AlGaN ultraviolet photoconductors grown on sapphire

Walker, D.; Zhang, X.; Kung, Patrick; Saxler, A.; Javadpour, Sirus; Xu, Jiao; Razeghi, Manijeh

doi:10.1063/1.115597

Cited by 177 publications

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“…Theoretical evidence for the semi-insulating character of AlN Antonella Fara, (1) Fabio Bernardini, (1) and Vincenzo Fiorentini (1,2) (1) Istituto Nazionale per la Fisica della Materia and Dipartimento di Fisica, Università di Cagliari, Italy (2) Walter Schottky Institut, Technische Universität München, 85748 Garching, Germany (April 5, 2018)…”

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“…Doping is still a central problem in this area, especially as recent developments tend to focus on heterojunction systems, and hence on alloys with large In and Al content. Doping of either kind has been shown to be increasingly difficult as the AlN fraction increases in Al x Ga 1−x N alloys [1]. On the other hand, theoretical studies of doping have been much less detailed for AlN [2][3][4] than for GaN [2,[5][6][7][8], and practically null for InN.…”

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Theoretical evidence for the semi-insulating character of AlN

Fara

Bernardini

Fiorentini

1999

Journal of Applied Physics

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We present ab initio density-functional calculations for acceptors, donors, and native defects in aluminum nitride, showing that acceptors are deeper (Be ∼ 0.25 eV, Mg ∼ 0.45 eV) and less soluble than in GaN; at further variance with GaN, both the extrinsic donors Si Al and C Al , and the native donor VN (the anion vacancy) are found to be deep (about 1 to 3 eV below the conduction). We thus predict that doped AlN will generally turn out to be semi-insulating in the normally achieved Al-rich conditions, in agreement with the known doping difficulties of high-x AlxGa1−xN alloys. [2,[5][6][7][8], and practically null for InN. Here we contribute to the discussion the results of accurate first principle density functional theory calculations for acceptors, donors, and vacancies in wurtzite AlN. We find that typical candidate acceptors, namely cation-substituting Be and Mg, are moderately but sizably deeper than in GaN. More interestingly, donors known to be shallow in GaN (Si, C, and the nitrogen vacancy) are in fact deep in AlN in their substitutional configuration. Our results are in general agreement with previous results, when comparable. An exception is the behavior of donors, which is at partial variance with previous studies [2,3], but seems to agree with experimental data [1] indicating serious ntype doping difficulties for AlN and high-x Al x Ga 1−x N alloys.Method -We use local-density-functional-theory (LDA) [9] ultrasoft-pseudopotential plane-wave calculations of total energies and forces in doped AlN wurtzite supercells typically encompassing 32 atoms, and with plane wave cutoff of 25 Ry (for further technical details see Refs. [6,7,10]), to predict from first principles the formation energies and thermal ionization energies of cation-substituting Be, Mg, Si, and C, and of the N and Al vacancies in AlN. The carriers concentration at temperature T due to an impurity or defect with thermal ionization energy ǫ, and a formation energy E form , isasssuming impurity incorporation or defect creation in thermal equilibrium at a growth temperature of T g , and with N s =2.44×10 22 cm −3 available cation or anion sites (i.e. half the theoretical atomic density of AlN). Thus, the largest the formation and ionization energies, the less efficient the doping. A non-zero formation entropy (neglected here) will of course enhance the dopant concentration. The formation energy for an impurity in charge state Q is (2) with µ e the electron chemical potential (synonimous with the Fermi energy E F in our T=0 calculations), E tot (Q) the total energy of the fully-relaxed defected supercell in charge state Q, E Q v its top valence band energy, n X and µ X the number of atoms of the involved species (X=Al, N, impurity) and their chemical potentials. The latter potentials are determined by the equilibrium conditions with AlN and the compounds (if any) of the specific impurity with Al or N. The structures and formation enthalpies of the solubility limiting compounds (Al 2 O 3 , Si 3 N 4 , Be 3 N 2 , Mg 3 N 2 , and d−C) are calc...

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Theoretical evidence for the semi-insulating character of AlN

Fara

Bernardini

Fiorentini

1999

Journal of Applied Physics

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“…Wide bandgap semiconductors are able to withstand the harsh environment and high temperature, thus, AlGaN/GaN devices could find many applications in such fields as: the military, aerospace, automotive and petroleum industries, engine monitoring, flame detection, and solar UV detection [7,8]. However, the thermal stability of AlGaN/GaN heterostructures and their chemical inertness also engender difficulties in ohmic contact formation.…”

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Study of interface reactions between Ti/Al/Ni/Au metallization and AlGaN/GaN heterostructures

Macherzyński

Paszkiewicz

2013

Open Physics

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Abstract:The electrical characteristics, and the range of interface metal-semiconductor reactions of Ti/Al/Ni/Au metallization with AlGaN/GaN heterostructures at various annealing temperatures ranging from 715 • C to 865 • C, have been investigated. The relation between the depth of the interface solid state reaction and the current-voltage (I-V) characteristics of the ohmic contact, have been studied. It was observed, that the transition from nonlinear to linear I-V behaviour occurred after the annealing at 805 • C. The structural changes in AlGaN/GaN heterostructures beneath the metallic contact after the thermal treatment, were investigated. After removing the metallization by etching, the atomic force microscope profiles and scanning electron microscope images, were studied to define the depth to which the interfacial solid state reactions between the metallization and the semiconductor structure take place. It was observed, that the changes in the heterostructures, caused by the interface m-s reactions, were observed up to a depth of 180 nm at 865 • C. In the worst case, this could result in the complete removal of the two-dimensional electron gas under the metallization of the ohmic contacts. To study the influence of the annealing process parameters on the properties of the two-dimensional electron gas, the van der Pauw Hall mobility measurement was performed.PACS (

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“…Since the first demonstration of solar-blind Al x Ga 1−x N photoconductors [1,2], AlGaN-based ultra-violet (UV) photodetectors with cut-off wavelengths smaller than 280 nm have proved their potential for solar-blind detection. They can be utilized in a number of civil and military applications.…”

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High-speed characterization of solar-blind Al_xGa₁_xN p–i–n photodiodes

Bıyıklı

Kimukin

Tut

et al. 2004

Semicond. Sci. Technol.

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We report on the temporal pulse response measurements of solar-blind Al x Ga 1−x N-based heterojunction p-i-n photodiodes. High-speed characterization of the fabricated photodiodes was carried out at 267 nm. The bandwidth performance was enhanced by an order of magnitude with the removal of the absorbing p+ GaN cap layer. 30 µm diameter devices exhibited pulse responses with ∼70 ps pulse width and a corresponding 3 dB bandwidth of 1.65 GHz.Since the first demonstration of solar-blind Al x Ga 1−x N photoconductors [1, 2], AlGaN-based ultra-violet (UV) photodetectors with cut-off wavelengths smaller than 280 nm have proved their potential for solar-blind detection. They can be utilized in a number of civil and military applications. Certain applications including missile warning and tracking systems or secure UV optical communication systems for space-to-space (inter-satellite) communication need highspeed solar-blind photodetectors [3]. To date, the fastest solar-blind AlGaN p-i-n detectors reported had ns-scale decay time constants [4,5]. In these AlGaN p-i-n detector reports, authors claim that the pulse response of their devices is limited by two factors: RC time constant and trapping mechanism (proportional to dislocation density). Recently we have reported very fast AlGaN solar-blind detectors with Schottky and MSM device structures, exhibiting GHz bandwidths [6,7]. In this paper, we report solar-blind AlGaN p-i-n photodiodes with high-speed performance in the GHz regime.The AlGaN p-i-n photodiode wafer was grown by MOCVD on sapphire substrate. The growth process started with an AlN buffer layer, followed by the growth of a 250 nm thick n-type doped GaN ohmic contact layer. Subsequently, a 100 nm thick unintentionally doped Al 0.45 Ga 0.55 N layer was grown as the active detector region. Finally, the 55 nm thick p+ ohmic contact layer was formed by a three step growth: a p+ Al 0.45 Ga 0.55 N layer, a p+ grading layer and a p+ GaN cap layer. The p-type doped grading and cap layers were grown in order to reduce carrier trapping at the AlGaN/GaN interface and to improve the p+ ohmic contact quality, respectively.The solar-blind devices were fabricated using a microwave compatible fabrication process. Figure 1 shows the cross-sectional device structure of the AlGaN p-i-n photodiode. Ohmic contacts were formed by annealed Ni/Au and Ti/Al alloys for p+ and n+ contacts, respectively. CCl 2 F 2 -based reactive ion etching (RIE) process was utilized for n+ ohmic and mesa isolation etch steps. The p-i-n detectors were passivated with a ∼120 nm thick Si 3 N 4 layer, grown by plasmaenhanced-chemical-vapour-deposition (PECVD). To make contact to the devices, thick (∼0.6 µm) interconnect metal pads were deposited in the final step of fabrication process. Microphotographs of completed AlGaN p-i-n photodiode samples captured by optical microscope and scanning electron microscope (SEM) are shown in figure 2.On-wafer dc and microwave measurements of the fabricated devices were carried out. To observe the effect of abs...

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AlGaN ultraviolet photoconductors grown on sapphire

Cited by 177 publications

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Theoretical evidence for the semi-insulating character of AlN

Theoretical evidence for the semi-insulating character of AlN

Study of interface reactions between Ti/Al/Ni/Au metallization and AlGaN/GaN heterostructures

High-speed characterization of solar-blind Al_xGa₁_xN p–i–n photodiodes

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AlGaN ultraviolet photoconductors grown on sapphire

Cited by 177 publications

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Theoretical evidence for the semi-insulating character of AlN

Theoretical evidence for the semi-insulating character of AlN

Study of interface reactions between Ti/Al/Ni/Au metallization and AlGaN/GaN heterostructures

High-speed characterization of solar-blind AlxGa1 xN p–i–n photodiodes

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High-speed characterization of solar-blind Al_xGa₁_xN p–i–n photodiodes