Electrical characterization of Mg-doped GaN grown by metalorganic vapor phase epitaxy

Abstract

We have applied frequency-dependent capacitance measurements and admittance spectroscopy on GaN:Mg to study the electronic states associated with Mg doping. Metalorganic vapor phase epitaxy GaN:Mg samples with two different Mg doping levels were grown and thermally annealed in nitrogen. Lateral dot-and-ring Schottky diodes using Au/Ti were fabricated. Frequency-dependent measurements on these diodes show that the capacitance is reduced at a higher frequency, most likely due to the inability of a deep center to maintain an equilibrium ionization state under a high-frequency modulation. Admittance spectroscopy, in which the conductance is monitored as a function of temperature, verifies the existence of one impurity-related acceptor level in the higher Mg-doped sample with an activation energy of 136 meV. For the lower Mg-doped sample, two acceptor levels at 124 and 160 meV were observed. We believe these levels are most probably associated with the Mg acceptor state itself, possessing energy levels which are very close to the results previously reported in the literature. © 1996 American Institute of Physics. ͓S0003-6951͑96͒01617-9͔The successful p-type doping in metalorganic vapor phase epitaxy ͑MOVPE͒ GaN using Mg has allowed the realization of blue light emitting p-n junction diodes. Treatments, such as electron-beam irradiation 1 or thermal annealing, 2 are usually required to activate the p-type conduction. Electrical characteristics of these Mg-related acceptors are, however, not well understood. The Mg acceptor binding energy has been reported to be 250 and ϳ160 meV from the donor-acceptor pair transition and from temperature-dependent photoluminescence ͑PL͒ experiments, respectively.3 The temperature-dependent Hall measurements revealed a Mg-related activation energy of 157 meV. 4 In this letter, we report the electrical characterization of GaN:Mg, using frequency-dependent capacitance measurements and admittance spectroscopy, in order to study the electronic states associated with Mg doping. It is well known that deep traps greatly affect the free carrier profiles, as determined by capacitance-voltage (C -V) measurements. 5Given the aforementioned energy level depths associated with Mg, as well as the fact that Mg-doping is responsible for most or all of the observed p-type conductivity, the small signal capacitance of Mg-doped GaN is expected to be dependent on measurement frequency. We had also found that deep level transient spectroscopy ͑DLTS͒ could not be successfully used to characterize these Mg-related deep levels, principally due to the low diode capacitance level at low measurement temperatures. In contrast, admittance spectroscopy is better suited for the study of these relatively shallow and/or fast levels. Mg-doped GaN samples were grown in a horizontal MOVPE reactor at 100 Torr using trimethyl gallium ͑TMGa͒, bis-cyclopentadienyl magnesium ͑Cp 2 Mg), and ammonia (NH 3 ) on ͑0001͒ sapphire substrates.7 The Mgdoped layer, ϳ2 m in thickness, was grown at 1050°C at a growth rate of ϳ2 m/h. Two ...

Thermal admittance spectroscopy of Mg-doped GaN Schottky diodes

“…From the study of the temperature dependence of the hole concentration obtained by Hall measurements on GaN layers, values of 112 to 190 meV are given, 4 -9 by admittance spectroscopy, values of 70 to 160 meV are cited. [2][3][4] In some cases, the value obtained by admittance measurements was lower than that obtained from the Hall method. 4 Photoluminescence studies of the donor-acceptor recombination yield again a different set of values between 150 and 250 meV.…”

“…2,5,29 Their energy positions depend on the growth method and sample preparation, but most are located at least at 100 meV above the valence band edge. In our study, their concentration N tB is rather weak as compared to the value of N tA , and their effect only appears at temperatures below 150 K. This may explain why acceptor levels with activation energies around 30 meV have not been detected in the past with the used experimental techniques.…”

“…The activation energy of the acceptor level related to the magnesium atoms has been measured by various experimental techniques leading to values between 70 and 250 meV, [2][3][4][5][6][7][8][9][10][11][12][13][14] which is much larger than for the impurities generally used as shallow dopants. As a consequence, Mg is not fully ionized at any temperature, and the resulting hole concentration is strongly temperature dependent.…”

“…[2][3][4][15][16][17][18][19] One interest of electrical studies is to confirm the presence of the Mg dopants and their electrical activity, and to deduce microscopic parameters such as ionization energies, capture cross sections, spatial concentration profiles, etc. Another interest is to contribute to the basic understanding of the conduction mechanism in these structures as the performances of the various electronic and optoelectronic devices depend on the efficiency of the current injection and of the carrier generation or carrier recombination.…”

Electrical Characterization of InGaN/GaN Multiple‐Quantum‐Well Structures by Thermal Admittance Spectroscopy

“…Activation energies related to defect levels can be obtained by a classical interpretation of TAS data [6,7]. Although one should be careful when using such a method [8], this type of analysis has been applied to QW structures, either to assess the presence of defects [9] or to determine QW parameters [10].…”