO layers are used as the current spreading layers for LEDs, the degradation of the efficiency with injection is significantly reduced to 27% compared to LEDs with semitransparent Ni/Au contacts having 50% efficiency degradation up to the same current density ∼3500 A/cm2. This can be due to presumably the reduction in current filamentation or current crowding.
The effect of ballistic and quasi-ballistic electrons on the efficiency droop of InGaN light emitting diodes
WILEY-VCH Verlag, phys. stat. sol. (RRL), Vol. 4, No. 8-9. (08 September 2010), pp. 194-196, doi:10.1002/pssr.201004147
The effect of hot electrons on electroluminescence of m -plane double heterostructure light emitting diodes (with a single 6 nm active layer of In0.20Ga0.80N) is investigated. Diodes with an electron blocking layer (Al0.15Ga0.85N) demonstrate from 3 to 5 times higher electroluminescence efficiency than those without a blocking layer. The lower electroluminescence efficiency in devices without the blocking layer is ex- plained in terms of electron overflow caused by ballistic and quasi-ballistic transport of injected electrons across the InGaN active region. The same mechanism explains the decrease, observed at high current densities, of the electroluminescence efficiency (efficiency droop) in the In0.20Ga0.80N diodes with the Al0.15Ga0.85N blocking layer. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
On carrier spillover in c- and m-plane InGaN light-emitting diodes
The International Society for Optical Engineering., Proceedings of SPIE In Photonics West: Integrated Optoelectronic Devices 2010, Vol. 7602, No. 1. (11 February 2010), 760224, doi:10.1117/12.845806
The internal quantum efficiency (IQE) and relative external quantum efficiency (EQE) in InGaN LEDs emitting at 400 nm with and without electron blocking layers (EBLs) on c-plane GaN and m-plane GaN were investigated in order to shed some light on any effect of polarization charge induced field on efficiency killer carrier spillover. Without an EBL the EQE values suffered considerably (by 80 %) for both orientations, which is clearly attributable to carrier spillover. Substantial carrier spillover in both polarities, therefore, suggests that the polarization charge is not the major factor in efficiency degradation observed, particularly at high injection levels. Furthermore, the m-plane variety with EBL did not show any discernable efficiency degradation up to a maximum current density of 2250 Acm-2 employed while that on cplane showed a reduction by ~ 40 %. In addition, IQE of m-plane LED structure determined from excitation power dependent photoluminescence was ~80 % compared to 50 % in c-plane LEDs under resonant and moderate excitation condition. This too is indicative of the superiority of m-plane LED structures, most probably due to relatively larger optical matrix elements for m-plane orientation.
Internal quantum efficiency of m-plane InGaN on Si and GaN
The International Society for Optical Engineering., Proceedings of SPIE In Photonics West: Integrated Optoelectronic Devices 2010, Vol. 7602, No. 1. (11 February 2010), pp. 76021N-76021N-6, doi:10.1117/12.843727
High brightness InGaN light emitting diodes (LEDs) require high quantum efficiency and its retention at high injection levels. The efficiency drop at a high injection levels in InGaN light emitting diodes (LEDs) has been attributed, e.g. to polarization field on polar c-plane InGaN and the heavy effective hole mass which impedes high hole densities and transport in the active quantum wells. In this study, we carried out a comparative investigation of the internal quantum efficiency (IQE) of InGaN active region in LED structures using resonant optical excitation for layers with polar (0001) orientation on c-plane sapphire, and nonpolar (1-100) m-plane orientation, the latter on specially patterned Si and on m-plane bulk GaN. Analysis of the resonant photoluminescence (PL) intensity as a function of the excitation power indicate that at comparable generated carrier concentrations the IQE of the m-plane InGaN on Si is approximately a factor of 2 higher than that of the highly optimized c-plane layer. At the highest laser excitation level employed (corresponding carrier concentration n ~ 1.2 x 1018 cm-3), the m-plane LED structure on Si has an IQE value of approximately 65%. We believe that the m-plane would remain inherently advantageous, particularly at high electrical injection levels, even with respect to highly optimized c-plane varieties. The observations could be attributed to the lack of polarization induced field and the predicted increased optical matrix elements.
InGaN staircase electron injector for reduction of electron overflow in InGaN light emitting diodes
AIP, Applied Physics Letters, Vol. 97, No. 3. (2010), 031110, doi:10.1063/1.3465658
Ballistic and quasiballistic electron transport across the active InGaN layer are shown to be responsible for electron overflow and electroluminescence efficiency droop at high current levels in InGaN light emitting diodes both experimentally and by first-order calculations. An InGaN staircase electron injector with step-like increased In composition, an “electron cooler,” is proposed for an enhanced thermalization of the injected hot electrons to reduce the overflow and mitigate the efficiency droop. The experimental data show that the staircase electron injector results in essentially the same electroluminescence performance for the diodes with and without an electron blocking layer, confirming substantial electron thermalization. On the other hand, if no InGaN staircase electron injector is employed, the diodes without the electron blocking layer have shown significantly lower (three to five times) electroluminescence intensity than the diodes with the blocking layer. These results demonstrate a feasible method for the elimination of electron overflow across the active region, and therefore, the efficiency droop in InGaN light emitting diodes.
Efficiency retention at high current injection levels in m-plane InGaN light emitting diodes
AIP, Applied Physics Letters, Vol. 95, No. 12. (21 September 2009), 121107, doi:10.1063/1.3236538
We investigated the internal quantum efficiency (IQE) and the relative external quantum efficiency (EQE) of m -plane InGaN light emitting diodes(LEDs) grown on m -plane freestanding GaN emitting at ∼ 400 nm for current densities up to 2500 A / cm 2 . IQE values extracted from intensity and temperature dependent photoluminescencemeasurements were consistently higher, by some 30%, for the m -plane LEDs than for reference c -plane LEDs having the same structure, e.g., 80% versus 60% at an injected steady-state carrier concentration of 1.2 × 10 18 cm − 3 . With increasing current injection up to 2500 A / cm 2 , the maximum EQE is nearly retained in m -plane LEDs, whereas c -plane LEDs exhibit approximately 25% droop. The negligible droop in m -plane LEDs is consistent with the reported enhanced hole carrier concentration and light holes in m -plane orientation, thereby enhanced hole transport throughout the active region, and lack of polarization induced field. A high quantum efficiency and in particular its retention at high injection levels bode well for m -plane LEDs as candidates for general lighting applications.
Internal quantum efficiency of c-plane InGaN and m-plane InGaN on Si and GaN
AIP, Applied Physics Letters, Vol. 95, No. 10. (07 September 2009), 101106, doi:10.1063/1.3224192
We investigated internal quantum efficiency (IQE) of polar (0001) InGaN on c -sapphire, and ( 1 1 ¯ 00 ) nonpolar m -plane InGaN on both m -plane GaN and specially patterned Si. The IQE values were extracted from the resonant photoluminescence intensity versus the excitation power. Data indicate that at comparable generatedcarrier concentrations the efficiency of the m -plane InGaN on patterned Si is approximately a factor of 2 higher than that of the highly optimized c -plane layer. At the highest laser excitation employed ( ∼ 1.2 × 10 18 cm − 3 ) , the IQE of m -plane InGaN double heterostructure on Si is approximately 65%. We believe that the m -plane would remain inherently advantageous, particularly at high electrical injection levels, even with respect to highly optimized c -plane varieties. The observations could be attributed to the lack of polarization induced field and the predicted increased optical matrix elements in m -plane orientation.
On carrier spillover in c- and m-plane InGaN light emitting diodes
AIP, Applied Physics Letters, Vol. 95, No. 20. (2009), 201113, doi:10.1063/1.3266833
The internal quantum efficiency (IQE) and relative external quantum efficiency (EQE) in InGaN light-emitting diodes (LEDs) emitting at 400 nm with and without electron blocking layers (EBLs) on c-plane GaN and m-plane GaN were investigated in order to shed some light on any effect of polarization charge induced field on efficiency killer carrier spillover. Without an EBL the EQE values suffered considerably (by 80%) for both orientations, which is clearly attributable to carrier spillover. Substantial carrier spillover in both polarities, therefore, suggests that the polarization charge is not the major factor in efficiency degradation observed, particularly at high injection levels. Furthermore, the m-plane variety with EBL did not show any discernable efficiency degradation up to a maximum current density of 2250 A cm−2 employed while that on c-plane showed a reduction by ∼ 40%. In addition, IQE of m-plane LED structure determined from excitation power dependent photoluminescence was ∼ 80% compared to 50% in c-plane LEDs under resonant and moderate excitation condition. This too is indicative of the superiority of m-plane LED structures, most probably due to relatively larger optical matrix elements for m-plane orientation.
Low-Temperature Synthesis of Large-Area, Free-Standing Nanorod Arrays on ITO/Glass and other Conducting Substrates
Advanced Materials, Vol. 20, No. 23. (02 December 2008), pp. 4470-4475, doi:10.1002/adma.200801253
Anodic alumina thin-film templates of unprecedented quality are fabricated on ITO/glass, silicon, and flexible substrates over 2 cm2 areas, of interest for devices. For the first time, free-standing nanorods of a variety of oxides and metals are reproducibly synthesized over large areas on ITO/glass by electrochemical deposition into the vertically aligned nanopores of the templates, followed by template removal.