We have investigated the spectral response of front-surface-illuminated GaN and AlGaN/GaN p-i-n ultraviolet photodetectors prepared by reactive molecular beam epitaxy on sapphire substrates. GaN homojunction p-i-n photodiodes exhibited a peaked response near the band edge. This enhanced response was absent in the AlGaN/GaN heterojunction p-i-n detectors. We analyzed the effect of p-layer thickness of the GaN p-i-n diodes on the magnitude of the peak photoresponse. The AlGaN/GaN photodiodes had a maximum zero-bias responsivity of 0.12 A/W at 364 nm, which decreased by more than 3 orders of magnitude for wavelengths longer than 390 nm. A reverse bias of −10 V raised the responsivity to 0.15 A/W without any significant increase in noise. The root-mean-square noise current in a 1 Hz bandwidth is ∼1.0 pA, corresponding to a noise-equivalent-power of ∼8.3 pW. We measured extremely fast decay times of 12 ns for the AlGaN/GaN and 29 ns for the GaN photodiodes.

A method of growing semi-insulating GaN epilayers by ammonia molecular beam epitaxy through intentional doping with carbon is reported. Thick GaN layers of high resistivity are an important element in GaN-based heterostructure field-effect transistors. A methane ion source was used as the carbon dopant source. The cracking of the methane gas by the ion source was found to be the key to the effective incorporation of carbon. High-quality C-doped GaN layers with resistivities greater than 106 Ω cm have been grown with high reproducibility and reliability. AlGaN/GaN heterostructures grown on the C-doped semi-insulating GaN-based layers exhibited a high-mobility two-dimensional electron gas at the heterointerface, with room-temperature mobilities typically between 1000 and 1200 cm2/V s, and liquid-nitrogen-temperature mobilities up to 5660 cm2/V s. The carrier density was almost constant, with less than 3% change over the measured temperature range.

The properties of carbon-doped GaN epilayers grown by molecular-beam epitaxy have been studied by temperature-dependent resistivity, Hall-effect measurements, x-ray diffraction, and by photoluminescence spectroscopy. Carbon doping was found to render the GaN layers highly resistive (>108 Ω cm) and quench the band edge excitonic emissions. Yellow luminescence is still present in carbon-doped GaN layers. The highly resistive state is interpreted as being caused by direct compensation by the carbon acceptors and by the consequently enhanced potential barrier at the subgrain boundaries. Evidence of dislocations joining to form potential barriers along the subgrain boundaries was observed in photoassisted wet etching experiments on electrically conducting GaN layers. GaN films grown on insulating carbon-doped base layers are of excellent transport and optical properties.

The mechanism of the UV photoenhanced wet etching of GaN is determined. The UV photoenhanced wet etching does not require an electrical contact to be made to the sample, and nitrides deposited on insulating substrates (such as sapphire) can be etched, unlike photoelectrochemical (PEC) wet etching. The present technique relies on adding an appropriate oxidizing agent, in this case, peroxydisulfate (S2O82−), to KOH solutions. In a similar mechanism to PEC wet etching, the regions of low defect density are preferentially etched, leaving regions of high electron recombination such as threading dislocations relatively intact. The threading dislocations may be physically broken off, either by stirring or by a postetch sonication of the sample in KOH solution. Smoothly etched surfaces can be obtained under the proper conditions. A noble metal mask acts in a catalytic manner, yielding etch rates approximately one order of magnitude greater than those observed using inert masks. The essential role of the free radicals, originating from the peroxydisulfate ion, in the etching reaction is confirmed. The etching reaction is more rapid for more heavily n-type doped samples, and insulating C-doped layers act as an etch stop layer.

High quality GaN epilayers have been grown on oxygen and zinc surfaces of ZnO (0001) substrates by reactive molecular beam epitaxy and the effect of the intermediate buffer layer on the structural and optical properties of the GaN films has been investigated. The optical and structural characterization of the GaN epilayers and ZnO substrates were performed using photoluminescence, reflectivity, x-ray double diffraction, atomic force microscopy, and transmission electron microscopy. The optical results indicated that GaN was grown with compressive strain due to the difference in thermal expansion coefficient between GaN and ZnO. The surface roughness has been reduced by using an intermediate low temperature GaN buffer layer. The low temperature photoluminescence spectra of GaN/ZnO epilayers did not reveal any sign of the well-known midgap yellow signal. Linear polarized reflectivity and photoluminescence indicated that GaN epilayer planes were not misoriented with respect to the ZnO substrate planes: this result was confirmed by x-ray double diffraction measurements.

Likely contamination of GaN films by impurities emanating from Al2O3, SiC, and ZnO substrates during growth has been studied by secondary ion mass spectrometry (SIMS) analysis. The defective near-substrate region allows impurities to incorporate more readily as compared to the more perfect crystal as evidenced by increased impurity levels in that region detected by SIMS. The SIMS measurements in GaN layers grown on SiC, ZnO, and sapphire showed large amounts of Si, Zn, and O, respectively, within a region wider than the defective near-substrate layer pointing to the possibility of impurity diffusion at growth temperatures. The qualitative trend observed is fairly clear and significant.

Articles you may be interested inGate frequency sweep: An effective method to evaluate the dynamic performance of AlGaN/GaN power heterojunction field effect transistors Appl. Phys. Lett. 105, 073507 (2014); 10.1063/1.4893607AlGaN/GaN field effect transistors for power electronics-Effect of finite GaN layer thickness on thermal characteristics Appl.Self-heating is an important issue for GaN heterostructure field effect transistors ͑HFETs͒, especially in high power applications. Here we report the temperature dependence of the dc characteristics of some GaN HFETs including the variation of the transconductance. We present the characteristics as a function of added power, instead of voltage bias, and use the temperature data to transform the power dependence into a dependence on the average device temperature. For similar devices on sapphire and SiC, at 20 V V DS and 0 V V G , the temperature increase for the same added power is ϳ2.7 times greater in the sapphire-based device.

Mg-doped GaN samples prepared by reactive molecular beam epitaxy have been investigated in an attempt to gain insight into the impurity incorporation and the origin of auto doping in otherwise undoped GaN films. The Hall and secondary ion mass spectroscopy data were utilized for the analysis of possible background impurities such as Si, O, and H in an effort to ascertain whether the background electron concentration is of impurity origin or native defect origin. The data appear to support the N vacancy as a possible cause of auto-n-type doping seen in undoped GaN. The effect of the ammonia flow rate on the incorporation of Mg atoms in GaN films and on the behavior of H were studied for layers grown on c-plane sapphire as well as 6H-SiC. Increased incorporation of Mg with larger ammonia flow rates is attributed to Ga vacancies and accompanying site selection. Moreover, p-GaN films grown under high ammonia flux are reported with a hole concentration, mobility, and resistivity of about 8ϫ10 17 cm Ϫ3 , 26 cm 2 /V s, and 0.3 ⍀ cm, respectively.