We present a deep-ultraviolet semiconductor laser diode that operates under current injection at room temperature and at a very short wavelength. The laser structure was grown on the (0001) face of a single-crystal aluminum nitride substrate. The measured lasing wavelength was 271.8 nm with a pulsed duration of 50 ns and a repetition frequency of 2 kHz. A polarization-induced doping cladding layer was employed to achieve hole conductivity and injection without intentional impurity doping. Even with this undoped layer, we were still able to achieve a low operation voltage of 13.8 V at a lasing threshold current of 0.4 A.

https://iopscience.iop.org/article/10.7567/1882-0786/ab50e0/pdf

A 271.8 nm deep-ultraviolet laser diode for room temperature operation

Solar-blind ultraviolet (UV) photodetectors (PDs) have attracted tremendous attention in the environmental, industrial, military, and biological fields. As a representative III-nitride material, AlGaN alloys have broad development prospects in the field of solar-blind detection due to their superior properties, such as tunable wide bandgaps for intrinsic UV detection. In recent decades, a variety of AlGaN-based PDs have been developed to achieve high-precision solar-blind UV detection. As integrated optoelectronic technology advances, AlGaN-based focal plane arrays (FPAs) are manufactured and exhibit outstanding solar-blind imaging capability. Considering the rapid development of AlGaN detection techniques, this paper comprehensively reviews the progress on AlGaN-based solar-blind UV PDs and FPAs. First, the basic physical properties of AlGaN are presented. The epitaxy and p-type doping problems of AlGaN alloys are then discussed. Diverse PDs, including photoconductors and Schottky, metal–semiconductor–metal (MSM), p-i-n, and avalanche photodiodes (APDs), are demonstrated, and the physical mechanisms are analyzed to improve device performance. Additionally, this paper summarizes imaging technologies used with AlGaN FPAs in recent years. Benefiting from the development of AlGaN materials and optoelectronic devices, solar-blind UV detection technology is greeted with significant revolutions. Summarizing recent advances in the processing and properties of AlGaN-based solar-blind UV photodetectors and focal plane arrays as well as AlGaN growth and doping techniques.

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Progress on AlGaN-based solar-blind ultraviolet photodetectors and focal plane arrays.

Polarization-induced nanowire light emitting diodes (PINLEDs) are fabricated by grading the Al composition along the c-direction of AlGaN nanowires grown on Si substrates by plasma-assisted molecular beam epitaxy (PAMBE). Polarization-induced charge develops with a sign that depends on the direction of the Al composition gradient with respect to the [0001] direction. By grading from GaN to AlN then back to GaN, a polarization-induced p–n junction is formed. The orientation of the p-type and n-type sections depends on the material polarity of the nanowire (i.e., Ga-face or N-face). Ga-face material results in an n-type base and a p-type top, while N-face results in the opposite. The present work examines the polarity of catalyst-free nanowires using multiple methods: scanning transmission electron microscopy (STEM), selective etching, conductive atomic force microscopy (C-AFM), and electroluminescence (EL) spectroscopy. Selective etching and STEM measurements taken in annular bright field (ABF) mode demons...

Mixed Polarity in Polarization-Induced p–n Junction Nanowire Light-Emitting Diodes

Polarization induced hole doping on the order of ∼1018 cm−3 is achieved in linearly graded AlxGa1−xN (x = 0.7 ∼ 1) layer grown by molecular beam epitaxy. Graded AlxGa1−xN and conventional Al0.7Ga0.3N layers grown on AlN are beryllium (Be) doped via epitaxial growth. The hole concentration in graded AlxGa1−xN:Be (x = 0.7 ∼ 1) layers demonstrates that polarization generates hole charges from Be dopant. The Al0.7Ga0.3N layer is not conductive owing to the absence of carriers generated from the Be dopant without the inducement of polarization. Polarization doping provides an approach to high efficiency p-type doping in high Al composition AlGaN.

Polarization induced hole doping in graded AlxGa1−xN (x = 0.7 ∼ 1) layer grown by molecular beam epitaxy

https://iopscience.iop.org/article/10.1149/2.0111704jss/pdf

Review—Group III-Nitride-Based Ultraviolet Light-Emitting Diodes: Ways of Increasing External Quantum Efficiency

We propose a type of pn-junction not formed by impurity-doping, but rather by grading the Al composition in an AlxGa1−xN thin film, resulting in alternating p and n conducting regions due to polarization charge. By linearly grading AlxGa1−xN from 0% to x (x ≤ 30%) and back to 0% Al, a polarization induced pn-junction is formed, even in the absence of any impurity doping. X-ray diffraction reciprocal space maps are used to determine the strain state of the different graded composition samples. Polarization induced doping also provides a solution to the problem of p-type doping efficiency for III-nitrides.

Polarization induced pn-junction without dopant in graded AlGaN coherently strained on GaN

High electron sheet concentrations of ∼1015 cm−2 result from polarization doping with compositionally graded AlGaN films grown on unintentionally doped GaN templates which exhibit background electron concentrations of ∼1016 cm−3. Similar graded films grown on semi-insulating (SI), free standing GaN substrates exhibited carrier concentrations two orders less. Transport studies of the as-grown materials using temperature dependent Hall effect revealed a very weak temperature dependence of the carrier concentration and mobility as compared to traditionally doped films using Si as a dopant. And qualitative modeling of the electron mobility indicates that alloy scattering and charged dislocation scattering are the most significant contributors to limiting the mobility over the entire temperature range.

Polarization doping: Reservoir effects of the substrate in AlGaN graded layers

Here, we report on initial experiments to use the space charge of the changing internal polarization field from graded AlGaN films in a simple p-n junction device. Our devices are fabricated from films which are graded from GaN to AlGaN then reverse graded back to GaN without the intentional addition of impurity dopants. Structural characterization of the films is reported through X-Ray diffraction, reciprocal space maps, and temperature dependent I-V measurements are used to demonstrate the rectifying behaviour of the device (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Paul Minor

Papers

Polarization induced pn-junction without dopant in graded AlGaN coherently strained on GaN

Polarization doping: Reservoir effects of the substrate in AlGaN graded layers

Polarization induced doping in graded AlGaN films

Polarization induced pn-junction without dopant in graded AlGaN coherently strained on GaN (vol 101, 122103, 2012)

Erratum: ``Polarization induced pn-junction without dopant in graded AlGaN coherently strained on GaN'' [Appl. Phys. Lett. 101, 122103 (2012)]