Conductivity and Hall-effect in highly resistive GaN layers

Abstract

Highly resistive GaN layers grown by molecular beam epitaxy are characterized by temperature dependent conductivity and Hall effect measurements. Samples with 300 Х3ϫ10 3 ⍀ cm show room temperature Hall mobility of 22 and 35 cm 2 V Ϫ1 s Ϫ1 and have a temperature dependence H ϳT x with xϭ0.9 and 0.5. This is in contradiction to a sample with 300 Х32 ⍀ cm which has a room temperature mobility of 310 cm 2 V Ϫ1 s Ϫ1 and a H ϳT x with xϭϪ1.4. The same activation energy of 0.23 eV, attributed to donor-like defects, is found for all three samples investigated. Temperature dependent conductivity data can be reasonably fitted considering band conduction. The presence of various hopping mechanisms is discussed. © 2000 American Institute of Physics. ͓S0003-6951͑00͒02825-4͔Highly resistive GaN buffer layers are important for field-effect transistor and high electron mobility transistor structures to avoid a parallel conductive channel that degrades device performance. However, not much is reported about their preparation and transport properties. Layers with resistivity of 14, 3ϫ10 3 , and 10 6 ⍀ cm were previously prepared by varying the stoichiometry during molecular beam epitaxial ͑MBE͒ growth.1 Another possibility to obtain high resistive GaN layers is by acceptor doping. MBE grown C-doped layers 2 and hydride vapor phse epitaxial Zn-doped layers 3 with resistivity of 10 6 and 10 12 ⍀ cm, respectively, have been reported. Unfortunately, carrier concentration and mobility data on samples with resistivity higher than 14 ⍀ cm are not present in the literature due to the use of conductive substrates, 3 or unmeasurable Hall coefficients. 1,4Hopping conduction, with extremely low Hall mobility H Ͻ1 cm 2 V Ϫ1 s Ϫ1, is assumed to be responsible for the latter case.1,5 When the concentrations of the autodoping centers and the deep defects are comparable, the layer becomes highly resistive and conduction by hopping among deep centers might occur. Additional models taking into account scattering on charged dislocations resulting in reduced mobility have been proposed. 6,7 To extend the knowledge of carrier transport behavior in GaN layers, we report in this letter on the temperature dependent conductivity and Hall effect measurements on highly resistive GaN layers grown by MBE.The undoped GaN layers were grown on sapphire substrates by MBE using a radio frequency atomic nitrogen plasma source.8 A low temperature AlN buffer was grown after nitridation of the sapphire. The substrate temperature was then increased to 750°C for the 2-m-thick GaN growth which was done under slightly Ga-rich flux ratios. High resolution x-ray diffraction measurements showed a reasonable layer quality with Х7 arc min full width at half maximum of the ͑0002͒ scan.9 Samples of about 6ϫ6 mm 2 were cut from each wafer and In contacts were alloyed at 850°C after appropriate surface cleaning. The ohmic behavior of the contacts was confirmed by current-voltage characteristics. Accurate temperature dependent conductivity and low magnetic field (Bϭ0.5 T͒ Hall eff...

Fast Growth of GaN Epilayers via Laser-Assisted Metal–Organic Chemical Vapor Deposition for Ultraviolet Photodetector Applications

“…We found that as the growth temperature increases from 930 to 960 and 990 °C, the mobility increases from 226 to 293 and 369 cm 2 V –1 s –1 and the carrier concentration decreases from 8.4 × 10 16 to 5.3 × 10 16 and 3.1 × 10 16 cm –3 , respectively. The mobility measured in our study is comparable to those reported in several studies for high-quality GaN films deposited using MOCVD and MBE at high temperatures. − The observed mobility increase with the increased temperature was also reported in the MOCVD GaN films . This can be attributed to the increased film thickness and the improved crystallinity; namely, the crystal defects in the GaN films affect their mobility by scattering the charge carriers. ,− …”

Conductivity and Hall effect characterization of highly resistive molecular-beam epitaxial GaN layers

“…However many questions concerning hopping in GaN are still unanswered. 1,2,11 Additionally, models taking into account scattering on charged dislocations resulting in lowering the mobility have been presented. [12][13][14] Lower instead of higher mobility at lower doping concentration, widely reported for low doped n-GaN, can be explained by the predominant influence of dislocation scattering in these layers.…”

Introduction

“…Unintentionally doped gallium nitride shows N-type in a variety of situations. The intrinsic carrier concentration of the gallium nitride sample reported in recent years can be reduced to about 10 16 cm À3 [26,27]. It is generally believed to be caused by nitrogen vacancies [28][29][30].…”